BiVO4 Thin Films Research Articles

Effect of morphology on the optical limiting of BiVO4 films

Precise control over the morphology and crystal structure of semiconductor-based optical materials is essential for optimizing their nonlinear optical properties. Morphological factors significantly affect the material’s interaction with light, influencing nonlinear absorption, harmonic generation, and optical confinement. In this study, the nonlinear optical (NLO) properties and optical limiting (OL) performance of bismuth vanadate (BiVO4) films depending on precursor solution pH condition, powder morphology and weight percentage of the BiVO4 powders in PMMA (10–30 wt.%) were investigated. The BiVO4 powders were synthesized via the hydrothermal method and the effect of precursor solution pH (pH of 1, 7.5 and 12) was investigated on the size and morphology of the BiVO4 powders. BiVO4 films were prepared using the spin coating method utilizing the BiVO4 powders and PMMA. The bandgap values of films were found to change from 3.39 to 3.56 eV depending on the BiVO4 growth solution pH. The nonlinear absorption coefficients (βeff), saturation intensity thresholds (ISAT) and optical limiting (OL) threshold were obtained using the open aperture (OA) Z-scan experiment. It has been observed that the nonlinear absorption and optical limitation performance of the BiVO4 films can be altered by changing the powder morphology and BiVO4 weight concentration. BiVO4 thin films fabricated using a precursor solution pH 12 and weight concentration of 30 wt.% showed the best performance among all fabricated films with the highest nonlinear absorption coefficient (βeff), saturation density threshold (ISAT) and lowest OL threshold value.

Modified successive ionic layer adsorption and reaction for interconnected bismuth vanadate nanograins: Highly active visible light harvesting photoanodes

A modified successive ionic layer adsorption and reaction (SILAR) method is developed for depositing interconnected bismuth vanadate (BiVO4) nanoparticles with improved visible light harvesting activity. Facile control of preparative parameters like deposition cycles, anionic precursor, pH and deposition temperature significantly altered BiVO4 surface morphology. BiVO4 thin films prepared with Na3VO4 anionic precursor show dispersed nanoparticles type morphology and that deposited with NaVO3 precursor displays 3D interconnected nanoparticles morphology. The BiVO4 photoanodes with 3D interconnected nanoparticles morphology exhibits an excellent photocurrent density of 5.19 mA cm−2 at 1.23 V vs. RHE with superior photostability and improved applied bias photon to current efficiency (ABPE) (1.50 %) compared to dispersed nanostructured BiVO4 photoanodes. The excellent photoelectrochemical (PEC) characteristics of BiVO4 photoanodes with 3D interconnected nanoparticle morphology can be ascribed to porous morphology, optimum thickness, narrow band gap energy and favorable electronic band structure for water splitting. The current study illustrates the usefulness of the modified SILAR approach for depositing 3D interconnected BiVO4 nanoparticle morphology in a single step that is superior and efficient to previously reported multistep processes.

Retraction Note: Visible light-enhanced photocatalytic dye degradation and hydrogen evolution performance of BiVO4 thin films prepared at various annealing temperatures

Retraction Note: Visible light-enhanced photocatalytic dye degradation and hydrogen evolution performance of BiVO4 thin films prepared at various annealing temperatures

Retraction Note: Influence of Fe doping on the photocatalytic dye degradation and H2 evolution properties of BiVO4 thin films under visible-light irradiation

Retraction Note: Influence of Fe doping on the photocatalytic dye degradation and H2 evolution properties of BiVO4 thin films under visible-light irradiation

Effect of annealing temperature on bismuth vanadate nano thin films for solar cell applications

Bismuth vanadate (BiVO4) has been used as the photoanode electrodes in ferroelectric solar cells owing to its unique combination of properties: a narrow bandgap among ferroelectrics, economic viability, negative conduction band edge, and remarkable stability. The present work explores the fabrication and characterization of BiVO4 thin films prepared via spin coating, with a specific focus on elucidating the influence of the annealing temperature (400–550 °C) on their structural, optical, and photovoltaic properties. From X-ray diffraction (XRD) analysis, it is observed that higher annealing temperatures have promoted the formation of larger grains, enhanced crystallinity, and induced a preferred crystal orientation characteristic of the monoclinic scheelite structure. Tauc plot analysis shows the dependence of the optical band gap on the annealing temperature for BiVO4 thin films. The band gap values have decreased slightly from 2.50 eV at 400 °C to 2.44 eV at 550 °C. This is indicated by the slightly narrowed bandgap, which influences the structure of the material or defect states at higher annealing temperatures. A narrower bandgap allows for the absorption of lower-energy light, potentially improving the light absorption efficiency of BiVO4 thin films in photoanode applications. The influence of annealing temperature on BiVO₄ thin film solar cell performance was investigated further through the analysis of open-circuit voltage (Voc), short-circuit current (Isc) and power conversion efficiency.

