J. Phys. Chem. B 2001, 105, 4647-4652
4647
Preparation and Characterization of Monodispersed YSZ Nanocrystals Guangsheng Pang,† Siguang Chen, Yingchun Zhu, Oleg Palchik, Yuri Koltypin, Afie Zaban, and Aharon Gedanken* Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel ReceiVed: January 25, 2001
A novel method for large-scale preparation of yttria-stabilized zirconia (YSZ) nanocrystals is presented. The hydrous YSZ colloidal nanoparticles are self-assembled on the surface of SrCO3 nanoparticles by a sonochemical method. After calcination, fully crystalline monodispersed YSZ nanoparticles are obtained, and the SrCO3 is washed out by 10% HNO3 solution. The agglomeration of YSZ particles is inhibited as the crystallization occurs on the surface and interface of SrCO3 nanoparticles. The nanocrystals are monodispersed with an average particle size of 4.7 nm and a high surface area of 165 m2/g. The quantum confinement effect is observed: the band gap increases from 4.13 eV for the agglomerated sample to 5.44 eV for the monodispersed YSZ nanocrystals.
Introduction The development of reliable and reproducible methods for preparing large quantities of high crystallinity and monodispersivity of inorganic nanocrystals remains an area of extensive interest. Monodispersed nanoscale zirconia powder is essential for most of the applications in ceramics and catalysis. The uniform distribution of the size of particles is very important for optical spectroscopy. The optical absorption of yttriastabilized zirconia (YSZ) has been reported as having a band gap in the range from 4 to 5.8 eV.1-3 This wide range may result from the fact that it is difficult to obtain good optical quality specimens with well-controlled microstructure. The electronic properties of nanoparticles differ from those of bulk materials.4 Quantum confinement is expected, and the absorption edge is shifted to a higher energy when the grain size decreases. For nanocrystalline YSZ thin films, only a small band gap energy shift of 0.25 eV is observed when the particles size changes from 100 to 1 nm.3 A large variety of methods have been used to prepare nanocrystalline zirconia or YSZ, such as sol-gel5,6 precipitation,7,8 hydrothermal,9 spray pyrolysis,10 and plasma synthesis.11 Precipitation of inorganic salt solutions provides an economical route for large quantity production of nanoscale zirconia particles since this approach uses inexpensive inorganic salts as a starting material.7,12,13 Formation of hydrous zirconia particles by a low temperature aqueous chemical method has been studied extensively.13-15 The as-prepared samples are usually amorphous and contain water, and further drying and calcination at high temperature are therefore indispensable for crystallization and dehydration. The calcined product is made up of aggregated crystalline particles. The particle size depends on the calcination temperature and the concentration of the stock solution.16 Several attempts have been made to prepare monodispersed zirconia nanocrystals. Changing the surface properties by replacing the hydroxyl group with methyl siloxyl surface group has been used to avoid the crystal growth.17 Methyl siloxyl surface group * Corresponding author. Fax: 972-3-535-1250; E-mail: gedanken@ mail.biu.ac.il. † Permanent address: Department of Chemistry, Jilin University, Changchun, 130023, China.
