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. 2022 Jun 24;7(27):22987–22996. doi: 10.1021/acsomega.2c00945

Synthesis and Performance Analysis of Photocatalytic Activity of ZnIn2S4 Microspheres Synthesized Using a Low-Temperature Method

Mohammad Imran 1, Waseem Ashraf 1, Aurangzeb Khurram Hafiz 1, Manika Khanuja 1,*
PMCID: PMC9280934  PMID: 35847261

Abstract

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In this paper, we report the synthesis of zinc indium sulfide (ZnIn2S4) microspheres synthesized via a low-temperature route, and the as-synthesized material was used for photocatalytic degradation of malachite green (MG), methyl orange (MO), and Direct Red 80 (DR-80) dyes. The as-synthesized material was characterized by powder X-ray diffraction and field-emission scanning electron microscopy for studying the crystal structure and surface morphology, respectively. Fourier transform infrared spectroscopy was performed to determine the functional groups attached. UV–Visible absorption spectrometry was done for light absorbance and band gap analysis, and Mott–Schottky analysis was performed to determine the nature and flat band potential of the material. A scavenger study was performed to analyze the active species taking part in the degradation process. The reusability of the material was tested up to four cycles to check the reduction in efficiency after each cycle. A time-correlated single-photon counting study was performed to observe the average lifetime of generated excitons during photocatalysis. It was found that the as-synthesized porous sample is more efficient in degrading the cationic dye than anionic dyes, which is further explained in the article.

Introduction

Increasing environmental pollution is one of the major problems for the whole world, in which the percentage of water pollution is much greater.1−3 In a survey by World Bank, nearly one-fifth of water pollution comes from textile industries, which generates more waste containing mainly dyes, during their manufacturing and dyeing processes. The wastewater of synthetic textile industries shows low biodegradability and strong color, whichhave features associated with the dyes, organic pollutants, and other toxic chemicals. These chemical effluents accumulate in water bodies, inhibiting photosynthesis, by which the production of carcinogenic and mutagenic byproducts occur, which cause severe harmful effects to the environment as well as living organisms.

Direct Red 80 (DR-80) and methyl orange (MO) are among a large group of synthetic industrial dyes, whereas malachite green (MG) is an aniline-based dyestuff (a triphenylmethane salt) made by Dobner and Fisher in 1877 that is available in the market. MG, MO, and DR-80 are mostly used for dyeing, printing of textiles, and killing of parasites and fungi in fish tanks. As a dyestuff, all of these dyes are soluble in water bodies and can cause lung cancer, colorectal cancer, asthma, kidney damage, and other diseases directly or indirectly. These anionic dyes are categorized in the azo class, which is the largest group of synthetic industrial dyes, and exhibit coloration because of the presence of certain functional groups in their molecular structure such as chromophores (−N=N−) and auxochromes (SO3–, =O).4−8

According to the literature survey, several methods have been used to remove the chemical effluents containing dyes due to their potential damage to the environment. Physical, chemical, and biological processes such as adsorption, microbial decomposition, and enzymatic decomposition have been used for a few decades to degrade these dyes. These processes exhibit certain inefficiencies, as they usually demand high cost and much time or generate toxic and carcinogenic intermediate products. Hence, the effective and alternate method for the degradation of these effluents is photocatalysis, which is a low-cost and lesser time-consuming and eco-friendly process, as it uses the solar spectrum {E = hν} to generate free radicals such as •OH and •O in the presence of photons that oxidize the effluents, which breaks these complex dyes into environment friendly byproducts.9

In this context, a lot of work has been performed on photocatalysis using various micro- and nanostructured catalysts, such as ZnO and TiO2, which are the most popular photocatalysts but have limited application due to their wide band gap (3.2 eV). This lies in the UV region utilizing only 5% of the solar spectrum. According to many reports, the efficiency of a photocatalyst can be enhanced by changing the morphology, surface area, band gap, and many other parameters.10 Similarly, other materials such as cadmium and lead chalcogenide have been studied extensively during the past two decades.11−15 Despite their exciting properties, they have limited applications, due to the toxicity of cadmium and lead. Therefore, the ternary ABpCq materials (A = Cu, Zn, and Ag, etc., B = Al, Ga, and In, etc., C = S, Se, and Te) and quaternary Cu2XYZ4-type materials (X = Mn, Fe, Co, Ni, and Zn, Y = Al, Ga, In, Si, Ge, and Sn, and Z = S, Se, and Te) semiconductors are receiving attention as promising alternatives.16−20

