Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting

Boosting Charge Transport in BiVO4 Photoanode for Solar Water Oxidation

The ability to regulate charge separation is pivotal for obtaining high efficiency of any photoelectrode used for solar fuel production. Vacancy engineering for metal oxide semiconductor photoelectrode is a major strategy but has faced a formidable challenge in bulk charge transport because of the elusive charge self-trapping site. In this work, a new deep eutectic solvent to engineer bismuth vacancies (Bi_vac ) of BiVO₄ photoanode is reported; the novel Bi_vac can remarkably increase the charge diffusion coefficient by 5.8 times (from 1.82 × 10^-7 to 1.06 × 10^-6 cm² s^-1 ), which boosts the charge transport efficiency. Through further loading CoBi cocatalyst to enhance charge transfer efficiency, the photocurrent density of BiVO₄ photoanode with optimal Bi_vac concentration reaches 4.5 mA cm^-2 at 1.23 V vs reversible hydrogen electrode under AM 1.5 G illumination, which is higher than that of previously reported O_vac engineered BiVO₄ photoanode where the BiVO₄ photoanode is synthesized by a similar procedure. This work perfects a cation defect engineering that enables the potential capability to equate the charge transport properties in different types of semiconductor materials for solar fuel conversion.

Enhanced surface kinetics and charge transfer of BiVO4 photoanode by Rh2O3 cocatalyst loading for boosted solar water oxidation

The Rh 2 O 3 /BiVO 4 composite photoanode exhibits enhanced surface reaction kinetics and charge transfer efficiency, enabling a photocurrent density of ca. 3.5 mA/cm 2 at 1.23 V (vs. RHE), which is about 3.89 times higher than that of the pristine BiVO 4 , with a lower onset potential of 0.29 V (vs. RHE).

Controlled Synthesis and Structural Modulation to Boost Intrinsic Photocatalytic Activity of BiVO4

Modulation of the structure, composition, and morphology through a simple refinement of the synthetic parameters are central to obtaining signature catalytic properties, enabling an understanding of the underlying growth and...

Temperature Effect on Photoelectrochemical Water Splitting: A Model Study Based on BiVO4 Photoanodes

Photoelectrochemical (PEC) water splitting is typically studied at room temperature. In this work, the temperature effect on PEC water splitting is studied using crystalline BiVO₄ thin film photoanode as a model system. Systematic temperature-dependent electrochemical study demonstrates that the PEC activity is boosted at elevated electrolyte temperatures and indicates that thermal energy plays a main role in improving charge carrier transport in the bulk of BiVO₄. Irreversible surface reconstruction is observed after PEC reactions at elevated temperature in the presence of hole scavengers, with regularly spaced stripes emerging on BiVO₄ grains. The surface-reconstructed photoanode exhibits up to 40% improvement in photocurrent densities and ∼ 0.25 V shift of photocurrent onset to favorable direction. Detailed investigation shows the formation of an amorphous layer without stoichiometric change at the reconstructed surface. This work provides insights of the temperature effect on the photoelectrode in solar water splitting and reveals the non-negligible effect of hole scavengers in photoelectrochemical measurement.

Charge-Compensated Doping Extends Carrier Lifetimes in SrTiO3 by Passivating Oxygen Vacancy Defects

Experiments reported that oxygen vacancies shorten the charge carrier lifetime of SrTiO₃ but that it is greatly improved upon Al and Na doping. Using nonadiabatic (NA) molecular dynamics, we demonstrate that the in-gap hole trap state created by an oxygen vacancy can be eliminated by charge-compensated doping when two Ti⁴⁺ ions or two Sr²⁺ ions are equally replaced by Al³⁺ or Na⁺ ions. Nevertheless, Al³⁺ and Na⁺ reduce the strength of NA coupling to a different extent, resulting in increased charge carrier lifetimes of 4.6 and 1.3 ns. The lifetimes are several times longer than that of the pristine system and 3 orders of magnitude longer than that of defective SrTiO₃, which is within 50 ps due to strong NA coupling. The weakly correlated electron and hole wave functions in doped systems accelerate decoherence, further delaying charge recombination. Our study rationalizes the complex charge-phonon dynamics in SrTiO₃ and proposes charge-compensated doping for the design of advanced visible-light photocatalysts.

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.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

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.

Phase-Controllable Growth Nix Py Modified CdS@Ni3 S2 Electrodes for Efficient Electrocatalytic and Enhanced Photoassisted Electrocatalytic Overall Water Splitting

The rational design and construction of cost-effective nickel-based phosphide or sulfide (photo)electrocatalysts for hydrogen production from water splitting has sparked a huge investigation surge in recent years. Whereas, nickel phosphides (Ni_x P_y ) possess more than ten stoichiometric compositions with different crystalline. Constructing Ni_x P_y with well crystalline and revealing their intrinsic catalytic mechanism at atomic/molecular levels remains a great challenge. Herein, an easy-to-follow phase-controllable phosphating strategy is first proposed to prepare well crystalline Ni_x P_y (Ni₃ P and Ni₁₂ P₅ ) modified CdS@Ni₃ S₂ heterojunction electrocatalysts. It is found that Ni₃ P modified CdS@Ni₃ S₂ (CdS@Ni₃ S₂ /Ni₃ P) exhibits remarkable stability and bifunctional electrocatalytic activities in both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory results suggest that P-Ni sites and P sites in CdS@Ni₃ S₂ /Ni₃ P, respectively, serve as OER and HER active sites during electrocatalytic water splitting processes. Moreover, benefiting from the advantageous photocatalyst@electrocatalyst core@shell structure, CdS@Ni₃ S₂ /Ni₃ P delivers an advantaged photoassisted electrocatalytic water splitting property. The champion electrical to hydrogen and solar to hydrogen energy conversion efficiencies of CdS@Ni₃ S₂ /Ni₃ P, respectively, reach 93.35% and 4.65%. This work will provide a general guidance for synergistically using solar energy and electric energy for large-scale H₂ production from water splitting.

