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.     

Preparation and photoelectrochemical properties of functional carbon nanotubes and Ti co-doped Fe2O3 thin films

Functional carbon nanotubes (CNTs) were incorporated into Ti-doped Fe₂O₃ thin films by a facile, one-step co-electrodeposition method. The films were characterized by X-ray diffraction, scanning electron microscopy, UV–visible absorption, and X-ray photoelectron spectroscopy. The introduction of CNTs results in a better absorption in visible region and greatly enhances the photoelectrochemical properties of the Ti–Fe₂O₃ films. The improved photoelectrochemical properties of the CNTs and Ti co-doped Fe₂O₃ films are due to the charge equilibration which interplays between the Ti–Fe₂O₃ and CNTs. The effect of CNTs to mediate fast charge transfer and to retard charge recombination rate in the composites is also demonstrated by kinetics analysis and electrochemical impedance spectroscopy. The influence of different groups-modified CNTs and different content of CNTs was also studied. The highest photocurrent is 4.5 mA/cm² at 1.23 V (vs. RHE) obtained by incorporating 0.10 mg/mL amino-group modified CNTs in the Ti–Fe₂O₃ film. The amino-functionalized CNTs doped film exhibits the highest photoelectric response compared with those doped by the pristine and acid-treated CNTs under the same conditions, which can be ascribed to the better hydrophilicity and dispersibility of the amino-functionalized CNTs.     

MWCNTs and Cu2O sensitized TiFe2O3 photoanode for improved water splitting performance

Fe₂O₃ and Cu₂O, both earth abundant materials are used in functionalizing Ti doped Fe₂O₃ photoanodes with Cu₂O and MWCNTs for improving photoelectrochemical performance for hydrogen generation. Pristine Ti doped Fe₂O₃ are fabricated by spray pyrolysis deposition method on the conducting ITO coated glass substrate. Two different modifications are adopted to improve the photoelectrochemical performance of pristine sample by subsequent deposition of multi walled carbon nano tubes (MWCNTs) alone and also in combination with Cu₂O. Better photoresponse in modified samples is attributed to increase in conductivity and promotion of electron transport to Fe₂O₃ layer due to presence of MWCNTs while formation of heterojunction also promotes charge transfer kinetics by effective separation of charge carriers. Offering high photocurrent density of 5.17 mA cm^?2 at 1 V vs SCE, high open circuit voltage (V_oc), least resistance, higher negative flat band potential (V_fb), TiFe₂O₃/(MWCNTs + Cu₂O), emerges as the most photoactive sample. High applied bias photon to current conversion efficiency (ABPE) value of 4.6% is obtained for the modified sample against 0.07% ABPE for TiFe₂O₃ photoanodes.     

Convex-nanorods of α-Fe2O3/CQDs heterojunction photoanode synthesized by a facile hydrothermal method for highly efficient water oxidation

Morphology controlling and surface modification of semiconductors is the key for efficient photoelectrochemical (PEC) water splitting systems. This work provides a new strategy for achieving morphology control and heterojunction construction simultaneously by one-step hydrothermal method. The α-Fe₂O₃/CQDs heterojunction photoanode with convex-nanorods morphology is successfully prepared by hydrothermal method in CQDs (Carbon Quantum Dots) aqueous contained iron precursor followed by low temperature annealing treatment. Compared with bare hematite photoanode, the α-Fe₂O₃/CQDs photoanode has 8.5 time higher photocurrent density (at 1.23 V vs. RHE) of 0.35 mA cm^?2 and a negative shift of onset potential about 300 mV. The enhanced photoelectrochemical response is attributed to the convex-nanorods which benefit higher absorbance of light and the formed α-Fe₂O₃/CQDs heterojunction, which can efficiently enhance the electron-hole separation and reduce the surface charge recombination. The morphology and properties of the sample were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fouriertrans form infrared spectroscopy (FTIR), UV–vis spectra, X-ray diffractometry (XRD), X-ray photoelectron spectra (XPS), and photoelectrical measurements.     

