A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of α-Fe2O3 Photoanode for Solar Water Splitting

Oxygen Vacancy: How Will Poling History Affect Its Role in Photoelectrocatalysis

Oxygen vacancy (VO) has been recognized to possess an effect to promote the charge separation and transfer (CST) in various n-type semiconductor based photoelectrodes. But how external stimulus will change this VO effect has not been investigated. In this work, external polarization is applied to investigate the effect of VO on the CST process of a typical ferroelectric BiFeO3 photoelectrode. It is found that negative poling treatment can significantly boost VO effect, while positive poling treatment will deteriorate the CST capability in BiFeO3 photoelectrodes. This poling history determined VO effect is rooted in the VO induced defect dipoles, wherein their alignment produces a depolarization electric field to modulate the CST driving force. This finding highlights the significance of poling history in functionalizing the VO in a photoelectrode.

Electrocatalytic FeFe2O4 embedded, spermine-imprinted polypyrrole (Fe/MIPpy) nanozymes for cancer diagnosis and prognosis

Developing synthetic materials, with enzyme-like molecular recognition capabilities, as functional receptors in electronic or electrochemical devices for the timely diagnosis of major diseases is a great challenge. Herein, we present the development of Fe/MIPpy nanozymes, characterized as enzyme-like artificial receptors, for the precise and non-invasive monitoring of cancer biomarkers in aqueous solutions and human saliva. Through the integration of PVA-stabilized FeFe2O4 nanocrystals in a molecularly imprinted conducting polypyrrole matrix, the Fe/MIPpy nanozymes demonstrate 424 nA cm-2 nM-1 sensitivity and 220 pM detection limit. Charge-transfer mechanisms, Fe/MIPpy-spermine interactions, and the principle of spermine recognition are investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The disposable Fe/MIPpy sensor operates wirelessly and offers rapid and remote quantification of spermine, making it a promising material for the development of cost-effective tools for non-invasive cancer diagnosis and prognosis.

Manipulating the surface states of BiVO4 through electrochemical reduction for enhanced PEC water oxidation

Bismuth vanadate (BiVO₄) is a prospective candidate for photoelectrochemical (PEC) water oxidation, but its commercial application is limited due to the serious surface charge recombination. In this work, we propose a novel and effective electrochemical reduction strategy combined with co-catalyst modification to manipulate the surface states of the BiVO₄ photoanode. Specifically, an ultrathin amorphous structure is formed on the surface of BiVO₄ after electrochemical reduction ascribed to the breaking of the surface metal-O bonds. Photoelectrochemical measurements and first-principles calculation show that the electrochemical reduction treatment can effectively reduce the surface energy, thereby passivating the recombined surface states (r-ss) and increasing the mobility of photogenerated holes. In addition, the FeOOH co-catalyst layer further increases the intermediate surface states (i-ss) of BiVO₄, stabilizes the surface structure and enhances its PEC performance. Benefiting from the superior charge transfer efficiency and the excellent water oxidation kinetics, the -0.8/BVO/Fe photoanode achieves 2.02 mA cm^-2 photocurrent at 1.23 V_RHE (2.4 times that of the original BiVO₄); meanwhile, the onset potential shifts 90 mV to the cathode. These results provide a new surface engineering tactic to modify the surface states of semiconductor photoanodes for high-efficiency PEC water oxidation.

Composite Indium Tin Oxide Nanofibers with Embedded Hematite Nanoparticles for Photoelectrochemical Water Splitting

Hematite is a classical photoanode material for photoelectrochemical water splitting due to its stability, performance, and low cost. However, the effect of particle size is still a question due to the charge transfer to the electrodes. In this work, we addressed this subject by the fabrication of a photoelectrode with hematite nanoparticles embedded in close contact with the electrode substrate. The nanoparticles were synthesized by a solvothermal method and colloidal stabilization with charged hydroxide molecules, and we were able to further use them to prepare electrodes for water photo-oxidation. Hematite nanoparticles were embedded within electrospun tin-doped indium oxide nanofibers. The fibrous layer acted as a current collector scaffold for the nanoparticles, supporting the effective transport of charge carriers. This method allows better contact of the nanoparticles with the substrate, and also, the fibrous scaffold increases the optical density of the photoelectrode. Electrodes based on nanofibers with embedded nanoparticles display significantly enhanced photoelectrochemical performance compared to their flat nanoparticle-based layer counterparts. This nanofiber architecture increases the photocurrent density and photon-to-current internal conversion efficiency by factors of 2 and 10, respectively.

