Photocatalytic degradation of methylene blue with Cu doped ZnS nanoparticles

Photocatalytic degradation of methylene blue with Cu doped ZnS nanoparticles

Journal of Luminescence 145 (2014) 6–12 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevier.co...

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Journal of Luminescence 145 (2014) 6–12

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Photocatalytic degradation of methylene blue with Cu doped ZnS nanoparticles Ruby Chauhan a, Ashavani Kumar b,n, Ram Pal Chaudhary a a b

Department of Chemistry, Sant Longowal Institute of Engineering & Technology, Longowal 148106, India Department of Physics, National Institute of Technology, Kurukshetra 136119 India

art ic l e i nf o

a b s t r a c t

Article history: Received 27 January 2013 Received in revised form 10 May 2013 Accepted 9 July 2013 Available online 19 July 2013

Cu doped ZnS nanoparticles (Zn1  xCuxS; where x ¼0.00, 0.03, 0.05 and 0.10) were synthesized by a chemical precipitation method. The synthesized products were characterized by X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope, ultraviolet-visible and photoluminescence spectrometer. The X-ray diffraction and high resolution transmission electron microscope studies show that the size of crystallites is in the range of 2–10 nm. XRD study revealed that the samples are composed of cubic phase without doping and at 3 mol% Cu doping concentration while at the doping of 5 mol% and 10 mol% Cu, phase transition from cubic blende to hexagonal phase occurs in ZnS. Photocatalytic activities of ZnS and 3, 5 and 10 mol% Cu doped ZnS were evaluated by decolorization of methylene blue in aqueous solution under ultraviolet and visible light irradiation. It was found that the Cu doped ZnS bleaches methylene blue much faster than the undoped ZnS upon its exposure to the visible light as compared to the ultraviolet light. The optimal Cu/Zn ratio was observed to be 3 mol% for photocatalytic applications. & 2013 Elsevier B.V. All rights reserved.

Keywords: Nanoparticle Scanning electron microscope High resolution transmission electron microscope Photocatalysis

1. Introduction With the rapid industrialization of the world, organic contaminants have become a major pollutant contributing to the degradation of the environment, and a considerable focus of research is oriented toward efficient removal or degradation processes of these contaminants. The discharge wastes containing dyes are toxic to microorganisms, aquatic life and human beings [1]. These deleterious effects of chemicals such as azo dyes, herbicides, and pesticides are actually present in rivers and lakes, and are in part suspected of being endocrine-disrupting chemicals (EDCs) [2–5]. Textile dyes and other industrial dyestuffs constitute one of the largest groups of organic compounds that represent an increasing environmental danger. Semiconductor photocatalysis offers the potential for complete elimination of toxic chemicals through its efficiency and potentially broad applicability [6]. Recently, transition-metal sulfides, in particular ZnS, have unique catalytic functions as a result of the rapid generation of electron–hole pairs by photoexcitation and the highly negative reduction potentials of excited electrons compared to those of TiO2 and ZnO [7,8]. A major drawback of pure ZnS is the large band gap means it can only be activated upon irradiation with photons of light in the UV domain, limiting the practical efficiency for solar applications. Therefore,

n

Corresponding author. Tel.: +91 1744 233495; fax: +91 1744 233050. E-mail address: [email protected] (A. Kumar).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.07.005

in order to enhance the solar efficiency of ZnS under solar irradiation, it is necessary to modify the nanomaterial to facilitate visible light absorption. Transition metals doping in ZnS has shown great promise in achieving visible light active (VLA) photocatalysis. The incorporation of transition metals in the ZnS crystal lattice may result in the formation of new energy levels between valence band (VB) and conduction band (CB), including a shift of light absorption towards the visible light region. Photocatalytic activity usually depends on the nature and the amount of doping agent. A favorable shift of optical response under the visible region occurs subsequent to the doping of transition metal or rare-earth metal ions, such as Ni2+ and Cu2+; therefore, ZnS nanoparticles can also be used as effective catalysts for photocatalytic evolution of H2 and photoreduction of toxic ions under visible-light irradiation. In recent years, sulfides such as CdS, ZnS, ZnxCd1  xS, multiwalled carbon naotube-Zn1  xCdxS, CdS quantum dots-sensitized Zn1  xCdxS and Zn1  xCuxS have been proved to be an efficient visible light driven photocatalyst for H2 production from water splitting [9–16]. In order to enhance the photocatalytic activity of photocatalyst, interfacial charge transfer reaction should be increased and electron–hole recombination decreases by modifying the properties of photocatalyst. Several methods have been developed such as increasing its charge to volume ratio, optimization of particle size, and doping of metals and non-metals. A dopant ion may act as an electron trap or hole trap. This would prolong the lifetime of the charge carriers, resulting in an enhancement in photocatalytic

