Alexander G. Agrios

Transparent Conducting Aerogels of Antimony-Doped Tin Oxide

Bulk antimony-doped tin oxide aerogels are prepared by epoxide-initiated sol-gel processing. Tin and antimony precursors are dissolved in ethanol and water, respectively, and propylene oxide is added to cause rapid gelation of the sol, which is then dried supercritically. The Sb:Sn precursor mole ratio is varied from 0 to 30% to optimize the material conductivity and absorbance. The materials are characterized by electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), nitrogen physisorption analysis, a four-point probe resistivity measurement, and UV-vis diffuse reflectance spectroscopy. The samples possess morphology typical of aerogels without significant change with the amount of doping. Calcination at 450 °C produces a cassiterite crystal structure in all aerogel samples. Introduction of Sb at 15% in the precursor (7.6% Sb by XPS) yields a resistivity more than 3 orders of magnitude lower than an undoped SnO2 aerogel. Calcination at 800 °C reduces the resistivity by an additional 2 orders of magnitude to 30 Ω·cm, but results in a significant decrease in surface area and pore volume.

Photocatalytic effects of titanium dioxide nanoparticles on aquatic organisms—Current knowledge and suggestions for future research

Nanoparticles are entering natural systems through product usage, industrial waste and post-consumer material degradation. As the production of nanoparticles is expected to increase in the next decade, so too are predicted environmental loads. Engineered metal-oxide nanomaterials, such as titanium dioxide, are known for their photocatalytic capabilities. When these nanoparticles are exposed to ultraviolet radiation in the environment, however, they can produce radicals that are harmful to aquatic organisms. There have been a number of studies that have reported the toxicity of titanium dioxide nanoparticles in the absence of light. An increasing number of studies are assessing the interactive effects of nanoparticles and ultraviolet light. However, most of these studies neglect environmentally-relevant experimental conditions. For example, researchers are using nanoparticle concentrations and light intensities that are too high for natural systems, and are ignoring water constituents that can alter the light field. The purpose of this review is to summarize the current knowledge of the photocatalytic effects of TiO2 nanoparticles on aquatic organisms, discuss the limitations of these studies, and outline environmentally-relevant factors that need to be considered in future experiments.

The Correlation of the Anodic and Cathodic Open Circuit Potential (OCP) and Power Generation in Microbial Fuel Cells (MFCs)

The microbial fuel cell (MFC) is a biotechnology capable of converting biodegradable organic compounds (e.g. hydrocarbon, protein) in wastewater into electricity [1]. An anaerobic biofilm catalyzes the oxidation of organic compounds in the MFC anode. The electrons and protons generated in the anode reach the cathode, which is exposed to air, where they react with oxygen. MFCs hold a great promise to generate electricity from wastewater treatment, a cost-effective approach for environment and energy sustainability [1]. This study focused on the correlation of the electrochemical properties (e.g. open circuit potential (OCP)) and the power generation in MFCs. The single chamber MFCs had a volume of 0.13 L (Figure 1). Raw wastewater was used as the anodic solution in order to simulate real conditions. Sodium acetate was added periodically as a substrate at the concentration of 3 g/L. Two MFCs with different anode geometric areas (10 and 40 cm 2 ) and the same cathode (5 cm 2 , platinum loading 0.5 mgPt/cm 2 ) were compared. Plain carbon cloths were used as the anodes, which were previously colonized by electrogenic bacteria by inoculation in raw wastewater for 4 weeks. Clean platinum-coated carbon cloth was used as the cathode. OCP (vs Ag/AgCl reference electrode) and power generation were measured weekly for 8 weeks. Biofilm growth on anodes and cathodes were also observed weekly. During the 8-week operational period, the anodic OCP values were around -475 to -500 mV, which were close to the theoretical for the sodium acetate oxidation (500 mV) (Figure 2). However, the cathodic OCP values dropped dramatically, especially in the 1 st week (Figure 2). The cathodic OCP values were 570-590 mV initially, but decreased to less than 180-200 mV after the 1 st week due to the colonization of aerobic bacteria on the cathode surfaces, which inhibited contact between of the water and platinum catalyst. The cathodic OCP values slowly decreased after the 1 st week, and were 50 mV in the 8 th

Fluoride additive in epoxide-initiated sol–gel synthesis enables thin-film applications of SnO2 aerogels

Aerogels of SnO2 were synthesized by an epoxide-initiated sol–gel method. Using ammonium fluoride in the precursor solution allowed for tunability of the aerogel morphology while no change in the conductivity was measured. In particular, aerogel shrinkage was decreased dramatically by the addition of the fluoride precursor. Unfluorinated aerogels showed severe shrinkage of 43% volume change upon supercritical drying compared to the original alcogel volume. Fluorinated samples exhibited a much less pronounced shrinkage at 7%. Multiple characterization methods converged to reveal the mechanism by which fluoride enables the morphological tunability. These findings enable the casting of SnO2 aerogels as thin films (which in the absence of fluoride these crack and delaminate due to shrinkage), opening potential uses in many optoelectronic devices including solar cells.

