Article pubs.acs.org/cm
Thermodynamics of the CoO−ZnO System at Bulk and Nanoscale Chengcheng Ma and Alexandra Navrotsky* Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis, Davis, California 95616, United States ABSTRACT: Enthalpies of formation of Cox Zn1−x O solid solutions (both bulk and nanophase materials) at 298 K have been determined using high-temperature oxide melt solution calorimetry in molten sodium molybdate (3Na2O·4MoO3) solvent at 973 K. Both the rocksalt and wurtzite phases show an approximately linear dependence of enthalpy of solution on composition, implying a zero heat of mixing in each phase, consistent with negligible lattice parameter changes on substitution of Co2+ for Zn2+. The surface energy of wurtzite Zn0.88Co0.12O solid solution was determined to be 2.33 ± 0.30 J/m2 (anhydrous surface) and 1.65 ± 0.25 J/m2 (hydrous surface), which are very close to values for ZnO. The wurtzite CoO surface energy was estimated to be similar. Here, we argue that, because of the lower surface energies of wurtzite phases than of rocksalt phases, the phase field of the wurtzite solid solution expands to higher CoO content at the nanoscale, suggesting that the reported extended solubility of CoO in ZnO nanoparticles represents thermodynamic stabilization and free energy minimization at the nanoscale. Conversely, the rocksalt Co1−xZnxO phase shows thermodynamic destabilization, lower zinc content, and easier oxidation (to Co3−xZnxO4 spinel phase) at the nanoscale than in the bulk. KEYWORDS: CoO, ZnO, solid solution, nanoparticles, surface energy, thermodynamics
■
INTRODUCTION The difference in thermodynamics between nanophase (nanoparticle, nanoceramic, or thin film) materials and their bulk counterparts gives rise to many novel interesting properties which we can call nanoscale effects (nanoeffects). In particular, there are changes in the stability of polymorphs1,2 and the pressure−temperature (P−T) positions of dehydration3 and oxidation−reduction equilibria4 brought about by differences in surface energies of different phase assemblages. Cobalt-doped ZnO is a dilute magnetic semiconductor candidate for next generation spintronic devices,5 which makes it an interesting system to study nanoeffects on solid solubility, phase stability, and magnetism. Extensive polymorphism involving changes between tetrahedral and octahedral coordination (wurtzite, sphalerite, and rocksalt phases for CoO, wurtzite, sphalerite, and high-pressure rocksalt phases for ZnO) make the CoO− ZnO system rich in fundamental thermodynamic and structural questions. Theoretical calcuation6 predicts that a ferromagnetic ground state with a high Curie temperature (TC) would be achievable for ZnO with a sufficient amount of transition metal ion dopants such as Mn2+, Fe2+, Co2+, or Ni2+. Cobalt is a major candidate and is chosen in this study not only because its divalent oxidation state is more stable, and hence allows more flexibility in synthesis conditions than Mn2+ and Fe2+, but also because of its high electron magnetic moment and large solubility in ZnO. For example, under equilibrium condition at 1323 K, there is 0.17 ± 0.01 mol fraction CoO in bulk ZnO, while there is only 0.03 ± 0.01 mol fraction NiO in bulk ZnO.7 Nevertheless, the equilibrium solubility of CoO in bulk ZnO (up to 1327 K) is still lower than the critical doping level predicted by theory6 to yield a stable ferromagnetic state (∼18 mol % Co substitution for Zn). However, the solubility can be further extended in the nanophase regime to satisfy the © 2012 American Chemical Society
threshold concentration requirement and enable ferromagnetic behavior. It has been reported that the apparent solubility of CoO in wurtzite ZnO can be increased to 40 mol % using organic spray pyrolysis at 773 K and roughly 70 mol % at 573 K.8,9 The products may be metastable, but solubility may also be extended by surface energy effects. In particular, the surface energy of rocksalt CoO4 has been determined to be 3.57 ± 0.30 J/m2 (anhydrous surface) and 2.82 ± 0.20 J/m2 (hydrous surface), while that for wurtzite ZnO is 2.55 ± 0.23 J/m2 (anhydrous surface) and 1.31 ± 0.07 J/m2 (hydrous surface).10 The lower surface energy of the wurtzite phase would suggest that, for small particles (nanophases), the extent of CoO solubility in the wurtzite phase would increase, while that of ZnO in the rocksalt phase would decrease. The main purpose of this study is to address such nanoeffects quantitatively by using directly measured energetics. We address the following questions. What causes the drastically increased apparent solubility of CoO in ZnO from 18% in bulk to 70% in nanophases?8,9 Are these effects thermodynamic or kinetic in nature? If thermodynamic, are the factors responsible for the large solubility increase of CoO in ZnO energetic, entropic, or both? Do the surface energies change when Co2+ is substituted for Zn2+ in wurtzite or Zn2+ for Co2+ in rocksalt? What are the heats of mixing in these solid solutions at bulk and nanoscales? Using high-temperature oxide melt solution calorimetry and water adsorption calorimetry, we provide quantitative thermodynamic insights into these questions. Low-temperature heat capacity measurements, currently in progress, will be reported separately. Received: February 15, 2012 Revised: May 8, 2012 Published: June 7, 2012 2311
dx.doi.org/10.1021/cm3005198 | Chem. Mater. 2012, 24, 2311−2315
Chemistry of Materials
Article
Table 1. Analytical and Calorimetric Data for Nanoparticles and Bulk Materials in the ZnO−CoO System enthalpy correction for nH2O
sample composition microprobe confirmed Zn0.955Co0.045O Zn0.912Co0.088O Zn0.884Co0.116O •0.15H2O Zn0.868Co0.132O Zn0.824Co0.176O Zn0.179Co0.821O Zn0.106Co0.894O Zn0.073Co0.927O Zn0.036Co0.964O ZnO CoO
■
nominal
surface area (m2 / mol)
measured drop solution enthalpy for oxide·nH2O (kJ/mol)
Zn0.96Co0.04O Zn0.92Co0.08O Zn0.88Co0.12O
bulk bulk 6037 (11 nm)
17.57 ± 0.68 16.89 ± 0.51 17.09 ± 0.58
Zn0.88Co0.12O Zn0.84Co0.16O Zn0.16Co0.84O Zn0.12Co0.88O Zn0.08Co0.92O Zn0.04Co0.96O ZnO CoO
bulk bulk bulk bulk bulk bulk bulk bulk
16.63 16.02 13.83 14.68 15.30 15.81 18.06 16.50
± ± ± ± ± ± ± ±
using liquid water as ref state (kJ/mol)
10.35
0.48 0.49 0.41 0.27 0.53 0.38 0.43 0.70
using water vapor adsorption data (kJ/mol)
14.51
corrected enthalpy of drop solution of anhydrous oxide for hydrated surface (kJ/mol)
for anhydrous surface† (kJ/mol)
6.74 ± 0.60
17.57 ± 0.68 16.89 ± 0.51 2.58 ± 0.70 16.63 16.02 13.83 14.68 15.30 15.81 18.06 16.50
± ± ± ± ± ± ± ±
0.48 0.49 0.41 0.27 0.53 0.38 0.43 0.70
Water Adsorption Calorimetry. Oxide nanoparticles are usually hydrated. The heat effect associated with water adsorption on nanoparticle surfaces was measured at room temperature using a Setaram Calvet microcalorimeter coupled with a Micromeritics ASAP 2020 analysis system as described previously.14 Samples were pressed into pellets and put into a forked silica tube and degassed under a static vacuum (
