Mechanistic Insights into Defect-Assisted Carrier Transport in Bismuth Vanadate Photoanodes

Understanding defect-assisted carrier transport is critical for optimizing the performance of solar water splitting devices. Here we analyze the mechanism of two distinct types of point defects, oxygen vacancies and hydrogen donors, in defining carrier transport and thus the photoelectrochemical (PEC) behavior in bismuth vanadate (BiVO4). While the conventional hydrogen annealing brings hydrogen donors as a dominant defect, we introduce a novel carbon monoxide treatment that does not introduce hydrogen but only generates more oxygen vacancies. Combined with PEC and solid-state transport characterizations, it is revealed that oxygen vacancies are more effective than hydrogen donor to improve electron transport both within BiVO4 domains and along structural boundaries, thus yielding larger front-illuminated photocurrent, larger film conductivity, and smaller polaron hopping barrier. This study provides mechanistic insights into defect engineering that can guide novel approaches to overcoming charge transport limitations in low-mobility semiconductors

Separation and Purification of Furfuryl Alcohol Monomer and Oligomers Using a Two-Phase Extracting Process

Aqueous two-phase extraction processes were applied for the first time for the separation and purification of furfuryl alcohol monomer and oligomers. Deionized water was used as the liquid–liquid extraction solvent whereas magnesium sulfate acted as the salting-out reagent. Furfuryl alcohol preferentially partitioned to the water phase and could be further extracted from its aqueous solution due to the decreased solubility in salt rich phase. Various influences, such as partition coefficient and extractability, were studied during the liquid–liquid extraction. The extraction by using deionized water resulted in a high oligomer content around ∼94 wt % in the separated furfuryl alcohol oligomer solution whereas the salting-out furfuryl alcohol showed a purity of ∼92 wt %

Anomalous Conductivity Tailored by Domain-Boundary Transport in Crystalline Bismuth Vanadate Photoanodes

Carrier transport in semiconductor photoelectrodes strongly correlates with intrinsic material characteristics including carrier mobility and diffusion length, and extrinsic structural imperfections including mobile charged defects at domain boundaries, which collectively determines the photoelectrochemistry (PEC) performance. Here we elucidate the interplay between intrinsic carrier transport, domain-boundary-induced conductivity, and PEC water oxidation in the model photoanode of bismuth vanadate (BiVO₄). In particular, epitaxial single-domain BiVO₄ and <i>c</i>-axis-oriented multidomain BiVO₄ thin films are fabricated using pulsed laser deposition to decouple the intrinsic and extrinsic carrier transport. In addition to the low intrinsic conductivity that is due to the small-polaron transport within BiVO₄ domains, we identify anomalously high electrical conductivity arising from vertical domain boundaries for multidomain BiVO₄ films. Local domain-boundary conduction compensates the inherently poor electron transport by shortening the transport distance for electrons diffused into the domain-boundary region, therefore suppressing the photocurrent difference between front and back illumination. This work provides insights into engineering carrier transport through coordinating structural domain boundaries and intrinsic material features in designing modulated water-splitting photoelectrodes

Enhancing CO Oxidation Activity <i>via</i> Tuning a Charge Transfer Between Gold Nanoparticles and Supports

Charge transfer from the supports to nanoparticles at the interface is one of the key factors to determine the catalytic performances of supported nanoparticles. In this work, we showed in a systematic way that the charge transfer from semiconductor supports to Au nanoparticle catalysts can lower the onset temperature toward CO oxidation. For this study, a novel Au/SiO2/Si composite system synthesized by the helium droplet deposition method with precisely tuned SiO2 layer thickness was fabricated to control the magnitude of interfacial charge transfer. With the support of X-ray photoelectron spectroscopy and numerical simulations, it was demonstrated that the Schottky barrier formed across the Au/SiO2/Si heterojunction led to a negative charge accumulation on the surface of Au nanoparticles. In turn, this additional charge can be transferred to the antibonding orbital of adsorbed O2 molecules to activate the O–O bonds, leading to enhanced CO oxidation. In addition to the charge transfer mechanism, the role of a strong electric field arising from the formation of the Schottky barrier was also explored to explain the observed enhancement of catalytic reactivity. Overall, this work highlights an important pathway for systematically tuning metal–support interactions to accelerate catalytic reactions and designing the next generation of nanocatalysts

Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of Methane: In Situ Identification of Ru^δ+–Ce³⁺ Interactions for Enhanced Conversion

The metal–oxide interaction changes the surface electronic states of catalysts deployed for chemical conversion, yet details of its influence on the catalytic performance under reaction conditions remain obscure. In this work, we report the high activity/stability of a ceria-supported Ru–nanocluster (<1 nm) catalyst during the dry reforming of methane. To elucidate the structure–reactivity relationship underlying the remarkable catalytic performance, the active structure and chemical speciation of the catalyst was characterized using in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), while the surface chemistry and active intermediates were monitored by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methane activates on the catalyst surface at temperatures as low as 150 °C. Under reaction conditions, the existence of metal–support interactions tunes the electronic properties of the Ru nanoclusters, giving rise to a partially oxidized state of ruthenium stabilized by reduced ceria (Ruδ+–CeO2–x) to sustain active chemistry, which is found to be very different from that of large Ru nanoparticles supported on ceria. The oxidation of surface carbon is also a crucial step for the completion of the catalytic cycle, and this is strongly correlated with the oxygen transfer governed by the Ruδ+–CeO2–x interactions at higher temperatures (>300 °C). The possible reaction pathways and stable surface intermediates were identified using DRIFTS including ruthenium carbonyls, carboxylate species, and surface −OH groups, while polydentate carbonates may be plain spectators at the measured reaction conditions

Development of a New Generation of Stable, Tunable, and Catalytically Active Nanoparticles Produced by the Helium Nanodroplet Deposition Method

Nanoparticles (NPs) are revolutionizing many areas of science and technology, often delivering unprecedented improvements to properties of the conventional materials. However, despite important advances in NPs synthesis and applications, numerous challenges still remain. Development of alternative synthetic method capable of producing very uniform, extremely clean and very stable NPs is urgently needed. If successful, such method can potentially transform several areas of nanoscience, including environmental and energy related catalysis. Here we present the first experimental demonstration of catalytically active NPs synthesis achieved by the helium nanodroplet isolation method. This alternative method of NPs fabrication and deposition produces narrowly distributed, clean, and remarkably stable NPs. The fabrication is achieved inside ultralow temperature, superfluid helium nanodroplets, which can be subsequently deposited onto any substrate. This technique is universal enough to be applied to nearly any element, while achieving high deposition rates for single element as well as composite core–shell NPs

Growth of Nanoparticles with Desired Catalytic Functions by Controlled Doping-Segregation of Metal in Oxide

The size and morphology of metal nanoparticles (NPs) often play a critical role in defining the catalytic performance of supported metal nanocatalysts. However, common synthetic methods struggle to produce metal NPs of appropriate size and morphological control. Thus, facile synthetic methods that offer controlled catalytic functions are highly desired. Here we have identified a new pathway to synthesize supported Rh nanocatalysts with finely tuned spatial dimensions and controlled morphology using a doping-segregation method. We have analyzed their structure evolutions during both the segregation process and catalytic reaction using a variety of in situ spectroscopic and microscopic techniques. A correlation between the catalytic functional sites and activity in CO₂ hydrogenation over supported Rh nanocatalysts is then established. This study demonstrates a facile strategy to design and synthesize nanocatalysts with desired catalytic functions