Synthesis and properties of amorphous SiO2 nanoparticles

Article abstract of DOI:10.1134/S0020168512040103

We have studied the synthesis of amorphous silica nanoparticles through fluorination processing of quartz sand. The results demonstrate that synthesis conditions influence the physicochemical properties of the resultant amorphous silica. We have obtained

Full text of DOI:10.1134/S0020168512040103

ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 4, pp. 355–360. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.S. Rimkevich, A.A. Pushkin, I.V. Girenko, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 4, pp. 423–428.
Synthesis and Properties of Amorphous SiO Nanoparticles
2
V. S. Rimkevich, A. A. Pushkin, and I. V. Girenko
Institute of Geology and Nature Management, Far East Branch, Russian Academy of Sciences,
Relochnyi per. 1, Blagoveshchensk, 675000 Russia
eꢀmail: vrimk@yandex.ru
Received August 18, 2011
Abstract—We have studied the synthesis of amorphous silica nanoparticles through fluorination processing
of quartz sand. The results demonstrate that synthesis conditions influence the physicochemical properties
of the resultant amorphous silica. We have obtained silicaꢀcontaining powders 17 to 89 nm in average particle
2
size and 92 to 508 m /g in specific surface area.
DOI: 10.1134/S0020168512040103
INTRODUCTION
rosilicate, (NH ) SiF6. To separate and collect volatile
4 2
reaction products, we used a twoꢀzone condenser made
of stainless steel and Teflon. Ammonia gas was absorbed
in a waterꢀfilled vessel. Amorphous silica nanoparticles
were obtained through precipitation, by adding aqueous
ammonia (NH OH) to an aqueous (NH ) SiF6 soluꢀ
In many areas of science and technology, the past
few decades have seen rapid growth in studies based on
the use of amorphous silica in relation to its state and
purity. Amorphous silica is widely used as a filler for
rubber, textile materials, paper, plastics, paints, pigꢀ
mented lacquers, pharmaceutical substances, and cosꢀ
metics; in the production of silicon and silicon ferroalꢀ
4
4 2
tion. The process was run in a laboratoryꢀscale Teflon
hydrolizer. NH HF2 was regenerated in a laboratoryꢀ
4
loys; in the growth of quartz single crystals for electronꢀ size evaporator/crystallizer system.
ics; in the production of quartz glass, refractories,
abrasives, and molds; and as an adsorbent and catalyst
in a number of technologies in the chemical, petroꢀ
chemical, food, and other industries.
The raw material, intermediate phases, and final
products were characterized by chemical analysis,
Xꢀray diffraction, spectral analysis, electron microsꢀ
copy, differential thermal analysis, and other techniques
Standard methods for the preparation of amorphous available at the Amur Mineralogical/Geochemical
silica employ complex, multistep processes and require Research Center, Institute of Geology and Nature
expensive chemicals and specific raw materials and Management, Far East Branch, Russian Academy of
apparatus [1, 2]. In this context, there is high current Sciences.
interest in developing an economically attractive proꢀ
Elemental analysis of samples was carried out on a
cess for the preparation of amorphous silica nanopartiꢀ
Bruker S4 Pioneer spectrometer. Fluorine and ammoꢀ
cles from quartz sand, a readily available, inexpensive
nia were determined after H SiF6 and NH3 removal, by
2
material containing little harmful impurities.
titrating the resultant solutions with thorium nitrate and
sulfuric acid, respectively. The phase composition of the
samples was determined by Xꢀray diffraction on an
MDꢀ10 EFA Xꢀray minidiffractometer using ICDD
PDF 2008 data. Microimpurities were determined by
emission spectroscopy on an STEꢀ1 crossed dispersion
spectrograph using pouring in a threeꢀphase arc and
evaporation from a carbon channel. Electronꢀmicroꢀ
scopic examination was carried out on a JEOL
JSMꢀ6390 LV scanning electron microscope and a
SOLVER R47 atomic force microscope (Vladivostok,
EXPERIMENTAL
We studied foundry quartz sand with the following
composition (wt %): SiO2, 95.80; Al O , 2.26; Fe O ,
2
3
2
3
0
.17; TiO , 0.23; Na O, 0.09; K O, 0.97; loss on igniꢀ
2 2 2
tion, 0.40 (Chalganskoe quartz–kaolin–feldspar
sand deposit, Amur oblast, Russia). In our experiꢀ
ments, we used quartz sand ground to a particle size
under 0.001 cm.
