sol gel spin coating technique - kanya maha vidyalayakmvjla.org/mrp.pdfstrutural properties of in-...
TRANSCRIPT
Synthesis and characterization of doped/undoped/co-doped ZnO nanoparticles prepared by
different techniques - Gas sensor/ Photocatalytic applications:
ZnO nanoparticles has been prepared by different techniques such as sol gel spin and dip
coating, thermal evaporation method, doctor blade technique, simple heat treatment method,
simple combustion method and rf magnetron sputtering technique. Herein synthesis of
nanoparticles has been done by varying the different parameters such as annealing
temperature, aging time, doping elements (Al, In, Sn, Sb, Ni, Er, Ce ), stabilizer, solvents, pH
value, codoping (In-Sn) and rare earth doping (Er, Ce, Eu, Nd) elements by varying atomic
weight % etc. and their characteristics has been studied (FTIR, FESEM, XRD, UV VIS,
DTA, LCR, RAMAN, Photoluminscence). On the basis of good optoelectronic properties the
prepared nanoparticles has been studied for gas sensor and photodegradation applications.
Sol gel spin coating technique:
Zinc nitrate hexahydrate with different molar concentrations
(0.20M,0.25M,0.10M,0.15M,0.05M,0.02M) as solute, dissolve this in different solvents as
methanol, 2 methoxy ethanol, ethanol and isopropyl alcohol at room temperature. Di
ethanolamine (DEA)/ Mono ethanolamine (MEA) used as stabilizer. The molar ratio of
stabilizer to zinc acetate was kept at 1.0. The resulting solution was stirred at 60-800c for 0.5/
1 hr. At last transparent solution (ZnO) has been observed. Different atomic weight
percentage of various types of doping can be added.
Zinc oxide films have been prepared by spin coating method on glass substrate, which
was cleaned thoroughly and dried. The spin coating time is 30 sec in the beginning 10sec,
spin speed was 1000rpm and in later 20 sec spin speed was 2000 rpm. After each coating
samples were dried at 80- 1000c for 10 min and pre annealed it at 300-400
0c for 10-15 min.
The procedure from spin coating to pre annealing was repeated for several times to make
desired thickness. At last all the samples post annealed at 5000c for an hour.
Simple heat treatment method:
ZnO thin films and powder has been prepared using thermal evaporation technique (deposit
on glass substrate) and simple Combustion method. Fig. shows the systematic scheme of
ZnO nanoparticles prepared by simple heat treatment and thermal evaporation method.
0.17M of Zn acetate dihydrate dissolve in 10 ml triple deionised water and then stirrer it at
700C for 30 min. After stirring the mixture, solution was filtered with whatman filter paper
(No. 1) and transferred into the crucible, placed in the muffle furnance at 4000C for 10-20
min. During heating, spongy like material is obtained. Then anneal it again for an hour at
5000C for an hour. Finally white ZnO powder is obtained. And Al as doping element is used
by varying different annealing temperature (125-2250C). and their different characteristics
has been studied.
Thermal evaporation Technique:
ZnO thin films and powder has been prepared using thermal evaporation technique (deposit
on glass substrate) and simple Combustion method. Fig. 1 (a). shows the systematic scheme
of ZnO nanoparticles prepared by simple heat treatment and thermal evaporation method.
0.17M of Zn acetate dihydrate dissolve in 10 ml triple deionised water and then stirrer it at
700C for 30 min. After stirring the mixture, solution was filtered with whatman filter paper
(No. 1) and transferred into the crucible, placed in the muffle furnance at 4000C for 10-20
min. During heating, spongy like material is obtained. Then anneal it again for an hour at
5000C for an hour. Finally white ZnO powder is obtained. Fig. 1(b). shows the experimental
set up of ZnO films prepared by thermal evaporation technique in two zone split furnance.
For this, prepared ZnO nanoparticles from simple heat treatment are taken as source and glass
slide as substrate by maintain source temperature at 8000C and substrate at 400
0C for an hour.
Fig. Systematic scheme of ZnO nanoparticles prepared by simple combustion and thermal
evaporation method.
