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Study of template-free synthesis hierarchical m-ZrO2 nanorods by hydrothermal method
Powder Technology 256 (2014) 71–74
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Study of template-free synthesis hierarchical m-ZrO2 nanorods by hydrothermal method Shahzad Ahmad Khan, Zhengyi Fu ⁎, Sahibzada Shakir Rehman, Muhammad Asif, Weimin Wang, Hao Wang State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 22 June 2013 Received in revised form 15 January 2014 Accepted 1 February 2014 Available online 10 February 2014 Keywords: Powders: chemical preparation Chemical properties Growth models ZrO2
a b s t r a c t The template-free synthesis of hierarchical m-ZrO2 nanorods by a simple hydrothermal method and its possible formation mechanism based on a series of chemical reactions have been proposed here. The traditional preparation methods of hierarchical ZrO2 nanorods are involved in expensive equipment, complicated process and high production cost. Owing to their physical and chemical properties hierarchical ZrO2 nanorods have received considerable attention. The results revealed that as synthesized products are composed of many nanorods with 100–200 nm in diameter and 3–5 μm in length. The final product after annealing is involved into hierarchical monoclinic ZrO2 (m-ZrO2) nanorods, the big nanorod was made up of many small nanorods with 30–40 nm in diameter and 350–450 nm in length. This discovery could open a new path to the template-free synthesis of hierarchical nanomaterials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ZrO2 has three crystal forms as follows: monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2) and cubic zirconia (c-ZrO2). The m-ZrO2 is stable below 1170 °C and c-ZrO2 exists stable above 2370 °C. The pure t-ZrO2 is believed to be stable in the range of 1170 °C to 2370 °C. Due to the martensitic phase transformation originated from volumetric change effect when tetragonal to monoclinic phase takes place [1,2], the partial stabilized t-ZrO2 has been used extensively in the ceramic because of its transformation toughening performance. The hierarchical zirconia (ZrO2) nanomaterials are widely used in many fields, including solid-state electrolytes, electro-optical materials and catalytic materials [3,4]. The hierarchical nanomaterials have recently attracted increasing interests owing to their unusual physical and chemical properties [5,6]. Especially, hierarchical zirconia (ZrO2) nanomaterials have good optical and electric properties, high dielectric constant, extremely low thermal conductivity, and relatively high thermal expansion coefficient [7,8]. The ZrO2 hollow porous microspheres could be synthesized using bio-templates Fan et al. found [9]. The hierarchical ZrO2 nanomaterials suffer from the fact that spontaneous and controlled growth is limited to certain methods, such as PECVD, reactive-spray atomization, sol–gel, hydrothermal, electro-deposition, etc. Liu et al. reported that stable lamellar zirconia was fabricated using the hydrothermal method and SDS template [10]. However, the traditional physical method is involved in expensive equipment, complicated process and high production cost. Although the traditional chemical method usually needs surfactant or organic template that results in complicated process and precise control. Thus, it is urgent to ⁎ Corresponding author. Tel.: +86 27 87865484; fax: +86 27 87215421. E-mail address: [email protected] (Z. Fu). 0032-5910/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2014.02.012
develop quick, simple and low-cost methods to make hierarchical ZrO2 nanomaterials. Recently, Huang et al. adopted a facile hydrothermal method to synthesize hierarchical TiO2 and Nb3O7F nanomaterials when they studied the chemical properties of boride ceramic [11,12]. The importance of this work lies in the demonstration of the synthesis of hierarchical m-ZrO2 nanorods. The zirconium diboride (ZrB2) and hydrogen peroxide (H2O2) were used as raw materials for the synthesis of hierarchical m-ZrO2 nanorods by facile hydrothermal method. To the best of our knowledge, there is still no research work that report about such system. The hierarchical m-ZrO2 nanorod formation mechanism, the natural properties of zirconium and a rational model have been proposed. 