Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemical Water Oxidation

Synergistic etching of nickel foam by Fe3+ and Cl- ions to synthesize nickel-iron-layered double hydroxide nanolayers with abundant oxygen vacancies for superior urea oxidation

Urea electrolysis is an appealing topic for hydrogen production due to its ability to extract hydrogen at a lower potential. However, it is plagued by sluggish kinetics and noble-metal catalyst requirements. Herein, we developed nickel-iron-layered double hydroxide (NiFe-LDH) nanolayers with abundant oxygen vacancies (OV) via synergistically etching nickel foam with Fe3+ and Cl- ions, enabling the efficient conversion of urea into H2 and N2. The synthesized OV-NiFe-LDH exhibits a lower potential (1.30 vs. reversible hydrogen electrode, RHE) for achieving 10 mA cm-2 in the urea oxidation reaction (UOR), surpassing most recently reported Ni-based electrodes. OV provides favorable conductivity and a large surface area, which results in a 4.1-fold in electron transport and a 5.1-fold increase in catalyst reactive sites. Density Functional Theory (DFT) calculations indicate that OV can lower the adsorption energy of urea, and enhance the bonding strength of *CONHNH, giving rise to improved UOR. This study provides a viable path toward economical and efficient production of high-purity hydrogen.

Fine-Tuning Filter Cake Sealing Performance: The Role of Particle Size in Hematite Weighted Water-Based Drilling Fluid

Three hematite grades with different particle sizes (i.e., large, medium, and small) were evaluated, and the selection criterion was median particle size. The investigation involves the following stages: rheology, filtration, and filter cake formation. Different rheological models including Bingham, Power law, Herschel-Bulkley, and Robertson-Stiff were implemented to find the optimum model for characterizing fluid behavior. The results showed that medium-sized hematite particles produced the highest filtration volume, filter cake thickness, and filter cake permeability. These results were confirmed when a varied pore distribution filtration medium was used. The NMR results showed the same trend where the highest reduction in core porosity was found when a medium-size particle distribution was used. There is a minimum alteration in the rheological behavior of the drilling fluid as the particle size was varied, and the drilling fluids showed a shear-thinning behavior and were best described by the Herschel-Bulkley model. Particle size ratio emerges as a key factor for controlling and enhancing the filtration properties and filter cake characteristics.

The application of plasma technology for the preparation of supercapacitor electrode materials

With the rapidly growing demand for clean energy and energy interconnection, there is an urgent need for rapid and high-capacity energy storage technologies to realize large-scale energy storage, transfer energy, and establish the energy internet. Supercapacitors, which have advantages such as high specific capacitance, fast charging and discharging rates, and long cycle lifetimes, are being widely used in electric vehicles, information technology, aerospace, and other fields. The performance of supercapacitors is crucially dependent on electrode materials. These can be categorized into electric double-layer capacitors and pseudocapacitors, primarily made from carbon and transition metal oxides, respectively. However, effectively monitoring the physicochemical properties of electrode materials during preparation and processing is challenging, which limits the improvement of supercapacitors' performance. Plasma materials preparation technology can effectively affect the materials preparation processing by energetic electrons, ions, free radicals, and multiple effects in plasma, which are easily manipulated by operation parameters. Therefore, plasma material preparation technology is considered a promising method to precisely monitor the physicochemical and electrochemical properties of energy storage materials and has been widely studied. This paper provides an overview of plasma materials preparation mechanisms, and details of the plasma technology application in the preparation of transition metal hybrids, carbon, and composite electrode materials, as well as a comparison with traditional methods. In conclusion, the advantages, challenges, and research directions of plasma materials preparation technology in the field of electrode materials preparation are summarized.

A facile and efficient method for preparing La-doped Co3O4 by electrodeposition as an efficient air cathode in rechargeable zinc-air batteries: Role of oxygen vacancies

A rechargeable zinc-air battery (ZAB) is a promising candidate for simple and low-cost energy storage systems. However, preparing the air cathode material using a binder-free method and a bifunctional catalyst is still the major challenge in the field. Herein, we demonstrate the effect of different La contents doped into the Co3O4 spinel structure in the presence of oxygen vacancies prepared by a facile and efficient electrodeposition technique on the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and ZAB performance. Incorporating the La dopant into the Co3O4 improves the OER and ORR performances and thus enhances the specific capacity and energy density of ZAB. The optimal La-doping amount in the CoLa-1 catalyst demonstrates high feasibility for practical application with a capacity of 780 ± 24 mAh/g and an energy density of 901 ± 39 mW g-1, significantly outperforming the pristine Co3O4. The stability and cycling tests reveal good durability performance after 300 cycles and 100 h of testing without degradation, which is much more stable than the benchmark Pt/C + RuO2 electrode. The performance enhancement is attributed to the synergetic effect of high active surface area, low charge transfer resistance, and optimal oxygen vacancies. A kinetic micromechanism is proposed to illustrate the importance of the oxygen vacancy amount in trapping oxygen gas and maximizing the number of ORR and OER reactions.

