Hydrothermal Formation of the Head-to-Head Coalesced Szaibelyite MgBO2(OH) Nanowires
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The significant effect of the feeding mode on the morphology and size distribution of the hydrothermal synthesized MgBO2(OH) is investigated, which indicates that, slow dropping rate (0.5 drop s−1) and small droplet size (0.02 mL d−1) of the dropwise added NaOH solution are favorable for promoting the one-dimensional (1D) preferential growth and thus enlarging the aspect ratio of the 1D MgBO2(OH) nanostructures. The joint effect of the low concentration of the reactants and feeding mode on the hydrothermal product results in the head-to-head coalesced MgBO2(OH) nanowires with a length of 0.5–9.0 μm, a diameter of 20–70 nm, and an aspect ratio of 20–300 in absence of any capping reagents/surfactants or seeds.
KeywordsNanowires Szaibelyite Magnesium borate hydroxide Mixing Hydrothermal
One-dimensional (1D) nanostructures, including nanotubes, nanorods, nanowires, and nanobelts, etc., have been paid extensive attention for their unique structures, fantastic properties, and great potential applications [1–5]. Among the multitudinous 1D nanostructures, nanowires have attracted extraordinary research interest for their multifunctionality as building blocks for bottom–up nanotechnology . On the other hand, the properties of 1D nanostructure were greatly dependent on the aspect ratio [7–9], longer or higher aspect ratio has now in many cases emerged as one of the focuses of the synthesis of 1D nanostructures. As a consequence, much effort has been devoted to the synthesis of 1D nanostructures with high aspect ratio, such as capping reagents or surfactants-assisted synthesis of high-aspect-ratio hydroxyapatite  and CdS  nanorods, vanadium oxide  and Te  nanobelts, Au  and aluminum borate  nanowires; seed-mediated synthesis of high-aspect-ratio Au nanorods  and ZnO nanowires and nanotubes ; process conditions-optimized synthesis of high-aspect-ratio titanate nanofibers/nanotubes , Cu nanowires , ZnO nanorods/nanowires , and magnesium oxysulfate nanowires .
One-dimensional nanostructured magnesium borates, including MgB4O7 nanowires , Mg3B2O6 nanotubes  and nanobelts , Mg2B2O5 nanowires [25, 26], nanorods  and whiskers [28–30], etc., have attracted much attention in recent years for their potential usage as reinforcements in the electronic ceramics , wide band gap semiconductors , antiwear additive , and plastics or aluminum/magnesium matrix alloys . Traditionally, 1D mirco-/nanostructured magnesium borates were prepared via chemical vapor deposition (CVD) [22–27] or molten salt synthesis (MSS) route [28–30] at high temperature within 850–1,250 °C or solution-based method under supercritical conditions (500–600 °C, 200–1,000 bar, 14 days) . In the recent work, we developed a flux-assisted thermal conversion route to the high crystallinity pore-free Mg2B2O5 nanowhiskers at a relatively low temperature as 650–700 °C  based on the former hydrothermal synthesis of MgBO2(OH) nanowhiskers . Apparently, it is of great significance to increase the aspect ratio of hydrothermal synthesized MgBO2(OH) nanowhiskers to acquire high-aspect-ratio 1D Mg2B2O5 nanostructures, considering somewhat unavoidable shrinkage or breakage in the thermal conversion .
MgBO2(OH) particles without morphology control were synthesized by the dissolution and phase transformation of 2MgO·2B2O3·MgCl2·14H2O at 180 °C for 72.0 h . Low-aspect-ratio MgBO2(OH) whiskers (average diameter: 30 nm, average length: 700 nm) coexisting with floccules and nanoparticles were formed by the hydrothermal reaction of MgO and B2O3 at 180 °C for 48.0 h . Most recently, MgBO2(OH) nanobelts have also been reported . In the previous work, uniform MgBO2(OH) nanowhiskers (diameter: 20–50 nm, length: 0.5–3 μm) were hydrothermally synthesized (240 °C, 18 h), using MgCl2· 6H2O, H3BO3, and NaOH as the reactants . Based on the understanding of the effect of the process parameters on the diameter, length, and aspect ratio of the hydrothermal product , herein we report for the first time the significant effect of the feeding mode on the morphology and size distribution of the hydrothermal product, which resulted in the head-to-head coalesced MgBO2(OH) nanowires with a length of 0.5–9.0 μm, a diameter of 20–70 nm, and an aspect ratio of 20–300 in absence of any capping reagents/surfactants or seeds. The feeding mode-intensified 1D preferential growth was also helpful for the wet chemistry based synthesis of other 1D nanostructured materials, especially for those with anisotropic crystal structures.
MgBO2(OH) nanowires were synthesized by a modified coprecipitation at room temperature followed by the hydrothermal treatment. In a typical procedure, 4 mol L−1 of NaOH was dropped into the solution containing 3 mol L−1 of H3BO3 and 2 mol L−1 of MgCl2 under vigorous magnetic stirring at room temperature, keeping the molar ratio of Mg:B:Na as 2:3:4. Thereafter, 40 mL of the slurry (Mg7B4O13·7H2O)  was put into a Teflon-lined stainless steel autoclave with a capacity of 70 mL. The autoclave was heated to 240 °C and kept under isothermal condition for 18.0 h, and then cooled down to room temperature naturally. The product was filtered, washed with deionized water for three times and dried in vacuum at 105 °C for 6.0 h. All of the reactants were analytical grade without further purification. To investigate the hydrothermal formation of the MgBO2(OH) nanowires, the dropping rate, droplet size, and amount of the NaOH solution and also the hydrothermal time were adjusted within the range of 0.5–1.0 drop per second (d s−1 hereafter), 0.02–0.12 mL per drop (mL d−1 hereafter), 3.5–7.0 ml, 2.0–18.0 h, respectively, whereas with other conditions kept the same.
