1. Introduction
Silica is one of the most fundamental mineral components found in the Earth’s soil. There are a number of different uses of silica as industrial and engineering materials to make products for improving our quality of life. One of the commonly occurring forms of crystalline silica is quartz because it is the most thermodynamically stable phase under ambient conditions. Quartz is a chemically inert and relatively hard crystalline mineral present in soils and rocks. It has two polymorphs of α- and β-quartz that have slightly different crystal symmetries[ 1]. In general, only α-phase is stable at ambient temperature and atmospheric pressure[2].
Hydrothermal growth of quartz crystals has been studied extensively for more than almost fifty years[3,4]. A few examples of synthesizing quartz nanocrystals (NCs) with sizes from hundreds of micrometers to tens of nanometers have been demonstrated via hydrothermal routes [5-8]. Unfortunately, the reported methods did not always guarantee excellent control over well-defined sizes, shapes, and crystal phases likely due to extremely fast nucleation and growth rates and rapid coarsening under highly alkaline hydrothermal conditions[5,7]. In this communication, we report an improved hydrothermal method to synthesize quartz NCs from precursors of amorphous silica nanoparticles (ASNs) dissolved in aqueous solutions. We show that as-synthesized NCs have a highly crystalline phase of α-quartz and have relatively uniform sizes ranging from 407.5 to 826.2 nm that predominantly depend on their reaction time from 6 to 24 h.
2. Materials and Methods
Quartz NCs were synthesized via a hydrothermal reaction based on the scheme shown in Figure 1. An aqueous solution of 0.225 g ASNs prepared via a slightly modified Stöber process[8-10] mixed with 27 mL triply distilled H2O and 3 mL 1N sodium hydroxide (NaOH) solution was heated at 100 ℃ and stirred at 400 rpm for 1 h. The solution turned from murky white to clear after ~20 min likely due to the complete dissolution of ASNs at pH ~12. After 1 h, ~1 mL of 1N hydrochloric acid (HCl) was added to eliminate excess hydroxyl ions by partially neutralizing the solution at pH ~7.0. The resulting solution was transferred to a 50 mL polyether etherketone (PEEK)-lined stainless steel autoclave and heated to ~250 ℃ over the time period of 3~ 24 h. Once the reaction was complete, the autoclave was slowly cooled to room temperature. The white precipitates were then filtered through a Nylon membrane with a pore diameter of 0.25 μm, washed with an excess of deionized water and absolute ethanol and dried in a vacuum oven at room temperature to yield ~120 mg of the NCs.
X-ray powder diffraction (XRD) data of the NCs were recorded by using a Philips X’Pert-MPD Diffractometer (Philips Inc., USA) with a monochromatized source of Cu Kα1 radiation (λ = 0.15405 nm) at 1.2 kW power (40 kV, 30 mA). Fourier transform infrared spectroscopy (FT-IR) was performed by using a Spectrum GX FT-IR spectrometer (Perkin Elmer Inc., USA) with both KBr pellets and attenuated total reflection (ATR) techniques. X-ray photoelectron spectroscopy (XPS) data of the NCs were acquired using an ESCALAB 250 spectrometer (Thermo Fisher Scientific Inc., USA) with a monochromatic X-ray source of Al anode Kα radiation (1486.6 eV) as an excitation source. The binding energy was calibrated to the C 1s line of carbon at 284.6 eV before the actual measurements. The morphologies of the NCs were investigated by field emission scanning electron microscopy (FESEM) performed on Zeiss Supra 25 microscope (Zeiss International, Germany) with an accelerating voltage ranged from 5 to 10 kV. Transmission electron microscopy (TEM) of the NCs was performed on Hitachi H-7600 TEM (Hitachi High-tech, Germany) operated at an accelerating voltage of 80 kV. High-resolution TEM (HRTEM) and selected area electron diffraction (SAED) analysis were performed on a TALOS F200X TEM (FEI & Thermo Fisher Scientific Electron Microscopy Solutions, USA) at an acceleration voltage of 200 kV. 29Si solid-state nuclear magnetic resonance (ssNMR) spectroscopy of the NCs and ASNs was performed on a 400MHz AVANCE III HD NMR spectrometer (Bruker Corporation) equipped with a 4 mm (outer diameter of zirconia rotor) magic angle spinning (MAS) probe at Korea Basic Science Institute (KBSI, Western Seoul Center, Korea). Spectra were obtained using direct excitation at 79.51 MHz with a 1.6 μs pulse width (pulse angle π/6), a 50 s recycle delay at ambient probe temperature (~25 ℃), and a sample spin rate of 11 kHz. Signals from 4900 scans were accumulated. 29Si NMR chemical shifts (δ in ppm) were referenced versus an external sample of tetrakis(trimethylsilyl) silane at -135.5 ppm with respect to tetramethylsilane (TMS) at 0.0 ppm.
