Nanostructured Photoelectrodes Research Articles

Constructing efficient Ti4O7/PbO2 heterostructured photoelectrode for water remediation

Photoelectrocatalytic oxidation (PEC) technology is considered to be an efficient process for the treatment of organic wastewater, and its performance depends on the characteristics of the PEC photoanodes. Therefore, the construction of heterogeneous nanostructured photoelectrodes based on suitable semiconductor materials for fast induced carrier transfer efficiency is essential for a high-performance PEC technique. Herein, titanium suboxide/lead (IV) oxide (Ti4O7/PbO2) ceramic photoelectrode with efficient PEC performance was synthesized by coupling PbO2 nanospheres with Ti4O7 through a hydrothermal method. To maximize the PEC performance of the ceramic electrodes, the Ti4O7/PbO2 nano-heterostructures were optimized by modulating the hydrothermal temperature. The optimized ceramic electrodes possessed a low Tafel slope, a low charge-transfer resistance and a good photocurrent response. Ti4O7/PbO2-110 exhibited an efficient PEC activity (approximately 92.21%) for the degradation of reactive brilliant blue KN-R. The efficient PEC performance of Ti4O7/PbO2-110 arose from the formation of a type II heterojunction between Ti4O7 and PbO2, which achieved efficient photogenerated carrier separation and facilitated the formation of intermediate active species. This work not only validates the efficient performance of Ti4O7/PbO2-110 for PEC water purification but also provides a reference strategy for the preparation of heterostructured ceramic photoelectrodes with a high PEC efficiency.

Light absorption enhancement in dye-sensitized solar cells using thin nanostructured photoelectrodes

As thin-film solar cells are the attraction of many research works nowadays, dye-sensitized solar cells (DSSCs) among other types hold the promise of excellent cost/performance ratio because of the relatively low material costs and simple processing conditions. In order to improve its relatively low performance, different approaches are being explored to enhance the photon-to-current conversion efficiency. Notably, one of the most accessible approaches is by modifying the optical characteristics of the photoelectrode of DSSCs to improve light trapping within the cell. Here, we introduce a cost-effective nanopatterning process done on the photo-active TiO2 photoelectrode layer for better light absorption. This simple technique resulted in increased cell short-circuit current density (from 4.664 to 5.963 mA/cm2) which consequently enhanced cell efficiency (from 2.243 to 2.819%). Furthermore, without compromising the absorption significantly, nanopatterned photoelectrodes with a smaller thickness can lead to reduced material costs and possibly allowing the construction of semitransparent cells. This work can contribute to an attainable efficient DSSC manufacturing route for practical applications.

Synergistic Microfluidic and Optical Performance Enhancements of Photoelectrodes for Space Applications

