Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor nanoparticles

Accepted Manuscript Title: Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor nanoparticles Author: N. Sreelekha K. Subramanyam D. Amaranatha Reddy G. Murali S. Ramu K. Rahul Varma R.P. Vijayalakshmi PII: DOI: Reference:

S0169-4332(16)30742-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.003 APSUSC 33004

To appear in:

APSUSC

Received date: Revised date: Accepted date:

28-1-2016 15-3-2016 1-4-2016

Please cite this article as: N.Sreelekha, K.Subramanyam, D.Amaranatha Reddy, G.Murali, S.Ramu, K.Rahul Varma, R.P.Vijayalakshmi, Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor nanoparticles, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research highlights •

Cu1-xCoxS nanoparticles were synthesized via chemical co-precipitation method.

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Structural, band gap, magnetization and photocatalysis studies were carried out.

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All the doped samples exhibited intrinsic room temperature ferromagnetism.

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Effect of magnetic properties on photocatalytic activity was analyzed.

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CuS:Co nanoparticles may find applications in photocatalytic and spintronic devices.

Graphical Abstract

Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor nanoparticles N. Sreelekhaa,b, K. Subramanyama,b, D. Amaranatha Reddyc, G. Muralid, S. Ramua, K. Rahul Varmae, R.P. Vijayalakshmia* a

Department of Physics, Sri Venkateswara University, Tirupati 517502, India Department of Physics, Raghu Engineering College, Visakhapatnam, Andrapradesh-531162, India. c Department of Chemistry and Chemical Institute for Functional Materials, Pusan National University, Busan,609735, Republic of Korea d Department of BIN Fusion Technology & Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk, Korea e Department of Mechanical Engineering, University of California, Berkeley, U.S.A b

ABSTRACT Pristine and Co doped covellite CuS nanoparticles were synthesized in aqueous solution by facile chemical co-precipitation method with Ethylene Diamine Tetra Acetic Acid (EDTA) as a stabilizing agent. EDAX measurements confirmed the presence of Co in the CuS host lattice. Hexagonal crystal structure of pure and Co doped CuS nanoparticles were authenticated by XRD patterns. TEM images indicated that sphere-shape of nanoparticles through a size ranging from 5-8 nm. The optical absorption edge moved to higher energies with increase in Co concentration as indicated by UV–Vis spectroscopy. Magnetic measurements revealed that bare CuS sample show sign of diamagnetic character where as in Co doped nanoparticles augmentation of room temperature ferromagnetism was observed with increasing doping precursor concentrations. Photocatalytic performance of the pure and Co doped CuS nanoparticles were assessed by evaluating the degradation rate of rhodamine B solution under sun light irradiation. The 5% Co doped CuS nanoparticles provide evidence for high-quality photocatalytic activity. Keywords: Chemical synthesis, Co doped CuS, RTFM, Photocatalytic dye degradation Corresponding author E-mail address: [email protected], Tel: + 91 9441408408

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1. Introduction In recent years, owing to distinctive optical, electrical, photovoltaic and catalytic properties of transition metal chalcogenide compounds are incredibly attractive for research. Among the plethora of chalcogenide based compounds, covellite phase copper monosulfide (CuS) p-type semiconductor is a non-toxic and significant class of material having budding applications in catalysis, energy storage and conversion, solar cell,

non volatile memory

devices, gas sensing, cold cathode, lithium ion batteries application and optoelectronic devices. In addition as a chalcogenide, CuS has significant attention in present days due to owned ample stoichiometric composition [Cu2S, Cu1.96S, Cu1.94S, Cu1.8S, Cu1.75S and CuS] and various morphology evolutions. In particular the band gap of CuxS can be speckled in a wide range (1.2 to 2.5 eV) through stoichiometric composition (x=1.0 -2.0), creating it a extremely enviable compound intended for spintronic and photovoltaic applications [1- 7]. The upcoming generations of gadgets, recognized as spintronic devices, are incredibly gorgeous owing to their multi-functionality and performance, which integrate the electronic, magnetic and photonic properties of the materials. From the spintronics point of view, it is require developing the dilute magnetic semiconductors through ferromagnetically polarized carriers at room temperature in such way that the spin in addition to charge of the carriers can be together with an exterior magnetic field to be in command of devices [8-11]. However transition metal ions doped CuS nanoparticles through hefty surface area are indisputably contemplated to be the desired form to make dilute magnetic semiconductors for spintronic application.

