Magnetically Separable MnxZn1-xFe2O4; (0.0 ≤ x ≤ 0.5) Nanostructures: Structural, Morphological, Opto-Magnetic, and Photocatalytic Properties

Article abstract of DOI:10.1080/15533174.2015.1004454

Mn²⁺ doped ZnFe₂O₄ nanocrystals have been successfully synthesized by one-pot autocombustion method. The synthesized Mn_xZn_1-xFe₂O₄ nanocrystals were characterized by XRD, FT-IR, HR-SEM, TEM, EDX, BET, DRS, PL, and VSM. The results indicated that the prepared spinel Mn_xZn_1-xFe₂O₄ nanocrystals showed high crystallinity and uniform size distribution with particle-like morphologies. The present study leads to enhance the photocatalytic activity of Mn_xZn_1-xFe₂O₄ with the addition of TiO₂. Having established that spinel Mn_xZn_1-xFe₂O₄ was photocatalytically active, the mixed oxide catalysts of Mn_xZn_1-xFe₂O₄ -TiO₂ were also tested for the photocatalytic degradation of 4-chlorophenol.

Full text of DOI:10.1080/15533174.2015.1004454

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Magnetically separable Mn Zn Fe O ; (0.0 ≤ x ≤
x
1-x
2 4
0.5) nanostructures: Structural, morphological,
opto-magnetic and photocatalytic properties
A. Manikandan & S. Arul Antony
To cite this article: A. Manikandan & S. Arul Antony (2015): Magnetically separable Mn Zn1-
x
Fe O ; (0.0 ≤ x ≤ 0.5) nanostructures: Structural, morphological, opto-magnetic and
x
2
4
photocatalytic properties, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-
Metal Chemistry, DOI: 10.1080/15533174.2015.1004454
To link to this article: http://dx.doi.org/10.1080/15533174.2015.1004454
Accepted author version posted online: 22
Dec 2015.
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Date: 30 December 2015, At: 03:51

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Magnetically separable Mn Zn Fe O ; (0.0 ≤ x ≤ 0.5) nanostructures: Structural,
x
1-x
2
4
morphological, opto-magnetic and photocatalytic properties
1
,*
1,#
A. Manikandan , S. Arul Antony
1
PG and Research Department of Chemistry, Presidency College, Chennai 600 005, India
*
Corresponding author Email: mkavath15@gmail.com
#
Corresponding author E-mail: antonypresidency15@gmail.com
Abstract
2
+
Mn doped ZnFe O nano-crystals (NCs) have been successfully synthesized by one-pot
2
4
auto combustion method. The as-synthesized Mn Zn Fe O NCs were characterized by XRD,
x
1-x
2
4
FT-IR, HR-SEM, TEM, EDX, BET, DRS, PL and VSM. The results indicated that the as-
prepared spinel Mn Zn Fe O NCs showed high crystallinity and uniform size distribution with
x
1-x
2
4
particle-like morphologies. The present study leads to enhance the photocatalytic activity of
Mn Zn Fe O with the addition of TiO . Having established that spinel Mn Zn Fe O was
x
1-x
2
4
2
x
1-x
2
4
photo-catalytically active, the mixed oxide catalysts of Mn Zn Fe O -TiO were also tested for
x
1-x
2
4
2
the photocatalytic degradation (PCD) of 4-Chlorophenol (4-CP).
Keywords
Ferrites; Nano-crystals; X-ray diffraction; Opto-Magnetic properties; Photo-catalyst
1
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1
. Introduction
Recently, spinel-type nanostructured semiconducting magnetic materials with the general
2
+
3+
2+
2+
2+
2+
2+
2+
formula M (Fe ) O : where M = Mn , Co , Ni , Cu , Zn , etc. have become an important
2
4
area in nano-science and nanotechnology, due to their unique physical and chemical properties
than that of their bulk counterparts [1]. These properties are exploited in technological
applications like magnetic devices, switching devices, microwave absorbers, active components
of ferrofluids, permanent magnets, color imaging, magnetic refrigeration, biomedicine, drug
delivery, magnetic resonance imaging (MRI), anti-cancer drugs, magnetic cell separation,
recording media and photo-catalyst [2-10].
ZnFe O is a normal spinel structure and has been reported as a semiconducting material,
2
4
2
+
3
where Zn preferably occupies the tetrahedral (A-) sites, due to their affinity for sp bonding
with O  anions leaving all the Fe ions on the octahedral (B-) sites. Also, ZnFe O is
2
3+
2
4
paramagnetic in nature at room temperature [11], due to weak super exchange interaction
3
+
3+
attributed to 90º angle in Fe -O-Fe [12]. ZnFe O is soft ferrite and are very useful in
2
4
microwave applications, high quality filter, radio frequency circuits, transformer cores,
read/write heads for high speed digital tapes and photo-catalyst [9]. Various techniques have
been used for the preparation of nano-crystalline spinel ferrites, such as co-precipitation, sol-gel,
mechanical alloying, hydrothermal, ball milling and ultra sonic cavitation [9, 13-15] methods.
Rath et al. [16] have reported the dependence on cation distribution of particle size, lattice
parameter and magnetic properties in nanosized Mn-Zn ferrite with different concentration of Zn
content by a hydrothermal precipitation method. Auzans et al. [17] have reported the properties
of Mn-Zn ferrite nanoparticles used for ionic and surfacted ferrofluids with different ratios of Zn
2
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content by co-precipitation method. Azam [18] has reported the effect of Co doping on structural
and electrical properties of Co Cu Fe O (0.0 ≤ x ≤ 0.5) nano-particles by sol-gel method. Azam
x
1-x
2
4
3
+
et al. [19] have reported the structural, optical and transport properties of Al doped BiFeO3
were synthesized by solution combustion method using citric acid as fuel at a temperature as low
as 200 ºC. Gozuak et al. [20] and Koseoglu [21] have reported the structural, electrical and
magnetic properties of Co Zn Fe O and Mn Ni Fe O nanoparticles respectively, were
x
1-x
2
4
x
1-x
2
4
prepared by a PEG-assisted hydrothermal method.
However, the above conventional methods meet some disadvantages such as, the
requirement of complicated equipment, high-energy consuming, and higher processing
temperature and also require rather long reaction time caused by the multiple steps (calcinations)
to complete the crystallization of ferrite nanostructures. For this reason, the introduction of a
simple one-pot urea-assisted auto combustion method is advantageous for the synthesis of
various ceramic powders, because the formation of nano-crystals, rapid processing time and low
energy cost [22-24]. The spinel ferrite structure allows, introduction of different metal ions,
which can change the optical, magnetic and electric properties considerably and these properties
of the ferrites depend on the distribution of the cations in A- and B- sites and on the range of the
deficiency in the chemical composition [25]. The magnetic properties of ferrites strongly depend
on the kind and content of substituting metal ions.
