ARTICLE pubs.acs.org/JPCC
Electrochemical Formation of Pt Nanoparticles on Multiwalled Carbon Nanotubes: Useful for Fabricating Electrodes for Use in Dye-Sensitized Solar Cells Hsin-Yi Wang,†,‡ Fu-Ming Wang,*,†,§ Yung-Yun Wang,‡ Chi-Chao Wan,‡ Bing-Joe Hwang,|| Raman Santhanam,^ and John Rick† †
Sustainable Energy Center, National Taiwan University of Science and Technology, Taipei, Taiwan Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan § Nanoelectrochemistry Laboratory, Graduate Institute of Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan ^ Maxwell Technologies, San Diego, California 92123, United States
)
‡
ABSTRACT:
In the present work, hydrophobic multiwalled carbon nanotubes (MWCNTs) were modified, by the substitution of poly(acrylic acid) (PAA) and inorganic acids (H2SO4 and HNO3), as hydrophilic MWCNTsPAA and MWCNTsAcid. The modified nanotubes were dispersed in an ethanol solution containing 3,4-ethylenedioxythiophene (EDOT). PtCl62 ions were reduced electrochemically to platinum (Pt) by electrons supplied by the polymerization of EDOT. The reduced platinum was deposited as Pt nanoparticles (nps) on the MWCNTs, to form MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps). Electrostatic interactions between carboxyl groups on the MWCNTs and EDOT monomers allowed the Pt(nps) to be adsorbed onto the MWCNTs. The quantity of Pt(nps) associated with the MWCNTsPAA-PEDOT-Pt(nps) composite was greater than that with the MWCNTsAcid-PEDOT-Pt(nps) composite, due to the substitution of the carboxyl group in MWCNTsPAA. UVvisible spectroscopy was used to measure the reducing ability of EDOT. FTIR-ATR, TGA, ZETA potential analysis, SEM, and TEM were used to analyze the morphologies and properties of the MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps) electrodes: the cell efficiencies of these new materials, when used to fabricate composite counter electrodes in dye-sensitized solar cells (DSSCs), were found to be 4.78% and 2.71%, respectively.
1. INTRODUCTION In 1991, Iijima observed string-like materials (later characterized as multiwalled carbon nanotubes (MWCNTs)) during arc dischargeevaporation fullerene (C60) synthesis.1 Subsequently, many groups have shown interest in these MWCNTs due to their unique and remarkable properties, such as their high mechanical strength,2 special temperature-resistivity characteristics,3 and electronic properties.4,5 Recently, homogeneous MWCNT film, r 2011 American Chemical Society
deposited onto conductive glass or flexible plastic substrates, has allowed the replacement of metals with conductive MWCNT composite materials.616 However, the hydrophobicity and poor adhesion properties of MWCNTs are disadvantages needing to be Received: February 7, 2011 Revised: March 21, 2011 Published: April 04, 2011 8439
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Figure 3. Basic configuration of (a) MWCNTsPAA and (b) MWCNTsAcid.
Figure 1. The illustrating sketches of (a) the device and (b) the preparing process of trielements composite electrode.
Figure 2. UVvis spectra of the mixed solution containing PtCl62 and EDOT. (a) Initial state (without polymerization) and (b) final state (with polymerization).
addressed if the materials are to be used in industrial applications. To enhance the adhesion of MWCNTs, adhesive agents such as N-methylpyrrolidone (NMP)17 and carboxymethylcellulose (CMC)18 are used with MWCNTs to attach them
onto the substrate. However, the use of these insulating adhesive agents reduces electrical conductivity. It is well-known that conductive polymers offer good adhesive properties and have a convenient fabrication process. In addition, conductive polymers offer greater electrical conductance than insulation binders. Attempts have been made to combine MWCNTs with a conductive polymer to improve conductivity. Binary films prepared using CNTs and a conducting polymer, poly-3,4-ethylenedioxythiophene (PEDOT), generally show a greater enhancement in conductivity when compared to those of other common polymeric materials.1922 The in situ fabrication of PEDOT/CNT composites by the polymerization of EDOT monomers, in the presence of CNT materials, results in the formation of a polymer skin of PEDOT which acts as a “conductive glue” and effectively assembles the CNTs into a conductive network.23 Additionally, Li et al. demonstrated that the EDOT conductive monomer can also act as a reducing agent.24 Recently, the preparation of noble metal nanoparticles (nps) on MWCNTs has been viewed as an attractive strategy because the noble metal and the MWCNT composite can provide a conductive framework. A common method of preparing metal(nps) on MWNCTs is to reflux the MWCNTs with a metal salt and a reducing agent for several hours at 110120 C. Another method is to oxidize the CNTs by acid treatment and disperse them into an ethanol solution. The addition of HAuCl4 then produces the CNT-Au(nps) adduct.25 However, the acidic treatment of growing functional groups on MWCNTs is inconvenient. Hsin et al. modified MWCNTs with poly(acrylic acid) (PAA), which rapidly introduced carboxyl groups without damaging the MWCNT structure.25,26 In addition, the carboxyl groups can be utilized as anchors connecting MWCNTs to metal electrodes, thereby enhancing their electro-conductivity.27 Dye-sensitized solar cells (DSSCs) are considered as potential candidates for the next generation solar cells due to their acceptable energy conversion efficiency and low manufacturing cost. MWCNTs can be used as electrodes28 or doped with an electrolyte29 to improve 8440
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Figure 4. ATR-FTIR spectra of (a) MWCNTsPAA and (b) MWCNTsAcid.
