Structure, microstructure and dielectric study of (ba0.6 sr0.4)(zr0.6ti0.4)o3...
10.1007_s11082-014-9975-2
1. 1 23
Optical and Quantum Electronics
ISSN 0306-8919
Opt Quant Electron
DOI 10.1007/s11082-014-9975-2
Effect of heating rate on structural and
optical properties of Si and Mg co-doped $$
hbox {ZrO}_{2}$$ ZrO 2 nanopowders
Nasrollah Najibi-Ilkhechi, Behzad
Koozegar-Kaleji & Esmaiel Salahi
2. 1 23
Your article is protected by copyright and all
rights are held exclusively by Springer Science
+Business Media New York. This e-offprint is
for personal use only and shall not be self-
archived in electronic repositories. If you wish
to self-archive your article, please use the
accepted manuscript version for posting on
your own website. You may further deposit
the accepted manuscript version in any
repository, provided it is only made publicly
available 12 months after official publication
or later and provided acknowledgement is
given to the original source of publication
and a link is inserted to the published article
on Springer's website. The link must be
accompanied by the following text: "The final
publication is available at link.springer.com”.
4. N. N. Ilkhechi et al.
melting point (3,253◦C), it is cubic (C-ZrO2) (Morava and Smolik 2007; Castro and Reyes
2002). When zirconia nanopowders is prepared via the sol–gel method, the tetragonal phase
can be stabilized at low temperatures, depending on pH and the hydrolysis catalysts used
in the synthesis (Gomez et al. 1998). The stabilization of doped zirconia by metals in the
tetragonal and cubic phases has been earlier reported. Stabilizers such as Y2O3, MgO, CaO,
Cr2O3, Fe2O3, NiO, and CuO are commonly doped into zirconia nanopowders to promote
the retention of the high temperature polymorphs. The main aim of using these metal oxides
as dopants is to obtain zirconia in the cubic and tetragonal phases with a high BET surface.
Also, increasing the content of stabilizers favors meta-stabilization of both tetragonal and
cubic polymorphs (Ramaswamy and Bhagwat 2004; Chraska and King 2000; Bhagwata and
Ramaswamy 2003; Karagedov and Shatskaya 2006; Adamski et al. 2007; Qu and Chu 2007;
Suciu et al. 2007).
Septawendar et al. (2013) reported that the nanoparticles of calcia-stabilized zirconia
consisted of the cubic and the tetragonal phases of zirconia after calcination at 500◦C. It has
been proved that using 25% ammonia solution as a catalyst stabilizes the tetragonal phase
of zirconia (Ca-SZA, calcia-stabilized zirconia) at 500–700◦C.
Aguilar et al. (2000) showed that polymorphic tetragonal-monoclinic transformation
region was observed in zirconia-rich compositions, meaning that the critical size of the
tetragonal-monoclinic transformation decreased as SiO2 content increased.
Tan et al. (2011) proposed synthesized pure cubic zirconia nanocrystallites by a simple
method of femtosecond-pulsed laser ablation in ammonia at different pulse powers, while a
mixture of tetragonal and monoclinic zirconia was obtained in water. Jayakumar et al. (2013)
Showed that monoclinic phase is found to be stable when particle size is bigger than 20nm,
tetragonal is found to be stabilized in the range of 7–20nm and the cubic phase is stabilized
as the particle size decreases to 6nm and less.
The optical properties (French et al. 1994) and especially the photoluminescence (PL)
properties of ZrO2 have been seldom reported, although PL has already been observed in a
ZrO2 sol and nanoparticle systems (Emeline et al. 1998). Ionic sizes of dopants also influence
crystallite sizes in doped ZrO2 samples. Small crystallite sizes of ZrO2 are usually obtained
when the ionic sizes of dopants are largely different from Zr4+ (Hartridge et al. 2001). The
electronic structures of ZrO2 are also altered by dopants because of introduced energy levels
(Kralik et al. 1998). ZrO2 is a direct band gap insulator with two direct band to band transitions
respectively at 5.2 and 5.79eV. The experimental values for ZrO2 (tetragonal)band gap in
the range of 5.8–6.6eV, according to the results reported by Emeline and Serpon (2001).
The dependence of the absorption coefficient (α) on the photon energy (hυ) in the band-edge
spectral region for a direct transition is shown by a relation: αhυ = Const (hυ − Eg)1/2,
where Eg is the band gap of the solid (Pankove 1971). The electronic structures of ZrO2
are also altered by dopants because of introduced energy levels (Kralik et al. 1998). UV–Vis
spectra for various zirconia nanostructures show a sharp absorption peak centered at about
290nm.
