Facile fabrication characterization and efficient photocatalytic activity of surfactant free zns cds and cus nanoparticles

Original Articleof surfactant free ZnS, CdS and CuS nanoparticles Department of Chemistry, University College of Science, Osmania University, Hyderabad, 500007, Telangana State, India a

Original Article

of surfactant free ZnS, CdS and CuS nanoparticles

Department of Chemistry, University College of Science, Osmania University, Hyderabad, 500007, Telangana State, India

a r t i c l e i n f o

Article history:

Received 26 February 2019

Received in revised form

1 August 2019

Accepted 19 August 2019

Available online xxx

Keywords:

Metal sulfide nanoparticles

Heterogeneous photocatalysis

Rate of reaction

Photostability

Reactive species

a b s t r a c t

In this research, some metal sulfide nanoparticles (NPs) including ZnS, CdS and CuS NPs were prepared

by a simple and low-cost co-precipitation method for the photocatalytic degradation of Bromothymol blue dye (BTB) under natural sunlight irradiation The synthesized materials were characterized by XRD, FT-IR, UV-vis DRS, PL, TGA, SEM and TEM techniques for the investigation of structural, electronic, thermal, and morphological properties The optical absorption and the band gaps of the ZnS, CdS, and CuS NPs were calculated as 3.62 eV, 2.21 eV, and 1.16 eV from the UV-vis DRS XRD results demonstrate the cubic structure of ZnS NPs, CdS NPs, and the hexagonal structure of CuS NPs in the polycrystalline nature The spherical shape and size of the NPs are observed in the range of 5e12 nm from the XRD and TEM analysis The FTIR spectra reveal that the functional groups are associated with the synthesized materials by the metal-sulfur (Zn-S, Cd-S, and Cu-S) vibration bands The CdS NPs exhibited a more

efficient photocatalytic activity for the BTB dye degradation than the ZnS and CuS NPs Similarly, the results on the photostability for the degradation of BTB indicate that the CdS NPs exhibited the activity and stability for up to 5 cycles which are better than those of the ZnS and CuS NPs, consistent with their tiny size and extremely effective reacting surface area Hence, the semiconducting materials are expected

to have the potential as a highly efficient, cost-effective and eco-friendly heterogeneous catalyst for industrial applications

© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

In recent years, the increase in population and the expansion of

human settlement lead to the development of process industries

that use a large number of pollutants including pesticides,

herbi-cides, nitrophenols, and dyes The presence of the highly toxic and

hazardous pollutants in water and wastewater released from the

chemical industries is a major concern The removal of pollutants

from water is a challenging issue The current processes, such as

coagulation,flocculation, adsorption, and biological oxidation

suf-fer from various drawbacks because they do not completely remove

the pollutants and are not cost-effective[1,2] The complications

related to all the above processes are that they do not completely

degrade the pollutants, but only change it from one to another

and simultaneously produce a large amount of toxic secondary

products[3] In addition, organic dyes are nontoxic themselves, but when they mix with water, the mixtures then easily form highly toxic complexes that degrade to form other toxic subsidiary prod-ucts To solve the above problems, advanced oxidation processes, such as photocatalysis, photo-ozonation, photo-Fenton process, etc and their combined operations have been significantly effective in the pollutant removal on the lab-scale and at industrial levels This

is because of the higher degradation, greener approaches, more cost-effectiveness, lower toxicity and greater ease of performances [1] The photocatalytic process results in the oxidation-reduction andfinally the degradation of a wide variety of organic pollutants through their interaction with photogenerated holes or reactive oxygen species, such as
OH and
O2 radicals [3] The usage of semiconductor catalysts in the photocatalysis process can be a good option for the degradation of a wide variety of pollutants[4]

In the II-VI group of semiconductors, nanoparticles of metal sulfides have garnered much attentions as important materials for the applications in solar cells[5], lithium-ion batteries[6], light-emitting diodes[7], photocatalysis[8], electrocatalytic H2 evolu-tion[9], and antibacterial activity[7] Among them, in recent years,

* Corresponding author.