decomposes in air at 350 °C resulting in SiO2 particles which serves as the “pinning” particles. Washing the precipitate with ethanol affects the crystallization process of the hydrous zirconia and the textural properties of the calcined zirconia.18 Lowering the agglomeration is ascribed to the presence of an ethoxide group on the surface,19 which is formed by replacing hydroxyl group and the coordinated water on the surface by ethanol. It has also been reported that weakly agglomerated nanoparticles of YSZ can be prepared by the decomposition of metal nitrate coated on carbon powder.20 Recently, several publications reported on the coating of nanoparticles on the surfaces of submicrospherical silica and alumina particles using the sonication method.21-25 We extend this method to preparing YSZ nanocrystals by self-assembling the hydrous YSZ colloidal particles on the surface of SrCO3 nanoparticles. SrCO3 particles are selected as the substrate for two reasons: first, they are thermally stable and their decomposition temperature is ca. 1100 °C, and second, they dissolve in dilute acid and can easily be washed out from the products. After drying and calcination at higher temperature, the hydrous YSZ particles dehydrate and crystallize on the surface of SrCO3 nanoparticles. Crystallized YSZ particles are separated by SrCO3 particles and aggregation is inhibited. This novel method is suitable for most cases in which inorganic nanocrystals are prepared by the precipitation method. It will yield in all cases a monodispersed calcined product. Experimental Section The reactants used for the preparation of YSZ are Y2O3, ZrO(NO3)2.xH2O, Sr(NO3)2, and NH4HCO3 (Aldrich). The experimental procedure is schematically described in Figure 1. Y2O3 and ZrO(NO3)2‚xH2O are dissolved in nitric acid and deionized water under heating and stirring, respectively. The solutions are then mixed together. The mole composition of the starting reaction mixture is 0.08Y2O3:0.92ZrO(NO3)2.xH2O, and the concentration of zirconium in the mixture solution at this point is 0.05 M. Ammonium hydroxide is dropped into the solution under sonication using a direct immersion titanium horn (Vibracell, 20 kHz, 100 W/cm2). The product is centrifuged and washed with deionized water. At this stage, the product is
10.1021/jp010334q CCC: $20.00 © 2001 American Chemical Society Published on Web 05/02/2001
4648 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Pang et al.
Figure 1. . Preparation procedure of YSZ nanocrystals.
referred to as as-prepared hydrous YSZ. The as-prepared hydrous YSZ is washed with ethanol and sonicated in ethanol for another 30 min. At this stage, the product is referred to as an ethanol-treated sample. Nano SrCO3 particles are prepared by dropping NH4HCO3 solution into 0.05 M Sr(NO3)2 solution under sonication and washed with water and ethanol. The SrCO3 nanopartiles are dissolved in ethanol and formed a suspension solution. The SrCO3 nanoparticles suspension solution is added into the hydrous YSZ colloidal solution under stirring. The proportion of hydrous YSZ and the SrCO3 is 1:2.5 in weight. The mixture solution is further sonicated for 10 min. The mixture solution is evaporated by heating and stirring, dried at 120 °C, and calcined at 600 °C. SrCO3 is washed out by dissolving it in 10% HNO3 solution under sonication. The solid product at this stage is referred to as sample C. The calcined sample of the as-prepared hydrous YSZ and the ethanol-treated hydrous YSZ are referred to as sample A and sample B, respectively. The transmission electron micrographs (TEM) are obtained by using JEOL-JEM 100SX microscopes. Samples for TEM are prepared by placing a drop of suspension solution on a copper grid (400 mesh, electron microscopy sciences) coated with carbon film and then being allowed to dry in air. Powder X-ray diffraction (XRD) is performed on a Rigaku 2028 diffractometer, with nickel filtered CuKR radiation. The particle size is calculated from the X-ray line broadening, using the Debye-Scherrer equation. Thermogravimetric analysis (TGA) is performed using a Mettler Toledo TGA/SDTA851 in the temperature range of 30-1100 °C in nitrogen atmosphere at a heating rate of 10 °C per minute. The particle size of sample C is measured by dynamic light scattering (DLS) with ethanol as solvent. The DLS measurement is performed with a Coulter N4 plus instrument at a wavelength of 632.8 nm of He-Ne at 23°. The particle size is estimated by SDP weight analysis. The specific surface area of the powders is measured via the Brunauer-Emmett-Teller (BET) method with nitrogen absorption. UV-vis diffuse reflectance spectroscopy (DRS) is measured by Cary 500 UV-visible spectophotometer at room temperature in the wavelength region between 200 and 800 nm. Results and Discussion The hydrous YSZ nanoparticles are precipitated by dropping ammonium hydroxide into a mixture of yttrium and zirconium solution under sonication. The as-prepared hydrous YSZ nanoparticles are amorphous and contain water. The particle size of
