ARTICLE pubs.acs.org/JPCC
Synthesis of TiO2 Nanoparticles on Plasma-Treated Carbon Nanotubes and Its Application in Photoanodes of Dye-Sensitized Solar Cells Shouwei Zhang,†,‡ Haihong Niu,‡ Yan Lan,† Cheng Cheng,† Jinzhang Xu,‡ and Xiangke Wang*,† † ‡
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
bS Supporting Information ABSTRACT: Carbon nanotubes (CNTs) have attracted much interest because of their special physicochemical properties. Herein, O2 plasma-treated CNTs (denoted as PS-CNTs) are incorporated within a TiO2 matrix (denoted as PS-CNTs/ TiO2) as photoanodes in dye-sensitized solar cells (DSSCs). The PS-CNTs/TiO2 composites provide more uniform holes and rough surface over the photoanode and also provide a greater degree of dye adsorption and lower levels of charge recombination, as compared to either chemical modified CNTs/TiO2 (denoted as CM-CNTs/TiO2) or TiO2 alone. The high dispersion of TiO2 on PS-CNTs can improve the electron conduction paths, leads to high electron transfer efficiency, and thereby results in the high performance of the DSSC devices. Herein, the PS-CNTs/TiO2-based working photoanode demonstrates a conversion efficiency of 6.34% in DSSCs, which is ∼75% higher than that of conventional TiO2-based devices.
1. INTRODUCTION Since Oregan and Gratzel1 made their breakthrough with a new type of solar cell in 1991, dye-sensitized solar cells (DSSCs) based on TiO2 nanocrystalline photoanodes have attracted considerable attention owing to their simple process and relatively high energy conversion efficiency. Compared to commercial silicon-based solar cells, DSSCs promise lower fabrication cost and offer higher benefits as compared with amorphous silicon solar cells. 25 Despite the fact that power conversion efficiencies of 11% have been attained with DSSCs, further improvements in the performance of these solar cells are still necessary.6,7 The transport of photoinduced electrons across the TiO2 nanoparticle network is the major concern to attain high overall light-to-electricity efficiency in the working electrodes. Rapid photoinduced electron transport in a DSSC electrode ensures the efficient collection when it competes with the charge recombination processes. In general, photogenerated electrons injected onto the mesoscopic film of a TiO2 photoanode must travel through the TiO2 nanoparticle network and encounter many grain boundaries. This random transit path enhances the probability of recombination with oxidizing species or I3 ions in the electrolyte, thus inhibiting the photocurrent and the photoconversion efficiency.810 Hence, the prevention of charge recombination is expected to result in a great improvement for the photoinduced electron transfer. Regulating the flow of photogenerated charge carriers is proposed to improve the electron recombination of DSSCs based on the semiconductor nanotube or nanowire architectural design concept.1113 When the semiconductors are assembled on the photoanode surface r 2011 American Chemical Society
and then are modified with dye molecules, they offer the possibility to improve the charge collection and transport.1116 The challenge is using 1D nanostructure networks as a support to anchor light-harvesting semiconductor and to assist electron transport to photoanode in DSSCs. Carbon nanotubes (CNTs) have been investigated intensively as semiconductor supports because of their unique electrical and electronic properties, wide electrochemical stability window, high surface area, adsorption, mechanical, and thermal properties.1726 The electrons injected from the excited dye into TiO2 nanoparticles are transferred through CNT scaffolds to generate a photocurrent. An illustration of the electron transfer between a photoexcited semiconductor nanoparticle and CNTs is presented in Figure S1 of Supporting Information.Within the past decade, the incorporation of CNTs in a nanocrystalline TiO2 working electrode has been studied extensively to increase the solar energy conversion efficiency of DSSCs.2729 However, the CNT loading may inhibit the improvement of cell performance due to serious CNT aggregation in the blended procedure. To improve their dispersion property, CNTs are functionalized by different methods to covalently attach chemical groups (e.g., carboxylic groups), such as nitric (HNO3) and sulfuric (H2SO4) acid oxidation, air oxygen, ozone oxidation, and plasma oxidation, resulting in the formation of carboxyl and carbonyl groups on the surfaces of CNTs. However, due to the rather harsh conditions involved, Received: July 3, 2011 Revised: September 26, 2011 Published: October 03, 2011 22025
dx.doi.org/10.1021/jp206267x | J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C most oxidation reactions result in the opening of the nanotube tips, detrimental damage of their sidewalls, or both. A particularly attractive option is the modification of CNTs while largely retaining their structural integrity. Compared to other chemical modification methods, plasma treatment is surface specific modification without destroying the bulk properties with the advantages of shorter reaction time and nonpolluting process. Herein, a homogeneous CNTs/TiO2 nanocomposite photoanode was prepared by using an acid-catalyzed solgel method. The oxygen-containing groups were introduced onto the surfaces of CNTs by using radio frequency (RF) inductively coupled O2 plasma (ICP) treatment to improve the dispersion of CNTs, which is important for the improvement of the conversion efficiency of DSSCs. The oxygen vacancies were reduced by O2 plasma treatment, which acts as electron trap sites to facilitate the recombination of injected conduction band electrons with I3 ions in the electrolyte.
