Electronically Coupled Uranium and Iron Oxide Heterojunctions as Efficient Water Oxidation Catalysts

[[abstract]]The most critical challenge faced in realizing a high efficiency photoelectrochemical water splitting process is the lack of suitable photoanodes enabling the transfer of four electrons involved in the complex oxygen evolution reaction (OER). Uranium oxides are efficient catalysts due to their wide range optical absorption (E g ≈ 1.8–3.2 eV), high photoconductivity, and multiple valence switching among uranium centers that improves the charge propagation kinetics. Herein, thin films of depleted uranium oxide (U3O8) are demonstrated grown via chemical vapor deposition effectively accelerate the OER in conjunction with hematite (α‐Fe2O3) overlayers through a built‐in potential at the interface. Density functional theory simulations demonstrate that the multivalence of U and Fe ions induce the adjustment of the band alignment subject to the concentration of interfacial Fe ions. In general, the equilibrium state depicts a type II band edge as the favored alignment, which improves charge‐transfer processes as observed in transient and X‐ray absorption (TAS and XAS) spectroscopy. The enhanced water splitting photocurrent density of the heterostructures (J = 2.42 mA cm−2) demonstrates the unexplored potential of uranium oxide in artificial photosynthesis.[[notice]]補正完

Equivalent Circuit Analysis of Impedance Response for Cr-doped TiO2-NTs towards NO2 Gas

High-temperature gas sensing requires the increase of sensitivity and reduction of cross-sensitivity. The use of TiO2-Nanotubular layers as gas sensors has shown that the selectivity and sensitivity can be influenced by doping with trivalent elements and by optimization of morphological aspects such as pore diameter and nanotube length. In this work, focus has been given on the understanding of the effect of doping and properties of nano-tubular TiO2-layers on sensing behavior and mechanism toward NO2 by using equivalent circuit modeling achieved by impedance spectroscopic measurements

Constructing Fe2O3/TiO2 core-shell photoelectrodes for efficient photoelectrochemical water splitting

In this study, plasma enhanced chemical vapor deposition (PECVD) was utilized to co-axially modify hydrothermally grown Fe2O3 nanorod arrays by depositing a TiO2 overlayer to create Fe2O3/TiO2 core-shell photoelectrodes. Comprehensive structural (XRD, SEM, TEM) and compositional (XPS) analyses were performed to understand the effects of the TiO2 shell on the PEC activities of the Fe2O3 core. It was revealed that the heterojunction structure formed between TiO2 and Fe2O3, significantly improved the separation efficiency of photo-induced charge carriers and the oxygen evolution kinetics. A maximum photocurrent density of similar to 900 mu A cm(-2) at 0.6 V vs. saturated calomel electrode (SCE) was obtained for the Fe2O3/TiO2 photoelectrodes, which was 5 and 18 times higher when compared to that of hydrothermally synthesized Fe2O3 and PECVD synthesized TiO2 electrodes, respectively. Moreover, the Fe2O3/TiO2 core-shell nanorod arrays displayed superior stability for PEC water splitting. During 5000 s PEC measurements, a steady decrease of the photocurrent was observed, mainly attributed to the evolution of oxygen bubbles adsorbed on the working electrodes. This observation was verified by the complete recovery of the PEC performance demonstrated for a second 5000 s PEC measurement carried out after a brief time interval (10 min) that allowed the electrode surface to regenerate

Plasma CVD grown Al2O3 and MgAl2O4 coatings for corrosion protection applications

The development of active corrosion protection systems for metallic substrates is an issue of prime concern. The objective of this work was to demonstrate the applicability of plasma enhanced CVD (PECVD) in growing protective top oxide layers over sol-gel deposited organosiloxane (R-SiOx; water based inorganic-organic hybrid coating formulation containing 3-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane, crosslinked with hexamethylmethoxymelamine) coating to improve its thermal stability, chemical inertness and corrosion resistance. [MgAl2((OBu)-Bu-t)(4)H-4] and [AlH2((OBu)-Bu-t)](2) were used as single source precursors for depositing MgAl2O4 and Al2O3 thin films respectively. The coating systems namely, Al2O3/R-SiOx (ALSI) and MgAl2O4/R-SiOx (MALSI) were comparatively analyzed towards their corrosion protection behaviour. The post annealed PECVD deposited films were uniform, transparent and exhibited good adhesion as confirmed by focused ion beam scanning electron microscope (FIB-SEM) cross sectional analysis. ALSI and MALSI bilayer systems were found to be stable over temperatures as high as 600 degrees C. Corrosion performance of the coating systems effectively demonstrated the protective function of ALSI and MALSI bilayer system. However, it was observed that ALSI outperforms MALSI in this application

Trace Amount of Platinum Supported on Carbonized Biomorphic Wood for Efficient Electrochemical Hydrogen Evolution in Alkaline Condition

Carbonized biomorphic wood (Bio-C), which features unique properties such as highly ordered microtexture, well-developed porous structure, and low charge transfer resistance, was prepared as a promising scaffold with trace amount of platinum (Pt) supported to efficiently electrocatalyze hydrogen evolution reaction (HER). Pt/Bio-C nanocomposites demonstrated superior HER activity with a current density reaching as high as similar to 58 mA cm(-2) at -0.2 V vs. reversible hydrogen electrode (RHE) in strongly alkaline condition

High Water-Splitting Efficiency through Intentional In and Sn Codoping in Hematite Photoanodes

