Facile fabrication, characterization and efficient photocatalytic activity of surfactant free ZnS, CdS and CuS nanoparticles

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.

Journal of Science: Advanced Materials and Devices (2019) 381e391 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Facile fabrication, characterization and efficient photocatalytic activity of surfactant free ZnS, CdS and CuS nanoparticles Dasari Ayodhya*, Guttena Veerabhadram 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 a b s t r a c t Article history: Received 26 February 2019 Received in revised form August 2019 Accepted 19 August 2019 Available online 28 August 2019 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 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/) Keywords: Metal sulfide nanoparticles Heterogeneous photocatalysis Rate of reaction Photostability Reactive species 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, herbicides, 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 suffer from various drawbacks because they not completely remove the pollutants and are not cost-effective [1,2] The complications related to all the above processes are that they not completely degrade the pollutants, but only change it from one to another and simultaneously produce a large amount of toxic secondary * 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 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 products 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 and finally the degradation of a wide variety of organic pollutants through their interaction with photogenerated holes or reactive oxygen species, such as OH and O2 À 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], lightemitting diodes [7], photocatalysis [8], electrocatalytic H2 evolution [9], and antibacterial activity [7] Among them, in recent years, 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/) 382 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 the photocatalysis has attracted intensive attention as promising candidates for the efficient degradation of toxic pollutants including dyes, pesticides, and antibiotics under sunlight irradiation to solve the various aqueous environmental pollution 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 decorated 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 3D flower-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 generation of electronehole pairs by the absorption of photons with energy 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 microwave 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 synthetic 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 characterization 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 synthesized samples was examined for 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 hỵ, OH and O2 species The prepared catalysts are considered as suitable for removal of highly toxic and hazardous organic materials for the environmental protection 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 in Table 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 The chemical structure, lmax, molecular weight, and solubility of BTB dye Dye Bromothymol blue (BTB) C27H28Br2O5S Structure lmax M.Wt (g/mol) Solubility (g/L) 614 nm 624.38 0.1 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 to yellowish (CdS), and light green to greenish black (CuS) The precipitation temperature was maintained constant with a waterjacketed reaction vessel using a water circulating thermostatic bath The reaction mixture was further stirred at room temperature for 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 80  C for h before further analysis 2.3 Characterization techniques The X-ray diffraction (XRD) study was done using a Philips X-ray diffractometer with Cu-Ka radiation (l ¼ 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 cmÀ1 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 383 of the synthesized materials were carried out on a Mettler Toledo Star system in the temperature range of 30e1000  C under the dynamic N2 gas atmosphere with a heating rate of 10  C minÀ1 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 in the darkness to verify the adsorption equilibrium between the BTB and the photocatalyst Then the suspension was taken under the sunlight irradiation on sunny days between 12 pm and pm After the suspension was exposed to the sunlight irradiation, a mL of sample was collected at a specific time interval 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) Â 100%, Fig The UV-vis DRS (a), PL (b), FTIR (c), and XRD (d) of the synthesized ZnS, CdS and CuS NPs 384 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 where, C0 and 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 cycles using the same procedure as mentioned above After each cycle of the photodegradation study, the photocatalyst was filtered from the suspension, and then washed with ethanol and double distilled water several times The catalyst was finally