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Elucidating the effects of high temperature mixing method under hydrothermal condition (HTMM) on grain refinements and assembling structures
Powder Technology 305 (2017) 440–446
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Elucidating the effects of high temperature mixing method under hydrothermal condition (HTMM) on grain refinements and assembling structures Qilin Gu a,b,c, Qiaomei Sun a,b, Kongjun Zhu a,⁎, Chuanxiang Zhang d, Jinsong Liu b, Jing Wang a, Jinhao Qiu a a
State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China c Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore d College of Materials Engineering, Nanjing Institute of Technology, Nanjing 211167, PR China b
a r t i c l e
i n f o
Article history: Received 29 June 2016 Received in revised form 12 October 2016 Accepted 15 October 2016 Available online 17 October 2016 Keywords: Hydrothermal synthesis Grain refinement Morphology controlling Barium strontium titanate Sodium niobate
a b s t r a c t Crystal size and microstructure are of great importance in determining the physical and chemical properties of functional materials, and refine powders, especially their assembled porous structures have potential application in ceramic fabrication, absorption, catalysts and drug delivery, due to their characters of high activity and large specific surface area. Herein, high temperature mixing method under hydrothermal condition (HTMM) was adapted to synthesize barium strontium titanate [(Ba, Sr)TiO3, BST] powders with various Ba/Sr ratios (x = 0.5, 1.0, 3.0 and 4.0). In comparison with conventional hydrothermal synthesis (CHS), the effects of HTMM on grain refinement and porous structure formation were exclusively investigated. XRD and SEM results indicated that, in the given condition, BST powders prepared by HTMM were much smaller than that by CHS, especially at a lower Ba/Sr ratio. Additionally, assembled porous architectures were constructed by HTMM. It's believed that the high temperature mixing process and continuous rotation contributed to the grain refinement and assembled porous structure, respectively. The assumption was further confirmed through the synthesis of sodium niobate (NaNbO3) powders by HTMM. It's demonstrated that HTMM is advantageous in preparation of refine powders and porous assembled architectures. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Both crystal size and microstructure are of great importance in determining the physical and chemical properties of functional materials [1,2]. It's notable that there exists a critical grain size, below which the materials will display significantly enhanced performance due to the unique characters of large surface areas, affluent surface atoms and extremely high activation [3]. The preparations for micro- or/and nanoparticles are mainly divided into two categories, i.e. top-down [4] and bottom-up [5]. Among them, the bottom-up methods, like co-precipitation [6,7], solvothermal [8], sol-hydrothermal [9,10] and microemulsion [11], show evident advantages in nanomaterial production, due to their low reaction temperature, compositional homogeneity and controllability, especially in comparison to the solid state reaction. As the representative bottom-up method, the facile hydrothermal syntheses have been employed to synthesize kinds of advanced materials. Although selective synthesis can be achieved by tuning the hydrothermal process parameters, including pH, raw material ratio, temperature and duration [12– ⁎ Corresponding author. E-mail address: [email protected] (K. Zhu).
http://dx.doi.org/10.1016/j.powtec.2016.10.033 0032-5910/© 2016 Elsevier B.V. All rights reserved.
14], one prominent issue is that the crystallization process occurs inevitably at certain temperature during the elevating period; the earlier formed crystals will suffer from a longer crystal growth process than the later ones, thus the final products would be lack of homogeneity in size distribution, and some large grains may disperse in small particles randomly. To obtain the size-uniform nanoparticle, several novel strategies have been designed deliberately. For instance, Friderichs et al. proposed a two-phase oil/water solvothermal environment method to prepare monodisperse SrTi1 − xZrxO3 nanocubes with an edge length of 10 nm [15]. Most remarkably, the size and shape of the nanoparticles depended on neither the Zr content nor reaction time. It's believed that oleate ion surfactants played a vital role in prohibiting the grain growth [15]. Another common approach is the introduction of organic capping-agents to prevent the crystal facets growth [16–18]. It's well-known that nucleation and crystal growth is the two key parts controlling the final grain size and distribution. However, previous works were mainly involved in the management of crystal growth process. Herein, we paid our special attention toward the nucleation process, and developed a high temperature mixing method under hydrothermal conditions (HTMM) to refine the grain size and modify
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its uniformity. In our previous work, HTMM was primarily designed to circumvent the intermediate phase, aiming to obtain the pure targets [19]. During this process, the self-designed double-chambered Teflonlinear enables the raw materials located separately, and the chemical reaction is triggered as it reaches the preset temperature. Actually, it's demonstrated that HTMM had unique advantages in circumventing the intermediate impurities, thereby improving the purity of final products [20]. ABO3-type perovskite structure compounds, barium strontium titanate [(Ba, Sr)TiO3, BST] and sodium niobate (NaNbO3), are widely investigated environmental-friendly piezoelectric and catalytic materials [21–24]. For example, BST nanopowders with an average grain size of 475 nm were prepared by a high-energy ball-milling method [25]. Taking the syntheses of BST and NaNbO3 as case study, in this work, we will elucidate the effects of HTMM on the grain refinement and morphology controlling from the view point of nucleation and crystal growth. The proposed HTMM would be a prevalent strategy to synthesize nanomaterials. 2. Experimental procedures 2.1. Chemicals Potassium hydroxide (KOH, 99.0% min) and titanium dioxide (TiO2, 99.0% min) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Barium chloride(BaCl2, 99.5% min) was purchased from Nanjing
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Reagent Co., Ltd. Strontium nitrate (Sr(NO3)2, 99.5% min) was purchased from Shanghai Xinbao Fine Chemical Factory. Tetrabutyl titanate (Ti(C4H9O)4, 98.0% min) was purchased from Shanghai Zhanyun Chemical Reagent Co., Ltd. Nitric acid (HNO3, 65% min) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Absolute ethyl alcohol (C2H5OH, 99.7% min) was purchased from Nanjing Ningshi Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH, 96% min) and niobium oxide (Nb2O5, 99.5% min) was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received without further purification.
