Photochemical preparation of atomically dispersed nickel on cadmium sulfide for superior photocatalytic hydrogen evolution

Journal Pre-proof Photochemical preparation of atomically dispersed nickel on cadmium sulfide for superior photocatalytic hydrogen evolution Huizhen Zhang, Yuming Dong, Shuang Zhao, Guangli Wang, Pingping Jiang, Jun Zhong, Yongfa Zhu

PII:

S0926-3373(19)30980-4

DOI:

https://doi.org/10.1016/j.apcatb.2019.118233

Reference:

APCATB 118233

To appear in:

Applied Catalysis B: Environmental

Received Date:

1 May 2019

Revised Date:

16 August 2019

Accepted Date:

24 September 2019

Please cite this article as: Zhang H, Dong Y, Zhao S, Wang G, Jiang P, Zhong J, Zhu Y, Photochemical preparation of atomically dispersed nickel on cadmium sulfide for superior photocatalytic hydrogen evolution, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118233

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Photochemical preparation of atomically dispersed nickel on cadmium sulfide for superior photocatalytic hydrogen evolution

Huizhen Zhang1, Yuming Dong*1, Shuang Zhao1, Guangli Wang1, Pingping Jiang1, Jun Zhong*2, Yongfa Zhu3

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1, International Joint Research Center for Photoresponsive Molecules and Materials, Key Laboratory of Synthetic and Biological Colloids, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China. E-mail:

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[email protected]

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2, Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow

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University, Suzhou 215123, China. E-mail: [email protected]

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3, Department of Chemistry, Tsinghua University, Beijing 100084, China.

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Graphical abstract

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Highlights  Atomically dispersed nickel modified cadmium sulfide superior photocatalyst was successfully prepared by a novel and precise photochemical route.  The Ni1/CdS hybrid catalyst performed outstanding activity and stability for HER under visible light, nature sunlight outdoors and aerobic conditions.  3. A possible mechanism on the enhanced photocatalytic activity was investigated. Abstract Up to now, hydrogen production with a low-cost and efficient system driven by sunlight still remains a great challenge. Herein, atomically dispersed Ni modified CdS

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nanorods (NRs) hybrid photocatalyst (Ni1/CdS) with Ni loading up to 2.85 wt% was prepared by a facile, rapid and scalable photochemical method. Under optimal

conditions, the highest rate for H2 evolution of Ni1/CdS photocatalyst is 630.1 mmol

g-1 h-1 under visible light, which is one of the most robust photocatalytic HER systems

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based on CdS currently. Furthermore, the Ni1/CdS catalyst exhibits good stability and durability for hydrogen evolution reaction (HER) and outstanding photocatalytic

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activity under sunlight and aerobic conditions, indicating the great practical value of present reaction system. Density functional theory (DFT) calculations reveal that the

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introduction of single Ni atom on the CdS can improve the hydrogen binding energy and electronic properties, thus greatly boosting the photocatalytic H2 production

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activity.

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Keywords: single-site, Ni, photocatalyst, water-splitting

1. Introduction

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Sunlight-driven chemical reaction can be performed efficiently under normal temperature and pressure conditions, which is one of the ideal routes to solve the energy and environmental problems and realize green chemistry. In the past decades, the photocatalytic hydrogen evolution reaction via water splitting has been extensively studied and popularly regarded as a promising and valuable strategy for converting inexhaustible solar energy into available chemical energy.[1-5] The photocatalytic process consists of three steps: effective absorption of sunlight, separation and transfer

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of photogenerated electrons and holes, surface oxidation and reduction. Previous studies have shown that the times required for the above three links are about 10-15 s, 10-13-10-11 s, 10-9-10-3 s, respectively.[6-8] It is obvious that the surface reaction is the speed control step of the entire process. And the surface reaction consumption and the recombination of the photogenerated charges are a pair of competition process, meaning that the lag of the surface reaction can easily lead to the backlog of photogenerated charges and further recombination, which limits the utilization efficiency of light energy. In view of the surface reaction, loading the reduction co-

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catalyst on the surface of the light-absorbing semiconductor material is an effective way to improve the photocatalytic hydrogen production efficiency.[9-13]

In recent years, many studies have demonstrated that transition metals[14,15] (Co, Ni, etc) and their compounds (phosphates,[16,17] sulfides,[18,19] hydroxides,[19,20] etc) can

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serve as efficient cocatalysts for HER. At present, there are several common methods

for preparing transition metal based cocatalysts: hydrothermal method, ion-exchange

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method, precipitation and calcination. These classical preparation routes play a key role in promoting the research and catalytic application of transition metal compounds.

