Photoluminescence Investigations and Band Gap Engineering in Environment Friendly ZnO Nanorods: Enhanced Water Treatment Application and Defect Model

Abstract

The inadvertent discharge of industrial effluents, mainly textile, contributes to the complex contamination load in water bodies. Textile dyes are the critical effluents and recalcitrant to traditional remediation procedures. Therefore, energy viable and environment friendly solutions are needed. In this study, we have synthesized zinc oxide nanorods (NRs) at various temperatures using modified thermal decomposition and evaluated its photocatalytic activities. Field effect scanning electron microscopy has confirmed rod-like morphology till TS = 500 °C and spherical morphology from TS = 600 °C onward. Photoluminescence spectra have shown a prominent defect peak in the synthesized ZnO, except for the NRs synthesized at 300 °C. Synthesized ZnO NRs and NPs have been employed to degrade crystal violet (CV) and congo red (CR) dyes. ZnO NRs have shown impressive photocatalytic performance with faster treatment time as compared to the earlier reports. Synthesis parameters are well correlated with the observed high efficiency and the band gap tailoring. Based on our findings, for the first time, we have proposed (i) defect model correlating synthesis parameters with defect states, (ii) systematic correlation of defect states with photocatalytic efficiency, and (iii) ZnO nanorods synthesized at 300 °C via an improved synthesis method as a promising photocatalytic solution to degrade the CV and CR dyes in contaminated water.

Full text

Photoluminescence Investigations and Band Gap Engineering in
Environment Friendly ZnO Nanorods: Enhanced Water Treatment
Application and Defect Model
Sumit Kumar, Jyoti Pandey, Ritika Tripathi, and Suchitra Rajput Chauhan*
Cite This: https://doi.org/10.1021/acsomega.3c03860
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ABSTRACT: The inadvertent discharge of industrial euents, mainly textile,
contributes to the complex contamination load in water bodies. Textile dyes are
the critical euents and recalcitrant to traditional remediation procedures.
Therefore, energy viable and environment friendly solutions are needed. In this
study, we have synthesized zinc oxide nanorods (NRs) at various temperatures
using modified thermal decomposition and evaluated its photocatalytic
activities. Field eect scanning electron microscopy has confirmed rod-like
morphology till TS= 500 °C and spherical morphology from TS= 600 °C
onward. Photoluminescence spectra have shown a prominent defect peak in the
synthesized ZnO, except for the NRs synthesized at 300 °C. Synthesized ZnO
NRs and NPs have been employed to degrade crystal violet (CV) and congo
red (CR) dyes. ZnO NRs have shown impressive photocatalytic performance
with faster treatment time as compared to the earlier reports. Synthesis
parameters are well correlated with the observed high eciency and the band
gap tailoring. Based on our findings, for the first time, we have proposed (i) defect model correlating synthesis parameters with
defect states, (ii) systematic correlation of defect states with photocatalytic eciency, and (iii) ZnO nanorods synthesized at 300 °C
via an improved synthesis method as a promising photocatalytic solution to degrade the CV and CR dyes in contaminated water.
1. INTRODUCTION
Textile industries, since their inception, have been discharging
hazardous euents and dyes into the water bodies through
dierent channels. A global survey has indicated a 40% deficit
in water availability to meet the estimated demand by the year
2030.
1
Thus, development of better treatment technologies,
techniques, and materials than conventional physiochemical
treatments like adsorption, coagulation, ozonation, floccula-
tion, and membrane filtration is needed. Conventional
treatment techniques either fail to completely demineralize
the euents
2−4
or generate other waste products. Among all
techniques for textile wastewater (TWW) remediation, the
semiconductor nanomaterial-based photocatalysis being envi-
ronment friendly is a plausible technology.
5
Soon, photo-
catalysts would be in the spotlight to remediate TWW due to
their solitary capability of breaking larger textile dye molecules
into smaller molecules and harmless byproducts via the
environment friendly route. In addition, photocatalysts utilize
photogenerated electrons and holes
6
to degrade euents, so it
is a clean and low-cost technique to combat these problems.
ZnO is well known due to its impressive benefits including
green properties, good chemical stability, biocompatibility,
distinct electronic structure, and low production cost, thus
becoming an obvious choice in photocatalytic fields.
5
Nano
ZnO due to its human friendly nature
7
becomes a potential
candidate for its utility in the wastewater treatment via
photocatalysis.
When the ZnO photocatalyst is exposed with suitable
radiations, it populates e−(s) and hole(s) in the CB and VB,
respectively. This initiates redox reaction with water and
oxygen resulting in generation of reactive oxygen species
(ROS), which in turn degrade dyes or organic euents into
CO2and minerals.
8
An avid recombination of these photo-
generated carriers is invariably detrimental to quantum
eciency of the photocatalyst to degrade dye molecules.
9−13
Solid surface adsorbed gases, segregated impurities, and defects
are sources/sinks of photogeneration carriers. Therefore,
control of defects and associated band tailoring is of
paramount importance for enhanced photocatalytic activity
of ZnO.