These types of semiconductors have been extensively studied because of their unique optoelectronic and catalytic properties.21−24 Among these compounds, zinc indium sulfide as one of the AB2C4 families has attracted considerable attention because of its high potential for application in photocatalysis, charge storage, and thermoelectricity.25−33 Many dyes have been degraded previously using zinc indium sulfide (ZnIn2S4) under visible light, but no reports are present on photocatalytic degradation of MG and DR-80 dyes using ZnIn2S4. Some of the reported results are mentioned in Table 1.

Table 1. Reported Results on Photocatalytic Degradation of Various Dyes Using ZnIn2S4 Materials.

materials/dose synthesis solvent reaction temperature and time synthesis method morphology pollutants/dose (ppm) photocatalytic degradation activity ref
ZnIn2S4/100 mg pyridine 160 °C, 16 h solvothermal method nano/micropeony (hexagonal) MB/50 η = 99.98% (90 min) (34)
ZnIn2S4/80 mg ethylene glycol 200 °C, 10 min microwave, solvothermal approach monodispersed spheres MB/10 — (35)
ZnIn2S4/40 mg water 80 °C, 6 h thermal solution method microsphere MO/10 C/C0 ≈ 0 (2.5 h) (36)
ZnIn2S4/40 mg water 80 °C, 6 h hydrothermal marigold-like microsphere MO/10 C/C0 ≈ 0 (3 h) (37)
CR/20 C/C0 ≈ 0 (5 h)
RhB/30 C/C0 ≈ 0 (180 min)
ZnIn2S4/40 mg water 120 °C, 10 h hydrothermal method nanoparticles (cubic) MO/20 cubic (K = 0.593 h–1) (38)
flower-like microspheres (hexagonal) hexagonal (K = 0.185 h–1)
ZnIn2S4/40 mg water 195 °C, 10 min microwave-assisted hydrothermal method marigold-like microspheres MO/10 η = 100% (2 h) or C/C0 ≈ 0 (2 h) (39)
ZnIn2S4/100 mg water 65 °C, 16 h hydrothermal method microspheres (hexagonal) MO/10 η = 100% (2.5 h) or C/C0 ≈ 0 (2.5 h) (40)
ZnIn2S4/0.1 g water 210 °C, 1 h solvothermal method nanoparticles (hexagonal) MO/40 K = 0.89 h–1, annealed (41)
ZnIn2S4/— water 200 °C, 12 h hydrothermal nanopowder Cr(VI)/— η = 99.7% (40 min) (42)
ZxIS3–x/1 mg (3 mL solution) water 280 °C, 60 months colloidal chemistry nanoplates R6G/(100 μmol/L)/(3 mL solution) η = 100% (10 min) (43)
ZnIn2S4/50 mg (100 mL DI) water 110 °C, 4 days one-step wet-chemical method nanowire MO/20 η = 76% (210 min) (44)
95 °C, 12 h nanotube η = 100% (210 min)
ZnIn2S4/60 mg (CH2OH)2 160 °C, 24 h biomolecule-assisted method flower-like hollow microspheres MO/40 (120 mL DI) 0.045 min–1 (k) (45)
ZnIn2S4/0.3 g (600 mL DI ethanol 160 °C, 24 h solvothermal method flower-like microspheres MO/25 99% (100 min) (46)
ZnIn2S4/0.04 mg (2 mL DI)   300 °C, 24 h thermolysis nanocrystals MB/(0.02 mmol/L) η = 66% (25 min) (47)
ZnIn2S4/100 mg (100 mL DI) water 60 °C, 6 h ionic liquid microemulsion-mediated hydrothermal method microsphere MO/10 η = 98.5% (10 min) (22)
ZnIn2S4/0.1 g (400 mL DI) water 80 °C, 6 h hydrothermal method   RhB/15 η = 97.8% (90 min (48)
MO/15 η = 5.6% (90 min)
ZnIn2S4/20 mg (100 mL DI) water 80 °C, 6 h low-temperature method marigold MG/10 η = 99.68 (30 min) this work
MO/10 η = 99.48 (90 min)
DR-80/10 η = 99.12 (75 min)
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ZnIn2S4 is an interesting material for photocatalysis. It is a direct semiconductor with band gap energies reported for different structures, sizes, and shapes of micro- and nanoparticles lying between 2 and 3 eV.43,49 ZnIn2S4 can be crystallized in two morphologies such as cubic and hexagonal lattices. The cubic ZnIn2S4 lattice structure exhibits remarkable thermoelectric properties,33 while the hexagonal ZnIn2S4 lattice variation shows photoluminescence and photoconductivity. Both polymorphs are photocatalytically active materials.38,50

