TiO2-based Nanomaterials with Photocatalytic Properties for the Advanced Degradation of Xenobiotic...

TiO2-based Nanomaterials with Photocatalytic Propertiesfor the Advanced Degradation of Xenobiotic Compoundsfrom Water. A Literature Survey

Mălina Răileanu & Maria Crişan & Ines Niţoi &Adelina Ianculescu & Petruţa Oancea &

Dorel Crişan & Ligia Todan

Received: 16 November 2012 /Accepted: 28 March 2013# Springer Science+Business Media Dordrecht 2013

Abstract In recent years, the photochemistry ofnano-semiconductor particles has been one of thefastest growing research areas in the physicalchemistry field. TiO2 is considered as the most thor-oughly investigated semiconductor in the literature, dueto its photocatalytic activity, excellent functionality,thermal stability, and non-toxicity. It seems to be the

most promising for the photocatalytic destruction oforganic pollutants. The challenge for scientific materialsis to find a processing method in which the crystallinephase as well as the size and morphology of TiO2

nanocrystals can be controlled. The concept of the pres-ent paper consists of a comprehensive study regardingthe level of knowledge in the synthesis of TiO2-basednanopowders and their application in the advanced deg-radation of aromatic nitrocompounds. The objectivesare related to: critical analysis of the synthesis tech-niques of the TiO2-based nanopowders, underliningthe importance of using the sol–gel method evaluationof the morphological and structural specific characteri-zation of these techniques; and a comprehensive studyof the operational parameters of the pollutant photocat-alytic degradation. The relative simple sol–gel method isthe most widely used, being considered as a versatilemeans of developing catalytic materials, as well as animportant experimental tool in understanding theirphysical and chemical properties. In order to enhanceTiO2 photocatalysis and to extend the response into thevisible domain, titanium has been doped with metals,nonmetals, and ionic components. A recent literaturesurvey concerning some transition metals-doping (Fe,Co, and Ni) of TiO2 nanopowders by the sol–gel methodwas also included.

Keywords Titanium dioxide . Fe- . Co- . Ni-dopedTiO2

. Nanopowders . Sol–gel process . Photocatalysis .

Xenobiotic compounds

Water Air Soil Pollut (2013) 224:1548DOI 10.1007/s11270-013-1548-7

M. Răileanu :M. Crişan (*) :D. Crişan : L. TodanIlie Murgulescu Institute of Physical Chemistry,Romanian Academy,202 Splaiul Independentei,060021 Bucharest, Romaniae-mail: [email protected]

M. Crişane-mail: [email protected]

I. NiţoiNational Research and Development Institutefor Industrial Ecology, ECOIND,71-73, Drumul Podu Dâmboviţei Street,060652 Bucharest, Romania

A. IanculescuDepartment of Oxide Materials Science and Engineering,“Politehnica” University of Bucharest,1-7 Gh. Polizu, P.O. Box 12–134, 011061 Bucharest,Romania

P. OanceaDepartment of Physical Chemistry, Faculty of Chemistry,University of Bucharest,4-12 Bd. Regina Elisabeta,Bucharest 030016, Romania

1 Introduction

1.1 General Overview

In the early 20th century, the production of carbon blackand fumed silica (obtained by flame hydrolysis ofsilicium tetrachloride) in the 1940s exemplified nano-particle technology much earlier before the currentnanotech fervor. This is because the earlier nanotech-nology was pursued empirically — the materials werediscovered, not designed. In recent years, nanomaterialshave been a focus of nanoscience and nanotechnology,representing an ever-growing multidisciplinary field ofstudy attracting tremendous interest, investment andeffort in research and development around the world.Nanoscale is fascinating because it is on this scale thatatoms and molecules interact and assemble into struc-tures that possess unique properties, which are depen-dent on the size of the particles. It is at this scale thatmolecular interactions, processes, and phenomena canbe controlled and directed to form the desired geome-tries of the materials building blocks with desirableproperties.

What makes nanoscale building blocks interesting isthat by controlling the size in the range of 1–100 nm andthe assembly of such constituents, one could alter andprescribe the properties of the assembled nanostructures.One might consider the nanoscale as the last “size” fron-tier for materials science. As Professor Roald Hoffmann,the Chemistry Nobel laureate puts it, "Nanotechnology isthe way of ingeniously controlling the building of smalland large structures, with intricate properties; it is the wayof the future, with incidentally, environmental benignnessbuilt in by design" (Lu and Zhao 2004).

Nanostructured materials may possess nanoscalecrystallites, long-range ordered or disordered struc-tures or pores spaces. Nanomaterials can be designedand tailor-made at the molecular level to have desiredfunctionalities and properties. Manipulating matter atsuch a small scale with precise control of its propertiesis one of the hallmarks of nanotechnology.

Most of the research in the broad field of nanoscienceis dedicated to the development of synthesis routes tonanoparticles and nanostructures. These efforts gaveaccess to nanomaterials with a wide range of composi-tions, monodisperse crystallite sizes, unprecedentedcrystallite shapes, and with complex assembly proper-ties (Djerdj et al. 2008). The development of high-speedcomputing (and hence modeling), of advanced

characterization techniques (such as atomic force mi-croscopy and scanning tunneling microscopy) and ofsynthesis routes (such as sol–gel processing) made pos-sible to design nanomaterials. Nanoparticles andnanomaterials represent an evolving technology thathas the potential to have an impact on an incrediblywide number of industries and markets. There are manynovel properties and applications of nanoparticles dem-onstrated: from catalysis, environmental remediation,biomedical to information displays, and electronics.

It is evident that the emergence of nanotechnologyis bound to have significant implications in the re-search field of photocatalysis. The nanostructured ma-terials and their remarkable properties will proveinvaluable in the development of novel photocatalystsand the enhancement of existing ones.

It is well known that among the various photocatalysts,titanium occupies a very important place, due to its highphotocatalytic activity, excellent functionality, high chem-ical stability, thermal stability, non-toxicity and inexpen-siveness. Its applications can be roughly divided into“energy” and “environmental” categories, many of whichdepend on the property interactions of TiO2materials withthe environment (Chen and Mao 2007).

All the mentioned properties are improved in thecase of nanostructured TiO2, which can be obtained bythe sol–gel method. An advantage of nanometer-sizedmaterials stems from the changes in electrochemicalpotentials of the photogenerated charge carriers thataccompany decreasing particle size. The sol–gel meth-od facilitates the preparation of the photocatalyst inboth forms (powders and coatings). Moreover, it isvery convenient for producing nanocomposites mate-rials in which different phases could be highly dis-persed in an inorganic matrix, that is for the doping oftitanium with different metal or non-metal ions.

Recently, titanium dioxide has been extensively usedfor the decomposition of environmental pollutants as apossible alternative to conventional water treatmenttechnologies. In order to clean the water from chemical-ly stable synthetic organic compounds, so-called ad-vanced oxidation processes (AOPs) have beendeveloped. Philippopoulos and Nikolaki (2010) consid-er the AOPs as a promising technology for the treatmentof wastewaters which contain non-easily removableorganic compounds. Photocatalysis has attracted muchattention as "an environmentally benign catalyst" be-cause photocatalysts possess a potential to oxidizeorganic compounds into nontoxic CO2 and H2O,

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decompose NOx, and reduce CO2 under UV light irra-diation. Hence, the photocatalytic system is often repre-sented as "an artificial photosynthesis" (Takeuchi et al.2012).

TiO2 photocatalysis belongs to AOP processes thatuse energy to produce highly reactive intermediates ofhigh oxidizing or reducing potential, which destroy thetarget compounds (Černigoj et al. 2006; Sobczyński andDobosz 2001). The reason for the increased interest inthis method is that the process can be carried out underambient conditions and may lead to total mineralizationof organic carbon to CO2 (Kiriakidou et al. 1999).

Organic chemicals which may be found as pollutantsin wastewater effluents from industrial or domesticsources must be removed or destroyed before dischargeto the environment. Such pollutants may also be found inground and surface waters which also require treatmentto achieve acceptable drinking water quality. The in-creased public concern with these environmental pollut-ants has prompted the need to develop novel treatmentmethods with photocatalysis gaining a lot of attention inthe field of pollutant degradation (Beydoun et al. 1999).Xenobiotics (nitroaromatic compounds) were identifiedin wastewater resulting from industrial activities, like:organic and inorganic chemicals, plastics, explosives,leather tanning, petroleum refining, nonferrous metals,pulp and paper, auto and other laundries and pesticidesmanufacture (Contreras et al. 2001; Lei et al. 2006;Rodriguez et al. 2002).

1.2 Objectives

The concept of this paper consists of a comprehensivestudy regarding the level of knowledge in the synthesisof TiO2-based nanopowders and their application in theadvanced degradation of aromatic nitrocompounds. Theobjectives are related to: critical analysis of the synthesistechniques of the TiO2-based nanopowders, underliningthe importance of using the sol–gel method, evaluationof the morphological and structural specific characteri-zation techniques of these powders, and a comprehen-sive study of the operational parameters of the pollutantphotocatalytic degradation.

2 Study of Un-doped and Doped TiO2 Systems

Titanium represents one of the most studied inorganiccompounds in chemistry. Its preparation, physical and

chemical properties together with its applications forall crystallographic forms (anatase, rutile and brookite)have been the object of numerous studies. The litera-ture supplies thousands of articles dedicated to TiO2,which include several reviews (Akpan and Hameed2010; Chen and Mao 2007; Diebold 2003; Fujishimaet al. 2000; Han et al. 2009; Hashimoto et al. 2005;Herrmann 2005; Hoffmann et al. 1995; Linsebigler etal. 1995; Litter 2005; Nakata and Fujishima 2012;Yoshida and Watanabe 2005) and books (Fujishimaet al. 1999; Herrmann 1999; Kaneko and Okura 2003;Ollis et al. 1989; Watanabe et al. 1993). It is wellknown that among the various photocatalysts, titaniumoccupies a very important place, due to its high pho-tocatalytic activity, excellent functionality, high chem-ical stability, thermal stability and non-toxicity. Thisaccounts for its importance in the field.

Interest in TiO2-based photocatalysis has been re-markable since Fujishima and Honda’s first reports inthe early 1970’s of UV-induced redox chemistryon TiO2 (Fujishima and Honda 1972). Recently,Fujishima et al. (2008) highlighted the astonishingnumber of publications involving heterogeneous pho-tochemical studies (in general), specifically those in-volving TiO2. Of their 2008 Surface Science Reportsreview, the number of publications has increased dra-matically over the last decade to the extent that of the~2,400 heterogeneous photochemistry papers pub-lished in 2008, roughly 80 % involved TiO2-basedmaterials. This remarkable level of publication activityreflects the potential for new applications emergingfrom research in this field. Numerous reviews existon the topics of heterogeneous photochemistry, TiO2-based photochemistry that blend in chemistry, physicsand engineering perspectives to the field. There arereviews involving the role of TiO2-based materials insuch subjects as photocatalytic water splitting andhydrogen production, photoelectrochemistry, dye sen-sitization and solar energy conversion, reactor designand process kinetics, and photochemical air and watertreatments (Fujishima et al. 2000; Henderson 2011;Pelaez et al. 2012).

2.1 Photocatalytic Effect

When TiO2 is irradiated by UV rays, pairs of electricalcharges–holes in the valency band and electrons in theconductivity band are created. The holes react withwater molecules or with the hydroxyl ions and

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hydroxyl radicals are formed, which are strong oxi-dants of the organic molecules (Trapalis et al. 2003).

In principle, a photocatalytic reaction may proceedon the surface of TiO2 powders via several steps:

(a) Production of electron–hole pairs, photogeneratedby exciting semiconductor with light energy

(b) Separation of electrons and holes by trap avail-able on the TiO2 surface

(c) A redox process induced by the separated electronsand holes with the adsorbants present on the surface

(d) Desorption of the products and reconstruction ofthe surface

It has been shown that the photocatalytic activity ofTiO2 is influenced by the crystal structure (anatase,rutile), surface area, size distribution, porosity, surfacehydroxyl group density, etc. (Lee et al. 1993). Thesecharacteristics influence the production of electron–holepairs, the surface adsorption–desorption and the redoxprocess. Electron–hole recombination is in direct com-petition with the trapping process (step b). The rate oftrapping and the photocatalytic activity of TiO2 will beenhanced by retarding the electron–hole recombination.The principal method of slowing electron–hole recom-bination consists of loading metals onto the surface ofthe TiO2 particles. The mechanisms of photocatalysisare discussed in recent reviews (Bhatkhande et al. 2001;Fox and Dulay 1993; Herrmann 2005; Hoffmann et al.1995; Litter 1999; Mills et al. 1993; Mills and Le Hunte1997; Stasinakis 2008).

A simplified mechanism for the photo-activationof a semiconductor catalyst is presented in Fig. 1(Herrmann 2005).

A model for photocatalytic mechanism is presentedbelow:

TiO2 + hν → e− + h+ 1. Photo-generation electron/holepairs

e−+h+→TiO2+(energy)M+e−→M(e−)

2. Electron–hole recombination.Metal attracts free electron fromTiO2 conduction band, slowsrecombination and promotesradical formation

h+ + H2O → OH• + H+ 3. Formation of radicals(symbol Ox)e− + O2 → O2

−•

O2−• + H+ → HO2•

TOC + Ox → TOC(partially oxidized species) +CO2 + H2O

4. Radical oxidation of organiccompounds

Titanium dioxide is a photoactive semiconductor.When it is illuminated with UV light, an electron fromits valence band is promoted to its conduction band,generating an electron deficiency or a hole in its valenceband, thus overloading its conduction band. The con-duction band in crystalline TiO2 is composed of unoc-cupied titanium 3d orbitals while the valence band isformed by filled oxygen 2p orbitals. It is confirmed thatthe oxygen vacancies can be treated as electron donors,which determine the n-type conductivity. Addition ofthe metal islands on the material surface is an importantway of improving the photocatalytic activity. When twomaterials (the pair TiO2 semiconductor and metal) areelectrically connected, electron migration from thesemiconductor to metal (that has a higher work functionthan the semiconductor), occurs until the two Fermilevels are aligned. In the same time the thermodynamicequilibrium is reached. The semiconductor–metal junc-tion is called Schottky barrier. It can serve as an efficient

Fig. 1 Mechanism forthe photo-activation of asemiconductor catalyst


electron trap preventing electron–hole recombinationin photocatalysis (Linsebigler et al. 1995; Woan etal. 2009). In the nanocrystalline porous TiO2 theoxygen adsorption has a great influence on photo-conductivity (Brajsa et al. 2004). The generated elec-tron–hole pairs react with water or oxygen toproduce radicals (Ox) in the surface of the semicon-ductor. As the organic species are adsorbed anddesorbed on the surface of the catalyst, the formedradicals readily react with the adsorbed species, oxi-dizing them and creating CO2 and H2O and partiallyoxidized species.

The below reaction describes the process for theimplementation of semiconductor photocatalysis inthe degradation of organic pollutants:

Studying the mechanism implied in this reaction,Philippopoulos and Nikolaki (2010) have given themost important steps of the process, as follows:

(a) Charge carrier generation (electrons–holes)

TiO2 þ hn ! hþ þ e�

(b) Charge carrier trapping

hþ þ TiIVOH ! TiIVOH�þ

e� þ TiIVOH ! TiIIIOHe� þ TiIV ! TiIII

(c) Charge carrier recombination

e� þ TiIVOH�þ ! TiIVOHhþ þ TiIIIOH ! TiIVOH

(d) Charge transfer to the interfacial region

e� þ TiIVOH�þ þ P ! TiIVOHþ P*

TiIIIOHþ O2 ! TiIVOHþ O2��

where P is the organic pollutant and P* representsthe oxidized form of P. Philippopoulos andNikolaki (2010) noted that the exact mechanismof the process, as well as the role of each compo-nent in the reaction still remain a research field.

2.2 Dopants

Compared to other photocatalysts, TiO2 is the mostpromising material, due to its specific properties:

– High reactivity under photon energy (l=300–390 nm)

– High chemical and biological inertness, whereasother catalysts (CdS, GaP) are degraded into sec-ondary toxic compounds

– Thermal stability and strong mechanical properties– Low cost

Despite titanium dioxide being the most promisingphotocatalyst, it can only be activated by light ofwavelength 390 nm or lower. The UV region (below390 nm) constitutes only ~4 % of the energy availablewithin the solar spectrum. Thus considerable researchis being carried out to extend this photocatalyst’sresponse into the visible part of the solar spectrum.The sensitization of TiO2 with a second component toenhance activity and shift the wavelength of irradia-tion into the visible region is a goal that has beeneagerly pursued (Beydoun et al. 1999).

Dye sensitization is an alternative method in whichdye (sensitizer) adsorbed on TiO2 surface gets excitedby adsorbing visible light and effects charge transitionat sub-band-gap excitation to permit photocatalyticprocess (Chatterjee and Mahata 2001). In the presenceof visible light, surface adsorbed 8-hydroxyquinoline(HOQ) acts as a sensitizer in the photocatalytic elec-tron transfer process (Chatterjee and Bhattacharya1999). Dye sensitization in photoelectrochemical sys-tems has been extensively explored, but cannot beused for detoxification of waste waters since the dyemolecules are also degradeable (Zhang et al. 2000).

