Photoluminescence and photocatalytic studies of metal ions (Mn and Ni) doped ZnS nanoparticles

Optical Materials 47 (2015) 7–17

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Photoluminescence and photocatalytic studies of metal ions (Mn and Ni) doped ZnS nanoparticles Jagdeep Kaur a, Manoj Sharma b, O.P. Pandey a,⇑ a b

School of Physics and Materials Science, Thapar University, Patiala 147004, Punjab, India Department of Nanotechnology, Sri Guru Granth Sahib World University, Fatehgarh Sahib 140406, Punjab, India

a r t i c l e

i n f o

Article history: Received 15 January 2015 Received in revised form 8 June 2015 Accepted 8 June 2015

Keywords: ZnS Doping Capping Photocatalysis

a b s t r a c t The present study deals with the structural, optical and photocatalytic studies of thioglycerol capped doped ZnS nanoparticles (NPs). Effect of two dopant metal ions (Ni and Mn) on photoluminescence emission and photocatalytic properties have been studied in detail. Zn1xMxS; M = Ni or Mn; x = 0, 0.01, 0.02, 0.03 and 0.04 NPs have been synthesized using simple chemical precipitation route. Structural and morphological studies have been done by using X-ray diffraction (XRD) technique and high resolution transmission electron microscopy (HRTEM). Capping of thioglycerol on the surface of doped ZnS has been confirmed by Fourier transform infrared (FTIR) studies. UV–Vis and photoluminescence studies have been carried out to study the effect of doping on optical properties of synthesized materials. Degradation of crystal violet has been carried out with the aim to investigate the effect of Ni or Mn doping on photocatalytic activity of ZnS. It has been observed that both the metal ions have decreased the photocatalytic activity of ZnS. The effect of photocatalytic reaction temperature on photocatalytic properties of one of the doped sample has also been investigated. It has been interpreted from the results that photocatalytic activity of doped semiconductor nanostructures is strongly dependant on their photoluminescence properties as well as on photocatalytic reaction temperature. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years, semiconductor (SC) nanostructures (NSs), especially II–VI SCs have attracted great interest due to their unique structural, optical and electronic properties which arise due to their large surface to volume ratio (S/V) and quantum confinement effect [1]. Among the most extensively studied II–VI SCs, ZnS is a well known multifunctional material with a wide band gap of 3.54 eV (cubic zinc blende phase). This material has been proved as a better photocatalyst due to rapid generation of electron–hole pairs with photoexcitation as it is a direct wide band gap SC material [2]. Further, it possesses high negative reduction potential of excited electrons due to its higher conduction band position in aqueous solution as compared to other extensively studied photocatalysts [3]. Also, this material show good photocatalytic activity due to trapped holes arising from surface defects on the sulphides [4]. It has also been utilized in UV detectors due to its high resistivity at ambient conditions and fast switching time upon UV light illumination, thus exhibiting the highest potential for a UV-light detector [5]. An important subset of semiconductor NSs are those ⇑ Corresponding author. E-mail address: [email protected] (O.P. Pandey). http://dx.doi.org/10.1016/j.optmat.2015.06.022 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

doped with a small percentage of dopants to alter their structural, magnetic, electronic and optical properties for various desired applications. Owing to its large band gap, ZnS can easily host different transition metal ions acting as luminescent centres [6]. A number of transition metal ions such as Mn, Cu, Ag, and Eu have been successfully doped into ZnS to obtain desired structural and optical properties for their potential application in related areas [1]. Dopant ions offer a way to trap charge carriers which result in better photocatalytic activity of the material [7]. However, in some publications, it is reported that some dopant ions enhance charge carrier recombination resulting in decreased photoactivity of the host material [8,9]. Porambo et al. [10] have reported that doping of ZnS nanocrystals with Mn leads to an initial increase in the apparent rate constants in case of degradation of 2-chlorophenol and then decreased at higher dopant concentrations. Mohamed [7], in his work, investigated the effect of Cu content on photocatalytic activity of ZnS thin films. Photocatalytic degradation of methylene blue was recorded maximum in case of Cu content of 3%. Ullah and Dutta [8] have investigated the role of Mn on photocatalytic activity of doped ZnO NPs. Photocatalytic activity of Mn doped ZnO nanoparticles (NPs) has been observed to decrease as compared to that of undoped ZnO NPs under UV irradiation. Whereas, under visible irradiation, Mn doped ZnO NPs have shown

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J. Kaur et al. / Optical Materials 47 (2015) 7–17

enhanced photocatalytic activity as compared to that of undoped ZnO NPs. Recently, Ashkarran [9] has observed decrease in photocatalytic degradation of three dyes (Rhodamine B, Bromocresol Green and Bromochlorophenol Blue) using Mn doped ZnS NPs as compared to that of undoped ZnS. From the above discussion, it is clear that dopant ions in SC NPs not always act as a trap for charge carriers (which ultimately enhances their photocatalytic activity), sometimes; these can enhance charge carrier recombination resulting in decreased photocatalytic activity. Although, photocatalytic studies of transition metal ion doped ZnS NPs have been carried out by various research groups, but, research related to investigate the effect of Mn and Ni doping on structural, optical and photocatalytic studies of ZnS NPs is still in its early stage and need more understanding. Also, it has been reported that there is a strong correlation between photoluminescence and photocatalytic properties of a material [11]. Hence, in the present work, thioglycerol capped ZnS NPs doped with Mn and Ni have been synthesized with the aim to study the effect of dopant ions on photoluminescence and photocatalytic properties of ZnS.

