Preparation of Highly Dispersed Nano-La2O3 Particles Using

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Preparation of Highly Dispersed Nano-La2O3 Particles Using Modified Carbon Black as an Agglomeration Inhibitor Guirong Wang, Yongshan Zhou, David G. Evans, and Yanjun Lin* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Nanoparticles readily agglomerate, especially during nucleation, growth, and calcination processes. In this work, modified carbon black (MCB) has been used to prevent particle agglomeration during the nucleation step in the preparation of a highly dispersed nano-La(OH)3 precursor by a coprecipitation reaction. The surface carboxyl groups formed on MCB after modification can adsorb and fix positively charged La3+ ions on the surface. Therefore, nano-La(OH)3 nuclei can be uniformly deposited on the MCB surface. After nucleation, La(OH)3 particles with a size of about 20 nm with a positive surface charge still interact strongly with the negatively charged MCB surface, which effectively prevents their agglomeration during the subsequent aging process. Furthermore, due to the release of CO2 over a wide temperature range from 400 to 700 °C during a subsequent calcination process, La2O3 particles obtained by calcination of the La(OH)3 precursor can be effectively isolated at high temperature and prevented from agglomerating. By using MCB as an agglomeration inhibitor in this way, highly dispersed La2O3 nanoparticles with a size of 50 nm having excellent photoluminescence ability can be prepared.

1. INTRODUCTION Nanomaterials have been widely studied by virtue of their potential applications in many scientific fields. It is well-known that the properties of nanomaterials are quite different from those of their bulk counterparts, and the properties of nanomaterials are both size- and shape-dependent. Nanoscale La2O3 has been widely used as a catalyst,1 superconductor,2 exhaust-gas convector,3 hydrogen storage material,4 etc. Many methods to prepare nanosized La2O3 have been reported, including hydrothermal,5−8 solvothermal,9 chemical vapor deposition,10 and template-assisted routes.11 However, agglomeration of the nanoparticles occurs in many cases and has an adverse effect on the properties of the final material. Therefore, it is important to develop an economical and simple method of preparing highly dispersed nano-La2O3 without agglomeration. In our laboratory we have developed a rotating fluid film reactor based on a modified colloid mill12−14 that has been used to synthesize several nanomaterials on both laboratory and industrial scales, including layered double hydroxides (LDHs),15−17 Mg(OH)2,18 and BaSO4.19 In this reactor, two solutions are fed in and form a liquid thin film, in which very small crystal nuclei are produced almost instantaneously by a coprecipitation reaction under very high speed shearing forces. After exiting the reactor, the nuclei undergo a separate aging process. In a conventional preparation of La(OH)3 using this reactor, when La(NO3)3·6H2O solution and NaOH solution were mixed in this reactor, the resulting La(OH)3 nuclei were very small with high surface energy. In contrast to the nuclei of LDHs, Mg(OH)2, or BaSO4, these La(OH)3 nuclei are unstable and rapidly agglomerate to form secondary particles. Therefore, it is very important to find a way of preventing the agglomeration of the La(OH)3 precursor during the nucleation process. Furthermore, agglomeration of the primary particles must also be inhibited in the subsequent aging and calcination processes used to convert them into La2O3 nanoparticles. The surface of carbon black (CB) has a number of carboxyl, quinonoid, phenolic, and lactone groups.20−22 The latter three © 2012 American Chemical Society

groups can be converted into carboxyl groups by treatment with strong nitric acid, thus enhancing the surface activity.23−25 The negatively charged surface carboxyl groups can adsorb positive ions and therefore make it possible for their uniform deposition on the surface and prevent agglomeration of the resulting nanoparticles. In this work, modified carbon black (MCB) samples with varying amounts of carboxyl groups were prepared and added to the solution of La(NO3)3·6H2O used in the preparation of the La(OH)3 precursor in the rotating fluid film reactor. The efficacy of MCB in inhibiting particle agglomeration in the nucleation, growth, and calcination processes used in the preparation of La2O3 was investigated by scanning electron microscopy (SEM), thermogravimetric−differential thermal analysis (TG−DTA), and laser particle size analysis.