The sand was fluorinated in a purposeꢀdesigned labꢀ Russia). Thermogravimetric scans were performed with
oratoryꢀscale apparatus, which included a nickel alloy an STA 449C Jupiter thermal analyzer at a heating rate
reactor, where the raw material was sintered with from 2 to
5 C/min, using lidded platinum crucibles (the
°
ammonium bifluoride (NH HF2), and the resultant initial sample weight was 0.10–0.15 g). The IR spectra
4
mixture was heatꢀtreated to give ammonium hexafluoꢀ of the raw material and amorphous silica were obtained
355

356
RIMKEVICH et al.
the formation of (NH ) SiF6; and synthesis of amorꢀ
DSC,
μ
V/mg
4 2
TG, %
00
EXO
phous silica nanoparticles at temperatures from 20 to
90°C.
1
–15.67%
0
90
80
70
60
50
40
30
20
10
Figure 1 shows differential scanning calorimetry
DSC) and thermogravimetry (TG) curves illustrating
the multistep reaction between quartz sand and ammoꢀ
nium bifluoride. Thermodynamic calculations of the
corresponding Gibbs energy change suggest that reacꢀ
tions (1) and (2) are possible even at room temperature
159.1
°
C
–11.44%
–0.5
(
–1.0
2
04.3°C
107.5°C
–1.5
–
63.04%
–2.0
(
Table 1).
The first step is reaction (1), which yields ammoꢀ
nium heptafluorosilicate, (NH ) SiF7. The DSC curve
–2.5
9
.07%
2
95.1°C
4
3
50
100
150
200
250
Temperature,
300
in Fig. 1 shows the corresponding endothermic peak at
07.5 . Raising the temperature leads to the formaꢀ
tion of a mixture of (NH ) SiF and ammonium
°C
1
°
C
Fig. 1. DSC and TG curves for the reaction between quartz
4
3
7
sand and ammonium bifluoride.
hexafluorosilicate by reaction (2). At 204.3
°
C, we
observe (NH ) SiF formation by reaction (3).
4
2
6
on an ETIR–Spectrum One spectrophotometer (Perkin
Elmer, USA). The specific surface area of the amorꢀ
phous silica powders was evaluated from nitrogen therꢀ
mal desorption using a Sorbtometr M specificꢀsurface
analyzer (manufactured at the Boreskov Institute of
Catalysis, Siberian Branch, Russian Academy of Sciꢀ
ences).
In the TG curve in Fig. 1 the weight loss accompanyꢀ
ing the endothermic event at 159.1 is 15.67%, which
corresponds to the water and ammonia removal by
reaction (1) (calculated weight loss, 16.36%). Raising
the temperature leads to the removal of HF and the
°
C
residual water and ammonia. At 204.3 C, the weight
°
loss evaluated from the TG curve is 11.44%, and the calꢀ
culated value is 10.93% [reactions (2) and (3)]. The
maximum calculated amount of ammonia, 7.17 wt %,
was found for reaction (3).
The thermodynamic parameters, rate constants and
activation energy of chemical reactions were calculated
using the Microsoft Excel 2007 application. Amorꢀ
phous silica nanoparticles were imaged using the NOVA
program and Microsoft Windows Paint application.
