Electrodeposited Cr-Doped α-Fe2O3 thin films active for photoelectrochemical water splitting

Polycrystalline hematite (α-Fe₂O₃) Chromium (Cr)-doped thin films were electrodeposited on fluorine-doped tin oxide-coated glass substrates. The electrodeposition bath comprised an aqueous solution containing FeCl₃·6H₂O, NaCl, and H₂O₂.Chromium was added to the electrolyte at such a proportion that the Cr/(Cr + Fe) ratio remained within the 2–8 at. % range. The as-deposited films were subsequently annealed in air at 650 °C for 2 h. The structure and morphological characteristics of the undoped and Cr-doped α-Fe₂O₃ thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectroscopy. Cr doping led the main XRD lines to shift to lower angles, which mostly resulted from substituting Fe³⁺ for Cr⁴⁺ ions that leads to α-Fe₂O₃ lattice contraction. The SEM observations showed that the roughness and aspect of surfaces changed with the Cr doping level. The photoelectrochemical (PEC) performance of the α-Fe₂O₃ films was examined by chronoamperometry and linear sweep voltammetry techniques. The Cr-doped films exhibited greater photoelectrochemical activity than the undoped α-Fe₂O₃ thin films. The highest photocurrent density was obtained for the 8% Cr-doped α-Fe₂O₃ films in 1 M NaOH electrolyte. All the samples achieved their best IPCE at 400 nm. The IPCE values for the 8 at.% Cr-doped hematite films were 20-fold higher than that of the undoped sample.This Cr-doped hematite films ‘excellent photoelectrochemical performance was mainly attributed to improved charge carrier properties. Such high photoactivity was attributed to the large active surface area and increased donor density caused by increasing the Cr doping in the α-Fe₂O₃ films.     

Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by Δμ_H(σ) = −1.124σ kJ/mol_H for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.     

Design of efficient Mn-doped α-Fe2O3/Ti-doped α-Fe2O3 homojunction for catalyzing photoelectrochemical water splitting

To produce clean chemical fuel of hydrogen efficiently, applying photocatalysts for conducting photoelectrochemical water splitting is indispensable. Hematite (α-Fe₂O₃) has been considered as one of the most effective photocatalysts for water oxidation due to excellent visible-light responses, high stability and source abundance properties, but low electrical conductivity and slow oxidation evolution kinetics limit its application. In this study, a novel α-Fe₂O₃ homojunction is constructed via doping Ti and Mn in two layers using two-step hydrothermal synthesis followed by one-step annealing process. Co-doping effect of Ti and Mn in α-Fe₂O₃ and growing sequence of Mn doped α-Fe₂O₃ (Mn:Fe₂O₃) and Ti doped α-Fe₂O₃ (Ti:Fe₂O₃) are also investigated to illustrate the efficient design of Mn:Fe₂O₃/Ti:Fe₂O₃ homojunction. The optimized Mn:Fe₂O₃/Ti:Fe₂O₃ electrode shows the highest photocurrent density of 2.10 mA/cm² at 1.60 V_RHE respectively comparing to those of 0.10, 1.20 and 0.22 mA/cm² for Ti:FeOOH, Ti:Fe₂O₃ and α-Fe₂O₃ electrodes. The outstanding performance of Mn:Fe₂O₃/Ti:Fe₂O₃ homojunction is attributed to the smaller charge-transfer resistance, higher carrier density, and less charge recombination. This work gives a rational design for hematite-based photocatalysts and successfully attains greatly improved photocatalytic ability for water oxidation. Development of homojunction using heteroatom doping in thus verified to be highly applicable on synthesizing promising photocatalysts.     

Cobalt doping of α-Fe/Al2O3 catalysts for the production of hydrogen and high-quality carbon nanotubes by thermal decomposition of methane

Fe–Co/Al₂O₃ catalysts were developed and tested in the catalytic decomposition of methane (CDM) for the synthesis of multi-wall carbon nanotubes (MWCNT) and the CO₂-free hydrogen production. While Fe (54.5–66.7 mol.%) is the main active phase for the carbon formation on the catalyst, Co acts as dopant aiming to improve its overall catalytic behaviour. Catalysts with Co contents of up to 18.2 M% showed the presence of α-Fe and Fe–Co crystallites with different size and lattice parameter. Fe_1-xCo_x alloy with bcc crystal system was identified only for Co contents of 14.0% and above, and presented a lattice constant lower than α-Fe, which would modify the carbon diffusion of the metal particle during the MWCNT growth. Co inhibited the Fe₃C formation during CDM resulting in higher carbon formations and longer activity times. This phase, shown in undoped catalysts, favored the presence of bamboo-type carbon nanotubes.     