Integrating Computation and Experiment to Investigate Photoelectrodes for Solar Water Splitting at the Microscopic Scale

ConspectusPhotoelectrochemical water-splitting is a promising and sustainable way to store the energy of the sun in chemical bonds and use it to produce hydrogen gas, a clean fuel. The key components in photoelectrochemical cells (PECs) are photoelectrodes, including a photocathode that reduces water to hydrogen gas and a photoanode that oxidizes water to oxygen gas. Materials used in photoelectrodes for PECs must effectively absorb sunlight, yield photogenerated carriers, and exhibit electronic properties that enable the efficient shuttling of carriers to the surface to participate in relevant water-splitting reactions. Discovering and understanding the key characteristics of optimal photoelectrode materials is paramount to the realization of PEC technologies.Oxide-based photoelectrodes can satisfy many of these materials requirements, including stability in aqueous environments, band edges with reasonable alignment with the redox potentials for water splitting, and ease of synthesis. However, oxide photoelectrodes generally suffer from poor charge transport properties and considerable bulk electron-hole separation, and they have relatively large band gaps. Numerous strategies have been proposed to improve these aspects and understand how these improvements are reflected in the photoelectrochemical performance. Unfortunately, the structural and compositional complexity of multinary oxides accompanied by the inherent complexity of photoelectrochemical processes makes it challenging to understand the individual effects of composition, structure, and defects in the bulk and on the surface on a material's photoelectrochemical properties. The integration of experiment and theory has great potential to increase our atomic-level understanding of structure-composition-property relationships in oxide photoelectrodes.In this Account, we describe how integrating experiment and theory is beneficial for achieving scientific insights at the microscopic scale. We highlight studies focused on understanding the role of (i) bulk composition via solid-state solutions, intercalation, and comparison with isoelectronic compounds, (ii) dopants for both the anion and cation and their interactions with oxygen vacancies, and (iii) surface/interface structure in the photocurrent generation and photoelectrochemical performance in oxide photoelectrodes. In each instance, we outline strategies and considerations for integrating experiment and theory and describe how this integration led to valuable insights and new directions in uncovering structure-composition-property relationships. Our aim is to demonstrate the unique value of combining experiment and theory in studying photoelectrodes and to encourage the continued effort to bring experiment and theory in closer step with each other.

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.

Interfacial Bridging Strategy for Charge Extraction/Injection in the BiVO4/CoSn-Layered Double Hydroxide p-n Heterojunction Approach Using Graphene Quantum Dots for Enhanced Water Oxidation Kinetics

The design of a photoanode with a bridging strategy that can enhance the charge injection and transport in a heterojunction can be an efficient approach to separate the photogenerated charge carriers and enhance the water oxidation kinetics. Aiming at such issues, herein we propose a BiVO₄/GQDs/CoSn-LDH (layered double hydroxide) photoanode, which leads to the formation of a p-n heterojunction with bridged graphene quantum dots (GQDs) to accelerate the photoelectrochemical (PEC) performance. The BiVO₄/GQDs/CoSn-LDH photoanode exhibits a maximum photocurrent density of 4.15 mA/cm², which is ∼3-fold higher than for the pristine BiVO₄ photoanode with an ∼250 mV cathodic shift in the onset potential. A faradaic yield of ∼91% confirms that the obtained photocurrent is mainly due to water oxidation. A mechanistic study based on the electrochemical impedance (EIS), charge separation, and charge injection efficacy measurements reveals that the introduction of GQDs between BiVO₄ and CoSn-LDH provides a continuous conducting network to extract holes from the BiVO₄ surface and efficiently inject into the CoSn-LDH surface for the water oxidation reaction.

Rational Design of Semiconductor Heterojunctions for Photocatalysis

Electronic structure calculations provide a useful complement to experimental characterization tools in the atomic-scale design of semiconductor heterojunctions for photocatalysis. The band alignment of the heterojunction is of fundamental importance to achieve an efficient charge carrier separation, so as to reduce electron/hole recombination and improve photoactivity. The accurate prediction of the offsets of valence and conduction bands in the constituent units is thus of key importance but poses several methodological and practical problems. In this Minireview we address some of these problems by considering selected examples of binary and ternary semiconductor heterojunctions and how these are determined at the level of density functional theory (DFT). The atomically precise description of the interface, the consequent charge polarization, the role of quantum confinement, the possibility to use facet engineering to determine a specific band alignment, are among the effects discussed, with particular attention to pros and cons of each one of these aspects. This analysis shows the increasingly important role of accurate electronic structure calculations to drive the design and the preparation of new interfaces with desired properties.

