Io

BVa

b

c

a

ARRAA

KTPd

1

a[ctTwiarwb

h

0d

Applied Catalysis B: Environmental 107 (2011) 140– 149

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

journa l h o me pa ge: www.elsev ier .com/ locate /apcatb

nfluence of gold particle size on the photocatalytic activity for acetone oxidationf Au/TiO2 catalysts prepared by dc-magnetron sputtering

ogdan Cojocarua, S tefan Neatua, Elena Sacaliuc-Pârvulescua,b, Francis Lévyb,asile I. Pârvulescua,∗, Hermenegildo Garciac,∗∗

University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4–12, 030016 Bucharest, RomaniaEcole Polytechnique de Lausanne, Department of Physics, 1015 Lausanne, SwitzerlandTechnical University of Valencia, Av. de los Naranjos s/n, 46022 Valencia, Spain

r t i c l e i n f o

rticle history:eceived 2 May 2011eceived in revised form 23 June 2011ccepted 3 July 2011vailable online 8 July 2011

eywords:itaniahotocatalysisc-magnetron gold sputtering

a b s t r a c t

Two series of Au/TiO2 materials with different gold content have been prepared by dc-magnetron sput-tering on ceramic shaped pure anatase or Degussa P25 TiO2. The time of deposition was varied between 1and 20 min in order to obtain different thickness and nanoparticle size of the gold films. For comparisonsamples with Au loadings in the range 0.3–0.9 wt% were prepared using the deposition–precipitationmethodology. The obtained materials were characterized by X-ray diffraction (XRD), X-ray photoelec-tron spectroscopy (XPS), DR-UV-Vis and atomic force and scanning electron microscopy techniques. Thephotocatalytic activity was checked in the photo-oxidation of acetone under both UV and visible irra-diation conditions. Several factors were found to influence the photoactivity. An optimal Au contentcorresponding to a maximum activity is observed and attributed to the occurrence of adequate tita-nia surface coverage and Au particle size. The support plays an important role and it was found thatpristine anatase on which gold (average particle size 7.7 nm) was deposed by dc-reactive sputtering

exhibits the maximum of the activity. Samples prepared by dc-sputtering were more active than sam-ples prepared by deposition–precipitation method. Also, the photocatalysts prepared using dc-reactivesputtering showed activity under both UV and visible light irradiation, while those prepared using thedeposition–precipitation technique are active only under UV light. The most likely mechanism of goldactivation of titania is that upon light absorption, gold nanoparticles inject electrons into the titaniaconduction band.

. Introduction

Environmental applications of TiO2 as photocatalyst havettracted a very large research interest over the last three decades1–4]. It was well established that TiO2 illuminated with UV lightan degrade to the point of achieving a high degree of mineraliza-ion for almost any organic pollutant. Environmental research oniO2 began in earnest with reports of the photocatalytic splitting ofater [5]. Because of the low efficiencies of water splitting, research

nterest gradually shifted to TiO2-based treatment of polluted airnd water. Today, TiO2 research continues reporting many envi-

onmental applications for the remediation of contaminated air orater. In the last years, the coupling of titania with other solids has

een reported to improve the photocatalytic performance of these

∗ Corresponding author. Tel.: +40 21 410 02 41; fax: +40 21 410 02 41.∗∗ Corresponding author. Tel.: +34 96 387 70 00.

E-mail addresses: vasile.parvulescu@g.unibuc.ro (V.I. Pârvulescu),garcia@qim.upv.es (H. Garcia).

926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcatb.2011.07.007

© 2011 Elsevier B.V. All rights reserved.

composites. Much more than that, the investigation of titania-metalmaterials revealed that the metal deposition process enhances theefficiency of photocatalytic redox processes [6].

Au/TiO2 materials have received particular attention since itwas demonstrated that they present a variety of optoelectronicand photonic applications [7–9] and have therefore been widelyemployed for photocatalytic as well as for catalytic applications[10–14]. Besides degradation of pollutants, photocatalytic reform-ing of hydrogen on TiO2 catalysts modified with palladium andgold [15] or photocatalytic water splitting [16] are important appli-cations of Au/TiO2 photocatalysis in the conversion of light intochemical energy. These unique optical and/or chemical propertiesresult from quantum size effects of the supported nanoparticleson titania, and interface and/or surface effects between nanoparti-cles and the semiconductor. There are recent contrasting reportsdescribing that the presence of Au nanoparticles on titania can

increase [16] or decrease [17] the photocatalytic activity of thissemiconductor and, therefore, it is now of current interest to ascer-tain the photocatalytic activity of Au/TiO2 for general reactions.This conflicting data may arise from the influence of the Au content

s B: En

asos

gIsthgtApaiott

2

2

hcSmtprww(LbsrbbgsfiiAnBa

0mTpssrT

2

5tb

was saturated with acetone and blown into the reactor with aflow rate of 5 cm3 min−1. The acetone saturated gas stream wassubsequently mixed in a gas mixer with 10% humidity containingair stream to maintain a constant level of humidity. The gas

B. Cojocaru et al. / Applied Catalysi

nd particle size on the photocatalytic activity and, for this rea-on, it is important to determine the relative photocatalytic activityf consistent sets of samples in which these two parameters areystematically varied [18].

