Photocurrent Onset Potential Research Articles

Maximizing Quantum Hole Collection on Anatase TiO2 Thin Films with (001) Facets for Photoelectrochemical Oxigen Evolution Reaction Efficiency

Hydrogen production from water, a key element in the renewable energy portfolio, offers a solution to reduce reliance on fossil fuels. Photoelectrochemical (PEC) hydrogen production, using light-absorbing semiconductors and catalysts, offers a clean and cost-effective method for harnessing solar energy. To enhance the efficiency of PEC, this study explores the use of anatase TiO2 thin film photoelectrodes as an alternative to expensive and rare catalysts, aiming to generate hydrogen as a clean fuel with oxygen as a byproduct.Anatase titanium dioxide is a well-established functional material widely applied in the fields of environment and energy. Recognized as one of the premier materials for photocatalysis and a promising photoanode material for water splitting, titanium dioxide (TiO2) finds use in diverse applications such as dye-sensitized solar cells, perovskite-based solar cells, and cleaning devices. Its band gap of 3.2 eV, stability, strong absorption in the UV region, and energetics of the valence and conduction band edges make anatase particularly suitable for PEC water splitting, especially for the oxygen evolution reaction (OER).Our research focuses on the development of anatase thin films with strong [001] texture and (001) facets at the surface of the film. The (001) facets are known to accumulate photogenerated hole states while the [001] orientation imparts a negative flat band potential. We hypothesize that these two properties, (001) facets and [001] texture, are ideal for OER. This study combines these two structural characteristics within the semiconductor TiO2 layer to enhance PEC performance and maximize the efficiency of hole collection during water oxidation.Anatase TiO2 thin films with (001) facets and [001] orientation were hydrothermally synthesized on a fluorine-doped tin oxide (FTO) substrate and annealed at 500°C. The films exhibited [001] orientation and (001) faceting as shown by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Photoelectrochemical measurements via cyclic voltammetry in a 0.1 M KOH and Na2SO3 solution demonstrated the films' hole collection efficiency under a 365 nm UV light source. The photoelectrochemical (PEC) and oxygen evolution reaction (OER) characteristics of anatase thin films were observed both under dark and light conditions. Prior to illumination, only the Ti3+/Ti4+ redox couple was observed with an onset potential of 0.01 V (RHE) and negligible OER at anodic potentials. When UV irradiated, the TiO2 thin films exhibited a photocurrent that increased through the anodic scan. The onset potential of photocurrent, measured in the presence of the hole scavenger, exhibited a negative shift ranging from -100 to -200 mV compared to the KOH onset potential. This shift is attributed to the film surface's limited efficiency in collecting holes for water oxidation at low potentials, as rapid recombination kinetics compete with water oxidation. Interestingly, the photocurrent observed with Na2SO3 electrolyte was 0.15 to 0.3 times more than what we saw with KOH electrolyte. Our preliminary results indicate a current density of ~2.0-2.5 mA cm-2 at 1.8 V and a hole collection efficiency of ~84 % at 1.23 V vs RHE, the thermodynamic water oxidation potential. In summary, we have made significant progress toward enhancing the PEC activity of anatase TiO2 thin films by optimizing the film structure for both reactivity and charge transport.

Suppressing substrate oxidation during plasma-enhanced atomic layer deposition on semiconductor surfaces

Plasma-enhanced atomic layer deposition (PE-ALD) is widely employed in microelectronics, energy, and sensing applications. Typically, PE-ALD processes for metal oxides utilize remote inductively coupled plasmas operated at powers of >200 W, ensuring a sufficient flux of oxygen radicals to the growth surface. However, this approach often leads to significant oxidation of chemically sensitive substrates, including most technological semiconductors. Here, we demonstrate that plasma powers as low as 5 W can effectively suppress substrate oxidation while maintaining the structural, optical, and electronic quality of the films. Specifically, we investigate the growth of titanium oxide (TiOx) using two commonly used metalorganic precursors, titanium isopropoxide and tetrakis(dimethylamino)titanium. Films deposited with 5 and 300 W oxygen plasma power are nearly indiscernible from one another, exhibiting significantly lower defect concentrations than those obtained from thermal ALD with H2O. The low plasma power process preserves desired physical characteristics of PE-ALD films, including large optical constants (n > 2.45 at 589 nm), negligible defect-induced sub-bandgap optical absorption (α < 102 cm−1), and high electrical resistivity (>105 Ω cm). Similar behavior, including suppressed interface oxidation and low defect content, is observed on both Si and InP substrates. As an example application of this approach, the assessment of InP/TiOx photocathodes and Si/TiOx photoanodes reveals a significant improvement in the photocurrent onset potential in both cases, enabled by suppressed substrate oxidation during low power PE-ALD. Overall, low power PE-ALD represents a generally applicable strategy for producing high quality metal oxide thin films while minimizing detrimental substrate reactions.