Doctor blade Technique
In order to develop nanoscale gas sensing device, doctor blade technique is used. These
nanoparticles were dispersed in deionised water to prepare the slurry for the fabrication of
nanodevice. The prepared paste was then coated on substrate (glass, silica, sapphire) by using
glass rod. For sensing device assembly pair of Au electrodes on the top of its surface and
microheater on its bottom surface is arranged. Synthesized nanoparticles were assembled on
the substrate by connecting Cu wires using silver paste to Au electrodes. To measure the gas
sensing properties static gas sensing set up is used.
Rf Magnetron Sputtering
ZnO based thin film has been prepared by rf magnetron sputtering technique by using
different substrates. Prior to the deposition, substrate was cleaned with distilled water,
followed by isopropyl alcohol and methanol in ultrasonic cleaner for 10 min, dried in
nitrogen gas. Sputtering system (IISC Bangalore) has ZnO target (99.99%) pure for 2 inch
diameter was sputtered in argon atmosphere for about 200 second to remove the surface layer
of target before deposition of film. Magnetron sputtering rf power of frequency 13.5 MHz,
the base pressure in the chamber prior to the deposition was maintained at 5x10-6
Torr,
controlling by pirani gauge, distance between target and substrate is 7.5 cm. The rf power
during the growth was kept constant at 100 Watt. This deposition was done under argon gas
flow of 200 sccm. After the deposition films were annealed at 6000C for an hour. Different
thickness of the films has been deposited by varying the deposition time.
Structural Properties:
XRD spectra corresponds to different molar concentration of doped/undoped by varying
annealing temperature is carried out and was found the preferential orientation of all the sol
gel spin coated films were along (002) plane but aluminium doped ZnO films for 0.15 M by
heat treatment shows decrease in intensity of (002) plane as compared to spin coated ZnO
films Fig. In case of spin coated films, the mixture first stirred for 1 hr at 70°C and then
cooled and this cycle was repeated two times and it was observed that intensity of plane (002)
was also found to decrease when the temperature was brought down by keeping the mixture
for I hr at 10°C and then again magnetically stirred at about same temperature (70°C) for 1hr
as shown in Fig shows the prominent peaks when the film was not quenched to lower
temperature before coating, but annealed at 4500C after coating.
The presence of prominent peaks showed the polycrystalline nature of films. This may be due
to the fact that when the mixture was suddenly quenched to lower temperature and then
heated again, the crystallanity of the films might get deteriorated due to formation of stress
due to varied temperature conditions.
XRD reveals that the most prominent peak is 101 and the crystal size lies in the range of 20-
75 nm. Which can be determined from scherrer formula
XRD spectra corresponding to annealing temperature from 400-600˚C were carried out and
was found that preferential orientation of all films were along (101) crystal plane at 2Ɵ =
36.48° at different annealing temperature. This shows that these films prepared by simple
combustion method are polycrystalline in nature and show a good c-axis orientation.
20 30 40 50 60 70 80
0
500
1000
1500
2000
Inte
ns
ity
(a
.u)
2 (degree)
100
002
101
102
110 103 112
20 30 40 50 60 70 800
200
400
600
800
1000
1200
1400
1600
1800
Inte
ns
ity
(a
.u)
2degree)
100
002
101
102
110
103 112
20 30 40 50 60 70 80
0
200
400
600
800
1000
1200
1400
Inte
ns
ity
(a
.u)
2(degree)
100
002
101
102
110003 200
30 40 50 60
0
100
200
300
400
500
600
In
ten
sity (
a.u
)
2degree)
(001)
(110)
(101) (102) (103)
Structural property of Er- ZnO nanoparticles
To examine crystallinity, the prepared Er- ZnO nanoparticles were characterized by X Ray
Diffraction (XRD). Fig. 2 shows the XRD pattern of prepared Er doped nanoparticles. All the
observed pattern shows the diffraction planes 2θ = 31.80˚, 34.44˚, 36.22˚, 47.59˚, 56.67˚,
62.91˚, 66.43˚, 68.09˚, 69.15˚and 77.04˚corresponding to hexagonal wurtzite ZnO planes of
(100), (002), (101), (102), (110), (103) (200), (112), (201) and (202) respectively. All the
prepared samples shows most prominent diffraction peak at 36.22˚. A weak reflection at
28.35˚ attributed to (222) plane for Er2O3 crystalline structure [9]. These observed diffraction
planes well matched with standard card number (JCPDS 36 -1451). The lattice spacing was
calculated from Bragg’s formula
2d sinθ = nλ
Where d is lattice spacing and θ is angle of incidence, λ is wavelength and n is diffraction
order.
