Metal Oxide Photoanodes Research Articles

Toward combined photo-electrochemical system for degradation of ceftriaxone contaminated water over Ti-based mixed metal oxide photoanodes performance evaluation and mechanism insights

BackgroundAs a growing environmental concern over the accumulation of antibiotics in aquatic environmets, the development of an efficient degradation process has been addressed. In this study, the application of the photo-electrochemical oxidation (PEO) process for the degradation of ceftriaxone was evaluated. MethodsExperiments were performed in an undivided cell equipped with Ti/IrO2 (0.1)-Ta2O5 (0.1)-TiO2 (0.8) and Ti/IrO2 (0.2)-Ta2O5 (0.2)-TiO2 (0.6) as anodes and Platinum (Pt) sheet as the cathode of the degradation process. Anodes were characterized using scanning electron microscopy (SEM), mapping energy dispersive X-ray (EDS-mapping), ultraviolet–visible diffuse reflectance spectroscopy (DRS), and atomic force microscopy (AFM). Cyclic voltammetry (CV) and photocurrent analysis were performed to consider the photo-electrochemical behavior of anodes. The effect of operational parameters, including initial pH (3–9), ceftriaxone initial concentration (C = 10–50 mg L−1), current density (I = 100–500 mA cm−2), and Na2SO4 as electrolyte concentration (Celectrolyte = 0.05–0.25 mg L−1) on ceftriaxone removal efficiency were determined. Significant findingsOutcomes of experiments revealed that under optimum conditions (pH = 6, C = 30 mg L−1, Celectrolyte = 0.1 mg L−1, and I = 300 mA cm−2), 98.6 % of degradation efficiency was achieved. The combined process resulted in 77.6 and 69.3 % total organic carbon removal of ceftriaxone on Ti/IrO2 (0.1)-Ta2O5 (0.1)-TiO2 (0.8) and Ti/IrO2 (0.2)-Ta2O5 (0.2)-TiO2 (0.6) after five hours of PEO process, respectively. Additionally, the feasible intermediates of ceftriaxone degradation were identified using Gas chromatography-mass spectroscopy (GC-MS) analysis.

Harnessing the Power of PM6:Y6 Semitransparent Photoanodes by Computational Balancement of Photon Absorption in Photoanode/Photovoltaic Organic Tandems: >7 mA cm−2 Solar Synthetic Fuels Production at Bias‐Free Potentials

This study first demonstrates the potential of organic photoabsorbing blends in overcoming a critical limitation of metal oxide photoanodes in tandem modules: insufficient photogenerated current. Various organic blends, including PTB7‐Th:FOIC, PTB7‐Th:O6T‐4F, PM6:Y6, and PM6:FM, were systematically tested. When coupled with electron transport layer (ETL) contacts, these blends exhibit exceptional charge separation and extraction, with PM6:Y6 achieving saturation photocurrents up to 16.8 mA cm−2 at 1.23 VRHE (oxygen evolution thermodynamic potential). For the first time, a tandem structure utilizing organic photoanodes has been computationally designed and fabricated and the implementation of a double PM6:Y6 photoanode/photovoltaic structure resulted in photogenerated currents exceeding 7 mA cm−2 at 0 VRHE (hydrogen evolution thermodynamic potential) and anodic current onset potentials as low as −0.5 VRHE. The herein‐presented organic‐based approach paves the way for further exploration of different blend combinations to target specific oxidative reactions by selecting precise donor/acceptor candidates among the multiple existing ones.

2D nanosheet SnS2 solution-processed photoanodes: Unveiling enhanced visible light absorption for solar fuels applications

