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Synthesis and characterization of hydrozincite and its conversion into zinc oxide nanoparticles
Journal of Alloys and Compounds 461 (2008) 66–71
Synthesis and characterization of hydrozincite and its conversion into zinc oxide nanoparticles Rizwan Wahab, S.G. Ansari, Young Soon Kim, M.A. Dar, Hyung-Shik Shin ∗ Thin Film Technology Laboratory, School of Chemical Engineering, Chonbuk National University, Chonju 561756, Republic of Korea Received 14 May 2007; received in revised form 3 July 2007; accepted 4 July 2007 Available online 19 July 2007
Abstract Faceted nanoparticles of zinc oxide were successfully achieved via thermal annealing of hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder at different annealing temperatures, i.e. 300, 500 700 and 900 ◦ C in air for 2 h using sol–gel method by refluxing for 6 h at 70 ◦ C. Zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O) was used as precursor with urea (NH2 CONH2 ) for the synthesis of hydrozincite. The morphological observations by field emission scanning electron microscope indicated increase in particle size from 20 to 300 nm with increase in the annealing temperatures. High resolution TEM and SAED measurement indicate the distance between two lattice fringes is ∼0.52 nm corresponding to (0 0 0 1) fringes. A standard peak of zinc oxide was observed at 457 cm−1 from the FTIR analysis. Thermo gravimetric analysis (TGA) revealed that the primary weight loss starts at ∼130 ◦ C due to solvent evaporation and the secondary weight loss due to phase transition from hydrated zinc oxide to zinc oxide was observed at ∼290 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrozincite; ZnO; Nanoparticles; Sol–gel
1. Introduction Research on nanostructured zinc oxide (ZnO) materials has reached to a level that it does not need much introduction on their synthesis and application due to their specific chemical, surface and microstructural properties [1–3]. The research achievements till date have shown that zinc oxide is rich in nanostructure amongst the various semiconducting materials. It exhibits wide band gap (3.37 eV) and large exciton binding energy (60 meV) presenting itself a promising material for the wide range of wellknown technological applications [4–6]. Still researchers aim to synthesize dimensionally controlled particles in large quantities and understand their properties to explore new applications [7]. In this regard, several methods are being used to prepare zinc oxide nanoparticles, such as evaporative decomposition of solution, template assisted method, wet chemical method, gas phase reaction and many more [8–11], with structure varying from nanowires, nanobelts, nanobridges, nanonails,
∗
Corresponding author. Fax: +82 63 270 2306. E-mail address: [email protected] (H.-S. Shin).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.029
nanoribbons, nanotubes, nanorings, nanorods to microflowers and columnar hexagonal-shaped rods/bars [12–18]. Additionally, the old known techniques such as metal organic chemical vapour deposition (MOCVD), spray pyrolysis, ion beam assisted deposition, laser-ablation, sputter deposition, template assisted growth and chemical vapour deposition are also investigated for the synthesis of nanostructures of zinc oxide [19–26]. For example, Umar et al. presented the synthesis of nanostructures of ZnO in the form of javelin-like nanorods on copper foil at 500 ◦ C, flower shaped on Si substrates at 400–500 ◦ C and aligned coaxial nanocolumns on steel alloy substrates at ∼490 ◦ C by thermal evaporation methods [27–30]. Similarly, Khan et al. presented very interesting hallow microspheres and microphone like microstructures via thermal evaporation methods [31,32]. Mahmud et al. [33] reported the preparation of nanomallets of zinc oxide from a catalyst-free combust-oxidised process in a blast furnace at a very high temperature 1150–1210 ◦ C. Jie et al. [34] presented the synthesis of high-density well-aligned ZnO nanorods arrays by the pulsed laser deposition method. ZnO nanotetrapods with hexagonal crown like structure are reported by Fa-Quan et al. [35]. Shen et al. reported the preparation of different types of zinc oxide morphologies using simple round to round metal vapour deposition route at 550 ◦ C [36]. Djurisic
R. Wahab et al. / Journal of Alloys and Compounds 461 (2008) 66–71
Fig. 1. The typical FESEM image of as-grown hydrozincite powder, prepared by the sol–gel method by refluxing for 6 h at at 70 ◦ C.