2.2. Conventional hydrothermal syntheses (CHS) In the typical hydrothermal synthesis of BST powders, TiO2 was used as starting materials. Firstly, TiO2 (0.04 M) and BaCl2 (0.16 M) were added into 55 ml KOH (0.6 M) aqueous solution. After stirring for 30 min, different amount of Sr(NO3)2, depending on the ratios of Ba/Sr (x = 0.5, 1.0, 3.0, 4.0) were dissolved in the mixture. Subsequently, the prepared precursors were transferred into Teflon-lined autoclave and kept at 200 °C for 10 h. And then, the resultant was naturally cooled to room temperature. The product was rinsed with de-ionized water and then anhydrous alcohol, and precipitated with centrifugation for 10 min at 3000 rpm to yield a white powder. Rinsing was repeated thrice to remove excess ions from the final product, and the precursor was dried at 80 °C overnight.
Fig. 1. FE-SEM images of BST powders synthesized by (a-c) CHS and (d-f) HTMM with different Ba/Sr ratios: (a) x = 0.5; (b) x = 3.0; (c) x = 4.0; (d) x = 0.5; (e) x = 3.0; (f) x = 4.0.
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a Cu Kα radiation (40 kV, 40 mA) source (λ = 0.154178 nm) at a scanning rate of 10 o/min in the 2θ range of 5°–60°. The morphology and microstructure of as-synthesized samples were observed using a Hitachi S4800 field emission scanning electron microscope (FE-SEM, Hitachi S4800, Japan). Energy dispersive X-ray spectrometer was used to analyze elemental composition and distribution. Nitrogen isothermal adsorption–desorption measurements are performed to determine the Brunauer–Emmett–Teller (BET) surface areas using an automatic surface area and pore analyzer (SSA-4300, Beijing Builder Corp., China) after treating the samples at 150 °C for 10 h. Using a 514.5 nm laser, Raman spectra were measured by a confocal laser micro-Raman spectroscopy system (Raman, LABRAM HR800) within the range of 100– 1000 cm− 1. UV–vis diffuse reflectance spectra were recorded on a UV–visible spectrophotometer (TU-1901, PGeneral Instrument Inc., China) at room temperature with BaSO4 as the reference and then converted into absorption spectra via Kubelka–Munk transformation.
Fig. 2. XRD patterns of BST powders synthesized by CHS (black color) and HTMM (blue color) with different Ba/Sr ratios (x = 0.5, 1.0, 3.0 and 4.0). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. High temperature mixing method under hydrothermal condition (HTMM) Teflon-liners with two separate chambers were fabricated for HTMM. To prevent the reaction between reactants, the starting compounds were divided into two groups and placed into the two chambers, respectively. Taking the preparation of BST powders as an example, one chamber was filled with BaCl2, Sr(NO3)2 and 6 ml KOH (0.6 M) aqueous solution, and the other one is TiO2 and 6 ml de-ionized water. The specific quantity of the reactants is consistent with the CHS, including the variation of Ba/Sr (x = 0.5, 1.0, 3.0, 4.0). Teflon-lined autoclaves were put up-right in a heating oven. Once the temperature reached 200 °C, the autoclaves were connected to the rotating electrical machine. In this way, the reactants in the separate chambers could be blended and the reaction would proceed drastically. The rotation was continued in the whole duration (10 h). Finally, the samples were naturally cooled down. The product was rinsed with de-ionized water and then anhydrous alcohol, and precipitated with centrifugation for 10 min at 3000 rpm to yield a white powder. Washing was repeated thrice to remove excess ions from the final product, and the precursor was dried at 80 °C for 24 h. In the syntheses of NaNbO3, similar process was conducted. Namely, NaAc and KOH aqueous solution were poured into the one chamber, while Nb2O5 and water was placed into the other chamber. The reaction was triggered at 220 °C and the autoclaving and rotating time was 16 h. The process parameters were set according to our previous work [26].
2.4. Characterization The crystal structure of the as-prepared sample was characterized by a Bruker X-ray diffractometer (XRD, Bruker D8 Advance, Germany) with
Fig. 3. Raman spectra of BST powders prepared by (a) CHS and (b) HTMM with different Ba/Sr ratios (x = 0.5, 1.0, 3.0 and 4.0).
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Fig. 4. (a) low-magnification and (b) high-magnification FE-SEM images of hydrothermally synthesized BST powders by HTMM using tetrabutyl titanate as Ti source (Ba/Sr = 3.0).
