Hydrogen Production by Photocatalytic Water Splitting over Pt

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Hydrogen Production by Photocatalytic Water Splitting over Pt/TiO2 Nanosheets with Exposed (001) Facets Jiaguo Yu,*,† Lifang Qi,† and Mietek Jaroniec*,‡ State Key Laboratory of AdVanced Technology for Material Synthesis and Processing, Wuhan UniVersity of Technology, Luoshi Road 122#, Wuhan 430070, People’s Republic of China, and Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242

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ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: June 17, 2010

Pt/TiO2 nanosheets with exposed (001) facets were fabricated by a simple hydrothermal route in a Ti(OC4H9)4HF-H2O mixed solution followed by a photochemical reduction deposition of Pt nanoparticles on TiO2 nanosheets under xenon lamp irradiation. The prepared samples were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption-desorption isotherms, UV-vis diffuse reflectance spectroscopy, and photoluminescence (PL) spectroscopy. Production of •OH radicals on the TiO2 surface was detected by the PL technique using coumarin as a probe molecule. The effects of Pt loading on the rates of photocatalytic hydrogen production of the as-prepared samples in ethanol aqueous solution were investigated and discussed. The results showed that the photocatalytic hydrogen production rates of TiO2 nanosheets from the ethanol aqueous solutions were significantly enhanced by loaded Pt on the TiO2 nanosheets, and the latter with a 2 wt % of deposited Pt exhibited the highest photocatalytic activity. All fluorinated TiO2 nanosheets exhibited much higher photocatalytic activity than Degussa P25 TiO2 and pure TiO2 nanoparticles prepared in pure water due to the synergistic effect of surface fluorination and exposed (001) facets. 1. Introduction Anxiety about depletion of fossil fuel reserves and the pollution caused by combustion of fossil fuels make hydrogen an attractive alternative energy source.1 Recently, hydrogen is mainly obtained by using nonrenewable resources (e.g., fossil fuels) or the high-energy consumption process, which are neither environmentally friendly nor economical.2 Therefore, there is a great interest in the development of sustainable and economic methods for the production of hydrogen. Since the first report by Fujishima and Honda3 on the photoelectrochemical water splitting on a TiO2 electrode, the photocatalytic water splitting has become a promising way for a clean, low-cost, and environmentally friendly production of hydrogen by using solar energy. In comparison to other semiconductor photocatalysts, TiO2 has been widely used because of its biological and chemical inertness, nontoxicity, low cost, availability, and longterm stability against photo and chemical corrosion.4–10 However, the photocatalytic efficiency of TiO2 for water splitting is limited due to the high recombination rate of photogenerated electron-hole pairs.11 To resolve this problem, many methods have been proposed to enhance the photocatalytic activity of TiO2, including the doping of transition metal or nonmetal ions, the deposition of noble metals, the surface sensitization of dyes and the preparation of composite semiconductors.4,7,12 Recently, anatase TiO2 single-crystalline nanosheets with high percentage of reactive (001) facets have attracted of a lot of attention.13–16 Both theoretical and experimental studies indicate that higher surface energy of (001) facets is more effective for dissociative adsorption of reactant molecules compared with the thermody* To whom correspondence should be addressed. E-mail: jiaguoyu@ yahoo.com (J.Y.); [email protected] (M.J.). † Wuhan University of Technology. ‡ Kent State University.

namically more stable (101) facets.17–20 Further investigations indicate that water molecules can chemically dissociate on the (001) surface but, contrarily, only physically adsorb on the (101) surface.19 Therefore, it is reasonable to infer that these (001) facets should be much more effective for water splitting than the (101) facets.21–23 Furthermore, the single-crystalline structure of TiO2 nanosheets with a low density of defects can also reduce the recombination rate of photogenerated electron-hole pairs on grain boundaries and crystalline defects, thus improveing the photocatalytic efficiency.24 However, to the best of our knowledge, there are only a few reports on the photocatalytic water splitting for hydrogen production by using TiO2 nanosheets with exposed (001) facets as water splitting photocatalysts.24,25 For example, Amano et al.24 reported fabrication of decahedral single-crystalline anatase particles with exposed (001) facets and sizes of 50-250 nm by a gas-phase process using TiCl4 as a titanium source and showed their enhanced photocatalytic activity for water splitting. The rate of H2 evolution by these particles was higher than that on the commercial-grade Degussa P25 TiO2 powder (Degussa P25).24 Degussa P25 is one of the best photocatalysts for water splitting among currently available commercial TiO2 photocatalysts,26a and only a small number of lab-made TiO2 samples show activity higher than that of Degussa P25.24,26b Furthermore, Lu and co-workers25a also reported an enhanced water splitting photoactivity for oxygendeficient anatase TiO2 sheets with exposed (001) facets,25a nitrogen-doped anatase TiO2 sheets with dominant (001) facets,25b and nanosized anatase TiO2 single crystals.25c However, their photocatalytic activity was not compared with that of Degussa P25.25 In this study, the TiO2 nanosheets with exposed (001) facets were first fabricated by a simple hydrothermal treatment of the mixed solution of tetrabutyl titanate and hydrofluoric acid, followed by loading different amounts of Pt onto TiO2 nanosheets