operating voltage was kept at 20 kV and tungsten filament is the source. Morphological study of the samples was done by high resolution transmission electron microscope (HRTEM) JEOL 2100F having operating voltage of 200 kV. FTIR spectrum was recorded using Bruker Alpha-T. Optical absorption spectra of the samples were recorded with a double beam UV–Visible spectrophotometer (Hitachi, Model-U-3900H). Photoluminescence (PL) emission spectra of samples has been recorded with Edinburgh Instruments FS920 spectrometer equipped with 450 W Xenon Arc Lamp and a cooled single photon counting photomultiplier (Hamamatsu R2658P). Photocatalytic experiments were carried out in dark using self designed photoreactor at room temperature [13].

2.4. Apparatus for photocatalytic study The photocatalytic degradation experiment was conducted in dark at room temperature using self designed photochemical reactor. Details of the photochemical reactor are given in our previous publication [13]. A known amount of catalyst (0.5 g/L) was mixed in 50 mL of dye solution (1 mg/L) for photocatalytic experiments.

2. Experimental 2.1. Materials and reagents Zinc acetate [Zn(CH3COO)2], nickel acetate [Ni(CH3COO)2], manganese acetate [Mn(CH3COO)2], sodium sulphide nonahydrate [Na2S9H2O], 1-thioglycerol [C3H8O2S] and crystal violet [C25N3H30Cl] were purchased from Sigma Aldrich and were of high purity (99.99%). All chemicals used in this work were of analytical grade and were used as such without further purification. Ultrapure water was used in the entire synthesis. 2.2. Synthesis ZnS NPs doped with 0, 1, 2, 3 and 4 wt.% of Mn and Ni (Zn1xMxS; M = Mn or Ni; x = 0, 0.01, 0.02, 0.03 and 0.04) were synthesized by simple chemical precipitation method as reported earlier with some modifications [12,13]. Solution of Zn(ac)2 and M(ac)2 was prepared in ultrapure water and were stirred for 30 min separately. After 30 min., solution of M(ac)2 was poured dropwise into the stirred solution of Zn(ac)2. Solution of Na2S was also prepared separately and was stirred for 30 min. To this solution, stirred solution of thioglycerol was added dropwise. After 30 min. of stirring, solution of Na2S was added into the above stirred solution. The reaction mixture was allowed to stir for 3 h at 80 °C followed by overnight ageing. The resulting solution was centrifuged to remove the unreacted species and excess capping agent. The washed precipitates were then dried at 80 °C for 24 h. and were crushed to obtain fine powder. For comparative study, undoped ZnS was also prepared using the above mentioned procedure. For the purpose of simplification, Zn1xNixS; x = 0.01, 0.02, 0.03 and 0.04 have been coded as N1, N2, N3 and N4 respectively. Similarly, Zn1xMnxS; x = 0.01, 0.02, 0.03 and 0.04 have been coded as M1, M2, M3 and M4 respectively unless otherwise specified. Undoped ZnS has been coded as NM0.

3. Results and discussion 3.1. XRD studies Fig. 1(a) and (b) shows the XRD patterns of ZnS doped with Ni and Mn ions respectively. As can be seen in the figure, all samples exhibit cubic zinc blende phase with no impurity phase indicating that Ni and Mn ions are successfully doped in the host lattice. However, the presence of Ni and Mn in prepared samples has been confirmed through Energy Dispersive X-ray (EDS) spectroscopy analysis. For instance, EDS patterns for one of the Ni and Mn doped ZnS are shown in Fig. 2(a) and (b) respectively. Diffraction peaks from (1 1 1), (2 2 0) and (3 1 1) planes match well with those of the b-ZnS (cubic) reported in the ICDD Powder Diffraction File No. 80-0020. In all the samples, broadening of diffraction peaks indicate nanosize formation of ZnS. It is to be noted that in Fig. 1(a) and (b), the XRD peaks of doped ZnS NPs became weaker and broader as compared to that of undoped ZnS. This suggests that the crystallinity of doped ZnS NPs is deteriorated with the increase in doping content in the source material. Decreased crystallinity of doped samples as compared to that of undoped ZnS indicates the increase in disorder due to incorporation of impurity ions. To investigate the effect of doping on ZnS host lattice, the most intense diffraction peak corresponding to (1 1 1) plane was selected (Fig. 3(a) and (b)). Larger line broadening in case of doped samples indicates their smaller particle size as compared to the undoped ZnS sample. A careful comparison of diffraction peak in the range 2h = 22–38° for all samples indicates that this peak is slightly shifted towards higher angle which indicates that Ni is successfully substituted for Zn in the host lattice. In case of Mn doped ZnS, no appreciable peak shift has been observed in any sample which may be attributed to the very small difference in ionic radius of Mn2+ (0.66) and Zn2+ (0.6). Crystallite size of NPs was calculated by following Scherrer’s equation

2.3. Characterization techniques The as prepared samples were characterized by X-ray diffraction (XRD) technique using Panalytical’s X’Pert Pro diffractometer with Cu Ka radiation. Elemental analysis of as prepared samples was done using scanning electron microscope (SEM; JEOL, JSM-6510LV) equipped with energy dispersive X-ray spectrometer (EDS; Oxford INCA) having Si(Li) detector. For measurement,

t¼

kk b cos h

ð1Þ

where k = 0.9, t is the crystallite size (Å), k(Å), the wavelength of Cu Ka radiation and b is the corrected half width of the diffraction peak [14]. Crystallite size for Ni and Mn doped ZnS came out in the range of 1.98–2.19 nm and 1.9–2.0 nm respectively.