2. EXPERIMENTAL METHODS 2.1. Materials. Analytical grade La(NO3)3·6H2O, nitric acid, and sodium hydroxide were purchased from the Beijing Chemical Co., Ltd. Deionized water was used in all the experimental processes. Carbon Black (CB) with a particle size of about 40 nm was purchased from the Tianjin Jin Qiu Shi Carbon Black Co., Ltd. 2.2. Preparation of La2O3. CB was modified by treatment with strong nitric acid (65 wt %) at 68 °C for different times followed by heating at 120 °C for 24 h in an oven to produce activated MCB.26 The La(OH)3 precursor was synthesized using the rotating fluid film reactor followed by a separate aging process. Separate 125 mL portions of aqueous solutions of 0.3 M NaOH and 0.1 M La(NO3)3·6H2O containing MCB were simultaneously added at the same rate to the rotating fluid film Received: Revised: Accepted: Published: 14692

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SEM micrographs of the same four La(OH)3 samples prepared with different amounts of MCB are shown in Figure 2. The primary particles in all four samples have similar sizes of about 20 nm, consistent with the values calculated from the XRD data. It is clear from Figure 2a that the crystals prepared in the absence of MCB show extensive agglomeration, growing into large blocks. The extent of agglomeration decreased with increasing amounts of added MCB, and there was no agglomeration of the crystals when the loading of MCB amount reached 10 wt % (Figure 2d), showing that MCB can effectively prevent agglomeration of La(OH)3 during the nucleation process. The secondary particle size distributions of the four La(OH)3 samples prepared with different amounts of MCB, as determined by laser particle size analysis, are shown in Figure 3. The particle size distribution of La(OH)3 prepared in the absence of MCB spans the range 0.8−10 μm, consistent with the SEM image in Figure 2a. With increasing addition of MCB, the intensity of the peak at a particle size distribution of 0.8−10 μm decreases and the intensity of the peak at a particle size distribution of 0.05−0.5 μm increases. For an MCB loading of 10 wt %, the particle size distribution is essentially unimodal, with one peak in the range 0.05−0.5 μm. The data in Figure 3 confirm that MCB prevents the agglomeration of La(OH)3 during the nucleation process. 3.1.2. Effect of MCB Surface Properties on the Agglomeration of La(OH)3. Carbon black (CB) was treated by strong nitric acid at 68 °C for different times from 1 to 12 h to prepare MCB with varying surface properties. The quantitative FT-IR spectra of the resulting MCB samples are presented in Figure 4. Only very weak absorption peaks due to carboxyl groups at about 1730 and 1212 cm−1 can be seen in the spectrum of the unmodified CB sample. After modification with strong nitric acid, the strength of the carboxyl peaks increased markedly with increasing treatment time up to 8 h, which indicates the gradual formation of an increasing number of carboxyl groups on the MCB surface. However, for a longer treatment time of 12 h the carboxyl peak intensity decreased markedly, suggesting that oxidative decomposition of carboxyl groups occurs. SEM images of La(OH)3 prepared with the addition of 10 wt % of the different MCB samples are shown in Figure 5. It can be seen that the primary particles of all the samples have a similar size of about 20 nm, which is similar to that of the samples shown in Figure 2. When La(OH)3 was prepared with unmodified MCB, the nanosized crystals showed extensive agglomeration, growing into a large block (Figure 5a). This indicates that the very small number of carboxyl groups on the surface of CB are not able to prevent agglomeration of the nanosized primary particles of La(OH)3 into larger secondary particles. After CB was modified by strong nitric acid for 1 h, agglomeration of La(OH)3 nanoparticles was greatly reduced (Figure 5b), showing that the negatively charged carboxyl groups on the surface are able to prevent cross-linking between the La(OH)3 nanoparticles. The agglomeration of La(OH)3 nanoparticles further decreases with increasing modification time of CB because of the increasing number of carboxyl groups on the surface, and when the modification time was 8 h, there was essentially no particle agglomeration (Figure 5d). However, for a longer modification time of 12 h, slight agglomeration of La(OH)3 nanoparticles was observed, consistent with the reduced number of carboxyl groups on the MCB surface observed by FT-IR spectroscopy.

