Heidi A. Springer

Oregon State University College of Engineering

Project Title: Self-assembled Mesoporous Metal Oxide Thin Films
Advisor: Prof. Hugh Hillhouse, Chemical Engineering

Introduction

Self-assembled mesoporous materials are porous materials (pore diameter = 2-50nm) formed by the condensation of an inorganic metal oxide solution around an ordered organic template. The organic template solution (aka surfactant) consists of amphiphilic molecules (molecules having both hydrophobic (water loathing) and hydrophilic (water loving) blocks) whose hydrophobic blocks associate to one another in order to minimize their contact with water. These associations of amphiphilic molecules are referred to as micelles.

Based on a simple geometric model; the shape of a micelle is determined by the shape of the amphiphilic molecules that create it. For instance, if the amphiphile is conical in shape, it will form a spherical micelle. Given the appropriate conditions, these micelles will pack together to form ordered mesostructures. See figure 1 below.

Once a firm mesostructure is formed, the organic template can be removed leaving behind a uniformly ordered porous material.

Mesoporous materials have a variety of applications as molecular sieves (molecule selection by size), gas sensors, and high surface area catalysts. Two of the metal oxides examined here, titania and tin oxide also have semiconducting properties making them strong candidates for use in (inexpensive) photovoltaic solar devices.

Project Objectives

  • Explore mesostructure self-assembly using titanium oxide (titania) and silica solutions.
  • Investigate the role of surfactant concentration in synthesizing mesoporous tin oxide thin films with ordered cubic structure.

Approach

  • Synthesis method for mesoporous tin oxide (SnO2) and titania (TiO2) films:
    • PEO-PPO-PEO triblock copolymer is dissolved in ethanol
    • MCl4** and water are added to the solution with stirring
    • Thin films are prepared using dip coating
    • Films are aged 12 hours at ambient conditions
    • Films are subjected to humidity and/or thermal treatment to aid condensation of the inorganic (metal oxide) framework
    • Calcination at 300-400oC
    **M represents the metal used, either Sn for tin (IV) or Ti for titanium (IV).

    Using tin oxide as the inorganic solution, two different PEO-PPO-PEO surfactants were tested as organic templates for a cubic ordered mesoporous film. Their trade name is Pluronic, manufactured by BASF. Both P123 (EO20PO70EO20) and F127 (EO106PO70EO106) were used.

    While maintaining a constant solution of anhydrous tin (IV) chloride, ethanol, and water; surfactant concentration was varied. The techniques used to identify the structure of the synthesized materials are x-ray diffraction (XRD) and transmission electron microscopy (TEM).

Findings

  • Ideal surfactant concentration for a cubic structured mesoporous material of tin oxide and Pluronic F127 appears to be at an ethylene oxide (hydrophilic block) to tin ratio of 1.4. [(EO):Sn = 1.4].
  • Tin oxide and Pluronic P123 displays well defined XRD peaks at (EO):Sn ratio of 0.6. However, the mesostructure appears to be lamellar rather than cubic. Data also indicates some affect from film aging time. Further experiments are needed.

Figure 1: Conical amphiphiles form spherical
micelles which pack into cubic mesostructures.
Figure 2: TEM image of mesoporous titania.