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Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 57– 63

Contents lists available at SciVerse ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h om epa ge: www.elsev ier .com/ locate / jphotochem

hotocatalytic activity enhancement of TiO2 thin films with silver doping underisible light

ulce Jocelyn Ramos Gutiérreza, Nini Rose Mathewsb, Susana Silva Martínezc,∗

Posgrado en Ingeniería y Ciencias Aplicadas, FCQeI-CIICAp, UAEM, MexicoCentro de Investigación en Energía, UNAM, Priv. Xochicalco S/N, Temixco, Morelos, MexicoCentro de Investigación en Ingeniería y Ciencias Aplicadas, UAEM, Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Morelos C.P. 62209, Mexico

a r t i c l e i n f o

rticle history:eceived 16 November 2012eceived in revised form 13 March 2013ccepted 17 April 2013vailable online 3 May 2013

eywords:

a b s t r a c t

TiO2 and silver doped TiO2 films (photoanodes) were prepared by the sol–gel method using silver contentsof 1%, 3% and 5% (w/w). Photoelectrocatalytic activity of these photoanodes was evaluated by monitor-ing the photocatalytic decomposition of methanol and basic orange II (BOII) in aqueous solution undervisible light illumination at pH 3 adjusted with HClO4 by the application of potential bias. Enhancedphotoelectrocatalytic activity was found by the application of 0.4 V (vs. SCE) bias potential under visiblelight illumination. A detrimental effect was observed on the photoelectrocatalytic activity of the TiO2

g–TiO2 photoanodesias potentialasic orange IIethanol

hotoelectrolysis

photoanodes in the presence of BOII, and is evident from the low photocurrents; however, the pres-ence of silver in the TiO2 films increased the photoelectrocatalytic activity. Also the photoelectrolysis ofmethanol removed over 80% of total organic carbon in 5 h of reaction under visible light and 0.4 V (vs. SCE)bias potential. Nevertheless, negligible TOC removal was observed for the BOII photoelectrolysis with orwithout bias potential and under visible light illumination or in dark. Less than 9% of discoloration wasachieved at the 3% Ag–TiO2 photoanode under visible light illumination and bias potential.

. Introduction

The application field of photocatalysis is broad and it has con-ributed in the development of new ways to prevent and removerganic pollutants. The photoelectric and photochemical propertiesf titanium dioxide as a catalyst have driven its research for vari-us applications such as solar energy conversion and degradation ofrganic compounds in wastewater [1–6]. Moreover TiO2 is an inex-ensive, nontoxic, chemically and biologically inert semiconductor7–10].

The use of TiO2 powder for the removal of contaminants inater and air has shown to be efficient due to its large surface

rea available for the reaction [11], however, the immobilizationf TiO2 particles on solid substrates present great advantages overystems which employ colloidal suspensions of TiO2 particles, sincey this way the difficulty of recovering the catalyst from treatedater can be solved [12–14]. TiO2 may be immobilized on a variety

f materials, such as glass spheres or sheets, the reactor walls, fiber-lass mesh [15], polyethylene and polypropylene films, or films and

ransparent porous TiO2 glass substrates [16], etc. Nevertheless, ifhe TiO2 is immobilized on substrates of electrically conductive

aterials, it is possible to use in electrochemical techniques [3,4],

∗ Corresponding author. Tel.: +52 01777 3297084; fax: +52 01777 3297984.E-mail addresses: ssilva@uaem.mx, silvamar2009@gmail.com (S.S. Martínez).

010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2013.04.021

© 2013 Elsevier B.V. All rights reserved.

which provide valuable information regarding to the rate of con-taminant destruction in comparison with the photocurrent passingthrough the working electrode. From this comparison, it is possibleto deduce the oxidation efficiency under any potential applied [17].

