TiO2 Composite Films Applied to

Article pubs.acs.org/JPCC

Effects of Graphene in Graphene/TiO2 Composite Films Applied to Solar Cell Photoelectrode Y. Kusumawati,†,‡ M. A. Martoprawiro,‡ and Th. Pauporté*,† †

Institut de Recherche Chimie-paristech, IRCP-ENSCP-CNRS UMR8247, 11 rue P. et M. Curie, 75005 Paris, France Inorganic and Physical Chemistry Division, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung (ITB), Jl. Ganesha 10, Bandung 40132, Indonesia

‡

S Supporting Information *

ABSTRACT: A simple and effective method has been developed to prepare a composite porous film that incorporates graphene sheets and anatase TiO2 nanoparticles. After sensitization, the films have been investigated as dyesensitized solar cell photoelectrodes. The cell performances showed that the incorporation of an optimized graphene content of 1.2 wt % increases the power conversion efficiency by 12% due to the enhancement of the short-circuit current density (Jsc). The photoelectrodes have been characterized by various techniques, and the cell functioning has been studied by impedance spectroscopy over a large applied potential range. The electronic structure, charge carrier lifetime (τn), transport/collection time (τtr), and electron transport parameters of the layers have been determined. We conclude that photoelectrodes with and without graphene show no limitation due to the transport of the I−/I3− redox shuttle. The rate of the charge transfer (recombination) parasitic reaction is unchanged with the presence of graphene. The electron transport in the photoelectrode is significantly faster for the composite film due to a quantified 60% increase in the layer conductivity. However, we have also shown that the charge-carrier collection efficiency is very high even without graphene, and that this parameter is not key to explain the cell-performance enhancement. Graphene also increases the film specific internal surface area. The composite films have a higher dye loading. They exhibit a better solar light absorption and a Jsc enlargement. liquid electrolyte.17 The research on improving the DSSC performances is ongoing because there are still many possibilities to overcome the cell shortcomings by optimizing each of its components. Adding graphene into the nanoparticle TiO2 porous photoanode network has been shown to be of great interest with the significant improvement of the cell efficiency by increasing the current density (Jsc) of the solar cell without significant decrease in the open circuit voltage (Voc) and fill factor (FF).20−32 Various techniques have been developed to prepare TiO2/ graphene composite such as sol−gel or dispersion techniques,23,26,29 solvothermal approaches,30 hydrothermal techniques,25,28,31−33 or electrospinning.22 It has been shown that graphene oxide is the best starting material to prepare the composite. By using graphene oxide it is possible to ensure the intimate attachment between graphene and TiO2.21,25 A second step is the reduction of graphene oxide in graphene. Various techniques have been reported including chemical,29,32 UVassisted photocatalytic processes,20 and thermal reduction treatments.21,34 The latter is of special interest in the case of

1. INTRODUCTION Graphene is a real 2-D material that can be prepared with a single atomic layer thickness. Since its experimental discovery in 2004,1 this material has drawn much attention, and the preparation, characterizations, and applications of graphene sheets have given rise to a large amount of literature.2−15 Graphene sheets can be prepared by various approaches including mechanical exfoliation,1 epitaxial growth,9,10 chemical vapor deposition,11,12 and chemical exfoliation.13,14 As a zero bandgap material with electrons that are just like massless relativistic particles, this material exhibits excellent electrical conduction properties in two dimensions and a high mobility of charge carriers (200 000 cm2 V−1 s−1). It is a strong compound with a breaking strength of 42 N·m−1, and it has a high thermal conductivity.2 Graphene can also be produced as very thin sheets with low visible light absorption properties and can be used as conducting transparent layer.6,15 It has also an extremely high theoretical specific surface area (SSA) of 2600 m2·g−1,7 and its lattice structure is stable up to 1500 °C.8 Dye-sensitized solar cells (DSSCs) have attracted considerable attention in recent years due to their low production cost and relatively high conversion efficiency.16−19 The most typical DSSC consists of a nanoparticle anatase TiO2 photoelectrode sensitized by an organic or organo-metallic dye and a platinum counter-electrode separated by an iodide−triiodide (I−/I3−) © 2014 American Chemical Society

