A novel low temperature vapor phase hydrolysis method for the production of nano-structured silica materials using silicon tetrachloride

Abstract

Here we report for the first time, a novel method of low temperature vapor phase hydrolysis for the production of nano-structured silica particles. Silica nanoparticles were obtained by the hydrolysis of silicon tetrachloride vapor with water vapor at a low temperature range (150–250 °C). The effects of reaction temperature and residence time on the specific surface area and size distribution were determined to obtain optimal synthesis conditions. Silica nanoparticles with a specific surface area of 418 m² g⁻¹ and an average size of 141.7 nm were obtained at a temperature of 150 °C and with a residence time of 5 s. The particle morphology, phase composition, chemical composition, thermal analysis, and chemical functional groups present were determined by TEM, XRD, XRF, TGA, and IR methods, respectively. Results indicated that silica synthesized by low temperature vapor phase hydrolysis method is an amorphous mesoporous material, with an approximately spherical shape, a mass friction demission of 2.29, and a high hydroxyl density of 13.03 nm⁻². This method provides a simple and environmentally benign way for the mass production of silica nanoparticles, as well as a quick method for the preparation of functional silica materials.

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Introduction

Silica nanoparticles are an advanced functional material widely used as additives for paints and rubbers because of their high specific surface area, high degree of dispersion, high purity, and ease of functionalization.^1,2 Recently, silica nanoparticles used as a catalyst support,^3–5 an adsorption material for capturing CO₂,^6,7 and in biomedical engineering for drug delivery^8,9 also have drawn much attention. In these applications, the particle morphology, size distribution, specific surface area, phase composition, and surface activity are considered to be the key characteristics of particles that affect their performance.

Silica nanoparticles produced by different methods have different properties. A range of methods are used to synthesize silica nanoparticles, among which the wet phase chemical method and vapor phase chemical method are the most commonly adopted. The wet chemical method is the process of simultaneous hydrolysis and condensation of silica gel precursors such as silicon tetrachloride, silicate, and tetraethyl orthosilicate (TEOS). Silica powders are obtained after a series of processes including precipitation, washing, and drying. In some studies, alcohol, acid, and ammonia have been added as surfactants and catalysts^10,11 to form uniform silica particles with better dispersibility. Other studies have focused on assistant methods, such as ultrasonication,¹² the microemulsion method,^13,14 the microwave method,¹¹ and the pressured carbonation technique¹⁵ to shorten the reaction time and optimize the reaction process. However, adding surfactants and catalysts may lead to a decline in product purity, and the corresponding washing process might increase the process complexity. The use of new assistant methods still cannot solve the problem of by-product pollution and low yields. Since the hydrogen–oxygen flame method was first invented by Deggusa in the 1960s,¹⁶ flame technology has been widely used to produce ultrapure silica nanoparticles by burning silicon tetrachloride in a hydrogen–oxygen flame. Many investigations of particle dynamics during the flame method have been undertaken in recent decades.^17–19 The silica powders obtained through this method, which is solvent-free and produces few by-products, exhibit low density, controlled size, and large specific surface area of up to 400 m² g⁻¹. However, the flame method requires high reaction temperature (up to 1800 °C),²⁰ large investment, and encounters sever corrosion to facilities.

The wet phase chemical method can result in low product quality, complex post-treatment processes, and low yields, whereas the vapor phase method involves the use of flame technology, which requires large investment and specific facilities. The development of alternative cheaper and environmentally benign routes to prepare silica nanoparticles with desirable properties is therefore of considerable interest. The direct vapor-phase hydrolysis method involves a hydrolysis reaction in the vapor phase, in absence of a flame. Kirkbir and Komiyama²¹ synthesized uniform and submicron-sized TiO₂ powder using titanium isopropoxide hydrolyzing with water vapor at a reaction temperature as low as 107 °C, and produced a specific surface area in the range of 55–228 m² g⁻¹. Klug, et al.²² prepared Fe₂O₃ with high photoactivity in a FeCl₃/H₂O vapor system at growth temperatures between 200–350 °C. John and Surender²³ synthesized rutile phase titania through vapor phase hydrolysis in a H₂O/C₂H₅OH/TiCl₄ vapor system at 353 K and 387 K. In Park's study,²⁴ SiCl₄ vapor was hydrolyzed with water vapor at 150 °C as the first step to form oxychloride particles. The sample was then converted into silica particles through further hydrolysis at 1000 °C to produce silica particles in a size range of 250–300 nm. Using the low temperature vapor phase hydrolysis method to synthesize silica nanoparticles from SiCl₄ in one step has never been previously reported.