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Figure 6. TGA analysis of (a) pristine MWCNTs, (b) MWCNTsPAA, and (c) MWCNTsAcid.
Table 1. Zeta Potential Analysis Data of Functionalized CNTs
Figure 5. Raman spectra of (a) pristine MWCNTs, (b) MWCNTsPAA, and (c) MWCNTsAcid at 785 nm excitation.
DSSC conductivity and redox catalytic activity. In this study, we electrochemically reduced H2PtCl6 to form Pt(nps) on modified hydrophilic MWCNTs in preference to the Pt sputtering counter electrode. We used two different methods to graft carboxyl groups on MWCNTs. The properties and morphologies of the composites were analyzed by various techniques, namely, FTIR-ATR spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), Zeta potential analysis, and transmission electron microscopy (TEM). DSSC performances were investigated under amplitude modulation (AM 1.5) illumination.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Multiwalled carbon nanotubes (MWCNTs), with a diameter of 4090 nm, were purchased from the Golden Innovation Business Co. (Japan). Fluorine-doped tin oxide (FTO) glass (NSG, 13 Ω/cm2), TiO2 used as photoanodes with a total thickness of 14 μm on FTO glass, and dye (N719, Peccell) were supplied by the Tripod Company (Taiwan). 4-tertButypryidine (0.5 M ) (Aldrich), 1 M 1,3-dimethylimidazoliumiodine
sample
potential (mV)
MWCNTsPAA
65
MWCNTsAcid
46
(Merck), 0.15 M iodine (J.T. Baker), and 0.1 M guanidine thiocyanate (Aldrich) in 3-methoxypropionitrile (Acros) were used as the testing electrolyte. 3,4-Ethylenedioxythiophene (EDOT) and H2PtCl6 were purchased from Aldrich and used without additional purification. 2.2. Study of the Reducing Ability of EDOT. H2PtCl6 (0.0025 g) and EDOT (6.4 103 mL) in DI water (10 mL) were ultrasonically vibrated for 20 min to achieve a good dispersion. UVvis spectroscopy was used to observe changes after dispersion of the mixed solution. 2.3. Sample Preparation. 2.3.1. MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps) Suspensions Preparation of MWCNTsPAA. MWCNTsPAA was prepared using MWCNTs (0.2 g) in a two-step process: (i) acrylic acid and DI water solution with volume ratio 1:9 were ultrasonically agitated for 30 min, and then (ii) benzoyl peroxide (0.03 g) dissolved in THF was slowly added with ultrasonic agitation, to promote acrylic acid polymerization. The concentrated MWCNTs suspension was washed three times with DI water until the pH approached pH 7. After filtration, the MWCNTs were ultrasonically dispersed in DI water (20 mL). Preparation of MWCNTsAcid. MWCNTs (0.2 g) were refluxed for 20 h in a solution comprising concentrated H2SO4 and HNO3 acids (volume ratio 3:1). After filtering, the solid obtained was washed three times with DI water, and then the MWCNTsAcid obtained were dispersed into DI water to prepare a 1% suspension. 2.3.2. Preparation of MWCNTsPAA-PEODT-Pt(nps), MWCNTsAcidPEDOT-Pt(nps), and Pure PEDOT Electrodes. EDOT (6.4 103 mL) was added to MWCNTsPAA-PEDOT-Pt(nps) (5 mL) and MWCNTAcid-PEDOT-Pt(nps) (5 mL) suspensions. Composite films were prepared at room temperature in a glass cell (1 1.5 3 cm3) (Figure 1). The cell was assembled with an FTO glass electrode and stainless steel counter electrode. Polymerization 8441
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Figure 7. Fabricating process of (a) MWCNTsPAA-PEDOT-Pt(nps) and (b) MWCNTsAcid-PEDOT-Pt(nps).