Boosting photoelectrochemical efficiency by near-infrared-active lattice-matched morphological heterojunctions

Photoelectrochemical catalysis is an attractive way to provide direct hydrogen production from solar energy. However, solar conversion efficiencies are hindered by the fact that light harvesting has so far been of limited efficiency in the near-infrared region as compared to that in the visible and ultraviolet regions. Here we introduce near-infrared-active photoanodes that feature lattice-matched morphological hetero-nanostructures, a strategy that improves energy conversion efficiency by increasing light-harvesting spectral range and charge separation efficiency simultaneously. Specifically, we demonstrate a near-infrared-active morphological heterojunction comprised of BiSeTe ternary alloy nanotubes and ultrathin nanosheets. The heterojunction's hierarchical nanostructure separates charges at the lattice-matched interface of the two morphological components, preventing further carrier recombination. As a result, the photoanodes achieve an incident photon-to-current conversion efficiency of 36% at 800 nm in an electrolyte solution containing hole scavengers without a co-catalyst.

Lessons learned from first-principles calculations of transition metal oxides

Transition metal oxides (TMOs) are an important class of materials with diverse applications, ranging from memristors to photoelectrochemical cells. First-principles calculations are critical for understanding these complex materials at an atomic level and establishing relationships between atomic and electronic structures, particularly for probing quantities difficult or inaccessible to experiment. Here, we discuss computational strategies used to understand TMOs by focusing on two examples, a photoanode material, BiVO₄, and an oxide for low-power electronics, La_1-xSr_xCoO₃. We highlight key aspects required for the modeling of TMOs, namely, the descriptions of how oxygen vacancies, extrinsic doping, the magnetic state, and polaron formation impact their electronic and atomic structures and, consequently, many of the observed properties.

Exciton-Phonon Interactions in Monolayer Germanium Selenide from First Principles

We investigate from first principles exciton-phonon interactions in monolayer germanium selenide, a direct gap two-dimensional semiconductor. By combining the Bethe-Salpeter approach and the special displacement method, we explore the phonon-induced renormalization of the exciton wave functions, excitation energies, and oscillator strengths. We determine a renormalization of the optical gap of 0.1 eV at room temperature, which results from the coupling of the exciton with both acoustic and optical phonons, with the strongest coupling to optical phonons at ∼100 cm^-1. We also find that the exciton-phonon interaction is similar between monolayer and bulk GeSe. Overall, we demonstrate that the combination of many-body perturbation theory and special displacements offers a new route to investigate electron-phonon couplings in excitonic spectra, the resulting band gap renormalization, and the nature of phonons that couple to the exciton.

Controlling Charge Carrier Trapping and Recombination in BiVO4 with the Oxygen Vacancy Oxidation State

The lack of an in-depth understanding of the intrinsic oxygen vacancy (O_V) defect properties in the photoanode BiVO₄ limits the further improvement of its photoelectrochemical water splitting performance. To address this issue, nonadiabatic molecular dynamics simulations are performed to study the impact of O_V on charge carrier lifetimes in BiVO₄. The simulations show that a neutral O_V gives rise to local structural distortions due to the formation of V-O-V bonds, forcing the electrons trapped on the nearer of the two V atoms to form two deep polaron-like V⁴⁺ hole traps. These localized midgap states greatly accelerate nonradiative electron-hole recombination compared to that of pristine BiVO₄, reaching a time scale of several nanoseconds in good agreement with experiments. The ionized O_V state restores the bandgap to its value in pristine BiVO₄ and restores the charge carrier lifetimes due to the fast loss of coherence time. Our study reveals the mechanism of the detrimental role of O_V in BiVO₄ and provides valuable insights for improving the performance of the BiVO₄ photoanode by ionizing the oxygen vacancy.

Suppressing Water Dissociation via Control of Intrinsic Oxygen Defects for Awakening Solar H2 O-to-H2 O2 Generation

BiVO₄ theoretically has a thermodynamic activity trend toward highly selective water oxidative H₂ O₂ formation, but it is more inclined to generate O₂ in practical. The influence of intrinsic oxygen vacancy (O_vac ), especially, on surface reactivity, has never been considered as a possible activity loss mechanism in the synthetic BiVO₄ . In this work, it is theoretically and experimentally demonstrated that the intrinsic surface O_vac is responsible for lower H₂ O₂ evolution activity via promoting water dissociation to form intermediate. Through an annealing process under a V₂ O₅ rich atmosphere, the surface O_vac can be eliminated that awakens the photoelectrochemical (PEC) water oxidative H₂ O₂ activity in a NaHCO₃ electrolyte, which achieves an average of 58.4%, and increases by up to 4.28 times of the one annealed in air. This work offers a general understanding of catalytic activity loss and may be extended to other photo or electrocatalysts for catalytic selectivity regulation.

