On the application of Marcus-Hush theory to small polaron chemical dynamics in oxides: its relationship to the Holstein model and the importance of lattice-orbital symmetries

The chemical dynamics of small polaron hopping within oxides is often interpreted through two-site variations on Marcus-Hush theory, while from a physics perspective small polaron hopping is more often approached from Holstein's solid-state formalism. Here we seek to provide a chemically oriented viewpoint, focusing on small polaron hopping in oxides, concerning these two phenomenological frameworks by employing both tight-binding modelling and first-principles calculations. First, within a semiclassical approach the Marcus-Hush relations are overviewed as a two-site reduction of Holstein's model. Within the single-band regime, similarities and differences between Holstein derived small polaron hopping and the Marcus-Hush model are also discussed. In this context the emergence of adiabaticity (or, conversely, diabaticity) is also explored within each framework both analytically and by directly evolving the system wavefunction. Then, through first-principles calculations of select oxides we explore how coupled lattice and orbital symmetries can impact on hopping properties - in a manner that is quite distinct typical chemical applications of Marcus-Hush theory. These results are then related back to the Holstein model to explore the relative applicability of the two frameworks towards interpreting small polaron hopping properties, where it is emphasized that the Holstein model offers an increasingly more appealing physicochemical interpretation of hopping processes as band and/or coupling interactions increase. Overall, this work aims to strengthen the physically oriented exploration of small polarons and their physicochemical properties in the growing oxide chemistry community.

Modification of Ti-doped Hematite Photoanode with Quasi-molecular Cocatalyst: A Comparison of Improvement Mechanism Between Non-noble and Noble Metals

Loading of molecular catalyst on the surface of semiconductors is an attractive way to boost the water oxidation activity. As active sites, molecular water oxidation cocatalysts show increasing attraction and application possibility. In order to compare the advantages between molecular catalysts with non-noble and noble metals, the loading of the Fe(salen) and Ru(salen) as cocatalyst precursors on the surface of Ti-Fe₂ O₃ was investigated Quasi-Fe(salen) and Ru(salen) improved the photocurrent density by 1.5 and 1.7 times compared to that of the original Ti-Fe₂ O₃ photoanode, respectively. The quasi-Fe(salen) could improve the conductivity and reaction kinetics on the photoanode surface. By contrast, the notable advancements could be attributed to more reaction sites for quasi-Ru(salen) as cocatalysts. Thus, non-noble quasi-Fe(salen) is a promising cocatalyst to replace the noble metal salen, and further optimization can be expected with regard to the precise control of reaction sites.

A High-Efficiency Hematite Photoanode with Enhanced Bonding Energy Around Fe Atoms

Hematite nanoarrays are important photoanode materials. However, they suffer from serious problems of charge transfer and surface states; in particular, the surface states hinder the increase in photocurrent. A previous strategy to suppress the surface state is the deposition of an Fe-free metal oxide overlayer. Herein, from the viewpoint of atomic bonding energy, it is found that the strength of bonding around Fe atoms in the hematite is the key to suppressing the surface states. By treating the surface of hematite with Se and NaBH₄ , the Fe₂ O₃ transforms to a double-layer nanostructure. In the outer layer, the Fe-O bonding is reinforced and the Fe-Se bonding is even stronger. Therefore, the surface states are inhibited and the increase in the photocurrent density becomes much faster. Besides, the treatment constructs a nanoscale p-n junction to promote the charge transfer. Improvements are achieved in onset potential (0.25 V shift) and in photocurrent density (5.8 times). This work pinpoints the key to suppressing the surface states and preparing a high-efficiency hematite nanoarray, and deepens our understanding of hematite photoanodes.

