Maximizing Photoelectrochemical Performance in Metal-Oxide Hybrid Composites via Amorphous Exsolution-A New Exsolution Mechanism for Heterogeneous Catalysis.

Exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle-perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond's low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

Growth Kinetics of Individual Au Spiky Nanoparticles Using Liquid-Cell Transmission Electron Microscopy.

Precise control over the size and morphology of the Au spiky nanoparticle (SNP) is essential to obtain narrow and tunable surface plasmon resonance (SPR). However, these challenges require a fundamental understanding of the particle growth mechanism and kinetics as well as its morphological transition, which can only be achieved by real-time observation at nanometer resolution. Here, we report in situ liquid cell transmission electron microscopy studies of single and multiple Au SNP growth at various conditions of such parameters as size and dose rate of electron beam and HAuCl4 solution concentration. The particle evolves via a mixture of reaction and Au formation-limited growth, followed by Au formation-limited growth-a transition from faceted to roughened surfaces, confirmed by the analysis with different beam sizes and the UV-vis spectra that feature a unique trend of short- and long-wavelength plasmon band shift. Quantitative analyses combined with a theoretical model determine the transition time (tc) of the two regimes and estimate the surface concentration (ci) of the particle with time. Interestingly, tc can be manipulated by the particle density, which alters the surface roughening rate, and the density is modulated by tuning the aforementioned parameters based on DLVO theory. These results suggest a new method for synthesizing a Au SNP whose size, morphology, SPR, and density can be sensibly manipulated without adding reducing or capping agents.

SnS2 Nanoparticles Embedded in BiVO4 Surfaces via Eutectic Decomposition for Enhanced Performance in Photoelectrochemical Water Splitting.

BiVO4 has garnered substantial interest as a promising photoanode material for photoelectrochemical water-splitting due to its narrow band gap and appropriate band edge positions for water oxidation. Nevertheless, its practical use has been impeded by poor charge transport and sluggish water oxidation kinetics. Here, a hybrid composite photoanode is fabricated by uniformly embedding SnS2 nanoparticles near the surface of a BiVO4 thin film, creating a type II heterostructure with strong interactions between the nanoparticles and the film for efficient charge separation. This structure forms via eutectic melting during atomic layer deposition of SnS2 with subsequent phase separation between SnS2 and BiVO4 at room temperature, offering greater advantages and flexibilities over conventional exsolution techniques. Furthermore, the SnS2/BiVO4 hybrid composite is coated with a thin amorphous ZnS passivation layer to accelerate charge transfer process and enhance long-term stability. The optimized BiVO4/SnS2/ZnS photoanode exhibits a photocurrent density of 5.44 mA cm-2 at 1.23 V versus RHE, which is 2.73 times higher than that of the BiVO4 photoanode, and a dramatic improvement in photostability retention at 1.23 V versus RHE, increasing from 55% to 91% over 24 hours. This method of anchoring nanoparticles onto host materials proves highly valuable for energy and environmental applications.

Concurrent Amorphization and Nanocatalyst Formation in Cu-Substituted Perovskite Oxide Surface: Effects on Oxygen Reduction Reaction at Elevated Temperatures.

The activity and durability of chemical/electrochemical catalysts are significantly influenced by their surface environments, highlighting the importance of thoroughly examining the catalyst surface. Here, Cu-substituted La0.6Sr0.4Co0.2Fe0.8O3-δ is selected, a state-of-the-art material for oxygen reduction reaction (ORR), to explore the real-time evolution of surface morphology and chemistry under a reducing atmosphere at elevated temperatures. Remarkably, in a pioneering observation, it is discovered that the perovskite surface starts to amorphize at an unusually low temperature of approximately 100 °C and multicomponent metal nanocatalysts additionally form on the amorphous surface as the temperature raises to 400 °C. Moreover, this investigation into the stability of the resulting amorphous layer under oxidizing conditions reveals that the amorphous structure can withstand a high-temperature oxidizing atmosphere (≥650 °C) only when it has undergone sufficient reduction for an extended period. Therefore, the coexistence of the active nanocatalysts and defective amorphous surface leads to a nearly 100% enhancement in the electrode resistance for the ORR over 200 h without significant degradation. These observations provide a new catalytic design strategy for using redox-dynamic perovskite oxide host materials.

Segmentation and Morphology Computation of a Spiky Nanoparticle Using the Hourglass Neural Network.

Morphological measurements of nanoparticles in electron microscopy images are tedious, laborious, and often succumb to human errors. Deep learning methods in artificial intelligence (AI) paved the way for automated image understanding. This work proposes a deep neural network (DNN) for the automated segmentation of a Au spiky nanoparticle (SNP) in electron microscopic images, and the network is trained with a spike-focused loss function. The segmented images are used for the growth measurement of the Au SNP. The auxiliary loss function captures the spikes of the nanoparticle, which prioritizes the detection of spikes in the border regions. The growth of the particles measured by the proposed DNN is as good as the measurement in manually segmented images of the particles. The proposed DNN composition with the training methodology meticulously segments the particle and consequently provides accurate morphological analysis. Furthermore, the proposed network is tested on an embedded system for integration with the microscope hardware for real-time morphological analysis.

