Mesostructured Thin Films Research Articles

Boron Nitride-Titania Mesoporous Film Heterostructures.

The fabrication of optically active heterostructures in the shape of mesostructured thin films is a highly challenging task. It requires an integrated process to allow in one-step incorporating the two-dimensional materials within the mesoporous ordered host without disrupting the pore organization. Hexagonal boron nitride (BN) nanosheets have been successfully introduced into titania mesoporous films using a template-assisted sol–gel synthesis and evaporation-induced self-assembly. Two types of BN sheets have been used, with and without defects, to investigate the role of defects in heterostructure properties. It has been found that the defects increase the ultraviolet radiation A (UVA) absorbance and enhance the photocatalytic response of the film. The BN sheets are optically transparent and do not exhibit any photocatalytic property but contribute to anatase crystallization via heterogeneous nucleation.

Photoluminescence of zinc oxide mesostructured films doped with Rhodamine 6G

The photoluminescence of thin films doped with fluorescent dyes, such as rhodamine 6G (Rh6G), is strongly dependent on their concentration and aggregation states. We have doped ZnO dense and mesostructured thin films with different concentrations of Rh6G. The films have been prepared via sol-gel using a block copolymer as template to obtain the mesostructured samples. Rh6G at high concentrations aggregates preferentially as not-fluorescent H-dimers in the dense samples and fluorescent J-dimers in the mesostructured films. The dense ZnO coatings show a shift of the typical excitation because of the formation of H-dimers. On the contrary, the mesostructured films exhibit an intense emission, typical of the Rh6G monomer, with a small contribution of the J-dimers. The optical response of the films has been characterized by UV–vis and steady state fluorescence spectroscopy. Controlling the aggregation states of the dyes within a fluorescent matrix has shown to be an important strategy for tuning the emission.

Synthesis of mesostructured thin films by ionic wind assisted electrostatic spray deposition

Thin films produced by electrostatic spray deposition (ESD) have nanometer-sized structures despite the initial sprayed droplets being typically a few tens of microns in diameter. The size and morphology of the resulting structures is not only affected by the solvent properties and drying kinetics, but also by Coulomb fission owing to the high surface charge density that the droplets build up upon evaporation. In this work we modulate the charge density of the droplets by inducing ionic wind along the spray, and produce mesoscopic structures. Using WO3 as an example, we show that the technique provides a practical way to control the morphology of thin films produced by ESD.

Mesostructured HSO3-functionalized TiO2-P2O5 sol-gel films prepared by evaporation induced self-assembly method with high proton conductivity

Ordered mesoporous coatings, especially those structured as channels in three dimensions, could be applied as thin proton conducting membranes for application in proton exchange membrane fuel cells (PEMFC). This kind of materials shows high proton conductivity due to their favourable architecture with small and interconnected porosity. Transparent and crack-free TiO2-P2O5 sol-gel coatings with thickness around 200nm were obtained by Evaporation induced self-assembly (EISA) method using Pluronic-127 as pore generating agent. X- Ray diffraction, SAXS and TEM analysis confirm the presence of ordered porous mesostructured films with a cubic Fd3m structures. Spectral ellipsometry and EEP measurements were used to determine the film thickness, refractive index, pore size, pore volume and specific surface area. EIS four-probe method was used to measure the proton conductivity. The textural properties (pore size in the range of mesoporous, high specific surface area and high pore volume) combined with inherent characteristics of sol-gel materials (high concentration of OH groups and release of protons mainly from Ti-OH and P-OH bonds) lead to a high proton conductivity, with maximum values around 0.11 and 0.20S/cm depending on the phosphorous precursors (PCl3 or H3PO4), measured at 140°C and 80%RH. Acid functionalisation of mesoporous TiO2-P2O5 thin-films, with HSO3 groups, increases the proton conductivity up to 0.79S/cm at 140°C and 80%RH presenting high long-term stability. This work shows a processing guideline to design functionalized-inorganic mesostructured thin-films with high proton conductivity at high temperature.

Getting order in mesostructured thin films, from pore organization to crystalline walls, the case of 3-glycidoxypropyltrimethoxysilane.

A new type of mesostructured hybrid organic-inorganic film has been synthesised by evaporation-induced self-assembly using 3-glycidoxypropyltrimethoxysilane as the precursor and a tri-block copolymer, Pluronic F127, as the template. The chemistry has been tuned to form bridged polysilsesquioxanes that self-organise into ordered lamellar structures. Controlled aging under highly basic conditions, which has been monitored by Raman and infrared spectroscopy, has been used to obtain the layered ordered hybrid structures in the precursor sol. The pH of the sol has been adjusted to form the micelles that act as templates during solvent evaporation. The self-assembly of the system has been studied in situ by small and wide angle X-ray scattering using a synchrotron light source, which has confirmed both the formation of hybrid layered structures and the long-range organization of the mesophase in the hybrid films. The present approach allows ordering the hybrid film on two different length scales; at the end of film processing, hybrid crystals are incorporated into the pore walls and the micelles are arranged within the films with long range order.

