Ucc

Ca

b

c

d

h

•

•

•

a

ARRAA

KGCCPC

h0

Journal of Hazardous Materials 304 (2016) 400–408

Contents lists available at ScienceDirect

Journal of Hazardous Materials

jo ur nal ho me p ag e: www.elsev ier .com/ locate / jhazmat

tilization of reduced graphene oxide/cadmium sulfide-modifiedarbon cloth for visible-light-prompt photoelectrochemical sensor foropper (II) ions

.Y. Fooa,∗, H.N. Lima,b,∗, A. Pandikumarc, N.M. Huangc, Y.H. Ngd

Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, MalaysiaFunctional Device Laboratory, Institute of Advance Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, MalaysiaLow Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, MalaysiaParticles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, NSW 2052, Australia

i g h l i g h t s

CdS/rGO-modified carbon cloth fordetection of copper(II).Two linear detection range of 0.1 �Mto 1.0 �M and 1.0 �M to 40.0 �М.Detection limit of 0.05 �M and0.50 �M.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 22 July 2015eceived in revised form 27 October 2015ccepted 4 November 2015vailable online 10 November 2015

eywords:rapheneadmium Sulfide

a b s t r a c t

A newly developed CdS/rGO/CC electrode was prepared based on a flexible carbon cloth (CC) substratewith cadmium sulfide (CdS) nanoparticles and reduced graphene oxide (rGO). The CdS was synthesizedusing an aerosol-assisted chemical vapor deposition (AACVD) method, and the graphene oxide was ther-mally reduced on the modified electrode surface. The existence of rGO in the CdS-modified electrodeincreased the photocurrent intensity of the CdS/rGO/CC-modified electrode by three orders of magni-tude, compared to that of the CdS/ITO electrode and two orders of magnitude higher than the CdS/CCelectrode. A new visible-light-prompt photoelectrochemical sensor was developed based on the compet-itive binding reaction of Cu2+ and CdS on the electrode surface. The results showed that the effect of the

2+

arbon Clothhotoelectrochemical sensoropper (II) Detection

Cu on the photocurrent response was concentration-dependent over the linear ranges of 0.1–1.0 �Mand 1.0–40.0 �M with a detection limit of 0.05 �M. The results of a selectivity test showed that thismodified electrode has a high response toward Cu2+ compared to other heavy metal ions. The proposedCdS/rGO/CC electrode provided a significantly high potential current compared to other reported values,and could be a practical tool fo

∗ Corresponding author. Fax: +60389435380.E-mail address: janetlimhn@gmail.com (H.N. Lim).

ttp://dx.doi.org/10.1016/j.jhazmat.2015.11.004304-3894/© 2015 Elsevier B.V. All rights reserved.

r the fast, sensitive, and selective determination of Cu2+.© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Photoelectrochemical (PEC) measurement is implemented indeveloping techniques for sensing platforms because of its low

dous M

pmcscTrp

egsaulnteeplpr

usvifwsTtwHccd

oaiHratsbatflhpaFeeutsr

aevt

C.Y. Foo et al. / Journal of Hazar

rocessing cost, simple instrumentation, and ability to provide aore accurate miniaturization method compared to other opti-

al and electrochemical detection methods [1,2]. In general, a PECensor functions by coupling photo-irradiation with electrochemi-al detection [3], thus providing promising analytical applications.he key features affecting the PEC sensor performances are mainlyegulated by the charge separation and transfer mechanism of thehotoactive component.

Currently, cadmium sulfide (CdS) is extensively used in photo-lectrochemical sensing applications because of its narrow bandap [4], effective light-harvesting media [5], and excellent chargeeparation properties [6]. CdS is a semiconductor material with

suitable band gap energy that shows prominent potential forses in solar cells [7], photochemical catalysis [8], and numerous

uminescence devices [9]. When exposed to light illumination, CdSanoparticles will absorb photons with an energy level higher thanhe band gap, which is 2.39 eV [4]. This allows the electrons to bexcited from the valance band to the conduction band, forminglectron–hole pairs. Once the excitation occurs, the electron–holeairs would either be recombined or transferred to the charge col-

ecting substrate (i.e., a carbon cloth substrate in this work), whichossesses a lower conduction band energy to generate a photocur-ent [10].

There have been reports on the synthesis of CdS nanostructuressing several methods such as DC sputtering, solvothermal route,onochemical treatment, chemical bath deposition, and chemicalapor deposition [9]. These synthesis methods tend to involve var-ous intricate procedures and complicated instrumentation. Apartrom that, the deposition of a CdS thin film on a glass substrateas recently reported using aerosol assisted chemical vapor depo-

ition (AACVD), a modification of chemical vapor deposition [11].hese AACVD methods are facile and highly versatile for the fabrica-ion of nanostructured compounds from a single precursor solutionith controlled stoichiometry and morphological characteristics.ence, the proposed AACVD method is a reliable method for fabri-ating CdS nanoparticles, and the structural properties can be easilyontrolled by selecting the solvent concentration and changing theeposition flow rate and temperature.

