Influence of surfactants on Fe2O3 nanostructure photoanode

Fe₂O₃ nanostructures photoanodes were prepared via sol–gel spin-coating method onto fluorine-doped tin oxide glass substrates using six different surfactants: polyethylene glycol (PEG-300), Triton X-100, pluronic F127, cetyltrimethylammonium bromide (CTAB), octadecyltrimethylammonium bromide (OTAB) and tetradecyltrimethylammonium bromide (TTAB). The resulting films have thickness from 520 ± 10 to 980 ± 10 nm after calcinations at 450 °C in the air. A comparative study of photocatalytic activity of thin films was performed. The photo-generated samples were determined by measuring the currents and voltages under illumination of UV–vis light. The highest photocurrent density of 1.77 mA/cm² at 1 V/SCE, under illumination intensity of 100 mW cm⁻² from a solar simulator with a global AM 1.5 filter, were produced by TTAB treated sample. The optical properties, morphology, surface roughness and structure of the films were also characterized by UV–visible spectroscopy, SEM, AFM and XRD. The results are consistent with photocatalytic performance: TTAB treated sample has the highest grain size and optical absorption. The improved performance of this sample can be attributed to the crystallinity process of TTAB, which leads to the larger grain size and highest photocatalytic activity. The study demonstrates that photoelectrochemical performance of metal oxide can be improved by simply changing surfactant. The results highlighted the superior performance of cationic surfactants over non-ionic surfactants in preparing Fe₂O₃ photoanodes by sol–gel method. Moreover, the study showed that decreasing hydrocarbon tail of cationic surfactants can increase the crystallite size and improve photocatalytic activity.     

Ultrathin hematite films deposited layer-by-layer on a TiO2 underlayer for efficient water splitting under visible light

Ultrathin hematite (α-Fe₂O₃) film deposited on a TiO₂ underlayer as a photoanode for photoelectrochemical water splitting was described. The TiO₂ underlayer was coated on conductive fluorine-doped tin oxide (FTO) glass by spin coating. The hematite films were formed layer-by-layer by repeating the separated two-phase hydrolysis-solvothermal reaction of iron(III) acetylacetonate and aqueous ammonia. A photocurrent density of 0.683 mA cm⁻² at +1.5 V vs. RHE (reversible hydrogen electrode) was obtained under visible light (>420 nm, 100 mW cm⁻²) illumination. The TiO₂ underlayer plays an important role in the formation of hematite film, acting as an intermediary to alleviate the dead layer effect and as a support of large surface areas to coat greater amounts of Fe₂O₃. The as-prepared photoanodes are notably stable and highly efficient for photoelectrochemical water splitting under visible light. This study provides a facile synthesis process for the controlled production of highly active ultrathin hematite film and a simple route for photocurrent enhancement using several photoanodes in tandem.     

Effects of Ni,Fe2O3, and CNT addition by reactive mechanical grinding on the reaction rates with H2 of Mg-based alloys

Ni, Fe₂O₃, and CNT were added to Mg. The content of the additives was about 20 wt % with that of Fe₂O₃ 6 wt%. The contents of about 20 wt % additives and 6 wt% Fe₂O₃ are known optimum ones to improve the reaction rates of Mg with H₂. Samples with compositions of 80 wt% Mg–14 wt% Ni–6 wt% Fe₂O₃ (named as Mg–14Ni–6Fe₂O₃), and 78 wt% Mg–14 wt% Ni–6 wt% Fe₂O₃–2 wt% CNT (named as Mg–14Ni–6Fe₂O₃–2CNT) were prepared by reactive mechanical grinding. The hydriding and dehydriding properties of these samples were then measured, and the effects of Ni, Fe₂O₃, and CNT addition on the hydriding and dehydriding rates of Mg-based alloys were investigated by comparing their hydrogen-storage properties with those of pure Mg and Mg–10 wt% Fe₂O₃.     

Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4

Novel photocatalysts, which consist of two visible light responsive semiconductors including graphite-like carbon nitride (g-C₃N₄) and Fe₂O₃, were successfully synthesized via electrodeposition followed by chemical vapor deposition. The morphology of the g-C₃N₄/Fe₂O₃ can be tuned from regular nanosheets to porous cross-linked nanostructures. Remarkably, the optimum activity of the g-C₃N₄/Fe₂O₃ is almost 70 times higher than that of individual Fe₂O₃ for photoelectrochemical water splitting. The enhancement of photoelectrochemical activity could be assigned to the morphology change of the photocatalysts and the effective separation and transfer of photogenerated electrons and holes originated from the intimately contacted interfaces. The g-C₃N₄/Fe₂O₃ composites could be developed as high performance photocatalysts for water splitting and other optoelectric devices.     

