CRYSTAL GROWTH & DESIGN
Hexagonal Nanodisks of Cadmium Hydroxide and Oxide with Nanoporous Structure Weidong Shi, Cheng Wang, Haishui Wang, and Hongjie Zhang* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
2006 VOL. 6, NO. 4 915-918
ReceiVed August 21, 2005; ReVised Manuscript ReceiVed February 15, 2006
ABSTRACT: In this work we demonstrate that hexagonal nanodisks of cadmium hydroxide with nanoporous structures could be fabricated by a facile hydrothermal treatment without using any templates or organic additives. With this method, the length of the hexagonal edge and thickness of the nanodisks can be adjusted through controlling the experimental conditions such as the pH value of the mother liquor and the initial concentration of the cadmium ion. On the basis of our experimental observations and understandings of the nanocrystal growth, the formation of the nanodisks is believed to mainly originate from the oriented attachment of small particles. Furthermore, the hexagonal Cd(OH)2 nanodisks can be converted to CdO semiconductors with similar morphology by calcinations. Introduction Anisotropic nanostructures can serve as building blocks to generate functional materials or device units with practical applications. These applications include energy conversion or storage, chemical and biological sensing, photonic and electronic apparatus, petroleum catalysis, etc.1 A number of anisotropic nanostructures, e.g. tubes, wires, rods, cubes, dendrites, disks, etc., have been fabricated through a variety of methods.2-6 To synthesize anisotropic structures, pioneering works made use of preshaped structures such as anodic porous alumina (APA), mesoporous silica, liquid crystalline phases, carbon nanotubes, etc. as templates so that the target materials could be “printed” from these templates.7-10 However, this method relies heavily on the templates and there are not many such templates; therefore, non-template-based approaches have been widely pursued in the past decade. Now it is realized that the morphology control can be accomplished by manipulating the nuclei formation and subsequent crystal growth through optimizing the synthetic conditions. Even so, the findings of most anisotropic nanostructures are still the serpentine bonus of tremendous efforts. In other words, the precise morphology control of nanomaterials and the following scale-up for industrial production remain a huge challenge for current research. In classic colloidal models, crystal growth can be categorized as either kinetically or thermodynamically controlled and is subject to Ostwald ripening during the synthetic courses. Usually it is the Ostwald ripening that determines the morphology of the final products, since it is believed that the initially formed nuclei are defect-free.11 During the Ostwald ripening process, larger particles tend to grow even larger at the expense of smaller particles, which means that the smaller particles dissolve into the mother liquor progressively. Specific morphology might evolve from this process, provided the synthetic conditions favor this particle evolution. Generally, nanoparticles have different crystal planes and each plane has a different surface energy. Those planes with high surface energy have a strong tendency to capture monomers from the mother liquor in order to reduce their surface energy. This leads to growth along those planes. The tendency varies according to the different crystal planes, * To whom correspondence should be addressed. Tel: +86-431-5262127. Fax: +86-431-5698041. E-mail:
[email protected].
eventually producing crystals with anisotropic morphology. Advantages of this approach could be, for example, that it is not necessary to use posttreatment in order to get rid of organic additives (described later) in practical applications and the method might be feasible for scale-up for industrial mass production.4 In addition to this, many additives, including both organic and inorganic species, have been investigated in liquidphase synthesis. These additives can bind to certain crystal planes and reduce the surface energy of these planes, which allows the crystals to grow along those planes that have weak binding capabilities with additives.5,6 The most widely used additives are organic surfactants such as CTAB, AOT, and organic amines. In this paper, we report the preparation of hexagonal cadmium hydroxide and oxide nanodisks with nanopores by optimizing the synthetic conditions without using any organic additives. Experimental Section Synthesis of Nanodisks. A 1.5 M NaOH aqueous solution was carefully added to a 0.01 M CdCl2 (18 mL) aqueous solution until the pH reached 14.0. A white colloidal suspension was formed progressively when the pH was less than 10.0. When the pH was greater than 10, a great deal of colloids appeared immediately. A 15 mL portion of the colloid solution thus obtained was transferred into a 20 mL Teflonlined stainless steel autoclave, which was then sealed. The autoclave was kept at 140 °C in an oven for hydrothermal treatment for 12 h. After that, the autoclave was cooled naturally to room temperature. The white precipitate (Cd(OH)2 products) was collected. After centrifugation, the supernatant alkaline solution was decanted, and then the products were washed three times with DD water and dried at 50 °C for 2 h. The conversion of the Cd(OH)2 thus obtained to CdO nanodisks was carried out in an oven in air at 350 °C for 3 h. Characterization. Characterization of the products was performed by X-ray diffraction (XRD, Rigaku-D/max 2500V with Cu KR radiation (λ ) 1.5418 Å)), transmission electron microscopy (TEM, JEOL JEM2010 with a tungsten filament at an accelerating voltage of 200 kV), and scanning electron microscopy (SEM, XL30 ESEM FEG). N2 adsorption-desorption isotherms were measured at liquid-nitrogen temperature (77 K) using a Quantachrome Instruments NONA 1000 instrument. Samples were degassed at 120 °C overnight before measurements. Specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) model, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett-Joyner-Halenda (BJH) model.
10.1021/cg050431z CCC: $33.50 © 2006 American Chemical Society Published on Web 03/14/2006
916 Crystal Growth & Design, Vol. 6, No. 4, 2006
Figure 1. XRD pattern of the Cd(OH)2 nanodisks obtained.
Figure 2. Low-magnification (a) and high-magnification (b) SEM images of Cd(OH)2 nanodisks and TEM images of a single Cd(OH)2 nanodisk parallel to the copper grid (c) and perpendicular to the copper grid (d).
Results and Discussion The prepared samples of cadmium hydroxide were characterized by X-ray diffractometry, and the pattern is shown in Figure 1. All the peaks in this figure can be readily indexed to a pure hexagonal phase of Cd(OH)2 (space group P3hm1 (No. 164)) with lattice constants a ) 3.495 Å and c ) 4.711 Å (JCPDS 31-0228). The morphologies of the product prepared by the procedure described in the Experimental Section are visualized by SEM and TEM, as shown in Figure 2. At lower magnification, the SEM image (Figure 2a) reveals that Cd(OH)2 products consist almost entirely of nanometer-scale disks with uniform size and well-defined hexagonal shape. A small portion (
