Flat Band Potential Research Articles

Synthesis and characterization of CuAl2O4 nanoparticles: Application for the removal of Eriochrome Black T under solar light irradiation

This work focuses on the use of the spinel CuAl2O4 as photocatalyst for the degradation of Eriochrome Black T (EBT), a potentially hazardous dye. CuAl2O4 is synthetized by co-precipitation and studied by physic-chemical and electrochemical properties. X-ray diffraction revealed the formation of a single phase with cubic symmetry with a homogeneous morphology. X-ray Photoelectron Spectroscopy (XPS) and FTIR spectroscopy provide a better understanding of the chemical composition and surface structure of CuAl2O4. Diffuse reflectance (DR) gave a band gap of 1.94 eV, attributed to Al3+ in octahedral configuration. The BET analysis led to a specific surface area of 37 m2/g with the availability of an appreciable surface for photochemical reactions. Chrono-amperometry indicated n-type conductivity of CuAl2O4, with electrons as dominant charges and an activation energy (Ea) of 0.17 eV. A flat band potential of 0.26 VSCE (Volt vs. Saturated Calomel Electrode) was calculated from the interfacial capacitance in Na2SO4 solution. The photoactivity of EBT degradation was assessed by using chemical oxygen demand (COD). A quasi-complete degradation of EBT was observed within 180 min under sunlight. The mineralization was carried out with an initial EBT concentration of 10 mg/L, a dose of 1 g/L at pH ∼ 8, similar to that of an aquatic environment. These experimental conditions are important to assess the applicability of this method in a real environment.

A Mott-Schottky analysis of mesoporous silicon in aqueous electrolyte solution by electrochemical impedance spectroscopy

Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. In recent years, it has been shown that filling the pores with aqueous electrolytes, in addition opens a particularly wide field for modifying and achieving active control of these functionalities, e.g., for electrochemo-mechanical actuation and tunable photonics, or for the design of on-chip supercapacitors. However, a mechanistic understanding of these new features has been hampered by the lack of a detailed characterization of the electrochemical behavior of mesoporous silicon in aqueous electrolytes. Here, the capacitive, potential-controlled charging of the electrical double layer in a mesoporous silicon electrode (pore diameter 7nm) imbibed with perchloric acid solution is studied by electrochemical impedance spectroscopy. Thorough measurements with detailed explanations of the observed phenomena lead to a comprehensive understanding of the capacitive properties of porous silicon. An analysis based on the Mott-Schottky equation enables the determination of essential parameters such as the flatband potential, the carrier concentration and the width of the space charge region. A comparison with bulk silicon shows that the flatband potential in particular is significantly altered by the introduction of nanopores, as it shifts from 1.4±0.1V to 1.9±0.2V.

Utilization of Electrocoagulated Sewage as a Photoelectrocatalyst for Water Splitting.

Electrocoagulation (EC) as a wastewater treatment process for the removal of pollutants has been demonstrated in numerous studies. However, proper management of solid waste generated after EC treatment is essential to minimize its environmental impact. Hence, more emphasis needs to be paid towards unused solid waste after EC treatment. The present study investigates the possibilities of utilizing waste released after the EC process as an electrocatalyst in the presence of sunlight. In this study, the sludge produced after domestic wastewater treatment by the EC process is collected and tested for water oxidation reaction under AM 1.5 illumination of simulated solar light. The sludge produced after EC treatment was characterized meticulously and confirmed to be the magnetite phase of iron oxide, which is used as a photoanode for photoelectrochemical (PEC) water splitting. The chemical composition of sludge is majorly dependent on the treatment time, which plays a crucial role in deciding the metal ions present in the sludge. After 30 min, which is the optimized time for EC treatment, sludge was studied as an efficient photoanode material. The band gap illumination of sludge (iron oxide) as working electrodes results in anodic current; the photocurrent appears at a bias of ca. 390 mV with respect to the flat-band potential. The PEC activity of waste is treatment-time dependent and decreases after reaching an optimal time of 30 min. A photocurrent density of 4.6 × 10-6 A cm-2 was found at the potential of 1.23 V (vs RHE) for sludge collected after 30 min of treatment time. It indicates that the sludge-derived photoanode has the potential to be an efficient component in PEC systems, contributing to the overall efficiency of water-splitting processes. Our experimental results show a new pathway of a "waste to energy" approach that aligns with the principles of circular economy and sustainable resource management.

