Syntheses of metal sulfide nanocrystals and their photocatalytic investigation

Abstract

Photocatalysis has emerged as one of the most promising technologies and represents a easy way to utilize the energy of natural sunlight and it can provide viable solutions for environmental remediation and alternative clean energy supply. In natural photosynthesis, plants use sunlight to rearrange chemical bonds of H2O to produce O2 and the reduced fuel. Therefore, the direct conversion of solar energy into chemical energy/fuels using artificial photosynthesis constitutes an ideal technology for harvesting and storing sunlight energy in the form of chemical fuels such as H2. The dihydrogen has been considered as the fuel of future and an environmental friendly alternative to fossil fuels. It is important to note that, the strong dependence on non-renewable energy resources like fossil fuels has resulted global warming and air pollution. Therefore it is utmost urgent to develop clean sources of energy which are pollution free. Thus, development of heterogeneous photocatalysts for efficient water splitting into H2 and O2 using sunlight constitutes the most promising process for carbon-neutral, sustainable energy. In this regard, semiconductor nanostructures have attracted immense attention due to their size-dependent unique optical and photocatalytic properties. The properties of these nanomaterials depend on their crystallite's size, morphology and structure. Therefore, controlling the size and morphology is crucial to tailor the properties of these nanostructures. Particularly, the metal sulfide nanocrystals (NCs) of CdS, Zn1-xCdxS and MoS2 have gained considerable interest owing to their promising photocatalytic applications for degradation of toxic organic pollutants and water splitting and H2 generation. Various inorganic and organic sulfur compounds have been employed as in situ source of S2- ions for the syntheses of metal sulfide nanocrystals (NCs). However, literature study revealed that it is quite difficult to obtain nanocrystals with smaller (~2-5 nm) size without the use of template/capping ligand. Though the use of template molecules/capping agents assist in the formation of nanostructures with well-defined morphology, the capping ligand-free NCs offer several advantages over capped-NCs. Towards development of metal sulfide nanostructures with controlled size, morphology, we sought to use organosulfur compounds wherein the release of S2- ions can be controlled by tuning the reaction temperature. In this regard, the present work investigates the development of facile, one-pot routes for controllable synthesis of metal sulfide (CdS, ZnS, PbS, CuS, Zn1-xCdxS and MoS2) NCs from new organosulfur compounds, 4,4'-dipyridyl disulphide (DPDS = (C5H4N)2S2) and dibenzyl disulphide (DBDS = (C7H7)2S2) which acts as temperature controlled in situ source of S2- ions. The synthesized metal sulfide NCs have been characterized by various techniques and their photocatalytic applications for degradation toxic organic pollutants (dyes) in water and water splitting into H2 generation has been investigated. For the first time, we have demonstrated application of Zn1-xCdxS NCs for visible-light-assisted reduction of niroaromatic pollutants in water by utilizing the hydrogen generated by water splitting as a source of protons. Further, development of hybrid nanostructures of MoS2/C3N4 and their visible-light-assisted photocatalytic properties for water splitting and H2 generation has been also been investigated.