Tube Formation in Nanoscale Materials

Nanoscale Res Lett (2008) 3:473–480 DOI 10.1007/s11671-008-9193-6 NANO REVIEW Tube Formation in Nanoscale Materials Chenglin Yan Æ Jun Liu Æ Fei Liu Æ Junshu Wu Æ Kun Gao Æ Dongfeng Xue Received: 10 October 2008 / Accepted: 17 October 2008 / Published online: 4 November 2008 Ó to the authors 2008 Abstract The formation of tubular nanostructures normally requires layered, anisotropic, or pseudo-layered crystal structures, while inorganic compounds typically do not possess such structures, inorganic nanotubes thus have been a hot topic in the past decade. In this article, we review recent research activities on nanotubes fabrication and focus on three novel synthetic strategies for generating nanotubes from inorganic materials that do not have a layered structure. Specifically, thermal oxidation method based on gas–solid reaction to porous CuO nanotubes has been successfully established, semiconductor ZnS and Nb2O5 nanotubes have been prepared by employing sacrificial template strategy based on liquid–solid reaction, and an in situ template method has been developed for the preparation of ZnO taper tubes through a chemical etching reaction. We have described the nanotube formation processes and illustrated the detailed key factors during their growth. The proposed mechanisms are presented for nanotube fabrication and the important pioneering studies are discussed on the rational design and fabrication of functional materials with tubular structures. It is the intention of this contribution to provide a brief account of these research activities. Keywords Nanotubes
Chemical synthesis
Nanostructures
Inorganic materials C. Yan
J. Liu
F. Liu
J. Wu
K. Gao
D. Xue (&) State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China e-mail: Introduction Recently, considerable attention has been focused on micro- and nanostructured materials due to their unique properties and potential applications in many aspects [1–5], among which nanotubes have been attracting special interests since Iijima’s identification of carbon nanotubes [5]. The tubular form is particularly attractive because it provides access to three different contact regions, inner and outer surfaces as well as both ends. However, for a long time the nanotube formation is generally limited to layered materials, through the bending of thin crystal flakes. Due to the weakness of interlayer interactions (van der Waals forces) and to the dangling bonds that can be eliminated by interlayer covalent bonds, nanotubes formation is very analogous to the case of carbon nanotubes based on a ‘‘rolling-up’’ mechanism [5]. A number of studies have been devoted to generating nanotubes from most kinds of materials [6–8], which clearly indicate that solid materials can be prepared as nanotubes by properly selecting proper preparation methods, for example, BN, V2O5, NiCl2, TiO2, and other materials with tubular structures [9–14]. Inorganic tubular structures become a symbol of the new and fast-developing research area due to their tremendous applications for over a decade. Inorganic nanotubes are less well studied, in part due to difficulties in well controlling their dimensions [15]. However, inorganic nanotubes still share many advantages of carbon nanotubes and can match increasing demand for various functions. Non-carbon materials [16], for example, titania nanotubes have been studied and show improved properties compared to colloidal or other forms of titania for applications in photocatalysis [17, 18], sensing [19], and photovoltaics [20, 21]. The past couple of decades have witnessed an exponential growth of activities in the synthesis of nanotubes, 123 474 driven by both excitement of understanding new science and the potential hope for applications and economic impacts. The numerous potential applications of inorganic nanotubes have been highlighted in a number of recent studies [17–21]. The present article reviews the classical methods and some recent contributions to the synthesis of nanotubes from inorganic materials that do not contain layered structure. We explicitly describe three different approaches for fabrication of tubular nanostructures, each approach is highlighted by at least one example. Classical Preparation Methods Rolling of Layered Materials for the Formation of Nanotubes It is widely accepted that solid materials from layered precursors can be prepared as nanotubes by carefully controlling experimental conditions, based on a ‘‘rolling-up’’ mechanism. Two-dimensional layered compounds such as WS2 [22], MoS2 [23], and other structural analogues either roll up to form nanoscrolls or grow in rolled-up form, resulting in formation of single-wall or multi-wall nanotubes in gas atmosphere. The driving force lies in the builtin asymmetry of the unit cell along one zone axis and the thermal stress existing at high temperature, which initiates the scrolling of the layered sheets with reduced interlayer forces at the edges. Figure 1 is the model showing the process for the scrolling formation mechanism. Similarly to the gas-action route, there have been significant research efforts devoted to nanotubes of layered or anisotropic crystal structured materials in solution, including WO3
H2O [24], Cu(OH)2 [25], SrAl2O4 [26], CeO2 [27], and CeO2-x [28]. The bending and roll-up of a thin layer to form tube is a thermally driven process. From a kinetic viewpoint, the rolling of layered structure may be initiated by a stress of either a structure or an electrical nature caused by the asymmetry of the layer. Though many nanotubes of layered or artificial lamellar structures have been successfully achieved, this strategy cannot be applied to non-layered materials. Fig. 1 Schematic illustration of nanotubes via rolling layered materials. a Formation of nanoplatelet. b An extension of reaction time results in the appearance of nanoscroll. c Nanotube formation through rolling nanoplatelet 123 Nanoscale Res Lett (2008) 3:473–480 Hard Templating Route for the Formation of Nanotubes Templating approach is an important method to fabricate inorganic hollow tubes via high-temperature process [29–32]. The graphical representation of formation process of tubular structures is show in Fig. 2. Chemical vapor deposition (CVD), atomic layer deposition (ALD), and other vapor phase deposition techniques have been successfully employed to create conformal coating against existing templates. After the formation of core-sheath structures, the templates can be selectively removed by different chemical reactivities of core and shell components. Yang et al. employed the first ‘‘eptitaxial casting’’ process to synthesize single-crystalline GaN nanotubes [33]. As illustrated in Fig. 2, ZnO nanowires are used as template for the deposition of GaN thin films using metalorganic CVD. ZnO nanowires can be (...truncated)