The scale-up of photoelectrochemical water splitting and CO2 reduction is currently hindered by the excessive band gap of metal oxide photoanodes. Metal sulfides 2D nanostructured morphology offer improved optoelectronic properties and catalytic capabilities in the oxygen evolution reaction (OER). This study demonstrates for the first time visible spectra absorption capabilities of SnxSy photoanodes, fabricated by facile non-vacuum techniques and post-annealed in a sulfur atmosphere, yielding SnS2 multilayer nanosheets with visible-light absorption down to 900 nm wavelengths. Optimal annealing (500 °C) resulted in >1 V photovoltages, photocurrents surpassing 1.6 mA/cm2 at 1.23VRHE, incident photon-to-current conversion efficiency (IPCE) of 75% at 330 nm. Photon conversion in the 500–900 nm range (down to 1.37 eV) is correlated with phase transformation from orthorhombic α-SnS to Sn2S3 and further hexagonal SnS2. IPCE is key for describing effective phase transformation and photon conversion at wavelengths below the dominant direct band gap for the SnS2, suggesting potential indirect transitions or confinement in the low-dimensional nanosheets. SnS2 nanosheets fabricated by solution-processed means, hold great potential for large-scale, cost-effective, and efficient photoelectrochemical devices.

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.

(Invited) Operando Optical Spectroscopy Analyses of Photoelectrochemical Water Oxidation Kinetics

Water oxidation is a key kinetic bottleneck in many photoelectrochemical systems for solar to fuels and chemicals. We have recently been developing an operando optical spectroscopic analysis to assay these reaction kinetics in metal oxide photoanodes. This technique, which we refer to as photoinduced absorption spectroscopy, is based around measuring the optical absorption of oxidising species (‘surface holes’) generated in photoanodes under anodic bias and pulsed (typically 5 s) light irradiation. These analyses have allowed us to observe a non-linear dependence of water oxidation kinetics on surface hole density, most typically a 3rd order rate law dependence under circa 1 sun irradiation conditions. In my talk I will talk about three different applications of this PIAS approach to investigate the function of BiVO4, Fe2O3 and TiO2 photoanodes. Firstly I will discuss how this non-linear dependence can be exploited to determine the photoelectrochemically active surface area of photoanodes as function of metal oxide nanostructuring. I will then go on to demonstrate that, when normalised for matched surface hole densities, water oxidation kinetics measured in under light irradiation or in the dark at strong anodic bias are identical, and discuss how this conclusion can be exploited to quantify the photovoltage under irradiation. Finally I will discuss deviation from rate law behaviour observed under > 1 sun irradiation conditions, and its mechanistic origin.

Accelerated Characterization of Electrode‐Electrolyte Equilibration

AbstractOperational durability is poorly characterized by traditional (photo)electrocatalyst discovery workflows, creating a barrier to scale‐up and deployment. Corrosion is a prominent degradation mechanism whose thermodynamics depend on the concentration of corrosion products in electrolyte. We present an automated system for characterizing the equilibration of (photo)electrodes with dissolved metals in electrolyte for a given electrode, pH, and electrochemical potential. Automation of electrode selection, electrolyte preparation, and electrolyte aliquoting enables rapid identification of self‐passivating electrodes and estimation of the equilibrium dissolved metals concentrations. The technique is demonstrated for metal oxide photoanodes in alkaline electrolyte, where BiVO4 is found to continually corrode, in agreement the literature. An amorphous Ni−Sb−O photoanode is found to passivate with a Ni‐rich coating on the order of 1 monolayer with less than 1 μM total dissolved metals in electrolyte, demonstrating its suitability for durable photoelectrochemical operation. The automation and throughput of the instrument are designed for incorporation in accelerated electrocatalyst discovery workflows so that durability can be considered on equal footing with activity.

Enhanced Broadband Light Harvesting in Ultrathin Absorbers Enabled by Epitaxial Stabilization of Silver Thin Film Mirrors.

Silver thin film mirrors are attractive candidates for use as specular back reflectors to enhance broadband light absorption via strong optical interference in ultrathin film semiconductor photoabsorbers. However, deposition of metal-oxide absorbers often requires exposure to high temperature in an oxygen atmosphere, conditions that cause thermal etching and degrade the specular reflectance of silver films. Here, we overcome this challenge and demonstrate that epitaxial growth of silver mitigates thermal etching under the high-temperature oxygen-containing environments that cause polycrystalline films to degrade. The degree of thermal etching resistance is related to the epitaxial film structure, where high-quality films completely prevent thermal etching, allowing for direct deposition of metal-oxide thin film photoabsorbers at elevated temperatures without any degradation of the optical properties of the silver layer. As a proof of concept for device applications, a metal-oxide photoanode for photoelectrochemical water splitting is fabricated by directly growing epitaxial SnO2 and Ti-doped α-Fe2O3 (hematite) thin films onto stabilized silver reflectors by pulsed laser deposition. The photoanode displays enhanced broadband light absorption due to strong interference effects enabled by the highly reflective silver film and demonstrates stable operation in a photoelectrochemical cell under conditions of water photo-oxidation in alkaline electrolyte.

