Modulating Photogenerated Electron Transfer and Hydrogen

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Modulating Photogenerated Electron Transfer and Hydrogen Production Rate by Controlling Surface Potential Energy on a Selectively Exposed Pt Facet on Pt/TiO2 for Enhancing Hydrogen Production Entian Cui†,‡ and Gongxuan Lu*,† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 10080, China ABSTRACT: We reported the results of the modulation of photogenerated electrons transfer and photocatalytic hydrogen evolution behaviors of Pt/TiO2 photocatalyst via controlling surface potential energy on a selectively exposed Pt facet for a highly efficient photocatalytic hydrogen generation from water. By photosensitization using Eosin Y as an antenna molecule, distinct differences in photocatalytic hydrogen evolution performances over Pt/TiO2 with different exposed facets ({100}, {100/111}, and {111}) of Pt under visible light irradiation were observed. Pt{111}/TiO2 photocatalyst exhibited a much higher photocatalytic hydrogen generation activity than those of Pt{100}/TiO2 and Pt{100/111}/TiO2. As evidenced by photoluminescence spectra, photoelectrochemical characterizations, electrochemical impedance spectra (EIS) measurements, and Mott−Schottky measurements, Pt nanoparticles with exposed {111} facets were more effective in trapping the electrons from the conduction band of TiO2 than that of {100} facets due to their higher Fermi level of {111} facets. In addition, Pt{111}/TiO2 exhibited much lower apparent activation energy for hydrogen generation than those of other samples because the fraction of Pt atoms located on edges and corners on Pt{111} nanoparticles was higher than that on Pt{100} nanoparticles. Therefore, Pt{111}/TiO2 can provide more reaction sites for water reduction. In addition, Pt{111}/TiO2 exhibits much lower apparent activation energy or hydrogen generation than those of other samples because this catalyst can provide more reaction sites for water reduction. The formation of hydrogen via recombination between chemisorbed H atoms is more likely to occur over Pt{111} facets because of the reasonable transition state geometry of chemisorbed H on Pt{111} facets. This study discloses the facet-dependent effect of noble-metal cocatalyst on semiconductors in photocatalytic water reduction and will give an insight into design and synthesis of high-efficient metal/semiconductor hybrid photocatalysts.

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deposition method3 of Pt had a significant effect on the photocatlytic activity of Pt/semiconductor catalysts. The surface structure of deposited Pt nanoparticles is also an important factor affecting these photocatalytic activities. It is known that the hydrogen evolution reaction (HER) in electrochemical catalysis over Pt nanoparticles is structuresensitive. For example, Conway and co-workers found that the kinetics of HER in alkaline electrolytes varied with crystal facets of Pt(hkl). These activity for the HER in alkaline solution increased in the sequence {100} < {111} < {110}, which resulted from the ΔHads differences of Pt for hydrogen overpotential (opd H) over different facets and the interatomic distance of nearest neighboring active sites on Pt.14 However, Marković found that the HER activity in alkaline solution followed another sequence {111} < {100} < {110}, and the activity was directly related to

INTRODUCTION Photocatalytic water splitting for hydrogen production over semiconductor photocatalysts driven by sunlight irradiation is one of the most promising ways to produce renewable energy.1−4 However, the fast recombination rate of photogenerated electrons and holes is one of the main drawbacks that restrict the photocatalytic efficiency for hydrogen generation. Commonly, to suppress the charge recombination for enhancing the hydrogen evolution efficiency, loading noble metals such as Pt, Pd, Au, Ag, etc. on semiconductor surfaces as cocatalyst is necessary,5−8 and Pt has been proven to be the most active cocatalyst material due to its lower overpotential for hydrogen production and suitable Femi level for accepting photogenerated electrons.9 Li and co-workers reported an extremely high QE of 93% over Pt- and PdS-coloaded CdS for hydrogen evolution in the presence of Na2S and Na2SO3 as sacrificial reagents.10,11 But the metal Pt is too expensive to be used widely. Thus, it is of importance to utilize Pt as a cocatalyst more efficiently. Studies have proven that the introduction order,12 loading amount,13 and © 2013 American Chemical Society

Received: June 2, 2013 Revised: November 26, 2013 Published: November 26, 2013 26415