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J. Kaur et al. / Optical Materials 47 (2015) 7–17

20

30

40

50

60

(a)

20

70 N4

30

40

50

60

(b)

M4

M3

N1

NM0

(111) (220)

20

Counts (arb. units)

Counts (arb. units)

N3

N2

30

40

50

M2

M1

NM0

(111) (220)

(311)

60

70

70

20

30

40

50

(311)

60

70

2θ (degree)

2θ (degree)

Fig. 1. XRD patterns of (a) Ni doped ZnS NPs and (b) Mn doped ZnS NPs in the range 20–70°.

Fig. 2. EDS patterns of (a) N4 and (b) M4.

then allowed to dry to get finally dispersed powder. Fig. 4(a) and (b) shows the TEM image of N2. In Fig. 4(a), nearly spherical particle of ZnS is clearly visible. Amorphous layer of organic capping agent is also visible in the image. This confirms the effective capping of thioglycerol on the surface of ZnS. Fig. 4(b) shows high resolution TEM (HRTEM) micrograph of the same sample. In this micrograph, lattice fringing is clearly observed which indicates the well defined crystal structure of ZnS. The value of d spacing has come out to be 0.32 nm which corresponds to the (1 1 1) lattice plane spacing of ZnS in cubic phase. Fig. 4(c) shows SAED pattern of the same sample. The value of d spacing calculated from this spectra came out to be 0.31, 0.19 and 0.16 nm which corresponds to (1 1 1), (2 2 0) and (3 1 1) lattice plane of cubic ZnS. This further confirms the cubic phase of ZnS. Fig. 5(a) shows TEM micrograph of M1. As observed in Fig. 5(a), the particles of doped ZnS are irregularly spherical. The NPs have grown due to overnight ageing and also the sample was prepared at 80 °C to make the doping efficient. Similar to Ni doped ZnS, amorphous layer corresponding to thioglycerol is also clearly seen around the NPs surface which confirms that effective capping is obtained on NPs surface. From the high-resolution TEM (HRTEM) image in Fig. 5(b), the lattice fringes can be clearly observed, suggesting the well-defined crystal structure. The fringe with lattice spacing of ca. 0.31 nm corresponds to the (1 1 1) plane of the cubic ZnS which is in good agreement with the XRD pattern. SAED pattern of the same sample has been shown in Fig. 5(c). The value of d spacing calculated from this spectra came out to be 0.31, 0.19 and 0.16 nm which corresponds to (1 1 1), (2 2 0) and (3 1 1) plane of cubic ZnS. These values agree well with American Society for Testing and Materials (ASTM) card no. 77–2100 of cubic structure of ZnS. Hence cubic phase of ZnS has also been confirmed from this spectra.

3.2. TEM analysis 3.3. FTIR analysis For TEM study, the as prepared powder of ZnS NPs was dispersed in ethanol and ultrasonicated for 10 min. One drop of this solution was dropped on carbon coated Cu grid. The solution was

Fig. 6 represents FTIR spectra of Ni and Mn doped ZnS. For comparative studies, FTIR spectra of undoped ZnS is also shown in the

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J. Kaur et al. / Optical Materials 47 (2015) 7–17

24

26

28

30

32

(a)

24

34 N4

26

28

30

32

(b)

M4

N1

M3

Counts (arb. units)

Counts (arb. units)

N3

N2

M2

M1

NM0

NM0

24

26

28

30

32

34

34

24

26

28

30

32

34

2θ (degree)

2θ (degree)

Fig. 3. XRD patterns of (a) Ni doped ZnS and (b) Mn doped ZnS in the range 22–38°.

same figure. As can be seen in figure, both samples exhibit IR spectra similar to that of undoped ZnS. Therefore, we can say that doping has no profound effect on IR response of as prepared samples. The peak positions for all samples along with peak assignments are given in Table 1. Pure thioglycerol exhibits S–H vibration band at 2557 cm1. This peak was not observed in any sample. It is attributed to the fact that the thiolate functions of the thioglycerol ligands are connected to the Zn2+ sites on the ZnS nanocrystals surfaces which confirm the capping of thioglycerol on ZnS surface [15]. 3.4. UV–Vis studies To investigate the effect of Ni doping on optical response of ZnS, UV–Vis studies are performed for all samples in the range of 250– 500 nm. The relation between reflectance R and absorption coefficient a as given by Kubelka–Munk method [20] is

FðRÞ ¼

ð 1  RÞ 2 a ¼ 2R S

ð2Þ

where F(R) is the Kubelka–Munk function, S is the scattering coefficient. From the above equation, F(R) can be assumed to be proportional to a [20]. Absorption spectra for all samples have been plotted by employing the above equation. As discussed in our recent publication [13], band gap for undoped ZnS came out to be 3.85 eV which is quite higher as compared to its bulk counterpart (3.54 eV). Fig. 7(a) and (b) shows optical response of Ni and Mn doped ZnS respectively. It is clearly seen in Fig. 7(a) that absorbance of Ni and Mn doped ZnS is significantly increased in near visible region as compared to that of undoped ZnS. The relation between the incident photon energy (ht) and the absorption coefficient (a) or F(R) is given by [20]

