H2O2 Treated CdS with Enhanced Activity and Improved Stability by a

Subscriber access provided by TULANE UNIVERSITY

Article

H2O2 treated CdS with enhanced activity and improved stability by a weak negative bias for CO2 photoelectrocatalytic reduction Zhengpeng Li, Hengbin Cheng, Yifei Li, Wei Zhang, and Ying Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06144 • Publication Date (Web): 19 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

H2O2 treated CdS with enhanced activity and improved stability by a weak negative bias for CO2 photoelectrocatalytic reduction Zhengpeng Li+, Hengbin Cheng+, Yifei Li, Wei Zhang and Ying Yu* Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, 152 Luoyu Road, Wuhan 430079, People’s Republic of China

+ The authors contributed equally to this work. * Corresponding author. Email: [email protected].

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: To convert greenhouse gas CO2 to available energy by using solar energy is a promising approach for addressing energy dilemma and global warming issues. Although CdS as a photocatalyst can achieve CO2 reduction to fuel, there are still two main problems of activity and stability to be solved. Herein, a simple hydrothermal method with the presence of a little quantity of H2O2 is used to prepare CdS with Cd vacancies, which can promote the separation of photogenerated electrons and holes for activity improvement. It is found that the best catalyst 0.4CdS demonstrates not only high CO selectivity over other carbonaceous products, but also a considerable CO production rate of 316 μmol g-1 h-1, which is 2.1 times as high as that of pure CdS for CO2 reduction under visible light irradiation. In addition, an efficient solution through an additional feeble negative voltage to improve the photostability of 0.4CdS is achieved, which makes this sample remain 94.2% of its original CO2 photoelectrocatalytic reduction performance after four cycles. Thus, this study provides a facile strategy to address the stability and activity issues of CdS under visible light irradiation, which is presumably suitable for improving the other semiconductors with low stability and activity for highly efficient CO2 reduction.

KEYWORDS: CO2 reduction; photocatalysis; photoelectrocatalysis; CdS; Cd vacancies


Page 2 of 38

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Global climate change and energy crisis will threaten the survival and development of human beings in the long term.1-4 Solar-powered conversion of greenhouse gas especially CO2 to storable value-added chemicals and fuels holds the promise both to become eco-friendly alternatives for sustaining energy demand and to alleviate adverse consequences of anthropogenic CO2 overproduction.5,6 The solar energy from sunlight is assimilated by a photocatalyst, resulting in the generation of electron and hole pairs.7 And the reductive nature of photogenerated electrons makes it possible to convert CO2 into valuable products such as carbon monoxide, methane and formic acid.7 What really matters in the photoconversion process is the photocatalyst. In consequence, the semiconductor materials with high efficiency and stability are highly desired. Since the pioneering work of photocatalytic splitting of water to hydrogen and oxygen over TiO2 was published by Honda and Fujishima in 1972, photocatalytic system has been utilized in energy conversion area.8 TiO2, a superstar photocatalyst, has been intensively focused on due to its bargain price, avirulence and stabilization. Unfortunately, its photoresponse range is within the ultraviolet portion of sunlight, which severely decreases the utilization of solar energy. Additionally, its inferior efficiency on account of the recombination of photogenerated electrons and holes also limit its applications. Thus, in recent years, other suitable semiconductors such as ZnO,9 g-C3N4,10 ZrO2,11 Cu2O,2 CdS,12,13 etc as efficient photocatalysts for CO2 reduction have attracted wide interest. Among them, CdS is one of the most studied



semiconductors due to its suitable bandgap of ~ 2.4 eV for effective sunlight absorption, and its favorable energy band structure, in which the conduction band is sufficiently negative for CO2 reduction to fuels.9 Furthermore, abundance and inexpensive price make it possible for CdS to be promisingly used for CO2 conversion to fuel under visible light irradiation. Despite the fact that CdS semiconductor exhibits attractive ability for solar light harvesting, there are several disadvantages that still limit its application. On the one hand, the fast recombination of photogenerated electrons and holes is a key factor that hampers the carrier rapid migration to catalyst surface for photocatalytic reactions due to its poor conductivity. Up to date, a series of methods have been put forward to reduce the recombination rate of photogenerated electrons and holes, for instance, heterojunction structure construction,9,14 cocatalyst deposition,15-17 carbonaceous material modification,18,19 etc. In addition, defect engineering is another significant way to tune the functional properties of sulfide, such as electron configuration, charge migration, and catalytic activity.20 In previous study, tuning anion vacancy species represented by oxygen vacancies is considered to be the effective approach since anion vacancies tend to be the centers for capturing photogenerated electrons in the photoreaction, on account of the fact that these electrons easily transfer from the conduction band to anion vacancies rather than recombine with holes.21 So far, cation vacancies have been seldom reported. However, the cation vacancies are deemed to promote the electrical conductivity and the mobility of electronic charge carriers.22 It has been reported that there is a new band transition above the top of original valence


Page 4 of 38

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


band (VB) for the semiconductor with abundant cation vacancies, which enlarges the width of VB.20 It is well known that the mobility of photogenerated holes is intrinsically determined by the VB width, which means that wider VB results in higher mobility of holes.10,23 Therefore, metal defects can improve the migration ability of photogenerated holes to facilitate the separation efficiency of electron-hole pairs. On the other hand, photocorrosion effect is another key drawback for CdS, in which the sulfur ion (S2-) is liable to be oxidized to sulfite ion (SO32-) or sulfate ion (SO42-) by photogenerated holes.24

Compared

with

photocatalytic

(PC)

system,

the

advantages

of

photoelectrocatalytic (PEC) have been confirmed, such as efficient separation efficiency of photogenerated carriers due to built-in potential in photoelectrodes.25 Additionally, the electrons input from external circuit are able to recombine with the holes in the valence band of the photocatalyst, which prevents the photocatalyst from being oxidized by photogenerated holes for stability improvement.26 In order to address the activity and stability issues for CdS, herein, a facile hydrothermal approach with the presence of hydrogen peroxide (H2O2) was used to fabricate CdS with abundant Cd vacancies, which is beneficial to improve catalytic activity due to stepped-up separation of electron-hole pairs. In addition, we also established a system of CO2 photoelectrocatalytic reduction, which worked in a threeelectrode system under simulated sunlight with a weak negative bias. Through the two ways, both the activity and stability of CdS have been greatly enhanced. To the best of our knowledge, there has been no report about CdS with Cd vacancies for CO2 reduction.