Another disadvantage of titanium dioxide is that theelectron–hole (charge carrier) recombination takes placein a time span of several nanoseconds and in the absenceof the promoters (e.g., Pt or RuO2) the catalytic activitydecreases. Depositing or incorporating metal ion dop-ants into the titanium dioxide particles can influence theperformance of these photocatalysts. This affects thedynamics of electron–hole recombination and interfa-cial charge transfer. The largest enhancement ofphotoactivity through doping was found in nanosizedparticles, in which the dopant ions are located within 1–2 nm of the surface (Beydoun et al. 1999). Moreover, itis very convenient to produce nanocomposites materialsin which different phases could be highly dispersed inan inorganic matrix, that is, to doped titanium withdifferent metal or non-metal ions. Recent studies (Choiet al. 1994; Gupta and Tripathi 2011; Hashimoto et al.2005; Herrmann 2005; Lu et al. 2012a, 2012b; Nie et al.

Organic pollutant O2TiO2, hv Eg CO2 H2O

inorganic matter


2009; Tan et al. 2011) pointed out the doping procedureas a possibility to enhance the efficiency of the photo-catalytic process. The dopant ions can behave as bothhole and electron traps or they can mediate interfacialcharge transfer (Choi et al. 1994). Also known as “im-purity doping”, the procedure ensures the extension ofthe spectral response of a wide band gap semiconductorto visible light. An advantage of using the sol–gel meth-od is the ability to control in a simple way the concen-tration of the dopant in the nanostructure of titaniumdioxide. An enhancement of photocatalytic activity ofthe noble metal (Pt, Pd, Au, Ag, etc.) modified TiO2 hasbeen explained in terms of a photoelectrochemicalmechanism in which the electrons generated by UVirradiation on the TiO2 semiconductor transfer to theloaded metal particles, while the holes remain in thesemiconductor, resulting in a decrease in the electron–hole recombination (Zhang et al. 2001). Choi et al.(1994) have conducted an in-depth study of the dopingeffect of 21 kinds of transitional metal ions on nano-crystalline TiO2. The dependence of photocatalytic ac-tivity versus the concentration of metal ions doping hasbeen established. It can be found that there is an opti-mum dopant concentration for the best photocatalyticperformance of the doped-TiO2 material. Above thisvalue, the metal dopants can act as electron–hole recom-bination centers which are detrimental to the photocat-alytic activity. Highly dispersed nanoparticles of noblemetals, such as Pt, Pd, Rh, Ru and Au, in mesoporoussupports as titanium, alumina and silica are widely usedas catalysts in organic synthesis, petrochemistry, etc. Ina study concerning the photocatalytic activity of noble-metal-loaded TiO2, Ranjit et al. (1996) suggest that anohmic contact is formed between the metal and semi-conductor. Hence electrons can easily flow to the metalsites on TiO2 under irradiation and the role of the metalis to act as an electron sink and thus to enhance theactivity. However, it is difficult to introduce metalnanoparticles into mesopores by traditional impregna-tion methods, because they tend to richly deposit onouter surface of mesoporous materials and moreover itis difficult to control the loading amount by impregna-tion (Yuan et al. 2006). The sol–gel method easilyallows the preparation of nanocomposite materials suchas inorganic matrices in which a metal phase could behighly dispersed.

Different dopants may not have the same effect ontrapping electrons and/or holes on the surface orduring interface charge transfer because of the

different positions of the dopant in the host lattice.Consequently, the photocatalytic efficiency would bedifferent for different types of dopants (Shah et al.2002). Selection of the dopants depends on the reactionof interest. Not all dopants work efficiently for all re-actions. For example, Fe3+ works well for the catalysisof CHCl3, but not for 2-chlorophenol (Burns et al.2002). The dopants hinder the crystalline structure for-mation, leading to densification and non-crystallinestructures as the dopant occupies and completesunsatisfied chemical and mechanical bonds.

3 Synthesis Techniques of the TiO2-BasedNanopowders

In their applications, TiO2 is present both as amor-phous and crystalline materials (as anatase, brookiteand rutile). Titanium dioxide in all its forms is one ofthe suitable industrial or engineering materials in ourdaily lives, e.g., as a white pigment for paints, cos-metics, drugs and foodstuffs. The anatase phase (witha body centered tetragonal structure) of TiO2 is pre-ferred in various applications such as photocatalysts,gas sensors, solar cells and electrochemical devices.The rutile phase of TiO2 (simple tetragonal) has foundapplications in capacitors, filters, power circuits andcondensers because of its high dielectric constant. Therutile polymorph is also preferred for cromophoreaddition. The brookite phase is considered an interest-ing candidate in photocatalytic applications but it isvery difficult to be prepared.

The photocatalytic behavior of TiO2-basednanopowders depends on their crystal structure, latticeparameters, lattice defects, internal strains, specificsurface area and size and morphology of particle.The increase of the surface area or decrease of thedimension of primary particles can improve the TiO2

performance in the most of its applications. The selec-tion of synthesis technique constitutes an importantfactor for the efficiency of the nanopowders in thephotocatalysis process. A lot of methods can be usedin synthesis of these materials. Remarkable progresshas been made in the last three decades in the scienceof processing ceramic materials as a result of theincreasing use of “wet chemistry” (Brinker andScherrer 1990; Corriu and Anh 2009; Jolivet et al.2000; Lee and Gouma 2012; Livage et al. 1988;Pierre 1992; Wright and Sommerdijk 2001).


3.1 Synthesis by Solid State Reactions

Solid state reactions are used especially for obtainingof components for the high-technique polycrystallineceramics through the classical ceramic method andceramics with additives.

However, the photocatalytic activity of TiOx pow-ders with different oxygen defects was little reportedin literature. Carbon or chromium doping was used tocontrol the degree of oxygen defects of samples. Thenovel carbon- or chromium-doped TiOx photocatalystwith different oxygen defects were synthesized bymechanochemical technique using a planetary ballmill followed by a heating process (Wang et al.2012). Mechanical ball milling procedure, an econom-ical and practical approach, was also employed byother authors (Aysin et al. 2011) to reduce the particlesize of titanium powders to nanoscale and silver load-ing to TiO2 powders for improving these photocata-lytic activities. Tryba (2008) and Wang et al. (2003)also applied the ball milling method to dissolve Fe intotitanium dioxide for obtaining Fe-doped TiO2 pow-ders. However, the TiO2 surface structure wasdestroyed by ball milling treatment, resulting in areduced photocatalytic activity (Tang et al. 2012).

3.2 Hydrothermal Method

The term “hydrothermal” is purely of geological origin.It was first used by the British geologist, Sir RoderickMurchison (1792–1871), to describe the action of waterat elevated temperature and pressure, in bringing aboutchanges in the earth’s crust leading to the formation ofvarious rocks and minerals. Hydrothermal processingcan be defined as any heterogeneous reaction in thepresence of aqueous solvents or mineralizers under highpressure and temperature conditions to dissolve andrecrystallize materials that are relatively insoluble underordinary conditions (Byrappa and Adschiri 2007). Thehydrothermal synthesis is currently applied for obtainingof different inorganic compounds in nanocrystalline state(Somiya and Roy 2000; Suchanek and Riman 2006).The hydrothermal method with the aqueous solvent asreaction medium is unpollutant, the reactions takingplace in a closed system, thus offering a relatively greenand inexpensive route with low crystallization tempera-ture. It is important that the hydrothermal obtained pow-ders can be produced with different microstructures,morphologies and different phase compositions, by

varying parameters as: temperature, pressure, time ofthe process, concentration of the precursors, concentra-tion and pH of solution (Byrappa and Adschiri 2007). Amixed phase of TiO2 brookite and anatase is obtained inthe acidic pH region (pH<6), while the pure anataseTiO2 forms under the alkaline conditions (pH>9) (Luet al. 2012a, b; Yu et al. 2006). Gupta and Tripathi (2012)recommend the following conditions for the synthesis ofTiO2 particles: T≤200 °C, p<100 bars; the use of Teflonliners in order to ensure the purity and the homogeneity;the use of NaOH, KOH, HCl, HNO3, HCOOH, andH2SO4 solutions as mineralisers. It was found thatHNO3 is preferable for obtaining mono-dispersednanoparticles of TiO2.

According to the literature data (Chen and Mao2007; Kolen’ko et al. 2003; Ovenstone andYanagisawa 1999; Wang et al. 1999b), the hydrother-mal synthesis of TiO2 nanocrystals starts, in general,from aqueous TiOSO4 solutions or amorphousTiO2⋅nH2O gels either in pure distilled water or inthe presence of different mineralizators as hydroxides,clorites and fluorites of alkaline metals of the differentpH values. The use of alkoxide precursors is alsosignaled (Aruna et al. 2000; Jeong et al. 2008; Phanet al. 2009). Castro-Lόpez et al. (2010) obtained Fe-doped TiO2 by hydrolysis of titanium butoxide inaqueous media, using isopropanol as co-solvent. Theadequate amount of iron (III) nitrate diluted in waterhas been added in order to obtain the Fe content of0.1 mol%. The final suspension was hydrothermallytreated with water stream in an autoclave at 120 °C,and 1,980 kPa for 3 h. After the extraction of water byevaporation and continuous stirring at 70 °C, the pow-der of Fe-doped TiO2 has been obtained.

Morphology control of the TiO2 nanostructure wasestablished using a simple hydrothermal method in thepresence of concentrated hydrochloric acid in differentamounts (Phan et al. 2009).

Ultrafine powders of TiO2 have been obtained byhydrothermal H2O2 oxidation starting from metallic Ti(Gupta and Tripathi 2012).

A microwave-assisted hydrothermal method wasemployed to synthesize nitrogen-doped titaniumnanoparticles (Zhang et al. 2009). Due to the highheating efficiency of microwave, rapid synthesis couldbe achieved in comparison with conventional hydro-thermal one.

The photocatalytic activity of TiO2 nanoparticlesobtained by hydrothermal procedure was also studied


(Liu et al. 2009; Nguyen et al. 2011; Ovenstone 2001;Ovenstone and Yanagisawa 2001; Yu et al. 2007).

3.3 Solvothermal Method

Solvothermal processes can be defined as chemical re-actions or transformations in a solvent under supercrit-ical conditions or near such a pressure–temperaturedomain. The specific physico-chemical properties ofsolvents in these conditions can, in particular, mark-edly improve the diffusion of chemical species(Demazeau 1999).

Chen and Mao (2007) consider the solvothermalprocessing to be versatile for the synthesis of a varietyof nanoparticles with narrow size distribution anddispersity. According to Gupta and Tripathi (2012),its advantages consist in the excellent chemical homo-geneity and in the possibility of obtaining uniquemetastable structures at low reaction temperatures.

The solvothermal method is an alternative route forsynthesis of TiO2 nanopowders; it is, in fact, a hydro-thermal one in which the organic solvent like metha-nol, butanol, toluene are used instead of water (Guptaand Tripathi 2012; Macwan et al. 2011). The morphol-ogy of nanoparticles, crystalline phase and their sur-face chemistry can be controlled by varying theprecursor composition, reaction temperature, pressure,solvent property and aging time. Generally, the pres-ence of surfactants in the solvothermal synthesis de-termines the decreasing of the average particle size ofTiO2 nanopowders (Lam et al. 2008).

Comsup et al. (2009) have synthesized nanocrys-talline TiO2 with average crystallite sizes 7 and 15 nmusing titanium n-butoxide and 1,4-butanediol as tita-nium precursor and solvent, respectively. Lin et al.(2010) pointed out the synthesis of anatase TiO2

nanoparticles by solvothermal reactions of Ti(O-iPr)4in alcohol using ionic liquid as additive [BMIM][Cl],1-butyl-3-methyl-imidazolium chlor, in different waterconcentrations. Ionic liquids are normally consideredto act as reaction media, templates or surfactants, dueto their special properties, such as negligible vaporpressure, low toxicity, low melting points, high chem-ical and thermal stability. The controlled hydrolysis ofTi(O-iPr)4 in the presence of ionic liquids to formtitanium oxo clusters plays a key role in the formationof anatase nanostructures. The more condensed titani-um oxo clusters may improve the formation of thenanostructures.

Laokiat et al. (2012) have obtained, by solvothermaltechnique, Fe(III)-doped TiO2 for the air treatment inorder to oxidate some contaminants such as volatilearomatic compounds: benzene, toluene, ethylbenzene,and xylene (BTEX). The synthesis was based on thesolvolysis of the titanium isopropoxide and metal ionswith isopropyl alcohol in the presence of polyethyleneglycol.

3.4 Precipitation and Co-precipitation Method

This is a conventional method used in general forobtaining of oxide materials (mono-oxides or compos-ites) with large particle size and consists in precipita-tion of hydroxides in basic medium followed bythermal treatment for obtaining a crystalline oxide.Usually, TiCl4 was dissolved in ion-exchanged water,and then an ammonia solution was added (Macwan etal. 2011). Wilska (1954) has noted the correlationbetween the crystal form of the hydrolyzed product(TiO2) and the method of hydrolytic precipitation.Ammonia precipitation and hydrolysis at boiling tem-perature were used for both chloride and sulfate solu-tions while the sodium hydroxide precipitation wasused only for the chloride solution. Additional infor-mation about these experiments was presented in thepaper.

A homogeneous precipitation method using ureawas employed to produce ultrafine rutile powders(Samuel et al. 2005; Šubrt et al. 2011). Urea is usedas a fuel, precipitating agent and as a resin former withformaldehyde. When urea is used along with nitratesalt of a cation and heated at 400 °C, the exotermicreaction between nitrate (oxidant reactant) and urea(fuel) leads to formation of corresponding nanocrys-talline oxide.

Synthesis of Fe-doped TiO2 powders by a conven-tional co-precipitation technique using ammonium hy-droxide as a hydrolyzing agent, starting from titaniumtetrachloride, was accomplished by Ganesh et al.(2012c). If TiO2 powder prepared by the same methodexists in the form of crystal pure as rutile phase, Fe-doped TiO2 powder (up to 10 wt.%) contains only theanatase phase. The same group of authors (Ganesh etal. 2012b) has obtained, also by co-precipitation meth-od, Ni-doped TiO2 powders. Even 0.1 % Ni is suffi-cient to transform TiO2 from rutile to anatase phaseand to enhance its Brunauer, Emmett and Teller (BET)surface area value from 23 to 41 m2/g. Different


amounts of Co-doped TiO2 (Co=0, 0.1, 0.2, 0.3, 0.4,0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0,8.0, 9.0, and 10 wt.%) powders have been prepared bya conventional coprecipitation technique using ammo-nia as the hydrolyzing agent and starting from TiCl4(Ganesh et al. 2012a).

Pulišova et al. (2010) have developed a simple meth-od to synthesize TiO2·nH2O nanoparticles using precip-itation from aqueous solutions containing TiOSO4 byammonia as a precipitation agent, succeeded by additionof hydrogen peroxide.

An alternative of this method is the precipitation inmicroemulsion medium. The advantage with themicroemulsion route is that the size of the particlescan be affected by the ratio of surfactant to water. Itoccurs in two forms: O/W (oil in water) and W/O(water in oil), which are analogous to aqueous andnon-aqueous mediums, respectively (Gupta andTripathi 2012). The synthesis of TiO2 nanoparticlesstarting from a TiCl4 solution emulsified into the oilphase has been reported by Deorsola and Valluri(2009). The precipitation of spherical and ultrafinetitanium particles occurred due to the high instabilityof the Ti precursor and the interactive contact amongnanodroplets.

3.5 Micelle and Inverse Micelle Methods

The micelle and inverse micelle methods are some ofthe commonly employed techniques to synthesizeTiO2-based nanomaterials. The most significant pa-rameters in controlling TiO2 nanoparticle size and sizedistribution are: the values of H2O/surfactant andH2O/titanium precursor ratios, the ammonia concen-tration, feed rate, and the reaction temperature.Literature data (Chen and Mao 2007) have shownamorphous TiO2 nanoparticles with diameters of 10–20 nm converted to the anatase phase at 600 °C and tothe rutile (the more thermodynamically stable phase)at 900 °C. TiO2 nanoparticles have also been obtainedas a result of the chemical reactions between a solutionof TiCl4 and ammonia in a reversed microemulsionsystem consisting of cyclohexane, poly(oxyethylene)5nonyle phenol ether, and poly(oxyethylene)9 nonyle phe-nol ether. The resulting amorphous TiO2 nanoparticleshave transformed into anatase when they were heated attemperatures between 200 °C and 750 °C and into rutileat temperatures higher than 750 °C. Chen and Mao(2007) give us other two examples: (a) the shuttle-like

crystalline TiO2 nanoparticles synthesized by hydrolysisof titanium tetrabutoxide in the presence of acids (HCl,HNO3, H2SO4, H3PO4) in NP-5 (Igepal CO-520)-cyclo-hexane reverse micelles at room temperature. The reac-tion conditions have largely controlled the crystalstructure, morphology, and particle size of the TiO2

nanoparticles. The key factors affecting the formation ofrutile at room temperature were: the acidity, the type ofacid used, and the microenvironment of the reverse mi-celles. (b) Another example was the TiO2 nanoparticlesobtained by the controlled hydrolysis of titaniumtetraisopropoxyde in reverse micelles formed in CO2

with ammonium carboxylate perfluoropolyether(PFPECOO−NH4

+) (MW 587) and poly(dimethyl aminoethyl methacrylate-block-1H,1H,2H,2H-perfluorooctylmethacrylate) (PDMAEMA-b-PFOMA) as surfactants.