In this work, we report for the first time, a novel method of low temperature vapor phase hydrolysis to produce high specific surface area silica nanoparticles. Silica nanoparticles were obtained by the hydrolysis of silicon tetrachloride vapor with water vapor in a low temperature range. The study considered the effects of reaction temperature and residence time on specific surface area and size distribution to determine the optimal synthesis conditions. The particle morphology, phase composition, chemical composition, thermal analysis, and chemical functional groups present were determined by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), thermal gravimetric analysis (TGA), and IR methods, respectively. The advantages of this method for synthesizing silica nanoparticles include inexpensive starting materials (silicon tetrachloride and water), an environmentally benign and simple process, a lower energy consumption, and a large-scale production capability.

Experimental section

Vapor phase hydrolysis system set up

Silica nanoparticles were synthesized by a vapor-phase hydrolysis system designed by the authors, which is consisted of three parts: the feed system, the core reaction system, and the product collection system. The feed system contained a carrier gas control system, two evaporators, and two automatic injection pumps. Silicon tetrachloride (99.9%, AR) and distilled water were injected into two evaporators, respectively, by two automatic injection pumps. The injection speed could be precisely controlled between 0.15 μL and 50 ml per minute. Nitrogen gas was used as a carrier gas to introduce vaporized silicon tetrachloride and water into a quartz glass tube of 800 mm in length and 40 mm in diameter surrounded by a furnace. The gas speed was controlled by a mass flow controller (MFC) and a float flow meter. Silicon tetrachloride vapor and water vapor were hydrolyzed in the quartz glass tube at a certain temperature in the core reaction system (see eqn (1)), where primary silica particles were formed.

SiCl₄ + 2H₂O → SiO₂ + 4HCl (1)

At the end of the tube, an Erlenmeyer flask with a branch pipe was set as the collector. Particles aggregated and deposited on the wall of the Erlenmeyer flask, and the residual gases passed through an alkali liquor layer to absorb hydrochloric acid produced by the hydrolysis process. After the reaction, the collector was removed and dried at 105 °C for 2 h before silica powder was collected.

Synthesis of silica nanoparticles

Hydrolysis processes were operated under different temperatures and at different residence times. The reaction temperature was set at 125 °C, 150 °C, 175 °C, 200 °C, 225 °C, 250 °C, 275 °C, and 300 °C by altering the temperature control system of the furnace. The flow speed of N₂ passing through silicon tetrachloride vapor was set to be 60 L h⁻¹ by the MFC, and the flow speed of N₂ passing through water vapor was set to be 200 L h⁻¹ by the float flow meter. The injection speeds of silicon tetrachloride and distilled water were set at 0.33 ml min⁻¹ and 1.0 ml min⁻¹, respectively. Silica produced under different temperatures was designated as SiO₂-125 °C, SiO₂-150 °C, SiO₂-175 °C, etc.

The residence time was controlled by the flow speed of the carrier gas. The temperature was set at 150 °C, and different residence times were operated. Silica produced under different residence times were designated as SiO₂-4 s, SiO₂-5 s, SiO₂-7 s, SiO₂-10 s, SiO₂-14 s, and SiO₂-18 s.

Characterization

Nitrogen adsorption–desorption isotherms at 77 K were measured by a Surface Area and Porosity Analyzer (Micromeritics, ASAP2020 HD88). The samples were degassed at 363 K for 60 minutes and held at 433 K for 120 minutes before analysis. A Barrett–Emmett–Teller (BET) model was used to calculate the specific surface area and a Barrett–Joyner–Halenda (BJH) model was used to calculate the pore volume distribution and the average pore size. The particle size distribution was obtained from a nanoparticle size analyzer (Beckman Coulter, DelsaNano C) with samples dispersed in distilled water at 3 wt‰ concentration after ultrasonic oscillation. The morphology of particles was observed using TEM (JEM-2011).

The phase composition was characterized by XRD equipment (Siemens, D8 Advance), using Cu-Kr radiation (λ = 0.15406 nm) with a 2θ range of 10° ∼ 70°. Chemical composition analyses were performed using an XRF analyzer (Shimadzu, XRF-1800).

