Catalysis Today 240 (2015) 242247

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Synerg o hM/TiO2 f T

S. BashirSABIC-Corpora

a r t i c l

Article history:Received 26 JaReceived in reAccepted 10 MAvailable onlin

Keywords:TiO2 (anatase,SynergismTiO2 XPS valenTiO2 XPS Ti2phydrogen prodTiO2 particle size

/rutil serierticleher iere ho the

depoc prodonditis is g syn

area when the initial starting semiconductor was anatase nano (while no synergism seen in the case ofthe micro crystals). Maximum rate observed was in the 1030% rutile range. Possible reasons for thesedifferences are discussed.

2014 Elsevier B.V. All rights reserved.

1. Introdu

Hydrogemethods intovoltaic anand photo ctions carrieincluding bband energyrecombinatand defects

TiO2, ammaterial, coare anataseof 3.0 eV at and rutile psurements [12] and in blonger life twater and

CorresponE-mail add

http://dx.doi.o0920-5861/ ction

n production from water can be obtained via manycluding solar thermal water splitting [1], combined pho-d electrolysis [2], photo electrochemical processes [3]atalysis [4]. Photocatalytic hydrogen production reac-d out on semiconductor materials relies on many factorsand gap energies, band edges (conduction and valence

positions), electrons and holes diffusion, electronholeion rates [4], bulk structure [5], surface structure [6,7]

[810].ong the most active photocatalytic semiconductornsists of several polymorphs the most common of them

with a band gap of 3.2 eV and rutile with a band gaproom temperature. The lifetime of electrons in anatasehases using time resolved microwave conductivity mea-(TRMC) was studied on powder [11] and single crystalsoth cases charge carriers in anatase were found to haveime than those in rutile. For hydrogen production fromorganic compounds the anatase phase is more active

ding author. Tel.: +966 (12) 2755060.resses: idrissh@sabic.com, h.idriss@ucl.ac.uk (H. Idriss).

than the rutile one [13,14]. To increase the electron life time metaldeposition on the semiconductor surfaces such as Ag, Rh, Au, Pt andPd, are routinely used while to increase the hole life-time organiccompounds such as alcohols and glycols are added into the aqueousmedia.

Mixed phase TiO2 anatase and rutile have however shown, inmany reports, superior activity than the expected arithmetic sumof that of anatase and rutile separately. This observation that fallsunder synergism has been the subject of considerable studies [15].For example the rate of hydrogen production from ethanol over1.5 wt.% Au/P25 (85% anatase and 15% rutile) is about two timeshigher than that seen over 2 wt.% Au/anatase alone and the latteris about 100 times more active than that of 2 wt.% Au/rutile; nomatter is the expression of the rate (per unit area, unit mass ornormalized to the XPS Au4f/Ti2p signal) [16]. Similar observationswere reported over Pt/TiO2 anatase, rutile and their mixtures [17].Others [18] studied the photocatalytic activity of anatase and rutilephase in the visible region. The activity of the mix phase undervisible light was explained as due to the lower band gap (3.0 eV)rutile polymorph where the photoexcitation mainly occurred inrutile phase with electrons being transferred to anatase phase. Nairet al. [19] proposed the interfacial model for the synergistic effectbetween anatase and rutile under UV and visible light. The transferof electrons from rutile to anatase in the presence of visible light and

rg/10.1016/j.cattod.2014.05.0342014 Elsevier B.V. All rights reserved.ism and photocatalytic water splitting tcatalysts: Effect of initial particle size o

, A.K. Wahab, H. Idriss

te Research and Innovation (CRI), KAUST, Thuwal 23955, Saudi Arabia

e i n f o

nuary 2014vised form 5 May 2014ay 2014e 4 July 2014

rutile)

ce band

uction

a b s t r a c t

In order to study the effect of anatasegen (often invoked as synergism) twoin both series is anatase but their pais ca. 1520 nm (nano) and in the otsemiconductors (nano and micro) wtransformation of the anatase phase tently prepared mixed phases Pt wasseries was tested for the photocatalytias a sacricial agent (under identical cproduction was seen in both cases; thphase transformation. However, stronydrogen overiO2

