Similar to natural photosynthetic systems, artificial photosynthetic systems require synergistic cooperation between light harvesting, charge separation and redox catalysis. Herein, a three-dimensional (3D) hierarchical photocatalyst is designed with a novel Z-scheme two-photon excitation, defined by the complementary absorption of higher energy and lower energy photons by cadmium sulfide nanowires (CdS NWs) and cobalt–benzimidazole (Co-bIm) coordination polymers (CBPs), respectively. Without any noble-metal co-catalyst, the microscopically integrated CdS–CBP photocatalysts demonstrated dramatically enhanced photocatalytic activities of H_2 evolution, which were up to 10.6 folds higher than those of pristine CdS NWs. Structurally, the intimate interfacial contact between the 3D CdS NW scaffold and the discrete CBP microstructures benefits their strong electronic interaction and efficient charge separation. Upon simultaneous light excitation, a tandem solid-state electron flow from CdS to CBP and then from metal (Co) to ligand (bIm) precisely catalyzes the reduction of pre-activated H atoms on the bIm ligands for efficient H_2 evolution

Chen, Junxiang

Chen, Linfeng

Chen, Shengli

Jin, Jingpeng

Peng, Tianyou

Richter, Matthias H.

Xia, Pengfei

Xu, Wei

Yu, Weilai

Zhang, Shuai

Caltech Authors - Main

 Supplementary Information 
Biomimetic Z-scheme Photocatalyst with a Tandem Solid-state 
Electron Flow Catalyzing H2 Evolution 
Weilai Yua,b*, Shuai Zhanga, Junxiang Chena, Pengfei Xiac, Matthias H. Richterb, 
Linfeng Chena, Wei Xua, Jingpeng Jina, Shengli Chena, Tianyou Penga* 
aCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, 
China 
bDivision of Chemistry and Chemical Engineering, California Institute of Technology, 
Pasadena, CA, 91125, USA 
cState Key Laboratory of Advanced Technology for Material Synthesis and 
Processing, Wuhan University of Technology, Wuhan, 430070, China 
Email: yuweilai93@whu.edu.cn; typeng@whu.edu.cn 
1 
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.
This journal is © The Royal Society of Chemistry 2018
2 
 
1. Experimental 
1.1 Materials syntheses 
All reagents were of analytical grade and used without further purification.  
Preparation of CdS NWs: CdS NWs were synthesized thorough a ethylenediamine-
assisted solvothermal method. In a typical synthesis, 3.00 g of cadmium nitrate 
tetrahydrate (Cd(NO3)2.4H2O) was dissolved in 65 mL of ethylenediamine under 
vigorous stirring in a 100 mL Teflon-lined autoclave. Then, 2.20 g of thiourea was 
added into the above solution and continuous stirring was maintained for 10 min until 
it was fully dissolved. The autoclave was sealed tightly and put into oven at 200 ℃ 
for 24 h. Afterwards, the obtained yellow precipitates were centrifuged, rinsed with 
ethanol and distilled water three times and dried in oven at 70 ℃ overnight.  
Preparation of CdS-CBP composites: the CdS-CBP composites were fabricated via 
a one-step procedure in solution phase. In a beaker, 0.291 g of cobalt nitrate 
hexahydrate (Co(NO3)2.6H2O) was dissolved in 30 mL of methanol to form a light red 
solution and 0.20 g of the above CdS NWs was then added into the solution. 
Meanwhile, in another beaker, 0.944 g of benzimidazole was dissolved in 30 mL of 
methanol to prepare a colorless transparent solution. After stirring for 1 h, the 
benzimidazole/methanol solution was rapidly poured into the suspension containing 
CdS NWs and Co2+ ions. Then the color of the suspension was observed to change 
immediately. The suspension was stirred for another specific period of time before the 
resulting solids was centrifuged and rinsed with ethanol three times. Different samples 
were named as CdS-CBP-xh, where x represented the stirring time (number of hours) 
for the syntheses of composites. For the purpose of comparison, pure CBPs were also 
3 
 
obtained in the absence of CdS NWs under the same conditions, which were named as 
CBP-xh. 
Method of loading 1 wt.% Pt co-catalyst onto CdS NWs: 0.2 g of CdS NWs was 
put in 50 mL aqueous solution containing 133.6 μL chloroplatinic (H2PtCl6) solution 
(0.077 M). The suspension was first maintained stirring in dark for 30 min. Then 0.1 g 
of NaBH4 was added into the suspension for reducing platinum and continuous 
stirring was kept for another 3 hours. After the reaction was complete, the containing 
solids were centrifuged, rinsed with water and ethanol three times and dried in oven at 
70 ℃ overnight. 
 