E-mail addresses: ayodhyadasari@gmail.com (D Ayodhya), gvbhadram@

osmania.ac.in (G Veerabhadram).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.08.006

2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).

the photocatalysis has attracted intensive attention as promising

candidates for the efficient degradation of toxic pollutants

including dyes, pesticides, and antibiotics under sunlight

irradia-tion to solve the various aqueous environmental polluirradia-tion issues

[8,10e13] In addition, the group of metal sulfide nanomaterials has

been developed with various shapes (flowers, rods, ribbons, etc.)

and sizes for the degradation of pollutants because their unique

structure, electronic, magnetic, and optical properties originate

from their large surface-to-volume ratio and the quantum

confinement effect [14,15] For example, ZnS NPs with various

morphologies, such as spherical,flower-likes, microspheres

deco-rated with nanoparticles and nanorods for the photocatalytic

degradation of the reactive blue 21 were synthesized by two

distinct, simple, and efficient methods [16] The CdS NPs were

synthesized successfully at low temperature via a catalyst-free

hydrothermal technique and used for the degradation of the two

anionic azo dyes, namely reactive red (RR141) and Congo red (CR)

azo dyes and the methylene blue [7,17] The monodispersed and

homogeneous 3Dflower-like CuS NPs were synthesized and exhibit

good photocatalytic properties for degrading organic pollutants

(methylene blue) in water [18] In addition, ZnS, CdS and CuS

semiconductors with direct band gap exhibit a high potential as

effective photocatalysts because of their ability of a rapid

genera-tion of electronehole pairs by the absorption of photons with

en-ergy equal to or more than their respective band gaps

To date, various types of nanoparticles, including metal sulfide

NPs were synthesized through various strategies, including the

sonochemical method[19], the solegel method[20], the thermal

and photochemical decomposition [20], the electrochemical

reduction[20], template methods[21], micro-emulsion methods

[22], solvothermal/hydrothermal methods[15,16,20], and the

mi-crowave method[23] The above processes are not only tedious in

their preparation procedures but also time and energy intensive

Therefore, we report herein a facile and rapid synthesis strategy

for the metal sulfide NPs using the co-precipitation method[24]

Compared with the previous approaches, our strategy possesses

several advantages, including low-cost, minimal number of

syn-thetic steps, and low energy/material consumption, making it

clean and environmentally benign The ZnS and CdS NPs exist in a

cubic crystal structure with a band gap of 3.66 eV and 2.42 eV,

respectively[16,17] CuS NPs exhibit a low reflectance in the visible

and relatively high reflectance in the NIR region, which makes it a

prime candidate for the solar energy absorption with the direct

band gap of 1.2e2.0 eV They have been extensively applied in the

industry, for instance, for the photocatalytic degradation of organic pollutants and biology markers[18,19] Therefore, ZnS, CdS and CuS NPs have been extensively focused for this photocatalytic application due to their high chemical stability, non-toxicity and environmental safety nature

Herein, we report the fabrication and inclusive characteriza-tion of surfactant free ZnS, CdS and CuS NPs synthesized via a simple co-precipitation technique The prepared materials show a spherical morphology with an average diameter in the range of

5e10 nm, formed through the assembly of many tiny particles as evidenced from the TEM analysis The photocatalytic activity of the ZnS, CdS and CuS NPs for the degradation of BTB under the sunlight irradiation was studied The recyclability of the synthe-sized samples was examined for 5 cycles and it was found that the catalyst is fairly active throughout the cycles The reactive species trapping experiments were also conducted for the degradation of BTB to examine the role of the hþ,
OH and
O2  species The

prepared catalysts are considered as suitable for removal of highly toxic and hazardous organic materials for the environmental protection

2 Experimental 2.1 Materials and methods All chemicals of analytical (AR) grade were used without further purification Double distilled water was used as a solvent for the preparation of the stock solutions The zinc acetate dihydrate, cadmium acetate dihydrate, copper acetate, ethanol, and BTB were purchased from Sigma Aldrich Chemicals, India The chemical structures and properties of the BTB dye are shown inTable 1 2.2 Synthesis of the ZnS, CdS and CuS NPs

The ZnS, CdS and CuS NPs were synthesized by using a simple co-precipitation method In a typical procedure, aqueous solutions were prepared by dissolving 0.125 M metal acetate (zinc acetate for ZnS NPs; cadmium acetate for CdS NPs and copper acetate for CuS NPs) in 40 mL of double distilled water, that was followed by the slowly dropping (addition of) an equal amount of aqueous sodium sulphide of the same mixing ratio (0.125 M in 40 mL) into the aforementioned acetate ones under continuous stirring The color

of the reaction mixtures was changed due to the metal sulphide precipitation, namely from colorless to white turbid (ZnS), colorless Table 1

The chemical structure,lmax , molecular weight, and solubility of BTB dye.