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nanoparticles as Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2, respectively. The loading amount of Pt for all three samples was 0.5 wt %. Photocatalytic Hydrogen Production Experiments. The photocatalytic activity test was carried out at a constant temperature of 20 °C. Photocatalytic hydrogen evolution experiments were performed in a 190 mL quartz flask with a flat window. The reactor was sealed with a silicone rubber septum for sampling. A 300 W xenon lamp equipped with a 420 nm cutoff filter was used as a light source. The reactant mixture was prepared by dispersing 100 mg of powder photocatalyst and 35.7 mg of Eosin Y (EY) in 100 mL of aqueous triethanolamine (TEOA, 15(v/v) %, pH 7) solution under ultrasonication and was purged with argon gas for 40 min before irradiation. During the photocatalytic process, 0.5 mL gas sample in the top of reactor was collected intermittently through the septum and the amount of hydrogen evolution was measured with gas chromatography (Aglient 6820, TCD, 13× column, Ar carrier). The rate constant of the reaction was determined at four different temperatures (0, 20, 40, and 50 °C). From the slope of the polts of ln K against 1/T, where T is the absolute temperature, the activation energy (Ea) of the reaction was determined (Ea = −slope/R). The apparent quantum efficiencies (AQEs) were measured under same photocatalytic reaction conditions with irradiation light through a band-pass filter (400, 430, 460, 490, and 520 nm). The photon flux of incident light was determined using a Ray virtual radiation actinometer (FU 100, silicon ray detector, light spectrum, 400−700 nm; sensitivity, 10−50 μV μmol−1·m−2·s−1). The AQEs were calculated from the ratio of the number of reacted electrons during hydrogen evolution to the number of incident photons according to the following equation:

the difference in activation energies, which was affected by underpotential deposition of hydrogen (Hupd) coverage.15 These different activity sequences in term of Pt exposed facets probably resulted from the specific experimental conditions such as pH, surface coverage, and the type of electrolyte anions.16 Norskov’s group found that the calculated activation barriers for HER on Pt{111} and Pt{100} surface were similar, about 0.85 eV at the equilibrium potential.17 It was verified by Liu’s group that the HER activities over Pt{111}and Pt{100} were almost same.18 That means that the differences of HER activities can be attributed to the different Pt single-crystal surface structures and the coverage of opd H. For example, the Hoshi group found that the exchange current density increased linearly with the increase of the step density, and they concluded that the stepped sites are active site for HER.19−21 Consider the different reported data of facet dependence of Pt on HER activities, to reveal the effect of the different exposed facet of Pt nanoparticles on hydrogen evolution from photocatalytic water splitting should be an important objective. In the present work, Pt samples with different facets were prepared and deposited on TiO2. By carefully comparing the photocatalytic activity difference of HER over Pt{100}/TiO2, Pt{100/111}/ TiO2, and Pt{111}/TiO2 samples, we found that the exposed facets of Pt nanoparticles over TiO2 affected significantly the electron transfer direction and hydrogen generation rate. Pt{111}/TiO2 photocatalyst exhibited the highest photocatalytic activity compared with those of Pt{100}/TiO2 and Pt{100/ 111}/TiO2 samples. The enhanced photocatalytic activity of Pt{111}/TiO2 resulted from the faster electrons transfer during hydrogen formation. The Fermi level of Pt{111} facets was more positive than that of Pt{100} facets, which led to the faster electron transfer from TiO2 to Pt nanoparticles. Therefore, the photocatalytic activity of Pt/TiO2 could be modulated by controlling the exposed facets of Pt nanoparticels.

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AQEs =

EXPERIMENMTAL SECTION Preparation of Pt Nanoparticles with Different Exposed Facets. Pt nanoparticles with different exposed facets were prepared by a reported colloidal method.22 Sodium polyacrylate (PA, Mw ≈ 5100) and K2PtCl4 were used as capping agent and metallic precursor, respectively. In a typical process, sodium polyacrylate was first added into 100 mL of K2PtCl4 (10−4 M) aqueous solution. The ratios of K2PtCl4 to PA were changed from 0.083 to 0.017 and 0.008 to obtain cubic Pt{100}, hexagonal Pt{100/111}, and tetrahedral Pt{111} nanoparticles, respectively. Then, this mixture solution was purged with highpurity Ar gas for 20 min to remove the dissolved oxygen followed by bubbling with H2 gas for 5 min to reduce the Pt precursor. Finally, the vessel was sealed for 24 h without stirring. After that, the obtained Pt nanoparticles were cleaned with strong basic aqueous solution and washed very carefully with water before using. Loading Pt Nanoparticles on TiO2 Photocatalyst. To prepare Pt modified TiO2 photocatalysts (Pt/TiO2), Pt nanoparticles with different exposed facets were loaded on TiO2 surface by a wet-absorption process as follows: (1) In 50 mL of deionized water, a certain amount of as-prepared Pt nanoparticles was added and ultrasonicated for 10 min; (2) TiO2 (P25) was dispersed into the above-mentioned Pt suspensions under ultrasonication and kept stirring for 24 h; (3) the mixture was filleted and washed three times with deionized water and methanol in sequence and finally annealed at 423 K for 2 h. The obtained products were labeled according to the used Pt

