final accept-Optical and structural properties of TiO2 nanopowders with Co-Ce doping at various temperature

Optical and structural properties of TiO2 nanopowders
with Co/Ce doping at various temperature
Nasrollah Najibi Ilkhechi1 • Mahnaz Alijani2 •
Behzad Koozegar Kaleji2
Received: 1 November 2015 / Accepted: 23 January 2016
Ó Springer Science+Business Media New York 2016
Abstract In this study, preparation of 2 mol% Ce and 4 mol% Co doped TiO2
nanopowders via sol–gel process have been investigated. The effects of Co and Ce doping
and calcination temperature (475–1000 °C) on the structural and optical properties of
titania nanopowders studied by X-ray diffraction (XRD), scanning electron microscope,
transmission electron microscope and UV–Vis absorption spectroscope. XRD results
showed that, Titania rutile phase formation in ternary system (Ti–Co–Ce) was inhibited by
Ce4?
and promoted by Co4?
co-doped TiO2 in high temperatures (500–700 °C) and
61 mol% anatase composition is retained even after calcination at 800 °C. The optical
absorption spectrum indicates that the TiO2 nanoparticles have a direct band gap of
3.21 eV. But optical band gap of the doped TiO2 (2 mol% Ce and 4 mol% Co) was found
to be 3.14–3.20 eV.
Keywords Optical materials Á Sol–gel growth Á X-ray diffraction Á Ce/Co doped TiO2
1 Introduction
The titanium dioxide has been widely used in the ﬁeld of pollutant degradation and
environment protection since photo-catalytic function of titania was discovered in 1972
(Gopal et al. 1997; Mark et al. 1983). The titanium dioxide has the advantage of not only
high photo-catalytic activity, but also good acid resistance, low cost, and no toxicity, which
& Mahnaz Alijani
nasrollah.najibi@gmail.com; mahnaz.alijani@icloud.com
& Behzad Koozegar Kaleji
b.kaleji@malayeru.ac.ir
1
Faculty of Material Engineering, Sahand University of Technology, Tabriz, Iran
2
Department of Materials Engineering, Faculty of Engineering, Malayer University,
P.O. Box: 65719-95863, Malayer, Iran
123
Opt Quant Electron (2016) 48:148
DOI 10.1007/s11082-016-0435-z
Downloaded from http://www.elearnica.ir

makes the titanium dioxide become one of the best photo-catalytic agents (Weast 1984;
Kostov 1973). TiO2 can catalytically decomposed a large number of organic and inorganic
pollutants under illumination of visible light (Fujishima and Honda 1972; Wang et al.
1997; Palmer and Eggins 2002). However, depending on the structural form, the photo-
catalytic activity of TiO2 has been found to vary. As known, anatase or the mixture phase
of anatase and rutile show the highest photo-catalytic activity (Hoffmann et al. 1995; Ohko
et al. 1998). Anatase with large surface area, high crystallinity and nanoscale crystallite
size exhibits a high photo-catalytic activity.
TiO2 doped with Ce, V, Cu, Sn, Nd, Fe, Cr, or Co shows a red shift in the absorption
band compared to pure TiO2, and considerable photo-catalytic activity under visible light
irradiation (Li et al. 2005; Kubacka et al. 2007; Jin et al. 2007; Cao et al. 2009; Xie and
Yuan 2004; Bouras et al. 2007; Kudo 2007).
In the studies employing Ce doped TiO2 in powder form, Ce doping was reported to
enhance the photocatalytic activity of TiO2 powder, when used less than 0.5 %. The
positive effect of Ce and also of other dopants was suggested as decreasing the particle size
and increasing the surface area, thus providing higher adsorption; introducing new elec-
tronic states into the band gap of TiO2, and separation of the charge carriers (Caimei et al.
2006; Stengl et al. 2009; Chen and Mao 2007).
Zhang and Liu (2008) found cerium doping could prohibit the recombination of the
photogenerated electron–hole pairs. Yan et al. (2006) reported preparation of Ce-doped
titania through sol–gel auto-igniting process. It exhibited strong absorption in the UV–Vis
range and a red-shift in the band-gap transition. However it was also reported that
superﬂuous dopants would act as recombination center (Paola et al. 2002). More recently,
Co doped TiO2 generates a wide interest as diluted magnetic semiconductor (DMS)
because of its ferromagnetic behavior above room temperature for low Co doping con-
centration and it exhibits the Curie temperature, TC—650 K (Janisch et al. 2005; Shinde
et al. 2003; Fukumura et al. 2004). This feature makes it a promising candidate for
fabricating various spintronic devices.
In this paper, the TiO2 nanocomposite, doped by 2 mol% Ce and 4 mol% Co, were
prepared 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.
2 Experimental procedures
2.1 Preparation of the nanopowders
The preparation of precursor solution for Co and Ce doped TiO2 nanopowder is described
as follows: TiO2, Ce2O3 and CoO2 sols were prepared, separately. Titanium (IV) butoxide
(TBT = Ti(OC4H9)4, Aldrich) was selected as titanium source. 10 ml of ethanol (EtOH,
Merck) and 4 ml of ethyl acetoacetate, which is as a sol stabilizer, were mixed, and then
4 ml of TBT was added by the rate of 1 ml/min to the mixture at the ambient temperature
(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
hydrolysis 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 Ce2O3 sol and CoO2 sol, cerium nitrate
hexahydrate (Ce(NO3)3Á6H2O, Merck), cobalt nitrate (Co(NO3)2Á6H2O, Merck) were
148 Page 2 of 9 N. N. Ilkhechi et al.
123

