Photocatalytic Activity of Nanocomposite Catalyst Films Based on Nanocrystalline...

Published: May 19, 2011

r 2011 American Chemical Society 12033 dx.doi.org/10.1021/jp201098t | J. Phys. Chem. C 2011, 115, 12033–12040

ARTICLE

pubs.acs.org/JPCC

Photocatalytic Activity of Nanocomposite Catalyst Films Based onNanocrystalline Metal/SemiconductorsFrancesca Petronella,† Elisabetta Fanizza,†,‡ Giuseppe Mascolo,§ Vito Locaputo,§ Luca Bertinetti,^

Gianmario Martra,^ Salvatore Coluccia,^ Angela Agostiano,†,‡ M. Lucia Curri,*,‡ and Roberto Comparelli*,‡

†Universit�a degli Studi di Bari�Dip. Chimica, Via Orabona 4, 70126 Bari, Italy‡CNR-IPCF, Istituto per i Processi Chimici e Fisici, Sez. Bari, c/o Dip. Chimica Via Orabona 4, 70126 Bari, Italy§IRSA-CNR, Istituto di Ricerca sulle Acque, Via F. De Blasio 5, 70132 Bari, Italy^Dipartimento di Chimica IFM&Centro Interdipartimentale di Eccellenza “Nanostructured Interfaces and Surfaces” (NIS), Universit�adi Torino, Via P. Giuria 7, 10125 Torino, Italy

bS Supporting Information

1. INTRODUCTION

Advanced oxidation processes (AOPs) represent a well-known approach for performing removal of recalcitrant organicpollutants in water and wastewater. These processes are em-ployed in the water treatment trains as a final polishing step afterbiological treatment or as a pretreatment in order to enhance thebiodegradability of the influent wastewater.1�3 The fundamentalconcept underlying the AOPs is the “in situ” generation ofhydroxyl radicals in order to exploit their strong reactivity,ultimately leading to the complete mineralization of the targetcompounds. AOPs that make use of ultraviolet (UV) light andcatalysts have proven to be very effective for the removal oforganic pollutants. In this perspective, TiO2 represents the mostused photocatalyst due to its chemical stability, commercialavailability, and excellent catalytic properties.4,5 Band-gap photo-excitation of semiconductors generates electron�hole pairs ableto generate the production of hydroxyl radicals in water.6 Mostextensive studies have been carried out in the literature by usingsemiconductor powder suspended in the target molecule aqu-eous solution,7 although the large-scale application of catalystslurries has been prevented by technological difficulties and highcost related to the catalyst recovery. On the other hand, theimmobilization of the catalysts onto substrates is known to cause

a drop in performance due to the reduction of the overall activesurface area.8 Nanostructured semiconductors are reasonably ex-pected to reduce such losses of performances due to their extremelyhigh surface-to-volume ratio that can greatly increase the density ofactive sites available for adsorption and catalysis. Furthermore, thesize-dependent band gap allows tuning electron�hole red-oxpotentials to achieve selective photochemical reactions. Finally,the reduced dimensions of the nanocatalysts are expected to allowthe photogenerated charges to readilymigrate on the catalyst surface,thus reducing the probability of undesired bulk recombination.9

Anatase TiO2, in both bulk and nanostructured form, is recog-nized as one of the most efficient, nontoxic, and inexpensivephotocatalysts,10 although several works report also on theeffectiveness of other nanosized semiconductors, such as ZnO,in removing organic targets in water matrixes.11�15

Two crucial processes are known to define the overall catalyticefficiency, namely, the competition between the recombinationand the trapping of the charge carriers, followed by the competi-tion between the recombination of the trapped carriers and the

Received: February 2, 2011Revised: May 19, 2011

ABSTRACT: The photocatalytic properties of anatase TiO2 nanorods(NRs) and noble metal�semiconductor nanocomposites (TiO2 NRs/Ag)prepared by colloidal chemistry routes and immobilized onto suitablesubstrates were investigated. Photocatalytic experiments were performedunder UV irradiation in order to test the degradation of a target compound(the azo dye, methyl red) in aqueous solution using TiO2 P25 Degussa as areference material. Absorbance spectroscopy and liquid chromatography/mass spectrometry (LC/MS) measurements pointed out that, according topH conditions, TiO2NRs andTiO2NRs/Ag presented a photoactivity up to1.3 and 2 times higher than TiO2 P25 Degussa, respectively. Notably, theTiO2 NRs/Ag-based catalysts demonstrated a photocatalytic activity 2-foldhigher than bare TiO2 NRs. Remarkably, only a negligible dependence on pH conditions was detected for the nanocompositecatalyst, whereas both TiO2NRs andTiO2 P25Degussa showedmuch higher photoactivity at acidic pH. In all the investigated cases,the identified byproducts pointed out the occurrence of the same reaction mechanism, basically relying on the hydroxyl radicalattaching on the benzene ring and on the homolytic rupture of the nitrogen�carbon bond of the dimethyl-amino moiety.