According to the rate constants and activation enerꢀ
gies in Table 2, reactions (1)–(3) were kinetically conꢀ
trolled. The rate of the process can be increased by raisꢀ
ing the temperature. In molten NH HF (126.8
°
C)
the reaction has the highest rate. Optimal process
conditions can be reached at 200 in 3.5 h. The
,
4
2
RESULTS AND DISCUSSION
°
C
The fluorination processing of quartz sand was studꢀ
ied experimentally in three steps: sintering of the raw
material with ammonium bifluoride at temperatures
ammonia volume evolved then reaches 99% of the
theoretical one [3].
from 100 to 200
°
C; heat treatment of the resultant powꢀ
During sintering [reactions (1)–(3)], the ammonia
der in the temperature range 300–500
°
C, resulting in gas and water vapor were captured in an aqueous soluꢀ
Table 1. Calculated Gibbs energy (kJ) of the chemical reactions underlying the fluorination of quartz sand and the synthesis
of silica nanoparticles
t
, °C
25
100
200
300
(1)
–1698.65
+ 3NH3 + 3HF
–1776.5
(3)
–1464.7
500
2
SiO + 7NH HF = 2(NH ) SiF + NH
↑
+ 4H O
↑
2
4
2
4 3
7
3
2
Δ
Δ
Δ
Δ
Δ
G
G
G
G
G
–39
–96.45
–306.05
–1851.85
–1988.5
–1662.3
138.6
2
SiO + 7NH HF = (NH ) SiF + (NH ) SiF + 4H O
↑
↑
↑
(2)
2
4
2
4 3
7
4 2
6
2
–36
256
–115.5
–354.5
SiO + 3NH HF = (NH ) SiF + 2H O
↑
+ NH3
↑
2
4
2
4 2
6
2
181.9
–49.9
(
NH ) SiF + 4NH4OH = SiO2
↓
+ 6NH F + 2H O (4)
4
2
6
4
2
–48.7
0.5
–19.1
20.3
59.7
2
NH F = NH HF + NH
↑
(5)
4
4
2
3
–12.0
–28.7
–45.4
–78.8
INORGANIC MATERIALS Vol. 48
No. 4
2012

SYNTHESIS AND PROPERTIES OF AMORPHOUS SiO NANOPARTICLES
357
2
tion to form aqueous ammonia, which was used in the Table 2. Rate constant
k and activation energy Ea for the
sintering of quartz sand with ammonium bifluoride and the
sublimation of the volatile compound ammonium hexafluꢀ
orosilicate
(
NH ) SiF hydrolysis step. The NH3 and HF released
4 2 6
during sintering [reaction (2)] reacted in the second
zone of the condenser to form ammonium fluoride,
NH4F, which was directed to the evaporator/crystalꢀ
lizer system.
t
,
°
C
k
, min–1
t
,
°
C
k
, min–1
quartz sand sintering
(NH ) SiF sublimation
4
2
6
According to Xꢀray diffraction and chemical analyꢀ
sis data, the powders obtained under reducing or inert
1
00
50
0.002202
0.005031
0.006382
0.008230
18.3
300
0.002268
0.010620
0.044069
0.061729
10.8
conditions at 200
and Al, Fe, Na, and K compounds, which formed simꢀ
ple fluorides. The (NH ) SiF thus obtained was puriꢀ
°
C in 3.5 h consisted of (NH ) SiF
4 2 6
1
350
450
500
4
2
6
175
00
prevailed at temperatures above 295.1 C, in agreement Ea, kJ/mol
fied by heat treatment. This led to ammonium hexafluꢀ
orosilicate sublimation, which began at 204.3 and
2
°
C
°
with previous data [4, 5].
