Europium doping effect on 3D flower-like SnO2 nanostructures: Morphological changes, photocatalytic performance and fluorescence detection of heavy metal ion contamination in drinking water

Article abstract of DOI:10.1039/c9ra03405f

Pure and Eu-doped (1, 3, 5, 7 & 10 mol%) SnO₂ nanostructures have been successfully synthesized by a facile and simple hydrothermal method. The properties of as-synthesized samples have been investigated by various analytical techniques. It was found that the morphology of as-synthesized flower-like SnO₂ nanostructures made of intermingled small-size agglomerated nanorods can be precisely controlled by varying the Eu dopant concentration in a reasonable range. Moreover, the photocatalytic activity of SnO₂ studied by the degradation of rhodamine-B (RhB) dye in aqueous media shows excellent performance on 10 mol% europium doping, which may be attributed to its specific morphology and larger surface area as seen from BET measurements. Furthermore, sensors based on 10 mol% Eu-doped SnO₂ nanostructures show the highest fluorescence quenching efficiency (0.23) as compared to pure SnO₂ and other doped samples for the lowest concentration of Cd²⁺ (10 ppb) in drinking water with a Limit of Detection (LOD) as low as 7 ppb; 0.007 μg mL^-1. The formation mechanism of Eu-doped SnO₂ nanostructures has been discussed in detail.

Full text of DOI:10.1039/c9ra03405f

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Europium doping effect on 3D flower-like SnO2
nanostructures: morphological changes,
photocatalytic performance and fluorescence
detection of heavy metal ion contamination in
drinking water
Cite this: RSC Adv., 2019, 9, 37450
*
Harsimranjot Kaur,
Harbhajan Singh Bhatti and Karamjit Singh
Pure and Eu-doped (1, 3, 5, 7 & 10 mol%) SnO2 nanostructures have been successfully synthesized by a facile
and simple hydrothermal method. The properties of as-synthesized samples have been investigated by
various analytical techniques. It was found that the morphology of as-synthesized flower-like SnO2
nanostructures made of intermingled small-size agglomerated nanorods can be precisely controlled by
varying the Eu dopant concentration in a reasonable range. Moreover, the photocatalytic activity of SnO2
studied by the degradation of rhodamine-B (RhB) dye in aqueous media shows excellent performance
on 10 mol% europium doping, which may be attributed to its specific morphology and larger surface
area as seen from BET measurements. Furthermore, sensors based on 10 mol% Eu-doped SnO2
nanostructures show the highest fluorescence quenching efficiency (0.23) as compared to pure SnO2
and other doped samples for the lowest concentration of Cd2+ (10 ppb) in drinking water with a Limit of
Detection (LOD) as low as 7 ppb; 0.007 mg mLꢀ1. The formation mechanism of Eu-doped SnO2
nanostructures has been discussed in detail.
Received 6th May 2019
Accepted 15th October 2019
DOI: 10.1039/c9ra03405f
rsc.li/rsc-advances
importance. Moreover, for environmental purication and organic
pollutant degradation, semiconductor photocatalysts receive
1. Introduction
continuous research attention for its advanced oxidation process
and moderate reaction conditions. Among various metal oxide
semiconductors, SnO2,6 CeO2,7 and Fe2O3 (ref. 1) are probably the
most common materials used for heavy metal ion detection and as
leading photocatalysts because of their photochemical stability,
low cost, non-toxicity and simple preparation.
Among them tin oxide (SnO2), an n-type semiconductor with
a wide direct band-gap (3.6 eV), has attracted extensive attrac-
tion due to its wide potential applications in photocatalysis,8
photodetectors,9 gas sensors,10 rechargeable batteries11 and
heavy metal ion detection.12 Usually, the morphologies of
nanosized metal oxide materials have an effect on their appli-
cations.13 Therefore, it is important to construct well controlled
morphologies which possess high surface to volume ratio and
have good performance and high reactivity.14
Till now, various SnO2 morphologies including nano-
particles,15 hollow spheres,16 nanosheets,17 nanoowers,18 poly-
hedrons19 and 3D hierarchical nanostructures20 have been
synthesized and extensively investigated. Among such classes of
materials, three dimensional (3D) nanostructures are deemed to
be the most effective, due to their large surface area, porous
nanostructures formed by the adjacent building blocks, low
density and better reactivity towards various applications such as
photocatalysis, sensing etc. Wang et al. have prepared various
In recent years, with prolonged urbanization and economic
development, the problems arising from volatile organic reagents,
toxic heavy metal ions, gases, and pollutants have become
increasingly serious. Therefore, it is necessary to detect such toxic
pollutants and chemicals without delay for the safety of people.