3. Results and discussion As a solid solution of BaTiO3 and SrTiO3, the physical properties of BST can be significantly affected by chemical composition. In this contribution, the effects of HTMM on the grain size and shape of BST with different Ba/Sr ratios were investigated. FE-SEM images of BST powders synthesized via CHS and HTMM were shown in Fig. 1(a–c) and Fig. 1(d–f), respectively. As shown in Fig. 1(a), the samples prepared by CHS at x = 0.5 presented an irregular aggregation with the average size of about 1 μm. For the same chemical composition, the samples prepared by HTMM, however, were sub-microsized cubes assembled by a mass of nanoparticles (Fig. 1(d)). The phenomenon became much evident at x = 3.0 as shown in Fig. 1(e), and the particles showed an average grain size of about 150 nm. On the contrary, the samples with x = 3.0 presented by CHS still presented the sub-microcubes. With the Ba/ Sr ratio increased to x = 4.0, both the size and shape of samples prepared by CHS and HTMM are somewhat comparable, where BST nanoparticles with an average size of 150 nm aggregated slightly. In addition, it's found in Fig. 1 that with the increasing of Ba/Sr ratio, the grain size of products reduced gradually, no matter which treatment method was adapted. Nevertheless, the samples obtained at x = 0.5 by HTMM showed comparable dimension to that at x = 3.0 by CHS. The
results indicated that HTMM can be more effective in grain refinement. Since the increase of Ba/Sr ratio can reduce the grain size, the effect of HTMM on grain refinement was discount at higher Ba/Sr ratio (e.g. x = 4.0). In fact, some of our previous work has revealed the grain refinement effect of HTMM. For instance, keeping the other parameters as constants, ZnO nanocrystals prepared by conventional method were about ten times larger than those prepared by HTMM [27]. Similar results were also observed in the hydrothermal synthesis of KNN cubes, where the grain size could be reduced from 5 μm to near 1 μm through replacing conventional method by HTMM [28]. The phase composition of the samples has been characterized by XRD and the results are presented in Fig. 2. For the samples prepared by CHS and HTMM, the main phase of all the samples was indexed to be perovskite structured (Ba, Sr)TiO3 with space group of Pm3̄ m. Considering the specific chemical composition of each sample, careful search and match were completed in Jade 5 software. It's found that the products with x = 0.5 and x = 1matched well with SrTiO3 (JCPDS number: 35–0734, a = b = c = 3.905 Å). With the increase of Ba/Sr ratio, the final product with x = 3.0 and x = 4.0 was assigned to Ba0.5Sr0.5TiO3 (JCPDS number: 34-0411, a = b = c = 3.947 Å) and Ba0.6Sr0.4TiO3 (JCPDS number: 35-0734, a = b = c = 3.965 Å), respectively. The compositional changes in final products can also be reflected
Fig. 5. (a) Illustration of hydrothermal process by HTMM: (1) the reactants are separately placed into each chamber to prevent their reaction during the elevating process; (2) Once the temperature reached the preset value, the vessels will be rotated by the connected electrical machine. (3) In this way, the starting compounds will be mixed at high temperature, and the chemical reaction will proceed rapidly. Numerous of crystal nucleus are constructed beneficial from the abundant dynamic energy. (b) While in CHS, the nucleation process is slow and inhomogeneous during the whole temperature-elevating period. (c) Accompanying with the rotation of vessels, the crystals in the chamber are rolling with the reaction medium, and hierarchical structures assembling by the as-formed crystals will be shaped.
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Fig. 6. (a) low-magnification and (b) high-magnification FE-SEM images of hydrothermally synthesized NaNbO3 powders by HTMM at 220 °C for 16 h.
in the peak shift of XRD pattern. With the gradual increase of Ba/Sr ratio, the diffraction peaks shifted slightly to the low angle, owing to the larger ion radius of Ba2+ (1.32 Å) than Sr2+ (1.14 Å) [29]. In addition to the dominated BST phase, as shown in Fig. 2, additional peaks around 2θ = 27°, 36°, 54° belonging to rutile TiO2 (JCPDS number: 21-1276, a = b = 4.593 Å, c = 2.959 Å) can be observed easily in the samples synthesized by CHS. On the contrary, the signals attributed to TiO2 impurity was quite weak in samples prepared by HTMM. Therefore, It's concluded that HTMM possesses improved reaction dynamics and can yield products with higher purity, in consistent with our previous works [30]. At the same time, it's found that the peak intensity of assynthesized samples became weakened with the Ba/Sr rising, regardless of the synthesis process. However, at a fixed Ba/Sr ratio, the peak intensities of the two samples were distinguished; that is, the peak intensity of samples prepared by CHS was stronger than that by HTMM, resulting from their larger grain size and better crystallinity. To further clarify the phase composition and crystallinity, Raman spectra of BST samples prepared by CHS and HTMM were provided. As shown in Fig. 3, the first-order Raman bands of rutile TiO2 around 144 cm−1 (B1g), 449 cm−1 (Eg), 611 cm−1 (A1g) were observed in all the samples [31,32]. The result further confirmed the appearance of TiO2 impurity, in consist with XRD results. At the same time, the appearance of first-order Raman bands TO2, TO3, TO4 and LO4, as marked by dashed line in Fig. 3(a), characterized the BST powders [33,34]. It's noticed that the intensity of Raman bands belonging to impurity TiO2 was higher than main products BST. In general, Raman spectra reflects the local vibration and short range structural character, while XRD pattern provides the information about phase component in macroscopical scale. In this regard, it's infered that there existed mass of anion motions with respect to stationary central cations [32]. Benefiting from the particular rotation process, the proposed HTMM may also have potential effects on morphological regulation, especially for the formation of complex architectures. Subsequently, instead of titanium oxide, tetrabutyl titanate was utilized as titanium source, and BST powders with x = 3.0 were prepared by HTMM. From the low-magnification SEM image (Fig. 4(a)), porous sphere particles with an average size about 300 nm were observed. The nut-like spheres were composed of numerous nanoparticles less than 100 nm, as shown in Fig. 4(b). Previously, a similar case with dandelion-like balls constructed from niobate fibers were hydrothermally observed by HTMM [30]. In addition, the BET surface areas of BST powders prepared by HTMM were measured to be 61.914 m2/g, which was seven fold larger than that prepared by conventional hydrothermal synthesis (8.415 m2/g). That is to say, these assembled architectures resulted from HTMM were abundant in surface, and were suitable for potential application in the absorption, catalysis and drug delivery fields. Although the advantages of HTMM in grain refinement and shape regulation have been recognized in the previous work, there still lacks a systematic understanding about this process. In combination with the above results, we attempted to elucidate the underlying mechanism for the grain refinement and morphology formation during HTMM. The