2. EXPERIMENTAL SECTION 2.1. Materials. The tetra-butyl ortho-titanate (TBOT, 98 wt %) was obtained from the Sinopharm Chemical Reagent Co., Ltd., China and used as received. All chemicals used in this study were reagent analytical grade, including anhydrate ethanol (EtOH, 99.7 wt %), nitric acid (HNO3, 6368 wt %), and other reagents for synthesis. 2.2. Preparation and Purification of CNTs. CNTs were synthesized using a chemical vapor deposition (CVD) method.30 Acetylene in a hydrogen flow was used as the starting material. NiFe nanoparticles were used as catalysts. The growth of CNTs was carried out at 760 °C. The obtained CNTs were then purified by a strong oxidant and calcined at 500 °C for 4 h in argon to remove carbon nanoparticles and carbonaceous impurities. The catalyst Ni and Fe in oxidized CNTs were measured by inductively coupled plasma-mass spectroscopy (ICP-MS), and the results showed that the contents of Ni and Fe in the CNTs were less than 0.01 and 0.03%, respectively. 2.3. Preparation of Chemical Modified CNTs (CM-CNTs). Chemical modification of CNTs was done as described in a previous paper.31 Typically, CNTs (500 mg) were mixed with H2SO4/HNO3 (100 mL, volume 1:1) solution, which was refluxed at 140 °C for 6 h to form a dark-brown suspension. After that, distilled water (1000 mL) was added, and the solution was then stirred for another several hours. A black solid was obtained after filtration and rinsed several times with distilled water. It was then dried under vacuum for 8 h. 2.4. Preparation of Plasma Surface Modified CNTs (PSCNTs). A plasma generator induced by a radio frequency (RF) ICP was used in this study. Prior to ignition of the O2 plasma, the pressure in the reactor was evacuated to 0.038 Torr. Pure O2 gas was then introduced into the reactor via a gas mass flow controller (MFC). Plasma ignition occurred at ∼0.26 Torr with a supplied power of 50 W. The CNTs were treated by O2 plasma for 40 min under continuous stirring, and thus plasma surface modified CNTs were obtained. 2.5. Synthesis of CNTs/TiO2 Nanocomposites. CM-CNTs (50 mg) or PS-CNTs (50 mg) were sonicated in EtOH (50 mL) for 15 min to disperse them well. Then the measured quantity of TBOT (50 mmol) was introduced into the CNTethanol solution. The mixture of TBOT and CNTs was sonicated for an additional 30 min to improve the interaction between the two reagents. Subsequently, the solution was stirred magnetically for
ARTICLE
30 min, and then nitric acid (4 mL) and deionized water (2 mL) were added to catalyze the hydrolysis and condensation reaction. The final mole ratio of the mixture of TBOT:EtOH:HNO3:H2O was 1.0:1.7:0.17:0.10. Finally, the mixture was loosely covered and kept stirring until a homogeneous gel was formed. The gel was aged in air for 24 h, and then the xerogel was crushed into a fine powder and dried at 60 °C in an oven for 24 h. Then, the powder was calcined at 450 °C for 1 h in air to obtain the CNTs/ TiO2 composites. 2.6. Preparation of CNTs/TiO2 Thin Films and DSSCs. The CNTs/TiO2 thin films were prepared by applying the nanocomposites on an electric conducting glass plate. Fluorine-doped tin oxide (FTO) was used as an electric conducting plate. The FTO glass plates were purchased from the Geao Co., Wuhan, China. The nanocomposite gels, which were obtained after the above reaction, were then used as the starting samples. A certain amount of CNTs/ TiO2 suspension, ethyl cellulose, and terpineol, were grinded together for 3 h in a mortar. This mixture was then transferred into a beaker and stirred with a magnet tip (1 min) and sonicated with ultrasonic horn [(2 min work + 2 min rest) 180 times]. Ethanol and water were removed from these suspensions by a rotary evaporator, and the slurry was obtained. This slurry was screen printed onto the support to generate a 0.25 cm2 (0.5 cm 0.5 cm) active area and dried at room temperature for 12 h. Subsequently, the electrodes were sintered at 450 °C for 30 min, generating a crack-free, high surface area of CNTs/TiO2 photoanode. For the DSSC fabrication, the sensitizing dye was applied to the electrodes by soaking the electrodes for 24 h in a solution of ruthenium dye (cis-bis(isothiocyanato) bis(2,20 -bipyridyl-4,40 dicarboxylato) ruthenium(Π) bis-tetrabutylammonium) (N719, Solaronix SA, Switzerland, 5 104 M)) in acetonitrile/t-butyl alcohol (v:v = 1:1). The DSSC comprised a sensitized working electrode, a platinized counter (Pt) electrode, and the electrolyte with a 25 μm thick thermal plastic hot-melt sealing foil (Surlyn1172, Dyesol, Australia) in between. The electrolyte used in this study consisted of N-methyl-Nbutyl imidazolium iodide (BMII, 0.6 M), iodine (0.03 M), guanidinium thiocyanate (0.1 M), and 4-tert-butylpyridine (TBP, 0.5 M) in the mixture of acetonitrile/valeronitrile (v/v = 85:15). 2.7. Characterization of CNTs/TiO2 Nanocomposites and the Photoanode. X-ray diffraction (XRD) analysis was performed on a D/Max-IIIA X-ray diffractometer (Rigaku Co., Japan), using Cu Kα (λ = 0.15418 nm) as the radiation source to identify the crystalline structure and average crystallite size of TiO2 particles. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a VG ESCALAB MKII spectrometer using a Mg Kα X-ray source (1253.6 eV, 120 W) at a constant analyzer. The energy scale was internally calibrated by referencing the binding energy (BE) of the C 1s peak at 284.60 eV for carbon. Raman spectroscopy analysis was carried out on an NR-1800 laser Raman spectrometer (JASCO, Japan). The particle morphology and microstructures of CNTs/TiO2 nanocomposites were observed using field emission scanning electron microscopy (FE-SEM, Sirion200, FEI Corp., Holland) and transmission electron microscopy (TEM, JEM-2011, JEOL, Japan). The N2 adsorptiondesorption isotherms at 77 K were measured with an adsorption instrument (TriStarII, Micromeritics Company, USA) to evaluate their pore structures and surface area. All samples were degassed at 180 °C before the measurements. The specific surface areas were determined using the Brunauer EmmettTeller (BET) equation, and the pore size distributions were analyzed using the BarretJoynerHalenda method. 22026
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Emission spectra of RF ICP (A). Raman spectra of CNTs before and after O2 plasma treatment for 40 min at plasma power of 50 W and pressure of 0.26 Torr (B). XPS analysis of CNTs before (C) and after (D) O2 plasma treatment for 40 min at plasma power of 50 W and pressure of 0.26 Torr.