The wide band gap has been reduced to 2.3 and 2.8eV, respectively, when 5wt.% of Fe3+
or V5+ is doped (Saadoune et al. 2003).
In this paper, ZrO2 nanopowders, doped by Si+4 and Mg+2, were prepared by sol–gel
method. The aim of the present work was to investigate the effect of cations presence at
different heating rate on the band gap energy of zirconia crystals by studying its XRD and
optical property.
123
Author's personal copy
5. Effect of heating rate on structural and optical properties of Si and Mg
2 Experimental procedures
2.1 Preparation of doped zirconia nanoparticles
The preparation of precursor solution for Si and Mg doped ZrO2 nanopowders was as fol-
lows: ZrO2, MgO and SiO2 sols were prepared, separately. For the preparation of ZrO2 sol,
ZrO(NO3) · H2O (Loba chemie) was used as precursor. 2ml Zirconyl Nitrate was added to
20ml ethanol then stirred well to synthesize pure ZrO2 sol. Ethanol was used as a media for
uniformly polymerization. After adding 3ml nitric acid (HNO3) to as catalyst to the solu-
tion, 5ml De-ionized water was added to the solution slowly to initiate hydrolysis process.
Solution was aged for 24h in order to complete all reactions.
ThechemicalcompositionoftheresultantsolutionwasZirconylNitrate:H2O: HNO3:Ethyl
aceto acetate (EAcAc):ethanol (EtOH)=1:2.5:1.5:2:10 in volume ratio. In order to prepare
MgO sol and SiO2 sol, tetraethoxysilane (Si(OC2H5)4, Aldrich) were dissolved in ethanol
with volume ratio of TEOS:ethanol = 1:8 and MgCl2:ethanol = 1:10 at ambient tempera-
ture with continuous stirring. For the co-doping ZrO2, Si doping was made for 10min after
Mg doping under continuous stirring at room temperature. Subsequently, the formed gel
was dried at 100◦C. The prepared samples were calcined at 800◦C for different heating
rate.
2.2 Characterization methods
XRD patterns and phase identification of the samples were recorded using X-ray diffraction
analysis (Philips, MPD-XPERT, λ:Cu Kα = 0.154nm). The samples were scanned in the 2θ
range of 20–70◦. The average crystallite size of nanopowders (d) was determined from the
XRD patterns, according to the Deby-Scherrer Eq. (1) (Wang et al. 2007)
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 Tetragonal and Cubic phases were taken from Tetragonal (1 0 1) and
Cubic (1 1 1) planes diffraction lines, respectively.
Microstructural characteristics such as particle size and morphology of synthesized pow-
ders were analyzed by high resolution transmission electron microscopy (TEM) (Philips
CM200) at the accelerating voltage of 100 kV.
2.3 Band gap energy measurement
The proper amounts of dispersant (HNO3) was added to 50ml distilled water, followed
by the addition of 0.01g of samples calcined at different heating rates for Z-20%Si, Z-
20%Mg and Z-20%Si-20%Mg. The pH of suspension was adjusted to a desired value and
the suspension was stirred for 30 min using a magnetic stirrer and subjected to a subse-
quent treatment in an ultrasonic bath for 60 min. The specimens were stirred again for 30
min using a magnetic stirrer. The dispersion stability of doped ZrO2 aqueous suspension
was evaluated by the absorbance of suspension using a mini1240 Shimadzu UV–visible
spectrometer.
123
Author's personal copy
6. N. N. Ilkhechi et al.
3 Results and discussion
3.1 X-ray diffraction studies of pure and doped zirconia nanopowders
XRD patterns of the doped ZrO2 nanopowders calcined at 800◦C for 2h with different heating
rates is shown in Fig. 1. Patterns show the effect of heating rate and doping on the phase
structures of ZrO2 nanopowders. All samples were identified as the mixture polymorphs
of tetragonal (JCPDS: No.050-1089), cubic (JCPDS: No.49-1642), and monoclinic phases
(JCPDS: No.80-0966). X-ray diffraction peaks at 2θ = 30.12◦, 30.27◦ and 28.1◦ are indexed
as (0 1 1), (1 1 1) and (−1 1 1) planes of tetragonal, cubic, and monoclinic phases, respectively.
In Fig. 1a, the XRD of 20%Si doped ZrO2 at different heating rate are shown.
