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A nickel complex, an efficient cocatalyst for both electrochemical and photochemical driven hydrogen production from water
Molecular Catalysis 448 (2018) 10–17
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A nickel complex, an efficient cocatalyst for both electrochemical and photochemical driven hydrogen production from water ⁎
Jia-Mei Leia, Qiu-Xia Penga, Su-Ping Luoa, Yin Liub, Shu-Zhong Zhana, , Chun-Lin Nib, a b
T
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College of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China College of Materials and Energy, Institute of Biomaterial, South China Agricultural University, Guangzhou 510642, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Nickel complex CdS materials Electrocatalytic system Photocatalytic system Hydrogen evolution
Inspired by that nickel complexes can act as molecular catalysts for hydrogen generation from organic acid or water, we choose a nickel(II) complex, [Bz-4-MePy]2[NiII(i-mnt)2] 1 (i-mnt2− = iso-maleonitriledithiolate, Bz-4MePy+ = 1-benzyl-4′-methylpyridinium ion) as a potential catalyst. Electro- and photochemical investigations show that this nickel complex can act as both an electrocatalyst and a photocatalyst for H2 generation from water via an unstable nickel hydride intermediate. A homogeneous electrocatalytic system containing complex 1 can afford 577.4 mol of H2 per mole of catalyst per hour (mol H2/mol catalyst/h) from neutral water at an overpotential (OP) of 837.6 mV. Together with CdS nanorod (CdS NR) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor in a pH 3.5 aqueous solution, under photoirradiation with blue light (λmax = 469 nm), complex 1 also can provide hydrogen with a turnover number (TON) of 55340 mol of H2 per mole of catalyst during first 60 h irradiation. The highest apparent quantum yield (AQY) is ∼26.8% at 420 nm.
1. Introduction To decrease the growth of CO2 emissions and its consequent effects on global climate change, many methods have been developed with the hope of using natural resources to provide renewable energy, such as hydrogen. Hopefully, hydrogen is an ideal clean energy source that can relieve these problems. Electrochemical or photochemical driven water reduction to dihydrogen is an important and simple method [1–3]. To improve the rate of these reduction reactions, it is necessary to introduce catalysts. As we know, hydrogenase enzymes, containing transition metal complexes (such as iron and nickel) can efficiently catalyze both the production and the oxidation of hydrogen [4,5]. It is impossible to get in large amounts for practical uses, as the stability is often limited outside of their native environment. My group has successfully developed a series of molecular electrocatalysts based on transition metal complexes [6–13]. However, the most viable method for large-scale growth in carbon-free energy is the light-driven splitting of water into dihydrogen and dioxygen [14–16]. Now, the key issue on the splitting of water is the design of an efficient catalyst for water reduction with high turnover rates, good stability and durability [17,18]. In a typical system, the molecular catalyst is combined with a molecular photosensitizer (PS), such as ruthenium(II) trisbipyridyl complex, Ru(bpy)3Cl2 and a sacrificial electron donor. However, these photosensitizers can suffer degradation during irradiation [19–24].
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Encouraged by Honda’s work that photocatalytic splitting of water can occur on TiO2 electrode [25], photocatalysis has demonstrated wide ranging potential applications in areas such as converting solar energy. To mimic natural photosynthesis by converting solar energy into chemical energy, the research on the photocatalytic splitting of water to produce hydrogen, has been carried out extensively [26–29]. Considering that the visible light accounts for about 43% of the solar radiation energy, while the ultraviolet light only contributes to about 4%, people have been focusing the studies on the design of visible-lightresponsive photosensitizers [30–33]. CdS materials are selected as photosensitizers for the conversion of solar energy into chemical energy under visible-light irradiation, because CdS has a narrow band gap (with an Eg of 2.4 eV). Moreover, the potential of its conduction band (CB) is more negative than the reduction potential of hydrogen proton (H+/H2), letting it more proper for the H2 generation [34–37]. However, the photocatalytic activity of CdS itself toward water reduction is very low due to high-rate charge recombination of photogenerated electron [38]. To suppress this recombination, the introduction of a cocatalyst, which can be loaded on CdS is an ideal method for improving the photocatalytic activity [39–43]. Generally, noble metals, such as Pt, Pd and Rh are proper candidates, because they can attract and trap photoelectrons and suppress the recombination of electron-hole pairs, which together improve the efficiency of electron utilization. It is necessary to
Corresponding authors. E-mail addresses: [email protected] (S.-Z. Zhan), [email protected] (C.-L. Ni).
https://doi.org/10.1016/j.mcat.2018.01.014 Received 18 July 2017; Received in revised form 20 December 2017; Accepted 10 January 2018 Available online 21 February 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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develop non-noble-metal catalysts, as noble metals are too expensive [41]. These considerations have led to the development of cocatalysts employing more abundant metals, and several transition metal complexes have been developed as cocatalysts to produce hydrogen [44,45]. In this paper, we report a new catalyst based on the nickel complex, [Bz-4-MePy]2[Ni(i-mnt)2] 1 for electrochemical and photochemical driven hydrogen production. 2. Experimental section 2.1. Materials and physical measurements The nickel complex, [Bz-4-MePy]2[Ni(i-mnt)2] 1 was prepared according to the literature procedure [46]. And the CdS nanorods (CdS NRs) was provided by using the reported method [47]. The procedures for electrochemical measurements were showed in “Supplementary Materials”. A Hitachi U-3010 spectrometer was used to measure UV–vis spectra. ESI–MS experiment was performed in the negative ion mode on an AB Sciex API 3200 Spectrometer. The luminescent spectra were recorded on a F-4500 fluorescence spectrophotometer. For the photocatalytic system, each sample was prepared in a flask of buffer solution with ascorbic acid, CdS NRs, and nickel complex 1. Then, the flask was sealed with a septum. At room temperature, blue light (469 nm) was used to irradiate each sample. After photocatalysis, a 0.50 mL aliquot of the headspace was removed and replaced with 0.50 mL of CH4. The headspace sample was injected into the gas chromatograph (GC). An Agilent Technologies 7890A gas chromatography instrument was used for GC experiments. Measurements and analysis for the chemical compositions and valence states of the photocatalysts were carried out by ESCALAB 250 Xi X-photoelectron spectroscopy (XPS) with monochromatic Al Kα (1486.6 eV) X-ray sources. Transmission Electron Microscopy (TEM) images were afforded by using a JEM-2010 electron microscope. Scanning electron microscopy (SEM) images were obtained on a Merlin emission gun SEM instrument. Measurements and analysis for the crystalline diffraction patterns of CdS NRs and the related components were carried out by using Bruker D8 Advance powder Xray diffraction.