reacting reactants are located in separate chambers, and will not contact with each other till the temperature is elevated to the pre-set value, as the situation (1) illustrated in Fig. 5(a). Once the system temperature reaches the preset value (i.e. 200 °C), the sealed autoclaves will be rotated by the connected electrical machine, as the situation (2) illustrated in Fig. 5(a). In this way, the reactants are mixed abruptly, and the involved reaction will proceed rapidly; namely, numerous of crystal nucleus will form in a short time, due to the large driving force available, which is illustrated as the state (3) in Fig. 5(a). Since the nucleation process is quite thorough and extensive, the crystal growth will be inhibited, thus the products with refined crystal size and narrow distribution will be finally resulted. On the contrary, during CHS, nucleation process occurs partially during the temperature-elevating period, once the driving force is large enough in some local points, as shown in Fig. 5(b). The earlier formed crystal nucleus could experience a longer period of grain growth during the subsequent heating and dwelling stage, thus the bigger crystals will be emerged in the final products, causing a broad size distribution. In terms of the assembled structure, it's believed to be promoted by the continuing rotation process. With the autoclave rotating continuously, the formed crystal nucleus in the chamber will move with the flowing of liquid medium; large numbers of crystals would rolling along the inner wall of liner. After constant rolling and collision, the crystals will contact with each other and get dense gradually. Eventually, the assembled architecture with round shape is formed. Such processes are illustrated as Fig. 5(c). To further verify the proposed mechanism, HTMM was adapted to synthesize other ABO3-type compounds. Based on our previous work [26], NaNbO3 were selected and the processing parameters were similar
Fig. 7. UV–vis absorption spectra of the BST samples prepared by different processes (CHS and HTMM) and Ti sources (TBOT: tetrabutyl titanate; the others are titanium oxide).
Q. Gu et al. / Powder Technology 305 (2017) 440–446 Table 1 The average particle size, band gap wavelength (λg) and band gap (Eg) of BST samples prepared at various conditions. Sample
1
2
3
4
5
Treatment Ba/Sr Ti source Particle size/nm λg/nm Eg/eV
CHS 0.5 TiO2 900 397 3.12
CHS 3.0 TiO2 500 399 3.10
HTMM 0.5 TiO2 500 397 3.12
HTMM 3.0 TiO2 200 398 3.11
HTMM 3.0 TBOT 400 393 3.15
to our previous work. After a reaction at 220 °C for 16 h, the microstructure of the as-prepared NaNbO3 is shown in Fig. 6. The irregular sphereshaped particles with a uniform size of 100 μm were observed in the low-magnification FE-SEM image as shown in Fig. 6(a). From the high-magnification FE-SEM image (Fig. 6(b)), we can see that the sphere-like architectures were assembled from numbers of plate-like particles, which were the main yields by CHS. It's notable that the size of the plates was quite smaller than the ones prepared by CHS [26]. The results in terms of dimensional reduction and assembled morphology can be interpreted by the proposed mechanism. Interestingly, a layer with 5 μm in thickness was coated partially on the surface of the sphere-like architecture (Fig. 6(b)). It's inferred that the layers were the later formed NaNbO3 crystals with poor crystalline, which can be evidenced by the weakened XRD pattern. It's well-acknowledged that much unsaturated coordination structures may reside on the surface of smaller sized particles, and the band gap of the finer particles becomes broaden. Therefore, the UV– vis absorbance spectra of the BST samples prepared at various conditions were collected and plotted as Fig. 7. The band gap wavelength (λg) and band gap (Eg) of BST samples were listed in Table 1. Regardless of the treatment method, CHS or HTMM, the BST powders with x = 3.0 manifested evident red shift compared with the ones with x = 0.5. This phenomenon can be understood as a result of composition differences, since the energy band gap of SrTiO3 is slightly larger that of BaTiO3 [35]. Analogously, Prabhas et al. pointed out that with the increasing Nb content in NaNbxTa1 − xO3 specimens, this onset absorption energy shifted from the NaTaO3 to the NaNbO3 state [36]. Note that the absorbance spectra of BST powders prepared by HTMM exhibited a slight blue shift when compared with the ones prepared by CHS. Additionally, as the titanium source was changed from titanium oxide to TBOT, their absorbance spectra showed an obvious shift to low wavelength. It's inferred that the observed blue shift may mainly result from the reduced grain size [37,38]. These results manifested that the HTMM can effectively reduce the grain size of resulted powders, especially when the reactant is soluble, in consistent with FE-SEM results.
4. Conclusion In this work, HTMM was adapted to prepare BST powders with various Ba/Sr ratios, and special attentions were paid to the advantages of HTMM in grain refinement and morphological regulation. XRD and SEM results indicated that HTMM outperformed in reducing the grain size of BST powders in comparison to CHS, especially at the lower Ba/ Sr ratio. When the titanium source was changed from titanium oxide to tetrabutyl titanate, the resulted BST powders showed porous architectures assembled from numerous nanoparticles. It's proposed that the HTMM aggravated the rapid nucleation, and inhibited the following grain growth, giving rise to the refine grains with uniform distribution. At the same time, it's the continuing rotation that promoted the assembling of grains, resulting in the formation of porous architectures. The proposed assumption has been verified through the synthesis of NaNbO3 powders, where smaller plates were obtained, and further assembled into the sphere-like porous architectures. The unique
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advantages in refine grains and assembled structures endow HTMM promising applications in absorption, catalysis and drug delivery field. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC No. 51372114 and 51672130), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and Astronautics) (Grant No. 0514Y01), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Also, Qilin Gu would like to thank the support from Nanjing University of Aeronautics and Astronautics PhD short-term visiting scholar project.
