Precipitation of ZnO Powders from Aqueous Solutions with Water-Soluble Polymers Y.-F. Gao,*,†,‡ H.-Y. Miao,§ H.-J. Luo,† and M. Nagai‡ Research Center for Industrial Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Changning, Shanghai 200050, China, AdVanced Research Laboratories, Musashi Institute of Technology, Tokyo 158-0082, Japan, and School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2187–2193
ReceiVed NoVember 10, 2007; ReVised Manuscript ReceiVed April 4, 2008
ABSTRACT: ZnO powders were synthesized in the presence of water-soluble polymers. These polymers have effects on the nucleation and growth of ZnO crystals in solution, resulting in difference in crystalline morphologies. The role of polymers is 2-fold: first, indiscriminating adsorption of Zn(II) ions onto the polymer surfaces, which leads to the site-specific supersaturation with respect to the nucleation of ZnO, and secondarily, morphological development by interactions of zinc species with polymers and their hydrolyzed products. The incorporation of polymers into crystals was also confirmed experimentally. After annealing, mesopores were present in powders. This study shows the possibility to synthesize inorganic/organic composites with special morphologies, and suggests a novel process to optimize structures for the achievement of advanced functions. Introduction Organic scaffolds play an important role in the biomineralization process to form biomineral crystals often with complex morphologies and well-defined crystalline features. Macromolecules and surfactants can bind to nanocrystal surfaces to produce inorganic-organic particles with shape and size specificity that can act as hybrid building blocks in aggregationbased pathways of crystal growth.1 In these studies, some traditional compounds such as CaCO3, BaSO4, BaCrO4 have been usually used as models.1–6 Organic additives such as citric acid,7 surfactants8 and watersoluble polymers9–16 have been involved in the crystallization of ZnO in solution. These additives can change the crystallization habits, resulting in the production of hierarchical structures. Biomimetic processes that use organic additives (soluble polymers or amino acids) to modify the growth of ZnO can develop various ZnO crystals from solution.9–16 In a pioneering work, water-soluble poly(ethylene oxide)-block-poly(methylacrylic acid) (PEO-b-PMAA) and poly(ethylene oxide-blockstyrene sulfonic acid) (P(EO-b-SSH)) were used as an admixture in the crystallization of zinc oxide from an aqueous solution of a zinc salt.9–12 The synergetic effects of the functional groups PEO (as solution blocks) and PMAA (as anchor blocks) lead to great modification in crystallization habits, as seen in the changes in morphology, crystalline orientation and size/size distribution of the final precipitates.9–12 Peng et al. described a novel route to the synthesis of ZnO nanorings, disks and disklike crystals on a large scale by using polymers as modifiers for crystal growth.13 The ZnO particles show morphologies similar to that synthesized by using an anionic surfactant as a template.14 Imai et al. conducted systematic and comprehensive studies on the self-organization of hierarchically structured inorganic crystals including ZnO in artificially mimicking biomineralization systems, with polymeric molecules such as a gel matrix and soluble anionic polymers.15 Bauermann et al. proposed a biofriendly process * Corresponding author. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Musashi Institute of Technology. § Harbin Institute of Technology.
to prepare ZnO nanocrystals in an aqueous solution of zinc nitrate with a buffer tris(hydroxymethyl)aminomethane and dissolved gelatin as the organic matrix at a pH close to neutral and 37 °C.16,17 The process yields either hexagonal plate16 or spherical ZnO nanocrystals.17 Hoffman et al. also suggested that the water-soluble copolymers are favorable for the formation of homogeneous, adhesive ZnO thin films and suppress the formation of undesired, larger zincite crystals.18 The polymers here are believed to have influences on the nucleation, growth and aggregation processes during ZnO formation. As discussed above, the role of polymers is considered primarily to change the growth rate along some crystallographic directions,1–7 or even acts as binder to construct hierarchical structures such as mesocrystals by using preformed particles as building blocks.1–5 The polymer is commonly considered to be adsorbed to different crystallographic planes of ZnO nuclei and/or nanocrystals, which changes the subsequent nucleation and growth rate along specific crystalline orientations, and should be responsible for the generation of the hierarchical structures. In general the polymers are considered to play their roles after the nucleation. However, considering the fundamental adsorption ability of inorganic species to polymer surfaces, the polymer may have more important effects than that being expected commonly, especially in the early stage of reactions. We have observed previously that the polymer affects the changes in the degree of supersaturation, therefore shortening the induction period in regard to nucleation, and increasing the growth rate of crystals. A similar phenomenon was also found by Taubert previously.10 These results suggest that the polymer induces the site-specific reaching of supersaturation and nucleation around polymer chains in the initial stage of reaction. The effects on the initial nucleation of ZnO should have a significant impact on further growth, which can result in the formation of different morphologies. In this paper, we report how polymers affect the growth, crystalline properties and morphologies of ZnO powders, and the incorporation of polymers into crystals, forming inorganic-organic composites. Because both materials choices and solution conditions have effects on the nucleation/growth of ZnO in solution, as an initial step to understand the role of polymers in the nucleation stage, two kinds of polymers with different physicochemical properties
10.1021/cg701112x CCC: $40.75 2008 American Chemical Society Published on Web 06/05/2008
2188 Crystal Growth & Design, Vol. 8, No. 7, 2008
Gao et al.
Figure 1. SEM graphs of ZnO powders obtained by collecting in solution with PAM-PAA as additive after reactions at 90 °C for 28 h. NH3 amount: (A) 0 mL; (B) 2 mL; (C) 5 mL; (D) 7 mL. The scale bars for A-D are 3.33, 1.5, 3, 6 µm, respectively.
were used in this study: poly(acrylamide-co-acrylic acid) partial sodium salt (PAM-PAA) and polyacrylamide (PAM) solution (50 wt % solution in water). Polyacrylamide contains both carbonyl and amide groups along its hydrophobic backbone. Polyacrylamide and its various anionic or cationic copolymers along with derivatives form one of the most popular series of water-soluble polymers in various industrial applications. In an aqueous solution PAM exhibits electroneutrality, while PAMPAA is anionic. Polyacrylamide is a usual additive in the watercleaning industry partially because it can adsorb many impurities in water. The difference in physics and chemistry of these two polymers should have variable effects on the nucleation and growth of ZnO in solution.
strongly stirred for about 20 min. Then, to these solutions an appropriate amount of NH3 was added with stirring. For the solutions with 5 and 7 mL of NH3, they became completely transparent after stirring. And then, to these solutions an appropriate amount of polymers was added. F-doped SnO2 (FTO, 2 × 2 cm2, resistivity:
