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New methods are needed to increase the activity and stability of earth-abundant catalysts for electrochemical water splitting to produce hydrogen fuel. Electrodeposition has been previously used to synthesize manganese oxide films with a high degree of disorder and a mixture of oxidation states for Mn, which has led to electrocatalysts with high activity but low stability for the oxygen evolution reaction (OER) at high current densities. In this report, we show that multipotential electrodeposition of manganese oxide under illumination produces nanostructured films with significantly higher stability for the OER compared to films grown under otherwise identical conditions in the dark. Manganese oxide films grown by multipotential deposition under illumination sustain a current density of 10 mA/cm2 at 2.2 V vs. RHE for 18 hours (pH 13). Illumination does not enhance the activity or stability of manganese oxide films grown using a constant potential, and films grown by multipotential deposition in the dark undergo a complete loss of activity within one hour of electrolysis. Electrochemical and structural characterization indicate that photoexcitation of the films during growth reduces Mn ions and changes the content and structure of intercalated potassium ions and water molecules in between disorder layers of birnessite-like sheets of MnOx, which stabilizes the nanostructured film during electrocatalysis. These results demonstrate that combining multiple external stimuli (i.e., light and an external potential) can induce structural changes not attainable by either stimulus alone to make earth-abundant catalysts more active and stable for important chemical transformations such as water oxidation. 

Materials with metastable phases can exhibit vastly different properties from their thermodynamically favored counterparts. Methods to synthesize metastable phases without the need for high-temperature or high-pressure conditions would facilitate their widespread use. We report on the electrochemical growth of microcrystals of bismuth selenide, Bi2Se3, in the metastable orthorhombic phase at room temperature in aqueous solution. Rather than direct epitaxy with the growth substrate, the spontaneous formation of a seed layer containing nanocrystals of cubic BiSe enforces the metastable phase. We first used single-crystal silicon substrates with a range of resistivities and different orientations to identify the conditions needed to produce the metastable phase. When the applied potential during electrochemical growth is positive of the reduction potential of Bi3+, an initial, Bi-rich seed layer forms. Electron microscopy imaging and diffraction reveal that the seed layer consists of nanocrystals of cubic BiSe embedded within an amorphous matrix of Bi and Se. Using density functional theory calculations, we show that epitaxial matching between cubic BiSe and orthorhombic Bi2Se3 can help stabilize the metastable orthorhombic phase over the thermodynamically stable rhombohedral phase. The spontaneous formation of the seed layer enables us to grow orthorhombic Bi2Se3 on a variety of substrates including single-crystal silicon with different orientations, polycrystalline fluorine-doped tin oxide, and polycrystalline gold. The ability to stabilize the metastable phase through room-temperature electrodeposition in aqueous solution without requiring a single-crystal substrate broadens the range of applications for this semiconductor in optoelectronic and electrochemical devices. 

Abstract As one of the most fundamental physical phenomena, charge density wave (CDW) order predominantly occurs in metallic systems such as quasi‐one‐dimensional (quasi‐1D) metals, doped cuprates, and transition metal dichalcogenides, where it is well understood in terms of Fermi surface nesting and electron‐phonon coupling mechanisms. On the other hand, CDW phenomena in semiconducting systems, particularly at the low carrier concentration limit, are less common and feature intricate characteristics, which often necessitate the exploration of novel mechanisms, such as electron‐hole coupling or Mott physics, to explain. In this study, we combined electrical transport, synchrotron X‐ray diffraction and density‐functional theory (DFT) calculations to investigate CDW order and a series of hysteretic phase transitions in a dilute d ‐band semiconductor, BaTiS 3 . Our experimental and theoretical findings suggest that the observed CDW order and phase transitions in BaTiS 3 may be attributed to both electron‐phonon coupling and non‐negligible electron‐electron interactions in the system. Our work highlights BaTiS 3 as a unique platform to explore CDW physics and novel electronic phases in the dilute filling limit and could open new opportunities for developing novel electronic devices. This article is protected by copyright. All rights reserved 

Oxygen vacancies in semiconductor photocatalysts play several competing roles, serving to both enhance light absorption and charge separation of photoexcited carriers as well as act as recombination centers for their deactivation. In this Letter, we show that single-molecule fluorescence imaging of a chemically activated fluorogenic probe can be used to monitor changes in the photocatalytic activity of bismuth oxybromide (BiOBr) nanoplates in situ during the light-induced formation of oxygen vacancies. We observe that the specific activities of individual nanoplates for the photocatalytic reduction of resazurin first increase and then progressively decrease under continuous laser irradiation. Ensemble structural characterization, supported by electronic-structure calculations, shows that irradiation increases the concentration of surface oxygen vacancies in the nanoplates, reduces Bi ions, and creates donor defect levels within the band gap of the semiconductor particles. These combined changes first enhance photocatalytic activity by increasing light absorption at visible wavelengths. However, high concentrations of oxygen vacancies lower the photocatalytic activity both by introducing new relaxation pathways that promote charge recombination before photoexcited electrons can be extracted and by weakening binding of resazurin to the surface of the nanoplates.