A key challenge in the field of electrochemical carbon dioxide reduction is the design of catalytic materials featuring high product selectivity, stability, and a composition of earth-abundant elements. In this work, we introduce thin films of nanosized metal-organic frameworks (MOFs) as atomically defined and nanoscopic materials that function as catalysts for the efficient and selective reduction of carbon dioxide to carbon monoxide in aqueous electrolytes. Detailed examination of a cobalt-porphyrin MOF, Al2(OH)2TCPP-Co (TCPP-H2 = 4,4',4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrabenzoate) revealed a selectivity for CO production in excess of 76% and stability over 7 h with a per-site turnover number (TON) of 1400. In situ spectroelectrochemical measurements provided insights into the cobalt oxidation state during the course of reaction and showed that the majority of catalytic centers in this MOF are redox-accessible where Co(II) is reduced to Co(I) during catalysis.

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.

TiO2 anatase plays a central role in energy and environmental research. A major bottleneck toward developing artificial photosynthesis with TiO2 is that it only absorbs ultraviolet light, owing to its large bandgap of 3.2 eV. If one could reduce the bandgap of anatase to the visible region, TiO2-based photocatalysis could become a competitive clean energy source. Here, using scanning tunneling microscopy and spectroscopy in conjunction with density functional theory calculations, we report the discovery of a highly reactive titanium-terminated anatase surface with a reduced bandgap of less than 2 eV, stretching into the red portion of the solar spectrum. By tuning the surface preparation conditions, we can reversibly switch between the standard anatase surface and the newly discovered low bandgap surface phase. The identification of a TiO2 anatase surface phase with a bandgap in the visible and high chemical reactivity has important implications for solar energy conversion, photocatalysis, and artificial photosynthesis.

Direct conversion of carbon dioxide to multicarbon products remains as a grand challenge in electrochemical CO reduction. Various forms of oxidized copper have been demonstrated as electrocatalysts that still require large overpotentials. Here, we show that an ensemble of Cu nanoparticles (NPs) enables selective formation of C-C products at low overpotentials. Densely packed Cu NP ensembles underwent structural transformation during electrolysis into electrocatalytically active cube-like particles intermixed with smaller nanoparticles. Ethylene, ethanol, and -propanol are the major C-C products with onset potential at -0.53 V (vs. reversible hydrogen electrode, RHE) and C-C faradaic efficiency (FE) reaching 50% at only -0.75 V. Thus, the catalyst exhibits selective generation of C-C hydrocarbons and oxygenates at considerably lowered overpotentials in neutral pH aqueous media. In addition, this approach suggests new opportunities in realizing multicarbon product formation from CO, where the majority of efforts has been to use oxidized copper-based materials. Robust catalytic performance is demonstrated by 10 h of stable operation with C-C current density 10 mA/cm (at -0.75 V), rendering it attractive for solar-to-fuel applications. Tafel analysis suggests reductive CO coupling as a rate determining step for C products, while -propanol (C) production seems to have a discrete pathway.

Halide perovskites are promising semiconductor materials for solutionprocessed optoelectronic devices. Their strong ionic bonding nature results in highly dynamic crystal lattices, inherently allowing rapid ion exchange at the solid-vapor and solid-liquid interface. Here, we show that the anion-exchange chemistry can be precisely controlled in single-crystalline halide perovskite nanomaterials when combined with nanofabrication techniques. We demonstrate spatially resolved multicolor CsPbX 3 (X = Cl, Br, I, or alloy of two halides) nanowire heterojunctions with a pixel size down to 500 nm with the photoluminescence tunable over the entire visible spectrum. In addition, the heterojunctions show distinct electronic states across the interface, as revealed by Kelvin probe force microscopy. These perovskite heterojunctions represent key building blocks for high-resolution multicolor displays beyond current state-of-the-art technology as well as high-density diode/transistor arrays.nanowire | halide perovskite | anion exchange | heterojunction S ignificant research efforts are currently directed toward lead halide-based perovskites, owing to their unusual optoelectronic and photovoltaic properties (1-5). In addition to polycrystalline thin films, various solution-based synthetic routes toward low-dimensional nanostructures of halide perovskites have been recently demonstrated, with control over size, shape, mixed halide composition, and consequently their band gap and emission wavelength (6-10). Interestingly, it has been shown that these materials, both in bulk and in nanocrystalline form, can undergo fast anion-exchange reactions at the solid-liquid or solid-gas interface, with a fine-tuning of the chemical composition and electronic properties (11)(12)(13)(14). The fast ion-exchange kinetics in halide perovskite are related to the low defect formation energy and the existence of a large number of vacancies, which make the ions highly mobile in the crystal lattice (15). As a result, the anionexchange reaction in halide perovskites favors forming homogeneous alloys instead of any kind of heterostructures as commonly observed in II-VI semiconductor compounds. If the exchange reaction can be localized at particular positions, then it is possible to produce substrates with well-defined patterns of semiconductor heterojunctions (16)(17)(18)(19)(20). The physical properties (optical, electrical, magnetic, etc.) of the heterostructure are fundamentally interesting, and the patterned semiconductor heterojunctions are essential for the fabrication of large-scale high-density integrated electronic and photonic devices.Compared with perovskite polycrystalline thin films and quantum dots, single-crystalline nanowires provide an ideal platform for producing and studying heterojunctions via ion-exchange chemistry because of the absence of grain boundaries and the unique one-dimensional geometry (21-24). The relatively thin diameter of the nanowire ensures rapid ion exchange in the radial direction, whereas the micrometer scale l...