Fig. 1. (a) CVs of 1.10 mM complex 1 in CH3CN with 0.10 M of [n-Bu4N]ClO4. (b) CVs of a 1.10 mM of complex 1 in CH3CN with varying concentrations of acetic acid. Conditions: Glassy carbon working electrode (1.0 mm diameter), Pt counter electrode, Ag/AgNO3 reference electrode, scan rate 100 mV/s.
3. Results and discussion electrochemistry, with the results plotted in Fig. 1a. Complex 1 displayed two quasi-reversible couples at 0.47 and −1.28 V, and one reversible wave at −1.70 V versus Ag/AgNO3, which can be assigned to the NiIII/NiII, NiII/NiI and NiI/Ni0 couples, respectively. According to Fig. S2, no significant change was found after several scans, indicating that this nickel complex is stable under these conditions.
3.1. General characterization The reaction of NiCl2·6H2O, K2(i-mnt)·H2O and 1-benzyl-4′-methylpyridinium bromide ([Bz-4-MePy]Br) provided the nickel(II) complex, [Bz-4-MePy]2[Ni(i-mnt)2] 1 (Scheme 1) [46]. This ion-pair complex was agreement with the following ESI–MS analysis. As shown in Fig. S1, complex 1 exhibited one ion at a mass-to-charge ratio (m/z) of 338.0, which is assigned to [Ni(i-mnt)2-H]−. Considering that nickel complexes can set up homogeneous electrocatalytic systems for hydrogen generation via an unstable hydride intermediate [48–51], we checked if this nickel complex also can act as an electrocatalyst for hydrogen generation. In CH3CN, at a glassy carbon electrode, this nickel complex displayed a rich redox
3.2. The electrocatalytic system for hydrogen evolution from acetic acid Next, acetic acid was selected as proton resource to test the electrocatalytic performance of the nickel complex. As shown in Fig. 1b, with the addition of varied content of acetic acid (from 0.0 to 1.336 mM), the strengths of peak currents emerging at −1.74 V versus Ag/AgNO3 increased systematically, indicating that the reduction of Ni Scheme 1. The synthesis MePy]2[Ni(i-mnt)2] 1.
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[Bz-4-
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Scheme 2. Possible electrocatalytic mechanism for proton reduction to hydrogen by [Bz-4-MePy]2[Ni(i-mnt)2] 1.
3.3. The electrocatalytic system based on complex 1 for hydrogen evolution from water
(I) to Ni(0) and protonation are responsible for hydrogen generation in this electrocatalytic system. Based on the above observations, ESI–MS analysis and literature precedent [52], a possible electrocatalytic cycle for proton reduction to hydrogen was put forward. From Scheme 2, the nickel(0) species, [Ni0(i-mnt)2]4− was obtained by two one-electron reductions of [NiII(imnt)2]2− 1. Then the introduction of hydrogen proton (H+) resulted in the NiII-H species, [H-NiII(i-mnt)2]3−. Finally, further introduction of hydrogen proton (H+) led to the formation of dihydrogen, and regenerated the starting sample. Next, a series of measurements for bulk electrolysis were employed to test the electrocatalytic activity, giving the results shown in Fig. S3a. For example, with the introduction of complex 1, this electrocatalytic system afforded 137 mC of charge during 2 min of electrolysis under −1.45 V versus Ag/AgNO3, with accompanying production of a gas. Gas chromatography (GC) was used to detect the H2 produced after a 2 h bulk electrolysis (Fig. S4). According to Fig. S3b, under same conditions without complex 1, the CPE experiment only gave a charge of 8 mC. This result shows that this nickel complex does indeed serve as an electrocatalyst for hydrogen production. According to Eq. (1) [53] and Eq. (2) [54], the catalytic activities were calculated, with the results listed in Fig. S5. For example, at overpotential (OP) of 941.6 mV, this electrocatalytic system afforded 71.41 mol of hydrogen per mole of catalyst (mol H2/mol catalyst/h) (Eq. (S1)).
Overpotential = Applied potential − E⊙HA = Applied potential − E⊙H+-(2.303RT / F )pK aHA
We further characterized the electrochemical behavior of the nickel complex in buffered aqueous solutions, where pH 3.0–7.0 which are the range associated with catalytic water reduction. According to Fig. S6, in the presence of 1, with decreasing pH values from 7.0 to 3.0, the strength of the reduction wave increased, and the onset of the catalytic wave were shifted to higher potentials, which are assigned to a catalytic process [55]. Note, at pH 7.0, complex 1 exhibited one quasi-reversible redox wave at −0.48 versus Ag/AgCl (Fig. S6-inset), which can be assigned to the NiII/NiI couple. Further investigation for the electrocatalytic activity of complex 1 in aqueous media came from bulk electrolysis in buffer with and without complex 1. As shown in Fig. S7a, under −1.45 V versus Ag/AgCl, this buffer solution without complex 1 provided only 49 mC during 2 min of electrolysis. Surprisingly, the introduction of complex 1 resulted in 921 mC of charge under identical conditions (Fig. S7b) and many gas bubbles, which were confirmed to be H2 by GC. According to Figs. S8ab, this nickel complex could afford 6.5 mL of H2 during a 1 h electrolysis with a Faradaic efficiency of 91%. Based on Eqs. (2) and (3) [56], turnover frequencies (TOFs) for electrocatalytic hydrogen production by complex 1 were calculated, giving the results listed in Fig. S9. For example, the electrocatalytic system could afford 959.2 mol H2/mol catalyst/h at an OP of 837.6 mV (Eq. S2). Overpotential = Applied (−0.059 pH)
potential-
E(pH) = Applied
potential(3)
(1) 3.4. Photocatalytic system based on complex 1 for H2 generation
TOF = ΔC/(F*n1*n2*t)
Next, we examined the photocatalytic performance for water reduction by building a heterogeneous system containing complex 1 as a cocatalyst, ascorbic acid (H2A) as an electron donor and CdS NRs as a photosensitizer. To understand the effect of pH of media on the activity of H2 production, a series of experiments were carried out. According to the data plotted in Fig. S10, the best pH for photocatalytic H2 generation mediated by the nickel complex (0.02 mM) was observed at pH 3.5, with a turnover number (TON) of 1860 mol of H2 (mol of cat)−1 during 2 h of irradiation.
