Thomas C. DeVore

Desorption of Nitric Acid From Boehmite and Gibbsite

Solid-state Fourier transform infrared spectroscopy (FTIR), evolved gas analysis-FTIR (EGA-FTIR), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) have been used to investigate the desorption of nitric acid from boehmite and from gibbsite. Samples containing between 3 and 36% of adsorbed nitric acid by mass were prepared by placing the mineral in a 70% nitric acid solution or by the adsorption of nitric acid vapors in humid air. FTIR established that water-solvated nitrate was the main species adsorbed on the surface of either mineral under these conditions. The water-solvated nitrate vaporized as nitric acid at approximately 400 K with an enthalpy of desorption of approximately 50 kJ/mol for both surfaces. A second nitric acid desorption occurred at approximately 450 K and had an enthalpy of desorption of 85 kJ/mol (95 kJ/mol) for boehmite (gibbsite). This was assigned as desorption of partially solvated aluminum hydroxylated nitrate. Monodentate and bridging nitrate were also observed on the boehmite. These species desorbed at approximately 725 K as NO2 and O2 with an enthalpy of reaction of approximately 55 kJ/mol of NO2 desorbed.

Improving the adhesion of Au thin films onto poly(methyl methacrylate) substrates using spun-cast organic solvents

Polymeric substrates are becoming more widely used in a variety of technologies such as flat panel displays, biosensors, and photovoltaic devices. The advantages of polymeric substrates include improved manufacturability, lower cost of fabrication, lower processing temperatures, and overall thermal budget. A critical processing step is the deposition of metal thin films onto the polymeric substrate to fabricate electrodes and interconnecting wires. These components are essential in sensors, catalysts, photonics, polymer electronics, micro total analysis systems, and microelectrodes. Vapor deposited gold Au thin films are widely used in many of these technologies. The material properties that make Au useful include its corrosion resistance, high infrared reflectivity, and outstanding electrical and thermal conductivities 11% and 34% better than Al, respectively . Unfortunately, Au is a relatively inert metal that has notoriously poor adhesion to polymers. Process engineers have developed extensive methods to deposit Au interconnects and electrodes in silicon-based microelectronics and microelectromechanical systems by using a vapor deposited adhesion layer. Typically, this layer is produced by deposition of a reactive metal such as Cr or Ti, which can form a chemical bond with polar atoms on the surface. The adhesion layer is generally thin 5 nm and is deposited prior to the Au film without breaking vacuum so that the surface of the adhesion film does not oxidize. This produces a thin metal film that is conformal and well bonded to the Si, SiO2, or other inorganic substrates.

Photoelectrochemical performance of W-doped BiVO4 thin films deposited by spray pyrolysis

Abstract. The effects of tungsten doping and hydrogen annealing on the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO4) photoanodes for solar water splitting were studied. Thin films of BiVO4 were deposited on indium tin oxide-coated glass slides by ultrasonic spray pyrolysis of an aqueous solution containing bismuth nitrate and vanadium oxysulfate. Tungsten doping was achieved by adding either silicotungstic acid (STA) or ammonium metatungstate (AMT) to the precursor. The 1.7- to 2.2-μm-thick films exhibited a highly porous microstructure. Undoped films that were reduced at 375°C in 3% H2 exhibited the largest photocurrent densities under 0.1  W cm−2 AM1.5 illumination, where photocurrent densities of up to 1.3  mA cm−2 at 0.5 V with respect to Ag/AgCl were achieved. Films doped with 1% or 5% (atomic percent) tungsten from either STA or AMT exhibited reduced PEC performance and greater sample-to-sample performance variations. Powder x-ray diffraction data indicated that the films continue to crystallize in the monoclinic polymorph at low doping levels but crystallize in the tetragonal scheelite structure at higher doping. It is surmised that the phase and morphology differences promoted by the addition of W during the deposition process reduced the PEC performance as measured by photovoltammetry.

Photoelectrochemical performance of W-doped BiVO4 thin-films deposited by spray pyrolysis

The effect of tungsten doping and hydrogen annealing treatments on the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO4) photoanodes for solar water splitting was studied. Thin films of BiVO4 were deposited on ITO-coated glass slides by ultrasonic spray pyrolysis of an aqueous solution containing bismuth nitrate and vanadium oxysulfate. Tungsten doping was achieved by adding either silicotungstic acid (STA) or ammonium metatungstate (AMT) in the aqueous precursor. The 1.7 μm – 2.2 μm thick films exhibited a highly porous microstructure. Undoped films that were reduced at 375 ºC in 3% H2 exhibited the largest photocurrent densities under 0.1 W cm-2 AM1.5 illumination. This performance enhancement was believed to be due to the formation of oxygen vacancies, which are shallow electron donors, in the films. Films doped with 1% or 5% tungsten from either STA or AMT exhibited reduced photoelectrochemical performance and greater sample-to-sample performance variations. Powder X-ray diffraction data of the undoped films indicated that they were comprised primarily of the monoclinic scheelite phase while unidentified phases were also present. Scanning electron microscopy showed slightly different morphology characteristics for the Wdoped films. It is surmised that the addition of W in the deposition process promoted the morphology differences and the formation of different phases, thus reducing the PEC performance of the photoanode samples. Significant PEC performance variability was also observed among films deposited using the described process.

Thermopile Sensors for the Detection of Airborne Pollutants

We have developed sensors for airborne pollutants using 36-junction thermopiles as the sensing platform. The thermopiles detect heat released when airborne pollutants react with chemical coatings applied to the sensing junctions. Our thermopiles consist of microfabricated thin film bismuth-antimony junctions on thin polyimide or polyethylene terephthalate drumhead membranes that are supported by aluminum substrates. The sensitivity of our thermopiles (> 5 V-s/J) was measured using solvent evaporation and acid-base reactions. A copper oxalate coating on the sensing junctions enables the detection of ammonia vapor. The integrated sensor output is proportional to the logarithm of the ammonia concentration in the air injected into the test chamber. Sub-ppm concentrations of ammonia in air can be detected Detection of acid vapors (e.g., HC1) is also described.