Photocatalytic activity of ZnMn2O4/TiO2 heterostructure under solar light irradiation: Experimental and theoretical study

This study focuses on the application of the hetero-junction ZnMn2O4/TiO2 as an efficient photocatalyst for the degradation of Rhodamine B (RhB), a cationic dye. ZnMn2O4 was synthesized by sol-gel auto-combustion, employing citric acid as fuel. The photocatalysts underwent characterization by X-ray diffraction, Fourier Transform infrared spectroscopy, Scanning Electronic Microscopy with integrated EDX and UV–Vis Diffuse Reflectance. Creating a p-n heterojunction could significantly boost photocatalytic efficiency. In this work, we use TiO2 as a n-type co-catalyst to enhance electron transfer efficiency. The ZnMn2O4 spinel exhibits direct optical transition at 1.97 eV, while TiO2 displays an indirectly allowed transition at 3.24 eV. Mott-Schottky plots revealed flat band potentials of -0.121 and +0.3 V/SCE for ZnMn2O4 and TiO2, respectively. Kinetic data of RhB adsorption were fitted using the pseudo-second order model. The heterojunction p-ZnMn2O4/n-TiO2 demonstrates a significant pollutant degradation rate, with 95% removal within 180 min under sunlight irradiation, surpassing the effectiveness of ZnMn2O4. To validate the experimental results, a theoretical investigation was conducted using density functional theory (DFT) and molecular dynamics (MD) simulations. Geometry optimization of the Rhodamine B molecule was studied in both gas and liquid phases, and various quantum chemical parameters were computed. Molecular dynamics simulations were additionally employed to examine the dynamic behavior of Rhodamine B adsorption on ZnMn2O4 (211) and TiO2 (101) surfaces in an aqueous environment.

Understanding the Internal Conversion Efficiency of BiVO4/SnO2 Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis.

This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (<5 nm) SnO2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample's thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott-Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential.

Operando Tracking the Interactions between CoOx and CeO2 during Oxygen Evolution Reaction

AbstractCeO2 greatly enhances the electrocatalytic oxygen evolution reaction (OER) activity of CoOx, though the enhancement mechanism beyond this synergy is yet to be understood. Here, operando hard X‐ray absorption spectroscopy (hXAS) is applied to monitor the Co K edge and Ce L3 edge in CoOx/CeO2 to shed light on the evolution of the Co and Ce oxidation states during OER. In addition, ex situ soft XAS (sXAS) characterizations provide information on the irreversible surface‐specific transformations of the Co L3 edge as well as of the O K edge. Combining the operando and ex situ spectroscopic characterizations with comprehensive electrochemical analyses, it is confirmed that CeO2 is not the active center for the OER. However, coupling CeO2 with CoOx introduces significant modifications in the Co and O species at the CoOx surface and alters the flat band potential (Efb), leading to more favorable Co oxidation state transformations during OER and possibly modifying the preferential reaction pathway. This work establishes the connections between electronic structures, Co oxidation state and the OER reaction mechanism for CoOx/CeO2 composites electrodes.

Associating Physical and Photocatalytic Properties of Recyclable and Reusable Blast Furnace Dust Waste.

