Iron-doping-enhanced photoelectrochemical water splitting performance of nanostructured WO3: a combined experimental and theoretical study

SnS2 Nanoparticles Embedded in BiVO4 Surfaces via Eutectic Decomposition for Enhanced Performance in Photoelectrochemical Water Splitting

BiVO4 has garnered substantial interest as a promising photoanode material for photoelectrochemical water-splitting due to its narrow band gap and appropriate band edge positions for water oxidation. Nevertheless, its practical use has been impeded by poor charge transport and sluggish water oxidation kinetics. Here, a hybrid composite photoanode is fabricated by uniformly embedding SnS2 nanoparticles near the surface of a BiVO4 thin film, creating a type II heterostructure with strong interactions between the nanoparticles and the film for efficient charge separation. This structure forms via eutectic melting during atomic layer deposition of SnS2 with subsequent phase separation between SnS2 and BiVO4 at room temperature, offering greater advantages and flexibilities over conventional exsolution techniques. Furthermore, the SnS2/BiVO4 hybrid composite is coated with a thin amorphous ZnS passivation layer to accelerate charge transfer process and enhance long-term stability. The optimized BiVO4/SnS2/ZnS photoanode exhibits a photocurrent density of 5.44 mA cm-2 at 1.23 V versus RHE, which is 2.73 times higher than that of the BiVO4 photoanode, and a dramatic improvement in photostability retention at 1.23 V versus RHE, increasing from 55% to 91% over 24 hours. This method of anchoring nanoparticles onto host materials proves highly valuable for energy and environmental applications.

Plasmonic nanocavity-modulated electrochemiluminescence sensor for gastric cancer exosomal miRNA detection

Plasmonic nanocavity possessing highly light field confinement and electromagnetic field enhancement can concentrate and enhance the luminescence signal. The plasmonic nanocavity has the great potential value in biosensing research and improve analytical sensitivity. In this work, we constructed a plasmonic nanocavity between circular Au nanoplate-film and spherical Au nanoparticle with tetrahedral DNA nanostructures. The nanocavity structure can regulate the local density of optical states and provide the field restriction to enhance the spontaneous ECL radiation of PEDOT-S dots. Additionally, Au nanoparticle acted as nanoantenna which localized and modulated ECL to directional emission. Because the plasmonic nanocavity effectively collected and redistributed ECL signal, the emission was enhanced by 5.9 times with polarized characteristics. The proposed plasmonic nanocavity-based ECL sensor was further used to detect exosomal miRNA-223-3p in ascites. The detection results indicated the novel sensing strategy can assist early diagnosis of peritoneal metastasis of gastric cancer.

Surface charge-directed borophene-phosphorous nitride nanodot heterojunction supports for enhanced photoelectrochemical performance

Complimentary surface-charged, nanosized 0D-0D hybrids of phosphorous nitride dots (PNDs) (ς = +9.5 mV) and borophene dots (BDs) (ς = -26.2 mV) having favourable band alignments are proposed for a type-II heterojunction. This hybrid model provides rapid carrier separation and carrier recombination resistance for enhanced PEC water oxidation.

Studies on the EPR parameters and local angular distortion for the tetragonal Cu2+ center in CaWO4 crystal

The electron paramagnetic resonance (EPR) parameters-g factors g_i (i = || and ⊥) and hyperfine structure constants A_i (M) and A_i (N), with M and N belonging to isotopes ⁶³ Cu²⁺ and ⁶⁵ Cu²⁺ -and local structure of Cu²⁺ ion occupying W⁶⁺ site in CaWO₄ crystal are theoretically studied based on the perturbation formulas of these parameters for a 3d⁹ ion under tetragonally elongated tetrahedra. In these formulas, the ligand orbital (LO) and spin-orbit coupling (SOC) contributions are included due to the shorter impurity-ligand distance R (≈1.83 Å) and hence the strong covalency of the studied [CuO₄ ]^6- cluster, and the related molecular orbital coefficients are quantitatively determined from the cluster approach in a uniform way; meanwhile, the required crystal field (CF) parameters for the tetragonally distorted tetrahedron (TDT) are estimated from the superposition model and the local structure of the impurity Cu²⁺ center. According to the calculation, the bond angle θ between the four equivalent Cu²⁺ -O^2- bonds and the C₄ axis in the CaWO₄ :Cu²⁺ is found to be about 2.1° smaller than that (θ₀  ≈ 54.74°) for an ideal tetrahedron due to the Jahn-Teller (JT) effect and the size mismatch. The fitted results agree well with the observed values, and the validity of the present assignment for the local structure of the Cu²⁺ center is also discussed.

