Photocatalytic, optical and magnetic properties of Fe-doped ZnO nanoparticles prepared by chemical route

Article abstract of DOI:10.1016/j.jallcom.2013.11.127

Polyvinyl pyrrolidone (PVP) capped Zn_1-xFe _xO (0.000001 ≤ x ≤ 0.1) nanocrystalline powders were prepared by chemical co-precipitation technique. Structural, optical and magnetic characterizations of the annealed samples were performe

Full text of DOI:10.1016/j.jallcom.2013.11.127

Accepted Manuscript
Photocatalytic, optical and magnetic properties of Fe-doped ZnO nanoparticles
prepared by chemical route
Khanesh Kumar, Mansi Chitkara, Inderjit Singh Sandhu, D. Mehta, Sanjeev
Kumar
PII:
S0925-8388(13)02868-5
DOI:
http://dx.doi.org/10.1016/j.jallcom.2013.11.127
JALCOM 29956
Reference:
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12 August 2013
12 November 2013
19 November 2013
Please cite this article as: K. Kumar, M. Chitkara, I.S. Sandhu, D. Mehta, S. Kumar, Photocatalytic, optical and
magnetic properties of Fe-doped ZnO nanoparticles prepared by chemical route, (2013), doi: http://dx.doi.org/
10.1016/j.jallcom.2013.11.127
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Photocatalytic, optical and magnetic properties of Fe-doped ZnO nanoparticles
prepared by chemical route
Khanesh Kumara, Mansi Chitkaraa, Inderjit Singh Sandhua, D. Mehtab, Sanjeev Kumarc,*
aNanomaterials Research Laboratory (NRL), Department of Applied Sciences, Chitkara
University, Rajpura-140401, Punjab (India)
bDepartment of Physics, Panjab University Chandigarh-160014 (India)
cDepartment of Physics, G.G.D.S.D. College, Chandigarh-160030 (India)
Abstract
Polyvinyl pyrrolidone (PVP) capped Zn1-xFexO (0.000001 ≤ x ≤ 0.1) nanocrystalline powders were
prepared by chemical co-precipitation technique. Structural, optical and magnetic characterizations
of the annealed samples were performed using X-ray powder diffraction (XRD), transmission
electron microscope (TEM), energy dispersive X-ray fluorescence (EDXRF) , Fourier-transform
infrared (FTIR) spectroscopy, UV–visible spectrophotometry, photoluminescence (PL) and
vibrating sample magnetometer (VSM) measurements. XRD and TEM studies reveal that the
synthesized ZnO nanocrystals have hexagonal wurtzite structure with average crystalline size ~8-
15 nm. EDXRF and FTIR measurements confirmed the doping and incorporation of Fe impurities
in ZnO nanostructures. Photocatalytic efficiency of the synthesized samples was determined by
degradation of methylene blue (MB) dye in aqueous solution under UV/sunlight exposure. The
dependence of photocatalytic behavior and luminescence efficiency on the dopant concentration
was studied in detail. Magnetic measurements performed at room temperature show that pure ZnO
exhibits diamagnetic behavior, ferromagnetic order was clearly observed at minute concentration
of Fe impurity (at 0.000010%) and large concentration of dopant (at 10%) leads to strong
paramagnetic component in all the Fe-doped ZnO samples.
Keywords: A. Nanostructured materials; B. Chemical synthesis; C. Catalysis; D. Magnetic
measurements
.
*Corresponding Author: Tel: +91-9465095825; fax +91-1762507085
E-mail address: sanjeevkumar@chitkara.edu.in
1

1. Introduction
Synthesis and characterizations of wide-band gap semiconductor nanomaterials with unique
photocatalytic, optical and magnetic properties have been the main research area over the last
decade. Most preferred wide-band gap II-VI semiconductors are TiO2, CdS, ZnS, ZnSe and
ZnO. ZnO is an n-type semiconductor with band gap (3.37 eV) and a large exciton binding
energy of 60 meV at room temperature. Furthermore, ZnO nanocrystals have shown wide
applications in ultra-violet absorbent [1], photocatalyst [2,3], gas sensor [4,5], photo-detector
[6,7] and anti-virus agent [8,9]. The photocatalytic activities of various photocatalysts have
been tested using aqueous solutions of several dyes [10-15] and other environmental pollutants
[16-19]. In many cases, ZnO has been reported to be more efficient than other photocatalysts
[10,15,17]. High photosensitivity, excellent mechanical characteristics and environmentally
safe nature together make ZnO as suitable photocatalyst to treat water from a number of organic
pollutants and solving environmental pollution problems. It has been noticed that the photocatalytic
activity of ZnO is mainly dependent on the electrons and holes which are produced by light
absorption. Additionally, the valence band position of a semiconductor and the incident photon
energy also plays an important role in deciding the photocatalytic activity of the semiconductor. The
wide band gap semiconductor materials assisted processes, for their activation, demand higher
photon energy, which could be near-UV or in UV range [20-21]. However, since UV occupies
merely ca. 4% of whole solar energy, the technology becomes difficult, if not impossible, to widen
the application. Activity of a photocatalyst nanoparticle can be elevated and extended in the visible
region using a number of strategies such as doping, sensitization, nanostructuration and phase mixing
[22]. The electronic and optical properties of semiconductor nanomaterials are tuned by quasi-stable
2