10.1021/jp104488b  2010 American Chemical Society Published on Web 07/13/2010

Hydrogen Production by Photocatalytic Water Splitting

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TABLE 1: Experimental Conditions for the Preparation of Samples

a

no.

RF

morphology

phasea

RPt

washing

composition

color

NS1 NS2 NS3 NS4 NS5 NS6 NS7 P25 NP

1 1 1 1 1 1 1 0 0

nanosheets nanosheets nanosheets nanosheets nanosheets nanosheets nanosheets nanoparticles nanoparticles

A A A A A A A A, R A

0 0.5 1 2 4 6 2 2 2

no no no no no no yes no no

TiO2/F TiO2/F/Pt TiO2/F/Pt TiO2/F/Pt TiO2/F/Pt TiO2/F/Pt TiO2/Pt TiO2/Pt TiO2/Pt

white light gray light gray gray black black gray gray gray

A and R denote anatase and rutile, respectively.

via photoreduction method. The effect of the Pt loading content on the rates of photocatalytic hydrogen production of the asprepared samples in ethanol-aqueous solutions was investigated and discussed. Furthermore, efficient hydrogen production and decomposition of pollutants (such as glycerol, triethanolamine, and glucose) in water were achieved on the prepared Pt-TiO2 nanosheets. Finally, the performance of the Pt-TiO2 nanosheets was compared with TiO2 nanoparticles and Degussa P25, showing that the photocatalytic activity of the former was higher than or at least comparable to that of Degussa P25. 2. Experimental Section 2.1. Sample Preparation. All the reagents were of analytical grade and were used without further purification. Anatase TiO2 nanosheets with exposed (001) facets were prepared by the hydrothermal method similar to that reported by Xie et al.16 In a typical synthesis, 25 mL of Ti(OC4H9)4 and 3 mL of hydrofluoric acid solution (with a concentration ca. 40 wt %) were mixed in a dried Teflon-lined autoclave with a capacity of 100 mL at ambient temperature, followed by hydrothermal treatment of the mixture at 180 °C for 24 h. The nominal molar ratios of HF to Ti(OC4H9)4, which hereafter was designated as RF, were 1 (as shown in Table 1). After hydrothermal reaction, the white precipitate was collected by centrifuge, washed with distilled water and ethanol for three times, and then dried in an oven at 80 °C for 6 h. To investigate the effect of particle morphology on the efficiency of photocatalytic water splitting, TiO2 nanoparticles were also prepared at the same conditions except 3 mL of hydrofluoric acid were replaced by 3 mL of distilled water (RF ) 0, as shown in Table 1). To further confirm the enhanced effect of surface fluorination on the photoactivity, the fluorinated surface of TiO2 nanosheets prepared at RF ) 1 was cleaned with diluted 0.1 M NaOH solution, resulting in the formation of F-free TiO2 nanosheets. The Pt/TiO2 catalysts were prepared by impregnation of the above-prepared TiO2-nanosheet powders (0.2 g) in 80 mL of H2PtCl6 aqueous solution with different concentrations (from 0, 0.06. 0.13, 0.26, 0.53, to 0.78 mM). The suspensions were stirred and followed by UV illumination (350 W Xe arc lamp) for 20 min at room temperature. After that, the precipitates were collected by centrifuge and rinsed with distilled water and ethanol for three times. The catalysts were finally dried in an oven at 80 °C for 12 h. The nominal weight ratios of Pt to Ti (RPt) were 0, 0.5, 1, 2, 4, and 6 wt %, and the obtained powders were labeled as NS1, NS2, NS3, NS4, NS5, and NS6, respectively. The detailed experimental conditions for the preparation of samples were listed in Table 1. The color of the prepared Pt/TiO2 samples gradually changed from white to black with increasing RPt after Xe arc lamp irradiation (see Table 1). For comparison, the photocatalysts were also prepared using F-free TiO2 nanosheets, commercial Degussa P25, and the