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activity. ZnS is II–VI compound semiconductor with direct and wide band gap of 3.68 eV at room temperature and widely used as an important component in ultraviolet light emitting diodes, solar cells, and photocatalysts in many situations including a visible light activated water splitting agent. It has two kinds of structures: zinc blende structure (cubic crystal) and wurtzite structure (hexahedron). This paper reports a simple route for the preparation of undoped and Cu doped ZnS nanoparticles, via the chemical precipitation method [17] in the presence of polyethylene glycol as capping agent. The prepared nanoparticles were characterized and then utilized as photocatalyst in the photodegradation of methylene blue as an organic cationic dye. Methylene blue [3,7-bis (dimethylamino)-phenothiazin-5-iumchloride] is a blue cationic thiazine dye which was used as a model dye to evaluate the photocatalytic activity of pure and Cu doped ZnS samples thermally treated at 100 1C in muffle furnace for 30 min.

2. Experimental 2.1. Chemicals For the preparation and photocatalytic activity of undoped and Cu doped ZnS nanoparticles, the chemicals used were polyethylene glycol [M ¼6000, OH(OCH2CH2)nH; PEG], zinc sulfate [M¼ 288, ZnSO4  7H2O], copper sulfate [M¼ 249.68, CuSO4  5H2O], Sodium sulfide [M ¼78, Na2S], methylene blue (MB) dye (M ¼319.85, C16H18N3SCl). All chemicals used were AR grade from Sigma-Aldrich and used without further purification.

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ensure the catalyst powder dispersed in the MB solution, the mixture was stirred for 10 min, and then kept in dark for an hour to achieve adsorption equilibrium. The sample was then transferred into the photoreactor for UV–vis exposure and the lamp was turned on and approximately 5 ml mixture of catalyst and MB solution was sampled from the photoreactor after fixed time interval. The sampling of the irradiated solution was performed up to 300 min at room temperature. The concentration of MB in the solutions was ascertained by referring to an absorption– concentration standard curve that was established by measuring the optical absorption of methylene blue at 665 nm by UV–vis spectrometer (CamSpec). 2.4. Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku mini desktop diffractometer using graphite filtered CuKα radiation (λ ¼1.54 Å) at 40 KV and 100 mA with a scanning rate of 31/min (from 2θ¼20–801). Optical absorption spectra were recorded on a UV–vis spectrometer (CamSpec). Photoluminescence (PL) study was carried out using a Fluorescence Spectrophotometer (Varian; Cary Eclipse) with an excitation wavelength 310 nm. Morphology and sizes of the product were determined by a scanning electron microscope (SEM: ZEISS EVO MA-10) equipped with an energy dispersive spectrometer (EDS: Oxford Link ISIS 300). High resolution transmission electron microscopy (HRTEM, model: Tecnai G2 F30 STWIN, field emission gun operated at 300 kV) was performed to study the internal microstructure and atomic scale configuration of lattice structure in real and reciprocal space.

2.2. Synthesis

3. Results and discussion

Nanoparticles of Cu doped ZnS (Zn1  xCuxS where x¼ 0.00, 0.03, 0.05, and 0.10) were prepared by the chemical precipitation method. Freshly prepared aqueous solutions of chemicals were used for the synthesis of nanoparticles. 0.1 M zinc sulfate, 0.1 M copper sulfate and 0.1 M sodium sulfide were used as reactant materials. Freshly prepared 50 ml of aqueous solution of 0.1 M sodium sulfide was mixed drop by drop in 50 ml of 0.1 M solution of zinc sulfate and 50 ml of 0.1 M solution of copper sulfate using vigorous stirring and then added 0.05 g of polyethylene glycol as a capping agent. After the completion of reaction, the solution was allowed to settle for sometimes and the supernatant solution was then discarded carefully. The remaining solution was filtered and washed several times with distilled water. The wet precipitate was dried in an oven at 80 oC for 2 h and then calcined at 100 oC for 30 min with a heating rate of about 10 oC/min in a muffle furnace.

