Hydrothermal Formation of the Head-to-Head Coalesced Szaibelyite MgBO2(OH) Nanowires

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.

MgBO 2 (OH) particles without morphology control were synthesized by the dissolution and phase transformation of 2MgOÁ2B 2 O 3 ÁMgCl 2 Á14H 2 O at 180°C for 72.0 h [35]. Low-aspect-ratio MgBO 2 (OH) whiskers (average diameter: 30 nm, average length: 700 nm) coexisting with floccules and nanoparticles were formed by the hydrothermal reaction of MgO and B 2 O 3 at 180°C for 48.0 h [36]. Most recently, MgBO 2 (OH) nanobelts have also been reported [37]. In the previous work, uniform MgBO 2 (OH) nanowhiskers (diameter: 20-50 nm, length: 0.5-3 lm) were hydrothermally synthesized (240°C, 18 h), using MgCl 2 Á6H 2 O, H 3 BO 3 , and NaOH as the reactants [33]. Based on the understanding of the effect of the process parameters on the diameter, length, and aspect ratio of the hydrothermal product [38], 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 MgBO 2 (OH) nanowires with a length of 0.5-9.0 lm, 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.

Experimental
MgBO 2 (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 H 3 BO 3 and 2 mol L -1 of MgCl 2 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 (Mg 7 B 4 O 13 Á7H 2 O) [33] 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 MgBO 2 (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 CuKa radiation (k = 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
According to the analysis of the precipitate obtained at room temperature [33], the corresponding coprecipitation leading to the slurry containing white precipitate Mg 7 B 4 O 13 Á7H 2 O can be written in ionic form as follows: NaOH aq: ð Þ ! Na þ aq: ð Þ þ OH À aq: ð Þ; ð3Þ The hydrothermal conversion can thus be expressed as follows, definitely showing the necessary basic medium for the hydrothermal formation of szaibelyite MgBO 2 (OH) phase [39]: The effect of the feeding mode, such as dropping rate or droplet size of the NaOH solution, on the morphology and size of the hydrothermal product was shown in Figs. 1 and 2, respectively, in case of appropriate initial concentration of NaOH (0.33 mol L -1 ), hydrothermal temperature (240°C), and time (18.0 h). When the dropping rate and droplet size were 1.0 d s -1 and 0.12 mL d -1 , respectively, the hydrothermal product was MgBO 2 (OH) with nonuniform 1D morphology (Fig. 1a), and the uniformity of the 1D morphology was improved on the whole with the droplet size decreased from 0.12 to 0.02 mL d -1 (Fig. 1ad). Similar phenomenon emerged when the dropping rate was altered to 0.5 d s -1 , whereas with the droplet size decreased within the range of 0.12-0.02 mL d -1 (Fig. 1eh). It was worth noting that, the morphology uniformity was greatly improved with the slowing down of the dropping rate from 1.0 to 0.5 d s -1 under the same droplet size, denoted as Fig. 1a, e, b, and f, etc. Most significantly, the uniform MgBO 2 (OH) nanowhiskers ( Fig. 1 h) were obtained while the dropping rate and droplet size were kept as 0.5 d s -1 and 0.02 mL d -1 , respectively, indicating the promotion of the morphology uniformity via the slow dropping rate and small droplet size of the dropwise added NaOH solution.
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 -1 both 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 -1 when 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 -1 for 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 -1 within 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 -1 under the same droplet size situation, especially for the small droplet size within the range of 0.02-0.05 mL d -1 .
The effect of the feeding mode on the hydrothermal product 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 enlarging the aspect ratio of the hydrothermal product thus could promote the 1D growth of the MgBO 2 (OH) nanostructures during the subsequent hydrothermal treatment. Since low concentration of the reactants, relatively long reaction time and high temperature favored the synthesis of MgBO 2 (OH) nanowhiskers with a longer size and higher aspect ratio [38], less amount of NaOH solution (4 mol L -1 ), in other words, lower initial concentration of NaOH (0.17 mol L -1 ) was employed in the room temperature coprecipitation so as to further increase the length and aspect ratio of the hydrothermal product, with the molar ratio of Mg:B:Na and also total volume of the mixed solution unchanged. The resultant well dispersed uniform nanowires (Fig. 3a) with high  (Fig. 3b, b 1 -b 2 ) were obtained, which consisted of pure phase of monoclinic MgBO 2 (OH) (PDF No. 39-1370) as shown in Fig. 3c. The interplanar spacings of 0.597 nm detected from the legible lattice fringes along the axis of the nanowire (Fig. 3b 1 ) was quite similar to that of the (200) planes of the standard MgBO 2 (OH), indicating the preferential growth direction of the nanowires parallel to the (200) planes, in agreement with that of the MgBO 2 (OH) nanowhiskers along the c-axis [38] and also the growth habit of the natural szaibelyite (MgBO 2 (OH)) [40]. The statistic data showed that the MgBO 2 (OH) nanowires had a length of 0.5-9.0 lm (approx. 