Different deposition techniques have been used to immobilizeTiO2 particles, some of these are the rotary evaporation, pulsedlaser deposition [18], sputtering [19], chemical vapor depositionof precursors [20], dip-coating [21], sol–gel [22], spin-coating [23],aerosol powder coating, coating electrophoretic [24], ion implan-tation [25], hydrothermal crystallization, etc. Among the abovementioned methods, sol–gel is the most competent due to its sim-plicity and low cost in the production of multicomponent uniformlayers [26].

In spite of the positive properties of TiO2, spectroscopic studiesshowed that ∼90% of the photogenerated electron–hole (e−–h+)pairs recombine rapidly after excitation, representing a quantumyield less than 10% [27]. Besides as a wide band semiconductor, itrequires the application of light with energy greater than 3.2–3.0 eVfor anatase and rutile respectively for the photoactivation. Theabsorption threshold corresponds to the UV light spectrum, whichis between ∼380 and ∼400 nm [28–32], causing a slow reactionrate, since according to data reported, the solar spectrum contains

only about 5% of UV radiation [33–36]. For this reason researcheshave focused on improving its performance by modifying its sur-face by deposition of noble metals [37]; thus moving its absorptionspectrum toward the visible region.

5 ry and Photobiology A: Chemistry 262 (2013) 57– 63

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8 D.J.R. Gutiérrez et al. / Journal of Photochemist

In this work, we studied the photoelectrocatalytic properties ofndoped and Ag doped TiO2 films on conductive glass depositedy sol–gel method and dip-coating technique. These photoanodesere used for the oxidation of methanol and basic orange II dye asodel compounds.

. Experimental

.1. TiO2 and Ag–TiO2 photoanode preparation

TiO2 and Ag–TiO2 thin films were prepared by the sol–gelethod using optically transparent conducting glass plates

10 mm × 25 mm × 1.1 mm ITO glass, Sigma–Aldrich). The ITOlates were cleaned with an alkali free detergent, and double dis-illed water in an ultrasonic bath. These ITO plates were dried prioro utilization, first under a stream of air and subsequently driedn an oven at 100 ◦C. Silver contents of 1%, 3% and 5% (w/w) weresed to prepare the Ag–TiO2 thin films. Precursor solutions for thinlms were prepared at room temperature following the proce-ure reported by Mathews et al. [38]; which briefly consisted ofreparing two solutions separately. A solution containing 0.8 mL ofeionized water, 0.5 mL of nitric acid (AR grade), 5 mL of ethanol99.9% pure, AR grade) and silver nitrate (AR grade) was addedrop wise into a homogeneous solution containing 6.5 mL of tita-ium butoxide (AR grade) and 17.5 mL of ethanol (99.9% pure, ARrade). Then the sol was stirred during 2 h. For film deposition theTO plates were submerged into the titania sols, perpendicularlyo the solution surface, with a speed of 0.76 mm/s and withdrawnt the same speed. The films were first dried for about 15 min atoom temperature and then oven dried at 100 ◦C for 5 min beforeore coatings were applied (three coatings). After drying, the thin

lm electrodes were sintered at 400 ◦C with a stream of air for h. The morphology and thickness of the TiO2 films were obtainedsing a high resolution scanning electron microscope (SEM) (LEO450 VP). The structural characterization was carried out usingigaku X-ray diffractometer with monochromatized CuK� radi-tion (� = 1.54056 A). X-ray diffraction spectra of the films wereecorded by scanning 2� in the range of 20–70◦, with grazing inci-ence angle of 0.5◦.

.2. Photoelectrochemical cell

The photoelectrochemical cell (PEC) was comprised by one com-artment three-electrode cell made of Pyrex glass, fitted with auartz glass window, and a visible light lamp. The area of the pho-oanodes (working electrodes) in contact with the solution was

cm2. The counter electrode was a 3 cm2 platinum mesh and a sat-rated calomel electrode (SCE) was used as a reference electrode. Aotentiostat (Epsilon BAS) interfaced with a PC (Dell, Intel Pentium) was used to measure the current–voltage data. The visible lightource used was a 35 W Xenon lamp (8000 K). The components ofhe PEC were enclosed inside a wooden black box to prevent theight from the surroundings.