Facile ionic transport in lead halide perovskites plays a critical role in device performance. Understanding the microscopic origins of high ionic conductivities has been complicated by indirect measurements and sample microstructural heterogeneities. Here, we report the direct visualization of halide anion interdiffusion in CsPbCl3–CsPbBr3 single crystalline perovskite nanowire heterojunctions using wide-field and confocal photoluminescence measurements. The combination of nanoscale imaging techniques with these single crystalline materials allows us to measure intrinsic anionic lattice diffusivities, free from complications of microscale inhomogeneity. Halide diffusivities were found to be between 10−13 and ∼10−12 cm2/second at about 100 °C, which are several orders of magnitudes lower than those reported in polycrystalline thin films. Spatially resolved photoluminescence lifetimes and surface potential measurements provide evidence of the central role of halide vacancies in facilitating ionic diffusion. Vacancy formation free energies computed from molecular simulation are small due to the easily deformable perovskite lattice, accounting for the high equilibrium vacancy concentration. Furthermore, molecular simulations suggest that ionic motion is facilitated by low-frequency lattice modes, resulting in low activation barriers for vacancy-mediated transport. This work elucidates the intrinsic solid-state ion diffusion mechanisms in this class of semisoft materials and offers guidelines for engineering materials with long-term stability in functional devices.

Within the last several years, metal halide perovskites such as methylammonium lead iodide, CHNHPbI, have come to the forefront of scientific investigation as defect-tolerant, solution-processable semiconductors that exhibit excellent optoelectronic properties. The vast majority of study has focused on Pb-based perovskites, which have limited applications because of their inherent toxicity. To enable the broad application of these materials, the properties of lead-free halide perovskites must be explored. Here, two-dimensional, lead-free cesium tin iodide, (CsSnI), perovskite nanoplates have been synthesized and characterized for the first time. These CsSnI nanoplates exhibit thicknesses of less than 4 nm and exhibit significant quantum confinement with photoluminescence at 1.59 eV compared to 1.3 eV in the bulk. Ab initio calculations employing the generalized gradient approximation of Perdew-Burke-Ernzerhof elucidate that although the dominant intrinsic defects in CsSnI do not introduce deep levels inside the band gap, their concentration can be quite high. These simulations also highlight that synthesizing and processing CsSnI in Sn-rich conditions can reduce defect density and increase stability, which matches insights gained experimentally. This improvement in the understanding of CsSnI represents a step toward the broader challenge of building a deeper understanding of Sn-based halide perovskites and developing design principles that will lead to their successful application in optoelectronic devices.

Electrochemical AFM is a powerful tool for the real‐space characterization of catalysts under realistic electrochemical CO2 reduction (CO2RR) conditions. The evolution of structural features ranging from the micrometer to the atomic scale could be resolved during CO2RR. Using Cu(100) as model surface, distinct nanoscale surface morphologies and their potential‐dependent transformations from granular to smoothly curved mound‐pit surfaces or structures with rectangular terraces are revealed during CO2RR in 0.1 m KHCO3. The density of undercoordinated copper sites during CO2RR is shown to increase with decreasing potential. In situ atomic‐scale imaging reveals specific adsorption occurring at distinct cathodic potentials impacting the observed catalyst structure. These results show the complex interrelation of the morphology, structure, defect density, applied potential, and electrolyte in copper CO2RR catalysts.