Crystal size can be found from Scherrer’s formula
Where D is crystalline size, K is constant (0.92), λ = 0.154 nm, mean wavelength of CuKα1
radiation, β is full width half maxima and θ is Bragg’s angle in radians. Fig. 2 shows the
intensity of (101) peak increases, indicating preferential orientation along c axis.
The value of lattice parameters a and c for Er doped ZnO nanoparticles were
calculated using the following equation
(1)
Where h,k and l are the Miller Indices, d is interplanar distance and a, c are lattice
parameters.
D is the density and it was calculated by using formula
D =
Table 1 shows the density of Er doped ZnO is double than ZnO (JCPDS 36-1451). Thus Er
doping has been demonstrated by the modification of lattice parameters in comparison with
pure ZnO. XRD peaks reveal a small shift towards higher angle which shows the variation in
lattice parameters. After a certain limit of doping concentration non uniform distribution of
dopant ions has been found. This shows the importance of lattice space distribution of Er in
the ZnO:Er.
Strutural properties of In- Sn co-doped ZnO nanoparticles by varying Aging time
The crystallinity of In-Sn co-doped ZnO nanoparticles (A1-A3) was analyzed by the
X- Ray Diffraction (XRD) method (Xpert Pro with CuKα, nickel metal is used asbeta filter).
Figure 2 shows the typical XRD pattern of prepared In-Sn codoped ZnO nanoparticles for
A1-A3. All the observed pattern shows the diffraction planes 2θ = 31.60˚, 34.54˚, 36.32˚,
47.49˚, 56.37˚, 62.71˚, 66.53˚, 68.18˚, 69.17˚, 77.04˚ corresponding to hexagonal wurtzite
ZnO planes of (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202)
respectively. All the prepared samples shows most prominent diffraction peak at 36.32˚.
These observed diffraction peaks are well matched with standard card number (JCPDS 36-
1451). However, compared to their result, crystallization nature in this study is much better
due to several peaks observed in XRD spectra.
FIG.2 XRD spectra of codoped ZnO nanoparticles (a) 0 hrs (b) 24 hrs and (c) 36 hrs.
The lattice spacing are calculated from Bragg’s formula
2d sinθ = nλ
Where d is lattice spacing and θ is angle of incidence, λ is wavelength and n is the diffraction
order.
Aging Time (hrs) Crystal Size (nm) Along diffraction plane
(100) (002) (101)
0 32.00 21.70 27.25
24 30.75 21.11 26.15
36 29.95 19.35 25.85
Table I. Crystal size of ZnO nanoparticles for (A1-A3).
Crystal size can be found from scherrer’s formula
Aging Time (hrs) Crystal Size (nm) Along diffraction plane
(100) (002) (101)
0 2.817 2.602 2.476
24 2.815 2.603 2.477
36 2.814 2.601 2.479
Table 2. Lattice spacing of ZnO nanoparticles for A1-A3.
Morphological Study:
Fig. shows the micrographs of ZnO crystallites prepared under different conditions and
techniques. Fig. shows the spin coated films on glass substrates. Here in this case before
spin coating the mixture were first stirred for 1 hr at 70°C and then cooled for I hr at 10°C
and then again magnetically stirred at about same temperature (70°C) for 1hr, the observed
crystallite width size was of order of 60-70 nm and also observed that few crystallites have
oriented more along length wise having size of order of 200-250nm whereas the crystallites
in which this process is not carried out the observed crystallites were of order of 110-130 nm
and more regular crystallites densely packed were observed. shows the crystallites of order of
40-50 nm for 0.15M concentration by heat treatment with the content of 1.8% Al as dopant
followed by annealing and it is observed that the size is more as compared to undoped. The
results obtained are in well agreement with XRD results. It is also observed that more ordered
and dense crystallites are observed in case of Aluminium doped under annealing.
