Journal of Materials Science: Materials in Electronics (2020) 31:4058–4072
https://doi.org/10.1007/s10854-020-02953-3
Enhancement of photocatalytic activity of ­ZrO2 nanoparticles
by doping with Mg for UV light photocatalytic degradation of methyl
violet and methyl blue dyes
G. Rajesh1 · S. Akilandeswari1,2
· D. Govindarajan1 · K. Thirumalai3
Received: 23 June 2019 / Accepted: 20 January 2020 / Published online: 3 February 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
In this study, pristine ­ZrO2 (600 °C) and Mg (0.02, 0.04, 0.06, 0.08 M)-doped ­ZrO2 nanoparticles were effectively synthesized
at about 10 nm size and achieved complete degradation of methyl violet and methyl blue dyes under UV irradiation. The
calcined products were analyzed by XRD, FTIR, UV-DRS, PL, FESEM-EDX, TEM-SAED pattern and XPS techniques.
­ rO2 nanoparticles were affirmed by
The tetragonal crystal structure of the pristine Z
­ rO2 and Mg (0.02–0.08 M)-doped Z
XRD analysis. The Mg (0.08 M)-doped ­ZrO2 nanoparticles were approximately in quasi-spherical morphology and high
agglomeration was confirmed by FESEM and TEM results. The surface defects and oxygen vacancies were analyzed by
PL spectroscopy. The Mg (0.08 M)-doped ­ZrO2 nanoparticles exhibited enlarged photocatalytic activity with 94% and 90%
degradation of methyl violet and methyl blue dyes under UV irradiation.
1 Introduction
The various organic dyes are increasingly applied in many
industries such as leather tanning, textile, plastic, paper,
food, pharmaceutical, cosmetics, photo-electrochemical
cells, hair coloring, rubber and agricultural research and
so on [1–3]. These types of industries subsequently discharge their colored wastewater containing pigments and
dyes. Colored wastewater (dyes) significantly attributed to
environmental pollution and health risk [4]. Almost these
colored wastewaters are toxic and non-biodegradable. Various physical, biological and chemical techniques have been
applied for its removals such as precipitation, activated carbon, ozonation, membrane separation, Fenton, photo-Fenton
catalytic reaction, bioremediation, adsorption, reverse osmosis, coagulation and ultra-filtration [1–3, 5]. Among these
techniques, photocatalysis is the most important approach
* S. Akilandeswari
akilaphy2010@gmail.com
1
Department of Physics, Annamalai University, Annamalai
Nagar, Chidambaram, Tamil Nadu, India
2
Department of Physics, Government College for Women
(Autonomous), Tamil Nadu, Kumbakonam 612001, India
3
Department of Chemistry, Government Arts College,
Tiruvannamalai, Tamil Nadu, India
because of it’s low cost, eco-friendliness and totally oxidizes
the pollutants to ­H2O and ­CO2 [1–3].
There are many materials applied for photocatalytic degradation including T
­ iO2 [6] ZnO [7], ­WO3 [8], ­ZrO2 [9],
­SnO2 [10], ­In2O3 [11], ­Fe2O3 [12], NiO [13], CuO [14]
CdS [15], and PbS [16]. Among these, zirconium oxide
­(ZrO2) is a most attractive material. ­ZrO2 is a wide band
gap (5.0–5.5 eV) semiconductor, and this oxide material
had detected broad application in ceramics, catalysts, gas
sensors, fuel cell, solid-state electrolytes, barrier coatings
and optical devices [17–21]. It exhibits excellent thermal,
mechanical, electrical and optical properties, such as low
thermal conductivity, high hardness, high fracture toughness, high refractive index, optical transparency, high corrosion resistance and polymorphic nature [17–22]. Zirconium oxide ­(ZrO2) exhibits three kinds of polymorphs
as follows: Monoclinic (m-ZrO2), tetragonal (t-ZrO2) and
cubic (c-ZrO2). The m-monoclinic (m-ZrO2) phase is stable below at < 1170 °C, t-tetragonal (t-ZrO2) phase is stable at 1170 − 2370 °C, and c-cubic (c-ZrO2) phase is stable
at > 2370 °C, respectively [23]. These crystal phases strongly
depend on thermal condition and preparation method [24].
Moreover, the t-tetragonal phase (t-ZrO2) has strongly
improved photocatalytic activity compared to c-cubic
(c-ZrO2) and m-monoclinic (m-ZrO2) phase zirconium oxide
­(ZrO2) materials [25, 26]. Pristine zirconia is essentially
a poor oxide ion conductor at lesser temperature. Hence,
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researchers are intent to make a new material, where elevated temperature zirconium oxide ­(ZrO2) cubic/tetragonal
(high ionic conductivity) phase structures stabilized at lesser
temperature by doping [27]. Many trivalent or divalent cations such as ­Y3+, ­Ce3+ or ­Ca2+, ­Mg2+ used as suitable dopant
materials [28]. Among all dopants, magnesium (Mg) is a
crucial dopant ion because of it’s low cost and eco-friendliness. Magnesium oxide exhibits excellent photocatalytic
activity and has recently been reported the most. In addition,
the type of support plays an important role in the photocatalytic properties and for a given reaction the activity and
selectivity of the catalyst can be enhanced by the use of a
suitable support. Because of their elevated thermal stability, zirconia and ceria oxides are often applied as supports.
Based on these aspects, we have decided to study the photocatalytic role of Mg ions in mixed metal oxides [29, 30].
Bryan et al. [31] have been studied on a greatly enhanced
resistance to optical damage in LiNbO, using Mg doping.
Then, the nanostructures demonstrated an enhanced photoconductivity thus decomposed the light-induced index
changes. Magnesium-doped LiNbO, nanostructures were
also applied as substrates for integrated optical devices.
Sweeney et al. [32] investigated the threshold level for
magnesium doping strongly influencing electron spin resonance, luminescence and optical absorption. Huafu et al.
[33] found the photovoltaic effect of LiNbO, which is only
small, changed for magnesium-doped products. Powell et al.
[34] affirmed that Mg doping enhances the photoconductivity of LiNbO, by one order of magnitude. Venkatachalam
et al. [35] investigated the that doping of ­TiO2 nanoparticles with B
­ a2+ and ­Mg2+ produces increased photocatalytic
activities than that of pure ­TiO2 nanoparticles. However, the
entry of metal ions into the ­TiO2 host lattice also results in
the formation of important lattice defects (oxygen vacancies) because of the charge compensation and the ionic radii
disparity between ­Mg2+ (or ­Ba2+) and T
­ i4+, which may put
huge uncertainties to the origin of photoactivities.
In addition, energy gap, crystallite size, surface morphology and oxygen vacancies (Vo) play a crucial role in the photoluminescence and photocatalytic activity of magnesiumdoped ­ZrO2 [28].
Various techniques have been adopted to fabricate ­ZrO2
nanoparticles such as sol–gel [36], hydrothermal method
[37], micro-emulsion [38], ball milling [39], precipitation
method [40] and combustion synthesis [41]. The precipitation method is an important technique because it is easy
preparation, eco-friendly and low cost [7]. The photocatalytic activity was assessed for the photodegradation of Rhodamine B (RhB) under UV radiation [42].
In the present study, pristine Z
­ rO2 and Mg (0.02, 0.04,
0.06, 0.08 M)-doped Z
­ rO 2 nanoparticles were prepared
by a facile precipitation method. The effective synthesised nanoparticles have been applied to photocatalytic
degradation of methyl violet (MV) and methyl blue (MB)
dyes under UV irradiation. The novelty of this work provides that the substitution of Mg in Z
­ rO2 increases with
decrease in the band gap, relevant blue-shift behavior, and
shows their advantage to enhance the photocatalytic properties. There are no reports on the increase with decrease
of the optical band gap energy and good photocatalytic
properties for Mg-doped ­ZrO2 nanoparticles with different Mg concentrations, these special properties could be
used for environmental cleanup as well as display devices.
2 Experimental procedure
2.1 Materials
Zirconium chloride octahydrate ­(ZrOCl2·8H2O), magnesium acetate tetrahydrate ­(CH3COO2 Mg·4H2O), sodium
hydroxide (NaOH), methyl violet (­C 25 H 30 CIN 3 ), and
methyl blue ­(C37H27N3O9S3Na2) for all chemicals applied
were of AR grade (analytical grade-AR) purchased from
SD Fine chemical and Merck. The obtained chemicals
used without any further purification. The glass wares
used throughout experimental work was washed in acid.
Ultrapure water was used for sample preparations and
dilutions.
2.2 Synthesis of pristine ­ZrO2 and Mg (0.02, 0.04,
0.06, 0.08 M) doped Z
­ rO2 nanoparticles
Pristine ­ZrO2 and Mg-doped Z
­ rO2 nanoparticles were synthesised by a facile precipitation method. First 0.2 M (0.32 g)
of ­ZrOCl2·8H2O was dissolved in 50 mL of ultrapure water.
The aqueous solution was stirring vigorously using magnetic
stirrer for 15 min. Thereafter, magnesium acetate tetrahydrate with dissimilar mole percentage of (0.02, 0.04, 0.06
and 0.08 M) powder dissolved in 20 mL of ultrapure water
was added into the above mixtures. The aqueous solution
was magnetically stirred for 30 min at room temperature.
Then, 0.1 M (0.2 g) of sodium hydroxide is dissolved in
50 mL of ultrapure water was poured drop by drop to the
homogenous mixture to adjust the pH range 11–12. After
NaOH injection, the entire aqueous solution was slowly
changed into white precipitate. Then, the entire white precipitate solution was stirred for 5 h with 80 °C. The obtained
white gel was washed three times with using ultrapure water
and ethanol to remove unwanted ions. The white gel precipitate was allowed to heat at 100 °C C for 5 h. Finally, the
products were calcined in a muffle furnace at 600 °C for 6 h
to get the final products. A similar process was adapted to
the preparation of pristine ­ZrO2 nanoparticles.
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2.3 Characterizations
made of highly polished aluminum and built in cooling
fan at the bottom. It is provided with the magnetic stirrer
at the center. Open borosilicate glass tube of 40 cm height
and 10 mm diameter was used as a reaction vessel with the
total light exposure length of 330 mm.