2 × no. of evolved hydrogen molecules × 100% no. of incident photons

Working Electrode Preparation and Photoelectrochemical Measurement. Photocurrent responses of photocatalyst samples were measured using an electrochemical analyzer (CHI660A) in a homemade standard three-compartment cell. Platinum foil was used as counter electrode and a saturated calomel electrode (SCE) as the reference electrode. For the preparation of working electrodes for electrochemical measurements, a homogeneous catalyst ink was first prepared by dispersing 4 mg of catalyst material and 80 μL of 5 wt % Nafion solution in 2 mL of H2O by ultrasonication. Then 400 μL of catalyst ink dispersions were drop-coated directly onto the precleaned indium tin oxide (ITO) glass surface (ca. 2 cm2) by microsyringe and dried under an infrared heat lamp. The geometrical surface areas of working electrode exposed to the electrolyte was a circular film of 1.6 cm2. TEOA solution (15 vol %, pH 7) was used as supporting electrolyte. A 300 W xenon lamp with optical cutoff filter (λ ≥ 420 nm) was used for excitation light. Electrochemical impedance spectra (EIS) measurements were also carried out in the above-mentioned three-electrode system and recorded over a frequency range of 0.005−105Hz with ac amplitude of 10 mV at 0.5 V. The Mott−Schottky plots were obtained at a fixed frequency of 1 kHz in the dark. Voltammograms were carried out at room temperature in the above-mentioned three-electrode system, which was deaerated by using Ar (99.99%). The electrolyte was 0.5 M H2SO4 solution. The active surface area of the Pt nanoparticles was determined by 26416

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Figure 1. TEM images and corresponding size distribution profiles of Pt{100} (A, D), Pt{100/111} (B, E), and Pt{111} (C, F) nanoparticles, respectively. The insets in panels A−C were the schematic diagram of Pt nanoparticles.

Figure 2. TEM and HRTEM images of Pt{100}/TiO2 (A, D), Pt{100/111}/TiO2 (B, E), and Pt{111}/TiO2 photocatalysts (C, F), respectively.

voltage of 300 kV. Photoluminescence spectra were determined by a FluoroMax-4 spectrofluorometer spectrometer. Nano LED diode emitting pulses at 460 nm with 1 MHz repetition rate and pulse duration of 1.3 ns was used as an excitation source. UV− visible DRS spectra were obtained on a HITACHI U-3310 spectrophotometer equipped with an integrating sphere accessory (BaSO4 was used as a reference).

measuring the charge involved in the so-called hydrogen adsorption region assuming 230 μC·cm−2 for the total charge after the subtraction of the double layer charging contribution recorded in 0.5 M H2SO4 solution. However, all potentials were corrected to a “constant-temperature” scale, and were referenced to the reversible hydrogen electrode at 1 atm of hydrogen at the same temperature in the same electrolyte. The Mott−Schottky plots were obtained at a fixed frequency of 1 kHz in the dark. Characterization. Transmission electron microscopy (TEM) images were taken with a Tecnai-G2-F30 field emission transmission electron microscope operating at accelerating

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RESULTS AND DISCUSSION A series of Pt nanoparticles with different exposed facets were prepared according to the method described previously.22 The 26417

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Figure 3. Voltammetric profiles for (A) Pt{100}/TiO2, (B) Pt{100/111}/TiO2, and (C) Pt{111}/TiO2 nanoparticles in 0.5 M H2SO4. Sweep rate: 50 mV s−1.

predominantly hexagonal, suggesting the coexistence of {100} and {111} Pt surface domains. The lattice spacing of 0.23 nm was observed on the HRTEM image (Figure 2E), corresponding to the (111) planes of Pt, which proved that there were {111} existing in hexagonal Pt nanoparticles. The particle size was 6.0 ± 1.5 nm (Figure 1E). Figure 1C showed a representative TEM image of the Pt{111} nanoparticles. The vast majority of Pt nanoparticles with tetrahedral and octahedral shapes were observed. In this case, the shape suggested the presence of a preferential {111} surface structure, corresponding to the lattice spacing of 0.23 nm (shown by Figure 2E). The Pt particle size of this sample was 6.5 ± 2.0 nm (Figure 1F). In addition, the images of Figure 2A−C showed that the Pt nanoparticles were in good dispersion on the surface of TiO2 particles. HRTEM images of Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 indicated that shapes of Pt nanoparticles loaded on TiO2 had no obvious change after calcinated at 423 K.23 Figure 3 showed the characteristic voltammetric profiles obtained for the preferentially oriented Pt nanoparticles in 0.5 M sulfuric acid as supporting electrolyte. The sharpness and the symmetry of the voltammetric peaks were clear evidence of the surface cleanliness. It can be seen from the Figure 3A that there were two well delineated peaks at 0.38 and 0.24 V corresponding to the coupling of hydrogen adsorption with the bisulfate anion desorption on (100) terrace sites and n(100) × (111) step sites, respectively;24 that was, these two peaks were the

