14516 Phys. Chem. Chem. Phys., 2011, 13, 14516–14522 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 14516–14522 Synthesis of Ge-imogolite: influence of the hydrolysis ratio on the structure of the nanotubes C. Levard, ab A. Masion,* ab J. Rose, ab E. Doelsch, bc D. Borschneck, ab L. Olivi, d P. Chaurand, ab C. Dominici, e F. Ziarelli, f A. Thill, bg P. Maillet bg and J. Y. Bottero ab Received 9th February 2011, Accepted 18th May 2011 DOI: 10.1039/c1cp20346k The synthesis protocol for Ge-imogolite (aluminogermanate nanotubes) consists of 3 main steps: base hydrolysis of a solution of aluminum and germanium monomers, stabilization of the suspension and heating at 95 1C. The successful synthesis of these nanotubes was found to be sensitive to the hydrolysis step. The impact of the hydrolysis ratio (from n OH /n Al = 0.5 to 3) on the final product structure was examined using a combination of characterization tools. Thus, key hydrolysis ratios were identified: n OH /n Al = 1.5 for the formation of nanotubes with structural defects, n OH /n Al = 2 for the synthesis of a well crystallized Ge imogolite and n OH /n Al 4 2.5 where nanotube formation is hindered. The capability of controlling the degree of the nanotube’s crystallinity opens up interesting opportunities in regard to new potential applications. Introduction Imogolites (Al 2 SiO 3 (OH) 4 ) are natural aluminosilicate nano- tubes. These nanotubes have inner and outer diameters of 1 and 2 nm, respectively, and their lengths range from a few tens of nanometres to several hundred nanometres. Imogolite is composed of a curved gibbsite (Al(OH) 3 ) layer on the outer surface and Si monomers linked to 6 Al octahedra inside the tube. 1 The first synthesis of imogolite was performed over 30 years ago. 2 The first step consisted of adjusting the pH of a dilute solution of non-complexing salts of aluminum and orthosilicic acid (n Al /n Si = 2; [Al] = 0.002 M) to pH 5 by the drop wise addition of sodium hydroxide (NaOH, 0.1 mol L À1 ). The second step was reacidification to pH 4.5 by the addition of acetic acid (CH 3 COOH, 0.1 mol L À1 ), resulting in the formation of imogolite precursors called proto-imogolite. Finally, the solution was heated at 96–100 1C, leading to the imogolite structure after 5 days. For an initial Al/Si molar ratio of 2, a hydrolysis ratio R (R = n OH /n Al ) of 2 to 2.8 was identified as optimal for the synthesis of imogolite, deviation from this range leading to the synthesis of allophane. 3 Allophanes are aluminosilicates with a chemical composition identical to imogolite but with a different structure. 4 Allophanes are described as hollow nanospheres with diameters ranging from 3–5 nm. In dilute systems (millimolar concentration), the hydrolysis ratio appears as the key factor determining the structure of the final formed species. With the fast expansion of nanotechnologies, there is a strong interest in the synthesis of nanotubes for their potentially interesting properties such as gas storage 5 and catalysis 6 ... In the case of imogolite, standard experimental conditions lead to the synthesis of only very modest amounts, thereby impeding its possible use in industrial applications. To increase the yield, the synthesis has been performed following the same procedure described above, with the initial reagent concentrations increased by two orders of magnitude (decimolar concentrations). The synthesis of imogolite under these conditions failed, 7 and allophanes have been synthesized instead regardless of the hydrolysis ratio. 8,9 However, a recent report describes the synthesis of imogolite from concentrated solutions at the cost of a considerably lengthened growth period. 10 A possible explanation that can account for the slower kinetics observed with decimolar concentrations is the precipitation of SiO 2 . The growth kinetics are then controlled by the solubility of this secondary phase. Analogue structures, in which Si has been substituted by Ge, have also been examined. Ge-imogolite has been successfully synthesized from dilute (millimolar) reagents: the structure is identical to imogolite except for an enlarged diameter (3.3 nm) a CEREGE, Aix-Marseille University, CNRS, IRD, Colle `ge de France, Europo ˆle Me ´diterrane ´en de L’Arbois, BP 80, 13545 Aix-en-Provence, France. E-mail: masion@cerege.fr b International Consortium for the Environmental Implications of Nanotechnology iCEINT, Europo ˆle de l’Arbois, 13545 Aix-en-Provence, France c CIRAD, UPR Recyclage et risque, F-34398 Montpellier, France d ELETTRA, Synchrotron Light Source, 34012 Trieste, Italy e CP2M, Aix-Marseille University, Avenue Escadrille Normandie Niemen, 13397 Marseille, France f Spectropole, Aix-Marseille University, Fe´de´ration des Sciences Chimiques CNRS-FR1739, av. Escadrille Normandie Nie ´men, 13397 Marseille cedex 20, France g CEA Saclay, IRAMIS, LIONS, 91191 Gif sur Yvette, France PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Stanford University on 12 November 2011 Published on 12 July 2011 on http://pubs.rsc.org | doi:10.1039/C1CP20346K View Online / Journal Homepage / Table of Contents for this issue