Determining the role of oxygen vacancies in the photoelectrocatalytic performance of WO3 for water oxidation

Surface Adsorption and Photoinduced Degradation: A Study of Spinel Ferrite Nanomaterials for Removal of a Model Organic Pollutant from Water

Spinel oxide nanocrystals are attractive materials for photoinduced advanced oxidation processes that degrade organic pollutants in water due to their chemical stability and tunability, visible light absorption, and magnetic recoverability. However, a systematic understanding of the structural and chemical factors that control the reactivity of specific spinel oxide nanocrystal materials toward photoinduced degradation processes is lacking. This Perspective illustrates these knowledge gaps through an investigation into the impacts of surface chemistry and composition of spinel ferrite nanocrystals of formula MFe2O4 (M = Mg, Fe, Co, Ni, Cu, Zn) on their ability to remove a model organic pollutant (methyl orange (MO)) from water. We identify two mechanisms by which the nanocrystals remove MO from water: (i) surface adsorption and (ii) photoinduced degradation under visible light irradiation in the presence of hydrogen peroxide via the photo-Fenton reaction. Nanocrystals that do not contain any surface ligands are more effective at removing MO from water than nanocrystals that contain surface ligands, despite our observation that the ligand-less nanocrystals do not form stable colloidal dispersions in water, while ligand-coated nanocrystals are colloidally stable. For many of the spinel ferrite compositions studied here, the fraction of methyl orange removal via adsorption to the nanocrystal surface in the absence of photoexcitation is larger than the fraction removed under irradiation. Our data indicate that the composition-dependent surface charge of the nanocrystals controls the degree of surface adsorption of the charged MO molecule. Overall, these results demonstrate that careful consideration of the impacts of surface chemistry on the behavior of spinel ferrite nanocrystals is required to accurately assess and subsequently understand their activity toward the photoinduced degradation of organic molecules.

Correlating activities and defects in (photo)electrocatalysts using in-situ multi-modal microscopic imaging

Photo(electro)catalysts use sunlight to drive chemical reactions such as water splitting. A major factor limiting photocatalyst development is physicochemical heterogeneity which leads to spatially dependent reactivity. To link structure and function in such systems, simultaneous probing of the electrochemical environment at microscopic length scales and a broad range of timescales (ns to s) is required. Here, we address this challenge by developing and applying in-situ (optical) microscopies to map and correlate local electrochemical activity, with hole lifetimes, oxygen vacancy concentrations and photoelectrode crystal structure. Using this multi-modal approach, we study prototypical hematite (α-Fe2O3) photoelectrodes. We demonstrate that regions of α-Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxygen vacancy concentration, with the film thickness and extended light exposure also influencing local activity. Our work highlights the importance of microscopic mapping to understand activity, in even seemingly homogeneous photoelectrodes.

A comprehensive review of oxygen vacancy modified photocatalysts: synthesis, characterization, and applications

Photocatalytic technology is a novel approach that harnesses solar energy for efficient energy conversion and effective pollution abatement, representing a rapidly advancing field in recent years. The development and synthesis of high-performance semiconductor photocatalysts constitute the pivotal focal point. Oxygen vacancies, being intrinsic defects commonly found in metal oxides, are extensively present within the lattice of semiconductor photocatalytic materials exhibiting non-stoichiometric ratios. Consequently, they have garnered significant attention in the field of photocatalysis as an exceptionally effective means for modulating the performance of photocatalysts. This paper provides a comprehensive review on the concept, preparation, and characterization methods of oxygen vacancies, along with their diverse applications in nitrogen fixation, solar water splitting, CO2 photoreduction, pollutant degradation, and biomedicine. Currently, remarkable progress has been made in the synthesis of high-performance oxygen vacancy photocatalysts and the regulation of their catalytic performance. In the future, it will be imperative to develop more advanced in situ characterization techniques, conduct further investigations into the regulation and stabilization of oxygen vacancies in photocatalysts, and comprehensively comprehend the mechanism underlying the influence of oxygen vacancies on photocatalysis. The engineering of oxygen vacancies will assume a pivotal role in the realm of semiconductor photocatalysis.

Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

Machine learning analysis to interpret the effect of the photocatalytic reaction rate constant (k) of semiconductor-based photocatalysts on dye removal

Photocatalytic reactions with semiconductor-based photocatalysts have been investigated extensively for application to wastewater treatment, especially dye degradation, yet the interactions between different process parameters have rarely been reported due to their complicated reaction mechanisms. Hence, this study aims to discern the impact of each factor, and each interaction between multiple factors on reaction rate constant (k) using a decision tree model. The dyes selected as target pollutants were indigo and malachite green, and 5 different semiconductor-based photocatalysts with 17 different compositions were tested, which generated 34 input features and 1527 data points. The Boruta Shapley Additive exPlanations (SHAP) feature selection for the 34 inputs found that 11 inputs were significantly important. The decision tree model exhibited for 11 input features with an R2 value of 0.94. The SHAP feature importance analysis suggested that photocatalytic experimental conditions, with an importance of 59%, was the most important input category, followed by atomic composition (39%) and physicochemical properties (2%). Additionally, the effects on k of the synergy between the metal cocatalysts and important experimental conditions were confirmed by two feature SHAP dependence plots, regardless of importance order. This work provides insight into the single and multiple factors that affect reaction rate and mechanism.

Defect Engineering of WO3 by Rapid Flame Reduction for Efficient Photoelectrochemical Conversion of Methane into Liquid Oxygenates

Photoelectrochemical (PEC) conversion is a promising way to use methane (CH4) as a chemical building block without harsh conditions. However, the PEC conversion of CH4 to value-added chemicals remains challenging due to the thermodynamically favorable overoxidation of CH4. Here, we report WO3 nanotube (NT) photoelectrocatalysts for PEC CH4 conversion with high liquid product selectivity through defect engineering. By tuning the flame reduction treatment, we carefully controlled the oxygen vacancies of WO3 NTs. The optimally reduced WO3 NTs suppressed overoxidation of CH4 showing a high total C1 liquid selectivity of 69.4% and a production rate of 0.174 μmol cm-2 h-1. Scanning electrochemical microscopy revealed that oxygen vacancies can restrain the production of hydroxyl radicals, which, in excess, could further oxidize C1 intermediates to CO2. Additionally, band diagram analysis and computational studies elucidated that oxygen vacancies thermodynamically suppress overoxidation. This work introduces a strategy for understanding and controlling the selectivity of photoelectrocatalysts for direct conversion of CH4 to liquids.

A non-invasive approach to the resistive switching physical model of ultra-thin organic-inorganic dielectric-based ReRAMs

The race to next-generation non-volatile memory is on, and ultra-thin (<5 nm) organic-inorganic hybrid dielectric-based ReRAMs are a top contender. However, their extremely small thickness hinders their processability through material characterization techniques, leaving gaps in our understanding of the resistive switching (RS) dynamics in the hybrid dielectric layer. Furthermore, the poor uniformity of key switching parameters remains a persistent issue in ReRAMs, which impedes any trends to be clearly defined through electrical characterization. This work uses electrical manipulation through a ramped-pulse series (RPS) method to improve the voltage and resistance fluctuations in the reset process of ultra-thin Al/Hf-hybrid/Ni devices. By analyzing their electrical behavior under different pulse and temperature conditions, we propose a comprehensive physical model that describes the operating mechanism of the device. Our results confirm the coexistence in the conductive filament (CF) of a hybrid metallic portion composed of Al and Hf3Al2 and an oxygen vacancy portion. The vacancies are found to play a significant role in RS, with most of them generated during the CF forming process and participating to different degrees in the filament rupture of the RPS-processed and non-RPS-processed devices via Joule heating, drift, and Fick forces. Additionally, we identify the cause of switching failure events to be based on the presence of an Al2O3 interlayer in the Al/Hf-hybrid interface.

Self-Diffusion versus Intentional Doping: Beneficial and Damaging Impact on Hematite Photoanode Interfaces

The comprehension of side effects caused by high-temperature thermal treatments in the design of (photo)electrodes is essential to achieve efficient and cost-effective devices for solar water splitting. This investigation explores the beneficial and damaging impacts of thermal treatments in the (photo)electrode design, unraveling the impact of self-diffusion and its consequences. The industrial-friendly polymeric precursor synthesis (PPS) method, which is known for its easy technological application, was chosen as the fabrication technique for hematite photoabsorbers. For substrate evaluation, two types of conductive glass substrates, aluminum borosilicate and quartz, both coated with fluorine-doped tin oxide (ABS/FTO and QTZ/FTO, respectively), were subjected to thermal treatments following the PPS protocol. Optical and structural analyses showed no significant alterations in substrate properties, whereas X-ray photoelectron spectroscopy (XPS) revealed the migration of silicon and calcium ions from the glass component to the FTO surface. This diffusion can be further mitigated by an oxide buffer layer. To track the potential ion diffusion on the photoabsorber surface and assess its effect on the photoelectrode performance, hematite was selected as the model material and deposited onto the glass substrates. From all the ions that could possibly migrate, only Si4+ and Ca2+ originating from the glass component, as well as Sn4+ from the fluorine-doped tin oxide (FTO), were detected on the surface of the hematite photoabsorber. Interestingly, the so-called "self-diffusion" of these ions did not result in any beneficial effect on the hematite photoelectrochemical response. Instead, intentional modifications showed more substantial impacts on the photoelectrochemical efficiency compared to unintentional self-diffusion. Therefore, "self-diffusion", which can unintentionally dope the hematite, is not sufficient to significantly impact the final photocurrent. These findings emphasize the importance of understanding the true effect of thermal treatments on the photoelectrode properties to unlock their full potential in photoelectrochemical applications.

