Alsalik, Yahya M.

Idriss, Hicham

Katsiev, Khabiboulakh

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provided by the addition of molecular oxygen in the gas phaseduring photo-oxidation36,37).A similar set of experiments was conducted on Au/TiO2(110) surfaces. A clean and oxidized surface was prepared foreach experiment with the methodology described in theExperimental Section. Figure 2 presents the XPS Au 4fcollected spectra of the “as prepared” surfaces and thoseobtained after the photocatalytic reaction. A nonlinear butgradual shift of the XPS Au 4f core levels from higher to lowerbinding energy was observed with increasing coverage. Thebinding energy of XPS Au 4f7/2 decreased from 84.8 eV for theinitial coverage of 0.11 at. % to 84.3 eV for the final coverage of0.50 at. %. This has been seen in many other works before.38The shift is attributed to a particle size effect,39 where, formetal clusters with a subnanometer size, the process ofphotoemission leaves an unscreened hole. After the photo-reaction of ethanol, the binding energies shift of XPS Au 4fcore level peaks was still noticed although with a slightdecrease of their fwhm, most likely due to a marginal sinteringas observed before by STM in similar reaction conditions.23The presence of Au clusters on top of TiO2(110) hasresulted in one main difference in the probed electronic stateof Ti 2p after reaction. A gradual disappearance of the reducedstates (Ti3+ cations) as a function of Au coverage occurred. Byabout 0.50 at. % of Au, almost all reduced states havedisappeared. The main spectra are presented in Figure 3 a andb together with Table 1. A gradual decrease of the, afterreaction Ti3+ cations, is noticed with increasing Au coverage. Itis important to mention that each experiment was conductedseparately where the Au atoms were removed from the surfaceof TiO2(110) by successive Ar+-sputtering and annealing.Once an oxidized clean surface is obtained Au cations weredosed again at a given coverage (Figure 2). The correspondingXPS C 1s spectra are presented in Figure S3. All spectracontain the signature of ethoxide species at about 285 and286.5 eV, as observed in Figure 1C. There is a minor signaldeveloping at 289−289 eV which is that of − COO- groupwith increasing gold coverage. The presence of an oxidizedroute may appear contradictory to the role of Au in acquiringthe electronic charges from Ti3+, yet it is explained consideringthe complete photoreaction steps on Au/TiO2. The ejection ofa CH3 radical during the reaction leads to a slight buildup offormates, as has been seen previously.40,41,38Although Figure S4 shows the presence of d-electrons due tothe creation of these reduced states, at about 1 eV below EF,the very weak signal-to-noise prevented quantitative analysis.The fraction of Ti surface atoms in the Ti3+ oxidation stateafter the photocatalytic reaction as measured using the Ti 2plines and as a function of Au coverage is shown in Figure 4a,b.Figure 4a shows the as computed peak areas of Ti3+ (theindividual spectra with the difference being highlighted areshown in Figure S2). A monotonic decrease of the % of Ti3+cations with increasing Au at. % is seen. At about 0.5 at. % ofAu, the Ti3+ peak area is virtually zero. Figure 4b presents theFigure 3. XPS Ti 2p of the fresh as prepared Au/TiO2 (110) rutile surface and the same after photocatalytic reaction with ethanol/water at roomtemperature. Ethanol/water (m/z 18/m/z 31 = 0.35) at a total pressure inside the chamber = 5 × 10−7 Torr under UV−vis excitation (320−630nm) and a flux of 690 mW cm−2.Table 1. At. % and Number of Au Atoms on a TiO2(110)Rutile Single Crystal, Together with the Fraction of Ti3+Cations Formed and Reacted upon the PhotocatalyticReaction of EthanolaAuat. %No. of goldatoms/cm2fractionof Ti3+fraction of Ti3+that has reactedNo. of atoms of Ti3+that has reacted/cm20 0 0.1 0 00.11 1.1 × 1012 0.089 0.011 5.5 × 10110.17 1.7 × 1012 0.094 0.006 3.0 × 10110.22 2.2 × 1012 0.09 0.01 5.0 × 10110.40 4.0 × 1012 0.084 0.016 8.0 × 10110.43 4.3 × 1012 0.066 0.034 1.7 × 10110.49 4.9 × 1012 0.042 0.058 2.9 × 1011aThe raw data are presented in Figures 2, 3, and S2.same data plotted taking into consideration the total number ofatoms of Au in the different experiments and the number ofTi3+ cations that have reacted. In other words, it is arelationship between the number of Au atoms that have“possibly” trapped electrons during the photoreaction and thenumber of Ti3+ that have disappeared. The shape of