.3. Photoelectrocatalytic activity

In order to study the photoelectrocatalytic activity of the thinlms, methanol and basic orange II (BOII) were used as modelompounds. All compounds and chemicals were used as receivedithout further purification. The pH of the solutions was adjustedith HClO4 or NaOH (AR grade). 30 mL of solutions containing dif-

erent concentrations of the model compounds under constant pH

ere used. The solution in the photo reactor was mixed with an

lectromagnetic stirrer. The distance between the vessel and the5 W Xenon lamp was fixed at 1 cm. The current (photocurrent)as recorded as a function of the applied bias potential (−0.5 to

Fig. 1. XRD patterns of the TiO2 nanocrystals with different content of Ag.

0.5 V vs. SCE) under dark and illumination conditions for the differ-ent solutions containing the model compounds. Oxidation of themodel compounds was also carried out by applying a bias potentialof 0.4 V vs. SCE under 5 h of illumination. Total organic carbon (TOC)was analyzed after the photoelectrocatalytic oxidation. TOC analy-sis was carried out using standard methods and standard reagents(20 mg L−1 and 150 mg L−1 TOC).

3. Results and discussion

3.1. Characterizations of the thin film structure

The XRD patterns for undoped and Ag doped TiO2 films areshown in Fig. 1, indicating that the major phase of TiO2 and Ag–TiO2films is an anatase structure. For Ag–TiO2 films, it can be seen thatthe peak intensity increases with the increase of Ag concentrationand no peaks of silver oxides were observed. Nevertheless, it wasreported the existence of Ag and Ag2O [39,40], and Ag, Ag+ and Ag2+

[40,41] on TiO2 surface after doping the TiO2 with silver ions. It hasalso been reported the presence of Ti3+ species on the TiO2 surfacewhich resulted from the incorporated materials of Ag or Ag2O onthe TiO2 coatings [39–41]. The Ti3+ species (vacancy sites) on theAg–TiO2 surface enhance the photooxidation of organic compoundsbecause these species can trap the photogenerated electrons [39].

Kuo et al. indicated that the major role of silver or silver oxideincorporated on TiO2 could be attributed to accelerate the for-mation of superoxide radical anion O2

− and also decrease theprobability of recombination of electrons and holes by scavengingthe electrons in the conduction band by silver dopants [39]. Wanget al., reported that the chemical states of Ag+ and Ag2+ are bene-fit to the photocatalytic ability of TiO2 films since doping the TiO2with silver ions (Ag+ and Ag2+) may lead to the formation of space-charge layer [41]. Thus, depending on the optimum dopant contentthe e−/h+ pairs are efficiently separated by the large electric filedbefore recombination [42].

The crystallite size (Table 1) was calculated by Scherer’s equa-tion using the FWHM from the main anatase peak (1 0 1) at2� = 25.22◦; the crystallite size of Ag–TiO2 films tends to increasewith the increasing amount of Ag ions. These results indicate thatduring the drying and annealing processes the Ag+ ions spreadingon the surface of anatase grains would gradually be reduced to Ag0

which increases the crystallite size [43,44]. It can be observed thatfor 5% Ag doped TiO2 the peaks (0 0 4), (2 1 1) and (2 0 4) are clearlymore defined.

D.J.R. Gutiérrez et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 57– 63 59

Table 1Lattice constants a and c of the TiO2 films with different Ag contents.

Film Crystalline size (nm) Lattice parameters

a (Å) c (Å)

TiO2 13.19 – –1% Ag–TiO2 15.27 3.7715 9.8630

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Table 2Band gap and flat band potential for the TiO2 and Ag–doped TiO2 thin film electrodes.

