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Hydrothermal preparation of nanocrystalline ZnO2
Particuology 10 (2012) 388–391
Contents lists available at SciVerse ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Short communication
Hydrothermal preparation of nanocrystalline ZnO2 Guipeng Feng a , Liyun Yang a,∗ , Tianxi Wang b , Jiamin Zhang b , Tianjun Lou b a b
Department of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang 453000, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
a r t i c l e
i n f o
Article history: Received 25 March 2011 Received in revised form 7 June 2011 Accepted 16 June 2011 Keywords: Zinc peroxide Nanocrystals Semiconductors Hydrothermal synthesis Photoluminescence
a b s t r a c t A green hydrothermal method was proposed for the synthesis of nanocrystalline ZnO2 , using Zn5 (CO3 )2 (OH)6 powder and 6 vol% H2 O2 aqueous solution as the starting materials. Characterization results from X-ray diffraction, Raman, high resolution transmission electron microscopy and selected area electron diffraction revealed that the products synthesized at 80–120 ◦ C for 6–18 h were pure cubic phase ZnO2 nanocrystals. Room temperature photoluminescence spectra of the as-synthesized ZnO2 nanocrystals displayed a wide and strong emission band in the visible region of about 525–570 nm upon laser excitation at 325 nm, which may have originated from their surface state and other crystal defects. © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction ZnO2 , an important semiconductor material with a wide band gap of about 4.1–4.5 eV (Chen et al., 2009), can be used as a luminophor (Bai & Liu, 2010; Cheng et al., 2009), photocatalyst (Hsu & Wu, 2005), and precursor to ZnO (Drmosh, Gondal, Yamani, & Saleh, 2010; Escobedo-Morales et al., 2011; Gondal, Drmosh, Yamani, & Saleh, 2009; Rosenthal-Toib, Zohar, Alagem, & Tsur, 2008; Sun, Hao, Wang, & Wang, 2007; Zhang, Wu, Hu, & Guo, 2005). Recently, great effort has been devoted to the synthesis of ZnO2 nanoparticles, driven by scientific understanding and the hope for better performance and novel applications (Bai & Liu, 2010; Chen et al., 2009; Cheng et al., 2009; Drmosh et al., 2010; EscobedoMorales et al., 2011; Gondal et al., 2009; Rosenthal-Toib et al., 2008; ˝ Szabó, & Dékány, 2009; Sun et al., 2007; Zhang et al., 2005). Sebok, However, most of the current methods are difficult to scale-up at low cost in an environmentally friendly way. Hydrothermal techniques have been widely used to prepare inorganic nanomaterials (Chen et al., 2010; Jiao, Wang, Ke, Sun, & Demir, 2011; Peng, Beysen, Li, Sun, & Yang, 2010; Zhang et al., 2010), not only inducing the formation of well-crystallized products at low temperatures (<200 ◦ C), but also controlling the size and shape of the products (Chen et al., 2010; Jiao et al., 2011; Peng et al., 2010; Zhang et al., 2010). We report the hydrothermal synthesis of nanocrystalline ZnO2 from Zn5 (CO3 )2 (OH)6 powder
∗ Corresponding author. Tel.: +86 0373 3682674. E-mail address: [email protected] (L. Yang).
and 6 vol% H2 O2 aqueous solution at 80–120 ◦ C, as well as the characterization of the resulting products by X-ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and room temperature photoluminescence (RTPL). Compared to current methods (Chen et al., 2009; Drmosh et al., 2010; EscobedoMorales et al., 2011; Gondal et al., 2009; Hsu & Wu, 2005; Lindroos & Leskela, 2000; Ortega-López, Avila-García, Albor-Aguilera, & ˝ et al., Sánchez Resendiz, 2003; Rosenthal-Toib et al., 2008; Sebok 2009; Zhang et al., 2005), our proposed method has at least three distinct advantages: (1) the source material, Zn5 (CO3 )2 (OH)6 (hydrozincite) is a natural ore, plentiful and cheap; (2) no need for controlling the solution pH with ammonia, ethylenediamine or NaOH, and (3) no release of toxic byproducts.
