Influence of the Electrostatic Interaction between a Molecular Catalyst

Jun 20, 2017 - Xiyang Liu , Fei Huang , Yide He , Yang Yu , Yong Lv , Yanhua Xu ... Zhe-Jian Yu , Wen-Ya Lou , Liang-Min Xia , Bai-Yang Lou , Xue-Fen ...
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Influence of the Electrostatic Interaction between a Molecular Catalyst and Semiconductor on Photocatalytic Hydrogen Evolution Activity in Cobaloxime/CdS Hybrid Systems Yuxing Xu,†,‡ Ruotian Chen,†,‡ Zhen Li,†,‡ Ailong Li,†,‡ Hongxian Han,*,† and Can Li*,† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Division of Solar Energy, Dalian National Laboratory for Clean Energy; Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), 457 Zhongshan Road, Dalian 116023, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The influence of the electrostatic interaction on photocatalytic H2 evolution activity in cobaloxime/cadmium sulfide (CdS) hybrid systems was studied by measuring the charges of the cobaloximes and the zeta potentials of CdS under different pH conditions (pHs 4−7). Cobaloxime/CdS hybrid systems may have potential as a valid model for the investigation of the electrostatic interaction between a molecular catalyst and semiconductor because the kinetics of methanol oxidation and the driving force of electron transfer from photoirradiated CdS to cobaloxime have little effect on the pH-dependent photocatalytic H2 evolution activity. Our experimental results suggest that electrostatic repulsion between cobaloxime and CdS disfavors the electron transfer from CdS to cobaloxime and hence lowers the photocatalytic H2 evolution activity. Whereas, electrostatic attraction favors the electron transfer process and enhances the photocatalytic H2 evolution activity. However, an electrostatic attraction interaction that is too strong may accelerate both forward and backward electron transfer processes, which would reduce charge separation efficiency and lower photocatalytic H2 evolution activity. KEYWORDS: electrostatic interaction, cobaloxime, cadmium sulfide (CdS), photocatalytic hydrogen evolution, charge transfer

1. INTRODUCTION The development of photocatalytic systems for water splitting and carbon dioxide reduction to produce solar fuels is thought to be one of the most potentially significant solutions to address current energy and environmental problems.1 To construct such an efficient photocatalytic system, cocatalysts are usually needed to achieve high photocatalytic efficiency.2 Semiconductor−molecular catalyst hybrid systems have been extensively studied as photocatalytic systems for water splitting and CO2 reduction because they possess the advantages of broad light absorption by the semiconductor and the high catalytic activity of the molecular catalyst.3 To design an efficient hybrid photocatalytic system, one should consider not only the correct energy level match between the semiconductor and molecular catalyst to have enough driving force to electron flow in the designed direction,4 but also the interaction between them, which may significantly affect the kinetics of charge transfer and separation. It has been reported that chemical bonding between a molecular catalyst and semiconductor can improve the photocatalytic activities of such hybrid systems.5−9 The confinement of [FeFe]-hydrogenase mimics in a chitosan environment was also reported to improve the photocatalytic H2 evolution activity.10 However, there is still a lack of detailed investigation on the effect of the electrostatic interaction © 2017 American Chemical Society

between a molecular catalyst and semiconductor on the photocatalytic performance of a semiconductor−molecular catalyst hybrid system. Tuning the electrostatic interaction between a molecular catalyst and semiconductor can be realized by varying the pH conditions. A change in pH may vary the charge of a molecular catalyst due to the protonated/deprotonated chemical groups and the zeta potential of the semiconductor. A change in pH may also affect the flat band (Efb) of the semiconductor, the redox potential of the molecular catalyst, and the reaction kinetics of the sacrificial reagent.11 Simultaneous changes in more than one important reaction parameter makes it difficult to clarify the interaction effect between a molecular catalyst and semiconductor on the photocatalytic activity, although the pH dependence of the photocatalytic activity of hybrid photocatalytic systems is well known.10,12−23 To investigate the effect of the electrostatic interaction between a molecular catalyst and semiconductor on photocatalytic activity, a valid model is necessary. Received: May 3, 2017 Accepted: June 20, 2017 Published: June 20, 2017 23230