Figure 8. TEM images showing the morphologies of (a) PEDOT/ MWCNTsPAA-PEDOT-Pt(nps) and (b) PEDOT/MWCNTsAcid-PEDOT-Pt(nps) electrodes.
Figure 9. SEM images reviewing the surface morphologies of (a) PEDOT/ MWCNTsPAA- PEODT-Pt(nps) and (b) PEDOT/MWCNTsAcid-PEODT-Pt(nps) electrodes.
was carried out using potentiostatic polarization (þ2.0 V) for 1 min, and the resulting films were dried in an oven at 60 C. The pure PEDOT electrode was prepared using the same method with EDOT (6.4 103 mL), ethanol (4 mL), and DI water (1 mL). 2.4. Methods of Analysis. The reducing ability of EDOT was determined using UVvisible spectroscopy (UVvis, Hitachi UV-300). Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Biorad, FTS-3500) and Zeta potential analysis were used to investigate the surface modification of MWCNTs. Raman spectroscopy (BHSM, DH-2) was used to determine MWCNT structure variation after acid treatment. Thermo-gravimetric (TGA, Perkin-Elmer, Diamond TG/DTA) analysis
was performed to study the thermal stability of MWCNTs in air (25 cm3 min1) from 30 to 800 C at a heating rate of 10 C min1. The electrode morphology was investigated by transmission electron microscopy (TEM) using a JEOL JEM2100 microscope and also by scanning electron microscopy (SEM) using a JSM-5600 microscope. Counter electrode catalytic effects toward tri-iodide reduction were determined using electrochemical impedance spectroscopy (PGSTAT320N, Autolab) by applying an alternating current voltage raging between 100 000 and 0.1 Hz with 5 mV amplitude in a symmetric cell. A 30 μm thick Surlyn film was used as an electrode spacer, and the electrode had an active area of 0.636 cm2. 8442
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Figure 12. Nyquist plots of (a) PEDOT, (b) MWCNTsAcid-PEDOTPt(nps), and (c) MWCNTsPAA-PEDOT-Pt(nps) electrodes. Figure 10. Cross-section SEM images of (a) dropped MWCNTsPAAPEDOT-Pt(nps), (b) dropped MWCNTsAcis-PEDOT-Pt(nps), (c) PEDOT/MWCNTsPAA-PEDOT-Pt(nps), and (d) PEDOT/MWCNTsAcidPEDOT-Pt(nps) electrodes.
Figure 11. (a) Symmetric cell configuration in EIS tests. Equivalent circuit (b) for PEDOT electrode and (c) for MWCNTAcid-PEDOTPT(nps) and MWCNTPAA-PEDOT-PT(nps) electrodes.
2.5. Cell Performance. The sintered TiO2 (0.283 cm2) elec-
trode was immersed in a 0.3 mM dye (N719)ethanol solution at 40 C for over 12 h. A 30 μm thick Surlyn film was used as a separator, and the volatile electrolyte was injected into the cell. DSSC performance was measured using a potentiostat/galvanostat with a computer-controlled digital source meter (Keithley 2400), under visible light illumination with a Newport solar simulator (AM 1.5, 100 mW cm2).