The realisation of long-term space missions as well as lunar habitats require reliable and efficient oxygen and chemical producing devices e.g., for life support, fuel generation and in-situ resource utilisation (ISRU). We have recently demonstrated that integrated semiconductor-electrocatalyst systems can be operated in microgravity environments generated for 9.2 s at the Bremen Drop Tower (Center of Applied Space Technology and Microgravity, ZARM) at terrestrial efficiencies, thus opening the possibility of utilising these photoelectrochemical (PEC) devices for chemical synthesis in space. 1,2 , 3 Our findings reveal that by altering the electrocatalyst surface morphology through nanostructuring, we can introduce catalytic hot-spots on the photoelectrode which - through their hydrophilic nature - can also enhance gas bubble detachment in microgravity and thus circumvent the near-absence of buoyancy.1,3 We have found that the surface hydrophilicity and electrocatalyst loading require however careful balancing as the most effective nanostructures for gas bubble desorption can also result in less catalyst material on the photoelectrode surface, which in turn leads to a non-optimal PEC device performance.Here, we present a combined theoretical and experimental microfluidic, optical and photoelectrocatalytic performance analysis of photoelectrodes with integrated, nanostructured electrocatalysts, aiming at providing a guideline for an optimal design for microgravity applications. Focusing on the hydrogen evolution reaction (HER) and a well-investigated photoabsorber for this reaction, p-type InP, we firstly built an optoelectronic model in COMSOL Multiphysics in pursuit of quantifying the optical performance enhancements of Rh and Pt electrocatalyst nanostructures such as nanopyramids, nanowires and sub-wavelength aperture holes.4 We then developed a microfluidic model using the same nanostructure geometries to predict the hydrophilicity of the device surface and subsequently, the gas bubble contact angle. The best-performing nanostructured photoelectrodes predicted in our models where then manufactured, photoelectrochemically tested and fully optically and spectroscopically characterised. Predicted local electric field enhancements through catalytic hotspot formation were moreover experimentally validated using scanning electrochemical cell microscopy (SECCM). Our findings indicate that our developed optoelectronic and microfluidic models can be utilised to design a theoretical framework for identifying ideal electrocatalyst nanostructures for microgravity applications.References Brinkert K, et al. Efficient solar hydrogen generation in microgravity environment. Nature Communications 9, (2018). Brinkert K, Mandin P. Fundamentals and future applications of electrochemical energy conversion in space. npj Microgravity 8, (2022). Akay Ö, et al. Releasing the Bubbles: Nanotopographical Electrocatalyst Design for Efficient Photoelectrochemical Hydrogen Production in Microgravity Environment. Advanced Science 9, 2105380 (2022). Tembhurne S, Haussener S. Integrated Photo-Electrochemical Solar Fuel Generators under Concentrated Irradiation. Journal of The Electrochemical Society 163, H988 (2016). Pan Y, Huang S, Li F, Zhao X, Wang W. Coexistence of superhydrophilicity and superoleophobicity: theory, experiments and applications in oil/water separation. Journal of Materials Chemistry A 6, 15057-15063 (2018).

Design and Development of Novel Nanostructured Visible-Light Active Photoelectrodes based on TiO2 Nanorods for Solar Hydrogen Generation

Design and Development of Novel Nanostructured Visible-Light Active Photoelectrodes based on TiO2 Nanorods for Solar Hydrogen Generation

Tantalum oxynitride nanotube film arrays for unconventional nanostructured photo-electrodes active with visible light

Tantalum-oxy-nitride nanotubes (TaOxNy NTs) ordered in 1D were successfully synthesized through controlled anodic oxidation of metallic Ta foils followed by high-temperature annealing under NH3 flow to prepare unconventional nanostructured photo-electrodes active with visible light. For the synthesis, a two-step anodization process was adopted: i) the first step in ethylene glycol with 0.33 % NH4F and 3 % H2O, ii) the second step in H2SO4 (98 %) with 1 % HF and 4 % H2O, by applying different voltages and anodization times. Scanning Electron Microscopy integrated with Energy Dispersive X-ray spectroscopy (SEM-EDX) revealed the NTs length ranging from 2 to 5 µm, and the inner tube diameter from 20 to 100 nm, in good correlation with the synthesis parameters. The N-content (from 2.7 % to 19.1 %) was dependent on the nitridation temperature (from 500° to 900°C), and the conversion of bare Ta2O5 into TaOxNy strongly influenced the absorption in the visible region with a bandgap shift from 4.0 to 2.1 eV, respectively. Photo-electrochemical and catalytic performances were evaluated with chronoamperometric measurements and tests in different photo-reactions using AM 1.5 G simulated sunlight: (i) degradation of methylene blue (MB), (ii) ethanol gas-phase photo-reforming and (iii) bias-assisted photoelectrochemical water splitting. The TaOxNy NTs film-type electrode obtained by nitriding at 800 °C showed the highest photocurrent value (0.1 mA∙cm-2), and the highest rates of MB degradation (k1 of 0.0189 min-1) and H2 photo-production.

