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Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References

The modified sol–gel technique was proposed to obtain ZnO nanoparticles with the sizes of 30–70 nm. The proposed method allows obtaining ultradispersed powdery composites with high chemical homogeneity, which are highly demanded raw products for the development of new functional materials of a wide destination range. Metal nitrates were used as the metal sources, and hexamethylenetetramine and monoethanolamine were used to sol stabilization. Acetylacetone was used as complexing agent to obtain ultradispersed ZnO powders. The obtained powders were characterized by use XRD, TEM, N2 adsorption–desorption, FTIR spectroscopy, and element analysis. According to the results, it can be observed that Bi2O3 or/and NiO with particle sizes of ≤5 and 8–20 nm, respectively, are dispersed uniformly on the surface of ZnO nanoparticles with sizes of 50–70 nm. The chemical nature of the decoration was confirmed by the presence of characteristic bands in the FTIR spectra, indicated the chemical interaction between all structural elements of compositions.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References

Ultradispersed ZnO powders are important raw products for new fine-grained ceramic materials for nanoelectronics, semiconductors with wide bandgap, and especially for the development of small electronic devices.[1, 2] However, production of the initial powders with high chemical homogeneity becomes particularly important in the development of the fine-grained ceramics with high operational characteristics. We suggest using, for this purpose, not just mixtures of ultradispersed metal oxide powders, but the powders consisting of ZnO nanoparticles, the surface of which was decorated with particles of other oxides. The scientific Periodicals contains quite many publications on decorating carbon and metal oxide nanotubes and SiO2 nanoparticles, the surface of which is decorated with gold or platinum,[3] silver[4] or metal oxide[5] particles. Physical methods are the most frequently used for the preparation of decorated nanostructures.[6-8] Much less attention is paid to decoration of inorganic particles, though a great fundamental and practical importance of such works. Apparently, this is due to the greater difficulties in the syntheses development.

It was reported that the nanostructured Bi2O3, NiO–Bi2O3 and NiO–ZnO powders exhibit high catalytic activity in the photodecolorization of methyl orange and methylene blue.[9-11] At the same time, it was found that UV–visible irradiation induces surface alteration of Bi2O3 leading to the formation of Bi2O3/Bi2O4−x nanocomposites with excellent photocatalytic activity.[10] The interphase interaction was shown in photocatalytic two-phase NiO–Bi2O3 nanocomposite.[11]

The possibility of using for varistor production the powders consisting of ZnO particles, the surface of which was covered with other metal oxides, is considered in some publications.[12-15] Previously, we reported about modified sol–gel technique of obtaining a wide variety of single ultradispersed metal oxide powders.[16-18] In this study, we report the results achieved with the development of the approach to obtaining of ultradispersed powdery compositions, consisting of ZnO nanoparticles, the surface of which was decorated with particles of Bi2O3 and/or NiO, which content in the compositions was 6 mol%. It has been shown how a way of obtaining a composite powder impact on the dispersion of its constituent oxides. The morphology and phase composition of Bi2O3- and/or NiO- decorated ZnO nanoparticles were investigated. These raw materials are very important for the developing of cost-effective technology of small size electronic devises.

2 Experimental Procedure

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References

Ultradispersed powders of oxides Bi2O3, NiO, and ZnO and composites based on them were synthesized by modified sol–gel technique using bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], nickel nitrate hexahydrate [Ni(NO3)2·6H2O)], and zinc nitrate hexahydrate [Zn(NO3)2·6H2O] as sources of metals, respectively. The appropriate amounts of the mineral salts were dissolved with stirring in deionized water to produce 0.05M solutions at temperature 60°C for 30 min. Hexamethylenetetramine was used for sol stabilization to obtain ZnO powders, and monoethanolamine (MEA) was used as sol stabilization to obtain Bi2O3 and NiO powders. Acetylacetone was used as complexing agent to obtain ultradispersed ZnO powders. The sol stabilizer to metal molar ratio value was 1:3. To obtain single-oxide powders, the synthesized sols were evaporated at temperature 80°C–90°C for the gel formation. During the subsequent heat treatment, these gels were calcined in air at 500°C for 3 h. The decoration of nanoparticles of calcined (500°C) ZnO powder with Bi2O3 and NiO particles was carried out using the same sol compositions that in the obtaining of single oxides. As-prepared Bi- or/and Ni-containing sols were formed and then evaporated at temperature 80°C–90°C in the presence of ZnO microsuspension obtained previously. So, gelation occurred directly on the ZnO nanoparticles surface, and then calcination was carried out according to the established schedule at 550°C for 1 h. In the obtained composite powders, the molar ratio values for Bi2O3/ZnO, NiO/ZnO, and Bi2O3/NiO/ZnO were 3/97, 3/97, and 3/3/94, respectively. The obtained powders were characterized by use XRD (DRON-3M with CuKα radiation, LNPO “Burevestnik”, St.-Petersburg, Russia), TEM (LEO 912ab_Omega, Carl Zeiss, Oberkochen, Germany), and N2 adsorption–desorption (NOVA 2200 Boyton Beach, PA, sample weight was 0.84–0.95 g) methods and element analysis (Aanalyst 400; Perkin Elmer, Waltham, MA). The refinement of the powders crystal structures was performed using the Rietveld method.[19]

