Perspective—A Brief Perspective on the Fabrication of Hierarchical Nanostructure for Solar Water Splitting Photoelectrochemical Cells

Abstract

Among various solar energy generation methods, a water splitting photoelectrochemical (PEC) cell have been anticipated as a one of the promising source to generate hydrogen by natural sunlight. Owing to unique electrical, electrochemical and optical properties, the nanostructured metal oxides were intensively investigated to adopt as a main material in the water splitting PEC cell. However, most of nanostructured metal oxides have an improper bandgap size/position for splitting water molecule, it needs to be engineered to shift a level of the energy bandgap. In this prospective review, we will briefly discuss solution-processed fabrication methods for water splitting applications. Current StatusSunlight-driven energy generation systems have been broadly studied to establish a clean and sustainable energy source. Water splitting photoelectrochemical (PEC) cell is one of the next-generation hydrogen generation technology.1-3 Since metal oxides have unique electrical, electrochemical and optical properties, it usually was adopted as a main material for the water splitting PEC cell. At this time, the metal oxides were fabricated to have nanoscale structure. Typically, the nanostructured materials have different chemical, electrical and optical characteristics, largely depended on the size. 4 One of the featured characteristics for the nanoscale materials is large surface to volume ratio, which indicates that it offers many chemical reaction sites. Moreover, the nanostructured materials own minimized defects and shortened migration distance for electrons/holes, which decreases electron-hole recombination and eventually leads high photoresponse efficiency.5 Nanomaterials were engineered as various shapes and structures such as thin film, 6 porous frameworks, 7,8 nature-mimicked branched systems 9,10 and hierarchical structures 11,12 to obtain maximize surface areas. Among the various structures, the hierarchical metal oxide nanostructure has been broadly studied for the electrochemical applications due to a simple structure form as well as a capability of modulation for the electrical property. Regarding the hierarchical structure is basically consisted of an inner core and an outer surrounding, the core material fabrication process firstly needs to be determined. At the early stage of research, metal oxide-based hierarchical nanostructures were generally synthesized via vapor-liquid-solid process, mostly conducted under vacuum environment. However, several pioneer researchers have introduced a hybrid fabrication process using both dry and wet methods to get hierarchical nanostructure. Synthesis of Metal Oxide Nanostructure by Hydrothermal Process for Water Splitting ApplicationsOver several decades, nanostructured metal oxides were synthesized by various vacuum-based synthesis methods such as evaporator, 13,14 sputtering, 15 chemical vapor deposition (CVD), [16][17][18][19] pulsed laser deposition (PLD) 20 and other techniques. Recently, a hydrothermal reaction was practiced to make metal oxide nanostructure z E-mai...

Plasmonic properties of an Ag@Ag2Mo2O7 hybrid nanostructure easily designed by solid-state photodeposition from very thin Ag2Mo2O7 nanowires

“…18,19 For example, the facile solution-based photoreduction process was successfully employed to design new hematite/Ag and ZnO/Ag hierarchical nanostructures which were applied as a photoanode in photo-electrochemical cells for water splitting applications. [20][21][22] A novel approach using a non-aqueous sol-gel process based on the low-temperature synthesis route has also been recently reported. 23 However these methods require a solid-liquid interface that often hinders a precise control of the NPs size distribution and provide an inhomogeneous coverage onto the semiconductor surface.…”