Wei Li Ong

Structural design of TiO2-based photocatalyst for H2 production and degradation applications

TiO2-based photocatalysts, being inexpensive and abundant, in conjunction with having high photostability and environmentally friendly characteristics, are the most extensively studied photocatalytic material for hydrogen production and pollutant degradation. However, its existing issues, such as wide bandgap, high overpotential and rapid recombination of photogenerated carriers limit its photocatalytic properties. The opportunities for structural development of a TiO2 nanomaterial towards highly efficient and pragmatic photocatalysis applications are evidently plentiful. Hence, in this review, we will look into critical structural engineering strategies that give favorable physicochemical properties such as improved light absorption, photostability, charge-carrier dynamics, increase surface area etc. that benefit photocatalysis functionalities. Amongst the various structural engineering options, we will be covering the most prevalent and elegant core–shell and hierarchical structural designs, which rationally combine the advantages of structural manipulation and multi-material composition engineering. This review aims to provide a comprehensive and contemporary overview, as well as a guide of the development of new generation TiO2 based photocatalysts via structural design for improved solar energy conversion technologies.

Green chemistry synthesis of a nanocomposite graphene hydrogel with three-dimensional nano-mesopores for photocatalytic H2 production

In this work, we have developed a nanocomposite graphene hydrogel (NGH) based on green chemistry, employing vitamin C (VC) to attain a supramolecular 3D network of hybrid nanostructured materials. Here, it is shown that the hydrogel is an appropriate and robust host for stable a TiO2 semiconductor catalyst sensitized with visible light responsive nanostructured particles. The NGH is tailored with well-defined nano-mesopores, a large surface area, a highly dispersive nanosheet–nanorods–nanoparticle composite, and enhance visible light absorption. Finally, we demonstrate practical applications of utilizing the NGH with water containing pores for photocatalytic H2 production. An important pragmatic consideration of using a NGH is the ease of separation and recovery of the nanosized catalyst after the photoreaction which would otherwise require extensive and expensive nanofiltration.

TiO2 Fibers Supported β-FeOOH Nanostructures as Efficient Visible Light Photocatalyst and Room Temperature Sensor

Hierarchical heterostructures of beta-iron oxyhydroxide (β-FeOOH) nanostructures on electrospun TiO2 nanofibers were synthesized by a facile hydrothermal method. This synthesis method proves to be versatile to tailoring of β-FeOOH structural design that cuts across zero-dimensional particles (TF-P), one-dimensional needles (TF-N) to two-dimensional flakes (TF-F). In addition, synthesizing such oxyhyroxide nanostructures presents the advantage of exhibiting similar functional performances to its oxides counterpart however, without the need to undergo any annealing step which leads to undesirable structural collapse or sintering. The as-prepared hierarchical heterostructures possess high surface area for dye adsorptivity, efficient charge separation and visible photocatalytic activity. Also, for the first time, hydrogen gas sensing has been demonstrated on β-FeOOH nanostructures at room temperature. The reported hierarchical heterostructures of β-FeOOH on TiO2 nanofibers afford multiple functions of photocatalysis and sensing which are highly promising for environment monitoring and clean up applications.

Self-Biased Hybrid Piezoelectric-Photoelectrochemical Cell with Photocatalytic Functionalities.

Utilizing solar energy for environmental and energy remediations based on photocatalytic hydrogen (H2) generation and water cleaning poses great challenges due to inadequate visible-light power conversion, high recombination rate, and intermittent availability of solar energy. Here, we report an energy-harvesting technology that utilizes multiple energy sources for development of sustainable operation of dual photocatalytic reactions. The fabricated hybrid cell combines energy harvesting from light and vibration to run a power-free photocatalytic process that exploits novel metal-semiconductor branched heterostructure (BHS) of its visible light absorption, high charge-separation efficiency, and piezoelectric properties to overcome the aforementioned challenges. The desirable characteristics of conductive flexible piezoelectrode in conjunction with pronounced light scattering of hierarchical structure originate intrinsically from the elaborate design yet facile synthesis of BHS. This self-powered photocatalysis system could potentially be used as H2 generator and water treatment system to produce clean energy and water resources.

Room temperature sequential ionic deposition (SID) of Ag2S nanoparticles on TiO2 hierarchical spheres for enhanced catalytic efficiency

Porous TiO2 hierarchical spheres with high surface area synthesized via a solvothermal method were successfully modified with an Ag2S co-catalyst by a sequential ionic deposition (SID) method at room temperature. The presence of Ag2S facilitated efficient charge separation, thus reducing recombination and enhancing the photocatalytic activity of the photocatalyst. The enhanced photocatalytic performance was demonstrated by water splitting where hydrogen (H2) gas was produced at an evolution rate of 708 μmol h−1 g−1 and methyl orange was degraded with a rate constant of 0.018 min−1. This is the first time that photocatalytic water splitting using a suspension system has been demonstrated on a Ag2S/TiO2 hierarchical heterostructure and the material shows stability in its photocatalytic performance despite being recycled several times. The composite material presents properties which are highly promising for the generation of clean energy and environmental clean up applications.