Significantly improve photoelectrochemical performance of Ti:Fe2O3 with CdSe modification and surface oxidation

Significant interest has been arisen to explore photoanodes for full optical absorption spectrums and good stability in photoelectrochemistry. Herein CdSe is used to modify Ti:Fe₂O₃ photoanode forming Ti:Fe₂O₃/CdSe heterojunction. Combining with an air annealing treatment, Ti:Fe₂O₃/CdSe exhibits a 6.5 times higher photocurrent density that of the pristine Ti:Fe₂O₃ to achieve 3.25 mA cm^?2 at 1.2 V vs. RHE. The photoelectrochemical (PEC) stability of Ti:Fe₂O₃/CdSe annealed in air shows great improvement comparable to both unannealed and annealed ones in Ar. The enhancement mechanisms for both heterojunction and annealing are explored for fundamental insights, which reveal that the surface oxide layer can significantly increase the PEC stability of Ti:Fe₂O₃/CdSe photoanode. X-ray photoelectron spectra and transmission electron microscope results further confirms the surface oxidation on CdSe layer after annealing in air.     

Iron oxide (n-Fe2O3) nanowire films and carbon modified (CM)-n-Fe2O3 thin films for hydrogen production by photosplitting of water

Iron oxide n-Fe₂O₃ nanowire photoelectrodes were synthesized by thermal oxidation of Fe metal sheet (Alfa Co. 0.25 mm thick) in an electric oven then tested for their photoactivity. The photoresponse of the n-Fe₂O₃ nanowires was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen, which is proportional to photocurrent density, J_p. The optimized electric oven-made n-Fe₂O₃ nanowire photoelectrodes showed photocurrent densities of 1.46 mA cm⁻² at measured potential of 0.1 V/SCE at illumination intensity of 100 mW cm⁻² from a Solar simulator with a global AM 1.5 filter. For the optimized carbon modified (CM)-n-TiO₂ synthesized by thermal flame oxidation the photocurrent density for water splitting was found to increase by two fold to 3.0 mA cm⁻² measured at the same measured potential and the illumination intensity. The carbon modified (CM)-n-Fe₂O₃ electrode showed a shift of the open circuit potential by −100 mV/SCE compared to undoped n-Fe₂O₃ nanowires. A maximum photoconversion efficiency of 2.3% at applied potential of 0.5 V/E_aoc was found for CM-n-Fe₂O₃ compared to 1.69% for n-Fe₂O₃ nanowires at higher applied potential of 0.7 V/E_aoc. These CM-n- Fe₂O₃ and n- Fe₂O₃ nanowires thin films were characterized using photocurrent density measurements under monochromatic light illumination, UV-Vis spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

Efficient photoelectrochemical water splitting of CaBi6O10 decorated with Cu2O and NiOOH for improved photogenerated carriers

Broadening the light absorption and accelerating the separation of photogenerated electron-hole pairs is of crucial importance for strongly enhancing the photoelectrochemical (PEC) water splitting performances of photoelectrode. In this paper, a novel CaBi₆O₁₀/Cu₂O/NiOOH photoanode for photoelectrochemical water splitting is prepared, where, the NiOOH acts as water oxidation catalyst to accelerate water oxidation taking place in the interfaces between electrode and electrolyte, Cu₂O is chosen to extend the absorption range of the light absorber, enhancing an efficient separation and transfer of the electron-hole pairs. This triple CaBi₆O₁₀/Cu₂O/NiOOH photoanode negatively shifts the onset potential and exhibits an improved photocurrent density 1.89 mA·cm^?2 at 1.23 V vs RHE, which is 1.4 and 4.8 times higher compared to CaBi₆O₁₀/Cu₂O and CaBi₆O₁₀, respectively. More importantly, the CaBi₆O₁₀/Cu₂O/NiOOH electrode shows excellent photoelectrochemical stability in comparison with CaBi₆O₁₀/Cu₂O after 2 h irradiation. The amazing photoelectrochemical performance is due to the broader light absorption spectrum, the improved photogenerated carriers separation, transfer and consumption. The research results demonstrate a promising ternary semiconductor structure, which can improve photoelectrochemical performance effectively. Moreover, these results also imply that the CaBi₆O₁₀/Cu₂O/NiOOH heterojunction structure has a great potential application for photoelectrochemical water splitting systems.     

Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts

In this study, magnetic Fe₃O₄ modified Ru/Carbon nanotubes (CNTs) catalysts were used to achieve the catalytic transfer hydrogenation of furfural (FF) to furfuryl alcohol (FFA), with alcohols as the solvent and hydrogen donors. According to the result of the catalyst characterization, Fe₃O₄ promoted the formation of Ru⁰ species. The effects of Fe₃O₄ loading and different hydrogen donors on the catalytic transfer hydrogenation of FF were tested, and the reaction parameters and catalyst stability were also analyzed. It is found that Fe₃O₄ effectively enhanced the activity of Ru/CNTs in catalytic transfer hydrogenation of FF, the catalytic activity was optimized at the Fe₃O₄ loading of 5 wt%, and the optimal hydrogen donor was i-propanol. Moreover, the Ru–Fe₃O₄/CNTs could be easily collected for further use and possessed excellent stability. The mechanism of the catalytic transfer hydrogenation of FF using Ru–Fe₃O₄/CNTs was discussed, and the corresponding catalyst activity groups included metal Ru sites and RuO_x-Fe₃O₄ Lewis acid sites, which account for the excellent catalytic activity of transfer hydrogenation.     

H2 evolution from a photoelectrochemical cell with n-Cu2O photoelectrode under visible light irradiation

H₂ evolution was observed for the first time from a photoelectrochemical cell using an n-type Cu₂O photoelectrode under visible light irradiation. Three-electrode configuration was used in the photoelectrochemical cell to observe H₂ evolution. AgCl/Ag calomel electrode and a platinum plate were used as the reference and counter electrodes, respectively. Fe²⁺/Fe³⁺ redox couple was used as the electrolyte. H₂ evolution was visible on the platinum electrode in the photoelectrochemical cell.     

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.     

Novel synthesis of Ti–Fe2O3/MIL-53(Fe) via the in situ process for efficient photoelectrochemical water oxidation

The slow kinetics of water oxidation has become a challenge for photoelectrochemical hydrogen production. Here, a novel organic-inorganic integrated photoanode system was constructed by using MIL-53(Fe) formed during the in-situ etching process as a cocatalyst to modify Ti–Fe₂O₃. The photocurrent density of Ti–Fe₂O₃/MIL-53(Fe) reaches 2.5 mA/cm², 10 times that of bare Ti–Fe₂O₃ at 1.23 V vs. RHE, and the water oxidation photocurrent onset potential shifts 105 mV negatively. Ti–Fe₂O₃/MIL-53(Fe) reaches 52% at 390 nm for IPCE. The excellent photoelectrochemical performance is due to iron oxide clusters boost charge separation and transfer, in-situ etching exposes more reactive sites, and the tight connection reduces interfacial resistance, which greatly accelerates the surface kinetics of Ti–Fe₂O₃. The in-depth understanding is provided for in-situ modification of photoanodes by metal organic frameworks in this work.     

Plasmon Au modified 3D-nanostructrue CoOx decorated hematite for highly efficient photoelectrochemical water splitting

Surface modification and interface engineering are efficient strategies to address the serious charge recombination and the sluggish water oxidation kinetics in photoelectrochemical water splitting. In this work, CoO_x decorated hematite nanosheets (Fe₂O₃/CoO_x) are deposited on Nickel foam by the in-situ hydrothermal process. Au nanoparticles are incorporated on Fe₂O₃/CoO_x semiconductors (Fe₂O₃/CoO_x/Au) by electrochemical deposition. In photoelectrochemical test, Fe₂O₃/CoO_x attains a photocurrent density of 1.87 mA cm^?2 at 1.23 V_RHE, which is 4.45 times that for α-Fe₂O₃. The onset potential of Fe₂O₃/CoO_x decreases by 266 mV compared with α-Fe₂O₃. The 3D-nanostructrue Fe₂O₃/CoO_x/Au attains a photocurrent density of 3.88 mA·cm^?2 at 1.23 V_RHE, which is 9.24 times that of ɑ-Fe₂O₃. The applied bias photon-to-current efficiency, charge separation and charge injection efficiency of Fe₂O₃/CoO_x/Au are improved. EIS studies show the co-modification of CoO_x and Au reduces charge transfer resistance. This strategy would provide a potential approach to promote light absorption and charge separation for photoelectrochemical catalyst.     