3 Results and Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References

Ultradispersed ZnO powder, prepared according to previously described method,[17, 18] has been used as the basis for the obtaining of decorated nanoparticles. Figure 1 shows the X-ray diffraction pattern for the calcined at 500°C ZnO powder with the average crystallite size of 29 nm calculated by the Rietveld method (Table 1). ZnO powder was well crystallized (100%) in the wurtzite modification with share microdeformations in amounts not exceeding 0.02%. TEM data indicate that the ZnO crystallite sizes are 20–40 nm (Fig. 2), which is confirmed by XRD pattern. According to N2 adsorption–desorption data, ZnO powder has micro–mesoporous structure, as evidenced by V type adsorption–desorption isotherms [Fig. 3(a)]. The mesoporous structure of ZnO was bimodal, and the pores with sizes of 2–3 and 15–50 nm were observed [Fig. 3(b)], while BET surface was equal to 9.4 ± 0.3 m2/g.

Table 1. Morphological Parameters of the Obtained Powders (According to XRD and N2 Adsorption–Desorption Data)
 Phase compositionContent (wt%)Average crystallite size (nm)BET surface (m2/g)
  1. a

    Confirmed by elemental analysis.

  2. b

    Determined by elemental analysis.

ZnOWurtzite100299.4 ± 0.3
Bi2O3α, ω-Bi2O394>1000
Bi06
NiONiO90407.2 ± 0.2
Ni0108.2 ± 0.3
Bi2O3/ZnOZnO83427.0 ± 0.2
Bi2O314<5 
Bi38ZnO58+δ3 
NiO/ZnOZnO9647 
NiO4a40 
Bi2O3/NiO/ZnOZnO8270 
Bi2O316<5 
NiO2b28 
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Figure 1. XRD pattern for ZnO powder

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Figure 2. TEM images of ZnO powder

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Figure 3. N2 adsorption–desorption curves (a) and pore sizes distribution (b) for ZnO powder.

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Decorated particles of Bi2O3/ZnO composite were obtained using as-prepared Bi-containing sol and calcined at 500°C mesoporous ZnO powder. So, sol–gel transition and formation of Bi-containing gel were carried out directly within mesopores and on the surface of ZnO nanoparticles using sol–MEA combination. The formation of Bi2O3 crystalline particles on the entire available surface of ZnO nanoparticles occurred during subsequent thermal treatment at 550°C.

The TEM microphotos (Fig. 4) clearly show that the surface of ZnO nanoparticles is coated by particles with sizes much smaller, then 5 nm. According to XRD data (Fig. 5), in addition to wurtzite modification ZnO phase (hexagon), the content of which was 83 wt%, the composite Bi2O3/ZnO (Zn/Bi atomic ratio value was equal to 94/6) contained 17 wt% Bi-containing crystalline phase in which besides the tetragonal lattice Bi2O3 was presented. In the last at least 3 wt% of sillenite Bi38ZnO58+δ (JCPDS card no. 41-0253) with a cubic crystal structure (a = 10.200 Å) could be present. Accordingly, the calculation based on XRD data the particle size of Bi2O3 phase was equal to 2–4 nm, which is several orders smaller than in a single Bi2O3 powder, obtained by the same method (Table 1).