1.2 Characterization 
X-ray diffraction (XRD) patterns were obtained on Miniflex600 X-ray diffractometer 
(Rigaku, Japan) with Cu Kα radiation at the accelerating voltage of 40 kV. A scan rate 
of 4° min-1 was applied to record the powder patterns for 2θ in the range of 5-60°. 
UV-vis diffusive reflectance spectra (DRS) were obtained on a MPC-3100 
spectrometer equipped with a integrating sphere (Shimadzu, Japan) by using die-
pressed disk samples. Specpure BaSO4 was used as the reflectance standard sample 
for measurements. Thermogravimetric analysis (TGA) of the samples were performed 
on DTG-60H analyzer (Shimadzu, Japan) from ambient temperature to 900 ℃ at a 
heating rate of 10 ℃ min-1. X-ray photoelectron spectroscopy (XPS) measurements 
were performed on a ESCALAB 250 Xi electron spectrometer (Thermo Fisher, USA) 
equipped with Al Kα radiation source operated at 200 W. All the binding energies 
were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The 
ultraviolet photoelectron spectroscopy (UPS) was performed on a Kratos Axis Ultra 
system with a base pressure of 1 × 10−9 Torr in the analysis chamber. Field-emission 
scanning electron microscopy (FE-SEM) images, EDX mapping and line scan profiles 
were obtained on JSM-7500F (JEOL, Japan) scanning electron microscope. 
Transmission electron microscopy (TEM) images and high-resolution transmission 
electron microscopy (HR-TEM) analyses were performed on JEM-2100HR (JEOL, 
Japan) at an accelerating voltage of 200 kV. Photoluminescence (PL) emission spectra 
were measured at room temperature on F-7000 fluorescence spectrometer (Hitachi, 
Japan) with excitation wavelength of 375 nm. Nitrogen adsorption-desorption 
isotherms were measured on an ASAP 2020 nitrogen adsorption apparatus 
(Micromeritics, USA). All of the samples were degassed at 180 ℃ before 
measurements. The BET surface area was determined by a multipoint BET method 
using the adsorption date in the relative pressure (P/P0) range of 0.05-0.25. The pore 
size distributions were determined using the desorption date by the Barrett-Joyner-
Halenda (BJH) method. Electron paramagnetic resonance (EPR) spectra are 
performed on an ESR spectrometer (MEX-nano, Bruker) with a modulation frequency 
of 100 kHz and a microwave power of 15 mW. The hydroxyl radicals (•OH) and 
superoxide anion radicals (•O2−) are trapped by the 5,5-dimethyl-l-pyrroline N-oxide 
(DMPO), producing the ESR signals of their adducts. The mixed solution of DMPO/
H2O and DMPO/CH3OH was prepared by the traditional method. Briefly, 10 mg as-
fabricated samples and 40 μL DMPO were dissolved in 1 mL deionized water under 
the stirring condition. Then the mixture was irradiated under a 300W xenon lamp for 
120s. After that, 0.1 mL mixture solution (containing DMPO-•OH) was quickly 
detected by the ESR spectrometer. The detection method of DMPO- •O2− was 
identical to the DMPO-•OH, except that the deionized water was replaced by the 
methanol (CH3OH). 
4 
1.3 Photocatalytic H2 production activity test 
Photocatalytic H2 production tests were performed in an outer irradiation-type photo-
reactor (Pyrex glass) connected to a closed gas-circulation system. A 300 W Xe lamp 
(PLS-SXE300, Beijing Trusttech Co. Ltd., China) was used as light source, which 
was collimated and focalized into 5 cm2 parallel faculae. The illuminated spectrum of 
incident light of the lamp is shown in Figure S11. Typically, the photoreaction is 
performed in 10 mL of aqueous suspension containing 5 mg of photocatalyst and 10 
vol% of triethanolamine (TEOA) as sacrificial reagent. Prior to irradiation, the 
suspension of photocatalyst was dispersed in an ultrasonic bath for 5 min and then the 
reactor was irradiated from top after the air had been thoroughly removed. The 