Increased Photoelectrochemical Performance of Vanadium Oxide Thin Film by Helium Plasma Treatment with Auxiliary Molybdenum Deposition

Helium (He) plasma treatment can be a versatile tool to fabricate nanostructures on a surface and change the surface chemistry. Herein, vanadium (V) thin film is exposed to helium plasmas and the changes in morphology and surface properties are investigated. The irradiation condition at which the surface morphology can be changed is identified. During the He plasma irradiation, molybdenum (Mo) from the sample cover deposits and formation of composite nanostructured photoelectrode with a combination of occurs. The photoelectrochemical (PEC) performance increases threefold through the plasma treatment. In addition to the increase in the surface area, the formation of the heterogenous/composite structure, oxygen vacancies, and reduced oxides such as decreases the charge‐transfer resistance and contributes to the improvement in the PEC performance.

Strategies for reducing the overpotential of one-dimensional Si nanostructured photoelectrodes for solar hydrogen production

Strategies to reduce the overpotential of 1D silicon nanostructured photoelectrodes for solar hydrogen generation: co-catalysts to improve the reaction kinetics, hetero- and homo-junctions to improve carrier collection and enhance photovoltage.

Nanomaterials Facilitating Conversion Efficiency Strategies for Microbial CO2 Reduction.

Microbial electro- and photoelectrochemical CO2 reduction represents an opportunity to tackle the environmental demand for sustainable fuel production. Nanomaterials critically impact the electricity- and solar-driven microbial CO2 reduction processes. This minireview comprehensively summarizes the recent developments in the configuration and design of nanomaterials for enhancement of the bacterial adhesion and extracellular electron transfer (EET) processes, based on the modification technologies of improving chemical stability, electrochemical conductivity, biocompatibility, and surface area. Furthermore, the investigation of incorporating non-photosynthetic microorganisms using advanced light-harvesting nanostructured photoelectrodes for solar-to-chemical conversion, as well as the current understanding of EET mechanisms occurring at photosynthetic semiconductor nanomaterials-bacteria biohybrid interface is detailed. The crucial factors influencing the performance of microbial CO2 reduction systems and future perspectives are discussed to provide guidance for the realization of their large-scale application.

Tandem Nanostructures: A Prospective Platform for Photoelectrochemical Water Splitting

A platform for efficient photoelectrochemical (PEC) water splitting must fulfil different requirements: the absorption of the solar spectrum should be maximized in use for charge carrier generation. To avoid recombination, fast separation of charge carriers is required and the energetic positions of the band structure(s) must be optimized with respect to the water splitting reactions. In these respects, constructing tandem nanostructures with rationally designed nanostructured units offers a potential opportunity to break the performance bottleneck imposed by the unitary nanostructure. So far, quite a few tandem nanostructures have been designed, fabricated, and employed to improve the efficiency of PEC water splitting, and significant achievements have been realized. This review focuses on the current advances in tandem nanostructures for PEC water splitting. Firstly, the state of the art for tandem nanostructures applied in PEC water splitting is summarized. Secondly, the advances in this field and advantages arising of employing tandem nanostructures for PEC water splitting are outlined. Subsequently, different types of tandem nanostructures are reviewed, including core‐shell tandem nanostructured photoelectrode, the two‐photoelectrode tandem cell, and the tandem nanostructures of plasmon related devices for PEC water splitting. Based on this, the future perspective of this field is proposed.

Reaction Kinetics of Photoelectrochemical CO2 Reduction on a CuBi2O4-Based Photocathode.

The CO2 reduction reaction (CO2RR) is an essential step in natural photosynthesis and artificial photosynthesis to provide carbohydrate foods and hydrocarbon energy in the carbon-neutral cycle. However, the current solar conversion efficiencies and/or product selectivity of the CO2RR are very sluggish due to its complicated multiple-step charge transfer reactions. Here, we systematically investigate the charge transfer reaction rate during CO2 reduction on CuBi2O4 photocathodes, where the surface is modified with 3-aminopropyltriethoxysilane (APTES). We discover that the surface amine group increases the charge separation rate, significantly enhancing the surface charge transfer reaction rate. However, the surface acidity has less influence on the first-order reaction, indicating that a rate-determining step (RDS) exists in the early stage of the photoelectrochemical cell (PEC) processes. Moreover, the intensity-modulated photocurrent spectroscopy (IMPS) confirms that both surface charge transfer and the recombination rate on APTES-coated CuBi2O4 are larger than bare CuBi2O4 while possessing comparable charge transfer efficiencies. Overall, the surface charge transfer reactions under the PEC condition require designing more effective nanostructured photoelectrodes and powerful characterization methods to intrinsically increase the charge separation and transfer rate while reducing the recombination rate.

