Influence of Manganese doping on Structural, optical and ethanol sensing of S...
10.1007_s11082-014-0033-x
1. 1 23
Optical and Quantum Electronics
ISSN 0306-8919
Volume 47
Number 7
Opt Quant Electron (2015) 47:1751-1763
DOI 10.1007/s11082-014-0033-x
Optical and structural properties of TiO $
$_{mathbf{2}}$$ 2 nanocomposite doped
by Si and Cu at high temperature
Nasrollah Najibi Ilkhechi, Fallah Dousi,
Behzad Koozegar Kaleji & Esmaiel Salahi
2. 1 23
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4. 1752 N. N. Ilkhechiet al.
(Fujishima and Honda 1972). Nano crystalline TiO2 photo-catalysts have attracted exten-
sive interest as promising materials in view of its applications in environmental pollu-
tion control and energy storage. Also, TiO2 can catalytically decompose a large num-
ber of organic and inorganic pollutants under illumination of visible light. The photo-
catalytic activity of TiO2 depends on its present phase. There are three crystalline
forms of TiO2: anatase, rutile and brookite. Anatase phase is metastable and has the
greater photo-catalytic activity; rutile has a high chemical stability but is less active
(Reddy et al. 2002; Jing et al. 2008; Wang 2007; Hoffmann et al. 1995; Ohko et al.
1998; Hermann 1999). Anatase with large surface area, high crystallinity and nanoscaled
crystallite size exhibits a high photo-catalytic activity. Many studies have focused on
doping metal oxides such as Fe2O3 (Sahni and Reddy 2007; Feng et al. 2004; Ismail
2005), SiO2 (Yu et al. 2002; Guan 2005; Ying and Chang 2006; Hong 2003; Awate et
al. 2005; Jung and Park 2004; Yuranova et al. 2005; Wang et al. 2005), V2O5 (Begin-
Colin et al. 1996), SnO2 (Kanai et al. 2004; Liu et al. 2002), Cr2O3 (Takeuchi et al. 2000),
ZnO (Liu et al. 2004), Nb2O5 (Zhang and Reller 2002), ZrO2 (Das et al. 2002; Vishwanathan
et al. 2004; Chary et al. 2005; Fu et al. 1996; Wu et al. 2005), CuO (Colon et al. 2006; Deng
et al. 2011; Masakazu 2000; Colon et al. 2006) and precious metals, such as Pt and Ag
(Subramanian et al. 2001; Rajeshwar et al. 2001).
Balachandran et al. (2010) prepared the TiO2/SiO2 core/shell nanocomposites by sol–gel
method and showed that the addition of SiO2 could effectively suppress the particle growth
and improve the stability of TiO2 hydrosol. It was also confirmed that the titania/silica mixture
had high thermal stability, which results in the suppression of phase transformation of titania
from anatase to rutile. Also, Sikong et al. (2008) reported that the addition of SiO2 seems to
exhibit a higher photocatalytic activity than pure TiO2, because of a large surface area effect.
Sensitization of Cu doped TiO2 with eosin improved the photo-catalytic activity under
visible light irradiation (Jin et al. 2007). Hussain et al. (2012) reported that copper loaded
S–TiO2 photo-catalyst showed high specific area, small crystallite size, narrow band gap
which contribute to their high photo-catalytic activity.
In our previous research, we studied the effect of doping Si (up to 20mol%) and Zr (up
to 20mol%) on photo-catalytic behavior of titania based nanopowders at high temperature.
We found that 20% of Si and 15mol% Zr shows the most significant improvement on photo-
catalytic behavior of TiO2 under visible irradiation and also, we could stability of anatase
phase up to 1,000◦C (Najibi Ilkhechi and Koozegar Kaleji 2014).
Inthispaper,theTiO2 nanocomposite,dopedby20mol%Siand5mol%Cu,wereprepared
by sol–gel method. The effect of the dopant cations and calcination temperature on the
structure and optical properties was studied in a systematic way. The efficiency of these
samples as photocatalysts for the degradation of MO, as organic compound model, under
visible light, was investigated.
