Rachel A. Caruso

Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 Beads Achieve Power Conversion Efficiencies Over 10%

Dye-sensitized solar cells employing mesoporous TiO(2) beads have demonstrated longer electron diffusion lengths and extended electron lifetimes over Degussa P25 titania electrodes due to the well interconnected, densely packed nanocrystalline TiO(2) particles inside the beads. Careful selection of the dye to match the dye photon absorption characteristics with the light scattering properties of the beads have improved the light harvesting and conversion efficiency of the bead electrode in the dye-sensitized solar cell. This has resulted in a solar to electric power conversion efficiency (PCE) of greater than 10% (10.6% for Ru(II)-based dye C101 and 10.7% using C106) for the first time using a single screen-printed titania layer cell construction (that is, without an additional scattering layer).

Synthesis of monodisperse mesoporous titania beads with controllable diameter, high surface areas, and variable pore diameters (14-23 nm).

Monodisperse mesoporous anatase titania beads with high surface areas and tunable pore size and grain diameter have been prepared through a combined sol-gel and solvothermal process in the presence of hexadecylamine (HDA) as a structure-directing agent. The monodispersity of the resultant titania beads, along with the spherical shape, can be controlled by varying the amount of structure-directing agent involved in the sol-gel process. The diameter of the titania beads is tunable from approximately 320 to 1150 nm by altering the hydrolysis and condensation rates of the titanium alkoxide. The crystallite size, specific surface area (from 89 to 120 m(2)/g), and pore size distribution (from 14 to 23 nm) of the resultant materials can be varied through a mild solvothermal treatment in the presence of varied amounts of ammonia. On the basis of the results of small-angle XRD, high-resolution SEM/TEM, and gas sorption characterization, a mechanism for the formation of the monodisperse precursor beads has been proposed to illustrate the role of HDA in determining the morphology and monodispersity during the sol-gel synthesis. The approach presented in this study demonstrates that simultaneous control of the physical properties, including specific surface area, mesoporosity, crystallinity, morphology, and monodispersity, of the titania materials can be achieved by a facile sol-gel synthesis and solvothermal process.

Hollow titania spheres from layered precursor deposition on sacrificial colloidal core particles.

The preparation of monodisperse hollow titania spheres with defined diameter, wall thickness and crystal phase is reported. The hollow spheres have been produced by the layered deposition of a water-soluble titania precursor, e.g. titanium(IV) bis (ammonium lactato) dihydroxide (TALH), in alternation with poly(diallydimethylammonium chloride) (PDADMAC) onto submicrometer-sized template particles e.g. polystyrene (PS) particles, followed by calcination at elevated temperatures. the layer-by-layer growth of the coating on the colloid particles was observed by microelectrophoresis and transmission electron microscopy (TEM). Calcination of the TALH/PDADMAC-coated particles resulted in intact, hollow titania spheres, as confirmed by scanning electron microscopy (SEM) and TEM. Calcining the coated particles at 450 or 950 DEG C resulted in hollow sphere consisting of titania in anatase or rutile form, respectively. Nanometer-level control over the sphere wall thickness was achieved by varying the number of layers deposited on the PS particles. The hollow titania spheres produced can be used in photonic applications, where hollow spheres of high refractive index materials are desired, and in catalysis.

Enhancing photocatalytic activity of titania materials by using porous structures and the addition of gold nanoparticles

Titanium dioxide is a photocatalyst that has attracted considerable attention for tackling pollution in liquid or gaseous environments. Titania has the benefits of high stability and low toxicity, it is abundant and therefore is relatively cheap. However, intrinsic issues in the material, in particular the recombination between the photon induced electron and hole pair, the wide band gap (∼3.2 eV), and the associated issues of nanoparticle separation (generally nanoparticle samples are required to achieve high surface areas) have hampered the full potential of this photocatalytic (PC) material. Here, recent progress in producing porous titania materials, the addition of gold nanoparticles (Au NPs) to the titania and the coupling of these two approaches to improve the PC properties are reviewed. Incorporating porosity within the titania material affords large surface areas without associated nanoparticulate separation issues, and increased accessibility for the organic pollutant to the active sites on the titania, thereby enhancing PC activity. Au NPs act as electron sinks to enhance the charge separation between the e−/h+ produced on photon absorption, hence improving the quantum yield of superoxide radicals, resulting in improved PC activity. Further enhancement can be achieved by coupling the porous structure of the TiO2 and the addition of Au NPs.

Surface-Metastable Phase-Initiated Seeding and Ostwald Ripening: A Facile Fluorine-Free Process towards Spherical Fluffy Core/Shell, Yolk/Shell, and Hollow Anatase Nanostructures†

Versatile synthetic method: Monodisperse anatase microspheres with various complex morphologies have been synthesized by using a versatile fluorine-free solvothermal process in the presence of ammonia. Unambiguous evidence related to surface seeding and a subsequent hollowing process revealed an Ostwald ripening evolution process.

