Article pubs.acs.org/Langmuir
Concentration Dependence on the Shape and Size of Sol−GelDerived Yttria-Stabilized Zirconia Ceramic Features by Soft Lithographic Patterning Sjoerd A. Veldhuis, Antony George, Maarten Nijland, and Johan E. ten Elshof* Inorganic Materials Science Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ABSTRACT: Typical surface areas of 5 × 5 mm2 were patterned with high-aspect-ratio micrometer- and submicrometer-sized structures of yttria-stabilized zirconia using a combination of micromolding in capillaries and sol−gel chemistry. The influence of precursor solution concentration and mold geometry on the final shape and dimensions of the patterned structures was investigated. At a precursor concentration of [Zr] = 0.724 mol/dm3, isolated objects− due to the controlled cracking of patterned films−such as crosses (height 1.4 μm, width 6.0 μm) and “dog bones” (height 800−900 nm, width 900 nm) or patterned films (height 450 nm) were obtained, depending on the mold geometry. Lower precursor concentrations led to differently sized and shaped structures, with changes in dimensions of more than an order of magnitude. Employing a precursor concentration of [Zr] = 0.036 mol/dm3 yielded isolated rings (height 100−150 nm, line width 20 nm) and squares (height 40 nm, line width 40 nm). A better understanding of the relationship between the precursor concentration, mold geometry, and observed coherent crack patterns in as-dried sol−gel structures may lead to new techniques in patterning isolated features.
1. INTRODUCTION
To our knowledge, no reports have been published on the micropatterning of YSZ. We describe here the patterning of arrays of isolated YSZ structures by a combination of micromolding in capillaries14 (MIMIC) and sol−gel processing. Both are known for their low cost, technical simplicity, and flexibility regarding shape and composition. In contrast to what is demonstrated in this study, MIMIC normally allows only the formation of continuous features and micropatterns. The aim of this research is to investigate the influence of the metal alkoxide precursor concentrations and PDMS mold geometry on the final shape and dimensions of the patterned YSZ structures. The sol−gel process comprises the hydrolysis and condensation of a stabilized metal alkoxide precursor into an amorphous gel and its crystallization after subsequent heat treatment. Because our aim was to realize microstructures with relatively high aspect ratios (i.e., ∼1 or higher), high precursor concentrations are beneficial because they lead to reduced volume shrinkage during solidification.7 We employed concentrated solutions in which the dimeric Zr clusters16 of the zirconium(IV) npropoxide precursor solution were stabilized by ligand exchange, the bidentate binding of 2-methoxyethanol,17 and the coordination number expansion of the Zr atom.17,18 The pH of the precursor solution was decreased by the addition of glacial acetic acid (HAc). At low pH values, yttrium is mostly
Yttria-stabilized zirconia (YSZ) is a high-performance, technologically important ceramic material because of its outstanding properties such as high mechanical strength, chemical resistance, high ion conductivity, and biocompatibility. For instance, YSZ is used in (oxygen) sensors,1 as a catalyst and catalyst support,2 as an electrolyte material in solid oxide fuel cells (SOFC),3 and in thermal barrier coatings.4 The structuring of YSZ films on the microscale and nanoscale could be a very beneficial method of fabricating electrolyte monoliths for micro-SOFC and oxygen sensor technology. Furthermore, an increase in the interfacial surface area between the electrodes and the electrolyte could lead to decreased oxygen ion transport resistance through the interface and thus to improved performance of the fuel cell. The large surface area of the patterned structures could also be beneficial when exploited in sensing devices. Extensive research has been done on the patterning of nanostructures and thin films from functional metal oxides.5−9 Micropatterned titanium dioxide (TiO2) and zinc oxide (ZnO) have attracted significant attention because of their applications in (dye-sensitized) solar cells,10 semiconductors,11 and lightemitting diodes.12 The patterning of thin-film zirconium oxide (ZrO2) has also attracted considerable attention, and new routes to nanopatterns and thinner films have been explored.13−15 Although these routes provide good control over the size and shape of the patterns, they often make use of expensive production techniques. © 2012 American Chemical Society
Received: July 18, 2012 Revised: September 29, 2012 Published: October 3, 2012 15111
dx.doi.org/10.1021/la302893s | Langmuir 2012, 28, 15111−15117
Langmuir
Article
present as Y3+ ions. Acetic acid can also act as a chelating agent and further stabilize the Zr clusters. Figure 1 shows a schematic diagram of the patterning process. An elastomeric mold with a micropatterned relief
thermal annealing (RTA), which causes surface modification and expels the liquid phase, trapped in the pores, from the gel. The main advantage of RTA over conventional thermal processing is that materials can be heated so quickly that densification is favored over crystallization and crystal growth is inhibited or postponed to higher temperatures than when slower heating rates are employed,19 yielding higher-density films and patterns.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Zirconium(IV) n-propoxide (Zr[(OC3H7)]4), 70 w/w% in propanol) and yttrium(III) nitrate hexahydrate (Y(NO3)3·6H2O, purity 99.9%) were purchased from Alfa Aesar GmbH. Glacial acetic acid (99.8%), 2-methoxyethanol (99.3%), and 1-propanol (99.9%) were acquired from Sigma-Aldrich. All chemicals were used as received from the suppliers without any further purification. Due to its high reactivity, zirconium(IV) n-propoxide was stored and handled in a water-free environment (
