Synthesis and enhanced photocatalytic property of Ni doped ZnS nanoparticles

Synthesis and enhanced photocatalytic property of Ni doped ZnS nanoparticles

Solar Energy 159 (2018) 434–443 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Synthesis ...

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Solar Energy 159 (2018) 434–443

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Synthesis and enhanced photocatalytic property of Ni doped ZnS nanoparticles

MARK



M. Jothibasa, , C. Manoharanb, S. Johnson Jeyakumara, P. Praveenc, I. Kartharinal Punithavathya, J. Prince Richarda a

PG & Research Department of Physics, T.B.M.L. College, Porayar 609307, Tamil Nadu, India Department of Physics, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India c PG & Research Department of Physics, St. Joseph’s College of Arts & Science (Autonomous), Cuddalore 607001, Tamil Nadu, India b

A R T I C L E I N F O

A B S T R A C T

Keywords: Ni doped ZnS Cubic structure Spherical shape Photocatalytic activity

Pure and Ni (0.5–2.0%) doped ZnS nanoparticles were prepared by an inexpensive solid state reaction method. The structural, functional, optical, morphological and chemical compositions of the products were characterized by XRD, FT-IR, UV–Vis, PL, SEM with EDX and TEM analyses. The X-ray diffraction results confirmed that the polycrystalline nature with cubic crystal structure of the nanoparticles. Also, using these data, crystallite size, dislocation density, micro-strain, stacking fault and lattice constant were calculated. The functional group associated with the vibration of a molecule was investigated by FTIR spectroscopy. The optical band gap was increased from 3.58 to 3.97 eV with increasing Ni dopant concentrations. The SEM and TEM images depict the nanosized particles with spherical shape morphology. The elemental composition of Ni-ZnS nanoparticles was examined by EDX analysis. The PL emission spectra show an intensity quenching upon Ni doping and exhibit green emission in the visible region. The photocatalytic activity results indicated that the Ni doping enhanced the photocatalytic activity of ZnS. Thus, Ni-ZnS could be effectively used as photocatalyst for degradation of environmental pollutant Methylene Blue dye.

1. Introduction Semiconductor nanocrystals are defined as materials consisting of hundreds to thousands of atoms. This field of material science has rapidly been developing and attracted extensive studies during the past two decades due to their unique structure, electronic, magnetic, and optical properties originated from their large surface-to-volume ratio and quantum confinement (Afzaal et al., 2010; Fang et al., 2011c). Doping of transition metals or rare earth ions is an effective way to adjust the color output of the semiconductor nanostructures, which is important for their applications in LEDs, lasers, etc. (Didosyan et al., 2004; Kikkawa and Awschalom, 1999; Zutic et al., 2004). ZnS is a commercially important II–VI semiconductor having a wide bandgap of 3.67 eV (bulk) and large exciton-binding energy (∼40 meV). A number of reports are available on the magnetic and luminescence properties of ZnS NPs with different transition metals (TM) with many conflicts (Bhargava et al., 1994; Borse et al., 1999; Divya et al., 2011; Kumar et al., 2013; Poornaprakash et al., 2016a,b; Reddy et al., 2007, 2014; Sambasivam et al., 2008, 2009). Poornaprakash et al. took Fe, Co, and Ni ions as dopants into ZnS NPs, since these three ions are important



TMs with abundant electron shell structures with ionic radius less than Zn, which means that these dopants can easily penetrate into the ZnS crystal lattice (Poornaprakash et al., 2017). The environmental hazard is mainly associated with toxic and nonbiodegradable wastes; which are responsible for different types of pollution i.e. soil, water and air pollution. Amongst this water soluble formulations are more dangerous as they create direct impact on living beings (Encinas et al., 1996). Mainly organic dyes in waste waters coming out from textile and other industries have the major contribution to this. Many dyes are nontoxic themselves but when they mix with water; they easily form highly toxic complexes in the water waste and thus pollute water. For example, when azo dyes dissolve in water their breakdown or intermediate products are benzidine, naphthalene and other aromatic compounds, which are carcinogenic or mutagenic. Approximately, 50–70% of the dyes are aromatic azo compounds and some of the azo dyes and their degradation products such as aromatic amines are highly carcinogenic (Pouretedal and Keshavarz, 2011). Also mutagenesis, teratogenesis, carcinogenesis, respiratory toxicity and reduced fertility in humans have been reported for many of the dyes. The textile industries contain considerable amounts of azo dyes, and

Corresponding author. E-mail addresses: [email protected]ffmail.com, [email protected] (M. Jothibas).

https://doi.org/10.1016/j.solener.2017.10.055 Received 23 October 2016; Received in revised form 28 September 2017; Accepted 19 October 2017 0038-092X/ © 2017 Elsevier Ltd. All rights reserved.

