A Study on the Influences of Processing Parameters on The
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Journal of Sol-Gel Science and Technology 36, 183–195, 2005 c 2005 Springer Science + Business Media, Inc. Manufactured in the United States. A Study on the Influences of Processing Parameters on the Growth of Oxide Nanorod Arrays by Sol Electrophoretic Deposition S.J. LIMMER∗,T.P. CHOU AND G.Z. CAO Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA [email protected] Received March 31, 2005; Accepted July 8, 2005 Abstract. Template-based sol electrophoretic deposition has been demonstrated as an attractive method for the synthesis of oxide nanorod arrays, including simple and complex oxides in the forms of amorphous, polycrystalline, and single crystal. This paper systematically studied a number of processing parameters to control nanorod growth by sol electrophoretic deposition. The influences of particle and template zeta potentials, condensation rate, deposition rate (or externally applied electric field), the presence of organic additives, and sol concentration on the growth of nanorod arrays were studied. It was found that higher zeta potential or electric field resulted in higher growth rates but less dense packing. Templates with charge opposite to that of the sol particles prevented formation of dense nanorods, sometimes resulting in nanotubes, depending on the field strength during electrophoresis. In addition, the pH of the sol and chelating additives were also varied and likely affected the deposition process by affecting the condensation reactions. Keywords: sol electrophoresis, electrophoretic deposition, sol-gel processing, template-assisted nanorod growth, oxide nanorods, nanorod arrays 1. Introduction The technique of sol electrophoretic deposition in templates provides a versatile method for forming The fabrication of nanostructures and nanomaterials nanorods of numerous simple and complex oxide with desired dimensions and compositions is an es- nanorods [5–9], and while sol electrophoretic depo- sential cornerstone of nanotechnology, and many syn- sition has been known for some time as a technique thesis techniques have been developed for the creation for the formation of films, only recently has it been of nanostructured materials [1]. In the area of nanos- adapted for the formation of nanorods [10, 11]. In con- tructured materials, so-called one-dimensional nanos- cept, the only requirement is that one applies an electric tructures (nanorods, nanowires) have attracted large field to an electrostatically stabilized sol. Nanoclus- amounts of interest for their potential applications, ters dispersed in such a sol would develop a surface along with the information they can provide about charge, and a double layer structure in the vicinity of the influence of size and dimensionality on proper- the nanoclusters would form due to a combination of ties [2, 3]. Nanorods of oxide materials are particu- electrostatic attraction, Brownian motion, and osmotic larly appealing [4], given the large number of func- force, as depicted in Fig. 1. An externally applied elec- tional properties that oxides exhibit. Thus, there is sub- tric field to such a colloidal system or sol would set stantial interest in methods for the synthesis of oxide the charged particles in motion in response to the elec- nanorods. tric field (Fig. 1), migrating such nanoclusters until the motion of such nanoclusters is stopped by either an ∗Present address: Materials Research Center, University of electrode or other physical barrier. However, there are Missouri-Rolla, Rolla, Missouri, USA a number of parameters that significantly influence the 184 Limmer, Chou and Cao Figure 1.Aschematic of the electrical double-layer surrounding a particle in a colloidal suspension that is responsible for the electrostatic stabilization of a colloid (left), and the electrophoretic motion and process involved in template-based sol electrophoretic deposition (right). growth process, the microstructure, and the morphol- follows. TiO2 sol was formed by dissolving titanium ogy of the resultant nanorod arrays. This has not been (IV) isopropoxide (97%, 30 mL) in glacial acetic acid studied or reported in detail. (60 mL), followed by the addition of deionized (DI) In this study, the influence of several of these pro- water (30 mL). Upon the addition of water, a white cessing parameters are reported, albeit less quantita- precipitate instantaneously formed. However, the sol tive or less conclusive, so as to provide some gen- became a clear liquid after ∼5 minutes of stirring. eral guidance to further study the synthesis of nanorod The preparation of SiO2 and PZT sols has been pub- arrays by sol electrophoresis. More specifically, we lished in Reference 10. The SiO2 sol was made by attempted to vary independently each of six differ- dissolving tetraethyl orthosilicate (98%, 21 mL) in a ent parameters: particle zeta