The UVvisible diffuse reflectance spectra (UVvis DRS) of the as-prepared samples were obtained on a UVvisible spectrophotometer (SoildSpec-3700DUV, Shimadzu Corp., Tokyo, Japan). BaSO4 was used as a reflectance standard in a UVvisible diffuse reflectance experiment. The amount of dye adsorbed into the photoanode was determined by desorbing the dye from the composites into a 0.1 M KOH aqueous solution, and measured the absorbance of the resulting solution using UVvis spectroscopy (UV-2550, Shimadzu, Japan). The photocurrent density photovoltage (JV) characteristics of DSCCs with an active area of 0.25 cm2 (0.5 cm 0.5 cm) were measured under AM 1.5 (100 mW cm2) illumination, which was provided by a solar simulator (Oriel Sol 3A, USA) with a Keithley 2420 source meter (Keithley, USA). The monochromatic photocurrentwavelength measurements were carried out by placing a monochromator, assisted by an automatic filter wheel, between the DSSCs and the light source (100 mW 3 cm2). The photocurrent of the cell at different incident wavelengths was also recorded. The incident monochromatic photon-to-electron conversion efficiency (IPCE), plotted as a function of excitation wavelength, was calculated from the following equation IPCEð%Þ ¼
1240Jsc λPin
where Jsc is the photocurrent density, λ is the wavelength of the incident light, and Pin is the power of the incident light.
3. RESULTS AND DISCUSSION 3.1. Optical Emission Spectroscopy Analysis. Optical emission spectroscopy (OES) is used for diagnostics of reactive plasmas. The main advantage of this method is the noninvasive character of measurements. Several wavelengths corresponding to atomic transitions in oxygen are used to analyze the plasma emission spectra. The most significant oxygen lines in our experimental conditions are at 656.5 and 777.2 nm (Figure 1A), which correspond to the de-excitation of the oxygen atoms, produced by the following ways: e + O2 f e + O* + O and e + O2 f e + O*, i.e., dissociative excitation or direct impact excitation of oxygen atoms, respectively.32 The active oxygen can be created in the bulk of plasma or on the surfaces of CNTs. The active oxygen species are highly reactive and interact with the surfaces of CNTs. The oxidation of CNTs is attributed to the existence of the active oxygen species. The peak intensities at the wavelengths of 656.5 and 777.2 nm are reduced significantly after CNTs are added in plasma reactor, which indicates that the oxygen plasma reacts with CNTs. 3.2. Raman Analysis. Raman spectroscopy has been extensively used to characterize various carbon materials, and this technique shows a high sensitivity to the disorder on the surfaces based on the optical skin depth.33 Figure 1B shows the Raman spectra of CNTs before and after O2 plasma treatment. For the untreated CNTs, the G peak at 1573.8 cm1 is the E2g2 mode corresponding to the movement in the opposite direction of two 22027
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
ARTICLE
Figure 2. XPS survey spectra of pristine TiO2 (A) and PS-CNTs/TiO2 (B) and high-resolution XPS Ti 2p spectrum of PS-CNTs/TiO2 (C). XRD patterns of pristine TiO2, CM-CNTs/TiO2, and PS-CNTs/TiO2 nanocomposites (D).
neighboring carbon atoms in the graphitic sheet. It indicates the presence of crystalline graphitic carbon in CNTs. The D peak at 1345.1 cm1 is an A1g breathing mode, which is generally attributed to the defects in the curved graphite sheet, sp3 carbon, or other impurities. The ratio of R value (R = ID/IG), where I corresponds to the peak area of the Lorentzian functions, allows us to estimate the relative extent of structural defects. The R values are 1.21 and 1.24 for the untreated and O2 plasma-treated CNTs. The increase of R for the O2 plasma treated CNTs can be attributed to the microstructure of carbon sheets in tubes and the increase of oxygen content. Compared to the untreated CNTs, a red shift of each peak position of the plasma-treated CNTs takes place, which is caused by the increased disorder and defect density in the plasma-treated CNTs.34,35 After O2 plasma treatment, the R value slightly increases due to the enhancement of surface defects and embedment of oxygen atoms. The slight increase of the R value and blue shift of peak positions of the plasma-treated CNTs indicate the change of the surface structure of CNTs and the introduction of oxygen atoms, but the O2 plasma does not destroy the integrity of CNT structure. 3.3. XPS Analysis. XPS technique can be used to identify groups attached to the surfaces of CNTs. Parts C and D of Figure 1 show the high-resolution XPS C1s spectra of CNTs before and after O2 plasma treatment for 40 min at plasma power of 50 W and pressure of 0.26 Torr. To further explain the process of plasma oxidation, the C 1s peak is deconvoluted into five Gaussian peak components: (1) the main peak at 284.7 ( 0.2 eV corresponds to the sp2-hybridized graphitelike carbon atoms (CdC); (2) the peak at 285.7 ( 0.2 eV is attributed to the sp3hybridized carbon atoms (C—C); (3) the peaks at 286.1 ( 0.2 eV, 287.2 ( 0.2 eV, and 289.7 ( 0.2 eV are considered to originate in carbon atoms bound to one and two oxygen atoms, because electronegative oxygen atoms induce a positive charge
on a carbon atom. Hence, they correspond to C—O (e.g., alcohol, ether), CdO (e.g., ketone, aldehyde), and O—CdO (e.g., carboxylic, ester) species, respectively.36,37 The quantitative analysis (Figure 1D as compared to Figure 1C) indicates that the sp2 CdC fraction decreases after plasma treatment, whereas the C—O, CdO, and O—CdO fractions increase. These changes suggest that the CdC bonds are oxidized and new COx groups are generated on the surfaces of CNTs by O2 plasma treatment. It is believed that various oxidative reactions occur during plasma treatment. After O2 plasma treatment, free radicals can be created on the treated surfaces, which can then couple with active species from the oxygen plasma environment.38,39 The chemical forms of surface elements in the composites were also investigated by XPS analysis. The XPS survey spectra of the pristine TiO2 and PS-CNTs/TiO2 hybrid materials are shown in parts A and B of Figure 2. The characteristic energy peaks of three kinds of atoms, i.e., oxygen, titanium, and carbon, can be observed in the spectra. The C 1s peak of the electrode with PS-CNTs is stronger than that without CNTs, confirming that the CNTs are successfully introduced into the photoanode. The photoelectron peak for Ti 2p appears clearly at a binding energy, Eb, of 460 eV, with O 1s at Eb = 531 eV and C 1s at Eb = 286 eV. The C 1s peak corresponds to the CNTs, while the Ti 2p and O 1s peaks can be attributed to Ti4+ and O2.40 Figure 2C displays the XPS spectrum of Ti 2p for PS-CNTs/ TiO2 composites. The XPS Ti 2p peaks appear at 459.6 eV (Ti 2p3/2) and 465.3 eV (Ti 2p1/2), slightly shifting toward higher binding energy as compared with those of the pure bulk anatase (TiO2).40 This implies that the Ti in TiO2 nanoparticles decorated on PS-CNTs is in a different environment from that of pure anatase (TiO2) and also suggests the strong interaction between TiO2 nanoparticles and PS-CNTs. The peak at 455 eV assigned to Ti2+ (TiO) and the peak at 456.7 eV assigned to Ti3+ 22028
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
ARTICLE
Figure 3. Illustration of the postulated structural morphology for the CNTs/TiO2 nanocomposites and FESEM and TEM images of the pristine TiO2, CM-CNTs/TiO2, and PS-CNTs/TiO2 nanocomposites.