e phases of TiO2 on the photo-catalytic production of hydro-s of Pt/TiO2 materials were prepared. The initial phase of TiO2

size is different. In one case the mean particle size of TiO2t is ca. 100 nm (micro). Before the deposition of Pt, the twoeated to elevated temperatures to obtain partial (and total)

rutile phase (UVvis, XRD, XPS-valence band). On this differ-sited (ca. 1 at.%; corrected XPS Pt4f/Ti2p = ca. 0.05) and eachuction of hydrogen from water in presence of ethanol (5 vol.%)ions). Based on rates per unit mass no synergism for hydrogenin part due to the decrease in the BET surface area during theergism was observed for hydrogen production rates per unit

S. Bashir et al. / Catalysis Today 240 (2015) 242247 243

migration of electrons from anatase to rutile in case of UV light wasreported; the formation of potential charge barrier at the interfacecontributed in the proposed mechanism.

In this study, the effect of anatase to rutile ratios on the photocat-alytic activitransformaent grades other with were anneacentages ofdepends onperature [2of initial paied here. Thanatase andfor the photsacricial aon TiO2, weover-potenwas micro the rate oncparticles wh

2. Experim

Two diffimpregnatiobtained fr(nano) and labeled as T(nano) powperatures inannealed isThese condmixed phaspared by disfrom Sigmasolution waprepared. Twas left at 7at 100 C forlowing the a(nano and m

The powa Philips Xbetween 10step time o(K = 1.5418rutile in the[28] and are

%Rutile = [

X-ray pThermo scieber was typneutralizatiwith respecband energsition condrate = 0.1 eVan EX06 ionsample currof approximfor analysis

0

100

200

300

400

500

600

700

800(a)

24

unit) (d) 37 % Rutile

0

200

400

600

800

000

200

24

2 theta (degrees)

a) XRD pattern of TiO2 (nanoinitially anatase) annealed for 1 h in theC range. (b) XRD pattern of TiO2 (microinitially anatase) annealed at

for different periods of times (in hours) to obtain the indicated rutile %.

ollected over the wavelength range of 250900 nm on ao Fisher Scientic UVvis spectrophotometer equipped withg mantis diffuse reectance accessory. Absorbance (A) andance (%R) of the samples were measured. The reectanceta was used to calculate the band gap of the samples usingc plot (KubelkaMunk function). BET surface areas of cata-ere measured using Quantachrome Autosorb analyzer by N2tion. Catalysts were evaluated for hydrogen production in a

volume Pyrex glass reactor. 25 mg of catalyst sample wasced into the reactor. The catalyst sample was then reducedhydrogen ow at 350 C for 1 h followed by purging withn gas for 30 min. Milli-Q deionized water (20 mL) and theial agent (1 mL i.e. 5% by volume) of ethanol were added intoctor. The nal mixture was subjected to constant stirring ini-nder dark condition for some time to get better dispersionlyst powder and the sacricial agent in the water mixture.ctor was then exposed to the UV light; a 100 Watt ultraviolet

H-144GC-100, Sylvania par 38) with a ux of ca. 2 mW/cm2 atnce of 10 cm with the cut off lter (360 nm and above). Prod-alysis was performed by gas chromatograph (GC) equippedermal conductivity detector (TCD) connected to Porapak Q

column (2 m) at 45 C and N2 was used as a carrier gas.

ults and discussion

was carried out to study the phase transformation ofe to rutile of TiO2 (nano and micro) powders. Fig. 1a and b

the gradual transformation of the anatase phase into rutilety coupled with crystallite (particle) size on the phasetion of anatase to rutile were investigated. Two differ-of TiO2 one with large crystallite size (micro) and thesmall crystallite size (nano) were used. TiO2 samplesled at different temperatures to get the varying per-

anatase and rutile. Transformation of anatase to rutile many factors including impurity content [20,21], tem-2,23], pressure [24] and particle size [25,26]. The effectrticle size and the annealing temperature were stud-e annealed TiO2 samples containing different ratio of

rutile were then impregnated with Pt and evaluatedocatalytic H2 production from water using ethanol as agent. We have opted to use Pt because it is well studiedll dispersed, easily reduced and has among the lowesttial [27]. We nd no synergism when the initial phasesize anatase particles but considerable synergism (one normalized to unit area) when the initial TiO2 anataseere of nano-size.