Nanostructured Au Electrode with 100 h Stability for Solar-Driven Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide.

Solar-to-chemical energy conversion is a potential alternative to fossil fuels. A promising approach is the electrochemical (EC) reduction of CO2 to value-added chemicals, particularly hydrocarbons. Here, we report on the selective EC reduction of CO2 to CO on a porous Au nanostructure (pAu) cathode in 0.1 M KHCO3. The pAu cathode anodized at 2.6 V exhibited maximum Faradaic efficiency (FE) for conversion of CO2 to CO (up to 100% at -0.75 V vs reversible hydrogen electrode (RHE)). Furthermore, commercial Si photovoltaic cells were combined with EC systems (PV-EC) consisting of pAu cathodes and IrO2 anodes. The triple-junction cell and EC system resulted in a solar-to-CO conversion efficiency (SCE) of 5.3% under 1 sun illumination and was operated for 100 h. This study provides a PV-EC CO2 reduction system for CO production and indicates the potential of the PV-EC system for the EC reduction of CO2 to value-added chemicals.

Direct Observation of Rhodium Ex-Solution from a Ceria Nanodomain and Its Use for Hydrogen Production via Propane Steam Reforming.

The ex-solution phenomenon has received attention as a promising technique to prepare highly durable heterogeneous catalysts. Perovskite materials have been mainly used as host oxides for ex-solution, but their small surface areas have limited their practical use. Here, Rh was ex-solved by reducing Rh-doped ceria solid solution, and nanosized Rh catalysts with a high surface area of 70.7 m2/g were prepared. The Rh nanoparticles ex-solved from the ceria nanodomains were directly monitored by in situ transmission electron microscopy. The Rh nanoparticles whose sizes are 2-3 nm were not coarsened during the propane steam reforming process carried out at 700 °C for 65 h, leading to high resistance against sintering and coke formation. On the contrary, the Rh catalyst simply deposited on CeO2 was significantly sintered after the reaction, and the size of Rh nanoparticles increased to 25 nm, resulting in severe coke formation. Our work shows that ex-solution from a ceria-based nanodomain can be a good way to prepare metal nanoparticle catalysts with a large surface area and excellent durability for gas-phase reactions at high temperatures.

Single-Atom Pt Stabilized on One-Dimensional Nanostructure Support via Carbon Nitride/SnO2 Heterojunction Trapping.

Catalysis with single-atom catalysts (SACs) exhibits outstanding reactivity and selectivity. However, fabrication of supports for the single atoms with structural versatility remains a challenge to be overcome, for further steps toward catalytic activity augmentation. Here, we demonstrate an effective synthetic approach for a Pt SAC stabilized on a controllable one-dimensional (1D) metal oxide nano-heterostructure support, by trapping the single atoms at heterojunctions of a carbon nitride/SnO2 heterostructure. With the ultrahigh specific surface area (54.29 m2 g-1) of the nanostructure, we obtained maximized catalytic active sites, as well as further catalytic enhancement achieved with the heterojunction between carbon nitride and SnO2. X-ray absorption fine structure analysis and HAADF-STEM analysis reveal a homogeneous atomic dispersion of Pt species between carbon nitride and SnO2 nanograins. This Pt SAC system with the 1D nano-heterostructure support exhibits high sensitivity and selectivity toward detection of formaldehyde gas among state-of-the-art gas sensors. Further ex situ TEM analysis confirms excellent thermal stability and sinter resistance of the heterojunction-immobilized Pt single atoms.

Dopant-Driven Positive Reinforcement in Ex-Solution Process: New Strategy to Develop Highly Capable and Durable Catalytic Materials.

The ex-solution phenomenon, a central platform for growing metal nanoparticles on the surface of host oxides in real time with high durability and a fine distribution, has recently been applied to various scientific and industrial fields, such as catalysis, sensing, and renewable energy. However, the high-temperature processing required for ex-solutions (>700 °C) limits the applicable material compositions and has hindered advances in this technique. Here, an unprecedented approach is reported for low-temperature particle ex-solution on important nanoscale binary oxides. WO3 with a nanosheet structure is selected as the parent oxide, and Ir serves as the active metal species that produces the ex-solved metallic particles. Importantly, Ir doping facilitates a phase transition in the WO3 bulk lattice, which further promotes Ir ex-solution at the oxide surface and eventually enables the formation of Ir particles (<3 nm) at temperatures as low as 300 °C. Low-temperature ex-solution effectively inhibits the agglomeration of WO3 sheets while maintaining well-dispersed ex-solved particles. Furthermore, the Ir-decorated WO3 sheets show excellent durability and H2 S selectivity when used as sensing materials, suggesting that this is a generalizable synthetic strategy for preparing highly robust heterogeneous catalysts for a variety of applications.