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Structures of [CoPy] and [Co-Cl2]. (b) X-ray powder diffraction (XRD) patterns of hexagonal phase CdS. (c) Zeta potentials of CdS nanoparticles suspended in methanol aqueous solution. (d) Mott−Schottky plots of CdS electrodes obtained in 0.2 M Na2SO4 methanol aqueous solutions at the pH range of 4−7. except that an aqueous solution of Na2S (200 mL, 0.14 mol/L) was added slowly to a Cd(OAc)2 solution (300 mL, 0.14 mol/L) under vigorous stirring. Pt/CdS was obtained by the photodeposition of Pt on CdS using H2PtCl6 as the Pt source in methanol aqueous solution (20 vol % and hereafter). CdS electrodes applied for the Mott− Schottky measurements were prepared by the dipping method. CdS electrodes applied for SPV measurements were prepared by the hydrothermal method, which involved immersion in various methanol aqueous solutions before SPV measurements. 2.3. Preparation of Cobaloximes. [Co(dmgH2)(dmgH)Cl2] ([Co-Cl2]) and [Co(dmgH)2pyCl] ([CoPy]) (Figure 1a) were synthesized by a facile coordination method.25 Typically, CoCl2· 6H2O (2.5 g, 0.0105 mol) and dimethylglyoxime (2.75 g, 0.0236 mol) were dissolved in 80 mL of acetone and stirred for ca. 5 min. The reaction mixture was filtered and the filtrate was left for 1 day to form green [Co-Cl2]. Finally, [Co-Cl2] was filtered, washed with acetone and 95 vol % alcohol aqueous solution, and dried under vacuum. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 2.34 (dimethylglyoxime). [CoPy] was synthesized by coordinating [Co-Cl2] with pyridine. To a suspension of [Co-Cl2] (3.3 g, 9.1 mmol) and pyridine (1.8 mL) in chloroform (85 mL), deionized water (30 mL) was added under vigorous stirring. After stirring for another 2 h, the aqueous layer was discarded and the chloroform layer was filtered and extracted with water until the washings were nearly colorless. Finally, the brown [CoPy] was obtained by evaporating the solution under vacuum. 1H NMR (400 MHz, CD3Cl): δ (ppm) = 8.29, 8.27 (2H, pyridine-αH); 7.72, 7.70, 7.68 (1H, pyridine-γH); 7.25, 7.23, 7.21 (2H, pyridine-βH); 2.40 (12H, dimethylglyoxime). 2.4. General Methods for CdS Characterization. The phase of the CdS nanoparticles was characterized by XRD on a Rigaku D/Max2500/PC powder diffractometer using Cu Kα radiation (operating voltage: 40 kV, operating current: 200 mA, scan rate: 5° min−1). The zeta potentials of the CdS nanoparticles were measured on a Malvern Zetasizer Nano ZS90. To meet the reaction conditions, ∼10 mg of

In this work, we employed a valid model to investigate the influence of the electrostatic interaction on photocatalytic H2 evolution activity by tuning the pH values of cobaloxime/ cadmium sulfide (CdS) hybrid systems using methanol as the electron donor. It was found that the electrostatic interaction can be tuned by varying the pH values of cobaloxime/CdS hybrid photocatalytic systems without significantly changing the kinetics of methanol oxidation and the thermodynamic driving force for electron transfer between CdS and cobaloximes. Our experimental results show that electrostatic attraction rather than electrostatic repulsion can improve the electron transfer and charge separation between CdS and cobaloximes, and as a result, improves the photocatalytic H2 evolution activity. However, electrostatic attraction that is too strong may hinder the charge separation efficiency due to accelerated backward electron transfer, as evidenced by the decay of surface photovoltage (SPV), hence reducing the photocatalytic H2 evolution activity.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents of analytical grade or chemical grade were used as received without further purification. Cd(OAc)2 (98%), CoCl2·6H2O (99.0%), pyridine (99.5%), ethanol absolute (99.7%), methanol anhydrous (99.5%), and N,N-dimethylformamide (99.0%, DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethylglyoxime (98%, dmgH2) and acetone (99.5%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Na2S· 9H2O (98%) was purchased from Shanghai Tongya chemical Technology Development Co., Ltd. 2.2. Preparation of CdS Nanoparticles, Pt/CdS, and CdS Electrodes. CdS nanoparticles were synthesized by a precipitation− hydrothermal method similar to that reported in the literature,24 23231

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

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Figure 2. CVs of (a) [CoPy] and (b) [Co-Cl2] measured in 0.5 M Na2SO4 aqueous solutions at pHs 4−7 after bubbling with Ar. The scanning rate was 100 mV s−1. The concentrations of cobaloximes were ca. 0.25 mM.