Received: June 1, 2023
Accepted: July 7, 2023
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A rigorous literature survey indicates that there are no
reports on the detailed XRD and photoluminescence
investigation of ZnO synthesized over a wide range of
temperature. There are number of legitimate motives for
such systematic investigation and its correlation with photo-
catalytic eciency. In this study, we have developed an
improved and environment friendly synthesis approach,
magnetic stirred-mechanical assisted thermal decomposition.
We have undertaken synthesis parameters, for instance,
synthesis temperature, and post-synthesis treatments for
realizing band gap engineering through the control of defects
and surface states. Also, the findings of our work establish the
role of defects in reducing degradation time of crystal violet
and congo red dyes.
2. EXPERIMENTAL DETAILS
Zinc acetate dihydrate is a preferred choice as a precursor for
the synthesis of ZnO nanomaterials pertaining to its low
decomposition temperature.
2.1. Synthesis: Magnetic Stirred Mechanical-Assisted
Thermal Decomposition (MSMATD) Method. Zinc
acetate was manually ground for 1 h and then heat-treated at
the desired temperature in the range from 260 to 700 °C (TS)
for 4 h followed by quenching to room temperature. The
samples that were heat-treated at TS≥500 °C were quenched
to room temperature (RT) in air from 425 °C, and samples
heat-treated at TS< 500 °C were air-quenched from TSto RT.
Obtained samples were crushed in a mortar and pestle for 2
min and then magnetically stirred in deionized (DI) water
(volume used = weight of powder ×10) for a 1 h duration.
Stirred mixtures were then filtered using Whatman filter paper
to obtain the solid powders followed by their drying at 70 °C
for 8−10 h.
2.2. Characterizations. X-ray diraction analysis con-
firmed the crystallographic phase of the synthesized material.
The theta−theta powder X-ray diraction method (Panalytical
EMPYREAN) was adopted to obtain the X-ray diraction
pattern. Diraction intensity was recorded at 45 kV applied
voltage and 45 mA current setting. The diraction pattern was
recorded for 2θbetween 20 and 80°. Lattice parameters were
determined via PowderX software.
14
Particle size of the
synthesized ZnO nanomaterials was determined from X-ray
diraction data via the Williamson−Hall plot. Morphology and
particle size of the synthesized samples were determined by
field eect scanning electron microscopy (FE-SEM, Sigma-300,
Carl Zeiss), and elemental composition was determined by
energy-dispersive X-ray spectroscopy (EDX). The band gap
was determined using diuse reflectance spectroscopy and, a
decrease in the concentration of dye solutions was determined
by UV−visible spectroscopy (Lambda-365, PerkinElmer). A
Witech Alpha 300 RAS system was employed to record the
photoluminescence emission spectrum at λ= 355 nm and 285
μW incident power. An emission signal was recorded using a
charge coupled detector.
2.3. Photocatalytic Testing of ZnO Nanomaterial
Synthesized via MSMATD against the Crystal Violet
(CV) Dye and Congo Red (CR) Dye. 2.3.1. Dye Solution.
Various dye solutions of dierent molarities using CV and CR
dyes and DI water were prepared for photocatalytic testing.
Here, the preparation method for one dye solution (CV, 10
μM solution) is described in detail. CV dye powder (1.2 mg)
was added to 300 mL of DI water followed by the magnetic
stirring of the resulted solution for 1 h at 250 RPM (rotations
per minute) at RT.
2.3.2. Photocatalytic Testing. The synthesized ZnO
photocatalyst was added to each test bottle containing 10
mL of solution in the range from 1 to 30 mg. The test bottles
were exposed to the 80 W mercury lamp. The eciency of the
synthesized ZnO nanophotocatalyst was determined in terms
of change in the concentration of the dye through UV−visible
absorption measurements.
3. RESULTS AND DISCUSSION
3.1. X-ray Diraction (XRD). The XRD patterns for all the
samples from 275 to 700 °C show the polycrystalline ZnO
phase corresponding to the wurtzite hexagonal crystal
structure. Diraction peaks due to (100), (101), (102),
Figure 1. (a) X-ray diraction pattern of the MSMATD synthesized ZnO nanomaterial. The sample prepared at 260 °C did not show the
formation of the ZnO phase. The peaks observed in the diraction pattern were identified as the ZnO wurtzite phase in samples synthesized at TS=
275−700 °C. (b) EDX spectrum of ZnO prepared at 300 °C. (c) Lattice parameters obtained from the indexing and refining program using XRD
data.
Table 1. Results Obtained from XRD, FE-SEM, and DRS
Analysis
based on feature indicated
in FE-SEM
TS
(°C)
diameter (evaluated
from XRD data)
(nm)
aspect
ratio/
diameter morphology
absorption
peak position
(nm)
275 21.8 nanorods 380.0
300 19.2 nanorods 380.0
400 9.1 nanorods 385.6
500 5.9 nanorods 386.1
600 114.5 ∼250 nm nanoparticle 386.1
700 116.5 ∼500 nm nanoparticle 386.1
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(110), (002), (103), (112), (201), (200), (004), and (202)
planes have been observed (Figure 1a). JCPDS file 36-1451
was employed for identifying the phase of the synthesized
ZnO. The spurious low intensity peaks observed between 2θ=
20 and 35°could be due to impurities in the sample or
intermediate product. Vijayalakshmi et al.
