Acid Treatment Enables Suppression of Electron-Hole Recombination in Hematite for Photoelectrochemical Water Splitting

We report a new strategy for efficient suppression of electron-hole recombination in hematite photoanodes. Acid-treated hematite show substantially enhanced photocurrent density compared to untreated samples. Electrochemical impedance spectroscopy studies reveal that the enhanced photocurrent is partly due to improved efficiency of charge separation. Transient absorption spectroscopic studies coupled to electrochemical measurements indicate that in addition to improved bulk electrochemical properties, acid treated hematite has significantly decreased surface electron-hole recombination losses due to a greater yield of the trapped photoelectrons being extracted to the external circuit

Nanostructured Hematite for Photoelectrochemical Water Splitting

Solar water splitting is an environmentally friendly reaction of producing hydrogen gas. Since Honda and Fujishima first demonstrated solar water splitting in 1972 by using semiconductor titanium dioxide (TiO2) as photoanode in a photoelectrochemical (PEC) cell, extensive efforts have been invested into improving the solar-to-hydrogen (STH) conversion efficiency and lower the production cost of photoelectrochemical devices. In the last few years, hematite (α-Fe2O3) nanostructures have been extensively studied as photoanodes for PEC water splitting. Although nanostructured hematite can improve its photoelectrochemical water splitting performance to some extent, by increasing active sites for water oxidation and shortening photogenerated hole path length to semiconductor/electrolyte interface, the photoactivity of pristine hematite nanostructures is still limited by a number of factors, such as poor electrical conductivities and slow oxygen evolution reaction kinetics. Previous studies have shown that tin (Sn) as an n-type dopant can substantially enhance the photoactivity of hematite photoanodes by modifying their optical and electrical properties. In this thesis, I will first demonstrate an unintentional Sn-doping method via high temperature annealing of hematite nanowires grown on fluorine-doped tin oxide (FTO) substrate to enhance the donor density. In addition to introducing extrinsic dopants into semiconductors, the carrier densities of hematite can also be enhanced by creating intrinsic defects. Oxygen vacancies function as shallow donors for a number of hematite. In this regard, I have investigated the influence of oxygen content on thermal decomposition of FeOOH to induce oxygen vacancies in hematite. In the end, I have studied low temperature activation of hematite nanostructures

Synthesis of urchin-like CdWO4 microspheres via a facile template free hydrothermal method

Urchin-like CdWO4 microspheres with hollow interiors have been successfully synthesized by a facile template free hydrothermal treatment method. The urchin-like CdWO4 hollow spheres are composed of radiatively assembled single-crystalline CdWO4 nanorods with lengths of several hundred nm and widths of 25 80 nm. The samples exhibit a blue-green emission in the range of 450 500 nm with the emission peak centered around 470 nm when excited at 293 nm. The effects of the preparation conditions such as the hydrothermal synthesis temperature, hydrothermal synthesis time, and the dosage of urea on the crystalline phase as well as morphology of final products have been systematically investigated. The successful synthesis of CdWO4 with a uniform urchin-like morphology relies on not only the choice of peroxo-polytungstic acid precursor, but also a controlled pH adjustment with the help of urea, both of which contribute to a homogenous nucleation and crystal growth process. It is also found that increasing the synthesis temperature, time and dosage of urea will enhance the photoluminescence (PL) emission of resultant materials. Our contribution provides a simple approach to synthesize CdWO4 materials with hierarchical architectures and potential applications as detectors in X-ray devices

A mechanistic study into the catalytic effect of Ni(OH)2 on hematite for photoelectrochemical water oxidation. Nanoscale 2013, 5, 4129–4133. [CrossRef] [PubMed

We report a mechanistic study of the catalytic effect of Ni(OH) 2 on hematite nanowires for photoelectrochemical water oxidation. Ni compounds have been shown to be good catalysts for electrochemical and photoelectrochemical water oxidation. While we also observed improved photocurrents for Ni-catalyst decorated hematite photoanodes, we found that the photocurrents decay rapidly, indicating the photocurrents were not stable. Importantly, we revealed that the enhanced photocurrent was due to water oxidation as well as the photo-induced charging effect. In addition to oxidizing water, the photoexcited holes generated in hematite efficiently oxidize Ni . We proposed that the catalytic mechanism of the Ni(II) catalyst for water oxidation is a two-step process that involves the fast initial oxidation of Ni 2+ to Ni . Finally, we elucidated the real catalytic performance of Ni(OH) 2 on hematite for photoelectrochemical water oxidation by suppressing the photoinduced charging effect. This work could provide important insights for future studies on Ni based catalyst modified photoelectrodes for water oxidation

Oxygen deficient α-Fe2O3 photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses.

Intrinsic doping of hematite through the inclusion of oxygen vacancies (VO) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe2O3-x photoelectrodes. Whilst it is widely accepted that the introduction of VO leads to improved conductivities, little else is verified regarding the actual mechanism of enhancement. Here we employ transient absorption (TA) spectroscopy to build a comprehensive kinetic model for water oxidation on α-Fe2O3-x . In contrast to previous suggestions, the primary effect of introducing VO is to block very slow (ms) surface hole - bulk electron recombination pathways. In light of our mechanistic research we are also able to identify and address a cause of the high photocurrent onset potential, a common issue with this class of electrodes. Atomic layer deposition (ALD) of Al2O3 is found to be particularly effective with α-Fe2O3-x , leading to the photocurrent onset potential shifting by ca. 200 mV. Significantly TA measurements on these ALD passivated electrodes also provide important insights into the role of passivating layers, that are relevant to the wider development of α-Fe2O3 photoelectrodes

Acid Treatment Enables Suppression of Electron-Hole Recombination in Hematite for Photoelectrochemical Water Splitting