Figure 3. Potentiometric titration plots of (a) [CoPy] and (b) [Co-Cl2] obtained by titrating the solution of cobaloximes in methanol aqueous solution from acidic to alkaline conditions. Conditions: 0.0099 g of [CoPy] (∼22.9 μmol, pre-dissolved in 0.5 mL DMF) and 0.0505 g of (∼139.9 μmol) [Co-Cl2] were dissolved in 25/50 mL methanol aqueous solution with original pH as 5.3 and 2.5, respectively. The insets are the structures of the corresponding main components of the cobaloximes at pHs 4−7. relatively low, hence the activity coefficients of the ions in the solution can be roughly regarded as γ = 1. 2.6. Adsorption Amount of Cobaloximes on CdS. The amounts of adsorbed cobaloximes on CdS under different pH conditions were found by measuring the concentrations of cobaloxime before and after the adsorption experiment using UV−visible (V-550; JASCO) absorption spectroscopy. 2.7. Photocatalytic H2 Evolution Measurement. Photocatalytic activities were measured in a Pyre reaction cell connected to gas chromatography (MS-5A column, thermal conductivity detector, Ar carrier, GC-7890A; Agilent) with a closed gas circulation and evacuation system. The reaction mixture (100 mL of methanol aqueous solution containing 0.25 mM cobaloxime and 100 mg of CdS) under different pH conditions was degassed and then irradiated for 1 h by a Xe lamp (300 W) equipped with an optical filter (λ ≥ 420 nm).

CdS nanoparticles were dispersed in 100 mL of methanol aqueous solution and the pH of the mixture was adjusted to designated values before measurement. The flat bands (Efb) of CdS under different pH conditions, obtained from Mott−Schottky plots (Princeton, Parstat 2273), were measured in 0.2 M Na2SO4 methanol aqueous solution. The pH values reported in this work were adjusted by diluted HNO3 and NaOH aqueous solutions, and the measured potentials were converted to V versus NHE (ENHE = ESCE + 0.244 V). The SPV data of the CdS electrodes were obtained by obtaining the difference in the surface potentials in light and dark conditions. The surface potentials were measured by photoassisted Kelvin probe force microscopy (KPFM) in an ambient atmosphere with the amplitude modulatedKPFM mode. KPFM was carried out in lift mode with a lift height of 100 nm. The surface potential in light conditions was measured under focused 480 nm monochromatic light with an intensity of 1 mW/cm2, which was split from a 500 W Xenon-arc lamp. 2.5. General Methods for Cobaloxime Characterization. NMR spectra were recorded on a Bruker 400 or 500 MHz NMR spectrometer. The cobaloximes were pre-dissolved in DMF (≤0.5 mL) for the measurements hereafter. The redox potentials of the cobaloximes (Co(II)/Co(I)) under different pH conditions were obtained from cyclic voltammograms (CVs) recorded on a CH660 (Shanghai Chenhua Limited, China) with a scanning rate of 100 mV s−1 in a 0.5 M Na2SO4 aqueous solution after bubbling with Ar. A glassy carbon electrode and Pt wire were used as the working electrode and counter electrode, respectively. The apparent pKa values of the cobaloximes were obtained using a potentiometric titration method by titrating the cobaloxime solutions from acidic conditions to alkaline conditions.26 The concentrations of the cobaloximes applied were

3. RESULTS 3.1. Characterization of Synthesized CdS and Cobaloximes. The XRD pattern confirmed that the synthesized CdS is in the hexagonal phase (Figure 1b). The zeta potential plots of CdS versus pH (Figure 1c) show that the isotropic electric point of the prepared CdS nanoparticles is ca. 4.4. Additionally, the CdS nanoparticles are negatively charged with similar zeta potentials at pHs 5−7, but are positively charged at ∼pH 4. Figure 1d shows the pH-dependent Mott−Schottky plots of CdS measured in methanol aqueous solution, and the inset is the Efb of CdS. It is obvious that the Efb of CdS stayed almost the same (around 0.71 V) at pHs 5−7, however the Efb 23232