Hematite (α-Fe2O3) with Oxygen Defects: The Effect of Heating Rate for Photocatalytic Performance

Hematite (α-Fe2O3) emerges as an enticing material for visible-light-driven photocatalysis owing to its remarkable stability, low toxicity, and abundance. However, its inherent shortcomings, such as a short hole diffusion length and high recombination rate, hinder its practical application. Recently, oxygen vacancies (Vo) within hematite have been demonstrated to modulate its photocatalytic attributes. The effects of Vo can be broadly categorized into two opposing aspects: (1) acting as electron donors, enhancing carrier conductivity, and improving photocatalytic performance and (2) acting as surface carrier traps, accelerating excited carrier recombination, and deteriorating performance. Critically, the generation rate, distribution, role, and behavior of Vo significantly differ for synthesis methods due to differences in formation mechanisms and oxygen diffusion. This complexity hampers simplified discussions of Vo, necessitating careful investigation and nuanced discussion tailored to the specific method and conditions employed. Among various approaches, hydrothermal synthesis offers a simple and cost-effective route. Here, we demonstrate a hydrothermal synthesis method for Vo introduction to hematite using a carbon source, where variations in the heating rate have not been previously explored in terms of their influence on Vo generation. The analyses revealed that the concentration of Vo was maximized at a heating rate of 16 °C/min, indicative of a high density of surface defects. With regard to photocatalytic performance, elevated heating rates (16 °C/min) fostered the formation of Vo primarily on the hematite surface. The photocatalytic activity was 7.1 times greater than that of the sample prepared at a low heating rate (2 °C/min). These findings highlight the crucial role of surface defects, as opposed to bulk defects, in promoting hematite photocatalysis. Furthermore, the facile control over Vo concentration achievable via manipulating the heating rate underscores the promising potential of this approach for optimizing hematite photocatalysts.

From doping to composites: zirconia (ZrO2) modified hematite photoanodes for water splitting

Herein, a ZrO2 added α-Fe2O3 photoanode that can split water at low applied potential is reported. First, the pristine hematite α-Fe2O3 photoanode was synthesized using an aerosol-assisted chemical vapour deposition (AACVD) method followed by modification with various amounts of ZrO2 (2 to 40%) in the form of thin films on conducting glass substrate. The XRD, Raman spectroscopy and scanning electron microscopy (SEM) analyses confirmed the presence of the monoclinic phase of ZrO2 in the composites with multifaceted particles of compact morphology. The optical analysis showed an increase in the absorbance and variation in band gap of the composites ascribed to the heterogeneity of the material. The photoelectrochemical studies gave a photocurrent density of 1.23 mA cm-2 at 1.23 V vs. RHE for the pristine hematite and remarkably higher value of 3.06 mA cm-2 for the optimized amount of ZrO2 in the modified α-Fe2O3 photoanode. To the best of our knowledge, this is the highest photocurrent reported for a ZrO2 containing photoanode. The optimized composite electrode produced nine times more oxygen than that produced by pristine hematite.

Surface Self-Transforming FeTi-LDH Overlayer in Fe2 O3 /Fe2 TiO5 Photoanode for Improved Water Oxidation

Integrating hematite nanostructures with efficient layer double hydroxides (LDHs) is highly desirable to improve the photoelectrochemical (PEC) water oxidation performance. Here, an innovative and facile strategy is developed to fabricate the FeTi-LDH overlayer decorated Fe2 O3 /Fe2 TiO5 photoanode via a surface self-transformation induced by the co-treatment of hydrazine and NaOH at room temperature. Electrochemical measurements find that this favorable structure can not only facilitate the charge transfer/separation at the electrode/electrolyte interface but also accelerate the surface water oxidation kinetics. Consequently, the as-obtained Fe2 O3 /Fe2 TiO5 /LDH photoanode exhibits a remarkably increased photocurrent density of 3.54 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE) accompanied by an obvious cathodic shift (≈140 mV) in the onset potential. This work opens up a new and effective pathway for the design of high-performance hematite photoanodes toward efficient PEC water oxidation.