Analysis of the photocurrent from CuO/TiO2 nanostructured photoelectrodes to elucidate the benefits of nanocrystal size decrease

Analysis of the photocurrent from CuO/TiO2 nanostructured photoelectrodes to elucidate the benefits of nanocrystal size decrease

An excellent H2 production photoelectrode based on mixed valence Sn3O4 nanoflake arrays treated by H2O2 hydrothermal reaction

To design nanostructured photoelectrodes with unique morphology and suitable band structure is a crucial step for potential photoelectrochemical application. For above purpose, the compact Sn3O4 nanoflakes with the smooth surface have been directly grown on carbon paper substrate by a simple hydrothermal method. It is found that the molar ratio of Sn2+ and Sn4+ ions in Sn3O4 nanoflakes can be modulated by the subsequent H2O2-assisted hydrothermal treatment. The effect of different molar ratio on the energy band has been investigated systematically, together with the evolution of the surface morphology of nanoflakes. Finally, a highly efficient photoelectrode based on Sn3O4 nanoflake has been prepared by the H2O2-assisted hydrothermal process, which is of larger active surface area and suitable band structure, and therefore exhibits the excellent photocurrent response and photocatalytic performance for H2 production. The photoelectrode based on Sn3O4 nanoflake displays enhanced photocurrent with 40 μA cm−1 at a basis of 0 V and higher H2 generation rate with 1.43 × 104 μmol h−1 g−1, much better than those of the original sample. Such superior performance can be probably attributed to the combined effect of unique porous nanoflake-structured, higher active surface area and suitable band structure.

Photoelectrochemical properties of plasma-induced nanostructured tungsten oxide

Helium (He)-induced nanostructured tungsten sheets were synthesized by He plasma irradiation under different plasma exposure durations. After calcination, nanostructured tungsten oxide samples were used as photoelectrodes to test photoelectrochemical (PEC) performance. The results showed that nanostructured WO3 photoanodes have higher PEC performance compared to the sample without nanostructures. The 15 min irradiated sample had the highest photocurrent density of 3.5 mA/cm2 under the thermodynamic potential of water oxidation (1.23 V vs. RHE). It was found that the oxide layer thickness and exposed crystal facet have a significant impact on PEC performance. The plasma synthesis technique has proved to be an effective method for preparing nanostructured WO3 photoelectrodes.

Fabrication of porous 1D WO3 NRs and WO3/BiVO4 hetero junction photoanode for efficient photoelectrochemical water splitting

Porous columnar nanostructured photoelectrodes may significantly affect the photoelectrochemical (PEC) water splitting efficiency. The glancing angle deposition (GLAD) technique is the most popular method to fabricate columnar nanostructured thin films. Here, we have fabricated vertically aligned porous 1D tungsten oxide nano-rod (1D WO3 NRs) thin films by DC magnetron sputtering method using glancing angle deposition with a constant rotation speed of 5 rpm. The depositions carried out at various deposition angles of α = 0°, 60°, 70°, 75°, 80°, and 85°. The deposited 1D WO3 NRs adopted a monoclinic structure oriented in the (002) crystalline plane perpendicular to the FTO substrate. Using systematic deposition conditions, the porous columnar 1D WO3 NRs were engineered to find the effect on the photoelectrochemical water splitting. Photocurrent density of the 1D WO3 NRs photoanodes was tested in near-neutral pH (0.5 M Na2SO4) electrolyte solutions. Under the optimum conditions, the 1D WO3NRs photoanode led to the premier photocurrent density of 1.04 mA/cm2 (1 V vs. Ag/AgCl) due to the optimum surface porosity in the 1D WO3 NRs. The individual layer of BiVO4 photoanode exhibited a photocurrent density of 1.24 mA/cm2 (1V vs. Ag/AgCl). Highest photocurrent density of 2.24 mA/cm2 with high photo-stability was observed in optimized heterostructure of 1D WO3 NRs/BiVO4 photoanode.