To date, there have been several reports about the depositionf nanoparticles on a conductive glass substrate (e.g., ITO, FTO,nd quartz crystal), including TiO2, ZnO, and CdS, using drop coat-ng [12], hydrothermal [13], and electrodeposition methods [14].owever, there has been little concern over the fact that the use of

igid and expensive substrates is believed to restrict the flexibilitynd functionality, which hinders the development of low-cost pho-oelectrochemical sensors for real-world applications. A flexibleubstrate has several advantages over a traditional rigid glass-ased substrate, in term of cost and processability. An enormousmount of effort has already been made by various research groupso design and fabricate various cost efficient electronic devices onexible substrates for practical applications [15–18]. On the otherand, a flexible electrode substrate such as carbon cloth (CC) canrovide many conducting channels for rapid electron transport andlso ease the diffusion of electrolyte in the electrode material [19].or instance, CC provides a large support area for the deposition oflectrochemically active nanoparticles to fabricate a functionalizedlectrode with a large electrochemically active surface area, hightilization efficiency for active materials, and an excellent massransport property compared to an ordinary rigid electrode sub-trate [20]. Therefore, it was investigated to replace an ordinaryigid substrate as a deposition medium in this study.

Graphene, on the other hand, is a promising nanomaterial with

2D structure made up of sp2-hybridized carbon atoms in anxtended honeycomb network. It has been substantially used inarious studies, especially in energy conversion and storage sys-ems, because of its nano-size thin layer properties, large coverage

aterials 304 (2016) 400–408 401

area, and high conductivity. Most importantly, it is also a “zeroband gap” semiconductor, which makes it an excellent electrodematerial for photocatalytic and photoelectrochemical applicationswhen coupled with a photoactive component. In order to function-alize the pristine graphene with photoactive semiconductors suchas CdS, the structural modification of the graphene has to be carriedout. Reduced graphene oxide (rGO), which is a graphene derivativewith characteristics similar to pristine graphene, was introducedto enhance the chemical properties of graphene. It contains oxy-gen functional groups such as hydroxyl, epoxy, and carbonyl in sp3

carbons, which cause the properties to vary from those of pristinegraphene. Furthermore, these oxygen functional groups influencethe adsorption and desorption of molecules. Therefore, variouskinds of chemical reactions can be modulated. Success in enhanc-ing the photoelectrochemical activity of TiO2 and ZnO has beenachieved by coupling them with graphene derivatives [12,21].

To the best of our knowledge, the fabrication of CdS nanoparti-cles with rGO on a flexible carbon cloth substrate has not previouslybeen reported. Herein, we propose a facile fabrication method fora CdS/rGO/CC electrode through an AACVD method using cad-mium acetate and thiourea as the Cd and S precursors, respectively.We present the results of a detailed investigation of the charac-teristics and photoelectrochemical performance of this electrode.To compare the photoelectrochemical performance, the fabrica-tion of pure CdS/CC and CdS/ITO electrodes was carried out. TheCdS/rGO/CC electrode showed a significant enhancement in pho-tocurrent generation under visible light illumination comparedwith the CdS/CC and CdS/ITO electrodes. The fabricated electrodewas applied as a photoelectrochemical sensor for Cu2+ ion detec-tion. The CdS/rGO/CC electrode showed a decrease in photocurrentintensity in the presence of Cu2+ ions due to the competitive bindinginteraction that occurred on the electrode surface. The system-atic reduction in photocurrent intensity depending on the Cu2+

concentration suggests that CdS/rGO/CC is a promising photoelec-trochemical sensor with high sensitivity and selectivity properties.

2. Experimental

2.1. Materials and characterization techniques

Graphite powder was purchased from Ashbury GraphiteMills Inc (code no. 3061), United States. Sulfuric acid (H2SO4,95–98%), phosphoric acid (H3PO4, 85%), charcoal activated powder(Chem–Pur), potassium permanganate (KMnO4, 99.9%), hydro-gen peroxide (H2O2, 30%), and hydrochloric acid (HCl, 37%) werepurchased from Systerm, Malaysia. Cadmium acetate dehydrate((CH3CO2)2.2H2O, 97%) and thiourea (CH4N2S, 96%) were obtainedfrom Fluka. Potassium sulfate (K2SO4), sodium sulfate (Na2SO4),magnesium chloride (MgCl2), barium chloride (BaCl2), zinc sul-fate (ZnSO4), manganese(II) sulfate (MnSO4), aluminum (III) sulfate(Al2(SO4)3), copper (II) sulfate (CuSO4), iron (III) sulfate (Fe2(SO4)3),nickel (II) sulfate (NiSO4), cobalt (II) sulfate (CoSO4) and sil-ver nitrate (AgNO3) were purchased from Merck, Germany, andtriethanolamine (TEA) was obtained from Sigma Chemical, Co.,Doubly distilled water was used throughout the experiment. Thecarbon cloth ELAT was obtained from NuVant, USA, while indium-doped tin oxide (ITO) glass was supplied by Xin Yang TechnologyLimited, China. Voltammetry experiments were carried out usinga Princeton potensiostat/galvanostat controlled by Versa Studiosoftware. The surface morphologies were analyzed using a field

emission scanning electron microscope (FEI Quanta SEM Model400F) equipped with an energy dispersive X-ray (EDX) accessory.The Raman spectra were recorded using a WITec Raman spec-trophotometer (Alpha 300R).