2. Materials and methods All the chemical reagents used are of analytical grade. The Zn5 (CO3 )2 (OH)6 powder, which has a lamellar morphology with a thickness of around 20–40 nm, was purchased from Sinopharm Chemical Reagent Co., Ltd. In a typical procedure, 0.5 g of Zn5 (CO3 )2 (OH)6 powder was first weighted into a Teflon-lined stainless steel autoclave of 50 mL-capacity, and then 40 mL of 6 vol% H2 O2 aqueous solution was added with stirring. The autoclave was sealed and heated at 80–120 ◦ C for 6–18 h, and then allowed to cool to room temperature naturally. The as-formed precipitates were centrifuged, washed with distilled water and ethanol, and dried in air at 80 ◦ C. The obtained products were characterized by XRD (German Bruker AXS D8 ADVANCE X-ray diffractometer),
1674-2001/$ – see front matter © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2011.06.010
G. Feng et al. / Particuology 10 (2012) 388–391
389
Fig. 1. XRD patterns of products obtained via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at (a) 80 ◦ C, (b) 100 ◦ C and (c) 120 ◦ C for 6 h, and (d) at 100 ◦ C for 18 h.
Raman (Britain Renishaw Invia Raman spectrometer, excitation at 532 nm, 10 mW), HRTEM and SAED (American FEI Tecnai G2 F30 S-TWIN field-emission transmission electron microscope), and RTPL (Britain Renishaw Invia Raman spectrometer equipped with a He–Cd laser, excitation at 325 nm). 3. Results and discussion Fig. 1(a)–(d) shows the XRD patterns of the products obtained via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at 80–120 ◦ C for 6–18 h. All the products in Fig. 1(a)–(d) displayed only the characteristic XRD peaks of cubic phase ZnO2 (JCPDS card No. 01-077-2414). Moreover, no XRD peak of monoclinic phase Zn5 (CO3 )2 (OH)6 was visible, indicating that the starting material, monoclinic Zn5 (CO3 )2 (OH)6 , had been completely transformed into ZnO2 under the current hydrothermal conditions. Fig. 2(a)–(d) shows the Raman spectra of the as-synthesized products obtained at different reaction conditions, all exhibiting four peaks at around 411, 472, 838 and 936 cm–1 , consistent with
Fig. 3. (a) TEM image of the ZnO2 nanocrystals synthesized at 100 ◦ C for 6 h, (b) HRTEM image and (c) SAED pattern of a typical ZnO2 nanocrystal.
Fig. 2. Raman spectra of products of hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at (a) 80 ◦ C, (b) 100 ◦ C and (c) 120 ◦ C for 6 h, or (d) at 100 ◦ C for 18 h.
the Raman spectra of ZnO2 nanoparticles reported in literature (Bai & Liu, 2010; Drmosh et al., 2010; Sun et al., 2007), but different from the Raman spectra of Zn5 (CO3 )2 (OH)6 (Yang, Feng, & Wang, 2010) and ZnO (Dong et al., 2010; Pradhan, Kumar, Ando,
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G. Feng et al. / Particuology 10 (2012) 388–391
2010; Moreira, Andrés, Varela, & Longo, 2009). The “broken” surface or dangling bonds and other crystal defects can form various animated energy levels localized within the forbidden gap of ZnO2 , giving rise to their visible RTPL. When the reaction temperature or the reaction time was increased, the structure of the resulting ZnO nanocrystals became more ordered and integrated, which can be inferred from their stronger and sharper XRD peaks in Fig. 1. So, their photoemission intensities related to the surface state and other crystal defects also decreased. 4. Conclusions
Fig. 4. RTPL spectra of ZnO2 nanocrystals obtained via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at (a) 80 ◦ C, (b) 100 ◦ C and (c) 120 ◦ C for 6 h, and (d) at 100 ◦ C for 18 h.