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Elucidating the effects of high temperature mixing method under hydrothermal condition (HTMM) on grain refinements and assembling structures Qilin Gu a,b,c, Qiaomei Sun a,b, Kongjun Zhu a,⁎, Chuanxiang Zhang d, Jinsong Liu b, Jing Wang a, Jinhao Qiu a a
State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China c Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore d College of Materials Engineering, Nanjing Institute of Technology, Nanjing 211167, PR China b
a r t i c l e
i n f o
Article history: Received 29 June 2016 Received in revised form 12 October 2016 Accepted 15 October 2016 Available online 17 October 2016 Keywords: Hydrothermal synthesis Grain refinement Morphology controlling Barium strontium titanate Sodium niobate
a b s t r a c t Crystal size and microstructure are of great importance in determining the physical and chemical properties of functional materials, and refine powders, especially their assembled porous structures have potential application in ceramic fabrication, absorption, catalysts and drug delivery, due to their characters of high activity and large specific surface area. Herein, high temperature mixing method under hydrothermal condition (HTMM) was adapted to synthesize barium strontium titanate [(Ba, Sr)TiO3, BST] powders with various Ba/Sr ratios (x = 0.5, 1.0, 3.0 and 4.0). In comparison with conventional hydrothermal synthesis (CHS), the effects of HTMM on grain refinement and porous structure formation were exclusively investigated. XRD and SEM results indicated that, in the given condition, BST powders prepared by HTMM were much smaller than that by CHS, especially at a lower Ba/Sr ratio. Additionally, assembled porous architectures were constructed by HTMM. It's believed that the high temperature mixing process and continuous rotation contributed to the grain refinement and assembled porous structure, respectively. The assumption was further confirmed through the synthesis of sodium niobate (NaNbO3) powders by HTMM. It's demonstrated that HTMM is advantageous in preparation of refine powders and porous assembled architectures. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Both crystal size and microstructure are of great importance in determining the physical and chemical properties of functional materials [1,2]. It's notable that there exists a critical grain size, below which the materials will display significantly enhanced performance due to the unique characters of large surface areas, affluent surface atoms and extremely high activation [3]. The preparations for micro- or/and nanoparticles are mainly divided into two categories, i.e. top-down [4] and bottom-up [5]. Among them, the bottom-up methods, like co-precipitation [6,7], solvothermal [8], sol-hydrothermal [9,10] and microemulsion [11], show evident advantages in nanomaterial production, due to their low reaction temperature, compositional homogeneity and controllability, especially in comparison to the solid state reaction. As the representative bottom-up method, the facile hydrothermal syntheses have been employed to synthesize kinds of advanced materials. Although selective synthesis can be achieved by tuning the hydrothermal process parameters, including pH, raw material ratio, temperature and duration [12– ⁎ Corresponding author. E-mail address: [email protected] (K. Zhu).
http://dx.doi.org/10.1016/j.powtec.2016.10.033 0032-5910/© 2016 Elsevier B.V. All rights reserved.
14], one prominent issue is that the crystallization process occurs inevitably at certain temperature during the elevating period; the earlier formed crystals will suffer from a longer crystal growth process than the later ones, thus the final products would be lack of homogeneity in size distribution, and some large grains may disperse in small particles randomly. To obtain the size-uniform nanoparticle, several novel strategies have been designed deliberately. For instance, Friderichs et al. proposed a two-phase oil/water solvothermal environment method to prepare monodisperse SrTi1 − xZrxO3 nanocubes with an edge length of 10 nm [15]. Most remarkably, the size and shape of the nanoparticles depended on neither the Zr content nor reaction time. It's believed that oleate ion surfactants played a vital role in prohibiting the grain growth [15]. Another common approach is the introduction of organic capping-agents to prevent the crystal facets growth [16–18]. It's well-known that nucleation and crystal growth is the two key parts controlling the final grain size and distribution. However, previous works were mainly involved in the management of crystal growth process. Herein, we paid our special attention toward the nucleation process, and developed a high temperature mixing method under hydrothermal conditions (HTMM) to refine the grain size and modify
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its uniformity. In our previous work, HTMM was primarily designed to circumvent the intermediate phase, aiming to obtain the pure targets [19]. During this process, the self-designed double-chambered Teflonlinear enables the raw materials located separately, and the chemical reaction is triggered as it reaches the preset temperature. Actually, it's demonstrated that HTMM had unique advantages in circumventing the intermediate impurities, thereby improving the purity of final products [20]. ABO3-type perovskite structure compounds, barium strontium titanate [(Ba, Sr)TiO3, BST] and sodium niobate (NaNbO3), are widely investigated environmental-friendly piezoelectric and catalytic materials [21–24]. For example, BST nanopowders with an average grain size of 475 nm were prepared by a high-energy ball-milling method [25]. Taking the syntheses of BST and NaNbO3 as case study, in this work, we will elucidate the effects of HTMM on the grain refinement and morphology controlling from the view point of nucleation and crystal growth. The proposed HTMM would be a prevalent strategy to synthesize nanomaterials. 2. Experimental procedures 2.1. Chemicals Potassium hydroxide (KOH, 99.0% min) and titanium dioxide (TiO2, 99.0% min) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Barium chloride(BaCl2, 99.5% min) was purchased from Nanjing
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Reagent Co., Ltd. Strontium nitrate (Sr(NO3)2, 99.5% min) was purchased from Shanghai Xinbao Fine Chemical Factory. Tetrabutyl titanate (Ti(C4H9O)4, 98.0% min) was purchased from Shanghai Zhanyun Chemical Reagent Co., Ltd. Nitric acid (HNO3, 65% min) was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. Absolute ethyl alcohol (C2H5OH, 99.7% min) was purchased from Nanjing Ningshi Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH, 96% min) and niobium oxide (Nb2O5, 99.5% min) was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received without further purification.