exposed facets of Pt nanoparticles were controlled by varying the Pt/PA ratio. Rapid initial reduction of Pt4+ formed the stable tetrahedral nanoparticles nuclei. At high polymer concentration (Pt/PA = 0.008), PA capped the tetrahedral nanoparticles, preventing the rapid growth of tetrahedral nuclei. PA also acted as a good buffer. It did not allow for an increase of [H3O+] and thus protected the capped tetrahedral nanoparticles from decapping, so Pt tetrahedral nanoparticles were obtained. At low polymer concentration (Pt/PA = 0.083), the buffer action was not high, which led to the removal of the polymer from the tetrahedral nanoparticles surface, allowing its further growth. Because the {111} facets were more catalytically active, the reduction of the Pt4+ into Pt atoms on this facet converted tetrahedral to hexagonal nanoparticles and then to cubic nanoparticles. Because the work function on Pt{100} was lower than that on Pt{111} by 0.5 eV, PA adsorbed strongly on Pt(100) facets, preventing the rapid growth of Pt{100} facets, and thus Pt cubic nanoparticles were obtained. Figures 1 and 2 separately gave the typical TEM and HRTEM images of Pt nanoparticles and Pt/TiO2 as-prepared. As can be observed from Figure 1A, a preferential cubic shape was obtained, which suggested the existence of a {100} preferential surface structure, which was confirmed by HRTEM characterization (Figure 2D). The lattice spacing parallel to the top and bottom facets was ca. 0.19 nm, corresponding to the (100) planes of cubic Pt. The particle size was 4.7 ± 1.6 nm (Figure 1D). The shape of the Pt nanoparticles shown in Figure 1B was 26418

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intensity on the photocatalytic performance on three samples was the same. We reported previously that the Eosin Y-sensitized TiO2 and RGO/Pt photocatalysts showed a high AQE of 9.3% for H2 evolution under visible light irradiation.27−29 Figure 5 showed the average rate and AQEs of hydrogen evolution catalyzed by Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 sensitized by Eosin Y as a photosensitizer under visible light irradiation. Domen et al. found that the smaller Rh cores gave higher activity than the larger ones, which can be attributed to the active area variation as the size of Rh nanoparticles varying.30 To demonstrate the facet effect more clearly, the normalized activities based on the electrochemical active surface area of Pt{111}/TiO2 sample were presented, as shown in Figure 6. Control experiments revealed that no hydrogen gas could be detected in the absence of either photosensitizer or light irradiation, suggesting that this reaction was photocatalytically under visible light irradiation. The EY-Pt{111}/TiO2 exhibited the highest initial hydrogen evolution rate (91.94 μmol·h−1· cm−2) in comparison with those of EY-Pt{100}/TiO2 (64.78 μmol·h−1·cm−2) and EY-Pt{100/111}/TiO2 (47.51 μmol·h−1· cm−2) (Figure 6A). These results indicated clearly that there was an effect of faceting of Pt nanoparticles on Pt/TiO2 samples. This difference was further confirmed by investigating wavelength dependence of photocatalytic hydrogen evolution, as shown in Figure 6B. It was apparently that EY-Pt{111}/TiO2 showed the highest AQEs for hydrogen evolution compared with those of EY-Pt{100}/TiO2 and EY-Pt{100/111}/TiO2 in the visible light range from 400 to 520 nm. The AQEs for all three photocatalysts first increase and then decrease with increasing wavelengths and the optimal wavelength capable of obtaining the highest AQE was found to be 460 nm. A maximum AQE of 28.5% was obtained under irradiation at 460 nm for EY-Pt{111}/TiO2, which was higher than those of EY-Pt{100}/TiO2 (18.8%) and EY-Pt{100/111}/TiO2 (23.4%). The photogenerated electron transfer and electron−hole recombination characteristics on the above photocatalysts were studied by photoluminescence spectra (PL) and fluorescence decay spectra. It should be noted that for all three photocatalytic systems mentioned above, the interaction between EY and TiO2 was considered to be the same. The photoluminescence and fluorescence decay characterization were obtained on Pt{100}/ TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 samples. The results were shown in Figure 7 and Table 2. As shown in Figure 7A, the intensity of PL decreased in the order Pt{100}/TiO2 < Pt{100/111}/TiO2 < Pt{111}/TiO2. Because the higher PL intensity represented the higher recombination efficiency of photogenerated electron−hole pairs,31 Pt{111}/TiO2 sample had the smallest PL intensity, indicating that Pt{111} nanoparticles could inhibit efficiently the recombination of photogenerated electron−hole pairs. To further probe the performance of photogenerated charges, we investigated the fluorescence decay behaviors of Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2. As shown in Figure 7B and Table 2, the lifetime of photogenerated electrons on the Pt{111}/TiO2 was much shorter (0.74 ns) than those on Pt{100/111}/TiO2 (1.39 ns) and Pt{100}/TiO2 (1.68 ns). The shorter lifetime was likely due to the faster transfer of photogenerated electrons and the faster reaction of photogenerated electrons at active sites on the Pt nanoparticles surface. Thus, it was reasonable to conclude that the significant difference in lifetime might mainly be attributed to the different electron transfer rates from TiO2 to Pt nanoparticles. To take into account the difference in these samples,