Superior sponge-like carbon self-doping graphitic carbon nitride nanosheets derived from supramolecular pre-assembly of a melamine-cyanuric acid complex for photocatalytic H2 evolution

The photocatalytic evolution of hydrogen (H₂) from water splitting is considered a promising route to overcome the energy crisis, and the key lies in the preparation of efficient photocatalysts. Herein, superior ordered sponge-like carbon self-doped graphitic carbon nitride (g-C₃N₄) nanosheets (SCCNS) were prepared via a combined strategy of melamine-cyanuric acid complex supramolecular pre-assembly and solvothermal pre-treatment using ethylene glycol (EG) aqueous solutions (EG:water = 50:50 vol.%) as a solvent and carbon doping source. The following pyrolysis converts the naturally arranged melamine-EG-cyanuric acid supramolecular intermediates to highly crystalline SCCNS with large specific surface areas. The optimal SCCNS-180 exhibits superior photocatalytic H₂ evolution activities (∼4393 and 11 320 μmol h^-1 g^-1) when irradiated with visible light and simulated sunlight; these values are up to ∼17- and ∼18-fold higher than that of bulk g-C₃N₄. The quantum efficiency of SCCNS-180 at λ = 420 nm can reach 6.0%. The excellent photocatalytic performance of SCCNS-180 derives from its distinct ordered sponge-like nanosheet structure with highly crystallinity and the carbon doping, leading to its improved optical absorption, accelerated photoinduced electron-hole pair transfer and separation rate and enlarged specific surface area (134.4 m² g^-1).

Enhancing Water-Splitting Efficiency Using a Zn/Sn-Doped PN Photoelectrode of Pseudocubic α-Fe2O3 Nanoparticles

α-Phase hematite photoelectrodes can split water. This material is nontoxic, inexpensive, and chemically stable; its low energy gap of 2.3 eV absorbs light with wavelengths lower than 550 nm, accounting for approximately 30% of solar energy. Previously, we reported polyhedral pseudocubic α-Fe₂O₃ nanocrystals using a facile hydrothermal route to increase spatial charge separation, enhancing the photocurrent of photocatalytic activity in the water-splitting process. Here, we propose a p-n junction structure in the photoanode of pseudocubic α-Fe₂O₃ to improve short carrier diffusion length, which limits its photocatalytic efficiency. We dope Zn on top of an Fe₂O₃ photoanode to form a layer of p-type semiconductor material; Sn is doped from the FTO substrate to form a layer of n-type semiconductor material. The p-n junction, n-type Fe₂O₃:Sn and p-type Fe₂O₃:Zn, increase light absorption and charge separation caused by the internal electric field in the p-n junction.

Two-Dimensional Materials and Composites as Potential Water Splitting Photocatalysts: A Review

Hydrogen production via water dissociation under exposure to sunlight has emanated as an environmentally friendly, highly productive and expedient process to overcome the energy production and consumption gap, while evading the challenges of fossil fuel depletion and ecological contamination. Various classes of materials are being explored as viable photocatalysts to achieve this purpose, among which, the two-dimensional materials have emerged as prominent candidates, having the intrinsic advantages of visible light sensitivity; structural and chemical tuneability; extensively exposed surface area; and flexibility to form composites and heterostructures. In an abridged manner, the common types of 2D photocatalysts, their position as potential contenders in photocatalytic processes, their derivatives and their modifications are described herein, as it all applies to achieving the coveted chemical and physical properties by fine-tuning the synthesis techniques, precursor ingredients and nano-structural alterations.