FESEM images reveals that the grain size is of the order of nanometer. Different
morphology is observed which is due to the variation in annealing temperature. The size of
grain increases with increase in temperature.
It is evident from the FESEM images that the prepared materials possessed
nanosphere shape having granular nature. It can be clearly seen from Fig. shows that samples
consist of ZnO nanostructures. The average width and length of ZnO nanoparticles are found
to be 18.7 nm and 20.1 nm for isopropyl alcohol and 19.8 nm and 21.7 nm for triple
deionized water. The typical size of nanoparticles is in the range of 40-50 nm.
Fig. 3 FESEM images of In-Sn co-doped ZnO nanoparticles.
Figure 3 shows the FESEM (FESEM- JSM6100 (Jeol)) images of pure and In – Sn co-doped
ZnO nanoparticles (A1-A3). Figure 3(a) shows morphology of pure ZnO. It is observed that
ZnO nanoparticles become more uneven with aging time. As for A1(0hrs) sample surface is
relatively rough and non uniform grain size is observed. However, surface roughness
decreases with increase in aging time. It is clear from figure 3 (b,c) that codoped ZnO
nanoparticles produce almost uniformly distributed rectangular shaped crystallites. Non
uniform circular rod shapes nanoparticles are observed in figure 3 (a,d). These different
Morphologies is greatly influenced by the time rate of photocatalytic activity. In the
photpcatalytic study rectangular shaped nanoparticles shows best photocatalytic efficiency
which is due to the large surface area to volume ratio of rectangular shaped ZnO
nanoparticles. This indicates that 24 hrs aging time is the ideal for preparation of ZnO based
nanoparticles. Aggregated rod shapes additive ZnO nanoparticles are observed with average
diameter 40- 90 nm. Large size is observed in case of pure ZnO nanoparticles. The average
width and length of codoped ZnO nanoparticles are found to be of the order of 19 nm and 21
nm for A1 (0hrs), 18 nm and 20 nm for A2 (24 hrs), 22 nm and 25 nm for A3 (36hrs) (Table
3). These results show that the synthesized nanoparticles have relatively rough surface and
non uniform grains. With the increase in aging time sol get more homogenous and stable.
Accordingly, the quality of prepared nanoparticles gets improved.
FTIR Characterization:
To detect the presence of functional group and phase transformation, IR spectra of samples
were taken in the transmittance mode. It gives information about the way in which molecules
are bonded.
3100—3450 Cm-1
OH mode
2400-3100 Cm-1
O-C-O
1020-1070 Cm-1
C=O
400-500 Cm-1
ZnO
FTIR study of Er-doped ZnO nanoparticles
UV VIS Spectra (Transmittance, Absorbance, Optical Band gap)
According to Beer Lambert’s Law
A= log (I0/I)= e.c.l,
Where, A is absorbance, I0 is the intensity of incident light at given wavelength, I is
transmitted intensity, L is path length, c is concentration of the solution and e is molar
absorbtivity. molar absorbtivity.
The absorption peaks in Fig. showed the strong absorption peak in the wavelength range
350-390 nm. Due to electron transitions from valance band to conduction band this can be
assigned to intrinsic band gap absorption of ZnO. From absorbance spectra it is clear that
absorbance is proportional to the concentration of the solution as described by Beer
Lambert’s law. According to Beer Lambert’s law, high absorbance means high concentration
of ZnO nanoparticles. Beer Lambert’s law states that
A= log (I0/I)= e.c.L
Here A is the absorbance. I0 is the intensity of incident light at given wavelength, I is
transmitted intensity, L is path length, c is concentration of solution and e is molar
absorbtivity.
The optical absorbance spectrum in Fig. showed that there is increase in absorbance with
annealing temperature. This above spectrum shows that ZnO crystals have a low absorbance
in the visible region, which is a characteristic of ZnO. The absorbance coefficient for direct
transition semiconductor is related to the optical band gap (Eg)
αhν = A(hν-Eg)1/2
where hν is photon energy, Eg is optical band gap, A is constant. Fig. Shows (αhν)2 plotted as
function of photon energy (eV) for ZnO. The extrapolation of straight line to αhν = 0 gives
the value of direct optical band gap. It is observed that optical direct transition lies in the
range of 3.00-3.27eV. analyzing the optical band gap data it is clear that band gap decreases
with increase in molar concentration of the solution. This decrease in band gap is due to the
stress relaxation. The decrease in stress with increase in molar concentration is attributed to
the densification and an increase in oxygen contents which reduces stress and relax optical
band gap.