The phase purity and crystalline size of the pristine
­Z rO 2 and Mg (0.02–0.08 M) doped ­Z rO 2 nanoparticles
are obtained using an X’PERT PRO XRD diffractometer with Cu Kα radiation (λ = 1.5406 Å). The functional
groups and chemical composition of the pristine ­ZrO2 and
Mg-doped ­Z rO 2 nanoparticles were obtained by FTIR
measurements using a PerkinElmer spectrometer (spectrum-1000). Raman analysis was performed by confocal
scanning spectrometer (Renishaw in VIA) with an exciting
source of wavelength 785 nm. The energy gap of the pristine ­ZrO2 and Mg-doped Z
­ rO2 nanoparticles were recorded
by UV–visible diffuse reflectance spectroscopy (Shimadzu
2450). The photoluminescence emission spectra were
obtained by Jobin-YVON, FLUOROLOG-FL3-11instrument and using 150 W xenon lamps. The surface morphology, chemical composition and particle size of pristine
­ZrO2 and Mg-doped Z
­ rO2 nanoparticles were investigated
using ZEISS Supra 40VP using field emission scanning
electron microscopy (FESEM) and energy-dispersive
X-ray spectrum (EDX). The surface morphology and particle size of the Mg (0.08 M)-doped ­ZrO2 nanoparticles
were recorded by JEOL 3010 instrument using transmission electron microscopy (TEM). The electronic states of
every element on the surface of the Mg (0.08 M)-doped
­ZrO2 nanoparticles are analyzed by X-ray photoelectron
spectroscopy (XPS) spectrum (PHI50000 Versa probe II
FEI Inc) using Al Kα (1486.6 eV) as radiation. Heber multilamp photoreactor model HML-MP 88 was used for photoreaction (Fig. 1). This model consists of eight medium
pressure mercury vapor lamps (8 W). The irradiation was
carried out using four parallel medium pressure mercury
lamps (4 × 8 W = 32 W) emitting 365 nm wavelength. It
has a reaction chamber with specially designed reflectors
2.4 Photocatalytic measurements
Photocatalytic activity of Mg (0.08 M)-doped ­Z rO 2
nanoparticles were assessed by photodegradation of
methyl violet and methyl blue (initial concentration
dyes = 3 × 10–4 M) dye aqueous solution under UV radiation. The photocatalyst degradation test was carried out
using a 8 W UV lamp with a wavelength of 365 nm. After
that, 5 mg of Mg (0.08 M)-doped ­ZrO2 catalyst was added
and the whole solution was magnetic stirred in dark room
for 30 min to permit the adsorption of dye aqueous solution on the catalyst surface. After specific time intervals,
6 mL quantity of the dyes solution was taken out and then
immediately centrifuged. The concentration of MV and
MB dye solution was analyzed using UV–Vis spectrometer
(Shimadzu, UV-1800). The maximum strong absorbance
of MV dye solution was measured at 291 and 663 nm and
the strong absorbance of MB dye solution was measured
at 312 and 624 nm.
The degradation rate of MV and MB dye was estimated
using the following equation:
The degradation efficiency =
C0 − Ct
× 100%,
C0
(1)
where C0 is the initial dyes (MV and MB) concentration, Ct is the dyes (MV and MB) concentration at a certain irradiation time (t).
Fig. 1 Schematic diagram of
Heber photoreactor
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and 4.42 nm, respectively. Table 1 shows the crystalline
size increases from 2.9, 4.25, 4.26, 4.28 and 4.42 nm with
increasing Mg dopant from 0.02, 0.04, 0.06 to 0.08 M,
respectively.
3.2 UV–Vis DRS analysis
Fig. 2 XRD pattern of pristine ­
ZrO2 and Mg (0.02, 0.04, 0.06,
0.08 M) doped Z
­ rO2 nanoparticles are calcined at 600 °C
3 Results and discussion
3.1 X‑ray diffraction analysis
The crystal phase, crystalline size and purity of the pristine ­ZrO2 and Mg (0.02–0.08 M)-doped Z
­ rO2 (600 °C)
nanoparticles were analyzed by XRD measurements. Figure 2 displays the XRD patterns of pristine ­ZrO2 and Mg
(0.02–0.08 M)-doped ­ZrO2 nanoparticles. Figure 2 shows
four main peaks at 2θ = 30°, 35°, 50° and 60° corresponding
to (101), (110), (112) and (121) tetragonal phase (JCPDS
no. 88-1007). No-phase transformation from tetragonal to
another phase was found by increasing Mg (0.02–0.08 M)
concentration. In addition, the dopant Mg ions had no consequence on the zirconium crystal structure. However, the
peak intensity of the Mg (0.02–0.08 M)-doped ­ZrO2 nanoparticles was shifted toward the lower angle side, when compared to the pristine ­ZrO2 sample, indicating the thriving
incorporation of the magnesium dopant into the zirconium
host matrix [25, 28, 43]. This may be reasonable for ionic
radius ­Mg2+ (0.72 Å) greater than that of Z
­ r4+ (0.84 Å).
Furthermore, since magnesium has the oxidation state of + 2,
it induces some oxygen vacancy in the tetragonal structure
[28, 43]. The average crystallite size (D) of both pristine
­ZrO2 and Mg (0.02–0.08 M) doped ­ZrO2 nanoparticles were
determined by the Scherrer’s relation [44].