Received: March 9, 2014 Revised: April 15, 2014 Published: April 21, 2014 9974

dx.doi.org/10.1021/jp502385p | J. Phys. Chem. C 2014, 118, 9974−9981

The Journal of Physical Chemistry C

Article

the photoanode preparation because the nanoparticle TiO2 film requires a final annealing heat treatment.21 The origin of the beneficial effect of graphene on solar cell Jsc remains unclear. (i) Reduced graphene oxide (RGO) has been reported to increase the photoelectrode surface area23,31 and to improve their dye loading.23,26,31,35 Increasing the surface area causes more dye attached to the surface of TiO2/graphene composite compared with pure TiO2 and increases the dye loading. (ii) The work function of graphene (−4.4 to 4.5 eV) is localized below the conduction band of TiO2 (−4.2 eV) and may favor the charge extraction from TiO2.23,35 (iii) Graphene is also reported to give higher conductive layers23 and then to enhance the charge transport properties of the photoanode.21,26,31 (iv) It has also been described to act as a structuring agent that facilitates the production of larger pores that improve the redox shuttle transport by diffusion.26 Although many reports have shown the improvement of DSSC performances by the addition of graphene, the effect of this additive on the cell functioning still needs to be clarified. Impedance spectroscopy (IS) is a powerful technique to extract the key cell functioning parameters and to understand the electron dynamics in the photoelectrode.36,37 In the present work, we have prepared anatase TiO2/graphene composite films using a simple sol−gel technique and graphene oxide single layer sheets (SGO) as the graphene source. The aim of the present study is to clarify the role of RGO in cell improvement. For this purpose, the dye loading determination as well as BET (Brunauer, Emmett and Teller) and optical measurements have been done and completed by an extensive IS study of the solar cells. The electronic structure, charge carrier lifetime (τn), transport/collection time (τtr), and electron transport parameters have been determined. We have then been able to accurately evaluate the real contribution of points i−iv on the observed enhancement of TiO2/RGO composite DSSC performances.

Table 1. Effect of Composite SGO Content on the Specific Internal Surface Area Determined by BET (SSA), on the Sensitized Layer Dye Concentration, and on the Maximum of Absorbance at 530 nm sample

graphene content (wt %)

SSA (m2/cm3)

TGr0 TGr6 TGr12 TGr30

0 0.6 1.2 3

114 115 124 110

dye concentration (mM) 95 111

absorbance at 530 nm (cm−1) 950 992 1235

was coated on the conducting glass substrates by the doctor blading technique, relaxed, and dried at 125 °C for 5 min. The step was repeated several times to achieve the accurate film thickness. The films were then annealed at 500 °C for 15 min. A final TiCl4 treatment consisted of immersing the TiO2 films in a 40 mmol L−1 TiCl4 solution at 70 °C for 30 min and annealing again at 500 °C. The film morphologies were examined with a high-resolution Ultra 55 Zeiss FEG scanning electron microscope (SEM) at an acceleration voltage of 10 kV. Their thicknesses were measured with a Dektak 6 M stylus profiler. For the film structural characterizations, a high-resolution X-ray diffractometer Siemens D5000 operated at 40 kV and 45 mA using the Cu Kα radiation with λ = 1.5406 Å was used. The Fourier transform infrared (FTIR) curves were measured with a Tensor 27 apparatus from Bruker. The investigated samples were mixed with dry KBr, pressed as a pellet, and measured in a transmission mode. The BET specific internal surface area of the films was determined from the adsorption isotherms of Kr at the boiling point of liquid nitrogen (∼77 K) using a Micromeritics ASAP 2010 apparatus.42,43 The film porosities were estimated by mass measurements equal to 0.62. The optical film properties (total transmission and total reflection) were recorded with a Carry 5000 UV−vis-NIR spectrophotometer equipped with an integrating sphere. 2.3. Solar Cell Preparation and Characterizations. The composite layers were immersed upon cooling in 0.5 mmol L−1 N719 dye in a mixture of acetonitrile and tert-butanol (1:1) and one equivalent of tetrabutylammonium hydroxide in the dark at room temperature for 24 h. For the counter electrode preparation, FTO glass substrates were cleaned by ultrasound in acetone and ethanol for 5 min each. Then, they were treated in a furnace for 30 min at 450 °C to remove organic contaminants. The Pt catalyst was deposited on the FTO glass by coating with a drop of H2PtCl6 solution (6 mg in 1mL of ethanol) subsequently heated at 400 °C for 20 min. This step was repeated once. The two electrodes were sealed with a 50 μm hot melt spacer (Surlyn, DuPont), and the internal space was filled with the electrolyte through a hole drilled in the counter electrode, which was subsequently sealed with Surlyn and an aluminum foil. The electrolyte employed was a solution of 0.6 mol·L−1 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 mol·L−1 LiI, 0.05 mol·L−1 I2, 0.10 mol·L−1 guanidinium thiocyanate, and 0.5 mol·L−1 4-tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85/15 volume ratio). The dye loading of the photoelectrode was determined by spectrophotometry after complete dye desorption in a 0.1 mol·L−1 KOH solution. The I−V curves were recorded by a Keithley 2400 digital sourcemeter, using a 0.01 V·s−1 voltage sweep rate. The solar cells were illuminated with a solar simulator (Abet Technology

