Photocurrent Generation and Conductivity Relaxation in Reduced

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Photocurrent Generation and Conductivity Relaxation in Reduced Graphene Oxide Cd Zn S Nanocomposite and its Photocatalytic Activity 0.75

0.25

Sankalpita Chakrabarty, Koushik Chakraborty, Arnab Laha, Tanusri Pal, and Surajit Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509575p • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 15, 2014

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The Journal of Physical Chemistry

Photocurrent Generation and Conductivity Relaxation in Reduced Graphene Oxide Cd0.75Zn0.25S Nanocomposite and its Photocatalytic Activity Sankalpita Chakrabarty1, Koushik Chakraborty1, Arnab Laha1,†, Tanusri Pal2,*, Surajit Ghosh1,* 1

Department of Physics & Technophysics, Vidyasagar University, Midnapore 721102, WB,

India 2

Department of Physics, Midnapore College, Midnapore 721101, WB, India

ABSTRACT: We report the photocurrent generation in reduced graphene oxide – cadmium zinc sulphide (RGO-Cd0.75Zn0.25S) nano composite material under simulated solar light irradiation, where the photocurrent increases linearly with increasing incident light intensity. We also report the temperature dependent electrical conductivity and conductivity relaxation in RGO-Cd0.75Zn0.25S composite. At low frequency the real part of conductivity is independent of frequency and above a characteristic crossover frequency the conductivity decreases with the increase in frequency which indicates the onset of a relaxation phenomenon. The dc conductivity of the RGOCd0.75Zn0.25S composite shows Arrhenius behaviour. From the scaling of real part of conductivity spectra we have observed that the dynamic process occurring at different temperatures have the same thermal activation energy. The RGO-Cd0.75Zn0.25S composite shows an enhancement of photo catalytic activity in comparison to control sample under simulated solar light irradiation to degrade Rhodamine B. The RGO sheets prolong the separation of photo induced electrons and holes in Cd0.75Zn0.25S, which hinder the electron-hole recombination and subsequently enhances the photocurrent generation and photocatalytic activity under simulated solar light irradiation.

KEYWORDS:

graphene-semiconductor

composite,

photocurrent

generation,

Electrical

conductivity, conductivity relaxation, photocatalysis 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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INTRODUCTION Solution processable reduced graphene oxide (RGO) is enjoying a great attention for large scale thin film device due to ease of material processing, low cost of fabrication, mechanical flexibility, and compatibility with various substrates.1-10 In particular, large surface area with a wide range of oxygen functionalities in RGO creates ability to make composites with other nano materials, including polymer, metal, metal oxide nano particle to form unique hybrid composites.11-15 For instance, when RGO is anchored with the photo-sensitive nano materials, such as, quantum dots, nano particles, the RGO serves as a continuous pathway for electron transfer process from the molecules and the composite is expected to be an extraordinary potential candidate for photo excitonic charge generation. With such advantages, researchers have devoted much effort to develop and construct optoelectronic devices such as photodetectors, solar cells, sensors and photocatalyst using inorganic semiconductor functionalized RGO composites.14-24 Lightcap et al.studied the electron and energy transfer from photo excited CdSe colloidal quantum dots to RGO and they also focused on the improvement of photo response of RGO-CdSe composite over control CdSe and GO-CdSe composites.18 A highly sensitive ultraviolet sensor based on ZnO nanorod/RGO composites is reported by Chang and co-workers.24 All these RGO based composites are optically active only for selective wavelength depending on their band gap energy. To this end, ternary chalcogenide nano materials are well studied due to tuneability of their optical band gap by changing the compositions.25-31 For an instance, Cd1-xZnxS has the potential to form a continuous series of solid solutions and allows to open up the possibility to vary the optical band gap systematically in a controlled way from VIS (CdS=2.42 eV) to UV (ZnS=3.7eV) region. The performance of an optoelectronic device strongly depends on the formation of excitons and their subsequent dissociation into free electrons and holes. The photo induced charge generation increases with increase of exciton diffusion length and lifetime. Better crystallinity structure offers higher diffusion length and lifetime of excitons32. In our recent studies on solvothermally synthesized CdS nano structures with different morphology, we have observed that the nanorod morphology shows a better crystallinity and subsequently an efficient photo induced charge generation33. In the present study, we have chosen specially nanorod structure of the alloyed semiconductor material and made a composite with RGO, to ensure their efficient photo induced charge generation as well as photocatalytic enhancement 2 ACS Paragon Plus Environment