12034 dx.doi.org/10.1021/jp201098t |J. Phys. Chem. C 2011, 115, 12033–12040

The Journal of Physical Chemistry C ARTICLE

interfacial charge transfer. Accordingly, improved charge separa-tion and inhibition of charge carrier recombination are essentialin enhancing the overall quantum efficiency for interfacial chargetransfer.16,17 This goal can be achieved by suitably modifying theproperties of the particles by selective surface treatments. In thisperspective, several approaches have been proposed in theliterature, including surface modification of the semiconductorparticles with red-ox couples or noble metals.18 For instance,metal nanoparticles (NPs) highly dispersed onto an active oxideare a classical example of bifunctional catalysts, in which chemi-sorptive activation of the substrate on the metal is enhanced bycharge transfer between the two different materials.19,20

We have previously demonstrated that anatase TiO2 spherical-like nanocrystals (NCs) prepared by hydrolytic routes possess amuch higher photocatalytic activity than commercially availableTiO2 P25 Degussa, when both catalysts are deposited onto atransparent substrate.10,12,13 Rodlike particles are expected to showhigher photocatalytic activity than spherical NCs having the sameradius due to the higher surface-to-volume ratio. These featureswould guarantee a high density of active sites available for surfacereactions as well as a high interfacial charge carrier transfer rate.However, the occurrence of surface trap statesmay compensate forthe increased delocalization of carriers in the rods.21,22

In this work, we aim to take a step further in the application ofthe TiO2 nanomaterials for photocatalysis. In particular, here, wehave considered a novel TiO2/metal nanocomposite and ex-plored the possible geometrical and reactivity effect of thecopresence of Ag NPs and TiO2 NRs, in one composite inter-related material. For this purpose, a recently proposed syntheticapproach for obtaining TiO2 nanorods (NRs) anatase in crystal-line phase (100% anatase), and with a tunable aspect ratio andsize distribution, has been used. It is thus possible to achieve aprecise control of NR reactivity, which depends on the exposedcrystalline plane.21,23 Such selective reactivity has been used topromote the TiO2 NR-assisted photoreduction of Agþ exclu-sively at the TiO2 NR tip corresponding to the Æ004æ surface,23thus leading to the formation of Ag NPs, tunable in size as afunction of reaction time, surrounded by TiO2 NRs that simul-taneously act as a photocatalyst, to promote the Ag NP nuclea-tion, and as capping agent, to prevent metal NP aggregation, andat the same time, to achieve their solubility in organic solvents. Itis worth to point out that the proposed system is characterized bythe absence of any specific ligand for Ag NPs, thus resulting in areadily available surface, ultimately able to enhance the efficiencyof the electron-transfer processes leading to the photocatalyticeffect. The high control on both metal and semiconductorgeometry allows effectively controlling the nanocompositephotocatalytic properties as a function of Ag NP size, aspreviously reported.19,22�24 Indeed, such Ag NPs demonstratedtheir high chemical stability and how their ability in storingelectrons could be tuned with size, thus allowing releasing ofelectrons to a suitable acceptor.24 In this work, for the first time,we exploited such an ability under oxidative conditions.

The photocatalytic activity of TiO2 NRs and of a noblemetal�semiconductor nanocomposite (TiO2 NRs/Ag) underUV irradiation has been investigated using the azo dye methylred as a model compound in aqueous matrixes at different pHvalues. The obtained results have been compared to the photo-catalytic activity of the most used commercially available catalyst,namely, TiO2 P25 Degussa, in the same experimental conditions,that is, deposited onto transparent substrates by casting. Thedegradation mechanism in the reaction carried out by using the

investigated catalysts has been also studied by monitoring thedegradation byproducts (BPs), in order to account for the possiblepeculiarity in the behavior of the specific material.