(2)
Where ΔC is the charge from the catalyst solution during controlledpotential electrolysis (CPE) minus the charge from the solution without catalyst during CPE; F is Faraday's constant, n1 is the number of moles of electrons required to generate one mole of H2, n2 is the number of moles of catalyst in solution, and t is the duration of electrolysis in + second. E⊙ is the standard potential for the solvated proton/dihyH drogen couple and KaHA is the dissociation constant of acetic acid. 12
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Fig. 2. Hydrogen evolution kinetics obtained upon continuous visible irradiation (λ = 469 nm) of a pH 3.5 buffer solution containing 0.07 mg mL−1 CdS, 0.12 M ascorbic acid, and 0.02 mM complex 1.
Fig. 5. pH values change of the catalytic system (0.12 M ascorbic acid, 0.07 mg mL−1 CdS NRs and 0.02 mM complex 1) during photolysis.
Fig. 3. Current responses versus time of the nickel complex/CdS NRs under visible-light irradiation (λ > 420 nm) at 0.0 V using Ag/AgCl as a reference electrode.
Fig. 6. XRD patterns of CdS NRs, complex 1 and the CdS NRs–1 composite photocatalysts (before and after irradiation).
complex 1 and a varying content of CdS NRs, the TON during 2 h of photolysis increased with increasing the concentrations of CdS NRs until a saturation value of 2050 mol of H2 (mol of cat)−1 was reached at 0.070 mg mL−1 (Fig. S11). Then, the TON decreased when the concentration of CdS NRs was set to more (Fig. S11). The photocatalytic systems containing 0.05 mg mL−1 CdS, 0.02 mM complex 1 and varying contents of ascorbic acid were designed to select the best ratio of ascorbic acid. According to Fig. S12, the TON used in 2 h increased with increasing the concentration of ascorbic acid until a highest value of 1960 mol of H2 (mol of cat)−1 was reached at 0.12 M. Then, the TON decreased when the concentration of ascorbic acid was set to more. The above observation and analysis resulted in an optimal threecomponent system, containing 0.07 mg mL−1 CdS NRs, 0.12 M ascorbic acid, and 0.02 mM complex 1. According to the data plotted in Fig. 2, H2 generation started immediately upon light irradiation and could last for about 120 h. For example, this system can achieve a TON of 58900 mol of H2 per mol of catalyst during first 80 h. Moreover, the photocatalytic system showed obvious photocurrents with good reproducibility, as expected when it was illuminated by visible light (Fig. 3). To understand the effect of photocatalyst concentrations on hydrogen production, a series of experiments were carried out with varying concentrations of complex 1. As shown in Fig. S13, under continuous irradiation, complex 1 provided H2 with a TON of 2320 mol of H2 (mol of cat)−1 at 0.05 mM. When the concentration of 1 was lowered to 0.01 mM, the TON increased significantly to 3900 mol of H2
Fig. 4. Photocatalytic H2 production of complex 1 under visible light (λ = 569 nm) and an apparent quantum yield (AQY) of complex 1 under monochromatic light (λ = 420 nm). The reaction system contained 0.070 mg CdS NRs, 0.020 mM complex 1, 0.12 M ascorbic acid (pH 3.5).
To obtain an optimal photocatalysis system, several measurements were carried out in parallel for comparison. First, we checked the effect of the concentrations of sacrificial reagent and photosensitizer on the photocatalytic H2 generation catalyzed by complex 1. For instance, to a photocatalytic system containing 0.12 M ascorbic acid, 0.020 mM
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Fig. 7. (a) XPS survey spectra of the photocatalyst samples. The black plot is for the mixture of CdS NRs and complex 1. (b) High resolution spectra of S 2p peaks at 161.344 eV and 162.518 eV. (c) High resolution spectra of Cd 3d peaks at 404.748 and 411.493 eV. (d) High resolution spectra of Ni 2p peaks at 854.092 and 871.290 eV.
Fig. 9. Mott-Schottky plots of CdS in 0.010 M K3Fe(CN)6/ K4Fe(CN)6 under dark conditions, FTO working electrode (1 cm2), Pt counter electrode, Ag/AgCl reference electrode.
the photocatalytic system for H2 generation were calculated, giving the results listed in Fig. 4. The average value of AQY was estimated to be ∼23.6% during 10 h of irradiation.
Fig. 8. Photoluminescence (PL) emission spectra of CdS NRs and complex 1/CdS NRs at the excitation wavelength of 450 nm.
(mol of cat)−1 (Fig. S13), showing that the concentration of complex 1 has a significant effect on the photocatalytic activity for H2 production. The dependence of TON on the catalyst concentration indicates that the formation of polynuclear species might be involved in the inactivation of complex 1 [57]. To obtain the apparent quantum yields (AQYs), the photocatalytic system was irradiated for 10 h under monochromic light with a bandpass filter (λ= 420 nm + 5 nm). According to Eq. (4) [58], the AQYs of
AQY (%) = (2·nH2 ·NA ·h·c)/(tirr ·λ·I·A)·100
(4)
nH2 is the hydrogen generation (mol H2), NA is the Avogadro constant, h is the Planck constant, c is speed of light, tirr is the irradiation time, I is the intensity, A is the irradiated area of the photoreactor, where, I is 5 mW cm−2, A is 19.63 cm2, tirr is 7200s. To identify components responsible for the photocatalytic H2 evolution in the photocatalytic system, we added any two of the three 14
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during photolysis. To solve these equations, several physical or physiochemical methods were employed. First, powder X-ray diffraction (XRD) technology was used to characterize this photocatalytic system, with the results listed in Fig. 6. Comparing with the XRD pattern of complex 1 (Fig. 6a), all the different peaks (Fig. 6c) were well matched with the pattern of CdS NRs (Fig. 6b), indicating that the introduction of the nickel complex does not affect the crystallinity of CdS NRs. According to Fig. 6d, the XRD signs were almost same as those before irradiation, indicating that CdS NRs is stable during 120 h of photolysis. To characterize the chemical composition and oxidation state of different atoms in the composite particles before and after irradiation, the XPS spectra of the samples were provided. From Fig. 7a, both spectra of the mixed complex 1/CdS NR sample before (black) and after irradiation (red) were quite similar, with the presence of Cd, S, O, and C elements. As shown in Fig. 7b, before photocatalysis, the mixed complex 1/CdS NR sample exhibited two main peaks at 161.344 eV and 162.518 eV, which can be assigned to S 2p in CdS NRs [59–61]. Fig. 7c demonstrated two obvious Cd 3d peaks located at 404.748 and 411.493 eV, which are consistent with the Cd character in CdS NRs [59]. After 120 h irradiation, the position or strength of both S 2 s and Cd 3d peaks remained almost constant, indicating that CdS NRs is stable as a photosensitizer during photocatalysis. The high resolution XPS spectrum of Ni 2p in a mixed complex 1/CdS NR sample was shown in Fig. 7d. Before irradiation, two appreciable Ni 2p peaks were observed at 854.092 and 871.290 eV, which are assigned to a Ni2+ ion. After 120 h irradiation, the strengths of peaks at both 854.092 and 871.290 eV decreased (Fig. 7d), indicating that the photoirradiation led the nickel complex to be decomposed. However, the adjustment of complex 1 back to the original 0.02 mM did not recover completely, indicating that the decomposition of the nickel complex is not the only reason for the loss of the photocatalytic activity. We also used UV–vis spectra to check the stability of the above photocatalytic system. According to Fig. S19, before irradiation, the three-component system afforded a main peak at 260 nm, which can be assigned to that of ascorbic acid. However, the strength of the peak at 260 nm decreased over a photolysis period of 120 h, indicating that the amount of ascorbic acid decreased after photolysis under visible light. These results suggest that the decompositions of complex 1 and H2A are the primary reason for the cease of hydrogen evolution after illumination.
