Jingfeng Han

A Tantalum Nitride Photoanode Modified with a Hole-Storage Layer for Highly Stable Solar Water Splitting**

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm(-2) , whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole-storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.

An Integrated Photoelectrochemical–Chemical Loop for Solar‐Driven Overall Splitting of Hydrogen Sulfide

Abundant and toxic hydrogen sulfide (H2 S) from industry and nature has been traditionally considered a liability. However, it represents a potential resource if valuable H2 and elemental sulfur can be simultaneously extracted through a H2 S splitting reaction. Herein a photochemical-chemical loop linked by redox couples such as Fe(2+) /Fe(3+) and I(-) /I3 (-) for photoelectrochemical H2 production and H2 S chemical absorption redox reactions are reported. Using functionalized Si as photoelectrodes, H2 S was successfully split into elemental sulfur and H2 with high stability and selectivity under simulated solar light. This new conceptual design will not only provide a possible route for using solar energy to convert H2 S into valuable resources, but also sheds light on some challenging photochemical reactions such as CH4 activation and CO2 reduction.

Cu2O/CuO photocathode with improved stability for photoelectrochemical water reduction

A p-type cuprous oxide (Cu2O) photocathode covered with a thin layer of cupric oxide (CuO) was prepared by fast annealing of a copper foil via H2–O2 flame. The as-prepared composite photocathode is characteristic of a junction structure formed between the (110) plane of Cu2O and the (111) plane of CuO. The Cu2O/CuO composite photocathode showed improved stability for photoelectrochemical (PEC) water reduction by increasing the coverage of CuO on Cu2O.

Manipulating the Interfacial Energetics of n-type Silicon Photoanode for Efficient Water Oxidation

The photoanodes with heterojunction behavior could enable the development of solar energy conversion, but their performance largely suffers from the poor charge separation and transport process through the multiple interfacial energy levels involved. The question is how to efficiently manipulate these energy levels. Taking the n-Si Schottky photoanode as a prototype, the undesired donor-like interfacial defects and its adverse effects on charge transfer in n-Si/ITO photoanode are well recognized and diminished through the treatment on electronic energy level. The obtained n-Si/TiOx/ITO Schottky junction exhibits a highly efficient charge transport and a barrier height of 0.95 eV, which is close to the theoretical optimum for n-Si/ITO Schottky contact. Then, the holes extraction can be further facilitated through the variation of surface energy level, with the NiOOH coated ITO layer. This is confirmed by a 115% increase in surface photovoltage of the photoanodes. Eventually, an unprecedentedly low onset potential of 0.9 V (vs RHE) is realized for water oxidation among n-Si photoanodes. For the water oxidation reaction, the n-Si/TiOx/ITO/NiOOH photoanode presents a charge separation efficiency up to 100% and an injection efficiency greater than 90% at a wide voltage range. This work identifies the important role of interfacial energetics played in photoelectrochemical conversion.

Cl- making overall water splitting possible on TiO2-based photocatalysts

Overall water splitting on a TiO2-based photocatalyst has been extensively investigated. However, in most cases, the products are not in a stoichiometric ratio, thus the reaction is not really overall water splitting. In this work, we found that in the presence of Cl−, the evolution of O2 and H2 over Pt/TiO2 can be successfully achieved, and the activity can be enhanced up to 3 times compared to having no Cl− present. Furthermore, the H2 : O2 ratio can be close to 2.0, i.e. the stoichiometric ratio of overall water splitting. It is proposed that the Cl− ion is involved with the reaction intermediate of O2 evolution from water oxidation. Our work not only reported overall water splitting on a TiO2-based photocatalyst, but also provided experimental evidence for understanding the possible reaction process and the mechanism of photocatalytic water splitting.

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.

Using Pd as a Cocatalyst on GaN–ZnO Solid Solution for Visible-Light-Driven Overall Water Splitting

Rhodium was reported to be an essential component of the efficient cocatalyst (Rh2−xCrxO3 or core/shell of Rh@Cr2O3) on GaN–ZnO for photocatalytic overall water splitting (POWS). Herein we demonstrate that the Rh can be replaced by less-expensive Pd deposited via atomic layer deposition and together with photodeposition of Cr2O3 shell, a comparable activity in POWS can be achieved even with much less Pd loading than Rh in Rh2−xCrxO3/GaN–ZnO under the same reaction conditions.Graphical Abstract

A new Pb(IV)-based photocathode material Sr2PbO4 with good light harvesting ability

Crystalline plumbate Sr2PbO4 has been synthesized by a high temperature flux method and characterized by X-ray powder diffraction analysis, UV-vis spectroscopy, theoretical studies and SEM analysis. The results show that Sr2PbO4 has indirect optical transitions with an energy of about 1.75 eV which can be assigned to the electron transfer from the state of O-2p to the mixed states of O-2p and Pb-6s. The photoelectrochemical properties of Sr2PbO4 were studied for the first time, indicating that this material has p-type conductivity. The measured current density at an intensity of 47 μA cm−2 makes Sr2PbO4 a promising photocathode material for visible-light-driven water splitting.