Interface Charge Transfer versus Surface Proton Reduction: Which Is

May 13, 2015 - University of Chinese Academy of Sciences, Yuquan Road 19A, Beijing ... alkaline solution; they thought it might be due to the ΔHads ...
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Interface Charge Transfer versus Surface Proton Reduction: Which Is More Pronounced on Photoinduced Hydrogen Generation over Sensitized Pt Cocatalyst on RGO? Zhen Li,†,‡ Qiansen Wang, Chao Kong,†,‡ Yuqi Wu,† Yuexiang Li,§ and Gongxuan Lu*,† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Zhong Road 18, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Yuquan Road 19A, Beijing 100049, China § Nanchang University, Xuefu Road 999, Nanchang 330031, China ABSTRACT: The roles of interface charge transfer and surface proton reduction on photocatalytic hydrogen generation were investigated using Pt cocatalyst on RGO as a model catalyst. The results indicated that the interface charge transfer between reduced graphite oxide (RGO) and Pt nanoparticles (Pt NPs) was more pronounced on photoinduced hydrogen generation over sensitized Pt cocatalyst on RGO than surface proton reduction reaction. The photogenerated electron interface transfer could be modulated by controlling surface potential energy on a selectively exposed Pt facet. By X-ray photoelectron spectroscopy (XPS) and the fluorescence lifetime experiments, we proved that Pt(100) NPs had a noticeably stronger interaction with RGO than Pt(111) NPs, which resulted in a faster electron transfer from RGO to Pt(100) NPs and eventually led to higher activity for hydrogen evolution than Pt(111)/RGO cocatalyst. The XPS results confirmed this interaction difference; that is, the different facets of Pt NPs loading on RGO coresponded to different binding energy. The binding energy of Pt(100) f7/2 shifted to higher energy side by 0.54 eV on RGO, while the binding energy of Pt(111) f7/2 shifted up only ∼0.31 eV. The results of fluorescence lifetime verified that the photogenerated electrons transferred faster from RGO to Pt(100) NPs than to Pt(111) NPs, indicating Pt(100) NPs more strongly interacted with RGO. This study disclosed the facet-dependent effect of noble-metal Pt cocatalyst on RGO in photocatalytic water reduction and will give insight into understanding the electron transfer between Pt NPs and RGO.

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INTRODUCTION Photocatalytic water splitting for hydrogen production by sunlight irradiation is one of the most promising routes for the development of clean and renewable energy sources.1−6 After extensive study in recent years, dye-sensitized photocatalytic hydrogen generation has been proved to be an effective route under visible-light irradiation. For example, Min et al. reported that the quick recombination of photogenerated hole and electron was one of the main obstacles related to low efficiency in the photocatalytic hydrogen generation. To improve the charge transfer, RGO, which exhibited exceptional electronic, optical, and thermal properties, was applied as a potential electron transfer material to effectively reduce the recombination and to achieve high activity of hydrogen evolution in dyesensitized photocatalytic systems.7−12 TiO2, CdxZn1−xS, and so on13−16 immobilized on the graphene achieved high effective hydrogen evolution rate. These results revealed that the RGO could enhance fast charge transfer of photogenerated charges of excited dye or semiconductors to hydrogen evolution sites and reduce the rate of charge recombination, leading to higher activity for photocatalytic H2 generation from water. However, some issues are still unclear for photocatalytic hydrogen generation, such as the roles of interface charge © XXXX American Chemical Society

transfer and surface proton reduction in photocatalytic hydrogen generation process. For surface proton reduction, the nature of metal is thought to decide the activity of hydrogen evolution. Commonly, different metals such as Pt, Ag, Au, Pd, and so on17−19 are used as cocatalysts. The different activities of various metals may be due to the Fermi level for accepting photogenerated electrons and overpotential for hydrogen generation.20,21 In fact, the same metal with different facets could exhibit different activity for hydrogen evolution.22,23 For example, Barber and his coworkers24 found that the activity of H2 generation increased in the sequence (100) < (111) in alkaline solution; they thought it might be due to the ΔHads differences for hydrogen overpotential (opd H) over different facets of Pt NPs. Cui et al.25 suggested that the activity of H2 generation over Pt(111) is higher than that over Pt(100), mainly due to the interatomic distance of nearest neighboring active sites on Pt NPs, but Ross et al.26 found that the activity of H2 evolution followed another sequence (111) < (100) in alkaline solution, resulting from the difference in activation Received: January 23, 2015 Revised: April 29, 2015

