Photochemical deposition of SERS active silver nanoparticles on silica gel and their application as...

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Journal of Colloid and Interface Science 272 (2004) 134–144www.elsevier.com/locate/jcis

Photochemical deposition of SERS active silver nanoparticles on sigel and their application as catalysts for the reduction of aromatic

nitro compounds

Subrata Kundu, Madhuri Mandal, Sujit Kumar Ghosh, and Tarasankar Pal∗

Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India

Received 10 June 2003; accepted 21 November 2003

Abstract

We report an impregnation technique for immobilization of silver(I) gelatin complex on silica gel. Subsequent UV exposure ofimpregnated silica gel deposited silver nanoparticles on the solid matrix. Conventional techniques (UV–visible spectroscopy, TEMand thermal analysis) have been used to identify and characterize silver particles on silica surfaces. The photoproduced silvhave shown unique SERS activity that authenticates the presence of silver nanoclusters in the silica matrix. Hence, the surfacematrix remains SERS-active for months. This surface activity of the silica matrix inspired us to successfully study the catalytic rednitro-compounds in aqueous, organic, and three different micellar media. Different thermodynamic parameters for the reductionhave also been evaluated. Catalytic activity of the particles in micelles is explained in the light of hydrophobic and electrostatic intbetween the substrate and the micelles. 2003 Elsevier Inc. All rights reserved.

Keywords: Silica gel; Silver nanoparticles; SERS; Nitro-compounds; Reduction; Micelle

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1. Introduction

Preparation and characterization of metal particles in“neglected dimension” have been found to be an activeof research because of their potential applications in chistry, physics, biology, medicine, and electronics. Theseticles, because of their size-dependent redox potentialquantum confinement, find applications in catalysis [1and optical devices [5]. Particularly, semiconductor [6] anoble metal nanoparticles [7,8] have been areas of resedue to their well-defined physical, chemical, and optoetronic properties.

In view of the importance of the surface structuresthese materials, much effort has been invested in creanew classes of materials through the modification of surstructures. Metallic nanoparticles, especially silver, havetracted maximum attention because they have already shpromise in catalysis [1–4] and SERS (surface enhancedman scattering) studies [9]. Incidentally, silver nanopartic

* Corresponding author.E-mail address: [email protected] (T. Pal).

0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2003.11.046

h

n-

have strong plasmon absorption in the visible regionhelps us to study the evolution and characterization ofver nanoparticles in solution. Several scientists have demstrated a large number of routes for the preparation ofnanoparticles, for example, wet chemical methods [10], ptoactivation processes [11],γ -radiolysis [12–15], laser pulsmethods [16], and sonochemical methods [17]. Howevehas been a difficult task to work with silver nanoparticin the nanosize regime, in comparison to gold, becausthe aerial oxidation and very high aggregation tendencsilver nanoparticles. In the nanometer size range theproperties of a metal change [18,19]. The reduction potial becomes progressively negative as the size of the mparticle goes to finely divided states, particularly in the nadomain [19]. Bulk silver has a reduction potential vaof +0.79 V for the Ag(I)

(aqueous)/Agmetal system, and for theAg(I)

(aqueous)/Agatom system, it is−1.80 V, measured againNHE. Similar changes of reduction potential values areobserved for gold and copper and expected for other m[20–22]. But the formation of metal atoms from its ionsvery difficult because of the large negative potential ofmetalion (aqueous)/metalatom system. So conventional stron

http://www.elsevier.com/locate/jcis

S. Kundu et al. / Journal of Colloid and Interface Science 272 (2004) 134–144 135

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reducing agents such as free radicals (E0 ∼ −1.0–1.5 V vsNHE), borohydride (E0 = −1.33 V vs NHE), and weak reducing agents such as hydrazine and alcohol may not rethe metal ions [23]. Thus, the uncertainty in forming atoas nucleation centers is understandable. Once a metalis evolved as a nucleation center, it can act as a catalysthe reduction of the remaining metal ions present in thelution via autocatalysis [19].

The metal clusters are formed as transient intermedduring the formation of metal colloids by the reductionmetal compounds in solution. In the initial stage of colloformation, metal atoms are produced, which subsequeagglomerate. The growth of particles can be written accing to the general equationMn−1 + Mx+ + xe = Mn, wheren is the agglomeration number. The redox potential of a cter depends upon the value ofn, as mentioned above. Athe value ofn increases, the cluster potential approachesconventional bulk potential value.

