Developing techniques for producing renewable and clean fuels is necessary to deal with increasing energy and environmental issues [1, 2]. Photocatalytic water splitting is one of the most attractive methods for utilizing solar energy for H2 production [3, 4]. However, the four-electron oxygen evolution reaction (OER) is generally considered to be the bottleneck for the water splitting reaction owing to its sluggish kinetics, multiple-electron transfer, and complicated O-O bond formation [5]. Numerous efforts have therefore been devoted to developing efficient water oxidation catalysts (WOCs) in the past few decades [6, 7], since the pioneering work of Gersten et al. [8].
Metal oxide semiconductors have been widely studied for photocatalytic water splitting, because of their suitable band structures and high stability [9]. BiVO4 with a bandgap of 2.4 eV has received much attention. However, BiVO4 suffers from excessive charge recombination, low charge transport efficiency, and slow water oxidation kinetics. Strategies such as morphology control, hybridizing with other materials, and doping have been attempted to improve the photocatalytic performance of BiVO4 [10]. WOCs can also effectively reduce the recombination rate of photoinduced charge carries, which in turn improves the photocatalytic water oxidation. Noble metal-based WOCs such as Ru(bpy)32+ and IrO2 have been widely employed for this purpose, and exhibit very high activities for water oxidation. However, their high cost and scarcity limit their scalable application. Catalysts consisting of more abundant elements such as Co, Ni, Fe, Mn, and Bi are more promising in this respect. In particular, transitional metal-based molecular complexes have attracted increasing attention in the past few years, because of their low cost, tailorable structures, and homogeneous properties. A promising approach is combining molecular WOCs with semiconductors to utilize the advantages of both molecular complexes and metal oxides for efficient photocatalytic oxygen evolution. Niu et al. [11] reported a system consisting of semiconductor quantum dots and an acetaminosalol cobalt(Ⅱ) complex, which exhibited considerable activity and good stability during 72 h of photocatalytic reaction.
In the current study, we report a hybrid system consisting of M(dca)2 (where M is a metal; dca is dicyanamide) as a WOC and BiVO4 as the photosensitizer, for photocatalytic oxygen evolution in aqueous solution. Both BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 exhibit better performance than pure BiVO4, with turnover numbers (TONs) of 3.46 and 2.03 after 6 h of photocatalytic reaction, respectively. BiVO4/Co(dca)2 also possesses considerable photocatalytic activity after 30 h under visible light irradiation. Surface photovoltage (SPV) and electrochemical measurements show that efficient hole transfer from BiVO4 to M(dca)2 results in high charge separation efficiency, which in turn promotes the water oxidation reaction kinetics. It is demonstrated that the water nucleophilic attack (WNA) pathway could be responsible for the photocatalytic water oxidation. These findings provide insights for constructing earth-abundant element-based photocatalytic systems for efficient and stable oxygen evolution in aqueous solution.
Co(NO3)2·6H2O, Ni(NO3)2·6H2O, NaN(CN)2, Bi(NO3)3·5H2O, NH4VO3, Na2S2O8, Na2SO3, urea, ethanol, and N, N'-dimethyl-formamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Fluorine-doped tin oxide (FTO) coated glass was purchased from Nippon Sheet Glass Co., Ltd, Japan. All materials were of analytical grade and used without further processing. Millipore deionized water (resistivity: 18.25 MΩ·cm) was used throughout all experiments.
BiVO4 was prepared by a simple homogeneous precipitation method according to the literature [5]. Typically, 19.4 g of Bi(NO3)3·5H2O was added into 100 mL of water and stirred for 20 min to form a homogeneous solution. Simultaneously, 4.68 g of NH4VO3 was dissolved into 100 mL of water to form another solution. The two solutions were mixed, followed by the addition of 7.5 g of urea and then heating to 90 ℃ under stirring. After the crystallization of BiVO4, the obtained slurry was stirred at 90 ℃ for 24 h to form the precipitate, which was then washed with water, collected by filtration, and dried at 45 ℃.
