Mechanical property and corrosion resistance of zirconia/polydopamine nanocomposite multilayer films fabricated via a novel non‐electrostatic layer‐by‐layer assembly technique

Zirconia/polydopamine (ZrO₂/PDA) nanocomposite multilayer films were constructed on Si substrate via a novel nonelectrostatic layer‐by‐layer (NELBL) assembly technique. The building block of this technique is the newly reported dopamine molecule, which can be attached to almost all material surfaces and undergo oxidation‐polymerization to form PDA layers; more importantly, the outer hydroxyl groups of the PDA layer can chelated with certain inorganic oxide nanoparticles to generate oxide films. Thus, ZrO₂/PDA nanocomposite multilayer films were fabricated by sequential NELBL deposition of PDA and ZrO₂ nanoparticles. The formation of the ZrO₂/PDA nanocomposite multilayer films was monitored by the water contact angle (WCA) and ellipsometric thickness measurements, while the microstructure of the fabricated films was analyzed by means of atomic force microscope (AFM), field emission scanning electron microscope (FESEM), X‐ray photoelectron spectrum (XPS), and X‐ray diffraction (XRD) analysis. The mechanical and anticorrosion behaviors of the annealed ZrO₂/PDA nanocomposite multilayers were found to be greatly enhanced as compared with that of the annealed homogeneous ZrO₂ film. The better mechanical and anticorrosion behaviors of the annealed ZrO₂/PDA nanocomposite multilayers than the annealed homogeneous ZrO₂ film may be closely related to their special microstructure. Namely, the organic–inorganic hybrid microstructure of the annealed ZrO₂/PDA nanocomposite multilayers may largely account for the increased nanohardness and corrosion resistance. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of a co-substituted A-site on structural characteristics and ferroelectricity of (Pb,Ba, Ca)TiO3 complex perovskites: analysis of local-, medium- and long-range order

Thin films of A-site co-substituted, PbTiO₃ (PTO) by Ba²⁺ and Ca²⁺, i.e., PBCT70, PBCT60, PBCT50 and PBCT40 were fabricated on Pt/Ti/SiO₂/Si substrates by chemical solution deposition. Structures of the samples were investigated from the viewpoint of local-, medium- and long-range order by X-ray absorption near structure (XANES), micro-Raman and infrared spectroscopies and by X-ray diffraction (XRD). The films thickness were determined by using field-emission scanning electron microscope. The experimental results demonstrate that BaO₁₂ clusters are the critical dominant ferroelectricity cause in PBCT thin films rather than CaO₁₂ clusters. XRD analysis which was applied to investigate the crystal symmetry indicates the absence of long-range structural distortion for samples at higher concentrations of Ba²⁺ and Ca²⁺. However, an analysis of structural medium- and local-range order such as infrared, micro-Raman and XANES spectroscopies revealed that symmetry changes are much more prominent; i.e., local structural distortions remain. Temperature-dependent dielectric permittivity measurements confirmed a decreasing ferroelectric-to-paraelectric phase transition temperature and showed a broad phase transition with an increase in BaO₁₂ and CaO₁₂ clusters. In addition, the lack of long-range polar ordering for the ferroelectric dipole caused by symmetry changes decreases the remanent polarization.     

Band gap tuning in nanocomposite ZrO2–SnO2 thin film achieved through sol–gel co-deposition method

Transparent SnO₂, nanocomposite ZrO₂–SnO₂ and ZrO₂ thin films were prepared by sol–gel dip-coating technique. X-ray diffraction (XRD) spectra showed a mixture of three phases: tetragonal ZrO₂ and SnO₂ and orthorhombic ZrSnO₄. X-ray photoelectron spectroscopy (XPS) gave Zr 3d, Sn 3d and O 1s spectra of the nanocomposite ZrO₂–SnO₂ thin film which revealed the presence of oxygen vacancies in the nanocomposite ZrO₂–SnO₂ thin film. Scanning electron microscopy (SEM) observations showed that microstructure of the nanocomposite ZrO₂–SnO₂ thin film consists of uniform dispersion of isolated SnO₂ particles in ZrO₂ matrix. The band gap for the ZrO₂ was estimated to be 5.51 eV and that for the nanocomposite ZrO₂–SnO₂ film was 4.9 eV. These films demonstrated the tailoring of band gap values which can be directly employed in tuning the band gap by simply changing the relative concentration of zirconium and tin elements. Photoluminescence (PL) spectra revealed an intense emission peak at 424 nm in the nanocomposite ZrO₂–SnO₂ film which indicate the presence of oxygen vacancies in ZrSnO₄.     