3.1. XRD studies Fig. 1 shows the XRD patterns of undoped and Cu doped zinc sulfide (Zn1  xCuxS, where x ¼0.00, 0.03, 0.05 and 0.10) powder samples calcined at 100 1C. The XRD patterns indicate that the samples are composed of cubic phase without doping and 3 mol% Cu doping concentration. While at the doping of 5 mol% and 10 mol% Cu, the samples are composed of mainly cubic and partially hexagonal phases. Peaks marked (C) and (H) correspond to cubic and hexagonal phases, respectively. The XRD pattern of calcined samples at 100 1C exhibit peaks at 2θ values 28.91, 33.01,

2.3. Photocatalytic activities of undoped and Cu doped ZnS nanoparticles The photocatalytic activity was evaluated by measuring the decomposition of the distilled water solution of methylene blue (MB) (with a concentration of 10 mg/l at pH 6.5) under ultraviolet (UV) and visible light irradiation. UV irradiation was carried out using a 125 W UV lamp, the irradiation wavelength of which was predominated at 365 nm. A 1000 W halogen lamp was used for the visible light irradiation. A specially designed photocatalytic reactor system made of double walled reaction chamber of glass tubes was used for photodegradation experiments. A UV or halogen lamp was kept inside the glass tube surrounded by a circulating water tube for cooling the lamp. The solution for photodegradation measurement was prepared by adding pure or 3, 5 and 10 mol% Cu doped ZnS (1 g/l) to 50 ml aqueous solution of methylene blue (MB; 10 mg/l at natural pH ¼ 6.5). In order to

Fig. 1. XRD patterns of undoped and 3, 5 and 10 mol% Cu doped ZnS nanoparticles calcined at 100 1C.

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48.11 and 57.11 which could be indexed to scattering from (111), (200), (220) and (311) planes. The measured d-spacing 3.08, 2.66, 1.88 and 1.61 Å correspond to the reflection from (111), (200), (220) and (311) crystal planes of the cubic structure. All the diffraction peaks agreed with the reported Joint Committee on Powder Diffraction Standards (JCPDS) Card no. 80-0020. The split in peaks at doping concentrations 0.05 and 0.10 mol% Cu is due to hexagonal or wurtzite phase formation [18]. The characteristic peaks of hexagonal phase at higher concentration of Cu at angles (2θ)¼ 28.4o, 30.51, and 47.51correspond to the reflections from (002), (101), (110) crystal planes. All the diffraction peaks agreed with the reported Joint Committee on Powder Diffraction Standards (JCPDS) Card no. 79-2204. XRD data reveals that the diffraction peak of Zn1  xCuxS at the (111) plane slightly shifts to higher angle and lattice constant decreases with increasing Cu concentration. The lattice parameters a and c for cubic structure were calculated by the following equation. For a ¼b¼ c (cubic) 2

2

2

dhkl ¼ a=ðh þ k þ l Þ1=2 where h, k and l are the Miller indices of the peak, and dhkl is the planar distance. The lattice parameters for ZnS and for Zn0.90Cu0.10S are a ¼b ¼c ¼5.35 Å and a¼ b¼c ¼5.34 Å, respectively. The lattice parameters of Zn0.90Cu0.10S are found slightly decreased due to the smaller ionic radius of Cu2+ (r Cu2+ ¼0.57 Å) than that of Zn2+ (r Zn2+ ¼0.60 Å) ions. The calculated values of crystallite size and lattice parameter are presented in Table 1 for undoped and 3, 5 and 10% Cu doped ZnS calcined at 100 1C. A definite line broadening of the diffraction peaks is an indication that the synthesized materials are in nanometer range. The mean crystalline size was calculated from the full-width at halfmaximum (FWHM) of XRD lines by using the Scherrer formula Dhkl ¼ 0:9λ=ðβhkl cos θÞ where D is the average crystalline diameter, λ is the wave-length in angstrom, β is the line width at half-maximum and θ is the Bragg angle. We used the most intense peak (111) in the XRD patterns to calculate the average crystallite size. The crystallite size of Zn1  xCuxS (x ¼0.00, 0.03, 0.05 and 0.10) nanoparticles is in the range of 2–3 nm calcined at 100 1C. 3.2. SEM and TEM studies Fig. 2 shows SEM image of Cu doped ZnS agglomerated particles calcined at 100 1C. The size of agglomerates is a broad distribution of the order of 1–2 mm, which was mainly assembled by nanoparticles. The SEM image shows the agglomerates of particles and not the crystallite size. It was not possible by SEM image to calculate the crystallite size due to the resolution limit. The more precise size distribution of nanocrystallites was performed by TEM. Fig. 3(a–d) shows HRTEM images of ZnS calcined at 100 1C. It in general the microstructure of nanoparticles was noted as ultra-fine particles with an average size of about 5–6 nm (Fig. 3). However the particles of size about 2 nm minimum and 8 nm maximum are also seen in the micrograph (Fig. 3(a)). At high

