Enhanced visible light photocatalytic reduction of

Powder Technology 279 (2015) 209–220

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Enhanced visible light photocatalytic reduction of organic pollutant and electrochemical properties of CuS catalyst Murugan Saranya a, Rajendran Ramachandran a,b, E James Jebaseelan Samuel b, Soon Kwan Jeong c,⁎, Andrews Nirmala Grace a,c,⁎ a b c

Centre for Nanotechnology Research, VIT University, Vellore 632 014, Tamil Nadu, India Photonics, Nuclear & Medical Physics Division, VIT University, Vellore-632014, Tamil Nadu, India Climate Change Technology Research Division, Korea Institute of Energy Research, Yuseong-gu, Daejeon 305-343, South Korea

a r t i c l e

i n f o

Article history: Received 25 November 2014 Received in revised form 17 March 2015 Accepted 27 March 2015 Available online 3 April 2015 Keywords: CuS Nanostructures Hydrothermal Photocatalyst Nitrobenzene 4-Nitrophenol

a b s t r a c t The present work states the removal of organic compound using CuS photocatalyst under visible light irradiation. The CuS photocatalysts were synthesized by hydrothermal method by using copper nitrate as copper precursor and thiourea, sodium thiosulphate as sulfur precursors via hydrothermal route. The reaction was carried out at 150 °C for 24 h using water as a solvent. The prepared CuS nanostructures were analyzed by X-ray diffraction, FE-SEM, photoluminescence (PL), UV–vis spectroscopy and Fourier transform infrared spectroscopy (FT-IR). A detailed mechanism on the effect of sulfur source on CuS morphology has been elucidated. The catalytic process efficiency mainly depends upon the adsorption and electron transfer between the dye molecules. The successive decrement in the absorbance at 400 nm shows the effective decrease in 4-nitrophenol pollutant as monitored by UV–vis spectrophotometer. The as prepared CuS catalyst showed highly efficient and versatile photocatalytic activities as well as excellent recyclability in degrading organic contaminant viz. nitrobenzene and 4-nitrophenol under visible light irradiation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Photocatalysis has gained more attention among environmental scientists for the application to issues like treatment of dyes, organic pollutants and purification of polluted water [1]. The organic pollutant causes a major threat to living systems. Nitro based organic compounds are the major by-products in various industrial sectors like leather, pharmaceutical products, agrochemical industries, printing and painting industries as coloring agents [2,3]. Particularly phenol and its derivatives have become common organic pollutant in industrial and agricultural effluents, which is toxic to the environmental ecosystem. A number of methods have been made to treat these organic pollutants like microbial degradation [4], chemical treatment [5], photocatalytic degradation [6], and waste water treatment [7]. From these reported methods, photocatalytic degradation is an emerging and cost effective method for the removal of organic pollutants. Usage of visible light in the treatment of waste water has been paid more attention in recent years as utilization of visible light is having enormous opportunity for future technology [8].

⁎ Corresponding authors at: Centre for Nanotechnology Research, VIT University, Vellore 632014, Tamil Nadu, India. Tel.: +91 416 2202412; fax: +91 416 2243092. E-mail addresses: [email protected] (S.K. Jeong), [email protected], [email protected] (A.N. Grace).

http://dx.doi.org/10.1016/j.powtec.2015.03.041 0032-5910/© 2015 Elsevier B.V. All rights reserved.