Blast furnace dust waste (BFDW) proved efficient as a photocatalyst for the decolorization of methylene blue (MB) dye in water. Structural analysis unequivocally identified α-Fe2O3 as the predominant phase, constituting approximately 92%, with a porous surface showcasing unique 10-30 nm agglomerated nanoparticles. Chemical and thermal analyses indicated surface-bound water and carbonate molecules, with the main phase's thermal stability up to 900 °C. Electrical conductivity analysis revealed charge transfer resistance values of 616.4 Ω and electrode resistance of 47.8 Ω. The Mott-Schottky analysis identified α-Fe2O3 as an n-type semiconductor with a flat band potential of 0.181 V vs. Ag/AgCl and a donor density of 1.45 × 1015 cm-3. The 2.2 eV optical bandgap and luminescence stem from α-Fe2O3 and weak ferromagnetism arises from structural defects and surface effects. With a 74% photocatalytic efficiency, stable through three photodegradation cycles, BFDW outperforms comparable waste materials in MB degradation mediated by visible light. The elemental trapping experiment exposed hydroxyl radicals (OH•) and superoxide anions (O2-•) as the primary species in the photodegradation process. Consequently, iron oxide-based BFDW emerges as an environmentally friendly alternative for wastewater treatment, underscoring the pivotal role of its unique physical properties in the photocatalytic process.

Voltage-Induced Inversion of Band Bending and Photovoltages at Semiconductor/Liquid Interfaces.

At semiconductor/liquid interfaces, the surface potential and photovoltages are produced by a combination of band bending and quasi-Fermi-level splitting at the semiconductor surface, which are usually treated in a qualitative fashion. As such, it is important to develop quantitative metrics for the band energies and photovoltaics at these interfaces. Here, we present a spectroscopic method for monitoring the photovoltages produced at semiconductor/liquid junctions. The surface reporter molecule mercaptobenzonitrile (MBN) is functionalized on the photoelectrode surface (p-type silicon) and is measured using in situ surface-enhanced Raman scattering (SERS) spectroscopy with a water immersion lens under electrochemical working conditions. In particular, the vibrational frequency of the C≡N stretch mode (ωCN) around 2225 cm-1 is sensitive to the local electric field in solution at the electrode/electrolyte interface via the vibrational Stark effect. Over the applied potential range from -0.8 to 0.6 V vs Ag/AgCl, we observe ωCN to increase from 2220 to 2229 cm-1 (at low laser power). As the incident laser power is increased from 83.5 μW to 13.3 mW, we observe additional shifts of ΔωCN = ±1 cm-1, corresponding to photovoltages produced at the semiconductor/liquid interface ΔV = ±0.2 V. Based on Mott-Schottky measurements, the flat band potential (FBP) occurs at -0.39 V vs Ag/AgCl. For applied potentials above the FBP, we observe ΔωCN > 0 (i.e., blue-shifts ∼1 cm-1) corresponding to positive photovoltages, whereas for applied potentials below the flat band potential, we observe ΔωCN < 0 (i.e., red-shifts ∼1 cm-1) corresponding to negative photovoltages. These spectroscopic observations reveal voltage-induced changes in the band bending at the semiconductor/liquid junction that, thus far, have been difficult to measure.

Optimal Selection for Redox Couples and Enhanced Performance through Magnetic Nanofluid Electrolyte in Solar Flow Batteries

The limited photoelectric conversion efficiency poses one of the critical constraints on commercializing solar flow batteries (SFBs). This study compares the chemical and photoelectrochemical properties of three commonly used redox couples. Additionally, magnetic Fe3O4 nanoparticles, for the first time, are introduced to optimize the electrolyte, and they are compared with the original electrolyte. Across different redox couples, the variations in semiconductor flat-band potentials and carrier concentrations result in changes in photoelectric current density. Notably, FeCl2/FeCl3 redox coupled with TiO2 photoelectrodes exhibits the highest photoelectric current density, reaching 75.7 µA cm−2. However, the trade-off of this electrolyte, i.e., providing high photocurrent while being unable to supply sufficient open-circuit voltage, imposes limitations on the practical application of SFBs. Alternatively, for TEMPO and 4-OH-TEMPO electrolytes, which can provide a higher open-circuit voltage, the electrochemical activity is enhanced, and the solution ohmic resistance is reduced by introducing magnetic nanoparticles to form a magnetic nanofluid. As a result, the photoanode’s photocurrent density increases by 36.6% and 17.0%, respectively, in the two electrolytes. The work reported here effectively addresses the current issue of low photocurrent density in SFBs and presents new optimization strategies for SFBs.