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DOI: 10.1021/acs.jpcc.5b00746 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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were filtered, washed with water several times, and finally redispersed in water with ultrasonic treatment prior to use. Preparation of Pt(111)/RGO and Pt(100)/RGO Photocatalysts. The Pt NPs were prepared according to the article.29 A solution of 1 × 10−4 M K2PtCl6 was prepared in 100 mL of water, and 0.08 mL of 0.1 M sodium polyacrylate was added. Then, Ar gas was used to replace the air of the solution for 20 min. The Pt ions were reduced by bubbling H2 gas at high flow rate through the solution for 5 min. The reaction vessel was then sealed, and the solution was left 12 h overnight. The next day, the solution turned lightly golden, and Pt(100) NPs were synthesized. The NPs of Pt(111) were prepared in the similar way, in which 0.4 mL of 0.1 M sodium polyacrylate was added. The Pt(111)/RGO and Pt(100)/RGO photocatalysts were prepared by the addition of 5 mL of 1.2 mg·mL−1 RGO; then, the mixture was tread by ultrasonic treatment for 30 min. Photocatalytic H2 Evolution Activity and AQE Measurements. Photocatalytic experiments were performed in a sealed Pyrex flask (150 mL) with a flat window (an efficient irradiation area of 14 cm2) and a silicone rubber septum for sampling at ambient temperature. The reaction system was constructed by adding 35 mg Eosin Y (EY 5 × 10−4 M) and 1.2 mL of triethanolamine (TEOA ∼10%, v/v), and the pH value of reaction resolution was adjusted by hydrochloric acid or sodium hydroxide solution. The light source was a 300 W Xe lamp, which was equipped with either a 420 nm cutoff filter or various band-pass filters. Photon flux of the 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). Prior to irradiation, the suspension of the catalysts was dispersed by ultrasonic treatment for 30 min and degassed by bubbling Ar gas for 20 min. The amount of hydrogen evolution was measured using gas chromatography (Agilent 6820, TCD, 13 × column, Ar carrier), and the AQE was calculated from the ratio of the number of reacted electrons during hydrogen evolution to the number of incident photons by the follow equation

energies, which was affected by underpotential deposition of hydrogen coverage. That means the role of electron transfer from RGO to active site might also be the key factor for significant activity for water splitting. The study of this problem may help further understanding of interface charge transfer versus surface proton reduction: which one is more pronounced on photoinduced hydrogen generation over sensitized Pt cocatalyst on RGO? In our present work, we proved that the interface charge transfer plays a more important role than surface proton reduction on photoinduced hydrogen generation over sensitized Pt cocatalyst on RGO. The H2 evolution activity over Pt(111) NPs photocatalyst without RGO is higher than that over Pt(100) NPs photocatalyst without RGO. After loading onto the RGO, however, the H2 evolution activity over Pt(100)/RGO photocatalyst is much higher than that over Pt(111)/RGO photocatalyst. It was about 4 times higher than that over Pt(100) photocatalyst. At same time, the H2 evolution activity over Pt(111)/RGO photocatalyst is only 2.3 times higher than that over Pt(111) photocatalyst. The different sequence was caused by the difference of interface charge transfer rate from RGO to Pt NPs. The photogenerated electron interface transfer rate from RGO to Pt NPs could be modulated by the interaction between RGO and Pt NPs due to the different interaction between RGO and Pt NPs. The interaction between Pt(100) and RGO was stronger than that between Pt(111) NPs and RGO.