Chemical sensors based on semiconductor oxides play a vital role
in many areas nowadays, such as in medical diagnosis, environ-
mental protection, and detection of toxic gases, pollutants and
heavy metal ions.1 Heavy metal ions have always been a signicant
threat to human health and the environment, among which Cd is
of major concern, mainly due to its presence at relatively high
concentrations in drinking water and its effects on human health.2
Furthermore, according to the International Agency for Research
and Cancer (IARC), Cd is found to be a human carcinogen.3 Cd
contaminated drinking water leads to chronic renal failure.2 It can
cause both acute and chronic intoxication.4 It is highly toxic to the
kidneys. Upon long exposure at low concentrations, it could
become deposited in the kidneys, leading to kidney disease, fragile
bones and lung damage.5 Therefore, developing selective and
sensitive methods to detect and quantify metal ions at low
concentrations for environmental monitoring is of considerable
Department of Physics, Punjabi University, Patiala-147002, Punjab, India. E-mail:
kaurharsimranjot027@gmail.com
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morphologies of as synthesized SnO2 nanomaterials for photo-
catalytic applications.21 Liu and co-workers reported the
synthesis of hierarchical SnO2 nanostructures made of ultrathin
nanosheets.14 Ghosh et al. reported the synthesis of poppy ow-
erlike SnO2 nanoower array on Sn–foil by solvothermal
method.22 Wu et al. reported the synthesis of novel SnO2 nano-
owers by PEG and citric acid assisted hydrothermal method.23
Chen et al. have prepared 3D SnO2 nanoowers with nanoporous
petals using a controlled structure-preserving thermal-oxidation
process.24 Tan et al. have prepared SnO2 hollow spheres via
surfactant assisted hydrothermal method.25 He et al. have
demonstrated the synthesis and gas sensing properties of 3D
SnO2 nanostructures.26 Chen et al. synthesized SnO2 nano-
polyhedrons assembled from ultrathin SnO2 nanowires by SDS
assisted hydrothermal process.19
2. Experimental section
A simple hydrothermal method along with mild operating
conditions (reaction temp. < 200 ꢁC) has been used for the
synthesis of pure and doped SnO2 nanostructures. The precise
control over its synthesis conditions (temperature, duration, pH
etc), low energy consumption and less air pollution reect the
simplicity of this method.
2.1 Synthesis of europium doped SnO2 samples
All the chemicals and organic-solvents are analytical grade
reagents and used without any further purication. To synthe-
size europium-doped SnO2 samples, rstly 4 mmol SnCl4$5H2O
and 8 mmol C6H5Na3O7$2H2O were dissolved into 30 mL of
ethanol–water (1 : 1) solution. The resulting solution was
vigorously stirred for 1 h with the help of magnetic stirrer. Aer
vigorous stirring for 1 h, the solution was loaded into Teon-
Although, several researchers have reported the synthesis,
properties and applications of nanorods assembled ower-like
3D SnO2 nanostructures but mostly they had followed compli-
ꢁ
lined stainless steel autoclave and heated to 180 C for 24 h.
cated experimental procedures as seen in literature.27,28
A
Aer heat treatment, the autoclave was naturally cooled down
to room temperature. Aer cooling, the resultant product was
washed several times with de-ionized water and ethanol. Finally,
simple hydrothermal method is used in the present research to
synthesize 3D ower-like SnO2 nanostructures. As the sensing
and photocatalytic properties of nanomaterials are highly
dependent upon their morphology and size, so by tuning the
morphology of nanorods assembled nanoowers to smaller size
dimension will signicantly enhance their sensing and photo-
catalytic performance.
ꢁ
the obtained precipitates were annealed at 600 C for 2 h. The
same procedure was followed for the preparation of europium-
doped SnO2 samples, with the addition of europium(III) nitrate
pentahydrate (Eu (NO3)3$5H2O) to tin(IV) chloride solution before
the addition of tri-sodium citrate (C6H5Na3O7$2H2O) and the
concentration of Eu was varied from (1–10) mol%. Finally,
europium-doped SnO2 samples were collected.
Therefore a challenging work is to control the morphology of
3D SnO2 nanostructures by tuning the shape and size of nano-
rods or nanosheets lling interspaces to smaller dimensions for
their enhanced signicant performance. Doping method has
been extensively used in order to modify the electronic structure
of nanoparticles, inducing narrow energy band gap to achieve
improved optical, morphological, photocatalytic29 and sensing
properties.30 Recent experimental studies have shown that the
introduction of rare earth elements like La, Nd, Sm, Yb, Ce etc. in
wide band-gap semiconductors such as ZnO, SnO2, TiO2 etc.
results in their improved sensing31 and photocatalytic perfor-
mance.32 Rare earth doped compounds have been studied
because of their fast mobility of oxygen ions and catalytic prop-
erties which are one of the important factors in chemical
sensing.31 Various authors had reported the effect of doping of
Yb,31 La,33 Y34 and Ce35 on structural, optical, morphological,
photocatalytic and sensing properties of SnO2 nanostructures. To
the best of our knowledge, no attention has yet been focused on
heavy-metal ion sensing and photocatalytic performance of Eu-
doped SnO2 nanostructures. On the contrary, researchers tend
to explore the synthesis, morphology, optical, electrical, gas-
sensing and photoluminescence properties of Eu doped
SnO2.36–39 So, it is necessary to investigate the photocatalytic and
heavy-metal ion sensing properties of Eu-doped SnO2 nano-
structures. Moreover, the photocatalytic activity of as-synthesized
Eu-doped SnO2 nanostructures has been examined using
rhodamine-B dye as a test contaminant in aqueous media.
2.2 Characterization
The crystallographic, morphological, compositional and optical
properties of as-synthesized samples were investigated by various
analytical techniques. The crystallographic parameters of the
prepared samples were examined by X-ray diffractometer (Rigaku
Miniex-600 powder X-ray diffractometer, Cu-Ka1, l ¼ 0.154 nm)
in the 2q range varying from 20–80ꢁ, with a scanning rate and
step size of 4000ꢁ minꢀ1 and 0.02ꢁ at (40 kV, 15 mA), respectively.
The morphological features of the samples were studied by FEI
Quanta 200 F SEM Field Emission Scanning Electron Microscope
(FESEM). The chemical and elemental composition of the
samples was investigated by Energy Dispersive Spectroscopy
(EDS), attached with FESEM, respectively. The functional groups
of the samples were determined by Fourier Transform Infrared
Spectroscopy (FTIR, PerkinElmer spectrometer) in the range of
4000–400 cmꢀ1. The surface analysis of the samples was carried
out by BET (Autosorb-I Quatachrome Corp.). The optical prop-
erties were investigated by UV-vis absorption spectroscopy
(Hitachi U-2900) and (PL) photoluminescence spectroscopy
(Fluoromax-4 spectrouorometer) at room temperature.
