Photoelectrochemical water splitting and gas ionisation sensing using metal oxide nanostructures

Energy harvesting directly from sunlight has attracted tremendous attention owing to its great potential for low-cost and clean hydrogen production. However, the current photoconversion efficiency from nanostructured metal oxides remains low due to a number of factors, such as low surface area, limited light absorption, poor electron mobility and high electron-hole recombination. In this research, a number of approaches have been carried out to overcome these difficulties. Firstly, changing the morphology of nanomaterials will help to increase the effective surface area of the photoanodes. ZnO nanotubes were prepared and the photoelectrochemical measurements revealed an efficiency of 3 times higher than their nanorod counterparts. In addition, the combination of ZnO nanorods with a 3D metal substrate, stainless steel mesh, showed a further enhancement in the water splitting efficiency by two-fold when compared with that on a planar substrate. Secondly, the hybridisation of two different metal oxides was studied by creating a heterojunction to improve the charge separation, extending the light absorption and increasing the total surface area of the electrode. In this work, both urchin-like ZnO nanorod arrays on TiO2 hollow hemispheres and 1D BiVO4/ZnO nanorod films displayed synergistic enhancement in photoelectrochemical water splitting efficiency. Thirdly, doped ZnO nanostructures with different optical and/or electrical properties were tailored for photoelectrochemical water splitting and gas ionisation sensing applications. The photoelectrochemical water splitting performances of the doped ZnO nanostructures was improved by at least 27% due to increased light absorption. Conductive Y-doped ZnO nanorods were prepared and applied in gas ionisation sensor application. The measurements revealed that both the selectivity and sensitivity of Y-doped ZnO nanorods were enhanced with respect to undoped ZnO nanorods. Furthermore, the effect of UV illumination on gas sensitivity was also investigated. In summary, different approaches and namomaterials have been adapted and demonstrated in this thesis, for the design of specific photoanodes/electrodes for specific applications

Transparent conductive oxides in photoanodes for solar water oxidation

Rational designs of the conductive layer below photocatalytic films determine the efficiency of a photoanode for solar water oxidation. Generally, transparent conductive oxides (TCOs) are widely used as a conductive layer. In this mini review, the fundamentals of TCOs are explained and typical examples of nanoscale TCOs are presented for application in photoelectrochemical (PEC) water oxidation. In addition, hybrid structures formed by coating other photocatalysts on nanoscale TCOs are discussed. In the future, the nanostructured electrode may inspire the design of a series of optoelectronic application

Ultra rapid direct heating synthesis of ZnO nanorods with improved light trapping from stacked photoanodes for high efficiency photocatalytic water splitting

An ultra rapid growth method for vertically aligned ZnO nanorod (NR) thin films on metal meshes is developed using a direct heating synthesis (DHS) technique. A typical nanorod growth rate of 10 µm/hr was achieved. The effects of the applied heating powers and growth durations on the morphologies of ZnO nanostructures were examined. High density surface defects were formed on the ZnO NRs, which is responsible for slow charge recombination and high efficiency in the photoelectrochemical (PEC) water splitting process. The light absorption for a photoanode was significantly improved by light trapping using a 3D stacked metal mesh photoanode structure. With the internal reflection between the stacked photoanodes, the final light leakage is minimised. The light absorption in the stacked photoanode is improved without restricting the charge transportation. In comparison with a single mesh photoanode and a chemical bath deposition (CBD) grown flat photoanode, the PEC water splitting efficiency from the stacked photoanode was increased by a factor of 2.6 and 6.1 respectively

Light trapping by porous TiO2 hollow hemispheres for high efficiency photoelectrochemical water splitting

Photocatalytic water splitting has recently received increasing attention as a green fuel source. The controlled nano-geometry of the photocatalytic material can improve light harvesting. In this study, as a proof of concept, hollow hemisphere (HHS)-based films of TiO2 material were created by a conventional electrospray method and subsequently applied for photoelectrochemical (PEC) water splitting. To preserve the morphology of the HHS structure, a hydrolysis precipitation phase separation method (HPPS) was developed. As a result, the TiO2 HHS-based thin films presented a maximum PEC water splitting efficiency of ca. 0.31%, almost two times that of the photoanode formed by TiO2 nanoparticle-based films (P25). The unique morphology and porous structure of the TiO2 HHSs with reduced charge recombination and improved light absorption are responsible for the enhanced PEC performance. Light scattering by the HHS was demonstrated with total reflection internal fluorescence microscopy (TRIFM), revealing the unique light trapping phenomenon within the HHS cavity. This work paves the way for the rational design of nanostructures for photocatalysis in fields including energy, environment, and organosynthesis

An enhanced gas ionization sensor from Y-doped vertically aligned conductive ZnO nanorods

