Environmentally Sustainable Fabrication of Cu1.94S-rGO Composite

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Environmentally Sustainable Fabrication of Cu S-rGO Composite for Dual Environmental Application: Visible Light Active Photocatalyst and Room Temperature Phenol Sensor Suvanka Dutta, Sourav Biswas, Ram Chandra Maji, and Rajnarayan Saha ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03186 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Sustainable Chemistry & Engineering

Environmentally Sustainable Fabrication of Cu1.94S-rGO Composite for Dual Environmental Application: Visible Light Active Photocatalyst and Room Temperature Phenol Sensor Suvanka Dutta, Sourav Biswas, Ram Chandra Maji, Rajnarayan Saha* Suvanka Dutta: [email protected] Sourav Biswas: [email protected] Ram Chandra Maji: [email protected] Department of Chemistry, National Institute of Technology, Durgapur, WB, India-713209 * Author to whom all the correspondence should be addressed. Email: [email protected]

Abstract In the present study a novel Cu1.94S-rGO composite is synthesized in ecofriendly way and its application as photocatalyst and phenol sensor is explored. The extensive microscopic and spectroscopic characterization tools were used to confirm the structures and chemical vicinity of synthesized djurleite nanocomposite. This highly permeable mesoporous composite is showing direct band gap of 2.17 eV which confer its semiconducting properties. Wet hydrogen peroxide catalytic oxidation method of photocatalysis was adopted here for the photo degradation of a model cationic and an anionic dye which results 90.76% and 79.44% removal efficiency respectively with the fixed catalyst dose and minimum additive of 0.55mM hydrogen peroxide dose. While the removal efficiency improved significantly by the addition of more amount of hydrogen peroxide. A significant reusability is also observed here in both kinds of dye molecules which contingent in its potential applicability. It also exhibits a good response toward phenol in the range of 0.2µM to 1.4 µM concentration by modified Cu1.94S-rGO/GCE . Sensing ability of such composites in different scan rate, different pH and different types of molecules are also studied. Spiking recovery process inferred that modified electrode can be really useful device for detection and quantification of phenol in real sample also. Surely this study opens up the new possibilities for the application of inorganic semiconductor materials in betterment of our near future.

ACS Paragon Plus Environment


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Keywords: Cu1.94S-rGO, Visible light active Photocatalyst, Dye degradation, Phenol Sensor, Cyclic voltammetry Introduction In the last few decades a major group of researchers are focusing on finding the ecofriendly synthesis route of fabricating materials for identification, quantification and total eradication of toxic organics in wastewater. This quest eventually found out that graphene or reduced graphene oxide (rGO) based nanocomposites is quiet efficient in water treatment and also as a sensor for different toxic pollutants.1-6 In this field UV light driven inorganic semiconductor materials are already explored in various combination of Metal oxides and Metal oxide rGO composites, 1, 5, 715

but not much attention paid to visible light driven photocatalyst where maximum portion of

the natural solar energy would be utilized. Recently, very few successful studies of chalcogenides-based visible light driven photocatalyst have been reported.

3-4, 6, 16-19

Among the

various metal chalcogenides, copper sulfide is a p-type semiconductor material with very narrow band gap and has been focus of considerable attention not only because of its excellent optical, electrical, physical and chemical properties but also due to its prospective applications in many fields, such as solar energy converter, optical filter, cathode material and superconductors at low temperature.20-25 It is believed that chemical composition and morphology of chalcogenides nanoparticles greatly affect the resulting properties and potential application. Here copper sulfide attained special interest because of its distinct properties arising from the diverse nonstoitiometric composition, complex structure and valance state.26-31 Copper sulfides can be sub divided into several groups depending on the Cu/S ratio. Though, the monosulfide group (1.6≤Cu/S≤2) of Copper i.e., Cu1.94S (djurleite), Cu1.8S (digenite), Cu1.6S (anilite) and Cu1.75S (geerite), are particularly popular because of their excellent semiconducting properties. Tailoring them onto the surface of reduced graphene oxide can enhance the inherent properties of these nanoparticles, as strong conjugation of rGO allows excellent electronic mobility and the conductive surface of sp2 carbon framework helps in shuttling the electrical charges between nanoparticles.32-33 Moreover, this composite can form mesoporous photocatalyst, which in turn have high surface area with exposure of higher reaction site to absorb dye molecules and light energy. Among the few reported

copper mono-sulfide and its reduced graphene oxide

composite, Mondal et al, synthesized Cu2-xS by using pyrazolyl-based thiolato single-source


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precursor and used as an effective photocatalyst, on the other way , Yu et al., prepared rGOCuS/Cu2S for visible light active photocatalyst.