2 Experimental procedures
2.1 Preparation of the doped titania nanopowders
The preparation of precursor solution for Si and Cu doped TiO2 nanopowder is described
as follows: TiO2, CuO and SiO2 sols were prepared, separately. Titanium (IV) butoxide
(TBT=Ti(OC4H9)4, Aldrich) was selected as titanium source. 10ml of ethanol (EtOH,
Merck) and 4ml of ethyl acetoacetate, which is as a sol stabilizer, were mixed, and then
4ml of TBT was added by the rate of 1ml/min to the mixture at the ambient temperature
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5. Optical and structural properties 1753
(25◦C). The solution was continuously stirred for 1 h, followed by dropping of HNO3 as
catalyst to the solution. Deionized water was added to the solution slowly to initiate hydrol-
ysis process. Solution was aged for 24 h in order to complete all reactions. The chemical
composition of the alkoxide solution was TBT:H2O:HNO3:EAcAc:EtOH=1:8:3:0.05:5 in
volume ratio. In order to prepare SiO2 sol and CuO sol, tetraethoxysilane (Si(OC2H5)4,
Aldrich) and (Cu(NO3)2 3H2O, Merck) were dissolved in EtOH with volume ratio of
Si(OC2H5)4:EtOH=2:13 and Cu(NO3)2 3H2O:EtOH=1:6 at ambient temperature with
continuous stirring. Si was doped 10min after Cu doping under continuous stirring at room
temperature for 30min. The formed gel was dried at 100◦C for 60min. Finally, the prepared
samples were calcined at desired temperatures (500, 600, 700, 800, 900, 1,000◦C) for 2h.
2.2 Characterization methods
Samples were recorded using X-ray diffraction analysis (Philips, MPD-XPERT, λ:Cu Kα =
0.154nm). The samples were scanned in the 2θ ranging of 20◦–60◦. The average crystallite
size of nanopowders (d) was determined from the XRD patterns, according to the Scherrer
equation (1) (Najibi Ilkhechi and Koozegar Kaleji 2014)
d = kλ/β cos θ (1)
where k is a constant (shape factor, about 0.9), λ the X-ray wavelength (0.154nm), β the full
width at half maximum (FWHM) of the diffraction peak, and θ is the diffraction angle. The
values of β and θ of anatase and rutile phases were taken from anatase (1 0 1) and rutile (1 1
0) planes diffraction lines, respectively. The amount of rutile in the samples was calculated
using the following Eq. (2) (Najibi Ilkhechi and Koozegar Kaleji 2014)
XR = (1 + 0.8 (IA/IR))−1
(2)
where XR is the mass fraction of rutile in the samples, and IA and IR are the X-ray integrated
intensities of (1 0 1) reflection of the anatase and (1 1 0) reflection of rutile, respectively. The
diffraction peaks of crystal planes (101), (200), and (105) of anatase phase in XRD patterns
were selected to determine the lattice parameters of the TiO2 and doped TiO2 nanopowders.
The lattice parameters were obtained by using the Eq. 3 (Najibi Ilkhechi and Koozegar Kaleji
2014)
(Bragg’s law): 2d(h kl)sin θ = λ
(1/dhkl)2
= (h/a)2
+ (k/b)2
+ (l/c)2
(3)
where d(hkl) is the distance between the crystal planes of (h k l); λ is the wavelength of X-ray
used in the experiment; 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 anatase form, a = b = c).
Morphology of the nanopowder was observed using scanning electron microscope (SEM,
XL30 Series) with an accelerating voltage of 10–30kV. TEM imaging was carried out using
Zeiss-EM10C-80kV instrument. FTIR absorption spectra were measured over the range of
4000–400cm−1 at room temperature.