the freshly prepared sample is ca. 2 nm according to the transmission electron micrographs (TEM) result, but it is not stable. In aqueous solution, the particle size increases after storage a few hours, which is due to coagulation and polymerization. When the as-prepared hydrous YSZ particles are dissolved in ethanol under sonication, the large shear gradients, which are generated by the liquid motion in the vicinity of the cavitation bubbles,26 prevent the coagulation of the particles. The YSZ particles are separated in ethanol and a very stable colloidal solution is formed. When the hydrous YSZ colloidal solution is mixed with the SrCO3 suspension, a homogeneous mixture is formed. The hydrous YSZ particles are self-assembled on the surface of SrCO3 nanoparticles. Thermogravimetric analysis (TGA) results show that both the as-prepared and ethanol-treated hydrous YSZ contain water. For the as-prepared sample, there is a weight loss of 30.9 wt % when the sample is heated from room temperature to 600 °C. Weight loss of 21.2 wt % when the sample is heated from room temperature to 230 °C is due to the loss of surface water and hydroxide. Weight loss of 9.7 wt % when the sample is heated from 230 °C to 600 °C is due to the loss of intraparticle water. The total weight loss of the ethanol-treated hydrous YSZ sample is 18.7 wt % from room temperature to 600 °C, which is significantly lower than that of the as-prepared sample. For the ethanol-treated sample, the weight losses from room temperature to 300 °C are mainly due to the loss of water. The weight losses from 300 °C to 400 °C are due to the oxidative decomposition of chemisorbed ethoxy group, according to the TG and DTA study of ethanol washing samples.18 The decrease of water content of the ethanol-treated sample indicates that there is a partial dehydrating process taking place during the sonication in ethanol. The pure SrCO3 nanoparticles decompose at 1000 °C. TGA curve of the mixture of ethanol-treated hydrous YSZ and SrCO3 reveals both the dehydration of YSZ and the decomposition of SrCO3. There is no weight loss between 400 °C and 600 °C. This means that the hydrous YSZ can be dehydrated and crystallized at this temperature range, whereas the SrCO3 nanoparticles do not decompose. Figure 2 shows the TEM micrographs of the YSZ samples. Sample A is ground before the TEM measurement. Figure 2a shows that sample A consists of aggregated particles with a particle size from a few nanometers to tens of nanometers (Figure 2a). On the contrary, sample B consists of weakly aggregated particle with a narrow distribution of particle sizes of ca. 6 nm (Figure 2b). The most important reasons for the aggregation of the hydrous YSZ are the coagulation of the
Preparation of Monodispersed YSZ Nanocrystals
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4649
Figure 2. TEM micrographs of (a) sample A; (b) sample B; (c) SrCO3 coated with ethanol-treated hydrous YSZ; and (d) sample C.
hydrous particles in aqueous solution and the condensation of the surface hydroxyl groups. After sonication in ethanol, the surface groups are replaced by ethoxide. Washing the precipitate with ethanol affects the crystallization process of the hydrous zirconia and the textural properties of the calcined zirconia.18 The degree of aggregation is significantly lower than that of sample A, which is prepared without washing and sonication in ethanol, but there is still obvious agglomeration as confirmed by the TEM micrograph. Figure 2c shows the TEM micrographs of the mixture of ethanol-treated hydrous YSZ and SrCO3. The
hydrous YSZ nanoparticles are coated on the surface of the SrCO3 nanoparticles. After calcination at 600 °C and washing with HNO3 solution, single-phase monodispersal YSZ nanoparticles with a particle size of ca. 4-5 nm are obtained (Figure 2d). The particle size of sample C was also checked by dynamic light scattering (DLS). When sample C dissolves in ethanol, it can be sustained for a few hours without precipitation. The DLS result of sample C indicates that the average particle size is 4.9 nm with a very narrow distribution from 3 to 6 nm. The particle
4650 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Pang et al. TABLE 1: BET Surface Areas and the Average Particle Sizes Estimated by BET, XRD, and DLS BET surface area (m2/g) sample A sample B sample C SrCO3
60 113 165 10
dBETa (nm)
dXRDb (nm)
dDLSc (nm)
17.4 8.8 6.0
16.4 6.3 4.7
4.9
a Derived from the BET surface area assuming a spherical shape of the particles. b Calculated from the X-ray line broadening. c Estimated by SDP weight analysis.