characteristic peaks of Pt{100}. Thus, the voltammetric profile clearly pointed out that these Pt nanoparticles had a (100) preferential surface structure and that a large part of these (100) sites were presented at the surface. The adsorption state around 0.5 V was observed at Figure 3B, which corresponded to the adsorption/desorption of bisulfate anions25 and was much more clearly than that in the previous cases. This feature was directly related to the presence of bidimensionally ordered (111) domains. The sharp peaks at 0.24 V was presented with a similar intensity in the voltammetric profile. These voltammetric data indicated that the surface structure of this type of Pt nanoparticles was mainly formed by (100) and (111) surface domains. There was also a peak at 0.5 V in Figure 3C. It was important to note that the peak at 0.25 V was very small. Therefore, the surface structure of this type of Pt nanoparticles was mainly formed by (111) surface domains. The Pt electrochemical active surface areas of Pt{100}/TiO2, Pt{100/ 111}/TiO2, and Pt{111}/TiO2 samples were 0.035, 0.040, and 0.034 cm2, respectively, which were obtained from the surface integrals of CV curves.25,26 The quantitative analysis of the site distribution for the different nanoparticles was measured by using Bi and Ge irreversible adsorption, and the results were presented in Table 1. These results were in good agreement with the qualitative analysis of the voltammograms. Table 1. Site Distribution on the Different Pt/TiO2 Samples Determined by Irreversibly Adsorbed Bi and Ge samples

(100) ordered domains/%

(111) ordered domains/%

Pt{100}/TiO2 Pt{100/111}/TiO2 Pt{111}/TiO2

24 43 16

8 38 46

Figure 4 gave the UV−vis diffuse reflectance absorption spectra of EY-Pt{100}/TiO2, EY-Pt{100/111}/TiO2, and EY-

Figure 4. UV−vis diffuse reflection spectra of EY-Pt{100}/TiO2, EYPt{100/111}/TiO2, and EY-Pt{111}/TiO2 samples.

Pt{111}/TiO2 samples. The three samples all had an obvious adsorption in the visible light range, which was caused by the adsorbed EY molecules. There were no obvious light absorption difference between EY-Pt{100}/TiO2, EY-Pt{100/111}/TiO2, and EY-Pt{111}/TiO2 samples. Therefore, the effect of light 26419

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Figure 5. (A) H2 evolution rates of Eosin Y (4.0 × 10−4 M)-sensitized Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 photocatalysts from 100 mL of 15% (v/v) TEOA aqueous solution under visible light irradiation (λ ⩾ 420 nm). (B) Apparent quantum efficiencies (AQEs) of hydrogen evolution for the above three photocatalysts under light irradiation with different wavelengths. Light source: 300 W Xe lamp with either a cut-off filter of 420 nm or a band-pass filter.

Figure 6. (A) Normalized H2 evolution rates of Eosin Y (4.0 × 10−4 M)-sensitized Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 photocatalysts from 100 mL of 15% (v/v) TEOA aqueous solution under visible light irradiation (λ ⩾ 420 nm). (B) Normalized apparent quantum efficiencies (AQEs) of hydrogen evolution for the above three photocatalysts under light irradiation with different wavelengths. Light source: 300 W Xe lamp with either a cut-off filter of 420 nm or a band-pass filter.

