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Ultrasonics - Sonochemistry

journal homepage: www.elsevier.com/locate/ultson

Sonochemically recovered silver oxide nanoparticles from the wastewater ofphoto film processing units as an electrode material for supercapacitor andsensing of 2, 4, 6-trichlorophenol in agricultural soil samples

Elanthamilan Elaiyappillaib, Sakthivel Kogularasua, Shen-Ming Chena,⁎,Muthumariappan Akilarasana, Christy Ezhilarasi Joshuab, Princy Merlin Johnsonb,⁎,M. Ajmal Alic, Fahad M.A. Al-Hemaidc, M.S. Elshikhc

a Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 106, Taiwan, ROCbDepartment of Chemistry, Bishop Heber College, Tiruchirappalli, Indiac Department of Botany and Microbiology, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

A R T I C L E I N F O

Keywords:SonochemicalRecovered silver oxide nanoparticles2, 4, 6-Trichlorophenol sensorSupercapacitorSoil contamination

A B S T R A C T

The present work describes the sensing application and supercapacitive behavior of silver oxide nanoparticlesrecovered from wastewater of photo film processing units via one-pot green sonochemical recovery process. Therecovered silver oxide nanoparticles (Ag2O NPs) were characterized by spectral techniques such as FT-IR,Raman, UV–Vis and analytical tools such as XRD, FE-SEM, TEM, EDX, XPS and BET. In view of Ag2O NPs aselectrode material with wide technological applications, the recovered Ag2O NPs were examined for theirsensing and supercapacitive behavior. The developed sensor was explored to detect 2, 4, 6-trichlorophenol, andas expected it shows moral parameters which are required of an effective sensor. Therefore, it was exploited forthe quantification of 2, 4, 6-trichlorophenol in soil samples from the agricultural area. Cyclic voltammetric (CV),Galvanostatic Charge-Discharge (GCD) and Electrochemical Impedance Spectroscopic (EIS) studies on the re-covered Ag2O NPs coated Ni foam electrode depicted the pronounced capacitive behavior. The GCD studiesrevealed an enhanced electrochemical performance, particularly with the large specific capacitance of 530 F/g ata current density of 1 A/g. The cyclic stability of the electrode material was identified with 88% retention inspecific capacitance even after 5000 GCD cycles. These results strongly proved that the recovered Ag2O NPs arepotential candidates for sensing and supercapacitor applications.

1. Introduction

Nano metallic silver and its composites have found wide applica-tions in electrochemical equipment, medical products, textiles, agri-cultural fields, food and food packaging industry because of theirelectronic, optical, magnetic, catalytic and antimicrobial properties[1–3]. Moreover, due to their noticeable photosensitivity, they arewidely used in photography [4]. When exposed to light, the silver ha-lide (AgBr), coated on photographic films is reduced to metallic silver.During the progress and fixing of the film, silver halide crystals that arenot exposed to light are removed/leached by thio solution from the filminto the processing solution [5]. Since the photo film processing solu-tion is used repeatedly, the wastewater is rich in silver. The wastewaterfrom the photographic film processing units is reported to contain asilver content of 1–12 g/L [6] (see Table 1).

However, silver is quoted as one of the most toxic/hazardous metalsby the regulatory bodies. Further, the film processing effluents maypollute the soil and water, if disposed of without treatment [6,7].Moreover, the world silver demand steadily increases by about 2–2.5%per annum [6]. Therefore the recovery of silver takes prime importancefrom a commercial point of view. Thus Photographic wastes comprisingof scrap films and effluents offer a significant resource for secondarysilver usage [8,9]. Hence, treatment of wastewater of photo film pro-cessing units may contribute significant economic as well as environ-mental benefits.

Many recovery methods for silver from photo film processing unitsare reported, which include electrolysis, metal replacement, adsorption,chemical precipitation, hydrogen peroxide treatment, membrane se-paration [10,11] enzymatic emulsion liquid membrane extractionmethods [12] etc. and have their own limitations such as high cost, use

https://doi.org/10.1016/j.ultsonch.2018.09.029Received 11 August 2018; Received in revised form 9 September 2018; Accepted 20 September 2018

⁎ Corresponding authors.E-mail addresses: smchen78@ms15.hinet.net (S.-M. Chen), pmej_68@yahoo.co.in (P.M. Johnson).

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Available online 25 September 20181350-4177/ © 2018 Elsevier B.V. All rights reserved.

