Mesoscopic objects, porous layers and nanocomposites—Possibilities of sol–gel chemistry

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Abstract

The goal of this study was to prepare mesoscopic objects, thin porous films and nanocomposite coatings with the use of sol–gel technique. Silica nanotubes, titania nanoparticles, porous titania and zirconia coatings as well as titania nanocomposites were successfully synthesized by changing the type of sol–gel precursor, sol composition and applying dip-coating deposition procedure in order to obtain thin films or coatings. All materials were visualized and characterized by the Atomic Force Microcscopy (AFM) technique. Moreover, characterization of titania nanocomposites was extended to the tribological tests performed by means of microtribometer operating in normal loads range of 30–100 mN.

The AFM analysis of mesoscopic objects and nanoparticles showed that the diameter of synthesized silica nanotubes was 60–70 nm and the size of titania nanoparticles was 43 nm. In case of porous layers the pore size in titania and zirconia coatings oscillated between 100 and 240 nm, however their shape and distribution were irregular.

Microtribological studies of nanocomposites revealed the moderate decrease of the coefficient of friction for samples containing 5, 15 and 5 wt.% of zirconia nanoparticles in titania coatings annealed at 100, 500 and 1000 °C respectively. An enhancement of antiwear properties was already observed for 1 wt.% of nanophase content, except the sample annealed at 500 °C. It was also found that the annealing at high temperatures is a primary factor which affects the reduction of friction and wear of titania coatings while the presence of nanoparticles has secondary effect.

Investigations in this study carried out with the use of the AFM technique highlighted the potential and flexibility of sol–gel approach in designing of various types of advanced materials in a form of mesoscopic objects, porous coatings and composite layers. Results collected in this study clearly demonstrated that sol–gel technique can be applied effectively in preparation of broad range of modern materials.

Introduction

The sol–gel method is a versatile technique commonly used in materials’ engineering allowing the fabrication of many metal oxides, composites, organic–inorganic hybrids and other materials in form of powders, thin films, fibers, tubes, rods and porous materials in mirco- and nanosize scale range. Details about the sol–gel chemistry may be found in Ref. [1].

Micro- but especially nanosized objects and nanostructured layers have recently experienced considerably interest in scientific communities. They offer, due to their dimensions, unique opportunities in investigations of the influence of the size on materials’ properties [2]. The sol–gel technique represents a suitable method of fabrication of these types of materials, while the AFM is an accessible technique in their research. Silica nanotubes find nanotechnological applications as sensors, actuators and as optoelectronic devices [3]. Porous thin films may be applied as catalysts, sensors but also as low-k materials in electronic devices [4]. Another example of advanced applications of porous materials, with pore size suitably tuned, are photonic crystals used in optoelectronic systems [5]. Coatings with embedded ceramic or metallic nanoparticles are extensively used in fabrication of new nanocomposite functional materials, antiwear coatings for tribological applications and protective layers in space applications [6]. As well, hybrid sol–gel organic–inorganic systems find a broad application in biocompatible systems and “intelligent” coatings having self-adaptive properties [7].

The goal of this work was to prepare various types of advanced materials in form of mesoscopic objects, porous coatings and composite layers in order to demonstrate the possibilities and flexibility of the sol–gel method. The AFM was applied as a suitable technique in imaging of several representative samples of above-mentioned types of materials such as: silica nanotubes prepared with the use of two various organic templates, titania nanoparticles, porous titania and zirconia thin films prepared with surfactants or polymer beads as templates, titania-based composite coatings containing ceramic nanoparticles and finally hybrid organic–inorganic films. Investigations of composite titania coatings containing ceramic nanoparticles also covered microtribological tests performed in mN normal load range.

Section snippets

Sample preparation

Silica nanotubes were prepared according to the method presented by Wang et al. [8]. In this procedure citric acid was used as a template. Another synthesis was carried out according to the procedure reported by Nakamura and Matsui [9], where dl-tartaric acid was applied as a template. In both procedures tetraethyl orthosilicate (TEOS) was used as a silica precursor. Resulted solids were dispersed in ethanol in ultrasonic bath and deposited on muscovite for further observations in the AFM.

Results and discussion

The AFM image of separated silica nanotubes prepared with the use of citric acid is presented in Fig. 1. The average diameter is 60–70 nm. This is in good agreement with values reported by Wang et al. [8], however it is less than the typical diameter of the majority of nanotubes obtained in his study (100–150 nm). It is hard to distinguish if edges of silica objects are opened and as consequence if we observe tubes or rather rods. It may be due to well-known artifacts of rounding shapes of

Conclusions

The broad range of advanced materials may be prepared by low cost and relatively simple sol–gel approach. Silica nanotubes and titania nanoparticles were presented as examples of mesoscopic objects. Creating of sol–gel coatings exhibiting an adjustable level of porosity and controlled arrangement and size of pores is easily achievable. In experiments presented in this study the surface hole structure observed by AFM indicates probably a porous interconnected network, which makes these films

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