thedependence has some logarithmic part (although it is bestfitted with an inverse cubic function) where at high enoughnumbers saturation has occurred. As can be seen, theasymptote is reached at about 1 Au atom per electronconsumed and there is little incentive to further increase theiramount, this is equivalent to about 0.50 Au at. %. This is in linewith almost all studies in photoreaction of alcohols using noblemetals on top of TiO2 powder and probably most other oxidesemiconductors; where the optimal catalyst compositions isoften composed of less than 2 wt % of the deposited noblemetal42−50 (1.5 wt % of Au on TiO2 is about 0.6 at. % Au).Form the derivative of the fitting function of the data, themaximum number of Au atoms needed to trap one electron atvery low coverage can be obtained and it is found to be about3. That is probably best explained statistically, at very lowcoverage electrons have less chance to travel far enough toreach gold atoms and are therefore tapped in their path(wavefucntion) and stay there for a period of time.Returning back to the initial question, do these Ti3+ cationsdisappear because one electron per Ti cation is transferred toAu clusters which in turn reduce H+ cations to atomichydrogen, or do H+ cations are reduced to atomic hydrogen onthe semiconductor surface and the presence of Au atoms is fortheir catalytic recombination to molecular hydrogen only?Probably putting the facts first would help addressing thequestion.1. Reduced Ti3+ cations are formed within TiO2 uponphotocatalytic reactions of a hole trapping, reactant suchas ethanol in this work.2. A metal is needed to make the reaction, in the absenceof a metal the rate is 1−2 orders of magnitude slowerand this seems to be independent from the nature of thesemiconductor in powder form, TiO2, SrTiO3, ZnO,CdS, and g-C3N4 to name a few.3. The presence of Au (as in this work) suppresses theformation of Ti3+ cations.4. A small surface coverage of a metal is needed (typically afew %) increasing the coverage does not increase thereaction rate (in some cases a mild decrease occurs).5. Hydrogen ions reduction is orders of magnitude slowerthan the electron transfer from rate from TiO2 to Au (orfrom a semiconductor to a noble metal in general).If the presence of Au clusters is only linked to the hydrogenatom recombination reaction one would not have expectedreaching such a fast asymptote as shown in Figure 4b. In aprevious work, we have observed on Au/TiO2 powder, agradual quenching of the PL that started to saturate at a muchhigher loading then the one observed here (about 10×higher).51 Others have also observed an almost total quenchingpf the PL signal at 0.15 ML Au/TiO2 powder.52 If one mayconsider the quenching of TiO2 photoluminescence (PL) as anindication of electron transfer from the semiconductor (TiO2)to the noble metal (Au) then the results reported in this workare dissimilar to PL results that were conducted in noncatalyticconditions. The fast-reaching saturation (in terms of metal%)both for removing Ti3+ cations and for making hydrogenmolecules seems to be linked to the “limited potential” of theseclusters to acquire excited electrons from the semiconductorand the “limited potential” of TiO2 to make Ti3+ cations. Sincethe hydrogen ion reduction is orders of magnitude slower thanthe electron transfer to Au clusters, the rate is thus limited bythe former step. In other words, there is no need for moremetal atoms to be deposited on the surface. This, and the time-discrepancy between both (the fast (and limited) electrontransfer to gold atoms and the slow electron transfer to H+)appear to be the two most important reasons for affecting theoverall reaction rate.Figure 4. Decrease in the Ti3+ peak area as a function of Au atomic% on TiO2(110) and (b) the number of Au atoms on rutile TiO2(110) as afunction of number of Ti3+ cations that has disappeared (with respect to the pristine surface) after the photocatalytic reaction with ethanol/water atroom temperature. Ethanol/water (m/z 18/m/z 31 = 0.35) at a total pressure inside the chamber = 5 × 10−7 Torr under UV−vis excitation (320−630 nm) and a total light flux of 690 mW cm−2.(22) Eder, M. M. J. Photochemistry and Photocatalysis of Alcoholson Bare and Metal Cluster-Loaded TiO2(110). 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Electron Transfer From a Semiconductor to a Metal and Its Implication on Photocatalysis for Hydrogen Production

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