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of toxic chemicals, high electricity consumption and low yield. Only afew reports are available on the recovery of silver nanoparticles fromthe film processing wastewater using electro-ultrasonication [13], inwhich sonication is followed by electrolysis. But sonochemical recoverymethod for silver has not been reported yet. Moreover, it is noteworthyto mention that the sonochemical method comes under the principles ofgreen chemistry. Ultrasonic sound wave assisted extraction of silveroxide nanoparticle is found to be an efficient one, because, when liquidsare treated with ultrasound, acoustic cavitation can deliver a suitableenvironment for chemical reactions under extreme condition, whichinvolve in rapid formation growth and finally implosive collapse ofbubbles in the liquid. During sonication, collapsing of bubbles createsstrong heat and high pressure within the short time [14–17].

Many research articles have reported the application of silver basedcomposites as an active electrode material for supercapacitor applica-tion [18–20]. Yuksel et al. reported the specific capacitance of 500.7 F/g by electro-depositing AgNW/MoO2 with a coaxial structure using 1MLiClO4 in Propylene Carbonate (PC) as the electrolyte [21]. Tang et al.reported the specific capacitance of 615 F/g for SSCNTs/PANI/Agcomposite. Shen et al. fabricated AgNWs/WO3 electrode with a specificcapacitance of 138.2 F/g [22]. The specific capacitance of 450 F/g wasreported by Kalambate et al. for the GNS/AgNPs/PPY composite elec-trode, fabricated via in situ oxidative polymerization [23].

To the best of our knowledge, there are no reports on the study ofthe supercapacitive behavior of silver oxide nanoparticles, recoveredfrom photo film processing units as an electrode material. Herein, aone-pot green method has been explored to recover Ag as Ag2O NPs

from wastewater of photo film processing units via sonolysis. The as-recovered Ag2O NPs coated electrode exhibited excellent electro-chemical response with the high specific capacitance of 530 F/g ascompared to the silver based electrode materials mentioned in litera-ture [24]. Hence, the present work focused on (i) a simple, low-cost,rapid, one-pot sonochemical route to recover Ag2O nanoparticles fromwastewater of photo-film processing units, (ii) selective electrochemicaldetection and quantification of 2, 4, 6-trichlorophenol (2, 4, 6-TCP) inagricultural soil samples (iii) the evaluation of the as-recovered Ag2ONPs for the electrochemical supercapacitor application and (iv) thefabrication of supercapacitor electrode material with high specific ca-pacitance and excellent stability.

2. Experimental section

2.1. Materials used

The photographic wastewater were collected from MRI scan centersand X-ray divisions of various hospitals located in Tiruchirappalli dis-trict, Tamil Nadu, India. NaOH was purchased from Merck, India.Sodium sulfate (Na2SO4), Poly (vinylidene fluoride) (PVDF), Carbonblack and N-methyl-2-pyrrolidone (NMP) were procured from AlfaAesar. All the chemicals were used as received.

2.2. Sonochemical recovery of Ag2O NPs from wastewater of photo filmprocessing units

Initially, 20ml of wastewater was filtered (Whatman No. 41) toeliminate any suspended and undissolved impurities (S1) and the fil-trate was sonicated in an ice-bath using an ultrasonicator for 30min toget a homogenous solution. During sonication, 1M NaOH (in aqueousmedium) was added in drops until the completion of the precipitation[25]. Finally, the resulting dark brown colored precipitate was washedthoroughly with double distilled water, dried, characterized and usedfor supercapacitor application.

2.3. Characterization techniques

The FT-IR spectrum of the as-recovered Ag2O NPs was recorded byPerkin Elmer FT-IR Spectrophotometer. Raman Spectrum was analyzedby Brucker RFS 27: Standalone FT-Raman spectrophotometer. UV–Visspectrum was recorded by JASCO Spectrophotometer. The surfacemorphological images, energy dispersive X-ray (EDX) spectrum of Ag2ONPs were analyzed by Field-Emission Scanning Electron Microscope(FE-SEM, Hitachi SU6600). Transition images were captured using TEM(JEOL 2100F). XRD and BET adsorption isotherm was performed byShimadzu XRD 6000 (Japan) and ASAP 2020 Porosimeter respectively.X-ray photoelectron spectra (XPS) were obtained by using ShimadzuESCA 3100.