Fig. 10. Electrochemical impedance spectroscopy Nyquist plots of CdS NRs and complex 1/CdS NRs with 0.010 M K3Fe(CN)6/K4Fe(CN)6 electrolyte in dark conditions.
components (ascorbic acid, CdS NRs, or complex 1) to a reaction flask to see if the H2 production can be formed. According to Fig. S14, a mixture of complex 1 and CdS NRs only afforded 0.62 μmol H2, the integration of ascorbic acid and CdS NRs led to an increase in H2 evolution (2.17 μmol), and 0.90 μmol H2 was produced when ascorbic acid and complex 1 was combined. Thus, the combination of the nickel complex, ascorbic acid and CdS NRs is essential for the photocatalytic activity in this reaction system. To test the effect of the (crystallinity) morphology of CdS on the catalytic activity, we investigated the photocatalytic activities of the systems with CdS clusters (Fig. S15) and CdS NRs (Fig. S16), respectively. As shown in Figs. S17–S18, CdS NR exhibited much better photocatalytic activity than that of CdS cluster. This can be attributed to nanosize effects: 1) the smaller size can cause larger surface area and more catalytic sites; 2) the nanometer-scale size can shorten carrier diffuse path and reduce the recombination of photogenerated electrons and holes in catalyst [43]. 3.5. Investigation for the stability and durability of the photocatalytic system As shown in Fig. 5, after a 120 h photolysis period, the pH had increased by 1.2 units (from 3.5 to 4.7), consistent with the accumulation of OH− by water reduction, 2H2O + 2e → H2 + 2OH−. However, this catalytic function could not be recovered completely when the solution pH was adjusted back to the original 3.5. This result can be attributed to the decomposition of CdS NRs, ascorbic acid, complex 1 or all of them
3.6. Photocatalytic mechanism It is apparent that the photocatalytic hydrogen evolution goes as follows: visible light absorption of the photosensitizer, formation of Scheme 3. Possible mechanism for H2 production by the photocatalytic system based on complex 1.
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Appendix A. Supplementary data
photo-generated charges, separation of electron–hole pair, and electron transfer. As shown in Fig. S20, the CdS NRs had a main absorption in the visible region with a band gap at ∼2.30 eV. Mixing the nickel complex with CdS NRs resulted in a red shift of the absorption onset and reduction of Eg of CdS NRs (Fig. S20b). The results indicate that the introduction of the nickel complex into CdS NRs can obviously improve the range and ability of visible light absorption of CdS NRs. To further look for the photocatalytic mechanism for hydrogen generation over the nickel complex modified CdS NRs, the photoluminescence (PL) spectra of the photocatalysts were provided. As shown in Fig. 8, CdS NRs exhibited two emission peaks at 536 and 680 nm, respectively. However, the introduction of the nickel complex into CdS NRs led to decrease of the peak intensities of CdS NRs at both 536 and 680 nm, and a lower possibility of electron–hole pair recombination because of the fast electron transfer from CdS NRs to complex 1 [62]. To understand the relationship between the electrocatalytic and photocatalytic performances, Mott−Schottky analysis were carried out, with the results shown in Fig. 9. According to Fig. 9, Mott−Schottky analysis gave a flat-band potential (Efb) of −0.58 V versus Ag/AgCl [63]. Based on that the nickel complex has a reductive potential at −0.48 V versus Ag/AgCl (Fig. S6-inset in the Supporting Information), the electron transfer from the excited CdS NRs to the nickel complex is thermodynamically favorable. Moreover, the electrochemical impedance spectroscopy was employed to investigate charge transfer properties and separation efficiency of photogenerated charge carriers. Compared with CdS NRs, the CdS NRs/complex 1 showed much smaller arc radius (Fig. 10), representing a faster interfacial charge transfer and higher separation efficiency of photogenerated charge carriers [64–66]. Based on the above observation, electrochemical analysis and literature precedents [67–69], we put forward a possible mechanism for the photocatalytic H2 production. As outlined in Scheme 3, the transfer of the photoexcited electrons from the conduction band (CB) of CdS NSs to the nickel(II) ion of complex 1 afforded the reduced nickel(0) species. Then, the addition of hydrogen proton formed the NiII-H species, a high reactive intermediate. Further introduction of hydrogen proton to the NiII-H species provided dihydrogen, and regenerated the starting nickel complex 1. Moreover, the photogenerated hole remaining in the CdS NSs was subsequently coupled with electrons come from H2A, thus preventing the recombination of photogenerated charge carries and letting the continuous reactions.
