J Mater Sci: Mater Electron (2021) 32:5790–5802

Effect of Zn variation in TiO2/ZnS nanocomposite

on photocatalysis for the degradation of the hazardous
crystal violet dye
G. Hannah Priya1, A. Alinda Shaly1, and J. Mary Linet1,*

1
Department of Physics, Loyola College, Chennai 600034, India

Received: 28 September 2020 ABSTRACT

Accepted: 10 January 2021 TiO2/ZnS nanocomposite was prepared by one-step hydrothermal method. The
Published online: zinc content in the TiO2/ZnS nanocomposite was varied from M = 0.25, 0.50
1 February 2021 and 0.75. The as-prepared TiO2/ZnS nanocomposite was subjected to various
morphological characterization such as X-ray diffraction (XRD), UV–Visible
Ó The Author(s), under spectroscopy, Fourier transform infrared spectroscopy (FTIR), Photolumines-
exclusive licence to Springer cence analysis, Scanning electron microscopy (SEM) and Transmission electron
Science+Business Media, LLC microscopy (TEM). The activity of photocatalysis was investigated by the
part of Springer Nature 2021 removal of crystal violet dye in aqueous medium. The crystalline size increased
with an increase in zinc quantity. The molar ratio of Zn (M = 0.25) showed
maximum photodegradation of 70.07% in TiO2/ZnS nanocomposite.

1 Introduction the three phases, rutile forms a stable structure,

whereas anatase and brookite remain in their
Among various semiconductor materials titanium
metastable forms [3, 4]. It is reported that anatase
dioxide (TiO2) and zinc sulfide (ZnS) have been
phase show the highest photocatalytic activity among
exploited in the field of environmental catalysis due
the three TiO2 polymorphs because it possesses high
to its non-toxicity, stability and photoactivity. The
potential energy of photogenerated electrons and a
quantum size effects of nanoscale semiconductors
large surface area. TiO2 is a large band semiconduc-
bring about changes in size, shape and optical
tor with bandgap of 3.2 eV (anatase), 3.02 eV (rutile)
properties from those of their corresponding bulk
and 2.96 eV (brookite) [5, 6]. TiO2 nanoparticles
materials. The enhanced properties of a nanoparticle
exhibit interesting properties like biocompatibility,
from that of its bulk have led to its applications in
photochemical stability and photoefficiency [7]. It has
numerous fields such as electronics, optoelectronics,
various applications in photovoltaic cells, ultra-thin
and photocatalysis [1, 2].
capacitors, temperature-sensing devices, electro-
Titanium dioxide (TiO2) belongs to the group of
chemical electrodes, solar cells, etc. [8, 9].
transition metal oxides. They have three structural
ZnS belongs to II-VI binary compound with two
forms, namely anatase, rutile and brookite. Among
polymorphs, namely Sphalerite and wurtzite.

Address correspondence to E-mail: linet.mary@gmail.com

https://doi.org/10.1007/s10854-021-05300-2
J Mater Sci: Mater Electron (2021) 32:5790–5802 5791