The hydroxyl content was calculated using a Thermogravimetric Analyzer (Mettler-Toledo, TGA/DSC 1 STAR^e) with a heating rate of 10 °C min⁻¹ from 50 °C to 1200 °C under the protection of argon gas. Chemical functional groups were determined by Fourier transform infrared spectrometer (Thermo Fisher Nicolet, NEXUS 870) with a scan wave number range of 400–4000 cm⁻¹.

Results and discussion

Effects of temperature on silica nanoparticles

Reaction temperature is a crucial factor affecting the specific surface area and average aggregate particle size of silica nanoparticles, as well as the process energy consumption. Fig. 1 shows that from 125 °C to 250 °C, the specific surface area of silica was maintained at ∼350 m² g⁻¹. From 250 °C to 300 °C, there was a decreasing trend in specific surface area of silica, to below 300 m² g⁻¹. According to Kruis et al.,¹⁸ silica nanoparticles formed in the vapor phase can be described by three processes: instantaneous chemical reaction, colliding to grow to molecular clusters and macroscopic particles (also called nucleation), and the aggregation of primary particles. The influence of these three processes on the formation of silica nanoparticles were discussed in the following sections.

Fig. 1 Specific surface area of silica synthesized by vapor phase hydrolysis under temperature range of 125–300 °C.

The enthalpy change and the Gibbs free energy change of reaction eqn (1) under different reaction temperatures were calculated, and the standard data related to the reaction were obtained from Lang's Handbook of Chemistry.²⁵ The hydrolysis reaction was determined to be a strong exothermic reaction since the value of Δ_fH was negative, i.e., from −139.3 kJ mol⁻¹ to −138.5 kJ mol⁻¹ when temperature range was 125–300 °C. With an increase in temperature, the value of Δ_fH increased slightly, although this may not have influenced the reaction. The negative value of Δ_fG indicates that the hydrolysis reaction can happen instantaneously, and increasing the temperature will aggravate the reaction, with the Δ_fG value decrease from −163.4 kJ mol⁻¹ to −184.7 kJ mol⁻¹. Calculations of the enthalpy change and Gibbs free energy change also suggest that spillages of SiCl₄ as an industrial by-product need be controlled for security considerations.²⁶ According to nucleation theory, the nucleation barrier of SiO₂ is relatively low, which makes it very easy for SiO₂ monomers to undergo nucleation, and form primary silica nanoparticles.¹⁶ The demarcation point is considered to be 250 °C. When the temperature is relatively low (below 250 °C), it is easier for water vapor to condense on the inner wall of the collector, and the micro-size droplets may provide an attachment to form a nucleation.²⁷ This was also confirmed by the experimental observations. When the temperature was higher than 250 °C, there were very few coagulated water droplets in the collector, and the condensation of water vapor was influenced by the temperature field of the hydrolysis system.

The diameter of silica aggregates at different temperatures was also investigated. SiO₂-150 °C and SiO₂-300 °C with high and low specific surface area, respectively, were selected to analyze the number diameter distribution. The results indicated that SiO₂-150 °C had a better size distribution with a lower average particle size, a narrower error range, and a lower polydispersity index (PDI) (see Fig. 2). The polydispersity index also indicated that silica synthesized by low temperature vapor phase hydrolysis was approximately monodispersed.

Fig. 2 Number size distribution of silica particles calculated by DLS method: (a) SiO₂-150 °C, PDI: 0.270; (b) SiO₂-300 °C, PDI: 0.281.

Fig. 3 showed the nitrogen adsorption–desorption isotherms and pore distribution of SiO₂-150 °C and SiO₂-300 °C. Samples of SiO₂-150 °C and SiO₂-300 °C (see Fig. 3a) both displayed an approximate type IV isotherm as defined by IUPAC. The presence of type H3 hysteresis loops indicates the existence of mesopores, and the unlimited adsorption at high P/P₀ suggests that aggregates of plate-like particles give rise to slit-shaped pores.^6,28 Compared with SiO₂-150 °C, the hysteresis loop for SiO₂-300 °C is shifted to a high P/P₀ direction, illustrating that the pores of SiO₂-300 °C are larger than those of SiO₂-150 °C. The average pore size calculated by BJH adsorption (9.12 nm for SiO₂-150 °C and 10.29 nm for SiO₂-300 °C) also supports this. The pore size distribution (see Fig. 3b) shows that SiO₂-150 °C and SiO₂-300 °C both have a relatively wide meso-pore size distribution and the peak of pore size blew 2 nm indicates the presence of micropores. Silica particles with a higher specific surface area have a higher pore volume.