energy states within the band gap [23,24]. Therefore, it is indispensable and urgent to develop a
particular photocatalyst sensitive to sunlight for waste water treatment.
Diluted magnetic semiconductors (DMSs) have also attracted considerable attention
due to their potential applications in the new emerging fields of optoelectronics, spin-
polarised light emitting diodes (LEDs) and photovoltics [25,26]. Numerous contradictory
results on the magnetic properties of transition metal (TM)-doped ZnO nanomaterials have
been reported in literature [27–36]. From all the experimental investigation on the
ferromagnetism of TM-doped ZnO; some measurements confirmed the presence of
ferromagnetism while other measurements report the absence of any magnetic order in the
samples prepared using different techniques. Few measurements [31,32] relate the origin of
room temperature ferromagnetism in TM-doped ZnO with magnetic cluster or secondary
magnetic phases. On the other hand some measurements [33,34] support an intrinsic
ferromagnetic origin. Till now, the origin of ferromagnetism in TM-doped ZnO remains very
controversial. Similar contradictory results were also observed in Fe-doped ZnO
nanomaterials [37-38, 39]. Sharma et al. [40] demonstrated that 1% Fe-doped ZnO samples
show diamagnetic character, while ferromagnetic nature is observed for 2% and 3% Fe-
doped samples, and higher doping of Fe, paramagnetic nature dominates. Room temperature
ferromagnetism in Fe-doped ZnO with additional Cu doping have been reported by Zhang et
al. [41] and Liu et al. [42]. Recently, Dhiman et al. [43] confirmed the subtle enhancement in
saturation magnetization with Fe doping in ZnO matrix. The reported controversial results
give an indication that room temperature ferromagnetism in DMSs is very sensitive to
preparation methods and need more detail experimental investigations.
3

The present work reports the photocatalytic, optical and magnetic properties of Zn1-
xFexO nanomaterials synthesized using chemical co-precipitation method. Photocatalytic
efficiency of the synthesized nanomaterials has been studied for the degradation of MB dye
both under UV/sunlight irradiation and well correlated with the luminescence quantum yield.
Moreover, the magnetic properties of the Zn1-xFexO nanoparticles were investigated using
vibrating sample magnetometer (VSM) measurements.
2. Experimental Procedure
2.1. Preparation of Zn1-xFexO nanoparticles:
The polyvinyl pyrrolidone (PVP) capped Zn1-xFexO (0.000001≤ x ≤ 0.1) nanoparticles were
synthesized using bottom-up wet chemical precipitation technique through hydrolysis and
oxidizing process. The aqueous solutions of analytic reagent grade chemicals: ZnCl2, FeCl3,
NaOH and polyvinylpyrrolidone (PVP) [(C6H9NO)n] were used in samples preparation
without further purification. In a typical synthesis of Fe-doped ZnO nanocrystals, 1.0 M
ZnCl2, 0.1M FeCl3, 2.0 M NaOH and 2 % PVP were prepared separately in deionized water.
The Zn and Fe precursor solutions were mixed in the stoichiometric proportion under
vigorous stirring. 4 ml of 2% PVP solution was added to total 50 ml volume, before
dropwise addition of 1.0 M ZnCl2 solution into 2.0 M NaOH solution. The PVP solution was
added to prevent the further growth of nanocrystals and whole reaction process was carried
out at room temperature. The resulting precipitate was filtered, thoroughly washed and then
dried in air at ambient temperature to obtain nanoparticles in powder form. Finally, the
synthesized powder was calcined in a muffle furnace at 600o C for 2 h under normal
atmospheric conditions.
4