Photoelectric properties of BiVO4 thin films deposited on fluorine doped tin oxide substrates by a modified chemical solution deposition process

BiVO₄ films deposited on Fluorine doped tin oxide glass substrates were successfully prepared by a modified chemical solution deposition process. Structure and optical spectrum analysis show that the resultant BiVO₄ films consist entirely of monoclinic scheelite structure and have a narrow band gap of ～2.66 eV. The films were investigated by photoelectrochemical and photovoltaic measurements with regard to hydrogen production and solar energy conversion under visible light. The BiVO₄ photoanodes show significantly higher visible light induced photoelectrochemical performance (～1.1 mA/cm² at 1.0 V vs. Ag/AgCl) than those reported ones, which is very promising for splitting water to H₂ and O₂. A Schottky BiVO₄ solar cell was also investigated for comparison with photoelectrochemical measurements. The correlation between the photoelectrochemical and photovoltaic behavior for BiVO₄ was explained. Our research should provide important support for the applications of BiVO₄ films or its modified forms such as doping and nanocomposite in heterojunction photoelectrochemical cells and solar cells with suitable energy level alignment at the interface.     

Photoelectrochemical evaluation of undoped and C-doped CdIn2O4 thin film electrodes

Undoped and C-doped cadmium indate (CdIn₂O₄) thin films and powders were synthesized, characterized, and evaluated for photoelectrochemical water splitting. Both undoped and C-doped CdIn₂O₄ samples have cubic lattices, and the presence of carbonate-type species was confirmed in the C-doped sample by XPS. Doping C into CdIn₂O₄ leads to a red shift (but no separate peak) in light absorption and band gap narrowing. The photocurrent densities of CdIn₂O₄ electrodes are at least three-fold greater than either CdO or In₂O₃ electrodes with equivalent film thickness. Carbon doping further improved the photocurrent densities by 33%. The photoelectrochemical performance of C-doped CdIn₂O₄ was optimized with respect to several synthetic parameters, including the C:In molar ratio and glucose concentration in the spray precursor solution, the calcination temperature, and the film thickness. The present work shows that CdIn₂O₄ is a promising photocatalyst and can be suitably doped to improve the electrochemical properties for solar conversion applications.     

Photocatalytic hydrogen production using Fe2O3-based core shell nano particles with ZnS and CdS

Stability and efficiency of photocatalysts are important to realize the practical applications of them for photocatalytic hydrogen production from industrial sulfide effluent. Novel, magnetically separable core–shell nano photocatalysts viz., CdS/Fe₂O₃, ZnS/Fe₂O₃ and (CdS + ZnS)/Fe₂O₃ were prepared and their hydrogen evolution activity under visible light was examined. The XRD result shows that CdS and ZnS were very well coated on the surface of the iron oxide core shell particles. The HR-TEM result also confirms the core shell formation. (CdS + ZnS)/Fe₂O₃ evolved higher volume of hydrogen than the other catalysts. It is ascribed to rapid migration of excited electrons from (CdS + ZnS) toward Fe₂O₃ suppressing electron hole annihilation compared to other catalysts. The catalysts can be easily recovered from the reaction medium using external magnetic bar and so the photocatalyst can be reused without any mass loss. Hence, it can be a potential catalyst for recovery of hydrogen from industrial sulfide containing waste streams.     

Magnetic field effects on selective catalytic reduction of NO by NH3 over Fe2O3 catalyst in a magnetically fluidized bed

Selective catalytic reduction (SCR) of NO from simulated flue gas by ammonia with Fe₂O₃ particles as the catalyst was performed using a magnetically fluidized bed (MFB). X-ray diffraction (XRD) spectroscopy and Brunauer–Emmett–Teller (BET) method were used to analyze Fe₂O₃ catalyst. Important effects of magnetic fields were observed in the SCR of NO by ammonia over Fe₂O₃ catalyst. The apparent activation energies of SCR were reduced by external magnetic fields, and the SCR activity of Fe₂O₃ catalyst was improved with the magnetic fields at low temperatures. Thus the scope of temperature with high efficiency of NO removal was extended from 493–523 K to 453–523 K by magnetic fields. Magnetic fields of 0.01–0.015 T were suggested for NO removal on Fe₂O₃ catalyst with MFB. The results suggested that the magnetoadsorption of NO onto Fe₂O₃ surface together with NH₂ and NO free radicals effects induced by the external magnetic fields both acted to improve the rate of SCR of NO on Fe₂O₃ catalyst. On the other hand, magnetic field effects were also attributed to improved gas–solid contact in MFB.     