34-35

As a sensor copper monosulfide showed

good detecting ability of glucose and hydrogen peroxide in the work of Lu et al, and also Vinocurov et al., explored comparative peroxide sensing ability of Cu2S and Cu2-xS.6, 36-37 Yet no photocatalysis or sensor application is still reported solely with copper sulfide djurleite or its rGO composite. Because of the smaller band gap of copper sulfide nanoparticles, wet hydrogen peroxide catalytic oxidation (WHPCO) process is most successful. High efficiency, economic, low air and water discharge of secondary pollutant made this WHPCO very popular. 38 However, the scalable synthetic routes to highly mono-disperse copper sulfide nanoparticles with a distinct shape and composition is very rare. Herein, copper sulfide djurleite and its mesoporous rGO composite are synthesized by facile hydrothermal method transforming the process of Zhang et al., 6 where the novel L-cysteine is used uniquely as sulfur source, reducing agent and the binder of metal sulfide on to rGO surface. Extensive characterization tools are used here to confirm the successful synthetic routes and determination of final structural conformation of synthesized nanomaterials. Because of small band gap of this composite, WHPCO method of photocatalysis is proved to be more effective in this study. As synthesized nanoparticles are checked for their photocatalysis property with two model dyes (cationic and anionic). Both the investigated dyes are extensively used in the textile, paper and leather industries and medicines. Both are mutagen, mitotic poisons and crystal violet is also a carcinogens.39-40 Due to the higher surface area of Cu1.94S-rGO composites, provides more amounts of active sites for catalysis and makes it superior photocatalyst in both cases. Here apart from the water treatment ability of these synthesized nanoparticles, we have tried to explore if they have any sensing property of any kind of water pollutants. Though, application of Cu1.94S-rGO composite in the field of sensors is less explored comparative to CuS-rGO. All those, CuS-rGO decorated electrodes showed effective sensing efficiency for glucose41 and hydrogen peroxide 37at room temperature. However, here we have first time reported the superior phenol sensing ability of Cu1.94S-rGO modified electrodes which have higher surface area and better electron transport ability. Phenols, the priority pollutant, mainly releases into the environment via various natural and anthropogenic activities. They are known to be very recalcitrant, carcinogenic and toxic in nature. But the sensing of such toxic material has been scarcely researched. 42-43 To the best of our knowledge, there has been no such systematic attempt also to correlate the dual applicability of such semiconducting materials



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till date. The superior applicability of such material is also evident from comparison with existing literature (Table S1 and Table S2). Experimental Section Materials Copper nitrate tri-hydrate, H2SO4, H3PO4. HCl, KMnO4 and Hydrogen Peroxide were procured from Merck India. L Cysteine was obtained from Sigma Aldrich. Methyl orange, Crystal violet dye powders and Graphite flakes were purchased from Loba Chemie (India). All the chemicals were used without any further purification. Synthesis of Graphene oxide Graphene oxide (GO) was synthesized by following previously reported room temperature synthetic protocol.44 Initially graphite flakes were mixed with a mixture of concentrated H2SO4 and H3PO4 (9:1). Subsequently KMnO4 was added drop-wise into the mixture with continuous stirring at room temperature. Then the mixture was kept for 3 days at room temperature with continuous stirring. After completion, H2O2 was added into the mixture till the color change from black to yellowish. Finally the solid mass was obtained by numbers of centrifugation and washing with water and HCl. Synthesis of Cu1.94S and Cu1.94S-rGO composite At the beginning 1.22 g of L-Cysteine was dissolved in 90 mL Distilled water. Then 10 mL of 1 (M) copper nitrate solution is added drop wise into this solution with continuous stirring. After 2 h of stirring at room temperature, the solution is transferred to a custom made teflon liner of 120 mL capacity and was put in stainless steel autoclave. Autoclaving is done 140˚ C temperature for 14 hours. After the reaction vessel cools down to room temperature the whole mass and solution filtered with GF/C filter paper by a vacuum filtration system. The product is washed with 500 mL of distilled water (18 mΩ) then with 100 mL of absolute ethanol. The obtained black powder is vacuum dried at room temperature and stored in an amber color 15mL culture bottle in a vacuum desiccator. Almost similar procedure was followed while preparing the Cu1.94S-rGO composite. Here 2.8 g of L-Cysteine was added in 90 mL GO and to this solution 10 mL of 1


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(M) copper nitrate solution added drop-wise. Rest of the process is as exact as the synthesis of Cu1.94S. Characterization The morphological studies were done using a field emission-scanning electron microscope (Bruker XFlash 6160 FESEM) and a high-resolution transmission electron microscope (TECHNAI TF20 ST HRTEM). Crystal structure of the as-grown materials was determined using Pananalytical X’pert Pro X-ray Diffractometer. UV–Vis absorption spectra of dispersed nanoparticles solution were recorded by a UV–Vis spectrophotometer (1601SHIMADZU). FTIR Spectra of the samples were recorded by Thermo Nicolet iS10 spectrometer using KBr pellets in the range 4000- 400 cm-1. pH of the dye solutions were measured using Orion star A214 Thermo Scientific pH meter. Brauner–Emmet–Teller (BET) surface area was obtained using a NOVA 1000e surface analyser of the maker Quantachrome Instrument, USA. LabRAM HR was used for Raman experiment. Thermogravimetric analysis (TGA) was carried out using Simultaneous Thermal Analyzer (STA 6000). Electron paramagnetic resonance (EPR) spectra were recorded using MiniScope MS 5000 Magnettech EPR spectrometer. Photoluminescence spectra were recorded using a Hitachi F-2500 spectrofluorimeter at room temperature. Photooxidation of Cationic dye Crystal violet (CV) and anionic dye Methyl orange (MO) was conducted separately in a batch process (dye structures given in ESI Figure S1). Details of the WHPCO method with the reaction cabinet design (ESI Figure S2) are given in supporting information section. Sensing activity was measured by CHI 660B (Chenhua, Shanghai), the detailed electrode system was explained in the supporting information section. Results and discussion Characterization of synthesized nanostructures A facile one pot, template-free hydrothermal synthetic route is reported here for fabrication of Cu1.94S and Cu1.94S-rGO which is schematically described in scheme 1 and scheme 2. We presume that the formation mechanism of Cu1.94S with considering the interaction of copper (II) ion with the precursor L-cysteine which is eco-friendly in-situ source of S2- ions. L-cysteine, is listed in essential amino acids, acts as potential chelating agent for any soft metal centres at neutral pH. This information leads us to form copper (I)-cysteine complex through hydrothermal synthesis procedure while starting from specific amount of copper (II) nitrate and L-cysteine