M(dca)2 was synthesized according to a method reported previously [12]. In a typical synthesis, 5 mmol of Co(NO3)2·6H2O or Ni(NO3)2·6H2O was dissolved in 20 mL of water; 10 mmol of NaN(CN)2 was added into 20 mL of DMF. Then, the NaN(CN)2 solution was slowly added into the Co(NO3)2 or Ni(NO3)2 aqueous solution. The resulting suspension was stirred at room temperature for 12 h. Finally, the resulting precipitate was collected by filtration, washed with ethanol, and dried at 60 ℃ in a vacuum oven.
Powder X-ray diffraction (XRD) patterns of samples were collected using an X'Pert PRO MPD X-ray diffractometer (PANalytical, Netherlands) with (u/2u) Bragg-Brentano geometry (Ni filter; Cu Kα radiation; λ = 0.1541874 nm). UV-Vis absorption spectra of M(dca)2 solutions were collected using a Cary 7000 UV-Vis-near-IR spectrophotometer with a blank solution as the reference. Fourier-transform infrared (FT-IR) spectra were collected using a Nicolet Magna 670 FT-IR spectrometer. SPV spectra were collected using a PL-SPS1000 spectrometer (Beijing Perfectlight Technology Co., Ltd., P. R. China) with a monochromatic light source [13] and an irradiation range from 400 to 600 nm. The amount of molecular catalyst present during the photocatalytic O2 evolution process was determined using inductively-coupled plasma mass spectrometry (ICP-MS, ICPE-9000*, Shimadzu, Japan). Typically, 0.5 mL aliquots of the photocatalytic system suspension were removed at intervals using syringe pumps, followed by high-speed centrifugation (15000 r/min for 10 min) to obtain supernatants for ICP-MS tests. X-ray photoelectron spectrometry (XPS) (AXIS Ultra DLD, Shimadzu/Kratos Analytical, Japan) was used to detect the surface chemical components and states of samples, with monochromatic Al Kα radiation (150 W, 15 kV, 1486.6 eV) under high vacuum (< 3.9 × 10-10 kPa).
Photocatalytic oxygen production experiments were carried out in a 100 mL Pyrex glass cell under visible light. The light source was a 300 W Xe arc lamp (PLSSXE300, Beijing Trusttech Technology Co., Ltd, P. R. China) equipped with a 420 nm cut-off filter. In a typical photocatalysis experiment, 15 mg of BiVO4 powder as a photosensitizer was well dispersed in an aqueous solution (70 mL) containing 5 mg of M(dca)2 as a WOC and 0.8 g of Na2S2O8 as a sacrificial electron acceptor. The reaction solution was purged with Ar for 30 min to remove air before irradiation. The evolved gas was detected by gas chromatography (GC, SP-2100, Beijing Beifen-Ruili Analytical Instrument Co., Ltd, P. R. China) equipped with a thermal conductivity detector (TCD) and TDX-01 column. Argon was used as the carrier gas. No oxygen production was detected in the control experiment in the absence of irradiation.
The amount of M(dca)2 adsorbed on BiVO4 was calculated using the following relationship [14]:
where Cs is the amount of M(dca)2 adsorbed on BiVO4 (mg/g), C0 is the concentration of M(dca)2 (mg/L) before adsorption, Ce is the equilibrium concentration of M(dca)2 in the supernatant (mg/L), Vs is the solution volume (L), and m is the mass of BiVO4 (g). The concentration of M(dca)2 was determined using the Bouguer-Lambert-Beer law.
The adsorption data was fitted to the Freundlich model:
or its logarithmic form:
where Kf is the Freundlich adsorption coefficient which indicates the adsorption capacity, and 1/n is the isotherm curvature which indicates the adsorptive intensity.