EXPERIMENTAL SECTION Materials Cadmium acetate (Cd(Ac)2•2H2O), thiourea (CH4N2S), H2O2 solution (30%), polyethylene glycol-4000 were purchased from Shanghai Guoyao Chemicals Co. Ltd., which were analytical reagents (AR) and used without further purification. All of aqueous solutions used in this study were prepared by deionized water. Preparation of H2O2 treated CdS nanoparticles In this study, H2O2 treated CdS were synthesized through a hydrothermal method, and a typical process was as follows. 3.6 mmol of Cd(Ac)2 • 2H2O and an equivalent amount of thiourea was dissolved in 80 mL deionized water and stirred vigorously for 30 min at room temperature and normal pressure. The above solution was transferred to polytetrafluoroethylene autoclave with 100 mL capacity. Subsequently, with vigorous stirring, a certain amount of H2O2 (10%) was slowly added dropwise to the solution. The additive amount of H2O2 (10%) was 0.2, 0.4 and 0.6 ml, and the correspondingly obtained samples were labeled as 0.2CdS, 0.4CdS and 0.6CdS, respectively. After stirring for 30 min, 80 mg of polyethylene glycol was added to the solution, followed by stirring for another 30 min. Finally, the hydrothermal synthesis reactor was put into an oven with temperature of 160 °C. After 48 h hydrothermal reaction, the autoclave was cooled down to room temperature. The resulting precipitate was centrifuged with ethanol and deionized water for 5 times, and then was placed in a vacuum oven at 80 ° C overnight. By adjusting the amount of H2O2, the samples with


Page 6 of 38

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


different amount of Cd vacancies were prepared. When preparing pure CdS, all the conditions were the same as those for the preparation of H2O2 treated samples, except that H2O2 was not added. Preparation of H2O2 treated CdS photoanodes The H2O2 treated CdS photoanodes were fabricated by the deposition of the prepared samples onto fluorine-doped tin oxide substrates (2 × 1 cm2) through a spin-coating method. Before the spin-coating experiment, 20 mg H2O2 treated CdS (0.2CdS, 0.4 CdS, 0.6CdS, and Pure CdS) was dispersed in a solution containing 300 μl of alcohol and 50 μl 5% nafion solution (Sigma-aldrich Co., Ltd.) by ultrasonication. Then four drops of the above solution were put onto fluorine doped tin oxide (FTO, AGC Fabritech Co., Ltd.) substrates. The FTO electrodes with H2O2 treated CdS were rapidly span by controlling revolving speed at 3500 rpm for 40 s, and then they were dried at 60 °C for 5 minutes. In order to ensure that each piece is uniform and with the same thickness, the above process was repeated twice. Materials Characterization The morphology of samples in the study was observed by a field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and a transmission electron microscopy (Titan G2, 200 kV). The phase constituents of the prepared samples were analyzed from X-ray powder diffraction (XRD) patterns detected from the PANalytical diffractometer (D/max 40kv) with Cu Kα radiation (  = 0.154598 nm). UV-vis diffuse reflectance spectra (UV-vis DRS) were collected on a PerkinElmer Lambda 35 spectrophotometer at 400 ~ 800 nm range with barium sulfate (BaSO4) as a reference.



X-ray photoelectron spectroscopy (XPS) (VG Multiab-2000) spectra was used to figure out the chemical states of the elements for the samples using a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and charge neutralizer. All of the binding energy were calibrated by the C 1s peak at 284.6 eV. Brunauer-Emmett-Teller (BET) specific surface area for the samples was analyzed by Micromeritics ASAP2020 nitrogen adsorption apparatus. LabRAMHR Raman system was used for Raman spectra measurement under Ar+ (532 nm) laser excitation. Fourier transform infrared (FTIR) spectra were recorded with a FTIR spectrometer (Nicolet iS50 FT-IR, USA) in the range of 525 ~ 4000 cm−1. Time-resolved photoluminescence spectra (TRPL) were obtained on a FLS980 fluorescence spectrometer (Edinburgh). Electrochemical Measurements A CHI660E (ShangHai Chen Hua Instruments Co., Ltd.) electrochemical station was used to conduct electrochemical measurements in a standard three-electrode cell with the sample on FTO glass as the working electrode, a Pt plate as the counter and the standard Ag/AgCl electrode as the reference. The transient photocurrent measurement was performed in 0.02 M Na2SO4 aqueous solution under the irradiation of a 350W xenon lamp (Lap Pu, XQ) with a 420 nm cutoff filter to remove the light with the wavelength less than 420 nm, in which the light on/off cycle was 50 s. The electrochemical impedance spectroscopy (EIS) was determined over the frequency range of from 10 mHz to 100 kHz with the potential of 0 V vs. Ag/AgCl and the Mott−Schottky plots were carried out at the frequency of 1.0 kHz. Photocatalytic and Photoelectrocatalytic Performance Tests


Page 8 of 38

Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO2 PEC reduction reactions were conducted in a sealed three-electrode electrochemical cell (250 mL) with two-compartments full of KHCO3 aqueous solution (170 mL, 0.1 M, pH = 6.8), divided by nafion membrane. The CO2 PC reduction reactions were carried out in a teflon hermetic container (250 mL) with 170 mL KHCO3 aqueous solution (0.1 M, pH = 6.8). Both PEC and PC reactors were equipped with a quartz window in the front. Prior to the reaction, the reactor was adequately purged with high-purity CO2 (99.99%) gas for 30 min to make sure that the O2 was removed completely. The external potentials (0 V, -0.3 V, -0.6 V, -0.9 V vs. Ag/AgCl) for CO2 PEC reduction reactions were powered by CHI660E electrochemical station with the samples on FTO glass as the working electrode, a Pt plate as the counter and the standard Ag/AgCl electrode as the reference. The average intensity of the 350 W xenon lamp with the 420 nm UV-cutoff filter was 1.06 mW/cm2. During the PEC and PC reactions, a 500 μL syringe was used for the collection of gaseous products, which was then promptly analyzed by two gas chromatograph instruments (GC2014, Serial Nos. C11484403556 SA and C11485013433 SA, FID and TCD respectively, Shimadzu, Japan) using nitrogen as carrier gas. Control trials were also conducted under identical conditions except that CO2 was replaced by N2 atmosphere or there was no light irradiation. The cycle experiments were performed under the same condition in each 4 h PEC and PC reduction with the same sample, in which fresh CO2 and electrolyte was used for each cycle. The quantity of the evolved products for each CO2 reduction was the average of three tests. All reactions were carried out at room temperature and normal atmospheric pressure.



RESULTS AND DISCUSSION

Figure 1. (a) XRD patterns and (b) corresponding enlarged parts in the range of 25.5 ~ 27.5° for pure CdS, 0.2CdS, 0.4CdS and 0.6CdS. High-resolution XPS spectra of (c) Cd 3d and (d) S 2p for pure CdS and 0.4CdS. (e) XPS spectra of Cd 3d for 0.4CdS collected at different etching times. (f) N2 adsorption−desorption isotherms for pure CdS and 0.4CdS.

