Flat Band Potential Research Articles

Preparation and Photoelectric Properties of Pr-Doped p-Cu2O Thin Films Photocatalyst Based on Energy Band Structure Regulation.

A Pr-doped p-Cu2O thin film was prepared on indium tin oxide conductive glass by electrochemical deposition; the effect of Pr doping on the structure, morphology, and physicochemical properties of p-Cu2O was investigated. The results show that with the increase in Pr doping amount, the particle size of p-Cu2O increases, the absorption boundary redshifts, and the band-gap width decreases. Pr doping increases the flat band potential and carrier concentration of p-Cu2O; when the doping amount is 1.2 mM, the carrier concentration reaches 1.14 × 1024 cm-3. Compared with pure p-Cu2O, the charge transfer resistance of Pr-doped p-Cu2O decreases and the photocurrent and open circuit voltage increase, indicating that the carrier transfer rate is accelerated, and the separation efficiency of photogenerated electrons and holes is effectively improved. The result of a norfloxacin photocatalytic degradation experiment showed that the degradation rate of norfloxacin increased from 52.3% to 76.2% and Pr doping effectively improved the photocatalytic performance of p-Cu2O. The main reasons for enhancing the photocatalytic performance are that the band gap of Pr-doped p-Cu2O decreases, the Fermi level of Cu2O is closer to the valence band position, the hole concentration near the valence band, and the oxidation capacity increases, and more h+ oxidize norfloxacin molecules. In addition, the Pr in Pr-Cu2O acts as a conductor to guide electrons on the guide band to the crystal surface, which increases the contact between photogenerated electrons and dissolved oxygen, which is conducive to the formation of the active species ·O2- and can effectively reduce the recombination of photogenerated carriers. In the process of photocatalytic degradation of norfloxacin, the main active species are ·O2-, ·OH, and h+, which play auxiliary roles. TOC tests show that the norfloxacin molecules can be effectively degraded into small molecule organic matter, CO2, and H2O in the presence of Pr-doped p-Cu2O.

Synergistic effect of surface oxygen vacancies and hydroxyl groups on Cu-doped TiO2 photocatalyst for hydrogen evolution

The intricate nature of the titanium dioxide (TiO2) photocatalytic system, influenced by Cu doping, often yields conflicting outcomes. This study addresses this complexity by examining the impact of Cu ion doping in nanostructured TiO2 to augment its photocatalytic efficacy in hydrogen generation from ethanol-water mixtures. The improved performance is primarily attributed to the generation of oxygen vacancies (Ovac) within the TiO2 lattice. By adjusting the thermal processing parameters, the chemical state of Cu is modulated to optimize photocatalytic performance. Solid-state nuclear magnetic resonance (SSNMR) and density functional theory (DFT) calculations confirm that the presence of Ovac and hydroxyl groups (OH−) surface species not only facilitates local spatial charge separation but also enhances light absorption, thereby improving photocatalytic hydrogen evolution reaction (HER) rate up to 20.2 mmol h−1 g−1 under UV light. Furthermore, a proposed mechanism involves the participation of Ovac and OH− species, supported by Mott-Schottky (M − S) plots and ultraviolet photoelectron spectroscopy (UPS), revealing that Cu–TiO2 exhibits a lower flat band potential (Efb) and smaller work function compared to TiO2. This research underscores the pivotal role of Cu ions and Ovac:OH− in advancing the photocatalytic activity of TiO2-based crystals, imparting noteworthy implications for sustainable hydrogen generation.

Performances of Nano-Structured Cu2O Thin Films Electrochemically Deposited on ITO Substrates in Lactate Bath as Liquid Petroleum Gas Sensors