Figure 3. XRD patterns of (a) the as-prepared hydrous YSZ; (b) the ethanol-treated sample; (c) sample A; (d) sample B; (e) sample C; and (f) the calcined mixture of hydrous YSZ and SrCO3.
size of the DLS result is consistent with the Powder X-ray diffraction (XRD) and TEM results. DLS result confirmed that the YSZ particles are monodispersed. The XRD patterns of the ethanol-treated sample (Figure 3b) are the same as those of the as-prepared hydrous YSZ sample (Figure 3a), which shows two broad humps at ca. 30 and 50 degree (2θ), respectively. The amorphous hydrous YSZ consists of tetrameric species of zirconium, which have a structure similar to that of cubic ZrO2. After partial dehydration of the as-prepared hydrous YSZ by sonication in ethanol, the colloidal particles retain their amorphous structure. This is in contrast to boiling the zirconium solution, which results in a crystalline phase.14 During the sonication process, partial dehydration of the hydrous YSZ particles takes place at the interfacial region around the cavitation bubbles. The high temperature in this region causes the formation of amorphous hydrous YSZ particles instead of the crystalline phases. After calcination at 600 °C, the hydrous amorphous YSZ is fully dehydrated and crystallizes in a cubic phase. Figure 3c-e shows the XRD patterns of sample A, B, and C, respectively. The particle sizes estimated from the broadening of the XRD are 17.4, 6.3, and 4.7 nm for the samples A, B and C, respectively. Figure 3f shows the XRD patterns of the calcined mixture of ethanol-treated hydrous YSZ and SrCO3. After calcination at 600 °C, the crystalline YSZ and SrCO3 retain as a simple mixture of the two solids. Brunauer-Emmett-Teller (BET) specific surface area results and the average particle sizes estimated by BET, XRD, and DLS are listed in Table 1. Sample A has a moderate surface area of 60 m2/g, which is consistent with the reported result.16 After treated in ethanol by sonication, the surface area of sample B significantly increases to 113 m2/g. The equivalent BET diameter (dBET) of 8.8 nm, which is derived by assuming a spherical particle shape, is larger than dXRD of 6.3 nm. This is
due to the agglomeration of the particles after calcination. Sample C has a very high surface area of 165 m2/g. The equivalent dBET of 6.0 nm is also larger than dXRD of 4.7 nm. The reason for relative larger dBET is due to a soft-agglomeration existing in sample C. The soft-agglomeration of sample C can be broken by sonication in ethanol according to the DLS results. On the contrary, the hard-agglomeration in sample B cannot be broken. The soft-agglomeration in sample C can be interpreted in terms of the van der Waals and electrostatic forces of the crystalline YSZ nanoparticles. For normal size particles, the van der Waals and electrostatic forces are negligibly small. However, when the particle size is on the nanometer scale, particles prefer to agglomerate since the van der Waals forces and electrostatic forces far exceed the single particle weight.27 The surface area of SrCO3 nanoparticles is 10 m2/g. When the hydrous YSZ nanoparticles and SrCO3 nanoparticles are mixed in the proportion 1:2.5 by weight, the hydrous YSZ nanoparticles are coated on the surface of SrCO3 nanoparticles in a mutilayer structure. It is possible to decrease the particle size of the dehydrated YSZ by increasing the proportion of SrCO3 in the mixture. The calcination temperature and the concentration of the stock solution have also been reported as influencing the particle size of zirconia.16 In our novel method, the particle sizes are insensitive to the preparation conditions. The average particle size, estimated by XRD, is in the range 4.9 ( 0.2 nm even as changing the concentration of zirconium from 0.01 to 0.1 M, the calcination temperature from 500 °C to 600 °C, and the proportion of hydrous YSZ and the SrCO3 from 1:2.5 to 1:10. The band gaps of the YSZ sample are studied by UV-vis diffuse reflectance spectroscopy (DRS). The Kubelka-Munk function, F(R) ) (1-R)2/2R, is used to determine the band gap by analyzing the DRS results. Figure 4 shows the plots of F(R) vs wavelength of sample A, B, and C. The band gaps are defined by extrapolation of the rising part of the plots to X axis (dotted line in Figure 4). The band gaps of sample