Ultrafast Photosynthetic Reduction of Elemental Sulfur by Au

J. Phys. Chem. B 2006, 110, 10771-10778

10771

Ultrafast Photosynthetic Reduction of Elemental Sulfur by Au Nanoparticle-Loaded TiO2 Tomokazu Kiyonaga, Tomohiro Mitsui, Motofumi Torikoshi, Manabu Takekawa, Tetsuro Soejima, and Hiroaki Tada* Department of Applied Chemistry, Faculty of Science and Engineering, Kinki UniVersity, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ReceiVed: March 13, 2006; In Final Form: April 17, 2006

Nanometer-sized gold particles with varying mean size from 3.2 to 12.2 nm were loaded on the surfaces of TiO2 particles in a highly dispersed state with the loading amount maintained constant (0.46 ( 0.02 mass %) using the deposition-precipitation method. Light irradiation (λex > 300 nm) to a deaerated ethanol TiO2 particle suspension containing elemental sulfur (S8) led to the energetically uphill reduction of S8 to H2S. It has been found that this reaction is dramatically enhanced with such a low level of Au loading on TiO2 and that the zero-order rate constant of reaction increases with decreasing mean size of Au nanoparticles (d). The effects of reaction parameters (substrate concentration, light intensity, temperature) on the rate of reaction were studied to infer the essential reaction mechanism. Further, a kinetic analysis has led to a conclusion that the increase in the rate of reaction with decreasing d results from the improvement of the charge separation efficiency.

Introduction Nanometer-sized Au particles have attracted much interest as a new catalyst since the discovery of their extraordinarily high activities for many important chemical reactions.1 The most important and interesting fact in this case is that the catalytic activity is highly sensitive to the Au particle size (d). For example, Au particles exhibit a maximum catalytic activity for low-temperature CO oxidation at d ≈ 3 nm, and the activity is completely lost at d > 10 nm.2 Another point is the selection of the metal oxide supports of Au nanoparticles appropriate for target reactions, e.g., the activity for CO combustion is on the order of TiO2 . Co3O4 ≈ Fe2O3 > SiO2 > Al2O3.3 Also, epoxidation of propylene strongly depends on the crystal structures of the TiO2 support, i.e., anatase is active, whereas rutile and amorphous are inactive.4 Thus, the thermal stability of Au nanoparticles on the TiO2 surface is of great importance; however, the information is deficient. In view of energy storage in a chemical form, this becomes more serious because the rapid progress of thermal endoergonic reactions requires high temperature to pass over the activation energy necessarily greater than the change of Gibbs free energy (∆G). On the other hand, this is not generally valid for photochemical reactions including heterogeneous photocatalytic reactions that occur via electronically excited states. In this respect, TiO2-photoinduced reactions are very interesting because fast uphill reactions can be achieved at low temperatures. The processes are classified into spontaneous “photocatalytic reactions” (∆G < 0) and nonspontaneous “photosynthetic reactions” (∆G > 0).5 Remarkable enhancement effects with loading of noble metals having large work function such as Pt (5.6 eV) and Ag (4.6 eV) have been widely observed in TiO2 photocatalytic reactions due to the increase in the efficiency of charge separation between photogenerated electronhole pairs.6 Regardless of the large work function of Au (5.4 eV), the study on the Au nanoparticle-loaded TiO2 (Au/TiO2) * To whom correspondence should be addressed. Tel: +81-6-6721-2332. Fax: +81-6-6727-4301. E-mail: [email protected].

