Photocurrent Generation and Conductivity Relaxation in ReducedGraphene Oxide Cd0.75Zn0.25S Nanocomposite and Its PhotocatalyticActivitySankalpita Chakrabarty,† Koushik Chakraborty,† Arnab Laha,†,§ Tanusri Pal,*,‡ and Surajit Ghosh*,†

†Department of Physics & Technophysics, Vidyasagar University, Midnapore 721102, West Bengal, India‡Department of Physics, Midnapore College, Midnapore 721101, West Bengal, India

*S Supporting Information

ABSTRACT: We report the photocurrent generation in reduced graphene oxide−cadmium zinc sulfide (RGO−Cd0.75Zn0.25S) nano composite material under simulated solarlight irradiation, where the photocurrent increases linearly with increasing incident lightintensity. We also report the temperature dependent electrical conductivity and conductivityrelaxation in RGO−Cd0.75Zn0.25S composite. At low frequency, the real part of conductivityis independent of frequency, and above a characteristic crossover frequency, theconductivity decreases with the increase in frequency, which indicates the onset of arelaxation phenomenon. The dc conductivity of the RGO−Cd0.75Zn0.25S composite showsArrhenius behavior. From the scaling of real part of conductivity spectra, we have observedthat the dynamic process occurring at different temperatures have the same thermalactivation energy. The RGO−Cd0.75Zn0.25S composite shows an enhancement of photocatalytic activity in comparison to control sample under simulated solar light irradiation todegrade Rhodamine B. The RGO sheets prolong the separation of photo induced electronsand holes in Cd0.75Zn0.25S, which hinder the electron−hole recombination and subsequently enhances the photocurrentgeneration and photocatalytic activity under simulated solar light irradiation.

■ INTRODUCTION

Solution processable reduced graphene oxide (RGO) isenjoying a great attention for large scale thin film device dueto ease of material processing, low cost of fabrication,mechanical flexibility, and compatibility with various sub-strates.1−10 In particular, large surface area with a wide range ofoxygen functionalities in RGO creates ability to makecomposites with other nano materials, including polymer,metal, metal oxide nano particle to form unique hybridcomposites.11−15 For instance, when RGO is anchored with thephotosensitive nano materials, such as, quantum dots, nanoparticles, the RGO serves as a continuous pathway for electrontransfer process from the molecules, and the composite isexpected to be an extraordinary potential candidate for photoexcitonic charge generation. With such advantages, researchershave devoted much effort to develop and constructoptoelectronic devices such as photodetectors, solar cells,sensors, and photocatalyst using inorganic semiconductorfunctionalized RGO composites.14−24 Lightcap et al. studiedthe electron and energy transfer from photo excited CdSecolloidal quantum dots to RGO, and they also focused on theimprovement of photo response of RGO−CdSe compositeover control CdSe and GO−CdSe composites.18 A highlysensitive ultraviolet sensor based on ZnO nanorod/RGOcomposites is reported by Chang and co-workers.24 All theseRGO-based composites are optically active only for selectivewavelength depending on their band gap energy.

To this end, ternary chalcogenide nano materials are wellstudied due to tuneability of their optical band gap by changingthe compositions.25−31 For an instance, Cd1−xZnxS has thepotential to form a continuous series of solid solutions andallows to open up the possibility to vary the optical band gapsystematically in a controlled way from VIS (CdS = 2.42 eV) toUV (ZnS = 3.7 eV) region. The performance of anoptoelectronic device strongly depends on the formation ofexcitons and their subsequent dissociation into free electronsand holes. The photo induced charge generation increases withincrease of exciton diffusion length and lifetime. Bettercrystallinity structure offers higher diffusion length and lifetimeof excitons.32 In our recent studies on solvothermallysynthesized CdS nano structures with different morphology,we have observed that the nanorod morphology shows a bettercrystallinity and subsequently an efficient photo induced chargegeneration.33 In the present study, we have chosen speciallynanorod structure of the alloyed semiconductor material andmade a composite with RGO, to ensure their efficient photoinduced charge generation as well as photocatalytic enhance-ment probabilities. Although there are a few reports on RGO−CdZnS composite materials which establish its potential forefficient photocatalytic hydrogen production,34,35 neither of

