Dielectric Sensing with Deposited Gold Bipyramids

Colloidal gold bipyramids with a narrow ensemble plasmon resonance in the near-infrared are adhered on glass or silicon substrates using polyelectrolytes. Atomic force microscopy and scanning electron microscopy show a monolayer of the deposited colloids that remains nonaggregated, with an optical density of ∼0.1 at the peak plasmon resonance. The substrates can be repeatedly immersed in various solvents. The near-infrared resonance shifts with the optical index of the solvent by ∼−0.62 eV/refractive index unit. The figure of merit for the ensemble absorption shift is comparable to the best values reported for single metallic colloidal particles

Temperature Effect on Photoelectrochemical Water Splitting: A Model Study Based on BiVO₄ Photoanodes

Photoelectrochemical (PEC) water splitting is typically studied at room temperature. In this work, the temperature effect on PEC water splitting is studied using crystalline BiVO4 thin film photoanode as a model system. Systematic temperature-dependent electrochemical study demonstrates that the PEC activity is boosted at elevated electrolyte temperatures and indicates that thermal energy plays a main role in improving charge carrier transport in the bulk of BiVO4. Irreversible surface reconstruction is observed after PEC reactions at elevated temperature in the presence of hole scavengers, with regularly spaced stripes emerging on BiVO4 grains. The surface-reconstructed photoanode exhibits up to 40% improvement in photocurrent densities and ∼ 0.25 V shift of photocurrent onset to favorable direction. Detailed investigation shows the formation of an amorphous layer without stoichiometric change at the reconstructed surface. This work provides insights of the temperature effect on the photoelectrode in solar water splitting and reveals the non-negligible effect of hole scavengers in photoelectrochemical measurement

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

Quantifying Bulk and Surface Recombination Processes in Nanostructured Water Splitting Photocatalysts via In Situ Ultrafast Spectroscopy

A quantitative description of recombination processes in nanostructured semiconductor photocatalystsone that distinguishes between bulk (charge transport) and surface (chemical reaction) lossesis critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity

Enhancing Water Splitting Activity and Chemical Stability of Zinc Oxide Nanowire Photoanodes with Ultrathin Titania Shells

Zinc oxide nanowire photoanodes are chemically stabilized by conformal growth of an ultrathin shell of titania through atomic layer deposition, permitting their stable operation for water splitting in a strongly alkaline solution. Because of the passivation of zinc oxide surface charge traps by titania coating, core/shell nanowire arrays supply a photocurrent density of 0.5 mA/cm² under simulated AM1.5G sunlight at the thermodynamic oxygen evolving potential, demonstrating 25% higher photoelectrochemical water splitting activity compared to as-grown zinc oxide wires. By thermally annealing the zinc oxide wire arrays prior to surface passivation, we further increase the photocurrent density to 0.7 mA/cm²the highest reported value for doped or undoped zinc oxide photoanodes studied under similar simulated sunlight. Photoexcitations at energies above the zinc oxide band gap are converted with efficiency greater than 80%. Photoluminescence measurements of the best-performing nanowire arrays are consistent with improved water splitting activity from removal of deep trap states

Surface-Energy Induced Formation of Single Crystalline Bismuth Nanowires over Vanadium Thin Film at Room Temperature

We report high-yield room-temperature growth of vertical single-crystalline bismuth nanowire array by vacuum thermal evaporation of bismuth over a choice of arbitrary substrate coated with a thin interlayer of nanoporous vanadium. The nanowire growth is the result of spontaneous and continuous expulsion of nanometer-sized bismuth domains from the vanadium pores, driven by their excessive surface energy that suppresses the melting point of bismuth close to room temperature. The simplicity of the technique opens a new avenue for the growth of nanowire arrays of a variety of materials

Unconventional Relation between Charge Transport and Photocurrent via Boosting Small Polaron Hopping for Photoelectrochemical Water Splitting

Doping in semiconductor photoelectrodes controls defect formation and carrier transport that critically determine the device performance. Here we report an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO4 photoanodes. Using the single-crystalline thin film approach, we identify that Mo doping significantly condenses the optimization regime between carrier transport and photon collection. For Mo-doped BiVO4 films, an unprecedentedly thin layer (50 nm), less than one-third of the pristine BiVO4 thickness, delivers larger photocurrents by overcoming the charge transport limitation, representing a regime not covered in conventional models. We provide direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of a small polaron, with the latter applying substantial impact on the photoelectrochemical performance. Density functional theory calculations reveal that fully ionized Mo dopants establish a strong electrostatic interaction with a small polaron, which helps reduce its hopping barrier by minimizing the local lattice expansion. Our results deliver mechanistic insights on the interplay between extrinsic doping and carrier transport, and provide guidance in developing advanced semiconductor photoelectrodes

Investigation of Electron Extraction and Protection Layers on Cu₂O Photocathodes

Many semiconductor photoelectrodes used for solar fuel production require the addition of buffer and protection layers to enhance their solar-to-fuel conversion efficiency and long-term stability. For example, Cu2O, which is the most efficient oxide-based photocathode but suffers from photocorrosion, has been assembled with various buffer and protection layers to suppress photocorrosion and use more photoexcited electrons for useful reactions such as water reduction to H2. However, the abilities of various buffer and protection layers to extract electrons from Cu2O have never been directly evaluated. Instead, their abilities were estimated based on the photocurrent for water reduction after adding a hydrogen evolution catalyst on top of them. In these evaluations, as the photocurrent is affected not only by the buffer or protection layer but also by the catalyst, the ability of the buffer or protection layer to extract electrons from Cu2O could not be accurately determined or compared. In this study, we demonstrate that 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), whose reduction rate is faster than the photocorrosion rate of Cu2O, can be used as an effective electron scavenger to directly evaluate any change caused by a buffer or protection layer in electron–hole separation in Cu2O. In particular, we compared the performances of ZnO and TiO2 layers on Cu2O for extracting electrons and suppressing photocorrosion. We also compared the performances of TiO2 layers prepared by electrodeposition and atomic layer deposition (ALD) to show that the deposition method can make a striking impact on the performance of the same TiO2 because it can affect the critical characteristics of the layer (e.g., defect levels, conductivity, interfacial atomic arrangements) that govern interfacial charge transfer in multilayer photoelectrodes

Cobalt Oxide-Coated Single Crystalline Bismuth Vanadate Photoanodes for Efficient Photoelectrochemical Chlorine Generation

Bismuth vanadate (BiVO4) is an outstanding photoanode material for photoelectrochemical water splitting. In this work, a series of single crystalline BiVO4 photoanodes are synthesized by pulsed laser deposition (PLD). Once coated with a thin layer of cobalt oxide (CoOx) cocatalyst, also by PLD, the photoanodes support efficient photoelectrochemical generation of chlorine (Cl2) from brine under simulated solar light. The activity of the chlorine generation reaction (ClER) is optimized when the thickness of CoOx is about 3 nm, with the faradic efficiency of ClER exceeding 60%. Detailed studies show that the CoOx cocatalyst layer is amorphous, uniform in thickness, and chemically robust. As such, the cocatalyst also effectively protects the underlying BiVO4 photoanodes against chlorine corrosion. This work provides insights into using artificial photosynthesis for byproducts that carry significant economic value while avoiding the energetically expensive oxygen evolution reactions