Imperfect makes perfect: defect engineering of photoelectrodes towards efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting for hydrogen evolution has been considered as a promising technology to solve the energy and environmental issues. However, the solar-to-hydrogen (STH) conversion efficiencies of current PEC systems are far from meeting the commercial demand (10%) due to the lack of efficient photoelectrode materials. The recent rapid development of defect engineering of photoelectrodes has significantly improved the PEC performance, which is expected to break through the bottleneck of low STH efficiency. In this review, the category and the construction methods of different defects in photoelectrode materials are summarized. Based on the in-depth summary and analysis of existing reports, the PEC performance enhancement mechanism of defect engineering is critically discussed in terms of light absorption, carrier separation and transport, and surface redox reactions. Finally, the application prospects and challenges of defect engineering for PEC water splitting are presented, and the future research directions in this field are also proposed.

Enhanced Oxygen Evolution Reaction Activity in Hematite Photoanodes: Effect of Sb-Li Co-Doping

Co-doping represents a valid approach to maximize the performance of photocatalytic and photoelectrocatalytic semiconductors. Albeit theoretical predictions in hematite suggesting a bulk n-type doping and a surface p-type doping would deliver best results, hematite co-doping with coupled cations possessing low and high oxidation states has shown promising results. Herein, we report, for the first time, Sb and Li co-doping of hematite photoanodes. Particularly, this is also a seminal work for the introduction of the highly reactive Sb⁵⁺ directly into the hematite thin films. Upon co-doping, we have a synergistic effect on the current densities with a 67-fold improvement over the standard. Via a combined investigation with profuse photoelectrochemical measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman analyses, we confirm the two doping roles of Sb⁵⁺ and Li⁺ as the substitutional and interstitial dopant, respectively. The improvements are attributed to a higher charge carrier concentration along with a lower charge transfer resistance at the surface.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Surface engineering of hematite nanorods photoanode towards optimized photoelectrochemical water splitting

Rapid charge recombination in hematite (Fe₂O₃) during photoelectrochemical water splitting is a major obstacle to achieving high-efficiency photoelectrodes. Surface defect engineering is considered as a viable strategy for enhancing photoelectrochemical activity of oxide photoanodes. Herein, a one-dimensional (1D) defective γ-Fe₂O₃ nanorods (DFNRs) photoanode is prepared using solvothermal and high-temperature hydrogenation strategies. The as-prepared DFNRs possess superior visible-light absorption capacity and exhibit excellent photoelectrochemical performance (0.98 mA cm^-2), with approximately three-fold higher photocurrent density than that of pristine Fe₂O₃ (FNRs, 0.32 mA cm^-2). The enhanced activity of the DFNRs results from the moderate formation of oxygen vacancy defects, which promotes spatial charge separation and transfer at the DFNRs/electrolyte interface, as well as the 1D nanorod structure, which favors rapid charge transfer. The surface of γ-Fe₂O₃ with hydroxyl (OH) groups provides sufficient surface-active sites. This result suggests that surface-oxygen deficiency of γ-Fe₂O₃ can not only expand the light absorption range but also facilitating photo-generated charge carriers separation. This surface engineering strategy provides an alternative method for preparing stable and highly active metal oxide photoanodes for photoelectrochemical water splitting.

Significantly Promoting the Photogenerated Charge Separation by Introducing an Oxygen Vacancy Regulation Strategy on the FeNiOOH Co-Catalyst

Semiconductor/co-catalyst coupling is considered as a promising strategy to enhance the photoelectrochemical (PEC) conversion efficiency. Unfortunately, this model system is faced with a serious interface recombination problem, which limits the further improvement of PEC performances. Here, a FeNiOOH co-catalyst with abundant oxygen vacancies on BiVO₄ is fabricated through simple and economical NaBH₄ reduction to accelerate hole transfer and achieve efficient electron-hole pair separation. The photocurrent of the BV (BiVO₄ )/Vo-FeNiOOH system is more than four times that of pure BV. Importantly, the charge transfer kinetics and charge carrier recombination process are studied by scanning photoelectrochemical microscopy and intensity modulated photocurrent spectroscopy in detail. In addition, the oxygen vacancy regulation proposed is also applied successfully to other semiconductors (Fe₂ O₃ ), demonstrating the applicability of this strategy.