photocatalytic reaction is only limited,7-11 and further, the Au particle size effect on the rate of reaction remains unknown. This paper reports an ultrafast Au/TiO2 photosynthetic reduction of elemental sulfur (S8) to H2S in ethanol solutions. The effects of several reaction parameters on the rate of reaction were studied in detail to infer the essential reaction mechanism. Particular emphasis was placed on the size control of Au nanoparticles and the size dependence of the rate of reaction. Experimental Section Materials and Characterization. As a sulfur source, S8 with a rhombic form (Kanto Chemicals, >99.5%) was used as received. Nanometer-sized Au particles were loaded on TiO2 particles with a crystal form of anatase and a specific surface area of 8.1 m2 g-1 (A-100, Ishihara Sangyo) by the depositionprecipitation method using HAuCl4 as a raw material.12 The pH of a 4.86 × 10-3 M aqueous solution (100 mL) of HAuCl4‚ 4H2O (Kanto Chemicals, >99.0%) was adjusted to 6.0 with 1 mol dm-3 NaOH. The solution turned from yellow to lighter yellow, accompanied by ligand exchange from [AuCl4]- to [Au(OH)4-xClx]-. To this solution, 10 g of the TiO2 particles were added to be magnetically stirred at 343 K for 1 h. The particles were washed with distilled water three times and then heated in air. The mean diameter of the Au nanoparticles (d) was controlled by heating temperature (673-873 K) and time (1-20 h). To preclude the influence of the crystallinity change on the photocatalytic activity of TiO2, all the TiO2 particles were used as a support of Au particles after heating 923 K for 4 h. The specific surface areas of the TiO2 particles and the value of d were determined by the Brunauer-Emmett-Teller method and transmission electron microscopy at an applied voltage of 300 kV (JEM-3010, JEOL), respectively. The amount of Au nanoprticles loaded (x) was quantified by inductively coupled plasma spectroscopy (ICPS-7500, Shimadzu). The surface areas of the metal nanoparticles were calculated from the density of bulk Au (19.3 g cm-3) and the values of x and d by assuming that every particle is hemisphere.

10.1021/jp061528e CCC: $33.50 © 2006 American Chemical Society Published on Web 05/13/2006

10772 J. Phys. Chem. B, Vol. 110, No. 22, 2006 Photocatalytic Reduction of Sulfur. Prior to the photocatalytic reaction of S8, its adsorption on TiO2 or Au/TiO2 was studied. Au/TiO2 particles (5.0 g) were added to the ethanol solutions with varying S8 concentration (0.5 L). After stirring for 24 h at 298 K, the particles were then separated from the solution by centrifugation. The concentrations of S8 in the supernatant were determined from the absorbance at 263.5 nm. The adsorption amount of S8 was calculated from the difference in the concentrations before and after adsorption. After TiO2 or Au/TiO2 particles had been dispersed in ethanol solutions of S8 (2.2 × 10-5 - 1.72 × 10-4 mol dm-3) and purged with argon for 30 min, irradiation (λex > 300 nm) was started using a high-pressure mercury lamp (H-400P, Toshiba) in a double jacket-type reaction cell (31 mm in diameter and 175 mm in length, transparent to light with λ > 300 nm). Since S8 hardly absorbs light of λ > 300 nm, Au/TiO2 is almost selectively excited under the conditions. The reaction amount of S8 was determined by the decrease in the absorbance at 263.5 nm in the same way as the quantification of its adsorption amount. After the gas generated during the reaction has been trapped in 0.1 mol dm-3 NaOH aqueous solutions, the electronic absorption spectra were recorded on an ultraviolet-visible spectrophotometer (U-4000, Hitachi) to identify and quantify it. Also, the quantitative analysis of the organic products was carried out by gas chromatography (Shimadzu GC-2014; fid column SHINCARBON A (3.2 mm φ × 3.1 m). N2 gas was flowed as a carrier gas at a pressure of 50 kPa, and the injection and column temperatures were 343 and 363 K, respectively. The light intensity integrated from 320 to 400 nm (I320-400) was measured by the use of a digital radiometer (DRC-100X, Spectroline) to be varied from 0.5 to 8.0 mW cm-2. The temperature of the suspension was kept constant (273-323 K) by circulating thermostated water through an outer jacket around the cell. After anatase-type TiO2 films with a film thickness of 65 ( 5 nm had been formed on SnO2-film-coated glass substrates (TiO2/SnO2) by a sol-gel method,13 Au nanoparticles were loaded on the surface of the TiO2 film by the depositionprecipitation method (Au/TiO2/SnO2). A photoelectrochemical cell (PEC) was designed with an Au/TiO2/SnO2 photoelectrode, a Pt counter electrode, and a Ag/AgCl reference electrode (in satd KCl). After a constant electrode potential had been reached by bubbling argon for 30 min in the dark, irradiation (λex > 300 nm, I320-400 ) 10 mW cm-2) was started using a 500 W Xe lamp as light source (Wacom HX-500). The electrode potential response with irradiation was followed for the PEC connected with a potentio/galvanostat (HZ-5000, Hokuto Denko) in deaerated 0.1 mol dm-3 Na2SO4 solutions as a function of the C2H5OH concentration. Molecular Orbital Calculations. The electronic absorption spectrum of S8 was calculated by the INDO/S method using a ZINDO program within a CAChe work system (CAChe Scientific).14 Results and Discussion Characterization of Au/TiO2. Au particles were loaded on TiO2 by the deposition-precipitation method, in which a precursor complex, [Au(OH)3Cl]-, was adsorbed from the aqueous solution on the TiO2 surfaces to be heated at 673873 K for 1-20 h. Figure 1 shows TEM images (left) and size distribution (right) of Au particles for the Au/TiO2 samples prepared under various heating conditions. Figure 2A shows the change in the mean diameter of Au particles (d) as functions of heating temperature (Tc) and time (tc). The d value increases with increases in Tc and tc from 3.2 nm [(Tc, tc) ) (673 K, 1 h)]

