Jingying Shi

Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3

When Alpha met Beta: a tuneable α-β surface phase junction on Ga(2)O(3) can significantly improve photocatalytic overall water splitting into H(2) and O(2) over individual α-Ga(2)O(3) or β-Ga(2)O(3) surface phases. This enhanced photocatalytic performance is mainly attributed to the efficient charge separation and transfer across the α-β phase junction.

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.

Visible light driven overall water splitting using cocatalyst/BiVO4 photoanode with minimized bias†

BiVO4 and many other semiconductor materials are ideal visible light responsive semiconductors, but are insufficient for overall water splitting. Upon loading water oxidation cocatalyst, for example Co-borate (denoted as CoBi) used here, onto BiVO4 photoanode, it is found that not only the onset potential is negatively shifted but also the photocurrent and the stability are significantly improved. And more importantly, PEC overall water splitting to H2 and O2 is realized using CoBi/BiVO4 as photoanode with a rather low applied bias (less than 0.3 V vs. counter electrode) in a two-electrode scheme, while at least 0.6 V is needed for bare BiVO4. This work demonstrates the practical possibility of achieving overall water splitting using the PEC strategy under a bias as low as the theoretical minimum, which is the difference between the flat band and proton reduction potential for a photoanode thermodynamically insufficient for water reduction. As long as the water oxidation overpotential is overcome with an efficient cocatalyst, the applied bias of the whole system is only used for that thermodynamically required for the proton reduction.

Enabling an integrated tantalum nitride photoanode to approach the theoretical photocurrent limit for solar water splitting

The feasibility of photoelectrochemical (PEC) water-splitting cells relies on the development of high-performance photoanodes. Significant progress has been made in the discovery of narrow bandgap semiconductors as promising photoanodes. However, the rational design of photoanode architecture that brings the potentials of narrow bandgap semiconductors into fruition for efficient PEC water oxidation still remains a key challenge. Herein, we show a highly efficient photoanode system consisting of a tantalum nitride (Ta3N5) semiconductor for light harvesting, hole-storage layers (Ni(OH)x/ferrhydrite) that mediate interfacial charge transfer from Ta3N5 to coupled molecular catalysts (Co cubane and Ir complex) for water oxidation and a TiOx blocking layer that reduces the surface electron–hole recombination. The integrated Ta3N5 photoanode exhibits a record photocurrent of 12.1 mA cm−2 at 1.23 V vs. the reversible hydrogen electrode (RHE), which is nearly its theoretical photocurrent limit under sunlight (12.9 mA cm−2), suggesting that almost each pair of photogenerated charge carriers in Ta3N5 has been efficiently extracted and collected for solar water splitting.

Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation

A composite Sr2TiO4/SrTiO3(La,Cr) heterojunction photocatalyst has been prepared by a simple in situ polymerized complex method. Upon Pt cocatalyst loading, this catalyst shows higher photocatalytic activity towards hydrogen production than individual SrTiO3(La,Cr) and Sr2TiO4(La,Cr) in the presence of methanol sacrificial reagent. Microscopic morphology studies show that well defined heterojunctions are formed by matching the lattice fringes of SrTiO3(La,Cr) and Sr2TiO4(La,Cr), and Pt was preferentially loaded on the surface of the Sr2TiO4(La,Cr) component in the composite Sr2TiO4/SrTiO3(La,Cr) photocatalyst. XPS and EPR analyses show that the composite photocatalyst also has the lowest amount of Cr6+ electron trapping sites. Band structure analysis by combining absorption spectroscopy and Mott–Schottky plots shows that, in the composite photocatalyst, the photogenerated electrons and holes tend to migrate from SrTiO3(La,Cr) to Sr2TiO4(La,Cr) and from Sr2TiO4(La,Cr) to SrTiO3(La,Cr), respectively. This kind of band structure can facilitate charge transfer and separation driven by the minor potential difference between the two components, which is further confirmed by the observation of long lived electrons in the time resolved FT-IR spectroscopic study. It is concluded that the superior photocatalytic activity of the composite heterojunction photocatalyst is due to efficient charge transfer and separation by well defined heterojunctions formed between SrTiO3(La,Cr) and Sr2TiO4(La,Cr), preferential loading of Pt nanoparticles on the Sr2TiO4(La,Cr) component, and the lowest amount of Cr6+ in the composite photocatalyst. The tailored design and synthesis of the composite heterojunction structure is a promising approach for the improvement of the photocatalytic activity of a photocatalyst.