2.3 Photo-catalytic activity measurement
The photocatalytic activity was evaluated by monitoring the degradation of MO solution
under visible illumination. In each experiment, 0.08g standard sample calcined at different
temperatures was dispersed in 50ml of MO solutions with concentration of 5 × 10−6 M. A
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6. 1754 N. N. Ilkhechiet al.
Fig. 1 XRD spectra of the pure and doping TiO nanopowders at 500◦C TiO2 (T), TiO2–5%Cu (TC), TiO2–
5%Cu–20%Si (TCS)
150W lamp was used as the light source. Before the test, all powders containing MO solutions
were magnetically stirred in dark for 1 h to establish the adsorption–desorption equilibrium.
Then, the solutions were the irradiated under visible light with constant stirring rate of
450rpm. After 40min irradiation, 5ml 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 was measured between 200 and 1,000nm UV–vis spectrophotometer. The extent of
the methyl orange decomposition was determined by measuring the value of the absorbance
value at 478nm (max absorption of (MO) using a UV–vis spectrometer). The degradation
rate (g (%)) of methyl orange was calculated by the following formula:
d(%) = (A0 − At )/A0 × 100 (4)
where A0 and At represent the initial equilibrium concentration and reaction concentration
of reactant at 478nm, respectively (4) (Najibi Ilkhechi and Koozegar Kaleji 2014).
3 Results and discussion
3.1 X-ray diffraction studies of the nanopowders
Figure 1 shows the XRD patterns of pure and doped TiO2 nanopowders with 5mol% of Cu
and 20mol% of Si heat treated at 500◦C for 2h. X-ray diffraction peak at 25.3◦ corresponds
to characteristic peak of crystal plane (1 0 1) of anatase, at 27.8◦ corresponds to characteristic
peak of crystal plane (1 1 0) of rutile in nanopowders. According to XRD patterns, the pure
and doped TiO2 (T) constituted of pure anatase phase. No characteristic peaks of SiO2 or CuO
were observed in doped TiO2 nanopowders which, suggests the incorporation of Si4+ and
Cu2+ into the TiO2 lattice (Balachandran et al. 2010; Hussain et al. 2012). The calculated
crystallite sizes of anatase, calculated by Scherrer formula, are reported in Table 1. The
decrease in crystal size can be attributed to the presence of Si–O–Ti and Cu–O–Ti in the
doped TiO2 nanopowders, which inhibits the growth of crystal grains. According to Table 1,
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7. Optical and structural properties 1755
Table 1 Characterization of pure and doped TiO2 (TC, TCS) at temperature 500◦C
Sample A% R% Crystallite
size (nm)
a = b (A0) c (A0) Cell volume
(A0)3
Sm2/g
dA dR
TiO2 (T) 100 – 9.47 – 3.798 8.944 126.357 183.2
T–5%Cu 100 – 8.35 – 3.667 12.086 162.519 207.6
T–5%Cu–20%Si 100 – 6.10 – 3.748 10.612 155.880 216.8
Fig. 2 XRD spectra of the T–5%Cu (TC) nanopowders at different temperatures. a 600◦C, b 700◦C, c
800◦C, d 900◦C, e 1,000◦C
Table 2 Characterization of 5%Cu doped TiO2 (TC) at different temperature
Calcination
temperature (◦C)
A% R% Crystallite
size (nm)
a = b (A0) c (A0) Cell volume (A0)3 Sm2/g
dA dR
600 100 – 10.19 – 3.796 9.431 136.00 150.9
700 100 – 13.89 – 3.770 6.019 85.54 110.7
800 – 100 – 17.93 4.679 2.997 65.62 79.6
900 – 100 – 24.50 4.681 2.991 65.53 58.3
1,000 – 100 – 32.36 4.664 3.000 65.26 44.1
the crystallite size and surface area of pure and doped TiO2 nanopowders at temperature
500◦C was as following: T>T–5%Cu (TC)>T–5%Cu–20%Si (TCS).