Operando Photo-Electrochemical Catalysts Synchrotron Studies

The attempts to develop efficient methods of solar energy conversion into chemical fuel are ongoing amid climate changes associated with global warming. Photo-electrocatalytic (PEC) water splitting and CO₂ reduction reactions show high potential to tackle this challenge. However, the development of economically feasible solutions of PEC solar energy conversion requires novel efficient and stable earth-abundant nanostructured materials. The latter are hardly available without detailed understanding of the local atomic and electronic structure dynamics and mechanisms of the processes occurring during chemical reactions on the catalyst–electrolyte interface. This review considers recent efforts to study photo-electrocatalytic reactions using in situ and operando synchrotron spectroscopies. Particular attention is paid to the operando reaction mechanisms, which were established using X-ray Absorption (XAS) and X-ray Photoelectron (XPS) Spectroscopies. Operando cells that are needed to perform such experiments on synchrotron are covered. Classical and modern theoretical approaches to extract structural information from X-ray Absorption Near-Edge Structure (XANES) spectra are discussed.

Atomistic Observation of Temperature-Dependent Defect Evolution within Sub-stoichiometric WO3-x Catalysts

Tunable crystalline defects endow WO_3-x catalysts with extended functionalities for a broad range of photo- and electric-related applications. However, direct visualization of the defect structures and their evolution mechanism is lacking. Herein, aberration-corrected and in situ transmission electron microscopy was complemented by theoretical calculations to investigate the effect of temperature on the defect evolution behavior during hydrogenation treatment. Low processing temperature (100-300 °C) leads to the occurrence of randomly distributed oxygen vacancies within WO_3-x nanosheets. At higher temperatures, oxygen vacancies become highly mobile and aggregate into stacking faults. Planar defects are prone to nucleate at the surface and develop in a zigzag form at 400 °C, while treating at 500 °C promotes the growth of {200}-type stacking faults. Our work clearly establishes that the atomic configuration of the defects in WO_3-x samples could be manipulated by regulating the hydrogenation temperature. This study not only expands our understanding of the structure-function relationships of sub-stoichiometric tungsten oxides but also unlocks their full potential as advanced catalysts by tuning stoichiometry in a controlled manner.

Novel WS2 /Fe0.95 S1.05 Hierarchical Nanosphere as a Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction

Fe_0.95 S_1.05 with high reactivity and stability was incorporated into WS₂ nanosheets via a one-step solvothermal method for the first time. The resulted hybrid catalyst has much higher catalytic activity than WS₂ and Fe_0.95 S_1.05 alone, and the optimal WS₂ /Fe_0.95 S_1.05 hybrid catalyst was found by adjusting the feed ratio. The addition of Fe_0.95 S_1.05 was proven to be able to enhance the hydrogen evolution reaction (HER) activity of WS₂ , and vice versa. At the same time, it was found that the catalytic effect of the hybrid catalyst was the best when the feed ratio was W : Fe=2 : 1. In other words, we confirmed that there is a synergistic effect between W- and Fe-based sulfide hybrid catalysts, and validated that the reason for the improved HER performance is the strong interaction between the two in the middle sulfur. WS₂ /Fe_0.95 S_1.05 -2 hybrid catalyst leads to enhanced HER activity, which shows a low overpotential of ∼0.172 V at 10 mA cm^-2 , low Tafel slope of ∼53.47 mV/decade. This study supplies innovative synthesis of a highly active WS₂ /Fe_0.95 S_1.05 hybrid catalyst for HER.

Hollow bimetallic selenide derived from a hierarchical MOF-based Prussian blue analogue for urea electrolysis

PBA@MOF–Ni/Se with a nanocube structure grown on a flower-shaped MOF–Ni template exhibits better performance in urea electrolysis.