Electrocatalytic oxygen and hydrogen evolution reactions at Ni3B/Fe2O3 nanotube arrays under visible light radiation

We demonstrate that arrays of Ni₃B/Fe₂O₃ nanotubes supported by Fe foil can simultaneously boost the kinetics of the oxygen evolution reaction and hydrogen evolution reaction after exposure to visible light radiation.

Activating the surface and bulk of hematite photoanodes to improve solar water splitting

A simple electrochemical activation treatment is proposed to improve effectively the photoelectrochemical performance of Nb,Sn co-doped hematite nanorods. The activation process involves an initial thrice cathodic scanning (reduction) and a subsequent thrice anodic scanning (oxidation), which modifies both the surface and bulk properties of the Nb,Sn:Fe2O3 photoanode. First, it selectively removes the surface components to different extents endowing the hematite surface with fewer defects and richer Nb-O and Sn-O bonds and thus passivates the surface trap states. The surface passivation effect also enhances the photoelectrochemical stability of the photoanode. Finally, more Fe2+ ions or oxygen vacancies are generated in the bulk of hematite to enhance its conductivity. As a result, the photocurrent density is increased by 62.3% from 1.88 to 3.05 mA cm-2 at 1.23 VRHE, the photocurrent onset potential shifts cathodically by ∼70 mV, and photoelectrochemical stability improves remarkably relative to the pristine photoanode under simulated sunlight (100 mW cm-2).

Layer dependence of the photoelectrochemical performance of a WSe2 photocathode characterized using in situ microscale measurements

Transition-metal dichalcogenide (TMD) materials are good candidates for photoelectrochemical (PEC) electrode materials because of their distinctive optoelectronic properties and catalytic activities. Monolayer WSe₂ is a p-type semiconductor with a direct bandgap that makes it a suitable PEC cathode material. In the present work, in situ PEC characterization of a single sheet device was carried out at the microscale to explore its performance. The PEC characteristics were found to be strongly related to the number of WSe₂ layers. Monolayer WSe₂ exhibited a dominant large current density and incident photo-to-current efficiency (IPCE) compared with those of multilayer WSe₂. Its PEC performance decreased with increasing number of layers. The photocurrent mapping results also revealed that the basal-plane sites and the edge sites on a monolayer WSe₂ sheet contributed equally to its catalytic activity, which is not consistent with traditional catalyst theory. The underlying mechanism is discussed.

The performance of WSe₂ PEC cathode increased with decreasing number of layers. Monolayer WSe₂ exhibited best PEC characteristics and IPCE efficiency. The basal-plane of WSe₂ sheet had the same PEC catalytic activity with the edge sites.

Direct electrochemical reduction of hematite decorated graphene oxide (α-Fe2O3@erGO) nanocomposite for selective detection of Parkinson's disease biomarker

An unusual approach is reported herein to fabricate magnetic hematite (α-Fe₂O₃) decorated electrochemically reduced graphene oxide (α-Fe₂O₃@erGO) nanocomposite. The method utilizes direct electrochemical reduction of self-assembled, ex-situ synthesized α-Fe₂O₃ anchored GO to erGO (α-Fe₂O₃@erGO) on glassy carbon electrode (GCE) for selective detection dopamine (DA), an important biomarker of Parkinson's disease. The formation of α-Fe₂O₃@erGO/GCE has been confirmed by XPS and Raman spectroscopy. α-Fe₂O₃@erGO modified GCE exhibits synergistic catalytic activity nearly 2.2 and 5 fold higher than α-Fe₂O₃@GO and other modified electrodes, respectively towards oxidation of DA. The fabricated sensor exhibited linear dynamic ranges over 0.25 - 100 µM in response to DA with a LOD of 0.024 µM (S/N = 3), LOQ of 0.08 µM (S/N = 10), and a sensitivity of 12.56 µA µM^-1 cm^-2. Finally, the practical analytical application of the proposed α-Fe₂O₃@erGO/GCE was investigated for the determination of DA in commercially available pharmaceutical formulation and human serum samples, and showed satisfactory recovery results towards DA.