Fig. 1 XRD pattern of a 20%Si doped ZrO2 b 20%Mg doped ZrO2 c 20%Si and 20%Mg co-doped ZrO2
at different heating rate. a 1◦C/min, b 4◦C/min, c 7◦C/min d 10◦C/min
123
Author's personal copy
7. Effect of heating rate on structural and optical properties of Si and Mg
Table 1 Characteristic of doped ZrO2 at different heating rates (d: crystallite size (nm))
Sample code 1 ◦C/min 4 ◦C/min 7 ◦C/min 10 ◦C/min
dT dM dC dT dM dC dT dM dC dT dM dC
Z-20%Si 13 — 21 12 — 27 62 26 35 — — 20
Z-20%Mg — — — 29 28 24 72 23 47 27 12 22
Z-20%Si-20%Mg 29 — 48 49 — 34 37 — 21 23 — 18
As heating rate increased, broad peaks were detected at lower heating rates became sharper
which shows the evolution of a crystalline phase up to heating rate of 7◦C/min and then
decreased. At higher heating rate (10◦C/min), the cubic zirconia was dominant and the major
phase. After annealin nanocrystalline non doped zirconia, the tetragonal phase transformed
to the monoclinic phase, a transformation that begins at approximately 450◦C and can be
completed between 800 and 1,000◦C. In the Si doped ZrO2, however, the formation of
Z(T) was detected via XRD pattern as the beginning in the thermal treatment at 800◦C and
remained up to heating rate of 7◦C/min. The presence of Z(T) and Z(M) was confirmed
by recording the XRD pattern at heating rates of 1–7◦C/min which is due to the modified
Zirconia crystallization behavior by addition of silicon oxide.
In the sample with the 20%Mg doped ZrO2 (Fig. 1b), the solubility limit for MgO in
zirconia increased and formed as cubic MgO (P) at 4–10◦C/min. The monoclinic phase
was formed at other heating rates. In Z-20%Mg sample (Fig. 1b), XRD showed a nearly
amorphous pattern, with traces of Monoclinic zirconia, indicated by a peak at 31.5◦. At
higher heating rate (7◦C/min), a substantial increase in intensity of the three phases was
observed. Results showed that the effect of Mg dopant on stability of monoclinic phase was
higher than Si.
The Z-20%Si-20%Mg (Fig. 1c) showed no periclase phase (MgO), and tetragonal and
cubic phase were deteced at all heating rates comparing to other samples (Fig. 1a, b). This
can prove that adding Si and Mg dopants inhibited the formation of cubic MgO and decreased
the intensity of monoclinic phase. Calcination of samples at higher heating rate (10◦C/min)
eliminated monoclinic phase and decreased the intensity of tetragonal and cubic phases. the
amorphous silica inhibited the formation of monoclinic and periclase phases at high heating
rates. Table 1 shows that the dependence of depending crystallite size with the heating rate.
3.2 UV–vis absorption
UV–Vis spectra were acquired in the wavelength region of 300–800nm for all samples Fig. 2
shows the UV–visible absorption spectra of doped ZrO2 nanoparticles annealed at different
heating rates. All the samples exhibit an absorption peak around 400–500nm.
To estimate the value of the direct band gap of doped ZrO2 nanoparticles from the absorp-
tion spectra, the Tauc relation was used (Manoj and Beena 2011).
(α h υ)1/n
= A(h υ −Eg) (2)
where A is a constant, Eg is the band gap of the material, α is the absorption coefficient and
the exponent ‘n’ depends on the type of transition (n =1/2 and 2 for direct and indirect band
gap semiconductors), hν is the photon energy, respectively. The best linear relationship was
obtained by plotting (αh ν)2 against photon energy (hν), indicating that the absorption edge
123
Author's personal copy
8. N. N. Ilkhechi et al.
Fig. 2 UV–Vis absorption spectra of the a 20%Si b 20%Mg c 20%Si and 20%Mg doped ZrO2 nanopowders
at different heating rate
Table 2 Band gap (eV) of doped ZrO2 at different heating rates
Sample code 1 (◦C/min) 4 (◦C/min) 7 (◦C/min) 10 (◦C/min)
Z-20%Si 3.8 3.9 3.1 4
Z-20%Mg 4.1 4 3 3.2
Z-20%Si-20%Mg 3.9 3.7 3.7 3.8
was due to a direct allowed transition. The line fit to the (αh ν)2 versus hν plot is obtained
by fitting a straight line to the linear portion of the curve. The value of optical band gap were
determined from the value of intercept of the straight line at α = 0. The values of band gap of
Si doped ZrO2 (Fig. 2a) calculated from Tauc plots were found to be 3.8, 3.9, 3.1 and 4eV for
the heating rates of 1, 4, 7, and 10◦C/min, respectively. It is clear from Tables 1 and 2 that the
band gap decreases with the increase in heating rate, while the crystallite size of tetragonal
and cubic phase increases. It has been observed that the band gap was maximum (4eV) when
the average crystallite was (20nm) at 10◦C/min and while it was minimum (3.1eV) with
average crystallite size of 41nm at 7◦C/min.