15
reported the
presence of both zinc hydroxide and zinc oxide phases in raw
ZnO powder and obtained a pure ZnO phase upon its
sintering at 900 °C. LukovicGolicet al.
16
prepared ZnO from
zinc acetate dihydrate via the sol−gel process and observed
peaks corresponding to zinc acetate, zinc oxide, and zinc
hydroxide acetates (ZHA) in the synthesized powder. The
authors obtained phase pure ZnO after washing the sample
with methanol to remove ZHA. Malhotra et al.
17
prepared
ZnO via phytoassisted precipitation using a Eupatorium
odoratum medicinal plant and observed peaks from inter-
mediate products along with ZnO in the XRD pattern. Peak
intensities in Figure 1a clearly indicate ZnO as the
Figure 2. (i−vii) FE-SEM of ZnO prepared by the MSMATD
method at 35,000×magnification. Indicated nanorods and nano-
particles have dimensions (in nm) as (ii) 1, 1368.56; 2, 1472.77; 3,
1450.65; (iii) 1020.53; (iv) 657.72; (v) 1, 595.101; 2, 521.87; (vi) 1,
252; 2, 147; 3, 204; (vii) 1, 417; 2, 501. At low temperatures, there
was a growth of nanorods, whereas at elevated temperatures, a
globular structure was formed.
Figure 3. (a) Room temperature diuse reflectance spectra of ZnO nanorods and nanoparticles prepared by the MSMATD method. (b) UV−
visible absorption spectra of ZnO nanoparticles/nanorods prepared by MSMATD, 1 h at 275 °C≤TS≤700 °C. The inset shows the dierential of
absorbance with respect to wavelength versus wavelength indicating the peak position. Table 1 contains all peak positions. (c) Kubelka−Munk
function versus energy plot for the ZnO nanorod prepared at 300 °C. Intercept of the dashed line shows the band gap. (d) Band gap values
obtained for all the samples prepared by MSMATD from 275 to 700 °C as a function of synthesis temperature.
Table 2. Results from PL Peak Fitting, Peak Position (in
nm), FWHM, Area under the Curve, and Normalized Area
under the Peaks with Respect to the Respective UV Peaks
Are 100%
TS
(°C) peak FWHM area under the
peak normalized area (in
%) E
(eV)
275 382.2 13.9 356.3 100.0 3.2
535.6 64.1 635.2 178.3 2.3
495.7 45.4 520.4 146.1 2.5
586.4 204.6 861.0 241.7 2.1
300 381.7 12.6 749.1 100.0 3.2
395.4 21.8 258.0 34.4 3.1
544.5 178.7 574.1 76.6 2.3
500.5 80.7 792.1 105.7 2.5
400 384.6 17.2 161.5 100.0 3.2
581.5 186.5 398.9 247.1 2.1
540.0 66.9 291.3 180.4 2.3
498.3 50.9 369.1 228.6 2.5
500 384.9 16.5 232.2 100.0 3.2
534.1 64.3 525.8 226.5 2.3
494.9 46.7 471.0 202.9 2.5
590.2 207.3 759.7 327.2 2.1
600 383.9 15.8 132.6 100.0 3.2
495.9 49.1 389.1 293.5 2.5
539.5 71.0 412.1 310.8 2.3
632.5 226.0 480.0 362.1 2.0
700 385.1 16.8 259.8 100.0 3.2
497.2 47.6 1063.1 409.2 2.5
535.7 67.8 1484.0 571.2 2.3
384.9 16.5 232.2 499.0 2.1
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predominant phase. Existence of multiphase induces the
change in morphology of nanoparticles.
18,19
The synthesized
samples in this study did not show any morphological change
due to the multiphasic nature of the samples (the Field Eect
Scanning Electron Microscopy (FE-SEM) section contains the
detailed discussion). EDX measurements have confirmed the
multiphase nature (Figure 1b). Existence of only Zn signal and
O signal peaks clearly indicates that the extra peaks observed in
the XRD pattern are due to intermediate phases (Figure 1b).
We avoided chemical washing in our synthesis process to
prevent from being hazardous to the environment. An
interactive program on “powder diraction data interpretation
and indexing”
14
was employed to determine the lattice
parameter of synthesized ZnO using XRD data. The R-factor
∼10−3and F>10 for all the samples are indicative of the
satisfactory refinement of the lattice parameters. Figure 1c
depicts the determined lattice parameters aand cas a function
of synthesis temperature (TS). The inset of the figure indicates
minimum in c/aratio at TS= 300 °C, though the dierence is
negligible.
The average particle size
20
of the ZnO nanoparticles was
determined (Table 1) from the Williamson−Hall plot using
the intercept value from βcos θversus 4 sin θplot (not
included here for the purpose of conciseness), as described
elsewhere.
21
Here, βrepresents the FWHM of XRD peaks for
the ZnO phase. XRD investigation has been further verified
with FE-SEM findings.