Synergistic Effect of Fe Doping and Plasmonic Au Nanoparticles on W18O49 Nanorods for Enhancing Photoelectrochemical Nitrogen Reduction

Photoelectrochemical (PEC) nitrogen fixation has opened up new possibilities for the production of ammonia from water and air under mild conditions, but this process is confronted by the inherent challenges associated with theoretical and experimental works, limiting the efficiency of the nitrogen reduction reaction. Herein, we report for the first time a novel and efficient photoelectrocatalytic system, which has been prepared by assembling plasmonic Au nanoparticles with Fe-doped W₁₈O₄₉ nanorods (denoted as WOF-Au). (i) The introduction of exotic Fe atoms into nonstoichiometric W₁₈O₄₉ can eliminate bulk defects of the W₁₈O₄₉ host, which resulted in narrowing bandgap energy and facilitating electron-hole separation and transportation. (ii) Meanwhile, Au nanoparticles combined with a semiconductor induce the localized surface plasmon resonance and generate energetic (hot) electrons, increasing electron density on W₁₈O₄₉ nanorods. Consequently, this plasmonic WOF-Au system shows an NH₃ production yield of 9.82 μg h^-1 cm^-2 at -0.65 V versus Ag/AgCl, which is ∼2.5-folds higher than that of the WOF (without Au loading), as well as very high stability, and no NH₃ formation was found for the bare W₁₈O₄₉ (WO). This high activity can be associated with the synergistic effects between the Fe dopant and plasmonic Au NPs on the host semiconductor W₁₈O₄₉. This work can bring some insights into the target-directed design of efficient plasmonic hybrid systems for N₂ fixation and artificial photocatalysis.

Effects of In Situ Co or Ni Doping on the Photoelectrochemical Performance of Hematite Nanorod Arrays

Co-doped and Ni-doped hematite (α-Fe2O3) nanorod arrays were prepared on fluorine-doped tin oxide (FTO) conductive glass via aqueous chemical growth, in which the doping and the formation of nanorods occurred simultaneously (i.e., in situ doping). These samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet (UV)–visible spectrophotometry, linear sweep voltammetry and Mott–Schottky (M–S) measurement. Results showed that the introduction of 5% Co or Ni into α-Fe2O3 (the molar ratio of dopant to Fe is 1:20) did not change its crystal phase, morphology, energy gap and flat band potential. Both the undoped and the doped α-Fe2O3 showed a direct band gap of 2.24 eV, an indirect band gap of 1.85 eV, and a flat band potential of −0.22 V vs. saturated calomel electrode (SCE). At an applied potential of 0.2 V vs. SCE, the Co-doped and the Ni-doped α-Fe2O3 exhibited a photocurrent of 1.28 mA/cm2 and 0.79 mA/cm2, respectively, which were 2.1 times and 1.3 times that of the undoped α-Fe2O3. After the Co or Ni doping, the charge carrier concentration increased from 1.65 × 1025 m−3 to 3.74 × 1025 m−3 and 2.50 × 1025 m−3, respectively. Therefore, the increase in the photocurrent of the doped α-Fe2O3 was likely attributed to their enhanced conductivity.

Aqueous Dual-Ion Battery Based on a Hematite Anode with Exposed {1 0 4} Facets

The aqueous rechargeable lithium battery (ARLB) is one of the most promising devices for large-scale grid applications. Currently, a key issue for ARLBs is to develop promising anode materials with favorable electrochemical performances. Here, for the first time, we demonstrate an aqueous battery that utilizes the reversible redox reaction with hydroxide ions (OH^- ) in the hematite (Fe₂ O₃ ) anode and a commercial Li ion intercalation compound in neutral solution as the cathode. The fabricated aqueous battery displays a reversible capacity of 92 mAh g^-1 . The morphology of the used Fe₂ O₃ anode with exposed {1 0 4} facets for this aqueous battery is unique and attractive. Importantly, with the dual-pH neutral-alkaline hybrid electrolyte, many excellent anode materials that previously could only work in alkaline electrolytes can now be successfully combined with commercial cathodes in neutral solutions, which may significantly enrich the range of anode materials for ARLBs. In addition, the reported battery configuration can be extended to other aqueous batteries beyond Li-ion ones with lower cost.

