1 To whom correspondence should be addressed. Fax: (091) 3222-55303. E-mail: sram@matsc.iitkgp.ernet.in. Journal of Solid State Chemistry 157, 40 } 49 (2001) doi:10.1006/jssc.2000.9034, available online at http://www.idealibrary.com on X-Ray Diffraction and IR Spectrum for Activated Surface Hydrolysis of Al Metal into AlO(OH) ' aH 2 O Nanocrystals in a New Monoclinic Crystal Structure S. Rana and S. Ram1 Materials Science Centre, Indian Institute of Technology, Kharagpur-721 302, India Received July 5, 2000; in revised form October 27, 2000; accepted December 1, 2000 Activated spontaneous surface hydrolysis of nascent Al metal in distilled water at room temperature results in AlO(OH) ' aH 2 O (boehmite) nanocrystals of an average 35 nm diameter. The nanocrystals are separated by 2 to 5 nm with a fairly sharp size distribution in the electron micrograph. Their X-ray di4rac- tion is analyzed by assuming a monoclinic crystal structure with lattice parameters a 5 0.866 nm, b 5 0.506 nm, c 5 0.983 nm, and b 5 94.563. It is a new structure of boehmite in comparison to the orthorhombic bulk structure. It involves 3.3 times the lattice volume in an orthorhombic lattice enclosed in a smaller surface area, S 5 3.7 3 10 3 m 2 / g, as compared to 4.5 3 10 3 m 2 / g in it. This structure forms in a fast Al surface hydrolysis reaction in this example because it undergoes a fast release of its total surface energy as a function of its fast growth. The molecular vibrational structure of AlO(OH) ' aH 2 O nanocrystals is ana- lyzed from their IR spectrum which has three characteristic bandgroups at 200 +1200 cm 21 , 1300 +2500 cm 21 , and 2800 + 4000 cm 21 . A considerably improved value of the O+H group stretching frequency by as much as 360 cm 21 relative to the bulk value appears for a proton localized structure in the nanocrystals. ( 2001 Academic Press Key Words: X-ray di4raction analysis; IR analysis; AlO(OH) ' aH 2 O nanocrystals; surface hydrolysis of Al metal; monoclinic boehmite crystal structure. INTRODUCTION AlO(OH) ' aH 2 O (boehmite) is one of the most important raw materials of aluminum and its oxides. It is the main component of many kinds of bauxites, and regarding the utility of the latter, the structure and properties of the boehmite component play a determining role. Moreover, synthetic boehmite is used in synthesizing electronically pure transition aluminas or sapphire and Al 2 O 3 composites (1}5). Its microstructure, i.e., particle morphology, size, and separation between particles, plays a major role in monitor- ing the microstructure and other properties of the product according to experimental conditions. An AlO(OH) ' aH 2 O precursor of con"ned particle size on a nanometer scale is useful for deriving a nanocrystalline or mesoporous Al 2 O 3 powder by its controlled thermal decomposition at moder- ate temperature. Today, the two classes of materials are used extensively as heterogeneous catalysts, as absorption media, and in several other optical and electronic devices and components (1}8). Their utility is manifested in their re"ned microstructures which allow molecules access to large internal surface areas and cavities that enhance catalytic activity and adsorption properties. Mesoporous materials are typically amorphous or nanocrystalline as per their con"ned dimension of particles and pores on a nanometer scale. Several methods, e.g., spray pyrolysis (1), gas condensa- tion (2), adiabatic combustion of Al3‘ salts (3), sol}gel or alkoxide method (4, 9), and surface hydrolysis of Al metal (5), are available for preparing a precursor of AlO(OH) ' aH 2 O. A special advantage of the last method, which is used here, over the other methods is that it easily provides separ- ated AlO(OH) ' aH 2 O particles of a controlled size as small as 20 to 50 nm. They are in a new monoclinic crystal structure in comparison to the D17 2h orthorhombic bulk crystal structure (9 }12). In this article, we report a structural analysis of AlO(OH) ' aH 2 O nanocrystals in monoclinic crystal struc- ture using X-ray di!raction and infrared spectrum. The results are discussed in correlation with X-ray di!raction (9 }12) and IR or Raman spectroscopy (11}13) of the bulk sample. A con"ned AlO(OH) ' aH 2 O structure in nanocrys- tals results in a modi"ed IR spectrum of the bulk sample. It presents a proton localized structure with an improved value of O}H group stretching frequency by as much as 360 cm~1 over the bulk value. 40 0022-4596/01 $35.00 Copyright ( 2001 by Academic Press All rights of reproduction in any form reserved.