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excitonic systems (Dong et al., 2007; Navaneethana et al., 2010). ZnS is a critical inorganic material for an assortment of potential applications including optical coatings, solar panels, opto-electronic modulators, photoconductors, sensors, transducers and photocatalytic applications. Broad investigation of photocatalytic properties has been performed in different transition metal ions such as oxide and sulfide nanoparticles (Arsha Kusumam et al., 2016; Kiruthiga et al., 2014; Rajabi and Farsi, 2015a; Zeng et al., 2016). In particular, ZnS nanomaterial has attracted wide attention, because of its unique band structure and high capability to decompose the organic pollutants. Hamid Reza Rajabi and Mohammad Farsi have prepared ZnS quantum dots by chemical preparation method, which exhibited a high photocatalytic activity on methylin violet (Rajabi and Farsi, 2016). Chen et al. (2013) have reported that the as-synthesized ZnS rods displayed the largest photocatalytic activity in the degradation of methyl orange. Clearly, the structure of photocatalyst is the fundamental factor to influence the performance, and ZnS photocatalyst with different structures will have potential applications in energy and environment areas. A number of methods have been explored to synthesize ZnS crystals, such as solvothermal synthesis (Chai et al., 2014), thermal decomposition method (Niasari et al., 2010), sol-gel (Bhattacharjee et al., 2002), microwave irradiation (Jiang and Zhu, 2004), co-precipitation method (Thielsch et al., 1996), gas phases condensed (Sanchez-Lopez and Fernandez, 1998) and solid state reaction method (Lu et al., 2004). In the recent past, solid state reaction method has been widely and successfully used as an ideal method to prepare inorganic nanoparticles (Park et al., 2014). In the present work, we have synthesized pure and Ni (0.5–2.0 at.%) doped ZnS nanoparticles using a simple solid state reaction method at low temperature utilizing zinc acetate dihydrate and thioacetamide as precursor materials. The prepared samples are characterized by various analytical tools such as XRD, FTIR, UV–VISIBLE, PL, SEM with EDX and TEM analyses. In addition, the photocatalytic properties of pure and Ni2+ doped ZnS nanoparticles have assessed deliberately on degradation of methylene blue dye (model pollutant) under sunlight illumination.