reactor using peristaltic pumps. The resulting slurry of La(OH)3 nuclei was aged in a three-necked flask at reflux temperature for 4 h. After washing and then drying at 80 °C for 12 h, the product was calcined in a muffle furnace at 900 °C for 2 h to produce La2O3 powder. 2.3. Characterization of Samples. Powder XRD patterns were recorded on a Rigaku XRD-6000 diffractometer, using Cu Kα radiation (λ = 0.154 18 nm) at 40 kV, 30 mA, with a scanning rate of 5°/min, in the 2θ range from 10° to 70°. Fourier transfer infrared (FT-IR) spectra were recorded on a Bruker Vector 22 spectrophotometer using the KBr disk method in the range from 4000 to 400 cm−1, with a resolution of 2 cm−1 and accumulation of 32 scans (for KBr disks, quantitative analysis was carried out with a weight ratio of sample to KBr of 1:100). SEM images were obtained using a Hitachi S-4700 scanning electron microscope operating at 20 kV. The specimens were prepared as follows: a sample was suspended in water and ultrasonicated for 10 min. A drop of the resulting suspension was then deposited onto a thin Si3N4 film. TG−DTA curves were obtained on a Beifen PCT-IA instrument in the temperature range 25−1000 °C in air. Fluorescence spectra were recorded on a Shimadzu RF-5301PC spectrofluorophotometer. A Malvern Mastersizer-2000 laser particle size analyzer was used for the analysis of agglomerate particle size distribution.

3. RESULTS AND DISCUSSION 3.1. Preparation of the La(OH)3 Precursor with MCB Added during the Nucleation Process. 3.1.1. Effect of Varying the Amount of MCB on the Agglomeration of the La(OH)3 Precursor. Figure 1 shows the XRD patterns of

Figure 1. XRD patterns of La(OH)3 prepared with addition of varying amounts of MCB: (a) 0 wt %, (b) 0.1 wt %, (c) 1 wt %, and (d) 10 wt %.

La(OH)3 samples prepared by nucleation in the rotating fluid film reactor in the absence of MCB (a) and increasing amounts of MCB from 0.1 wt % (b), through 1 wt % (c) to 10 wt % (d). The modification time of the MCB used here was 8 h. In each case, the main peaks at about 15.7°, 27.9°, 39.4°, and 48.6° 2θ can be indexed to the characteristic (100), (101), (201), and (211) reflections of La(OH)3, on the basis of the standard pattern (JCPDS No. 36-1481). The samples with different MCB loading have similar average crystal size calculated using the Scherrer formula,27 which are (a) 22.5 nm, (b) 23.7 nm, (c) 24.9 nm, and (d) 21.8 nm. 14693

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Figure 2. SEM micrographs of La(OH)3 prepared with addition of varying amounts of MCB: (a) 0 wt %, (b) 0.1 wt %, (c) 1 wt %, and (d) 10 wt %.

with the latter peak being the main one, indicative of extensive agglomeration. When the MCB was treated by strong nitric acid for 1 h, the smaller particle size distribution peak became dominant while the peak due to the larger particles decreased in intensity. When the MCB modified for 8 h was used, the particle size distribution was unimodal with a peak at about 0.05−0.5 μm. However, when the MCB modified for 12 h was used, a bimodal particle size distribution was again observed, showing that agglomeration had occurred. This trend in particle size distribution measured by laser particle size analysis is in accordance with the SEM data. From the above analysis of FT-IR, SEM, and particle size distribution data, we can see that the extent of agglomeration of La(OH)3 is closely related to the number of the surface carboxyl groups on MCB. This may be explained in terms of the adsorption and fixing of La3+ cations by the negatively charged surface carboxyl groups, which makes it possible for their uniform deposition on the MCB surface. Furthermore, the ζ-potential of La(OH)3 was found to be positive in the range of pH employed. Therefore, the surface positive charge of the La(OH)3 nuclei allows them to be also anchored on the MCB surface by electrostatic attraction, which contributes to the prevention of La(OH)3 agglomeration during the nucleation process. This process is illustrated schematically in Figure 7. When MCB was modified for 8 h, it had the maximum number of surface carboxyl groups and the strongest ability to adsorb and fix La3+ cations. Therefore, La(OH)3 can be uniformly deposited on the MCB surface. When the amount surface carboxyl groups on MCB was not sufficient to adsorb most of the La3+ and La(OH)3 nuclei, a large number remain in solution and agglomeration will occur. 3.2. Inhibition Effect of MCB on the Agglomeration of the La(OH) 3 Precursor during the Crystallization Process. Although the modified MCB is able to prevent agglomeration of the La(OH)3 particles during nucleation, it is

Figure 3. Particle size distribution of La(OH)3 prepared with addition of varying amounts of MCB: (a) 0 wt %, (b) 0.1 wt %, (c) 1 wt %, and (d) 10 wt %.