In this article, we present the low-temperature solution method synthesis of hexagonal ZnIn2S4 microspheres and its ability to act as photocatalyst for the reduction of organic dyes in the aqueous solution. The microspheres were synthesized with a fixed amount of zinc, indium, and sulfur precursors. The structural and optical properties and photocatalytic activity on MG, MO, and DR-80 were studied.

Synthesis of ZnIn2S4 Microspheres

All chemicals used were analytical grade. In a normal reaction, ZnSO4.7H2O (4.0 mmol) and In2(SO4)3 (4.0 mmol) were added by stoichiometric ratio, and thioacetamide C2H5NS (TAA) (20.0 mmol) was mixed in a conical flask with 250 mL capacity containing 80 mL of distilled water. Then the conical flask was put into a water bath and maintained at 80 °C for 6 h after sealing it with a ground glass stopper. After completing the reaction, the conical flask was cooled to room temperature. The yellowish precipitate was collected and washed with deionized water, absolute ethanol, and acetone several times. The final sample was dried in a vacuum oven at 60 °C for 6 h; the powder was collected for characterization, and further photocatalytic studies were performed. Figure 1 shows the synthesis of ZnIn2S4.

Figure 1.

Figure 1

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Schematic diagram of the synthesis of ZnIn2S4 microspheres.

Characterizations

X-ray powder diffraction (XRD) was performed with a XRD Smart Lab Guidance Rigaku diffractometer (40 kV, 40 mA) using Cu Kα X-ray radiation (λ = 1.5418 Å) at a scanning rate of 0.02 deg/s. The obtained pattern was used to analyze the phase constituents in samples. UV–vis spectra were obtained with a Varian Cary 500 UV–vis–NIR spectrometer to analyze the energy band gap using the Tauc plot. The Mott–Schottky plot was obtained to find the flat band potential (EFB) using Metrohm auto lab. The scanning electron microscopy (SEM) images were captured on a Zeiss Sigma field-emission scanning electron microscope (FE-SEM), at an accelerating voltage of 10 kV to examine the morphology of the obtained sample. Energy-dispersive X-ray spectrometry (EDX) was carried out to analyze the chemical composition using an EDX spectrometer attached to the same microscope. Fourier transform infrared spectroscopy (FTIR) was analyzed using Vertex 70 V, Bruker spectrometer to study the functional groups and bond structure. Time-correlated single-photon counting (TCSPC) was performed on a Horiba DeltaFlex-01-DD measurement spectrometer to evaluate the lifetime of charge carriers at an excitonic emission wavelength of 401 nm. The biexponential kinetic model was used to fit the decay curves of the sample, and the best fitting was done with χ2 equal to 2.712. All of the measurements were carried out at room temperature.

Photocatalytic Activity Measurement

The photocatalytic degradation activity of hexagonal structure-type material ZnIn2S4 was performed in an aqueous solution on anionic and cationic dyes such as anionic MO, DR-80, and cationic MG in the presence of sunlight. In this activity, each dye such as MO, MG, and DR-80 was taken (10 ppm) separately in three different 250 mL capacity beakers with 20 mg of ZnIn2S4 containing 100 mL of distilled water. Before irradiation, the solution was stirred in the dark for 30 min to ensure the establishment of an adsorption and desorption equilibrium. During irradiation, nearly 2–3 mL of the suspension was collected, centrifuged, and filtered through a Millipore filter to separate the photocatalyst particles. The filtrate was placed on a Varian Cary 500 Scan UV–Vis–NIR spectrophotometer to analyze the absorption peaks at a maximum absorption wavelength of dye. The abatement percentage of the dye concentration is described as C/C0, where C is the value of the absorption peak point of dye at each irradiated time interval and C0 is the value of the absorption peak point of the dye when adsorption and desorption equilibrium was achieved.

