Gel Thin Films Research Articles

Influence of substrate composition on corrosion protection of sol–gel thin films on magnesium alloys in 0.6M NaCl aqueous solution

The corrosion protection behaviour of organic–inorganic hybrid thin films on AZ31 and AZ61 magnesium alloy substrates has been studied. These films were prepared by a sol–gel dip-coating method. The organopolysiloxane precursors were γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethoxysilane (TMOS). An attempt was made to determine the possible relationships between the degradation of the sol–gel film and composition of the metal substrate during the exposure of the metal/coating system to 0.6 M NaCl aqueous solutions. For this purpose electrochemical impedance spectroscopy (EIS) and hydrogen evolution measurements were applied. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the sol–gel films formed on the surface of AZ61 alloy were far more perfect and uniform than those formed on the AZ31 alloy. This behaviour was attributed to the effect of the native oxide film initially present on the surface of the AZ61 alloy, which inhibited the attack of magnesium. Results indicated that the sol–gel coated AZ61 substrate tended to develop corrosion products slower than the sol–gel coated AZ31 substrate, trend that could change by prolonging exposure time. After 6 days of immersion, a clear inhibitive effect of the corrosion products formed during the test was observed in the case of the sol–gel coated AZ31, but not with the coated AZ61 alloy substrate, a phenomenon explained by the carbonate enrichment observed by XPS.

Rhodamine B nanocrystals: elaborations, characterizations and functionalizations for biosensing applications

A biological sensor based on fluorescent organic nanocrystals (NCs) of Rhodamine B grown in sol–gel thin films was developed. The original signalization function is based on fluorescence contrasts of NCs, which exhibit a simple fluorescence signature, good photostability and higher fluorescence intensities compared to dispersed dye molecules. Thanks to a well-controlled dissolution process of the sol–gel surface, accurately followed by atomic force microscopy, the NCs were made emerging just a few nanometers above the silicate thin films to be directly accessible to biological macromolecules. Thus, hairpin-shaped DNA, functionalized by a probe-molecule (DNA probe), has been grafted onto nanocrystal surfaces leading to a fluorescence quenching by Forster resonance energy transfer. After hybridization of these hairpin-shaped DNA probes with their complementary DNA-target, the molecular probes and NCs are pulled apart, stopping thus the quenching. This “turn-on” of nanocrystal fluorescence allows thus a label-free DNA detection. The preparation methodology of the signalization function, its functionalization by hairpin-shaped DNA probes and first DNA-sensor experiments are presented.

Photocatalytic antibacterial activity of TiO2–SiO2 thin films: The effect of composition on cell adhesion and antibacterial activity

• The effect of composition of TiO 2 –SiO 2 thin films on cell adhesion was studied. • The effect of surface composition on surface free energy was studied. • SiO 2 addition favors for cell adhesion. • Strong relation was found between the bacterial adhesion and antibacterial activity. Colloidal solutions of SiO 2 –TiO 2 mixed oxides having different SiO 2 contents were synthesized by sol–gel technique and thin films were deposited over glass slides by using dip coating technique. The samples were characterized by XRD, SEM, AFM and methylene blue adsorption methods. The surface free energy of thin films was measured by sessile drop method by using diiodomethane, glycerol and water as probe liquids. The photocatalytic antibacterial activity of the samples was determined for Escherichia coli XL1-blue by using artificial solar irradiation. The surface characterization studies revealed the presence of small crystallites of anatase phase, enhanced dispersion of TiO 2 particles and increasing surface free energy at elevated SiO 2 content. Highest photocatalytic antibacterial activity was obtained over 92 wt% SiO 2 containing sample and strong correlation was found between the bacterial adhesion and antibacterial activity. Surface-adhered E. coli cells having direct contact with TiO 2 particles were inactivated at a higher rate than suspended cells.

Free-standing SnO2/MWCNT nanocomposite anodes produced by different rate spin coatings for Li-ion batteries

Free standing SnO2/multiwalled carbon nanotube (MWCNT) nanocomposite anode materials were prepared for Li-ion batteries by sol–gel technique. Firstly, SnO2 precursor sols were synthesized after removing chloride ions. Then the sols coated on MWCNT buckypapers, which used as substrates to form nanocomposite electrodes by spin coating method. Sintered nanocomposite structures were then characterized by field emission gun-scanning electron microscopy (FEG-SEM), energy dispersive X-ray spectrometer (EDS), and X-ray diffraction (XRD) analyses. Electrochemical tests were performed for the produced electrodes, assembled as CR2016 cells. The effect of spin rate on the anode capacity was investigated. Coating on the MWCNT buckypapers was thought to use as mechanical support to prevent electrode failure and prevented the formation of cracks of the sol–gel thin film on the MWCNT surfaces. The results showed beneficial effects to prevent mechanical disintegration and subsequent anode pulverization of SnO2 anodes because of huge volume increase during lithium intercalation. The results indicated that the nanocrystalline SnO2/MWCNT composites are suitable for applying as an anode electrode for Li-ion batteries to increase electrochemical energy storage performance.