Bromothymol blue (BTB)

C 27 H 28 Br 2 O 5 S

to yellowish (CdS), and light green to greenish black (CuS) The

precipitation temperature was maintained constant with a

water-jacketed reaction vessel using a water circulating thermostatic

bath The reaction mixture was further stirred at room temperature

for 4 h After the stirring, the precipitates were collected by vacuum

filtration and then washed with ethanol and double distilled water

alternately 3e4 times to remove the impurities After that, the

precipitates were dried at 80C for 6 h before further analysis

2.3 Characterization techniques

The X-ray diffraction (XRD) study was done using a Philips X-ray

diffractometer with Cu-Karadiation (l¼ 1.5406 Å) at 40 kV and

30 mA for 2q values over 10-80 at room temperature for the

observation of the crystalline phases The absorption and emission

properties with band gap calculations of the synthesized samples

were evaluated using a Shimadzu UV-3600 spectrophotometer and

a Shimadzu RF-5301PC spectrofluorometer with an excitation

wavelength of 325 nm The functional groups involved in the

materials were investigated by a Bruker FTIR spectrophotometer

with the mixing of KBr pellet in the wavenumber region

400e4000 cm1 The morphology and the size of the materials

were investigated by scanning electron microscope (SEM; Zeiss

Evo18) and a transmission electron microscope (TEM; Tecnai G2)

operating at 200 KV The thermogravimetry analysis (TGA) studies

of the synthesized materials were carried out on a Mettler Toledo Star system in the temperature range of 30e1000 C under the

dynamic N2gas atmosphere with a heating rate of 10C min1 2.4 Photocatalytic activity measurement

The photocatalytic performance of the synthesized materials was investigated by the degradation of the BTB dye in an aqueous solution under the sunlight irradiation In order to prepare the reaction suspension, 20 mg of the catalyst was added to 100 mL of the BTB dye aqueous solution with an initial concentration of

10 mg/L The aqueous solution was magnetically stirred for

20 min in the darkness to verify the adsorption equilibrium be-tween the BTB and the photocatalyst Then the suspension was taken under the sunlight irradiation on sunny days between 12

pm and 1 pm After the suspension was exposed to the sunlight irradiation, a 3 mL of sample was collected at a specific time in-terval and then the sample was centrifuged in order to remove the catalyst from the sample The absorbance of the BTB solution was measured at 614 nm using an UV-vis spectrometer, which corresponds to its maximum absorption wavelength The percentage of the photodegradation was determined from the equation:

Photodegradation efficiency (%) ¼ (1 e C/C0) 100%,

Fig 1 The UV-vis DRS (a), PL (b), FTIR (c), and XRD (d) of the synthesized ZnS, CdS and CuS NPs.

where, C0and C represent the concentration of the BTB dye aqueous

solution after magnetically stirring in the darkness (concentration

at time (t)¼ 0) and the concentration of the dye aqueous solution at

the different time (t) of the photo-irradiation, respectively

2.5 Photocatalyst stability test

The recycling process permits to find out the stability and

reusability of the nanomaterials The stability of the synthesized

ZnS, CdS and CuS photocatalysts was investigated for the purpose of

practical applications in industries The photocatalytic experiment

was repeated up to 5 cycles using the same procedure as mentioned

above After each cycle of the photodegradation study, the

photo-catalyst wasfiltered from the suspension, and then washed with

ethanol and double distilled water several times The catalyst was finally dried at 80C for 6 h and then reused for the next run.

