Armand J. Atanacio

Controlled pore structure modification of diatoms by atomic layer deposition of TiO2

Diatoms produce diverse three-dimensional, regular silica structures with nanometer to micrometer dimensions and hold considerable promise for biological or biomimetic fabrication of nanostructured materials and devices. The unique hierarchical porous structure of diatom frustules is in particular attractive for membrane applications in microfluidic systems. In this paper, a procedure for pore size modifications of two centric diatom species, Coscinodiscus sp. and Thalassiosira eccentrica (T. eccentrica) using the atomic layer deposition (ALD) of ultrathin films of titanium oxide (TiO2) is described. TiO2 is deposited by sequential exposures to titanium chloride (TiCl4) and water. The modified diatom membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDAX), and secondary ion mass spectrometry (SIMS). These techniques confirmed the controlled reduction of pore sizes while preserving the shape of the diatom membrane pores. Pore diameters of diatom membranes can be further tailored for specific applications by varying the number of cycles and by changing their surface functionality.

Mercury vapor sensor enhancement by nanostructured gold deposited on nickel surfaces using galvanic replacement reactions

Anthropogenic mercury emission is a serious global environmental problem because of its toxicity to humans, plants and wildlife. In order to control these emissions, accurate and reliable online continuous mercury monitoring systems (CMMs) are critical. Such systems can notify appropriate authorities or provide feedback signals to a process control system in time, thus making them an integral part of monitoring and controlling Hg emissions. We demonstrate how nanostructured gold can easily be deposited in small quantities on nickel electrode based QCMs using galvanic replacement (GR) reactions with the resultant surface having excellent Hg monitoring properties. The developed GR surfaces were observed to have higher sensitivity and selectivity in the presence of interfering gas species (NH3 and H2O), as well as to have ∼80% higher mercury sorption capacity than the most efficient mercury sorbents reported to date. Investigations towards the Hg-sensing capabilities of the resultant Ni–Au surface based Hg sensors showed ∼50% better sensitivity and detection limit over control Au films. Furthermore, the GR based QCMs were found to self-regenerate without changing the operating temperature of the sensor, undergoing Hg desorption with sensor recoveries of 93.7–99.3% following Hg exposure at an operating temperature of 90 °C. Surface depth profile analysis of the Ni–Au electrode surfaces showed that the high recovery rate of the sensors was primarily due to the Ni–Au structures, which unlike continuous Au thin-films more commonly used for Hg sensing applications, do not accumulate Hg at the sensitive-layer–substrate interface. Furthermore, the GR Ni–Au surfaces were found to be highly selective towards Hg vapor in the presence of NH3 and H2O interfering gas species which makes them potentially suitable for operating in harsh industrial effluent environments.

Effect of indium segregation on the surface versus bulk chemistry for indium-doped TiO2

This work reports the effect of indium segregation on the surface versus bulk composition of indium (In)-doped TiO(2). The studies are performed using proton-induced X-ray emission (PIXE), secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS). The results of XPS analysis indicate that annealing of In-doped TiO(2) containing 0.3 atom % In at 1273 K in the gas phase of controlled oxygen activity [p(O(2)) = 75 kPa and 10 Pa] results in a surface enrichment of 2.95 and 2.61 atom % In, respectively. The obtained segregation data are considered in terms of the transport of indium ions from its titanium sites in the bulk phase to the surface where these ions are incorporated into interstitial sites. The effect of oxygen activity on the segregation-induced surface enrichment is considered in terms of the formation of a low-dimensional surface structure and a sublayer, which are charged negatively. The latter is formed as a result of strong interactions between titanium vacancies and interstitial indium ions, leading to the formation of defect complexes. The data obtained in this work may be used for engineering of TiO(2)-based semiconductors with enhanced performance in solar energy conversion.