2. Materials and method The ZrB2 (99.5%) grain sizes range from 1 to 2 μm and H2O2 (30%, AR) was purchased from Alfa Aesar Inc. (US) and Sinopharm Chemical Reagent Co. Ltd. (China), respectively. All the chemicals were used without further purification. Firstly, ZrB2 (1 g) and H2O2 (10 mL) were added into an aqueous solution (50 mL), respectively. Then the mixed solution was transferred into a 100 mL Teflon-line autoclave, followed by sealing, maintaining at the 170 °C for 24 h and naturally cooling down at room temperature. After centrifugation, the collected precipitate was washed with deionized water and absolute ethanol for several times, respectively. Finally, the as-synthesized products were dried at the 80 °C for 12 h and annealed at 700 °C in air for 8 h. The morphologies, size of the as-synthesized and annealed samples were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). The composition of the samples was analyzed by an energy dispersive X-ray detector (EDX, Thermo Noran VANTAG-ESI, 120 KV). The phase of the products was analyzed by X-
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ray powder diffraction (XRD, Rigaku Ultima II, Japan) with Cu Kα irradiation and a dynamic thermal analyzer (TG/DTA, METZSCH Instrument Co. Ltd., STA449, Germany). 3. Results and discussion 3.1. Morphology, composition and structure Fig. 1 presents a set of typical field emission scanning electron microscopy (FESEM) images of the as-synthesized and annealed samples under a strong ultrasonic wave treatment for 1 h shows different morphologies. The FESEM images of Fig. 1(a) and (b) revealed that the assynthesized sample consists of a large quantity of nanorods with 100– 200 nm in diameter and 3–5 μm in length. The sample annealed at 700 °C was involved into hierarchical nanorod, the big rod was composed of many small connected nanorods with diameters of 30–40 nm and lengths of 350–450 nm [Fig. 1(c) and (d)]. The EDX of the annealed sample as shown in Fig. 2, revealed that Zr and O were present in the annealed sample with different percentages that is in agreement with the results of XRD (Fig. 3). The phases of the as-synthesized and annealed samples were examined by XRD as shown in Fig. 3. The as-synthesized product, 1D ZrO2 ∙ nH2O nanorod with broadening and weak peak has a poor crystallinity. The reason may be attributed to amorphous intermediate product, that will be further explained in the following formation mechanism. The samples annealed at 700 °C can be indexed as standard patterns of m-ZrO2 (JCPDS 65-1025). The narrow line widths indicate a high crystallinity of the material. The same result can be obtained even if the annealing temperature is continuously increased up to 850 °C. Annealing is believed to have a great impact on the final phase of hydrothermal products. The TG/DTA from Fig. 4, is used to characterize the structure of assynthesized product under air atmosphere. The TG curve revealed that there are two obvious stages of weight loss. In the first stage, the weight
loss from 25 °C to 475 °C amounts to about 14.98% which corresponds to the release of the water molecules namely, absorbed water and structural water as well as a little decomposition of boric acid crystal to diboron trioxide (B2O3). From the sharp exothermic peak at 256 °C the DTA curve contributes to the release of absorbed water. The exothermic peak at 466 °C is assigned to a rapid release of bound hydroxyl groups, the decomposition of hydrous ZrO2 into amorphous ZrO2 [13]. The temperature ranges from 475 °C to 870 °C indicate 2.25% weight loss, which could be attributed to the volatilization of B2O3 by-product in the second stage. It is revealed that weak exothermic peak around 656 °C may result from the transformation of amorphous ZrO2 into m-ZrO2 , indicating that the main process for m-ZrO2 formation could be completed above 700 °C. It is well known that B2O3 would volatilize at over 600 °C and to remove a little B2O3 completely the sample was annealed at 700 °C for 8 h.
3.2. Formation mechanism The prepared m-ZrO2 nanorods have a hierarchical structure. In this research work, we synthesized hierarchical m-ZrO2 nanorods by the facile hydrothermal method. However, the formation of hierarchical m-ZrO2 nanorod was little. Guo et al. also found that zirconium hydroxide was changed to amorphous ZrO2 at 460 °C and m-ZrO2 at 600 °C, respectively [13]. Oliver et al. found that 1D ZrO2 could be synthesized from the zirconium hydroxyfluoride chain structure after annealing at over 460 °C [14]. Potyomkin and Sukharev thought that it is easy for ZrO2 to form amorphous hydrous zirconia under an acid environment [15]. 2ZrB2 þ 5O2 →2ZrO2 þ 2B2 O3
ð1Þ
ZrO2 þ H2 O2 →ZrO3 þ H2 O
ð2Þ
Fig. 1. FESEM (a & b) as-synthesized samples, (c & d) annealed samples at 700 °C.