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probabilities. Although there are a few reports on RGO-CdZnS composite materials which establish its potential for efficient photocatalytic hydrogen production,34,35 neither of these studies explores the photo induced charge generation in large area thin film optoelectronic devices and photocatalytic dye degradation efficiency under simulated solar light irradiation. The temperature dependent electrical conductivity and the conductivity relaxation phenomena in RGO based composite materials also remain unaddressed. Here, we report the photocurrent generation and photocatalytic activity of RGO-Cd1xZnxS

nanorod composite under simulated solar light irradiation, where the composite was

synthesized by simple, cost effective one-pot solvothermal route. The value of ‘x’ was chosen 0.25, to make the material more functional in the visible side of the total solar spectrum. The structural and morphological characteristics of our synthesized materials were studied by X-Ray Diffraction (XRD) analysis, Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM). The reduction of Graphene Oxide (GO) was confirmed by X-ray Photoelectron spectroscopy. The optoelectronic properties of our thin film device were investigated under simulated solar light irradiation and it exhibits much higher photo sensitivity than pure RGO. The temperature dependent electrical conductivity and the conductivity relaxation mechanism were also studied in the frequency range from 20 Hz to 2 MHz and analyzed in the frame work of Drude conductivity formalism.36-40 The dc conductivity of our composite material shows Arrhenius behaviour. From the scaling of conductivity spectra we have observed that the dynamic process occurring at different temperatures have the same thermal activation energy. As per our knowledge, this is the first time report on frequency dependent electrical conductivity and conductivity relaxation in RGO based composite material. The photocatalytic activity of the RGO-Cd0.75Zn0.25S nanocomposite was also observed under the simulated solar light irradiation taking Rhodamine B (RhB) as a dye material. Furthermore, the possible mechanism for photocurrent generation and the photocatalytic activity of the RGO-Cd0.75Zn0.25S composite system are also proposed.

EXPERIMENTAL SECTION Materials. Graphite powder, sodium nitrate [NaNO3], potassium persulfate [K2S2O8], phosphorus pentoxide [P2O5], zinc acetate dihydrate [Zn(CH3COO)2, 2H2O], cadmium acetate dihydrate [Cd(CH3COO)2, 2H2O], thiourea [NH2CSNH2], RhB were purchased from Sigma 3 ACS Paragon Plus Environment


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Aldrich, sulfuric acid [H2SO4], Potassium permanganate [KMnO4], hydrogen peroxide [H2O2], hydrochloric acid [HCl], ethylenediamine [EN, NH2CH2CH2NH2] were purchased from Merck. All the materials were analytical grade and used as received without further treatment. Materials Preparation. GO was prepared by modified Hummers method.41,42 Typically, Graphite powder (2g), K2S2O8 (1g), 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 oil bath for 6h under stirring condition. This preoxidized graphite was then cleaned by DI water and dried at 60oC. 0.2 g as-prepared preoxidized graphite powder and 0.1 g NaNO3 were added to 5 mL of concentrated H2SO4 in an ice-bath, and then KMnO4 (0.6 g) was added gradually under stirring condition. After reaching the room temperature, mixture was transferred into another bath at 35 °C for 3h, and then 10mL of DI water was added slowly to the mixture. External heating was introduced to maintain the reaction temperature at 80 °C for 1h. The suspension was further diluted by 30 mL DI water at the same condition and the reaction was terminated by adding 12 mL H2O2 (3 wt %) at room temperature. The volume of the mixture was increased to 100 ml by adding DI water and the mixture was kept under stirring condition for next 12 hours. A dilute solution of HCl [H2O:HCl = 10:1] was added with the mixture under stirring condition for 5h to remove the metallic ions. The resulting solids were washed with distilled water until the pH reaches to 6 and dried at 60°C overnight. For one pot synthesis of RGO-Cd0.75Zn0.25S, an appropriate amount of Zn(CH3COO)2, 2H2O (0.25mM), Cd(CH3COO)2, 2H2O (0.75mM), and NH2CSNH2 (3mM) were taken in a teflon-lined stainless steel autoclave. Up to 80% of the total volume of the autoclave was filled with EN and water mixture with 2:1 volume ratios (EN:water 2:1). The zinc-cadmium and sulphur sources were used in 1:3 molar ratios. The resultant mixture was stirred for a few minutes and 60 mg GO was added to the solution, followed by sonication for 15 min. Then the autoclave was sealed properly and placed inside a preheated oven at 175°C and the reaction was continued for 8 h. After getting the normal temperature, the resulting precipitates were collected by centrifugation and washed with DI water and ethanol for several times. To get the powder form of RGO-Cd0.75Zn0.25S the sample was dried in a vacuum oven at room temperature for 6 h. Pure Cd0.75Zn0.25S nanorod was synthesized as controlled sample by using same experimental protocol except adding GO. Materials Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku-Miniflex X-ray diffractometer with CuKα radiation (λ=0.15418 nm) at 30 kV and 10 4 ACS Paragon Plus Environment