Aerosol Spray Assisted Synthesis of Ni Doped BaTiO3 Hollow Porous Spheres/Graphene as Photoanode for Water Splitting

A key challenge in photoelectrochemical water splitting is to fabricate nanostructured photoelectrodes with desirable properties. Herein, Ni doped BaTiO3 hollow spheres (HS) supported on reduced graphene oxide (RGO) nanocomposite were successfully synthesized by aerosol spray drying method. Former spheres consisted of small nanoparticles with size 5–20 nm possessing high porosity and large area. Characterizations were done by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscopy (FESEM). Ni doped BaTiO3 HS/RGO nanocomposites as photoanodes exhibited an enhanced photoelectrochemical performance and long-term stability compared to BaTiO3 HS and BaTiO3 HS/RGO. The photocurrent density of Ni doped BaTiO3 HS/RGO photoanode revealed 8.14 μA cm−2 which is 37% more than BaTiO3 HS. From electrochemical impedance spectroscopy (EIS) analysis, Ni doped BaTiO3 HS/RGO exhibited the lowest charge transfer resistance in comparison with BaTiO3 HS and BaTiO3 HS/RGO which leads to enhancement in the electrocatalytic activity. The successful doping of BaTiO3 HS by Ni and its loading over RGO increases the separation rate of photogenerated electrons and holes and improved the light harvesting. Ni doped BaTiO3 HS/RGO as photo-electrocatalyst exhibits excellent activity toward water oxidation capability and presents a new approach for high-efficient future photo-electrocatalysis.

Outstanding stability and photoelectrochemical catalytic performance of (Fe, Ni) co-doped Co3O4 photoelectrodes for solar hydrogen production

In this work, pure and (Fe, Ni) co-doped Co3O4 nanostructured photoelectrodes of different doping levels and thicknesses were manufactured at constant substrate temperature (450 °C) using the spray pyrolysis technique. In addition to the chemical compositions; the structural, optical, electrical, and photoelectrochemical (PEC) properties were investigated through the use of various analysis techniques. By increasing the codpoing ratio to 6%, the low energy band gap is decreased from 1.43 to 1.3 eV and the high energy bandgap is increased from 2.63 to 2.87 eV, in addition to the reduction in particle size from 30.2 to 12.0 nm. The high energy gap vanishes by increasing the codoped film's spread volume to 60 ml. X-ray photoelectron spectroscopy of 6%(Fe, Ni)-60ml Co3O4 confirms the existence of Ni2+,3+ and Fe2+,3+. Among the studied photoelectrodes, the 6%(Fe, Ni)-60ml Co3O4 photoelectrode displays a photocatalytic hydrogen output rate of 150 mmol/h.cm2 @-1V in 0.3M Na2SO4 electrolyte. The photocurrent density of 6%(Fe, Ni)-60ml photoelectrode reached up to 13.6 mA/cm2@-1V with an IPCE (incident photon to current conversion efficiency) of ∼42%@405 nm and STH (solar to hydrogen conversion efficiency) of ∼11.37%, which are the highest values yet for Co3O4-based photocatalysts. The value of ABPE(applied bias photon-to-current efficiency) is 0.34%@(-0.28V and 636 nm). Interestingly, this photoelectrode shows a photogenerated current density of −0.14 mAcm−2 at 0 V and a PEC current onset over 0.266V. The thermodynamic parameters, corrosion parameters, PEC surface areas, Tafel slopes, and impedance spectroscopies are also being studied to confirm and classify the PEC H2 production mechanism. The 6%(Fe, Ni)-60ml Co3O4 photoelectrode stability/reusability shows only a 6.6% reduction in PEC performance after ten successive runs at -1V with a corrosion rate of 1.2 nm/year. This work offered a new codoping strategy for the design of a highly active Co3O4 based photocatalyst for the generation of solar light-driven hydrogen.