ZnFe O has potential applications in the conversion of sunlight and sensitive towards
2
4
2
+
3+
visible light [12]. The conduction mechanism in ZnFe O is due to the Fe and Fe ions present
2
4
in B- sites as suggested by Feteira et al. [26]. Mostly, nano-sized ZnFe O has been extensively
2
4
studied by universal researchers, because of its fashionable size dependent physical and chemical
3
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properties as compared to bulk materials [1]. Among the other semiconductors, ZnFe O appears
2
4
to be a highly promising photo-catalyst [9]. Many perovskite or spinel type complex oxides have
been found to have visible-light-driven photoactivity. Among them, spinel ZnFe O with a
2
4
relatively narrow band gap energy of 1.9 eV, concerned significant interest in the conversion of
solar energy, photochemical hydrogen production from water and photo-catalysts, due to its
visible-light response, good photochemical stability and low cost [27, 28]. The PCD of 4-chloro
2
+
phenol (4-CP) has been investigated for different concentration of Mn doped ZnFe O catalyst
2
4
with TiO and the observed photocatalytic activity is correlated with the physicochemical
2
properties. It has been reported that a coupled semiconductor of TiO -ZnFe O exhibits visible
2
2
4
light photocatalytic activity as ZnFe O can absorb visible light and transfer the photogenerated
2
4
electron to TiO , due to the comparable conduction band potentials of these two catalysts [29].
2
However, it is known that ZnFe O alone is photocatalytically inactive under visible-light
2
4
irradiation. A core-shell type cobalt ferrite coated with silica and titania exhibited photo-
degradation activity for methyl orange comparable to that of heated sample and P-25 TiO [30].
2
Though, this coupled photocatalyst showed visible light activity for the photo-degradation of
organic pollutants, it is seen that its visible light activity is privileged. Chung et al. [31] have
synthesized TiO -NiFe O and TiO -SiO -NiFe O magnetic nano-composites by a multistep
2
2
4
2
2
2
4
spray pyrolysis process and found that the titania-silica layered particles showed better
photocatalytic activity for methylene blue degradation compared to titania alone on nickel ferrite.
Lee et al. [32] synthesized barium ferrite-silica-titania composite photocatalyst by precipitation
method showed photocatalytic activity for the degradation of Procion red MX-5B dye under UV
illumination and the photocatalytic activity increased after heat treatment at 773 K due to the
4
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improved crystallinity of the heat treated sample. Xu et al. [27] have reported the photocatalytic
properties of visible-light-active ZnFe O -doped TiO photocatalyst. Hankare et al. [10] have
2
4
2
reported TiO -Al O -ZnFe O for photocatalytic activity for the degradation of methyl red and
2
2
3
2
4
thymol blue as compared to TiO -ZnFe O composite and ZnFe O . Fu et al. [33] have reported
2
2
4
2
4
magnetically separable ZnFe O -graphene nanocomposite photocatalyst with different graphene
2
4
content was prepared by a facile one-step hydrothermal method and the photodegradation rate of
methylene blue was 88% after visible light irradiation for only 5 min and reached up to 99%
after irradiation for 90 min. Xiong et al. [34] report a facile strategy to fabricate a CdS-MFe O
2
4
nanocomposite with differing ferrite content via a two-step hydrothermal method and the
photocatalytic activities of as-prepared photocatalysts were evaluated by the degradation of
rhodamine B and 4-CP in aqueous solution under visible-light irradiation.
When metal was doped into the spinel ZnFe O , more surface defects are produced,
2
4
which hindered the recombination of photo-induced electron-hole pairs. This contributed to the
improvement of the photocatalytic activity. Even though, TiO is most extensively investigated
2
photo-catalyst, civilizing its quantum efficiency is still a major challenge. The enhancement of
photocatalytic activity of TiO can be achieved by doping with metals, the addition of electron
2
donors to the reaction system, and the addition of other semiconductor oxides in order to reduce
the charge carrier recombination [35]. It has been reported that the addition of Mn Zn Fe O to
x
1-x
2
4
TiO can increase the surface area and hydroxyl groups in Mn Zn Fe O -TiO mixed catalysts
2
x
1-x
2
4
2
which, in turn, improves the quantum efficiency of TiO [36]. Moreover, Mn Zn Fe O and
2
x
1-x
2
4
Mn Zn Fe O -TiO binary oxide catalysts have been well investigated for their photocatalytic
x
1-x
2
4
2
properties. Such advanced Mn Zn Fe O -TiO mixed oxides extend their application through
x
1-x
2
4
2
5
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the generation of new catalytic sites, due to a strong interaction between them. Therefore, it is
highly interesting and desirable to study the photocatalytic activity of Mn Zn Fe O -TiO
2
x
1-x
2
4
mixed oxides.
2
. Experimental
2
.1. Materials and methods
All the chemicals used in this study were of analytical grade obtained from Merck, India
and were used as received without further purification. Zinc nitrate (Zn(NO ) ·6H O, 98%),
3
2
2
manganese nitrate (Mn(NO ) ·4H O, 98%), ferric nitrate (Fe(NO ) ·9H O, 98%) and urea
3
2
2
3 3
2
(
CO(NH ) ) as the fuel are used for this auto combustion method. The samples were prepared
2 2
2
+
with the addition of Mn of different molar ratios (Mn Zn Fe O ; x = 0.0, 0.1, 0.2, 0.3, 0.4 and
x
1-x
2
4
0
.5) to ZnFe O . Metal nitrates and urea were dissolved separately in 20 ml of de-ionized water
2 4
and poured into a beaker and stirred using magnetic stirrer for 15 minutes to obtain clear
solution. In case of ZnFe O , the precursor solution was taken in the silica crucible and treated in
2
4
an air furnace at 400°C for 2h at a heating rate of 5°C/min, and cooled at the same rate, it
became solid. The preparation procedure of urea assisted auto combustion method was given in
Scheme 1. Initially, the mixture was subjected to boiling and underwent evaporation followed by
the decomposition with the evolution of gases. When the solution reached the point of
spontaneous combustion, it vaporized and instantly became a solid. After the completion of the
combustion reaction, the solid powder was obtained and then it was washed with ethanol and
dried at 70ºC for 1 h. The as-prepared nanocrystals, Mn Zn Fe O with x = 0.0, 0.1, 0.2, 0.3 0.4
x
1-x
2
4
and 0.5, were labeled as ZnFe O , Mn Zn Fe O , Mn Zn Fe O , Mn Zn Fe O ,
2
4
0.1
0.9
2
4
0.2
0.8
2
4
0.3
0.7
2
4
Mn Zn Fe O and Mn Zn Fe O , respectively.