above-mentioned TiO2 nanoparticle powders as raw materials by photoreduction deposition of Pt (RPt ) 2); these three samples were labeled as NS7, P25, and NP, respectively. 2.2. Characterization. X-ray diffraction (XRD) patterns obtained on an X-ray diffractometer (type HZG41B-PC) using Cu KR irradiation at a scan rate of 0.05° 2θ s-1 were used to determine the phase structures of the samples studied. The average crystallite sizes of anatase and rutile grains were quantitatively calculated using Scherrer formula after correcting the instrumental broadening. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analyses were conducted on a JEM-2100F electron microscope (JEOL, Japan) using a 200 kV accelerating voltage. X-ray photoelectron spectroscopy (XPS) measurements were done on a VG ESCALAB MKII XPS system with Mg KR source and a charge neutralizer. All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The Brunauer-Emmett-Teller (BET) surface area (SBET) of the powders was analyzed by nitrogen adsorption using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All samples were degassed at 100 °C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method using adsorption data in the relative pressure (P/P0) range of 0.05-0.25. Desorption data were used to determine the pore size distribution via the Barret-Joyner-Halender (BJH) method, assuming a cylindrical pore model.27 The volume of adsorbed nitrogen at the relative pressure (P/P0) of 0.972 was used to determine the pore volume and average pore size. Photoluminescence (PL) spectra were measured at room temperature on a fluorescence spectrophotometer (F-7000, Hitachi, Japan). The excitation wavelength was 315 nm, the scanning speed was 1200 nm/min, and the PMT voltage was 700 V. The width of excitation slit and emission slit were both 5.0 nm. UV-vis diffused reflectance spectra of the samples were obtained for the dry-pressed film samples using a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV-vis diffuse reflectance experiment. 2.3. Photocatalytic H2 Production Activity. The photocatalytic H2 production experiments were performed in a 100-mL Pyrex flask at ambient temperature and atmospheric pressure, and three openings of the flask were sealed with silicone rubber septum. A 350-W Xe arc lamp was used as light source (20 cm far away from the photocatalytic reactor). The focused intensity on the flask was ca. 20 mW/cm-2. In a typical photocatalytic experiment, 20 mg of catalyst was suspended in 80 mL of ethanol (20 mL) and water (60 mL) solution under magnetic stirring and a side irradiation. Before irradiation, suspensions of the catalyst were dispersed in an ultrasonic bath, and nitrogen was bubbled through the reaction mixture for 40 min to remove the dissolved oxygen to ensure that the reaction system is under

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Figure 2. XRD patterns of the samples prepared at RPt ) 0 (NS1), 0.5 (NS2), 1 (NS3), 2 (NS4), 4 (NS5), and 6 (NS6). Figure 1. TEM (a and c) and HRTEM (b and d) images of the samples prepared at RPt ) 0 (NS1) (a and b) and RPt ) 2 (NS4) (c and d).

anaerobic conditions. A 0.4 mL of gas was sampled intermittently through the septum, and hydrogen was analyzed using a gas chromatograph (GC-14C, Shimadzu, Japan, TCD, nitrogen as a carrier gas, and 5 Å molecular sieve column). Prior usage all glassware was rigorously cleaned and carefully rinsed with distilled water. To further test photocatalytic H2 production activity of the prepared samples and to extend their environmental application, aqueous solutions of glycerol (0.1 M), triethanolamine (8 mM), and glucose (0.5 M) as model pollutants were used as the substitute of ethanol aqueous solution to evaluate the photocatalytic H2-production efficiency of the TiO2 sample. 2.4. Analysis of Hydroxyl Radicals (•OH). The production of •OH on the surface of the UV-illuminated TiO2 samples was detected by a photoluminescence (PL) method using coumarin as a probe molecule. Coumarin readily reacts with •OH to produce highly fluorescent product, 7-hydroxycoumarin (7HC) (umbelliferone) (as shown in Supporting Information, Figure S1).28,29 Experimental procedure was similar to the measurement of photocatalytic activity except 60 mL of water was replaced by 60 mL of coumarin aqueous solution (1 × 10-3 M). The PL spectra of generated 7HC were measured on a Hitachi F-7000 fluorescence spectrophotometer. After UV-light irradiation for 1 h, the reaction solution was filtrated to measure the increase in the PL intensity around 456 nm excited by 332 nm light. 3. Results and Discussion 3.1. Morphology and Phase Structures. TEM and HRTEM were used to observe the changes in the morphology of the TiO2 nanosheets in the absence and presence of Pt. Figure 1a shows a typical TEM image of TiO2 nanosheets (NS1) fabricated by hydrothermal method without Pt deposition. The product consists of well-defined sheet-shaped structures having a rectangular outline with an average side length of ca. 50-80 nm and thickness of ca. 6 nm. HRTEM image (Figure 1b) indicates that the lattice spacing parallel to the top and bottom facets is ca. 0.235 nm, corresponding to the (001) planes of anatase TiO2. On the basis of the TEM results, the percentage of exposed (001) facets on the TiO2 nanosheets is ca. 75%. Figure 1c shows TEM image of Pt/TiO2 nanosheets prepared at RPt ) 2 (NS4). Pt nanoparticles with average diameter of ca.

