Influence of morphology on the photocatalytic and fiber optic ammonia gas sensing performance of tin oxide nanostructures by a novel microwave irradiation method

Article abstract of DOI:10.1007/s13738-019-01702-6

Tin oxide hexagonal-shaped nanodisks (NDs) and nanowires (NWs) were successfully prepared by one-step microwave irradiation method using cetyltrimethylammonium bromide (CTABr) and polyethylene glycol (PEG) as surfactants. The XRD pattern indicates that SnO₂ is crystalline with tetragonal rutile structure. TEM micrographs confirm SnO₂ nanodisks, approximately 100?nm in width and 20?nm in thickness, and a straight single SnO₂ nanowire with a diameter of about 30?nm and length up to several micrometers for CTAB- and PEG-assisted samples, respectively. The elemental composition and oxidation state were also confirmed through EDS and XPS analyses. Effect of morphology on the photocatalytic performance of SnO₂ nanostructures was studied toward degradation of methylene blue (MB) and rhodamine B (RhB) under visible light irradiation. For SnO₂ NDs, it was observed that 99% and 97% of MB and RhB dyes were degraded in 100?min of irradiation time. In contrary, the SnO₂ NWs showed high sensitivity (71.4?counts/ppm), fast response (35?min) and recovery time (25?min) toward ammonia gas compared to SnO₂ NDs. This could be attributed to large surface area and high adsorption of ammonia molecules on the SnO₂ surface.

Full text of DOI:10.1007/s13738-019-01702-6

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01702-6
ORIGINAL PAPER
Infuence oꢀ morphology on the photocatalytic and ꢁber optic
ammonia gas sensing perꢀormance oꢀ tin oxide nanostructures
by a novel microwave irradiation method
M. Parthibavarman1 · S. Sathishkumar · S. Prabhakaran
2
3
Received: 14 October 2018 / Accepted: 20 May 2019
©
Iranian Chemical Society 2019
Abstract
Tin oxide hexagonal-shaped nanodisks (NDs) and nanowires (NWs) were successfully prepared by one-step microwave
irradiation method using cetyltrimethylammonium bromide (CTABr) and polyethylene glycol (PEG) as surfactants. The
XRD pattern indicates that SnO is crystalline with tetragonal rutile structure. TEM micrographs conꢀrm SnO nanodisks,
2
2
approximately 100 nm in width and 20 nm in thickness, and a straight single SnO nanowire with a diameter of about 30 nm
2
and length up to several micrometers for CTAB- and PEG-assisted samples, respectively. The elemental composition and
oxidation state were also conꢀrmed through EDS and XPS analyses. Eꢁect of morphology on the photocatalytic performance
of SnO nanostructures was studied toward degradation of methylene blue (MB) and rhodamine B (RhB) under visible light
2
irradiation. For SnO NDs, it was observed that 99% and 97% of MB and RhB dyes were degraded in 100 min of irradia-
2
tion time. In contrary, the SnO NWs showed high sensitivity (71.4 counts/ppm), fast response (35 min) and recovery time
2
(
25 min) toward ammonia gas compared to SnO NDs. This could be attributed to large surface area and high adsorption of
2
ammonia molecules on the SnO surface.
2
Keywords SnO nanostructures · Microwave irradiation · Catalyst · Fiber optic · Ammonia gas · High sensitivity
2
Introduction
and surface defect concentration. Tin oxide, an important
n-type semiconductor with a wide band gap of 3.65 eV, has
been the most considered material used in a broad range
of application such as photocatalysts [5, 6], solar cell [7],
sensors [8, 9], electrode materials for Li-ion batteries [10],
optical electronic devices [11, 12] and transparent conduc-
tors [13].