Low Onset‐Potential Z‐Scheme Ta3N5‐based Photoanode with Enhanced Light Harvesting and Charge Transport

AbstractSimultaneous enhancement of light harvesting and charge transport in photoelectrochemical (PEC) systems is a major challenge to achieving high solar‐to‐hydrogen efficiency. Here, a Ta3N5‐Si Z‐scheme system is constructed to facilitate charge transport pathways from generation to catalysis, taking advantage of the exquisite bandgap and band position of Ta3N5. The tailored Ta3N5‐Si junction with an NbNx electron mediator effectively establishes a Z‐scheme charge transport and enhances the driving force for water oxidation, reducing the onset potential by an increment in photovoltage. Moreover, the nitrogen‐doped CoFeOx co‐catalyst boosts hole dynamics and kinetics at the surface level, resulting in improved hole extraction for water oxidation catalysis. The synergy between the above strategies cooperatively expedites the charge separation and transport in a Ta3N5 photoanode, which decreases the photocurrent onset potential from 0.69 to 0.27 V versus the reversible hydrogen electrode, a reduction of 420 mV. This result represents one of the lowest onset potentials observed for Ta3N5‐based photoanodes. A systematic approach to enhancing photovoltage and photocurrent expands the design concept of metal nitride‐based PEC devices.

Defect Engineering of Ta3N5 Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping

AbstractWhile Ta3N5 shows excellent potential as a semiconductor photoanode for solar water splitting, its performance is hindered by poor charge carrier transport and trapping due to native defects that introduce electronic states deep within its bandgap. Here, it is demonstrated that controlled Ti doping of Ta3N5 can dramatically reduce the concentration of deep‐level defects and enhance its photoelectrochemical performance, yielding a sevenfold increase in photocurrent density and a 300 mV cathodic shift in photocurrent onset potential compared to undoped material. Comprehensive characterization reveals that Ti4+ ions substitute Ta5+ lattice sites, thereby introducing compensating acceptor states, reducing the concentrations of deleterious nitrogen vacancies and reducing Ta3+ states, and thereby suppressing trapping and recombination. Owing to the similar ionic radii of Ti4+ and Ta5+, substitutional doping does not introduce lattice strain or significantly affect the underlying electronic structure of the host semiconductor. Furthermore, Ti can be incorporated without increasing the oxygen donor content, thereby enabling the electrical conductivity to be tuned by over seven orders of magnitude. Thus, Ti doping of Ta3N5 provides a powerful basis for precisely engineering its optoelectronic characteristics and to substantially improve its functional characteristics as an advanced photoelectrode for solar fuels applications.

(Invited) Dilute Anion Alloyed III-Nitride Nanowires for Photoelectrochemical Water Splitting