Nematic ordering of SWNT in meso-structured thin liquid films of polystyrenesulfonate.

The formation of nematic-like islands of single-walled carbon nanotubes (SWNT) in polystyrenesulfonate (PSS) dispersions confined into nanometrically thin films is reported. The SWNT are observed to assemble into orientationally ordered phases, where the intertube distance, as measured via transmission electron microscopy at cryogenic temperatures, matches the polyelectrolyte's bulk correlation length deduced from X-ray scattering. The micrometers-long islands of orientationally ordered carbon nanotubes are observed in both SWNT and double-walled carbon nanotubes (DWNT) but not in specimens prepared from similar dispersions of multiwalled carbon nanotubes (MWNT). These observations, together with relaxation and rheological experiments, suggest that the orientational ordering may result from coupling between confinement of the polymer-wrapped SWNT and DWNT and the microstructure of the solvated polyelectrolyte.

Using Three-Dimensional 3D Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) Analysis To Probe Pore Deformation in Mesoporous Silica Films

In the past decade, remarkable progress has been made in studying nanoscale objects deposited on surfaces by grazing-incidence small-angle X-ray scattering (GISAXS). However, unravelling the structural properties of mesostructured thin films containing highly organized internal three-dimensional (3D) structures remains a challenging issue, because of the lack of efficient algorithms that allow prediction of the GISAXS intensity patterns. Previous attempts to calculate intensities have mostly been limited to cases of two-dimensional (2D) assemblies of nanoparticles at surfaces, or have been adapted to specific 3D cases. Here, we demonstrate that highly organized 3D mesoscale structures (for example, porous networks) can be modeled by the combined use of established crystallography formalism and the Distorted Wave Born Approximation (DWBA). Taking advantage of the near-zero intensity of symmetry-allowed Bragg reflections, the casual extinction or existence of certain reflections related to the anisotropy of the form factor of the pores can be used as a highly sensitive method to extract structural information. We employ this generic method to probe the slightly compressed anisotropic shape and orientation of pores in a mesoporous silica thin film having P63/mmc symmetry.

Formation of mesostructured thin films at the air–liquid interface

The growth of free-standing surfactant-templated films of inorganic oxides at the air-solution interface is an attractive route to manufacture unsupported mesostructured membranes for a range of potential applications. So far this synthesis method has been relatively neglected due to the fragility of the initial films. More recent work to understand the mechanism of formation has led to development of thicker, more robust films, as well as providing new information on the general formation mechanisms of mesoporous materials whether in film or particulate form. This review describes the properties of silica and other inorganic oxide films templated by surfactants and grown at the air-solution interface, their formation mechanisms and implications for further development of these materials.

A Facile Approach for Controlling the Orientation of One-Dimensional Mesochannels in Mesoporous Titania Films

Controlling of the orientation of mesochannels in mesostructured thin films is important for the development of novel molecular devices and, in particular, generating vertically aligned mesochannels with respect to the substrate plane is extremely challenging for nonsiliceous materials. We describe a facile and highly effective air flow method, which is able to control the unidirectional alignment of titania mesochannels in a desired direction (e.g., parallel, perpendicular, or oblique) on a large scale, via manipulation of the air flow rate and incident angle. The titania mesochannels were characterized by TEM, SEM, SAXRD, and GISAXS. The unidirectional, vertically aligned mesostructured titania films were found to exhibit excellent ion conductivity.

Mesostructured thin films as electrocatalysts with tunable composition and surface morphology

Among the most challenging issues in technologies for electrochemical energy conversion are the insufficient activity of the catalysts for the oxygen reduction reaction, catalyst degradation and carbon-support corrosion. In an effort to address these barriers, we aimed towards carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored physical properties. We present here a new class of metallic materials with tunable near-surface composition, morphology and structure that have led to greatly improved affinity for the electrochemical reduction of oxygen. The level of activity for the oxygen reduction reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk polycrystalline Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.

