Copyright © 2010 American Scientific Publishers Journal of All rights reserved Nanoscience and Nanotechnology Printed in the United States of America Vol.10, 1–13, 2010 Effect of Alkali Metal Hydroxides on the Morphological Development and Optical Properties of Ceria Nanocubes Under Hydrothermal Conditions View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by DSpace@IZTECH Institutional Repository Özlem Kepenekçi, Mehtap Emirdag-Eanes∗, and Mustafa M. Demir∗ Izmir Institute of Technology, Department of Chemistry, Gülbahçe köyü, Urla, 35430 I˙zmir, Turkey Nanocrystalline cerium(IV) oxide (CeO2, ceria) particles were produced via the hydrothermal treat- ment of cerium nitrate hexahydrate with various alkali metal hydroxides (MOH: M = Li, Na, K). Experimental conditions such as [MOH], reaction temperature, and reaction time were studied. Particle morphology as well as size of crystallites was precisely controlled by choice of experimen- tal conditions. While rod-shaped particles were obtained at 120 C, well-defined nanocubes were formed at higher temperatures regardless of the choice of MOH. Examination of particle growth kinetics, in the final stages of crystallization, showed that particle growth rate is controlled by two dif- ferent mechanisms. Grain boundary diffusion controls the particle growth in the presence of NaOH with an activation energy of 113.8 kj/mol and surface diffusion for LiOH ad KOH with the activation energy of 43.0–150.9 kj/mol, respectively. In addition, the particles exhibit strong violet and blue emissions at 400 nm and 370 nm. The former emission originates from excitation of a wide band gap of CeO2. The latter one is attributed to the trivalency of the cerium ion and appears to be sensitive to all the experimental conditions studied. Both extending reaction time and increasing temperature reduce the intensity of the 370 nm emission and increase the intensity of the 400 nm RESEARCH ARTICLE emission. Keywords: Ceria, Crystal Growth, Growth Kinetics, Nanoparticles, Semiconductor. 1. INTRODUCTION ceria particles.As a wet-chemical technique, the hydrother- mal process has attracted a lot of attention since desired Nanoscale semiconductor particles are of great interest due morphology can readily be produced at high purity by con- to their unique morphology dependent (size and shape) trolling parameters such as reaction time, reaction tem- physical properties.1–3 Therefore, synthetic methods have perature, concentration, and type of reactant.Previously, been developed to tailor the morphology of these parti- Yang et al.15 reported the synthesis of cube-shaped CeO cles making them useful in practical applications.In par- 2 starting from cerium(III) nitrate and NaOH at different ticular, there is an increasing interest in preparation of temperatures over 24 h.The authors studied the effect of cerium oxide (ceria) that has recently been used in various [NaOH] on the cubes and found that the size of the par- fields such as an oxygen ion conductor in solid-oxide fuel ticles increased as the concentration was increased.The cells4 and as an oxygen pump due to its high oxygen ion effect of pH on the crystallization of nanosized CeO was conductivity.5 Ceria has also received much attention in 2 16 the area of three-way catalysts6 as oxygen promoters and studied by Li W–H et al. It was claimed that the crys- 7 8 tallites synthesized in an acidic hydrothermal medium are in the area of catalysts for H2 production from fuels. Moreover, ceria has gained much attention for develop- larger and more monodisperse than the ones produced in ment of ultraviolet (UV) absorbent material, its absorption a neutral or alkaline medium.The effect of the counter- at about 400 nm being the strongest among all oxides.9 anions of cerium source on CeO2 nanostructures was inves- Several techniques including hydrothermal,10 11 reverse- tigated by Hu et al.17 They reported that the synthesized 12 13 − micellular, sonochemical, and those involving homoge- nanorods converted into cubes by addition of NO3 ions 14 − neous precipitation have been used for the preparation of under hydrothermal conditions.When NO 3 ions were present in the solution, the dissolution-recrystallization pro- ∗ − Authors to whom correspondence should be addressed. cess was largely improved.NO 3 ions act as both capping J. Nanosci. Nanotechnol. 