Nowadays it is a challenge to scientists to stabilize mnanoparticles in a solid matrix, especially to use themcatalysts. Thus their agglomeration or precipitation fromlution is prevented. So studies have been directed toimprovement of solid matrices. Different templates, mtrixes, LB films, porous aluminates [24,25], porous cbonate membranes [26], lithographically processed m[27], micelles [28], different ligands, organic polymers [2etc., are used to stabilize the metal nanoparticles. ReceFendler and his group have prepared layer by layer sassembly of silver nanoparticles capped by graphite onanosheets [30]. Stathotos and Lanos have depositednanoparticles on mesoporous TiO2 films [31]. Gedanken andhis group have presented deposition of various nanopartsuch as iron/iron oxide [32], CdS [33], Eu2O3 [34], and sil-ver [35] nanoparticle on silica spheres. The silica sphequalify as a hard spherical substrate for the following rsons:

(1) tight size distribution can be achieved over a wirange;

(2) surface silanol composition and the extent of hydrobonding can be modified by thermal treatment to chatheir reactivity;

(3) silanol groups can form covalent links;(4) isotropic interaction in an aqueous or organic susp

sion, which helps to form ordered arrays on substrat

In this report, the deposition of gelatin-stabilized silvnanoparticles onto a silica matrix (SNSM) after impregtion and subsequent UV irradiation of a silica matrix cotaining silver(I)–gelatin complex is presented for the fitime. Again, the presence of silver clusters in the SNsamples has been examined by conventional techniqueit has become a candidate for SERS studies. Finally, thever nanoparticles on the silica matrix (here it is immobilizSNSM) have been successfully exploited as a solid catafor the reduction of aromatic nitro-compounds. The deta

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kinetic aspects of reduction of the aromatic nitro-compouby NaBH4 in the presence of trace amount of SNSM as clyst have been reported in both aqueous and micellar meSo the present work is related to a simple and straighward preparation technique for silver catalyst (SNSM) isolid matrix and its use in the reduction of nitro-compoun

2. Experimental

2.1. Reagents and instruments

All reagents were of analytical reagent grade and wused without any further purification. Column chromatoraphy silica gel of∼100 mesh size (SRL, India), gelatpowder (Oxo, London), AgNO3 (Merck, India), and ascorbic acid, N2H4, and NaBH4 (all from BDH, India) were usedas received. 4-nitrophenol (4-NP), 4-nitroaniline (4-NA),aminophenol (4-AP), and 2-nitrophenol (2-NP) were recrtallized using petroleum ether and ethyl acetate. Poly(oethylene) isooctyl phenyl ether (Triton X-100 or TX-100sodium dodecyl sulfate (SDS), and cetyltrimethylammnium bromide (CTAB) was purchased from Aldrich. Actonitrile (CH3CN) was purchased from S.D. Fine ChemicaIndia. Double-distilled water was used throughout theperiment. NaBH4 solution was prepared in ice-cold distillewater freshly each time before use.

Photoirradiations were carried out in a photoreactorted with ordinary germicidal lamps of wavelength 365 n(Philips, India). The photoreactor can produce a flux650 lux. The flux was monitored using a digital lux mter (Model LX-101), Taiwan. The photoreactor (intensitylight) was calibrated with an Ophir power meter (NOVA dplay and 30-A-SH sensor). The number of photons absoper unit area of the sample per second from the photoreaof 100 lux is 3.03× 1015. All UV–visible absorption spectra were recorded in a Shimadzu (Kyoto, Japan) UV-digital spectrophotometer equipped with 1-cm quartz ceTEM and EDAX analysis were performed with an instrment H-9000 NAR, Hitachi, using an accelerating voltaof 300 kV. Very thin epoxy resin was used to hold the saple powder on a 200 mesh copper grid. Thermal analwas performed with Metler Toledo TGA/SDTA 851 ThermAnalyzer, Switzerland. SERS spectra were recorded wiSA, Inc. HR-320 spectrograph equipped with a PrinceInstrument charge-coupled device (REICCD) and the msurements were done with a Kr laser (200 mW) using anm entrance slit. 1,2-Dimethyl phenyl isonitrile (DMPNCwas used as the SERS probe.

2.2. Preparation of silver–gelatin solution and itsimpregnation in silica gel

The reagent solution was prepared by dissolving 0.gelatin powder (OXO, London) in 100 ml of warm distillewater (80◦C), which was previously boiled, with consta

136 S. Kundu et al. / Journal of Colloid and Interface Science 272 (2004) 134–144

l
Scheme 1. Schematic presentation of photoreduction of silver(I)–gelatin complex on silica gel surface to study the evolution of silver nanopartices on thesilica matrix for SERS study.
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stirring. Then 0.01698 g of solid AgNO3 was added so thathe final concentration of AgNO3 in the gelatin solution became 10−3 M. A few drops of 2 M NaOH solution was addeto the mixture for removing turbidity if appeared in the slution. Then the solution was warmed at 50–60◦C for 5 minand the pH of the solution was adjusted to about 8.0all practical purposes [36]. The solution was then cooleroom temperature. Silica gel was added to this solutionder stirring condition. Under these conditions, silver gelacomplex became homogeneously impregnated over theica gel surface. The whole mixture appeared like a sluand then turned into a paste. The paste was dried in auum desiccator for 6–8 h. The dried mass was whiteremained so for months in the dark.