Tuning the Electronic Structure of Monoclinic Tungsten Oxide Nanoblocks by Indium Doping for Boosted Photoelectrochemical Performance

Photoelectrochemical (PEC) water oxidation, a desirable strategy to meet future energy demands, has several bottle-necks to resolve. One of the prominent issues is the availability of charge carriers at the surface reaction site to promote water oxidation. Of the several approaches, metal dopants to enhance the carrier density of the semiconductors, is an important one. In this work, we have studied the effect of In-doping on monoclinic WO₃ nanoblocks, growing vertically over fluorine-doped tin oxide (FTO) without the aid of any seed layer. X-ray photoelectron spectroscopy (XPS) data reveals that In³⁺ ions are partially occupying the W⁶⁺ ions in In-doped WO₃ photoanode. In³⁺ ions are offering better performance by adding additional charge carriers for amplifying the expression of the number of carriers. The maximum current density value of 2.18 mA/cm² has been provided by the optimized In-doped WO₃ photoanode with 3 wt% indium doping at 1.23 V vs. RHE, which is ∼3 times higher than that of undoped monoclinic WO₃ photoanode. Mott-Schottky (MS) analysis reveals charge carrier density (N_D ) for In-doped WO₃ photoanode has been enhanced by a factor of 3. An average Faradic yield of ∼90 percent has been achieved which can serve as a model system using In³⁺ as a dopant for an inexpensive and attractive method for enhanced WO₃ based PEC water oxidation.

Photocatalytic reduction of Cr(VI) using Mg-doped WO3 nanoparticles

The hydrothermal synthesis method was employed for the fabrication of pristine tungsten trioxide (WO₃) and that of varying dopant percentages (1, 3 and 5% m/m) of magnesium (Mg-WO₃). The optical and structural properties of the synthesized materials were characterized using DRS, XRD, FTIR, TEM, BET, FESEM, XPS, PL, and Raman. Rectangular shaped nanostructures were observed through FESEM, wherein confirmed as monoclinic with the aid of XRD, FTIR and Raman analysis. Visualization of the doping was carried out using HRTEM imagery, which was also confirmed by a slight increase (0.0069 nm) of d spacing. As a consequence, band gaps were diminished and band edge positions were shifted. Band edge position shifts were confirmed via XPS analysis (0.08 eV). The point of zero charge was observed to shift towards positive upon doping at working pH 1 and 3.75 pH was the highest recorded. The rate of recombination was greatly reduced upon doping was observed through PL analysis. This was supported by DFT calculations, in which case the reduction of the rate of recombination was attributed to the introduction of Mg orbital. An improved efficiency was observed via the photo reduction of Cr(VI) metal ion in waste water, in which case, 97% reduction was attained.

Engineering the Morphology and Crystal Phase of 3 D Hierarchical TiO2 with Excellent Photochemical and Photoelectrochemical Solar Water Splitting

Owing to their unique characteristics, hierarchical TiO₂ nanostructures have several advantages in solar-fuel production. In this work, a single-step approach has been developed to control both the crystal phase and morphology of TiO₂ with 3 D urchin-like structure via a surfactant-free solvothermal route. The growth of 3 D hierarchical structure with phase-engineered band alignment, the role of the H₂ O/HCl ratio, and fine-tuning of the reaction parameters are investigated systematically. An optimum ratio of anatase (41 %) to rutile (59 %) in the mixed-phase TiO₂ (AR-2) results in excellent photocatalytic H₂ generation activity of 5753 μmol g^-1 after 5 h of irradiation with apparent quantum yields of 20.9 % at 366 nm and 4.5 % at 420 nm. The superior performance of AR-2, attributed to efficient separation of charge carriers through the phase junction, is apparent from the transient photocurrent response and photoluminescence studies. The 3 D urchin-like pure rutile TiO₂ (R-1) composed of nanorods shows enhanced photocatalytic activity compared with pure anatase and pure rutile TiO₂ nanoparticles, and this demonstrates the role of morphology. The best-performing mixed-phase 3 D TiO₂ shows excellent durability up to 25 h and is shown to produce 3522 μmol g^-1 of H₂ under natural sunlight, which highlights its potential for long-term application.