S.A. Khan et al. / Powder Technology 256 (2014) 71–74
73
Fig. 2. EDX spectrum of the annealed sample at 700 °C.
B2 O3 þ 3H2 O→2H3 BO3
ð3Þ
H3 BO3 þ H2 O2 →HBO3 þ 2H2 O
ð4Þ
ZrB2 þ 3O2 þ 2H2 O2 →ZrO3 þ 2HBO3 þ H2 O
ð5Þ
ZrO3 þ 3H2 O→ZrðOHÞ4 þ H2 O2
ð6Þ
ZrðOHÞ4 þ ðn−2ÞH2 O→ZrO2 ∙nH2 OðamorphousÞ
ð7Þ
The ZrB2 doesn't react with oxygen below 540 °C. However, the reaction can be realized under a certain pressure and low-temperature as shown in Eq. (1). Subsequently, the oxidized products can further react with H2O2 and H2O according to Eqs. (2)–(4). ZrB2 can be oxidized
Fig. 3. Typical XRD spectrum of the samples before and after annealing at 700 °C.
to ZrO3 and HBO3 according to Eq. (5) under a certain pressure and H2O2 environment. The Zr4+ in aqueous solutions does not form cationic complexes even in an acid solution [15], but it could react with H2O and form Zr(OH)4. Zr(OH)4, nonexistent in a stable structure, dehydrates to amorphous ZrO2 ∙ nH2O at ambient temperature [13]. The final product ZrO3 subsequently, was hydrolyzed and formed amorphous ZrO2 ∙ nH2O precipitation according to Eq. (6) and Eq. (7). In this research work according to above experimental observation and reaction mechanism, we proposed a three-step sequential model to explain the formation mechanism of hierarchical m-ZrO2 nanorod. The schematic formation process of hierarchical m-ZrO2 is shown [Fig. 5(a)]. ZrB2 was oxidized to Zr(OH)4 in the first step. Zr(OH)4 interact each other through H-bond and dehydrate so that the surface energy arrives at the minimum by eliminating the surface in the second step. Subsequently, the precipitation could absorb water molecule in a symmetrical position as shown in Fig. 5(b). After embedding of the H2O molecule, the sterically hindered effect of H2O species restricts the 2D or 3D growth. The product would be amorphous ZrO2 ∙ nH2O 1D nanorod after aging and growth for a long time. The XRD pattern of assynthesized 1D ZrO2 ∙ nH2O nanorod is amorphous in nature and as compared to the annealed samples this model is in good agreement with XRD results. Then the ZrO2 ∙ nH2O nanorods are decomposed
Fig. 4. Typical TG/DTA curves of the as-synthesized sample.
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Fig. 5. (a) Hierarchical m-ZrO2 nanorod formation. (b) Reaction of amorphous ZrO2 ∙ nH2O.
because of the dehydroxylation and could finally reorganized into hierarchical m-ZrO2 form after annealing (step 3). 4. Conclusions The formation of such system is very important to the synthesis of 1D nanomaterials without a template. It has been shown that a simple hydrothermal method is suitable to prepare hierarchical m-ZrO2 nanorods. The results are useful in understanding (1) the chemical properties of ZrB2 and ZrO2, (2) the design of “its” derivatives for mZrO2 nanomaterials, and (3) the formation mechanism which may serve as another example of 1D nanomaterials. Acknowledgment This work has been financially supported by the Ministry of Science and Technology of China (S2010GR0771) and the National Natural Science Foundation of China (51161140399). References [1] Lu. Gui-Ying, Richard Lederich, Wole Soboyejo, Residual stresses and transformation toughening in MoSi2 composites reinforced with partially stabilized zirconia, J. Mat. Sci. Eng. A 210 (1996) 25–41. [2] Guy Anné, Stijn Put, Kim Vanmeensel, Dongtao Jiang, Jef Vleugels, Omer Van der Biest, Hard, tough and strong ZrO2–WC composites from nanosized powders, J. Eur. Ceram. Soc. 25 (2005) 55–63. [3] G. Sumana, M. Das, S. Srivastava, B.D. Malhotra, A novel urea biosensor based on zirconia, J. Thin Solid Films 519 (2010) 1187–1191.