Effect of Interfacial SiOx Defects on the Functional Properties of Si-Transition Metal Oxide Photoanodes for Water Splitting.

The transfer of photogenerated charges through interfaces in heterojunction photoanodes is a key process that controls the efficiency of solar water splitting. Considering Co3O4/SiOx/Si photoanodes prepared by physical vapor deposition as a representative case study, it is shown that defects normally present in the native SiOx layer dramatically affect the onset of the photocurrent. Electron paramagnetic resonance indicates that the signal of defects located in dangling bonds of trivalent Si atoms at the Si/SiOx interface vanishes upon vacuum annealing at 850 °C. Correspondingly, the photovoltage of the photoanode increases to ≈500 mV. Similar results are obtained for NiO/SiOx/Si photoanodes. Photoelectrochemical analysis and impedance spectroscopy (in solution and in the solid state) indicate how the defect annealing modifies the Co3O4/SiOx/Si junction. This work shows that defect annealing at the solid-solid interface in composite photoanodes strongly improves the efficiency of charge transfer through interfaces, which is the basis for effective solar-to-chemical energy conversion.

Bodipy and Dipyrrin as Unexpected Robust Anchoring Groups on TiO2 Nanoparticles

Devising cost effective methods for efficiently capturing and storing solar energy is among the grand challenges of science (1). We are using insights from studies of natural photosynthetic systems (2) to develop bioinspired materials for photo-electrochemical water oxidation and solar fuel production by using molecular catalysts and dyes attached to mesoporous metal oxide photoanodes. Covalent attachment of molecules to metal oxide surfaces typically demands the presence of an anchoring group that in turn requires synthetic steps to introduce (3). BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) chromophores have long been used in dye-sensitized solar cells, but carboxylic acid groups typically had to be installed to act as surface anchors. We now find that even without the introduction of such anchors, the unmodified BODIPY can bind to TiO2 surfaces via its BF2 group through boron–oxygen surface bonds. Dipyrrin, the parent molecule of BODIPY, is also capable of binding directly to TiO2 surfaces, likely through its chelating nitrogen atoms. These binding modes prove to be even more robust than that of an installed carboxylate and offer a new way to attach molecular complexes to surfaces for (photo)electrocatalytic applications since, once bound, we show that surface bound BODIPY and dipyrrin derivatives exhibit ultrafast photoinjection of electrons into the conduction band of TiO2 (4). Directing Matter and Energy: Five Challenges for Science and the Imagination, U.S. Department of Energy, Washington, DC, December 2007.D.J. Vinyard and G.W. Brudvig, Annu. Rev. Phys. Chem. (2017) 68, 101.K.L. Materna, R.H. Crabtree and G.W. Brudvig, Chem. Soc. Rev. (2017) 46, 6099.J.A. Jayworth et al., Dalton Trans. (2022) 51, 14260.

(Invited) Water Oxidation Catalysis with Atomically Defined Active Sites on Nanostructured Materials for Solar Energy Applications