Synergistic photocatalytic degradation mechanism of BiOClxI1-x-OVs based on oxygen vacancies and internal electric field-mediated solid solution

The easy recombination of photoexcited electron-hole pairs is a serious constraint for the application of photocatalysts. In this work, a range of BiOClxI1-x solid solutions with abundant oxygen vacancies (BiOClxI1-x-OVs) were synthesized. In particular, the optimal BiOCl0.5I0.5-OVs sample exhibited almost 100% removal of bisphenol A (BPA) within 45 min visible light exposure, which was 22.4, 3.1 and 4.5 times greater than BiOCl, BiOCl-OVs and BiOCl0.5I0.5, respectively. Besides, the apparent quantum yield of BPA degradation reaches 0.24%, better than some other photocatalysts. Benefiting from the synergism of oxygen vacancies and solid solution, BiOCl0.5I0.5-OVs gained an enhanced photocatalytic capacity. Oxygen vacancies induced an intermediate defective energy level in BiOClxI1-x-OVs materials, promoting the generation of photogenerated electrons and the molecular oxygen adsorption to produce more active oxygen radicals. Meanwhile, the fabricated solid solution structure enhanced the internal electric field between BiOCl layers, achieving rapid migration of photoexcited electrons and effective segregation of photoinduced charge carriers. Thus, this study provides a viable idea to solve the problems of poor visible light absorption of BiOCl-based photocatalysts and easy reorganization of electrons and holes in the photocatalysts.

Anomalous Dependence of Photocarrier Recombination Time on the Polaron Density of TiO2(110)

Polarons play a crucial role in energy conversion, but the microscopic mechanism remains unclear since they are susceptible to local atomic structures. Here, by employing ab initio nonadiabatic dynamic simulations, we investigate electron-hole (e-h) nonradiative recombination at the rutile TiO2(110) surface with varied amounts of oxygen vacancies (Vo). The isolated Vo facilitates e-h recombination through forming polarons compared to that in the defect-free surface. However, aggregated Vo forming clusters induce an order-of-magnitude acceleration of polaron diffusion by enhancing phonon excitations, which blocks the defect-mediated recombination and thus prolongs the photocarrier lifetime. We find that photoelectrons are driven to migrate toward the top surface due to polaron formation. Our results show the many-body effects of defects and polaron effects on determining the overall recombination rate, which has been ignored in the Shockley-Read-Hall model. The findings explain the controversial experimental observations and suggest that engineering Vo aggregation would instead improve photocatalysis efficiencies in polaronic materials.

The Role of Lattice Defects on the Optical Properties of TiO2 Nanotube Arrays for Synergistic Water Splitting

In this study, we report a facile one-step chemical method to synthesize reduced titanium dioxide (TiO2) nanotube arrays (NTAs) with point defects. Treatment with NaBH4 introduces oxygen vacancies (OVs) in the TiO2 lattice. Chemical analysis and optical studies indicate that the OV density can be significantly increased by changing reduction time treatment, leading to higher optical transmission of the TiO2 NTAs and retarded carrier recombination in the photoelectrochemical process. A cathodoluminescence (CL) study of reduced TiO2 (TiO2-x) NTAs revealed that OVs contribute significantly to the emission bands in the visible range. It was found that the TiO2 NTAs reduced for a longer duration exhibited a higher concentration of OVs. A typical CL spectrum of TiO2 was deconvoluted to four Gaussian components, assigned to F, F+, and Ti3+ centers. X-ray photoelectron spectroscopy measurements were used to support the change in the surface chemical bonding and electronic valence band position in TiO2. Electron paramagnetic resonance spectra confirmed the presence of OVs in the TiO2-x sample. The prepared TiO2-x NTAs show an enhanced photocurrent for water splitting due to pronounced light absorption in the visible region, enhanced electrical conductivity, and improved charge transportation.

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.