and Leung provide a detailed overview on the optical properties of ZnO nanostructures [37]. Chemical routes were equally investigated to explore the synthesis of nanostructures of ZnO. For example, Music et al. presented the synthesis of zinc oxide nanoparticles of about 100 nm in size using ZnCl2, Zn(CH3 COO)2 ·2H2 O,
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Zn(NO3 )2 ·6H2 O and sodium carbonate by refluxing at 90 ◦ C for 24 h and annealing at 600 ◦ C for 8 h in air [38]. Wang et al. presented the synthesis of zinc oxide nanoparticle using solvothermal method at 180 ◦ C for 24 h with zinc acetate dihydrate and urea and found particle size ranging from 50 to 200 nm [39]. Wu et al. changed the morphology from particles to rod of ZnO when the temperature increased from 70 to 90 ◦ C by sol–gel method using zinc nitrate hexahydrate (Zn(NO3 )2 ·2H2 O) and alkali sodium hydroxide at 90 ◦ C [40]. Ni et al. presented the synthesis of zinc oxide nanoparticles of ∼80 nm in size, by the solution–combustion method using zinc acetate di-hydrate and ethylene glycol [41]. In this paper we report the facile and convenient route for the synthesis of zinc oxide nanoparticles of ∼20 nm in size from hydrozincite powder. The hydrozincite was synthesized by sol–gel method with refluxing time of 6 h at 70 ◦ C. A systematic study is presented to show the effect of annealing (from 300 to 900 ◦ C) on the conversion and morphology of the resulting power. The formation mechanism of zinc oxide nanoparticles and its growth habit is also presented. 2. Experimental Zinc acetate dihydrate (Zn(CH3 COOH)2 ·2H2 O) and urea (NH2 CONH2 ) were used for the synthesis of zinc oxide nanoparticles by using sol–gel method. The chemicals were purchased from Aldrich Chemical Corporation and used
Fig. 2. (a and b) Low and high magnification FESEM images of annealed hydrozincite powder at 300 ◦ C. Whereas (c and d) Low and high magnification images of annealed hydrozincite powder at 500 ◦ C.
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R. Wahab et al. / Journal of Alloys and Compounds 461 (2008) 66–71
Fig. 3. (a and b) Low and high magnification FESEM images of the annealed hydrozincite powder at 700 ◦ C and (c and d) low and high magnification images of annealed hydrozincite powder at 900 ◦ C. without further purification. In a typical experiment: 0.3 M zinc acetate dihydrate (Zn(CH3 COOH)2 ·2H2 O) and 2 M urea (NH2 CONH2 ) were dissolved in 100 ml deionized water and stirred for 30 min. The solution was then transferred in a three-necked refluxing pot and refluxed for 6 h at 70 ◦ C. The solutions pH was measured as 12.6 by the expandable ion analyzer (EA 940, Orian, United
Kingdom), before transferring to the flask. During refluxing temperature was measured and controlled by k-type thermocouple and a PID controller. After refluxing, the white aqueous solution was washed with methanol three or four times to remove ionic impurities and dried at room temperature for further analysis. The dried samples were annealed in air ambient at
Fig. 4. (a) Low magnification TEM image of annealed hydrozincite powder at 300 ◦ C and (b) presents the typical high resolution transmission electron microscopy (HRTEM) of annealed hydrozincite powder; the image clearly show the distance between two lattice fringes is ∼0.52 nm. The corresponding SAED pattern (inset) indicates the crystallinity of the synthesized products.
R. Wahab et al. / Journal of Alloys and Compounds 461 (2008) 66–71
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four different temperatures, i.e. 300, 500, 700 and 900 ◦ C for 2 h each. After annealing powder samples were characterized for morphological and chemical properties. For morphological observations, using field emission SEM (FESEM, Hitachi S-4700), the powder was uniformly sprayed on carbon tape and coated with thin platinum layer (∼10 nm). For the transmission electron microscopic (TEM, JEM-2010, Japan, 200 kV,) measurement, powder was sonicated in ethanol for 5 min thereby dipping copper grid in the solution and drying at room temperature. Information about the crystallinity and phases were obtained by the X-ray pow˚ in the Bragg angle ranging der diffractometer (Rigaku, Cu K␣, λ = 1.54178 A) between 20◦ and 65◦ . The composition of the synthesized power was obtained by the Fourier transform infrared (FTIR) spectroscopy in the wavelength range of 400–4000 cm−1 . Thermo gravimetric study (TG) was conducted by TGA 2050 thermo-gravimeter with the heating rate of 20◦ min−1 .