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mA. Morphology and crystal structures of the nanorods decorated graphene sheet were obtained from transmission electron microscopic (TEM) and high-resolution TEM (HRTEM) studies by JEOL 2010, operated at 200 KV. X-ray photoelectron spectroscopy (XPS) experiments were conducted using a ULVAC −PHI 5000 Versa Probe II spectrometer with an Al Kα radiation source of photon energy 1486.6 eV which was operated at 25 Watt and 15 kV at a vacuum typically below 1 × 10−10 Torr. The pass energy was set at 117.4 eV and 29.35 eV for survey and high resolution spectra respectively. The peak deconvolution was performed using Gaussian components. UV-Visible spectra of Cd0.75Zn0.25S and RGO-Cd0.75Zn0.25S were recorded on Shimadzu UV-1700 UV- Visible Spectrophotometer. The band gap of control Cd0.75Zn0.25S was calculated by Kubelka-Munk method43,44 using the relation αℎν = (ℎν − ) / where A is a constent, α is the absorption coefficient, hν is the photon energy, Eg is the band gap of the material. Device Fabrication and Opto-electrical Transport Measurements. The photocurrent generation under simulated solar light irradiation illumination in the RGO-Cd0.75Zn0.25S composite thin film was investigated by fabricating a photodetector where thin film was prepared by simple drop-casting method from the RGO-Cd0.75Zn0.25S dispersed in isopropyl alcohol on a pre-cleaned glass substrate. The sample was left in a fume hood for a few hours to dry. Pair of parallel electrodes of channel length of 4 mm and channel width of 4 mm was drawn by using conducting silver paint (Ted Pella). The room temperature I - V characteristics under dark and illuminated conditions were carried out in ambient conditions by a Keithley 2611A sourcemeter using standard two probe method. Data were collected by LabTracer 2.0 interfaced with the data acquisition card. A xenon lamp (solar-light simulator, Newport, Oriel) of maximum power of 150W was employed as the illumination source. The optical power was measured with a calibrated silicon photodiode. Electrical Conductivity Measurement. For electrical measurements, the dried RGOCd0.75Zn0.25S composite sample was ground into more fine powder by agate mortar and pestle then compressed into pellets with area and thickness of 0.39 cm2 and 0.15 cm respectively by applying a pressure of ∼ 6 Ton cm-2 at room temperature. The ac conductivity of our RGOCd0.75Zn0.25S composite material was evaluated by measuring the frequency-dependent capacitance C(ω) and conductance G(ω) of the pallet with an Agilent E4980A LCR meter within the frequency range of 20 Hz to 2 MHz and temperature range 300−413 K. 5 ACS Paragon Plus Environment


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Evaluation of Photocatalytic Activity. Photocatalytic reactions were carried out in a photoreaction vessel containing an aqueous solution of organic dye (RhB) under simulated solar light (AM 1.5, 100mW/cm2, Newport) at ambient temperature and atmospheric pressure. In a typical photocatalytic experiment, appropriate amount of photocatalyst was added to 20 µM aqueous solution of RhB and made catalyst concentration of 1 gm L-1. Before irradiation, the dye solution with photocatalyst was magnetically stirred in dark for 2 h to let the photocatalyst be dispersed uniformly, and the absorbance spectrum of the sample was recorded in the spectrophotometer (Simadzu, UV-1700 UV- Visible Spectrophotometer), marked as zero time reading (t = 0). The solution was then exposed to the simulated solar light irradiation. The reaction vessel was kept under a water jacket to maintain the room temperature, and also to ensure that degradation was only the result of photocatalytic activity without having any thermal effect. Samples were collected at intervals of 15min and centrifuged to remove the catalyst particles. The supernatant solution was then analyzed by UV-VIS spectrophotometer to monitor the degradation of RhB dye. The percentage of degradation was calculated at intervals for different times by normalizing the peak intensity in term of the starting (t = 0) peak intensity of RhB with catalyst. The degradation efficiency of the photocatalyst can be defined as follows:45 Degradation Efficiency (%) = 1 −

C × 100% C

Where C0 is the concentration at t=0 and C is residual concentration at different irradiation intervals of RhB. The photodegradation of RhB follows pseudo-first-order kinetics, which can be expressed as follows45 ln