TABLE 2: Effects of RPt on the Physical Properties and Photocatalytic Activity of the Samples

samples

RPt

crystalline size (nm)

NS1 NS2 NS3 NS4 NS5 NS6 NS7 P25 NP

0 0.5 1 2 4 6 2 2 2

14.9 15.1 15.2 15.0 14.9 14.8 15.1 30.1 12.0

SBET (m2/g)

pore volume (cm3/g)

average pore size (nm)

activity µmol h-1

108 101 96 94 93 84 104 41 105

0.53 0.46 0.40 0.41 0.38 0.35 0.38 0.40 0.10

16.0 14.8 14.6 14.4 13.2 13.0 12.4 35.2 3.6

3.0 236.6 306.5 333.5 308.7 270.9 185.0 223.4 170.2

2-4 nm are clearly observed due to their high electron density and good immobilization on the surface of TiO2 nanosheets. HRTEM image (Figure 1d) of Pt nanoparticles indicates that the lattice fringes of 0.227 nm match the crystallographic planes of Pt (111). The energy dispersive X-ray spectroscopy (EDX, not shown here) indicates that the NS4 is composed of Ti, O, F, and Pt elements. Ti, O, and F elements come from surfacefluorinated TiO2 nanosheets, and Pt orginates from the Pt nanoparticles loaded on TiO2 nanosheets. XRD was used to investigate the changes in the phase structure and crystallite size of the samples prepared at different RPt ratios. Figure 2 shows a comparison of the XRD patterns of samples NS1, NS2, NS3, NS4, NS5, and NS6. As can be seen that only anatase phase of TiO2 (JCPDS No. 21-1272, space group: I41/amd (141)) is observed for pure TiO2 nanosheets (NS1) and Pt/TiO2 nanosheets prepared at RPt < 4 (NS2, NS3, and NS4). Two weak and broad diffraction peaks at 2θ about 39.8 and 46.3° can be observed for NS5 and NS6 with higher RPt ratios, indicating the presence of metallic Pt (JCPDS No. 65-2868, space group: Fm3m j (225)). For lower RPt, no diffraction peaks of Pt were observed mainly due to low loading of this element. The average crystallite sizes of pure TiO2 nanosheets and Pt/TiO2 nanosheets calculated using Scherrer’s equation for the main (101) diffraction peak of anatase are listed in Table 2. As can be seen all the samples have almost the same crystallite size (ca. 15 nm), implying that the deposition of Pt does not alter the crystallite size and morphology (see Figure 1) of TiO2 nanosheets. This finding is not surprising because low-temperature photoreduction deposition of Pt has no enough energy to stimulate the growth of TiO2 nanosheets.



Figure 3. Nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution curves (inset) for the samples prepared at RPt ) 0 (NS1) and RPt ) 2 (NS4).

