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The photocurrent of BiVO4 is limited by surface recombination not surface catalysis, which is currently reported as the main restrict factor for high efficiency photo-electrochemical (PEC) water splitting. To solve this problem, an ultrathin Ni:FeOOH (∼8 nm) modified nanoporous BiVO4 photoanode (BiVO4@Ni:FeOOH) was constructed with core-shell structure for PEC water oxidation. Attributed to the p-n hetero-junction formed between BiVO4 and Ni:FeOOH, the photocurrent density of BiVO4@Ni:FeOOH increased by a factor of 11 (2.86 mA cm−2 at 1.23 V vs. RHE), together with ∼180 mV negative shift of onset potential under AM 1.5 G irradiation (100 mW cm−2) in comparison to the pristine BiVO4. More importantly, detailed insight into the fate of the photo-generated charge carriers at the surface is investigated. Intensity modulated photocurrent spectroscopy (IMPS) is used to investigate the surface carrier dynamics of BiVO4 and BiVO4@Ni:FeOOH. IMPS results and hole scavenger measurement (HSM) certify the main role of Ni:FeOOH is to improve surface recombination by largely decreasing the surface recombination rate constant (krec), not surface catalysis. This work demonstrates Ni:FeOOH can facilitate local surface kinetics and reduce recombination rates as well and be used in other photoelectrodes especially the photoanodes with surface defects for PEC water splitting.

Investigation on the mechanism and kinetics of charge transfer at semiconductor/electrolyte interface is significant for improving the photoelectric conversion efficiency and developing novel and high-efficiency photovoltaic devices. Scanning electrochemical microscopy (SECM), as a powerful analytical technique, has a potential advantage of high spatial and temporal resolution. It has been expanded into a broad range of research fields since the first inception of SECM in 1989 by Bard groups, which includes biological, enzymes, corrosion, energy conversion and storage (such as solar cells, hydrogen and battery). Herein, we review the basic principles and the development of SECM, and chiefly introduce the recent advances of SECM investigation in photoelectrochemical (PEC) cells including solar cells and PEC water splitting. These advances include rapid screening of photocatalysts/photoelectrodes, interfacial reaction kinetics and quantitation of reaction intermediates, which is significant for evaluating the performance, choosing catalysts and developing novel composite photoanodes and high efficiency devices. Finally, we briefly describe the development trends of SECM in energy research.

Background: Chang-Kang-Fang formula (CKF), a multi-herb traditional Chinese medicinal formula, has been clinically used for treatment of irritable bowel syndrome with diarrhea (IBS-D). Though we have reported the compounds of CKF and the therapeutic effect on IBS-D rats, the exact mechanism underlying is still not clear. The aim of this study is to clearly define the effect of CKF on IBS-D by regulating gut microbiota and gut-brain axis. Method: We investigated the effects of CKF on IBS-D rat model, established by psychosocial stress (restraint) combined with the peripheral stimulation (senna leaf gavage) stress. The changed of body weight and the number of fecal pellets was investigated during the experiment. The effect on intestinal sensitivity was assessed based on the abdominal withdrawal reflex (AWR) scores and the intestinal permeability, the expression of ZO-1, measured by immunohistochemistry. The effect of CKF on gut-brain axis was evaluated by the expression of 5-HT through immunohistochemistry. The composition of gut microbiota was detected through 16sRNA. Results: Administration of CKF significantly have shown the therapeutic effect on IBS-D rats, involving decreased the score of AWR and increased the number of pellets, though there were no different on body weight change. In addition, CKF could upregulated the expression of ZO-1 in colon and downregulated the expression of 5-HT in colon and brain Moreover, CKF could rebalance the gut microbiota of IBS-D, increasing the abundance of Lactobacillu, Allobaculum, Roseburia and Lachnospiraceae_NK4A136. Conclusion: CKF potentially alleviates IBS-D through regulating gut microbiota and gut-brain axis.

With the development of clean and renewable energy, hydrogen produced via photoelectrochemical (PEC) water splitting has attracted considerable attention. However, to develop the photoanodes with stable and excellent PEC ability is still a big challenge. In our work, TiO2 nanorods decorated with boron doped g-C3N4 (BCN/TiO2) is fabricated via thermal polymerization method to improve the PEC performance. The BCN/TiO2 displays 4-fold increase of the photocurrent density (1.01 mA cm−2) at 1.23 V vs. RHE under irradiation (100 mW cm−2, AM 1.5 G). And the onset potential of BCN/TiO2 exhibits a negative shift with 100 mV. Attributed to the broad light absorption of BCN and hetero-junction forming between BCN and TiO2, the IPCE value is increased to 87.8% in 380 nm, and the charge separation and transfer efficiency are both increased. Doping metal-free inorganic material with heteroatoms is a simple and efficient strategy to increase the light absorption within visible light and charge transfer efficiency in PEC and photocatalytic applications.

Ni,Fe-based (oxy)hydroxides have been one of the most active catalysts for the oxygen evolution reaction. In this article, reduced graphene oxide supported Ni doped FeOOH (RGO/Ni:FeOOH) was prepared for electrochemical and photoelectrochemical (PEC) water oxidation. The RGO/Ni:FeOOH exhibited a lower over-potential (260 mV at 10 mA cm−2) and smaller Tafel slope (32.3 mV dec−1) than that of the FeOOH and Ni:FeOOH. Such significant enhancement is attributed to Ni doping and RGO, which reduce the over-potential, improve the conductivity and enlarge surface areas. Besides, RGO/Ni:FeOOH decorated the TiO2 nanorods (NRs) was also fabricated for photoelectrochemical (PEC) water oxidation, which exhibited a higher photocurrent density and lower onset potential than that of TiO2 NRs the bare under illumination due to the synergistic effect of RGO and Ni:FeOOH. These results demonstrate the RGO/Ni:FeOOH has great promising as a co-catalyst to improve the PEC performance.