4 rdous

2

(dimwA2brcvea

2

G5ewitflrd

2

tAuaed

wwssmMpBA

3

3

dTspfinotti

02 C.Y. Foo et al. / Journal of Haza

.2. Deposition of CdS nanoparticles on CC through AACVD

CdS nanoparticles were deposited on the carbon cloth substrates1 cm × 1.5 cm) using a homemade AACVD assembly. Prior to theeposition, the carbon cloth substrates were ultrasonically cleaned

n acetone, ethyl alcohol, HCl (1.0 M), and deionized water. Cad-ium acetate (0.05 M) and thiourea (0.1 M) for the Cd and S sourcesere continuously stirred until completely dissolved in methanol.rgon gas was passed through the aerosol mist at a flow rate of25 mL/min to carry the aerosol droplets into the reaction cham-er. Deposition was conducted at 400 ◦C for 60 min. The end of theeaction chamber was covered with aluminum foil to enhance theirculation of the aerosol mist. The exhaust from the reactor wasented directly into the extraction system of the fume hood. Thelectrode was then rinsed with doubly distilled water and dried inn oven at 80 ◦C. This sample was described as CdS/CC.

.3. Fabrication of CdS/rGO/CC electrode

The CdS/rGO/CC electrode was fabricated as follows. Initially,O was synthesized via a modified Hummer’s method [21]. Then,0 �L of 0.8 mg/mL GO was drop-cast onto the surface of a CdS/CClectrode and allowed to air dry at room temperature. The GOas reduced using a thermal annealing treatment. The anneal-

ng temperature was set at 250 ◦C at a rate of 10 ◦C/min, andhen the specific temperature was kept for 60 min under an argonow of 225 mL/min. The heated substrate was naturally cooled tooom temperature with the protection of the argon flow. This wasenoted as CdS/rGO/CC.

.4. Photoelectrochemical studies

All of the photoelectrochemical studies were performed in ahree-electrode system containing Pt as the counter electrode andg/AgCl as the reference electrode. A CdS/rGO/CC electrode wassed as a working electrode, and a 150 W hydrogen lamp was useds the visible light source. KCl (0.1 M) was used as the supportinglectrolyte, and 0.5 M of TEA was used as the sacrificial electrononor.

Different concentrations of Cu2+ ions from 0.1 �M to 40.0 �Mere added to the electrolyte, and the photocurrent intensitiesere measured using a chronoampherometry technique. For the

electivity test, 1.0 �M quantities of different prepared metal ionolutions were added to the solution, and the photocurrent waseasured. For the selectivity studies, each stock solution of K+, Na+,g2+, Ba2+, Zn2+, Mn2+, Al3+, Cu2+, Fe3+, Ni2+, Co2+ and Ag+ ions was

repared by dissolving a suitable amount of K2SO4, Na2SO4, MgCl2,aCl2, ZnSO4, MnSO4, Al2(SO4)3, CuSO4, Fe2(SO4)3, NiSO4, CoSO4 org(NO3) in water, respectively.

. Results

.1. Morphological studies of CdS/rGO/CC

Highly dispersed spherical CdS nanoparticles were successfullyeposited on the carbon cloth substrate using the AACVD method.he carbon cloth before the treatment is shown in Fig. S1. Fig. 1Ahows a low-magnification FESEM image of a carbon cloth sam-le after the AACVD treatment. It can be seen that the carbon clothbers were fully covered by numerous CdS nanoparticles. The mag-ified FESEM image shown in Fig. 1B further indicates that the

btained CdS nanoparticles had a spherical structure. It appearshat these small spherical nanoparticles are closely packed togethero form a larger uniform and hierarchical architecture. A compar-son of Fig. 1C and D with Fig. 1A and B shows that there was no

Materials 304 (2016) 400–408

notable change before and after adding rGO to the electrode sys-tem. However, the magnified view of a localized region in Fig. 1Dclearly shows that a translucent rGO sheet appeared around the CdSnanoparticles. It is noteworthy that no rGO was deposited on thecarbon cloth fibers but wrapped tightly around the CdS nanoparti-cles after the drop casting process. It is estimated from Fig. 1D thatthe diameter of the CdS nanoparticles was similar to that of theCdS/CC electrode and also confirms the narrow size distribution onthe carbon cloth fibers, with an average diameter of 200–300 nm,as shown in the histogram (Fig. 1E and F), suggesting that the rGOdid not affect the morphology and size of the CdS nanoparticles.Fig. 1G shows the EDX analysis results for the CdS/CC electrode andproves that the deposited nanoparticles consisted of CdS. A highC intensity can be noticed in the CdS/rGO/CC electrode (Fig. 1H),which shows that the CdS nanoparticles were completely coveredby the rGO nano-sheet.