The composition and structure of the samples were identified by an X-ray powder diffractometer (XRD, D/max-RB, Rigaku, Japan) using CuKα radiation (λ = 1.54178 Å). The morphology of the samples were examined with a field emission scanning electron microscopy (FESEM, JSM 7401F, JEOL, Japan) and a high resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan). The particle size of that contained in the precursor slurry was detected via a malvern particle size analyzer (MICRO-PLUS, MALVERN, England). And the average diameter and length of the hydrothermal product were estimated by direct measuring about 200 particles from the typical FESEM images taken at 1.0 kV with the magnifications of 15,000–40,000.
Results and Discussion
Size variation of the hydrothermal product with the droplet size of the NaOH solution showed that the average length and diameter of the hydrothermal product derived from dropping rate of 0.5 and 1.0 d s−1both decreased slightly with the decrease of the droplet size from 0.12 to 0.07 mL d−1, which however both began to increase when the droplet size further decrease from 0.05 to 0.02 mL d−1(Fig. 2a–b). Meanwhile, within the same range of the droplet size as 0.02–0.05 mL d−1, the average length and diameter of the hydrothermal product increased with the decrease of the dropping rate from 1.00 to 0.5 d s−1. The specific evolution trend of the average length and diameter of the hydrothermal product (Fig. 2a–b) determined the corresponding change of the average aspect ratio of the hydrothermal product with the droplet size of the NaOH solution (Fig. 2c). Remarkably, the average aspect ratio of the hydrothermal product significantly increased for the dropping rate of 0.5 d s−1when the droplet size decreased from 0.05 to 0.02 mL d−1(Fig. 2c), similar to the significant increase of the average length and diameter for the same dropping rate within the same range of the droplet size (Fig. 2a–b). To further investigate the effect of the feeding mode, the variation of the particle size of the precursor obtained after the accomplishment of the NaOH feeding was monitored, which revealed a decrease of the precursor particle size with the decrease of the droplet size from 0.12 to 0.02 mL d−1(Fig. 2d). Notably, a significant decrease of the particle size emerged as the droplet size decreased from 0.07 to 0.02 mL d−1for the dropping rate of 0.5 d s−1, in contrast with a steady decrease of the particle size for the dropping rate of 1.0 d s−1within the whole droplet size range. Besides, the precursor particle size decreased with the slow-down of the dropping rate from 1.0 to 0.5 d s−1under the same droplet size situation, especially for the small droplet size within the range of 0.02–0.05 mL d−1.
Comparatively, head-to-head overlapped or side-by-side attached growth phenomena were not readily observed in the morphology evolution of the hydrothermal products obtained at 240 °C for various time originated from the room temperature coprecipitation in case of relatively big droplet size and fast dropping rate of the NaOH solution . Thus, the droplet size and dropping rate of the dropwise added NaOH solution played a key role in the formation of the small size nanoparticles of the hydrothermal precursor (slurry containing Mg7B4O13·7H2O) and further formation of the high aspect ratio hydrothermal product. Small droplet size and slow dropping rate under vigorous stirring are favorable for the creation of the low supersaturation, which favors the 1D preferential growth of the nanocrystals with anisotropic crystal structures [5, 21], similar to the double-injection method for the synthesis of magnesium oxysulfate nanowires . Consequently, the low supersaturation originated from the room temperature coprecipitation in case of small droplet size and slow dropping rate of the dropwise added NaOH solution promoted the formation of the small size precursor particles and further formation of the short and thin MgBO2(OH) nanowhiskers, resulting in subsequent head-to-head overlapped or side-by-side attached growth and finally head-to-head coalesced MgBO2(OH) nanowires. However, the extended experiments showed that, with other conditions kept the same, longer hydrothermal time such as 30.0 h was not favorable for the formation of longer MgBO2(OH) nanowires, which led to broad leaf-like MgBO2(OH) nanostructures with distinct wide distribution of the diameter due to excess side-by-side attached growth . Moreover, unlike some other nanowires synthesized in presence of capping reagents or surfactants , MgBO2(OH) nanowires were obtained in absence of any surfactants, and neither hexadecyl trimethyl ammonium bromide (CTAB) nor sodium dodecyl benzene sulfonateon (SDBS) have been proved effective for the formation of high aspect ratio MgBO2(OH) nanowhiskers.
In summary, the significant effect of the feeding mode on the morphology and size distribution of the hydrothermal synthesized MgBO2(OH) indicated that, slow dropping rate (0.5 d s−1) and small droplet size (0.02 mL d−1) of the dropwise added NaOH solution were favorable for promoting the 1D preferential growth and thus enlarging the aspect ratio of the 1D MgBO2(OH) nanostructures. The joint effect of the low concentration of the reactants and feeding mode resulted in the head-to-head coalesced MgBO2(OH) nanowires with a length of 0.5–9.0 μm, a diameter of 20–70 nm, and an aspect ratio of 20–300 in absence of any capping reagents/surfactants or seeds. The feeding mode-promoted 1D preferential growth was also helpful for the wet chemistry based synthesis of other 1D nanostructured materials, especially for those with anisotropic crystal structures.
This work is supported by the National Natural Science Foundation of China (No. 50574051, 50874066).