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4. Conclusion We have designed successfully new catalytic systems based on the nickel complex for both electrochemical and photochemical water reduction to dihydrogen. The electrocatalytic system can afford 577.4 mol H2/mol catalyst/h from neutral water at an OP of 837.6 mV. The photocatalytic system containing CdS NRs as a photosensitizer, the nickel complex as a cocatalyst, and H2A as a sacrificial electron donor can provide hydrogen with a TON of 55340 mol of H2 per mole of catalyst during first 60 h irradiation and work for about 120 h. Compared with CdS cluster as a precursor, CdS NR shows much better photocatalytic activity, indicating that the crystallinity of photosensitizer significantly affects the photocatalytic activity of the system. The studies also show that the decompositions of complex 1 and H2A are the primary reason for the cease of hydrogen evolution after illumination. These findings may offer a new method for the design of effective non-noble-metal catalyst for water reduction. Acknowledgements This work was supported by the National Science Foundation of China (Nos. 20971045, 21271073 and 21372088) and the Science and Technology Project (No.2016A010103025) from Guangdong Science and Technology Department. 16
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
A nickel complex, an efficient cocatalyst for both electrochemical and photochemical driven hydrogen production from water ⁎
Jia-Mei Leia, Qiu-Xia Penga, Su-Ping Luoa, Yin Liub, Shu-Zhong Zhana, , Chun-Lin Nib, a b
T
⁎
College of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China College of Materials and Energy, Institute of Biomaterial, South China Agricultural University, Guangzhou 510642, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Nickel complex CdS materials Electrocatalytic system Photocatalytic system Hydrogen evolution
Inspired by that nickel complexes can act as molecular catalysts for hydrogen generation from organic acid or water, we choose a nickel(II) complex, [Bz-4-MePy]2[NiII(i-mnt)2] 1 (i-mnt2− = iso-maleonitriledithiolate, Bz-4MePy+ = 1-benzyl-4′-methylpyridinium ion) as a potential catalyst. Electro- and photochemical investigations show that this nickel complex can act as both an electrocatalyst and a photocatalyst for H2 generation from water via an unstable nickel hydride intermediate. A homogeneous electrocatalytic system containing complex 1 can afford 577.4 mol of H2 per mole of catalyst per hour (mol H2/mol catalyst/h) from neutral water at an overpotential (OP) of 837.6 mV. Together with CdS nanorod (CdS NR) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor in a pH 3.5 aqueous solution, under photoirradiation with blue light (λmax = 469 nm), complex 1 also can provide hydrogen with a turnover number (TON) of 55340 mol of H2 per mole of catalyst during first 60 h irradiation. The highest apparent quantum yield (AQY) is ∼26.8% at 420 nm.
1. Introduction To decrease the growth of CO2 emissions and its consequent effects on global climate change, many methods have been developed with the hope of using natural resources to provide renewable energy, such as hydrogen. Hopefully, hydrogen is an ideal clean energy source that can relieve these problems. Electrochemical or photochemical driven water reduction to dihydrogen is an important and simple method [1–3]. To improve the rate of these reduction reactions, it is necessary to introduce catalysts. As we know, hydrogenase enzymes, containing transition metal complexes (such as iron and nickel) can efficiently catalyze both the production and the oxidation of hydrogen [4,5]. It is impossible to get in large amounts for practical uses, as the stability is often limited outside of their native environment. My group has successfully developed a series of molecular electrocatalysts based on transition metal complexes [6–13]. However, the most viable method for large-scale growth in carbon-free energy is the light-driven splitting of water into dihydrogen and dioxygen [14–16]. Now, the key issue on the splitting of water is the design of an efficient catalyst for water reduction with high turnover rates, good stability and durability [17,18]. In a typical system, the molecular catalyst is combined with a molecular photosensitizer (PS), such as ruthenium(II) trisbipyridyl complex, Ru(bpy)3Cl2 and a sacrificial electron donor. However, these photosensitizers can suffer degradation during irradiation [19–24].
⁎
Encouraged by Honda’s work that photocatalytic splitting of water can occur on TiO2 electrode [25], photocatalysis has demonstrated wide ranging potential applications in areas such as converting solar energy. To mimic natural photosynthesis by converting solar energy into chemical energy, the research on the photocatalytic splitting of water to produce hydrogen, has been carried out extensively [26–29]. Considering that the visible light accounts for about 43% of the solar radiation energy, while the ultraviolet light only contributes to about 4%, people have been focusing the studies on the design of visible-lightresponsive photosensitizers [30–33]. CdS materials are selected as photosensitizers for the conversion of solar energy into chemical energy under visible-light irradiation, because CdS has a narrow band gap (with an Eg of 2.4 eV). Moreover, the potential of its conduction band (CB) is more negative than the reduction potential of hydrogen proton (H+/H2), letting it more proper for the H2 generation [34–37]. However, the photocatalytic activity of CdS itself toward water reduction is very low due to high-rate charge recombination of photogenerated electron [38]. To suppress this recombination, the introduction of a cocatalyst, which can be loaded on CdS is an ideal method for improving the photocatalytic activity [39–43]. Generally, noble metals, such as Pt, Pd and Rh are proper candidates, because they can attract and trap photoelectrons and suppress the recombination of electron-hole pairs, which together improve the efficiency of electron utilization. It is necessary to
Corresponding authors. E-mail addresses: [email protected] (S.-Z. Zhan), [email protected] (C.-L. Ni).
https://doi.org/10.1016/j.mcat.2018.01.014 Received 18 July 2017; Received in revised form 20 December 2017; Accepted 10 January 2018 Available online 21 February 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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develop non-noble-metal catalysts, as noble metals are too expensive [41]. These considerations have led to the development of cocatalysts employing more abundant metals, and several transition metal complexes have been developed as cocatalysts to produce hydrogen [44,45]. In this paper, we report a new catalyst based on the nickel complex, [Bz-4-MePy]2[Ni(i-mnt)2] 1 for electrochemical and photochemical driven hydrogen production. 2. Experimental section 2.1. Materials and physical measurements The nickel complex, [Bz-4-MePy]2[Ni(i-mnt)2] 1 was prepared according to the literature procedure [46]. And the CdS nanorods (CdS NRs) was provided by using the reported method [47]. The procedures for electrochemical measurements were showed in “Supplementary Materials”. A Hitachi U-3010 spectrometer was used to measure UV–vis spectra. ESI–MS experiment was performed in the negative ion mode on an AB Sciex API 3200 Spectrometer. The luminescent spectra were recorded on a F-4500 fluorescence spectrophotometer. For the photocatalytic system, each sample was prepared in a flask of buffer solution with ascorbic acid, CdS NRs, and nickel complex 1. Then, the flask was sealed with a septum. At room temperature, blue light (469 nm) was used to irradiate each sample. After photocatalysis, a 0.50 mL aliquot of the headspace was removed and replaced with 0.50 mL of CH4. The headspace sample was injected into the gas chromatograph (GC). An Agilent Technologies 7890A gas chromatography instrument was used for GC experiments. Measurements and analysis for the chemical compositions and valence states of the photocatalysts were carried out by ESCALAB 250 Xi X-photoelectron spectroscopy (XPS) with monochromatic Al Kα (1486.6 eV) X-ray sources. Transmission Electron Microscopy (TEM) images were afforded by using a JEM-2010 electron microscope. Scanning electron microscopy (SEM) images were obtained on a Merlin emission gun SEM instrument. Measurements and analysis for the crystalline diffraction patterns of CdS NRs and the related components were carried out by using Bruker D8 Advance powder Xray diffraction.