Film Band gap (eV) Flat band potential (V) at pH 3

TiO2 3.21 −0.4041% Ag–TiO2 3.19 −0.398

3% Ag–TiO2 16.96 3.7862 9.66975% Ag–TiO2 17.75 3.7936 9.6037

The lattice parameters of the anatase phase which is having aetragonal unit cell were calculated from the peak positions (1 0 1)nd (2 0 0), given in Table 1; and these values are close to theeported bulk values of anatase phase (a = 3.7842 A, c = 9.5146 A)38]. However, the lattice parameters of pure TiO2 films were notetermined because of the absence of the (2 0 0) peak. Table 1 showshat the lattice constant a increases with the increase of Ag, while

decreases.Fig. 2 shows the SEM images of the TiO2 and Ag–TiO2 thin films

nnealed at 400 ◦C for 2 h. The Ag content (%, w/w, Ag+) was (a) 0%,b) 1%, (c) 3% and (d) 5%. The average thickness of the TiO2 filmsas approximately 276–307 nm. The SEM images show uniform

urfaces of the TiO2 and Ag–TiO2 films suggesting that silver ionsere evenly distributed onto the TiO2 nanoparticles during the dip-

oating process [45].

.2. Photoelectrochemical characterization of the TiO2 andg–TiO2 films

The photoelectrochemical characterization of the films was car-ied out by recording the photocurrent as a function of appliedotential (between −0.5 to 0.5 V vs. SCE), under visible light illu-ination (Fig. 3A) and dark (Fig. 3B). The solution used was water

t pH 3 (adjusted with HClO4). The Fig. 3A shows a higher limitinghotocurrent for Ag–TiO2 under illumination which increases with

ncreasing the silver content in the film up to 3% and then it

ecreases but it is still higher than that on TiO2 film.

Zheng et al. also reported an optimal silver content to obtainigh photocurrents and suggested that the Ag loading has an effectn the amount of free carriers in the catalyst, which in turn affect

Fig. 2. SEM images of the TiO2 and Ag–TiO2 thin films annealed at 400 ◦C

3% Ag–TiO2 3.11 −0.3575% Ag–TiO2 3.05 −0.326

the photocatalytic activities of TiO2 thin films [45]. These photocur-rents represent the photocatalytic activity of the TiO2 and Ag–TiO2films for water oxidation [45–48]. Also, the higher photocurrentsobserved for the Ag–TiO2 indicates (a) that the presence of silver inthe films confers higher photoelectrocatalytic activity of the TiO2catalyst toward the visible light, which is confirmed by the lowerband gap values obtained as silver increases in the film (see Table 2)[42], and (b) that the photogenerated electrons could be morerapidly transferred toward the counter electrode [49–52] becausethe deposited Ag (with the appropriate content of Ag) can trapthe photogenerated electrons assisting the external electric fieldto migrate them from the TiO2 film anode toward the counter elec-trode [49], thus lowering the electron–hole recombination rates[52].

The main role of the applied bias potential is to prevent thebuildup of free electrons released by photohole surface capturereaction, which, in turn, minimizes the recombination of photo-generated electrons and holes [50]. In dark (Fig. 3B), the currenton the undoped TiO2 is negligible, whereas the linear scan voltam-mograms (LSV) for the 1% Ag–TiO2 films show small increase incurrent between 0.150 and 0.250 V and for the 3% and 5% Ag–TiO2films two well defined peaks at higher positive potentials of about0.375 V vs. SCE were observed. This behavior is explained by theelectrochemical oxidation of silver on the Ag–TiO2 surface as theoxidation state of the silver in the films was Ag(0), as reported byZheng et al. [45]; these authors found that Ag+ in the TiO2 thin filmwas converted into Ag(0) atom or cluster after annealing at 500 ◦C[45–48,51]. Additionally, in dark no current flows at 0 V in con-

trast with the current observed under illumination. It is observed(Fig. 3A) that the saturation photocurrent becomes practically inde-pendent at potentials ≥ 0 V vs. SCE. Table 2 shows the band gapvalues for all the films tested. It can be observed that the band gap

for 2 h: (a) TiO2, (b) 1%Ag–TiO2, (c) 3%Ag–TiO2, and (d) 5%Ag–TiO2.