Preparation and characterization of α-Fe2O3 supported clay as a novel photocatalyst for hydrogen evolution

Improvement in the hydrogen evolution is reported over α-Fe₂O₃ supported on Algerian natural clay. The hetero-system is prepared by impregnation and calcination at 450 °C. It was characterized by X-ray diffraction, SEM analysis, FTIR spectroscopy and photo electrochemistry. The hematite Fe₂O₃ crystallizes in the corundum structure and exhibits n-type conductivity with a flat band potential of −0.88 V_SCE. Hence, the photo electrons located in Fe₂O₃-CB (−1 V_SCE) have high ability to reduce water into hydrogen. α-Fe₂O₃ gets effectively dispersed in the clay and the photoactivity increases with increasing its content. SO₃²⁻, working as hole scavenger, provides an absolute protection against the photo corrosion and favors the charges separation. The best performance of H₂ evolution occurs at alkaline pH on 10% Fe₂O₃/clay with a liberation rate 0.121 μmol/mg/min and a quantum efficiency of 1.2%.     

Understanding hydrogen diffusion mechanisms in doped α-Fe through DFT calculations

Hydrogen embrittlement is detrimental to structural metals during applications. Herein, we explore the hydrogen diffusion mechanisms in doped α-Fe using first-principles calculations. We prove that the hydrogen trap is a thermodynamically spontaneous process, and doping will decrease the hydrogen adsorption energy due to the change of adsorption sites. Furthermore, hydrogen diffusion from surface to subsurface will determine the diffusion rate. Mo, Mn and C are beneficial to the increase of the energy barrier of hydrogen diffusion from the surface to subsurface and in the bulk. The current work provides a promising path towards enhancing the hydrogen diffusion barrier in α-Fe.     

Various nickel doping in commercial Ni–Mo/Al2O3 as catalysts for natural gas decomposition to COx-free hydrogen production

Non-oxidative decomposition of natural gas to CO_x-free hydrogen production over commercial nickel-molybdate hydrotreating catalysts with different Ni loading from 5 to 40wt% were studied at 700 °C. The catalysts were characterized by XRD, BET, TEM, Raman spectroscopy and TG-DTA analysis. The catalytic decomposition activities showed that a tremendous hydrogen production (∼90%) was obtained over 20–40wt%Ni/Mo–Al₂O₃ catalysts. Moreover, all catalysts exhibited excellent durability up to 9 h with stable catalytic activity toward H₂ production. Although the increase of Ni content reduces the catalyst surface area, the H₂ productivity and longevity increases with increased Ni content, i.e., the catalytic decomposition activity primarily depends on the active Ni sites which overcompensates the surface deficiencies. TEM, TGA and XRD data of used catalysts indicated that a higher thermal stability and graphitization degree of multi-walled carbon nanotubes were obtained on all Ni containing catalysts. Higher metal loading produced carbon nanofibers beside CNTs due to increment of particle size and long reaction time.     

Effects of surface oxide films on hydrogen permeation and susceptibility to embrittlement of X80 steel under hydrogen atmosphere

Hydrogen embrittlement (HE) induced by hydrogen permeation is a serious threat to the hydrogen transmission pipeline. In this study, oxide films were prepared on X80 steel by applying high-temperature oxidation, blackening treatment and passivation in concentrated H₂SO₄, and their effects on hydrogen permeation and HE susceptibility of X80 substrate were studied by conducting hydrogen permeation tests and slow strain rate tension (SSRT) tests. A numerical diffusion model was established to quantitatively determine the resistance of these oxide films to hydrogen permeation. Results showed that the oxide film prepared by high-temperature oxidation presented the highest resistance to hydrogen permeation with the ?_m/?_f value of 3828, and the corresponding HE index decreased from 38.07% for bare X80 steel to only 4.00% for that covered with oxide film. The characteristic of the corresponding fracture surfaces changed from brittle features such as quasi cleavage facets and secondary cracks to typical ductile dimple feature.     