2.6 Reactive species trapping experiments

In order to further study the main active species and the pho-tocatalytic mechanism of the degradation process, trapping ex-periments were carried out The generation of various active species was detected during the above mentioned photocatalytic process For this, several scavengers, including ammonium oxalate (AO, 1 mmol L1), tert-butyl alcohol (t-BuOH, 5 mmol L1), and p-benzoquinone (p-BQ, 1 mmol L1) were added into the BTB dye solution before the sunlight irradiation to quench hþ,
OH and
O2, respectively Further, the photocatalytic degradation process of BTB

Fig 2 (aec) High-resolution SEM and (def) HR-TEM images the synthesized ZnS, CdS and CuS NPs.

dye was similarly performed as a degradation test using

synthe-sized samples under sunlight irradiation

3 Results and discussion

3.1 UV-visible absorption study

The optical absorption and bandgap measurements of the

nanoparticles were made from the diffuse reflectance spectra of

ZnS, CdS and CuS NPs and the results are shown inFig 1(a) The

band gap property of the synthesized catalysts was evaluated using

the Plank's equation (E ¼ h c/l) with the wavelength at the

maximum absorption of the sample derived from the UV-vis

ab-sorption spectra The optical abab-sorption edges were estimated to be

about 350 nm for ZnS, 572 nm for CdS, and>800 nm for CuS NPs

[12,19,20] The optical band gaps of the synthesized ZnS, CdS and

CuS NPs were calculated to be 3.62 eV, 2.21 eV and 1.16 eV,

respectively, which are close to the corresponding values in

pre-vious reports[3,4,14,25,26] The calculated band gaps of the

sam-ples are higher than their bulk counterparts This can be explained

by the quantum confinement effect of the nanoparticles and the

increase in the band gap occurring due to the increase in size of the

synthesized materials Hence, the UV-vis absorption spectrum

analysis evidently signifies that the absorption of the synthesized

materials is in the visible and NIR region of the electromagnetic

spectrum Therefore, we have concluded that because of the strong

absorption in the visible region, it is predictable that the prepared

materials might be photocatalytically active under the visible light

irradiation

3.2 Photoluminescence study Photoluminescence study is a useful technique to probe the recombination and the transfer of photo-induced electrons (e) and holes (hþ)[27].Fig 1(b) presents the PL spectra of the ZnS, CdS and CuS samples at room temperature, taken with the excitation wavelength of 325 nm As it shows the emission bands are observed

at 452 nm, 512 nm and 461 nm, which can be attributed to the ZnS,

Fig 3 The energy dispersive X-ray analysis (EDAX) spectra of the synthesized (a) ZnS, (b) CdS, and (c) CuS NPS.

Fig 4 The TGA curve of the synthesized ZnS, CdS and CuS NPs.

CdS, and CuS NPs, respectively The emission in the visible region is

generally ascribed to the band of the acceptor transition while the

deep level emission is usually attributed to the structural defects

and the impurities in the system[19] As an evidence, the prepared

samples exhibit the longest lifetime of photogenerated charges,

which can be ascribed to the inhibited recombination of charges, or

in other words, the effective separation of eand hþ The previous

studies have reported that this blue shift can likely be attributed to

many factors, e.g., surface effects and the agglomeration of

nano-particles [11] Furthermore, the change was observed in the

recombination rate that may enhance the photocatalytic

perfor-mance of the synthesized samples

3.3 FTIR analysis

In order to identify the presence of the functional group

vibra-tion bands in the synthesized materials, FTIR spectroscopy was

utilized The FTIR spectra of the synthesized ZnS, CdS and CuS NPs

taken in the range of wavenumbers 4000-400 cm1at room

tem-perature are shown inFig 1(c) The FT-IR spectra of all samples

show a strong absorption band at 3500-3000 cm1, which is a

characteristic band of the associated hydroxyl groups The other peaks at 1630-1500 cm1correspond to the stretching vibrations of

a hydroxyl group and representing the water as moisture[28] The peak observed between 750 and 500 cm1indicates the metal-sulphur stretching bands in ZnS, CdS, and CuS NPs In ZnS sam-ple, the broadband around 3421 cm1 represents the stretching vibrations of the OeH group, whereas the bands around 2342 cm1 correspond to CO2mode These CO2bands may arise due to some trapped CO2in the air ambience during the FTIR characterization [29] Various higher wave number impurity bands are due to the surface adsorbed water from the precursors during the synthesis process or during the characterization[29]

