Morphology evolution of NaTaO3 submicrometer single-crystals: from cubes to quasi-spheres

ARTICLES SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 10 April 2015 | doi: 10.1007/s40843-015-0041-6 Sci China Mater 2015, 58: 281–288 Morphology evolution of NaTaO3 submicrometer single-crystals: from cubes to quasi-spheres Wei Chen, Qin Kuang* and Zhaoxiong Xie* Surface structure control of functional nano-/micro-crystallites has attracted great attention because many important physicochemical properties depend on their surface. Guided by the supersaturation-dependent surface structure evolution strategy we proposed recently, NaTaO3 submicrometer crystals with morphologies of cubes, corner truncated cubes, edge and corner truncated cubes, and quasi-spheres can be synthesized by changing the volume ratio of ethylene glycol to water and the amount of NaOH in the composite solvent. Under low supersaturation condition, NaTaO3 cubic crystals with low energy {100} facets were obtained. As the supersaturation increases, the corners and edges of NaTaO3 cubic crystals, which possess higher surface energy, were gradually truncated. Surprisingly, quasi-sphere crystallites formed under extremely high supersaturation condition, which is difficult to be explained by the classical crystal growth theories. By analyzing the formation work of two-dimension crystal nuclei, we concluded that the crystal growth tend to be isotropic at extremely high supersaturation, which well explained the formation of the quasi-sphere crystallites. INTRODUCTION In the past decade, many efforts have been devoted to controlling the crystal surface structures for specific purpose, since many physicochemical properties of crystals strongly depend on their exposed surface [1–5]. Despite the great success in morphology-controlled syntheses of nanocrystals, our comprehension of the formation mechanism of specific surface of crystals in nanoscale is far from enough. Especially, it is still a great challenge to explore some universal, precise surface-controlling strategies that apply to most crystalline materials from metallic to ionic and even molecular crystals. Establishing the mechanistic proposals on the basis of thermodynamics and kinetics principles and concepts, which are universal for the growth of macroscopic and microscopic crystals, is the first step towards a rational design and systematic approach [6,7]. Recently, based on thermodynamics and the Thomson−Gibbs equation, we concluded that the surface energy of crystal face is in proportion to the supersaturation of crystal growth units during the crystal growth, which can effectively guide the control of the surface structure of some crystals [8]. NaTaO3 is a semiconducting metal oxide with perovskite structure and it has important applications in many fields such as photocatalysis and sensing [9−12]. To enhance the performance of NaTaO3 in the applications, many improvement strategies, by which the bulk or surface structures of NaTaO3 can be modified reasonably, have been developed to date [9,13−16]. Both photocatalysis and sensing occur on the surface of the functional materials, so the performance of NaTaO3 can be significantly enhanced through optimizing the surface structure of NaTaO3 crystals. For example, Jiang et al. [17] reported that NaTaO3 cubic nanocrystals, which possessed very high effective surface area by self-assembling into mesocrystal-like architectures, showed outstanding photocatalytic performance for water splitting. Unfortunately, rare success has been made in rationally engineering specific surface of NaTaO3 crystals. No matter in macroscopic scale or in nanoscale, NaTaO3 crystals mostly grow up into cubic or quasi-cubic morphologies enclosed with {100} facets [10,16−18]. This growth habit of NaTaO3 is inherently determined by the perovskite-type crystals in which {100} facets have the lowest surface energy and lead to the best thermodynamic stability. Therefore it is desirable to study how to synthesize morphology controllable NaTaO3 crystals with specific surface structures and make it effective in improving the performance of NaTaO3-based materials. In this paper, guided by the supersaturation dependent surface evolution strategy, we successfully controlled the growth of perovskite NaTaO3 crystals via controlling the supersaturation by tuning ratios of ethylene glycol (EG)/ water composite solvents and concentrations of NaOH in the reaction solution. It was found that the morphologies of NaTaO3 crystals evolved from cubes to corner truncated cubes, edge and corner truncated cubes, and finally to quasi-spheres with increasing the supersaturation. Noticeably, the quasi-sphere shape of single crystal particles obtained at high supersaturation condition cannot be explained by the classical crystal growth theories, such as the Wulff con- State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China * Corresponding authors (emails: (Xie Z); (Kuang Q)) 281 April 2015 | Vol.58 No.4 © Science China Press and Springer-Verlag Berlin Heidelberg 2015 ARTICLES SCIENCE CHINA Materials struction theorem. We found that the formation work of two-dimensional crystal nuclei during the crystal growth can well explain the growth of quasi-sphere shape of single crystal. EXPERIMENTAL Chemicals Tantalum (V) oxide (Ta2O5, 99.99%) was purchased from Beijing InnoChem Science & Technology Co., Ltd. Ammonium hydroxide (NH3·H2O, 25%), hydrofluoric acid (HF, 40%) and ethylene glycol (EG, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH, 96%) was purchased from Guanghua Sci-Tech Co., Ltd. All the reagents were used without further purification. Syntheses of NaTaO3 Submicrometer Crystals Commercial Ta2O5 was pre-treated by HF aqueous solution (40%) according to the previously reported procedure [19]. Firstly, 0.50 g Ta2O5 and 3.0 mL of HF aqueous solution were mixed in a Teflon-lined autoclave with a capacity of 20 mL. Then the autoclave was sealed and heated at 140°C for 60 min to dissolve Ta2O5. After being cooled to room temperature naturally, 20.0 mL of NH3·H2O was dropwise added into the resulting mixture, and then white precipitates generated. Finally, the white precipitates were collected by centrifugation, washed by deionized water and ethanol for several times, and dried in oven at 60°C. This product was named as Ta2O5·nH2O and used as the precursor of NaTaO3 submicrometer crystals. NaTaO3 submicrometer crystals with different morphologies were prepared in the composite solvent of EG and water with different ratios. In a typical synthesis of cubic NaTaO3 crystals, 0.010 g Ta2O5·nH2O and 0.40 g (10 mmol) NaOH were dispersed in 1.0 mL of EG and 13.0 mL of deionized water by ultrasonication. Then, the mixture was transferred into a Teflon-lined autoclave of 20 mL, sealed and heated at 200°C for 12 h before it was (...truncated)