Facile synthesis of tungsten carbide nanorods and its application as counter electrode in dye sensitized solar cells

Materials Science in Semiconductor Processing 39 (2015) 292–299

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Facile synthesis of tungsten carbide nanorods and its application as counter electrode in dye sensitized solar cells P. Vijayakumar a, M. Senthil Pandian a, S.P. Lim b, A. Pandikumar b, N.M. Huang b, S. Mukhopadhyay c, P. Ramasamy a,n a

SSN Research Centre, SSN College of Engineering, Kalavakkam 603110, Chennai, India Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia c Centre of Excellence for Green Energy and Sensor System, Indian Institute of Engineering Science and Technology, Howrah 711103, West Bengal, India b

a r t i c l e in f o

Keywords: Tungsten carbide nanorods Counter electrode Electrochemical properties Large surface area Catalytic activity

abstract Tungsten carbide nanorods (WC-NRs) are synthesized by pseudomorphic transformation of chemically synthesized W3O8 nanorods using a high-temperature method. The WC-NRs was introduced into dye sensitized solar cell (DSSC) as counter electrode (CE) catalyst to replace the expensive platinum (Pt). The synthesized WC-NRs were characterized by field emission scanning electron microscopy (FESEM), BET surface area analysis and powder Xray diffraction (PXRD) measurements. The electrochemical properties of WC-NRs counter electrode were studied using electrochemical impedance spectroscopy (EIS) techniques. The photovoltaic performance of the DSSC with WC-NRs counter electrode was evaluated under simulated standard global AM 1.5G sunlight (100 mW/cm2). The solar to electrical energy conversion efficiency (η) of the WC-NRs with binder and binder free based DSSC was found to be 1.92% and 0.59% respectively. The cell performance can be attributed to the WC-NRs network, catalytic redox activity and 1-D efficient charge-transfer network. Such WC-NRs configuration as CE provides a potential feasibility for counter electrodes in DSSC applications. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction The dye sensitized solar cells (DSSCs) have attracted considerable attention as cost-effective photovoltaic systems from the time of their discovery. DSSCs offer advantages over bulk Si-based solar cells, which are widely used in photovoltaic systems at present. DSSCs can provide transparent and flexible solar cells and operate at high efficiencies at low light intensities, including scattered, n Corresponding author. Mobile: þ91 9283105760; Tel.: þ 91 44 27469700; fax: þ91 44 27475166. E-mail address: [email protected] (P. Ramasamy).

http://dx.doi.org/10.1016/j.mssp.2015.05.023 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

angled or shaded light [1–3]. Consequently, it is generally accepted that DSSCs are well suited for both building integrated photovoltaic (BIPV) systems and portable flexible solar cells. Due to low fabrication cost, its permanence, environmental compatibility and simple process, interest in DSSC has grown considerably. Although the cost of DSSC fabrication is 20% compared with silicon solar cell, for practical application the improvement of efficiency is inevitable [4]. During the DSSC operation process, oxidized dyes are regenerated by the iodide in the electrolyte and finally triiodide converted to iodide at the counter electrode (CE). The function of the counter electrode is to collect electrons