Sphalerite displays a cubic form having a bandgap of nanocomposite is illustrated using a schematic
3.54 eV and wurtzite shows a hexagonal structure representation.
with a bandgap of 3.91 eV [10]. These large bandgaps
of TiO2 and ZnS serves as a drawback since their
practical applications are limited to the UV region of
the electromagnetic spectrum which accounts to only
4–5% of the solar spectrum and they have high
electron–hole recombination rate. These imperative
drawbacks were significantly reduced via prepara-
tion of a nanoscale coupled TiO2/ZnS semiconductor
using hydrothermal method. Hydrothermal method
is a simple, cost-effective technique in which the
average particle size can be controlled by changing
the processing time and temperature.
The combination of TiO2/ZnS nanocomposite by
precipitation cum sol–gel method revealed enhanced
optical, electrical and morphological characteristics
when compared to synthesized TiO2, ZnS and TiO2-
P25 nanoparticles; furthermore, the photodegrada-
tion of phenol red sodium salt (PRSS) were found to
be more effective for TiO2/ZnS nanocomposite [11].
Prasannalakshmi et al. (2017) synthesized TiO2/ZnS The same procedure was again followed by
nanocomposite using sol–gel method by varying the changing the molar concentration of Zn = 0.50 and
molarity of zinc sulfide nanoparticles [12]. Literature 0.75 M, respectively.
references show that the synthesis of TiO2/ZnS
nanocomposite was done mainly using sol–gel 2.2 Characterization techniques
method, and there is no report available on one-step
The powder X-ray diffraction analysis was done
hydrothermal synthesis of TiO2/ZnS nanocomposite
using Bruker D8 advanced XRD system. The Cu ka
in research literature. Furthermore, the photocatalytic
radiation of wavelength 1.5406 Å was used as an
degradation of Crystal Violet (CV) dye has not been
X-ray source in the sample. The crystallite size of the
performed yet using TiO2/ZnS nanocomposite.
particle was calculated using Scherrer’s and Wil-
In the present investigation, TiO2/ZnS nanocom-
liamson–Hall equation. The UV–Visible spectroscopy
posite was synthesized by varying the molar con-
(Jasco V- 670) was recorded at room temperature in
centration of Zn (M = 0.25, 0.50 and 0.75) via one-step
the wavelength range of 200–800 nm. The reflectance
hydrothermal method. TiO2/ZnS nanocomposites
spectrum of the sample was transformed into
with variation in Zn are represented as A1 = 0.25 M,
absorption spectra using Kubelka–Munk function.
A2 = 0.50 M and A3 = 0.75 M. The as-prepared
Bandgap can be calculated using reflectance spectra.
nanocomposites were characterized, and the results
The Fourier transform infrared spectrometer (Shi-
are discussed below.
madzu Irtracer-100) was used to determine the vari-
ous functional groups of the sample. Scanning
electron microscope (SEM) was used to determine the
2 Experimental
surface morphology of the samples. Transmission
2.1 Synthesis of TiO2/ZnS nanocomposite electron microscope (TEM) was used to find the
particle size, crystallinity and lattice spacings of the
All the chemicals are analytical grade and they have sample. Photoluminescence (PL) emission spectrum
been used as received without any further purifica- of the samples was recorded with (Horiba Jobin
tion. The chemicals were obtained from Sigma- Yvon—Fluorolog F3-111 spectrofluorometer).
Aldrich. The synthesis process of TiO2/ZnS
5792 J Mater Sci: Mater Electron (2021) 32:5790–5802

3 Results and discussion

3.1 X-ray diffraction analysis

Powder X-ray diffraction (XRD) was used to analyze

the crystal structure, phase identification, crystallite
size and to identify the presence of TiO2 and ZnS
nanoparticles in the sample. The crystal phases were
found with 2h values ranging from 20 to 80°. The
diffraction peaks exemplify the presence of TiO2 and
ZnS, thus verifying the formation of TiO2/ZnS
nanocomposite. The major peaks at 2h = 25.25°,
38.12° and 48.0° are associated to the (101), (004),
(200) anatase TiO2 crystalline plane (JCPDS card no.
84–1285). The (220) and (400) planes at 2h values 47.8°
and 69.3° represent the cubic ZnS planes (JCPDS card
no. 05—0566). Table 1 shows the crystallite size of
TiO2/ZnS nanocomposites. The crystallite size of A1
is small when compared to A2 and A3 nanocom-
posites. This indicates that 0.25 M of zinc offers a
decreased crystallite size which thereby leads to a
large surface area. The presence of a large surface Fig. 1 XRD pattern of TiO2/ZnS (A1, A2 and A3) nanocomposite
area leads to an increase in the photocatalytic activity.
The powder XRD pattern of A1, A2 and A3 is shown observe the lattice strain, Williamson–Hall plot (W–
in Fig. 1. H) analysis comes into effect as it can be used to
The average crystallite size of TiO2/ZnS estimate the crystallite size and strain of the
nanocomposite was derived using Scherrer’s nanocomposite.
equation: The lattice strain gets induced into the sample by
Kk various factors such as imperfections in the crystal
D¼ ð1Þ lattice and presence of impurities in the sample. The
bhkl cos h
lattice strain is expressed using the following
where k is the wavelength of Cu Ka radiation, bhkl is relation:
the full width half maximum (FWHM) of the
bhkl
diffraction peak, h is the diffraction and K is the e¼ ð2Þ
4 tan h
dimensionless shape factor that is equal to 0.9.
The Scherrer’s equation is only used to find the Kk
cos hbhkl ¼ þ 4e sin h ð3Þ
crystallite size of the nanocomposite, it will not con- D
sider the lattice strain that occurs in the sample due to Equation (3) gives the Williamson–Hall equation.
reduced crystallite size [13]. Among the various The y-intercept and microstrain e was estimated by
methods such as Pseudo-Voigt function [14], Rietveld linear fitting of the data. The calculated crystallite
refinement [15], Williamson–Hall analysis [16] and size and lattice strain is shown in Table 1.
Warren–Averbach analysis [17] which can be used to