A series of H2O2 treated CdS samples, 0.2CdS, 0.4CdS and 0.6CdS, were synthesized with the presence of different amount of H2O2. XRD patterns for 0.2CdS, 0.4CdS and 0.6CdS, along with pure CdS prepared under the same conditions, are shown in Figure 1a. It can be distinctly found that the diffraction peaks were characteristic of hexagonal CdS (PDF no. 2-549). There was no significant difference for the patterns of all the samples, and no impurity peak attributed to CdO, CdSO4 or


Page 10 of 38

Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CdSO3 was identified.27-29 As can be seen from Figure 1b, a comparison of the XRD patterns in the range of 25.5 ~ 27.5° assigned to the (002) plane of hexagonal CdS was displayed. The peak at around 26.6° for pure CdS sample shifted to the high degree for H2O2 treated CdS samples and the offset was more obvious with the increase of H2O2 content, which could be attributed to present macro residual stress caused by the reflection of lattice distortion mainly due to the presence of vacancies.30,31 To further investigate the surface composition and bonding states of the H2O2 treated CdS, high resolution X-ray photoelectron spectroscopy (XPS) spectra were collected. The survey spectrum of 0.4CdS was discerned in Figure S1 in Supporting Information and the Cd 3d high-resolution XPS spectrum for pure CdS was resolved into two sub-peaks with binding energy of 405.1 and 411.9 eV, ascribed to the spin orbit of Cd 3d 5/2 and Cd 3d 3/2, respectively (Figure 2c). The spin orbit split of the two peaks of Cd 3d orbital was 6.79 eV, which verifies that the valence state of Cd element in the pure CdS was Cd2+.5,32 As for 0.2CdS and 0.4CdS, it is worth noting that two new peaks at 405.7 and 412.5 eV were emerged, attributed to Cd vacancies.33,34 It is known that Cd vacancies engineering is very critical to tune the functional property of CdS, such as electronic structure related to photocatalytic performance. The influence of Cd vacancies on the width of the valence band for CdS may make it better for the mobility of holes to reduce the recombination of photogenerated electrons and holes and then improve their migration to the surface for PC reaction. Compared with 0.2CdS (Figure S2 in Supporting Information), the ratio (0.414) of the area for the peak of Cd vacancies to that of Cd2+ became larger for 0.4CdS (0.632), which means that



with the addition of more H2O2, more Cd vacancies were formed for CdS. A detailed comparison with the atomic ratio of Cd to S is presented in Table S1 in Supporting Information, which shows that the amount of Cd atoms in CdS became less with the increase of H2O2 amount. Furthermore, the symmetrical signal peaks centered at 161.4 eV and 162.7 eV in S 2p spectrum for pure CdS (Figure 1d) were observed, corresponding to S 2p 1/2 and S 2p 3/2 spin orbits, respectively. It indicates that there was only S2- state in the sample.24 For the sample of 0.4CdS, the peaks slightly shifted toward a lower binding energy, which might be due to the presence of Cd vacancies. In the hydrothermal process, S2- may be oxidized to a higher valence state 𝑥 (-2 < 𝑥 < 0) by an appropriate amount of H2O2. Since no peak corresponding to S or CdSO3 was detected in XRD patterns, 𝐶𝑑𝑥𝑆 (-2 < 𝑥 < 0) was present. In such a system with 2

insufficient Cd demand, Cd vacancies are likely to be formed.35,36 To further probe the distribution of Cd vacancies inside 0.4CdS, an Ar ion gun in the chamber of the XPS instrument was applied to remove the surface atoms of the samples. The Cd 3d XPS spectra for 0.4CdS collected at the sputtering time of every minute are shown in Figure 1e. It is found that there was no change for the Cd 3d peak under different etching time, which indicates that Cd vacancies were evenly distributed in the sample, not only on the surface but also in the bulk. Additionally, there was also no change for the S 2p peak as well, which is shown in Figure S3 in Supporting Information. From Table S1 in Supporting Information, we can also find that the Cd:S ratio for 0.4CdS hardly changed under different etching time, which reveals again that the distribution of Cd vacancies in the sample was uniform.


Page 12 of 38

Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To further ascertain the aperture structure of the 0.4CdS sample, BET surface area was detected using N2 adsorption−desorption measurement. The data are presented in Figure 1f. A representative type III H2 hysteresis loop curve for 0.4CdS was demonstrated. Additionally, the specific surface area of 0.4CdS was 12.36 m2 g−1, which was much larger than that of pure CdS (8.37 m2 g−1). It can be attributed to the smaller crystallite size for 0.4CdS (Figure S4 in Supporting Information) than that for the pure CdS, for which the smaller the particle radius, the larger the specific surface area.37 The larger specific surface area for 0.4CdS is able to provide more surface active sites and promote the adsorption of reactant and intermediate molecules, which can be beneficial for CO2 reduction reaction.2, 38 SEM test was performed to explore the details about the morphology of pure CdS and 0.4CdS. Figures 2a and 2c show that both 0.4CdS and pure CdS were assembled by particles. As shown in the SEM images with different magnifications in Figures 2a ~ 2b, a cauliflower-like morphology consisted of irregular particles with a large size of about 2 ~ 3 μm was observed for pure CdS. When H2O2 was present during hydrothermal synthesis, the SEM images with different magnifications for 0.4CdS displayed a fern-like morphology with the advantage of one-dimensionalstructure as its “leaves” (Figures 2c ~ 2d). The individual one-dimensional branch of 0.4CdS may be benefit for the efficient carrier transfer to the sample surface for redox reaction.2,39 The fern-like morphology of 0.4CdS was further confirmed by TEM analysis in Figure 2e. High resolution TEM (HRTEM) image for 0.4CdS (Figure 2f) exhibited the lattice fringe of about 0.335 nm, corresponding to the (002) plane of hexagonal CdS, in



Figure 2. SEM images for (a ~ b) pure CdS, (c ~ d) 0.4CdS. (e) TEM image and (f) HRTEM image for 0.4CdS.

accordance with the XRD spectra in Figure 1a. Substantially, compared with the pure CdS, the 0.4CdS sample with a diameter size of 600 ~ 800 nm possessed a rough surface, which may provide a large specific surface area and contribute to trap the incident light efficiently,40 consistent with the BET data. Moreover, the SEM image of the lateral view of 0.4CdS photoanode (Figure S5 in Supporting Information) displayed that the film with a thickness of approximately 2.75 μm was uniformly coated on FTO substrate. The UV−vis diffuse reflectance spectrum (DRS) is used to explore the light absorption performance and band structure of semiconductor materials. By using a Kubelka−Munk transformation, bandgaps can be calculated through Tauc plots. It can be found in Figure 3a that pure CdS absorbed the light with the wavelength shorter


Page 14 of 38

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) UV-vis diffuse reflectance spectra for pure CdS, 0.2CdS, 0.4CdS and 0.6CdS (the insert shows the plot of [F(R)]2 versus photo energy). (b) Mott-Schottky plots. (c) Band structure alignments of pure CdS and 0.4CdS. (d) Photocurrent density and (e) electrochemical impedance spectra (EIS) of pure CdS, 0.2CdS, 0.4CdS and 0.6CdS. (f) Time-resolved PL spectra (excitation at 400 nm, emission at 511 nm) for pure CdS and 0.4CdS.

than ∼550 nm, which indicates that it has photoresponse in visible light region. In comparison with pure CdS, 0.4CdS showed an apparently decreased absorption in visible light region since the light absorption edge shifted toward short wavelength range. On the basis of the Kubelka−Munk function, the DRS measurements were translated and plotted against the energy of photon (the insert in Figure 3a). It can be seen that the bandgap for 0.4CdS was estimated to be 2.47 eV, which was larger than that for pure CdS (2.32 eV). The enlarged bandgap may lead to an upshift of the



