UC Santa Cruz

Metal oxide semiconductors are ubiquitous in modern energy conversion, energy storage, and electronic technologies, yet their practical performance is frequently limited by poor electrical conductivity, defect-mediated charge recombination, structural instability, and thermodynamic constraints on compositional engineering. This dissertation addresses these challenges across four interconnected studies that share a common thesis: that rational manipulation of crystal defects through heteroatom doping, post-process surface treatments, and hydrogen incorporation, can unlock otherwise inaccessible performance in metal oxide systems relevant to photoelectrochemistry, multiferroics, electrochemical lithium recovery, and beyond.Chapter 2 investigates the persistent problem of dopant oxide formation in n-type doped hematite (α-Fe2O3), a leading photoanode and energy storage material. Through a combination of extended X-ray absorption fine structure spectroscopy, powder X-ray diffraction, and electrochemical measurements, it is shown that dopant oxide phases (SnO2, Fe8Ge3O18) form fractionally above a theoretically predicted critical dopant concentration that depends on dopant identity and synthesis temperature. Two remediation strategies are evaluated: rapid thermal quenching to kinetically trap dopants within the hematite lattice, and post-process alkaline washing to selectively dissolve surface-segregated dopant oxides. While the washing procedure successfully removed approximately 90% of total measured dopant, it did so non-selectively, removing beneficial lattice-incorporated dopant alongside the targeted oxide phase. This resulted in degraded photoelectrochemical and energy storage performance, revealing that dopant spatial distribution (lattice-incorporated versus surface-segregated oxide) is a decisive parameter governing device performance. Chapter 3 develops and compares four wet-chemical synthesis routes for bismuth ferrite (BiFeO3, BFO), a rare room-temperature multiferroic perovskite of interest for optoelectronic, ferroelectric, photon upconversion, and energy storage applications. Potentiostatic co-electrodeposition, Pechini sol-gel deposition, solution drop-casting, and xerogel processing are evaluated for their ability to produce phase-pure, doped BFO thin films and nanoparticle powders. The xerogel route proved most versatile, reproducibly yielding phase-pure nanoparticles accommodating transition metal (Sn, Ge) and lanthanide (Yb, Er) dopants. Preliminary photoluminescence measurements of Yb/Er co-doped BFO revealed complete quenching of the near-band-edge emission, motivating further investigation of BFO as a host crystal for photon upconversion via energy transfer upconversion mechanisms. Electrochemical characterization established baseline pseudocapacitive behavior in aqueous electrolytes and demonstrated the expected multi-step conversion mechanism in lithium-ion battery configurations, while dielectric measurements revealed systematic conductivity improvements with Sn doping consistent with small polaron hopping conduction. Chapter 4 addresses the global challenge of lithium supply security by developing gallium-doped lithium manganese oxide (LiMn2O4, LMO) electrodes for electrochemical lithium extraction from dilute aqueous sources. Structural characterization by XRD, TEM, and XPS demonstrates that Ga³⁺ substitution at the Mn octahedral site induces measurable lattice contraction, suppresses Jahn-Teller distortion, and progressively increases the Mn⁴⁺:Mn³⁺ ratio. Electrochemical evaluation in 50 mM LiCl, a concentration representative of the most dilute industrially relevant brines, reveals that LGMO-5 (5% Ga) retains 100% of its initial capacity over 100 cycles and 85% over 1000 continuous galvanostatic cycles, a dramatic improvement over undoped LMO, which loses approximately half its capacity within 100 cycles. Inductively coupled plasma optical emission spectroscopy confirms that Mn dissolution in LGMO-5 was approximately 40 times lower than in undoped LMO, directly linking the stability enhancement to suppressed Mn disproportionation. This 1000-cycle aqueous stability substantially exceeds published cycling lifetimes for doped LMO in electrochemical lithium recovery. Chapter 5 provides a comprehensive review of hydrogen treatment methods for metal oxides and their effects on structural, optical, electronic, and magnetic properties. Post-process hydrogen annealing, proton-electron co-doping, electrochemical hydrogenation, and in-process incorporation during chemical and physical vapor deposition synthesis are critically compared. The review synthesizes evidence that hydrogen treatment can narrow bandgaps through disorder-induced band tail states, enhance electrical conductivity through extended carrier lifetimes or improved mobilities, and even induce insulator-to-metal electronic phase transitions. Open questions regarding the long-term stability of hydrogen-induced modifications, the mechanistic distinction between surface hydrogenation and bulk interstitial doping, and the scalability of treatment processes are identified as priorities for continued research. Collectively, this dissertation demonstrates that defect engineering is a versatile and powerful approach to overcoming intrinsic limitations of metal oxide semiconductors. The findings contribute both fundamental understanding of dopant behavior, phase stability, and defect-property relationships, and practical advances toward improved photoanodes, multiferroic materials, lithium recovery electrodes, and hydrogen-treated functional oxides.