Microwave-assisted exfoliation method to develop platinum-decorated graphene nanosheets as a low cost counter electrode for dye-sensitized solar cells

Abstract

Graphene nanosheets (GNs) are prepared from natural graphite by a simple ecofriendly microwave-assisted exfoliation technique. The as prepared GNs are decorated with platinum (Pt) nanoparticles by a simple chemical reduction method and used as a low-cost counter electrode (CE) material for dye-sensitized solar cells (DSSCs). Structure and morphology of the prepared GNs and Pt-decorated GNs (Pt–GNs) are evaluated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The electrochemical behavior of GNs and Pt–GNs are compared with std. Pt using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies. These studies indicate that the Pt–GNs-based counter electrode offered superior electrocatalytic activity towards the I⁻/I₃⁻ redox mediator with enhanced charge transfer rate and exchange current density at the electrode/electrolyte interface over std. Pt and GNs-based counter electrodes. DSSCs are fabricated with std. Pt, GNs and Pt–GNs to determine the photovoltaic performance under 1 Sun illumination (100 mW cm⁻², AM 1.5). It is found that the cell fabricated with 1 wt% Pt-decorated GNs as counter electrode showed an 11% improvement in photovoltaic cell efficiency compared with the cell assembled with std. platinum and other reported graphene–Pt-based composites as counter electrodes.

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Introduction

Dye-sensitized solar cells have acquired considerable attention as an alternative photovoltaic device for conventional solar cells due to their high performance, low cost, flexibility, light weight, simple fabrication process and several industrial and commercial applications.^1–3 The first DSSC was developed by Gratzel et al. in 1991. To date, the maximum efficiency achieved with DSSC is 13%,⁴ which can act as a significant alternative to conventional silicon-based solar cells. In general, DSSC is composed of a wide band gap n-type semiconductor, i.e. TiO₂, which is coated on a transparent conducting oxide (TCO) as photoanode that has a monolayer of adsorbed dye molecules, an electrolyte with a redox couple (I₃⁻/I⁻) dissolved in an organic solvent and a counter electrode.⁵ When the photo-electrochemical cell is exposed to light, the electrons in the dye get excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The electron is then transferred to the photoanode and the dye is regenerated by oxidation of I⁻ to I₃⁻ in the electrolyte. The main tasks of the counter electrode material are to collect the electrons from the external circuit and to regenerate the redox couple of electrolyte. Typically, a counter electrode material requires good electrocatalytic activity, high conductivity and a large surface area, and it should also speed up the reduction reaction of tri-iodide into iodide (I₃⁻ + e⁻ → I⁻).⁶ Platinum is exploited as the CE material in the majority of DSSCs because of its superior electrocatalytic performance and conductivity. Nevertheless, Pt is an expensive and rare metal, and its electrocatalytic activity diminishes with prolonged operation. This is due to the formation of platinum iodide (PtI₄) by corrosion of Pt in the triiodide-containing electrolyte.⁷ Moreover, platinized CEs require thermal annealing or sputtering, which enhances the cost of mass production and hence hinders the commercialization of DSSCs. In order to make a cell inexpensive, the conventional platinum counter electrode is replaced by relatively more abundant alternate materials. Various transition metal oxides, chalcogenides, carbides, nitrides, phosphides and their composites have been introduced as counter electrode materials.⁸ To date, various allotropes of carbonaceous materials such as mesoporous carbon, activated carbon, carbon nanotubes (CNTs), graphite, graphene and conducting polymers and their composites have also been investigated as counter electrode materials for DSSCs.^9–14 Recently, graphene has drawn more attention than the other carbonaceous materials owing to its interesting properties such as good electrochemical activity, strong mechanical strength, high electrical, thermal conductivity, electrochemical stability and large surface area.