3. RESULTS AND DISCUSSION 3.1. Reducing Capability of EDOT. UVvisible spectroscopy measurements were carried out to confirm EDOT reducing ability. It has been reported that the UVvisible absorption band ranges of H2PtCl6 and EDOT solutions are from 260 to 350 nm and 260 to 290 nm, respectively.30 The absorption band of a H2PtCl6 and EDOT mixed solution is from of 260 to 335 nm as shown in Figure 2a. Figure 2b shows the UVvis spectra of the H2PtCl6 and EDOT solution after ultrasonic polymerization. In this figure, the PtCl62 ion absorption band is no longer present, suggesting that the PtCl62 ion had been reduced to Pt metal. In addition, a new absorption band that resulted from the formation of PEDOT and caused the solution color to become deep blue was observed from 290 to 1000 nm. 3.2. Characterization of MWCNTsPAA and MWCNTsAcid. Figure 3 shows schematic representations of the MWCNTsPAA and MWCNTsAcid structures. Figure 4 compares the ATRFTIR spectra of MWCNTsPAA and MWCNTsAcid. The absorption band appearing at around 17001720 cm1 originates from CdO stretching, and the broad band appearing at 24003400 cm1 is due to the OH group in carboxylic acid, indicating the poly(acrylic acid) (PAA) and organic acid substitution. Raman spectroscopy is widely used to characterize MWCNT structure since this technique is very sensitive to MWCNT structural disorder. Raman spectra of MWCNT, MWCNTsPAA, and MWCNTsAcid are presented in Figures 5a, b, and c. These figures show the D band at 1317 cm1 and the G band at 1579 cm1, resulting from the defects in nanotube and CC stretching vibrations, respectively. The intensity ratio (ID/IG) of MWCNTsPAA is 0.58, which is much lower than that of MWCNTsAcid (ID/IG = 1.23). This dramatic change reflects the extensive damage which occurrs when strong acids functionalize MWCNTs.31 In addition, the blue-shift spectra commonly observed for hydrophilic MWCNTs result from increased disorder and defects in MWCNTs.32 The greater blue-shift of MWCNTsAcid indicates that the structure of the MWCNTsAcid has a considerable number of defects and disordered carbon atoms. Thermogravimetry analysis (TGA) curves of MWCNT, MWCNTsPAA, and MWCNTsAcid are shown in Figure 6. Figures 6a and b show that pristine MWCNTs decompose at 8443
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Table 2. EIS Simulation Results of PEDOT, MWCNTsAcid-PEDOT-Pt(nps), and MWCNTsPAA-PEODT-Pt(nps) Electrodes component PEDOT
Rs (ohm cm2) 17.2
RCT1 (ohm cm2) -
Cdl1 (F) -
RCT2 (ohm cm2) 174.7
5
MWCNTsAcid-PEDOT-Pt(nps)
18.9
6.09
6.11 10
MWCNTsPAA-PEDOT-Pt(nps)
20.9
2.74
2.837 106
17.58 4.563
Cdl2 (F)
τ (s)
1.594 106
0.7294
1.375 104
0.7197
3.712 102
2.812 103
Figure 13. Bode representation of the phase angle of (a) PEDOT, (b) MWCNTsAcid-PEDOT-Pt(nps), and (c) MWCNTsPAA-PEDOT-Pt(nps) electrodes.
Figure 14. IV curves of DSSCs employing (a) PEDOT, (b) PEDOT/ MWCNTsPAA-PEDOT-Pt(nps), and (c) PEDOT/MWCNTsAcid-PEDOT-Pt(nps) electrodes.
720 C and that MWCNTsPAA has a stair-shaped line between 250 and 700 C, representing the decomposition of PAA at 250 C, prior to the onset of MWCNT weight loss at 750 C. In addition, MWCNTsPAA shows greater thermal stability than MWCNT and MWCNTsAcid. The TGA profiles demonstrate that the structure of the hydrophilic MWCNTs, prepared by grafting PAA, is more stable than MWCNTs treated with strong acid. Furthermore, Table 1 indicates that the surface charge of MWCNTsPAA is greater than that of MWCNTsAcid. Zeta potential analysis reveals that MWCNTsPAA has a greater number of carboxyl groups than MWCNTsAcid, causing the surface charge of the MWCNTsPAA to be more negative than that of the MWCNTsAcid. 3.3. Morphology of MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps). Figure 7 describes the fabrication process for the MWCNTsPAA-PEODT-Pt(nps) and the MWCNTsAcid-PEODT-Pt(nps). The EDOT monomer was used as a reducing agent to reduce Pt ions for the synthesis of Pt(nps) on hydrophilically modified MWCNTs. The MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps) surface morphologies were observed using TEM. Figure 8a shows that the Pt(nps) are presented on the surface of the MWCNTsPAA and homogeneously dispersed. Typically, the presence of many PAA carboxyl groups, which provide negative charges, prevents the Pt particle size from exceeding 2 nm. The diameter of Pt (nps) formed on the MWCNTsAcid surface is 23 nm as shown in Figure 8b. Interestingly, the quantity of Pt(nps) on MWCNTsPAA was greater than on MWCNTsAcid. This result suggests the Pts(nps) have been attracted by the negative charge available on the CNT’s surface. Due to the carboxyl groups, a greater number of negatively charged sites are available for Pt nanoparticle growth. The TEM images shown in Figure 8 are consistent with the results obtained from Zeta potential analysis.