3.4 XRD analysis The powder X-ray diffraction (XRD) measurements were carried out to determine the crystalline nature of the synthesized mate-rials The powder XRD patterns of ZnS, CdS and CuS NPs are dis-played inFig 1(d) The exhibited XRD patterns are well matched with the cubic (JCPDS Card No: 05-0566), cubic zinc blende (JCPDS Card No: 10-0454) and hexagonal (JCPDS Card No: 79-2321) phases

Fig 5 The UV-vis absorption spectra of BTB dye degradation using (a) absence of catalyst (photolysis), (b) ZnS NPs, (c) CuS NPs and (d) CdS NPs under 60 min of sunlight irradiation.

for ZnS, CdS, and CuS NPs, respectively The planes (1 0 0), (1 0 2), (1

0 3), (0 0 6), (1 1 0) and (1 0 8) indicate the covellite phase with the

hexagonal crystal structure for CuS NPs and the (1 11), (2 2 0) and (3

1 1) crystal planes indicate the cubic phase for ZnS and CdS NPs

Furthermore, the peak broadening as observed implies either the

amorphous nature or thefine crystalline behavior of the samples

As can be seen, no impurity peak is observed The crystallite size (D)

of the catalysts was calculated from the width of the most intense

peak using the DebyeeScherrer equation:

D¼ kl/bcosq,

where, k is a constant (k¼ 0.96),lis the wavelength of the X-ray

(0.15418 nm),bis the full-width at half maximum (FWHM), andqis

the diffraction angle The average crystallite size of ZnS, CdS, and

CuS samples was found to be 10 ± 0.2 nm, 7 ± 0.3 nm and

11± 0.2 nm, respectively

3.5 Morphology study

The morphology, shape and average grain diameter of the

syn-thesized materials were examined by the SEM and TEM analysis The

samples for these analysis were prepared on a carbon-coated copper

grid by just dropping a very small amount of the aqueous sample

solutions on the grid, the extra solution was then removed using a

blotting paper, and then the as attained particles on the SEM grid

were allowed to dry by putting it under a mercury lamp for 5 min As

it is seen, the SEM and TEM images reveal that the synthesized

materials possess unequal shapes, including spherical shape, which

results in high surface areas.Fig 2(aec) shows the representative

SEM images of the ZnS, CdS, and CuS NPs with sizes of roughly less

than 100 nm To further investigate the exact particle diameter of the

synthesized materials, TEM analysis was carried out and the taken

images are shown inFig 2(def) The TEM samples were prepared by

dip-coating a 400 mesh carbon-coated copper grid in a dilute sample

solution and then allowing the solvent to evaporate It could be found

that the ZnS, CdS, and CuS NPs appeared in the likely spherical

shapes with the average particle diameter of about 8± 1.5, 6 ± 1.1 and

10± 0.8 nm, respectively These values are consistent to the

crys-tallite sizes calculated from the XRD line broadening Moreover, the

particle sizes are larger than those of their crystallites due to the

reunion of the nano-sized crystallites

The energy-dispersive X-ray (EDAX) spectra of three different

metal sulfide nanoparticles (ZnS, CdS, and CuS) are shown in

Fig 3(aec), respectively As can be seen from Fig 3, the EDAX

analysis of the ZnS, CdS, and CuS samples indicates the presence of

zinc (Zn), cadmium (Cd), copper (Cu), and sulphur (S) as the major

constituents (Fig 3), confirming the presence of the constituent

elements in the synthesized nanoparticles In addition, the EDAX

analysis of the CuS NPs shows slightly intense peaks of the carbon

(C) and oxygen (O) elements due to the absorption of excessive

contamination on the surface of the nanoparticles Similarly, a

strong peak is observed in Zn or Cd or Cu and S in the atomic ratio of

ZnS, CdS, and CuS NPs, calculated from the quantified action,

peaked at the 1:1 ratio No other impurity elements have been

observed in the synthesized samples suggesting the high purity of

the samples

3.6 Thermogravimetric analysis

The TGA measurements were performed for the investigation

of the thermal stability of the ZnS, CdS and CuS NPs (seeFig 4) in

the range 30e1000C at a 10C/min heating rate under the N2

atmosphere The TGA curve of the samples exhibits several

noticeable mass loss steps which can be explained as the stepwise

decomposition and degradation processes of the unreacted mol-ecules beginning from the desorption of adsorbed atmospheric components and lasting to the absolute ash formation[30] As shown inFig 4, the residual solvents evaporate at below 100C and the samples thermally exfoliated at around 200Ce1000C.