Synthesis and photoelectrochemical study of BiWxV1−xO4+x/2 films by a polymer-assisted method

A series of BiW_xV_1−xO_4+x/2 films were coated on fluorine-doped tin oxide (FTO) glass by a polymer-assisted method and examined as photoelectrodes for photoelectrochemical measurements under Xe lamp light irradiation in a 0.5 M Na₂SO₄ solution. The compositions, structural, optical and morphologic properties of the films were characterized by XPS, XRD, UV–vis and SEM. The results showed the successfully synthesized films and their photoelectrochemical activities, revealing that the amount of tungsten had an important effect on the photoelectrochemical activities of BiW_xV_1−xO_4+x/2 films and the highest incident photon to current conversion efficiency (IPCE) was obtained when x equaled 0.1.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.     

Improved separation efficiency of photogenerated carriers for Fe2O3/SrTiO3 heterojunction semiconductor

To investigate the mechanisms of the improvement on separation efficiency of photogenerated carriers, a Fe₂O₃/SrTiO₃ heterojunction semiconductor with an improved separation efficiency was successfully prepared. The heterojunction semiconductor was characterized with X-ray diffraction (XRD), UV–vis absorption spectrum, scanning electron microscope (SEM) and surface photovoltage (SPV) spectroscopy. The energy band diagrams of Fe₂O₃ and SrTiO₃ were determined with X-ray photoelectron spectroscopy (XPS), based on which the conduction band offset (CBO) between Fe₂O₃ and SrTiO₃ was quantified to be 1.26 ± 0.03 eV. The recombination of photogenerated carriers was investigated with photoluminescence (PL) spectrum, which indicates that the formation of Fe₂O₃/SrTiO₃ decreases the recombination. Thus the improved separation efficiency is mainly due to the energy difference between the conduction band edges of Fe₂O₃ and SrTiO₃, and the decreased electron-hole recombination for Fe₂O₃/SrTiO₃.     

Efficient indium tin oxide/Cr-doped-TiO2 multilayer thin films for H2 production by photocatalytic water-splitting

Cr-doped-TiO₂ thin films, with three different Cr concentrations (2, 5.5, and 9 at.%), have been synthesized by radio-frequency magnetron sputtering in order to sensitize TiO₂ in visible light. UV–visible spectra showed that maximum narrowing (2.1 eV) of the TiO₂ band gap is obtained for the highest Cr concentration. However, negligible photocurrent was measured with Indium Tin Oxide (ITO)/Cr-doped-TiO₂ (9 at.%) single bilayer sample due to the increased recombination rate of the photo-generated charges on the defects associated to the Cr³⁺ ions. To lower the charge recombination rate in the Cr-doped-TiO₂, multilayer films with different numbers of ITO/Cr-doped-TiO₂ (9 at.%) bilayers (namely, 3-, 4-, 5-, 6- and 7-bilayers) were deposited by keeping the total thickness of TiO₂ constant in each multilayer film. When the multilayer films were exposed to visible light, we observed that the photocurrent increases as function of the number of bilayers by reaching the maximum with 6-bilayers of ITO/Cr-doped-TiO₂. The enhanced photocurrent is attributed to: 1) higher absorption of visible light by Cr-doped-TiO₂, 2) number of space charge layers in form of ITO/TiO₂ interfaces in multilayer films, and 3) generation of photoelectrons just in/or near to the space charge layer by decreasing the Cr-doped-TiO₂ layer thickness. The reduced charge recombination rate in multilayer films was also confirmed by studying the photocurrent kinetic curve. The superior photocatalytic efficiency of the 6-bilayers film implies higher hydrogen production rate through water-splitting: we obtained indeed 24.4 μmol/h of H₂ production rate, a value about two times higher than that of pure TiO₂ (12.5 μmol/h).     

