Nonepitaxial Thin-Film InP for Scalable and Efficient Photocathodes

To date, some of the highest performance photocathodes of a photoelectrochemical (PEC) cell have been shown with single-crystalline p-type InP wafers, exhibiting half-cell solar-to-hydrogen conversion efficiencies of over 14%. However, the high cost of single-crystalline InP wafers may present a challenge for future large-scale industrial deployment. Analogous to solar cells, a thin-film approach could address the cost challenges by utilizing the benefits of the InP material while decreasing the use of expensive materials and processes. Here, we demonstrate this approach, using the newly developed thin-film vapor–liquid–solid (TF-VLS) nonepitaxial growth method combined with an atomic-layer deposition protection process to create thin-film InP photocathodes with large grain size and high performance, in the first reported solar device configuration generated by materials grown with this technique. Current–voltage measurements show a photocurrent (29.4 mA/cm²) and onset potential (630 mV) approaching single-crystalline wafers and an overall power conversion efficiency of 11.6%, making TF-VLS InP a promising photocathode for scalable and efficient solar hydrogen generation

Reactive Sputtering of Bismuth Vanadate Photoanodes for Solar Water Splitting

Bismuth vanadate (BiVO₄) has attracted increasing attention as a photoanode for photoelectrochemical (PEC) water splitting. It has a band gap in the visible light range (2.4–2.5 eV) and a valence band position suitable for driving water oxidation under illumination. While a number of methods have been used to make BiVO₄ photoanodes, scalable thin film deposition has remained relatively underexplored. Here, we report the synthesis of BiVO₄ thin films by reactive sputtering. The use of separate Bi and V sputtering targets allows control of the Bi/V ratio in the film. Under optimized, slightly V-rich conditions, monoclinic phase BiVO₄ with photoactivity for water oxidation is obtained. The highest photocurrents, ca. 1 mA cm^–2 at the reversible O₂/H₂O potential with simulated AM 1.5G illumination, are obtained with bilayer WO₃/BiVO₄, where the WO₃ serves as a hole-blocking layer

Amorphous Si Thin Film Based Photocathodes with High Photovoltage for Efficient Hydrogen Production

An amorphous Si thin film with TiO₂ encapsulation layer is demonstrated as a highly promising and stable photocathode for solar hydrogen production. With platinum as prototypical cocatalyst, a photocurrent onset potential of 0.93 V vs RHE and saturation photocurrent of 11.6 mA/cm² are measured. Importantly, the a-Si photocathodes exhibit impressive photocurrent of ∼6.1 mA/cm² at a large positive bias of 0.8 V vs RHE, which is the highest for all reported photocathodes at such positive potential. Ni–Mo alloy is demonstrated as an alternative low-cost catalyst with onset potential and saturation current similar to those obtained with platinum. This low-cost photocathode with high photovoltage and current is a highly promising photocathode for solar hydrogen production

Field-Effect Transistors Built from All Two-Dimensional Material Components

We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator, and metal layers. Specifically, MoS₂ is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. This transistor exhibits n-type behavior with an ON/OFF current ratio of >10⁶, and an electron mobility of ∼33 cm²/V·s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. A WSe₂–MoS₂ diode with graphene contacts is also demonstrated. The diode exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystalline, layered materials with atomically uniform thicknesses are stacked on demand, without the lattice parameter constraints. The results demonstrate the promise of using an all-layered material system for future electronic applications

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

19.2% Efficient InP Heterojunction Solar Cell with Electron-Selective TiO₂ Contact

We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO₂ deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO₂ film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm² and a high power conversion efficiency of 19.2%

Deterministic Nucleation of InP on Metal Foils with the Thin-Film Vapor–Liquid–Solid Growth Mode

A method for growth of ultralarge grain (>100 μm) semiconductor thin-films on nonepitaxial substrates was developed via the thin-film vapor–liquid–solid growth mode. The resulting polycrystalline films exhibit similar optoelectronic quality as their single-crystal counterparts. Here, deterministic control of nucleation sites is presented by substrate engineering, enabling user-tuned internuclei spacing of up to ∼1 mm. Besides examining the theory associated with the nucleation process, this work presents an important advance toward controlled growth of high quality semiconductor thin films with unprecedented grain sizes on nonepitaxial substrates

Role of TiO₂ Surface Passivation on Improving the Performance of p‑InP Photocathodes

The role of TiO₂ thin films deposited by atomic layer deposition on p-InP photocathodes used for solar hydrogen generation was examined. It was found that, in addition to its previously reported corrosion protection role, the large valence band offset between TiO₂ and InP creates an energy barrier for holes reaching the surface. Also, the conduction band of TiO₂ is well-aligned with that of InP. The combination of these two effects creates an electron-selective contact with low interface recombination. Under simulated solar illumination in HClO₄ aqueous electrolyte, an onset potential of >800 mV vs RHE was achieved, which is the highest yet reported for an InP photocathode

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

General Thermal Texturization Process of MoS₂ for Efficient Electrocatalytic Hydrogen Evolution Reaction

Molybdenum disulfide (MoS₂) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS₂ because the pristine MoS₂ mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS₂ is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS₂, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS₂ surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS₂ across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS₂. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm² of HER current density