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solution. At first copper (II) nitrate on reaction with cysteine reduces to Cu (I) and simultaneously cysteine itself oxidizes to cystine (disulfide form) as thiols are strong reducing agent.45-48 According to soft-hard acid base (HSAB) principle CuI is soft cation and sulfur is soft anion, so form stable [CuI(µ2-S-cysteine)]2 complex. During autoclaving at 140°C, the complex disintegrates, and controlled amount of S2- ions are released upon breakage of C-S bonds (which SCys coordinated to metal centre) of cysteine and ultimately forms Cu1.94S nuclei and selfaggregated to minimize the surface energy. In presence of GO sheets similar types of selfaggregation occur at the carboxyl functional sites due to electrostatic interaction.

HOOC CuII(NO3)2.3H2O

NH2 HOOC

SH

H2N

HOOC S CuII

CuII NH2 S

H2N

HOOC S CuI

CuI NH2 S

COOH

COOH

H2N

S CuI

CuI NH2 S COOH

Cu1.94S

Scheme 1. Synthesis mechanism of Cu1.94S


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Scheme 2. Synthesis mechanism of Cu1.94S-rGO composite

As synthesized Cu1.94S and Cu1.94S-rGO composite were characterised using various optical and spectroscopic technique. SEM and TEM images evidently confirm that the successful synthesis of Cu1.94S and Cu1.94S-rGO (Figure 1). Here figure 1a and 1d depict the SEM and TEM images of synthesized GO sheets. The self-aggregation process of Cu1.94S nuclei leads to formation of Cu1.94S nanoclusters with average particle size 20-30 nm which is obtained from TEM images (Figure 1b and 1e). However a slight reduction in size of Cu1.94S observes in presence of GO sheets (Figure 1c and 1f). It also observes that Cu1.94S nanoclusters are unevenly distributed on the surface of GO sheet, indicates the electrostatic interaction originated form carboxyl moieties which mainly situated on the surface of GO. EDAX analysis (ESI Figure S3) efficiently confirms the purity and atomic percentage of copper and sulfur content which are closely matched with djurleite. The average atomic percentage ratio of Cu and S is 1.932:1.



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Figure 1. SEM micrograph of (a) GO, (b) Cu1.94S, (c) Cu1.94S-rGO and TEM image of (d) GO, (e) Cu1.94S, (f) Cu1.94S-rGO X-ray diffraction on powder samples was performed for studying crystal structure of Cu1.94SrGO composite, Cu1.94S nanoparticles and GO and the obtained difractogram is produced in Figure 2a. Peak at 10.8° is the characteristic peak of the (002) plane of GO. Whereas, this peak is absent in Cu1.94S-rGO composite, indicating the effective reduction GO into reduced graphene oxide (rGO). The obtained peak positions correspond to djurleite type patterns of the sample and highly coincide with JCPDS card no. 01-071-1383 (ESI Figure S4). Characteristics broad signals correspond to the Bragg angle for different planes for the crystalline djurleite, which are in accordance with the previous research.49 Although XRD peaks suggest the djurleite structure of synthesized nanoparticles, but to find the exact oxidation state of copper in the nanoparticles we have done EPR analysis (Figure 2b and 2c). Cu2-xS and Cu2-xS-rGO in air shows isotropic signal (S= ½) at 298K (red trace in b and violet trace c) with g= 2.17 and 2.08 respectively whereas CuCl2 at 298 K (black dotted trace in b and c) gives axial signal with g= 2.18 and g= 2.07. Interestingly intensity of the spectrums of


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Cu2-xS and Cu2-xS-rGO less than CuCl2 because in CuCl2 , copper in +2 oxidation state (d9, EPR active) but in Cu2-xS and Cu2-xS-rGO copper in +1 oxidation state (d10, EPR silent).50 Interestingly in N2 atmosphere (when not exposed in air after formation) both Cu2-xS and Cu2-xSrGO are EPR silent (wine trace in b and purple trace in c). So it is confirmed that in Cu2-xS and Cu2-xS-rGO all copper in +1 oxidation state like monovalent Cu2S, but due to Jahn Teller distortion, formation of stabilized d9 system (+2 oxidation state due to surface oxidation) efficiently shows isotropic signal. Intensity of the signal increase at 77K (orange trace in b and magenta trace in c) due to increase of population in the ground state for both Cu2-xS and Cu2-xSrGO.

