Plinio Innocenzi

Infrared spectroscopy of sol–gel derived silica-based films: a spectra-microstructure overview

Infrared (IR) spectroscopy is one of the most popular analytical techniques used to characterize sol-gel silica materials in their different stages. The method represents, in particular, a simple and versatile tool to investigate the microstructural evolution in gels and films, as a function of temperature and synthesis parameters. Several studies have shown that sol-gel IR absorption spectra exhibit, with respect to silica melt glass spectra, some specific features closely related to the peculiarities of sol-gel processing. Furthermore, because of the differences between silica bulk gels and films the spectra- microstructure correlation must be especially evaluated for thin films. IR spectroscopy has been used to evaluate residual porosity, Si-O-Si bonding rearrangements during drying and firing stages and to model the microstructure evolution during film processing. Some questions are still, however, arising around the interpretations of the IR spectra, in particular about the presence of cyclic species in the microstructure and disorder-induced vibrational modes. An attempt is made here to present an overview of the different relationships existing between IR spectra and microstructure of sol-gel silica films as they actually appear from current literature.

Structure and properties of sol-gel coatings from methyltriethoxysilane and tetraethoxysilane

Acid catalyzed solutions of methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) were used to obtain bulk materials and silica coatings of about 2 μm, after densification at 500°C. The structural evolution as a function of MTES content and heat treatment was studied.A higher content of MTES was found to enhance the maximum thickness of the coatings free of cracks. Critical thickness and shrinkage of the films were measured. MTES was also found to affect porosity and to play an important role in avoiding fractures in the films.

Microstructural and optical properties of sol-gel silica-titania waveguides

Abstract Two micron silica-titania coatings made by single step sol-gel dip-coating were prepared as planar waveguides. Acid catalyzed solutions of methyltriethoxysilane (MTES) mixed with tetraethoxysilane (TEOS) and tetrabutoxytitanate were used as precursors. Purely inorganic and crackfree silica-titania coatings were obtained after annealing at 500°C. The waveguides had propagation losses, 0.3 dB/cm, of the same order as thin silica-titania films prepared without MTES. Films were studied by infrared spectroscopy. Rutherford backscattering spectrometry, nuclear reactions analysis and elastic recoil detection analysis. The waveguide structural composition after annealing at 500°C was found to be similar to MTES and TEOS derived silica-titania coatings. The waveguides were characterized by measuring refractive index, porosity and shrinkage, with thermal treatment. The MTES derived films showed a higher shrinkage during annealing but the same refractive index and porosity as the TEOS derived waveguides.

Optical and surface properties of inorganic and hybrid organic-inorganic silica-titania sol-gel planar waveguides

Inorganic and hybrid organic–inorganic silica (SiO2)–titania (TiO2) planar waveguides, with a relative molar composition 70–30, have been fabricated by sol–gel dip-coating. The composition and the density of the film samples have been measured by Rutherford backscattering spectrometry, nuclear reaction analysis and elastic recoil detection analysis. The measured compositions have been found to agree with the nominal composition of the sols. The film density of the inorganic samples increased with the firing temperature in the range 1.80–2.58. A root mean square roughness (rms) <1 nm was measured on the surface of the samples by atomic force microscopy. A refractive index in the range 1.61–1.54 was measured (λ=632.8 nm). The inorganic waveguides had smaller losses (∼0.3 dB cm−1) compared to the hybrid waveguides (∼1 dB cm−1).

Humidity sensors based on mesoporous silica thin films synthesised by block copolymers

Abstract The application of mesostructured thin films to fabricate electrochemical sensors requires the control of dimension, shape and distribution of pores in the material. Silica mesoporous thin films were deposited via dip coating on silicon and alumina substrates with interdigitated electrodes. Mesostructured films were obtained by sol-gel self-assembled process using di-block, tri-block or star-block copolymers: 2-D hexagonal mesoporous phases in silica were formed. After deposition the films were calcined in air to remove the surfactant and were characterised by Fourier transform infrared spectroscopy and low angle X-ray diffraction. Current variations with relative humidity were measured using different applied d.c. voltage; I/V characteristics were performed at various relative humidity values. Moreover the dependence of response from temperature and behaviour during cyclic test in dry–wet conditions was studied. The electrical response was found to be dependent on dimension of pores and their surface. Electrical characterisation upon exposure to humidity shows that the mesoporous structure is easily accessible by external environment, and the films prepared by non-ionic surfactants exhibit good performances in comparison with commercial humidity sensors.

