EDTA-assisted synthesis of amorphous BiSx nanodots for improving photocatalytic hydrogen-evolution rate of TiO2

https://doi.org/10.1016/j.jallcom.2021.161425Get rights and content

Highlights

  • A novel and efficient BiSx/TiO2 was prepared by an EDTA-assisted two-step method.

  • BiSx/TiO2 showed a higher H2-evolution rate than TiO2 and crystalline Bi2S3/TiO2.

  • A mechanism on the unsaturated S atoms of amorphous BiSx nanodots was proposed.

Abstract

Compared with crystalline metal sulfides, amorphous metal sulfides can expose more unsaturated S atoms as hydrogen-production active centers to enhance photocatalytic hydrogen-evolution activity of photocatalysts. Therefore, it is of great significance to explore simple and mild strategies to fabricate new amorphous metal sulfides for improving H2-production performance. In this work, the amorphous BiSx nanodots with the size of 0.5–2 nm as a novel and effective H2-evolution cocatalyst were successfully and homogeneously loaded on TiO2 surface to extremely facilitate the photocatalytic H2-production performance. The BiSx/TiO2 photocatalyst was obtained by an EDTA-assisted two-step process, which was including the adsorption of the Bi(Ш)-EDTA ions on TiO2 and the in-situ formation of amorphous BiSx nanodots. It is found that the BiSx/TiO2(1.0 wt%) photocatalyst achieved the maximum photocatalytic hydrogen-production performance (803.2 μmol h−1 g−1, AQE= 3.86%), which was 83.6 and 1.6 folds of the blank TiO2 and crystalline Bi2S3-modified TiO2 (c-Bi2S3/TiO2), respectively. Importantly, the above amorphous BiSx nanodots can serve as a general cocatalyst to promote the H2-generation activity of other typical photocatalysts (g-C3N4 and CdS). The boosted photocatalytic H2-production performances are attributed to that the amorphous BiSx nanodots can provide more unsaturated active S atoms as the efficient H2-evolution active sites for promoting H2-evolution rate. This study provides a new insight for exploiting novel metal sulfide cocatalyst in the field of photocatalytic hydrogen evolution.

Introduction

Utilizing semiconductor photocatalysts to convert solar energy into hydrogen energy is considered to be one of the prospective strategies for solving the problem of energy crisis [1], [2], [3], [4], [5], [6]. Among the semiconductor photocatalysts, TiO2 has been widely used in the field of photocatalytic water splitting due to its non-toxic properties, superior chemical stability and abundant resource [7], [8], [9], [10]. Unfortunately, its photocatalytic H2-evolution performance is very limited because of the high recombination rate of photogenerated charges and slow interfacial hydrogen-production reaction [11], [12]. To improve the H2-production efficiency of TiO2, cocatalyst modification on TiO2 surface is deemed as one of the efficient methods because cocatalysts can rapidly transfer the photogenerated electrons and provide hydrogen-evolution active sites [13], [14], [15]. In recent years, various metal compounds as photocatalytic hydrogen-evolution cocatalysts such as metal .phosphides (Ni2P, Co2P, Cu3P, etc.) [16], [17], [18], metal carbides (Mo2C, WC, etc.) [19], [20], and metal sulfides (NiSx, CoS2, MoS2, etc.) [21], [22], [23] have been widely reported. In particular, metal sulfides are broadly applied to improve the photocatalytic hydrogen-production performance owing to the unsaturated S atoms on the edge of metal sulfides with a strong ability for adsorbing hydrogen ions, which can be served as effective interfacial hydrogen-evolution active sites [24], [25], [26]. However, the reports on hydrogen-evolution cocatalysts of metal sulfides mainly focus on transition-metal sulfides, and the researches about non-transition metal sulfides are relatively insufficient. Therefore, it is of great significance to further develop new non-transition metal sulfide cocatalysts to promote the photocatalytic H2-production activity of photocatalysts.