½FðRÞht

1=n

¼ Aðht  Eg Þ

ð3Þ

where A is a constant and Eg is the band gap of the material and the exponent n depends on the type of the transition i.e. 2, 3, 1/2, 1/3

values corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions. As shown in Fig. 8(a) and (b), square of the absorbed energy (F(R)ht) were plotted against photon energy (ht) to determine the energy for direct gap transition. Band gap energies for all samples were determined by extrapolating the straight portion of the graph on ht axis at F(R) = 0. From Fig. 8(a), it is observed that band gap of Ni doped ZnS has decreased from N1 to N3 whereas for N4, it has increased slightly. The observed red shift in the absorption band edge with nickel doping in ZnS may be due to the sp–d exchange interactions between the band electrons and the localized d-electrons of the Ni2+ ions. In case of N4, band gap is larger than N3, which may be explained by the Burstein–Moss shift [21,22]. According to the Burstein–Moss shift, at high doping content, the Fermi level shifts into the conduction band. This results in the absorption transition from valence band to the Fermi level in the conduction band, instead of from the top of the valence band to the bottom of the conduction band due to the donor electron filling of the conduction band [22]. Hence, the changes of transition levels lead to the energy gap broadening and resulted in increased band gap for N4. In their work, Sabri et al. [23] have observed the initial red shift of absorption wavelength at lower Mn content in Mn doped ZnO NPs whereas the blue shift has been observed at higher Mn content. They have also attributed this phenomenon to the Burstein-Moss shift. In case of Mn doped ZnS, it is observed that band gap of Zn1xMnxS decreased from M1 to M4. The band gap shrinkage in doped samples is quite obvious due to the introduction of impurity levels in the host lattice and sp–d exchange interactions between the band electrons and the localized d-electrons of the Mn2+ ions. Variation in band gap with Ni and Mn doping has been shown in Fig. 9. Band gap values along with corresponding absorption wavelengths are given in Table 2. 3.5. Photoluminescence studies To investigate the effect of doping on emission characteristics of ZnS, PL spectra of all the samples have been recorded. Fig. 10

J. Kaur et al. / Optical Materials 47 (2015) 7–17

11

recombination. Photoexcited electrons are preferentially transferred to nickel ion induced trapping centres as compared to anion vacancy defect centres. Hence, in the present case, similar to the results observed by Borse et al. [26], photoexcited electrons are preferentially transferred to nickel ion induced trapping centres as compared to anion vacancy defect centres which acted as non-radiative recombination centres thereby reducing the emission intensity. At higher dopant concentrations (N3 and N4), red shift in emission intensity peak has been observed. This suggests that trap states have been created by Ni ion between the valance band and conduction band. Therefore, a least quantum of energy has been provided to the nearest neighbour atom or lattice whenever the electron makes transition from the conduction band to trap state. As a result, the emission energy has been reduced thereby shifting the emission spectrum towards longer wavelength. Murugadoss et al. [27] have observed red shift of 445 nm

Fig. 4. (a) TEM image, (b) HRTEM image and (c) SAED pattern of N2.

shows the emission spectra of Ni doped ZnS. It is clearly observed in Fig. 10 that as doping content in the host lattice is increased, emission intensity keeps on decreasing and is minimum in case of N4. In ZnS:Ni2+ semiconductor nanomaterials, the lowest multiplet term 3F of the free Ni2+ ion split into 3T1, 3T2 and 3A2 through the anisotropic hybridization. Due to the d–d optical transitions of Ni2+, the luminescent centre of Ni2+ is formed in ZnS [24]. In their work, Podlowski et al. [25] have reported that d–d transitions within Ni2+ ion in Ni2+ doped CdS and ZnS are essentially non radiative due to which emission intensity is decreased. In case of N3 and N4, overall emission intensity has sharply decreased and ZnS related emission has also suppressed. Borse et al. [26] have also observed luminescence quenching due to incorporation of Ni ions in ZnS lattice. According to them, Ni ions have acted as electron trapping centres which results in non-radiative

Fig. 5. (a) TEM image, (b) HRTEM image and (c) SAED pattern of M1.

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J. Kaur et al. / Optical Materials 47 (2015) 7–17

3500

3000

2500

2000

2500

2000

1500

1000

1500

1000

1.00

Transmittance (arb. units)

0.99

M3 1.00

0.99

N3 1.00

0.99

NM0 3500

3000

[29]. Contrary to this, the addition of Mn2+ ions to the outside of preformed ZnS results in ultraviolet emission [30]. Hence, in the present case, it is confirmed that Mn2+ is being substituted in the host lattice. This emission peak keeps on increasing with the increase in doping content. No luminescence quenching has been observed even at higher Mn dopant concentration which is in line with the previously reported results. As reported by Bol et al. [31], concentration quenching is caused by energy migration from donor to traps, which are present in a low concentration. Quenching requires efficient high donor concentrations for migration of energy via an array of donors to traps as migration is limited to very short distances. In case of Mn2+, the transition involved in the energy migration is spin forbidden 6A1–4T1 transition within the 3d5 configuration of the Mn2+ ion. Hence, non-radiative energy transfer between neighbouring Mn2+ ions via dipole-dipole interaction is not efficient. As this transition is forbidden and only transfer between nearest neighbour Mn2+ ions is expected hence, concentration of Mn2+ ions at which energy migration is expected to be observed is high. In case of M1 and M2, the peak position is at 607 nm. At higher dopant content i. e. in case of M3 and M4, this emission peak is slightly red shifted to 610 nm. Many research groups have reported this red shift in Mn2+ related emission in ZnS [32–35]. Cruz et al. [32] have attributed this red shift to decrease in the density of surface states with increasing particle size or by strong electron-phonon coupling in quantum dots. Hu and Zhang [33] have ascribed this shift due to hybridization of

-1

Wavenumber (cm ) 8

Fig. 6. FTIR spectra of thioglycerol capped undoped and doped ZnS.