2.3. Evolution of silver particles on silica gel

It was reported earlier that silver(I) gelatin solution pduced and stabilized silver nanoparticles in aqueous pwith a tight size distribution. Here we prepared silvnanoparticles onto the silica matrix by a UV photoirradiatprocess. A vacuum-dried 0.05 g white (unexposed) silvegelatin-impregnatedsample was mechanically spread alas a monolayer on a flat glass container and was irradin the photoreactor under UV light of 650 lux for 20 miThe white solid sample turned brownish black due todeposition of silver on the silica surface and was storethe dark in a vacuum desiccator. A longer exposure time

-

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not cause any deterioration of the size of the nanopartiand catalytic properties did not alter. The presence ofnanoparticles on silica gel was characterized by UV–visspectrophotometry after extraction of the silver particlewater. Routine TEM, EDAX studies, and thermal analywere done for characterization of silver nanoparticlesthe whole procedure is shown in Scheme 1. Again, exceSERS activity involving SNSM samples indirectly authencated the presence of silver clusters in the silica matrix.

2.4. Procedure for the reduction of nitro-compounds usingsolid silver as catalyst, SNSM

In a standard quartz cuvette having a 1-cm path len2 ml of water and 20 µl of nitro-compound (final concetration in solution: 4.3 × 10−4 M) were taken. To it wasadded 300 µl of aqueous NaBH4 (0.1 M). It took 5 minfor the peak of the colorless product to appear (inductime, IT) in the blue region after the addition of (0.0053solid silica gel impregnated silver catalyst, SNSM. Thengradual decoloration of the yellow solution (due to nitcompounds) and the formation of the product were obsethrough UV–visible spectrophotometry (Fig. 1). After tyellow color was completely discharged, i.e., the complereaction was, the peak due to nitro-compoundswas no loobserved. On the other hand, the appearance of a newat 430 nm was noticed 5 min after of the completion ofreaction.


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Fig. 1. UV–visible spectra for the successive reduction of 4-NP by SNScatalyst. Conditions:[4-NP] = 4.3 × 10−4 M; [NaBH4] = 1.3 × 10−2 M;amount of SNSM catalyst= 0.0053 g. Time of interval is 1 min. The peaaround 403 nm is the decrease of absorbance value due to 4-nitropheion and 295 nm is the increase of absorbance value due to formatio4-amino phenol.

3. Results and discussion

3.1. Binding of silver with gelatin

Gelatin is widely used in photographic industries [3On the other hand, gelatin stabilizes small particles vwell and prevents particle coagulation. Aqueous solutiogelatin forms a colorless, 1:1 weak complex with silver(I)alkaline medium, which has been found to be a very imptant inexpensive analytical reagent in recent years [40,The interaction of silver(I) with gelatin is considerably ifluenced by pH. The binding of silver ions with the –NH2group of gelatin may be

Ag+ + H2N· · ·COOH+ OH− � H2N· · ·COOAg+ H2O

and

H2N· · ·COOAg+ OH− � AgHN· · ·COO− + H2O,

or

AgHN· · ·COO− + OH− � [Ag–N· · ·COO]−2 + H2O

and

[Ag–N· · ·COO]−2 + H2O

� Ag+ + OH− + HN−· · ·COO−,

where the gelatin skeleton is represented by dotted linesfree silver ions are reduced to silver metal, presumablythe latent image formation in photography [42]. The redtion takes place through a series of steps [35]: the photoeration of electron–hole pairs; the reduction of silver catito atoms by some fraction of the electrons; the subseqbuildup of atoms to evolve clusters. It was observed that 1

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of 0.5% gelatin solution can effectively bind with 453 µgsilver(I) ion at pH∼ 8 and this was verified by the expusion of free silver by dialysis [40] at 30◦C. Mild reducingagents such as ascorbic acid, hydrazine, formaldehydebon monoxide, and hydrogen sulfide reduce the complegelatin-stabilized silver(I) solution with an absorption maimum at 415 nm. This process of generation of silvergiven birth to a good number of analytical methods [43–to detect different reducing agents present in water andsamples. It has been shown that at a carefully chosencentration the gelatin moiety becomes a good compromfor binding silver ions as well as stabilizing silver nanopticles if produced in solution by wet chemical methods [1This is due to slow reduction of the silver(I)–gelatin compby the reducing agents as mentioned.