2.3 Photocatalytic experiment
Furthermore, for sensing application, Eu-doped SnO2 nano- The photocatalytic degradation of rhodamine-B (RhB) dye was
structures have been effectively used for the detection of toxic performed using pure and Eu-doped SnO2 samples as the nano-
heavy metals like Cd2+ ion in water with the lowest concentration photocatalysts under UV irradiation exposure at room temper-
up to 10 ppb (parts per billion) level.
ature. An array of 4 UV tubes of 18 W each was served as the light
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source to provide UV-vis illumination. To carry out the experi- Billion (PPB) were prepared from stock solution by quantitative
ment, rstly 40 mg of nano-photocatalyst was added into dilution. To demonstrate the effect of Cd2+ ion on Eu-doped
100 mL of rhodamine-B (RhB) solution (3.2349 mg Lꢀ1). Prior to SnO2 NPs, 500 mL portions of several concentrations of Cd2+
UV irradiation exposure, the suspension was stirred in dark for ion (0–100 ppb) were added one at a time to 3 mL of Eu-doped
60 min to reach an adsorption–desorption equilibrium among SnO2 NPs solution with continuous stirring and the resulting
rhodamine-B (RhB) dye, nano-photocatalyst and water. At mixture was allowed to stand for 15 min at room temperature
certain time intervals of irradiation, 3–5 mL of samples were and the photoluminescence intensity of the resulting solution
collected and analyzed with UV-vis absorption spectrophotom- was recorded on spectrouorometer at 340 V.
eter (Hitachi U-2900) to record the remnant concentration of
rhodamine-B (RhB) by monitoring the corresponding absor-
bance of the characteristic peak at 554 nm.
3. Results and discussion
3.1 The structural characteristics
2.4 Detection of Cd2+ ion contamination in water by sensing
The crystallographic features of as-prepared samples are
the uorescence property of Eu-doped SnO2 nanostructures
investigated by X-ray diffractometer (XRD) as shown in Fig. 1(a).
The uorescence detection of aqueous Cd2+ in water was All the diffraction peaks are well indexed to tetragonal rutile
studied using europium-doped SnO2 NP solution at room structure of SnO2 (JCPDS no. 41-1445, a ¼ b ¼ 0.47382 nm and c
temperature. Various concentrations of Cd(NO3)2 (10 ppb, 20 ¼ 0.31871 nm, space group P42/mnm (136)). Also, no diffraction
ppb, 30 ppb, 40 ppb, 50 ppb, 80 ppb, 100 ppb) at Parts Per peak due to any other impurity has been detected in the spectra,
Fig. 1 XRD patterns of pure and Eu-doped SnO2 with different doping amount (a) 2q in the range of 20–80ꢁ; (b) comparison the shift of (110) and
(101) peaks; (c) plots showing the variation of lattice parameters with % Eu content and (d) plot showing the variation of crystallite size and
compressive strain with Eu (mol%) content.
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which indicates the purity of as-synthesized europium doped recorded, as shown in Fig. 2(a) and (b). The peaks of tin (Sn),
SnO2 samples. The crystallographic parameters calculated from oxygen (O) and europium (Eu) are clearly observed in the spectra
XRD patterns and Debye's Scherrer formula are listed in Table 1. which conrm the presence of europium (Eu) in SnO2 host
A decrease in the value of average crystallite size (D) and lattice lattice. Pure SnO2 comprises 38.31% oxygen (O), 61.69% tin (Sn)
parameters a, b and c has been observed with the increase in the and Eu-doped SnO2 samples comprises 55.92% oxygen (O),
doping concentration, indicating the successful incorporation 34.81 tin (Sn) and 9.27% europium (Eu) by weight. The
of europium ions into SnO2 host lattice which leads to lattice morphological features of as-prepared samples were investi-
distortion and suppressing the growth of larger SnO2 nano- gated by Field Emission Scanning Electron Microscopy
crystallites.40 To investigate the effect europium doping on the (FESEM), as shown in Fig. 3(a). As conrmed by FESEM images,
crystallinity of as-prepared 3D ower-like SnO2 nanostructures, pure SnO2 comprises ower-like morphology, grown in very
the 2q for (110) and (101) diffraction peaks has been monitored. high density. Further, it is observed that the ower-like struc-
Fig. 1(b) shows the comparison in the shi of 2q for (110) and tures are made of small-size agglomerated irregular-shape
(101) diffraction peaks which indicates a slightly right shi in nanorods, which intermingle with each other having their one
Eu-doped SnO2 samples, as compared with pure SnO2 and the side connected with the side surface of other nanorods. The
maximum the concentration is, the greater is the shi observed diameter of the nanorods is about 21–25 nm. The morphology
in the spectra. The right shi observed in Eu-doped SnO2 of 1% Eu-doped SnO2 is shown in Fig. 3(b). Compared with the
samples indicates the presence of compressive strain (as listed undoped sample, the products are composed of SnO2
in Table 1), which is calculated using the following equation:41
b cos q
1
h sin q
¼
þ
(1)
l
d
l
where q is the Bragg angle of the peak, l is the X-ray wavelength
and b is the Full Width Half Maximum (FWHM) in radians. As
seen in Fig. 1(d), an increase in the amount of compressive strain
has been examined from ꢀ0.031 to ꢀ0.046 with the decrease in
the value of average crystallite size for Eu-doped SnO2 samples.
