Declan E. McCormack

Growth of well-defined ZnO microparticles by hydroxide ion hydrolysis of zinc salts

The morphology of the microcrystalline zinc oxide formed by reaction of zinc salts with sodium hydroxide depends critically on the reaction conditions. To understand this, the nature of the solid product has been probed by scanning electron microcopy (SEM) and X-ray diffraction (XRD) and the concentration of zinc remaining in the solution has been determined at regular intervals throughout the reaction. Two general preparative procedures have been followed. The first, which involves simply heating an aqueous solution containing Zn(NO3)2 (0.025 M) and sodium hydroxide (0.375 M) to 101 °C and maintaining it at that temperature for periods of up to 8 hours, produces star-like microcrystals. The star-like morphology is apparently caused by multiple crystal twinning at the onset of growth, and the size and shape of the microcrystals have been found to be affected by both the reaction stoichiometry and the type of zinc salt counterion. In the second method, which produces needles of zinc oxide (lengths up to 9.5 µm—aspect ratio >6), the reagents are stirred at room temperature for two hours before heating the mixture to 101 °C and maintaining it at that temperature for up to 24 hours. Pre-stirring allows the formation of Zn(OH)2, which is shown by XRD and SEM to transform to ZnO upon heating.

The effect of processing conditions on varistors prepared from nanocrystalline ZnO

Nanoparticles of ZnO were prepared by the reaction of ethanolic solutions of zinc acetate and oxalic acid followed by drying (80 °C) and calcination (500 °C). Subsequently varistor materials were fabricated from this nanoparticular ZnO via two separate routes:- a) from a “core shell” material using metal salts as additives; b) by using a conventional solid state mixing of metal oxides. Sintering (1050 °C) and subsequent electrical studies were carried out for each of these samples and they were compared with commercial varistor samples prepared under similar conditions. “Core shell” type varistor material showed considerably higher breakdown voltage (Vc = 850 ± 30 V mm−1) as compared to a sample prepared by mixing with metal oxides (Vc = 683 ± 30 V mm−1) or commercial varistor discs (Vc = 507 ± 30 V mm−1). The high breakdown voltage obtained is attributed to the formation of more varistor-active grain boundaries per unit area.

Rapid microwave synthesis of mesoporous TiO2 for electrochromic displays

The fabrication of paper quality electrochromic displays based on the viologen modified TiO2 electrodes (Vio2+/TiO2) requires a cost-effective, energy efficient and rapid synthesis of mesoporous TiO2 with high yield in short reaction time. A straightforward and industrially viable process for the preparation of mesoporous nanocrystalline titania (meso-nc-TiO2) for NanoChromics™ display device applications by the use of microwave synthesis is presented here. Spherical aggregates of meso-nc-TiO2 were rapidly achieved using titanium butoxide, deionised water and common alcohols (isopropanol, ethanol and butanol) at comparatively low microwave power intensity (300 W) for 2 min irradiation. The material has been characterised by a range of different techniques such as XRD, Raman spectroscopy, SEM and BET surface area analysis. These materials possess surface areas up to 240 m2 g−1, which is significantly higher than similar traditional sol–gel or commercial samples. This meso-nc-TiO2 prepared was used as the working electrode for an electrochromic display device with Sb doped SnO2 as the counter electrode material on an ITO coated conducting glass. A working prototype of a NanoChromics™ display was successfully fabricated using this approach.

Self-assembled arrays of ZnO nanoparticles and their application as varistor materialsElectronic supplementary information (ESI) available: XRD plots and FESEM images. See http://www.rsc.org/suppdata/jm/b4/b400927d/

Linear arrays of ZnO nanoparticles have been successfully prepared by a simple sol–gel condensation reaction involving chemical modifiers, followed by drying (80 °C) and calcination (500 °C). The calcined material (nano-array ZnO) is composed of approximately spherical nanoparticles of average diameter 21 ± 3 nm, self-assembled to form arrays extending in length to 2–4 µm. The morphology of the ZnO is found to depend sensitively on the amounts of chemical modifiers present. In their absence the ZnO produced (nano-ZnO) is an unstructured agglomerate of nanoparticles. The mechanism for formation of these linear arrays has been investigated by examining the intermediates formed at 80 °C and 250 °C using XRD and TEM and by following the decomposition reactions using TGA and DSC. Varistors prepared from the nano-array ZnO by sintering (1050 °C) with appropriate mixtures of metal oxides showed a breakdown voltage of 786 ± 30 V mm−1, which is substantially higher than that of samples prepared under similar conditions from either micron-sized commercial ZnO (507 ± 30 V mm−1) or from nano-ZnO (683 ± 30 V mm−1).