Semiconducting materials are widely used in many applications because of their unique physical, chemical, magnetic, surface electronic and optical properties due to their small size when compared to their bulk materials [9,10]. Chalcogenides such as CuS, ZnS, CoS, SnS, PbS and CdS are commonly used semiconducting materials [11,12]. Among the different chalcogenides, CuS is an important semiconducting nanomaterial with direct band gap with various potential applications in photoelectric devices, lithium batteries, chemical sensors, thermoelectric cooling materials, high capacity cathode materials, solar radiation absorbers, solar cells, optical filters, superionic materials, photocatalyst, and so forth [13–19]. So far, various morphologies of CuS like nanorods, nanoflakes, nanotubes, nanospheres, nanowires, nanoplatelets, flower like structures, nanotubes and nanoribbons have been reported [20–23]. A wide variety of preparatory routes has been reported like microwave [24], co-precipitation [25], sol–gel [26], solvothermal/hydrothermal [27–29], chemical vapor deposition [30], template assisted growth [31], and polyol method [32]. Apart from these various techniques, hydrothermal route is preferred. The hydrothermal method is the most appealing method because it could be operable at less temperature with high efficiency and could be scaled up. The use of surfactant and template may increase the complexity of the reaction process, which results in impurity of the products. It is thus necessary to develop a facile, surfactant and template free method for the CuS synthesis. However, the template and surfactant usage in the reaction systems will increase the complexity of reaction, expenses,

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time and impurity of the products. Thus, developing a simple, high purity, template free and surfactant free products is still a significant challenge. In fact, the hollow and complex structures prepared from hard templating routes mostly suffer from disadvantages related to their high cost and tedious synthetic procedures. Herein we report the synthesis of well-defined hierarchical architectures of CuS with good control over size and shape by template and surfactant free synthetic approach. Further the influence of two sulfur sources thiourea (TU) and sodium thiosulphate (STS) have been systematically investigated and necessary mechanisms have been detailed. Furthermore, the photocatalytic properties of the prepared CuS nanostructures are studied for the organic contaminant degradation. The as-prepared CuS catalyst was tested for the reduction of 4-nitrophenol (4-NP) and nitrobenzene (NB) contaminants. 2. Experimental 2.1. Materials Copper nitrate trihydrate (99%), thiourea (98%), sodium thiosulphate (98%), 4-nitrophenol, and nitrobenzene were purchased from SD FineChem, India, and used as received. All the chemicals were of analytical grade and used without further purification. 2.2. Synthesis of CuS photocatalyst

using a CHI 660C series electrochemical workstation. The optical properties of CuS were measured by DRS UV–vis spectrophotometer (HITACHI U-2800) using powder samples. The photo luminescence property was studied by fluorescence spectrometer (JOBINYOON HORIBA - FLUOROLOG) and the functional groups were studied by FT-IR spectroscopy (SHIMADZU AFFINITY-1). 2.4. Photocatalytic performance The photodegradation experiment was carried out in a homemade reactor setup as given in Scheme 1. The reactor wall is made of borosilicate cylinder with cooling water connections. The visible source used in the experiment is 150 Watt immersive tungsten lamp. The organic pollutants with photocatalyst were stirred continuously by magnetic pellet. The sample was collected periodically via sampling port to monitor the catalytic properties of the CuS catalyst. The photocatalytic degradation efficiency was calculated by the below equation, Degradation Rate ð%Þ ¼

  C  100 1− Co

ð1Þ

where, ‘Co’ is the initial concentration before degradation and ‘C’ is final concentration after time ‘t’. 2.5. Fabrication of electrodes for electrochemical measurement

Typically, when 1 mmol of Cu (NO3)2·3H2O was dissolved in 40 ml of DI water, a green color solution was formed. To it, 2.5 mmol thiourea was added with vigorous stirring so that the reactants are well dispersed. Then, the solution was transferred to a 300 ml Teflon-lined autoclave. The autoclave was sealed and kept at 150 °C for 24 h. After the reaction time, the resulting solution was allowed to cool down to room temperature naturally. Then the solution was washed with distilled water and absolute ethanol. The resulting product was dried in vacuum at 60 °C to obtain the final product. The same procedure was repeated with sodium thiosulphate instead of thiourea as sulfur source.

About 5 mg of CuS catalyst was dispersed in 1 ml of ethanol and sonicated for 10 min. Then the solution was dropped on a glassy carbon electrode and dried. A three electrode system viz. glassy carbon as a working electrode, platinum wire as counter and calomel as reference electrode was used. The EIS measurement was performed in the electrolyte mixture of 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture as redox probe in 0.1 M KCl solution. The impedance spectra were recorded over the frequency range of 105–0.1 Hz. The electrochemical characteristics were tested with the same electrode setup used for EIS measurement in 1 M KOH electrolyte at two different scan rates.