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In addition to that from water purification technology point of view, photocatalysis is an intensifying field and its utility lies in the areas of elimination of unfavorable organic compounds, wastewater treatment and exclusion of air pollutant in an enclosed environment [12-13]. In our habitual life dyes are most important group of organic compounds, which are locate in massive amount of functions. The greater part of the pigments and artificial dyes are contaminated, non eco-friendly and opposed to direct decomposition by sunlight and as a result, they emerge as a separation of importunate toxins [14-15].The release and subsequent gathering of these contaminated amalgams in water media create unsightly hazards for the surroundings. So, the great fashionable as well as capable inexpensive process toward eliminate the dyes is to leave behind the waste stream in excess of a high surface area activated carbon, which adsorbs the dyes before to their leave go of into main stream irrigate bodies [16]. On the other hand, photocatalytic action of the dye by means of stellar energy in the prevalence of a appropriate photocatalyst be able to support in elimination of pigment in the direction of favorable byproducts. Furthermore photocatalytic activity assists for reduce number of steps required in textile waste matter management. Herein high regard, as a host matrix CuS being a risk-free and economical with stable underneath atmospheric circumstances, would be perfect to exploit in hygienic technology. Moreover, due its outstanding characteristics and suitable band gap in the visible region, CuS is identified as a best photocatalyst material. A photocatalyst with good magnetic properties at nanoscale permits the use of the process of magnetic separation, which is one of the most efficient and simple method for removing suspended solids from wastewater without the need for further separation processes. The magnetic photocatalyst allows its use as a suspended material, providing the advantage to have a high surface area for reaction. Hence the overall aspire of this paper is to produce a dilute

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magnetic semiconductor nanoparticles for spintronics devices and to provide the magnetic photocatalysts for attaining the organic contaminants degradation and an easy removal and recovery from slurry systems. Much attention has been paid by more research groups for the synthesis of various CuS based nanostructures, for the photocatalytic application. Recently CuS photocatalytic activity has fascinated enormous attention owing to their potential applications in the degradation of dye as major ecological contaminants. Typically, the degradation of organic dyes in water suspension is exploited as a probe reaction to estimate the catalytic activity of photocatalytic performance. X. Meng et al. [2] reported hierarchical CuS hallow nanospheres and their structure enhanced visible light photocatalytic properties. Recently Qu wei Shu et al. [14] reported controlled synthesis of CuS caved super structures and their application to the catalysis of organic dye degradation in the absence of light [14]. Jianguo Yu et al. [15] reported ion exchange synthesis and enhanced visible light photo activity of CuS/ZnS nano composite hallow spheres. Y. Zang et al. [17] reported biomolecule assisted, environmentally friendly one pot synthesis of CuS/RGO nanocomposites with enhanced photocatalytic performance for degradation of rhodamine B. Amrita Ghosh et al [18] also reported a simple electrochemical route to deposit Cu7S4 thin films and their photocatalytic properties. Furthermore, in recent years, CuS nanostructures with various morphologies, such as nanoparticles [19-20], nanowires [21-22], nanorods [23-24], nanoribbons [25], nanoplates [26], nanoflakes [27], nanoflowers [28] and hierarchical tubular structures [29] have been synthesized. Numerous methods have been engaged to prepare CuS nanostructures, including chemical co-precipitation method [30], combustion method [31], hydrothermal or solvothermal methods [32, 29], electrospinning technique [4], solid state reaction method [33] sonochemical method [34] and template-assisted

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method [14]. We note that, to the best of our acquaintance, there are no reports on Co doped CuS nanoparticles on the subject of applied and academic interest. In preceding work, our research group found well defined room temperature ferromagnetic hysteresis loop for 1% Cr dopant concentration in SnO2 lattice beside its structural and optical properties [35]. In recent times we have also reported the effect of Co co-doping, Mn co-doping and Cu co-doping on the structural, optical and magnetic properties of SnO2: Cr DMS nanoparticles [36-38]. In the present work Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles were prepared by chemical co-precipitation technique and the result of Co doping on the morphological, structural, optical, magnetic and photocatalytic properties is reported. Besides, the effects of magnetic properties on photocatalytic activity of prepared samples were examined. Herein as a dopant Co2+ (0.058 nm) is known to inhibit the crystallite size and it is expected to substitute at Cu2+ (0.065 nm) site easily in the CuS host matrix and it has a soaring magnetic moment. 2. Experimental 2.1 Preparation of Cu1-xCoxS nanoparticles Pure and 1%, 3%, 5% and 7% Co doped CuS nanoparticles stabilizing with EDTA were

prepared

by

facile

chemical

co

precipitation

method.

