J. Adv. Ceram.｜Construction and photocatalytic hydrogen production performance of CdS/MoO₂@Mo₂C-MXene heterojunction

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Research abstract

Hydrogen is a clean, environmentally friendly, non-polluting gas and is considered to be a promising energy source. Among many hydrogen production methods, semiconductor photocatalytic splitting of water for hydrogen production is a convenient and low-cost continuous hydrogen production method. Due to the narrow band gap, suitable energy band structure and good charge transfer ability, CdS has been proven to be one of the excellent semiconductor photocatalysts for hydrogen production. However, the fast recombination of photogenerated electron-hole pairs limits the photocatalytic performance of CdS. Coupling CdS with other materials to form heterojunctions can achieve efficient separation of photogenerated electron-hole pairs and suppress their recombination through electron trapping and proper adjustment of the energy band structure. Some noble metals, such as Pt, Au, Pd, etc., have been proved to be used as cocatalysts, but the expensive price, extreme scarcity and complex synthesis process of noble metals limit their commercial application as photocatalysts. Therefore, it is of great significance to develop high-performance CdS-based photocatalysts by coupling with low-cost materials such as transition metal sulfides, transition metal oxides, graphene oxide, and hydroxides.

The 2D transition metal carbide/nitride MXene is obtained by stripping off the A element in the MAX phase ceramic material, and the 3D morphology is transformed into a 2D sheet structure. During the etching process, according to different etchants on the MXene surface Various functional groups are attached. Therefore, due to the hydrophilic functional groups, excellent electrical conductivity, and exposed active sites on the surface of MXene, this class of materials shows great potential in many fields. After the first report of MXene as a cocatalyst in 2017, a large number of studies on MXene in the field of photocatalytic hydrogen production have emerged. However, most of the literature reports on photocatalytic hydrogen production are based on Ti3C2 MXene, because Ti3C2 MXene, as the first member of the MXene family, has been widely studied by researchers due to its simple preparation process and outstanding performance in many fields with the development of more than ten years.

Mo2C MXene is a new member of the MXene family, and its corresponding three-dimensional bulk material Mo2C itself has good catalytic performance. Mo2C MXene has a two-dimensional structure with high specific surface area, which should theoretically have better photocatalytic performance. However, MXene acts as a cocatalyst, and its photocatalytic activity decays during photocatalytic irradiation. This is due to the poor stability of MXene when exposed to light. In order to solve the above problems and enhance the photostability of MXene, 2D MXene surface can be realized by covering a stabilizing layer, and the stabilizing layer should be thin enough, with high electrical conductivity and potential to promote the current carrying between MXene and CdS Sub-fast transfer.

Therefore, an easy-to-implement experimental method was designed in this paper, namely, MoO2 nanolayers were grown in situ on the surface of Mo2C MXene, and then CdS was composited on the surface of MoO2. MoO2 is a metal oxide with high stability and high conductivity, and its introduction between Mo2C MXene and CdS can achieve enhanced and stabilized photocatalytic activity. Using this method, a novel CdS/MoO2@Mo2C composite photocatalyst was prepared, and MoO2 and Mo2C MXene as dual cocatalysts effectively enhanced the photocatalytic hydrogen production performance of CdS.

Introduction

Recently, the team of Professor Zhou Aiguo from Henan University of Technology used a two-step hydrothermal method to prepare a CdS/MoO2@Mo2C composite photocatalyst. In the first stage, Mo2C MXene was oxidized by hydrochloric acid, and MoO2 nanolayer was grown in situ on the surface of Mo2C MXene to form a MoO2@Mo2C binary mixture. Then in the second hydrothermal process, cadmium nitrate and thiourea were used as cadmium source and sulfur source, respectively, and the released Cd2+ and S2- were electrostatically self-assembled on the surface of Mo2C MXene to form CdS nanoparticles, and then CdS nanoparticles were formed in a certain concentration of ethylenediamine. It grows in a thorn-like shape in the environment. In the photoelectrochemical test, CdS/MoO2@Mo2C showed good visible light absorption performance, photogenerated carrier separation efficiency and charge transfer efficiency. Especially compared to Mo2C MXene and MoO2 as single cocatalysts, respectively, the dual cocatalyst MoO2@Mo2C shows its advantages. MoO2 with strong metallicity between CdS and Mo2C MXene plays the role of electron transfer bridge in this system, which can effectively promote the transfer of photogenerated electrons excited from CdS. On the band structure, the strong Schottky heterojunction formed by the combination of CdS and MoO2@Mo2C hinders the recombination of photogenerated carriers. Coupled with the good catalytic activity of Mo2C MXene itself, the performance of Mo2C MXene as a cocatalyst for hydrogen production was further improved. The results of photocatalytic experiments show that the optimal ratio of CdS/MoO2@Mo2C (5 wt.%) can achieve a hydrogen production rate of 22672 μmol g-1 h-1 under visible light irradiation at a light intensity of ≥420 nm and 80 mW cm-2. , which is about 21% higher than the previously reported hydrogen production rate of CdS/Mo2C.