80% within 1-5 lm), a diameter of 20-70 nm (approx. 68% within 30-50 nm), and an aspect ratio of 20-300 (approx. 78% within 20-100) (Fig. 3d-f). Apparently, the length and aspect ratio of the resultant MgBO 2 (OH) nanowires were much higher than those of the MgBO 2 (OH) nanowhiskers [33].
To investigate the formation of the nanowires, the morphology evolution of the hydrothermal products acquired at 240°C for various time were tracked (Fig. 4ac), in case of slow dropping rate (0.5 d s -1 ), small droplet size (0.02 mL d -1 ), and low initial concentration of the NaOH (0.17 mol L -1 ) during the room temperature coprecipitation. Short and thin nanowhiskers having grown for 2.0 h (Fig. 4a) tended to be attached with each other either head-to-head or side-by-side (denoted as dotted circles), and the nanowhiskers became longer with fewer attached phenomena observed as the time prolonged to 6.0 h (Fig. 4b). Finally, MgBO 2 (OH) nanowires with high aspect ratio and sometimes curved 1D morphology were obtained when hydrothermally treated for 18.0 h, owing to the previous head-to-head or side-by-side attachment growth of the individual nanowhiskers (Fig. 4c). Further, TEM observations on the joint sections of the nanowires indicated that, either the seemingly straight nanowires (Fig. 4 d 1 -d 2 ) or curved ones (Fig. 4 d 3 -d 4 ) were formed via the head-to-head overlapped or side-by-side attached growth of the nanowhiskers. Particularly, the legible lattice fringes parallel to the axis of the nanowire (Fig. 4e 1 -e 2 ) with the detected interplanar spacings of 0.597 nm revealed that the MgBO 2 (OH) nanowires tended to be attached with one other in a direction approx. along the (200) planes, leading to the seemingly straight or slightly curved nanowires.
The formation of the MgBO 2 (OH) nanowires could thus be depicted, as shown in Fig. 5. Tiny amorphous irregular Mg 7 B 4 O 13 Á7H 2 O [33] nanoparticles derived from the coprecipitation at room temperature with small droplet size and slow dropping rate of the dropwise added NaOH solution gradually dissolved and further converted to short and thin crystalline 1D MgBO 2 (OH) nanostructures (i.e., nanowhiskers) with the hydrothermal temperature continuously increased to 240°C. With time going on under the isothermal condition (240°C), short and thin MgBO 2 (OH) nanowhiskers began head-to-head overlapped or side-byside attached growth, due to the necessity of reducing the On the other hand, the late growth of the overlapped 1D MgBO 2 (OH) nanostructures into the coalesced nanowires might be attributed to the joint effect of the oriented attachment [41][42][43] and Ostwald ripening [44,45], which however needed further in-depth investigation. 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 [38]. 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 Mg 7 B 4 O 13 Á7H 2 O) 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 [21]. 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 MgBO 2 (OH)   nanowhiskers, resulting in subsequent head-to-head overlapped or side-by-side attached growth and finally head-tohead coalesced MgBO 2 (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 MgBO 2 (OH) nanowires, which led to broad leaf-like MgBO 2 (OH) nanostructures with distinct wide distribution of the diameter due to excess side-by-side attached growth [39]. Moreover, unlike some other nanowires synthesized in presence of capping reagents or surfactants [5], MgBO 2 (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 MgBO 2 (OH) nanowhiskers.

Conclusion
In summary, the significant effect of the feeding mode on the morphology and size distribution of the hydrothermal synthesized MgBO 2 (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 MgBO 2 (OH) nanostructures. The joint effect of the low concentration of the reactants and feeding mode resulted in the head-to-head coalesced MgBO 2 (OH) nanowires with a length of 0.5-9.0 lm, 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.