The optimization of surface morphology of Au nanoparticles on WO3 nanoflakes for plasmonic photoanode

Among many candidates for photoanode materials of photoelectrochemical (PEC) cell, nanostructured tungsten trioxide (WO₃) is regarded as one of the most promising materials due to its superior electrical properties and adequate bandgap (∼2.8 eV) and band edge position. WO₃ nanoflakes (WO₃ NFs), which have merits on its high surface area and crystallinity, have been actively studied for this manner but solar-to-hydrogen efficiency of WO₃ NFs based photoanode is still not sufficient both in light absorption and charge separation. Plasmon-induced enhancement using Au nanoparticles is excellent approach for both the efficiency of light absorption and charge separation of WO₃. However, it still needs optimization on its amount, shape, coverage, and etc. Here, we synthesized WO₃ NFs by solvothermal growth and decorated gold nanoparticles on these nanoflakes by e-beam evaporation and rapid thermal annealing process in a row. By this process, a large-area AuNPs/WO₃ nanocomposite structure with various size, interparticle distance, and coverage of AuNPs were fabricated. These AuNPs/WO₃ NFs type photoanode achieve high light absorption both in UV and visible range and consequently higher photocurrent density. The optimized AuNPs/WO₃ nanocomposite photoanode exhibits 1.01 mA cm^-2 of photocurrent density, which is increased to 19.8% compared with bare WO₃ nanoflakes. Field emission-scanning electron microscope, x-ray diffraction, UV-vis spectrometer analysis were measured to analyze the morphology and crystallinity and relationship between structure and PEC performance.

Green Synthetic Fuels: Renewable Routes for the Conversion of Non-Fossil Feedstocks into Gaseous Fuels and Their End Uses

Innovative renewable routes are potentially able to sustain the transition to a decarbonized energy economy. Green synthetic fuels, including hydrogen and natural gas, are considered viable alternatives to fossil fuels. Indeed, they play a fundamental role in those sectors that are difficult to electrify (e.g., road mobility or high-heat industrial processes), are capable of mitigating problems related to flexibility and instantaneous balance of the electric grid, are suitable for large-size and long-term storage and can be transported through the gas network. This article is an overview of the overall supply chain, including production, transport, storage and end uses. Available fuel conversion technologies use renewable energy for the catalytic conversion of non-fossil feedstocks into hydrogen and syngas. We will show how relevant technologies involve thermochemical, electrochemical and photochemical processes. The syngas quality can be improved by catalytic CO and CO2 methanation reactions for the generation of synthetic natural gas. Finally, the produced gaseous fuels could follow several pathways for transport and lead to different final uses. Therefore, storage alternatives and gas interchangeability requirements for the safe injection of green fuels in the natural gas network and fuel cells are outlined. Nevertheless, the effects of gas quality on combustion emissions and safety are considered.

Engineering surface and morphology of La/WO3 for electrochemical oxygen reduction

Lanthanum derived crystal and defect engineering for enhanced ORR performance in WO₃.

Tri-functional molecular relay to fabricate size-controlled CoOx nanoparticles and WO3 photoanode for an efficient photoelectrochemical water oxidation

Heterojunction and element doping to couple light-harvesting semiconductors with catalytic materials have been widely employed for photoelectrochemical (PEC) water splitting.

Designing WO3/CdIn2S4 type-II heterojunction with both efficient light absorption and charge separation for enhanced photoelectrochemical water splitting

WO₃ is a typical photoanode material for photoelectrochemical (PEC) water splitting. However, the PEC activity of WO₃ photoanode is limited by its poor visible light absorption as well as severe carrier recombination at the electrode/electrolyte interface. Herein, we integrate small-band-gap CdIn₂S₄ nanoplates with hydrothermally grown WO₃ nanowall arrays to form into a three-dimensional (3D) WO₃/CdIn₂S₄ heterojunction through a chemical bath deposition process. The synthesis parameters of CdIn₂S₄, including reaction time and temperature, have been tuned to optimize the PEC performance. The WO₃/CdIn₂S₄ composite photoanode prepared at 50 °C for 5 h exhibits the highest photocurrent of 1.06 mA cm^-2 at 1.23 V versus reversible hydrogen electrode without the presence of holes scavenger, which is about 5.9 times higher than that of bare WO₃ photoanode. The band alignment between WO₃ and CdIn₂S₄ is confirmed by the ultraviolet-visible light absorption spectra and ultraviolet photoelectron spectra. The PEC performance enhancement is attributed to the enhanced light absorption benefiting from the small band gap of CdIn₂S₄ and efficient charge separation originating from the type-II alignment between WO₃ and CdIn₂S₄.