Received: September 22, 2014Revised: November 10, 2014

Article

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these studies explores the photo induced charge generation inlarge area thin film optoelectronic devices and photocatalyticdye degradation efficiency under simulated solar lightirradiation. The temperature dependent electrical conductivityand the conductivity relaxation phenomena in RGO basedcomposite materials also remain unaddressed.Here, we report the photocurrent generation and photo-

catalytic activity of RGO−Cd1−xZnxS nanorod composite undersimulated solar light irradiation, where the composite wassynthesized by simple, cost-effective one-pot solvothermalroute. The value of “x” was chosen 0.25, to make the materialmore functional in the visible side of the total solar spectrum.The structural and morphological characteristics of oursynthesized materials were studied by X-ray Diffraction(XRD) analysis, Transmission Electron Microscopy (TEM),and High Resolution TEM (HRTEM). The reduction ofGraphene Oxide (GO) was confirmed by X-ray Photoelectronspectroscopy. The optoelectronic properties of our thin filmdevice were investigated under simulated solar light irradiation,and it exhibits much higher photo sensitivity than pure RGO.The temperature dependent electrical conductivity and theconductivity relaxation mechanism were also studied in thefrequency range from 20 Hz to 2 MHz and analyzed in theframework of Drude conductivity formalism.36−40 The dcconductivity of our composite material shows Arrheniusbehavior. From the scaling of conductivity spectra, we haveobserved that the dynamic process occurring at differenttemperatures have the same thermal activation energy. As perour knowledge, this is the first report on frequency dependentelectrical conductivity and conductivity relaxation in RGObased composite material. The photocatalytic activity of theRGO−Cd0.75Zn0.25S nanocomposite was also observed underthe simulated solar light irradiation taking Rhodamine B (RhB)as a dye material. Furthermore, the possible mechanism forphotocurrent generation and the photocatalytic activity of theRGO−Cd0.75Zn0.25S composite system are also proposed.

■ EXPERIMENTAL SECTIONMaterials. Graphite powder, sodium nitrate [NaNO3],

potassium persulfate [K2S2O8], phosphorus pentoxide [P2O5],zinc acetate dihydrate [Zn(CH3COO)2, 2H2O], cadmiumacetate dihydrate [Cd(CH3COO)2, 2H2O], thiourea[NH2CSNH2], RhB were purchased from Sigma-Aldrich,sulfuric acid [H2SO4], Potassium permanganate [KMnO4],hydrogen peroxide [H2O2], hydrochloric acid [HCl], ethyl-enediamine [EN, NH2CH2CH2NH2] were purchased fromMerck. All the materials were analytical grade and used asreceived without further treatment.Materials Preparation. GO was prepared by modified

Hummers method.41,42 Typically, Graphite powder (2 g),K2S2O8 (1 g), and P2O5 (1 g) were added to 20 mL of 98%H2SO4 at 80 °C and the mixture was kept at 80 °C in an oilbath for 6 h under stirring. This preoxidized graphite was thencleaned by DI water and dried at 60 °C. 0.2 g as-preparedpreoxidized graphite powder and 0.1 g NaNO3 were added to 5mL of concentrated H2SO4 in an ice-bath, and then KMnO4(0.6 g) was added gradually under stirring condition. Afterreaching room temperature, mixture was transferred intoanother bath at 35 °C for 3 h, and then 10 mL of DI waterwas added slowly to the mixture. External heating wasintroduced to maintain the reaction temperature at 80 °C for1 h. The suspension was further diluted by 30 mL DI water atthe same condition, and the reaction was terminated by adding