Fig. 2. SEM image of 5 mol% Cu doped ZnS nanoparticles calcined at 100 1C.

magnifications the size of the nanoparticles with clear faceting was seen (Fig. 3(b–d)). It was observed that within individual particles, there were well oriented planes revealing the single crystalline nature of individual particles. In Fig. 3(b), a single particle of size about 5.6 nm stacked with 111 planes of cubic crystal of ZnS with interplanar spacing of 0.308 nm has been displayed. In another micrograph (Fig. 3(c)) a single facetted particle of size about 6.2 nm consisted of 220 planes with interplanar spacing of 0.188 nm is shown. Fig. 3(d) exhibits a collection of few nanoparticles with shared boundaries having interplanar spacings of 0.308, 0.188 and 0.166 nm. Cu2+ doping has no observable influence on the particle size of ZnS. 3.3. Optical studies Fig. 4 shows UV–vis absorption spectra of Cu doped zinc sulfide (Zn1  xCuxS where, x¼0.00 and 0.05) samples calcined at 100 1C. Optical absorption spectra show weak absorption peak at around 280 nm, which is fairly blue-shifted from the absorption edge of the bulk ZnS (345 nm). ZnS has good absorption for light in the wavelength of 220–350 nm. The absorption edge shifted towards the shorter wavelength side in 5 mol% Cu doped ZnS nanoparticles. Manifacier model is used to determine the absorption coefficient from the absorbance data [19]. The fundamental absorption, which corresponds to the transmission from valance band to conduction band, is employed to determine the band gap of the material. The direct band gap energy can be estimated from a plot of (αhν)2 vs. photon energy (hν). The energy band gap was determined by using the relationship αhν ¼ AðhνEg Þn ðαhνÞ2 ¼ AðhνEg Þ

Table 1 Crystallite size and lattice parameter of undoped and 3, 5 and 10 mol% Cu doped ZnS nanoparticles calcined at 100 1C. % of doping Average crystallite size for Lattice of Cu sample calcined at 100 1C (nm) parameter a (Å) 0 3 5 10

3 2 3 3

5.35 5.34 5.35 5.34

where hν ¼photon energy, α ¼absorption coefficient (α ¼4πk/λ; k is the absorption index or absorbance, λ is the wavelength in nanometer), Eg ¼energy band gap, A¼ constant, n ¼1/2 for the allowed direct band gap. The exponent n depends on the type of transition and it may have values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively [20]. The value of direct band gap was determined by extrapolating the straight line portion of (αhν)2 vs. hν graph to the hν axis; as shown in Fig. 5.

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0.308 nm (hkl:111)

5.6 nm 50 nm

0.308 nm (hkl:111) 0.188 nm (hkl:220)

0.188 nm (hkl:220)

6.2 nm

0.161 nm (hkl:311)

Fig. 3. (a–d) HRTEM images of ZnS nanoparticles calcined at 100 1C. (111), (220) and (311) lattice fringes of denoted area (d111 ¼ 0.308 nm, d220 ¼0.188 nm and d311 ¼0.166 nm).

