Effect of Surfactants on the Structure and Morphology of Magnesium Borate Hydroxide Nanowhiskers Synthesized by Hydrothermal Route
© to the authors 2009
Received: 5 August 2009
Accepted: 26 September 2009
Published: 13 October 2009
Magnesium borate hydroxide (MBH) nanowhiskers were synthesized using a one step hydrothermal process with different surfactants. The effect surfactants have on the structure and morphology of the MBH nanowhiskers has been investigated. The X-ray diffraction profile confirms that the as-synthesized material is of single phase, monoclinic MgBO2(OH). The variations in the size and shape of the different MBH nanowhiskers have been discussed based on the surface morphology analysis. The annealing of MBH nanowhiskers at 500 °C for 4 h has significant effect on the crystal structure and surface morphology. The UV–vis absorption spectra of the MBH nanowhiskers synthesized with and without surfactants show enhanced absorption in the low-wavelength region, and their optical band gaps were estimated from the optical band edge plots. The photoluminescence spectra of the MBH nanowhiskers produced with and without surfactants show broad emission band with the peak maximum at around 400 nm, which confirms the dominant contribution from the surface defect states.
Nanostructured materials have received great interest due to their fascinating physical, optical, electrical, and thermoelectric properties as well as their potential applications in nanodevices [1–4]. Metal borates are considered among the most important of these materials because of their unique properties, such as their light weight, high strength, high heat-resistance, corrosion-resistance, and high coefficient of elasticity, etc. Hence, the nanoscale metal borates are ideal for exploring their potential applications in the fields of nanocomposites, nanomechanics, and nano-electronics. Among the various metal borates, aluminum borate is perhaps the best known ceramic material with chemical stability, enhanced mechanical properties and potential applications in high-temperature composites . Magnesium borate is another remarkable ceramic material that shows excellent mechanical and thermal properties.
Magnesium borate hydroxide (MgBO2(OH)), also known as the Szaibelyite, is a widely available translucent mineral in nature and is used as the main source of boron in industry [6, 7]. The Szaibelyite is also an important source of anhydrous magnesium borate . Magnesium borate can be used as thermo-luminescent phosphor , antiwear and friction reducing additive , ferro-elastic material , which is a candidate for tunable laser applications , and can be used as a luminescent material for fluorescent discharge lamps, cathode ray tube screens, and X-ray screens . Recently variety of magnesium borate nanostructures such as nanorods , nanowires [15, 16], nanobelts , nanoparticles  and nanotubes  have been fabricated by different synthesis techniques including thermal evaporation, chemical vapor deposition, ethanol supercritical fluid drying technique, and thermal evaporation in IR-irradiation heating furnace [10, 14–18]. However, in all these reported routes, the synthesis was performed at high temperatures (750–1,100 °C).
The synthesis of nanoparticles with controlled size and shape results in new electronic and optical properties, which is suitable for many electronic and optoelectronic applications . The use of surfactants as stabilizers has advantages with the fact that these surface-active chemicals possess sufficient strength to effectively control the particle size growth. The surfactants support to have particles with “monodisperse” size distribution and increased aspect ratio, and they also effectively prevent the particles from agglomeration [20–23]. Over the decades, the hydrothermal process has proved to be one of the most successful methods for synthesizing low dimensional materials. However, there exist very few reports on the synthesis of nanostructured MgBO2(OH) using the hydrothermal method [8, 24–26]. In addition, the conversion of magnesium borate hydroxide to anhydrous magnesium borate is rarely reported [7, 21]. Zhu et al. [8, 24, 25] reported the hydrothermal synthesis of MgBO2(OH) nanowhiskers using MgCl2, H3BO3 and NaOH as the starting materials with molar ratio of Mg:B:Na as 2:3:4 at 240 °C for 18 h. Zhu et al.  also investigated the effect of the dropping rate of NaOH into the precursor solution, droplet size, and amount of the NaOH solution and the hydrothermal reaction time on the hydrothermal formation of the MgBO2(OH) nanowhiskers with other synthesis parameters kept constant. The morphology preservation and crystallinity improvement in the thermal conversion of the hydrothermal synthesized MgBO2(OH) nanowhiskers to Mg2B2O5 nanowhiskers was investigated in the temperature range of 650–700 °C and was kept under isothermal condition for 2.0–4.0 h . Xu et al.  demonstrated the growth of magnesium borate (Mg2B2O5) nanorods at 400 °C (supercritical condition) by solvothermal route and explained that the temperature of 200 °C was not sufficient for synthesizing the well-defined nanostructures. In addition, the synthesis of the magnesium borate nanorods needs the assistance of surfactants/capping agents. In their work, the MgBO2(OH) columnar-like particles were synthesized at 320 °C with ethanol and water as solvents. In the present work, the MBH nanowhiskers with regular shape and size were successfully synthesized at a reaction temperature of 200 °C (H2O as solvent) without using any surfactants/capping agents. Additionally, the effect of surfactants on the structure and surface morphology of the MBH nanomaterials is studied. Optical properties including UV–vis absorption and photoluminescence (PL) of the MBH nanowhiskers are also investigated.