12 mL H2O2 (3 wt %) at room temperature. The volume of themixture was increased to 100 mL by adding DI water, and themixture was kept under stirring condition for next 12 h. Adilute solution of HCl [H2O:HCl = 10:1] was added with themixture under stirring condition for 5 h to remove the metallicions. The resulting solids were washed with distilled water untilthe pH reaches to 6 and dried at 60 °C overnight. For one potsynthesis of RGO−Cd0.75Zn0.25S, an appropriate amount ofZn(CH3COO)2, 2H2O (0.25 mM), Cd(CH3COO)2, 2H2O(0.75 mM), and NH2CSNH2 (3 mM) were taken in a Teflon-lined stainless steel autoclave. Up to 80% of the total volume ofthe autoclave was filled with EN and water mixture with 2:1volume ratios (EN:water 2:1). The zinc−cadmium and sulfursources were used in 1:3 molar ratios. The resultant mixturewas stirred for a few minutes and 60 mg GO was added to thesolution, followed by sonication for 15 min. Then the autoclavewas sealed properly and placed inside a preheated oven at 175°C and the reaction was continued for 8 h. After getting thenormal temperature, the resulting precipitates were collected bycentrifugation and washed with DI water and ethanol forseveral times. To get the powder form of RGO−Cd0.75Zn0.25Sthe sample was dried in a vacuum oven at room temperature for6 h. Pure Cd0.75Zn0.25S nanorod was synthesized as controlledsample by using same experimental protocol except adding GO.

Materials Characterization. Powder X-ray diffraction(XRD) patterns were recorded on a Rigaku-Miniflex X-raydiffractometer with Cu Kα radiation (λ = 0.154 18 nm) at 30kV and 10 mA. Morphology and crystal structures of thenanorods decorated graphene sheet were obtained fromtransmission electron microscopic (TEM) and high-resolutionTEM (HRTEM) studies by JEOL 2010, operated at 200 kV. X-ray photoelectron spectroscopy (XPS) experiments wereconducted using a ULVAC −PHI 5000 Versa Probe IIspectrometer with an Al Kα radiation source of photon energy1486.6 eV, which was operated at 25 W and 15 kV at a vacuumtypically below 1 × 10−10 Torr. The pass energy was set at117.4 and 29.35 eV for survey and high resolution spectra,respectively. The peak deconvolution was performed usingGaussian components. UV−visible spectra of Cd0.75Zn0.25S andRGO−Cd0.75Zn0.25S were recorded on Shimadzu UV-1700UV−Visible Spectrophotometer. The band gap of controlCd0.75Zn0.25S was calculated by Kubelka−Munk method43,44

using the relation αhν = A(hν − Eg)1/2 where A is a constent, α

is the absorption coefficient, hν is the photon energy, Eg is theband gap of the material.

Device Fabrication and Opto-electronic TransportMeasurements. The photocurrent generation under simu-lated solar light irradiation illumination in the RGO−Cd0.75Zn0.25S composite thin film was investigated byfabricating a photodetector where thin film was prepared bysimple drop-casting method from the RGO−Cd0.75Zn0.25Sdispersed in isopropyl alcohol on a precleaned glass substrate.The sample was left in a fume hood for a few hours to dry. Pairof parallel electrodes of channel length of 4 mm and channelwidth of 4 mm was drawn by using conducting silver paint(Ted Pella). The room temperature I−V characteristics underdark and illuminated conditions were carried out in ambientconditions by a Keithley 2611A sourcemeter using standardtwo probe method. Data were collected by LabTracer 2.0interfaced with the data acquisition card. A xenon lamp (solar-light simulator, Newport, Oriel) of maximum power of 150 Wwas employed as the illumination source. The optical powerwas measured with a calibrated silicon photodiode.

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Electrical Conductivity Measurement. For electricalmeasurements, the dried RGO−Cd0.75Zn0.25S compositesample was ground into more fine powder by agate mortarand pestle then compressed into pellets with area and thicknessof 0.39 cm2 and 0.15 cm respectively by applying a pressure of∼6 Ton cm−2 at room temperature. The ac conductivity of ourRGO−Cd0.75Zn0.25S composite material was evaluated bymeasuring the frequency-dependent capacitance C(ω) andconductance G(ω) of the pallet with an Agilent E4980A LCRmeter within the frequency range of 20 Hz to 2 MHz andtemperature range 300−413 K.Evaluation of Photocatalytic Activity. Photocatalytic