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from the external circuit and catalyze the reduction of the redox couple [5]. Most commonly, the counter electrodes in DSSCs have been prepared by platinum (Pt) vacuum deposition or thermal annealing of a Pt precursor on a transparent conductive oxide (TCO) substrate to reduce the over potential for reduction of I3 to I  in redox electrolytes [6]. Pt has high conductivity, catalytic activity and stability, for the counter electrode, Pt nanoparticles have generally been accepted as providing the optimal solution. However, Pt is expensive and rare and it is already in high demand as a catalyst in a variety of chemical and electrochemical fields. Pt is one of the most expensive components that will hinder the commercialization of DSSCs [7]. Alternatively, other materials including carbon materials (graphite, activated carbon, carbon black, single-wall carbon nanotubes) [8–13] are attempted to achieve the similar performance like Pt based DSSCs. Although carbon based counter electrodes show high catalytic activity, their chief weakness is the poor bonding strength between carbon material and the substrate (FTO glass) [14]. This may cause instability in those DSSCs using carbon CEs. Poly(3,4-ethylenedioxythiophene) (PEDOT) [15–19], poly(3,3-diethyl-3,4-dihydro-2H-thieno (-3,4-b) (1,4) dioxepine) (PProDOTEt2) [20], polypyrrole [21] and polyaniline [22] have been introduced. For these polymer catalysts, the film thickness was important, because it affected both catalytic activity and resistance. Recently inorganic materials, such as CoS, TiN and WO2 have been proposed as CE catalysts all of which show great potential [23]. Transition-metal carbides are considered as a potential substitute for Pt because of their low cost, high catalytic activity, selectivity and good thermal stability under rigorous conditions [24]. Tungsten carbide is a versatile material exhibiting good hardness, a high melting point, durability and conductivity. Lee et al., also prepared tungsten carbide polymer-derived WC (WC-PD) and microwave-assisted WC (WC-MW) by different synthesis methods. The reported energy conversion efficiency was 6.61% and 7.01% respectively [25]. The aim of this work is to synthesize tungsten carbide nanorods and to apply for DSSC as novel WC-NRS based CE catalysts for the regeneration of the Iodolyte redox electrolyte. The WC-NRs based counter electrodes were prepared and performance of binder free based DSSC, was compared with the performance of binder based DSSC. Binder based DSSC shows better efficiency. 2. Experimental methods 2.1. Materials Sodium tungstate dihydrate ((Na2WO4  2H2O), ACS Reagent, Z99%), ammonium sulfate ((NH4)2SO4, Bioxtra, Z99%) and zirconium (IV) oxide (99%, 5 mm) powders were purchased from Sigma Aldrich. Glucose (C6H12O6, 99%) was purchased from Alfa Aesar. Titanium dioxide (TiO2) (Degussa P25) was received from Systern. N719 Ruthenizer 535-bisTBA and Iodolyte Z-100 were bought from Solaronix. Indium doped tin oxide (ITO) conducting glass slides (7 Ω/sq cm) were purchased from Xin Yan Technology Limited, China.

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2.2. Synthesis of tungsten carbide nanorods 2.2.1. Synthesis of W3O8 nanorods The schematic representation for the synthesis of WCNRs is shown in Fig. 1. W3O8 nanorods were synthesized by hydrothermal method using (NH4)2SO4 as a capping agent as follows. Na2WO4  2H2O and (NH4)2SO4 were taken in the 1:2 M ratio, dissolved in deionized water (15 ml) and then HCl (3 M) aqueous solution was added to adjust the pH value to 2. The obtained solution was transferred into a teflon lined stainless autoclave where the reaction was maintained at 180 1C for 8 h. Then the precipitate was isolated by filtration and then washed sequentially with water and ethanol for three times. Finally the solution was centrifuged and resultant precipitate was dried at 60 1C. 2.2.2. Synthesis of WC nanorods In a typical synthesis procedure of WC-NRs, suitable amounts of as-prepared W3O8-NRs and glucose (molar ratio W/C¼1:12.8) were dissolved in deionized water (15 ml) and vigorously stirred for 20 min. The mixture was hydrothermally treated in a sealed teflon lined stainless steel autoclave at 180 1C for 8 h to form carbon coated W3O8 nanorods. The as-prepared precursors were calcinated at 900 1C under a flow of H2/Ar (V H2 =V Ar ¼ 1: 3, 300 ml min  1) for 3 h and finally WC-NRs were obtained. 2.3. Preparation of WC-NRs counter electrode 2.3.1. Binder free WC-NRs preparation 200 mg of WC-NRs powder was finely ground in mortar and then mixed into the 8 ml of ethanol and vigorously stirred overnight until homogeneous suspension was obtained. The resultant homogeneous WC-NRs suspension was coated onto the conducting side of ITO glass substrate by spin coating method. The WC-NRs coated films were dried at room temperature and heated at 450 1C for 1 h. 2.3.2. WC-NRs prepared with the mixture of TiO2 and ZrO2 binder A mixture of 200 mg WC-NRs material and 4 g zirconium dioxide (ZrO2) pearl was dispersed in 8 mL of isopropanol. The mixture was then milled for 4 h using mortar and pestle. Finally the prepared material was ultrasonically dispersed for 48 min (6 segments) and 50 mg of TiO2 (P25, Degussa) was prepared by the same method. The WC-NRs with the mixture of TiO2 and ZrO2 binder coated films were dried at room temperature and the product was heated at 450 1C for 1 h. 2.4. Fabrication of DSSCs and evaluation of their performance The TiO2 modified photoanode (prepared by doctor blade technique [26]) was immersed into the ethanolic solution of 0.3 mM N719 (Ruthenizer 535-bisTBA) dye for 24 h at room temperature. The dye-adsorbed photoanode was withdrawn from the solution and immediately cleaned with ethanol. A WC-NRs (with binder and binder free) modified ITO plate was placed on dye-absorbed