Modification of mesoporous TiO2 electrodes by surface treatment with titanium(IV), indium(III) and zirconium(IV) oxide precursors: preparation, characterization and photovoltaic performance in dye-sensitized nanocrystalline solar cells

Post-treatment of titanium dioxide (TiO2) films for use in dye-sensitized solar cells has been carried out with titanium(IV), indium(III) and zirconium(IV) oxide precursor solutions. The nanostructured electrodes were characterized using nitrogen gas sorption (NGS), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), energy dispersive x-ray spectroscopy (EDX), field emission scanning electron microscopy (FEGSEM) and high resolution transmission electron microscopy (HRTEM). The change in the nanostructure was quantified and the thicknesses of the core?shell coatings determined. An evaluation of the dependence of thickness by HRTEM concluded that one coating step of either the indium or zirconium precursor gave thicknesses of 0.5?nm, with EDX and XPS confirming the presence of either In or Zr at the TiO2 electrode surface, respectively. These working electrodes were then used to fabricate dye-sensitized nanocrystalline solar cells (DSSCs) whose performance was tested under AM1.5G 100?mW?cm?2 illumination. TiCl4 post-treatment was found to improve the photovoltaic efficiencies from 3.6% to 5.3%. Single coatings of either In2O3 or ZrO2 on the TiO2 working electrode resulted in an increased efficiency from 3.6% up to 5.0%. Thinner coatings gave the highest solar cell efficiency. The drop in performance was mainly due to a decrease in short circuit current density (Jsc) with the greater shell thicknesses. ZrO2-coated TiO2 electrodes subjected to microwave heat treatment using a 2.45?GHz microwave produced the highest efficiencies (5.6%) largely due to an increase in short circuit current from 11.4 to 13.3?mA?cm?2.

Silica films with bimodal pore structure prepared by using membranes as templates and amphiphiles as porogens

Extended porous silica films with thicknesses in the range of 60 to 130 μm and pores on both the meso- and macroscale have been prepared by simultaneously using porous membrane templates and amphiphilic supramolecular aggregates as porogens. The macropore size is determined by the cellulose acetate or polyamide membrane structure and the mesopores by the chosen ethylene-oxide-based molecular self-assembly (block copolymer or non-ionic surfactants). Both the template and the porogen are removed during an annealing step leaving the amorphous silica material with a porous structure that results from sol–gel chemistry occurring in the aqueous domains of the amphiphilic liquid-crystalline phases and casting of the initial template membrane. The surface area and total pore volume of the inorganic films vary from 473 to 856 m2 g–1, and 0.50 to 0.73 cm3 g–1, respectively, depending on the choice of template and porogen. The combined benefits of both macro- and mesopores can potentially be obtained in one film. Such materials are envisaged to have applications in areas of large molecule (biomolecule) separation and catalysis. Enhanced gas and liquid flow rates through such membranes, due to the presence of the larger pores, also makes them attractive as supports for other catalytic materials.

Increased nanopore filling: Effect on monolithic all-solid-state dye-sensitized solar cells

A vacuum pore-filling method is described to fabricate solid-state dye-sensitized solar cells (DSSCs) based on nanocomposite polymer electrolytes. Scanning electron microscopy and energy dispersive x-ray spectrometry analyses suggest that incomplete pore filling of the nanostructured TiO2 electrodes is a major efficiency-limiting factor in the fabrication process of solid-state DSSC. Application of the vacuum method resulted in DSSC with much improved energy conversion efficiencies when compared to devices made via conventional drop-casting technique. The energy conversion efficiency gain increased steadily with increasing TiO2 film thicknesses. It is expected that the vacuum pore-filling technique will be applicable to a wide range of hole-transport materials.

High performance LiFePO4 electrode materials: influence of colloidal particle morphology and porosity on lithium-ion battery power capability

Porous colloidal particles of LiFePO4 have been prepared using water based synthesis methods in the presence of tri-block copolymer amphiphiles. A systematic investigation into the synthesis parameters revealed the importance of porosity, particle size, crystallinity and carbon content on the electrochemical properties. Mesopore formation and particle connectivity were critical for efficient electrolyte access for high power LiFePO4 electrode materials. Samples performed well at high rates with discharge capacities of 124 mA h g−1 at 5 C and 113 mA h g−1 at 10 C achieved. Discharge capacities of 164 mA h g−1 were obtained at 0.1 C rates which are close to the theoretical capacity for LiFePO4 of 170 mA h g−1.

Sonochemical formation of colloidal platinum

Abstract The ultrasound irradiation of aqueous solutions of PtCl2−6 was found to produce colloidal platinum of about 3 nm in diameter. The presence of aliphatic alcohols significantly enhanced the reduction process. It is shown that the extent of reduction of PtCl2−6 in the presence of alcohol is directly related to the Gibbs surface excess of the alcohol at the air-water interface. The significance of this is discussed in relation to ultrasound induced cavitation in solution.