dissolved in EtOH with volume ratio of Co(NO3)2Á6H2O: EtOH = 1:30 and Ce(NO3)3-
6H2O: EtOH = 1:35 at ambient temperature with continuous stirring. Ce was doped
20 min after Co doping under continuous stirring at room temperature for 40 min. The
formed gel was dried at 100 °C for 60 min. Finally, the prepared samples were calcined at
desired temperatures (475, 600, 700, 800, 900, 1000 °C) for 2 h.
2.2 Characterization methods
Samples were recorded using X-ray diffraction analysis (Philips, MPD-XPERT, k:Cu
Ka = 0.154 nm). The samples were scanned in the 2h ranging of 20°–70°. The average
crystallite size of nanopowders (d) was determined from the XRD patterns, according to
the Scherrer equation (Klug and Alexander 1974)
d ¼ kk=b cos h ð1Þ
where k is a constant (shape factor, about 0.9), k the X-ray wavelength (0.154 nm), b the
full width at half maximum (FWHM) of the diffraction peak, and h is the diffraction angle.
The values of b and h 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 equation (Klug and Alexander 1974)
XR ¼ 1 þ 0:8 IA=IRð Þð ÞÀ1
ð2Þ
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) (Klug
and Alexander 1974)
Bragg’s lawð Þ : 2dðhklÞ sin h ¼ k; 1=dhklð Þ2
¼ h=að Þ2
þ k=bð Þ2
þ l=cð Þ2
ð3Þ
where d(hkl) is the distance between the crystal planes of (h k l); k is the wavelength of
X-ray used in the experiment; h 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 10–30 kV. TEM imaging was carried
out using Zeiss-EM10C-80kV instrument.
Nitrogen adsorption isotherms were measured at 77 K using a N2 adsorption analyzer
(Micromeritics, ASAP 2020). The Brunauer, Emmett, and Teller (BET) model was used to
estimate the surface area of the samples according to the N2 adsorption data.
2.3 Band gap energy measurement
The proper amounts of mentioned dispersant (HNO3) was added to 50 ml distilled water
followed by the addition of 0.01 g of samples calcined at different temperature for TiO2
and T-2 %Ce-4 %Co. pH of suspension was adjusted to a desired value, then the sus-
pension was stirred for 30 min using a magnetic stirrer and subjected to a subsequent
treatment in an ultrasonic bath for 60 min. The specimens were stirred again for 30 min
using a magnetic stirrer. Moreover, the dispersion stability of doped and pure TiO2 aqueous
Optical and structural properties of TiO2 nanopowders with Co… Page 3 of 9 148
123

