The document discusses nanocrystalline cristobalite powders doped with 10% copper. X-ray diffraction analysis showed that pure silicon dioxide consisted of quartz at 800°C, while the doped powder was nearly amorphous. At 900°C, the doped powder showed traces of cristobalite and tridymite phases. Increased temperature caused these phases to grow, with the most rapid cristobalite growth between 1100-1200°C. Photocatalytic tests showed the doped powder had higher degradation of methyl orange under visible light compared to pure silicon dioxide.

1. 1 23
Silicon
ISSN 1876-990X
Silicon
DOI 10.1007/s12633-015-9363-y
Temperature Stability and Photocatalytic
Activity of Nanocrystalline Cristobalite
Powders with Cu Dopant
Nasrollah Najibi Ilkhechi & Behzad
Koozegar Kaleji

2. 1 23
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3. Silicon
DOI 10.1007/s12633-015-9363-y
ORIGINAL PAPER
Temperature Stability and Photocatalytic Activity
of Nanocrystalline Cristobalite Powders with Cu Dopant
Nasrollah Najibi Ilkhechi1 · Behzad Koozegar Kaleji1
Received: 16 March 2015 / Accepted: 24 September 2015
© Springer Science+Business Media Dordrecht 2015
Abstract SiO2 nanoparticles doped by 10 % mol Cu
were prepared via a sol-gel method under process control.
The effects of doping and calcination temperature on the
structural and photo-catalytic properties of SiO2 nanopow-
ders have been studied by X-ray diffraction (XRD), scan-
ning electron microscopy (SEM), transmission electron
microscopy (TEM) and UV-Vis absorption spectroscopy.
Cristobalite and tridymite crystalline phases were found
at a calcinations temperature range of 900∼1200 ◦C and
amorphous phase was formed at a temperature of 800 ◦C
for doped SiO2. The photocatalyst activity was evaluated
by photocatalytic degradation kinetics of aqueous methyl
orange (MO) under visible radiation. The results show that
the photocatalytic activity of the 10 % mol Cu doped SiO2
nanopowders have a larger degradation efficiency than pure
SiO2 under visible light at 900 ◦C temperature.
Keywords SiO2 nanopowders · Sol–gel · Degradation ·
Cu dopant
1 Introduction
The fact that SiO2 can exist in numerous crystalline and
amorphous forms, and its status as one of the most preva-
lent compounds on earth, has stimulated a long history of
Nasrollah Najibi Ilkhechi
nasrollah.najibi@gmail.com
Behzad Koozegar Kaleji
b.kaleji@malayeru.ac.ir
1 Department of Materials Engineering, Faculty of Engineering,
Malayer University, P.O.Box: 65719-95863, Malayer, Iran
experimental and theoretical investigations. It has been the
contention of Fl¨orke [1, 2] that tridymite is not realy a phase
in the system SiO2, but that it depends on the presence of
impurities for its existence. Tuttle and England [3], however,
demonstrated the formation of tridymite in the presence
of no impurity other than water. Hill and Roy [4] estab-
lished the existence of a unique stable tridymite phase in the
system SiO2, and they further pointed out the relationship
between this and metastable forms of tridymite. In a parallel
study, Hill and Roy [5] investigated chemically pure cristo-
balites and showed that highly ordered 3C cristobalite is
only formed at high temperatures, and that the phase which
is frequently formed within the tridymite stability range is
a disordered cristobalite. The latter study showed also that
the degree of order in the cristobalite structure is most easily
detected by observing variation in the cristobalite inversion
temperature.
A number of novel materials have been investigated,
from organometallics, catalysts [6], semiconductors [7], car-
bonaceous materials [8], to optical fibers, aiming to develop
a new generation of nanotechnology-based mesosystems
[9]. The development of nanostructured materials repre-
sents special significance for optical applications. Among
the materials widely investigated due to their optical prop-
erties, ion-doped silicas have proved to be competitive
candidates [10]. In addition, ion-doped silicas are very
attractive materials for selective separation and adsorption
of chemicals for sensing [11]. However, the topology and
properties of its surface are the most relevant in chemical
applications because they control how molecules interact
with and adsorb or anchor onto it. Uses of SiO2 as an
active phase-support in catalysis are numerous due to the
large surface area and the good mechanical and thermal
properties of this oxide. A SiO2 surface under equilib-
rium and normal conditions is terminated with oxygen
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4. Silicon
atoms for which dangling bonds resulting from the for-
mation of the surface are saturated with hydrogen atoms.