3. Results and Discussion
Figure 2 shows XRD, FT-IR, XPS, and 29Si ssNMR measurements of α-quartz NCs. Figure 2A shows the XRD patterns of the as-synthesized NCs. All the peaks in the XRD pattern (red curve) were consistent with that of bulk α-quartz (blue curve, Joint Committee on Powder Diffraction Standards (JCPDS) card no. 46-1045). FT-IR spectrum of the as-synthesized α-quartz NCs (red curve, Figure 2B) exhibited IR absorption bands at 3438 and 3243 cm-1 and they were assigned to the hydroxy stretching vibration of adsorbed water and surface silanol[11]. The band at 1638 cm-1 was assigned to the bending vibration of adsorbed water[11]. IR bands at 1103, 800, and 474 cm-1 were attributed to asymmetric and symmetric stretching and symmetric bending of Si-O-Si functional moieties[12]. In general, the FT-IR spectrum obtained from α-quartz NCs was comparatively analogous to one from ASNs precursor except for the fingerprint region from 1000 to 400 cm-1 and a missing band at 945 cm-1, which was likely due to fewer free hydroxy groups bound to the surface Si of α-quartz NCs. Survey XPS spectra of α-quartz NCs and ASNs (Figure 2C) likewise provided very similar binding energies (BEs) of Si 2p peaks at 103.1 eV and O 1s peak at 532.1 eV. However, the results of 29Si ssNMR spectroscopy explicitly showed that they were dissimilar in the chemical environments of their silica networks. 29Si ssNMR spectrum of ASNs (black curve, Figure 2D) showed three characteristic peaks at -92.24, -101.12, and -110.86 ppm, which were assigned to dihydroxy terminated Q2 [(-O)2Si(OH)2], monohydroxy terminated Q3 [(-O)3Si(OH)], and nonhydroxy terminated Q4 [(-O)4Si] sites, respectively. On the contrary, 29Si ssNMR spectrum of α-quartz NCs (red curve, Figure 2D) rather showed a major Q4 peak at -107.65 ppm with much narrower spectral width, which was likely due to both a predominant population of nonhydroxy terminated sites and less change in Si-O-Si angles in highly ordered silica networks of the crystalline NCs[8].
Figure 3 shows FESEM and HRTEM micrographs of the NCs. As-synthesized NCs displayed a similar shape and morphology of bulk quartz with an apparent crystal shape that has a six-sided prism with two six-sided pyramids at both ends (inset image of Figure 3B). Based on FESEM measurements of Figure 3A, the average size of the NCs synthesized from 6 h reaction time was measured to be 407 ± 16 nm with a slightly skewed distribution (inset histogram of Figure 3A). HRTEM micrograph of typical α-quartz NCs (Figure 3C) showed lattice fringes exhibiting highly crystalline domains of (101) and (011) planes separated from domains of (100) by grain boundary. Likewise, the degree of crystallinity was revealed by a set of ordered spots that were assigned to (100), (101), and (011) planes (inset image of Figure 3C), which was also confirmed by calculated d-spacings from independent measurements of SAED. These d-spacings of 0.426, 0.334, and 0.334 nm were in excellent agreement with the (100), (101), and (011) reflections of XRD patterns shown in Figure 2A, respectively.
Figure 3D shows the measured reaction time dependence of the length and width of the NCs. A few aspects of the growth process are worth addressing. First, immediately after the start of the hydrothermal reaction, the products formed over the period of 0~3 h appeared to have amorphous structures confirmed by XRD and 29Si ssNMR measurements. Second, the results of XRD measurements revealed the emergence of the crystallinity of α-quartz after the induction period of ~3 h, implying that the structure and order of the silica network likely began to emerge in the products of the NCs. Third, their growth exhibited a power-law dependence on the reaction time t of the NC formation with the exponent α of the form Atα close to ~0.5. Currently, we do not fully understand the mechanism of the growth of the NCs. It is likely that the mechanism may follow classical principles of nucleation and growth of colloidal nanoparticles and nanocrystals. Further studies on the growth of the NCs are still ongoing and their in-depth analyses will shed light on the elucidation of the growth mechanism. The complete analyses of the growth kinetics will be reported in the near future.
4. Conclusions
We successfully synthesized α-quartz NCs from ASN precursor solutions under mild hydrothermal conditions of ~250 ℃ and autogenic pressure. The structure, morphology, and composition of the NCs were characterized using FT-IR, ssNMR, XPS, XRD, FESEM, and HRTEM measurements systematically. As-synthesized NCs exhibited highly crystalline nanostructures of α-quartz and their average size can be tuned in a relatively narrow range from 407.5 to 826.2 nm with reaction time. Additional studies of characterizing the processes’ growth kinetics will be required to understand the mechanism responsible for the formation of quartz NCs in the near future. In summary, our results provide a facile means of producing quartz NCs of high crystal quality and relatively narrow size distributions, offering potential uses for technological applications in optoelectronics, sensing, and rechargeable battery devices.
