Development of Solid Solution Materials for Solar Energy Conversion

Lee, Kyureon

Graduate Thesis Or Dissertation

Development of Solid Solution Materials for Solar Energy Conversion Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/mk61rh063

Abstract

Of the many efforts to develop new sustainable and renewable energy resources, hydrogen production from solar driven water splitting has been intensely explored. For this application, it is essential to find and develop semiconductor materials that are able to generate visible light-induced charge carriers at energies suitable for water splitting, which is the ultimate goal of the projects in this dissertation.

A bulk scale (Ga_1-xZn_x)(N_1-xO_x), solid solution of GaN and ZnO, is a promising photocatalyst for overall water splitting under visible illumination. Even though it exhibits composition-dependent visible absorption, the value of x (ZnO content) that can be obtained for the bulk (Ga_1-xZn_x)(N_1-xO_x) is limited in the conventional synthetic route. Thus, it was desirable to develop a new synthetic route for (Ga_1-xZn_x)(N_1-xO_x) with a wide range of x values in order to investigate the potential of (Ga_1-xZn_x)(N_1-xO_x) in solar energy absorption. In addition, nanoscale semiconductors generally show better photocatalytic properties than their bulk counterparts in many aspects. The work described in Chapter 2 and 3 details the synthesis, reaction mechanism, and structural and optical characterizations of (Ga_1-xZn_x)(N_1-xO_x) nanocrystals with a wide range of compositions. We examined how the band gap depends on composition and observed a minimum of 2.25 eV at x= 0.87. Finally, we find that (Ga_1-xZn_x)(N_1-xO_x) nanocrystals exhibit photoelectrochemical water Oxidation activity that is higher than that of bulk (Ga_1-xZn_x)(N_1-xO_x) with the same composition.

Cobalt mixed-metal spinel Oxides, Co(Al_1-xGa_x)₂O₄, have been predicted to exhibit favorable properties as photocatalysts for solar energy conversion. Chapter 4 focuses on the synthesis and structural and optical characterizations of Co(Al_1-xGa_x)₂O₄ with a range of 0 ≤ x ≤ 1. UV-vis absorbance data revealed that the absorption onset energies decreased monotonically with increasing x. Finally, the photocatalytic activities were evaluated via photodegradation of methyl orange. Remarkably, low energy (<2.5 eV) ligand-field transitions contributed between 46–72% of the photoactivity of Co(Al_0.5Ga_0.5)₂O₄.

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