Branched nanostructures for photoelectrochemical water splitting

Abstract

IntroductionSunlight is a free, non-polluting and abundant renewable source of clean energy. The amount of solar energy that strikes the Earth yearly is ~10,000 times the total energy that is consumed on our planet. Converting solar energy into other easily usable forms has attracted considerable interest in the last several decades. Among different solar energy conversion technologies, photoelectrolysis has the ability to split water through solar energy to produce hydrogen (a chemical fuel) without any emission of by-products.1-6 The demonstration of metal oxides as photoanodes for photoelectrochemical (PEC) water splitting was pioneered in 1972 by However, the conversion efficiency today remains unsatisfactory, even lower than that of photovoltaics (PVs), and is limited mainly by the low performance of PEC electrodes. An efficient PEC cell requires electrode materials that can provide rapid charge transfer at the electrode/electrolyte interface, long-term stability and efficient harvesting of a wide range of the solar spectrum, and that are low-cost and non-toxic as well. In the last several decades, however, no breakthroughs on PEC electrode materials have been achieved yet in either the material design of photoelectrode or material stability. 10With recent advances in nanoscience and nanotechnology, nanomaterials and their designs should be promising candidates for constructing photoelectrodes because of their extremely small feature size, large surface area, large surface-to-volume ratio and short diffusion length for carrier transport. 1,3,7,10 It is believed that nanostructured materials can provide beneficial effects including quantum dot sensitisation, band structural modification, the domination of crystal facets and plasmonic association. Therefore, to develop better photoelectrodes and more efficient PEC and PV devices, one of the main strategies is nanostructuring by exploiting scaling laws and specific effects at the nanoscale to enhance the efficiency of existing semiconductors and metal oxides. As a result, there has been intensive research directed worldwide by taking advantages of nanomaterials to make PEC devices with higher solar-to-hydrogen conversion efficiency.It is well known that specific surface area, defined by surface-tovolume ratio, depends on feature size and geometry. The surface-tovolume ratio is, to a first-order approximation, inversely proportional to the feature size as 1/r.11 Highly symmetric objects, such as spheres, have low specific surface area. With equal volume, a rod has a larger surface-to-volume ratio than a spherical particle, and

Anisotropic3DColloidal Quantum Nanostructures

“…In addition, 3D heterostructures can allow for more efficient charge separation and collection due to the close proximity between the photogenerated current carriers and the semiconductor-electrolyte interface. 182 97 ZnO semiconducting material actively absorbs light only in the UV region that contributes less than 5% of the total energy of the solar spectrum. Therefore, this material normally needs to be sensitized by more narrow-bandgap semiconductors such as CdS, CdSe, and other quantum dots.…”

“…Therefore, this material normally needs to be sensitized by more narrow-bandgap semiconductors such as CdS, CdSe, and other quantum dots. 182 Yong et al reported new nanostructured photoelectrodes where hierarchical ZnO/WO x nanostructures provided effective charge collection paths and CdSe/CdS cosensitizers enhanced visible light harvesting ( Figure 16). These photoelectrodes produced a high photocurrent density of 11 mA cm −2 at −0.5 V bias versus SCE for hydrogen generation.…”

Silver-doped SnO2nanostructures for photocatalytic water splitting and catalytic nitrophenol reduction

“…At present, certain wide gap metal oxides, such as ZnO, TiO 2 , ZrO 2 , and SnO 2 , have been assessed to be suitable and desirable candidates to carry out photocatalytic hydrogen generation, because they are inexpensive and have high chemical stability and good photocatalytic H 2 -evolution activity. 37–42 However, the energy gap of these metal oxides constrains the absorption capability to the high energy region (UV) of solar light, which results in a moderately small productivity of H 2 . 43–45 Unlike ZnO and TiO 2 , SnO 2 suffers from low photocatalytic H 2 activity due to its low conduction band potential and flash charge recombination.…”

Microwave Popped Co(II)-Graphene Oxide Hybrid: Bifunctional Catalyst for Hydrogen Evolution Reaction and Hydrogen Storage

“…Development of low cost functional materials has attracted intensive research interest in recent years for eco-friendly energy conversion and energy conservation processes. 1 Among these processes, catalysts play vital roles in applications like water splitting and hydrogen spillover. [2][3][4] While the hydrogen spillover can facilitate hydrogenation reactions, it is also useful for hydrogen storage.…”