Organic Thin Films Enable Retaining the Oxidation State of Copper Catalysts during CO2 Electroreduction.

A key challenge in electrocatalysis remains controlling a catalyst's structural, chemical, and electrical properties under reaction conditions. While organic coatings showed promise for enhancing the selectivity and stability of catalysts for CO2 electroreduction (CO2RR), their impact on the chemical state of underlying metal electrodes has remained unclear. In this study, we show that organic thin films on polycrystalline copper (Cu) enable retaining Cu+ species at reducing potentials down to -1.0 V vs RHE, as evidenced by operando Raman and quasi in situ X-ray photoelectron spectroscopy. In situ electrochemical atomic force microscopy revealed the integrity of the porous organic film and nearly unaltered Cu electrode morphology. While the pristine thin film enhances the CO2-to-ethylene conversion, the addition of organic modifiers into electrolytes gives rise to improved CO2RR performance stability. Our findings showcase hybrid metal-organic systems as a versatile approach to control, beyond morphology and local environment, the oxidation states of catalysts and energy conversion materials.

Nanoscale Electron Transfer Variations at Electrocatalyst-Electrolyte Interfaces Resolved by in Situ Conductive Atomic Force Microscopy.

Rational innovation of electrocatalysts requires detailed knowledge of spatial property variations across the solid-electrolyte interface. We introduce correlative atomic force microscopy (AFM) to simultaneously probe, in situ and at the nanoscale, electrical conductivity, chemical-frictional, and morphological properties of a bimetallic copper-gold system for CO2 electroreduction. In air, water, and bicarbonate electrolyte, current-voltage curves reveal resistive CuOx islands in line with local current contrasts, while frictional imaging indicates qualitative variations in the hydration layer molecular ordering upon change from water to electrolyte. Nanoscale current contrast on polycrystalline Au shows resistive grain boundaries and electrocatalytically passive adlayer regions. In situ conductive AFM imaging in water shows mesoscale regions of low current and reveals that reduced interfacial electric currents are accompanied by increased friction forces, thus indicating variations in the interfacial molecular ordering affected by the electrolyte composition and ionic species. These findings provide insights into how local electrochemical environments and adsorbed species affect interfacial charge transfer processes and support building in situ structure-property relationships in catalysis and energy conversion research.

Potential-Dependent Morphology of Copper Catalysts During CO2 Electroreduction Revealed by In†Situ Atomic Force Microscopy.

Electrochemical AFM is a powerful tool for the real-space characterization of catalysts under realistic electrochemical CO2 reduction (CO2 RR) conditions. The evolution of structural features ranging from the micrometer to the atomic scale could be resolved during CO2 RR. 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 CO2 RR in 0.1 m KHCO3 . The density of undercoordinated copper sites during CO2 RR 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 CO2 RR catalysts.

Intrinsic anion diffusivity in lead halide perovskites is facilitated by a soft lattice.

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.

Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation.

Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption.

Strongly Quantum Confined Colloidal Cesium Tin Iodide Perovskite Nanoplates: Lessons for Reducing Defect Density and Improving Stability.

Within the last several years, metal halide perovskites such as methylammonium lead iodide, CH3NH3PbI3, 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, (CsSnI3), perovskite nanoplates have been synthesized and characterized for the first time. These CsSnI3 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 CsSnI3 do not introduce deep levels inside the band gap, their concentration can be quite high. These simulations also highlight that synthesizing and processing CsSnI3 in Sn-rich conditions can reduce defect density and increase stability, which matches insights gained experimentally. This improvement in the understanding of CsSnI3 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.

Thermochromic halide perovskite solar cells.

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.

Copper nanoparticle ensembles for selective electroreduction of CO2 to C2-C3 products.

Direct conversion of carbon dioxide to multicarbon products remains as a grand challenge in electrochemical CO2 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 C2-C3 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 n-propanol are the major C2-C3 products with onset potential at -0.53 V (vs. reversible hydrogen electrode, RHE) and C2-C3 faradaic efficiency (FE) reaching 50% at only -0.75 V. Thus, the catalyst exhibits selective generation of C2-C3 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 CO2, 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 C2-C3 current density 10 mA/cm2 (at -0.75 V), rendering it attractive for solar-to-fuel applications. Tafel analysis suggests reductive CO coupling as a rate determining step for C2 products, while n-propanol (C3) production seems to have a discrete pathway.

Spatially resolved multicolor CsPbX3 nanowire heterojunctions via anion exchange.

Halide perovskites are promising semiconductor materials for solution-processed 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 CsPbX3 (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.

Metal-organic frameworks for electrocatalytic reduction of carbon dioxide.

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.

TiO2 anatase with a bandgap in the visible region.

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.

Convergent and divergent two-dimensional coordination networks formed through substrate-activated or quenched alkynyl ligation.

Metal coordination assemblies of the symmetric bi-functional 4,4'-di-(1,4-buta-1,3-diynyl)-benzoic acid are investigated by scanning tunnelling microscopy on metal surfaces. The formation of long-range ordered, short-range disordered and random phases depends on the competition between the convergent and divergent coordination motifs of the individual functional groups and is crucially influenced by the substrate.

Atomic-scale observation of multiconformational binding and energy level alignment of ruthenium-based photosensitizers on TiO2 anatase.

Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dye's energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO2, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence. Here we demonstrate multiconformational adsorption and energy level alignment of single N3 dyes on TiO2 anatase (101) revealed by scanning tunnelling microscopy and spectroscopy. The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection. These findings emphasize the critical role of the interfacial coupling and suggest that further designs of dye-sensitized solar cells should target a higher selectivity in the dye-substrate binding conformations in order to ensure efficient electron injection from all photosensitizers.

Highly adaptable two-dimensional metal-organic coordination networks on metal surfaces.

The formation of extended two-dimensional metal-organic coordination networks (2D-MOCNs) showing high adaptability to surface step edges and structural defects is revealed by scanning tunneling microscopy. Rod-like 4,4'-di-(1,4-buta-1,3-diynyl)-benzoic acid (BDBA) and iron atoms assemble into extended 2D-MOCNs on Au(111) and Ag(100) surfaces. Independent from the chosen substrate and its surface symmetry the MOCN grows continuously over multiple surface terraces through mutual in-phase structure adaptation of network domains at step edges as well as on terraces. The adaptability of the MOCNs is mainly ascribed to the high degree of conformational flexibility of the butadiynyl functionality of the ligand. Despite their flexibility, the MOCNs exhibit considerable robustness against annealing at high temperatures. The findings show that mesoscale self-assembled functional architectures with a high degree of substrate error tolerance can be realized with metal coordination networks.