& Leung, 2009; Wermelinger, Mornaghini, Hinderling, & Spolenak, 2010). Fig. 3(a) shows the TEM image of the ZnO2 powder synthesized at 100 ◦ C for 6 h, while Fig. 3(b) and (c) shows, respectively the HRTEM image and SAED pattern of this product. Fig. 3(a) shows that this product consists of nanocrystals with a size of 5–10 nm. The clear lattice fringes in the HRTEM image (Fig. 3(b)) reveal that the synthesized ZnO2 is well-crystallized. The measured interplanar spacing of about 0.244 nm corresponds to the (2 0 0) crystal plane of cubic ZnO2 . Besides, the annular SAED pattern in Fig. 3(c) further confirms the formation of cubic polycrystalline ZnO2 . Because the starting materials in our synthesis of ZnO2 were only Zn5 (CO3 )2 (OH)6 powder and 6 vol% H2 O2 aqueous solution (H2 O2 used as O2 2− source), and ZnO2 was the sole resulting solid after the hydrothermal reaction at 80–120 ◦ C for 6–18 h, it is believed that the mechanism of ZnO2 formation may be described as: Zn5 (CO3 )2 (OH)6 + 5H2 O2 → 5ZnO2 + 8H2 O + 2CO2 Since no valence change of Zn and other elements occurred, the formation of ZnO2 was simple precipitation. Although there was no available information about the solubility constant of ZnO2 , it is believed that the solubility of ZnO2 in water is smaller than that of Zn5 (CO3 )2 (OH)6 . This may be inferred from the facts that Zn5 (CO3 )2 (OH)6 first dissolves in 5 vol% ammonia solution to form Zn(NH3 )4 2+ , subsequently the addition of H2 O2 into the Zn(NH3 )4 2+ solution can produce a precipitate of ZnO2 , implying that the transformation reaction of Zn5 (CO3 )2 (OH)6 → Zn(NH3 )4 2+ → ZnO2 in water is thermodynamically allowable. Therefore, Zn5 (CO3 )2 (OH)6 has a tendency to transform into ZnO2 in the H2 O2 aqueous solution, and this transformation process can be completed under the hydrothermal conditions of 80–120 ◦ C for 6–18 h. Fig. 4 shows the RTPL spectra of the as-synthesized ZnO2 nanocrystals upon laser excitation at 325 nm, showing all products displaying each a wide and strong emission band in the visible region of 525–570 nm. However, the photoemission intensity of ZnO2 nanocrystal decreased with increasing reaction temperature and reaction time. As previously reported, there was inevitably a large amount of “broken” surface or dangling bonds and other crystal defects (structural disorder at short and intermediate ranges, etc.) in the nanocrystalline products (Anicete-Santos et al., 2007; Li, Zhang, Wang, & Zhang, 2011; Macario, Moreira, Andrés, & Longo,
Nanocrystalline ZnO2 powder was synthesized via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at 80–120 ◦ C for 6–18 h. The proposed method was simple, green and cost-effective, and suitable for large-scale production of multifunctional ZnO2 nanocrystals. The as-synthesized ZnO2 nanocrystals exhibited a wide and strong RTPL band in the visible region of 525–570 nm upon laser excitation at 325 nm, suggesting their use as a photoluminescent material. References Anicete-Santos, M., Orhan, E., de Maurera, M. A. M. A., Simões, L. G. P., Souza, A. G., Pizani, P. S., et al. (2007). Contribution of structural order-disorder to the green photoluminescence of PbWO4 . Physical Review B, 75, 165105. Bai, H. & Liu, X. (2010). Green hydrothermal synthesis and photoluminescence property of ZnO2 nanoparticles. Materials Letters, 64, 341–343. Chen, W., Lu, Y. H., Wang, M., Kroner, L., Paul, H., Fecht, H. J., et al. (2009). Synthesis, thermal stability and properties of ZnO2 nanoparticles. The Journal of Physical Chemistry C, 113, 1320–1324. Chen, J. S., Zhu, T., Hu, Q. H., Gao, J. J., Su, F. B., Qiao, S. Z., et al. (2010). Shape-controlled synthesis of cobalt-based nanocubes, nanodiscs, and