Defective TiO2 Nanotube Arrays for Efficient Photoelectrochemical Degradation of Organic Pollutants

Oxygen vacancies (OVs) are one of the most critical factors that enhance the electrical and catalytic characteristics of metal oxide-based photoelectrodes. In this work, a simple procedure was applied to prepare reduced TiO₂ nanotube arrays (NTAs) (TiO_2-x) via a one-step reduction method using NaBH₄. A series of characterization techniques were used to study the structural, optical, and electronic properties of TiO_2-x NTAs. X-ray photoelectron spectroscopy confirmed the presence of defects in TiO_2-x NTAs. Photoacoustic measurements were used to estimate the electron-trap density in the NTAs. Photoelectrochemical studies show that the photocurrent density of TiO_2-x NTAs was nearly 3 times higher than that of pristine TiO₂. It was found that increasing OVs in TiO₂ affects the surface recombination centers, enhances electrical conductivity, and improves charge transport. For the first time, a TiO_2-x photoanode was used in the photoelectrochemical (PEC) degradation of a textile dye (basic blue 41, B41) and ibuprofen (IBF) pharmaceutical using in situ generated reactive chlorine species (RCS). Liquid chromatography coupled with mass spectrometry was used to study the mechanisms for the degradation of B41 and IBF. Phytotoxicity tests of B41 and IBF solutions were performed using Lepidium sativum L. to evaluate the potential acute toxicity before and after the PEC treatment. The present work provides efficient PEC degradation of the B41 dye and IBF in the presence of RCS without generating harmful products.

Synergistic role of hydrogen treatment and heterojunction in H-WO3-x/TiO2-x NT/Ti foil-based photoanodes for photoelectrochemical wastewater detoxification and antibacterial activity

The process of photoelectrochemical wastewater detoxification is limited by significant charge recombination, which is difficult to suppress with efficient single-material photoanodes. We demonstrated the effectiveness of hydrogen treatment in evaluating charge separation properties in WO_3-x/TiO_2-x NT/Ti foil heterojunction photoanodes. The influence of varying hydrogen annealing (200-400 °C) on the structural and photoelectrochemical properties of WO₃/TiO₂ NS/NT heterojunction is studied systematically. Additionally, after hydrogen treatment of pristine WO₃/TiO₂ NT/Ti foil photoanodes, substoichiometric H-WO_3-x/TiO_2-x NT-300 achieved the 1.21 mA/cm² photocurrent density, which is 8.06 and 3.27 times than TiO₂ NT and WO₃/TiO₂ NT. The hydrogen-treated H-WO_3-x/TiO_2-x NT-300 electrode exhibits 3 times greater bulk efficiencies than the WO₃/TiO₂ NT electrode due to the production of oxygen vacancies at the interface. Additionally, optimum H-WO_3-x/TiO_2-x NS/NT-300 photoanode exhibited 93.8% E. coli and 99.8% BPA decomposition efficiencies. The present work shows the effectiveness of microwave-assisted H-WO_3-x/TiO_2-x NT heterojunction photoanodes for organic decomposition and antibacterial activity in a neutral environment without surface-loaded co-catalysts.

The Piezoresponse in WO3 Thin Films Due to N2-Filled Nanovoids Enrichment by Atom Probe Tomography

Tungsten trioxide (WO₃) is a versatile n-type semiconductor with outstanding chromogenic properties highly used to fabricate sensors and electrochromic devices. We present a comprehensive experimental study related to piezoresponse with piezoelectric coefficient d₃₃ = 35 pmV^-1 on WO₃ thin films ~200 nm deposited using RF-sputtering onto alumina (Al₂O₃) substrate with post-deposit annealing treatment of 400 °C in a 3% H₂/N₂-forming gas environment. X-ray diffraction (XRD) confirms a mixture of orthorhombic and tetragonal phases of WO₃ with domains with different polarization orientations and hysteresis behavior as observed by piezoresponse force microscopy (PFM). Furthermore, using atom probe tomography (APT), the microstructure reveals the formation of N₂-filled nanovoids that acts as strain centers producing a local deformation of the WO₃ lattice into a non-centrosymmetric structure, which is related to piezoresponse observations.

Enhancement of ZnO@RuO2 bifunctional photo-electro catalytic activity toward water splitting

Catalytic materials are the greatest challenge for the commercial application of water electrolysis (WEs) and fuel cells (FCs) as clean energy technologies. There is a need to find an alternative to expensive and unavailable platinum group metal (PGM) catalysts. This study aimed to reduce the cost of PGM materials by replacing Ru with RuO₂ and lowering the amount of RuO₂ by adding abundant and multifunctional ZnO. A ZnO@RuO₂ composite in a 10:1 molar ratio was synthesized by microwave processing of a precipitate as a green, low-cost, and fast method, and then annealed at 300°C and 600°C to improve the catalytic properties. The physicochemical properties of the ZnO@RuO₂ composites were investigated by X-ray powder diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), UV-Vis diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) spectroscopy. The electrochemical activity of the samples was investigated by linear sweep voltammetry in acidic and alkaline electrolytes. We observed good bifunctional catalytic activity of the ZnO@RuO₂ composites toward HER and OER in both electrolytes. The improved bifunctional catalytic activity of the ZnO@RuO₂ composite by annealing was discussed and attributed to the reduced number of bulk oxygen vacancies and the increased number of established heterojunctions.