Metal-Organic Framework-Derived Fe/Co-based Bifunctional Electrode for H2 Production through Water and Urea Electrolysis

Hollow-structured Fe_x Co_2-x P, Fe_x Co_3-x O₄ , and Prussian blue analogue (FeCo-PBA) microbuilding arrays on Ni foam (NF) are derived from Co-based metal-organic frameworks (Co-MOF) using a simple room temperature and post-heat-treatment route. Among them, Fe_x Co_2-x P/NF shows excellent bifunctional catalytic activities by demonstrating very low oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) overpotentials of 255/114 mV at a current density of 20/10 mA cm^-2 respectively, whereas Fe_x Co_3-x O₄ /NF and FeCo-PBA/NF demand higher overpotentials. Remarkably, for water electrolysis, Fe_x Co_2-x P/NF requires only 1.61 V to obtain 10 mA cm^-2 . In contrast to water electrolysis, urea electrolysis reduces overpotential and simultaneously purifies the urea-rich wastewater. The urea oxidation reaction at the Fe_x Co_2-x P/NF anode needs just 1.345 V to achieve 20 mA cm^-2 , which is 140 mV less than the 1.48 V potential required for OER. Moreover, the generation of H₂ through urea electrolysis needs only 1.42 V to drive 10 mA cm^-2 .

Nicotinamide adenine dinucleotide immobilized tungsten trioxide nanoparticles for simultaneous sensing of norepinephrine, melatonin and nicotine

Herein, we report the anionic surfactant, ethylene diamine tetraacetic acid (EDTA), mediated synthesis of WO₃ nanoparticles and its subsequent modification through gamma irradiation (GI) and electrochemical immobilization with nicotinamide adenine dinucleotide (NAD). Glassy carbon electrode (GCE) modified with GI-WO₃ NPs and the enzyme NAD exhibited strong electro-oxidation of three important biomolecules such as norepinephrine (NEP), melatonin (MEL) and nicotine (NIC) in 0.1 M phosphate buffer saline (PBS) at physiological pH of 7. Square wave voltammetry (SWV) studies exhibited three well-defined peaks at potentials of 120, 570 and 840 mV, corresponding to the oxidation of NEP, MEL and NIC respectively, indicating that simultaneous determination of these compounds is feasible at the NAD/GI EDTA-WO₃/GCE. The proposed sensor displayed a wide linear range of 0.010-1000 μM with the lowest detection limit of 1.4 nM for NEP, 2.6 nM for MEL and 1.7 nM for NIC respectively. Furthermore, the modified electrode was successfully applied to detect NEP, MEL and NIC in pharmaceutical and cigarette samples with excellent selectivity and reproducibility.

Structural, Optical, Band Edge and Enhanced Photoelectrochemical Water Splitting Properties of Tin-Doped WO3

The substitutional doping of tungsten oxide (WO3) with metal ions demonstrates a promising approach to enhance its photoelectrochemical (PEC) water splitting efficiency. In this article, the substitutional doping of Sn ions into WO3 lattice and its effect on optical, electrical, band edge, and PEC water splitting properties are explored. Sn-doped WO3 thin films were synthesized using a facile hydrothermal method. The characterization data reveal that the doping of Sn alters the morphology, induces multiple crystal phases, effects the crystal orientation, reduces the band gap, and increases the carrier density of WO3. With the uniform distribution of Sn ions in WO3 and the decreased charge transfer resistance at the electrode/electrolyte interface, the doped WO3 show notable enhancement in its PEC activity compared to the undoped WO3. The band edge study revealed that the introduction of Sn in WO3 lattice causes an increase in the energy distance between the valence band edge and Fermi level and, at the same time, induces a downward shift in both the valence and conduction band edges towards higher potentials with respect to reversible hydrogen electrode (RHE). Conclusively, this work shows significant and new insights about Sn-doped WO3 photoanodes and their influence on PEC water splitting efficiency.

Effect of oxygen deficiency on the excited state kinetics of WO3 and implications for photocatalysis

Using WO₃ as a model material, we investigate how different oxygen vacancy concentrations affect trapping of photogenerated charges and photocatalytic reactions in metal oxides.

