Info

and catalytic activity of LaNiO3 were reported. The result of TEM showed that the ultrasound could cause a decrease of particle size. The average particle size of LaNiO3 prepared by sonochemistry is 20 nm. The specific surface area of LaNiO3 is 11.27 m2 • g-1 by BET. The O-ads/O-ads on the surface are 1.25 (ultrasound) and 1.01 (without ultrasound) by XPS. This shows that ultrasound could lead to increased surface content oxide and an increased number of surface crystal oxygen vacancies. The result of TPR showed that LaNiO3 prepared by sono-chemistry has a lower reduction temperature and a larger ratio of surface oxygen to crystal oxygen. The result of the evaluation of catalytic activity showed that ultrasound could increase the catalytic activity of LaNiO3 for NO decomposition.

An example of a more complicated mixed metal oxide system is the perovskite, La07Sr03FeO3. This material was synthesized by coprecipitation using both a Five Star Technologies cavitational processor [74] and conventional highspeed mechanical mixing under otherwise similar conditions for comparison purposes. The XRD results for materials obtained from both processes are summarized in Figure 16. The material obtained via the classical precipitation technique is shown at the top of the figure. This pattern shows that large amounts of separate phase compounds are present in addition to the desired perovskite material. The other four XRD patterns were processed for materials obtained by cavitational synthesis under various process conditions. It can be seen that all four of these materials exhibited high phase purity. Another multimetallic perovskite, LaCoO3, has also been synthesized using sonochemical processing [75]. XRD analysis confirmed the phase purity of this material with a crystallite grain size of 33 nm. The SEM photograph of this material is shown in Figure 17. The base crystallites that are present in the large agglomerates of this material

4500 i 4000-r 3500 -

4500 i 4000-r 3500 -

0 10 20 30 40 50 60 70 80 90 Diffraction Angle 20

Figure 16. Example of a complicated mixed metal oxide system is the perovskite La07Sr03FeO3. This material was synthesized by coprecipitation using both a Five Star Technologies cavitational processor and conventional high-speed mechanical mixing under otherwise similar conditions for comparison purposes. The XRD results for materials obtained from both processes are summarized in the following figure. The material obtained via the classical precipitation technique is shown at the top of the figure. This pattern shows that large amounts of separate phase compounds are present in addition to the desired perovskite material. The next four XRD patterns were processed for materials obtained by cavitational synthesis under various process conditions. It can be seen that all four of these materials exhibited high phase purity. Reprinted with permission from [75], W. R. Moser et al., Worcester Polytechnic Institute. © Five Star Technologies.

0 10 20 30 40 50 60 70 80 90 Diffraction Angle 20

Figure 16. Example of a complicated mixed metal oxide system is the perovskite La07Sr03FeO3. This material was synthesized by coprecipitation using both a Five Star Technologies cavitational processor and conventional high-speed mechanical mixing under otherwise similar conditions for comparison purposes. The XRD results for materials obtained from both processes are summarized in the following figure. The material obtained via the classical precipitation technique is shown at the top of the figure. This pattern shows that large amounts of separate phase compounds are present in addition to the desired perovskite material. The next four XRD patterns were processed for materials obtained by cavitational synthesis under various process conditions. It can be seen that all four of these materials exhibited high phase purity. Reprinted with permission from [75], W. R. Moser et al., Worcester Polytechnic Institute. © Five Star Technologies.

Figure 17. LaCoO3, also synthesized using the Five Star processing technologies. XRD analysis confirmed the phase purity of this material with a crystallite grain size of 33 nm. The SEM photograph of this material is shown below. The base crystallites that are present in the large agglomerates of this material are all quite similar in size. The use of such materials with a narrow distribution of primary particles could be quite useful in the development of an effective pore structure for catalytic applications. Reprinted with permission from [75], W. R. Moser, Five Star Technologies. © Five Star Technologies. BHR Group (1999).

Figure 17. LaCoO3, also synthesized using the Five Star processing technologies. XRD analysis confirmed the phase purity of this material with a crystallite grain size of 33 nm. The SEM photograph of this material is shown below. The base crystallites that are present in the large agglomerates of this material are all quite similar in size. The use of such materials with a narrow distribution of primary particles could be quite useful in the development of an effective pore structure for catalytic applications. Reprinted with permission from [75], W. R. Moser, Five Star Technologies. © Five Star Technologies. BHR Group (1999).

are all quite similar in size. The use of such materials with a narrow distribution of primary particles could be quite useful in the development of an effective pore structure for catalytic applications.