The aim of the present study was to investigate the properties ofold-doped titania catalysts prepared by dc-magnetron sputtering.n this process, atoms from a target (cathode) connected to a voltageource were sputtered off by positively charged ions originating inhe plasma created between the cathode and the anode (substrateolder). These atoms further react with the particles of the reactiveas and deposit on the substrate in oxidized form. The main advan-age of this technique is reproducibility and the ease in which theu content on the TiO2 support can be controlled. The preparedhotocatalysts were tested in the photocatalytic decomposition ofcetone, a volatile organic compound that creates serious problemsn insufficiently aerated indoor environments. Besides the wide usef acetone as industrial solvent in varnishes, lacks and glues, ace-one is also con, acetone is also consider as a model compound toest the activity of photocatalysts for other VOCs.

. Experimental

.1. Catalysts preparation

Thin films constituted by gold nanoparticles of different sizesave been deposited onto two different types of ceramic-shapedommercially available TiO2 supports (pure titania anatase fromachteleben Chemie GmbH and TiO2 Degussa P25) using the dc-agnetron sputtering technique. Before the deposition procedure,

he anatase support (present in powder form) was pressed in roundlatelets with a diameter of 0.5 cm and ∼0.3 cm thickness. Theutile-anatase support was pressed in thin square form plateletsith a side of 1.5 cm and ∼0.3 cm thickness. The deposition of goldas made in pure Ar atmosphere at a pressure of 0.5 Pa using an Au

99.99%) target (diameter 35 mm) provided from Metals Researchtd. in direct dc mode with a discharge of 300 V. The distanceetween the substrate holder and the gold target inside the depo-ition chamber was 11 cm. The supports temperature was kept atoom temperature during deposition. The supports were polishedefore the deposition of Au films. The time of deposition was variedetween 1 and 20 min in order to obtain different film thickness ofold nanoparticles onto TiO2 supports. To ensure a uniform depo-ition, the support was rotated during the sputtering process. Thinlms of Au were also deposited on Si substrates for thickness cal-

bration. The resulting samples were denoted Au/TiO2(A)-n andu/TiO2(AR)-n, were n indicates the thickness of the gold coating inm and A, R denote the anatase and rutile allotropic forms of TiO2.efore catalytic tests, part of the samples was calcined under argont 400 ◦C.

For comparison samples with gold loadings in the range.3–0.9 wt% were prepared by the deposition–precipitationethod using TiO2 Degussa P25 as support. In this procedure 1 g of

iO2 support was added to an aqueous solution of HAuCl4 (0.2 M)reviously adjusted at pH 8.5 with a NaOH solution (0.2 M). Thelurry was maintained at 75 ◦C, under vigorous stirring for 5 h. Aftertirring, the sample was filtered, washed with deionised water toemove chlorides, and then dried under vacuum at 80 ◦C for 48 h.hese samples were denoted as Au/TiO2 (DP).

.2. Catalysts characterization

The thickness of the samples was measured with an Alphastep00, Surface profiler, Tencor Instruments. The crystalline struc-ure of the Au nanoparticles on TiO2 supports was investigatedy X-ray diffraction (XRD) in both geometries, Bragg–Brentano

vironmental 107 (2011) 140– 149 141

and grazing incidence ( = 5◦), with a Rigaku diffractometer usingmonochromatized Cu K� radiation). Scanning electron microscopy(SEM) was used to study the surface morphology of Au/TiO2 usinga Philips XL 30 FEG microscope. The surface morphology wasdetermined in air by atomic force microscopy (AFM, TopometrixExplorer). The nature of the Au–Ti bonding in fresh and calcinedsamples was investigated by X-ray photoelectron spectroscopy(XPS, Kratos Axis). Different energy regions were scanned to obtainthe spectra for carbon, oxygen, titanium and gold. The bindingenergy scale was corrected for surface charging by taking the C1s peak of contaminant carbon as a reference at 284.8 eV. Fit-ting, integration, and XPS data were prepared using a commercialsoftware package (CASAXPS, CASA software Limited). DR-UV-Vismeasurements were carried out with an Analytic Jena Specord 250spectrophotometer using an integrating sphere accessory and MgOas reference. The slit was set at 4 nm. DR-UV-Vis spectra of the cat-alysts were recorded in reflectance units and were transformed inKubelka–Munk remission function F(R).

2.3. Photocatalytic tests

Photocatalytic tests were performed in a flow system, using awater-cooled quartz reactor. All the catalysts were irradiated usinga HPK 125 W high-pressure mercury vapor UV-lamp from HeraeusNoblelight with a maximum emission at 365 nm and a 40 W visiblelamp F74-765 from Tunsgram (Fig. 1) [19]. The catalyst plateletswith the same surface area around 0.785 cm2 for Au/TiO2(A) and2.25 cm2 for Au/TiO2(AR) or 50 mg of Au/TiO2 (DP) were placedinside the reactor perpendicularly to the light propagation direc-tion. Light intensity at the distance where the catalysts were placedwas 810 Lx as measured with an 840006 Speer Scientific luxmeter.The quartz walls of the photoreactor determined lower limit ofentering light (about 365 nm cut-off filter). Purified and dried air

Fig. 1. Emission spectra of the lamps used in photocatalytic tests (a) UV lamp; (b)visible lamp.