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EXPERIMENTAL SECTION Preparation of Graphite Oxide (GO). GO was prepared from graphite by using a modified Hummers method.27,28 Typically, graphite powder (100 g) was added to an 80 °C mixture solution of concentrated H2SO4 (150 mL), P2O5 (50 g), and K2S2O8 (50 g). The resultant mixture was isolated and allowed to cool to room temperature. Then, the mixture was diluted with distilled water (7.5 L) and the product was filtered and washed with distilled water until the filtrate pH become neutral. The product was dried in air at room temperature for 24 h. Subsequently, the preoxidized graphite (20g) and NaNO3 (10g) were added to cold concentrated H2SO4 (0 °C, 460 mL). KMnO4 (60g) was then added gradually with stirring and cooling so that the temperature of the mixture was kept below 20 °C. The mixture was then stirred at 35 °C for 2 h. Distilled water (920 mL) was slowly added to the mixture, followed by stirring for 15 min. The reaction was terminated by adding distilled water (2.8 L) and then H2O2 solution (50 mL, 30%). The product was filtered, washed repeatedly with HCl (1:10, v/ v) until sulfate could not be detected with BaCl2, and then dried in a vacuum oven at 40 °C for 24 h. Preparation of Aqueous Dispersion of Reduced Graphite Oxide (RGO). Aqueous dispersions of RGO (1.2 mg·mL−1) were prepared by reducing grapheme oxide with sodium borohydride (NaBH4) as a reductant. In a typical synthetic procedure, 500 mg of graphite oxide powder was dispersed into 250 mL of distilled water with the ultrasound treatment (25 kHz, 250 W) until the solution became clear. The obtained yellow-brown dispersions of grapheme oxide were then heated to 95 °C in an oil bath under magnetic stirring. After stirring for few minutes, 2 g of NaBH4 was added to the aqueous dispersions, and the reduction reaction was maintained at this temperature for 10 h, over which the color of the solution gradually changed into dark black, indicating the reduction of GO. After the reaction, the obtained dispersions

AQE[%] = 2 × number of evolved H 2 molecules /number of incident photos × 100

Working Electrode Preparation and Photoelectrochemical Measurement. Photocurrent responses of photocatalyst samples were measured on an electrochemical analyzer (CHI660A) in a homemade standard three-compartment cell, consisting of an organic glass enclosure with a quartz window, and a 1.2 cm diameter opening opposite the window to the work electrode was clamped. The working electrodes were prepared by drop-coating sample suspensions directly onto the precleaned indium tin oxide glass (ITO glass) surface by microsyringe with an infrared heat lamp to speed drying. The surface of working electrode exposed to the electrolyte was a circular film with the geometrical surface areas of 1 cm2. Platinum foil was used as a counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The supporting electrolyte was TEOA mixed with 0.1 M Na2SO4 aqueous solution. A 300 W Xe lamp with an optical cutoff filter (λ ≥ 420 nm) was used for excitation light source. The unbiased anodic photocurrent was investigated with an amperometric current−time technique. Characterization. Transmission electron microscopy (TEM) images were taken with a Tecnai-G2-F30 field-emission transmission electron microscope operating at accelerating B

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interaction between Pt NPs and RGO, which could determine the interface transfer rate of electrons from RGO to Pt. The effect of pH on activity of different crystal surface Pt NPs loading on the RGO was also studied, and the results are shown in Figure 2. It is shown that the activity is insensitive on

voltage of 300 kV. The accelerating voltage and current were 40 kV and 30 mA, respectively. XPS analysis was performed using a VG Scientific ESCALAB210-XPS photoelectron spectrometer with a Mg KR X-ray resource. The fluorescence decay times were measured using the Horiba Jobin Yvon Data Station HUB operating in time-correlated single photon counting mode (TCSPC) with the time resolution of 200 ps. Nano LED diode emitting pulses at 460 nm with 1 MHz repetition rate was used as an excitation source. Light-scattering Ludox solution was used to obtain the instrument response function (prompt). The time ranges are 0.055 ns/channel in 4096 effective channels. Horiba Jobin Yvon DAS6 fluorescence decay analysis software was used to fit the model functions to the experimental data.

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RESULTS AND DISCUSSION The time courses of H2 evolution catalyzed by RGO, Pt(100), Pt(111), Pt(100)/RGO, and Pt(111)/RGO in 10% (v/v) TEOA aqueous solution under visible-light irradiation (λ ≥ 420 nm) at pH 7 are shown in Figure 1. No H2 is detected when

Figure 2. Effect of pH on photocatalytic activity of Pt(111)/RGO and Pt(111)/RGO photocatalysts sensitized by EY (5.0 × 10−4 mol/L) for hydrogen evolution in 100 mL of 10% (v/v) triethanolamine (TEOA) aqueous solution (pH 7) under visible-light irradiation (λ ≥ 420 nm).

