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

A Review of Electrical Assisted Photocatalytic Technologies for the Treatment of Multi-Phase Pollutants

This article reviews the fundamental theories and reaction mechanisms of photocatalytic technologies with the assistance of electrical field for degrading multi-phase pollutants. Photo(electro)catalysis including photocatalytic oxidation (PCO) and photoelectrocatalytic oxidation (PECO) have been a potential technologies applied for the treatment of organic and inorganic compounds in the wastewaters and waste gases, which has been treated as a promising technique by using semiconductors as photo(electro)catalysts to convert light or electrical energy to chemical energy. Combining photocatalytic processes with electrical field is an option to effectively decompose organic and inorganic pollutants. Although photocatalytic oxidation techniques have been used to decompose multi-phase pollutants, developing efficient advanced oxidation technologies (AOTs) by combining photocatalysis with electrical potential is urgently demanded in the future. This article reviews the most recent progress and the advances in the field of photocatalytic technologies combined with external electrical field, including the characterization of nano-sized photo(electro)catalysts, the degradation of multi-phase pollutants, and the development of electrical assisted photocatalytic technologies for the potential application on the treatment of organic and inorganic compounds in the wastewaters and waste gases. Innovative oxidation techniques regarding photo(electro)catalytic reactions with and without oxidants are included in this review article.

High Energy Capacitors Based on All Metal-Organic Frameworks Derivatives and Solar-Charging Station Application

High energy and efficient solar charging stations using electrochemical capacitors (ECs) are a promising portable power source for the future. In this work, two kinds of metal-organic framework (MOF) derivatives, NiO/Co₃ O₄ microcubes and Fe₂ O₃ microleaves, are prepared via thermal treatment and assembled into electrochemical capacitors, which deliver a relatively high specific energy density of 46 Wh kg^-1 at 690 W kg^-1 . In addition, a solar-charging power system consisting of the electrochemical capacitors and monocrystalline silicon plates is fabricated and a motor fan or 25 LEDs for 5 and 30 min, respectively, is powered. This work not only adds two novel materials to the growing categories of MOF-derived advanced materials, but also successfully achieves an efficient solar-ECs system for the first time based on all MOF derivatives, which has a certain reference for developing efficient solar-charge systems.

Carbon-incorporated porous honeycomb NiCoFe phosphide nanospheres derived from a MOF precursor for overall water splitting

Carbon-incorporated porous honeycomb NiCoFe phosphide nanospheres were successfully prepared, exhibiting excellent performance for overall water splitting.

Nanostructured TaON/Ta3N5 as a highly efficient type-II heterojunction photoanode for photoelectrochemical water splitting

A heterostructured TaON/Ta₃N₅ photoanode exhibits a 350 mV negative shift of photocurrent onset potential to 0.65 V versus the reversible hydrogen electrode.

Local vs Nonlocal States in FeTiO3 Probed with 1s2pRIXS: Implications for Photochemistry

Metal-metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron-titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core-hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

Enhanced photoelectrochemical water splitting by oxides heterojunction photocathode coupled with Ag

A novel one-dimensional Co₃O₄/CuO/Ag composite structure film was directly grown on indium tin oxide glass substrate by a simple hydrothermal method and electrodeposition method. The film was employed for the first time as a photocathode for photoelectrochemical (PEC) water splitting to generate hydrogen. The photocurrent density of the Co₃O₄/CuO/Ag composite structure achieved -5.13 mA cm^-2 at -0.2 V vs. RHE, which is roughly 12.8 times that of 1D Co₃O₄ nanowires and 3.31 times Co₃O₄/CuO heterojunction photocathodes. The enhanced PEC performance of this Co₃O₄/CuO/Ag composite structure ascribes increased light-harvesting and light-absorption, distensible photoresponse range, decreased interface charge transfer resistance, and improved photogenerated electron-hole pairs transfer and separation.

Efficient all p-type heterojunction photocathodes for photoelectrochemical water splitting

Co₃O₄ nanostructures with different morphologies are directly grown on an ITO substrate and Sb₂S₃ is loaded onto these to construct a Co₃O₄/Sb₂S₃ heterojunction, which is used as an all p-type photocathode for PEC water splitting for the first time.