The BiVO4 electrode was used as the working electrode, which was prepared according to reported procedures with some modifications [12]. First, FTO-coated glass was successively washed by sonication in acetone, alcohol, and then deionized water for 15 min each. A 5 mg of BiVO4 was dispersed in a mixture of 1.5 mL of water, 1.5 mL of ethanol, and 100 μL of Nafion solution (5 wt%) by sonication for 30 min, to obtain a stable suspension. Then, 250 μL of the resulting suspension was deposited on the FTO glass (2×2 cm), which was then dried at room temperature.
Electrochemical measurements were carried out using a potentiostat (273A, Princeton Applied Research Company, USA) and a three-electrode cell. All electrochemical measurements were carried out in the dark in a solution of 0.5 mol/L Na2SO4 (70 mL, pH = 6.8), which was purged with N2 for 20 min prior to measurements. Linear sweep voltammetry (LSV) tests were carried out from 0.4 to 1.8 V vs. Ag/AgCl. Electrochemical impedance spectroscopy (EIS) measurements were carried at 1.4 V vs. Ag/AgCl. Mott-Schottky (M-S) measurements were carried out in the dark at a frequency of 1000 Hz. Pt and a Ag/AgCl (saturated KCl) electrode were employed as the counter and reference electrodes, respectively.
Photoelectrochemical measurements were carried out using a potentiostat (273A, Princeton Applied Research Company, USA) with a three-electrode cell. All photoelectrochemical measurements were carried out in a solution of 0.5 mol/L Na2SO4 (70 mL, pH = 6.8), which was purged with N2 for 20 min prior to measurements. The separation efficiency and injection efficiency were tested with the addition of 0.1 mol/L Na2SO3 as a hole scavenger under visible light irradiation. Pt and a Ag/AgCl (saturated KCl) electrode were employed as the counter and reference electrodes, respectively.
BiVO4 was synthesized by a homogenous precipitation method according to literature [5]. The crystal structure was confirmed from the XRD pattern (Fig. S1). The main peaks at 2θ = 18.5° and 29.4° agreed well with standard data for the scheelite-monoclinic structure (JCPDS 14-0688). M(dca)2 was prepared by a simple solution method [12], and the molecular structure of M(dca)2 is shown in Fig. S2. The crystal and chemical structures and optical properties of M(dca)2 were characterized by XRD and FT-IR spectroscopy (Fig.s S3-S6). All measured data agreed well with that from literature reports [12]. This confirmed the synthesis of M(dca)2 (M = Co, Ni), which would serve as WOCs for photocatalytic oxygen evolution. As shown in Fig. S7, the BiVO4 sample absorbed light from the UV to visible region (λ < 530 nm), while M(dca)2 showed negligible absorption. This indicated that BiVO4 acted as the photosensitizer in the photocatalytic system.
To investigate the adsorption behavior of M(dca)2 on BiVO4, a series of adsorption tests based on UV-vis measurements were carried out. As shown in Fig. 1(a), the adsorption of M(dca)2 on BiVO4 was fitted well with the Freundlich equation. The adsorption coefficients (Kf) for Co(dca)2 and Ni(dca)2 were 3.14 and 0.014, respectively. These results indicated that Co(dca)2 more efficiently adsorbed on BiVO4 than Ni(dca)2. This indicated a stronger interaction between BiVO4 and Co(dca)2, which should yield more efficient charge transfer and thus higher photocatalytic performance. The mode of adsorption of M(dca)2 on BiVO4 was determined from FT-IR spectra. As shown in Fig.s S8 and S9, the peak at 2360-2184 cm-1 corresponded to the C≡N stretching vibration of dicyanamide ligands [15], while those at 1647-1640 cm-1 corresponded to the C=O stretching vibration of DMF [16] anchored with M(dca)2 during synthesis. All these FT-IR peaks did not shift after the hybridization of M(dca)2 with BiVO4. This indicated that M(dca)2 was adsorbed on BiVO4 via physical interactions rather than via chemical bonds.