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

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ACS Applied Materials & Interfaces

Figure 4. Amounts of adsorbed (a) [CoPy] and (b) [Co-Cl2] on CdS after stirring for ∼1.5 h in the dark at pHs 4−7. Methanol aqueous solutions (50 mL) containing 0.25 mM cobaloxime and 50 mg of CdS were used to meet the photocatalytic reaction conditions.

Figure 5. Amount of evolved hydrogen and TOF values of adsorbed cobaloximes under different pH conditions obtained from the (a) [CoPy]/CdS and (b) [Co-Cl2]/CdS hybrid systems. The TOF values were estimated from the amounts of adsorbed cobaloximes on CdS. Reaction conditions: 100 mg of CdS; 0.25 mM cobaloxime; 100 mL of methanol aqueous solution; 300 W Xe lamp (λ ≥ 420 nm).

Additionally, both the −2 and −3 charged forms could be found for [Co-Cl2] at pHs 5−6. 3.2. Adsorption of Cobaloximes on CdS. The amounts of [CoPy] and [Co-Cl2] adsorbed on CdS showed different trends when the pH of the cobaloxime/CdS hybrid systems increased from 4 to 7. When the pH value increased from 4 to 7, the amounts of [CoPy] adsorbed on CdS monotonously increased from ca. 0.00350 to ca. 0.0168 mmol/g, as shown in Figure 4a. However, the adsorbed amount of [Co-Cl2] decreased monotonously from pH 4 to 7, as shown in Figure 4b. The results indicate that these two molecular complexes possess completely different pH-dependent behaviors when they are combined with CdS in methanol aqueous solution. 3.3. Photocatalytic Performance of Cobaloxime/CdS Hybrid Systems. The photocatalytic H2 evolution amounts of cobaloxime/CdS hybrid systems largely depend on the pH conditions of the reaction mixture. Figure 5a shows that the H2 evolution amounts of [CoPy]/CdS decreased with an increase in the pH of the reaction mixture from 4 to 5, and increased when the pH value further increased from 5 to 7. The photocatalytic performances were also evaluated using turnover frequencies (TOFs) estimated from the amounts of adsorbed cobaloxime on CdS. The TOF of [CoPy] at pH 4 (ca. 41.1 h−1) was much higher than those of [CoPy] at pHs 5−7, and the TOFs were relatively constant (ca. 10 h−1) with a very small decreasing trend when the pH value further increased from 5 to 7 (Figure 5a). Figure 5b shows that the H2 evolution activity of [Co-Cl2]/ CdS decreased as pH increased from 4 to 7, whereas the TOF of [Co-Cl2] remained almost the same (ca. 5.0 h−1) in the pH range of 4−6 and then decreased at pH 7 (ca. 3.6 h−1).

of CdS at pH 4 (0.68 V) was a little more positive than that at pHs 5−7. The CVs of the cobaloximes measured at pHs 4−7 are shown in Figure 2. At pH 4, both cobaloximes showed relatively more positive Co(II)/Co(I) redox potentials, which were −0.66 and −0.62 V for [CoPy] and [Co-Cl2], respectively. However, at pHs 5−7, the Co(II)/Co(I) redox potentials were similar for [CoPy] and [Co-Cl2], and were around −0.69 and −0.70 V for [CoPy] and [Co-Cl2], respectively. Figure 3 shows the potentiometric titration curves of [CoPy] and [Co-Cl2] recorded from acidic to alkaline conditions in methanol aqueous solution. The amounts of NaOH consumed between the inset lines were around 25.0 and 115.2 μmol for [CoPy] (∼22.9 μmol) and [Co-Cl 2 ] (∼139.9 μmol), respectively, indicating that the potentiometric titration method applied in this work is reliable for the characterization of these molecular complexes. Considering that the original pH value of 0.92 mM [CoPy] in methanol aqueous solution was ca. 5.3, the apparent pKa value of [CoPy] indicated in Figure 3a is pKa1 = 7.6. The original pH value of ∼2.8 mM [Co-Cl2] in methanol aqueous solution was ca. 2.5, an apparent pKa2 value of [CoCl2] around 3.3 could be calculated. Therefore, the apparent pKa value of [Co-Cl2] indicated in Figure 3b is pKa3 = 5.6. According to the obtained pKa values, the main components of [CoPy] and [Co-Cl2] under different pH conditions can be inferred. [CoPy] keeps its neutral molecular form in the pH range of 4−7 with a rather small proportion of negatively charged components under higher pH conditions at pH 7. However, [Co-Cl2] changed to its corresponding −2 and −3 charged deprotonated forms at pHs 4 and 7, respectively. 23233