The TiO2 nanomaterials prepared using the micelleand inverse micelle methods normally have an amor-phous structure, requiring calcination to induce thecrystallinity. This can be considered a disadvantagebecause this process can lead to the growth and ag-glomeration of TiO2 nanoparticles. However, the crys-tallinity can be improved by annealing in the presenceof the micelles at temperatures much lower than thoserequired for the traditional calcination treatment in thesolid state, which is an advantage. In conclusion, thementioned techniques are able to produce highly crys-talline TiO2 nanoparticles with unchanged physicaldimensions and minimal agglomeration.

3.6 Combustion Method

Although the combustion method of preparing nano-crystalline materials appears to be a breaking down(destructive) process, it is, in fact, a building-up one asthe product nuclei are formed initially and grow (Patilet al. 2002). Solution combustion synthesis (SCS) in-volves a self-sustained reaction in homogeneous solu-tion of different oxidizers (e.g., metal nitrates) andfuels (e.g., urea, glycine, hydrazides). Depending onthe type of the precursors, as well as on conditionsused for the process organization, the SCS may occuras either volume or layer-by-layer propagating com-bustion modes (Aruna and Mukasyan 2008).

Nanosized titanium was obtained by the combustionof aqueous solutions containing stoichiometric amountsof titanyl nitrate, TiO(NO3)2 and fuels such as glycine,hexamethylentetramine, and oxalyldihydrazide (ODH)(Hegde et al. 2005; Nagaveni et al. 2004). The


controlled hydrolysis of titanium isopropoxide underice-cold (4 °C) conditions with vigorous stirring giveswhite precipitate of TiO(OH)2. The precipitate waswashed several times in distilled water and thendissolved in nitric acid to get a clear, transparent titanylnitrate solution. This solution was used as precursor forthe synthesis of TiO2.

During combustion, the temperature reaches about650 °C for a short period of time (1–2 min) making thematerial crystalline. Since the time is so short, growthin the size of TiO2 particle and phase transition torutile is hindered. For synthesizing metal-substitutedtitania, transition metals in the form of nitrates aretaken and mixed in the solution. The material is usedin the as-synthesized form for catalytic studieswithout any further processing.

Nano-sized TiO2 was prepared also by a micro-wave-assisted combustion method using as precursortitanyl nitrate and urea as fuel (Rasouli et al. 2012).

To obtain optimum and reproducible nano-sizedmaterials, a citrate–nitrate autocombustion methodwas used (Choi et al. 2007; Macwan et al. 2011).Titanyl nitrate solution was prepared using commer-cial TiO2 powder, hydrofluoric acid (HF) and concen-trated nitric acid (HNO3). An optimized citrate/nitrateratio was used to produce TiO2 nanoparticles. Thisprocedure is a modified Pechini method (Macwan etal. 2011; Pechini 1967). The Pechini process is a sol–gel technique, in which a chelated complex is formedbetween the mixed cations and carboxylic acidgroups such as citric acid, the cations being uniform-ly distributed throughout the gel structure. By ther-mal treatment, this complex is broken, and cationsare oxidized to crystalline metal oxides.

3.7 Thermal Plasma Process

This method is used to synthesize nano-sized TiO2

powders starting from titanium tetrachloride, TiCl4,in a thermal plasma reactor (Macwan et al. 2011).The plasma torch consists of tungsten cathode andcopper anode, and the plasma was generated by Argas. TiCl4 was injected into the plasma region throughthe injection block by Ar carrier gas.

Well-crystallized iron (III)-doped TiO2 nanopowderswith controlled Fe3+ doping concentration and uniformdopant distribution, have been synthesized with plasmaoxidative pyrolysis byWang et al. (2006). Under visiblelight irradiation, the Fe3+-doped TiO2 with an iron

doping concentration of ~1 at.% had the highest photo-catalytic reactivity due to the narrowing of band gap sothat it could effectively absorb the light with longerwavelength.

3.8 Flame Spray Pyrolysis (FSP)

Un-doped TiO2 (Skandan et al. 1999; Teleki et al. 2006)and TiO2 nanoparticles doped with 1–5 at.% Nb wereproduced in a single step by FSP (Phanichphant et al.2011). FPS is a very promising technique for synthesisof high purity nanosized materials with controlled sizeand high surface area in one step. Titanium isopropoxideand niobium (IV) 2-ethylhexanoate were used as tita-nium and niobium precursors, respectively. Both pre-cursors were dissolved in xylene and acetonitrile inequal volumes with the total metal atom concentrationmaintained at 0.5 mol/l. The precursor was fed intoan FSP reactor by a syringe pump and was dispersedinto droplets by an oxygen flux.

Visible light-active Fe-doped TiO2 was preparedusing the one-step FSP technique described by Teohet al. (2007). They obtained homogeneous powdersfor Fe/Ti ratios approximately up to 0.05.

3.9 Immobilization of TiO2 Nanopowders in PolymerMatrix

Direct immobilization of nano-sized TiO2 could be apractical route for the preparation of these materials inthe shape of nanoparticles dispersed into a polymer ordirectly sprayed onto the surface during polymeriza-tion. Furthermore, these materials could be used toremove contaminants from polluted water. The proce-dure for direct immobilization of titania nanoparticlessuspension in the form of anatase phase into ε-caprolactone solution in chloroform, or the directspraying of TiO2 suspension onto partly solidifiedpolymer (polycaprolactone) by solvent-cast processeswere signaled by Sökmen et al. (2011). Photocatalyticproperties of the obtained material were tested byphotocatalytic removal of methylene blue as modelcompound and the antimicrobial properties were in-vestigated using Candida albicans as model microor-ganism. Thermogravimetric analysis revealed thatpolymer did not exhibit major deformation after a longtime of irradiation, only a slight shifting of the meltingpoint being observed. The same group of authors(Sivlim et al. 2012) have tested the photocatalytic


properties of the samematerials at the removal of organicpollutant 4-chlorophenol (4-CP) from aqueous solutionwithout additional pH adjustment, using a UV-A light(365 nm) source. Removal of the pollutant was moreeffective in the presence of polymers with high molecu-lar mass. By leaving under light for a while the materialcan be regenerated and reused for further treatments.

3.10 Biological Synthesis

Today, it is recognized that many organisms can pro-duce inorganic materials, both intra- and extracellular-ly. The interaction between the biological structuresand the inorganic particles represents one of thenewest and most exciting areas of research becausethe preparation of nanomaterials using microorgan-isms is an eco-friendly approach. Bacteria, fungi, andyeasts are all capable of producing minerals throughmicrobial processes, thus inspiring novel syntheticroutes for metal and semiconductor nanomaterials.TiO2 has become one of the most extensively studiednon-biological inorganic materials possible to be syn-thesized by biomolecules.

There are literature data (Gupta and Tripathi 2012)announcing TiO2 nanoparticles obtained using both thefungus Fusarium oxisporum and the gram-positive bac-teria Lactobacillus sporogenes. An eco-friendly methodfor the synthesis of TiO2 nanoparticles has also beenreported, starting from titanium isopropoxide solution,using nyctanthes leaves extract. The leaves weretransformed into a fine powder (by cutting, grinding,and sieving) which was mixed with ethanol andextracted under reflux conditions at 50 °C. As a resultof the reaction between the ethanolic leaf extract and theTi alcoxide, TiO2 nanoparticles have been obtained,which were separated by centrifugation and calcinated.Their morphology was spherical and the particle sizewas in the range of 100–150 nm.

3.11 Sonochemical Method

Sonochemical synthesis is considered as a technique forgenerating novel materials with unusual properties andenvironment-friendly. Ultrasounds have proved to bevery useful in the synthesis of a wide range of nano-structured materials. Their chemical effects do not comefrom the direct interaction with the molecular species.The sonochemistry arises from acoustic cavitation thatis the formation, growth, and implosive collapse of

bubbles in a liquid. The cavitational collapse producesintense local heating (~5,000 K), high pressures(~1,000 atm), and huge heating and cooling rates(>109 K/s). The sonochemical synthesis allows the ma-jor control of crystalline structure, size and morphologyof particles. This is why it has been applied in order toprepare various TiO2 nanomaterials. Thus, by choosinga suitable precursor and appropriated experimental con-ditions, it was possible to prepare large amounts (>90 %yield) of pure anatase phase nanoparticles. The as-prepared anatase nanoparticles can be converted intomixtures of anatase-rutile TiO2 by thermal treatment.The structural changes are observed at 550 °C. Thecrystal size also changes, increasing from 5 nm foranatase to 35 nm when the anatase coexists with therutile phase (Hernández-Perez et al. 2012).

Highly photoactive TiO2 nanoparticle photocatalystswith anatase and brookite phases have been prepared byhydrolysis of titanium tetraisopropoxide in pure water orin a mixture of EtOH/H2O (1:1) under ultrasonic radia-tion. Depending on the reaction temperature and theused precursor, both anatase and rutile TiO2

nanoparticles as well as their mixtures can be synthe-sized using this technique (Chen and Mao 2007). Aramiet al. (2007) have prepared TiO2 nanoparticles withmean diameter of about 20 nm, average crystallite sizeof 15 nm and BET specific surface area of 78.88 m2/g.Nano-sized TiO2 with anatase phase at a relatively lowtemperature (75 °C) and in a short time has also beenobtained by Ghows and Entezari (2010). The synthesiswas carried out by the hydrolysis of titanium tetra-isopropoxide in the presence of water, ethanol and dis-persant under ultrasonic irradiation at low intensity. Asonochemical process has also been used by Choi andHan (2012) in order to produce transition-metal-dopedTiO2 nanoparticles.

3.12 Sol–Gel Method

Considered as unique and fascinating from both a sci-entific, and practical point of view, the sol–gel processhas gained in recent years increasing importance in thematerials science field, being unanimously recognizedfor the uniqueness of its advantages in preparing somespecial materials and biomaterials with remarkableproperties (electric, magnetic, optic or of sensing, etc.).

Solution chemistry offers many possible routes for“chemical manipulation” and allows various combina-tions in the synthesis of solids of diverse structures,


compositions and morphologies (Jolivet et al. 2000).The motivation for sol–gel processing is primarily thepotentially higher purity and homogeneity and thelower processing temperature associated with sol–gelscompared with traditional glass melting or ceramicpowders methods (Hench and West 1990). Due topossibilities of leading the chemical reactions occur-ring in solution through the sol–gel procedure, theobtaining of nanosized materials with predeterminedproperties is a relatively new technology. One of theadvantages of preparing of oxide materials by the sol–gel method is the possibility to control their micro-structure and homogeneity because the most applica-tions are focused on homogeneous materials.

Molecular precursors are transformed into an oxidenetwork by hydrolysis and condensation reactions.Two routes are currently used depending on the natureof the molecular precursors: metal alkoxides in organ-ic solvents or metal salt in aqueous solutions (Livageand Ganguli 2001).

Ward and Ko (1995) consider the sol–gel method asa versatile means in developing catalytic materials, aswell as an important experimental tool in understand-ing their physical and chemical properties. Due to thegreat technological interest on titanium products invarious areas, considerable attention has recently beendevoted to improved methods of synthesis by sol–geltechniques. TiO2 gels have been known for a longtime. They can be obtained either by dissolving sodi-um titanate in concentrated HCl followed by adding aweak base such as K2CO3, (NH4)2CO3 or Na2CO3 inorder to avoid high pH variations, or throughthermohydrolysis of TiCl4 or TiO(NO3)2 under acidicconditions. The colloidal particles are crystalline andhave anatase or rutile structure depending on the pHand the nature of the counter-ions (Livage et al. 1988).

Most recent studies are based on sol–gel procedureusing alkoxide precursors Ti(OR)4.Monolithic TiO2 gelscan be synthesized from Ti(OR)4 where R is an organicradical, such as ethyl (Et), propyl (n-Pr), isopropyl (i-Pr),n-butyl (n-Bu), and secondary butyl (s-Bu), usingsubstoichiometric hydrolysis ratios (1 < h < 4) andanorganic acid catalysts (HCl, HNO3). The preparationroutes and the effect of the different parameters duringthe hydrolysis–condensations reactions have been de-tailed in a recent study (Crişan et al. 2011b).

Among the transition metals, titanium, as Ti oxides,has found wide applications in sol–gel technology. Thisis due to its high refractive index, good optical

transmission in VIS and NIR regions, good physicaland chemical stability, high electrical resistance anddielectric constant, the catalytic and photocatalytic apti-tude and semiconductive properties. In their applica-tions, TiO2 resulted from sol–gel process is presentboth as amorphous gel and crystalline material as ana-tase, brookite and rutile. The temperature at which ana-tase completely transforms in rutile depends on manyfactors such as the method of preparation, the impuritiespresent in anatase, the oxygen–metal coordination in theprecursor, the oxygen–metal bond length in the precur-sor gel, and the texture and primary particle size ofanatase (Kumar et al. 1993). Both these phases have atetragonal symmetry (anatase has a body centered te-tragonal structure whereas rutile has simple tetragonal).The anatase phase of TiO2 is known for its applicationsas photocatalysts, gas sensors, solar cells and electro-chemical devices. The rutile phase of TiO2 has foundapplications in capacitors, filters, power circuits andcondensers because of its high dielectric constant.

The diversity of the sol–gel TiO2-based materials isvery high due to the possibility to control the hydro-lysis–polycondensation parameters and the densifica-tion process. The effect of molecular structuralvariation in the gel, by modification of the reactionparameters, can be evaluated in principle by establish-ment of the properties of the resulted oxide materials.

The sol–gel process can provide submicron, mono-disperse TiO2 powders, no matter of the used precur-sor (inorganic or organic). The inorganic methodsupposes the thermohydrolysis of titanile sulfate(TiOSO4) or of titanium chloride (TiCl4) in the pres-ence of sodium sulfate (Na2SO4) (Duncan andRichards 1976; Santacesaria et al. 1986) or tartaricacid (Dhage et al. 2004). In both cases, monodispersespheres with diameters around 0.4 nm are obtained.

The hydrolysis of alkoxides (the organic method)represents the most common way for the preparationof TiO2 particles. The chemical reactions can be sche-matically presented as follows:

Ti ORð Þ4 þ xH2O ! Ti ORð Þ4�x OHð Þx þ xROH hydrolysisð Þð3:1Þ

Ti ORð Þ4�x OHð Þx þ Ti ORð Þ4 ! ORð Þ4�xTiOxTi ORð Þ4�x

þ xROH condensationð Þ ð3:2Þ

The oversimplified description of the sol–gel chem-istry implies two key ideas: (1) the formation of a gel as


a result of the condensation of the partially hydrolyzedspecies into a three-dimensional polymeric network, and(2) any factors which affect these reactions can have animpact on the properties of the resulted gel. As hydro-lysis and polycondensation take place at the same time,being in competition, there is possibility of changing, insome manner, their relative rate. There are many factorswhich influence the process. Ward and Ko (1995) con-sider that just the control of these factors (generallyreferred to as sol–gel parameters) makes the differencebetween the sol–gel and other preparation methods. Alist of the most representative parameters includes bothtypes of precursor and solvent, precursor concentration,water and catalyst (acid or base) content, the pH of thesolution, the temperature, and the presence of additives.They all affect the structure of the initial gel and asconsequence the properties of the material in all subse-quent processing steps. Other important parameters ofthe sol–gel processing refer to the aging (the timebetween the formation of the gel and its drying)and to the drying conditions which can stronglyaffect the sol–gel product.

In the case of the sol–gel powders, the sol–gel pro-cess refers to the processing into solution of some solidmaterials which do not deposit, so they are not precip-itates. They can be sols (composed from discrete unitswhich remain dispersed in the liquid) or gels (made upfrom solid tridimensional networks spread into thewhole liquid matrix). The result of the succession ofreactions above presented assumes the following trans-formations of the precursors in an aqueous medium:hydrolysis → polymerization → nucleation → growth.In the case of un-ionized precursors, as alkoxides, thehydrolysis and polymerization–condensation reactionslead to some linear polymeric structures containing M–O–M bonds, favored by a slow hydrolysis compared tothe polymerization (Pierre 1992).

The advantages of the sol–gel method in the prepa-ration of mono-component materials refer to the veryhigh purity due to the quality of the available precursorsand to the possibility to tailor the textural properties ofthe product, especially the surface area and the pore sizedistribution. For the multi-component systems the fol-lowing specific advantages can bementioned: the abilityto control both structure and composition at molecularlevel, the possibility to introduce several components ina single step, and the power to impose kinetic con-straints on a system and thereby stabilize metastablephases. Furthermore, the controlled shape and size

(usually, mono-disperse), and the nanometer size ofthe particles must be mentioned.

A systematic and exhaustive study of the literaturedata regarding sol–gel TiO2 nanomaterials have beenpresented in a recent work of the authors (Crişan et al.2011b). Therefore, now we mention only the mostrecent papers in the field: Ahmed (2012), Ahmed etal. (2011), Akpan and Hameed (2010), Fröschl et al.(2012), He (2011), Ho et al. (2012), Khataee andMansoori (2012), Lopes et al. (2012), Macwan et al.(2011), and Tseng et al. (2010).

