Open Access Publications from the University of California

## Surface Structure Probe of Transition Metal-Based Oxygen Evolving Systems with Spectroscopy

• Author(s): Doan, Hoang Quoc
• et al.
Abstract

The goal of a carbon free H$_{2}$ economy using a photoelectrochemical cell to split water is a worthwhile endeavor to solve the looming energy crisis. Many scientists have taken up this cause and an abundance of studies exists characterizing, optimizing, and creating materials to integrate into a full artificial photosynthetic system. However, despite widespread attention, a viable industrial-scaled photoelectrochemical device has yet to emerge due to low efficiencies, slow kinetics, and high energetic barriers. To that end, going back to the fundamentals may be a necessary step to understand what is causing the bottleneck, particularly within the heterogeneous water oxidation catalysts. Herein, the surface electronic structure of transition metal-based semiconductors and their 3$d$ valence electrons that actively participate in the oxygen evolution process are investigated using a number of spectroscopic techniques in an effort to unravel the mechanism and uncover important material properties. I focus on three major properties: 1) photo-excited carrier dynamics affecting the excited state electronic structure, 2) ground state electronic structure including covalent and atomic parameters, and 3) surface state-mediated interfacial hole transport.

First, the transient electronic structure of Co$_{3}$O$_{4}$, a promising water oxidation catalyst, is probed via transient absorption spectroscopy. With selective excitation of key optical transitions, both inter- and intravalence transitions involving the 3$d$ electrons, the kinetics and spectrum are investigated. A wide range of pump and probe wavelengths, spanning the ultraviolet to the visible to the near infrared, are employed. Despite this range of pump and probe energies, the carrier dynamics were largely unaffected. Additionally, the kinetics and spectra show a unique independence to fluence and sample morphology.

The kinetics reveal a photo-excited carrier density that quickly thermalizes when excited across the charge transfer transitions and converts into $d-d$ excitations. The recombination from these localized midgap $d$ states occurs at a longer, nanosecond time scale.

In addition to perturbing the system via photo-excitation, the electronic structure of Co$_{3}$O$_{4}$ was tuned using an applied potential in a technique called spectroelectrochemistry. Spectroelectrochemistry reinforced the results of the transient absorption spectroscopy and confirmed the identity of the midgap $d$ states through determination of the energetics of these 3$d$ states as well as assignment of whether electrons or holes induced the absorptions or bleaches observed within the transient spectrum.

Taken together, the results suggest a special type of intrinsic hole trap center that is a potentially promising long-lived state for utilization in photo-activation. Further, since the photo-excited hole is efficiently localized at these 3$d$ sites, the most likely water oxidation reaction intermediate is an oxidized cobalt center, \emph{i.e.} a Co(IV)=O species.

The second investigation probes the ground state 3$d$ electronic structure of active and non-active water oxidation catalysts using X-ray absorption spectroscopy to determine an electronic structure-activity relationship. A set of molecular, homogeneous cobalt polyoxometalates serves as model systems for extracting electronic structure parameters, such as metal valence state (\emph{i.e.} 2+ or 3+), metal coordination environment (\emph{i.e.} tetrahedral or octahedral environments), structural distortions and covalency between the metal and oxygen ligands, from cobalt L-edge spectra. No definitive structure-activity relationship could be established because X-ray absorption spectroscopy could not distinguish between the number of metal atoms within the molecular structure or the identity of the heteroatom surrounding the metal(s) and ligands. These properties are what defines the extent of catalytic activity and point to the importance of using more sensitive techniques such as resonant inelastic X-ray scattering. However, ligand field multiplet theory was able to simulate well the experimental Co L-edge spectra for the well-defined model systems and report the ligand field parameter 10Dq to within an accuracy of $\pm$1~eV and the strength of electron-electron interactions to within $\pm$5$\%$ of atomic values. These parameters were subsequently applied to characterize a lesser-defined heterogeneous sample, a Co$_{3}$O$_{4}$ thin film.

Lastly, the final chapter addresses the surface structure dynamics under \emph{in situ} conditions, \emph{i.e.} during the influence of water, on GaN. GaN, a widely studied semiconductor for integration as a photoanode in the water oxidation reaction, possesses a unique surface chemistry and a mobility that is experimentally accessible. Using surface sensitive transient grating spectroscopy, a quantitative value for the hole mobilities of both an undoped and a n-doped GaN film were determined in air and at a semiconductor/electrolyte interface. It was found that interfacial carrier mobility is highly dependent on the surface intermediates. For n-doped GaN, a large density of dark surface states in the form of various adsorbed species exist that can localize holes and, further, allow the holes to hop from metal site to metal site along the surface, possibly assisted by proton-coupled electron transfer with the water molecules. This charge transfer pathway during dark equilibration between the semiconductor electrochemical potential and water oxidation potential more than doubles the interfacial hole diffusivity compared to its value in air. This was confirmed under both acidic and neutral conditions, 0.1~M HBr (pH = 2) and 0.1~M Na$_{2}$SO$_{4}$ (pH = 7). In contrast, the hole diffusivity of undoped GaN with no significant surface state density is unchanged with introduction of an electrolyte solution.

The results suggest that charge transport and surface reactivity, which are generally treated independently, are connected phenomena.