2.4. Electrochemical measurements

The recovered Ag2O NPs were further analyzed by CyclicVoltammetry (CV), Galvanostatic Charge-Discharge (GCD) and elec-trochemical impedance spectroscopic (EIS) studies by PrincetonApplied Research (VSP-1) Electrochemical Analyzer. A three-electrodeassembly was used for all the measurements in aqueous 1M KOH as theelectrolyte. A mixture of 80 wt% active material (Ag2O NPs), 10 wt%acetylene black and 10wt% PVDF in N-methyl-2-pyrrolidone wascoated on the nickel foam with a definite area of 1 cm2 and was used asthe working electrode, counter electrode – platinum wire and referenceelectrode – Ag/AgCl (satd. KCl) were used. The calculated total mass ofthe Ag2O NPs in the working electrode was 5mg.

Table 1Comparison of the results of the developed sensor (Ag2O NPs/GCE) with pre-viously reported 2, 4, 6-TCP sensors.

Electrode LOD/µM Linear range (µM) Method Ref.

aCS@Ag@GO 0.0097 0.03–35 DPV [42]HS-bb-CD/AuNPs/CSAM/ITO 0.001 0.003–0.028 DPV [43]dCNTs-OH/PtNPs/eRhB/GCE 1.55 5.0–175 AMP [44]fBCP/gerGO/GCE 0.005 0.05–1.0

1.0–130LSV [45]

Carbon paste-poly(o-phenylendiamine)

0.1 µM 0.1–1.0 AMP [46]

hemin/Cu-hMOF 0.005 0.01–9 DPV [47]Imprinted microgel/CNT/SPE 25 – DPV [48]Ag2O NPs/GCE 0.0027 0.005–1261.6 DPV This

work

aCS: Carbon Sphere; bb-CD: beta-cyclodextrin; cSAM: Self-assembled mono-layer; dCNTs-OH: Hydroxylated carbon nanotubes; eRhB: Rhodamine B; fBCP:bromocresol purple; gerGO: electrochemically reduced graphene oxide; hMOF:metal-organic framework.

Fig. 1. XRD pattern of Ag2O NPs.

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3. Results and discussion

3.1. FT-IR, Raman and UV–Vis absorption spectrum of Ag2O NPs

The FT-IR spectrum of the as-recovered Ag2O NPs (S2) showedstrong bands at 508 and 568 cm−1, which correspond to Ag-Ostretching mode of Ag2O NPs [26,27]. The broad and small peaks at

3400 and 1573 cm−1 can be ascribed to the stretching and bendingvibrations of O–H bond, due to the adsorbed water molecules on thesurface of Ag2O NPs [28]. FT-Raman spectrum of Ag2O NPs (S3) dis-played bands at 228, 355 and 388 cm−1, which can be ascribed to thestretching vibrational modes of Ag-O bond and it further confirms Ag2ONPs formation [29,30].

UV–Vis absorption spectrum of Ag2O NPs (S6A) exhibited a strong

Fig. 2. (A-D) FE-SEM images of Ag2O NPs.

Fig. 3. BET analysis (A), pore size distribution curve of Ag2O NPs (B).

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absorption maximum at 245 nm, which confirms the existence of Ag2ONPs as reported in earlier works [31]. The optical band gap energy (Eg),was calculated from the Tauc plot and it was found to be 4.80 eV (S6B).

3.2. XRD analysis of Ag2O NPs

The XRD pattern of the synthesized Ag2O NPs (Fig. 1) exhibitedmajor diffraction peaks at 26.5°, 34.2°, 46.2°and 77.3°, matching with(1 1 0), (0 0 3), (0 0 4) and (3 1 1) planes respectively, and it

corresponds to face-centered cubic (FCC) lattice of Ag2O NPs. (JCPDScard No: 42-0874). The average particle diameter of Ag2O NPs wasfound to be ≈21 nm using the Scherrer equation (1) [32,33].

= =d kλβ θ

d kλβ θcos

·cos (1)

where k is the Scherrer constant, which is the shape factor taken equalto 0.9, λ is the X-ray wavelength, β is the line broadening at half themaximum intensity (FWHM) in radians, and θ is the Bragg angle.