2. EXPERIMENTAL SECTION 2.1. Film Preparation. A viscous paste was created using anatase TiO2 particles mixed with single-layer graphene oxide (SGO) sheets prepared by the modified Hummer’s method.38,39 1 g of TiO2 nanoparticles prepared according to ref 40, 4.06 g of terpineol, 5 mL of ethanol, and various amounts of SGO were mixed together under vigorous agitation. 281 mg of ethyl cellulose (EC) powder (5−15 mPa·s) and 219 mg of EC (30−50 mPa·s) were dissolved in 4.5 mL of ethanol. The two solutions were then mixed together and sonicated several times using an ultrasonic horn. Ethanol and water were then removed from the solution in a rotary-evaporator at an initial temperature of 58 °C to create a viscous paste.19,41 The initial SGO weight to the total SGO plus TiO2 weight ratio was varied at 0, 0.6, 1.2, and 3%. The related prepared porous layers are denoted TGr0, TGr6, TGr12, and TGr30, respectively, as summarized in Table 1. FTO glass substrates (TEC15, Pilkington) were cleaned with soap and rinsed with distilled water. They were subsequently treated in an ultrasonic bath in acetone for 5 min and in ethanol for 5 min. The substrates were dried and placed in a 450 °C furnace for 30 min. After cooling, they were immersed in a 40 mmol L−1 aqueous solution of TiCl4 at 70 °C for 30 min and washed with water and ethanol. The number of TiCl4 treatment of FTO/glass substrate was investigated and the best cell performance was achieved by doing it twice, as shown in Figure S1 of the Supporting Information. A layer of TiO2/SGO paste 9975



Article

Sun 2000) filtered to mimic air mass AM 1.5G conditions. The illuminated surface was delimited by a black mask. The power density was calibrated to 100 mW.cm−2 by the use of a reference silicon solar cell. The cells, with a surface area of 0.36 cm2, were characterized by impedance spectroscopy (IS). The spectra were measured in the dark, over a large potential range, by a Solartron FRA1255 frequency response analyzer coupled with a PAR273 EGG potentiostat. The AC signal was 10 mV and the frequency range was 100 kHz to 0.05 Hz. The spectra were fitted and analyzed using the Zview modeling software (Scribner). The electrochemical IS results were corrected for IR-Drop, as described elsewhere.36 The corrected voltage is noted Vcor. The reproducibility of the presented effects of graphene content on the cell performances and impedance response has been checked on several cells prepared with several paste batches.

3. RESULTS AND DISCUSSION The interaction between TiO2 nanoparticles and RGO has been described to occur through physisorption, electrostatic binding, or charge transfer interaction.28 In the present work, the coupling of RGO and TiO2 has been achieved by a long and vigorous mixing of SGO and TiO2 in solution including an ultrasonic treatment and subsequent thermal annealing of the mixture to get a composite film. Finally, a treatment in a TiCl4 solution was performed to improve the intimate mixing of the two compounds, enhance the nanoparticle necking, and fully cover the RGO sheets. The reduction of SGO to RGO is an important step to ensure the interaction between TiO2 and graphene, and several processes for reducing SGO have been reported in the literature.20−23 In the present case, SGO reduction was achieved during the thermal treatments of the composite layers.21 FTIR measurements were conducted to control the RGO/ TiO2 interaction and the efficient reduction of SGO. Figure 1 displays the transmission spectra of SGO, of the initial layer before annealing, and of the porous layer after annealing. The peak 4 at 1650 cm−1 is assigned to the skeletal CC vibration in graphene and the peak 5 at ∼1720 cm−1 is the CO (carbonyl) stretching contribution of COOH groups.25,44,45 Figure 1 shows that peak 5, due to carbonyl groups, disappears after annealing. The CC peak remains present, as expected for graphene reduction.25,44,46 A small vibration peak is observed at 1450 cm−1 (peak 6) for the composite film, assigned to C−O; it may indicate the interaction between TiO2 and the RGO carbon skeleton.25 If we compare the SGO spectrum and that of TiO2/RGO, we observe an absorption band in the 2950−2850 cm−1 range due to C−H stretching (peak 7). It is absent on the SGO reference spectrum and confirms the reduction of SGO to RGO.25,42 The absorption band at 600−900 cm−1 (peak 1) of TiO2/RGO is induced by the Ti−O−Ti stretching vibration modes and is characteristic of crystalline TiO2. The layers with and without RGO were observed by SEM (Figure S2 in the Supporting Information). The films are porous and made of interconnected nanoparticles. No significant morphological change could be found with graphene addition in the TiO2 layer. In both cases, XRD patterns are indexed by the anatase TiO2 phase (Figure S3 in the Supporting Information).19 No diffraction peak at 26.1° assigned to the graphite phase could be observed in Figure S3 in the Supporting Information because RGO is present at low concentration in the composite layer. The SSAs for films

Figure 1. (A) FTIR spectra of TiO2/graphene composite after (a) and before (b) annealing. (c) Ethylcellulose and (d) SGO. (B) Zoom view of panel A.