reactions were carried out in a photoreaction vessel containingan aqueous solution of organic dye (RhB) under simulatedsolar light (AM 1.5, 100 mW/cm2, Newport) at ambienttemperature and atmospheric pressure. In a typical photo-catalytic experiment, appropriate amount of photocatalyst wasadded to 20 μM aqueous solution of RhB and made catalystconcentration of 1 g L−1. Before irradiation, the dye solutionwith photocatalyst was magnetically stirred in dark for 2 h to letthe photocatalyst be dispersed uniformly, and the absorbancespectrum of the sample was recorded in the spectrophotometer(Simadzu, UV-1700 UV- Visible Spectrophotometer), markedas zero time reading (t = 0). The solution was then exposed tothe simulated solar light irradiation. The reaction vessel waskept under a water jacket to maintain the room temperature,and also to ensure that degradation was only the result ofphotocatalytic activity without having any thermal effect.Samples were collected at intervals of 15 min and centrifugedto remove the catalyst particles. The supernatant solution wasthen analyzed by UV−vis spectrophotometer to monitor thedegradation of RhB dye. The percentage of degradation wascalculated at intervals for different times by normalizing thepeak intensity in term of the starting (t = 0) peak intensity ofRhB with catalyst.The degradation efficiency of the photocatalyst can be

defined as follows:45

= − ×⎛⎝⎜

⎞⎠⎟

CC

deg. eff. (%) 1 100%0

where deg. eff. is the degradation efficiency, C0 is theconcentration at t = 0, and C is residual concentration atdifferent irradiation intervals of RhB.The photodegradation of RhB follows pseudo-first-order

kinetics, which can be expressed as follows:45

=CC

ktln 0

where k (min−1) is the degradation rate constant.

■ RESULTS AND DISCUSSION

Materials Characterization. The XRD patterns of thesynthesized RGO−Cd0.75Zn0.25S composites and controlledCd0.75Zn0.25S are shown in Figure 1A. All the XRD peakpositions of Cd0.75Zn0.25S in RGO−Cd0.75Zn0.25S composite aresimilar to the previously reported values of Cd0.75Zn0.25S, whichindicates the formation of crystalline alloy semiconductor inRGO−composite.28 Although the XRD peak intensity hasslightly decreased for RGO−Cd0.75Zn0.25S composite comparedto the controlled Cd0.75Zn0.25S sample, no peak shift indicatesthat RGO does not make any significant change in thecrystalline phase of Cd0.75Zn0.25S. Moreover, no diffraction peakfor RGO is observed in the RGO−Cd0.75Zn0.25S composite dueto very low diffraction intensity of graphitic planes compared tothe crystallinity of Cd0.75Zn0.25S.The morphology and the microstructures of the synthesized

sample were analyzed by TEM and shown in Figure 1B. TEMimages show that the RGO−Cd0.75Zn0.25S nanocompositeconsisted of two-dimensional RGO sheet anchored withCd0.75Zn0.25S nanorod. The wrinkles, observed on RGO sheetindicate the formation of single layer.7 It is clearly observed thatthe Cd0.75Zn0.25S nanorods are well spread over on the RGOsheet and the average length of the nanorod is ∼100 nm andwidth ∼10−20 nm. The HRTEM image [inset of Figure 1B]clearly indicates that the Cd0.75Zn0.25S in RGO−Cd0.75Zn0.25Shave high crystallinity nanorod with lattice spacing is around0.334 nm and can be assigned as the (002) lattice plane ofCd0.75Zn0.25S. This is also in well agreement with the XRDpattern (Figure 1A).The effective reduction of GO sheets in RGO−Cd0.75Zn0.25S

nanorod compound was confirmed by the deconvolution of theC 1s peak of XPS spectra. The S, C, Cd, O and Zn peaks arealso clearly observed in the survey spectra of XPS (Figure S1 inthe Supporting Information, SI). All the results are similar topreviously reported values.29,34