N1 N2 N3 N4

(a) 7

Table 1 Peak assignment of various bonds in thioglycerol capped undoped and doped ZnS. Peak position (cm1)

Peak assignment

NM0

N3

M3

3265 2885 2810 2346 1642 1540 1416 1033 902

3254 2880 2807 2343 1652 1537 1415 1029 879

3217 2873 2802 2345 1675 1533 1421 1030 898

F(R) (arb. units)

6 5 4 3 2 1 0 250

300

350

400

450

500

Wavelength (nm)

4

F(R) (arb. units)

emission peak in ZnS with Ni doping. Luminescence quenching at higher concentrations of Ni ion has been attributed to increase in the radiationless transitions by the higher concentration of Ni ions. It is worth mentioning that source of Ni ions selected also affects luminescence spectra of Ni doped ZnS. Yang et al. [28] have reported a single peak (520 nm) using NiSO4 as a source of Ni ions. But Murugadoss and Kumar [27] have observed two peaks using Ni(CH3COO)2 as a source of Ni ions. In the present case, Ni(CH3COO)2 has been used as a source of Ni ion but no new peak has been observed with respect to undoped ZnS in luminescence spectra. Fig. 11(a) shows the photoluminescence emission spectra of Mn doped ZnS NPs in the range 350–800 nm. As can be seen in the figure, unlikely to undoped ZnS which exhibit emission spectra in blue region, all Mn doped samples exhibit sharp well defined emission peak in orange–red region of visible spectra. Simultaneously, intensity of emission peaks in blue region has also increased. When Mn2+ ions are substituted for host cation sites, the mixing between the s–p electrons of the host ZnS and the d electrons of Mn2+ occurs and makes the forbidden transition of 4T1–6A1 partially allowed, resulting in the characteristic emission of Mn2+

(b)

M1 M2 M3 M4

3

2

1 O–H stretching [15] C–H symmetrical stretching [15] C–H asymmetrical stretching [15] Interference from CO2 [16] O–H bending [15] COO [17] –CH2–S [18] C–C stretch [18] O2 stretching and bending [19]

0 250

300

350

400

450

500

Wavelength (nm) Fig. 7. Absorption spectra of (a) Ni doped ZnS, (b) Mn doped ZnS plotted by Kubelka Munk method.

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J. Kaur et al. / Optical Materials 47 (2015) 7–17

1000

N1 N2 N3 N4

(a)

[F(R)hυ]2

800 600 400 200 0 3.0

3.5

4.0

4.5

Band gap (eV) M1 M2 M3 M4

(b)

300

[F(R)hυ]2

250

3.6. Photocatalytic studies

200 150 100 50 0 3.5

3.0

4.0

4.5

Band gap (eV) Fig. 8. Tauc plots for (a) Ni doped ZnS and (b) Mn doped ZnS to determine band gap.

s–p states of ZnS and d states of Mn2+ ion. According to Koda et al. [34] when a metal ion occupies a certain position in a crystal, the crystal field strength affecting that particular ion increases. Therefore in case of Mn2+ doped ZnS, with increasing concentration of Mn2+ in ZnS, more Mn2+ ions enter into the ZnS host and the space containing the ions becomes smaller resulting in increased crystal field. Hence, the transition energy between the 4T1 and 6 A1 levels of Mn2+ decreases and it may cause a red shift in the PL spectra. Peng et al. [35] have suggested that the red shift of

Photocatalytic degradation of crystal violet has been done to investigate the effect of doping on photocatalytic activity of ZnS NPs. As a demonstration, spectral changes taking place in absorbance of dye during photochemical reaction catalysed by N1 and M1 are shown in Fig. 12(a) and (b) in time interval of 60 min. The characteristic absorption peak of this dye at 590 nm has been selected to monitor the photocatalytic degradation of the dye. By Beer Lambert law, the decrease in concentration of dye is recorded at different intervals of time to measure degradation rate. Fig. 13(a) and (b) shows the comparative degradation behaviour of dye catalysed with Ni and Mn doped ZnS respectively in terms of change in concentration with respect to the initial concentration. As can be seen in figure, all Ni doped samples show lower degradation as compared to that of undoped sample. N4 recorded the lowest degradation among all the doped samples. In case of Mn doped ZnS, at lower doping content i. e. up to M2, lower degradation of crystal violet as compared to that of undoped ZnS is observed. At higher doping concentrations i. e. in case of M3 and M4, photocatalytic degradation slightly increased but is still less than that of undoped ZnS. M2 recorded the lowest degradation among all the Mn doped samples. Since, in this study, concentration of solute is very low (1 mg/L), so Langmuir–Hinshelwood kinetics model [37] can be simplified to pseudo first order kinetic model equation.