the reaction pH. The Pt(100)/RGO and Pt(111)/RGO photocatalysts all exhibit high activity on H2 generation within the pH range from 5 to 9; however, the activity of H2 generation reduces rapidly in the strong alkaline environment over both photocatalysts, and it might result from the decrease in H+ concentration. In addition, the activity of H2 generation over both photocatalysts drops quickly in the strong acidic environment, which is likely due to the protonation of TEOA and EY in acidic solution. That protonation process would decrease the electron-donating properties of TEOA32 and the variation of absorption performance of EY in the visible-light region. Additionally, it is easy to observe that the activity of H2 generation over Pt(100)/RGO is significantly higher than that over Pt(111)/RGO in all runs regardless of different pH values. This result also implies that Pt(100)/RGO photocatalyst is better than Pt(111)/RGO photocatalyst on H2 generation in terms of different interaction between Pt NPs and RGO, leading to the different interface transfer rate of electrons from RGO to Pt. Apparent quantum efficiencies (AQEs) of Pt(100)/RGO and Pt(111)/RGO photocatalysts were examined under a wide range of visible-light irradiation from 430 to 550 nm, as shown in Figure 3. The AQEs of 11.61 and 12.69%, corresponded to Pt(111)/RGO and Pt(100)/RGO photocatalysts, respectively, and are reached at 520 nm, which is located near the wavelength of the highest absorption of EY (518 nm) in the visible-light range;9,10 however, the highest AQEs of 11.99 and 13.78% for Pt(111)/RGO and Pt(100)/RGO were achieved at 490 nm. This result was due to the interaction between Eosin Y molecules and RGO, which could impact the absorption performance of Eosin Y for visible light. This interaction is mainly noncovalent with the π−π stacking and ester-like linkage reactions between EY and RGO.33,34 This result indicated the charge could transfer directly and quickly from excited EY to RGO. In addition, the AQEs of Pt(100)/RGO

Figure 1. H2 evolution from EY (5.0 × 10−4 mol/L) photosensitized systems catalyzed by RGO, Pt(100), Pt(111), Pt(111)/RGO, and Pt(111)/RGO in 100 mL of 10% (v/v) triethanolamine (TEOA) aqueous solution (pH 7) under visible-light irradiation (λ ≥ 420 nm).

RGO is used alone, indicating that RGO is inactive for hydrogen evolution. The activity of H2 evolution increases rapidly after loading Pt NPs onto the RGO, implying that Pt is the reactive sites for H2 generation; however, the Pt NPs selfexhibit minor activity for hydrogen evolution without RGO, as only 269.4 and 202.2 μmol H2 are produced in 1.5 h over Pt(111) and Pt(100) photocatalysts, respectively. It reveals that RGO is the pivotal material for efficient photocatalytic hydrogen evolution over Pt NPs cocatalyst.25 Here the RGO might play important roles in transferring electron efficiently from photosensitizer Eosin Y to Pt NPs and retarding the recombination of photoexcited pairs.30,31 Simultaneously, an obvious difference in activity of H2 evolution over Pt(100)/ RGO and Pt(111)/RGO photocatalysts can be found. 801.4 μmol H2 could be produced over Pt(111)/RGO in 1.5 h; however, only 632.3 μmol H2 was generated over Pt(100)/ RGO under the same conditions. The H2 evolution activity over Pt(111) NPs photocatalyst is higher than that over Pt(100) NPs photocatalyst without RGO. After loading onto the RGO, however, the H2 evolution activity over Pt(100)/ RGO photocatalyst is much higher than that over Pt(111)/ RGO photocatalyst. It is about 4 times higher than that over Pt(100) photocatalyst. At same time, the H2 evolution activity over Pt(111)/RGO photocatalyst is only 2.3 times higher than that over Pt(111) photocatalyst. Compared with Pt NPs without RGO, the difference might due to the different C