C = kt C

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

RESULTS AND DISCUSSION Materials Characterization. The XRD patterns of the synthesized RGO-Cd0.75Zn0.25S composites and controlled Cd0.75Zn0.25S are shown in Figure 1A. All the XRD peak positions of Cd0.75Zn0.25S in RGO-Cd0.75Zn0.25S composite are similar to the previously reported values of Cd0.75Zn0.25S which indicates the formation of crystalline alloy semiconductor in RGOcomposite.28 Although the XRD peak intensity has slightly decreased for RGO-Cd0.75Zn0.25S 6 ACS Paragon Plus Environment

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composite compared to the controlled Cd0.75Zn0.25S sample, no peak shift indicates that RGO does not make any significant change in the crystalline phase of Cd0.75Zn0.25S. Moreover, no diffraction peak for RGO is observed in the RGO-Cd0.75Zn0.25S composite due to very low diffraction intensity of graphitic planes compared to the crystallinity of Cd0.75Zn0.25S. The morphology and the microstructures of the synthesized sample were analysed by TEM and shown in Figure 1B. TEM images show that the RGO-Cd0.75Zn0.25S nanocomposite consisted of two dimensional RGO sheet anchored with Cd0.75Zn0.25S nanorod. The wrinkles, observed on RGO sheet indicate the formation of single layer.7 It is clearly observed that the Cd0.75Zn0.25S nanorods are well spread over on the RGO sheet and the average length of the nanorod is ~ 100 nm and width ~ 10-20 nm. The HRTEM image [inset of Figure 1B] clearly indicates that the Cd0.75Zn0.25S in RGO-Cd0.75Zn0.25S have high crystallinity nanorod with lattice spacing is around 0.334 nm and can be assigned as the (002) lattice plane of Cd0.75Zn0.25S. This is also in well agreement with the XRD pattern (Figure 1A). The effective reduction of GO sheets in RGO-Cd0.75Zn0.25S nanorod compound was confirmed by the deconvolution of the C1s peak of XPS spectra. The S, C, Cd, O and Zn peaks are also clearly observed in the survey spectra of XPS (Figure S1 in Supporting Information). All the results are similar to previously reported values.29,34 A comparison of UV−visible absorption spectra of RGO-Cd0.75Zn0.25S and the controlled sample are demonstrated in Figure 1C. The direct bandgap energy of controlled Cd0.75Zn0.25S is calculated as 2.86 eV according to the Kubelka−Munk function43,44 and estimated by extrapolating the straight portion of the (αhν)2 versus photon energy (hv) curve to absorption axis is zero, as shown in the inset of Figure 1C. Furthermore, the presence of RGO in the RGO-Cd0.75Zn0.25S composite reduces the reflection of light and increases the absorption of the RGO-Cd0.75Zn0.25S composite material in comparison to the controlled Cd0.75Zn0.25S. The absorption edge of RGO-Cd0.75Zn0.25S composite is quite similar to that of control Cd0.75Zn0.25S which depicts that RGO is not incorporated into the lattice of Cd0.75Zn0.25S and a similar observation was reported by Zhang et al.34 Photocurrent Generation in RGO-Cd0.75Zn0.25S Composite under Simulated Solar Light Irradiation. The schematic illustration of our photodetector device along with the electrical transport measurement setup is shown in Figure 2A. Figure 2B shows the Current (I) - Voltage (V) characteristics of the device under dark and illumination condition where illumination level 7 ACS Paragon Plus Environment


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varies from 90 to 160 mW cm-2. All the curves pass through origin and show linear variation of current with applied voltage. At V = 2V, channel current changes from 93 µA to 159 µA under illuminated light intensity of 160 mW cm-2. The photocurrent (IPh) was calculated by subtracting the dark current (ID) from current under illumination (IL) [IPh = IL − ID]. The performance of a photodetector is determined by values of photosensitivity P, (the ratio of photocurrent to dark current), and our thin film device shows 71% of P under illuminated intensity of 160 mW cm-2. We have measured P for our device for different intensities and variation of P with intensity is shown in inset of Figure 2B. The solid line is a linear fit of the experimental data which indicates that P increases linearly with the intensity of illuminated light intensity. Similar linear dependence of P with intensity was also observed in RGO-PbS decorated photodetector22 and carbon nanotube film.46 We have also studied the dynamical photo response of our thin film device. The variation of photocurrent with time upon exposure to simulated solar light intensity of 150 mW/cm2 on photodetector devices at bias voltage of 2V is presented in inset of Figure 2C. The irradiation was turned ON and OFF periodically with 100 sec interval of time. The devices responded 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 source being turned on and off and highly reproducible upon repeated ON/OFF of the illumination demonstrating the stability of the fabricated device. It can be seen that when illuminated by the light source, the current increases slowly until it reaches a steady state (153 µA) and slowly recovers the dark current (95 µA) when the light is switched off. The dynamic response of our device to the simulated solar light source shows exponential behaviour and can be well described by I(t) = Idark + A exp -(t-t0)/τd for decay , where τd is the time constant for the dynamical photo response, and t-t0 is the time when light was switched off, Idark is the dark current and A is the scaling constant. This is shown in Figure 2C. The open squares are the experimental data points and solid line is a fit to the above equation. The time constant for decay of current was calculated about 27 seconds from the fit for RGO-Cd0.75Zn0.25S thin film. The slow time response may be due to the charge trapping between different RGO sheets and at the Cd0.75Zn0.25S/RGO interfaces. The size of RGO sheets and the ratio of RGO and Cd0.75Zn0.25S may play an important role for improvement of time response of our thin film device. We have tried to explain photocurrent generation mechanism of our system through band diagram which is shown in Figure 2D. When the RGO-Cd0.75Zn0.25S thin film is illuminated by simulated solar light, 8 ACS Paragon Plus Environment