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However, it is a pity for these traditional methods that the cocatalysts are randomly distributed on the surface of light-absorbing semiconductors, which is unable to achieve

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cocatalyst position match. By contrast, many studies have demonstrated that the cocatalysts (Pt, Au, Ag et al) can be precisely deposited on the electronic enrichment area by photochemical reduction route, realizing the position match, which is of

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advantage to the charge transfer and helpful for efficient and stable hydrogen evolution.[22-25] Therefore, by means of photochemical method, transition metal based

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cocatalysts promise to be deposited on the outlet points of photo-generated electrons and present more excellent hydrogen production activity due to more efficient charge transfer.

Compared with noble metals, transition metal based cocatalysts is difficult to be prepared by photo-reduction methods, and the reports are rare. A small number of reported photochemical reduction method existed in the form of transition metal based cocatalysts synthesized by nanoparticles,[26-28] and only the atoms on the surface could 3

participate in the photocatalytic hydrogen evolution process, resulting in low atom efficiency. Reducing the size of transition metals to isolated atom affords an ideal approach to realize the maximum atom efficiency and understand the structure-activity relationship of catalysts from the atomic and molecular levels.[29-31] A growing number of studies have shown that single-atom catalysts exhibit significantly different activity, selectivity and stability from conventional nanocatalysts due to their particular structure. Recently, a few studies have prepared Pt, Pd single-atom catalysts by photochemical reduction method.[32-34] However, as far as we know, there is still no report on the

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preparation of atomically dispersed transition metal catalysts using the photochemical strategy.

Herein, we report a facile and rapid photochemical method to prepare a highly stable,

atomically dispersed Ni catalyst (denoted as Ni1/CdS) on CdS nanorods (NRs) with Ni

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loading up to 2.85% (Figure 1). The Ni1/CdS catalyst exhibits extremely high catalytic activity under visible light. And no significant decrease in catalytic activity was

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observed after long-term evolution of H2 for 16 hours under visible light (λ > 420 nm). Furthermore, the Ni1/CdS hybrid catalyst also exhibits outstanding photocatalytic HER

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activity under sunlight and aerobic conditions, indicating the great practical value of present reaction system. Synchrotron radiation based X-ray absorption spectroscopy

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(XAS) was used to verify that Ni atoms have been atomically anchored on the surface of CdS NRs. Moreover, density functional theory (DFT) calculations demonstrates that the introduction of Ni atoms can optimize H binding and improve charge density, and

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thus promote the reaction rate of water splitting into hydrogen.

2. Experimental Section 2.1 Materials

Cadmiumchloride (CdCl2·2.5H2O, 99.0%), thiourea (CH4N2S, 99.0%), ethylenediamine (H2NCH2CH2NH2, 99.0%), nickel acetate (C4H6NiO4·4H2O, 98.0%), triethanolamine (C6H15NO3, 98.0%), sodium sulfide (Na2S·9H2O, 99.0%), sodium sulfite (Na2SO3, 99.0%), absolute ethanol (C2H5OH), absolute 4

methanol (CH3OH), lactic acid (C3H6O3, 85.0%), and sodium sulfate (Na2SO4, 99.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Nafion® perfluorinated resin solution was purchased from sigma-aldrich. All reagents were used without further purification. 2.2 Synthesis of CdS NRs CdS NRs were prepared using a hydrothermal synthesis method. Typically, 10.13 mmol CdCl2·2.5H2O and 30.38 mmol NH2CSNH2 were dispersed in 30 mL ethylenediamine and then transferred to a 50 mL autoclave and further

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maintained at 160 oC for 48 h. The yellow precipitates were collected and washed with absolute ethanol and distilled water. 2.3 Synthesis of the Ni1/CdS NRs catalysts.

The Ni1/CdS NRs composite was prepared by a photochemical method. In

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detail, 50 mg CdS NRs, 1 mL Ni(CH3COO)2 aqueous solution (12.5 mg mL-1), 1 mL thiourea aqueous solution (38 mg mL-1) and 8 mL ultrapure water were mixed

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in a 25 mL flask and dispersed through several minutes sonication. Then the mixed system was purged with nitrogen for 40 min to remove air and illuminated

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irradiation for 20 minutes under a 300 W Xenon lamp at room temperature. The deposition content of Ni was adjusted by changing the time of illumination.

