Colloidal Ternary Metal Oxide Semiconductor Nanocrystals

Ternary metal oxide semiconductors are a richly diverse class of materials with optical, electronic, chemical, and magnetic properties that vary widely depending on composition and stoichiometry. These materials are therefore useful in a variety of applications ranging from solar energy conversion  to information storage to thermoelectrics. As with other semiconductors, shrinking ternary metal oxide materials down to the nanoscale can cause new or enhanced properties to emerge due to increased surface area-to-volume ratio or quantum confinement effects. However, most methods for synthesizing nanocrystalline ternary metal oxide materials rely on solvothermal or sol-gel techniques that result in products with broad distributions in nanocrystal size and shape, or cannot easily access nanocrystal diameters smaller than 10 nm. We are working to develop methods for solution-phase syntheses of colloidal ternary metal oxide semiconductor nanocrystals at ambient pressure that provide precise control over size, shape, and surface chemistry. Initial target materials have small band gaps and band-edge potentials capable of photocatalyzing desirable transformations related to solar fuels production, such as reduction of protons or carbon dioxide.

Electrochemistry of Nanostructured Metal Oxide Semiconductors

Due to their high surface area and general redox stability, nanostructured metal oxide semiconductor films can be excellent photoelectrodes for desirable photoelectrochemical transformations, such as reduction of carbon dioxide and oxidation of water. The nanoscopic interface between the metal oxide semiconductor and liquid electrolyte largely determines the photoelectrochemical behavior of these photoelectrodes, but the structure and chemistry of this interface are poorly understood and therefore difficult to control. We are exploring how the photoelectrochemical properties of nanostructured metal oxide semiconductors evolve as a function of their surface area-to-volume ratio and surface chemistry. Our goal is to reveal structure-function relationships between the morphology and chemistry of nanostructured oxide materials and their photoelectrochemical behavior.

Excited State Dynamics of Nanostructured Photoelectrodes

Several processes, such as bulk recombination, surface trapping, and surface recombination, prevent or hinder the transfer of photogenerated charge carriers from semiconductor photoelectrodes to substrates or coupled catalysts. These processes often occur on timescales too fast to measure by electrochemical techniques, which makes them difficult to characterize under the operating conditions of a photoelectrochemical cell. We will use time-resolved diffuse reflectance spectroscopy to measure the excited state dynamics of semiconductor photoelectrodes in operando, and thereby uncover mechanisms by which these processes influence the time-averaged photoelectrochemical performance of the electrode.