Aqueous solutions of silver(I)–gelatin complex canimpregnated easily and then dried onto silica gel matrwhere silver remains as silver ions for months while kepthe dark. The stability of the silver(I)–gelatin complex icreased when it was present on a solid matrix, unlikesolution of the silver(I)–gelatin complex. After impregntion of the silver(I)–gelatin complex and subsequent dryof the silica gel matrix, it was shown that silver(I) ions dposited onto the silica matrix. UV irradiation of the dry siligel matrix containing silver(I) ions produced silver nanopticles (SNSM). From BET analysis the pore specific vume and specific surface area of SNSM were calculate0.46086439 cm3/gm and 321.25177×104 cm2/gm, respec-tively. The evolution of silver nanoparticles onto the somatrix is like a “development” process [39] in photogrphy. This photoreduction process does not require any hreducing agent or expert hand to obtain stable silver nanoticles (in turn catalyst particles). Due to the inherent opaof the silica matrix, UV light could not pass through tsolid material and 20 min irradiation time was necessaryquantitative reduction of silver(I) to silver nanoparticlesthe silica surfaces. However, higher concentrations of sinitrate deposit silver nanoparticles on the silica gel maunder UV irradiation with lower (<20 min) exposure timeOverexposure did not cause any alteration of the surstructure of the nanoparticle aggregates on the solid maunlike the aggregates of silver in solution [49].

3.2. Characterization of silver nanoparticles

3.2.1. UV–visible absorption studyThe UV–visible absorption spectrum (Fig. 2) demo

strates the presence of silver nanoparticles in solutionrecord the UV–visible spectra 0.05 g of the SNSM samwas mixed with 5 ml of water and the mixture was sonicafor about 15 min. Due to sonication the Ag(0) nanoparticpassed into the aqueous medium along with gelatin andcolor of the aqueous layer became yellow-tinted. UV–visiabsorption spectra of this solution (Fig. 2a) exhibit antense, broad absorption peak around 435 nm (λmax) due tosurface plasmon excitation of silver particles and indic


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Fig. 2. Absorption spectra of the aqueous solution obtained after sonicof impregnated silica gel (a) after irradiation (plasmon band due to snanoparticles is around 435 nm) and (b) before irradiation (silver (I)–gecomplex).

dispersed Ag(0) in aqueous gelatin solution. This spectwas compared with the spectrum of the solution preparea similar procedure with the unexposed silica gel imprnated with silver(I)–gelatin complex (Fig. 2b). The UV irrdiation produced Ag(0) in the solid silica gel matrix. Hoever, from the leached-out solution obtained from theexposed impregnated solid silica gel matrix no peak aro∼400 nm was observed; hence Ag(0) was absent. A hiAgNO3 concentration in the silica matrix and subsequUV-irradiation, of course, darkened the color of the silmatrix and in that case increased absorbance of the solowing to the presence of larger amounts of silver nanopcles was noted.

3.2.2. Energy-dispersive X-ray analysis (EDAX)The presence of silver, silicon, carbon, sodium, and o

gen were examined with EDAX measurement. The EDspectrum was utilized to obtain a quantitative estimatethe Ag present and authenticated the complete reductioAg(I) to Ag(0) upon UV exposure. For the light elemensuch as oxygen only a rough estimate could be madecause EDAX is a bulk analysis, and oxygen from the sigel spheres also contributes to the oxygen signal. The ssilicon, and Ag/Si ratio in all samples were identical. Tsilver content in all samples was about 6.95%. This valuclose to the molar ratio of AgNO3/silica gel in the startingsolution.

3.2.3. Electron microscopy studies (TEM)It has been stated that the color of the Ag(I)–gela

complex after impregnation in silica gel was white, buturned blackish brown after photoirradiation. The transmsion electron micrographs for both the samples (exposedunexposed) are shown in Fig. 3. We observed sphericaver nanoparticles of∼16± 4 nm diameters (Fig. 3a) for thexposed silica gel matrix but no trace of silver particles frthe unexposed one (Fig. 3b). The size distributions ofparticles are shown in the histogram. In some placesparticles were found to remain as aggregate, and thisdue to the presence of gelatin that glued the silica geltrices, causing aggregation. Higher amounts of gelatin (increased such aggregation.

f

-

,

Fig. 3. Transmission electron micrograph (TEM) with histogram of (a)ver nanoparticles after 20 min irradiation and (b) before irradiationsilver particles are observed).

3.2.4. Thermal analysis studyThermal analysis of both the samples in air (exposed

unexposed) clearly indicated two main features: ancho


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of silver particles on the silica surfaces and very slownealing of the small silver particles into larger silver partic(60–80 nm, not shown). The annealing of silver particstarted at 220◦C and ended at∼500◦C (depicted by anexotherm). The broad exotherm was due to the presof a complex gelatin moiety in the matrix. The presenof gelatin helped the anchoring of silver that takes plat about 140◦C. Both these phenomena were distinctly oserved by broad exothermic peaks [35], but the temperawas somewhat higher in the presence of gelatin. The wand gelatin loss were observed for both the samples (cated by broad endothermic peaks) in the regions 80–12◦Cand 540–740◦C, respectively.