60 D.J.R. Gutiérrez et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 57– 63

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ecreases with increasing silver content and the flat band potentialncreases (is less negative) as silver content in the films increases.

The magnitude of the photocurrents observed in the TiO2 andg–TiO2 films in aqueous solutions containing no other species

han the inert supporting electrolyte can be related to the ratef production of the powerful OH• radicals resulting from waterxidation by the photogenerated holes [53]. When organic com-ounds are added in the solution, the photocurrents are expected to

ncrease, because these compounds especially alcohols, are gener-lly more efficient photohole scavengers than water; therefore thexidation of organic molecule is thermodynamically more favor-ble than water oxidation at the TiO2 electrodes [54], and undergo

relatively rapid and irreversible oxidation [55,56]. Fig. 4 showsn enhancement of the photocurrent on the films under visibleight illumination in the presence of 10 × 10−3 mol L−1 methanolFig. 4A) and 1 × 10−6 mol L−1 BOII dye (Fig. 4B) in aqueous solu-

ions at pH 3 adjusted with HClO4. The photocurrent enhancements observed for both organics but with different pattern when 5%ilver content is on the film. Well defined limiting photocurrentsFig. 4A) are clearly seen by the oxidation of water and methanol,

Fig. 4. Photovoltammograms recorded at a 10 mV s−1 potential scan rate (towardpositive potentials) of TiO2 and Ag–TiO2 under visible light illumination in (A)10 × 10−3 mol L−1 methanol and (B) 1 × 10−6 mol L−1 BOII aqueous solutions at pH 3.

with the highest photocurrent plateau achieved on the 3%Ag–TiO2film. In the case of water and BOII oxidation a pronounced peak isobserved on the 5% Ag–TiO2 film at ∼0.375 V vs. SCE which can beascribed to the electrochemical oxidation of silver on the Ag–TiO2surface; this same peak (small bump, around the same potential),shown in Fig. 3A, is also observed in the absence of organic com-pounds, which confirms such behavior.

Figs. 5 and 6 show the saturation photocurrent profiles at biaspotential of 0.4 V (vs. SCE)) for the TiO2 films with different Ag con-tent obtained from the current-potential LSV under visible lightillumination for the methanol and BOII systems as a function of theorganic concentration. Fig. 5 shows a rapid increase in the satu-ration photocurrent with the increase in methanol concentrationand then the rate slows and reaches almost constant value wherethe saturation photocurrent becomes practically independent ofmethanol concentration; this is observed in the case of all films. Thisis because at high concentration the methanol molecules reach-

ing the TiO2 film surface were not completely mineralized to CO2;also, the positive effects on photocurrent at high concentrations ofmethanol can be related to the number of photogenerated holestransfer, steps and intermediates during mineralization [3,4].

D.J.R. Gutiérrez et al. / Journal of Photochemistry and Photobiology A: Chemistry 262 (2013) 57– 63 61

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ig. 5. Saturation photocurrents (obtained at 0.4 V vs. SCE from the LSV) of TiO2 andg–TiO2 under visible light illumination as a function of methanol concentration inqueous solutions at pH 3.

