Surface Decorated Ni Sites for Superior Photocatalytic Hydrogen Production
Wenhuan Huang, Tingting Bo, Shouwei Zuo, Yunzhi Wang, Jiamin Chen, Samy Ould-Chikh*, Yang Li, Wei Zhou*, Jing Zhang, Huabin Zhang*
Prof. W. Huang, Mrs. J. Chen
Key Laboratory of Chemical Additives for China National Light Industry, College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China
Dr. W. Huang, Dr. S. Zuo, Dr. S. Ould-Chikh, Mr. Y. Zhi, Dr. Y. Li, Prof. H. Zhang.
KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
E-mail: [email protected];[email protected] Dr. T. Bo, Prof. W. Zhou
Department of Applied Physics, Tianjin Key Laboratoryof Low Dimensional Materials Physics and Preparing Technology, Faculty of Sci-ence, Tianjin University, Tianjin 300072, P. R. China E-mail: [email protected]
Prof. J. Zhang
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
Experimental methods
Preparation of CdS nanowires. CdS nanowires were prepared by a previously reported method.
[S1] 3.8 g of Cd(NO3)2∙4H2O and 2.8 g of thiourea were dissolved in 100 ml of ethanediamine.
The solution was transferred into a 200 mL Teflon-lined stainless steel autoclave and kept at 180
oC for 24 h. After cooling down to room temperature, the obtained CdS NWs were collected by centrifugation and washed with deionized water and ethanol several times, which were then dried at 60 oC in a vacuum oven.
Preparation of CdS@Ni. 100 mg of as-prepared CdS NWs was added into 20 mL of ethanol to form a suspension. Then, 5 mg of Ni(NO3)2·6H2O was added. The suspension was kept at 70 oC under stirring for 2 h to evaporate theethanol solution. The product was collected and calcinated at 300 oC for 1 h in N2 atmosphere to enhance the interaction between the isolated Ni atoms and CdS nanowires.
Photocatalytic measurements. Photocatalytic performance was evaluated by a gas-closed circulation system equipped with a 300 W Xe lamp as the light source. A UV-cut filter was used to remove the UV light from the light source. 200 mg of the sample was dispersed in 100 mL of lactic acid aqueous solution for the reaction. The generated H2 was characterized by a gas chromatography.
X-Ray Absorption Data Collection, Analysis, and Modeling: Ni K-edge X-ray absorption spectra were acquired at beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF) in fluorescence mode at room temperature using a Si (111) double-crystal monochromator. The storage ring of BSRF was operated at 2.5 GeV with a maximum current of 250 mA in decay mode. The energy was calibrated using Ni foil; the intensities of the incident and fluorescence X-ray were monitored by using standard N2-filled ion chamber and Ar-filled Lytle-type detector,
respectively. To achieve the best signal-to-noise ratio, the powdered samples were uniformly mixed with boron nitride powder and pressed to a pellet, which was sealed in a cell holder with Kapton windows for X-ray absorption fine structure (XAFS) measurement. A detuning of about 20% by misaligning the silicon crystals was also performed to suppress the high harmonic content.
The XAFS raw data were background-subtracted, normalized, and Fourier transformed by the standard procedures with the ATHENA program. Least-squares curve fitting analysis of the EXAFS χ(k) data was carried out using the ARTEMIS program, with the theoretical scattering amplitudes, phase shifts, and the photoelectron mean free path for all paths calculated by ab initio code FEFF9. All fits were performed in the R space with k- weight of 3. Due to the important non-MT effects in asymmetrical or sparse systems, the Ni K-edge theoretical XANES calculations were carried out with the FDMNES code in the framework of full-potential finite difference method (FDM). The energy dependent exchange-correlation potential was calculated in the real Hedin–Lundqvist scheme, and then the spectra were convoluted using a Lorentzian function with an energy- dependent width to account for the broadening due both to the core-hole width and to the final state width.
Energy transfer efficiency, ΦET. The energy transfer efficiency, ΦET were calculated using the following equation: 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 are τD = 1/ko and τD-A = 1/(ko+ ke), respectively.
Computational methods and details: In this work, all calculations of surface systems were
calculated by the first principles based on the Vienna Ab initio Simulation Package (VASP).[2-3]
The generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof is used and a vacuum thickness more than 15 Å is adopt to separate it from its periodic model.[4]
The plane wave cutoff energy is set as 400 eV and the van der Waals (vdW) interactions are included by using the PBE-D2 functional.[5] Furthermore, the convergence criterion of total energy and residual force are 10−5 eV and 0.03 eV/Å, respectively.Moreover, the Brillouin zone was sampled with a gamma-centered scheme of 3 × 3 × 1 k-points. In addition, because of the polarization in the heterojunction, the dipole correction in its normal direction is included throughout the calculation.[6] Furthermore, we use the addition of pseudohydrogen atoms to the upper and lower surfaces of the systems to counteract the polarization effect. Here, a CdS (001) surface system was used (total atomic number 24). For the doped system, it is constructed by replacing a surface Cd atom from the pure phase surface system and labeled CdS@Ni.