3.2.5. Surface-enhanced Raman scattering (SERS) studySince the discovery of surface-enhanced Raman sca

ing (SERS), exhaustive efforts have been devoted in a nber of laboratories toward the development of practical sstrates to investigate and utilize the SERS effect [50,51].cause and the theory of SERS are still a subject of muchbate. It is presumed that the oscillating electric field of a libeam induces an oscillating dipole in a colloidal metal pticle. This oscillating dipole can be thought of as the fconduction electrons oscillating up and down in phase wthe oscillating field. If the natural frequency of oscillationthe conduction electrons (the plasma oscillation frequematches the frequency of the light, the plasma oscillais in resonance with the light, and the induced dipolecomes large. SERS study requires an active substratethe order of enhancement of SERS intensity depend

-

d

the microstructure and proximity of the individual particof the substrate [52,53]. Silver is one of the best matals for making SERS-active surfaces since the behavioits dielectric constant near the Fröhlich frequency givesto intense surface plasmon absorption in the visible walength region. Silver aggregate of an appropriate dimenhas been shown to be the material best suited to SERSies.

Here in our study, 1,2-dimethyl phenyl isonitri(DMPNC) in methanol was used as the SERS probe. Itsorbed well on the surface of silver nanoparticles (methaevaporates out) and showed good SERS intensity. Whwas adsorbed the NC stretching vibration near 2125 cm−1

shifted to about 2175 cm−1. A band near 2175 cm−1 in-dicates SERS of the adsorbed molecule rather than noRaman of free molecules in solution. The frequency wasdependent on which metal DMPNC is adsorbed.

Spectra for the background, i.e., the solution of DMPin methanol, showed the NC stretching band at 2121 cm−1.After its addition onto the silver particle (solid) presentgelatin matrix, the NC stretching band shifted to 2180 cm−1

(Fig. 4a) and several other bands very close to thosebackground one. This vast shift from 2121 to 2180 cm−1

vouched for definite SERS spectra, not normal Raman.gle dispersed 15-nm silver particles should have typiclow or no SERS enhancement. The observed strong Ssignal most likely arose from aggregated silver particlesled to an enhancement [37,38]. In Fig. 4, two SERS sptra of DMPNC are shown, one using freshly prepared siparticles (Fig. 4a) and another using aged (∼3 months old)

Fig. 4. Surface enhanced Raman scattering (SERS) spectra of 1,2-dimethyl phenyl isonitrile (DMPNC) on the photoproduced silver nanoparticles (∼16±4 nm)anchored onto the silica gel surface in the presence of gelatin (a) just after UV irradiation and (b) after keeping the irradiated sample for∼3 months. TheX-axis is wavenumber (cm−1) and theY -axis implies count.


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silver particles (Fig. 4b). From these spectra it is clearfreshly prepared SNSM was much more sensitive to SEbut after a long time, SERS activity decreased due to theface oxidation of silver particles present in SNSM sampThis study involving SNSM depicted the presence of silon the silica matrix well and left an avenue to study otmolecules also.

In aqueous media, high concentrations of gela(∼0.5%)-stabilized silver nanoparticles did not show insttaneous SERS signals [54]. This is due to the prefereoccupation of silver surfaces by the gelatin moiety. Howethe silica gel matrix helped to see the SERS signal even wlarger amounts of gelatin remained present. Gelatin prothe silver surfaces; thus the nanocluster surfaces remastable for months together to show some SERS activity eafter aging for 2 months. Therefore, the SNSM matrix hpens to be a promising candidate for SERS. Individusilica, gelatin, or both of them together are no good forsame study. The SERS activity involving SNSM particis due to the presence of silver in them. Silver on theica matrix remains active to serve the purpose as a subsfor SERS without the need of any electrolyte for silver pticle aggregation [37,38]. In turn, it may be said that SEof DMPNC on SNSM unequivocally speaks the presencsilver in the catalyst.

3.3. Reduction of 4-NP with SNSM catalyst

In-depth study was undertaken on the NaBH4 reductionof 4-NP in the presence of silica gel impregnated with ssilver particles (SNSM) as catalysts. For the investigatthe UV–visible spectra of the nitrophenols in water wstudied. It has been observed that only 4-NP in aquemedium has a maximum absorption (λmax) at 317 nm. Butthere was a red shift of the peak of 4-NP from 317 to 403observed immediately after the addition of NaBH4. This wasdue to the formation of 4-nitrophenolate ions in alkaline cdition caused by the addition of NaBH4. The peak at 403 nmremained unchanged even for a couple of days in thesence of any catalyst. Addition of 0.0053 g of the cata(SNSM) into the reaction mixture caused a gradual fadof the characteristic yellow color due to 4-NP and finacomplete bleaching of the yellow color of the 4-NP sotion was observed. This decolorization was quantitativmonitored spectrophotometrically with time and it has bnoted as a successive decrease of peak height. This is dthe reduction of 4-NP to 4-AP. After the completion of treduction, a new peak appeared at 430 nm due to silvermon absorption. The successive decrease of the intensthe absorption spectrum and a blue shift of the peak f403 to 387 nm (Fig. 1) was observed during the coursthe reaction. In the intermediate stage of reduction no pat 430 nm due to the Ag0 plasmon band was observed. Thmay be the peak due to the Ag0 plasmon band that remainemasked within the absorption band of the nitro-compouBut finally a clear absorption band at 430 nm due to the0

d

e

to

-f

Fig. 5.1H NMR spectrum of 4-aminophenol produced from the reducof 4-NP by SNSM as catalyst (solvent D20).