2. EXPERIMENTAL SECTION

2.1. Materials. All chemicals were of the highest purityavailable and were used as received without further purification.Silver nitrate (AgNO3, 99.998%), titanium tetraisopropoxide(Ti(OPri)4 or TTIP, 99.999%), trimethylamino-N-oxide dihy-drate ((CH3)3NO 3 2H2O or TMAO, 98%), and oleic acid(C18H33CO2H or OLEA, 90%) were purchased from Aldrich.All solvents used were of analytical grade and purchased fromAldrich. Commercial TiO2 is TiO2 ‘‘Degussa P25’’ (TiO2 P25),and methyl red is 2-(4-dimethylamino-pheny-lazo)-benzoic acid,C. I. 13020, MeRed.2.2. Catalyst Preparation. TiO2 NRs. Organic-capped TiO2

NRs (100% anatase) have been synthesized by hydrolysis ofTTIP using technical grade OLEA as a surfactant at lowtemperatures (100 �C) upon direct injection of large aqueousbase volumes into OLEA:TTIP mixtures, as reported elsewhere.21

Briefly, TTIP has been hydrolyzed by reacting with an excess ofaqueous base solution. TMAO dihydrate has been used ascatalysts for polycondensation, and the growth has been carriedout for 5 days. Anatase TiO2 NRs have been readily precipitatedupon addition of an excess of ethanol, and the resulting pre-cipitate has been isolated by centrifugation and washed threetimes with ethanol to remove the excess of surfactant. At thisstage, the OLEA-coated TiO2 NRs have been easily redispersedin CHCl3, without any further growth or irreversible aggregation.

Figure 1. TEM micrographs of (A) TiO2 NRs and (B) TiO2 NRs/Agnanocomposites. (C) High-resolution detail of a Ag particle directly incontact with a TiO2 NR.



The TiO2 NR size has been determined by TEM images(Figure 1A). A detailed structural and morphological character-ization of the as-prepared TiO2 NRs can be found elsewhere.21

TiO2 NRs/Ag Nanocomposites. The photocatalytic activityand the phase selective reactivity of preformed anatase TiO2NRshave been exploited to prepare TiO2 NRs/Ag nanocompositesaccording to a previously proposed photocatalytic approach.23

The peculiar strength point of such a photocatalytic route relieson the fact that the TiO2 NRs act as stabilizers for Ag NPs innonpolar media without addition of specific organic ligands forthe metal; therefore, the metal surface can be readily available forcatalysis. Ag NPs have been produced upon photocatalyticreduction of AgNO3 promoted by photogenerated electrons inTiO2 NRs under UV irradiation (light source, high-pressure 200W mercury lamp; λ > 250 nm). Such a TiO2NRs/Ag nanocom-posite can be uniquely obtained in the presence of TiO2 NRs,because, by using TiO2 P25 Degussa, or spherical TiO2 NCs, astable TiO2NRs/Ag nanocomposite23 cannot be obtained.Briefly, a quartz cuvette has been filled with a solution

containing the desired concentration of TiO2 NRs (expressedwith reference to the parent species, TTIP) and AgNO3 in adeaeratedCHCl3/EtOHmixture (EtOHcontent was 10% (v/v)).The TiO2/AgNO3 molar ratio was been 50:1. Aliquots have beenextracted at scheduled time intervals via a syringe under exclusionof air and properly diluted (1:10) with deaerated solvent (theCHCl3/EtOH mixture) for absorption measurements and fortransmission electronmicroscopy (TEM) investigations (Figure 1B).An optically clear, dark brown solution has been obtained afterUV irradiating for 2 h. A detailed characterization of thenanocomposite can be found elsewhere.23

2.3. CatalystCharacterization. UV�visAbsorbanceSpectroscopy.UV�Vis absorption spectra and reflectance spectra were recordedwith a UV�Vis-near IR Cary 5 (Varian) spectrophotometer.Transmission ElectronMicroscopy (TEM).Observations of the

catalysts by high-resolution transmission electron microscopy(HRTEM) were performed with a JEOL 3010-UHR operated at300 kV. Samples were prepared by dropping a dilute solution ofthe NCs in chloroform onto carbon-coated copper grids andthen allowing the solvent to evaporate.From TEM images, the size of TiO2 NRs and Ag NPs was

measured, and histograms of the size distribution were built,measuring the dimension of at least 100 particles for each sample.The average length and/or diameter, Lm and/or dm, respectively,for TiO2 NRs and NPs, were calculated by using the formula, forexample, for TiO2NRs, Lm = ∑iLifi, with Li being themean lengthof the size class containing a fraction of particles, fi = ni/ntot = ni/∑ini. The ‘‘theoretical’’ specific surface area of the NRs (supposedto be cylinders with a fixed radius, r = 5 nm, as revealed by TEMmicrographs, and length Li) was calculated with the formula,SSANR = 2(∑iLifi þ r)/(FNRr∑iLifi) m2/g, where FNR is thedensity of the anatase (3.9 cm3/g). For Ag NPs (supposed to bespheres with a radius of ri), the specific surface was calculatedwith the formula, SSANP = 3∑iri