New designs of hydrophobic and mesostructured Ultra Low k materials with isolated mesopores

ABSTRACTIn this work, hydrophobic mesostructured organosilica thin films, exhibiting isolated mesopores (~ 7 nm), have been successfully deposited by spin-coating using different polystyrene-block-polyethylene oxide copolymers (PS-b-PEO) as structure-directing agents and methyltriethoxysilane (MTES) as organosilica precursor. Different ordered mesostructures (Face Centered Cubic, 2D or 3D Hexagonal and Body Centered Cubic) can be achieved by controlling different synthesis parameters. X-Ray Diffraction (XRD) and Grazing Incidence Small Angle X-Ray Scattering (GISAXS) techniques were used to investigate the mesostructure evolution through thermal and UV treatments. Swelling and shrinkage were evidenced by in-situ XRD and X-Ray Reflectivity measurements during the thermal removal of the meso-templates. Infrared spectroscopy and 29Si NMR were additionally used to investigate the microstructure evolution. The film porosity was estimated thanks to Ellipsometry Porosimetry (EP). Correlation between mechanical properties through nanoindentation measurements and the mesostructure ordering is discussed as well as assessments of the dielectric constant k by mercury contact probe.

Multiple thin film formation from dilute mixtures of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB)

Dilute mixtures of the water soluble polymer polyethyleneimine (PEI) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) form mesostructured thin films at the air/solution interface. In this paper we show that these films form spontaneously, reaching an equilibrium composition. When the film is removed, a new solid film rapidly reforms, with a similar morphology when inspected by Brewster angle microscopy (BAM). Successive removal of films lead to a series of structurally similar films, until the amount of surfactant in solution approaches the lower limit of film forming concentration. The results obtained using surfactant-selective potentiometry suggest that the incipient polymer:surfactant aggregates are in a position to migrate to the surface rapidly after film removal, prior to mesostructure formation. X-ray reflectivity indicates that films formed at different PEI:CTAB compositions retain mesophase regular structures with the same d-spacing feature, equal to 52.2 Å. Grazing incidence diffraction measurements indicate that films are composed of small crystallites arranged in a random powder fashion, developing a rough surface morphology evidenced by BAM. The central finding is that PEI:CTAB films form when the amount of CTAB in solution is higher than a critical film formation concentration, very close to the critical aggregation concentration (CAC), allowing the formation of several equilibrated films from the same growing solution.

Hybrid functional mesostructured thin films with photo-oxidative properties in the visible range

Hybrid mesostructured thin films functionalised with organic photosensitiser molecules demonstrated high efficiency for the decontamination of polluted atmosphere via singlet oxygen production.

Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-1 films

Mesoporous silica films and MFI-type pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium ( o-Ps) into vacuum. Positronium time-of-flight (TOF) spectroscopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of o-Ps emitted from the films. Evidence for an efficient o-Ps emission was found in both the mesoporous and silicalite-1. A 3γ fraction in the range of 31–36 % was found in the films with the highest o-Ps yield in each type of porous material, indicating that 40–50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with 2–3 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous films of 300–400 nm thickness.

Nanostructured Silica Thin Films Self-Assembled with Electron Donors and Acceptors to Measure Electron Tunneling

Surfactant-templated, mesostructured thin films are synthesized such that photoelectron donors and electron acceptors are separated spatially in the different regions of the thin film. A photoelectron donor is placed within the silica framework by using a silylated derivative of the well-known tris(bipyridine)ruthenium(II) cation. Selective placement of the electron acceptor is achieved by using a surfactant derivative of methyl viologen. Luminescence decay traces and luminescence spectra are collected for the electron donor in the presence of varying amounts of the electron acceptor. Because of the spatial separation of the donor and acceptor noncontact electron transfer occurs and the electron-transfer rate decreases exponentially with the distance separating the donor and acceptor. Luminescence decay traces are calculated and fit to the experimental data in order to extract a value for the contact quenching rate, k0 (s(-1)), as well as the exponential decay constant beta (A(-1)) which governs how fast the electron-transfer rate decreases as a function of the donor-acceptor distance. The value beta = 2.5 +/- 0.4 A(-1) shows that the mesostructured material is an excellent insulator, better than frozen organic glasses or proteins and approaching that of vacuum. Combining deliberate placement methods, spectroscopy, and calculations has made possible the first measurement of beta for the silica region of mesoporous thin films.

