Assigning Surface Hole Polaron Configurations of Titanium Oxide Materials to Excited-State Optical Absorptions.

For water splitting, a comprehensive understanding of the underlying reaction intermediates and pathways is crucial for optimizing catalyst design. Among the most well-known active photoanodes for the oxygen evolution half-reaction are TiO-based materials. A hole polaron, which consists of a metal-oxide distortion around trapped holes, has been suggested as a local reactive oxygen configuration.

Optical and Plasmonic Properties of High-Electron-Density Epitaxial and Oxidative Controlled Titanium Nitride Thin Films.

The present paper reports on the fabrication, detailed structural characterizations, and theoretical modeling of titanium nitride (TiN) and its isostructural oxide derivative, titanium oxynitride (TiNO) thin films that have excellent plasmonic properties and that also have the potential to overcome the limitation of noble metal and refractory metals. The TiNO films deposited at 700 °C in high vacuum conditions have the highest reflectance ( = ∼ 95%), largest negative dielectric constant (ε = -161), and maximal plasmonic figure of merit (FoM = -ε/ε) of 1.2, followed by the 600 °C samples deposited in a vacuum ( = ∼ 85%, ε = -145, FoM = 0.

Do Small Hole Polarons Form in Bulk Rutile TiO?

Hole transport and localization through small polarons are essential to the performance of TiO in photocatalysis applications. The existence of small hole polarons in bulk rutile TiO has been, however, controversial with contradicting evidence from theory and experiments. Here, we use first-principles computations and more specifically a Koopmans' compliant hybrid functional and charge correction to study small hole polarons in bulk rutile.

Phenomenology of Intermediate Molecular Dynamics at Metal-Oxide Interfaces.

Reaction intermediates buried within a solid-liquid interface are difficult targets for physiochemical measurements. They are inherently molecular and locally dynamic, while their surroundings are extended by a periodic lattice on one side and the solvent dielectric on the other. Challenges compound on a metal-oxide surface of varied sites and especially so at its aqueous interface of many prominent reactions.

Formation of the oxyl's potential energy surface by the spectral kinetics of a vibrational mode.

One of the most reactive intermediates for oxidative reactions is the oxyl radical, an electron-deficient oxygen atom. The discovery of a new vibration upon photoexcitation of the oxygen evolution catalysis detected the oxyl radical at the SrTiO3 surface. The vibration was assigned to a motion of the sub-surface oxygen underneath the titanium oxyl (Ti-O●-) created upon hole transfer to (or electron extraction from) a hydroxylated surface site.

Free energy difference to create the M-OH intermediate of the oxygen evolution reaction by time-resolved optical spectroscopy.

Theoretical descriptors differentiate the catalytic activity of materials for the oxygen evolution reaction by the strength of oxygen binding in the reactive intermediate created upon electron transfer. Recently, time-resolved spectroscopy of a photo-electrochemically driven oxygen evolution reaction followed the vibrational and optical spectra of this intermediate, denoted M-OH. However, these inherently kinetic experiments have not been connected to the relevant thermodynamic quantities.

Coherent Acoustic Interferometry during the Photodriven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate (Ti-OH*).

The oxygen evolution reaction (OER) from water requires the formation of metastable, reactive oxygen intermediates to enable oxygen-oxygen bond formation. Conversely, such reactive intermediates could also structurally modify the catalyst. A descriptor for the overall catalytic activity, the first electron and proton transfer OER intermediate from water, (M-OH*), has been associated with significant distortions of the metal-oxygen bonds upon charge-trapping.

The electron-transfer intermediates of the oxygen evolution reaction (OER) as polarons by spectroscopy.

The conversion of diffusive forms of energy (electrical and light) into short, compact chemical bonds by catalytic reactions regularly involves moving a carrier from an environment that favors delocalization to one that favors localization. While delocalization lowers the energy of the carrier through its kinetic energy, localization creates a polarization around the carrier that traps it in a potential energy minimum. The trapped carrier and its local distortion-termed a polaron in solids-can play a role as a highly reactive intermediate within energy-storing catalytic reactions but is rarely discussed as such.

Detecting the Photoexcited Carrier Distribution Across GaAs/Transition Metal Oxide Interfaces by Coherent Longitudinal Acoustic Phonons.

A prominent architecture for solar energy conversion layers diverse materials, such as traditional semiconductors (Si, III-V) and transition metal oxides (TMOs), into a monolithic device. The efficiency with which photoexcited carriers cross each layer is critical to device performance and dependent on the electronic properties of a heterojunction. Here, by time-resolved changes in the reflectivity after excitation of an n-GaAs/p-GaAs/TMO (CoO, IrO) device, we detect a photoexcited carrier distribution specific to the p-GaAs/TMO interface through its coupling to phonons in both materials.

The Formation Time of Ti-O and Ti-O-Ti Radicals at the n-SrTiO/Aqueous Interface during Photocatalytic Water Oxidation.

The initial step of photocatalytic water oxidation reaction at the metal oxide/aqueous interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO these one-electron intermediates are radicals located in Ti-O (oxyl) and Ti-O-Ti (bridge) groups arranged perpendicular and parallel to the surface respectively, and form electronic states in the band gap of SrTiO. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band.

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts.

X-ray absorption studies of the geometric and electronic structure of primarily heterogeneous Co, Ni, and Mn based water oxidation catalysts are reviewed. The X-ray absorption near edge and extended X-ray absorption fine structure studies of the metal K-edge, characterize the metal oxidation state, metal-oxygen bond distance, metal-metal distance, and degree of disorder of the catalysts. These properties guide the coordination environment of the transition metal oxide radical that localizes surface holes and is required to oxidize water.

Distorted Tetrahedral Co in KH[CoWO]·xHO Probed by 2p3d Resonant Inelastic X-ray Scattering.

The Co 2p X-ray absorption spectroscopy and high-energy-resolution (∼0.09 eV fwhm) 2p3d resonant inelastic X-ray scattering (RIXS) spectra of the single-cobalt-centered polyoxometalate KH[CoWO]·xHO were measured. The low-energy dd transition features at 0.

Detecting the oxyl radical of photocatalytic water oxidation at an n-SrTiO3/aqueous interface through its subsurface vibration.

Although the water oxidation cycle involves the critical step of O-O bond formation, the transition metal oxide radical thought to be the catalytic intermediate for this step has eluded direct observation. The radical represents the transformation of charge into a nascent catalytic intermediate, which lacks a newly formed bond and is therefore inherently difficult to detect. Here, using theoretical calculations and ultrafast in situ infrared spectroscopy of photocatalysis at an n-SrTiO3/aqueous interface, we reveal a subsurface vibration of the oxygen directly below, and uniquely generated by, the oxyl radical (Ti-O(•)).

How surface potential determines the kinetics of the first hole transfer of photocatalytic water oxidation.

Interfacial hole transfer between n-SrTiO3 and OH(-) was investigated by surface sensitive transient optical spectroscopy of an in situ photoelectrochemical cell during water oxidation. The kinetics reveal a single rate constant with an exponential dependence on the surface hole potential, spanning time scales from 3 ns to 8 ps over a ≈1 V increase. A voltage- and laser illumination-induced process moves the valence band edge at the n-type semiconductor/water interface to continuously change the surface hole potential.