(Ti2O3) are not observed. Therefore, it can be concluded that only titanium dioxide exists in the composites. 3.4. Characterization of CNT/TiO2 Photoanode. Figure 2D presents the XRD patterns of the pristine TiO2, CM-CNTs/ TiO2, and PS-CNTs/TiO2 nanocomposites. The XRD patterns show that only the anatase phase can be identified in the nanocomposites. These results imply that the TiO2 nanocrystalline structure was retained with CNTs in the TiO2 matrix. Furthermore, the peaks at 2θ positions of 26.0° and 43.4°, which are the characteristics of carbon nanotubes,41 are not observed in the XRD patterns. The interpretation of this phenomenon is that the main peak of the CNTs at 26.0° might over lap with the main peak of anatase crystallites at 25.4° since these two peaks are very close. Moreover, the TiO2 crystallinity is much higher than that of CNTs, which causes the TiO2 to shield the CNT peaks.25 Notably, inset in Figure 2D, the width of the XRD peak at 25.4 is slightly broadened for the CNTs/TiO2 nanocomposites as compared to pristine TiO2. This implies that the introduction of CNTs may markedly change the TiO2 crystalline size. The TiO2 crystalline size in the nanocomposite is determined from the half width of the peaks according to Scherrer’s formula (d = 0.9λ/β cos θ, where d is the grain size, λ is the X-ray wavelength, β is full width at half-maximum intensity (fwhm) of (101) peak in radians, and θ is the Bragg’s diffraction angle). The crystalline size of pristine TiO2 is 15.1 nm, whereas that of TiO2 in nanocomposites declines to 13.8 nm, which is caused by the interaction between the hydroxyl group in the hydrolyzed titania precursor and the chemical groups (such as OH and COOH)
on the surface defects of CNTs. In this case, the defects of CNTs may be regarded as sites of grain growth for TiO2 crystallites in CNTs/TiO2 nanocomposites. Hence, the introduction of CNTs into the TiO2 matrix may reduce the size of the crystallized TiO2 on CNTs and thus suppress the agglomeration of TiO2 particles in the sintering procedure, reducing the TiO2 crystalline size. This change of TiO2 crystalline size will provide the nanocomposite electrodes with a greater number of contact points between sintered TiO2 particles and at the interface between the TiO2 and the underlying substrate (FTO glass). This might cause more efficient charge separation and faster photoinduced electron transfer.42 Moreover, smaller crystalline sizes are wellknown to result in larger surface area, and this would significantly provide more sites for the adsorption of dye molecules. Consequently, a working electrode based on the CNTs/TiO2 nanocomposites should possess higher conversion efficiency than pristine TiO2-based working electrode for DSSCs. 3.5. Morphological Analysis of Incorporating CNTs into the TiO2 Matrix. Figure 3 shows the FESEM microphotographs of the photoanodes. The FESEM images indicate that the differences between the nanocomposite morphologies with various CNTs are consistent with the results of the roughness. The introduction of CNTs into the TiO2 matrix causes better dispersion of TiO2 particles and smaller crystalline size, consequently forming a structure with high porosity and coarse surface.24,43,44 Hence, for the morphology analysis of the nanocomposites, the FESEM images provide the formation of more porous structure due to 22029
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
ARTICLE
Table 1. BET Surface Area (SBET), Total Pore Volume (Vp), Average Pore Diameter (Dp), and TiO2 Crystal Size (dTiO2) of Samples sample raw CNTs
SBET (m2/g)
Vp (cm3/g)a
Dp (nm) dTiO2 (nm)b
219.08
1.20
22.18
61.52
0.31
6.71
15.1
CM-CNTs/TiO2
143.06
0.39
7.61
14.4
PS-CNTs/TiO2
143.09
0.51
6.82
13.8
pristine TiO2
a
The total pore volume was evaluated for a P/P0 ratio of 0.99. b TiO2 crystal size was calculated by Scherrer’s equation (applicable from 1 to 100 nm).