Stable and Efficient Photoinduced Charge Transfer of MnFe2O4/Polyaniline Photoelectrode in Highly Acidic Solution

Tailoring conductive polymers with inorganic photocatalysts, which provide photoinduced electron-hole generation, have significantly enhanced composites leading to excellent photoelectrodes. In this work, MnFe2O4 nanoparticles prepared by a hydrothermal method were combined with polyaniline to prepare mixed (hybrid) slurries, which were cast onto flexible FTO to prepare photoelectrodes. The resulting photoelectrodes were characterized by XRD, FESEM, HRTEM and UV-VIS. The photoelectrochemical performance was investigated by linear sweep voltammetry and chronoamperometry. The photocurrent achieved by MnFe2O4/Polyaniline was 400 μA/cm2 at 0.8 V vs. Ag/AgCl in Na2SO4 (pH = 2) at 100 mW/cm2, while polyaniline alone achieved only 25 μA/cm2 under the same conditions. The best MnFe2O4/Polyaniline displayed an incident photon-to-current conversion efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) of 60% at 405 nm wavelength, and 0.17% at 0.8 V vs. Ag/AgCl, respectively. High and stable photoelectrochemical performance was achieved for more than 900 s in an acidic environment.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO₄) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO₄ photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO₄. In this review article, the fundamental properties of BiVO₄ are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO₄ photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO₄ photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO₄ photoanodes are presented, providing guidelines for the design of efficient BiVO₄ photoanodes for solar fuel production.

Boosting the quantum efficiency of the BiVO4 photoanode by increasing the oxygen vacancies for highly-efficient solar water oxidation

BiVO₄ (BVO) is a promising photoanode material for photoelectrochemical (PEC) water splitting. However, it is severely restricted by its short charge diffusion length and poor charge transport. Introducing oxygen vacancies into BVO is an effective method to solve these problems because they serve as surface electron capture sites and facilitate charge separation. In this work, a novel gas reaction method using chemical vapor deposition was used to produce abundant oxygen vacancies in single-crystal BVO. Oxygen vacancies in BVO acted as hole donors. This method effectively reduced the surface agglomeration and produced uniform BVO crystals. The optimized BVO photoanode achieved a photocurrent density of 2.44 mA cm^-2 (1.23 V vs. RHE) and an incident photon-to-current efficiency of 90% (450 nm). This work provides an effective strategy to prepare high-performance BVO photoanodes by chemical vapor deposition, electrodeposition and thermal evaporation.

BiO(OH)xI1-x solid solution with rich oxygen vacancies: interlayer guest hydroxyl for improved photocatalytic properties

A series of BiO(OH)_xI_1-x solid solution (SS) catalysts were successfully prepared by ion exchange of I^- and OH^- between the [Bi₂O₂]²⁺ layers. The morphology and microstructure were studied in depth using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) method, etc. Tunable absorption in the visible-light region was achieved by changing the proportion of OH^- to I^-. Due to the etching effect of OH^-, oxygen vacancies (OVs) greatly increased for the SS catalysts, and were confirmed by X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), and electron paramagnetic spectroscopy (EPR). The unique composition of OH^-, I^-, OV, and [Bi₂O₂]²⁺ layers in BiO(OH)_xI_1-x materials resulted in diverse photoexcitations. The BiO(OH)_0.45I_0.55 photocatalyst displayed a 10-fold-improved 2-chlorophenol (2-CP) degradation rate compared to BiOI. The interfacial reaction process by the photoinduced valence-band holes and conduction-band electrons proved to be a more efficient pathway for organic pollutant degradation by the BiO(OH)_xI_1-x SS photocatalyst. The OVs in the SS photocatalyst facilitated photoexcited and electron migration and transformation.

Greatly enhanced oxidative activity of δ-MnO2 to degrade organic pollutants driven by dominantly exposed {-111} facets

The reactivity of oxidizing materials is highly related to the exposed crystal facets. Herein, δ-MnO₂ with different exposure facets were synthesized and the oxidative activities of the as-prepared materials were evaluated by degrading phenol in water without light. The degradation rate of phenol by δ-MnO₂-{-111} was significantly higher than that by δ-MnO₂-{001}. δ-MnO₂-{-111} also displayed high degradation efficiency to a variety of other organic pollutants, such as ciprofloxacin, bisphenol A, 3-chlorophenol and sulfadiazine. Comprehensive characterization and theoretical calculation verified that the {-111} facet had high density of Mn³⁺, thus displaying enhanced direct oxidative capacity to degrade organic pollutants. In addition, the dominant {-111} facet promoted adsorption/activation of O₂, thus favored the generation of superoxide radical (O₂^•-), which actively participated in the degradation of pollutants. The phenol degradation kinetics could be divided into two distinct phases: the rapid phase (k1_obs = 0.468 min^-1) induced by Mn³⁺ and the slower phase (k2_obs = 0.048 min^-1) dominated by O₂^•-. The synergistically promoted non-radical and radical based reactions resulted in greatly enhanced the oxidative activity of the δ-MnO₂-{-111}. These findings deepen the understanding of facet-dependent oxidative performance of materials and provided valuable insights into the possible practical application of δ-MnO₂ for water purification.