General and Robust Photothermal-Heating-Enabled High-Efficiency Photoelectrochemical Water Splitting

The ability of photoanodes to simultaneously tailor light absorption, charge separation, and water oxidation processes represents an important endeavor toward highly efficient photoelectrochemical (PEC) water splitting. Here, a robust strategy is reported to render markedly improved PEC water splitting via sandwiching a photothermal Co₃ O₄ layer between a BiVO₄ photoanode film and an FeOOH/NiOOH electrocatalyst sheet. The deposited Co₃ O₄ layer manifests compelling photothermal effect upon near-infrared irradiation and raises the temperature of the photoanodes in situ, leading to extended light absorption, enhanced charge transfer, and accelerated water oxidation kinetics simultaneously. The judiciously designed NiOOH/FeOOH/Co₃ O₄ /BiVO₄ photoanode renders a superior photocurrent density of 6.34 mA cm^-2 at 1.23 V versus a reversible reference electrode (V_RHE ) with outstanding applied bias photon-to-current efficiency of 2.72% at 0.6 V_RHE . In addition to the metal oxide, a wide variety of metal sulfides, nitrides, and phosphides (e.g., CoS, CoN, and CoP) can be exploited as the heaters to yield high-performance BiVO₄ -based photoanodes. Apart from BiVO₄ , other metal oxides (e.g., Fe₂ O₃ and TiO₂ ) can also be covered by photothermal materials to impart significantly promoted water splitting. This simple yet general strategy provides a unique platform to capitalize on their photothermal characteristics to engineer high-performing energy conversion and storage materials and devices.

Hierarchical Nanoporous BiVO4 Photoanodes with High Charge Separation and Transport Efficiency for Water Oxidation

To fabricate high efficiency photoanodes for water oxidation, it is highly required to engineer their nanoporous architecture and interface to improve the charge separation and transport efficiency. By focusing on this aspect, we developed hierarchical nanoporous BiVO₄ (BV) from solution processed two-dimensional BiOI (BI) crystals. The orientation of the BI crystals was controlled by changing the solvent volume ratios of ethylene glycol (EG) to ethanol (ET), which resulted in different hierarchical and planar BV morphologies through a chemical treatment followed by thermal heating. The morphology with optimal particle dimension, connectivity, and porosity can offer a highly enhanced electrochemically active surface area (ECSA). The hierarchical BV owning a maximum ECSA showed the best photoelectrochemical (PEC) performance in terms of the highest photocurrent density and charge separation efficiency. However, to further improve the performance of the electrode, conformal and ultrathin SnO₂ underlayers were deposited by a powerful atomic layer deposition technique at the interface to effectively block the defect density, which significantly improved the photocurrents as high as 3.25 mA/cm² for sulfite oxidation and 2.55 mA/cm² for water oxidation at 0.6 V versus the reversible hydrogen electrode (RHE). The electrode possessed record charge separation efficiency of 97.1% and charge transfer efficiency of 90.1% at 1.23 V_RHE among to-date reported BiVO₄-based photoanodes for water oxidation. Furthermore, a maximum applied bias photon-to-current efficiency (ABPE) of 1.61% was found at a potential as low as 0.6 V_RHE, which is highly promising to make a tandem cell. These results indicate that the construction of the hierarchical nanoporous photoanode with an enhanced ECSA and its proper interface engineering can significantly improve the PEC performance.

Role of surface termination in forming type-II photocatalyst heterojunctions: the case of TiO2/BiVO4

In this work we investigate TiO₂ and BiVO₄ nanostructures by means of density functional theory (DFT) calculations, to provide an estimate of the band alignment in TiO₂/BiVO₄ interfaces, highly active in photo-electrochemistry and photocatalytic water splitting. Calculations were carried out with both DFT range separated and self-consistent dielectric dependent hybrid functionals (HSE06 and PBE0_DD). The impact of systems' size has been investigated. The converged electronic levels of TiO₂ and BiVO₄ surfaces have been used to predict the band alignment in TiO₂/BiVO₄ heterostructures. Results indicated that when TiO₂ (101) surface is matched with BiVO₄ (110), a type-II alignment is obtained, where the band edges of BiVO₄ are higher in energy that those of TiO₂. This picture is favorable for charge-carriers separation upon photoexcitation, where electrons move toward TiO₂ and holes toward BiVO₄. On the contrary, if TiO₂ (101) is interfaced to BiVO₄ (010) the offset between the band edges is negligible, thus reducing the driving force toward separation of charge carriers. These results rationalize the dependence on the facet's exposure of the observed photocatalytic performances of TiO₂/BiVO₄ composites, where the TiO₂ (101)/BiVO₄ (110) interface outperforms the TiO₂ (101)/BiVO₄ (010) one.

Energy Transfer-Induced Photoelectrochemical Improvement from Porous Zeolitic Imidazolate Framework-Decorated BiVO4 Photoelectrodes

BiVO₄ , which is a representative photoanode material for photoelectrochemical water splitting, intrinsically restricts high conversion efficiency, owing to faster recombination, low electron mobility, and short electron diffusion length. While the photocurrent density of typical BiVO₄ corresponds to only 21.3% of the maximum photocurrent density (4.68 mA cm^-2 ), decoration of the BiVO₄ photoanode with zeolitic imidazolate framework-67 (ZIF-67) exhibits a synergetic effect to raise the overall photocatalytic ability at the BiVO₄ surface region to a higher level via the energy-transfer process from BiVO₄ to ZIF-67. The hybrid ZIF-67/BiVO₄ photoanode follows two convenient photoelectrochemical pathways: 1) energy-transfer-induced water oxidation reaction in ZIF-67 and 2) water oxidation reaction by direct contact between the BiVO₄ surface and electrolytes. Compared to the moderate photocurrent density (≈1 mA cm^-2 ) of single-layer BiVO₄ , the proposed ZIF-67/BiVO₄ photoanodes show a remarkably high photocurrent (2.25 mA cm^-2 ) with high stability, despite the lack of hole scavengers in the electrolyte. Furthermore, the absorbed photon-to-current efficiency of the ZIF-67/BiVO₄ photoanode is ≈2.5 times greater than that of BiVO₄ . This work proposes a promising solution for efficient water oxidation that overcomes the intrinsic material limitations of BiVO₄ photoelectrodes by using energy transfer-induced photon recycling and the decoration of porous ZIFs.