Fig. 3 (a) N₂ adsorption–desorption isotherms at 77 K of SiO₂-150 °C and SiO₂-300 °C (b) BJH dV/d log (D) pore volume distribution of SiO₂-150 °C and SiO₂-300 °C, calculated by the adsorption branch of the isotherms.

The effects of reaction temperature on silica nanoparticles suggest that silica with a high specific surface area (higher than 350 m² g⁻¹) can be synthesized at a low temperature range of 125–250 °C, which is significantly low compared to that in the oxygen–hydrogen flame method (1400–1800 °C). The specific surface area of silica is not sensitive to temperature in low temperature ranges, which is very promising for industrial production. Silica synthesized by low temperature vapor phase hydrolysis is approximately monodisperse, and silica with a higher specific surface area also has a better size distribution and higher pore volume.

Effects of residence time on silica nanoparticles

Table 1 shows the specific surface area, pore volume, average pore width, primary particle size and aggregate particle size of samples with different residence times at 150 °C. According to BET theory, the average primary particle size is calculated by d_p = 6000/ρ_p S_BET, where S_BET is the specific surface area, and ρ_p is 2.2 g cm⁻¹.^3,20 Average aggregate particle size was obtained from a nanoparticle size analyzer using dynamic light scattering (DLS) method. When reaction temperature was 150 °C, samples with different residence times from 4–18 s all had a high specific surface area (>330 m² g⁻¹), and there was a peak in specific surface area of 418.0 m² g⁻¹ when the residence time was 5 s. When the residence time was longer than 5 s, the specific surface area significantly decreased. The average DLS particle size of all the samples, which also refers to the size of particle aggregates, was less than 200 nm. The lowest peak of aggregate particle size (141.7 nm) occurred when the residence time was 5 s. Variation in the average particle size derived from the BET calculations and the aggregate particle size determined by the DLS method displayed the same trend.

Table 1 Silica properties at different residence times^a

Residence time/s	SBET/m^2 g^−1	Pore volume/cm^3 g^−1	Adsorption average pore width/nm	dp/nm	Average DLS particle size/nm	np
a dp refers to the average primary particle size of silica; np is the average number of primary particles to form an aggregate.
4	397.7	0.73	7.35	6.86	147.1	1107
5	418.0	0.84	8.08	6.53	141.7	1147
7	391.1	0.77	7.89	6.97	149.1	1099
10	346.6	0.74	8.51	7.87	180.6	1357
14	344.2	0.68	7.88	7.92	175.9	1230
18	333.7	0.73	8.75	8.17	186.1	1147

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For silica samples with different residence times at the same temperature, a mass balance calculation was performed for the process of formation an aggregate, as follows in eqn (2):

n_p primary particles → an aggregate

where ρ_a refers to the density of silica aggregates (tapping density), g L⁻¹; d_a is the average particle diameter of silica aggregates, nm; and n_p is the average number of primary particles to form an aggregate, dimensionless. To measure the tapping density, a container of known volume was filled with silica powder and gently tapped until no more powder could be added. The powder was then weighed to obtain the tapping density of 247 g L⁻¹. This is a non-standard technique but gives reproducible results.²⁹ Since the value of ρ_p and ρ_a were determined, the n_p at different residence times were calculated respectively, and the n_p values are listed in Table 2. Matsoukas and Friedlander proposed a theory to describe the relationship between primary particle size and aggregate particle size,^18,30 (see eqn (3))where D_f is the mass friction dimension. For rods, D_f = 1; for disks, D_f = 2; for spheres, D_f = 3. The value of D_f was calculated as 2.293 ± 0.007 using a statistical method, indicating that silica synthesized by vapor phase hydrolysis under the same temperature has the same n_p value and a uniform primary particle morphology falling in between disks and spheres, which also corresponded to the nitrogen adsorption–desorption isotherms suggesting that the synthesized silica was aggregates of plate-like particles.