 ln

Ct C0

 ¼ kT

ð4Þ

where Ct is the concentration of dye after irradiation in selected time interval t, C0 is the initial concentration of dye, k is the first order rate constant, and T is irradiation time. Fig. 14(a) and (b) shows plot between ln(Ct/C0) and irradiation time to determine the value of rate

3.84

Energy gap (eV)

the peak is an effect of doping. When Mn2+ enters the ZnS host lattice, it introduces lattice distortion (due to difference in ionic radii of both ions) that would influence the energy level structure of ZnS. In the region from 350 to 500 as shown in Fig. 11(b), there are emission peaks located at 380, 393 and 416 nm in case of M3 and M4. Emission peaks located at 380 and 393 nm are due to radiative recombination of interstitial sulphur. Emission peak observed at 416 nm in case of M3 and M4 is attributed to interstitial zinc [36]. Peng et al. [35] have reported the enhancement in both the blue emission and the orange emission with increase in Mn2+ concentration. They have explained that the Mn2+ emission grows without the expense of host related emission. But the enhancement in host related emission is negligible as compared to that of impurity ions. As no well-defined emission peak is observed in the blue region in case of M1 and M2, hence energy transfer between the ZnS host and the Mn2+ impurity in these particular samples is very efficient. Hence, these results show that the doped Mn2+ ions in ZnS are the major luminescent component and can drastically change emission properties of the host ZnS.

3.80 Table 2 Calculated band gap values and corresponding absorption wavelengths of undoped and doped ZnS.

3.76

3.72

Ni Mn

3.68 0

1

2

3

Doping content (%) Fig. 9. Variation in band gap of ZnS with Ni and Mn doping.

4

Sample

Band gap (eV)

Absorption wavelength (nm)

NM0 N1 N2 N3 N4 M1 M2 M3 M4

3.85 3.84 3.82 3.75 3.78 3.76 3.73 3.71 3.69

322.60 322.60 325.23 331.30 328.67 330.42 333.07 334.87 336.68

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J. Kaur et al. / Optical Materials 47 (2015) 7–17

21000

0.16

Absorbance (arb. units)

Intensity (arb. units)

Intensity (arb. units)

N3 N4

3000

28000

2000

1000

0

14000

400

500

600

Wavelength (nm)

N1 N2 N3 N4

7000

(a) N1

0.12 Dye

0.08

0 min. 60 min. 120 min.

0.04

180 min.

0.00

0 400

500

600

700

400

500

Wavelength (nm) Fig. 10. Photoluminescence emission spectra of Ni doped ZnS at kexc = 320 nm. Inset shows zoomed spectra of N3 and N4.

(a)

M1 M2 M3 M4

2000000

610 nm

1500000

0.16

Absorbance (arb. units)

Intensity (arb. units)

2500000

607 nm

1000000

0.12 Dye 0 min.

0.08

60 min. 120 min. 180 min.

0.04

0.00

500000

400

500

600

700

Wavelength (nm) 400

500

600

700

800

Wavelength (nm) M1 M2 M3 M4

(b) Intensity (arb. units)

700

(b) M1

0

393 nm

100000

380 nm

Fig. 12. Absorbance spectra of crystal violet degraded in the presence of (a) N1 and (b) M1.

where Ct is the concentration of dye after irradiation in selected time interval, C0 is the initial concentration of dye. As shown in Fig. 15(a) and (b), % degradation in presence of undoped ZnS is maximum (85.35%) among all the photocatalysts. Degradation percentage values for all samples have been summarized in Table 3. The order of photo degradation rate is obtained as

416 nm 50000

NM0 > N1 > N2 > N3 > N4 NM0 > M4 > M1 > M3 > M2

0 350

400

450

500

Wavelength (nm) Fig. 11. Photoluminescence emission spectra of Mn doped ZnS in the range (a) 350– 800 nm and (b) 350–500 nm.

constant k. In case of Ni doped ZnS, N4 has shown minimum value of rate constant (k = 0.0038 min1) whereas in case of Mn doped ZnS, M2 has recorded the lowest rate (k = 0.0063 min1). The degradation percentage has been calculated as

 %D ¼

600

Wavelength (nm)

1

 Ct  100 C0

ð5Þ

Decreased photocatalytic activity of Ni doped ZnS NPs may be attributed to the changed absorption characteristics of ZnS on doping with Ni. When semiconductor is photoexcited, electrons are promoted to conduction band leaving holes in valence band. These charge carriers may recombine (radiatively or non-radiatively) or may be trapped in centres created by dopant ions or intrinsic defects. These trapped charge carriers may also recombine or may be transported to the surface to participate in the reactions which are occurring on the surface of photocatalyst. In the present study, Ni ions have formed trap states within the energy gap of ZnS. The obtained results are in accordance with the PL results which are discussed in Section 3.5. Photoluminescence intensity has been quenched by Ni doping. There is a strong correlation between the photoluminescence and photocatalytic properties of an optically active material. As discussed in PL section, emission intensity decreases with the increase