Optimizing Photovoltaic Efficiency Through Di-Indium Tri-Selenide Thin Film Solar Cells

This study focuses on investigating the photovoltaic efficiency of chemically deposited In2Se3 thin films for solar energy conversion. Thin film solar cells have several advantages, including reduced material usage, cost-effectiveness, and the potential for improved efficiency. Our research specifically explores the viability of chemically deposited In2Se3 thin films for converting solar energy into electrical energy through chemical processes. The photovoltaic (PV) cell configuration consists of In2Se3 thin films as the photoanode and a sulfide-polysulphide (0.15 M) solution as the electrolyte. The structure of the cell is n-In2Se3 | NaOH (0.15 M)+ S (0.15 M)+Na2S (0.15 M) |C (graphite). To comprehensively analyze the performance of the thin films, we employ various characterization techniques. These include studying the current-voltage characteristics under both dark and light conditions, capacitance–voltage in the dark, barrier height measurements, power output analysis, photo response evaluation, and spectrum response examinations. Through these analyses, we have discovered that In2Se3 exhibits n-type conductivity. Noteworthy results include a junction ideality factor of 3.43, a flat band potential of −0.680 V, and a barrier height of 0.193 eV. The power output characteristics provide crucial insights into the performance of the PV cell. We measured the open circuit voltage, short circuit current, fill factor, efficiency, and photo response lighted ideality factor, which were found to be 320 mV, 25.4 μA, 36.46%, 0.69%, and 2.56, respectively. This research significantly contributes to our understanding of the photovoltaic efficiency of di-indium tri-selenide thin film solar cells. It offers valuable insights for optimizing their performance in solar energy conversion applications.

Effect of the Heterovalent Sc3+ and Nb5+ Doping on Photoelectrochemical Behavior of Anatase TiO2

In this study, we explored the effect of either Nb or Sc doping at a concentration range of 0.0–1.0 at.% on the physical–chemical and photoelectrochemical behavior of TiO2 anatase electrodes. This behavior was characterized by work function, flat band potential, donor density, spectral dependence of photocurrent and stationary photocurrent measurements. All experimental results are interpreted in terms of the formation of the shallow delocalized polaron states in the case of Nb doping and deep acceptor states induced by Sc doping on TiO2 anatase.

Stable Photoelectrochemical Reactions at Solid/Solid Interfaces toward Solar Energy Conversion and Storage.

Electrochemistry has extended from reactions at solid/liquid interfaces to those at solid/solid interfaces. However, photoelectrochemistry at solid/solid interfaces has been hardly reported. In this study, we achieve a stable photoelectrochemical reaction at the semiconductor-electrode/solid-electrolyte interface in a Nb-doped anatase-TiO2 (a-TiO2:Nb)/Li3PO4 (LPO)/Li all-solid-state cell. The oxidative currents of a-TiO2:Nb/LPO/Li increase upon light irradiation when a-TiO2:Nb is located at a potential that is more positive than its flat-band potential. This is because the photoexcited electrons migrate to the current collector due to the bending of the conduction band minimum toward the negative potential. The photoelectrochemical reaction at the semiconductor/solid-electrolyte interface is driven by the same principle as those at semiconductor/liquid-electrolyte interfaces. Moreover, oxidation under light irradiation exhibits reversibility with reduction in the dark. Thus, we extend photoelectrochemistry to all-solid-state systems composed of solid/solid interfaces. This extension would enable us to investigate photoelectrochemical phenomena uncleared at solid/liquid interfaces because of low stability and durability.

Unlocking a Type-II CoO@BiVO4 Heterostructure for Wastewater Purification.