Liquid-Metal-Induced Hydrogen Insertion in Photoelectrodes for Enhanced Photoelectrochemical Water Oxidation

Fast charge separation and transfer (CST) is essential for achieving efficient solar conversion processes. This CST process requires not only a strong driving force but also a sufficient charge carrier concentration, which is not easily achievable with traditional methods. Herein, we report a rapid hydrogenation method enabled by gallium-based liquid metals (GBLMs) to modify the prototypical WO₃ photoelectrode to enhance the CST for a PEC process. Protons in solution are controllably embedded into the WO₃ photoanode accompanied by electron injection due to the strong reduction capability of GBLMs. This process dramatically increases the carrier concentration of the WO₃ photoanode, leading to improved charge separation and transfer. The hydrogenated WO₃ photoanode exhibits over a 229% improvement in photocurrent density with long-term stability. The effectiveness of GBLMs treatment in accelerating the CST process is further proved using other more general semiconductor photoelectrodes, including Nb₂O₅ and TiO₂.

Nb/N Co-Doped Layered Perovskite Sr2TiO4: Preparation and Enhanced Photocatalytic Degradation Tetracycline under Visible Light

Sr2TiO4 is a promising photocatalyst for antibiotic degradation in wastewater. The photocatalytic performance of pristine Sr2TiO4 is limited to its wide bandgap, especially under visible light. Doping is an effective strategy to enhance photocatalytic performance. In this work, Nb/N co-doped layered perovskite Sr2TiO4 (Sr2TiO4:N,Nb) with varying percentages (0−5 at%) of Nb were synthesized by sol-gel and calcination. Nb/N co-doping slightly expanded the unit cell of Sr2TiO4. Their photocatalytic performance towards antibiotic (tetracycline) was studied under visible light (λ > 420 nm). When Nb/(Nb + Ti) was 2 at%, Sr2TiO4:N,Nb(2%) shows optimal photocatalytic performance with the 99% degradation after 60 min visible light irradiation, which is higher than pristine Sr2TiO4 (40%). The enhancement in photocatalytic performance is attributed to improving light absorption, and photo-generated charges separation derived from Nb/N co-doping. Sr2TiO4:N,Nb(2%) shows good stability after five cycles photocatalytic degradation reaction. The capture experiments confirm that superoxide radical is the leading active species during the photocatalytic degradation process. Therefore, the Nb/N co-doping in this work could be used as an efficient strategy for perovskite-type semiconductor to realize visible light driving for wastewater treatment.

Improved Carrier Lifetime in BiVO4 by Spin Protection

Mechanistic understanding of the effect bulk defects have on carrier dynamics at the quantum level is crucial to suppress associated midgap mediated charge recombination in semiconductors yet many questions remain unexplored. Here, by employing ab initio quantum dynamics simulation and taking BiVO₄ with oxygen vacancies (Ov) as a model system we demonstrate a spin protection mechanism for suppressed charge recombination. The carrier lifetime is significantly improved in the high spin defect system. The lifetime can be optimized by tuning the Ov concentration to minimize the nonradiative relaxation. Our work addresses literature ambiguities and contradictions about the role of bulk Ov in charge recombination and provides a route for defect engineering of semiconductors with enhanced carrier dynamics.

Laser-Reduced BiVO4 for Enhanced Photoelectrochemical Water Splitting

The present study proposes a laser irradiation method to superficially reduce BiVO4 photoelectrodes and boost their water oxidation reaction performance. The origin of this enhanced performance toward oxygen evolution reaction (OER) was studied using a combination of a suite of structural, chemical, and mechanistic advanced characterization techniques including X-ray photoelectron (XPS), X-ray absorption spectroscopy (XAS), electrochemical impedance spectroscopy (EIS), and transient absorption spectroscopy (TAS), among others. We found that the reduction of the material is localized at the surface of the sample and that this effect creates effective n-type doping and a shift to more favorable energy band positions toward water oxidation. This thermodynamic effect, together with the change in sample morphology to larger and denser domains, results in an extended lifetime of the photogenerated carriers and improved charge extraction. In addition, the stability of the reduced sample in water was also confirmed. All of these effects result in a two-fold increase in the photocurrent density of the laser-treated samples.

