Solution-grown chalcogenide semiconducting materials for water purification and splitting

Abstract

The earth-abundant chalcogenides materials have shown a good presence in photovoltaic devices, but limited investigations are in the area of photocatalytic and water splitting toward the production of hydrogen. The Cu2FeSnS4 (CFTS) has lesser change to form bimetallic phases during growth and better stoichiometry can be achieved. The solution-based approach is investigated to grow the various type of materials morphologies. Due to tunability of the surface properties of solution grown CFTS, it become interesting to understand their effect on various properties of materials and subsequently over photocatalytic and photo-electrochemical water splitting properties. The water purification and photo-electrochemical water splitting both process are related to photogeneration and separation of the electron under photoexcitation. Hence, the development of efficient materials for solar (photo) driven catalytic process is the central theme of the thesis work. Synthesis of Cu2FeSnS4 (CFTS) particles by solvothermal process and effect of temperature, reaction time, solvents on structural, morphological, and optical properties are investigated. The variation in reaction temperature varies the crystallinity of the CFTS particles and synthesis at 200ºC results in better crystallinity. The temperature variation also results in a significant difference in morphology (aggregate particles, sphere-sheet and intermixed with the aggregate-sheet) of CFTS particles. The bandgap of the CFTS particles also gets changed from 1.42 eV to 1.5 eV after varying the synthesis temperature. Similarly, the change in synthesis time leads to form porous-sphere, sheet-flakes, and intermixed flower-sheet morphologies of CFTS particles and variation in crystallinity. There is variation in the elemental composition is observed when different the synthesis is carried out at a different temperature and for different time duration. However, annealing in the Sulphur atmosphere leads better crystalline CFTS in the stannite phase. Further, variation in solvent also shows a significant impact on morphologies of CFTS particles, flower, larger grain structure, aggregate particles, and agglomerated particles are obtained when different solvents are used. The different solvent results in the slightly different elemental composition of the CFTS particles. The bandgap of CFTS particles also varies from 1.43 eV to 1.7 eV when different solvents are used. Different sulphur precursors namely thiourea, thioacetamide, sulphur powder and sodium sulphide are also used for the synthesis of CFTS particles. The CFTS particles have noticeable variation in morphology, crystallinity, and elemental composition when different sulphur precursors are used. The thiourea results in better crystallinity, stoichiometry and larger size grains of CFTS particles. In addition, different morphologies of CFTS particles (with surface controlled) are obtained when different surfactants i.e. thioglycolic acid (TGA), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) are used. The PVP as a surfactant result in a better crystalline phase, while impurity phases are observed in TGA, PVA. The significant change in morphology (porous particles, highly porous particles having nanosheets-nanoparticles on the surface, and uniform spherical) of CFTS particles are obtained by changing the surfactant in the synthesis process. The application of Cu2FeSnS4 (CFTS) particles in water purification is studied via photocatalytic and adsorption approaches. In one of the studies, the CFTS particles synthesized using different Sulphur precursors are tested for water purification. The CFTS particles synthesized using thiourea have shown the best results, and approximately 70% methylene blue dye is degraded in 60 min under white light excitation. In another study, porous CFTS particles are adopted to study the removal of organic acid fuchsin dye pollutant from wastewater by adsorption process. The porous spheres of CFTS particles have shown approximately (89.25 ± 2.21) % of acid fuchsin (AF) dye adsorption within 10 min and the value reaches (97.12 ± 0.76) % in 60 min. The high adsorption capacity i.e. (123.12 ± 2.09) mg/g is obtained for porous spherical CFTS particles. The adsorption isotherm and kinetic studies reveal that the Langmuir isotherm and pseudo-second-order kinetic model can explain the dye adsorption. The highest adsorption capacity (128.12 mg/g) and 98% acid fuchsin (AF) dye adsorption observed within 60min when porous sphere CFTS is used as an adsorbent. Further, the porous CFTS particles exhibit good stability and reusability of the adsorbent for wastewater purification. Further, the application of Cu2FeSnS4 (CFTS) particles in water splitting towards hydrogen production is also investigated via electrocatalytic and photo-electrochemical methods. In the first study, different morphologies of CFTS particles are investigated to test the electrocatalytic ability for hydrogen evolution reactions (HER). CFTS particles with highly porous surface having nanosheet nanoparticles on surface (grown using PVP) exhibit excellent electrocatalytic performance. While photo-electrochemical water splitting performance of CFTS is tested with Ni-doped TiO2 nanorod structure. The earth-abundant and visible light-sensitive Cu2FeSnS4 layer is decorated on Ni-TiO2 nanorods (CFTS/Ni-TiO2 NRs) using a wet chemical approach. In which Ni-doped TiO2 nanorod are hydrothermally grown. These heterostructure photoanodes demonstrated a significant increase in photocurrent from 0.730 mA/cm2 to 2.09 mA/cm2 (at 1.23V vs RHE). The lifetime of photogenerated charge carriers also improves and CFTS/Ni-TiO2 NRs exhibit excellent photo-electrochemical properties with high stability; hence this heterostructure can be a potential candidate for solar energy device applications.