Basic Sol Research Articles

New LbL‐TiO2/ZnO Compact Films to Improve Performance of Dye‐Sensitized Solar Cells

AbstractThe preparation of nanostructured thin films consisted of TiO2/ZnO bilayers (BL) and their application in dye‐sensitized solar cells (DSCs) are reported. The film was prepared by the Layer‐by‐Layer (LbL) technique using, respectively, acid and basic sols of TiO2 and ZnO nanoparticles. The particles in the sols were characterized by dynamic light scattering (DLS) and the compact morphology of the LbL films were probed by scanning electronic (SEM) and atomic force (AFM) microscopies. The photoelectrochemical parameters of DSCs with 0, 15 and 20 BL were assessed by current‐voltage (JxV) curves, incident photon‐to‐current efficiency (IPCE), electron lifetime and electrochemical impedance spectroscopy (EIS). These results revealed a significant improvement (up to 67%) on the DSC conversion efficiency promoted by LbL film underlayers, with enhancements ascribed to an efficient blocking effect preventing the FTO/I3− charge recombination and improved contact between the mesoporous‐TiO2 and the conducting substrate (FTO).

A novel route to prepare hydrophobic and durable antireflective hybrid silica coating by sol-gel method

The application of antireflective coatings to the glass covers of solar thermal collectors, allows increasing the efficiency of the whole system. The work presented here describes the room temperature synthesis of highly transparent and hydrophobic silica coatings using tetraethylorthosilicate (TEOS) as a precursor and polydimethylsiloxane (PDMS) as a hydrophobic modifying agent via a simple dip coating technique. Then, the films were characterized by measuring contact angle and optical transparency, Fourier transform infrared (FTIR) spectroscopy, Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) measurements. A new two-step method by double layer of acidic and basic sols was employed. Results showed the maximum transmittance of the antireflective solar glass with double layer coating is about 95.02% at 565 nm wavelength, which is about 4% higher than the substrate glass. The durability of the materials used in solar systems is a key point since they should keep their initial properties during the operational lifetime. In this work, the stability of the optical properties of the films after one year at room temperature in an environment has been achieved, thanks to the application of a hydrophobic treatment and two step-catalyzed sols to the coating. The addition of PDMS to the silica sols improved the hydrophobicity of the coating, and prevented to some extent the coating from cracking which occurred in a pure inorganic thick antireflective coating. It was observed that the obtained silica films become hydrophobic with the introduction of the hydrophobic organic group and static water contact angle (97°) was obtained for the silica film prepared with double layer of acidic/basic coating.

Characterization and photocatalytic activity of TiO2 by sol–gel in acid and basic environments

The sol–gel synthesis of water-based sols of nanocrystalline TiO2 in a large pH range (1.3–10.6) was carried out by acid hydrolysis of titanium(IV) isopropoxide with acetic acid or malonic acid acting also as complexing agents. Subsequent peptization was achieved in acid medium, in the case of acetic acid, and in basic medium (through triethylamine), in the case of malonic acid. The TiO2 particles were characterized using X-ray diffraction, Raman, high-resolution transmission electron microscopy and dynamic light scattering–electrophoretic light scattering. Methyl orange (MeO) and methylene blue (MB) were used as model contaminants to investigate the degradation activity of the different sols under UV irradiation. MB molecules adsorbed on the TiO2 surface of the basic sol were found as trimeric species. The basic TiO2 sol was effective for the degradation of both dyes, whereas the acid sols gave satisfactory results only for MeO.