3.2. XPS Analysis. To analyze and determine chemical composition of the prepared Pt/TiO2 nanosheets and to identify the chemical status of Pt in the samples, the XPS analysis was carried out. For NS1, the Ti, O, F, and C elements are observed and the corresponding photoelectron peaks appear respectively at binding energies of 458.8 (Ti2p), 531 (O1s), 684.4 (F1s), and 285 eV (C1s) (see Figure S2 of Supporting Information). The atomic ratio of Ti:O is about 1:2, which is in a good agreement with the nominal atomic composition of TiO2. The C element is visible due to the residual carbon from precursor solution and the hydrocarbon present in XPS instrument itself. The XPS spectrum of F1s core electrons with the binding energy at 684.4 eV is ascribed to F- ions physically adsorbed on the surface of TiO2.11,30 The previous investigation indicates that the formation of Ti-F bond can significantly lower the surface energy of the (001) facets and make them more stable than (101) facets, thus resulting in the formation of anatase TiO2 single crystals with a large percentage of reactive (001) facets.13a A more detailed analysis indicates that NS4 contains not only Ti, O, F, and C but also Pt elements. Figure S2b of Supporting Information shows high-resolution XPS spectrum of Pt 4f for NS4. The binding energies corresponding to Pt 4f7/2 and Pt 4f5/2 are 70.7 and 74.0 eV, respectively, indicative of metallic Pt. The 3.33 eV difference between the binding energies of Pt 4f7/2 and Pt 4f5/2 peaks is also characteristic of metallic Pt4f states. For many H2-production catalytic reactions, metallic platinum can act as reactive sites and provide much more active sites than other states of platinum such as PtO and PtO2.31 3.3. BET Surface Areas and Pore Size Distributions. Figure 3 shows nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution curves (inset in Figure 3) for the NS1 and NS4 samples. Nitrogen adsorption-desorption isotherms measured on these two samples show hysteresis loops at relative pressures close to unity, indicating the presence of large mesopores (about 15-16 nm), which categorize them as type IV according to IUPAC classification;27 however, the shape of adsorption branches at relative pressures close to unity resemblances somewhat type II,27 suggesting the presence of macropores too (see discussion bellow). The shapes of hysteresis loops are of type H3, associated with mesopores present in aggregates of platelike particles, giving rise to slitlike pores,27 which is consistent with the TEM results (Figure 1). The isotherm for NS1 shows high adsorption at relative pressures (P/P0) approaching 1.0, suggesting the formation of large

Figure 4. UV-vis absorbance spectra (a) and the corresponding photos (b) of suspensions in water for the samples prepared at RPt ) 0 (NS1), 0.5 (NS2), 1 (NS3), 2 (NS4), 4 (NS5), and 6 (NS6) and P25 prepared at RPt ) 2 without F.

mesopores and macropores. In fact, the mesopores and macropores are formed due to aggregation of nanosheets, because the singlecrystal nanosheets are nonporous. Such organized porous structures might be extremely useful in photocatalysis because they possess efficient transport pathways to reactant and product molecules. The pore-size distributions (inset in Figure 3) indicate that NS1 and NS4 exhibit wide pore-size distributions from 2 to 60 nm, further confirming the existence of mesopores and macropores. Table 2 shows that all Pt/TiO2 nanosheet samples have smaller average pore sizes than pure TiO2 nanosheets, which is due to the presence of Pt nanoparticles with sizes of ca. 2-4 nm, resulting in a slight reduction of pore dimensions. Table 2 also indicates that the specific surface areas, pore volume increase and the average pore size slightly decrease with increasing RPt, due to the deposition of Pt nanocrystals. It is easy to understand a decrease in the BET surface area and pore volume of the Pt/TiO2 samples with increasing RPt, because the specific surface area (m2/g) and pore volume (cm3/g) are expressed per gram of the samples, which contain some amount of heavy nonadsorbing Pt. The densities of Pt and TiO2 (anatase) are 21.45 and 3.84 g/cm3, respectively. Consequently, the densities of the Pt/TiO2 samples increase with increasing RPt, resulting in the reduction of the BET surface area and pore volume. However, the observed reduction of the average pore size is caused by two factors related to: (i) the formation of smaller pores due to aggregation of Pt nanoparticles and (ii) the incorporation of Pt nanoparticles into mesopores and macropores. 3.4. UV-Vis and PL Apectra. Figure 4a shows a comparison of UV-vis absorbance spectra of samples NS1, NS2, NS3, NS4, NS5, and NS6. For all the samples, a significant increase in the absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic bandgap absorption of anatase TiO2 (∼3.2 eV). For P25, the absorption edge shifts toward longer wavelengths due to the lower bandgap of P25, which contains

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Figure 5. PL spectra of the samples prepared at RPt ) 0 (NS1), 0.5 (NS2), 1 (NS3), 2 (NS4), 4 (NS5), and 6 (NS6).

