3 April 1998 Ž . Chemical Physics Letters 286 1998 88–92 The ‘silica garden’: a hierarcharical nanostructure Craig Collins a , Wuzong Zhou a , Alan L. Mackay b , Jacek Klinowski a, ) a Department of Chemistry, UniÕersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK b Department of Crystallography, Birkbeck College, UniÕersity of London, Malet Street, London WC1E 7HX, UK Received 25 November 1997; in final form 15 January 1998 Abstract The bulk of the ‘silica garden’, made by seeding a solution of sodium silicate with crystals of aluminium nitrate, is an Ž amorphous aluminosilicate. Transmission electron microscopy reveals a hierarchic substructure involving fine tubes f44 . nm in diameter composed of amorphous silica rods. It is also possible that the macroscopic fibres seen by scanning electron microscopy contain periodic arrays of microtubules f4.4 mm in diameter. q 1998 Elsevier Science B.V. 1. Introduction The ‘silica garden’ is the spectacular tubular growth obtained when an aqueous solution of sodium Ž . silicate water glass is seeded with crystals of cer- w x tain water-soluble metal salts 1–7 . The experiment has long been used to excite the interest of the non-scientific public in chemistry, and is included in most young chemist sets, under such names as ‘crystal wonder’. The first written reference to the Ž . silica garden is an account by Glauber 1684 of an ‘iron tree’ produced with crystals of ferrous sulphate wx wx 8 . Clunies Ross 1 , who described the growth of the silica garden using various metals salts, could not explain the diversity of form the gardens assume, but suggested that the process is driven by osmotic pressure, a hypothesis which has since been con- w x firmed 3–7 . As soon as the salt crystal is dropped into the silicate solution, a semipermeable gel mem- brane, probably composed of a hydrous metal silicate wx 5 , is formed around it. Osmotic pressure drives ) Corresponding author. E-mail: jk18@cam.ac.uk water across the membrane, leading to the formation of colloidal hydroxide on the crystal side and silica wx gel on the solution side 5 . The membrane swells and then bursts open, ejecting concentrated salt solu- tion into the silicate, which immediately forms a precipitate. The tip of the resulting shoot usually contains a small bubble of trapped gas, probably air wx 2 . The membrane acts as an osmotic pump, with the salt solution flowing up the inside of the fibre until the crystal is completely dissolved. We have examined in detail the growth, morphology and com- Ž . position of the silica garden made using Al NO . 3 3 2. Experimental Sodium silicate and aluminium nitrate nonahy- Ž . drate, Al NO P 9H O, were obtained from Aldrich. 3 3 2 Ž . Distilled water 50 ml was added to 20 ml of 6.4 M Ž . with respect to silica sodium silicate at room tem- perature in 100 ml glass beakers 5 cm in diameter under vigorous stirring. The pH of the solution was Ž . Ž . f 12. Al NO P 9H O 4g was then seeded into 3 3 2 Ž . the solution so that the silicate was in excess and 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0009-2614 98 00081-5