3. Result and discussion Fig. 1 shows the typical FESEM image of as grown hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder, where plate like structure is clearly evident. These plates convert to nanosized particles when annealed at various temperatures. Fig. 2(a and b) shows the low and high magnification FESEM image of the annealed hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder at 300 ◦ C in air for 2 h where nanoparticles are seen clearly. A mixed shape (circular and elongated) can be seen with particle sizes of around 20–30 nm. Annealing at 500 ◦ C results in agglomeration as aggregated particles can be seen in Fig. 2(c and d), with particle size of about 50–60 nm. The variation in shape at this annealing temperature is much lesser than that at 300 ◦ C. Fig. 3(a and b) shows the FESEM image of annealed hydrozincite powder at 700 ◦ C. An increase is particle size from 50 to 100 nm is observed with increased heating, with grains changing from circular to faceted grains. Fig. 3(c and d) presents the FESEM image of annealed hydrozincite powder at 900 ◦ C, where the particles size has further increased to ∼250 nm. The nanoparticles are seen merging with each other and forming neck between the particles. Such neck formation, may lead to the densification of the particles. A fine-tuning in annealing process may help in channelizing the ZnO nanoparticles, which needs further studies. Detailed structural information was further obtained by the transmission electron microscopy (TEM) equipped with the selected area electron diffraction (SAED) pattern setup. Fig. 4(a) shows the low magnification TEM image of the ZnO nanoparticles obtained after annealing hydrozincite at 300 ◦ C. TEM observations, consistent with the FESEM observation (Fig. 2(a and b)), shows that the nanoparticles are around 10–20 nm in size and are nearly spherical in shape. The corresponding SAED pattern confirmed that the synthesized products are single crystalline (shown as inset in Fig. 4(a)). Fig. 4(b) shows the high resolution TEM (HRTEM) image of these nanoparticles. The lattice fringe between two adjacent planes is about 0.52 nm apart which is equal to the lattice constant of wurtzite ZnO, confirming the wurtzite hexagonal phase. The corresponding SAED pattern (inset in Fig. 4(b)) is consistent with the HRTEM observation. Fig. 5 shows the X-ray diffraction pattern of as grown hydrozincite powder. The indexed peaks in the spectrum are closely matched with the available powder diffraction JCPDS data for hydrozincite (19-1458, 89-1397, and 89-0511).
Fig. 5. The X-ray diffraction pattern of as-grown hydrozincite powder prepared by the sol–gel method at 70 ◦ C in 6 h refluxing time. Marked (*) points indicate the unidentified peaks.
Fig. 6 represents the X-ray diffraction patterns of the annealed hydrozincite powder at different annealing temperatures (300–900 ◦ C). Fig. 6(a) shows the diffraction pattern for hydrozincite powder annealed at 300 ◦ C with broad peaks of ZnO along 1 0 1 0 , 0 0 0 2 and 1 0 1 1 . The broad peaks clearly depicts that the dimension of particles is very less supporting FESEM observations (Fig. 2(a and b)). Fig. 6(b–d) presents the diffraction patterns of hydrozincite powder annealed at 500, 700 and 900 ◦ C. All of the indexed peaks in the obtained spectrum are well matched with that of bulk ZnO (JCPDS Card No. 36–1451) which confirms that the synthesized powder is single crystalline, possesses wurtzite hexagonal phase. No other peaks related to impurities like zinc carbonate, zinc hydroxide were detected. The composition of the synthesized powder was analyzed by the FTIR spectroscopy. Figs. 7 and 8 shows the FTIR spectrum acquired in the range of 400–4000 cm−1 at room temperature. Fig. 7 shows the FTIR spectra of as grown hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder. This spectrum shows the basic features of zinc carbonate group. The broadband between 3455 and 3358 cm−1 belongs to the adsorbed H2 O molecule. The peak centered at 2369 and 2221 cm−1 denotes the presence of CO2 .
Fig. 6. (a–d) Typical X-ray diffraction pattern at different annealing temperatures, i.e. 300, 500, 700 and 900 ◦ C, respectively.
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R. Wahab et al. / Journal of Alloys and Compounds 461 (2008) 66–71
Fig. 7. Typical FTIR spectrum of as-grown hydrozincite powder prepared by the sol–gel method at 70 ◦ C in 6 h refluxing time.
The peaks at 1683 and 1624 represents the C O and C O in the spectrum [17]. The two IR bands at 1464 and 1393 cm−1 are assigned to the carbonate group [38]. The bands corresponding to υ1 stretching frequency centered at 1046 cm−1 whereas the band at 835 and 777 cm−1 shows the υ2 and υ4 stretching frequencies. These bands show the chemical bonding, crystal structure and relative intensities of the IR bands of the carbonate [42–45]. The peaks centered at 578, 552 and 514 cm−1 shows the presence of zinc oxide. The peaks position centered at 1342, 1155, 937 and 681 cm−1 could not be identified. According to Ghose [46], the structure of two oxygen atoms of the carbonate group are bonded to an octahedral and tetrahedral zinc atom, where as the third oxygen is hydrogen bonded to three OH groups. The carbonate groups in hydrozincite (Zn5 (CO3 )2 (OH)6 ) shows the three types of interaction in chemical bonding. Fig. 8(a) represents the FTIR spectrum of annealed hydrozincite powder. The band at 450 cm−1 is correlated to zinc oxide. The bands at 3200–3600 cm−1 correspond to O H mode of vibration. The stretching mode of vibration of C O is observed at 1531 cm−1 . The bands at 1642 and 2355 cm−1 are due to the C O and CO2 groups. When the annealing temperature was
Fig. 9. Typical TGA analysis of as-grown hydrozincite powder from room temperature to 700 ◦ C.