Figure 2. (a) XRD pattern for Cu1.94S, Cu1.94S-rGO and GO; X-Band EPR spectra measured in solid state (b) of Cu2-xS at 298K (red trace), Cu2-xS at 77K (orange trace), Cu2xS

in N2 at 298K (wine trace), (c) of Cu2-xS-rGO at 298K (violet trace), Cu2-xS-rGO at 77K

(magenta trace), Cu2-xS-rGO in N2 at 298K(purple trace). Dotted black traces in b and c are CuCl2 in solid state at 298K. The thermal stabilities of the as-grown samples are shown in Figure S5. The TGA curve of the Cu1.94S (red trace in Figure S5), shows minimal weight loss (12%) in the entire temperature range of 35°C to 800°C which can be attributed to the loss of surface bound water. In case of Cu1.94S-rGO composite up to 250oC, the weight loss may be related with loss of surface bound water. However, the weight loss from 250oC to 800°C can be attributed to the removal of oxygen-containing groups of rGO and the disintegration of the carbon framework in the



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composite. TGA analysis shows that the rGO mass ratio of the Cu1.94S-rGO is ~31 wt%. The successful formation of Cu1.94S and Cu1.94S-rGO composites are confirmed by FT-IR study also (Figure 3a). Peaks in the range of 400 cm-1 to 650 cm-1 are the indicator of Cu-S bond formation; as this region is the fingerprint region of metal sulfide bond. FT-IR spectra of GO also present in the Figure 3a as a control. Strong and broad peak at ~ 3425 cm-1 and 1716 cm-1 can be assigned to the stretching vibrations of O–H and C=O groups which are presented on the surface of GO sheets. The intensity of absorption peaks of the oxygen-containing groups in GO sheets dramatically decreased due to deoxygenation in case of formation of Cu1.94S-rGO composite. The peak at 1716 cm−1 that arises from C=O stretching mode, completely disappears in Cu1.94SrGO composite and the peak of the unsaturated and saturated C-OH group has shifted from 1027 cm-1 to 1047 cm−1 and 1082 cm-1 to 1092 cm-1 respectively. The acuity of the peaks may be ascribed to the presence of unreacted side chain hydroxyl groups, which also specifies that the C=O functional group in GO is decomposed after the hydrothermal reduction treatment. Eventually it suggests a partial reduction of GO sheets during formation of Cu1.94S-rGO composites; such result is corroborated with previously explained XRD data. Cu-S stretching node also observed at 618 cm-1 in Cu1.94S-rGO composites, suggest the successful bond formation of Cu-S on GO sheets.51-52 For more understanding the reduction of GO sheets and coupling of Cu1.94S on to the GO, we performed Raman spectroscopy of control GO and Cu1.94SrGO composites in Figure 3b. Raman spectra clearly show two signature bands of a carbon system; a G-band which corresponds to the stretching of sp2 hybridized carbon atoms and a Dband which corresponds to the disordered carbon atoms and defects. The intensity ratio of the bands (ID/IG) indicates the amount of disorder and defects in the structure. It is observed that G and D bands of GO are located at 1598 and 1352 cm-1, whereas in case of as synthesized Cu1.94SrGO these 2 bands are found to be at 1585.78 and 1353.89 cm-1. Moreover, the intensity ratio (ID/IG) was also significantly changed from 0.95 to 1.19, indicating the reduction of GO during the hydrothermal process which efficiently enhanced the defect sites.53-55 Further peak at 471 cm1

indicates attachment of Cu1.94S on rGO surface.

To be a noble contender for a photocatalyst, a greater catalyst surface area along with lager pore size and pores volume will be essential resulting in higher photo-catalytic activity, so the specific surface area of a photo-catalyst is a most significant factor. In general larger specific surface are can provide more active catalytic sites for photo-catalysis reaction. Here the Brunauer–Emmett–


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Teller (BET) sorptometry was conducted to determine the specific surface area of synthesized Cu1.94S and Cu1.94S-rGO composites by nitrogen adsorption-desorption measurements in Figure 3c. The BET surface area of Cu1.94S is found be 16.12 m2/g, whereas the same for Cu1.94S-rGO composites is 109.95 m2/g. The higher surface area of the Cu1.94S-rGO composite can be attributed to the presence of exfoliated rGO sheets. The N2 adsorption-desorption curve of Cu1.94S presents a type III isotherm indicating lateral interaction between molecules are strong. However, Cu1.94S-rGO composite exhibits a type IV isotherm due to unrestricted multi-layer formation of GO sheets, and the hysteresis loop at 0.45 to 1 is associated with the capillary condensation at mesopores. It also confers that additional adsorption is occurring as the interaction of adsorbate with adsorbed layer is greater than the interaction with the adsorbent surface. The pore size calculation is done by BJH (Barret- Joyner- Halenda) method and average pore radius were found to be ~ 2.98 nm for Cu1.94S-rGO and 39.25 nm for Cu1.94S. The unique double pore structures along with considerably high surface area make Cu1.94S-rGO composites as a favourable candidate for photo-catalysis.