nanoflowers and their comparative lithium-storage properties. ACS Applied Materials & Interfaces, 2, 3628–3635. Cheng, S., Yan, D., Chen, J. T., Zhuo, R. F., Feng, J. J., Li, H. J., et al. (2009). Soft-template synthesis and characterization of ZnO2 and ZnO hollow spheres. The Journal of Physical Chemistry C, 113, 13630–13635. Dong, J. J., Zhang, X. W., You, J. B., Cai, P. F., Yin, Z. G., An, Q., et al. (2010). Effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films: Identification of hydrogen donors in ZnO. ACS Applied Materials & Interfaces, 2, 1780–1784. Drmosh, Q. A., Gondal, M. A., Yamani, Z. H. & Saleh, T. A. (2010). Spectroscopic characterization approach to study surfactants effect on ZnO2 nanoparticles synthesis by laser ablation process. Applied Surface Science, 256, 4661–4666. Escobedo-Morales, A., Esparza, R., García-Ruiz, A., Aguilar, A., Rubio-Rosas, E. & Pérez, R. (2011). Structural and vibrational properties of hydrothermally grown ZnO2 nanoparticles. Journal of Crystal Growth, 316, 37–41. Gondal, M. A., Drmosh, Q. A., Yamani, Z. H. & Saleh, T. A. (2009). Synthesis of ZnO2 nanoparticles by laser ablation in liquid and their annealing transformation into ZnO nanoparticles. Applied Surface Science, 256, 298–304. Hsu, C.-C. & Wu, N. L. (2005). Synthesis and photocatalytic activity of ZnO/ZnO2 composite. Journal of Photochemistry and Photobiology A: Chemistry, 172, 269–274. Jiao, Z., Wang, J., Ke, L., Sun, X. W. & Demir, H. V. (2011). Morphology-tailored synthesis of tungsten trioxide (hydrate) thin films and their photocatalytic properties. ACS Applied Materials & Interfaces, 3, 229–236. Li, J., Zhang, Y. C., Wang, T. X. & Zhang, M. (2011). Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3 )2 ·4H2 O and TiO2 nanocrystals. Materials Letters, 65, 1556–1558. Lindroos, S. & Leskela, M. (2000). Growth of zinc peroxide (ZnO2 ) and zinc oxide (ZnO) thin films by the successive ionic layer adsorption and reaction-SILARtechnique. The International Journal of Inorganic Materials, 2, 197–201. Macario, L. R., Moreira, M. L., Andrés, J. & Longo, E. (2010). An efficient microwaveassisted hydrothermal synthesis of BaZrO3 microcrystals: Growth mechanism and photoluminescence emissions. CrystEngComm, 12, 3612–3619. Moreira, M. L., Andrés, J., Varela, J. A. & Longo, E. (2009). Synthesis of fine micro-sized BaZrO3 powders based on a decaoctahedron shape by the microwave-assisted hydrothermal method. Crystal Growth & Design, 9, 833–839. Ortega-López, M., Avila-García, A., Albor-Aguilera, M. L. & Sánchez Resendiz, V. M. (2003). Improved efficiency of the chemical bath deposition method during growth of ZnO thin films. Materials Research Bulletin, 38, 1241–1248. Peng, D., Beysen, S., Li, Q., Sun, Y. & Yang, L. (2010). Hydrothermal synthesis of monodisperse ␣-Fe2 O3 hexagonal platelets. Particuology, 8, 386–389. Pradhan, D., Kumar, M., Ando, Y. & Leung, K. T. (2009). Fabrication of ZnO nanospikes and nanopillars on ITO glass by templateless seed-layer-free
G. Feng et al. / Particuology 10 (2012) 388–391 electrodeposition and their field-emission properties. ACS Applied Materials & Interfaces, 1, 789–796. Rosenthal-Toib, L., Zohar, K., Alagem, M. & Tsur, Y. (2008). Synthesis of stabilized nanoparticles of zinc peroxide. Chemical Engineering Journal, 136, 425–429. ˝ D., Szabó, T. & Dékány, I. (2009). Optical properties of zinc peroxide and zinc Sebok, oxide multilayer nanohybrid films. Applied Surface Science, 255, 6953–6962. Sun, M., Hao, W., Wang, C. & Wang, T. (2007). A simple and green approach for preparation of ZnO2 and ZnO under sunlight irradiation. Chemical Physics Letters, 443, 342–346. Wermelinger, T., Mornaghini, F. C. F., Hinderling, C. & Spolenak, R. (2010). Correlation between the defect structure and the residual stress distribution