Based upon the absorbance spectra optical band gap of the prepared ZnO nanoparticles can
be calculated from the equation.
Eg
=
Here λ is the maximum wavelength of well defined absorbance peaks.
The absorbance spectra of these films (for different aging time) were measured in the range
of 200-750 nm. “Figs” showed the strongest absorbance peak in the range of 350-380 nm.
The transmittance spectra can be found from absorbance spectra by using relation
Absorbance = 2 – log10 T (1)
The transmittance of the sample can be defined as the ratio of photon that passes through the
sample over the incident number of photons. The UV absorption peak and Peak shifts from
361 nm to 358 nm were observed due to size difference.
Table 1. Effect on optical band gap with different molar concentration
Molar concentration Technique Used Optical band gap (eV)
0.10M Sol Gel 3.07
0.10M Simple Heat Treatment 3.21
0.15M Sol Gel 3.02
0.15M Simple Heat Treatment 3.02
0.15M Simple Heat Treatment a) 3.02, b) 3.16, c) 3.27
Electrical Properties:
To study the semiconductor in the field of electronics various methods are proposed by
different researchers. Fig. showed the variation in capacitance and applied frequency
(0.2MHz- 3.6MHz). Low value of capacitance is observed at high frequency. This is due to
the fact that more value of capacitance resulting from interface states in equilibrium with
ZnO nanoparticles can follow the ac signal. It has also observed that the more value of
capacitance has been observed in case of thermal evaporation technique in comparison with
simple heat treatment method.
Variation of capacitance with frequency of ZnO nanostructure.
Many semiconductors has been investigated to study their utilization in the field of
electronics, by using different solvents the study of capacitance and dielectric constant as
function of frequency is one of the convenient method.
The value of dielectric constant can be found from the formula
C = (€0 €r A)/d .
Where C is capacitance, €0 is dielectric constant, A is the area of circular pellet and d is its
thickness. It was observed from the Fig that the dielectric constant decreases with frequency.
This variation with frequency is due to charge transport relaxation time.
Thermal properties:
Thermal analysis starts from room temperature up to 10000C at the heating rate of 5
0 C/ min
in the presence of nitrogen gas. The associated peak near 1500C is associated with OH group.
An exothermic peak near 3500C is related to crystallization of ZnO. These results confirm
that the prepared ZnO films have good thermal stability, which can be used for the electronic
applications such as for the fabrication of solar cell and gas sensor.
Thermal analysis of the ZnO has been carried out from DTA. This analyses start from
room temperature up to 10000C at the heating rate 100C/ min in the presence of nitrogen
gas. Fig. showed exothermic and endothermic peaks. The exothermic peak appearing in
the range 90-1100C is attributed to the loss of absorbed water. Exothermic peak near around
300- 3400C is due to decomposition and crystallization of the precursor. An exothermic peak
in the DTA curve up to 6500C which is due to Wurtzite phase and possibly related to the
crystallization of ZnO. These results confirmed that ZnO composite have good thermal
Stability. This gives the information that ZnO nanoprticles are helpful for the fabrication of
solar cell based devices.
Gas sensing property:
Fig. shows the response of doped/ undoped ZnO films to different ethanol vapour gas
concentrations. It is observed from the figure that the response of both the samples was
enhanced with gas concentration and more response is observed in case of Al doped ZnO. At
various concentrations of ethanol gas very less change in response and recovery time. The
calculated value of response and recovery time is 15s and 20s. The rapid response and
recovery time of the sensor reveals it has potential applications.
Photocatalytic applications
Fig. represents the absorbance spectra recorded from 200-650 nm wavelength for DR-
23 dye solutions irradiated at different time interval in the presence of UV radiations and
ZnO photocatalyst. A continuous decrease in the absorbance intensity clearly confirms the
fact that as synthesized ZnO nanoparticles are acting as photocatalyst for the degradation of
the dye under UV irradiations. A/Ao value almost approaches to zero after 60 min of UV
irradiation of the aqueous suspensions of DR-23 dye and ZnO photocatalyst.