Fig. 1. (a) CVs of 1.10 mM complex 1 in CH3CN with 0.10 M of [n-Bu4N]ClO4. (b) CVs of a 1.10 mM of complex 1 in CH3CN with varying concentrations of acetic acid. Conditions: Glassy carbon working electrode (1.0 mm diameter), Pt counter electrode, Ag/AgNO3 reference electrode, scan rate 100 mV/s.
3. Results and discussion electrochemistry, with the results plotted in Fig. 1a. Complex 1 displayed two quasi-reversible couples at 0.47 and −1.28 V, and one reversible wave at −1.70 V versus Ag/AgNO3, which can be assigned to the NiIII/NiII, NiII/NiI and NiI/Ni0 couples, respectively. According to Fig. S2, no significant change was found after several scans, indicating that this nickel complex is stable under these conditions.
3.1. General characterization The reaction of NiCl2·6H2O, K2(i-mnt)·H2O and 1-benzyl-4′-methylpyridinium bromide ([Bz-4-MePy]Br) provided the nickel(II) complex, [Bz-4-MePy]2[Ni(i-mnt)2] 1 (Scheme 1) [46]. This ion-pair complex was agreement with the following ESI–MS analysis. As shown in Fig. S1, complex 1 exhibited one ion at a mass-to-charge ratio (m/z) of 338.0, which is assigned to [Ni(i-mnt)2-H]−. Considering that nickel complexes can set up homogeneous electrocatalytic systems for hydrogen generation via an unstable hydride intermediate [48–51], we checked if this nickel complex also can act as an electrocatalyst for hydrogen generation. In CH3CN, at a glassy carbon electrode, this nickel complex displayed a rich redox
3.2. The electrocatalytic system for hydrogen evolution from acetic acid Next, acetic acid was selected as proton resource to test the electrocatalytic performance of the nickel complex. As shown in Fig. 1b, with the addition of varied content of acetic acid (from 0.0 to 1.336 mM), the strengths of peak currents emerging at −1.74 V versus Ag/AgNO3 increased systematically, indicating that the reduction of Ni Scheme 1. The synthesis MePy]2[Ni(i-mnt)2] 1.
11
of
[Bz-4-
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Scheme 2. Possible electrocatalytic mechanism for proton reduction to hydrogen by [Bz-4-MePy]2[Ni(i-mnt)2] 1.
3.3. The electrocatalytic system based on complex 1 for hydrogen evolution from water
(I) to Ni(0) and protonation are responsible for hydrogen generation in this electrocatalytic system. Based on the above observations, ESI–MS analysis and literature precedent [52], a possible electrocatalytic cycle for proton reduction to hydrogen was put forward. From Scheme 2, the nickel(0) species, [Ni0(i-mnt)2]4− was obtained by two one-electron reductions of [NiII(imnt)2]2− 1. Then the introduction of hydrogen proton (H+) resulted in the NiII-H species, [H-NiII(i-mnt)2]3−. Finally, further introduction of hydrogen proton (H+) led to the formation of dihydrogen, and regenerated the starting sample. Next, a series of measurements for bulk electrolysis were employed to test the electrocatalytic activity, giving the results shown in Fig. S3a. For example, with the introduction of complex 1, this electrocatalytic system afforded 137 mC of charge during 2 min of electrolysis under −1.45 V versus Ag/AgNO3, with accompanying production of a gas. Gas chromatography (GC) was used to detect the H2 produced after a 2 h bulk electrolysis (Fig. S4). According to Fig. S3b, under same conditions without complex 1, the CPE experiment only gave a charge of 8 mC. This result shows that this nickel complex does indeed serve as an electrocatalyst for hydrogen production. According to Eq. (1) [53] and Eq. (2) [54], the catalytic activities were calculated, with the results listed in Fig. S5. For example, at overpotential (OP) of 941.6 mV, this electrocatalytic system afforded 71.41 mol of hydrogen per mole of catalyst (mol H2/mol catalyst/h) (Eq. (S1)).
Overpotential = Applied potential − E⊙HA = Applied potential − E⊙H+-(2.303RT / F )pK aHA
We further characterized the electrochemical behavior of the nickel complex in buffered aqueous solutions, where pH 3.0–7.0 which are the range associated with catalytic water reduction. According to Fig. S6, in the presence of 1, with decreasing pH values from 7.0 to 3.0, the strength of the reduction wave increased, and the onset of the catalytic wave were shifted to higher potentials, which are assigned to a catalytic process [55]. Note, at pH 7.0, complex 1 exhibited one quasi-reversible redox wave at −0.48 versus Ag/AgCl (Fig. S6-inset), which can be assigned to the NiII/NiI couple. Further investigation for the electrocatalytic activity of complex 1 in aqueous media came from bulk electrolysis in buffer with and without complex 1. As shown in Fig. S7a, under −1.45 V versus Ag/AgCl, this buffer solution without complex 1 provided only 49 mC during 2 min of electrolysis. Surprisingly, the introduction of complex 1 resulted in 921 mC of charge under identical conditions (Fig. S7b) and many gas bubbles, which were confirmed to be H2 by GC. According to Figs. S8ab, this nickel complex could afford 6.5 mL of H2 during a 1 h electrolysis with a Faradaic efficiency of 91%. Based on Eqs. (2) and (3) [56], turnover frequencies (TOFs) for electrocatalytic hydrogen production by complex 1 were calculated, giving the results listed in Fig. S9. For example, the electrocatalytic system could afford 959.2 mol H2/mol catalyst/h at an OP of 837.6 mV (Eq. S2). Overpotential = Applied (−0.059 pH)
potential-
E(pH) = Applied
potential(3)
(1) 3.4. Photocatalytic system based on complex 1 for H2 generation
TOF = ΔC/(F*n1*n2*t)
Next, we examined the photocatalytic performance for water reduction by building a heterogeneous system containing complex 1 as a cocatalyst, ascorbic acid (H2A) as an electron donor and CdS NRs as a photosensitizer. To understand the effect of pH of media on the activity of H2 production, a series of experiments were carried out. According to the data plotted in Fig. S10, the best pH for photocatalytic H2 generation mediated by the nickel complex (0.02 mM) was observed at pH 3.5, with a turnover number (TON) of 1860 mol of H2 (mol of cat)−1 during 2 h of irradiation.