Results and Discussion

Structural Analysis

The XRD pattern of the sample prepared at 80 °C for 6 h is shown in Figure 2. The diffraction peaks of the 2θ angle at 21°, 27.6°, 30.1°, 39.5°, 47.2°, 52.2°, and 55.6° correspond to the (006), (102), (104), (108), (110), (116), and (022) planes of hexagonal ZnIn2S4 (JCPDS No. 03-065-2023), respectively, and a small diffraction peak of 2θ angle at 33.6° (shown in Figure 2through a letter c) corresponds to the (400) plane of cubic ZnIn2S4 (JCPDS No. 00-048-1778). Crystallographic planes of hexagonal ZnIn2S4 (JCPDS No. 03-065-2023) and cubic ZnIn2S4 (JCPDS No. 00-048-1778) indicate the formation of nearly pure hexagonal ZnIn2S4. The particle size of the as-synthesized microsphere was calculated by Scherrer’s formula using eq 1.51

graphic file with name ao2c00945_m001.jpg 1

where Dhkl (nm) symbolizes the crystallite size of a particular hkl plane, λ is the Cu Kα radiation wavelength of X-ray radiation (λ = 1.540 Å), β denotes the full width at half-maxima (FWHM) of the hkl plane peak, and θ (radian) denotes Bragg’s diffraction angle.

Figure 2.

Figure 2

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XRD patterns of ZnIn2S4 microspheres.

In the hexagonal lattice, the relationship between interplanar spacing d of hkl planes and the lattice constants a and c is as shown in eq 2.

graphic file with name ao2c00945_m002.jpg 2

The lattice constants a and c of ZnIn2S4 microspheres were calculated using (006) and (110) planes, as shown in Table 2. The calculated lattice parameters agreed with the standard values a = 3.85 Å and c = 24.68 Å corresponding to the hexagonal ZnIn2S4 sample.

Table 2. Lattice Constants (a, c) and Average Crystallite Size (Dhkl) for ZnIn2S4 Microspheres.

sample a (Å) (lattice constant) c (Å) (lattice constant) crystallite size (Dhkl) (nm) average d-spacing
ZnIn2S4   25.50 4.91 d006 = 4.25
3.94     d110 = 1.92
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Surface Morphology

The FE-SEM images of ZnIn2S4 microspheres are shown in Figure 3a–c, which show clear microsphere morphologies containing nanoflower-like petals around it in Figure 3b. Microspheres look like puffy flowers with densely packed petals. The average dimensions of microspheres have a ∼1 μm radius and ∼18 nm thick petals, as clear from the Figure 3c.

Figure 3.

Figure 3

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(a–c) Low- and high-resolution SEM images of ZnIn2S4 microspheres. (d) EDX spectra of ZnIn2S4 microspheres.

Energy-dispersive X-ray spectroscopy was used to check the chemical composition of the as-synthesized sample ZnIn2S4 shown in Figure 3d. The EDX spectra verified the existence of Zn, In, and S elements in the ZnIn2S4 microsphere. The chemical composition mapping of the ZnIn2S4 microsphere is given in Table 3.

Table 3. Composition Analysis of ZnIn2S4 Microspheres.

element weight concentration (%) atomic concentration (%)
Zn 5.11 4.92
In 64.48 35.37
S 30.40 59.71
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UV–Visible Absorption and Band Gap

Optical absorption and band gap of the prepared sample were analyzed by UV–vis absorption spectroscopy using a Varian Cary 500 UV–Vis–NIR spectrometer. The UV–Vis absorption spectra were plotted in terms of absorption versus wavelength (nm), as shown in Figure 4a, and the Tauc plot (hν vs (αhν)2) of the prepared sample ZnIn2S4 microsphere is shown in Figure 4b, where h is the Planck constant (h = 6.626 × 10–34 J·s), ν is the frequency, and α is the absorption constant, defined as α = 2.303A/t, where A and t are absorbance and thickness of the cuvette, respectively. The band gap of the ZnIn2S4 microspheres was estimated as Eg = 2.1 eV with the help of a Tauc plot.