plasmon band appeared. This reduction can be well valized by the disappearance of the 403-nm peak withappearance of new peak at 295 nm (Fig. 1). The peakto 4-AP appeared after 5 min of addition of SNSM cataparticles in the reaction mixture. It can be clearly said tthe peak at 295 nm is due to 4-AP because the samewas observed for an authentic 4-AP solution (6.0×10−5 M)under identical experimental conditions, where the reducof 4-NP is carried out in the presence of silver nanoparticThe formation of 4-AP as the sole product was also cfirmed from the1H NMR spectrum (in D2O) of the product(shown in Fig. 5). The spectrum was compared with thaan authentic sample of 4-AP. The appearance of broadnals in the spectrum atδ 6.60 and 4.30 was due to the presence of aromatic protons and amino protons. The hydrproton was not found in the spectrum because of the soD2O. Co-TLC studies of the reaction mixture and an authtic sample of 4-AP also supported the formation of 4-AThe reduced solution finally changed to blackish color a2 h. This was due to aerial oxidation of 4-AP in the alkline medium. But this blackening could be arrested for 5–when the solution was kept under N2 atmosphere. The ratof reaction has been observed to be first-order with respe4-NP and the activation energy of the nanoparticle-catalyreaction was calculated to be 33.7 kJ mol−1 when the reaction was carried out at four different temperatures.

3.4. Effect of varying amounts of Ag0 (solid) particleson reduction of 4-NP

The role of metal particles is very much important inreaction. To check the effect of the amount of Ag0 we variedthe amount of SNSM in the reaction mixture, keeping otparameters constant. An aqueous solution containingof water, 4-NP (20 µl, 4.3 × 10−4 M), and NaBH4 (300 µl,0.1 M) was taken in a quartz cuvette. Now varying amouof solid catalyst were added to the reaction mixture. Timdependent reduction of 4-NP was studied spectrophotorically by the successive decrease in the absorbanceat 403 nm and the successive increase in the absorbvalue at 295 nm. With the increasing amount of cata(silver nanoparticles in solid matrix, SNSM) the rate w


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Fig. 6. Plot of rate against amount of SNSM catalyst for the reductio4-NP. Conditions: [4-NP]= 4.3 × 10−4 M; [NaBH4] = 1.3 × 10−2 M;amount of SNSM catalyst varied from 0.0053 to 0.042 g.

observed to be increased. Rate values are plotted agvarying amounts of catalyst as shown in Fig. 6. At the eof the reduction and after prolonged standing and sontion, a peak for silver hydrosol became visible at 430 nSo it is also concluded that 4-AP did not shift the plasmpeak position of silver hydrosol, but after standing, the silnanoparticle leaches out in water from the added SNSM

3.5. Effect of NaBH4 and substrate (4-NP) concentrationfor reduction of 4-NP

The reduction of 4-NP was tested with varying amouof NaBH4. Here it was found that with the increase in cocentration of NaBH4 the induction time, IT (time requireto observe visual change, i.e., the start of the reducof 4-NP), decreased and consequently the initial ratecreased. Actually NaBH4 served here as the reducing agfor nitro-compounds to produce the corresponding amcompounds. A little NaBH4 is always decomposed in threaction medium during the course of reaction. So ancess amount of NaBH4 was always employed. As soonwe added the NaBH4, the metal particles started the caalytic reaction by relaying electrons from the donor BH−

4to the substrate 4-NP (acceptor) only after the adsorptioboth onto the particle surfaces. Here, as the catalyst pcles are held on the solid surfaces, there was no chanagglomeration of silver nanoparticles due to the presenclarger amounts of NaBH4. That is, the electrolytic effect duto the added NaBH4 for aggregation of nanoparticles is nobserved here. Only larger amounts of NaBH4 reduce theIT. Moreover, gelatin prevents the particle from coagulatiHowever, agglomeration of silver particles has been repowith excess of NaBH4 (electrolytic effect) and that happenwhen the particles are evolved in situ in the aqueous pin the reaction mixture just at the start of catalysis.

The added silver particles (SNSM) remained consthroughout the kinetic studies and even for succesbatches of reductions. It was confirmed from the almconstant absorption value at 430 nm (λmax for the plas-mon absorption band due to silver nanoparticles) after

t

f

Fig. 7. Plot of ln[A] against time for the reduction of 4-NP using SNSas a catalyst in the presence of variable concentrations of NaBH4. Con-ditions: [4-NP]= 4.3 × 10−4 M; amount of SNSM catalyst= 0.0053 g;[NaBH4] = 0.9 × 10−2 M (A); 1.3 × 10−2 M (B); 2.2 × 10−2 M (C);2.8× 10−2 M (D).

complete reduction of 4-NP. A careful observation ofUV–visible spectrum of the reduction revealed that initiafor the first batch of reduction, the reaction has some IT,after that the reaction was found to follow first-order retion kinetics with respect to 4-NP.