Photoconductive UV detectors based heterostructures of Cd and Mg doped ZnO sol gel thin films

The photoconductive properties of single (CdZnO/MgZnO, Li:CdZnO/MgZnO) and double (MgZnO/ZnO/MgZnO, CdZnO/ZnO/MgZnO) heterostructures were studied by sol gel method at different annealing temperatures. These structures were analysed by X-ray diffraction (XRD) and photoluminescence (PL). The photoconductive measurements were carried out by semiconductor parametric analyzer. The hexagonal wurtzite structure was retained with good crystalline nature. The UV related strong near band edge (NBE) emission, ultra violet and blue green emission peaks were observed from PL spectra. Photocurrent was observed to have enhanced as a result of annealing temperature and Li substitution in the doped ZnO lattice. The increase of responsivity at ten orders of magnitude was found. Meanwhile, the dark current was found to have increased slightly due to the adsorption of O2 and H2O molecule on the surface of the film. Sub-linear behaviour of photocurrent was also observed.

A Fast Room-Temperature Poling Process of Piezoelectric Pb(Zr<SUB>0</SUB><SUB>.45</SUB>Ti<SUB>0</SUB><SUB>.55</SUB>)O<SUB>3</SUB> Thin Films

The effect of two poling processes on the ferroelectric and piezoelectric properties of sol–gel and pulsedlaser-deposited Pb(Zr0.45Ti0.55)O3 (PZT) thin films has been investigated as a function of the poling field, poling temperature and poling time. In the case of dc-electric field poling at an elevated temperature (200 °C), the remnant polarization and effective piezoelectric coefficient are found to increase with and saturate at high dc-poling field (400 kV/cm) and long poling time (30 minutes). The room-temperature poling process using ac electric field poling, shows the same trend with poling field but much shorter poling times (100 seconds), with only a slightly lower saturation value of polarization. It is suggested that in room-temperature poling screening charges are merely rearranged, whereas in high temperature poling these charges are largely removed. A much larger improvement in the properties of sol–gel PZT thin films is found, as compared to those deposited using pulsed laser deposition (PLD), indicating that a poling process is required for sol–gel films.

Aluminum-doped zinc oxide sol–gel thin films: Influence of the sol's water content on the resistivity

Thin films of indium tin oxide (ITO) have gained substantial interest due to their optical and electrical properties. Since ITO is an expensive material and indium is a rare element, considerable attempts have been made to replace it by, e.g., aluminum-doped zinc oxide (ZnO:Al). The production of ZnO:Al is less cost-intensive, especially if the sol–gel technique is applied, while its properties are comparable to those of ITO. In this study, we demonstrate that the electrical properties of ZnO:Al thin films can be improved considerably by the addition of small amounts of ultrapure water to the dip coating solution during the preparation. The lowest resistivity obtained with a film prepared from a sol containing 0.2M water is 2.8·10−3Ωcm. Optical modeling thus indicates an improvement of the free carrier mobility of films prepared from sols in the presence of additional water. The films prepared have an average thickness of 340nm and a solar transmittance above 85% after annealing in a forming gas atmosphere. Clearly, the addition of water to the sol has a positive impact on the resistivity of the final ZnO:Al thin film. We suggest the observed increase of the free carrier mobility to be due to an improved electron transfer at the grain boundaries between the spherical nanoparticles.