Figure 4.

Figure 4

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(a) UV–vis absorption, (b) Tauc plot, and (c) Mott–Schottky (MSK) plot of ZnIn2S4.

To understand the process of charge transfer in the ZnIn2S4 microsphere semiconductor and the flat band potential (EFB), the Mott–Schottky (MSK) analysis was performed. The EFB was obtained by extrapolating the positive slope of the MSK plot to the x-axis, as shown in Figure 5c, and the positive slope of the as-synthesized photocatalyst depicts the n-type electronic nature of the semiconductor. Hence, due to the n-type nature, the degradation efficiencies of the cationic dyes are much better than those of anionic dyes, which can be seen through rate kinetic studies, as well, in Figure 6. The value of the EFB was obtained as 1.7 V versus the normal hydrogen electrode (NHE).

Figure 5.

Figure 5

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FTIR spectra of ZnIn2S4 microsphere.

Figure 6.

Figure 6

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Time-dependent UV–vis spectra of different dyes using 20 mg of ZnIn2S4 photocatalyst (a–c) dark reaction of ZnIn2S4 with DR-80, MG, and MO, respectively, (d) MO, (e) MG, and (f) DR-80. (g) C/C0 versus time graph of the three dyes. (h) Photodegradation kinetics.

The position of the conduction band edge potential ECB and the valence band edge potential EVB for the ZnIn2S4 microsphere was calculated by using the Butler and Ginley equation, as given below in eqs 3 and 4.52

graphic file with name ao2c00945_m003.jpg 3
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where X represents the absolute electronegativity of the semiconductor determined as the geometrical mean of electronegativity of the constituent atoms (i.e, XZnIn2S4 = 4.84), Ee (4.5 eV) represents the free electron’s energy on the hydrogen (H2) scale, and Eg represents the band gap energy of the semiconductor. The obtained values of XZnIn2S4, Ee, Eg, EVB versus NHE, and ECB versus NHE corresponding to ZnIn2S4 are presented in Table 4 and Figure 8.

Table 4. Energy Band Structure Parameters of As-Synthesized ZnIn2S4 Microsphere.

XZnIn2S4 EFB Ee (eV) band gap Eg (eV) EVB (eV) vs NHE ECB (eV) vs NHE
4.84 –1.7 4.5 2.1 1.39 –0.71
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Figure 8.

Figure 8

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Proposed mechanism of photocatalytic degradation and charge transfer.

Optical Studies

FTIR analysis was carried out to find the functional groups and impurities, attached to the surface of the catalyst, as shown in Figure 5. A broad strong peak present at 3400–3550 cm–1 represents OH and NH stretching vibrations. Peaks present at 2922 and 2852 cm–1 represent aliphatic CH stretching vibration of CH, CH2, or CH3. The peak at 1635 cm–1 was assigned to the deformation vibration of N–H. The C=C stretching vibration occurs at 1650–1450 cm–1. The peak present at 1161 cm–1 occurs due to the hydroxyl group. FTIR spectra confirm that our synthesized sample does not contain any impurity contents.

Photocatalytic Activity

An absolute study of as-synthesized material ZnIn2S4 with three different dyes, i.e., MO, MG, and DR-80, was conducted to test the photocatalytic behavior and the degradation efficiencies of organic dyes, as shown in Figure 6.

The procedure to study photocatalytic behavior is as follows. First, the dark reaction (in the absence of light) was performed using DR-80 aqueous dye solution, such that ZnIn2S4 catalyst was allowed to react with the dye in the absence of light. The UV–vis absorption spectra were calculated for time t = 0 and 30 min in the dark, as shown in Figure 6a–c. It was clear from the figure that there was no change in the absorption intensity even after 30 min, confirming that the degradation does not occur in the dark due to the surface adsorption phenomenon.