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Fig. 1. Schematic representation for the preparation of WC-NRs.

photoanode and then clamped firmly together. The redox electrolyte (Iodolyte Z-100) solution was introduced into the cell assembly by capillary action. An active area of 0.5 cm  0.5 cm was used to measure the cell performance. A 150 W Xenon arc lamp (Newport, Model 69907) containing simulated AM 1.5G filter with a manual shutter was used as a light source for the experiments. The photocurrent density–photovoltage (J–V) profile is used to evaluate the photovoltaic parameters, such as open-circuit voltage (Voc), short-circuit photocurrent density (Jsc), maximum photovoltage (Vmax) and maximum photocurrent density (Jmax). Fill factor (FF) and power conversion efficiency (η) of the DSSC can be obtained from the Eqs. (1) and (2), respectively FF ¼ V max J max =V oc J sc

ð1Þ

η ¼ V oc J sc FF=I s

ð2Þ

where Is is the intensity of the incident light. 2.5. Characterization techniques The morphology and chemical composition were studied using a Carl Zeiss SIGMA field emission scanning electron microscope fitted with energy-dispersive X-ray spectroscopic (EDX) accessory. X-ray diffraction pattern was recorded on a PANalytical-X'Pert PRO X-ray diffractometer with CuKα radiation. Brunauer–Emmett–Teller (BET) surface area analysis was carried out using Quantachrome Autosorb 1-C/TDP surface area analyzer via

nitrogen (N2) adsorption–desorption, using a multi-point method after degassing the samples by flowing nitrogen. The electrochemical and photocurrent measurements were carried out using a computer-controlled Versa STAT-3 electrochemical workstation (Princeton Applied Research, USA). 3. Results and discussions 3.1. Structural and morphological properties of WC-NRs X-Ray powder diffraction (XRD) analysis was used to determine the phases of the samples. Fig. 2(a) and (b) indicates W3O8 formed which is identified by the peaks at the 2θ of 23.81,27.91,28.51,34.31,37.71,47.91,50.21,56.11, 56.21,58.31,63.41 and 71.41. Those Peaks are corresponding to the (001), (200), (130), (040), (131), (002), (330), (331), (202), (061), (401) and (332) facets of W3O8, in good agreement with the Orthorhombic W3O8-NRs (JCPDS Card no.: 81-2262; a¼6.386 Å, b¼ 10.43, c¼ 3.800 Å, SG:C222). The peaks at the 2θ of 31.51, 35.61, 48.21, 65.71, 73.11 and 75.21 are corresponding to the (001), (100), (101), (002), (111) and (200) facets of WC-NRs, in good agreement with the hexagonal WC-NRs (JCPDS Card no.: 51-0939; a ¼2.906 Å, c ¼2.837 Å, SG:P6m2). The formation of WC is known to depend on the amounts of carbon sources, heating rate, and especially, temperature [27]. The main phase observed is WC, although significant amounts of W2C were seen after synthesis from precursor W3O8/ glucose (W/C) [28]. The diffraction peaks at the 2θ of

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W O -NRs

100k

200 nm

WC-NRs

100k

200 nm

Fig. 3. FESEM images of as prepared (a) W3O8-NRs and (b) WC-NRs.

Fig. 2. PXRD patterns of as prepared (a) W3O8-NRs and (b) WC-NRs.

34.41, 39.51, 52.21, 61.61, and 69.41 are corresponding to the (002), (102), (221), (132) and (321) facets of Orthorhombic W2C. These peaks matched with JCPDS Card no: 89-2371(a ¼4.728 Å, b¼6.009, c¼5.193 Å, SG:Pbcn). Carbon-coated W3O8-NRs were used as the precursors for WC-NRs synthesis. This way of introducing carbon within nanorod-structured W3O8 facilitates the reaction by ensuring a uniform distribution of carbon onto W3O8-NRs and improving their contact. Fig. 3(a) shows the FESEM image of as prepared W3O8-NRs. The W3O8-NRs have rod like morphology with the length of 300–400 nm and diameter 33 nm. The nanorods show no alignment and majority of nanorods are similar in size. The WC-NRs show well defined morphology (Fig. 3(b)) with 350–400 nm length and 94 nm diameter. WC-NRs have been obtained due to the gradual chemical conversion of W3O8-NRs, beginning at the exterior surface and diffusing inward through solid-state carburization towards the nanorod center. From the perspective of the pseudomorphic transformation, solid-state carburization is preferred for the morphological retention of the W3O8-NRs rather than carburization by carbonaceous gases, which had