Developing a semiconductor-based heterostructure photoanode is crucial in improving the photoelectrocatalytic (PEC) efficiency for degrading refractory organic pollutants. Nevertheless, the PEC performance of the photoanodes is usually restricted by electron/hole pair recombination, oxygen evolution, and slow electron transfer. Herein, a novel CoO@BiVO4 nanowire array film (Ti/CoO@BiVO4) with n-type semiconductor characteristics was prepared via a straightforward hydrothermal method. The optimized Ti/CoO@BiVO4 electrode exhibited excellent PEC decolorization efficiency of active brilliant blue KN-R (∼92.8%) and long-term stability, outperforming recent reports. The insight reason for enhancing the PEC degradation efficiency of the Ti/CoO@BiVO4 electrodes can be attributed to the large electrochemical active area, low charge transfer resistance, and negative flat band potential. The formation of a type-II heterostructure was investigated between CoO and BiVO4 further to promote the generation and separation efficiency of electron/hole pairs, indicating that the optimized Ti/CoO@BiVO4 electrode has the potential for the water PEC degradation ability and superior service life.

Physical and photo-electrochemical characterization of Ca2Fe2O5 for metronidazole antibiotic degradation under sunlight

This study aims to investigate and describe the properties of the brownmillerite Ca2Fe2O5, synthesized from nitrates precursors. The X-ray diffraction showed that the single phase crystallizes in an orthorhombic symmetry (Space Group: Pcmn) with a specific surface area of 8.16 m2 g−1. The d-d transition of Fe3+: in octahedral coordination gives a direct optical band gap (2.02 eV) as determined by diffuse reflectance spectroscopy in agreement with the brown color of the oxide. The first one is due to the charge transfer O2−: 2p → Fe3+: t2g. This enables the material to exploit a large part of the solar radiation for photocatalytic purposes. The chemical stability of Ca2Fe2O5 is demonstrated over the pH range (5–12). The capacitance-potential plot revealed n-type conduction; a flat band potential of (Efb) 0.40 VSCE and electron concentration (NA) of 8.57 × 1024 m−3 were obtained in Na2SO4 (0.1 M) solution. The intensity-potential (J - E) profile showed electrochemical stability with oxygen intercalation in the layered lattice. The conduction band is more anodic than the level of O2/O2•, allowing the successful oxidation of Metronidazole by •O2− radical under visible light. The photodegradation of Metronidazole (MNZ) was successfully achieved at an initial concentration of 10 mg/L under solar radiation and a photocatalyst dose of 1 g/L at a free pH after 120 min of exposure and obeys a first-order kinetic with a rate constant of (33.7 × 10−3 min−1).

Hole Extraction Beyond the Depletion Layer in Ge-Doped Hematite Photoanode

As extensively discussed in the literature, photoelectrochemistry water splitting has enormous potential for the generation of green hydrogen. However, to obtain an efficient and economically viable photoelectrochemistry cell, it is necessary to use efficient semiconductor materials from the photogenerated charge separation point of view. In n-type semiconductors, based on transition metal oxides, where the diffusion length (Lp) is small compared to the absorption depth (α-1), i.e., αLp<< 1, the charge separation process will occur in the depletion layer region. Since this layer is typically tens of nanometers in size, only a fraction of the material exposed to light will contribute to the water photo-oxidation process, thus drastically restricting the efficiency of these semiconductor oxides. Hematite (α-Fe2O3) is an n-type semiconductor oxide widely explored as a photoanode for water photo-oxidation and is known to be a material dominated by the charge separation process in the depletion layer, following the Gartner-Butler model. In this way, we will only have photocurrent in Vappl>Vfb (Vappl is the applied potential, and Vfb is the flat band potential). Here we will explore the possibility of extracting holes at potentials smaller than Vfb in Ge-doped hematite photoanodes through rigorous measurements of Vfb and absorbed photon-to-current conversion efficiency (APCE) as a function of Vappl. We determined Vfb based on the Gartner–Butler analysis in the presence of a sacrificial reagent (H2O2) under different wavelength excitation and observed a dependence of the Vfb with the wavelength. The APCE measurements as a function of Vappl-Vfb clearly show the existence of photocurrent at potentials below Vfb, strongly suggesting that we are extracting holes even before the formation of the depletion layer (before the band bending). We estimate that the holes arrive from a region approximately 3nm close to the electrolyte-semiconductor interface. This phenomenon was only observed for Ge-doped hematite and not for pristine hematite.