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the activity with the contact area and exposed area of Pt NPs. The total exposed area of Pt(100) and Pt(111) NPs without RGO is 0.078 and 0.148 m2, respectively. The corresponding rate of hydrogen generation (the amount of H2/surface area) is 2592.3 μmol·m−2 in 1.5 h over Pt(100) and 1820.3 μmol·m−2 in 1.5 h over Pt(111); however, the total exposed areas of Pt(100) and Pt(111) NPs with RGO are 0.065 and 0.111 m2, respectively. Similarly, the rate of hydrogen generation (the amount of H2/exposed area) is 12 330.0 μmol·m−2 in 1.5 h over Pt(100)/RGO, and that is 5696.5 μmol·m−2 in 1.5 h over Pt(111)/RGO. It suggests that the rate of hydrogen generation increases 4.7 and 3.1 times after loading on RGO over Pt(100)/RGO and Pt(111)/RGO, respectively. These results indicated that the exposed area and the exposed facets could not account for excellent activity. Considering the total contact areas of Pt(100) and Pt(111) NPs with RGO are 0.013 and 0.037 m2, respectively, the rate of hydrogen generation (the amount of H2/contact area) is 61 650.0 μmol·m−2 in 1.5 h over Pt(100)/RGO, and that is 17 089.5 μmol·m−2 in 1.5 h over Pt(111)/RGO; these results imply that the contact area is not the key for excellent activity. Therefore, the interaction between different facets of Pt NPs and RGO becomes the possible reason for the excellent activity of hydrogen generation; that is, the charge transfer is faster between Pt(100) NPs and RGO than that between Pt(111) NPs and RGO. TEM images of Pt(111)/RGO and Pt(100)/RGO were taken to study the morphology and structure of the photocatalysts, and the results are shown in Figure 4. The TEM images showed that Pt NPs were highly dispersed on RGO, and the surface of cubic NPs and tetrahedral NPs corresponds to the facets of Pt(100) and Pt(111) (in Figure 4A−C), respectively. To further validate the specific planes of Pt NPs, we carried out the high-resolution TEM (HRTEM), and the results are shown in Figure 4D,E; the lattice spacing of 0.19 nm could be assigned to the (100) facet of Pt NPs and the lattice spacing of 0.23 nm could be assigned to the (111) facet of Pt NPs. The result implied that different preferential surface structures of Pt NPs were synthesized. To further make sure the preferential surface of different photocatalysts, we tested the voltammograms, and the results are shown in Figure 5. The adsorption state around 0.44 V can be easily observed in Figure 5A, which corresponds to the adsorption/desorption of bisulfate anions.38 This feature is directly related to the presence of bidimensionally ordered Pt(111) NPs. Thus, it is clear that these Pt NPs have a (111) preferential surface structure and that a large part of (111) sites are presented on the surface. Compared with Figure 4A, the peak at 0.44 V is very week and is negligible in Figure 5B, but there is a major peak at 0.02 V. It corresponds to the coupling of hydrogen adsorption with the bisulfate anion desorption on (100) terrace sites.39 This indicates that the surface structure of this type of Pt photocatalysts is mainly formed by (100) surface. The XPS spectra of Pt photocatalysts were performed to study the surface chemical state of Pt NPs. It can be seen from Figure 6 that the binding energy is significantly different in different crystal surface of Pt photocatalysts. The binding energy of Pt(100) f7/2 is 70.94 eV; nevertheless, that of Pt(111) f7/2 is 71.14 eV which was 0.20 eV lower compared with the binding energy of Pt(100) f7/2. It indicated the surface chemical states were significantly different in those two crystal surface of Pt photocatalysts and it might have direct relationship to the activity of H2 generation on Pt NPs; however, after the Pt NPs

Figure 3. AQEs of H2 evolution for EY (5.0 × 10−4 mol/L) photosensitized systems catalyzed by Pt(111)/RGO and Pt(111)/ RGO photocatalysts. The system was irradiated by a 300 W Xe lamp with a cutoff filter of 420 nm and a bandpass filter.

are higher than those of Pt(111)/RGO photocatalysts in the entire experimental wavelength range. It further implied that the different crystal surfaces had different interaction between Pt NPs and RGO, which led to the different AQEs in the same wavelength over different photocatalysts. It is in good agreement with the activity of H2 evolution over both photocatalysts. Fluorescence lifetime measurements are also carried out to probe the interactions between reactants, and the results are shown in Table 1. The emission decay of EY in TEOA solution Table 1. Decay Parameters of EY in the Presence of RGO, Pt(111)/RGO and Pt(111)/RGOa EY systems EY EY-RGO EY-Pt(111)/RGO EY-Pt(100)/RGO

lifetime (ns)

preexponential factors B

1.29 τ1 = τ2 = τ1 = τ2 = τ1 = τ2 =

B1 B1 B2 B1 B2 B1 B2

1.12 2.07 1.16 2.18 1.17 2.24

= = = = = = =

1 0.8034 0.1966 0.8062 0.1938 0.7664 0.2336

average lifetime (ns)