3.3. FE-SEM, TEM and EDX analysis of Ag2O NPs

(Figs. 2 and S4) showed the results of the surface morphological andnanostructural analysis using FE-SEM and EDX respectively. It is ob-served that the silver oxide nanoparticles formed nanosheet likestructure, arranged in layer by layer manner. At higher magnifications(Fig. 2C and D), it is observed that the agglomerated Ag2O NPs havedisordered arrangement with interspaces between them, which couldprovide a very good electrode-electrolyte interface for exchange of ionsfrom the electrolyte [34]. The TEM images of Ag2O NPs are displayed in(Fig. S5), which confirms that the Ag2O NPs were of nano sized. EDXanalysis of the as-recovered Ag2O NPs showed the formation of silveroxide (Fig. S7). Furthermore, the elemental analysis showed the highestproportion of silver (Ag) and oxygen followed by S, Na, C and Ca alsopresent in trace amounts.

3.4. BET analysis

The N2 adsorption/desorption isotherm plot and the pore size dis-tribution of the Ag2O NPs are shown in (Fig. 3A and B) respectively.

Fig. 4. XPS survey spectrum of Ag2O NPs. (Inset: Ag 3d).

Fig. 5. (A) CVs of bare GCE, and Ag2O NPs in 0.1M KCl containing 5mM [Fe(CN)6]3/4−. (B) Ag2O NPs calibration plots of ipc/ipa vs V1/2. (C) CVs obtained for bareGCE (a) and Ag2O NPs modified electrode (b) in pH 5 comprising 50 µM 2, 4, 6-TCP. (D) The plot of pH vs potential (red) and pH vs current (blue). (For interpretationof the references to colour in this figure legend, the reader is referred to the web version of this article.)

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The surface area evaluated from the BET isotherms for the Ag2O NPswas 20.1 m2/g. In this analysis, Type III isotherm was observed, whichrevealed the occurrence of mesopores (2–50 nm) formed between Ag2Onanosheets. Further, it was confirmed by the pore size distributioncurve (Fig. 3B). This mesoporous network can deliver low resistancepathways, which may increase the charge transport as well as powercapability. The large specific surface and pore size are the key reasonsfor high specific capacitance and high rate charge/discharge ability ofas-recovered Ag2O NPs. Therefore, it is noted that the recovered Ag2ONPs have excellent electrochemical performance [35,36].

3.5. XPS spectra

The as-prepared Ag2O NPs were examined by XPS. The (Fig. 4),shows the full range spectrum of Ag2O NPs. In the (Inset), each Ag 3dlevel can be deconvoluted into two splitting peaks, and the corre-sponding binding energies are 368.3 and 367.5 eV which were ascribedto Ag 3d3/2 and Ag 3d5/2, and other typical peaks were also denoted anddisplayed in the (Fig. 4). The peak shape of the Ag 3d5/2 and Ag 3d3/2levels suggests that a single Gaussian–Lorentzian peak function cannotbe applied to fit each Ag 3d level successfully. The binding energiesobserved for the Ag2O are consistent with those of previous literature,respectively [37]. Finally, the existence of the recovered Ag2O NPs wasconfirmed through the displayed XPS spectrum.

3.6. Electrochemistry of recovered Ag2O NPs modified GCE

The recovered Ag2O NPs decorated GCE was inspected to determine

the surface area. The CVs of bare GCE and Ag2O NPs/GCE obtained inKCl (0.1M) encompassing the redox probe [Fe(CN)6]3/4− (5 mM) aredisplayed in (Fig. 5A). On comparing the anodic-to-cathodic peak po-tential separation (ΔEp) through the redox peak current of the twoelectrodes, the Ag2O NPs/GCE exhibits substantial ΔEp value comparedto that of bare GCE. The calculated ΔEp of unmodified GCE and Ag2ONPs /GCE were 129mV, and 103mV, correspondingly. At the sametime, cathodic-to-anodic peak current ratio was found to be equivalentto 1 accounting for the reversible reaction. Here, the ensuing ratio ofIpc/Ipa were measured as 0.94 and 0.99 for unmodified GCE and Ag2ONPs/GCE, respectively. The EASA (electroactive surface area) of thesynthesized microstructure was measured through Randles–Sevcikequation (2) [38,39], as described below. Meanwhile, by altering thescan rates, the kinetic studies were investigated in [Fe(CN)6]3/4−

system (Fig. 5B).

