Visible-light photocatalytic degradation of methylene blue with Fe doped CdS nanoparticles

Applied Surface Science 270 (2013) 655–660

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Visible-light photocatalytic degradation of methylene blue with Fe doped CdS nanoparticles Ruby Chauhan a , Ashavani Kumar b,∗ , Ram Pal Chaudhary a a b

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

a r t i c l e

i n f o

Article history: Received 24 July 2012 Received in revised form 9 January 2013 Accepted 11 January 2013 Available online 24 January 2013 Keywords: Nanoparticle UV–vis spectrometer Scanning electron microscope Transmission electron microscope Photocatalysis

a b s t r a c t Fe doped CdS nanoparticles (Cd1−x Fex S; where x = 0.00, 0.03, 0.05 and 0.10) were synthesized by a chemical precipitation method. The synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman and UV–vis spectrometer. The XRD and TEM measurements show that the size of crystallites is in the range of 2–10 nm. With increased the Fe doping concentration, the position of the Raman bands shifted towards higher wavenumbers and their intensities decreased drastically. Optical measurements indicated that the absorption band edge shifted towards longer wavelength upon Fe doping. Direct allowed band gap of undoped and Fe doped CdS nanoparticles measured by UV–vis spectrometer were 2.3 and 2.2 eV at 100 ◦ C, respectively. Photocatalytic activities of CdS and Fe doped CdS were evaluated by irradiating the solution of methylene blue (MB) and sample under visible light. It was found that Fe doped CdS bleaches MB much faster than undoped CdS upon its exposure to the visible light. The optimum Fe/Cd ratio was observed to be 3 mol% for photocatalytic applications. In contrast, little degradation was observed for the pure CdS powder. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Environmental problems associated with organic pollutants and toxic water pollutants provide the impetus for sustained fundamental and applied research in the area of environmental remediation. Semiconductor photocatalysis offers the potential for complete elimination of toxic chemicals through its efficiency and potentially broad applicability [1]. Most of the dyestuffs that the textile and paper industries produced are difficult to decompose, due to the relatively stable chemical structures of aromatic dyes and hence cause severe contaminations to the drinking water and irrigation systems in some areas. Recently, transition-metal sulfides, in particular ZnS and CdS, have been intensively studied because of their unique catalytic functions compared to those of TiO2 [2,3]. These studies have revealed that CdS nanocrystals (NCs) are good photocatalysts as a result of the rapid generation of electron–hole pairs by photoexcitation. The photocatalytic properties occur not only in the photoreductive production of H2 from water and the photoreduction of CO2 [4], but also in the phototransformation of various organic substrates such as the oxidative formation of carbon–carbon bonds from organic electron donors, cis–trans photoisomerization of alkenes, and the photoreduction of

∗ Corresponding author. Tel.: +91 1744 233495; fax: +91 1744 233050. E-mail address: [email protected] (A. Kumar). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.110

aldehydes and their derivatives [5]. A favorable shift of the optical response into the visible region occurs subsequent to the doping of transition metal or rare-earth metal ions, such as Ni2+ and Cu2+ ; therefore, CdS nanoparticles can also be used as effective catalysts for photocatalytic evolution of H2 and photoreduction of toxic ions under visible-light irradiation [6]. One strategy that has been investigated for improving the activity of particulate photocatalysts is to decrease the particle size down to the nanoscale regime [7–9]. Decreasing the average particle size increases the specific surface area and thus increases the number of active surface sites where photogenerated charge carriers are able to react with adsorbed molecules to form free radicals. However, decreasing the particle size of a photocatalyst also increases the rate of surface charge recombination. As a result, the activity of particulate photocatalyst does not increase monotonically with decreasing particle size [8,9]. It is thus evident that the attainment of high activity in nanoparticulate photocatalyst requires a means of inhibiting charge carrier recombination. In recent years, metal chalcogenides such as cadmium sulphide have attracted considerable attention due to their potential applications in electronic, optical and photocatalytic degradation [10,11]. CdS is II–VI compound semiconductor with direct and band gap of 2.42 eV at room temperature and also has cubic and hexagonal wurtzite structure with lattice spacing a = b = c = 5. 83 A˚ for cubic ˚ c = 6.68 A˚ for hexagonal, so it is most promising canand a = 4.21 A, didate among II–VI compounds for detecting visible radiation. CdS