increased from 300 to 500, 700 and 900 ◦ C, only a broad band of zinc oxide was observed (460, 465 and 448 cm−1 , respectively) as can be seen in Fig. 8(b–d). The FTIR observations support the X-ray diffraction results. Thermo-gravimetric analysis is a technique by which one can measures the mass loss with respect to the temperature. Fig. 9 shows the weight loss of the annealed powder sample from room temperature to 700 ◦ C, for as-synthesized powder. The primary weight loss was observed at 103 ◦ C due to the solvent evaporation. Phase transition or secondary weight loss was observed at 365 ◦ C with a percentage weight loss of about 5.66%. 3.1. Expected reaction mechanism First the zinc acetate di-hydrate was dissolved in deionized water and then 2 M urea solution was introduced, that resulted in a clear solution without precipitation. Upon, refluxing urea decomposes in to carbon dioxide and ammonia as per Eq. (1). The evolved carbon dioxide reacts with water and forms carbonic acid (H2 CO3 ) (Eq. (2)). The free carbonate ion (CO3 2− ) ions would react with the zinc ion (Zn2− ) forming ZnCO3 (Eq. (3)). With increase in thermal energy (refluxing temperature), a suspension of zinc complex appeared due to the interaction of urea with hydroxyl (OH− ) and zinc (Zn2− ) ion. NH2 CONH2 + H2 O → 2NH3 + CO2
(1)
CO2 + H2 O → H2 CO3
(2)
H2 CO3 → 2H+ + CO3 2−
(3)
5Zn2+ + 2CO3 2− + 6OH− → (Zn5 (CO3 )2 (OH)6 ) (as grown) At 300◦ C
Zn5 (CO3 )2 (OH)6 −→ 5ZnO + 2CO2 ↑ +3H2 O Fig. 8. Typical FTIR spectra of annealed zinc oxide nanoparticles (a) at 300 ◦ C, (b) at 500 ◦ C, (c) at 700 ◦ C and (d) at 900 ◦ C.
(4) (5)
As grown basic hydrozincite compound Zn5 (CO3 )2 (OH)6 , white precipitate, was subjected to the thermal treatment at dif-
R. Wahab et al. / Journal of Alloys and Compounds 461 (2008) 66–71
ferent temperatures, i.e. 300, 500, 700 and 900 ◦ C for 2 h in air. According to Wiedemann et al. [47], the kinetics and thermal decomposition of hydrozincite compound (Zn5 (CO3 )2 (OH)6 ) is dependent on the experimental atmosphere, i.e. annealing or heating condition. As grown particles of hydrozincite (Zn5 (CO3 )2 (OH)6 ) was seen converted into zinc oxide nanoparticles after annealing (300 ◦ C and above). CO2 and water evolves on heating, as per the above equations. Annealing forms solid bonds between particles when they are heated. This bond reduces the surface energy of the crystals by removing free surfaces, with the secondary elimination of grain boundary area via grain growth. With extended heating, it is possible to reduce the pore volume, leading to compact shrinkage. At 300 ◦ C the particles appears spherical, elongated due to the shrinkage. At 500 ◦ C spherical particles of zinc oxide contacts with each other at random orientation. Adherence occurs due to weak vander Waals forces. The closer the particles approach one another, the greater the bonding force. At 700 ◦ C, the particles of irregular shapes are visible, due to the presence of amorphous fraction. The precise inspection of these particles, revealed that these are actually hexagone of irregular shapes. The rate of crystallization increases with increasing temperature at 900 ◦ C; particles merge with each other and form bigger micro-level range particles. The size of the particles increases in the range of 250–300 nm. 4. Conclusion In conclusion, we presented the synthesis of hydrozincite plate like-particles by the sol–gel method and upon heating at 300 ◦ C these particles convert to nanoparticles of zinc oxide, of around 20 nm size. But at higher temperature 500 ◦ C–900 ◦ C the size of zinc oxide nanoparticles increases upto 300 nm due to agglomeration. FTIR bands at 1046, 835 and 777 cm−1 clearly confirms that Zinc carbonate was found in the solution. After annealing the peaks at 460, 465 and 448 cm−1 in the FTIR spectrum shows its conversion to zinc oxide. The rate of conversion of complex hydrozincite (Zn5 (CO3 )2 (OH)6 ) to zinc oxide is dependent on the annealing temperature. Acknowledgements This work is supported by KMOST (research Grant No. 2004-01352) and KOSEF (research Grant No. R01-2004-00010792-0), and also by the grant of Post Doc program, Chonbuk National University (01—the second half term of 2006). S.G. Ansari acknowledges the Korean Federation of Science and Technology (KOFST) and KRF for Brain-Pool fellowship. We also would like to thank Mr. Kang Jong-Gyun, Center for University-Wide Research Facilities, Chonbuk National University for his cooperation in microscopic observations and KBSI, Korea, Jeonju branch for access to their FESEM setup. References [1] Z.L. Wang, Mater. Today (June) (2004) 26. [2] Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, S.J. Pearton, J.R. LaRoche, Mater. Sci. Eng. R47 (2004) 1–47.