Table 1 Average crystallite size of A1, A2 and A3 (TiO2/ZnS) nanocomposite

Sample name TiO2/ZnS Crystallite size D (nm) Crystallite size W–H plot (nm) Lattice strain (e)

A1 9.41 nm 9.69 nm 6.342 9 10–3

A2 13 nm 13.56 nm 2.3935 9 10–3
A3 20.8 nm 22.0 nm 1.1327 9 10–3
J Mater Sci: Mater Electron (2021) 32:5790–5802 5793

Thus, the crystallite size calculated using Scherrer’s transmission in a nanomaterial. Tauc plot for
equation and Williamson–Hall plot showed a absorption spectrum is used to find the bandgap (Eg)
reduced crystallite size for A1 when compared with of a nanomaterial. The bandgap of a nanomaterial
A2 and A3. This reduced crystallite size thus allows can also be found using the values of Ephoton (Epho-
for an increased surface area [18]. Figure 2 shows the ton = 1240/k eV) extrapolated to a = 0 (a is absorp-
Williamson–Hall plot of A1, A2 and A3 tion coefficient). The corresponding result gives an
nanocomposite. absorption energy which corresponds to a bandgap
By comparing the 2 h values from XRD to the (Eg). Absorption spectrum was chiefly utilized to
standard JCPDS values, it was confirmed that anatase detect the degradation of dye compounds in an
and zinc blende structure formed the TiO2/ZnS effluent solution by the process of photocatalysis.
nanocomposite. The presence of these polymorphs Absorbance vs wavelength graph was plotted to find
were further verified by calculating the lattice the degradation of the dye in the solution. Reflectance
parameter ‘a’ and ‘c’ for tetragonal structure of TiO2 spectroscopy is used to find the bandgap of a mate-
and ‘a’ for cubic structure of ZnS. The standard lattice rial with the assistance of Kubelka–Munk function.
constants of TiO2 was a = 3.782 Å and c = 9.502 Å, The Kubelka–Munk function F(R) is obtained from
and ZnS was a = 5.345 Å matches well with the the following equation:
recorded XRD [19, 20] (Table 2).
ð1  R Þ2
FðRÞ ¼ ð4Þ
2R
3.2 UV–visible analysis
where R is the reflectance (%). It is found that
UV–Visible spectroscopy is an important characteri- Kubelka–Munk function is directly proportional to
zation technique that enables the study of optical the absorption coefficient; and the effect that is
properties such as absorption, reflection and brought about by the scattered light is eradicated.