All chemicals used for the synthesis of magnesium borate hydroxide nanostructures were analytical grade (Fisher Scientific) and used without further purification. In a typical synthesis, 3.846 g of magnesium nitrate hexahydrate (MgNO3·6H2O) and 0.568 g of sodium borohydrate (NaBH4) were separately mixed in 10 mL distilled water. Then the two solutions were put together and placed in ultrasonicator (Branson, Model 2510, 40 kHz) for about 30 min to get homogeneous and clear solution. The solution was put into a Teflon liner (30 mL capacity) up to 80% of the total volume. The Teflon lined autoclave was sealed and placed in a furnace and maintained at 200 °C for 24 h (in air). After the completion of the hydrothermal reaction, the autoclave was cooled down to room temperature naturally. The precipitate was filtered and washed repeatedly with distilled water and ethanol (100% Reagent alcohol, Fisher Scientific) and was later dried at 100 °C for 4 h. The procured powders were used further for various characterizations. The above synthesis procedure was repeated for 1.5 M MgNO3·6H2O and 1.5 M NaBH4 with the addition of 0.184 g (0.1 M) Cetyl trimethylammonium bromide (CTAB), 0.144 g (0.1 M) sodium dodecyl sulfate (SDS) and 2 mL Triton X-100, respectively, at 200 °C for 24 h. The surfactants CTAB, SDS and Triton are cationic, anionic and non-ionic, respectively. The magnesium borate hydroxide nanowhiskers synthesized with and without surfactant were annealed at 500 °C for 4 h in air. The heating rate of 6 °C/min and cooling rate of 0.5 °C/min were maintained constant for each of the nanowhiskers synthesis. The magnesium borate hydroxide nanowhiskers fabricated without surfactant and with CTAB, SDS and Triton are termed as MBH-NON, MBH-CTAB, MBH-SDS and MBH-Triton, respectively. To investigate the effect of synthesis process on the nanostructure formation, the MgBO2(OH) samples were also produced by heating the starting materials in open beaker at 150 °C for 6 h in air.
Surface morphology analysis of the MBH nanostructures was performed by a field emission scanning electron microscope (SEM, JEOL JSM-6330F, 15 kV). X-ray diffraction (XRD) measurements were carried out using Siemens D5000 diffractometer equipped with Cu anode operated at 40 kV and 40 mA. The XRD patterns were collected with step size of 0.01° and a scan rate of 1 s/step. UV–vis spectra were obtained from Perkin-Elmer Lambda 900 UV/Vis/NIR spectrometer, and the PL spectra were recorded from Horiba Jobin-Yvon FluoroLog FL3-22 spectrofluorometer. For the spectroscopic analysis, magnesium borate hydroxide powders were added to NaOH solution for a better dispersion, and the solution was taken into a quartz cell (1 cm optical path length) at room temperature.
Results and Discussion
X-ray Diffraction Analysis
Surface Morphology Analysis
Effect of Annealing on the Crystal Structure and Surface Morphology
UV–Vis Absorption and Photoluminescence
UV–vis absorption spectra of the as-synthesized MBH nanowhiskers formed with the assistance of surfactants are shown in Fig. 8b. When compared with the MBH-NON nanowhiskers, the nanostructures synthesized with surfactants show optical absorption peaks with maximum intensity at around 350 nm (photon energy of 3.55 eV), which can be related to the absorption in the band gap region. The optical band gap values estimated from the optical band edge plots ((αhγ)2vs. hγ, not shown) for MBH-CTAB, MBH-SDS, MBH-Triton-In Autoclave and MBH-Triton-In open beaker are 2.64, 2.7, 2.5 and 2.62 eV, respectively. The large difference in the optical band gap for MBH-NON nanowhiskers and MBH nanowhiskers synthesized with various surfactants arises due to the fact that the surfactants can induce the formation of intermediate surface defect states in the band gap region . The reduced optical band gap values can also be assigned to the increased aspect ratio of the nanowhiskers with the addition of surfactants (MBH-CTAB and MBH-Triton). The increase in the absorption peak intensity and band gap for MBH-SDS nanowhiskers in comparison with the MBH-CTAB and MBH-Triton-In Autoclave can be assigned to the reduced particle size . MBH-Triton-In open beaker urchin-like nanowhiskers also shows increased band gap than the MBH-Triton-In Autoclave nanowhiskers due to the smaller particle size.
Magnesium borate hydroxide nanowhiskers of various shape and size were synthesized by hydrothermal route with and without using surfactants. The crystal structure and surface morphology of the MBH nanowhiskers are studied. XRD patterns reveal that the as-prepared nanowhiskers are pure MgBO2(OH) with monoclinic phase. The MBH samples synthesized with surfactants formed nanowhiskers with improved aspect ratio. The present synthesis technique produces MgBO2(OH) nanowhiskers with controlled shape and size at relatively low temperatures. The thermal annealing shows significant influence on the crystal structure and surface morphology of the MBH nanowhiskers. The MBH nanowhiskers synthesized with surfactants show reduced optical band gap (2.5–2.7 eV) than the MBH-NON sample (4.15 eV), which can be attributed to the increased aspect ratio and presence of surface defects with the addition of surfactants. The room temperature PL spectra of the MBH nanowhiskers synthesized with and without using surfactants show broad luminescence band at around 400 nm, which can be attributed to the violet emission originating from the surface defect states.
This work was supported by the National Science Foundation under grant DMR-0548061. We would like to thank Dr. Dezhi Wang for his help with the TEM characterization.
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