The investigations are focused on structural and surface morphological features and their influence on electrical and wetting characteristics against liquid petroleum gas (LPG) sensing of synthesized nanostructured cuprous oxide (Cu2O) thin films grown by electrochemical deposition (ECD) on indium tin oxide (ITO) glass substrates in lactate bath (pH 10). p-Cu2O films revealed a cubical-tetrahedron nano-scale polycrystalline grain distribution (average grain size: 64.2 nm, inter planer spacing: 0.24 nm) that exhibited dominance in the crystallographic plane (111). LPG sensing evaluations of p-Cu2O/ITO done at 70 °C constant temperature with 100% LPG at 5 cc min−1 flow rate, showed excellent gas sensor response, recovery, and stability over time due to its moderate wetting behavior (average contact angle: ∼86°). AC impedance measurements carried out at room temperature demonstrated an ionic to electronic conduction variation from 100 Hz to 1 MHz, due to adsorption/desorption of LPG molecules (CnH2n+2) and oxygen species on the nano-particles surface. Cu2O/ITO flat band potential was found to be higher than its’ ambient state after exposure to LPG with an increment in acceptor density. Overall, this efficient LPG sensing could be attributed to the interfacial properties of Cu2O thin films and ITO substrates, that provide an optimized fabrication process of nano-structured Cu2O thin films.

Evidence of silicon substrate effect on improved electrolytic performance of Sb-SnO2 thin films/polycrystalline Si/chromate solution structure

The photo-electrochemical properties of pure SnO2 and Sb-doped SnO2 grown on Si substrate (Sb(%)/SnO2/Si) thin films synthesized by the Atmospheric Pressure Chemical Vapor Deposition (APCVD) were studied. The nature of the substrate and its chemical composition were assessed. SnO2 and Sb(%)/SnO2/Si films crystallize in a tetragonal rutile-structure and the Sb-doping does not affect the growth direction of the diffracted peaks but increases the crystallites size. The lower Sb/Sn ratio (∼0.2) revealed a better homogeneity of SnO2 onto Silicon than the glass substrate. Indeed, the AFM images showed that Si gives a smoother Sb/SnO2 film. The electrochemical measurements were studied with the variation of Sb(%)/SnO2/Si. The Capacitance-Potential (C-2 – E) characteristics confirmed the n-type nature induced by the compensation mechanism Sb5+/Sn4+, the flat band potentials of −0.83 and −0.85 V for SnO2/Si and Sb(0.85 %)/SnO2/Si respectively were obtained and both values are cathodic enough to reduce Cr(VI). The variation of the impedance with the frequency and the effect of the substrate on the transport properties showed that the grain boundaries dominate the conduction mechanism, influenced by the microstructures of the films (crystallography and texture). Sb(%)/SnO2/Si/Cr(VI) solution provided variable electrochemical properties, and an appreciable difference in the light absorbance confirmed the chromate reduction (λmax = 350 nm, 16 ppm) to less hazardous form Cr3+; a conversion of 80 % was obtained under solar light.

Influence of B-atom in g-C3N4 matrix to enhance the photocatalytic dinitrogen to ammonia conversion

Boron-doped metal-free catalysts (B-g-C3N4) were successfully synthesized via the facile thermal polymerization method. In B-doped g-C3N4 synthesis, boric acid was varied viz; 1%, 3%, 6%, and 9% as compared to 2 g of melamine. 6% B-doped g-C3N4 was found to be the optimized catalyst showcasing enhanced performance for photocatalytic nitrogen reduction reaction (NRR). XRD and FT-IR results revealed that B doping did not change the basic chemical structure of g-C3N4. The presence of boron in the synthesized materials was confirmed via EDS, Elemental mapping, and XPS analysis. Boron doping also increased the carrier lifetime of g-C3N4 confirmed by time-resolved photoluminescence analysis. 6% B-g C3N4 showed the best photo-catalytic ammonia production among all the synthesized catalysts. It could produce ammonia up to 345.35 μmol.L−1.h−1 as compared to pure g-C3N4 (112.06 μmol. L −1.h −1). There was no loss of N2 reduction activities after 5 cycles of catalyst reuse. Mott-Schottky, photocurrent, and EIS were used to determine the flat band potential, electron recombination rate, and charge transfer resistance. Post N2 reduction, XRD, and XPS analysis of pure g-C3N4 and 6% B-g-C3N4 were almost identical revealing high stability of the doped g-C3N4. The doped boron enhances the optical and electrical properties and enhances photocatalytic activity for the production of NH3.