Pt{100/111}/TiO2 (curve e) and Pt{111}/TiO2 (curve d) electrodes under light irradiation, indicating the introduction of Pt{111} nanoparticles benefited the charge transfer; that was, Pt{111} facets were more effective in trapping the electrons from the TiO2 conduction band. Additionally, we have conducted the Mott−Schottky experiments for Pt{100}/TiO2, Pt{100/111}/ TiO2, and Pt{111}/TiO2 samples, as shown in Figure 10. All three samples showed a positive slope in the Mott−Schottky plots, as expected for the n-type semiconductor. Importantly, the Pt{111}/TiO2 sample exhibited the smallest slopes of the Mott− Schottky plot compared with Pt{100}/TiO2 and Pt{100/111}/ TiO2, suggesting that the photogenerated electrons had the biggest charge transfer rate, which led to the smallest electron− hole recombination rate.32 These results further confirmed that compared with Pt{100} and Pt{100/111} nanoparticles, Pt{111} nanoparticles could promote the charge transfer faster and thus significantly enhance the photocatalytic hydrogen production activity.

the different electron transfer rates may result from the different exposed facets of Pt nanoparticles. To provide additional evidence for the above inference about the relationship between exposed facets of Pt nanoparticles and photogenerated electron transfer, the transient photocurrent responses of EY-sensitized Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 electrodes coated on ITO were recorded for several on−off cycles under visible light irradiation. The results were shown in Figure 8. It was apparently that the photocurrent of EY-Pt{111}/TiO2/ITO electrode was the largest, followed by EY-Pt{100/111}/TiO2/ITO electrode and then the EYPt{100}/TiO2/ITO electrode, indicating the fastest photogenerated electron transfer occurred on the interface of Pt{111} and TiO2. Electrochemical impedance spectra (EIS) analysis was a powerful tool in studying the charge transfer process. The EIS Nynquist plots of the three samples were shown in Figure 9. The Pt{111}/TiO2 sample (curve f) showed the smallest semicircle in the middle-frequency region in comparison with those of 26420

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Figure 7. (A) Photoluminescence spectra (excitation wavelength: 280 nm) of Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2. (B) Fluorescence decay profiles of different Pt/TiO2 samples excited at 280 nm.

Table 2. Parameters of Fluorescence Decays of Pt{100}/ TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 Samples systems

lifetime τ (ns)

χ2

Pt{100}/TiO2 Pt{100/111}/TiO2 Pt{111}/TiO2

1.68 1.39 0.74

1.01 1.04 1.11

Figure 9. EIS Nyquist plots of Pt/TiO2 film electrodes. The curves of a− f represent Pt{100}/TiO2 (dark), Pt{100/111}/TiO2 (dark), Pt{111}/ TiO2 (dark), Pt{100}/TiO2 (light), Pt{100/111}/TiO2 (light), and Pt{111}/TiO2 (light), respectively.

Figure 8. Transient photocurrent−time profiles of EY (4.0 × 10−4 M)sensitized Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 coated on ITO glass in 15% (v/v) TEOA at pH 7 under chopped light irradiation of 420 nm.

The stability test of EY-Pt{100}/TiO2, EY-Pt{100/111}/ TiO2, and EY-Pt{111}/TiO2 catalysts has been carried out, and the result was shown in Figure 11. It was found that after 5 h reaction, the hydrogen production over three photocatalysts all reached a maximum. And at the same time, the dye solution bleached with increasing reaction time. The same phenomenon was also reported in a previous study.33 The hydrogen evolution activity of these three catalysts can be revived by the concurrent addition of dye and TEOA, indicating that the decomposition of sacrificial donor was responsible for the activities decline. Therefore, the Pt/TiO2 catalysts were rather stable during the photocatalytic hydrogen evolution process The different photogenerated electron transfer rates between Pt{100}/TiO2 and Pt{111}/TiO2 were mainly owing to the

Figure 10. Mott−Schottky plots of Pt{100}/TiO2, Pt{100/111}/TiO2, and Pt{111}/TiO2 samples.

difference of energy levels of Pt{100} and Pt{111} facets.34 The Fermi level of Pt{111} and Pt{100} facets were about −1.29 and −1.95 eV, respectively;35,36 that was, the difference between 26421

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Figure 11. Stability tests of EY-sensitized Pt{100}/TiO2, Pt{100/111}/ TiO2, and Pt{111}/TiO2 photocatalytic systems in 100 mL of 15% (v/ v) TEOA aqueous solution under visible light irradiation (λ ⩾ 420 nm). The reaction was continued for 28 h, with evacuation every 7 h: adding EY and TEOA after every evacuation. The system was irradiated by a 300 W Xe lamp with an optical cut-off filter of 420 nm.