2.3. Characterization 3. Results and discussion The crystallographic and the phase structures were identified by X-ray diffraction (XRD, Philips X'Pert Pro, Cu-Kα: λ = 0.154 nm). Morphology of the products and particle size were analyzed by field emission-scanning electron microscopy (HITACHI SU6600 SEM). The electrochemical properties of the material were tested

During the growth of CuS nanostructures, the synthesis was optimized by use of two different sulfur sources. This was done to know the effect of the same on the morphology of the products. In this regard, TU and STS were used as sulfur sources to grow CuS nanostructures. A

Scheme 1. Schematic representation of reactor set-up used in the photocatalytic experiment.

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the relatively high pressure generated during the high temperature (250 °C) hydrothermal synthesis leads to the desulfurization of some CuS to Cu2S. This is obviously due to the excess amount of thiosulfate in the reaction medium [34]. 3.2. Morphological characterization

Fig. 1. XRD patterns of prepared CuS catalyst from different sulfur precursors a) CuS/TU and b) CuS/STS.

detailed morphological difference and its importance to photocatalysis are given in the following reactions. 3.1. XRD characterization Fig. 1 shows the XRD pattern of CuS nanostructures grown from two different sulfur sources a) thiourea and b) sodium thiosulphate at 150 °C for 24 h. The diffraction peaks were indexed with standard hexagonal CuS phase with JCPDS card no. 06-0464 and the cell parameters are a = 3.792 and c = 16.344. From the XRD graph, it can be seen that the resultant product is of pure phase [33]. The broader diffraction peaks show the smaller size of crystalline formation. The XRD graph clearly shows peaks at (101), (102), (103) (006), (110), (108) and (116) as in accordance with the JCPDS file indicating that CuS exists as covellite phase. However, the product obtained for sodium thiosulphate sulfur source has presence of some Cu2S peak [JCPDS card no. 89-2670] along with the major hexagonal CuS diffraction peaks. We believe that

3.2.1. SEM analysis FE-SEM images of CuS nanostructures grown from two different sulfur sources are given in Figs. 2 and 3. To know the changes on the morphology of the products, the reaction was carried out at 150 °C for 24 h using two sulfur sources. Fig. 2(a–d) shows the corresponding FE-SEM images of CuS nanostructures with thiourea as sulfur source. From the figure, it can be seen that the structures are uniform and well defined, in which formation of nanoplates self-assembled to form cylindrical like structures could be observed [35]. The width of the platelets ranged around 40–110 nm. CuS easily forms nanoplates with hexagonal shape because of intrinsic structural features of covellite [36]. Fig. 3(a–d) shows the SEM images of CuS nanostructures with sodium thiosulphate as sulfur source grown at 150 °C for 24 h at different magnifications. From the images, it can be seen that the structures are composed of hexagonal platelets self-assembled to form spherical like structures [37]. It suggests that the hexagonal nanoplate formation might be partly related to the intrinsic anisotropic structural characteristic of CuS nanostructures. As compared to TU, the morphology was not well-defined and uniform in the case of STS sulfur source. This could be due to the week coordination of STS with Cu in CuS nanostructures. 3.2.2. Reaction mechanism The growth mechanism is postulated based on the above results. It is well known that under hydrothermal treatment, complex formation of copper–thiourea aqueous systems and nanomaterial formation will occur. It is speculated that CuS nanostructure formation might follow three steps: first, a large amount of CuS primary particles were produced when copper–thiourea complexes were solvothermally treated.

Fig. 2. FE-SEM images of CuS with TU as sulfur source (a–c) EDX spectra (d).

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Fig. 3. FE-SEM images of CuS with STS as sulfur source (a–c) EDX spectra (d).