The process had a large rate constant and low activaꢀ
tion energy (Table 2). The best temperature for ammoꢀ
because the solvent rapidly vaporized at higher temperꢀ
atures and the quality of the product was insufficiently
nium hexafluorosilicate formation was 500 C: the calꢀ
°
high at temperatures below 20
°
C. Holding the suspenꢀ
culated theoretical amount of this compound was
sion for 1 h at temperatures from 20 to 90
°
C
contributed
reached in 0.5 h. The volatile compound (NH ) SiF
4
2
6
to gel stabilization and considerably improved the filterꢀ
ability of the gel. The suspension was then filtered and
washed with distilled water with mechanical stirring,
and the filtrate was dried to constant weight.
was captured and collected in the first zone of the conꢀ
denser. The resultant fluorides remained precipitated,
without subliming.
Electronꢀmicroscopic examination showed that the
ammonium hexafluorosilicate consisted of platelets less
Figure 2 plots the degree of extraction of amorphous
than 100 nm in thickness, which formed fibrous, pyraꢀ silica against initial ammonium hexafluorosilicate conꢀ
midal, and dendritic crystalline aggregates. According centration in aqueous solution, which was found as
to spectrochemical analysis data, the (NH ) SiF was
4
2
6
а
= (аp/а0)С,
very pure: the contents of metallic impurities (Al, Fe,
Mn, Mg, Cu) were within 10–5 to 10 wt %. A second where ap is the practical yield of SiO gel, а0 is the theꢀ
–3
2
sublimation cycle reduced the content of harmful oretical yield of SiO2 gel, and
С is the amorphous silica
impurities to under 10– to 10 wt %.
8
–6
content of the SiO2 gel.
Amorphous silica nanoparticles were synthesized at
It can be seen in Fig. 2 that the degree of extraction
of amorphous silica reaches a maximum, 92.36 wt %, in
dilute (3 wt % (NH ) SiF6), nearly ideal solutions at
temperatures from 20 to 90 C in an aqueous solution of
°
ammonium hexafluorosilicate (3 to 33 wt %), which
was reacted with aqueous ammonia (25 wt % NH3) to
give a suspension at pH 8–9. The suspension was left to
stand at temperature for 0.5–1.5 h.
4
2
α
1
0
0
0
0
0
.0
Ammonium hexafluorosilicate hydrolyzes in an
aqueous alkaline solution by reaction (4) (Table 1).
Reaction (4) takes place at room temperature, with
.9
.8
.7
.6
.5
Δ
G25 = –48.7 kJ. Its Gibbs energy increases with temꢀ
perature: ΔG100 = –19.1 kJ.
Boiling down an aqueous NH4F solution leads to
ammonium bifluoride crystallization by reaction (5).
The ammonium bifluoride can be used again as a raw
material.
2
1
It is unreasonable to use ammonium hexafluorosiliꢀ
cate contents below 3 wt %, which lead to the formation
0
10
20
30
Weight percent
of difficultꢀtoꢀfilter SiO2 gel. Raising the (NH ) SiF6
4 2
content to above 33 wt % reduces the yield of the proꢀ
cess and impairs the quality of the final product. The
Fig. 2. Degree of extraction of amorphous silica against
initial ammonium hexafluorosilicate concentration in
temperature range used, 20–90
°
C, was selected
aqueous solution at (1) 25 and (2) 80°C.
INORGANIC MATERIALS Vol. 48
No. 4 2012

358
RIMKEVICH et al.
N
, %
3
3 nm
(
а)
2
1
1
0
nm
5
nm
2
0
0
0
0
5
40
20
0
1
5
0
0
40
50
nm
40
50
1
0
20 30 40 50
Particle size, nm
30
30
40
30
40
30 nm
20
2
0
20
10
1
0
20
0
10
0
10
N, %
1
7 nm
(b)
60
nm
50
nm
70
50
30
10
40
30
20
10
0
60
4
0
20
0
30
40
40
nm
3
0
nm
30
20
20
30
2
5
1
0
20
10
20
1
5
0
10
5
10 15 20 25 30
Particle size, nm
0
10
5
0
0
0
N
, %
0
0
61 nm
5
4
3
2
1
(
c)
nm
60
nm
40
0
0
0
0
20
0
20
70
6
0 60 50
60
nm
5
0
50
40
40
40
30
20
10
50
40
30
30
30
20
10
0
2
0
10
20
nm
d)
nm
0
1
0
4
0 80 120 160 200 240
Particle size, nm
0
(
N, %
8
9 nm
4
3
2
1
0
nm
00
5
3
1
0
0
0
0
2
0
0
0
100
0
1
00
140
80
120
60
100
nm
160
4
0
80
80
120
nm
20
60
40
80
0
40
40
2
0
0
4
0 80120160200240280320
Particle size, nm
0
Fig. 3. Effect of synthesis conditions on the particle size distribution and threeꢀdimensional images of amorphous silica nanoparꢀ
ticles.