2.2. Conventional hydrothermal syntheses (CHS) In the typical hydrothermal synthesis of BST powders, TiO2 was used as starting materials. Firstly, TiO2 (0.04 M) and BaCl2 (0.16 M) were added into 55 ml KOH (0.6 M) aqueous solution. After stirring for 30 min, different amount of Sr(NO3)2, depending on the ratios of Ba/Sr (x = 0.5, 1.0, 3.0, 4.0) were dissolved in the mixture. Subsequently, the prepared precursors were transferred into Teflon-lined autoclave and kept at 200 °C for 10 h. And then, the resultant was naturally cooled to room temperature. The product was rinsed with de-ionized water and then anhydrous alcohol, and precipitated with centrifugation for 10 min at 3000 rpm to yield a white powder. Rinsing was repeated thrice to remove excess ions from the final product, and the precursor was dried at 80 °C overnight.
Fig. 1. FE-SEM images of BST powders synthesized by (a-c) CHS and (d-f) HTMM with different Ba/Sr ratios: (a) x = 0.5; (b) x = 3.0; (c) x = 4.0; (d) x = 0.5; (e) x = 3.0; (f) x = 4.0.
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a Cu Kα radiation (40 kV, 40 mA) source (λ = 0.154178 nm) at a scanning rate of 10 o/min in the 2θ range of 5°–60°. The morphology and microstructure of as-synthesized samples were observed using a Hitachi S4800 field emission scanning electron microscope (FE-SEM, Hitachi S4800, Japan). Energy dispersive X-ray spectrometer was used to analyze elemental composition and distribution. Nitrogen isothermal adsorption–desorption measurements are performed to determine the Brunauer–Emmett–Teller (BET) surface areas using an automatic surface area and pore analyzer (SSA-4300, Beijing Builder Corp., China) after treating the samples at 150 °C for 10 h. Using a 514.5 nm laser, Raman spectra were measured by a confocal laser micro-Raman spectroscopy system (Raman, LABRAM HR800) within the range of 100– 1000 cm− 1. UV–vis diffuse reflectance spectra were recorded on a UV–visible spectrophotometer (TU-1901, PGeneral Instrument Inc., China) at room temperature with BaSO4 as the reference and then converted into absorption spectra via Kubelka–Munk transformation.
Fig. 2. XRD patterns of BST powders synthesized by CHS (black color) and HTMM (blue color) with different Ba/Sr ratios (x = 0.5, 1.0, 3.0 and 4.0). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. High temperature mixing method under hydrothermal condition (HTMM) Teflon-liners with two separate chambers were fabricated for HTMM. To prevent the reaction between reactants, the starting compounds were divided into two groups and placed into the two chambers, respectively. Taking the preparation of BST powders as an example, one chamber was filled with BaCl2, Sr(NO3)2 and 6 ml KOH (0.6 M) aqueous solution, and the other one is TiO2 and 6 ml de-ionized water. The specific quantity of the reactants is consistent with the CHS, including the variation of Ba/Sr (x = 0.5, 1.0, 3.0, 4.0). Teflon-lined autoclaves were put up-right in a heating oven. Once the temperature reached 200 °C, the autoclaves were connected to the rotating electrical machine. In this way, the reactants in the separate chambers could be blended and the reaction would proceed drastically. The rotation was continued in the whole duration (10 h). Finally, the samples were naturally cooled down. The product was rinsed with de-ionized water and then anhydrous alcohol, and precipitated with centrifugation for 10 min at 3000 rpm to yield a white powder. Washing was repeated thrice to remove excess ions from the final product, and the precursor was dried at 80 °C for 24 h. In the syntheses of NaNbO3, similar process was conducted. Namely, NaAc and KOH aqueous solution were poured into the one chamber, while Nb2O5 and water was placed into the other chamber. The reaction was triggered at 220 °C and the autoclaving and rotating time was 16 h. The process parameters were set according to our previous work [26].
2.4. Characterization The crystal structure of the as-prepared sample was characterized by a Bruker X-ray diffractometer (XRD, Bruker D8 Advance, Germany) with
Fig. 3. Raman spectra of BST powders prepared by (a) CHS and (b) HTMM with different Ba/Sr ratios (x = 0.5, 1.0, 3.0 and 4.0).
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Fig. 4. (a) low-magnification and (b) high-magnification FE-SEM images of hydrothermally synthesized BST powders by HTMM using tetrabutyl titanate as Ti source (Ba/Sr = 3.0).