The promising characteristics of doped photocatalystsregarding replacement of UV artificial light sourceswith solar polychromatic light, represents a challengefor researches in the synthesis of photocatalysts andenvironmental fields. An advantage of using the sol–gel method is the ability to control in a simple way theconcentration of the dopant in the nanostructure oftitanium dioxide. Our recent studies regarding thesol–gel doped TiO2 nanomaterials with S (Crişan etal. 2007; Crişan et al. 2008c; Răileanu et al. 2009), Ag(Răileanu et al. 2009), Pd (Crişan et al. 2008a, b), Au(Crişan et al. 2011a) will be completed with the pres-ent one regarding doping with transitional metals. Thisis the reason why we present here an exhaustive newliterature survey regarding doping with Fe, Co and Ni(Tables 1, 2 and 3). The modification of TiO2 bydoping with transition metals provides a successfulalternative by which some researchers have realizedthe complete degradation of some organic dyes inwastewater treatment (Han et al. 2009).

The sol–gel combustion is a new method whichcombines the advantages of the chemical sol–gel pro-cess (high purity, good homogeneity, low processingtemperature) with those of the combustion synthesis(low energy requirements, simple equipment, shortoperation time). It is based on the gelling, followedby combustion, of an aqueous solution which containssalts of the desired metals and an inorganic fuel suchas acetylene black. As a result, a voluminous andfluffy product is obtained with a large surface area.The sol–gel combustion hybrid method allowsobtaining nanosized powders. The nano-crystallizedTiO2 powders prepared by Han et al. (2008) are suchan example. The product synthesized at 500 °C hasparticles with 15–20 nm sizes and a BET surface areaof 242.7 m2 g−1.

A nonhydrolytic alternative of the sol–gel process re-fers to the sol method (Chen and Mao 2007) that usually


Tab

le1

The

chem

ical

compo

sitio

nsandtheexperimentalcond

ition

sof

preparationforsol–gelFe-do

pedTiO

2po

wders

from

literature,

together

with

thecorrespo

ndingdrying

cond

ition

s,thermal

treatm

entsandtheresultedcharacteristics(specificsurfaceandparticle

size,respectiv

ely)

Reactants

Dopantcontent

Reaction

conditions

Gelation

time

Drying

conditions

Therm

altreatm

ent(t.t.)

Specificsurface

(m2g−

1 )Particle

size

(nm)

Reference

Precursors

Solvents

Catal.

T(°C)

pHt(h)

T(°C)

t(h)

T(°C)

t(h)

Beforet.t.

After

t.t.

TiO

2Fe

TiO

2Fe

TiCl 4

Fe(acac) 31

–H2O

NH4OH

0.5–5[w

t.%Fe]

09–9.5

0.5

110

24500

2441.8

1,000–10,000

Navío

etal.1999

Ti(OPri )

4FeSO4

isoP

rOH

isoP

rOH

–Ti/Fe=

1/0.15

(MR)

––

––

–500

2–

–3–6

Tonejcet

al.2001

Ti(OBu)

4Fe(NO3) 3·9H2O

EtOH

–Fe/Ti=0;1;2;3;4;

5[at.%

]r.t.

––

9012

400;500;

600

2–

36–114

–Wanget

al.2004

Ti(OPri )

4Fe(NO3) 3·9H2O

–H2O

HNO3

upto1.8[wt.%

Fe(III)]

253

Piera

etal.2003

Ti(OPr)4

Fe 2(SO4) 3·xH2O

EtOH

+TEA

EtOH

+H2O

–0.1–0.5[at.%

Fe]

––

–100

3;3.5;

4.5

460;520;

580

––

–18–20

Chenet

al.2008

NH4OH

09

110

24500

6Akpan

etal.2010

TiCl 4;

Ti(OiPr)4

BA;BUT

6.8–7.4

Djerdjet

al.2008

Ti(OBu)

4FeC

l 3(FeC

l 2)

n-Octanol

n-Octanol+H2O

––

230

––

––

560

––

––

Zaleska

2008

Ti(OBu)

4Fe(NO3) 3·9H2O

EtOH

H2O

HCl

0.4–1.0[m

ol%

Fe(III)/TiO

2]

r.t.

–48

8024

500;700;

900

2–

73–85

9.2–24.7

Jamalluddin

and

Abdulah

2011

Ti(OBu)

4FeC

l 2EtOH

+H2O

H2O

+EtOH

HCl

1;5;

10[w

t.%Fe/TiO

2]

703

–80

10200;400;

600;800

4–

–4.4–78

Wanget

al.2001

Ti(OiPr)4

Fe(NO3) 3

EtOH

+HNO3

H2O

HNO3

0.025;0.05;010;

0.50;1.00;2.00

[mol%

Fe2O

3/TiO2]

r.t.

324

110

1550

49.6–42.9

–19.5–30.3

Luu

etal.2010

Ti(n-OBu)

4FeC

l 2·2H2O

H2O

+EtOH

+HCl

HCl

1[w

t.%Fe]

703

24–

–200;400;

600;800

4–

3–200

–Lόpez

etal.2001

––

––

–0.5;5;10;20;30;

40[%

Fe]

––

––

–300;400;

600;800;

1150

––

0.19

–291

–Sedneva

etal.2012

Ti(OBu)

4FeCl 2·4H2O

;FeCl 3·6H2O

;Fe(N

H4)2(SO

4)2

EtOH

+H2O

H2O

+EtOH

HCl;NH3

(aq.)

5[w

t.%Fe]

703;

9–

7024

600

434–79

––

Pecchiet

al.2002

Ti(OiPr)4

FeC

l 3EtOH

EtOH

+H2O

NH3(aq.)

0.3;0.5;0.8;1.0

[mol%]

r.t.

3–4

0.5

105

24400

2–

––

Kokila

etal.2011

Ti(OiPr)4

Fe(CH3COO) 2

EtOH

CH3COOH

–0.01;0.05;0.15

[Fe/Ti

(MR)]

70–

8;9

60–

500;580;

800

––

2.67–

102.30

9.7–16.2

Šijaković-Vujičić

etal.2004

Ti(SO4) 2

Fe(NO3) 3

H2O

H2O

Na 2CO3

1.44;2.06;4.69;

9.16

[wt.%

Fe]

806.5

–120

10650

10–

––

Luet

al.2008

Ti(OiPr)4

Fe(acac) 3

2-PrO

H2-PrO

HHNO3

0.1;0.5[%

Fe]

801.5; 6.5

––

–500;

600

2;2+

18–

15–21

17–69

Marugán

etal.

2009

Ti(OBu)

4Fe(NO3) 3

EtOH

H2O

CH3COOH

0.1–1.0[at.%

]–

–48

8048

480

338.17

––

Sun

etal.2011

Ti(OiPr)4

Fe(NO3) 3

EtOH

EtOH

–1;5;10

[%Fe]

––

––

–450;

470

0.08

––

–Glaspelland

Manivannan2005

Ti(OiPr)4

Fe(NO3) 3·

6H2O

EtOH

H2O

HNO3

calculable

50;150

1.6

72–

–400

4–

98–

Haet

al.2006

Ti(OiPr)4

Fe(NO3)3·9H2O

EtOH

H2O

––

––

––

––

––

–


Tab

le1

(con

tinued)

Reactants

Dopantcontent

Reaction

conditions

Gelation

time

Drying

conditions

Therm

altreatm

ent(t.t.)

Specificsurface

(m2g−

1)

Particle

size

(nm)

Reference

Precursors

Solvents

Catal.

T(°C)

pHt(h)

T(°C)

t(h)

T(°C)

t(h)

Beforet.t.

After

t.t.

TiO

2Fe

TiO

2Fe

0.1;0.5;1.0;

2.0;5.0;10

[Fe/Ti

at.%]

Wetchakun

etal.

2007

Ti(OBu)

4Fe(NO3) 3

isoPrO

HH2O

HNO3

0.1[mol%

Fe]

r.t.

1.5

–70

4400

241

––

Castro-Lόpez

etal.

2010

Ti(OiPr)4

Fe(NO3)3·9H2O

EtOH

H2O

NH4OH

2[m

ol%

Fe]

––

–60

24500

215

<40

Seabraet

al.2011

Ti(OiPr)4

Fe 2O3

EtOH

+H2O

EtOH

–0.5–10

[wt.%

Fe]

r.t.

–0.5

8024

400

2–

143–180

–Lezneret

al.2012

Ti(OiPr)4

Fe(NO3)3·9H2O

HNO3

H2O

EDTA

+NH3

(aq.)

5.0[at.%

Fe]

r.t.

––

6024

400

3–

134

8.1–20.1

Pongw

anet

al.2012

Ti(SO4) 2

Fe(NO3) 3

H2O

H2O

Na 2CO3

5,10,15[wt.%

Fe]

r.t.+80

6.5

–120

10650

10–

–13

Zhu

etal.2005a

Ti(OiPr)4

Fe(NO3) 3

2-PrO

HH2O

HNO3

0.11–1.76[wt.%

Fe]

––

2–20

110

12550

243.2–12.1

––

Ranjitand

Viswanathan1997

Ti(OBu)

4Fe(acac) 3

n-Butanol

n-Butanol

CH3COOH+

acethylacetone

0.5[at.%

Fe]

752–2.5

–100

–450

3–

–400,000–

500,000

Cerneaet

al.2007

Ti(OiPr)4

Fe(NO3)3·9H2O

EtOH

EtOH

–0.5/1;1/1;1.5/1

[Fe/Ti

(MR)]

––

––

–300;500

40;40

––

–Bersani

etal.2000

TMET

Fe(NO3)3·9H2O

MEtOH

MEtOH

–2/100;5/100;

10/100

[Fe/Ti

(MR)]

––

––

–1,000

––

––

Bersani

etal.2000

Ti(OiPr)4

FeC

l 3·6H2O

EtOH

H2O

HNO3

0.1–1[at.%

Fe]

r.t.+45+70

1.5

–70

24200–700

(400)

1–

113

–Choiet

al.2010a

Fe(acac) 3

iron

acethy

lacetonate,MRmolar

ratio

,r.t.room

temperature,TEAtriethanolam

ine,

BAbenzyl

alcoho

l,BUT2-bu

tano

ne,EDTA

ethy

lenediam

inetetraacetic

acid,TMET

titanium

tetram

etho

xyetho

xyde,MEtOH

metho

xy-ethanol


Tab

le2

The

chem

ical

compo

sitio


ition

sof

preparationforsol–gelCo-do

pedTiO

2po

wders

from

literature,

together

with

thecorrespo

ndingdrying

cond

ition

s,thermal

treatm

ents

andtheresulted

characteristics(specificsurfaceandparticle

size,respectively)

Reactants

Dopantcontent

Reaction

conditions

Gelation

time

Dryingconditions

Therm

altreatm

ent

(t.t.)

Specificsurface

(m2g−

1 )Particlesize

(nm)

Reference

Precursors

Solvents

Catal.

T(°C)

pHt(h)

T(°C)

t(h)

T(°C)

t(h)

Beforet.t.

After

t.t.

TiO

2Co

TiO

2Co

TiCl 4

Co(III)2,4-

pentane-

dionate

EtOH

EtOH

–0.0085;0.017;

0.0255;0.034;

0.085[mol%

Co(III)]0.5;1.0;

1.5;2.0;5[wt.%

Co(III)]

r.t.

––

––

200–900

1–

––

Barakat

etal.

2005

Ti(diisopropoxide)

bis(2,4-pentane-

dionate)

Co(CH3COO) 2·

4H2O

PVP

PVP

–Ti0.93Co 0.07O

2Ti1−x

Co xO2(x=0.01–

0.15)

27–

–100

–550

3–

–10–15

Maensiriet

al.

2006

Ti(OPri )

4CoC

l 2Toluene

+MOEH

Toluene

+MOEH

–0–6.7(12)[m

ol%

CoO

]–

––

––

––

––

–Westin

etal.2004

Ti(OPr)4

Co(NO3)2·6H2O

EtOH+TEA

EtOH+H2O

–0.00002–0.2[at.%

Co]

––

–100

3;3.5;

4.5

460;520;

580

––

––

Chenet

al.2008

TiCl 4

Co(NO3) 2

H2O

H2O

NH4NO3;

NH3;HCl

0.2;0,6;1.0[%

Co]

807

––

–450–850

1–

––

Hai

etal.2009

Ti(OBu)

4Co(NO3)2·6H2O

n-Butanol

EtOH

HNO3

0.25;0.50;1.0[%

Co/Ti

(MR)]

r.t.

––

––

400;500;

600;700

2;3; 4;6

––

8.3–

27.2

Chenetal.2010

Ti(OBu)

4Co(NO3) 2

EtOH+H2O

EtOH+H2O

CH3COOH

2.5–

15[%

Co]

––

–110

–450

3–

81.4–

103.1

8.63

–10.1

Liu

etal.2011

Ti(OBu)

4CoC

l 2EtOH

EtOH+H2O

HCl

Ti1−xCo xO2−δ(x=

0.001;0.005;

0.01;0.02;0.05)

r.t.

–120

60–

200–1,000

2–

–6–95

Lim

etal.2007

Ti(OiPr)4

Co(NO3) 2

CH3COOH

+H2O

H2O

–0.05;0.1;0.5;1.0;

2.0;5.0[mol%

Co/TiO2]

70–

12100

–500

2–

–8–13

Ham

adanianetal.

2010

Ti(OBu)

4CoC

l 2·6H2O

EtOH

H2O

CH3COOH

Calculable

r.t.

––

100

–500

2–

–20

Rahim

iet

al.

2012

TiCl 4

Co(III)2,4-

pentane-

dionate

EtOH

EtOH

–0.004–0.14

[mol%

Co(III)]

r.t.

––

––

200–900

1–

39.7

25Barakat

etal.

2004

Ti(OBu)

4Co(NO3) 2

EtOH

H2O

CH3COOH

0.1–

1.0[at.%

]–

–48

8048

480

330.08

––

Sun

etal.2011

Ti(OiPr)4

Co(NO3) 2

isoP

rOH

H2O

CH3COOH

Calculable

r.t.

–24x30.5x6

––

552

––

2–6

Pacheco

etal.

2004

Ti(OiPr)4

Co(CH3COO) 2·

4H2O

2-ME+EG

2-ME+EG

–Co 0

.04Ti 0.96O2

––

––

–320;420;

450;470;

520;620;

720

1;3;

6.5

––

–Geet

al.2005

Ti(OiPr)4

Co(NO3)2·6H2O

EtOH

H2O

HNO3

1[w

t.%Co]

50;180

1.73

96–

–400;

500

5–

84–

Bae

etal.2007

Ti(OiPr)4

Co(NO3) 2

EtOH

EtOH

–1;5;10

[%Co]

––

––

–450;

470

0.08

––

–Glaspelland

Manivannan

2005

Ti(OiPr)4

Co(NO3)3·6H2O

EtOH

H2O

HNO3

calculable

50;150

1.6

72–

–400

4–

84–

Haet

al.2006

Ti(OiPr)4

Co(NO3)2·6H2O

isoPrOH+EG

H2O

–calculable

75–

–110

24550

4–

––


Tab

le2

(con

tinued)

Reactants

Dopantcontent

Reaction

conditions

Gelation

time

Dryingconditions

Therm

altreatm

ent

(t.t.)

Specificsurface

(m2g−

1 )Particlesize

(nm)

Reference

Precursors

Solvents

Catal.

T(°C)

pHt(h)

T(°C)

t(h)

T(°C)

t(h)

Beforet.t.

After

t.t.

TiO

2Co

TiO

2Co

T-Thienprasert

etal.2011

Ti(OiPr)4

CoC

l 2·6H2O

EtOH+

CiAc+

AcA

c

EtOH

–Co xTi1–xO

2(x=0.01;

0.03;0.05;0.08;

0.10)1;3;5;8;

10[at.%

Co]

120+80

–18

200

1400;600;

800

1;1;

1–

–31

Karim

ipouret

al.

2011

Ti(OEt)4

Co(NO3)6H

2OEtOH+H2O

H2O

–20

[wt.%

Co/TiO2]

r.t.

––

110

12450

4–

––

Suriyeetal.2005a

Ti(OEt)4

Co(acac) 2

EtOH+H2O

acetone

–5[w

t.%Co]

70–

––

––

––

81–

Aguirre

etal.

2006

Ti(OiPr)4

CoC

l 2EtOH

H2O

HNO3

0.1–1[at.%

Co]

r.t.+45+

701.5

–70

24200–700

1–

102

–Choietal.2010a

r.t.room

temperature,PVPpo

lyviny

lpy

rrolidon

e,MOEH

metho

xy-ethanol,TEAtriethanolam

ine,

MRmolar

ratio

,2-ME2-metho

xyethano

l,EG

ethy

lene

glycol,CiAccitric

acid,

AcA

cacethy

lacetone

aSol–gel

combinedwith

wetness

impregnatio

nmetho

d


Tab

le3

The

chem

ical

compo

sitio


ition

sof

preparationforsol–gelNi-do

pedTiO

2po

wders

from

literature,

together

with

thecorrespo

ndingdrying

cond

ition

s,thermal

treatm

entsandtheresultedcharacteristics(specificsurfaceandparticle

size,respectiv

ely)

Reactants

Dopantcontent

Reaction

conditions

Gelation

time

Drying

conditions

Therm

altreatm

ent(t.t.)

Specificsurface

(m2g−

1)

Particle

size

(nm)

Reference

Precursors

Solvents

Catal.

T(°C)

pHt(h)

T(°C)

t(h)

T(°C)

t(h)

Beforet.t.

After

t.t.

TiO

2Ni

TiO

2Ni

Ti(OPri )

4Ni(N

O3)26H

2OEtOH

H2O

HNO3

8[w

t.%Ni]

r.t.

