Miao Zhong

Surface Modification of CoOx Loaded BiVO4 Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation

Photoelectrochemical (PEC) devices that use semiconductors to absorb solar light for water splitting offer a promising way toward the future scalable production of renewable hydrogen fuels. However, the charge recombination in the photoanode/electrolyte (solid/liquid) junction is a major energy loss and hampers the PEC performance from being efficient. Here, we show that this problem is addressed by the conformal deposition of an ultrathin p-type NiO layer on the photoanode to create a buried p/n junction as well as to reduce the charge recombination at the surface trapping states for the enlarged surface band bending. Further, the in situ formed hydroxyl-rich and hydroxyl-ion-permeable NiOOH enables the dual catalysts of CoO(x) and NiOOH for the improved water oxidation activity. Compared to the CoO(x) loaded BiVO4 (CoO(x)/BiVO4) photoanode, the ∼6 nm NiO deposited NiO/CoO(x)/BiVO4 photoanode triples the photocurrent density at 0.6 V(RHE) under AM 1.5G illumination and enables a 1.5% half-cell solar-to-hydrogen efficiency. Stoichiometric oxygen and hydrogen are generated with Faraday efficiency of unity over 12 h. This strategy could be applied to other narrow band gap semiconducting photoanodes toward the low-cost solar fuel generation devices.

Fabrication of hierarchical ZnO architectures and their superhydrophobic surfaces with strong adhesive force.

Grid-structured ZnO microsphere arrays assembled by uniform ZnO nanorods were fabricated by noncatalytic chemical vapor deposition, taking advantage of morphologies of alumina nanowire pyramid substrates and ZnO oriented growth habits. Every ZnO microsphere (similar to the micropapilla on a lotus leaf surface) is assembled by over 200 various oriented ZnO nanorods (similar to the hairlike nanostructures on mircopapilla of a lotus leaf). This lotus-leaf-like ZnO micro-nanostructure films reveal superhydrophobicity and ultrastrong adhesive force to liquid. The realization of this hierarchical ZnO nanostructure film could be important for further understanding wettability of biological surfaces with micro-nanostructure and application in microfluidic devices.

Highly Active GaN‐Stabilized Ta3N5 Thin‐Film Photoanode for Solar Water Oxidation

Ta3 N5 is a very promising photocatalyst for solar water splitting because of its wide spectrum solar energy utilization up to 600 nm and suitable energy band position straddling the water splitting redox reactions. However, its development has long been impeded by poor compatibility with electrolytes. Herein, we demonstrate a simple sputtering-nitridation process to fabricate high-performance Ta3 N5 film photoanodes owing to successful synthesis of the vital TaOδ precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta3 N5 by forming a crystalline nitride-on-nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm-2 was obtained with a CoPi/GaN/Ta3 N5 photoanode at 1.2 VRHE under simulated sunlight, with O2 and H2 generated at a Faraday efficiency of unity over 12 h. Our vapor-phase deposition method can be used to fabricate high-performance (oxy)nitrides for practical photoelectrochemical applications.

ZnO dense nanowire array on a film structure in a single crystal domain texture for optical and photoelectrochemical applications

A single crystal domain texture quality (a unique in-plane and out-of-plane crystalline orientation over a large area) ZnO nanostructure of a dense nanowire array on a thick film has been homogeneously synthesized on a-plane sapphire substrates over large areas through a one-step chemical vapor deposition (CVD) process. The growth mechanism is clarified: a single crystal [02(-)1] oriented ZnAl(2)O(4) buffer layer was formed at the ZnO film and the a-plane sapphire substrate interface via a diffusion reaction process during the CVD process, providing improved epitaxial conditions that enable the synthesis of the high crystalline quality ZnO nanowire array on a film structure. The high optoelectronic quality of the ZnO nanowire array on a film sample is evidenced by the free exitonic emissions in the low-temperature photoluminescence spectroscopy. A carrier density of ~10(17) cm(-3) with an n-type conductivity of the ZnO nanowire array on a film sample is obtained by electrochemical impedance analysis. Finally, the ZnO nanowire array on a film sample is demonstrated to be an ideal template for a further synthesis of a single crystal quality ZnO-ZnGa(2)O(4) core-shell nanowire array on a film structure. The fabricated ZnO-ZnGa(2)O(4) sample revealed an enhanced anticorrosive ability and photoelectrochemical performance when used as a photoanode in a photoelectrochemical water splitting application.