Ionic Liquids Assisted Synthesis of ZnO Nanostructures: Controlled Size, Morphology and Antibacterial Properties

Systematic analysis about the exploitation of imidazolium based ionic liquids (ILs), [BMIM] BF4 [IL1], [EMIM] BF4 [IL2] and [BMIM] PF6 [IL3] as the morphological template on the basic sol–gel method adopted synthesis of nanostructured zinc oxide (ZnO) is presented. X-ray diffraction (XRD), particle size analysis (PSA) and scanning electron microscopy (SEM) have been employed for the characterization of structure and morphology of the synthesized ZnO particles. Well-defined capsule like shaped morphology with lower nanosize is observed for the ZnO nanoparticles with IL1 than those with IL2 and IL3. This confirms that IL1 served as an effective templating material due to their unique properties. Especially the effective aggregation of ZnO particles with a self-organized frame of IL1 was the essential factor to produce the lower nanosized ZnO with capsule shaped structure. The synthesized ZnO samples with IL2 and IL3 fabricated the flake like shaped and rod like shaped morphologies in the range of nanoscale. The formed ZnO nanoparticles with IL2 exhibit higher nanosize than the ZnO nanoparticles produced by IL1, owing to shorter length of alkyl group in its cation which restricts steric effect and permits the nanoparticles to grow longer. Even though IL3 produced the discrete ZnO nanorods, the hydrophobic nature of IL3 created the higher nanosize than the ZnO nanoparticles formed by other two ionic liquids. Antibacterial properties of the synthesized ZnO nanostructures were investigated against Staphylococcus aureus (gram positive) and Escherichia coli (gram negative) bacteria by Agar diffusion test method. Microbial experiments indicate that the synthesized ZnO samples show a wide spectrum of antimicrobial activities and performed better against S. aureus than E. coli with the same concentration of ZnO.

PVP-assisted synthesis of dense silica-coated graphite with electrically insulating property

In order to endow graphite with an electrical insulating property, graphite flakes were coated with dense amorphous silica by polyvinylpyrrolidone (PVP) as a coupling agent between graphite and silica. First, PVP was physically adsorbed on graphite in water. Later on, silica layers were grown on the PVP-adsorbed graphite by a basic sol–gel reaction of tetraethyl orthosilicate (TEOS) in ethanol, resulting in dense amorphous silica-coated graphite. This silica-coated graphite is categorized as an electrical insulator with a high surface resistivity (2.14×1012Ω/sq), originating from very high surface coverage of the silica on the graphite.

Formation of hybrid nano-crystals in organic–inorganic films from a basic sol

Organic–inorganic films containing hybrid nanocrystals have been prepared by sol–gel processing in controlled conditions. We have systematically changed the temperature and the aging time of a precursor sol containing an organically modified alkoxide bearing an epoxy group, 3-glycidoxypropyltrimethoxysilane, to obtain a controlled crystallization of hybrid layered structures in hybrid films. The precursor sol has been aged at different temperatures, from 5 to 60 °C, and for 1, 2 or 3 days; the films have been deposited from the aged sol and immediately after characterized by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. We have observed that the formation of the hybrid crystals can be obtained only when at least 50% of the epoxies are opened and a larger silica condensation is achieved. These conditions are reached after aging at 60 °C for 1 day, or at longer aging times when the sol is aged at lower temperatures. Transmission electron microscopy and optical polarized images have confirmed the formation of the hybrid crystals.

Electrochemical behaviour of silica basic hybrid coatings deposited on stainless steel by dipping and EPD

The aim of this work is the characterisation of the corrosion behaviour of stainless steel (AISI 304) substrates coated by dipping and electrophoretic deposition (EPD) from a sol–gel basic sol. Particulate silica sols (labelled NaSi) were prepared by basic catalysis from ethyltriethoxysilane (TEOS), methyltriethoxysilane (MTES) and sodium hydroxide. Coatings between 2 and 10 μm were prepared by using concentrated and diluted sols by dipping and EPD process and the corrosion behaviour of the coated substrates were studied through potentiodynamic and impedance spectroscopy measurements (EIS). Potentiodynamic studies of coatings produced by dipping reveal a strong dependence of the protective properties with the concentration of the sol. This behaviour was confirmed by EIS showing that only the coatings obtained from concentrated sol present enough protective properties. On the contrary, EPD coatings prepared from diluted NaSi sol showed an excellent corrosion resistance, maintaining a pure capacitive behaviour for long periods of immersion. EPD deposition is thus proposed as a good alternative method for obtaining thicker and denser coatings with good protective properties from dilute and stable sols.