These surface hydroxyl groups are especially important
because they are the points on the surface onto which
molecules adsorb and react. Also, SiO2 exhibits a good
hydrophilic property for cotton textile surfaces and self
cleaning glasses [12–27]. Furthermore, cotton fabric is
an ideal place for settling and growing pathogenic bac-
teria because of its porous and hydrophilic structure. So,
antibacterial finishing is also of importance, especially in
some specific applications like medical usage. There are
many antibacterial agents used in this field, including metal
nanoparticles like silver and copper [28–35]. The latter is
the most familiar antibacterial agent used for centuries.
Like many other particles, the desired properties of cop-
per may be improved by reducing its size to nano-scale.
Hence, these nanoparticles can be developed and applied
in various new fields, such as water purification, medical
science, human tissue, antifouling and antibacterial agents,
etc. [34].
The optical properties of Cu-doped silica gels in the form
of xerogels, coatings and powders have been extensively
studied [36]. The structural evolutions of copper-doped
porous silica gels treated at different temperatures have also
been studied [37].
Roy et al. have prepared xCu-(1 - x) S iO2 (x = 0.3,
0.7) nanocomposites in bulk form by the sol-gel method
followed by hot pressing [38]. In their study the gels were
reduced in a hot presser for a few minutes at 500 ◦C. The
average diameter of the resulting copper particle ranges
from 86 to 175 ˚A.
In the present work, Cu has been selected as an additive
to increase the photocatalytic activity and stability phases
of silica at the visible light wavelengths. The nanopowders
were prepared by the sol–gel process. Developing more sta-
ble cristobalite phase SiO2 nanopowders at low temperature
was achieved in this study.
2 Experimental Procedures
2.1 Preparation of the Doped Silica Nanopowders
The preparation of the precursor solution for Cu doped SiO2
nanopowder is as follows: SiO2 and CuO sols were pre-
pared, separately. Tetraethoxysilane (TEOS = Si(OC2H5)4,
Aldrich) was selected as the silica source. 10 ml of ethanol
(EtOH, Merck) and 4 ml of ethyl acetoacetate, which is a sol
stabilizer, were mixed, and then 4 ml of TEOS was added
at the rate of 1 ml/min to the mixture at the ambient tem-
perature (25 ◦C). The solution was continuously stirred for
1 h, followed by dropwise addition of HNO3 as catalyst
to the solution. Deionized water was added to the solu-
tion slowly to initiate the hydrolysis process. The solution
was aged for 24 h in order to complete all reactions. The
chemical composition of the alkoxide solution was TEOS:
H2O: HNO3: EAcAc: EtOH = 1: 8: 3: 0.05: 5 in volume
ratio. In order to prepare the CuO sol, copper nitrate tri-
hydrate (Cu(NO3)2.3H2O, Aldrich) was dissolved in EtOH
with volume ratio of Cu(NO3)2.3H2O:EtOH = 1:6 at ambi-
ent temperature with continuous stirring. Then, a mixture
of SiO2 and CuO sols was made with mol ratios of CuO
at ambient temperature. The mol ratio of Cu in the mix-
ture was 0,10 mol %. The formed gel was dried at 100 ◦C
for 60 min. Finally, the prepared samples were calcined
at desired temperatures (800, 900, 1000, 1100, 1200 ◦C)
for 2 h.
2.2 Characterization Methods
Samples were studied using X-ray diffraction analysis
(Philips, MPD-XPERT, λ:Cu Kα = 0.154 nm). The
samples were scanned in the 2θ range of 20◦–70◦. The
average crystallite size of nanopowders (d) was deter-
mined from the XRD patterns, according to the Scherrer
Eq. 1 [39]
d = kλ/β cos θ (1)
Where k is a constant (shape factor, about 0.9), λ the X-ray
wavelength (0.154 nm), β the full width at half maximum
(FWHM) of the diffraction peak, and θ is the diffrac-
tion angle. The values of β and θ of quartz, cristobalite
and tridymite phases were taken from quartz-hexagonal
(101), cristobalite-cubic (111) and planes diffraction lines,
respectively.
The diffraction peaks of crystal planes (101), (102), (200)
and (202) of quartz and cristobalite phases in XRD patterns
were selected to determine the lattice parameters of the SiO2
and doped SiO2 nanopowders. The lattice parameters were
obtained by using the Eqs. 2–4 [39]
(Bragg’s law) : 2d(h kl) sin θ = λ (2)
(1/dh k l)2
= (h2
+ k2
+ l2
)/a2
(3)
(1/dh k l)2
= (h/a)2
+ (k/b)2
+ (l/c)2
(4)
Where d(hkl) is the distance between the crystal planes of
(h k l); λ is the wavelength of X-ray used in the exper-
iment; θ is the diffraction angle of the crystal plane (h
k l); h k l is the crystal plane index; and a, b, and c
are lattice parameters (in quartz and cristobalite forms,
a = b =c).