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Finally, the dark-green products were obtained by centrifugation and washing repeatedly with distilled water and alcohol, and dried under N2 flowing. The acquired samples were named as Ni(OH)2-T/CdS, where T referred to the

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illumination time (min) under UV-vis light. 2.4 Synthesis of Ni(OH)2 NPs/CdS

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The Ni(OH)2 modified CdS nanorods photocatalysts were fabricated by a

precipitation method. In a typical process, 200 mg of CdS nanorods were dispersed in 50 mL of 0.25 M NaOH aqueous solution, and then 3.5 mL of 0.05 M Ni(NO3)2 aqueous solution was added dropwise under stirring. The obtained mixture was stirred for 2 h at room temperature. After that, the precipitate was washed repeatedly with deionized water and ethanol. Finally, the acquired precipitate was dried at 60 oC for 12 h. 5

2.5 Photocatalytic H2 production tests The prepared of the photocatalysts were added to 50 mL of aqueous solution containing sacrificial agent and dispersed well by ultrasonication. Then the mixed system was purged with nitrogen for 40 min to remove air and illuminated under a 300 W Xe lamp equipped with a 420 nm cut-off filter. Hydrogen evolution was measured by gas chromatography (FULI GC9790 using a 5 Å molecular sieve column, argon as a carrier gas) with a thermal conductivity detector (TCD).

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2.6 Photoelectrochemical measurements Photoelectrochemical measurements were performed on a CHI600E

electrochemical analyzer (Chenhua Instruments Co., Shanghai, China) in a

standard three-electrode system with a working electrode, a Pt net as the counter

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electrode and Ag/AgCl as the reference electrode. A 300 W Xe lamp with a cut-

off filter (λ > 420 nm) served as the light source and a 0.5 M Na 2SO4 solution

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(pH = 7) was used as the electrolyte. Preparation of the working electrodes was as follows: 5 mg pure CdS (or Ni1/CdS) was dispersed in a mixture solution of

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25 μL Nafion® perfluorinated resin solution and 225 μL ethanol. 30 μL of resultant suspension was uniformly dropped onto the surface of a 1×1 cm2 FTO

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plate. And then, the working electrodes were dried at room temperature. 2.7 Characterizations

The powder X-ray diffraction (XRD) patterns were recorded on a D8 X-ray

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diffractometer (Bruker AXS, German). The scanning rate was 2°min -1 in 2θ. An environmental scanning electron microscope (Hitachi S-4800) was employed to

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investigate microscopic feature. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were collected on a JEM-2100 transmission electron microscope (JEOL, Japan) to examine the morphology and size of sample. The TEM-EDX images were obtained with a Tecnai G2 F30 transmission electron microscope (FEI, USA) equipped with a Rontec EDX system. To detect surface species of sample, X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ESCALAB 6

250 XI (Thermo, USA) X-ray photoelectron spectrometer with Al Kα line as the excitation source (hv = 1484.8 eV) and adventitious carbon (284.8 eV for binding energy) was used as reference to correct the binding energy of sample. UV-vis diffuse relectance spectroscopy (DRS) was performed on a UV-3600 (Shimadzu, Japan) spectrophotometer. Photoluminescence (PL) spectra and time-resolved photoluminescence emission spectra were recorded by using a fluorescence spectrophotometer (Edinburgh Instruments, FLS980). In order to explore the charges features, the surface photovoltage (SPV) spectra was measured by self-

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made equipment. X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) data were collected at Taiwan Light

Source (Beamline 01C1, 17C1), Taiwan Photon Source (44A1) and Shanghai Synchrotron Radiation Facility (Beamline 14w).

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2.8 Computational methods

DFT calculations were performed in the Vienna ab initio simulation package

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(VASP). A spin-polarized GGA PBE functional, all-electron plane-wave basis sets with an energy cutoff of 400 eV, and a projector augmented wave (PAW)

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method were adopted. CdS is simulated using a surface model of p (4 × 4) unit cell periodicity. A (3 × 3 × 1) Monkhorst-Pack mesh was used for the Brillouin-

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zone integrations to be sampled. Electronic density-of states (DOS) of (4 × 4) supercells were calculated using a higher 9 × 9 × 1 k-point mesh. The conjugate gradient algorithm was used in the optimization. The convergence threshold was

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set 1*10-4 eV in total energy and 0.05 eV/ Å in force on each atom. The binding