relatively high rates of nucleation. Besides, the excess surface carbon is also likely to be responsible for increased retention of the WC-NRs [29]. The nanorods may act as onedimensional conductors and thus the nanorods are expected to be efficient in electron transfer to the electrolyte. The energy dispersive X-ray (EDX) analysis of W3O8-NRs and WC-NRs is shown in Fig. 4(a). The emission peaks corresponding to the elements O and W were observed at 0.57 (O), 1.4, 1.8, 7.4, 8.4, 9.6, 11.3 (W) respectively. The energy dispersive X-ray analysis of WC-NRs showed the emission peaks corresponding to the elements C and W at 0.3 (C), 1.4, 1.8, 2, 7.4, 8.5, 9.7, 10 (W), respectively (Fig. 4(b)). For studying the pore structures of W3O8-NRs and WCNRs samples and understanding their influence on the catalytic performance, the BET surface area and porosity were analyzed in detail. Fig. 5(a) and (b) shows nitrogen adsorption–desorption isotherm of the W3O8-NRs and WC-NRs. The pore size distribution curve of the W3O8NRs and WC-NRs is inserted in Fig. 5(a) and (b). Based on these measurements, the specific surface area and the average pore diameter calculated by the BET method and by the BJH method are about 56.8670.5 m2/g and 401.030271.3208 m2/g and 1.986 Å and 7.249 Å, respectively. The total pore volume was about 0.282 cm3/g and 0.238 cm3/g. W3O8-NRs and WC-NRs exhibit representative type I and type II isotherms with H3 and H2 hysteresis loop, as shown in Fig. 5(a) and (b) respectively. The surface of the W3O8-NRs is composed of mesopores. W3O8-NRs compared to WC-NRs have very low surface area.

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Fig. 4. EDAX spectra of as prepared (a) W3O8-NRs and (b) WC-NRs.

Increased surface area plays crucial role for counter electrode in DSSC applications [30]. The synthesized WC-NRs have a highly porous structure and large specific surface area, which may be the key factor to enhance the performance for DSSC. In addition, the mesoporous structure can be considered as an effective electron transport network that facilitates the collection and transfer of electrons from the external circuit and subsequent regeneration of the redox couple. Further, the network is also favorable for the mass transport of the redox couple [31]. 3.2. Photovoltaic performance of DSSC with WC-NRs counter electrode The DSSC was fabricated using the WC-NRs counter electrode and its photovoltaic performance was evaluated under simulated AM 1.5G solar irradiation of 100 mW/cm2. The working mechanism of the fabricated DSSC is shown in Fig. 6. During light illumination, the dye molecules are excited which simultaneously inject photogenerated electrons into the conduction band of TiO2. Then, the electrons move through the TiO2 surface towards the ITO conducting glass and reach the load via external circuit. At the same time the photogenerated holes reach the WC-NRs counter

electrode with the aid of I  /I3 redox couple in the electrolyte. The better performance of the WC-NRs counter electrode can be explained as follows: the counter electrode consists of irregular arrangement of WC-NRs (Fig. 6), which exhibits the probability of directed electron pathway along the long axis of the nanorods. These 1D nanorods facilitate rapid transport of photogenerated holes to the conducting glass through straight conduction pathway along the direction parallel to the axis of the nanorods that leads directly to the electron collecting electrode. The highest electrochemical activity for regeneration of the I  /I3 redox couple due to the 1D nanofeatures, provided good electron transport [32,33]. The current density–photovoltage (J–V) characteristics of the fabricated DSSC are shown in Fig. 7(a). The photovoltaic parameters, such as short circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF) and power conversion efficiency (η) of the DSSC with binder free WC-NRs based solar cell were found to be 5.44 mA/cm2, 370 mV, 0.29 and 0.59% respectively. But in the case of mixture of TiO2 and ZrO2 binder based solar cell, the value of JSC, VOC, FF and η were found to be 5.92 mA/cm2, 807 mV, 0.40 and 1.92% respectively. The investigation of TiO2 bonding with FTO glass had been carried out by Ito et al. [34]. On the

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200 W O -NRs

Quantity adsorbed (cm3 /g)