A comparison of UV−visible absorption spectra of RGO−Cd0.75Zn0.25S and the controlled sample are demonstrated inFigure 1C. The direct bandgap energy of controlledCd0.75Zn0.25S is calculated as 2.86 eV according to theKubelka−Munk function43,44 and estimated by extrapolatingthe straight portion of the (αhν)2 versus photon energy (hν)curve to absorption axis is zero, as shown in the inset of Figure1C.Furthermore, the presence of RGO in the RGO−

Cd0.75Zn0.25S composite reduces the reflection of light andincreases the absorption of the RGO−Cd0.75Zn0.25S composite

Figure 1. (A) XRD patterns of Cd0.75Zn0.25S and RGO−Cd0.75Zn0.25S. (B) TEM image of RGO−Cd0.75Zn0.25S (HRTEM image of Cd0.75Zn0.25S isalso shown in the inset). (C) Optical absorption spectra of Cd0.75Zn0.25S and RGO−Cd0.75Zn0.25S. Plot of (αhν)2 vs photon energy for controlCd0.75Zn0.25S is shown in the inset of (C).

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material in comparison to the controlled Cd0.75Zn0.25S. Theabsorption edge of RGO−Cd0.75Zn0.25S composite is quitesimilar to that of control Cd0.75Zn0.25S which depicts that RGOis not incorporated into the lattice of Cd0.75Zn0.25S and a similarobservation was reported by Zhang et al.34

Photocurrent Generation in RGO−Cd0.75Zn0.25S Com-posite under Simulated Solar Light Irradiation. Theschematic illustration of our photodetector device along withthe electrical transport measurement setup is shown in Figure2A. Figure 2B shows the Current (I)−Voltage (V) character-

istics of the device under dark and illumination condition whereillumination level varies from 90 to 160 mW cm−2. All thecurves pass through origin and show linear variation of currentwith applied voltage. At V = 2 V, channel current changes from93 μA to 159 μA under illuminated light intensity of 160 mWcm−2. The photocurrent (IPh) was calculated by subtracting thedark current (ID) from current under illumination (IL) [IPh = IL− ID]. The performance of a photodetector is determined byvalues of photosensitivity P, (the ratio of photocurrent to darkcurrent), and our thin film device shows 71% of P underilluminated intensity of 160 mW cm−2. We have measured P forour device for different intensities and variation of P withintensity is shown in inset of Figure 2B. The solid line is a linearfit of the experimental data which indicates that P increaseslinearly with the intensity of illuminated light intensity. Similarlinear dependence of P with intensity was also observed inRGO−PbS decorated photodetector22 and carbon nanotubefilm.46 We have also studied the dynamical photo response ofour thin film device. The variation of photocurrent with timeupon exposure to simulated solar light intensity of 150 mW/cm2 on photodetector devices at bias voltage of 2 V is presentedin the inset of Figure 2C. The irradiation was turned ON andOFF periodically with 100 s interval of time. The devicesresponded to the illumination as soon as the source was turned“ON”. After turning off the light, the photocurrent decreases

with time. The plot is shown for three cycles of the light sourcebeing turned on and off and highly reproducible upon repeatedON/OFF of the illumination demonstrating the stability of thefabricated device. It can be seen that when illuminated by thelight source, the current increases slowly until it reaches asteady state (153 μA) and slowly recovers the dark current (95μA) when the light is switched off. The dynamic response ofour device to the simulated solar light source shows exponentialbehavior and can be well described by I(t) = Idark + A exp −(t −t0)/τd for decay, where τd is the time constant for the dynamicalphoto response, and t − t0 is the time when light was switchedoff, Idark is the dark current, and A is the scaling constant. This isshown in Figure 2C. The open squares are the experimentaldata points and solid line is a fit to the above equation. Thetime constant for decay of current was calculated about 27 sfrom the fit for RGO−Cd0.75Zn0.25S thin film. The slow timeresponse may be due to the charge trapping between differentRGO sheets and at the Cd0.75Zn0.25S/RGO interfaces. The sizeof RGO sheets and the ratio of RGO and Cd0.75Zn0.25S mayplay an important role for improvement of time response of ourthin film device. We have tried to explain photocurrentgeneration mechanism of our system through band diagramwhich is shown in Figure 2D. When the RGO−Cd0.75Zn0.25Sthin film is illuminated by simulated solar light, excitons aregenerated at RGO and Cd0.75Zn0.25S. The excitons aresubsequently dissociated into free electrons and holes at theRGO−Cd0.75Zn0.25S interfaces, different defect states of RGOand few at the interface of electrode and composites. For a puresemiconductor material, it has a tendency of recombination ofelectron−hole species that decreases the photocurrentgeneration. In RGO−Cd0.75Zn0.25S, due to anchoring ofCd0.75Zn0.25S onto the basal plane on RGO, the electron−hole recombination process will be retarded, and the electronswill diffuse to the positive electrode through the interconnectedRGO sheets while the holes to the negative electrode resultingin an enhancement of photocurrent generation in ourcomposite materials.The photocurrent generation in a photodetector can be