X-ray photoelectron spectroscopy-based valence band spectra of passive films on titanium

Titanium (Ti) is always covered by thin passive films. Thus, valence band (VB) spectra, obtained using X-ray photoelectron spectroscopy (XPS), are superpositions of the VB spectra of passive films and that of the metallic Ti substrate. In this study, to obtain the VB spectra only of passive films, angular resolution (for eliminating the substrate Ti contribution) and argon ion sputtering (for removing passive films) were used along with XPS. The passive film on Ti was determined to consist of a very thin TiO₂layer with small amounts of Ti₂O₃, TiO, hydroxyl groups, and water with a thickness of 5.9 nm. The VB spectra of Ti were deconvoluted into four peak components: a peak at ~1 eV, attributed to the Ti metal substrate; a broad peak in the 3–10 eV range, mainly attributed to O 2p (~6 eV) and O 2p-Ti 3d hybridized states (~8 eV), owing to the π (non-bonding) and σ (bonding) orbitals in the passive oxide film; and a peak at ~13 eV, attributed to the 3σ orbital of O 2p as OH⁻or H₂O. The VB region spectrum between approximately 3 and 14 eV from Ti is originating from the passive film on Ti. In particular, characterization of VB spectrum obtained with a takeoff angle of less than 24° is effective to obtain VB spectrum only from the passive film on Ti. The property as n-type semiconductor of the passive film on Ti is probably higher than that of rutile TiO₂ceramics.     

Microstructural and chemical transformation of thin Ti/Pd and TiD<Subscript><Emphasis Type="BoldItalic">y</Emphasis></Subscript>/Pd bilayer films induced by vacuum annealing

Using a combination of scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction and X-ray photoelectron spectroscopy (XPS), we made a comparative study of the high-temperature annealing impact on thin titanium deuteride (TiD _y ) films covered by an ultrathin Pd layer, and on Ti/Pd bilayer films. The bilayer films were prepared under ultrahigh vacuum conditions and were in situ annealed using the same annealing procedure. It was found that the surface and the bulk morphology of both films undergo different annealing-induced transformations, leading to an extensive intermixing between the Ti and Pd layers and the formation of a new PdTi₂ bimetallic phase. Energy-filtered TEM imaging and energy-dispersive X-ray spectrometry analysis, as well as XPS depth profiling all provided evidence of a different distribution of Pd and Ti in the annealed TiD _y /Pd film compared with the annealed Ti/Pd film. Our results show that thermal decomposition of TiD _y , as a consequence of annealing the TiD _y /Pd film, modifies the intermixing process, thereby promoting Ti diffusion into the Pd-rich top layer of the TiD _y film and thus providing a more likely path for the formation of the PdTi₂ phase than in an annealed Ti/Pd film. Figure Figure Microstructural and chemical characterisation of thin TiDy/Pd film after annealing     

Low energy ion bombardment of Ti and TiNx films

The influence of low energy ion bombardment on TiN_x film growth and film properties was investigated. The discharge was characterized using Langmuir probe technique as well as energy resolved mass spectrometry with a plasma monitor (Hiden HAL 301 S/EQP). The deposited films were investigated by means of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Increasing the N₂ gas flow as well as increasing the negative substrate voltage at constant gas flow effect an increase of the N/Ti ratio in the films determined by XPS. The influence of the energy flux to the surface due to ion bombardment was mainly recognized in the substructure of the films. In addition, pure Ti films were modified by nitrogen ion bombardment after deposition using an ion gun. An increase of the N/Ti ratio was observed with increasing ion energy. Finally saturation is reached.     