Devising cost effective methods for efficiently capturing and storing solar energy is among the grand challenges of science (1). We are using insights from studies of natural photosynthetic systems (2) to develop bioinspired materials for photo-electrochemical water oxidation and solar fuel production by using molecular catalysts and dyes attached to mesoporous metal oxide photoanodes. Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications. We are developing anchoring chemistry to attach molecular water-oxidation catalysts to metal oxide surfaces (3), which not only greatly increases the stability of the molecular catalyst but also improves the catalytic performance of the oxide material. Our progress on the development and characterization of molecular iridium, copper and manganese water-oxidation catalysts (4-5), along with their application for photoelectrochemical water oxidation (6) and solar fuel production, will be discussed. Directing Matter and Energy: Five Challenges for Science and the Imagination, U.S. Department of Energy, Washington, DC, December 2007.D.J. Vinyard and G.W. Brudvig, Annu. Rev. Phys. Chem. (2017) 68, 101.J.L. Troiano, R.H. Crabtree and G.W. Brudvig, ACS Appl. Mater. Interfaces (2022) 14, 6582.S.W. Sheehan et al., Nature Comm. (2015) 6, 6469.K.J. Fisher et al., ACS Catalysis (2017) 7, 3384.Y. Zhao et al., Proc. Natl. Acad. Sci. U.S.A. (2018) 115, 2902.

Enhanced photoelectrochemical water oxidation by micro− nano bubbles: Measurements and mechanisms

Micro-nano bubbles (MNBs) have attracted far-ranging attention owing to their special properties, such as large surface area, high surface energy and production of reactive oxygen species (ROS), which enable potential applications of MNBs in photocatalytic sewage treatment. Here, we proposed combining MNBs and a photoelectrochemical (PEC) reaction system to achieve efficient PEC water oxidation. The results indicated that the metal oxide photoanodes (TiO2, BiVO4 and WO3) achieved a 10%−26% improvement in photocurrent density and markedly enhanced efficiencies of charge injection and separation in MNB electrolytes. The improved PEC performance in MNBs is attributed in part to the detachment effect on the photoanode surface, which can promote oxygen bubble departure from the electrode and expose more active sites, and in part to the generated hydroxyl radical OH., which can capture photogenerated electrons quickly and increase the energy band bending between the photoanode and the electrolyte, resulting in accelerated hole transport efficiency and a reduced reaction barrier at the interface. This work illustrates a new design strategy combining MNBs to improve the performance of PEC application systems.

Effect of carbon dot on photovoltaic performance of n-TiO2/p-NiO and n-TiO2/p-CuO heterojunctions in Dye-Sensitized solar cells

Carbon quantum dot (C-dot) behaves as a photosensitizer and dye-adsorber. Here, effect of C-dot was investigated on dye (N719)-photosensitized solar cells (DSSCs) consisting of n-type TiO2 and p-type metal oxide (NiO or CuO) photoanodes. Power conversion efficiency (PCE) of pristine TiO2 DSSC was enhanced double with adding 10 wt% NiO. Furthermore, the doping of 10 wt% C-dot into TiO2/NiO(10 wt%) nanocomposites increased PCE up to 2.88 times. Then the 14.32% PCE was achieved, since coexistence of C-dot and N719 accelerates the faster and rich charge separation and charge transfer at TiO2/NiO heterojunction. On the other hand, PCE of DSSC consisting of n-TiO2 and p-CuO semiconductors was slightly higher than those of n-TiO2/p-NiO DSSC, but PCE of n-TiO2/p-CuO DSSC decreased after Cdot was added. The calculated energy level alignments exhibit that electrons photogenerated on Cdot and N719 successfully transfer to cathode through NiO and TiO2 in TiO2/NiO/C-dot DSSC, while electrons in a conduction band of TiO2 are recombined with holes in HOMO of C-dot with similar energy level in TiO2/CuO/C-dot nanocomposites. This investigation demonstrates that C-dot provides photo-generated charges and accelerates charge transfer, but it also raises charge recombination, depending on energy level balance with semiconductor components and influencing the performance of DSSC.

Surface Oxygen Species in Metal Oxide Photoanodes for Solar Energy Conversion

Converting and storing solar energy directly as chemical energy through photoelectrochemical devices are promising strategies to replace fossil fuels. Metal oxides are commonly used as photoanode materials, but they still encounter challenges such as limited light absorption, inefficient charge separation, sluggish surface reactions, and insufficient stability. The regulation of surface oxygen species on metal oxide photoanodes has emerged as a critical strategy to modulate molecular and charge dynamics at the reaction interface. However, the precise role of surface oxygen species in metal oxide photoanodes remains ambiguous. The review focuses on elucidating the formation and regulation mechanisms of various surface oxygen species in metal oxides, their advantages and disadvantages in photoelectrochemical reactions, and the characterization methods employed to investigate them. Additionally, the article discusses emerging opportunities and potential hurdles in the regulation of surface oxygen species. By shedding light on the significance of surface oxygen species, this review aims to advance our understanding of their impact on metal oxide photoanodes, paving the way for the design of more efficient and stable photoelectrochemical devices.