To investigate the possibility of charge transfer between BiVO4 and M(dca)2, SPV spectra of BiVO4, BiVO4/Co(dca)2, and BiVO4/Ni(dca)2 were collected. As shown in Fig. 1(b), the addition of Co(dca)2 or Ni(dca)2 caused the amplitude of BiVO4 to significantly increase. A higher amplitude generally indicates more efficient charge separation [17, 18]. It was therefore reasonable that photogenerated charge separation in BiVO4 should be enhanced when M(dca)2 was introduced. BiVO4/Co(dca)2 showed better charge separation behavior than BiVO4/Ni(dca)2. This indicated that Co(dca)2 was a better WOC than Ni(dca)2 in the present hybrid systems.
Photocatalytic tests for oxygen evolution in the BiVO4/M(dca)2 hybrid systems were carried out in a fully aqueous solution containing Na2S2O8 as a sacrificial electron acceptor under visible light irradiation (λ > 420 nm). As shown in Fig. 2(a), both BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 exhibited higher performance than pure BiVO4, which demonstrated the catalytic role of M(dca)2 in the water oxidation process. In comparison to pure BiVO4 which showed a photocatalytic oxygen evolution rate of 252.2 μmol/(h·g) during 6 h of reaction, the BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 systems exhibited higher photocatalytic performances, with oxygen evolution rates of 508.1 and 297.7 μmol/(h·g) and TONs (vs. M(dca)2) of 3.46 and 2.03, respectively. The BiVO4/Co(dca)2 system showed a photocatalytic oxygen evolution rate 1.7 times higher than that of the BiVO4/Ni(dca)2 system. This could be explained by the higher surface charge transfer of BiVO4/Co(dca)2 as evidenced by SPV analysis, and further evidenced in the following electrochemical measurements. The stability of BiVO4/Co(dca)2 was monitored throughout a 30 h photocatalytic experiment. Hybridizing Co(dca)2 with BiVO4 enhanced the oxygen evolution rate and resulted in good photocatalyst stability. Under visible light irradiation, BiVO4/Co(dca)2 showed a relatively stable oxygen evolution rate during the 30 h photocatalytic reaction, as shown in Fig. 2(b), producing 171.95 μmol of oxygen after the 30 h (TON = 13, vs. Co(dca)2). The stability of BiVO4/Co(dca)2 was much higher than that of reported BiVO4/molecular catalyst photocatalytic systems, for which activities usually last < 10 h [17, 19].
The stabilities of the molecular catalysts during the photocatalytic process were further investigated by ICP-MS. As shown in Fig. S10, the amount of Co(dca)2 in the photocatalytic solution showed negligible decrease during the 30 h photocatalytic oxygen evolution test. This confirmed the high stability of Co(dca)2, with no evidence of the formation of insoluble Co species. After the photocatalytic test, the powder in the suspension was collected by filtration and analyzed by XPS. As shown in Fig. S11, the intensity of the Co 2p XPS peak was negligible for the powder residue of the BiVO4/Co(dca)2 system and was mainly due to trace Co(dca)2 physically absorbed on BiVO4. This adsorbed Co(dca)2 suppressed the formation of cobalt oxides on the BiVO4 surface during the photo-oxidation reaction. The N 1s peak of the powder residue after photocatalytic reaction was observed at ~400 eV, and showed no obvious shift compared with that of pristine Co(dca)2 (Fig. S12). This result also supported the conclusion that there was minimal change in the structure of the molecular catalyst during the photocatalytic test, since the catalyst was the only N source. Similar results were observed for the BiVO4/Ni(dca)2 system as shown in Fig.s S13 and S14, which confirmed the good stability of Ni(dca)2 as a WOC.
To better understand the increased oxygen generation activity of BiVO4 when M(dca)2 was added, electrochemical tests were carried out to investigate the effects of M(dca)2 on the charge transfer behavior at the BiVO4/electrolyte interface.