80
°
C
and drops to 71.47 wt % at an (NH ) SiF content
Eа = R
Δ
ln
а/
Δ
(1/Т)
4
2
6
of 33 wt % and the same temperature. At 25
degree of extraction of amorphous silica is 82.35 wt % at
wt % (NH ) SiF and drops to 68.53 wt % as the
°
C, the
for each pair of data points in Fig. 2. The average Eа is
1
.6 kJ/mol, indicating that reaction (4) is diffusionꢀ
3
4
2
6
limited.
(
NH ) SiF content increases to 20 wt %. At this temꢀ
4 2 6
The size of amorphous silica nanoparticles was
perature, our studies of amorphous silica extraction
determined on a SOLVER R47 atomic force microꢀ
scope (Vladivostok, Russia). The nanoparticles were
suspended in highꢀpurity isopropanol placed in a disꢀ
posable plastic vessel. The suspension was applied to a
mica substrate using a mechanical pipette with replaceꢀ
able tips, which allowed us to avoid mixing of different
were limited by the (NH ) SiF solubility, which was
4
2
6
20 wt %. Thus, we identified conditions that maximized
amorphous silica extraction, which is an important
issue in the preparation of this valuable component
from various types of mineral raw materials.
From experimental data processing, we obtained the nanoparticles. In our analyses, we used an NSG 03 canꢀ
degree of extraction of amorphous silica as a function of tilever probe, with a nominal tip radius of 10 nm.
temperature in the form of an Arrhenius equation [6]:
Figure 3 illustrates the effect of hydrolysis conditions
[
reaction (4)] on the particle size distribution and preꢀ
а
=
а0ехр( а
Е /RT).
sents threeꢀdimensional images of nanoparticles. The
nanoparticles have the smallest average size (17 nm) at
The activation energy was found as
INORGANIC MATERIALS Vol. 48
No. 4
2012

SYNTHESIS AND PROPERTIES OF AMORPHOUS SiO NANOPARTICLES
359
2
0.5
0.4
0.3
1
2
3
0
.2
.1
0
4
H O
2
H O
2
0
SiO2
SiO2
500
Wavenumber, cm
3500
3000
2500
2000
1500
1000
0
–
1
Fig. 4. IR spectra of the (
of aqueous solution was (
1
) quartz sand and (2–4) amorphous silica powders prepared at 80
°C. The initial (NH ) SiF content
4 2 6
2) 33, ( ) 20, and ( ) 3 wt %.
3
4
an initial (NH ) SiF content of 3 wt % and synthesis
The spectral range 400 to 1600 cm–1 characterizes
(Fig. 3b). At a synthesis temperaꢀ the structure of atomic groups in the samples studied.
and ammonium hexafluorosilicate conꢀ Both the parent quartz sand and amorphous silica powꢀ
tents of 20 and 33 wt %, the average particle size is 61 ders had absorption bands in the range = 900–
4
2
6
temperature of 80
°
C
ture of 80
°
C
ν
–
1
(
Fig. 3c) and 89 nm (Fig. 3d), respectively. At an 1200 cm , corresponding to stretches of SiO2 tetraheꢀ
(NH ) SiF content of 3 wt %, lowering the synthesis
dra. The absorption bands in the range 720–780 cm–1
increases the average particle size are due to stretching modes of the SiO2 group, and the
4
2
6
temperature to 25
to 33 nm (Fig. 3a).