During the past decades, nanostructured metal oxides have
fascinated great interest due to their exclusive properties and
potential applications. Semiconducting oxides, with exclu-
sive and proꢀcient properties [1], have recently fascinated
serious attention in the ꢀelds of catalytically decomposing
[
2] and detecting various organic compounds [3] that are
Generally, nanodimensional with large surface area
would oꢁer great opportunities to investigate new physi-
cal and chemical applications including gas sensors and
photocatalysis. Recently, many eꢁorts have been made to
usually volatile and toxic. Based on the adsorption–reac-
tion–desorption process, both photocatalytic and gas sens-
ing reactions generally happen on the surfaces [4], and the
performance frequently depends on the eꢂcient surface area
improve the sensing and catalytic behavior of SnO using
2
doping [14–17], adding catalysts [18–20] and construct-
ing the heterojunctions [21, 22]. Among these, surfactant-
assisted synthesis of metal oxides inꢃuences the properties
of obtained powder, particularly the shape, size, crystal
morphology and the degree of crystallinity. These factors
manipulate the catalytic and gas sensing performance of
the metal oxide semiconductors. Trunk et al. [23] have pre-
*
M. Parthibavarman
varmanphysics85@gmail.com
1
2
3
PG and Research Department of Physics, Chikkaiah Naicker
College, Erode, Tamilnadu 638 004, India
Department of Physics, Mahendra Engineering College,
Namakkal, Tamilnadu 637 503, India
pared SnO nanorods by a hydrothermal method using tin
2
chloride, liquid ammonia, sodium hydroxide and cetyltri-
Centre for Crystal Growth, VIT University, Vellore,
Tamilnadu 632 014, India
methyl ammonium bromide (CTAB) as starting materials
Vol.:(0
1
123456789)
3

Journal of the Iranian Chemical Society
and studied their gas sensing and photocatalytic properties.
Malik et al. [24] have prepared ꢃower like SnO using CTAB
2
as surfactant and studied the photocatalytic degradation of
various organic pollutants such as MO, MB and RhB. In an
endeavor to assist and improve the photocatalytic and gas
detecting behavior of SnO nanostructures, we have using
2
two surfactants such as CTAB- and PEG-assisted SnO
2
nanostructures (NDs and NWs) and studied their structural,
morphological, catalytic and ammonia gas sensing proper-
ties synthesized by microwave method. Compared with the
above processes, the microwave method has sparked much
interest due to their operation simplicity, eꢁective, low-cost
route to synthesis, less time-consuming (about 10 min) and
for large-scale production.
Experimental procedure
Fig. 1 Powder XRD pattern of SnC and SnP samples, respectively
Synthesis of SnO nanostructures
2
For the preparation of nanostructured SnO powder, 2.25 g
Table 1 Shows the average grain size, lattice parameters and volume
2
of SnC and SnP samples, respectively
tin (II) tetrachloride was dissolved in 50 ml distilled water
under strong stirring for 30 min. Aqueous ammonia was
added dropwise to the resulting solution in order to modulate
the pH at 10. The pH of the sol was found to be 6.5 before
Sample Grain
size
Morphology Lattice param- Cell volume (Å3)
eters
(
nm)
a (Å) b (Å)
adding of NH OH. The sol was then left for constant stir-
4
SnC
SnP
43
65
Nanodisks
Nanowires
4.7012 3.1856 70.40
4.7087 3.1891 70.81
ring for about 30 min. Aqueous solution of 0.5 g CTAB and
PEG-400 was then added in the above solution to obtain an
emulsiꢀed solution and stirred for 2 h followed by addition
of 15 ml of ethanol. Then, the solid gel was transferred into
a Teꢃon-lined house hold microwave oven and maintained
at 120 °C for 15 min. The Teꢃon-lined bowl has been used
to transfer the solution to microwave oven. The temperature
of the sol was found to be 70 °C during the microwave irra-
diation process. The precipitate was collected and washed
with deionized water. The gel was further dried at 60 °C
for overnight. The CTAB- and PEG-assisted samples were
named as SnC and SnP. The color of the ꢀnal product was
pale yellow and gray for SnC and SnP, respectively.
Debye–Scherrer’s formula [26] and found to be 43 nm and
65 nm for SnC and SnP samples, respectively. Figure 2a
shows the SEM image of SnC sample. From the SEM
image, the nanodisks are clearly shown. The nanodisks
are about 100 nm in diameter and 20 nm in width. It is
fascinating to note that most of the nanodisks display hex-
agonal shape, but some broken nanodisks were also seen
in the micrographs. Figure 2b, c shows the corresponding
TEM images, and hexagonal-shaped nanodisks was further
conꢀrmed. Meanwhile, high-resolution images (HRTEMs)
are shown in Fig. 2c with detailed lattice structure. The
interplanar distances are 2.63 Å and 3.44 Å, corresponding
Results and discussion
Figure 1 shows the wide-angle XRD patterns of SnC and
SnP samples, respectively. All the diꢁraction peaks can be
to the (101) and (110) planes of rutile SnO , respectively.