2fi/(FNP∑iri3fi), where FNP is thedensity of the Ag (10.49 cm3/g).25�27 For the TiO2 NRs/Agnanocomposites, the total area SSA resulted from the sum of thesurface area of Ag nanoparticles and TiO2 NRs weighted for theirrespective weight percent: SSANR/Ag =wNRSSANRþwNPSSANP.2.4. Photocatalysis Experiments. In a typical experiment,

0.001 mmol of TiO2-based catalysts has been deposited onto aglass slide (with a surface of 3.6 cm2) by dropping the properamount of catalyst solution and letting the solvent to evaporate,thus leading to nearly transparent films. TiO2 P25 films have

been obtained by a catalyst CHCl3 suspension at the same molarconcentration of the synthesized catalyst. The concentration ofthe catalyst solution has been calculated by atomic absorptionspectroscopy for both the TiO2 NRs and the TiO2 NRs/Agnanocomposites. The main characteristics of the catalysts aresummarized in Table 1. The transparent glass support has beensuitably shaped in order to fit a 1 cm � 1 cm quartz cell andpositioned against the cuvette wall, further with respect to thelight beam. As a radiating source, a medium-pressure 200 Wmercury lamp (λ > 250 nm) has been used. All experiments havebeen performed under ambient atmosphere, keeping the systemunder vigorous stirring. At a fixed illumination time, samples ofthe solution have been withdrawn from the reaction batch inorder to monitor the degradation course by UV�vis absorptionspectroscopy and LC analysis. The desired pH has been obtainedby adding the proper amount of 0.1M HCl.2.5. Analytical Investigation. Dye decoloration has been

monitored by UV�vis absorption spectra by measuring theabsorbance intensity at the maximum wavelength of the dye atthe desired pH.BP identification as well as determination of residual dye

concentration has been performed by LC/MS and LC/MS-MSusing an Acquity chromatographic system, equipped with both anautosampler and a photodiode array detector (Waters), interfacedto an API 5000 mass spectrometer (Applied Biosystem Sciex) bymeans of a turbo ion spray interface. The 5 μL samples have beeninjected by a Rheodyne valve and a 10 μL loop and eluted at0.3 mL/min through a HSS T3 C18 column, 2.1 � 150 mm 1.8μm,with the following gradient: from95/5A/B (A, water/MeOH95/5 þ ammonium acetate 1.5 mM; B, MeOH þ ammoniumacetate 1.5 mM) to 0/100 in 12 min, which has been thenmaintained for 3 min. The MS interface conditions have beenthe following: needle voltage, 4500 V; declustering potential, 120V; mass range, 50�500 Th; scan time, 0.3 s; nebulizer gas flow(air), 1.5 L/min; curtain gas flow (nitrogen), 1 L/min; auxiliary gasflow (air) delivered by a turbo-V heated probe, 6 L/min at 450 �C.Injections have been performed in duplicate. BPs have beenidentified using a multiple reaction monitor (MRM) LC/MS-MS method in which the [M þ H]þ ion of each targetcompound (MeRed and BPs) has been fragmented in thecollision cell at 30 V, and a suitable product ion was thenmonitored in the third quadrupole of the MS system. Determi-nation of dissolved Ti and Ag concentrations has been per-formed by using graphite furnace absorption spectroscopyusing a GFS97 instrumentation (Thermo).

3. RESULTS AND DISCUSSION

3.1. Catalyst Characteristics. The morphological and struc-tural investigation of the photocatalysts has been carried out bytransmission electronmicroscopy. In Figure 1, typicalmicrographs

Table 1. Main Characteristics of the Catalysts

average size crystalline phase surface area

TiO2 P25 25�30 nm 30% rutile 50( 15 m2/g

70% anatase

TiO2 NRs 3 � 18 nm 100% anatase 240( 5m2/g

TiO2 NRs/Ag 3 � 18 nm

(TiO2 NRs)

8 nm (Ag NPs)

100% anatase

(TiO2 NRs)

235( 5m2/g



of theNRs and the nanocomposites are presented. Bare TiO2NRs(Figure 1A) of about 3 nm in diameter and with an average lengthof 18 nm are visible, exhibiting a high aspect ratio (up to 1:10) andoften organized in head-to-tail chains. Figure 1B shows the TiO2