Hsieh et al. reported a standalone graphene as CE for DSSC that exhibited lower efficiency than Pt-based counter electrodes. This can be attributed to the lower amount of defective sites due to its highly oriented graphitic structure. This low efficiency can be avoided by increasing the number of defect sites or by forming composites.^15,16 Graphene has been composited with metal nanoparticles, conducting polymers and chalcogenides for use as a counter electrodes for DSSCs.^17–19 Recently, Kim et al. prepared aqueous dispersible graphene–Pt composite using a one-pot chemical reduction method and showed a 0.68% improvement in cell efficiency with 10% Pt loading in comparison with a pure platinized CE.²⁰ Cheng et al. prepared a graphene–Pt composite by growing graphene by chemical vapor deposition (CVD), followed by sputtering of Pt and achieved an improvement of 1.78% in photo-conversion efficiency (PCE).²¹ Dao et al. synthesized a graphene–Pt nanohybrid using a modified Hummers method, followed by reduction of Pt under Ar plasma and achieved an improvement of 5% in PCE.²² Tjoa et al. synthesized a graphene–Pt composite by a photochemical technique and achieved an improvement of 8%.²³ To date, the maximum achieved improvement in photovoltaic cell efficiency for graphene–Pt composites is not more than 8% when compared to platinum.^24–26 In the above-mentioned methods, graphene oxide is mainly synthesised using a modified Hummers method or the Staudenmaier method. These methods are time-consuming, involve toxic and harsh chemicals that are detrimental to environment and also require higher annealing temperature for exfoliation.^27,28 There are a very few reports available for the preparation of exfoliated graphene and metal (Fe, Pt and Pd) decorated graphene by a microwave irradiation method which are used as electrocatalysts in glucose biosensors.^29–31 However, there is no report on the use of microwave-assisted exfoliated graphene nanosheets as a counter electrode for DSSCs.

Hence, in the present investigation, graphene nanosheets were prepared using a microwave-assisted exfoliation method, and then decorated with platinum by a simple chemical reduction method and used as a counter electrode for a DSSC. This method lowers the cost and time consumption for production. The structure and morphology of Pt–GNs was confirmed by XRD, Raman spectroscopy and SEM. The electrochemical behavior of the prepared Pt-decorated GNs was evaluated by cyclic voltammetry and AC-impedance techniques. Photovoltaic performance of DSSC fabricated with 1 wt% of Pt-decorated GNs as counter electrode was compared with the DSSC fabricated with std. Pt and GNs as counter electrodes.

Experimental

Materials used

Natural graphite AR grade (Himedia), ammonium persulfate (Rankem), hydrogen peroxide (Merck), chloroplatinic acid (Sigma-Aldrich), isopropanol (Merck) and hydrazine (Merck) were used in this work. All these chemicals were analytical regent and used without further purification.

Preparation of graphene nanosheets (GNs)

GNs were prepared from natural graphite by a simple microwave-assisted exfoliation method using an optimized wt ratio of natural graphite and ammonium persulfate (1 : 1) in a 50 ml glass beaker. To this, 5 ml of 30% hydrogen peroxide was added. This reaction mixture was sonicated for 5 minutes and then placed in a domestic microwave oven (2.45 GHz, LG, India) and irradiated at 500 W for 160 s. Under the microwave irradiation, the intercalation and exfoliation of the graphite occurred rapidly with fuming and lightening without affecting the sp² carbon of the basal plane.³⁰ Once the lightening was over, we ensured that the reaction was completed.

Preparation of Pt decorated GNs

1 wt% of Pt-decorated GNs were prepared by a simple chemical reduction method using chloroplatinic acid. Initially, an appropriate amount of GNs were dispersed in deionized water, and then a required amount of chloroplatinic acid salt in isopropanol was added slowly with constant stirring and continued further for 12 h. To this, hydrazine was added drop by drop at pH 10 with constant stirring for 1 h. Once the reaction was completed, the mixture solution was vacuum filtered and dried at 80 °C overnight.

Physical and electrochemical characterization

The crystalline structure of GNs and platinum-decorated GNs were determined by X-ray diffraction studies (Rigaku, Model: Ultima IV). A confocal micro-Raman spectrometer with a laser beam of 514 nm was used to identify the fundamental properties and microstructure of carbonaceous materials (Renishaw, Model: RM 2000). The surface morphologies of the prepared GNs and 1 wt% of Pt-decorated GNs were observed by scanning electron microscopy (Hitachi, Model: S-3400N). The amount of Pt decorated on the GNs was estimated by energy dispersive X-ray spectroscopy (EDX), which is equipped with a SEM. The particle size and crystalline quality of the Pt-decorated GNs were confirmed by transmission electron microscopy and SAED studies (Philips, Model: CM200). The electrochemical impedance and cyclic voltammetry measurements were performed with electrochemical workstation (VSP, Bio-Logic, France). The cyclic voltammetry study was performed in a three-electrode cell to investigate the electrocatalytic properties of counter electrode towards the reduction of triiodide ion in the redox mediator. It consisted of the prepared CE material with an active area of 0.25 cm² as working electrodes, Pt as counter electrode with an active area of 1 cm² and Ag/AgCl as the reference electrode and 0.01 M LiI, 0.001 M I₂ and 0.1 M LiClO₄ in acetonitrile as an electrolyte in the potential range of −1.0 to 1.0 V at various scan rates. The electrochemical impedance measurements (EIS) were carried out using symmetric cells for std. Pt, GNs and Pt–GN electrodes. The Surlyn tape was used between two symmetrical electrodes as the spacer to calculate the charge transfer resistance (R_ct) in the frequency range from 100 kHz to 1 mHz. The Tafel polarization measurements were performed at the scan rate of 50 mV s⁻¹ using the same symmetrical cells to verify the electrocatalytic activity of the CE by knowing its exchange current density.