LSV curves of BiVO4, BiVO4/Co(dca)2, and BiVO4/Ni(dca)2 films were recorded in Na2SO4 (0.5 mol/L), as shown in Fig. 3(a). Both the BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 systems showed much higher current densities than BiVO4. The onset potential of BiVO4/Co(dca)2 for the OER was 1.78 V, which was more negative than those of BiVO4/Ni(dca)2 (1.91 V) and BiVO4 (2.06 V). The LSV results indicated that M(dca)2 were superior WOCs to BiVO4, which agreed well with the photocatalytic results. M-S measurements were carried out to further investigate the interfacial energetics in terms of interfacial band bending. BiVO4 is a typical n-type semiconductor, so its energy band bends upward at the BiVO4/electrolyte interface [20]. Thus, there is a driving force for hole transfer from the bulk to the surface for water oxidation [21]. As shown in Fig. 3(b), the flat band potential of BiVO4 was negatively shifted by 78 and 49 mV when Co(dca)2 and Ni(dca)2 were added, respectively. This resulted in increased band bending and thus an enhanced driving force for hole transfer, as shown in Fig. 3(d), which benefited the water oxidation reaction. Stronger band bending occurred at the Co(dca)2-engineered BiVO4/electrolyte interface vs. the Ni(dca)2-engineered interface, as demonstrated by the more cathodic shift of the flat band potential in the BiVO4/Co(dca)2 system. This implied that the Co(dca)2-engineered BiVO4/electrolyte interface energetics favored more efficient interfacial hole transfer. EIS tests were also carried out to investigate the interface charge transfer behaviors. The Nyquist plot of BiVO4/M(dca)2 showed a much smaller semicircle diameter than the plot of pure BiVO4, as shown in Fig. 3(c). This indicated the smaller interfacial resistivity for charge carrier transfer with the BiVO4/electrolyte interface energetics engineered by M(dca)2 [22]. The inset in Fig. 3(c) shows the equivalent circuit for the obtained Nyquist plots. The charge transfer resistances (Rct) of the three samples followed the order BiVO4/Co(dca)2 (507 Ω) < BiVO4/Ni(dca)2 (1747 Ω) < BiVO4 (5130 Ω). This indicated that the most favorable OER kinetics were obtained for the BiVO4/Co(dca)2 system, in good agreement with the photocatalytic and LSV results. The M-S and EIS results showed that photogenerated hole transfer from BiVO4 to the electrolyte was greatly promoted by the addition of M(dca)2, and that Co(dca)2 promoted interfacial charge transfer and then catalyzed the OER more efficiently than Ni(dca)2.
To evaluate the effects of M(dca)2 on the surface charge recombination of BiVO4 during the photocatalytic water oxidation reaction, the charge separation efficiency and charge injection efficiency were measured for films of BiVO4, BiVO4/Co(dca)2, and BiVO4/Ni(dca)2 as photoanodes. The photocurrent density (JPEC) can be expressed as:
where Jlim is the theoretical photocurrent limit under 1 sun irradiation, which is estimated from the bandgap of the semiconductor, ηabs is the light absorption efficiency, ηsep is the charge separation efficiency, and ηinj is the charge injection efficiency at the semiconductor/electrolyte interface. If all absorbed photons are converted to photocurrent (i.e. ηsep = 100%, ηinj = 100%), then the unity converted photocurrent density Jabs is expressed as:
The Jlim and Jabs for BiVO4 were 7.5 and 6.02 mA/cm2, respectively, according to reported data from the literature [23]. When Na2SO3 was introduced as the hole scavenger, the charge injection could be considered to be very fast, and ηinj could be considered to be 100%. Thus, the resulting photocurrent Jsep can be expressed as:
Thus, we obtain expressions for ηsep and ηinj:
The charge separation efficiencies (ηsep) of the three samples were similar, as shown in Fig. 4(a). This indicated that the addition of M(dca)2 did not greatly improve the bulk charge separation in BiVO4. This result in turn confirmed that M(dca)2 as a WOC mainly affected the charge transfer behavior at the BiVO4/electrolyte interface. As shown in Fig. 4(b), the hole injection efficiency (ηinj) of BiVO4 was only ~10% at 1.8 V vs. RHE. This indicated excessive charge recombination at the surface of pure BiVO4. A very slight enhancement in ηinj was observed for BiVO4/Ni(dca)2, which was evidenced in the slight increase in photocatalytic activity of the BiVO4/Ni(dca)2 system vs. pure BiVO4. In comparison, the ηinj of BiVO4/Co(dca)2 was much higher than that of BiVO4/Ni(dca)2, reaching ~40% at 1.8 V vs. RHE. These results demonstrated that hole transfer at the BiVO4 surface could be efficiently enhanced by adding Co(dca)2, which led to the much higher photocatalytic activity of the BiVO4/Co(dca)2 system.
