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Fundamental properties of alkoxide-derived titania gel films providing plastics surface with high refractive index

  • Invited Paper: Sol-gel and hybrid materials for optical, photonic and optoelectronic applications
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Abstract

In order to obtain the information of the fundamental properties of alkoxide-derived titania gel films on plastics, we prepared the films on polycarbonate (PC) substrates by spin-coating, dried them at 120 °C for 24 h, and evaluated their refractive index, uncracking critical thickness, flexibility, chemical durability and adhesion to the substrate. First, dried gel films were prepared from solutions of various types of alkoxides, i.e., from Ti(OR)4-HNO3-H2O-ROH solutions (R = C2H5, C3H7i, or C4H9n) with or without CH3COOH as a chelating agent (CA). Among them, those prepared from Ti(OC3H7i)4 had higher refractive indices than the others. Next, dried gel films were prepared from Ti(OC3H7i)4-HNO3-H2O-(C2H5OH or C3H7iOH) with or without CA, where HCOOH, CH3COOH, CH3COCH2COCH3 were employed as CA. The addition of CA in solutions tended to lower the refractive index of the dried gel films. The uncracking critical thickness was ca. 30–80 nm, with larger critical thickness given by the addition of CA in solutions, which was attributed to the lower degrees of the increase in in-plane tensile stress during heating. The critical thickness on PC substrates was found to be smaller than that on Si(100) substrates (ca. 100–150 nm) possibly due to the larger thermal expansion coefficient of PC than Si(100). Cross-cut adhesion test showed that all these Ti(OC3H7i)4-derived dried gel films have excellent adhesion to PC substrates. Chemical durability was evaluated on these dried films by soaking them in water and ethanol at room temperature for 30 min. No significant changes in thickness or refractive index on soaking were detected under such a limited test condition. These dried films on PC substrates were shown to be bent without cracking until the radius of curvature decreased down to about 16 mm.

Graphical abstract

Highlights

  • Titania gel films were prepared by spin-coating on polycarbonate substrates from Ti(OC3H7i)4-HNO3-H2O-(C2H5OH or C3H7iOH) solutions with or without chelating agents, HCOOH, CH3COOH and CH3COCH2COCH3, and dried at 120 °C for 24 h.

  • The uncracking critical thickness was ca. 30–80 nm, which was smaller than that on Si(100) substrates (100–150 nm), while the chelating agents contributed to an increase in critical thickness.

  • Irrespective of presence or absence and of the type of CA, the adhesion of the dried films to PC substrates, which was evaluated by cross cut adhesion test, was excellent, and showed no significant changes in thickness and refractive index on soaking in water and ethanol at room temperature for 30 min.

  • The dried films on PC substrates could be bent without cracking until the radius of curvature decreased down to about 16 mm.

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Acknowledgements

We thank Mr. Yuma Ohta and Mr. Naoki Tamura, Department of Chemistry and Materials Engineering, Kansai University, for their help in examining the reproducibility of IR absorption spectra of the samples.

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  1. Department of Chemistry and Materials Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, 564-8680, Japan

    Nana Matsushita & Hiromitsu Kozuka

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  1. Nana Matsushita
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  2. Hiromitsu Kozuka
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Correspondence to Hiromitsu Kozuka.

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Matsushita, N., Kozuka, H. Fundamental properties of alkoxide-derived titania gel films providing plastics surface with high refractive index. J Sol-Gel Sci Technol 104, 673–684 (2022). https://doi.org/10.1007/s10971-022-05878-x

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