Figure 13. Arrhenius plots obtained from EY (4.0 × 10−4 M)photosensitized systems catalyzed by Pt{100}/TiO2, Pt{100/111}/ TiO2, and Pt{111}/TiO2 100 mL of 15% (v/v) TEOA aqueous solution under visible light irradiation (λ ⩾ 420 nm) under different temperatures.

Pt{111} facet Fermi level and TiO2 conduction band was 0.66 eV larger than that between the Pt{100} facet and TiO2, which suggested that electron transfer from the conduction band of TiO2 to {111} facets were more feasible thermodynamically (Figure 12, solid curve), leading to the faster accumulation of

different bond enthalpies, desorption energies, and adsorption geometries. Pt{111} nanoparticles had more surface atoms at corners and edges in comparison with Pt{100} and Pt{100/111} nanoparticles; that was, the number of coordinative unsaturated atoms on Pt{111} nanoparticle surface was the largest,37,38 so the apparent energy of Pt{111}/TiO2 was the lowest. Marković previously found decreasing of the cocatalyst size would induce additional positive effects because decreasing particle size increased the number of coordinative unsaturated atoms.39 HRE was known to be a structure-sensitive reaction, so it was strongly conceivable that the increase of the number of unsaturated atoms and/or facets with high surface energy accelerate the rate of hydrogen evolution over smaller cocatalyst nanoparticles.15,24,31,40,41 Domen demonstrated that in Rh/ Cr2O3/(Ga1−xZnx)(N1−xOx) catalyst, smaller Rh cores gave a higher activity than the larger ones because of the increase of active sites for hydrogen evolution.31 Likewise, the number of active sites on the Pt nanoparticles surface also had positive effects on the Pt/TiO2 photocatalytic activity. The Pt{111} nanoparticles had sharp edges and corners. It was expected that atoms at these locations were likely to be chemically active, which were considered to be photocatalytic active sites. The fraction of atoms at the edges and corners of tetrahedral Pt nanoparticles was ca. 35%, making it the highest photocatalytic active. However, for cubic Pt nanoparticles, most surface atoms were located on {100} facets, which were known to be the least active, and the fraction of atoms at edges and corners was only ∼6% (the size of cubic nanoparticle being similar to that of tetrahedral nanoparticles),41 resulting in the number of chemisorbed H atoms on Pt{111} facets much more than these on Pt{100} facets. HER involves the following four processes: (1) chemisorbed H production by water reduction; (2) H diffusion; (3) H2 production by chemisorbed H recombination; (4) H 2 desorption. As mentioned above, the chemisorbed H was produced on Pt{111} and Pt{100} facets, and the number of chemisorbed H atoms on Pt{111} facets was much more than that on Pt{100} facets. The chemisorbed H recombination (H− H recombination) was the rate-determining step.24,42 The distance (dH−H) between neighboring chemisorbed H atoms

Figure 12. Schematic diagram of different energy levels for Pt{100} (solid curve) and Pt{111} facets (dotted curve).

electrons on the {111} facets for water reduction. Overall, the exposed facets of Pt nanoparticles had great influence on electron transfer and the recombination processes of photogenerated charges and thus affected the photocatalytic hydrogen evolution activity. The activation energy was then obtained from the slope of the linear ln k vs 1000/T dependence (Ea = −slope/R). As can be seen in Figure 13, the apparent activation energy was the smallest for the EY-Pt{111}/TiO2 sample (7.4 kJ/mol) and the highest for the EY-Pt{100}/TiO2 sample (12.3 kJ/mol). The apparent activation energy of the EY-Pt{100/111}/TiO2 sample was intermediate (10.8 kJ/mol). The surface atomic arrangement of Pt nanoparticles also had an important impact on hydrogen formation. The lower apparent activation energies obtained with the Pt{111}/TiO2 sample may be attributed to an increase of surface atoms on corner and edge sites. Edge and corner atoms exhibited open coordination sites that may result in significantly 26422

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Figure 14. (A) Schematic diagram of H2 on neighboring P atoms. (B) Chemisorbed H atoms connecting on the Pt{100} facet. (B-1) Atomic configurations of chemisorbed H atoms connecting on the Pt{100} facet. (C) Chemisorbed H atoms connecting on the Pt{1111} facet. (C-1) Atomic configurations of chemisorbed H atoms connecting on the Pt{111} facet. Color code: H, white spheres; Pt, blue spheres.