D=
𝜅𝜆
𝛽cos𝜃
(2)
where D is the crystalline size, k = 0.9 is the fixed number,
λ is the wavelength of radiation (Cu Kα = 0.15405 nm), θ
is the Bragg angle, β is the full width half maximum. The
crystalline sizes of the pristine ­ZrO2 and Mg (0.02–0.08 M)doped ­ZrO2 NPs are determined to be 2.9, 4.25, 4.26, 4.28
The UV-DRS spectra of pristine Z
­ rO 2 and Mg
(0.02–0.08 M)-doped ­ZrO2 nanoparticles are displayed in
Fig. 3a, b. The pristine Z
­ rO2 and Mg (0.02–0.08 M)-doped
­ZrO2 (600 °C) nanoparticles have exhibited high-optical
quality in the UV region since complete reflectance in the
220 to 450 nm range. As the concentration of magnesium
ions is increased, the peaks’ intensity slightly increases with
decrease and it is significantly shifted to the lower wavelength region. Finally, increase in the concentration of magnesium ions substituted into Z
­ rO2 lattice, all dopant absorption are slight red shifted with a peak intensity slightly
narrowed in the reflectance owing to quantum confinement.
The zirconia nanoparticles exhibit a large energy gap and the
energy gap slightly increases and decreases with enhancing
Mg content owing to the formation of mid-band states upon
the incorporation of dopant ion [28]. Moreover, magnesium
ion incorporation into Z
­ rO2 host lattice thus creates oxygen
vacancies in zirconium oxide crystal [28, 45].
The energy gap considering direct transitions of pristine
­ZrO2 and Mg (0.02–0.08 M) Z
­ rO2 nanoparticles are computed using the following equation.
𝛼=
A(hv − Eg )1∕2
hv
(3)
,
where α is the absorption coefficient, A is constant, Eg is
the energy gap, ν is the light frequency, respectively. The
energy gap of the products was computed by a linear plot of
(αhν)2 versus photon energy (hν). The estimated energy gap
5.12, 5.45, 5.38, 5.28 and 5.03 eV values for pristine ­ZrO2
and magnesium (0.02–0.08 M)-doped Z
­ rO2, respectively.
Finally, it is clearly seen that the energy gap of magnesiumdoped products first increases and decreases gradually when
Table 1 The effect of Mg on the crystallite size, phase and band gap
of ­ZrO2 nanoparticles are calcined at 600 °C
S. no
Samples
Crystallite
size (nm)
Phases
Band gap
(Eg) (eV)
1
2
3
4
5
Pristine ­ZrO2
ZrO2: Mg (0.02 M)
ZrO2: Mg (0.04 M)
ZrO2: Mg (0.06 M)
ZrO2: Mg (0.08 M)
2.9
4.25
4.26
4.28
4.42
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Tetragonal
5.12
5.45
5.38
5.28
5.03
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Fig. 3 a UV-DRS reflectance spectra of pristine ­ZrO2 and Mg (0.02, 0.04, 0.06, 0.08 M) doped ­ZrO2 nanoparticles, b band gap energy of pristine ­ZrO2 and different Mg doped Z
­ rO2 nanoparticles
compared to that of pristine ­ZrO2 product. It can be clearly
illustrated in Table 1.
3.3 Photoluminescence spectroscopy
The photoluminescence analyzes provided information
about addition impurities, charge transfer, energy gap and
the surface defect. The PL emission spectra of the pristine
­ZrO2 and magnesium-doped Z
­ rO2 nanoparticles recorded
at 220 nm of excitation wavelength, it can be seen clearly
in Fig. 4. The pristine ­ZrO 2 nanoparticles show sharp
and weaker UV emission peaks at 337 nm and 279, 337,
398 nm with two broader and weaker visible emission peaks
at 419 nm and 460 nm. It can be clearly seen in Fig. 4a.
Moreover, the Mg (0.02–0.08 M)-doped ­ZrO2 nanoparticles
show sharp and weaker UV emission peaks at 331 nm and
274, 337, 395 nm with one weaker and two broader visible
emission peaks at 410 nm and 438 nm respectively. It can
be seen in Fig. 4b. The weak peak observed at 395 nm corresponded to excitation from the ionized oxygen vacancies
(F and F centers) from the CB to the VB. The weak UV
emission peak is observed owing to the radioactive recombination of photogenerated hole with an electron occupying
the oxygen vacancy (Vo) and it was related with the NBE
transition, which was due to the elevated crystal quality of
the synthesized products [25, 46]. The other weak emission peaks observed at 438 and 460 nm may be attributed to
defect states or surface defects. Occasionally, characteristic
visible emission peaks below the fundamental edge have
been observed in Z
­ rO2 which is mainly owing to the Vo [36,
42]. Generally, oxygen vacancy, oxygen anti-site, cation
vacancy, cation anti-site, cation interstitial and oxygen interstitial defects are mainly on the preparation method, thermal condition, heating atmosphere and the type of excitation
wavelength used to the sample [37, 47]. The PL emission
peak intensity of the Mg (0.08 M)-doped ­ZrO2 nanoparticles was considerably reduced compared to that of pristine
­ZrO2, signifying that defects can reduce the recombination
rate of electron–hole pairs. Additionally, magnesium-doping
­ZrO2 nanoparticles enhanced the separation of the photogenerated electron–hole pairs and the lifetime of electron–hole
pairs. Hence, the photocatalytic activity of the magnesium
(0.08 M) doping of Z
­ rO2 nanoparticle photocatalyst activities may be enhanced [25].
3.4 Functional group analysis
FT-IR spectra of pristine Z
­ rO2 and Mg (0.02–0.08 M)doped ­Z rO 2 nanoparticles are delivered in the range
4000–400 cm−1 in Fig. 5. The pristine Z
­ rO2 nanoparticles
show broad, sharp and weak absorption peaks at 3411, 2978,
2937, 1571, 1425, 1300, 961, 808, 617, 509 and 451 cm−1.