3.6. Kinetic study of the reduction of 4-NP catalyzedby silver nanoparticle

In a typical set of experiments, the time of addition4-NP was kept constant and the NaBH4 concentration wasvaried. With varying concentrations of NaBH4, the plots ofln[A] versus time are shown in Fig. 7. In our experimenthave always used a particular concentration of NaBH4 to getthe reaction condition independent of the NaBH4 concentra-tion, i.e., in the region where parallel lines forK values wereobtained. Here NaBH4 plays a crucial role in the reductioof 4-NP. Under the influence of strong nucleophiles suchBH−

4 ions the reduction potential of silver particles is loered. Upon the addition of BH−4 ions the following changewere observed:

(a) the surface of the silver particles becomes vulneraboxidation and results in a coating of silver oxide onsilver surface [55];

(b) then BH−4 again reduces the oxide layer and regener

the fresh surface of the silver particles;(c) nitrophenolate ion gets an avenue to be adsorbed

the regeneration of the fresh surface of particles andstituting the BH−

4 ion from the surface.

The presence of an oxide layer on the catalyst surshould be responsible for the observed IT. The inductime became longer with a dilute NaBH4 solution. It wasalso observed that the IT in an oxygen atmospheregreater and in a nitrogen atmosphere it was less and neble. This clearly speaks for the surface oxidation/poisonof silver nanoparticles in an oxygen atmosphere. The


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onthsfere into

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hash acidb-e.g.,PthelEx-

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the

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versible formation and dissolution of silver nanoparticlessolution in the presence of NaBH4 [55] have demonstratethis. As soon as the surface gets renewed it becomesrounded by BH−4 ions. The substrate 4-nitrophenolate icompetes with BH−4 ion to substitute/co-adsorb along wiBH−

4 ion onto the metal particles. Then the electron trantakes place. The progress of the reaction stopped with thtroduction of trace amount of alkali or cyanide solution inthe reaction mixture. This was observed at any stage oreaction. It was due to the change in surface propertiesof the silver surfaces.

3.7. Reduction of nitrophenols with other reducing agents

Keeping other conditions the same, the reactionalso been tested using other mild reducing agents suchydrazine (0.1 M) and alkaline solution of ascorbic a(0.1 M). In our experimental condition, we failed to oserve the reduction of a series of nitro-compounds,2-NP, 4-nitroaniline (4-NA), 2,4-dinitrophenol, and 4-Nwith ascorbate anion or hydrazine. The reduction ofabove nitro-compound by N2H4 in the presence of metasolutions of Pd, Pt, Ru, etc., also did not take place.planation has already been put forward for N2H4-assistedreduction of different dyes [21], in the presence of Ru,and Pt as catalyst. So with ascorbate anion, hydrazinmolecular hydrogen no reduction was observed evenone week in the presence of SNSM particle as catalyst.

3.8. Reduction of other nitro-compounds

Reduction of two different nitro-compounds, e.g., 2-and 4-NA, was also studied using NaBH4 in the presenceof SNSM catalyst. Under identical reaction conditions,yellow color due to 2-NP and 4-NA at the 4.3 × 10−4 Mconcentration was discharged slowly during the coursreduction. The absorbance values at the respectiveλmax po-sitions of the compounds decreased gradually in the pence of NaBH4 (1.2 × 10−2 M) and the catalyst. Duringthe course of reduction two new peaks gradually appefor 4-NA at 300 and 209 nm, both of which are duep-phenylenediamine in alkaline aqueous solution [57]. Silarly, in aqueous alkaline solution, a peak was observe406 nm due to 2-nitrophenolate ion and the peak gradudisappeared during the course of NaBH4 reduction. A newpeak at 298 nm appeared due to the 2-AP. TLC, IR,NMR spectra of all the products were compared withpure compounds. A comparison of the reduction rateslowed the order 4-NP> 2-NP> 4-NA.

3.9. Studies in micelles

It is well known that micelles increase the suspendiity of metal nanoparticles (stabilization) and thus helpstudy the reaction kinetics [3] for heterogeneous catalyAgain micelles sometime refresh the surfaces of cataparticles [55,56,58], and are used in various ratios to ctrol the particle shape and size [59]. However, the ex