The record efficiencies in photovoltaic devices and hydrogen production via photoelectrochemical water splitting have been achieved in both cases by employing complex architectures of III-V alloys 1, which has been enabled by the outstanding charge carrier transport properties, band gap and band edge engineering flexibility, as well as highly mismatch growth that can be achieved in III-V alloys by controlling their chemical composition.It has been extensively demonstrated that Molecular Beam Epitaxy (MBE) and Metal-Organic Physical Vapor Deposition (MOPVD) techniques can produce III-V multinary alloys with compositions beyond miscibility gaps due to their “far from equilibrium” nature. And although great progress has been made towards improving the growth rate in these techniques, III-V device synthesis cost is still significantly higher (one order of magnitude) than Si solar cells manufacturing technologies 2. In this context, it is necessary to develop novel synthesis methods that yield high crystalline quality III-V alloys, higher throughput, and lower operational cost. For instance, Hydride/Halide Vapor Phase Epitaxy HVPE-grown GaAs top cell incorporated in a GaAs/Si tandem cell is an example of an optimized photovoltaic device with 31.4% efficiency, showcasing the uttermost potential of this growth method 2.More recently, Halide Vapor Phase Epitaxy growth of GaSbzP1-z polycrystalline free-standing films with up to 6.7% antimony incorporation and growth rates around 500 µm/h has been reported3. Linear sweep voltammetry measurements in 1M H2SO4 using GaSbzP1-z photoanodes showed lower onset potentials and higher fill factors compared to single crystal GaP. These results have been attributed to the visible light absorbing capabilities (direct band gaps between 2.2 and 2.5 eV), and lower charge transfer resistance at the semiconductor/electrolyte interface.On the other hand, a novel method for gallium nitridation involving plasma-activated nitrogen species dissolution into molten gallium has enabled the growth of single crystal GaN4. Furthermore, dilute anion Ga-Nitride alloys have been synthesized with the same technique but including other V-group species ions, i.e., Sb+ and Bi+, that can substitute nitrogen forming ternary GaSbxN1-x and GaBiyN1-y.In this presentation, we show the progress with dilute anion alloyed III-Nitride nanowires, GaSbxN1-x and GaBiyN1-y, synthesized by dissolving gallium, antimony or bismuth and plasma-activated nitrogen species into copper or gold droplets. Antimony and bismuth incorporation levels determined through x-ray diffraction c-plane peak shift, x and y, reached levels as high as 5.6 and 8.8 at%, respectively, resulting in direct band gap energies around 2.0 eV, determined through diffuse reflectance and photocurrent spectroscopy measurements. The visible light absorbing nanowire films exhibited photocurrent onset potentials around 0.0 V vs RHE when tested in 1 M sulfuric acid solution and 3-electrode setup with Ag/AgCl reference and Pt counter electrodes. 2-electrode chronoamperometry measurements with p-Si and n-type dilute alloys showed stable photocurrent densities up to 8.5 mA*cm-2 in 30 h. These results show the promising potential of dilute anion alloyed III-Nitrides as stable photoanodes for water splitting applications, and demonstrate the advantage of this novel synthesis technique over MOCVD that in previous studies has enabled the obtention of 0-D or 1-D GaSbxN1-x structures with up to 7% anion incorporation, while compromising crystalline quality.5

Cooperative catalytic behavior of Bi2S3 and ZrO2 nanoparticles on Bi2O3 photoanodes for promoted stability and solar driven photoelectrochemical hydrogen production

Photoelectrochemical water splitting via solar energy consumption is recognized as a strategy to address fossil resources and global warming concerns. Here, we reveal the co-catalytic effect of ZrO2 and Bi2S3 nanoparticles on nanostructured bismuth oxide (Bi2O3) electrodes synthesized by electrochemical deposition and an in-situ photoelectrochemical transformation process to fabricate heterostructured Bi2S3/Zr-Bi2O3 electrodes. The cooperative interaction between ZrO2 and Bi2S3 nanoparticles leads to a 12-fold enrichment in Bi2O3 electrode photocurrent density. The cooperation between the above features has considerably reduced the photocurrent onset potential, and improved the separation of charge carriers and optical features of the Bi2O3:Zr/Bi2S3 electrodes resulting in attaining an applied bias photon-to-current efficiency (ABPE) of 1.22 %. Through these examinations, it is possible to create a variety of nanostructured electrode films with adjustable optical and electronic features for solar cells, electrochemical energy storage, and PEC.

Multifunctional Role of Ag-Substitution in Enhancing the Photoelectrochemical Properties of LaFeO3 Photocathodes.

