Shengyang Wang

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.

Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts

Plasmonic photocatalysis, stemming from the effective light absorbance and confinement of surface plasmons, provides a pathway to enhance solar energy conversion. Although the plasmonic hot electrons in water reduction have been extensively studied, exactly how the plasmonic hot holes participate in the water splitting reaction has not yet been well understood. In particular, where the plasmonic hot holes participate in water oxidation is still illusive. Herein, taking Au/TiO2 as a plasmonic photocatalyst prototype, we investigated the plasmonic hot holes involved in water oxidation. The reaction sites are positioned by photodeposition together with element mapping by electron microscopy, while the distribution of holes is probed by surface photovoltage imaging with Kelvin probe force microscopy. We demonstrated that the plasmonic holes are mainly concentrated near the gold-semiconductor interface, which is further identified as the reaction site for plasmonic water oxidation. Density functional theory also corroborates these findings by revealing the promotion role of interfacial structure (Ti-O-Au) for oxygen evolution. Furthermore, the interfacial effect on plasmonic water oxidation is validated by other Au-semiconductor photocatalytic systems (Au/SrTiO3, Au/BaTiO3, etc.).

Bridging surface states and current–potential response over hematite-based photoelectrochemical water oxidation

The relation between surface states (SS) and the current–potential (j–V) response is unravelled in an electrochemical way. The photocurrent is initiated at the potential where SS turn vacant, the slope of j–V curves depends on the density of SS. The distribution of SS impacts the profiles of j–V curves of a hematite photoanode.

Dual Extraction of Photogenerated Electrons and Holes from a Ferroelectric Sr0.5Ba0.5Nb2O6 Semiconductor.

The separation of photogenerated charges is a critical factor in photocatalysis. Recently, anomalous photovoltaic (APV) field effects (Voc ∼ 10(3) V/cm) in ferroelectrics, with their strong driving force for charge separation, have attracted much attention in photocatalysis and photoelectrocatalysis. However, it is still unknown whether photogenerated electrons and holes can be simultaneously extracted by the strong driving force toward the surface of ferroelectrics and can become available for surface reactions. This issue becomes critically important in photocatalysis because the surface reaction utilizes both the electrons and holes that reach the surface. In this work, a model lateral symmetric structure, metal/Sr0.5Ba0.5Nb2O6/metal (metal = Ag or Pt), as an electrode was fabricated. The dual extractions of photogenerated electrons and holes on the two opposite metal electrodes were achieved, as revealed by photovoltaic and ferroelectrical hysteresis measurements and photoassisted Kelvin probe force microscopy (KPFM). It was found that the high Schottky barriers of the two opposite Sr0.5Ba0.5Nb2O6-Pt electrodes are key factors that alter the two space charge regions (SCRs) by a poling effect. The resulting built-in electrical fields with parallel directions near both electrodes significantly enhance the charge separation ability. Our model unravels the driving force of charge separation in ferroelectric semiconductors, thus demonstrating the potential for highly efficient charge separation in photocatalysis.

Promoting Charge Separation and Injection by Optimizing the Interfaces of GaN:ZnO Photoanode for Efficient Solar Water Oxidation

Photoelectrochemical water splitting provides an attractive way to store solar energy in molecular hydrogen as a kind of sustainable fuel. To achieve high solar conversion efficiency, the most stringent criteria are effective charge separation and injection in electrodes. Herein, efficient photoelectrochemical water oxidation is realized by optimizing charge separation and surface charge transfer of GaN:ZnO photoanode. The charge separation can be greatly improved through modified moisture-assisted nitridation and HCl acid treatment, by which the interfaces in GaN:ZnO solid solution particles are optimized and recombination centers existing at the interfaces are depressed in GaN:ZnO photoanode. Moreover, a multimetal phosphide of NiCoFeP was employed as water oxidation cocatalyst to improve the charge injection at the photoanode/electrolyte interface. Consequently, it significantly decreases the overpotential and brings the photocurrent to a benchmark of 3.9 mA cm-2 at 1.23 V vs RHE and a solar conversion efficiency over 1% was obtained.