3.2. Raman spectral studies of CC/CdS-rGO

Raman spectroscopy was used to investigate the significantstructural changes from GO to rGO and also the deposition of CdSon the carbon cloth using the AACVD method. Fig. 2 shows theRaman spectra for CdS/GO/CC and CdS/rGO/CC, where the D and Gbands for the CdS/GO/CC electrode were centered at approximately1353.06 cm−1 and 1593.71 cm−1, respectively. The G band wascaused by the in-plane bond stretching of the sp2 bond, whereas theD band was related to different types of defects [22]. The reductionof GO to rGO using the thermal method shifted the wavenumbersof the D and G bands to lower values, which were 1340.64 cm−1 and1577.62 cm−1, respectively. This is because the thermal treatmentcaused a decrease in the size of the sp2 domain, and increased theedge planes, as well as the expansion of the disorder in the pre-pared rGO. Moreover, the 2D peak that appeared at 2704.46 cm−1

showed that the number of rGO layers was increased comparedto GO after the thermal reduction process. It was noticed that theintensity ratios of the D–G bands (ID/IG) for GO and rGO were1.00 and 1.12, respectively, which indicated that the GO was ther-mally reduced to rGO with the annealing temperature of 250 ◦C.The thermal treatment caused a dramatic increase in the structuraldefects, which was attributable to the evolution of the COx (x = 1,2) species. The removal of oxygen from COx is an essential stepin the reduction process, because the oxygen atoms are isolatingthe sp2 clusters in the GO, and a reduction process by removing Oleads to better connectivity among the existing graphene structuresthrough the formation of new sp2 clusters [23]. CdS is consideredto be a Raman active compound [24]. Therefore, the Raman spec-tra of both electrodes were found to have two significant peaksat 307.37 cm−1 and 759.35 cm−1, which are attributed to the first-order and second-order longitudinal optical phonon (LO) modesof CdS, respectively [25]. The appearance of rGO is more transpar-ent than that of GO. Hence, a higher intensity of LO peaks can beobserved in the CdS/rGO/CC electrode compared to the CdS/GO/CCelectrode.

3.3. Photoelectrochemical properties of CdS/rGO/CC electrode

The substrate used for the deposition of CdS affected the pho-tocurrent and stability of the electrode. From Fig. 3A, a smallphotocurrent intensity of 67 �A was obtained under light illumi-nation for the CdS/ITO electrode. A promotion of the photocurrentintensity to 83 �A was obtained from the CdS/CC electrode, whichcan be assigned to the better conductance of the carbon cloth

compared to the ITO glass. However, a further increase in the pho-tocurrent intensity by 248% was observed after the introduction ofrGO into the system. The photocurrent generated after the intro-duction of rGO was exceedingly high compared to the previous

C.Y. Foo et al. / Journal of Hazardous Materials 304 (2016) 400–408 403

F catioT C andH

rseev

ig. 1. FESEM images of CdS/CC and CdS/rGO/CC electrodes (A and C) at low magnifihe nanoparticle distributions are presented in the histograms in E and F for CdS/C).

eported CdS-modified electrode, which was around 206 �A and

table [10,26,27]. In principle, the unique 2D “mat” structure andxtraordinary electron-transport property of rGO can enhance thefficiency of CdS by covering it fully and intimately, which pro-ides a sufficient interfacial contact and maximizes the electron

n. Higher magnification of these two electrodes are shown in B and D, respectively. CdS/rGO/CC, respectively. EDX analysis results for CdS/CC and CdS/rGO/CC (G and

conductivity [28]. This is directly evidenced by the FESEM analy-

sis results, whereby an intimate integration of the rGO sheets andCdS nanoparticles was observed. In this case, the excellent electronconductivity of rGO could be effectively utilized, which efficientlyenhanced the lifetime of the photo-generated electron–hole pairs.

404 C.Y. Foo et al. / Journal of Hazardous

AeCw

Fr(e

Fig. 2. Raman spectra of CdS/CC and CdS/rGO/CC electrodes.

s a result, the photocurrent intensity generated by the CdS/rGO/CC

lectrode was significantly higher than those of the CdS/CC anddS/ITO electrodes. From Fig. 3B, the photocurrent intensityas steady and reproducible after several on–off cycles of light

ig. 3. Photocurrent intensities of CdS/ITO, CdS/CC, and CdS/rGO/CC electrodes,espectively, measured in KCl electrolyte with 0.5 M TEA and bias potential of 0.1 VA). Time-based photocurrent intensities of the CdS/rGO/CC, CdS/CC and CdS/ITOlectrodes measured in the same electrolyte with light on and off, respectively (B).