dried at 80  C for 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 photocatalytic mechanism of the degradation process, trapping experiments 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, mmol LÀ1), tert-butyl alcohol (t-BuOH, mmol LÀ1), and pbenzoquinone (p-BQ, mmol LÀ1) were added into the BTB dye solution before the sunlight irradiation to quench hỵ, OH and O 2, respectively Further, the photocatalytic degradation process of BTB Fig (aec) High-resolution SEM and (def) HR-TEM images of the synthesized ZnS, CdS and CuS NPs D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 385 dye was similarly performed as a degradation test using synthesized samples under sunlight irradiation 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 in Fig 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 absorption spectra The optical absorption 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 previous reports [3,4,14,25,26] The calculated band gaps of the samples 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 Fig The TGA curves of the synthesized ZnS, CdS and CuS NPs 3.2 Photoluminescence study Photoluminescence study is a useful technique to probe the recombination and the transfer of photo-induced electrons (e) and holes (hỵ) [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 The energy dispersive X-ray analysis (EDAX) spectra of the synthesized (a) ZnS, (b) CdS, and (c) CuS NPS 386 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 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 e and hỵ The previous studies have reported that this blue shift can likely be attributed to many factors, e.g., surface effects and the agglomeration of nanoparticles [11] Furthermore, the change was observed in the recombination rate that may enhance the photocatalytic performance of the synthesized samples characteristic band of the associated hydroxyl groups The other peaks at 1630-1500 cmÀ1 correspond to the stretching vibrations of a hydroxyl group and representing the water as moisture [28] The peak observed between 750 and 500 cmÀ1 indicates the metalsulphur stretching bands in ZnS, CdS, and CuS NPs In ZnS sample, the broadband around 3421 cmÀ1 represents the stretching vibrations of the OeH group, whereas the bands around 2342 cmÀ1 correspond to CO2 mode These CO2 bands may arise due to some trapped CO2 in 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] 3.3 FTIR analysis 3.4 XRD analysis In order to identify the presence of the functional group vibration 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 cmÀ1 at room temperature are shown in Fig 1(c) The FT-IR spectra of all samples show a strong absorption band at 3500-3000 cmÀ1, which is a The powder X-ray diffraction (XRD) measurements were carried out to determine the crystalline nature of the synthesized materials The powder XRD patterns of ZnS, CdS and CuS NPs are displayed in Fig 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 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 sunlight irradiation D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 387 for ZnS, CdS, and CuS NPs, respectively The planes (1 0), (1 2), (1 3), (0 6), (1 0) and (1 8) indicate the covellite phase with the hexagonal crystal structure for CuS NPs and the (1 11), (2 0) and (3 1) crystal planes indicate the cubic phase for ZnS and CdS NPs Furthermore, the peak broadening as observed implies either the amorphous nature or the fine 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: decomposition and degradation processes of the unreacted molecules beginning from the desorption of adsorbed atmospheric components and lasting to the absolute ash formation [30] As shown in Fig 4, the residual solvents evaporate at below 100  C and the samples thermally exfoliated at around 200  Ce1000  C At the final stage of TGA, 32.43%, 21.62%, and 24.32% weight loss were observed at 1000  C, of ZnS, CdS, and CuS NPs, respectively, due to the thermal degradation of the unstable components in the presence of samples D ¼ k l / b cosq, 3.7 Photocatalytic activity where, k is a constant (k ¼ 0.96), l is the wavelength of the X-ray (0.15418 nm), b is the full-width at half maximum (FWHM), and q is the diffraction angle The average crystallite size of ZnS, CdS, and CuS samples was found to be 10 ± 0.2 nm, ± 0.3 nm and 11 ± 0.2 nm, respectively 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 in Fig 5(aed) From Fig 5(a), it can be seen that the BTB dye exhibits poor photodegradation performances (approximately 5%) in the absence of a catalyst under 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 activities than the absence of a catalyst under h sunlight irradiation 3.5 Morphology study The morphology, shape and average grain diameter of the synthesized 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 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 in Fig 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 ± 1.5, ± 1.1 and 10 ± 0.8 nm, respectively These values are