2
The SAED pattern (Fig. 2c) showed bright and sharp spots,
evidenced to SnO with a tetragonal structure, which are
which have been indexed single-crystalline nature. Fig-
2
reliable with the standard values of Joint Committee on
Powder Diꢁraction Standards (JCPDS card no. 41-1445).
No other impurity phases such as surfactant or other tin
species were detected, which demonstrate samples with
relatively high-purity crystal. Moreover, the calculated
lattice parameters and volume are good persistent with
the already reported values [25] and the results are shown
in Table 1. The average grain size was calculated from
ure 2d shows the typical SEM image of SnO NWs. It can
2
be seen that SnO nanowires have a diameter of ~ 30 nm
2
and a length up to several tens of micrometers. Figure 2e
shows a bright-ꢀeld TEM image of a single SnO nanow-
2
ire. The high-resolution TEM image and its SAED pattern
conꢀrmed that the SnO nanowire is single crystalline and
2
the interplanar spacing is 2.64 Å, which corresponds to
the (101) plane of the rutile structure of SnO . Figure 2f
2
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Journal of the Iranian Chemical Society
Fig. 2 a SEM image of SnC, b TEM image of SnC, c HRTEM image of SnC and SAED pattern (inset), d SEM image of SnP, e TEM image of
SnP and SAED pattern (inset) and f EDX spectra of SnP
shows EDX spectra of the as‐synthesized SnO NWs. The
of oxide ions. Compared with SnC, the peaks at 633 and
2
−
1
spectra indicate the presence of only Sn and O elements,
778 cm of the SnP Raman spectrum become widen,
which might be ascribed to the eꢁects of oxygen deꢀ-
regardless of position in the SnO nanostructure. The XPS
2
study was carried out to know the valance state of the
sample, and the results are presented in Fig. 3. Disparate
elements present in the compound are exposed in the sur-
vey spectrum (Fig. 3a). In order to get the more feature
about the valance state and the chemical composition of
Sn 3d and O 1s, high-resolution spectra were examined
ciency and crystal distortion of SnO NWs, and this may
2
be attributed to the formation of oxygen deꢀciency during
the microwave irradiation process. Figure 4b shows the PL
spectra of SnC and SnP samples, respectively. For SnC
sample, there are two emission peaks located at 385 nm
and 420 nm, which is related to UV and blue–green emis-
sion, respectively. The UV emission is possibly owing
to oxygen deꢀciencies, tin interstitials or dangling bond
as shown in Fig. 3b–d. The Sn 3d spectrum of SnO NDs
2
(
Fig. 3b) shows two symmetrical peaks at 486.2 eV and
4
94.2 eV, which are attributed to Sn 3d and Sn 3d
,
and structural defects. The blue-green emission of SnO
5
/2
3/2
2
respectively. Similarly, the Sn 3d spectra of SnO NWs
is known to be correlated to defect levels within the band
2
slightly deviated to higher energy side. The peak appeared
gap of SnO associated with Sn interstitials [31]. In SnP
2
at 486.4 eV and 494.8 eV, which is related to Sn 3d and
sample, a broad yellow emission peak at about 588 nm was
5
/2
Sn 3d , respectively (Fig. 3c). Also the binding energies
observed from SnO nanowires, which might be related to
3
/2
2
found for O 1s core level for SnC (530.3 eV) are clearly
distinguishable from the O 1s of SnP (531.3 eV) and are in
good agreement with the previously reported values [27,
the defect levels associated with O vacancies or Sn inter-
stitials as previously reported [32]. Figure 5 shows the
UV–Vis DRS of both SnC and SnP samples, respectively.