2.2. Characterization:
The crystalline size of pure and Fe-doped ZnO nanoparticles has been determined from XRD
spectra recorded using powder XRD (PAN-Analytic) setup equipped 3050/60 goniometer
and Cu anode X-ray tube. The XRD scans for the powder samples were performed in the 2θ
range 20-80o keeping step size 0.001 for the Cu K X-ray radiation (λ = 1.5418 Å-1) at 40 mA- 45
kV generator setting. TEM micrographs at an accelerating voltage of 160 KV with
magnification of ×30 K and ×30 K were taken using Hitachi (H-7500) TEM facility available
at Panjab University, Chandigarh, India. Elemental and compositional analyses of the
nanostructures were carried out using EDXRF spectrometer involving Mo anode X-ray tube
(PAN-Analytic) as an excitation source and LEGe (Canberra made, FWHM = 150 eV at 5.89
keV) as photon detector. FTIR studies were performed using TAG FTIR spectrometer. The
optical absorption measurements were carried out using UV-visible absorption
spectrophotometer (Systronic PC based Double Beam Spectrophotometer: 2202). For UV-
visible absorption studies 0.01 g of pure and Fe-doped ZnO nanoparticles powder were
dispersed in 10 mL of ethanol solution, and further used for characterization. Energy resolved
PL spectra of the synthesized samples were recorded using micro Raman & photo luminescence
spectrometer (Renishaw’s made) equipped with multiple laser excitations from UV to near IR
region. Magnetic measurements were performed using a vibrating sample magnetometer (VSM-
735).
2.3 Photocatalytic measurements:
The photocatalytic activities of Zn1-xFexO nanomaterials were examined by studying
degradation of MB dye aqueous solution under UV/sunlight exposure. The natural pH of the
5

aqueous dye solution is 6.5 and can be adjusted by adding H2SO4 or NaOH solutions. The
photocatalytic studies under UV light were performed using UV-photoreactor [dimension of
3 ft × 1.5 ft (height × length)] equipped with two 18 W UV-tubes (Orial made 66001). The
distance between the reactor setup and UV-tubes was ~10 cm. The photocatalytic reaction
under UV light were carried out with ~112 mg nanopowder mixed with 250 ml of 32 ppm
MB dye solution in a glass reactor with surface area 200 cm2. The aqueous suspension was
put under constant stirring in dark for 1h, so that the MB dye atoms were adsorbed on the
surface of nanocrystals. The stable suspension was then exposed to the UV-radiation with
continuous magnetic stirring. About 10 ml of suspension solution was sampled at different
time interval and centrifuged. The supernatant after centrifugation was analyzed by the UV–
Visible spectrophotometer. The rate of decolorization of the test contaminant was noticed
from the optical absorption spectra. The degradation efficiency (ε) of dye is calculated form
equation
Co − C
ε =
×100
(1)
Co
where Co is the initial concentration of the dye and C is the concentration after UV-photons
irradiation.
According to the Langmuir-Hinshelwood kinetics model [44], the photocatalytic
process of MB dye can be expressed as apparent pseudo-first-order kinetic equation:
Co
ln
= κt
(2)
C
where k is the apparent pseudo-first-order rate constant, Co is original MB concentration and
C is MB concentration in aqueous solution at time t.
6

3. Results and discussion
3.1 Crystallographic Characterization:
The XRD patterns for all the Zn1-xFexO powder samples synthesized from hydrolysis of
ZnCl2 are shown in Fig. 1. The narrow and sharp peaks positioned at typical 2θ values are
indexed on the basis of JCPDS card No. 36-145, and represent the successful synthesis and
crystalline nature of ZnO naomaterials. All recorded reflections support the hexagonal
wurtzite structure of ZnO with lattice parameters a = b = 3.249 Å and c = 5.206 Å. The XRD
patterns for all the synthesized samples are exactly same without any visible secondary
phases or impurity peaks which demonstrates that the dopant is well integrated into the
lattice sites during the synthesis process. However, the XRD peaks are shifted to lower
angles with increasing Fe dopant concentration compared to those of pure ZnO. The minor
peak shift is usually assigned to successful incorporation of dopant ions in host matrix [45].
The average crystalline size (D) of the synthesized nanomaterials was calculated using the
Scherrer’s formula [46],
0.9λ
D =
(3)
β cosθ
where β is full width half maxima (FWHM) of the diffraction peak, θ is the Bragg peak angle
and λ is the wavelength of Cu Kα radiation. The estimated crystalline sizes of Zn1-xFexO
nanoparticles at most intense crystallographic plane <110> are found 15 nm, 13 nm, 10nm,
11nm, 8nm and 9 nm for x = 0.100000, 0.010000, 0.001000, 0.000100, 0.000010 and
0.000001, respectively.
The particle size and morphology of the synthesized ZnO nanocomposites were
analyzed from TEM micrographs [Fig. 2a and b]. The micrograph [Fig. 2b] reveals that the
7