Earth-abundant LaFeO3 is a promising p-type semiconductor for photoelectrochemical cells due to its stable photoresponses, high photovoltages and appropriate band alignments, but the photoelectrochemical properties of LaFeO3 , especially the incident-photon-to-current conversion efficiency, need to be further improved. Herein, we propose to partially substitute La3+ of LaFeO3 with Ag+ to enhance the photoelectrochemical performance of LaFeO3 . The combined experimental and computational studies show that Ag-substitution improves surface charge transfer kinetics through introducing active electronic states and increasing electrochemically active surface areas. Furthermore, Ag-substitution decreases grain boundary number and increases majority carrier density, which promotes bulk charge transports. Ag-substitution also reduces the bandgap energy, increasing the flux of carriers involved in photoelectrochemical reactions. As a result, after 8 % Ag-substitution, the photocurrent density of LaFeO3 is enhanced by more than 6 times (-0.64 mA cm-2 at 0.5 V vs RHE) in the presence of oxygen, which is the highest photocurrent gain compared with other cation substitution or doping. The corresponding photocurrent onset potential also demonstrates a positive shift of 30 mV. This work highlights the versatile effects of Ag-substitution on the photoelectrochemical properties of LaFeO3 , which can provide useful insights into the mechanism of enhanced photoelectrochemical performance by doping or substitution.

High‐Performance Silver‐Doped Porous CuBi2O4 Photocathode Integrated with NiO Hole‐Selective Layer for Improved Photoelectrochemical Water Splitting

AbstractCuBi2O4(CBO) has received considerable attention owing to its ideal optical bandgap and positive photocurrent onset potential. However, CBO photocathodes exhibit poor charge carrier separation and transfer across the conducting substrate interface. Herein, a systematic incorporation of Ag+‐cations into nanosized porous CBO network (ACBO) using a simple pulsed‐electrodeposition method. In ACBO photocathode, the Ag+‐ions replace the Bi3+‐ions, thereby building an increased hole concentration, which further signifies the photogenerated electron‐hole separation. Additionally, introducing a low‐cost NiO hole‐selective layer between ACBO and the conducting substrate enables a back‐interface‐aided hole‐extraction and electron blocking, resulting in an improved charge transfer across the back interface. Compared with an unmodified CBO, the NiO/ACBO photocathode exhibits a three‐fold enhanced photocurrent performance. This enhances the photocurrent originating from the incorporation of a substantial amount of Ag+‐ions into the CBO structure, leading to an increased acceptor density as well as the formation of an appropriate hole‐selective layer across the back‐contact. The absorption percentage, time‐resolved photoluminescence, and photoelectrochemical impedance spectroscopy measurements unveil the potential light harvesting, charge separation, and transfer characteristics of the NACBO photoelectrode, respectively. Through this systematic study, an efficient and simple strategy is determined for developing ternary metal oxide‐based photocathode/photoanode systems for sustainable energy applications.

Photoelectrochemistry of a photosystem I – Ferredoxin construct on ITO electrodes

In this study, photobioelectrodes based on a ferredoxin-modified photosystem I (PSI-Fd) from Thermosynechococcus vestitus have been prepared and characterized regarding the direct electron transfer between PSI-Fd and the electrode. The modified PSI with the covalently linked ferredoxin (Fd) on its stromal side has been immobilized on indium-tin-oxide (ITO) electrodes with a 3-dimensional inverse-opal structure. Compared to native PSI, a lower photocurrent and a lower onset potential of the cathodic photocurrent have been observed. This can be mainly attributed to a different adsorption behavior of the PSI-Fd-construct onto the 3D ITO. However, the overall behavior is rather similar to PSI. First experiments have been performed for applying this PSI-Fd photobioelectrode for enzyme-driven NADPH generation. By coupling the electrode system with ferredoxin-NADP+-reductase (FNR), first hints for the usage of photoelectrons for biosynthesis have been collected by verifying NADPH generation.