1 s B: En

flpaiaTerafccdmbie

3

3

c

42 B. Cojocaru et al. / Applied Catalysi

ow was maintained constant during the experiments using aump. The reactor was coupled to a Fisher–Rosemount on-line gasnalyzer, equipped with a CO2 detector. The gas analyzer was cal-brated before each test. Before UV irradiation, acetone-containingir was purged through the reactor over the catalysts for 20 min.he start of UV irradiation was taken as the initial moment ofach catalytic experiment. Samples of the gas phase inside theeactor were taken at regular intervals with a gas-tight syringe andnalyzed using Trace GC 2000 equipment with a DSQ MS systemrom Thermo Electron. The activity was expressed as steady-stateonversion % per surface area of the catalysts. The conversion wasalculated as the number of moles of acetone transformed in CO2ivided by the number of moles of acetone present in the gasixture. In order to check the reproducibility of the photocatalytic

ehavior and the stability of the catalysts, each sample was testedn several runs keeping the exposure time at least 200 min for eachxperiment.

. Results and discussions

.1. Preparation and morphology of Au/TiO2 photocatalysts

Sputtering is a very practical and reliable technique of surfaceoating. Sputtering of noble metals on a flat surface can lead to

Fig. 2. AFM images of the Au/TiO2: (a) Au/TiO2(A)-59 nm, (b) Au/TiO2(A)-15 n

vironmental 107 (2011) 140– 149

the formation of a continuous thin film of uniform thickness. Incontrast, in the case studied here in which gold sputtering is madeover a compressed wafer of a nanoparticulate support, the resultingsample is constituted by a layer of gold nanoparticles deposited onthe TiO2 nanoparticles. The morphology of Au/TiO2 samples afterdifferent sputtering times is exemplified in Fig. 2. In all cases, atypical globular surface texture due to sputtered gold following themorphology of the support was observed. AFM images indicate thatAu/TiO2(A)-15 nm was the sample with the lowest roughness andthis was interpreted as indicating the complete coverage of the TiO2surface by gold.

3.2. Crystal phase and Au particle size

The X-ray diffraction patterns of the Au/TiO2 samples are pre-sented in Figs. 3 and 4. Comparison with the pattern of TiO2 anatase(Fig. 3) reveals that most of the reflections of Au/TiO2(A) stem fromthe support TiO2. In addition X-ray diffraction pattern collectedfor Au/TiO2(AR)-n (Fig. 4) is in agreement with this sample beingconstituted by anatase (A) with some rutile (R) phases as deduced

from the observation of characteristic rutile peaks at 2� = 27.3◦

and 36◦. In addition, three peaks at 2� around 38.2◦, 44.4◦ and64.6◦ were observed revealing the presence of metallic gold par-ticles on the TiO2 supports. They are assigned to the diffraction

m, (c) Au/TiO2 (A)-235 nm, (d) Au/TiO2(AR)-34 nm, (e)TiO2(A)-support.

B. Cojocaru et al. / Applied Catalysis B: Environmental 107 (2011) 140– 149 143

F(5

laads(wfUftArtwq1vfceas

F(

ig. 3. X-ray diffraction patterns of Au/TiO2(A)-n catalysts: (a) TiO2(A) support,b) Au/TiO2(A)-5 nm, (c) Au/TiO2(A)-10 nm, (d) Au/TiO2(A)-15 nm, (e) Au/TiO2(A)-9 nm, (f) Au/TiO2(A)-67 nm, (g) Au/TiO2(A)-235 nm.

ines of (1 1 1), (2 0 0), (2 2 0) planes of polycrystalline Au. Sincen overlap occurred between the (1 1 1) diffraction peak of Aund the (1 0 1) diffraction peak of rutile the crystallite size wasetermined from the 44.4◦ diffraction peak. The average crystalliteize of Au was calculated by applying the Debye–Scherrer formulaD = 0.9�/ˇcos�), where D is the average crystallite size, � is theavelength of the X-ray radiation (Cu K� = 0.154056 nm), is the

ull width at half-maximum (FWHM), and � is the diffraction angle.sing this formalism, after 2 min, it was determined a 5.3 nm size

or Au/TiO2(AR)-15 and 7.7 nm for Au/TiO2(A)-15. The increase ofhe sputtering time led to an increase of the particle size. Thus, afteru being sputtered for a longer time to the TiO2 support the Aueflections become more intense indicating an increase of the crys-allinity. For samples Au/TiO2(A)-n, the Au crystallites increasedith the sputtering time from 7.7 to 12.8 nm along with the Au

uantity, this corresponding to a thickness of the gold layer of5 nm (Fig. 5). For the Au/TiO2(AR)-n samples, the crystallite sizearied from 6.2 to 7.7 nm while the gold layer thickness increasedrom 25 to 34 nm. Compared to Au/TiO2(A)-n samples, the same

rystallite size was determined for a thickness twice larger. How-ver, after 20 min the thickness is very similar for both supportsnd it corresponds to a complete coverage of titania (the crystalliteize is about 12.7 nm). For Au/TiO2 (DP), XRD patterns (Fig. 6) did

ig. 4. X-ray diffraction patterns of Au/TiO2(AR)-n catalysts: (a) TiO2(AR) support,b) Au/TiO2(AR)-11 nm, (c) Au/TiO2(AR)-25 nm, (d) Au/TiO2(AR)-34 nm.