Kiyonaga et al.

Figure 1. TEM images of Au/TiO2 particles prepared under various heating conditions (left) and their size distribution of the Au particles (right): Tc, tc ) (A) 673 K, 1 h; (B) 773 K, 4 h; (C) 873 K, 4 h; (D) 873 K, 20 h.

to 12.2 nm [(Tc, tc) ) (873 K, 20 h)], while the amount of Au loaded is invariant (x ) 0.46 ( 0.02 mass %). As a result, the number density of Au particles on the TiO2 surface decreases with increasing Tc and tc. Figure 2B shows the relation between the mean number of Au particles loaded per one TiO2 particle (N) and d: Each N value was calculated from the corresponding x value. When the Au particle size decreases, the N value slowly increases in the range of d larger than 5 nm, sharply rising up at d < 5 nm. These facts indicate that the heating process at Tc > 673 K activates the surface migration of Au nanoparticles to be unified partially on the TiO2 surface. The small standard deviation (1.5 ( 0.7 nm) and relative standard deviation (24 ( 4%) of d for the samples except sample (d) are a reasonably good indication of size monodispersity of Au nanoparticles. The N value for sample (d) is so small that the fairly large relative standard deviation of 36.9% is likely caused by that of the TiO2 particle size. In this manner, this preparation method enables to control the mean diameter of Au nanoparticles in a fairly wide range with their loading amount maintained constant. This also provides information on the thermal stability of Au nanoparticles on the TiO2 surface, which is crucial for the design of thermally activated Au/TiO2 catalytic reactions.

Ultrafast Photosynthetic Reduction of S8

J. Phys. Chem. B, Vol. 110, No. 22, 2006 10773

Figure 3. (A) Electronic absorption spectra of TiO2 (a) and Au/TiO2 (b-d): d, x ) (b) 4.4 nm, 0.48 mass %; (c) 7.9 nm, 0.46 mass %; (d) 12.2 nm, 0.45 mass %. (B) Plots of λmax vs d. Figure 2. (A) Plots of d as functions of Tc and tc. (B) Relation between N and d.

Figure 3A shows electronic absorption spectra of TiO2 (a) and Au/TiO2 having different d values ((b) d ) 4.4 nm, x ) 0.48 mass %; (c) d ) 7.9 nm, x ) 0.46 mass %; (d) d ) 12.2 nm, x ) 0.45 mass %). In the spectrum of TiO2, a broad and intense absorption band due to its interband transition is situated at λ < 385 nm, while Au/TiO2 has an additional absorption peak in the range from 540 to 590 nm assignable to the surface plasmon absorption of Au nanoparticles. Also, the intensity of the surface plasmon absorption increases as the d value increases. In the Mie theory, the extinction coefficient of small metal particles is given by the summation over all electric and magnetic multipole oscillations causing the absorption and scattering of the interacting electromagnetic filed. When the particle size is much smaller than the wavelength of the absorbing light, only the dipole term can be assumed to contribute to the absorption.15 In this quasi-static regime (dipole approximation), the extinction coefficient of the small metallic particles is proportional to the volume, which qualitatively explains the increase in the surface plasmon absorption intensity with increasing d. Figure 3B shows the absorption maximum wavelength of the Au surface plasmon band (λmax) as a function of d. The red-shift amount monotonically increases as the d