Figure 3: (a) FTIR of Cu1.94S, Cu1.94S-rGO and GO; (b) Raman of Cu1.94S-rGO and GO; (c) N2 adsorption-desorption isotherms of Cu1.94S-rGO NC; Insets represent the N2 adsorption-desorption isotherms of Cu1.94S The optical property of the dispersion was ascertained by UV-visible spectroscopy. Figure 4a represents the absorption spectra of bearing λmax= 432 nm for Cu1.94S-rGO as the reminiscent of absorption while Figure S6 shows the absorption spectra of GO where λmax= 230 nm is the characteristic peak. Distinct increased absorption in the region of near infra-red (NIR) region can



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be due to the formation of Cu1.94S-rGO nanoparticles which is evocative for interband transitions, valance band to unoccupied state.56-58 The photoluminescence of Cu1.94S and Cu1.94SrGO is important because of its photovoltaic, photoconductive and rectifying response under illumination. The room temperature PL activity was determined with the ethanolic suspension of Cu1.94S and Cu1.94S-rGO in Figure 4b (Pl of Cu1.94S given in ESI Figure S7a). From the reported work it can be suggested that the nature of the emission spectrum depends on the morphology and inherent structure of sample .51, 59 Here, upon exciting the nanoparticles at 350 nm, 3 peaks generates at 405, 427 and 454 nm which originates from direct bandgap.49,

60

The distinct

absorption in the visible zone and increased absorption in the NIR region (Figure 4a) indicative of the presence of both direct and indirect band gap. Now, these band gap energies are calculated from the equation “αhυ = A (hυ- Eg)n”, here A is a constant, hν is photon energy, Eg is the allowed energy gap, n =2 for direct band gap and ½ for allowed indirect transition. For direct band gap a plot of (αhυ)2 vs hυ based on the direct transition shown in Figure 4c and for indirect band gap (αhυ)1/2 vs hυ plotted in Figure 4d . The extrapolated value of hυ at α=0 gives the absorption edge energy, corresponding to indirect band gap of 1.31 eV and direct band gap 2.17 eV (Direct band gap of Cu1.94S estimation given in ESI Figure S7b). Direct bandgap is higher than the bulk Cu1.94S (1.85 eV), indicating the quantum confinement effect of nanoscale size. 58, 61


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Figure 4: Optical measurement of as-synthesized nanoparticles; (a) Absorbance spectra of Cu1.94S-rGO composite, in inset absorbance spectra of Cu1.94S; (b) PL spectrum while excited at 350 nm and deconvolution of the peaks; (c) Plot of (α αhυ) υ)1/2 vs photon energy hυ υ for determination of indirect band gap; d. Plot of (α αhυ) υ)2 vs photon energy hυ υ for determination of direct band gap Photocatalytic activity of synthesized nanoparticles To establish the prospective application of synthesized Cu1.94S and Cu1.94S-rGO composite, in degradation of organic pollutants, we have examined the photocatalytic activities by choosing the photodegradation of an anionic dye (Methyl Orange) and a cationic dye (Crystal Violet) in the presence of Hydrogen peroxide. Although methyl orange is weakly toxic but crystal violet is environmentally recalcitrant dye, potent carcinogen and mitotic poison. Two different sets of blank experiments for the two dyes, using only visible light, H2O2, Catalyst in dark and the same combination in light were performed (ESI Figure S8a and b). Figure 5 exhibits, photocatalytic activity of Cu1.94S and Cu1.94S-rGO, where maximum absorption originates at 464 nm due to MO



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and 590 nm for Crystal Violet. It is important to note that the intensity of the absorption spectra decreases as the exposing time increases from 0 to 60 min. With regard to the reduction of the absorbance and absence of any new peak, we can assume that intermediates with stronger absorbance than both of the dyes are not formed or those are unstable under the experimental condition. However ESI Figure S8a and b clearly suggest better photodegradation efficiency of Cu1.94S-rGO in comparison with Cu1.94S. Mainly under light irradiation, due to the semiconducting nature of Cu1.94S, valence band electrons of are excited to conduction band while holes are created in the valence band. Normally, these charge carriers both electrons and holes, quickly recombine, and only a fraction are contributed in the photocatalytic reaction process, causing in low reactivity. The detailed chemical reactions and radicals formations are explained in ESI Figure S9. However, embedding of Cu1.94S on to rGO sheets, the catalytic activity was enhanced which governed by several factors such as surface area, light response range, absorption capacity and recombination time. Since, due to the higher surface area of Cu1.94S-rGO composites, more electrons which get excited into the conduction band of Cu1.94S would lay on the surface and creating more amounts of active sites for catalysis. Furthermore, it can be assumed that graphene oxide sheets in Cu1.94S-rGO composites may enhances the recombination time also for photogenerated electron–hole pairs. This is supported by delocalized π structure nature of GO sheets, which facilitates the transfer of photo induced electrons and can perform as an excellent electron acceptor leading to hole-electron separation. So it can be speculated that Cu1.94S/rGO heterojunction has good catalytic activity mainly due to the higher surface area and synergistic effect of both nanomaterials. Scheme 3a exhibits the illustration of plausible mechanism of photocatalysis under visible light irradiation on Cu1.94S-rGO composites. Now for determining the reaction rates of heterogeneous photocatalytic degradation, LangmuirHinshelwood pseudo first order kinetics model was extensively used here. According to this model the reaction rate is proportional to the surface coverage of the dye molecules on the substrate. Now the plot of concentration vs time leads to a linear diagram so that its slope equals to the rate constant of photo degradation. Comparatively much better result observes in case of Cu1.94S-rGO (0.0019 min-1 Methyl Orange dye and 0.0022 for Crystal Violet dye) than Cu1.94S (0.0007 min-1 Methyl Orange dye and 0.0010 for Crystal Violet dye).