180 160 140 120 100 80 60 40 20

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Fig. 5. (a) N2 adsorption/desorption isotherm of W3O8-NRs and the inset indicates pore-size distribution obtained by the BJH method; (b) N2 adsorption/desorption isotherm of WC-NRs and the inset indicates poresize distribution obtained by the BJH method.

surface of TiO2 and FTO/ITO glass, there are many hydroxyl groups (–OH). In the sintering process, an ether bond is formed, and the TiO2 particles adhere to the FTO/ITO glass substrate firmly. Mixed oxides have been investigated as potential electrochemical capacitor or supercapacitor materials [35,36]. We added TiO2 þZrO2 to WC-NRs to improve the bonding strength with the FTO glass substrate and we obtained improved results which has been included. Structural, morphological, surface area, electrochemical and photovoltaic studies of the binder material (TiO2 þZrO2) will be reported along with several other binder materials in our future work. Recently, the efficiency (η) of 2.19% was achieved by our group [37] for sputtered Pt counter electrode. However, WC-NRS CE based DSSC shows less efficiency than Pt based DSSC. But low cost WC-NRS will have the prominent role in production of DSSC modules. The applicability of the WC-NRs as CE in DSSC indicates the efficient regenaration of the electrolyte through pinholes and nanopores present in the 1-D nanostructure. The stability and sustainability of DSSC with binder free WC-NRs CE was studied and the

results are shown in Fig. 7(b). The observed photocurrent– time profile of DSSC is in good agreement with their J–V characteristics. As soon as the light is turned ‘on’, the photocurrent rises quickly to a maximum value and showed steady-state current as well. When the light is turned ‘off’, sudden fall in the photocurrent appears and no current in the dark. The steady-state photocurrentmaximum remains same after several ‘on–off’ cycles which revealed that cell has good stability. Electrochemical impedance spectroscopic (EIS) measurement was conducted at scanning frequencies from 0.01 to 100,000 Hz to analyze the electrochemical characteristics of the DSSC with binder free WC-NRs CE. The Nyquist plot (Fig. 7(c)) showed several regions from the high frequency regions to the low frequency regions which correspond to different kinds of resistances: (a) Ohmic serial resistance (Rs) due to the outside circuit resistance (between the substrates resistance and lead connections); (b) charge-transfer resistance (Rct) originates from the electron transfer ability at the electrode/electrolyte interface; (c) electrical double-layer capacitance (Cdl); and (d) diffusion impedance (Zw) of the triiodide/iodide redox couple in the electrolyte [38–40]. The Bode phase plot (plot of phase shift (θ) against frequency (Hz)) for the DSSC with WC-NRs CE showed increased phase angle value in the lower frequency and decreased phase angle value at higher frequency (Fig. 7(d)) which suggests that the prepared WC-NRs show higher conductivity and it could be suitable candidate for DSSC application. Further, an increase in the value of |Z| ohms at the lower frequencies region (10  1 and 102 Hz) in the Bode amplitude plot (plot of log impedance (|Z|) against frequency (Hz)) (Fig. 7(e)) suggests the enhanced catalytic activity of the WC-NRs CE. 4. Conclusions In this study, the tungsten carbide nanorods (WC-NRs) were synthesized by pseudomorphic transformation of chemically synthesized W3O8-NRs. The synthesized WC-NRs were characterized by PXRD, FESEM, EDX and BET surface area analysis. The electrochemical properties were studied with Nyquist plot, Bode phase plot and Bode amplitude plot using EIS measurements. Large surface area WC-NRs was found to be good catalytic material. The WC-NRs (with binder

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6 WC-NRS ( Binder free) WC-NRS (With Binder)

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Jsc =5.924 mA/cm Voc = 0.807 FF =0.40 η (%)=1.92

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Frequency (Hz) Fig. 7. (a) Photocurrent density vs. voltage curve obtained for WC-NRs (binder free, with binder) counter electrode based DSSC under simulated AM 1.5G sunlight (100 mW/cm2) irradiation. (b) Photocurrent vs. time profiles obtained for the WC NRs counter electrode based DSSC under illumination on–off condition. (c) Nyquist plot, (d) Bode phase plot and (e) Bode amplitude plot obtained for the DSSC with WC-NRs counter electrode.

and binder free) based DSSC showed the overall conversion efficiency (η) of 1.92% and 0.59%, respectively. The cell performance can be attributed to the WC-NRs network,

catalytic activity and 1-D efficient charge-transfer network. Such WC-NRs as counter electrode provides a potential feasibility for counter electrodes in DSSC applications.

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