Amorphous FeOOH Oxygen Evolution Reaction Catalyst for Photoelectrochemical Water Splitting

Reaching the goal of economical photoelectrochemical (PEC) water splitting will likely require the combination of efficient solar absorbers with high activity electrocatalysts for the hydrogen and oxygen evolution reactions (HER and OER). Toward this goal, we synthesized an amorphous FeOOH (a-FeOOH) phase that has not previously been studied as an OER catalyst. The a-FeOOH films show activity comparable to that of another OER cocatalyst, Co-borate (Co–B_i), in 1 M Na₂CO₃, reaching 10 mA/cm² at an overpotential of ∼550 mV for 10 nm thick films. Additionally, the a-FeOOH thin films absorb less than 3% of the solar photons (AM1.5G) with energy greater than 1.9 eV, are homogeneous over large areas, and act as a protective layer separating the solution from the solar absorber. The utility of a-FeOOH in a realistic system is tested by depositing on amorphous Si triple junction solar cells with a photovoltaic efficiency of 6.8%. The resulting a-FeOOH/a-Si devices achieve a total water splitting efficiency of 4.3% at 0 V vs RHE in a three-electrode configuration and show no decrease in efficiency over the course of 4 h

(Mg,Fe)CO3_Electricalconductivity_dataset.xlsx

Electrical conductivity of FeCO3 and Fe0.65Mg0.35CO3 at 126-2000 K and 0-83 GPa</p

Pressure-Dependent Light Emission of Charged and Neutral Excitons in Monolayer MoSe₂

Tailoring the excitonic properties in two-dimensional monolayer transition metal dichalcogenides (TMDs) through strain engineering is an effective means to explore their potential applications in optoelectronics and nanoelectronics. Here we report pressure-tuned photon emission of trions and excitons in monolayer MoSe₂ via a diamond anvil cell (DAC) through photoluminescence measurements and theoretical calculations. Under quasi-hydrostatic compressive strain, our results show neutral (X⁰) and charged (X^–) exciton emission of monolayer MoSe₂ can be effectively tuned by alcohol mixture vs inert argon pressure transmitting media (PTM). During this process, X^– emission undergoes a continuous blue shift until reaching saturation, while X⁰ emission turns up splitting. The pressure-dependent charging effect observed in alcohol mixture PTM results in the increase of the X^– exciton component and facilitates the pressure-tuned emission of X^– excitons. This substantial tunability of X^– and X⁰ excitons in MoSe₂ can be extended to other 2D TMDs, which holds potential for developing strained and optical sensing devices

Coupling-Assisted Renormalization of Excitons and Vibrations in Compressed MoSe₂–WSe₂ Heterostructure

Vertical heterostructures (HSs) constructed with two-dimensional (2D) materials is expected to generate fascinating properties due to interlayer coupling between neighboring layers. However, interlayer coupling can be easily obscured by cross-contamination during transfer processes, rendering their experimental demonstration challenging. Here, we explore the coupling-assisted renormalization of excitons and vibrations in a mechanically fabricated MoSe₂–WSe₂ HS through high-pressure photoluminescence, Raman spectra, and density functional theory calculations. Accompanied by the interlayer coupling enhancement, the excitonic and vibrational renormalizations involving dimensionality and composition variations were achieved. A cycle of 2D–3D–2D excitonic evolution was disclosed and pressure-induced emergence of X^– exciton of MoSe₂ in HS was found reflecting the band structure transition in the MoSe₂–WSe₂ HS. The Raman spectra reveals that the coupled A₂″ vibrations of WSe₂ and MoSe₂ in HS was stiffened and out-of-plane A₁′ vibrations of WSe₂ and MoSe₂ in HS got coherent upon pressure modulation. This coupling-assisted renormalization in MoSe₂–WSe₂ HS can be extended to other 2D layered HSs, which indicates the possibility to design a flexible HS with controlled excitonic and vibrational system for light-emitting diodes, excitonic, and photovoltaic devices

Pressure-Modulated Conductivity, Carrier Density, and Mobility of Multilayered Tungsten Disulfide