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Synthesis and characterization of hydrozincite and its conversion into zinc oxide nanoparticles Rizwan Wahab, S.G. Ansari, Young Soon Kim, M.A. Dar, Hyung-Shik Shin ∗ Thin Film Technology Laboratory, School of Chemical Engineering, Chonbuk National University, Chonju 561756, Republic of Korea Received 14 May 2007; received in revised form 3 July 2007; accepted 4 July 2007 Available online 19 July 2007
Abstract Faceted nanoparticles of zinc oxide were successfully achieved via thermal annealing of hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder at different annealing temperatures, i.e. 300, 500 700 and 900 ◦ C in air for 2 h using sol–gel method by refluxing for 6 h at 70 ◦ C. Zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O) was used as precursor with urea (NH2 CONH2 ) for the synthesis of hydrozincite. The morphological observations by field emission scanning electron microscope indicated increase in particle size from 20 to 300 nm with increase in the annealing temperatures. High resolution TEM and SAED measurement indicate the distance between two lattice fringes is ∼0.52 nm corresponding to (0 0 0 1) fringes. A standard peak of zinc oxide was observed at 457 cm−1 from the FTIR analysis. Thermo gravimetric analysis (TGA) revealed that the primary weight loss starts at ∼130 ◦ C due to solvent evaporation and the secondary weight loss due to phase transition from hydrated zinc oxide to zinc oxide was observed at ∼290 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrozincite; ZnO; Nanoparticles; Sol–gel
1. Introduction Research on nanostructured zinc oxide (ZnO) materials has reached to a level that it does not need much introduction on their synthesis and application due to their specific chemical, surface and microstructural properties [1–3]. The research achievements till date have shown that zinc oxide is rich in nanostructure amongst the various semiconducting materials. It exhibits wide band gap (3.37 eV) and large exciton binding energy (60 meV) presenting itself a promising material for the wide range of wellknown technological applications [4–6]. Still researchers aim to synthesize dimensionally controlled particles in large quantities and understand their properties to explore new applications [7]. In this regard, several methods are being used to prepare zinc oxide nanoparticles, such as evaporative decomposition of solution, template assisted method, wet chemical method, gas phase reaction and many more [8–11], with structure varying from nanowires, nanobelts, nanobridges, nanonails,
∗
Corresponding author. Fax: +82 63 270 2306. E-mail address: [email protected] (H.-S. Shin).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.029
nanoribbons, nanotubes, nanorings, nanorods to microflowers and columnar hexagonal-shaped rods/bars [12–18]. Additionally, the old known techniques such as metal organic chemical vapour deposition (MOCVD), spray pyrolysis, ion beam assisted deposition, laser-ablation, sputter deposition, template assisted growth and chemical vapour deposition are also investigated for the synthesis of nanostructures of zinc oxide [19–26]. For example, Umar et al. presented the synthesis of nanostructures of ZnO in the form of javelin-like nanorods on copper foil at 500 ◦ C, flower shaped on Si substrates at 400–500 ◦ C and aligned coaxial nanocolumns on steel alloy substrates at ∼490 ◦ C by thermal evaporation methods [27–30]. Similarly, Khan et al. presented very interesting hallow microspheres and microphone like microstructures via thermal evaporation methods [31,32]. Mahmud et al. [33] reported the preparation of nanomallets of zinc oxide from a catalyst-free combust-oxidised process in a blast furnace at a very high temperature 1150–1210 ◦ C. Jie et al. [34] presented the synthesis of high-density well-aligned ZnO nanorods arrays by the pulsed laser deposition method. ZnO nanotetrapods with hexagonal crown like structure are reported by Fa-Quan et al. [35]. Shen et al. reported the preparation of different types of zinc oxide morphologies using simple round to round metal vapour deposition route at 550 ◦ C [36]. Djurisic
R. Wahab et al. / Journal of Alloys and Compounds 461 (2008) 66–71
Fig. 1. The typical FESEM image of as-grown hydrozincite powder, prepared by the sol–gel method by refluxing for 6 h at at 70 ◦ C.