Bismuth sulfide, a typical non-transition metal sulfide, has aroused great interest in the field of photocatalysis, hydrogen evolution, and solar cells owing to its efficient carrier capturing capability, low cost, nontoxicity, and good chemical stability [27], [28], [29]. For instance, Liu et al. found that the crystalline Bi2S3 nanowires with average length of 1.5 µm by a solvothermal process (200°C, 2 h) promoted the photoelectrochemical (PEC) hydrogen generation activity and stability of TiO2 photoanode [30]. Li et al. reported that the Bi2S3/Cd0.5Zn0.5S photocatalyst was prepared through a microwave-assisted method in the microwave reactor (500 W, 120°C) and represented an obviously boosted H2-production performance compared with the pure Cd0.5Zn0.5S [31]. Majhi et al. described that the crystalline Bi2S3 nanoparticles with size ranging between 15 and 25 nm were used to construct Bi2S3/b-Bi2O3/ZnIn2S4 ternary photocatalyst via a reflux route, which exhibited prominent visible-light photocatalytic activity for aqueous phase tetracycline degradation, Cr(VI) reduction, and H2 evolution [32]. These works proved that Bi2S3 could be acted as an efficient cocatalyst to distinctly enhance the H2-evolution activity, primarily because the active unsaturated S atoms on Bi2S3 worked as effective H2-production reaction sites to adsorb H+ ions in the solution. However, the above-mentioned preparation methods about bismuth sulfide usually require relatively extreme conditions (high temperature, high pressure, or multiple synthetic procedures), resulting in that the pre-prepared crystalline Bi2S3 exhibited large particles (>10 nm) and limited interfacial active sites. Compared to the crystalline Bi2S3 with a large size, the small and amorphous BiSx nanodots (<2 nm) with more unsaturated S atoms as active centers are expected to be helpful for hydrogen production, which a similar point has been proven in our previous work [33]. Therefore, it is necessary and significant to explore simple and mild methods to synthesize amorphous BiSx nanodots for the boosted hydrogen-generation performance of photocatalysts.

In this work, the amorphous BiSx nanodots as a hydrogen-production cocatalyst were modified on the TiO2 surface by an EDTA-assisted two-step method, including the absorption of the Bi(Ш)-EDTA ions on the TiO2 and the in-situ conversion of BiSx. In this case, the strong complexation between EDTA and Bi3+ ion easily generates stable Bi(Ш)-EDTA ions and prevents the rapid hydrolysis of Bi3+, which can be of great benefit to the following formation of amorphous BiSx nanodots. The prepared BiSx/TiO2 samples displayed a markedly enhanced hydrogen-evolution performance in comparison with the blank TiO2. Based on test results, a possible mechanism of BiSx for promoting hydrogen production was proposed, that is, the unsaturated S of amorphous BiSx can act as the interfacial active centers to facilitate the hydrogen-production rate of TiO2. This work can provide a new synthesis strategy for developing the new amorphous metal-sulfide nanodot cocatalysts for optimizing photocatalytic H2-generation activity of photocatalysts.

Section snippets

Chemicals

Titanium dioxide (TiO2, P25), bismuth nitrate (Bi(NO3)3·5 H2O), thioacetamide (TAA), edetic acid (EDTA), cadmium nitrate (Cd(NO3)2·4 H2O), melamine (C3H6N6), sodium sulfide (Na2S·9 H2O), and ethyl alcohol were purchased from Shanghai Chemical Reagent Ltd. (P.R. China). All chemicals were of analytic grade and used without further purification.

Synthesis of BiSx/TiO2 photocatalyst

The BiSx/TiO2 photocatalysts were prepared via the EDTA-assisted two-step methods including the absorption of Bi(Ш)-EDTA ions on the TiO2 and the in-situ

Synthetic strategy of BiSx/TiO2

The fabrication of the BiSx/TiO2 samples can be simplified into two steps, including the adsorption of Bi(Ш)-EDTA ions on the TiO2 and the in-situ formation of amorphous BiSx nanodots, as shown in Fig. 1. In general, the strong complexation between EDTA and Bi3+ ions can easily generate Bi(Ш)-EDTA ions due to the high stability constant of Bi(Ш)-EDTA (Kf = 1022.8) [38], which can prevent the hydrolysis of Bi3+ (Fig. 1C-1). When the positive Bi(Ш)-EDTA ions are added into the negative TiO2

Conclusions

In summary, the amorphous BiSx nanodots with the size of 0.5–2 nm are successfully and homogeneously decorated on the TiO2 to form the BiSx/TiO2 via a two-step strategy, including the adsorption of the Bi(Ш)-EDTA ion on TiO2 and the in-situ formation of amorphous BiSx nanodots. In comparison to blank TiO2, the BiSx/TiO2 photocatalysts exhibited a remarkably enhanced hydrogen-production activity. In particular, the BiSx/TiO2(1.0 wt%) photocatalyst achieved the greatest photocatalytic H2

CRediT authorship contribution statement

Lin Dong: Methodology, Validation, Data curation, Writing – original draft. Ping Wang: Conceptualization, Supervision, Resources, Funding acquisition, Writing – review & editing. Huogen Yu: Conceptualization, Supervision, Resources, Writing – review & editing.

Declaration of Competing Interest

The authors declare that we 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 supported by the National Natural Science Foundation of China (51872221).

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