(2)
Where ΔC is the charge from the catalyst solution during controlledpotential electrolysis (CPE) minus the charge from the solution without catalyst during CPE; F is Faraday's constant, n1 is the number of moles of electrons required to generate one mole of H2, n2 is the number of moles of catalyst in solution, and t is the duration of electrolysis in + second. E⊙ is the standard potential for the solvated proton/dihyH drogen couple and KaHA is the dissociation constant of acetic acid. 12
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Fig. 2. Hydrogen evolution kinetics obtained upon continuous visible irradiation (λ = 469 nm) of a pH 3.5 buffer solution containing 0.07 mg mL−1 CdS, 0.12 M ascorbic acid, and 0.02 mM complex 1.
Fig. 5. pH values change of the catalytic system (0.12 M ascorbic acid, 0.07 mg mL−1 CdS NRs and 0.02 mM complex 1) during photolysis.
Fig. 3. Current responses versus time of the nickel complex/CdS NRs under visible-light irradiation (λ > 420 nm) at 0.0 V using Ag/AgCl as a reference electrode.
Fig. 6. XRD patterns of CdS NRs, complex 1 and the CdS NRs–1 composite photocatalysts (before and after irradiation).
complex 1 and a varying content of CdS NRs, the TON during 2 h of photolysis increased with increasing the concentrations of CdS NRs until a saturation value of 2050 mol of H2 (mol of cat)−1 was reached at 0.070 mg mL−1 (Fig. S11). Then, the TON decreased when the concentration of CdS NRs was set to more (Fig. S11). The photocatalytic systems containing 0.05 mg mL−1 CdS, 0.02 mM complex 1 and varying contents of ascorbic acid were designed to select the best ratio of ascorbic acid. According to Fig. S12, the TON used in 2 h increased with increasing the concentration of ascorbic acid until a highest value of 1960 mol of H2 (mol of cat)−1 was reached at 0.12 M. Then, the TON decreased when the concentration of ascorbic acid was set to more. The above observation and analysis resulted in an optimal threecomponent system, containing 0.07 mg mL−1 CdS NRs, 0.12 M ascorbic acid, and 0.02 mM complex 1. According to the data plotted in Fig. 2, H2 generation started immediately upon light irradiation and could last for about 120 h. For example, this system can achieve a TON of 58900 mol of H2 per mol of catalyst during first 80 h. Moreover, the photocatalytic system showed obvious photocurrents with good reproducibility, as expected when it was illuminated by visible light (Fig. 3). To understand the effect of photocatalyst concentrations on hydrogen production, a series of experiments were carried out with varying concentrations of complex 1. As shown in Fig. S13, under continuous irradiation, complex 1 provided H2 with a TON of 2320 mol of H2 (mol of cat)−1 at 0.05 mM. When the concentration of 1 was lowered to 0.01 mM, the TON increased significantly to 3900 mol of H2
Fig. 4. Photocatalytic H2 production of complex 1 under visible light (λ = 569 nm) and an apparent quantum yield (AQY) of complex 1 under monochromatic light (λ = 420 nm). The reaction system contained 0.070 mg CdS NRs, 0.020 mM complex 1, 0.12 M ascorbic acid (pH 3.5).
To obtain an optimal photocatalysis system, several measurements were carried out in parallel for comparison. First, we checked the effect of the concentrations of sacrificial reagent and photosensitizer on the photocatalytic H2 generation catalyzed by complex 1. For instance, to a photocatalytic system containing 0.12 M ascorbic acid, 0.020 mM
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Fig. 7. (a) XPS survey spectra of the photocatalyst samples. The black plot is for the mixture of CdS NRs and complex 1. (b) High resolution spectra of S 2p peaks at 161.344 eV and 162.518 eV. (c) High resolution spectra of Cd 3d peaks at 404.748 and 411.493 eV. (d) High resolution spectra of Ni 2p peaks at 854.092 and 871.290 eV.
Fig. 9. Mott-Schottky plots of CdS in 0.010 M K3Fe(CN)6/ K4Fe(CN)6 under dark conditions, FTO working electrode (1 cm2), Pt counter electrode, Ag/AgCl reference electrode.
the photocatalytic system for H2 generation were calculated, giving the results listed in Fig. 4. The average value of AQY was estimated to be ∼23.6% during 10 h of irradiation.
Fig. 8. Photoluminescence (PL) emission spectra of CdS NRs and complex 1/CdS NRs at the excitation wavelength of 450 nm.
(mol of cat)−1 (Fig. S13), showing that the concentration of complex 1 has a significant effect on the photocatalytic activity for H2 production. The dependence of TON on the catalyst concentration indicates that the formation of polynuclear species might be involved in the inactivation of complex 1 [57]. To obtain the apparent quantum yields (AQYs), the photocatalytic system was irradiated for 10 h under monochromic light with a bandpass filter (λ= 420 nm + 5 nm). According to Eq. (4) [58], the AQYs of
AQY (%) = (2·nH2 ·NA ·h·c)/(tirr ·λ·I·A)·100
(4)
nH2 is the hydrogen generation (mol H2), NA is the Avogadro constant, h is the Planck constant, c is speed of light, tirr is the irradiation time, I is the intensity, A is the irradiated area of the photoreactor, where, I is 5 mW cm−2, A is 19.63 cm2, tirr is 7200s. To identify components responsible for the photocatalytic H2 evolution in the photocatalytic system, we added any two of the three 14
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during photolysis. To solve these equations, several physical or physiochemical methods were employed. First, powder X-ray diffraction (XRD) technology was used to characterize this photocatalytic system, with the results listed in Fig. 6. Comparing with the XRD pattern of complex 1 (Fig. 6a), all the different peaks (Fig. 6c) were well matched with the pattern of CdS NRs (Fig. 6b), indicating that the introduction of the nickel complex does not affect the crystallinity of CdS NRs. According to Fig. 6d, the XRD signs were almost same as those before irradiation, indicating that CdS NRs is stable during 120 h of photolysis. To characterize the chemical composition and oxidation state of different atoms in the composite particles before and after irradiation, the XPS spectra of the samples were provided. From Fig. 7a, both spectra of the mixed complex 1/CdS NR sample before (black) and after irradiation (red) were quite similar, with the presence of Cd, S, O, and C elements. As shown in Fig. 7b, before photocatalysis, the mixed complex 1/CdS NR sample exhibited two main peaks at 161.344 eV and 162.518 eV, which can be assigned to S 2p in CdS NRs [59–61]. Fig. 7c demonstrated two obvious Cd 3d peaks located at 404.748 and 411.493 eV, which are consistent with the Cd character in CdS NRs [59]. After 120 h irradiation, the position or strength of both S 2 s and Cd 3d peaks remained almost constant, indicating that CdS NRs is stable as a photosensitizer during photocatalysis. The high resolution XPS spectrum of Ni 2p in a mixed complex 1/CdS NR sample was shown in Fig. 7d. Before irradiation, two appreciable Ni 2p peaks were observed at 854.092 and 871.290 eV, which are assigned to a Ni2+ ion. After 120 h irradiation, the strengths of peaks at both 854.092 and 871.290 eV decreased (Fig. 7d), indicating that the photoirradiation led the nickel complex to be decomposed. However, the adjustment of complex 1 back to the original 0.02 mM did not recover completely, indicating that the decomposition of the nickel complex is not the only reason for the loss of the photocatalytic activity. We also used UV–vis spectra to check the stability of the above photocatalytic system. According to Fig. S19, before irradiation, the three-component system afforded a main peak at 260 nm, which can be assigned to that of ascorbic acid. However, the strength of the peak at 260 nm decreased over a photolysis period of 120 h, indicating that the amount of ascorbic acid decreased after photolysis under visible light. These results suggest that the decompositions of complex 1 and H2A are the primary reason for the cease of hydrogen evolution after illumination.