and Leung provide a detailed overview on the optical properties of ZnO nanostructures [37]. Chemical routes were equally investigated to explore the synthesis of nanostructures of ZnO. For example, Music et al. presented the synthesis of zinc oxide nanoparticles of about 100 nm in size using ZnCl2, Zn(CH3 COO)2 ·2H2 O,
67
Zn(NO3 )2 ·6H2 O and sodium carbonate by refluxing at 90 ◦ C for 24 h and annealing at 600 ◦ C for 8 h in air [38]. Wang et al. presented the synthesis of zinc oxide nanoparticle using solvothermal method at 180 ◦ C for 24 h with zinc acetate dihydrate and urea and found particle size ranging from 50 to 200 nm [39]. Wu et al. changed the morphology from particles to rod of ZnO when the temperature increased from 70 to 90 ◦ C by sol–gel method using zinc nitrate hexahydrate (Zn(NO3 )2 ·2H2 O) and alkali sodium hydroxide at 90 ◦ C [40]. Ni et al. presented the synthesis of zinc oxide nanoparticles of ∼80 nm in size, by the solution–combustion method using zinc acetate di-hydrate and ethylene glycol [41]. In this paper we report the facile and convenient route for the synthesis of zinc oxide nanoparticles of ∼20 nm in size from hydrozincite powder. The hydrozincite was synthesized by sol–gel method with refluxing time of 6 h at 70 ◦ C. A systematic study is presented to show the effect of annealing (from 300 to 900 ◦ C) on the conversion and morphology of the resulting power. The formation mechanism of zinc oxide nanoparticles and its growth habit is also presented. 2. Experimental Zinc acetate dihydrate (Zn(CH3 COOH)2 ·2H2 O) and urea (NH2 CONH2 ) were used for the synthesis of zinc oxide nanoparticles by using sol–gel method. The chemicals were purchased from Aldrich Chemical Corporation and used
Fig. 2. (a and b) Low and high magnification FESEM images of annealed hydrozincite powder at 300 ◦ C. Whereas (c and d) Low and high magnification images of annealed hydrozincite powder at 500 ◦ C.
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Fig. 3. (a and b) Low and high magnification FESEM images of the annealed hydrozincite powder at 700 ◦ C and (c and d) low and high magnification images of annealed hydrozincite powder at 900 ◦ C. without further purification. In a typical experiment: 0.3 M zinc acetate dihydrate (Zn(CH3 COOH)2 ·2H2 O) and 2 M urea (NH2 CONH2 ) were dissolved in 100 ml deionized water and stirred for 30 min. The solution was then transferred in a three-necked refluxing pot and refluxed for 6 h at 70 ◦ C. The solutions pH was measured as 12.6 by the expandable ion analyzer (EA 940, Orian, United
Kingdom), before transferring to the flask. During refluxing temperature was measured and controlled by k-type thermocouple and a PID controller. After refluxing, the white aqueous solution was washed with methanol three or four times to remove ionic impurities and dried at room temperature for further analysis. The dried samples were annealed in air ambient at
Fig. 4. (a) Low magnification TEM image of annealed hydrozincite powder at 300 ◦ C and (b) presents the typical high resolution transmission electron microscopy (HRTEM) of annealed hydrozincite powder; the image clearly show the distance between two lattice fringes is ∼0.52 nm. The corresponding SAED pattern (inset) indicates the crystallinity of the synthesized products.
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four different temperatures, i.e. 300, 500, 700 and 900 ◦ C for 2 h each. After annealing powder samples were characterized for morphological and chemical properties. For morphological observations, using field emission SEM (FESEM, Hitachi S-4700), the powder was uniformly sprayed on carbon tape and coated with thin platinum layer (∼10 nm). For the transmission electron microscopic (TEM, JEM-2010, Japan, 200 kV,) measurement, powder was sonicated in ethanol for 5 min thereby dipping copper grid in the solution and drying at room temperature. Information about the crystallinity and phases were obtained by the X-ray pow˚ in the Bragg angle ranging der diffractometer (Rigaku, Cu K␣, λ = 1.54178 A) between 20◦ and 65◦ . The composition of the synthesized power was obtained by the Fourier transform infrared (FTIR) spectroscopy in the wavelength range of 400–4000 cm−1 . Thermo gravimetric study (TG) was conducted by TGA 2050 thermo-gravimeter with the heating rate of 20◦ min−1 .