Dual strategy of ultrasound and doping enhance Mott Schottky heterojunction carrier exchange for efficient oxygen evolution

Metal–organic frameworks (MOFs) materials are highly efficient materials for catalytic hydrogen production from electrolytic water, but the electronic configuration severely limits catalytic activity. In this study, UE-S-FeBTC was synthesized on metal substrate nickel iron foam (NIF) by one-step electrodeposition to construct Mott Schottky heterojunction. Chemical S doping mixes the p-π conjugation in the overall d-π conjugation system, effectively modulating the electronic configuration of FeBTC and enhancing carrier diffusion at the interface. Moreover, physical ultrasound of the synthesis process results in a tighter contact interface, thus improving the carrier exchange between the semiconductor and the substrate, which causes a further enhancement of flat band potential (FBP). The material exhibits good catalytic activity, requiring only 306 mV overpotential to drive 100 mA cm−2 and remains stable at 200 mA cm−2 for up to 12 h. Excellent catalytic activity derived from closer contact at the Mott Schottky heterojunction interface and the larger specific surface area formed by sonication resulting in the loading of more active sites. The stronger FBP also effectively improves the carrier density at the catalytic active site. This study provides a new design idea for the construction of Mott Schottky heterojunction catalysts with strong FBP.

Electrocatalytic Mechanisms for an Oxygen Evolution Reaction at a Rhombohedral Boron Monosulfide Electrode/Alkaline Medium Interface.

Rhombohedral boron monosulfide (r-BS) with a layer stacking structure is a promising electrocatalyst for an oxygen evolution reaction (OER) within an alkaline solution. We investigated the catalytic mechanisms at the r-BS electrode/alkaline medium interface for an OER using hybrid solvation theory based on the first-principles method combined with classical solution theory. In this study, we elucidate the activities of the OER at the outermost r-BS sheet with and without various surface defects. The Gibbs free energies along the OER path indicate that the boron vacancies at the first and second layers of the r-BS surface (VB1 and VB2) can promote the OER. However, we found that the VB1 is easily occupied by the oxygen atom during the OER, degrading its electrocatalytic performance. In contrast, VB2 is suitable for the active site of the OER due to its structure stability. Next, we applied a bias voltage with the OER potential to the r-BS electrode. The bias voltage incorporates the positive excess surface charge into pristine r-BS and VB2, which can be understood by the relationship between the OER potential and potentials of zero charge at the r-BS electrode. Because the OH- ions are the starting point of the OER, the positively charged surface is kinetically favorable for the electrocatalyst owing to the attractive interaction with the OH- ions. Finally, we qualitatively discuss the flat-band potential at a semiconductor/alkaline solution interface. It suggests that p-type carrier doping could promote the catalytic performance of r-BS. These results explain the previous measurement of the OER performance with the r-BS-based electrode and provide valuable insights into developing a semiconductor electrode/water interface.

Unleashing the power of solar light: WO3 nanorods decorated onto BiVO4 dendrites for tetracycline detoxification and water splitting.

The coupling of different oxide materials in a nanohybrid enables the customization of their optical and charge transport properties, leading to improved interfacial charge segregation and migration. In this study, BiVO4/WO3 (BVW), a sunlight-driven photocatalyst with distinct mole ratios was synthesized via a facile hydrothermal approach. The resultant catalyst exhibits a nanorods shape morphology decorated onto dendrite-like matrix and is studied for photocatalytic elimination of tetracycline (TC) and photoelectrocatalytic (PEC) H2 production. The effect of illumination time, solution pH, photocatalyst concentration, and mole ratios of BiVO4 to WO3 on the photocatalytic abatement of TC were tested sequentially as effective operating factors. Under optimal condition, 3:1 BiVO4:WO3 (31BVW) nanohybrid demonstrated a maximum degradation efficacy of 96.2% (rate constant ~0.0241 min-1), which is much better than its individual components and commercial TiO2-P25 (50.9%). The resultant by-products of TC decomposition were analyzed using GC-MS to explain the degradation mechanism. Moreover, as a photoanode, 31BVW showed a high photocurrent density of 0.64 mA/cm2 at 1.23 V vs RHE and a steady photocurrent for ~6 h under chronoamperometry study at1.23 V vs RHE. However, bare BiVO4 and WO3 exhibited the photocurrent density of 0.001 mA/cm2, and 0.015 mA/cm2, respectively at 1.23 V vs RHE. The Mott-Schottky analysis of 31BVW confirms their n-type behavior, with a calculated flat band potential of -0.067 V. The hydrogen production rate was theoretically calculated as 4.56 mmolcm-2 s-1 from chronoamperometric measurements. The photocatalyst's efficacy in TC degradation was further established via its reusability upto 7 cycles. Post degradation characterization of catalyst confirms its stability in lieu of practical usage. Comparative studies with existing literature revealed the superiority of reported photocatalysts in both applications. Overall, the binary BVW photocatalyst shows great potential for removing detrimental contaminants as well as H2 production via PEC water splitting due to efficient charge separation, reduced recombination, high surface area, and widen absorption window of the nanohybrid.

Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting.

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe2O3) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe2O3-Fe3O4-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe2O3 nanosheet layer serves as a photoanode, whereas the conducting Fe3O4 layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe2O3 grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm-2 at 1.23 VRHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 1017 cm-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Photoelectrochemical passivation of undoped n-InP by ultra-thin polyphosphazene film: Towards a perfect photoanode?

For the first time, the photoanodic behavior of undoped n-InP (≈ 1015 atoms.cm−3) is studied in liquid ammonia at low temperature (-55 °C) under atmospheric pressure. Under illumination, the low doping level provides unique conditions to monitor charge transfer at the interface since only photo-generated holes are involved in the passivation mechanism. Specific photoanodic transient phenomena are observed due to the photo-holes availability at the interface which depends on light intensity. Cycle voltammograms differ strongly from the initial ones in the dark. Photo-electrochemical transformations of InP responses are followed by cycle voltammetry for two conditions of illumination; under low and high luminous flux intensity. In spite of a pure photo-holes process for both cases, voltammograms are highly dependent on light intensity while impedance measurements present the same strong modifications. One-volt positive shift of the flat band potential is indeed observed. However, whatever is the luminous flux intensity, a low photo-anodic charge is consumed (≈ 1 mC.cm−2) and the same ultra-thin film of polyphosphazene onto n-InP is shown by XPS analyses. Properties of this new interface is discussed in term of energy diagram and photo-holes transfer ability. Under these experimental conditions used, (J ≤ 100 µA.cm−2 and Q ≈ 50 mC.cm−2), the passivated interface behaves as a photoanode toward ammonia oxidation since the ultra-thin film is maintained onto InP.

Facile synthesis, characterization, and photocatalytic performance of Bi1-xLaxFeO3 (0 ≤ x ≤ 0.25) perovskite nanoparticles

In this study, we report the combustion-based synthesis and characterization of La-substituted BiFeO3 (BFO) perovskite nanoparticles. Undoped BFO NPs (x = 0) were found to have a single perovskite phase with a rhombohedral structure, while La-BFO NPs (x = 0.05 to 0.25) with Bi3+, La3+, Fe2+, and O2– ions species showed an average crystalline size (≈ 16–22 nm increase) and a reduced band gap. Morphological analyses revealed that La-BFO NPs have an irregular shape, with a propensity towards aggregation and the formation of fused grains with well-defined grain boundaries. The magnetization-field hysteresis curves demonstrated that La-BFO NPs exhibit ferromagnetic behavior at ambient temperature. The surface alterations were assessed using the Zeta potential, and it was discovered that the La-doping BFO perovskite system can generate a negative shift in the flat-band potential. Furthermore, the photocatalytic degradation (PCD) of rhodamine B by La-BFO NPs in the presence of visible light was investigated. When a little amount of H2O2 was added during photocatalysis, La-BFO NPs demonstrated photo-Fenton-like catalytic activity, leading to 99.50% PCD efficiencies.

Surface Modification of Nickel Titanate Nanocubes with Ultra‐small Palladium and Platinum Nanocrystals to Promote Solar Hydrogen Generation under Visible Light