Photocatalytic Properties of TiO2: Evidence of the Key Role of Surface Active Sites in Water Oxidation

Photocatalytic activity of oxide semiconductors is commonly considered in terms of the effect of the band gap on the light-induced performance. The present work considers a combined effect of several key performance-related properties (KPPs) on photocatalytic activity of TiO2 (rutile), including the chemical potential of electrons (Fermi level), the concentration of surface active sites, and charge transport, in addition to the band gap. The KPPs have been modified using defect engineering. This approach led to imposition of different defect disorders and the associated KPPs, which are defect-related. This work shows, for the first time, a competitive influence of different KPPs on photocatalytic activity that was tested using oxidation of methylene blue (MB). It is shown that the increase of oxygen activity in the TiO2 lattice from 10(-12) Pa to 10(5) Pa results in (i) increase in the band gap from 2.42 to 2.91 eV (direct transitions) or 2.88 to 3 eV (indirect transitions), (ii) increase in the population of surface active sites, (iii) decrease of the Fermi level, and (iv) decrease of the charge transport. It is shown that the observed changes in the photocatalytic activity are determined by two dominant KPPs: the concentration of active surface sites and the Fermi level, while the band gap and charge transport have a minor effect on the photocatalytic performance. The effect of the defect-related properties on photoreactivity of TiO2 with water is considered in terms of a theoretical model offering molecular-level insight into the process.

Fabrication, Structural Characterization and Testing of a Nanostructured Tin Oxide Gas Sensor

A nanostructured SnO2 conductometric gas sensor was produced from thermally evaporated Sn clusters using a thermal oxidation process. SnO2 clusters were simultaneously formed in an identical process on a Si3N4 membrane featuring an aperture created by a focused ion beam (FIB). Clusters attached to the vertical edges of the aperture were imaged using a transmission electron microscope. The original morphology of the Sn cluster film was largely preserved after the thermal oxidation process and the thermally oxidized clusters were found to be polycrystalline and rutile in structure. NO2 gas sensing measurements were performed with the sensor operating at various temperatures between 25degC and 290degC. At an operating temperature of 210degC, the sensor demonstrated a normalized change in resistance of 3.1 upon exposure to 510 ppb of NO2 gas. The minimum response and recovery times for this exposure were 45 s and 30 s at an operating temperature of 265degC. The performance of the SnO2 sensor compared favorably with previously published results. Finally, secondary ion mass spectrometry and X-ray photoelectron spectroscopy were used to establish the levels of nitrogen present in the films following exposure to NO2 gas.

Photosensitive oxide semiconductors for solar hydrogen fuel and water disinfection

Abstract Hydrogen is expected to become a commonly used energy carrier on the global scale in the near future. However, hydrogen as a fuel is environmentally friendly only when generated from water using renewable energy, such as solar energy. Therefore, intensive research aims to develop a new generation of solar materials, which may be used for the production of hydrogen fuel from water using solar energy. The highly promising candidates for solar energy conversion are photosensitive oxide semiconductors (POSs), particularly the TiO2-based semiconductors, which may be used for converting solar energy into the chemical energy required for hydrogen generation from water, as well as water purification (removal of microbial agents and toxic contaminants from water). The present work considers an R&D strategy for developing TiO2-based systems capable of converting solar energy into the chemical energy via water oxidation. The effect of surface versus bulk semiconducting properties on the performance of POSs is considered in terms of partial and total water oxidation. The progress requires modification of the key performance-related properties (KPPs) in order to enhance the light-induced reactivity of the POSs with water. The most recent approach in the development of POSs with enhanced performance is deposition of metallic islets of different size and shape in order to induce a plasmonic effect. The development of high-performance POSs can be achieved through a multidisciplinary approach. It is shown that defect disorder has a critical effect on the light-induced reactivity of POSs and the solar energy conversion. Therefore, defect engineering may be applied in the development of high-performance POSs. This work considers the hurdles in the development of high-performance POSs for specific applications and formulates the key questions that must be addressed to overcome these hurdles. The concepts developed for TiO2 may be expanded for other metal oxides.