3.2. Field Eect Scanning Electron Microscopy (FE-
SEM). FE-SEM images (Figure 2) show the variation in shape
and size of synthesized ZnO formed at dierent temperatures.
At low temperatures, nanorods (NRs) were formed, whereas at
higher temperatures (TS≥600 °C), there was a tendency to
form pseudo spheroid nanoparticles (NPs). The shape and size
of the photocatalyst are some of its most conducive
characteristics toward the favorable outcome of the light-
driven process in water remediation. The formation of flakes at
lower temperature is in agreement with Sunainea et al.
22
These
authors showed sheet-like morphology having 200 μm size of
zinc acetate. These sheets on calcination at dierent heating
rates led to the formation of NRs and NPs.
22
Additionally, they
reported a change in morphology of nanomaterials with
variation in the heating rate, viz., NRs at a low heating rate of
40 °C/h and NPs at high heating rates of 80, 120, and 200 °C/
h.
22
Deebansok et al. demonstrated a change in ZnO
morphology like flakes and spheres by varying the precursor
concentration during the hydrothermal synthesis process.
23
The progressive heating temperature expedites bonding
between particles rendering the change in morphology from
rods to spheroid. This phenomenon was reported earlier when
pseudo spherical shaped ZnO nanoparticles were prepared by
the direct precipitation method at 550 °C for 2 h by Chen et
al.
24
While sintering the sample at 900 °C with an objective to
obtain phase pure ZnO, Vijayalakshmi et al.
15
also observed
changes in morphology and reported spherical structures.
The aspect ratio of the synthesized NRs was determined
using the longest and distinctly observed NRs for all the
Figure 4. (a) PL spectra of ZnO nanorods and nanoparticles synthesized via the MSMATD approach at dierent synthesis temperatures. (b) (i, ii)
Gaussian peaks fitted to a broad band in the visible region and sharp peak in the UV region for ZnO nanoparticles and nanorods prepared by
MSMATD at 275 °C≤TS≤700 °C (only two graphs are shown for conciseness, and the corresponding energy level diagrams are shown in panels
(iii, iv)). The red line shows overall fitting, black dot curve, 3.2 eV peak; green dash curve, 2.5 eV peak; green color short dash curve, 2.3 eV peak;
green dot curve, 2.1 eV peak; violet dot curve, 3.14 eV peak. (iii) Various transition states of ZnO NRs and NPs synthesized at 275, 400, 500, 600,
and 700 °C. (iv) Various transition states of ZnO NRs synthesized at 300 °C, ST, surface traps. (c) Observed PL intensities corresponding to 2.1,
2.3, and 2.5 eV peaks from MSMATD synthesized nanorods and nanoparticles over 275−700 °C. The yellow dotted line indicates the normalized
area under the PL peak in the UV range.
Figure 5. Proposed defect model presenting defect distribution in two
semi-infinite grains sharing the same grain boundary.
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samples synthesized at the specific temperatures (Figure 2 and
Table 1). NPs synthesized at 700 °C were comparatively
bigger in size as compared to NPs synthesized at 600 °C.
Raoufi reported an increase in pseudo spherical nanoparticle
size for ZnO prepared by the precipitation method and
annealed for 4 h at 250−550 °C.
25
The particle sizes as
evaluated from XRD (Table 1) are dierent from those
obtained from FE-SEM analysis. Such dierences occur when
the sample contains particles of varying sizes, and XRD analysis
is known to provide average particle size. A very marginal
increase in particle size with an increase in temperature from
600 to 700 °C is seen via XRD (Table 1). Thus, the NPs
formed at 600 and 700 °C are not homogeneous with respect
to particle size and consist of a high number of fine particles
Figure 6. (a) Change in the absorption signal from the CV dye after regular interval of time for CO= 30 μM and the photocatalyst amount
(MSMATD synthesized ZnO) = 2 g/L; ZnO is prepared at 300 °C. (b) Pseudo first-order reaction of the CV dye under visible light exposure using
ZnO synthesized at TS= 260 −700 °C. (c) Rate constant, Kapp, for crystal violet dye degradation, degraded using ZnO synthesized at 300 °C and
using a photocatalyst amount of 2 g/L, CO= 30 μM. (d) CV dye degradation with ZnO synthesized at dierent temperatures. CO= 30 μM,
photocatalyst amount = 2 g/L; 10 mL of dye solution, picture taken after 120 min of exposure. For ZnO synthesized at 300 °C, η= 97.30% (in 120
min) and 93.18% (in 90 min).
Figure 7. (i) Eect of the amount of the MSMATD synthesized ZnO photocatalyst on degradation of the CV dye at λmax = 587 nm, TS= 300 °C
(results of some amounts are shown for the sake of clarity). (ii) Eect of the amount of the MSMATD synthesized ZnO photocatalyst on
degradation of the CR dye at λmax = 496 nm, TS= 300 °C for various initial dye concentrations. (iii) Catalyst loading experiment: CV dye
degradation with ZnO synthesized at 300 °C with dierent photocatalyst amounts. CO= 12 μM, photocatalyst amount of 0.1−3 g/L; 10 mL of dye
solution, ZnO synthesized at 300 °C, picture taken after 45 min of exposure. For ZnO in 2 g/L amount, η= 88.97% (in 45 min) and 87.05% (in 30
min). (iv) Catalyst loading experiment: CR dye degradation with ZnO synthesized at 300 °C with dierent catalyst amounts. CO= 12 μM,
photocatalyst amount of 0.5−3 g/L; 10 mL of dye solution, ZnO synthesized at 300 °C, picture taken after 25 min of exposure. For ZnO in a 2 g/L
amount, η= 88.6% (in 25 min) and 54.78% (in 15 min).