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2. Experimental

1200

900

(311)

1000

o

Fe doped CdS at 100 C (Cd1-xFexS)

(220)

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(111)

is used as window material for heterojunction solar cells to avoid the recombination of photogenerated carriers which improves the solar cell efficiency because of its wide band gap and stability [12]. It has also application in light emitting diodes [13], photo detectors [14], sensors [15], and electrically driven lasers [16]. CdS particles were successfully synthesized in a variety of media, such as nonaqueous solvents [17–19], reversed micelles [20,21], vesicles [22], zeolites [23,24] and other methods [25]. This paper reports a simple route for the preparation of undoped and Fe doped CdS nanoparticles, via chemical precipitation method [26] in the presence of polyethyleneglycol as capping agent. The prepared nanoparticles were characterized and then utilized as photocatalyst in the photodegradation of methylene blue as 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 Fe doped CdS samples thermally treated at 100 ◦ C in muffle furnace for 30 min.

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800 700

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600 500 400

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300 200 100 0 20

X=0.00 30

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60

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

2.1. Chemicals For the preparation and photocatalytic activity of undoped and Fe doped CdS nanoparticles, the chemicals used were polyethylene glycol (M = 6000, OH(OCH2 CH2 )n H; PEG), cadmium sulphate (M = 769.52, 3CdSO4 ·8H2 O), ferrous sulphate (M = 278, FeSO4 ·7H2 O), Sodium sulphide (M = 78, Na2 S), methylene blue (MB) dye (M = 319.85, C16 H18 N3 SCl). All chemicals used were AR grade from Sigma Aldrich and used without further purification.

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 upto 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).

2.2. Synthesis

2.4. Characterization

Nanoparticles of Fe doped CdS (Cd1−x Fex S where x = 0.00, 0.03, 0.05, 0.10) were prepared by chemical precipitation method. Freshly prepared aqueous solutions of chemicals were used for the synthesis of nanoparticles. 0.1 M cadmium sulphate, 0.1 M ferrous sulphate and 0.1 M sodium sulphide were used as reactant materials. Freshly prepared 50 ml of aqueous solution of 0.1 M sodium sulphide was mixed drop by drop in 50 ml of 0.1 M solution of cadmium sulphate and 50 ml of 0.1 M solution of ferrous sulphate using vigorous stirring and then added 0.5 g of polyethylene glycol as a capping agent. 0.5 ml sulphuric acid was added to the water before adding the ferrous sulfate heptahydrate to prevent the formation of ferric iron. 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 and thoroughly ground and then calcined at 100 ◦ C in muffle furnace.

X-ray diffraction (XRD) patterns were recorded on a Rigaku mini desktop diffractometer using graphite filtered Cu K␣ radia˚ at 40 kV and 100 mA with a scanning rate of 3◦ /min tion ( = 1.54 A) (from 2 = 20◦ to 80◦ ). Raman measurements were performed on Micro Raman spectrophotometer with low temperature photoluminescence. Optical absorption spectra were recorded on a Shimadzu double beam double monochromator spectrophotometer (UV-2550), equipped with an integrated sphere assembly ISR-240A in the range of 190–900 nm. Morphology and sizes of the product were determined by scanning electron microscope (SEM: ZEISS EVO MA-10) equipped with an energy dispersive spectrometer (EDS: Oxford Link ISIS 300) and TEM measurements carried out using an H-7500 model (Hitachi Ltd., Tokyo, Japan). Diluted nanoparticles suspended in absolute ethanol were introduced on a carbon coated copper grid, and were allowed to dry in air for conducting TEM studies. 3. Result and discussion