InP nanopore arrays for photoelectrochemical hydrogen generation

We report a facile and large-scale fabrication of highly ordered one-dimensional (1D) indium phosphide (InP) nanopore arrays (NPs) and their application as photoelectrodes for photoelectrochemical (PEC) hydrogen production. These InP NPs exhibit superior PEC performance due to their excellent light-trapping characteristics, high-quality 1D conducting channels and large surface areas. The photocurrent density of optimized InP NPs is 8.9 times higher than that of planar counterpart at an applied potential of +0.3 V versus RHE under AM 1.5G illumination (100 mW cm(-2)). In addition, the onset potential of InP NPs exhibits 105 mV of cathodic shift relative to planar control. The superior performance of the nanoporous samples is further explained by Mott-Schottky and electrochemical impedance spectroscopy ananlysis.

Bulky crystalline BiVO4 thin films for efficient solar water splitting

We report a new fabrication method of flat, uniform BiVO4 films on an electrically conductive transparent indium tin oxide (ITO) film based on a solution process for depositing bismuth precursor films and a high-temperature calcination process with an organic vanadium precursor. The synthesised BiVO4 films, composed as a monolayer of crystallites (diameter ≤ 1 μm) fixed on the ITO, realised half-cell solar-to-hydrogen energy conversion efficiencies of over 1.5% by the aid of impregnated CoOx and atomic-layer-deposited NiO when tested as oxygen-evolving photoanodes for water splitting under solar simulator AM 1.5G illumination. Stoichiometric oxygen and hydrogen were generated with Faraday efficiencies of unity over 12 h at 0.6–0.9 VRHE. This morphology of our bulky semi-transparent BiVO4 films exhibited state-of-the-art solar water splitting performances.

Vertically aligned ZnO–ZnGa 2 O 4 core–shell nanowires: from synthesis to optical properties

Dense and vertically aligned ZnO–ZnGa2O4 core–shell nanowires were synthesized in large scale on a-plane sapphire substrates by a simple two-step chemical vapor deposition method. The synthesized ZnO–ZnGa2O4 core–shell nanowires were connected through their base by a thick underlayer of the same material realizing electrical contact of the nanostructured array. X-ray diffraction and transmission electron microscopy analyses of the core–shell nanowires reveal that the ZnO cores and ZnGa2O4 shells of the core–shell nanowires are of single-crystal quality and have aligned crystallographic orientations. The ultraviolet–visible diffuse reflectance spectra of the core–shell nanowires showed two sharp edges corresponding to near-band-edge absorption contributed by the ZnO cores and the ZnGa2O4 shells. Moreover, the room-temperature photoluminescence spectra of the core–shell nanowires gave three UV emission peaks coming from the ZnGa2O4 shells and the ZnO cores. The dense and vertically aligned ZnO–ZnGa2O4 core–shell nanowires showing promising photoelectric properties offer an ideal structure for light harvesting applications such as a photoanode in a photoelectrochemical water splitting cell.

Stability of hydrogen incorporated in ZnO nanowires by plasma treatment.

The stability of hydrogen in ZnO is studied using hydrogenated nanowires by plasma treatment. Enhanced near band edge UV emission and reduced defect level green emission is observed after hydrogen plasma treatment. Through thermal stability tests, this effect is found to be stable at room temperature and nearly stable up to ~500 K, but begins to deteriorate at higher temperature. The study of the irradiation stability of the hydrogen in ZnO nanowires shows that the hydrogen is stable under an electron beam with an accelerating voltage lower than 5 kV, but is not stable under 10 kV or under an intensive laser beam. The results could benefit the further understanding of the role of hydrogen in ZnO and light-emitting devices based on hydrogenated ZnO.

Oxygen-deficient WO3−x@TiO2−x core–shell nanosheets for efficient photoelectrochemical oxidation of neutral water solutions

Preparation of highly active, stable and earth-abundant photoanodes for water oxidation is an important strategy to meet the demand of developing clean-energy technologies. In this paper, efficient and stable photoanodes based on oxygen-deficient black WO3−x@TiO2−x core–shell nanosheets with precisely controlled shell thickness have been fabricated for photoelectrochemical (PEC) conversion from neutral water solutions. The black WO3−x@TiO2−x core–shell nanosheet photoanode with the shell thickness of ∼15 nm achieved around 8 times higher photocurrent density (∼3.20 mA cm−2) than the pure WO3 photoanode at 1.23 V vs. the RHE. An improved onset potential with long-term PEC durability was also realized with the obtained black WO3−x@TiO2−x core–shell nanosheet photoanodes. The promoted PEC water oxidation performance was likely to be originated from enhanced light absorption, interfacial charge transfer and charge separation in these WO3−x@TiO2−x nanosheets which were revealed by finite-difference time-domain simulations and specific band alignment, along with optical and electrochemical spectroscopic evidence. In a word, such black WO3−x@TiO2−x nanosheet photoanodes suggest many exciting opportunities for PEC water splitting toward highly efficient solar fuel generation and many other PEC sensing applications.

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.