huge amount of inorganic salts. So, this issue is needed to be monitored closely and sort out urgently. In the recent year, photocatalytic processes that decompose organic contaminant into simple inorganic species have attracted high attention to sort out various aqueous environmental pollution issues. II–VI compound semiconductors with direct band gap exhibit high potential as effective photocatalyst due to ability of rapid generation of electronhole by absorption of photons with energy equal to or more than to its band gap. Such generated electron-hole pairs can produce free radicals in the system to redox the compounds absorbed on the surface of a photocatalyst. The resulting free radicals (%OH) are a very efficient oxidizer of organic materials and can degrade pollutants in the medium. Hence, developments of novel II–VI group semiconductors as photocatalyst and tailoring of existence ones for decoloration of toxic organic dyes have received considerable attention. Amongst the various II–VI compound semiconductor, zinc sulfide (ZnS) has been extensively focused for this application due to its high chemical stability, non-toxicity and environmental safety nature. It exists in two crystalline forms, namely sphalerite (cubic phase) with a band gap of 3.66 eV and wurtzite (hexagonal phase) with a band gap of 3.77 eV at room temperature (Acharya et al., 2013; Fang et al., 2011a; Ong and Chang, 2001). The fundamental interest in ZnS is due to the presence of polar surfaces, excellent transport properties, good thermal stability and high electronic mobility, etc. In addition to this, ZnS based system has been widely explored as a photocatalyst, due to its high energy conversion efficiency, the relatively negative redox potential of its conduction band carbon dioxide reduction and splitting for H2 evolution (Fang et al., 2011b; Wang et al., 2015; Zhang et al., 2011; Zhao et al., 2007). The rate of oxidation and reduction in the photocatalytic activities decide its photocatalytic efficiency. In order to improve the photocatalytic efficiency, photogenerated electron-hole pair recombination is needed to be delayed. During the past several decades foreign elements (transition metals and nonmetal elements) as dopant have been attempted to develop visible light driven photocatalysis, which introduce impurity levels in the forbidden band and results in the enhanced absorption in visible region. There are various reports on enhancement in photocatalytic efficiency of ZnS by adding metals as dopants e.g. Mn2+ (Ashkarran, 2014; Chitkara et al., 2011), Co2+ (Tong et al., 2015; Chena et al., 2010), Ni2+ (Rajabi and Farsi, 2015b), Cu2+ (Chauhan et al., 2014), Cd2+ (Jia et al., 2011). However, it is expected that photocatalytic activities drastically decrease as the formation of recombination centres for photogenerated e− and h+. Few report claims decrease in photodegradation rate of ZnS with dopants. Still there are dilemmas about the role of dopants in photocatalytic efficiency of ZnS. So the questions whether (i) such doping can be successfully applied to photocatalysis, (ii) the photocatalytic activity is possible in presence of visible light under natural atmospheric conditions; all these queries are still unanswered. This makes us curious to investigate doping effect on ZnS. Rohini and Smita selected the isovalent metals cations Mn2+, Co2+, Cu2+ and Cd2+ randomly as dopants. By fine variation of their percentage starting from 1 to 10% at atomic scale, their effect on photocatalytic activities of ZnS are systematically studied for two model dyes Cango Red (CR) and Malachite Green (MG). According to our knowledge it is a first attempt to study photocatalysis activity of doped-ZnS nanoparticles system for CR and MG dye under natural environmental conditions i.e. without irradiating in UV–Vis radiation or intense solar light (Rohini and Smita, 2016). Lately, semiconductor nanoparticles (NPs) have pulled in an extraordinary interest in view of their size, tunable structural and optical properties emerging of quantum confinement effect. As a standout amongst the most imperative semiconductors zinc sulfide (ZnS) has been exceptionally referred to for quite a while as an adaptable and phenomenal phosphor host material and it has a wide band-gap of 3.8 eV and a small Bohr radius (2.4 nm), which make it a superb contender for investigating the natural recombination processes in dense

2. Experimental section 2.1. Synthesis of Ni doped ZnS nanoparticles In the present study Ni doped ZnS nanoparticles were prepared by solid state reaction method using chemical reagents obtained from commercial sources in analytical reagent (AR) grade and used without any further purification. In a typical synthesis of ZnS, appropriate amount of Zn(CH3COO)2·2H2O, NiCl2 (0.5–2.0 at.%) and thiourea were first mixed and grounded thoroughly to obtain a homogeneous mixture, and then transferred into a crucible. The content of Ni varied from 0.5 to 2.0 at.%. The crucible which contained the reactant was heated in a furnace at 400 °C for 4 h in nitrogen atmosphere. The reaction mixture was allowed to cool to room temperature naturally and the resultant products were washed with de-ionized water for several times to remove the impurities. The obtained products were dried in hot air oven at 100 °C. 2.2. Characterizations The crystalline phase purity of Ni doped ZnS was examined by X-ray diffraction (XRD) using SHIMADZU-XRD 6000 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å), high speed rate of 1000°/min and high-precision angle reproducibility (0.0002°) provide fast measurement and highly reliable data at room temperature. The chemical structure was investigated by SHIMADZU-IR PRESTIGE 21 Fourier Transform Infrared spectrophotometer (FTIR) with high spectral resolution 0.5 cm−1 in which the IR spectrum was recorded by diluting the milled powder in KBr in the wavelength between 4000 and 400 cm−1 to assess the presence of functional groups in Ni doped ZnS. 435

(220)

(111)