0
.4
0.6
2
4
0.5
0.5
2
4
6
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The expected urea assisted auto combustion reaction may be as follows,
Zn(NO ) .6H O + 2Fe(NO ) .9H O + 6.66CO(NH )
→
3
2
2
(S)
3 3
2
(S)
2 2 (S)
ZnFe O
+ 37.33H O ↑ + 6.66CO2 (g)↑ + 10.66N2 (g)↑
2 (g)
2
4 (S)
2
.2. Characterization techniques
The structural characterization of Mn-doped ZnFe O (Mn Zn Fe O with x = 0.0, 0.1,
2
4
x
1-x
2
4
0
.2, 0.3, 0.4 and 0.5) nanocrystals were performed using Rigaku Ultima X-ray diffractometer
equipped with Cu-Kα radiation (λ = 1.5418 Å). Structural refinements using the Rietveld method
was carried out by PDXL program; both refined lattice parameters and crystallite size of the
obtained ferrites were reported. The surface functional groups were analyzed by Perkin Elmer
FT-IR spectrometer. Morphological studies and energy dispersive X-ray analysis of pure and
Mn-doped ZnFe O nanocrystals have been performed with a Jeol JSM6360 high resolution
2
4
scanning electron microscopy. The transmission electron micrographs were carried out by
Philips-TEM (CM20). The surface area was derived from the N adsorption-desorption isotherms
2
using liquid nitrogen at 77 K using an automatic adsorption instrument (Quantachrome Corp.
Nova-1000 gas sorption analyzer). The UV-Visible diffuse reflectance spectrum was recorded
using Cary100 UV-Visible spectrophotometer to estimate their band gap energy (E ). The
g
photoluminescence properties were recorded at room temperature using Varian Cary Eclipse
Fluorescence Spectrophotometer. Magnetic measurements were carried out at room temperature
using a PMC MicroMag 3900 model vibrating sample magnetometer equipped with 1 Tesla
magnet.
2
.3. Photocatalytic reactor setup and degradation procedure
7
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Photocatalytic degradation (PCD) experiments were carried out in a self-designed
photocatalytic reactor and the schematic diagram of the photocatalytic reactor is shown in
Scheme 2. The cylindrical photocatalytic reactor tube was made up of quartz/borosilicate with a
dimension of 36 cm height and 1.6 cm diameter. The top portion of the reactor tube has ports for
sampling, gas purging and gas outlet. The aqueous 4-Chloro phenol (4-CP) solution containing
appropriate quantity of ZnFe O nano photo-catalysts was taken in the quartz/borosilicate tube
2
4
and subjected to aeration for thorough mixing and placed inside the reactor setup. The lamp
housing has low pressure mercury lamps (88 W) emitting either 254 or 365 nm with polished
anodized aluminum reflectors and black cover to prevent UV leakage. Prior to photocatalytic
experiments, the adsorption of 4-CP on pure ZnFe O and Mn-doped ZnFe O nano photo-
2
4
2
4
catalyst was carried out by mixing 100 ml of aqueous solution of 4-CP with fixed weight of the
respective photo-catalyst. This slurry was equilibrated for 30 min in a magnetic stirrer. The PCD
was carried out by mixing 100 ml of aqueous 4-CP solution and fixed weight of ZnFe O nano
2
4
photo-catalyst. PCD of 4-CP was also carried out with Mn Zn Fe O -TiO mixed oxides. A
x
1-x
2
4
2
known amount of commercial TiO (Degussa P-25) was added to a known amount of Mn Zn
2
x
1-
Fe O and finely ground in a mortar and pestle for 30 min so as to obtain a mixture of Mn Zn
x
x
2
4
x
1-
Fe O -TiO in the required mol%. The finely ground mixture was then used directly for the
2
4
2
PCD experiments. No heat treatment of the mixture was carried out. Therefore, the mixed oxides
were actually physical mixtures and not solid solutions. Therefore, the interactions of Mn Zn
x
1-
Fe O with TiO can be assumed to be those taking place at the grain boundaries. The PCD
x
2
4
2
efficiency was also calculated for pure oxides (ZnFe O and TiO ) and also for mixed oxides
2
4
2
(
ZnFe O -TiO ,
Mn Zn Fe O -TiO ,
Mn Zn Fe O -TiO ,
Mn Zn Fe O -TiO ,
0.3 0.7 2 4 2
2
4
2
0.1
0.9
2
4
2
0.2
0.8
2
4
2
8
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Mn Zn Fe O -TiO and Mn Zn Fe O -TiO ). The experiments were performed at room
0
.4
0.6
2
4
2
0.5
0.5
2
4
2
temperature and the pH of the reaction mixture was adjusted (2 to 11.0 using 1 M of H SO and
2
4
NaOH solution). Prior to irradiation, the slurry was aerated for 30 min to reach adsorption
equilibrium followed by UV irradiation. Aliquots were withdrawn from the suspension at
specific time intervals and centrifuged immediately at 1500 rpm. Then it was filtered through a
0
.2 μm millipore filter paper to remove suspended particles. The extent of 4-CP degradation was
monitored using UV-Vis spectrophotometer (Perkin-Elmer, Lamda 25). The effect of the pH of
the solution was studied by adjusting the pH of the phenol solution containing the catalyst, using
dilute HCl and NaOH (both from Merck, India). The pH of the solution was measured using a
HANNA Phep (ModelH198107, 0.2-0.5 pH unit accuracy) digital pH meter. The extent of
mineralization was determined using a total organic carbon analyzer (TOC - Shimadzu VCPN).
The PCD efficiency (η) was calculated from the following expression, η = C -- C /C X 100 (or)
i
t
i
η = TOC -- TOC /TOC X 100, where, C or TOC , initial concentration of 4-CP, C or TOC ,
i
t
i
i
i
t
t
concentration of 4-CP after ‘t’ minutes.
3
. Results and discussion
3
.1. Structural analysis
The structural phase and purity of the as-prepared Mn Zn Fe O NCs were confirmed
x
1-x
2
4
by analyzing the X-ray powder diffraction (XRD) patterns. Fig. 1 shows the XRD patterns of
Mn Zn Fe O NCs prepared with various Mn substitutions (x = 0.0, 0.1, 0.2, 0.3 0.4 and 0.5).
x
1-x
2
4
The main diffraction peaks (2θ) appear at about 29.76, 35.73, 36.92, 42.72, 53.16, 57.04, 62.13,
1.02, 74.14 and 78.83°, which are attributed to the (220), (311), (222), (400), (422), (511),
7
(440), (620), (533) and (444) planes, respectively show the characteristics of a single-phase cubic
9
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spinel structure with Fd3m space group (JCPDS card no. 22-1012) [37]. The observed diffraction
peaks indicates well defined spinel phase, which was developed during the combustion. During
the heating process, naturally oxidation and formation of metal oxide is very common.
Moreover, the control of furnace atmosphere during complete combustion is also very crucial.
However, some peaks of α-Fe O phase can also be observed at 2θ = 24.02 and 42.12° in each
2
3
diffraction pattern, it is mainly due to the partial oxidation of ZnFe O and Mn-Zn ferrites [38].
2
4
The main diffraction peak of cubic spinel ferrite at (311) plane was considered as a measure of
its degree of crystallinity.