Figure 5. Diffuse reflectance UVvis spectra of the pristine TiO2 (a), CM-CNTs/TiO2 (b), and PS-CNTs/TiO2 (c) photoanodes. Figure 4. (A) N2 adsorptiondesorption isotherms and (B) pore size distribution of the pristine CNTs, CM-CNTs/TiO2, and PS-CNTs/ TiO2 nanocomposites.
the contributions of the CNTs. A reasonable interpretation is also postulated in Figure 3 to further understand the effect of CNTs on the morphology changes. It is suggested that the introduction of PS-CNTs causes high dispersion of TiO2 nanoparticles, thus leading to more uniform pores in TiO2-based thin film, which will result in higher surface area for dye adsorption and rough surface structures.24,43,44 However, the CM-CNTs may cause the formation of CNT aggregation. The good dispersion of TiO2 particles would not be achieved, leading to the formation of large and irregular pore sizes. It is proposed that these irregular pores with large sizes may affect the improvement of surface area effectively, and excessive increases the roughness. The TEM images of the CNTs/TiO2 photoanodes (Figure 3) confirmed the existence of a continuous thin layer coating covering the entire surface of CNTs. The diameter of the raw CNTs was 6070 nm, and the thickness of the TiO2 layer was estimated to be 1530 nm. The average diameters were estimated from enlargements of TEM images by considering each tube section and averaging the measurements. One can see that the TiO2 nanoparticles are well-dispersed on the surfaces of PSCNTs, whose surface contains an abundance of hydroxyl groups, carboxyl groups, and carbonyl groups by plasma treatment, to form an adlayerlike pattern with a uniform particle size of about 1314 nm. These nanoparticles do not aggregate to form large
clusters, and there is good contact between the TiO2 nanoparticles and PS-CNTs. The TEM image of PS-CNTs/TiO2 also shows the rare appearance of bare CNTs in the composites, which confirms the intactness of the CNT tubular structure after the plasma treatment. However, the good dispersion of TiO2 particles are not achieved onto the surfaces of CM-CNTs as one can see from the TEM image. On the basis of these results, it is suggested that a working electrode with a higher porous microstructure of PS-CNTs will adsorb more dyes due to a larger adsorption area. The result also implies that the cell performance of DSSCs could be improved.43,44 3.6. BET Surface Area (SBET) and Pore Properties Measurements. The N2 adsorptiondesorption isotherms and the pore size distribution for raw CNTs, CM-CNTs/TiO2, and PSCNTs/TiO2 composites are shown in Figure 4. The values of N2BET surface areas, pore volumes, and average pore diameters are listed in Table 1. All the isotherms can be ascribed to type IV, indicating the presence of a mesoporous pore structure. The pore size distribution plot of the raw CNTs exhibits a wide pore size distribution ranging from 5 to 100 nm. The pristine TiO2 presents a monomodal pore size distribution centered at 8.71 nm. When CNTs are composited with TiO2, the pore size distributions of the composites become narrow. The high distribution centers at 8.68 and 8.83 nm can be attributed to TiO2, while the pores bigger than 15 nm in CNTs are obviously reduced for all the composites. The reduction in pore volume of the composites compared to that of raw CNTs strongly 22030
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
Figure 6. The UVvis spectra of dye desorbed from composite working electrodes using 0.1 M KOH aqueous solution: the pristine TiO2 (a), CM-CNTs/TiO2 (b), and PS-CNTs/TiO2 (c) photoanodes.
suggests that the deposited TiO2 particles may block the CNT pores. From Table 1, it is interesting to note that the BET surface area of CNTs/TiO2 is increased significantly as compared with that of pristine TiO2, while CM-CNTs/TiO2 and PS-CNTs/TiO2 have similar surface areas. The presence of the CNTs in the nanocomposites hinders particle growth, resulting in a slight decrease of crystal size. Smaller crystalline sizes are well-known to result in larger surface area, which is confirmed by the BET analysis. 3.7. UVVis Analysis. Diffused reflectance spectrum is a useful measurement to reveal light scattering ability of samples.45,46 Figure 5 shows the diffused reflectance UVvis spectra of pristine TiO2, CM-CNTs/TiO2, and PS-CNTs/TiO2 photoanodes. The CNT-based photoanodes absorption covers the whole range of the measured UVvis region due to the introduction of CNTs. This indicates that the incorporation of CNTs is good for the light-absorbing properties of the composites. The absorption threshold of pristine TiO2 is about 385 nm, corresponding to the band gap of anatase (∼3.2 eV). It presents a strong absorption band only in the UV region which is attributed to the bandband transition. However, with the incorporation of CNTs, which are gray in color, the composites exhibit strong visible light absorption. Further observation indicates that there is no visible change in the absorption edges of the CNTs/TiO2 composite photoanodes in comparison to pristine TiO2, implying that CNTs are not incorporated into the lattice of TiO2 but only contact with its surface, namely, ruling out the lattice doping of carbon into TiO2 host. This is due to the fact that the loading amount of CNTs is low.45 Compared with CM-CNTs/TiO2 and pristine TiO2, PS-CNTs/TiO2 composites show a red shift, indicating the decrease of electron ionization potential.46 This phenomenon suggests that PS-CNTs/TiO2 can achieve charge transfer between CNTs and TiO2 easily.46 The UVvis absorption of dye solutions formed by the desorption of dye from composite films using 0.1 M KOH aqueous solution are shown in Figure 6. As compared with the pristine TiO2 photoanode, it is evident that the CNTs/TiO2 composite photoanode has higher adsorption of dye. This is due to the high specific surface area of CNTs, providing a large surface area for the anchoring of TiO2 particles. It is interesting to
ARTICLE
