Novel ZnS-ZnO composite synthesized by the solvothermal method through the partial sulfidation of ZnO for H2 production without sacrificial agent
Graphical abstract
Introduction
At present, there is a greater energy demand and a strong environmental problem due to the increase of population and industrial development, so it is of interest to promote new technologies to generate clean energy. Sunlight and water are renewable natural resources on earth and can be harnessed for the generation of clean fuels such as H2 [1]. Unique properties of hydrogen make it a versatile, green chemical energy carrier suitable for all types of heat engines and other equipments. Taking into account this fact, the development of novel and efficient technologies to produce hydrogen, based on systems mimicking natural photosynthesis is of great interest to solve problems of energy shortage.
The use of semiconductor materials in photocatalysis is an alternative to obtain H2 without damaging the environment. The water splitting through photocatalysis includes three steps; the first, is the light absorption by the semiconductor to form the electron-hole pairs, in the second step, the migration of the charge carriers on the active sites located on the semiconductor surface, or, the recombination of the photogenerated electron-hole pairs occurs in the bulk material and finally, a redox reaction is carried out on the material surface, where the holes oxidize the H2O molecule to transform it into O2 and protons, while the electrons reduce the H+ to form H2 [2,3]. The positions of the valence and the conduction band play an important role in the water splitting process, the valence band must be more positive than the oxygen evolution reaction “OER” potential (+0.82 V relative to a SHE (standard hydrogen electrode) at pH 7.0), and the conduction band must be more negative than the hydrogen evolution reaction “HER” potential (−0.41 V relative to a SHE at pH 7.0) [4]. The production of molecular hydrogen (H2) by photocatalytic water splitting is energetically disadvantaged due to the rapid recombination of the electron-hole pairs and the slow kinetic for OER; however, the use of sacrificial agents increases the production rate of H2 [5]. The field of photocatalytic H2 production using carbon-compounds as sacrificial agents has gained significant attention. Among them, methanol has been widely used as a sacrificial agent for H2 production by photocatalysis, owing to its simple structure that helps to understand the reaction mechanism, and this molecule acts like a hole-scavenger. Also, this alcohol can be easily mineralized producing less toxic substances [6]. It has been reported that alcohols have a strong tendency to react with holes and thus suppress the recombination and therefore an increase on the H2 production is observed. An important point to note here is that, in the presence of alcohols, the lifetimes of photo-generated holes are significantly shorter (10−12–10−9 s) than those of photo-generated electrons (10−5–10−3 s) [7], since the alcohols act as a hole trap avoiding the recombination process.
A key problem in photocatalytic water splitting over disperse semiconductor photocatalysts is the recombination of the electron-hole pairs in the bulk or on the surface of semiconductors. Nevertheless, improved spatial separation of photogenerated charges allows to increase the quantum efficiency of hydrogen production [8]. The combination of different semiconductors to create composites is an alternative to overcome the recombination and create new materials with enhanced photocatalytic properties for H2 production since the separation of the charge carriers is favored due to the formation of heterojunctions [3,9]. Recently, several types of photocatalytic composite materials have been used to improve the photocatalytic activity, for example, Bi2O3-TiO2 [10], MoS2/RGO layers as cocatalyst on CdS [11], WS2-CdS [12], among others [13]. In the present work, the use of ZnS coupled with ZnO throughout a heterojunction is proposed. The position of the valence and conduction band of ZnS make possible to carry out the photocatalytic process for the H2 production [14]; the addition of ZnO may contribute to the separation of the electron-hole pairs to increase the H2 production. It has been reported that ZnS-ZnO is an efficient composite for the photocatalytic production of H2 using sacrificial agents [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]; however, in this work it is shown that ZnS-ZnO composite is also capable to generate H2 without using any sacrificial agent. This composite can be obtained by several synthesis routes, for example, it can be obtained from its precursors (Zn2+ and S2−) [25], from the partial sulfidation of ZnO [26], and partial oxidation of ZnS [17], the last two methods have the inconvenient that have to be carried at high temperatures (>200 °C); however in this work the solvothermal method has been employed to obtain the ZnS-ZnO material by the partial sulfidation of ZnO at low temperatures (≤100 °C). These composites proved to be efficient for the H2 production without using any sacrificial agent, which has not been previously reported for this type of materials.
ZnS-ZnO composites were synthesized through the partial sulfidation of ZnO at 80, 100, and 150 °C. The material with the higher photocatalytic activity was the composite synthesized at 100 °C with an atomic ratio of S/Zn = 0.71. The high activity was attributed to the formation of a heterojunction, which improves a high charge transfer, in other words, a greater accumulation of electrons and holes is generated at the conduction and the valence band, respectively. Additionally, using methanol as sacrificial agent, the photocatalytic activity increased up to five times the H2 production for the most active photocatalyst, since holes are rapidly transferred to methanol molecules in the interface avoiding the recombination, enhancing the quantum yield [27]. The ZnS-ZnO composites were efficient for the H2 production with and without sacrificial agent in the presence of UV light (λ = 254 nm).
Section snippets
ZnO synthesis
The ZnO was obtained from the calcination of Zn5(CO3)2(OH)6 at 300 °C (see the Fig. 1S at the supplementary information); this was obtained by the precipitation method. Zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, Meyer, 99%) and urea (NH2CONH3, Reasol, 99%), in a molar ratio 1:3, were dissolved in 600 mL of distilled water. This mixture was kept under vigorous stirring at 90 °C for 72 h. The precipitated powder was filtered and washed using distilled water, and subsequently, dried at 100 °C for
X-ray diffraction
Fig. 1A shows the crystallographic planes of ZnO, ZS80, ZS100, and ZS150 materials. ZnO shows the diffraction planes corresponding to the hexagonal zincite phase (data base code amcsd 0005203). The peaks were detected at 2θ° at 31.8, 34.4, 36.3, 47.5, 56.6, 62.9, 66.4, and 68.0°, corresponding to the plans (100), (002), (101), (102), (110), (103), (200), and (112), respectively. Additionally, peaks were found at 28.5, 47.6, and 56.4° for the sulfided materials, corresponding to the
Conclusions
In this work a new method of synthesis is proposed to obtain the ZnS-ZnO composite active for H2 production with and without sacrificial agent. The sulfided materials were synthesized by the solvothermal method at 80, 100, and 150 °C; the ZS100 material was the most efficient, and presented an H2 production of 247 μ mol h−1 g−1 without sacrificial agent and using methanol as sacrificial agent, the hydrogen evolution increases up to five times, 1242 μ mol h−1 g−1, using a UV Hg lamp of 254 nm.
Acknowledgments
The authors acknowledge to CONACyT for the scholarship given during the development of this project (CVU/No. of scholar 507712/286023 and 442565/269185). The authors thank to CONACyT for the support granted through the project CB-2015-01 256410.
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2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :Among several metal oxides, zinc oxide has been interested by scientific researchers because of its good redox capability and less toxic [28–30]. But zinc oxide semiconductor photocatalysts have the shortcoming of poor photocatalytic H2O decomposition for H2 activity owing to the recombination of photogenerated carriers and holes [31–33]. Therefore, it is important to research excellent ZnO-based photocatalysts to suppresses this phenomenon nowadays.