A, B, and C are 4.13 eV (300 nm), 5.10 eV (243 nm), and 5.44 eV (228 nm), respectively. The band gap of the agglomerated YSZ (4.13 eV) is similar to the literature value of the single-crystal YSZ.1,2 Although impurities, defects, and surface states also affect the change of band gap,29 these effects are neglected here because all the samples are precipitated in the same composition and calcined at the same temperature (600 °C). The inset of Figure 4 shows the DRS results of the YSZ samples. The absorption edge is significantly shifted to higher energy when the particle sizes decrease from 17.4 nm of sample A to 6.3 nm of B and further to 4.7 nm of C. It is worth noting that there are two obvious absorption bands for sample B. This phenomenon is also reported in the literature28 and is attributed to color centers of polaron-like defects.29 Our result, however, shows that this phenomenon is more likely to be due to the quantum confinement effect. Weakly agglomerated YSZ nanoparticles exist in sample B according to the TEM results. The DRS curve of
Preparation of Monodispersed YSZ Nanocrystals
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4651 for 10 min. Our results indicate that an additional absorption band observed in the zirconia spectrum is due to the quantum confinement effect. Under certain preparing conditions, the zirconia product can be identified as consisting of two distinct types of particles, nonagglomerated and agglomerated. As a result, the observed DRS spectrum is composed of two different absorption bands. The heavily agglomerated particles have a band gap similar to that of the bulk material, whereas the nonagglomerated particles exhibit a quantum confinement effect and the increase in the band gap energy. Conclusion
Figure 4. Plots of F(R) vs wavelength of (a) sample A; (b) sample B; and (c) sample C. The inset show the DRS result.
In summary, fully crystalline yttria-stabilized zirconia (YSZ) nanocrystals are prepared by a novel method. The agglomerations of YSZ particles are inhibited, while crystallization occurs on the surface and interface of SrCO3 nanoparticles. This method is simple, inexpensive, and easy to scale-up. The YSZ nanocrystals are monodispersed with an average particle size of 4.7 nm and a high surface area of 165 m2/g. The quantum confinement effect is observed, and the band gap increases from 4.13 eV for the heavily agglomerated YSZ sample to 5.44 eV of the monodispersed YSZ nanoparticles with a particle size 4.7 nm. Acknowledgment. Prof. A. Gedanken thanks the BMBF, Germany, for financial support through the Energy Program. Dr. Chen, Dr. Zhu, and Dr. Pang thank the Kort 100 Scholarship Foundation for supporting their postdoctoral fellowship. The authors thank Antje Vo¨lkel for the assistance in analytical ultracentrifuge (AUC) measurements. The authors also thank Prof. Arlene Wilson-Gordon for editorial assistance. References and Notes
Figure 5. DRS result of (a) sample B; the sediment of sample B after storage in water for (b) 0.2 h; (c) 0.2-1 h; (d) 1-24 h, (e) >24 h, (f) sample A; and (g) the suspension particles of sample A after storage for 10 min in water.
sample B are composed of the absorption bands both of the nonagglomerated particles (similar to sample C) and agglomerated particles (similar to sample A). The weakly agglomerated particles can be separated from heavily agglomerated particles by sonication in water. After storing for a certain period, the larger agglomerated particles settle down first, and the upper suspension solution mainly consists of smaller agglomerated or nonagglomerated particles. The particle size decreases as the storage time increases. The proportions of the sediment for sample B are ca. 93, 4, 2, and 1 wt % for storage time of 0.2 h, 1 h, 24 h, and > 24 h, respectively. The sediment after storage of 24 h is obtained by centrifuging. Figure 5 shows the DRS result of sample A and B and the sediment of them after storage in water for a certain time. We observe a blue shift of the absorption band due to the confinement effect resulting from the change of the particle size. Although sample A is hardly agglomerated, there are also ca. 2 wt % of relatively smaller particles separated after storage of the suspension of sample A
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