2.3. Photocatalytic activities of undoped and Fe doped CdS nanoparticles

3.1. XRD studies

A specially designed photocatalytic reactor system made of double walled reaction chamber of glass tubes was used for photodegradation experiments. A visible halogen lamp (1000 W) was kept inside the glass tube surrounded by a circulating water tube for cooling the halogen lamp. The solution for photodegradation measurement was prepared by adding pure or 3, 5 and 10 mol% Fe doped CdS (1 g/l) to 50 ml aqueous solution of methylene blue (MB; 10 mg/l at natural pH 6.5). In order to 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 visible

Fig. 1 shows the XRD patterns of Fe doped cadmium sulphide (Cd1−x Fex S, where x = 0.00, 0.03, 0.05 and 0.10) powder samples calcined at 100 ◦ C. The XRD patterns indicate that all the samples were composed of single (cubic) phase. The XRD pattern of Fe doped CdS samples calcined at 100 ◦ C exhibit peaks at 2 values 26.5◦ , 43.9◦ and 51.9◦ which could be indexed to scattering from (1 1 1), (2 2 0) and (3 1 1) planes with measured d-spacing 3.35, 2.05 and 1.75 A˚ of all crystal planes of the cubic structure. All the diffraction peaks are in agreement with the reported JCPDS card no. 80-0019. X-ray diffraction data reveals that the diffraction peak of Cd1−x Fex S at (1 0 0) plane slightly shifts to larger

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broadening shift of Raman bands associated with particle size. According to the Hiesenberg uncertainty principle, the particle size and phonon position hold the following relationship: XP ≥

Fig. 2. SEM image of 5 mol% Fe doped CdS nanoparticles calcined at 100 ◦ C.

angle and lattice constant decreases with increasing Fe concentration. The lattice parameters for CdS are a = b = c = 5.75 A˚ and ˚ respectively. The lattice parameCd0.90 Fe0.10 S are a = b = c = 5.72 A, ters of Cd0.90 Fe0.10 S are found slightly decreased due to the smaller ˚ than that of Cd2+ (rCd2+ = 0.96 A). ˚ ionic radius of Fe2+ (rFe2+ = 0.57 A) 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 Debye–Scherrer formula: Dhkl =

0.9 ˇhkl cos 

where D is the average crystalline diameter,  is the wave-length in angstrom, ˇ is the line width at half-maximum and  is the Bragg angle. We used the most intense peak (1 1 1) in the XRD patterns to calculate the average crystallite size. The crystallite sizes of Cd1−x Fex S (x = 0.00, 0.03, 0.05 and 0.10) nanoparticles are in the range of 2–3 nm calcined at 100 ◦ C. 3.2. SEM and TEM studies Fig. 2 shows SEM image of 5 mol% Fe doped CdS agglomerated particles calcined at 100 ◦ C. It is clear from Fig. 2 that the morphology of agglomerates is in cubic shape. The size of agglomerates is a broad distribution of the order of 1–2 ␮m, 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) shows TEM image of 5 mol% Fe doped CdS calcined at 100 ◦ C. It can be observed by image analysis (Fig. 3(a)) that the particle size of Fe doped CdS is non-uniform and is about 2–10 nm in near agreement with the size estimated from XRD (ca. 2–3 nm). HRTEM and lattice imaging reveal that the nanocrystals are cubic with a d spacing of 0.33 nm, corresponding to the (1 1 1) plane of cubic CdS (Fig. 3(b) and (c)). XRD and TEM studies show that there is no any observable effect on the particle size by 5% Fe2+ doping in CdS. 3.3. Raman studies Raman spectroscopy is a powerful tool to investigate the structural properties of nanostructures, monitoring the unusual band

h2 , 4

(1)