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in ZnO visualized by TEM and Raman microscopy. Materials Letters, 64, 28–30. Yang, L. Y., Feng, G. P. & Wang, T. X. (2010). Green synthesis of ZnO2 nanoparticles from hydrozincite and hydrogen peroxide at room temperature. Materials Letters, 64, 1647–1649. Zhang, H., Li, H., Guo, Y., Li, G., Fan, C., Wang, K., et al. (2010). Mild hydrothermal preparation of a layered metal hydroxide salt with microtube/rod morphology. Particuology, 8, 192–197. Zhang, Y. C., Wu, X., Hu, X. Y. & Guo, R. (2005). Low-temperature synthesis of nanocrystalline ZnO by thermal decomposition of a “green” single-source inorganic precursor in air. Journal of Crystal Growth, 280, 250–254.
Contents lists available at SciVerse ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Short communication
Hydrothermal preparation of nanocrystalline ZnO2 Guipeng Feng a , Liyun Yang a,∗ , Tianxi Wang b , Jiamin Zhang b , Tianjun Lou b a b
Department of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang 453000, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
a r t i c l e
i n f o
Article history: Received 25 March 2011 Received in revised form 7 June 2011 Accepted 16 June 2011 Keywords: Zinc peroxide Nanocrystals Semiconductors Hydrothermal synthesis Photoluminescence
a b s t r a c t A green hydrothermal method was proposed for the synthesis of nanocrystalline ZnO2 , using Zn5 (CO3 )2 (OH)6 powder and 6 vol% H2 O2 aqueous solution as the starting materials. Characterization results from X-ray diffraction, Raman, high resolution transmission electron microscopy and selected area electron diffraction revealed that the products synthesized at 80–120 ◦ C for 6–18 h were pure cubic phase ZnO2 nanocrystals. Room temperature photoluminescence spectra of the as-synthesized ZnO2 nanocrystals displayed a wide and strong emission band in the visible region of about 525–570 nm upon laser excitation at 325 nm, which may have originated from their surface state and other crystal defects. © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction ZnO2 , an important semiconductor material with a wide band gap of about 4.1–4.5 eV (Chen et al., 2009), can be used as a luminophor (Bai & Liu, 2010; Cheng et al., 2009), photocatalyst (Hsu & Wu, 2005), and precursor to ZnO (Drmosh, Gondal, Yamani, & Saleh, 2010; Escobedo-Morales et al., 2011; Gondal, Drmosh, Yamani, & Saleh, 2009; Rosenthal-Toib, Zohar, Alagem, & Tsur, 2008; Sun, Hao, Wang, & Wang, 2007; Zhang, Wu, Hu, & Guo, 2005). Recently, great effort has been devoted to the synthesis of ZnO2 nanoparticles, driven by scientific understanding and the hope for better performance and novel applications (Bai & Liu, 2010; Chen et al., 2009; Cheng et al., 2009; Drmosh et al., 2010; EscobedoMorales et al., 2011; Gondal et al., 2009; Rosenthal-Toib et al., 2008; ˝ Szabó, & Dékány, 2009; Sun et al., 2007; Zhang et al., 2005). Sebok, However, most of the current methods are difficult to scale-up at low cost in an environmentally friendly way. Hydrothermal techniques have been widely used to prepare inorganic nanomaterials (Chen et al., 2010; Jiao, Wang, Ke, Sun, & Demir, 2011; Peng, Beysen, Li, Sun, & Yang, 2010; Zhang et al., 2010), not only inducing the formation of well-crystallized products at low temperatures (<200 ◦ C), but also controlling the size and shape of the products (Chen et al., 2010; Jiao et al., 2011; Peng et al., 2010; Zhang et al., 2010). We report the hydrothermal synthesis of nanocrystalline ZnO2 from Zn5 (CO3 )2 (OH)6 powder
∗ Corresponding author. Tel.: +86 0373 3682674. E-mail address: [email protected] (L. Yang).
