Fabrication and Characterization of Few Layered Graphene Sheet Decked with CeO₂ Nano Particles for Dye Sensitized solar cell Application

Fabrication and Characterization of Few Layered Graphene Sheet Decked with CeO₂ Nano Particles for Dye Sensitized solar cell Application

1Satish Bykkam

Nano Electronics Laboratory, Center for Nano Science & Technology,

Institute of Science & Technology, Jawaharlal Nehru Technological University

Hyderabad-500085, Telagana State, India

3Ch. Shilpa Chakra

Nano Electronics Laboratory, Center for Nano Science & Technology,

Institute of Science & Technology, Jawaharlal Nehru Technological University Hyderabad-500085, Telagana State, India

2K. Venkateswara Rao Nano Electronics Laboratory,

Center for Nano Science & Technology, Institute of Science & Technology,

Jawaharlal Nehru Technological University Hyderabad-500085, Telagana State, India

4T. Dayakar

Nano Electronics Laboratory,

Center for Nano Science & Technology, Institute of Science & Technology,

Jawaharlal Nehru Technological University Hyderabad-500085, Telagana State, India

Abstract The present research work describes a simple and low cost process for the synthesis of Few Layered Graphene (FLG) sheet Decked by CeO2 Nanoparticles (NPs) using Ultrasonic Assisted Synthesis (UAS) method. The synthesized sample is characterized by Field Emission Scanning Electron Microscope (FE-SEM), High Resolution Transmission Electron Microscope (HR-TEM), and X-ray Diffraction (XRD). The prepared FLG/CeO2 nanocomposite is used as a photoanode in Dye-sensitized solar cell (DSSC) application. The current density-voltage (J-V) characteristics were measured with N719 dye under AM 1.5G, 100 m W/m2 of the solar simulator. The efforts showed that the optimized Power Conversion Efficiency (PCE) is observed to be 2.17% in FLG/CeO2 nanocomposite photoanode, compared to pure CeO2 photoanode.

Keywords Few-Layered Graphene, FLG/CeO2 nanocomposite, photoanode, Dye-sensitized solar cells, power conversion efficiency.

1. INTRODUCTION

   Dye-sensitized solar cells (DSSCs) have retained considerable attention during the last decade because of their prominent and promising solar energy conversion system and the potential of becoming a low cost alternative to silicon- based solar cells [1, 2]. In the configuration of DSSC, the dye plays an critical role in harvesting solar energy and converts it into electrical energy with the help of semiconducting photoanode materials. Therefore, the efficiency mainly depends on the type of the dye used in the solar cell and acts as sensitizers [3, 4]. Up to now the Numerous metal complexes and organic dyes utilized as sensitizers; the best study example is the Gratzel group reported that the ruthenium-bipyridyl dyes which show a high conversion efficiency of about 11% [5]. Buscaino et al. showed the remarkable power conversion efficiency of 11.2% by using ruthenium(II) dye [(C4H9)N]2 [Ru(4carboxy-4`-carboxylate- 2,2`bipyridine)2 (NCS)2 (n719) as sensitizer and the

   photovoltaic data of the N719 sensitizer adsorbed on TiO2 films in DSSC [6].

   Graphene and metal oxides composites have attracted considerable attentions towards the researchers [7-9] due to their unique properties such as two dimensional structure, high chemical inertness, high electrical conductivity, and large specific surface and also many potential applications [10-13]. In such nanocomposites Graphene plays as an effective support. Moreover, in the case of a uniform metal oxide distribution in the plane of the graphene sheets makes charge transport phenomena possible, which may give rise to novel properties, different from those of the constituent components. This is demonstrated by recent studies of composited based on graphene and a number of metal oxides: ZnO, TiO2, SnO2 and others [14-16].

   Among metal oxides,cerium oxide (CeO2) were received considerable interest because of their high transparency electro-optical performance in the visible and near IR region [17, 18]. It is an n-type semiconductor whose band gap has been reported to vary from 2.7 to 3.4 eV depending on the method of synthesis and also have variable applications like fuel cells, solar cells, catalysis, and medical [19-22]. The controlled synthesis of CeO2 nanoparticles less than 10 nm in size is of key importance because the catalytic activity of crystalline CeO2 strongly depends on its surface structure. However, despite of their high catalytic activity, unsupported CeO2 nanocrystallites have a tendency to form aggregates, in which the particles lose some of their properties. Thus, a new composite material consisting of graphene/CeO2 NPs might allow one to create a high performance in DSSC application. In this paper, we report a novel process for synthesis of FLG/CeO2 nanocomposite through UAS method. In which, graphene oxide (GO) is varied with Cerium (III) nitrate hexahydrate as a precursor material and the resulting solution is ultrasonicated for getting fine dispersion. During the

   process GO was simultaneously converted into FLG and nano size CeO2 NPs decked on FLG sheet. The prepared FLG/ CeO2 nanocomposite used as a photoanode in DSSC.