containing various graphene concentrations have been determined by krypton adsorption measurements44 and are reported in Table 1. The SSA maximum is found for 1.2 wt % of initial SGO. High concentration of graphene is detrimental for the SSA parameter. In Figure 2, the absorbance spectra show that the higher SSA of TGr12 layer corresponds to a higher absorbance of the sensitized layers. The N719 dye concentrations in TGr0 and TGr12 films after sensitization have been titrated by spectrophometry. They are reported in

Figure 2. Absorbance spectra of the TiO2/graphene composite films after sensitization by N719. TGr0 (a), TGr6 (b), and TGr12 (c). 9976



Article

Table 2. Effect of Composite SGO Content on the Cell Performance Characteristics (100 mW·cm−2, AM 1.5G filtered illumination) and on α, T0, and β Parameters Determined by IS Technique sample

a

graphene content (wt %)

TGr0

0

TGr6 TGr12

0.6 1.2

TGr30

3.0

VOC (V)

JSC (mA·cm−2)

FF (%)

η (%)

0.71 0.69a 0.71 0.71 0.68a 0.70

11.0 13.0a 11.5 13.6 14.4a 11.6

74.1 74.8a 71.6 66.6 76.8a 69.4

5.78 6.70a 5.83 6.42 7.49a 5.65

α

T0 (K)

β

0.28a

1046a

0.62a

0.28a

1046a

0.66a

Cells investigated by impedance spectroscopy.

wavelength region above 750 nm where light is no more absorbed by the N719 dye. IS is a powerful technique that has been employed to a large extent to investigate the kinetic of electrochemical and photoelectrochemical processes occurring in many functional systems, including DSSCs, in which coupled processes are involved.47−54 The impedance spectra of TGr0 reference cell and TGr12 cell (which yielded the best performances) were measured in the dark over a large applied potential range to extract the kinetic data of the photoelectrodes at variable densities of state (noted g). Examples of spectra are presented in Figure S5 in the Supporting Information. They all showed a characteristic low-middle frequency semicircle due to the resistance to charge transfer (recombination) (Rct) across the sensitized oxide−electrolyte interface coupled to the total electrode capacitance (denoted as Cμ). At high frequency, a second semicircle was found due to the resistance (RPt) and capacitance (CPt) of the charged counter-electrode/electrolyte interface. In the intermediate frequency range, a ∼45° straightline segment could be observed at not too high applied voltage, which is characteristic of the electron transport by diffusion and is modeled by a transport resistance, Rtr in Figure 4a. The highfrequency series resistance, Rs, is due to the electrical contacts.

Table 1 and show a significantly higher dye loading in the presence of RGO. DSSCs have been prepared with composite layers containing various amounts of graphene. The I−V curves are presented in Figure S4 in the Supporting Information, and the cell characteristics are gathered in Table 2. In Figure 3, the cell

Figure 3. Effect of graphene content on the solar cell characteristics (AM 1.5G filtered 1 sun illumination). (a) η and Jsc; (b) Voc and FF.

parameters are displayed as a function of SGO content. The best efficiency is found for TGr12 cell with a reproducible overall conversion efficiency increase of 12% compared with bare TiO2. The efficiency curve presents a volcano shape that follows the variation of Jsc with the graphene content (Figure 3a). The Voc is almost constant, and the FF slightly decreases with SGO content (Figure 3b). The volcano shape is in good agreement with previous reports.21 η and Jsc decrease at high graphene content due to visible/near-infrared light absorption by RGO that shields light absorption by the dye and reduces the number of photogenerated electrons.21 The light absorption by graphene is confirmed in Figure 2 in the

Figure 4. (a) Equivalent electrical circuit used to model the impedance spectra. (b) Variation of Rct (triangles) and Cμ (dots) with the corrected applied voltage. The red symbols are for the TGr0 cell and the blue symbols are for the TGr12 cell. 9977



Article

The full equivalent circuit used to fit the spectra is presented in Figure 4a. We note that for TGr0 and TGr12 cells the IS spectra did not exhibit a clear Warburg loop at low frequency, even at high applied voltage. (See Figure S5a in the Supporting Information.) This clearly excludes a performance limitation due to diffusion of the I−/I3− redox shuttle. Therefore, point iv in the Introduction cannot explain the solar cell improvement in the TiO2/graphene composite photoelectrode. Cμ is a chemical capacitance due to trap states localized below the conduction band minimum. It is commonly believed that these states are mainly located at or near the particle surface and at the necks between adjacent particles.55 Cμ is reported as a function of Vcor in Figure 4b. It varies more or less exponentially, and the experimental data have been fitted by the relationship ⎡ qV ⎤ ⎡ qV ⎤ Cμ = C0, μ exp⎢α cor ⎥ = C0, μ exp⎢ cor ⎥ ⎣ kBT ⎦ ⎣ kBT0 ⎦

treatment aims at covering graphene with a thin layer of TiO2 and then avoids the recombination parasitic reaction. Rct show a logarithmic variation that follows the relationship ⎡ qV ⎤ R ct = R 0,ct exp⎢ −β cor ⎥ kBT ⎦ ⎣