about 25% rutile. The Pt loading obviously influences visiblelight absorption of TiO2. Comparing with NS1 (pure TiO2), the absorption spectra of all Pt/TiO2 samples show an enhanced absorption in the visible-light region. Also, the samples show a stronger absorption with increasing RPt. The enhancement of visible-light absorption peak is due to the fact that the incident photon frequency is resonant with the collective excitations of the conduction electrons of noble-metal nanoparticles, called the localized surface plasmon resonance (LSPR).32 Therefore, it is not surprising that the absorption in the visible-light region increases with increasing RPt because of the increase of number and/or amount of Pt nanoparticles. The above TEM and XPS results also confirm that metallic Pt nanoparticles are successfully loaded onto TiO2 nanosheets by photochemical reduction method. It is interesting to note that NS4 and P25 exhibit similar absorption in the visible-light region due to the same content of loaded Pt (RPt ) 2). Figure 4b shows the corresponding photos of all the samples dispersed in water, indicating that their colors enhance from white to black with increasing RPt, further confirming the presence of nanosized metallic Pt in Pt/TiO2 nanosheets. The PL emission spectra are used to investigate the efficiency of charge carrier trapping, migration, transfer and separation and to understand the fate of photogenerated electrons and holes in semiconductor since PL emission results from the recombination of free carriers.10 Figure 5 presents a comparison of the PL spectra of the samples NS1, NS2, NS3, NS4, NS5, and NS6. A considerable fluorescence decrease (or quenching) is observed. The quenching of fluorescence indicates transfer of photogenerated electrons from TiO2 to Pt and the favorable (or close) contact between Pt and TiO2 nanosheets.33 For all the samples, three main emission peaks appear at about 396, 450, and 468 nm, which are equivalent to 3.13, 2.76, and 2.65 eV, respectively. The strongest PL peak at about 396 nm is attributed to the emission of bandgap transition with the energy of light approximately equal to the bandgap energy of anatase (387.5 nm). In addition, there are four small peaks observed in the wavelength range from 440 to 500 nm. These PL signals are due to excitonic PL, which mainly result from surface oxygen vacancies and defects of the TiO2 samples. The PL peaks at 450 and 468 nm are attributed to band edge free excitons, and the other two peaks at 482 and 492 nm are due to bound excitons.34 The band-band PL intensity of pure TiO2 and Pt/ TiO2 samples reveals a significant decrease with increasing RPt. This indicates that the Pt/TiO2 nanosheets have relative low recombination rate of electrons and holes under UV light

Yu et al. irradiation. This is ascribed to the fact that the electrons are excited from the valence band to the conduction band and then migrate to Pt nanoparticles, which prevents the direct recombination of electrons and holes. Generally, low recombination rate of electrons and holes is often associated with high photocatalytic activity.33,34a 3.5. Photocatalytic Activity. Photocatalytic H2 production on various samples was evaluated using ethanol as scavenger. Control experiments indicated no appreciable H2 production in the absence of either irradiation or photocatalyst, suggesting that H2 was produced by photocatalytic reactions on TiO2 samples under UV illumination. Table 2 lists a comparison of photocatalytic activity (or photocatalytic H2-production rate) of the TiO2 samples prepared with varying RPt and Degussa P25 loaded with Pt to achieve RPt ) 2. As can be seen RPt has a significant influence on the photocatalytic activity of the TiO2 samples. At RPt ) 0, NS1 shows a negligible photocatalytic activity (or no photocatalytic activity) because of the rapid recombination between photogenerated electrons and holes in pure TiO2. Also, the presence of a large overpotential in the production of H2 on the TiO2 surface and the fast backward reaction (recombination of hydrogen and oxygen into water) make TiO2 inactive in photocatalytic water splitting in the absence of Pt. Usually, deposition of noble metals on semiconductor surface can greatly enhance the separation of photogenerated electrons and holes, prolong their lifetime, and meanwhile suppress the reverse reaction between O2 and H2, which finally results in enhancement of photocatalytic activity.35 It should be noted that in the presence of a small amount of Pt, the activity of NS2 is markedly enhanced. The photocatalytic activity of the samples increased further with increasing RPt from 0.5 to 2. The highest photocatalytic activity, 333.5 µmol h-1, was achieved for NS4 (RPt ) 2); this value exceeds that of P25. It is well-known that Degussa P25 has an excellent photocatalytic activity in pollutant degradation and photocatalytic H2 production.36 The H2 production rate of Pt-P25 (RPt ) 2) was determined to be 223.4 µmol h-1 at the same experimental conditions as those in the case of NS4. To the best of our knowledge, this is the first report showing that the surfacefluorinated TiO2 nanosheets with exposed (001) facets exhibit excellent photocatalytic activity for water splitting. A further increase of RPt causes a reduction of the photocatalytic activity. This is probably due to increase in the opacity and light scattering of NS5 and NS6 (see Figure 3), leading to a decrease of irradiation passing through the reaction suspension solution.37 Another possible explanations are: (i) partial blockage of the TiO2 surface active sites due to excessive Pt and (ii) deterioration of the catalytic properties of metal nanoparticles due to their enlargement.38 Therefore, it is not surprising that the photocatalytic activity of NS5 and NS6 is smaller. The effects of surface fluorination and morphology on the photoactivity are illustrated in Figure 6, which presents a comparison of this activity for NS4, NS7, and NP. It was shown previously that the adsorbed fluorine ions on the surface of TiO2 can be easily removed by calcination at 600 °C or alkaline washing in a NaOH solution without altering the crystal structure and morphology.13a,16 In this study, the fluorinated TiO2 nanosheets were washed using 0.1 M NaOH solution or distilled water, and then their photocatalytic activity was tested. Experimental results indicate that water washing has no obvious influence on the photocatalytic activity of TiO2 nanosheets. This is not difficult to understand because water washing has no influence on the morphology and surface composition of TiO2 nanosheets. However, when the fluorinated TiO2 nanosheets