15

J. Kaur et al. / Optical Materials 47 (2015) 7–17

NM0 N1 N2 N3 N4

1.0

Ct/C0

0.8 0.6 0.4 0.2

(a) 0.0 -50

0

50

100

150

200

Irradiation time (min.) NM0 M1 M2 M3 M4

1.0

Ct/C0

0.8 0.6

In photoluminescence, efforts are being made to make the radiative recombination dominant to achieve high quantum yield. In this way, photoluminescence and photocatalytic processes are different. When semiconductor is photoexcited, electrons are promoted to conduction band leaving holes in valence band. These charge carriers may recombine (radiatively or non radiatively) or may be trapped in centres created by dopant ions or intrinsic defects. These trapped charge carriers may also recombine or may be transported to the surface to participate in the reactions which are occurring on the surface of photocatalyst. As in case of M1 and M2, there is efficient energy transfer from the host ZnS to Mn2+ ion, hence no host related peak is observed. Due to the efficient transfer of energy resulting in radiative recombination, there are no charge carriers available which can participate in photocatalytic process. When ZnS is photoexcited, the electrons jump to conduction band and are quickly transported to the impurity states created by Mn2+ where due to hybridization of sp orbitals of ZnS and d orbital of Mn2+, spin forbidden transition takes place (discussed in Section 3.5) resulting in orange–red emission. The more efficient this transfer will be, the less charge carriers will be available for photocatalytic process. Hence photocatalytic activity is decreased in M1 and M2. However, at higher dopant concentrations (M3 and M4), ZnS related emission peaks have appeared indicating less efficient transfer from host to impurity ion. Hence some charge carriers will be available for the degradation of dye. That is why a slight increase in photocatalytic activity in presence of M3

0.4 0.2

(b) -50

0

50

100

150

-0.5

200

Irradiation time (min.) Fig. 13. Variation of Ct/C0 with irradiation time for (a) Ni doped ZnS and (b) Mn doped ZnS.

ln(Ct/C0)

0.0

(a)

0.0

-1.0

NM0 N1 N2 N3 N4

-1.5

-2.0 -50

0

50

100

150

200

Irradiation time (min.)

(b)

0.0

-0.5

ln (Ct/C0)

in dopant content which indicates that non-radiative recombination is taking place thereby preventing the charge carriers to participate in the photocatalytic process. Hence, we can say that in case of Ni doped ZnS, non-radiative e–h+ recombination dominates over interfacial charge transfer [38]. Kaneva et al. [38] have reported the deceleration of photocatalytic process in the presence of 1–15% Ni doped ZnO thin films both under UV and visible illumination. They have stated that Ni2+ may act like a p-type dopant into ZnO, working as a charge-carrier recombination centre thus enhancing the recombination of electron–hole pairs thereby decreasing the effective charge carrier concentration. The decreasing of effective charge carrier concentration has lowered the band bending on the crystallite surface, which has decreased the driving force acting on photogenerated electrons and holes. This decrease in driving force has reduced charge carrier separation thereby quenching the photocatalytic activity. Decreased photocatalytic activity of Mn doped ZnS NPs is explained by the photoluminescence results. It is generally observed in Mn2+ doped ZnS that spin forbidden transition related to Mn2+ states give rise to sharp intense peak in orange red region. This transition happens due to efficient transfer of energy from host ZnS to Mn2+ ion which implies that efficient radiative recombination has been resulted from this transition. However, in a photocatalytic process, maximum charge separation is needed to make the charge carriers available to participate in the processes which are taking place on the surface of semiconductor photocatalyst.

-1.0

NM0 M1 M2 M3 M4

-1.5

-2.0 -50

0

50

100

150

200

Irradiation time (min.) Fig. 14. Plot between ln(Ct/C0) and irradiation time to determine the value of rate constant k.

J. Kaur et al. / Optical Materials 47 (2015) 7–17

40

Irr

20

ad 1 8 0 ia 1 2 0 t io

n

tim

60 0

e

(m

in

(N1)

N2

N3

N4 0

(NM0)

(a) .)

60 40

Ir

0 ra 2 0 5 0 di 1 0 0 1 a

tio

n

tim

20

50

M1

0

e

-5 0

(m

in

M2

M3

(1-C /C )* t 0 100

80

0.16

M4 0

NM0

(b)

.)

Fig. 15. Degradation percentage of crystal violet in the presence of (a) Ni doped ZnS and (b) Mn doped ZnS at different irradiation times.

and M4 has been observed. Very few reports are available which demonstrate the effect of Mn2+ doping on photocatalytic activity of ZnS NPs. The presented results are somewhat similar to that observed by Ashkarran [9]. He has also reported the decrease in photocatalytic activity of ZnS with Mn doping. The model pollutant selected in his work is rhodimine B, bromocresol green and bromochlorophenol blue. In his results, photocatalytic activity is observed to be absent where photoluminescence is observed. These results indicate that although Mn2+ ion can improve the overall quantum efficiency of ZnS in visible region but can deteriorate its photocatalytic activity.

Table 3 First order rate constants for the photocatalytic degradation of crystal violet and corresponding percent degradation under UV–Vis illumination using as prepared photocatalysts. Photocatalyst

Adj. R2

k (min1)

(1  Ct/C0) ⁄ 100

NM0 N1 N2 N3 N4 M1 M2 M3 M4

0.89 0.74 0.72 0.70 0.69 0.99 0.99 0.99 0.98

0.0082 0.0053 0.0047 0.0042 0.0038 0.0078 0.0063 0.0075 0.0078

85.35 73.88 69.43 64.33 61.08 81.72 73.63 80.25 81.78

Absorbance (arb. units)