2. EXPERIMENTAL

   1. Synthesis of FLG/CeO2 nanocomposite

      The GO was prepared according to modified hummers method in the presence of the ultrasonic process [23]. The FLG/CeO2 nanocomposite synthesized by using an Ultrasonic Qsonica sonicator (Model no: Q500, 20 KHz Frequency, 500

      W) at 45% amplitude. In this process, 0.5 g of GO was dispersed in 200 ml ethanol to get a dark brown color solution by stirring for 30 min, then 2 ml of hydrazine monohydrate solution was added to the resulting dispersion solution. Then the mixture was stirred for 60 min at 95°C. Finally, an appropriate amount of Cerium (III) nitrate hexahydrate was added into the above GO solution, resulted solution was transferred into 500 ml vessel and placed in an ultrasonic chamber for 2 hrs sonication at room temperature. Finally, the solution was filtered and washed 3 times with distilled water and dried in hot air over at 90°C for 5 hours to evaporate the solvents. The subsequent powder was calcination at 600°C for

      2 hours in a muffle furnace. The obtained FLG/CeO2 nanocomposite with different weight percentages (FLG1.0,

      2.0 and 3.0 wt %) by changing the amount of the FLG. For evaluation, Pure CeO2 NPs was synthesized by the same way using above method except that GO was not added.

   2. Fabrication of FLG/CeO2 photoanode

   In the primary part, 2.0 g of CeO2 nanopowder was dispersed in 20 ml of ethanol and subjected to Ultrasonication bath for 30 min. After that the solution was pulverized in a porcelain motor and pestle to get a stable colloidal dispersion. In which Ethanol acts as a dispersing agent because it prevents the coagulation of CeO2 NPs which affects the porosity of the film. Furtherly, the paste was diluted by the slow addition of 1 ml PEG (MW 10000) to maintain viscosity and concentration of the paste. Finally, a few drops of a detergent (Triton X-100) were added to overcome the surface tension property of the paste, to facilitate even spreading and to avoid subsiding the formation of surface cracks. In the finishing part, to obtain a FLG/CeO2 nanocomposite paste at a different weight percentage of FLG (1.0,2.0 and 3.0 wt %), the method of Sacco et al. [24] is followed. Briefly, FLG/CeO2 nanocomposite powder is also dissolved in ethanol and subjected to Ultrasonication bath for 30 min, to get a stable colloidal dispersion the same above process is used.

   FTO conductive glass with a sheet resistance of (sheet resistance =9 Ohms/sq, and 80% transmittance in te visible region) were first cleaned with a detergent solution using an ultrasonic bath for 15 min, rinsed with water and ethanol, and then dried. CeO2 and FLG/CeO2 nanocomposite pastes were deposited on the FTO conductive glass by doctor blade technique in order to obtain CeO2 and FLG/CeO2 film. The film on the substrate was annealed at 450°C for 30 min.

3. CHARACTERIZATIONS

   The surface morphology analyzed by field emission scanning electron microscopy (FESEM, Model no. Carl Zeiss Merlin Compact 6027), particle size and decoration of the CeO2 NPs on the FLG is performed by High resolution transmission electron microscopy (HRTEM, Model no: JEOLJEM 200CX). The crystal phase and composition of FLG/CeO2 nanocomposite investigated by XRD (XRD, Model no: Bruker D8 advanced) with CuK radiation (k=1.540Ã…). The current density-voltage (J-V) parameters of the DSSCs were measured by under the illumination of a simulated AM1.5G solar light from the 450-W Xenon lamp (model no: Oriel Class 3 A) using a solar simulator with Keuthhley 2440 source meter.

4. RESULTS AND DISCUSSION

   Fig 1shows FESEM image of the CeO2 NPs and FLG (1.0,

   2.0 and 3.0 wt %) CeO2 nanocomposite. The surface morphology of the CeO2 NPs (Fig. 1(a)) is observed as onion resembles the texture which reflects its layer structure. After ultrasonic treatment, the morphology is retained and the surface area covered with CeO2 NPs decked on FLG sheet which is confirmed by FESEM as shown in Fig 1(b-d). The Vander walls, tent the FLG to aggregate back to the graphite structure, decked with NPs and functional groups helpful to overcome these interactions.

   Fig.1. FESEM images of (a) CeO2 NPs (b) FLG (1.0 wt %) /CeO2 nanocomposite (c) FLG (2.0 wt %) /CeO2 nanocomposite (d) FLG (3.0 wt

   %) /CeO2 nanocomposite.