––

––

300

221

14–

Gonçalves

etal.2006

Ti(OPr)4

NiSO46H

2O

EtOH

EtOH

–0.00002–0.2

[at.%

Ni]

––

–100

3;3.5;4.5

460;520;580

––

––

Chenet

al.2008

Ti(OPri )

4NiCl 26H

2O

i-Propanol+

AcA

cH2O

+i-propanol

HCl

2.5;

5[w

t.%Ni]

r.t.

––

806

500

1–

–18.5–20.3

Hermaw

anetal.2011

Ti(OPri )

4Ni(N

O3)26H

2OEtOH

H2O

HNO3

0.1–0.5[at.%

Ni]

r.t.

1.5

–70

24400

1–

––

Choiet

al.2010b

Ti(OBu)

4Ni(CH3CO2) 2

CH3OH

H2O

HNO3

0.25

[Ni/T

i]–

––

––

––

–81

–ChoiandSuh

2007

Ti(OPri )

4Ni(N

O3)26H

2OEtOH

H2O

HNO3

0.1–1[at.%

Ni]

r.t.+45

+70

1.5

–70

24200–700

1–

112

–Choiet

al.2010a

Ti(OBu)

4NiCl 26H

2O

EtOH

+PEG

EtOH

HCl

0;0.2;0.8;1.5;2

[mol%

Ni]

––

–100

10450

3–

–11.5–14

Zhang

2011

Ti(OBu)

4Ni(N

O3)26H

2OEtOH

+H2O

EtOH

+H2O

–0.5;

1.5;

3;6

[wt.%

Ni]

––

–70

12400

4–

87–95

7.8–14.4

Rodríguez-González

etal.2011

TiCl 4

Ni(NO3) 2·6H2O

H2O

–NH3(aq.)

calculable

–7

–50

1.5

300

1.5

––

–Wanget

al.2009

r.t.room

temperature,AcA

cacethy

lacetone,PEG

poly-ethyleneglycol


involves the reaction of TiCl4 with different oxygendonor molecules such as metal alkoxides [Ti(OR)4] ororganic ethers (ROR), according to reactions 3 and 4:

TiX4 þ Ti ORð Þ4 ! 2TiO2 þ 4RX ð3:3Þ

TiX4 þ 2 ROR ! TiO2 þ 4RX ð3:4ÞThe bridges Ti–O–Ti are formed as a consequence

of the condensation between Ti–Cl and Ti–OR. Thealcoxide groups can also proceed from the reaction ofTiCl4 with alcohols or ethers. The particle growthstrongly depends on temperature and the TiCl4 con-centration influences the particle size.

Surfactants have been widely used in the preparationof various nanoparticles with good size distribution anddispersity. Added into the reaction mixture as cappingagents, different surfactants, such as acetic acid andacetylacetone, can lead to monodispersed TiO2 synthe-sized nanoparticles. In the surfactant-mediated shape evo-lution of TiO2 nanocrystals in nonaqueous media, it wasestablished that the shape of the TiO2 nanocrystals can bemodified by changing the surfactant concentration.

4 Morphological and Structural CharacterizationTechniques

4.1 Structure and Phase Composition

4.1.1 X-ray diffraction (XRD)

X-ray scattering techniques are a family of non-destructive analytical techniques which reveal informationabout the crystal structure, chemical composition, andphysical properties of single crystals and polycrystallinepowders and thin films. These techniques are based onobserving the scattered intensity of an X-ray beam hittinga crystalline (long range ordered) sample as a function ofincident and scattered angle, polarization, and wavelengthor energy (Guinebretière 2007; Dinnebier and Billinge2008; Pecharsky and Zavalij 2005).

XRD permits users to:

& Measure the average spacing between layers orrows of atoms

& Determine the orientation of a single crystal or grain& Determine the lattice parameters of a crystalline

compound, knowing its unit cell symmetry& Find the crystal structure of an unknown material

& Measure the size, shape and internal stress of smallcrystalline regions

& Identify the crystalline constituents and to estimatetheir amount in a multi-phase ceramic sample

4.1.2 Infrared (IR) spectroscopy

IR spectroscopy is the spectroscopy that deals with theIR region of the electromagnetic spectrum, that is,light with a longer wavelength and lower frequencythan visible light. It covers a range of techniques,mostly based on absorption spectroscopy. As with allspectroscopic techniques, it can be used to identifyand study the structuring of crystalline and amorphousmaterials. A common laboratory instrument that usesthis technique is a Fourier transform infrared (FTIR)spectrometer.

The IR portion of the electromagnetic spectrum isusually divided into three regions; the near-, mid- andfar-infrared, named for their relation to the visiblespectrum. The higher-energy near-IR, approximately14,000–4,000 cm−1 (0.8–2.5 μm wavelength) can ex-cite overtone or harmonic vibrations. The mid-infrared,approximately 4,000–400 cm−1 (2.5–25 μm) may beused to study the fundamental vibrations and associa-ted rotational–vibrational structure. The far-infrared,approximately 400–10 cm−1 (25–1,000 μm), lying ad-jacent to the microwave region, has low energy andmay be used for rotational spectroscopy. The namesand classifications of these sub-regions are conven-tions, and are only loosely based on the relative mo-lecular or electromagnetic properties (Nakamoto1986).

IR spectroscopy is very useful mainly for structuralcharacterization of amorphous or quasi-crystalline pre-cursor powders or intermediates, for which other in-vestigation techniques as XRD are not effective.

4.2 Thermal Behavior of Precursor Powders

4.2.1 Thermogravimetry (TG)

TG is the technique in which the mass of a substanceis measured as a function of temperature while thesubstance is subjected to a controlled temperatureprogramme. The record is the thermogravimetric orTG curve; the mass should be plotted on the ordinatedecreasing downwards and temperature (T) or time (t)on the abscissa increasing from left to right.


4.2.2 Differential Thermal Analysis (DTA)

DTA is the technique in which the temperature differ-ence between a substance and a reference material ismeasured as a function of temperature whilst the sub-stance and reference material (thermally inactive overthe temperature range of interest) are subjected to thesame controlled temperature programme. The record isthe differential thermal or DTA curve; the temperaturedifference (ΔT) should be plotted on the ordinate withendothermic reactions downwards and temperature ortime on the abscissa increasing from left to right.

4.2.3 Differential Scanning Calorimetry (DSC)

DSC is the technique in which the difference in energyinputs into a substance and a reference material ismeasured as a function of temperature whilst the sub-stance and reference material are subjected to a con-trolled temperature programme. Two modes, power-compensation differential scanning calorimetry (power-compensation DSC) and heat-flux differential scanningcalorimetry (heat-flux DSC), can be distinguisheddepending on the method of measurement used.

DTA is very useful for identifying the physical andchemical processes which take place at the heating ofsome polymeric or molecular precursor powders (pre-pared by wet chemical methods) which will be convertedin oxide compounds of interest, while TG is mostly usedfor establishing the appropriate thermal treatment condi-tions, in order to ensure a complete decomposition of theprecursors and/or quantitative reactions between the po-tential intermediates (Hatakeyama and Liu 1998).

DSC is usually employed to emphasize particular(structural, electric or magnetic) phase transitions andtheir reversible character during the heating/coolingprocess.

4.3 Particle Size Distribution of Powders (PSD)

PSD analysis is a measurement designed to determineand report information about the size and range of aset of particles representative of a material. Particlesize and distribution analysis of a sample can beperformed using a variety of techniques, each withadvantages and disadvantages, depending on the sam-ple properties and question at hand.

PSD is usually expressed from the technique bywhich it is determined. Many factors influence the

choice of technique such as detection limits, particlesize range, sample presentation, sample concentration,solubility, and particle shape.

4.3.1 Laser-Diffraction Analysis (Low Angle LaserLight Scattering)

The most time-efficient and robust way to obtain PSDanalyses is by using a laser-diffraction analyzer. Laserdiffraction is used to detect particle sizes in the range of~0.1 to 2,000 μm equivalent spherical diameter(depending on the instrument) using light scattering the-ory. The refractive and absorption indices for the materialmust be known for accurate measurements to be made.

Powders can be measured by laser diffraction either inwater or air, with aggregation being reduced using waterand treatment with ultrasound. One sample takes appro-ximately 5 min to run. Samples should be dried andsieved to exclude particles >2,000 μm (2 mm) diameter.

4.3.2 Sieving

Sieving is a simple, portable, inexpensive and widelyused method of classifying powders according to theirphysical size alone, independent of other physical orchemical properties, by using a series of woven wireor punch plate sieves arranged in decreasing order ofaperture size. Sieving can be performed manually orby machine agitation. Key variables that influencesieving results include particle shape, presence of veryfine particles, initial sieve loading, time and method ofagitation and aggregation of the powder. Reproducibilityis often poor due to these variables.

4.4 Specific Surface Area and Porosity of Powdersand Ceramics

4.4.1 Nitrogen Gas Adsorption

Specific surface area αBET is defined as the ratio A/ms

(unit: m2 g−1) between the absolute surface area A of asolid and its mass ms (sample weight). The surfacearea includes all parts of accessible inner surfaces(mainly pore wall surfaces).

Precise measurement of the specific surface area ofsolids by gas adsorption (nitrogen, krypton) accordingto the BET method as described in the standard ISO9277:2010 and in the IUPAC Recommendations of1984 (Sing et al. 1985) was done by volumetric static


measurement of the adsorption isotherm in the rangefrom 0.1 to 1,400 m2 g−1. Originally, the method wasconsidered only applicable to type II or type IV ad-sorption isotherms (nonporous solids or macro andmesoporous solids with pore widths >2 nm) accordingto the IUPAC classification. As described in the sec-ond edition of the international standard ISO 9277(issued 2010) the method has been extended to micro-porous adsorbents (pore widths below 2 nm).

The BET method involves the determination of theamount of the adsorbate or adsorptive gas required tocover the external and the accessible internal pore sur-faces of a solid with a complete monolayer of adsorbate.This monolayer capacity can be calculated from theadsorption isotherm by means of the BET equation.The gases used as adsorptives have to be only physicallyadsorbed by weak bonds at the surface of the solid (vander Waals forces) and can be desorbed by a decrease ofpressure at the same temperature. Themost common gasis nitrogen at its boiling temperature (77.3 K). In thecase of a very small surface area (below 1 m2 g−1), thesensitivity of the instruments using nitrogen is insuffi-cient and krypton at 77.3 K should be used.

In order to determine the adsorption isotherm volumet-rically, known amounts of adsorptive are admitted step-wise into the sample cell containing the sample previouslydried and outgassed by heating under vacuum. Theamount of gas adsorbed is the difference of gas admittedand the amount of gas filling the dead volume (free spacein the sample cell including connections). The adsorptionisotherm is the plot of the amount gas adsorbed (inmmol g−1) as a function of the relative pressure p/p0.

By means of this technique, several parameters canbe determined:

& Single-point and multi-point BET surface areas& Langmuir surface areas& Pore volume and pore area distributions in the

mesopore and macropore ranges, or in user-defined pore size ranges

& Average pore size

& Total micropore volume, using the t-plot and αS

plot methods

One drawback is that the time used for a singleanalysis can be hours. With nitrogen adsorption, onlyopen pores are determined, and the cylindrical poremodel is assumed in pore size distribution measure-ments. The desorption isotherm in the characterizationof pore size distribution is affected by the pore network;when pressure is reduced, liquid will evaporate fromlarge open pores, but pores of the same size that areconnected to the surface with narrower channels remainfilled (Allen 1997). This changes the shape of the poresize distribution. The samples come into contact withthe temperature of liquid nitrogen (−196 °C) duringanalysis, which may destroy them.

4.4.2 Mercury Porosimetry

Mercury porosimetry is an extremely useful characteri-zation technique, especially for porous materials.Compared to alternative pore size characterizationmethods (e.g., gas sorption) mercury porosimetry basedon a simpler principle (the Washburn equation) is muchfaster, producing full pore size distributions in minutescompared to hours in gas sorption tests and, perhapsmore importantly, is unique in that it covers a very widerange of pore sizes, including large pores >0.5 μm thatare difficult or impossible to probe reliably by othertechniques. Thus, with this method pores between ~3and ~200 μm (mesopores and macropores) can be in-vestigated (Giesche 2006; Leon y Leon 1998).

Mercury porosimetry provides information aboutthe total pore volume, percent porosity, average poresize, pore size distribution, specific surface area of thepores (assuming a cylindrical shape for the pores),bulk density and apparent density of a ceramic sampleand transport properties of the pore structure.

With the method, only pores that reach the surface ofthe sample can be determined. The sample must be dry,because mercury cannot intrude into the sample whenvoids are filled with another liquid. Samples with a finepore structure are difficult to degas, and adsorbed layersreduce effective pore diameter and pore radius values.

During measurement, high pressures to force mer-cury into small pores may compress the sample (Allen1997; Palmer and Rowe 1974; Dees and Polderman1981; Johnston et al. 1990; Ek and Newton 1998;Webb and Orr 1997). This effect can be shown

Fig. 2 Pore diameter ranges determined with mercuryporosimetry and nitrogen adsorption


especially in samples containing closed pores (Webband Orr 1997) and is observed as a too large volume ofsmall or medium sized pores.

Non-capillary pore structure and limitations of theWashburn equation in determination of the smallestpores are the reasons for the differences between poresize distributions determined with mercury porosimetryand nitrogen adsorption (De Wit and Scholten 1975).However, total pore volume and pore surface area re-sults are not dependent on pore shape (Rootare andPrenzlow 1967), and the shape of pore size distributionis not remarkably different from the true distribution inspite of the assumption of a circular cross section of thepores (Ritter and Drake 1945).

Comparison of Nitrogen Adsorption and MercuryPorosimetry Methods Pore structure analysis by mer-cury porosimetry is faster than by nitrogen adsorption.In mercury porosimetry and nitrogen adsorptiondeterminations, two different physical interactions takeplace. Both methods are based on surface tension, capi-llary forces and pressure. With mercury porosimetry,large pores at the intrusion phase are determined first,while with nitrogen adsorption, the smallest pores aremeasured first at the adsorption phase (Webb and Orr1997). The determination range of high-pressure mer-cury porosimetry is wider (pore diameter 3 nm–200 μm)than that of nitrogen adsorption (0.3–300 nm), andmercury porosimetry determines larger pores that areout of the detection range of nitrogen adsorption(Fig. 2). With nitrogen adsorption, the smallest poresthat are out of range of mercury porosimetry, can bedetermined. However, results of the two methods can becompared. The comparable parameters are total porevolume, volume pore size distribution and specific sur-face area/total pore surface area. Although the pore sizerange that can be determined with adsorption isnarrower than that obtained with mercury porosimetry,it is more widely used (Allen 1997).

4.5 Morphology of Powders and Microstructureof Ceramics

Electron microscopy (EM) techniques represent themost effective investigation methods to analyze themorphology of particles, mesocrystals and/or self-assembled structures, as well as the microstructureof thin films and micro- or nano-structured bulkceramics.

EM can be defined as a specialized field of sciencethat employs the electron microscope as a tool anduses a beam of electrons to form an image of a specimen(Bozzola and Russell 1992; Heath 2005). In contrast tolight microscopy (LM), which uses visible light as asource of illumination and optical (glass) lenses to mag-nify specimens in the range between approximately 10to 1,000 times their original size, EM is operated in thevacuum and focuses the electron beam and magnifiesimages with the help of electromagnetic lenses. Theelectron microscope takes advantage of the muchshorter wavelength of the electron (e.g., l=0.005 nmat an accelerating voltage of 50 kV) when compared tothe wavelengths of visible light (l=400–700 nm)(Flegler et al. 1993). When the accelerating voltage isincreased in EM, the wavelength decreases and resolu-tion decreases. In other words, increasing the velocity ofelectrons results in a shorter wavelength and increasedresolving power (Flegler et al. 1993).

When an electron beam interacts with the atoms in asample, individual incident electrons undergo two typesof scattering— elastic and inelastic. In the former, onlythe trajectory changes and the kinetic energy and velo-city remain constant. In the case of inelastic scattering,some incident electrons will actually collide with anddisplace electrons from their orbits (shells) around nu-clei of atoms comprising the sample. This interactionplaces the atom in an excited (unstable) state. Specimeninteraction is what makes EM possible. The inelasticinteractions are utilized when examining thick or bulkspecimens (scanning electron microscopy [SEM]).

4.5.1 Scanning electron microscopy

When a sample is bombarded with electrons, the stron-gest region of the electron energy spectrum is due tosecondary electrons. The secondary electron yield de-pends on many factors, and is generally higher for highatomic number targets and at higher angles of incidence.Secondary electrons are produced when an incidentelectron excites an electron in the sample and loses mostof its energy in the process. The excited electron movestowards the surface of the sample undergoing elastic andinelastic collisions until it reaches the surface, where itcan escape if it still has sufficient energy.

Production of secondary electrons is very topogra-phy related. Due to their low energy (5 eV) onlysecondaries that are very near the surface (<10 nm)can exit the sample and be examined. Any changes in


topography in the sample that are larger than thissampling depth will change the yield of secondariesdue to collection efficiencies. Collection of these elec-trons is aided by using a "collector" in conjunctionwith the secondary electron detector.

Backscattered electrons consist of high-energyelectrons originating in the electron beam that arereflected or back-scattered out of the specimen inter-action volume. The production of backscattered elec-trons varies directly with the specimen's atomicnumber. This differing production rates causes higheratomic number elements to appear brighter than loweratomic number elements. This interaction is utilized todifferentiate parts of the specimen that have differentaverage atomic number.