value varies from 3.2 to 12.2 nm: λmax ) -0.246d2 + 8.53d + 517 (R2 ) 0.9997). Both blue shifts and red shifts of λmax with an increase in size of metal nanoparticles have thus far been observed, depending on the systems.16 Then, the peak position of the surface plasmon oscillation cannot simply be related to a size effect within the quasi-static regime. Au/TiO2 Photosynthetic Reduction of S8. Ethanol S8 solutions with the concentrations less than 1.72 × 10-4 mol dm-3 were prepared by dissolving rhombic S8 crystal (eq 1). Figure 4A shows the electronic absorption spectral change of a 1.72 × 10-4 mol dm-3 deaerated S8 ethanol solution in the presence of Au (d ) 4.8 nm, x ) 0.62 mass %)/TiO2 with irradiation (λex > 300 nm): tp denotes irradiation time. The S8 solution has an absorption peak at 263.5 nm (max ) 6.06 × 103 mol-1 dm3 cm-1) with a shoulder near 275 nm. ZINDO calculations were performed for an S8 molecule to yield the electronic absorption spectrum. The calculated spectrum resembled the experimental spectrum, although the absorption peaks in the former were shifted toward shorter wavelength by ca. 60 nm with respect to those in the latter (data not shown). From spectral analysis, the two absorption bands were assigned to the n f σ* transitions. Noticeably, TiO2 is almost selectively excited under the present irradiation conditions. Upon the

10774 J. Phys. Chem. B, Vol. 110, No. 22, 2006

Kiyonaga et al. irradiation, the intensity of the S8 absorption monotonically decreases, while no new absorption appears.

dissolution process: S8 (rhombic) f S8 (in ethanol) (1) Figure 4B shows time courses for the photoinduced reaction of S8 with varying initial concentrations (C0) in the presence of TiO2 (a) and Au (d ) 4.8 nm, x ) 0.62 mass %)/TiO2 (b-d) at 298 K. The decrease in the S8 concentration with adsorption in the dark was negligibly small under the conditions. When light is irradiated to a 1.72 × 10-4 mol dm-3 deaerated S8 ethanol solution with TiO2 (a), the concentration slowly decreases with increasing tp. As a result of such a low level of Au loading (b), the reaction is drastically enhanced, and the S8 completely disappears within 15 min at C0 < 1.72 × 10-4 mol dm-3. Regardless of x and C0, the zero-order rate law holds for the reaction, of which the rate constant, k, increases with the Au loading by a factor of as much as 9.4, being very weakly dependent on C0 in the Au/TiO2 system. Both the irradiation and TiO2 (or Au/TiO2) were necessary for the reaction to occur. Only H2S was detected as a reduction product by gas chromatography, whereas the dehydrogenation of C2H5OH proceeded in the absence of S8. In the electronic absorption spectrum of aqueous NaOH solutions trapping H2S evolved during the reaction, an absorption peak appeared at 230 nm due to S2ions. The ratio of the mole number of S2- ions generated to that of S8 consumed was ca. 8 irrespective of tp, indicating that S8 is stoichiometrically reduced to H2S. Also, the generation of CH3CHO was confirmed as an oxidation product in the gas phase by gas chromatography. Figure 4C shows adsorption isotherms of S8 from the ethanol solutions on TiO2 (a) and Au/TiO2 (b) at 298 K. The adsorption amounts on TiO2 and Au (d ) 3.2, x ) 0.38 mass %)/TiO2 are expressed by the number of sulfur atoms adsorbed per unit surface areas of TiO2 and Au. The adsorption selectivity of S8 on the Au surface of Au/TiO2 reached 99.6%, while the affinity of the TiO2 surface for alcohols is known to be very strong.17 X-ray photoelectron spectroscopic studies showed that a part of S8 molecules undergoes dissociative adsorption on the Au surface.18 Also, the adsorption energy of sulfur on the Au surface was calculated to be ca. 250 kJ mol-1 by the density functional theoretical (DFT) calculations for an S-Au10 model cluster.19 Further, previous work showed that Au nanoparticles loaded on TiO2 act as reduction sites in the Au/TiO2 photocatalysis due to the photoinduced electron transfer from TiO2 to Au.20 Thus, the great affinity between Au and sulfur would lead to the attainment of the saturated adsorption during the reaction; this explains the zero-order and weak C0 dependence of the reaction rate and the fact that the reduction of sulfur is predominant over that of H+ on the Au surfaces of Au/TiO2. The assumption that the surface Au atoms are reduction sites provided a turnover number at tp ) 5 min of more than 15 in every system. These results prove that Au/TiO2 operates as an excellent photocatalyst in this reaction (eq 2). Since this overall reaction is energetically uphill (eq 3), it may be of importance and interest also from the perspective of energy conversion from light energy to chemical energy. Figure 4. (A) The change in the electronic absorption spectra of an S8 ethanol solution in the presence of Au/TiO2 with irradiation under deaerated conditions: C0 ) 1.72 × 10-4 mol dm-3. (B) Time courses for the photoinduced reaction of S8 in the presence of TiO2 (a) and Au/TiO2 (b-d) as a function of C0 at 298 K (λex > 300 nm, I320-400 ) 3.7 mW cm-2): (a) C0 ) 1.72 × 10-4 mol dm-3; (b) C0 ) 1.72 × 10-4 mol dm-3; (c) C0 ) 8.6 × 10-5 mol dm-3; (d) C0 ) 4.3 × 10-5 mol dm-3. (C) The adsorption isotherms of S8 from the ethanol solutions on TiO2 (a) and Au/TiO2 (b) at 298 ( 1 K.

