A hematite photoelectrode grown on porous and conductive SnO₂ ceramics for solar-driven water splitting

Photoelectrochemical water splitting using solar energy is a highly promising technology to produce hydrogen as an environmentally friendly and renewable fuel with high-energy density. This approach requires the development of appropriate photoelectrode materials and substrates, which are low-cost and applicable for the fabrication of large area electrodes. In this work, hematite photoelectrodes are grown by aerosol assisted chemical vapour deposition (AA-CVD) onto highly-conductive and bulk porous SnO2 (Sb-doped) ceramic substrates. For such photoelectrodes, the photocurrent density of 2.8 mA cm-2 is achieved in aqueous 0.1 M NaOH under blue LED illumination (λ = 455 nm; 198 mW cm-2) at 1.23 V vs. RHE (reversible hydrogen electrode). This relatively good photoelectrochemical performance of the photoelectrode is achieved despite the simple fabrication process. Good performance is suggested to be related to the three-dimensional morphology of the porous ceramic substrate resulting in excellent light-driven charge carrier harvesting. The porosity of the ceramic substrate allows growth of the photoactive layer (SnO2-grains covered by hematite) to a depth of some micrometers, whereas the thickness of Fe2O3-coating on individual grains is only about 100–150 nm. This architecture of the photoactive layer assures a good light absorption and it creates favourable conditions for charge separation and transport.</p

Photoelectrocatalytic production of solar fuels with semiconductor oxides: materials, activity and modeling

Oxide photoelectrochemistry has been under continuous development over the last half century. These decades have witnessed the use of electrodes of different nature (from single crystals to nanoparticulate films), new electrode materials (including ternary and multinary transition metal oxides), and different strategies for improving their efficiency and stability (e.g. doping or protective layers). Although the very high initial expectations for using oxide electrodes in solar energy conversion were not fully met, substantial efforts have been devoted to reach an in-depth understanding of the processes limiting their functioning, providing firm bases for further developments. In this article, we review our main contributions in this field; in particular, we focus on the water photooxidation (i.e. oxygen evolution reaction), water photoreduction (i.e. hydrogen evolution reaction) and full water splitting processes (in a tandem cell) with binary and ternary oxides, including metal hydroxides as co-catalysts. We emphasize the importance of modeling and obtaining mechanistic insights and we conclude with a reflection on the main issues to be tackled in this field, which in our opinion should experience major advances in the coming years.Continued support from the Spanish Ministry of Science and Innovation (MICINN) is gratefully acknowledged, in particular through the current project RTI2018-102061-B-I00 (FONDOS FEDER). Financial support from the Generalitat Valenciana through project PROMETEO/2020/089 is also thanked

Fabrication of Ordered and High-Performance Nanostructured Photoelectrocatalysts by Electrochemical Anodization: Influence of Hydrodynamic Conditions

Nanostructured semiconductor metal oxides, such as TiO2, WO3, Fe2O3 or ZnO, are being widely investigated for their use as photoanodes, due to their higher surface areas in contact with the electrolyte, which increases the efficiency of photoelectrochemical processes. Metal oxide nanostructures have been synthesized by a number of different techniques. Anodization is one of the simpler methods used to synthesize nanostructured photoanodes, and the morphology and size of nanostructures can be designed by adequately controlling anodization parameters. Besides, these nanostructures are directly bound to the metallic back contact, improving significantly the efficiency of electron collection. It has been observed that hydrodynamic conditions during anodization (using a rotating disk electrode, RDE) greatly influenced the morphology of nanostructures and, therefore, their photoelectrochemical performance. The objective of this chapter is to review the innovative nanostructures with high-aspect ratios that can be fabricated by anodization under different hydrodynamic conditions

III-V Semiconductor Nanostructures for Photoelectochemical Water Splitting

The desire for the development of renewable energy technologies is ever growing to sustain global socio-economic growth and meet future technological developments due to declining fossil fuel reserves and growing environmental concerns of their by-products. Although photovoltaics is well established as a renewable technology to generate clean energy, it is intermittent in nature and hence storing solar energy for short and long-term applications is still challenging. Hydrogen generation via photoelectrochemical (PEC) water splitting is one of the promising routes to secure a sustainable, green, storable and portable form of energy. III-V semiconductors have gained intense research interest for PEC water splitting applications owing to their outstanding properties such as variable band gaps to capture the entire solar spectrum, high absorption coefficients and high crystalline quality. In addition, nanostructures possess several essential attributes towards achieving efficient water splitting such as enhanced light absorption, reduced carrier transfer length and large surface area. This thesis report on GaN and InP nanopillar (NP) photoelectrodes fabricated using a top-down approach for PEC water splitting. This work involves the fabrication of large area GaN and InP NPs using inductively coupled plasma (ICP) etching of the respective wafers masked by a self-organized random mask technique, followed by a study of their PEC performance. NP photoelectrodes exhibited a remarkable improvement in PEC performance compared to their planar counterparts due to the enhanced absorption and increased semiconductor/electrolyte interface area. The PEC performance of the GaN NP photoanodes was shown to be influenced by doping concentration, NP dimensions such as diameter and length, and band gap engineering of the GaN NPs. The PEC performance of the InP NPs was strongly dependent on the surface damage of NPs, which was eliminated by wet treatment of the NPs in sulfur-oleylamine (S-OA) solution. Finally, long-term photo-stability was demonstrated for both NP photoelectrodes

