Ailong Li

Enhancing charge separation on high symmetry SrTiO3 exposed with anisotropic facets for photocatalytic water splitting

One of the challenging issues in photocatalytic overall water splitting is to efficiently separate the photogenerated charges and the reduction and oxidation catalytic sites on semiconductor-based photocatalysts. It has been reported that the photogenerated charge can be separated between different facets of a semiconductor crystal with low symmetry. However, many semiconductor crystals possess high symmetry (such as the cubic phase) and expose isotropic facets, which are not suitable for charge separation between the facets. Herein, using a nanocrystal morphology tailoring strategy, we synthesized the exposed facets of high symmetry SrTiO3 nanocrystals from isotropic facets (6-facet SrTiO3) to anisotropic facets (18-facet SrTiO3), which leads to the exposure of different crystal facets. We found that the reduction and oxidation catalytic sites can be separately distributed only on the anisotropic facets of 18-facet SrTiO3 nanocrystals, but randomly distributed on every facet of 6-facet SrTiO3 nanocrystals. Based on these findings, the selective distribution of dual-cocatalysts on the anisotropic facets of 18-facet SrTiO3 nanocrystals leads to a fivefold enhancement of apparent quantum efficiency. The superior performance can be attributed to the charge separation between anisotropic facets and the separation of the reduction and oxidation catalytic sites to reduce the charge recombination. These findings will be instructive for the rational design of a high efficiency photocatalytic system for solar energy conversion.

Substrate–Electrode Interface Engineering by an Electron-Transport Layer in Hematite Photoanode

The photoelectrochemical water oxidation efficiency of photoanodes is largely limited by interfacial charge-transfer processes. Herein, a metal oxide electron-transport layer (ETL) was introduced at the substrate-electrode interface. Hematite photoanodes prepared on Li(+)- or WO3-modified substrates deliver higher photocurrent. It is inferred that a Li-doped Fe2O3 (Li:Fe2O3) layer with lower flat band potential than the bulk is formed. Li:Fe2O3 and WO3 are proved to function as an expressway for electron extraction. Via introducing ETL, both the charge separation and injection efficiencies are improved. The lifetime of photogenerated electrons is prolonged by 3 times, and the ratio of surface charge transfer and recombination rate is enhanced by 5 times with Li:Fe2O3 and 125 times with WO3 ETL at 1.23 V versus reversible hydrogen electrode. This result indicates the expedited electron extraction from photoanode to the substrate can suppress not only the recombination at the back contact interface but also those at the surface, which results in higher water oxidation efficiency.

Decorating mesoporous silicon with amorphous metal–phosphorous-derived nanocatalysts towards enhanced photoelectrochemical water reduction

Developing earth-abundant catalysts as alternatives to noble metals and facile approaches that can integrate catalysts to photoelectrodes for the hydrogen evolution reaction (HER) are critical for the successful application of solar-driven water splitting devices. Herein, we presented a facile and universal synthetic route that can incorporate a series of amorphous metal–phosphorous-derived (denoted as M–P) HER catalysts with p-Si under ambient conditions. An onset potential of +0.3 V vs. reversible hydrogen electrode (RHE) and a photocurrent density of ca. 20 mA cm−2 at 0 V vs. RHE were obtained under simulated AM 1.5G solar illumination (100 mW cm−2), and are among the best values ever reported for Si photocathodes decorated with noble-metal-free catalysts. This excellent performance is ascribed to the drastically reduced charge transfer resistance across the p-Si and electrolyte due to the combination of a high quality semiconductor/catalyst interface and highly active cocatalysts with an amorphous nature. Moreover, the M–Ps/p-Si photocathodes demonstrated excellent stability due to the protection afforded by the M–Ps catalysts that were intimately adhered to p-Si.

Design and Fabrication of a Dual-Photoelectrode Fuel Cell towards Cost-Effective Electricity Production from Biomass

A photo fuel cell (PFC) offers an attractive way to simultaneously convert solar and biomass energy into electricity. Photocatalytic biomass oxidation on a semiconductor photoanode combined with dark electrochemical reduction of oxygen molecules on a metal cathode (usually Pt) in separated compartments is the common configuration for a PFC. Herein, we report a membrane-free PFC based on a dual electrode, including a W-doped BiVO4 photoanode and polyterthiophene photocathode for solar-stimulated biomass-to-electricity conversion. Air- and water-soluble biomass derivatives can be directly used as reagents. The optimal device yields an open-circuit voltage (VOC ) of 0.62 V, a short-circuit current density (JSC ) of 775 μA cm-2 , and a maximum power density (Pmax ) of 82 μW cm-2 with glucose as the feedstock under tandem illumination, which outperforms dual-photoelectrode PFCs previously reported. Neither costly separating membranes nor Pt-based catalysts are required in the proposed PFC architecture. Our work may inspire rational device designs for cost-effective electricity generation from renewable resources.

Influence of the Electrostatic Interaction between a Molecular Catalyst and Semiconductor on Photocatalytic Hydrogen Evolution Activity in Cobaloxime/CdS Hybrid Systems

The influence of the electrostatic interaction on photocatalytic H2 evolution activity in cobaloxime/cadmium sulfide (CdS) hybrid systems was studied by measuring the charges of the cobaloximes and the zeta potentials of CdS under different pH conditions (pHs 4-7). Cobaloxime/CdS hybrid systems may have potential as a valid model for the investigation of the electrostatic interaction between a molecular catalyst and semiconductor because the kinetics of methanol oxidation and the driving force of electron transfer from photoirradiated CdS to cobaloxime have little effect on the pH-dependent photocatalytic H2 evolution activity. Our experimental results suggest that electrostatic repulsion between cobaloxime and CdS disfavors the electron transfer from CdS to cobaloxime and hence lowers the photocatalytic H2 evolution activity. Whereas, electrostatic attraction favors the electron transfer process and enhances the photocatalytic H2 evolution activity. However, an electrostatic attraction interaction that is too strong may accelerate both forward and backward electron transfer processes, which would reduce charge separation efficiency and lower photocatalytic H2 evolution activity.

Conversion of Biomass Derivatives to Electricity in Photo Fuel Cells using Undoped and Tungsten-doped Bismuth Vanadate Photoanodes.

The photo fuel cell (PFC) is a promising technology for simultaneously converting solar energy and bioenergy into electricity. Here, we present a miniature air-breathing PFC that uses either BiVO4 or W-doped BiVO4 as the photoanode and a Pt/C catalyst as the air-breathing cathode. The PFC exhibited excellent performance under solar illumination and when fed with several types of biomaterial. We found the PFC performance could be significantly enhanced using W-doping into the BiVO4 photoanode. With glucose as the fuel and simulated sunlight (AM 1.5 G) as the light source, the open-circuit voltage increased from 0.74 to 0.92 V, the short-circuit current density rose from 0.46 to 1.62 mA cm(-2) , and the maximum power density was boosted from 0.05 to 0.38 mW cm(-2) , compared to a PFC using undoped BiVO4 as the anode.

Enhancing photoresponsivity of self-powered UV photodetectors based on electrochemically reduced TiO2 nanorods

Electrochemically reduced TiO2 nanorod arrays (R-NRAs) have been used for the first time to construct a self-powered, visible light blind ultraviolet (UV) photodetector. The fabricated R-NRAs device demonstrated superior photodetector performance with high photon-to-current efficiency of up to 22.5% at an applied bias of 0 V. The enhancement is attributed to a disordered surface layer which greatly improves the charge separation and transfer efficiency at the electrode/electrolyte interface.