Optical absorption spectrum was recorded in the range 300–1200 nm using JASCO-V-670 spectrophotometer (Resolution: 0.1 nm). The photoluminescence spectrum (PL) was studied at room temperature using Fluorolog FL3-22 HORIBA JOBIN YVON spectrofluorometer (Photomultiplier response linearity: 2 × 106 cps in photon counting mode and dispersion: 2.1 nm/mm) with an excitation wavelength of 325 nm. The morphology and composition of the product was investigated by SEM (JEOL-JSM 6390, Resolution: 3.0 nm, ACC V 30 kV, WD 8 MM, SEI) with EDX (Oxford Instruments-INCA PENTA FET X3 with resolution of 125 eV) analysis. The morphology of the sample was further examined by transmission electron microscope (TEM: Tecnait20) operating at 200 kv with high resolution (Point: 0.27 nm; Line: 0.144 nm).

(311)

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2.0% Ni

Intensity (a.u)

1.5% Ni

1.0% Ni

2.3. Photocatalytic activity The photocatalytic activity of pure and Ni doped ZnS samples were evaluated by monitoring the photodegradation of methylene blue (MB) in aqueous solution, respectively. MB was selected because of its strong adsorption to metal oxide surfaces, well defined optical absorption and good resistance to light degradation. The photocatalytic experiments were carried out on sunny days between 11 am and 2 pm. The 0.03 g of photocatalyst of pure and Ni doped ZnS was charged into 100 ml of 10 mg/l MB aqueous solution, respectively. The suspensions were magnetically stirred for 30 min to attain adsorption–desorption equilibrium between dye and ZnS. The mixed solutions were irradiated using sunlight. The solutions were then taken out every 30 min (up to 180 min) and the photocatalyst was separated from the mixture solution by centrifugation immediately, and then the UV–Vis absorption of the clarified solutions was analyzed with a UV–Vis spectrophotometer (Shimadzu, UV-1800). The absorbance of MB solution was measured at 664 nm, which corresponds to its maximum absorption wavelength.

0.5% Ni

0% Ni

10

20

30

40

50

60

70

80

2θ (Degree) Fig. 1. X-ray diffractograms of cubic undoped and Ni doped ZnS nanoparticles prepared at different doping percentages.

D=

Kλ βcosθ

(1)

where D is the average crystallite size (Å), K is the shape factor (0.9), λ is the wavelength of X-ray (1.5406 Å) Cu Kα radiation, θ is the Bragg angle and β is the corrected line broadening of the nanoparticles. The estimated values of the crystallite sizes using this relation are given in Table 1. From this table, we could find that the particle size increases with increase of Ni doping content up to 1.5%, and then decreases for further doping (2% of Ni). This result shows that the average crystallite size of nano Ni-ZnS is about 2.84 nm. The lattice parameters ‘a’, ‘b’ and ‘c’ for the cubic structure (α = β = γ, a = b = c = 90°) can be determined by the following equation,

3. Results and discussion 3.1. Crystallographic analysis The XRD analysis is an important tool for analyzing the structural properties of the sample. Fig. 1 shows the X-ray diffraction patterns for ZnS nanoparticles with different doping percentages of Ni. Three prominent peaks corresponding to (1 1 1), (2 2 0) and (3 1 1) reflection planes of ZnS were observed respectively at 2θ of 29.04°, 48.06° and 57.11°. The diffraction peaks from (1 1 1), (2 2 0) and (3 1 1) planes have just showed up in the pattern and all other high-angle peaks have submerged out of sight. On comparing with the standard samples (JCPDS Card No: 05-0566), the X-ray diffractrogram and 2θ values of ZnS were observed to be in genuinely great assertion, thus containing the cubic crystal structure (Murugadoss and Rajesh Kumar, 2014). The broadening in the diffraction peaks might be due to the size effect, therefore the crystallite size is in the nano-regime. These nanocrystals have lesser lattice planes compare to the bulk, which contribute to the broadening of the peaks in the diffraction pattern. It could also arise due to lack of sufficient energy needed by atoms to move to a proper site in forming the crystallite. The width of peaks becomes large as the particles become smaller (Ashokkumar et al., 2015), and also width of peaks becomes small as the particles become large. From the XRD patterns, it is observed that peak positions shift towards higher 2θ value with increasing Ni doping indicating a increase in the lattice parameters (5.38–5.40 Å). Which is due to the smaller ionic radius of Ni2+ (0.83 Å) compared to that of Zn2+ (0.88 Å). This confirms that Ni atoms are added to the host lattice of ZnS (Cai et al., 2013). No characteristic peak of Ni has been observed that means Ni dopant does not affect the crystal structure of ZnS. The crystallite size has been inferred from 2θ of the (1 1 1) diffraction peak on the basis of the Scherrer’s relation,

d=

a h2

+ k2 + l2

(2)

where h, k and l are the Miller indices of the peak. The structural parameters are calculated from the following equations (Jothibas et al., 2016; Manoharan et al., 2015),