The WNA and radical coupling (RC) mechanisms are generally considered to be responsible for water oxidation catalyzed by molecular catalysts [24, 25]. To determine a possible reaction mechanism for BiVO4/Co(dca)2, we carried out a series of photocatalytic tests. As shown in Fig. S15, the initial oxygen generation rate of BiVO4/Co(dca)2 exhibited a linear relationship with the Co(dca)2 concentration. This indicated a WNA mechanism, in which hydroperoxo species from a two-electron oxidation are attacked by water to generate oxygen.
Based on the above analysis, a possible reaction pathway for the BiVO4/Co(dca)2 system can be proposed, and is shown in Scheme 1. Electrons and holes are typically generated in BiVO4 under visible light irradiation. Photogenerated electrons at the conduction band (CB) were consumed by the S2O82- sacrificing agent. Holes at the valence band (VB) could be transferred to M(dca)2 absorbed on BiVO4, to catalyze oxygen evolution. According to the reported Pourbaix diagram for Co(dca)2 [12], the [CoⅡ-OH2]2+/[CoⅢ-OH]2+ and [CoⅢ-OH]2+/[CoⅣ-OH]3+ couples are responsible for the water oxidation reaction of Co(dca)2, and the potentials of CoⅡ-CoⅢ and CoⅢ-CoⅣ were 1.56 and 1.87 V (vs. RHE) at neutral pH, respectively. Based on the WNA mechanism discussed above [24], a possible reaction pathway for BiVO4/Co(dca)2 is proposed, as shown in the right part of Scheme 1. The Co(dca)2 ligated a water molecule to form [CoⅡ-OH2]2+, which was deprotonated to form [CoⅢ-OH]2+. The generated [CoⅢ-OH]2+ could be converted to [CoⅣ-OH]3+ by photogenerated holes to catalyze oxygen generation. Thus, the photocatalytic redox cycle of oxygen evolution was established.
We constructed a hybrid system with BiVO4 as the light harvester and a transitional metal complex (M(dca)2, M = Co, Ni) as the water oxidation catalyst, for photocatalytic oxygen evolution under visible light irradiation in fully aqueous solution using persulfate as a sacrificial agent. Introducing Co(dca)2 and Ni(dca)2 enhanced the photocatalytic performance of BiVO4. The oxygen evolution rates were 508.1 and 297.7 μmol/(h·g) for the BiVO4/Co(dca)2 and BiVO4/Ni(dca)2 systems, respectively. The excellent photocatalytic improvement, especially for the BiVO4/Co(dca)2 system, was ascribed to the Co(dca)2-engineered BiVO4/electrolyte interface energetics and Co(dca)2-catalyzed surface water oxidation. These two factors led to a decreased energy barrier for hole transfer from the bulk to the surface, which promoted the water oxidation reaction kinetics. These findings provide a means for hybridizing a semiconductor and molecular catalyst for active and stable photocatalytic water oxidation.
The financial support from the National Natural Science Foundation of China (51672210, 51323011, 51236007). S.H.Shen was supported by the Foundation for the Author of National Excellent Doctoral Dissertation of China (201335), the National Program for Support of Top-notch Young Professionals and the "Fundamental Research Funds for the Central Universities".
2018,
Vol. 39 Issue (3): 502-509 
PDF
2018,
Vol. 39 Issue (3): 502-509 PDF