The Mg (0.02–0.08 M)-doped Z
­ rO2 (600 °C) nanoparticles
show broad, sharp and weak absorption peaks at 3701 (0.06,
0.08 M), 3445, 1638, 1449, 975, 869, 577 and 471 cm−1.
Broadband 3701.9 (0.06, 0.08 M), 3445.6 and 3411 cm−1
was owing to the stretching vibration of surface O–H group
and the interlayer H
­ 2O molecules [27, 48]. The weak and
sharp peak at 1638 and 1571 cm−1 was owing to the bending
vibration of O–H group resulting from the absorption of the
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Fig. 4 Photoluminescence spectra of pristine ­ZrO2 (a) and Mg (0.02, 0.04, 0.06, 0.08 M) ­ZrO2 (600 °C) nanoparticles (b) are calcined at 600 °C
wave number region. The small broad and weak peaks at
975, 961, 869, 808, 617, 577, 509, 471 and 451 cm−1 are
assigned to zirconium and oxygen (Zr–O) band stretching
mode [24, 25].
3.5 Raman spectroscopy
Fig. 5 FTIR spectra of pristine ­
ZrO2 and Mg (0.02, 0.04, 0.06,
0.08 M)-doped ­ZrO2 nanoparticles
­H2O molecules on the surface of ­ZrO2 [49]. The observed
peaks at 1571, 1449, 1425 and 1300 cm−1 correspond to the
zirconium hydroxide (Zr–OH) bond [25, 49]. Then, very
small peaks at 2978 and 2937 cm−1 are attributed to the
C=H vibrations and after increase in Mg concentration, the
peaks vanished completely. When Mg ion is introduced into
­ZrO2 host lattice, the absorption peaks shifted toward higher
The crystal structures and phase composition of the pristine
and Mg (0.02–0.08 M)-doped ­ZrO2 nanoparticles can be
determined by the Raman spectroscopy. The Raman spectra
of pristine and Mg (0.02–0.08 M)-doped ­ZrO2 nanoparticles
recorded in the range of 100 to 1800 cm−1 are shown in
Fig. 6. The main vibration peaks are located at 145, 259,
448, and 629 cm−1 are assigned to the vibration modes of
tetragonal ­ZrO2 [50, 51], while peaks located at 1082, 1250,
1343 and 1528 cm−1 correspond to multiphonon processes
[52, 53]. The very weak peaks can be assigned to the Ramanactive modes of B1g symmetry species at (~ 145 cm−1), Eg
at (~ 263 cm−1), B1g at (~ 312 cm−1), Eg at (~ 456 cm−1) and
Eg at (629 cm−1). The peak positions well match with the
reported Raman vibration peaks for the t-phase of Z
­ rO2 [54].
These are in well agreement with the obtained XRD results
[55]. We have not found any other peaks signal related to Mg
or its oxide phases that more confirms the well incorporation
of Mg ions into the ­ZrO2 host matrix [25].
The most significant observation in the Raman vibration
spectra of the samples is the light broadness and shifting to
a lower wavenumber side with Mg loading [56, 57]. Any
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reaction rate and charges on the particles, etc. Separation of
particles generally depends on the particle charge and concentration of dopant ions in the host matrix. When the concentration of M
­ g2+ ions exceeds the vital value, the charge
repulsion between the particles were disabled which leads
to coagulation and results in particle agglomeration [63, 64].
3.6.1 Compositional analysis
Fig. 6 Raman spectra of pristine Z
­ rO2 and Mg (0.02, 0.04, 0.06,
0.08 M)-doped ­ZrO2 nanoparticles
The chemical composition of the obtained Mg (0.08 M)doped ­ZrO2 nanoparticles were analyzed using EDX as
displayed in Fig. 7c. These analyzed result displayed the
presences of magnesium, oxygen and zirconium by the
appearance of Mg, O, Zr peaks. It indicates the purity of the
product and without any other signal present in the product. The percentage of the elements ratio are displayed in
Table 2.
3.7 TEM analysis
characteristic Raman vibration is associated with Zr-O bending and stretching vibration [57]. The Eg peaks are owing
to the symmetric bending stretching vibration of O–Zr–O
in ­ZrO2, the B1g peak is owing to the symmetric stretching
vibration of O–Zr–O [57]. The ionic size of M
­ g2+ (0.72 Å)
4+
is smaller than that of ­Zr (0.84 Å), and hence, doping of
this ion will distort the lattice structure of Z
­ rO2; since there
is charge difference between ­Mg2+ and ­Zr4+, doping of Mg
generates Vo in the lattice of ­ZrO2 to maintain the charge
neutrality [58]. If doping occurs on the substitutional position on the Z
­ r4+ site, the Zr-O-Zr bond will be disturbed
and a new Mg-O-Zr bond will be formed. Therefore, disturbance of the Zr–O–Zr bonds and the formation of new
Mg–O bonds will affect the Raman-active modes and will
result in the slight broadening and shifting of the peaks,
which is considered to be the result of increasing surface
defect states, for instance oxygen vacancies. Owing to the
generation of these Vo, the lattice is contracted and the peaks
are shifted to a lower frequency region. Perker and Siegel in
their work described that Vo is responsible for the broadening and shifting of the Raman peaks [59]. On the other hand,
some people reported that the quantum size effect has a role
to play in the shifting and peak broadening [60].