consistent to the crystallite 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 (see Fig 4) in the range 30e1000  C at a 10  C/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 Fig (aeb) The photocatalytic degradation efficiency plots of BTB degradation using ZnS, CdS and CuS NPs 388 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 The degradation effect was characterized by monitoring the absorption peak of the BTB centered at 614 nm by the UV-vis spectrophotometer Fig clearly demonstrates that the maximum absorption peak of the BTB decreases with the increasing irradiation time This illustrates that the BTB dye concentration 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 activity in the degradation of BTB being 83.42% in 60 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 degradation percentage (%D) of the BTB dye using the ZnS, CdS and CuS NPs can be calculated and the results are plotted in Fig 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 in Fig 7(aed) The cycled samples after the 5th cycle represent a similar structure and intensity in their XRD patterns 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 The kinetic plots of the photocatalytic degradation of BTB dye using the synthesized ZnS, CdS and CuS NPs under sunlight irradiation Fig (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 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 389 Table 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, minÀ1) Half-life time, (t1/2, min.) R2 BTB ZnS NPs CdS NPs CuS NPs mg 20 mg 20 mg 20 mg 5.12 63.88 83.42 46.23 0.0007 0.0182 0.0343 0.0096 990 38.08 20.19 72.18 0.99 0.99 0.97 0.98 Table A comparison of the present work with the recent investigations of photocatalytic degradation of dyes using various catalysts Sample Dye Light source Amount of catalyst Conc of dye % of Efficiency Ref ZnS ChPA/ZnS ChPA/ZnS ZnS ZnS ZnS CdS CdS NPs CdS NRs CdS NSs CuS ZnS CdS CuS MB MO CR MB MO RhB RhB MB MB MB MB BTB BTB BTB Sunlight Sunlight Sunlight Vis-light Vis-light Vis-light Vis-light UV light UV light UV light UV light Sunlight Sunlight Sunlight 0.03 g 0.2 g 0.2 g 10 mg 10 mg 10 mg 0.1 g 0.5 g/L 0.5 g/L 0.5 g/L 0.025 g 20 mg 20 mg 20 mg 10 mg/L Â 10À5 M Â 10À5 M 10 mg/L 10 mg/L 10 mg/L 10 mg/L 0.5 mg/L 0.5 mg/L 0.5 mg/L 0.5 g/L 10 mg/L 10 mg/L 10 mg/L 72.13 69 75 49 12 88 46.5 29 31 38 43 63.88 83.42 46.23 [32] [33] [33] [34] [34] [34] [35] [36] [36] [36] [37] This work This work This work 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 Several measured experimental results have been fitted by the pseudo-first-order kinetic model [31]: ln (A0/At) ¼ k t, where, k is the rate constant, A0 is the initial absorbance of the BTB dye, and At is the absorbance of the BTB at time t According to the pseudofirst-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 the fitting correlation coefficient (R2) and the results are tabulated in Table For a better comparison, the synthesized ZnS, CdS, and CuS NPs exhibited excellent photocatalytic activity for the degradation of several dyes than the other similar catalysts The data are summarized in Table [32e37] 3.7.3 Photostability and determination of active species In order to further study the photostability of the synthesized CdS NPs, a five-cycle experiment was carried out and the results are presented in Fig 9(a) It could be observed that after five cycles, there is no obvious deterioration in the photodegradation performance, 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 cycle experiments Furthermore, reactive species trapping experiments were performed to investigate the reactive oxygen species in the photocatalytic process and the results are shown in Fig 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 OÀ , 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-benzoquinone (p-BQ) in scavenging hỵ, OH and OÀ , 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 Fig (a) The photostability and (b) reactive species trapping plots of the photocatalytic degradation of BTB dye using high efficient CdS NPs 390 D Ayodhya, G Veerabhadram / Journal of Science: Advanced Materials and Devices (2019) 381e391 is reduced to 34% This means OÀ 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 important roles in the photocatalytic process Conclusion In summary, ZnS, CdS and CuS NPs were successfully synthesized 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 cmÀ1 are 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 superoxide 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 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 Acknowledgements The authors would like to thank DST-FIST, New Delhi, India for providing necessary analytical facilities and sincere thanks to the 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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 D Ayodhya, G Veerabhadram / Journal of Science:... 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. .. 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,