Furthermore, only a right shi towards higher angle has been
observed in the case of Eu-doped SnO2 samples along with no
peak broadening (as seen in Fig. 1(b)), which indicates the
presence of homogenous strain in Eu-doped SnO2 samples, as in
the case of homogenously strained crystallite, the crystallites in
the sample were compressed by the same amount, which causes
only a shi in diffraction peak but no peak broadening. More-
over, a decrease in the values of lattice parameters has been
observed with the increase in doping concentration as seen in
(Table 1 and Fig. 1(c)). According to vegard's law, the subsitu-
tional incorporation of the dopant ions into host lattice, leads to
the shrinkage of lattice, while the dopant incorporation in the
interstitial sites between Sn and O atoms, however, will expand
the lattice.42,43 So, the results predicted from Table 1, indicates
the subsitutional doping of europium ions into SnO2 host lattice.
3.2 The morphological and compositional analysis
To study the elemental composition as-prepared of Eu-doped
SnO2 samples, the EDS spectra of the samples have been
Fig. 2 (a) EDS spectra of pure SnO2; (b) 10 mol% Eu-doped SnO2.
Table 1 Crystallographic parameters of pure and Eu-doped SnO2 samples calculated from XRD patterns
Lattice constant
a ¼ b (A)
Lattice constant
c (A)
Crystallite
size D (nm)
˚
˚
Specimen
Strain (h)
Pure SnO2
4.7636
4.7566
4.7333
4.7318
4.7243
4.7216
4.7382
3.1949
3.1938
3.1862
3.1826
3.1822
3.1819
3.1871
33.58
33.44
32.09
29.17
25.67
20.02
—
ꢀ0.031
ꢀ0.033
ꢀ0.036
ꢀ0.039
ꢀ0.042
ꢀ0.046
1 mol% Eu–SnO2
3 mol% Eu–SnO2
5 mol% Eu–SnO2
7 mol% Eu–SnO2
10 mol% Eu–SnO2
Std JCPDS 41-1445
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Fig. 3 FESEM images of (a) pure SnO2; (b) 1 mol%; (c) 3 mol%; (d) 5 mol%; (e) 7 mol% and (f) 10 mol% Eu-doped SnO2 samples.
nanoheets, in place of nanorods, with the average diameter of lattice results in SnO2 nanorods of longer length (Fig. 3(f)) as
about ꢂ21 ꢃ 3 nm. 3% Eu-doped SnO2 nanostructures, as compared to nanorods observed in the case of (Eu : SnO2 7%)
shown in Fig. 3(c) consist of hollow triangle-shape SnO2 nano- with an average diameter reduced to ꢂ15 ꢃ 3 nm.
sheets, with an average diameter of ꢂ21 ꢃ 4 nm. Mix
Interestingly, from FESEM images, it has been observed that
morphology of small spheres and SnO2 nanorods has been the morphology of SnO2 nanorods changes rst into nano-
observed in the case of 5% Eu-doped SnO2 SnO2 (Fig. 3(d)). By, sheets, then from nanosheets to nanorods alongwith spheres
increasing the Eu dopant concentration from 5 to 7%, the and nally results into nanorods of longer length, aer the
morphology comprises only SnO2 nanorods, in place of nano- doping of Eu element into pure SnO2. The basic reason behind
rods alongwith spheres (as observed in 3% Eu doped SnO2), this is the subsitutional incorporation of Eu ions into SnO2 host
with an average diameter of ꢂ19 ꢃ 2 nm as seen in Fig. 3(e). lattice, which modies the local structure of host lattice, thereby
Maximum dopant concentration of Eu (10 mol%) in SnO2 host leading to the variation in the surface energy of some crystal
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planes, which further changes their growth rate.44 Therefore, with tin precursor plays an important role to control the
europium serves as a structure-modifying agent in the growth of morphology of as-synthesized samples. When added along with
nanorods. It can be observed from Fig. 3(a)–(f) that the diameter tin precursor, forms a complex with tin ions because of its
and thickness of various morphologies of nanostructures is strong coordination ability and will lead to decrease in the
reduced gradually with the increase in the amount of Eu concentration of free tin ions in the solution. The as-formed
doping, which is the result of suppression of grain boundary new complex slows down the rate of reaction, thereby forming
migration and increasing the energy barrier for grain growth.45 3D ower-like nanostructures.14 Furthermore, it has been
examined that the SnO2 morphology could be successfully
controlled by varying the Eu dopant concentration in a reason-
able range, which indicates the subsitutional doping of euro-
pium ions in SnO2 host lattice.
3.3 Growth mechanism
The formation mechanism of pure and europium-doped SnO2
nanostructures is explained in Fig. 4. Firstly, the nucleation of
tiny SnO2 nanocrystals takes place from the hydrolysis of tin
precursor. Aer nucleation process, the tiny crystals of SnO2
aggregated and grew into SnO2 nanorods by oriented attach-
ment process.46 Aer that, the new particles formed will further
grow into another nanorod by spontaneously landing on as-
formed nanorods, in order to control their total surface
energy,47,48 forming ower-like SnO2 nanostructures as illus-
trated in Fig. 4(a). Here, sodium citrate, which is added along
Firstly, compound nuclei will be formed by introducing
europium ions with different molar concentrations (1.3.5.7 &
10%) into the precursor solution, due to subsitutional incor-
poration of europium ions into SnO2 host lattice which takes
place only with the addition of europium ions into precursor
solution with the same molar ratio (1 : 1). This will lead to the
generation of double-ionized oxygen vacancies, which further
changes the surface energy and density of charges of each
crystal face49 and thereby leading to different growth rates of
Fig. 4 (a) Schematic formation mechanism of pure SnO2 nanostructures; (b) (1, 3, 5, 7 & 10 mol%) Eu-doped SnO2 nanostructures.