photocatalytic process: S8 (in ethanol) + 8C2H5OH Au/TiO2

9 8 8H2S + 8CH3CHO (2) hν (λ > 300 nm) ex

overall process: S8 (rhombic) + 8C2H5OH f 8H2S + 8CH3CHO ∆G° ) 67.2 kJ mol-1 (3)



TABLE 1: Comparison of Half-Lives for Various TiO2 and Metal-Loaded TiO2 Photocatalytic Reactions no

t1/2/ min

substrate

C 0/ mol dm-3

photocatalyst (PC)

PC/ g dm-3

S/ m2 g-1

T/ K

1

S8

4-7.5

1.72 × 10-4

Au/TiO2 (A-100)

4

8.1

298

2

rhodamine 6G

50-60

3.0 × 10-5

TiO2 (A-100)

1

8.1

294

3

bis(2-dipyridyl)disulfide

t4/5 ) 100

5.41 × 10-5

TiO2 (A-100)

1

8.1

303

4


100

5.41 × 10-5

Ag/TiO2 (A-100)

1

8.1

303

5


40

5.41 × 10-5

Au/TiO2 (A-100)

1

8.1

303

6

pentachlorophenol

20

4.5 × 10-5

TiO2 (P-25)

2

55

308-328

7

4-chlorophenol

14

4.7 × 10-5

TiO2 (P-25)

2

55

308-328

8

3,4-dichlorophenol

45

1.1 × 10-4

TiO2 (P-25)

2

55

308-328

9

2,4,5-trichlorophenol

55

1.0 × 10-4

TiO2 (P-25)

2

55

308-328

chlorobenzene

90

4.0 × 10-4

TiO2 (P-25)

2

55

308-328

10-5

TiO2 (P-25)

2

55

308-328

10 11

1,2,4-trichlorobenzene

24

5.5 ×

12

toluene

40-50

1.0 × 10-3

TiO2 (P-25)

2

55

303

13

Metolachlor

21

1.76 × 10-5

TiO2 (P-25)