In vitro and in vivo antibacterial performance of Zr & O PIII magnesium alloys with high concentration of oxygen vacancies

The effects of dual Zr and O plasma immersion ion implantation (Zr & O PIII) on antibacterial properties of ZK60 Mg alloys are systematically investigated. The results show that a hydrophobic, smooth, and ZrO₂-containing graded film is formed. Electrochemical assessment shows that the corrosion rate of the plasma-treated Mg alloy decreases and the decreased degradation rate is attributed to the protection rendered by the surface oxide. In vitro and in vivo antibacterial tests reveal Zr & O PIII ZK60 presents higher antibacterial rate compared to Zr PIII ZK60 and untreated control. The hydrophobic and smooth surface suppresses bacterial adhesion. High concentration of oxygen vacancies in the surface films are determined by X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectra (UV–vis DRS) and electron paramagnetic resonance (EPR) and involved in the production of reactive oxygen species (ROS). The higher level of ROS expression inhibits biofilm formation by down-regulating the expression of icaADBC genes but up-regulating the expression of icaR gene. In addition, Zr & O PIII improves cell viability and initial cell adhesion confirming good cytocompatibility. Dual Zr & O PIII is a simple and practical means to expedite clinical acceptance of biodegradable magnesium alloys.

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A ZrO₂-containing graded film with high concentration of oxygen vacancies was formed via PIII.

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S. aureus adhesion was suppressed due to enhanced hydrophobicity and decreased roughness.

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High concentration of oxygen vacancies lead to the upregulation of ROS expression.

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ROS mediates biofilm-associated genes expression to inhibit biofilm formation in vitro and in vivo.

Promoting Photoelectrochemical Water Oxidation on Ti-Doped Fe2O3 Nanowires Photoanode by O2 Plasma Treatment

Surface electron traps on semiconductor photoanodes mediate surface recombination and deteriorate the photoelectrochemical (PEC) water oxidation performance of the photoanode. Developing convenient methods to reduce surface electron traps is therefore essential for high efficiency PEC water oxidation on semiconductor photoanodes, particularly for nanostructured photoanodes with large surface area. Herein, we employ a O2 plasma treatment to boost the PEC water oxidation performance of Ti-doped Fe2O3 (Ti-Fe2O3) nanowires photoanodes, aiming to reduce surface oxygen vacancies, the dominant electron traps on Ti-Fe2O3 surface. X-ray diffraction (XRD), scanning electron microscopy and spectroscopic analyses show that the oxygen plasma treatment changes the structural, morphological and optical properties negligibly, but it does reduce the content of surface oxygen vacancies, as estimated from O1s X-ray photoelectron spectroscopy spectra. An optimal O2 plasma treatment (200 W, 70 s) increases the photocurrent density of the Ti-Fe2O3 nanowire photoanode to 2.14 mA·cm−2 (1.23 V vs. RHE) under air mass 1.5G simulated solar light, which is 1.95 times higher than the pristine Ti-Fe2O3 nanowire photoanode. The surface hole transfer efficiency is also improved by 1.66 times due to the reduced surface recombination. The work suggests that O2 plasma treatment is a convenient but effective method to boost the PEC water oxidation performance of Ti-Fe2O3 photoanode and might be applicable to other semiconducting oxide photoanodes for high efficiency PEC water splitting.

Oxygen Vacancy Engineering in Titanium Dioxide for Sodium Storage

Titanium dioxide (TiO₂ ) is a promising anode material for sodium-ion batteries (SIBs) due to its low cost, natural abundance, nontoxicity, and excellent electrochemical stability. Oxygen vacancies, the most common point defects in TiO₂ , can dramatically influence the physical and chemical properties of TiO₂ , including band structure, crystal structure and adsorption properties. Recent studies have demonstrated that oxygen-deficient TiO₂ can significantly enhance sodium storage performance. Considering the importance of oxygen vacancies in modifying the properties of TiO₂ , the structural properties, common synthesis strategies, characterization techniques, as well as the contribution of oxygen-deficient TiO₂ on initial Coulombic efficiency, cyclic stability, rate performance for sodium storage are comprehensively described in this review. Finally, some perspectives on the challenge and future opportunities for the development of oxygen-deficient TiO₂ are proposed.