Cu(CO2CH3)2·2H2O,

Co(CH3COO)2.4H2O and Na2S chemicals are of AR grade were used as precursors devoid of supplementary purification. The precursor’s solutions (0.2 M) in aqueous media were prepared separately as per the stoichiometric ratio and swirled over 20 min. Sodium sulphide precursor solution was added drop wise to the mixture of copper and cobalt cationic precursor solutions. After 15 min 0.5 ml of stabilizing agent (EDTA) was added to the final solution and was stirred for 8 h. Resultant precipitate were washed number of times with methanol and de ionized water 5

followed by dried out at 110oC over 8 h and subsequently prepared in to better-quality nanopowders. 2.2. Characterization The samples of bare and 1%, 3%, 5% and 7% Co doped CuS nanoparticles were subjected to different characterization studies. Scanning electron microscopy (SEM) with EDAX attachment (CARL-ZEISS EVO MA 15) was used to observe the morphological and chemical composition analyses. In addition that Agilent 7700 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was also used to estimate the Co dopant concentration in host matrix. Seifert 3003 TT X-ray Diffractometer with Cu-Kα radiation with a wavelength of 1.542 Å was utilized to investigate the structural properties and estimate the crystallite size from XRD patterns. The particle size along with structure evidences were done by Phillips TECHNAI FE 12, Transmission Electron Microscope (TEM). JASCO-V-670 spectrophotometer was exploited for optical absorption measurements. Fourier transform infrared spectra were recording using Thermo Nicolet FTIR-200 thermo Electron Corporation. Micro Raman spectrometer (LABRAM HR 800) was used at room temperature in the range of 400–600 cm-1 for note the Raman modes. Magnetic measurements were recorded at room temperature with a Lakeshore Vibrating Sample Magnetometer, VSM 7410. 2.3 Measurement of photocatalytic activity The photocatalytic abilities of the Cu1-xCoxS (x= 0.00, 0.01, 0.03, 0.05 and 0.07) were assessed via the decomposition of Rhodamine B (RhB) under simulated-solar irradiation. A solar simulator equipped with an AM 1.5 G filter and 150 W Xe lamp (Abet Technologies) was used as the light source. In support of photocatalysis analysis, the photocatalyst (100 mg)

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was suspended in a 100 mL aqueous solution of RhB (C0= 10 mg L−1). Prior to turning on the light source, the reaction system was magnetically stirred in the dark for 30 min to reach absorption–desorption equilibrium between the photocatalyst and the dyes. Magnetic stirring was continued after light irradiation in order to keep the photocatalyst particles suspended throughout the measurements. At given time intervals of illumination, ∼5 mL aliquots of the mixture were removed and centrifuged at 5000 rpm for 15 min to separate the photocatalyst powder. Subsequent to centrifugation, the UV–vis spectrum of the supernatant was recorded to monitor the adsorption and degradation behavior. The characteristic absorption peak of RhB at 554 nm was used to assess the extent of degradation. The degradation efficiency of the photocatalyst was defined as follows: Degradation efficiency (D (%)) = A (RhB)0 – A (RhB)t A (RhB)0 Where A(RhB)0 and A(RhB)t are the absorbances of RhB at 554 nm in the dark and under lightirradiation (at t minutes), respectively.

3. Results and discussion 3.1 Elemental analysis The composition regarding CuS: Co samples were analyzed through Energy Dispersive Analysis of X-rays (EDAX) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Fig. 1 shows the energy dispersive X-ray analysis (EDAX) of pure and 5% Co doped CuS nanoparticles. Expected stoichiometric ratios of Cu, S and Co elements were found from the EDAX and (ICP-MS) analysis. EDAX study revokes the presence of other elements in prepared nanoparticles. Table.1 gives the target and estimated compositions of Co in CuS nanoparticles.

