Au nanodots@thiol-UiO66@ZnIn2S4 nanosheets with significantly enhanced visible-light photocatalytic H2 evolution: The effect of different Au positions on the transfer of electron-hole pairs

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Highlights

  • A new Au@UiOS@ZIS photocatalyst was elaborately designed for the first time.

  • Au NDs were anchored in the pore space of thiol-functionalized UiO66 MOF.

  • ZnIn2S4 nanosheets were wrapped around the UiO66 MOF containing Au NDs.

  • Au positions had a grateful effect on the transfer of photogenerated charge carriers.

  • A transfer channel of electrons from ZnIn2S4 to UiOS and then to Au was established.

Abstract

Herein, a new type of photocatalysts, Au nanodots@thiol-UiO-66@ZnIn2S4 nanosheets (Au@UiOS@ZIS), is elaborately designed for the photocatalytic H2 evolution from water splitting, where Au nanodots (NDs) are anchored in the pore space of thiol-functionalized UiO66 metal-organic framework (MOF) and ZnIn2S4 nanosheets are wrapped around the UiO66 MOF containing Au NDs. It is found that the different Au positions have a grateful effect on the transfer of photogenerated charge carriers. In Au@UiOS@ZIS, the photoexcited electrons transfers from ZnIn2S4 to UiOS and then to Au NDs, establishing a smooth transmission channel of electrons. As expected, the optimal sample (Au4@UiOS@ZIS40) presents a high photocatalytic H2 production rate of 391.6 μmol/h (10 mg of catalyst) under visible light irradiation, which is 435.1, 61.2 and 10.2 times higher than that of the pure UiOS, ZnIn2S4 and UiOS@ZIS, respectively.

Introduction

In the past few years, semiconductor-based photocatalytic hydrogen production has attracted wide attention due to its potential for improving energy and environmental problems [1,2]. To date, various types of photocatalysts, such as metal oxides [3,4], metal sulfides [5,6], and metal-free semiconductors [[7], [8], [9]], etc. have been successfully developed for photocatalytic H2 production. Among various semiconductors used, ZnIn2S4 as a ternary chalcogenide is suitable enough to drive H2 generation reaction under visible light irradiation owing to its good visible-light absorption ability, excellent chemical stability and suitable conduction band position [[10], [11], [12]]. Unfortunately, the photocatalytic H2 production activity of pristine ZnIn2S4 is low owing to the weak separation ability and limited transfer efficiency of photogenerated electrons and holes. Despite many efforts have been devoted to enhance the photocatalytic activity of ZnIn2S4 [13,14], including the formation of composites with other materials, such as porous materials, noble metals, and semiconductors, etc. [15,16], the development of efficient ZnIn2S4-based photocatalysts is still a serious challenge.

As a type of orderly porous materials, metal organic frameworks (MOFs) have recently attracted widespread interest and demonstrated potential applications in various fields, including photocatalysis [[17], [18], [19], [20], [21], [22], [23], [24]]. On the one hand, the outer surface of MOF with many active sites can act as effective charge transfer medium to avoid the problem of poor charge separation in a single semiconductor. As reported in early publications [25,26], narrow band gap semiconductors (e.g., ZnIn2S4) were excited by visible light to generate electrons and injected them into the lowest unoccupied molecular orbital (LUMO) of n-type semiconductor-like MOFs. On the other hand, the MOF pore space functions as an ordered nanoreactor, and synthesizing functional species and encapsulating them in the MOF pores in these reactors to avoid structural defects and restrains the recombination of charge carriers. Meng et al. [27] co-immobilized CdS quantum dots (QDs) and carbon nanodots (NDs) in the cages of MIL-101, and they found that the photogenerated charge carriers can be produced in the CdS@MIL-101 heterostructure and the carbon NDs mainly serve as an electron collector to prolong carrier lifetime. Although much effort has been expended, the photocatalytic efficiency of metal-sulfide/MOF has still serious barrier until now owing to the rapid recombination of electron-hole pairs. Thus, for metal-sulfide/MOF, it has become an urgent task to explore a powerful method to suppress the recombination of charge carriers.

