Jacques Robichaud

Controlled Growth of WO 3 Nanostructures with Three Different Morphologies and Their Structural, Optical, and Photodecomposition Studies

Tungsten trioxide (WO3) nanostructures were synthesized by hydrothermal method using sodium tungstate (Na2WO4·2H2O) alone as starting material, and sodium tungstate in presence of ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O] or cobalt chloride (CoCl2·6H2O) as structure-directing agents. Orthorhombic WO3having a rectangular slab-like morphology was obtained when Na2WO4·2H2O was used alone. When ferrous ammonium sulfate and cobalt chloride were added to sodium tungstate, hexagonal WO3nanowire clusters and hexagonal WO3nanorods were obtained, respectively. The crystal structure and orientation of the synthesized products were studied by X-ray diffraction (XRD), micro-Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM), and their chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS). The optical properties of the synthesized products were verified by UV–Vis and photoluminescence studies. A photodegradation study on Procion Red MX 5B was also carried out, showing that the hexagonal WO3nanowire clusters had the highest photodegradation efficiency.

Porous orthorhombic tungsten oxide thin films: synthesis, characterization, and application in electrochromic and photochromic devices

Porous orthorhombic tungsten oxide (o-WO3) thin films, stabilized by nanocrystalline anatase TiO2, are obtained by a sol–gel based two stage dip coating method and subsequent annealing at 600 °C. An Organically Modified Silicate (ORMOSIL) based templating strategy is adopted to achieve porosity. An asymmetric electrochromic device is constructed based on this porous o-WO3 layer. The intercalation/deintercalation of lithium ions into/from the o-WO3 layer of the device as a function of applied coloration/bleaching voltages have been studied. XRD measurements show systematic changes in the lattice parameters associated with structural phase transitions from o-WO3 to tetragonal LixWO3 (t-LixWO3) and a tendency to form cubic LixWO3 (c-LixWO3). These phase transitions, induced by the Li ions, are reversible, and the specific phase obtained depends on the quantity of intercalated/deintercalated Li. Raman spectroscopy data show the formation of t-LixWO3 for an applied potential of 1.0 V and the tendency of the system to transform to c-LixWO3 for higher coloration potentials. Optical measurements show excellent contrasts between colored and bleached states. An alternate photochromic device was constructed by sensitizing the o-WO3 layer with a ruthenium based dye. The nanocrystalline anatase TiO2 in the o-WO3 layer has led to an enhanced photochromic optical transmittance contrast of ∼51% in the near IR region. The combination of the photochromic and electrochromic properties of the synthesised o-WO3 layer stabilized by nanocrystalline anatase TiO2 opens up new vista for its application in energysaving smart windows.

Systematic synthesis and analysis of change in morphology, electronic structure and photoluminescence properties of pyrazine intercalated MoO3 hybrid nanostructures

High aspect ratio molybdenum trioxide (MoO3) nanorods grown along the [100] direction were successively synthesized by a simple hydrothermal method. We used sodium molybdate and hydrochloric acid as starting materials and from their reaction we obtained MoO3 nanorods of high aspect ratio. The dimensions of the nanorods were found to be uniform in size, with well-defined boundaries. The intercalation of an organic molecule (pyrazine) into these nanorods has resulted in single-crystalline MoO3 microstructures, with a change in their length and breadth of a few orders. Pyrazine has acted as a stitching molecule and has bound the nanorods together along their length to form micron sized single crystalline MoO3. The presence of pyrazine and its intercalation was confirmed by a uniform shift in the XRD [0k0] peak positions. As the size of the pyrazine is similar to the van der Waals gap of the orthorhombic MoO3 crystal, it seemed to fit well within the gap and thereby helped to bind the nanorods along the [0k0] direction. The Raman ring deformation modes, at 714 and 996 cm−1, have also supported the intercalation of the pyrazine in the van der Waals gap. The deintercalation process was done by calcinating the sample at 400 °C and the removal of pyrazine was confirmed by TGA and XRD measurements. The influence of pyrazine in the valence band electronic density of states (DOS) of MoO3 was also analyzed by XPS and XES methods. The replacement of oxygen at the van der Waals gap by nitrogen from the intercalating pyrazine caused a shift in the valence band towards the Fermi level. A photoluminescence study was also conducted, reflecting the intercalation effect on the emission characteristics of the MoO3 nanostructures.

Crack-free 2D-inverse opal anatase TiO2 films on rigid and flexible transparent conducting substrates: low temperature large area fabrication and electrochromic properties

Two-dimensional (2D) inverse opal (IO) TiO2 films, synthesized by colloidal crystal templating, such as polystyrene (PS) spheres, are particularly interesting because of their potential applications in sensors, solar cells, and electrochromic devices. For these applications, high crystallinity is essential for device performance. Usually, to achieve the IO structure with high crystallinity, the PS opal template is first removed by calcination at a temperature of ∼400 °C, and subsequently to crystallize amorphous TiO2, a temperature higher than 400 °C is needed. This results in cracks and collapse of the macroporous framework. Furthermore, this route is limited to thermally stable substrates, such as glass, which is a significant drawback as the increasing development of technologies and modern electronics requires the design of inexpensive, lightweight, and efficient optoelectronic devices on flexible substrates. In order to circumvent these problems, we developed a ‘dynamic-hard-template’ infiltration strategy for the fabrication of large-area crack-free nanocrystalline (NC) anatase 2D-TiO2 IO films on rigid transparent conducting substrates and on ITO coated flexible polyethyleneterephthalate (ITO/PET) substrates, by using various sizes of PS spheres. According to this strategy, first a dynamic opal 2D film of PS spheres is self-assembled on the surface of water, followed by the infiltration of preformed anatase TiO2 nanoparticle sol from the bottom into the PS opal crystal as the guest material, thus eliminating the need for high temperature crystallization. The obtained floating PS/TiO2 opal composite film is deposited on ITO-coated glass and ITO/PET substrates. An optimized low temperature chemical method is adopted to remove the PS template to yield NC anatase 2D-TiO2 IO films. The films obtained on ITO/PET substrates were successfully used as an active electrode in the fabrication of a flexible electrochromic device.