A Cobalt@Cucurbit[5]uril Complex as a Highly Efficient Supramolecular Catalyst for Electrochemical and Photoelectrochemical Water Splitting

A host-guest complex self-assembled through Co2+ and cucurbit[5]uril (Co@CB[5]) is used as a supramolecular catalyst on the surface of metal oxides including porous indium tin oxide (ITO) and porous BiVO4 for efficient electrochemical and photoelectrochemical water oxidation. When immobilized on ITO, Co@CB[5] exhibited a turnover frequency (TOF) of 9.9 s-1 at overpotential η=550 mV in a pH 9.2 borate buffer. Meanwhile, when Co@CB[5] complex was immobilized onto the surface of BiVO4 semiconductor, the assembled Co@CB[5]/BiVO4 photoanode exhibited a low onset potential of 0.15 V (vs. RHE) and a high photocurrent of 4.8 mA cm-2 at 1.23 V (vs. RHE) under 100 mW cm-2 (AM 1.5) light illumination. Kinetic studies confirmed that Co@CB[5] acts as a supramolecular water oxidation catalyst, and can effectively accelerate interfacial charge transfer between BiVO4 and electrolyte. Surface charge recombination of BiVO4 can be also significantly suppressed by Co@CB[5].

BiNV bond: A hole-transfer bridge for high-efficient separation and transfer of carriers

Addressing the inherent holes transport limitation of BiVO₄ photoanode is crucial to achieve efficient photoelectrochemical (PEC) water splitting. The construction of the hole-transfer bridge between co-catalysts and BiVO₄ photoanode could be an effective way to overcome sluggish hole-transfer kinetics of BiVO₄ photoanode. Herein, C_xN_y/BiVO₄ photoanode was prepared by coupling carbon nitride hydrogel (CNH) containing unsaturated N on the BiVO₄ photoanode during annealing. C_xN_y/BiVO₄ photoanode exhibited excellent PEC performance and stability. Photoelectrochemical tests proved that the coupling of C_xN_y accelerated holes transfer and enhanced oxygen evolution kinetics. X-ray photoelectron spectroscopy (XPS) and theoretical calculations confirmed the existence of the BiNV bond between BiVO₄ photoanode and C_xN_y, which could serve as the hole-transfer bridge to significantly accelerate separation and transfer of carriers driven by the interfacial electric field. Moreover, it was found that the coupling of C_xN_y effectively inhibited the dissociation of metal ions through changing their coordination environment, resulting in the excellent stability of C_xN_y/BiVO₄ photoanode. This result provides unique insights into vital roles of the interfacial structure, which might have a significant impact on the construction of PEC devices.

A nano-surface monocrystalline BiVO4 nanoplate photoanode for enhanced photoelectrochemical performance

A nano-surface monocrystalline BiVO₄ photoanode with a large surface area is prepared by a low-cost and simple etching process.

Role of Alkali Metal in BiVO4 Crystal Structure for Enhancing Charge Separation and Diffusion Length for Photoelectrochemical Water Splitting

Alkali metal (Na or K) doping in BiVO₄ was examined systematically for enhancing bulk charge separation and transport in addition to improving charge transfer from the surface. The alkali metal-doped BiVO₄ thin film photoanodes having nanostructured porous grain surface morphology exhibited better photocurrent density than pristine BiVO₄. In particular, Na:BiVO₄/Fe:Ni/Co-Pi photoanode showed a significantly improved photocurrent of 3.2 ± 0.15 mA·cm^-2 in 0.1 M K₂HPO₄ electrolyte at 1.23 V_RHE under 1 sun illumination. The depth-dependent Doppler broadening spectroscopy measurements confirmed the significant reduction in Bi- and V-based defect density with Na metal doping, and this led to a higher bulk diffusion length of charge pairs (four times that of the pristine one). Na doping led to reduced surface defects resulting in improved surface charge transfer based on cyclic voltammetry experiments. The density functional theory calculations confirmed the improved performance in Na-doped BiVO₄ photoanodes achieved through interband formation and reduction in the band gap.

Synthesis of SrTiO3 submicron cubes with simultaneous and competitive photocatalytic activity for H2O splitting and CO2 reduction

Single crystalline strontium titanate (SrTiO3) submicron cubes have been synthesized based on a molten salt method. The submicron cubes showed superior photocatalytic activity towards both water splitting and carbon dioxide reduction, in which methane (CH4) and hydrogen (H2) were simultaneously produced. The average production rate of methane up to 8 h is 4.39 μmol g-1 h-1 but drops to 0.46 μmol g-1 h-1. However, the average production rate of hydrogen is 14.52 before 8 h but then increases to 120.23 μmol g-1 h-1 after 8 h. The rate change of the two processes confirms the competition between the H2O splitting and CO2 reduction reactions. Band structure and surface characteristics of the SrTiO3 submicron cubes were characterized by diffuse reflective UV-Vis spectroscopy, Mott-Schottky analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The results reveal that the simultaneous and competitive production of methane and hydrogen is due to a thermodynamics factor, as well as the competition between the adsorption of carbon dioxide and water molecules on the surface of the faceted SrTiO3. This work demonstrates that SrTiO3 photocatalysts are efficient in producing sustainable fuels via water splitting and carbon dioxide reduction reactions.