(3)

where β can be considered an empirical estimation of the reaction order. β values determined from the fits are gathered in Table 2 and are 0.62 and 0.66 with and without graphene, respectively. β value lower than 1 is an empirical way that is believed to describe sublinear recombination kinetics that takes into account the fact that electrons may be transferred from occupied levels located in the energy gap.58,59 From Rct and Cμ, the lifetimes of electrons in the photoelectrodes have been calculated according to τn = RctCμ and are reported in Figure 6a. The presence of graphene is not

(1)

where kB is the Boltzmann constant, T the absolute temperature, and q the elementary charge. α is a parameter that accounts for the depth of the trap energy distribution below the conduction band. Both with and without RGO in the photoelectrode, α values are similar and equal to 0.28 (Table 2). The α value for anatase TiO2 solar cells is classically reported in the literature to range between 0.2 and 0.4.56,57 According to eq 1, the trap state depth can also be expressed as a temperature, T0, of 1046 K (Table 2). From Cμ, the density of states due to the traps, g, has been calculated g (Vcor) =

Cμ(Vcor) qAd(1 − p)

(2)

where A is the geometric area of the cell, d is the oxide layer thickness (11 μm), and p is the film porosity (0.62). The g function presented in Figure 5 is similar in the presence and in

Figure 6. (a) Variation of τn (full symbols) and τtr (open symbols) with the corrected applied voltage. Red symbols and lines: TGr0 cell; blue symbols and lines: TGr12 cell. (b) Variation of the photoelectrode conductivity with the density of states, g. Red: TGr0 cell; Blue: TGr12 cell.

Figure 5. Distribution of the density of states, g(Vcor). The dot symbols are for the TGr0 cell and the diamond symbols for the TGr12 cell.

detrimental for the recombination, and the same τn is found for TGr0 and TGr12 cells. Again, because of the TiCl4 final treatment, we can suppose that the SGO sheets are covered by TiO2 and are not involved in recombination processes. Some papers have reported that because of the respective band positions between TiO2 and graphene, electron transfer must be favored between these two components (point ii in the Introduction).23,35 A reduction of the recombination parasitic

the absence of SGO; therefore, SGO has no clear effect on the density of state in the layer. The curve extrapolation lead to a bottom of the TiO2 conduction band localized at ∼0.75 to 0.80 V. (See ref 36 for details.) Rct is plotted versus Vcorr in Figure 4b for the two cells. The Rct values are very similar; then, the rate of the charge transfer (recombination) parasitic reaction is unchanged with the presence of graphene. The final TiCl4 9978



Article

order of 11 micrometers, and they explain the very high ηcoll. We note longer values found in the presence of SGO.

reaction may then be expected if the SGO is isolated from the electrolyte layer by TiO2. However, our IS data do not show such an effect. The transport times have been calculated according to τtr = RtrCμ and are plotted as a function of the DOS in Figure 6a. Graphene has a strong influence on this parameter because much shorter transport times are found in its presence. The conductivity of TiO2 and composites’ material in the photoelectrodes was determined from the charge transport resistance by σn =

d A(1 − p)R tr

4. CONCLUSIONS In conclusion, we have developed a simple and effective technique for the preparation of mesoporous TiO2/RGO composite films and integrated the layers in DSSCs. The effect of RGO content on the cell performances has been investigated, and we have shown that the use of 1.2 wt % of graphene reproducibly enhances the solar cell overall conversion efficiency by ∼12%. The interaction between TiO2 and graphene and the thermal reduction of SGO to RGO have been shown. The IS technique has been used to closely examine the effect of RGO on the cell functioning. The photoelectrodes with and without graphene show no limitation due to the transport of the I−/I3− redox shuttle. The rate of the charge transfer (recombination), a parasitic reaction, is unchanged with the presence of graphene. The electron transport in the photoelectrodes is significantly faster for the composite film due to a quantified 60% increase in the layer conductivity in the presence of 1.2 wt % RGO. However, we have also shown that the charge collection efficiency is very high, even without RGO, and that this parameter is not key to explain the cell performance enhancement. RGO increases the film SSA. Therefore, a higher dye loading occurs in the presence of graphene and gives rise to a better solar light absorption and to the enlargement of Jsc. However, we have noted that for RGO content above 1.2 wt % light absorption by graphene becomes detrimental and shields the light absorption by the dye.

(4)

In Figure 6b, we compare the conductivity of TGr0 and TGr12 photoelectrodes as a function of g. The conductivity increases rapidly with this parameter. Moreover, the IS confirms that introducing RGO in the layer has a beneficial effect on the material conductivity. From Figure 6b, we measured an increase in conductivity by 60% due to the high 2D conductivity of reduced graphene sheets. The charge collection efficiency has been calculated using the following classical relationship60,61 1 ηcoll = τ 1 + τtr

(

n

)

(5)

The results are displayed in Figure 7. The conductivity improvement of composite photoelectrode due to RGO favors

■

ASSOCIATED CONTENT

S Supporting Information *

Effect of FTO substrate treatment on the I−V curves. SEM views of the films. I−V curves of the TiO2/graphene DSSCs. XRD patterns of TiO2/graphene layers. Typical IS spectra of TiO2/graphene DSSCs. Dn versus Vcor and Ln versus Vcor curves. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: (33)1 55 42 63 83. E-mail: [email protected].