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5. Silicon
Fig. 1 XRD spectra of the pure
and doped SiO2 nanopowders at
800 ◦C a Pure SiO2 (S) b Cu
doped SiO2 (SC=S-10 %Cu)
Morphology of the nanopowder was observed using a
scanning electron microscope (SEM, XL30 Series) with an
accelerating voltage of 10–30 kV. TEM imaging was carried
out using a Zeiss-EM10C-80 kV instrument.
2.3 Photo-Catalytic Activity Measurement
The photocatalytic activities of the catalysts were evaluated
by the degradation of organic dyes including MO under vis-
ible light irradiation. Typically, approximately 0.08 g of cat-
alyst was added to 50 ml of aqueous MO solutions (10 ppm).
Prior to irradiation, the suspensions were stirred in the dark
for 1 h to obtain a colloidal solution. The photocatalytic
experiment was conducted at room temperature in a cylin-
drical glass vessel equipped with a magnetic stirrer under
a visible light positioned horizontally and 10 cm above
the colloid surface. The glass vessel was illuminated by
150 W lamps. The entire arrangement was placed in a box
sealed with aluminum foil to avoid the passage of light into
the box. Prior to irradiation, the colloidal solutions were
magnetically stirred in the dark to establish the absorption
equilibrium of MO in solution. Under ambient conditions
and stirring, the solution in the glass vessel was exposed
to visible light. Then, the solutions were irradiated under
visible light with a constant stirring rate of 450 rpm. After
40 min irradiation, 5 ml of supernatants were taken from
the suspension by a syringe filter unit to scan the UV–
Vis absorption spectrum. The UV–Vis absorption spectra of
samples were measured between 200 and 1000 nm with a
UV–Vis spectrophotometer.
The extent of the MO decomposition was determined by
measuring the value of the absorbance value at 478 nm [max
absorption of (MO)] using a UV–Vis spectrometer.
The degradation rate [d (%)] of MO was calculated by
Eq. 5 [39]
d(%) = (A0 − At )/A0 × 100 (5)
Where d is degradation rate, At is absorption after radiation
and A0 is absorption before radiation.
3 Results and Discussion
3.1 X-ray Diffraction Studies of the Nanopowders
Figure 1 shows the XRD patterns of pure and doped SiO2
nanopowders with 10 % mol of Cu heat treated at a tem-
perature of 800 ◦C for 2 h. Results show that the crystal
phase composition of the nanopowders depended on the cal-
cination temperature. The x-ray diffraction peak at 26.5◦
corresponds to the characteristic peak of the crystal plane
(101) of quartz, and the peak at 22.4◦ corresponds to the
characteristic peak of the crystal plane (111) of cristo-
balite in nanopowders. According to XRD patterns, the
pure SiO2 consisted of pure quartz phase at a calcina-
tion temperature of 800 ◦C but for the S-10 %Cu sample
(Fig. 1b), XRD showed a nearly amorphous pattern, with
traces of quartz silica. The results show that the addi-
tion of Cu+2 into SiO2 has a promoting effect on quartz
to cristobalite and tridymite crystalline phase transforma-
tion at a temperature of 900 ◦C (Fig. 2). No characteristic
peaks of CuO were observed in doped SiO2 nanopowders at
Fig. 2 XRD spectra of the S–20 %Cu (SC) nanopowders at different
temperatures a 900 ◦C b 1000 ◦C c 1100 ◦C d 1200 ◦C
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6. Silicon
Table 1 Characterization of 10 % Cu doped SiO2 (SC) at different temperature
Calcination temperature Phase Crystallite size (nm) a=b ( ˚A) c ( ˚A) Cell volume ( ˚A)3 S m2/g
dC dQ dT
S-800 ◦C Q – 23.14 – 5.100 5.637 146.618 97.8
SC-800 ◦C A – – – – – –
SC-900 ◦C C-T 14.11 – 20.96 7.051 7.051 350.551 183.6
SC-1000 ◦C C-T 17.64 – 29.35 7.085 7.085 355.769 146.6
SC-1100 ◦C C-T 33.60 – 13.35 7.064 7.064 352.485 100.7
SC-1200 ◦C C-T 35.29 – 17.26 6.996 6.996 342.503 73.2
Q: Quartz C: Cristobalite T: Tridymite A: Amorphous
different calcination temperature which, suggests the incor-
poration of Cu+2 into the SiO2 lattice [40, 41]. Increased
temperature to 1000 ◦C, caused slightly expanded tridymite
and cristobalite structures to be formed. The most rapid
cristobalite growth as indicated by the (111) reflection
peak intensity occurred between 1100 and 1200 ◦C tem-
peratures; however, some suggestion of partial conver-
sion of cristobalite to tridymite at lower temperatures was
observed.