Vacuum annealing phenomena in ultrathin TiD y /Pd bi-layer films evaporated on Si(100) as studied by TEM and XPS

Using a combination of TEM and XPS, we made an analysis of the complex high-temperature annealing effect on ultrathin titanium deuteride (TiD _y ) films evaporated on a Si(100) substrate and covered by an ultrathin palladium layer. Both the preparation and annealing of the TiD _y /Pd bi-layer films were performed in situ under UHV conditions. It was found that the surface and bulk morphology of the bi-layer film as well as that of the Si substrate material undergo a microstructural and chemical conversion after annealing and annealing-induced deuterium evolution from the TiD _y phase. Energy-filtered TEM (EFTEM) mapping of cross-section images and argon ion sputter depth profiling XPS analysis revealed both a broad intermixing between the Ti and Pd layers and an extensive inter-diffusion of Si from the substrate into the film bulk area. Segregation of Ti at the Pd top layer surface was found to occur by means of angle-resolved XPS (ARXPS) and the EFTEM analyses. Selected area diffraction (SAD) and XPS provided evidence for the formation of a new PdTi₂ bimetallic phase within the top region of the annealed film. Moreover, these techniques allowed to detect the initial stages of TiSi phase formation within the film–substrate interlayer.     

XPS depth profile study of porous zirconia films deposited on stainless steel by spray pyrolysis: the problem of substrate corrosion

Zirconia (ZrO₂) films of tissue‐like structure and narrow pore size distribution have been deposited by spray pyrolysis using aqueous zirconyl chloride octahydrate (ZrOCl₂·8H₂O) precursor solutions. Stainless‐steel sheets, protected or unprotected by a ZnO barrier layer, have been used as the substrate material held at 473 K. The ZnO barrier layers have been deposited on the stainless steel held at 523 K by spray pyrolysis using a zinc acetate precursor. Their property of corrosion protection to stainless steel has been proved by electrochemical polarization measurements in 0.5 M NaCl solution. A complementary study of XPS (depth profiling, mapping) and x‐ray diffraction has shown that the unprotected steel substrates were corroded during ZrO₂ film post‐annealing in air at T ≥ 773 K, whereas steel substrates protected with a compact barrier layer of crystalline ZnO before ZrO₂ film deposition did not show surface corrosion even after annealing up to 997 K. Copyright © 2004 John Wiley & Sons, Ltd.     

Achieving ultra-low friction of a-C:H film grown on 9Cr18Mo steel for industrial application via programmable high power pulse magnetron sputtering

Owing to the high hardness and hydrogen passivation of carbon bonds, hydrogenated diamond-like carbon (a-C:H) film has shown promising potential to achieve ultra-low friction and wear on steel surfaces. Here, a-C:H film was successfully deposited on 9Cr18Mo steel via programmable high power pulse magnetron sputtering and potential application for industrial was evaluated. The a-C:H films against different mating materials of GCr15 steel balls, Al₂O₃, Si₃N₄, ZrO₂, and a-C:H-coated GCr15 balls all showed ultra-low friction under a normal load of 5 N in a dry ambient air environment. Among them, self-mating tribo-system a-C:H films on steel surfaces and a-C:H-coated steel balls achieve best friction performance; the principal reason is that both contacting surfaces coated with a-C:H film have the lower electron affinities compared with other tribo-systems. However, the differences of coefficient of friction (COF) for uncoated-GCr15, Al₂O₃, ZrO₂, Si₃N₄, and a-C:H(GCr15) balls can be attributed to different sizes of clustering in wear debris. This work provides new insights on synthesis and industry application of the a-C:H films with ultra-low friction properties.     