0.09

55

298

In general, the rate of TiO2 photocatalytic reactions depends on many parameters including the crystallinity and surface area of TiO2, the kind of substrates, and their initial concentration, light intensity, and temperature. Table 1 summarizes half-lives (t1/2) in various TiO2 and metal-loaded TiO2 photocatalytic reactions together with the experimental conditions:21 Initial substrate concentration (C0) is listed because the t1/2 is also a function of C0 for the reaction whose reaction order is other than first. When the t1/2 values for the reactions carried out under comparable conditions are compared (reactions 1-5), the value of this reaction is smaller than those of the other reactions by more than 1 order of magnitude. Also, even for the reactions performed under the light intensities greater than 20 times (reactions 6-13), the t1/2 values are larger than that of the present system. Thus, this reaction deserves to be called “ultrafast photosynthetic reaction”. The dependence of the Au (d ) 4.8 nm, x ) 0.62 mass %)/ TiO2 photosynthetic reduction of S8 on light intensity (I) was examined at C0 ) 1.72 × 10-4 mol dm-3: I is the light intensity integrated from 320 to 400 nm. As Figure 5A shows, the rate of the zero-order reaction significantly increases with increasing I. Figure 5B shows the plots of k vs I, indicating that the k value linearly increases with increasing I at I < 1 mW cm-2 to deviate negatively from the straight line at I > 1 mW cm-2. Similar results were previously reported in various photocatalytic reaction systems.22 Figure 6A shows the temperature effect on the Au (d ) 4.8 nm, x ) 0.62 mass %)/TiO2 photosynthetic reaction at C0 ) 1.72 × 10-4 mol dm-3. The rate of the zeroorder reaction monotonically increases as the temperature increases. The Arrhenius plot shown in Figure 6B yields an activation energy (Ea) of 16.8 kJ mol-1, which is only onefourth of the ∆G° value for the overall reaction. This value is further smaller than that reported for a similar reaction, the Au/ TiO2-photocatalyzed reduction of bis(2-dipyridyl)disulfide to 2-mercaptopyridine by water (Ea ) 19.7 kJ mol-1), where the water oxidation by the holes in the valence band (vb) of TiO2 was concluded to be the rate-determining step.3 If this is true also for the present reaction, its lower Ea would result from the better reducing ability of C2H5OH than water as indicated below.

irradiation conditions

refs

λex > 300 nm, I320-400 ) 3.7 mW cm-2 λex > 420 nm, I320-400 ) 2.2 mW cm-2 λex > 300 nm, I320-400 ) 4.6 mW cm-2 λex > 300 nm, I320-400 ) 4.6 mW cm-2 λex > 300 nm, I320-400 ) 4.6 mW cm-2 λex > 330 nm, I340-830 ) 100 mW cm-2 λex > 330 nm, I340-830 ) 100 mW cm-2 λex > 330 nm, I340-830 ) 100 mW cm-2 λex > 330 nm, I340-830 ) 100 mW cm-2 λex > 330 nm, I340-830 ) 100 mW cm-2 λex > 330 nm, I340-830 ) 100 mW cm-2 λex ) 365 nm, I ) 220 mW λex > 290 nm, I ) 75 mW cm-2

this study ref 20a ref 8 ref 8 ref 8 ref 20b ref 20b ref 20b ref 20b ref 20b ref 20b ref 20c ref 20d

Figure 7 shows the dark and photopotentials of the SnO2 electrodes overlaid with Au (d ) 3.2 nm, x ) 0.38 mass %)/ TiO2 films in deaerated aqueous C2H5OH solutions as a function of C2H5OH concentration (Ce). In this case, since SnO2, TiO2, and Au are in direct contact each other, their Fermi energies (EF) are regarded to be approximately equal at equilibrium in the dark. Upon irradiation, the EF jumps in the negative direction by ca. 0.3 eV without C2H5OH. Noticeably, the amount of the shift in EF increases with an increase in Ce, reaching ca. 0.4 eV at Ce ) 1 mol dm-3. A recent kinetic and DFT study has shown that this Fermi energy upward shift is the driving force for the reductive sulfur desorption from the Au surfaces preadsorbed with sulfur atoms.19 In this system, the photogenerated vb holes oxidize C2H5OH or H2O adsorbed on the TiO2 surface, while the conduction band (cb) electrons are transferred to Au particles to reduce H+ to H2 on their surfaces. The Fermi energy upward shift is thought to be induced by the fact that the rate of the oxidation process overwhelms that of the reduction process because of the strong oxidation power of the vb holes at the initial state of the reaction. Further, the increase in the Fermi energy upward shift with increasing Ce can be explained in terms of the current doubling effect of C2H5OH23 as well as its larger reducing ability than water. Essential Reaction Mechanism. These results allowed us to present a plausible reaction scheme for the Au/TiO2 photosynthetic reduction of S8 in ethanol (Scheme 1). In the dark, S8 undergoes chemisorption on the Au surfaces of Au/TiO2 in a close-packed state, accompanied by the partial S-S bond cleavage (Step 1). Illumination of TiO2 (λex > 300 nm) gives rise to the excitation of electrons in the vb to the cb by interband transition (Step 2). A portion of the excited electrons flows into Au with a large work function (Step 3), while the other electron-hole pairs vanished by recombination (Step 4). The oxidation of C2H5OH adsorbed on the TiO2 surface by the vb holes yields the CH3CHOH radical and H+ (Step 5). The CH3CHOH radical has a potential sufficient to directly inject another electron to the cb of TiO2. This current doubling process affords CH3CHO and H+ (Step 6), and the cb electron is further transferred to Au (Step 7). The accumulation of electrons in