ental

erent series of 1 wt.% Pt/TiO2 were prepared by weton. TiO2 powder sample containing pure anatase phaseom Sigma Aldrich of about 20 nm size labeled as TiO2the one obtained from Fisher Scientic of about 0.1 miO2 (micro) were used as the starting materials. TiO2der was annealed isochronally (for 1 h) at different tem-

the 700800 C range while TiO2 (micro) powder wasothermally at 1000 C for different intervals of time.itions were found to be the optimum ones to obtain thee from each sample. Metal precursor solution was pre-solving calculated amount of metal salt (PtCl2 obtained

Aldrich) in 1 N HCl. The calculated amount of precursors then impregnated to each of the annealed sampleshe impregnated mixture was subjected to stirring and080 C overnight. The resulting slurry was then dried

24 h, followed by calcination at 350 C for 5 h in air. Fol-bove mentioned procedure, two series of 1 wt.% Pt/TiO2icro) were obtained.der XRD patterns of the samples were recorded onpert-MPD X-ray powder diffractometer. A 2 interval

and 90 was used with a step size of 0.010 and af 0.5 s. The X-ray, Ni-ltered Cu K radiation sourceA), operated at 45 mA and 40 kV. The percentages of

mixed phase were calculated by the expression below presented in Table 1.

1

(A/R) 0.884 + 1] 100

hotoelectron spectroscopy was conducted using antic ESCALAB 250 Xi, The base pressure of the cham-ically in the low 1010 to high 1011 mbar range. Chargeon was used for all samples. Spectra were calibratedt to C1s at 285.0 eV. Pt4f, O1s, Ti2p, C1s and valencey regions were scanned for all materials. Typical acqui-itions were as follows: pass energy = 30 eV and scan

per 200 ms. Ar ion bombardment was performed with gun at 1 kV beam energy and 10 mA emission current;ent was typically 0.91.0 nA. Self-supported oxide disksately 0.5 cm diameter were loaded into the chamber

. UVvis absorbance spectra of the powdered catalysts

(b)

Cou

nts/

s (a

rbitr

ary

1

1

Cou

nts/

s (a

rbitr

ary

unit)

Fig. 1. (7008001000 C

were cThermprayinreect(%R) dathe Taulysts wadsorp100 mLintroduunder nitrogesacricthe reatially uof cataThe realamp (a distauct anwith thpacked

3. Res

XRDanatasshows28.52827.52726.52625.525.5

2 theta (degrees)

(c) 29 % Rutile

(b) 10 % Rutile

(a) 3 % Rutile

28.52827.52726.52625.525.5

(f) 78% Rutile

(e) 68% Rutile

(d) 25% Rutile

(c) 7.6% Rutile

(b) 1.2% Rutile

(a) 0.5% Rutile

244 S. Bashir et al. / Catalysis Today 240 (2015) 242247

Table 1Rutile content, BET surface area, and particles size of TiO2 (initially of nano (ca. 20 nm) and micro (ca. 100 nm) sizes).

Sample Temperature (C) % Rutile BET(m2/g) d (Anatase) (nm) d (Rutile) (nm)

TiO2 nano

0 55 22 032 838 3840 7645 8748 97

TiO2 micro

93 97 108

103 11495 101

100 118110 122

when anatadifferent tem1 h) in case 7 h at 1000imental sec(with theseations in th

The anatof anatase ppeak at 2 =(1 1 0) wereshifts of anaing annealinvalues of anet al. [25] resizes of 12 n

Comprestetragonal aface stress b0.15 A in thcles at 800

direction wlattice consincrease in32 nm) to 7TiO2(microexcept withare shifting

It has betransforms the large patemperaturin the smallarger surfathe TiO2 (na10 m2/gCataof anatase tproceeds thin activatioat 8.2 nm antor was invsize in the the initial pperature offrom the chversion) warutile also dalso varies 299, 236, aand 12 nm pannealing t

KubelkaMunk function, F(R) = (1R)2/(2R), was used to cal-the band gap of the materials. The Tauc plot of the quantity

E)1/2 against the radiation energy was used for measure-Fig. 2a and b shows the band gaps of 1 wt.% Pt/TiO2 (nano)catalysts, the pure anatase has the band gap of 3.2 eV ande increase in rutile content band gap decreases up to 3.0 eVe maximum rutile content of 37%. Fig. 4 shows the band gap.% Pt/TiO2 (micro) series. The same trend was observed inse as well. With the increase in rutile content the band gap

to decrease and at higher rutile content (up to 78%), slightlythan 3 eV.