Second is the formation of a large amount of CuS primary particles and later aggregation of primary particles takes place. Finally, these particles assemble preferentially into larger diverse structures. Also, a higher temperature of 150 °C facilitates the homogeneous formation of CuS nanostructures with both nucleation and growth phase at a controllable manner and well-defined morphology. Under hydrothermal reaction conditions, when the reaction time was made longer, the primary nanostructures grew after the nucleation stage to secondary structures and oriented itself homogenously to well defined cylindrical like structures. The stability of nanomaterials is an imperative factor that has to be taken into account during its synthesis. It is well known that strong intermolecular forces such as Vander Waals attraction, π–π interaction, etc., contribute to the aggregation of nanoparticles.  2þ 2þ Cu þ nTu þ mSolvent→ CuðTuÞn ðSolÞm  2þ Solvothermal CuðTuÞn ðSolÞm  → CuS↓

comparable to the other reports [39,40], where the uses of surfactants were reported. In the case of thiosulphate as sulfur source, numerous hexagonal plates are self-assembled to form spherical shaped nanostructures with the thickness of each plates ranging between 28–60 nm as seen in Fig. 3. The sulfur source plays an important role deciding the different

ð2Þ

The reaction of thiourea–Cu(II) ions can be easily observed by the color change of the reaction solution from green to pale white in color, indicating that thiourea–copper(II) complexes were formed. Due to the Ostwald ripening, the formation of nucleation phase takes place initially and these nuclei will aggregate and forms the building blocks with diverse morphology [38]. Thus it is a challenge to obtain stable dispersion and to achieve this; stabilizers are normally used during the synthesis of nanoparticles. At room temperature, the prepared material was very stable with no sign of precipitation and without the use of any stabilizers. In this work, very stable dispersions of CuS nanomaterials were obtained without any stabilizers and the dispersions were stable for months. The structural features showed unique morphology with different hierarchical structures without any surfactant, which is

Fig. 4. UV–vis absorbance spectra of CuS catalyst from different sulfur precursors a) CuS/TU catalyst and b) CuS/STS catalyst.

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Fig. 5. Band gap spectra of CuS catalyst a) CuS/TU catalyst and b) CuS/STS catalyst.

morphology of the products. From the above results, two-step formation mechanism is possible for the copper source and sodium thiosulphate sulfur source as follows,   þ 2H2 O→ CuðS2 O3 ÞðH2 OÞ2   2þ 2− 2− Cu þ 2S2 O3 → CuðS2 O3 Þ2   þ 2− CuðS2 O3 ÞðH2 OÞ2 →CuS↓ þ SO4 þ 2H þ H2 O  2− þ 2− þ 6H2 O→CuS þ 3SO4 þ 12H CuðS2 O3 Þ2 Cu

2þ

þ S2 O3

2−

ð3Þ

A careful examination of the image shows a four-fold symmetry and shows different ferns arising from a common branch. When compared to the two sulfur sources, in thiourea sulfur source, the particles are well-defined and uniformly assembled. 3.2.3. Effect of sulfur source It is also worth mentioning here that sulfur sources have great effects on the final morphologies of CuS nanomaterials. As we know, different sulfur sources have different release rate of S2 − ions. Once sodium thiosulphate (STS) sulfur source was introduced into water, it dissociated into Na+ and S2− immediately, and therefore the formation of CuS nuclei occurred rapidly. As a result, CuS nuclei grew in an explosive way to form CuS nanoparticles because of the high concentration

of S2−. STS can readily release S2−, but the concentration of S2− is a little lower than S2 − released from thiourea (TU) sulfur source. When STS was introduced into the reaction system, weak anisotropic growth resulted in the small nanoplates. As an outcome, the CuS nuclei grew in an uncontrollable manner to form CuS nanoparticles, due to the high concentration of S2−.  2þ 2þ CuðNH2 CSNH2 Þx ⇔Cu þ xNH2 CSNH2 NH2 CSNH2 þ 3H2 O→H2 S þ 2NH 2þ þ Cu þ H2 S→CuS þ 2H