The size, shape, crystallinity and lattice interplane distance
of the Mg (0.08 M)-doped Z
­ rO2 nanoparticles were further
carried out using TEM with SAED pattern measurement.
The TEM, histograms and SAED pattern micrographs of
the Mg (0.08 M)-doped Z
­ rO2 nanopaticles are displayed in
Fig. 8. TEM images of Mg (0.08 M)-doped ­ZrO2 nanoparticles under different magnifications are displayed in Fig. 8a,
b The TEM images display the quasi-spherical in shape
and some of them place slight agglomerations. The highmagnification micrograph was displayed in Fig. 8c. These
images can clearly depict lattice fringes pattern at 0.27 nm
corresponds to the crystalline (101) plane of tetragonal
­ZrO2 [25]. The SAED image of Mg (0.08 M) doped Z
­ rO2
nanoparticles exhibits clear ring structures and then slight
crystalline spot as denoted in Fig. 8d. In addition, the Mg
(0.08 M)-doped ­ZrO2 nanoparticles had confirmed the polycrystalline behavior; it is evident from the SAED and XRD
images. The average size of the Mg (0.08 M) doped Z
­ rO2
product is calculated using the histogram. The average size
of the Mg (0.08 M) doped ­ZrO2 nanoparticles is computed
to the 4.7 nm, respectively. It can be clearly seen in Fig. 8e.
3.6 FESEM analysis
3.8 X‑ray photoelectron spectroscopy
The surface morphology, size and shape of the Mg (0.08 M)doped ­ZrO2 nanoparticles were determined by field emission
scanning electron microscope analysis. FESEM micrographs
of Mg (0.08 M) doped Z
­ rO2 nanoparticles under dissimilar magnifications images are shown in Fig. 7a, b. Photographs show that Mg (0.08 M)-doped ­ZrO2 nanoparticles are
slightly uniform, quasi-spherical particles and high agglomeration was formed [61, 62]. The agglomeration of the particles is caused by the following factors, namely impurities,
The surface composition and chemical state of the elements
in the Mg (0.08 M)-doped ­ZrO2 were characterized by XPS.
The XPS spectrum of Mg (0.08 M)-doped ­ZrO2 nanoparticles mainly shows the composition of Co, Mg, O and Zr
species can be observed from the survey, no other elements
detected, indicating the formation of zirconium oxide. It
clearly displayed in Fig. 9a. The characteristic of C 1s peak
at 285 eV, in binding energy (C=C/C–C) results arises from
the surface contamination of hydrocarbons or unreacted
13
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4065
Fig.7 a, b FESEM images, c EDX spectrum of Mg (0.08 M) doped ­ZrO2 nanoparticles
Table 2 EDX analysis
the elemental ratio of Mg
(0.08 M)-doped ­ZrO2 NPs
Element
Weight%
Atomic%
OK
Mg K
Zr L
33.04
11.89
55.06
65.40
15.49
19.11
zirconium precursor [65, 66]. Figure 9b shows the high resolution Zr 3d spectrum of the Mg (0.08 M)-doped Z
­ rO2 nanoparticles. Two main peaks’ position Zr 3­ d5/2 and Zr 3­ d3/2 are
to be found at 180.25 eV and 182.57 eV, respectively: hence,
the form of zirconium was Z
­ r4+ in zirconium dioxide [67,
68]. It is well agreed with FTIR and EDX results.
Figure 9c shows the high-resolution Mg 2p lines. The
high-resolution magnesium XPS shows a shoulder and
smaller peaks (two peaks) assigned to the 48.2 eV and
51.1 eV, respectively. One at 48.2 eV associated with
Mg(0) and the other one at 51.1 eV associated with M
­ g2+
corresponding to magnesium oxide (MgO) [69, 70]. This
result indicates the presence of ­Mg2+ valence states, typically for Mg (0.08 M) doped ­ZrO2 (600 °C) can be attributed to the presence of ­Mg2+ replacing ­Zr4+ lattice [70,
13
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Fig.8 a–c TEM different magnification images of Mg (0.08 M) doped ­ZrO2 nanoparticles, d SAED pattern and e histogram
71]. Figure 9d shows the high-resolution O 1s spectrum
of Mg (0.08 M)-doped ­ZrO2 (600 °C) nanoparticles. Two
types of oxygen peaks presented at 528.3 and 534.2 eV,
respectively. The peak position at 528.3 eV was due to
13
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Journal of Materials Science: Materials in Electronics (2020) 31:4058–4072
4067
Fig. 9 XPS spectra of Mg (0.08 M)-doped Z
­ rO2 nanoparticles, a survey spectrum, b Zr 3d, c Mg 2p and d O 1s
the lattice oxygen and second peak presented at 534.2 eV
could be due to hydroxyl oxygen [72, 73].
