Combinatorial Discovery of Lanthanum–Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuel Applications

Abstract

Abstract:Oxynitrides with the photoelectrochemical stability of oxides and desirable band energetics of nitrides comprise a promising class of materials for solar photochemistry. Challenges in synthesizing a wide variety of oxynitride materials has limited exploration of this class of functional materials, which we address using a reactive co-sputtering combined with rapid thermal processing method to synthesize multi-cation-multi-anion libraries. We demonstrate the synthesis of a La x Ta 1-x O y N z thin film composition spread library and its characterization by both traditional thin film materials characterization and custom combinatorial optical spectroscopy and X-ray Absorption Near Edge Spectroscopy (XANES) techniques, ultimately establishing structure-chemistry-property relationships. We observe that over a substantial La-Ta composition range the thin films crystallize in the same perovskite LaTaON 2 structure with significant variation of anion chemistry. The relative invariance in optical band gap demonstrates a remarkable decoupling of composition and band energetics so that the composition can be optimized while retaining the desirable 2 eV band gap energy. We also demonstrate the intercalation of diatomic nitrogen into the La 3 TaO 7 structure, which gives rise to a direct-allowed optical transition at 2.2 eV, less than half the value of the oxide's band gap. These findings motivate further exploration of the visible light response of this material that is predicted to be stable over a wide range of electrochemical potential.

Combinatorial Synthesis of Oxysulfides in the Lanthanum–Bismuth-Copper System

“…The RTP in Ar atmosphere is expected to be sufficiently short in duration to mitigate reactions between the bulk of the thin film and the atmosphere. This assumption was confirmed in our previous study of oxynitrides, , although we suspect that the near-surface will change S and/or O stoichiometry during RTP. While the limited extent of such composition changes has a small effect on XRF measured composition and negligible effect on measured XRD signal, for purposes of studying surface-sensitive properties, including photocatalysis, the beneficial or deleterious effects of heterogeneous surface layers must be evaluated.…”

Quantification of Material Fluorescence and Light Scattering Cross Sections Using Ratiometric Bandwidth-Varied Polarized Resonance Synchronous Spectroscopy

“…Light/matter interactions are the most fascinating phenomena in nature that have remained as central research topics since the inception of modern science. − Applications exploiting light/matter interactions encompass essentially every imaginable scientific discipline that ranges from molecular sciences to space exploration. − Depending on their optical properties in the UV–vis wavelength region, all materials can be divided into the following five categories: (1) approximate pure light absorbers with no significant light scattering and emission, (2) pure light scatterers with no significant light absorption and emission, (3) simultaneous light absorbers and scatterers with no significant photon emission, such as metal nanoparticles, − (4) simultaneous light absorbers and emitters with no significant photon scattering, all the way to (5) simultaneous light absorbers, scatterers, and emitters, including synthetic fluorescent nanoparticles and supramolecules . Many emerging functional materials, especially nanoscale photonic materials, are highly optically complicated because they can simultaneously absorb, scatter, and emit light in the same wavelength regions. − Reliable determination of the material photon extinction, absorption, scattering, and emission activities is critical for material characterizations, designs, and applications.…”

Functional Oxides for Photoneuromorphic Engineering: Toward a Solar Brain

“…Oxynitrides (such as La-Ta-N-O) are another family of oxides being studied as the absorber in photovoltaic systems where the introduction of nitrogen greatly results in a reduction of the oxide band gap (e.g. LaTaON 2 vs La 3 TaO 7 ) [298] . It possesses a narrow bandgap energy of 2.1 eV that allows for the absorption of up to 40% of solar spectrum.…”