Silica-Zirconia Sol–Gel Coatings Obtained by Different Synthesis Routes

Homogeneous xSiO2-(1−x)ZrO2 coatings have been prepared onto glass-slides, monocrystalline Si and stainless steel (AISI 304) using sols prepared via acid and basic catalysis. Zirconium tetrabutoxide (TBOZr), zirconium n-propoxide (TPZ), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) were used as precursors of zirconia and silica, respectively. The different parameters involved in the synthesis procedure, as molar ratios H2O/alkoxides, NaOH/alkoxides, and sintering temperature have been analysed, correlating the stability and rheological properties of the sols. The evolution and structure of the sols and coatings have been studied by FTIR. Coatings have been prepared by dipping from acid and basic sols. Electrophoretic Deposition (EPD) technique has also been used to prepare coatings onto stainless steel from basic particulate sols in order to increase the critical thickness. A maximum thickness of 0.5 μ m was reached by both dipping and EPD process for 75SiO2: 25 ZrO2 composition. The critical thickness decreases with ZrO2 amount depending strongly of the drying conditions. Si–O–Zr bonds have been identified by FTIR, indicating the existence of mixed network Si–O–Zr in the coatings obtained by the different routes. Crystallisation of ZrO2(t) was only observed at high sintering temperature (900∘C) by FTIR and confirmed by DRX.

Effect of chemisorbed surface species on the photocatalytic activity of TiO2 nanoparticulate films

TiO2 sols prepared in acidic and basic medium were deposited into films by a spin coating method. Photodegradation experiments showed that photocatalytic activity of the films prepared from acidic sol was much higher than that from basic sol. It is identified that there are more chemisorbed species of CO2 on the surface of the TiO2 films from the basic sol than on the surface of the TiO2 films from the acidic sol. The chemisorbed species of CO2 reduce the concentration of active species such as hydroxyl group and bridging oxygen on surface of the TiO2 film and contribute to the formation of surface electron traps in the band gap which are detrimental to charge separation, thus lowering the photocatalytic activity.

Surface photovoltage spectra and photoelectrochemical properties of semiconductor-sensitized nanostructured TiO2 electrodes

Three kinds of TiO2 nanostructured thin films and their CdS-sensitized films, consisting of different sizes of TiO2 nanoparticles prepared with different methods, have been investigated. The surface photovoltage spectra (SPS) measurements indicate that the density of surface states on TiO2 is likely dependent upon the details of prepared methods. TiO2 particles prepared from basic sol have more surface states than that prepared from acidic sol. When the TiO2 thin films prepared using the TiO2 sols were sensitized by CdS particles, the SPS responses relative to the surface states on TiO2 from 350 to 800 nm were decreased. The photoelectrochemical properties of nanostructured TiO2 electrodes suggest that the fewer the surface states and the smaller the particle sizes of TiO2, the larger the photocurrent response. For CdS sensitized TiO2 thin film electrode, it is shown that the semiconductor sensitization is an efficient way to decrease the influence of surface states on the charge separation, and can improve the intensity of photocurrent response.

99/00946 Drying of coals by using supercritical carbon dioxide : Iwai, Y. et al. Ind. Eng. Chem. Res., 1998, 37, (7), 2893–2896

Shrinking behavior of tin oxide gels during solvent exchange in CO2 supercritical drying was investigated. The drying process consisted of two solvent-exchanging steps, including a first replacement of water by acetone and a second of acetone by liquid CO2. Gels prepared from a basic sol with a ζ-potential of −75 mV exhibited extensive shrinkage during both steps, while those from an acidic hydrous precipitate of lower potential (|ζ-potential|<35 mV) shrank only during the second step. The shrinkage during the first step correlates with the ξ-potential reduction with increasing acetone concentration, while that during the second step for both gels can be attributed to osmotic compressive pressure that arises from an increase in the liquid–solid (gel skeleton) interfacial energy with increasing CO2 concentration. Dramatically different aerogel microstructures were obtained by controlling the gel surface potential in conjunction with different combinations of solvent-evaporating and supercritical drying.