Intrinsic and boron-enhanced hydrogen diffusion in amorphous silicon formed by ion implantation

The concentration dependence of H diffusion in amorphous Si (a-Si) formed by ion implantation is reported for implanted H profiles. An empirical relationship is proposed which relates the diffusion coefficient to the H concentration valid up to 0.3 at. %. B-enhanced H diffusion is observed and shows trends with temperature typically associated with a Fermi level shifting dependence. A modified form of the generalized Fermi level shifting model is applied to these data.

Reducing mortality risk by targeting specific air pollution sources: Suva, Fiji

Health implications of air pollution vary dependent upon pollutant sources. This work determines the value, in terms of reduced mortality, of reducing ambient particulate matter (PM2.5: effective aerodynamic diameter 2.5μm or less) concentration due to different emission sources. Suva, a Pacific Island city with substantial input from combustion sources, is used as a case-study. Elemental concentration was determined, by ion beam analysis, for PM2.5 samples from Suva, spanning one year. Sources of PM2.5 have been quantified by positive matrix factorisation. A review of recent literature has been carried out to delineate the mortality risk associated with these sources. Risk factors have then been applied for Suva, to calculate the possible mortality reduction that may be achieved through reduction in pollutant levels. Higher risk ratios for black carbon and sulphur resulted in mortality predictions for PM2.5 from fossil fuel combustion, road vehicle emissions and waste burning that surpass predictions for these sources based on health risk of PM2.5 mass alone. Predicted mortality for Suva from fossil fuel smoke exceeds the national toll from road accidents in Fiji. The greatest benefit for Suva, in terms of reduced mortality, is likely to be accomplished by reducing emissions from fossil fuel combustion (diesel), vehicles and waste burning.

Determination of niobium diffusion in titania and zirconia using secondary ion mass spectrometry

Abstract This paper provides an outline for the use of secondary ion mass spectrometry (SIMS) in the determination of diffusion data in metal oxides. The focus is on the determination of Nb bulk and grain boundary diffusion coefficients in TiO2 and zirconia. Specifically, the diffusion of Nb in TiO2 and yttria doped (10 mol.-%) ZrO2 (10YSZ) has been assessed. The following bulk diffusion coefficients D 93Nb were obtained D 93Nb =(1·03±0·051) × 10−18 m2 s−1 10YSZ(1273K) D 93Nb =(1·91±0·096) × 10−16 m2 s−1 TiO2(1273K) The grain boundary diffusion parameter for Nb grain boundary diffusion in 10YSZ was also determined D 93Nb δα =(7·48 ± 0·37) × 10−25 m2 s−1 10YSZ(1273K) The Nb grain boundary diffusion coefficient D′93Nb was determined to be D′93Nb =(3·99 ± 0·20) × 10−16 m2 s−1 10YSZ(1273K)

Effect of niobium segregation on the surface properties of titanium dioxide

The present paper considers the effect of segregation on the performance of photo-electrode materials for photo-electrochemical water splitting. This phenomenon, which alters the surface composition of a material during processing at elevated temperatures, has the capacity to dominate interfacial charge transfer between the photo-electrode and the electrolyte. As the present understanding of segregation in metal oxides is limited, this paper aims at addressing the need to collect empirical data which can be used for the development of novel materials. In the present investigation, Nb surface segregation was investigated at 1273 K under high and low oxygen activity using secondary ion mass spectrometry (SIMS). A calibration procedure was used to enable quantifiable data and Nb was observed to segregate strongly, especially at high oxygen activity. While this was attributed to the defect disorder, it remained unclear whether gas/solid equilibrium was achieved, and consequently whether the observed behaviour represents equilibrium segregation. Irrespectively, the observed behaviour clearly illustrates how the surface composition of a metal oxide can be altered through the control of segregation. This must be considered in the pursuit of high performance photo-electrode materials for water splitting under sunlight.