Nopphawan Phonthammachai

Particle Size Effect of Silver Nanoparticles Decorated Single Walled Carbon Nanotube Electrode for Supercapacitors

Well dispersed silver nanoparticles AgNPs of different sizes 1–13 nm on single walled carbon nanotubes SWCNTs were synthesized by a facile room-temperature deposition–precipitation process. The morphology and microstructure of samples examined by the transmission electron microscopy showed a monodispersed silver particle decorated SWCNT of 2 wt % as determined by the Rietveld phase analysis of powder X-ray diffraction patterns. The chemical state of silver determined from the binding energies of high resolution Ag 3d peaks from X-ray photoelectron spectroscopy revealed a silver Ag 0 oxidation state. Electrochemical properties were studied using cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance studies. Silver decorated SWCNTs demonstrated to be effective bifunctional charge collectors and active electrode materials for a supercapacitor, exhibiting a higher specific capacitance 106 F g �1

Synthesis of Contiguous Silica−Gold Core−Shell Structures: Critical Parameters and Processes

A direct process for preparing contiguous gold shells (15-25 nm thick) over amorphous silica spheres (200 nm) is described. In this method, gold seeds are synthesized from HAuCl(4) in a dilute NaOH solution using deposition-precipitation with subsequent metallization by sodium borohydride (NaBH(4)). The ease of dispersing gold nanocrystals on spheres of bare silica and spheres after grafting with ammonia was studied as a function of pH (4-8), reaction temperature (65-96 degrees C), and time (5-30 min). Additional parameters requiring optimization included the quantity of NaBH4 and the HAuCl(4) in K(2)CO(3) solution to silica volume ratio. The evolution of gold nanocrystal growth was monitored by transmission electron microscopy, and the bathochromic shift of ultraviolet-visible absorption was correlated with shell perfection and thickness.

Simple route to monodispersed silica-titania core-shell photocatalysts.

A monodispersed silica-titania core-shell photocatalyst was synthesized via a sol-gel route without the need of pH adjustment, cationic polyelectrolytes, or surfactants in a process where silica spheres were impregnated with hydrolyzed titanium tetrabutoxide, incubated at room temperature, and then condensed using an ethanol/water (1:1) solvent. Four coating cycles in a 10% v/v titania sol produced homogeneous titania shells. The quality of catalysts was assessed quantitatively using Rietveld analysis of powder X-ray diffraction patterns combined with X-ray fluorescence spectrometry. During calcination, the anatase-to-rutile transformation was delayed to 1000 degrees C, which is approximately 300 degrees C higher than usually observed. The thermal stability and surface area of titania were enhanced through the slow crystal growth of anatase. The photocatalytic activity of the core-shell photocatalysts calcined at 400-600 degrees C was found to be proportional to the thickness of titania but did not directly correlate with the surface area.

Synthesis of gold nanoshells based on the depositionprecipitation process

The synthesis of gold nanoshells involves the preparation of precursor seed particles consisting of nanoparticulate gold loaded on a dielectric core as scaffolds on which the layer of gold shell can be grown. The common route in preparing these seed particles involves a two-step technique of synthesizing colloidal gold particles followed by attaching them to amine functionalized dielectric core. In this paper, we present the use of a singlestep deposition-precipitation (DP) process, commonly used in the catalytic field, as a feasible alternative route to seeding gold hydroxide nanoparticles onto a silica core and growing a layer of gold shell around the core without the need for prior synthesis of colloidal gold. The various factors such as pH, temperature and time of reaction, as well as the effect of amine functionalization of silica on the deposition of gold were investigated. Compared to bare silica nanoparticles whose low isoelectric point render the DP process of seeding nanoparticulate gold unfavorable, amine functionalization of the silica surface is able to alter its isoelectric point to facilitate the deposition of gold hydroxide nanoparticles and increase the uniformity and density of seeding. We have also established that by varying the pH and time of reaction, it is possible to control the size of the gold hydroxide nanoparticles and density of seeding. The highest seeding density was achieved at pH 8 where the surface charge on the amine terminated silica surface favored the attraction of complex gold anions and the hydrolysis of these complex anions at elevated temperature produced the insoluble gold hydroxide precipitate which was readily deposited on the silica. As the time of reaction was increased from 3 min to 60 min, these gold hydroxide nanoparticles also increased in size from 2 nm to 7 nm. We have also shown the progressive growth of the gold hydroxide seeds to eventually form gold nanoshells with a complete layer of gold shell with tunable optical response.