We report a strategy for efficient suppression of electron–hole recombination in hematite photoanodes. Acid‐treated hematite showed a substantially enhanced photocurrent density compared to untreated samples. Electrochemical impedance spectroscopy studies revealed that the enhanced photocurrent is partly due to improved efficiency of charge separation. Transient absorption spectroscopic studies coupled to electrochemical measurements indicate that, in addition to improved bulk electrochemical properties, acid‐treated hematite has significantly decreased surface electron–hole recombination losses owing to a greater yield of the trapped photoelectrons being extracted to the external circuit

Photoenhanced Electrochemical Interaction between <i>Shewanella</i> and a Hematite Nanowire Photoanode

Here we report the investigation of interplay between light, a hematite nanowire-arrayed photoelectrode, and <i>Shewanella oneidensis</i> MR-1 in a solar-assisted microbial photoelectrochemical system (solar MPS). Whole cell electrochemistry and microbial fuel cell (MFC) characterization of <i>Shewanella oneidensis</i> strain MR-1 showed that these cells cultured under (semi)­anaerobic conditions expressed substantial <i>c</i>-type cytochrome outer membrane proteins, exhibited well-defined redox peaks, and generated bioelectricity in a MFC device. Cyclic voltammogram studies of hematite nanowire electrodes revealed active electron transfer at the hematite/cell interface. Notably, under a positive bias and light illumination, the hematite electrode immersed in a live cell culture was able to produce 150% more photocurrent than that in the abiotic control of medium or dead culture, suggesting a photoenhanced electrochemical interaction between hematite and <i>Shewanella</i>. The enhanced photocurrent was attributed to the additional redox species associated with MR-1 cells that are more thermodynamically favorable to be oxidized than water. Long-term operation of the hematite solar MPS with light on/off cycles showed stable current generation up to 2 weeks. Fluorescent optical microscope and scanning electron microscope imaging revealed that the top of the hematite nanowire arrays were covered by a biofilm, and iron determination colorimetric assay revealed 11% iron loss after a 10-day operation. To our knowledge, this is the first report on interfacing a photoanode directly with electricigens in a MFC system. Such a system could open up new possibilities in solar-microbial device that can harvest solar energy and recycle biomass simultaneously to treat wastewater, produce electricity, and chemical fuels in a self-sustained manner

Increased Interleukin-23 in Hashimoto’s Thyroiditis Disease Induces Autophagy Suppression and Reactive Oxygen Species Accumulation

Hashimoto’s thyroiditis (HT) represents the most common organ-specific autoimmune disease. Inflammatory factors and reactive oxygen species (ROS) play detrimental roles during the pathogenesis of HT. In this study, we found that thyroid follicular cells (TFCs) from HT patients expressed an elevated level of interleukin-23 (IL-23), which contributed to autophagy suppression and ROS accumulation. Additionally, IL-23-induced autophagy suppression and ROS accumulation in human TFCs was attributed to AKT/mTOR/NF-κB signaling pathway activation. Inhibition of either IL-23 by a specific neutralization antibody, or mTOR by rapamycin, or NF-κB by IKK-16, significantly reversed the autophagy suppression and ROS accumulation. These results demonstrate a key role for IL-23 in HT pathogenesis and provide a potential therapeutic strategy against IL-23 or its signaling pathway in HT

LiCl/PVA Gel Electrolyte Stabilizes Vanadium Oxide Nanowire Electrodes for Pseudocapacitors

Here we report a new strategy to improve the electrochemical stability of vanadium oxide electrodes for pseudocapacitors. Vanadium oxides are known to suffer from severe capacitance loss during charging/discharging cycling, due to chemical dissolution and ion intercalation/deintercalation-induced material pulverization. We demonstrate that these two issues can be addressed by using a neutral pH LiCl/PVA gel electrolyte. The function of the gel electrolyte is twofold: (i) it reduces the chemical dissolution of amphoteric vanadium oxides by minimizing water content and providing a neutral pH medium and (ii) it serves as a matrix to maintain the vanadium oxide nanowire network structure. Vanadium oxide nanowire pseudocapacitors with gel electrolyte exhibit excellent capacitance retention rates of more than 85% after cycling for 5000 cycles, without sacrificing the electrochemical performance of vanadium oxides

Photohole Induced Corrosion of Titanium Dioxide: Mechanism and Solutions

Titanium dioxide (TiO₂) has been extensively investigated as photoanode for water oxidation, as it is believed to be one of the most stable photoanode materials. Yet, we surprisingly found that TiO₂ photoanodes (rutile nanowire, anatase nanotube, and P25 nanoparticle film) suffered from substantial photocurrent decay in neutral (Na₂SO₄) as well as basic (KOH) electrolyte solution. Photoelectrochemical measurements togehter with electron microscopy studies performed on rutile TiO₂ nanowire photoanode show that the photocurrent decay is due to photohole induced corrosion, which competes with water oxidation reaction. Further studies reveal that photocurrent decay profile in neutral and basic solutions are fundamentally different. Notably, the structural reconstruction of nanowire surface occurs simultaneously with the corrosion of TiO₂ in KOH solution resulting in the formation of an amorphous layer of titanium hydroxide, which slows down the photocorrosion. Based on this discovery, we demonstrate that the photoelectrochemical stability of TiO₂ photoanode can be significantly improved by intentionally coating an amorphous layer of titanium hydroxide on the nanowire surface. The pretreated TiO₂ photaonode exhibits an excellent photocurrent retention rate of 97% after testing in KOH solution for 72 h, while in comparison the untreated sample lost 10−20% of photocurrent in 12 h under the same measurement conditions. This work provides new insights in understanding of the photoelectrochemical stability of bare TiO₂ photoanodes