Published: August 12, 2011 r2011 American Chemical Society 18111 dx.doi.org/10.1021/jp2050614 | J. Phys. Chem. C 2011, 115, 18111–18121 ARTICLE pubs.acs.org/JPCC Structural and Electronic Properties of (Al 2 O 3 ) n Clusters with n =1À10 from First Principles Calculations Amol B. Rahane, †,‡ Mrinalini D. Deshpande,* ,† and Vijay Kumar § † Department of Physics, HPT Arts and RYK Science College, Nasik, Maharashtra-422 005, India ‡ Department of Physics, University of Pune, Pune, Maharashtra-411 007, India § Dr. Vijay Kumar Foundation, 1969 Sector 4, Gurgaon, Haryana-122 001, India 1. INTRODUCTION Aluminum oxide, Al 2 O 3 , also known as alumina is a very important ceramic material that has wide technological applica- tions ranging from electronics, optics, biomedical, and me- chanical engineering to catalyst support. Its great usefulness rests primarily on its external hardness (15 GPa), high melting point (2327 K), and low electrical conductivity (10 À12 S/m at 20 o C). Also aluminum is widely used in everyday life and oxidation of its surface happens naturally in the presence of atmospheric oxygen. Therefore, it is common to protect alumi- num surfaces by anodization. Such protective thin coatings may have amorphous structures. In bulk, alumina exists in different phases. In the corundum phase of alumina also known as α-Al 2 O 3 , Al ions have an octahedral nearest neighbor environ- ment of oxygen whereas oxygen atoms are tetracoordinated. In the γ-phase of Al 2 O 3 , Al ions have tetrahedral and octahedral neighborhoods and it is widely used as support in catalysis. Understanding the nanoparticles of alumina can throw light on the initial processes occurring on surfaces and also in the development of ceramic materials from constituent nanoparti- cles. With the current rapid developments in nanoscience and nanotechnologies, it is of interest to understand nanostructures of a material like alumina and the processes occurring at the atomic scale in such small systems. Clusters of aluminum oxide have, consequently, been studied both theoretically 1À19 and experimentally 17À23 to better understand the relationship be- tween atomic structures, bonding nature, and other proper- ties. Most of these studies are, however, on small clusters of (Al 2 O 3 ) n with n up to about 5 that have been more extensively studied. 1À3,7,11,17À19 For larger clusters, the structures and properties have not been well established. Some researchers 4,8,14 have studied cage structures, and in some other studies, 5,16 amorphous structures have been suggested to be favored. Not only would a proper knowledge of the atomic structures be important for understanding the nanostructures of alumina, but also such a study is likely to have wider implications for nano- particles of other oxides and related materials. In the present work, we focus on the study of the atomic and electronic structures of alumina clusters (Al 2 O 3 ) n with n =1À10. The rest of the paper is organized as follows. Section 2 describes the computational method. The results are presented and discussed in section 3, and a summary of the results is given in section 4. 2. COMPUTATIONAL DETAILS The calculations have been performed using projector aug- mented wave (PAW) pseudopotential method 24,25 as implemen- ted in Vienna Ab initio Simulation Package (VASP) 26,27 with- in the framework of the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof. 28 The clusters were placed in a cubic supercell with an edge length of 24 Å, and periodic boundary conditions were imposed. The cutoff energy Received: May 31, 2011 Revised: August 10, 2011 ABSTRACT: The atomic structures, growth behavior, and electronic properties of (Al 2 O 3 ) n , n =1À10, clusters have been studied within the framework of density func- tional pseudopotential theory and generalized gradient approximation for the exchangeÀ correlation energy. The lowest energy isomers of these clusters show preference for 4-membered Al 2 O 2 and 6-membered Al 3 O 3 rings. There are 3-, 4-, and 5-fold coordinated Al atoms and 2-, 3-, and 4-fold coordinated oxygen atoms. The atomic structures have similarity with that of the γ-Al 2 O 3 phase and the average coordinations of Al and O atoms in clusters are much lower from the values in the ground state of α-Al 2 O 3 (corundum structure). In general, isomers with cage structures lie significantly higher in energy compared with the lowest energy structures we have obtained. The bonding characteristics for clusters of different sizes is studied using Bader charge analysis. It is found that with increasing size, the charge transfer from Al atoms to oxygen increases toward the value in bulk. Further, the infrared (IR) and Raman spectra have been calculated. For n = 4, a comparison of the calculated IR spectra for a few isomers with the available experimental results on cation shows the possibility of the occurrence of a mixture of isomers in experiments. The Raman spectra of these isomers are, however, quite different. Therefore, it is suggested that measurements on Raman spectra could give a clear indication of the isomer present in experiments.