Fig. 10. Electrochemical impedance spectroscopy Nyquist plots of CdS NRs and complex 1/CdS NRs with 0.010 M K3Fe(CN)6/K4Fe(CN)6 electrolyte in dark conditions.
components (ascorbic acid, CdS NRs, or complex 1) to a reaction flask to see if the H2 production can be formed. According to Fig. S14, a mixture of complex 1 and CdS NRs only afforded 0.62 μmol H2, the integration of ascorbic acid and CdS NRs led to an increase in H2 evolution (2.17 μmol), and 0.90 μmol H2 was produced when ascorbic acid and complex 1 was combined. Thus, the combination of the nickel complex, ascorbic acid and CdS NRs is essential for the photocatalytic activity in this reaction system. To test the effect of the (crystallinity) morphology of CdS on the catalytic activity, we investigated the photocatalytic activities of the systems with CdS clusters (Fig. S15) and CdS NRs (Fig. S16), respectively. As shown in Figs. S17–S18, CdS NR exhibited much better photocatalytic activity than that of CdS cluster. This can be attributed to nanosize effects: 1) the smaller size can cause larger surface area and more catalytic sites; 2) the nanometer-scale size can shorten carrier diffuse path and reduce the recombination of photogenerated electrons and holes in catalyst [43]. 3.5. Investigation for the stability and durability of the photocatalytic system As shown in Fig. 5, after a 120 h photolysis period, the pH had increased by 1.2 units (from 3.5 to 4.7), consistent with the accumulation of OH− by water reduction, 2H2O + 2e → H2 + 2OH−. However, this catalytic function could not be recovered completely when the solution pH was adjusted back to the original 3.5. This result can be attributed to the decomposition of CdS NRs, ascorbic acid, complex 1 or all of them
3.6. Photocatalytic mechanism It is apparent that the photocatalytic hydrogen evolution goes as follows: visible light absorption of the photosensitizer, formation of Scheme 3. Possible mechanism for H2 production by the photocatalytic system based on complex 1.
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Appendix A. Supplementary data
photo-generated charges, separation of electron–hole pair, and electron transfer. As shown in Fig. S20, the CdS NRs had a main absorption in the visible region with a band gap at ∼2.30 eV. Mixing the nickel complex with CdS NRs resulted in a red shift of the absorption onset and reduction of Eg of CdS NRs (Fig. S20b). The results indicate that the introduction of the nickel complex into CdS NRs can obviously improve the range and ability of visible light absorption of CdS NRs. To further look for the photocatalytic mechanism for hydrogen generation over the nickel complex modified CdS NRs, the photoluminescence (PL) spectra of the photocatalysts were provided. As shown in Fig. 8, CdS NRs exhibited two emission peaks at 536 and 680 nm, respectively. However, the introduction of the nickel complex into CdS NRs led to decrease of the peak intensities of CdS NRs at both 536 and 680 nm, and a lower possibility of electron–hole pair recombination because of the fast electron transfer from CdS NRs to complex 1 [62]. To understand the relationship between the electrocatalytic and photocatalytic performances, Mott−Schottky analysis were carried out, with the results shown in Fig. 9. According to Fig. 9, Mott−Schottky analysis gave a flat-band potential (Efb) of −0.58 V versus Ag/AgCl [63]. Based on that the nickel complex has a reductive potential at −0.48 V versus Ag/AgCl (Fig. S6-inset in the Supporting Information), the electron transfer from the excited CdS NRs to the nickel complex is thermodynamically favorable. Moreover, the electrochemical impedance spectroscopy was employed to investigate charge transfer properties and separation efficiency of photogenerated charge carriers. Compared with CdS NRs, the CdS NRs/complex 1 showed much smaller arc radius (Fig. 10), representing a faster interfacial charge transfer and higher separation efficiency of photogenerated charge carriers [64–66]. Based on the above observation, electrochemical analysis and literature precedents [67–69], we put forward a possible mechanism for the photocatalytic H2 production. As outlined in Scheme 3, the transfer of the photoexcited electrons from the conduction band (CB) of CdS NSs to the nickel(II) ion of complex 1 afforded the reduced nickel(0) species. Then, the addition of hydrogen proton formed the NiII-H species, a high reactive intermediate. Further introduction of hydrogen proton to the NiII-H species provided dihydrogen, and regenerated the starting nickel complex 1. Moreover, the photogenerated hole remaining in the CdS NSs was subsequently coupled with electrons come from H2A, thus preventing the recombination of photogenerated charge carries and letting the continuous reactions.
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4. Conclusion We have designed successfully new catalytic systems based on the nickel complex for both electrochemical and photochemical water reduction to dihydrogen. The electrocatalytic system can afford 577.4 mol H2/mol catalyst/h from neutral water at an OP of 837.6 mV. The photocatalytic system containing CdS NRs as a photosensitizer, the nickel complex as a cocatalyst, and H2A as a sacrificial electron donor can provide hydrogen with a TON of 55340 mol of H2 per mole of catalyst during first 60 h irradiation and work for about 120 h. Compared with CdS cluster as a precursor, CdS NR shows much better photocatalytic activity, indicating that the crystallinity of photosensitizer significantly affects the photocatalytic activity of the system. The studies also show that the decompositions of complex 1 and H2A are the primary reason for the cease of hydrogen evolution after illumination. These findings may offer a new method for the design of effective non-noble-metal catalyst for water reduction. Acknowledgements This work was supported by the National Science Foundation of China (Nos. 20971045, 21271073 and 21372088) and the Science and Technology Project (No.2016A010103025) from Guangdong Science and Technology Department. 16
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