Effects of native defects and cerium impurity on the monoclinic BiVO4 photocatalyst obtained via PBE+U calculations

In this article, we report a periodic density functional theory (DFT) investigation on the formation of the native defects and cerium doping in monoclinic BiVO₄ (m-BiVO₄) and their effect on the electronic structures, using the Perdew-Burke-Ernzerhof functionals corrected for on-site Coulombic interactions (PBE+U). From the point defect formation energies and transition levels, the Bi_vac (Bi vacancy), V_vac (V vacancy), O_int (O interstitial) and Ce_V (Ce doping on V site) defects in m-BiVO₄ are identified as shallow acceptors. For Ce doping in m-BiVO₄, the substitution of Bi by Ce is energetically favorable in the single positively charged state (Ce) under Bi/V-poor conditions, while the substitution of V by Ce is in the single negatively charged state (Ce) under O-rich conditions. The calculated electronic structures suggest that Ce degrades the activity by an unoccupied deep level in the gap region, mainly composed of Ce 4f orbitals, which makes this defect as the photogenerated electron-hole recombination center, in good agreement with the experimental results. For Ce, no localized state exists within the calculated band gap. Its formation energy is sensitive to the chemical potentials and Fermi energy, suggesting that the Bi/V-poor and O-rich conditions are desirable to eliminate the deep-level states and improve photocatalysis. Our results provide insights into enhancing the photocatalytic activity of m-BiVO₄ for energy and environmental applications through the rational design of defect-controlled synthesis conditions.

The role of oxygen defects in metal oxides for CO2 reduction

The abuse of fossil fuels release large amount of CO2, causing intense global warming. Using photoreduction and electroreduction to convert CO2 into highly valuable fuels such as CO and CH4 can effectively solve this problem. However, due to the limited activity and selectivity, pristine catalyst materials cannot meet the requirements of practical applications, which means that some modifications to these catalysts are necessary. In this review, a series of research reports on oxygen defect engineering have been introduced. First, the methods of preparing oxygen defects by heat treatment, doping, and photoinduction combined with influencing factors in the preparation are introduced. Subsequently, common characterization methods of oxygen defects including EPR, Raman, XPS, EXAFS, and HRTEM are summarized. Finally, the mechanisms of introducing oxygen defects to improve CO2 reduction are discussed, and include enhancing light absorption, improving CO2 adsorption and activation, as well as promoting stability of the reaction intermediates. The summary of research on oxygen defects provides guidance for researchers who focus on CO2 reduction and accelerate the realization of its industrial applications in the future.

The Fast-Track Water Oxidation Channel on BiVO4 Opened by Nitrogen Treatment

BiVO₄ is one of the most promising photoanode materials for water-splitting systems. Nitrogen incorporation into a BiVO₄ surface overcomes the known bottleneck in its charge-transfer kinetics into the electrolyte. We explored the role of nitrogen in the surface charge recombination and charge-transfer kinetics by employing transient photocurrent spectroscopy at the time scale of surface recombination and water oxidation kinetics, transient absorption spectroscopy, and X-ray photoelectron spectroscopy. We attributed the activity enhancement mechanism to the accelerated V⁵⁺/V⁴⁺ redox process, in which incorporated nitrogen suppresses a limiting surface recombination channel by increasing the oxygen vacancies.

Metallic Woodpile Nanostructures for Femtomolar Sensing of Alzheimer's Neurofilament Lights

Alzheimer's disease (AD), the most common age-related neurodegenerative disorder, accompanies a massive degradation of neurons including axonal injury. Being an axonal neuron-specific protein, neurofilament light (NfL) is a blood biomarker that reflects the neurodegeneration in AD, but no attempt has been made yet to develop sensing platforms that target NfLs in blood serum or plasma. Here, we report three-dimensional cross-stacked Pt nanowire arrays for the ultrasensitive photoelectrochemical (PEC) detection of NfLs. We constructed a woodpile-like Pt nanowire array (PtWP)-based biocathode by printing multilayer Pt nanowire arrays in an orthogonal configuration and conjugating them with NfL-specific DA2 antibodies. According to our collective electrochemical analyses, the five-layered PtWP electrode modified with DA2 antibodies exhibited high oxygen reduction activities due to the large electrochemical active surface area and the effective electron transfer properties. We have combined the DA2-PtWP biocathode with a water-oxidizing, iron oxyhydroxide-deposited bismuth vanadate (FeOOH/BiVO₄) photoanode to assemble a bias-free PEC detection system. Powered by a white-light-emitting diode, the unbiased PEC platform accurately recognizes NfLs in blood plasma with the limit-of-detection of 38.2 fg/mL and limit-of-quantification of 853 fg/mL, which is 40 times lower than the NfL levels in AD patients' blood.