On the basis of different solvents used for the fabrication of ZnO nanoparticles, above
mentioned detailed reveals that nanoaprticles which was prepared by TDW as solvent has
better properties than IPA. So the synthesis by using deionized water used for the
photocatalytic application for DR 23 dye. Fig. (a). represents the absorbance spectra recorded
from 200-650 nm wavelength for DR-23 dye solutions irradiated at different time interval in
the presence of UV radiations and ZnO photocatalyst. A continuous decrease in the
absorbance intensity clearly confirms the fact that as synthesized ZnO nanoparticles are
acting as photocatalyst for the degradation of the dye under UV irradiations. A/Ao value
almost approaches to zero after 60 min of UV irradiation of the aqueous suspensions of DR-
23 dye and ZnO photocatalyst (Fig.(b)). The corresponding percentage photocatalytic
degradation of DR-23 dye as a function of UV irradiation time is shown in Fig. (c) Complete
degradation (100%) of the said dye was observed within 60 min of UV irradiations in the
presence of ZnO nanoparticles.
Langmuir– Hinshelwood kinetic model was used to investigate the kinetic studies of
the photodegradation process (Eq 3) is applied.
Where Co = initial concentration of the DR-23 dye and C = the concentration of DR-23 dye at
irradiation time ‘t’.
When graph between C
Cln o and irradiation time ‘t’ gives a straight line passing through
origin as shown in Fig. d. The slope of the line passing through the origin gives the rate
constant k for the photocatalytic process. Pseudo first order rate constant ‘k’ for the
photodegradation of DR-23 dye was found to be 0.0631 min-1
with correlation constant (R2)
of 0.99596.
Fig. (a) UV–Vis absorbance spectra (b) variations of A/Ao (c) percentage degradation of DR-
23 after different time intervals of UV irradiation and (d) pseudo-first order rate kinetics for
the photodegradation of DR-23dye.
Photocatalytic activity of Er-doped ZnO nanoparticles
Fig. 1 shows the systematic designed photoreactor with water circulating unit (to maintain
constant temperature) and an opening for O2 supply and another opening for withdrawing the
sample after regular interval of time. A 125 W UV lamp was used as source for UV
irradiation. This photocataytic experiment was used for evaluating the photocatalytic
applications of Er doped ZnO nanoparticles against DR 31 dye. A 100 ml aqueous solution of
ktC
Cln o
35 ppm DR 31 dye was prepared in deionised water. 0.080g of Er- ZnO nanoparticles was
suspended in to the dye solution as photocatalyst. Prior to UV irradiation, the resulting
suspension was ultrasonicated for 35 min for proper homogeneity of photocatalyst as well as
to maintain adsorption desorption equilibrium. A 5 ml of sample solution was taken out from
photoreactor after a regular interval of time followed by centrifugation at 3000 rpm for 10
min in order to remove ZnO suspension from the solution. Percentage degradation was
calculated using eq. 1
Percentage degradation =
(1)
where, Ao is initial absorbance of dye and A is absorbance of dye solution after UV light
irradiation.
3.2. Photocatalytic degradation of prepared Er-doped ZnO nanoparticles
For the photocatalytic applications, DR-31 (35 ppm) was chosen as target dye for Er doped
ZnO nanoparticles. Photocatalytic degradation was performed under UV light. Fig. 6
represents absorbance spectra recorded from 300-750 nm for DR-31 dye under regular
interval of time. Er-ZnO as photocatalyst plays a vital role for photodegradation application,
which is mainly due to adsorption of dye molecules. Decrease in absorbance intensity clearly
confirmes that Er doped ZnO are acting as photocatalyst for the degradation of dye. A strong
absorption band at 509 nm represents maximum wavelength for DR-31 dye. Fig.6 shows the
devreasing trend of absorption for each Er doped ZnO photocatalyst. Complete degradation
of dye was observed in 60 min under UV irradiation. Light excitation causes some photo
stimulated elsto optic effects [21]
Fig 7 (a) shows a corresponding plot between relative change in absorption
intensity (A/A0) and irradiation time by varying Er concentration. From detailed experiment,
it was observed that Er doped ZnO nanoparticles with 2.5% doping element shows drastically
decrease in absorbance with increase in irradiation time. Fig 7 (b) represents variation in
percentage photodegradation with irradiation time. With increasing Er doping concentration
up to 2.5% there is significantly increase in percentage degradation. However, with increase
in further concentration the percentage (3.0%, 3.5%) degradation was decreased. The
obtained results confirmed that 2.5% of Er into ZnO is important for complete degradation
(99.1%) in 60 min of DR-31 dye. Thus EZ2 is the best composition for the photodegradation
of said dye. Table 2 represents Er doped ZnO nanoparticles exhibits better photocatalytic
performance as compared to other Er- ZnO as photocatalyst reported in literature.
