TiO2 Nanostructures: Recent Physical Chemistry Advances■ TITANIUM DIOXIDETiO2 has emerged as one of the most fascinating materials inthe modern era. It has succeeded in capturing the attention ofphysical chemists, physicists, material scientists, and engineersin exploring distinctive semiconducting and catalytic properties.Inertness to chemical environment and long-term photo-stability have made TiO2 an important component in manypractical applications and in commercial products. From drugsto doughnuts, cosmetics to catalysts, paints to pharmaceuticals,and sunscreens to solar cells, TiO2 is used as a desiccant,brightener, or reactive mediator. The U.S. Food and DrugAdministration permits up to 1% TiO2 as an inactive ingredientin food products. While there are no known health effects, arecent study found 3−6 year old children are the most affectedgroup of people that consume TiO2 particles from foodproducts.1 Many new properties of TiO2 have been exploredduring the past few years. This virtual issue (http://pubs.acs.org/page/jpccck/titanium-dioxide.html) highlights a few keyphysical chemistry advances made with TiO2 nanostructures.Photocatalysis. TiO2 is a large bandgap semiconductor

that is commonly investigated in rutile (bandgap 3.0 eV) andanatase (bandgap 3.2 eV) phases. Its response to UV light hasled to the emergence of the photocatalysis research field.2−4 Ithas been investigated extensively for its super hydrophilicityand use in environmental remediation and solar fuel production.Bandgap excitation of TiO2 causes charge separation, followed byscavenging of electrons and holes by surface adsorbed species(Scheme 1A). Alternatively, the photocatalytic activity in the

visible can be extended by coupling with a sensitizing dye orshort bandgap semiconductor (Scheme 1B). By controllingthe surface treatment and medium conditions, it is possible totune the photocatalytic properties for desired applications.TiO2 is very effective for mineralization of contaminants fromair and is used in commercial products such as self-cleaningglass and ceramic tiles. On the other hand, practical use ofTiO2 for remediation of chemical contaminants from waste-water remains a challenge because of adverse catalystpoisoning effects. Challenges remain to extend the photo-

catalytic response in the visible region. In spite of extensiveefforts to dope TiO2 with C, N, S, and transition metal ions,photocatalytic activity in the visible has remained quite low.Recent perspective articles highlight issues and challenges relatedto photocatalysis.2−4

Solar Cells. Mesoscopic TiO2 film is a major component ofdye-sensitized solar cells,5,6 organic photovoltaics,7 andquantum dot sensitized solar cells.8,9 Mesoscopic TiO2 film,which is employed as a substrate in these next-generation solarcells, assists in capturing electrons from an excited sensitizer orquantum dot and transports the electrons to the collectingelectrode surface (Scheme 2). A recently developed quantitative

model explains the dependence of the photovoltaic response ofthe dye-sensitized solar cell on the nonlinear charge recombinationrate (Un = krn

β, with β < 1) and interprets the diffusion lengthmeasurements obtained from various techniques.10 Significantstrides have been made to achieve efficiencies in the 6−12% rangefor sensitized liquid junction solar cells. Various spectroscopystudies have been conducted to probe the ultrafast charge injectionfrom an excited sensitizer into TiO2 and charge recombinationprocesses to identify the factors that limit energy conversionefficiency.11,12

Designing Nanostructure Architectures. TiO2 nano-structures of different shapes have been designed to tailoroptical and electronic properties. While anodization of Ti foil isa popular technique to prepare TiO2 nanotubes, hydrothermalmethods provide convenient routes to prepare 1- and 2-dimensional nanostructures.13 The ability to design anatasenanocrystals with the most active [001] facet has enabledresearchers to systematically probe the catalytic activity.14

Semiconductor−metal composite nanostructures in core−shell or coupled geometry provide a convenient way to enhancethe photocatalytic activity by facilitating reduction processes.15

Electron storage, as well as localized plasmon resonance of

Published: June 7, 2012

Scheme 1. Photocatalytic Activation Using TiO2Nanoparticles: (A) Direct Bandgap (UV) Excitation and (B)Sensitized Charge Injection Using a Visible Light Absorber(Adapted From Reference 4)

Scheme 2. Principle of Operation of Liquid Junction Dye orQuantum Dot Sensitized Solar Cell Using Mesoscopic TiO2Films

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metal nanoparticles embedded in the TiO2 core or placed in theclose proximity of the TiO2 nanostructure, have been shown toimprove photocatalytic and photovoltaic performances.16 To date,only limited spectroscopic studies have been conducted toestablish a hole transfer process aided by an oxidation catalystsuch as IrO2.