Figure 6d–f (abs/time) illustrates the photocatalytic degradation of MO, MG, and DR-80, respectively, using 20 mg of ZnIn2S4 and 10 ppm dyes in 100 mL of deionized water. The samples were collected at t = 0, 15, 30, 45, 60, 75, and 90 min to study the time-dependent photocatalytic activities.

The C/C0 versus time graph is plotted as shown in Figure 6g. It is observed that the cationic dye MG is more degraded than anionic dye MO and azo dye DR-80. The C/C0 is the ratio of absorbance intensity at any recorded time (say, t = 0, 15, 30, 45, 60, 75, and 90 min) to the absorbance intensity at zero time (t = 0 min). Here, the photocatalytic degradation process follows the first-order rate kinetics as defined by the given eqs 5, 6, and 7.

graphic file with name ao2c00945_m005.jpg 5
graphic file with name ao2c00945_m006.jpg 6
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where C stands for the intensity of absorbance at any recorded time (t), C0 is the initial intensity of absorbance at time (t = 0 min) and k is the rate constant of reaction. The rate constant k was determined by plotting a graph between −ln(C/C0) versus time (t) and by drawing the corresponding slope. The rate constants were observed to be 0.55, 0.199, and 0.065 min–1 for MO, MG, and DR-80, respectively, using 20 mg of the catalysts and 10 ppm of the above-mentioned dyes, as shown in Figure 6h.

As obvious from Figure 7, ∼57.54, 99.68, and 90.79% of the dyes (MO, MG, and DR-80, respectively) were degraded in the first 30 min using ZnIn2S4 photocatalyst, and from the rate constants, it was also observed that the ZnIn2S4 photocatalyst was more active for the cationic group, such as MG, than for the anionic group, such as MO and DR-80.

Figure 7.

Figure 7

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Comparative efficiencies of the three dyes MO, MG, and DR-80 using 20 mg of ZnIn2S4 and 10 ppm dyes.

The photocatalytic degradation efficiency was calculated using the following formula, as shown in eq 8.

graphic file with name ao2c00945_m008.jpg 8

where η is degradation efficiency, C is the intensity at a recorded time (t), and C0 is the intensity at time (t = 0 min).

The photocatalytic degradation efficiencies (η) were 99.43% for MO, 99.68% for MG, and 99.12% for DR-80 within 90, 30, and 75 min, respectively, as shown in Figure 7, using 20 mg of ZnIn2S4 photocatalyst and 10 ppm dyes in 100 mL of DI water.

Scavenger Study and Degradation Mechanism

Active species trapping experiment was performed to get a deep insight into the primary and secondary species responsible for the degradation of MG, MO, and DR-80 dyes during the photocatalysis process. Different scavengers like tetrabutanol (TBA) for •OH, 1,4-benzoquinone (BQ) for •O2–, and potassium iodide (KI) for both •OH and h+ were used in this experiment. As can be seen from Figure 9, the TBA and KI have a least considerable effect on the degradation process, suggesting •OH and h+ as secondary species in the degradation, whereas BQ shows a great impact on the degradation process, suggesting •O2 as the main primary species helping in the degradation process.

Figure 9.

Figure 9

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Scavenger study of zinc indium sulfide microsphere photocatalysts using BQ, KI, and TBA scavengers.

The ZnIn2S4 microspheres, cationic dye MG, and anionic dyes MO and DR-80 were used to study the mechanism of photocatalytic degradation, as shown in Figure 8. When the incident photon, with an energy equal to or greater than the energy band gap of ZnIn2S4 (Eg = 2.1 eV) (hν ≥ Eg) strikes the surface of the photocatalyst, the valence band electron (e–) moves toward the conduction band after absorbing energy from incident photon, leaving behind a hole (h+). Thus it creates an electron–hole pair. A reaction between hole (h+) and water (H2O) takes place, yielding hydroxyl radical (•OH) and H+ ions, as shown in the following eqs 9, 10, and 11.

graphic file with name ao2c00945_m009.jpg 9
graphic file with name ao2c00945_m010.jpg 10
graphic file with name ao2c00945_m011.jpg 11