Fabrication of DSSC

First, the ITO glass plates were cleaned sequentially with acetone, ethanol and deionized water in an ultrasonic water bath to remove organic impurities, and then dried in air. The ITO glass plates were selected due to their low surface roughness and high electrical conductivity as compared with FTO glass plates.^32,33 Scotch tape was employed as a spacer to control the film thickness and to provide an uncoated area for electrical contact.³⁴ The TiO₂ paste (Dyesol Ltd) was spread over the spacer between the scotch tape and conducting glass substrate using the doctor blade technique, and then dried in air at 30 °C for 10 min and sintered at 450 °C for 30 min to remove any organic matter. The thickness of the photo anode film was ca. 12 μm and its area was 0.20 cm². The sintered photoanode was cooled down to 80 °C and immersed in a solution containing 3 × 10⁻⁴ M of dye, di-tetrabutyl ammonium cis-bis(isothiocyanato)bis (2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) for 24 h. This dye-sensitized TiO₂ photoanode was then cleaned with anhydrous ethanol to remove excess dye and dried in moisture-free air. The counter electrodes were prepared by a slurry-coating procedure. The mixture containing exfoliated graphene or Pt–GNs and poly (vinylidenefluoride) in the ratio of 95 : 5 wt% in N-methyl-2-pyrrolidone was finely ground together using a mortar. Subsequently, the paste was coated with 10 μm thickness on pretreated ITO plate using the doctor blade technique, and then dried for 12 h at 80 °C in a vacuum oven.

The TiO₂ photoanode was assembled with various counter electrodes such as std. Pt paste (Dyesol Ltd.), GNs and 1 wt% of Pt-decorated GNs separately using a hot press at 110 °C. The I⁻/I₃⁻ redox electrolyte, which consisted of 0.5 M of LiI, 0.05 M of I₂, 0.5 M of TBP and 0.5 M of 1-butyl-3-methylimidazolium iodide (ionic liquid) in acetonitrile, was injected into the cells through two small holes drilled in the counter electrode. The holes were then covered and sealed with small squares of Surlyn tape.³⁵

Photovoltaic performance

Photovoltaic performance of the fabricated DSSCs was evaluated using a calibrated AM 1.5 solar simulator (Newport, Oriel instruments USA 150W, model: 67005) with a light intensity of 100 mW cm⁻² calibrated using standard mono-crystalline silicon solar cell (Newport, Oriel instruments, model: 91150V) and a computer-controlled digital source meter (Keithley, Model: 2420). The light-to-electricity conversion efficiency (η) of the assembled cells was calculated from the measured photoelectrochemical parameters such as fill factor (FF), open circuit voltage (V_oc), short circuit current density (J_sc) and incident optical power (P_in).

Results and discussion

The XRD patterns of the natural graphite, GNs and 1 wt% of Pt-decorated GNs are shown in Fig. 1. The characteristic high intensity (002) peak appears at 2θ = 26.7° for the natural graphite with high magnitude of nearly 400 000, and one more small intense peak appears at 2θ = 55° confirms a highly organized crystalline structure of hexagonal graphite.³⁶ In contrast, similar peaks are found in GNs with sharp and low intense peaks at 2θ = 26.6° and 55.1°, indicating the formation of exfoliated graphene nanosheets.³⁷ The peak height with decreased magnitude of several orders suggests that the exfoliated graphene nanosheets size is reduced to nanometer level. The interplanar spacing (0.332 nm) of GNs was also measured from the XRD pattern. The X-ray diffraction pattern of 1 wt% of Pt-decorated GNs exhibits the characteristic sharp diffraction peaks at 2θ = 39.8° (111) and at 2θ = 46.2° (200), which specifies the face-centered cubic (fcc) platinum crystalline lattice, in good agreement with the JCPDS data.³⁸ This confirms that the platinum precursor, H₂PtCl₆ has been reduced completely to platinum by hydrazine.