4þ

þ CO3

2−

ð3Þ

For thiourea as sulfur sources, the strong complex action between Cu2 +, thiourea and water led to the formation of Cu–Tu complexes. The reaction of thiourea–Cu(II) ions can be easily observed by the color change of the reaction solution from green to pale white in color, indicating that thiourea–copper(II) complexes were formed. This process decreased the formation speed of CuS greatly and resulted in the anisotropic growth of CuS. Therefore, nanoplates and nanorods were the constituting units of the hierarchical structures. Furthermore, it can be concluded that the release rate of S2− is also the determinant factor on the morphology of the metal sulfide nanocrystals. When STS was used, the products were grown in an isotropic mode and their final

Fig. 6. Photoluminescence spectra of CuS catalyst from different sulfur precursors a) TU and b) STS.

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Fig. 7. FT-IR spectra of CuS catalyst from different sulfur precursors a) CuS/TU catalyst and b) CuS/STS catalyst.

morphology was small nanoparticles. When Tu were used, anisotropic growth played a vital role in the synthesis of metal sulfide nanocrystals, accordingly, the final products were nanorods, nanoplates or their assemblies [41]. At longer reaction times, the nanoplates protrude further, leading to the construction of hierarchical structures built of wellordered and oriented nanoplates [42]. So, sulfur sources have a great influence on the morphology formation of CuS nanostructures. The release of S2− ions depends on the different sulfur sources. Hence the release rate of S2− rate plays a crucial factor on the morphology of the metal sulfide nanostructures.

3.3. Optical property of CuS nanostructures As an indirect band gap material, the optical properties of CuS were studied further to know the effect of sulfur source on the band gap. The UV–vis spectrum of CuS nanostructures is given in Fig. 4. From the figure, it can be clearly seen that CuS samples show absorption spectrum in the region of 400 to 800 nm. In both cases, the absorption spectra exhibited a shoulder around 620 nm. These results suggested that CuS could be a promising photocatalytic material, absorbing the visible

light as seen from the graph. With the help of absorption spectra, the optical band gap is calculated by the below equation [43]. To calculate the band gap energy, (αhυ)2 vs. (αυ) was plotted, where ‘α’ is the absorption coefficient, ‘hυ’ is photon energy, ‘A’ is a constant, ‘Eg’ is the band gap, and ‘n’ is either 2 for direct transition or ½ for indirect transition. Thus, by extrapolating a straight line at the linear part of the curve gives energy band gap.   n ðαhνÞ ¼ A hν−Eg

ð4Þ

According to the above equation, based on the direct transition, the band gaps of the as-obtained CuS are 2.08 eV for thiourea and thiosulphate sulfur sources as shown in Fig. 5(a) and (b). There was not much difference observed in the band gap for both the sulfur sources. Photoluminescence spectrum of CuS nanostructures at 150 °C is given in Fig. 6. For the excitation value of 400 nm, CuS showed a broad emission peak around 543 nm for both the samples, which is different from Jiang's results where CuS has no emission peaks in the range of 400–700 nm [44]. There is a strong emission peak at 543 nm for both

Fig. 8. Time dependent UV–vis absorbance spectrum on nitrobenzene degradation at different time durations using a) CuS/TU catalyst and b) CuS/STS catalyst.

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Fig. 9. (a) Variation of nitrobenzene concentration after the absorption equilibrium and (b) kinetic fit plot of ln (C/Co) vs time for different photocatalysts at different time intervals.

the sulfur sources which is different from the reported literatures [45], but it is consistent with the PL results reported by Roy and Srivastava [46]. The emission is the same for both the sulfur sources with only the intensity of the peak altered. This could be due to change in the size effect of the products, which in turn increases the content of the surface oxygen vacancy and defects. The morphology and size of the products are also being responsible for the PL spectra of the products. To further know the bonding, FT-IR was recorded for the CuS samples. Fig. 7 shows the FT-IR spectra of the as-prepared CuS catalyst. The broad absorption peak around 3452 and 3167 cm− 1 corresponds to the –OH group of H2O, indicating the existence of water absorbed on the surface of the sulfide products. The peaks centered at around 1625 cm− 1 and 1402 cm−1 belong to the C = O stretching mode of the absorbed CO2. The absorbed H2O and CO2 on the surface of the sulfide obtained from the decomposition of thiourea prepared by hydrothermal or solvothermal methods have also been reported previously [47,48]. The peaks at 1118 cm−1 correspond to C–O stretching and the

