Ultrathin MoSe2 three-dimensional nanospheres as high carriers transmission channel and full spectrum harvester toward excellent photocatalytic and photoelectrochemical performance

https://doi.org/10.1016/j.ijhydene.2019.12.217Get rights and content

Highlights

  • MoSe2 ultrathin nanospheres with three-dimensional network structure were prepared.

  • The photocatalytic and photoelectrochemistry activity of the MoSe2 ultrathin nanospheres is largely enhanced.

  • The MoSe2 ultrathin nanospheres lower internal resistance and higher carrier transport and separation efficiency.

Abstract

MoSe2 ultrathin nanospheres with three-dimensional network structure (MSS) were prepared by improved solvothermal method. These MoSe2 nanospheres are only 10 nm in size and actually composed of ultra-thin MoSe2 nanosheets with a thickness of only 2–3 molecular layers. Compared with the MoSe2 nanosheets (6–8 molecular layer thicknesses) of the three-dimensional flower structure (MSF) prepared by ordinary hydrothermal method, the MSS are thinner resulting in higher specific surface area of 5 times than that of MSF, and the light absorption ability at all UV–vis spectrum is stronger. The photocatalytic and photoelectrochemistry results show that the photocatalytic activity of MSS is 17 times that of the MSF, and the photoelectrochemical performance is twice. The results of electrochemical impedance spectroscopy and fluorescence spectroscopy confirmed that the MoSe2 ultra-thin nanospheres with three-dimensional network structure have lower internal resistance and higher carrier transport and separation efficiency. In the most important three aspects that determine the photoelectrochemical performance of photocatalyst: specific surface area, light absorption capacity, carrier transport and separation efficiency, MSS exceed MSF. This three-dimensional network nanospheres structure can improve the performance of MoSe2. This research successfully demonstrates the enhancement of the properties of MoSe2 two-dimensional materials through structural regulation.

Graphical abstract

Ultrathin MoSe2 three-dimensional nanospheres show excellent photocatalytic and photoelectrochemical performance.

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Introduction

New energy materials are those that convert renewable energy into electricity. Among these renewable energy sources, hydrogen energy is an ideal energy source. Hydrogen is rich in energy and can be converted from solar energy, bioenergy, chemical energy and et al. Moreover, hydrogen energy has high energy density and green pollution free, and is expected to be widely used in new energy vehicles and personal electronic equipment[1]. There are many ways to obtain hydrogen. Photocatalysis is an efficiency approach. Solar energy is the most ideal source of hydrogen energy, and photocatalysts can convert solar energy to hydrogen energy. After the photocatalyst absorbs solar energy, it excites electron-hole pairs, which can reduce hydrogen ions into hydrogen. This transformation is green and pollution-free, and it does not consume other energy sources[[2], [3], [4]]. So much of the research on hydrogen energy materials is now focused on the development of highly efficient photocatalysts.

The earliest and most widely used photocatalyst was TiO2. TiO2 has many advantages, such as high photocatalytic activity and chemical stability, large crust content, no pollution and no toxicity[[5], [6], [7], [8]]. However, the disadvantages of TiO2 are also obvious: the large band gap (3.0–3.2 eV) result in no absorption of visible light and the internal resistance is high so that carrier transport is difficult[[9], [10], [11], [12], [13], [14], [15]]. Therefore, the hydrogen production efficiency of TiO2 is not high, and it is more suitable for photocatalytic degradation. In response to these circumstances, many narrow bandgap semiconductors have also been studied for photocatalytic hydrogen production[16]. A lot of recent studies are metal sulfides. Metal sulfides have two advantages over metal oxides. One is that the band gap is generally narrow, so that the visible light absorption efficiency is high, and the internal resistance is small result in high carrier mobility. Second, many metal sulfides have a two-dimensional structure and a large specific surface area, which is suitable for use as a catalytic reaction[[17], [18], [19], [20]]. We also did a lot of metal sulfides for photocatalysis research in the early stage[[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. Metal selenide also has two major advantages of metal sulfides. Otherwise, as the metalloid character of selenium is weaker than that of sulfur, the metal selenide has weaker ionic bond which causes narrower band gap and higher absorbance of visible light. Moreover, the carriers’ mobility of metal selenide will also be much higher than that of metal sulfide due to the narrower band gap which can lead to better performance. Among them, MoSe2 is a two-dimensional material with a band gap of only 1.7 eV, which can absorb ultraviolet light and visible light in the whole band and has good photoelectrochemical performance[[32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]]. However, the existing chemically synthesized MoSe2 two-dimensional nanosheets are generally thicker, reaching 6–10 molecular layers, resulting in a low specific surface area[[48], [49], [50], [51], [52]]. If a thinner MoSe2 nanosheet can be obtained, its photoelectrochemical performance can be further improved.

We synthesized the ultrathin MoSe2 nanospheres by a solvothermal method. This MoSe2 nanosphere is actually constructed by some pleated ultrathin MoSe2 two-dimensional nanosheets, and the microscopic appearance is like a lot of small balls gathered together. Compared with ordinary hydrothermally synthesized flower-like MoSe2 two-dimensional nanosheets, this MoSe2 nanosphere has a thickness of only 2–3 molecular layers, which is much lower than the thickness of 6–8 molecular layers of ordinary MoSe2 nanosheets, so it has a large specific surface area. Its photocatalytic activity is 17 times than ordinary MoSe2 nanosheets, and the enhancement of photocurrent is twice. The results confirmed that the ultrathin MoSe2 nanospheres have excellent photocatalytic and photoelectrochemical properties and are suitable for photocatalytic hydrogen production.

Section snippets

Synthesis of samples

  • 1)

    Preparation of MoSe2 nanoflowers (MSF). MoSe2 nanoplates were prepared by a simple hydrothermal progress. First, 2 mmol Na2MoO4·H2O were dissolved in 30 mL deionized water and the mixed solution was marked as A solution. Then 4 mmol Se powder and 8 mmol NaBH4 were dispersed in 30 mL deionized water. The solution was heated to 80 °C and continuously stirred until the Se powder was completely dissolved. The solution contained Se and NaBH4 was marked as solution B. Secondly, solution A and B were

Structure and morphology

The XRD spectra of the two MoSe2 samples are shown in Fig. 2. The four main diffraction peaks can be distinguished on the two MoSe2 spectra. The positions of the four diffraction peaks correspond to the standard pattern of hexagonal MoSe2 (JCPDS No. 29–0914). The four peaks correspond to (002), (100), (103), and (110) peaks, respectively[35,47]. Otherwise, there are two low peaks at 26.6 and 65.6° can be identified as (004) and (200) peaks. It is worth noting that although the peak positions

Conclusion

MoSe2 ultrathin nanospheres with three-dimensional network structure (MSS) were prepared by improved solvothermal method. These MoSe2 nanospheres are only 10 nm in size and actually composed of ultra-thin MoSe2 nanosheets with a thickness of only 2–3 molecular layers. Compared with the MoSe2 nanosheets (6–8 molecular layer thicknesses) of the three-dimensional flower structure (MSF) prepared by ordinary hydrothermal method, the MSS are thinner resulting in higher specific surface area of 5

Acknowledgements

This work was supported by Chinese National Natural Science Foundation (No. 51602086, 51702073, 61602142 and 61072015), Zhejiang Provincial Natural Science Foundation of China (No. Y20B030030).

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