AbstractModification of nickel titanate nanocubes with metal nanocrystals is a viable approach to build a Schottky heterojunction for efficient hydrogen generation. The deconvoluted XPS spectrum of Pt 4 f (peaks at 72.0 and 75.5 eV corresponding to Pt° and Pt2+) and Pd 3d3/2 (335.4 and 336.7 eV corresponding to Pd° and Pd2+) and presence of lattice fringes in HRTEM at 2.19 Å of Pd°, reveal the formation of heterojunction in Pd−NiTiO3 and Pt−NiTiO3 nanocubes. The solar hydrogen generation investigation exhibits 2‐fold enhancement in HER (130 and 165 μmol g−1 h−1 while using Pt−NiTiO3 and Pd−NiTiO3, respectively) than that of bare NiTiO3 (88 μmol g−1 h−1). The creation of heterojunctions between titanates and metal nanoparticles, facilitating efficient transport of photo‐generated electron to empty or partially filled d or f orbitals of metals, thereby lowering electron‐hole recombination rate, as revealed by shorter average lifetime 29 ns (Pd−NiTiO3) than 64 ns (NiTiO3). Further, the unison of faster charge transfer kinetics as revealed by the Nyquist plot, more negative flatband potential (Efb −0.3 vs. RHE) leading to appropriate band bending, reduced overpotential requirement, higher oxygen vacancies (19.46 %) and uniform dispersion of metal atoms on NiTiO3 surfaces that are acting as trapping centers etc. are enabling improved hydrogen generation in the case of Pd−NiTiO3.

Terminal Deuterium Atoms Protect Silicon from Oxidation.

In recent years, the hybrid silicon-molecular electronics technology has been gaining significant attention for applications in sensors, photovoltaics, power generation, and molecular electronics devices. However, Si-H surfaces, which are the platforms on which these devices are formed, are prone to oxidation, compromising the mechanical and electronic stability of the devices. Here, we show that when hydrogen is replaced by deuterium, the Si-D surface becomes significantly more resistant to oxidation when either positive or negative voltages are applied to the Si surface. Si-D surfaces are more resistant to oxidation, and their current-voltage characteristics are more stable than those measured on Si-H surfaces. At positive voltages, the Si-D stability appears to be related to the flat band potential of Si-D being more positive compared to Si-H surfaces, making Si-D surfaces less attractive to oxidizing OH- ions. The limited oxidation of Si-D surfaces at negative potentials is interpreted by the frequencies of the Si-D bending modes being coupled to that of the bulk Si surface phonon modes, which would make the duration of the Si-D excited vibrational state significantly less than that of Si-H. The strong surface isotope effect has implications in the design of silicon-based sensing, molecular electronics, and power-generation devices and the interpretation of charge transfer across them.

Enhancing the Photocatalytic Performance of BiVO4 for Micropollutant Degradation by Fe and Ag Photomodification

Wider application of BiVO4 (BVO) for photocatalytic water treatment is primarily limited by its modest photocatalytic effectiveness, despite its appropriately narrow band gap for low-cost, sunlight-facilitated water treatment processes. In this study, we have photomodified an isotype BVO, consisting of a tetragonal zircon and monoclinic scheelite phase, with Fe (Fe@BVO) and Ag (Ag@BVO) ionic precursors under UV illumination in an aqueous ethanol solution in order to assess their effect on the opto-electronic properties and effectiveness for the removal of ciprofloxacin (CIP). Fe@BVO failed to demonstrate enhanced effectiveness over pristine BVO, whereas all Ag@BVO achieved improved CIP degradation, especially 1% Ag@BVO. At pH 4 and 6, 1% Ag@BVO demonstrated nearly 24% greater removal of CIP than BVO alone. Photomodification with Fe created surface oxygen vacancies, as confirmed by XPS and Mott–Schottky analysis, which facilitated improved electron mobility, although no distinct Fe-containing phase nor Fe-doping was detected. On the other hand, the introduction of mid-band gap states by oxygen vacancies decreased the reducing power of the photogenerated electrons as the flat band potentials were shifted to more positive values, thus likely negatively impacting superoxide formation. In contrast, Ag-photomodification (Ag@BVO) resulted in the formation of Ag2O/AgO and Ag nanoparticles on the surface of BVO, which, under illumination, generated hot electrons by surface plasmon resonance and enhanced the mobility of photogenerated electrons. Our research underscores the pivotal role of photogenerated electrons for CIP degradation by BiVO4-based materials and emphasizes the importance of appropriate band-edge engineering for optimizing contaminant degradation.