10776 J. Phys. Chem. B, Vol. 110, No. 22, 2006

Figure 5. (A) Light intensity dependence of the Au/TiO2 photosynthetic reduction of S8: (a) I ) 0.5 mW cm-2; (b) I ) 1.0 mW cm-2; (c) I ) 2.0 mW cm-2; (d) I ) 3.7 mW cm-2; (e) I ) 8.0 mW cm-2. (B) Plots of the rate constant vs light intensity.

the cb increases the Fermi energy of TiO2 (EF(TiO2)) and further that of Au (EF(Au)) to EF′(TiO2) and EF′(Au) in the photostationary state, respectively. When the EF′(Au) exceeds the reduction potential of sulfur, its desorption from the Au surface produces H2S (Step 8).19 The hot electron mechanism driven by the excitation of Au nanoparticles24 can be excluded because no reaction was observed when only the Au surface plasmon was excited under the visible-light irradiation (λex > 400 nm). This reaction may recall a previous study on the RuO2/CdSphotocatalyzed H2 production from H2S aqueous solutions.25 However, in that case, S2- ions acted as an electron donor to participate in the hole oxidation process, in contrast to the present system where S2- ions, or H2S, are generated via the reduction process. In this reaction scheme, the essential factors for this ultrafast photosynthetic reaction can be summarized as follows: The first is the efficient charge separation due to the photoinduced electron transfer from TiO2 to Au (charge separation effect). The second factor is that the oxidant (S8) and the reductant (ethanol) are selectively and abundantly supplied to reduction sites (Au surface) and oxidation sites (TiO2 surface), respectively (reasonable delivery effect). The third is that the desorption of sulfur atoms adsorbed on the surface of

Kiyonaga et al.

Figure 6. (A) Temperature dependence of the Au/TiO2 photosynthetic reduction of S8: (a) 273 K; (b) 283 K; (c) 298 K; (d) 323 K. (B) Arrhenius plot for the Au/TiO2 photosynthetic reduction of S8.

Au due to its photoinduced Fermi energy upward shift, followed by the subsequent liberation of the reduction product, H2S, into the gas phase restricts readsorption of the desorbed sulfur on the Au/TiO2 photocatalyst (restriction of product readsorption). Au Particle Size Dependence of the Photocatalytic Activity. The crystallinity of TiO2 particles, which is one of the important factors affecting the photocatalytic activity, increases as the postannealing temperature (Tc) increases. To preclude this influence, all the TiO2 particles were subjected to the preparation of Au/TiO2 after the preheating at 923 K for 4 h (see Experimental Section). Figure 8A shows the values of k and turnover frequency (TOF) for the Au/TiO2 photosynthetic reduction of S8 as a function of d: C0 ) 1.72 × 10-4 mol dm-3. The k value increases with a decrease in d in the range from 3.2 to 12.2 nm. The application of the steady-state approximation for the photogenerated charge carriers leads to eq 4 accounting for the zero-order rate law and the weak dependence of k on C0 in this reaction.

-d[S8]/dt ) k ) Iφ{kcs/(krec[h+ vb(TiO2)] + kcs)}

(4)

+ 2 where krec[h+ vb(TiO2)] + kcskox[C2H5OH] [hvb(TiO2)] - kcsIφ



Figure 7. Dependence of the electrode potentials (U/V vs Ag/AgCl) of SnO2 electrodes covered with Au/TiO2 films in the dark (a) and under irradiation (b) as a function of the C2H5OH concentration (Ce).