Mesostructured Silica Supports for Functional Materials and Molecular Machines

AbstractMesostructured silica thin films and particles prepared by surfactant‐templated sol–gel techniques are highly versatile substrates for the formation of functional materials. The ability to deliberately place molecules possessing desired activities in specific spatially separated regions of the nanostructure is an important feature of these materials. Such placement utilizes strategies that exploit the physical and chemical differences between the silica framework and the templated pores. As an example of placement of pairs of molecules, donor and acceptor molecules can be targeted to different regions of mesostructured thin films and energy transfer between them can be measured. The results not only demonstrate the spatial separation but also are used as a molecular ruler to measure the average distance between them. Mesostructured silica is also an excellent support for molecular machines. Molecules that undergo large amplitude motion, when attached to the silica, can function as impellers and nanovalves when activated by light, electrical (redox) and chemical (pH, competitive binding) energy. Derivatized azobenzene molecules, attached to pore walls by using one of the placement strategies, function as impellers that can move other molecules through the pores. Rotaxanes and pseudorotaxanes, placed at pore entrances, function as gatekeepers that can trap and release molecules from the pores when stimulated. Deliberately placed functional molecules on and in mesostructured silica offer many possibilities for both fundamental studies on the nanoscale and for applications in fields as diverse as fluidics, biological drug delivery and controlled release.

Niobia-stabilised anatase TiO 2 highly porous mesostructured thin films

The thermal stability of anatase TiO 2 mesoporous thin films has been considerably increased up to 800 °C by substituting a fraction of Ti cations by Nb. Mesoporous mixed oxide films Ti 1− x Nb x O 2, with x = Nb/(Nb + Ti) ranging from 0 to 1, and homogeneous crystalline walls have been successfully prepared from chloride precursors and non-ionic block copolymer templates. They all exhibit a uniaxially distorted Im3m bcc mesostructure with the a lattice parameter ranging from 14 to 19 nm. Films have been characterised by simultaneous in situ SAXS and WAXS investigations during thermal treatment. Transmission electron microscopy and UV–visible ellipsometry were used to confirm the structure, the porosity and the good optical quality. A high porosity of 40% (pore size estimated at 7 nm) was maintained for the 10% Nb-doped films above the temperature of crystallisation and a bandgap of 3.40 eV has been measured after annealing at 500 °C.

Formation of mesoporous silica thin films on oxide substrates by casting

The preparation of mesoporous silica thin films with bi-dimensional hexagonal pores whose channel axis oriented parallel to the plane of the film on oxide substrates has been studied using casting, X-ray diffraction (XRD), scanning electron spectroscopy (SEM), N 2 adsorption–desorption and thermogravimetry (TG). XRD and SEM observations show that the surfactant–silica mesostructured thin films are synthesized on the substrates using a cetyltrimethylammonium bromide (CTMABr)–tetraethyl orthosilicate (TEOS) gel under acidic conditions at ambient temperature. XRD and N 2 adsorption–desorption studies reveal that the powdered mesoporous silica composite remains structurally stable and ordered after template removal and that the supported thin films are plausibly stable after template removal but less stable and ordered than the powdered composite. TG analysis indicates that the template supercritical fluid extraction (SFE) efficiency reaches 87%. The influences of the surfactant-substrate and film-substrate interactions on the formation and structural stability of the thin films are discussed.

Formation of mesostructured thin films at the air/water interface

Mesostructured thin films are known to form at the interface between either air or a solid surface and a dilute acidic solution containing the silica precursor, and surfactant template. These films are transparent, and, when grown at the air/solution interface, self-supporting. We have been studying the formation processes of these films to try to understand and thus control the formation of both the mesoscale and the macroscale structure. The films are characterised using a variety of in situ methods including X-ray reflectivity techniques and Brewster angle microscopy, to study both the large and small scale structure development processes. As a result of our investigations we have suggested a coacervation driven mechanism for film growth and application of this understanding now drives our further development of these films. Here we present results concerning the effect of surfactant headgroup type, structure and counterions on the development of well-organised mesostructures, and also on the macroscopic film characteristics.

Mesostructured thin films deposited by PECVD from TMGe

The plasma enhanced CVD (PECVD) is proposed as an effective tool for preparation of the mesostructured thin films. The technique is compatible with silicon technology, which makes it interesting from potential applications point of view. Thin layers deposited by this method, using tetramethylgermane (TMGe) as a source compound, can be deposited in forms characterized by various structures in nanometer-scale range. Depending on the process parameters both homogenous distribution of carbon and germanium atoms in the material as well as specific nanostructure with germanium and carbon rich regions can be fabricated. The structure differences have an obvious impact on the electronic properties of the material and result in two kinds of electronic structure—namely amorphous insulator (a-I) and amorphous semiconductor (a-S), respectively. The two-level structure has been observed in the a-S films. The first level consists of germanium grains (few nanometers in size) embedded in the carbon matrix. The grains are no randomly distributed in the material but agglomerate forming domains of about 60 nm in size defining the second level structure. The paper concentrates on the deposition technology which leads to the a-S films and presents results of investigations implying the existence of the mesostructure in this material. Among applied experimental techniques the most interesting results have been achieved by Raman spectroscopy, impedance spectroscopy and charge sensitive scanning probe microscopy (SPM).