Superoxide anion and singlet oxygen dominated faster photocatalytic elimination of nitric oxide over defective bismuth molybdates heterojunctions

Establishing an ideal photocatalytic system with efficient reactive oxygen species (ROS) generation has been regarded as the linchpin for realizing efficient nitric oxide (NO) removal and unveiling the ROS-mediated mechanism. In this work, a novel oxygen-deficient 0D/1D Bi_3.64Mo_0.36O_6.55/Bi₂MoO₆ heterojunctions (BMO-12-H) were successfully synthesized under the enlightenment of clarified crystal growth mechanism of bismuth molybdates. Because of the synergies between defect-engineering and heterojunction-construction, BMO-12-H demonstrated improved photoelectrochemical properties and O₂ adsorption capacity, which in turn facilitated the ROS generation and conversion. The enhancement of •O₂^- and ¹O₂ endowed BMO-12-H with strengthened NO removal efficiency (59%) with a rate constant of 12.6*10^-2 min^-1. A conceivable NO removal mechanism dominated by •O₂^- and ¹O₂ was proposed and verified based on the theoretical calculations and in-situ infrared spectroscopy tests, where hazardous NO was oxidized following two different exothermic pathways: the •O₂^--induced NO → NO₃^- process and the ¹O₂-induced NO → NO₂ → NO₃^- process. This work offers a basic guideline for accelerating ROS generation by integrating defect-engineering and heterojunction-construction, and provides new insights into the mechanism of efficient NO removal dominated by •O₂^- and ¹O₂.

Depleted Oxygen Defect State Enhancing Tungsten Trioxide Photocatalysis: A Quantum Dynamics Perspective

Oxygen vacancies generally create midgap states in transition metal oxides, which are expected to decrease the photoelectrochemical water-splitting efficiency. Recent experiments defy this expectation but leave the mechanism unclear. Focusing on the photoanode WO₃ as a prototypical system, we demonstrate using nonadiabatic molecular dynamics that an oxygen vacancy suppresses nonradiative electron-hole recombination, because the defect acts as an electron reservoir instead of a recombination center. The occupied midgap electrons prefer to be populated a priori compared to the band edge transition because of a larger transition dipole moment, converting to depleted/unoccupied trap states that rapidly accept conduction band electrons and then cause trap-assisted recombination by impeding the bandgap recombination regardless of oxygen vacancy configurations. The reported results provide a fundamental understanding of the "realistic" role of the oxygen vacancies and their influence on charge-phonon dynamics and carrier lifetime. The study generates valuable insights into the design of high-performance transition metal oxide photocatalysts.

Understanding the "Anti-Catalyst" Effect with Added CoOx Water Oxidation Catalyst in Dye-Sensitized Photoelectrolysis Cells: Carbon Impurities in Nanostructured SnO2 Are the Culprit

In 2017, we reported a dye-sensitized, photoelectrolysis cell consisting of fluorine-doped tin oxide (FTO)-coated glass covered by SnO₂ nanoparticles coated with N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI) dye and then a photoelectrochemically deposited CoO_x water oxidation catalyst (WOCatalyst), FTO/nano-SnO₂/PMPDI/CoO_x. This system employed nanostructured SnO₂ stabilized by a polyethyleneglycol bisphenol A epichlorohydrin (PEG-BAE) copolymer and other C-containing additives based on a literature synthesis to achieve a higher surface area and thus greater PMPDI dye absorption and resultant light collection. Surprisingly, the addition of the well-established WOCatalyst CoO_x resulted in a decrease in the photocurrent, an unexpected "anti-catalyst" effect. Two primary questions addressed in the present study are (1) what is the source of this "anti-catalyst" effect? and (2) are the findings of broader interest? Reflection on the synthesis of nano-SnO₂ stabilized by PEG-BAE, and the large, ca. 10:1 ratio of C to Sn in synthesis, led to the hypothesis that even the annealing step at 450 °C in of the FTO/SnO₂ anode precursors was unlikely to remove all the carbon initially present. Indeed, residual carbon impurities are shown to be the culprit in the presently observed "anti-catalyst" effect. The implication and anticipated broader impact of the results of answering the two abovementioned questions are also presented and discussed along with a section entitled "Perspective and Suggestions for the Field Going Forward."