Similarly, another reaction between electrons (e–) and oxygen (O2) occur, yielding superoxide radicals. These radicals react with toxins and reduce them into harmless or less toxic byproducts, as shown in eqs 12 and 13.

graphic file with name ao2c00945_m012.jpg 12
graphic file with name ao2c00945_m013.jpg 13

Reusability Study

The reusability and the cyclic stability are always an important part of the photocatalysts. The stability and the reusability of the ZnIn2S4 microspheres were determined for four cycles, as shown in Figure 10. In the first cycle, the efficiency is 99.32% using 50 mg of ZnIn2S4 in 10 ppm DR-80 dye in 100 mL of solution. Photocatalysts were separated from the DR-80 dye solution by filtration after each cycle, washed, dried, and reused in the next cycle. The light irradiation time (30 min), dye solution quantity, and concentration were kept the same for each cycle. ZnIn2S4 microspheres possessed good reusability and showed 70.83% efficiency after four cycles. Such decreased efficiency was due to waste of photocatalyst during filtration after each cycle. Therefore, the as- synthesized ZnIn2S4 microsphere is an effective and stable photocatalyst for the degradation of DR-80 dye.

Figure 10.

Figure 10

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Cyclic stability of ZnIn2S4 microspheres in photocatalytic degradation of DR-80 dye for four cycles under visible light irradiation.

Time-Correlated Single-Photon Counting (TCSPC)

Time-resolved transient and steady-state decay were analyzed to study the generation, transfer, and decay of the excitons. Biexponential fitting of the decay curve (Figure 11) was performed using eq 14, with a least χ2 value of 2.712:

graphic file with name ao2c00945_m014.jpg 14

To obtain the lifetimes of τ1 and τ2, I(t) is the intensity, τ1 is the decay times for faster nonradiative processes, whereas τ2 is the decay time for slower radiative decay process. It can be seen from Table 5 that τ1 contributes 89.22% to the deactivation of excited states, which suggests that the degradation process happens due to the nonradiative pathways and radiative pathways having the very least role in the degradation process of dyes.

Figure 11.

Figure 11

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Time-correlated single-photon count spectra of ZnIn2S4.

Table 5. Lifetimes of Generated Carriers.

  lifetime (ns)
 relative intensities (%)
  
samples τ1 τ2 A1 A2 χ2
ZnIn2S4 0.0480152 1.13987 89.22% 10.78% 2.712
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Conclusions and Future Work

An analytical study was performed to understand the photocatalytic performance of ZnIn2S4 on malachite green, methyl orange, and Direct Red-80. The ZnIn2S4 sample was synthesized using a low-temperature solution method inside a water bath. Synthesized microspheres were found to be micron size and indicate the formation of a nearly pure hexagonal ZnIn2S4 structure. Marigold flower-like morphology was obtained, which is depicted from the FE-SEM micrographs. Photocatalytic performance was largely dependent on ZnIn2S4 structure, morphology, and optical characteristics. Degradation efficiency (η) of 99.68, 99.48, and 99.12 in 30, 90, and 75 min for MG, MO, and DR-80, respectively, was achieved, which makes ZnIn2S4 a potential candidate for degradation of both anionic and cationic dyes. Band gap energy calculated using the Tauc plot was Eg = 2.1 eV corresponding to 590 nm wavelength, which makes the photocatalyst suitable in the visible region. MSK analysis was done to find the flat band potential of ZnIn2S4. An average crystallite size of 4.91 nm was obtained from XRD analysis. A reusability study suggests a ∼5–10% decrease in efficiency after consecutive cycles, where a part of a reduction in efficiency is due to the quantity of sample decreasing during the collection process after each cycle. All of these observations and analyses suggest ZnIn2S4 is a potential and suitable candidate for degrading both anionic and cationic dyes. In the future, we will work on the synthesis of ZnIn2S4 with some other materials to form a Z-scheme and heterostructure-based photocatalysts to further extend the domain of the photocatalyst in degrading heavy metals and more complex dyes.

Acknowledgments

This work supported by Science and Engineering Research Board [SERB (No. ECR/2017/001222)] to one of the authors (M.K.) is highly appreciated.

The authors declare no competing financial interest.

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