presented as follows:47

σ η τν

= = =I qL

P

hv BPEWDph

p optd opt

where, τp is the photo generated carrier lifetime, W is theeffective channel width, L is the length, D is the devicethickness, q is the elementary charge, E is the electric fieldinside the photodetector, vd is the drift velocity, hν is thephoton energy, B ( = qτηvd/hνL) is the proportionality factor,and Popt is the incident optical power. Our experimental resultsare in conformity with the above linear relationship betweenphotocurrent generation as well as P and incident opticalpower.

Electrical Conductivity and Conductivity Relaxation inRGO−Cd0.75Zn0.25S Composite. To get a better insight of theelectron transport mechanism in our synthesized RGO−Cd0.75Zn0.25S composite material, the in-depth analysis offrequency dependent electrical conductivity has been carriedout and as per our knowledge, this is the first time reporting offrequency dependent conductivity of RGO based compositematerials. The response of our RGO−Cd0.75Zn0.25S nano-composite material to a frequency dependent electric field canbe defined by the complex total conductivity σ*(ω) = σ′(ω) +iσ″(ω).

Figure 2. (A) The cartoon of our photo detector device along with theelectrical transport measurement setup. (B) I−V characteristics forRGO−Cd0.75Zn0.25S nanocomposite thin film device under darkcondition and under different illumination intensity. The variation ofphoto sensitivity with illuminated intensity of our thin film device isshown in the inset. (C) The decay of current when the illuminatedlight was turned off. Current versus time for several cycles as the lightwas turned on and off is shown in the inset. (D) Photocurrentgeneration mechanism in our thin film device.

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The conduction of electrons in the RGO sheets will behindered due to the presence of immobile Cd0.75Zn0.25Snanorods on the basal plane of RGO and electrons will bounceand rebounce off Cd0.75Zn0.25S nanorods and the carbon atomsof the RGO sheets. Figure 3A is the schematic representation of

the electronic motion in RGO while bouncing and rebouncingoff Cd0.75Zn0.25S nanorods and carbon atoms. The motion ofelectrons under the influence of frequency dependent electricfield inside the RGO−Cd0.75Zn0.25S composite materials can beexplained by Drude conduction mechanism.36−40 For betterappreciation of this mechanism, the basic principle ofconduction inside our composite material in ac field ispresented below.Let, the momentum of an conduction electron at any time t

be p(t) under the influence of an external force f(t). Two thingscan happen in the next time interval dt: (i) they can undergocollision with large size Cd0.75Zn0.25S molecule or carbonmolecule with a probability of [dt/τc], and will lose allmomentum and emerge with a random momentum dp = f(t) dt[as (dp/dt) = f(t)], and their contribution to total momentumof system will be [dt/τc] × [f(t) dt], where τc is the mostprobable conductivity relaxation time. Otherwise, (ii) they donot suffer collision with probability [1 − dt/τc], and theirmomentum after time dt will be [p(t) + f(t) dt], then theircontribution to total momentum of system is [1 − dt/τc]·[p(t)+ f(t) dt].Thus, the net momentum at time t + dt is as follows:

τ τ

τ

+ = + − +

= − +

⎡⎣⎢

⎤⎦⎥

⎡⎣⎢

⎤⎦⎥

⎡⎣⎢

⎤⎦⎥

p t tt

f t tt

p t f t t

p tt

p t f t t

( d )d

[ ( ) d ] 1d

[ ( ) ( ) d ]

( )d

( ) [ ( ) d ]

c c

c

On simplification, the above relation appears as follows:

τ=

−+

pt

p tf t

dd

( )( )

c (1)

In an alternating electric field E(t), [E(t) = E0e−iωt], eq 1

becomes the following:

τ=

−− ω−p

tp t

eEdd

( )e i t

c0

Using the relation p(t) = p0e−iωt the solution of eq 1 will be,

p(t) = −eEτc/(1−iωτc).If n is the electron concentration and m is the mass of

electron, then the current density is as follows:

τωτ

σ ω=−

=−

= *Jnep t

mne

m iE t E

( )(1 )

( ) ( )2

c

c

σ ωτωτ

σωτ

* =−

=−

nem i i

( )(1 ) (1 )

2c

c

dc

c

where σdc is the dc conductivity. The real part of the ACconductivity [σ′(ω)] can be written as follows:

σ ωσω τ

′ =+

( )1

dc2

c2

(2)

The above two equations (eqs 1 and 2) are similar to theequation for AC electrical conductivity of a metal.48 Figure 3Bshows the isotherms for real part of the total conductivity σ′(ω)= G(ω)l/A in the angular frequency domain ω, at differenttemperatures, shown in the inset, where l and A are thethickness and area of the sample, respectively. Ourexperimental variation of σ′(ω) with applied frequency is inwell agreement with that of eq 2. At low frequency, the real partof conductivity was independent of frequency and correspondsto the dc conductivity (σdc), and above a characteristiccrossover frequency the conductivity decreases with theincrease in frequency. The transition from the frequencyindependent to the frequency-dependent conductivity indicatesthe onset of a relaxation phenomenon, which is here explainedin the framework of the Drude formalisms. Similar behavior wasobserved for all temperatures. The σdc and τc were calculatedfrom the fitting of eq 2 with experimentally observed σ′(ω) vsω curves, and the fitting for a particular temperature 373 K isshown in the inset of Figure 3B. We have observed that σdcexhibited Arrhenius behavior, and the activation energy wascalculated as 24 meV from the straight-line fits and is shown inFigure 3C. The frequency corresponding to the τc gives themost probable conductivity relaxation frequency ωc = 1/τc.To investigate the temperature-dependence of relaxation

process, we have scaled the conductivity spectra for differenttemperatures. In this scaling process, the σ′(ω) was scaled byσdc, while the frequency axis was scaled by ωc. We haveobserved that the conductivity spectra for different temper-atures superpose perfectly on a single master curve (Figure3D), which implies that the dynamic process occurring atdifferent temperatures have the same thermal activation energy.A similar temperature independent relaxation mechanism wasobserved in the disorder system.49−51

Photocatalytic Degradation of RhB over RGO−Cd0.75Zn0.25S Composite under Simulated Solar LightIrradiation. To explore the potential application of thecomposite in the environment, the photocatalytic activity ofRGO−Cd0.75Zn0.25S composite on the degradation of aqueous

Figure 3. (A) Schematic diagram of electron conduction process inRGO−Cd0.75Zn0.25S composite material. (B) The frequency depend-ence of electrical conductivity for different temperatures. Theconductivity spectrum for 373 K with fitting of eq 2 is shown in theinset. (C) The Arrhenius behavior of DC Conductivity with its straightline fit. (D) Scaling of conductivity spectra for different temperatures.