3.9 ESR spectroscopy
ESR is a useful tool to investigate the electronic configuration, oxidation state, site occupancy of the dopant ions more
critically and to understand the charge carrier concentration
and also to know magnetic exchange coupling at a microscopic level. Therefore, this technique is used to study the
nature of doped M
­ g2+ ions in the Z
­ rO2 lattice. The ESR
spectra of the Mg (0.08 M)-doped ­ZrO2 nanoparticles are
displayed in Fig. 10. It is clearly seen that the Mg (0.08 M)doped ­ZrO2 nanoparticles have a strong broad symmetric in
the range of g = 2.02–2.22, while a slightly sharp and weak
intense peak signal appeared at g = 4.23 in the case of Mg
(0.08 M)-doped ­ZrO2 nanoparticles. The spectrum exhibits
strong broad and slightly sharp but weak absorption peaks
associated with ferromagnetic (FM) and paramagnetic-resonant (PMR) fields at 1000 and 3900 Gauss, respectively.
A strong and broad peak signal appearing at the lesser field
is due to ferromagnetic resonance, which develops from
charge transition with the ground level of the FM domain
and a slightly sharp but narrow peak developing from the
PM states of some surface defects is observed [74, 75]. The
ESR peaks are also slightly broad, which clearly indicates
the presence of dipolar interaction among neighboring ­Mg2+
ions that lead to enhancing in the width of the ESR peaks
[76]. The ESR peak signal at g = 2.02 to 2.22 range was
reported and it can be attributed to lattice defects or oxygen
vacancies [74, 77]. These results are in good agreement in
previous oxide material results [78–80].
13
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Fig. 10 Room temperature ESR spectrum of Mg (0.08 M)-doped
­ZrO2 nanoparticles
Fig. 12 Time-dependent UV–Vis absorption spectra of the photocatalytic degradation of MB in the presence of Mg (0.08 M)-doped ­ZrO2
nanoparticles
Fig. 11 Time-dependent UV–Vis absorption spectra of the photocatalytic degradation of MV in the presence of Mg (0.08 M)-doped ­ZrO2
nanoparticles
3.10 Photocatalytic studies
The photocatalytic activities of Mg (0.08 M)-doped ­ZrO2
nanoparticles were assessed by the degradation of MV and
MB dye solution under UV light irradiation. The MV and
MB was taken as representative harmful dyes to assess the
photocatalytic performance, which displayed weak and
stronger absorption peaks at 298, 578 nm and 312, 597 nm.
It is clearly displayed in Figs. 11 and 12. The main absorption peaks at 298, 578 nm and 312, 597 nm are observed to
diminish gradually between 0 to 70 min. Once the irradiation
Fig. 13 Plot of (C/Co) versus time for the photodegradation of MV
and MB dyes, MV with Mg (0.08 M) doped ­ZrO2 and MB with Mg
(0.08 M)-doped ­ZrO2 nanoparticles under UV irradiation
process was initiated the strong absorption peaks intensity
of MV and MB dye solution gradually narrowed with different time intervals. The aqueous solution of MV and MB
dyes is irradiated for 70 min (without catalysts), the photodegradation of MV and MB dyes solution is detected < 7%
and < 6%, respectively. The Mg (0.08 M)-doped ­ZrO2 catalyst is inserted to the MV and MB dyes solution, the photodegradation of MV and MB dyes increases up to 94% and
90% for different time intervals. It is clearly displayed as in
Fig. 13.
The pristine Z
­ rO 2 Mg (0.02 M) doped Z
­ rO 2 , Mg
(0.04 M)-doped ­ZrO2 and Mg (0.06 M)-doped Z
­ rO2 were
13
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4069
used for photodegradation of MV and MB dyes under the
same conditions and the photodegradation efficiencies are
32, 66, 75 and 85% for MV and 31, 62, 74 and 83% for
MB, respectively (not shown). Among them, Mg (0.08 M)doped ­ZrO2 catalyst exhibits the highest photocatalytic activity with 94% degradation of MV dye solution. Magnesium
plays a crucial role in the enhancement of MV dye degradation. The photocatalytic activity results prove that the doping
amount of magnesium has a large influence on the catalytic
activity. From the results, the photocatalytic activity was
gradually decreased with large amount of magnesium content. This might be due to the possibility that excess magnesium may act as recombination centers and cover the active
sites on the zirconium oxide surface and thereby decrease
the efficiency of charge separation. Hence, it was crucial to
attain a balance between the active trapping sites, which preferring the inhibition of the recombination of electron–hole
pairs and less trapped parts, which foremost to lower capacity for the separation of interfacial charge transfer. From the
results, the photocatalytic activity of Mg (0.08 M)-doped
­ZrO2 catalyst was effectively improved [28].
various times to eliminate any organic/inorganic substances adsorbed on the Mg (0.08 M)-doped Z
­ rO2 catalyst surface. The recovered Mg (0.08 M)-doped Z
­ rO2 catalyst was dried in a hot air oven at 100 °C for 90 min and
applied for each run. The fresh dye solution of MV and
MB was made every run. The reusability test was repetitive up to four times while keeping all other parameters
constant. Figure 14 clearly shows the results of MV and
MB photodegradation for four runs. Mg (0.08 M)-doped
­ZrO 2 catalyst exhibits significant photostability as both
MV and MB dyes solutions. The whole photodegradation
occurs in the first, second, third and fourth cycle runs,
respectively, for 70 min.
The whole photodegradation occurs in the 1st run
(94%), 2nd run (93%), 3rd run (93%) and 4th run (90%)
for MV and 1st run (90%), 2nd run (90%), 3rd run (88%)
and 4th run (87%) for MB, respectively. The results from
recycling runs are shown in Table 3. In addition, the
results indicated that the Mg (0.08 M)-doped Z
­ rO2 prepared in the current study had fine photocatalytic stability
and permitted for possible repeat utilization. To affirm, the
mineralization of MV and MB, the photodegradation were
analyzed by chemical oxygen demand values. The percentage of COD reduction is given in Table 4. After 70 min of
irradiation with Mg (0.08 M)-doped ­ZrO2, 93.5 and 89.7%
of COD reduction is obtained. This result indicates almost
complete mineralization of the dyes.