At thefinal stage of TGA, 32.43%, 21.62%, and 24.32% weight loss were observed at 1000C, of ZnS, CdS, and CuS NPs, respectively, due to the thermal degradation of the unstable components in the presence of samples

3.7 Photocatalytic activity 3.7.1 Photocatalytic degradation of BTB dye The photocatalytic activities of the synthesized ZnS, CdS and CuS NPs were evaluated by the degradation reaction of the BTB dye aqueous solution under the sunlight irradiation, and the results are shown inFig 5(aed) FromFig 5(a), it can be seen that the BTB dye exhibits poor photodegradation performances (approximately 5%)

in the absence of a catalyst under 1 h sunlight irradiation Fig 5(bed) presents the photodegradation curves of the BTB dye using ZnS, CuS, and CdS NPs, respectively Comparatively, the synthesized ZnS, CdS, and CuS NPs have higher photocatalytic ac-tivities than the absence of a catalyst under 1 h sunlight irradiation

Fig 6 (aeb) The photocatalytic degradation efficiency plots of BTB degradation using ZnS, CdS and CuS NPs.

The degradation effect was characterized by monitoring the

ab-sorption peak of the BTB centered at 614 nm by the UV-vis

spec-trophotometer Fig 5 clearly demonstrates that the maximum

absorption peak of the BTB decreases with the increasing

irradia-tion time This illustrates that the BTB dye concentrairradia-tion decreases

in the presence of catalysts and the sunlight irradiation at the same

time It indicates also that the decrease in the absorption of the

mixed solution is due to the destruction of the homo and hetero

aromatic rings present in the dye molecules, which is confirmed by

the lower intensities of the absorbance peak of BTB Among the

synthesized materials, CdS NPs show the best photocatalytic

ac-tivity in the degradation of BTB being 83.42% in 60 min under

sunlight irradiation This is higher than that of the ZnS (63.88%) and

CuS (46.23%) materials under similar reaction conditions The

exhibited enhanced photocatalytic activity of CdS NPs is due to the

small size of particles and large effective surface area The

degra-dation percentage (%D) of the BTB dye using the ZnS, CdS and CuS

NPs can be calculated and the results are plotted inFig 6(a,b) As it

is clearly shown, the CdS NPs exhibit enhanced photocatalytic

performance compared to those of ZnS and CuS

The XRD and SEM measurements of the synthesized ZnS, CdS,

and CuS NPs were carried out in the process of the photocatalytic

degradation of BTB for evaluating the stability of a catalyst and the

results are shown inFig 7(aed) The cycled samples after the 5th

cycle represent a similar structure and intensity in their XRD

pat-terns and SEM images, confirming the desirable stability of the

prepared ZnS, CdS, and CuS NPs The above-mentioned results reveal the excellent reusability and the performance longevity of the synthesized samples under the sunlight irradiation, making

Fig 7 (a) XRD spectra of the synthesized ZnS, CdS, and CuS NPs; SEM images of the (b) ZnS, (c) CdS, and (d) CuS NPs after the treatment of a photocatalytic reaction.

Fig 8 The kinetic plot of the photocatalytic degradation of BTB dye using the syn-thesized ZnS, CdS and CuS NPs under sunlight irradiation.

them promising candidates for practical environment technological

applications including wastewater treatment

3.7.2 Evaluation of the kinetic rate constants

The quantitative investigation of the reaction kinetics of the BTB

dye degradation by the synthesized photocatalysts is presented in

Fig 8 Several measured experimental results have beenfitted by

the pseudo-first-order kinetic model[31]: ln (A0/At)¼ k t, where, k

is the rate constant, A0is the initial absorbance of the BTB dye, and

Atis the absorbance of the BTB at time t According to the

pseudo-first-order rate equation, the rate constant (k) for the BTB dye

degradation at the absence of catalysts and in the presence of

catalysts including ZnS, CdS, and CuS NPs was determined The plot

of ln (A0/At) as a function of the irradiation time gives the rate

constant values (k) and thefitting correlation coefficient (R2) and

the results are tabulated inTable 2 For a better comparison, the

synthesized ZnS, CdS, and CuS NPs exhibited excellent

photo-catalytic activity for the degradation of several dyes than the other

similar catalysts The data are summarized inTable 3 [32e37]