χ2

1.29 1.31

0.948 1.001

1.36

0.999

1.42

0.999

a Concentrations of dyes and photocatalysts were 1 × 10−6 mol/L. Single-exponential fit for EY. Double-exponential fit for EY-RGO, EYPt(111)/RGO, and EY-Pt(111)/RGO. Average lifetime was determined according to the reported method.33.

is single-exponential, suggesting that there is only one emitting species (monomeric EY molecules). After the addition of RGO, the emission decay of EY-RGO in TEOA solution is twoexponential. The long-lived (2.07 ns) and short-lived (1.12 ns) components are probably due to the EY molecules interacting with RGO and the monomeric EY molecules.35,36 This result suggests that there is an interaction between EY and RGO37 and the RGO can rapidly transfer the electron from excited EY. By the addition of Pt NPs, the lifetime is further prolonged, and the average lifetimes of Pt(111)/RGO and Pt(100)/RGO photocatalysts increase by 0.5 and 1.1 ns, respectively. It further demonstrates that the interaction between Pt NPs and RGO is different with different crystal surfaces. Namely, the interaction between Pt(100) and RGO is stronger than that between Pt(100) and RGO, which leads to longer lifetime. To prove the excellent activity of hydrogen generation is caused by interaction between Pt NPs and RGO, we compare D

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Figure 4. TEM images of Pt/RGO (A), Pt(100) NPs (B), and Pt(111) NPs (C). HRTEM images of Pt(100) NPs (D) and Pt(111) NPs (E).

Figure 6. Pt 4f XPS spectra of Pt(100), Pt(111), Pt(111)/RGO, and Pt(111)/RGO photocatalysts.

interaction between the RGO and the different crystal faces of Pt NPs, the binding energies of same plane whether or not loaded on the RGO were further analyzed. The binding energy of Pt(100) shifted from 70.94 to 71.48 eV after loading on RGO, ∼0.54 eV increasing, whereas Pt(111) shifted from 71.14 to 71.45 eV after loading on RGO, ∼0.31 eV increasing. These results indicated that the interaction between Pt(100) NPs and RGO was obviously stronger than that between Pt(111) NPs and RGO, and the charge transfer was easier and faster between Pt(100) and RGO, which lead to higher activity of H2 generation. To further investigate the interaction between Pt NPs and RGO, we examined the transient photocurrent responses of Pt(111)/RGO and Pt(100)/RGO photocatalysts coated on ITO for several one-off cycles of intermittent irradiation (60 s), as shown in Figure 7. The higher photocurrent of Pt(100)/ RGO photocatalyst implies the fast electron transfer from RGO to Pt(100) NPs. In this process, the excited-state EY could be reductively quenched by the TEOA to form EY− species on the

Figure 5. Voltammetric profiles for (A) Pt(111) and (B) Pt(100) NPs in 0.5 M H2SO4. Sweep rate: 1 mV/s.

loading onto the RGO, the binding energies of Pt NPs were similar, which were 71.48 and 71.45 eV, corresponding to Pt(100) f7/2 and Pt(111) f7/2, respectively. To compare the E

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Figure 7. Transient photocurrent time profiles of Pt(111)/RGO (B) and Pt(100)/RGO coated on ITO glass in mixed solution of 10% (v/ v) TEOA and 0.1 M Na2SO4 at pH 7 under visible-light irradiation (≥420 nm).

Figure 9. Stability tests of H2 evolution over EY-sensitized Pt(111)/ RGO and Pt(111)/RGO photocatalysts. The reaction was continued for 450 min, with evacuation every 90 min: (1) first run; (2) second run; (3) third run; (4) fourth run was collected by centrifuging from the reaction mixture. The recycled photocatalysts were mixed with TEOA solution and fresh EY and evacuated. (5) Fifth run.