Effect of oxygen vacancies on photoelectrochemical properties of amphoteric semiconductor Bi4Ti3O12 photoelectrode

Bi4Ti3O12 (BTO) photoelectrodes were fabricated by a sol-gel spin coating method and oxygen vacancy (OV) concentrations in BTO were regulated through a following heat-treatment in different atmospheres. Based on the results of XRD, XPS and HRTEM, OVs have been introduced in BTO after Ar processing. An interesting p/n type amphoteric photo-response characteristic was discovered in BTO as demonstrated by the LSV, CV plots and open-circuit photovoltage measurement. The typical amphoteric photo-response characteristic of BTO should be related to its special Fermi level structure and accordingly minor band bending in the interface between BTO and the electrolyte. The positive photocurrent density of Ar-BTO is 0.75 μA/cm2 at 0.5 V vs. Ag/AgCl, which is 4.4 times over that of BTO, and its photocurrent onset potential showed an obvious negative shift (∼0.1 V) in relative to BTO. The charge carrier densities of Ar-BTO was over 60% higher than that of BTO. The enhanced photoelectrochemical properties of Ar-BTO was ascribed to its abundant OVs which effectively inhibited recombination, boosted carrier injection efficiency and activated electrode surface as demonstrated by the EIS and PL characterizations. These findings provide new perspective for fabricating new type bismuth-based photoelectrodes and optimizing its photoelectrochemical performance.

Bi2 S3 -Cu3 BiS3 Mixed Phase Interlayer for High-Performance Cu3 BiS3 -Photocathode for 2.33% Unassisted Solar Water Splitting Efficiency.

To realize practical solar hydrogen production, a low-cost photocathode with high photocurrent density and onset potential should be developed. Herein, an efficient and stable overall photoelectrochemical tandem cell is developed with a Cu3 BiS3 -based photocathode. By exploiting the crystallographic similarities between Bi2 S3 and Cu3 BiS3 , a one-step solution process with two sulfur sources is used to prepare the Bi2 S3 -Cu3 BiS3 blended interlayer. The elongated Bi2 S3 -Cu3 BiS3 mixed-phase 1D nanorods atop a planar Cu3 BiS3 film enable a high photocurrent density of 7.8mA cm-2 at 0V versus the reversible hydrogen electrode, with an onset potential of 0.9 VRHE . The increased performance over the single-phase Cu3 BiS3 thin-film photocathode is attributed to the enhanced light scattering and charge collection through the unique 1D nanostructure, improved electrical conductivity, and better band alignment with the n-type CdS layer. A solar-to-hydrogen efficiency of 2.33% is achieved under unassisted conditions with a state-of-the-art Mo:BiVO4 photoanode, with excellent stability exceeding 21h.

Photochemically Etching BiVO4 to Construct Asymmetric Heterojunction of BiVO4 /BiOx Showing Efficient Photoelectrochemical Water Splitting.

BiVO4 as a promising semiconductor candidate of the photoanode for solar driven water oxidation always suffers from poor charge carrier transport property and photo-induced self-corrosion. Herein, by intentionally taking advantage of the photo-induced self-corrosion process, a controllable photochemical etching method is developed to rationally construct a photoanode of BiVO4 /BiOx asymmetric heterojunction from faceted BiVO4 crystal arrays. Compared with the BiVO4 photoanode, the resulting BiVO4 /BiOx photoanode gains over three times enhancement in short-circuit photocurrent density (≈3.2mAcm-2 ) and ≈75mV negative shift of photocurrent onset potential. This is due to the formation of the strong interacted homologous heterojunction, which promotes photo-carrier separation and enlarges photovoltage across the interface. Remarkably, the photocurrent density can remain at ≈2.0mAcm-2 even after 12h consecutive operation, while only ≈0.1mAcm-2 is left for the control photoanode of BiVO4 . Moreover, the Faraday efficiency for water splitting is determined to be nearly 100% for the BiVO4 /BiOx photoanode. The controllable photochemical etching process may shed light on the construction of homologous heterojunction on other photoelectrode materials that have similar properties to BiVO4 .