Materials 304 (2016) 400–408

illumination, with no overshoot at the beginning or end of theon–off cycle, indicating that the direction of electron diffusion wasfree from grain boundaries, which can create traps to hinder elec-tron movement and slow down the photocurrent generation [29].

For subsequent investigations, CdS/rGO/CC, with its outstandingphotoelectric behavior, was employed for the optimization of thedeposition and analysis condition. Three parameters were studiedto maximize the photocurrent generation: the reaction tempera-ture, applied potential, and the TEA concentration. First, the effectof the reaction temperature was studied, and it was found that thephotocurrent of the modified electrode increased with an increasein the reaction temperature from 100 ◦C to 500 ◦C, as shown inFig. 4A. The deposition of CdS nanoparticles on the carbon cloth sub-strate was sped up with an increase in the reaction temperature,which resulted in an increase in the CdS nanoparticles depositedon the substrate and hence enhanced the photocurrent. However,a higher temperature (500 ◦C) caused the color of CdS to changefrom yellow to orange (Fig. 4A inset), which provided an unstablephotocurrent response, and hence resulted in a low photocurrentintensity. The color change at high temperature was due to thechange in the crystal lattice of the CdS nanoparticles. There aretwo lattice forms of CdS: cubic and hexagonal. The metastablecubic CdS nanoparticles are orange in color and have poor elec-trical conductivity properties, whereas the yellow highly stablehexagonal CdS has excellent electrical conductivity and hence issuitable for photoelectrochemical use [30]. Next, the effect of theapplied potential on the photocurrent generation was studied, andthe result is shown in Fig. 4B. The photocurrent intensity generatedby the CdS/rGO/CC electrode was studied at bias potentials from−0.1 V to 0.2 V using a 0.1 M KCl electrolyte. A significantly highphotocurrent was obtained at 0.1 V, and a further increase in theapplied potential led to a decrease in the photocurrent intensity.

Thus, a bias potential of 0.1 V was taken as the optimum biaspotential to generate photocurrent from the CdS/rGO/CC elec-trode. A lower potential will also hinder the interference of otherreductive species coexisting in the sample. Finally, different con-centrations of TEA (0.1–0.9 M) were added to the electrolyte tostudy their effect on the photocurrent generation, and the enhance-ment effect started to diminish after 0.7 M TEA. This might havebeen due to the excess electrons donated from the TEA, whichwould cause a back electron transfer and lower the photocurrentgeneration [31]. TEA is a tertiary aliphatic amine, which acted asa sacrificial electron donor in this photo-induced electron transferprocess. This helped to scavenge the holes generated from the pho-toexcitation process and hindered the recombination process forthe electron–hole pairs during the photoelectrochemical process.However, they would not be restored in the subsequence reactionprocess but were destroyed by the irreversible oxidation reaction.

To study the effect of the photoelectrochemical response ofthe modified electrode, the current–voltage (I–V) curves of theCdS/rGO/CC electrode were obtained in the presence and absenceof light illumination at room temperature, as shown in Fig. 5.It can clearly be seen that the CdS/CC-modified electrode hada typical I–V curve, but was significantly outperformed by theCdS/rGO/CC-modified electrode, which showed a two-orders-of-magnitude enhancement. This result was further evidenced bythe EIS analysis results, whereby the recombinant resistance ofCdS/rGO/CC was significantly lower than that of the CdS/CC elec-trode. Nyquist plots of CdS/rGO/CC and CdS/CC in the frequencyranges of 10 mHz and 10 kHz are shown in Fig. 6. At the low fre-quency range, CdS/CC shows a higher diffusivity resistance, as seenin more gradual slope for the higher Z’, indicating that the elec-

tron transfer is not essential on the electrode surface. On the otherhand, CdS/rGO/CC has a much smaller diffusion resistance becausethe rGO was intimately integrated with the CdS nanoparticles,which facilitated the electron diffusion. This would further enhance

C.Y. Foo et al. / Journal of Hazardous Materials 304 (2016) 400–408 405

Fti

tt

ctt[fwiti

electrons migrated from the valance band (VB) to the conductionband (CB) of the CdS nanoparticles. The presence of rGO with a CBslightly lower than that of the CdS [32] created a stepwise migration

ig. 4. Effects of deposition temperature (A), potential bias (B), and TEA concen-ration (C) on photocurrent intensity using 0.1 M KCl electrolyte under visible lightllumination condition.

he electron transfer ability, hinder the charge recombination, andherefore show a higher photocurrent generation.