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energy of H2 molecule was calculated from the relation: Eads(H+) = E(CdS+H+) - (E(CdS) + 1/2 E(H2))

where Eads(H+) is the total energy of an adsorbate bound to CdS, E(H2) is the total energy of H2, and E(CdS) is the total energy of the bare CdS. The free energy of the adsorbed state was calculated as: △ GH* = Eads(H*) + EZPE - T△ S where Eads(H*) is the hydrogen binding energy, and EZPE is the difference corresponding to the zero point energy between the adsorbed state and the gas 7

phase. As the vibrational entropy of H* in the adsorbed state is small, the entropy of adsorption of 1/2 H2 is S(H) ≈ -0.5S0(H2), where S0(H2) is the entropy of H2 in the gas phase at the standard conditions. Therefore the overall corrections were taken as in: △ GH* = Eads(H*) + 0.24 eV

2.9 Energy transfer efficiency (ΦET) The energy transfer efficiency (ΦET) is calculated using the following equation:

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ΦET = ke / (kr +knr + ke) = ke / (ko + ke), (Eq. 1) where kr, knr, and ke = radiative decay, non-radiative decay, and energy transfer constants, respectively. The ko and ke values were found from the lifetimes for

donor molecule (τD) and donor molecule in the presence of acceptor (τD-A), which

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are τD = 1/ko and τD-A = 1/(ko+ ke), respectively.

3.1 Structure and composition

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3. Results and discussion

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CdS nanorods were synthesized by a solvothermal method [35] and used as the support and photo-active material. Ni(CH3COO)2 (nickel source) and thiourea (sacrifice agent) were introduced into an aqueous dispersion of CdS NRs to allow

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the adsorption of Ni species. After 20 min of irradiation, the Ni 1/CdS catalyst was collected and washed thoroughly with water and ethanol. X-ray powder

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diffraction (XRD) patterns (Figure S1) of pure CdS and Ni1/CdS composite give only the typical diffraction peaks of CdS, which are indexed to the hexagonal

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phase (PDF#77-2306),[36,37] even with the loading content of Ni as high as 2.85 wt% analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images show that CdS NRs have an average diameter of about 50 nm, length of 1-3 μm (Figure S2). After the decoration of isolated nickel atoms over CdS NRs, no obvious nanoparticles (NPs) can be observed on these rod-like structures (Figure 2a,b). Besides, HRTEM investigation shows that lattice fringe

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of the Ni1/CdS is identical to that of CdS, indicating that the modification of nickel do not change the original structure of CdS (Figure 2c). TEM/EDX data of Ni1/CdS clearly demonstrates the existence of Cd, S and Ni elements, implying the nickel is successfully anchored on CdS (Figure 2d). Synchrotron radiation based XAS spectra of Ni1/CdS NRs were collected to confirm the atomic dispersion of Ni over CdS NRs. Figure 3a shows the X-ray absorption near edge structure (XANES) spectra of NiS, Ni 1/CdS NRs and Ni(OH)2 reference at Ni K-edge, while Figure 3b shows the corresponding

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Fourier transform curves of extended X-ray absorption fine structure (EXAFS) data. The XANES spectra reveal that the Ni in Ni1/CdS NRs has a chemical state

similar to Ni(OH)2 instead of NiS. The EXAFS curve of Ni1/CdS NRs in Figure 3b clearly shows the formation of Ni-O bonds. Moreover, the Ni1/CdS sample

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shows almost no feature at the position for Ni-Ni bonds, strongly suggesting the formation of isolated nickel sites over CdS NRs.

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In order to further evaluate the surface species and chemical states of Ni1/CdS, X-ray photoelectron spectroscopy (XPS) analysis was performed and studied.

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The XPS survey spectrum of Ni1/CdS (Figure S3a) shows the existence of Cd, S, Ni and O elements. In Cd 3d spectrum of pure CdS (Figure S3b), two peaks at

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binding energy of 404.8 and 411.5 eV, are assigned to 3d 5/2 and 3d3/2 of Cd2+ in CdS NRs, respectively.[38] In addition, the S 2p high resolution XPS spectrum of pure CdS (Figure S3c) shows two peaks at 161.1 and 162.3 eV, which match well

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with the literature values for the binding energies of sulfide ions in CdS nanorods.[38] The main peak of Ni 2p at 856.7 eV for Ni1/CdS is close to the

