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J. Phys. Chem. C 2008, 112, 14286–14291
Dynamic Stable Nanostructured Metal Oxide Fractal Films Grown on Flat Substrates Lan Chen,† Ju Xu,‡ Peter Fleming,† Justin D. Holmes,†,‡,§ and Michael A. Morris*,†,‡,§ Materials Section and Supercritical Centre, Department of Chemistry, UniVersity College Cork, Cork, Ireland, Tyndall National Institute, Cork, Ireland, and Centre for Research on AdaptiVe Nanostructures and NanodeVices (CRANN), Trinity College Dublin, Dublin 2, Ireland ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: June 04, 2008
Well-organized, dynamic stable nanostructured oxide films, especially transition-metal oxide films have applications in areas of wide scope but are difficult to be grown from solution directly due to the hightemperature requirement in most formation processes which usually destroys the dynamic stable nanostructures. A novel synthetic method associated with a spin coating procedure broke the confinement and produced many different dendrite-like nanostructured fractal films directly on flat substrates from solutions at room temperature, for example, ZnO, Al2O3, TiO2, Nb2O5, WO3, and so forth. In two dimensions, theses fractal films show abundant surface structures like cypress leaf, daisy, fish bone, fern leaf, tree root, and so forth, and their growth and dimensions comply with the reaction-limited aggregation (RLA) mechanism. Experimental results show that the morphology and dimensionality of the FFs is strongly dependent on the chemical compositions and the interactions between particles and the substrate surfaces. The stronger the interaction between the particles and the surfaces, the more even the films, while the stronger the interaction among the particles, the thicker the films. Introduction Nanostructured metal oxide films have a variety of applications, such as solar energy cells,1,2 photocatalysts,3–5 electrochromic materials,6,7 sensors,8,9 display devices,10,11 and so forth. In all of the above applications, the structure and dimensional extension of the surface are very important factors for the efficiency of the applications. Modification of the surface extension may be achieved either by increasing the system size or by roughening the surface. To roughen the oxide surface, sol-gel coatings, vapor, or oxidation of metal deposits are the most used methods.12–14 Highly ramified two-dimensional (2D) nanostructured oxide fractal films (FFs) synthesized directly from solutions have never been reported due to a lack of appropriate casting techniques for the dynamic stable thin films in the low-temperature range, and consequently, experimental and theoretical study of these processes are scarce. Any dynamic stable fractal aggregation can be considered as a nonequilibrium, thermodynamic unstable intermediate state between order and disorder where the relatively ordered states are quenched and kept stable under some dynamic conditions, for example, low temperature, but are destroyed and change to disorder under some thermodynamic conditions, for example, annealing or calcination in elevating temperature. In most cases, the fractal growth is controlled by the mass (heat) transfer rate, and a diffusion-limited aggregation (DLA) model is applicable. A few papers have reported a reaction-limited fractal aggregation (RLA) where the reaction rate is the rate-determining factor.15,16 Spin coating with high revolution speeds, which can produce powerful radial diffusive driving forces, is an ideal tool to investigate the behaviors in the RLA regime. In our previous attempt, we obtained free-standing metal oxide materials by the ion exchange reactions between alkali * To whom correspondence should be addressed. E-mail:
[email protected]. † University College Cork. ‡ Tyndall National Institute. § Trinity College Dublin.
TABLE 1: Metal Oxide Colloidal Solutions and the Corresponding Coated Films no. of as-synthesized colloid MCln/ Na2O/ methanol/ coated layers solutions mM mM ml C-ZnO-25
1
1
40
C-ZnO-50
2
2
40
C-ZnO-100
3
3
30
C-TiO2-50
1
2
20
C-Al2O3-25
1
1.5
40
C-Nb2O5-50
1
2.5
20
C-WO3-12.5
0.5
1.5
20
1 2 3 1 2 3 1 2 3 2 3 2 3 2 3 2 3
formed films on substrates F-ZnO-25-1 F-ZnO-25-2 F-ZnO-25-3 F-ZnO-50-1 F-ZnO-50-2 F-ZnO-50-3 F-ZnO-100-1 F-ZnO-100-2 F-ZnO-100-3 F-TiO2-50-2 F-TiO2-50-3 F-Al2O3-25-2 F-Al2O3-25-3 F-Nb2O5-50-2 F-Nb2O5-50-3 F-WO3-12.5-2 F-WO3-12.5-3
metal oxides and transition-metal or other main group metal chlorides. Sub-10 nm ZnO, Al2O3, TiO2, Nb2O5, and WO3 nanoparticles with either monodispersed or polydispersed sizes were synthesized directly from anhydrous solutions.17 In an attempt to extend the previous work, this time, we used flat substrates, for example, polished Si(100) wafers and normal glass slides incorporated in the syntheses. Instead of the slow filtration of the as-formed oxide colloidal solutions on filter papers, the spin coating was used to evaporate the solvents at a very fast speed, and the evenly coated thin films were formed in only 40 s. Here, we show how these dynamic stable metal oxide fractal films can be deposited directly onto flat substrates at room temperature (RT), and these FFs, in two dimensions, exhibit abundant dendrite nanostructures with different chemical composition.
10.1021/jp803539s CCC: $40.75 2008 American Chemical Society Published on Web 07/15/2008
Metal Oxide Fractal Films Grown on Flat Substrates Experimental Section Preparation of solutions for spin coating oxide nanoparticles was conducted according to the previously developed procedure.17 The result of the preparation is a solution in which welldefined oxide nanoparticles are formed in a highly dispersed form. Briefly, 1 mmol of anhydrous metal chlorides (MCln, n ) 2, 3, 4, 5, and 6 for Zn, Al, Ti, Nb, and W metal elements, respectively) was dissolved into 10 mL of anhydrous methanol to which an equivalent amount of Na2O (in 10 mL of anhydrous methanol) was added under rigorous stirring, which was sonicated in methanol for 20 min prior to the addition. Metal oxide colloidal solutions with different compositions and concentrations are listed in Table 1. One drop of the assynthesized colloidal solutions was transferred onto a piece of Si(100) substrate or a glass slide (2 cm × 2 cm), which was sonicleaned in ethanol and dried under air at 60 °C overnight prior to use. The spinning procedure began with a 3 s-1 revolution speed for 10 s, and the speed was increased to 50 s-1 and maintained constant for 30 s. Finally, the oxide films were air-dried at 60 °C overnight. The oxide colloidal solutions used and the corresponding as-coated oxide films are summarized in Table 1. The spin coater used here is a P6708 coating system (Indianapolis, U.S.A.), and the surface analysis (SEM) of the films was carried out on a JSM-5510 apparatus (JEOL) using a beam voltage of 5 kV (15 kV for EDX analysis) and a DME-2452 DS50 atomic force microscope (AFM). The purity of the materials was examined using total external reflection X-ray fluorescence spectroscopy (TXRF, Bruker S2 PICOFOX) and X-ray photoelectron spectroscopy (XPS, Vacuum Science Workshop ESCABase). TXRF results showed