The enhanced efficiency of the CdS/rGO/CC-modified electrodeould be attributed to the bridging effect and the charge separa-ion and transport of the rGO. CdS nanoparticles can bind withhe surface of the rGO nanosheets through van der Waals forces32]. Therefore, the rGO was introduced to the electrode sur-ace. This enables the CdS nanoparticles to integrate intimately

ith the carbon cloth, which encapsulated them. This improved

nterfacial contact could facilitate the electron transfer betweenhe CdS and carbon cloth and prevent possible current leakagen the CdS/rGO/CC-modified electrode. In addition, when the CdS

Fig. 5. LSV of (A) CdS/rGO/CC-modified electrode and (B) CdS/CC-modified electrodein KCl electrolyte with 0.5 M TEA under different light illumination conditions at scanrate of 5 mV s−1.

nanoparticles were connected with the rGO, the structure of theenergy levels was altered. Stepwise energy levels were formed,and the electron-transfer processes of the CdS/rGO/CC electrodeare illustrated in Fig. 7. During light illumination, the photo-excited

Fig. 6. EIS analysis results for both electrodes with 0.5 M TEA under visible lightillumination at scan rate of 5 mV s−1.

406 C.Y. Foo et al. / Journal of Hazardous Materials 304 (2016) 400–408

F trode

s

ostteat

3

pFrsP

ig. 7. Schematic illustration of photoelectrochemical response of CdS/rGO/CC electepwise electron transfer in the absence (A insert) and presence (B insert) of Cu2+.

f electrons from the CdS nanoparticles to the carbon cloth sub-trate, while the holes were transported to the electrolyte. Indeed,he photo-excited electrons could be continuously extracted andransferred to the rGO, which minimized the recombination oflectron–hole pairs. Furthermore, rGO played an important role asn electron shuttle between the CdS and carbon cloth electrode dueo its high conductance compared to CdS nanoparticles.

.4. Photoelectrochemical sensing of Cu (II) ions

The presence of copper (II) ions in the electrolyte decreased thehotocurrent intensity of the CdS/rGO/CC electrode, as shown in

ig. 8A. As the concentration of the Cu2+ increased, the photocur-ent response of the CdS/rGO/CC electrode gradually decreased. Thetable photocurrent response of the proposed electrode acted as aEC sensor for Cu2+ ion detection. It has been reported that the

without (A) and with (B) Cu2+ under visible light illumination. The inserts show the

presence of Cu2+ in a CdS solution will cause a competitive bindingeffect with Cd2+ and also reduce Cu2+ to Cu+ under light illumina-tion [33]. As a consequence, the formation of CuxS (x = 1,2), whichhave lower solubilities than CdS, could occur on the surface of theCdS/rGO/CC electrode due to the competitive displacement of sur-face Cd2+ by Cu+ and Cu2+. Moreover, the generation of CuxS onthe electrode surface also provided a lower energy level, which ledto the recombination of the excited electrons in the conductionband and holes in the valance band [26]. Because of the existence ofelectron–hole pair recombination centers, the photocurrent inten-sity of CdS/rGO/CC decreased in the electrolyte containing Cu2+.This decrease in the photocurrent intensity of the CdS/rGO/CC elec-

trode was highly dependent on the Cu2+ concentration. Fig. 8Bshows that the photocurrent drop against the Cu2+ concentrationconsisted of two linear segments with different gradients, indi-cating two different detection ranges of analyte concentration;

dous Materials 304 (2016) 400–408 407

0sswmoTw

itahrettmiaa

Ft1tr

Table 1Comparison of proposed work with some typical detection methods for Cu2+ usingCdS materials.

No. Detection method LDRa LODb Ref.

1 Photoluminesence 0.0–10.0 �M 0.50 �M [34]0.001–3.0 �M 0.50 nM [35]

2 Electrochemiluminesence 0.1–10.0 �M 20.0 nM [36]3 Fluoresence 0.1–5.0 �M 0.02 �M [37]

0.01–20.0 �M 0.10 �M [38]7.5 nM–314.0 �M 0.10 �M [39]

4 Photoelectrochemical 1.0–38.0 �M 0.55 �M [40]0.1–1.0 �M 0.05 �M This

work1.0–40.0 �M 0.50 �M

a Linear Range of Detection.b Limit of Detection.

C.Y. Foo et al. / Journal of Hazar

.1–1.0 �M for the first linear segment, and 1.0–40.0 �M for theecond linear segment. The corresponding R2 value for both theegments are 0.9982 and 0.9948, respectively. The detection limitsere calculated to be 0.05 �M and 0.50 �M for segment I and seg-ent II, respectively, using 3�/S, where � is the standard deviation

f a blank sample, and S is the slope of the linear calibration plot.he photocurrent responses of the CdS/CC and CdS/ITO electrodesere displayed in Fig. S2.