3.3. Kinetic study of photocatalytic degradation using Er doped ZnO nanoparticles
The kinetic study for Er doped ZnO nanoparticles (photocatalyst) for photodegradation of
DR-31 dye was studied by using Langmuir – Hinshelwood Kinetic model [22] by eq. 3.
These all observations are under UV irradiations.
ln
(3)
Where C0 is initial concentration of DR-31 dye and C is concentration of DR- 31 dye at
irradiation time ‘t’. K is Pseudo first order rate constant.
Fig. 8 exhibits plot of ln (Co/C) vs irradiation time and it has been found that for
degradation of DR-31 dye for each sample obeyed pseudo-first order reaction kinetics. Fig. 9
represents the variation of rate constant for Er doped ZnO nanoparticles (EZ1-EZ4). It is
clearly observed from the bar diagram that pseudo-first order rate constant is maximum for
EZ2 sample as compared to other Er-doped ZnO nanomaterials. Linear regression coefficient
(R), (R)2 half life time and rate constant was summarised in Table 3. The maximum value of
K for the said dye was found to be 0.07010 (min)-1
and half life time was minimum for EZ 2.
These studies confirmed that different concentration of Er is responsible for the
photodegradation of dye.
In the photocatalytic activity, DR-31 dye molecules are physiorbed or
chemisorbed onto the surface of ZnO nanoparticles. Er doped ZnO is expected to increase
surface defects, which is responsible for the enhancement of photocatalytic efficiency. Er
ions can also act as effective electron scavenger to trap the conduction band electrons, which
reduce the probability of electron hole recombination. This means that Er ions on the surface
of ZnO nanoparticles may act as scavenger. In this process, the reactive species are O2-, HO2 ,
H2O2. In order to find responsible species for degradation of dyes, this scavenger study was
performed. For Er doped nanoparticles, the Er ions incorporated in ZnO are responsible to
absorb excited electrons from conduction band of ZnO. At higher Er concentration, the
number of free electrons decreases, this causes decreasing photdegradation.
Absorption spectra vs wavelength for photocatalytic degradation of DR doped ZnO as photocatalyst:
EZ1, EZ2, EZ3 and EZ4.
Percentage degradation of DR-31 dye.
Pseudo-first order kinetics for DR respectively
Gas sensing application for Sn doped ZnO nanoparticles
Sn doped ZnO nanoparticles were characterized in standard gas sensing set up wherein a particular
amount of host gas was injected into the chamber with air as background. Before exposure to organic
gas, oxygen atoms are adsorbed into ZnO surface; it takes electrons from surface and become O-
(release oxygen). This O- ion helps to create the depletion layer on the host surface. Addition of Sn
help to completes the ZnO structure. This allows more oxygen to be adsorbed, which can enhance the
response.
Fig. 8 clearly shows that variation in temperature increased the gas sensor response, which
increased sensitivity. The ethanol sensing mechanism of the sample is explained as follows.
Adsorption is a surface defect. It forms the ionic species (O2-
and O-) on sample surface. kinetic
reaction before and after the ethanol exposure is described in equations (3)-(5) [29].