NRs after the photocatalytic reduction of AgNO3; the morpholo-gical and dimensional features of theNRs appear unchanged, whilethe presence of Ag spherical NPs (dark spots in Figure 1B) can beobserved. The size of the metal particles ranges between 5 and10 nm, and their average diameter is 8.0 nm. It is worth noting that,in the nanocomposites, theTiO2NRs tend to assemble around theAg NPs to form circular aggregates. High-resolution images(Figure 1C) reveal that NRs are single crystals of anatase, whereasmetal NPs are mostly polycrystalline Ag with each crystallinedomain being 2�3 nm in size; Ag NPs are directly in contact withone ormoreTiO2NRs through an interface extendedover 2�4 nm.In addition from TEM images and according to geometrical andstatistical considerations, wewere able to estimate the surface area ofTiO2 NRs (240 m

2/g) and the TiO2/Ag nanocomposite (235 m2/

g). The slightly lower surface area of the nanocomposite accountsfor a loss of a few active sites due to the contact betweenAgNPs andTiO2 NRs. The crystalline phase and surface area of TiO2 P25 havealready been reported elsewhere.28,29 The main charcteristics of thecatalysts are summarized in Table 1.3.2. Photocatalysis Experiments.TiO2 NRs and TiO2 NRs/

Ag nanocomposites have been tested in the UV-induced photo-degradation of MeRed (C. I. 13020; initial concentration, 3 �10�5M) and compared with TiO2 P25 Degussa, with all catalystsbeing deposited onto transparent substrates by casting. Theselection of the methyl red dye as the target compound repre-sents a convenient choice, because its degradation mechanismhas been extensively investigated and is thus suitable to detectpossible alternative reaction pathways occurring in the presenceof the metal NP-based composite. Photocatalytic tests have beentypically carried out for 3 h. Blank experiments performed in theabsence of catalysts and/or without UV irradiation have only

resulted in a negligible decoloration of the solution. The degra-dation course has been monitored by recording UV�vis absor-bance spectra and following the decay of the main dye absorptionpeak (this trend will be referred to as “decoloration”). Such anapproach is based on the assumption that the concentration ofBPs absorbing at the same wavelength of the parent dye shouldbe negligible with respect to MeRed concentration, in considera-tion of the good agreement between the results obtained byoptical and chromatographic investigation of the reaction timecourse, as we previously reported in the case of spherical TiO2

and ZnO NCs.10,11

Previous studies demonstrated an inherent instability of TiO2

NC films (also in the case of TiO2 P25) in strong alkalineconditions;10 accordingly, the efficiency of the catalysts has beenevaluated at acidic and nearly neutral pH values only (pH 2.5 and6.2). The percentage of decoloration as a function of pH value forthe TiO2 P25, the TiO2 NRs, and the TiO2 NRs/Ag nanocompo-sites, respectively, is reported in Figure 2A�C. For all the investi-gated catalysts, the highest decoloration percentage has beenachieved at pH 2.5, with all the catalysts being able to catalyze acomplete decoloration. In the case of pH 6.2, only the TiO2 NRs/Ag catalyst has been able to promote a complete decoloration(in 120 min), whereas the decoloration extent achieved by usingTiO2 NRs and TiO2 P25 has been only 65% and 48%, respectively.The photocatalytic activity of the three catalysts has been

evaluated by plotting the percentage of decoloration after 60 min(that is the reaction time needed by the most photoactivecatalysts to achieve a complete decoloration) as a function ofthe catalyst (Figure 3) for both the pH values tested. TiO2 NRsand TiO2 NRs/Ag-based catalysts present a much higher cata-lytic efficiency than TiO2 P25, at either acidic or neutral pH. Infact, after 60 min, TiO2 NRs catalyzed a decoloration of 91% atpH 2.5 and 47% at pH 6.2, whereas the decoloration percentageattained by TiO2 P25 has been 70% and 40% at pH 2.5 and pH6.2, respectively. Anyway, the most striking result was that the

Figure 2. Time course evolution of decoloration as a function of pHvalue in the case of TiO2 P25 (A), TiO2NRs (B), and TiO2NRs/Ag (C)assisted experiments evaluated by monitoring the absorbance intensityat 523 nm (pH 2.5) and 430 nm (pH 6.2). Reported data are presentedas mean values ( standard deviation obtained from the analysis of fivereplicates.

Figure 3. Percentage of MeRed decoloration as a function of pHestimated by absorbance spectra. Irradiation time: 60 min. Reporteddata are presented as mean values ( standard deviation obtained fromthe analysis of five replicates. In the top panels (A�D), the photographsof the vials after the irradiation are reported for all the investigatedconditions.