product consist of spherical particles with the average size ~14 nm, which is in good
agreement with that estimated by Scherer formula based on the XRD pattern. Selected area
electron diffraction (SAED) pattern [inset Fig. 2a] also confirms the crystallinity of ZnO
nanoparticles.
3.2 FTIR and energy dispersive X-ray fluorescence (EDXRF) studies:
The presence of various chemical functional groups and the formation of PVP capped ZnO
nanoparticles are also supported by FTIR spectra (Fig. 3) in KBr matrix. The common
absorption peaks at 3446, 2362, 1118 and 548 cm-1 are found in the spectra of all samples.
The broad peak at 3446 cm-1 is attributed to absorption due to hydroxyl (-OH) group [47].
The photocatalytic activity of any photocatalyst is closely related to the amount of –OH
groups because the –OH groups can provide an electron to the photogenerated hole and
-1
•
transfer to reactive OH radicals. Strong peak at 2362 cm is assigned to the existence of
atmospheric CO2 in the samples [48]. The peak at 1118 cm-1 is attributed to OH stretching of
H2O molecules in ZnO lattice [49]. Weak absorption peak at 1625 cm-1 correspond to OH
bending vibration due to small amount of water in ZnO nanomaterials [48]. The
characteristic peaks at 548 cm-1 in all the spectra are assigned to the stretching mode of ZnO.
The weak absorption peak at 620 cm-1 in the Zn1-xFexO (x = 0.100000) sample is
characteristic of a Fe-O stretch [50]. Hence, we believe that Fe-ions have successfully
incorporated into the crystal lattice of ZnO. Other major characteristic peaks are due to PVP
capping agent and coupling agent (KH-570) used in the infrared spectroscopy.
Energy dispersive X-ray fluorescence (EDXRF) technique was used to analyze the
elemental and composition of the synthesized nanomaterials. Typical EDXRF spectrum of
8

the as-formed Zn1-xFexO (x = 0.100000) nanocrystals is shown in Fig. 4. The EDXRF
spectrum shows characteristic X-ray peaks of Zn and Fe element from nanomaterial samples,
and Ar present in air. EDXRF analysis also confirms the non existence of any foreign
impurity and homogeneous distribution of the constituent elements in the synthesized
samples. The inset (b) of Fig. 4 represents the Fe incorporation in nanoparticles as a function
of the initial cations ratio in the starting solution. The analytical experimental results exhibit
good agreement with the calculated values. The experimental results of the Fe/Zn ratio are
calculated from the peak areas of the Fe-Kα and Zn- Kα characteristics X-rays, respectively.
The estimation of Fe content at higher dilution, i.e. for x ≤ 0.000100 is not possible due to the
detection limit of EDXRF set up.
3.3 Photocatalytic Behavior:
MB dye is a hetrocyclic aromatic compound with the molecular formula C16H18N3SCl. It is
highly toxic chemical commonly used for dyeing cotton, wood and silk. Large amount of
MB dyes is released as coloured wastewater in the ecosystem by various textile industries.
The released wastewater is source of toxic pollution and hazardous for aquatic life and
human beings. The complete mineralization of MB dyes with cost-effective process is
important to avoid environmental contaminations. The pure or doped wide-band gap
semiconductor materials have been used as photocatalysts for dye degradation.
Photocatalytic properties of PVP capped Zn1-xFexO nanostructures were evaluated from
monitoring the degradation of MB dye under UV/sunlight irradiation using optical
absorption spectroscopy. Absorption spectra of MB dye with and without UV radiation
exposure are shown Fig. 5 for comparison. Each spectrum shows absorption peaks at 243
9