The average crystallite size of the as-prepared NCs was calculated from the diffraction
peak of the (311) plane in the XRD profile, in accordance with Debye-Scherrer formula:
0

.89
cos
L 
where ‘L’ is the average crystallite size, ‘λ’, the X-ray wavelength (0.1542 nm), ‘β’, full width at
half maximum (FWHM) and ‘θ’, the Bragg angle of the (311) plane. The calculated crystallite
size of the samples is summarized in Table 1. It has been observed that the crystallite size is
decreased from 23.25-17.53 nm with increase in Mn content, upto x = 0.0-0.3. However, and
further increase the Mn content (x = 0.4 and 0.5) the crystallite size is decreased from 17.92-
1
8.36 nm. It has a minimum crystallite size of 17.53 nm for 0.3 Mn content (Mn Zn Fe O ).
0.3 0.7 2 4
The results show a significant amount of XRD line broadening, a characteristic of nano-phase
materials.
In order to further analyze the structural change, the measured XRD patterns of the NCs
were simulated based on the Rietveld refinement method. Rietveld XRD data analysis of the as-
1
0
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prepared NCs is shown in Fig. 2. It was designed to refine simultaneously both the structural and
microstructural parameters (Lattice constant and Crystallite size). During the refinements, the
goodness of fit is defined by the reliability factor S = R /R , where R and R , are, respectively,
wp
e
wp
e
the R -weighted and the R -expected patterns. The obtained values of the lattice parameter and
wp
ep
crystallite size from the Rietveld analysis are shown in Fig. 3 and Table 1. The lattice parameter
of pure ZnFe O is found to be equal to 8.442 Å, which is in good accordance with the standard
2
4
value of 8.441 Å (JCPDS No. 22-1012). The lattice parameter increases with increasing Mn
content from 8.442 to 8.451 Å, thus obeying Vegard’s law. The Vegard’s law is based on the
change due to ionic radii of replacing and replaced ions and predicts the linear change in the
lattice parameter for the spinel system with the substitution of different ions. The ionic radius of
2
+
2+
Mn ion (0.91 Å) is higher than that of Zn ion (0.82 Å), and thus causes an effective increase
in the lattice constant. A linear increase in the lattice constant thus indicates that the Mn ions are
replacing the Zn ions in ZnFe O matrix and also confirms that Mn is occupied in the lattice of
2
4
spinel ferrite [39, 40].
3
.2. Fourier transform infrared (FT-IR) spectroscopy
Fourier transform infrared (FT-IR) spectra were recorded on a Perkin Elmer FT-IR
spectrometer using the KBr pellet method. FT-IR spectra of Mn Zn Fe O (x = 0.0, 0.1, 0.2, 0.3
x
1-x
2
4
0
.4 and 0.5) nano-crystals are shown in Fig. 4. From the FT-IR spectra, it is found that two

1
strong absorption peaks observed at lower frequency (825 and 430 cm ) can be assigned to the
M-O (Zn, Mn and Fe) bonds at A and B sites [27, 41]. This can be attributed to the high degree
of crystalline nature of the Mn Zn Fe O nano-crystals. The obtained results were good
x
1-x
2
4

1
agreement with Rietveld refinement XRD analysis. A broad vibration band at ~ 3440 cm is
1
1
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associated with the OH stretching vibrations of water molecules, indicating higher amount of
surface hydroxyl groups (OH), while those at ~ 1630 cm1 are associated with their bending
mode. Generally, the H O and CO molecules are easily chemisorbed onto Mn Zn Fe O
2
2
x
1-x
2
4
nanocrystals surface when exposed to the atmosphere. However, in this present study, we have

1
observed two bands at ~ 825 and ~ 430 cm was confirmed the M-O (Zn, Mn and Fe) bonds
stretching vibrations in A- and B- sites of magnetic spinel Mn Zn Fe O NCs. Similar values
x
1-x
2
4
are reported earlier [27, 41, 42].
.3. Scanning electron microscopy (SEM) studies
The morphological characteristics of the spinel Mn Zn Fe O NCs were investigated by
3
x
1-x
2
4
the high resolution scanning electron microscopy (HR-SEM) and the obtained results are shown
in Fig. 5. Fig. 5a-c shows the HR-SEM images of pure ZnFe O , Mn Zn Fe O and
2
4
0.3
0.7
2
4
Mn Zn Fe O samples, respectively. HR-SEM images indicated that the as-prepared samples
0
.5
0.5
2
4
consist of well crystallized homogeneous spherical shaped particle-like nanocrystals, which are
agglomerated together, due to the presence of magnetic interactions among the particles [43].
3
.4. Transmission electron microscopy (TEM) studies
To provide a further evidence for the formation of spherical shaped particle-like nano-
crystal morphology of the as-synthesized Zn Mn Fe O samples, high resolution transmission
1
-x
x
2
4
electron microscopy (HR-TEM) analysis was carried out and is shown in Fig. 6. Fig. 6a shows
the HR-TEM image of undoped ZnFe O particle-like nanocrystals with diameter ranging from
2
4
2
3-25 nm. Fig. 6b,c shows the HR-TEM images of Mn Zn Fe O and Mn Zn Fe O NCs,
0.3 0.7 2 4 0.5 0.5 2 4
respectively, with diameter ranging from 15-21 nm. It is obvious that the spherical shaped
particle-like nano-crystals are uniform in size, which is consistent with the average crystallite
1
2
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size obtained from the peak broadening in XRD and Rietveld analysis. The observed average
particle size is close to the values obtained using the Scherer equation. The nano-sized features
are noticeably visible in Fig. 6a-c, which was found to be highly crystalline as evident form HR-
TEM image displayed in Fig. 6d. Fig. 6d yields an inter-planar distance d spacing of 0.25 nm can
be assigned to the (311) lattice plane of the cubic spinel ZnFe O nanocrystals [44]. It clearly
2
4
showed that the nano-crystals are agglomerated, due to the presence of the magnetic interactions
among the particles. Selected area electron diffraction pattern (SAED) of ZnFe O nano-crystals
2
4
are shown in Fig. 6a (inset), which implies that the as-prepared samples are single crystalline in
nature.
3
.5. Energy dispersive X-ray (EDX) analysis
The elemental composition and purity of the as-prepared nanocrystals were recorded at
2
+
room temperature by EDX analysis. Fig. 7 shows the EDX results of pure and Mn -doped
ZnFe O (Mn Zn Fe O : x = 0.0 and 0.5) nano-crystals. Fig. 7a shows the peaks of Fe, Zn and
2
4
x
1-x
2
4
O elements in pure ZnFe O and Fig. 7b, shows the peaks of Fe, Zn, Mn and O elements for Mn-
2
4
doped ZnFe O samples, and there is no other peaks, which is confirmed the as-prepared samples
2
4
are pure and there is no other impurity. The above mentioned results confirm the formation of
pure and Mn-doped ZnFe O phase. The percentage of Mn/Zn values obtained is given in the
2
4
inset of Fig. 7a, b. Pure ZnFe O and Mn-doped ZnFe O did not deviate from their initial
2
4
2
4
2
+
stoichiometry and matched well with the initial degree of Mn substitutions in ZnFe O spinel.