Fig. 1 XRD patterns of natural graphite, GNs and Pt decorated GNs.

Raman spectroscopy is a useful tool to distinguish the degree of ordered and disordered carbon structures. The Raman spectra of GNs and 1 wt% of Pt–GNs are shown in Fig. 2. The G band is usually assigned to the E_2g phonon of sp² C atoms that is connected to the graphitic hexagon-pinch mode, and the D band raised from a breathing mode of point photons of A_1g symmetry that corresponds to the defects in graphene sheet. Raman spectra of GNs show a G band at 1579 cm⁻¹ and D band at 1373 cm⁻¹ and a weak second-order 2D band at 2722 cm⁻¹.^39–41 The disorder-induced band (D band) is found at 1350 cm⁻¹ for the prepared Pt–GNs with a very low intensity, suggesting that it is nearly defect free. The ratio between the intensity of D and G bands is generally used to predict the presence of defects, and the size of the in-plane sp² domain. The intensity ratio of I_D/I_G (R) for Pt–GNs is found to decrease (0.89) relative to GNs (1.1). This significantly decreased R value of Pt–GNs is attributable to the decrease in size of the in-plane sp² domains and a partially ordered crystal structure of Pt–GNs.

Fig. 2 Raman spectra of GNs and Pt decorated GNs.

The microstructures of exfoliated graphene and Pt-decorated GNs are shown in Fig. 3(a and b). From Fig. 3(a), it can be seen that GNs exhibited a flat, smooth and very thin-layered structure but the sheets are randomly crumpled, indicating that the ordered thick-layered structure of the natural graphite precursor has been disrupted by the microwave irradiation.⁴² From Fig. 3(b), it can be seen that the presence of Pt nanoparticles on GNs and these Pt nanoparticles are evenly dispersed throughout the GNs. This may help to enhance the electrocatalytic redox behavior of I₃⁻/I⁻. Fig. 3(c) shows the EDX spectrum of the Pt-decorated GNs. The amount of Pt decorated on the GNs was confirmed by EDX and found to be ∼1.02 wt%.

Fig. 3 SEM image of (a) GNs, SEM image of (b) Pt decorated GNs (c) the EDX spectrum of Pt decorated GNs.

TEM images of Pt-decorated GNs are shown in Fig. 4(a–c). It can be seen that the fine Pt nanoparticles in the range of 2–3 nm were well dispersed over the GNs. SAED pattern of Pt–GNs is shown in Fig. 4(d). The SAED pattern was indexed to the (111), (200) and (220) reflections of face-centered cubic platinum, which is consistent with the results of the XRD pattern. The randomly weak and diffuse diffraction rings represent the loss of long-range ordering in the GNs. In addition, the sharp diffraction spots revealed the polycrystalline structure of Pt nanoparticles.

Fig. 4 (a–c) TEM images of Pt decorated GNs at different magnifications and (d) SAED pattern of Pt decorated GNs.

Cyclic voltammetry analysis is an effective tool to investigate the electro-catalytic activity of the prepared counter electrode by the relation between ion (I⁻/I₃⁻) diffusivity and reaction kinetics using a three-electrode system. The higher electrocatalytic activity towards the I₃⁻/I⁻ redox process is an essential factor for efficient CEs. The cyclic voltammograms show oxidation and reduction kinetics of I₃⁻/I⁻ redox couple as follows:

3I⁻ ⇌ I₃⁻ + 2e⁻ oxidation (1)

I₃⁻ + 2e⁻ ⇌ 3I⁻ reduction (2)

The relative positive pair (right) of anodic peak is due to the oxidation reaction and the negative pair of cathodic peak (left) is associated with the reduction reaction. The negative pair of cathodic peaks is more significant because it has a direct impact on the photovoltaic performance of DSSCs. The cyclic voltammograms are obtained for std. Pt, GNs and 1 wt% of Pt-decorated GNs at the scan rate of 50 mV s⁻¹ in the potential range of −1 to 1 in an electrolyte solution contains 0.01 M LiI, 0.001 M I₂ and 0.1 M LiClO₄ in acetonitrile are shown in Fig. 5. The cyclic voltammogram of Pt-decorated GNs has a higher anodic and cathodic peak current densities; the larger electrochemical active surface area of the electrode is due to the larger enclosed redox reaction area of the CV curve. This suggests an improved electrocatalytic redox behavior towards the reduction of I₃⁻ and oxidation of I⁻. The relatively lesser oxidation to reduction peak separation and a positive side shifting in the reduction peak were observed in 1 wt% of Pt-decorated GNs than the std. Pt and GNs based CEs. This indicates faster electron transfer kinetics, high electrocatalytic activity and reduction of overpotential for I₃⁻ reduction of 1 wt% Pt-GNs based CEs. It revealed that the addition of platinum onto GNs has enhanced the electrocatalytic activity rather than GNs and std. Pt-based CE materials.