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along with a small number of large particles. This is also
evident in FE-SEM images of the NPs formed at 600 and 700
°C (Figure 2). Also, previously reported green synthesis
procedures showed non-uniform particle size growth of
ZnO.
26,27
The aspect ratio is increasing with a decrease in synthesis
temperature from 500 down to 275 °C. This is in agreement
with earlier report
28
showing an increase in diameter and
decrease in length of nanorods while increasing calcination
temperature from 350 to 450 °C during mechanical-assisted
thermal decomposition of Zinc acetate dihydrate. On the other
hand, Fu et al. showed the increase in the aspect ratio of ZnO
NRs with an increase in the reagent concentration.
29
Thus, the
morphology, particle size, and aspect ratio were found to vary
with variation in synthesis temperature (TS). The eect on the
band gap was analyzed via diuse reflectance spectroscopy in
the next section.
3.3. Diuse Reflectance Spectroscopy (DRS). For all
NPs and NRs prepared at TS≥275 °C, a strong reflection was
observed (Figure 3a) in visible range (above ∼386 nm). With a
decrease in synthesis temperature from 700 to 275 °C (Table 1
and Figure 3b), the absorbance peak shifted to lower
wavelength. A sharp shift for ZnO prepared below TS= 400
°C points to a blue shift with lowering of TS. This visible blue
shift could be due to the confinement eect. Kumar Jangir et
al.
19
also reported a blue shift with lowering particle size of
ZnO nanoparticles prepared by various techniques. The band
gap energy of the synthesized ZnO nanoparticles and nanorods
was evaluated using Kubelka−Munk (KM) relation as
described earlier.
17,30−32
The intercept of the Kubelka−
Munk (KM) function versus energy yielded a band gap value
as shown for the sample prepared at 300 °C (Figure 3c).
The reduction in band gap (Figure 3d) of ZnO nanoma-
terials as compared to bulk ZnO (in 3.3 eV) could be due to
zinc and oxygen vacancies, zinc and oxygen interstitials, and
dislocation and stacking faults. The presence of defects in
synthesized ZnO NRs and NPs was investigated via photo-
luminescence spectroscopy.
3.4. Photoluminescence (PL) Spectroscopy. We
investigated optical properties of synthesized ZnO to under-
stand its application as the photocatalyst in the textile dye
Figure 8. (i) Apparent rate constant for degradation of the CV and CR dye using ZnO NRs synthesized at 300 °C, photocatalyst amount = 2 g/L.
(ii) 1/Kapp vs the initial dye concentration to obtain adsorption and photocatalytic degradation of the CV and CR dye using nanorods synthesized
at 300 °C, photocatalyst amount = 2 g/L.
Table 3. Kinetic Parameters of Photocatalytic Degradation
of the CV and CR Dye
a
dyes CV CR
R0.936 0.981
KO82.17% 68.53%
K17.82% 31.46%
Kapp =KOK(from the L-H model) 0.08204 min−10.0227 min−1
Kapp experimental value at CO= 10
μM0.08175 min−10.04409 min−1
a
CV and CR are exposed to visible light under the eect of NRs
synthesized at 300 °C and used 2 g/L.
Table 4. Comparison of the Photocatalytic Degradation Parameter in This Work with Existing Literature for the CV and CR
Dye
photocatalyst dye dye conc.
(μM) dye solution
(mL) photocatalyst
amount (g/L) time duration
(min) η(%) rate const.
(min−1) source ref
ZnO CV 12.256 100 0.08 240 82 0.00739 UV, Hg vapor
lamp,125 W 55
ZnO flower
nanomaterial CV 24.51 0.1 80 ∼96 UV, Xe arc lamp,
300 W 44
pristine ZnO CV 24.51 100 0.05 60 40 tungsten
incandescent lamp 56
ZnO CV ∼12 10 2 (i) 30 , (ii) 45 (i) 87, (ii)
89 0.08175 Hg lamp80 W this research
paper
ZnO CR 71.77 50 0.05 50 25 0.0054 Xe lamp, 350 W 55
ZnO CR 28.7 300 0.5 60 95.02 UV-A, 365 nm 54
ZnO@0.25Ag thin
film CR 21.54 8 thin film 120 91.9 0.0211 350 W Xe lamp 57
ZnO thin film CR 10 40 thin film (2 cm ×2
cm) 2880 75.6 florescent lamp 58
ZnO CR ∼12 10 2 25 89 0.5535 Hg Lamp80 W this research
paper
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contaminated water remediation. The nanoparticle size, defect
in the crystalline structure, and surface states aect the PL
properties of ZnO.