4.4.2. Spinel Oxides

Shin and co-workers [76] demonstrated the use of ultrasound in accelerating the crystallization of nanosize ferrite powders. The effects of ultrasonic waves on the crystallization were studied for ferrite powders prepared using the coprecipitation method. The process of crystallization was evaluated employing XRD. The ferrite powder prepared without ultrasonic irradiation by the coprecipitation method was amorphous. However, the ferrite prepared with ultrasonic irradiation for 5 h was observed to crystallize, and the crystallization of the ferrite powder became more enhanced in proportion to the time of ultrasonic irradiation. The ferrite powder prepared with ultrasonic irradiation for 25 h had a higher crystallinity and a larger specific surface area than the ferrite powder calcined at 500 0 C for 2 h after preparing with coprecipitation. The prepared ferrite powders were a nanosize crystal phase. Jeevanandam and co-workers [77] reported a nanosized nickel aluminate spinel with the aid of ultrasound radiation by a precursor approach. Sonicating an aqueous solution of nickel nitrate, aluminum nitrate, and urea yields a precursor which, on heating at 950 0 C for 14 h, yields nanosized NiAl2O4 particles with a size of ca. 13 nm and a surface area of about 108 m2 • g-1. Vijayakumar and co-workers [78] reported the synthesis of Fe3O4 by sonica-tion of iron (II) acetate in water under an argon atmosphere. The prepared Fe3O4 nanoparticles are superparamagnetic, and their magnetization at room temperature is very low (<1.25 emu • g-1).

4.4.3. Complex Oxides

Shafi and Gedanken [79] prepared garnet phases by the sonochemical decomposition of the solutions of organic precursors, for the synthesis of nanostructured crystalline single-phase BaFe12O19 particles. Nanosized amorphous precursor powders for BaFe12O19, were prepared by the sonochemical decomposition of the solutions of Fe(CO)5 and Ba[OOCCH(C2H5)C4H9]2 in decane, under air at 273 K. The amorphous phase, when heated, shows the formation of single-phase BaFe12O19, and the TEM micrograph revealed near uniform platelets with sizes less than 50 nm. The observed magnetization measured up to a field of 1.5 kG of the nanocrystalline BaFe12O19 sample (48 emu • g-1) was significantly lower than that for the reported multidomain bulk particles (72 emu • g-1), reflecting the ultrafine nature of the sample. In a further study, Shafi and co-workers [79] reported the formation of Olympic-ring-like colloidal hexaferrite particles without the use of a surfactant (Fig. 18). The intersection of two rings is amazing, as this is in direct contradiction to the proposed mechanism for the ring formation, based on the dry hole formation on an evaporating thin film completely wetted to the substrate. The creation of this unique feature is attributed to the interplay of magnetic forces with the regular particle-substrate interactions.

4.4.4. Mixed Oxides

The nonhydrolytic reaction of FeCl3 with (Z-C3H7O)4Ti induced by sonication at 353 K forms bimetallic oxides, in which the iron and titanium ions are linked together by oxygen; the product remains amorphous, even when heated at 773 K for 2 h. When heated to 1073 K, the iron ions in

Figure 18. Transmission electron micrographs of amorphous BaFe12O19 nanoparticles prepared by a sonochemical decomposition technique showing Olympic rings formation. The creation of this unique feature is attributed to the interplay of magnetic forces with the regular particle-substrate interactions. Reprinted with permission from [79], K. V. P. M. Shafi et al., J. Phys. Chem. B 103, 3358 (1999). © 1999, Royal Society of Chemistry.

Figure 18. Transmission electron micrographs of amorphous BaFe12O19 nanoparticles prepared by a sonochemical decomposition technique showing Olympic rings formation. The creation of this unique feature is attributed to the interplay of magnetic forces with the regular particle-substrate interactions. Reprinted with permission from [79], K. V. P. M. Shafi et al., J. Phys. Chem. B 103, 3358 (1999). © 1999, Royal Society of Chemistry.

the bimetallic oxide exist in the form of crystalline Fe2TiO5. The Fe2O3 in the products obtained by sonicating the mixture of Fe(CO)5 and (C2H5O)4Ti at 313 K is amorphous; in contrast, when pure Fe(CO)5 is sonicated under the same conditions, the presence of titanium compounds affects the crystallization of Fe2O3. The amorphous Fe2O3 is easily reduced and reacts with TiO2, forming FeTiO3 in the presence of a reducing agent and calcination under vacuum. The particles of the as-prepared products are all of nanometric size, but they aggregate to form Fex Oy-TiO2 [80].