[4] S.R. Li, M.S. Li, C.X. Zhang, S.P. Wang, X.B. Ma, J.L. Gong, Steam reforming of ethanol over Ni/ZrO2 catalysts: effect of support on product distribution, J. Int. J. Hydrog. Energy 37 (2012) 2940–2949. [5] K. Miszta, J. de Graaf, G. Bertoni, D. Dorfs, R. Brescia, S. Marras, L. Ceseracciu, R. Cingolani, R. van Roij, M. Dijkstra, L. Manna, Hierarchical self-assembly of suspended branched colloidal nanocrystals into superlattice structures, J. Nat. Mater. 10 (2011) 872–876. [6] R.M. Erb, R. Libanori, N. Rothfuchs, A.R. Studart, Composites reinforced in three dimensions by using low magnetic fields, J. Sci. 335 (2012) 199–204. [7] C. Zhang, C.X. Li, J. Yang, Z.Y. Cheng, Z.Y. Hou, Y. Fan, J. Lin, Tunable luminescence in monodisperse zirconia spheres, J. Langmuir 25 (2009) 7078–7083. [8] D. Min, N. Hoivik, G.U. Jensen, F. Tyholdt, C. Haavik, U. Hanke, Dielectric properties of thin-film ZrO2 up to 50 GHz for RF MEMS switches, J. Appl. Phys. A. 105 (2011) 867–874. [9] X.J. Fan, X.Q. Song, X.H. Yang, L.X. Hou, Facile fabrication of ZrO2 hollow porous microspheres with yeast as bio-templates, J. Mater. Res. Bull. 46 (2011) 1315–1319. [10] C. Liu, S.S. Zhao, X.J. Ji, B. Wang, D.X. Ma, A novel reflux-hydrothermal synthesis of thermally stable lamellar crystalline zirconia via SDS template, J. Mater. Chem. Phys. 133 (2012) 579–583. [11] F. Huang, Z. Fu, A.H. Yan, W. Wang, H. Wang, Y. Wang, J. Zhang, Q. Zhang, Facile synthesis, growth mechanism, and UV–vis spectroscopy of novel urchin-like TiO2/TiB2 heterostructures, J. Cryst. Growth Des. 9 (2009) 4017–4022. [12] F. Huang, Z.Y. Fu, W.M. Wang, H. Wang, Y.C. Wang, J.Y. Zhang, Q.J. Zhang, S.W. Lee, K. Niihara, Synthesis of shape-controlled Nb3O7F/NbB2 heterostructure: a new idea to synthesize binary hybrid materials by incomplete reaction, J. Mater. Res. Bull. 45 (2010) 739–743. [13] G.Y. Guo, Y.L. Chen, W.J. Ying, Thermal, spectroscopic and X-ray diffractional analyses of zirconium hydroxides precipitated at low pH values, J. Mater. Chem. Phys. 84 (2004) 308–314. [14] D.P. Brennan, P.Y. Zavalij, S.R.J. Oliver, A one-dimensional zirconium hydroxyfluoride, [Zr(OH)2 F3][enH], J. Solid State Chem. 179 (2006) 665–670. [15] V.A. Potyomkin, Y.I. Sukharev, Formation of liotropic features of zirconium oxyhydrate gels, J. Chem. Phys. Lett. 371 (2003) 626–633.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Study of template-free synthesis hierarchical m-ZrO2 nanorods by hydrothermal method Shahzad Ahmad Khan, Zhengyi Fu ⁎, Sahibzada Shakir Rehman, Muhammad Asif, Weimin Wang, Hao Wang State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 22 June 2013 Received in revised form 15 January 2014 Accepted 1 February 2014 Available online 10 February 2014 Keywords: Powders: chemical preparation Chemical properties Growth models ZrO2
a b s t r a c t The template-free synthesis of hierarchical m-ZrO2 nanorods by a simple hydrothermal method and its possible formation mechanism based on a series of chemical reactions have been proposed here. The traditional preparation methods of hierarchical ZrO2 nanorods are involved in expensive equipment, complicated process and high production cost. Owing to their physical and chemical properties hierarchical ZrO2 nanorods have received considerable attention. The results revealed that as synthesized products are composed of many nanorods with 100–200 nm in diameter and 3–5 μm in length. The final product after annealing is involved into hierarchical monoclinic ZrO2 (m-ZrO2) nanorods, the big nanorod was made up of many small nanorods with 30–40 nm in diameter and 350–450 nm in length. This discovery could open a new path to the template-free synthesis of hierarchical nanomaterials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ZrO2 has three crystal forms as follows: monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2) and cubic zirconia (c-ZrO2). The m-ZrO2 is stable below 1170 °C and c-ZrO2 exists stable above 2370 °C. The pure t-ZrO2 is believed to be stable in the range of 1170 °C to 2370 °C. Due to the martensitic phase transformation originated from volumetric change effect when tetragonal to monoclinic phase takes place [1,2], the partial stabilized t-ZrO2 has been used extensively in the ceramic because of its transformation toughening performance. The hierarchical zirconia (ZrO2) nanomaterials are widely used in many fields, including solid-state electrolytes, electro-optical materials and catalytic materials [3,4]. The hierarchical nanomaterials have recently attracted increasing interests owing to their unusual physical and chemical properties [5,6]. Especially, hierarchical zirconia (ZrO2) nanomaterials have good optical and electric properties, high dielectric constant, extremely low thermal conductivity, and relatively high thermal expansion coefficient [7,8]. The ZrO2 hollow porous microspheres could be synthesized using bio-templates Fan et al. found [9]. The hierarchical ZrO2 nanomaterials suffer from the fact that spontaneous and controlled growth is limited to certain methods, such as PECVD, reactive-spray atomization, sol–gel, hydrothermal, electro-deposition, etc. Liu et al. reported that stable lamellar zirconia was fabricated using the hydrothermal method and SDS template [10]. However, the traditional physical method is involved in expensive equipment, complicated process and high production cost. Although the traditional chemical method usually needs surfactant or organic template that results in complicated process and precise control. Thus, it is urgent to ⁎ Corresponding author. Tel.: +86 27 87865484; fax: +86 27 87215421. E-mail address: [email protected] (Z. Fu). 0032-5910/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2014.02.012
develop quick, simple and low-cost methods to make hierarchical ZrO2 nanomaterials. Recently, Huang et al. adopted a facile hydrothermal method to synthesize hierarchical TiO2 and Nb3O7F nanomaterials when they studied the chemical properties of boride ceramic [11,12]. The importance of this work lies in the demonstration of the synthesis of hierarchical m-ZrO2 nanorods. The zirconium diboride (ZrB2) and hydrogen peroxide (H2O2) were used as raw materials for the synthesis of hierarchical m-ZrO2 nanorods by facile hydrothermal method. To the best of our knowledge, there is still no research work that report about such system. The hierarchical m-ZrO2 nanorod formation mechanism, the natural properties of zirconium and a rational model have been proposed. 2. Materials and method The ZrB2 (99.5%) grain sizes range from 1 to 2 μm and H2O2 (30%, AR) was purchased from Alfa Aesar Inc. (US) and Sinopharm Chemical Reagent Co. Ltd. (China), respectively. All the chemicals were used without further purification. Firstly, ZrB2 (1 g) and H2O2 (10 mL) were added into an aqueous solution (50 mL), respectively. Then the mixed solution was transferred into a 100 mL Teflon-line autoclave, followed by sealing, maintaining at the 170 °C for 24 h and naturally cooling down at room temperature. After centrifugation, the collected precipitate was washed with deionized water and absolute ethanol for several times, respectively. Finally, the as-synthesized products were dried at the 80 °C for 12 h and annealed at 700 °C in air for 8 h. The morphologies, size of the as-synthesized and annealed samples were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). The composition of the samples was analyzed by an energy dispersive X-ray detector (EDX, Thermo Noran VANTAG-ESI, 120 KV). The phase of the products was analyzed by X-
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S.A. Khan et al. / Powder Technology 256 (2014) 71–74
