Design and Preparation of Silicate based Nanoparticles for Efficient Photocatalysis and Electrocatalysis

Sarkar, Debashrita (2021) Design and Preparation of Silicate based Nanoparticles for Efficient Photocatalysis and Electrocatalysis. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Debashrita Sarkar (13IP007)) 13IP007.pdf - Submitted Version Restricted to Repository staff only Download (14MB)

Abstract

The global population growth has led to burdensome reliability on various manufactured products such as paper, plastics, electronics, textile, chemicals among many others which are directly discharged in water as industrial waste. Presently, heavy metal toxicity, germination of superbugs, chemical effluents are some of the rising water pollution effects having a direct detrimental effect on human and other life forms. The ideal solution would be to develop photocatalyst which can selectively destroy and remove the toxicants present in the wastewater with ability to remove various kinds of pollutants. Currently, there is a dearth of highly efficient, multipurpose photocatalyst which are cheap, environmentally friendly and possess highly efficient catalytic ability. Also, the incessant use of natural resources, especially coal, natural gas and petroleum for driving the energy requirements of the industrialization is another major setback to the environment. Their slow replenishment rate and associated greenhouse gas emission area threat towards a sustainable future. In this regard, hydrogen as fuel is being increasingly envisaged as a promising alternative due to its high energy density and environmentally benign properties which can be generated using water electrolysis. Currently, there is a need to develop cheap, environment friendly, durable electrocatalyst and understand the various mechanisms to improve the electrocatalytic activity of electrocatalysts. The above challenges have led us to develop highly efficient silicate-based catalyst owing to their ubiquitous availability along with unique structural features rendering them high surface area, internal polarization, layered arrangement which are extremely beneficial for photocatalytic and electrocatalytic applications. Chapter 1 provides a general introduction of the challenges related to incessant population growth leading to water pollution and energy scarcity and role of catalytic pathways in overcoming these crises. This is followed by brief introduction on photocatalytic and electrocatalytic pathways, their mechanism and methods to improve their efficiency with the shortcomings of state-of-the-art catalyst. The structural attributes of metal silicates are mentioned as well as their potential role as efficient photocatalyst and electrocatalyst has been discussed. Chapter 2 delves into the design of defects as oxygen vacancies into Bi₂SiO₅ nanoparticles by doping Eu³⁺ ions. The defect rich structure possesses a high surface area, favors monoclinic phase and shows excellent initial sorption rate for dye removal. The oxygen vacancy sites acted as binding sites for pollutant molecules and Eu²⁺/Eu²⁺ redox couple hindered the recombination rate facilitating high photocatalytic activity as well. Eu³⁺-doped Bi₂SiO₅ nanoparticles are shown to photocatalytically degrade cationic, anionic, neutral and pharmaceutical wastes. Chapter 3 illustrates the role of synergetic effect of halide ions and oxygen vacancy sites on Bi2SiO5 nanoparticles leading to (1) enhancement in light absorption towards visible spectrum from ultraviolet, (2) reduced bandgap, (3) surge in charge migration, (4) boost in the photon to electric energy conversion and (5) increased surface area of the Bi₂SiO₅ NPs. Oxygen vacancy rich halide incorporated Bi₂SiO₅ NPs showed up to 12 times improved photocatalytic rate over pristine towards toxic dye pollutants and high yield for benzaldehyde synthesis from benzyl alcohol. Chapter 4 discuss the role of S incorporation in improving the oxygen evolution reaction of iron-doped cobalt phyllosilicate. The heteroatom incorporated cobalt phyllosilicate shows high TOF value of 1.05 s⁻¹ and high stability during chronopotentiometry. Interestingly, after chronopotentiometry, a decrease in the overpotential from 340 mV to 260 mV is noted to maintain a current density of 10 mA/cm2 which is attributed to (1) facilitating Co²⁺/Co³⁺ by Fe doping, (2) faster catalyst activation due to lower metal-sulphur bond energy compared to metaloxygen bond energy (3) higher pore diameter that enables faster diffusion of reactants and products, and (4) lower charge transfer resistance of sulphur incorporated iron-doped cobalt phyllosilicate than pristine. Chapter 5 focusses on the role of redox inactive silicate anion towards improving the electrocatalytic activity of cobalt phyllosilicate. The highly stable electrodes displayed a reduction in overpotential from 400 mV to 300 mV after 60h of chronopotentiometry. This unique behaviour was attributed to the etching phenomenon of SiO4 4- group in the alkaline medium during the electrocatalysis. The etching of redox inactive silicate species increases the accessibility of active Co ions which are present even in bulk and improves the electrocatalytic activity.

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