Electrical, Photoelectrochemical, and Photoelectron Spectroscopic Investigation of the Interfacial Transport and Energetics of Amorphous TiO₂/Si Heterojunctions

Solid-state electrical, photoelectrochemical, and photoelectron spectroscopic techniques have been used to characterize the behavior and electronic structure of interfaces between n-Si, n⁺-Si, or p⁺-Si surfaces and amorphous coatings of TiO₂ formed using atomic-layer deposition. Photoelectrochemical measurements of n-Si/TiO₂/Ni interfaces in contact with a series of one-electron, electrochemically reversible redox systems indicated that the n-Si/TiO₂/Ni structure acted as a buried junction whose photovoltage was independent of the formal potential of the contacting electrolyte. Solid-state current–voltage analysis indicated that the built-in voltage of the n-Si/TiO₂ heterojunction was ∼0.7 V, with an effective Richardson constant ∼1/100th of the value of typical Si/metal Schottky barriers. X-ray photoelectron spectroscopic data allowed formulation of energy band-diagrams for the n-Si/TiO₂, n⁺-Si/TiO₂, and p⁺-Si/TiO₂ interfaces. The XPS data were consistent with the rectifying behavior observed for amorphous TiO₂ interfaces with n-Si and n⁺-Si surfaces and with an ohmic contact at the interface between amorphous TiO₂ and p⁺-Si

Discovery and Characterization of a Pourbaix-Stable, 1.8 eV Direct Gap Bismuth Manganate Photoanode

Solar-driven oxygen evolution is a critical technology for renewably synthesizing hydrogen- and carbon-containing fuels in solar fuel generators. New photoanode materials are needed to meet efficiency and stability requirements, motivating materials explorations for semiconductors with (i) band-gap energy in the visible spectrum and (ii) stable operation in aqueous electrolyte at the electrochemical potential needed to evolve oxygen from water. Motivated by the oxygen evolution competency of many Mn-based oxides, the existence of several Bi-containing ternary oxide photoanode materials, and the variety of known oxide materials combining these elements with Sm, we explore the Bi–Mn–Sm oxide system for new photoanodes. Through the use of a ferri/ferrocyanide redox couple in high-throughput screening, BiMn₂O₅ and its alloy with Sm are identified as photoanode materials with a near-ideal optical band gap of 1.8 eV. Using density functional theory-based calculations of the mullite Bi³⁺Mn³⁺Mn⁴⁺O₅ phase, we identify electronic analogues to the well-known BiVO₄ photoanode and demonstrate excellent Pourbaix stability above the oxygen evolution Nernstian potential from pH 4.5 to 15. Our suite of experimental and computational characterization indicates that BiMn₂O₅ is a complex oxide with the necessary optical and chemical properties to be an efficient, stable solar fuel photoanode

Reduction of Aqueous CO₂ to 1‑Propanol at MoS₂ Electrodes

Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS₂ or thin-film MoS₂ electrodes yields 1-propanol as the major CO₂ reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and <i>t</i>-butanol were also produced. At an applied potential of −0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO₂ to 1-propanol were ∼3.5% for MoS₂ single crystals and ∼1% for thin films with low edge-site densities. Reduction of CO₂ to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e^– and 18 H⁺ in a process that involves the formation of 2 C–C bonds. NMR analyses using ¹³CO₂ showed the production of ¹³C-labeled 1-propanol. In all cases, the vast majority of the Faradaic current resulted in hydrogen evolution via water reduction. H₂S was detected qualitatively when single-crystal MoS₂ electrodes were used, indicating that some desulfidization of single crystals occurred under these conditions