Table 2 Weightlessness analysis of SiO₂-5 s

Temperature/°C	50–62	62–160	160–370	370–592	592–900	Total
Water type	Free water	Adsorbed water	Hydroxyls			—
			Geminal	Vicinal	Isolated	—
Weight loss (%)	1.27	3.02	3.18	3.52	1.44	12.43
Hydroxyl number (mmol g^−1)	—	—	3.53	3.91	1.60	9.04
Hydroxyl density (nm^−2)	—	—	5.09	5.63	2.30	13.03

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Property determination

Morphology. A change in reaction temperature and residence time would result in differences in the specific surface area and average DLS particle size, but would generate little difference in morphology because the reaction system was determined. TEM images of silica with different specific areas did not show a perceivable distinction because both the specific surface areas and the DLS particle size are average values of silica in groups. We selected a sample of SiO₂-5 s, of which the specific surface area was the highest, to analyze the morphology. Fig. 4 shows the TEM images of SiO₂-5 s at different magnifications. At a 0.2 μm scale, a group of silica aggregates can be seen which are substantially uniformly distributed (Fig. 4a). At a 100 nm scale, a rough image of a silica aggregate can be seen with a size of 160 nm, which is consistent with the results of the average DLS particle size (141.7 nm) (Fig. 4b). Fig. 4c shows the three-dimensional amorphous morphology of an aggregate formed by a group of primary particles at a 50 nm scale. The primary particle size is shown to be roughly 10 nm. The detailed morphology of primary particles can be seen in Fig. 4d in a 10 nm scale. The results indicate that primary silica particles are translucent, and have a plate-like spherical shape, which also supports the results obtained from nitrogen adsorption–desorption isotherms and the D_f value.

Fig. 4 TEM images of SiO₂-5 s. (a) a group of silica aggregates; (b) rough image of a silica aggregate; (c) three-dimensional morphology of an aggregate formed by a group of primary silica particles; (d) morphology of primary silica particles.

Phase and chemical composition. The X-ray pattern in Fig. 5 confirms that the powders are silica with few other impurities detected. The peak with a wide 2θ range also indicates that the silica produced by low temperature vapor phase hydrolysis method is amorphous. XRF results show that the silica is in high purity (99.93%), meeting the purity requirements for commercial fume silica. Very few impurities were detected, included 0.0326% Fe₂O₃, 0.0269% Cr₂O₃, and 0.0085% Br, and these three impurities showed a good reproducibility in different samples. Cr₂O₃ and Br might have been introduced by SiCl₄ and the presence of Fe₂O₃ might have been caused by the process of sample collection, considering that a spatula made of iron was used to collect samples. The level of most impurities was below the detection limit of the XRD equipment.

Fig. 5 XRD pattern of silica particles produced by the low temperature vapor phase hydrolysis method.

Thermal analysis. Fig. 6 shows TGA and DTG data profiles for silica synthesized by the low temperature vapor phase hydrolysis method. The DTG curve shows four low peak values, which represent the lowest rate of weight loss during the temperature range nearby. The weight loss process can be divided into five stages. The first stage (1.27% weight loss) occurred over a temperature range of 50–62.15 °C, and is likely to be free water. The second stage (3.02% weight loss) involves the loss of adsorbed water in the temperature range of 62.15–161.62 °C. When the temperature was higher than 161.62 °C, which is close to the 177.5 °C used by Jal et al.,³¹ silica starts to sequentially loss different kinds of hydroxyl groups in genimal, vicinal, and isolated forms.³² Genimal hydroxyl refers to the condition when two hydroxyl groups are present in a single silica atom, while two hydroxyl groups present in two adjacent silica atoms are referred to as vicinal hydroxyl. Isolated hydroxyl is present in a single silica atom with no hydrogen bonds formed with other silanols of the same silica atom.³¹ Usually isolated hydroxyls are so stable that they are only slowly dehydroxylated at high temperatures above 600 °C,³² with the low peak of 592 °C in Fig. 6 also supporting this. The weight loss due to the loss of these three kinds of hydroxyl is 8.04%, reaching a total weight loss of 12.43% when the loss from the first two stages is also accounted for. When the temperature reaches 900 °C, there is almost no weight loss. By considering the weight loss over different temperature ranges, the hydroxyl content N_OH(SiO₂) can be calculated by eqn (4):where M_H2O is the molecular weight of water, g mol⁻¹. Hydroxyl density D_OH(SiO₂) is given by eqn (5):^33,34where N_A is the Avogadro number, dimensionless; and S_BET refers to the specific surface area measured by the BET method, m² g⁻¹. The calculations of weight loss, hydroxyl content, and density are listed in Table 2. The total hydroxyl density was 13.03 nm⁻², which is higher than fume silica (S220, 3.5 OH nm⁻²)³² and precipitate silica (7.68 OH nm⁻²).³¹ A higher hydroxyl density may provide a higher reactivity on silica surface, and for this purpose a series of silylating agents can be covalently bonded to the activated silanol groups, which may then be utilized to applications mentioned in the introduction section.