where X is the particle size, P is the phonon momentum distribution, and  is the reduced Planck’ constant. As the particle size decreases, the phonon is increasingly confined within the particle, and the phonon momentum distribution increases. This situation leads to a broadening of the momentum of scattered phonon according to the law of conservation of the scattered phonon according to the law of conservation of momentum, causing a peak broadening as well as a shift of the Raman bands [27]. Fig. 4 shows the Raman spectra of the undoped and 5 mol% Fe doped CdS nanoparticles. The Raman spectra at 297 and 599 cm−1 correspond to the 1 LO (longitudinal optical) and 2 LO phonon modes of CdS which were consistent with reported values [28] with no other peaks related with impurities. Raman modes of Fe doped CdS were slightly red shifted in comparison to Raman modes of CdS as the ionic radius of Fe is less than that of Cd. In addition, with Fe doping there is an additional factor of lattice softening because Fe replaces the Cd sites, leading to overall shift of the Raman modes of lower phonon energies. With increase the doping concentration, the position of the Raman bands shifts towards higher wavenumbers and their intensities decrease drastically. The observation can be attributed to the reduction of particle size and increasing force constants in the Fe doped samples [29]. 3.4. Optical studies Fig. 5 shows UV–vis diffuse reflectance spectra of Fe doped cadmium sulphide (Cd1−x Fex S where, x = 0.00, 0.03, 0.05, 0.10) samples calcined at 100 ◦ C. It can be seen that the intensive absorptions are present in ultraviolet–visible range of about 240–515 nm. The absorption edge shifted towards the longer wavelength side in 3, 5 and 10 mol% Fe doped CdS nanoparticles. Manifacier model is used to determine the absorption coefficient from the absorbance data [30]. 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 (˛h)2 versus photon energy (h). The energy band gap was determined by n 2 using the relationship:ahv = A(hv − Eg ) (ahv) = A(hv − Eg )where h = photon energy, ˛ = absorption coefficient (˛ = 4k/; k is the absorption index or absorbance,  is the wavelength in nm), Eg = energy band gap, A = constant, n = 1/2 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 [31]. The value of direct band gap was determined by extrapolating the straight line portion of (˛h)2 vs. h graph to the h axis; as shown in Fig. 6. The direct band gap decreases from 2.3 eV to 2.2 eV with 3, 5 and 10% Fe doped samples calcined at 100 ◦ C. 3.5. Photocatalytic response of CdS and Fe doped CdS nanoparticles To get the response of photocatalytic activities of undoped and Fe doped CdS 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 max 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. Methylene blue shows most intense

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Fig. 3. (a) TEM and (b) HRTEM images of 5 mol% Fe doped CdS nanoparticles calcined at 100 ◦ C (c) (1 1 1) lattice fringes of denoted area (d1 1 1 = 0.33 nm).

absorption peak at 665 nm. The percentage of decolorization efficiency of samples has been calculated as: Efficiency(%) = 100 ×

 (A − A)  0 A0

= 100 ×

 (C − C)  0 C0

where A0 , A, C0 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. Fig. 7(a) and (b) shows the time-dependent UV–vis absorption spectra of methylene blue during photoirradiation with undoped and 3 mol% Fe doped CdS under visible light.

Pure methylene blue dissolved in water shows small degradation in 300 min when irradiated with visible light. This smaller degradation of MB with OH radical originated from water. The rate of decolorization was recorded with respect to the change in intensity of absorption peak at 665 nm for methylene blue. Photodegradation of methylene blue (MB) was carried with CdS and Fe doped CdS by irradiating mixture of photocatalyst and MB with visible light. It was observed that Fe doped CdS decolorizes methylene blue faster than undoped CdS. It was also observed that CdS degraded about 100% of methylene blue within 300 min while 3 5 and 10 mol% Fe doped CdS degraded about 100% of methylene

R. Chauhan et al. / Applied Surface Science 270 (2013) 655–660

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Photocatalysis with CdS under visible light

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-1

Raman shift (cm )

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Wavelength (nm)

Fig. 4. Raman spectra of pure and 5 mol% Fe doped CdS nanoparticles calcined at 100 ◦ C.

(b)

Photocatalysis with Fe doped CdS under visible light 0.8

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Fe doped CdS at 100 C Absorbance (a.u.)

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Fig. 7. Time-dependent UV–vis absorption spectra for decolorization of methylene blue using 5 mol% Fe doped CdS under Visible light.

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Wavelength (nm) Fig. 5. UV–vis diffuse reflectance spectra (DRS) of undoped and 3, 5 and 10% Fe doped CdS nanoparticles calcined at 100 ◦ C.

1.00E+012 o

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(αhν) (a.u.)