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

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ACS Applied Materials & Interfaces 3.4. Steady-State SPV and Its Decay of CdS Electrodes. The effects of the electrostatic interactions between [CoCl2] and CdS were studied by steady-state SPV measurement. Figure 6 demonstrates that the SPV of CdS, derived from the

electron donors, such as triethanolamine, ascorbic acid, lactic acid, and methanol, methanol with a pKa value of ca. 15.529 in water can maintain its molecular form over a wide pH range. However, the surface state of CdS and the main component of cobaloximes can be influenced by the pH values of reaction conditions. Therefore, the electrostatic interaction between cobaloxime and CdS can be tuned by adjusting the pH value of the reaction mixture. Such a situation ensures that cobaloxime/ CdS hybrid systems using methanol as the electron donor can be applied as an ideal model for the study of the effect of the electrostatic interaction between a molecular catalyst and semiconductor on photocatalytic H2 evolution activity. As shown in Figure 5, the photocatalytic H2 evolution amounts of the cobaloxime/CdS hybrid systems are highly influenced by the pH conditions. Pt has been reported to be a suitable proton reduction catalyst for CdS in many photocatalytic systems.24,30 Herein, Pt/CdS was applied as a reference catalyst to study methanol oxidation by photoirradiated CdS under different pH conditions. It was found that the photocatalytic H2 evolution amounts of Pt/CdS were relatively low compared with those of the hybrid cobaloxime/ CdS systems, and increased with increasing pH from 4 to 7 (Figure S1). Our previous time-resolved PL results of CdS in [CoPy]/CdS also demonstrated that the PL decay of CdS is much more efficient in aqueous solution with both CdS and [CoPy] than that in methanol aqueous solution with CdS only at both pHs 6 and 10.11 The above results indicate that the overall photocatalytic reaction in the cobaloxime/CdS hybrid systems is induced by proton reduction when using methanol as the hole scavenger at pHs 4−7. Therefore, the kinetics of forward and backward electron transfer between photoirradiated CdS and cobaloximes, which are associated with proton reduction reactions, are the main factors affecting photocatalytic performance. This makes it convenient for us to correlate the electrostatic interaction between cobaloxime and CdS with photocatalytic H2 evolution activity under different pH conditions. The generation of the key intermediate species of Co(I) for the catalytic reduction of a proton to H2 from cobaloximes with a Co3+ center is usually considered to be a sequential twoelectron reduction process. Because the second electron reduction of cobaloxime (Co(II)/Co(I)) is more difficult than the first electron reduction of cobaloxime (Co(III)/ Co(II)), the overall photocatalytic activities of cobaloxime/CdS hybrid systems might be largely determined by the driving force of the second electron reduction of cobaloxime by photoirradiated CdS. Table 1 lists the energy level differences of experimentally measured Efb values of CdS and the Co(II)/ Co(I) redox potentials of cobaloxime for the second reduction

Figure 6. Steady-state SPV and its decay profile of CdS electrodes immersed in methanol aqueous solutions with/without 0.25 mM [CoCl2] at pHs 4 and 5 before the measurement. The inset is the normalized SPV. CdS electrodes soaking under designated pH conditions with or without [Co-Cl2] were denoted as CdS−[CoCl2]-pH or CdS-pH, respectively.