Fig. 2 Williamson–Hall plot

of A1, A2 and A3
nanocomposite
5794 J Mater Sci: Mater Electron (2021) 32:5790–5802

Table 2 Lattice parameter and volume of unit cell for TiO2/ZnS (A1, A2 and A3) nanocomposite

Sample name TiO2/ZnS Lattice parameter a (Å) Lattice parameter c (Å) Volume of the unit cell (Å)3

A1 TiO2 (Tetragonal) a = 3.7973 c = 9.4460 TiO2 V = 136.20

ZnS (Cubic) ZnS
a = 5.4156 V = 158.83
A2 TiO2 (Tetragonal) a = 3.7835 c = 9.4593 TiO2 V = 135.40
ZnS (Cubic) ZnS V = 160.01
a = 5.4290
A3 TiO2 (Tetragonal) a = 3.7801 c = 9.4549 TiO2 V = 135.10
ZnS (Cubic) ZnS V = 158.61
a = 5.4131

The optical absorbance coefficient (a) using Tauc that the bandgap increases when the Zn content was
plot is expressed by the following equation: increased. The increased level of Zn leads to an

n uneven coupling of the TiO2/ZnS nanocomposite.
A hm  Eg
a¼ ð5Þ Thus, the increased Zn content may cause agglom-
hm
eration of ZnS on top of TiO2 nanoparticles.
since F(R) is equivalent to absorption coefficient (a).
From Eqs. (4) and (5), we obtain the equation to find 3.3 Fourier transform infrared spectroscopy
the bandgap: (FTIR)

n
½FðRÞhm ¼ A hm  Eg ¼ aðhmÞ ð6Þ
The functional groups present in the FTIR spectrum
where A is a constant factor, h indicates the Planck’s of TiO2/ZnS (A1, A2 and A3) nanocomposite is
constant and m represents frequency of the incident shown in Fig. 5. The FTIR spectrum was recorded in
photon. The direct and indirect transitions have the the range of 4000 to 450 cm-1 to determine the
value of n = 2 and 1/2, respectively. The transition absorbance and the nature of the bonds of TiO2/ZnS.
values differ depending on the electronic transitions 3100 to 3600 cm-1 shows the (Ti–OH) stretching
that occur. modes of hydroxyl groups due to the absorption of
The UV–Visible reflectance spectrum of TiO2/ZnS H2O onto the surface of the nanocomposite. The
nanocomposite was employed to estimate the band- peaks at 1004.91, 983.70 and 981.77 cm-1 around
gap properties. The reflectance measurements were 1000 cm-1 range indicates the presence of ZnS that
recorded in the wavelength range of 250–800 nm at originates from the resonance interaction between
room temperature. The reflectance peak of TiO2/ZnS
(A1, A2 and A3) nanocomposite is shown in Fig. 3.
As the Zn content increases, there is a shift in the
reflectance peak toward the lower wavelength region
known as blue shift or hypsochromic effect.
The direct and indirect bandgap energy of TiO2/
ZnS found using K–M plot is shown in Fig. 4a and b.
Pure TiO2 (anatase) nanoparticles exhibit indirect
bandgap and ZnS has direct bandgap energy. Thus
the coupling of these two semiconductors may lead
to the possibility of formation of both indirect and
direct bandgaps. The direct bandgap energy of
A1 = 3.70 eV, A2 = 3.73 eV and A3 = 3.76 eV. The
indirect bandgap energy of A1 = 2.88 eV,
A2 = 2.95 eV and A3 = 3.04 eV. From the bandgap
values obtained using the K–M function, it is evident
Fig. 3 Reflectance spectra of TiO2/ZnS nanocomposite
J Mater Sci: Mater Electron (2021) 32:5790–5802 5795

Fig. 4 a, b, c Direct bandgap

and d, e, f indirect bandgap
energy of TiO2/ZnS
nanocomposite

vibrational modes of sulfur ions in the nanocrystal 3.4 Photoluminescence analysis

system [21]. The peaks at 1612.49, 1616.35 and
1618.28 cm-1 shows the presence of C = C stretching The structural defect states and the amount of charge
and C-O stretching vibration mode. The peak at carrier recombination which takes part in the radia-
1112.93 cm-1 occurs due to the presence of C-O tive de-excitation state was determined using PL
stretching of secondary alcohol. The peaks at 1400.32 analysis. Figure 6 shows the PL spectrum of TiO2/
and 1402.25 cm-1 could be ascribed to the S = O ZnS nanocomposite excited at 325 nm.
stretching confirming the presence of sulfur. 619.15 The emission bands were recorded in the spectral
and 621.08 cm-1 attributes to the stretching vibra- region between 345 and 630 nm. The PL emission
tions of Zn–S bond [22]. shows a UV emission at 373 nm and three visible
5796 J Mater Sci: Mater Electron (2021) 32:5790–5802