Figure 9a shows the surface morphology of MWCNTs PAAPEDOT-Pt(nps) film on FTO glasses, which confirms that upon electro-polymerization of EDOT the MWCNTs PAA PEDOT-Pt(nps) is indeed fixed to the FTO substrate. An integral CNT structure seen in Figure 9a supports the results obtained from TGA analysis. Figure 9b shows that the MWCNTs Acid-PEDOT-Pt(nps) are shorter than MWCNTs PAA PEDOT-Pt(nps); however, a greater quantity of MWCNTs Acid are fixed onto the FTO. This is due to the reflux in strong acid and the fact that the cleaved MWCNTsAcid fragments are shorter than MWCNTsPAA. Thus, MWCNTsAcid are easily transferred to FTO glass via electro-polymerization of EDOT. Figure 10 shows cross-sectional SEM images of the different electrodes. The morphologies of both MWCNTs PAA PEDOT-Pt(nps) and MWCNTs Acid-PEDOT-Pt(nps) electrodes dropped on FTO glasses are rough and disordered as shown in Figures 10a and b. However, the morphology of the MWCNTs PAA -PEDOT-Pt(nps) electrode prepared by electro-polymerization is flat and smooth with a thickness of approximately 1.6 μm (Figure 10c). Figure 10d shows a uniformly coated MWCNTsAcid -PEDOT-Pt(nps), in which the FTO glass and the film thickness is 2 μm. Since MWNCTs treated with concentrated acid fragment into many short tubes, MWCNTsAcid are more easily attached to FTO glass by electro-polymerized EDOT than in the case with MWCNTsPAA . 3.4. Analysis of Electrochemical Catalytic Effect. Figure 11a illustrates the cell configuration used to measure the catalytic ability of the electrode with respect to tri-iodide ion reduction.33 A typical equivalence circuit used to interpret a symmetric cell employing pristine PEDOT electrodes is shown in Figure 11b. This figure 8444
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Table 3. Cell Performances of DSSCs Employing PEDOT, PEDOT-/MWCNTsPAA-Pt(nps), and PEDOT-/MWCNTsAcidPt(nps) Electrodes counter electrode PEDOT PEDOT/MWCNTsAcid-PEDOT-Pt(nps) PEDOT/MWCNTsPAA-PEDOT-Pt(nps)
ISC (mA cm2)
VOC (V)
ff
η (%)
5.52
0.54
0.56
1.65
6.67 11.01
0.55 0.63
0.69 0.74
2.71 4.78
includes several regions representing various resistances: (i) series resistance (RS), representing the outside circuit resistance (FTO resistance and lead connections) and the resistance to electrons transferring within the electrolyte, (ii) charge transfer resistance (RCT) describing electron transfer ability during tri-iodide reduction at the counter PEDOT electrodes, (iii) electrical double-layer capacitance (Cdl), and (iv) diffusion impedance (ZD) representing the impedance caused by electrolyte convection. The diffusion distance (d) and time constant (τ) of redox couple on the electrode surface are given by " ZDðf Þ ¼ ZDð0Þ
pffiffiffiffiffiffiffi# tanh jωτ pffiffiffiffiffiffiffi jωτ
1 ¼ D d2 τ The equivalent circuit shown in Figure 11c was used to simulate the MWCNTsPAA-PEDOT-PT(nps) and MWCNTsAcid-PEDOT-PT(nps) composite electrodes. It should be mentioned that there were two RC parallel circuits in the equivalence circuit for the complex surface structure and the interface double layer caused by MWCNTs. In the first RC parallel circuit, the higher frequency region was caused by MWCNTs and Pt(nps) due to their high electrochemical activity, while the second RC parallel circuit in a lower frequency region resulted from PEDOT. The RCT1 of the electrochemical reaction indicates that Pt is absorbed on the PEDOT counter-electrode and reveals its catalytic activity. The Nyquist plots of the PEDOT, MWCNTsPAA-PEDOT-Pt(nps), and MWCNTsAcid-PEODT-Pt(nps) electrodes are shown in Figure 12, while Table 2 shows the fitting valves after simulation. The RCT2 values of MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps) are smaller than those of the pristine PEDOT electrode, indicating that MWCNTs-PEDOT-Pt(nps) composites outperform PEDOT electrodes because of the high electrical conductivity of MWCNT and the activity of the