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reported value for Ni(OH)2 (Figure S3d).[39] And the broad peak at 861.6 eV of Ni 2p is satellite peak. The above results further confirm that as-obtained Ni1/CdS contains Ni-O bonds. The results are also in good agreement with the XAS data. In addition, the Cd 3d and S 2p peaks of Ni1/CdS NRs shift to the higher binding energy compared with that of bare CdS (Figure S3b,c), suggesting that modification of atomically dispersed nickel causes the decrease in the electron density of CdS NRs, because of strong interfacial interactions between them. 9

3.2 Photocatalytic performance Above all, we identified optimal photo-reduction deposition time for the photocatalytic HER (Figure 4a). At first, the H2 production gradually enhanced with the increase of the photo-reduction time. When it was increased to 20 minutes, the sample performed the highest H2 production rate, which was about 30 times higher than that of pure CdS, indicating that single Ni atom is an effective cocatalyst for CdS. However, when the deposition time was over 20 min, the H2 evolution rates began to show a tendency to decline, which can be

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ascribed to the aggregation of Ni(OH)2 (Figure S4 and Figure S5). In order to highlight the advantages of isolated nickel atoms in photocayalytic HER, the

hydrogen evolution activity of Ni1/CdS was examined with reference to pure CdS

and Ni(OH)2 NPs/CdS fabricated by precipitation method (Figure S6). The

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hydrogen evolution rate of 326.7 mmol g−1 h−1 was observed for Ni1/CdS (2.85 wt% Ni loading), which was nearly 35 times as high as bare CdS did and 13.5

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times higher than that of Ni(OH)2 NPs/CdS (2.88 wt% Ni loading) (Figure 4b). The above results clearly demonstrate that the single-atom nickel is responsible

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for the outstanding photocatalytic activities of the Ni1/CdS. Then, we studied the photocatalytic performances of pure CdS and Ni 1/CdS in

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different kinds of hole scavengers (Figure S7) Photocatalytic H2 production activities are all improved markedly after single-atom nickle deposition compared to native CdS in four aqueous solutions of 20 vol% methanol, 20 vol%

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triethanolamine, 20 vol% lactic acid and 0.75 M/1.05 M of Na2S/Na2SO3, indicating that the Ni1 can serve as an excellent cocatalyst for CdS whether in

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acidic, neutral, or alkaline environment. Further, it is easy to find that the HER activity of Ni1/CdS in Na2S/Na2SO3 aqueous solution is optimum, compared with that in the other three environments. Beyond this, the influence of Na 2S/Na2SO3 concentrations on the photocatalytic activity of Ni1/CdS photocatalyst has been investigated (Figure S8). As the concentration of Na2S/Na2SO3 increased from 0.5 M/0.7 M to 1.5 M to 2.1 M, the HER rate gradually improved from 184.9 mmol g-1 h-1 to 630.1 mmol g-1 h-1, possibly because higher concentration of hole 10

scavenger brings about faster transfer of the holes, which is equivalent to suppressing the recombination of photogenerated carriers. These results are compared favorably to those values of previously reported catalysts based on CdS (Table S1). In addition, the stability and durability of the Ni1/CdS NRs were measured and investigated (Figure 4c). After long-term evolution of H2 for 16 hours under visible light (λ > 420 nm), the photocatalytic H2 evolution rate has remained fundamentally unchanged, indicating the good stability of this material for HER.

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Also, in order to investigate the practical applications of Ni1/CdS photocatalyst, hydrogen production experiments were carried out under sunlight (Figure S9) and aerobic conditions, respectively. As illustrated in Figure 4d, the amount of

H2 evolution rapidly increased to about 907 mmol g -1 with the passage of the

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sunlight irradiation time. After around 2:00 that afternoon, the HER rate began to decline, probably caused by the solar dimming and lower concentration of the

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sacrifice agent. More interestingly, the Ni1/CdS hybrid photocatalyst showed outstanding photocatalytic activity under aerobic conditions (Figure S10). The

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H2 evolution rate under aerobic conditions was about on a level with that under an inert atmosphere. Therefore, atomically dispersed Ni on the electronic

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enrichment area is an efficient and stable cocatalyst for hydrogen evolution. 3.3 Photochemical formation of Ni1/CdS We carried out some experiments to investigate the formation mechanism of

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the Ni1/CdS according to the specific experimental conditions (Table S2). The system of experiment A included Ni(CH3COO)2, thiourea and irradiation for 20