Page 16 of 38

conduction band position for the reducibility enhancement of CdS.23 In order to prove it, the conduction band positions of the pure CdS and different H2O2 treated CdS samples calculated from the Mott−Schottky (MS) plots are shown in Figure 3b. The positive slope displays that the pure CdS and H2O2 treated CdS were n-type semiconductors with electron conduction. According to the MS plots, the flat-band potential ( EFB ) was estimated to be -1.1 and -1.3 V (vs. Ag/AgCl) for pure CdS and 0.4CdS, respectively, which can be transformed to the normal hydrogen electrode (NHE) scale for the above measured value via Equation 1.41

ENHE  E Ag/ AgCl  0.05916  pH  E A0g/ AgCl

1

where ENHE is the transformed potential vs. NHE, E Ag/ AgCl the experimentally measured potential vs. Ag/AgCl reference electrode, and E A0g/ AgCl the normal potential of Ag/AgCl at 298 K (0.1976 V). Then the conduction-band ( ECB ) edge can be calculated based on Equation 2. EFB  ECB  k BT ln  NV / N A 

 2

in which N A is the carrier density estimated from Equation 3.42 N A   2 / e0 0   d 1/ C 2  / dV 

1

 3

where e0 is the electron charge,  the dielectric constant of CdS (  CdS  8.7),  0 the permittivity of vacuum, C the interfacial capacitance, and V

the applied

potential. And NV , the effective density of states in the valence band, can be indicated as Equation 4.2 NV  2  2 m k BT / h 2 

3/2

 4

in which m is the effective carrier mass ( mhole  0.58m0 ), k B the Boltzmann constant,


Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


T the absolute temperature, and h the Planck constant. The estimated date was 0.636 V for pure CdS, in accordance with previous reports43, and that was -0.831 V for 0.4CdS. The energy position for the calculated conduction band was above the reduction potential of CO2,3 which might furnish a tractive force for CO2 photoreduction thermodynamically. Table S2 in Supporting Information shows the conduction and valence band positions relative to NHE calculated from Mott-Schottky plots and UVvis diffuse reflectance spectra, and the bandgap for the pure CdS and CdS treated with different amount of H2O2 samples. Compared with the pure CdS, the conduction band potential of 0.4CdS significantly up-shifted. It infers that the reducibility of the photogenerated electrons for 0.4CdS was stronger than that for pure CdS, which is beneficial for CO2 PC and PEC reduction.44 To further verify this conclusion, XPS valence-band spectra were performed to analyze the band edges of the pure CdS and 0.4CdS. As can be seen from Figure S6 in Supporting Information, the valence band positions were calculated to be 1.874 and 1.684 V relative to NHE for pure CdS and 0.4CdS, respectively. The calculating process for XPS valence band spectra is displayed in Supporting Information and the detailed data of the conduction and valence band positions calculated from XPS valence-band spectra and UV-vis diffuse reflectance spectra for the H2O2 treated CdS samples are shown in Table S3 in Supporting Information. It can be found that 0.4CdS possessed conduction band position more negative than that of CdS as well, resulting in the higher reducibility of 0.4CdS, in agreement with the trend calculated from the Mott−Schottky plots. The band



Page 18 of 38

structure alignments of pure CdS and 0.4CdS are schematically illustrated in Figure 3c, which demonstrates that the conduction band ( ECB ) edges of the pure CdS and 0.4CdS were located above the reduction potential of CO2.3 In summary, the conduction band positions of the pure CdS and 0.4CdS calculated from both the MS plots and XPS valence band spectra are more negative than the reduction potential required for CO2 reduction to CO, CH4 and H2, as shown in Equations 5 ~ 7.2,3

CO2  2 H   2e   CO  H 2O CO2  8 H   8e   CH 4  2 H 2O 2 H   2e   H 2

E0  0.53 V vs NHE at pH 7 E0  0.24 V vs NHE at pH 7

E0  0.41 V vs NHE at pH 7

 5  6

7

In addition, it is worth emphasizing that the photogenerated holes can play a key role as well in CO2 conversion along with the photogenerated electrons since the holes may oxidize water for oxygen evolution, and S2- for the formation of SO32- and SO42-, or else they recombine with electrons.24 From Figure 3c, it is found that the valence band edges of the pure CdS and 0.4CdS were situated below the oxidation potential of water, which is 0.81V at pH = 7.45 Hence, the O2 formation is proved to be a thermodynamically permissible process in this case, and the redox cycle of the CO2 photoreduction reaction can be accomplished. The light-harvesting property of the semiconductors was detected by plotting the photocurrent density under visible light irradiation (  > 420 nm) against irradiation time (Figure 3d). 0.6CdS showed the highest photocurrent density, about 4.6 times as much as that of the pure CdS. The addition of H2O2 during the sample preparation process resulted in the improvement of photocurrent densities for the final obtained


Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples, which was 1.6 and 3 times as much as that of 0.2CdS and 0.4CdS, respectively. This significant improvement suggests the enhanced light-harvesting performance due to the presence of Cd vacancies, facilitating the efficient photogenerated electron and hole separation and transfer. In addition, it is worth noting that the instant photocurrent under irradiation was extremely large for 0.6CdS, but it decreased rapidly on account of photocorrosion resulted from the oxidation of S2- to SO32- and SO42- by photogenerated holes. As the amount of H2O2 increased, the photocorrosion of the prepared CdS became more serious, which might be due to the reason that Cd vacancies enabled rapid transfer of photogenerated electrons and holes and thus, on the contrary, more photogenerated holes participated in the reaction of S2- oxidization.46 Furthermore, electrochemical impedance spectra (EIS) without light illumination was collected to investigate interfacial charge transfer process (Figure 3e). The interfacial charge transfer resistance is related to the diameter of the semicircle of Nyquist plot. While the pure CdS showed very large diameter due to its low conductivity, the semicircle diameter for 0.6CdS was the smallest in that for all of the H2O2 treated samples, implying much reduced resistance induced by the presence of Cd vacancies for the fast transport of charge carriers. These PEC results affirm that the prolonged lifetime and stepped-up separation of photogenerated electron-hole pairs were realized for the H2O2 treated CdS samples with Cd vacancies, which can be greatly propitious for CO2 reduction reactions. The separation of photogenerated electron-holes pairs of pure CdS and 0.4CdS has been further confirmed by the time-resolved photoluminescence spectra (TRPL) as shown in Figure 3f. The spectrum curves for the two samples were



Page 20 of 38

fitted through a bi-exponential function and the expression is as follow.47

I  t   A1 exp  t /  1   A2 exp  t /  2 

8

where  1 and  2 is the shorter lifetime and the longer lifetime of the decay times respectively, and A1 and A2 the amplitudes of photoluminescence. And the average lifetime can be calculated as follow.