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RhB solution (Figure 4A) was performed and compared withthat of controlled Cd0.75Zn0.25S nanorod (SI) under simulated

solar light irradiation. With increasing illumination time, thecatalysis reaction starts and the gradual decreasing in absorptionpeak intensity occurs and it leads to the subsequent degradationof the organic dye. A small blue shift of the RhB absorbancemaximum with increasing illuminated time is observed. Theblue shift is related to the N-deethylation of RhB to rhodamine(Rh), the major intermediate part of the degradation of RhB.52

The photo degradation efficiency of RhB reached 90% in thepresence of RGO−Cd0.75Zn0.25S catalyst after 150 min which is64% in the presence of controlled Cd0.75Zn0.25S within the sametime duration under simulated solar light irradiation (Figure4B). To investigate the kinetics of the degradation of RhBunder irradiation, we have plotted ln (C0/C) with irradiationtime which is presented in Figure 4C. It can be seen from thefigure that the photo degradation follows first order kinetics45

and the photocatalytic activity of RGO−Cd0.75Zn0.25S nanorodcomposites (k = 1.6 min−1) is about 2.3 times higher that ofcontrolled Cd0.75Zn0.25S nanorod (k = 0.7 min−1). The highphotocatalytic activity of RGO−Cd0.75Zn0.25S nanorod compo-sites compared to controlled Cd0.75Zn0.25S nanorod is expectedas RGO sheets play an crucial role toward efficient chargeseparation and charge transportation and help for creation ofhydroxyl radicals OH● the key factor for destruction of RhBdyes through oxidation. The schematic representation of themechanism causes the enhanced photodecomposition of thedye by the RGO−Cd0.75Zn0.25S composite is shown in Figure4D and the mechanism for creation of hydroxyl radicals OH●

under simulated solar light irradiation are summarized in the SI.

■ CONCLUSIONSIn summary, we have investigated the optoelectronic transportproperties of large area RGO−Cd0.75Zn0.25S composite films

under dark and simulated solar light illumination. Theenhancement of photocurrent under 160 mW cm−2 illumina-tion intensity is 71% compared to the dark current, and thetime constant of the dynamical photo response is 27 s. We havealso studied the electrical conductivity and the conductivityrelaxation phenomena for different temperatures. The dcconductivity of our composite material shows Arrheniusbehavior and a temperature independent conductivity relaxa-tion phenomenon in our RGO based composite material hasbeen observed. The photocatalytic activity of the RGO−Cd0.75Zn0.25S nanocomposite has increased by 2.3 times incompared to controlled Cd0.75Zn0.25S samples. We haveobserved that RGO plays an important role to efficient chargeseparation, which hinders the electron−hole recombination andenhanced the photocurrent generation and photocatalyticactivity of our composite material under simulated solar lightirradiation.

■ ASSOCIATED CONTENT*S Supporting InformationXPS analysis, imaginary part of frequency dependentconductivity and its scaling, photocatalytic activity ofCd0.75Zn0.25S for the degradation of RhB under simulatedsolar light irradiation, and mechanism for photo catalyticactivity. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: surajit@mail.vidyasagar.ac.in.* E-mail: tanusri.pal@gmail.com.

Author Contributions§Undergraduate Student.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Department of Science andTechnology (DST), New Delhi, India via grant SR/FTP/PS-066/2010. We are also thankful to the University GrantsCommission (UGC), New Delhi, India and DST, New Delhi,India for providing special assistance and infrastructural supportto our department via SAP and FIST program, respectively. Weexpand our thanks to the Department of Physics andMeteorology, IIT Kharagpur for providing DST-FIST fundedXPS facility.

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Figure 4. (A) UV−vis absorption spectra of RhB with RGO−Cd0.75Zn0.25S for different time of simulated solar light irradiation. (B)The comparison of the photo degradation efficiency as a function oftime under simulated solar light irradiation over Cd0.75Zn0.25S andRGO−Cd0.75Zn0.25S. (C) Plot of ln(C0/C) as a function of simulatedsolar light irradiation time for the photocatalysis of RhB solutioncontaining Cd0.75Zn0.25S and RGO−Cd0.75Zn0.25S. (D) Schematicdiagram of charge carrier transfer from Cd0.75Zn0.25S to RGO sheetsand dye degradation mechanism in RGO−Cd0.75Zn0.25S photocatalystunder simulated solar light irradiation.

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