Fig. 5. Au crystallite size variation with film thickness for Au/TiO2(A) andAu/TiO2(AR) samples.

not showed diffraction lines corresponding to gold, which can becorrelated with the good dispersion and the small particle size ofthe gold particles as confirmed by TEM (around 3.4 nm, Fig. S1).

The thickness of the deposited gold layer, as measured with theAlphastep profiler, varied between 5 and 235 nm. Table 1 compilesthe thickness of the investigated photocatalysts for the differentdeposition times. As expected the increase of the deposition timewas accompanied by an increase of the thickness. In addition theincrease of the Au layer over 15 nm led to the complete coverage oftitania. Identical deposition times led to similar film thickness onboth supports.

3.3. Au oxidation state in Au/TiO2 photocatalysts

X-ray photoelectron spectroscopy experiments were performedin order to investigate the oxidation state of Au and the interactionwith the TiO2 surface. At the same time they provided informa-tion on the effect of the temperature treatment on the Au/TiO2materials. Fig. 7 presents the XPS spectra in the region of the Au4f7/2 level for three different Au/TiO2 photocatalysts with thick-nesses of Au layer between 5 and 15 nm. The position of the Au4f7/2 peak at values lower than 84 eV confirms the fact that inde-

pendently of the deposition time, Au is found as metallic Au0. Thepresence of cationic Au is known to occur at values higher than84 eV. The thickness of the Au layer does not change the position ofthe band at 83.3 eV [20]. Shifting to lower eV values compared to

Fig. 6. X-ray diffraction patterns of Au/TiO2(DP) catalysts: (a) 0.3 wt%; (b) 0.5 wt%;(c) 0.7 wt% and (d) 0.9 wt%.

144 B. Cojocaru et al. / Applied Catalysis B: Environmental 107 (2011) 140– 149

FA

8atfbTetT

Fig. 8. XPS of Ti 2p3/2 and Ti 2p1/2 for Au/TiO2(A)-n catalysts: (a) Au-TiO2-5 nm; (b)

TF

ig. 7. XPS of Au 4f5/2 and Au 4f7/2 for Au/TiO2(A)-n catalysts: (a) Au-TiO2-5 nm; (b)u/TiO2-10 nm; (c) Au/TiO2-15 nm, line-before heating; dot-after heating.

4 eV of metallic Au, was previously explained [21], to be caused by negative charge transfer from the framework oxygen of TiO2 tohe metallic gold. The relative small difference between the workunction of Au (5.42 eV) and that of TiO2 (4.7 eV) must be responsi-le for some charge transfer at the metal–semiconductor interface.

his polarization should favor charge separation by photochemicalxcitation of gold at its surface plasmon band. Figs. 8 and 9 presenthe XPS spectra of Ti and O from the same samples. The presence ofi4+ from TiO2 framework is characterized by the peak of Ti 2p3/2 at

Au/TiO2-10 nm; (c) Au/TiO2-15 nm, line-before heating; dot-after heating.

able 1ilm layer thickness of the investigated photocatalysts for the different deposition times.

Time of Au deposition (min) 0 1 1.5 2 5 7 12 14.5 20

Film thickness Au/TiO2(A) (nm) 0 5 10 15 24 33 59 67 235Film thickness Au/TiO2(AR) (nm) 0 5 11 15 25 34 60 68 235

B. Cojocaru et al. / Applied Catalysis B: Environmental 107 (2011) 140– 149 145

Fig. 9. XPS of O 1s1/2 and O 1s2/2 for Au/TiO2(A)-n catalysts: (a) Au-TiO2-5 nm; (b)Au/TiO2-10 nm; (c) Au/TiO2-15 nm, line-before heating; dot-after heating.

Table 2XPS atomic surface concentration of Au, Ti, and O from the analysis of Au/TiO2(A)-nsamples before and after thermal treatment at 400 ◦C.

Sample Heating O 1s (%) Ti 2p (%) Au 4f (%)