nm, 286 nm and 664 nm and absorption peak at 664 nm was used to track the photocatalytic
degradation process. No significant changes in the recorded spectra have been noticed
spectra after 3 h irradiation, which indicate that pure MB solution cannot be easily degraded
by UV light. The absorption spectra of MB dye solution mixed with Zn0.990000Fe0.010000
O
nanocrystals under different durations of UV and sunlight irradiation are shown in Fig. 6a
and (b), respectively. These results clearly reveal that the degradation of MB solution was
induced by the addition of Zn1-xFexO photocatalyst. Decolorization of MB dye occurs at a
slower rate under UV light in comparison to that of solar light irradiation.
Proposed photocatalytic degradation scheme for MB dye under UV/sunlight light
irradiation in presence of Fe-doped ZnO catalyst is schematically shown in Fig. 7. Irradiation
of ZnO particles with UV/sunlight photons results in the excitation of an electron (e-) from
its valence band to the conduction band leaving behind a positively charged hole (h+) in
valance band. The e- and h+ charge carrier can recombine with the emission of heat energy, or
they can migrate to the particle surface to form photocatalytic active centers. The valence
band h+ reacts with the chemisorbed H2O molecules to form reactive species such as
hydroxyl (•OH) radicals. Likewise, conduction band e- results superoxide (•O2) radicals from
the trapping of electron by oxygen atom. The presence of oxygen prevents recombination by
trapping electron through the formation of superoxide ion maintaining electrical neutrality
within the ZnO particle. Hydroxyl radicals are known powerful oxidizing agents and can
react with dye molecules to cause their complete degradation. The complete mineralization
−
of MB molecule results inorganic products such as SO42− ions, NO3 ions, CO2 and H2O. The
libration CO2 gas was identified by saturated BaCl2 solution test via white BaCO3 precipitate
10

formation. The photocatalytic activity reduces considerably, if electron–hole recombination
takes place within the same timescale as required for oxidation and redox processes.
Therefore, separation of electron-hole pairs with lifetime of the order of femtoseconds is a
vital parameter to determine the photocatalytic activity. The separation time for electron-hole
recombination can be increased by various intrinsic or extrinsic defects. The intrinsic defects
include oxygen vacancies in nanostructured (reducible oxides), and extrinsic defects are
doping of the nanomaterial with a suitable impurities. It has been noticed that pure ZnO is a
typical n type semiconductor and always associated with intrinsic defects like large quantities
of oxygen vacancies. The loss of oxygen from ZnO lattice at high anneal temperature can
produce addition oxygen vacancies. In case of Fe-doped ZnO nanostructures, Fe-ions enter
into the crystal lattice of ZnO and leading the formation of Zn vacancy defects (VZn) and
more oxygen vacancy defects (Vo). Therefore, the defects on the surface of catalysts are also
responsible for the photogenerated electron-hole pairs under UV/sunlight irradiation.The
retardation of the electron-hole recombination will increase the photocatalytic efficiency of
the ZnO nanocrystals photocatalysts and, consequently, accelerate hydroxyl radical
formation which will enhance the rate of MB degradation.
The sensitized photodegradation curves and plots of ln(C0/Ct) versus the UV
irradiation time (t) in case of Fe-doped ZnO, pure ZnO and ZnS nanopowders are shown in
Fig. 8a and b, respectively. Likewise, Fig. 9a and b represents the sensitized
photodegradation curves and concentration plots in presence of various photocatalysts under
sunlight irradiation. Relationship between ln(C0/Ct) and t divulge that photodegradation of
MB follows the pseudo-first order kinetics. The photocatalytic degradation of MB dye was
evaluated from the apparent rate constants (k) calculated from the linear fitted curves using
11

Eq. 2. Higher the value of apparent rate constants (k), more will be the photocatalytic
performance. The values of first-order rate constants (k) and degradation efficiency (ε) for
various photocatalysts are listed in Table 1. It is clearly seen from Table 1 that the
degradation efficiency (ε) and rate constant (k) is generally higher for Fe-doped ZnO
catalysts as compared to that of pure ZnO and ZnS. The MB dye solution degraded to
maximum extent (ε = 41% and k = 6.02 × 10-3 min-1) in case of Zn0.990000Fe0.010000
O
nanocrystals, whereas it degraded to minimum extent (ε = 10% and k = 1.68 × 10-3 min-1) by
Zn0.999990Fe0.000010O photocatalyst after 105 min UV-light irradiation. Increasing order of the
photocatalytic activity of the synthesized nanoparticles is pure ZnO < Zn0.999990Fe0.000010O <
Zn0.999900Fe0.000100O < Zn0.999000Fe0.001000O < Zn0.900000Fe0.100000O < Zn0.999999Fe0.000001O <
Zn0.990000Fe0.010000O. The value of rate constant (k) increases with increasing the doping
concentration of Fe ions up to optimum loading (1%) and then decreases. Hence, there exists
optimal Fe concentration (1%) for which photocatalytic activity is maximum. It is concluded
that 1% doping of Fe may be more efficient for separating the photoinduced electron-hole
pairs. There could be several reasons for the variations in the degradation of MB dye with
dopant ion concentrations. The increase in degradation efficiency with increase in Fe-dopant
concentration may be due to the increase in total surface area, i.e., number of active sites
available on photocatalyst surface for free radical formation. On the other hand , the decrease
in degradation efficiency above optimum concentration of dopant ion, is due to reduction in
light penetration through solution, increase in light scattering, loss in active surface area by
agglomeration [51] and de-excitation of active molecules by collision with ground state
molecules [52,53]. In addition, optimum concentration of dopant ion is necessary for
12