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3.2 Morphological studies The SEM images of the as-prepared Cu1-xCoxS (x = 0.00, 0.01, 0.03 and 0.05) nanoparticles are shown in Fig. 2(a)–(d). From the Fig 2(a)–(d) it is clear that the morphology of Co doped nanoparticles is dissimilar from that of the bare nanoparticles. In pure CuS nanoparticles less agglomeration can be seen where as the Co doped CuS nanoparticles are mainly composed of a huge amount of inhomogeneous agglomerated territories made up of sphere like shaped particles. Besides as the Co dopant concentration increase the morphology alters as well as shape of the particle becomes more or less spherical with the grain size allocation becoming uniform. The modifications in the morphological characteristics as the Co content is increased may be ascribed to the disparity in the ionic fractions of the bond which may be correlated to the bonds electro negativity [39]. In order to observe the morphology and size of the synthesized nanoparticles TEM analysis were carried out. Fig. 3 (a-b) shows TEM images of pure and 5% Co doped CuS nanoparticles. The average size of the nanoparticles found to be in the span of 5-8 nm. 3.3. Structural analysis The purity, phase and crystallanity of our pure and Co doped CuS nanoparticles were resolute by X-ray diffraction technique (XRD). Fig. 4 shows the XRD patterns of the Cu1xCoxS

(x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. All the peaks could be indexed to the

hexagonal phase of CuS (space group P63/mmc) with lattice parameters a=3.972 Å and c= 16.38 Å, which are well matched with the standard JCPDS card No. 06-0464. The peaks at 2θ = 27.29o, 29.34o, 31.79o, 48.28o and 59.32o were indexed to the (101), (102), (103) (110) and (116) planes of hexagonal structure of CuS respectively. No peaks analogous to other phases or impurities could be seen within the resolution limit yet in samples of highest dopant content (7 at %). The broadness of the peaks specifies the formation of nanosized products. Auxiliary, the 8

locations of XRD peak move in the direction of higher 2θ values by increasing Co dopant concentration, it could be ascribed to the lesser ionic radii of Co2+ (0.58 nm) as correlated to Cu2+(0.065 nm). Since, consequence results sustain the substitution of Cu2+ ion by the Co2+ ion in the as prepared Co doped CuS nanoparticles. The average nanocrystallite size (D) was estimated from the full width at half maximum (FWHM) of the most prominent XRD (110) broadening peak using the Debye-Scherer formula, D=0.89λ/βcosθ, where λ is the wavelength of X-ray radiation, β is the full width at half maximum of the peak at diffraction angle θ [40] and the average crystallite sizes were found to be reduced with increasing the Co doping concentration which is in the span of 5-10 nm. It is good agreement with the particle size estimated from the TEM analysis. Herein the doping of the Co ion in CuS host matrix not only reduces the particle size but also degenerate the crystallanity of the nanoparticles. As the Co doping concentration increases the intensity of the XRD peak decreases and FWHM increases, which is due to the degradation of the crystallanity. This means that even though Co ions reside in the regular site of Cu2+, it produces crystal defects in the region of the dopant and the charge imbalance generate from this defect, changes the stoichiometry of the samples. 3.4 Optical absorption studies In order to evaluate the energy band gap values of prepared samples optical absorption measurements were carried out. Fig. 5 shows the UV-visible optical absorption spectra Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. From the figure it illustrates that the material can absorb radiation in the spectral region of 400-700 nm. Optical absorption edge for the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) samples lies at 632.6, 652.6, 629.4, 626.26 and 610.8 nm and related band gap energies of nanoparticles were found to be 1.96 eV, 1.90 eV, 1.97 eV, 1.98 eV and 2.03 eV respectively. This blue shift in the direction