SCHEME 1: A Plausible Reaction Scheme for the Au/ TiO2 Photosynthetic Reduction of S8 in Ethanol

) 0 is valid: φ denotes the excitation efficiency of TiO2, and krec, kcs, and kox stand for the rate constants of charge recombination, charge separation, and ethanol oxidation by h+ vb(TiO2), respectively. Further, the substitution of [h+ vb(TiO2)] into eq 4 yields eq 5.

I/k ) 1/φ + (krec/φ kcskox[C2H5OH])1/2I1/2

(5)

Equation 5 explains the nonlinear I-dependence of k, suggesting that the recombination of photogenerated charge carriers becomes serious enough to cause the negative deviation of k at I > 1 mW cm-2 (Figure 5B). The increase in k with decreasing d can be caused by the resulting increase in the Au surface area. The effect of the Au surface area on the photocatalytic activity of Au/TiO2 is clearly shown by the TOF data in Figure 8A, i.e., the TOF significantly increases with increasing d in the same range. When the d value increases (i.e., the N value decreases), the mean number of excited electrons distributed to one Au particle should increase. Thus, the increase in the TOF with increasing d is ascribable to the greater electron pool effect of larger Au particles. Kamat et al. have observed a slight

Figure 8. (A) Au size effect on the Au/TiO2 photosynthetic reduction of S8. (B) Plots of I/k vs I1/2 at 298 K: (a) Au (d ) 3.2 nm, x ) 0.44 mass %); (b) Au (d ) 4.4 nm, x ) 0.48 mass %); (c) Au (d ) 7.9 nm, x ) 0.46 mass %); (d) Au (d ) 12.2 nm, x ) 0.45 mass %). (C) Plots of σ-1 vs d.

increase in EF′ with decreasing d in a suspension system consisting of TiO2 and Au nanoparticles (3 < d < 8 nm), which means that the reducing power of Au/TiO2 under irradiation increases as a result of decrease in d.26 This may provide another

10778 J. Phys. Chem. B, Vol. 110, No. 22, 2006 possible explanation for the increase in k with decreasing d (Figure 8A). However, the photoinduced Fermi energy upward shift should be enhanced with increases in both the energy level gap of the Au band and the electron number trapped by one Au particle. Since the former increases and the latter decreases (Figure 2B) on downsizing Au particles, the trend that the EF′ increases with decreasing d would not always be valid. Figure 8B shows plots of I/k vs I1/2 for the Au/TiO2 having different d values: (a) Au (d ) 3.2 nm, x ) 0.44 mass %); (b) Au (d ) 4.4 nm, x ) 0.48 mass %); (c) Au (d ) 7.9 nm, x ) 0.46 mass %); (d) Au (d ) 12.2 nm, x ) 0.45 mass %). In each case, the plot gives a straight line, which supports the validity of the proposed reaction mechanism (Scheme 1). As eq 5 shows, the reciprocal of the slope (σ-1) proportional to kcs/krec becomes a good criterion of charge separation efficiency. As shown in Figure 8C, the plot of σ-1 vs d indicates that the σ-1 value increases with an increase in d. As a result of an increase in d (i.e., decrease in N), the diffusion length necessary for excited electrons to be trapped by one of Au nanoparticles increases, which would further increase the recombination probability. Consequently, the increase in k with decreasing d can mainly be attributed to the resulting enhancement of charge separation efficiency. Conclusions The loading of Au nanoparticles on TiO2 has drastically enhanced the photosynthetic reduction of S8 due to the synergy of the charge separation effect, reasonably delivery effect, and restriction of product readsorption. The rate of reaction has been shown to increase with decreasing mean size of Au nanoparticles mainly because of the better charge separation effect. This study would present useful guidelines for the design of the highly efficient (photo)catalytic reactions of Au/TiO2. Acknowledgment. The authors express sincere gratitude to Dr. Tetsuro Kawahara (Nippon Sheet Glass Co.) for helpful discussion. This work was supported in part by a Grant-in-Aid for Scientific Research (C) no. 16550169 from the Ministry of Education, Science, Sport, and Culture, Japan. H.T. acknowledges Ishihara Techno Co. Ltd. for the gift of TiO2 (A-100) sample. References and Notes (1) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301.

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