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various crystal faces. Then, these primary SnO2 nanocrystals observed with an increase in europium dopant concentration
oriented grow with the driving force of decreasing surface from 0 to 10 mol%. The band-gap of pure and Eu-doped SnO2
energy into different structures through aggregation mecha- samples has been calculated using Tauc relation, as given
nism50 and Ostwald ripening process,51 under the inuence of below:
europium (Eu) as illustrated in Fig. 4(b).
ahn ¼ A(hn – Eg)n
(2)
3.4 Functional group analysis
where a is the absorption coefficient, A is constant, Eg is the
band-gap of semiconductor and n ¼ 1/2 for direct band-gap
semiconductor.56 Fig. 6(b) shows the Tauc plot of (ahn)2 versus
the photon energy (eV) of europium-doped SnO2 samples. The
measured values of band-gap are found to be 3.60, 3.65, 3.80,
3.87, 3.90 and 4.07 eV for pure, 1%, 3%, 5%, 7% and 10% Eu-
doped SnO2 samples. It has been found that with an increase
in europium-doping concentration from 0 to 10 mol%, the
band-gap increases. 10% europium-doped SnO2 samples
possess larger band-gap value, which may be attributed to the
variation in lattice defects.
Fig. 5 shows the FTIR spectra of as-synthesized samples. The
bands in 680–721 cmꢀ1 region corresponds to the O–Sn–O
bending vibration, while the bands observed in 471–618 cmꢀ1
region are due to the antisymmetric vibration of Sn–O–Sn mode
of tin oxide. The broad absorption peak at ꢂ3426 cmꢀ1 attrib-
uted to the O–H stretching vibration.52 The peak at 1110 cmꢀ1 is
due to Sn–OH vibration.53 Peaks in the region 1448–1480 cmꢀ1
are attributed to the CH2 bending vibrations. 1700–1729 cmꢀ1
peaks corresponds to C]O stretch.
Moreover, a small shi in Sn–O–Sn peak has been observed
in the spectra, which can be explained by Hooke's law54 which
states that the vibration frequency is indirectly proportional to
the reduced mass i.e. the vibrational frequency shis to the
lower value with an increase in the reduced mass. A slight shi
of Sn–O–Sn peak to lower value has been observed with
a decrease in doping concentration from 10 mol% to 0 mol%.
In order to investigate the presence of defects in as-
synthesized
europium-doped
SnO2
samples,
the
3.5 Optical properties
To investigate the optical properties of europium doped SnO2
samples, the UV-vis absorption spectra has been recorded as
shown in Fig. 6(a). The absorbance of light by nanoparticles is
greatly inuenced by the presence of impurity centers.55 All the
samples exhibit absorbance in 300–340 nm region. Moreover,
a blue-shi in the absorption edges of the samples has been
Fig. 5 FTIR spectra of (a) pure; (b) 1 mol%; (c) 3 mol%; (d) 5 mol%; (e) Fig. 6 (a) UV-vis absorption spectra; (b) Tauc plot of band gap energy
7 mol% and (f) 10 mol% Eu-doped SnO2 samples.
for europium-doped SnO2 samples.
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ultraviolet region (350 nm) and other strong band in visible
region (666 nm). The rst peak at 350 nm is assigned to Near
Band Edge (NBE) emission, which is due to the radiative
recombination of photo-generated charge carriers. Apart from
this peak, another visible emission peak at 666 nm has been
observed, which is attributed to oxygen defects related shallow
and deep trap centers. Mostly, oxygen vacancies are known to be
the most common defects present in metal oxide semi-
+
conductors, existing in three different charge states;57 VO0 , VO
and VO2+. The peak at 666 nm is attributed to the visible emis-
sion from deep trap centers, whose origin can be assigned to the
radiative recombination of electrons in deep level states with
the photo-excited holes in the valence band.
It has been observed that, with an increase in Eu dopant
concentration from 1 to 10 mol%, the intensity of strong band
at 666 nm gradually increases, while that of weak band at
350 nm tends to merge with each other.
During subsitutional incorporation of Sn4+ ions by Eu3+ ions,
ꢀ1 charge of the substituted Sn4+ ion has to be compensated
somewhere from the lattice in the form of oxygen vacancy. The
oxygen vacancies thus created will act as radiative luminescence
centers giving rise to defect levels, thereby enhancing the photo-
luminescence intensity of europium-doped SnO2 doped samples.
Fig. 7 PL emission spectra (inset; enlarged spectra) of pure and
europium-doped SnO2 samples.
photoluminescence emission spectra have been recorded,
keeping the excitation wavelength at 330 nm, as shown in Fig. 7.
The emission spectra consist of two regions, one weak band in
Fig. 8 (a) Nitrogen adsorption and desorption isotherms of pure SnO2; (b) 10% europium-doped SnO2; (c) pore size distribution curve of pure
and (d) 10% Eu-doped SnO2.
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Increase in photoluminescence intensity will dominate the shown inset in Fig. 8(a) and (b), which reveal the pore diameter
photocatalytic activity. But, in our study, 10 mol% europium- for pure and doped SnO2 as 2.0 and 2.2 nm, respectively. The
doped SnO2 sample exhibits highest photoluminescence larger surface area of 10 mol% Eu-doped SnO2 leads to its
intensity as well as highest photocatalytic activity (as seen in enhanced photocatalytic activity.
