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.

Developing a MySQL Database for the Provenance of Black Tiger Prawns (Penaeus monodon).

As the demand for seafood increases, so does the incidence of seafood fraud. Confirming provenance of seafood is important to combat fraudulent labelling but requires a database that contains the isotopic and elemental "fingerprints" of authentic seafood samples. Local isotopic and elemental databases can be scaled up or combined with other databases to increase the spatial and species coverage to create a larger database. This study showcases the use of isotopic and elemental fingerprints of the black tiger prawn (Penaeus monodon) to develop a database that can be used to securely store the data necessary for determining provenance. The utility of this database was tested through querying and building seven different datasets that were used to develop models to determine the provenance of P. monodon. The models built using the data retrieved from the database demonstrated that the provenance of P. monodon could be determined with >80% accuracy. As the database was developed using MySQL, it can be scaled up to include additional regions, species, or methodologies depending on the needs of the users. Combining the database with methods of determining provenance will provide regulatory bodies and the seafood industry with another provenance tool to combat fraudulent seafood labelling.

Role of Oxygen Vacancy Ordering and Channel Formation in Tuning Intercalation Pseudocapacitance in Mo Single-Ion-Implanted CeO2-x Nanoflakes.

Metal oxide pseudocapacitors are limited by low electrical and ionic conductivities. The present work integrates defect engineering and architectural design to exhibit, for the first time, intercalation pseudocapacitance in CeO2-x. An engineered chronoamperometric electrochemical deposition is used to synthesize 2D CeO2-x nanoflakes as thin as ∼12 nm. Through simultaneous regulation of intrinsic and extrinsic defect concentrations, charge transfer and charge-discharge kinetics with redox and intercalation capacitances together are optimized, where reduction increases the gravimetric capacitance by 77% to 583 F g-1, exceeding the theoretical capacitance (562 F g-1). Mo ion implantation and reduction processes increase the specific capacitance by 133%, while the capacitance retention increases from 89 to 95%. The role of ion-implanted Mo6+ is critical through its interstitial solid solubility, which is not to alter the energy band diagram but to facilitate the generation of electrons and to establish the midgap states for color centers, which facilitate electron transfer across the band gap, thus enhancing n-type semiconductivity. Critically, density functional theory simulations reveal, for the first time, that the reduction causes the formation of ordered oxygen vacancies that provide an atomic channel for ion intercalation. These channels enable intercalation pseudocapacitance but also increase electrical and ionic conductivities. In addition, the associated increased active site density enhances the redox such that the 10% of the Ce3+ available for redox (surface only) increases to 35% by oxygen vacancy channels. These findings are critical for any oxide system used for energy storage systems, as they offer both architectural design and structural engineering of materials to maximize the capacitance performance by achieving accumulative surface redox and intercalation-based redox reactions during the charge/discharge process.

Baseline characterisation of source contributions to daily-integrated PM2.5 observations at Cape Grim using Radon-222.

We discuss 15 years (2000-2015) of daily-integrated PM2.5 samples from the Cape Grim Station. Ion beam analysis and positive matrix factorisation are used to identify six source-type fingerprints: fresh sea salt (57%); secondary sulfate (14%); smoke (13%); aged sea salt (12%); soil dust (2.4%); and industrial metals (1.5%). An existing hourly radon-only baseline selection technique is modified for use with the daily-integrated observations. Results were not significantly different for days on which >20 hours were below the baseline radon threshold compared with days when all 24 hours satisfied the baseline criteria. This relaxed daily baseline criteria increased the number of samples for analysis by almost a factor of two. Two radon baseline thresholds were tested: historic (100 mBq m-3), and revised (50 mBq m-3). Median aerosol concentrations were similar for both radon thresholds, but maximum values were higher for the 100 mBq m-3 threshold. Back trajectories indicated more interaction with southern Australia and the Antarctic coastline for air masses selected with the 100 mBq m-3 threshold. Radon-only baseline selection using the 50 mBq m-3 threshold was more selective of minimal terrestrial influence than a similar recent study using wind direction and back trajectories. The ratio of concentrations between terrestrial and baseline days for the primary sources soil, smoke and industrial metals was 3.4, 2.6, and 5.5, respectively. Seasonal cycles of soil dust had a summer maximum and winter minimum. Seasonal cycles of smoke were of similar amplitude for terrestrial and baseline events, but of completely different shape: peaking in autumn and spring for terrestrial events, compared to summer for baseline conditions. Seasonal cycles of industrial metals had a summer maximum and winter minimum. A significant fraction of the Cape Grim baseline smoke and industrial metal contributions appeared to be derived from long-term transport (>3 weeks since last terrestrial influence).

Chemical characterisation and source identification of atmospheric aerosols in the Snowy Mountains, south-eastern Australia.

Characterisation of atmospheric aerosols is of major importance for: climate, the hydrological cycle, human health and policymaking, biogeochemical and palaeo-climatological studies. In this study, the chemical composition and source apportionment of PM2.5 (particulate matter with aerodynamic diameters less than 2.5µm) at Yarrangobilly, in the Snowy Mountains, SE Australia are examined and quantified. A new aerosol monitoring network was deployed in June 2013 and aerosol samples collected during the period July 2013 to July 2017 were analysed for 22 trace elements and black carbon by ion beam analysis techniques. Positive matrix factorisation and back trajectory analysis and trajectory clustering methods were employed for source apportionment and to isolate source areas and air mass travel pathways, respectively. This study identified the mean atmospheric PM2.5 mass concentration for the study period was (3.3±2.5)µgm-3. It is shown that automobile (44.9±0.8)%, secondary sulfate (21.4±0.9)%, smoke (12.3±0.6)%, soil (11.3±0.5)% and aged sea salt (10.1±0.4)% were the five PM2.5 source types, each with its own distinctive trends. The automobile and smoke sources were ascribed to a significant local influence from the road network and bushfire and hazard reduction burns, respectively. Long-range transport are the dominant sources for secondary sulfate from coal-fired power stations, windblown soil from the inland saline regions of the Lake Eyre and Murray-Darling Basins, and aged sea salt from the Southern Ocean to the remote alpine study site. The impact of recent climate change was recognised, as elevated smoke and windblown soil events correlated with drought and El Niño periods. Finally, the overall implications including potential aerosol derived proxies for interpreting palaeo-archives are discussed. To our knowledge, this is the first long-term detailed temporal and spatial characterisation of PM2.5 aerosols for the region and provides a crucial dataset for a range of multidisciplinary research.

Bulk diffusion of niobium in single-crystal titanium dioxide.

The present work reports the tracer diffusion coefficient for (93)Nb in rutile TiO(2) single crystals using secondary ion mass spectrometry (SIMS). The determined tracer diffusion coefficient exhibited the following temperature dependence in air ( p(O2) = 21 kPa) over the range 1073-1573 K: D93(Nb) = (4.7 m2 s(-1))x10(-7+/-0.4) exp ((-244 +/- 9 kJ mol-1)/RT) Through comparison to the self-diffusion of (44)Ti in rutile TiO(2), (93)Nb is interpreted to diffuse via the interstitialcy mechanism. The obtained tracer diffusion data are useful for ensuring compositional control during the processing of Nb-doped TiO(2)-based semiconductors using solid-state reactions between Nb(2)O(5) and TiO(2).

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.