Atmospheric pressure microplasmas in ZnO nanoforests under high voltage stress

Atmospheric pressure ZnO microplasmas have been generated by high amplitude single pulses and DC voltages applied using micrometer-separated probes on ZnO nanoforests. The high voltage stress triggers plasma breakdown and breakdown in the surrounding air followed by sublimation of ZnO resulting in strong blue and white light emission with sharp spectral lines and non-linear current-voltage characteristics. The nanoforests are made of ZnO nanorods (NRs) grown on fluorine doped tin oxide (FTO) glass, poly-crystalline silicon and bulk p-type silicon substrates. The characteristics of the microplasmas depend strongly on the substrate and voltage parameters. Plasmas can be obtained with pulse durations as short as ∼1 μs for FTO glass substrate and ∼100 ms for the silicon substrates. Besides enabling plasma generation with shorter pulses, NRs on FTO glass substrate also lead to better tunability of the operating gas temperature. Hot and cold ZnO microplasmas have been observed with these NRs on FTO glass substr...

Blue and white light emission from zinc oxide nanoforests.

Summary Blue and white light emission is observed when high voltage stress is applied using micrometer-separated tungsten probes across a nanoforest formed of ZnO nanorods. The optical spectrum of the emitted light consistently shows three fine peaks with very high amplitude in the 465–485 nm (blue) range, corresponding to atomic transitions of zinc. Additional peaks with smaller amplitudes in the 330–650 nm range and broad spectrum white light is observed depending on the excitation conditions. The spatial and spectral distribution of the emitted light, with pink–orange regions identifying percolation paths in some cases and high intensity blue and white light with center to edge variations in others, indicate that multiple mechanisms lead to light emission. Under certain conditions, the tungsten probe tips used to make electrical contact with the ZnO structures melt during the excitation, indicating that the local temperature can exceed 3422 °C, which is the melting temperature of tungsten. The distinct and narrow peaks in the optical spectra and the abrupt increase in current at high electric fields suggest that a plasma is formed by application of the electrical bias, giving rise to light emission via atomic transitions in gaseous zinc and oxygen. The broad spectrum, white light emission is possibly due to the free electron transitions in the plasma and blackbody radiation from molten silicon. The white light may also arise from the recombination through multiple defect levels in ZnO or due to the optical excitation from solid ZnO. The electrical measurements performed at different ambient pressures result in light emission with distinguishable differences in the emission properties and I–V curves, which also indicate that the dielectric breakdown of ZnO, sublimation, and plasma formation processes are the underlying mechanisms.

Attachment of Pt nanoparticles to a metal oxide surface using a thiol–carboxyl bifunctional molecule

HYPOTHESIS Molecular attachments to platinum have received far less study than binding to gold. Of particular interest is whether the binding of bifunctional molecules, containing both thiol and carboxyl groups, can attach platinum to surfaces such as metal oxides. EXPERIMENTS Attachment of 4-mercaptobenzoic acid (4-MBA) to bulk and nanoparticulate platinum was studied by cyclic voltammetry (CV), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Platinum nanoparticles were attached to TiO2 using 4-MBA and probed for Pt loading and electrocatalytic activity. FINDINGS Currents for reduction and oxidation of a standard redox couple on a Pt wire are sharply decreased when the Pt is previously exposed to 4-MBA, indicating bonding. This effect is not observed for benzoic acid. The absence of the SH stretching vibrational mode in Raman spectra of 4-MBA-modified Pt nanoparticles is consistent with sulfur-bonding of the molecules to the nanoparticle surface. High-resolution XPS studies of S and Pt core electrons show the formation of SPt bonds. Therefore, 4-MBA binds to Pt via the S atom but not via the carboxyl group, enabling Pt attachment to other surfaces such as metal oxides. 4-MBA increased both the amount of Pt bound to a TiO2 surface and the rate of a redox reaction on the surface.

Morphological Characterization of ALD and Doping Effects on Mesoporous SnO2 Aerogels by XPS and Quantitative SEM Image Analysis

Atomic layer deposition (ALD) is unsurpassed in its ability to create thin conformal coatings over very rough and/or porous materials. Yet although the coating thickness on flat surfaces can be measured by ellipsometry, characterization of these coatings on rough surfaces is difficult. Here, two techniques are demonstrated to provide such characterization of ALD-coated TiO2 over mesoporous SnO2 aerogel films on glass substrates, and insights are gained as to the ALD process. First, X-ray photoelectron spectroscopy (XPS) is used to determine the coating thickness over the aerogel, and the results (0.04 nm/cycle) agree well with ellipsometry on flat surfaces up to a coating thickness limit of about 6 nm. Second, quantitative analysis of SEM images of the aerogel cross section is used to determine porosity and roughness, from which coating thickness can be inferred. The analysis reveals increasing porosity from the aerogel/air interface to the aerogel/substrate interface, indicating a thicker ALD coating near the air side, which is consistent with tortuous diffusion through the pores limiting access of ALD precursors to deeper parts of the film. SEM-derived porosity is generally useful in a thin film because bulk methods like nitrogen physisorption or mercury porosimetry are impractical for use with thin-film samples. Therefore, in this study SEM was also used to characterize quantitatively the morphologogical changes in SnO2 aerogel thin films due to doping with Sb. This study can be used as a methodology to understand morphological changes in different types of porous and/or rough materials.

Antimony Doped Tin Oxide Aerogels for Applications in Energy Conversion and Energy Storage

1. Dept of Civil & Environmental Engineering, University of Connecticut, 261 Glenbrook Rd, Storrs, CT, 06269 2. Dept of Materials Science & Engineering, U. of Connecticut, 191 Auditorium Rd, Storrs, CT 06269-3222 3. Sandia National Laboratory, Materials Characterization Dept, POB 5800,MS 0886, Albuquerque, NM 87185 4. Dept of Chemical & Biomolecular Engineering, U. of Connecticut, 191 Auditorium Rd, Storrs, CT 06269-3222