In the case BOII (Fig. 6) the opposite is observed on the undopediO2 film (inset of this figure) in which the addition of BOII exhibitsn unfavorable effect on photocurrent which worsens as the BOIIoncentration increases; similar behavior was reported by Jiangt al. [3]. This phenomenon could be attributed to the accumulationf intermediates nearby the film surface which might act as recom-ination centers for photogenerated holes and electrons [3]. On theontrary for Ag–TiO2 films a remarkable increase in the saturationurrent is observed with the increase in BOII concentration. Theaturation current increases on the 1% and 5% Ag–TiO2 films up to aoncentration of 240 × 10−6 mol L−1 BOII and then it decreases forigher BOII concentrations. The saturation photocurrent on the 3%g–TiO2 film is practically constant for [BOII] ≤ 90 × 10−6 mol L−1;t higher BOII concentrations the saturation photocurrent shows aapid decrease rate. The increase in photocatalytic activity of TiO2lms with the Ag content is more remarkable in the case of BOIIystem up to certain concentration suggesting lower electron–holeecombination rates. The decrease in the saturation current after aertain BOII concentration on the Ag–TiO2 films may be attributed

o that the BOII molecules or their by-products might diffuse awayr be continually accumulated near the TiO2 and Ag–TiO2 filmurface. Such phenomenon was caused by the difference in thenteractions of the organic compounds and their partially degraded

ig. 6. Saturation photocurrents (obtained at 0.4 V vs. SCE from the LSV) of TiO2

inset) and Ag–TiO2 under visible light illumination as a function of BOII concentra-ion in aqueous solutions at pH 3.

photocatalysis (�, unbias potential) of methanol as a function of silver content inthe TiO2 films in 60 × 10−3 mol L−1 methanol concentration in aqueous solutions atpH 3 under visible light illumination.

intermediates with the TiO2 surface [5]. Finally, the BOII concen-tration range, in which the saturation photocurrent decreased, wasmuch lower than that for methanol.

3.3. Photoelectrolysis of the organic compounds under visiblelight

A set of experiments were carried out of bulk photoelectrolysisfor the organic compounds at prolonged constant potential (0.4 Vvs. SCE, up to 5 h) under visible light illumination using the TiO2 andAg–TiO2 photoanodes at pH 3 (adjusted with HClO4). The photocat-alytic oxidation of methanol was also carried out under the sameconditions without applying potential to the films. Fig. 7 showsthe results for TOC removal of 60 × 10−3 mol L−1 methanol aque-ous solution under visible light illumination, with (�) and without(�) bias potential, as a function of silver loading on the TiO2 filmphotoanodes. Higher TOC removals were acquired for the biasedpotential systems on the TiO2 film photoanodes in comparison withthose for the unbiased potential (photocatalysis). The bias poten-tial increases the TOC removal of methanol at the TiO2 films andthe silver loading on the films also contributes on removing fur-ther the TOC under visible light illumination. This behavior may beexplained by the following reasons: (a) the methanol moleculeseither undergo direct oxidation by scavenging holes from thevalence band or some of its oxidized intermediates inject elec-trons in the conduction band which is depleted of electrons due tothe external positive bias (“current doubling” effect) [3,53] and (b)the presence of silver in the films confers further positive effect onthe oxidation of methanol by increasing the photocatalytic activitytoward the visible light. The oxidation of methanol was also carriedout in the absence of visible light irradiation applying only a biaspotential of 0.4 V (vs. SCE) a less than 7% of TOC was decreased at5 h of the electrolysis.

In the case of the BOII photoelectrolysis with or without biaspotential and under visible light illumination or in dark, negligi-ble TOC removal was observed. Less than 9% of discoloration wasachieved at the 3% Ag–TiO2 photoanode under visible light illumi-nation and bias potential.