= × vIp 2.69 10 n AD C5 3/2 1/2 1/2 (2)

The electroactive surface area of unmodified GCE and Ag2O NPs/GCE were calculated as 0.093 cm2 and 0.127 cm2 by identifying theslope of Ipa vs. v1/2. Thus, in comparison with bare/GCE, Ag2O NPs/GCE was observed to possess many electroactive sites. Moreover, theCVs acquired at altered scan rates explained the pseudo capacity of theAg2O NPs/GCE. The greater charge diffusion polarization of Ag2O NPs/GCE was further confirmed by the intensified peak current.

3.7. Voltammetric response of Ag2O NPs modified GCE towards 2, 4, 6-TCP

In (Fig. 5C), the CVs recorded at unmodified GCE and Ag2O NPs

Fig. 6. (A) CVs obtained at Ag2O NPs/GCE containing 2, 4, 6-TCP (25 µM to 250 µM) in 0.1M pH 5 solution. (B) Calibration plot of [2, 4, 6-TCP]/µM vs Current (µA).(C) CVs obtained at Ag2O NPs/GCE in 0.1M pH 5 solution containing 100 μM of 2, 4, 6-TCP at different scan rates (20–300mV s−1). (D) (Scan rate)1/2 (Vs–1)1/2 vs.peak currents (μA).

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/GCE in pH 5 comprising 50 µM 2, 4, 6-TCP are displayed. It is note-worthy to mention that the analyte was detected well at Ag2O NPs/GCE which may be due to fast electron transferences and superiorelectrocatalytic properties, as substantiated by the intensified anodicpeaks at minimized over-potential. As shown in (Fig. 5C), the in-tensified peak currents obtained at Ag2O NPs/GCE specifies the cata-lytic activity attained by the abundant electroactive sites available withlarger surface area and higher conductivity of Ag2O NPs/GCE owing toits electronic structure, which afford a robust circumstance for the highelectrochemical performance. To compare the efficiency of the re-covered Ag2O NPs with the commercial Ag2O, the CV studies wereperformed, which are displayed in (Fig. S11). The obtained results

illustrates the better catalytic activity of recovered Ag2O NPs.

3.8. Effect of pH on Ag2O NPs/GCE

By using Ag2O NPs/GCE the effect of pH on the electrochemicaloxidation of 2, 4, 6-TCP was examined. On changing the pH from 3 to11, the oxidation peak current fluctuated. (Fig. 5D blue) specifies theplot of different pH vs. current, and it evidently proves that pH 5 has thehighest oxidation peak current. Thus, it was confirmed that pH 5 is anideal pH for 2, 4, 6-TCP recognition and all the electrochemical studieswere done at pH 5. The plot of pH against the peak potential (Ep)portrays an outstanding linearity for 2, 4, 6-TCP detection (Fig. 5D red).The conquered slope value got for 2, 4, 6-TCP as (−45mV) is adjacentto the theoretical value of the Nernst equation [40,41]. This proves thatthe equal number of electrons and protons were involved in 2, 4, 6-TCPoxidation at Ag2O NPs/GCE.

3.9. Electrochemical performance of Ag2O NPs modified GCE towards 2, 4,6-TCP

The electrochemical performance of the modified electrode Ag2ONPs/GCE towards 2, 4, 6-TCP was verified by sequentially increasingthe concentrations, as demonstrated in (Fig. 6A), and there was a linearincrease in anodic peak current with increasing concentration of 2, 4, 6-TCP (Fig. 6B). And on altering the scan rates in an ascending mode, theoxidation peak current of 2, 4, 6-TCP got intensified (Fig. 6C), which

Fig. 7. (A) DPV response of Ag2O NPs/GCE for successive addition of different concentrations (50 nM to 150 µM) of [2, 4, 6-TCP] at pH 5. (B) Linear plot for currentresponse vs. [2, 4, 6-TCP]. (C) DPV selective response of Ag2O NPs/GCE towards 50 μM of [2, 4, 6-TCP]. (D) Stability of the sensor during its continuous use for30 days. CV response of Ag2O NPs/GCE film modified electrode towards 100 μM of 2, 4, 6-TCP in 0.1M PBS (pH 5) was monitored for the given number of days.

Table 2Comparison of the determination of 2, 4, 6-TCP in agricultural soil samples bythe developed and HPLC method.

Sample Spiked(µM)

Found (µM) Relativeerror (%)

aRSD (%) AverageRecovery

(%)HPLC DPV

Agricultural Soil 0 bND bND – – 97.135.0 4.91 4.79 2.44 3.5610 9.86 9.71 1.52 2.3920 19.89 19.61 1.40 2.59

a RSD=Relative standard deviations of three individual measurements.b ND=Not detected.

