Standing porous ZnO nanoplate-built hollow microspheres and kinetically controlled dissolution/crystal growth mechanism
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iping Cai,a) Guozhong Wang, and Changhao Liang Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China (Received 16 October 2011; accepted 28 December 2011)
The standing porous nanoplate-built ZnO hollow microspheres with micro/nanostructure are fabricated based on a modified hydrothermal strategy, using citrate as structural director, and subsequent annealing treatment. The hollow spheres are composed of the vertically standing and cross-linked single crystalline porous nanoplates with the exposed surface of nonpolar (1010) planes. Experiments have revealed the structural evolution: the formation of amorphous spheres in the initial reaction stage, followed by surface crystallization and nanoplate outward growth accompanied by inward dissolution of the amorphous spheres. Citrate in the precursor solution plays a dominant role in the formation of such porous ZnO hollow spheres. A model is presented, based on citrate-induced amorphous sphere formation and kinetically controlled dissolution and crystal growth. The model describes the formation of the hollow spheres, thermodynamically and kinetically.
I. INTRODUCTION
The micro/nanostructured materials (microsized objects with nanostructures) are usually stable against aggregation. These materials present high activities and high specific surface areas.1 They have attracted much attention in the fields of adsorbents,2 catalysts,3 and sensors.4 ZnO is also an environmental friendly material whose surface has functional groups, such as hydroxyl groups, which can be the active sites for adsorption.5,6 ZnO with micro/ nanostructures could thus be a good material for environmental treatment. Micro/nanostructured ZnO shows structurally enhanced photocatalytic and adsorptive performances compared with the conventional ZnO nanomaterials.3,7 For instance, Lu et al.8 prepared threedimensional Ag/ZnO hollow spheres, which showed significantly enhanced photocatalytic effect. Our previous work has demonstrated that pyramid-like hierarchical micro/ nanostructured ZnO had much stronger photocatalytic effect than the ZnO nanoneedles, nanoparticles, nanosheets, and even TiO2 (P25).3 In addition, the porous plate-like micro/nanostructured ZnO showed significantly enhanced adsorption performance as compared to the commercial a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.15 J. Mater. Res., Vol. 27, No. 6, Mar 28, 2012
ZnO nanoparticles.7 Recently, micro/nanostructured ZnO hollow objects, such as flower-like,9 tube-like,10 coralline-like11 and tabour-like,12 etc., have attracted attention due to their unique hierarchical structures. The formation of the hollow structures was attributed to conventional hard templates,13 galvanic replacement,14,15 Kirkendall effect,16 micelle templating,17 and Ostwald ripening.18 Herein, we report a novel hollow micro/nanostructure, or the porous nanoplate-assembled micro/n
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