3a presents the XPS spectra of Pt4f of 1 wt.% Pt loaded onmally annealed TiO2 (micro) powder samples. The chem-mpositions of Pt on the surface, the ratio of Pt4f/Ti2p and2p were calculated using the corrected area under the XPS

of Pt4f7/2 and Pt4f5/2 and are presented in Table 2. Plat-as mostly present in the oxidized form in all samples (Pt2+).ak position at 72.672.8 eV corresponds to Pt4f7/2 of Pt2+

hat at 75.976.1 eV were assigned to Pt4f5/2 of Pt2+. The peak

0.0E+

5.0E-

1.0E-

1.5E-

2.0E-

2.5E-

3.0E-

1.5E-

2.0E-720 3 30 740 10 20 760 29 16 780 37 13.5 800 56 11

1000

0.5 9.8 1.2 5.4 7.6 5.2

25 6 68 4.5 78 4.5

se powder was subjected to annealing isochronally atperatures between 720 C and 780 C (annealing time:

of TiO2 (nano) and isothermally for different time (1 toC) in case of TiO2 (micro). As indicated in the exper-tion these conditions were found to be the optimum

samples) for making the mixed phases, with large vari-e ratio anatase to rutile.ase to rutile ratio was calculated by taking the intensityhase (1 0 1) peak at 2 = 25.30 and rutile phase (1 1 0)

27.40. The peak positions of anatase (1 0 1) and rutile well in agreement with others [29] except for the peaktase (1 0 1) and rutile (1 1 0) in TiO2 (nano) with increas-g temperature. Peak shifts of 0.3 were observed in 2atase (1 0 1) and rutile (1 1 0) from 720 C to 780 C. Liported similar observations with initial anatase particlem and 23 nm in the temperature range of 700800 C.sion in the lattice strain along the c direction of thenatase caused larger angular peak shifts due to the sur-uild up at higher temperature. Reduction of 0.06 A ande lattice constant c for 23 nm and 12 nm anatase parti-C, with negligible changes in the lattice constant along aas reported. In the present study, the reduction in bothtants a (0.047 A) and c (0.131 A) were observed with

annealing temperature from 720 C (crystallite size:80 C (crystallite size: 45 nm) for TiO2 (nano) powder.) samples have not shown major peak position shifts

the rutile content of 7.6% and 25%, in which the peaks at lower 2 angles.en reported that the small particle size TiO2 anataseinto rutile at relatively low temperature compared torticle size [30]. The increase in transformation at lowere was reported due to the increase in nucleation sitesl crystalline size of anatase [26] and consequently toce areas [25]. In our case the initial BET surface area ofno; 55 m2/gCatal.) is much higher than the TiO2 (micro;

l.). Zhang et al. found the phase transformation kineticso rutile to be size dependent [31]. The transformationrough particle contacts and there was a slight variationn energy with an initial particle size (around 218 kJ/mold 198 kJ/mol at 21.4 nm) and the pre-exponential fac-

Theculate (F(R) ment. photo with thwith thof 1 wtthis castartedlower

Fig.isotherical coO1s/Ticurvesinum wThe pewhile t

(a)

(b)

Tauc

uni

ts u

nitsersely proportional to the fourth power of the particletemperature range of 520600 C. In this study, witharticle of TiO2 nano of about 25 nm and annealing tem-

720 to 780 C for 1 h, the activation energy (calculatedanges in the rutile to anatase ratios up to ca. 30% con-s found to be 490 kJ/mol. Transformation of anatase toepends on preparation method so the activation energywith the synthesis conditions. Activation energies ofnd 180 kJ/mol for initial anatase particle size of 23, 17repared by metal organic chemical vapor deposition at

emperatures of 700 to 800 C have been reported [25].