and 6 vol% H2 O2 aqueous solution at 80–120 ◦ C, as well as the characterization of the resulting products by X-ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and room temperature photoluminescence (RTPL). Compared to current methods (Chen et al., 2009; Drmosh et al., 2010; EscobedoMorales et al., 2011; Gondal et al., 2009; Hsu & Wu, 2005; Lindroos & Leskela, 2000; Ortega-López, Avila-García, Albor-Aguilera, & ˝ et al., Sánchez Resendiz, 2003; Rosenthal-Toib et al., 2008; Sebok 2009; Zhang et al., 2005), our proposed method has at least three distinct advantages: (1) the source material, Zn5 (CO3 )2 (OH)6 (hydrozincite) is a natural ore, plentiful and cheap; (2) no need for controlling the solution pH with ammonia, ethylenediamine or NaOH, and (3) no release of toxic byproducts.
2. Materials and methods All the chemical reagents used are of analytical grade. The Zn5 (CO3 )2 (OH)6 powder, which has a lamellar morphology with a thickness of around 20–40 nm, was purchased from Sinopharm Chemical Reagent Co., Ltd. In a typical procedure, 0.5 g of Zn5 (CO3 )2 (OH)6 powder was first weighted into a Teflon-lined stainless steel autoclave of 50 mL-capacity, and then 40 mL of 6 vol% H2 O2 aqueous solution was added with stirring. The autoclave was sealed and heated at 80–120 ◦ C for 6–18 h, and then allowed to cool to room temperature naturally. The as-formed precipitates were centrifuged, washed with distilled water and ethanol, and dried in air at 80 ◦ C. The obtained products were characterized by XRD (German Bruker AXS D8 ADVANCE X-ray diffractometer),
1674-2001/$ – see front matter © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2011.06.010
G. Feng et al. / Particuology 10 (2012) 388–391
389
Fig. 1. XRD patterns of products obtained via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at (a) 80 ◦ C, (b) 100 ◦ C and (c) 120 ◦ C for 6 h, and (d) at 100 ◦ C for 18 h.
Raman (Britain Renishaw Invia Raman spectrometer, excitation at 532 nm, 10 mW), HRTEM and SAED (American FEI Tecnai G2 F30 S-TWIN field-emission transmission electron microscope), and RTPL (Britain Renishaw Invia Raman spectrometer equipped with a He–Cd laser, excitation at 325 nm). 3. Results and discussion Fig. 1(a)–(d) shows the XRD patterns of the products obtained via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at 80–120 ◦ C for 6–18 h. All the products in Fig. 1(a)–(d) displayed only the characteristic XRD peaks of cubic phase ZnO2 (JCPDS card No. 01-077-2414). Moreover, no XRD peak of monoclinic phase Zn5 (CO3 )2 (OH)6 was visible, indicating that the starting material, monoclinic Zn5 (CO3 )2 (OH)6 , had been completely transformed into ZnO2 under the current hydrothermal conditions. Fig. 2(a)–(d) shows the Raman spectra of the as-synthesized products obtained at different reaction conditions, all exhibiting four peaks at around 411, 472, 838 and 936 cm–1 , consistent with
Fig. 3. (a) TEM image of the ZnO2 nanocrystals synthesized at 100 ◦ C for 6 h, (b) HRTEM image and (c) SAED pattern of a typical ZnO2 nanocrystal.