Rapid fabrication of oxygen defective α-Fe2O3(110) for enhanced photoelectrochemical activities

Defect engineering is increasingly recognized as a viable strategy for boosting the performance of photoelectrochemical (PEC) water splitting using metal oxide-based photoelectrodes. However, previously developed methods for generating point defects associated with oxygen vacancies are rather time-consuming. Herein, high density oxygen deficient α-Fe₂O₃ with the dominant (110) crystal plane is developed in a very short timescale of 10 minutes by employing aerosol-assisted chemical vapor deposition and pure nitrogen as a gas carrier. The oxygen-defective film exhibits almost 8 times higher photocurrent density compared to a hematite photoanode with a low concentration of oxygen vacancies which is prepared in purified air. The existence of oxygen vacancies improves light absorption ability, accelerates charge transport in the bulk of films, and promotes charge separation at the electrolyte/semiconductor interface. DFT simulations verify that oxygen-defective hematite has a narrow bandgap, electron-hole trapped centre, and strong adsorption energy of water molecules compared to pristine hematite. This strategy might bring PEC technology another step further towards large-scale fabrication for future commercialization.

Morphology-dependent interfacial interactions of Fe2O3 with Ag nanoparticles for determining the catalytic reduction of p-nitrophenol

In this work, we fabricated three kinds of Ag/Fe₂O₃ model catalysts with different morphologies to study the interfacial interactions between Ag and Fe₂O₃, and how they affected the catalytic activity in hydrogenation of p-nitrophenol was explored. The hydrothermal method was used to synthesize the metal oxide supported silver catalyst, with various morphologies including nanoplates (NPs), nanospheres (NSs), and nanocubes (NCs). The crystal structure, morphology and surface elements of the composite were investigated by various measurements, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The catalytic activity was also evaluated by the reduction of p-nitrophenol to p-aminophenol. It was found that the activities of the above catalysts varied with the morphology of the support. Among them, Ag/Fe₂O₃ NPs promoted the highest performance, Ag/Fe₂O₃ NSs were slightly inferior, and Ag/Fe₂O₃ NCs were the worst. At last, we ascribed the remarkable activity of Ag/Fe₂O₃ NPs to the strong metal-support interactions between Ag and Fe₂O₃.

Tuning of oxygen vacancy-induced electrical conductivity in Ti-doped hematite films and its impact on photoelectrochemical water splitting

Titanium (Ti)-doped hematite (α-Fe₂O₃) films were grown in oxygen-depleted condition by using the spray pyrolysis technique. The impact of post-deposition annealing in oxygen-rich condition on both the conductivity and water splitting efficiency was investigated. The X-ray diffraction pattern revealed that the films are of rhombohedral α-Fe₂O₃ structure and dominantly directed along (012). The as-grown films were found to be highly conductive with electrons as the majority charge carriers (n-type), a carrier concentration of 1.09×10²⁰ cm⁻³, and a resistivity of 5.9×10⁻² Ω-cm. The conductivity of the films were reduced upon post-deposition annealing. The origin of the conductivity was attributed firstly to Ti⁴⁺ substituting Fe³⁺ and secondly to the ionized oxygen vacancies (V_O) in the crystal lattice of hematite. Upon annealing the samples in oxygen-rich condition, V_O slowly depleted and the conductivity reduced. The photocurrent of the as-grown samples was found to be 3.4 mA/cm⁻² at 1.23 V vs. RHE. The solar-to-hydrogen efficiency for the as-grown sample was calculated to be 4.18% at 1.23 V vs. RHE. The photocurrents were found to be significantly stable in aqueous environment. A linear relationship between conductivity and water-splitting efficiency was established.