Figure 7. Effect of corrected applied voltage on the charge collection efficiency, ηcoll.

Notes

The authors declare no competing financial interest.

■

the charge transport and then the photoelectrode charge collection efficiency (point iii in the Introduction). However, this parameter is very high both in the presence and in the absence of graphene. Therefore, when the I−/I3− redox shuttle is used, the cell Jsc and η improvements cannot be explained by the higher electrical conductivity of the photoelectrode. It is explained by the larger SSA and higher dye loading previously shown. Using TiO2/RGO composite photoelectrode enlarges the visible-near-infrared light absorption by the sensitizer. From τn and τtr measured by IS, we have also estimated the effect of RGO on the diffusion coefficient of the electrons, noted Dn, and on the mean diffusion length of the charge carriers, noted Ln. The results are reported in Figures S6 and S7 in the Supporting Information, respectively. As expected, Dn is enlarged in the presence of RGO. Ln values are always much larger than the photoelectrode film thickness, which is on the

ACKNOWLEDGMENTS Y.K. acknowledges Campus France and the Higher Education Ministry of Indonesian (DIKTI) government for financial support in the framework of the DDIP collaboration programme. Jiri Rathousky (J. Heyrovsky institute of Physical-Chemistry, Prague, Czech Republic) is acknowledged for Kr adsorption experiments.

■

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388.

9979



Article

(3) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (4) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611−622. (5) Novoselov, K. S. Nobel Lecture: Graphene: Materials in the Flatland. Rev. Modern Phys. 2011, 83, 837−849. (6) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308− 1308. (7) Yoo, E.; Kim, J.; Hosono, E.; Zhou, H.; Kudo, T.; Honma, I. Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries. Nano Lett. 2008, 8, 2277− 2282. (8) Bai, J.; Huang, Y. Fabrication and Electrical Properties of Graphene Nanoribbons. Mater. Sci. Eng., R 2010, 70, 341−353. (9) Robinson, J.; Weng, X.; Trumbull, K.; Cavalero, R.; Wetherington, M.; Frantz, E.; LaBella, M.; Hughes, Z.; Fanton, M.; Snyder, D. Nucleation of Epitaxial Graphene on SiC(0001). ACS Nano 2010, 4, 153−158. (10) Huang, H.; Chen, W.; Chen, S.; Wee, A. T. S. Bottom-up Growth of Epitaxial Graphene on 6H-SiC(0001). ACS Nano 2008, 2, 2513−2518. (11) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706−710. (12) Park, H. J.; Meyer, J.; Roth, S.; Skakalova, V. Growth and Properties of Few-Layer Graphene Prepared by Chemical Vapor Deposition. Carbon 2010, 48, 1088−1094. (13) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558−1565. (14) Liang, Y.; Frisch, J.; Zhi, L.; Norouzi-Arasi, H.; Feng, X.; Rabe, J. P.; Koch, N.; Müllen, K. Transparent, Highly Conductive Graphene Electrodes from Acetylene-Assisted Thermolysis of Graphite Oxide Sheets and Nanographene Molecules. Nanotechnology 2009, 20, 434007. (15) Zhang, H.; Babichev, A. V.; Jacopin, G.; Lavenus, P.; Julien, F. H.; Egorov, A. Y.; Zhang, J.; Pauporté, T.; Tchernycheva, M. Characterization and Modeling of a ZnO Nanowire Ultraviolet Photodetector with a Graphene Transparent Contact. J. Appl. Phys. 2013, 114, 234505. (16) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Film. Nature 1991, 353, 737. (17) Kalyanasundaram, K. Dye-Sensitized Solar Cells; EPFL Press: Lausanne, Switzerland, 2010. (18) Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, K.; Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629. (19) Magne, C.; Dufour, F.; Labat, F.; Lancel, G.; Durupthy, O.; Cassaignon, S.; Pauporté, T. Effects of TiO2 Nanoparticle Polymorphism on Dye-Sensitized Solar Cell Photovoltaic Properties. J. Photochem. Photobiol., A 2012, 232, 22−31. (20) Kim, S. R.; Parvez, M. K.; Chhowalla, M. UV-Reduction of Graphene Oxide and its Application as an Interfacial Layer to Reduce the Back-Transport Reactions in Dye-Sensitized Solar Cells. Chem. Phys. Lett. 2009, 483, 124−127. (21) Wang, H.; Leonard, S. L.; Hu, Y. H. Promoting Effect of Graphene on Dye-Sensitized Solar Cells. Ind. Eng. Chem. Res. 2012, 51, 10613−10620. (22) Peining, Z.; Nair, A. S.; Shengjie, P.; Shengyuan, Y.; Ramakrishna, S. Facile Fabrication of TiO2-Graphene Composite with Enhanced Photovoltaic and Photocatalytic Properties by Electrospinning. ACS Appl. Mater. Interfaces 2012, 4, 581−585.