Organic-inorganic hybrid perovskite quantum dots with high PLQY and enhanced carrier mobility through crystallinity control by solvent engineering and solid-state ligand exchange.

The photoluminescence quantum yield (PLQY) and charge carrier mobility of organic-inorganic perovskite QDs were enhanced by the optimization of crystallinity and surface passivation as well as solid-state ligand exchange. The crystallinity of perovskite QDs was determined by the Effective solvent field (Esol) of various solvents for precipitation. The solvent with high Esol could more quickly countervail the localized field generated by the polar solvent, and it causes fast crystallization of the dissolved precursor, which results in poor crystallinity. The post-ligand adding process (PLAP) and post-ligand exchange process (PLEP) increase the PLQY of perovskite QDs by reducing non-radiative recombination and the density of surface defect states through surface passivation. Particularly, the post ligand exchange process (PLEP) in the solid-state improved the charge carrier mobility of perovskite QDs in addition to the PLQY enhancement. The ligand exchange with short alkyl chain length ligands could improve the packing density of perovskite QDs in films by reducing the inter-particle distance between perovskite QDs. The maximum hole mobility of 6.2 × 10-3 cm2 V-1 s-1, one order higher than that of pristine QDs without the PLEP, is obtained at perovskite QDs with hexyl ligands. By using PLEP treatment, compared to the pristine device, a 2.5 times higher current efficiency in perovskite QD-LEDs was achieved due to the improved charge carrier mobility and PLQY.

Simple eco-friendly synthesis of the surfactant free SnS nanocrystal toward the photoelectrochemical cell application.

A simple, low cost, non-toxic and eco-friendly pathway for synthesizing efficient sunlight-driven tin sulfide photocatalyst was studied. SnS nanocrystals were prepared by using mechanical method. The bulk SnS was obtained by evaporation of SnS nanocrystal solution. The synthesized samples were characterized by using XRD, SEM, TEM, UV-vis, and Raman analyses. Well crystallized SnS nanocrystals were verified and the electrochemical characterization was also performed under visible light irradiation. The SnS nanocrystals have shown remarkable photocurrent density of 7.6 mA cm-2 under 100 mW cm-2 which is about 10 times larger than that of the bulk SnS under notably stable operation conditions. Furthermore, the SnS nanocrystals presented higher stability than the bulk form. The IPCE(Incident photon to current conversion efficiency) of 9.3% at 420 nm was obtained for SnS nanocrystal photoanode which is strikingly higher than that of bulk SnS, 0.78%. This work suggests that the enhancement of reacting area by using SnS nanocrystal absorbers could give rise to the improvement of photoelectrochemical cell efficiency.

Substrate-mediated strain effect on the role of thermal heating and electric field on metal-insulator transition in vanadium dioxide nanobeams.

Single-crystalline vanadium dioxide (VO2) nanostructures have recently attracted great attention because of their single domain metal-insulator transition (MIT) nature that differs from a bulk sample. The VO2 nanostructures can also provide new opportunities to explore, understand, and ultimately engineer MIT properties for applications of novel functional devices. Importantly, the MIT properties of the VO2 nanostructures are significantly affected by stoichiometry, doping, size effect, defects, and in particular, strain. Here, we report the effect of substrate-mediated strain on the correlative role of thermal heating and electric field on the MIT in the VO2 nanobeams by altering the strength of the substrate attachment. Our study may provide helpful information on controlling the properties of VO2 nanobeam for the device applications by changing temperature and voltage with a properly engineered strain.

High-performance photoconductivity and electrical transport of ZnO/ZnS core/shell nanowires for multifunctional nanodevice applications.

We report the electrical and optical properties of ZnO/ZnS core/shell nanowire (NW) devices. The spatial separation of charge carriers due to their type II band structure together with passivation effect on ZnO/ZnS core/shell NWs not only enhanced their charge carrier transport characteristics by confining the electrons and reducing surface states in the ZnO channel but also increased the photocurrent under ultraviolet (UV) illumination by reducing the recombination probability of the photogenerated charge carriers. Here the efficacy of the type-II band structure and the passivation effect are demonstrated by showing the enhanced subthreshold swing (150 mV/decade) and mobility (17.2 cm2/(Vs)) of the electrical properties, as well as the high responsivity (4.4×10(6) A/W) in the optical properties of the ZnO/ZnS core/shell NWs, compared with the subthreshold swing (464 mV/decade), mobility (8.9 cm2/(Vs)) and responsivity (2.5×10(6) A/W) of ZnO NWs.