Synergistic photoelectrochemical reduction of Cr(VI) and oxidation of organic pollutants by g-C3N4/TiO2-NTs electrodes

The g-C3N4/TiO2-NTs electrodes were synthesized by a dip-coating procedure followed by high-temperature annealing used in photoelectrochemical process. From the results, a simultaneous and rapid reduction of Cr(VI) and degradation of phenol in Cr(VI)/phenol system was observed with photoelectrocatalytic activity under UV-visible light irradiation than photocatalytic and electrocatalytic activities. The different kinds of Cr(VI)/organic pollutants systems were also investigated systematically. In addition, different scavengers were also added in Cr(VI)/phenol and Cr(VI)/benzyl alcohol systems to indicate that the hydroxyl radicals and superoxide radicals were the most major active species for the denomination of Cr(VI) and organic pollutants. The intermediates of phenol and benzyl alcohol were also detected during the reaction in order to deduce the photoelectrocatalysis mechanism underg-C3N4/TiO2-NTs electrodes that the charge separation was improved and subsequently electron-transfer efficiency was higher.

Passivation of hematite nanorod photoanodes with a phosphorus overlayer for enhanced photoelectrochemical water oxidation

Hematite (i.e., α-Fe2O3) nanorod photoanodes passivated with a phosphorus overlayer have been fabricated by decomposing sodium hypophosphite (NaH2PO2) at a low temperature over the hematite nanorod surface. Extensive scanning electron microscopy, transmission electron microscopy, x-ray diffractometry and UV-vis spectroscopy characterizations confirm that conformal deposition of an amorphous phosphorus overlayer does not change the crystal structure, morphology, and optical absorption properties of hematite photoanodes. X-ray photoelectron spectroscopy reveals that phosphorus in the deposited overlayer exists in an oxidized state. Comprehensive steady-state polarization, transient photocurrent response, and impedance spectroscopy measurements as well as Mott-Schottky analysis manifest that the phosphorus overlayer is able to effectively passivate surface states and suppress electron-hole recombination, substantially enhancing the photocurrent for water oxidation. Combining the phosphorization treatment with two-step thermal activation, a photocurrent density of 1.1 mA cm(-2) is achieved at 1.23 V versus reversible hydrogen electrode under illumination of 100 mW cm(-2), ca 55 times higher than that of the non-activated pristine hematite photoanode measured under the same conditions. The simple and fast phosphorization strategy we present here can be readily applied to passivate surfaces of other semiconductor photoelectrodes to improve their photoelectrochemical performance.

Enhanced Bulk and Interfacial Charge Transfer Dynamics for Efficient Photoelectrochemical Water Splitting: The Case of Hematite Nanorod Arrays

Charge transport in the bulk and across the semiconductor/electrolyte interface is one of the major issues that limits photoelectrochemical (PEC) performance in hematite photoelectrodes. Efficient charge transport in the entire hematite is of great importance to obtaining high photoelectrochemical properties. Herein, to reach this goal, we employed both TiO2 underlayer and overlayer deposition on hematite nanorod films, followed by a fast annealing treatment. The TiO2 underlayer and overlayer not only serve as dopant sources for carrier density increase but also reduce charge recombination at the fluorine-doped tin oxide (FTO)/hematite interface and accelerate charge transfer across the hematite/electrolyte interface. This synergistic doping and interface modifying effects give rise to an enhanced photoelectrochemical water oxidation performance of hematite nanorod arrays, generating an impressive photocurrent density of 1.49 mA cm(-2) at 1.23 V vs RHE. This is the first report on using both underlayer and overlayer modification with the same material to improve charge transport through the entire electron transport path in hematite, which provides a novel way to manipulate charge transfer across the semiconductor interface for a high-performance photoelectrode.