 average 

Ai i 2 Ai i

9

It is noteworthy that the shorter lifetime, longer lifetime and average lifetime of 0.4CdS was 2.12, 14.97 and 9.09 ns, which was 1.32, 13.91 and 5.26 ns for the pure CdS, respectively. In general, the shorter lifetime (  1 ) is in connection with quasi-free excitons whereas the longer lifetime (  2 ) is deemed to originate from the formation of photogenerated electrons and holes.48 The calculated results illuminate that the lifetime of photogenerated electrons and holes for 0.4CdS are markedly prolonged compared with that of the pure CdS, originated from the recombination suppression of of photogenerated carriers and the separation acceleration of photogenerated electronholes pairs due to the presence of Cd vacancies. The above-mentioned results indicate that it is effective for H2O2 presence during preparation process to adjust intrinsic, optical and electronic properties of the semiconductor, which is supposed to improve the performance of CdS for target PC and PEC reactions. To evaluate the activity of H2O2 treated CdS for CO2 photoreduction, CO2 PC reduction reactions were conducted under visible light ( 

> 420 nm)

irradiation in the presence of CO2 and water (Figure 4a). The products of CO, H2 and CH4 were detected by gas chromatography after 4 h visible light irradiation, in which


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Yield of CO, H2 and CH4 evolved from CO2 PC reduction for pure CdS, 0.2CdS, 0.4CdS and 0.6CdS under visible light irradiation for 4 h. (b) Yield of CO, H2 and CH4 evolved from CO2 PEC reduction at different potentials for 0.4CdS under visible light irradiation for 4 h. (c) Yield of CO, H2 and CH4 evolved from CO2 PC and PEC (-0.3 V vs. Ag/AgCl) reduction for 0.4CdS in comparison with that for the same sample measured at different control conditions. (d) Normalized yield of CO, H2 and CH4 evolved from CO2 PC and PEC (-0.3 V vs. Ag/AgCl) reduction for pure CdS and 0.4CdS. (e) Stability of pure CdS and 0.4CdS for PC and PEC (-0.3 V vs. Ag/AgCl) evolution of CO.

CO was predominant. As can be seen from Figure 4a, the pure CdS only exhibited a moderate activity (151 μmol h-1 g-1) for the conversion reaction of CO2 to CO in the established catalytic system but H2O2 treated CdS samples produced CO with the substantially enhanced amount. The CO formation rate for 0.4CdS, in particular,



reached 316 μmol g-1 h-1, much higher than any other H2O2 treated CdS photocatalysts, consistent with the calculated conduction band position (Table S2 and Table S3 in Supporting Information). Additionally, the apparent quantum efficiencies (AQEs) of CO was calculated to be 0.33% for 0.4CdS at 0 = 400 nm. When excessive H2O2 were added during the hydrothermal process, the catalytic performance of the prepared 0.6CdS sample decreased, which may be owing to severe photocorrosion derived from S2- oxidation by the presence of more photogenerated holes. Another interesting finding is that photocorrosion may be the key reason that limits the performance of CdS as shown in Figure 4e, in which the results of repeated CO2 photoreduction experiments for four cycles under the same conditions are demonstrated. The activity of the pure CdS decreased to 71.2% of that for the first cycle, resulted from the serious photocorrosion through self-oxidation by photogenerated holes. Similarly, the yield of CO decreased to 198 μmol h-1 g-1 in the fourth cycle for 0.4CdS, about 62.9% of that for the first run. Its photocorrosion extent was even more serious than that for pure CdS (Figure S7 in Supporting Information), which is due to the reason that more photogenerated carriers migrate to the photocatalyst surface to participate in the redox reaction owing to the presence of Cd vacancies, corresponding to more photogenerated holes available for S2- oxidation. In order to improve the photostability of 0.4CdS, a negative bias was added to establish a CO2 PEC reduction system, in which external input electrons may be combined with photogenerated holes to prevent S2- from hole oxidation for the stability improvement of CdS.26,49 The PEC performance of the pure CdS and 0.4CdS was


Page 22 of 38

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


measured in the same cell as that for CO2 PC reduction reactions, in which there was the connection of different electrodes and the accession of the weak negative bias. Thereafter, the catalytic activity of 0.4CdS was tested at different potentials under visible light irradiation (  > 420 nm), which showed a maximum amount for CO at potential of -0.3 V versus Ag/AgCl electrodes (389 μmol h-1 g-1 (Figure 4b). This performance surpasses that for the most CdS based photocatalysts reported previously (Table S4). Therefore, -0.3 V vs. Ag/AgCl was considered as the optimal reaction potential and applied in the subsequent measurements. The decline of CO production at -0.9 V may be due to the competitive reaction of hydrogen production from water splitting.50 The identical PEC test was conducted for the pure CdS (Figure S8 in Supporting Information). It can be found that the best potential was -0.6 V. However, the evolved CO amount at this potential was almost half of that at -0.3 V for 0.4CdS. Thus, the PEC performance of the H2O2 treated CdS is much better than that for the pure CdS. In addition, control experiments were conducted under the same conditions in the absence of either visible light or CO2 (N2 was used instead of CO2). The result showed that no appreciable carbon containing product was detected, which suggests that the formed CO was indeed derived from CO2 reduction (Figure 4c). Besides, the activity of electrocatalytic reduction of CO2 at -0.3 V vs. Ag/AgCl for 0.4CdS was also measured for comparison. There was no CO, CH4 or H2 detected, implying that electrocatalysis had no contribution for CO2 reduction in this system and the applied negative voltage only acted as an electron donor to capture photogenerated holes.26,49 Accordingly, O2 was also detected in the PEC system (Figure S9 in Supporting



Information), in which the yield for 0.4CdS exceeded that for pure CdS. What’s more, the PEC performance at -0.3 V vs. Ag/AgCl was superior to the PC, demonstrating that a negative bias is of benefit for CO2 reduction. It may be originated from facilitating the separation of electrons and holes, and allowing electrons to migrate rapidly to the catalyst surface for CO2 reduction reaction.49 It should be pointed out that specific surface area was also a key factor to influence the catalytic reaction. Therefore, the reaction rate was normalized using the specific surface area to rule out its effect as shown in Figure 4d. After excluding the effect of specific surface area, the activity of 0.4CdS sample was still much higher than that of pure CdS, which identifies that the enhanced photocatalytic performance is not resulted from the morphology, specific surface area and crystallinity, but from the electron band structure, low recombination rate and fast carrier migration. Substantially, the normalized yield of PEC reaction was higher than that of PC for both the pure CdS and 0.4CdS. Additionally, the PEC activity at -0.3 V vs. Ag/AgCl for both the pure CdS and 0.4CdS was able to be maintained after four cycles, just about 4.1% and 5.8% loss respectively in comparison with that for the first loop, inferring that the enhanced photostability of CdS may be due to the sufficient combination between the additional electrons from external circuit and photogenerated holes. So as to explore the specific structure and composition changes for pure CdS and 0.4CdS during the CO2 reduction for PC and PEC reactions, SEM images were taken for the pure CdS sample after 8 h CO2 PC reduction, 0.4CdS sample after 8 h PC and PEC reaction as presented in Figure S10 and Figures 5a ~ 5b, respectively. It is found