17 A better understanding of interfacial charge transferkinetics is important in designing more efficient photocatalysts forwater splitting reactions.4

The development of carbon nanostructure composites isanother emerging area that has led to the design of photo-catalysts with tailored properties.18 In particular, grapheneoxide-based TiO2 composites have been shown to enhancephotocatalytic and photoelectrochemical properties.19,20

Graphene’s ability to capture and shuttle electrons providesa unique opportunity to develop catalyst mats with multi-functional characteristics (Scheme 3).21

Surface Studies. Photon-stimulated reactions of O2 on the[110] surface of reduced rutile TiO2 have enabled researchersto explore the diverse photochemistry of adsorbed oxygenspecies.22 The storage of electrons in rutile TiO2, which resultsfrom trapping at interstitial Ti species or oxygen vacancies (Ti3+

sites), dictates the overall photocatalytic properties.23 The-oretical studies using density functional theory (DFT) havebeen applied to probe interactions of surface-to-adsorbedspecies and obtain a better understanding of the surface bindingaspects and reactivity.24

TiO2 has been extensively studied worldwide, resulting inmore than 13 600 publications in 2010−2011 (Source:Thomson Scientific Web of Science). During this same period,the Journals of Physical Chemistry A/B/C and Letters publishedmore than 750 papers on this topic, thus showing JPC’spreeminence in disseminating TiO2-related scientific advances.Opportunities for TiO2 now lie in more challenging areas, suchas energy conversion and storage. TiO2 nanostructures haverecently shown promise in designing Li-ion and Na-ion storagebatteries.25 Shape- and size-controlled TiO2 nanostructures willcontinue to provide the base architecture to construct light-harvesting assemblies and facilitate photoinduced chargeseparation processes.

Prashant V. Kamat, Deputy EditorThe Journal of Physical Chemistry Letters,University of Notre Dame,Notre Dame, Indiana 46556, United States

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Scheme 3. Illustration of Selective Oxidation and ReductionProcesses in a TiO2−Graphene Oxide−Metal NanoparticleComposite (Adapted from Reference 21)

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(21) Kamat, P. V. Graphene-Based Nanoarchitectures. AnchoringSemiconductor and Metal Nanoparticles on a Two-DimensionalCarbon Support. J. Phys. Chem. Lett. 2010, 1, 520−527, DOI: 10.1021/jz900265j.(22) Petrik, N. G.; Kimmel, G. A. Electron- and Hole-MediatedReactions in UV-Irradiated O2 Adsorbed on Reduced RutileTiO2(110). J. Phys. Chem. C 2011, 115, 152−164, DOI: 10.1021/jp108909p.(23) Chretien, S.; Metiu, H. Electronic Structure of Partially ReducedRutile TiO2(110) Surface: Where Are the Unpaired ElectronsLocated? J. Phys. Chem. C 2011, 115, 4696−4705, DOI: 10.1021/jp111209a.(24) Deskins, N. A.; Rousseau, R.; Dupuis, M. Defining the Role ofExcess Electrons in the Surface Chemistry of TiO2. J. Phys. Chem. C2010, 114, 5891−5897, DOI: 10.1021/jp101155t.(25) Xiong, H.; Slater, M. D.; Balasubramanian, M.; Johnson, C. S.;Rajh, T. Amorphous TiO2 Nanotube Anode for Rechargeable SodiumIon Batteries. J. Phys. Chem. Lett. 2011, 2, 2560−2565, DOI: 10.1021/jz2012066.

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