FEATURE ARTICLE Nanocrystals as Stoichiometric Reagents with Unique Surface Chemistry Kenneth J. Klabunde,* Jane Stark, Olga Koper, Cathy Mohs, Dong G. Park, 1 Shawn Decker, Yan Jiang, Isabelle Lagadic, and Dajie Zhang Department of Chemistry, Kansas State UniVersity, Manhattan, Kansas 66506 ReceiVed: January 12, 1996; In Final Form: March 28, 1996 X Nanocrystals of MgO and CaO have been prepared by a modified aerogel/hypercritical drying/dehydration method. For nanocrystalline MgO (AP-MgO) surface areas ranged from 250 to 500 m 2 /g, whereas for AP- CaO 100-160 m 2 /g. These materials have been compared with more conventional (CP) microcrystalline samples of lower surface area with regard to (1) morphology (AP-samples (autoclave preparation) are tiny polyhedral crystallites, while CP-samples (conventional preparation) are larger, hexagonal platelets and cubes); (2) residual surface OH (AP-samples have less acidic OH, which are more isolated from each other; (3) acid gas adsorption (AP-samples adsorb more SO 2 and CO 2 at low pressures and room temperature and prefer monodentate rather than bidentate adsorption modes, but at higher pressures CP-samples adsorb more SO 2 and HCl apparently due to the formation of more well ordered multilayers); (4) destructive adsorption of organophosphorus compounds and chlorocarbons (AP-samples are superior due to higher surface areas and higher surface reactivities), and (5) very thin layers of transition metal oxides on the MgO and CaO nanocrystals that significantly enhance destructive adsorption capacities to the point where [M x O y ]AP-MgO and [M x O y ]- AP-CaO become stoichiometric in reaction with CCl 4 . The data are conclusive that the nanocrystals are more reactive than the microcrystals, and this is mainly attributed to morphological differences, including defects. However, intrinsic electronic effects due purely to “smallness” cannot be ruled out. I. Introduction It has now been amply demonstrated that a host of properties depend on the particular size of particles in the nanoscale regime (Table 1). Band gaps can change, 2 coercive force in magnetic materials can be manipulated, 3 melting points can decrease with size, 4 surfaces can be more reactive, 5 and nanostructural metal specimens can have increased hardness by a factor of 5 or more. 6 A huge new field of science is in the making. Considering that the multitude of solid state chemicals that have been made or will be made can have size dependent properties opens an almost infinite number of possibilities. It is clear that inter- disciplinary research efforts are necessary in this developing field. However, chemists must take the lead since synthesis, purification, and studies of bonding are the important first steps. Nanoparticulate materials almost always must be prepared in the laboratory, are usually desired in a narrow size range (monodispersed), and are often air and moisture sensitive; thus skillful manipulation is necessary. Furthermore, the challenges of scaled-up synthesis in environmentally benign ways still await us. II. Classes of Nanoparticles The regime of nanoparticles falls between the classic fields of chemistry (0.5 to 1 or 2 nm, or 1 to 100 atoms) and solid state physics (10 nm up to bulk, or 10 000 atoms to 6 × 10 23 in bulk). 8 For the intermediate region of 2-10 nm, neither quantum chemistry nor classic laws of physics hold. When strong chemical bonding is present, delocalization of valence electrons can be extensive, and this delocalization can vary with size; this in turn can lead to different chemical and physical properties. Materials of interest can be segregated into metals, semicon- ductors, and insulator particles. With metals and semiconduc- tors, which possess strong metallic or covalent bonds, property changes with size are well documented and appreciated for what these changes represent: progression from “quantum behavior” (descrete molecular orbitals) to “band behavior” (a near continuum of energy states). However, in the case of insulator particles, quantum size effects are less obvious and so far are generally related to changed surface chemistry. Indeed, it may turn out that surface effects in nanoparticles will be extremely important in terms of new technologies. In spherical nanoparticles, for example at a size of 3 nm, 50% of the atoms or ions are on the surface (Figure 1), allowing the X Abstract published in AdVance ACS Abstracts, July 1, 1996. TABLE 1: Examples of Unique Properties of Nanoparticles property description lead references magnetism magnetic properties of nanoparticles 3f,g oxide-metal core/shell particles 3b-l optical band gap changes in semiconductors 2a-f plasma resonances in metals 2g-k photoluminescence in indirect band gap semiconductors 2c-e melting points sodium gas phase clusters 4c gold particles 4d metals and molecular crystals 4e,g-l semiconductor crystals 4f surface chemistry destructive adsorption of CCl 4 5a adsorption of acid gases 5b,c,d reaction with AlEt3, pyridine 5e mechanical increased hardness of metals 6c (consolidated increased ductility of ceramics 6b nanostructural materials) increased plasticity 6a,b metallic behavior one electron energy gaps 7 12142 J. Phys. Chem. 1996, 100, 12142-12153 S0022-3654(96)00224-9 CCC: $12.00 © 1996 American Chemical Society