steady-state contact potentials of CdS with light off and on, is largely influenced by the pH values of the soaking solution and [Co-Cl2]. CdS electrodes soaking under designated pH conditions with or without [Co-Cl2] were denoted as CdS− [Co-Cl2]-pH or CdS-pH, respectively. The SPV influenced by pH can be attributed to the variation of surface charge states of CdS under different pH conditions. The SPV values of CdS− [Co-Cl2] under all investigated pH conditions were higher than those of bare CdS, indicating that the photogenerated charge separation of CdS was facilitated by the coupling of CdS with [Co-Cl2]. In addition, the lifetime of the photogenerated charges can be evaluated by the decay of SPV. A simplified estimation of the lifetimes of photogenerated charges can be applied by recording the time when the SPV value decays to its 1/e. The normalized SPV and a given value of 1/e (the light was turned off at 1.9 s) are shown in the inset of Figure 6. When CdS was coupled with [Co-Cl2], the lifetime of the photogenerated charges decreased from 2.2 to 1.3 s at pH 4, whereas it increased from 3.0 to 4.1 s at pH 5. The results strongly suggest that the recombination rate of photogenerated charges in CdS−[Co-Cl2] is much faster at pH 4 compared with that at pH 5.

4. DISCUSSION The apparent photocatalytic H2 evolution amounts of semiconductor−molecular catalyst hybrid systems may be affected by many factors, such as the driving force for electron transfer from semiconductor to molecular catalyst,4 the amount of molecular catalyst adsorbed on the semiconductor surface, and the reaction kinetics of the hole scavenging sacrificial reagent.14,24 In cobaloxime/CdS hybrid systems, the flat band of CdS is known to be constant at a certain pH range,11,27,28 and the redox potential of cobaloxime (Co(II)/Co(I)) changes little over the pH range of 5−10.11 Further, amongst the most-used

Table 1. Flat Bands of CdS (Efb(CdS)), Co(II)/Co(I) Redox Potentials (E1/2[Co(II)/Co(I)]) of the Cobaloximes, and the Electron Transfer Driving Forces (ΔG) in the Cobaloxime/ CdS Hybrid Systems under Different pH Conditions ΔG (V)

E1/2[Co(II)/Co(I)] (V) pH pH pH pH pH 23234

4 5 6 7

Efb(CdS) (eV)

[CoPy]

[Co-Cl2]

[CoPy]/CdS

[Co-Cl2]/CdS

−0.68 −0.71 −0.71 −0.71

−0.66 −0.69 −0.69 −0.69

−0.62 −0.70 −0.70 −0.70

−0.02 −0.02 −0.02 −0.02

−0.06 −0.01 −0.01 −0.01

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

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Table 2. Zeta Potentials of CdS Nanoparticles, Charges of Cobaloximes, and the Electrostatic Interactions between Thema charges of cobaloximes and their electrostatic interaction with CdS pH pH pH pH pH a

4 5 6 7

charges of CdS + − − −

[CoPy] 0 0 0 0

(A) (A) (A) (−1) (A)

reduced-[CoPy] −1 −1 −1 −1

(A) (R) (R) (−2) (R)

[Co-Cl2] −2 −2 −3 −3

(A) (−3) (R) (−2) (R) (R)

reduced-[Co-Cl2] −3 −3 −4 −4

(A) (−4) (R) (−3) (R) (R)

Electrostatic attraction and electrostatic repulsion are denoted as A and R, respectively.