generating effective potential difference to efficiently separate the electron-hole pair at high
concentration [54].
It is worth mentioned that degradation of MB dyes assisted by semiconductor
materials demand higher photon energy in UV-range and such studies have been largely
documented in literature. The UV source is hazardous, expensive and results material
deterioration. In order to develop a sunlight sensitive photocatalyst, we either need an
appropriately band-gap engineered material, or we need the material to possess these
characteristics which are brought about by its defect nature. Therefore, the photodegradation of
MB dye was also studied under sunlight illumination by using natural light in the month of
April, 2013 in Rajpura town near Chandigarh (Latitude: 30.48oN and Longitude: 76.59oE),
India. In case of sunlight exposure, MB dye solution degraded to maximum extent (ε = 73%
and k = 3.48 × 10-2 min-1) in case of Zn0.990000Fe0.010000O sample, whereas it degraded to the
minimum extent (ε = 71% and k = 9.8 × 10-3 min-1) by Zn0.999900Fe0.000100O photocatalyst after
70 min of sunlight irradiation. The results are in agreement with the earlier findings that ZnO
canutilize visible light effectively for the degradation of organic compounds in water [55].
3.4 Optical behavior:
The photoluminescence studies have been widely used to understand the fate of electron-
hole pairs and efficiency of charge carrier trapping in semiconductor particles. Fig. 10 shows
the photoluminescence (PL) emission spectra for Zn1-xFexO nanoparticles at excitation
wavelength of 325 nm. The pure ZnO sample exhibit a strong and wide PL signals ranging
from 423-850 nm and centered at 706 nm. All other Fe-doped samples also show broad
emission spectra ranging from 520-850 nm. Many experimental studies have investigated the
13

nature of luminescence properties of ZnO nanostructures, but it is not clear so far [56, 57].
The PL behavior cannot be explained only on the basis of particle size; however, it is related
to the preparation methods used for nanomaterials synthesis. Moreover, the surface oxygen
vacancies and other defects of ZnO nanoparticles are also attributed to optical properties. It
has been observed that stronger the PL signal, higher the content of surface oxygen vacancies
and other defects. In present measurements, PL intensity gradually decreased as Fe content
increased, and was highest for the Zn0.999990Fe0.000010O sample. These results demonstrate that
the content of surface oxygen vacancy arrived at the highest degree in case of Zn1-xFexO
photocatalysts for x = 0.000010. The decreasing order of PL intensities of the synthesized
nanostructures is pure ZnO > Zn0.999990Fe0.000010O > Zn0.999900Fe0.000100O > Zn0.999000Fe0.001000
O
> Zn0.900000Fe0.100000O > Zn0.999999Fe0.000001O > Zn0.990000Fe0.010000O. The order of PL intensities
is just opposite to the order of photocatalytic activities. It is generally believed that the lower
the PL intensities, the higher the separation rate of photo-induced charge carriers. Hence
photocatalytic activity is closely related with the lifetime of photoexcited electron–hole pairs
and the most effective electron-hole separation of Zn1-xFexO nanoparticles will be expected to
achieve the highest photocatalytic activity.
The UV-visible absorption spectra of PVP capped Zn1-xFexO (x = 0.100000, 0.010000
and x = 0.000001) have been shown inset Fig. 10. It is clear from the spectra; weak
absorption starts from 500 nm and becomes strong near 375 nm (3.30 eV), which indicates
synthesized nanomaterials can absorb UV and some portion of visible light.
3.5 Magnetic behavior:
The magnetic behavior of ZnO particles doped with different concentration of Fe impurity
can be obtained from room temperature vibrating sample magnetometer (VSM) measurements.
14