Fig. 5. (αhν)2 vs. photon energy (hν) for undoped and 5 mol% Cu doped ZnS nanoparticles calcined at 100 1C. Fig. 4. Optical absorption spectra of undoped and 5 mol% Cu doped ZnS nanoparticles calcined at 100 1C.

The direct band gap increases from 3.6 eV to 3.8 eV with 5 mol% Cu doped samples calcined at 100 1C. 3.4. Photoluminescence studies Fig. 6 shows that the PL spectra of undoped as well as 3, 5 and 10 mol% Cu doped ZnS calcined at 100 1C with excitation wavelength of 310 nm. The PL spectrum of undoped as well as 3, 5 and 10 mol% Cu doped ZnS nanoparticles show emission peak at 423 nm. The PL peak at 423 nm (2.93 eV) is a well known peak due to the radiative transition of electrons from shallow trap states (ST) near the conduction band to sulfur vacancies (VS) residing

near the valence band [21]. Excitonic PL intensity of 3, 5 and 10 mol% Cu doped ZnS is less than undoped ZnS. The lower PL intensity indicated reduces of recombination rate of electrons and holes. Detailed mechanism of various processes involved in Zn1  xCuxS nanocrystals upon excitation is shown in Fig. 7. Usually, the luminescence mechanism of Cu2+ doped ZnS is described as follows: Cu2+ ions formed deep trap energy levels between valence band (VB) and conduction band (CB) of ZnS. By absorbance of external energy, electrons are excited from VB to CB and relax to shallow defect levels formed by impurity ions. The activated electrons will recombine with holes left in the CB or the holes transferred to Cu2+ trap energy levels by radiative transitions which give out photons. Acting as electron and or hole traps is the most important function of dopant (Cu). Addition of Cu2+ ions

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1.1 1.0

Absorbance (a.u.)

0.9

Photocatalysis with ZnS under UV light 0 min 15 min

30 min

0.8

60 min

0.7

120 min

0.6

240 min

0.5

90 min

180 min

300 min

0.4 0.3

66 % reduction (300 min)

0.2

600

700

Wavelength (nm) Fig. 6. PL spectra of undoped and 3, 5 and 10 mol% Cu doped ZnS nanoparticles at 100 1C.

0.9 Photocatalysis with Cu doped ZnS under UV light

0.8

Absorbance (a.u.)

0 min

0.7

15 min

0.6

30 min 60 min 90 min

0.5 0.4 0.3

Fig. 7. Process involved in Zn1  xCuxS nanocrystals upon excitation. e  , electron; h+, hole; hv, photon energy; CB, conduction band; ST, shallow trap; VB, valence band; and Vs, sulfur vacancy.

120 min 180 min 240 min 300 min 53 % reduction (300 min)

0.2 0.1

550

600

650

700

750

Wavelength (nm)

lengthens the lifetime of charge carriers, which results the decrease of recombination rate of electron–hole pairs [22–25]. Various charge carrier recombination and charge carrier trapping processes are also shown in Fig. 7. 3.5. Photocatalytic response of ZnS and Cu doped ZnS nanoparticles To get the response of photocatalytic activities of undoped and 3, 5 and 10 mol% Cu doped ZnS the absorption spectra of exposed samples at various time intervals were recorded and the rate of decolorization was observed in terms of change in intensity at λmax at 665 nm of the dye. MB was used as a test contaminant since it has been extensively used as an indicator for the photocatalytic activities owing to its absorption peaks in the visible range [26,27]. Methylene blue shows most intense absorption peak at 665 nm. The percentage of decolorization efficiency of samples has been calculated as Efficiency (%)¼100  [(Ao–A)/Ao] ¼100  [(Co–C)/Co] where Ao, A, Co and C are initial absorbance, absorbance after irradiation at various time intervals, initial concentration of solutions and concentration of dyes after irradiation at various time interval, respectively. Pure methylene blue dissolved in water shows small degradation in 300 min when irradiated with ultraviolet and visible light. This smaller degradation of MB with OH  radical originated from water [28]. Firstly, photodegradation of methylene blue (MB) was performed with ZnS and 3, 5 and 10 mol % Cu doped ZnS calcined at 100 1C by irradiating mixture of photocatalyst and MB with UV light. Fig. 8(a) and (b) shows the

Fig. 8. Time-dependent UV–vis absorption spectra for decolorization of methylene blue using (a) undoped and (b) 3 mol% Cu doped ZnS under ultraviolet light.

