Michael Wark

Anchoring of Functional Dye Molecules in MCM-41 by Microwave-Assisted Hydrothermal Cocondensation.

Covalent anchoring of functional dyes in the pores of a mesoporous silicate host Si-MCM-41 (shown in the picture) is achieved with the microwave-assisted cocondensation presented here. The short reaction time (20 min) ensures that no dye degrades during the hydrothermal synthesis.

Formation of CdS nanoparticles within functionalized siliceous MCM-41

Cadmium sulfide nanoparticles were prepared within the siliceous MCM-41 mesoporous molecular sieve, whose surface was functionalized with 3-mercaptopropyltrimethoxysilane at different temperatures. Their size, estimated from diffuse reflectance UV/VIS spectra, decreases with increasing number of introduced thiol groups acting as anchors.

Nanoparticles of Mesoporous SO3H-Functionalized Si-MCM-41 with Superior Proton Conductivity

Nanometer-sized mesoporous silica particles of around 100-nm diameter functionalized with a large amount of sulfonic acid groups are prepared using a simple and fast in situ co-condensation procedure. A highly ordered hexagonal pore structure is established by applying a pre-hydrolysis step in a high-dilution synthesis approach, followed by adding the functionalization agent to the reaction mixture. The high-dilution approach is advantageous for the in situ functionalization since no secondary reagents for an effective particle and framework formation are needed. Structural data are determined via electron microscopy, nitrogen adsorption, and X-ray diffraction, proton conductivity values of the functionalized samples are measured via impedance spectroscopy. The obtained mesoporous SO(3)H-MCM-41 nanoparticles demonstrate superior proton conductivity than their equally loaded micrometer-sized counterparts, up to 5 x 10(-2) S cm(-1). The mesoporosity of the particles turns out to be very important for effective proton transport since non-porous silica nanoparticles exhibit worse efficient proton transport, and the obtained particle size dependence might open up a new route in rational design of highly proton conductive materials.

Photochromism of spiropyran in molecular sieve voids: effects of host–guest interaction on isomer status, switching stability and reversibility

Spiropyran or its preferred configurational isomers, respectively, are incorporated in faujasite (NaY, HY and DAY) cages by in situ synthesis, and in Si-MCM-41 channels by wetness impregnation. Luminescence spectra of the colored isomers indicate the non-aggregated incorporation of merocyanine forms. High quantum yields for the photochromism (∼75%) demonstrate the ready access of the chromophores for the photons of the laser pulses, used for the photoinduced switching between the different configurational isomers. The strong retardation of the thermal relaxation rate from the photoproduct cis-merocyanine to the thermodynamically preferred trans-merocyanine in the faujasite hosts (HY, DAY) in comparison to spiropyrans in SiO2 or Al2O3 is attributed to an increase of the rotation barriers by the imposed spatial restrictions. Among the different faujasites the dealuminated Y-zeolite (DAY) exhibits the lowest relaxation rates of the photoproduct and the highest switching reversibilities between the trans- and the cis-merocyanine.

Platinum-filled oxidic nanotubes

SiO2 and TiO2 nanotubes filled with platinum nanoparticles were synthesized in a sol–gel process. The formation of the oxides proceeds around a precipitated platinum salt acting as template, i.e., the single-crystal fibers of the salt direct the structure and size of the nanotubes. Wire-like arrangements of encapsulated platinum clusters are formed from the template during calcination.

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity

Abstract We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20–25 and 35–45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.

Particle size and photoabsorption of NaX encapsulated CdS and PbS

Abstract Preparation of CdS and PbS from metal ion-exchanged zeolites by reaction with H 2 S results in the formation of sulfide particles with sizes up to 10 nm. Distinct sulfide dispersions with narrow particle size distributions can be obtained and identified by photoabsorption spectroscopy and electron microscopy. The decrease of the rate of photocorrosion with decreasing size of a sacrificial donor molecule indicates the state of the sulfide phase encapsulated in the zeolite.

Low-Temperature Preparation of Crystalline Nanoporous TiO2 Films by Surfactant-Assisted Anodic Electrodeposition

Highly porous, fully crystalline TiO 2 films were prepared without high-temperature treatment by cathodic electrodeposition using the surfactant sodium dodecyl sulfate as a structure-directing agent. X-ray diffraction and transmission electron microscopy show the almost exclusive presence of rutile in the films. Calcination in air leads to substantial texture changes, namely grain coarsening, the removal of micropores, and the creation of a large amount of mesopores, leading to higher efficiency of the calcined films in dye-sensitized solar cells compared to as-deposited films.

New SnO2 Nano-Clusters Obtained by Sol-Gel Route, Structural Characterization and Their Gas Sensing Applications

SnO2 is a well known and widely studied sensor material for the detection of CO and flammable gases like H2. Here we discuss the use of porous networks of SnO2 nanoparticles for an optical detection of the reducing gas CO. Nano-sized SnO2 clusters were prepared by the sol-gel method using an organically modified Sn precursor. After thermal treatment at 550°C the mean diameters of the primary SnO2 nanoparticles constructing the network were estimated to ∼25 nm and ∼15 nm, respectively, for particles obtained in acid and basic catalysis. The reversible redox behavior of SnO2 nano-clusters in reducing and oxidizing atmospheres (CO, O2) was studied optically by in-situ DR-UV/VIS spectroscopy.

Optical gas sensing by semiconductor nanoparticles or organic dye molecules hosted in the pores of mesoporous siliceous MCM-41

The presence of different gas atmospheres is optically detected by use of molecular sieves modified with metal oxide clusters or dye molecules. The reversible redox behaviour of tin dioxide clusters, embedded in the regular pores of mesoporous siliceous MCM-41 in reducing and oxidizing atmospheres (CO, H2, NH3, O2) is studied by in-situ diffuse reflectance (DR) UV/Vis spectroscopy. By impregnation with Sn precursors a carpet of SnO2 clusters is formed on the inner pore walls of the MCM-41 due to strong interactions of the tin oxide species with the silanol groups of the MCM-41 matrix. The response time of hosted two-dimensional SnO2 layers for the registration of CO by optical detection is short and concentrations down to 20 ppm CO in air and 50 ppm of H2 or NH3 in air or Ar, respectively, can be monitored. Hydrocarbons adsorbed in the pores of siliceous MCM-41 alter, depending on their chain length, the optical scattering of the mesoporous matrix. Since the gas adsorption depends on the partial pressure of the hydrocarbons this effect can be used to detect hydrocarbons and to monitor their concentration with the help of embedded light absorbing guests. This sensing, however, is only possible with powdered samples and by measuring in diffuse reflectance according to the Kubelka–Munk formalism. The concentration of SO2 in a gas atmosphere can be deduced from the quenching of the fluorescence of rhodamine dye molecules anchored in the pores of siliceous MCM-41.