Fig. 10(a)). The reason behind the highest photocatalytic activity
of 10% europium-doped SnO2 sample is its larger surface area
(as seen from BET; Fig. 8(b)) and more UV absorbance (as seen
3.7 Photocatalytic degradation of RhB dye by europium-
doped SnO2 samples
in Fig. 6(a)). Larger surface area and absorption of light by
In order to examine the photocatalytic performance of
10 mol% europium-doped SnO2 leads to the presence of more
europium-doped SnO2 samples for the degradation of organic
photo-generated charge carriers on the surface and the carriers
pollutants, photocatalytic degradation experiments of RhB dye
which are not trapped by the defect states may migrate towards
has been carried out under UV exposure. The time-dependent
absorption spectra of dye solution has been recorded aer
every 10 min time interval of UV exposure to measure the
the surface of nanoparticles through lattice, thereby enhancing
the probability of charge transfer to the surface and dominating
the radiative recombination between the carriers leading to
photocatalytic performance of as-synthesized Eu-doped SnO2
enhancement in photocatalytic activity.
samples. Fig. 9(a)–(f) shows the variation in the time-dependent
absorption spectra of RhB dye in the presence of Eu-doped SnO2
samples. As seen in Fig. 9, the intensity of absorption maxima (l
3.6 Nitrogen adsorption–desorption isotherms
The simple explanation for the enhanced photocatalytic activity ¼ 553 nm) of RhB dye has been greatly inuenced by all the
of the europium-doped SnO2 samples is their increased surface prepared pure and doped samples and decreases rapidly with
area. To study the surface analysis of pure SnO2 by europium the extension of exposure time. Fig. 10(a) shows the degradation
doping, nitrogen adsorption–desorption isotherm curves has of RhB dye as a function of time by pure and europium-doped
been recorded for pure and 10 mol% europium-doped SnO2. As SnO2 samples under UV exposure of 70 min. It can be seen
shown in Fig. 8(a) and (b), the isotherm exhibits a type V H2 that aer 70 min of UV irradiation exposure, 10 mol% europium
hysteresis loop. The surface area of pure SnO2 from BET anal- doped SnO2 exhibits the highest percentage of degradation
ysis is found to be 30.50 m2 gꢀ1, whereas it is increased to 58.01 (ꢂ90%) as compared to other samples as shown in Fig. 10(b).
m2 gꢀ1 in the case of 10 mol% Eu-doped SnO2. The pore size The main reason behind the remarkable photocatalytic activity
distribution curves of pure and 10 mol% Eu-doped SnO2 are of 10% Eu-doped SnO2 nanostructures may be attributed to its
Fig. 9 Time dependent absorption spectra of rhodamine-B dye degraded in the presence of (a) pure SnO2; (b) 1% Eu-doped SnO2; (c) 3% Eu-
doped SnO2; (d) 5% Eu-doped SnO2; (e) 7% Eu-doped SnO2 and (f) 10% Eu-doped SnO2 samples.
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specic morphology and smaller thickness of nanorods, as this
leads to the adsorption of large no of reactive species and
oxygen molecules by providing more active sites and allowing
effective transport for the reactant molecules to get to the active
sites. Moreover, the larger surface area of the 10% Eu-doped
SnO2 (as seen from BET measurements; Fig. 8(b)) can help to
diffuse the reactant molecules to the active sites, which further
results into the enhancement of loading dye molecules and
excellent photocatalytic degradation.6 Furthermore, larger
surface to volume ratio of 10% Eu-doped SnO2 sample leads to
more surface oxygen vacancies, thereby increasing its surface
activity and hence photocatalytic performance.
Fig. 11 shows the proposed schematic mechanism of RhB
degradation by europium-doped SnO2 nanophotocatalysts under
UV light exposure. The overall degradation process is as follows:
at rst, SnO2 absorbs photon energy greater than or equal to its
band gap, which leads to the excitation of electrons from valence
band to conduction band, generating electron–hole pairs. The
excited electrons from conduction band of SnO2 may in turn
migrate to surface deposited europium (Eu) nanoparticles
because of higher energy level of conduction band of SnO2 as
compared to Fermi level (Efe) of Eu–SnO2 and hence get trapped
by the defect states introduced by europium doping. Therefore,
europium nanoparticles will act as temporary trap centers for
photogenerated electrons, preventing their recombination with
holes. Moreover, UV irradiation exposure leads to the excitation
of rhodamine-B (RhB) dye molecules, which are adsorbed onto
the surface of SnO2 nanostructures. Upon excitation, the RhB dye
molecules will transfer the electrons from LUMO into the
conduction band of SnO2. The photogenerated electrons
produced from above mentioned processes will than react with
oxygen molecules (O2) molecules adsorbed on the surface of
photocatalyst, forming superoxide anion radicals (O2cꢀ). The as
formed O2cꢀ anion radicals will than react with (eCBꢀ) electrons
and proton (H+), forming hydrogen peroxide (H2O2). The
hydrogen peroxide (H2O2), by taking an electron will dissociate
into highly reactive hydroxyl radical (OH). Futhermore, the pho-
togenerated holes will react with water molecules (H2O) forming
Fig. 10 (a) RhB degradation as function of time by europium-doped
SnO2 samples; (b) plot showing the variation of degradation rate (%) by
all the samples w.r.t time.
Fig. 11 Proposed schematic mechanism of RhB degradation over the surface of europium-doped SnO2 nanophotocatalyst under UV
illumination.
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Fig. 12 PL emission spectra of the aqueous suspension of (a) pure SnO2; (b) 1 mol%; (c) 3 mol%; (d) 5 mol%; (e) 7 mol% and (f) 10 mol% Eu-doped
SnO2 samples in the presence of Cd2+ at different concentrations; (g) calibration fitting curve of the linearity of Stern–Volmer plot for Cd2+ at
different concentrations in the aqueous suspension of pure SnO2; (h) 1 mol%; (i) 3 mol%; (j) 5 mol%; (k) 7 mol% and (l) 10 mol% Eu-doped SnO2
samples.