Microwave-assisted hydrothermal synthesis of perovskite NaTaO3 nanocrystals and their photocatalytic properties

A fast and facile process for the preparation of perovskite NaTaO3 nanocrystals with Ta2O5 and NaOH as starting materials by a microwave-assisted hydrothermal (MHT) technique is reported. By pretreating the Ta2O5 powder by ball milling, pure-phase NaTaO3 can be synthesized under quite mild conditions through the formation of an intermediate pyrochlore Na2Ta2O6 phase, while much longer times are required in a conventional hydrothermal (CHT) process. After loading with NiO co-catalyst, the NaTaO3 nanocrystals prepared by the MHT method showed photocatalytic activity for overall water splitting more than two times greater than those prepared under CHT conditions.

Integrating a dual-silicon photoelectrochemical cell into a redox flow battery for unassisted photocharging

Solar rechargeable flow cells (SRFCs) provide an attractive approach for in situ capture and storage of intermittent solar energy via photoelectrochemical regeneration of discharged redox species for electricity generation. However, overall SFRC performance is restricted by inefficient photoelectrochemical reactions. Here we report an efficient SRFC based on a dual-silicon photoelectrochemical cell and a quinone/bromine redox flow battery for in situ solar energy conversion and storage. Using narrow bandgap silicon for efficient photon collection and fast redox couples for rapid interface charge injection, our device shows an optimal solar-to-chemical conversion efficiency of ∼5.9% and an overall photon–chemical–electricity energy conversion efficiency of ∼3.2%, which, to our knowledge, outperforms previously reported SRFCs. The proposed SRFC can be self-photocharged to 0.8 V and delivers a discharge capacity of 730 mAh l−1. Our work may guide future designs for highly efficient solar rechargeable devices.

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.

Efficient Hole Extraction from a Hole-Storage-Layer-Stabilized Tantalum Nitride Photoanode for Solar Water Splitting

One of the major hurdles that impedes the practical application of photoelectrochemical (PEC) water splitting is the lack of stable photoanodes with low onset potentials. Here, we report that the Ni(OH)x/MoO3 bilayer, acting as a hole-storage layer (HSL), efficiently harvests and stores holes from Ta3N5, resulting in at least 24 h of sustained water oxidation at the otherwise unstable Ta3N5 electrode and inducing a large cathodic shift of ≈600 mV in the onset potential of the Ta3N5 electrode.

Photoelectrochemical Water Splitting Promoted with a Disordered Surface Layer Created by Electrochemical Reduction

The recent discovery of colored TiO2 indicated that the disordered surface layer over the TiO2 particles/photoelectrodes is beneficial for higher photocatalytic performance; however, the role of the disordered surface TiO2 layer is not well understood. Here, we report an electrochemical strategy for tuning the surface structure of TiO2 nanorod arrays (NRAs) and try to understand the role of the disordered surface TiO2 layer. Photoelectrodes of TiO2 NRAs with a disordered shell were prepared by an electrochemical reduction method. The photocurrent of the NRAs with a disordered shell can reach as high as ∼1.18 mA/cm(2) at 1.23 V, which is 2.2 times of that of the pristine TiO2 NRAs. Our results show that the surface disordered layer not only improves the bulk charge separation but also suppresses the charge recombination at the electrode/electrolyte interface, acting as an efficient water oxidation cocatalyst of photoelectrochemical cell for solar water splitting.