Oxygen vacancies are widely used to tune the light absorption of semiconducting metal oxides, but a photophysical framework describing the impact of such point defects on the dynamics of photogenerated charges, and ultimately on catalysis, is still missing. We herein use WO₃ as a model material and investigate the impact of significantly different degrees of oxygen deficiency on its excited state kinetics. For highly oxygen-deficient films, photoelectron spectroscopy shows an over 2 eV broad distribution of oxygen vacancy states within the bandgap which gives rise to extended visible light absorption. We examine the nature of this distribution using first-principles defect calculations and find that defects aggregate to form clusters rather than isolated vacancy sites. Using transient absorption spectroscopy, we observe trapping of photogenerated holes within 200 fs after excitation at high degrees of oxygen deficiency, which increases their lifetime at the expense of oxidative driving force. This loss in driving force limits the use of metal oxides with significant degrees of sub-stoichiometry to photocatalytic reactions that require low oxidation power such as pollutant degradation, and highlights the need to fine-tune vacancy state distributions for specific target reactions.

Electrochromic semiconductors as colorimetric SERS substrates with high reproducibility and renewability

Electrochromic technology has been actively researched for displays, adjustable mirrors, smart windows, and other cutting-edge applications. However, it has never been proposed to overcome the critical problems in the field of surface-enhanced Raman scattering (SERS). Herein, we demonstrate a generic electrochromic strategy for ensuring the reproducibility and renewability of SERS substrates, which are both scientifically and technically important due to the great need for quantitative analysis, standardized production and low cost in SERS. This color-changing strategy is based on a unique quantitative relationship between the SERS signal amplification and the coloration degree within a certain range, in which the SERS activity of the substrate can be effectively inferred by judging the degree of color change. Our results may provide a first step toward the rational design of electrochromic SERS substrates with a high sensitivity, reproducibility, and renewability.

Electrochromic technology has diverse cutting-edge applications, but it has never been used to overcome the critical problems in the field of surface-enhanced Raman scattering (SERS). Here, the authors demonstrate a generic electrochromic strategy for ensuring the reproducibility and renewability of SERS substrates.

WO3/BiVO4 Type-II Heterojunction Arrays Decorated with Oxygen-Deficient ZnO Passivation Layer: A Highly Efficient and Stable Photoanode

In the present work, we report a ternary WO₃/BiVO₄/ZnO photoanode with boosted PEC efficiency and stability toward highly efficient water splitting. The type-II WO₃/BiVO₄ heterojunction arrays are firstly prepared by hydrothermal growth of WO₃ nanoplate arrays onto the substrates of fluorine-doped tin oxide (FTO)-coated glass, followed by spin-coating of BiVO₄ layers onto the WO₃ nanoplate surfaces. After that, thin ZnO layers are further introduced onto the WO₃/BiVO₄ heterojunction arrays via atomic layer deposition (ALD), leading to the construction of ternary WO₃/BiVO₄/ZnO photoanodes. It is verified that the ZnO thin layer in the WO₃/BiVO₄/ZnO photoanode contains abundant oxygen vacancies, which could act as an effective passivation layer to enhance the charge separation and surface water oxidation kinetics of photogenerated carriers. The as-prepared WO₃/BiVO₄/ZnO photoanode produces a photocurrent of 2.96 mA cm^-2 under simulated sunlight with an incident photon-to-current conversion efficiency (IPCE) of ∼72.8% at 380 nm at a potential of 1.23 V versus RHE without cocatalysts, both of which are comparable to the state-of-the-art WO₃/BiVO₄ counterparts. Moreover, the photocurrent of the WO₃/BiVO₄/ZnO photoanode shows only 9% decay after 6 h, suggesting its high photoelectrochemical (PEC) stability.

Synthesis of a WO3 photocatalyst with high photocatalytic activity and stability using synergetic internal Fe3+ doping and superficial Pt loading for ethylene degradation under visible-light irradiation

Synthesis of a WO₃ photocatalyst with high photocatalytic activity and stability using synergetic internal Fe³⁺ doping and superficial Pt loading for ethylene degradation under visible-light irradiation.