123
Author's personal copy
9. Effect of heating rate on structural and optical properties of Si and Mg
Fig. 3 SEM images of doped ZrO2. a Z-20%Si, b Z-20%Mg, c Z-20%Si-20%Mg at heating rate 7◦C/min
Theabsorptionspectraof20%MgdopedZrO2 nanoparticlesisshownFig.2b.Weobtained
the maximum band gap to be 4.1eV at heating rate of 1◦C/min. The decreasing trend hap-
pened at heating rates od 4, 7, and 10◦C/min for 20%Mg doped ZrO2 samples as 4, 3 and
3.2eV, respectively. Average crystallite for minimum band gap was (47nm) at 7◦C/min.
The band gap of the 20%Mg and 20%Si co-doped ZrO2 nanoparticles is calculated to
be 3.7–3.9eV from the UV–Visible absorption spectrum (Fig. 2c). Light absorption leads
to formation of electron in the conduction band and a positive hole in the valence band. In
small particles they are confined to potential wells of small lateral dimension and the energy
difference between the position of the conduction band and a free electron, which leads to a
quantization of their energy levels (Manoj and Beena 2011).
3.3 SEM and TEM analysis of the doped ZrO2 nanopowders
The surface morphological study of the doped ZrO2 nanoparticles was carried out using SEM
micrograph. Figure 3 shows the SEM image of the doped ZrO2 nanoparticles with heating
rate of 7◦C/min. It can be seen that the size of the co doped ZrO2 is smaller than Si or Mg
doped. Figure 3 indicates that SEM micrographs are consistent with the XRD results.
The particle morphologies of doped ZrO2 were observed by TEM, and the micrographs
are shown in Fig. 4. The Si/Mg co-doped particles are more dispersed than doped ZrO2
with particle sizes of 5–25nm (Fig. 4c). The Si/Mg doped ZrO2 nano-crystalline powders
prepared in non-aqueous system contained less hydroxyl groups than those synthesized in
aqueous system which causes less aggregation among nanocrystallites.
123
Author's personal copy
10. N. N. Ilkhechi et al.
Fig. 4 TEM images of doped ZrO2 a Z-20%Si, b Z-20%Mg, c Z-20%Si-20%Mg at heating rate 7◦C/min
4 Conclusions
Si and Mg co-doped ZrO2 nanoparticles were successfully synthesized using sol–gel method.
The XRD patterns confirm the formation of tetragonal, cubic, and monoclinic phases of ZrO2
nanoparticles. No impurity phase was observed in XRD patterns of co-doped or Si doped. In
Mg doped sample, a small amount of impurity phase corresponding to MgO (periclase) was
observed. The optical studies confirmed that the band gap of the doped samples decreases
initially up to 7◦C/min after which it started to increase at heating rate of 10◦C/min. The
Mg doping in ZrO2 improves the optical properties of the ZrO2 nanoparticles.
References
Adamski, A., Tabor, E., Gil, B., Sojka, Z.: Interaction of NO and NO2 with the surface of CexZr1−xO2 solid
solutions—influence of the phase composition. Catal. Today. 119, 114–119 (2007)
Aguilar, D.H., Torres-Gonzalez, L.C., Torres-Martinez, L.M.: A study of the crystallization of ZrO2 in the
sol–gel system: ZrO2 − SiO2. J. Solid State Chem. 158, 349–357 (2000)
Bhagwata, M., Ramaswamy, A.V.: Rietveld refinement study of nanocrystalline copper doped zirconia. Mater.
Res. Bull. 38, 1713–1724 (2003)
Castro, L., Reyes, P.: Synthesis and characterization of sol–gel Cu-ZrO2 and Fe-ZrO2 catalysts. J. Sol–Gel
Sci. Technol. 25, 159–168 (2002)
Chraska, T., King, A.H.: On the size-dependent phase transformation in nanoparticulate zirconia. Mater. Sci.
Eng. A. 286, 169–178 (2000)
Emeline, A., Kataeva, G.V., Litke, A.S., Rudakova, A.V., Ryabchuk, V.K., Serpone, N.: Spectroscopic and
photoluminescence studies of a wide band gap insulating material: powdered and colloidal ZrO2 sols.