Dual-Axial Gradient Doping (Zr and Sn) on Hematite for Promoting Charge Separation in Photoelectrochemical Water Splitting

One of the crucial challenges to enhance the photoelectrochemical water-splitting performance of hematite (α-Fe₂ O₃ ) is to resolve its very fast charge recombination in bulk. Herein, we describe the design and fabrication of dual-axial gradient-doping on 1D Fe₂ O₃ nanorod arrays with Zr doping for x-axial and Sn doping for y-axial directions to promote the charge separation. This dual-axial gradient-doping structure fulfills the requirements of a greater electron-carrier concentration for increasing conductivity as well as a higher charge-separation efficiency across the dual-axial direction of Fe₂ O₃ nanorods, ultimately showing an excellent photocurrent density of 1.64 mA cm^-2 at 1.23 V vs. RHE, which is 26.3 times more than that of the bare Fe₂ O₃ . Furthermore, the remarkably improved photocurrent density, when comparing the uniform Zr-doped Fe₂ O₃ nanorod arrays (1.0 mA cm^-2 at 1.23 V vs. RHE) with dual-axial gradient-doped (Zr and Sn) Fe₂ O₃ nanorod arrays, highlights the additional charge-separation effect resulting from gradient codoping of Zr and Sn. Hence, this promising design may provide guidelines for dual-axial gradient doping into photoelectrodes to realize efficient PEC water splitting.

Surface, Bulk, and Interface: Rational Design of Hematite Architecture toward Efficient Photo-Electrochemical Water Splitting

Collecting and storing solar energy to hydrogen fuel through a photo-electrochemical (PEC) cell provides a clean and renewable pathway for future energy demands. Having earth-abundance, low biotoxicity, robustness, and an ideal n-type band position, hematite (α-Fe₂ O₃ ), the most common natural form of iron oxide, has occupied the research hotspot for decades. Here, a close look into recent progress of hematite photoanodes for PEC water splitting is provided. Effective approaches are introduced, such as cocatalysts loading and surface passivation layer deposition, to improve the hematite surface reaction in thermodynamics and kinetics. Second, typical methods for enhancing light absorption and accelerating charge transport in hematite bulk are reviewed, concentrating upon doping and nanostructuring. Third, the back contact between hematite and substrate, which affects interface states and electron transfer, is deliberated. In addition, perspectives on the key challenges and future prospects for the development of hematite photoelectrodes for PEC water splitting are given.

Efficient Photoelectrochemical Water Splitting by g-C3N4/TiO2 Nanotube Array Heterostructures

Well-ordered TiO₂ nanotube arrays (TNTAs) decorated with graphitic carbon nitride (g-C₃N₄) were fabricated by anodic oxidization and calcination process. First, TNTAs were prepared via the anodic oxidation of Ti foil in glycerol solution containing fluorinion and 20% deionized water. Subsequently, g-C₃N₄ film was hydrothermally grown on TNTAs via the hydrogen-bonded cyanuric acid melamine supramolecular complex. The results showed that g-C₃N₄ was successfully decorated on the TNTAs and the g-C₃N₄/TNTAs served as an efficient and stable photoanode for photoelectrochemical water splitting. The facile deposition method enables the fabrication of efficient and low-cost photoanodes for renewable energy applications.

Visible-Light-Induced Water Splitting Based on a Novel α-Fe2O3/CdS Heterostructure

In this work, CdS nanoparticles were grown on top of a hematite (α-Fe₂O₃) film as photoanodes for the photoelectrochemical water splitting. Such type of composition was chosen to enhance the electrical conductivity and photoactivity of traditionally used bare hematite nanostructures. The fabricated thin film was probed by various physicochemical, electrochemical, and optical techniques, revealing high crystallinity of the prepared nanocomposite and the presence of two distinct phases with different band gaps. Furthermore, photoassisted water splitting tests exhibit a noteworthy photocurrent of 0.6 mA/cm² and a relatively low onset potential of 0.4 V (vs reversible hydrogen electrode) for the composite electrode. The high photocurrent generation ability was attributed to the synergistic interplay between conduction and valence band (VB) levels of CdS and α-Fe₂O₃, which was further interpreted by J-V curves. Finally, electrochemical impedance spectroscopy investigation of the obtained films suggests that the photogenerated holes could be transferred from the VB of α-Fe₂O₃ to the electrolyte more efficiently in the hybrid nanostructure.