°
C
= 500 cm–1 and lower frequencies arise from
bands at
Our results demonstrate that, by varying the initial bending modes of this group.
NH ) SiF content of aqueous solution from 3 to
ν
(
3
8
4
2
6
A comparative analysis of the spectra indicates that
the amorphous silica powders prepared at 80 and low
solution concentrations consist of smaller particles in
C and high
3 wt % and the synthesis temperature from 25 to
, one can increase the specific surface area of
°
C
0°C
2
the amorphous silica powder from 92 to 508 m /g and comparison with the samples prepared at 80
°
3
the specific pore volume from 0.096 to 0.225 cm /g.
solution concentrations.
According to chemical analysis data, the silica powꢀ
ders contained trace levels of fluorine and 2.53 to
CONCLUSIONS
7
.65 wt % water. The loss on ignition ranged from 3.73
The present results demonstrate that the fluorinaꢀ
tion processing of ground quartz sand involves three
steps: chemical interaction of the raw material with
ammonium bifluoride at temperatures from 100 to
to 5.76 wt %, in agreement with previous data [7]. Specꢀ
trochemical analysis indicated that the amorphous silꢀ
ica powders contained very little harmful impurities
(less than 1 ppm by weight). The Xꢀray diffraction patꢀ
2
00
the resultant powder in the temperature range 300–
00 , resulting in the formation of highꢀpurity
°
C, which yields sintered powder; heat treatment of
terns of the powders showed an amorphous halo, with
no crystalline reflections.
5
°
C
The structure of the amorphous silica nanoparticles ammonium hexafluorosilicate; and synthesis of amorꢀ
was studied by infrared spectroscopy [8]. Figure 4 preꢀ phous silica nanoparticles from an aqueous (NH ) SiF
4
2
6
sents the IR spectra of the parent quartz sand and amorꢀ
phous silica. The line of the quartz sand lies above the
lines of the amorphous silica powders, attesting to strucꢀ
tural transformations and suggesting that the particle
size of the powders depends on synthesis conditions. All
solution at temperatures from 20 to 90 C. Synthesis
°
conditions influence the physicochemical properties of
the resultant amorphous silica and the degree of silica
extraction. We have developed an economically attracꢀ
tive process for the preparation of amorphous silica
nanoparticles from readily available, inexpensive quartz
of the IR spectra show wellꢀdefined H O stretching ( =
ν
2
–1
–1
3
250–3550 cm ) and bending (
ν
= 1600–1650 cm ) sands, which involves almost complete ammonium bifꢀ
luoride recycling.
bands.
INORGANIC MATERIALS Vol. 48
No. 4 2012

360
RIMKEVICH et al.
ACKNOWLEDGMENTS
This work was supported in part by the Russian
Foundation for Basic Research (project no. 11ꢀ05ꢀ
0357a) and the Far East Branch of the Russian
icaꢀContaining Raw Material, Russ. J. Appl. Chem.
011, vol. 84, no. 3, pp. 345–350.
. Mel’nichenko, E.I., Krysenko, G.F., Epov, D.G., and
Marusova, E.Yu., Thermal Properties of (NH ) SiF ,
,
2
4
4
2
6
0
Russ. J. Inorg. Chem., 2004, vol. 49, no. 12, pp. 1803–
806.
. Medkov, M.A., Krysenko, G.F., and Epov, D.G., Comꢀ
bined Processing of Datolite Concentrate with Ammoꢀ
nium Bifluoride, Vestn. Dal’nevost. Otd. Ross. Akad.
Nauk, 2010, no. 5, pp. 63–66.
1
Academy of Sciences (project no. 09ꢀ3Aꢀ02041).
5
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INORGANIC MATERIALS Vol. 48
No. 4
2012