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Figure 4. TEM images of Bi2O3/ZnO powder. Triple junctions are marked with white circumferences.

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Figure 5. XRD pattern for Bi2O3/ZnO powder.

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According to the N2 adsorption–desorption data, Bi2O3/ZnO powder saved mesoporous structure (Fig. 6(a)], similar to the original structure of ZnO (Fig. 3(a)], but it has smaller pore volume. Although the pore size distribution also changed, the pores with sizes less than 3.4 nm almost disappeared, and the proportion of mesopores with sizes 3.5–4.0 nm decreased many times. At the same time, the pore size distribution in the region of more than 5 nm has become wider; however, their total volume was saved [Figs. 3(b) and 6(b)]. This fact demonstrates that the formation of Bi2O3 particles on the surface of ZnO nanoparticles occurred not only on the external surface but also within the mesopores. In this case, the BET surface of the composite is also reduced from 9.4 ± 0.3 (for initial ZnO) to 7.2 ± 0.2 m2/g (for Bi2O3/ZnO).

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Figure 6. N2 adsorption–desorption curves (a) and pore sizes distribution (b) for Bi2O3/ZnO powder.

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The decorated particles of composite NiO/ZnO were obtained using Ni-containing as-synthesized sol and calcined at 500°C mesoporous ZnO powder with the average particle size of 29 nm. Sol–gel transition was carried out within the mesopores space and on the surface of ZnO particles using sol-MEA combination. The formation of NiO particles on the surface of ZnO nanoparticles occurred during the subsequent thermal treatment at 500°C. TEM microphotos in Fig. 7 show that ZnO nanoparticles surface was covered with significantly smaller crystallites (sizes ~10 nm), which are also differs greater density from the ZnO particles. According to XRD data (Fig. 8), besides wurtzite-type crystal phase ZnO (96 wt%) composite contained 4 wt% nanocrystalline NiO (atomic ratio value Zn/Ni = 97/3).

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Figure 7. TEM images of NiO/ZnO powder.

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Figure 8. XRD pattern for NiO/ZnO powder.

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The crystal lattice parameters of the obtained ZnO were a = 3.249 and c = 5.205 Å, which practically exactly matches the reference data for wurtzite (JCPDS card no. 0-3-0888). This means that NiO is almost undissolved in ZnO crystal lattice. At the same time, the crystal lattice parameter of the cubic Ni-containing phases is not fully consistent with the known for NiO- 4.1768 Å (JCPDS card no. 04-0835) and its value is 4.1960 Å. The calculation by the Rietveld method showed that such interplanar spacing may correspond to Ni0.84Zn0.16O composition that indicates the formation of substitution solution based on the NiO crystal lattice. Apparently, there has been the formation of the NiO crystal lattice, within which the positions of Ni2+ (3d8 4s0) ions were partially isomorphically substituted by Zn2+ (3d10 4s0) ions. The observed increase in the NiO lattice parameter occurred due to the difference in the size of the Ni2+ and Zn2+ ions, equal to 0.69 and 0.74 Å, respectively. As well as in the case of Bi2O3/ZnO powder NiO dispersion in composite was multiples greater than its dispersion in a single powder (Table 1).

According to the N2 adsorption–desorption data, NiO/ZnO composite powder obtained by decoration of ZnO nanoparticles with NiO saved mesoporous structure similar to the structure of the original ZnO (Fig. 3), but with smaller pore volume (Fig. 9). At the same time pore size distribution also changed: pores with sizes of less than 6 nm completely disappeared (Fig. 9(b)]. Only mesopores with sizes more than 6 nm were preserved and the pore size distribution was wider than one for the initial ZnO powder (Figs. 3(b) and 9(b)]. The total pore volume as compared with the initial ZnO powder decreased significantly (Figs. 3 and 9). This indicates that the formation of NiO particles occurs not only on the geometric surface of ZnO particles but also inside the mesopores. BET surface of NiO/ZnO composite powder was equal to 8.2 ± 0.3 m2/g, which is also less than the BET surface of the initial ZnO equal to 9.4 ± 0.3 m2/g.

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Figure 9. N2 adsorption–desorption curves (a) and pore size distribution (b) for NiO/ZnO powder.