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excitons are generated at RGO and Cd0.75Zn0.25S. The excitons are subsequently dissociated into free electrons and holes at the RGO-Cd0.75Zn0.25S interfaces, different defect states of RGO and few at the interface of electrode and composites. For a pure semiconductor material, it has a tendency of recombination of electron-hole species that decreases the photocurrent generation. In RGO-Cd0.75Zn0.25S, due to anchoring of Cd0.75Zn0.25S onto the basal plane on RGO, the electronhole recombination process will be retarded and the electrons will diffuse to the positive electrode through the interconnected RGO sheets while the holes to the negative electrode resulting in an enhancement of photocurrent generation in our composite materials. The photocurrent generation in a photodetector can be presented as47 %&' = σ () = *

η τ& ,-&. /0 = 1,-&. + ℎν

Where, τp is the photo generated carrier lifetime, W is the effective channel width, L is the length, D is the device thickness, q is the elementary charge, E is the electric field inside the photodetector and vd is the drift velocity, hν is the photon energy and B ( = qτηvd/hνL) is the proportionality factor and Popt be the incident optical power. Our experimental results are in conformity with the above linear relationship between photocurrent generation as well as P and incident optical power. Electrical Conductivity and Conductivity Relaxation in RGO-Cd0.75Zn0.25S Composite. To get a better insight of the electron transport mechanism in our synthesized RGO-Cd0.75Zn0.25S composite material, the in-depth analysis of frequency dependent electrical conductivity has been carried out and as per our knowledge, this is the first time reporting of frequency dependent conductivity of RGO based composite materials. The response of our RGO-Cd0.75Zn0.25S nanocomposite material to a frequency dependent electric field can be defined by the complex total conductivity σ*(ω) = σ′(ω) + iσ″(ω). The conduction of electrons in the RGO sheets will be hindered due to the presence of immobile Cd0.75Zn0.25S nanorods on the basal plane of RGO and electrons will bounce and rebounce off Cd0.75Zn0.25S nanorods and the carbon atoms of the RGO sheets. Figure 3A is the schematic representation of the electronic motion in RGO while bouncing and re-bouncing off Cd0.75Zn0.25S nanorods and carbon atoms. The motion of electrons under the influence of frequency dependent electric field inside the RGO-Cd0.75Zn0.25S composite materials can be

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explained by Drude conduction mechanism.36-40 For better appreciation of this mechanism, the basic principle of conduction inside our composite material in ac field is presented 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 things can happen in the next time interval dt: (i) they can undergo collision with large size Cd0.75Zn0.25S molecule or carbon molecule with a probability of [dt/τc], and will lose all momentum and emerge with a random momentum dp = f(t) dt [as

0& 0.

= f(t)],

and their contribution to total momentum of system will be [dt/τc]×[f(t)dt], where τ2 is the most probable conductivity relaxation time. Otherwise, (ii) they do not suffer collision with probability [1-dt/τc], and their momentum after time dt will be [p(t)+ f(t)dt], then their contribution to total momentum of system is [1-dt/τc].[p(t)+ f(t)dt] Thus the net momentum at time t+dt is: 3(4 + 64) = 7

64

τ2

8 9:(4)64; + 71 −

64

τ2

8 93(4) + :(4)64; = 3(4) − 7

64

τ2

8 3(4) + 9:(4)64;

On simplification the above relation appears as: 0& 0.

=

Photocurrent Generation and Conductivity Relaxation in Reduced

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