Amine‐Rich Hydrogels Enhance Solar Water Oxidation via Boosting Proton‐Coupled Electron Transfer

AbstractPhotoelectrochemical (PEC) water oxidation is a highly challenging task that acts as a bottleneck for efficient solar hydrogen production. It is because each cycle of water oxidation is composed of four proton‐coupled electron transfer (PCET) processes and conventional photoanodes and cocatalysts have limited roles in enhancing the charge separation and storage rather than in enhancing catalytic activity. In this study, a simple and generally applicable strategy to improve the PEC performance of water oxidation photoanodes through their modification with polyethyleneimine (PEI) hydrogel is reported. The rich amine groups of PEI not only allow the facile and stable modification of photoanodes by crosslinking but also contribute to improving the kinetics of PEC water oxidation by boosting the PCET. Consequently, the PEC performance of various photoanodes, such as BiVO4, Fe2O3, and TiO2, is significantly enhanced in terms of photocurrent densities and onset potentials even in the presence of notable cocatalyst, cobalt phosphate. The present study provides new insights into and strategies for the design of efficient photoelectrodes and PEC devices.

Synthesis of ZnO Nanorods-Based Photoanode and Electrochemical Characterization for Azoic Dyes Degradation: Reactive Red 239 Study Case

The use of solar energy as an energy vector has been a subject of great interest and study in recent years, showing vast improvements in the efficiencies associated with the conversion to electrical energy. This interest has extended to various applications, among which the production of green hydrogen from the decomposition of water and in environmental remediation processes stand out.The main motivation in focusing the use of solar energy in photoelectrocatalytic degradation processes is the negative impact that the disposal of colored water has on the different effluents, causing a deterioration of the quality of these ecosystems and the death of many forms of life. In this sense, the application of semiconductors based on zinc oxide (ZnO) supported on FTO conductive glass was studied for the treatment of wastewater from the textile industry, contaminated with reactive red azo dye 239.The main motivation in focusing the use of solar energy in photoelectrocatalytic degradation processes is the negative impact that the disposal of colored water effluents has on the different ecosystems, causing a deterioration of the quality of these ecosystems and the death of many forms of life. In this sense, the application of semiconductors based on zinc oxide (ZnO) supported on FTO was studied for the treatment of wastewater from the textile industry, contaminated with reactive red azo dye 239.In the present writing, a process for the synthesis of a ZnO-based semiconductor was defined. This precedence was performed directly on the substrate of 15x15x1.1mm FTO conductive glass plates. Prior to the synthesis process, a pretreatment of the glass is performed. The pretreatment of the plates consists of washing with neutral soap, then immersion in 0.01 M nitric acid (HNO3) for 10 minutes, and finally, cleaning with deionized water in ultrasound for 10 minutes. The synthesis of the nanostructured ZnO was performed in two steps. First, a chronoamperometric electrodeposition of the ZnO seeds on the substrate at a fixed potential of -1.38 V vs. Ag/AgCl as reference, and a platinum electrode as counter electrode, was performed at 70ºC for 200 seconds with a Gamry® Interface 1000 potentiostat, then subjected to a heating period at 400ºC for 2 hours. After electronucleation of the seeds and heat treatment, an additional step is carried out, which is the chemical growth of ZnO, waiting for the formation of ZnO nanorods in the (100), (002) and (101) planes typical of the wurtzite structure, this is done in a zinc nitrate solution at 90ºC, pH > 13.5, for 75 minutes.In the semiconductor characterization, different electrochemical techniques such as cyclic voltammetry, electrochemical impedance spectroscopy and anodic linear scanning were evaluated. At the same time, the response of the photoanode to illumination with a 350 W xenon arc lamp was studied, showing the remarkable effect of light on the generation of charge carriers, decreasing recombination, and increasing photocurrents. On the other hand, the density of charge carriers in the semiconductor was evaluated and defined, as well as the position of the valence and conduction bands and their respective band gap.The charge carrier density was determined from Mott-Schottky, finding densities of 6.25x1021 and 3.72x1021 carriers/cm3, for frequencies of 500 and 1000 Hz, respectively. In parallel, the flat band potential, which under certain circumstances of charge carrier density can approximate the conduction band potential, was evaluated. This was estimated from three methods, Mott-Schottky (500 and 1000 Hz), photocurrent onset potential and open circuit potential, finding values of -0.086 and -0.124 (at 500 and 1000 Hz), -0.121 and -0.161 V vs. Ag/AgCl, respectively. The determination of the bad gap was performed from UV-VIS, obtaining a value of 3.13 eV.Finally, the discoloration of the solution was evaluated from UV-VIS measurements. This analysis was performed on a problem solution that resembles the concentration of dye and salts of a textile industry effluent. A ZnO electrode was used, with a defined working area of 1.30 cm2, and a dye volume of 25 mL. The experiment was carried out for a time period of 4 hours, and an applied potential of 1 V vs. Ag/AgCl/Cl- 3M, reaching a decolorization of 23.2% of the initial solution.