3. Result and discussion Fig. 1 shows the typical FESEM image of as grown hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder, where plate like structure is clearly evident. These plates convert to nanosized particles when annealed at various temperatures. Fig. 2(a and b) shows the low and high magnification FESEM image of the annealed hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder at 300 ◦ C in air for 2 h where nanoparticles are seen clearly. A mixed shape (circular and elongated) can be seen with particle sizes of around 20–30 nm. Annealing at 500 ◦ C results in agglomeration as aggregated particles can be seen in Fig. 2(c and d), with particle size of about 50–60 nm. The variation in shape at this annealing temperature is much lesser than that at 300 ◦ C. Fig. 3(a and b) shows the FESEM image of annealed hydrozincite powder at 700 ◦ C. An increase is particle size from 50 to 100 nm is observed with increased heating, with grains changing from circular to faceted grains. Fig. 3(c and d) presents the FESEM image of annealed hydrozincite powder at 900 ◦ C, where the particles size has further increased to ∼250 nm. The nanoparticles are seen merging with each other and forming neck between the particles. Such neck formation, may lead to the densification of the particles. A fine-tuning in annealing process may help in channelizing the ZnO nanoparticles, which needs further studies. Detailed structural information was further obtained by the transmission electron microscopy (TEM) equipped with the selected area electron diffraction (SAED) pattern setup. Fig. 4(a) shows the low magnification TEM image of the ZnO nanoparticles obtained after annealing hydrozincite at 300 ◦ C. TEM observations, consistent with the FESEM observation (Fig. 2(a and b)), shows that the nanoparticles are around 10–20 nm in size and are nearly spherical in shape. The corresponding SAED pattern confirmed that the synthesized products are single crystalline (shown as inset in Fig. 4(a)). Fig. 4(b) shows the high resolution TEM (HRTEM) image of these nanoparticles. The lattice fringe between two adjacent planes is about 0.52 nm apart which is equal to the lattice constant of wurtzite ZnO, confirming the wurtzite hexagonal phase. The corresponding SAED pattern (inset in Fig. 4(b)) is consistent with the HRTEM observation. Fig. 5 shows the X-ray diffraction pattern of as grown hydrozincite powder. The indexed peaks in the spectrum are closely matched with the available powder diffraction JCPDS data for hydrozincite (19-1458, 89-1397, and 89-0511).
Fig. 5. The X-ray diffraction pattern of as-grown hydrozincite powder prepared by the sol–gel method at 70 ◦ C in 6 h refluxing time. Marked (*) points indicate the unidentified peaks.
Fig. 6 represents the X-ray diffraction patterns of the annealed hydrozincite powder at different annealing temperatures (300–900 ◦ C). Fig. 6(a) shows the diffraction pattern for hydrozincite powder annealed at 300 ◦ C with broad peaks of ZnO along 1 0 1 0 , 0 0 0 2 and 1 0 1 1 . The broad peaks clearly depicts that the dimension of particles is very less supporting FESEM observations (Fig. 2(a and b)). Fig. 6(b–d) presents the diffraction patterns of hydrozincite powder annealed at 500, 700 and 900 ◦ C. All of the indexed peaks in the obtained spectrum are well matched with that of bulk ZnO (JCPDS Card No. 36–1451) which confirms that the synthesized powder is single crystalline, possesses wurtzite hexagonal phase. No other peaks related to impurities like zinc carbonate, zinc hydroxide were detected. The composition of the synthesized powder was analyzed by the FTIR spectroscopy. Figs. 7 and 8 shows the FTIR spectrum acquired in the range of 400–4000 cm−1 at room temperature. Fig. 7 shows the FTIR spectra of as grown hydrozincite (Zn5 (CO3 )2 (OH)6 ) powder. This spectrum shows the basic features of zinc carbonate group. The broadband between 3455 and 3358 cm−1 belongs to the adsorbed H2 O molecule. The peak centered at 2369 and 2221 cm−1 denotes the presence of CO2 .
Fig. 6. (a–d) Typical X-ray diffraction pattern at different annealing temperatures, i.e. 300, 500, 700 and 900 ◦ C, respectively.
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Fig. 7. Typical FTIR spectrum of as-grown hydrozincite powder prepared by the sol–gel method at 70 ◦ C in 6 h refluxing time.
The peaks at 1683 and 1624 represents the C O and C O in the spectrum [17]. The two IR bands at 1464 and 1393 cm−1 are assigned to the carbonate group [38]. The bands corresponding to υ1 stretching frequency centered at 1046 cm−1 whereas the band at 835 and 777 cm−1 shows the υ2 and υ4 stretching frequencies. These bands show the chemical bonding, crystal structure and relative intensities of the IR bands of the carbonate [42–45]. The peaks centered at 578, 552 and 514 cm−1 shows the presence of zinc oxide. The peaks position centered at 1342, 1155, 937 and 681 cm−1 could not be identified. According to Ghose [46], the structure of two oxygen atoms of the carbonate group are bonded to an octahedral and tetrahedral zinc atom, where as the third oxygen is hydrogen bonded to three OH groups. The carbonate groups in hydrozincite (Zn5 (CO3 )2 (OH)6 ) shows the three types of interaction in chemical bonding. Fig. 8(a) represents the FTIR spectrum of annealed hydrozincite powder. The band at 450 cm−1 is correlated to zinc oxide. The bands at 3200–3600 cm−1 correspond to O H mode of vibration. The stretching mode of vibration of C O is observed at 1531 cm−1 . The bands at 1642 and 2355 cm−1 are due to the C O and CO2 groups. When the annealing temperature was
Fig. 9. Typical TGA analysis of as-grown hydrozincite powder from room temperature to 700 ◦ C.