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specifies that the described experiment was diffusion-controlled. Thelinearity was confirmed through the graphical plot drawn against theanodic peak current and the square root of the scan rate, as exposed in(Fig. 6D).

3.10. Differential pulse voltammetric determination of 2, 4, 6-TCP

To determine the analytical efficacy of Ag2O NPs/GCE, differentialpulse voltammetry was executed for the detection of 2, 4, 6-TCP. In(Fig. 7A), the concentration of 2, 4, 6-TCP was increased from 50 nM to150 µM. For each addition of 2, 4, 6-TCP oxidation peak current in-creased linearly without any larger deviation (Fig. 7B). Therefore, Ag2O

NPs/GCE can be manifested for the trace level detection of 2, 4, 6-TCP.This modified electrode yields the wider linear range of50 nM–1261.6 µM, with the very low detection limit of 2.7 nM. Thesensitivity towards 2, 4, 6-TCP was calculated as3.9802 μA µM−1 cm−2. Therefore, Ag2O NPs/GCE owns the typicalparameters, which are necessary for an effective sensor.

3.11. Selectivity, stability and reproducibility

To evaluate the selectivity of Ag2O NPs/GCE modified electrode,DPV study was carried out, with the co-existence of the indicatedchemicals as the interferents. (Fig. 7C) shows the selective signal

Fig. 8. (A) The real sample analysis of [2, 4, 6-TCP] in soil samples. (B) Calibration plot for current response vs [2, 4, 6-TCP].

Fig. 9. (A) CV of Ag2O NPs at 5mV/s in aqueous 1M KOH. (B) CV of Ag2O NPs at various scan rate. (C) Variation of specific capacitance of Ag2O NPs with scan rate.

Fig. 10. (A) GCD Profile of Ag2O NPs at 1 A/g Current Density in aqueous 1M KOH. (B) GCD Profile of Ag2O NPs at different Current Density. (C) Variation ofSpecific Capacitance of Ag2O NPs with Current Density.

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obtained at Ag2O NPs/GCE modified electrode on the addition of 50 µMof 2, 4, 6-TCP, with 150 µM concentration of above said interferents inpH 5. Finally, the electrode responds too specifically to 2, 4, 6-TCPamong the pool of interferents.

To examine the wearing capacity of Ag2O NPs/GCE, its responseswere investigated. As expected, the sensor retained 98.2% of the initialresponse after its daily usage (Fig. 7D). To inspect the reproducibility,as shown in (S8-C, D) seven individual GCEs were fabricated with Ag2ONPs and the CVs were obtained in pH 5 (0.1 M) comprising 50 µM of 2,4, 6-TCP, and the achieved RSD was 3.14%. (S8-A, B) shows the CVresponses of Ag2O NPs/GCE in 50 μM of 2, 4, 6-TCP in pH 5 with anindividual electrode to examine the reusability of the developed elec-trode.

4. Real sample investigation

The applicability of the equipped sensor was tested with the sampleof agricultural soil. The real sample preparation and spiking proceduresare explained below. The recovered amount of 2, 4, 6-TCP in found andrecoveries of the real samples are charted in (Table 2).

4.1. Agricultural soil sample

To follow the sampling procedure, first the soil was collected fromthe agricultural area near Xindian River, Taipei and soil was allowed todry in open air for 5 days. Next, it was sieved thoroughly and 20 g ofsoil was taken separately in each of the vials. Then, desired

concentrations of working analyte were spiked into the soil samples andshaken well for 20–30min and permitted to dry for a day. Finally, thesoil samples were added with pH 5 and centrifuged (2000 rpm) thriceand the supernatant was collected to extract the whole of the spikedanalyte and filtered through a nylon filter. The samples were taken forHPLC-UV characterization for a comparison, and also the obtainedextract was used for executing the DPV method to validate the preparedsensor. The Ag2O NPs/GCE, (Fig. 8A, B) shows the good results by at-taining the linear range of 100 nM–683.7 µM, with the lowest detectionlimit of 16.86 nM and the sensitivity of the 2, 4, 6-TCP detector in thesoil sample was 2.085 μA µM−1 cm−2.

5. Elecrochemical supercapacitor measurement

5.1. Capacitance measurement from cyclic voltammetry

The electrochemical supercapacitive performance of the Ag2O NPselectrode was investigated through the CV and the specific capacitancewas calculated from the CV measurements using Eq. (3).