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of higher energy is a direct result of the quantum confinement effect associated with the small particle size in nanoregime. The measured energy band gap values are very close to those reported literature [17, 41-42]. There are no reports of band gap studies on Co doped CuS nanoparticles for comparison. The above results implicates that the pure and Co doped CuS nanoparticles have suitable band gap values for the photocatalytic degradation of organic pollutants under visible/solar light irradiation. 3.5 FTIR analysis A Fourier transform infrared spectra (FTIR) is a multi functional technique which provides comprehensive information regarding the elemental constituents and chemical bonding of the prepared nanoparticles. Fig. 6 shows the FTIR spectra of EDTA capped Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. The spectral band at 2347 cm−1 in Fig. 6 is correlated with the NH3+ stretching vibration of the amino acid of inner salts. The peak located at 1569 cm−1 is attributed to N–H bending. The absorption peaks found at 1220 cm−1, 1116 cm−1 are endorsed to the stretching vibrations of C=C and C=S bonds. Finally the peaks appearing at 623 cm−1 in all synthesized samples are attributed to the Cu–S stretching vibration of host matrix [43]. Hence, FTIR spectra of the Co doped samples of this work are matter-of-fact to be the almost similar patterns as that of pristine CuS sample. It strongly suggest that the Co is substituted inside the lattice of CuS without alter their parent structure. 3.6 Raman analysis Raman spectroscopy is an reliable instrument for the analysis of lattice defect identification, doping precursor concentration, changes in crystallanity and anisotropic properties of the nanometric crystalline materials. It is notable CuS with a hexagonal structure belongs to the space group P63/mmc. Fig. 7 (a, b) shows the Raman spectra of Cu1-xCoxS (x =

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0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. From the Fig.7 it is found that a strong intense peak at 477 cm−1 is assigned to the A1g longitudinal optical mode (LO) related with S-S stretching vibrational mode of the covellite system and it is good agreement with the reported Raman spectra of hexagonal CuS nanostructures [21, 44]. The observed sharp Raman peaks and their shifting towards higher wavelengths as contrast to the bare CuS nanoparticles be a sign of quantum confinement effects. Auxiliary, no significant Raman modes related to supplementary phases were noticed in the Co doped CuS samples. 3.7. Magnetic studies Fig. 8 (a, b) shows the room temperature magnetization versus magnetic field (M-H) plots of the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles in which the diamagnetic data of background have been subtracted and magnetic measurements were carried out in the range of ±15000 Gauss. It is clear from the Fig. 8 (a) undoped CuS nanoparticles exhibits diamagnetic in nature due to the non existence of unpaired electrons, whereas for x= 0.01 and 0.03 Co doped samples exhibits vortex behavior of anti-ferromagnetic and ferromagnetic nature having the hysteresis loops at low field area. Further, at x = 0.05 strong ferromagnetic behavior was observed with maximum saturation magnetization (Ms) 0.015 emu/g and for x = 0.07 ferromagnetism is found to be decrease with increasing Co content. Nevertheless, the number of mechanisms might be responsible for the source of ferromagnetism similar to performance in chalcogenide based nanocompounds at room temperature but RTFM still ambiguous and remains contentious. Although magnetic nature in nanocrystalline materials is observed due to imperfections in prepared crystalline solids, precipitated magnetic segment of the magnetic atoms and substitute interactions’ among the local moments of dopant magnetic impurities. Therefore for insightful of the origin of ferromagnetism, requires analysis of

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experimental circumstances, realistic structural and magnetic properties, in the direction of set up unambiguous root-result correlation. On the other hand XRD patterns and Raman studies could not be detected formation of any secondary phases like Co based oxides or sulfides and Co related clusters. Hence, ambiguously the consequence ferromagnetic behavior in Co doped CuS samples might be ascribed to the replacement of Co2+ in consign of Cu2+ in CuS host without alter their original structure. Further due to the anti-ferromagnetic interactions between the nearest neighboring Co-Co atoms at high Co dopant concentration in CuS create the weak of ferromagnetism. There is no magnetic report on Co doped CuS of any form for comparison. This is the first report on magnetic studies of Co doped CuS nanoparticles. The above results suggest that Co doped CuS nanoparticles may be a potential dilute magnetic semiconductors for spintronic device applications. 3.8. Photocatalytic activity and stability It is the massive importance’s to find out the adsorption process of the organic pollutants on the catalyst surface to clarify the mechanism of photocatalytic reactions, which can make easy their applications in contaminate destruction. Among the commonly used dyes such as methylene blue (MB), rhodamine B (RhB), rose Bengal, methyl orange, Congo red (CR), anthraquinone, phthalocyanine and indophenols; rhodamine B (RhB) (C28H31ClN2O3) is one of the representative pollutant to evaluate the photocatalytic performance of the as prepared Co doped CuS nanocatalysts under the solar light irradiation [2, 14]. Fig. 9 (a, b-e) shows changes in the UV–vis absorption spectra of rhodamine B (RhB) aqueous solution in the presence of Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. The typical absorption peak at 554 nm, which is related to RhB (554 nm) was preferred to observe the catalytic degradation process. From the Fig. 9 (a, b-e) it is found that the absorption intensity of 12