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Figure 5: Time dependent absorption spectral changes in MO and Crystal Violet Dye; (a) MO dye degradation with only Cu1.94S-rGO and Visible light (inset focus on reduction of λmax of the dye); (b) Crystal Violet Dye degradation with only Cu1.94S-rGO and Visible light (inset focus on reduction of λmax of the dye); Using the WHPCO method (c) MO dye degradation with Cu1.94S-rGO under visible light in presence of H2O2; (d) Crystal Violet Dye degradation with Cu1.94S-rGO under visible light in presence of H2O2. The narrow bandgap (2.17 eV) of Cu1.94S-rGO composite made it responsive to wider spectral range making it visible light and solar light active. Now it is observed that removal efficiency is quite lower in degrading the anionic MO dye comparative to the cationic Crystal Violet dye (Figure 5). Here, Figure 5a and 5b represent the photocatalytic degradation of the MO and crystal violet dye in presence of the photocatalyst and the visible light source only. Though the photocatalyst is visible light active but the concentration of photo-generated radicals are not enough for effective photocatalytic performance in both the cases. For better understanding λmax



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areas were focussed in the inset. Introduction of WHPCO method using hydrogen peroxide supplies sufficient amount of radicals rendering high photocatalytic degradation for both the dyes (Figure 5c and 5d). The synthesized photocatalyst Cu1.94S-rGO inherits negative charge because of surface bound OH- ions. Hence, the cationic Crystal Violet dye getting attached to the surface of catalyst by electrostatic attraction. Furthermore, π-π interaction between Crystal Violet dye and localized π orbitals of conjugated aromatic rings of rGO sheets also help in concentrating the dye molecules on the surface of the catalyst.62-63 Now under the light irradiation the concentrated dye molecules were encouraged to react with photogenarated active species, leading to the photocatalytic degradation towards Crystal Violet molecule (RE 90.76% with 0.55 mM HP dose). On the other hand, the elaborated structure of the anionic dye molecule, even this dye becomes polarizable, for possible electron transfer. As result MO dye degradation efficiency becomes comparatively lower (79.44% RE with 0.55 mM HP dose).

Figure 6. Time dependent removal efficiency with increased dose of hydrogen peroxide and fixed photo catalyst dose; (a) Anionic MO dye degradation; (b) Cationic Crystal Violet dye degradation It is observed that in all cases overall improvement in the photo-catalytic property of the prepared samples in presence of H2O2 whereas alone was not efficient for degradation of the dyes (ESI Figure S8). This is mainly attributed to the better electron accepting property of H2O2 than


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molecular oxygen and may increase the photo-catalysis rate by reducing the enhancement of electron-hole recombination and generation of active hydroxyl radicals. Removal efficiency and rate of degradation further increases with increase in H2O2 concentration with certain photocatalyst dose (Figure 6a and b) as the H2O2 dose is directly related to the number of hydroxyl radicals generated in the reaction medium due to decomposition of more amount of H2O2.

Figure 7. Plot of ln (C0/Ct) versus irradiation time (t) with fixed photocatalyst dose of varied dose of hydrogen peroxide (a) Anionic MO dye; (b) Cationic Crystal Violet Dye Upon increasing the concentration of H2O2 from 0.55 mM to 2.2 mM of the reaction medium degradation rate also increases from 0.0428 min-1 to 0.168 min-1 for MO dye (Figure 7a) and 0.0568 min-1 to 0.1661 min-1 for Crystal Violet dye (Figure 7b). So we can propose that the improvement in photodegradation of both cationic and anionic dyes relies on synergistic effect of both H2O2 and synthesized photo-catalysts. FTIR spectra of used Cu1.94S-rGO (it was separated by centrifugation after washing with distilled water and dried at 45° C for 24 hour) shows no identifiable peak of the dyes, indicating dye molecules are not simply absorbed on to the surface of the photocatalyst (ESI Figure S10). The reusability cycle of the photocatalyst after the heterogeneous reaction carries much of importance. Both the dye removal experiment was reperformed for 4 times using same Cu1.94S-rGO composite (ESI Figure S11) which was separated by centrifugation followed by washing with 5% dilute hydrochloric acid and distilled water and oven dried at 45° C for 24 hours. It is found that an excellent recyclability of Cu1.94S-rGO composite in both of the cases where the loss in removal efficiency for Crystal Violet is only