∫=Cs

idVS V m·Δ · (3)

where ∫ idV is the integral area of one full cycle of CV curve, ΔV po-tential window (V), ‘m’ mass of the active material (mg) and S is thescan rate (mV/s).

(Fig. 9A) shows the CV curve of Ag2O NPs at the scan rate of 5mV/swithin a potential window of 0 to 1 V. The quasi-rectangular shape ofthe CV curves indicates the pseudocapacitive behavior of Ag2O NPs.Further, a well-defined redox peak from the reversible redox reaction ofAg(I) was observed for Ag2O NPs coated electrode material [34]. Thehigh specific capacitance value was found to be 402 F/g for Ag2O NPselectrode at a scan rate of 5mV s−1. In order to compare the efficiencyof as-recovered Ag2O NPs, CV measurement was also performed for thecommercially available Ag2O NPs. The commercially available Ag2ONPs exhibited the specific capacitance of 62 F/g at 5mV/s scan rate(Fig. S9), which suggested that the recovered Ag2O NPs delivered ex-cellent performance than the commercial one.

(Fig. 9B) shows the CV curves of Ag2O NPs electrode performed atdifferent scan rates. The specific capacitance values were 402, 364, 328,315, 271, 240, 215, 177 and 150 F/g at the scan rate of 5, 10, 20, 25,50, 75, 100, 150 and 200mV/s respectively. (Fig. 9C) represents thespecific capacitance curve for Ag2O NPs against scan rate, which re-vealed that the increase of scan rate reduces the specific capacitancemainly by ion exchange mechanism [49]. Thus CV analysis proved thatthe Ag2O NPs electrode is a suitable candidate for supercapacitors [50].The specific capacitance value (402 F/g) observed in the present workis found to be greater than that of the previously reported Ag2O NPs orcomposites, some higher specific capacitance values have been reportedfor the silver-based electrode materials by some authors [51,52], butthey are for nanocomposites with increased matrices such as polymers

Fig. 11. Nyquist plot of Ag2O NPs (Inset: Equivalent Circuit).

Fig. 12. (a) First Cycle. (b) 5000th Cycle of GCD curve at 3 A/g current density. (c) Specific capacitance retention.

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or polymer with dopants or polymer-based carbon materials. Ad-ditionally, if the recovered Ag2O NPs are incorporated with a polymeror high surface area associated carbon support, it may increase thespecific capacitance values compared with the reported values. Hence,the simple, less time consuming and effective recovery route is thesignificant advantage of the present work.

5.2. Capacitance measurement from GCD

The Galvanostatic Charge-Discharge (GCD) property significantlyinfluences the evaluation of the rate capability of the electrode material(Fig. 10A). The GCD curves at different current densities (1, 2, 3, 4 and5 A/g) are shown in (Fig. 10B). The (Fig. 10C) showed a non-linearbehavior, which is dissimilar from the EDLC based electrodes, wherethe GCD profile was triangular in shape. The non-linear shape of theGCD plot for Ag2O NPs exhibits the three altered portions such as arapid IR drop (ii) a linear section from the presence of the EDL and (iii)a curved variation [53]. The specific capacitance of the electrode wasmeasured by galvanostatic charge/discharge studies using the eqn (4)[44].

=Cs I tm V

ΔΔ (4)

where I(A) is discharge current density and Δt (s) is the discharge time,ΔV (V) is the potential window and m (mg) is mass of the active ma-terial.

The Ag2O NPs electrode exhibited the high specific capacitance of530 F/g at a current density of 1 A/g, whereas the Csp values werefound to be 328, 142, 84, 60 F/g at the current densities of 2, 3, 4 and5 A/g respectively. The discharge profile of the supercapacitor elec-trode was found to have a dependence on the applied current and si-milar shape was observed at different current densities [54]. Moreover,the specific capacitance value of 60 F/g at the high current density of5 A/g (Fig. 10c) revealed that an effective charge transport by the Ag2ONPs. (Fig. 10C) also suggests that the Csp values decreased with in-creasing current density, thereby following the same fashion as ob-served in the case of CV analysis. There is a difference between thespecific capacitance measurements from the CV and GCD curves. Be-cause the capacitance was measured at a particular potential for CV,whereas the GCD derived specific capacitance was calculated for anaverage potential range of 0–1 V (vs. Ag/AgCl).