presence of vibrational peaks at 619 and 621 cm− 1 indicates the presence of Cu–S stretching modes [49]. 3.4. Photocatalytic effect of CuS catalyst on nitrobenzene and 4-nitrophenol Due to the interesting morphologies, the prepared CuS were tested for their photocatalytic properties towards the degradation of organic compounds. In this regard nitrobenzene and 4-nitrophenol were chosen as organic contaminants and the experiment was carried out under visible light irradiation. An aqueous solution of pollutant and catalyst mixture was stirred in dark to obtain equilibrium on the catalyst surface. After that the aqueous solution was exposed under visible light. The samples were taken out at different time durations, centrifuged to separate the CuS catalyst and their concentration was monitored by UV–vis spectrometer. Fig. 8 shows the UV–vis absorbance spectra of CuS catalyst at different time intervals. As the irradiation time prolonged the nitrobenzene absorbance peak at 250 nm starts decreasing as the

Fig. 10. Time dependent UV–vis absorbance spectrum on 4-nitrophenol degradation at different time durations using a) CuS/TU and b) CuS/STS catalysts.

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Fig. 11. (a) Variation of 4-nitrophenol concentration after the absorption equilibrium and (b) kinetic fit plot of ln (C/Co) vs time for different photocatalysts at different time intervals.

irradiation time continues. The photocatalytic activity increases with increasing the reaction time. Absorption peak of nitrobenzene completely disappeared after 60 min for CuS/TU catalyst. When compared to the CuS/TU sample, the CuS/STS sample showed a lesser degradation rate. It is clearly seen from the graph that there is only negligible decrease for the absorbance peak for without catalyst. Fig. 9(a) depicts the degradation activity of different catalysts and its corresponding C/Co ratios vs time is shown in Fig. 9b. As observed from the graph, in the absence of catalyst, there was only a negligible degradation. Without catalyst there no much decrease in the nitrobenzene concentration. Further, the CuS catalyst was also tested for the photodegradation of 4-NP under the same experimental conditions. The corresponding degradation graph is given in Fig. 10. Fig. 11(a) shows the 4-NP degradation activity of CuS nanostructures by plotting C/Co graph as a function of time where, C and Co are the initial and final concentrations of the pollutant at time t. After the irradiation period, the pollutant solution without catalyst shows only negligible change. Results revealed that the catalytic property of CuS/TU is high when compared to the CuS/STS catalyst. Thus in both the cases, CuS prepared from TU as sulfur source shows a higher degradation than prepared from STS. This could be due to the uniform and unique morphology structure. For comparison the photocatalytic activity also performed in dark for nitrobenzene

and 4-nitrophenol pollutants. Fig. S1 shows the photodecolorization of CuS/TU sample for 4-nitrophenol and nitrobenzene contaminants performed in the dark. But the results show only lesser degradation rate about η = 25.6% for 4-NP and η = 19.4% for nitrobenzene contaminant. 3.5. Kinetic study The study of adsorption kinetic parameters helps to better understand the adsorption rate. The adsorption kinetics on the CuS/TU catalyst was evaluated by pseudo first-order model [50]. A high correlation coefficient (CR) value indicates the successive fitting of the kinetic model. The amount of pollutant absorbed can be calculated by the below equation:

qt ¼

ðC o −C t ÞV m

ð5Þ

where qt is the amount of adsorbed adsorbent at time t in mg/g. Co and Ct are the initial and final concentrations of the pollutant at time t in mg/l respectively. V and m are the volume of organic pollutant and the mass of adsorbent in grams.

Fig. 12. Adsorption kinetics of CuS/TU catalyst for a) nitrobenzene and b) 4-nitrophenol.