TiO2 NRs/Ag-based catalysts appeared even more effective thanthe bare TiO2 NRs (although the surface areas of the twocatalysts are comparable, Table 1). Indeed, the compositecatalyst achieved a nearly complete degradation in 60 min only(95% and 80% at pH 2.5 and 6.2, respectively). As alreadyreported above, the catalytic activity of the metal-modified TiO2

catalysts presented a poor dependence on pH value. Conversely,both TiO2 NRs and P25 Degussa were much more effective atpH 2.5. In summary, the observed photocatalytic activities can besummarized as follows: TiO2 NRs/Ag > TiO2 NRs > TiO2 P25,irrespective of the experimental conditions.Noble metals deposited on semiconductor particles are known

to improve the photocatalytic electron-transfer processes at thesemiconductor interface.30 It is reasonable to expect that elec-trons accumulated in a large extent within the metal NPs, thusimproving the efficiency of electron�hole separation.23,24 In theTiO2 NRs/Ag nanocomposite, the absence of any specific ligandfor Ag NPs results in a readily available surface, able to enhancethe efficiency of the electron-transfer processes. In addition, itcan be also proposed that, provided that TiO2 NRs are in closecontact with each other, the resulting TiO2 network, which formsfrom the deposition and the solvent evaporation, can act as an“antenna system”31,32 that could be able to transfer the photo-generated electrons from the photoexcitedNRs to the neighbors,up to reach a TiO2�Ag interface, where, finally, they are injectedinto the Ag NPs. As a consequence, the TiO2 NRs/Ag systemresults is characterized by a high photocatalytic reductionefficiency.19,24 The peculiar Ag NP surface chemistry, that is,the lack of a specific ligand molecule and the consequent closecontact with TiO2 NRs in the nanocomposite, has been furtherdemonstrated in an experiment of photocatalysis assisted by aTiO2 NRs�Ag NPs mixture obtained by simply mixing pre-synthesized organic-coated Ag NPs and TiO2 NRs. Interestingly,the TiO2 NRs�Ag NPs mixture showed a photocatalytic activityhigher than that of the bare TiO2 NRs, but lower than that of theTiO2 NRs/Ag nanocomposite, thus confirming the enhancedability of the TiO2 NRs/Ag nanocomposite in promotingelectron transfer (see the Supporting Information).In the presence of an efficient acceptor, such as O2, electrons

accumulated in Ag NPs could be released, thus leading to theformation of •O2

�, which can finally lead to •OH. On the otherhand, photoinduced holes (hþ) in TiO2 can be scavenged by OH

�

or H2O to generate •OH.6,33 Therefore, Ag NPs can play a two-foldrole: (i) to reduce the e�/hþ recombination as a consequence of theelectron transfer from TiO2 NRs to Ag NPs and (ii) to enhance thephotoreduction efficiency owing to the Fermi level equilibration,thus ultimately leading to a higher yield of •OH. Nonetheless, alsobare TiO2 NRs are much more photoactive than TiO2 P25. Such ahigher photoactivity can be ascribed to the extremely higher surface-to-volume ratio of TiO2 NRs (240 vs 50 m2/g), although oncedeposited onto glass, the catalyst available surface area could bereduced. However, the different surface chemistries of the twocatalysts could also influence the photoactivity. Indeed, residualOLEA molecules could be present on the TiO2 NR surface despiteseveral washings. A recently reported multifrequency electronparamagnetic resonance (EPR) study of OLEA-capped TiO2

NRs and dots in comparison with TiO2 P25 carried out with andwithout UV irradiation has allowed identifying the location of thedifferent sites at which the charges are trapped.22 These experi-ments, taking advantage of the unique ability of the EPR spectros-copy to unambiguously identify the species involved in the chargeseparation processes, have demonstrated that, in the OLEA-

modified TiO2 NCs, the hole trapping site is a carbon-centeredradical. The formation of such a carbon radical could somehowstabilize the excited state ofOLEA-TiO2NRs, because its generationwould involve the trapping of TiO2 holes. Thus, the charge pairwould be instantaneously spatially separated (leaving the holelocated on the donating capping ligand and the electron positionedin the conduction band of the semiconductor). Such a chargeseparation is, in principle, able to extend the lifetime of thephotogenerated charges, preventing their recombination, thus en-hancing the probability of 3OH generation in the aqueous phase.The obtained results also indicate a dependence of the TiO2

NRs and TiO2 P25 photoactivity on pH conditions. Such adependence of TiO2 photocatalytic activity on pH value(irrespective of its size and shape) has been previously ascribedto the modification in the ionization state of the titania surfacedue to the occurrence of acid�base equilibria:

Ti�OHþHþ / Ti�OH2þ pKa1 ð1Þ

Ti�OHþOH� / Ti�O� þH2O pKa2 ð2ÞThe pHZPC (zero point charge) of nanosized TiO2 is known to

be around 5.534 so that the surface is positively or negativelycharged at low or high pH, respectively. This behavior can beexpected to primarily influence the adsorption of the dye on thecatalyst, thus affecting the overall photocatalytic process.35,36 Asthe adsorption of MeRed on the TiO2 surface is supposed to bedriven by its carboxylic moiety (pKa MeRed = 5.3), a Coulombicrepulsion should occur at extreme pH values. Taking intoaccount such considerations, a loss in photocatalytic perfor-mances should be expected at acidic pH. Conversely, the optimalpH value for all the investigated catalysts can be found: pH 2.5.The pH value may also influence the amount of •OH radicals

formed, because of the following chain reactions:

O2 þ e� f O2•� ð3Þ

O2•� þHþ / HO2

• pKa ¼ 4:88 ð4Þ

HO2• þHO2

• f H2O2 þO2 ð5Þ

O2•� þHO2

• f HO2� þO2 ð6Þ

HO2� þHþ f H2O2 ð7Þ

H2O2 þ e� f HO• þOH� ð8Þ

H2O2 þO2� f HO• þHO•� þO2 ð9Þ

H2O2 þ hν f 2•OH ð10Þ

HO2• formation at pH higher than pKa values leads to an

inversion of reaction 4, resulting in a lack of HO2•, which,

consequently, inhibits the formation of H2O2 (eq 5), which isa further source of •OH (eqs 8�10).37,38 Nevertheless, such amechanism does not completely explain the behavior of thetested catalysts, because, in the case of the TiO2NRs/Agnanocomposite, the photocatalytic activity is barely dependenton pH.The higher photoactivity observed at acidic pH could be

reasonably accounted for by the presence of a positive charge



on the titania surface at acidic pH, which could allow thephotogenerated electrons to reach more readily the catalystsurface, thus preventing detrimental electron�hole recombina-tion to a higher extent, as previously proposed in the case of thespherical-like TiO2 NCs.

10 The surprisingly high photocatalyticperformances of TiO2 NRs/Ag nanocomposites are in goodagreement with such a hypothesis. Indeed, as metal NPs areconsidered to be highly electronegative, their Fermi level can

shift to negative potentials, thus leading to charging effects ororiginal chemical reactivity.39 The acidity of the hydroxyl groupson the TiO2NR surface could be enhanced in the presence of themetal NPs, thus accounting for the surprisingly comparableeffectiveness of the TiO2 NRs/Ag catalyst at acidic and nearlyneutral pH values.30

Wemonitored the stability of the catalysts under UV irradiationto evaluate the possible leaching of Ti or Ag in the aqueoussolution during the degradation course. Analytical determinations,carried out by atomic absorption spectroscopy, revealed that Tiand Ag concentrations in water after the photodegradationexperiment for all the catalysts tested were below the detectionlimit. It can be thus concluded that the catalysts are stable underreaction conditions within the investigated temporal range. In thecase of the TiO2 NRs/Ag nanocomposite, reflectance spectra ofthe cast catalyst have been also recorded before and after thephotodegradation and compared to the spectrum of the as-prepared catalyst solution (Figure 4) to check possible aggregationphenomena or Ag NPs resizing upon deposition and irradiationunder oxidative conditions. In fact, the position and full width atmedium height (fwmh) of the plasmon band gives an indication ofthe metal NP size and shape, as well as the occurrence ofaggregation phenomena.40,41 The position of the plasmon banddoes not undergo any significant modification before and after thephotocatalytic experiment. No considerations can be done on theline shape or on the intensity of the reflectance spectra as they arestrongly dependent on the sampled region of the slide.3.3. Byproduct Formation. LC/MS analyses in positive ions

were carried out in order to investigate BP formation and theirtime evolution. Analytical results revealed the presence of thesame BPs irrespective of the employed catalyst (Table 2). Theobserved [MþH]þ ions are consistent with the presence of BPsderiving from the addition of one or two OH groups on the

Figure 4. Absorbance spectra of TiO2 NRs/Ag in CHCl3/EtOHsolution (A) and cast onto a glass slide before (B) and after (C) thephotodegradation experiment.