Page 24 of 38

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. SEM images for 0.4CdS (a) after 8 h CO2 PC reduction and (b) after 8 h CO2 PEC (-0.3 V vs. Ag/AgCl) reduction. (c) XRD patterns and (d) FTIR spectra of 0.4CdS before and after 8 h CO2 PC and PEC (-0.3 V vs. Ag/AgCl) reduction. (e) Schematic diagram of proposed process for CO2 PEC reduction over 0.4CdS under visible light irradiation at negative bias.

that the morphology changed after CO2 PC reduction for both the pure CdS and 0.4CdS. However, the architecture was relatively intact for 0.4CdS even after CO2 PEC reduction, verifying the high photostability of 0.4CdS in the PEC system. In addition, XRD and FTIR characterizations were performed to analyze the chemical composition and crystalline structure of the semiconductors after CO2 PC and PEC reduction. As shown in Figure 5c, two new peaks were shown up for 0.4CdS after CO2 PC reduction, which was assigned to CdSO3.29 It was the main product from CdS under visible light



irradiation since the XRD patterns of 0.4CdS before the reaction showed no impurity peak. On the contrary, the XRD patterns for 0.4CdS had almost no change after 8 h CO2 PEC reaction, indicating that photoelectrocatalysis is helpful to ease photocorrosion so that the stability under visible light is greatly improved. Similarly, another technique such as FTIR was also used to investigate the influence of long time illumination on the structure of 0.4CdS after PC and PEC reduction (Figure 5d). Compared with the FTIR spectrum of the original 0.4CdS, there were two new peaks at 935 and 1120 cm-1 for the same sample after CO2 PC reduction, which corresponded to CdSO3.29,51 However, there was no new peak in that of 0.4CdS after photoelectrocatalysis under the same condition. Therefore, all of the above data confirm the excellent stability of 0.4CdS during CO2 PEC reduction, which positively accounts for the continuously good performance of CO2 reduction for a prolonged period of time. Based on the above results, a probable mechanism of the CO2 PEC reaction over 0.4CdS is proposed as displayed in Figure 5e. When 0.4CdS was illuminated by visible light with energy level more than its bandgap, electrons and holes were generated. Adscititious electrons under relatively weak negative voltages were able to combine with photogenerated holes,26,49 improving the separation efficiency of photogenerated carriers. Additionally, the Cd vacancies for 0.4CdS were served as an electron collector and transporter to make a contribution for the photogenerated electron-hole separation as well, and thus for efficiently extending the lifetime of the charge carriers.23 Consequently, the photogenerated electrons (e-) might migrate rapidly from the conduction band of 0.4CdS to the surface to reduce the adsorbed CO2 molecules, which


Page 26 of 38

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was captured and activated on 0.4CdS surface to evolve CO and CH452 of two and eight electron reaction products, respectively. In the meantime, the proton (H+) in the system was reduced by electrons as well, leading to the formation of H2,26 which is the main competitive reaction for CO2. Moreover, the hydroxyl anions (OH-) in aqueous solution moved onto the platinum electrode (counter electrode) and was oxidized to oxygen (Figure S9 in Supporting Information) due to the fact that the valence band edge of 0.4CdS is more positive than the oxidation potential of H2O/O2. On the basis of the above discussion, the complete redox process related to the CO2 PEC reduction was efficiently accomplished.

CONCLUSIONS In summary, a high efficient CO2 PEC reduction reaction under visible light irradiation has been achieved over 0.4CdS synthesized by a facile hydrothermal method. Cd vacancies play a significant role in the enhancement of CO2 PC and PEC reduction reaction due to their functions for the improvement of the separation and transfer of photogenerated electrons and holes. Additionally, PEC system serves as photogenerated hole consumption system to improve the photostability of 0.4CdS and further facilitate the separation and transfer of photogenerated electrons and holes. Profited for the aforementioned advantages, the CO evolution rate from CO2 PC reduction for 0.4CdS is 316 μmol h-1 g-1, with an AQE value of 0.33% for CO at 0 = 400 nm, which is 2.1 times as much as that of the pure CdS. Moreover, 0.4CdS not only possesses excellent performance with the CO evolution rate of 389 μmol h-1 g-1 from



CO2 PEC reduction reaction for high selectivity, but also exhibits outstanding photostability with 94.2% activity retention after four cycles at -0.3 V vs. Ag/AgCl in the same system. This study engineers CdS into a highly efficient catalyst as well as provides a simple and efficient strategy to enhance the stability of photocorrosive semiconductors.

ASSOCIATED CONTENT Supporting Information XPS survey spectrum for 0.4CdS, XPS spectra of Cd 3d for 0.2CdS and of S 2p for 0.4CdS at different etching times, the ratio of Cd:S for different samples, calculated crystallite size for different samples, SEM image of the lateral view of 0.4CdS electrode, the bandgap data for different samples calculated from Mott-Schottky plots and UV-vis diffuse reflectance spectra, XPS valence-band spectra of pure CdS, 0.2CdS, 0.4CdS and 0.6CdS, the bandgap data for different samples calculated from XPS valence-band spectra and UV-vis diffuse reflectance spectra, the stability comparison for different samples, yield of CO, H2 and CH4 from CO2 PEC reduction at different potentials for pure CdS, and yield of O2 from CO2 PEC reduction at -0.3 V vs. Ag/AgCl for pure CdS and 0.4CdS, SEM image for the pure CdS after 8 h CO2 PC reduction, PC and PEC performance comparison of 0.4CdS with that of previous reported catalysts, and bandgap calculation based on XPS valence band spectrum.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]; tel: 86-27-67867037. Notes


Page 28 of 38

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The first and second author contributed equally to this study. The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (Nos. 21573085 and 51872108), Wuhan Planning Project of Science and Technology (No. 2018010401011294) and self-determined research funds of CCNU from the colleges’ basic research and operation of MOE (No. CCNU18TS034).

REFERENCES (1) Yu, J. G.; Low, J. X.; Xiao, W.; Zhou, P.; Mietek, J. Enhanced photocatalytic CO2reduction activity of anatase TiO2 by coexposed {001} and {101} facets. J. Am. Chem. Soc., 2014, 136, 8839–8842. (2) Yu, L.; Li, G. J.; Zhang, X. S.; Ba, X.; Shi, G. D.; Li, Y.; Wong, P. K.; Jimmy, C. Y.; Yu, Y. Enhanced activity and stability of carbon-decorated cuprous oxide mesoporous nanorods for CO2 reduction in artificial photosynthesis. ACS Catal., 2016, 6, 6444–6454. (3) Zhao, S.; Jin, R. X.; Jin, R. C. Opportunities and challenges in CO2 reduction by gold- and silver-based electrocatalysts: from bulk metals to nanoparticles and atomically precise nanoclusters. ACS Energy Lett., 2018, 3, 452−462. (4) Yu, L.; Xie, Y. L.; Zhou, J. Q.; Li, Y.; Yu, Y.; Ren, Z. F. Robust and selective electrochemical reduction of CO2: the case of integrated 3D TiO2@MoS2 architectures and Ti–S bonding effects. J. Mater. Chem. A, 2018, 6, 4706–4713.