Langmuir. 14, 5011–5022 (1998)
123
Author's personal copy
11. Effect of heating rate on structural and optical properties of Si and Mg
French, R.H., Glass, S.J., Ohuchi, F.S., Xu, Y.N., Ching, W.Y.: Experimental and theoretical studies on the
electronic structure and optical properties of three phases of ZrO2. Phys. Rev. B 49, 5133–5142 (1994)
Gomez, R., Lopez, T., Bokhimi, X., Munoz, E., Boldu, J.L., Novaro, O.: Dehydroxylation and the crystalline
phases in sol–gel zirconia. J. Sol–Gel Sci. Technol. 11, 309–319 (1998)
Hartridge, A., Krishna, M.G., Bhattacharya, A.K.: Temperature and ionic size dependence on the structure and
optical properties of nanocrystalline lanthanide doped zirconia thin films. Thin Solid Films. 384, 254–260
(2001)
Jayakumar, S., Ananthapadmanabhan, P.V., Thiyagarajan, T.K., Perumal, K., Mishra, S.C., Suresh, G., Su,
L.T., Tok, A.I.Y.: Nanosize stabilization of cubic and tetragonal phases in reactive plasma synthesized
zirconia powders. J. Mater. Chem. Phys. 140, 176–182 (2013)
Karagedov, G.R., Shatskaya, S.S.: Nature of a mechanically stimulated phase change in zirconia. Chem.
Sustain. Dev. 14, 345–353 (2006)
Kralik, B., Chang, E.K., Louie, S.G.: Structural properties and quasiparticle band structure of zirconia. Phys.
Rev. B 57, 7027–7036 (1998)
Li, Y.W., He, D.H., Cheng, Z.X., Su, C.L., Li, J.R., Zhu, M.: Effect of calcium salts on isosynthesis over ZrO2
catalysts. J. Mol. Catal. A 175, 267–275 (2001)
Manoj, S., Beena, B.: Synthesis and characterization of novel nanocrystalline zirconium (IV) tungstate semi-
conductor. J. Nano Electro. Phys. 3(1), 179–184 (2011)
Morava, P., Smolik, J.: Vapor phase synthesis of zirconia fine particles from zirconiumtetra-tert-butoxide.
Aerosol. Air. Qual. Res. 563–577 (2007)
Park, S., Vohs, J.M., Grote, R.: Direct oxidation of hydrocarbons on Cu/CeO2. J. Nat. 404, 265–267 (2000)
Qu, F.F., Chu, W.: Catalytic combustion of methane over nano ZrO2-supported copper-based catalysts. Chin.
Chem. Lett. 18, 993–996 (2007)
Ramaswamy, V., Bhagwat, M.: Structural and spectral features of nano-crystalline copper-stabilized zirconia.
Catal. Today. 97, 63–70 (2004)
Saadoune, I., Cora, F., Alfredsson, M., Catlow, C.R.A.: Computational study of the structural and electronic
properties of dopant ions in microporous AlPOs. 2. Redox catalytic activity of trivalent transition metal
ions. J. Phys. Chem. B 107, 3012–3018 (2003)
Septawendar, R., Purwasasmita, B.S., Sutardi, S.: Effect of the hydrolysis catalyst NH4OH on the preparation
of calcia stabilized zirconia with sugar as a masking agent at low temperatures. J. Aust. Ceram. Soc. 49,
101–108 (2013)
Suciu, C., Hoffmann, A.C., Vik, A., Goga, F.: Effect of calcination conditions and precursor proportions on the
properties of YSZ nanoparticles obtained by modified sol–gel route. Chem. Eng. J. 138, 608–615 (2007)
Tan, D., Lin, G., Liu, Y., Teng, Y., Zhuang, Y., Zhu, B., Zhao, Q., Qiu, J.: Synthesis of nanocrystalline cubic
zirconia using femtosecond laser ablation. J. Nanopart. Res. 13, 1183–1190 (2011)
Wang, L., Liu, Q., Chen, M., Liu, Y., Cao, Y., He, H., Fan, K.: Structural evolution and catalytic properties
of nanostructured Cu/ZrO2 catalysts prepared by oxalate gel-coprecipitation technique. J. Phys. Chem. C
111, 16549–16557 (2007)
Zhang, Q., Sheen, J., Wang, J., Wu, G., Chen, L.: Sol–gel derived ZrO2 − SiO2 highly reflective coatings. Int.
Inorg. Mater. 2, 319–323 (2000)
123
Author's personal copy