Fig. 6 TGA/DTG thermo-grams of silica nanoparticles synthesized from low temperature vapor phase hydrolysis.

Chemical functional groups. Fig. 7 shows a comparison of the IR spectra of untreated silica synthesized by low temperature vapor phase hydrolysis and silica calcinated in a furnace at 900 °C for 120 minutes. The two samples are referred to as SiO₂-untreated and SiO₂-900 °C, respectively. Several transmitted peaks of SiO₂-untreated are apparent, where the intense bands at 466.7 cm⁻¹, 801.5 cm⁻¹, and 1097.3 cm⁻¹ can be attributed to the bending vibration, symmetric stretching vibration, and asymmetric stretching vibration of the Si–O–Si bond, respectively. The band assigned to 951.5 cm⁻¹ represents the bending vibration and stretching vibration of the Si–OH bond. The band around 1634.8 cm⁻¹ can be ascribed to be the bending vibration of the H–O–H bond in molecule water. A broad band around 3000–3800 cm⁻¹ is caused by the stretching vibration of different kinds of hydroxyls and the remaining adsorbed water.^35,36 The existence of these characteristic peaks indicates the presence of the Si–O–Si band, the Si–OH band, and the H–O–H band in silica, which proves that untreated silica consists of Si–O bonds with surface hydroxyls. When silica is treated at 900 °C for 120 minutes, the intensity and number of characteristic peaks are changed. The vibration of the Si–O–Si bond does not display any variation, whereas the intensity of the bands around 3403.6 cm⁻¹ and 1634.8 cm⁻¹ significantly decrease, and the peak at 951.5 cm⁻¹ disappears completely, which indicates the reduction of the H–O–H bond. This result is consistent with the conclusions of the TGA, because most hydroxyls can be removed when the temperature reaches 900 °C. Hydroxyls are removed in pairs because two hydroxyls are needed to form water molecular, hence theoretically not all hydroxyls can be removed, and the existence of the band at 3403.6 cm⁻¹ in SiO₂-900 °C also confirms this.

Fig. 7 IR spectra of silica produced by the low temperature vapor phase hydrolysis method (a) untreated SiO₂-5 s; (b) SiO₂-5 s, calcination in 900 °C for 120 minutes.

Conclusions

In summary, a novel method of low temperature vapor phase hydrolysis was used for the first time to synthesize nano-structured silica materials using silicon tetrachloride as a cheaper precursor. The reaction temperature and residence time were investigated to obtain the optimal conditions for synthesis. When the temperature was 150 °C and the residence time was 5 s, silica nanoparticles with an average specific average surface area of 418 cm² g⁻¹ and an average size of 141.7 nm were obtained. The specific surface area of silica is not sensitive to temperature in the low temperature range of 125–250 °C. Silica nanoparticles with a higher specific surface area had a better size distribution and higher pore volume, and a change of residence time from 4–18 s did not change of degree of aggregation. A series methods were used to determinate the properties of silica nanoparticles. TEM results revealed that silica aggregates were substantially uniformly distributed, in three-dimensional morphology with an average size of ∼150 nm, which were formed by primary particles of ∼10 nm that had a plate-like spherical shape. This was also supported by the nitrogen adsorption–desorption isotherms and the D_f value of 2.293 ± 0.007. The XRD and XRF results indicated the product was silica, which was in an amorphous state with a purity of 99.93%. Thermal analysis indicated that silica could loss weight owing to the loss of water and different hydroxyls when heated. The hydroxyl density was calculated to be 13.03 nm⁻², indicating a high reactivity of the silica surface. IR spectra data also confirmed the existence of surface hydroxyls. The low temperature vapor phase hydrolysis method provides a simple and environmentally benign way for the mass production of nano-structured silica materials, as well as a quick method for the preparing of functional silica materials.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

The authors gratefully acknowledge the Hi–Tech Research and Development Program (863) of China for financial support (grant no. 2012AA06A116).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Estimate of residence time, thermodynamic calculations, BJH pore volume distribution, TEM images, X-ray diffraction patterns. See DOI: 10.1039/c3ra47018k

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