2010, Vol. 10, No. xx 1533-4880/2010/10/001/013 doi:10.1166/jnn.2010.3608 1 Effect of Alkali Metal Hydroxides on the Morphological Development and Optical Properties Kepenekçi et al. agent and an oxidizer during the formation of the CeO2 Spectrometer under excitation at 290 nm.All absorbance cubes.The influence of several other anions (such as Br −, and fluorometry measurements were done with the particles − 2− I , and SO4 ) was also investigated showing that the basic- suspended in deionized water.Due to the low dispersibility ity of the hydrothermal medium had a direct influence on of CeO2 nanoparticles in water, the samples were ultrason- the size and shape of the particles formed.However, there ically agitated for 10 min before measurement. is lack of information on how of alkali bases affect the structure and morphology of CeO2 particles.Such bases, also known as mineralizers, are used as a source of alkalin- 3. RESULT AND DISCUSSION ity for the crystallization of metal oxides and increase the 3.1. Structural and Morphological Characterization solubility of reagents to the levels sufficient for reactivity of CeO particles in a reasonable time.18 Here, we investigate the influence of 2 type and concentration of alkali base (LiOH, NaOH, KOH) Hydrothermal crystallization, in general, is based on a as well as reaction time and reaction temperature on the dissolution-precipitation mechanism of a precursor pre- particle morphology and crystal growth of CeO2 particles. pared from a metal salt using an alkaline solution as an X-ray diffractometry (XRD), scanning electron microscopy 19 oxygen source. In the particular case of CeO2,itwas (SEM), high resolution transmission electron microscopy claimed that the precursor is amorphous, and that crys- (HR-TEM), UV-vis spectroscopy, and photoluminescence talline CeO2 is obtained when a hydrothermal process is (PL) spectroscopy were utilized for characterization.We applied where the precursor is homogenously dissolved have demonstrated that the size and shape of the particles and subsequently reprecipitated.15 However, in this study, can be controlled through a combination of time and tem- we have demonstrated that the precursor is not amorphous perature; and the growth kinetics can be modulated by the and that the crystallization process begins during mix- type of metal hydroxides. ing of the reactant solutions at RT prior to hydrothermal treatment.Figure 1(a) presents the XRD pattern of a rep- 2. EXPERIMENTAL PROCEDURE resentative precursor prepared in the presence of NaOH. The reflections are well indexed for Ce2O3 •, CeO2 , 2.1. Experimental Method and as well as unreacted Ce(NO33 ♣.The presence of sharp reflections of both Ce O and CeO clearly suggests The cerium oxide nanocrystals were produced from the 2 3 2 that the precursor already contains the crystalline product reaction of 0.5 g cerium nitrate hexahydrate (Ce(NO33 · 6H2O) and a 10% solution of alkali metal hydroxide (MOH: M = Li, Na, K).The aqueous alkaline solutions were rapidly added to an aqueous solution of cerium nitrate inside a 23 mL teflon-lined stainless steel auto- CeO2 Ce O clave up to 50% of total available volume.The hydrother- 2 3 Ce(NO ).6H O mal reactions were carried out at various temperatures 3 2 from 120 Cto240C with the reaction times being var- iedfrom1hto24h.Each system was then allowed to cool to room temperature.The resulting particles were RESEARCH ARTICLE washed with 15 mL of water and separated by cen- (d) trifugation at 6000 rpm for 20 min.The particles were obtained by drying the precipitates at 60 C in air overnight. (c) Intensity/a.u 2.2. Characterization The crystal structure of the products was identified by (b) X-ray powder diffraction (XRD recorded on a Philips-X- perPro powder diffractometer with Cu K radiation of 0.15406 Å). The morphology of the CeO2 nanocrystals was observed by scanning electron microscope (SEM-Philips (a) XL-30S FEG, 5 kv) and transmission electron microscope 30 40 50 60 (TEM-FEI Tecnai F-20 microscope).The optical proper- 2θ ties were measured with a UV/VIS spectrometer (Var- Fig. 1. (a) X-Ray Diffraction pattern of precursor precipitated prior to ian Cary 50) equipped witha1cmquartz cuvette holder hydrothermal treatment.X-Ray Diffraction patterns of CeO 2 powders for liquid samples.Photoluminescence (PL) measurements synthesized at 240 C for 24 h in the presence of (b) KOH, (c) NaOH, were performed on a Varian Cary Eclipse Fluorescence (d) LiOH. 2 J. Nanosci. Nanotechnol. 10, 1–13, 2010 Kepenekçi et al. Effect of Alkali Metal Hydroxides on the Morphological Development and Optical Properties (Supplementary Fig.S1).When hydrothermal treatment is applied at 240 C for 24 h the signals of Ce(NO33 and Ce2O3 disappeared and the pure CeO2 phase was detected, regardless of the alkali base used.Particle mor- phology (size and shape) can be controlled by the type and concentration of MOH, reaction time, and reaction temperature. 3.1.1. Type of Alkali Base The effect of alkali metal hydroxides on the morphology of the precipitated CeO2 particles was examined.It was found that the formation and growth of CeO2 nanocubes were influenced significantly by the type of MOH used. The spectra (in Fig.1) labeled as b, c, and d refer to the CeO2 particles prepared in the presence of LiOH, NaOH, and KOH, respectively.All reflections in the spectra can be indexed to a face-centered cubic structure (JCPDS no 81-0792) with a lattice parameter of 5.412 Å.