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min under UV-vis light, which was the typical preparation of Ni1/CdS. Compared with experiment A, the differences of the experiment B, C, D and E were the absence of thiourea, Ni(CH3COO)2, irradiation and CdS, respectively. The photocatalytic H2 evolution rates were investigated (Figure S11). The HER rate of sample A mounted up to 142.7 mmol g-1 h-1, which was much higher than that of sample B (11.8 mmol g-1 h-1), sample C (4.6 mmol g-1 h-1) and sample D (4.8 mmol g-1 h-1). Besides, after 20 min irradiation, no any precipitation was 11

generated from experiment E, indicating the unique role of photo-active material. The experimental results showed that the every precursor and irradiation were all indispensable to construct the Ni1/CdS NRs composite photocatalyst. Furthermore, we measured the amount of H2 evolution of the pristine CdS, system A, system B and system C in Table S2 (Figure S12). Firstly, 50 mg pure CdS NRs were dispersed 10 mL ultrapure water and did not produce hydrogen after irradiation for an hour. In contrast, the amount of H2 production of the system C gradually enhanced with the increase of the irradiation time, indicating

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that thiourea can be served as a hole scavenger. The system B also showed HER activity, possibly attributing to the small amount deposition of Ni species. In

particular, the amount of H2 production of the system A reached 695.9 μmol g -1 after irradiation for 20 min under UV-vis light, which was much higher than that

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of the system B and C. When the irradiation time was over 20 min, the H 2 evolution rate began to decline. Therefore, the possible formation mechanism of

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the Ni1/CdS is as follows. Under irradiation, CdS NRs are excited by light to produce electrons and holes. Thiourea acts as a scavenger to consume the holes.

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The hydrogen ions are reduced by electrons to hydrogen. Meanwhile, the left hydroxyl ions react with adsorbed Ni2+ to form Ni(OH)2. Within 20 minutes, Ni

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existed in isolated sites, as shown in Figure 1. As the irradiation time further increased, the hydroxyl ions and nickle ions will accumulate on isolated sites and

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Ni(OH)2 nanoparticles formed. The reaction equations can be as follows: CdS + hv → CdS (eCB- + hVB+) 2H+ + 2e- → H2 Ni2+ + 2OH- → Ni(OH)2 TU (NH2CSNH2) + hVB+ → TU+

3.4 Charge-separation performances of photocatalysts UV-visible diffuse reflectance spectra were obtained to investigate the optical absorption of pure CdS and Ni1/CdS NRs samples (Figure 5a). After the coupling of Ni1 onto the CdS NRs surface, the absorption edge has remained unchanged. Further, the band gap energies of pure CdS and Ni1/CdS NRs are the exact same 12

amount and estimated to be 2.41 eV according to Tauc plots of (αhν)2 against the photon energy (hν) according to the formula αhν = A(hν - Eg)n/2, indicating no Ni doping in the CdS structure (Figure S13). And there is an increase in visible absorption over the wavelength range of 500-800 nm relative to the CdS sample. In order to clarify whether the enhanced absorption of visible light contributed positively to the HER activity of the Ni1/CdS, verification experiments were performed and the results were shown in Figure 5a. The hydrogen evolution rates of Ni1/CdS at different wavelengths are in agreement well with the UV-vis

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absorption spectrum of pure CdS, not that of Ni 1/CdS NRs, illustrating that the enhanced visible absorption due to nickel is next to useless for its high activity. As a result of the analysis mentioned above, the Ni plays the part of the cocatalyst in this photocatalytic system.

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It is well known that the separation and transfer efficiency of photogenerated

carriers is considered crucial to the photocatalytic activity. So a series of

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characterizations were performed to investigate the charge separation and migration process. First, steady-state photoluminescence spectra of pure CdS,

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Ni(OH)2 NPs/CdS and Ni1/CdS composite were performed at an excitation wavelength of 405 nm, as shown in Figure 5b. The Ni1/CdS sample exhibits

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remarkably damped emission at about 490 nm with respect to that of Ni(OH)2 NPs/CdS and pristine CdS, attributing to the fast transfer of electrons from CdS NRs to Ni1, which can suppress the photogenerated carriers recombination and

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improve the photocatalytic activity. Time-resolved fluorescence spectra were recorded and the decay curves were fitted with exponentials to obtain the decay