Advances in the synthesis of ZnO nanomaterials for varistor devices

ZnO based varistors are widely used for overvoltage protection in many electrical and electronic circuits, at voltages ranging from a few to over a million volts. By careful control of the microstructure, through nano-structuring by chemical routes, it should be possible to produce varistors with high breakdown voltage (Vc), as this is proportional to the number of active grain boundaries in the sintered body. This property is particularly important for the production of the small-sized varistors needed for modern electronic instruments such as tablet computers and mobile phones. The current review will outline the recent advances in the chemical processing (e.g. sol–gel, combustion synthesis plasma pyrolysis, micro-emulsion synthesis and precipitation routes) of varistors from ZnO nanomaterials and the properties of these materials. Uncontrolled grain growth at higher temperature is highlighted as a major challenge for obtaining desirable electrical properties for nano-varistors. Various novel sintering techniques such as step-sintering, spark plasma and microwave sintering methods are expected to deliver a varistor with controlled grain growth and optimum electrical characteristics.

Antibacterial properties of F-doped ZnO visible light photocatalyst

Nanocrystalline ZnO photocatalysts were prepared by a sol-gel method and modified with fluorine to improve their photocatalytic anti-bacterial activity in visible light. Pathogenic bacteria such as Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) were employed to evaluate the antimicrobial properties of synthesized materials. The interaction with biological systems was assessed by analysis of the antibacterial properties of bacteria suspended in 2% (w/w) powder solutions. The F-doping was found to be effective against S. aureus (99.99% antibacterial activity) and E. coli (99.87% antibacterial activity) when irradiated with visible light. Production of reactive oxygen species is one of the major factors that negatively impact bacterial growth. In addition, the nanosize of the ZnO particles can also be toxic to microorganisms. The small size and high surface-to-volume ratio of the ZnO nanoparticles are believed to play a role in enhancing antimicrobial activity.

Microheterogeneous catalysis in modified electrodes

Abstract Analytical expressions quantifying the transport and kinetics in conducting polymer modified electrodes containing a homogeneous distribution of spherical microparticulate catalysts are presented. In particular the dependence of the flux on the number of catalytic particles per unit volume, the layer thickness, the substrate concentration, the particle radius and the electrode potential are outlined.

Silver doped perfluoropolyether-urethane coatings: Antibacterial activity and surface analysis

The colonisation of clinical and industrial surfaces with pathogenic microorganisms has prompted increased research into the development of effective antibacterial and antifouling coatings. There is evidence that implanted biomedical surfaces coated with metallic silver can be inactivated by physiological fluids, thus reducing the bioactivity of the coating. In this work, we report the biofilm inhibition of Staphylococcus epidermidis using a room temperature processed silver doped perfluoropolyether-urethane coating. The release of silver ions from these fluoropolymers over a six-day period inhibited bacterial encrustation - as observed by scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) analysis indicated differences in carbon, fluorine and sodium surface composition between silver doped and undoped fluoropolymers after exposure to nutrient rich media. These silver doped perfluoropolyether coatings also exhibited antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii; suggesting potential use in preventing transmission of pathogenic and opportunistic microbes on environmental surfaces in healthcare facilities. The broad-spectrum antibacterial activity of these silver release coatings may be exploited on biomaterials surfaces to combat the development of resistant Gram-negative Enterobacteriaceae that can occur during prophylactic treatment for urinary tract infections.

Microwave-assisted synthesis of ZnO micro-javelins†

The microwave (MW)-assisted formation of ZnO micro-javelins from zinc nitrate and urea in aqueous solution is described. The particles (named as ‘micro-javelins’ because of their high aspect ratio and needle-like tips) grow hexagonally with well-defined facets in the 〈010〉 direction and pointed tips in (0001) direction. Powder X-ray diffraction patterns show the appearance of a strikingly dominant (1000) orientation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations reveal the morphological evolution of these hexagonal ZnO particles with time. The effect of precursor concentrations, counterion type and MW irradiation power and their consequent influence on pH and Zn2+ ion concentration are investigated. A mechanism for the formation of the micro-javelins is postulated. The microwave induced supersaturation of Zn(OH)+ species under the weakly basic pH condition and the initial growth through the (000) direction (oxygen-rich face) are proposed to be the key factors that dictate the formation of these ZnO micro-javelins. The present one-step microwave process is a straightforward and a reproducible method for the bulk synthesis of defect-free ZnO micro-javelins, which would find potential applications in microelectronic devices (e.g. lasers, cantilevers in surface probing equipment, etc.).

High performance ZnO varistors prepared from nanocrystalline precursors for miniaturised electronic devices

An industrially viable solution-based processing route using minimal amounts of solvent has been used to prepare bulk quantity nanopowders (average particle size 15 ± 3 nm) for the fabrication of ZnO varistors. The xerogels, calcined powders and sintered materials were fully characterised. The preparation of varistors from nanopowders has been optimised by studying the effect of temperature on grain growth, densification and breakdown voltage. The varistors are prepared by sintering at 1050 °C for 2 hours, a temperature that is significantly lower than that used in the current industrial process. Highly dense varistor discs prepared from the sintered material produce devices, with a breakdown voltage 85% higher than that of commercial varistors, making them suitable for use in miniaturised electronic circuitry. Improved performance of these materials has been attributed to the small grain size and better dispersion of additives on ZnO grains.