0.0E+

5.0E-

1.0E-

Tauc

Fig. 2. (a) Plowith differentphotocatalysts00

10

09

09

09

09

09

3.53.43.33.23.13.02.92.82.72.62.5eV

0% Rutile3 % Rutile10 % Rutile29% Rutile37 % Rutile

09

090.5% Rutile1.2% Rutile7.6% Rutile25% Rutile68% Rutile78% Rutile00

10

09

3.53.43.33.23.13.02.92.82.72.62.5eV

ts of Tauc units versus (eV) for 1 wt.% Pt/TiO2 (nano) photocatalysts % or rutile. (b) Plots of Tauc units versus (eV) for 1 wt.% Pt/TiO2 (micro).

S. Bashir et al. / Catalysis Today 240 (2015) 242247 245

(a)

(b)

Fig. 3. (a) XPSspectra of Pt4f

position at at 74.8 is atdifference iexpected vapronounced

Fig. 3b dTiO2(nano)was carriedples. The oxmainly Pt2+

spond to Ptcorrespondfor 5 min, tchange to m

Table 2XPS Pt 4f, Ti2p and O1s for the Pt/TiO2 anatase (micro) series with different rutile %.

le Atomic (%) Pt4f/Ti2p O1s/Ti2p% Ruti

0.5

1.2

7.6

25 spectra of Pt4f of 1 wt.% Pt/TiO2 (micro) at different rutile %. (b) XPS of 1 wt.% Pt/TiO2 (nano90% anatase) fresh and 5 min Ar+ sputtered.

71.5 is related to Pt4f7/2 of metallic Pt peak while peaktributed to Pt4f5/2 of Pt0. The peak positions and theirn the binding energy (E = 3.3) are well within thelues [32]. The metallic % of Pt (in general) was more

in the sample containing mainly anatase.isplays the XP spectra of Pt4f of 1 wt.% Pt loaded on

annealed at 740 C for 1 h (90% anatase). XPS analysis out for the as-prepared and the Ar ions sputtered sam-idation states of Pt in the freshly prepared samples wereand Pt4+. The peak positions at 72.7 and 76.0 eV corre-4f7/2 and Pt4f5/2 of Pt2+ while those at 75.0 and 78.2

to Pt4f7/2 and Pt4f5/2 of Pt4+. After Ar ion sputteringhe sample was reduced and the oxidation states of Ptetallic platinum; peaks positions shifted to the lower

68

78

binding enethe differen

Figs. 4aover the tw(micro) serrespect to least amou1 wt.% Pt/Tsponding h5.3 108 mgen with 1(anatase, mon the rutilwas seen oof the nanwith increadecreased; series the raat 25%) witthat synergcompared t

It is unclof Pt/TiO2(nprepared frson might materials. Mmost mustparticle sizhas been stvalence bansingle crystTiO2 (1 0 1) ever it is clehybridizatioent shapes.peaks with case of anatbinding enesame contrhas addresshybrid funcPt4f 1.1 0.05 2.2Ti2p 24.2O1s 53.3C1s 21.3Pt4f 1.6 0.07 2.2Ti2p 23.5O1s 52.4C1s 22.3Pt4f 1.1 0.05 2.3Ti2p 23.4O1s 52.8C1s 22.5Pt4f 1.2 0.05 2.3Ti2p 22.8O1s 52.4C1s 23.5Pt4f 0.9 0.04 2.3Ti2p 24.1O1s 56.1C1s 18.9Pt4f 1.0 0.04 2.3Ti2p 23.7O1s 55.1C1s 20.2

rgy at 71.7 and 75.0 for Pt4f7/2 and Pt4f5/2 of Pt0 withce in the splitting binding energies of 3.3 eV.