Pt(nps). The RS of MWCNTsPAA-PEDOT-Pt(nps) is higher than the others due to the PAA insulation. However, due to the complete structure of MWCNTs and the high Pt (nps) loading, the MWCNTsPAAPEDOT-Pt(nps) electrode showed a lower RCT1 (2.74 ohm cm2).This result reflects the higher catalytic activity of MWCNTsPAAPEDOT-Pt(nps) compared to the MWCNTsAcid-PEDOT-Pt(nps) electrode (6.09 ohm cm2). The thickness of the MWCNTsPAAPEDOT-Pt(nps) electrode is less than that of the MWCNTsAcidPEDOT-Pt(nps)—hence its higher catalytic activity. Additionally, the diffusion time constants of the PEDOT (0.7294 s), MWCNTsAcidPEDOT-PT(nps) (0.7197 s), and the MWCNTsPAA-PEDOTPt(nps) (2.812 103 s) electrodes suggest that the redox couples (I and I3) can effectively migrate to the counter electrode. The dependence on frequency is seen more clearly in the Bode representation shown in Figure 13 for the magnitude and phase angle of the Randles circuit. The phase angle, expressed as it tends toward zero at high and low frequencies, indicates that the current and potential are in phase which
represents that almost no capacitance and conductance exist in the system. Zj 1 j ¼ tan Zr Re According to the results of Figure 13, the composite electrode of MWCNTsPAA-PEDOT-Pt(nps) shows excellent properties regarding the intrinsic behavior far from capacitance. However, the pristine electrode of PEDOT reveals an existence of CPE (constant phase element) behavior at high frequency which will bring about non-Faradiac reaction eliminating the cell efficiency. 3.5. Cell Performances. Figure 14 and Table 3 provide IV curves and DSSC parameters, respectively. The MWCNTsPAAPEDOT-Pt(nps), MWCNTsAcid-PEDOT-Pt(nps), and pristine PEDOT electrodes show significant differences in efficiency, i.e., 4.78%, 2.71%, and 1.65%, respectively. Additionally, the ISC and VOC values of the MWCNTsAcid-PEDOT-Pt(nps) electrode are much higher than that of the other two electrodes. The ff of PEDOT, MWCNTsAcid-PEDOT-Pt(nps), and MWCNTsPAAPEDOT-Pt(nps) electrodes both show improvements, i.e., from 0.56 to 0.69 and 0.74, respectively, resulting in an enhanced efficiency.
4. CONCLUSION MWCNTsPAA and MWCNTsAcid containing carboxyl functionality have been successfully prepared. MWCNTsPAA, grafted with PAA during polymerization, had greater structural integrity than MWCNTsAcid, formed by reflux in strong acid. MWCNTsPAA showed increasing thermal stability due to the presence of numerous carboxyl groups. ZETA potential analysis confirmed that the MWCNTsPAA surface had a greater negative charge than MWCNTsAcid. The growth of the Pt particle size was controlled by the negative surface charge: the resulting diameter of the Pt particles was typically less than 3 nm. With the PEDOT electro-polymerization process, both the MWCNTsPAA-PEDOT-Pt(nps) and the MWCNTsAcid-PEODT-Pt(nps) composites were bound to FTO glass using the EDOT monomer to form a PEDOT binder, while the composite mounted on FTO glass provided MWCNTsPAA-PEDOT-Pt(nps) and MWCNTsAcid-PEDOT-Pt(nps) electrodes. DSSCs incorporating the MWCNTsPAA-PEDOT-Pt(nps) and the MWCNTsAcid-PEDOT-Pt(nps) electrodes showed impressive efficiency enhancements of 4.78% and 2.71%, respectively. The novel DSSC electrodes fabricated using these composite materials have many potential applications in a variety of electrochemical devices. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: þ886-2-27303755. Fax: þ886-2-27303733; þ886-2-27376922. 8445
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’ ACKNOWLEDGMENT The authors are grateful for financial support of this research from the National Science Council of Taiwan R.O.C. under Grant NSC 100-2923-E-011-001-MY3 and for technical assistance from the Sustainable Energy Center at the National Taiwan University of Science and Technology.
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dx.doi.org/10.1021/jp201220t |J. Phys. Chem. C 2011, 115, 8439–8446