ray powder diffraction (XRD, Rigaku Ultima II, Japan) with Cu Kα irradiation and a dynamic thermal analyzer (TG/DTA, METZSCH Instrument Co. Ltd., STA449, Germany). 3. Results and discussion 3.1. Morphology, composition and structure Fig. 1 presents a set of typical field emission scanning electron microscopy (FESEM) images of the as-synthesized and annealed samples under a strong ultrasonic wave treatment for 1 h shows different morphologies. The FESEM images of Fig. 1(a) and (b) revealed that the assynthesized sample consists of a large quantity of nanorods with 100– 200 nm in diameter and 3–5 μm in length. The sample annealed at 700 °C was involved into hierarchical nanorod, the big rod was composed of many small connected nanorods with diameters of 30–40 nm and lengths of 350–450 nm [Fig. 1(c) and (d)]. The EDX of the annealed sample as shown in Fig. 2, revealed that Zr and O were present in the annealed sample with different percentages that is in agreement with the results of XRD (Fig. 3). The phases of the as-synthesized and annealed samples were examined by XRD as shown in Fig. 3. The as-synthesized product, 1D ZrO2 ∙ nH2O nanorod with broadening and weak peak has a poor crystallinity. The reason may be attributed to amorphous intermediate product, that will be further explained in the following formation mechanism. The samples annealed at 700 °C can be indexed as standard patterns of m-ZrO2 (JCPDS 65-1025). The narrow line widths indicate a high crystallinity of the material. The same result can be obtained even if the annealing temperature is continuously increased up to 850 °C. Annealing is believed to have a great impact on the final phase of hydrothermal products. The TG/DTA from Fig. 4, is used to characterize the structure of assynthesized product under air atmosphere. The TG curve revealed that there are two obvious stages of weight loss. In the first stage, the weight
loss from 25 °C to 475 °C amounts to about 14.98% which corresponds to the release of the water molecules namely, absorbed water and structural water as well as a little decomposition of boric acid crystal to diboron trioxide (B2O3). From the sharp exothermic peak at 256 °C the DTA curve contributes to the release of absorbed water. The exothermic peak at 466 °C is assigned to a rapid release of bound hydroxyl groups, the decomposition of hydrous ZrO2 into amorphous ZrO2 [13]. The temperature ranges from 475 °C to 870 °C indicate 2.25% weight loss, which could be attributed to the volatilization of B2O3 by-product in the second stage. It is revealed that weak exothermic peak around 656 °C may result from the transformation of amorphous ZrO2 into m-ZrO2 , indicating that the main process for m-ZrO2 formation could be completed above 700 °C. It is well known that B2O3 would volatilize at over 600 °C and to remove a little B2O3 completely the sample was annealed at 700 °C for 8 h.
3.2. Formation mechanism The prepared m-ZrO2 nanorods have a hierarchical structure. In this research work, we synthesized hierarchical m-ZrO2 nanorods by the facile hydrothermal method. However, the formation of hierarchical m-ZrO2 nanorod was little. Guo et al. also found that zirconium hydroxide was changed to amorphous ZrO2 at 460 °C and m-ZrO2 at 600 °C, respectively [13]. Oliver et al. found that 1D ZrO2 could be synthesized from the zirconium hydroxyfluoride chain structure after annealing at over 460 °C [14]. Potyomkin and Sukharev thought that it is easy for ZrO2 to form amorphous hydrous zirconia under an acid environment [15]. 2ZrB2 þ 5O2 →2ZrO2 þ 2B2 O3
ð1Þ
ZrO2 þ H2 O2 →ZrO3 þ H2 O
ð2Þ
Fig. 1. FESEM (a & b) as-synthesized samples, (c & d) annealed samples at 700 °C.
S.A. Khan et al. / Powder Technology 256 (2014) 71–74
73
Fig. 2. EDX spectrum of the annealed sample at 700 °C.