Enhanced Photoelectrochemical Activity by Autologous Cd/CdO/CdS Heterojunction Photoanodes with High Conductivity and Separation Efficiency

The development for hydrogen from solar energy has attracted great attention due to the global demand for clean, environmentally friendly energy. Herein, autologous Cd/CdO/CdS heterojunctions were prepared in a carefully controlled process with metallic Cd as the inner layer and CdO as the interlayer. Further research revealed that the transportation and separation of photogenerated pairs were enhanced due to low resistance of the Cd inner layer and the type II CdO/CdS heterojunction. As a result, the optimized Cd/CdO/CdS heterojunction photoanode showed outstanding and long-term photoelectrochemical activity for water splitting, with a current density of 3.52 mA cm^-2 , or a benchmark specific hydrogen production rate of 1.65 μmol cm^-2  min^-1 at -0.3 V versus Ag/AgCl, by using the environmental pollutants of sulfide and sulfite as sacrificial agents.

Sequentially surface modified hematite enables lower applied bias photoelectrochemical water splitting

We achieve a low onset potential of 0.49 V using heavily doped Fe_2−xSn_xO₃surface passivation layer and NiOOH dual surface treatments.

Effect of graphene incorporation in carbon nanofiber decorated with TiO2 for photoanode applications

The photovoltaic performance of dye-sensitized solar cells (DSSCs) using a photoanode fabricated with graphene incorporated carbon nanofibers with a TiO₂ layer on their surfaces is reported.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Facile One-Step Synthesis of Hybrid Graphitic Carbon Nitride and Carbon Composites as High-Performance Catalysts for CO2 Photocatalytic Conversion

Utilizing and reducing carbon dioxide is a key target in the fight against global warming. The photocatalytic performance of bulk graphitic carbon nitride (g-C3N4) is usually limited by its low surface area and rapid charge carrier recombination. To develop g-C3N4 more suitable for photocatalysis, researchers have to enlarge its surface area and accelerate the charge carrier separation. In this work, novel hybrid graphitic carbon nitride and carbon (H-g-C3N4/C) composites with various carbon contents have been developed for the first time by a facile one-step pyrolysis method using melamine and natural soybean oil as precursors. The effect of carbon content on the structure of H-g-C3N4/C composites and the catalytic activity for the photoreduction of CO2 with H2O were investigated. The results indicated that the introduction of carbon component can effectively improve the textural properties and electronic conductivity of the composites, which exhibited imporved photocatalytic activity for the reduction of CO2 with H2O in comparison with bulk g-C3N4. The highest CO and CH4 yield of 22.60 μmol/g-cat. and 12.5 μmol/g-cat., respectively, were acquired on the H-g-C3N4/C-6 catalyst with the carbon content of 3.77 wt % under 9 h simulated solar irradiation, which were more than twice as high as that of bulk g-C3N4. The remarkably increased photocatalytic performance arises from the synergistic effect of hybrid carbon and g-C3N4.