RhB decreases through solar irradiation under the presence of prepared nanocatalysts and this turn revealed that the degradation of dye on the surface of nanoparticles. Furthermore, the results shows that for x= 0.05 Co doped CuS sample is more photoactive than pristine CuS nanoparticles. The time-dependent degradation ratios (Ct/C0) and the degradation efficiencies of RhB for Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles were determined and are presented in Fig. 10(a) and (b). From the figure it is clear that the concentration of RhB is not decomposed in the absence of the photocatalyst. This reveals that the degradation of RhB practically does not happen without a photocatalyst on illumination with sunlight. In the existence of pure CuS nanoparticles after a time span of 60 min, 74% degradation of RhB is achieved. This reveals that CuS nanoparticles have excellent photocatalytic ability under sun light. Nevertheless, Co doped CuS nanoparticles exhibited an significant degree of photocatalytic degradation ability, it took only 60 min for the complete degradation and it demonstrated itself to be far better than the CuS alone. Among the Co doped CuS nanoparticles, 5% Co doped CuS sample shows the optimum photocatalytic degradation ability. The main reason for high degradation rate of CuS by Co doping might be due to the tiny dimension of synthesized nanoparticles. Hence due to nanoregime of the particles creates large surface area helps to enhance the photocatalytic reactive locations and promote the efficiency of the electron–hole separation. In Co doped CuS nanoparticles, Co ions create new trapping sites which can act as electron pool and inhibits the photogenerated electrons-hole pair recombination. Thus, the charge carriers may migrate to the surface where they participate in oxidoreduction reaction with dye species and help in the improvement of photocatalytic performance of Co doped samples.

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Pseudo-first order kinetics plots for RhB degradation under the presence of the Cu1xCoxS

(x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanophotocatalysts as shown in Fig.10(c). The

kinetics replica is expressed as -ln(Ct/C0) = kt, where Ct is the RhB concentration in the aqueous solution at time t (mg L−1), C0 is the initial RhB concentration and k is the apparent pseudo-first order rate constant (min−1). The value of apparent pseudo-first order rate constant was estimated by a linear fit of the plot of -ln(Ct/C0) versus reaction time t. The k value was found to be 0.020759, 0.02758, 0.032277, 0.06470 and 0.03883 min-1 for pure CuS, 1% Co doped CuS, 3% Co doped CuS, 5% Co doped CuS and 7% Co doped CuS correspondingly. Though 5% Co doped CuS sample exhibited good photocatalytic performance, where k was 0.06470 min-1 which is 3.11 times higher than that of pure CuS. Hence, the above results are perceptibly suggested that Co doped CuS nanoparticles are efficiently improved the photocatalytic activity under photo-irradiation. In order to prove the existence of the hydroxyl radicals (OH-), superoxide radicals (.O2-) and generation of the identical amount of holes (h+) in the VB of Co doped CuS nanoparticles for the RhB degradation process, t-butyl alcohol [TBA], benzoquinone [BQ] and ethylene diamine tetra acetic acid [EDTA] were used as scavengers for OH-, .O2- and h+ respectively [46-48]. From the Fig. 11 of scavengers experiment, it is clear that the addition of TBA and BQ to the host matrix dramatically suppressed the degradation efficiency indicated that OH- and .O2- radicals played significant role in the reaction system. Further, the addition of EDTA slightly reduces the degradation rate which indicates that h+ played negligible role in the degradation process. Hence, based on the above results it is clear that superoxide radicals (OH-, .