potential, template zeta mixture of ethanol (8 mL) and DI water (3 mL). A potential, condensation rate, deposition rate, presence small amount of hydrochloric acid (1N, 0.09 mL) was of stabilizing additives, and sol concentration. While added to the sol to adjust the pH to ∼3 and the sol some of these parameters are not strictly indepen- was stirred for 2 hrs at room temperature. The sil- dent (concentration and deposition rate, for exam- ica sol thus formed was rather stable, and took sev- ple), we separated the influences out as much as eral weeks to gel at room temperature. The prepara- possible. tion of PZT sol was described in detail in Reference 20. Briefly, lead (II) acetate (24.48 g) is dissolved in glacial acetic acid (15.2 mL), heating to 110◦C for ∼15 2. Experimental min to dehydrate the lead acetate. The sample is then allowed to cool back to room temperature. Because In much of this study, TiO2 was used as a model system of the volatility of PbO, an excess amount of lead (5 to examine the various processing parameters. Since mol%) is used in the fabrication of this sol. Then tita- SiO2 sol can be either particulate or polymeric and nium (IV) isopropoxide (97%, 8.4 mL) and zirconium Pb(Zr,Ti)O3 (PZT) is a complex oxide, both systems (IV) n-propoxide (70%, 14.72 mL) are mixed together were also used. The preparation of TiO2 sol has been for ∼10 min at room temperature, and added to the reported in Ref. 12, similar to the process reported pre- lead solution once it has cooled to room temperature. viously [10, 11]. Briefly, the sols were prepared as Deionized water (16 mL) is then added to initiate and A Study on the Influences of Processing Parameters on the Growth of Oxide Nanorod Arrays 185 sustain hydrolysis and condensation reactions, and the means of X-ray diffraction (XRD), scanning electron sol is stirred for ∼15 min at room temperature. Lastly, microscopy (SEM, JSM 5200), and transmission elec- lactic acid (3.4 mL), glycerol (4.8 mL), and ethylene tron microscopy (TEM, EM420T). glycol (3.6 mL) are added to adjust the viscosity and stability of the sol. Such prepared sols have a concen- tration of about 5 vol% PZT, and are stable for several 3. Results and Discussion weeks at room temperature. These standard sols were occasionally modified by adjusting the concentration or 3.1. Growth of Nanorods pH, or by adding stabilizers, which is discussed in detail below. Figure 2 shows typical SEM images of TiO2 nanorods Both polycarbonate (PC) and anodic alumina grown in PC membranes. The size of the nanorods is (AAM) templates, consisting of arrays of parallel pore dependent on the pore size of the templates. Nanorods channels with either 50, 100, or 200 nm diameters were grown in 200, 100 and 50 nm templates pores resulted used. For growth in PC templates, the PC membrane in nanorods with 180, 90 and 45 nm diameters, respec- and the working electrode (aluminum) in a polypropy- tively. Figure 3 shows typical SEM images of TiO2 lene filter holder were held in place with a silicone gas- nanorods grown in AAM with 200 nm pores. Both ket. This assembly contacts the sol. The Pt counter elec- templates are very convenient to use for the growth trode was also placed in the sol, parallel to the working of nanorod arrays by electrophoretic deposition, and electrode. Growth in AAM templates involved attach- each offers some advantages and disadvantages. The ing the membrane to a working electrode, which was advantage of using PC as the template is its easy han- held parallel to the Pt counter electrode in a bath of dling and easy removal by means of pyrolysis at ele- the sol. The electric fields applied for the growth of vated temperatures, but the flexibility and large ther- nanorod arrays varied in the range of 0–6 V/cm. De- mal expansion coefficient of PC are prone to distortion tailed set-up can be found in our previous publications and breakage of nanorods during the subsequent heat [7, 10–12]. After the growth of nanorod arrays, the treatment and removal of the template. The advantage filled templates were dried at 100◦C for 24 hours and of using AAM as the template is its rigidity and re- then fired at 500◦C for 1 hour to densify and crystallize sistance to high temperatures, allowing the nanorods the grown nanorods. In the case of PC, the templates to densify completely before removal. This would re- are removed upon firing at 500◦C. For the removal of sult in a large surface area of fairly free-standing and AAM templates, wet-chemical etching in 40% sodium unidirectionally-aligned nanorod arrays. In addition, hydroxide (NaOH) or potassium hydroxide (KOH) in the AAM templates consist of a much higher pore den- water is required. sity, and thus allow for the growth of a higher density of Details of zeta potential measurements and spec- nanorods. The rigidity is also important for the attach- trometry analyses will be presented when discussing ment of nanorods to desired substrates.