Visible Light Driven Ultrasensitive and Selective NO2 Detection in Tin Oxide Nanoparticles with Sulfur Doping Assisted by l-Cysteine

In the pandemic era, the development of high-performance indoor air quality monitoring sensors has become more critical than ever. NO₂ is one of the most toxic gases in daily life, which induces severe respiratory diseases. Thus, the real-time monitoring of low concentrations of NO₂ is highly required. Herein, a visible light-driven ultrasensitive and selective chemoresistive NO₂ sensor is presented based on sulfur-doped SnO₂ nanoparticles. Sulfur-doped SnO₂ nanoparticles are synthesized by incorporating l-cysteine as a sulfur doping agent, which also increases the surface area. The cationic and anionic doping of sulfur induces the formation of intermediate states in the band gap, highly contributing to the substantial enhancement of gas sensing performance under visible light illumination. Extraordinary gas sensing performances such as the gas response of 418 to 5 ppm of NO₂ and a detection limit of 0.9 ppt are achieved under blue light illumination. Even under red light illumination, sulfur-doped SnO₂ nanoparticles exhibit stable gas sensing. The endurance to humidity and long-term stability of the sensor are outstanding, which amplify the capability as an indoor air quality monitoring sensor. Overall, this study suggests an innovative strategy for developing the next generation of electronic noses.

In-Situ Generated CsPbBr3 Nanocrystals on O-Defective WO3 for Photocatalytic CO2 Reduction

Metal halide perovskite (MHP) nanocrystals (NCs) have shown promising application in photocatalytic CO₂ reduction, but their activities are still largely restrained by severe charge recombination and narrow solar spectrum response. Assembly of heterojunctions can be beneficial to the charge separation in MHPs while the assembly process usually brings native interfacial defects, impeding efficient charge separation between two materials. Herein, an in-situ generation strategy was developed to prepare CsPbBr₃ /WO₃ heterojunction, using WO₃ nanosheets (NSs) as growing substrate for the growth of CsPbBr₃ NCs. The developed CsPbBr₃ /WO₃ heterojunction exhibited a high-quality interface, greatly facilitating charge transfer between two semiconductors. The hybrid photocatalyst displayed an excellent activity toward CO₂ reduction, which was about 7-fold higher than pristine CsPbBr₃ NCs and 3.5-fold higher than their assembled counterparts. The experimental results and theoretical simulations revealed that a Z-scheme mechanism with a favorable internal electric field was responsible for the good performance of CsPbBr₃ /WO₃ heterojunction. By using O-defective WO₃ NSs as a near-infrared (NIR) light absorber, the CsPbBr₃ /WO₃ heterojunction could harvest NIR light and showed an impressive activity toward CO₂ reduction. This work demonstrates a new strategy to design MHP-based heterojunctions by synergistically considering the interface quality, charge transfer mode, interfacial electric field, and light response range between two semiconductors.

Cytotoxicity of ZnO Nanoparticle Under Dark via Oxygen Vacancy Dependent Reactive Oxygen Species Generation

Antimicrobial and cytotoxic effect of zinc oxide nanomaterials are popularly thought due to zinc ion leaching, but the exact mechanism of cytotoxicity is controversial and not fully understood. Recent works...

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.

In situ constructed oxygen-vacancy-rich MoO3-x /porous g-C3N4 heterojunction for synergistically enhanced photocatalytic H2 evolution

A simple method was developed for enhanced synergistic photocatalytic hydrogen evolution by in situ constructing of oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunctions. Introduction of a MoO_3−x precursor (Mo(OH)₆) solution into g-C₃N₄ nanosheets helped to form a porous structure, and nano-sized oxygen-vacancy-rich MoO_3−xin situ grew and formed a heterojunction with g-C₃N₄, favorable for charge separation and photocatalytic hydrogen evolution (HER). Optimizing the content of the MoO_3−x precursor in the composite leads to a maximum photocatalytic H₂ evolution rate of 4694.3 μmol g⁻¹ h⁻¹, which is approximately 4 times higher of that of pure g-C₃N₄ (1220.1 μmol g⁻¹ h⁻¹). The presence of oxygen vacancies (OVs) could give rise to electron-rich metal sites. High porosity induced more active sites on the pores' edges. Both synergistically enhanced the photocatalytic HER performance. Our study not only presented a facile method to form nano-sized heterojunctions, but also to introduce more active sites by high porosity and efficient charge separation from OVs.

In situ growth method to construct a nano-sized oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunction. MoO_3−x derived OV traps and porous g-C₃N₄ nanosheet derived short migration distance and plentiful edge active sites.