The guideline value of copper ions in drinking water standard-zed by WHO’s Guidelines for Drinking-water Quality was ∼30 �Mo prevent acute gastrointestinal effects from copper and providen adequate margin of safety in populations with normal copperomeostasis. Table 1 compares the various methods used by othersesearchers for copper ion detection. The LOD from the CdS/rGO/CClectrode was far below the guideline value, which indicated thathe proposed photoelectrochemical sensor was sensitive enougho detect a trace amount of copper in drinking water. Further-

ore, the application of a photoelectrochemical route for copperon detection has rarely been reported, and this facile and economicpproach can be easily integrated into other real-world sensor

pplications with promising results.

ig. 8. Effect of Cu2+ on photocurrent intensity of CdS/rGO/CC electrode as concen-ration increases from (a) 0.1, (b) 0.2, (c) 0.4, (d) 0.8, (e) 1.0, (f) 2.0, (g) 4.0, (h) 8.0, (i)0.0, (j) 20.0 to (k) 40.0 �M upon light illumination (A). Linear relationship betweenhe Cu2+ concentration and photocurrent intensity, where Io and I are photocurrentesponses in the absence and presence of Cu2+, respectively (B).

Fig. 9. Effects of various metal ions with concentration of 1.0 �M on photocurrentintensity of CdS/rGO/CC electrode under light illumination.

3.5. Selectivity of sensor electrode

The selectivity of the CdS/rGO/CC electrode was proven by intro-ducing different metal ions such as K+, Na+, Mg2+, Ba2+, Zn2+, Mn2+,Al3+, Cu2+, Fe3+, Ni2+, Co2+ and Ag+ into the electrolyte at 1.0 �Mconcentrations and measuring the photocurrent. From Fig. 9, it canbe seen that Mg2+, Mn2+, Ni2+ and Ag+ could slightly decrease thephotocurrent intensity of the CdS/rGO/CC electrode due to the for-mation of a metal sulfide compound on the CdS surface. The metalcompound formed possessed a lower band energy, which causedthe recombination of electron–holes pair and eventually loweredthe photocurrent generated. However, the solubility product, Ksp ofCuS, was much smaller (8 × 10−37) than those of MnS, NiS, and oth-ers sulfide compounds. Thus, the interference of other metal ionswas negligible [41].

4. Conclusion

In conclusion, a CdS/rGO/CC electrode synthesized using theAACVD method could produce a significantly high photocurrent ata relatively low applied potential under light illumination. Intro-ducing rGO to the electrode system enhanced the photocurrentintensity by increasing the surface interaction of the CdS and car-bon cloth and also helped in the charge separation and transportof the CdS/rGO/CC electrode, which hindered the recombination ofelectron–hole pairs. By utilizing the phenomenon of metal sulfide

formation, a sensitive and selective photoelectrochemical strategywas designed to detect a trace amount of Cu2+ based on the compet-itive binding reaction between Cu2+ and CdS. This proposed methodhas the advantages of a wide detection range, low cost, facile

4 rdous

fAcp

A

((E

A

t0

R

[

[

[[

[[

[

[

[[[

[

[

[

[

[

[[

[[

[

[[

[[

[

[[

[

08 C.Y. Foo et al. / Journal of Haza

abrication, rapid response, high sensitivity, and high selectivity.s a result, the CdS/rGO/CC electrode showed propitious appli-ation potential for photoelectrochemical sensing, as well as forhotovoltaic cells.

cknowledgements

This research work was supported by Putra Grant IPBGP-IPB/2014/9440701) and High Impact Research GrantUM.C/625/1/HIR/MOHE/SC/21) from the Ministry of Higherducation of Malaysia.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.11.04.

eferences

[1] G.L. Wang, J.J. Xu, H.Y. Chen, S.Z. Fu, Biosens. Bioelectron. 25 (2009) 791–796.[2] Y.-F. Liu, J.-X. Chen, M.-Q. Xu, g.-C. Zhao, Int. J. Electrochem. Sci. 9 (2014) 9.[3] X. Li, Q. Yang, H. Hua, L. Chen, X. He, C. Hu, Y. Xi, J. Alloys Compd. 630 (2015)

94–99.[4] E.C. Ekuma, L. Franklin, G.L. Zhao, J.T. Wang, D. Bagayoko, Phys. B: Condens.

Matter 406 (2011) 1477–1480.[5] A.J. Nozik, Phys. E 14 (2002) 115–120.[6] Q. Shen, X. Zhao, S. Zhou, W. Hou, J.-J. Zhu, J. Phys. Chem. C 115 (2011)

17958–17964.[7] H. Li, X. Liu, Solar Energy 115 (2015) 603–612.[8] X. Guo, C. Chen, W. Song, X. Wang, W. Di, W. Qin, J. Mol. Catal. A: Chem. 387

(2014) 1–6.[9] A.M.A. Al-Hussam, S.A.-J. Jassim, J. Assoc. Arab Univ. Basic Appl. Sci. 11 (2012)

27–31.10] Z. Qian, H.J. Bai, G.L. Wang, J.J. Xu, H.Y. Chen, Biosens. Bioelectron. 25 (2010)

2045–2050.

11] A.A. Memon, M. Dilshad, N. Revaprasadu, M.A. Malik, J. Raftery, J. Akhtar, Turk.

J. Chem. 39 (2015) 169–178.12] F.T. Johra, W.-G. Jung, Appl. Cata. A: Gen. 491 (2015) 52–57.13] F. Xu, D. Bai, S. Han, D. Wu, Z. Gao, K. Jiang, Sensors Actuators B: Chem. 203

(2014) 89–94.