3. Results and discussion As a solid solution of BaTiO3 and SrTiO3, the physical properties of BST can be significantly affected by chemical composition. In this contribution, the effects of HTMM on the grain size and shape of BST with different Ba/Sr ratios were investigated. FE-SEM images of BST powders synthesized via CHS and HTMM were shown in Fig. 1(a–c) and Fig. 1(d–f), respectively. As shown in Fig. 1(a), the samples prepared by CHS at x = 0.5 presented an irregular aggregation with the average size of about 1 μm. For the same chemical composition, the samples prepared by HTMM, however, were sub-microsized cubes assembled by a mass of nanoparticles (Fig. 1(d)). The phenomenon became much evident at x = 3.0 as shown in Fig. 1(e), and the particles showed an average grain size of about 150 nm. On the contrary, the samples with x = 3.0 presented by CHS still presented the sub-microcubes. With the Ba/ Sr ratio increased to x = 4.0, both the size and shape of samples prepared by CHS and HTMM are somewhat comparable, where BST nanoparticles with an average size of 150 nm aggregated slightly. In addition, it's found in Fig. 1 that with the increasing of Ba/Sr ratio, the grain size of products reduced gradually, no matter which treatment method was adapted. Nevertheless, the samples obtained at x = 0.5 by HTMM showed comparable dimension to that at x = 3.0 by CHS. The
results indicated that HTMM can be more effective in grain refinement. Since the increase of Ba/Sr ratio can reduce the grain size, the effect of HTMM on grain refinement was discount at higher Ba/Sr ratio (e.g. x = 4.0). In fact, some of our previous work has revealed the grain refinement effect of HTMM. For instance, keeping the other parameters as constants, ZnO nanocrystals prepared by conventional method were about ten times larger than those prepared by HTMM [27]. Similar results were also observed in the hydrothermal synthesis of KNN cubes, where the grain size could be reduced from 5 μm to near 1 μm through replacing conventional method by HTMM [28]. The phase composition of the samples has been characterized by XRD and the results are presented in Fig. 2. For the samples prepared by CHS and HTMM, the main phase of all the samples was indexed to be perovskite structured (Ba, Sr)TiO3 with space group of Pm3̄ m. Considering the specific chemical composition of each sample, careful search and match were completed in Jade 5 software. It's found that the products with x = 0.5 and x = 1matched well with SrTiO3 (JCPDS number: 35–0734, a = b = c = 3.905 Å). With the increase of Ba/Sr ratio, the final product with x = 3.0 and x = 4.0 was assigned to Ba0.5Sr0.5TiO3 (JCPDS number: 34-0411, a = b = c = 3.947 Å) and Ba0.6Sr0.4TiO3 (JCPDS number: 35-0734, a = b = c = 3.965 Å), respectively. The compositional changes in final products can also be reflected
Fig. 5. (a) Illustration of hydrothermal process by HTMM: (1) the reactants are separately placed into each chamber to prevent their reaction during the elevating process; (2) Once the temperature reached the preset value, the vessels will be rotated by the connected electrical machine. (3) In this way, the starting compounds will be mixed at high temperature, and the chemical reaction will proceed rapidly. Numerous of crystal nucleus are constructed beneficial from the abundant dynamic energy. (b) While in CHS, the nucleation process is slow and inhomogeneous during the whole temperature-elevating period. (c) Accompanying with the rotation of vessels, the crystals in the chamber are rolling with the reaction medium, and hierarchical structures assembling by the as-formed crystals will be shaped.
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Fig. 6. (a) low-magnification and (b) high-magnification FE-SEM images of hydrothermally synthesized NaNbO3 powders by HTMM at 220 °C for 16 h.
in the peak shift of XRD pattern. With the gradual increase of Ba/Sr ratio, the diffraction peaks shifted slightly to the low angle, owing to the larger ion radius of Ba2+ (1.32 Å) than Sr2+ (1.14 Å) [29]. In addition to the dominated BST phase, as shown in Fig. 2, additional peaks around 2θ = 27°, 36°, 54° belonging to rutile TiO2 (JCPDS number: 21-1276, a = b = 4.593 Å, c = 2.959 Å) can be observed easily in the samples synthesized by CHS. On the contrary, the signals attributed to TiO2 impurity was quite weak in samples prepared by HTMM. Therefore, It's concluded that HTMM possesses improved reaction dynamics and can yield products with higher purity, in consistent with our previous works [30]. At the same time, it's found that the peak intensity of assynthesized samples became weakened with the Ba/Sr rising, regardless of the synthesis process. However, at a fixed Ba/Sr ratio, the peak intensities of the two samples were distinguished; that is, the peak intensity of samples prepared by CHS was stronger than that by HTMM, resulting from their larger grain size and better crystallinity. To further clarify the phase composition and crystallinity, Raman spectra of BST samples prepared by CHS and HTMM were provided. As shown in Fig. 3, the first-order Raman bands of rutile TiO2 around 144 cm−1 (B1g), 449 cm−1 (Eg), 611 cm−1 (A1g) were observed in all the samples [31,32]. The result further confirmed the appearance of TiO2 impurity, in consist with XRD results. At the same time, the appearance of first-order Raman bands TO2, TO3, TO4 and LO4, as marked by dashed line in Fig. 3(a), characterized the BST powders [33,34]. It's noticed that the intensity of Raman bands belonging to impurity TiO2 was higher than main products BST. In general, Raman spectra reflects the local vibration and short range structural character, while XRD pattern provides the information about phase component in macroscopical scale. In this regard, it's infered that there existed mass of anion motions with respect to stationary central cations [32]. Benefiting from the particular rotation process, the proposed HTMM may also have potential effects on morphological regulation, especially for the formation of complex architectures. Subsequently, instead of titanium oxide, tetrabutyl titanate was utilized as titanium source, and BST powders with x = 3.0 were prepared by HTMM. From the low-magnification SEM image (Fig. 4(a)), porous sphere particles with an average size about 300 nm were observed. The nut-like spheres were composed of numerous nanoparticles less than 100 nm, as shown in Fig. 4(b). Previously, a similar case with dandelion-like balls constructed from niobate fibers were hydrothermally observed by HTMM [30]. In addition, the BET surface areas of BST powders prepared by HTMM were measured to be 61.914 m2/g, which was seven fold larger than that prepared by conventional hydrothermal synthesis (8.415 m2/g). That is to say, these assembled architectures resulted from HTMM were abundant in surface, and were suitable for potential application in the absorption, catalysis and drug delivery fields. Although the advantages of HTMM in grain refinement and shape regulation have been recognized in the previous work, there still lacks a systematic understanding about this process. In combination with the above results, we attempted to elucidate the underlying mechanism for the grain refinement and morphology formation during HTMM. The
