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

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

Composite Perovskites of Cesium Lead Bromide for Optimized Photoluminescence

The halide perovskite CsPbBr₃ has shown its promise for green light-emitting diodes. The optimal conditions of photoluminescence and the underlying photophysics, however, remain controversial. To address the inconsistency seen in the previous reports and to offer high-quality luminescent materials that can be readily integrated into functional devices with layered architecture, we created thin films of CsPbBr₃/Cs₄PbBr₆ composites based on a dual-source vapor-deposition method. With the capability of tuning the material composition in a broad range, CsPbBr₃ is identified as the only light emitter in the composites. Interestingly, the presence of the photoluminescence-inactive Cs₄PbBr₆ can significantly enhance the light emitting efficiency of the composites. The unique negative thermal quenching observed near the liquid nitrogen temperature indicates that a type of shallow state generated at the CsPbBr₃/Cs₄PbBr₆ interfaces is responsible for the enhancement of photoluminescence

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

Surface Passivation of TiO₂ Nanowires Using a Facile Precursor-Treatment Approach for Photoelectrochemical Water Oxidation

We developed a facile precursor-treatment approach for effective surface passivation of rutile TiO₂ nanowire photoanode to improve its performance in photoelectrochemical (PEC) water oxidation. The approach was demonstrated by treating rutile TiO₂ nanowires with titanium precursor solutions (TiCl₄, Ti­(OBu)₄, or Ti­(OiP)₄) followed by a postannealing process, which resulted in the additional deposition of anatase TiO₂ layer on the nanowire surface. Compared to pristine TiO₂, all the precursor-treated TiO₂ nanowire electrodes exhibited a significantly enhanced photocurrent density under white light illumination. Among the three precursor-treated samples, Ti­(OBu)₄-treated TiO₂ nanowires achieved the largest enhancement of photocurrent generation, which is approximately a 3-fold increase over pristine TiO₂. Monochromatic incident photon-to-electron conversion efficiency (IPCE) measurements showed that the improvement of PEC performance was dominated by the enhanced photoactivity of TiO₂ in the UV region. The photovoltage and electrochemical impedance spectroscopy (EIS) measurements showed that the enhanced photoactivity can be attributed to the improved charge transfer as a result of effective surface state passivation. This work demonstrates a facile, low-cost, and efficient method for preparing highly photoactive TiO₂ nanowire electrodes for PEC water oxidation. This approach could also potentially be used for other photoconversion applications, such as TiO₂ based dye-sensitized solar cells, as well as photocatalytic systems

Stabilized TiN Nanowire Arrays for High-Performance and Flexible Supercapacitors

Metal nitrides have received increasing attention as electrode materials for high-performance supercapacitors (SCs). However, most of them are suffered from poor cycling stability. Here we use TiN as an example to elucidate the mechanism causing the capacitance loss. X-ray photoelectron spectroscopy analyses revealed that the instability is due to the irreversible electrochemical oxidation of TiN during the charging/discharging process. Significantly, we demonstrate for the first time that TiN can be stabilized without sacrificing its electrochemical performance by using poly­(vinyl alcohol) (PVA)/KOH gel as the electrolyte. The polymer electrolyte suppresses the oxidation reaction on electrode surface. Electrochemical studies showed that the TiN solid-state SCs exhibit extraordinary stability up to 15 000 cycles and achieved a high volumetric energy density of 0.05 mWh/cm³. The capability of effectively stabilizing nitride materials could open up new opportunities in developing high-performance and flexible SCs

Computational and Photoelectrochemical Study of Hydrogenated Bismuth Vanadate

We demonstrate hydrogenation as a facile method to significantly enhance the performance of BiVO₄ films for photoelectrochemical water oxidation. Hydrogenation was performed for BiVO₄ films by annealing them in hydrogen atmosphere at elevated temperatures between 200 and 400 °C. Hydrogen gas can reduce BiVO₄ to form oxygen vacancies as well as hydrogen impurities. DFT calculation predicted that both oxygen vacancies and hydrogen impurities are shallow donors for BiVO₄ with low formation energies. These defects could increase the donor densities of BiVO₄ without introducing deep trap states. Electrochemical impedance measurements showed that the donor densities of BiVO₄ films were significantly enhanced upon hydrogenation. Hydrogen-treated BiVO₄ (H-BiVO₄) photoanodes achieved a maximum photocurrent density of 3.5 mA/cm² at 1.0 V vs Ag/AgCl, which is 1 order of magnitude higher than that of air-annealed BiVO₄ obtained at the same potential. The enhanced photoactivities were attributed to increased donor densities of H-BiVO₄, which facilitates the charge transport and collection

Controlled Synthesis of AlN/GaN Multiple Quantum Well Nanowire Structures and Their Optical Properties

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal–organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)_<i>m</i> (<i>m</i> = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340–360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures

Morphology and Doping Engineering of Sn-Doped Hematite Nanowire Photoanodes

High-temperature activation has been commonly used to boost the photoelectrochemical (PEC) performance of hematite nanowires for water oxidation, by inducing Sn diffusion from fluorine-doped tin oxide (FTO) substrate into hematite. Yet, hematite nanowires thermally annealed at high temperature suffer from two major drawbacks that negatively affect their performance. First, the structural deformation reduces light absorption capability of nanowire. Second, this “passive” doping method leads to nonuniform distribution of Sn dopant in nanowire and limits the Sn doping concentration. Both factors impair the electrochemical properties of hematite nanowire. Here we demonstrate a silica encapsulation method that is able to simultaneously retain the hematite nanowire morphology even after high-temperature calcination at 800 °C and improve the concentration and uniformity of dopant distribution along the nanowire growth axis. The capability of retaining nanowire morphology allows tuning the nanowire length for optimal light absorption. Uniform distribution of Sn doping enhances the donor density and charge transport of hematite nanowire. The morphology and doping engineered hematite nanowire photoanode decorated with a cobalt oxide-based oxygen evolution reaction (OER) catalyst achieves an outstanding photocurrent density of 2.2 mA cm^–2 at 0.23 V vs Ag/AgCl. This work provides important insights on how the morphology and doping uniformity of hematite photoanodes affect their PEC performance

Role of Hydrogen in Defining the n‑Type Character of BiVO₄ Photoanodes

The roles of hydrogen impurity and oxygen vacancy defects on defining the conductivity, and hence photoelectrochemical (PEC) performance characteristics, of monoclinic scheelite bismuth vanadate (BiVO₄) are investigated using a combination of experiment and theory. We find that elemental hydrogen is present as an impurity in as-synthesized BiVO₄ and that increasing its concentration by annealing in H₂ at temperatures up to 290 °C leads to near-complete elimination of majority carrier transport limitations, a beneficial shift in the photoanodic current onset potential, and improved fill factor. Magnetic resonance measurements reveal that hydrogen can be incorporated in at least two different chemical environments, which are assigned to interstitial and substitutional sites. Incorporation of hydrogen leads to a shift of the Fermi level toward the conduction band edge, indicating that n-type character is correlated with increased hydrogen content. This finding is in agreement with theory and reveals that hydrogen acts as a donor in BiVO₄. Sub-bandgap photoluminescence is observed from as-synthesized material and is consistent with deep electronic states associated with oxygen vacancies. Hydrogen treatment leads to reduced emission from these states. These findings support the conclusion that hydrogen, rather than oxygen vacancies, is dominant in determining the n-type conductivity of BiVO₄. These findings have important implications for controlling the electronic properties and functional characteristics of this promising photoanode material