Fig. 11 illustrates the variation of magnetization (M) as a function of applied field (H) in case
of Zn1-xFexO (x = 0.000000, 0.000001, 0.000010 and 0.000100) nanoparticles. It is clear from
Fig. 11 undoped ZnO, i.e., Zn1-xFexO (x = 0.000000) shows diamagnetic behavior. However,
for x = 0.000010 and x = 0.000100, well-defined hysteresis loops with noticeable coercivity
were observed. Likewise, Fig. 12 depicts magnetization of Zn1-xFexO (x = 0.001000,
0.010000 and 0.100000) nanoparticles at room temperature. For x = 0.001000 and x =
0.010000, the loops are S-shaped, which indicate the ferromagnetic nature of the samples.
However, in case of x = 0.100000 the loop is not saturated and going up with increasing
field. More information about the nature and magnetic behavior of all the synthesized
samples is given in Table 2. From the present results, it is interesting to note that pure ZnO is
diamagnetic in nature, while minute concentration of Fe impurity results ferromagnetic
behavior and large concentration of Fe doping leads to strong paramagnetic component [58].
The origin of ferromagnetism in transition metal doped ZnO until now is still not clear. A
number of studies indicate that ferromagnetism in transition metal doped ZnO may come
from secondary magnetic phases or magnetic precipitation [59]. Other groups [38,60],
however, reported that the room-temperature ferromagnetism in Fe-doped ZnO could be due
to the oxidation of Fe in mixed valence (Fe 2þ and Fe 3þ ) states. The exchange interaction
between conductive electrons and local spin polarized electrons is responsible for the
magnetic behavior of Fe-doped ZnO nanoparticles.
In present measurements, we have not seen the presence of any secondary phases in
the XRD patterns of Fe-doped ZnO nanoparticles. The observed ferromagnetism in our
samples is not due to the impurity clusters or oxides of the dopant ions. However, we assume
that the origin of ferromagnetism in Zn1-xFexO nanoparticles is intrinsic property which is
15

highly correlated with structural defects [61,62]. The presence of structural defects such as
VZn and Vo can be further proved from the results of PL and photocatalytic measurements.
Thus, the magnetic coupling between Fe-ions is ferromagnetic mediated by VZn and Vo, and
this may account for observed room temperature ferromagetism [63]. Addition to
ferromagnetism, it has been observed that for x = 0.100000 in Zn1-xFexO, M-H loops do not
saturate even at high magnetic field of 10 kOe. Non-saturation of MH loop at higher applied
field is a commonly observed phenomenon in semiconductor/metal nanocomposites [64].
Recently, Rumpf et al. [64] also observed such non-saturation behavior for ferromagnetic
semiconductor and attributed it to the point like defects and magnetic anisotropy.
4. Conclusions
Nanocrystalline ZnO particles doped with different concentration of Fe impurity have been
successfully synthesized using chemical co-precipitation method. We have investigated their
photocatalytical, optical and magnetic properties as a function of dopant concentrations. The
preliminary results of photocatalytic experiments suggested that photocatalytic performance
of Fe-doped ZnO nanoparticles is much better than pure ZnO under both under UV and
sunlight light irradiation. Photocatalytic performance of the synthesized materials was
dependent on the Fe concentration and 1% Fe-doped ZnO (Zn0.990000Fe0.010000O) nanoparticles
was most active photocatalyst for the degradation of MB dye. Luminescence efficiency and
photocatalytic behavior dependence on the dopant concentration have been presented in a
fantastic and elaborative way. Moreover linear fitted photo-catalytic curves and PL spectra
reported in the present investigation will be very beneficial for future applications of these
nanocrystals in opto-electronics industries. From the magnetic measurements at room
16

temperature, it has been found that pure ZnO is diamagnetic in nature and specific
magnetization values were found to depend on dopant concentration.
Acknowledgments
The authors express their gratitude to Dr. Devinder Mehta, Professor, Department of Physics,
Panjab University, Chandigarh for the use of EDXRF facility and Dr. Ravi Kumar, Professor,
Department of Material Sciences, National Institute of Technology, Hamirpur for energy resolved
spectroscopic studies. Special thanks to Dr. R.K. Kotnala, Chief Scientist, National Physical
Laboratory, New Delhi for VSM measurements. The Regional Scientific Instruments Centre (RSIC),
Punjab University, Chandigarh is gratefully acknowledged for XRD and TEM studies.
17

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22

Table 1
Photocatalytic decolorization rate constants of MB dye using Zn1-xFexO photocatalysts under
UV/sunlight irradiation.
Samples
Decolorization under UV light Decolorization
under
sunlight
irradiation
irradiation
Rate
Degradation Correlation Rate
Degradation Correlation
Zn1-xFexO
constant efficiency
co-efficient constant
(R2 value) (k)
min-1
efficiency
co-efficient
(k)
in (ε) in %
in (ε) in %
(R2 value)
min-1
x = 0.100000
x = 0.010000
x = 0.001000
x = 0.000100
x = 0.000010
x = 0.000001
x = 0.000000
Pure ZnS
2.31E-3
6.02E-3
4.89E-3
4.25E-3
1.68E-3
2.29E-3
5.81E-4
2.08E-3
18 (7)
41 (5)
38 (4)
34 (3)
10 (2)
17 (6)
7 (1)
0.956
0.932
0.998
0.860
0.687
0.947
0.528
0.991
1.67E-2
3.48E-2
2.22E-2
9.8E-3
51
73
37
71
91
68
62
9
0.848
0.988
0.865
0.817
0.895
0.953
0.934
1
1.81E-2
1.00E-2
1.44E-2
1.06E-3
17
23