and b and Fig. 5 show the production of hydrogeno series, 1 wt.% Pt/TiO2 (nano) and 1 wt.% Pt/TiO2

ies. Hydrogen production rates were normalized withBET surface area of each catalyst. In both cases, thent of hydrogen was observed with pure rutile phase ofiO2 (nano) and 1 wt.% Pt/TiO2 (micro) with the corre-ydrogen production rate of 2.6 108 mol/m2 min andol/m2 min, respectively. The rate of reaction for hydro-

wt.% Pt/TiO2 (anatase, nano) as well as 1 wt.% Pt/TiO2icro) was two orders of magnitudes higher than thate phase (12 106 mol/m2 min). Similar observationn Au/TiO2 anatase and rutile series (14). In the caseo series however, the rate (per unit area) increasedsing the content of rutile up to 1030%, afterward itwith further increase in rutile %. In the case of the microte had a monotonic decrease (albeit with an osculationh increasing the rutile phase %. So it can be concludedism on this micro series is not noticeable at least when

o the initial starting materials (pure anatase phase).ear why in the case of catalysts prepared from one seriesano), synergistic effect is seen whereas with catalystsom TiO2(micro), no synergism is seen. One possible rea-be the effect of crystallite size and crystallinity of theany reasons can be invoked for this observation but

be related to geometric and electronic effects of thee. The valence band region of TiO2 anatase and rutileudied by many authors in some details. While manyd analysis were conducted on the rutile TiO2 (1 1 0)al [33], far less was done on its counterpart anatasebecause of lack of available single crystals [34,35]. How-ar because of the different oxygen and titanium orbitalsns the valence band (and conduction band) has differ-

XPS valence band reveals the presence of two main O2pbinding energy centered at about 4.5 and 7.5 eV. In thease the high binding energy peak is larger than the lowrgy peak while in the case of rutile they are of about theibution (or the other way around). A recent work [36]ed the projected density of states using the screenedtional (HSE06); this approach produces more accurate

246 S. Bashir et al. / Catalysis Today 240 (2015) 242247

(a)

(b)

Fig. 4. (a) H2 production from 1 wt.% PtTiO2 (nanoinitially anatase) with increas-ing rutile content. (b) H2 production for 1 wt.% Pt/TiO2(microinitially anatase) withincreasing rutile content.

0.0E+00

5.0E-07

1.0E-06

1.5E-06

2.0E-06

2.5E-06

3.0E-06

0 20 40 60 80 100

mol/m

in.m

2 cat

%Rul e

TiO2 nano

TiO2 micro

Fig. 5. Rates of H2 production over 1 wt.% Pt/TiO2 (nano and micro size) series.

Fig. 6. XPS of Ti2p3/2 of 1 wt.% Pt/TiO2 rutile, 1 wt.% Pt/TiO2 anatase and 1 wt.%Pt/TiO2 (anatase/rutile).

structural and band gap information than standard density func-tional approaches (such as LDA and GGA). The authors indicate ashift in the VBM of the anatase compared to rutile based on theircomputation and experimental (XPS) analyses. Based on this obser-vation we hof the mategive a full ato see if invalence line

Fig. 6 prhybrid mateTi2p of ruticompared tthat no conFWHM of this larger thashift is in lFigs. 7 and 8TiO2 and abinding enesame initialspectra to thin the spectabove the Othat of the sTi3d states VB with itsing, as expeabout 1 eV

Fig. 7. X-ray vand 1 wt.% Pt/ave analyzed our core and valence band levels of somerials studied in this work. The objective here is not toccount of the electronic structure of the materials butdeed there are noticeable movements in the core ands of the composed material.esents the XPS Ti2p of pure rutile, pure anatase and therial. The spectra were aligned to the C1s at 285.0 eV. Thele is found to be about 0.2 eV lower in binding energyo that of anatase. Their narrow FWHM gives condencetribution of reduced states is present. Interestingly thee Ti2p3/2 of the material composed of anatase and rutilen that of the rutile alone or anatase alone. The positionine with what is reported in Fig. 3 of reference [36].

present the VB region of pure rutile TiO2, pure anatasenatase/rutile TiO2. The spectra are aligned to the O2srgy and the base line is shifted so all spectra have the

offset for better comparison. We have opted to align thee O2s to prevent the effect of any possible non-linearityrometer because the C1s region is relatively far (260 eV2p region). We have also compared the fresh material toputtered one to see into any effect due to the presence ofassociated with oxygen defects. Fig. 7 presents the rutile

nger print of the O2p shape evident. Ar ions sputter-cted, results in the appearance of lines extending frombelow the Fermi level (shaded area). TiO2 composed ofalence band of 1 wt.% Pt/TiO2 rutile before and after Ar ions sputteringTiO2 (anatase/rutile).