Tungsten disulfide (WS₂) is a layered transition metal dichalcogenide (TMD) that differs from other two-dimensional (2D) compounds such as graphene due to its unique semiconducting, tunable-band-gap nature. Multilayered WS₂ exhibits an indirect band gap <i>E</i>_g of ∼1.3 eV, along with a higher load-bearing ability that is promising for strain-tuning device applications, but the electronic properties of multilayered WS₂ at higher strain conditions (<i>i</i>.<i>e</i>., static strain >12%) remain an open question. Here we have studied the structural, electronic, electrical, and vibrational properties of multilayered WS₂ at hydrostatic pressures up to ∼35 GPa experimentally in a diamond anvil cell and theoretically using first-principles <i>ab initio</i> calculations. Our results show that WS₂ undergoes an isostructural semiconductor-to-metallic (S–M) transition at approximately 22 GPa at 280 K, which arises from the overlap of the highest valence and lowest conduction bands. The S–M transition is caused by increased sulfur–sulfur interactions as the interlayer spacing decreases with applied hydrostatic pressure. The metalization in WS₂ can be alternatively interpreted as a 2D to 3D (three-dimensional) phase transition that is associated with a substantial modulation of the charge carrier characteristics including a 6-order decrease in resistivity, a 2-order decrease in mobility, and a 4-order increase in carrier concentration. These distinct pressure-tunable characteristics of the dimensionalized WS₂ differentiate it from other TMD compounds such as MoS₂ and promise future developments in strain-modulated advanced devices

Improved Visible Light Harvesting of WO₃ by Incorporation of Sulfur or Iodine: A Tale of Two Impurities

We report the incorporation of sulfur or iodine into monoclinic tungsten trioxide (S:WO₃ or I:WO₃ respectively), with the aim to improve its visible light-harvesting ability. Films were synthesized by spray pyrolysis with either ammonium sulfide or iodide added to the aqueous WO₃ precursor solutions. Red shifts of the absorption spectra were observed with S and I incorporation (from ∼2.7 to 2.6 and 2.1 eV respectively), likely due to the formation of intragap impurity bands. S:WO₃ samples exhibited enhanced photoelectrochemical (PEC) performance at low S concentrations, but this quickly deteriorated with increasing S content. Incident photon conversion efficiency (IPCE) data showed that this initial improvement was driven by improved collection efficiency at longer wavelengths. Conversely, photocurrent decreased at all levels of I addition. IPCE measurements for these films showed only a marginal increase in efficiency at longer wavelengths, indicating that the extra absorbed photons did not contribute significantly to the photocurrent. Time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiling revealed a uniform distribution of S throughout the S:WO₃ films, but showed surface segregation of I in the I:WO₃ samples. Raman and X-ray photoelectron spectrometry (XPS) showed that S and I substituted for oxygen, but in the case of S, other pathways such as interstitial incorporation and cation substitution could not be ruled out. The complexities of intentionally adding nonmetal impurities to metal oxide systems are highlighted in the context of the existing body of literature

Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO₄ Single Crystals: Intrinsic Behavior of a Complex Metal Oxide

Bismuth vanadate (BiVO₄) is a promising photoelectrode material for the oxidation of water, but fundamental studies of this material are lacking. To address this, we report electrical and photoelectrochemical (PEC) properties of BiVO₄ single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO₄) grown using the floating zone technique. We demonstrate that a small polaron hopping conduction mechanism dominates from 250 to 400 K, undergoing a transition to a variable-range hopping mechanism at lower temperatures. An anisotropy ratio of ∼3 was observed along the <i>c</i> axis, attributed to the layered structure of BiVO₄. Measurements of the ac field Hall effect yielded an electron mobility of ∼0.2 cm² V^–1 s^–1 for Mo and W:BiVO₄ at 300 K. By application of the Gärtner model, a hole diffusion length of ∼100 nm was estimated. As a result of low carrier mobility, attempts to measure the dc Hall effect were unsuccessful. Analyses of the Raman spectra showed that Mo and W substituted for V and acted as donor impurities. Mott–Schottky analysis of electrodes with the (001) face exposed yielded a flat band potential of 0.03–0.08 V versus the reversible H₂ electrode, while incident photon conversion efficiency tests showed that the dark coloration of the doped single crystals did not result in additional photocurrent. Comparison of these intrinsic properties to those of other metal oxides for PEC applications gives valuable insight into this material as a photoanode