Molybdenum-doped BiVO4 thin films deposited through chemical spray pyrolysis for ammonia sensing at room temperature

Molybdenum-doped BiVO4 thin films deposited through chemical spray pyrolysis for ammonia sensing at room temperature

Epitaxial Strain Modulation of Photoelectrochemical Properties of Epitaxial Bismuth Vanadate Photoanodes

Solar fuels derived from photoelectrochemical water splitting have the potential to significantly contribute to achieving sustainability in our global energy infrastructure. Transition metal oxide (TMO) photoabsorbers are interesting candidates as photoelectrodes due to their excellent bandgap tunability, relative stability and cost-effectiveness. Further improvements in the photoelectrochemical performance of such photoelectrodes would entail strategies beyond the discovery of new compositions and structures [1]. Strain engineering in particular is a powerful tool to alter the material’s electronic structure and the resulting properties. For instance, optoelectronic properties (i.e., light absorption characteristics) [2] and surface catalytic properties [3] could be modulated by lattice-mismatch imposed epitaxial strain. Much less understood, however, is how epitaxial strain modulation affects the band bending and surface electronic structure; these properties dictate the photogenerated carrier separation at the photoelectrode/electrolyte interface, and thus the photoelectrochemical performance.To gain further insight into the impact of epitaxial strain on the photoelectrochemical performance of TMO photoelectrodes, monoclinic bismuth vanadate (BiVO4) was chosen as a material platform for study given its desirable (bandgap ~2.4 eV, favorable band edge positions and relatively long minority diffusion lengths) and undesirable (localized carrier transport, photocorrosion) characteristics [1]. High crystalline quality epitaxial BiVO4 thin films were used as a model system in order to minimize complications arising from grain boundaries and different crystalline orientations inherent in polycrystalline systems. BiVO4 films with an epitaxial indium tin oxide (ITO) conducting layer were deposited on (001)-oriented yttrium-stabilized zirconia (YSZ) single crystalline substrates. YSZ substrates with different dopant concentrations were used to impose different epitaxial strain states on the BiVO4 films. This strategy allowed us to conduct extensive photoelectrochemical measurements (voltammograms, incident photon-to-current efficiency, impedance spectroscopy) and x-ray photoelectron spectroscopic (XPS) measurements as a function of the strain state. Further investigation with varying BiVO4 film thickness was also performed to decouple any thickness-dependent contributions from the epitaxial strain effects. Through strain tensor decomposition analysis, Tauc bandgaps correlates well with deviatoric strains, which in turn seem to suggest the role of volume-preserving lattice distortions. Finally, we will show that the observed trends in photocurrents, incident photon-to-current efficiency spectra, flatband potentials obtained from Mott-Schottky analysis, and surface electronic structure obtained from XPS can be rationalized in terms of lattice-induced distortions on the BiO8 dodecahedra.

Understanding Performance Limitations in Water Splitting Photoelectrodes at the Nanoscale

Photoelectrochemical (PEC) water splitting is a promising route for efficient conversion of solar energy into chemical fuels. In this context, economically viable photosystems are often based of semiconductor thin films which are polycrystalline or nanostructured with highly complex architectures e.g. with boundaries between differently orientated grains, different crystal facet orientations, as well as locally varying composition or phase. These nano- to micrometer properties often control critical processes, such as efficiency and stability, of the macroscale system. To understand the impact of nanoscale properties on the macroscopic performance, we aim at resolving local structural, chemical, and optoelectronic heterogeneities and at elucidating their effect on light-driven processes.In this work, we use a correlative approach to elucidate the nanoscale properties of polycrystalline BiVO4 thin films – a promising material for solar water splitting. Mapping of the local charge transport properties by photoconductive atomic force microscopy (pc-AFM) reveals the tolerance to grain boundaries.[1] Interestingly, scanning nearfield infrared microscopy shows strongly varying absorption from VO4 stretching modes between grains and at grain boundaries which correlate with the heterogeneities in the local photoconductivity across the polycrystalline thin films. Furthermore, local temperature-dependent current-voltage spectroscopy show that the low intrinsic bulk conductivity of BiVO4 limits the electron transport through the film, and that the transport mechanism can be attributed to space charge limited current (SCLC) in the presence of trap states.[1] Performing the same measurements in-situ in controlled gas-phase environment reveals the influence of chemical interactions of adsorbed oxygen and water on charge transport and interfacial charge transfer.[2] For BiVO4, we demonstrate that adsorbed oxygen acts as a surface trap state for electrons, which enhances the built-in potential and depletes the BiVO4 layer. Overall, combining insights from different nanoscale techniques generates a comprehensive picture of charge separation, transport, and transfer at the nanoscale. The gained nanoscale understanding of energy materials enables the rational design of durable and efficient solar fuel devices.[1] Eichhorn et al., Nat. Commun. 9, 2597(2018).[2] Eichhorn et al., ACS Appl. Mater. Interfaces, 10, 41, 35129 (2018).

