Functionalization of Halloysite Clay Nanotubes by Grafting with γ-Aminopropyltriethoxysilane Peng Yuan, †,‡ Peter D. Southon, ‡ Zongwen Liu, § Malcolm E. R. Green, ‡ James M. Hook, | Sarah J. Antill, ‡ and Cameron J. Kepert* ,‡ Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, School of Chemistry, The UniVersity of Sydney, New South Wales 2006, Australia, Australian Key Centre for Microscopy and Microanalysis, The UniVersity of Sydney, New South Wales 2006, Australia, and School of Chemistry, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia ReceiVed: June 27, 2008; ReVised Manuscript ReceiVed: August 8, 2008 Surface modification of natural halloysite clay nanotubes with γ-aminopropyltriethoxysilane (APTES) was investigated. Untreated and modified samples were characterized by nitrogen adsorption, X-ray diffraction, elemental analysis, thermogravimetry, transmission electron microscopy, atomic force microscopy, MAS nuclear magnetic resonance ( 29 Si, 13 C, 29 Al), and Fourier transform infrared spectroscopy. The modification mechanism was found to include not only the direct grafting of APTES onto the hydroxyl groups of the internal walls, edges and external surfaces of the nanotubes but other processes in which oligomerized APTES condensed with the directly grafted APTES to form a cross-linked structure. The thermal and evacuation pretreatment conditions were found to play an important role in controlling the extent and mechanism of the modification. The extent of modification is also strongly affected by the morphological parameters of the original clay samples. This study demonstrates that the surface chemistry of halloysite nanotubes is readily modified, enabling applications in nanocomposites, enzyme immobilization and controlled release. 1. Introduction The modification of oxide surfaces by coupling with func- tionalized organosilanes is applicable to the fields of catalysis, adsorption, electrochemistry, chromatography and nanocom- posite materials. Of particular interest is the use of ordered mesoporous silica materials such as SBA-15 1 and MCM-41 2 as supports for enzyme immobilization, heavy metal adsorption, and heterogeneous catalysis. 3 The mechanisms and applications of the interaction between organosilanes and mesoporous materials have been widely discussed and reviewed. 4-6 In recent years, the functionalization of clay minerals with organosilane has attracted considerable interest, due mainly to the increasing requirements from the area of polymer-clay nanocomposites. 7 Functionalization of clays with organosilanes has been explored as a way to improve clay dispersal in a polymer matrix, thus increasing the mechanical properties of resultant polymer-clay nanocomposites. Further, the binding properties of organosilane-functionalized clays have been investigated for the treatment of heavy metal contaminants. 8,9 The clay minerals investigated are principally lamellar clays, most notably laponite, 10,11 smectite, 12 kaolinite, 13,14 and montmo- rillonite, 15,16 and include also the tubular aluminosilicate clay, imogolite. 17 In the present study, organosilane functionalization of another tubular clay material, halloysite (Al 2 (OH) 4 Si 2 O 5 · 2H 2 O), was investigated. As a naturally occurring hydrated polymorph of kaolinite, halloysite has a similar structure and composition, but the unit layers are separated by a monolayer of water mol- ecules. 18 As a result, hydrated halloysite has a basal spacing (d 001 ) of 10 Å, which is ∼3 Å larger than that of kaolinite. The interlayer water is weakly held, so that halloysite-(10 Å) can readily transform to halloysite-(7 Å) (also known as metahal- loysite) by dehydration. 19,20 Tubular halloysite has a highly unusual meso/macroscopic superstructure, which results from the wrapping of the clay layers around onto themselves to form hollow cylinders under favorable geological conditions. This wrapping process is driven by a mismatch in the periodicity between the oxygen sharing tetrahedral SiO 4 sheets and adjacent octahedral A1O 6 sheets in the 1:1 layer. 21-24 In each halloysite nanotube (HNT), the external surface is composed of siloxane (Si-O-Si) groups, whereas the internal surface consists of a gibbsite-like array of aluminol (Al-OH) groups. The schematic representations 21 of the crystalline structure of halloysite-(10 Å) and the structure of a single tubular halloysite particle is shown in Figure 1. In contrast to kaolinite, very little is known of the chemical and physical properties of halloysite, and there are few applica- tions that aim to utilize the beautiful hollow tubular structure of this mineral. This is despite considerable interest in a range of other synthetic nanotubular solids such as carbon nanotubes (CNTs), and despite the existence of significant deposits in Australia, China, Guyana, Mexico and Brazil. 20 Generally, the length of halloysite nanotubes varies from ca. 0.02 to 30 µm 20 and the external diameter from ca. 30 to 190 nm, with an internal diameter range of ca. 10-100 nm. 21,25 These dimensions differ considerably from those of imogolite (0.25-0.35 µm in length and ∼0.82 nm in internal diameter). The very large diameter of the halloysite lumen makes it potentially suitable for the accommodation of a range of guests. For instance, halloysite has been shown to be capable of entrapping and subsequently releasing small pharmaceutical * To whom correspondence should be addressed. Phone: +61 2 9351 5741. Fax: +61 2 9351 3329. E-mail: c.kepert@chem.usyd.edu.au. † Chinese Academy of Sciences. ‡ School of Chemistry, The University of Sydney. § Australian Key Centre for Microscopy and Microanalysis, The Uni- versity of Sydney. | The University of New South Wales. J. Phys. Chem. C 2008, 112, 15742–15751 15742 10.1021/jp805657t CCC: $40.75  2008 American Chemical Society Published on Web 09/12/2008