Size-dependent properties of CeO 2 Ày nanoparticles as studied by Raman scattering Jonathan E. Spanier, Richard D. Robinson, Feng Zhang, Siu-Wai Chan, and Irving P. Herman* Department of Applied Physics and Applied Mathematics, and the Materials Research Science and Engineering Center, Columbia University, New York, New York10027 Received 16 April 2001; published 29 November 2001 The combined effects of strain and phonon confinement are seen to explain why the Raman peak near 464 cm -1 in CeO 2-y nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric on the low-energy side as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles. DOI: 10.1103/PhysRevB.64.245407 PACS numbers: 63.22.+m, 63.20.Kr, 63.20.Dj I. INTRODUCTION CeO 2 is of interest as a catalyst in vehicle emissions systems, 1 for cracking heavy oil in zeolites, 2 as a potentially useful solid oxide fuel cell electrolyte material, 3 and for gas sensors, 4 optical coatings, 5 high-T c superconductor structures, 6 silicon-on-insulator structures, and high storage capacitor devices. 7,8 Some applications may benefit from us- ing monodisperse CeO 2 nanoparticles, due to either possibly new properties in the nanodimension or the greater control in uniform structures. Li et al. 9 have prepared and characterized monodisperse CeO 2 nanoparticles. Wu et al. 10 have used extended x-ray- absorption fine structure to study the local atomic structure around Ce ions in CeO 2 nanoparticles. Electron diffraction has shown that a decrease in the size of CeO 2 nanoparticles is accompanied by a significant increase in the lattice parameter. 11,12 Such changes in lattice constant with particle size have also been confirmed by x-ray diffraction in Ref. 13. The authors in Ref. 12 explained this increase in terms of an associated reduction in the valence of the Ce 4 + ions to Ce 3 + ions caused by an increasing molar fraction of oxygen va- cancies. In a nanoparticle system that naturally possesses an enormous surface area per unit volume, such vacancies can enhance the ability of a volume of this material to store and release oxygen. When used as an additive to catalysts in automotive emissions systems, these nanoparticles can fur- ther enhance the range of fuel/air mixtures at which CO can be oxidized and NO x can be reduced. Previous Raman studies of CeO 2 nanoparticles at room temperature, RT have demonstrated that the Raman peak energy decreases and the linewidth increases with decreasing particle size. It has been suggested that these dependences be used to measure particle size rapidly. However, Ref. 14 could not explain these dependences using a phonon-confinement model, and suggested that phonon relaxation could be differ- ent, i.e., faster, with smaller nanoparticle size and that this could account for the Raman-spectrum changes with size. Some features in the CeO 2 nanoparticles Raman spectrum have been attributed to significant concentrations of impurity atoms or vacancies. 15 A detailed Raman analysis of CeO 2 nanoparticles is pre- sented here for a range of particle sizes and preparations. The increasing lattice constant strain relative to the bulk for successively smaller particles is seen to explain much of the Raman-spectrum changes with particle size, when the disper- sion in the particle-size distribution and phonon confinement are also included. If the rate of optical-phonon decay to acoustic phonons or coefficient of thermal expansion at room temperature were to vary with particle size, then the Raman peak energy and linewidth would also vary differently with temperature for different particle sizes. Accordingly, the Ra- man spectrum is also studied as a function of temperature for different particle sizes. II. EXPERIMENTAL PROCEDURE Solutions of 0.04M Ce(NO 3 ) 3 and 0.5M HMT (C 6 H 12 N 4 , hexamethylenetetramine reagents were mixed at room temperature with continuous stirring, producing nucle- ation and growth of CeO 2 -y particles. Solutions were al- lowed to mix for different controlled lengths of time 5–24 h and then placed in a centrifuge, yielding nanoparticles. The resulting particle size, disperison, and shapes were de- termined by transmission electron microscopy TEM. The nanoparticle size was controlled by the length of the reaction time. To obtain the largest particles the mixing reaction was carried out for 12–24 h prior to centrifugation and the par- ticles were then sintered in air at atmosphere at different temperatures 400– 800 °C for 8–16 h. 16 The lattice param- eter a was determined from fitting the x-ray diffraction peak position and the mean particle diameter from the peak width using the Scherrer formula, x 0 =0.94 / B cos  B , where  is the wavelength of the Cu K 1 line,  B is the angle between the incident beam and the reflecting lattice planes, and B is the width in radians of the diffraction peak. The size dis- persion is approximately Gaussian with a full width at the 1/e 2 points,  x , which is 44% of the mean diameter. Further PHYSICAL REVIEW B, VOLUME 64, 245407 0163-1829/2001/6424/2454078/$20.00 ©2001 The American Physical Society 64 245407-1