Nanostructured ZrO2 and Zr‐C‐N Coatings from Chemical Vapor Deposition of Metal‐Organic Precursors

Nanocrystalline zirconium carbonitride (Zr‐C‐N) and zirconium oxide (ZrO₂) films were deposited by chemical vapor deposition (CVD) of zirconium‐tetrakis‐diethylamide (Zr(NEt₂)₄) and ‐tert‐butyloxide (Zr(OBu^t)₄), respectively. The films were deposited on iron substrates and characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). The Zr‐C‐N films show blue, golden brown or bronze colours, with colour stability depending upon the precursor composition (pure metal amide or mixed with Et₂NH). The deposition temperature showed no pronounced effect on the granular morphology of the Zr‐C‐N films. The XRD data of the films correspond to the formation of carbonitride phase whereas the XPS analyses revealed a strong surface oxidation and incorporation of oxygen in the film. The films deposited using a mixture of Zr(NEt₂)₄ and Et₂NH showed higher N content, better adhesion and scratch resistance when compared to films obtained from the CVD of pure Zr(NEt₂)₄. Subject to the precursor composition and deposition temperature (550‐750 °C), the microhardness values of Zr‐C‐N films were found to be in the range 2.11‐5.65 GPa. For ZrO₂ films, morphology and phase composition strongly depend on the deposition temperature. The CVD deposits obtained at 350 °C show tetragonal ZrO₂ to be the only crystalline phase. Upon increasing the deposition temperature to 450 °C, a mixture of tetragonal and monoclinic modifications was formed with morphology made up of interwoven elongated grains. At higher temperatures (550 and 650 °C), pure monoclinic phase was obtained with facetted grains and developed texture.     

BiVO4‐TiO2 Composite Photocatalysts for Dye Degradation Formed Using the SILAR Method

Composite photocatalyst films have been fabricated by depositing BiVO₄ upon TiO₂ via a sequential ionic layer adsorption reaction (SILAR) method. The photocatalytic materials were investigated by XRD, TEM, UV/Vis diffuse reflectance, inductively coupled plasma optical emission spectrometry (ICP‐OES), XPS, photoluminescence and Mott–Schottky analyses. SILAR processing was found to deposit monoclinic‐scheelite BiVO₄ nanoparticles onto the surface, giving successive improvements in the films′ visible light harvesting. Electrochemical and valence band XPS studies revealed that the prepared heterojunctions have a type II band structure, with the BiVO₄ conduction band and valence band lying cathodically shifted from those of TiO₂. The photocatalytic activity of the films was measured by the decolourisation of the dye rhodamine 6G using λ>400 nm visible light. It was found that five SILAR cycles was optimal, with a pseudo‐first‐order rate constant of 0.004 min^?1. As a reference material, the same SILAR modification has been made to an inactive wide‐band‐gap ZrO₂ film, where the mismatch of conduction and valence band energies disallows charge separation. The photocatalytic activity of the BiVO₄–ZrO₂ system was found to be significantly reduced, highlighting the importance of charge separation across the interface. The mechanism of action of the photocatalysts has also been investigated, in particular the effect of self‐sensitisation by the model organic dye and the ability of the dye to inject electrons into the photocatalyst′s conduction band.     

Photoelectrochemical Properties of Sol-Gel Processed Ag-TiO2 Nanocomposite Thin Films

Ag-TiO₂ thin films were prepared with a sol-gel route, using titanium isopropoxide and silver nitrate as precursors, at 0, 0.03 and 0.06 Ag/Ti nominal atomic ratios. After drying at 80°C, the films were fired at 300°C, 500°C, and 600°C for 30 min and 5 h. Glancing angle X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS), with depth profiling of the concentration, were used to study the films. XPS analysis showed the presence of C and N as impurities in the nanocomposite films. Their concentration decreased with increasing the firing temperature. Chemical state analysis showed that Ag was present in metallic state, except for the outer layer where it was present as Ag⁺. For the films prepared with a Agt/Ti concentration of 0.06, depth profiling measurements of the film fired at 300°C showed a strong Ag enrichment at the outer surface, while composition remained almost constant within the rest of the film, at Ag/Ti atomic ratio of 0.02. Two layers were found for the films heated to 500°C, where the Ag/Ti ratios were 0.015 near the surface and 0.03 near the substrate. The photoelectrochemical properties of Ag-TiO₂ were studied for thin films deposited on ITO substrates. Photocurrents of Ag-TiO₂ nanocomposite electrodes fired at 300°C were observed even at visible light, for wavelengths longer than 400 nm.     