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On TEM microphoto in Fig. 10, it can be clearly seen that ZnO nanoparticles surface is coated with finer Bi2O3 particles, which have been discussed above (Fig. 4). It is also possible to observe the triple junctions formed in low-temperature sintering with the participation of Bi2O3–ZnO eutectic (Fig. 10). A similar image already has been observed in the study of the morphology of Bi2O3/ZnO composite (Fig. 4). Also NiO crystallites present, similar to ones observed in the microphotos of NiO/ZnO composite (Fig. 7).

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Figure 10. TEM image of Bi2O3/NiO/ZnO powder. Triple junctions are marked with white circumferences.

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According to XRD data, the sizes of Bi2O3 and NiO particles on the surface of ZnO nanoparticles (Fig. 11) were about 1.5–2.5 times higher than in the similar binary composites that corresponded to TEM investigation (Figs. 4, 7, and 10). Also, there are some interesting features of the powder phase composition revealed by XRD data (Fig. 11). Thus, according to the synthesis conditions and the results of elemental analysis, the NiO content is 2 wt%. At the same time Ni0.84Zn0.16O phase was not detected in the ternary composite Bi2O3/NiO/ZnO, although its presence has been shown reliably in NiO/ZnO composite.

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Figure 11. XRD pattern for Bi2O3/NiO/ZnO powder.

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According to N2 adsorption–desorption (−196°C) on Bi2O3/NiO/ZnO powder after decorating of ZnO nanoparticles retained the mesoporous structure similar to the mesoporous structure of the initial ZnO, however, with the decrease in the pore volume [Figs. 12(a) and 3(a), respectively]. The pore size distribution also changed as the pores with sizes less than 5 nm have disappeared completely (Fig. 12). Only mesopores larger than 5 nm were preserved, and the pore size distribution became wider [Fig. 12(b)] than in initial ZnO powder [Fig. 3(b)]. The total pore volume in comparison with the initial powder ZnO significantly decreased (compare Figs. 12 and 3). This suggests that the formation of Bi- and Ni-containing phases on the ZnO surface occurred not only on the surface, but inside the mesopores too. BET surface of composite Bi2O3/NiO/ZnO powder was 7.0 ± 0.2 m2/g, that also less than BET surface of initial ZnO (9.4 ± 0.3 m2/g). As in the case of Bi2O3/ZnO composites, the formation of the solid solution Bi38ZnO58+δ occurred, however, Ni0.84Zn0.16O did not formed, or that phase was X-ray amorphous. Figure 13 shows FTIR spectroscopy results for three obtained composites NiO/ZnO, Bi2O3/ZnO, and NiO/Bi2O3/ZnO. It has been shown, that three-component composite spectrum was not a superposition of the binary samples spectra. So, all characteristic bands of NiO/ZnO and Bi2O3/ZnO spectra were shifted in NiO/Bi2O3/ZnO spectrum, which indicated the chemical interaction between structural elements of all three oxides and confirm the formation of the decorated ternary system.

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Figure 12. N2 adsorption–desorption curves (a) and pores sizes distribution (b) for Bi2O3/NiO/ZnO powder.

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Figure 13. FTIR spectra of the decorated ZnO powders.

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4 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References

It was shown that modified sol–gel technique has opened a wide avenue for the obtaining of metal oxides nanoparticles decorated by others oxides. The development method allows to obtain the powders consisting of decorated ZnO nanoparticles in large quantities with a high reproducibility of physicochemical properties and a quantitative yield. Thus, Bi2O3 and NiO crystallite growth on the ZnO nanoparticle surface was carried under controlled conditions. The chemical nature of the decoration was confirmed by the presence of characteristic bands in the FTIR spectra of composite, indicated the chemical interaction between all structural elements of compositions. It is very important for obtaining chemically homogeneous ultradispersed powdery raw products for the development of new functional materials for various applications.

5 Acknowledgments

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References

The authors gratefully acknowledge support from the Russian Foundation for Basic Research, grant nos. 12-08-31052_mol_a and 13-03-00350_a, Drs. S.S. Abramchuk and S.A. Pisarev for TEM investigation, Dr. E.V. Shelerkhov for XRD investigation, and Dr. S.V. Kutsev for N2 adsorption–desorption investigation.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. 5 Acknowledgments
  8. Conflicts of interest
  9. References
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