The XRD patterns of 5mol% of Cu doped TiO2, nanopowders calcined at different tem-
peratures are shown in Fig 2. Samples heated at 500–700◦C show only anatase phase and
at 800–1,000◦C the samples showed rutile phase. Result show that the addition of Cu has a
promoting effect on the transformation of anatase to rutile crystalline phase (Najibi Ilkhechi
and Koozegar Kaleji 2014; Jin et al. 2007). All the samples were identified as the mixture
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8. 1756 N. N. Ilkhechiet al.
Fig. 3 XRD spectra of the T–5%Cu–20%Si (TCS) nanopowders at different temperatures. a 600◦C, b
700◦C, c 800◦C, d 900◦C, e 1,000◦C
polymorphs of anatase or rutile without any impurity phase. Which is due to the ionic radius
of Cu2+ ion (1.9, 0.58Å) approach those of Ti4+ ion (1.6, 0.66Å) in TiO2 and the Cu2+
ions will replace lattice Ti4+ ions and thus occupy lattice Ti4+ positions in the doping reac-
tive process. It is clear from Table 2 that the crystallite size increased but the surface area
decreased with increase the calcinations temperature for anatase (600◦C, 700◦C) and rutile
(800–1,000◦C) nanoparticles.
The XRD patterns of 5mol% Cu and 20mol% Si doped TiO2 nanopowders calcined at
different temperatures are shown in Fig. 3, indicated that the crystal phase composition of the
photocatalysts depended on the calcination temperature. When calcined at 600◦C, the amount
of crystallites was too little, and hence no remarkable diffraction peaks were produced in the
XRD profiles (Fig. 3a). As the calcination temperature increased to 700◦C, the difractions
(101) and (200) peaks of anatase TiO2 appeared (Fig. 3b). However, because the sample was
not well crystallized, other diffraction peaks were too weak to be indexed. The XRD patterns
for samples calcined at 800, 900 and 1,000◦C (Fig. 3c–e) showed that the TiO2 phase in
the photocatalyst transformed from the anatase to the rutile structure according to the (110),
(101), (111) and (220) peaks and the other main peaks (111) at 2 θ = 21.98◦ of cristoballite
SiO2 could also be identified in Fig. 3d. Moreover, the diffraction peaks of doped TiO2 and
SiO2 became sharper and more intense with the increase in calcination temperature, which
indicated the progress of the formation and growth of the crystallites.
The characteristic of all samples reported at Table 3. It is obvious that the surface area of
the co-doped TiO2 higher than other sample. Compared with those of TiO2 nanocomposite,
for Si doped T–5%Cu, intensity and average rutile crystallite sizes decreased. It is found that
the Si dopants can inhibit the rutile grain growth.
3.2 FTIR Analysis of pure and doped TiO2 nanopowders
Figure4showstheFTIRspectrumofthenanocrystallineTiO2 powdercalcinedattemperature
500◦Cintherangeof400–4,000cm−1.Metaloxidesgenerallygiveabsorptionbandsinfinger
print region below 1,000cm−1arising from inter-atomic vibrations.
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9. Optical and structural properties 1757
Table 3 Characterization of T–5%Cu–20%Si (TCS) at different temperature
Calcination
temperature (◦C)
A% R% Crystallite
size (nm)
a = b (A0) c (A0) Cell volume (A0)3 Sm2/g
dA dR
600 100 – 7.18 – 3.787 8.391 120.338 214.2
700 100 – 11.62 – 3.810 9.740 141.386 132.3
800 – 100 – 15.11 4.601 2.960 50.268 94.5
900 – 100 – 21.68 4.679 2.992 65.503 65.8
1,000 – 100 – 29.17 4.610 2.966 63.033 48.9
Fig. 4 FTIR spectra of pure and doped TiO2 nanoparticles calcined at temperatures 500◦C. a T, b TC, c TCS
The infrared spectra (Fig. 4) of pure and doped TiO2 exhibited the following bands:
(i) 3,442.27 and 3,332.02cm−1 due to inter molecular structure and the O–H band
(Salavati-Niasari et al. 2008).