Au/TiO2(A)-5 nm No 54 19 27Yes 58 21 21

Au/TiO2(A)-10 nm No 40 13 47Yes 51 17 32

Fig. 10. SEM images of Au/TiO2(A)-15 nm

Au/TiO2(A)-15 nm No 10 3 87Yes 46 15 39

457 eV, while the two peaks of O 1s1/2 (529 eV) and O 1s2/2 (531 eV)are assigned to the framework oxygen and the hydroxyl groupsfrom the surface, respectively. Fig. S2 in the supporting informa-tion shows the best deconvolution of the XPS O peaks showing thecontribution of these two types of oxygen in each photocatalyst.The effect of the calcination temperature was tested by heatingin argon the Au/TiO2 samples at 400 ◦C for 2 h and recording theXPS spectra after that. The analysis of Au 4f7/2 spectra before andafter the heating indicates that the pre-treatment affects the dis-tribution of Au on the surface of TiO2 but not its oxidation state.At 400 ◦C the decrease of the relative Au/Ti XPS ratio accounts forthe sintering of the crystallites (Table 2). The effect was previouslyobserved [22], and is dependent on the amount of deposited gold.The decrease of intensity of the Au 4f7/2 peak is more pronouncedin the case of Au/TiO2-5 nm and Au/TiO2-10 nm while for Au/TiO2-15 nm the effect is not very pronounced. Such a behavior can beexpected because small particles aggregate more rapidly. As resultof the calcination the charge transfer between the support TiO2and gold is accentuated and this is suggested by the shift to lowerbinding energies of the Au 4f7/2 band. As it is showed in Fig. 10,this effect is more pronounced for Au/TiO2-15 nm which sufferedalready a Au re-organization after heating.

Au 4f7/2 level XPS spectra of Au/TiO2 (DP) samples are presentedin Fig. 11. The binding energies of the Au 4f7/2 level in these catalysts(around 83 eV) suggest a more reduced oxidation state comparedto samples prepared by dc-sputtering method.

3.4. Optical absorption of Au/TiO2 photocatalysts

DR-UV-Vis spectra of Au/TiO2(A) samples present the plas-mon resonance band at 520–550 nm attributed to metallic gold(Fig. 12A). Along with the increase of the layer thickness a shiftto lower wavelengths is observed [23]. The shift may be explainedin terms of difference in the size of the gold particles. The assign-ment of the band at around 320 nm to (Au)n

ı+ [24] is open to doubt,since the samples contain only Au0 as revealed by XPS analysis.The band may be overlapped with the band attributed to TiO2.

The spectra of Au/TiO2(AR) (Fig. 12B) present an inverse behav-ior, with a shift of the plasmon from 480 to 550 nm. This can beexplained only by a different interaction between gold and support,rutile phase altering the electronic transfer. DR-UV-Vis spectra of

(a) before heating, (b) after heating.

146 B. Cojocaru et al. / Applied Catalysis B: Environmental 107 (2011) 140– 149

Amft

Fl

Fig. 11. XPS of Au 4f5/2 and Au 4f7/2 for Au/TiO2(DP) catalysts.

u/TiO2 (DP) samples present the plasmon resonance band with aaxima shifted at 556 nm for 0.3 wt% Au and at to around 578 nm

or the other samples (Fig. 13). These variations in the position ofhe absorption maxima of the surface plasmon bands of gold can

ig. 12. DR-UV-Vis spectra of (A) Au/TiO2(A)-n, and (B) Au/TiO2(AR)-n photocata-ysts.

Fig. 13. DR-UV-Vis spectra of Au/TiO2(DP) samples: (a) 0.3 wt%; (b) 0.5 wt%; (c)0.7 wt% and (d) 0.9 wt%.

be considered a reflection of the different particle size depend-ing on the gold loading as well as differences in the gold–titaniainterface.

3.5. Photocatalytic behavior

3.5.1. UV irradiationThe photocatalytic oxidation of acetone started after first attain-

ing the adsorption equilibrium. Considering that the reactionbegins only after switching on the UV lamp, the catalysts reached

the steady state after approximately 70 min (Fig. 14). The conver-sion increased rather fast during the first 40 min, and then slowlyreached a constant value.

Fig. 14. Evolution of the photocatalytic activity (m moles mineralized acetone/cm2

photocatalyst) of the (A) Au/TiO2(A)-n, and (B) Au/TiO2(AR)-n photocatalysts inphotodecomposition of acetone.

s B: Environmental 107 (2011) 140– 149 147

i

T

h

h

T

T

Aaaleiht(tntmram

atrgadptwLrwlraodvldlp

titdddm

o

T

tas

B. Cojocaru et al. / Applied Catalysi

The photocatalytic process is supposed to occur after the follow-ng mechanism:

iO2 + h� → TiO2(ecb− + hvb

+) (1)

vb+ + H2Oads → OH• + H+ (2)

vb+ + OH− → OH• (3)

iO2(ecb−) + Au → TiO2–Au(e−) (4)

iO2–Au(e−) + O2 → TiO2–Au + O2•− (5)

ccording to the Eq. (1), the photoexcitation of the samples gener-tes electron–hole pairs, creating the potential for both reductionnd oxidation processes to occur at the surface of the photocata-yst. Although a number of possible degradation pathways can benvisaged, the formation and subsequent reactions of hydroxyl rad-cals, generated from the oxidation of adsorbed water molecules orydroxide ions by photoexcited titania, are generally accepted ashe predominant degradation pathways of organic substrates (Eqs.2) and (3)). The chromatographic analysis of the gas phase insidehe photoreactor indicated that, under the investigated conditions,o CO or intermediate decomposition products are detectable onhese catalysts independently of the type of titania support or the

etal loading and only CO2 is observed. This is consistent withecent findings that conclude that photoinduced degradation ofcetone takes place more rapidly than the decomposition of inter-ediate species adsorbed on the surface [25].In order to check the evolution of the photocatalytic process,

fter 200 min of UV light irradiation, all the samples were submit-ed to an extraction procedure with CH2Cl2. After analyzing theesulting solution, different results are obtained as a function of theold loading on the support and the support nature. In addition tocetone, traces of formaldehyde and acetaldehyde (as intermediateecomposition products of acetone) were detected on the sam-les with small gold content. The yield to acetaldehyde was fiveimes smaller for the (A) samples compared with the (AR) sampleshich suggest different oxidation capabilities of the two supports.