Bulk and surface modified polycrystalline CuWO4 films for photoelectrochemical water oxidation

Polycrystalline CuWO4 film is an emerging photoanode material for photoelectrochemical (PEC) water splitting with a small bandgap to absorb visible light and excellent stability in a neutral electrolyte. However, its PEC performance is quite low mainly due to its poor charge transfer characteristics. To enhance the performance of the CuWO4 photoanode, two modification strategies are employed; SnO2 as an electron transfer layer to improve the bulk charge separation efficiency of CuWO4 and cobalt phosphate as a co-catalyst to augment the surface charge separation efficiency at the interface of CuWO4||electrolyte. The two modifications enhance the PEC activity two times to 0.13 mA/cm2 @ 1.23 VRHE for water oxidation and 0.24 mA/cm2 @ 1.23 VRHE for hole scavenger oxidation, and exhibit an excellent stability in a neutral electrolyte. Although the performance is still very low compared to well-developed metal oxide photoanodes, this work shows possibility of further improvement with further developments of synthesis method as well as applying other elaborate modification strategies.

Fe Substitutions Improve Spectral Response of Bi2WO6-Based Photoanodes

The quest for a scalable solar fuel technology has stimulated a concerted effort to develop a metal oxide semiconductor for solar-driven photoelectrochemical water oxidation (oxygen evolution reaction) in an efficient and durable manner. So far, the search for such a metal oxide photoanode has highlighted the promise of Bi-based oxides, which have also been extensively studied for other photocatalyst applications. Bi2WO6 is a durable photocatalyst whose primary shortcoming is a 2.8 eV band gap that limits utilization of the solar spectrum. Improvements in visible photoresponse upon incorporation of Fe have been reported in the photocatalysis literature, motivating our use of high-throughput synthesis and photoelectrochemistry to determine the spectral photoresponse for Bi–W–Fe oxides synthesized under nonequilibrium conditions based on thermal oxidation of metallic films. Photoactivity down to 2 eV was achieved over a broad range of compositions, with detailed characterization of optimal compositions revealing that Fe incorporation increases the valence band position by 0.75 eV. Density functional theory calculations of Fe substitutions on W sites in Bi2WO6 are consistent with this valence band shift, providing a plausible explanation for the experiments. This Fe-mediated band tuning yields a ca. 2 eV band gap while retaining a turn-on potential for photoanodic current near 0.4 V versus RHE, which combined with the operational durability motivates continuous study and development of this promising class of metal oxide photoanodes.

Magnetron Sputtered Al Co-Doped with Zr-Fe2O3 Photoanode with Fortuitous Al2O3 Passivation Layer to Lower the Onset Potential for Photoelectrochemical Solar Water Splitting

In this paper, we investigate the magnetron sputtering deposition of an Al-layer on Zr-doped FeOOH (Zr-FeOOH) samples to fabricate a Zr/Al co-doped Fe2O3 (Al-Zr/HT) photoanode. An Al-layer is deposited onto Zr-FeOOH through magnetron sputtering and the thickness of the Al deposition is regulated by differing the sputtering time. Electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, Mott-Schottky and time-resolved photoluminescence spectra analyses were used to study, in depth, the correlations between sputtered Al-layer thicknesses and PEC characteristics. High-temperature quenching (800 °C) assists in diffusing the Al3+ in the bulk of the Zr-doped Fe2O3 photoanode, whilst an unintended Al2O3 passivation layer forms on the surface. The optimized Al-Zr/HT photoelectrode achieved 0.945 mA/cm2 at 1.0 VRHE, which is 3-fold higher than that of the bare Zr/HT photoanode. The Al2O3 passivation layer causes a 100 mV cathodic shift in the onset potential. Al co-doping improved the donor density, thus reducing the electron transit time. In addition, the passivation effect of the Al2O3 layer ameliorated the surface charge transfer kinetics. The Al2O3 passivation layer suppressed the surface charge transfer resistance, consequently expediting the hole migration from photoanode to electrolyte. We believe that the thickness-controlled Al-layer sputtering approach could be applicable for various metal oxide photoanodes to lower the onset potential.