Separating the Effects of Band Bending and Covalency in Hybrid Perovskite Oxide Electrocatalyst Bilayers for Water Electrolysis

Testing single crystalline epitaxial thin films as model systems for metal oxide electrocatalysts in the oxygen evolution reaction (OER) enables differentiating the OER descriptors such as crystal facet orientation, surface chemical composition, electronic structure and band bending at the interface to the electrolyte [1,2,3]. We designed single crystalline perovskite oxide bilayer structures to systematically tune the surface electronic structure and the band bending at the electrode/electrolyte interface to understand their role in the OER.The Co−O covalency in perovskite oxide cobaltites such as La1 − x SrxCoO3 is believed to impact the electrocatalytic activity during the OER. Additionally, space charge layers through band bending at the interface to the electrolyte may affect the electron transfer into the electrode, complicating the analysis and identification of true OER activity descriptors. Here, we separate the influence of covalency and band bending in hybrid epitaxial bilayer structures of highly OER-active La0.6Sr0.4CoO3 and undoped and less-active LaCoO3. Ultrathin LaCoO3 capping layers of 2−8 unit cells on La0.6Sr0.4CoO3 show intermediate OER activity between La0.6Sr0.4CoO3 and LaCoO3 evidently caused by the increased surface Co−O covalency compared to single LaCoO3 as detected by X-ray photoelectron spectroscopy. A Mott−Schottky analysis revealed low flat band potentials for different LaCoO3 capping layer thicknesses, indicating that no limiting extended space charge layer exists under OER conditions as all catalyst bilayer films exhibited hole accumulation at the surface [1].The combined X-ray photoelectron spectroscopy and Mott−Schottky analysis of atomically defined bilayer structures thus enables us to differentiate between the influence of the covalency and intrinsic space charge layers, which are indistinguishable in a single physical or electrochemical characterization. Our results emphasize the prominent role of transition metal oxygen covalency in perovskite electrocatalysts and introduce a bilayer approach to fine-tune the surface electronic structure [1].[1] L. Heymann, M. L. Weber, M. Wohlgemuth, M. Risch, R. Dittmann, C. Baeumer, F. Gunkel (2022) Separating the Effects of Band Bending and Covalency in Hybrid Perovskite Oxide Electrocatalyst Bilayers for Water Electrolysis, ACS Appl. Mater. Interfaces[2] M. Wohlgemuth, M. L. Weber, L. Heymann, C. Baeumer, F. Gunkel (2022) Activity-Stability Relationships in Oxide Electrocatalysts for Water Electrolysis, Front. Chem.[3] M. L. Weber, G. Lole, A. Kormanyos, A. Schwiers, L. Heymann, F. D. Speck, T. Meyer, R. Dittmann, S. Cherevko, C. Jooss, C. Baeumer, F. Gunkel, (2022) Atomistic Insights into Activation and Degradation of La0.6Sr0.4CoO3-δ Electrocatalysts under Oxygen Evolution Conditions Figure 1

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.

Photocatalyst Based on Nanostructured TiO2 with Improved Photocatalytic and Antibacterial Properties.

