Synthesis and Study of Cu and Sn Based Nanostructured Metal Chalcogenides for Energy Storage and Photocatalytic Applications

Abstract

Rapid growth of population and global economy lead to substantial depletion of fossil fuels, which generated two pressing concerns: first, energy crisis due to exhaustion of existing fossil fuel reservoirs and second, environmental pollution due to rapid increase in greenhouse gas emission. Hence, to save the future, it is a matter of urgency to develop sustainable, environmental friendly material for energy harvesting and energy storage from renewable sources such as solar energy, wind energy etc. Nanostructured metal chalcogenides (MCs), particularly, copper and tin based metal sulfides, being nontoxic and earth abundant, are found to be potential as well as sustainable materials for several energy related applications ranging from solar cells to supercapacitors, catalysis to thermoelectrics, and so on. Although substantial study have been carried out on copper, tin based binary and ternary metal sulfides and their nanocomposites, still underlying mechanism of formation of different phases and morphology with varying solvents, capping agents and sulfur sources are least understood. Understanding of the mechanism will help in the control synthesis of a particular phase with a particular morphology and is highly required for various applications. Moreover, evaluation of the energy storage properties of these compounds are least studied. In this thesis, several copper and tin based binary, doped binary, ternary and their nanocomposites have synthesized and studied by following hot injection as well as reflux method. Attempts have been made to understand the role of different phase formation by varying solvents, capping agents and sulfur sources. Mechanistic path is proposed for different phase formation in Cu-Sb-S ternary system with varying sulfur sources. Further, the energy storage and photocatalytic applications of these materials are evaluated.
The research work started with one of the simplest binary nano metal chalcogenide i.e. CuS, which present the synthesis and study of CuS with different morphology by varying the solvent, capping agent and reaction condition. It has been observed that by varying the capping agent the morphology of CuS significantly changed. By adding MPA, it is observed that the growth of nanoparticles cease to form 30-40 nm in size. However, addition of PVP as capping agent, biconcave shape submicron crystals of CuS were obtained. Without any capping agent, copper and sulfur precursor in ethylene glycol as solvent form nanosheets which aggregates to give nanoflowers. CuS nanoflowers show good electrochemical properties. Further to improve the properties we have synthesized nanocomposite with varying Ni:CuS ratio and their electrochemical properties were evaluated. It is observed that Ni:CuS with molar ratio of 0.6:1 provide optimum supercapacitance properties compared to other compositions.
To further modify the electrochemical properties, we have incorporated a third element, Sb and synthesized various ternary compounds in Cu-Sb-S system. The effect of solvent, capping agent, sulfur sources and varying concentration of metal precursor on the formation of different phases and the electrochemical properties in these phases have been studied. By varying solvent, capping agent, sulfur source and concentration of metal precursor different phases such as CuSbS2, Cu3SbS3 and Cu3SbS4 are obtained. With equal moles of metal precursor, when elemental sulfur is used as sulfur precursor, nanoplates of pure CuSbS2 obtained, while thioacetamide is used as sulfur precursor, CuSbS2-Cu3SbS4 nanocomposite obtained with Cu3SbS4 nanoparticle decorated on the surface of CuSbS2 nanoplates. However, by changing the molar ratio of metal precursor to 3:1 with thioacetamide (TA) as sulfur precursor, Cu3SbS3-Cu3SbS4 nanocomposites are obtained. The ease of reduction of thioacetamide as compared to sulfur at high temperature in presence of oleylamine (OLA), promotes the oxidation of antimony from (III) to (V) state and the formation of Cu3SbS4 phase containing Sb(V). Moreover, it is observed that 25% enhancement of specific capacitance value of the CuSbS2-Cu3SbS4 nanocomposite as compared to the parent CuSbS2 nanoplates at a current density of 2 A/g. In summary, it is observed that both Ni-CuS and CuSbS2-Cu3SbS4 nanocomposite show enhanced electrochemical properties as compared to their parent binary or ternary compound.
After understanding the effect of sulfur sources on stabilization of various phases in Cu–based ternary metal chalcogenides, the work is further extended on Sn based metal chalcogenides. SnS is one of the simplest metal chalcogenide which we have focused to synthesize and study its photo catalytic activity for dye degradation. To enhance its properties and activity towards dye degradation we have further carried out doping and composite formation on SnS. In general, SnS and doped SnS has emerged as a promising energy material owing to its remarkable optical and electrical properties. Mn-substitution in SnS revealed a change in the size-cum-morphology of nanocrystals, while cuboid or plate shape nanocrystals were observed for SnS, Mn-substitution results in spherical shape. The optical band gap of Sn1−xMnxS shows a red-shift (by 0.15 eV) up to 10% Mn-concentration, however further substitution leads to contrasting effect. Mn-substitution in SnS enhances the photocatalytic activity on the degradation of Congo red dye in visible region. The scavenger study suggested that the photo generated hole, hydroxyl and super oxide radicals were the principal reacting species for the degradation of Congo red dye. Further, with small amount of Mn substitution at Sn site, a dilute magnetic semiconductorhas been developed. Sn1−xMnxS with x = 0.10 shows a weak-antiferromagnetic ordering at 28 K. Observation of enhanced photocatalytic activity of Mn doped SnS nanocrystals motivated us to study further SnS based nanocomposite and their applications in photocatalysis. Ag-SnS nanocomposite was synthesized via hot injection method. Different Ag-SnS nanocomposites were obtained by varying Ag and SnS concentration. It is observed that with increasing Ag concentration in the composite, the photocatalytic efficiency can be increased. Photocurrent measurement supported the observed increase in photocatalytic properties of Ag-SnS nanaocomposite with higher amount of Ag. The mechanistic study indicated that superoxide radicals and holes are the active species, which are largely responsible for the photocatalytic degradation of CR dye. So the photocatalytic activity of SnS could be enhanced by both doping with Mn and composite formation with silver metal.