Photocatalytic Water Splitting-The Untamed Dream: A Review of Recent Advances

Photocatalytic water splitting using sunlight is a promising technology capable of providing high energy yield without pollutant byproducts. Herein, we review various aspects of this technology including chemical reactions, physiochemical conditions and photocatalyst types such as metal oxides, sulfides, nitrides, nanocomposites, and doped materials followed by recent advances in computational modeling of photoactive materials. As the best-known catalyst for photocatalytic hydrogen and oxygen evolution, TiO₂ is discussed in a separate section, along with its challenges such as the wide band gap, large overpotential for hydrogen evolution, and rapid recombination of produced electron-hole pairs. Various approaches are addressed to overcome these shortcomings, such as doping with different elements, heterojunction catalysts, noble metal deposition, and surface modification. Development of a photocatalytic corrosion resistant, visible light absorbing, defect-tuned material with small particle size is the key to complete the sunlight to hydrogen cycle efficiently. Computational studies have opened new avenues to understand and predict the electronic density of states and band structure of advanced materials and could pave the way for the rational design of efficient photocatalysts for water splitting. Future directions are focused on developing innovative junction architectures, novel synthesis methods and optimizing the existing active materials to enhance charge transfer, visible light absorption, reducing the gas evolution overpotential and maintaining chemical and physical stability.

Core-Shell Vanadium Modified Titania@β-In2S3 Hybrid Nanorod Arrays for Superior Interface Stability and Photochemical Activity

Core-shell rutile TiO2@β-In2S3 and modified V-TiO2@β-In2S3 were synthesized to develop bilayer systems to uphold charge transport via an effective and stable interface. Morphological studies revealed that β-In2S3 was deposited homogeneously on V-TiO2 as compared to unmodified TiO2 nanorod arrays. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectrometry studies verified the presence of various oxidation states of vanadium in rutile TiO2 and the vanadium surface was utilized for broadening the charge collection centers in host substrate layer and hole quencher window. Subsequently, X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectra confirmed the rutile phases of TiO2 and modified V-TiO2 along with the phases of crystalline β-In2S3. XPS valence band study explored the interaction of valence band quazi Fermi levels of β-In2S3 with the conduction band quazi Fermi levels of modified V-TiO2 for enhanced charge collection at the interface. Photoelectrochemical studies show that the photocurrent density of V-TiO2@β-In2S3 is 1.42 mA/cm(2) (1.5AM illumination). Also, the frequency window for TiO2 was broadened by the vanadium modification in rutile TiO2 nanorod arrays, and the lifetime of the charge carrier and stability of the interface in V-TiO2@β-In2S3 were enhanced compared to the unmodified TiO2@β-In2S3. These findings highlight the significance of modifications in host substrates and interfaces, which have profound implications on interphase stability, photocatalysis and solar-fuel-based devices.

Fabrication of Au@CdS/RGO/TiO2 heterostructure for photoelectrochemical hydrogen production

Schematic energy band illustration of a novel Au@CdS/RGO/TiO₂ heterostructure as a photoelectrode for PEC hydrogen generation via water splitting.

Enhancement of photochemical heterogeneous water oxidation by a manganese based soft oxometalate immobilized on a graphene oxide matrix

Enhancement of photochemical water oxidation using a graphene oxide matrix for [Na₁₇[Mn₆P₃W₂₄O₉₄(H₂O)₂]·43H₂O@GO] soft-oxometalate is shown.

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment

Thermal treatment of ultrathin films of hematite (α-Fe2 O3 ) under an atmosphere of 5 % H2 in Ar is presented as a means of activating α-Fe2 O3 towards the photoelectrochemical splitting of water. Spin-coated films annealed in air exhibited no photoactivity, whereas films treated in hydrogen exhibited a photocurrent response. X-ray photoelectron spectroscopy and UV/Vis absorption spectroscopy results showed that the H2 -treated films contain oxygen vacancies, which suggests improved charge transport. However, Tafel slopes, scan-rate dependent measurements, and kinetic analyses performed by using H2 O2 as a hole scavenger suggested that surface modification may also contribute to their induced photoactivity. Electrochemical impedance spectroscopy results revealed the buildup of a surface trap capacitance at the point of photocurrent onset for the hydrogen-treated films under illumination. A decrease in charge trapping resistance was also observed, which suggests improved transport of charges away from the surface.

Promotional effects of cetyltrimethylammonium bromide surface modification on a hematite photoanode for photoelectrochemical water splitting

A hematite nanorod array was treated with cetyltrimethylammonium bromide (CTAB) surfactant by a simple hydrothermal method.

