Bibo Xia

One-step sol–gel preparation of PDMS–silica ORMOSILs as environment-resistant and crack-free thick antireflective coatings

Polydimethylsiloxane (PDMS)–silica ORMOSIL (organically modified silicate) sols were prepared by a one-pot sol–gel process using tetraethyl orthosilicate as an inorganic precursor and hydroxyl-terminated PDMS as an organic modifier, and antireflective (AR) coatings were prepared with these sols by dip coating. It was found that the addition of PDMS to silica sols significantly increased the viscosity and the controllable viscosity range of the silica sols, affording us a simple route to prepare very thick AR coatings with controlled thickness by a single deposition step. The coated fused silica substrates retained very high transmission of up to 99.9% at a determined wavelength. The addition of PDMS to the silica sols improved the hydrophobicity and abrasion-resistance of the coating, and prevented to some extent the coating from cracking which occurred in a pure inorganic thick AR coating. The PDMS–silica ORMOSIL AR coatings are stable after standing for 2 months at room temperature in an environment of 95% relative humidity. The relationship between structure and properties of the coatings is discussed considering the particle growth mechanism.

Sol–gel silica antireflective coating with enhanced abrasion-resistance using polypropylene glycol as porogen

Abstract Silica antireflective (AR) coatings with high transmittance and enhanced abrasion-resistance were synthesized by sol–gel process using polypropylene glycol (PPG) as porogen. The effects of molecular weight of PPG and weight ratio of PPG to SiO2 on the refractive index and abrasion-resistance of the coating were systematically studied and compared with those of polyethylene glycol (PEG). Experimental data showed that the refractive index decreased with increasing the weight ratio to SiO2 and molecular weight of both PEG and PPG, but PPG was much more effective than PEG. In the case of same molecular weight, PPG modified coating has the higher porosity than PEG modified one. When the weight ratio of PPG to SiO2 is in a low level, the PPG-containing silica AR coatings exhibit the good abrasion-resistance. PPG is liquid at room temperature and the better solubility than PEG. These effective and economic AR coatings with enhanced abrasion-resistance have potential value in the field of solar thermal collectors.

Preparation of porous silica films in a binary template system for double-layer broadband antireflective coatings

Porous silica films with a refractive index varying from 1.44 to 1.15 were prepared and applied to realize a double-layer broadband AR coating. The porous silica films were obtained using tetraethylorthosilicate (TEOS) as precursor and a binary template as porogen which was composed of cetyltrimethylammonium bromide (CTAB) and polypropylene glycol (PPG). The molar ratio of the two templates was optimized, and a hypothetical mechanism for the cooperative assembly of silicates with the binary template system was proposed. The two layers had the same skeleton despite different porosity with the top layer much more porous than the bottom one. The double-layer double-wavelength AR coating had excellent optical properties with transmittances of 99.6% and 99.8% at 532 nm and 1064 nm, respectively.

Preparation of mechanically stable triple-layer interference broadband antireflective coatings with self-cleaning property by sol–gel technique

Antireflective coatings usually suffer from poor functional durability during outdoor service. The incorporation of additional properties, such as broadband antireflection, self-cleaning and mechanical stability, is of great importance. In the current work, we synthesized broadband AR coatings via the design concept of triple-layer interference coating. SiO2–TiO2, TiO2 and nanoporous TiO2–SiO2 composite thin films were employed as bottom, middle and top layers. Substrates (BK-7 glass) coated with the triple-layer films attained consistent high transmittances at 500–700 nm and the average transmittance at that region was 99.4%. The triple-layer films exhibited an outstanding superhydrophilic property with water contact angle of 2° in 0.5 s, and the superhydrophilicity could last for 30 days in the absence of UV illumination. These triple-layer coatings also showed a good ability to decompose organic substances under UV irradiation. The synergistic effect between superhydrophilicity and photocatalysis provides the films with self-cleaning effect. Meanwhile, the abrasion resistance test of the triple-layer films indicated favorable robustness and good adhesion of the films to substrates. This multifunctional film can have potential applications in various areas, like solar cell panels, green houses and architectural glasses.