Surface Engineering of Nanomaterials for Photo-Electrochemical Water Splitting

Photo-electrochemical water splitting represents a green and environmentally friendly method for producing solar hydrogen. Semiconductor nanomaterials with a highly accessible surface area, reduced charge migration distance, and tunable optical and electronic property are regarded as promising electrode materials to carry out this solar-to-hydrogen process. Since most of the photo-electrochemical reactions take place on the electrode surface or near-surface region, rational engineering of the surface structures, physical properties, and chemical nature of photoelectrode materials could fundamentally change their performance. Here, the recent advances in surface engineering methods, including the modification of the nanomaterial surface morphology, crystal facet, defect and doping concentrations, as well as the deposition of a functional overlayer of sensitizers, plasmonic metallic structures, and protective and catalytic materials are highlighted. Each surface engineering method and how it affects the structural features and photo-electrochemical performance of nanomaterials are reviewed and compared. Finally, the current challenges and the opportunities in the field are discussed.

Refining Defect States in W18O49 by Mo Doping: A Strategy for Tuning N2 Activation towards Solar-Driven Nitrogen Fixation

Photocatalysis may provide an intriguing approach to nitrogen fixation, which relies on the transfer of photoexcited electrons to the ultrastable N≡N bond. Upon N₂ chemisorption at active sites (e.g., surface defects), the N₂ molecules have yet to receive energetic electrons toward efficient activation and dissociation, often forming a bottleneck. Herein, we report that the bottleneck can be well tackled by refining the defect states in photocatalysts via doping. As a proof of concept, W₁₈O₄₉ ultrathin nanowires are employed as a model material for subtle Mo doping, in which the coordinatively unsaturated (CUS) metal atoms with oxygen defects serve as the sites for N₂ chemisorption and electron transfer. The doped low-valence Mo species play multiple roles in facilitating N₂ activation and dissociation by refining the defect states of W₁₈O₄₉: (1) polarizing the chemisorbed N₂ molecules and facilitating the electron transfer from CUS sites to N₂ adsorbates, which enables the N≡N bond to be more feasible for dissociation through proton coupling; (2) elevating defect-band center toward the Fermi level, which preserves the energy of photoexcited electrons for N₂ reduction. As a result, the 1 mol % Mo-doped W₁₈O₄₉ sample achieves an ammonia production rate of 195.5 μmol g_cat^-1 h^-1, 7-fold higher than that of pristine W₁₈O₄₉. In pure water, the catalyst demonstrates an apparent quantum efficiency of 0.33% at 400 nm and a solar-to-ammonia efficiency of 0.028% under simulated AM 1.5 G light irradiation. This work provides fresh insights into the design of photocatalyst lattice for N₂ fixation and reaffirms the versatility of subtle electronic structure modulation in tuning catalytic activity.

n-Type Cu2O/α-Fe2O3 Heterojunctions by Electrochemical Deposition: Tuning of Cu2O Thickness for Maximum Photoelectrochemical Performance

Here, we report the enhanced photoelectrochemical performance of surface modified hematite thin films with n-type copper oxide nanostructures (Cu2O/Fe2O3) obtained through simple electrochemical deposition method. The thickness and amount of cuprous oxide layer were varied by simply changing the number of electrodeposition cycles (viz. 5, 10, 25, 50 and 100) in order to understand its thermodynamic and kinetic influence on the photoelectrochemical activity of the resultant nano-heterostructures. Structural and morphological characteristics of the obtained Cu2O/Fe2O3 films have been studied by absorption spectroscopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analysis. Electrochemical investigations such as linear sweep voltammetry, Mott–Schottky analysis, and electrochemical impedance spectroscopy suggested the formation of n-type Cu2O layers over the hematite films with varying charge-carrier densities, ranging from 0.56×1019 to 3.94×1019 cm−3, that varies with the number of cycles of electrochemical deposition. Besides, the thickness of deposited cuprous oxide layer is noted to alter the net electrochemical and photo-electrochemical response of the base material. An interesting, peak event was recorded for a particular thickness of the cuprous oxide layer (obtained after 25 cycles of electrochemical deposition) below and above which the efficiency of catalyst was impaired. The heterojunction obtained thus, followed well known Z-scheme and gave appreciable increment in the photocurrent response.