3.4. Effects of the external anodic bias on the photocatalyticactivity of the Ag/TiO2 photoanodes

The silver content in the Ag/TiO2 films may be affected by theexternal anodic bias due to the possibility of silver release by oxida-tion and silver dissolution after the repeated use of the photoanodes

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2 D.J.R. Gutiérrez et al. / Journal of Photochemist

uring the photoelectrocatalytic process. The Ag/TiO2 photoanodesere repeatedly used during the processes of photoelectrolysis. Sil-

er content affectation from the photoanodes was indirectly knowny measuring the reproducibility of the photocurrent of the filmsnder visible light illumination in the presence of 10 × 10−3 mol L−1

ethanol. The photocurrents recorded in Fig. 4A were taken aseference to assess the photocatalytic activity of the Ag/TiO2 pho-oanodes. This practice was performed before and after usinghe Ag/TiO2 photoanodes for prolonged times. The well definedimiting photocurrents by the oxidation of water and methanolas in Fig. 4A) were reproduced inside an experimental error of3%. These results showed that the photocatalytic activity of thehotoanodes was unaffected by the external anodic bias applieduring the treatment of the organics. Thus, the amount of silvereleased from the films was considered negligible since no effectas observed on the photocatalytic activity of the Ag/TiO2 photoan-

des. Similar results were reported by He et al. [49]. These authorseported that the COD removal efficiencies basically remainednchanged in the process of five-time batch runs when an anodicias of 0.8 V vs. SCE was applied [49], (this potential was twice thealue we used in our study). In their resulting solution of the fifthun, no Ag+ ion was observed with ICP analysis which suggestedhat the oxidation of Ag can be negligible for the photoelectroca-alytic (PEC) process in the presence of 0.8 V vs. SCE anodic bias,n agreement with our findings. Additionally, these authors con-ucted a pulse PEC, as preliminary experiment, in which PEC andhotocatalysis (PC) processes were alternately conducted at reg-lar intervals (4 min PEC process and 1 min PC process) using auch higher potential (4.2 V vs. SCE anodic bias), in which the oxi-

ized Ag+ ions were expected to be re-photoreduced to Ag on theiO2 film. This based on that Ag+ ions are easily photoreduced sincehey exhibit high photoreduction activity with a redox potential ofoAg+/Ag

= 0.7995 V [49]. They observed that the COD removal effi-

iency was much higher than that of PEC process with a constantnodic bias and no apparent decrease in COD removal efficiencyn the repeated batch runs was observed. These authors concludedhat such behavior can be attributed to the re-photoreduction ofhe oxidized Ag+, as expected [49].

. Conclusions

In summary, we have successfully get Ag–TiO2 films by theol–gel process. XRD pattern of pure TiO2 and Ag–TiO2 films showsingle phase of anatase. It was found that Ag doping favors to theransformation of amorphous TiO2 into anatase phase. The crystal-ite size of TiO2 particles in the films annealed at 400 ◦C increasedrom 13.19 to 17.75 nm with increasing silver content. Band gap ofhe films decreased from 3.21 to 3.05 eV with the increase of Ag ions.he photoelectrocatalytic oxidation process at TiO2 and Ag–TiO2hin film photoanodes has been studied using methanol and BOII as

odel compounds. At high methanol concentrations the saturationhotocurrent becomes practically independent of methanol con-entration for all TiO2 and Ag–TiO2 photoanodes. The opposite wasbserved at high BOII concentrations for the TiO2 film; while, theresence of silver in the TiO2 film showed an increase in the satu-ation photocurrent. The maximum saturation photocurrent (reac-ion rate) depends on the surface concentration of the organic com-ounds (methanol and BOII) at high bias potential. Under an appliedias higher TOC removal (mineralization) from the methanol oxi-ation during the photoelectrolysis was observed compared with

he unbiased condition at 5 h of reaction. Regarding to the BOII pho-oelectrolysis with or without bias potential and under visible lightllumination or in dark, negligible TOC removal was observed. Lesshan 9% of discoloration was achieved at the 3% Ag–TiO2 photoan-de under visible light illumination and bias potential.

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Photobiology A: Chemistry 262 (2013) 57– 63

Acknowledgments

The authors thank Secretaria de Educacion Publica throughPROMEP (Programa de Mejoramiento a Profesores) program forsponsorship this project and CONACyT for the grant given to theDJRG to support her postgraduate studies.

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