Carbon nitride derived carbon and nitrogen Co-doped CdS for stable photocatalytic hydrogen evolution

https://doi.org/10.1016/j.surfin.2021.101262Get rights and content

Abstract

The narrow band-gap and long life-time of photo-generated charges endow CdS with much higher photocatalytic hydrogen evolution (PHE) activity over other photocatalysts. Thus, CdS based materials are considered to be most promising photocatalysts for PHE activity and stability. In this work, graphitic carbon nitride (GCN) is used as sacrifice agent to synthesize is nitrogen and carbon co-doped CdS nanoparticles with of uniform small size and higher specific surface area via hydrothermal reaction. Besides, co-doping of nitrogen and especially carbon leads to electron penetration to CdS results in tuned band structure of doped CdS with down-shifted conducting band and valance band positions. Moreover, the doped heteroatoms act as charge trapping centers that enhance charge separation within CdS particles endowing their photo-generated charges with longer life-time in compared with that of bare CdS. Benefited from non-metallic heteroatoms doping, obviously increased PHE activity and stability of CdS under visible light irradiation are achieved.

Introduction

For the requirement of sustainable development to reduce emission of carbon dioxide and pollutants to environment, functional nanomaterials for new energy application technologies such as electrocatalysis and photocatalysis technologies have been intensively studied for decades [1,2]. Within all discovered semiconductor materials, CdS is known to be a famous semiconductor material for photocatalytic hydrogen evolution (PHE) from water under visible light irradiation [3]. Its narrow band-gap, which allows wide range visible light absorption, and long life-time of photo-generated charges endow CdS with much higher PHE activity than other photocatalysts such as TiO2, WO3 and graphitic carbon nitride (GCN) [4], [5], [6]. However, PHE by pure CdS still suffers poor photocatalytic efficiency, photo-induced electron-hole pairs combine on the CdS surface, and photo-corrosion that leads to poor stability of high surface area CdS based nanoparticle catalysts. Thus, efficiency and durability of CdS based hydrogen evolution catalysts for effective converting solar energy into hydrogen still needs to be improved [7], [8], [9].

Great efforts have been made to promote PHE activity and stability of CdS based materials via effective structure design, such as solid solution, Type-I and Type-II heterojunction, Z-scheme heterojunction. Type-II heterojunction has been widely studied for its effectiveness in separation photo-generated electron/hole pairs [10]. WO3 [11], [12], [13], [14], [15], ZnO [16], [17], [18], TiO2 [19,20], Cu2O [21], [22], [23], [24], [25], and some other semiconductor compounds [26], [27], [28], [29], [30] with higher valance band position than CdS were composited with CdS forming Type II hetero-junction to improve extraction of photo-generated holes from CdS to them. The formation of type II hetero-junction in these composites not only improves utilization of photo-generated charges for hydrogen evolution but also effectively prevents photocatalytic oxidation of CdS surface and preserves reduction reaction on CdS surface. As a result, photo-corrosion of CdS is effectively avoided and PHE stability of these CdS based composites is dramatically enhanced [31], [32], [33], [34]. Z-Scheme heterostructure of CdS based composites with quenched photo-generated holes on CdS and photo-generated electrons on second semiconductor are also found to be effective in improving its PHE activity and stability. Zou and co-authors report a composite of uniformly distributed CdS nanoparticles on BiVO4 nanosheets to form Z-scheme structure, which increases the carrier transfer efficiency [35]. Zhang and co-authors designed a CdS/NH2-MIL-125(Ti) composite with Z-scheme heterojunction, of which the special structure effectively promoted the separation and transfer of carriers [36]. Xue and co-authors embedded CdS on WO3/WS2 nanometer plate, of which the novel ternary composite with Z-Scheme heterostructure provides a new channel for the transfer of photo-generated electrons [37]. Yang and co-authors prepared CdS/Pt/N-NaNbO3 ternary composite and used Pt as an electronic bridge to improve the separation and transmission of photo-generated electrons and holes [38]. Hu and co-authors synthesized a novel CdS/polyimide composite, which is an effective application of polymers in photocatalysis [39]. Wei and co-authors designed a MoS2-CdS/WO3-MnO2 composite with double Z-scheme heterojunctions and demonstrated the great potential of fabricating double Z-scheme photocatalytic system [40].