In order to study the effect of Ni atom doping on hydrogen evolution reaction (HER) performance of CdS system, the corresponding Gibbs free energy was calculated based on the computational hydrogen electrode (CHE) proposed by Nørskov et al.[7-8] According to this method, the Gibbs free energy of the proton and electron pair (H+ + e−) is equal to half of the free energy of gaseous hydrogen (H2) at standard reaction conditions. Moreover, the change Gibbs free energy (G) of each species can be obtained from the following equation:
G = EDFT + EZPE - TS (1)
where EDFT is the electronic energy calculated from DFT calculations, EZPE is the zero-point energy the adsorbed species and S is the entropy contributionat room temperature,respectively.
In addition, the work function, j, is defined as follows:
j = Evac – EF (2)
where Evac denotes the energy of a stationary electron in a vacuum near the surface. EF represents the Fermi energy of structures.
Figure S1. XRD pattern of CdS nanowires.
Figure S2. (a-c) STEM images of CdS nanowires in different resolution.
Figure S3. XRD pattern of CdS@Ni nanowires.
Figure S4. The Raman spectra of CdS and CdS@Ni nanowires.
Figure S5. The N2 sorption isotherms examined at 77 K for pure CdS nanowires and CdS@Ni nanowires.
Figure S6. EXAFS spectra and Fourier-transformed magnitudes of CdS@Ni. The experimentally measured spectra match well with the calculated spectra for both samples. The best-fit parameters are shown in Table S1.
Figure S7. The linear combination fitting for the position of absorption edges in XANES curves.
Figure S8. H2 evolution rates over CdS@Ni in different solutions with varied volume ratios of lactic acid.
Figure S9. Gas generation rates in the catalytic system under different reaction conditions.
Figure S10. XRD pattern of CdS@Ni after catalysis.
Figure S11. (a) Fourier transform magnitudes of Co K-edge EXAFS spectra in R space for CdS@Ni after catalysis, and (b) calculated spectra are matched very well for all the samples. (d) The normalized Ni-edge XANES spectra of samples.
Figure S12. Proposed mechanisms of photocatalytic hydrogen evolution reactions as well as related photoexcited dynamics over CdS@Ni under visible light irradiation.
Figure S13. Electrochemical impedance spectra for CdS@Ni and CdS nanowire.
Figure S14. Structural model of the CdS (a) and CdS@Ni (b).
Table 1. Fitting results of Ni foil EXAFS
Sample Shell N R σ2 R factor (%)
Ni foila Ni-Ni 12 2.48 0.006 0.1
CdS@Nib Ni-S 4.3 2.26 0.004 0.3
CdS@Ni after
catalysisc Ni-S 4.6 2.24 0.005 0.3
Fitting range: a, 2.8≤k (/Å)≤14.0 and 1.3≤R (Å)≤3.0; b, 3.2≤k (/Å)≤9.8 and 1.4≤R (Å)≤3.0. c, 3.2≤k (/Å)≤9.8 and 1.2≤R (Å)≤2.8.
Table S2. Comparison of various cocatalysts for photocatalytic hydrogen evolution with the CdS as the catalyst.
Cocatalyst Light source Reaction condition Activity Ref
Isolated Ni atom 300 W Xe lamp, λ > 400 nm
20 vol% lactic aqueous solution
0.452 mmol/h This work Cobaloximes 300 W Xe lamp,
λ > 400 nm
10 vol% TEOA, 80 vol%
acetonitrile aqueous
0.946 mmol/h [S9]
WS2 300 W Xe lamp, λ > 400 nm
10 vol% lactic aqueous solution
0.420 mmol/h [S10]
CNT 300 W Xe lamp,
λ > 420 nm
Na2S and Na2SO3 aqueous solution
0.498 mmol/h [S11]
Single-Layer WS2
300 W Xe lamp, λ > 420 nm
10 vol% lactic aqueous solution
43 µmol/h [S12]
Au particles 300 W Xe lamp, λ > 500 nm
10 vol% lactic aqueous solution
8.65 µmol/h [S13]
Co2P LED light DL-mandelic acid 2.4 µmol/h [S14]
Titanium-oxo clusters
300 W Xe lamp, λ > 420 nm
Na2S and Na2SO3 aqueous solution
0.98 mmol/h [S15]
Isolated Pt atom 300 W Xe lamp, λ > 420 nm
20 vol% lactic aqueous solution
2.35 mmol/h [S16]
Ni(OH)2 300 W Xe lamp, λ > 420 nm
10 vol% TEOA aqueous solution
0.252 mmol/h [S17]
ZnO 300 W Xe lamp Na2S and Na2SO3 aqueous solution
1.2 mmol/h [S18]
MoOx 300 W Xe lamp, λ > 400 nm
20 vol% lactic aqueous solution
573.6 µmol/h [S19]
Table S3. Best fitted parameters of time-resolved PL spectra.
Sample Component Lifetime (s) Intensity (%) Decay
Lifetime (s)
CdS τ1
τ2 2.162 10-9
1.633 10-8 53.75
46.25 8.715 10-9
CdS@Ni τ1
τ2 τ3
2.554 10-10 1.850 10-9 8.052 10-9
76.89 14.50
8.61 1.158 10-9
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