Fig. 5 Cyclic voltammograms of std. Pt, GNs and Pt decorated GNs at a scan rate of 50 mV s⁻¹ in 0.01 M LiI, 0.001 M I₂ and 0.1 M LiClO₄ as the supporting electrolyte.

The characteristics of the cyclic voltammogram depend on the rate of electron transfer process and the scan rate. Cyclic voltammograms were carried out for the prepared Pt–GNs CE material at various scan rates (5, 10, 25, 50, 100 and 200 mV s⁻¹) are shown in Fig. 6. The oxidation and reduction peak current density has a linear relationship to the square root of the scan rate, as shown in Fig. 7. When the scan rate increased, the cathodic peak gradually shifted to the negative direction and the corresponding anodic peak shifted to the positive direction (Fig. 6). The peak shift indicates that the diffusion limitation of the redox reaction of I₃⁻/I⁻ at the Pt–GNs counter electrode. This phenomenon inferred that no specific interaction occurred between I₃⁻/I⁻ and the Pt–GNs CE.⁴³

Fig. 6 Cyclic voltammograms of Pt decorated GNs as a function of different scan rates.

Fig. 7 The reduction and oxidation current densities of the Pt–GNs electrode versus the square root of different scan rates.

EIS measurements were carried out using the symmetric cell system to determine the interfacial resistance of the counter electrode material in an electrolyte solution, which has major influence on the performance of DSSC. The high-frequency region response is assigned to electrochemical charge transfer behavior occurring at the CE/electrolyte boundary. The Nyquist plots of symmetrical cells fabricated using std. Pt, GNs and 1 wt% of Pt-decorated GNs counter electrode materials are compared with the corresponding equivalent circuit in Fig. 8. The EIS parameters, such as solution resistance (R_s) and charge transfer resistance (R_ct), were calculated from the Nyquist plot and are listed in Table 1. It can be seen that the 1 wt% of Pt–GNs has lower charge transfer resistance (2.65 Ω) than std. Pt (3.12 Ω). This indicates that the charge transfer processes occurred faster by utilizing the maximum surface of Pt–GNs rather than the surface of std. Pt. As a result, the higher anodic and cathodic peak current densities were obtained in the cyclic voltammogram.⁴⁴ The lower values of R_ct also lead to better photovoltaic performance.

Fig. 8 Nyquist plots of std. Pt, GNs and Pt decorated GNs obtained using a symmetrical cell.

Table 1 The EIS parameters for various counter electrodes

Electrodes	Rs (Ω)	Rct (Ω)	J0 (mA cm^−2)
Pt (standard)	5.07	3.12	4.12
GNs	6.60	6.98	1.84
Pt–GNs	7.20	2.65	4.85

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The interfacial charge transfer properties of iodide/triiodide redox couple on the counter electrode were evaluated by Tafel plots. The logarithmic current density (J) as a function of voltage (V) at room temperature is shown in Fig. 9. Theoretically, the Tafel plot can be divided into three zones with respect to the overpotential, such as a polarization zone at low overpotential (<120 mV), a Tafel zone at medium overpotential (with a sharp slope) and the higher potential region at the diffusion zone. The exchange current density (J₀) and the limiting diffusion current density (J_lim) are important parameters to elucidate the electrocatalytic activity of the electrodes that can be calculated from the Tafel zone and diffusion zone, respectively. The J₀ can be determined from the intersection of the extrapolated intercepts of the linear region of the anodic and cathodic curves when the overpotential is zero. The 1 wt% of Pt–GNs electrode exhibits larger value of exchange current density and J_lim, suggesting the higher electrocatalytic activity compared to std. Pt and GNs.²⁶

Fig. 9 Tafel curves of the symmetrical cells fabricated with two identical std. Pt–GNs and Pt–GNs electrodes.