The result was published online in the top international journal Journal of Advanced Ceramics (impact factor 11.534) with the title: Construction and performance of CdS/MoO2@Mo2C-MXene photocatalyst for H2 production.

Doctoral student Jin Sen is the first author, and Professor Zhou Aiguo is the corresponding author.

Graphical guide

Image summary

Figure 1. (a) XRD patterns of Mo2Ga2C, Mo2C MXene and MoO2@Mo2C-MXene. (b) XRD patterns of CMMx samples with different additions of MoO2@Mo2C-MXene and XRD patterns of standard wurtzite phase CdS.

Figure 2. Microstructural characterization of the sample. (a–c) SEM images of Mo2C MXene, MoO2@Mo2C-MXene and CdS/MoO2@Mo2C samples, respectively. (d) TEM image of MoO2@Mo2C-MXene after sonication. (e-f) High-resolution TEM images of MoO2@Mo2C-MXene corresponding to the yellow and red boxed regions in (d), and the inset is the electron diffraction pattern. (g) TEM image of CdS/MoO2@Mo2C. (h) High-resolution TEM image of CdS/MoO2@Mo2C after breaking the particles. (i) TEM image of the white boxed area in the close-up observation image (h). (j) STEM image and corresponding elemental map.

Figure 3. XPS characterization. (a) High-resolution XPS spectra of MoO2@Mo2C-MXene and CMM5 of Mo 3d. (b–c) High-resolution XPS spectra of S 2p and Cd 3d in CdS and CMM5, respectively.

Figure 4. Schematic diagram of the synthesis of CdS/MoO2@Mo2C nanoparticles.

Figure 5. (a) Hydrogen production versus time for the 20 mg CdS/MoO2@Mo2C sample under visible light irradiation. (b) Hydrogen production rate for each CMMx sample. (c) Cyclicity experiment of hydrogen production by CMM5.

Figure 6. (a) UV-Vis diffuse reflectance spectrum. (b) Photoluminescence spectrum. (c) Electrochemical impedance spectroscopy. (d) Transient photocurrent response.

Figure 7. (a) Tauc plot and (b) Mott-Schottky plot of CdS, CMM5 and CdS/Mo2C. (c) Schematic illustration of the band structures of CdS, CMM5 and CdS/Mo2C.

Figure 8. The band structure changes before and after CdS composite MoO2@Mo2C-MXene and Mo2C MXene and the schematic diagram of the separation and transfer of photogenerated electrons and holes in CdS/MoO2@Mo2C and CdS/Mo2C under visible light irradiation.

Summarize

In this work, CdS/MoO2@Mo2C revival photocatalysts were successfully prepared by a two-step hydrothermal method. In this system, CdS grows on the surface of MoO2@Mo2C-MXene, forming a thorny ball structure. The best CdS/MoO2@Mo2C (CMM5) shows an ultra-high photocatalytic hydrogen yield of 22672 μmol g-1 h-1 under visible light, which is 11.8 times higher than that of pure CdS and 21 times higher than that of CdS/Mo2C. %. The experimental results show that CdS/MoO2@Mo2C has better photoelectrochemical properties than CdS/Mo2C with only a single cocatalyst due to the presence of MoO2 as an electron bridge, which accelerates the transfer of photogenerated electrons. The band structure reveals that the conduction band position of CdS becomes more negative after compounding MoO2@Mo2C-MXene, forming a “higher” Schottky heterojunction than CdS/Mo2C, which can more effectively hinder electrons from returning to CdS The formation of carrier recombination. In addition, the band gap of CdS is narrowed after compounding MoO2@Mo2C-MXene, which is beneficial to the absorption of visible light. Taken together, these factors jointly affect and enhance the hydrolysis hydrogen production performance of this novel photocatalytic system with binary cocatalysts.

Literature link

https://doi.org/10.1007/s40145-022-0621-3

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