Fig. 6 Photoluminescence spectra of TiO2/ZnS (A1, A2 and A3)

nanocomposite

largely varied by making changes in the synthesis

conditions.
Fig. 5 FTIR transmission spectrum of TiO2/ZnS (A1, A2 and A3)
nanocomposite 3.6 HR-TEM micrographs

emissions at 427, 467 and 576 nm, respectively. There The HR-TEM image of the as-prepared samples with
is a free exciton transition at 373 nm. The sharp peak varying Zn content is shown in Fig. 8. The HR-TEM
at 427 nm belongs to the violet emission results depict the morphology of TiO2/ZnS (A1, A2
(410–430 nm) which is attributed to the recombina- and A3) nanocomposite. A1 nanocomposite exhibits a
tion of self-trapped excitons localized in TiO6 octa- high crystalline TiO2 coupled ZnS nanocomposite.
hedra (anatase) structure [8]. The green emission at The TiO2 particles revealed tetragonal nanorod for-
576 nm has been reported to be associated with the mation and ZnS was deposited on TiO2. The average
oxygen vacancies and impurities, the most common particle size of A1 was found to be 14.41 nm which is
defect in PL [23]. in close agreement with the crystallite size attained
from Scherrer’s method and Williamson–Hall
3.5 HR-SEM analysis method.
For a high percentage of ZnS in A2 and A3, the
The chemical composition, shape and particle size of capping of TiO2 with ZnS was not uniform and there
TiO2/ZnS nanocomposite was analyzed using SEM is an uneven nucleation of TiO2 and ZnS nanoparti-
analysis. The particle size of the nanocomposite A1, cles. These factors led to the agglomeration of ZnS on
A2 and A3 were obtained for 120,000 9 magnifica- the nanocomposite. Thus, there is a drastic change in
tion. The observed SEM image of the nanocomposite the morphology of A2 and A3 when the composition
A1, A2 and A3 showed an average particle size of of Zn was increased in the nanocomposite.
19.63, 20.04 and 22.70 nm, respectively. The small A1, A2 and A3 nanocomposite consisted of both
particle size of the coupled semiconductor (A1) TiO2 and ZnS lattice fringes. An alternate TiO2 and
exhibited a large surface area and well-defined ZnS lattice fringes were found in the TEM images,
mesopores in Fig. 7. As the Zn content in TiO2/ZnS indicating the formation of TiO2 coupled ZnS
nanocomposite (A2 and A3) was increased, the par- nanocomposite. The nanocomposite (A2 and A3)
ticle size was found to increase, and the nanocom- showed a lattice spacing of d = 0.35 nm for anatase
posite shows a non-uniform distribution of densely TiO2 (101) plane and d = 0.268–0.27 nm for cubic ZnS
arranged particles which are spherical in morphol- (200) plane. The lattice fringe d = 0.19 nm in A3
ogy. The shape and size of the nanocomposites can be nanocomposite is assigned to (220) spacing of the
J Mater Sci: Mater Electron (2021) 32:5790–5802 5797