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ꢀ
Table 2 LOD and quenching efficiencies of europium-doped SnO2
samples
H2O2 + eCB / OH + OHc
(8)
(9)
OHc + RhBc+ / CO2 + H2O
Limit of detection
Quenching efficiency
at (10 ppb)
Samples
(mg mLꢀ1) (LOD)
0% Eu : SnO2
1% Eu : SnO2
3% Eu : SnO2
5% Eu : SnO2
7% Eu : SnO2
10% Eu : SnO2
0.009; 9 ppb
0.010; 10 ppb
0.010; 10 ppb
0.010; 9 ppb
0.008; 8 ppb
0.007; 7 ppb
0.19
0.17
0.12
0.10
0.20
0.23
3.8 The quenching effect of Cd2+ ion on PL of europium-
doped SnO2 samples
In the present study, the optical response of pure and
europium-doped SnO2 samples has been tested with regard to
the possible uorescence variations against the addition of
different concentrations of Cd2+ ion (0, 10, 20, 30, 40, 50, 80 and
100 ppb; 0–100 ppb; 0–0.10 mg mLꢀ1) in the aqueous solution at
pH ¼ 6. The prepared aqueous solution of europium-doped
SnO2 samples exhibited a signicant response to Cd2+ ion.
Fig. 12(a)–(f) show the PL emission spectra at excitation wave-
length lex ¼ 330 nm of the reference aqueous suspension of
europium-doped SnO2 samples in the presence of Cd2+ at
different concentrations. As illustrated in Fig. 12, Cd2+ effi-
ciently quenched the uorescence signal ‘F’, which decreases
gradually with an increase in ion concentration without having
any appreciable change in the spectral prole of all the samples.
The quenching of the uorescence intensity can be quantied
by a Stern–Volmer plot which describes the relationship
between uorescence intensity and quencher concentration.
Fig. 12(g)–(l) shows the Stern–Volmer plots of the quenching
experiment (calibration curve of F0/F versus Cd2+ ion concen-
tration in the range 0–0.10 mg mLꢀ1; 0–100 ppb) by europium-
hydroxyl radicals (OHc). The hydroxyl radicals (OHc) formed are
known to be the powerful oxidizing agents, which will react with
dye molecules leading to de-mineralization of products into
harmless substances i.e. CO2 and H2O. The reactions of whole
process are summarized as follow:58
ꢀ
+
)
SnO2 + hn / SnO2 (eCB + hVB
(3)
(4)
(5)
(6)
(7)
eꢀ + O2 / O2
ꢀ
Eu3+ + 3eꢀ / Eu
h+ + H2O / H+ + OHc
O2c + eCB + 2H+ / H2O2
ꢀ
Table 3 Permissible limits for different parameters of drinking water set by different agencies i.e. (WHO), (ISI), (USEPA), (CPCB) and (ICMR)62
Parameters
USEPA
WHO
ISI
ICMR
CPCB
pH (mg Lꢀ1; mg mLꢀ1
)
6.5–8.5
—
6.5–8.5
0.1
6.5–8.5
0.3
6.5–9.2
1.0
6.5–8.5
1.0
Iron (mg Lꢀ1; mg mLꢀ1
)
Sulphate (mg Lꢀ1; mg mLꢀ1
Calcium (mg Lꢀ1; mg mLꢀ1
Cadmium (mg Lꢀ1; mg mLꢀ1
Magnesium (mg Lꢀ1; mg mLꢀ1
Copper (mg Lꢀ1; mg mLꢀ1
Arsenic (mg Lꢀ1; mg mLꢀ1
Lead (mg Lꢀ1; mg mLꢀ1
Zinc (mg Lꢀ1; mg mLꢀ1
Mercury (mg Lꢀ1; mg mLꢀ1
Chromium (mg Lꢀ1; mg mLꢀ1
)
)
—
—
0.005
—
1.3
0.05
—
—
0.002
0.1
—
75
0.005
50
1.0
0.05
0.05
5.0
0.001
—
150
75
0.01
30
0.05
0.05
0.10
5.0
0.001
—
400
200
0.01
—
400
200
No relaxation
100
1.5
No relaxation
No relaxation
15.0
No relaxation
No relaxation
)
)
)
)
1.5
0.05
0.05
0.10
0.001
—
)
)
)
)
Table 4 Comparison of measured heavy metal ions in different places63
Measured concentration (mg Lꢀ1; mg mLꢀ1
)
Location
Cd
Ni
Hg
Mn
Pb
Cu
Fe
Co
Sources
Gombe
India
Ethiopia
Pakistan
Dutse
—
—
0.003
—
0.00
0.043
—
—
—
—
0.12
—
—
—
0.273
—
0.003
—
0.351
0.015
0.02
—
BDL
0.433
0.015
0.62
—
—
1.676
1.824
—
—
0.018
—
0.041
64
65
66
67
63
0.002
0.017
0.067
0.003
2.283
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Fig. 13 Bar graphs representing fluorescence quenching efficiency obtained for Eu-doped SnO2 solution with Cd2+ at (a) 0.01 mg mLꢀ1; (b) 0.02
mg mLꢀ1; (c) 0.03 mg mLꢀ1; (d) 0.04 mg mLꢀ1; (e) 0.05 mg mLꢀ1; (f) 0.08 mg mLꢀ1; (g) 0.10 mg mLꢀ1 and (h) comparison of quenching efficiencies of
Eu-doped SnO2 samples at different Cd2+ ion concentrations.