B2 O3 þ 3H2 O→2H3 BO3
ð3Þ
H3 BO3 þ H2 O2 →HBO3 þ 2H2 O
ð4Þ
ZrB2 þ 3O2 þ 2H2 O2 →ZrO3 þ 2HBO3 þ H2 O
ð5Þ
ZrO3 þ 3H2 O→ZrðOHÞ4 þ H2 O2
ð6Þ
ZrðOHÞ4 þ ðn−2ÞH2 O→ZrO2 ∙nH2 OðamorphousÞ
ð7Þ
The ZrB2 doesn't react with oxygen below 540 °C. However, the reaction can be realized under a certain pressure and low-temperature as shown in Eq. (1). Subsequently, the oxidized products can further react with H2O2 and H2O according to Eqs. (2)–(4). ZrB2 can be oxidized
Fig. 3. Typical XRD spectrum of the samples before and after annealing at 700 °C.
to ZrO3 and HBO3 according to Eq. (5) under a certain pressure and H2O2 environment. The Zr4+ in aqueous solutions does not form cationic complexes even in an acid solution [15], but it could react with H2O and form Zr(OH)4. Zr(OH)4, nonexistent in a stable structure, dehydrates to amorphous ZrO2 ∙ nH2O at ambient temperature [13]. The final product ZrO3 subsequently, was hydrolyzed and formed amorphous ZrO2 ∙ nH2O precipitation according to Eq. (6) and Eq. (7). In this research work according to above experimental observation and reaction mechanism, we proposed a three-step sequential model to explain the formation mechanism of hierarchical m-ZrO2 nanorod. The schematic formation process of hierarchical m-ZrO2 is shown [Fig. 5(a)]. ZrB2 was oxidized to Zr(OH)4 in the first step. Zr(OH)4 interact each other through H-bond and dehydrate so that the surface energy arrives at the minimum by eliminating the surface in the second step. Subsequently, the precipitation could absorb water molecule in a symmetrical position as shown in Fig. 5(b). After embedding of the H2O molecule, the sterically hindered effect of H2O species restricts the 2D or 3D growth. The product would be amorphous ZrO2 ∙ nH2O 1D nanorod after aging and growth for a long time. The XRD pattern of assynthesized 1D ZrO2 ∙ nH2O nanorod is amorphous in nature and as compared to the annealed samples this model is in good agreement with XRD results. Then the ZrO2 ∙ nH2O nanorods are decomposed
Fig. 4. Typical TG/DTA curves of the as-synthesized sample.
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S.A. Khan et al. / Powder Technology 256 (2014) 71–74
Fig. 5. (a) Hierarchical m-ZrO2 nanorod formation. (b) Reaction of amorphous ZrO2 ∙ nH2O.
because of the dehydroxylation and could finally reorganized into hierarchical m-ZrO2 form after annealing (step 3). 4. Conclusions The formation of such system is very important to the synthesis of 1D nanomaterials without a template. It has been shown that a simple hydrothermal method is suitable to prepare hierarchical m-ZrO2 nanorods. The results are useful in understanding (1) the chemical properties of ZrB2 and ZrO2, (2) the design of “its” derivatives for mZrO2 nanomaterials, and (3) the formation mechanism which may serve as another example of 1D nanomaterials. Acknowledgment This work has been financially supported by the Ministry of Science and Technology of China (S2010GR0771) and the National Natural Science Foundation of China (51161140399). References [1] Lu. Gui-Ying, Richard Lederich, Wole Soboyejo, Residual stresses and transformation toughening in MoSi2 composites reinforced with partially stabilized zirconia, J. Mat. Sci. Eng. A 210 (1996) 25–41. [2] Guy Anné, Stijn Put, Kim Vanmeensel, Dongtao Jiang, Jef Vleugels, Omer Van der Biest, Hard, tough and strong ZrO2–WC composites from nanosized powders, J. Eur. Ceram. Soc. 25 (2005) 55–63. [3] G. Sumana, M. Das, S. Srivastava, B.D. Malhotra, A novel urea biosensor based on zirconia, J. Thin Solid Films 519 (2010) 1187–1191.
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