33
In general, RT PL of ZnO exhibits a
sharp (UV range) and a broad transition (visible range). The
sharp transition corresponds to the band-to-band transition
and the broad emission is due to dopant, impurities, and/or
point defects such as zinc interstitial and oxygen vacancies in
the ZnO.
17,34
The PL spectra of MSMATD synthesized ZnO NRs and
NPs showed two emission peaks, one in the UV range
(between 382 and 385 nm, see Table 2) and second in the
visible range corresponding to defect states (Figure 4a and
Table 2).
The broad peak in the visible range was reported earlier for
ZnO NPs and NRs prepared using an aqueous chemical
growth method,
35
wet chemical approach, and green approach
(like green tea leaves).
19
In synthesized ZnO, narrow UV
emission observed at 382−385 nm was due to the band-to-
band transition in synthesized NRs and NPs. The narrow peak
in the UV range indicates good optical properties
36
of the
synthesized NRs and NPs. It is well comprehended that
intrinsic defects are lesser in well-ordered grains, further
enhancing the luminous eciency of the UV band. A similar
result was also concluded by Godlewski et al.,
37
demonstrating
the intense edge emission from the GaN film for well-resolved
grains unlike the emission from the structure-less overgrowth
film. The emission spectra of the ZnO nanomaterial depend on
the synthesis process.
38−42
The emission band for synthesized
ZnO NRs and NPs prepared at TS= 275−700 °C shows an
asymmetric broad band in the green domain of a visible
spectrum. For interpreting defect chemistry in MSMATD-
synthesized NRs and NPs and their role on photocatalysis, the
broad band was fitted to Gaussian peaks with a slight change in
the peak position (0.05 eV). This slight change was due to the
varied local environment of the defect center in dierent
samples. Similar observations of change in the Gaussian peak
position due to varied local environments were reported by
Vanheusden et al.
43
and Ye et al.
36
A higher number of fitted peaks in the PL spectra indicate
the presence of various defect states (Figure 4b). The peak
fitting graphs corresponding to only two synthesis temper-
atures are shown for conciseness (Figure 4b(i, ii)). The
parameters obtained from fitted peaks, viz., peak position,
FWHM, area under the curve, and relative intensities of peak,
are tabulated (Table 2). Here, area under the peak was
calculated, considering that the UV peak is 100%.
3.4.1. Proposed Defect Model. In the undoped ZnO,
oxygen vacancies are tremendously present due to low
formation enthalpy, which result in green emissions.
44
In
ZnO nanomaterial, oxygen vacancies may exist in their charged
state as
36
the (i) Vo
ostate (oxygen vacancy, which has captured
two electrons); (ii) Vo
+(single-positive charged wrt to the
lattice); and (iii) Vo
2+ (doubly positively charged wrt to the
lattice).
Band bending occurs at the grain boundaries that is crucial
in terms of defect chemistry and eventually in photocatalytic
eciency. When grain boundaries have lower chemical
potential than the grains, a potential barrier develops at the
boundary.
45
Gupta and Carlson proposed an atomic model
46
assuming donor-like positively charged defects in the depletion
layer. Close to the grain boundary, there would be a depletion
region where most of oxygen vacancies were expected to be in
the Vo
+state. Band bending would create an electron depletion
region, so most of the vacancies would be in the Vo
2+
diamagnetic state. Meanwhile, in bulk, both Vo
2+ and Vo
+
would be higher, and their ratios would be as per synthesis
temperature, with majority of the defects expected to be in the
Vo
+paramagnetic state (Figure 5). This is in agreement with the
observed PL intensity (Figure 4c) for 2.1, 2.3, and 2.5 eV
emissions in our experiments. At higher TS, the synthesized
ZnO nanomaterial has a globular structure of larger
dimensions and thus the large grain size indicates a less
surface to volume ratio. Hence, there would be more
proportion of bulk as compared to the depletion region. This
finding is in agreement with high observed 2.1 and 2.5 eV
emissions in PL spectra (indicating higher Vo
2+ and Vo
+defects).
Nanorods prepared at TS= 300 °C showed a sharp UV peak
indicating well-ordered grains with lesser intrinsic defects. At
TS= 300 °C, there was possibly a reduction in barrier height
due to Zni
++VZn
−= Zni
0+VZn
0at the grain boundary, and band
bending could be ignored. A similar reduction in the barrier
was proposed for ZnO varistors.
46
Consequently, due to
reduced depletion width, a dip was expected in Vo
2+ defect
states and hence supported the absence of 2.1 eV peak
emission in PL spectra (see Figure 4c).
3.5. Application: Textile Dye Degradation Test.
Photocatalytic eciency was evaluated against CV and CR
dyes using MSMATD synthesized ZnO photocatalysts. The
following equation was used to obtain degradation eciency
= ×
C C
C
(%) 100
to
o
(1)
Here, COis the initial concentration of the dye and Ctis the
concentration of the dye after photocatalytic action of ZnO
after a certain duration of time.