Microstrain, ε =

βcosθ 4

Dislocation density, δ =

(3)

1 D2

2π2 ⎡ ⎤ Stacking fault, SF = ⎢ β 1⎥ 2 45(3tan θ) ⎣ ⎦

(4)

(5)

The structural parameters including dislocation density (δ), microstrain (ε) and stacking fault (SF) of cubic ZnS nanoparticles are summarized in Table 1. The lattice defects like δ, ε and SF showed a decreasing trend with increasing doping content from 0 to 2.0% of Ni, which may be due to the improvement of crystallinity as well as the high orientation along (1 1 1) direction (Fig. 1). This type of change in δ 436

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Table 1 Structural properties of Pure and Ni doped ZnS nanoparticles. Ni doping (%)

2θ (degree)

Crystallite size (D) (nm)

Dislocation density (δ) ×1014 lines/m2

Micro strain (Ɛ)

Lattice constant (Å)

Stacking fault (SF)

0 0.5 1.0 1.5 2.0

28.546 28.631 28.673 28.866 28.831

2.641 2.975 3.018 3.049 3.037

0.1443 0.1129 0.1097 0.1075 0.1083

0.0137 0.0114 0.0119 0.0118 0.0119

5.383 5.387 5.352 5.408 5.395

0.028 0.024 0.024 0.023 0.047

Table 2 Tentative vibrational assignments of Ni doped ZnS nanoparticles.

ZnS:2.0%Ni

Transmittance (a.u)

ZnS:1.5%Ni

Wave numbers (cm−1)

Assignments

3349.84 2045.00 1531.23 1384.91 604.25 508.00

O–H stretching CO2 molecules O–H bending Carboxyl and methylene groups Cubic ZnS

3.3. Optical study ZnS:1.0%Ni

3.3.1. Evaluation of band gap energy The absorption spectra of undoped and Ni doped ZnS nanoparticles were illustrated in Fig. 3. The absorption edge at around 383 nm is due to the phase of zinc sulfide. The absorption co-efficient is calculated using the formula,

ZnS:0.5%Ni

α=

2.303A l

where A is the absorbance and l is the path length. The value of optical band gap is determined from the absorption spectra using the Tauc relation,

ZnS

α hν = A(hν−Eg) n 4000

(6)

3500

3000

2500

2000

1500

1000

500

(7)

where α is the absorption co-efficient, A is the constant having separate value for different transitions, hν is the photon energy and Eg is the band gap energy. The value of n depends upon the nature of transition. The values of n for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions are 1/2, 2, 3/2 and 3, respectively. The band gap energies are found to be a negative number for n = 2, 3/2 and 3, and hence the relationship fitting to the ZnS is n = 1/2, which confirms the allowed direct transition. Fig. 4 shows the curves of (αhν)2 versus hν for pure and Ni doped ZnS nanoparticles prepared at different doping percentages. The Eg values are obtained by extrapolating the straight line portions of the graph to the X-axis. The measured energy band gaps from these plots are represented in Table 3. From this table, it can be observed that the Eg values varied from 3.38 to 3.97 eV respectively for pure and Ni doped ZnS nanoparticles. Among the Ni-ZnS products, the nanoparticles synthesized with 1.5% Ni dopant exhibits higher band gap energy. This is the optimum Ni doping percentage of ZnS nanoparticles and that can be used for further investigation. After the electronic absorption process, electrons located in the maximumenergy states in the valence band revert to minimum-energy states in the conduction band under the same point in the Brillouin zone (Santana et al., 2011). The exponential optical absorption edge and the optical band gap energy are controlled by the degree of structural disorder in the lattice. The abatement in the band gap value can be attributed to defects and local bond distortion as well as intrinsic surface states and, interfaces which yield confined electronic levels within the forbidden band gap (La Porta et al., 2013).

