Elsevier

Thin Solid Films

Volume 333, Issues 1–2, 25 November 1998, Pages 25-28
Thin Solid Films

The growth of TiO2 thin films on mixed self-assembly monolayers from solution

https://doi.org/10.1016/S0040-6090(98)00760-3Get rights and content

Abstract

The chemical synthesis of TiO2 thin films on mixed self-assembly monolayers (SAMs) is reported. The mixed SAMs with different ratios of -SH and -SO3H groups were obtained when oxidized in H2O2/HOAc solution for various times. Atomic force microscopy (AFM) demonstrated various growing modes of thin films that may be induced by -SH or other oxide states of sulfur groups in the mixed self-assembly monolayers. X-ray photoelectron spectroscopy (XPS) results illustrated that different oxidation states of titanium exist in the TiO2 films on the mixed SAMs.

Introduction

There has been a growing interest in the method to prepare oxide thin films and other thin films on organic self-assembly monolayers from evaporation or from aqueous solution because thin films of excellent quality can be fabricated by very simple procedures [1], [2], [3], [4]. Thin films of titanium dioxide (TiO2) have many useful electrical and optical properties, for example a high refractive index and permittivity, and excellent transmittance of visible light, etc. These properties make them useful in many fields [5], [6], [7].

We have successfully prepared TiO2 thin films on glass substrates through a self-assembly procedure [8]. SAMs play an important role in the deposition of TiO2 films because they not only provide the chemical functionality necessary to initiate growth of the inorganic films thus controlling film morphology and the size of particles grown at the film–solution interface [9], but also they may easily accommodate lattice mismatch so a specific crystallographic orientation of the films can be fabricated. However, the sulfonate (-SO3H) groups were obtained by oxidizing the terminal groups (-SH) in H2O2/HOAc solution and incomplete oxidization of -SH can make the SAMs mixed with different functional ones, which influence the growth of the thin films. In this paper, the growth of TiO2 thin films on such mixed SAMs has been studied.

Section snippets

Materials

γ-Mercaptopropyltrimethoxysilane (MPTS) was kindly supplied by Chemicals Factory of Shuguang, and was distilled at reduced pressure before use. The soda glass wafers used were obtained from a commercial source. Benzene was dried before use. Other reagents were of analytical grade.

Preparation of mixed self-assembly monolayers

The wash of substrates and the preparation of MPTS self-assembly monolayers have been described previously [8]. The terminal groups (-SH) surface of self-assembly monolayers were oxidized into mixed monolayers in a

Mixed self-assembly monolayers

The terminal groups (-SH) of self-assembly monolayers can be oxidized into sulfonate ones (-SO3H) by dipping in a solution of 30% H2O2/HOAc [8]. The self-assembly monolayers are composed of mixed functional groups of -SH and -SO3H when the oxidization is not complete. Table 1 lists the contact angles of -SH surface and after oxidization for different times. The decrease of contact angle from 70 to 31° indicated that various ratios of -SH groups were oxidized with increasing oxidization time.

Conclusions

The chemical synthesis of TiO2 thin films deposited on mixed self-assembly monolayers are reported. The mixed SAMs with different ratios of -SH groups and -SO3H were obtained when oxidized in H2O2/HOAc solution for various times. The existence of -SH groups or other oxide states of sulfur groups in the mixed self-assembly monolayers could induce various growths of particles demonstrated by AFM with size more than 200 nm. XPS results illustrated that different oxidation states of titanium (such

Acknowledgements

This work was supported by National Natural Science Foundation of China (no.69371032). The assistance of Prof. Yang Weiyi and Mr. Chen Wenfeng with XPS is gratefully acknowledged.

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