doped SnO2 samples. For a static quenching process, plotting describing static quenching with regression coefficient R2
¼
relative uorescence intensity (F0/F) against the concentration 0.997, 0.997, 0.996, 0.996, 0.995, 0.996 for 0%,1%, 3%, 5%, 7% &
(Q) should produce a linear line.59 The graphs shows the onset 10% europium-doped SnO2 samples. Here F0 is the uorescence
of good linearity from 0–0.10 mg mLꢀ1 Cd2+ concentration range intensity of the samples at initial time t ¼ 0 and F is the
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uorescence intensity at time t. The Limit of Detection (LOD) parameters such as cadmium (Cd), mercury (Hg), iron (Fe), lead
has been calculated based on IUPAC denition (3s of the (Pb), arsenic (As), chlorides, uorides, nitrite etc. are compared
reagent blank signal divided by the slope of the calibration for different agencies in Table 3. The permissible limit of heavy
curve, while s represents standard deviation60) of all the metal ions and other parameters set by different agencies does
samples, listed in Table 2. The LOD value of Cd2+ for 10% Eu- not show uniformity. For cadmium (Cd), the permissible limit
doped SnO2 sample (0.007 mg mLꢀ1; 7 ppb) has found to be set by different agencies are (mg Lꢀ1; mg mLꢀ1): USEPA 0.005,
well below the lowest possible concentration of our experiment WHO 0.005, ISI 0.01, ICMR 0.01 and CPCB no relaxation.62
(10 ppb, 0.010 mg mLꢀ1), as compared to other samples. More- According to ISI and ICMR, the permissible limit of Cd in
over, the quenching efficiency of all the samples has been drinking water is 0.01 mg mLꢀ1; 10 ppb. Moreover, the level of
calculated using the following equation:61
different heavy-metal ions like Cd, Ni, Hg, Mn, Pb, Cu, Fe and
Co in drinking water in different places has been evaluated by
various researchers (Table 4) by collecting the drinking water
samples and the results were analyzed using Atomic Absorption
Spectrometer (AAS). The results showed that ꢀco1ncentration of
cadmium (Cd) ranges from 0.0 to 0.06 mg L ; mg mLꢀ1, for
cobalt (Co) ranges from 0.027 to 0.08 mg Lꢀ1; mg mLꢀ1, for
copper (Cu) ranges from 0.0 to 0.08 mg Lꢀ1; mg mLꢀ1, for iron
(Fe) ranges from 0.0 to 23.26 mg Lꢀ1; mg mLꢀ1, for lead (Pb)
ranges from 0.0 to 0.19 mg Lꢀ1; mg mLꢀ1, for manganese (Mn)
ranges from 0.035 to 1.79 mg Lꢀ1; mg mLꢀ1, for mercury (Hg)
ranges from 1.380 to 3.15 mg Lꢀ1; mg mLꢀ1 and for nickel (Ni)
ranges from 0.00 to 0.09 mg Lꢀ1; mg mLꢀ1, among which the
value for cadmium (Cd) exceeded the recommended level set by
USEPA and WHO (0.005 mg Lꢀ1; mg mLꢀ1).63 This implies that
drinking water of these sampling regions is polluted and if used
without any treatment, can cause polluted and if used without
any treatment, can cause serious health effects to people around
these areas. The detection limit of the sensor 10% Eu-doped
SnO2 (0.007 mg mLꢀ1; 7 ppb) is lower than the ISI and ICMR
limit of Cd in drinking water (10 ppb; 0.01 mg mLꢀ1) which
indicates that it can be used as an efficient sensors for the
detection of Cd (present below 10 ppb; 0.01 mg mLꢀ1) in
drinking water in such polluted areas.
QE ¼ 1 – I/I0
(10)
where, I0 is the initial uorescence intensity in the absence of
analyte and I is the uorescence intensity in the presence of
corresponding analyte. Fig. 13(a)–(h) shows the bar graphs
corresponding to uorescence quenching efficiency obtained
for pure and europium-doped SnO2 samples with Cd2+
concentration range (0–0.010 mg mLꢀ1). Among all the samples,
only 10% europium-doped SnO2 caused maximum uorescence
quenching at lowest level of Cd2+ concentration (10 ppb; 0.010
mg mLꢀ1) with an efficiency of about 0.23 as compared to other
samples (as seen in Fig. 13(h)), which shows that 10%
europium-doped SnO2 sample exhibits highest sensitivity
towards the lowest ppb level of Cd2+ concentration (0.010 mg
mLꢀ1; 10 ppb), with LOD of 0.007 mg mLꢀ1; 7 ppb, which might
be attributed to smaller size and thickness of Eu-doped SnO2
nanorods.
It has been found that permissible limits of different heavy
metal ions in drinking water has set by different agencies i.e.
(WHO), (ISI), (USEPA), (CPCB) and (ICMR). But, a bias has been
observed in the permissible limits of drinking water set by these
different agencies. Permissible limits for drinking water
Fig. 14 (a) Selectivity bar diagram of the fluorescence quenching of 10% Eu-doped SnO2 (3 mL) towards Cd2+ and other heavy metal ions with
a concentration of (0.10 mg mLꢀ1); (b) the fluorescence intensity of 10% europium-doped SnO2 (3 mL) in the presence of 0.10 mg mLꢀ1 of various
metal ions with an added of 0.10 mg mLꢀ1 Cd2+
.
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SAIF, Panjab University; Chandigarh, SAI Lab Thapar Univer-
sity; Patiala, IIT Madras, IIT Kanpur for providing the necessary
laboratory facilities to carry out this work.
3.9 Sensing of 10% Eu-doped SnO2 towards other heavy-
metal ions
The uorescence property of 10% Eu-doped SnO2 in the pres-
ence of different heavy-metal ions like Cd2+, Ca2+, Ba2+, Cu2+,
Zn2+, Sr2+, Na+, K+, Hg2+, Co2+ and Pb2+ has been tested as seen
in Fig. 14(a). The uorescence intensity has been quenched in
the presence of Cd2+ ions whereas for other heavy-metal ions
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