Mechanistic insight into photochemical and photoelectrochemical degradation of organic pollutants with the use of BiVO4 and BiVO4/Co-Pi

In this work we clarify the role of hydroxyl radicals (•OH), superoxide anion radicals (O2•−) and the holes in photocatalytic and photoelectrocatalytic degradation of organic pollutants on BiVO4, and critically discuss different mechanisms of the process reported in the literature. The generation of O2•−anion radicals was monitored by the reaction with Nitro Blue Tetrazolium chloride (NBT), while the formation of •OH was controlled by transformation of terephthalic acid into fluorescent 2-hydroxyterephtalic acid. The band diagram of BiVO4 was constructed on the base of the Efb determined from the measurements of open circuit potential (OCP) and photocurrent onset potential, and the mechanism of photodegradation of caffeine (CAF) on BiVO4 was proposed. It was shown that the photodegradation rate constant of CAF may be increased from 2.1·10−3 to 4.6 10−3 min−1 by modification of BiVO4 surface with cobalt phosphate (Co-Pi) co-catalyst. The presence of Co-Pi resulted in the increase of the electron life-time and suppression of the electron-hole recombination in BiVO4. The 26-fold increase of the degradation rate constant, to the value 5.5·10−2 min−1, was achieved by application of an external potential of 0.6 V vs Ag/AgCl to the BiVO4 electrode and the mechanism of photoelectrocatalytic process was proposed.

Photoelectrochemical Methods for Flat Band Potential Estimation: Case Studies of ZnO Nanorods and TiO2 Compact Films

Semiconductor materials play a major role in the use of solar energy. ZnO and TiO2-based nanomaterials have been broadly used in photocatalytic applications, such as water splitting and environmental remediation. In order to determine the thermodynamic feasibility in a specific application, it’s important to determine the electronic band structure of these materials. This determination of the energetics in the semiconductor can be conducted from different approaches, usually by first determining the bandgap and conduction band edge. The bandgap determination is made through well-defined and standardized processes, unlike the conduction band, where the discrepancy is found between the values reported by different authors under the same conditions. In this article a comparison is made between the Mott-Schottky, photocurrent onset potential, and open-circuit potential (OCP) methods, as techniques of determining the flat band potential, taking as case studies the two semiconductor materials mentioned above. This comparison is followed by a discussion of the difficulties that may arise during experimentation and the possible difference between the values reported by each method.

Ni2P nanocrystals modification on Ta:α-Fe2O3 photoanode for efficient photoelectrochemical water splitting: In situ formation and synergistic catalysis of Ni2P@NiOOH cocatalyst

The application potential of hematite (α-Fe2O3) photoanode for photoelectrochemical (PEC) water splitting is restricted by its poor conductivity and severe carrier recombination. Herein, a coupling modification strategy of tantalum (Ta) doping and Ni2P nanocrystals modification is developed to realize the simultaneous optimization of photocurrent density and onset potential of α-Fe2O3 photoanode. The resulting Ni2P/Ta:α-Fe2O3 photoanode exhibits a photocurrent density of 2.98 mA/cm2 at 1.23 V vs. RHE, which is 2.76-fold higher than that of pristine α-Fe2O3. Characterization results show that Ta-doping improves the conductivity and carrier density, while Ni2P nanocrystals optimizes the hole injection efficiency and water oxidation kinetics of photoanode. Notably, Ni2P nanocrystals form a core–shell structure (Ni2P@NiOOH) in situ during the PEC reaction. Density functional theory calculations indicate that the adsorption sites for Ni2P@NiOOH cocatalyst are mainly Ni atoms at the interface between NiOOH and Ni2P, with adjacent Ni or P atoms from Ni2P core also participating in the reaction, and that the synergistic catalysis of Ni2P and NiOOH lowers the energy barrier for the key *OOH intermediates formation. Finite element simulations of the current density distribution show that Ni2P core with high conductivity exhibit a significant current-collector effect, accelerating carrier migration in Ni2P@NiOOH. This work contributes to the understanding of the catalysis of Ni2P-derived composite oxygen evolution reaction catalysts and provides a reference for the rational design of photoanode for efficient PEC water splitting.