A stable and highly sensitive gas ionization sensor (GIS) constructed from vertically aligned, conductive yttrium–doped ZnO nanorod (YZO NR) arrays is demonstrated. The conductive YZO NRs are synthesized using a facile one-pot hydrothermal method. At higher Y/Zn molar ratio, the aspect ratio of the YZO NRs is increased from 11 to 25. Doping with yttrium atoms decreases the electrical resistivity of ZnO NRs more than 100 fold. GIS measurements reveal a 6-fold enhancement in the sensitivity accompanied with a significant reduction in breakdown voltage from the highly conductive YZO NRs. Direct correlations between the resistivity of the NRs and GIS characteristics are established

Thickness control in electrophoretic deposition of WO3 nanofiber thin films for solar water splitting

Electrophoretic deposition (EPD) of ground electrospun WO3 nanofibers was applied to create photoanodes with controlled morphology for the application of photoelectrochemical (PEC) water splitting. The correlations between deposition parameters and film thicknesses were investigated with theoretical models to precisely control the morphology of the nanostructured porous thin film. The photoconversion efficiency was further optimized as a function of film thickness. A maximum photoconversion efficiency of 0.924% from electrospun WO3 nanofibers that EPD deposited on a substrate was achieved at a film thickness of 18 µm

Marangoni ring-templated vertically aligned ZnO nanotube arrays with enhanced photocatalytic hydrogen production

Here we report on a novel approach for the synthesis of vertically aligned hexagonal ZnO nanotube arrays. Ring-like structures were formed on substrate using the polymer-based seeding solution through the Marangoni mechanism, guiding the growth of ZnO nanotubes with aqueous chemical bath deposition. Photoelectrochemical hydrogen generation by water splitting on a ZnO nanotube anode is about three times as efficient as that on a similar ZnO nanorod anode

Enhanced photoelectrochemical water splitting by a 3D hierarchical sea urchin-like structure: ZnO nanorod arrays on TiO2hollow hemisphere

A hierarchical sea urchin-like hybrid metal oxide nanostructure of ZnO nanorods deposited on TiO2 porous hollow hemisphere with a thin zinc titanate interface layer is specifically designed and synthesized forming a combined type I straddling and type II staggered junctions. The HHSs, synthesized by electrospinning, facilitate light trapping and scattering. The ZnO nanorods offer a large surface area for improved surface oxidation kinetics The interface layer of zinc titanate (ZnTiO3) between the TiO2 HHSs and ZnO nanorods regulates the charge separation in a closely coupled hierarchy structure of ZnO/ZnTiO3/TiO2. The synergistic effects of improved light trapping, charge separation, and fast surface reaction kinetics result in a superior photoconversion efficiency of 1.07% for the photoelectrochemical (PEC) water splitting with an outstanding photocurrent density of 2.8 mA cm-2 at 1.23 V vs. RHE.</p

Light trapping by porous TiO2 hollow hemispheres for high efficiency photoelectrochemical water splitting

Photocatalytic water splitting has recently received increasing attention as a green fuel source. The controlled nano-geometry of the photocatalytic material can improve light harvesting. In this study, as a proof of concept, hollow hemisphere (HHS)-based films of TiO2 material were created by a conventional electrospray method and subsequently applied for photoelectrochemical (PEC) water splitting. To preserve the morphology of the HHS structure, a hydrolysis precipitation phase separation method (HPPS) was developed. As a result, the TiO2 HHS-based thin films presented a maximum PEC water splitting efficiency of ca. 0.31%, almost two times that of the photoanode formed by TiO2 nanoparticle-based films (P25). The unique morphology and porous structure of the TiO2 HHSs with reduced charge recombination and improved light absorption are responsible for the enhanced PEC performance. Light scattering by the HHS was demonstrated with total reflection internal fluorescence microscopy (TRIFM), revealing the unique light trapping phenomenon within the HHS cavity. This work paves the way for the rational design of nanostructures for photocatalysis in fields including energy, environment, and organosynthesis

Enhanced photoelectrochemical water splitting by a 3D hierarchical sea urchin-like structure: ZnO nanorod arrays on TiO2hollow hemisphere

A hierarchical sea urchin-like hybrid metal oxide nanostructure of ZnO nanorods deposited on TiO2 porous hollow hemisphere with a thin zinc titanate interface layer is specifically designed and synthesized forming a combined type I straddling and type II staggered junctions. The HHSs, synthesized by electrospinning, facilitate light trapping and scattering. The ZnO nanorods offer a large surface area for improved surface oxidation kinetics The interface layer of zinc titanate (ZnTiO3) between the TiO2 HHSs and ZnO nanorods regulates the charge separation in a closely coupled hierarchy structure of ZnO/ZnTiO3/TiO2. The synergistic effects of improved light trapping, charge separation, and fast surface reaction kinetics result in a superior photoconversion efficiency of 1.07% for the photoelectrochemical (PEC) water splitting with an outstanding photocurrent density of 2.8 mA cm-2 at 1.23 V vs. RHE.</p