Energetics and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3/FeAl with two different interfaces

To provide insights into the interface structure of hydrogen permeation barrier of α-Al₂O₃/FeAl and its effect on stability and diffusion of hydrogen isotopes, the thermodynamics and kinetics of H diffusion in α-Al₂O₃ (001)/FeAl (111) slab with Al/O and Al/Fe/O interfaces have been studied by the density functional theory. Hexagonal alumina layers above the FeAl plane in interface region are predicted. The interfacial binding involves cation–anion and metal–metal interactions. H-surface interaction on the α-Al₂O₃/FeAl slab resembles that on pure α-Al₂O₃ (001) slab, and the H interstitials in the α-Al₂O₃ part of the slab with the Al/O interface are significantly less stable than in bulk of α-Al₂O₃ slab, whereas that with the Al/Fe/O interface are slightly more stable. H diffusion into the α-Al₂O₃ part of both slabs must overcome a larger barrier of about 1.66–2.02 eV at surface-to-subsurface step, as pure α-Al₂O₃ case. For the bulk path, the migration of H atom can occur more readily in the α-Al₂O₃ part of the slab with the Al/O interface compared to that with the Al/Fe/O interface. Thus α-Al₂O₃/FeAl barrier with interface region of the Al, Fe mix-oxide is predicted to be much effective at protection against H permeation of the underlying steel.     

The role of hydrogen in the edge dislocation mobility and grain boundary-dislocation interaction in α-Fe

The atomistic mechanisms of dislocation mobility depending on the presence of hydrogen were investigated for two edge dislocation systems that are active in the plasticity of α-Fe, specifically ½<111>{110} and ½<111>{112}. In particular, the glide of the dislocation pile-ups through a single crystal, as well as transmission of the pile-ups across the grain boundary were evaluated in bcc iron crystals that contain hydrogen concentrations in different amounts. Additionally, the uniaxial tensile response under a constant strain rate was analyzed for the aforementioned structures. The results reveal that the presence of hydrogen decreases the velocity of the dislocations – in contrast to the commonly invoked HELP (Hydrogen-enhanced localized plasticity) mechanism -, although some localization was observed near the grain boundary where dislocations were pinned by elastic stress fields. In the presence of pre-exisiting dislocations, hydrogen-induced hardening was observed as a consequence of the restriction of the dislocation mobility under uniaxial tension. Furthermore, it was observed that hydrogen accumulation in the grain boundary suppresses the formation of new grains that leads to a hardening response in the stress-strain behaviour which can initiate brittle fracture points.     

Mechanisms for adsorption,dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: The role of crystal orientation

The mechanisms of adsorption of hydrogen on α-Al₂O₃(1-102) surface and of its diffusion in bulk are investigated, using first principles thermodynamics and kinetics, and compared with similar results obtained for the diffusion of hydrogen on α-Al₂O₃(0001) surface. Because of the different oxygen environments on both surfaces, the H binding energies on the (1-102) surface are 0.3–1.2 eV smaller than in the (0001) surface. The H₂ binding energies on (1-102) and (0001) surfaces are resembled. We have identified four main mechanisms, leading to dissociation of H₂, H migration on the surface, H diffusion into and inside the bulk. Equilibrium constant and activation barrier show that H₂ dissociation is the most favorable process and significant diffusion of H into the bulk can occur more readily from the (1-102) surface compared to the (0001) surface. Based on the hydrogen interaction with α-Al₂O₃(1-102) surface, a mechanism of α-Al₂O₃ suppressing H-permeation is identified.     

Enhanced electrocatalytic performances of α-Fe2O3 pseudo-nanocubes for oxygen reduction reaction in alkaline solution with conductive coating

Exploring of high-efficient, low-cost and eco-friendly catalysts for oxygen reduction reaction (ORR) is one of the vital issues for fuel cells and metal-air batteries. Herein, α-Fe₂O₃ pseudo-nanocubes have been synthesized with a facile solvothermal method, and then α-Fe₂O₃@NC composite catalysts were prepared through chemical polymerization of pyrrole on α-Fe₂O₃, following with pyrolyzed in nitrogen atmosphere. The synthesized composite catalysts display enhanced ORR catalytic performances, including the positive shifting of the onset potential, improving of the limited current density with long-term stability compared to those of the pristine α-Fe₂O₃. The enhanced electrocatalytic performances could be ascribed to the low intrinsic and charge transfer resistances, the high content of pyridinic-N and/or FeN, the emergence of graphitic-N and abundant oxygen vacancy on the composite surface. This study here implies that the catalytic activity and stability of metal oxides with poor conductivity could be controlled and improved by simply coating with a nanoscale conductive layer, which shows promising potential applications as precious-metal free catalysts for various metal-air batteries and fuel cells.     