Study on photocurrent of bilayers photoanodes using different combination of WO3 and Fe2O3

Bilayer photoanodes were prepared onto glass substrates (FTO) in order to improve generated photocurrents using UV-vis light by water splitting process. A comparative study of photocatalytic was performed over the films surface using Fe₂O_3, WO₃ and mixture of bicomponents (Fe₂O₃:WO₃). Different types of films were prepared using Fe₂O_3, WO₃ and bicomponents (mixture) on FTO substrates. The films were grown by sol gel method with the PEG-300 as the structure-directing agent. The photo-generated of the samples were determined by measuring the currents and voltages under illumination of UV-vis light. The morphology, structure and related composition distribution of the films have been characterized by SEM, XRD and EDX respectively. Photocurrent measurements indicated surface roughness as the effective parameter in this study. The deposited surfaces by bicomponents or mixture are flat without any feature on the surface while the deposited surfaces by WO₃ appears rough surface as small round (egg-shaped particles) and cauliflower-like. The surface deposited by Fe₂O₃ show rough no as well as WO₃ surface. The deposited surfaces by WO₃ reveal the higher value of photocurrent measurement due to surface roughness. Indeed, the roughness can be effective in increasing contact surface area between film and electrolyte and diffuse reflection (light scattering effect). The solution (Fe₂O₃:WO₃) shows the low photocurrent value in compare to WO₃ and Fe₂O₃ hat it may be due to decomposition the compound at 450 ± 1 °C to iron-tungstate Fe₂(WO₄)₃.     

Photoelectrochemical and structural characterization of carbon-doped WO3 films prepared via spray pyrolysis

Carbon-doped tungsten trioxide (WO₃) films were produced using a spray-pyrolysis methodology, with glucose used as the carbon dopant source. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis, scanning electron microscopy, and solid-state nuclear magnetic resonance. The photoelectrochemical activity was evaluated under near UV–visible light and visible light only irradiation conditions. The presence of carbonate-type species in the C-doped sample was confirmed by XPS and SSNMR. The C-doped WO₃ electrodes exhibited photocurrent densities up to 1.6 mA/cm² in 1 M HCl electrolyte and as high as 2.6 mA/cm² with the addition of methanol as a sacrificial agent. A high contribution (∼50%) of the photocurrent density was observed from visible light. C-doped WO₃ produced approximately 50% enhanced photocurrent densities compared with the undoped WO₃ electrode synthesized using the same procedures. The photoelectrochemical performance was optimized with respect to several synthetic parameters, including dopant concentration, calcination temperature and film thickness. These results indicate the potential for further development of WO₃ photocatalysts by simple wet chemical methods, and provide useful information towards understanding the structure and enhanced photoelectrochemical properties of these materials.     

Li4Ti5O12/CNTs composite anode material for large capacity and high-rate lithium ion batteries

Li₄Ti₅O₁₂ sub-micro crystallites are synthesized by ball-milling and one-step sintering under different heat treatment temperature from 700 °C to 900 °C. The composite electrode of Li₄Ti₅O₁₂/carbon nanotubes (CNTs) is prepared by mixing powders of Li₄Ti₅O₁₂ and CNTs in different weight ratios. Before mixing, in order to disperse CNTs in Li₄Ti₅O₁₂ particles preferably, the CNTs are cut and dispersed by hyperacoustic shear method and the composite electrodes of low resistance of about 20–30 Ω are obtained. The composite electrodes have steady discharge platform of 1.54 V and large specific capacity, initial discharge capacities are 168, 200, 196, 176 mAh g⁻¹ in different Li₄Ti₅O₁₂:CNTs weight ratios of 94:1, 92:3, 90:5, 88:7 respectively at 0.1 C discharge rate for the Li₄Ti₅O₁₂ synthesized in an optimized heat treatment temperature of 800 °C. In our experimental range, the composite electrode in a CNTs weight ratio of 3:92 shows the best performance under different discharge rate such as the initial capacity is 200 mAh g⁻¹ with discharge capacities retention rate of nearly 100%. Its capacity is about 151 mAh g⁻¹ under 20 C rate discharge condition with excellent high-rate performance. There is almost no decline after 20th cycles under 10 C rate discharging condition.     