4.5.2 Transmission, High-Resolution and ScanningTransmission Electron Microscopy(TEM/HRTEM/STEM)

TEM is a microscopy technique whereby a beam ofelectrons is transmitted through an ultra thin specimen,interacting with the specimen as it passes through. Animage is formed from the interaction of the electronstransmitted through the specimen; the image is mag-nified and focused onto an imaging device, such as afluorescent screen, on a layer of photographic film, orto be detected by a sensor such as a CCD camera.

For TEM observations, thin samples are requireddue to the important absorption of the electrons in thematerial. Therefore, while powders investigation in-volves no particular problems, the analysis of consol-idated materials requires several thinning steps,including: sample cutting, mechanical grinding andpolishing, dimple grinding and ion milling.

High acceleration voltage reduces the absorptioneffects but can cause radiation damage. At these ac-celeration tensions, a maximum thickness of 60 nm isrequired for TEM and HRTEM observations andquantifications. The main capabilities of a transmis-sion electron microscope of high performance refer to:

& Conventional diffraction contrast imaging (brightfield/dark field) and selected area/convergentbeam diffraction for crystallographic analysis.

& High-resolution phase contrast imaging used forstudying the atomic structure of defects, interfacesas well as imaging individual grains a few nano-meters in diameter.

& Chemical analysis using energy dispersive X-ray(EDX) and electron energy loss spectroscopy(EELS). EELS can also be used to probe thebonding environment of atoms (e.g., sp2/sp3

bonding in carbon).& Energy filtered TEM (EFTEM) for rapid mapping

of chemical elements over a large region.

Fig. 3 TEM images ofun-doped (a) and S-dopedTiO2 (b) powders, thermallytreated 1 h at 673 K

Fig. 4 TEM images of un-doped TiO2 gel, thermally treated 1 h at 573 K (a) and of Ag-doped TiO2 gels, thermally treated 1 h at 573 K(b) and 773 K (c), respectively


& Scanning transmission electron microscopy(STEM) option with high angle annular dark field(HAADF) imaging.

Figures 3, 4 and 5 represent a significant example inwhat the role of TEM is concerned in the TiO2-basedpowders characterization. They comparatively presentthe morphology and the size of particles of some un-doped and S-, Ag-, and Pd-doped TiO2 sol–gel powders,respectively (Răileanu et al. 2009; Crişan et al. 2008b).

The scanning transmission electron microscope(STEM) is an invaluable tool for the characterizationof nanostructures, providing a range of different imagingmodes with the ability to provide information on ele-mental composition and electronic structure at the ulti-mate sensitivity, that of a single atom. The STEMworkson the same principle as the normal scanning electronmicroscope (SEM), by forming a focused beam of elec-trons that is scanned over the sample while some desiredsignal is collected to form an image (Crewe et al. 1970).The difference with SEM is that thin specimens are usedso that transmissionmodes of imaging are also available.

Although the need to thin bulk materials down toelectron transparency can be a major task, it is oftenunnecessary for nanostructured materials, with samplepreparation requiring nothing more than simply sprin-kling or distributing the nanostructures onto a commer-cially available thin holey carbon support film. No longand involved grinding, polishing, or ion milling is re-quired, making the STEM a rapid means for nanostruc-ture characterization (Pennycook et al. 2006).

5 Operational Parameters of PollutantPhotocatalytic Degradation

The pollution of water resources and aquatic environmentcaused by xenobiotic compounds, which are synthetic

chemical substances, is an important problem of pastyears, especially in the case of toxic and refractory pol-lutants like nitroaromatic compounds. Due to their vari-ety (nitrobenzenes, nitrophenols, nitrotoluenes), toxicityand persistence these pollutants has a direct impact uponthe ecosystems health and present an immediate threat tohuman beings via contaminating surface and groundwater supplies. Therefore, it has become a challenge toachieve the effective removal of nitroaromatic pollutantsfrom wastewater in order to assure a sustainable manage-ment of water resources.

Semiconductor assisted photocatalysis (Gaya andAbdullah 2008) is included among economic and envi-ronmental friendly water treatment technology for effec-tive removal of organic pollutants. These days, TiO2 hasbecome an environmental decontamination photocalalystfor a large variety of organics, including nitroaromaticcompounds as it is presented in Table 4. This fact isrelated to its characteristics like: high chemical stability,ability to be activated by sunlight and efficient catalysisof pollutant degradation reactions, easiness of synthesisand use without risks for environmental and humans.

It has been demonstrated that catalyst characteris-tics and concentration, nature and concentration oftarget pollutants, light intensity and wavelength, oxy-gen concentration, oxidants addition, pH of aqueousmedium and the pollution matrix are the main param-eters affecting the efficiency and rate of pollutantdegradation. Each of these parameters will bediscussed in the following sections.

5.1 Catalyst Characteristics and Concentration

As was presented before, the nitroaromatics degrada-tion can be performed by photoinduced processassisted by bare or metal doped TiO2 catalysts. Thelast one usually presents enhanced pollutant photo-oxidation efficiency.

Fig. 5 TEM images ofun-doped (a) and Pd-dopedTiO2 (b) powders, thermallytreated 1 h at 673 K


The effect of metal ion dopants on the photocata-lytic activity is a complex problem due to the fact thatcatalyst activity modification is the result of changesthat occur in: interfacial charges transfer, light absorp-tion capacity of TiO2 and catalyst adsorption capacityof pollutant molecules.

Among the parameters which affect the catalystefficiency in a pollutant degradation process, the

nature of metal dopants can induce different effectson photocatalytic activity. Di Paola et al. (2002a) showthat in the case of transitional metal dopants, the initialdegradation rate (r0) of pollutant (i.e., 4-nitrophenol)depends on catalyst relative rate constant for photo-electron–hole recombination (kr) and catalyst point ofzero charge (PZC), which presents various values fordifferent dopants (Table 5).

Table 4 Application of TiO2-assisted photocatalysis on nitroaromatic pollutants degradation

Pollutant Catalyst Irradiation source Reference

Type Concentration (g/l)

2-, 3-, 4-Nitrophenols TiO2 (Degussa P25) 0.4 Hg lamp (125 W) l=254 nm Di Paola et al. 2003

2-Nitrophenol TiO2 (Degussa P25) 2 UV lamp l=365 nm Wang et al. 1999a, b

4-Nitrophenol TiO2 (Degussa P25) 0.2 Hg lamp (125 W) l=254 Lopez and Gomez 2011

Mo-TiO2 nm

4-Nitrophenol TiO2 (Degussa P25) 1 Hg lamp (125 W) Facchim 2000

Pt-TiO2

4-Nitrophenol TiO2 1.4 Hg lamp (125 W) Di Paola et al. 2002a

M-TiO2

M: Mo, W, Fe, V,Cr, Cu, Co

4-Nitrophenol TiO2 2.5 Hg lamp (450 W) l>340 nm Dieckmann and Gray 1996

4-Nitrophenol TiO2 0.4 UV lamp l=340 nm Zhao et al. 2010

Fe-TiO2

Trinitrophenol TiO2 (Degussa P25) 1 Solar simulator (Xenon lamp) Katsoni et al. 2011

Nitrobenzene TiO2 0.2 Xenon lamp (1500 W) Pelizzetti et al. 1993

ZnO 0.5 Minero et al. 1994

Nitrobenzene TiO2 (Degussa P25) 0.05–0.4 Sunlight Bhatkhande et al. 2003

Nitrobenzene TiO2 (Degussa P25) 0.05–0.3 UV lamp l=365 nm,l=254 nm

Bhatkhande et al. 2004

Nitrobenzene TiO2 (Degussa P25) 0.2 Xenon lamp (1,500 W)l=340–400 nm

Piccinini et al. 1997

Nitrobenzene TiO2 0.2 Hg lamp (125 W) Rajesh et al. 2011

M-TiO2

M: Li, Pd, Sr, Mg

2-, 3-, 4-nitrotoluene TiO2 (Fujititan TP-2) 4 Super-high pressure Hglamp 500 W)

Vohra and Tanaka 2002

Pt- TiO2 3

Trinitrotoluene TiO2 0.2 Medium-pressure Hg lamp(450 W)

Schmelling and Gray 1995

l>340 nm

Trinitrotoluene TiO2 (Degussa P25) 0.1 Hg lamp (200 W) Wang and Kutal 1995

Mono-, di-, tri-nitrobenzenes TiO2 (Degussa P25) 1 Xenon lamp (150 W) Dillert et al. 1995

Mono-, di-, tri-nitrotoluenes

Mono-, di-, tri-nitrophenols TiO2 (FujititanTP-2) 0.075 Hg lamp (500 W) Tanaka et al. 1997

Di-, tri-nitrobenzenestrinitrotoluene

TiO2 (Degussa P25) 0.1 Hg lamp l=254 nm Nahem et al. 1997

Trinitrotoluene TiO2 (Degussa P25) 1 Xenon lamp (150 W) Dillert et al. 1996

Trinitrobenzene


Presented data reveal the decrease of initial pollut-ant degradation rate in the following order: W-TiO2 >Mo-TiO2 > Cu-TiO2 > Co-TiO2 > Fe-TiO2 > Cr-TiO2 >V-TiO2, which is in accordance with increase ofphotoelectron–hole recombination rate constants, fordifferent doped metal ions. This behavior is ascribedto the photogenerated band-gap electrons and holes,which can migrate to the surface and govern redoxreactions with adsorbed substrates, in competitionwith their disappearance due to mutual recombination.Because certain surface sites can trap the photoexcitedelectrons before their recombination with the holes,the increased lifetime of these charges favor thepollutant oxidation by holes or by radical species(●OH, O2

●−) generated on the surface of catalystdue to holes and electrons reactions with water,OH− and O2, respectively.

The mentioned data show that Cr-TiO2 and V-TiO2

present the lowest photoactivity and also the lowestPZC values. Possible explanation of their photoreactivitycan be provided by taking into account the pollutantnature, acid–base character and electronic properties.It is obvious that acidic compounds like 4-nitrophenolwill present a poor interaction with acidic catalystsurface which disfavor pollutant adsorption and

decreases catalyst photoactivity. On the other hand,the maximum photoactivity for W-TiO2 catalyst is inaccordance with PZC=6.6 value, which is not too acidand with kr value which is the lowest among the dopedcatalyst presented. The beneficial effect due to the pres-ence of tungsten has been assigned to the formation ofW(V) species by transfer of a photogenerated electronfrom TiO2 to W(VI). Subsequently W(V) could beoxidized to W(VI) by transferring electrons to adsorbedO2 (Di Paola et al. 2002b).

Substitutional doping of TiO2 with Fe has a controver-sial influence on the catalyst photoactivity. Some papersshow that Fe(III) behaves like an electron/hole recombi-nation center leading to increase of kr value (Navio et al.1999; Serpone et al. 1994). Other studies indicate thatdoping with proper concentration of Fe(III) can drasticallyincrease the charge lifetime, which can be extended tominutes and even hours (Gennari and Pasquevich 1998).

Studies regarding the influence of Fe content in Fe-TiO2 doped catalyst used for the degradation of 4-nitrophenol revealed that photocatalytic activity is de-creasing with increasing of Fe(III) load between 1 % and8 % onto TiO2 at the beginning of irradiation, while after45 min it increases by further increasing of Fe(III) con-tent, except for the 1 % Fe-TiO2 (Zhao et al. 2010). Thisbehavior is in relation with high iron ions leaching intosolution. In spite of this aspect all doped catalysts showto be more photoactive than bare TiO2 and the optimalamount of iron is 1 %. After 60 min of irradiance, usingthis catalyst, the pollutant and total organic carbon(TOC) removal efficiencies are 100 % and 67.5 %, re-spectively. The beneficial effect of iron presence onphotocatalytic activity can be explained by the followingreactions which emphasize that transition metal ions areacting as hole–electron traps, especially at low concen-tration level (Ambrus et al. 2008; Yu et al. 2009):

Fe3þ þ e� ! Fe2þ ð5:1Þ

Table 5 Some properties of transitional metal doped catalystand initial photooxidation rates of 4-nitrophenol

Catalyst PZC (pH) kr (cm3 s−1) r0x 10

10

(M s−1 m−2)

Co-TiO2 7.7 2.5 2.3

Cr-TiO2 3.3 2.3 1.2

Cu-TiO2 7.6 2.3 3.1

Fe-TiO2 7.4 4.1 2.0

Mo-TiO2 5.8 2.1 3.9

V-TiO2 5.4 3.1 1.1

W-TiO2 6.6 1.9 6.4

Table 6 Structural andelectronic properties of bareTiO2 and doped catalysts

Catalyst Bare TiO2 Li-TiO2 Mg-TiO2 Pd-TiO2 Sr-TiO2

Anatase phase content (%) 100 76 91 89 90

Crystallite size (nm) 28 30 28 29 28

BET surface area (m2 g−1) 38 15 24 33 24

Average pore diameter (nm) 11.2 17.7 16.8 12.1 14.5

Ionic radius of doped metal (Å) – 0.76 0.72 0.86 1.12

Band-gap (eV) 3.03 3.10 3.14 3.20 3.14


Fe2þ þ O2ðadsÞ ! Fe3þ þ O2�� ð5:2Þ

Fe2þ þ Ti4þ ! Fe3þ þ Ti3þ ð5:3Þ

Fe3þ þ hvbþ ! Fe4þ ð5:4Þ

Fe4þ þ OH� ! Fe3þþ�OH ð5:5Þ

As was mentioned before, iron ions can also act asrecombination centers for the hole–electron pairswhen their concentration is high, according to thefollowing reactions (Ambrus et al. 2008):

Fe3þ þ e� ! Fe2þ ð5:6Þ

Fe2þ þ hvbþ ! Fe3þ ð5:7Þ

It has been also proved that Fe doping induced abatho-chromic shift by decrease of band-gap or in-troduction of intra-band-gap states, results in morevisible light absorption, with a positive effect onphotocatalytic activity and also on operation costsavings for practical application of photocatalysis inwater treatment process using solar light as irradia-tion source. This advantage is limited by the level ofused transition metal, because the excess of deposit-ed iron on TiO2 can form Fe(OH)2+ species, with agreat absorbance of incident light in UV–VIS do-main in respect to bare TiO2. This higher adsorptionis responsible (together with other factors) for thedecrease of the photocatalytic activity of Fe dopedcatalyst (Qi et al. 2005).

Rajesh et al. (2011) who have dealt with the Li, Mg,Pd and Sr doping of TiO2 demonstrated that ion radiusof metal doped is another parameter that can modifythe catalyst photoactivity. Studying the photooxidationof nitrobenzene in aqueous solution (4×10−4 M) underUV light irradiation, they found that after 4 h irradi-ance, pollutant degradation efficiency was 60 %,78 %, 69 %, 79 % and 74 % with bare TiO2, Li-TiO2, Mg-TiO2, Pd-TiO2 and Sr-TiO2, respectively,for 0.2 g/l catalyst dose. These findings revealed thathigher ionic radii of doped metal assure better nitro-benzene conversion. The lithium doped catalystshowed also the highest initial rate (1.3×10−7 M s−1)of nitrocompound degradation, although surface areaof this catalyst was observed to be lower than all othercatalysts, as presented in Table 6 (Rajesh et al. 2011).

Catalyst concentration in the photocatalytic watertreatment system affects the overall pollutant degradationrate in a true heterogeneous catalytic regime, wherethe dosage of TiO2 is directly proportional to theoverall photocatalytic reaction rate. A linear depen-dency occurs until catalyst concentration increasesabove a limit value when the degradation rate be-comes independent of catalyst concentration (Gayaand Abdullah 2008). The limit values mainly resultsfrom following factors: aggregation of catalyst parti-cles at high concentration causing a decrease in thenumber of surface active sites and increase in opacityand light scattering of catalyst particles, leading todecrease in the passage of irradiation through thereactor (Chan and Ray 1998).

A large number of studies have described also theeffect of TiO2 concentration on the pollutant degrada-tion efficiency. Zhao et al. (2010) reported the influenceof this parameter in the case of 4-nitrophenol photocat-alytic degradation on 1 % Fe-TiO2 doped catalyst at 1 hirradiation. The degradation yield increased to 95 % byincreasing catalyst concentration up to 0.1 g/l, and thenit was nearly constant from 0.1 to 0.4 g/l. Further in-creasing of catalyst concentration above 0.4 g/l resultedin a decrease to 75 % of the degradation yield. It wasobserved that a very small dose of doped catalyst issufficient for advanced degradation of 1.4×10−4 M pol-lutant concentration. The photocatalytic degradation ofthe same pollutant using 0.2–4 g/l dose of Degussa P25TiO2 was reported by Chan and Ray (1998). They foundan optimal catalyst concentration of 2 g/l, whichemphasized that among other factors (reactors geometryand UV light sources applied) the higher photoactivityof doped catalyst compared with un-doped TiO2 has animportant influence on the pollutant degradation effi-ciency. The Degussa P25 TiO2 was also used for degra-dation of 1.6×10−4 M nitrobenzene and the optimumcatalyst level reported was 3 g/l (Bhatkhande et al. 2003,2004). The different catalyst loading requested for thetwo nitrocompounds degradation is due to the highsensitivity of nitrophenol to the •OH radicals attackcompared with nitrobenzene.

