Full Length ArticleSystematic optimization of promoters in trace SnS2 coating SnO2 nano-heterostructure for high performance Cr(VI) photoreduction
Graphical abstract
SnS2 is obtained from SnO2 in-situ transformed in the process of trace vulcanization. Strong interfacial interaction between SnO2 and SnS2 has promoted the separation of photoinduced electrons and holes effectively and provided more electrons to reduce the Cr(VI) to Cr(III).
Introduction
Growing concern about the seriousness of water pollution urgently demands the development of a eco-friendly technology for the removal of hazardous materials, especially Cr(VI), from contaminated water [1], [2], [3], [4]. Among numerous approaches to removal of Cr(VI), photocatalytic Cr(VI) reduction, especially under visible light radiation, may be the most economical, clean, recyclable and effective mean for the future water treatment [5], [6], [7], [8]. However, in most cases, photocatalysis is not active for Cr(VI) reduction owing to the sluggish photoreduction kinetics. As a remedy, the nano-heterojunction photocatalytic materials were developed for their remarkable superiorities in accelerating photoreduction kinetics. Nano-heterojunction formed by chemically distinct semiconductors in a single nanostructure has created a revolution for unique optical properties and diverse functionalities [9], [10], [11], [12], [13]. The strong interfacial interaction at the nanoscale could prodcue novel properties nonexistent in the individual component and show great potential for enhancing the performance [14], [15]. Therefore, tremendous efforts have been devoted to construct the heterojunction in recent years. However, heterogeneous structure is easily detached during the photoreaction progress, resulting in degenerative catalytic activity and cycling performance. In-situ synthesis technology is a breakthrough method to form the heterostructure with strong interfacial interaction [16]. The sturdy interface can dominate the transfer direction of photoinduced charges and effectively separate photogenerated electrons and holes for promoting photocatalytic activity [17].
As a superior photocatalyst with excellent physicochemical properties, SnO2 displays many significant merits, such as chemical stability, non-toxicity, low-cost, etc [15], [18]. SnO2 possesses a low valence-band edge potential which endows the holes in the band with high oxidation ability [13]. Furthermore, SnO2 also act as an electron acceptor owe to its high electron mobility and more positive conduction band [19]. Unfortunately, due to its wide bandgap (Eg = 3.5–3.8 eV), SnO2 could only harvest ultraviolet light (λ < 420 nm) which accounts for less than 4% of the whole solar energy, thus greatly restricting the utilization of solar energy in practical application [19], [20], [21], [22], [23], [24]. Developing a novel photocatalyst which can efficiently harvest more solar light is urgently desired. Till now, multiple SnO2-based heterojunction photocatalyst have been studied to broaden the scope of optical absorption, such as SnO2/g-C3N4, Ag3PO4/SnO2, SnO2/Cu2O, and SnO2/Co3O4 [25], [26], [27], [28], [29], [30]. In-situ growth of a semiconductor component with narrow band gap on the surface of SnO2 nanocrystals achieves a wide photochemical response in visible region. Among various photocatalysts, SnS2 can be a promising candidate for the in-situ formation of Sn-based heterojunction catalysts due to its narrow band gap (2.25 eV) and easily coating on the adjacent SnO2 by vulcanization reaction [31].
Herein, we prepared a heterojunction catalyst with SnS2-shell coating on the SnO2 porous nanosphere by trace vulcanization process. The photocatalytic properties were improved by deliberately introducing strong interfacial interaction, which was investigated in detail below. Combining our kinetic analyses and the literature data ever reported, we conclude that interfacial interaction associated with the content of SnS2 is the key that enhances the separation and transfer of photogenerated charge carriers. The method of systematically regulating the interface to achieve effective separation and transfer of photogenerated charge carriers in SnS2/SnO2 system is first reported in this work, which may be extended to other heterojunction material systems for important environmental applications.
Section snippets
Materials
The raw materials for the sample synthesis are K2SnO3·3H2O (from Aldrich), H2C2O4, polyvinyl pyrrolidone (PVP), CH3CSNH2 (from Aldrich) with analytical grade that were used without further purification.
Synthesis of SnO2 nanospheres
The SnO2 nanospheres with different grain size were prepared via traditional hydrothermal method. Briefly, K2SnO3·3H2O, ranging from 0.5 to 5 mmol, was firstly dissolved in 60 mL distilled water with the assistance of magnetic stirring to form clearly solution, and then 1.5 g C2H2O4 and 0.2 g PVP
Morphologies of catalysts and their heterojunction catalysts
To verify that SnS2/SnO2 heterojunction catalysts have been successfully prepared by a facile and scalable hydrothermal-assisted route via in-situ trace vulcanization, the as-prepared samples are examined by TEM measurements. TEM images in Fig. S2a and b demonstrates that the particles in sample TOS-C4 and SnO2 are nearly monodispersed porous nanospheres with diameter ranging from 25 to 40 nm. Comparatively, fully vulcanized sample SnS2 (TOS-C20) is comprised of large-size nanosheets and some
Conclusion
Monodispersed SnS2/SnO2 heterojunction catalysts were prepared by in-situ trace vulcanization using a facile and scalable hydrothermal method. The molar ratio of S to Sn, particle size of SnO2 precursor and vulcanization time were applied to adjust the SnS2 content of heterojunction catalyst for better understanding the preferred photocatalytic performance. When the SnS2 content was lower than 19.5%, the interface structure and interfacial interaction can be controllably manipulated by
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (21671077, 21771075, 21871106 and 21571176). We thank the group of Tengfeng Xie from Jilin University for supporting this work.
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