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times. As shown in Figure 5c, fluorescence decays of Ni1/CdS and Ni(OH)2 NPs/CdS exhibits more rapid decay than that of bare CdS. The average radiative lifetimes of them are 1.40 ns, 2.70 ns and 6.19 ns, respectively, indicating isolated nickel atoms produce better trapping ability than that of Ni nanoparticles (Table S3). And based on the acquired time-resolved fluorescence decay lifetimes, we calculated the energy transfer efficiency (ΦET) to analyze the energy transfer behavior quantitatively (Eq. 1). The ΦET of Ni1/CdS (78.4%) is significantly 13

higher than that of Ni(OH)2 NPs/CdS (58.4%), demonstrating that isolated nickel atoms can realize more efficient directional migration of charge carriers. In addition, surface photovoltage (SPV) spectra were determined and analyzed (Figure 5d). Pure CdS, Ni1/CdS and Ni(OH)2 NPs/CdS display an obvious positive photovoltage response in the range of 300-500 nm, but the photovoltage values of them are almost zero when the wavelength exceeded 520 nm, again indicating that the band gap of CdS is not changed after cocatalyst modification and the enhanced visible absorption is invalid. Compared to bare CdS, the

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Ni1/CdS and Ni(OH)2 NPs/CdS present obviously increased SPV signals, especially for that of Ni1/CdS, suggesting that the photo-excited charge carriers are more easy to separate and transfer to the surface of Ni 1/CdS. The enhanced efficiency of charge separation and transfer was also verified by the

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photocurrent-time responses and Electrochemical Impedance Spectroscopy (EIS) Nyquist plots of pure CdS and Ni1/CdS. The Ni1/CdS produced a higher

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photocurrent and smaller EIS arc radius than CdS under the same condition (Figure S14). The isolated nickel atoms loading can greatly improve the charge

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transfer and separation of CdS, thus enhancing the photocatalytic H 2-evolution activity.

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3.5 Proposed photocatalytic mechanism

The above experimental results demonstrate that atomically dispersed Ni cocatalyst leads to significantly enhanced photocatalytic H2 production. We further performed

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density functional theory (DFT) based first principles theoretical calculations to gain the fundamental insight into the role of atomical Ni in Ni1/CdS hybrid photocatalyst.

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First, as a key descriptor of the HER activity, the adsorption free energy of H* (△GH*) was investigated. As shown in Figure 6a, the calculated △GH* values for Ni1/CdS, Ni(OH)2 NPs/CdS, and CdS are 0.56, 0.66 and 1.34 eV, respectively. Obviously, the Ni1/CdS possesses the lowest value of △GH*, which will guarantee both the most efficient electron-proton acceptance to form H* and the fastest hydrogen desorption, being consistent with the best experimental HER performance compared to Ni(OH)2 NPs/CdS and pure CdS. In addition, the density of states (DOS) was calculated to 14

further study the electronic structures of the photocatalysts (Figure 6b-d). We found that the introduction of Ni species on CdS results in the generation of several new energy levels in Ni1/CdS, Ni(OH)2 NPs/CdS. Furthermore, compared with bare CdS NRs, the DOS of Ni1/CdS and Ni(OH)2 NPs/CdS have an obvious increase at the conduction band edge and are closer to the Fermi level, leading to a higher carrier density for the advantage to charge transfer during HER process. In order to intuitively verify the above conclusions, corresponding distribution of charge densities have also been investigated (Figure 7a-c). In comparison with pure CdS, the charge density of

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Ni1/CdS and Ni(OH)2 NPs/CdS get significant improvement, mainly originating from Ni and S atoms, indicating that the introduction of Ni species are favorable to receive

electrons and boost the photocatalytic reaction activity. Remarkably, the Ni 1/CdS has a

higher charge density than Ni(OH)2 NPs/CdS, demonstrating that atomical Ni have

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evident advantage in improving photocatalytic hydrogen production performance

compared to Ni(OH)2 NPs. At the same time, the differential charge density maps

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(Figure 7d,e) showed that atomically dispersed Ni can enrich more electrons, in good agreement with the above results. Therefore, we speculate that the superior

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photocatalytic HER activity of Ni1/CdS may be due to the effect of single Ni atom on optimizing H binding and electronic properties.

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4. Conclusions

In summary, Ni1/CdS photocatalyst was successfully prepared by a simple and rapid

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photochemical method. The photocatalytic hydrogen evolution rate can mount up to 630.1 mmol g-1 h-1 under visible light. Furthermore, the Ni1/CdS hybrid catalyst

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exhibited good stability and durability for HER. More interestingly, the photocatalytic system showed remarkable HER performance under sunlight and aerobic conditions, indicating the great practical value of present reaction system. The single Ni atoms could optimize H binding and electronic properties, resulting in an emerging and greatly efficient photocatalytic performance for hydrogen evolution.