Structural and chemical characterisation of titanium deuteride films covered by nanoscale evaporated palladium layers

Thin titanium deuteride (TiD_y) films, covered by an ultra-thin palladium layer, have been compared with the corresponding titanium and palladium films using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The TiD_y layers were prepared under ultra-high vacuum (UHV) conditions by precisely controlled deuterium sorption at 298 K on a Ti film evaporated onto a Si(100) substrate. Both Ti and TiD_y films were then covered in situ by a nanoscale Pd layer. It was found that a 10- to 12-nm-thick Pd layer protects the TiD_y films efficiently against extensive air interaction. The morphology of both the surface and bulk Pd/TiD_y (Ti) films have been observed using SEM and cross-sectional TEM analysis, respectively. A polycrystalline bulk morphology in both Ti and TiD_y films accompanied by a fine-grained Pd surface was observed. High-magnification cross-sectional TEM images reveal the TiD_y film to be plastically deformed leading to an increase in the roughness of the top Pd layer. Complex structures, including Moiré patterns, have been identified within the Pd/TiD_y interface. The chemical nature of this interface has been analysed after partial sputtering of the Pd top layer using XPS. Besides TiD_y and Pd, TiO and PdO were found to be the main chemical species in the interface region of the Pd/TiH_y film. The XPS valence-band spectra of the Pd/TiD_y interface reveal electronic features characteristic of a Pd–Ti bimetallic structure.     

TCO-free dye solar cells based on Ti back contact electrode by facile printing method

The back contact dye solar cells (BCDSCs), in which the TCO(Transparent Conductive oxide) is omitted, have a potential for use of intact low-cost general substrates such as glass, metal foil and papers. Herein, we introduce a facile manufacturing method of a Ti back contact electrode (Ti BCE) for the BCDSCs. We found that the polylinkers such as poly(butyl titanate) have a strong binding property to make Ti particles connect one with another. A porous Ti film, which consists of Ti particles of ≤ 10? size connected by a small amount of polylinkers, has an excellent low sheet resistance of 10 Ω sq^-1 for an efficient electron collection for DSCs. This Ti BCE can be prepared by using a facile printing method under normal ambient conditions. Conjugating the new back contact electrode technology with the traditional monolithic structure using the carbon counter electrode, we fabricated TCO-less DSCs. These four-layer structurered DSCs consist of a dye-adsorbed nanocrystalline TiO₂ film on a glass substrate, a porous Ti back contact layer, a ZrO₂ spacer layer and a carbon counter electrode in a layered structure. Under AM 1.5 G and 100 mWcm^?2 simulated sunlight illumination, the four-layer structurered DSCs with N719 dyes and I^-/I₃^-redox electrolytes achieved PCEs up to 5.21 %.     

Sol-Gel Preparation and Characterization of Ag-TiO2 Nanocomposite Thin Films

Ag-TiO₂ thin films were prepared with a sol-gel route, using titanium isopropoxide and silver nitrate as precursors, at 0.03 and 0.06 Ag/Ti nominal atomic ratios. After drying at 80°C, the films were fired at 300°C and 500°C for 30 min. The films were analysed by X-ray diffraction (XRD) with glancing angle, and X-ray photoelectron spectroscopy (XPS), with depth profiling of the concentration. XPS analysis showed the presence of C and N as impurities in the nanocomposite films. Their concentration decreased with increasing the firing temperature. Chemical state analysis showed that Ag was present in metallic state, except for the very outer layer where it was present as Ag⁺. For the films prepared with a Ag/Ti concentration of 0.06, depth profiling measurements of the film fired at 300°C showed a strong Ag enrichment at the outer surface, while composition remained almost constant within the rest of the film, at 0.019. For the films heated to 500°C, two layers were found, where the Ag/Ti ratios were 0.015 near the surface and 0.026 near the substrate.     