Table 2. Proposed Chemical Structures of Investigated BPs



benzene ring (BPs 3, 4, and 5) and with the homolytic rupture ofthe N�Cbond on the amino-methyl moiety with the subsequentinsertion of one or two H atoms (BPs 1, 2, 3, and 5).The temporal evolution of the identified BPs (Figure 5) for the

reaction carried out at pH 6.2 in the presence of the TiO2NRsand the TiO2NRs/Ag reveals the nearly complete consumptionof the compounds within the investigated time range. Indeed,such evidence is in good agreement with the colorless appearanceof the reaction mixture at the end of the experiment. All profilesshow an initial increase of the concentration, followed by aprogressive decrease during the reaction course, according to atypical bell-like evolution. It is important to point out that all thetrends should be considered semiquantitative, due to the lack ofrelevant standards.In the case of the reaction assisted by TiO2 NRs, BPs 1 and 4

exhibit a maximum after 60 min of irradiation, respectively, whileBPs 2 and 3 show their maximum after 2 h of reaction,corresponding with the disappearance of BPs 1 and 4. On theother hand, the TiO2NRs/Ag catalyzed reactions lead to thecomplete degradation of all the investigated BPs after reachingtheir maximum concentration after 60 min of irradiation.In the case of both the TiO2 NRs and the TiO2NRs/Ag NPs

assisted experiments, BP 5 resulted as a transient species,appearing at a long irradiation time (>60 min) and with a peakarea value much lower than those measured for the other BPs.Such a behavior could be compatible with a low chemical stabilityproduct; in fact, such a product appears at 60 min, then quicklyevolves, disappearing at a longer time.The chemical structure of the observed BPs and their time

evolution suggest that the degradation takes place via two mainmechanisms, as already observed in the case of spherical anataseTiO2 NCs.10 The first mechanism involves the homolyticrupture of the nitrogen�carbon bond of the amine group,leading to the substitution of the methyl group with a hydrogenatom (BPs 1 and 2). The second mechanism involves hydroxylgroup substitution on benzene rings (BPs 3 and 4). It can be

reasonably proposed that OH substitution takes place first on thebenzene ring carrying the dimethyl amino group due to itscapability to stabilize the intermediate hydroxyl-benzene radical,as opposed to the other benzene ring that carries a �COOHmoiety. Such an OH substitution could occur repeatedly (BP 5)up to the opening of the aromatic ring, thus ultimately yieldingpolar low molecular weight compounds (thus accounting for thequick appearance/disappearance of BP 5), that is, organic acids,which are not amenable to LC-MS analysis. The presence of BPs3 and 5 suggests that the two proposed mechanisms areindependently active.

4. CONCLUSIONS

Colloidal nanocrystalline-based catalysts (namely, TiO2 NRsand TiO2 NRs/Ag nanocomposites) were synthesized andexploited in the photocatalytic degradation of organic com-pounds in water matrixes induced by UV light. TiO2 NRs andTiO2 NRs/Ag nanocomposites were tested for the first timeunder oxidative conditions and demonstrated to be much moreeffective than commercial catalysts. Interestingly, TiO2 NRs/Agnanocomposites showed a higher efficiency than TiO2 NRs andTiO2 P25 Degussa, irrespective of the pH conditions, thanks tothe copresence of the metal NPs. Indeed, Ag NPs preventsomehow the strong loss in performances of TiO2 NRs observedat neutral pH, also in the case of the commercially availablecatalyst, this being a very important feature in the perspective of areal application.

Finally, the preliminary investigation of the obtained BPsindicates that the degradation is likely to occur via a UV-inducedhomolytic rupture of the nitrogen�carbon bond and through aconcomitant OH substitution on the benzene ring, repeatedlyoccurring until the ring opening.

The ensemble of results indicates that the novel compositematerial brings a true advance in photocatalytic degradationprocesses, thus representing a promising and valuable candidateto test for viability for application at a larger scale.

’ASSOCIATED CONTENT

bS Supporting Information. Additional photocatalytic experi-ments assisted by organic-coated Ag NPs/TiO2 NRsmixed catalysts.Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (M.L.C.), [email protected] (R.C.).

’ACKNOWLEDGMENT

This work was partially supported by Apulia Region FundedProjects PS_083 and PE_049 within the Scientific ResearchFramework Program 2006 and by RELA-VALBIOR, Network ofLaboratories in the Apulia Region Framework Program forScientific Research (Italy). The authors gratefully thank MichelaCorricelli for helping in photocatalytic experiments.

’DEDICATIONzDedicated to the memory of Sara Diomede.

Figure 5. Evolution profiles of the identified BP normalized peak areaduringMeRed degradation catalyzed by the TiO2NRs (A) and the TiO2

NRs/Ag (B). All the data were normalized with respect to theirmaximum peak area value.



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