(ii) 511.02 and 511.04cm−1 which can be attributed to the Ti–O stretching and Ti–O–Ti
binding stretching modes(Kapusuz et al. 2013).
(iii) 472.02 and 471.69cm−1 which can be attributed to the vibrations of Cu–O (Najibi
Ilkhechi and Koozegar Kaleji 2014).
(iv) Band around 1,091.29cm−1 due to Si–O–Si bending modes (Queeney et al. 2004).
3.3 Photocatalytic evaluation
The reported optical band gap (Eg) in Fig. 5 (inset) has been calculated using the UV–vis
spectra by Najibi Ilkhechi and Koozegar Kaleji (2014):
αhυ = A hυ − Eg
n
(5)
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10. 1758 N. N. Ilkhechiet al.
Fig. 5 Tauc plots of pure TiO2 (T), T–5%Cu (TC) and T–5%Cu–20%Si (TCS) nanopowders calcined at
500◦C
Fig. 6 Tauc plots of T–5%Cu (TC) nanopowders calcined at different temperatures
where hυ is the photon energy, A and n are constants. For allowed direct transition n = 1/2,
direct forbidden transition n = 3/2 and indirect allowed transition n = 2. The optical band
gap Energy (Eg) is found by extrapolating the straight line portion of (αhυ)1/2 with the
abscissa axis (hυ) in the vicinity of the fundamental optical transition for pure and doped
nanopowders. It can be seen from Tauc plots (Fig. 5) that band gap of pure TiO2 nanoparticles
is 3.12eV. Also, the values of band gap calculated from Tauc plots were found to be 2.65 and
2.83eV for TC and TCS, respectively at temperatures 500◦C. It indicates a decrease in the
energy band gap for Cu doped (inset in Fig. 5). It has been reported that metal doping could
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11. Optical and structural properties 1759
Fig. 7 Tauc plots of T–5%Cu–20%Si (TCS) nanopowders calcined at different temperatures
Fig. 8 Photocatalytic degradation of MO determined by pure and doped TiO2 nanopowders after 40min
visible irradiation
form a dopant energy level within the band gap of TiO2 (Lia et al. 2001; Li and Li 2001). The
figure shows that the Si doped TiO2 nanocomposite increased the band gap energy, because
band gap energy of SiO2 higher than TiO2 nanoparticles.
The UV–vis DRS of TC and TCS calcined at 600–1,000◦C are shown in Figs. 6 and 7,
which showed that the band gap of samples decreased with the increase of the calcination
temperature. The largest reduction band gap is observed for 5mol% of Cu doped TiO2
at temperature 1,000◦C (2eV). This large reduction band gap may be attributed to those
impurities incorporated into the host (TiO2) structure which create extra energy levels within
the band gap. When the calcination temperature is below 700◦C, the “red-shift” of absorption
edge wavelength can be attributed to the growth of TiO2 crystallites. At 800–1,000◦C, the
“red-shift” is due to the increased crystallite size and phase transformation from anatase to
rutile, leading to the decrease of band gap energy.
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Fig. 9 SEM-EDX images of pure and doped TiO2 (TC,TCS) nanopowders calcinations at 500◦C. a T, b TC,
c TCS, d EDX–TC and e EDX–TCS nanopowders
Figure 8 shows the results of photo-catalytic decomposition of MO solution caused by
degradation of MO in contact with nanopowders with Si and Cu dopants at different cal-
cination temperatures. According to Fig. 8, the order of photo-catalytic activity of TiO2
nanopowders at 40min under the visible irradiation was as following: TCS>TC>T, which
suggests that the doping enhances the photo-catalytic activity of TiO2. This enhanced pho-
tocatalytic activity is because of suppressed recombination of photogenerated electrons and
holes.