time-dependent UV–vis absorption spectra of methylene blue during photoirradiation with undoped and 3 mol% Cu doped ZnS under ultraviolet light. It was observed that the undoped ZnS decolorizes methylene blue faster than Cu doped ZnS. It was also observed that undoped ZnS degraded about 66% of methylene blue in 300 min while 3 mol% Cu doped ZnS degraded about 53% of methylene blue within 300 min, respectively under UV light. Secondly, photodegradation of methylene blue (MB) was carried out with ZnS and 3, 5 and 10 mol% Cu doped ZnS calcined at 100 1C by irradiating mixture of photocatalyst and MB with visible light. Fig. 9(a) and (b) shows the time dependent UV–vis absorption spectra of methylene blue during photoirradiation under visible light in the presence of undoped and 3 mol% Cu doped ZnS nanoparticles, respectively. It was observed that undoped and 3 mol% Cu doped ZnS degraded 100% MB under visible light irradiation within 240 and 180 min, respectively. Figs. 10 and 11 show the photodegradation rate of MB under UV and visible light in the presence of undoped and 3, 5 and 10 mol% Cu doped ZnS (inset shows its ln Co/C vs. time graph). The photocatalytic degradation of methylene blue was observed to follow the first-order decay kinetics, ln (Co/C) ¼kt or ln (Ao/A)¼ kt, where Ao, A, Co and C are initial absorbance, absorbance after irradiation at various time interval (t), initial concentration of solution and concentration of dye after irradiation at various time interval (t), respectively [29]. Its ln (Co/C) plot shows a linear

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11

1.8 Photocatalysis with ZnS under visible light 0 min

1.4

1.2

1.2

1.0

30 min 60 min

0.8

______ ZnS ______ 3 mol% Cu doped ZnS ______ 5 mol% Cu doped ZnS ______ 10 mol% Cu doped ZnS

1.6

1.4

C/CO

Absorbance (a.u.)

1.6

1.0 0.8

90 min

180 min

0.4 0.2

0.6

120 min

0.6

0.4

240 min

0.2

100 % reduction (300 min)

500

550

600

650

700

750

0.0 0

Wavelength (nm)

50

100

150

200

250

Time (min) 1.2

Photocatalysis with Cu doped ZnS under visible light

1.0

30 min

Fig. 11. Photodegradation of methylene blue (MB) under visible light irradiation in the presence of ZnS and 3, 5 and 10 mol% Cu doped ZnS samples (inset shows its ln Co/C vs. time graph).

Absorbance (a.u.)

0 min

60 min

0.8 90 min

0.6

120 min

0.4 0.2

180 min

100 % reduction (300 min)

500

550

600

650

700

750

Wavelength (nm) Fig. 9. Time-dependent UV–vis absorption spectra for decolorization of methylene blue using (a) undoped and (b) 3 mol% Cu doped ZnS under visible light.

1.4

______ ZnS ______ 3 mol% Cu doped ZnS ______ 5 mol% Cu doped ZnS ______ 10 mol% Cu doped ZnS

1.3 1.2 1.1

C/Co

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0

50

100

150

200

250

300

Time (min) Fig. 10. Photodegradation of methylene blue (MB) under ultraviolet light irradiation in the presence of ZnS and 3, 5 and 10 mol% Cu doped ZnS samples (inset shows its ln Co/C vs. time graph).

relationship with the irradiation time. The calculated rate constant (k) for ZnS was 0.00302 min  1 and rate constants for 3, 5 and 10 mol% Cu doped ZnS were 0.00238, 0.00231 and 0.00226 min  1, respectively. It is clear that the doping of Cu ion in ZnS decreases