iterature assigned such differences to the occurrence of differenteaction pathways depending on the titania crystal phase. Thus, itas reported that on anatase the formate pathway is dominant

eading to no or very small amounts of acetaldehyde, while onutile two parallel pathways are possible producing both acetatend formate [25]. For gold layers larger than 15 nm irrespectivef TiO2 support, instead of acetone, formic and acetic acids wereetermined after percolation of the surface with CH2Cl2. Obser-ation of these products indicates that as the depth of the goldayer increases the complete oxidation of acetone to CO2 becomesisfavored. These results could be rationalized considering that a

arge coverage of the semiconductor by Au nanoparticles limits thehotocatalytic activation of acetone on titania surface.

The trapped photogenerated electrons on gold nanoparticles areransferred to adsorbed molecular oxygen to produce O2

•− rad-cals (Eqs. (4) and (5)) [26]. These radicals are enough reactiveo generate also an advanced oxidation of the primary interme-iate decomposition products (formaldehyde and acetaldehyde,epending on the support) to carbon dioxide. The possibility ofirect advanced photo-oxidation of organic compounds on plas-onic activated gold nanoparticles is already known [27,28].However a direct electron transfer from TiO2 to chemisorbed

xygen on open titania surface cannot be out-ruled:

iO2(ecb−) + O2 → TiO2 + O2

•− (6)

An important issue in Au/TiO2 photocatalysts is to determinehe influence of gold loading and particle size on the photocat-lytic activity. These data demonstrate that, indeed, there is atrong correlation between the gold loading, the gold particle size

Fig. 15. The photocatalytic activity (m moles mineralized acetone/cm2 photocata-lyst) of the (A) Au/TiO2(A)-n, and (B) Au/TiO2(AR)-n photocatalysts after 200 min UVirradiation in photodecomposition of acetone.

and morphology, the TiO2 surface coverage and the presence ofresidual hydroxyl groups. The cumulative effect of all these factorsdetermines the existence of an optimal gold loading exhibiting themaximum photocatalyst efficiency. Fig. 15 presents the photocat-alytic behavior of the investigated catalysts in the decompositionof acetone after 200 min of UV light irradiation. As it can beobserved, both titania supports present similar trend on its pho-tocatalytic activity. The photoactivity of the investigated samplesfurther begins to increase with the increase of the metal loading,reaching a maximum value for the Au/TiO2-15 nm photocatalysts.Based on the above observations, the photocatalytic behavior ofthese samples may be assigned to a better dispersion of gold on thetitania surface. Once the deposition time increases negative effectsappear. The decrease of the conversion for higher metal loadingsis directly connected to the merging of gold agglomerates withthe formation of the continuous gold film that hinders all the Eqs.(4)–(6). In conclusion, the highest conversions are exhibited by theAu/TiO2 samples with the optimum surface coverage coincidingwith smoothest surface, as given by AFM and an intermediate par-ticle size. Taking into account that the photocatalytic process takesplace on the titania surface, an increased roughness of the surface,correlated with narrower pores, seems to be unfavorable for theacetone access to the photocatalytically active sites. The heatingof the catalysts at 400 ◦C produces a slight increase of the con-version, generally smaller than 10% compared with that reachedusing non-thermally exposed samples. This behavior is caused by

an accentuated charge transfer between the support TiO2 and gold,as it was observed by XPS.

As commented earlier, the photocatalytic results obtained alsoindicate that there is an appropriate size of gold nanoparticles

1 s B: Environmental 107 (2011) 140– 149

tcegowttontotoctnthotcr

oIopcsoAatsra

ttio(laooA(admbo

3

imvrtT

A

48 B. Cojocaru et al. / Applied Catalysi

o achieve a maximum photocatalytic activity [28]. The electronsannot be transferred from bulk gold to adsorbed O2 because thenergy level of adsorbed oxygen is higher than that of gold. So, bulkold cannot contribute to an increase of the photocatalytic activityf titania. Because the Fermi energy of the metal particles increasesith the decrease of the size, due to a quantum size effect, gold par-

icles with an appropriate size can possess an energy level betweenhat of the conduction band of titania and that of the adsorbedxygen. Thus, the photoelectrons can be captured only by the goldanoparticles and subsequently transferred to the adsorbed oxygenhus leading to an effective separation of charges, and an increasef the photocatalytic activity of titania. When the size of gold par-icles becomes too large, their Fermi energy will be lower than thatf the adsorbed oxygen and as a consequence the photoelectronsannot be transferred to the adsorbed oxygen. Also, for the casehe size of gold particles is too small, the photoelectrons also can-ot be transferred from the bottom of the titania conduction bando the gold particles because the Fermi energy of gold particles isigher than that of adsorbed titania conduction band. Therefore,nly gold nanoparticles with an appropriate size are effective forhe enhancement of the photocatalytic activity of titania. In con-lusion, the dc reactive sputtering technique offers a practical andeproducible methodology to prepare such supported particles.