Also results show that for Cu+2 doped SiO2 nanopar-
ticles a cristobalite phase is formed at low calcination
temperature.
The increase in temperature improves the crystallinity of
samples. Simultaneously, the peaks became narrower as the
temperature was increased showing the increase in crystal-
Fig. 3 Photocatalytic degradation of MO determined by SiO2
nanoparticles with 10 % mol Cu dopant after 40 min visible irradiation
at different temperatures
lite size. Crystallite size, lattice parameters and cell volume
at different calcination temperatures are shown in Table 1.
It can be seen that the characteristic peaks of cristobalite
become stronger and more intense with increasing calcina-
tion temperature. It is clear that the crystallite size increased
but the lattice parameters, cell volume, and surface area
decreased with increase in the calcination temperatures. As
the temperature increased from 800–1200 ◦C, the crystallite
size of the powder greatly increased up to 35.29 nm.
3.2 Photocatalytic Evaluation
Figure 3 shows the degradation efficiencies of MO for the
different catalysts under visible irradiation. The MO degra-
dation on the un-doped SiO2 is less in comparison with
doped SiO2 photocatalyst loaded with Cu, this degrada-
tion increases with increasing temperature (up to 900 ◦C).
About 55 % MO is degraded after 40 min irradiation in the
presence of S–10 %Cu, while 20 % MO is degraded by
un-doped SiO2.
According to Fig. 3, the degradation extended towards
the visible range, and the degradation was enhanced by
increasing temperature in the wave length of 460 nm (max
absorption edge for MO, not shown here). It was appar-
ent that doping SiO2 was highly effective on degradation
of MO solution since the pure SiO2 had photocatalytic
activity. Addition of Cu would improve visible light attrac-
tion, and hence improve the photocatalytic activity under
visible light irradiation. The maximum degradation hap-
pened at a calcination temperature 900 ◦C. Degradation of
MO generally improved with increasing crystallization and
surface area. Higher calcination temperatures could pro-
mote the crystallization and effectively remove the bulk
defects sites for the recombination of the photo-induced
electron–hole pairs.
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7. Silicon
Fig. 4 SEM-EDX images of
pure and doped SiO2 (SC)
nanopowders prepared by
sol–gel process via calcinations
at 800 ◦C a pure SiO2 b doped
SiO2 and c EDX spectra of SC
nanopowders
3.3 SEM-EDX and TEM Analysis of Pure and Doped
SiO2 Nanopowders
The SEM images of pure and doped SiO2 nanopowders cal-
cined at 800 ◦C temperature are shown in Fig. 4. It can
be clearly seen that the microstructures of the powders are
strongly affected by doping. The image of pure SiO2 pow-
ders calcined at 800 ◦C as shown in Fig. 4a is an irregular
structure comprising flakes. It can be seen from Fig. 4b that
doped SiO2 has a slightly lower particle size and porosity
compared to pure SiO2.
In addition to SEM analysis, EDX analysis was per-
formed on powders in order to investigate the chemical
structure. The analyses revealed the existence of Si as the
main element. The EDX data of doped SiO2 in Fig. 4c
shows a peak around 1.7 keV. The peaks of Cu are distinct in
Fig. 4c at 0–1.5 keV. These results confirmed the existence
of cations in the solid catalysts.
The particle size and shape of pure and doped SiO2
nanoparticles which were calcined at a temperature of
800 ◦C were investigated by TEM and are shown in Fig. 5.
The TEM images illustrate that almost all the particles have
spherical shape and the effect of doping on particle size
was considerable. A decrease of particle size with doping
of Cu, is attributed to inhibited crystal growth. The TEM
results are in good agreement with XRD data measured
using Scherrer’s equation and surface area data as presented
in Table 1.
Fig. 5 TEM micrograph of S
and SC calcined at 800 ◦C
temperature a S -800 ◦C b
SC-800 ◦C
a b
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8. Silicon
4 Conclusion
This study focused on the effects of calcination temper-
ature and Cu dopant on phase transformation, crystallite
size, and photo-catalytic activity of silica nanopowders. It
was shown that the crystal phases and crystallite sizes of
doped SiO2 nanopowders largely depend on the calcina-
tions temperature. The cristobalite and tridymite crystalline
phases were found at a calcinations temperature range of
900∼1200 ◦C and amorphous phase was formed at a tem-
perature of 800 ◦C. The photo-catalytic activity of the doped
nanopowders is higher than that of pure SiO2 nanopow-
ders. Cu+2 substitution for Si+4 in the SiO2 lattice results
in a decrease in the rate of photogenerated electron–hole
recombination that is responsible for the enhancement in
photo-catalytic degradation rate.
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