Table 2
Magnetic characteristics of Zn1-xFexO nanoparticles at room temperature.
Samples
(Zn1-xFexO)
Magnetic
behavior
Saturated
magnetization
(emu/g)
-
Coercive field
(Oe)
x = 0.100000
x = 0.010000
x = 0.001000
x = 0.000100
x = 0.000010
x = 0.000001
x = 0.000000
Paramagnetic
-
Ferromagnetic 0.172
Ferromagnetic 0.0242
Ferromagnetic 0.0139
Ferromagnetic 0.0173
Ferromagnetic 0.0002
-
-
21.314
12.972
-
-
Diamagnetic
-
24

Figure captions
Fig. 1 XRD spectra of PVP capped ZnO and Zn1-xFexO (0.000001≤ x ≤ 0.1) nanoparticles.
Fig. 2 TEM micrographs of Zn0.900000Fe0.100000O nanocrystals.
Fig. 3 FTIR spectra of PVP capped Zn1-xFexO (x = 0.000000, 0.000001, 0.000010, 0.000100,
0.001000, 0.010000 and 0.100000) nanocrystals. The spectra are shifted vertically for clarity.
Fig. 4 Typical EDXRF spectrum of Zn0.900000Fe0.100000O nanocrystals. The inset (a) represents
lower energy part of the spectrum and (b) shows the Fe incorporation in the
nanoparticles as a function of the initial cation ratio in the starting solution.
Fig. 5 UV-visible spectra of the aqueous MB dye solution after exposure with UV/sunlight-light.
Fig. 6 Absorption spectra of MB dye in presence of Zn0.990000Fe0.010000O nanocrystals under (a)
different duration of UV light irradiation and (b) different duration of sunlight irradiation.
Fig. 7 Schematic presentation of various photocatalytic processes involved in MB dye
degradation.
Fig. 8 (a) Sensitized photodegradation curves and (b) plots of ln(C0/Ct) versus the irradiation
time (t) in presence of various photocatalysts under UV-light irradiation.
Fig. 9 (a) Sensitized photodegradation curves and (b) plots of ln(C0/Ct) versus the irradiation
time (t) in presence of various photocatalysts under sunlight irradiation.
Fig. 10 Room temperature PL spectra of Zn1-xFexO (x = 0.000000, 0.000001, 0.000010,
0.000100, 0.001000, 0.010000 and 0.100000) nanocrystals.
Fig. 11 Room temperature M-H curves for pure ZnO and Zn1-xFexO (x = 0.000100, 0.000010
and 0.000001) nanoparticles.
Fig. 12 Room temperature M-H curves for Zn1-xFexO (x = 0.100000, 0.010000 and 0.001000)
nanoparticles.
25

Fig. 1
Zn0.900000Fe0.100000
O
Zn0.990000Fe0.010000
O
Zn0.999000Fe0.001000
O
Zn0.999900Fe0.000100
O
Zn0.999990Fe0.000010
O
Zn0.999999Fe0.000001
Pure ZnO
70
O
30
40
50
60
80
Angle (2θ)

Fig. 2
(
b
)
( a )
1
0
0
n
m
2
0
n
m

Fig. 3
F
T
I
R
s
p
e
c
t
r
a
o
f
Z
n
-xF
e
O
n
a
n
o
c
o
m
p
o
s
i
t
s
1
x
x
=
0
.
0
0
0
0
0
0
x
x
=
=
0
.
.
0
0
0
0
0
0
0
0
1
1
0
0
0
x
x
=
=
0
0
.
.
0
0
0
0
1
1
0
0
0
0
0
0
x
x
=
=
0
0
.
.
0
1
0
0
0
0
0
0
0
0
0
1
3
5
0
0
3
0
0
0
2
5
0
0
2
0
0
0
1
5
0
0
1
0
0
0
5
0
0
-
1
W
a
v
e
n
u
m
b
e
r
(
c
m
)

Fig. 4
10
8
(a)
(b)
50
40
30
20
10
6
4
2
0
150
200
250
300
350
400
0.00
0.05
0.10
Channels
Fe/Zn ratio
200
300
400
500
Channels