As an emerging technique for rapid detection, Surface-enhanced Raman scattering (SERS) has received much practical attention. Here, we report a uniformly distributed silver nanoparticles on well-defined tungsten oxide nanoflakes hierarchical nanostructures (Ag/WO3-x) as flexible solid SERS substrate. The vertically aligned Ag/WO3-x nanoflakes were prepared by in situ redox reaction without reductant additives, ensuring the clean surface and free of extra interferences with good signal-to-noise ratio. Using rhodamine 6G (R6G) as probe molecule, the enhancement factor of developed Ag/WO3-x nanoflakes SERS substrate was found to be high as 2.41 × 107 with an ultrasensitive detection concentration of 10-12 M. Additionally, Ag/WO3-x nanoflakes SERS substrate exhibited good reproducibility and long-time stability. Meanwhile, the pesticide thiram with a 1.3 pM limit of detection (LOD) can be readily determined via developed SERS substrate. To illustrate the potential practical value of the prepared substrates, 2.4 ppb spiked thiram on apple peel surface were both successfully detected. Notably, the developed SERS substrates could be cut into small pieces for detecting trace chemical or biological molecules and the flexible utilization of substrates would be improved dramatically. As a result, vertically aligned Ag/WO3-x nanoflakes with well-defined shape and hierarchical structure have great potential as solid SERS substrate in the field of fast detection and trace analysis.

Utilization of solar energy to split water via photoelectrochemical (PEC) technique is a desirable technology to address the development of sustainable carbon-free energy source. The key challenge of this advanced technique is to design a photoelectrode with both high solar to chemical efficiency and fast charge transfer and separation efficiency. Herein, the core-shell heterostructure of TiO2@PCN (P doped g-C3N4) decorating with Au nanoparticles (Au NPs) photoanode (Au/TiO2@PCN) is successfully fabricated for high-efficiency PEC water splitting. The resulting integrated Au/TiO2@PCN photoanode exhibits significantly enhanced PEC activities both in UV and visible light region with AM 1.5 G irradiation (100 mW cm−2). Each component of Au/TiO2@PCN plays a vital role in promoting the PEC performance. The TiO2@PCN core-shell heterostructure could remarkably facilitate the charge transfer and separation efficiency with the effect of an internal built-in electric field. The introduction of Au NPs could further enhance the charge transfer efficiency and improve the PEC activity of TiO2@PCN in visible light, which is attributed to the localized surface plasmon resonance (LSPR) effect. Accordingly, the Au/TiO2@PCN presents pronounced photocurrent density of 2.03 mA cm−2 (1.23 V vs. RHE), which is four-fold higher than that of TiO2 NRs (0.51 mA cm−2). This work supports a new approach to combine the advantages of the core-shell heterostructure and the LSPR effect of noble metal for high-performance PEC water oxidation.

Nitrogen doping via high-temperature ammonization is a frequently used strategy to extend the light harvesting capacity of wide-bandgap catalysts in the visible region. Under such a reductive atmosphere, the reduction of transition metals is supposed to occur, however, this has not been thoroughly studied yet. Here, by combining chemically-controlled doping and subsequent liquid exfoliation, ultra-thin [Nb3O8]- nanosheets with separate N doping, reduced-Nb doping and N/reduced-Nb codoping were fabricated for comparative studies on the doping effect for photocatalytic hydrogen evolution. Layered KNb3O8 was used as the starting material and the above-mentioned three doping conditions were achieved by high-temperature treatment with urea, hydrogen and ammonia, respectively. The morphology, crystal and electronic structures, and the catalytic activity of the products were characterized thoroughly by means of TEM, AFM, XRD, XPS, EPR, absorption spectroscopy and photocatalytic hydrogen evolution. Significantly, the black N/reduced-Nb co-doped monolayer [Nb3O8]- nanosheets exhibit the mostly enhanced photocatalytic hydrogen generation rate, indicating a synergistic doping effect of the multiple chemical-design strategy. The modified electronic structure of [Nb3O8]- nanosheets and the role of exotic dopants in bandgap narrowing are put forward for the rational design of better photocatalysts with reduced-metal self-doping.

Hydrogen production by catalytic water splitting using sunlight holds great promise for clean and sustainable energy source. Despite the efforts made in the past decades, challenges still exist in pursuing solid catalysts with light harvesting capacity, large surface areas and efficient utilities of the photo-generated carrier at the same time. Here, we demonstrate a multiple structure design strategy to achieve highly enhanced photocatalytic performance on hydrogen production from water splitting in Dion-Jacobson perovskites KCa2Nan-3NbnO3n+1. Specifically, chemical doping (N/Nb4+) of the parent oxides via ammoniation improved the ability of sunlight harvesting efficiently; Subsequent liquid exfoliation of the doped perovskites yielded ultra-thin [Ca2Nan-3NbnO3n+1]- nanosheets with greatly increased surface areas. Significantly, the maximum hydrogen evolution appears in the n=4 nanosheets, which suggests the most favorable thickness for charge separation in such perovskite-type catalysts. The optimized black N/Nb4+-[Ca2NaNb4O13]- nanosheets show greatly enhanced photocatalytic performance, as high as 973 μmol/h with Pt-loading, on hydrogen evolution from water splitting. As a proof-of-concept demonstration, this work highlights the feasibility of combining various chemical strategies towards better catalysts and precise thickness-control of two-dimensional materials.