Thermodynamic and Kinetic Influence of Oxygen Vacancies on the Solar Water Oxidation Reaction of α-Fe2O3 Photoanodes

To reveal the role of oxygen vacancies in the solar water oxidation of α-Fe₂O₃ photoanodes, the kinetic and thermodynamic properties that are closely related to the water oxidation reaction of the α-Fe₂O₃ photoanode containing oxygen vacancies were investigated. Compared with the pristine α-Fe₂O₃ photoanode, the presence of surface oxygen vacancies can improve the water oxidation activity and stability of the α-Fe₂O₃ photoanode simultaneously, but the bulk oxygen vacancies have a negative effect on the water oxidation performance of the α-Fe₂O₃ photoanode. In thermodynamics, our investigations revealed that the presence of surface oxygen vacancies narrows the space charge region width of the α-Fe₂O₃ photoanode, which could boost the charge separation and transfer efficiency of the α-Fe₂O₃ photoanode during water oxidation. Because the surface property and hydrophilicity of α-Fe₂O₃ are modified owing to the presence of surface oxygen vacancies, the water oxidation kinetics of the α-Fe₂O₃ photoanode with surface oxygen vacancies is obviously boosted. Our findings in the present work provide comprehensive understanding of the thermodynamic and kinetic differences for α-Fe₂O₃ photoanodes with and without oxygen vacancies for solar water oxidation.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

High-Throughput Screening and Surface Interrogation Studies of Au-Modified Hematite Photoanodes by Scanning Electrochemical Microscopy for Solar Water Splitting

Au-modified hematite photoanode was screened for photoelectrochemical (PEC) water oxidation by the scanning electrochemical microscopy (SECM) technique with a scanning probe of the optical fiber for visible light irradiation of the photoanode substrate. The Au-modified hematite exhibited an enhancement in the photocurrent up to 3% (at. %), and the performance drop was observed with 4-10% (at. %) of Au modification. Subsequently, pristine and Au-modified hematite thin-film photoanodes were fabricated by the spin-coating method to confirm the results of SECM. The PEC response confirms that 3% (at. %) of Au is the optimum concentration to provide the best enhancement of PEC water oxidation with a ∼6-fold increase compared to the pristine hematite sample. Direct Au oxidation, charge recombination, and strong light absorption by Au are responsible for the decrease in PEC performance when the Au percentage is above 3%. The pristine and Au-modified hematite materials were also characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Au was found to exist in the form of embedded metallic nanoparticles in the modified hematite. Mott-Schottky analysis of the bulk samples confirms an improvement in charge carrier density for the Au-modified hematite. Additionally, there was little plasmonic enhancement as evidenced by UV-vis spectroscopy, with a minimal contribution toward photoactivity. Surface interrogation SECM quantitatively probed the reactive surface states (RSSs) such as OH^• formed on hematite and Au-modified hematite surfaces during water oxidation. The coverage of RSSs was found to increase with the substrate potential. The interrogated charge under the dark condition for the 3% Au-modified hematite sample is higher than the pristine hematite sample because of the enhanced electronic conductivity of the hematite film.

Oxygen-Vacancy-Dominated Cocatalyst/Hematite Interface for Boosting Solar Water Splitting

Surface suppression is one of critical issues for semiconductors in photoelectrochemical (PEC) water splitting. Deposition of oxygen evolution cocatalysts on photoanodes can improve the oxygen evolution rate, but still it has some limits in some cases. In this work, we propose a new and simple precipitation approach to transform the surface of hematite into iron phosphate (Fe-Pi). Further, Ar-plasma treatment on Fe-Pi/Fe₂O₃ introduces oxygen vacancies on the phosphorous and photoanode. A surface phosphate treatment accelerates the transfer of holes from the bulk to the surface. Besides, creating oxygen vacancy defects on Fe-Pi/Fe₂O₃ can significantly increase the reactivity of active sites, leading to the remarkable enhancement in oxygen evolution reaction activity and PEC performance. The resulting photoanode has a current density of 2.71 mA cm^-2 at 1.23 V_RHE and 3.5 mA cm^-2 at 1.50 V_RHE under simulated solar light condition. The reduced surface recombination by Fe-Pi layer and Ar-plasma treatment is confirmed by electrochemical analysis. These findings give a great potential of the use of a combination strategy for cocatalyst deposition and optimizing the performance of hematite.

In Situ Formation of WO3-Based Heterojunction Photoanodes with Abundant Oxygen Vacancies via a Novel Microbattery Method

Non-stoichiometric ratio semiconductor materials have exhibited excellent performance in energy conversion and storage fields. However, the hydrogen treatment method that is commonly used to introduce oxygen vacancies is expensive and dangerous. In this paper, a novel microbattery method using Zn powder and Fe powder as reductant has been developed to synthesize the oxygen vacancy modified WO_{3- x} films and oxygen-deficient heterojunction films (ZnWO_{4- x}/WO_{3- x} and Fe₂O_{3- x}/WO_{3- x}) at room temperature. The as-prepared WO_{3- x} and ZnWO_{4- x}/WO_{3- x} heterojunction films exhibit improved photoelectrochemical performance. It is worth noting that this microbattery method can quickly introduce oxygen vacancies into semiconductor materials, including powders and films at room temperature.