note that the amount of dye adsorbed by the PS-CNTs/TiO2 photoanode is higher than that of the CM-CNTs/TiO2 photoanode, although the surface areas of PS-CNTs/TiO2 and CMCNTs/TiO2 are very similar. In general, a working electrode with large surface area benefits the working efficiency of DSSCs. It could efficiently increase the quantity of dye-sensitizer adsorption and the contact area between the electrolyte and the electrode and thus enhances the photoelectron transfer rate and photoelectron conversion efficiency. However, for CM-CNTs loading, the electrode performance is not as high as PS-CNTs loading. The result may be ascribed to at least two reasons: First, a strong acid interaction between the CNT surfaces and most oxidation reactions in the chemical modification processes result in the structure damage of CNTs. It decreases the ability of building network of CM-CNTs/TiO2 composites, even the conductive network and electron transport paths may be destroyed completely; second, most of the large and irregular pores on the surface of CM-CNTs/ TiO2 photoanode inhibits dye uptake. The photoanode made of PS-CNTs/TiO2 shows the highest dye adsorption, which is in good agreement with the morphology of the composites. It is suggested that more uniform pores are advantageous for dye penetration and adsorption, while large and irregular pores are inaccessible. The PS-CNTs generate a larger anchoring surface area for TiO2, thereby enhancing dye adsorption. The structure of the PS-CNTs/TiO2 composites minimizes the aggregation and results in a more uniform pore distribution. After plasma treatment, the smooth CNT surfaces become rough, and the surface defects of PS-CNTs are enhanced. The integrity of the PS-CNT patterns is not damaged. Thereby, the plasma treatment method will lead to the existence of favorable conductive network, while the chemical method does not. In particular, the confinement of network makes the electron transport to be less diffusive and more directional, providing fast transport paths. In another word, the conductive network of CNTs may collect electrons and transport them fast enough to compete with the electronhole recombination of self-trapped excitations in TiO2. 3.8. Analysis of DSSC Performance Using Various Working Electrodes. The cell performances of DSSCs based on CNTs/ TiO2 photoanode s were measured under 1 Sun AM 1.5 simulated sunlight. Figure 7A shows the characteristics of the DSSCs fabricated using various working electrodes. The results are listed in Table 2, and the spread of the cells efficiency data are shown in Figure S2 of the Supporting Information. The DSSCs with the PS-CNTs/TiO2 photoanode exhibited a photocurrent density (Jsc) of 11.04 mA 3 cm2, a photovoltage (Voc) of 0.85 V, and a FF of 0.68, yielding a conversion efficiency (η) of 6.34%. For the DSSCs with CM-CNTs/TiO2 and pristine TiO2 photoanode fabricated using the same method, the values of Jsc, Voc, FF, and η were 8.56 mA 3 cm2, 0.83 V, 0.66, and 4.66% for CM-CNTs/ TiO2 as photoanode and 6.48 mA 3 cm2, 0.84 V, 0.65, and 3.63% for pristine TiO2 as photoanode. The value of photovoltaic conversion efficiency is 3.63% for the pristine TiO2 photoanode, which is lower than the previous work.29 However, in the recent publications,28,47 TiO2 was used to prepare solar cells, and low efficiencies of 3.94% (SBET = 82 m3/g)28 and 3.1% (SBET = 49.6 m3/g)47 were achieved. The low efficiency may be attributed to the fact that the SBET and the pore volume of the pristine TiO2 is low. There is no doubt that the low values of the SBET and the pore volume lead to low dye adsorption, which is invidious for enhancing the conversion efficiency. The PS-CNTs/TiO2 photoanode showed a marked improvement in photocurrent as compared with the pristine TiO2 22031
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
ARTICLE
Table 2. Characteristics of JV Curves of DSSCs with Various Working Electrodes electrode
Jsc (mA 3 cm2) Voc (V) FF η (%) improvement (%)
pristine TiO2 CM-CNTs/TiO2 PS-CNTs/TiO2
6.48
0.84
0.65 3.63
8.56 11.04
0.83 0.85
0.66 4.66 0.68 6.34
28 75
Table 3. Properties of the CNTs/TiO2 NanocompositeBased DSSCs optimum CNT process 28
loading (wt %)
efficiency improvement (%)a
(%)
blending (solgel)
0.1
3.94 f 4.71
20
blending49 (hydrolysis) blending22 (solgel)
0.8 0.1
2.88 f 4.04 3.32 f 4.97
40 49
blending29 (hydrothermally)
not given
4.90 f 7.37
50
2.87 f 4.62
61
0.5
3.63 f4.66
28
0.5
3.63 f 6.34
75
blending50 (solgel)
0.3
this work (solgel, CM-CNTs) this work (solgel, PS-CNTs) a
The efficiency (η) was measured under simulated solar light (AM 1.5, 100mW/cm2).
Figure 7. Photocurrent densityphotovoltage curves (A) and the IPCE spectra (B) of the DSSC prepared with pristine TiO2, CMCNTs/TiO2, and PS-CNTs/TiO2 photoanodes.
photoanode. The high specific surface area and improved electron transfer pathway provided by the structured CNTs led to high cell performance and fast charge transport in DSSC devices. Also, the oxygen-containing groups introduced on the surfaces of CNTs by O2 plasma treatment are hydrophilic and do not form aggregation, which can form strong complexes with dyes and thereby enhances the adsorption capacity of dyes. The Voc value of DSSCs prepared with CM-CNTs/TiO2 photoanode was lower than that of DSSCs prepared with pristine TiO2 photoanode. This may be the result of the poor adhesion of the sensitized TiO2 to the surface of CM-CNTs, which could lead to increased charge recombination. An increase in the dark current consequently caused a decrease in Voc.48 The DSSCs prepared with PS-CNTs/TiO2 photoanode showed a remarkable improvement in its performance characteristics as compared with the pristine TiO2 photoanode, providing an increase of ∼70% in photocurrent (6.48 to 11.04 mA 3 cm2) and ∼75% improvement in conversion efficiency (3.63 to 6.34%) without any decrease in Voc. These results also imply that the PS-CNTs loading, dye adsorption, and charge recombination are all important factors in controlling the photovoltaic properties and photo energy conversion efficiency. Herein, we achieved good adhesion at the interface and a high occupied surface area in the matrix, thereby produced the high performance of DSSCs with PS-CNTs/TiO2 photoanode.