Preparation of sponge-like porous SiO2 antireflective coatings with excellent environment-resistance by an acid-catalysed sol–gel method

Silica coatings with high-transmittance and good environment-resistance properties were prepared by an acid-catalyzed sol–gel method without template. Hexamethyldisilazane (HMDS) was added into an acid-catalyzed silica sol which acted not only as the catalyst but also as a blocking agent. The effect of different contents of HMDS has been investigated. The sponge-like silica coating was obtained when the weight ratio of HMDS to SiO2 was higher than 90%. The addition of HMDS enabled these silica antireflective coatings to possess excellent hydrophobicity properties. When the weight ratio of HMDS to SiO2 was 100%, the silica coating exhibited transmittance as high as 99.64% at a given wavelength and static water contact angle of 143.0°. After 48 hours of environment aging testing at 40%, 60% and 80% RH, these coatings showed a minute decrease upon maximum transmittance, from 99.64% to 99.56%, 99.49% and 99.38%, respectively. The sponge-like HMDS-containing coatings exhibited easy-to-clean properties, which would have great potential application in laser systems.

Preparation of silica coatings with continuously adjustable refractive indices and wettability properties via sol–gel method

Silica coatings with continuously adjustable refractive indices and wettability properties were prepared through a sol–gel base-catalyzed process. Adjustment of the molar ratio of water (H2O) to tetraethylorthosilicate (TEOS) was utilized to change the hydrolysis degree of the precursors, and hence change the morphology of the silica particles. With the increase in the H2O/TEOS molar ratio, the morphology of the silica particles changed from a linear net-work structure to a bead-like structure and then to a granular particle structure. A particle growth mechanism was proposed and verified by characterization. As the H2O/TEOS molar ratio increased from 0.3 to 21.0, the refractive indices of the silica coatings increased from 1.132 to 1.328. Meanwhile, a varied H2O/TEOS molar ratio also modulated the surface wettability of the silica coatings. The static water angle of the silica coatings decreased from 145° to 6° by increasing the H2O/TEOS molar ratio from 0.3 to 21.0. Different hydrophilic and hydrophobic coatings could be obtained by simply controlling the H2O/TEOS molar ratio. Silica coatings with different refractive indices and hydrophobic (or hydrophilic) properties were obtained at different H2O/TEOS molar ratios.

Thermal-induced durable superhydrophilicity of TiO 2 films with ultra-smooth surfaces

AbstractSuperhydrophilic surfaces without the need of other stimuli are usually realized by constructing a rough morphology. However, constructing rough surfaces usually require specialized equipment or complicated processing. Besides, rough surfaces can cause undesirable scattering, which strongly limits the use in optical devices. In this article, we prepared superhydrophilic TiO2 films with ultra-smooth surfaces using simple sol-gel dip-coating method. The hydrophilicity of the TiO2 films varied with different post-heat treatments. The films heat-treated at 400 °C exhibited a durable superhydrophilicity and anti-fogging property. This superhydrophilicity was attributed to the decrease of surface hydrophobic alkoxy groups and the formation of point defects, i.e., Ti3+ and oxygen vacancies, which are favourable for dissociative water adsorption. The amount of surface organic groups was influenced by autophobicity effects, further hydrolysis and decomposition of residual alkoxy groups. Additionally, the wettability behaviours of the films were also explained from the perspective of the surface energy. These results can benefit the design and manufacture of anti-fogging and self-cleaning superhydrophilic TiO2 films. The TiO2 films exhibited intrinsic superhydrophilicity and anti-fogging property; the superhydrophilicity can maintain 30 days.HighlightsThe TiO2 films exhibited durability, superhydrophilicity and anti-fogging property.The superhydrophilicity films with smooth surface were capable of being optical materials.The mechanism of the superhydrophilicity was well studied.