the photodegradation of the MB solution under the UV light. Cu doped ZnS shows lower photodegradation efficiency of MB as compared to the ZnS under UV light exposure due to the faster recombination of electron–hole pair as a result of the absorption characteristics caused by the Cu2+ doping [30]. The calculated rate constant for ZnS was 0.0121 and rate constants for 3, 5 and 10 mol % Cu doped ZnS were 0.0145, 0.0135 and 0.0126 min  1, respectively. This increase rate constant for ZnS may be due to additional light absorption above 400 nm by the ZnS particles or by an enhanced electron transfer from MB to the conduction band of ZnS. It is clear that the doping of Cu ion in ZnS increases the photodegradation rate of the MB solution under the visible light. It has been shown that the photocatalytic activity of Cu doped ZnS is strongly dependent on the dopant concentration. Optimal Cu concentration was 3 mol% as the prepared samples are only cubic phase. At 3 mol% concentration of Cu; ions serve as shallow trapping sites for the charge carriers and increases the photocatalytic efficiency by separating the arrival time of electron and hole at the surface. When the doping concentration becomes high (5 and 10 mol%; cubic and hexagonal phase), the possibility of charge trapping is high, and as such, the charge carriers may recombine through quantum tunneling. At higher concentration of Cu2+ ions, a great many crystal defects could be induced which may serve as recombination centers to reduce the photoactivity. Thus, increasing the amount of Cu (x ¼0.05 and 0.10) causes a phase transition from cubic blende to hexagonal phase ZnS and photocatalytic activity decreases due to high charge trapping and many crystal defects. It is also evident that the doping of Cu in ZnS enhances the photocatalytic activity of ZnS and hence Cu doped ZnS is more capable of degrading MB with the visible light irradiation. The addition of Cu metal on the ZnS photocatalyst surface can also enhance the photocatalytic degradation activity due to lower crystal size, higher surface area, higher efficiency for the electron–hole regeneration and charge trapping.

3.6. Mechanism of photocatalytic activity The complexities of the role of transition metal dopant ion are that it can participate in all of these processes. Acting as electron and/or hole traps is the most important function of dopant. The trap of charge carriers can decrease the recombination rate of electron–hole pairs and consequently, increase the lifetime of

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charge carriers. The process of charge trapping is as follows: ZnS þ nþ

M

hν-ZnSðe CB 

þ e -M

ðn1Þþ

þ

þ hVB Þ

ðReductionÞ

Mðn1Þþ þ O2 -Mðn1þ1Þþ þ O 2

Acknowledgments Authors are thankful to all for the technical support in getting SEM and HRTEM. We are also thankful to the Director NIT, Kurukshetra for providing the XRD and UV–vis facilities in physics department.

þ

Mnþ þ h -Mðnþ1Þþ ðOxidationÞ Mðnþ1Þþ þ OH -Mðnþ11Þþ þ OH where M is the transition metal (Cu) ion dopant and n is the valency of dopant (Cu) ion. The energy level of Mn+/M(n  1)+ lies below the conduction band edge and the energy level of Mn+/ M(n+1)+ lies above the valence band edge. Thus, the energy level of Cu metal ion affects the trapping efficiency. The trapping of electrons makes it easy for holes to transfer onto the surface of semiconductor and react with OH  in the MB solution and form active OH hydroxyl radicals to participate the destruction of MB. Hydroxyl radicals (OH) have been deemed to be major active species during the photocatalytic oxidation reaction [31–34].

4. Conclusions Undoped and Cu doped ZnS nanoparticles (Zn1  xCuxS where x ¼0.00, 0.03, 0.05 and 0.10) were successfully synthesized using the chemical precipitation method. XRD study revealed that all the samples were composed of mainly cubic phase. At higher concentration of Cu (x ¼0.05 and 0.10) small traces of hexagonal phase are formed. HRTEM and XRD studies show that the size of crystallites is about 2–10 nm. Absorption edge shifted towards the shorter wavelength side in 5 mol% Cu doped ZnS nanoparticles. The band gap values of the as prepared Cu doped ZnS samples were found to increase as compared to undoped ZnS. The ZnS capped with PEG restricted the agglomeration of the particles. Optimal Cu concentration was 3 mol% as the prepared samples are only cubic phase. At 3 mol% concentration of Cu ions serve as shallow trapping sites for the charge carriers and increases the photocatalytic efficiency by separating the arrival time of electron and hole at the surface. At higher concentrations of Cu cubic and small traces of hexagonal phase are formed which decrease the photocatalytic activity of Zn1  xCuxS nanocrystals as the recombination of trapped carrier dominates the interfacial charge transfer. Cu doped ZnS exhibited excellent photocatalytic activity for the photodegradation of MB under visible light as compared to undoped ZnS.

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