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Broadband absorption in the visible spectrum is essential in optoelectronic applications that involve power conversion such as photovoltaics and photocatalysis. Most ultrathin broadband absorbers use parasitic plasmonic structures that maximize absorption using surface plasmons and/or Fabry–Perot cavities, which limits the weight efficiency of the device. Here, we show the theoretical and experimental realization of an unpatterned/planar semiconductor thin-film absorber based on monolayer transition-metal dichalcogenides. We experimentally demonstrate an average total absorption in the visible range (450–700 nm) of >70% using <4 nm of semiconductor absorbing materials scalable over large areas with vapor phase growth techniques. Our analysis suggests that a power conversion efficiency of 15.54% and a specific power >300 W g–1may be achieved in a photovoltaic cell based on this metamaterial absorber.

The thermodynamic and structural properties of 1 nm thick films of CeVO4and LaVO4supported on γ-Al2O3were examined and compared to the corresponding bulk rare-earth vanadates. The films were prepared by atomic layer deposition (ALD) on high-surface-area powders and then examined after either oxidation or reduction at 1073 K. Both CeVO4and LaVO4uniformly covered the γ-Al2O3in the form of two-dimensional crystallites. Thin-film LaVO4reversibly transitioned between a perovskite structure upon reduction to a monazite structure upon oxidation, similar to what is observed for the bulk material; however, coulometric titration (CT) showed that the P(O2) at which equilibrium is established between these two phases was 7 orders of magnitude higher for the thin-film material. Thin-film CeVO4also formed a perovskite phase upon reduction but reversibly transitioned to a fluorite phase upon oxidation rather than the zircon phase, which is formed for bulk CeVO4. Multiple techniques showed that the oxidation states of both Ce and V in the thin-film CeVO4were +4, which is also different from bulk CeVO4. Reasons for why the properties of the thin-film, rare-earth vanadates differ from their bulk counterparts are discussed.

Electron microscopy studies on stannous oxide (α-SnO), a metastable two-dimensional layered material, have revealed exciting aspects of the structure and phase stability in the tin–oxygen system. Single-crystalline SnO sheets with an “inverse-pyramid” morphology (thickness of ∼30 nm and lateral dimension in the micron scale) have been synthesized using a simple wet chemical technique. Electron diffraction from the sheets reveals a secondary metastable phase, namely, o-SnO2, to be present coherently with the single-crystalline SnO. The orientation relationship between the two phases and their variants has been investigated. Atomic-resolution imaging reveals the presence of the coherent secondary o-SnO2phase and its two rotational variants on the surface. A detailed cross-sectional analysis of the sheets reveals that the SnO phase is completely encapsulated by a thin layer of the metastable orthorhombic phase. The metastable SnO phase is thus stabilized by the presence of the encapsulating o-SnO2on its surface; the metastable o-SnO2phase itself is stabilized by the underlying SnO substrate. The temperature dependence of resistance, I–Vcharacteristics, and the Schottky barrier height of the lithographically formed SnO/Cr–Au contact on the single crystals have been studied. The transport mechanism is found to be governed by the variable-range hopping (VRH) process. The present study clarifies several unexplained features of the reported microstructures in the SnO system and provides some new insights into the mutual stabilization of metastable phases in the oxides of Sn that could be applicable for other systems as well.

Development of highly efficient, earth-abundant, and stable bifunctional electrocatalysts is pivotal for designing viable next-generation metal–air batteries. Cobalt-based phosphates provide a treasure house to design electrocatalysts, with a wide range of cation substitutions to further enhance their electrocatalytic activity. In particular, phosphates with distorted geometry show favorable binding efficiency toward water molecules with low overpotential. In the present work, zinc-substituted cobalt phosphate ZnCo2(PO4)2was investigated. Its crystal structure was solved to a monoclinic framework built with CoO6octahedra and distorted CoO5/ZnO5trigonal bipyramid leading to efficient bifunctional electrocatalytic activity. It offers robust structural stability with onset potential values of 0.87 V (vs reversible hydrogen electrode (RHE)) and 1.50 V (vs RHE) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, respectively, comparable to the precious metal catalysts. The origin and stability of the bifunctional activity were probed by combining ex situ diffraction and electron microscopy corroborated by ab initiocalculations. Overall, zinc-substituted cobalt phosphate [ZnCo2(PO4)2] forms a potential bifunctional electrocatalyst with tunable local cobalt coordination that can be harnessed for metal–air batteries.

Economic and efficient bifunctional electrocatalysts are pivotal in realization of rechargeable (hybrid) metal-air batteries. It is ideal to employ noble-metal free bifunctional electrocatalysts that are not selective towards oxygen evolution and reduction (OER and ORR) activities. This work unveils cobalt-based tetraphosphate K2Co(PO3)4as an economic bifunctional electrocatalyst acting as cathode for rechargeable hybrid sodium-air batteries. Autocombustion route led to the development of homogeneous, carbon-coated, spherical K2Co(PO3)4nanoparticles enabling active site exposure to incoming guest molecules (O2, OH-). This monoclinic compound exhibited superior oxygen evolution activity with low overpotential (ca. 0.32 V) surpassing the commercial RuO2catalyst. Tetraphosphate K2Co(PO3)4was successfully implemented in hybrid Na-air batteries delivering reversible cycling with roundtrip efficiency over 70%. DFT study revealed this catalytic activity stem from the most active and stable surface (001) and half-metallic nature of Co in K2Co(PO3)4. Cobalt tetraphosphates can be harnessed to design low cost electrocatalysts for hybrid sodium-air batteries.