Hybrid sol–gel thin films doped with a pH indicator: effect of organic modification on optical pH response and film surface hydrophilicity

Optical sensors for application in innovative wearable sensing systems such as textile-integrated systems and wireless sensor platforms rely on the development of low-cost multifunctional materials compatible with standard fabrication technologies. We are developing optically responsive pH sensitive sol–gel coatings for integration with a mobile wireless smart tag sensing system. For this application, we have fabricated a range of thin pH sensitive films using bromocresol green (BCG) indicator immobilised in inorganic–organic silica hybrid matrices prepared by a sol–gel method and deposited by spin-coating onto glass substrates. The surface hydrophilicity of the films were varied by using the inorganic sol–gel precursor tetraethoxysilane together with either methyltriethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane or glycidoxypropyltrimethoxysilane as organically modified sol–gel precursors, co-polymerised in different ratios. Spectral characterisation of the films was performed using visible absorption spectroscopy. The shift in absorption maxima and other spectral changes of the different matrices have been identified, and the apparent pK app values of the immobilised BCG pH indicator determined. The surface wettability properties of the films have been studied by measuring the contact angle of water, formamide and diiodomethane which has allowed the estimation of the surface free energy (SFE) using three different models: Owens–Wendt, Wu and van Oss-Chaudhury-Good. It is shown that the SFE of the hybrid films is directly related to the type and the degree of organic modification, which in turn has a significant effect on the pH response-time of these sensing films.

Conductive sol–gel films

We report that silver nanowires (AgNW) entrapped in a series of sol–gel thin films – zirconia, alumina, titania and silica – provide conductive, highly flexible, highly adhesive, transparent thin films with excellent thermal and electrical stability. These films were subjected to intensive study and characterization including resistance, percolation analysis, critical bending angle analysis, critical thermal stability, transparency, and more. Remarkably, when resistance is kept constant, all of the studied parameters follow the same order – either increasing or decreasing – of ZrO2, SiO2, Al2O3, and TiO2. A detailed interpretation, based on the particle-sizes of the elementary sol–gel building blocks, is provided. In particular we found that the AgNWs re-enforce the ceramic matrices, thus providing excellent electroconductive stability with low silver concentration and high degree of homogeneity of the dopant distribution. Having in mind that current materials for films with these properties are often accompanied by disadvantages (such as brittleness, scarcity and poor adhesion) we offer the new conductive films as an interesting alternative to be considered.

In vitro Assessment of Photocatalytic Titanium Oxide Surface-modified Stainless Steel and Nickel Titanium Orthodontic Wires for its Antiadherent and Antibacterial Properties against Streptococcus mutans

Objective: To assess the antiadherent and antibacterial pro­ perties of surface-modified stainless steel and nickel titanium orthodontic wires with photocatalytic titanium oxide against Streptococcus mutans (S. mutans). Materials and methods: This study was done on 80 specimens of orthodontic wires each of 5 cm in length. The specimens were divided into eight test groups, four groups were used for anti ­ adherent test, and other four groups were used for antibacterial test. Each group consisted of 10 specimens. The groups contain­ ing uncoated stainless steel and uncoated nickel titanium wires acted as control group for their respective experimental group, containing coated stainless steel and nickel titanium wires. Surface modification of both stainless steel and nickel titanium orthodontic wires with photocatalytic titanium oxide was carried out by sol gel thin film dip coating method. Orthodontic wires were then subjected to microbiological tests for assessment of the antiadherent and antibacterial properties of photocatalytic titanium oxide coating against S. mutans. Results: Photocatalytic TiO2 coating on the wires prevented the adhesion of S. mutans to the orthodontic wires, hence demonstrating antiadherent property against S. mutans. Also, photocatalytic TiO2 coating on the surface of orthodontic wires demonstrated antibacterial effect against S. mutans. Conclusion: Surface modification of orthodontic wires with photocatalytic titanium oxide was found to have antiadherent and antibacterial properties against S. mutans.

Protein-templated gold nanoclusters sequestered within sol–gel thin films for the selective and ratiometric luminescence recognition of Hg2+

Sequestration of bovine serum albumin (BSA)-stabilized gold nanoclusters (AuNCs@BSA) prepared using microwave assistance within sol-gel-derived mesoporous silica films permits the selective and highly sensitive quenchometric detection of aqueous Hg(2+) (limit of detection = 600 pM) with luminescence signal arising from oxidized BSA allowing for an analytically robust and reliable ratiometric detection. Overall, this work highlights a number of important advances, including the highest luminescence quantum yield reported to date for a protein-templated luminescent noble metal nanocluster (13%) made possible using a microwave-mediated synthesis followed by cold incubation. We also demonstrate the clear advantage of exploiting the luminescence signal arising from oxidized BSA as an internal reference to generate selectivity of response to Hg(2+). A careful Stern-Volmer quenching analysis reveals the persistence of two unique quenching sites for AuNCs@BSA entrapped within a sol-gel-derived glass, a minor population of which is unquenchable. Finally, based on these AuNCs@BSA nanosensors, we advise a path forward for paper-based indicator strip detection of heavy metals in aqueous streams, the implementation of which can be performed using the unaided eye, making it a meaningful approach for routine screening and in resource-limited situations.