Black Si-doped TiO2 nanotube photoanode for high-efficiency photoelectrochemical water splitting

Black Si-doped TiO₂ (Ti–Si–O) nanotubes were fabricated through Zn metal reduction of the Ti–Si–O nanotubes on Ti–Si alloy in an argon atmosphere. The nanotubes were used as a photoanode for photoelectrochemical (PEC) water splitting. Both Si element and Ti³⁺/oxygen vacancies were introduced into the black Ti–Si–O nanotubes, which improved optical absorption and facilitated the separation of the photogenerated electron–hole pairs. The photoconversion efficiency could reach 1.22%, which was 7.18 times the efficiency of undoped TiO₂. It demonstrated that a Si element and Ti³⁺/oxygen vacancy co-doping strategy could offer an effective method for fabricating a high-performance TiO₂-based nanostructure photoanode for improving PEC water splitting.

Black Si-doped TiO₂ (Ti–Si–O) nanotubes were fabricated through Zn metal reduction of the Ti–Si–O nanotubes on Ti–Si alloy in an argon atmosphere.

Ultrafast NH3 Sensing Properties of WO3@CoWO4 Heterojunction Nanofibres at Room Temperature

Highly selective detection, quick response times (<5 s), and superior response (|Rn – Ra|/Ra = 1.17) to NH3 gas, particularly at room temperature (RT), are still enormous challenges in gas sensor applications. In this paper, a rational design and facile synthesis for a NH3 sensor have been proposed. Massage ball-like WO3@CoWO4 (Co-W) nanofibres (NFs) were prepared by a facile one-step synthesis utilising an electrospinning approach, followed by appropriate calcination. A Co-W NF sensor with a Co-to-W atomic ratio of 3 : 10 (Co-W-3), which consisted of nano-sized WO3 protrusions (10–15 nm) on submicrometre-sized single crystal CoWO4 particles (100–150 nm) exhibited excellent gas-sensing properties at RT due to the single crystal CoWO4–CoWO4 homojunction structure and distinct massage ball-like WO3–CoWO4 heterojunction. The approach developed in this work will be important for the low-cost and large-scale production of a Co-W-3 ultrafast sensing material with highly promising applications in gas sensors.

Gold Nanoparticles Decorated Hematite Photoelectrode for Sensitive and Selective Photoelectrochemical Aptasensing of Lysozyme

Photoelectrochemical aptasensor (PECAS) is a new and promising detection platform with both high sensitivity and good selectivity. Exploration of new photoelectrode materials and establishment of effective charge transfer channel between photoelectrode and aptamer are the main challenges in this field. In this work, an efficient PECAS based on Au nanoparticles (NPs) decorated Fe₂O₃ nanorod photoelectrode is rationally designed, fabricated, and exhibited excellent sensitivity and selectivity for detection of lysozyme (Lys) with an ultralow detection limit of 3 pM and wide detection range from 10 pM to 100 nM. The Au NPs not only act as anchor to establish an efficient charge transfer channel between the photoelectrode and the aptamer, but also help to enhance the PEC performance through adjusting the carrier density of Fe₂O₃. The rationally designed photoelectrode opens up a distinctive avenue for promoting the PECAS to be a versatile analysis method.

Three-Dimensional WO3 Nanoplate/Bi2S3 Nanorod Heterojunction as a Highly Efficient Photoanode for Improved Photoelectrochemical Water Splitting

The rational design of semiconductor photoanodes with sufficient light absorption, efficient photogenerated carrier separation, and fast charge transport is crucial for photoelectrochemical (PEC) water splitting. Incorporating a small-band-gap semiconductor to a large-band-gap material with matched energy band position is a promising route to improve the light harvesting and charge transport. Herein, we report the fabrication of a three-dimensional heterojunction with uniform Bi₂S₃ nanorods on WO₃ nanoplates by hydrothermal process and chemical bath deposition. The seed layer strategy was used to assist the growth of Bi₂S₃ nanorods for perfect interface contact between WO₃ and Bi₂S₃. The as-prepared WO₃/Bi₂S₃ composite exhibited a much enhanced photocurrent (5.95 mA/cm² at 0.9 V vs reversible hydrogen electrode), which is 35 and 1.4 times higher than those of pristine WO₃ and WO₃/Bi₂S₃ composite without a seed layer, respectively. In addition, higher incident photon-to-current conversion efficiency (68.8%) and photoconversion efficiency (1.70%) were achieved. The enhancement mechanism was investigated in detail, and the sufficient light absorption, efficient charge transport, and high carrier density simultaneously contribute to the improved PEC activity. These findings will open up new opportunities to develop other highly efficient heterostructures as photoelectrodes for PEC applications.