suspension was evaluated by the absorbance of suspension using a model mini1240 Shi-
madzu UV–Vis spectrometer.
3 Results and discussion
3.1 X-ray diffraction studies of the nanopowders
Figure 1 shows the XRD patterns of pure and doped TiO2 (T) samples calcined at
475–1000 °C for 2 h. According to the XRD patterns, the pure (T) and doped TiO2 were
crystallized in anatase phase and there are no other characterization peaks of impurities in
samples within the detection of X-ray diffractometry. By comparing the relative intensity
of the diffraction peaks, it can be seen that the intensity of (101) plane decreased and the
peak position (2h) is decreased after doping which indicates that dopant cations are suc-
cessfully doped into TiO2 crystal lattice. For the TCeCo samples calcined at 475–700 °C,
only the main peak of the TiO2 anatase phase, ca. 25.2° (2h), was barely observable. The
diffractogram of the TCeCo-800 °C reference compound presents two crystalline phases:
anatase and rutile. When calcination temperature was increased to 900–1000 °C only the
rutile phase was identiﬁed.
The calculated crystallite size and lattice parameter of anatase, calculated by Scherrer
formula, are reported in Table 1. Based on the Table 1, average crystallite size and lattice
parameters are related to different cation dopants. It clearly shows that the average crys-
tallite size is decreased from 21.8 to 15.9 nm by the addition of Ce and Co dopant. The
Fig. 1 XRD pattern of pure and Co/Ce co-doped TiO2 at different temperatures (475–1000 °C)
123

decrease in crystallite size can be attributed to the presence of Ce–O–Ti and Co–O–Ti in
the Ce and Co doped TiO2 nanopowders which inhibits the growth of crystal grains.
The Ce6?
radius (0.825 A˚ ) is bigger than Ti4?
radius (0.66 A˚ ) but Co4?
radius (0.58 A˚ )
is smaller than Ti4?
and both factors could led to slight induced stress in TiO2 lattice. Thus,
the c lattice parameters increase relative to that of TiO2. Based on the data in Table 1,
surface area of the Co and Ce co-doped nanopowders is higher than pure TiO2.
It is clear that the crystallite size was increased but the lattice parameters, cell volume,
and surface area has decreased with increase the calcination temperatures for anatase
(475–800 °C) and rutile (900–1000 °C) nanoparticles. The crystallite size of the anatase
increased from 21.8 to 39.1 nm when the temperature was raised to 700 °C. Table 1 shows
that after calcination of the T-2 %Ce-4 %Co sample at 475–1000 °C, a minimum surface
area of 26.677 m2
/g was measured. It is common that the surface area decreases at ele-
vating temperatures. With increasing temperature, the particles are simply growing to
reduce their free energy (i.e. maximizing the volume to surface ratio). They may also shift
from being more amorphous to more crystalline in the process.
3.2 Optical evaluation
For the study of the optical properties of the synthesized TiO2 nanoparticles, the band gap
and the type of electronic transition were determined, which were calculated by means of
the optic absorption spectrum. When a semiconductor absorbs photons of energy larger
than the gap of the semiconductor, an electron is transferred from the valence band to the
conduction band where there occurs an abrupt increase in the absorbency of the material to
the wavelength corresponding to the band gap energy. The relation of the To estimate the
value of the direct band gap of TiO2 nanoparticles from the absorption spectra we used the
Tauc relation given below (Rajeshwar et al. 2001)
ahtð Þ1=n ¼ A ht À Egð Þ ð4Þ
where a is absorption coefﬁcient, A a constant (independent from m) and n the exponent
that depends on the quantum selection rules for the particular material. A straight line is
obtained when (ahm)2
is plotted against photon energy (hm), which indicates that the
absorption edge is due to a direct allowed transition (n = 1/2 for direct allowed transition).
The TiO2 is activated with photons of energy of a longitude close to 400 nm which
involves a band gap of 3.2 eV; the literature reports a 3.23 eV value for anatase phase.
Table 1 Characteristic of pure and doped TiO2 at different temperature (d crystallite size (nm), %A amount
of anatase phase, %R amount of rutile phase, da crystallite size of anatase, dr crystallite size of rutile, a &
b & c lattice parameter of samples, Vu.c unit cell volume, BET: speciﬁc surface area of powders)
Sample %A %R dA
(nm)
dR
(nm)
a = b
(A˚ )
c (A˚ ) Vu.c
(A˚ )3
BET
(m2
/g)
T-475 °C 100 – 21.8 – 3.822 10.610 155.71 71.4
TCe(2 %)Co(4 %)-475 °C 100 – 15.9 – 3.487 9.680 117.70 98.1
TCe(2 %)Co(4 %)-600 °C 100 – 35.7 – 3.806 11.180 161.94 43.6
TCe(2 %)Co(4 %)-700 °C 100 – 39.1 – 3.813 10 145.38 39.8
TCe(2 %)Co(4 %)-800 °C 61 39 35.5 35.6 3.822 10.54 153.96 42.1
TCe(2 %)Co(4 %)-900 °C – 100 – 43.77 4.384 2.948 167.03 32.2
TCe(2 %)Co(4 %)-1000 °C – 100 – 47.57 4.582 2.961 62.16 29.6
123