Fe doped CdS at 100 C _________undoped CdS _________3 % Fe _______5 % Fe ________10 % Fe

0.00E+000 1.0

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hν (eV) Fig. 6. (˛h)2 vs. photon energy (h) for undoped and 3, 5 and 10% Fe doped CdS nanoparticles calcined at 100 ◦ C.

blue within 180, 180 and 240 min. This faster degradation rate of MB under visible light irradiation using Fe doped CdS is attributed to the increase in defect sites caused by Fe doping, leading to an enhanced optical absorption in the visible region. Fig. 8 shows the photodegradation rate of MB under visible light in presence of undoped and 3, 5 and 10 mol% Fe doped CdS (inset shows its ln C0 /C vs. time graph). The photocatalytic degradation of methylene blue was observed to follow the first-order decay kinetics [32]. The result shows that the photocatalytic decolorization of dye in CdS and Fe doped CdS can be described by the first order kinetic model, ln (C0 /C) = kt or ln (A0 /A) = kt, where A0 , A, C0 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. Its ln (C0 /C) plot shows a linear relationship with the irradiation time. The calculated rate constant (k) for CdS was 8.79 × 10−3 min−1 and for 3, 5 and 10 mol% Fe doped CdS was 1.04 × 10−2 , 9.8 × 10−3 and 9.1 × 10−3 min−1 , respectively. It is clear that the doping of Fe ion in CdS increase the photodegradation of the MB solution under the visible light. It has been shown that the photo catalytic activity of Fe doped CdS is strongly dependent on the dopant concentration. Doping of small (3 mol%) concentration of Fe increases the lifetime of excited charge carriers, which results the enhanced photo catalytic activity. When the doping concentration becomes too high (10 mol%), a great many crystal defects could be induced, which may serve as recombination centres to reduce the photoactivity. It is evident that doping of CdS with Fe enhances photocatalytic

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CdS 3 mol% Fe doped CdS 5 mol% Fe doped CdS 10 mol% Fe doped CdS

1.2

C/Co

1.0 0.8 0.6 0.4 0.2 0.0 0

50

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Time (min) Fig. 8. Photodegradation of methylene blue in presence of undoped and 3, 5 and 10 mol% Fe doped CdS under visible light (inset shows its ln Co/C vs. time graph).

activities of CdS, and hence Fe doped CdS is capable of degrading MB with the visible light irradiation. Here we assumed that Fe doped CdS generates electron–hole pair at the tail states of conduction band and valence band under visible light irradiation. The electron transfers to the adsorbed MB molecule on the particle surface. The excited electron from the photocatalyst conduction band enters in to the molecular structure of MB and complete decomposition of MB. Hole at the valence band generates hydroxyl radical via reaction with water, might be used for the oxidation of other organic compounds. This clearly demonstrates that the Fe doped CdS can be as potential photocatalyst for the water and environmental detoxification from organic pollutants which can operate at visible light 4. Conclusions Undoped and Fe doped CdS nanoparticles (Cd1−x Fex S where x = 0.00, 0.03, 0.05 and 0.10) were successfully synthesized using chemical precipitation method. SEM revealed nearly cubic morphology of Fe doped CdS agglomerated particles. XRD and TEM studies show that the size of crystallites is about 2–10 nm. The CdS capped with PEG restricted the agglomeration of the particles. The optical measurement yields energy band gaps and reveals that the absorption edge shifted towards the longer wavelength side in Fe doped CdS nanoparticles. The band gap value of as prepared Fe doped CdS samples were found to decrease as compared to undoped CdS. Fe doped CdS exhibited excellent photocatalytic activity for the photodegradation of MB under visible light in comparison to undoped CdS. Acknowledgements Authors are thankful to all technical staff for support in getting SEM, TEM and UV–vis spectra. We are also thankful to the Director NIT, Kurukshetra for providing the XRD and UV visible facilities in physics department. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chemical Reviews 95 (1995) 69–96. [2] I. Salem, Recent studies on the catalytic activity of titanium zirconium and hafanium oxides, Catalysis Reviews – Science and Engineering 45 (2003) 205–296.

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