Figure 4. FTIR spectra of MCB prepared by modification with nitric acid for different times: (a) 0 h, (b) 1 h, (c) 4 h, (d) 8 h, and (e) 12 h.

The particle size distributions of La(OH)3 prepared with the addition of 10 wt % of the different MCB samples are shown in Figure 6. Two particle size distribution peaks at about 0.05−0.5 and 0.5−10 μm were observed when unmodified CB was used, 14694

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Figure 5. SEM micrographs of La(OH)3 synthesized using MCB prepared by modification with nitric acid for different times: (a) 0 h, (b) 1 h, (c) 4 h, (d) 8 h, and (e) 12 h.

crystallite sizes of the samples, calculated by the Scherrer formula, were 38.8 and 39.6 nm. Compared with the average size calculated from the XRD patterns in Figure 1, it can be seen that, as expected, the particle size of La(OH)3 increases after the aging process. SEM micrographs of La(OH)3 after crystallization with and without addition of MCB are shown in Figure 9. Consistent with the XRD data, both samples have a similar primary particle size of about 35 nm. However, the blocks of La(OH)3 prepared without MCB (Figure 2a) undergo even more extensive agglomeration, growing into larger blocks (Figure 9a). In contrast, in the presence of 10 wt % MCB, the nanoparticles of La(OH)3 grow uniformly without agglomeration from about 20 nm (Figure 2d) to 42 nm (Figure 9b), which shows that MCB is effective in preventing agglomeration of the La(OH)3 during the crystallization process. This can be explained in terms of the separation of the positively charged La(OH)3 particles by the negatively charged MCB particles, which prevents cross-linking and growth of the nanoparticles during crystallization. The particle size distributions of La(OH)3 crystallized with and without added MCB are shown in Figure 10. The size distribution of the former sample is mainly within the range 0.8−10 μm, indicating extensive agglomeration, consistent with Figure 9a. In contrast, the size distribution of the latter sample

Figure 6. Particle size distribution of La(OH)3 synthesized using MCB prepared by modification with nitric acid for different times: (a) 0 h, (b) 1 h, (c) 4 h, (d) 8 h, and (e) 12 h.

also necessary to ensure that the La(OH)3 nuclei do not undergo agglomeration during the subsequent aging (crystallization) process. Separate samples of La(OH)3 nuclei prepared in the absence of MCB and in the presence of 10 wt % of MCB modified by nitric acid for 8 h were aged at reflux temperature for 4 h. The XRD patterns of the resulting samples are shown in Figure 8. It can be seen that the diffraction peaks become sharper after aging, indicative of relatively well-formed crystals, and no peaks of impurity phases can be observed. The average 14695

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Figure 7. Schematic representation of the uniform deposition of La(OH)3 on the MCB surface.

Figure 10. Particle size distributions of La(OH)3 after crystallization prepared (a) without MCB and (b) with MCB.

Figure 8. XRD patterns of La(OH)3 after crystallization prepared (a) without MCB and (b) with MCB.

has a single peak at about 0.05−0.5 μm, indicative of very little agglomeration, consistent with Figure 9b. 3.3. Inhibition Effect of MCB on the Agglomeration of La2O3 during the Calcination Process. Samples of La(OH)3 prepared with and without addition of MCB were calcined at 900 °C for 2 h. The XRD patterns of the product are shown in Figure 11. All the diffraction peaks can be indexed as hexagonal La2O3 (JCPDS No. 83-1348), without any evidence of impurity phases. The intense narrow peaks indicate that the La2O3 samples are highly crystalline. According to the Scherrer formula, the average crystallite size of La2O3 is calculated to be about 52 nm (prepared without MCB) and 54 nm (prepared with MCB), showing that the particle sizes of the precursors increase during calcination due to sintering. The TG−DTA curves of La(OH)3 prepared with MCB are presented in Figure 12. Four weight-loss stages can be seen in the decomposition process. The first stage below 150 °C involves the removal of adsorbed water. The second and fourth stages, with two endothermic peaks at around 365 and 852 °C,

Figure 11. XRD patterns of La2O3 obtained by calcination of La(OH)3 precursors prepared (a) without MCB and (b) with MCB.

result from the two-step dehydration and decomposition of La(OH)3. The third stage from about 400 to 700 °C, with a strong exothermic peak at around 453 °C, results from the combustion of MCB.