Viscoelastic characterization of three different sol–gel derived silica gels

Three different sol–gel derived silica gels having different gelation mechanism derived both from the acidic and basic sols were investigated. The aggregation processes are monitored by rheological measurements and the viscoelastic behaviour near the gel point is used to determine the critical scaling exponents for each case using frequency dependence of the loss and storage moduli ( G″ and G′). The scaling exponents are further analysed in order to obtain information about particle–particle/aggregate interactions in the system. The three aggregation processes forming gels are suggested to fit to the known aggregation theories for the sol–gel transition. The analysed data including estimated fractal dimensions provide information about the gelation process with respect to aggregation. The mechanisms of aggregation have an influence on the gel structure and the data obtained give an insight about the structure formation from a sol to a gel. The dried gels are additionally characterized by atomic force microscopy (AFM).

Tin oxide gel shrinkage during CO 2 supercritical drying

Shrinking behavior of tin oxide gels during solvent exchange in CO 2 supercritical drying was investigated. The drying process consisted of two solvent-exchanging steps, including a first replacement of water by acetone and a second of acetone by liquid CO 2. Gels prepared from a basic sol with a ζ-potential of −75 mV exhibited extensive shrinkage during both steps, while those from an acidic hydrous precipitate of lower potential (| ζ-potential|<35 mV) shrank only during the second step. The shrinkage during the first step correlates with the ξ-potential reduction with increasing acetone concentration, while that during the second step for both gels can be attributed to osmotic compressive pressure that arises from an increase in the liquid–solid (gel skeleton) interfacial energy with increasing CO 2 concentration. Dramatically different aerogel microstructures were obtained by controlling the gel surface potential in conjunction with different combinations of solvent-evaporating and supercritical drying.

Colloid stability of electrostatically stabilized sols

The dependence of the stability of SiO2 and Al2O3 sols on the pH of their medium has been studied. Vapor adsorption isotherms on powders were prepared from acidic and basic sols, the immersion enthalpy (heat) of samples containing preadsorbed water have been determined and, also, the reversibility of the sol ⇌ xerogel transformation, i.e. the peptizability of the powders, has been investigated.

Synthesis of Ag-colloids in sol-gel derived SiO2-coatings on glass

Ag colloid-containing coatings on soda lime glass and fused silica are prepared via the sol-gel process. To incorporate Ag+-ions in the coatings homogeneously, they are stabilized by a functionalised silane (aminosilane) and then mixed with the basic sol prepared from 3-glycidoxypropyl trimethoxysilane (GPTS) and tetraethoxysilane (TEOS). Crack-free and transparent coatings with a thickness of 0.5 to 1.2 μm, are obtained by heat treatment between 120°C and 600°C. The Ag-colloid formation was monitored by UV-VIS spectroscopy as a function of temperature. The investigations reveal that the substrate has a deciding influence on the Ag-colloid formation caused by alkali diffusion from the substrate into the coating. High resolution transmission electron microscopy (HRTEM) investigations prove that poly-crystalline AgxOy-nanoparticles are formed during thermal densification in the coatings and that this change is accompanied by a vanishing of the yellow colour of the coatings. A post-heat treatment in a reducing atmosphere (90% N2, 10% H2) turns back the yellow colour and single-crystalline Ag-colloids can be detected by HRTEM. A suitable choice of the temperature and time conditions allows the control of the colloid size during heat treatment in a reducing atmosphere. For comparison, ion-exchange experiments have been carried out which showed that a spontaneous Ag-colloid formation was achieved in the soda lime substrate at 400°C. Since Ag containing SiO2-coatings remained colourless after thermal treatment between 400°C and 600°C in air, on soda lime substrates, a remarkable diffusion of Ag+ into the substrate was excluded.