Fig. 7 SEM micrograph of

TiO2/ZnS (A1, A2 and A3)
nanocomposite

cubic ZnS phase. Thus, the lattice planes obtained in treating waste water contamination owing to its
from HR-TEM in Fig. 9 are well consistent with the low cost, high efficiency, color removal, long term
XRD results. thermodynamic stability and effective oxidation of
The hkl planes of TiO2/ZnS nanocomposite (A1, intractable pollutants by decomposition and reduc-
A2 and A3) were found using selected area electron tion to free hydroxyl radicals [24].
diffraction (SAED) in Fig. 10. The bright spots indi- Crystal Violet dye (CV) or gentian violet is a dark
cate the polycrystalline nature of the nanocomposite. green colored synthetic cationic dye. It is highly toxic
The fast fourier transform (FFT) was used to measure in nature. It is used as a coloring agent for acrylic
the distance between the atomic planes. The SAED fiber, paint, printing ink, textile industry and paper
rings represent alternate TiO2 and ZnS rings. The dyeing. The major drawback of CV dye includes
plane (101), (004) and (105) correspond to the irritation of the respiratory tracts, vomiting, diarrhea
tetragonal TiO2 with an anatase phase. The planes and an extended exposure to CV dye may damage
(200) and (220) represent the cubic ZnS phase. the mucous membrane, kidney failure, liver damage,
cancer and permanent blindness [25, 26]. The pho-
3.7 Photocatalytic activity todegradation of various dyes using TiO2/ZnS
nanocomposite is given in Table 3.
Textile, paper and plastic industries exploit extensive The aqueous medium for the photocatalytic
amount of water to color their products and release degradation of CV dye was prepared by adding 0.1 g
them into the environment, thereby contaminating of the catalyst to 100 mL of distilled water containing
the extensive water resources in nature. This polluted appropriate amount of CV dye. The CV dye with
dye water is hazardous to the environment and 0.1 g of catalyst was stirred in dark for an hour; this
causes various health issues to humans and aquatic provides an adsorption/desorption equilibrium. The
life. Among various advanced oxidation processes effluent when stirred in the dark allows a maximum
(AOP), photocatalytic oxidation is the most effective
5798 J Mater Sci: Mater Electron (2021) 32:5790–5802

Fig. 8 TEM images of TiO2/

ZnS (A1, A2 and A3)
nanocomposite

Fig. 9 HR-TEM image

showing lattice fringes of
TiO2/ZnS (A1, A2 and A3)
nanocomposite
J Mater Sci: Mater Electron (2021) 32:5790–5802 5799

Fig. 10 SAED pattern of A1,

A2 and A3 nanocomposite

adsorption of CV dye onto the surface of the catalyst. A2 = 58.6% and A3 = 49.67%. The A1 nanocomposite
After stirring for 1 h in the dark, incandescent light had an excellent adsorption capacity when compared
(tungsten lamp of 100 W) was illuminated onto the to A2 and A3 which is because of the decrease in
effluent medium. A cooling setup was arranged crystallite size and increase in surface area. It is well
around the beaker to avoid heating. known that the photocatalytic degradation process
The degradation rate of TiO2/ZnS nanocomposite occurs mostly on the surface of the photocatalyst.
was calculated using the equation: Thus, the large surface area and the decreased
C0 C bandgap play a significant role in the degradation
Degradation rate ð%Þ ¼  100 ð7Þ process. The results obtained suggest that A1 is a
C0
better photocatalyst than A2 and A3.
where C0 is the initial concentration of the dye and C
is the final concentration of the dye.
3.7.1 Kinetic parameters in the photodegradation process
To study the rate of decomposition of CV using the
prepared TiO2/ZnS nanocatalyst, absorbance spectra The photons from the incandescent light source
were recorded for every 15 min of irradiation of enable the excitation of electrons and generation of
incandescent light onto the sample (Fig. 11). The rate holes. The photogenerated holes (h?) get trapped to
of degradation of CV for TiO2/ZnS occurs at the H2O molecule and forms a reactive hydroxyl
k = 583 nm. The rate of degradation of CV dye for an radical (.OH), whereas the electrons gets captured by
irradiation time of 60 min was calculated using Eq. 7 the O2 and .O2- is formed. O2- and .OH radicals play
and it is graphically represented in Fig. 12. As the an effective role in the degradation of CV dye. The
irradiation time was prolonged the absorption spec- continuous transfer of electrons and holes among the
tra showed a gradual decrease. Thus the increase in TiO2–ZnS nanocomposite increases the lifetime of
degradation % is directly proportional to the irradi- charge carriers and reduces the recombination rate of
ation time. The degradation % of A1 = 70.07%,
5800 J Mater Sci: Mater Electron (2021) 32:5790–5802

Table 3 Photocatalytic degradation of various dyes using TiO2/ZnS nanocomposite

Synthesis method Pollutant removed Light source Removal Maximum time References
efficiency utilized

Sol–gel method Brilliant green Sun light 100% 25 min [12]