A third-generation hydrogen peroxide biosensor based on horseradish peroxidase immobilized by sol–gel thin film on a multi-wall carbon nanotube modified electrode

A third-generation H2O2biosensor was developed by using tetraethoxy-silicone sol–gel film to immobilize horseradish peroxidase (HRP) on a multi-walled carbon nanotubes (MWNTs) modified glass carbon electrode.

TiO2 Gel Thin Film Doped Ce and Sm Preparation and Cyclic Voltammetry Characteristics

TiO2 Gel Thin Film Doped Ce and Sm Preparation and Cyclic Voltammetry Characteristics

Modification of polyurethane‐coated fabrics by sol–gel thin films

AbstractSol–gel derived silica and hybrid films from tetraethoxysilane (TEOS) and 3‐glycidyloxypropyltrimethoxysilane (GLYMO) were deposited by dip‐coating, in order to find the best treatment. GLYMO‐based treatments preserved textile feel of the material. The coatings were characterized by infrared spectroscopy (IR), scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and water contact angle measurement. IR spectra confirmed the existence of silicate network and successfully conducted modification for both precursors. DSC and TGA showed that the conditions of hydrolysis had greater influence on TEOS than GLYMO‐based treatments. Both treatments shifted the degradation onset to higher temperatures. SEM images showed that polyurethane surface and pores were completely covered and filled by silica or hybrid thin films forming a composite organic–inorganic coating. Hydrophobic effect was preserved for all the samples, except for a single treatment of the GLYMO, due to its polar epoxy group. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2014,131, 39914.

An investigation into relative humidity measurement using an aluminosilicate sol–gel thin film as the active layer in an integrated optical Bragg grating refractometer

We have demonstrated that a planar integrated optical Bragg grating sensor chip, fabricated using a direct UV-writing approach, can perform as an all-optically accessed relative humidity sensor. Using a simple mesoporous silica thin film enabled operation across the range 0–100%RH, however a thickness decrease of 25% caused by hydrolytic degradation of the silica thin film resulted in a permanent decrease in sensitivity after 1–2 weeks use. Grafting the thin-film with aluminium oxide is shown here to prevent hydrolytic degradation with long-term sensitivity maintained. Aluminium oxide grafting offers the additional benefit of enhanced sensitivity, up to 3.5 times that of the silica analogue; the sensitivity of the aluminium oxide modified thin film device=0.69±0.05%RH/pm, in the range 0–60%RH within a gas flow system, whilst the sensitivity of the silica thin film sensor device=2.47±0.18%RH/pm. Long-term measurements, in static ambient lab conditions, were in excellent agreement with a commercial humidity sensor and also indicated that the thermal compensation afforded by the on-chip reference gratings was sufficient to prevent any drift in calibration. As with all humidity sensors, contamination of the sensor occurred. But for the aluminium oxide device, this was easily removed by rinsing with common organic solvents and the sensitivity was repeatedly and reproducibly restored. The device is accessed using optical fibre technology and is intrinsically safe in flammable environments.

Sol–gel thin-film based mesoporous silica and carbon nanotubes for the determination of dopamine, uric acid and paracetamol in urine

This work describes the preparation, characterization and application of a hybrid material composed of disordered mesoporous silica (SiO2) modified with multiwalled carbon nanotubes (MWCNTs), obtained by the sol–gel process using HF as the catalyst. This hybrid material was characterized by N2 adsorption–desorption isotherms, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission microscopy (HR-TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). This new hybrid material was used for the construction of a thin film on a glassy carbon electrode. The modified electrode using this material was designated SiO2/MWCNT/GCE. The electrocatalytic properties of the electrode toward dopamine, uric acid and paracetamol oxidation were studied by differential pulse voltammetry. Well-defined and separated oxidation peaks were observed in phosphate buffer solution at pH 7.0, in contrast with the ill-defined peaks observed with unmodified glassy carbon electrodes. The electrode had high sensitivity for the determination of dopamine, uric acid and paracetamol, with the limits of detection obtained using statistical methods, at 0.014, 0.068 and 0.098µmolL−1, respectively. The electrode presented some important advantages, including enhanced physical rigidity, surface renewability by polishing and high sensitivity, allowing the simultaneous determination of these three analytes in a human urine sample.

