Designing of Transition Metal Oxide and Chalcogenide Nanostructures for Potential applications in Photoelectrochemical Devices

Abstract

Abstract Designing of Transition Metal Oxide and Chalcogenide Nanostructures for Potential applications in Photoelectrochemical Devices

Supriya Ankush Patil Dept. of Chemistry The Graduate School Hanyang University

The growing need for energy by the human society and depletion of conventional energy sources demands a renewable, safe, infinite, low-cost and omnipresent energy source. One of the most suitable ways to solve the foreseeable world’s energy crisis is to use the power of the sun. Photovoltaic devices are especially based on metal oxides and chalcogenides of wide interest as they can convert solar energy to electricity. Among different types of nanomaterials, metal oxides and transition metal chalcogenide semiconductor nanostructures exhibit unique electronic, chemical and physical properties. The inherent properties of metal chalcogenide nanostructures can be further tuned depending upon their application to particular energy conversion device applications.The development of very simple, cost-effective and environment friendly methods, suitable for the large-scale synthesis of these, is therefore of great demand. Using metal oxdies (MOs) and transition metal chalcogenides (TMCs) nanostructurs, this thesis describe a methodology for improved power conversion efficiency of solar to electrical energy conversion devices. Chapter 1: This introduces the innovative, novel simple developed methodologies for metal oxides and metal chalcogenides nanostructure. Additionaly, a brief history of research plan on the clean energy demand and the goal of this research.Furthermore, discussion about historical background of photoelectrochemical cell and dye sensitized solar cell (DSCs), with structure and operating principle of DSCs for understanding the important aspects of energy conversion and storage. Chapter 2: We demonstrate an extremely simple but highly effective strategy for the synthesis of various functional metal oxides (MOs) such as ZnO, In2O3, Bi2O3, and SnO2 nanoparticles with various distinct shapes at room temperature via a solid-state reaction method. The method involves only mixing and stirring of the corresponding metal salt and base together in the solid phase, which yields highly crystalline metal oxides within 5-10 min of reaction time. The obtained paste can be directly doctor bladed onto a variety of substrates for photoelectrochemical applications. The proposed method does not require a sophisticated instrumental setup or harsh conditions, and the method is easily scalable. Hence, it can be applied for the cost-effective and large-scale production of MOs nanoparticles with high crystallinity. This method is easily scalable and can be employed for large scale production of crystalline metal oxide powders. As an example for potential applications of these metal oxides, we demonstrated the application of ZnO nanoplates as a photoanode in DSCs. Photoanodes composed of small nanoplates were spongy in nature, ensuring better connections among the nanostructures for the electron transport in the ZnO photoanode, and for the diffusion of redox couples in the electrolyte. We systematically studied the mechanism and the effect of the annealing temperature on DSCs. The device with the low-temperature treated ZnO photoanode obtained using the solid-state reaction has demonstrated a power conversion efficiency of 5%, which is certainly non-ignorable. Chapter 3: In the next part of the thesis, based on a coordination chemistry approach, the present work reports on the synthesis of thin films of various cobalt hydroxycarbonate nanostructures such as nanobeams, nanoneedles, and bending nanorods using three different cobalt precursors viz. Cl-, NO3- and CH3COO-. After pyrolysis in air, the hydroxycarbonate nanostructures are transferred into 1-D arrays of Co3O4 nanorods. The obtained 1-D Co3O4 nanostructures are then transformed into the corresponding analogous shaped 1-D arrays of porous cobalt sulfide (CoS1.0365) nanostructures using a wet chemical transformation method based on an ion exchange approach. As a proof-of-concept demonstration for the application, various shaped CoS1.0365 nanorod films synthesized are investigated as a Pt-free counter electrode in dye-sensitized-solar cells (DSCs). Among the various nanostructures, the thicker nanorod film synthesized using a chloride precursor has demonstrated the best electrocatalytic behavior toward triiodide reduction, which led to a short circuit current density of 18.04 mA cm-2 and energy conversion efficiency of 7.4% of the DSC. This photovoltaic performance is highly competitive to a current density of 18.26 mA cm-2 and energy. Conversion efficiency of 7.7% exhibited by the standard Pt counter electrode. This finding suggests the present strategy as a low cost deposition method to facilitate the application of non-precious metal-free counter electrodes in high performance DSCs. Chapter 4: Additionally, we have developed a novel, low-temperature, and general solution-based protocol, which works equally for deposition of most of the Metal sulfides (MS) nanostructured thin films.We present for the first time, the use of a general solution-based protocol chemically synthesized pristine MoS2 counter electrode (CE) in DSCs. DSCs based on a mechanically robust MoS2 CE with a molybdenite mineral-type structure exhibited a competitive power conversion efficiency of 7.01%, which is largely comparable to DSCs with Pt CEs (PCE = 7.31%). The MoS2 CE presented in this work is quite promising and is a prospective candidate to replace highly expensive Pt as a CE, owing to its comparable catalytic properties and most importantly, the ease of fabrication at extremely low temperatures. The films obtained by this protocol were crystalline and highly uniform. The films can be deposited on any desirable conductive as well as non-conductive substrates. Chapter 5: Furthermore, development of solution based anion exchange transformation mehods for 1-D nanotubes of Cobalt telluride nanostructures.Basically, tellurization is performed at high temperature in inert gas atmosphere, which is an expensive and complicated process. In an effort to develop an alternative strategy of tellurization, herein we report a thin film formation of self-standing cobalt telluride nanotubes on various conducting and non-conducting substrates using a simple binder-free synthetic strategy based on anion exchange transformation from a thin film of cobalt hydroxycarbonate nanostructures in aqueous solution at room temperature.After the ion exchange transformation of nanostructures, the film shows conversion from insulator to highly electrical conductive semimetallic characteristic. When used as a counter electrode in I3-/I- redox electrolyte based dye sensitized solar cells, the telluride film exhibits an electrocatalytic reduction activity for I3- with a demonstration of solar-light to electrical power conversion efficiency of 8.10%, which is highly competitive to the efficiency of 8.20% exhibited by a Pt-film counter electrode.It is believed that the ability of the proposed aqueous solution phase anion exchanged transformation strategy to generate 1-D nano-scaled structures of cobalt telluride offers new opportunities to synthesize various metal telluride nanostructures, and thus opens new prospects to extend their potential applications in various fields.