Published: June 10, 2011 r2011 American Chemical Society 13577 dx.doi.org/10.1021/jp201795y | J. Phys. Chem. C 2011, 115, 13577–13583 ARTICLE pubs.acs.org/JPCC Mechanism of Ni 1Àx Zn x O Formation by Thermal Treatments on NiO Nanoparticles Dispersed over ZnO Fernando Rubio-Marcos,* ,†,‡ Cristina V. Manzano, § Juli  an J. Reinosa, || Juan J. Romero, || Pascal Marchet, † Marisol S. Mart  in-Gonz  alez, § and Jos  e F. Fern  andez || † Laboratoire de Science des Proc  ed es C eramiques et de Traitements de Surface, UMR 6638 CNRS, Universit  e de Limoges, Centre Europ een de la C eramique, 12, rue Atlantis, 87068 Limoges Cedex, France ‡ Departamento de F  isica de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain § IMM—Instituto de Microelectr onica de Madrid (CNM-CSIC), Issac Newton 8, PTM, E-28760, Tres Cantos, Madrid Spain ) Instituto de Cer  amica y Vidrio, CSIC, Kelsen 5, 28049 Madrid, Spain 1. INTRODUCTION Transition metal oxides (TMOs) doped by different elements are versatile functional ceramics which may exhibit electric, optical, catalytic, and magnetic properties. For example, ZnO doped by 5% Mn have been theoretically predicted by Dietl to present room temperature ferromagnetism (RTFM). 1 The ex- ceptional physicochemical characteristics of transition metals, like the several possible oxidation and spin states, the similarity in ionic radii, and the tendency to occupate the same site into the crystalline structure are behind this versatility. As a consequence, these compounds find applications in a wide spectrum of technological fields. One deceptively simple mixed-metal oxide system, Ni 1Àx Zn x O, is based on the very stable rock salt NiO lattice and contains Zn 2+ substituting for Ni 2+ . 2À5 Both metals are formally M 2+ , their stable oxidation state in solid-state oxides, but the octahedral coordination of the rock salt structure is unusual for zinc, which is typically found tetrahedrally coordinated. 6 Then this unusual coordination of Zn 2+ ion in the nickel oxide lattice can potentially result in new and interesting properties. NiO adopts a rock salt crystal structure with a unit cell lattice parameter of a o = 4.1777 Å at 300 K. 7 The material is antiferro- magnetic, with a N eel temperature of 523 K, 8 and the alignment of the unpaired electron spins in alternating (111) planes causes a slight distortion to rhombohedral symmetry of approximately 1% along the Æ111æ direction. 9,10 Although Ni 2+ is 3d 8 , pure NiO is an insulator with a band gap of 4.2 eV 11 resulting from the strong Coulomb interactions among the highly correlated d-electrons and the localization of these electrons through hybridization with adjacent oxygen 2p levels. The Ni 1Àx Zn x O solid solution is known to be nominally face- centered cubic (fcc) rock salt for zinc concentrations up to the solubility limit (about 0.3), although, as occurring for the parent NiO structure, a slight rhombohedral distortion lowers the symmetry to the R3m space group. 12 Magnetic studies 12 have been performed on the Ni 1Àx Zn x O solid solution and showed that the antiferromagnetic structure persists throughout the range 0 e x e 0.3, with N eel temperature decreasing with increasing zinc content. The solid solution follows Vegard’s law, 13 in which the unit cell parameter increases linearly with zinc concentration from a o = 4.1777 Å for x = 0 to 4.2107 Å for x = 0.3, indicating a homogeneous solution in which Zn 2+ ions randomly substitute for Ni 2+ in bulk cation lattice positions. Since Zn 2+ configuration is 3d 10 while Ni 2+ is 3d 8 , substituting Zn 2+ for Ni 2+ can be anticipated to affect both the magnetic and the electronic structures of the parent NiO lattice. The present study explores the Ni 1Àx Zn x O solid solution in the system formed by ZnO supported NiO nanoparticles submitted to different thermal treatments. It is shown that Zn 2+ ions diffuse into NiO lattice producing a rock salt solid solution at surprisingly low temperatures due to the high surface reactivity of the nickel oxide nanoparticles. The structural and Received: February 23, 2011 Revised: June 7, 2011 ABSTRACT: The formation of Ni 1Àx Zn x O rock salt solid solution is obtained by thermal treatments applied to NiO nanoparticles supported on ZnO micrometric particles. The high vapor pressure of ZnO produces a Zn-rich atmosphere during thermal treatment. The Zn ions tend to be adsorbed by the highly reactive NiO nanoparticles. When heated to tem- peratures high enough, over 500 °C, the Zn ions react with the NiO nanoparticles forming the rock salt material. By variation of the treatment temperature, the composition of this rock salt structure can be varied through the whole solubility range.