The profile of pollutant degradation efficiency versuscatalyst concentration is a consequence of the increasednumber of catalyst particles that will lead to the increaseof absorbed photons and pollutant molecules number. Inthis context, the degradation efficiency will be enhancedwith increasing catalyst concentration due to the in-crease of total surface area available for contaminant


adsorption. Excess catalyst concentration is leading towater suspension opacity, which decreases light pene-tration. It is also reported that above certain catalystlevel, the number of TiO2 surface active sites maybecome almost constant due to decreased light penetra-tion, increased light scattering and loss in surface areabecause catalyst aggregation (particle–particle interac-tion). The occurrence of all these phenomena has asresult the decrease of pollutant degradation efficiency(Haque and Muneer 2007; Saquib et al. 2008a, 2008b).For any application, the photocatalytic reactor should beoperated at optimum catalyst concentration to avoidexcess catalyst and to ensure efficient photons absorp-tion and pollutant degradation.

5.2 Pollutant Nature and Concentration

Various studies have reported that reactivity of aro-matic compounds via TiO2 can be drastically affectedby the nature of target pollutant. Among the structuralpollutant characteristics which influenced its photolyt-ic degradability the electronic nature of substituents,their number and position in the aromatic ring areincluded (Parraa et al. 2003).

Tanaka et al. (1997) studying the photocatalytic deg-radation of mono-, di-, and trinitrophenols comparedwith phenols (2×10−4 M), using TiO2 in suspension(3 g/l) and a super-high pressure mercury lamp(500 W), as irradiance source, have found that all pol-lutants disappeared before 90 min following a pseudo-first-order kinetics. The degradation rate were in theorder: phenols > 4-nitrophenol > 2,4-dinitrophenol> 2,4,6-trinitrophenol as indicated by the values ofapparent rate constant (kI) which are decreasing from15.5×10−3 to 3.3×10−3 min−1. This behavior is in ac-cordance with other studies that show that the kinetic ofthe photocatalytic degradation of aromatic pollutants isfaster for the compounds with electron-donating sub-stituents like hydroxyl group in comparison withelectron-withdrawing substituents like nitro group.

Minero et al. (1994) also found that 2-nitrophenol iseasily photocatalytic oxidized compared with nitro-benzene, as it was revealed by rate constant values ofthe two compounds which are 0.27 and 0.17 min−1,respectively. This effect depends on electronegativityof functional group and their influence on the electricdensity of the aromatic ring. This is because pollutantphotocatalytic degradation process is based on elec-trophilic hydroxyl radical attack which is faster when

there is a greater electronic density on aromatic ring.Taking into account these aspects makes it easy toexplain the degradation suppressing effect observedfor the nitro group which increases with their number.

The effect of substituents nature can be analyzedmore quantitatively by using Hammett constants (σ),as D’Oliveira et al. (1993) had done for the study ofchlorine substituent on the photocatalytic degradationof chlorophenols. This constant represents the effectthat different substituents have on the electronic char-acter of a given aromatic system. A positive value of σindicates an electron-withdrawing (deactivating)group and a negative value indicates an electron-donating (activating) group. In the case of phenol,mononitrophenol, dinitrophenol and trinitrophenol σvalues increases from 0 to 2.34, which is in a goodcorrelation with the decrease of apparent pseudo-first-order rate constant established by Tanaka et al. (1997).

The same negative influence of nitro groups towardsthe attack of electrophilic reagent on the aromatic mol-ecule was emphasized also by Dillert et al. (1995) intheir study regarding degradation of nitrobenzenes andnitrotoluenes (1×10−4 M) in TiO2 aqueous suspension(1 g/l) using a xenon lamp (150 W) equipped with afilter to minimize radiation with wavelengths shorterthan 320 nm. The following order of reactivitywas identified: nitrotoluene > nitrobenzene >dinitritoluene > dinitrobenzene > trinitrotoluene >trinitrobenzene that confirmed the influence of in-creased number of deactivating nitro group and alsounderscores that methyl group of the toluene enhancedphotocatalytic reactivity of aromatic compounds.

Another study by the same authors (Dillert et al. 1996)regarding photocatalytic oxidation of 2,4,6-trinitrotolu-ene and 1,3,5-trinitrobenzene, performed in identicalworking conditions, show a superior initial degradationrate of the first compound (r0=1.3×10

−4 M min−1) com-pared with the second one (r0=2.2×10

−5 M min−1),which is consistent with the increased reactivity to deg-radation caused by the presence of methyl substituent.

It is also stressed that position of substituents can beanother structural characteristic that affects the pollutantreactivity. In order to establish the influence of the sub-stituents position on nitroaromatic compound reactivityin TiO2 suspension (0.4 g/l), Di Paola et al. (2003) studiedthe photocatalytic degradation of 2-,3-, and 4-nitrophenol(1×10−3 M) using a medium pressure mercury lamp(125 W). The three isomers have revealed different reac-tivity with the values of rate constant situated between


2.5×10−3 and 4.4×10−3 min−1, in the following order: 4-nitrophenol >2-nitrophenol >3-nitrophenol, in agreementwith the results of Dieckmann et al. (1992).

Various reactivity orders are reported in other papers,but can be due to differences in experimental conditionssuch as the catalyst used, the lamp type (Maurino et al.1997), the pH (Wang et al. 1999a, b). Kumar and Davis(1997) showed that 3-nitrophenol was photooxidizedwith greater difficulty than the two other isomers due tohigh stability of the surfaces species produced. Anotherpossible explanation offered by Vohra and Tanaka (2002)is related to the effective resonance structure of 2- and 4-nitrophenols, whilst 3-nitrophenol, due to the position ofthe nitro group, cannot delocalize the charges. As aconsequence, nitro group at the ortho and para positionaffects the electrophilic reaction of hydroxyl radical moresignificantly than at the meta position and 2- and 4-nitrophenols are faster oxidized than 3-nitrophenol.

As the effect of pollutant concentration is of impor-tance in any water treatment process, it is necessary toinvestigate its influence on process efficiency for thesuccessful application of the photocatalytic degradationmethod. Bhatkhande et al. (2003) studying the influenceof nitrobenzene initial concentration on pollutant degra-dation in TiO2 aqueous suspension (3 g/l) observed thattotal pollutant removal was achieved within 2 h of irradi-ance for (0.8–1.6)×10−3 M, whereas 4 h were required toeliminate high pollutant initial concentration (2.4×10−3–4×10−3 M). In other words, at the same irradiation timethe percentage of degraded pollutant is smaller if its initialconcentration is higher. The same behavior was recordedby Tanaka et al. (1997) in the case of other nitrocompoundlike 4-nitrophenol, using 2.3 g/l catalyst loading. Thedegradation efficiency decreases from 64 % to 24 %while pollutant initial concentration increases between2×10−4 and 1×10−3 M, at 30 min irradiation.

It should be noted that Chan and Ray (1998) observeddifferent concentration versus time profiles for variousinitial concentrations of the same pollutant which couldbe correlated, by the following exponential equation:

C ¼ C0 exp �kappt� �

;

where C is the pollutant concentration at irradiationtime t, C0 is the initial pollutant concentration, andkapp is the apparent rate constant (min−1). Because thisrate constant decreases with the increase in pollutantconcentration, the degradation process follows apseudo-first-order kinetic, as Di Paola et al. (2003)also emphasized for the three isomers of nitrophenol.

In the case of 2-nitrophenols, the authors show thatrate constant decreases from 1×10−2 to 2.7×10−3 min−1 for an increase of initial pollutant concen-tration between 0.3×10−3 and 1×10−3 M. The inversedependence of rate constants on the pollutant initialconcentration can be justified by assimilating thephotodegradation rate with Langmuir–Hinshelwood(L–H) kinetics.

Carp et al. (2004) taking into consideration that deg-radation rate of substrate present a saturation trend,show that this behavior can be assigned to the followingaspects: (1) the main step of photocatalytic degradationis a surface process, therefore a high adsorption capacityis associated with reaction favoring; this means that athigh pollutant initial concentration, all catalytic sites areoccupied and a further increase of concentrations doesnot affect catalyst surface concentration, therefore, thismay results in a decrease of the first-order rate constant.(2) Photogeneration and migration of electron–holepairs and their interaction with pollutant occurs in series,therefore, each of them may become rate-determiningstep for overall process; at low pollutant concentrationthe second step dominates the process and the degrada-tion rate increases linearly with pollutant loading, whileat high pollutant concentration the first step willgoverning the process and the degradation rate increasesslowly with pollutant loading. (3) Intermediates gener-ated during the degradation process also influence therate constant of parent pollutant; a higher initial pollut-ant concentration yield to a increased concentration ofadsorbed intermediates which will affect the overall rate.

Reported data suggest that ensuring advanced degra-dation of recalcitrant pollutants as polynitroaromatic de-rivates imposes the use of catalyst with increasedphotocatalytic activity like transitional metal-doped TiO2

or/and prolonged irradiation time for a given light inten-sity, catalyst loading and pollutant initial concentration.

5.3 Light Intensity and Wavelength

The initial electron–hole formation rate depends onthe light intensity (Marugan et al. 2008; Pareek et al.2008). To achieve a high photocatalytic reaction rate, arelatively high light intensity is required to adequatelyassure each TiO2 surface active sites with sufficientphotons energy (Chong et al. 2010).

In many studies, the organic conversion in thepresence of UV wavelength (l<400 nm) obeyed thelinear proportionality correlation to the incident


radiant flux (Glatzmaier et al. 1990, 1991). Dillert etal. (1995) investigated the degradation of nitrotoluenesand nitrobenzenes compounds in irradiated aqueousTiO2 suspensions varying the pH of the suspensionsand the light intensity. The disappearance rate of theorganic solute followed the first-order kinetics. Theinitial rate of the 1,3,5-trinitrobenzene photodegradationfor light intensities of 2.9 and 3.9 μmol photons min−1

were 0.35 and 0.39 μmol l−1 min−1, respectively. Thisindicates that the increased light intensity enhances bothhydroxyl radicals’ generation and pollutant removal.The same effect of light intensity increase, between 0.8and 4.0 μmol photons min−1, on initial reaction rate wasobserved in the case of 2,4,6-trinitrotoluene photocatalyt-ic degradation at pH=5 and pH=9. Bhatkhande et al.(2004) compared the degradation rate of nitrobenzeneover Aldrich-TiO2 under the wavelengths of 253 and365 nm and found that degradation rate at 253 nm israther higher than at 365 nm. This behavior is due to thepromotion in the conduction band of TiO2 of electronswith higher kinetic energy at 254 nmwavelength that canmove to the solid–liquid interface more easily and sup-press electron–hole recombination compared with365 nm wavelengths (Jing et al. 2011). Photocatalyticdegradation of nitrobenzene with Degussa P-25 as cata-lyst and using UV lamps emitting lights at lmax=253 nmand lmax=365 nm has shown almost similar behavior(Bhatkhande et al. 2004). This is also in agreement withthe results of Hofstadler et al. (1994).

Comparative studies of nitrobenzene photocatalyticdegradation using concentrated solar radiations and arti-ficial UV light in the same reactor have been performedby Bhatkhande et al. (2003). They show that nitroben-zene was completely removed in 4 h by solar irradiationand in 1 h using artificial UV lamp. This is due to the factthat in the artificial light the percent of useful component(UV) is substantially higher compared with solar light(Bhatkhande et al. 2004). Overcoming of this disadvan-tage can be accomplished by using of transitional metal-doped TiO2 with improved photoactivity in visible do-main (Shen et al. 2009), allowing replacement of artifi-cial UV lamps (power consumers) with solar light andleading to drastically diminishing of power consume andwater treatment costs.

5.4 Dissolved Oxygen

Oxygen plays an important role for semiconductorassisted photocatalytic degradation of organic

compounds. Dissolved molecular oxygen assures suffi-cient electron scavengers to trap the excited conduction-band electron and prevents charges recombination(Chong et al. 2009). Dissolved oxygen may be involvedin the formation of other reactive oxygen species andstabilization of radical intermediates, mineralization anddirect photocatalytic reaction (Chong et al. 2010).According to Okamoto et al. (1985), oxygen is theprimary acceptor of the conduction band electrons withformation of superoxide ion radical:

O2 þ eCB ! O2�� ð5:8Þ

Hydroperoxyl and hydroxyl radicals can be alsogenerated by the following reactions:

O��2 þ H2O ! OH� þ HO�

2 ð5:9Þ

HO�2 þ HO�

2 ! H2O2 þ O2 ð5:10Þ

O��2 þ H2O2 ! �OHþ OH� þ O2 ð5:11ÞMoreover, O2 appears to be the logical source of the

reactive oxygen species required for the stoichiometricproduction of CO2 (Wang and Kutal 1995). The presenceof dissolved oxygen is also suggested to induce the cleav-age mechanism for aromatic rings in organic pollutant.

Wang and Kutal (1995) investigated the oxygen effecton the photocatalytic degradation of 2.2×10−4 M 2,4,6-trinitrotoluene by 0.1 g/l P25 catalyst. The irradiation for6 h of N2-bubbled aqueous suspension of TiO2 withpollutant content provided rapid disappearance ofnitroaromatic compound, but less than 6 % of TOCconversion into CO2. In comparison, O2 bubbled sampleirradiated only 4 h assures 60 % conversion of TOC toCO2. These data demonstrate that rapid loss of trinitro-toluene under anaerobic conditions does not result fromits mineralization. Dieckmann and Gray (1996) studiedphotocatalytic degradation of 4-nitrophenol in the pres-ence of O2, and reported that complete disappearance ofpollutant is achieved after 3 h of irradiation. The initialpseudo-first order rate constant was 3.4×10−2 min−1. Inthe absence of oxygen in the same working conditions,the pseudo-first order rate constant of pollutant degrada-tion decreases to 6×10−3 min−1. This emphasis thatdissolved O2 plays a key role for TiO2-assisted photocat-alytic degradation of organic pollutants, due to its posi-tive effect on the diminishing of unfavorable electron–hole recombination which affect the process perfor-mance. However, is necessary to be noted that too much


increase of O2 level has an inhibiting effect on the pol-lutant adsorption on active sites, due to highly hydroxyl-ation of catalyst surface. As consequence the establishingof optimal O2 content must be performed for efficientapplication of the process as water treatment method.

5.5 Addition of Oxidant

The addition of external oxidants such as H2O2, KBrO3

and (NH4)2S2O8 into a semiconductor suspension hasbeen shown to increase the photocatalytic degradationof pollutant by (1) preventing the electron–hole recom-bination through accepting the conduction band electron,(2) increasing the hydroxyl radical concentration and (3)generating of other oxidizing species which increases theintermediate compounds degradation efficiency (Quamaret al. 2005a, b; Saquib and Muneer 2002; Saquib et al.2008a, 2008b; Faisal et al. 2007; Haque and Muneer2007). The order of enhancement is reported to beUV/TiO2/H2O2>UV/TiO2/BrO3

−>UV/TiO2/S2O82−

(Jing et al. 2011). The role of H2O2 addition in theincreasing of the oxidizing radical species concentrationsis illustrated by the following equations:

H2O2 þ e� ! �OHþ OH� ð5:12Þ

H2O2 ��Yhn HO��2 þ Hþ ð5:13Þ

H2O2 ��Yhn �OHþ �OH ð5:14ÞZhao et al. (2010) investigated the effect of H2O2

addition from 0.98 to 7.9 mM on the photocatalyticdegradation of 4-nitrophenol using 1 % Fe-TiO2 dopedcatalysts. They observed that pollutant degradation effi-ciency increases with the increase of the H2O2 concen-tration up to 5.9 mM. This effect can be attributed to theincrease of the concentration of hydroxyl radicals gen-erated by photolysis of H2O2. For higher concentrationno significant additional improvement was observed.Moreover, excess addition of hydrogen peroxide intothe suspensions results in decrease of reaction rate, asDillert et al. (1996) showed in their studies concerningthe photocatalytic degradation of trinitrotoluene andtrinitrobenzene in H2O2 presence. This behavior em-phasized that oxidant addition is a concentration limit-ing process, because H2O2 is also a hydroxyl radicalscavenger and its higher concentrations determinatesquenching of this species and decreasing of degrada-tion rate.

Because both dissolved O2 and H2O2 are electronacceptors in heterogeneous photocatalysis, addition ofextra oxidant is usually applied only for low oxygen level,but this method involved supplementary treatment cost.

5.6 pH

This is one of the most important operating parametersthat affects the photocatalytic degradation of organic con-taminants since it determines the charging of the surface ofphotocatalyst, hydrophobicity, the size of its aggregates,net charge of pollutants and the amount of generatedhydroxyl radical (Bahnemann et al. 2007). Different pHwill affect the charge density of the TiO2 catalyst (Rinconand Pulgarin 2004), as shown in the following reactions:

pH < PZC : TiOHþ Hþ ! TiOH2þ ð5:15Þ

pH > PZC : TiOHþ OH� ! TiO� þ H2O ð5:16Þ

The value of PZC for TiO2 (Degusa P25) is widelyreported at pH ~6.25 (Zhu et al. 2005b).