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method and reused for its catalytic property. After four cycles, there was no significant loss of photocatalytic activity, which indicates that CuS catalysts were not deactivated and having excellent stability and recyclability properties. 3.6. Phocatalytic mechanism

Fig. 13. Cycling degradation of nitrobenzene and 4-nitrophenol pollutant for CuS/TU catalyst.

The pseudo-first-order was analyzed to better understand the adsorption mechanism of organic pollutant onto the CuS/TU catalyst. The pseudo-first-order equation is given as follows: ln ðqe −qt Þ ¼ ln qe −k1 t

ð6Þ

where qe and qt are the sorption capacity at equilibrium and time t in mg/g and k1 is the pseudo-first-order rate constant. The values of ln (qe − qt) were linearly fitted with t. This plot give maximum linear regression coefficient (R2 = 0.926 for nitrobenzene and R2 = 0.889 for 4nitrophenol pollutant) indicating the kinetics fitted with the pseudofirst-order model and the corresponding kinetic plot is given in Fig. 12. The overall comparison of photodegradation % of organic pollutants in the presence of CuS catalyst and P25-TiO2 samples is given in Fig. S2. For comparison the degradation was performed with commercial P25TiO2 sample without any catalyst. The photocatalyst effect for CuS/TU catalyst showed higher degradation rate for the two organic pollutants than the CuS/STS catalyst and commercial P25-TiO2. To further know the stability and recyclability properties of CuS catalyst, it has been tested by repeating the degradation process of the CuS photocatalyst. Fig. 13 shows the degradation plot vs number of cycles. After each cycle, the CuS catalyst can be easily separated by precipitation and centrifuge

The schematic degradation mechanistic of CuS catalyst with band energy level with respect to NHE is shown in Scheme 2. Under visible light illumination, the activated electron excited from valence band (VB) is transferred to conduction band (CB) by creating holes in the valence band. These charge carriers will recombine quickly and forms photoexcited e−/h+ pairs. These photoexcited charge carriers may either recombine or absorbed by other species such as oxygen or water by forming reactive oxygen species such as hydroxyl and superoxide radical anions. The holes react with H2O to produce hydroxyl radicals while electrons react with O2 to form superoxide radical anions and H2O2 [51]. These radicals will break and destroy the various pollutants thus forming non-toxic CO2 and H2O [52]. The conduction band of CuS is −1.14 eV and the valance band of CuS is 0.94 eV. The energy levels are favorable for photo-induced electrons to transfer, which can efficiently separates these electron and delay the charge recombination in the electron transfer process, these resulting in enhanced photocatalytic performance. The band energy diagram of CuS photocatalyst is given in Scheme 3. CuS/TU catalyst with more number of nanoplates as compared to CuS/STS catalyst absorbs more number of photons and produce electron–hole pairs under visible light irradiation [53]. Furthermore, these nano-sized hexagonal platelets could decrease the electron–hole pair recombination resulting in enhanced photocatalytic properties of organic contaminants [54] and thus accounting for higher photocatalytic activity in CuS/TU. The degradation mechanism of CuS photocatalyst is proposed as follows: [55] þ

CuS þ hv→h −

þ

OH þ h

þ

H2 O þ h −

O2 þ e

VB

−

þe

CB

• VB →OH

• VB →OH

CB →O2

ð7Þ ð8Þ

þH

þ

•−

Scheme 2. Schematic representation of degradation mechanism for the 4-nitrophenol pollutant with CuS photocatalyst under visible light irradiation.

ð9Þ ð10Þ

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3.7. EIS measurements and electrochemical property

Scheme 3. Band energy diagram of CuS photocatalyst.