(23) Tang, Y. B.; Lee, C. S.; Xu, J.; Liu, Z. T.; Chen, Z. H.; He, Z.; Cao, Y. L.; Yuan, G.; Song, H.; Chen, L.; Cheng, H. L.; Zhang, W. J.; Bello, I.; Lee, S. T. Incorporation of Graphene in Nanostructured TiO2 Films via Molecular Grafting for Dye-Sensitized Solar Cell Application. ACS Nano 2010, 4, 3482−3488. (24) Song, J.; Yin, Z.; Yang, Z.; Amaladass, P.; Wu, S.; Ye, J.; Zhao, Y.; Deng, W. Q.; Zhang, H.; Liu, X. W. Enhancement of Photogenerated Electron Transport in Dye-Sensitized Solar Cells with Introduction of a Reduced Graphene Oxide-TiO2 Junction. Chem.Eur. J. 2011, 17, 10832−10837. (25) Shen, J.; Yan, B.; Ma, H.; Li, N.; Ye, M. One Step Hydrothermal Synthesis of TiO2- Reduced Graphene Oxide Sheets. J. Mater. Chem. 2011, 21, 3415−3421. (26) Fan, J.; Liu, S.; Yu, J. Enhanced Photovoltaic Performance of Dye-Sensitized Solar Cells Based on TiO2 Nanosheets/Graphene Composite Films. J. Mater. Chem. 2012, 22, 17027−17036. (27) Kim, S. R.; Parvez, M. D. K.; Chhowalla, M. UV-Reduction of Grapheme Oxide and Its Application as an Interfacial Layer to Reduce the Back Transport Reactions in Dye-Sensitized Solar Cells. Chem. Phys. Lett. 2009, 483, 124−127. (28) Cheng, G.; Akhtar, M. S.; Yang, O.-B.; Stadler, F. J. Novel Preparation of Anatase TiO2-Reduced Graphene Oxide Hybrids for High Performance Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 6635−6642. (29) Yang, N.; Zhai, J.; Wang, D.; Chen, Y.; Jiang, L. TwoDimensional Graphene Bridges Enhanced Photoinduced Charge Transport in Dye-Sensitized Solar Cells. ACS Nano 2010, 4, 887−894. (30) He, Z.; Guai, G.; Guo, C.; Loo, J.; Li, C. M.; Tan, T. T. Nanostructure Control of Graphene-Composited TiO2 by a One Step Solvothermal Approach for High Performance Dye-Sensitized Solar Cells. Nanoscale 2011, 3, 4613−4616. (31) Fang, X.; Li, M.; Guo, K.; Liu, X.; Zhu, Y.; Sebo, B.; Zhao, X. Graphene-Compositing Optimization of the Properties of Dyesensitized Solar Cells. Sol. Energy 2014, 101, 176−181. (32) Xiang, Q.; Yu, J.; Jaroniec, M. Enhanced Photocatalystic H2 production Activity of Graphene-Modified Titania Nanosheets. Nanoscale 2011, 3, 3670−3678. (33) Xiang, Q.; Yu, J. Graphene-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2013, 4, 753−759. (34) Jung, I.; Dikin, D. A.; Piner, R. D.; Ruoff, R. S. Tunable Electrical Conductivity of Individual Graphene Oxide Sheets Reduced at ‘Low’ Temperatures. Nano Lett. 2008, 8, 4283−4287. (35) Sun, S.; Gao, L.; Liu, Y. Enhanced Dye-Sensitized Solar Cell Using Graphene-TiO2 Photoanode Prepared by Heterogeneous Coagulation. Appl. Phys. Lett. 2010, 96, 083113. (36) Magne, C.; Moehl, T.; Urien, M.; Grätzel, M.; Pauporté, T. Effects of ZnO Film Growth Route and Nanostructure on Electron Transport and Recombination in Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 2079. (37) Pauporté, T.; Magne, C. Impedance Spectroscopy Study of N719-sensitized ZnO-Based Solar Cells. Thin Solid Films 2014, DOI: 10.1016/j.tsf.2013.11.121. (38) Hummer, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (39) Guo, C. X.; Yang, H. B.; Sheng, Z. M.; Lu, Z. S.; Song, Q. L.; Li, C. M. Layered Graphene/Quantum Dots for Photovoltaic Devices. Angew. Chem., Int. Ed. 2010, 49, 3014. (40) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Grätzel, C.; Nazeeruddin, M. K.; Grätzel, M. Fabrication of thin film dye-sensitized solar cell with solar to electric power conversion efficiency over 10%. Thin Solid Films 2008, 516, 4613. (41) Magne, C.; Cassaignon, S.; Lancel, G.; Pauporté, T. Brookite TiO2 Nanoparticle Films for Dye-Sensitized Solar Cells. ChemPhysChem 2011, 12, 2461−2467. (42) Pauporté, T.; Rathousky, J. Electrodeposited Mesoporous ZnO Thin Films as Efficient Photocatalysts for the Degradation of Dye Pollutants. J. Phys. Chem. C 2007, 111, 7639−7644. (43) Pauporté, T.; Rathousky, J. Growth Mechanism and Photocatalytic Properties for Dye Degradation of Hydrophobic Mesoporous 9980