[

[

[

Materials 304 (2016) 400–408

14] K. Zarebska, M. Skompska, Electrochim. Acta 56 (2011) 5731–5739.15] Z. Fan, H. Razavi, J.W. Do, A. Moriwaki, O. Ergen, Y.L. Chueh, P.W. Leu, J.C. Ho,

T. Takahashi, L.A. Reichertz, S. Neale, K. Yu, M. Wu, J.W. Ager, A. Javey, Nat.Mater. 8 (2009) 648–653.

16] H. Su, M. Zhang, Y.-H. Chang, P. Zhai, N.Y. Hau, Y.-T. Huang, C. Liu, A.K. Soh,S.-P. Feng, ACS Appl. Mater. Interfaces 6 (2014) 5577–5584.

17] J. Liu, Y. Qiao, C.X. Guo, S. Lim, H. Song, C.M. Li, Bioresour. Technol. 114 (2012)275–280.

18] L. Yang, Z. Hong, J. Wu, L.-W. Zhu, RSC Adv. 4 (2014) 25556–25561.19] S. Wang, R.A.W. Dryfe, J. Mater. Chem. A 1 (2013) 5279.20] L.C. Wu, Y.J. Chen, M.L. Mao, Q.H. Li, M. Zhang, ACS Appl. Mater. Interfaces 6

(2014) 5168–5174.21] N.M. Huang, H.N. Lim, C.H. Chia, M.A. Yarmo, M.R. Muhamad, Int. J. Nanomed.

6 (2011) 3443–3448.22] M. Sookhakian, Y.M. Amin, R. Zakaria, W.J. Basirun, M.R. Mahmoudian, B.

Nasiri-Tabrizi, S. Baradaran, M. Azarang, J. Alloys Compd. 632 (2015) 201–207.23] N.-J. Song, C.-M. Chen, C. Lu, Z. Liu, Q.-Q. Kong, R. Cai, J. Mater. Chem. A 2

(2014) 16563–16568.24] H.L. Lee, I.A. Mohammed, M. Belmahi, M.B. Assouar, H. Rinnert, M. Alnot,

Materials 3 (2010) 2069–2086.25] K.K. Nanda, S.N. Sarangi, S.N. Sahu, S.K. Deb, S.N. Behera, Phy. B: Condens.

Matter 262 (1999) 31–39.26] G.-L. Wang, J.-J. Xu, H.-Y. Chen, Nanoscale 2 (2010) 1112–1114.27] P. Wang, X. Ma, M. Su, Q. Hao, J. Lei, H. Ju, Chem. Commun. 48 (2012)

10216–10218.28] Y. Zhang, Z.-R. Tang, X. Fu, Y.-J. Xu, ACS Nano 5 (2011) 7426–7435.29] M. Sookhakian, Y.M. Amin, S. Baradaran, M.T. Tajabadi, A.M. Golsheikh, W.J.

Basirun, Thin Solid Films 552 (2014) 204–211.30] F. Liu, Y. Lai, J. Liu, B. Wang, S. Kuang, Z. Zhang, J. Li, Y. Liu, J. Alloys Compd. 493

(2010) 305–308.31] Y. Tamaki, K. Koike, T. Morimoto, O. Ishitani, J. Catal. 304 (2013) 22–28.32] A. Cao, Z. Liu, S. Chu, M. Wu, Z. Ye, Z. Cai, Y. Chang, S. Wang, Q. Gong, Y. Liu,

Adv. Mater. 22 (2010) 103–106.33] A.V. Isarov, J. Chrysochoos, Langmuir 13 (1997) 3142–3149.34] K.M. Gattas-Asfura, R.M. Leblanc, Chemical communications (2003), http://dx.

doi.org/10.1039/b308991f, 2684.35] Y. Hao, L. Liu, Y. Long, J. Wang, Y.-N. Liu, F. Zhou, Biosens. Bioelectron. 41

(2013) 723–729.36] J. Wang, X. Jiang, Sensors Actuators B: Chem. 207 (2015) 552–555, Part A.37] J. Chen, A. Zheng, Y. Gao, C. He, G. Wu, Y. Chen, X. Kai, C. Zhu, Spectrochim.

Acta Part A: Mol. Biomol. Spectrosc. 69 (2008) 1044–1052.38] Y.-h. Zhang, H.-s. Zhang, X.-f. Guo, H. Wang, Microchem. J. 89 (2008) 142–147.

39] K. Zhang, J. Guo, J. Nie, B. Du, D. Xu, Sensors Actuators B: Chem. 190 (2014)

279–287.40] F. Huang, F. Pu, X. Lu, H. Zhang, Y. Xia, W. Huang, Z. Li, Sensors Actuators B:

Chem. 183 (2013) 601–607.41] C.A. Young, E.J. Dahlgren, R.G. Robins, Hydrometallurgy 68 (2003) 23–31.