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~6.8% and for Methyl Orange is ~5.1%, after 4 cycles. SEM and TEM images of Cu1.95S-rGO composite after 4 cycles, also support its excellent reusability as no structural deformation is observed in the nanocomposite (Figure S12). Electrochemical determination of phenol sensing ability After establishing the synthesized Cu1.94S-rGO nanoparticles as a superior material for photocatalytic dye degradation, electrochemical performance of the modified GC electrodes by Cu1.94S-rGO were then explored by cyclic voltammetry. The modified electrode was tested using K3[Fe(CN)6] solution as redox probes by cyclic voltammetry. Figure 8a represents cyclic voltagrams of the bare Glassy Carbon Electrode (GCE), as prepared Cu1.94S/GCE and Cu1.94SrGO/GCE recorded in 0.02 M KCl solution containing 1 mM K3[Fe(CN)6] at scan rate 50mV/S at 298K using 1M Ag/Ag+ reference electrode and Pt wire as counter electrode. In comparison with the insignificant CV of the bare GCE (Figure 8a black trace), the GCE electrode modified by Cu1.94S (Figure 8a red trace) shows much higher redox response of [Fe(CN)6]3-/[Fe(CN)6]4-, which could be attributed to the augmentation of electron transfer of Cu1.94S to the electrode. Furthermore, in case of Cu1.94S-rGO /GCE (Fig. 8a violet trace), the redox current of [Fe(CN)6]3/[Fe(CN)6]4- sharply increases due to the enhancement of electron transfer led by the conductive core of Cu1.94S nanoparticles and the rGO matrix indicating greatly improved sensing ability. As the redox peaks are clearly shown in the CV for Cu1.94S-rGO /GCE, the quasi reversibility of the system can be presumed from ratio of anodic peak current vs cathodic peak current, ipa/ipc= 0.86.64 The CV of bare GCE, Cu1.94S/GCE, Cu1.94S-rGO/GCE in 20mM PBS (phosphate buffer solution) in presence 1µM phenol solution produced in Figure 8b. With the given potential phenol undergoes electro-oxidation forming the phenoxy radical (C6H5OH to C6H5O°). The detailed reactions are explained in the ESI Figure S13. This electro-oxidation increase due to enhancement in electron transfer in Cu1.94S-rGO/GCE as the highest current height for oxidation peak at 0.95 V observed, whereas for bare GCE shows oxidation peak at 0.61V with very low current height in comparison. The enhanced current response for Cu1.94S-rGO/GCE may be ascribed to the smaller size, higher surface area and better electron transfer of Cu1.94S-rGO NPs. In between Cu1.94S-rGO/GCE and Cu1.94S/GCE, the Cu1.94S/GCE electrode shows much lower current height at 0.82V for these oxidation. This may be attributed due to the larger particles size i.e., lower specific surface area (lower active sites) of the Cu1.94S NPs. It also assumes that cathodic peak current is achieved due to oxidation of phenol on electrode surface (Scheme 3b).


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So the modified Cu1.94S-rGO/GCE not only stimulates the transmission rate and adhesive ability of the marked object on the surface of the electrode but also assists and hastens the conversion of chemical to electrical signal, along with this large surface area and fast electron transfer rate of the rGO which provides much support to the decent sensor. 65-66

Figure 8. (a) Cyclic Voltagrams of the bare GCE and the modified Cu1.94S/GCE and Cu1.94S-rGO /GCE

in 0.02M KCl solution containing 1 mM K3[Fe(CN)6]; (b) Cyclic

voltagrams of phenol using GCE, modified Cu1.94S/GCE, modified Cu1.94S-rGO/GCE as working electrode at scan rate 50mV/s at 298K using aqueous Ag/Ag+ reference electrode and Pt wire as counter electrode Moreover, electrochemical mechanism can be acquired from the relationship between the peak current and scan rate. Hence in Figure 9a represents the CV of 1µM phenol at the Cu1.94SrGO/GCE with different scan rate: 10, 30, 50, 70, 90, 110, 130 and 150 mV/s. This study resulted in increased oxidation peak current with the elevation of scan rate linearly in the range of 10-150mV/s following Cottrell equation as shown in Figure 9a inset. Though little anodic shift is observed at higher scan rate which is proves the quasi reversible nature of the oxidation. The result indicates that the oxidation of phenol on Cu1.94S-rGO/GCE was a typical adsorptioncontrolled process. Consequently, it can be used for qualitative analysis of micro-level of phenol



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on to the surface of Cu1.94S-rGO/GCE. In order to decrease background current and obtained high sensitivity, the scan rate was chosen as 50 mV/s for the further experiment.

Figure 9. (a) Cyclic voltagrams of phenol at different scan rate 10, 30, 50, 70, 90, 110, 130, 150 mV/s; (b) Cyclic voltagrams of phenol, 2-nitrophenol and 2-amino phenol, Scan rate 50 mV/s , in both the cases using modified Cu1.94S-rGO/GCE as working electrode, platinum wire as counter electrode, aqueous Ag-Ag+ as reference electrode. Concentration of phenols is 1 µM for all experiments. For evaluating the specific recognition ability of the sensor, the CVs of Cu1.94S-rGO/GCE in PBS solution containing different phenolic compounds such as Phenol, 2-amino phenol, 2-nitro phenol were recorded respectively, which are shown in Figure 9b. Obviously the CVs of Cu1.94SrGO/GCE recorded in different phenolic solutions shows quite similar current height at around 0.80V-0.95V, i.e., only one peak at 0.80V - 0.95V conforming to phenol oxidation is present, representing the specific recognition ability of Cu1.94S-rGO/GCE to phenol. Though cathodic shift is observed for 2-amino phenol as amino group is electron donating group, which favours phenol to phenoxy radical oxidation (ESI Figure S13). For 2-nitro phenol little anodic shift was observed due to electron withdrawing effect of nitro group which disfavour the oxidation in comparison with un-substituted phenol.