This study shows an easy way to use electrochemistry and plasma layering to make Cobalt-Blue-TiO2 nanotubes that are better at catalysing reactions. Once a titanium plate has been anodized, certain steps are taken to make oxygen vacancies appear inside the TiO2 nanostructures. To find out how the Co deposition method changed the final catalyst's properties, it was put through electrochemical tests (to find the charge transfer resistance and flat band potential) and optical tests (to find the band gap and Urbach energy). The catalysts were also described in terms of their shape, ability to stick to surfaces, and ability to inhibit bacteria. When Cobalt was electrochemically deposited to Blue-TiO2 nanotubes, a film with star-shaped structures was made that was hydrophilic and antibacterial. The band gap energy went down from 3.04 eV to 2.88 eV and the Urbach energy went up from 1.171 eV to 3.836 eV using this electrochemical deposition method. Also, photodegradation tests with artificial doxycycline (DOX) water were carried out to see how useful the study results would be in real life. These extra experiments were meant to show how the research results could be used in real life and what benefits they might have. For the bacterial tests, both gram-positive and gram-negative bacteria were used, and BT/Co-E showed the best response. Additionally, photodegradation and photoelectrodegradation experiments using artificial doxycycline (DOX) water were conducted to determine the practical relevance of the research findings. The synergistic combination of light and applied potential leads to 70% DOX degradation after 60 min of BT/Co-E irradiation.

Physical and photo-electrochemical properties of the hetero-junction ZnCo2O4/ZnO prepared by nitrate route: Application to photodegradation of Indigo Carmine

The semi conducting and photoelectrochemical properties of prepared ZnCo2O4 by nitrate route are investigated. The X-ray diffraction pattern of the red compound indicates a spinel type structure crystallizing in a cubic symmetry (a = 8.0989 Å). The direct optical transition of 2.10 eV, measured by diffuse reflectance spectroscopy, is assigned to unfilled Co3+: 3d orbital splitting in octahedral field (t2g → eg). The electrical conduction occurs by small lattice polaron hopping with an activation energy of 0.19 eV and a hole mobility µ300K (=3.5 × 10−4 cm2 V−1 s−1), thermally activated. ZnCo2O4 exhibits a chemical stability over the pH range (4 and 12) and a flat band potential (Efb) of 0.12 V, was deduced from the capacitance – potential (C−2 – E) plot. The positive slope characteristics p type conduction with a hole density of 1.6 × 1017 cm−3 due to mixed valences Co3+/2+. The energy of the conduction band (2.96 eV / −1.79 VSCE) makes the spinel attractive for the photo-oxidation of dyes by O2− radicals. The powder is successfully tested in the photo degradation of Indigo Carmine (10 10 mg/L) over the hetero-junction upon visible and solar lights. The best performance of the hetero- junction (ZnCo2O4 / ZnO) is obtained with (10 %/90 %) percentage with an abatement of 93 %, larger than the spinel alone (49 %) under sunlight. The kinetic follows pseudo-first order with a half-life of 23 min.

Perovskite BaSnO3 nanoparticles for solar-driven bi-functional photocatalytic activity: PEC water splitting and Wastewater treatment

The highly crystalline n-type BaSnO3 nanoparticles with size ∼30 nm, a larger surface area of ∼15.11 m2·g−1, and a wide optical band gap of 3.24 eV have been synthesized using a co-precipitation route for applications in photoelectrochemical water splitting and organic dye degradation. The spin-coated films of BSO nanoparticles photoanodes exhibited a photocurrent density of 20 μA·cm−2 at a forward bias potential of 0.6 V, a donor carrier density of 2.56 × 1018 cm−3 with a flat band potential of −0.82 V, and four-fold increment in the photocurrent density at a small bias voltage of 0.3 V, under AM 1.5 illumination (100 mW·cm−2). Pseudo-first-order kinetic study showed 96.49 and 95.38 % degradation performance of the Methylene Blue and Crystal Violet dyes at rate constant of 0.0128 and 0.0127 min−1, respectively, under constant irradiation for 240 min in the presence of BSO NPs, which is the highest degradation efficiency reported so far for BSO photocatalyst. The scavenger study revealed the significant role of h+ and ·OH− radicals in the degradation process. Liquid chromatography-mass spectroscopy provides intermediates obtained during the photocatalytic degradation of dyes. The present study focuses on the bifunctional photocatalytic performance of the BSO NPs as a potential candidate for PEC water splitting and wastewater treatment.