Hierarchically branched Fe2O3@TiO2 nanorod arrays for photoelectrochemical water splitting: facile synthesis and enhanced photoelectrochemical performance

Highly photoactive and durable photoanode materials are the key to photoelectrochemical water splitting. In this paper, hierarchically branched Fe2O3@TiO2 nanorod arrays (denoted as Fe2O3@TiO2 BNRs) composed of a long Fe2O3 trunk and numerous short TiO2 nanorod branches were fabricated and used as photoanodes for water splitting. Significant improvement of photoelectrochemical water splitting performance was observed based on Fe2O3@TiO2 BNRs. The photocurrent density of Fe2O3@TiO2 BNRs reaches up to 1.3 mA cm(-2) at 1.23 V versus RHE, which is 10 times higher than that of pristine Fe2O3 nanorod arrays under the same conditions. Furthermore, an obvious cathodic shift in the onset potential of photocurrent was observed in the Fe2O3@TiO2 BNRs. More significantly, the Fe2O3@TiO2 BNRs are quite stable even after 3600 s continuous illumination, and the photocurrent density shows almost no decay. Finally, a tentative mechanism was proposed to explain the superior performance of Fe2O3@TiO2 BNRs for PEC water splitting and discussed in detail on the basis of our experimental results.

Morphology evolution of single-crystalline hematite nanocrystals: magnetically recoverable nanocatalysts for enhanced facet-driven photoredox activity

We have developed a new green chemical approach for the shape-controlled synthesis of single-crystalline hematite nanocrystals in aqueous medium. FESEM, HRTEM and SAED techniques were used to determine the morphology and crystallographic orientations of each nanocrystal and its exposed facets. PXRD and HRTEM techniques revealed that the nanocrystals are single crystalline in nature; twins and stacking faults were not detected in these nanocrystals. The structural, vibrational, and electronic spectra of these nanocrystals were highly dependent on their shape. Different shaped hematite nanocrystals with distinct crystallographic planes have been synthesized under similar reaction conditions, which can be desired as a model for the purpose of properties comparison with the nanocrystals prepared under different reaction conditions. Here we investigated the photocatalytic performance of these different shaped-nanocrystals for methyl orange degradation in the presence of white light (λ > 420 nm). In this study, we found that the density of surface Fe(3+) ions in particular facets was the key factor for the photocatalytic activity and was higher on the bitruncated-dodecahedron shape nanocrystals by coexposed {104}, {100} and {001} facets, attributing to higher catalytic activity. The catalytic activity of different exposed facet nanocrystals were as follows: {104} + {100} + {001} (bitruncated-dodecahedron) > {101} + {001} (bitruncated-octahedron) > {001} + {110} (nanorods) > {012} (nanocuboid) which provided the direct evidence of exposed facet-driven photocatalytic activity. The nanocrystals were easily recoverable using an external magnet and reused at least six times without significant loss of its catalytic activity.

Visible-light photocatalytic performances of TiO2nanobelts decorated with iron oxide nanocrystals

Anatase TiO₂nanobelts supported on Ti sheet and decorated by hematite Fe₂O₃nanocrystals were prepared by an easy-to-achieve three-step soft chemistry route and evaluated for its visible-light photocatalytic performances.

Integration of TiO2 photoanode and perovskite solar cell for overall solar-driven water splitting

Fully integrated device based on TiO₂ photoanode and perovskite solar cell for overall solar-driven water splitting.

Dendritic α-Fe2O3/TiO2 nanocomposites with improved visible light photocatalytic activity

A branch-like α-Fe₂O₃/TiO₂ heterostructure has been synthesized controllably through an electrospinning method combined with a hydrothermal approach.

Graphene-intercalated Fe2O3/TiO2 heterojunctions for efficient photoelectrolysis of water

Interfacial modification of α-Fe₂O₃/TiO₂ multilayer photoanodes by intercalating few-layer graphene (FLG) was found to improve water splitting efficiency due to superior transport properties, when compared to individual iron and titanium oxides and heterojunctions thereof.

Interface reacted ZnFe2O4 on α-Fe2O3 nanoarrays for largely improved photoelectrochemical activity

ZnFe₂O₄/α-Fe₂O₃ heterojunctions were successfully synthesized by depositing ZnO films on α-Fe₂O₃ nanorods through the atomic layer deposition technique, followed by annealing in air.