Electrochemical Properties of BaTaO2N Photocatalyst with Visible‐Light‐Driven Water Splitting Capability

AbstractA BaTaO2N (BTON) particulate photocatalyst enables solar water splitting in response to visible light irradiation at wavelengths of up to 640 nm. The specific nitridation conditions produced distinct types of BTON particles with the capability of one‐step overall water splitting (Active‐BTON) and without the overall water splitting activity (Inactive‐BTON). Unveiling the intrinsic differences between the active‐ and inactive‐BTON particles is crucial for obtaining more in‐depth information about the water splitting activity. Herein, we investigated the electrochemical (EC) and photoelectrochemical (PEC) properties of these BTON photocatalysts using the particulate‐based photoelectrodes for water splitting. EC measurements, including Mott–Schottky analysis, revealed that the flat band potential of Active‐BTON is located at a potential that is more positive than that of Inactive‐BTON, whereas the carrier concentration of Active‐BTON is one‐tenth lower than that of Inactive‐BTON. Irrespective of the pH value of the 1.0 M potassium phosphate aqueous solution, the Active‐BTON‐based photoelectrodes showed a higher photocurrent than that of Inactive‐BTON under simulated AM 1.5G solar illumination. The PEC performance of the BTON was found to be limited by the electrocatalytic activity of the CoOx co‐catalyst, specifically the electrolyte pH.

CoMn2O4: A new spinel photocatalyst for hydrogen photo-generation under visible light irradiation

CoMn2O4 synthetized by co-precipitation, was successfully tested as photo-catalyst in the H2 photo-evolution under soft experimental conditions. The single phase CoMn2O4 was confirmed by X-ray diffraction (XRD), FTIR spectroscopy and SEM analysis. The black oxide exhibits a good activity in the visible range with a forbidden band (Eg) of 1.61 eV. The latter is obtained from the diffuse reflectance spectrum and assigned to d-d transition of the normal spinel. The BET analysis of CoMn2O4 gave a specific surface area of 24 m2/g. The electrical and photo-electrochemical properties have also been studied; the flat band potential is determined from the capacitance measurement (Efb = 0.17 VSCE). According to the chrono-amperometry curve, CoMn2O4 exhibits p-type conduction with an activation energy of 0.18 eV relative to the Arrhenius-type law. The product turns out to be photo-electrochemically active under visible light, CoMn2O4-CB (-0.88 VSCE) offers the possibility of spontaneous H2-generation under soft conditions {pH ∼ 13, 50 °C, [S2O3-2] = 0.025 mol/L, dose CoMn2O4 = 1.25 mg/mL}. An evolution rate of 0.175 μmol mg−1 min−1 of H2 was obtained with a quantum efficiency of 1.47% under a radiation flux of 19 mW cm−2 (1.38 × 1019 photons s−1).

Photoelectrochemical Methods for the Determination of the Flat-Band Potential in Semiconducting Photocatalysts: A Comparison Study.

In addition to the band gap of a semiconducting photocatalyst, its band edges are important because they play a crucial role in the analysis of charge transfer and possible pathways of the photocatalytic reaction. The Mott-Schottky method using electrochemical impedance spectroscopy is the most common experimental technique for the determination of the electron potential in photocatalysts. This method is well suited for large crystals, but in the case of nanocatalysts, when the thickness of the charged layer is comparable with the size of the nanocrystals, the capacitance of the Helmholtz layer can substantially affect the measured potential. A contact between the electrolyte and the substrate, used for deposition of the photocatalyst, also affects the impedance. Application of other photoelectrochemical methods may help to avoid concerns in the interpretation of impedance data and improve the reliability of measurements. In this study, we have successfully prepared five visible-light active photocatalysts (i.e., N-doped TiO2, WO3, Bi2WO6, CoO, and g-C3N4) and measured their flat-band potentials using four (photo)electrochemical methods. The potentials are compared for all methods and discussed regarding the type of semiconducting material and its properties. The effect of methanol as a sacrificial agent for the enhanced transfer of charge carriers is studied and discussed for each method.

Robust SrTiO3 Passivation of Silicon Photocathode by Reduced Graphene Oxide for Solar Water Splitting.