2
4
It is interesting to note that the urea-assisted auto combustion method completely favors the
formation of spinel ZnFe O . A small peak was appeared at 2.1 KeV for all the samples, which
2
4
1
3
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indicated the presence of Au (gold) peak that has been used as a sputter coating, while preparing
the sample for HR-SEM analysis for the better visibility of the surface morphology.
3
.6. Diffuse reflectance spectroscopy (DRS) studies
To study the effect of crystallite size and crystallinity on the optical properties of spinel
ZnFe O semiconductor materials, the UV-Visible diffuse reflectance spectroscopy (DRS)
2
4
measurement was carried out at room temperature for all compositions. The band gap energy Eg,
can be approximately calculated from the optical diffuse reflectance data by the Kubelka-Munk
2
function, F(R) = (1-R) /2R, where ‘R’ is the diffuse reflectance. A graph is plotted between
2
[
F(R)hυ] and hυ, and the intercept obtained is the band gap energy (Fig. 8a). The estimated band
gap (E ) values of Mn Zn Fe O (x = 0.0, 0.1, 0.2, 0.3 0.4 and 0.5) nano-crystals are 1.95, 2.01,
g
x
1-x
2
4
2
.18, 2.27, 2.31, and 2.40 eV, respectively (Fig.8b and Table 1). It clearly shows that there is
2
+
increase in the band gap energy of Mn -doped ZnFe O samples, when compared to the
2
4
undoped ZnFe O nanoparticles (E = 1.95 eV). This is mainly attributed to the reduction in size
2
4
g
of ZnFe O NCs (Quantum confinement effect) as Mn content increase. Thus, the use of size
2
4
quantized doped semiconductor materials may result in increased photocatalytic activity. The
band-gap (E ) value of bulk ZnFe O is 1.9 eV, and hence there is a blue shift for all
g
2
4
compositions. This may be due to the additional sub-band-gap energy levels that are induced by
the abundant surface and interface defects in the agglomerated nano-crystals [45]. The shift in Eg
of the Mn Zn Fe O nano-crystals with the decreasing crystallite size is the result of quantum
x
1-x
2
4
confinement effects arising from the small size regime [46]. Similar results were reported in
other literatures, in which the values of the band gap energy given are 1.9 and 2.23 eV [29, 47].
1
4
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Fan et al. reported similar band gaps (2.1, 2.0 and 19 eV) for pure ZnFe O with three different
2
4
ranges of particle sizes [48].
However, the band gap values are increases with increase in the concentration of Mn
2
+
content. The band gap values are affected by various factors, such as particle size, crystallinity,
structural parameter (lattice constant), nature and composition of impurities (dopant). The
increase in the band gap of our synthesized Mn Zn Fe O (x = 0.0, 0.1, 0.2, 0.3 0.4 and 0.5)
x
1-x
2
4
nano-crystals may be attributed to the more increase in the structural parameter (lattice constant)
with the increase in the Mn content in comparison to the undoped ZnFe O (Tables 1). Morever,
2
4
the obtained band gap values of pure and Mn-doped ZnFe O nano-crystals are higher than that
2
4
of bulk (1.9 eV) and estimated band gap values of zinc ferrite nanowires [47], which are 2.23 and
.73 eV, respectively, and are similar to the ZnFe O films [49]. The band gap energy of the
1
2
4
nano-crystals increases with decrease in the particle size, due to the quantum confinement effect.
3
.7. Photoluminescence (PL) studies
The photoluminescence (PL) spectroscopy technique have been widely used to examine
the structure and properties of the active sites on the surface of metal oxides. PL spectrum is an
effective approach to study the electronic structure, optical and photochemical properties of
semiconductor materials. From the PL spectrum, one can obtain the information such as surface
oxygen vacancies and defects, as well as the efficiency of the charge carrier trapping,
immigration and transfer. The relationship between the PL spectrum and photochemical activity
can be explained based on the PL phenomena of excited energy higher or equal to the band gap
energy of semiconductor [50, 51].
1
5
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Photoluminescence (PL) spectra was recorded at room temperature to detect the optical
2
+
properties of Mn Zn Fe O nanostructures for various substitutions of Mn ions (x = 0.0, 0.1,
x
1-x
2
4
0
.2, 0.3 0.4 and 0.5) and are shown in Fig. 9a,b. All the samples were excited by using the
excitation wavelength at 410 nm and the PL spectra shows the characteristic near-band-edge
NBE) emission at around 428 nm. The broadband visible emission band was observed in the
(
entire PL spectrum at around 428 nm can be assigned to intrinsic defects, mainly interstitial zinc
defects, and is attributed to the recombination of electrons deeply trapped in oxygen vacancies
with photo-generated holes [52]. Spinel ZnFe O has been known to have the strong absorption
2
4
2
+
in the UV-Visible range. As we increase the concentration of Mn dopant into ZnFe O , overall
2
4
intensities of the PL peak for all compositions decreases (x = 0.1, 0.2, 0.3 0.4 and 0.5). This may
2
+
2+
be explained as follows: (i) With increase in the Mn concentration and leaving of the Zn
2
+
3+
cations in the spinel structure, Mn ions occupy the A- sites and transfer the Fe to the B- sites.
(
ii) These changes cause a decrease in the structural isotropy of the as-synthesized nanoparticles.
iii) The sample Mn Zn Fe O NCs have biggest saturation magnetization (M ) than other
(
0
.5
0.5
2
4
s
samples and have a structural isotropy in the A- sites. (iv) The occupation of half of the A- sites
2
+
3+
3+
by the Mn ions and the rest by the Fe ions causes the decrease in transitions of the Fe ions
in the A- sites. However, there are two minor peaks detected by the PL spectrometer at around
4
85 and 580 nm, for Mn Zn Fe O NCs respectively. However, the PL intensity of the emission
x 1-x 2 4
2
+
bands at 428 nm is increases with the increase in the Mn substitution in ZnFe O . This can be
2
4
attributed to the appearance of new electronic levels between the conduction and the valence
band and might be due to the increase in intrinsic defects. Thus, the employed urea-assisted one-
2
+
pot combustion method can produce plenty of defects in pure ZnFe O and Mn doped ZnFe O
4
2
4
2
1
6
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NCs. All the PL emission bands were slightly blue shifted and also broadened with higher PL
intensity (Fig. 9b), due to the increase in Mn content. This can be attributed to the increased
concreteness of surface defects, because of the presence of Mn dopants. Therefore, the band gap
and PL of ZnFe O NCs can be manipulated by controlling their particle size and doping with
2
4
metal ions, thus signify the capabilities of these nanostructures for assorted applications in
various fields.
3
.8 Magnetic measurements
Nano-structured spinel Mn Zn Fe O (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) photocatalyst
x
1-x
2
4
exhibited typical magnetic behavior. The hysteresis loop (M-H curve) for Mn Zn Fe O
x
1-x
2
4
nanocrystals are shown in Fig. 10, which clearly indicated that the prepared Mn Zn Fe O
x
1-x
2
4
based photocatalyst can be separated from water by the application of external magnetic field.