The charge transfer resistance (R_ct) may be correlated with the exchange current density (J₀), while the triiodide is reduced to iodide at the counter electrode. The exchange current density is calculated from the charge transfer resistance using eqn (3):

where R, T, n and F represent the gas constant, temperature, numbers of electrons transferred in the reduction reaction and the Faraday constant, respectively. Here, value of n is 2 because two electrons are involved in the electrochemical reduction reaction. The exchange current density calculated from charge transfer resistance data showed a higher value of J₀ for 1 wt% of Pt-decorated GNs (4.85 mA cm⁻²).

The photovoltaic performance parameters, such as V_oc, J_sc, maximum voltage (V_max), maximum current (J_max) and FF of DSSC, under illumination for various counter electrodes can be calculated using the following equations:

The results of voltage–current density curves for the assembled cells using std. Pt, GNs and 1 wt% of Pt-decorated GNs are shown in Fig. 10. Their corresponding photovoltaic parameters are compared with similar reported systems are given in Tables 2 and 3. From these tables, it can be observed that Pt-decorated GNs exhibited an 11% improvement in photovoltaic cell efficiency than the std. Pt, which is higher than that of other similar reported systems.^22–24,26 This is due to the high electrocatalytic redox behavior and higher charge transfer rate of 1 wt% of Pt-decorated GNs. In contrast to std. Pt, GNs showed a lower PCE but higher than the previously reported values.^15–17 This can be attributed to large number of defective sites that have been created by the present method, which lowers the charge transfer resistance and hence enhances the electrocatalytic activity of the prepared GNs.³¹

Fig. 10 I–V characteristics of DSSCs based on various std. Pt, GNs and Pt decorated GNs counter electrodes.

Table 2 Comparison of photovoltaic performances of DSSCs based on Graphene Pt obtained using different methods as counter electrodes

Counter electrode	Method of synthesis	Voc (V)	Jsc (mA cm^−2)	FF	Cell efficiency η (%)		% of improvement in PCE	Ref.
					Std. Pt	Pt–graphene
Graphene–Pt	Modified Hummers method/dry plasma reduction	0.71	16.66	0.66	8.18	8.56	5.0	22
Graphene–Pt	Modified Hummers method/facile photochemical synthesis	0.72	14.10	66.9	6.29	6.77	8.0	23
Graphene–Pt	Electrochemical deposition	0.73	15.84	0.65	7.03	7.57	8.0	24
Graphene–Pt	Staudenmaier method/polyol reduction	0.69	17.77	0.70	8.58	8.79	2.5	25
Pt–GNs	Microwave assisted synthesis/chemical reduction	0.69	10.90	0.68	4.60	5.10	11.0	This work

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Table 3 Comparison of photovoltaic performances of DSSCs based on graphene obtained using different methods as counter electrodes

Counter electrode	Method of synthesis	Voc (V)	Jsc (mA cm^−2)	FF	Efficiency %	Ref.
Graphene	Modified Hummers method	0.64	6.12	0.56	2.19	15
Graphene	Electrophoretic deposition	0.70	5.60	0.60	2.30	16
Graphene	Thermal exfoliation	0.62	3.94	0.42	1.01	17
Graphene	Electrochemical deposition	0.56	14.32	0.26	2.03	24
GNs	Microwave-assisted synthesis	0.73	6.90	0.64	3.20	This work

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Conclusion

A simple and rapid microwave-assisted exfoliation method, followed by a simple chemical reduction method, is used to prepare Pt-decorated GNs CE using natural graphite flakes and chloroplatinic acid. Raman spectroscopy confirmed the improved microstructure of Pt-decorated GNs as compared to GNs. By decorating platinum onto GNs, the electrocatalytic redox behavior of the electrode is enhanced by decreasing the charge transfer resistance of CE. Pt-decorated GNs achieved 11% improvement in photovoltaic cell efficiency (η = 5.1%), which is higher than that reported for other similar systems. A very small amount of Pt addition (1 wt%) onto GNs raised the cell efficiency (5.1%). The photovoltaic performance of DSSC using 1 wt% Pt-decorated GNs showed a higher short circuit current of 10.9 mA cm⁻² with a fill factor of 0.68. Thus, the superior photovoltaic cell performance of 1 wt% of Pt-decorated GNs could be used as a low cost alternative counter electrode for DSSCs.

Acknowledgements

The authors gratefully acknowledge CSIR (Ref. no. 01(2359)/10/EMR-II), New Delhi, for the financial support and CIF, Pondicherry University, India for providing their instrumentation facilities.

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