The relative sharp absorption peak around 330–380 nm for
each product was noted. For SnC sample, the absorption
edge was found to be 350 nm, and further, it was shifted
toward the higher wavelength side (red shift) for SnP sam-
ple. We have using Kubelka–Munk (K–M) model [33] to
2
8]. Hence, the XPS data clearly show that both SnC and
SnP samples consist of only SnO . Figure 4a shows the
2
room-temperature Raman spectra of SnC and SnP sam-
−
1
ples, respectively. The peaks present at 632 and 776 cm
are attributed to A and B modes, which are relevant to
1
g
2g
the symmetric and asymmetric stretching of Sn–O bonds,
ꢀnd out the band gap (E ) of the product. A graph is plot-
g
−
1
2
respectively [29, 30]. The peak at 467 cm express to E
ted between [F (R ) hυ] and hυ, and the intercept value
g
α
doubly degenerate mode, create from the vibration mode
is the direct band gap energy [34]. It was reported that the
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Journal of the Iranian Chemical Society
Fig. 3 a XPS survey spectra, b Sn 3d spectra of SnC, c Sn 3d spectra of SnP, d O 1s spectra of both SnC and SnP
grain size can mainly inꢃuence the band gap energy of
b shows the characteristics degradation proꢀle of SnC and
SnP photocatalyst samples, respectively. When the irradia-
tion time increases from 0 to 100 min, the MB degradation
eꢂciency was found to be 99 and 94% for SnC and SnP sam-
ples, respectively. Similarly, the RhB eꢂciency was found
to be 94 and 90% for SnC and SnP samples, respectively.
First-order kinetics model [36] has been used to calculate
the rate constant of the catalysts. Figure 7c, d shows the
kinetic ꢀt for the degradation of MB and RhB under vis-
ible light irradiation. After a simple calculation, k value of
SnO nanoparticles, which decreased from 3.51 to 3.31 eV
2
and the mean particle size increased from 43 to 65 nm.
The detailed photocatalytic activity was illustrated in our
previously published work [35]. The photocatalytic activity
of SnO catalyst powders was examined by the variation in
2
the absorption spectra of the MB and RhB dye solutions
throughout their photodegradation process. Figure 6a–d
shows changes in adsorption spectra of MB and RhB dye
solutions in the existence of the SnC and SnP photocata-
lyst irradiated by a visible light for diꢁerent time. It illus-
trates that the concentration of MB and RhB decreases as
the irradiation time increases by measuring the intensity of
−
1
MB is found to be 0.0537 and 0.0573 min . Similarly, the
−
1
RhB is 0.0424 and 0.0459 min for SnC and SnP sam-
ples, respectively. In outlook of the practical applications
typical absorption peak (λ =664 and 554 nm). The entire
for photocatalysts, the recyclable and stability of the SnO
max
2
degradation of the dye solution occurred within 100 min,
nanostructures are most signiꢀcance. We carried out photo-
and the major absorption peak of MB (λ =664 nm) and
catalytic experiments using the SnO catalyst powders for
max
2
RhB (λ =554 nm) almost vanished completely. Figure 7a,
seven cycles to degrade the MB and RhB dyes under visible
max
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Journal of the Iranian Chemical Society
Fig. 4 a Raman spectra and b photoluminescence spectra of SnC and SnP samples, respectively
Fig. 5 UV-DRS spectra of SnC and SnP samples, respectively, a reꢃectance spectra b K–M model
light irradiation, as shown in Fig. 8a, b. For SnC sample, the
photocatalytic degradation eꢂciency of the MB is slightly
dropped from 99 to 95.8% after seven cycles (i.e., it only
reduces by 3.2%), whereas the SnP sample is also found to
be small variation (94% to 91%) in the eꢂciency. The result
the surface of photocatalysis under visible light illumina-
tion. The electrons and holes react with the reactive species,
which results in decomposition of organic pollutants. Then,
the oxidative species degrade the organic pollutant (MB)
into the small molecules like CO , H O, etc. A desirable
2
2
reveals that the SnO catalyst powder is relatively stable and
surface area beneꢀts the photocatalytic activity as it allows
more organic substances to be attached to the surface of the
photocatalyst. According to SEM and TEM observations,
2
can be easily reused in the photocatalytic process. Figure 8c
shows the schematic demonstration for MB photocatalytic
mechanism of SnO catalyst in visible light illumination. In
SnO NDs have large surface area and high crystalline nature
2
2
general, the precise surface area plays a major role through-
out the photocatalytic degradation of dye molecules. On the
foundation of the photocatalytic mechanism [37], it is well
that jointly results in better photocatalytic activity.