Nanocrystalline sol-gel prepared titania films by Raman, FTIR, XRD, and atomic force microscopy

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure and morphology of the heat-treated titania films was examined by Raman and FTIR spectroscopy, x-ray diffraction and atomic force microscopy (AFM). The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have studied the size of the crystallites in TiO2 films and its dependence on the type of complexing agent used.

Photocatalytic properties of nanocrystalline titanium dioxide films in the degradation of domoic acid in aqueous solution: potential for use in molluscan shellfish biotoxin depuration facilities

Domoic acid (DA) is a water-soluble marine neurotoxin produced and released by certain species of the diatom genus Pseudo-nitzschia. Present in coastal waters, it can be a threat to public health and marine life, and can result in severe economic losses to the molluscan shellfish and crustacean harvesting industries. Here we report on the efficiency of nanocrystalline (NC) titania (TiO2) thin films used as a photocatalyst in the ultraviolet light photodegradation of DA. Titanium dioxide thin films produced by a sol-gel dip-coating method in the presence of polyethylene glycol of different molecular weights (200, 400 and 600) were deposited on glass substrates and crystallised at 90°C. The films were characterised using spectroscopic ellipsometry, Fourier transform infrared spectrometer (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The photocatalytic activity measurements were carried out by immersing the NC TiO2 films in a DA solution (2500 ng ml−1) and then exposing them for various times at room temperature to UVA irradiation (λ = approximately 350 nm). The degradation of DA, quantified by HPLC analysis, was not significant when using daylight or ultraviolet light irradiation alone, whereas the NC TiO2 films prepared at low temperature proved to be a very efficient photocatalyst when used in conjunction with UVA light. The effectiveness of the photodegradation was improved by increasing molecular weight of polyethylene glycol, which increased the thickness of the film. The presence, transformation and degradation of three DA isomers were observed. The approach may eventually be practical for destroying DA in seawater used by aquaculture industry depuration facilities.

Implication of Porous TiO2 Nanoparticles in PEDOT:PSS Photovoltaic Devices

Recent developments in the synthesis of mesoporous nanocrystalline titanium oxide (TiO2) have opened up several new opportunities in the construction of efficient photovoltaic (PV) cells. In this chapter, we describe principles involved in the construction of organic photovoltaic cell and the influence of the main parameters for the case of PEDOT:PSS polymers. At the outset different architectures of the PEDOT:PSS photovoltaics are described and later incorporation of the TiO2 nanoparticles in their architectures is carefully considered. A special attention is given to the technique of photovoltaic cell preparation and the basic principles of their functionality. A significant emphasis has been devoted to the current–voltage characteristics of photovoltaic cells, thus constructed with and without nanocrystalline TiO2, as well as to the ways it can be further improved. We also describe the synthesis of mesoporous TiO2 nanoparticles using titanium alkoxides as precursor and polyethylene glycol (PEG) of different molecular mass as templating agent to induce mesoporosity. The interaction of PEG with titanium alkoxides in polar and nonpolar solvents was studied in detail by macro-Raman spectroscopy, solid-state NMR and MALDI-TOF-MS. The removal of PEG as well as the crystallization process was obtained by hot water treatment at low temperature (90 °C). The mesoporosity was retained after further calcination up to 500 °C. Porosity, morphology, and microstructures of the resultant products were characterized by SEM, nitrogen adsorption–desorption measurements, micro-Raman spectroscopy and XRD. The mesostructure of the TiO2 particles facilitates enhanced transport of electrons, resulting in improved photovoltaic efficiency. Influence of TiO2 nanoparticles on the photovoltaic efficiency of the ITO/PEDOT:PSS/fluorine copolymers/polythiophene:TiO2/Al architecture is analyzed. Influence of TiO2 NP on the principal parameters of different PV architecture is discussed. The analysis of the efficiency is performed using both experimental and theoretical quantum chemical approach.

A polystyrene/WO3 opal composite monolayer film as a building block for the fabrication of 3D WO3 inverse opal films

Large area polystyrene (PS)/WO3 opal composite monolayers were successfully fabricated via a modified “dynamic-hard-template” infiltration strategy. These composite monolayers were then used as building blocks for the synthesis of three-dimensional (3D) WO3 inverse opal (IO) films in a bottom-up approach. To achieve this, a PS/WO3 opal composite monolayer serves as a support upon which another PS/WO3 opal composite monolayer is added, and so on, one over the other, rendering possible, after removal of the PS spheres template, the fabrication of large area 3D WO3 IO films. Similarly, bilayer, and trilayer WO3 IOs with large area, having the same pore sizes from layer to layer, can be obtained by repeatedly applying this strategy. This approach also allows control over hierarchical porosity and film thickness by simply changing the diameter of the sacrificial colloidal template and/or the inorganic precursor used in each layer. 3D WO3 IO films fabricated with this technique exhibit good electrochemical reversibility, cycling stability, and increased coloration efficiency (CE) as the number of WO3 IO layers is increased.