Figure 6. Comparison of the photocatalytic production of H2 from ethanol aqueous solutions for NS4 (nanosheets with F and RPt ) 2), NS7 (nanosheets without F and RPt ) 2), and NP (nanoparticles without F and RPt ) 2). Inset shows schematic diagram for generation and transfer of photogenerated e--h+ pairs in F-TiO2 under UV irradiation.

were washed with a 0.1 M diluted NaOH solution, the photocatalytic activity of NS7 drastically decreased (see Figure 6). This is due to the fact that F- ions adsorbed on the surface of TiO2 nanosheets were removed due to the following ligand exchange reaction5

tTi - F + OH- f tTi - OH + F-

(1)

This implies that F- ions play an important role in the enhancement of photocatalytic H2-production activity. This also further confirms that the surface fluorination is beneficial for photocatalytic activity of TiO2 nanosheets and/or photocatalytic H2 production.5,10,30 Our previous work indicates that the Fions on the surface of TiO2 can greatly reduce the recombination rate of photogenerated electrons and holes by acting as an electron-trapping sites to trap the photogenerated electrons due to its strong electronegativity and then transfer electrons to the adsorbed metallic Pt (see the inset of Figure 6).30 The XPS results (not shown here) further confirm the transfer of electrons from F to Pt. After loading Pt on fluorinated TiO2 sheets, the binding energy of the F 1s peak has a slight shift (from 684.26 to 684.44 eV).25a Note that the photocatalytic activity of NS7, consisting of fluorine-free TiO2 nanosheets, is still slightly higher than that of pure TiO2 nanoparticles (NP) because of the presence of (001) facets in the former, which are very reactive due to their special electronic and surface properties.22,23 Table 2 and Figure 6 show that all fluorinated TiO2 nanosheets exhibit much higher photocatalytic activity than Degussa P25 and pure TiO2 nanoparticles prepared in water due to the synergistic effect of surface fluorination and exposed (001) facets on the photoactivity. Photocatalytic production of H2 by water splitting has been regarded as one of most promising approaches since Fujishima and Honda discovered photoelectrochemical formation of H2 over a TiO2 electrode.3 However, because of the reversible reaction of H2 and O2 or the recombination of photogenerated electrons and holes on semiconductor surface, the photocatalytic efficiency is usually low. To achieve higher efficiency of photocatalytic water splitting, many researchers in this field used electron donors as sacrificial agents, which can react irreversibly