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The effect of pH on the oxidation peak at around 0.80V – 0.95V is also studied (shown in Figure 10a). Phenol itself shows weekly acidic nature in aqueous medium. Thus, in the acidic (pH 5) medium electro-oxidation gets encumbered and an anodic shift of the peak along with little increase of current height is observed. These little shift can be easily neglected for the detection of phenol in the concentration range of 2 × 10-7 M ~14 × 10-7 M. Figure 10b presents the CV of different concentration of phenol at scan rate 50 mV/s using modified Cu1.94S-rGO/GC as working electrode, platinum ware counter electrode, aqueous Ag-Ag+ as reference electrode. A linear relationship was fitted in the concentration range of 2 × 10-7 M ~14 × 10-7 M with relation coefficient of 0.993 (Figure 10b inset).

a)

b)

Figure 10. (a) Cyclic voltagrams of phenol at a conc. of 1.0 µM at different pH; (b) Cyclic voltagrams of phenol at conc. range 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4 µM, in both the cases using modified Cu1.94S-rGO /GCE as working electrode, platinum wire counter electrode, aqueous Ag-Ag+ as reference electrode. Scan rate 50 mV/s. The reusability and stability of the sensor was further studied by cyclic voltammetry of the used Cu1.94S-rGO/GCE sensor in PBS solution containing 1×10-6 M phenol. Figure S14 shows the peak position and current height of phenol oxidation remains almost same nearly about 7 times repeat of the experiment with the same used sensor. The calculated relative standard deviation of



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current height is 2.11% after repeat for 7 times. This demonstrates a good reusability of the fabricated sensor. The current response to phenol of prepared sensor after long storage was tested as well to study the stability of the sensor. The modified electrode retains 99% of its initial response after 5 days and 95 % after 10 days. Then it can be inferred that this sensor exhibits good stability and can be used for the detection of phenol. In order to confirm the applicability of the synthesize nanomaterials as a sensor, the fabricated electrode was used to detect phenols in two real samples, which were collected from tap water and canal water. Eventually both of them are spiked with 0.3, 0.5 and 1µM phenol and used as a sample. The results are listed in Table 1 clearly indicate that the prepared Cu1.94S-rGO/GCE is a reliable and effective electrochemical sensor for phenol detection in presence of other impurities.

Scheme 3. (a) Photo degradation mechanism of Cu1.94S-rGO composite under visible light irradiation and (b) electrochemical sensor mechanism for phenol detection by Cu1.94SrGO/GCE modified electrode. Table 1. Phenol detection in real samples

Sample

Tap water

Phenol added

Peak Current

Phenol found

Recovery

(µ µM)

(µ µA)

(µ µM)

(%)

0.3

215

0.3071± 0.0287

~100

0.5

321

0.5095±0.0490

~100


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River water

1

565

0.9878±0.0705

98.78

0.3

213

0.3024±0.0309

~100

0.5

319

0.5028±0.0527

~100

1

561

0.9782±0.0887

97.82

Conclusions In conclusion, we clearly demonstrated a facile synthetic procedure of djurleite copper sulfide and djurleite copper sulfide-reduced graphene oxide composites using a biomolecule as sulfur source cum reducing agent. As prepared composite showed excellent photocatalytic activity in visible light and sensor property. Because small band gap of this composite WHPCO method of photocatalysis was proved to be more effective in this case. A novel surface molecularly imprinted electrochemical senor for phenol with specific recognition and high sensitivity was also established. So the present study not only opens the new horizon for the synthesis of ecofriendly approach of chalcogenides based rGO composites, but also explored its fundamental importance in application oriented field. Supporting Information Dye structure, photocatalysis chamber and WHPCO method and sensing activity measurement method, EDX analysis of Cu1.94S-rGO, XRD matching analysis with reference file, TGA of the Cu1.94S and Cu1.94S-rGO, UV-Vis spectrogram of graphene oxide, Pl spectrum and bandgap estimation of Cu1.94S, control experiments of photocatalysis, mechanism of photocatalysis, FTIR of dyes and used photocatalyst, reusability cycles of Cu1.95S-rGO photocatalyst, SEM and TEM images of Cu1.94S-rGO after 4 catalytic cycle, Mechanism of phenol sensing, reusability graph for the sensor electrode, Comparison with existing literature in photo catalysis, Comparison with existing literature in electro-chemical sensor. Notes The authors declare no competing financial interest. Acknowledgements



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Table of Content A novel semiconducting and highly permeable Cu1.94S-rGO composite explored for achieving sustainability through Photocatalyst for textile dye degradation and electrochemical sensor material for water soluble phenols


Environmentally Sustainable Fabrication of Cu1.94S-rGO Composite

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