-1

Wavenumber (cm ) Fig. 2. FT-IR spectra of pure ZnS and different levels (0–2.0%) of Ni2+ doped ZnS nanocrystals.

and ε might be due to the recrystallization process in the polycrystalline nanoparticles. 3.2. FTIR spectroscopy FTIR transmittance spectra of undoped and Ni2+ doped ZnS nanoparticles synthesized at optimized temperature in air for 4 h is shown in Fig. 2. The ZnS nanoparticles demonstrate the attributes of the formation of high virtue products and portray the peaks correspond to ZnS. From the FTIR spectra (Fig. 2), several peaks at 3349, 2045, 1531, 1384, 604 and 508 cm−1 can be observed. The broad weak peaks at 3349 cm−1 must be related to the stretching and bending modes of trace amounts of adsorbed water on the particles (Wegmuller, 1987). The peaks at 2340 and 2045 cm−1 are ascribed to the existence of CO2 molecules. The peak at 1384 cm−1 region could be attributed to nitrate groups (Martinez-Sabater et al., 2009). The peaks appearing at 617 and 508 cm−1 are associated to Zn-S vibration and are characteristic of cubic ZnS (Kuppayee et al., 2011). The infrared absorption frequencies and the tentative vibrational assignments of pure and Ni doped ZnS nanoparticles are depicted in Table 2. 437

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Fig. 3. Optical absorption spectra of undoped and Ni doped ZnS nanoparticles.

doped ZnS nanoparticles (representative sample) has been investigated by EDX spectra (Fig. 6a & b). The EDX analysis of undoped ZnS (Fig. 6a) confirms the presence of Zn and S in near stoichiometric ratio and the atomic percentage of these elements are 56.59 and 43.41 at.%, respectively. Whereas, Fig. 6(b) (Ni-ZnS) shows the existence of the dopant Ni along with Zn and S with at.% of 0.74, 59.00 and 40.26, respectively. The size and morphology were further explored by TEM. The typical TEM photographs and selected area electron diffraction (SAED) patterns of Ni doped ZnS nanoparticles are illustrated in Fig. 7. It could be found that the nanoparticles appeared similar spherical shape with the average particle diameter of about 10.67 and 12.08 nm, demonstrating that Ni dopant could influence the increase of ZnS particle size. These values were consistent to the crystallite sizes calculated from the XRD

3.4. Morphological characterization Nanoparticles analysis using SEM supplemented by EDX was carried out for the cubic undoped and Ni doped ZnS to establish the morphology, grain size, shape and to confirm their chemical composition. SEM observed the nano-sized ZnS grains as large surface area with well-defined mesopores and the images are shown in Fig. 5(a–d). The surface of undoped ZnS was covered with small grains of about 15.32–19.04 nm in size. As the Ni doping content increases the particle size of ZnS also increases slightly to higher nanometers. A closer examination of these pictures reveals a well-defined particle-like morphology, having plenitude of spherical shaped particles. EDX analysis is an important analytical tool to determine the composition of the sample. The elemental composition of undoped and Ni (1.5%) 438

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Fig. 4. The plot of (αhν)2 versus hν for undoped and Ni doped ZnS nanoparticles.

line broadening. Moreover, particle sizes were larger than their crystallite ones due to the reunion of nano-sized crystallites. The Ni-ZnS particle size had been slightly increased; this might be due to the agglomeration of Ni on ZnS nanoparticles surface during the drying/annealing process. A possible reason for the substantial increase in the particle size might be due to the coalescence of nickel nanoparticles on the surface of semiconductor particles. The TEM observations clearly confirm that Ni nanoparticles are well dispersed on porous ZnS surface. The ZnS nanoparticles have very high surface area, strong hydrogen bonding through –OH groups and high surface free energy, thus they

Table 3 Calculated band gap energies of Pure and Ni doped ZnS nanoparticles. Ni Doping (%)

Band gap energy (eV)

Undoped 0.5 1.0 1.5 2.0

3.38 3.58 3.73 3.97 3.53

439

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Fig. 5. (a–d) SEM images of Undoped and Ni doped ZnS nanoparticles at different magnifications.