Enhanced photocurrent by MOFs layer on Ti-doped α-Fe2O3 for PEC water oxidation

Low photocurrent density of hematite (α-Fe₂O₃) originating from the inherent defects usually hinders its application in photoelectrochemical (PEC) water oxidation. In this paper, the synergetic effect of increase of oxygen vacancies and in-situ constructing heterojunction by coating MOFs on the α-Fe₂O₃ nanoarrays gives rise to the boosted photocurrent of α-Fe₂O₃ from 0.25 mA/cm² to 2.1 mA/cm² at 1.23 V (vs. RHE). The results showed that the appropriate energy band structure engineered by the presence of MOFs layer not only facilitated the PEC water oxidation, but also enhanced the light absorption performance. With inducing oxygen vacancies in further, the intrinsic conductivity of photoanode can be well ameliorated. The value of carrier density is improved one order higher to promote charge transfer between the interfaces and raise the carrier separation efficiency as a result.     

Role of V doping in core–shell heterostructured Bi2Te3/Sb2Te3 for hydrogen evolution reaction

Non-noble metal-based materials as low-cost hydrogen evolution reaction (HER) catalysts are key materials for sustainable hydrogen energy production. Bismuth and antimony chalcogenides are among the hopeful candidates to achieve this goal. In this work, a V-doped Sb₂Te₃ encapsulated Bi₂Te₃ core-shell electrocatalyst (Bi₂Te₃/V_x-Sb₂Te₃) has been synthesized by a two-step solvothermal method. V doping adjusts the electronic structure of catalyst, dramatically enhances electric double layer capacitance (C_dl) of the catalyst, decreases charge transfer resistance (R_ct) of the catalyst and increases carrier concentration of the catalyst. Therefore, the V doping method increases the active sites on the surface of the material, and promotes the charge transfer and electron transport in the HER process. In addition, V doping can also adjust the hydrophilicity of the material surface, promote the release of hydrogen, and quickly re-expose the active sites. Bi₂Te₃/V_x-Sb₂Te₃ electrocatalysts exhibit brilliant HER activity and high stability in both acidic and alkaline electrolytes. This study uses the strategy of V doping to control the electronic structure of materials, which will provide suggestions for the design and preparation for other high-activity catalysts.     

Mechanism for adsorption,dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: A density functional theory study

Toward understanding physical interaction of hydrogen isotopes with α-Al₂O₃ barrier, adsorption, dissociation and diffusion of hydrogen in α-Al₂O₃(0001) slab have been investigated by density functional theory (DFT) and rate theory. H₂ molecule, with parallel configuration, preferentially absorbs on a top Al atom site of first atomic layer on α-Al₂O₃(0001) surface, while H atom strongly bonds at a top O atom site of the second atomic layer, H atoms recombine into molecules on top Al atom sites of the third atomic layer. The barrier for H₂ exothermic dissociation on surface is 0.79 eV. The potential energy pathways of H diffusion in α-Al₂O₃ are studied, predicting that H atom diffusion preferentially occurs via surface path rather than bulk path involving elementary reorientation and hopping steps. The surface-to-subsurface diffusion is significantly endothermic except for the surface and subsurface-to-bulk path. Mechanism, in well agreement with experimental result, of α-Al₂O₃ resisting hydrogen permeation has proposed.     

High-performance H2 gas sensor based on Mn-doped α-Fe2O3 polyhedrons from N,N-dimethylformamide assisted hydrothermal synthesis

Transition elements doping metal oxide is of great significance for developing gas sensors with high performance. To achieve this goal, α-Fe₂O₃ rod and Mn-doped α-Fe₂O₃ polyhedrons are successfully synthesized by a facile N, N-dimethylformamide assisted hydrothermal method and characterized by several physical techniques. The morphology of α-Fe₂O₃ is evolved by introducing Mn elements. The performance of the α-Fe₂O₃ rod and Mn-doped α-Fe₂O₃ polyhedrons as H₂ gas sensors are studied and compared. The Mn/α-Fe₂O₃-5 polyhedrons sensor exhibit the best performance at optimum operating temperature of 300 °C. The response and recovery times are 10 s/24 s for H₂ with a concentration of 200 ppm, which is much faster in contrast to those recently reported H₂ gas sensors. Furthermore, α-Fe₂O₃ rod and Mn-doped α-Fe₂O₃ polyhedrons show abnormal n-p transition sensing behavior with the change of operating temperature. This work provides a facile method for synthesizing α-Fe₂O₃ based sensing materials for various sensors applications.     