   The further morphology, particle size and microstructure of FLG (1.0, 2.0 and 3.0 wt %)/CeO2 nanocomposite were studied by conventional TEM and high resolution transmission electron microscopy (HRTEM). Fig 2 shows electron microscopic images of CeO2 NPs (Fig 2 (a)), FLG (1.0, 2.0 and 3.0 wt %) / CeO2 nanocomposite, the size distribution of the CeO2 NPs (Fig 2 (b)) and selected area diffraction angle pattern of CeO2 NPs as shown in (Fig 2 (c)). It is seen from the image of CeO2 NPs are uniformly decked on the FLG sheet (Fig 2 (d-f)).

   Fig.2.TEM, HRTEM images of (a) CeO2 NPs, (b) size distribution of CeO2 NPs,(c) SAED pattern of CeO2 NPs, (d) FLG (1.0 wt %) /CeO2 nanocomposite, (e) FLG (2.0 wt %) /CeO2 nanocomposite , (f) FLG (3.0 wt

   %) /CeO2 nanocomposite.

   The phase and composition of CeO2 NPs and FLG (1.0, 2.0 and 3.0 wt %) / CeO2 nanocomposite was determined by X- ray diffraction (Fig 3). The CeO2 NPs characteristic peaks are observed at 28.5°, 33°, 47.7°, 56.3°, 69.4°, 77° and 79°

   corresponding to the (111), (200), (220), (311), (400), (331) and (420) planes, respectively. The CeO2 phase (cubic structure) the results are well matched with standard JCPDS database (JCPDS no. 34-0394). A small peak is observed at (2 =25.3°) which represents the (002) plane FLG sheet. These results indicated that the formation of FLG/CeO2 nanocomposite.

   Fig.3. The XRD pattern of CeO2 NPs FLG (1.0, 2.0 and 3.0 wt %) /CeO2 nanocomposite

   The current density-voltage (J-V) curves of CeO2NPs and FLG (1.0,2.0 and 3.0 wt %) /CeO2 nanocomposite photoanode as shown in Fig 4. The DSSCs characteristics of VOC, JSC, FF, PCE values are described in Table1.There is a clear enhancement current density (Jsc) from 6.90 to 8.62 mA/cm2 and an increase of open circuit voltage (Voc) from

   0.51 to 0.53V.The PEC enhancement from 1.75 to 2.17 % respectively. The appropriate amount of FLG (1.0 wt%) introduces in CeO2 NPs the increase in the number of photo generated electrons, for making CeO2 NPs conductive and the

   electrons are easily moved through the FLG sheet further leading to reduction of recombination rate. Finally, the highest efficiency was achieved in the FLG (1.0wt%)/CeO2 photonaode.

   Fig.4. J-V curves of CeO2 and FLG(1.0, 2.0 and 3.0 wt%)/CeO2 photoanode based DSSC.

   Photoanode	Voc (V)	Jsc (mA/cm2)	FF (%)	PEC (%)
   CeO2 NPs	0.51	6.90	49.6	1.75

   Photoanode	Voc (V)	Jsc (mA/cm2)	FF (%)	PEC (%)
   CeO2 NPs	0.51	6.90	49.6	1.75

   Table-1: Photovoltaic (parameters) of the DSSCs based on CeO2 and

   FLG (1.0 wt %) / CeO2 0.53 8.62 45.3 2.17

   FLG (2.0 wt %) / CeO2 0.50 5.84 48.5 1.43

   FLG (3.0 wt %) / CeO2 0.49 5.53 48.1 1.31

   FLG(1.0, 2.0 and 3.0 wt%)/CeO2 photoanode .

5. CONCLUSION

   Successfully prepared FLG/CeO2 nanocomposite by simple and low cost technique, Ultrasonic assisted synthesis (UAS) method. The characteristic features obtained FLG/CeO2 nanocomposite were explored by various characterization techniques such as surface morphology, structural properties were investigated. The fabrication of FLG (1.0, 2.0 and 3.0 wt %) /CeO2 nanocomposite used photoanode in DSSC. A noteworthy enhancement of (2.17%) in the power conversion efficiency (PCE) was achieved in DSSC in FLG (1.0%)

   /CeO2 nanocomposite to compare CeO2 NPs under A.M 1.5G solar simulated.

6. ACKNOWLEDGMENTS

Satish Bykkam sincerely thanks to University Grants Commission (UGC) Government of India, for financial support through the Rajiv Gandhi NationFellowship (RGNF) (File no: F1- 17.1/2012-13/RGNF-2012-13-SC-

AND-30114. And also thanks to the Center for Nanoscience and Technology (CNST), IST, JNT University for providing facilities.

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