O2 (gas) O2 (ads) (3)
O2 (ads) + e- O2
- (ads) (4)
O2- (ads) + e
- 2O
- (ads) (5)
Fig. 8 shows the sensor’s response to ethanol and acetone with variation of operating temperature at
300˚C, 400˚C and 500˚C respectively. These measurements help to indentify the operating
temperature of sensor device for exposure of different gases. This figure shows good linearity as a
function of operating temperature for ethanol and acetone gas. It has clearly seen from Fig.8 that the
2.0% (SZ3) Sn dopant yields the best sensing response at 400˚C.
Table 2 shows that for all the doping concentrations, highest sensitivity was observed at an
operating temperature of 400˚C. Sensitivity response has almost same value for variations in working
temperature. This phenomenon indicates that same response may be expected for other organic gases.
Fig. 9 shows the sensitivity of Sn doped ZnO based sensor towards ethanol and acetone gas at
different operating temperatures (300˚C, 400˚C, 500˚C). The sensitivity at 5% volume of ethanol and
acetone gas concentration was 86.8% and 84.4% respectively. The response and recovery time was
found to be 22s and 31s for SZ3 (ethanol). This figure reveals that at exposure of gases, sensitivity of
sensor increased upto 400˚C and thereafter it decreased which is due to the fact that at low
temperature, a low response of sensor is observed because the gas molecules do not have different
thermal energy to react with absorbed oxygen species. Comparison of gas sensing properties is
observed in Table 3. The increasing sensitivity has positive correlation with Sn dopant up to SZ3 (2%
Sn) and after that it decreases. This result has positive correlation with XRD, FESEM and UV Vis
spectroscopy results as described above. Therefore optimum amount of SZ3 (2 at.wt%) Sn yields to
maximum sensitivity.
Fig. 8. Sensor response to 5% volume of ethanol (E1-E3) at 300-5000C and acetone (A1-A3)
at 300-500˚C for SZ1-SZ4 respectively.
0 500 1000 1500 2000 2500 3000
0
10
20
30
40
50
60
70
80
90
Temp 4000C
Re
sp
on
se
(%
)
Time (s)
SZ 1
SZ 2
SZ 3
SZ 4
AcetoneA2
0 500 1000 1500 2000 2500 3000
0
10
20
30
40
50
60
70
80
Temp 3000C
Re
sp
on
se
(%
)
Time (s)
SZ 1
SZ 2
SZ 3
SZ 4
Ethanol
E1
0 500 1000 1500 2000 2500 3000-10
0
10
20
30
40
50
60
70
80
90
Temp 4000C
Re
sp
on
se
(%
)
Time (sec)
SZ 1
SZ 2
SZ 3
SZ 4
EthanolE2
0 500 1000 1500 2000 2500 3000-10
0
10
20
30
40
50
60
70
80
Temp 5000C
Re
sp
on
se
(%
)
Time (s)
SZ 1
SZ 2
SZ 3
SZ 4
EthanolE3
0 500 1000 1500 2000 2500 3000
0
10
20
30
40
50
60
70
80
Temp 3000C
Re
sp
on
se
(%
)
Time (S)
SZ 1
SZ 2
SZ 3
SZ 4
AcetoneA1
0 500 1000 1500 2000 2500 3000
0
10
20
30
40
50
60
70
80
90
Temp 5000C
Re
sp
on
se
(%
)
Time (s)
SZ 1
SZ 2
SZ 3
SZ 4
Acetone
A3
Fig. 9. Sensitivity of Sn doped ZnO based sensor at different temperature for SZ1-SZ4 towards
ethanol and acetone gases.
Temp (0C) Response (%)
Ethanol Acetone
SZ 1 SZ 2 SZ 3 SZ 4 SZ 1 SZ 2 SZ 3 SZ 4
300 60 67 72 45 42 64 74 51
400 64 71 86 53 61 69 83 50
500 55 63 72 42 57 64 45 43
Table 2 Response of sensors towards Ethanol and Acetone.
300 350 400 450 50040
50
60
70
80
90
S
en
sit
ivit
y (
%)
Temperature (0C)
SZ 1
SZ 2
SZ 3
SZ 4
Ethanol
300 350 400 450 50020
30
40
50
60
70
80
90
Se
ns
itiv
ity
(%
)
Temperature (0C)
SZ 1
SZ 2
SZ 3
SZ 4
Acetone