The values of band gap of Co and Ce co-doped TiO2 (Fig. 2) calculated from Tauc plots
were found to be 3.20, 3.18, 3.17, 3.15, 3.14 and 3.13 eV for the heating rates of 5 °C/min at
different temperature. It is vision from Table 2 that the band gap decreases with the increase
in temperature, while the crystallite size of anatase and rutile phase increases. It has been
observed that the band gap was maximum (3.21 eV) when the average crystallite was
(21.8 nm) at 475 °C and while it was minimum (3.14 eV) with average crystallite size of
47.57 nm at 1000 °C. A signiﬁcant decrease can be assigned to absorption of light caused by
the excitation of electrons from the valence band to the conduction band of titania.
3.3 SEM and EDX analysis of pure and doped TiO2 nanopowders
The surface morphological study of the TiO2 photocatalyst was carried out using SEM
images. Fig. 3 shows the SEM images of pure and doped TiO2 nanoparticles. It can be seen
that the aggregated packing of doped TiO2 nanoparticles was formed at 475 °C. It can be
seen from Fig. 3b that the doped TiO2 calcined at 475 °C have slightly lower particles size
than pure TiO2 (Fig. 3a). Also, this image shows the uniform particles which are
agglomerated together. It can be clearly seen that the microstructures of the powders are
strongly affected by doping and calcination temperatures which is due to aggregation of
particle size. The EDX data of doped TiO2 in Fig. 3c shows two peaks around 4.5 keV.
Fig. 2 UV–Vis absorption spectra of pure and doped TiO2 nanopowders at different temperature
Table 2 Band gap (eV) of pure
and doped TiO2
at different
temperature (Eg
optical band gap,
kedge
wavelength absorption
edge)
Sample kedge (nm) Eg (eV)
T-475 387 3.21
TCe(2 %)Co(4 %)-600 °C 388 3.20
TCe(2 %)Co(4 %)-700 °C 390 3.18
TCe(2 %)Co(4 %)-800 °C 391 3.17
TCe(2 %)Co(4 %)-900 °C 394 3.15
TCe(2 %)Co(4 %)-1000 °C 395 3.14
123

The intense peaks are assigned to the bulk TiO2 and the less intense one to the surface
TiO2. The peaks of Co and Ce are distinct in Fig. 3c at 5–8 keV. The less intense peak is
assigned to dopant in the TiO2 lattices. These results conﬁrmed the existence of cations in
of the solid catalysts.
4 Conclusions
This study focused on the effects of calcination temperature and Ce and Co dopants on
phase transformation, crystallite size, and optical properties of Titania nanopowders. The
nanopowders were prepared from precursor solutions via sol–gel method and calcinations
at temperature range of 475–1000 °C. Crystallite size of pure TiO2 tends to increase at
higher calcination temperatures. Doping Ce and Co in TiO2 was effective on crystal phases
the nanopowders. Optical properties of TiO2 is greatly inﬂuenced by its crystallinity, grain
size, surface areas, and surface hydroxyl content. Co and Ce inhibited the growth of
Fig. 3 SEM images of pure (a), doped TiO2 (b), and EDX (c) of doped TiO2 calcination temperature at
475 °C
123

crystallite size of anatase and the amorphous anatase transformation as well as the sub-
sequent anatase–rutile transformation.
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