Nanoscale Control of Metal Oxideand Carbonaceous Functional Materials

The controlled fabrication of nanometer scale devices is of fundamental concern for numerous technologies, from separations to electronics and catalysis. The complexity of device architectures calls for the development of synthetic methods that independently control each feature: pore dimensions, wall thickness, and any subsequent functional nanomaterial layers (e.g. photoactive electrocatalysts). Precision control over these orthogonal methods can be used to integrate 3D and 1D nanostructures. This dissertation presents the development of techniques useful in fabricating highly controlled nanoscale devices. The growth of single-phase bismuth vanadate (BiVO4) by atomic layer deposition (ALD) is demonstrated for the first time, allowing for the conformal growth of ultrathin BiVO4 on arbitrary substrates. A new tin oxide underlayer (SnO2) was developed to act as a hole-blocking underlayer concomitantly with ultrathin BiVO4 is to fabricate space-efficient photoanodes on a high-aspect ratio 3D substrate, combining the advantages gained by reducing BiVO4 thickness and preserving optical thickness. The heterojunction SnO2/BiVO4 space-efficient photoanode achieved the highest reported applied-bias photon-to-charge efficiency for any photoanode material synthesized via ALD. Lastly, the first demonstration of persistent micelle templates (PMT) with carbonaceous materials is reported, demonstrating independent control over important feature sizes, such as wall thickness and pore size, to adjust the capacity and charge/discharge rates of carbon-based supercapacitors

Co3O4 Nanopetals on Si as Photoanodes for the Oxidation of Organics

Cobalt oxide nanopetals were grown on silicon electrodes by heat-treating metallic cobalt films deposited by DC magnetron sputtering. We show that cobalt oxide, with this peculiar nanostructure, is active towards the photo-electrochemical oxidation of water as well as of organic molecules, and that its electrochemical properties are directly linked to the structure of its surface. The formation of Co3O4 nanopetals, induced by oxidizing annealing at 300 \ub0C, considerably improves the performance of the material with respect to simple cobalt oxide films. Photocurrent measurements and electrochemical impedance are used to explain the behavior of the different structures and to highlight their potential application in water remediation technologies

Recent Progress and Approaches on Carbon-Free Energy from Water Splitting

Sunlight is the most abundant renewable energy resource, providing the earth with enough power that is capable of taking care of all of humanity’s desires—a hundred times over. However, as it is at times diffuse and intermittent, it raises issues concerning how best to reap this energy and store it for times when the Sun is not shining. With increasing population in the world and modern economic development, there will be an additional increase in energy demand. Devices that use daylight to separate water into individual chemical elements may well be the answer to this issue, as water splitting produces an ideal fuel. If such devices that generate fuel were to become widely adopted, they must be low in cost, both for supplying and operation. Therefore, it is essential to research for cheap technologies for water ripping. This review summarizes the progress made toward such development, the open challenges existing, and the approaches undertaken to generate carbon-free energy through water splitting.[Figure not available: see fulltext.]. © 2019, © 2019, The Author(s).Different approaches for efficient carbon-free energy from water splitting are summarized.Step-wise evolution of water splitting research is highlighted with current progress.It describes the open challenges of charge transport properties and future research direction. © 2019, The Author(s)

Nanowire Photoelectrochemistry.

Recent applications of photoelectrochemistry at the semiconductor/liquid interface provide a renewable route of mimicking natural photosynthesis and yielding chemicals from sunlight, water, and air. Nanowires, defined as one-dimensional nanostructures, exhibit multiple unique features for photoelectrochemical applications and promise better performance as compared to their bulk counterparts. This article reviews the use of semiconductor nanowires in photoelectrochemistry. After introducing fundamental concepts essential to understanding nanowires and photoelectrochemistry, the review considers answers to the following questions: (1) How can we interface semiconductor nanowires with other building blocks for enhanced photoelectrochemical responses? (2) How are nanowires utilized for photoelectrochemical half reactions? (3) What are the techniques that allow us to obtain fundamental insights of photoelectrochemistry at single-nanowire level? (4) What are the design strategies for an integrated nanosystem that mimics a closed cycle in artificial photosynthesis? This framework should help readers evaluate the salient features of nanowires for photoelectrochemical applications, promoting the sustainable development of solar-powered chemical plants that will benefit our society in the long run

Recent Advances in BiVO4- and Bi2Te3-Based Materials for High Efficiency-Energy Applications

This chapter provides recent progress in developments of BiVO4- and Bi2Te3-based materials for high efficiency photoelectrodes and thermoelectric applications. The self-assembling nanostructured BiVO4-based materials and their heterostructures (e.g., WO3/BiVO4) are developed and studied toward high efficiency photoelectrochemical (PEC) water splitting via engineering the crystal and band structures and charge transfer processes across the heteroconjunctions. In addition, crystal and electronic structures, optical properties, and strategies to enhance photoelectrochemical properties of BiVO4 are presented. The nanocrystalline, nanostructured Bi2Te3-based thin films with controlled structure, and morphology for enhanced thermoelectric properties are also reported and discussed in details. We demonstrate that BiVO4-based materials and Bi2Te3-based thin films play significant roles for the developing renewable energy