produced H2 was analyzed with a gas chromatograph (GC, SP6890, TCD detector, 5 
Å molecular sieve columns and Ar carrier). During photocatalytic activity stability 
test, due to the volume loss of suspension in reactor, the suspension was refreshed 
with TEOA solution after each cycle to restore its volume to 10 mL. 
1.4 Photoelectrochemical measurements
Measurements of photocurrent response, electrochemical impedance spectra (EIS) 
and Mott-Schottky plots were performed on a CHI 660D electrochemical workstation 
(Chenhua Instrument, Shanghai, China) in a conventional three-electrode 
configuration, including a working electrode, counter electrode and reference 
electrode. The working electrodes were prepared with as-prepared photocatalysts with 
an active area of ca. 1.1 cm2. A Pt wire and Ag/AgCl electrode (immersed in saturated 
KCl solution) were used as the counter electrode and reference electrode, respectively. 
5 
6 
Low-power LED with wavelength of 420 nm (3 W, Shenzhen LAMPLIC Science Co. 
Ltd China) were used as the light source with a focused intensity of ca. 80.0 mW.cm-2. 
0.5 M Na2SO4 aqueous solution was used as the electrolyte in photoelectrochemical 
measurement. Typically, the working electrodes were prepared as follows: 10 mg of 
sample was ground with ethanol to form a slurry. Then the slurry was coated onto a 
pre-cleaned 2 cm x 1.2 cm F-doped SnO2-coated glass (FTO glass) electrode via a 
doctor blade technique. Last, the prepared electrodes were dried naturally overnight 
before measurements. 
1.5 Computational Details 
Geometry optimization and total energy calculations were performed within DFT 
framework as implemented with DMol3 code. The spin is unrestricted during the 
calculation. The PBE exchange-correlation functional within the generalized gradient 
approximation (GGA) was introduced. The all electron method was adopted. A 
double numerical basis set was used together with polarization function (DNP). A 
convergence criterion of 1×10-6 Ha (1 Ha=27.21 eV) on the total energy has been 
performed in the self-consistent-field (SCF) procedures. The Broyden-Fletcher-
Goldfarb-Shanno (BFGS) algorithm was used in performing the geometry 
optimizations. The convergence tolerances of energy, displacement and forces were 
1×10-5 Ha, 0.005 Å and 0.002 Ha/ Å, respectively in geometry optimization. A 
smearing of 0.005 Ha to the orbital occupation is applied to achieve accurate 
electronic convergence. 
7 
Figure S1. TEM image of CdS NWs 
8 
Figure S2. (a-b) FE-SEM images of the CdS-CBP-1h composite, showing the rough 
and jaggy edges of CBP microstructures  
9 
Figure S3. EDS mapping profile of the CdS-CBP-6h composite 
Figure S4. Line scan profile of the CdS-CBP-6h composite 
10 
Figure S5. TGA curves of pristine CdS (black), pure CBP 6h (red) and CdS-CBP 6h 
composite (blue) 
As shown in Figure S5, pure CBP 6h is very stable at temperature below 400 ℃ 
with only negligible weight loss. This showed that the amount of residual methanol in 
the CBP could be ignored, indicating the formation of CBP coordination polymer did 
not require the support of external solvent molecule. Then from 400 ℃ to 570 ℃, its 
weight percentage underwent drastic decrease from nearly 100 % to 25 %, 
representing the thermal decomposition of bIm and the oxidation of cobalt (Ⅱ) into 
Co3O4. Pure CdS underwent a decrease of weight percentage to 87 % at temperature 
above 700 ℃, which is consistent with the transformation of CdS into CdO.  
In this regard, the different temperature ranges for the weight loss of pure CdS 
and pure CBP can be used to estimate the composition of CdS-CBP 6h composite. 
The starting temperature for the weight loss of CdS-CBP 6h increased to 550 ℃, 
showing an increase of thermal stability. As denoted by dashed line, the change of 