Variation on anthracite combustion efficiency with CeO2 and Fe2O3 addition by Differential Thermal Analysis (DTA)

Effects of CeO₂ and Fe₂O₃ on anthracite combustion efficiency were investigated using differential thermal analysis (DTA). Based on heat release (Q_D) of anthracite as well as anthracite with CeO₂ and anthracite with Fe₂O₃ additions against α-Al₂O₃ in DTA experiment, effects of additives CeO₂ and Fe₂O₃ on anthracite combustion efficiency were evaluated. Under the same experimental conditions, heat releases of raw anthracite, anthracite with CeO₂ and anthracite with Fe₂O₃ were 11.04 kJ/g, 11.30 kJ/g and 11.42 kJ/g, respectively, indicating that anthracite combustion efficiency was improved by addition of CeO₂ and Fe₂O₃. To confirm the above results, carbon transfer was monitored using Thermogravimetric analysis Fourier transform infrared (TGA-FTIR) and Carbon-Sulfur analyzer during catalytic combustion process. The results indicated that CO₂ emission was increased, whereas CO emission and residual carbon of ash were decreased, being in accordance with the results of DTA. Finally, according to analyses of ignition temperature and catalytic combustion process, the possible mechanism of catalytic combustion of anthracite was proposed.     

Oxygen-releasing step of ZnFe2O4/(ZnO + Fe3O4)-system in air using concentrated solar energy for solar hydrogen production

The oxygen-releasing step of the ZnFe₂O₄/(ZnO + Fe₃O₄)-system for solar hydrogen production with two-step water splitting using concentrated solar energy was studied under the air-flow condition by irradiation with concentrated Xe lamp beams from a solar simulator. The spinel-type compound of ZnFe₂O₄ (Zn-ferrite) releases O₂ gas under the air-flow condition at 1800 K and then decomposes into Fe₃O₄ () and ZnO with a nearly 100% yield (ZnFe₂O₄ = ZnO + 2/3Fe₃O₄ + 1/6O₂). The ZnO was deposited as the thin layer on the surface of the reaction cell wall. A thermodynamic study showed that the ZnO was produced by the reaction between the O₂ gas in the air and the metal Zn vapor generated from ZnFe₂O₄. With the combined process of the present study on the O₂-releasing step and the previous one on the H₂ generation step (ZnO + 2/3Fe₃O₄ + 1/3H₂O = ZnFe₂O₄ + 1/3H₂) for the ZnFe₂O₄/(ZnO + Fe₃O₄)-system, solar H₂ production was demonstrated by one cycle of the ZnFe₂O₄/(ZnO + Fe₃O₄)-system, where the O₂-releasing step had been carried out in air at 1800 K and the H₂ generation step at 1100 K.     

Characterization of Fe2O3/CeO2 oxygen carriers for chemical looping hydrogen generation