5.3. Capacitance measurement using EIS

For the supercapacitor application, the electrochemical reactionkinetics is an important factor. Electrochemical impedance measure-ment for the Ag2O NPs coated electrode (Fig. 11) and bare nickel foam(Fig. S10) were taken from the Nyquist plot, from 0.1 Hz to 100 kHz atthe open circuit potential. From this plot, the Rs (the ionic resistance ofelectrolyte) and Rct (intrinsic resistance) values for Ag2O NPs werefound to be 3.2Ω and 2.5Ω and for bare nickel foam the values werefound to be 4.10Ω (Rs) and 2.90Ω (Rct). Normally, an Equivalent SeriesResistance (ESR) can be calculated by combining Rs and Rct values. TheESR value for Ag2O NPs was found to be 5.7Ω. The Warburg impedancevalue obtained was 3.2Ω, corresponding to the electrolyte ion diffusioninto electrode material. The Ag2O NPs displayed a minimum ESR value,which accelerates the penetration of the electrolyte into the activeelectrode, and increases the electrode-electrolyte interfaces. Further,the depressed semicircular portion at high-frequency region for Ag2ONPs indicates that the electrode has high conductivity [55]. This studyalso confirms that the as-recovered Ag2O NPs electrode is a suitablealternate material for supercapacitor applications.

5.4. Stability test

The cyclic stability of the Ag2O NPs was analyzed by repeating

constant current charge/discharge test for 5000 cycles in 1M KOH at acurrent density of 3 A/g. From (Fig. 12A-C), the specific capacitance ofAg2O NPs was found to be 142 F/g at first cycle (Fig. 12a) and it de-creased to 124 F/g after 5000 cycles (Fig. 12b), with retention of 88%specific capacitance (Fig. 12c). The excellent stability of the as-re-covered Ag2O NPs can be mainly ascribed to the Ag2O nanosheetstructure with abundant mesopores, which provides more active sites,and thus enhances the efficient electrolyte ions transportation duringcharge–discharge process. The redox reaction of Ag (I) to metallic silveris also the important reason behind the stability of the Ag2O coatedelectrode. These parameters revealed that the as-recovered Ag2O NPsfeatures good performance as a supercapacitor material.

6. Conclusions

A simple, one-pot, eco-friendly, sonochemical method to recoverAg2O NPs from wastewater of photo film processing unit was reported.The recovered silver oxide nanoparticle was characterized by variousspectral and analytical techniques. Further, it was evaluated for thesupercapacitor application. The as-synthesized Ag2O NPs displayedhigh ionic conductivity due to nanosheet like morphology with theenhanced surface area (20.1m2/g). Subsequently, the unique featureeffectively contributed to the movement and intercalation of electrolyteions into the interspace of the electrode material. Upon examining thesensing abilities of Ag2O NPs/GCE for 2, 4, 6-TCP, its shows a widerlinear range 50 nM-1261.6 µM with the appreciable sensitivity of3.9802 μA µM−1 cm−2, and also it possesses a very low detection limitof 2.7 nM. So, the developed sensor was probed for the detection of 2, 4,6-TCP in agricultural soil samples and as expected it showed a very lowdetection limit of 16.86 nM, in the real soil samples. Further electro-chemical investigation of the nickel foam electrode coated with the as-synthesized Ag2O NPs showed an excellent reversibility and fast chargetransfer and subsequently a significant electrochemical performancewith a high specific capacitance of 530 F/g at a current density of 1 A/gwas achieved. It was further observed that the capacitance retentionvalue of 88% even when subjected to 5000 charge/discharge cycles.These findings indicated that the as–recovered Ag2O NPs from waste-water of photo film processing units is quite suitable and a promisingelectrode material for high-performance supercapacitor. The perfor-mance of the supercapacitor may be improved by introducing the do-pants like polymer/carbon-based materials.

Acknowledgment

The authors extend their appreciation to the Deanship of ScientificResearch at King Saud University for funding this work through re-search group no (RG-1439-84). This work was supported by theMinistry of Science and Technology, Taiwan (MOST 107-2113-M-027-005-MY3). The Authors gratefully acknowledge the management ofBishop Heber College, Tiruchirappalli-620017, Tamil Nadu, India, forproviding Instrumentation through DST-FIST-HAIF facilities and UGCfor the financial assistance under Minor Project Scheme (No. F MRP-6429/16(SERO/UGC)).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2018.09.029.

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