4.5. Nanometal Chalcogenides

Xu H. and co-workers [81] reported a novel method for the preparation of copper monosulfide (CuS) and nickel monosulfide (NiS) nanoparticles via a sonochemical route from an aqueous solution containing metal acetate [Cu(CH3COO)2 or Ni(CH3COO)2] and thioacetamide (TAA) in the presence of triethanolamine (TEA) as a complexing agent under ambient air. The as-prepared nanoparticles have a regular shape, a narrow size distribution, and high purity. It is found to be a mild, convenient, and efficient method for the preparation of CuS and NiS nanoparticles. In fact, a sonochemical route has been successfully established to prepare copper selenide nanocrystals with different phases. It is found that the ratio of [Cu2+ ]/[SeSO3- ] determines the phases of the products. The size of Cu2-xSe nanocrystals can be controlled by using different complexing agents. This route is proved to be a convenient, mild, and energy-efficient route for the preparation of copper selenide nanocrystals with different phases. Mdleleni and co-workers reported the sonochemical synthesis of nanostructured molybdenum sulfide [82]. MoS2 is best known for its layered structure and as a desulfurization catalyst. MoS2 was prepared by irradiating solutions of molybdenum hexacarbonyl and sulphur in 1,2,3,5-tetramethylbenzene with high-intensity ultrasound. The MoS2 was amorphous as initially prepared (see Fig. 19), but subsequently crystallized upon heating at 725 K for 10 h under an atmosphere of flowing He. The sonochemically prepared MoS2 catalyzes the HDS of thiophene with activities roughly fivefold better than conventional MoS2 and comparable to those observed with RuS2, one of the best

Figure 19. Morphology of MoS2 prepared by (a) conventional and (b) sonochemical method. Reprinted with permission from [82], M. M. Mdleleni et al., J. Am. Ceram. Soc. 120, 6189 (1998). © 1998, American Chemical Society.

Figure 19. Morphology of MoS2 prepared by (a) conventional and (b) sonochemical method. Reprinted with permission from [82], M. M. Mdleleni et al., J. Am. Ceram. Soc. 120, 6189 (1998). © 1998, American Chemical Society.

prior catalysts. Nikitenko et al. have reported the synthesis of amorphous WS2 by the ultrasound irradiation of a W(CO)6 solution in diphenylmethane (DPhM) in the presence of a slight excess of sulfur at 90 0 C under argon. Heating the amorphous powder at 800 0 C under argon yields WS2 nanorods and their packings. The average size of WS2 nanorods was found to be 3-10 nm and 1-5 fim in thickness and length, respectively [83].

Vijayakumar and co-workers [84] reported the preparation of Ag2S and CuS-polyvinyl actetate composites. Li et al. developed nanocrystalline silver tellurides in organic solvent systems by high-intensity ultrasonic irradiation at room temperature. Ag2Te and Ag7Te4 are prepared in an ethylenediamine system and an ethanol system, respectively. The ultrasonic irradiation and the solvents are both important in the formation of products [85]. Nanospherical Ag2S/PVA and nanoneedles of CuS/PVA composite have been prepared by sonochemical irradiation of an ethylenediamine-water solution of elemental sulfur, silver nitrate, or copper acetate in the presence of polyvinyl alcohol. The particles are 25 and 225 nm for Ag2S/PVA and CuS/PVA nanocomposites, respectively. Both Ag2S and CuS, being narrow-bandgap nanocrystalline semiconductors, find significant applications in many technological applications, ranging from microelectronics to nonlinear optics, optoelectronics, catalysis, and photoelectrochemistry. Bandgaps of 1.05 and 2.08 eV have been estimated for Ag2S/PVA and CuS/PVA nanocomposites.

ZnSe nanoparticles about 3 nm in size have been prepared by the sonochemical irradiation of an aqueous solution of seleno urea and zinc acetate under argon [86]. This sonochemical method was found to be a general method for the preparation of other selenides as well. Figure 20 shows the X-ray diffraction pattern of sonochemically processed as-prepared powders after 1, 2, and 3 h of sonication. Zhu and co-workers also reported [87] that PbSe nanoparticles about 12 nm in size were prepared by a pulse sonoelec-trochemical technique from an aqueous solution of sodium seleno-sulfate and lead acetate. The effects of changing the various parameters on particle size were discussed, and possible explanations were offered. A bandgap of 1.10 eV was estimated from optical measurement of the nanoparticles.

20(Deg)

Figure 20. X-ray diffraction patterns of the as-prepared ZnSe after sonication of (a) 1, (b) 2, and (c) 3 h. Reprinted with permission from [86], J. Zhu et al., Chem. Mater. 12, 73 (2000). © 2000, American Chemical Society.

0 0

Post a comment