Solution Processing of Topochemically Converted Layered WO3 for Multifunctional Applications

Solution processing of nanomaterials is a promising technique for use in various applications owing to its simplicity and scalability. However, the studies on liquid-phase exfoliation (LPE) of tungsten oxide (WO₃ ) are limited, unlike others, by a lack of commercial availability of bulk WO₃ with layered structures. Herein, a one-step topochemical synthesis approach to obtain bulk layered WO₃ from commercially available layered tungsten disulfide (WS₂ ) by optimizing various parameters like reaction time and temperature is reported. Detailed microscopic and spectroscopic techniques confirmed the conversion process. Further, LPE was carried out on topochemically converted bulk layered WO₃ in 22 different solvents; among the solvents studied, the propan-2-ol/water (1 : 1) co-solvent system appeared to be the best. This indicates that the possible values of surface tension and Hansen solubility parameters for bulk WO₃ could be close to that of the co-solvent system. The obtained WO₃ dispersions in a low-boiling-point solvent enable thin films of various thickness to be fabricated by using spray coating. The obtained thin films were used as active materials in supercapacitors without any conductive additives/binders and exhibited an areal capacitance of 31.7 mF cm^-2 at 5 mV s^-1 . Photo-electrochemical measurements revealed that these thin films can also be used as photoanodes for photo-electrochemical water oxidation.

ZIF-8 derived ZnO/TiO2 heterostructure with rich oxygen vacancies for promoting photoelectrochemical water splitting

Due to the serious recombination of electron-hole and weak photoresponse ability, achieving highly efficient photoelectrochemical (PEC) water splitting activity for TiO₂ photoelectrode has become a key issue. In this paper, we reported a new method for preparing ZnO/TiO₂ photoelectrodes by electrostatic adsorption from zeolitic imidazolate framework-8 (ZIF-8) as the precursor. ZIF-8 was combined with TiO₂ nanorods (NRs) through electrostatic interaction and then calcined to obtain ZnO/TiO₂ heterojunction photoelectrodes with abundant oxygen vacancies (O_vac). The introduced ZnO with O_vac provides a large number of active sites which enhanced the electrical conductivity and charges separation of ZnO/TiO₂ photoelectrode. The optimal photocurrent density of ZnO/TiO₂ photoelectrodes at 1.23 V versus (vs.) reversible hydrogen electrode (RHE) under illumination (100 mW/cm²) has reached 1.76 mA/cm², almost 2.75 times that of the pure TiO₂. Meanwhile, the incident photon-to-electron conversion efficiency (IPCE) of the best photoelectrode has increased to 58.2% at 390 nm, the charge injection (η_injection) and separation (η_separation) efficiency have reached to 93.53% and 51.62% (1.23 V vs. RHE), respectively.

The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO3 photoanodes for water oxidation

WO3 photoanodes offer rare stability in acidic media, but are limited by their selectivity for oxygen evolution over parasitic side reactions, when employed in photoelectrochemical (PEC) water splitting. Herein, this is remedied via the modification of nanostructured WO3 photoanodes with surface decorated PdO as an oxygen evolution co-catalyst (OEC). The photoanodes and co-catalyst particles are grown using an up-scalable aerosol assisted chemical vapour deposition (AA-CVD) route, and their physical properties characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorption spectroscopy. Subsequent PEC and transient photocurrent (TPC) measurements showed that the use of a PdO co-catalyst dramatically increases the faradaic efficiency (FE) of water oxidation from 52% to 92%, whilst simultaneously enhancing the photocurrent generation and charge extraction rate. The Pd oxidation state was found to be critical in achieving these notable improvements to the photoanode performance, which is primarily attributed to the higher selectivity towards oxygen evolution when PdO is used as an OEC and the formation of a favourable junction between WO3 and PdO, that drives band bending and charge separation.

Potential and pitfalls: On the use of transient absorption spectroscopy for in situ and operando studies of photoelectrodes

Here, we discuss the application, advantages, and potential pitfalls of using transient UV/Vis (ultraviolet-visible) absorption spectroscopy to study photoelectrodes for water splitting. We revisit one of the most commonly studied water oxidation photoanodes (α-Fe₂O_3-x) to provide commentary and guidelines on experiment design and data analysis for transient absorption (TA) studies of photoelectrodes within a photoelectrochemical cell. We also assess the applicability of such in situ TA studies to understand photoelectrodes under operating conditions. A major limitation is that most, if not all, past in situ TA studies have been carried out using only pulsed light sources to generate carriers, with the electrode held in the dark at other times, which is shown to be a poor model for operating conditions. However, with a simple modification of existing TA experiments, a simple operando TA measurement is reported.

Impact of the Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes

Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. While these photoanodes exhibit very different current/voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential toward less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intra-bandgap states, indicating these intra-bandgap states do not contribute directly to water oxidation.