Beside the heterostructure construction strategy, a large number of recent studies have shown that metal or nonmetal elements doping in CdS forming solid solution can promote charge separation within photocatalysts. Shi and co-authors utilized P-doped CdS with rich S vacancies to extend the endurance of photo-generated electrons [41]. The heteroatoms introduce impurity levels into forbidden gap, which leads to the change of band gap structure in photocatalysts. Yu and co-authors show that Zinc solubilized in CdS forming Zn0.5Cd0.5S with broader band-gap than bare CdS has higher PHE activity than pure CdS [42]. Li and co-authors have reported that doping CdS with P forms an intermediate gap at the bottom of the conduction band of CdS, which is beneficial to prolong the life of photo-generated electrons [43]. Shi and co-authors reported that introduce Se into CdS improved the position of the Fermi level of CdS and greatly inhibited the recombination of electron hole pairs [44]. In addition, metal ions defects can become carrier traps, thus extending carrier life within photocatalysts. Zhang and co-authors demonstrated that doping CdS with Ni can not only narrow the badgap but also but also accelerate carriers'separation [45]. Poornaprakash and co-authors doping CdS with Er which showed very small grain size(≤ 10 nm) and a large number of carriers existed [46]. R Singh and co-authors successfully imported Ag2+ and Cu2+ doped CdS to explore the beneficial changes of structure and band edge potentials on photocatalysis [47]. Ma and co-authors reported a Cu and In co-doped CdS with largely improved photocatalytic hydrogen evolution activity by the hetero-phase junction with multi-arm nanorod of effectively enhanced charge separation [48]. Therefore, it is an advisable choice to modify CdS by heteroatoms doping especially dual element doping. At present, non-metallic elements doping especially C&N double element doping/size control double aims synthetic strategy reported in published works are few. Exploring new approach to achieve above goal would be a valuable work in developing CdS based photocatalysts.

In this work we have adopted a strategy of preparing nitrogen and carbon co-doped CdS in the presence of graphitic carbon nitride (GCN) under hydrothermal process. GCN synthesized via pyrolysis of urea under 550 °C for 3 h is used as sacrifice substrate to anchor Cd2+ ions for nucleation and growth of small size CdS nanoparticle and the decomposition of GCN during the growing process of CdS under hydrothermal reaction provides nitrogen and carbon sources for their doping in CdS matrix. As a result, GCN is fully decomposed and final C/N co-doped CdS (CN-CdS) was obtained. The co-doping of C and N elements is conducive to electron penetration into CdS, and the carrier separation efficiency is enhanced, which promotes its stability. The lower conduction band and valence band also enhance the hydrogen production activity. PL and its decay spectrums have shown that CN-CdS has displayed further reduced recombination of photo-generated charges while the life-time of them is increased. PHE activity of CN-CdS is found to be 3.8 times higher than bare CdS and 140 times higher than GCN. Moreover, the cyclic stability of CN-CdS is also found to be largely improved.

Section snippets

Synthesis of catalysts

GCN is prepared via thermal annealing 10 urea in covered corundum crucible and heated to 550 °C with temperature increase rate of 10 °C•min−1 then preserves for 3 h in air atmosphere and finally cooled to room temperature in a tube furnace naturally. Synthesis of XCN-CdS (X = 1, 2 and 3 represent 1, 2 and 3 g GCN added in hydrothermal reaction process) is carried out by a simple hydrothermal reaction. Typically, 5 mmol sodium citrate is dissolved in 40 ml deionized water to form a homogeneous

Results and discussion

CN-CdS composites are prepared via hydrothermal method as illustrated in Scheme 1. GCN synthesized via thermal pyrolysis of urea under 550 °C for 3 h is used as a substrate for nucleation and growth of CdS. The GCN substrate gradually decomposes during hydrothermal process providing nitrogen and carbon atoms for their doping in CdS to form CN-CdS. As can be seen in transmission electron microscopy (TEM) image (Fig. 1A) of CdS particles synthesized without GCN under hydrothermal reaction have

Conclusions

In conclusion, we use graphitic carbon nitride to synthesis carbon and nitrogen co-doped CdS. The graphitic carbon nitride act as template for CdS nucleation in the initial step and then decomposes providing carbon and nitrogen atoms to dope into CdS crystal matrix. It is found that carbon and nitrogen doping in CdS matrix leads to electron penetration to Cd and S of CdS particle resulting in down-shift of conducting band and valance band position, effective depresses photo-generated charges

Author statement

Yanchao Zhu: Conceptualization, Methodology. En Li: photocatalytic activity measurement and analysis. Hechuang Zhao: materials synthesis. Shengwei Shen: photophysical analysis. Jinghui Wang: TEM characterization and analysis: Zaozao Lv: surface state analysis. Muye Liu: electrochemical characterization and analysis. Yue Wen: BET characterization and analysis, Luhua Lu: Conceptualization, Writing-Original draft preparation, Writing-Reviewing and Editing. Jinghai Liu: Writing-Reviewing and Editing

CRediT authorship contribution statement

Yanchao Zhu: Conceptualization, Writing - review & editing, Writing - original draft. En Li: Methodology. Hechuang Zhao: Methodology. Shengwei Shen: Methodology. Jinghui Wang: Conceptualization, Writing - review & editing, Writing - original draft, Funding acquisition, Methodology. Zaozao Lv: Methodology. Muye Liu: Methodology. Yue Wen: Methodology. Luhua Lu: Conceptualization, Writing - review & editing, Writing - original draft, Funding acquisition. Jinghai Liu: Conceptualization, Writing -

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.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21303129, 21303080, 21961024, 21961025 and 51102218), and Natural Science Foundation of Zhejiang Province, China (LZ16E020001). Inner Mongolia Natural Science Foundation (2018JQ05).

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