Development of a robust photocathode using low-cost and high-performing materials, e.g., p-Si, to produce clean fuel hydrogen has remained challenging since the semiconductor substrate is easily susceptible to (photo)corrosion under photoelectrochemical (PEC) operational conditions. A protective layer over the substrate to simultaneously provide corrosion resistance and maintain efficient charge transfer across the device is therefore needed. To this end, in the present work, we utilized pulsed laser deposition (PLD) to prepare a high-quality SrTiO3 (STO) layer to passivate the p-Si substrate using a buffer layer of reduced graphene oxide (rGO). Specifically, a very thin (3.9 nm ∼10 unit cells) STO layer epitaxially overgrown on rGO-buffered Si showed the highest onset potential (0.326 V vs RHE) in comparison to the counterparts with thicker and/or nonepitaxial STO. The photovoltage, flat-band potential, and electrochemical impedance spectroscopy measurements revealed that the epitaxial photocathode was more beneficial for charge separation, charge transfer, and targeted redox reaction than the nonepitaxial one. The STO/rGO/Si with a smooth and highly epitaxial STO layer outperforming the directly contacted STO/Si with a textured and polycrystalline STO layer showed the importance of having a well-defined passivation layer. In addition, the numerous pinholes formed in the directly contacted STO/Si led to the rapid degradation of the photocathode during the PEC measurements. The stability tests demonstrated the soundness of the epitaxial STO layer in passivating Si against corrosion. This study provided a facile approach for preparing a robust protection layer over a photoelectrode substrate in realizing an efficient and, at the same time, durable PEC device.

Biodegradation of Oxide Nanoparticles in Apoferritin Protein Media: A Systematic Electrochemical Approach

AbstractFunctional oxide nanoparticles are extensively utilized in the last decades for biomedical purposes due to their unique functional properties. Nevertheless, their biodegradation mechanism by biological species, particularly by proteins at oxide/protein interfaces, still remains limited. Here, a systematic approaches is provided to investigate electrochemical behavior, electronic properties, and biodegradation mechanism of cobalt ferrite (CFO) and cobalt ferrite‐bismuth ferrite (CFO‐BFO) core‐shell nanoparticles in apoferritin‐containing media. Scanning Kelvin probe force microscopy results indicate that the presence of a thin shell (≈5 nm) of BFO on CFO causes a significant increase in surface potential. The potentiodynamic polarization measurements in different solutions showed higher anodic current densities for both samples when decreasing pH and increasing apoferritin concentration. Notably, CFO‐BFO exhibits lower anodic current densities than CFO due to a slightly higher flat band potential and lower donor density distribution on CFO‐BFO than on CFO, which results in lower electrochemical activity. Long‐term monitoring reveals that biodegradation of both nanoparticles is accelerated by high apoferritin concentrations and acidic media, resulting in the decrease of electrochemical potentials and impedance values, and enhancement of metal ion release. Thus, this systematic biodegradation study can help to predict the lifespan and toxicity of these functional nanoparticles in biological environments.

VFe@Ni/Ni3N Mott-Schottky heterojunction induced electronic modulation for efficient alkaline water splitting

Designing affordable and productive bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been a long-term goal of anion exchange membrane water electrolysis (AEMWE). Heteroatoms doping and heterostructure construction are generally considered as effective methods for electrocatalysts modification. Herein, V and Fe co-doped Ni/Ni3N (VFe@Ni/Ni3N) electrocatalyst with a Mott-Schottky heterostructure was prepared by hydrothermal and NH3 reduction methods. The formation of Ni/Ni3N Mott-Schottky heterojunction can effectively trigger the self-driven carrier transfer on the heterointerfaces, thus accelerating charges achievement to the surface active sites and advancing the catalytic efficiency. Meanwhile, the co-doping of V and Fe lowers the flat-band potential of Ni/Ni3N, leading to greater electron density, which increases the electron transport potentiality and facilitates the adsorption and reduction of H* in the HER process. Therefore, a remarkable performance for HER with overpotentials of 47 and 180 mV at 10 and 200 mA cm−2 in alkaline solution was achieved. Significant progress has also been made in OER and the full water electrolysis. Furthermore, for the AEMWE with VFe@Ni/Ni3N, a low applied cell voltage of only 1.75 V was needed to reach a high current density of 600 mA cm−2 with an impressive long-term stability of 100 h. This study supplies an innovative strategy of electronic modulation by co-doping and Mott-Schottky heterojunction construction, which is referential for the advancement of economic electrocatalysts for sustainable energy systems.