Study on parylene/SiO<Subscript>2</Subscript> composite films for protection of KDP crystals

In this paper, parylene/SiO₂ composite films were reported to protect KDP crystals, indispensable cells in ICF experiments, from moisture. FTIR, UV-NIR spectra and XPS were used to analyze the properties of films. Laser damage threshold of films was also measured. With porous silica coating on surface of parylene film, the transmittance of dual layers can be raised to more than 91%. KDP crystals with poly(p-xylylene)/SiO₂ coating could work well in ambient atmosphere for more than half a year.     

New Sol-Gel Routes to Organic-Inorganic Hybrid Materials: Modification of Metal Alkoxide by Phosphonic or Phosphinic Acids

Hybrid materials were synthesized by modification of Ti(O ⁱ Pr)₄ by diphenyl phosphinic (DPPA) or phenyl phosphonic acid (PPA). Different P/Ti and H₂O/Ti ratios have been investigated. The samples were characterized using FTIR and ³¹P MAS NMR spectroscopies, elemental analysis (EDX), and N₂ adsorption-desorption experiments. Both PPA and DPPA act as chemical modifiers. Depending on the H₂O/Ti and P/Ti ratios, and on the nature of the modifier (PPA or DPPA), molecular Ti oxo-alkoxo-phosphonates or phosphinates, stable sols, gels, or precipitates were obtained. In addition, the P/Ti ratio was retained during the hydrolysis whatever the H₂O/Ti ratio.     

Highly increased capacitance and thermal stability of anodic oxide films on oxygen-incorporated Zr-Ti alloy

Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm^?3 ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO₂ phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform, and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m^?2, which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO₂ phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application.     

Perfluorinated polymer surfaces comprising SF5-terminated long-chain perfluoroacrylate

A new perfluorinated acrylate monomer containing the SF₅(CF₂)₆-perfluorinated side chain was synthesized and polymerized into films. Bulk monomer characterization is consistent with the molecular structure based on FTIR, mass spectrometry and NMR analyses. The surface properties of polymer coatings were studied with aqueous wetting (contact angle) and X-ray photoelectron spectroscopy (XPS) methods. The surface composition is shown to be highly enriched in the terminal side chain SF₅-chemistry and exhibits properties consistent with a highly apolar, non-wetting perfluorinated polymer surface. Depth-dependent XPS studies using angular-resolved methods (ADXPS) confirmed a non-stoichiometric enrichment of sulfur and fluorine at the film ambient interface, consistent with a surface presence of the terminal SF₅-group and possibly film structural anisotropy in the surface zone. Time-of-flight (TOF) secondary ion mass spectrometry (SIMS) analysis supplements the XPS data by showing the presence of all expected SF₅-acrylate chemistry components in the outer 15 Å of the film surface.     

Effect of α-Hydroxyketones as Chelate Ligands on Dip-Coating of Zirconia Thin Films

The stabilization effects of -hydroxyketones such as acetoin, acetol, and their imine derivatives of monoethanolamine (MEA) on the 2-propanol solution of metal alkoxides (Al, Ti, Zr, Nb and Ta) were investigated. The hydroxyketones, especially acetoin which has a moderate boiling point around 150°C, were found to have a stabilization effect on the alkoxides of Ti^IV, Nb^V and Ta^V already in their molar ratios of 2, while the Zr^IV alkoxide was stabilized at the molar ratio of 4. By sol-gel processing using this acetoin-stabilized solution, crystallized ZrO₂ thin films could be prepared at temperatures as low as 400°C, which was lower than that from a diethanolamine (DEA)-stabilized precursor (450°C). The films prepared in this study showed a high transparency (more than 90%, UV-VIS spectroscopy), suggesting that the use of chelate ligands with low boiling points is a good way to lower the crystallization temperature of sol-gel-derived ZrO₂ thin films. The Zr complexes of the imines made by the reaction between the hydroxyketones and MEA were stable and photosensitive (<500 nm), making it possible to pattern ZrO₂ films by photo-irradiation of their gel films.