Fe₂O₃ is currently the most proper active metal oxide for chemical looping hydrogen generation (CLHG). However, supports are necessary to improve the reactivity and redox stability. CeO₂ can enhance the oxygen mobility, leading to high redox reactivity and carbon deposition resistance, which can be an excellent alternative support for oxygen carriers. In this paper, Fe₂O₃/CeO₂ oxygen carriers prepared by the co-precipitation method with different Fe₂O₃ loadings were investigated on a batch fluidized bed regarding the hydrogen yield and purity, redox reactivity and stability in CLHG with CO as fuel. The results showed that Fe6Ce4 is the best given comprehensive performance with no CO or CO₂ observed in the obtained hydrogen (detection limit 0.01% in volume). The oxygen mobility property for the reducible support CeO₂ and the physical contact between un-integrated Fe₂O₃ and CeO₂ could improve the reduction of Fe₂O₃. In addition, the formation of the hematite-like solid solution and perovskite-type CeFeO₃ could bring about abundant oxygen vacancies and promote the oxygen mobility, which contributes to the elimination of carbon deposition, counteracts the negative effect of serious sintering and guarantees the reactivity and redox stability of the Fe₂O₃/CeO₂ oxygen carriers. The Fe₂O₃/CeO₂ oxygen carriers were characterized by carbon monoxide temperature-programmed reduction measurement and X-ray diffraction patterns, and Fe6Ce4 was also selected to be characterized by scanning electron microscopy images and energy dispersive X-ray spectrometer analysis.     

Synergy between isolated-Fe3O4 nanoparticles and CNx layers derived from lysine to improve the catalytic activity for oxygen reduction reaction

In order to seek heterogeneous electrocatalyst with efficient catalytic activity for oxygen reduction reaction (ORR), Fe₃O₄-CN_x composite reported in our previous work was studied as electrocatalyst for ORR and showed poor catalytic activity. To improve the catalytic activity, Fe₃O₄-CN_x composite is modified by the CN_x layers derived from lysine through pyrolysis. The physical characterization show that the coral-shaped morphology of the resultant composite (Fe₃O₄-CN_x-Lys) is still retained, while the degree of its graphitic crystalline increases. Besides, Fe₃O₄-CN_x-Lys has 364.7 m² g⁻¹ of surface area with hierarchical porous structure. Electrochemical tests show that the catalytic activity Fe₃O₄-CN_x-Lys for ORR is not only higher than those Fe₃O₄-CN_x, XC-72-Lys derived from lysine and XC-72 Vulcan carbon but also comparable to that of commercial Pt/C (20 wt%).     

Up-conversion fluorescent carbon quantum dots decorated covalent triazine frameworks as efficient metal-free photocatalyst for hydrogen evolution

Photocatalytic hydrogen production holds great promise for alleviating the energy shortage through effective photo-to-chemical conversion, and the development of visible-light responsive, low-cost and sustainable photocatalysts remains key priority. In this study, carbon quantum dots/covalent triazine-based framework (CQDs/CTF) non-metallic photocatalyst was constructed through a simple impregnation method for photocatalytic H₂ evolution. Upon 0.24% CQDs loading, a three-fold enhanced H₂ production activity of 102 μmol?g^?1?h^?1 was achieved compared with pristine CTF-1 (34.5 μmol?g^?1?h^?1). Photoluminescence and photoelectrochemical study revealed carbon quantum dots served as the electron libraries, which was conducive to facilitate electron capture and promote the separation of photoinduced electron-hole pairs in CTF-1. Notably, the excitation-independent up-conversion fluorescent characteristics of CQDs endowed the catalysts broadened visible-light response range and higher solar energy utilization efficiency. This study deepens insights into the mechanism of CQDs modification and paves a trustworthy strategy for harvesting visible-light-driven metal-free photocatalyst with highly-active and robust performance.     

Morphological, optical and photocatalytic properties of TiO2–Fe2O3 multilayers

Morphological, optical and photocatalytic properties of TiO₂, Fe₂O₃ and TiO₂–Fe₂O₃ samples (formed by 1, 3 and 5 coatings) were studied. The layers were deposited on glass substrate by the sol–gel method. The catalytic activity of the samples was studied by the photodecomposition of methylene blue (MB) under visible light illumination. The FTIR results indicate that all samples present surface OH radicals that are bound either to the Ti or Fe atoms. This effect is better visualized at larger number of coatings in the TiO₂–Fe₂O₃/glass systems. Also, two mechanisms are observed during the photodecomposition of the MB.     

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.     

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.