Fig. 6. (a and b) EDX spectra of undoped and Ni doped ZnS nanoparticles.

tend to be aggregated. The SAED pattern (Fig. 7d) exhibits the well defined electron diffraction spots, confirming the crystalline nature of the cubic phase of ZnS nanocrystals. Meanwhile, Ni-ZnS (Fig. 7d) shows circular rings corresponding to diffraction planes of ZnS. From these patterns, it is clear that the crystallinity of the nanoparticles increases with the Ni ions incorporation as the dot pattern started appearing, this confirms the formation polycrystalline nature of the product. Hence, the discrete bright spots reveal the well-crystallized tetragonal form.

concentration, the dopant transition metal (Ni) acts as electron trapping centers and hence non-radiative recombination process increases (Kannadasan et al., 2014). As a result, quenching of intensity takes place at higher doping concentration. In ZnS:Ni nanoparticles, a green emission peak around 585 nm was observed, which is in good agreement with earlier results in ZnS:Cu nanostructures (Peng et al., 2006). The green emission arises from the recombination between the shallow donor level (sulfur vacancy) and Ni related acceptor center.

3.5. Photoluminescence study

3.6. Photocatalytic activity

Fig. 8 demonstrated the room-temperature photoluminescence spectra of pure and Ni doped ZnS nanoparticles with various Ni doping fixation. The spectra of the considerable number of samples were recorded in the wavelength scope of 450–750 nm. The Fig. 8 revealed that the solid bandedge discharge at around 585 nm and the power of an outflow crest was clearly enhanced while expanding the Ni-doping fixation up to 1.5% in ZnS nanoparticle. This is a direct result of expansion in thickness of free exciton and showed the change in crystalline nature of ZnS nanoparticles. Be that as it may, the force of band-edge emanation was decreased while expanding the Ni-doping focus above 1.5%. At higher doping

The photocatalytic performance is known to be dependent on the crystallinity, surface area, and morphology and it might be enhanced by abating the recombination of photogenerated electron-hole pairs, extending the excitation wavelength to a lower energy range, and increasing the amount of surface-adsorbed reactant species. As a rule, the procedure for photocatalysis starts when supra-band gap photons are specifically assimilated therefore producing electron-hole pairs in the semiconductor particles. This is followed by diffusion of the charge carriers to the surface of the particle where the interaction with water molecules would produce highly reactive species of peroxide (O2−) and hydroxyl radical (OH%) 440

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Fig. 7. (a–c) TEM photographs of Ni-ZnS nanoparticles and (d) corresponding SAED pattern.

h+ + H2O → H+ + *OH− OH− + Methylene blue → Degradation products * O2− + Methylene blue → Degradation products *

The degradation percentage (%D) of the MB dye can be calculated from the following equation (Mi et al., 2013),

% D=

C0−Ct × 100 C0

(8)

where C0 is the initial concentration of dye and Ct is the concentration of dye after irradiation in selected time intervals (0–120 min). Several experimental results suggest that the rates of photocatalytic oxidation of various contaminants over illuminated ZnS occur via pseudo firstorder kinetics (Hoang et al., 2012).

ln(C0 /Ct ) = kKt=kt (or) Ct = C0 e−kt Fig. 8. . PL spectra of undoped and Ni (0.5–2.0%) doped ZnS nanoparticles.

(9)

where k is the reaction rate constant and K is the adsorption co-efficient of the reactant. A plot of ln (C0/Ct) versus time represents a straight line; the slope equals the apparent first-order rate constant k. Figs. 9a and 9b show the absorption spectra of MB using pure and Ni doped ZnS catalyst as a function of wavelength (400–800 nm) for various time intervals 0, 30, 60, 90, 120, 150 and 180 min. The degradation effect was characterized by monitoring the absorption peak of MB centered at 664 nm. The plots clearly demonstrate that the maximum absorption peak decreases with increasing irradiation time. This illustrates that the MB dye concentration decreases in the presence of pure and Ni doped ZnS catalysts and solar light illumination. The decrease in the absorption of the mixed solution was due to the destruction of the homo and hetro-poly aromatic rings present in the dye molecules or due to rapid degradation of MB, which is confirmed by the lower intensities of the absorbance peak of MB. The effect of pure and Ni doped ZnS catalysts on percentage degradation of the MB dye has been examined by varying the time interval from 0180 min and the results are presented in Table 4. The percentage degradation increases rapidly with the increase in the time for MB-catalyst solutions. The maximum degradation of MB dye took place in 180 min of irradiation with sunlight using pure and Ni doped ZnS catalysts was