Exploring the electronic structure of BiVO4 thin films using energy-resolved electrochemical impedance spectroscopy

Bismuth vanadate (BiVO4) is a promising photoelectrochemical water-splitting semiconductor. This study focuses on elucidating the electronic structure of BiVO4 by analyzing energy-resolved electrochemical impedance spectroscopy (ER-EIS) data. The primary objective is to demonstrate the effectiveness of the ER-EIS, which is typically applied to organic semiconductors, in estimating the band edge positions of BiVO4 films deposited on FTO substrates.The impedance spectra analysis as a function of an applied overpotential permits an estimation of the density of states, where the limits correspond to the valence (VB) and conduction band (CB) edge positions that determine the redox reactions the semiconductor can promote. The energy difference between the CB and VB corresponds to the material's band gap (Eg). The calculated Eg using ER-EIS closely matches the reported optical properties of the material, and the energy positions of the VB and CB align with the reported values for BiVO4.

Chemically synthesized facet-controlled visible light active BiVO4 thin films for photoelectrochemical water splitting

Chemically synthesized facet-controlled visible light active BiVO4 thin films for photoelectrochemical water splitting

Molybdenum-doped BiVO4 thin films: Facile preparation via hot-spin coating method and the relationship between surface statistical parameters and photoelectrochemical activity

Molybdenum-doped BiVO4 thin films were uniformly coated on indium-doped tin oxide (ITO) substrates via a facile modified hot spin coating (HSC) technique. The prepared layers were used as photoanode in a photoelectrochemical (PEC) cell. Different percentage of Mo dopant was examined to maximize the photo-current density (J) of the layers. The highest J value (872 ± 8 μA/cm2) was obtained by 5 atomic% of Mo doping. After that, the surface topographies of these samples were changed by varying the initial precursor concentration from 27 to 80 mM. The relation between surface topographies and the PEC activity of Mo-doped BiVO4 thin films was investigated from microscopic point of view by calculating the surface roughness exponent of α, and a mechanism for the PEC activity of Mo-doped BiVO4 photoanodes was proposed accordingly. The sample with a small roughness exponent provided a surface with jagged microscopic fluctuations which may trap the air molecules between the electrolyte and sample surface, hindering the fine atomic interaction for photo-generated electron-hole transition. Therefore, the layer with the highest roughness exponent (2α = 0.48 ± 0.03), which means the smoother microscopic surface and better interfacial contact with the electrolyte, exhibited the best PEC activity.

Toward preparation of large scale and uniform mesoporous BiVO4 thin films with enhanced photostability for solar water splitting

The preparation methods of photocatalyst thin films play a critical role in increasing their efficiency. Ease of fabrication, the ability to commercialize, production on a large scale, reproducibility, and preparing the layers with stable performance are some of the most important criteria for a synthesis method to prepare a high-performance phot-electrode. A straightforward, cost-effective and reproducible hot spin coating (HSC) method was introduced in this work, to achieve these goals for BiVO4 thin films. The most important HSC parameters, namely substrate revolution speed (ωs), substrate temperature (Ts), and combustion time (Ct), were investigated systematically. The BiVO4 thin films prepared via the HSC method under the optimized parameters of ωs = 1200 rpm, Ts = 200 °C, and Ct = 15 s exhibited a uniform mesoporous surface texture (average pore diameter of 13.4 nm) and high adhesion (5B) to the substrate. Interestingly, the HSC method was capable of preparing super-hydrophilic BiVO4 layers on a large scale (tested up to 8 × 8 cm2) without any surface pinholes. The practical application of the obtained layers was examined inside a photoelectrochemical (PEC) cell. The optimized BiVO4 electrode generated a remarkable and robust photo-current density of ∼500 μA/cm2 without any change after 30 days. The preparation of BiVO4 thin films with such interesting properties via the facile HSC method makes it a promising technique to prepare highly efficient photo-anodes for practical use on a large scale.