TGA curve slope at ca. 700 ℃ indicated the beginning of weight loss of CdS. The 
weight loss at the above critical point was 76 %, which should be mainly attributed to 
11 
the thermal decomposition of CBP. Thus, the weight content of CBP within the CdS-
CBP 6h composite was calculated to be 32 %. 
12 
Figure S6. Nitrogen adsorption-desorption isotherms and pore size distributions (inset) of 
pristine CdS (black), pure CBP 6h (red) and CdS-CBP 6h (blue) 
Table S1. Specific surface area, pore volume and pore diameter of pristine CdS, pure 
CBP 6h and CdS-CBP 6h 
Sample SBET/m2 g-1 Vpore/cm3 g-1 dpore/nm 
CdS 17 0.046 10.7 
CBP 6h 4 0.008 9.2 
CdS-CBP 6h 12 0.03 10.2 
13 
Figure S7. XPS valence band spectrum for CBP 6h, showing the VB is 1.06 eV below its 
Fermi level 
Table S2. Performance comparison with recent efforts of CdS-based photocatalysts 
for H2 production 
Photocatalyst 
Sacrificial 
agent 
Co-
catalyst 
Champion H2 
production 
rate/umol/h/g 
Improvement 
factor over 
CdS 
Ref 
CdS-CBP TEOA none 97600 10.6 
this 
work 
CdS-WO3 Na2S/Na2SO3 Pt 15522 2 1
CdS-g-C3N4 Na2S/Na2SO3 Pt 5690 2
CdS-Bi2MoO6 Na2S/Na2SO3 Pt 7370 5.5 3
CdS-Au-g-C3N4 methanol Pt 19.02 4
CdS-TiO2 methanol 1028 5
CdS-SiC Na2S/Na2SO3 259 6
CdS-Ti3C2 lactic acid 14342 7
CdS-Phosphorene lactic acid 11192 11.6 8
CdS-BiVO4 Na2S/Na2SO3 Pt 57000 5.2 9
CdS-MoS2 Na2S/Na2SO3 MoS2 1750 2.03 10 
CdS-BiVO4 Na2SO3 Pt 23060 2.03 11 
CdS-CoPi lactic acid 13300 2.6 12 
CdS-red P Na2S/Na2SO3 Pt 923 2.53 13 
CdS/WO3 lactic acid Pt 2900 5 14 
14 
Figure S8. Comparison of PL spectra of pristine CdS NWs (black), pure CBP 6h (red) and 
CdS-CBP-6h composite (black) 
Figure S9. Mott-Schottky (MS) plots for (a) CdS and (b) CBP 6h measured in 0.5 M 
Na2SO4 (aq) electrolyte. 
15 
Figure S10. Comparison of XRD patterns for the CdS-CBP 6h sample after 1-
hour and 15-hour photocatalysis 
Figure S11. The illuminated spectrum of incident light for the lamp used in this work, 
provided by the factory. 
16 
References 
1. T. Hu, P. Li, J. Zhang, C. Liang and K. Dai, Applied Surface Science, 2018, 442, 20-29.
2. W. Jiang, X. Zong, L. An, S. Hua, X. Miao, S. Luan, Y. Wen, F. F. Tao and Z. Sun, ACS
Catalysis, 2018, 8, 2209-2217. 
3. J. Lv, J. Zhang, J. Liu, Z. Li, K. Dai and C. Liang, ACS Sustainable Chemistry & Engineering,
2018, 6, 696-706. 
4. X. Ding, Y. Li, J. Zhao, Y. Zhu, Y. Li, W. Deng and C. Wang, APL Materials, 2015, 3, 104410.
5. A. Meng, B. Zhu, B. Zhong, L. Zhang and B. Cheng, Applied Surface Science, 2017, 422,
518-527.
6. Y. Peng, G. Han, D. Wang, K. Wang, Z. Guo, J. Yang and W. Yuan, International Journal of
Hydrogen Energy, 2017, 42, 14409-14417. 
7. J. Ran, G. Gao, F.-T. Li, T.-Y. Ma, A. Du and S.-Z. Qiao, Nature Communications, 2017, 8,
13907. 
8. J. Ran, B. Zhu and S. Z. Qiao, Angewandte Chemie International Edition, 2017, 56, 10373-
10377. 
9. L. Zou, H. Wang and X. Wang, ACS Sustainable Chemistry & Engineering, 2017, 5, 303-309.
10. S. Ma, J. Xie, J. Wen, K. He, X. Li, W. Liu and X. Zhang, Applied Surface Science, 2017, 391,
580-591.
11. F. Q. Zhou, J. C. Fan, Q. J. Xu and Y. L. Min, Applied Catalysis B: Environmental, 2017, 201,
77-83.
12. T. Di, B. Zhu, J. Zhang, B. Cheng and J. Yu, Applied Surface Science, 2016, 389, 775-782.
13. Z. Shi, X. Dong and H. Dang, International Journal of Hydrogen Energy, 2016, 41, 5908-
5915. 
14. L. J. Zhang, S. Li, B. K. Liu, D. J. Wang and T. F. Xie, ACS Catalysis, 2014, 4, 3724-3729.


Biomimetic Z-scheme photocatalyst with a tandem solid-state electron flow catalyzing H_2 evolution

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Biomimetic Z-scheme photocatalyst with a tandem solid-state electron flow catalyzing H_2 evolution

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