(Invited) Local and Macroscopic Probes of Semiconductor/Electrocatalyst Photochemical Interfaces

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. These measurements are compared and contrasted to a new approach to “wirelessly” measure interfacial for different contact materials by analyzing shifts in element-specific shifts in x-ray photoelectron emission energies. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

(Invited) Light-Induced Structural Deformations in BiVO4 Photoanodes

Monoclinic scheelite bismuth vandate (BiVO4) stands as one of the best performing thin film metal oxide photoanodes investigated to date. However, these desirable photoelectrochemical performance characteristics are surprising given the driving force for free charge carriers to self-trap as polarons. For the case of electrons, localization at vanadium sites leads to formation of small electron polarons with a large, thermally activated hopping barrier of several hundred meV. For the case of holes, several experimental and theoretical studies have indicated polaron formation. However, despite the importance of photogenerated hole extraction for photoanode function, significant uncertainties remain regarding the degree of hole localization, the associated structural configurations, and the transport barriers. In this work, we apply time resolved optical pump/X-ray diffraction probe experiments to investigate light excitation-induced structural transformations of BiVO4. At low pump fluences, we observe a distinct, non-thermal response that indicates increased symmetry and a partial relaxation of the monoclinic distortion within the BiO8 dodecahedra. We rationalize this finding in terms of the underlying electronic structure of BiVO4, photo-induced depopulation of valence band states, and the nature of the monoclinic-to-tetragonal phase transformation. The resulting structural change is expected to increase hole localization and play a key role in the formation and structure of photo-induced hole polarons within the material. Complimentary electronic transport and optical pump-probe spectroscopies provide additional support for this interpretation and shed light on the underlying mechanisms of polaron formation, transport, and recombination in this promising photoanode material.

Generalized Method to Extract Carrier Diffusion Length from Photoconductivity Transients: Cases of BiVO4 , Halide Perovskites, and Amorphous and Crystalline Silicon

Long diffusion lengths of photoexcited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Time-resolved photoconductance measurements are often used to determine diffusion lengths in conventional semiconductors. However, effects such as polaron formation or multiple trapping can lead to time-varying mobilities and lifetimes that are not accounted for in the conventional calculation of the diffusion length. Here, a generalized analysis is presented that is valid for time-dependent mobilities and time-dependent lifetimes. The diffusion length is determined directly from the integral of a photoconductivity transient and can be applied regardless of the nature of carrier relaxation. To demonstrate our approach, photoconductivity transients are measured from 100 fs to 1 µs by the combination of time-resolved terahertz and microwave spectroscopy for BiVO4, one of the most studied metal oxide photoanodes for photoelectrochemical water splitting. The temporal evolution of charge carrier displacement is monitored and converges after about 100 ns to a diffusion length of about 15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a−Si:H, c−Si, and halide perovskite, which underlines its potential to determine the diffusion length in a wide range of semiconductors, including disordered materials.Received 14 April 2022Revised 2 August 2022Accepted 12 August 2022DOI:https://doi.org/10.1103/PRXEnergy.1.023008Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCarrier generation & recombinationDiffusionElectrical conductivityOptical conductivityPhotoconductivityPhotocurrentPhysical SystemsAmorphous materialsAmorphous semiconductorsComplex materialsEnergy applicationsEnergy materialsThin filmsTechniquesMicrowave techniquesTerahertz spectroscopyCondensed Matter, Materials & Applied Physics

(Invited) Nanoscale Electrocatalyst/Semiconductor Interfaces As Charge-Carrier-Selective Contacts in Photocatalytic and Photoelectrochemical Systems

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.