Ferroelastic twin domain patterns and polar domain walls of BiVO4 thin films via phase-field simulations

Polar domain walls in centrosymmetric ferroelastics (e.g., BiVO4 films) give rise to inhomogeneity and offer promises for solar energy conversion applications via expediting charge-carrier separation. However, the origin of polar domain walls and the relationship between them and spontaneous strain from structural phase transition remain elusive at the mesoscale in BiVO4 films. Here, from the perspective of phenomenological thermodynamics, the relationship between four variants of monoclinic (M) phase and the corresponding lattice parameters during the tetragonal-monoclinic (T-M) phase transition is revealed. Using phase-field simulations, it suggests that the formation of four variants of ferroelastic twin domains arises from spontaneous strain from T-M phase transition. Moreover, it is demonstrated that (i) polar domain walls in BiVO4 films originate from the flexoelectric effect induced by strain gradient across the domain walls during phase transition process; (ii) shear strain gradient dominates the contribution to the polarization vectors at the domain walls; (iii) 180° head-to-head or tail-to-tail in-plane polarization configurations emerge at the adjacent domain walls. The stability of several types of ferroelastic domain walls is also analyzed from the point of view of total free energy. These results not only provide a fundamental understanding of the origin of polar domain walls and the formation mechanisms of ferroelastic twin patterns, but also stimulate potential applications of polar domain walls of non-polar BiVO4 films in energy conversion.

Effect of sulfur doping on the photocatalytic performance of sputtered BiVO4 thin films

BiVO4 thin films doped with various concentrations of sulfur were fabricated using RF sputtering followed by post-deposition sulfurization. The incorporation of sulfur in the samples was calculated to be approximately 8–11 at% from the S2s peak in their X-ray photoelectron spectra. The optical bandgap of sulfur-doped BiVO4 was generally smaller than that of the undoped sample. BiVO4 films doped with ∼8 at% sulfur showed the highest photoelectrochemical performance compared to the undoped sample. Almost similar minority-carrier lifetimes in undoped and low sulfur-doped BiVO4, measured by time resolve photoluminescence, suggest that the crystal qualities in terms of the recombination properties are roughly the same for both cases. Thus, although further investigation may be necessary, the improved photocurrent in 8 at% sulfur-doped BiVO4 in our study can roughly be attributed to the decrease in the bandgap, which facilitates more photoexcited carriers to contribute to the photoelectrochemical reaction. A further increase in sulfur doping above 10 at% distorted the BiVO4 local crystal structure, inducing defects, thus resulting in a lower photocurrent.

Crystalline phase control of BiVO4 thin films using RF sputtering

A selective fabrication method for monoclinic-scheelite (m-s) BiVO4 and tetragonal-zircon (t-z) BiVO4 thin films using radio fRequency (RF) sputtering from a single target was developed. The kinetic energy of the sputtered atoms was controlled by varying the sputtering power to obtain BiVO4 films with m-s and t-z crystalline phases. Although the band gap of the t-z BiVO4 phase (3.0 eV) was larger than that of m-s BiVO4 (2.5 eV), the deposited t-z BiVO4 films showed a comparable photocurrent density (1.5 mA cm−2) at 1.23 V versus the reversible hydrogen electrode (400 W Xe lamp). This was mainly because of the reduced sputtering damage in the t-z BiVO4 crystal, which originated from the low sputtering power as well as the deep valence-band position in t-z BiVO4 that enabled the efficient utilization of the photocarriers. This work provides insights into crystalline phase control using the particle kinetic energy in sputtering.

Co-sputtered Mo:BiVO4 thin film-based transparent photovoltaic and photoelectrochemical activity for see-through energy systems

Recently, the photovoltaic (PV) and photoelectrochemical (PEC) cell has engrossed cumulative attention for clean and low-cost sustainable energy. Here, we propose transparent PV (TPV) and PEC (TPEC) based on sputtered BiVO4 semiconducting thin films to unite electric power and hydrogen generation for see-through energy systems. The TPV and TPEC devices performed better under white light illumination when doped with Mo, and showed average visible transmittance of ∼60%. Such enhancements are due to the Mo doping in BiVO4, which offers better optoelectronic properties suitable to TPV and TPEC devices than BiVO4 thin films. Mo:BiVO4-based TPV-TPEC device shows a high open circuit voltage of 0.43 V with a fill factor value of 33.8% and a short-circuit current density of 0.415 mA/cm2, attributed to the enhanced absorption for blue wavelength photons, indicating stable and safe onsite power production. The structural, optical, and electrical performances of BiVO4 films were studied and compared with its Mo:BiVO4. These co-sputtered Mo:BiVO4-based TPV and TPEC devices can further enhance the robust power-generating systems for see-through onsite energy production.

