Search for: All records

Mixed metal oxyhalides are an exciting class of photocatalysts, capable of the sustainable generation of fuels and remediation of pollutants with solar energy. Bismuth oxyhalides of the types Bi4MO8X (M = Nb and Ta; X = Cl and Br) and Bi2AO4X (A = most lanthanides; X = Cl, Br, and I) have an electronic structure that imparts photostability, as their valence band maxima (VBM) are composed of O 2p orbitals rather than X np orbitals that typify many other bismuth oxyhalides. Here, flux-based synthesis of intergrowth Bi4NbO8Cl–Bi2GdO4Cl is reported, testing the hypothesis that both intergrowth stoichiometry and M identity serve as levers toward tunable optoelectronic properties. X-ray scattering and atomically resolved electron microscopy verify intergrowth formation. Facile manipulation of the Bi4NbO8Cl-to-Bi2GdO4Cl ratio is achieved with the specific ratio influencing both the crystal and electronic structures of the intergrowths. This compositional flexibility and crystal structure engineering can be leveraged for photocatalytic applications, with comparisons to the previously reported Bi4TaO8Cl–Bi2GdO4Cl intergrowth revealing how subtle structural and compositional features can impact photocatalytic materials. 

Abstract Material properties strongly depend on the nature and concentration of defects. Characterizing these features may require nano- to atomic-scale resolution to establish structure–property relationships. 4D-STEM, a technique where diffraction patterns are acquired at a grid of points on the sample, provides a versatile method for highlighting defects. Computational analysis of the diffraction patterns with virtual detectors produces images that can map material properties. Here, using multislice simulations, we explore different virtual detectors that can be applied to the diffraction patterns that go beyond the binary response functions that are possible using ordinary STEM detectors. Using graphene and lead titanate as model systems, we investigate the application of virtual detectors to study local order and in particular defects. We find that using a small convergence angle with a rotationally varying detector most efficiently highlights defect signals. With experimental graphene data, we demonstrate the effectiveness of these detectors in characterizing atomic features, including vacancies, as suggested in simulations. Phase and amplitude modification of the electron beam provides another process handle to change image contrast in a 4D-STEM experiment. We demonstrate how tailored electron beams can enhance signals from short-range order and how a vortex beam can be used to characterize local symmetry. 

Abstract Transition metal dichalcogenides (TMDs) are known for their layered structure and tunable functional properties. However, a unified understanding on other transition metal chalcogenides (i.e. M 2 X) is still lacking. Here, the relatively new class of copper-based chalcogenides Cu 2 X (X = Te, Se, S) is thoroughly reported. Cu 2 X are synthesized by an unusual vapor–liquid assisted growth on a Al 2 O 3 /Cu/W stack. Liquid copper plays a significant role in synthesizing these layered systems, and sapphire assists with lateral growth and exfoliation. Similar to traditional TMDs, thickness dependent phonon signatures are observed, and high-resolution atomic images reveal the single phase Cu 2 Te that prefers to grow in lattice-matched layers. Charge transport measurements indicate a metallic nature at room temperature with a transition to a semiconducting nature at low temperatures accompanied by a phase transition, in agreement with band structure calculations. These findings establish a fundamental understanding and thrust Cu 2 Te as a flexible candidate for wide applications from photovoltaics and sensors to nanoelectronics. 

Abstract Chemical vapor deposition growth of metal carbides is of great interest as this method provides large area growth of MXenes. This growth is mainly done using a melted diffusion based process; however, different morphologies in growth process is not well understood. In this work, we report deterministic synthesis of layered (non-uniform c -axis growth) and planar (uniform c -axis growth) of molybdenum carbide (Mo 2 C) using a diffusion-mediated growth. Mo-diffusion limited growth mechanism is proposed where the competition between Mo and C adatoms determines the morphology of grown crystals. Difference in thickness of catalyst at the edge and center lead to enhanced Mo diffusion which plays a vital role in determining the structure of Mo 2 C. The layered structures exhibit an expansion in the lattice confirmed by the presence of strain. Density functional theory shows consistent presence of strain which is dependent upon Mo diffusion during growth. This work demonstrates the importance of precise control of diffusion through the catalyst in determining the structure of Mo 2 C and contributes to broader understanding of metal diffusion in growth of MXenes. 

Aqueous phosphate pollution can dramatically impact ecosystems, introducing a variety of environmental, economic, and public health problems. While novel remediation tactics based on nanoparticle binding have shown considerable promise in nutrient recovery from water, they are challenging to deploy at scale. To bridge the gap between the laboratory-scale nature of these nanostructure solutions and the practical benchmarks for deploying an environmental remediation tool, we have developed a nanocomposite material. Here, an economical, readily available, porous substrate is dip coated using scalable, water-based processes with a slurry of nanostructures. These nanomaterials have tailored affinity for specific adsorption of pollutants. Our Phosphate Elimination and Recovery Lightweight (PEARL) membrane can selectively sequester up to 99% of phosphate ions from polluted waters at environmentally relevant concentrations. Moreover, mild tuning of pH promotes at will adsorption and desorption of nutrients. This timed release allows for phosphate recovery and reuse of the PEARL membrane repeatedly for numerous cycles. We combine correlative microscopy and spectroscopy techniques to characterize the complex microstructure of the PEARL membrane and to unravel the mechanism of phosphate sorption. More broadly, through the example of phosphate pollution, this work describes a platform membrane approach based on nanostructures with specific affinity coated on a porous structure. Such a strategy can be tuned to address other environmental remediation challenges through the incorporation of other nanomaterials.