Acknowledgements 15

The authors acknowledge the support from Taiwan Light Source, Taiwan Photon Source and Shanghai Synchrotron Radiation Facility for the XAS experiments. The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 21676123, 21575052, U1732110), the Natural Science Foundation of Jiangsu Province (No. BK20161127), the Fundamental Research Funds for the Central Universities (JUSRP51623A), the Opening Foundation of Shandong Provincial Key Laboratory of Clean Production of Fine Chemicals (ZDSYS-KF201504) from Shandong Normal University and MOE &

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SAFEA for the 111 Project (B13025). The authors also thank Dr. Jinze Lv for his kind help on SPV technology. This work was supported by National Firstclass Discipline Program of Food Science and Technology (JUFSTR20180301)

and Postgraduate Research & Practice Innovation Program of Jiangsu Province

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(KYCX18_1815).

16

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Captions for figures

Figure 1. Schematic illustration of the preparation of Ni1/CdS by photochemical method. Figure 2. (a) SEM image, (b) TEM image and (c) HRTEM image of Ni1/CdS composite; (d) TEM/EDX result of Ni1/CdS NRs. The inset image in part c is selected-area electron diffraction (SAED) pattern.

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Figure 3. (a) XANES spectra of NiS, Ni1/CdS NRs and Ni(OH)2 reference at Ni K-edge; (b) The corresponding Fourier transform curves of EXAFS data.

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Figure 4. The photocatalytic activity of the photocatalysts under different conditions. (a) The H2 evolution rates of photocatalysts with different photoreduction deposition time under visible light (λ > 420 nm). The system contained 5.0 mg photocatalyst, 2 mL lactic acid and 8 mL water. (b) Time dependent hydrogen evolution under visible light (λ > 420 nm) and enhancement of evolution over Ni1/CdS (2.85 wt% Ni loading), Ni(OH)2 NPs/CdS with 2.88 wt% Ni content fabricated by precipitation method and pure CdS. The system contained 2.0 mg photocatalyst, 0.75 M Na2S and 1.05 M Na2SO3 in 50 mL aqueous solution. (c) Long-term evolution of H2 for 16 hours under visible light (λ > 420 nm). The system contained 1.0 mg Ni1/CdS, 0.83 M Na2S and 1.16 M Na2SO3 in 50 mL water. (d) Photocatalytic hydrogen evolution under sunlight irradiation in the presence of 2.0 mg Ni1/CdS NRs photocatalyst in a 50 mL aqueous solution containing 1.25 M Na2S and 1.75 M Na2SO3 in Wuxi city on November 1, 2017. Outdoor temperature: 12~22 ℃ , time: 9:50-16:20.

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Figure 5. (a) UV-Vis diffuse reflectance spectra of pristine CdS and Ni1/CdS, and the H2 evolution rate of Ni1/CdS (The system contained 5.0 mg photocatalyst, 2 mL lactic acid and 8 mL water using a 300 W Xe lamp with different band-pass filters as the light source.). (b) Comparison of photoluminescence spectra of pristine CdS, Ni(OH)2 NPs/CdS and Ni1/CdS composite. (c) Time-resolved photoluminescence spectra of pristine CdS, Ni(OH) 2 NPs/CdS and Ni1/CdS at an excitation wavelength of 405 nm. (d) Surface photovoltage spectra of pristine CdS, Ni(OH)2 NPs/CdS and Ni1/CdS. Figure 6. (a) Gibbs free energy for H* adsorption on different catalysts of Ni1/CdS, Ni(OH)2 NPs/CdS, and CdS. Calculated density of states of Ni1/CdS (b), Ni(OH)2 NPs/CdS (c), and CdS (d). The black dashed line denotes the position of the Fermi level. Figure 7. The charge density distribution of conduction band edge of Ni1/CdS (a), Ni(OH)2 NPs/CdS (b), and CdS (c). And the differential charge density maps of 19

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Ni1/CdS (d) and Ni(OH)2 NPs/CdS (e). The yellow and blue region indicate electron accumulation and depletion, respectively.

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Figures

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a 2.0

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Experimental condition: Sunny, 12-22 Wuxi city, November 1, 2017 0

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