Furthermore, the as-prepared photocatalyst can be redispersed into water after eliminating the
applied magnetic field (Fig. 10 inset). These magnetically responsive photocatalysts have main
advantage over the conventional photocatalysts, because they can be recovered and recycled
readily. Similar values are reported earlier by Shahid et al. [53].
The magnetic hysteresis (M-H) curves of the as-prepared Mn Zn Fe O (x = 0.0, 0.1,
x
1-x
2
4
0
.2, 0.3, 0.4 and 0.5) nanocrystals obtained from room temperature VSM measurement with the
applied field ranging from -10000 to +10000 Oe is shown in Fig. 10. From the VSM
measurements, saturation magnetization (M ), remanent magnetization (M ) and coercivity (H )
s
r
c
values are evaluated and are reported in Table 2. These curves are typical for a soft magnetic
material and indicate hysteresis ferromagnetism in the field ranges of ±10 KOe for the
Mn Zn Fe O , Mn Zn Fe O and Mn Zn Fe O respectively, whereas the samples of
0
.3
0.7
2
4
0.4
0.6
2
4
0.5
0.5
2
4
1
7
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ZnFe O , Mn Zn Fe O and Mn Zn Fe O show a superparamagnetic behavior. ZnFe O is
2
4
0.1
0.9
2
4
0.2
0.8
2
4
2
4
2
+
2+
a soft magnetic material. When non-magnetic Zn in ZnFe O is substituted by magnetic Mn
2
4
ions, there is a drastic change in magnetic properties such as M , M and H . Outside these ranges
s
r
c
2
+
the value of M increases with increasing the concentration of Mn . The values M were
s
s
obtained to be 6.928±03, 24.33±08, 37.77±15, 45.17±13, 55.24±02 and 67.38±11 emu/g at ±10
kOe for the samples of ZnFe O , Mn Zn Fe O , Mn Zn Fe O , Mn Zn Fe O ,
2
4
0.1
0.9
2
4
0.2
0.8
2
4
0.3
0.7
2
4
Mn Zn Fe O and Mn Zn Fe O , respectively. From the results, it is clearly understood that
0
.4
0.6
2
4
0.5
0.5
2
4
the magnetic properties of the samples are affected by the composition, cation distribution and
nature of dopant [54].
However, the paramagnetic behavior of ZnFe O was observed in bulk [55] and
2
4
nanoparticles [56]. In the case of undoped ZnFe O present a superparamagnetic behavior, which
2
4
3
+
is due to the increase of analomous Fe cation distribution in the A- sites upon nanosizing [57].
It can be seen that the M values of spinel ferrites in this work are difference from those prepared
s
by other methods. The difference of the magnetic properties in general depends on particle size,
crystallinity, nature and composition of the dopant, magnetization direction, and so on. Also,
these results point to that the magnetic properties of spinel ferrites are related to their preparation
methods. However, hysteresis curve showed a typical superparamagnetic behavior of the as-
prepared pure ZnFe O , Mn Zn Fe O and Mn Zn Fe O nanocrystals with lower
2
4
0.1
0.9
2
4
0.2
0.8
2
4
remanance and coercivity. The absence of ferromagnetism of the above nanocrystals may be due
3
+
to the non-equilibrium distribution of Fe ions in A- and B- sites. The non-saturation observed
in M-H loop and the absence of the hysteresis, lower values of M and H indicates the
r
c
superparamagnetic nature of the ferrite nanocrystals, which relax back their spins by rotation on
1
8
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removal of applied magnetic field so as to give a zero net magnetic moment. The smaller value
of M may be due to lattice defects, weaker magnetic super exchange interaction between A- and
s
B- sites and random orientation of spin on the surface of nanocrystals [58]. For Mn Zn Fe O ,
0
.3
0.7
2
4
Mn Zn Fe O and Mn Zn Fe O nanocrystals, a hysteresis was observed, thus indicating the
0
.4
0.6
2
4
0.5
0.5
2
4
2
+
ferromagnetism. This is due to the increase in the magnetic nature of the Mn concentration.
The change in the value of M is most probably due to the difference in the cation sharing at the
s
A- and B- sites.
2
+
3+
2+
3+
2
The cation distribution of Mn-Zn ferrite is [Zn 1-xFe
] [Mn Fe
] O ; where
1-x B 4
1+x A
x
2
+
the term within the square brackets indicates the tetrahedral (A-) and octahedral (B-) sites. Mn
ions have a tendency to go into the B-sites in the crystal lattice, because of the favorable fit of
2
+
charge distribution of this ion in the crystal field of the B- site. On other hand, Zn ions have a
3
preference for the A- site, due to their readiness to form covalent bonds involving sp hybrid
2
+
2+
orbitals [59]. However, the non-magnetic Zn (0 μB) are replaced by the magnetic Mn (5 μB)
2
+
ions, which increase the M when, the magnetic Mn ions introduced in the lattice, it starts to
s
3
+
occupy the B- sites and replace Fe ions from the B- sites, which then start migrating towards
3
+
the A- sites. Therefore, Fe ions partially occupy the A- and B- sites [60]. The weakening of A-
B exchange coupling, due to this migration results in the increase in magnetic moment.
The value of M for pure ZnFe O is 6.928±03 emu/g, which increases with the increase
s
2
4
2
+
in Mn concentration (x = 0.1-0.5, M = 24.33±08 to 67.38±11 emu/g). Kumar et al. [61]
s
2
+
reported magnetic nature of Ni ion doped zinc ferrite (Ni Zn Fe O where x = 0.1, 0.3, 0.5)
x
1-x
2
4
nanoparticles synthesized by a chemical co-precipitation method. They reported the doping
2
+
effect of Ni ions on the M and the values are increased (23.95 to 37.75 emu/g) with increasing
s
1
9
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Ni content (x = 0.1-0.5). Koseoglu et al. [62] reported the M value of 5 emu/g for ZnFe O
4
s
2
magnetic nanoparticles prepared by a PEG-assisted route.
Moreover, the M values of the as-prepared ZnFe O samples is smaller than that
s
2
4
observed for the bulk ZnFe O (70 emu/g) [63]. These differences in the M values between the
2
4
s
nanoferrites and bulk ferrites, which can be attributed to the finite size effects [64]. Also, the
magneto-crystalline anisotropy of the particles is dependent on the crystallinity of the
nanocrystals. In this present auto combustion method, the nanocrystals are prepared at lower
temperatures (400 ºC/2h), and it has produced moderately crystallized particles. Therefore, a
large amount of crystal defects can occur within the spinel lattice, and will cause a considerable
reduction of magnetic moment inside the particles, as a result of the magnetocrystalline
anisotropy distortion. Also, the very low values of H and M , indicate that they are also soft
c
r
magnets.
