Thye Foo Choo

Gold catalysed growth of silicon nanowires and core–shell heterostructures via solid–liquid–solid process and galvanic displacement

Abstract Silicon nanowires (SiNWs) were first synthesised using Au coated p type Si (100) substrate via the solid–liquid–solid (SLS) process. The growth parameters were selectively varied to achieve various stages of growth for studying their effects on the morphology and microstructures of the nanowires (NWs). The SLS growth of SiNWs is discussed in the context of the experimental conditions used. Straight NWs of large aspect ratios, good crystallinity and morphology were generally obtained at a growth temperature of 1000°C along with some worm-like amorphous structures. Te–Si NW core–shell structures were subsequently obtained via post-growth galvanic displacement of the SiNWs in an acidic HF electrolyte containing ions. The core–shell structures obtained were decorated with Te nanoparticles. This increases the NW surface areas and should have great potential in non-reflecting, photovoltaic and thermoelectric applications. Growth study on the SiNWs and Te–Si core–shell structures is presented using various microscopy, diffraction and probe based techniques for structural, morphological and chemical characterisations.

Fabrication and Characterization of ZnO Nanofibers by Electrospinning

ZnO nanofibers were successfully prepared by electrospinning a precursor mixture of polyvinylpyrrolidone (PVP)/zinc acetate, followed by calcination treatment of the electrospun composite nanofibers. The effect of applied voltage to the morphology of nanofibers was studied. Both PVP/Zn acetate and ZnO nanofibers were characterized by FESEM and XRD. The results show that the diameter of the nanofibers changed with applied voltage. Results found that the optimum calcined temperature was 500°C to produce continuous ZnO nanofibers.

The Effects of Substrate Orientation on Galvanic Growth of ZnO Structures

Electrochemical route has been a favorite technique for the fabrication of ZnO as it is relatively cheap and capable of producing abundance amount of materials. In this paper, we investigate the morphologies of hydrothermally synthesized ZnO structures assisted by galvanic process on conducting Au-coated silicon substrate. To induce the galvanic process, the substrate was in contact with aluminum so that the difference in the reduction potentials between the two materials provided the driving force for the formation of the ZnO structures. The galvanic process was found to promote compact and anisotropy growth of ZnO nanorods along the (001) plane. It was also found that substrate orientations in the electrolyte solution had an important bearing on the morphologies of ZnO. Well aligned periodic hexagonal arrays of ZnO nanorods of homogeneous diameters were obtained on gold-coated Si substrate with face-down orientation in the electrolyte solution.

Hydrogen sensing enhancement of zinc oxide nanorods via voltage biasing

The capability of zinc oxide (ZnO) as a hydrogen sensing element has been pushed to its limits. Different methods have been explored to extend its sensing capability. In this paper, we report a novel approach which significantly improves the hydrogen sensing capability of zinc oxide by applying a bias voltage to ZnO nanorods as the sensing elements. Zinc oxide in the form of aligned nanorods was first synthesized on an Au-coated Si(111) substrate using a facile method via the galvanic-assisted chemical process. The sensing performance of the zinc oxide nanorods was investigated in response to the applied biasing voltage. It was found that the sensitivity, response time and detection limit of the ZnO sensing elements were dramatically improved with increasing bias voltage. A 100% increment in sensing response was achieved for the detection of 2000 ppm hydrogen gas when the bias voltage was increased from −2 to −6 V with 70% reduction in response and recovery times. This remarkable sensing performance is attributed to the reaction of hydrogen with chemisorbed oxygen ions on the surface of the ZnO nanorods that served as the electron donors to increase the sensor conductance. Higher reverse bias voltages sweep the electrons faster across the electrodes. This shortened the response time and, at the same time, depleted the electrons in the sensor elements and weakens oxygen adsorption. The oxygen ions could then be readily removed by hydrogen, leading to a higher sensitivity of the sensors. This, therefore, envisages a way for high-speed hydrogen gas sensing with high detection sensitivities.