was an important influencing factor on the recombination process besides the binding energy for H on the metal surface and the diffusion energy barrier of chemisorbed H.14 dH−H was determined by the surface geometry of Pt nanoparticles. The H− H recombination probably occurred only when the distance between the neighboring chemisorbed H was within a reasonable interval (dH−H < 1.8 Å).43 Based on the bond lengths of rPt−Pt (2.77 Å), rPt−H (1.70 Å), and rH−H (0.90 Å),44 the model of H2 molecular chemisorbed on neighboring Pt atoms was created (Figure 14A), and ∠HPtPt was calculated to be 57.8°. On the basis of the calculated result, the probable models of chemisorbed H connecting the symmetric sites of Pt{100} and Pt{111} were presented in Figure 14B,C, which also represented the possible geometry of neighboring chemisorbed H. There were three different sites of the Pt{111} surface, namely a top site (above a Pt atom), a four-hollow site (between four Pt atoms), and a bridge site (between two Pt atoms), and four different sites of the Pt{100} surface, a top site (above a Pt atom), a four-hollow site (between four Pt atoms), and two bridge sites (between two Pt atoms). Figure 14B-1 and Figure 14C-1 showed the atomic configurations geometry of chemisorbed H atoms connecting on the Pt{100} facet and the Pt{111} facet, respectively. The angles between the chemisorbed H atom and Pt surface were 57.8° (four-hollow site and bridge site) and 90.0° (top site). Table 3 showed the calculated value of dH−H of probable H−H recombination models. The H−H recombination on the atop sites and hollow sites was spatially unfavorable for the dH−H being larger than 1.8 Å (d38−39, d40−41, d42−43, and d44−45 for Pt{100} facet and d37−38, d41−42, d42−43, and d41−43 for Pt{111} facet), relative to the neighboring sites (d36−37 for Pt{100} facet and d35−36 and d39−40 for Pt{111} facet). Compared with the Pt{100}facet, there were more suitable adsorption sites on the

Table 3. Distance between Two Chemisorbed H Atoms Connecting the Neighboring Pt Atoms of Pt{100} Facet and Pt{111} Facet crystal facet

rPt−Pt (Å)

angle (deg)

H atoms

dH−H (Å)

Pt{100} facet

2.77 3.92 4.80 3.92 2.77 2.77 3.92 2.77 2.77 3.92

57.8 57.8 57.8 90.0 90.0 57.8 57.8 57.8 90.0 90.0

A36−37 A38−39 A40−41 A42−43 A44−45 A35−36 A37−38 A39−40 A41−42 A41−43

0.96 2.04 3.05 3.92 2.77 0.96 2.04 0.95 2.77 3.92

Pt{111} facet

Pt{111} facet for H−H recombination, which was related to the special atoms arrangement of Pt{111} facet. In the view of statistics, the H−H recombination was more likely to proceed on Pt{111} facet, indicating that transition state geometry affect significantly the photocatalystic activity of Pt/TiO2 photocatalyst.

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CONCLUSION In this study, we have demonstrated that the exposed facet of Pt nanoparticles supported on TiO2 has significant influence on the photocatalytic activity of HER in a dye-sensitized photocatalytic system under visible light irradiation. EY-Pt{111}/TiO2 sample exhibited the highest photocatalytic hydrogen evolution activity (203.8 μmol·h−1), followed by EY-Pt{100/111}/TiO2 sample (160.7 μmol·h−1), and then EY-Pt{100}/TiO2 sample (107.7 μmol·h−1), and the same sequence as the order of PL intensity 26423

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and photocurrent. Pt nanoparticles with exposed {111} facets was more effective in trapping electrons from TiO2 conduction band than those of {100/111} and {100} facets. The difference between Pt{111} facet Fermi level and TiO2 conduction band was 0.66 eV larger than that between Pt{111} facet and TiO2, indicating that the photogenerated electrons transfer from TiO2 to Pt{111} facet was more feasible thermodynamically. Pt{111}/ TiO2 (7.4 kJ·mol−1) exhibited much lower apparent activation energy for hydrogen generation than those of Pt{100/111}/ TiO2 (10.8 kJ·mol−1) and Pt{100}/TiO2 (12.3 kJ·mol−1) due to a larger number of unsaturated Pt atoms at the corners and edges sites of Pt{111} nanoparticles, which provided more reaction sites for water reduction. The recombination of chemisorbed H atoms was the rate-determining step for HER, which was more likely to occur on Pt{111} facets for its reasonable transition state geometry of chemisorbed H on Pt{111} facets. This study revealed the possibility of improving the photocatalytic activity by designing the cocatalyst with special facets.

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AUTHOR INFORMATION

Corresponding Author

*G. Lu: tel, +86-931-4968 178; fax, +86-931-4968 178; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the 973 and 863 Programs of the Department of Sciences and Technology of China (2013CB632404, 2012AA051501, 2009CB22003) and the NSF of China (grant no. 21173242), respectively.

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