Resistive Switching and Polarization Reversal of Hydrothermal-Method-Grown Undoped Zinc Oxide Nanorods by Using Scanning Probe Microscopy Techniques

This paper reports the localized electrical, polarization reversal, and piezoelectric properties of the individual hexagonal ZnO nanorods, which are grown via the hydrothermal method and textured with [0001] orientation. The studies are conducted with conductive atomic force microscopy (c-AFM) and piezoresponse force microscopy (PFM) techniques. The correlation between the resistance switching and polarization reversal is discussed. The c-AFM results show that there is less variation on the set or reset voltage in nanorod samples, compared to that of the ZnO thin film. With increasing aspect ratio of the nanorods, both set and reset voltages are decreased. The nanorods with low aspect ratio show unipolar resistance switching, whereas both unipolar and bipolar resistance switching are observed when the aspect ratio is larger than 0.26. The PFM results further show the ferroelectric-like property in the nanorods. Comparing with that of the ZnO thin film, the enhanced piezoresponse in the nanorods can be attributed to the size effect. In addition, the piezoresponse force spectroscopy (PFS) experiments are conducted in ambient air, synthetic air, and argon gas. It shows that the depolarization field in the nanorod may be due to the moisture in the environment; moreover, the increased piezoresponse may relate to the absence of oxygen in the environment. It is also shown that the piezoelectric responses increase nonlinearly with the aspect ratio of the nanorods. By comparing the piezoresponse hysteresis loops obtained from the nanorod samples of as-grown, air-annealed and vacuum-annealed, it is found that the oxygen vacancies are the origin of the polarization reversal in ZnO nanorods. Finally, the tradeoff between the electrical and ferroelectric-like properties is also observed.

Patterned growth of vertically-aligned ZnO nanorods on a flexible platform for feasible transparent and conformable electronics applications

Despite the attractiveness of low temperature hydrothermal processes, the synthesis of vertical ZnO nanostructures has mostly been limited to rigid substrates. Moreover, patterned growth of nanostructures is also commonly carried out on rigid substrates, since conventional optical lithography is not easily applied to polymeric substrates, as focusing and reaction of the substrate with the organic solvent used in the lithography process prove to be a challenge. Here, we demonstrate the limited work on laser writing lithography patterned growth instead of the commonly used soft lithography patterned growth of nanorods on the transparent flexible substrate polyethylene terephthalate (PET) with a practical device demonstration. The visibly-transparent nanorods on the PET platform constitute a superior structural integrity with ohmic electro-conductivity even in a highly bent state. Accordingly, this can pave the way towards integration of vertically-aligned 1D nanostructures on a flexible platform for a transparent, conformable, shock-proof and lightweight product.

Highly flexible solution processable heterostructured zinc oxide nanowires mesh for environmental clean-up applications

We report the fabrication of a fully solution-processed ZnO nanowires array on flexible stainless steel mesh. ZnO nanowires of uniform dimensions are radially and densely assembled over a large area of the mesh. Various metal and metal oxide nanoparticles are photochemically deposited onto the ZnO nanowires and the corresponding effects on the photocurrent are investigated. Furthermore, the stability and robustness of the heterostructured ZnO nanowires grown on the mesh are evaluated by assessing the photocurrent in response to on/off cycles as well as undergoing various bending configurations. Finally, the heterostructured nanowire mesh is preliminarily tested for photodegradation of organic compound and separation of oil–water mixture. The multifunctional heterostructured nanowire mesh has shown potential applications for environmental clean-up purposes.

Modeling and Experimental Study of a Low-Frequency-Vibration-Based Power Generator Using ZnO Nanowire Arrays

A piezoelectric power generator based on ZnO nanowire arrays (NWAs) is proposed for scavenging energy from low-frequency ambient vibration. A low-temperature and low-cost hydrothermal method is employed to grow the uniform ZnO nanowires with the diameters of ~ 70 nm and lengths of ~2.5 . A theoretical model is derived to better understand piezoelectric-energy harvesting and to predict the output performance of cylindrical ZnO NWAs. The open-circuit output voltages of the resonant generators at normal and shear modes are 1.1 and 3.59 mV, respectively.

Ammonia plasma modification towards a rapid and low temperature approach for tuning electrical conductivity of ZnO nanowires on flexible substrates

Though the fabrication of ZnO nanostructures is economical and low temperature, the lack of a facile, reliable and low temperature methodology to tune its electrical conductivity has prevented it from competing with other semiconductors. Here, we carried out surface modification of ZnO nanowires using ammonia plasma with no heat treatment, and studied their electrical properties over an extended time frame of more than a year. The fabrication of flexible devices was demonstrated via various methods of transferring and aligning as-synthesized ZnO nanowires onto plastic substrates. Hall measurements of the plasma modified ZnO nanowires revealed p-type conductivity. The N1s peak was present in the X-ray photoelectron spectrum of the surface modified ZnO, showing the presence of ammonia complexes. Low temperature photoluminescence showed evidence of acceptor-bound exciton emission. The resulting electrical devices, a chemical sensor and p-n homojunction, show the tunable electrical response of the surface modified ZnO nanowires.