In comparison to the optimum results in other studies on CNTs/TiO2-based DSSC devices,22,28,29,49,50 the PS-CNTs/ TiO2 photoanode in this study exhibits a greater improvement (∼75%, increases from 3.63 to 6.34%) for the solar conversion efficiency (as shown in Table 3). This result confirms that the photoanode prepared with the PS-CNTs with oxygen-containing groups is a promising method to prevent the aggregation of CNTs. This plasma surface modification causes CNTs to disperse well in the TiO2 matrix and thereby increases the conversion efficiency of DSSCs. To establish the utility of the working electrodes, the IPCE of DSSCs with various working electrodes were measured to identify the effects of the nanocomposites on the generation of photocurrent. As expected, a substantial improvement in IPCE was observed for the DSSCs with PS-CNTs/TiO2 photoanode (Figure 7B). The increase in IPCE was due to the high degree of dye adsorption and low charge recombination in PS-CNTs/ TiO2 photoanode. The IPCE spectrum of PS-CNTs/TiO2 photoanode showed an improvement in the conversion efficiency in the red region of light (600700 nm), suggesting that the pores introduced through the incorporation of PS-CNTs acted as light-harvesting centers. Compared to CM-CNTs/TiO2 and pristine TiO2 photoanodes, the maximum IPCE (IPCEmax) value was 52% at the wavelength of 520 nm for PS-CNTs/TiO2. This increase in IPCEmax may be due to four factors: (1) As CNTs are a 1D nanomaterial, the TiO2 particles can anchor in it well. The photoinduced electrons can be captured by the CNTs easily. Because CNTs are excellent electron carriers, the rapid photoinduced electron transport is produced, and the recombination and back electron transfer are suppressed as a result. (2) More porous structures of the photoanode are formed due to the contribution of CNTs, which increases the light scattering as shown in the SEM images (Figure 3). (3) The higher dye adsorption caused by the introduction of PS-CNTs. (4) PSCNTs disperse very well in TiO2 matrix and do not form aggregation in the blended procedure. The oxygen-containing 22032
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C
ARTICLE
Figure 8. Energy diagram illustration of charge injection from excited sensitizer (S*) into TiO2 and transport of photoinjected electrons to the electrode surface without (A) and with (B) the CNT network. Operational principle of the device: The introduced CNT paths perform as an electron acceptor and transfer the electrons quickly. The recombination and back reaction are suppressed.
groups introduced on the surfaces of PS-CNTs are hydrophilic and improve the dispersion of PS-CNTs. For the reasons mentioned above, the incorporating PS-CNTs into a TiO2 nanocrystalline photoanode might be a useful and effective method for improving the photoelectron conversion efficiency of DSSCs. 3.9. Operational Principle of the Device. Because of the excellent electrical conduction, the CNT path behaves as an electron transfer channel, which can transport the photoinduced electron quickly,17 and the energy level of CNTs is between the conduction band (CB) of TiO2 and FTO.51 Under illumination, the CB of semiconductor TiO2 receives the electrons from the photoexcited dye. Because the TiO2 is anchored with CNTs, and the CNTs are homogeneous in the system, the excited electrons are captured by the CNTs without any obstruction. The electrons transferred into the CNT network are quickly transported to the collecting electrode surface, minimizing the possibility of charge recombination at grain boundaries. The incorporation of a CNT network in the TiO2 thin film thus helps to transport electrons through its conductive scaffold and to generate higher photocurrent (Figure 8). Hence, the recombination and back reaction are suppressed.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures depicting the photoinduced electron transfer and charge equilibration between an excited semiconductor nanoparticle and CNTs and a histogram of conversion efficiency distribution of the photoanodes. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 86-551-5582788. Fax: 86-551-5591310. E-mail: xkwang@ ipp.ac.cn.
’ ACKNOWLEDGMENT The authors acknowledge the financial support from National Natural Science Foundation of China (Grant Nos. 21077107 and 20971126) and the Ministry of Science and Technology of China (Grant Nos. 2011CB933700 and 2007CB936602). ’ REFERENCES
4. CONCLUSIONS Herein we described a simple and effective method to improve the dispersion property of CNTs by introducing oxygencontaining groups using the O2 plasma technique. The PSCNTs/TiO2 composite photoanode improved the attachment of TiO2 particles and provided a number of advantages over photoanodes formed of either CM-CNTs/TiO2 or pristine TiO2 alone. The PS-CNTs/TiO2 composite photoanode provided a greater degree of dye adsorption and lower levels of charge recombination. The surface morphology and distribution of pores were improved through the incorporation of PSCNTs. The photocurrent density of DSSCs using PS-CNTs/ TiO2 photoanode increased ∼70% and the conversion efficiency increased ∼75% as compared to those of DSSCs using pristine TiO2 photoanode. The result confirmed that utilizing PS-CNTs/TiO2 to prepare the photoanode would be a promising method to prevent the aggregation of CNTs in TiO2 matrix. The plasma technique is a useful method to improve the conversion efficiency of DSSCs.