Synthesis of porous NiO materials with preferentially oriented crystalline structures with enhanced stability as lithium ion battery anodes

A simple strategy is described for the synthesis of nickel oxide embedded in a carbonaceous matrix (NiO/C) using a templated agarose gel thin film, in an attempt to produce an electrode with a large reversible capacity and long cycle stability. The as-prepared films are directly deposited onto stainless steel substrates from a solution of the Ni2+ precursors. Scanning electron microscopy images indicate that the as-synthesized NiO/C has a porous and interconnected structure. The results of X-ray diffraction and Fourier transform-infrared spectroscopy analyses confirm the preferential (111) growth of NiO and the presence of carbonaceous materials. As an anode material for lithium ion batteries, this novel structure plays a positive role in producing a material with a large reversible capacity, high conductivity, and long cyclic stability. The high reversible capacity is maintained at an elevated current density. Even after 100 cycles, the NiO/C anodes deliver more than 600 mAh g−1 at a current density of 718 mA g−1, which is significantly higher than the capacity of commercial graphite anodes. The results indicate the existence of a synergetic effect between the porous NiO layers and the conductive matrix in the composite.

High-Energy-Density Sol–Gel Thin Film Based on Neat 2-Cyanoethyltrimethoxysilane

Hybrid organic-inorganic sol-gel dielectric thin films from a neat 2-cyanoethyltrimethoxysilane (CNETMS) precursor have been fabricated and their permittivity, dielectric strength, and energy density characterized. CNETMS sol-gel films possess compact, polar cyanoethyl groups and exhibit a relative permittivity of 20 at 1 kHz and breakdown strengths ranging from 650 V/μm to 250 V/μm for film thicknesses of 1.3 to 3.5 μm. Capacitors based on CNETMS films exhibit extractable energy densities of 7 J/cm(3) at 300 V/μm, as determined by charge-discharge and polarization-electric field measurements, as well as an energy extraction efficiency of ~91%. The large extractable energy resulting from the linear dielectric polarization behavior suggests that CNETMS films are promising sol-gel materials for pulsed power applications.

Enhancement of optical conductivity in the ultra-violet region of Cs doped ZnO sol gel thin films

Nano crystalline cesium (Cs) doped ZnO thin films were deposited on glass substrate by sol gel spin coating method with 1–3 mol.% doping concentration and different annealing temperatures. The deposited films were characterized by X-ray diffraction (XRD), Hall Effect, Photoluminescence (PL) and UV–Visible studies. XRD measurements reveal that all the samples abound in the wurtzite structure with polycrystalline nature. An increase in crystalline size from 19.60 to 44.54 nm is observed with the increase of doping concentration. Electrical conductivity of Cs doped ZnO films were observed from Hall effect measurements and the maximum carrier concentration obtained is 7.35 × 1018 cm−3. The near band emission (384 nm) peak intensity increases with the increase of Cs doping concentration and a maximum intensity 55,280 was observed for CZ3 film from PL spectrum. Also a low energy near infrared (NIR) emission peak centered at 1.62 eV appears for the Cs doped ZnO films. The average transmission of CZ film is 88 % and the absorption edge is red shifted with the increase of Cs doping concentration and also the optical conductivity increases in the UV region.

Uniform Thin Films of CdSe and CdSe(ZnS) Core(Shell) Quantum Dots by Sol–Gel Assembly: Enabling Photoelectrochemical Characterization and Electronic Applications

Optoelectronic properties of quantum dot (QD) films are limited by (1) poor interfacial chemistry and (2) nonradiative recombination due to surface traps. To address these performance issues, sol-gel methods are applied to fabricate thin films of CdSe and core(shell) CdSe(ZnS) QDs. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with chemical analysis confirms that the surface of the QDs in the sol-gel thin films are chalcogen-rich, consistent with an oxidative-induced gelation mechanism in which connectivity is achieved by formation of dichalcogenide covalent linkages between particles. The ligand removal and assembly process is probed by thermogravimetric, spectroscopic, and microscopic studies. Further enhancement of interparticle coupling via mild thermal annealing, which removes residual ligands and reinforces QD connectivity, results in QD sol-gel thin films with superior charge transport properties, as shown by a dramatic enhancement of electrochemical photocurrent under white light illumination relative to thin films composed of ligand-capped QDs. A more than 2-fold enhancement in photocurrent, and a further increase in photovoltage can be achieved by passivation of surface defects via overcoating with a thin ZnS shell. The ability to tune interfacial and surface characteristics for the optimization of photophysical properties suggests that the sol-gel approach may enable formation of QD thin films suitable for a range of optoelectronic applications.