Article

ZnO/SDS Films prepared by Electrodeposition. Microporous Mesoporous Mater. 2009, 117, 380−385. (44) Jiang, B.; Tian, C.; Zhou, W.; Wang, J.; Xie, Y.; Pan, O.; Ren, Z.; Dong, Y.; Fu, D.; Han, J.; Fu, H. In Situ Growth of TiO2 in Interlayers of Exanded Graphite for the Fabrication of TiO2-Graphene with enhanced Photocatalytic Activity. Chem.Eur. J. 2011, 17, 8379− 8387. (45) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The Structure of Suspended Graphene Sheets. Nature 2007, 446, 60−63. (46) Desai, J.; Alexandrer, K.; Riga, A. Characterization of Polymeric Dispersions of Dimenhydrinate in Ethyl Cellulose for Controlled Release. Int. J. Pharm. 2006, 308, 115−123. (47) Pauporté, T.; Durand, R. Impedance Spectroscopy Study of Electrochromism in Sputtered Iridium Oxide Films. J. Appl. Electrochem. 2000, 30, 35−41. (48) Pauporté, T.; Andolfatto, F.; Durand, R. Some Electrocatalytic Properties of Anodic Iridium Oxide Nanoparticles in Acidic Solution. Electrochim. Acta 1999, 45, 431−439. (49) Pauporté, T.; Finne, J.; Kahn-Harari, A.; Lincot, D. Growth by Plasma Electrolysis of Zirconium Oxide films in the Micrometer Range. Surf. Coat. Technol. 2005, 199, 213−219. (50) Bisquert, J. Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer. J. Phys. Chem. B 2002, 106, 325. (51) Bisquert, J.; Vikhrenko, V. S. Interpretation of the Time Constants Measured by Kinetic Techniques in Nanostructured Semiconductor Electrodes and Dye-Sensitized Solar Cells. J. Phys. Chem. B 2004, 108, 2313. (52) Wang, Q.; Moser, J. E.; Grätzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 14945−14953. (53) Adachi, M.; Sakamoto, M.; Jiu, J.; Ogata, Y.; Isoda, S. Determination of Parameters of Electron Transport in Dye-Sensitized Solar Cells using Electrochemical Impedance Spectroscopy. J. Phys. Chem. B 2006, 110, 13872−13880. (54) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, E.; Kuang, D.; Zakeeruddin, S. M.; Grätzel, M. Correlation Between Photovoltaic Performance and Impedance Spectroscopy of DyeSensitized Solar Cells based on Ionic Liquids. J. Phys. Chem. C 2007, 111, 6550−6560. (55) Park, K.; Zhang, Q.; Myers, D.; Cao, G. Charge Transport Properties in TiO2 Network with Different Particle Sizes for Dye Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 1044. (56) Peter, L. M. Characterization and Modeling of Dye-Sensitized Solar Cells. J. Phys. Chem. C 2007, 111, 6601−6612. (57) Wang, Q.; Ito, S.; Grätzel, M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J.; Bessho, T.; Hachiro, I. Characteristics of High Efficiency Dye-Sensitized Solar Cells. J. Phys. Chem. B 2006, 110, 25210−25221. (58) Bisquert, J.; Mora-Sero, I. Simulation of Steady-State Characteristics of Dye-Sensitized Solar Cells and the Interpretation of the Diffusion Length. J. Phys. Chem. Lett. 2010, 1, 450−456. (59) Villanueva-Cab, J.; Wang, H.; Oskam, G.; Peter, L. M. Electron Diffusion and Back Reaction in Dye-Sensitized Solar Cells: The Effect of Nonlinear Recombination Kinetics. J. Phys. Chem. Lett. 2010, 1, 748−751. (60) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595. (61) Guérin, V. M.; Magne, C.; Pauporté, T.; Le Bahers, T.; Rathousky, J. Nanoporous Electrodeposited versus Nanoparticulate ZnO Porous Films of Similar Roughness for Dye Sensitized Solar Cell Application. ACS Appl. Mater. Interfaces. 2010, 2, 3677−3685.

9981


TiO2 Composite Films Applied to

Recommend Documents