Bhatkhande et al. (2004), studying the pH effect onthe photocatalytic degradation of nitrobenzene (4×10−3 M) by TiO2 suspension (0.5 g/l), found it had anegligible effect on the pollutant photocatalytic degra-dation because this one does not have a group (e.g., OH,NH2) which can react with acid/base and ionize underdifferent pH values. In another study, Bhatkhande et al.(2003) reported that in both acidic and alkaline ranges,there is a relatively small variation of photocatalyticnitrobenzene degradation efficiency (95–98.5 %) andTOC removal (88–92 %) at 2.4×10−3 M initial nitro-benzene concentration and catalyst loading of 3 g/l.However, maximum pollutant and TOC removal wasachieved at pH between 6.5 and 7, which is close to thePZC of TiO2. In this pH domain, the pollutant, its maindegradation intermediates (phenolic derivates) and cat-alyst are neutral; consequently no charges repulsionoccurs and catalyst presents the highest adsorption ca-pacity which is favoring degradation and mineralizationreactions. The same negligible influence of pH on theinitial rate was reported by Dillert et al. (1995) in theirstudies regarding photocatalytic degradation of trinitro-toluene and other nitroaromatic compounds (1×10−4M)in TiO2 suspension (1 g/l).

A different behavior was reported by Venkatachalamet al. (2007) in the case of 4-nitrophenol (1.8×10−3 M).They found that degradation efficiency at pH=5 washigher than at pH=9. Pollutant degradation is


encouraged in the acidic condition due to an enhancedadsorption of 4-nitrophenol on the TiO2 surface. Inaddition, minimization of electron–hole recombinationin the acidic solution is other factor for the enhanceddegradation efficiency (Jing et al. 2011). In the alkalineconditions both 4-nitrophenol and TiO2 surface carriednegative charge and therefore the degradation efficiencywas lower. Zhao et al. (2010), using a 1 % Fe-TiO2

doped catalyst (0.1 g/l), showed that the degradationefficiency of the same pollutants (1.4×10−4 M) de-creased rapidly with increase in pH value. They explainthat besides the decrease of pollutant adsorption at pH>8, another factor which affects its degradation is CO2

production during the pollutant and intermediates’ deg-radation and its conversion to CO3

2− and HCO3− under

strong alkaline conditions. These anions are able to reactwith hydroxyl radical by the following reaction(Behnajady and Modirshahla 2006)

CO2�3 þ �OH ! CO��

3 þ �OH k ¼ 3:9� 108M�1s�1

ð5:17Þ

HCO�3 þ �OH ! CO��

3 þ H2O k ¼ 8:5� 106M�1s�1

ð5:18Þ

and generate inorganic radical anions with lower reac-tivity than hydroxyl radical which end up in a decreaseof the organic compound degradation.

The complex effect of pH in the degradation pro-cess requires establishment of its optimal values spe-cific for certain water advanced oxidation systems(catalyst/target pollutant/pollution matrix) in order toensure efficient photocatalytic water treatment.

5.7 Water Pollution Matrix

It is known that real wastewater or natural water arequite complex systems, containing both inorganic andorganic compounds; therefore, the basic understand-ing of the influence of these compounds on the pho-tocatalytic performance is essential to put in practice aTiO2 water treatment process.

Crittenden et al. (1996) reported that photocatalystdeactivation was usually observed when this process isapplied in the treatment of a water with inorganic saltscontent. This behavior is related to the inhibitory effectof inorganic ions on the surface of TiO2 semiconductor,because pollutant photocatalytic degradation is a surfaceprocess and adsorption of ions on catalyst may be incompletion with pollutant adsorption affecting the

system performance (Minero et al. 1992; Calza andPelizzetti 2001). At the same pH values, the differentanion adsorption degree is dependent on their nature andon the exchange reactions with hydroxyl groups, so isexpected that various anions differently affect the pol-lutant degradation efficiency. Bhatkhande et al. (2003)studied the effect of the presence of common anions likechloride, carbonate, bicarbonate, sulfate and nitrate(0.1 M) on the photocatalytic degradation of nitroben-zene (2.4×10−3 M) in TiO2 suspension (3 g/l), using aUV lamp. They found the following trend: Cl− >HCO3

−

> CO3−2 > SO4

−2 > NO3−. The first three anions have a

more pronounced influence on degradation process, thepollutant efficiency decreases from 97% (in the absenceof anions) to 90–93 % (in the presence of anions). Thisis due to their effect on adsorption of nitrobenzene onthe catalyst surface, like in the case of chloride anionsand their ability to act as hydroxyl radical scavengersespecially for bicarbonate and carbonate anions. In thelast case, carbonate radical, generated from the reactionof bicarbonate and carbonate anions with hydroxyl rad-ical, is a weak oxidizing agent in comparison withhydroxyl radical and the pollutant degradation efficien-cy is decreasing (Abdullah et al. 1990). Sulfate andnitrate have no effect on adsorption of pollutant andconsequently the 97 % nitrobenzene removal efficiencywas obtained both in their presence and absence. Thesame behavior of bicarbonate, chloride and nitrate an-ions was reported by Schmelling and Gray (1995) in thecase of photocatalytic degradation of trinitrotoluene(2.2×10−4 M) in TiO2 suspension (0.25 g/l) at pH=8.5, using a 340-nm UV lamp. In the presence of 1×10−2 M anion concentration, the value of pseudo-first-order rate constant is 8.0×10−2 min−1 compared with8.7×10−2 min−1 recorded in their absence. This effect ismore pronounced when anion concentration increases,as in the case of 0.1 M bicarbonate concentration whenrate constant value is 6.5×10−2 min−1. The rate of trini-trotoluene mineralization is also decreased by bicarbo-nate. At the lowest bicarbonate concentration thepercentage of dissolved organic carbon (DOC) mineral-ized after 60 min of irradiation was reduced by 26 %,while at higher bicarbonate concentration the DOC re-moval after 180 min was only 45 %, as compared to90 % DOC mineralization in the absence of anions.Reported data show once again the hydroxyl radicalscavenging effect of bicarbonate anion, which affectnot only the parent pollutant degradation but also thephoto-oxidation of degradation intermediates, as


retardation of DOC mineralization illustrated. Thesefindings are in accordance with the increase of someintermediates concentration in irradiated samples, liketrinitrobenzene, in the presence of bicarbonate anion,reported by the author.

Comparing the effect of common anions onnitroaromatics degradation versus other aromatic com-pounds like phenols, benzoic acid and hydroxyl-benzoic acids, it should be noted that their presenceis not so harmful for strongly adsorbing species suchas nitrobenzenes and nitrotoluenes (Yamalkav et al.2001; Ajmera et al. 2002; Subramaniam et al. 2000).

Among other inorganic species, which can be usuallypresented in the matrix of a wastewater, metal ions are ofspecific interest for the application of photocatalysis aswater treatment process, because both beneficial anddetrimental effects of their presence are reported. Itshould be underlined that these effects are stronglydependent on the nature of metal ions and their concen-tration (Litter 1999).

A general consensus of numerous studies show thata certain level of Cu2+, Fe2+, Al3+ may decrease thephotocatalytic reaction rate while Ca2+, Mg2+ andZn2+ may have negligible effects (Chong et al.2010). This is because the last cations are presentedin the maximum oxidation states resulting in theirinability to retard pollutant photocatalytic degradation.

Controversies on the effect of Cu2+ on the pollutantdegradation rate are presented in literature. Brezova etal. (1995) showed that Cu2+ enhances the aromaticcompounds degradation rate until an optimal concen-tration value is reached. In accordance with this, San etal. (2002) revealed that Cu2+ concentrations lowerthan 10−4 M can enhance the 4-nitrophenolphotcatalytic degradation rate, while further increasesof metal ion level has a detrimental effect on pollutantdegradation. Other authors mentioned a negative ef-fect on the aromatic compounds degradation for anyCu2+concentration (Wei et al. 1992).

Sclafani et al. (1991) observed that the presence ofFe2+ leads to decreases of some aromatic compoundsdegradation due to its competition with organic sub-strate for oxidizing species. Another detrimental effectof Fe2+ is fouling of catalyst surface with rustyorange color which changes with Fe(OH)3 forma-tion. The deposition of insoluble metal hydroxideon the TiO2 surface causes a drastic diminishing ofits photoactivity by blocking the accessibility of bothphotons and pollutant to the catalyst, having as result

decreasing of pollutant degradation rate (Choi et al.1994). The influence of various Fe2+ concentrations(0.01–0.1 M) on photocatalytic degradation of nitro-benzene was also studied by Bhatkhande et al. (2003).They observed that pollutant and TOC removal atdifferent metal ion levels and in its absence is almostthe same. However, only for 0.01 M of Fe2+ concen-tration a slight increase in the removal efficiency ofpollutant and its degradation intermediates wasrecorded. This behavior is in accordance with the re-sults reported by Sclafani et al. (1990) which revealedthe improvement of aromatic pollutants degradationrate in the presence of low concentration of FeSO4.

Despite the negligible effect of some anions or evenpositive effect of some cations on pollutant degrada-tion, in order to prevent photocatalyst deactivation inthe water matrix with high inorganic loading, pre-treatment of effluent by conventional process likeionic exchange, complexation, precipitation, coagula-tion–flocculation, settling should be applied.

6 Kinetics and Mechanism of PollutantPhotocatalytic Degradation

6.1 Kinetics Modeling

Langmuir Hinshelwood (L–H) kinetics is the mostcommon used to explain the heterogeneous catalyticprocesses. Numerous assumptions for the rate ofphotomineralization of organic compounds with irra-diated TiO2 follows the L–H law for the four possiblesituations: (1) the reaction takes place between twoadsorbed substances, (2) the reactions occurs betweena radical in solution and an adsorbed substrate mole-cule, (3) the reaction takes place between a radicallinked to the surface and a substrate molecule in solu-tion and (4) the reaction occurs with the both speciesbeing in solution (Konstantinou and Albanis 2004).

It is well known that the rate of photocatalytic reac-tion depends upon the •OH concentration (Kamble et al.2004; Turchi and Ollis 1990). The reaction between•OH radicals and organic molecules on the catalystsurface is considered to be the rate determining step.The surface second order rate for decomposition ofnitrobenzene may be written in terms of L–H kineticsas (Heredia et al. 2001; Bhatkhande et al. 2004).

r ¼ k0 0θOH�θC; ð6:1Þ


where k″ is the surface second order rate constant,θOH� is the fractional site coverage by hydroxylradicals and θC is the fraction of sites covered bynitrobenzene.

θOH� ¼ KO2PO2

1þ KO2PO2

ð6:2Þ

θC ¼ KC

1þ KC þPiKiIi

; ð6:3Þ

where KO2 , K and Ki are equilibrium adsorption con-stants and I are the intermediates of nitrobenzenedegradation. Equation 6.3 can be modified by thefollowing assumption:

KC þXi

KiIi ¼ KC0 ð6:4Þ

Such an assumption can be made when the adsorp-tion coefficients for all organic molecules present insolution are equal. Replacing Eq. 6.4 into Eq. 6.3,Eq. 6.1 can be written as:

r ¼ k0 0 KO2PO2

1þ KO2PO2

� KC

1þ KC0ð6:5Þ

Because the oxygen partial pressure remained con-stant, the fractional sites coverage by hydroxyl radi-cals could be considered constant. Therefore,

k0 0 KO2PO2

1þ KO2PO2

¼ constant ¼ kC ð6:6Þ

Equation 4 becomes:

r ¼ kCKC

1þ KC0¼ kobsC ð6:7Þ

Equation 6.7 is a first-order kinetics in respect tothe pollutant concentrations. The values of kC and Kcan be obtained using a linearized form of Eq. 6.7,where 1/r is plotted against 1/C:

1

kobs¼ 1

kCKþ C0

kCð6:8Þ

It was reported that the real K value obtained fromthe linearized plot 1/r against 1/C is significantlysmaller (Malato et al. 2009). This was explained bythe differences in adsorption–desorption phenomenaduring dark and illuminated period.

When concentration C0 is low the equation can besimplified to an apparent first-order rate constant (Eq. 6.9):

LnC0

C

� �¼ kKt ¼ kobst ð6:9Þ

A plot of ln C0/C versus time represents a straightline, slope of this line which upon linear regressionequals the apparent pseudo-first-order constant kobs.

6.2 Mechanism of Pollutant Degradation

Depending of working conditions, photocatalytic con-version of an aromatic compound can occur by differ-ent routes which can lead to various intermediateswith more or less toxic characteristics than parentpollutant, so knowing of degradation pathways is ofgreat importance for implementation of this AOP aswater treatment method.

Many references concerning mechanism ofnitroaromatics degradation in TiO2 suspension (DiPaola et al. 2003; Carp et al. 2004; Tanaka et al. 1997;Dillert et al. 1996; Wang et al. 1998) are generally agreethat the process occurs in two stages: one rapid related tobreaking of aromatic ring and the other slower whichconsists in oxidation of aliphatic chains. Figure 2 illus-trates the pathways for the photocatalytic degradation ofnitrobenzene, based on the mechanisms proposed indifferent studies (Minero et al. 1994; Carp et al. 2004).

OH

OHO2N

OH.

OH.

OH

OH

OH

NH2

NO2

OH.

. NO2

O-H+

CO2 , NO2- , NO3

- , NH4+

ring opening

OH

Fig. 6 The pathways of photocatalytic nitrobenzene degradation


The first stage is the most important because im-plies removal/conversion of nitro group which is re-sponsible for the toxicity of parent pollutant.

As presented in Fig. 6 (Minero et al. 1994; Carp et al.2004), the most important step of photocatalytic degra-dation is hydroxylation of nitrobenzene which impliesthe addition of hydroxyl radical to the aromatic ringyielding to a cyclohexadienyl radical which undergoesfurther oxidation to nitrophenol derivatives. As Mineroet al. (1994) showed the direct attack of hydroxyl radicalat the position carrying the nitro group is no possiblebecause phenol was not detected as intermediate.

The three isomers of nitrophenol formed in 2:1:1(ortho/meta/para) ratio of concentration are furtherdegraded by hydroxyl radical attack which occur atthe ring position activated by the presence of the twosubstituents, namely ortho and para positions in respectto the hydroxyl group. The resulted intermediates are 3-nitrocatechol, 4-nitrocatechol and nitrohydroquinone(Di Paola et al. 2003).

Dieckmann and Gray (1996) showed that anotherroute of nitrophenol conversion is electrophilic substi-tution of nitro group by hydroxyl radical, with elimi-nation of nitrous acid. In the case of 4-nitrophenol thisreaction yield to1,4-benzosemiquinone as intermedi-ate, which disproportionates into hydroquinone and1,4-benzoquinone.

Maurino et al. (1997) have reported that conductionband electron direct reduction of the nitro group innitrophenols with 4-aminophenol resulting can be animportant degradation pathway especially in acid pH,because amine formation requires the presence of H+.Because both 4-nitrophenol and oxygen compete forelectrons consumption, the reduction of 4-nitrophenolis also favored in less oxygenated system.

The polyhydroxylated intermediates resulted fromthe first stage are further oxidized with aromaticring opening leading to different aliphatic derivateslike mesoxalic acid semialdehyde, oxalic acid,hydroxymaleic acid, glyoxal, glyoxalic acid,acetic acid and formic acid (Minero et al. 1994).All these intermediates are mineralized to CO2

under oxidizing condition and prolonged irradiation(hours). Beside CO2, inorganic nitrogen species arealso formed. Nitrite ions are the main productformed at short irradiation times as the result ofdenitration of the nitrophenols. Their concentrationquickly reaches a maximum and then decreasesrapidly as result of oxidation to nitrate ions under

hydroxyl radical attack (Di Paola et al. 2003). Anothernitrogen species formed are ammonium ions resultedfrom aminophenol hydroxylation. Dieckmann andGray (1996) found that nitrate accounted for ~93 %of the total denitration products of 4-nitrophenol atpH=8.5, while ammonium ions only 7 % fromthe total nitrogen, confirming that oxidation is themain route of nitroaromatic compounds degradation.

In the case of nitrotoluenes photocatalytic degrada-tion, the main intermediates are nitrobenzenes whichrevealed that first attack of hydroxyl radicals takes placeon the methyl group substituted to the aromatic ring.The oxidation of methyl group produces nitrobenzoicacids and subsequently nitrobenzenes by decarboxyl-ation, as has been observed for di- and tri-nitrotoluenes(Dillert et al. 1995; Schmelling and Gray 1995).

Presented data emphasize the complex mechanism ofnitroaromatic compounds photocatalytic degradationwhich involves both oxidation and reduction reactions.The main degradation route is hydroxyl radical attackleading to polyhydroxylated benzenes with better bio-degradable characteristic than parent pollutant and itsprimary intermediates. The aliphatic acids and alde-hydes are the secondary intermediates resulted frompolyhydroxylated benzenes ring cleavage by oxidativepathway. Final degradation products are: CO2, nitrateions and ammonium ions at low concentration.

7 Conclusions

The paper represents a comprehensive study regarding thelevel of knowledge in the synthesis and characterization ofTiO2-based nanopowders, as well as of the operationalparameters of the pollutant photocatalytic degradation. Arecent literature survey underlining the importance of thesol–gel procedure for the un-doped and doped TiO2-basednanopowders preparation was also included.

TiO2-assisted photocatalysis is an efficient degra-dation method of nitroaromatic compounds which canbe applied for the conversion of toxic pollutants intobiodegradable intermediates or for their mineraliza-tion, in a wastewater treatment flow.

Acknowledgments This work was supported by a grant of theRomanian National Authority for Scientific Research, CNDI–UEFISCDI, project number PN-II-PT-PCCA-2011-3.1-0031.

Author contribution All the authors of this present paperhave equal contribution to its elaboration.


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