Electrical and structural characterisation of mesoporous silica thin films as humidity sensors

Abstract Mesoporous silica thin films have been prepared using a self-assembling process employing cetyltrimethylammonium bromide (CTAB) as the organic template. The films were deposited by dip-coating on silicon and alumina substrates. Thermal treatment at 250 and 450°C was used to remove the organic template. Fourier transformed infra-red spectra demonstrated that calcination is complete at 450°C. Electrical measurements under dry and wet conditions were performed for films deposited on alumina substrates with comb-type gold electrodes. The films showed an increase in current intensity as a function of relative humidity. No memory effects were observed after cyclic testing in dry–wet conditions. Electrical characterisation indicates that the mesophase is easily accessible by the external environment.

Mesoporous silica thin films for alcohol sensors

Silica mesoporous films using cetyltrimethylammonium bromide surfactant as a template have been prepared. The films have been deposited on silicon and alumina substrates by dip-coating and calcined at 250 and 450 � C. The films were characterized by Xray diffraction analysis, Fourier transform infrared spectroscopy and Rutherford backscattering spectrometry. The films have been shown to maintain the mesophase after calcination, at 250 � C the presence of residual surfactant has been observed. A density of 1.46 and 1.65 g cm � 3 has been measured by Rutherford backscattering spectrometry in the 250 and 450 � C calcined samples, respectively. These densities are much lower than the value (2.07 g cm � 3 ) measured in a reference silica film obtained via sol–gel and with the same thermal history. The electrical response of the silica mesoporous films has been investigated with different concentrations of alcohols. The sensitivity of the material to changes in the atmospheres of ethanol, methanol, 2-propanol, butanol-1 and exane has been tested. The results have shown a large sensitivity to the alcohols at room temperature and the possibility to discriminate between the different alcoholic species. # 2001 Elsevier Science Ltd. All rights reserved.

Mechanical Properties of 3-Glycidoxypropyltrimethoxysilane Based Hybrid Organic-Inorganic Materials

Hybrid organic-inorganic materials were synthesized from acid catalysed sols of tetraethyl orthosilicate, 3-glycidoxypropyltrimethoxysilane and titanium or zirconium alkoxides. The mechanical properties of these materials were measured in different conditions of preparation. The elastic modulus E was determined by a resonance method and by Knoop microindentation. After a thermal treatment at 125°C for 120 h, E was around 3–5 and 1–2 GPa for the samples synthesized with titanium butoxide or zirconium butoxide, respectively. An increase in E in the samples cured for longer times was observed. Knoop microhardness also increased with the heating time and was larger in samples synthesized from titanium alkoxides than zirconium alkoxides. The two methods gave results in good agreement when applied to samples treated for shorter times. In the other samples Knoop microindentation gave a larger value of E compared to the resonance vibration method. Hardness to elastic modulus ratio, H/E, was evaluated by Knoop microindentation. The elastic recovery at the longest heat treatment time was similar to that of soda-lime glasses. Fracture toughness was measured by three points flexural test, a KIc in the range of 0.4–0.5 MPa m1/2 was evaluated for samples treated during 168 h.

Coating of metals by the sol-gel dip-coating method

The sol-gel technique to deposit coatings on metals by dipping was studied. Different metals (Cu, Ni, Fe, Al) were coated with ZrO2, SiO2, TiO2 and B2O3-SiO2 solutions. The influence of some dipping and solution parameters was related with the quality of the obtained coatings. A scale of quality was determined to classify the sol-gel coatings on metals. Both fresh and aged solutions, as-received and abraded substrates were used. The protection against oxidation was evaluated by XRD and by measuring the weight gain upon thermal treatment.

Zirconia-ormosil films doped with PbS quantum dots

Abstract Zirconia-ormosil (organically modified silicates) films doped with different concentrations of PbS quantum dots were synthesized by sol–gel processing. A sol containing 3-(trimethoxysilil)propylmethacrylate as matrix precursor was mixed with a PbS colloidal doping sol. The control of the particle size was obtained by using 3-mercaptopropyltrimethoxysilane as capping agent in the PbS colloidal sol. The microstructure of the films was investigated by Fourier transform infrared spectroscopy, Rutherford backscattering spectrometry, X-ray diffraction and high resolution transmission electron microscopy. Linear and non-linear optical properties were studied by absorption spectrophotometery and non-linear m-line technique. A curing temperature of 130°C allowed a good degree of densification without deterioration of the sulfide particles and the change in refractive index from the substrate was sufficient to support waveguide modes. The doping concentration affected the mean particle size, shifting the linear absorption to shorter wavelengths for films doped with a smaller amount of PbS. The non-linear refractive index n 2 was measured at 1.064 μm. A negative and fully reversible non-linear effect was measured, with n 2 values in the range 10 −15 –10 −16 m 2 /W, depending on the concentration of PbS quantum dots.