O2

•−

2O2

−

þe

•−

CB

þ

þ 2H →H2 O2

þ

þ 2H →O2 þ H2 O2 −

H2 O2 þ e Dye þ h

þ

−

CB →OH

VB →Dye •

•

•þ

•

þ OH

→Final product

Dye þ OH →Dye →Final product

ð11Þ ð12Þ ð13Þ ð14Þ ð15Þ

To further show that CuS/TU has more conductive properties than CuS/STS, electrochemical impedance measurements were done. The obtained Nyquist plots for electrodes in the frequency range of 0.01 Hz to 100 kHz are shown in Fig. 14. It could be seen from the figure that there was no semicircle region at higher frequency in both electrodes, which is probably due to the low faradic charge-transfer resistance [56]. The intercept between the impedance plot and the real impedance (Z′) axis at higher frequency gives the solution resistance (Rs), which reflects the ionic conductivity of the electrolyte. The CuS/TU electrode showed lower solution resistance of 112 Ω than CuS/STS electrode (206 Ω) suggesting the better ionic conductivity between electrolyte and CuS/Tu. Fig. S3 shows the Nyquist plot of bare glassy carbon electrode. A higher Rs value (363 Ω) of bare electrode suggested that the CuS modified electrodes reduced the overall solution resistance and improved the electrochemical performance. Thus results show that CuS/ TU might have more active points due to its unique morphology and hence contributing to the good electrochemical properties. The fitting circuits of electrodes are given in inset of Fig. 14. Bode phase angle plots of CuS/Tu and CuS/STS electrodes are given in Fig. 15. Both electrodes exhibited a phase angle close to −50°, indicating the capacitive nature of electrodes. Three slopes at different frequency regions have been observed from the graph of total impedance versus frequency. In both the electrodes, a high slope at lower frequency region (1– 0.01 Hz) is observed, which is probably due to the resistive behavior of electrode surface interfaces [57]. The slope values at mid frequency region (100–1 Hz), is mainly due to the capacitive component and the lower slope value for CuS/TU electrode suggests the more conductive performance than CuS/STS based electrode. At higher frequency region (100–105 Hz), the electrodes exhibited both the resistive and capacitive behavior. The high conductivity for CuS prepared from TU could be due to the structural effects. The material was observed to be well structured and defined, and this might have an effect on the electron transport process. The EIS results suggest that CuS catalyst prepared from thiourea surfactant act as excellent charge carrier transporter and hence enhanced the photocatalytic activity towards nitrobenzene and 4-nitrophenol organic compounds.

Fig. 14. Nyquist plots of (a) CuS/TU and (b) CuS/STS based electrodes.

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Fig. 15. Bode plots of phase angle and bode plots of total impedance versus frequency of (a) CuS/TU and (b) CuS/STS based electrodes.

4. Conclusions CuS nanostructures are successfully synthesized by hydrothermal route using copper nitrate as a copper source and thiourea and sodium thiosulphate as sulfur sources respectively at 150 °C for 24 h. XRD results suggest the presence of pure hexagonal phase of CuS nanocrystals. The morphology difference of the CuS nanostructures synthesized using two different sulfur sources has been studied in detail. Results showed that the sulfur source plays a crucial role on the formation of CuS morphology and size of the CuS nanostructures. The prepared CuS photocatalyst was tested for its photocatalytic properties of nitrobenzene and 4-nitrophenol organic compounds. Results revealed that CuS catalyst showed enhanced degradation properties under visible light irradiation. The electrochemical impedance measurement showed better electrochemical conductivity for CuS/TU nanostructures. Conflict of interest The authors declare no conflict of interest. Acknowledgments The authors gratefully acknowledge the financial support of VIT University, Vellore, India for supporting this work under the research associate fellowship. This work was also supported under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B5-2434) South Korea. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2015.03.041. References [1] Z. Wan, G. Zhang, Controlled synthesis and visible light photocatalyitc activity of Bi12GeO20 uniform microcrystals, Sci. Rep. 4 (2014) 6298. [2] N. Sahiner, H. Ozay, O. Ozay, N. Aktas, A soft hydrogel reactor for cobalt nanoparticle preparation and use in the reduction of nitrophenols, Appl. Catal. B Environ. 101 (2010) 137–143. [3] T.S. Dhas, V.G. Kumar, L.S. Abraham, V. Karthick, K. Govindaraju, Sargassum myriocystum mediated biosynthesis of gold nanoparticles, Spectrochim. Acta A 99 (2012) 97–101.

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