Another observation of this study emerges from the comparisonf the two types of supports which were used in the present study.t is worth to note that both types of titania, either pure anataser a combination of anatase with some rutile are typically used inhotocatalysis and it is of interest to assess which the is the mostonvenient for the present application. If we consider the exposedurface of the catalyst, the conversion divided by the surface areaf the platelets shows an activity of about three folds higher foru/TiO2(A) catalysts than for Au/TiO2(AR) catalysts (Fig. 15). Such

behavior could be due to the fact that rutile acts as a sink forhe electrons generated in anatase. The premise serves to physicaleparation of the electron and the hole and thereby it depress theate of their recombination [1,29] and in the same time it preventsn efficient exchange of electrons between gold and titania.

Literature already reported examples of photo-oxidation of ace-one on gold supported on titania photocatalysts prepared by otherechniques [30]. In this paper we compared under the same exper-mental conditions Au/TiO2 samples prepared by sputtering withther prepared via deposition–precipitation method. On Au/TiO2DP) samples the conversion of acetone had a maximum for a goldoading of 0.5 wt%. The samples with 0.7 and 0.9 wt% Au showed

reduced catalytic activity that can be assigned to the presencef larger crystallites. Compared with the sputtered samples, theverall catalytic activity of these samples is smaller than that ofu/TiO2(A) and Au/TiO2(AR) (Fig. 16). However, on the Au/TiO2

DP) samples the steady conversion is reached quite rapidly, i.e.fter about 25 min of irradiation. It appears that although theeposition–precipitation is a simpler preparation method, dc-agnetron sputtering is superior providing active photocatalysts

y controlling the amount of gold and the dispersion on the surfacef support.

.5.2. Visible irradiationSputtered and deposited samples were also tested using visible

rradiation. Although the activate oxygen chemisorption on gold isostly facilitated by the UV irradiation [31], it may also occur under

isible light activation. In these conditions, gold has an additionalole as light harvester. Thus gold nanoparticles are absorbing pho-

ons and then injecting electrons into the titania conduction band.he photochemical reaction is thus initiated (Eqs. (6)–(8)):

u + h�(visible) → Au+(e−) (7)

Fig. 16. The overall photocatalytic activity of the Au/TiO2(DP) photocatalysts after200 min UV irradiation in photodecomposition of acetone.

TiO2 + Au+(e−) → TiO2(e−) + Au (8)

However, the role of the support is determinant and TiO2 isexhibiting a positive effect. Thus, in the case of visible light sourcea much slower reaction occurred on silica compared to titania [32].It results that for supported gold nanoparticles having similar sizedistributions the catalytic behavior is different depending on thesupport nature. Such a behavior confirms the mechanism involvingthe electron transfer from the conduction band of the semiconduc-tor to metal and back [32]. Literature suggests that titania and goldnanoparticles serve as independent adsorption and reaction sitesfor oxygen and acetone molecules [30].

It is interesting to note that the activity of the tested catalystsprepared by dc-sputtering after 200 min of visible light irradia-tion was approximately 20% lower than that of the same catalyststested using a UV source. This relative decrease in the photocat-alytic activity depending on the wavelength is relatively minor,considering that titania is inactive upon visible light irradiationand clearly illustrates the potential of using gold nanoparticlesto photosensitize titania using visible light. However, the effectof the presence of Au nanoparticles depends dramatically on thepreparation procedure and, thus, it was observed that Au/TiO2(DP) catalysts presented no activity under visible light irradia-tion. This contrasting behavior with respect to the visible-lightphotocatalytic activity of Au/TiO2 samples prepared by sputter-ing or deposition–precipitation clearly exemplifies the importanceof the preparation procedure and should be related to the differ-ent properties of the interface between Au and TiO2 nanoparticles,minimizing charge recombination. Surface techniques are neces-sary to get insight into the nature of the interface depending on thepreparation procedure.

4. Conclusions

The dc reactive sputtering deposition method is a very simpleand practical technique for the production of Au/TiO2 photocat-alysts with controlled nanoparticle size and layer thickness. Theresults in the photocatalytic oxidation of acetone demonstrate thephotocatalytic activity of these Au/TiO2 samples strongly dependson the gold loading and titania crystal phase, being an optimal of Aucontent for maximum activity as consequence of the titania cover-age and Au particle size. Thus, for the Au/TiO2(AR) and Au/TiO2(A)series, the highest activity was recorded for a particle size of 5.3 and

7.7 nm, respectively. The catalysts prepared using dc-reactive sput-tering showed activity under both UV and visible light irradiationconditions, while those prepared using deposition–precipitationtechnique only under UV activation, pointing out the importance

s B: En

oThainceam

A

s9C

A

t

R

[

[

[[

[[[

[

[[[[[[[

[[

[

[[

[

B. Cojocaru et al. / Applied Catalysi

f the exact preparation protocol to obtain highly active samples.he experiments showed that gold has an additional role as lightarvester. In this quality gold nanoparticles are absorbing photonsnd injecting electrons into the titania conduction band, effect-ng charge separation with the positive holes located on the goldanoparticles and the electrons in the TiO2 conduction band. Theombination of appropriate loading, particle size and partial TiO2xposure favors the photocatalytic activity of these materials whichppears to be higher than that of samples prepared by differentethods.

cknowledgements

The authors kindly acknowledge NATO’s Scientific Affairs Divi-ion in the framework of the Science for Peace Programme Sfp81476 for the financial support. Bogdan Cojocaru wish to thankNCSIS PNII PD 13/2010 for financial support.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.apcatb.2011.07.007.

eferences

[1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)69–96.