Methylene blue Sun light
Microemulsion-mediated Parathion-methyl UV light 96.5% 60 min [1]
solvothermal method Visible light 74.5%
Chemical deposition Acid blue 113 UV-A 99% 27.32 min [2]
radiation
Precipitation cum sol–gel method Phenol red sodium salt Sun light 94.9% 150 min [27]
4-nitrophenol UV light 60.9%
Sun light 28.4%
One-step hydrothermal method Crystal violet Incandescent 70.07% 60 min Present
light study

Fig. 11 Photocatalytic
degradation of CV dye using
TiO2/ZnS (A1, A2 and A3)
nanocomposite

electrons and holes [28–30]. The CV dye gets oxidized mechanism using TiO2–ZnS nanocomposite is illus-
by the hydroxyl radicals and it is thereby broken trated in Fig. 13.
down into less harmful by-products. Degradation The elemental steps that occur during the degra-
efficiency was found to be greater for A1 when dation mechanism of CV dye using TiO2–ZnS
compared to A2 and A3 because of the narrow nanocomposite is represented by the equations given
bandgap and small particle size. The increase in the below:
zinc content increases the recombination of charge

ðTiO2  ZnSÞ þ hm ! TiO2  ZnS hþ 
VB þ eCB ð8Þ
carriers and thereby decreases the photodegradation
of CV dye [31–33]. The photocatalytic degradation
J Mater Sci: Mater Electron (2021) 32:5790–5802 5801

ln CC0 and time. The pseudo first-order reaction
kinetics model is shown in Fig. 14. The pseudo first-
order reaction rate constant for TiO2/ZnS is calcu-
lated using the slope obtained from the plot. The
pseudo first-order rate constant for
A1 = 1.72 9 10–2 mg/cm2, A2 = 1.11 9 10–2 mg/cm2
and A3 = 0.963 9 10–2 mg/ cm2. There was a
decrease in rate constant value when the zinc level
was increased. Thus the kinetic result of TiO2/ZnS
was in agreement with the percentage of CV dye
removal.

4 Conclusion
Fig. 12 Degradation % of Crystal violet dye for TiO2/ZnS (A1, In this work, TiO2/ZnS nanocomposite was synthe-
A2 and A3) nanocomposite sized by one-step hydrothermal method. The resul-
tant nanocomposite was prepared by varying the
hþ 
VB þ H2 O ! OH þ H
þ
ð9Þ molar ratios of Zinc (0.25, 0.5 and 0.75 M). The
e þ O2 ! 
O ð10Þ powder XRD analysis confirmed the presence of
2
tetragonal TiO2 and cubic ZnS nanoparticle in the
Dye þ  OH ! oxidized products þ degraded by sample. The bandgap obtained using UV–visible
 products spectra showed a decrease in bandgap for A1 when
ð11Þ compared to A2 and A3. The increase in bandgap is
caused by the occurrence of an uneven coupling as
Langmuir–Hinshelwood kinetic expression was
the zinc content increased. FTIR spectrum confirmed
used to determine the photocatalytic degradation
the presence of the functional groups in the sample.
kinetics of the synthesized TiO2/ZnS nanocomposite
Photoluminescence emission peaks show the exis-
using pseudo first-order reaction kinetics,
  tence of violet and green emissions. The morpho-
C0 logical analysis using SEM imaging of TiO2/ZnS
ln ¼ kt ð12Þ
C nanocomposite exhibited mesopores, densely arran-
ged spherical particles with particle size ranging
where Co is the initial concentration of crystal violet
from 19 * 22 nm. The HR-TEM micrograph has lat-
dye at time t = 0, C is the concentration of crystal
tice fringes of TiO2 and ZnS, hence confirming the
violet dye after irradiation for a certain time interval
and k is the photodegradation rate constant which
can be obtained from the slope of a graph between

Fig. 13 Photocatalytic degradation mechanism of TiO2/ZnS

nanocomposite Fig. 14 Pseudo first-order reaction kinetics model
5802 J Mater Sci: Mater Electron (2021) 32:5790–5802

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