Langmuir−Hinshelwood (L-H)
47
is the most popular
kinetic model that can explain the photocatalytic degradation
rate (r) of the textile dye. According to the L-H model, for
mono-molecular reaction (that is photocatalytic reaction of the
dye adsorbed in Langmuir’s fashion)
19,48,49
= =
+
rC
t
K KC
KC
d
d 1
O
(2)
where tis the reaction time, KOis the photocatalytic reaction
rate constant, Kis the adsorption coecient of the textile dye
on the photocatalyst (ZnO), and Cis the concentration of the
dye in the bulk at equilibrium, after the dye is adsorbed by a
photocatalyst obeying a Langmuir isotherm and the adsorption
equilibrium is maintained during the photocatalytic reaction.
The above equation is often modified depending on the
reaction condition. For a low concentration, KC ≪1, the
above equation gets simplified to the pseudo first-order
reaction equation
= = =rC
t
K KC K C
d
dO app
(3)
where Kapp is the apparent pseudo first-order rate constant.
The above equation is useful for assessing reaction kinetics
when the reactant concentration (dye concentration) and
adsorption coecient or both are small. Taking integration on
both sides for the above equation
=
C
C
K t
dd
app
(4)
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G

+ =C C K tln 1 app
(5)
Applying the initial condition (at t= 0 s, C=CO) results
in
47,50,51
= =
C
C
K t C C eln or t
K t
O
app O app
(6)
The plot of Ctversus degradation time, t, shall exhibit an
exponentially decaying curve. Fitting of Ctversus tcurve shall
yield Kapp.
If KC > 1, then eq 2 reduces to the pseudo zero-order
reaction model
=K
C
t
d
dO
, which on integration results in Ct
=CO−KOt. In this case (high concentration), the
concentration of the dye is expected to vary linearly with
time. The parameter KOand Kcan be obtained by linearizing
eq 2, that is, reciprocal of eq 2
= +
r K KC K
1 1 1
O O
(7)
From eqs 3 and 7
= +
K K K
C
K
1 1
app O O
(8)
Thus, plotting
K
1
app
against the equilibrium concentration
should be a straight line with slope
K
1
O
and intercept
KK
1
O
3.5.1. Eect of Synthesis Temperature and Catalyst
Dosage. The eect of the amount of the synthesized ZnO
NRs or NPs used in photocatalysis was evaluated on the CV
and CR dye degradation by measuring absorption curves via
UV−visible spectroscopy. The absorption peak decreased
progressively with exposure time, indicating a decrease in the
dye concentration in the solution due to the photocatalytic
eect (Figure 6a). The exponential fitting of the concentration
versus exposure time (t) plots was used to compute the
apparent kinetic reaction rate constant values (Figure 6b).
The photocatalytic degradation fits the pseudo first-order
reaction as discussed above and is shown in Figure 6b. The
degradation profile of the CV dye under visible light exposure
(Figure 6b) showed a correlation with synthesis temperature of
photocatalyst nanomaterials (over TSfrom 260 to 700 °C).
The ZnO photocatalyst synthesized at TS= 300 °C was found
to be most appropriate for photocatalytic degradation (Figure
6c,d). ZnO NRs synthesized via MSMATD showed surface
trap states (as discussed under the Photoluminescence (PL)
Spectroscopy section; Figure 4b(iv)). The active surface traps
led to delayed recombination time, which enhanced photo-
catalytic eciency.
After finding the optimum synthesis temperature of the
photocatalyst, the eect of the photocatalyst amount was
investigated for CV and CR dyes (Figure 7(i,ii)). The
decolorizing eciencies generally improves upon increasing
the amount of the photocatalyst to an optimum amount for a
certain illumination time.
52
The optimized dosage facilitates
not only the best degradation eciencies
53
but also its cost
eectiveness.
Our findings suggested that the rate constant (Kapp) is
influenced by the ZnO amount. Initially, there was an increase
in the rate constant with an increase in dose due to the
proportional increase in the photocatalytic sites generating
more radicals. The ZnO photocatalyst per liter of dye solution
(2 g) was optimum dose (Figure 7(i−iv)) to achieve the
highest CV and CR degradation with the MSMATD
synthesized ZnO photocatalyst. Further, an increase in dose
showed saturation in the rate constant. This could be due to
the blockage of photocatalytic activity of one NR by another.
In the existing work, any rotation, shaking, or stirring was not
undertaken during the photocatalytic test. Shaking or stirring
during the photocatalytic test will be taken in extended work.
It is worth mentioning that decolouration is not the
confirmation of complete mineralization of the dye. Other
analytical characterizations like chromatographic techniques
were not in the scope of this study and may be undertaken
during subsequent research work.
3.5.2. Eect of the CV and CR Dye Concentration. The
impact of the initial dye concentration on photocatalytic
degradation of the CV and CR dye was investigated by varying
the initial dye concentration over a wide range. Photocatalytic
performance of synthesized NRs was inversely proportional to
the initial dye concentration under the same experimental
conditions (Figure 8(i)). For the CV and CR dye, the apparent
rate constant with the initial concentration was seen to decay
exponentially. For the CR dye, the apparent rate constant
initially decreased sharply with an increase in the concentration
and led to saturation. Interestingly, 30 μM onward, the CV dye
showed a faster decrease in Kapp, and for the CR dye, it was
already low (Figure 8(i)). This abatement (for the CR dye)
could be due to the decreased absorption of photons on the
photocatalyst surface owing to the increased dye concentration
or adsorption on the catalyst surface, which in turn reduced
radicals’ generation.