interactions. Herein, the TOFs of the cobaloximes determined by the amount of generated H2 per amount of adsorbed cobaloxime were applied to further investigate the influence of electrostatic interaction on photocatalytic H2 evolution activity (Figure 5). For the [CoPy]/CdS hybrid system, although the driving force remains constant at pHs 4−7 (ΔG = −0.02 eV, Table 1), and the electrostatic interaction between [CoPy] and CdS is a weak electrostatic attraction at the pH range of 4−7, the TOF of [CoPy] is much higher at pH 4 (TOF = 41.2 h−1) compared with those obtained at pHs 5−7 (TOF ≅ 10 h−1) (Figure 5a). It should be noted that the electrostatic interaction between reduced-[CoPy] and CdS is electrostatic attraction at pH 4, however it is electrostatic repulsion at pHs 5−7 (Table 2). Furthermore, the electrostatic differences between reduced[CoPy] and CdS correlate well with the TOF changes of [CoPy] in the [CoPy]/CdS hybrid system at pHs 4−7. These results strongly suggest that electrostatic attraction rather than electrostatic repulsion is more favorable for efficient charge transfer and separation in the [CoPy]/CdS hybrid systems, which is a similar finding to that of other photocatalytic or photoelectrochemical systems that show enhanced performance due to electrostatic attractions between the main components.33−38 Similar electrostatic interaction effects were also observed in the [Co-Cl2]/CdS hybrid system over the pH range of 4−7. Although the driving force of electron transfer from photoirradiated CdS to [Co(II)-Cl2] remains constant at pHs 5−7 (ΔG = −0.01 eV, Table 1), the TOF values of [Co-Cl2] are almost the same (∼5.0 h−1) at pHs 5 and 6 due to the similar strengths of the repulsive interactions between CdS and [CoCl2] and reduced-[Co-Cl2], however the decrease in TOF (3.6 h−1) at pH 7 is due to the enhanced electrostatic repulsion intensity between CdS and [Co-Cl2] (pKa3 = 5.6) and reduced[Co-Cl2] due to the more negative charge of [Co-Cl2] (−3) at pH 7 compared with that (−2/−3) at pHs 5 and 6, as shown in Table 2. At pH 4, the driving force for electron transfer from the photoirradiated CdS to [Co(II)-Cl2] is much larger (ΔG = −0.06 eV) and the interaction between [Co-Cl2] and CdS is electrostatic attraction, however the TOF value of [Co-Cl2] is almost the same as those at pHs 5 and 6. It is well known that the successive two-electron reduction of Co(III) to Co(I) is necessary to reduce protons to H2 during the photocatalytic H2 evolution process under most conditions,39−42 and the second electron transfer process for the reduction of Co(II) to Co(I) is much slower than the first electron transfer process for the reduction of Co(III) to Co(II) in a cobaloxime−semiconductor hybrid system.31 Therefore, the backward electron transfer from reduced-[Co-Cl2] to photoirradiated CdS needs to be considered when the electrostatic attraction between them is too strong. Although a much higher TOF was obtained for the [CoPy]/CdS hybrid system at pH 4 compared with the TOF at

of cobaloxime (Co(II)/Co(I)) over the pH range of 4−7, which determine the driving forces for the reduction of Co(II)species to Co(I)-species by photoirradiated CdS (ΔG = e[Efb − E1/2[Co(II)/Co(I)]]). The driving force for the second electron reduction of [CoPy] remains the same (ΔG = −0.02 eV) at pHs 4−7 in the [CoPy]/CdS hybrid system, that is, the kinetics of charge transfer to obtain the Co(I) key intermediate species from the photoirradiated CdS to [CoPy] is almost the same under different pH conditions over the pH range of 4−7, according to Marcus theory.31 If the driving force is the sole determining factor of the photocatalytic reactions, the photocatalytic H2 evolution amounts under different pH conditions should be similar. However, as shown in Figure 5, with the increase of the pH value from 5 to 7, the amount of H2 evolution increases for [CoPy]/CdS and decreases for the [CoCl2]/CdS hybrid systems, even though these two hybrid systems possess a similar driving force for electron transfer (ΔG = −0.01 eV for [Co-Cl2]/CdS). Therefore, the driving force cannot explain the observed pH-dependent photocatalytic H2 evolution activities. The interactions between cobaloxime and CdS under the reaction conditions are electrostatic in nature (the van der Waals force interaction is a kind of weak electrostatic attraction). The formation of a chemical bond between cobaloxime and CdS can be excluded as the CdS samples with adsorbed [CoPy] and [Co-Cl2] all showed negligible H2 evolution activities at pH 4 (Figure S2). Table 2 lists the zeta potentials of CdS, the main charges of the cobaloximes, and the attractive or repulsive consequences of the electrostatic interactions between cobaloxime and CdS over the pH range of 4−7. The zeta potentials of CdS are positive at pH 4 and negative at pHs 5−7. The main component of [CoPy] maintains its original neutral form with a rather small part of negatively charged components under higher pH conditions at pH 7. Therefore, the electrostatic interaction between [CoPy] and CdS can be qualitatively defined as a weak electrostatic attraction over the pH range of 4−7. For the [Co-Cl2]/CdS hybrid system, electrostatic attraction can be inferred due to the opposite charge of CdS (δ ≅ +4) and [Co-Cl2] (−2) at pH 4, whereas electrostatic repulsion can be inferred at pHs 5−7 due to the negative charges of CdS and [Co-Cl2]. Considering that neutrally charged [CoPy] is converted to reduced-[CoPy], which is negatively charged (−1), at the initial stage of the photocatalytic H2 evolution process,32 the charges of reduced[CoPy] and reduced-[Co-Cl2], and the consequences of the electrostatic interaction between the reduced cobaloximes and CdS are also listed in Table 2. According to the summarized electrostatic interactions between cobaloxime and CdS at pHs 4−7 listed in Table 2, it is obvious that the photocatalytic H2 evolution amounts of the [CoPy]/CdS and [Co-Cl2]/CdS hybrid systems (Figure 5) and the adsorption amounts of [CoPy] and [Co-Cl2] on CdS (Figure 4) cannot be solely ascribed to electrostatic 23235