Group IV Oxides for Perovskite Solar Cells

Nanostructured Group IV oxides (ZrO2 and HfO2) have been synthesized and their structural, optical, and energy parameters have been studied. The obtained nanopowders have been used to fabricate nanostructured thin-film photoelectrodes for perovskite solar cells (PSCs). FTO/cTiO2/HfO2/CH3NH3PbI3/Spiro-MeOTAD/Au cell architecture has been developed. Photovoltaic characteristics and photoconversion efficiency for PSCs based on mesoscopic hafnium and zirconium oxides have been compared to the analogous characteristics for known titania-based PSCs. It has been demonstrated that the differences in the photovoltaic characteristics of PSCs are due to the morphology of the mesoporous HfO2 and ZrO2 layers, as well as to the specific features of charge transfer and accumulation at the perovskite/photoelectrode interface. The results obtained indicate good prospects for the application of nanostructured Group IV oxide materials with Eg > 5 eV as photoelectrodes for PSCs. Charge hopping mechanism in very wide bandgap photoelectrodes and its advantages for the development of high-efficiency PSCs is discussed.

Photoelectrochemical water splitting using one-dimensional nanostructures

This review highlights the research on 1D nanostructured photoelectrodes for solar-driven PEC water splitting based on various strategies to enhance their performances.

Spatioselective Deposition of Passivating and Electrocatalytic Layers on Silicon Nanowire Arrays.

Metal–silicon nanowire array photoelectrodes provide a promising architecture for water-splitting because they can afford high catalyst loading and decouple charge separation from the light absorption process. To further improve and understand these hybrid nanowire photoelectrodes, control of the catalyst amount and location within the wire array is required. Such a level of control is currently synthetically challenging to achieve. Here, we report the synthesis of cm2-sized hybrid silicon nanowire arrays with electrocatalytically active Ni–Mo and Pt patches placed at defined vertical locations within the individual nanowires. Our method is based on a modified three-dimensional electrochemical axial lithography (3DEAL), which combines metal-assisted chemical etching (MACE) to produce Si nanowires with spatially defined SiO2 protection layers to selectively cover and uncover specific areas within the nanowire arrays. This spatioselective SiO2 passivation yields nanowire arrays with well-defined exposed Si surfaces, with feature sizes down to 100 nm in the axial direction. Subsequent electrodeposition directs the growth of the metal catalysts at the exposed silicon surfaces. As a proof of concept, we report photoelectrocatalytic activity of the deposited catalysts for the hydrogen evolution reaction on p-type Si nanowire photocathodes. This demonstrates the functionality of these hybrid metal/Si nanowire arrays patterned via 3DEAL, which paves the way for investigations of the influence of three-dimensional geometrical parameters on the conversion efficiency of nanostructured photoelectrodes interfaced with metal catalysts.

Unveiling the Effects of Nanostructures and Core Materials on Charge-Transport Dynamics in Heterojunction Electrodes for Photoelectrochemical Water Splitting.

Understanding the photogenerated charge-transport dynamics of metal oxide electrodes is the key to providing a strategy for practical improvement in the photoelectrochemical reaction activity. Here, we analyze the electron transport of a 3D bicontinuous SnO2/BiVO4 nanostructured photoelectrode by intensity-modulated photocurrent spectroscopy. We compare this electrode with 3D WO3/BiVO4 and planar-type bilayer SnO2/BiVO4 electrodes. In the results, we observe an order of magnitude faster electron transport in the 3D electrodes relative to the bilayer electrode. Moreover, we observe trap-limited transport on widely applied WO3/BiVO4 electrodes but confirm rapid trap-free transport on 3D SnO2/BiVO4. We also characterize the effect of electron transport on the water-splitting reaction. The electron-transport rate is directly related to the charge-separation efficiency in the water-splitting reaction. The fast transport time of the 3D SnO2/BiVO4 leads to the achievement of a significantly higher charge separation efficiency of 94%.