TiO2/Bi2S3 core–shell nanowire arrays for photoelectrochemical hydrogen generation

This paper demonstrates the procedure for construction of nontoxic TiO₂/Bi₂S₃ core–shell NWA photoanodes for PEC hydrogen generation.

A green approach for preparing doped TiO(2) single crystals

Doped TiO2 with metal, nonmetal, and rare earth elements has shown a great potential in energy and environmental applications, but it is difficult to dope well-defined TiO2 single crystals (SCs) with {001} exposed facet due to their high crystallinity. In this work, we developed a green and general approach to prepare the {001}-exposed TiO2 SCs doped with various elements, on the basis of recycling the wasted ethylene glycol electrolyte from the anodic oxidation for TiO2 nanotube preparation. All six representative elements (i.e., metal, nonmetal, and rare earth types) could be successfully doped into the TiO2 SCs without breaking their single-crystalline structure and exposed high-energy facet. The electronic properties of the doped TiO2 SCs were significantly improved. All the doped TiO2 SCs exhibited a superior photoactivity under visible-light irradiation for degrading rhodamine B, a typical organic pollutant. The prepared doped TiO2 SCs have a promising potential in environmental and energy applications.

Cobalt-phosphate-assisted photoelectrochemical water oxidation by arrays of molybdenum-doped zinc oxide nanorods

We report the first demonstration of cobalt phosphate (Co-Pi)-assisted molybdenum-doped zinc oxide nanorods (Zn(1-x)Mo(x)O NRs) as visible-light-sensitive photofunctional electrodes to fundamentally improve the performance of ZnO NRs for photoelectrochemical (PEC) water splitting. A maximum photoconversion efficiency as high as 1.05% was achieved, at a photocurrent density of 1.4 mA cm(-2). More importantly, in addition to achieve the maximum incident photon to current conversion efficiency (IPCE) value of 86%, it could be noted that the IPCE of Zn(1-x)Mo(x)O photoanodes under monochromatic illumination (450 nm) is up to 12%. Our PEC performances are comparable to those of many oxide-based photoanodes in recent reports. The improvement in photoactivity of PEC water splitting may be attributed to the enhanced visible-light absorption, increased charge-carrier densities, and improved interfacial charge-transfer kinetics due to the combined effect of molybdenum incorporation and Co-Pi modification, contributing to photocatalysis. The new design of constructing highly photoactive Co-Pi-assisted Zn(1-x)Mo(x)O photoanodes enriches knowledge on doping and advances the development of high-efficiency photoelectrodes in the solar-hydrogen field.

Enhanced hematite water electrolysis using a 3D antimony-doped tin oxide electrode

We present herein an example of nanocrystalline antimony-doped tin oxide (nc-ATO) disordered macroporous "inverse opal" 3D electrodes as efficient charge-collecting support structures for the electrolysis of water using a hematite surface catalyst. The 3D macroporous structures were created via templating of polystyrene spheres, followed by infiltration of the desired precursor solution and annealing at high temperature. Using cyclic voltammetry and electrochemical impedance spectroscopy, it was determined that the use of this 3D transparent conducting oxide with a hematite surface catalyst allowed for a 7-fold increase in active surface area for water splitting with respect to its 2D planar counterpart. This ratio of surface areas was evaluated based on the presence of oxidized trap states on the hematite surface, as determined from the equivalent circuit analysis of the Nyquist plots. Furthermore, the presence of nc-ATO 2D and 3D "underlayer" structures with hematite deposited on top resulted in decreased charge transfer resistances and an increase in the number of available active surface sites at the semiconductor-liquid junction when compared to hematite films lacking any nc-ATO substructures. Finally, absorption, transmission, and reflectance spectra of all of the tested films were measured, suggesting the feasibility of using 3D disordered structures in photoelectrochemical reactions, due to the high absorption of photons by the surface catalyst material and trapping of light within the structure.