Figure 8 shows that the Near 72% of MO was decomposed in the presence of T–5%Cu
after visible irradiation for 40min, while the values in the presence of T–5%Cu–20%Si and
T are 80 and 27%, respectively, highest photo degradation of MO is for the TCS sample
calcined at a temperature of 700◦C. When Si and Cu are introduced to TiO2 nanopowders,
the TiO2 nanocomposite are different from pure TiO2 nanopowders in both physical and
chemical characteristics, such as phase types (Figs. 1, 2, 3), surface areas, crystallite size
(Tables 1, 2, 3), and chemical compositions. A optimum calcination temperature of doped
TiO2 shows higher activity than pure TiO2. When the calcination temperature of doped TiO2
increased to 700◦C the photocatalytic reaction rate tends to increase and then decreased at
higher calcination temperature, because at high calcination temperature samples have rutile
structure and gave lower degradation efficiency compared with the samples calcined at lower
temperatures. Our results are in good agreement with those obtained in a previous study. On
the basis of our previous results, a nanocomposite of two phases of the same semiconductor
and good crystallization of anatase both lead to an improvement in photo activity. Thus,
the sample calcined at 700◦C have higher crystallization and intensity of anatase phase that
which is due to the photocatalytic activity increased (Najibi Ilkhechi and Koozegar Kaleji
2014).
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13. Optical and structural properties 1761
Fig. 10 TEM micrograph of T, TC and TCS calcined at 500◦C. a T, b TC, c TCZ
3.4 SEM-EDX and TEM analysis of pure and doped TiO2 nanopowders
Figure 9 shows surface morphology of the pure and doped TiO2 nanoparticles at temperature
500◦C. Figure 9 presents the images of doped TiO2 nanoparticles which illustrate the small
size (100–200nm) of the un doped particle. This result confirmed that width of the anatase
peak diffraction from XRD indicating the smaller crystalline size at 500◦C (Fig. 1). In
addition to SEM analysis, EDX analysis was performed on powders in order to investigate
the chemical structure. The analyses revealed the existence of Ti as the main elements. The
EDX data of doped TiO2 in Fig. 9d, e shows two peaks around 4.5keV. The intense peaks
are assigned to the bulk TiO2 and the less intense one to the surface TiO2. The peaks of Cu
and Si are distinct in Fig. 9d, e at 0–2.6keV less intense peak is assigned to dopant in the
TiO2 lattices. These results confirmed the existence of cations in the solid catalysts.
The particle size and shape of pure and doped TiO2 nanoparticles, which calcined at
temperature 500◦C were investigated by TEM and shown in Fig. 10. The TEM images
illustrate that almost of the particle in spherical shape and effect of doped on particle size
was considerable. An decrease of particle size with dopant of Si and Cu, attributed to inhibited
the crystal growth. The TEM results were in good agreement with XRD data measured using
Scherrer’s equation and surface area data as presented in Table 1.
4 Conclusion
This study focused on the effects of calcination temperature and Si/Cu dopants on phase
transformation, crystallite size, and photo-catalytic activity of titania nanopowders. It was
shown that the crystal phases and crystallite sizes of doped TiO2 nanopowders. Largely
depend on the calcinations temperature. Crystalline anatase single phase was found at a
calcinations temperature range of 500–700◦C and rutile phase were formed at a temperature
of 800–1,000◦C. Also, results confirmed the existence of cristoballite phase for 20mol% Si
doped T–5%Cu at temperature 1,000◦C.
The photo-catalytic activity of the doped nanopowders is higher than that of pure TiO2
nanopowders. Si4+ and Cu2+ substitution for Ti4+ in the TiO2 lattice results in a decrease in
the rate of photogenerated electron–hole recombination that is responsible for the enhance-
ment in photo-catalytic degradation rate.
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14. 1762 N. N. Ilkhechiet al.
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