60

(1-C /C )*10 t 0 0

80

3.6.1. Effect of photocatalytic reaction temperature Effect of reaction temperature on photocatalytic reaction rate has also been investigated for one of the doped sample i.e. M3. Fig. 16(a) and (b) shows the absorbance spectra of crystal violet at room temperature which is for 25 °C and 60 °C respectively. Fig. 17 shows the comparative degradation behaviour of dye catalysed with M3 at 25 °C and 60 °C in terms of change in concentration with respect to the initial concentration. Inset of the figure shows plot between ln(Ct/C0) and irradiation time to determine the value of rate constant k. It has been observed that a small increment in photochemical reaction temperature has drastically enhanced the rate of photochemical reaction rate. The value of rate constant for photocatalytic reaction at 60 °C obtained from linear fit of the plot as given in the inset of Fig. 17 came out to be 0.0104 min1 which is greater than the rate constant calculated for photocatalytic reaction at room temperature (0.0075 min1). Although irradiation is believed to be the primary source of generation of e/h+ pairs at ambient temperature (because the band gap is too high to overcome by thermal excitation), but from the observed results, it is concluded that photocatalytic reaction temperature can enhance generation of e/h+ pairs thereby resulting in increased reaction rate. Further experiments based on the above study are being carried out in our lab.

(a) M3 Reaction temp.=R.T. (~25 °C)

0.12

Dye 0 min. 60 min. 120 min. 180 min.

0.08

0.04

0.00 400

500

600

700

Wavelength (nm) 0.16

Absorbance (arb. units)

16

(b) M3 Reaction temp. =60 °C

0.12

0.08

0.04

0.00 400

Dye 0 min. 60 min. 120 min. 180 min.

500

600

700

Wavelength (nm) Fig. 16. Absorbance spectra of crystal violet degraded in the presence of M3 at photocatalytic reaction temperature of (a) 25 °C and (b) 60 °C.

J. Kaur et al. / Optical Materials 47 (2015) 7–17

0.0

1.0 ln(C t/C0)

-0.4

0.8

-0.8 -1.2 -1.6

Ct/C0

-2.0

0.6 -50

25 °C 60 °C 0

50

100

150

200

Irradiation time (min.)

0.4 0.2

25 °C 60 °C

0.0 -50

0

50

100

150

200

Irradiation time (min.) Fig. 17. Variation of Ct/C0 with irradiation time. Inset shows the plot between ln(Ct/C0) and irradiation time to determine the value of rate constant k.

4. Conclusions ZnS NPs doped with Ni and Mn ions have been successfully prepared via simple chemical precipitation route. Incorporation of dopant ions has been confirmed from the results of XRD, EDS, UV–Vis and PL studies. Band gap of doped ZnS NPs has decreased as compared to that of undoped ZnS. Both the dopant ions have decreased the overall photocatalytic activity of ZnS. Decrease in photocatalytic activity towards crystal violet has been attributed to the non radiative and radiative recombination of charge carriers in case of Ni and Mn doped ZnS NPs respectively. Non availability of charge carriers on the reaction sites has decreased photochemical reaction rate in case of doped ZnS NPs. However, increase in photocatalytic reaction temperature has drastically enhanced the photochemical reaction rate. The present study concludes the negative role of Ni and Mn dopant ions in photocatalytic activity of ZnS. Acknowledgement Authors are thankful to DRDO – India for funding this work through grant no. ERIP/ER/0703659/M/01/1287 dated 03-02-2011. References [1] C. Corrado, Y. Jiang, F. Oba, M. Kozina, F. Bridges, J.Z. Zhang, Synthesis, structural, and optical properties of stable ZnS:Cu, Cl nanocrystals, J. Phys. Chem. A 113 (2009) 3830–3839. [2] D. Jiang, L. Cao, W. Liu, G. Su, H. Qu, Y. Sun, B. Dong, Synthesis and luminescence properties of core/shell ZnS:Mn/ZnO nanoparticles, Nanoscale Res. Lett. 4 (2009) 78–83. [3] X. Li, C. Huan, H. Liu, J. Xu, B. Wan, X. Wang, ZnS nanoparticles self-assembled from ultrafine particles and their highly photocatalytic activity, Physica E 43 (2011) 1071–1075. [4] S. Yanagida, Y. Ishimaru, Y. Miyake, T. Shiragami, C. Pac, K. Hashimoto, T. Sakata, Semiconductor photocatalysis-ZnS-catalyzed photoreduction of aldehydes and related derivatives: two-electron-transfer reduction and relationship with spectroscopic properties, J. Phys. Chem. C 93 (1989) 2516– 2582. [5] X. Fang, Y. Bando, M. Liao, T. Zhai, U.K. Gautam, L. Li, Y. Koide, D. Golberg, An efficient way to assemble ZnS nanobelts as ultraviolet-light sensors with enhanced photocurrent and stability, Adv. Funct. Mater. 20 (2010) 500–508. [6] J. Cao, J. Yang, Y. Zhang, L. Yang, Y. Wang, M. Wei, Y. Liu, M. Gao, X. Liu, Z. Xie, Optimized doping concentration of manganese in zinc sulfide nanoparticles for yellow–orange light emission, J. Alloys Compd. 486 (2009) 890–894. [7] S.H. Mohamed, Photocatalytic, optical and electrical properties of copperdoped zinc sulphide thin films, J. Phys. D: Appl. Phys. 43 (2010) 035406– 035413. [8] R. Ullah, J. Dutta, Photocatalytic degradation of organic dyes with manganesedoped ZnO nanoparticles, J. Hazard. Mater. 156 (2008) 194–200.

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