Figure 9. SEM micrographs of La(OH)3 after crystallization prepared (a) without MCB and (b) with MCB. 14696

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Figure 14. TEM micrograph of La2O3 obtained by calcination of a precursor prepared with MCB.

Figure 12. TG and DTA curves of La(OH)3 prepared with MCB.

SEM micrographs of La2O3 derived from the calcination of La(OH)3 precursors prepared with and without MCB are shown in Figure 13. The primary particle size of both samples is about 50 nm, consistent with the XRD data. It is clear in Figure 13a that the La2O3 obtained by calcination of the precursor prepared without MCB undergoes considerable agglomeration, to an even greater extent than that of the corresponding precursor. However, the La2O3 particles obtained by calcination of the precursor prepared with MCB are highly dispersed without agglomeration, as is shown in Figure 13b. This may be attributed to the release of CO2 during the combustion of MCB, which effectively isolates the La2O3 particles at high temperature during the calcination process. A TEM micrograph of La2O3 derived from the calcination of a La(OH)3 precursor prepared with MCB is shown in Figure 14. After calcination, MCB has been removed by combustion. It is clear from this figure that the La2O3 particles are highly dispersed without agglomeration. The particle size distributions of La2O3 obtained from the calcination of La(OH)3 prepared with and without MCB are shown in Figure 15. The particle size distribution of the sample prepared without MCB varies over a broad range (0.8−10 μm) while the presence of MCB leads to a narrow distribution of small particles in the size range 0.1−0.7 μm. This indicates that MCB prevents the agglomeration of La2O3 during the calcination at high temperature, resulting in highly dispersed La2O3 nanoparticles.

Figure 15. The particle size distribution of La2O3 obtained by calcination of precursors prepared (a) without MCB and (b) with MCB.

3.4. Photoluminescence Properties of La2O3. It is wellknown that La2O3 is a photoluminescent material.28 The room temperature photoluminescence spectra obtained with an excitation wavelength of λex = 286 nm are shown in Figure 16. The strongest emission band located at 370 nm is a typical green band. Wang et al.29 have suggested it may be attributed to recombination of a delocalized electron close to the conduction band with a single charged state of a surface oxygen vacancy. Fluorescence efficiency30 is defined as Iflu/Aex (the ratio of the integral peak areas of the fluorescence emission and the absorbance at the excitation wavelength). The values of log(Iflu/ Aex) for La2O3 samples prepared with and without MCB are 4.65 and 3.76, respectively. The stronger fluorescence intensity of the sample prepared with MCB can be attributed to its

Figure 13. SEM micrographs of La2O3 obtained by calcination of precursors prepared (a) without MCB and (b) with MCB. 14697

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Figure 16. Room temperature photoluminescence spectra of La2O3 obtained by calcination of precursors prepared (a) without MCB and (b) with MCB.

higher surface-to-volume ratio and consequent higher density of single ionized oxygen vacancies,31 resulting from the small particle size and high particle distribution.

4. CONCLUSIONS MCB can be used as an agglomeration inhibitor to prepare nano-La2O3 by a coprecipitation method. La3+ cations can be adsorbed uniformly onto the MCB surface by electrostatic interaction with surface carboxyl groups, which favors uniform deposition of La(OH)3 nuclei. In the aging (crystallization) stage, the adsorption of La(OH)3 nuclei with a positive surface charge on the MCB surface also prevents agglomeration of La(OH)3 particles. Furthermore, MCB decomposes at about 400−700 °C and gives off carbon dioxide gas during the calcination of the precursor, which effectively prevents agglomeration of the La2O3 product at high temperature. Therefore, highly dispersed La2O3 nanoparticles with a size of ∼50 nm having excellent photoluminescence ability can be prepared. It should be possible to extend this method to the preparation of many other highly dispersed nanomaterials by simple and economical coprecipitation methods.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-64412125. Fax: +86-10-64425385. E-mail: linyj@ mail.buct.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Technologies R&D Program (Grant No. 2011BAE28B01) and the National Natural Science Foundation (Grant No. 21036001).

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REFERENCES

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