Facile Synthesis of Ultrafine Hematite Nanowire Arrays in Mixed Water-Ethanol-Acetic Acid Solution for Enhanced Charge Transport and Separation

Nanostructure engineering is of great significance for semiconductor electrode to achieve high photoelectrochemical performance. Herein, we report a novel strategy to fabricate ultrafine hematite (α-Fe₂O₃) nanowire arrays in a mixed water-ethanol-acetic acid (WEA) solvent. To the best of our knowledge, this is the first report on direct growth of ultrafine (∼10 nm) α-Fe₂O₃ nanowire arrays on fluorine-doped tin oxide substrates through solution-based fabrication process. The effect of WEA ratio on the morphology of nanowires has been systematically studied to understand the formation mechanism. Photoelectrochemical measurements were conducted on both Ti-treated α-Fe₂O₃ nanowire and nanorod photoelectrodes. It reveals that α-Fe₂O₃ nanowire electrode has higher photocurrent and charge separation efficiencies than nanorod electrode if the carrier concentration and space-charge carrier width are in the same order of magnitude. Normalized by electrochemically active surface area, the Ti-treated α-Fe₂O₃ nanowire electrode obtains 6.4 times higher specific photocurrent density than nanorod electrode. This superiority of nanowires arises from the higher bulk and surface charge separation efficiencies, which could be partly attributed to reduced distance that holes must transfer to reach the semiconductor-liquid junction.

Hierarchical CdMoO4 nanowire–graphene composite for photocatalytic hydrogen generation under natural sunlight

Herein, a facile in situ solvothermal technique for the synthesis of a CdMoO₄/graphene composite photocatalyst for hydrogen generation under natural solar light.

Low temperature fabrication of Fe2O3 nanorod film coated with ultra-thin g-C3N4 for a direct z-scheme exerting photocatalytic activities

We engineered high aspect ratio Fe₂O₃ nanorods (with an aspect ratio of 17 : 1) coated with g-C₃N₄ using a sequential solvothermal method at very low temperature followed by a thermal evaporation method. Here, the high aspect ratio Fe₂O₃ nanorods were directly grown onto the FTO substrate under relatively low pressure conditions. The g-C₃N₄ was coated onto a uniform Fe₂O₃ nanorod film as the heterostructure, exhibiting rational band conduction and a valence band that engaged in surface photoredox reactions by a direct z-scheme mechanism. The heterostructures, particularly 0.75g-C₃N₄@Fe₂O₃ nanorods, exhibited outstanding photocatalytic activities compared to those of bare Fe₂O₃ nanorods. In terms of 4-nitrophenol degradation, 0.75g-C₃N₄@Fe₂O₃ nanorods degraded all of the organic pollutant within 6 h under visible irradiation at a kinetic constant of 12.71 × 10⁻³ min⁻¹, about 15-fold more rapidly than bare Fe₂O₃. Further, the hydrogen evolution rate was 37.06 μmol h⁻¹ g⁻¹, 39-fold higher than that of bare Fe₂O₃. We suggest that electron and hole pairs are efficiently separated in g-C₃N₄@Fe₂O₃ nanorods, thus accelerating surface photoreaction via a direct z-scheme under visible illumination.

The engineered high aspect ratio of Fe₂O₃ nanorods coated with g-C₃N₄ demonstrates z-scheme mechanism, showing the best performance in 4-nitrophenol photodegradation and H₂ evolution.

Direct coating of a g-C3N4 layer onto one-dimensional TiO2 nanocluster/nanorod films for photoactive applications

We report highly stable photoanode, coating of metal-free graphitic carbon nitride (g-C₃N₄) onto titanium dioxide(TiO₂) nanorods with secondary nanoclusters.

Enhanced photoelectrochemical water oxidation performance of a hematite photoanode by decorating with Au–Pt core–shell nanoparticles

The charge transfer process of the AuPt/α-Fe₂O₃ composite photoanode for photoelectrochemical water oxidation.