3.11 Reusability of the catalyst
The photo-stability and reusability of the catalyst is a significant factor for commercial applications, so they optimized Mg (0.08 M)-doped ­ZrO2 catalyst was analyzed for
reusability. Photocatalytic experiments were accomplished
under the better reaction conditions using Mg (0.08 M)doped ­Z rO 2 catalyst four times. After each photoreaction, the Mg (0.08 M)-doped Z
­ rO2 catalyst was filtrated
and thoroughly washed with ethanol and ultrapure water
Table 3 Catalyst reusability
S.no
No. of cycle
% of MV removal
% of MB
removal
1
2
3
4
1
2
3
4
94
93
93
90
90
90
88
87
Table 4 COD measurement
Fig. 14 Reusability of Mg (0.08 M)-doped Z
­ rO2 on MV and MB degradation: dye concentration = 3 × 10–4 M
S. no
Time (min)
% of COD reduction
MV dye: Mg
(0.08 M)-doped ­ZrO2
% of COD reduction
MB dye: Mg
(0.08 M)-doped
­ZrO2
1
2
3
4
5
6
7
10
20
30
40
50
60
70
17
27
38
49
63
85
93
23
25
31
54
62
74
89
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Journal of Materials Science: Materials in Electronics (2020) 31:4058–4072
3.12 Active species test
and superoxide radicals further interact with the MV and
MB dyes to degrade it entirely (Eqs. 8, 9). When the Mg
ions inserted into Z
­ rO2 host lattice, new energy levels were
created within the energy gap. Improvement of photocatalytic activity for Mg (0.08 M) doped ­ZrO2 product may
be due to its reduced energy gap, large surface area and
finally electronic states. The above mechanisms for formation of radicals, degradation of dyes are summarized in
the below [25].
To find out the active species involved in photocatalytic
degradation, reactions were carried out via the trapping
experiments. A series of quenchers consisting of acryl amide
(AA), sodium azide (SA) they as a scavenger for super oxide
radicals, ammonium oxalate (AO) as a holes scavenger and
benzoic acid (BA) as a scavenger for hydroxyl radical were
added to the MV and MB solution before the addition of
nanoparticles to determine the dominant active species.
The addition of BA (hydroxyl radical scavenger) causes a
decrease in photocatalytic degradation efficiency and hence
it can be assumed that it is the main active species (Fig. 15).
On the contrary, the addition of AO (hole scavenger), and
the BA (hydroxyl radical scavenger) led to extreme decrease
in the efficiency in that order. Hence it can be concluded
that hydroxyl radical and holes are the major species in this
catalyst system influencing the photodegradation.
Mg − ZrO2 + h𝜈 → e− (CB) + h+ (VB)
(4)
O2 + e− →. O−2
(5)
h+ + OH− →. OH
(6)
h+ + H2 O → H+ +. OH−
(7)
3.13 Photodegradation mechanism
OH− + methyl violet and methyl blue → degradation products
(8)
The overall photodegradation process can be described
via the following. Under irradiation by UV light, electron ­(e−) and hole (­ h+) pairs were generated in the Z
­ rO2
(Eq. 4). These electrons simultaneously jump to the near
CB in the Mg energy levels and generate ­h+ in the VB,
and the photogenerated e­ − then react with available O
­ 2
molecules to produce superoxide radicals ( .O2−) (Eq. 5).
In addition, the photogenerated holes are permitted to
react with surface hydroxyl groups to form highly reactive hydroxyl radical (OH) (Eq. 6). The generated holes
can lead to dissociation of ­H2O molecules in the aqueous
solution, ­producing.OH− radicals (Eq. 7). These hydroxyl
.
Fig. 15 The degradation of MV and MB for Mg (0.08 M) doped Z
­ rO2
photocatalysts to various scavengers
O−2 + methyl violet and methyl blue → degradation products
(9)
The processes leading to photocatalytic degradation of
methyl violet and methyl blue and the schematic representation over the ­ZrO2 are shown in Fig. 16.
Fig. 16 Schematic representation of the photocatalytic degradation
process of MV and MB dyes in the Mg (0.08 M)-doped Z
­ rO2 catalysts process in the particle
13
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4 Conclusion
In summary, pristine ­ZrO2 and Mg-doped ­ZrO2 nanoparticles have been synthesized by a facile precipitation method.
The tetragonal crystal phase was affirmed by the XRD
study. The TEM and FESEM images affirmed that the Mg
(0.08 M)-doped ­ZrO2 nanoparticles were quasi-spherical and
slightly uniform in size. Energy gap estimated by UV-DRS
spectroscopy was detected to be 5.1, 5.45, 5.38, 5.28 and
5.03 eV, respectively. Further, under ultraviolet irradiation
92% and 89% MV and MB dyes degradation was determined
within 70 min for Mg (0.08 M)-doped ­ZrO2 photocatalyst.
The Mg (0.08 M)-doped Z
­ rO2 catalyst is found to be reusable. COD measurements affirm the complete mineralization
of MV and MB dye molecules.
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