Figure 9a, b shows the output spectral characteristics
of the SnO NDs (nanodisks) and SnO NWs (nanow-
2
2
*
−
known that the highly reactive OH and O are created on
ires) samples exposed to ammonia gas for various
2
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Journal of the Iranian Chemical Society
Fig. 6 Visible light absorbance of MB (λ =664 nm) as a function of irradiation time a SnC and b SnP; visible light absorbance of RhB
max
(
λ
=554 nm) as a function of irradiation time c SnC and d SnP
max
concentrations (0 to 500 ppm). The spectra exhibit two
peaks around at 707 and 762 nm for SnC sample, whereas
these peaks were slightly diverged and located at 709 and
found to be about 45 min and 35 min for SnC sample.
Similarly, the SnP was found to be 35 min and 25 min,
respectively. The gas sensing mechanism of SnO NWs
2
7
65 nm for SnP sample. Figure 9c depicts the plot between
is shown in Fig. 10. In the presence of air are the outer
the spectral peak intensity (762 nm) and vapor pressure
of ammonia gas for SnC and SnP samples, respectively.
The gas sensitivity was calculated by using the formula,
sensitivity = counts/ppm (gas) [38]. The calculated sen-
sitivity was found to be 51.2 counts/1000 ppm and 71.4
counts/1000 ppm for SnC and SnP samples, respectively.
Figure 9d shows the time response and recovery curve of
the SnC and SnP samples, respectively (707 nm). This
showed good reversibility and stability. The response
time was denoted as time duration, the sensor signal (at
medium oxygen molecules adsorbed on the surface of the
−
2−
−
sensing material as O , O and O ions by trapping elec-
2
trons from its conduction band, reduces the carrier con-
centration, whereas, in ammonia environment, ammonia
molecules reacting with these surface absorbed oxygen
ions results in the formation of CO and H O, which leads
2
2
to increase the electron concentration. This eventually
increases the conductivity of the SnO NWs. Moreover,
2
SnO NWs, it is expected that the electronic transport
2
properties of the entire SnO NWs will change eꢁectively
2
7
62 nm) took for raising from 10 to 90% of the maximum,
due to the gas adsorption. In the proposed sensor, when
the gas is present, the evanescent wave absorption may be
higher or lower than the reference value [39].
and for recovery time, the intensity fell from 90 to 10% of
the maximum. The response time and recovery time were
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Journal of the Iranian Chemical Society
Fig. 7 a, b MB and RhB temporal degradation proꢀle of SnC and SnP samples, respectively, c degradation proꢀle of kinetic ꢀt for the degrada-
tion of MB and d kinetic ꢀt for the degradation of RhB under visible light irradiation for SnC and SnP
Conclusions
stability (~ 3.2% variation) up to 7 cycles when compared
to SnO NWs. This might be owing to photogenerated
2
SnO nanostructures with diꢁerent morphologies were
holes which are reactive species contributing to the excel-
lent photocatalytic degradation of the dyes. All these
results show that the SnO2 NWs-coated clad modiꢀed
sensor is more suitable potential candidate for ammonia
detection. In our work, this new kind of SnO2 nanostruc-
tures oꢁers new insights into the treatment of organic con-
taminants by photocatalytic degradation and monitoring
the gas leak in chemical industry.
2
successfully synthesized through a simple microwave irra-
diation method by using CTAB and PEG as surfactants.
XRD and TEM results suggest that SnO is tetragonal
2
rutile-type structure with nanodisks and naowires mor-
phology, respectively. The photodegradation eꢂciency of
SnO NDs with MB solution is very high under visible
2
light illumination such as, high eꢂciency (99%) and more
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Journal of the Iranian Chemical Society
Fig. 8 a Recycling test of MB b RhB (seven cycle segment) for SnC and SnP samples, c photocatalytic mechanism of SnO under visible light
2
irradiation
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Journal of the Iranian Chemical Society
Fig. 9 a, b Spectral response of SnC and SnP samples toward vari-
ous concentration of ammonia gas, c variation of peak intensity with
ammonia gas concentration for SnC and SnP samples at the wave-
length of 762 nm and d time response and recovery curve of ammo-
nia gas concentration for SnC and SnP samples at 500 ppm
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Journal of the Iranian Chemical Society
Fig. 10 Schematic representa-
tion for the proposed gas sens-
ing mechanism of SnO NWs
2
1
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Compliance with ethical standards
Conꢀlict oꢀ interest The authors declare that they have no conꢃict of
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