Light-Directed Growth of Semiconductor Nanomaterials by Photoelectrodeposition

In this work, the chemical and physical properties of semiconductor microcrystals and metastable polymorphs were transformed through photoexcitation during electrochemical synthesis. Interfacial charge transfer using photogenerated carriers were used to successfully control the growth of metal oxide semiconductors and change their morphology, composition improve their catalytic activity. The main material systems studied in this thesis are electrodeposited copper oxide (Cu2O) and manganese oxide (MnOx) semiconductor films. Illumination can direct the shape transformation of Cu2O crystals at the nanoscale by facet-selective charge transfer. During photoelectrodeposition on Cu2O microcrystals with well-developed facets, light mediates the deposition of copper metal selectively on the {100} facets while the crystal interior is etched at {111} facets to form a shell structure. This process reveals that the combination of an applied electrochemical bias and illumination can control the facet-dependence of photochemical reactions on the surface of Cu2O. Illumination can also enhance the catalytic activity of MnOx films through transforming their composition and morphology during growth. MnOx films synthesized under illumination display a significantly higher activity for the oxygen evolution reaction (OER), better stability, and a lower onset potential compared to MnOx films synthesized in the dark. Structural and electrochemical characterization reveal that MnOx films formed under illumination undergo a phase change to develop structural features that increase their stability and activity for the OER. This thesis provides insights into the spatial separation of photo-induced charge carriers on the surface of semiconductors during photoelectrochemical synthesis. Photoelectrodeposition has been shown as a novel method to control redox reactions at preferred surfaces of the nanomaterials. It can tailor the morphology and control the oxidation states of transition metal ions in metal oxide semiconductors and enhance desirable properties including catalytic activity. Thus, photoelectrochemical synthesis helps us design functional materials at the nanoscale