The Encapsulation of precious metal nanoparticles in MOFs may be one of the effective ways to enhance the separation of photoexcited electrons and holes. Pt NPs have been loaded into the phosphorescent MOF frameworks, and the resulting Pt@MOF presented an effective photocatalytic hydrogen evolution [28]. Xiao et al. [29] reported that the decoration of Pt NPs in n-type-semiconductor MOFs could introduce sinks to trap the migrated electrons, and the holes spread freely to the MOF surface, therefore the separation of charge carriers was improved due to the Schottky barrier. Fu et al. [30] firstly encapsulated sub-nm Pt nanoclusters into Zr-porphyrin frameworks, and the resultant photocatalyst dramatically promoted electron-hole separation and 1O2 generation to achieve synergistic effect. Moreover, the decorated Au NPs on MOFs is also a hopeful method to obviously improve the separation and migration of charge carriers [[31], [32], [33], [34], [35]], and the Au NPs with high dispersion and small size are the key to shorten the migration distance of photoexcited charge carriers to the internal surface of MOFs, in which thiols have been confirmed to have a strong dispersion and adsorption effects on the grown Au NPs [[36], [37], [38], [39]].

With the above points in mind, in this work, we attempted to construct a new composite photocatalyst, Au nanodots@thiol-UiO-66@ZnIn2S4 nanosheets (abbreviated as Au@UiOS@ZIS), by a series of carefully designed steps. Firstly, thiol-functionalized UiO-66 MOFs (UiO-66-(SH)2, abbreviated as UiOS) were synthesized by an oil bath method. Secondly, Au NDs were encapsulated in the pores of UiOS by the adsorption force of thiol, resulting in Au@UiOS. Subsequently, the ZnIn2S4 (shorted as ZIS) nanosheets with visible-light response were wrapped around Au@UiOS, resulting in Au@UiOS@ZIS (the synthetic process is schematically showed in Scheme 1). As a result, the Au@UiOS@ZIS exhibited an impressive photocatalytic H2 production rate of 391.6 μmol/h under visible light, which is 435.1, 61.2 and 10.2 times higher than that of pristine UiOS (0.9 μmol/h), ZnIn2S4 (6.4 μmol/h) and UiOS@ZIS (39.2 μmol/h), respectively. The causes for obviously improved photocatalytic hydrogen production were further clarified in detail.

Section snippets

The synthesis of UiOS MOF

The synthesis of UiOS MOF was prepared in a Schlenk bottle under Ar gas conditions. In the first place, ZrCl4 (96 mg), 2,5-disulfanylterephthalic acid (BDC-(SH)2) (95 mg) and acetic acid (3.4 mL) were dispersed in N, N-dimethyl formamide (DMF) (16 mL). The next, the pretreated solution in the bottle was evacuated, until the end, the system has no oxygen and with Ar gas protection. After that, covering the bottle tightly and heating it in a preheated oil bath for 24 h at 120℃. After natural

The characterizations of photocatalysts

Fig. 1 shows the XRD patterns of the samples. The XRD peak of UiOS MOF is in good agreement with the simulation, indicating the synthesis of UiOS successfully. The intensity of the peak at 7−9° for the Au4@UiOS sample reduced due to the filling of Au NDs comparing with UiOS, and no clear diffraction peaks of Au NDs were found, probably owing to their low content and small size. UiOS@ZIS series samples show mixed characteristic diffraction peaks of hexagonal ZnIn2S4 (JCPDS No. 65-2023) and

Conclusions

In summary, a novel photocatalyst was successfully synthesized for the first time by means of two procedures: Au NDs were encapsulated in a thiol-MOF by electrostatic attraction, and then ZnIn2S4 nanosheets were wrapped around the MOF by a facile solvothermal method to achieve a spatially charge separated structure. The optimal sample of Au4@UiOS@ZIS40 presented the highest hydrogen production rate of 391.6 μmol/h (10 mg of catalyst), which is 435.1, 61.2 and 10.2 times higher than that of the

CRediT authorship contribution statement

Siman Mao: Investigation, Writing - original draft. Jian-Wen Shi: Supervision, Funding acquisition. Guotai Sun: Writing - review & editing. Dandan Ma: Methodology. Chi He: Formal analysis. Zengxin Pu: Data curation. Kunli Song: Validation. Yonghong Cheng: Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was sponsored by the National Natural Science Foundation of China (21972110), the Natural Science Foundation of Shaanxi Province, China (2019JM-154), and the State Key Laboratory of Electrical Insulation and Power Equipment, China (EIPE19123). SEM, TEM, XPS and ICP-MS were carried out at Analysis and Test Center of Xi’an Jiaotong University. We thank Zijun Ren, Jiao Li, Jiamei Liu and Guoqing Zhou for their help in using SEM, TEM, XPS and ICP-MS analyses, respectively.

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