.9 Photocatalytic properties
It has been usually accepted that the crystallinity, particle size, concentrations of doping
3
metal ions and morphologies of the nano-materials are significant factors that influence its
photocatalytic performance. Li et al. [65] have previously reported that photocatalytic activity of
ZnFe O nanostructures mainly determined by its particle morphology. Jia et al. [66] reported
2
4
that the photocatalytic activity of porous ZnFe O nanostructures obviously relies on surface
2
4
properties and surface defects. Therefore, it is important to prepare Mn Zn Fe O -TiO (0.0 ≤ x
x
1-x
2
4
2
≤
≤
0.5) NCs and understand the chemistry occurring at TiO supported in Mn Zn Fe O (0.0 ≤ x
2 x 1-x 2 4
0.5) NCs surface in order to control and tune the photocatalytic properties of the pure and Mn-
doped ZnFe O NCs. Hence, undoped ZnFe O -TiO and Mn doped ZnFe O -TiO NCs surface
2
4
2
4
2
2
4
2
2
0
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characterization and the correlation of structure with observed photocatalytic properties remains
important. Thus, we made an attempt to reveal the relationship between optical and
photocatalytic properties of pure metal oxides (ZnFe O and TiO ) and Mn doped mixed metal
2
4
2
oxides of ZnFe O -TiO (Mn Zn Fe O -TiO , Mn Zn Fe O -TiO , Mn Zn Fe O -TiO ,
2
4
2
0.1
0.9
2
4
2
0.2
0.8
2
4
2
0.3
0.7
2
4
2
Mn Zn Fe O -TiO and Mn Zn Fe O -TiO ) NCs and a series of experiments were carried
0
.4
0.6
2
4
2
0.5
0.5
2
4
2
out with 4-CP in aqueous suspension with the light of wavelength 365 nm.
.9.1 Effects of TiO supported and Mn doping on the photocatalytic activity of Mn Zn
1-
3
2
x
Fe O (0.0 ≤ x ≤ 0.5) NCs
x
2
4
The influence of TiO supported and Mn doping on ZnFe O -TiO NCs on the PCD
2
2
4
2
efficiency is evaluated as shown in Fig. 11. The PCD efficiency of pure ZnFe O is very low
2
4
when compared with TiO . The PCD efficiency of TiO supported ZnFe O (ZnFe O -TiO
2
2
2
4
2
4
2
NCs) is higher than pure ZnFe O NCs. However, the PCD efficiency of Mn Zn Fe O -TiO
2
4
x
1-x
2
4
2
(0.0 ≤ x ≤ 0.5) NCs increases with an increase in the concentration of Mn loading and showed a
maximum activity at x = 0.3 (Mn Zn Fe O -TiO ) and then decreases on further Mn doping to
0
.3
0.7
2
4
2
x = 0.5 (Mn Zn Fe O -TiO ). The reason can be explained as follows: As smaller particle size
0
.5
0.5
2
4
2
distribution of Mn Zn Fe O -TiO NCs, more surface defects are produced as demonstrated in
0
.3
0.7
2
4
2
PL spectra, and a space charge layer could be formed on the surface. Therefore, the migration of
the photo-induced electrons and holes toward surface defects is sensible [67]. Hence, the
separation efficiency of the electron-hole pairs of Mn Zn Fe O -TiO NCs with more oxygen
0
.3
0.7
2
4
2
defects should be more than that of ZnFe O -TiO NCs. Therefore, the improvement in the PCD
2
4
2
efficiency of ZnFe O -TiO NCs as Mn doping increases was recognized to the small particle
2
4
2
size and higher defect application. Nevertheless, excessive amounts of Mn-dopants (x = 0.4 and
2
1
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0
.5) can retard the photocatalysis process, because excess amount of dopants deposited on the
surface of ZnFe O -TiO increases the recombination rate of free electrons and energetic holes,
2
4
2
thus inhibiting the photo-degradation process. Hence, further increase in Mn doping (x = 0.5),
results in the decrease of PCD efficiency. However, the sample Mn Zn Fe O -TiO NCs
0
.3
0.7
2
4
2
shows maximum activity for 4-CP photo-degradation, which is attributed to a gradual
2
+
crystallization of zinc ferrite and further increase in the Mn doping results in an obvious
decrease in activity, which is due to the increase of the grain size [68].
However, in our present study shows that the photocatalytic activity of single phase
ZnFe O is enhanced, when it is coupled with TiO to form a composite catalyst and it is further
2
4
2
enhanced when an addition of Mn is present in the Mn Zn Fe O -TiO composite catalyst.
x
1-x
2
4
2
Though, the band gap of ZnFe O is smaller than that of TiO and it is a visible light active
2
4
2
catalyst, it exhibits lower photocatalytic activity, due to its lower valence band potential
compared to TiO . When TiO and ZnFe O are coupled and irradiated with UV-Vis light, the
2
2
2
4
photocatalytic activity is improved, though the charge carriers can migrate to ZnFe O , due to the
2
4
higher VB potential of TiO . The increased activity of the composite, when Mn is present is due
2
to the decrease in the migration of the charge carriers from TiO to ZnFe O .
2
2
4
3
.9.2 Effect of surface area on the photocatalytic activity
In order to have an idea of the adsorbance capability of the nanocrystals, BET surface
area analysis was determined using N adsorption/desorption studies. BET surface areas of
2
ZnFe O , TiO , ZnFe O -TiO , Mn Zn Fe O -TiO , Mn Zn Fe O -TiO , Mn Zn Fe O -
2
4
2
2
4
2
0.1
0.9
2
4
2
0.2
0.8
2
4
2
0.3
0.7
2
4
TiO , Mn Zn Fe O -TiO and Mn Zn Fe O -TiO nanocrystals are 37.25, 48.52, 42.47,
2
0.4
0.6
2
4
2
0.5
0.5
2
4
2
2
5
6.42, 61.69, 75.64, 73.26, and 72.16 m /g, respectively. The surface area is increased with
2
2
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increasing the concentration of Mn content (x = 0.3) and the sample Mn Zn Fe O -TiO was
0.3
0.7
2
4
2
2
found to be higher surface area (75.64 m /g) than other samples, and further increase the Mn
content (x = 0.4 and 0.5) the surface area decreased. Therefore, it is believed that the high surface
area of Mn Zn Fe O -TiO NCs could enhance the photocatalytic properties than that of other
0
.3
0.7
2
4
2
samples (Fig. 11). The BET surface area and pore structure play an important role in improving
catalyst performance.
In general, a high specific surface area has a favorable effect on the activity for photo-
catalysts. However, the photocatalytic performance is increases with increasing the surface area
as shown in Fig. 11. It is found that the high specific surface area of Mn Zn Fe O -TiO NCs
0
.3
0.7
2
4
2
was beneficial to photocatalytic activity via attractive the adsorption of 4-CP, which is the
determining step in the heterogeneous photocatalytic reaction. Usually, photocatalyst with a high
specific surface area would offer more surface active sites and photocatalytic reaction centers,
resulting in the enhancement of photocatalytic performance. Thus the higher surface area of
Mn Zn Fe O -TiO NCs may lead to higher photocatalytic activity, as compared to other
0
.3
0.7
2
4
2
samples [69]. In addition, when the size of particles decreases, the amount of the dispersion of
particles per volume in the solution will increase, which resulting the enhancement of the photon
absorbance. Besides, as the particle size decreases, the number of active surface sites increases.