(5) Zhou, M.; Wang, S. B.; Yang, P. J.; Huang, C. J.; Wang, X. C. Boron carbon nitride semiconductors decorated with CdS nanoparticles for photocatalytic reduction. ACS Catal., 2018, 8, 4928–4936. (6) Chu, S.; Ou, P. F.; Pegah, G.; Srinivas, V.; Zhou, B. W.; Ishiang, S.; Song, J.; Mi, Z. T. Photoelectrochemical CO2 reduction into syngas with the metal/oxide interface. J. Am. Chem. Soc., 2018, 140, 7869–7877. (7) Zhao, G. X.; Huang, X. B.; Wang, X. X.; Wang, X. K. Progress in the catalyst exploration for heterogeneous CO2 reduction and utilization: a critical review. J. Am. Chem. Soc., 2017, 5, 21625–21649. (8) Akira, F.; Kenichi, H. Electrochemical photolysis of water at a semiconductor electrode. Nature, 1972, 238, 37–38. (9) Ma, D. D.; Shi, J. W.; Zou, Y. J.; Fan, Z. Y.; Ji, X.; Niu, C. M.; Wang, L. Z. Rational design of CdS@ZnO core-shell structure via atomic layer deposition for drastically enhanced photocatalytic H2 evolution with excellent photostability. Nano Energy, 2017, 39, 183–191. (10) Liu, G.; Niu, P.; Sun, C. H.; Sean, C. S.; Chen, Z. G.; Lu, G. Q.; Cheng, H. M. Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3N4. J. Am. Chem. Soc., 2010, 132, 11642–11648. (11) Yoshiumi, K.; Tsunehiro, T.; Takuzo, F.; Satohiro, Y. Photoreduction of carbon dioxide with hydrogen over ZrO2. Chem. Commun., 1997, 9, 841-842. (12) Shen, L. J.; Luo, M. B.; Liu, Y. H.; Liang, R. W.; Jing, F. F.; Wu, L. Noble-metalfree MoS2 co-catalyst decorated UiO-66/CdS hybrids for efficient photocatalytic H2


Page 30 of 38

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


production. Appl. Catal. B-Environ., 2015, 166, 445–453. (13) Qin, N.; Xiong, J. H.; Liang, R. W.; Liu, Y. H.; Zhang, S. Y.; Li, Y. H.; Li, Z. H.; Wu, L. Highly efficient photocatalytic H2 evolution over MoS2/CdS-TiO2 nanofibers prepared by an electrospinning mediated photodeposition method. Appl. Catal. B-Environ., 2017, 202, 374–380. (14) Wang, X. W.; Yin, L. C.; Liu, G.; Wang, L. Z.; Riichiro, S; Lu, G. Q.; Cheng, H. M. Polar interface-induced improvement in high photocatalytic hydrogen evolution over ZnO–CdS heterostructures. Energy Environ. Sci., 2011, 4, 3976–3979. (15) Shang, L.; Tong, B.; Yu, H. J.; Geoffrey, I. W.; Zhou, C.; Zhao, Y. F.; Muhammad, T.; Wu, L. Z.; Tung, C. H.; Zhang, T. R. CdS nanoparticle‐decorated Cd nanosheets for efficient visible light‐driven photocatalytic. Adv. Energy Mater., 2016, 6, 1501241. (16) Wei, R. B.; Kuang, P. Y.; Cheng, H.; Yi, B. C.; Long, J. Y.; Zhang, M. Y.; Liu, Z. Q. Plasmon-enhanced photoelectrochemical water splitting on gold nanoparticle decorated ZnO/CdS nanotube arrays. ACS Sustainable Chem. Eng., 2017, 5, 4249−4257. (17) Rama, K. C.; Jeong, Y. D.; Misook, K. Smart hybridization of Au coupled CdS nanorods with few layered MoS2 nanosheets for high performance photocatalytic hydrogen evolution reaction. ACS Sustainable Chem. Eng., 2018, 6, 6445−6457. (18) Li, Q.; Guo, B. D.; Yu, J. G.; Ran, J. R.; Zhang, B. H.; Yan, H. J.; Gong, J. R. Highly efficient visible-light-driven photocatalytic hydrogen production of CdSCluster-decorated graphene nanosheets. J. Am. Chem. Soc., 2011, 133, 10878–



Page 32 of 38

10884. (19) Kyeong, M. C.; Kyoung, H. K.; Kangho, P.; Chansol, K.; Sungtak, K.; Ahmed, A. S.; Issam, G.; Hee, T. J. Amine-functionalized graphene/CdS composite for photocatalytic reduction of CO2. ACS Catal., 2017, 7, 7064–7069. (20) Wang, S. B.; Pan, L.; Song, J. J.; Mi, W. B.; Zou, J. J.; Wang, L.; Zhang, X. W. Titanium-defected undoped anatase TiO2 with p‑type conductivity, roomtemperature ferromagnetism, and remarkable photocatalytic performance. J. Am. Chem. Soc., 2015, 137, 2975–2983. (21) Bing Wang; Xiaohui Wang; Lei Lu; Chenguang Zhou; Zhenyu Xin; Jiajia Wang; Xiao-kang Ke; Guodong Sheng; Shicheng Yan; and Zhigang Zou, Oxygenvacancy-activated

CO2

splitting

over

amorphous

oxide

semiconductor

photocatalyst. ACS Catal., 2018, 8, 516–525. (22) Bak, T.; Nowotny, M. K.; Sheppard, L. R.; Nowotny, J. Effect of prolonged oxidation on semiconducting properties of titanium dioxide. J. Phys. Chem. C, 2008, 112, 13248–13257. (23) Guan, M. L.; Xiao, C.; Zhang, J.; Fan, S. J.; An, R.; Cheng, Q. M.; Xie, J. F.; Zhou, M.; Ye, B. J.; Xie, Y. Vacancy associates promoting solar-driven photocatalytic activity of ultrathin bismuth oxychloride nanosheets. J. Am. Chem. Soc., 2013, 135, 10411–10417. (24) Wang, M. Y.; Cai, L. J.; Wang, Y.; Zhou, F. C.; Xu, K.; Tao, X. M.; Chai, Y. Graphene-draped semiconductors for enhanced photocorrosion resistance and photocatalytic properties. J. Am. Chem. Soc., 2017, 139, 4144–4151.


Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Lu, W. W.; Jia, B.; Cui, B. L.; Zhang, Y.; Kaisheng, Y.; Zhao, Y. L.; Wang, J. J. Efficient photoelectrochemical reduction of CO2 to formic acid with functionalized ionic liquidas absorbent and electrolyte. Angew. Chem. Int. Ed., 2017, 56,11851– 11854. (26) Kevin, S.; Roel, K. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater., 2016, 1, 15010. (27) Quan, F. Y.; Zhang, J. L.; Li, D. H.; Zhu, Y. K.; Wang, Y.; Bu, Y. Y.; Qin, Y. M.; Xia, Y. Z.; Sridhar, K.; Yang, D. J. Biomass as a template leads to CdS@Carbon aerogels

for

efficient

photocatalytic

hydrogen

evolution

and

stable

photoelectrochemical cells. ACS Sustainable Chem. Eng., 2018, 6, 14911−14918. (28) Ristic, M.; Popovic, S.; Music, S. Formation and properties of Cd(OH)2 and CdO particles. Mater. Lett., 2004, 58, 2494–2499. (29) Maryam, M.; Haneih, H. Synthesis and growth mechanism of CdO nanoparticles prepared from thermal decomposition of CdSO3 nanorods. J. Mater. Sci.-Mater. Electron., 2016, 27, 6480–6487. (30) Huang, G. L.; Zhu, Y. F. Enhanced photocatalytic activity of ZnWO4 catalyst via fluorine doping. J. Phys. Chem. C, 2007, 111, 11952–11958. (31) Huang, G. L.; Zhang, S. C.; Xu, T. G.; Zhu, Y. F. Fluorination of ZnWO4 photocatalyst and influence on the degradation mechanism for 4-Chlorophenol. Environ. Sci. Technol., 2008, 42, 8516–8521. (32) Chai, Z. G.; Zeng, T. T.; Li, Q.; Lu, L. Q.; Xiao, W. J.; Xu, D. S. Efficient visible light-driven splitting of alcohols into hydrogen and corresponding carbonyl



Page 34 of 38

compounds over a Ni-modified CdS photocatalyst. J. Am. Chem. Soc., 2016, 138, 10128–10131. (33) Dae, S. K.; Yong J. C.; Jeunghee, P.; Jungbum, Y.; Younghun, J.; Myung-Hwa, J. (Mn, Zn) Co-doped CdS nanowires. J. Phys. Chem. C, 2007, 111, 10861–10868. (34) Arthur, V.; Prasenjit, G.; Shouvik, D. Cadmium vacancy minority defects as luminescence centers in size and strain dependent photoluminescence shifts in CdS nanotubes. J. Phys. Chem. C, 2014, 118, 21604–21613. (35) Sutassana, N. P.; Smith, M. F.; Kwiseon, K.; Du, M. H.; Wei, S. H.; Zhang, S. B.; Sukit, L. First-principles study of native defects in anatase TiO2. Phys. Rev. B, 2006, 73, 125205. (36) Zhang, S. X.; Satishchandra, B. O.; Yu, W. Q.; Gao, X. Y.; Liu, T.; Saurabh,

G.;

Gour, P. D.; Andrew, T. W.; Richard, L. G.; Thirumalai, V. Electronic manifestation of cation-vacancy-induced magnetic moments in a transparent oxide semiconductor: anatase Nb:TiO2. Adv. Mater., 2009, 21, 2282–2287. (37) Lynn, M. W.; John, W. M. Reactive surface area of skeletal carbonates during dissolution: effect of grain size. J. Sediment. Petrol., 1984, 54, 1081–1090. (38) Zhou, W.; Li, W.; Wang, J. Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K. F.; Wang, L.; Fu, H. G.; Zhao, D. Y. Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst. J. Am. Chem. Soc., 2014, 136, 9280–9283. (39) Zhang, Z. H.; Zhang, L. B.; Mohamed, N. H.; Zhang, H. N.; Wang, P. Plasmonic gold nanocrystals coupled with photonic crystal seamlessly on TiO2 nanotube photoelectrodes for efficient visible light photoelectrochemical water splitting.


Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nano Lett., 2013, 13, 14–20. (40) Yu, J. G.; Yu, H. G.; Cheng, B.; Zhou, M. H.; Zhao, X. J. Enhanced photocatalytic activity of TiO2 powder (P25) by hydrothermal treatment. J. Mol. Catal. A-Chem., 2006, 253, 112–118. (41) Li, M. L.; Zhang, L. X.; Fan, X. Q.; Zhou, Y. J.; Wu, M. Y.; Shi, J. L. Highly selective CO2 photoreduction to CO over g-C3N4/Bi2WO6 composites under visible light. J. Mater. Chem. A, 2015, 3, 5189–5196. (42) Yang, X. Y.; Abraham, W.; Wang, G. M.; Alissa, S.; Robert, C. F.; Qian, F.; Zhang, J. Z.; Yat, L. Nitrogen-doped ZnO nanowire arrays for photoelectronchemical water splitting. Nano Lett., 2009, 9, 2331–2336. (43) Li, Q.; Li, X.; Wageh, S.; Ahmed, A. G.; Yu, J. G. CdS/graphene nanocomposite photocatalysts. Adv. Energy Mater., 2015, 5, 1500010. (44) Fu, J. W.; Xu, Q. L.; Low, J. X.; Jiang, C. J.; Yu, J. G. Ultrathin 2D/2D WO3/gC3N4 step-scheme H2-production photocatalyst. Appl. Catal. B-Environ., 2019, 243, 556–565. (45) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev., 2009, 38, 253−278. (46) Yu, H. G.; Zhong, W.; Huang, X.; Wang, P.; Yu, J. G. Suspensible cubic-phase CdS nanocrystal photocatalyst: facile synthesis and highly efficient H2-evolution performance in a sulfur-rich system. ACS Sustainable Chem. Eng., 2018, 6, 5513−5523. (47) Ge, M. Z.; Cao, C. Y.; Li, S. H.; Tang, Y. X.; Wang, L. N.; Qi, N.; Huang, J. Y.;



Zhang, K. Q.; Al-Deyab, S. S.; Lai, Y. K. In situ plasmonic Ag nanoparticles anchored TiO2 nanotube arrays as visible-light-driven photocatalysts for enhanced water splitting. Nanoscale, 2016, 8, 5226-5234. (48) Susanginee, N.; Lagnamayee, M.; Kulamani, P. Visible light-driven novel gC3N4/NiFe-LDH composite photocatalyst with enhanced photocatalytic activity towards water oxidation and reduction reaction. J. Mater. Chem. A, 2015, 3, 18622– 18635. (49) Xie, S. J.; Zhang, Q. H.; Liu, G. D.; Wang, Y. Photocatalytic and photoelectrocatalytic reduction of CO2 using heterogeneous catalysts with controlled nanostructures. Chem. Commun., 2016, 52, 35-59. (50) Shi, G. D.; Yu, L.; Ba, X.; Zhang, X. S.; Zhou, J. Q.; Yu, Y. Copper nanoparticle interspersed MoS2 nanoflowers with enhanced efficiency for CO2 electrochemical reduction to fuel. Dalton Trans., 2017, 46, 10569-10577. (51) Hooi, L. L.; Issam, A. M.; Mohammed, B.; Mohamed, B. A.; Hervé, R.; Marc, A. Thermal and optical properties of CdS nanoparticles in thermotropic liquid crystal monomers. Materials, 2010, 3, 2069-2086. (52) Chang, X. X.; Tuo, W.; Gong, J. L. CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts. Energy Environ. Sci., 2016, 9, 2177-2196.


Page 36 of 38

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The photoelectrocatalytic system has been built up, in which greenhouse gas CO2 can be sustainably converted to available energy on the photoanode with CdS with Cd vacancies on the surface by using solar energy.




Page 38 of 38

H2O2 Treated CdS with Enhanced Activity and Improved Stability by a

Recommend Documents