O2-) are responsible for the decomposition of RhB dye into the harmless compounds and

mineral acids [46, 47]. Based on the above experimental results the possible mechanism for the

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enhancement of catalytic activity may explain as follows. When the Co doped CuS nanocatalysts are irradiated by sunlight with photon energy equivalent to the band gap of CuS, electrons (e-) in the VB can be excited to the CB with instantaneous generation of the identical amount of holes (h+) in the VB. The photo generated electrons and holes in the CuS nanocatalyst might involve in chemical reactions. The positive charge holes in the valence band can react with H2O or hydroxide ions assimilate at the surface of particle to produce hydroxyl radicals concurrently the electron in the conduction band can decrease O2 to produce superoxide radicals (.O2-) and subsequently other reactive oxygen species. These photo generated holes and hydroxyl radicals (OH-) as an oxidation agent are tremendously reactive in the direction of organic compounds and this is favorable for the dye degradation [49]. A schematic diagram that represents the charge-transferring process in the CuS:Co nanostructures is illustrated in Fig. 12. From the above inferences, plausible degradation reactions were as follows CuS: Co + hν → CuS: Co (e-) + CuS: Co (h+) CuS: Co (h+) + H2O → CuS: Co + .OH + H+ CuS: Co (h+) + O2 → CuS: Co + .O2 .

O2 + H+ →H2O2

H2O2 + .O2 → OH- + .OH + O2 .

OH + organic molecules → H2O+ CO2 + mineral acids

.

O2 + organic molecules → H2O+ CO2 + mineral acids In favor of practical applications the stability of a photocatalyst is essential, in

consequence the 5% Co doped CuS photocatalyst was recycled in RhB degradation tests under the same conditions over 5 cycles of 60 min each as shown in Fig. 10 (d). Fig. 10 (d) reveals that the photocatalytic activity didn’t turn down rapidly after five cycles. However the slight

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decrease of the activity of the 5% Co doped sample may be due to the slight solubility of CuS in aqueous solutions, resulting in catalyst loss. This designates that stable and efficient performance of the 5% Co doped CuS photocatalyst was maintained during RhB reduction, which makes the photocatalyst potentially applicable for water treatment. 4. Conclusions In précis, bare as well as Co doped CuS nanoparticles stabilized with EDTA were prepared through facile co precipitation method. EDAX, XRD and Raman analyses indicated that the synthesized nanoparticles are in crystalline scenery exclusive of any other impurity segments. TEM analysis revealed the creation of nanoparticles with average particle sizes in the span of 5–8 nm. The doping of 5% Co doped CuS nanoparticles showed distinct ferromagnetic hysteresis loop as well as with increasing the dopant concentration the ferromagnetic nature was decreased. Magnetic studies signified that 5% Co doped CuS nanoparticles have excellent potential application in spintronic devices. In addition to that 5% Co doped CuS dilute magnetic semiconductor nanoparticles were potentially photocatalysts suitable for water treatment. Acknowledgments The authors (N. Sreelekha and K. Subramanyam) are highly grateful to the Raghu Engineering College, Visakhapatnam, Andhra Pradesh, India, for providing the financial support. References [1] C. H. Lai, M. Y. Lu and L. J. Chen, Metal sulfide nanostructures: synthesis, properties and applications in energy conversion and storage, J. Mater. Chem., 22 (2012)19–30. [2] X. Meng, G. Tian, Y. Chen, R. Zhai, J. Zhou, Y. Shi, X. Cao, W. Zhou H. Fu,

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solar-light-induced catalyst for organic dye degradation in water, Appl. Surf. Sci. 358 (2015) 159–167 . [49] D. Amaranatha Reddy, Seunghee Lee, Jiha Choi, Seonhwa Park, Rory Ma, Haesik Yang, Tae Kyu Kim, Green synthesis of AgI-reduced graphene oxide nanocomposites: Toward enhanced visible-light photocatalytic activity for organic dye removal, Appl. Surf. Sci. 341 (2015) 175–184. Table captions Table. 1 Table .1 Elemental composition of Cu1-xCoxS nanoparticles from EDAX and ICP-MS. Figure captions Fig. 1 (a, b) EDAX spectra of Cu1-xCoxS (x = 0.00 and 0.05) nanoparticles. Fig. 2 (a-d) SEM images of Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 3 (a, b) TEM images of Cu1-xCoxS (x = 0.00 and 0.05) nanoparticles. Fig. 4 X-ray diffraction pattern of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 5 Optical absorption spectra of the Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 6 FTIR spectra of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 7 Raman spectra of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 8 (a) Room temperature M-H plots of Cu1-xCoxS (x = 0.00) nanoparticles. Fig. 8 (b) Room temperature M-H plots of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 9 (a) Changes in the UV–vis absorption spectra of rhodamine B (RhB) aqueous solution in the presence of Cu1-xCoxS (x = 0.00) nanoparticles. 22