-

-

s

r

mechanism of interaction between metal particles andsurfactant stabilizer has not yet been resolved [60,61]to study the interactions of the substrates and catalyst pcles, the reduction of 4-NP was performed in three diffemicelles. In micelles a weak hydrophobic force operatestween the substrate (4-nitrophenolate ion) and the micThus hydrophobic interaction inhibits the easy approacthe substrate toward the metal nanoparticle surfaces. Mover, micelles build wrapper-like barriers surroundingmetal particles, whereas BH−4 tends to remain outside thdynamic barrier (BH−4 has no hydrophobic part). Hencelowering in the reduction rate in micelles in comparisonthe reduction rate in aqueous medium for a particular mis understandable. When the aqueous system is replacan anionic micellar solution, such as SDS, adsorptionthe phenolate ion onto the nanoparticle surface is notturbed. This happened due to the negatively charged micsurface, which prevented the incorporation of the subs(4-NP) into the SDS micelle. So the reduction rate in Sbecame comparable to that in aqueous medium. In cacationic micelles (CTAB), however, due to the presenceboth hydrophobic and hydrophilic interactions betweenphenolate ion and the positively charged micellar surfadsorption of the substrate onto the catalyst particle wasdered/restricted so that the rate decreased to a consideextent. In a nonionic micelle (TX-100), on the other haan intermediate rate was observed because of only oneof interaction, hydrophobic in nature, that became operain nonionic micelles between the substrate (4-NP) anduncharged micellar surface. Thus the reaction rate in tdifferent micelles followed the order H2O ∼ SDS> TX-100> CTAB, and the absorbance of 4-NP as a functiontime in three different types of micelle (just above CMC) hbeen presented in Fig. 8. The different rates of the reactiodifferent concentrations of the three types of surfactant hbeen shown in Table 1.

The location of the substrate, phenolate ion in micelcould be quantified from the shift of acid–base equilibri

Fig. 8. Absorbance vs time (min) plot for the reduction of 4-NP prompby SNSM as catalyst in TX-100, CTAB, and SDS micelle. Contions: final concentration of TX-100= CTAB = SDS= 7.6 × 10−3 M;[4-NP] = 7.6×10−5 M; amount of SNSM catalyst= 0.0053 g; [NaBH4] =1.14× 10−2 M.


andnted

teruc-sil-is isilverthisd-

in

ant,

C

ratech-

re-

hodrixM,fi-ag-

ted

[62]. Here, the location of the substrate (4-NP), BH−4 , and

SNSM particles in the three different micellar surfacesthe electron transfer reaction were schematically presein Fig. 9.

3.10. Reaction in organic solvent (CH3CN)

The reduction of 4-NP was studied in detail in waand aqueous micellar medium. After the complete redtion of 4-NP, a plasmon peak due to gelatin-stabilizedver nanoparticles appeared in the reaction medium. Thdue to the presence of some number of leached-out snanoparticles in the dispersed medium. To overcomeleaching, i.e., the mixing of catalyst particles with the prouct in aqueous medium, an organic solvent, CH3CN, wasthought of as the dispersion medium. In CH3CN the induc-tion time for the reduction was 12 min relatively higher

Table 1Different initial reaction rate values in three different types of surfacteach at three different concentrations

Surfactant Initial rate Initial rate Initial rate(min−1) at (min−1) at (min−1) atbelow CMC just above CMC far above CM(10−5 M) (10−2 M) (10−1 M)

SDS 2.7× 10−2 7.6× 10−2 9.7× 10−3

TX-100 2.7× 10−2 4.7× 10−2 9.1× 10−3

CTAB 1.8× 10−2 6.8× 10−3 2.4× 10−4

Conditions: [4-NP]= 7.6 × 10−5 M, [NaBH4] = 1.14× 10−2 M, amountof SNSM catalyst = 0.0053 g.

comparison to the IT observed for aqueous medium andof reduction was also slower (Fig. 10). Interestingly, leaing of silver particles did not take place at all in CH3CN,which makes the separation of SNSM particles from theaction medium easy.

4. Conclusion

In conclusion, we have presented a new simple metof making silver nanoparticle catalysts in silica mat(SNSM). The characterization of the particle by TEEDAX, UV–visible spectroscopy, thermal analysis, andnally from SERS study revealed the presence of silver

Fig. 10. Absorbance vs time (min) plot for the reduction of 4-NP prompby SNSM as catalyst in aqueous medium and in CH3CN. Conditions:[4-NP] = 4.3 × 10−4 M; [NaBH4] = 1.3 × 10−2 M; amount of SNSMcatalyst= 0.0053 g.

Fig. 9. Schematic representation of electron transfer process between BH−4 and 4-NP− via SNSM as catalyst in three different micelles.


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gregates on silica surfaces. SNSM particles were found tactive for the reduction of several aromatic nitro-compouto the corresponding amino derivatives and separation ocatalyst particles happens to be easy. The procedure isful for large-scale synthesis. This study gives a clear insof heterogeneous catalysis even in micellar media.

Acknowledgments

S. Kundu and S.K. Ghosh are grateful to the Departmof Science and Technology (DST), New Delhi, for financsupport and M. Mandal thanks the Council of Scientific aIndustrial Research (CSIR), New Delhi, for financial astance. The authors also thank Professor J.A. Creightonhis help with the SERS studies and for useful suggestio

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Photochemical deposition of SERS active silver nanoparticles on silica gel and their application as...

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