Fig. 2. Raman spectra of products of hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at (a) 80 ◦ C, (b) 100 ◦ C and (c) 120 ◦ C for 6 h, or (d) at 100 ◦ C for 18 h.
the Raman spectra of ZnO2 nanoparticles reported in literature (Bai & Liu, 2010; Drmosh et al., 2010; Sun et al., 2007), but different from the Raman spectra of Zn5 (CO3 )2 (OH)6 (Yang, Feng, & Wang, 2010) and ZnO (Dong et al., 2010; Pradhan, Kumar, Ando,
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G. Feng et al. / Particuology 10 (2012) 388–391
2010; Moreira, Andrés, Varela, & Longo, 2009). The “broken” surface or dangling bonds and other crystal defects can form various animated energy levels localized within the forbidden gap of ZnO2 , giving rise to their visible RTPL. When the reaction temperature or the reaction time was increased, the structure of the resulting ZnO nanocrystals became more ordered and integrated, which can be inferred from their stronger and sharper XRD peaks in Fig. 1. So, their photoemission intensities related to the surface state and other crystal defects also decreased. 4. Conclusions
Fig. 4. RTPL spectra of ZnO2 nanocrystals obtained via hydrothermal treatment of Zn5 (CO3 )2 (OH)6 powder in 6 vol% H2 O2 aqueous solution at (a) 80 ◦ C, (b) 100 ◦ C and (c) 120 ◦ C for 6 h, and (d) at 100 ◦ C for 18 h.
& Leung, 2009; Wermelinger, Mornaghini, Hinderling, & Spolenak, 2010). Fig. 3(a) shows the TEM image of the ZnO2 powder synthesized at 100 ◦ C for 6 h, while Fig. 3(b) and (c) shows, respectively the HRTEM image and SAED pattern of this product. Fig. 3(a) shows that this product consists of nanocrystals with a size of 5–10 nm. The clear lattice fringes in the HRTEM image (Fig. 3(b)) reveal that the synthesized ZnO2 is well-crystallized. The measured interplanar spacing of about 0.244 nm corresponds to the (2 0 0) crystal plane of cubic ZnO2 . Besides, the annular SAED pattern in Fig. 3(c) further confirms the formation of cubic polycrystalline ZnO2 . Because the starting materials in our synthesis of ZnO2 were only Zn5 (CO3 )2 (OH)6 powder and 6 vol% H2 O2 aqueous solution (H2 O2 used as O2 2− source), and ZnO2 was the sole resulting solid after the hydrothermal reaction at 80–120 ◦ C for 6–18 h, it is believed that the mechanism of ZnO2 formation may be described as: Zn5 (CO3 )2 (OH)6 + 5H2 O2 → 5ZnO2 + 8H2 O + 2CO2 Since no valence change of Zn and other elements occurred, the formation of ZnO2 was simple precipitation. Although there was no available information about the solubility constant of ZnO2 , it is believed that the solubility of ZnO2 in water is smaller than that of Zn5 (CO3 )2 (OH)6 . This may be inferred from the facts that Zn5 (CO3 )2 (OH)6 first dissolves in 5 vol% ammonia solution to form Zn(NH3 )4 2+ , subsequently the addition of H2 O2 into the Zn(NH3 )4 2+ solution can produce a precipitate of ZnO2 , implying that the transformation reaction of Zn5 (CO3 )2 (OH)6 → Zn(NH3 )4 2+ → ZnO2 in water is thermodynamically allowable. Therefore, Zn5 (CO3 )2 (OH)6 has a tendency to transform into ZnO2 in the H2 O2 aqueous solution, and this transformation process can be completed under the hydrothermal conditions of 80–120 ◦ C for 6–18 h. Fig. 4 shows the RTPL spectra of the as-synthesized ZnO2 nanocrystals upon laser excitation at 325 nm, showing all products displaying each a wide and strong emission band in the visible region of 525–570 nm. However, the photoemission intensity of ZnO2 nanocrystal decreased with increasing reaction temperature and reaction time. As previously reported, there was inevitably a large amount of “broken” surface or dangling bonds and other crystal defects (structural disorder at short and intermediate ranges, etc.) in the nanocrystalline products (Anicete-Santos et al., 2007; Li, Zhang, Wang, & Zhang, 2011; Macario, Moreira, Andrés, & Longo,
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