Elsevier

Thin Solid Films

Volume 315, Issues 1–2, 2 March 1998, Pages 111-117
Thin Solid Films

Preparation of TiO2 films on self-assembled monolayers by sol–gel method

https://doi.org/10.1016/S0040-6090(97)00759-1Get rights and content

Abstract

TiO2 gel films were deposited on three kinds of substrates: (1) a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS), (2) a trimethylsilylated (TMS) glass surface and (3) a bare SiO2 glass surface using Ti(OC3H7i)4-derived titania sols. It was found that OTS SAM induces precipitation of anatase phase at a rather low temperature of 200°C and accelerates anatase-to-rutile phase transformation when a mole ratio of H2O:Ti(OC3H7i)4 as low as 0.5 was employed for sol preparation. These effects possibly result from the interaction between the ordered methyl groups of the OTS SAM and the hydrophobic groups of the titania polymers at the gel film/OTS SAM interface.

Introduction

From the viewpoint of energy saving and environmental protection, low-temperature materials synthesis has been highlighted in the past several decades. Recent developments in low-temperature crystal growth involve the use of Langmuir monolayers and self-assembled monolayers (SAMs) of highly ordered surfactant molecules as the molecular templates for the oriented nucleation of inorganic crystals 1, 2. It has been recognized that the structural information can be transferred from Langmuir monolayers and SAMs to growing crystals 3, 4, and the interaction between the highly ordered terminal groups of the monolayers and the ions in solution can control the crystal nucleation and growth events [5].

SAMs are ordered molecular assemblies formed by the chemical adsorption of an active surfactant on a solid surface, which have attracted much attention in the field of material science in the last decade [6]. Ordered molecular arrangements can be supplied by SAM adsorption even on solid surfaces of random atomic arrangements such as those of glass substrates, which provides a great potential for controlled inorganic crystal growth. This simple process makes SAMs inherently manufacturable and thus technologically attractive for surface engineering. Goss et al. [7]applied SAMs as the molecular adhesive to the fabrication of vapor-deposited gold electrodes on a glass substrate. Campbell et al. [8]utilized SAMs with different terminal functional groups as templates to induce the nucleation of calcium oxalate monohydrate. Feng and Bein [9]reported the oriented growth of zinco-phosphate zeolite crystals on gold surfaces modified with metal phosphonate multilayer films. Shin et al. [10]utilized SAMs with –OH and –SO3H functional groups fabricated on single-crystal Si wafers as substrates for the deposition of TiO2 thin films from aqueous solutions. Few studies on SAMs with hydrophobic terminal groups, however, were reported in the inorganic crystal growth process.

In the present study, the idea of applying SAMs as templates to induce the oriented nucleation of inorganic crystals was introduced to the sol–gel method for the first time. It is well known that the sol–gel synthesis involves hydrolysis of metal alkoxides by water, which is followed by polymerization reaction leading to metal oxides or hydrated oxides [11]. Conditions such as water:alkoxide ratio, solvent, catalyst, reaction temperature, and nature of alkyl groups of alkoxide are all known to affect the kinetics of hydrolysis reactions and in turn to modify the structural and physical characteristics of the resultant material obtained by heat treatment. Among the oxides usually prepared by the sol–gel method, TiO2, which is a very important material for its gas sensitive properties, excellent dielectric properties and catalysis applications, was chosen as a model inorganic crystal in the present study. Octadecyltrichlorosilane (OTS) SAMs with ordered methyl groups on its surface were utilized as the templates. Self-assembled, tightly packed, defect free OTS monolayers provide an ideal model surface for studying the interactions between organic surfaces and metal oxides. For a better understanding of the effects of the ordered OTS SAM as a substrate for the preparation of TiO2 films, SiO2 glass substrates of trimethylsilylated (TMS) surface with disordered methyl groups as well as bare silica surface were also used as the substrates. Based on the comparison of the crystallization behavior of TiO2 films deposited on these different substrates (bare SiO2 glass surface, TMS surface, and OTS SAM), the effect of SAM on the crystallization of TiO2 gel films was discussed, especially focusing on different water:alkoxide ratio of the starting solutions.

Section snippets

Pretreatment of the glass substrates

The pretreatment (cleaning) of the substrate surfaces is a critical step in the preparation of adsorbed monolayers. The fused SiO2 glass plates (25 mm×38 mm×1 mm) were cleaned first using surfactant by hand with finger sacks carefully and rinsed with water. Then the glass plates were placed on a stainless steel rack and ultrasonicated in ion-exchanged water for 10 min and in a boiling ethanol:chloroform mixture (1:1 by volume) for 10 min. Hydroxylation of the cleaned glasses was performed by

Thermal stability of OTS SAMs and TMS surfaces

Fig. 2 shows the relationship between the contact angles of OTS SAMs and TMS surfaces and the heat-treatment temperature. The contact angle represents the hydrophobicity of the surface. It can be seen that OTS SAMs and TMS surfaces show high hydrophobic properties up to 200°C and 400°C, respectively. That is, these organic surfaces are stable up to those temperatures. They, however, start to decompose at higher temperatures, gradually loosing their hydrophobic properties.

Crystallization behavior of the TiO2 films

Fig. 3 shows the XRD

Factors affecting the crystallization behavior

Crystallization of the TiO2 gel film (r=0.5) took place at 200°C when the OTS SAM was used as the substrate, while the films remained amorphous at this temperature when deposited on the bare and the TMS glass surfaces. The crystallization temperature found in the film on the OTS SAM is much lower than those usually observed for alkoxide-derived TiO2 films deposited on glass substrates. Anatase-to-rutile transformation began around 700°C and the anatase phase disappeared completely at 900°C in

Conclusion

In this study, TiO2 thin films were fabricated on different substrates by the sol–gel process using titanium isopropoxide catalyzed with HCl. The interaction between the ordered methyl groups of the OTS SAM and the hydrophobic groups of the titania polymer has been verified to provide a great potential for the rearrangement of the polymer to a highly ordered assembly, resulting in crystallization of anatase at rather low temperatures and easy transformation of anatase to rutile phase. This

Acknowledgements

The authors thank Dr. Takashi Monde, NEOS, for the contact angle measurement.

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