reacting reactants are located in separate chambers, and will not contact with each other till the temperature is elevated to the pre-set value, as the situation (1) illustrated in Fig. 5(a). Once the system temperature reaches the preset value (i.e. 200 °C), the sealed autoclaves will be rotated by the connected electrical machine, as the situation (2) illustrated in Fig. 5(a). In this way, the reactants are mixed abruptly, and the involved reaction will proceed rapidly; namely, numerous of crystal nucleus will form in a short time, due to the large driving force available, which is illustrated as the state (3) in Fig. 5(a). Since the nucleation process is quite thorough and extensive, the crystal growth will be inhibited, thus the products with refined crystal size and narrow distribution will be finally resulted. On the contrary, during CHS, nucleation process occurs partially during the temperature-elevating period, once the driving force is large enough in some local points, as shown in Fig. 5(b). The earlier formed crystal nucleus could experience a longer period of grain growth during the subsequent heating and dwelling stage, thus the bigger crystals will be emerged in the final products, causing a broad size distribution. In terms of the assembled structure, it's believed to be promoted by the continuing rotation process. With the autoclave rotating continuously, the formed crystal nucleus in the chamber will move with the flowing of liquid medium; large numbers of crystals would rolling along the inner wall of liner. After constant rolling and collision, the crystals will contact with each other and get dense gradually. Eventually, the assembled architecture with round shape is formed. Such processes are illustrated as Fig. 5(c). To further verify the proposed mechanism, HTMM was adapted to synthesize other ABO3-type compounds. Based on our previous work [26], NaNbO3 were selected and the processing parameters were similar
Fig. 7. UV–vis absorption spectra of the BST samples prepared by different processes (CHS and HTMM) and Ti sources (TBOT: tetrabutyl titanate; the others are titanium oxide).
Q. Gu et al. / Powder Technology 305 (2017) 440–446 Table 1 The average particle size, band gap wavelength (λg) and band gap (Eg) of BST samples prepared at various conditions. Sample
1
2
3
4
5
Treatment Ba/Sr Ti source Particle size/nm λg/nm Eg/eV
CHS 0.5 TiO2 900 397 3.12
CHS 3.0 TiO2 500 399 3.10
HTMM 0.5 TiO2 500 397 3.12
HTMM 3.0 TiO2 200 398 3.11
HTMM 3.0 TBOT 400 393 3.15
to our previous work. After a reaction at 220 °C for 16 h, the microstructure of the as-prepared NaNbO3 is shown in Fig. 6. The irregular sphereshaped particles with a uniform size of 100 μm were observed in the low-magnification FE-SEM image as shown in Fig. 6(a). From the high-magnification FE-SEM image (Fig. 6(b)), we can see that the sphere-like architectures were assembled from numbers of plate-like particles, which were the main yields by CHS. It's notable that the size of the plates was quite smaller than the ones prepared by CHS [26]. The results in terms of dimensional reduction and assembled morphology can be interpreted by the proposed mechanism. Interestingly, a layer with 5 μm in thickness was coated partially on the surface of the sphere-like architecture (Fig. 6(b)). It's inferred that the layers were the later formed NaNbO3 crystals with poor crystalline, which can be evidenced by the weakened XRD pattern. It's well-acknowledged that much unsaturated coordination structures may reside on the surface of smaller sized particles, and the band gap of the finer particles becomes broaden. Therefore, the UV– vis absorbance spectra of the BST samples prepared at various conditions were collected and plotted as Fig. 7. The band gap wavelength (λg) and band gap (Eg) of BST samples were listed in Table 1. Regardless of the treatment method, CHS or HTMM, the BST powders with x = 3.0 manifested evident red shift compared with the ones with x = 0.5. This phenomenon can be understood as a result of composition differences, since the energy band gap of SrTiO3 is slightly larger that of BaTiO3 [35]. Analogously, Prabhas et al. pointed out that with the increasing Nb content in NaNbxTa1 − xO3 specimens, this onset absorption energy shifted from the NaTaO3 to the NaNbO3 state [36]. Note that the absorbance spectra of BST powders prepared by HTMM exhibited a slight blue shift when compared with the ones prepared by CHS. Additionally, as the titanium source was changed from titanium oxide to TBOT, their absorbance spectra showed an obvious shift to low wavelength. It's inferred that the observed blue shift may mainly result from the reduced grain size [37,38]. These results manifested that the HTMM can effectively reduce the grain size of resulted powders, especially when the reactant is soluble, in consistent with FE-SEM results.
4. Conclusion In this work, HTMM was adapted to prepare BST powders with various Ba/Sr ratios, and special attentions were paid to the advantages of HTMM in grain refinement and morphological regulation. XRD and SEM results indicated that HTMM outperformed in reducing the grain size of BST powders in comparison to CHS, especially at the lower Ba/ Sr ratio. When the titanium source was changed from titanium oxide to tetrabutyl titanate, the resulted BST powders showed porous architectures assembled from numerous nanoparticles. It's proposed that the HTMM aggravated the rapid nucleation, and inhibited the following grain growth, giving rise to the refine grains with uniform distribution. At the same time, it's the continuing rotation that promoted the assembling of grains, resulting in the formation of porous architectures. The proposed assumption has been verified through the synthesis of NaNbO3 powders, where smaller plates were obtained, and further assembled into the sphere-like porous architectures. The unique
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advantages in refine grains and assembled structures endow HTMM promising applications in absorption, catalysis and drug delivery field. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC No. 51372114 and 51672130), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and Astronautics) (Grant No. 0514Y01), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Also, Qilin Gu would like to thank the support from Nanjing University of Aeronautics and Astronautics PhD short-term visiting scholar project.
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