surface of electrode and solution continuously. The strong interaction could induce the charge transfer and lower the charge recombination. The electrochemical H2 evolution activities of Pt(111)/RGO and Pt(100)/RGO photocatalysts deposited on ITO glass were also carried out using the linear sweep voltammetry (LSV) technique. As shown in Figure 8, the

could revive to only 72.3% of its original activity by the concurrent addition of dye and TEOA. It implies that the stability of Pt(111)/RGO photocatalyst is not as good as that of the Pt(100)/RGO photocatalyst. These results might be related to the interaction between Pt NPs and RGO because Pt(100) NPs have a strong interaction with RGO, which makes them firmly fixed on the RGO, but Pt(111) NPs have a relatively weak interaction with RGO, which leads to its easier detachment from the RGO; therefore, the stability of Pt(100)/ RGO photocatalyst is more excellent than that of Pt(111)/ RGO photocatalyst. On the basis of the above results, the reaction process of photocatalysis H2 evolution over Pt/RGO photocatalysts can be explained according to the proposed mechanism, as depicted in Scheme 1. The EY dye molecules adsorb on the surface of Scheme 1. Proposed Photocatalytic Mechanism for Hydrogen Evolution over Pt(111)/RGO and Pt(111)/RGO Photocatalysts under Visible-Light Irradiation

Figure 8. LSV curves of Pt(111)/RGO and Pt(100)/RGO photocatalysts coated on ITO glass in mixed solution of 10% (v/v) TEOA and 0.1 M Na2SO4 at pH 7.

cathode current corresponding to the reduction of water to H2 was lower with the increase in the applied potential.40 The cathode currents of Pt(100)/RGO/ITO electrode were higher than those of Pt(111)/RGO/ITO electrode. These results proved that the interaction between RGO and Pt(100) NPs was stronger, which could efficiently enhance the reduction of water to H2. The stabilities of H2 evolution over Pt(111)/RGO and Pt(100)/RGO photocatalysts were also tested, as shown in Figure 9. In the first three runs, there were not any fresh additives in the reaction systems, except for inert gas (Ar gas) displacement, the result showed that the activity of H2 generation over Pt(111)/RGO and Pt(100)/RGO photocatalysts gradually reduced, it was mainly because of the consumption of TEOA and EY.8,9 In the fourth run, both photocatalysts were collected from the reaction mixture by centrifugation and addition of fresh EY and TEOA. For the Pt(100)/RGO photocatalyst, the fourth activity was renewed and nearly identical to that of the first run. These results indicated the Pt(100)/RGO photocatalyst exhibited excellent stability; however, the activity of Pt(111)/RGO photocatalyst

RGO by the noncovalent π−π stacking interaction, which absorbs light photons to form a singlet excited-state EY1* under visible-light irradiation and then produces the lowest-lying triplet excited-state EY 3* via an efficient intersystem crossing. EY3* can be reductively quenched by TEOA and produce EY−• and oxidative donor (TEOA+).24,41 The e− of EY−• species is transferred to RGO due to its electron-transport characteristics, which lead to spatially separation of photogenerated charges.7 The accumulated electrons on the RGO will transfer to Pt NPs; F

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The Journal of Physical Chemistry C however, the transmission rate of e− is different with the distinct facets of Pt NPs. It transfers faster between RGO and Pt(100) NPs than between RGO and Pt(111) NPs. Proton adsorbed on the Pt NPs could be reduced to form H2 by the electron.

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CONCLUSIONS In this work, a new discovery, interface charge transfer between RGO and Pt NPs is more pronounced on photoinduced hydrogen generation over sensitized Pt cocatalyst on RGO than surface proton reduction, was presented. The photogenerated electron transfer rate could be adjusted by the different interaction between RGO and different exposed facets of Pt NPs, which was achieved by controlling surface potential energy on a selectively exposed Pt facet. The XPS result revealed that the different facets of Pt NPs loading on RGO had different binding energy shift. The binding energy of Pt(100) f7/2 shifted positively 0.54 eV after interacting with RGO, and the binding energy of Pt(111) f7/2 shifted positively only 0.31 eV, revealing that Pt(100) NPs had an obvious stronger interaction with RGO than Pt(111) NPs, which led a faster electron transfer from RGO to Pt(100) NPs and eventually led to higher activity for hydrogen evolution than Pt(111)/RGO cocatalyst. This study revealed the possibility of improving the photocatalytic activity by designing the cocatalyst with special facets to promote the electron-transfer rate between activity site and RGO.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-931-4968 178. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSF of China (grant nos. 21173242 and 21433007) and the 973 and 863 Programs of 60 Department of Sciences and Technology of China (2013CB632404, 2012AA051501), respectively.

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DOI: 10.1021/acs.jpcc.5b00746 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b00746 J. Phys. Chem. C XXXX, XXX, XXX−XXX