Effective Visible Light Exploitation by Copper Molybdo-tungstate Photoanodes

The need for stable oxide-based semiconductors with a narrow band gap, able to maximize the exploitation of the visible light portion of the solar spectrum, is a challenging issue for photoelectrocatalytic (PEC) applications. In the present work, CuW1-x Mo x O4 (E g = 2.0 eV for x = 0.5), which exhibits a significantly reduced optical band gap E g compared with isostructural CuWO4 (E g = 2.3 eV), was investigated as a photoactive material for the preparation of photoanodes. CuW0.5Mo0.5O4 electrodes with different thicknesses (80-530 nm), prepared by a simple solution-based process in the form of multilayer films, effectively exhibit visible light photoactivity up to 650 nm (i.e., extended compared with CuWO4 photoanodes prepared by the same way). Furthermore, the systematic investigation on the effects on photoactivity of the CuW0.5Mo0.5O4 layer thickness evidenced that long-wavelength photons can better be exploited by thicker electrodes. PEC measurements in the presence of NaNO2, acting as a suitable hole scavenger ensuring enhanced photocurrent generation compared with that of water oxidation while minimizing dark currents, allowed us to elucidate the role that molybdenum incorporation plays on the charge separation efficiency in the bulk and on the charge injection efficiency at the photoanode surface. The adopted Mo for W substitution increases the visible light photoactivity of copper tungstate toward improved exploitation and storage of visible light into chemical energy via photoelectrocatalysis.

Promoting Active Electronic States in LaFeO3 Thin-Films Photocathodes via Alkaline-Earth Metal Substitution

The effects of alkaline-earth metal cation (AMC; Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) substitution on the photoelectrochemical properties of phase-pure LaFeO₃ (LFO) thin-films are elucidated by X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance, and electrochemical impedance spectroscopy (EIS). XRD confirms the formation of single-phase cubic LFO thin films with a rather complex dependence on the nature of the AMC and extent of substitution. Interestingly, subtle trends in lattice constant variations observed in XRD are closely correlated with shifts in the binding energies of Fe 2p_3/2 and O 1s orbitals associated with the perovskite lattice. We establish a scaling factor between these two photoemission peaks, unveiling key correlation between Fe oxidation state and Fe-O covalency. Diffuse reflectance shows that optical transitions are little affected by AMC substitution below 10%, which are dominated by a direct bandgap transition close to 2.72 eV. Differential capacitance data obtained from EIS confirm the p-type characteristic of pristine LFO thin-films, revealing the presence of sub-bandgap electronic state (A-states) close to the valence band edge. The density of A-states is decreased upon AMC substitution, while the overall capacitance increases (increase in dopant level) and the apparent flat-band potential shifts toward more positive potentials. This behavior is consistent with the change in the valence band photoemission edge. In addition, capacitance data of cation-substituted films show the emergence of deeper states centered around 0.6 eV above the valence band edge (B-states). Photoelectrochemical responses toward the hydrogen evolution and oxygen reduction reactions in alkaline solutions show a complex dependence on alkaline-earth metal incorporation, reaching incident-photon-to-current conversion efficiency close to 20% in oxygen saturated solutions. We rationalize the photoresponses of the LFO films in terms of the effect sub-bandgap states on majority carrier mobility, charge transfer, and recombination kinetics.

Physical Insights into Band Bending in Pristine and Co-Pi-Modified BiVO4 Photoanodes with Dramatically Enhanced Solar Water Splitting Efficiency

Herein, a novel method is introduced to synthesize 3D hierarchically assembled BiVO₄ nanosheet photoanodes. Despite the fact that the obtained photoanodes inherit the intrinsic properties of 2D and 3D structures, they generate low photocurrent under simulated solar light at 1.0 sun. Upon modification with the cobalt-phosphate (Co-Pi) cocatalyst, the photocurrent is dramatically enhanced from 0.41 to 3.32 mA cm^-2 at 1.23 V_RHE. Charge-transfer kinetic studies by intensity-modulated photocurrent spectroscopy indicated that the low photocurrent response is mainly due to the high density of surface states, which pin the Fermi level and suspend the band bending. The Co-Pi loading passivates these surface states, unpins the Fermi level, and thus resumes the band bending. It also greatly enhances the rate constant of charge transfer and the overall efficiency, evincing that Co-Pi exhibits a dual function (i.e., passivation and catalysis). The current results explicitly disclose the role of the Co-Pi cocatalyst in photoelectrochemical solar water splitting on BiVO₄.

Electrochemical Oxidation of Metal-Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of Fe2TiO5 Photoanodes

A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Because catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type Fe₂TiO₅ photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water-splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in Fe₂TiO₅. Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe₂TiO₅ has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of Fe₂TiO₅. The combined experimental and theoretical results presented in this study greatly improve our understanding of Fe₂TiO₅ for use as a photoanode.