3.7.3 Photostability and determination of active species

In order to further study the photostability of the synthesized

CdS NPs, afive-cycle experiment was carried out and the results are

presented inFig 9(a) It could be observed that afterfive cycles,

there is no obvious deterioration in the photodegradation

perfor-mance, indicating the excellent photostability of CdS NPs under the

sunlight irradiation The similar photostability performance is

observed in the degradation of the BTB dyes by the synthesized ZnS

and CuS NPs under the sunlight irradiation using 5 cycle

experi-ments Furthermore, reactive species trapping experiments were

performed to investigate the reactive oxygen species in the

pho-tocatalytic process and the results are shown inFig 9(b) In the

typical photocatalytic mechanism, a series of oxidation-reduction

reactions take place and in addition to electrons, holes, reactive

oxygen species, such as
O2,
OH, etc are generated[38e40] To be

studied is the role of the effect of the various species, such as

ammonium oxalate (AO), tert-butyl alcohol (t-BuOH) and

p-ben-zoquinone (p-BQ) in scavenging hþ,
OH and
O2, respectively, in

the degradation of the BTB dye using the synthesized materials

Fig 9(b) shows that in the presence of p-BQ the rate of degradation

Table 2

The calculated parameters of the photocatalytic degradation of the BTB dye in the absence and presence of catalysts.

S.No Sample Amount of catalyst % of degradation Rate constant, (k, min1) Half-life time, (t 1/2 , min.) R 2

Table 3

The comparison of present work with the recent investigations of photocatalytic degradation of dyes using various catalysts.

Fig 9 (a) The photostability and (b) reactive species trapping plots of the photo-catalytic degradation of BTB dye using high efficient CdS NPs.

is reduced to 34% This means
O2  free radicals are the major

attacking species However, the surface free radicals play another

important role than those in the bulk The greater inhibition of the

reaction through the p-BQ than in the case of AO and t-BuOH as

radical scavenger indicates that the
OH plays a more important

role in the photocatalytic degradation of BTB However, it indicates

that the hydroxyl radicals, holes, superoxide radicals play

impor-tant roles in the photocatalytic process

4 Conclusions

In summary, ZnS, CdS and CuS NPs were successfully

synthe-sized using a simple and low-cost co-precipitation method without

any surfactant for the degradation of BTB dye under the natural

sunlight irradiation The synthesized materials were characterized

by XRD, FTIR, UV-vis DRS, PL, TGA, SEM, and TEM analysis The SEM

and TEM analyses revealed that the prepared materials are in the

nearly spherical shape with particle sizes in the range of 5e12 nm

From FTIR spectra, we have pointed out that the peaks at around

800-500 cm1are associated with metal-sulphur vibrational bands

for the confirmation of the ZnS, CdS, and CuS NPs The optical band

gap of synthesized materials was determined from the absorption

edge from the UV-vis DRS The photocatalytic measurements reveal

that the excellent degradation efficiency and the high reaction rate

constant for the BTB degradation were found better using CdS NPs

(83.42%) than those for the nanoparticles of ZnS (63.88%) and CuS

(46.23%), all due to the small size of particles and, thus large

effective reacting surface area The photocatalytic processes were

limited by the active species of hydroxyl radicals, holes, and

su-peroxide radicals as indicated from the reactive species trapping

experiments through the p-BQ than AO and t-BuOH as radical

scavengers The photo reusability results exhibit the maximum

efficiency after 5 cycles, indicating that the synthesized materials

show improved stability Therefore, the synthesized materials are

considered promising photocatalysts for further degradation of

toxic pollutants, including pesticides, phenols, and other organic

compounds in industries

Competing interest

The authors declare to no competing interests

Funding

Not applicable

Acknowledgements

The authors would like to thank DST-FIST, New Delhi, India for

providing necessary analytical facilities and sincere thanks to the

Head, Department of Chemistry, Osmania University for providing

infrastructure and other necessary facilities

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