[2] A.L. Linsebigler, G. Lu, J.T.J. Yates, Chem. Rev. 95 (1995) 735–758.[3] A. Mills, S. Le Hunte, J. Photochem. Photobiol. A 108 (1997) 1–35.[4] U. Stafford, K.A. Gray, P.V. Kamat, Heterogen. Chem. Rev. 3 (1996) 77–104.[5] A. Fujishima, K. Honda, Nature 238 (1972) 37–38.[6] A. Dawson, P.V. Kamat, J. Phys. Chem. B 105 (2001) 960.

[

[

vironmental 107 (2011) 140– 149 149

[7] S.W. Boettcher, J. Fan, C.-K. Tsung, Q. Shi, G.D. Stucky, Acc. Chem. Res. 40 (2007)784.

[8] T. Kiyonaga, T. Mitsui, M. Torikoshi, M. Takekawa, T. Soejima, H. Tada, J. Phys.Chem. B 110 (2006) 10771–10778.

[9] J.J. Pietron, R.M. Stroud, D.R. Rolison, Nano Lett. 2 (2002) 545.10] G. Wua, T. Chena, W. Sua, G. Zhoua, X. Zonga, Z. Leia, C. Lia, Int. J. Hydrogen

Energy 33 (2008) 1243.11] W. Yan, B. Chen, S.M. Mahurin, E.W. Hagaman, S. Dai, S.H. Overbury, J. Phys.

Chem. B 108 (2004) 2793.12] W.-C. Li, M. Comotti, F. Schuth, J. Catal. 237 (2006) 190.13] K. Dai, T. Peng, H. Chen, R. Zhang, Y. Zhang, Environ. Sci. Technol. 42 (2008)

1505.14] H. Tada, T. Kiyonaga, S.-i. Naya, Chem. Soc. Rev. 38 (2009) 1849.15] M. Bowker, P.R. Davies, L. Saeed Al-Mazroai, Catal. Lett. 128 (2009) 253–255.16] C.G. Silva, R. Juarez, T. Marino, R. Molinari, H. Garcia, J. Am. Chem. Soc. 133

(2011) 595–602.17] J.T. Carneiro, C.C. Yang, J.A. Moma, J.A. Moulijn, G. Mul, Catal. Lett. 129 (2009)

12–19.18] A. Primo, A. Corma, H. Garcia, Phys. Chem. Chem. Phys. 13 (2011) 886–910.19] http://www.msscientific.de/hpk125wlamp.htm.20] P. He, M. Zhang, D. Yang, J. Yang, Surf. Rev. Lett. 13 (2006) 51–55.21] S. Arrii, F. Morfin, A.J. Renouprez, J.L. Rousset, J. Am. Chem. Soc. 126 (2004) 1199.22] L. Armelao, D. Barreca, G. Bottaro, Chem. Mater. 16 (2004) 3331–3338.23] M.M. Stella, C.B. Louise, J. Anjela, L.F. Daniel, Chem. Mater. 10 (1998) 1214–1220.24] M. Magureanu, N.B. Mandache, J. Hu, R. Richards, M. Florea, V.I. Parvulescu,

Appl. Catal. B: Environ. 76 (2007) 275–281.25] A. Mattson, L. Österlund, J. Phys. Chem. C 114 (2010) 14121–14132.26] A.A. Ismail, D.W. Bahnemann, I. Bannat, M. Wark, J. Phys. Chem. C 113 (2009)

7429–7435.27] P.S.S. Kumar, R. Sivakumar, S. Anandan, J. Madhavan, P. Maruthamuthu, M.

Ashokkumar, Water Res. 42 (2008) 4878–4884.28] B. Tian, J. Zhang, T. Tong, F. Chen, Appl. Catal. B: Environ. 79 (2008) 394–401.29] R.I. Bickley, T. Gonzalez-Carreno, J.S. Lees, L. Palmisano, R.J.D. Tilley, J. Solid State

Chem. 92 (1991) 178–190.30] S.V. Awate, A.A. Belhekar, S.V. Bhagwat, R. Kumar, N.M. Gupta, Int. J. Photoen-

ergy (2008), Article ID 789149.31] S. Neatu, B. Cojocaru, V.I. Parvulescu, V. Somoghi, M. Alvaro, H. Garcia, J. Mater.

Chem. 20 (2010) 4050–4054.32] E. Kowalska, O.O.P. Mahaney, R. Abea, B. Ohtani, Phys. Chem. Chem. Phys. 12

(2010) 2344–2355.