Crystal growth promotion and interface optimization enable highly efficient Sb2Se3 photocathodes for solar hydrogen evolution

Antimony selenide (Sb2Se3) is an encouraging photocathode candidate that satisfies essential requirements for efficient solar hydrogen evolution, and also gains significant progress over the last few years. However, simultaneously achieving its satisfied photocurrent density (Jph) and onset potential (Von) remains a fundamental challenge. In this work, a thermodynamic/kinetic combination reaction process involving in-situ selenization of Sb precursor was established to synthesize a compact, large-grained, and preferentially [hk1]-oriented Sb2Se3 absorber layer. In parallel, indium (In3+) cation dopant was introduced to regulate both optical and electrical properties of CdS buffer layer. Such absorber and interface engineering exertions prospectively suppressed recombination losses and remarkably improved the planar-type Mo/Sb2Se3/CdS (In)/TiO2/Pt photocathode performance, owing to the enhanced charge carrier generation, separation, and transport. Consequently, the champion device delivers highly stimulating Jph of 35.7 mA cm−2, Von of 0.54 VRHE, also a record half-cell solar-to-hydrogen conversion efficiency of 5.6%, paving a bright avenue for its practical solar hydrogen evolution applications.

Fluorine-doped CuBi2O4 nanorod arrays for enhanced photoelectrochemical oxygen reduction reaction toward H2O2 production

CuBi2O4 (CBO) semiconductor composed of low cost and environmentally friendly Cu, Bi, and O elements possesses relatively narrow band gap, appropriate energy band position, and very positive photocurrent onset potential, which make it promising for photoelectrochemical (PEC) O2 reduction reaction (ORR). However, low separation and transfer efficiency of photogenerated carriers and poor product selectivity limit its further application in PEC ORR toward H2O2 production. In this work, we report F− ions doped CuBi2O4 (F-CBO) nanorod arrays with enhanced PEC ORR performance for H2O2 generation. PEC ORR activity of CBO nanorod arrays was obviously enhanced through decreasing the sizes of nanorods. Furthermore, the doping of F− ions into CBO lattice promoted the separation and transfer of photogenerated carriers and significantly improved the selectivity of ORR toward H2O2 path. As a result, 0.85 mmol/L of H2O2 could be achieved on the F-CBO photocathode within 45 min under simulated AM1.5G sunlight illumination (100 mW cm−2), which is 2.8 times higher than that obtained on the pristine counterpart. The roles of fluorine doping were revealed based on experimental results and density functional theory (DFT) calculations. The combination of morphology and surface doping engineering provides a new strategy for designing highly efficient photocathod for PEC ORR to H2O2 production.

Heterojunction interface engineering enabling high onset potential in Sb2Se3/CdS photocathodes for efficient solar hydrogen production

Sb2Se3 has emerged as an ideal photocathode candidate profiting from its superior optoelectronic properties, and has gained rapid development in photocurrent generation. However, achieving both high photocurrent density (Jph) and onset potential (Von) is of immense importance. In this work, self-assembled growth of Sb2Se3 with large crystal grains, benign orientation, and accurate composition was first fulfilled via a combination reaction involving sputtered and selenized Sb precursor. Then Mo/Sb2Se3/CdS/Pt photocathodes were constructed. In addition to the selenization condition-dependent Sb2Se3 film quality that influenced device performance, an additional Sb2Se3/CdS heterojunction post-annealing has demonstrated a strong positive effect. Thanks to the band alignment modification, charge transport strengthening, and surface wettability improvement, the champion device delivered Jph of 16.25 mA cm−2, Von of 0.52 VRHE, and HC-STH conversion efficiency of 2.58%. Such an interface engineering can pave the way for fabricating high Von Sb2Se3 photocathode to broaden its scope of solar hydrogen production applications.