Enhanced photocatalytic dye degradation and hydrogen evolution performance of Cu encapsulated BiVO4 under visible light irradiation

Monoclinic Bismuth Vanadate (BiVO4) has recently attracted a lot of interest for being one of the most effective photocatalyst materials for waste water treatment and it could also capture a wide spectrum of visible light. In this study, Sol Gel dip coating procedures were used to fabricate BiVO4 thin films encapsulated with Cu at various concentrations (1, 2, 3, and 4 wt%) and used as the photocatalysts, and a number of analytical techniques were employed to determine the films' physiochemical properties. The monoclinic BiVO4 crystalline structure was identified using XRD and Raman spectroscopy. FESEM images indicate that the morphology changes from amorphous to a nano flake like structure when Cu is encapsulated with BiVO4, and improved BET surface area was observed for BiVO4 thin films with increasing Cu concentration. The electronic structure was described using XPS, and optical absorption investigations revealed that the red shift within the absorption edge, which shows the band gap energy, was reduced for Cu-encapsulated BiVO4, allowing successful photogeneration and hydrogen (H2) evolution under visible light. The photocatalytic activity of BiVO4:Cu was investigated using the decomposition of Methylene blue (MB) dye under visible light. This investigation demonstrated that MB dye completely degrades within 120 min. It has been shown that BiVO4:Cu (4 wt%) thin films had higher degradation efficiencies with first order kinetics. This Cu-encapsulated BiVO4 monoclinic and nanoflake structure demonstrated strong optical absorption, improved charge carrier transport abilities, and high penetration depth, as well as maximum photodegradation efficiency and H2 evolution.

Scalable synthesis of BiVO4 thin films via anodic plating and thermal calcination

Fabrication of high-quality semiconductor thin films has long been a subject of keen interest in the photocatalytic field. Here, we report a facile, solution-based anodic plating and calcination for large-scale synthesis of BiVO4 thin films on indium tin oxide coated glass for use as photoanodes in solar water splitting. Using Na2SO3 as a sacrificial reagent, continuous solar H2 production with 94% Faradaic efficiency was obtained over 6 h of photoelectrochemical water splitting.

Band alignment and photocatalytic activity of monoclinic BiVO4 (0 1 0) and (1 0 0) films with SrTiO3

The crystal plane with the highest electron and hole mobility in a material is very important to physical and chemical properties. Herein, the BiVO4 (BVO) thin films exposed (010) or (100) planes which pointed out as the facets with the highest electron and hole mobility, and formed the interface with SrTiO3 (STO) were fabricated by magnetron sputtering. The band edge offsets calculation results of oriented BVO and STO show that the conduction band and valence band positions of the BVO (010) plane are 0.23 eV higher than those of (100). The average generation rates of H2O2 in BVO(100) are 5.3 × 10−2 and 3.8 × 10−1 µmol h−1 under 440 nm monochromatic light and full arc Xe lamp irradiation, which are 1.8 and 1.6 times higher than those in BVO(010), respectively.

Characterization and photoelectrochemical evaluation of BiVO4 films developed by thermal oxidation of metallic Bi films electrodeposited

This paper reports the synthesis, characterization, and photoelectrochemical evaluation of BiVO4 thin films. Bi thin films were obtained by electrodeposition and then thermally oxidized. Before thermal oxidation, the vacuum annealing temperature of the metallic Bi films significantly changed the final properties. Increasing the annealing temperature of the samples reduced their thickness and increased compaction and the growth in grain size. At the same time, this increase reduced the energy band gap, improved the electrical properties, including the charge carrier absorption and transport, and broadened the Urbach energy. The best performance was obtained with a film vacuum annealed at 10 mTorr and 200 °C, while the thin film annealed at 250 °C showed a deterioration of the material. The energy band gap ranged from 2.44 to 2.53 eV, while the calculated carrier density ranged from 6.24 × 1017 to 3.70 × 1018 cm−3. Based on Mott-Schottky plots, these materials exhibited Vfb of 0.441–0.611 V vs. NHE, making them suitable for photoanodes in solar water splitting and promising routes for hydrogen production.