Synthesis of high performance hydroxyapatite-gold catalysts for CO oxidation

Catalysts for low temperature CO oxidation were prepared by decorating hydroxyapatite (HAp) ceramic foam scaffolds with highly dispersed gold nanocrystals using a deposition-precipitation (DP) process. Catalytic activity, microstructure and crystallinity were studied as a function of reagent pH (4–12) and aging time (10, 30, 60 min) for powders and porous supports. Superior products with small (≤ 5 nm) gold crystals distributed homogeneously over HAp foam were obtained at pH 8–9. Larger crystal sizes and colloidal gold agglomeration appeared at longer aging times. The optimized catalyst prepared by reaction at pH 9 for 30 min showed 100% CO conversion to CO2 at 150°C. The Au-HAp composite demonstrated excellent durability by retaining structural and crystallographic integrity with no loss of activity when tested at 65°C out to 166 h.

Robust Gold-Decorated Silica−Titania Pebbles for Low-Temperature CO Catalytic Oxidation

A deposition-precipitation (DP) process was used to prepare silica-titania core-shell pebbles decorated with nanocrystalline gold suitable for low-temperature catalytic oxidation of carbon monoxide (CO). The microstructure, phase content, crystallography, and catalytic activity were correlated with the pH (3-8), aging time (15, 30, 60 min), and heat treatment employed for gold crystallization (200-400 degrees C). A homogeneous metal distribution, high gold loading (3.7-4.4 wt %), and superior interfacial adhesion between gold and titania were obtained when the support pebbles were prepared at 600 degrees C, a temperature lower than that required for the anatase-to-rutile transformation. Nucleation and growth of {111} faceted gold was favored at mid-pH (6.4-8), while smaller crystals (<7.5 nm) were obtained at short aging times (<or=60 min) and low growth temperatures (<or=300 degrees C). Catalytic activity was optimized by homogeneously dispersing gold nanocrystals (3 nm) using pH 6.4 and an aging time of 30 min. These robust materials may offer superior activity and lifetimes when deployed in fluidized bed catalytic cracking units.

X-ray absorption spectroscopy studies of phase transformations and amorphicity in nanotitania powder and silica–titania core–shell photocatalysts

The local environment of titanium in nanocrystalline sol-gel synthesized titania, cobaltiferous titania and silica–titania core–shell photocatalysts was investigated using X-ray absorption spectroscopy (XAS). Anatase reconstructively transforms to rutile via a persistent amorphous phase that is retained, in part, up to 1273 K. In nanotitania, temperature-dependent trends in Ti order correlation observed by XAS parallel the development of amorphous content extracted from powder X-ray diffraction patterns, such that amorphicity shows a transient maximum at ∼873 K with the onset of rutile crystallization. Cobaltiferous and core–shell materials behaved similarly, but with anatase retained to 973 and 1273 K, respectively. In the former, cobalt redox reactions may stabilize anatase to higher temperatures by ready charge-balancing during the loss of hydroxyl and the formation of oxygen vacancies. In the core–shell architecture, higher Ti coordination and interatomic distance variance in the first- and second-nearest-neighbour shells are maintained to 1273 K by interaction of a substantially aperiodic TiO6 network with the glassy silica substrate, which inhibits crystallization of rutile from the amorphous intermediate. Comparisons are also drawn with the commercial P25 catalyst. The overall transformation mechanism can be summarized as gel → non-stoichiometric anatase → amorphous titania → rutile. Smaller anatase crystals and a higher average Ti—Ti coordination environment in the core–shell structure may enhance photocatalytic activity directly, by creating larger specific surface areas and hosting reactive defects, or indirectly, by inhibiting exciton annihilation in aperiodic titania and delaying the crystallization of less photoactive rutile.