Free-Standing Electrospun W-Doped BiVO4 Porous Nanotubes for the Efficient Photoelectrochemical Water Oxidation

While bismuth vanadate (BiVO₄) has emerged as a promising photoanode in solar water splitting, it still suffers from poor electron-hole separation and electron transport properties. Therefore, the development of BiVO₄ nanomaterials that enable performing high charge transfer rate at the interface and lowering charge recombination is urgent needed. Herein, cobalt borate (Co-B) nanoparticle arrays anchored on electrospun W-doped BiVO₄ porous nanotubes (BiV_0.97W_0.03O₄) was prepared for photoelectrochemical (PEC) water oxidation. One-dimensional, free-standing and porousBiV_0.97W_0.03O₄/Co-B nanotubes was synthesized through electrospun and electrodeposition process. BiV_0.97W_0.03O₄/Co-B arrays exhibit a unique self-supporting core-shell structure with rough porous surface, providing abundant conductive cofactor (W) and electrochemically active sites (Co) exposed to the electrolyte. When applied to PEC water oxidation. BiV_0.97W_0.03O₄/Co-B modified FTO electrode displays high incident photon-to-current conversion efficiency (IPCE) of 33% at 405 nm (at 1.23 V vs. RHE) and its photocurrent density is about 4 times to the pristine nanotube. The higher PEC water oxidation properties of BiV_0.97W_0.03O₄/Co-B porous nanotubes may be attributed to the effectively suppress the electron-hole recombination at electrolyte interface due to its self-supporting core-shell structure, the high electrocatalytic activity of Co and the good electrical conductivity of BiV_0.97W_0.03O₄ arrays. This work offers a simple preparation strategy for the integrated Co-B nanoparticle with BiV_0.97W_0.03O₄ nanotube, demonstrating the synergistic effect of co-catalysts for PEC water oxidation.

Photocatalytic and Photoelectrochemical Systems: Similarities and Differences

Photocatalytic and photoelectrochemical processes are two key systems in harvesting sunlight for energy and environmental applications. As both systems are employing photoactive semiconductors as the major active component, strategies have been formulated to improve the properties of the semiconductors for better performances. However, requirements to yield excellent performances are different in these two distinctive systems. Although there are universal strategies applicable to improve the performance of photoactive semiconductors, similarities and differences exist when the semiconductors are to be used differently. Here, considerations on selected typical factors governing the performances in photocatalytic and photoelectrochemical systems, even though the same type of semiconductor is used, are provided. Understanding of the underlying mechanisms in relation to their photoactivities is of fundamental importance for rational design of high-performing photoactive materials, which may serve as a general guideline for the fabrication of good photocatalysts or photoelectrodes toward sustainable solar fuel generation.

Microwave hydrothermal synthesis and upconversion properties of BiVO4 nanoparticles

Nanomaterials are the subject of extensive investigations due to their applications in medicine, multimodal imaging, volumetric displays, and photonics. Here, lanthanide-doped bismuth vanadate (BiVO₄) upconverting nanoparticles (UCNPs) have been reported. The nanoparticles have been synthesized by a microwave hydrothermal method. As-synthesized nanoparticles are highly crystalline in the tetragonal zircon phase with particles about 200 nm in size. Under 980 nm excitation, intense multicolor visible and near-infrared upconversion emissions are observed. Moreover, broadband infrared downshifting emissions are also observed. Time-resolved emission measurements have been carried out to investigate the involved upconversion and energy transfer mechanism. The BiVO₄-based UCNPs may provide a new class of nanomaterials for multifunctional applications.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar-Driven Water Splitting

Hydrogen (H2) has a significant potential to enable the global energy transition from the current fossil-dominant system to a clean, sustainable, and low-carbon energy system. While presently global H2 production is predominated by fossil-fuel feedstocks, for future widespread utilization it is of paramount importance to produce H2 in a decarbonized manner. To this end, photoelectrochemical (PEC) water splitting has been proposed to be a highly desirable approach with minimal negative impact on the environment. Both semiconductor light-absorbers and hydrogen/oxygen evolution reaction (HER/OER) catalysts are essential components of an efficient PEC cell. It is well documented that loading electrocatalysts on semiconductor photoelectrodes plays significant roles in accelerating the HER/OER kinetics, suppressing surface recombination, reducing overpotentials needed to accomplish HER/OER, and extending the operational lifetime of semiconductors. Herein, how electrocatalyst coupling influences the PEC performance of semiconductor photoelectrodes is outlined. The focus is then placed on the major strategies developed so far for semiconductor/electrocatalyst coupling, including a variety of dry processes and wet chemical approaches. This Review provides a comprehensive account of advanced methodologies adopted for semiconductor/electrocatalyst coupling and can serve as a guideline for the design of efficient and stable semiconductor photoelectrodes for use in water splitting.

Quasi-degenerate states and their dynamics in oxygen deficient reducible metal oxides

The physical and chemical properties of oxides are defined by the presence of oxygen vacancies. Experimentally, non-defective structures are almost impossible to achieve due to synthetic constraints. Therefore, it is crucial to account for vacancies when evaluating the characteristics of these materials. The electronic structure of oxygen-depleted oxides deeply differs from that of the native forms, in particular, of reducible metal oxides, where excess electrons can localize in various distinct positions. In this perspective, we present recent developments from our group describing the complexity of these defective materials that highlight the need for an accurate description of (i) intrinsic vacancies in polar terminations, (ii) multiple geometries and complex electronic structures with several states attainable at typical working conditions, and (iii) the associated dynamics for both vacancy diffusion and the coexistence of more than one electronic structure. All these aspects widen our current understanding of defects in oxides and need to be adequately introduced in emerging high-throughput screening methodologies.