responsible for the degradation of adsorbed organic molecules. The process of photocatalytic degradation of methylene blue over ZnS catalyst can be described as follows. The first step involves adsorption of the dye onto the surface of ZnS nanostructure sample. Exposure of dye adsorbed ZnS nanostructures with sunlight leads to generation of electron-hole (e−-h+) pairs in ZnS. The photogenerated electrons in the conduction band of ZnS interact with the oxygen molecules adsorbed on ZnS to form superoxide anion radicals (*O2−). The holes generated in the valence band of ZnS react with surface hydroxyl groups to produce highly reactive hydroxyl radicals (*OH). These photogenerated holes can lead to dissociation of water molecules in the aqueous solution, producing radicals. The highly reactive hydroxyl radicals (*OH) and superoxide radicals (*O2−) can react with methylene blue dye adsorbed on ZnS nanostructures and lead to its degradation. ZnS + hυ → e− (CB) + h+ (VB) O2 + e− → *O2− h+ + OH− → *OH 441

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Fig. 9a. UV–Vis absorption spectra of MB with respect to irradiation time using ZnS photocatalyst.

Fig. 10. (a and b) Rate constant (k) and regression (R2) for the pure and Ni doped ZnS nanoparticles.

Table 5 Values of apparent rate constants k (min−1) and regression for the degradation of 10 mM MB in the presence of pure and Ni doped ZnS nanoparticles. Fig. 9b. UV–Vis absorption spectra of MB with respect to irradiation time using Ni-ZnS photocatalyst.

Samples

Rate constants (min−1)

R2

Pure ZnS Ni-ZnS

0.010 0.013

0.961 0.906

Table 4 The effect of methylene blue (MB) dye degradation by pure and Ni doped ZnS photocatalysts. Time (min)

0 30 60 90 120 150 180

These nanoparticles were characterized structurally by XRD analysis and found to have cubic crystal structure. The crystallite size of the ZnS nanoparticles increased with increase of Ni doping percentage. The lattice defects such as dislocation density, microstrain and stacking fault of ZnS are not considerably altered on Ni doping. Thus, the host lattice of ZnS has not affected by Ni content. From FTIR spectra, we investigated that the peaks at around 600 cm−1 are associated to Zn-S vibration. The optical band gap energy of ZnS nanoparticles has increased with Ni doping due to the blue shift in the absorption edge. The morphological analyses SEM and TEM depicted that the prepared nanoparticles are nearly spherical in shape and particle size within the nanometer scale. EDX spectrum confirms the presence of Zn, S and Ni in appropriate proportions. We found from the PL spectra, that the intensity of the emission peak increased with the increasing Ni content in the ZnS nanoparticles and exhibit green emission band around 585 nm. The photocatalytic measurement reveals that the percentage of degradation and reaction rate constant has increased by Ni doped on ZnS nanostructures. Therefore, we conclude that the Ni-ZnS nanoparticles are the promising photocatalyst on degradation of methylene blue dye taken as model contaminant for this present study.

% Degradation of MB Dye Pure ZnS

Ni-ZnS

0 30.82 37.46 46.03 63.70 69.07 72.13

0 51.66 64.61 70.51 70.98 86.79 87.38

72.13% and 87.38%, respectively. While in the case of time at 30 min, only 30.82% and 51.66% degradation was observed for pure and Ni doped ZnS catalysts, respectively. According to the pseudo-first-order rate equation, the rate constant (k) for MB degradation by pure and Ni doped ZnS was determined. The plot of ln (C0/Ct) as a function of irradiation time gives the rate constant values 0.010 min−1 and 0.013 min−1 (Fig. 10a&b). Moreover, the fitting correlation co-efficient (R2) is also determined to be 0.961 and 0.906, respectively for pure and Ni doped ZnS catalysts and tabulated (Table 5).

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The nanoparticles of pure and Ni doped ZnS were successfully synthesized using well known inexpensive solid state reaction method. 442

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