increased from 300 to 500, 700 and 900 ◦ C, only a broad band of zinc oxide was observed (460, 465 and 448 cm−1 , respectively) as can be seen in Fig. 8(b–d). The FTIR observations support the X-ray diffraction results. Thermo-gravimetric analysis is a technique by which one can measures the mass loss with respect to the temperature. Fig. 9 shows the weight loss of the annealed powder sample from room temperature to 700 ◦ C, for as-synthesized powder. The primary weight loss was observed at 103 ◦ C due to the solvent evaporation. Phase transition or secondary weight loss was observed at 365 ◦ C with a percentage weight loss of about 5.66%. 3.1. Expected reaction mechanism First the zinc acetate di-hydrate was dissolved in deionized water and then 2 M urea solution was introduced, that resulted in a clear solution without precipitation. Upon, refluxing urea decomposes in to carbon dioxide and ammonia as per Eq. (1). The evolved carbon dioxide reacts with water and forms carbonic acid (H2 CO3 ) (Eq. (2)). The free carbonate ion (CO3 2− ) ions would react with the zinc ion (Zn2− ) forming ZnCO3 (Eq. (3)). With increase in thermal energy (refluxing temperature), a suspension of zinc complex appeared due to the interaction of urea with hydroxyl (OH− ) and zinc (Zn2− ) ion. NH2 CONH2 + H2 O → 2NH3 + CO2
(1)
CO2 + H2 O → H2 CO3
(2)
H2 CO3 → 2H+ + CO3 2−
(3)
5Zn2+ + 2CO3 2− + 6OH− → (Zn5 (CO3 )2 (OH)6 ) (as grown) At 300◦ C
Zn5 (CO3 )2 (OH)6 −→ 5ZnO + 2CO2 ↑ +3H2 O Fig. 8. Typical FTIR spectra of annealed zinc oxide nanoparticles (a) at 300 ◦ C, (b) at 500 ◦ C, (c) at 700 ◦ C and (d) at 900 ◦ C.
(4) (5)
As grown basic hydrozincite compound Zn5 (CO3 )2 (OH)6 , white precipitate, was subjected to the thermal treatment at dif-
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ferent temperatures, i.e. 300, 500, 700 and 900 ◦ C for 2 h in air. According to Wiedemann et al. [47], the kinetics and thermal decomposition of hydrozincite compound (Zn5 (CO3 )2 (OH)6 ) is dependent on the experimental atmosphere, i.e. annealing or heating condition. As grown particles of hydrozincite (Zn5 (CO3 )2 (OH)6 ) was seen converted into zinc oxide nanoparticles after annealing (300 ◦ C and above). CO2 and water evolves on heating, as per the above equations. Annealing forms solid bonds between particles when they are heated. This bond reduces the surface energy of the crystals by removing free surfaces, with the secondary elimination of grain boundary area via grain growth. With extended heating, it is possible to reduce the pore volume, leading to compact shrinkage. At 300 ◦ C the particles appears spherical, elongated due to the shrinkage. At 500 ◦ C spherical particles of zinc oxide contacts with each other at random orientation. Adherence occurs due to weak vander Waals forces. The closer the particles approach one another, the greater the bonding force. At 700 ◦ C, the particles of irregular shapes are visible, due to the presence of amorphous fraction. The precise inspection of these particles, revealed that these are actually hexagone of irregular shapes. The rate of crystallization increases with increasing temperature at 900 ◦ C; particles merge with each other and form bigger micro-level range particles. The size of the particles increases in the range of 250–300 nm. 4. Conclusion In conclusion, we presented the synthesis of hydrozincite plate like-particles by the sol–gel method and upon heating at 300 ◦ C these particles convert to nanoparticles of zinc oxide, of around 20 nm size. But at higher temperature 500 ◦ C–900 ◦ C the size of zinc oxide nanoparticles increases upto 300 nm due to agglomeration. FTIR bands at 1046, 835 and 777 cm−1 clearly confirms that Zinc carbonate was found in the solution. After annealing the peaks at 460, 465 and 448 cm−1 in the FTIR spectrum shows its conversion to zinc oxide. The rate of conversion of complex hydrozincite (Zn5 (CO3 )2 (OH)6 ) to zinc oxide is dependent on the annealing temperature. Acknowledgements This work is supported by KMOST (research Grant No. 2004-01352) and KOSEF (research Grant No. R01-2004-00010792-0), and also by the grant of Post Doc program, Chonbuk National University (01—the second half term of 2006). S.G. Ansari acknowledges the Korean Federation of Science and Technology (KOFST) and KRF for Brain-Pool fellowship. We also would like to thank Mr. Kang Jong-Gyun, Center for University-Wide Research Facilities, Chonbuk National University for his cooperation in microscopic observations and KBSI, Korea, Jeonju branch for access to their FESEM setup. References [1] Z.L. Wang, Mater. Today (June) (2004) 26. [2] Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, S.J. Pearton, J.R. LaRoche, Mater. Sci. Eng. R47 (2004) 1–47.
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