“Eigen-periodic”-in-space surface heating in conduction with application to conductivity measurement of thin films

A two-dimensional heat conduction problem in Cartesian coordinates subject to a periodic-in-space boundary condition is analyzed by the Green’s functions approach. It is pointed out that when the frequency of the spatial periodic heating equates one of the natural frequencies (eigenvalues) of the system, the solution of the 2D heat conduction problem can be written down very simply as the product of the periodic surface condition (termed the “eigen-periodic”) by the solution of a 1D fin problem along the nonhomogeneous direction. This result suggests a novel and simple algebraic equation for determining the thermal conductivity of thin films placed on substrates under steady state conditions. High space frequencies of the sinusoidal heating, larger than the deviation frequency, are used to make negligible the thermal deviation effects due to the presence of the substrate.     

Deep surface reconstruction of Co-based cocatalyst on Ti-doped α-Fe2O3 photoanode for highly efficient water oxidation

Transition metal-based cocatalysts play important roles in promoting the surface kinetics of hematite (α-Fe₂O₃) photoanode. However, their performances are restricted by the shallow reconstruction process for generating highly efficient metal oxyhydroxides, where the oxygen evolution reaction (OER) occurs. Therefore, a Brnsted base-regulated strategy is developed to promote the in situ surface reconstruction of cocatalysts on Ti-doped α-Fe₂O₃ (Ti–Fe₂O₃) under photoelectrochemical conditions. After deep surface reconstruction by electrochemical activation, the CoWO₄ cocatalyst decorated Ti–Fe₂O₃ photoanode (a-CoWO₄/Ti–Fe₂O₃) delivers a photocurrent density of 0.88 mA cm^?2 at 1.23 V_RHE, which is about 3.0 times of activated Ti–Fe₂O₃ (a-Ti-Fe₂O₃) and 1.5 times of activated CoO_x/Ti–Fe₂O₃. Tungstate promotes the surface reconstruction of cobalt-based cocatalyst, resulting in a significant increase in bulk charge separation efficiency (η_sep) and surface charge injection efficiency (η_inj). Moreover, the type-II heterojunction between CoWO₄-derived CoOOH and a-Ti-Fe₂O₃ drives the rapid separation and transfer of photogenerated electron-hole pairs, and enhances the performance of Ti–Fe₂O₃ photoanode.     

Enhanced photoelectrochemical performance of an α-Fe2O3 nanorods photoanode with embedded nanocavities formed by helium ions implantation

Hematite is a prospective semiconductor in photoelectrochemical (PEC) water oxidation field due to its suitable bandgap for the solar spectrum absorption. Nevertheless, the low transfer and separation efficiency of the charge carriers are restricted by its diffusion length of hole which is 2–4 nm and further reduce the PEC performance. Here, we report an innovative method, by introducing nanocavities into the α-Fe₂O₃ nanorod arrays photoanodes through helium ions implantation, to improve the charge carriers' transfer and separation efficiency and further to enhance water oxidation performance. The result indicates that, the photocurrent density of nanocavities embedded α-Fe₂O₃ photoanode (S2-A sample) reaches 1.270 mA/cm² at 1.6 V vs. RHE which is 1-fold higher than that of the pristine α-Fe₂O₃ (0.688 mA/cm²) and the photocurrent density of S2-A sample reaches 0.652 mA/cm² at 1.23 V vs. RHE. In this work, the ion implantation combined with post annealing method is found to be a valid method to improve the photoelectrochemical performance, and it also can be further used to modify the other semiconductor photoelectrodes materials.     

Recent trends in development of hematite (α-Fe2O3) as an efficient photoanode for enhancement of photoelectrochemical hydrogen production by solar water splitting

Solar-assisted water splitting using photoelectrochemical (PEC) cell is an environmentally benign technology for the generation of hydrogen fuel. However, several limitations of the materials used in fabrication of PEC cell have considerably hindered its efficiency. Extensive efforts have been made to enhance the efficiency and reduce the hydrogen generation cost using PEC cells. Photoelectrodes that are stable, efficient and made of cost-effective materials with simple synthesizing methods are essential for commercially viable solar water splitting through PEC technology. To this end, hematite (α-Fe₂O₃) has been explored as an excellent photoanode material to be used in the application of PEC water oxidation owing to its suitable bandgap of 2.1 eV that can utilize almost 40% of the visible light. In this study, we have summarized the recent progress of α-Fe₂O₃ nanostructured thin films for improving the water oxidation. Strategic modifications of α-Fe₂O₃ photoanodes comprising nanostructuring, heterojunctions, surface treatment, elemental doping, and nanocomposites are highlighted and discussed. Some prospects related to the challenges and research in this innovative research area are also provided as a guiding layout in building design principles for the improvement of α-Fe₂O₃ photoanodes in photoelectrochemical water oxidation to solve the increasing environmental issues and energy crises.