(1) Oregan, B.; Gratzel, M. Nature 1991, 353, 737. (2) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825. (3) Gratzel, M. Prog. Photovolt. Res. Appl. 2000, 8, 171. (4) Lagemaat, J.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (5) Schlichthorl, G.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 103, 782. (6) Meyer, G. J. Inorg. Chem. 2005, 44, 6852. (7) Gratzel, M. Inorg. Chem. 2005, 44, 6841. (8) Gratzel, M. J. Photochem. Photobiol. A 2004, 164, 3. (9) Heimer, T. A.; Heilweil, E. J.; Bignozzi, C. A.; Meyer, G. J. J. Phys. Chem. A 2000, 104, 4256. (10) Kamat, P. V.; Haria, M.; Hotchandani, S. J. Phys. Chem. B 2004, 108, 5166. (11) Sirbuly, D. J.; Law, M.; Yan, H.; Yang, P. J. Phys. Chem. B 2005, 109, 15190. (12) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (13) Klimov, V. I. J. Phys. Chem. B 2006, 110, 16827. (14) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. 22033
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
The Journal of Physical Chemistry C (15) Galoppini, E.; Rochford, J.; Chen, H.; Saraf, G.; Lu, Y.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. B 2006, 110, 16159. (16) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. Nano Lett. 2007, 7, 2183. (17) Kongkanand, A.; Domınguez, R. M.; Kamat, P. V. Nano Lett. 2007, 7, 676. (18) Miller, A. J.; Hatton, R. A.; Silva, S. R. P. Appl. Phys. Lett. 2006, 89, 123115. (19) Ago, H.; Petrisch, K.; Schaffer, M. S. P.; Windle, A. H.; Friend, R. H. Adv. Mater. 1999, 11, 1281. (20) Miller, A. J.; Hatton, R. A.; Chen, G. Y.; Silva, S. R. P. Appl. Phys. Lett. 2007, 90, 023105. (21) Jang, S. R.; Vittal, R.; Kim, K. J. Langmuir 2004, 2, 9807. (22) Lee, T. Y.; Alegaonkar, P. S.; Yoo, J. B. Thin Solid Films 2007, 515, 5131. (23) Wang, W.; Serp, P.; Kalck, P.; Faria, J. L. J. Mol. Catal. A 2005, 235, 194. (24) Jitianu, A.; Cacciaguerra, T.; Benoit, R.; Delpeux, S.; Beguin, F. Carbon 2004, 42, 1147. (25) Yu, Y.; Yu, J. C.; Yu, J. G.; Kwok, Y. C.; Che, Y. K.; Zhao, J. C.; Ding, L.; Ge, W. K.; Wong, P. K. Appl. Catal., A 2005, 289, 186. (26) Wang, W.; Serp, P.; Kalck, P.; Faria, J. L. Appl. Catal., B 2005, 56, 305. (27) Brown, P.; Takechi, K.; Kamat, P. V. J. Phys. Chem. C 2008, 112, 4776. (28) Yu, J. G.; Fan, J. J.; Cheng, B. J. Power Sources 2011, 196, 7891. (29) Muduli, S.; Lee, W.; Dhas, V.; Mujawar, S.; Dubey, M.; Vijayamohanan, K.; Han, S. H.; Ogale, S. ACS Appl. Mater. Inter. 2009, 9, 2030. (30) Colomer, J. F.; Piedigrosso, P.; Willems, I.; Journet, C.; Bernier, P.; Van Tendeloo, G.; Fonseca, A.; Nagy, J. B. J. Chem. Soc., Faraday Trans. 1998, 94, 3753. (31) Yu, R. Q.; Chen, L. W.; Liu, Q. P.; Lin, J. Y.; Tan, K. L.; Ng, S. C.; Chan, H. S. O.; Xu, G. Q.; Hor, T. S. A. Chem. Mater. 1998, 10, 718. (32) Hrachova, V.; Kanka, A. Vacuum 1997, 48, 689. (33) Sveningsson, M.; Morjan, R. E.; Nerushev, O. A.; Sato, Y.; Backstrom, J.; Campbell, E. E. B.; Rohumund, F. Appl. Phys. A: Mater. Sci. Process. 2001, 73, 409. (34) Lee, S.; Peng, J. W.; Liu, C. H. Carbon 2008, 46, 2124. (35) Yu, K.; Zhu, Z.; Zhang, Y.; Li, Q.; Wang, W.; Luo, L.; Yu, X.; Ma, H.; Li, Z.; Feng, T. Appl. Surf. Sci. 2004, 225, 380. (36) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; Mclaughlin, J.; Brown, N. M. D. Carbon 2005, 43, 2951. (37) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (38) Tang, T.; Lu, N.; Wang, J. K.; Ryu, S. K.; Choi, H. S. J. Phys. Chem. C 2007, 111, 1820. (39) Steen, M. L.; Butoi, C. I.; Fisher, E. R. Langmuir 2001, 17, 8156. (40) Song, Z.; Hrbek, J.; Osgood, R. Nano Lett. 2005, 5, 1327. (41) Lee, S. W.; Sigmund, W. M. Chem. Commun. 2003, 6, 780. (42) Yu, Y.; Ma, L. L.; Huang, W. Y.; Du, F. P.; Yu, J. C.; Yu, J. G.; Wang, J. B.; Wong, P. K. Carbon 2005, 43, 651. (43) Yen, C. Y.; Lin, Y. F.; Hung, C. H.; Tseng, Y. H.; Ma, C. C.; Chang, M. C.; Shao, H. Nanotechnology 2008, 19, 045604. (44) Li, X. H.; Niu, J. L.; Zhang, J.; Li, H. L.; Liu, Z. F. J. Phys. Chem. B 2003, 107, 2453. (45) Yu, J. G.; Ma, T. T.; Liu, S. W. Phys. Chem. Chem. Phys. 2011, 13, 3491. (46) Yu, J. G.; Li, Q. L.; Shu, Z. Electrochim. Acta 2011, 56, 6293. (47) Chou, C. S.; Lin, Y. J.; Yang, R. Y.; Liu, K. H. Adv. Powder Technol. 2011, 22, 31. (48) Charlier, J. C. Chem. Res 2002, 35, 1063. (49) Kim, S. L.; Jang, S. R.; Vittal, R.; Lee, J.; Kim, K. J. J. Appl. Electrochem. 2006, 36, 1433.
ARTICLE
(50) Yen, C. Y.; Lin, Y. F.; Liao, S. H.; Weng, C. C.; Huang, C. C.; Hsiao, Y. H.; Ma, C. C. M.; Chang, M. C.; Shao, H.; Tsai, M. C.; Hsieh, K. C.; Tsai, C. H.; Weng, F. B. Nanotechnology 2008, 19, 375305. (51) Yu, H.; Zhang, S.; Zhao, H.; Xue, B.; Liu, P.; Will, G. J. Phys. Chem. C 2009, 113, 16277.
22034
dx.doi.org/10.1021/jp206267x |J. Phys. Chem. C 2011, 115, 22025–22034