Tubular morphology preservation and doping engineering of Sn/P-codoped hematite for photoelectrochemical water oxidation

Tubular hematite with high-concentration, uniform doping is regarded as a promising material for photoelectrochemical water oxidation. However, the high-temperature annealing commonly used for activating doped hematite inevitably causes deformation of the tubular structure and an increase in the trap states. In the present work, Sn-doped tubular hematite on fluorine-doped tin oxide (FTO) is successfully obtained at 750 °C from a Sn-coated FeOOH tube precursor. Sn/P codoping, which is rarely considered for hematite, is also achieved via a gas phase reaction in phosphide atmosphere. The tubular morphology allows the dopant to diffuse from both the inner and outer surfaces, thus decreasing the doping profile in the radial direction. The even distribution of Sn and P synergetically increases the carrier density of hematite by one order of magnitude, which shortens the width of the depletion layer to ca. 2.3 nm (compared with 19.3 nm for the pristine sample) and leads to prolonged carrier lifetime and efficient charge separation. In addition, this codoping protocol does not introduce additional surface trap states, as evidenced by the increased charge injection efficiency and surface kinetic analysis using intensity modulated photocurrent spectroscopy (IMPS). As a result, the morphology- and doping-engineered hematite exhibits photocurrents of 0.9 mA cm-2 at 1.23 V and 3.8 mA cm-2 at 2.0 V vs. RHE under AM 1.5 G illumination (100 mW cm-2) in 1.0 M NaOH, representing 4.5-fold and 4.8-fold enhancements, respectively, compared with the photocurrents of undoped hematite. The present method is shown to be effective for preparing multi-element-doped hematite nanotubes and may find broad application in the development of other nanotubular photoelectrodes with or without doping for efficient and robust water oxidation.

Oxygen-deficient WO3via high-temperature two-step annealing for enhanced and highly stable water splitting

In contrast with conventional one-step annealed WO₃, high-temperature two-step annealed WO₃ contains a higher concentration of oxygen deficiency, leading to more efficient charge separation and improved photocatalytic ability.

Defective Fe3+ self-doped spinel ZnFe2O4 with oxygen vacancies for highly efficient photoelectrochemical water splitting

Defective Fe³⁺ self-doped spinel ZnFe₂O₄ with abundant oxygen vacancies exhibits largely enhanced photoelectrochemical performance.

Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts

The catalytic conversion of dinitrogen (N₂ ) into ammonia under ambient conditions represents one of the Holy Grails in sustainable chemistry. As a potential alternative to the Haber-Bosch process, the electrochemical reduction of N₂ to NH₃ is attractive owing to its renewability and flexibility, as well as its sustainability for producing and storing value-added chemicals from the abundant feedstock of water and nitrogen on earth. However, owing to the kinetically complex and energetically challenging N₂ reduction reaction (NRR) process, NRR electrocatalysts with high catalytic activity and high selectivity are rare. In this contribution, as a proof-of-concept, we demonstrate that both the NH₃ yield and faradaic efficiency (FE) under ambient conditions can be improved by modification of the hematite nanostructure surface. Introducing more oxygen vacancies to the hematite surface renders an improved performance in NRR, which leads to an average NH₃ production rate of 0.46 μg h^-1  cm^-2 and an NH₃ FE of 6.04 % at -0.9 V vs. Ag/AgCl in 0.10 m KOH electrolyte. The durability of the electrochemical system was also investigated. A surprisingly high average NH₃ production rate of 1.45 μg h^-1  cm^-2 and a NH₃ FE of 8.28 % were achieved after the first 1 h chronoamperometry test. This is among the highest FEs reported so far for non-precious-metal catalysts that use a polymer-electrolyte-membrane cell and is much higher than the FE of precious-metal catalysts (e.g., Ru/C) under comparable reaction conditions. However, the NH₃ yield and the FE dropped to 0.29 μg h^-1  cm^-2 and 2.74 %, respectively, after 16 h of chronoamperometry tests, which indicates poor durability of the system. Our results demonstrate the important role that the surface states of transition-metal oxides have in promoting electrocatalytic NRR under ambient conditions. This work may spur interest towards the rational design of electrocatalysts as well as electrochemical systems for NRR, with emphasis on the issue of stability.