3.5.3. Eect Rate Constant and Adsorption Coecient.
Figure 8(i) is replotted as 1/Kapp versus the initial dye
concentration to obtain the adsorption and photocatalytic
degradation contributions (Figure 8(ii)); 1/Kapp versus the
initial dye concentration yielded a line with high correlation
coecients, R= 0.9811 and 0.936 for CR and CV dyes,
respectively. The photocatalytic reaction rate constant, KO, and
adsorption coecient of the textile dye on ZnO NR were
determined from the slope (= 1/KO) and intercept (= 1/KOK),
respectively. The values of KOand Kwere 0.615 μM/min and
0.1334 1/μM, respectively (for the CV dye), and 0.2224 μM/
min and 0.1021 1/μM, respectively (for the CR dye).
Table 3 shows the kinetic parameters of photocatalytic
degradation of CV and CR dyes. The product KOK=Kapp for
CV was obtained as 0.082 min−1and matched exactly (%
dierence = 0.35%) with the experimental Kapp value (= 0.0817
min−1) at 10 μM, indicating that photocatalytic degradation
can be satisfactorily described by the L-H model for the CV
dye. However, for the CR dye, the product KOK=Kapp is
0.0227, which was dierent than the experimentally obtained
value of Kapp = 0.04409 min−1(% dierence = 94.22%). The
correlation coecient is greater than 0.9 indicating validity of
the fit, but there is high dierence in Kapp values obtained from
the L-H model and the experimental results. This suggests that
the L-H model would not be suitable to describe photo-
catalytic CR dye degradation using MSMATD synthesized
ZnO nanomaterials. On the other hand, the photocatalytic CV
dye degradation using MSMATD synthesized ZnO is
satisfactorily explained by the L-H kinetic model over a wide
range of CV dye concentrations. This finding is in agreement
with Elaziouti and Ahmed whose study on CR dye degradation
concluded the L-H model to be inappropriate for explaining
photocatalytic degradation using commercially obtained
ZnO.
54
Based on our results, we propose MSMATD
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H

synthesized NRs prepared at 300 °C as the best for
photocatalytically degrading the CV and CR dye in the
smallest time. Existing photocatalytic research work on CV and
CR utilizing ZnO is presented in Table 4 for comparison.
4. CONCLUSIONS
An improved, environment friendly methodology has been
established to obtain high eciency ZnO photocatalyst
nanorods. Photocatalysis rate strongly depends on defect
occurrence and thus on the synthesis process and parameters.
ZnO nanorods synthesized via magnetic stirred mechanical-
assisted thermal decomposition at 300 °C exhibited the highest
photocatalysis eciency against crystal violet and congo red
dyes. The degradation kinetics of both dyes are fast with 87
and 89% eciencies in 30 and 25 min for the CV and CR dye,
respectively. Moreover, the comparison with previous studies
clearly indicates the better degradation in terms of degradation
time with low energy consumption. Photocatalytic degradation
of the crystal violet dye obeyed the Langmuir−Hinshelwood
model unlike the congo red dye. Defect occurrences have been
correlated with the photocatalytic eciency of pristine ZnO. A
defect model based on morphological and photoluminescence
investigations has been proposed for the synthesized ZnO.
■AUTHOR INFORMATION
Corresponding Author
Suchitra Rajput Chauhan −Centre for Advanced Materials
and Devices (CAMD), School of Engineering and
Technology, BML Munjal University, Gurgaon 122413
Haryana, India; orcid.org/0000-0003-1782-3102;
Phone: 91-120-4806860; Email: suchitra.rajput.chauhan@
iitdalumni.com,rajput.suchitra@gmail.com; Fax: 91-120-
480688
Authors
Sumit Kumar −Centre for Advanced Materials and Devices
(CAMD), School of Engineering and Technology, BML
Munjal University, Gurgaon 122413 Haryana, India
Jyoti Pandey −Centre for Advanced Materials and Devices
(CAMD), School of Engineering and Technology, BML
Munjal University, Gurgaon 122413 Haryana, India
Ritika Tripathi −Centre for Advanced Materials and Devices
(CAMD), School of Engineering and Technology, BML
Munjal University, Gurgaon 122413 Haryana, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.3c03860
Author Contributions
S.R.C. performed the experiments, data analysis, conceptual-
ization, funding acquisition, writing of the original draft,
review, editing, and supervision; J.P. performed the XRD result
evaluation and documentation; R.T. performed the FE-SEM
analysis and documentation; S.K. performed the PL measure-
ment, literature tables, and documentation. All authors have
read and agreed to the published version of the manuscript.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The Department of Science & Technology - Science and
Engineering Research Board (SERB) [grant number CRG/
2020/006144] financially supported this study. S.R.C.
thankfully acknowledges Mr. Akshit Malhotra and Dr. Ashwini
Chauhan (Central University Tripura) for FE-SEM, EDAX
measurements, and English language corrections.
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