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21473189), the 973 National Basic Research Program of China (No. 2014CB239401), and the Pioneer Initiative (B) Project of the Chinese Academy of Sciences (No. XDB17030200).

pHs 5−7 due to the electrostatic attraction between reduced[CoPy] and CdS, a relatively low TOF was obtained for the [Co-Cl2]/CdS hybrid system at pH 4, which might result from more accelerated backward electron transfer owing to much stronger electrostatic attraction as reduced-[Co-Cl2] (−3) is more negative than reduced-[CoPy] (−1) at pH 4. It can be seen from the decay profile of SPV (Figure 6) that the lifetime of the photogenerated charges in CdS−[Co-Cl2]-pH 4 (1.3 s) is shorter than that in CdS-pH 4 (2.2 s), whereas the lifetime of the photogenerated charges is longer in CdS−[Co-Cl2]-pH 5 (4.1 s) than that in CdS-pH 5 (3.0 s). This further confirms the accelerated recombination kinetics of the photogenerated charges when CdS is combined with [Co-Cl2] at pH 4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06154. These include the preparation of CdS, Pt/CdS, and CdS electrodes for Mott−Schottky and SPV measurements; the measurement of the adsorption amounts of cobaloxime on CdS, and the photocatalytic performances of Pt/CdS and Ads-cobaloxime-CdS (PDF)

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REFERENCES

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5. CONCLUSIONS In this work, the effects of the electrostatic interaction on the photocatalytic H2 evolution activities of cobaloxime/CdS hybrid systems ([CoPy]/CdS and [Co-Cl2]/CdS) under mild pH conditions (at pHs 4−7) have been studied in detail. It was found that the driving force of electron transfer from photoirradiated CdS to cobaloxime to produce the Co(I) intermediate species and the kinetics of methanol oxidation by photoirradiated CdS have little effect on the pH-dependent photocatalytic H2 evolution activity of the cobaloxime/CdS hybrid systems over the pH range of 4−7. As such, the cobaloxime/CdS hybrid system may serve as an ideal model for the investigation of the effects of electrostatic interactions between a molecular catalyst and semiconductor on photocatalytic H2 evolution activity. The influence of the adsorption amounts of cobaloxime on CdS was also excluded by using TOF values estimated from the photocatalytic H2 evolution amounts and the adsorption amounts of cobaloxime on CdS. Our study demonstrates that electrostatic attraction between cobaloxime and CdS generally enhances the photocatalytic H2 evolution activity (TOF) compared with electrostatic repulsion, however electrostatic attraction that is too strong may hinder the charge separation efficiency due to acceleration of the backward electron transfer process, which results in a mediocre photocatalytic activity. An optimal electrostatic interaction between the molecular catalyst and semiconductor is necessary to achieve high photocatalytic activities.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.H.). *E-mail: [email protected] (C.L.). ORCID

Hongxian Han: 0000-0002-2522-1817 Can Li: 0000-0002-9301-7850 Notes

The authors declare no competing financial interest. 23236

DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b06154 ACS Appl. Mater. Interfaces 2017, 9, 23230−23237