with the formed oxygen or photogenerated holes.7 But, if the sacrificial donors are more expensive than the produced H2, the use of electron donors is not of great value. A good strategy is to use organic wastes and pollutants in water as electron donors.7 This will result in production of clean energy source, H2, with simultaneous removal of environmental pollutants. Therefore, to further evaluate photocatalytic H2 production performance using organic pollutants as sacrificial agents, we examined photocatalytic activity of NS4 using glycerol, triethanolamine, and glucose as model electron donors or scavengers; the photocatalytic H2 production rates of these systems were 169.1, 140.2, and 133.6 µmol h-1, respectively, and were lower than those in ethanol, indicating the usefulness of these pollutants as sacrificial agents. This implies that the photocatalytic production of hydrogen and a simultaneous decomposition of pollutants can occur in a system and involves the reduction of photogenerated electrons and oxidation of holes, respectively. However, previous reports mostly involve the use of single electrons (for H2-production) or holes (for decomposition of pollutants). In this study, the synergistic use of photogenerated electrons (for energy production) or holes (for environmental purification) will provide a new insight into photocatalytic activity. Further studies are required to explain the detailed relationship between structure of organic molecules and photocatalytic H2 production activity. 3.6. Hydroxyl Radical Analysis. To understand photocatalytic mechanism and detect the involved active species in the photocatalytic H2 production, hydroxyl radicals (•OH) were detected on the surface of UV-illuminated NS1, NS4, and NS7 in an anaerobic environment by the PL technique using coumarin as a probe molecule. For the purpose of comparison, hydroxyl radicals (•OH) were also detected in pure water for NS1. Figure 7a shows a comparison of the PL spectra of 1 × 10-3 M coumarin aqueous solutions under UV irradiation for 1 h in the absence of O2 for NS1, NS4, and NS7. No PL peak is observed for NS1 in the absence of O2 and Pt. This indicates that no •OH radicals are produced. However, for NS1, in the presence of O2 and in pure water, a great increase in the PL intensity at about 456 nm is observed with increasing irradiation time, indicating the production of •OH radicals (as shown in Figure 7b). The above results imply that the •OH radicals are not the main active species in the absence of O2 and Pt. This can be easily understood by the following suggested mechanism. In the presence of O2 and in pure water, the electrons are excited from the valence band to the conduction band and then transferred to the adsorbed O2 on the surface of TiO2 to form the superoxide radical anions, (O2-•), which prevent the direct recombination of electrons and holes; meanwhile, the holes react with OH-/H2O to produce •OH radicals. So, the •OH radicals are easily detected. However, in the absence of O2, electrons can not react with O2. In this case the rates of electrons and holes recombination are faster than that of the reaction of holes and OH-/H2O. So, it is not surprising that no •OH radicals are observed in the absence of O2 (as shown in Figure 8a). However, for NS4 and NS7, a small PL peak at about 456 nm is observed in the presence of Pt, indicating the production of the •OH radicals. This also implies that Pt is an effective electron acceptor. The production of •OH radicals for NS4 and NS7 can be assigned to the fact that the excited electrons from the valence band to the conduction band can migrate to Pt nanoparticles and then react with H+ ions adsorbed to form H2. The Pt nanoparticles on TiO2 nanosheets produce a Schottky barrier, which facilitate the electron capture (see Figure 8b). Accumulation of holes at the valence band of anatase leads to the

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Yu et al. H2.25c This difference is probably due to different method used for removal of surface fluorine and different experimental conditions. 4. Conclusion Pt/TiO2 nanosheets with exposed (001) facets can be fabricated by hydrothermal treatment of tetrabutyl titanate and hydrofluoric acid mixture followed by the photochemical reduction deposition of Pt nanoparticles on TiO2 nanosheets. The photocatalytic H2-production activity of these nanosheets can be significantly enhanced by loading Pt reaching the highest activity at RPt ) 2. All fluorinated TiO2 nanosheets exhibit much higher photocatalytic activity than Degussa P-25 TiO2 and pure TiO2 nanoparticles prepared in pure water due to the synergistic effect of surface fluorination and exposed (001) facets. The surface-fluorinated Pt/TiO2 nanosheets are also of great interest for solar cells, photonic and optoelectronic devices, sensors, catalysis, biomedical engineering and nanotechnology. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (50625208, 20773097, and 20877061) and by the National Basic Research Program of China (2007CB613302).

Figure 7. (a) PL spectra of 1 × 10-3 M coumarin ethanol aqueous solutions in an anaerobic environment under UV irradiation for 1 h for the samples prepared at RPt ) 0 (NS1), RPt ) 2 (NS4), and RPt ) 2 without F (NS7); (b) PL spectral changes observed during illumination of NS1 prepared at RPt ) 0 in a 1 × 10-3 M coumarin aqueous solution in air and each PL spectrum was recorded every 15 min.

Figure 8. Transfer and separation of photogenerated electrons and holes on the surface of TiO2 in an anaerobic environment and in the absence (a) and presence (b) of Pt.

production of surface hydroxyl radical •OH, which is responsible for the oxidative decomposition of ethanol or other pollutants.33,34a Photogenerated electrons are effectively accumulated on Pt nanocrystal particles without recombining with holes. This causes a significant enhancement of the photocatalytic activity in H2 production.33,34a Further observation indicates that NS4 has a stronger PL peak (or greater formation rate of •OH radicals) than NS7, indicating that the surface fluorination can enhance the transfer of photogenerated electrons (see inset of Figure 6), reduce the recombination of photogenerated electronhole pairs, and prolong their lifespan, which are beneficial for the enhancement of photocatalytic activity. Our previous investigation also indicates that the formation rate of •OH radicals on F-TiO2 is much greater than that on pure TiO2.15,30 Note that this result differs from the recent report by Liu et al. because the latter refers to the removal of the surface fluorine species by calcination at 600 °C resulting in a substantially enhanced activity for generating •OH radicals and producing

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JP104488B

Hydrogen Production by Photocatalytic Water Splitting over Pt

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