Therefore, the photocatalytic activity of Mn Zn Fe O -TiO NCs is higher than other samples,
0
.3
0.7
2
4
2
due to that with very smaller particle size distribution would be a potentially efficient
photocatalyst.
3
.9.3 Effect of phenol concentration on PCD efficiency
2
3
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The rate of photo-catalytic degradation of 4-CP was strongly depended on the initial
concentration of 4-CP and it is found that the efficiency is decreased from 95.5% to 72.3% with
the increase in the initial 4-CP concentration from 50 to 250 mg/L after 210 min (Fig. 12). This
may be suitable to the fact that as the initial concentration of 4-CP increases, more and more 4-
CP molecules are adsorbed on the surface of Mn Zn Fe O -TiO (0.0 ≤ x ≤ 0.5) NCs
x
1-x
2
4
2

photocatalyst, but the number of ˙OH and ˙O radicals formed on the surface of Mn Zn Fe O -
2
x
1-x
2
4
TiO (0.0 ≤ x ≤ 0.5) NCs and the irradiation time is constant. The reactive species (˙OH and
2

˙
O ) required for the degradation of the contaminant on the catalyst surface remains constant for
2
a given light intensity, catalyst amount and duration of irradiation. Hence, the available ˙OH
radicals are scarce for the photocatalytic degradation pollutant at higher concentrations.
Therefore, relative number of ˙OH and ˙O  radicals available for attacking 4-CP molecules
2
becomes less, and consequently, PCD efficiency decreases. Also, as the concentration of 4-CP
increases, the photons get interrupted before they can reach the photocatalyst surface; hence,
absorption of photons by the photocatalyst decreases, and consequently, the PCD efficiency
reduces [70]. As a result, the phenol (4-CP) degradation rate decreases as the concentration
increases [71].
3
.9.4 Effect of the catalyst dosage
Blank experiments were carried out without photocatalyst to examine the extent of
degradation (Photolysis). There was no evidence of PCD of 4-CP in aqueous solution in the
absence of Mn Zn Fe O -TiO (0.0 ≤ x ≤ 0.5) NCs. When aqueous solution of 4-CP containing
x
1-x
2
4
2
ZnFe O -TiO NCs was irradiated with UV light, PCD of 4-CP was observed. The PCD of 4-CP
2
4
2
was found to increase with increase in the amount of ZnFe O -TiO NCs up to 30 mg/100 mL,
2
4
2
2
4
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and further increase in ZnFe O -TiO NCs photocatalyst amount showed negative effect as
2
4
2
illustrated in Fig. 13. This may be due to that the increase in the amount of ZnFe O -TiO NCs
2
4
2
catalyst increases the number of active sites on the photocatalyst surface, which in turn increase
-
the number of hydroxyl (˙OH) and superoxide (˙O ) radicals to degrade 4-CP. When the
2
concentration of ZnFe O -TiO NCs catalyst increases above the optimum value, the degradation
2
4
2
is decreases, due to the interception of the light by the suspension. Sun et al. [72] have reported
that as the excess of catalyst (turbidity) prevent the illumination of light, ˙OH radical, a primary
oxidant in the photocatalytic system decreased and hence the efficiency of the degradation
reduced. Furthermore, the increase in catalyst concentration beyond the optimum may result in
the agglomeration of catalyst particles; hence, the part of the catalyst surface becomes
unavailable for photon absorption, and thereby, PCD efficiency decreases [73]. In addition, an
increase in the substrate concentration can lead to the production of the intermediates, which
may adsorb on the surface of the catalyst. Unhurried distribution of the generated intermediates
from the catalyst surface can effect in the deactivation of active sites on the photocatalyst and
result in a reduction of the degradation rate.
Generally, 4-CP photocatalytic degradation was occurred by hydroxyl radicals attack at
the phenyl group of 4-CP. Therefore, in the present study, the PCD of 4-CP is believed to be
initiated through attacks by hydroxyl radicals at the phenyl group of 4-CP, which may result in
the formation of mono hydroxylated or dihydroxylated 4-CP. This is followed by the cleavage of
the two phenyl groups into intermediates. Finally, the mineralization to CO would have
2
occurred via oxidative processes involving the cleavage of intermediates to aliphatic acids as
shown in Scheme 3. The proposed degradation path way is given in Scheme 3. In addition,
2
5
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hydroquinone was the major intermediates when 4-CP was oxidized at pH 2 and 4, while no
aromatic intermediates were detected at pH 8. This may be due to the fact that hydroquinone in
acidic medium can exist in equilibrium with quinone as shown in the Scheme 3. Hence, the
mineralization of hydroquinone by photocatalytic degradation at acidic pH is rather difficult
[
74]. However, the concentration of hydroquinone and chlorocatechol was low in the pH range
of 5-9, implying that the large amounts of non-aromatic intermediates may be produced when 4-
CP was photocatalytically degraded as reported earlier [75]. Therefore, PCD efficiency at pH 8 is
reasonably high.
4
. Conclusions
In summary, high-crystalline self-assembled pure ZnFe O and Mn -doped ZnFe O
4
2
+
2
4
2
NCs have been successfully synthesized by a facile, low-cost, urea assisted auto-combustion
route. It was found that the structural, morphological, optical and magnetic properties are
2
+
2+
sensitively dependent on the incorporation of Mn ions in the Zn lattice site. The present
study, leads to enhance the photocatalytic activity of spinel Mn Zn Fe O with the adition of
x
1-x
2
4
TiO . Mn Zn Fe O -TiO mixed oxide catalysts were tested for the photocatalytic degradation
2
x
1-x
2
4
2
of 4-CP, and the sample Mn Zn Fe O -TiO mixed oxides showed activity that was
0
.3
0.7
2
4
2
comparable to the activity of pure ZnFe O and TiO . Addition of TiO in Mn Zn Fe O NCs,
2
4
2
2
x
1-x
2
4
which enhances the photocatalytic degradation of 4-CP. The photocatalytic activity of
Mn Zn Fe O -TiO NCs was higher than the other Mn-doped ZnFe O , pure ZnFe O , and
0
.3
0.7
2
4
2
2
4
2
4
commercial TiO (Degussa P-25) at 365 nm than 254 nm, due to the very smaller particle size
2
distribution would be a potentially efficient photocatalyst and high crystallinity, and surface
defects, which significantly influence on the enhanced photocatalytic activity.
2
6
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Acknowledgement
One of the authors, A. Manikandan is thankful to CSIR, New Delhi, for the award of
Senior Research Fellowship (CSIR-SRF).
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