The effects of intermittent thin ITO layers on the water-splitting efficiency of alpha-Fe2O3 films grown by PECVD on FTO substrates are reported. The alpha-Fe2O3 was codoped with indium and tin by temperature-driven ionic transport and diffusion from the ultrathin ITO layer sputtered between the alpha-Fe2O3 layer and FTO substrate. The alpha-Fe2O3/ITO/FTO photoanodes showed a remarkable interdependence between the thickness of the ITO layer and PEC efficiency. Hematite photoanodes with a 32 nrn thick ITO underlayer showed the highest photocurrent density of 2.5 mA cm(-2), corresponding to an approximate 3-fold enhancement over pristine alpha-Fe2O3 at 1.23 V vs RHE, whereas the thinner (8 nm) ITO underlayer yielded the lowest onset potential at 0.6 V vs RHE. Although the electrode with a thicker 72 nm ITO underlayer showed a higher onset potential of 0.9 V vs RHE, it still showed an enhancement in the photocurrent density at higher bias voltages. alpha-Fe2O3 was also deposited on metallic titanium substrates with intermittent sputtered tin and ITO layers. The codoping with indium and tin from ITO was observed to yield greatly enhanced performance when compared with both alpha-Fe2O3 alone and tin-doped alpha-Fe2O3. Transient absorption decays in the sub-nanosecond time scale were not affected by the doping, indicating that the doping had little effect on the primary charge carrier generation and recombination. On the other hand, fewer trapped electrons on the microsecond to millisecond time scale and a greatly increased amount of long-lived surface photoholes were observed for the ITO-doped samples. The transient absorption results imply that the large increases in photoelectrochemical efficiency were obtained due to higher electron mobility, which reduces recombination and leads to more efficient electron extraction from the electrodes

Critical role and modification of surface states in hematite films for enhancing oxygen evolution activity

Hematite films deposited by plasma-enhanced chemical vapor deposition of iron pentacarbonyl [Fe(CO)(5)] in an oxygen plasma were modified by postdeposition (i) oxygen plasma treatment and (ii) short annealing treatments to reduce the defects and to modify the (sub)surface states and consequently the photoelectrochemical properties. The oxygen plasma treatment resulted in the increase of particle size and augmented surface roughening by densification of grains. Moreover, it induced saturated surface states with reactive oxygen species (O-, OH-), evident in the X-ray photoelectron spectroscopy (XPS). Under standard illumination (1.5 AM; 100 mW/cm(2); 150 W xenon lamp), when compared to the pristine hematite coating (0.696 mA/cm(2) at 1.23 V versus RHE and 0.74 V-onset) the oxygen plasma-treated films showed severe deterioration in photocurrent density of 0.035 mA/cm(2) and an anodic shift in the onset potential (1.10 V-onset) due to oxygen rich surface. In a second set of experiments, the oxygen plasma-treated hematite films were briefly annealed (10 min at 750 degrees C) and the signals of Fe 2p and O 1s recovered to higher binding energies, indicating the formation of oxygen vacancies. In addition, a superior photocurrent density value of max. 1.306 mA/cm(2) at 1.23 V versus RHE to that of the pristine hematite photoanode with 0.74 V-onset was obtained. Transient absorption spectroscopy further elucidated that the oxygen plasma-induced electron trap states acting as recombination centers that are unfavorable for photoelectrochemical activity. The alteration in Fe:O stoichiometry and thus photocurrent density are corroborated by determination of water oxidation rates in annealed (7.1 s(-1)) and oxygen plasma treated (2.5 s(-1)) samples

Electronically-Coupled Phase Boundaries in α-Fe2O3/Fe3O4 Nanocomposite Photoanodes for Enhanced Water Oxidation

Photoelectrochemical (PEC) water splitting reactions are promising for sustainable hydrogen production from renewable sources. We report here, the preparation of alpha-Fe2O3/Fe3O4 composite films via a single-step chemical vapor deposition of [Fe((OBu)-Bu-t)(3)](2) and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidation activity (0.48 mA/cm(2) at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm(2)) standard illumination was found for hybrid films with a thickness of 11 mu m. This phenomenon is attributed to an improved electron transport resulting from a higher magnetite content toward the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of alpha-Fe2O3/Fe3O4 nanocomposite photoanodes is attributed to the close proximity and establishment of 3D interfaces between the weakly ferro- (Fe2O3) and ferrimagnetic (Fe3O4) oxides, which in view of their differential chemical constitution and valence states of Fe ions (Fe2+/Fe3+) can enhance the charge separation and thus the overall electrical conductivity of the layer

Magnetic Field-Assisted Control of Phase Composition and Texture in Photocatalytic Hematite Films

We report the influence of external magnetic fields applied parallel or perpendicular to the substrate during plasma chemical vapor deposition (PECVD) of hematite (alpha-Fe2O3) nanostructures. Hematite films grown from iron precursors show pronounced changes in phase composition (pure hematite vs. coexistence of hematite and magnetite) and crystallographic textures depending upon whether PECVD is performed with or without the influence of external magnetic field. Static magnetic fields created by rod-type (RTMs) or disk-type magnets (DTMs) results in hematite films with anisotropic or equiaxed grains, respectively. Using RTMs, a superior photoelectrochemical (PEC) performance is obtained for hematite photoanodes synthesized under perpendicularly applied magnetic field (with respect to substrate), whereas parallel magnetic field results in the most efficient hematite photoanode in the case of DTM. The experimental data on microstructure and functional properties of hematite films show that application of magnetic fields has a significant effect on the crystallite size and texture with preferred growth and/or suppression of grains with specific texture in Fe2O3 films. Investigations on the water splitting properties of the hematite films in a photoelectrochemical reactor reveal that photocurrent values of hematite photoanodes are remarkably different for films deposited with (0.659 mA cm(-2)) or without (0.484 mA cm(-2)) external magnetic field