Fig. 9 (b-e) Changes in the UV–vis absorption spectra of rhodamine B (RhB) aqueous solution in the presence of Cu1-xCoxS (x = 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 10. (a) Photocatalytic degradation of the RhB under irradiation of the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanocatalysts by simulated sunlight. (b) Degradation efficiency of the RhB by Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles as a function of time. (c) The pseudo-first order kinetics plots for RhB degradation in the presence of the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanophotocatalysts. (d) Reusability of the Cu1-xCoxS (x = 0.05) nanophotocatalysts during the solar light degradation of the RhB solution. Fig. 11 The photocatalytic degradation of RhB with different active species scavengers (EDTA: disodium ethylene diamine tetraacetate, BQ: benzoquinone, and TBA: t-butyl alcohol). Fig. 12 The schematic diagram of band gap matching and photogenerated charge separation of Co doped CuS nanoparticles and its photocatalytic process.

23

Table. 1 Target composition

Estimated composition from

Estimated composition

(at.% )

EDAX (at.%)

from ICP-MS (ppm)

Cu

S

Co

Cu

S

Co

Cu

Co

50.0

50

0.0

49.79

50.21

0.00

486350

---

49.0

50

1.0

48.26

50.83

0.91

479172

8351

47.0

50

3.0

46.98

49.97

3.05

463651

28734

45.0

50

5.0

44.25

51.10

4.65

438760

42632

43.0

50

7.0

42.26

50.95

6.79

415397

64753

Table .1 Elemental composition of Cu1-xCoxS nanoparticles from EDAX and ICP-MS.

24

Fig.1

Fig. 1 (a, b) EDAX spectra of Cu1-xCoxS (x = 0, and 0.05) nanoparticles.

25

Fig. 2

Fig. 2 (a-d) SEM images of Cu1-xCoxS (x = 0.00, 0.01, 0.03 and 0.05) nanoparticles.

26

Fig. 3

Fig. 3 (a, b) TEM images of Cu1-xCoxS (x = 0.00 and 0.05) nanoparticles.

27

Fig. 4

Fig. 4 X-ray diffraction pattern of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles.

28

Fig. 5

Fig. 5 Optical absorption spectra of the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Inset shows that optical absorption spectra of the pure CuS nanoparticles.

29

Fig. 6

Fig. 6 FTIR spectra of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles.

30

Fig. 7

Fig. 7 (a) Raman spectra of Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles. Fig. 7 (b) Extraction of Raman spectra for Cu1-xCoxS (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticles.

31

Fig. 8 (a)

Fig. 8 (a) Room temperature M-H plots of pure CuS nanoparticles.

32

Fig. 8 (b)

Fig. 8 (b) Room temperature M-H plots of Cu1-xCoxS (x = 0.01, 0.03, 0.05 and 0.07) nanoparticles.

33

Fig. 9 (a)

Fig. 9 (a) Changes in the UV–vis absorption spectra of rhodamine B (RhB) aqueous solution in the presence of Cu1-xCoxS (x = 0.00) nanoparticles.

34

Fig. 9 (b-e)

Fig. 9 (b-e) Changes in the UV–vis absorption spectra of rhodamine B (RhB) aqueous solution in the presence of Cu1-xCoxS (x = 0.01, 0.03, 0.05 and 0.07) nanoparticles.

35

Fig. 10 (a-d)

Fig. 10. (a) Photocatalytic degradation of the RhB under irradiation of the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanocatalysts by simulated sunlight. (b) Degradation efficiency of the RhB by Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles as a function of time. (c) The pseudo-first order kinetics plots for RhB degradation in the presence of the Cu1-xCoxS (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanophotocatalysts. (d) Reusability of the Cu1-xCoxS (x = 0.05) nanophotocatalysts during the solar light degradation of the RhB solution.

36

Fig. 11

Fig. 11 The photocatalytic degradation of RhB with different active species scavengers (EDTA: disodium ethylene diamine tetraacetate, BQ: benzoquinone, and TBA: t-butyl alcohol).

37

Fig. 12

Fig. 12 The schematic diagram of band gap matching and photogenerated charge separation of Co doped CuS nanoparticles and its photocatalytic process.

38