Photoresponse and H2 production of topographically controlled PEG assisted Sol–gel WO3 nanocrystalline thin films

WO₃ thin films were fabricated by sol–gel method using polyethylene glycol (PEG) as dispersing agent. Physical and photoelectrochemical properties of the synthesized nanocrystalline films were studied by varying weight ratio of PEG to tungsten precursor (x). Based on AFM observations and statistical modeling of the WO₃ surface, the thickness of the films increased by increasing the amount of x with a nearly linear fashion while the surface roughness reached to a saturated value. However, the film synthesized with x = 4 showed a chaotic surface behavior. Optical analysis revealed that by increasing the x, transmittance of the films decreased while their band gap energies remained unchanged. According to XRD results, variation of x did not change structure of the nanocrystalline film while XPS analysis indicated a better stoichiometry for the films with higher x values. A less charge transport life time was obtained for films with higher x values, but an enhanced photoresponse of the films and also hydrogen production via water splitting reaction were observed by increasing the amount of x. On the other hand, the charge transfer resistance of the samples reduced from 6.5 kΩ to 1.2 kΩ by addition of PEG to the sol from x = 0 to x = 2.     

Fabrication of all solid-state lithium-ion batteries with three-dimensionally ordered composite electrode consisting of Li0.35La0.55TiO3 and LiMn2O4

A composite electrode between three-dimensionally ordered macroporous (3DOM) Li_0.35La_0.55TiO₃ (LLT) and LiMn₂O₄ was fabricated by colloidal crystal templating method and sol–gel process. A close-packed PS beads with the opal structure was prepared by filtration of a suspension containing PS beads. Li–La–Ti–O sol was injected by vacuum impregnation process into the voids between PS beads, and then was heated to form 3DOM-LLT. Three-dimensionally ordered composite material consisting of LiMn₂O₄ and LLT was prepared by sol–gel process. The prepared composite was characterized with SEM and XRD. All solid-state Li-ion battery was fabricated with the LLT–LiMn₂O₄ composite electrode as a cathode, dry polymer electrolyte and Li metal anode. The prepared all solid-state cathode exhibited a volumetric discharge capacity of 220 mAh cm⁻³.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Reduction characteristics of CuFe2O4 and Fe3O4 by methane; CuFe2O4 as an oxidant for two-step thermochemical methane reforming

The reduction characteristics of CuFe₂O₄ and Fe₃O₄ by methane at 600–900 °C were determined in a thermogravimetric analyzer for the purpose of using CuFe₂O₄ as an oxidant of two-step thermochemical methane reforming. It was found that the addition of Cu to Fe₃O₄ largely affected the reduction kinetics and carbon formation in methane reduction. In the case of CuFe₂O₄, the reduction kinetics was found to be faster than that of Fe₃O₄. Furthermore, carbon deposition and carbide formation from methane decomposition were effectively inhibited. In case of Fe₃O₄, Fe metal formed from Fe₃O₄ decomposed methane catalytically, that lead to the formation of graphite and Fe₃C phases. It is deduced that Cu in CuFe₂O₄ enhanced reduction kinetics, decreased reduction temperature and prevented carbide and graphite formation. Additionally, methane conversion and CO selectivity in the syngas production step with CuFe₂O₄ were in the range of 33.5–55.6% and 54.9–59.6%, respectively.     

An electrochemical strategy of doping Fe into TiO2 nanotube array films for enhancement in photocatalytic activity

Highly ordered Fe³⁺-doped TiO₂ nanotube array films were fabricated directly by the electrochemical anodic oxidation of pure titanium in an HF electrolyte solution containing iron ions. The morphology, structure and composition of the as-prepared nanotube array films were characterized by SEM, Raman and XPS. The effects of dopant amount on the morphologies, structure, photoelectrochemical property and photoabsorption of the TiO₂ nanotube array film were investigated. The results showed that Fe³⁺ was successfully introduced into the nanotube array film. Compared with the undoped TiO₂ nanotube array film, the photocurrent of Fe³⁺-doped TiO₂ nanotube array films increased obviously. The absorption edge of Fe³⁺-doped TiO₂ nanotube array films appeared to be red shifted. The photocatalytic activity of Fe³⁺-doped TiO₂ nanotube array films was evaluated by the removal of methylene blue (MB) aqueous solution. A maximum enhancement of photocatalytic activity was achieved for Fe³⁺-doped TiO₂ nanotube array film prepared in 0.10 M Fe(NO₃)₃+0.5% HF electrolyte under UV irradiation, which attributes to the effective separation of photogenerated electron–hole upon the substitutional introduction of appropriate Fe³⁺ amount into the anatase TiO₂ structure.