Production of Mullite Ceramic Bodies from Kaolin Processing Waste and Aluminum Hydroxide

Small pellets with differing mullite contents were prepared by a conventional ball-milling and dry-powder pressing technique, followed by firing at temperature of 1300°C and 1600°C for four hours. Kaolin processing waste and high purity aluminum hydroxide were used as starting materials. The sintered samples were examined using X-ray diffraction to determine the weight percent of each identified phase. The results showed that the percentage of mullite in kaolin processing waste can be increased by introducing additional aluminum hydroxide. It was found that sintered samples yielded best results when derived from both types of kaolin processing waste with 40wt% of aluminum hydroxide and a firing temperature of 1600°C.

Large-Area Synthesis and Microstructural Investigations of Silicon Nanowires and Te/Bi2Te3-Si Core-Shell Structures

Large-area randomly-oriented silicon nanowires (SiNWs) were synthesized using Au-coated p-type Si (100) substrates via the solid-liquid-solid (SLS) process under different growth conditions. Microstructural studies on the NWs produced showed that straight crystalline nanowires of large aspect ratios were generally obtained at a growth temperature of 1000°C along with some worm-like amorphous structures. Large-area vertically aligned silicon nanowire (SiNW) arrays on p-type (001) Si substrates were also synthesized in an aqueous solution containing AgNO3 and HF by self-selective electroless etching. Diameters of the SiNWs produced from both methods varied from 50 nm to 350 nm and their lengths generally extended from several to approximately a few tens of µm depending on the growth conditions used. Te-Si and Bi2Te3-Si core-shell structures were subsequently obtained via galvanic displacement of SiNWs in acidic HF electrolytes containing HTeO2+ and Bi3+/HTeO2+ ions. The reactions were basically a nanoelectrochemical process due to the difference in redox potentials between the materials. The modified SiNWs of core-shell structures had roughened surface morphologies and, therefore, higher surface-to-bulk ratios compared to unmodified SiNWs. They should have potential applications in sensors, photovoltaic and thermoelectric nanodevices. Microstructural studies on the SiNWs and core-shell structures produced are presented using various microscopy, diffraction and probe-based techniques for characterization.

Aqueous synthesis of silicon nanowire arrays and core-shell structures via electroless nanoelectrochemical process

A solution growth of silicon nanowires (NWs) and core-shell structures is highly desirable due to low growth temperature and large-area synthesis capability. Silicon demonstrates novel electrochemical properties in aqueous solutions containing hydrofluoric acid which have raised significant research interests due to the complex electrochemical etching behavior involved. In the present study, we have demonstrated the synthesis of large-area vertically aligned silicon nanowire (SiNW) arrays (Fig. 1 and 2) in an aqueous solution containing AgNO3 and HF on p-type (001) Si substrate by self-selective electroless etching process [1]. The fabrication process was rather simple and rapid compared to the well-known Vapor-Liquid-Solid (VLS) growth [2] via chemical-vapor deposition (CVD) and other high-vacuum techniques. In this work, the temperature of electrolyte and etching duration were varied in order to achieve different stages of nanowire formation. Diameters of the SiNWs obtained varied from 50 nm to 200 nm and their lengths ranged from several to approximately a few tens of µm, depending on the reaction time and the electrolyte conditions used. Te-Si and Bi2Te3-Si core-shell structures (Fig. 3) were subsequently obtained via galvanic displacement of SiNWs in acidic HF electrolytes containing Bi3+ and Bi3+/HTeO2+ ions respectively. The reactions were basically a nanoelectrochemical process due to the difference in redox potentials between the materials. The modified SiNWs of the core-shell structures exhibit roughened surface morphologies and, therefore, have higher surface-to-bulk ratios compared to the unmodified SiNWs, which should have potential applications in sensor, photovoltaic and thermoelectric nanodevices. Growth study on the SiNWs and core-shell structures produced is presented using various microscopy, diffraction and probe-based techniques for structural, morphological and chemical characterizations.