S. Bashir et al. / Catalysis Today 240 (2015) 242247 247

Fig. 8. X-ray(anatase/rutile

anatase/rutthat of anapure anatasanatase is aposite mateone can sim(anatase/ruthat of rutil

There is ergism not is related torial. Becauselectronhothe larger cbetter photparticles thcenter and re-combinaalytic reacti100 nm sizeTiO2 of abofrom the bution for TiO100 nm sizethe 20 nm stransform parea, but deincrease in ptherefore lethe rutile ption. In othea better cryinterface an

4. Conclus

The phoethanol wa

size). The normalized rate of hydrogen production per unit areawas seen to increase when both phases (rutile and anatase up toa ratio of about 1/3) were present in the case of TiO2 nano but didnot in the case of TiO2 micro. In other words synergism was seenbetween the two phases for small particle sizes of TiO2 but not forlarge particle sizes. Two effects were discussed as a cause of thisbehavior: electronic and structural. XPS-valence band structure ofthe Pt/TiO2 (anatase and rutile) appears to simply track those ofthe pure phases and may not shed light on the electronic changesassociated with the valence band maxima of the materials. On theother had some changes occur in the anatase crystallite strain inthe case of TiO2 nano in the range where synergism occurs.

nces

teinfecConujishiNi, M.(2007Gioco

Wilso Wilsoundurigueinseb

QuaholbeaXu, Y.s. Rev. JungMurdoorca, onnel2.Scott

Sprip://dxhang, . Hu545. Nair119. Shankada

am. Soetch

hangang,

tter 13Li, C. N. GribossmeFu, L.A653oseph. KimD.-H. 313hangMouldctron valence band of 1 wt.% Pt/TiO2 anatase and 1 wt.% Pt/TiO2) before and after Ar ions sputtering.

ile is also displayed with the O2p shape dominated bytase. Fig. 8 presents similar spectra but starting frome. Inspection of both gures indicate that the VBM oft a lower energy than that of rutile and that the com-rials is somewhere in between. Based on these guresply indicate that the VBM of the composite material

tile) falls somewhere in between that of anatase ande.however another plausible explanation related to syn-based (at least directly) on its electronic structure. This

the degree of crystallinity or perfection of the mate-e the more crystalline is the material the less likelyle recombination will occur. It has been reported thatrystallite size usually have smaller defects leading toocatalytic activity while in case of smaller crystallinee defects are more prominent. The defects near thethe boundaries in the materials promote electron holetions which is the key limitation in case of photocat-ons [37]. Fig. 5 indicate that pure anatase TiO2 of about

is performing per unit area better than pure anataseut 20 nm size. This means that charge carrier diffusionlk to the surface is not a major factor in photoreac-

2 particles in this size range. It may also mean that the anatase TiO2 has fewer defects, per unit volume, thanize TiO2. Upon heating the small anatase particles toart of the phase to rutile the rate increases per unitcreases overall because the increase in particle size. Thearticle size results in a better degree of crystallinity andss electronhole recombination centers. In this processhase is formed but may little contribute into the reac-r words the increase in the rate may be also be due tostallinity of TiO anatase and not directly related to the

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[1] A. S[2] R. M[3] A. F[4] M.

11 [5] J.L. [6] J.N.[7] J.N.[8] S. K

Rod[9] A. L

[10] E.L.[11] C. C[12] M.

Phy[13] H.S[14] M.

J. Ll[15] K. C

11[16] M.

etyhtt

[17] J. Z[18] D.C

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Cer[22] N. W[23] H. Z[24] Z. W

Ma[25] W. [26] A.A[27] J. R[28] X.

647[29] K. J[30] C.-S

Yi, 134

[31] H. Z[32] J.F.

ele2atase/rutile.

ions

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Synergism and photocatalytic water splitting to hydrogen over M/TiO2 catalysts: Effect of initial particle size of TiO21 Introduction2 Experimental3 Results and discussion4 ConclusionsReferences