ARTICLE pubs.acs.org/JPCC
Understanding the Enhancement in Photoelectrochemical Properties of Photocatalytically Prepared TiO2-Reduced Graphene Oxide Composite Nicholas J. Bell,† Yun Hau Ng,† Aijun Du,‡ Hans Coster,§ Sean C. Smith,‡ and Rose Amal*,† †
ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia ‡ Centre for Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, QLD 4072, Australia § School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia ABSTRACT: Solution-phase photocatalytic reduction of graphene oxide to reduced graphene oxide (RGO) by titanium dioxide (TiO2) nanoparticles produces an RGO-TiO2 composite that possesses enhanced charge transport properties beyond those of pure TiO2 nanoparticle films. These composite films exhibit electron lifetimes up to four times longer than that of intrinsic TiO2 films due to RGO acting as a highly conducting intraparticle charge transport network within the film. The intrinsic UV-active charge generation (photocurrent) of pure TiO2 was enhanced by a factor of 10 by incorporating RGO; we attribute this to both the highly conductive nature of the RGO and to improved charge collection facilitated by the intimate contact between RGO and the TiO2, uniquely afforded by the solution-phase photocatalytic reduction method. Integrating RGO into nanoparticle films using this technique should improve the performance of photovoltaic devices that utilize nanoparticle films, such as dye-sensitized and quantum-dot-sensitized solar cells.
’ INTRODUCTION Mesoporous films of fused TiO2 nanoparticles serve a dual purpose in excitonic solar cells (XSCs): first, as a high surface area substrate for photovoltaic materials (sensitizers) such as organic dyes in dye-sensitized solar cells (DSSCs) or semiconducting nanoparticles in quantum dot solar cells; and second, as a conductor of photogenerated electrons from the sensitizer to the cell anode. The films excel at the first function (typical BrunauerEmmettTeller (BET) surface area measurements of TiO2 films range between 60 and 90 m2 g1 13); it is in the second function as a charge conductor that performance must be improved. Charge trapping sites within the TiO2 network and recombination at the particle boundaries conspire to annihilate the photogenerated electrons and undermine the cell’s efficiency. The problem is compounded by the massive surface area of the particleelectrolyte interface that is otherwise a highly desirable feature of the XSC (recombination increases with interface area). This is a particular problem in films with ordered layers of discretely sized nanoparticles in which the arrangement increases light scattering and absorption within the film and minimizes back-transport at the substratefilm interface but with the drawback of introducing more interfacial boundaries.4 Several strategies to improve charge transport within mesoporous nanoparticle films have been reported such as improving interconnectivity within TiO2 nanoparticle networks or annealing r 2011 American Chemical Society
or combining TiO2 with hole-transporting materials.5 Of these approaches, incorporating conducting carbon materials into the nanoparticle films appears particularly promising. Composites of carbon nanotubes and TiO2 have been shown to increase the photoconversion efficiency of TiO2 by a factor of 2.6,7 Fullerene (C60) and its derivatives have also been used in organicinorganic cells for electron capture.8,9 Graphitic carbon of various structures have also been found to enhance electron transport,10 however graphene, the newest member of the carbon family, remains relatively unexplored in this area. Pristine graphene possesses many unique properties that predisposes it for use in photoconversion devices: its two-dimensional structure transmits 95% of visible and UV light11(thus minimizing light blocking), and its potentially ballistic electron transport capability results in effectively zero conduction resistance within the graphene structure.12 The few reports to date show that graphene-incorporated TiO2 films do show enhanced efficiencies in XSCs and photocatalytic reactions. Chhowalla and co-workers reported an improved DSSC efficiency from 4.89 to 5.26% when graphene was incorporated into the device, although they attributed the effect to reduced back-transport (the “leakage” Received: November 30, 2010 Revised: February 20, 2011 Published: March 09, 2011 6004
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The Journal of Physical Chemistry C of electrons collected at the DSSC anode back to the electrolyte, that is, an internal short-circuit).13 Li et al. have found that a composite of Degussa P25 TiO2 and graphene is a good candidate for dye degradation.14 We have recently demonstrated that introducing reduced graphene oxide into a semiconductor nanoparticle matrix boosts its photoelectrochemical and photocatalytic performance.15 In all of these studies however, the mechanism behind such enhanced performance has not been explored. Whether it is due to a faster electron transport, or reduced recombination at the nanoparticle-electrolyte interface, or both, remains unclear at this stage. In this paper, we use a photocatalytic synthesis method to incorporate reduced graphene oxide (RGO) into a TiO2 film to demonstrate a 10-fold increase the photoconversion efficiency of the film. We present current transient analysis and electrochemical impedance spectroscopy (EIS) studies of this composite and show that the photoconversion enhancement is due to a quadrupling of the electron lifetime within the composite film.
’ EXPERIMENTAL DETAILS Graphene oxide (GO) was synthesized according to Hummers’ method by reacting commercial graphite powder (Aldrich, 2 μm) with a mixture of H2SO4 (Aldrich, 99%), NaNO3 (Aldrich), and KMnO4 (Aldrich) followed by addition of H2O2 (Aldrich, 30%).16 The solid product was filtered and washed repeatedly with 1 M HCl and deionized water, then dried under vacuum at room temperature and ground into a fine brown powder. The resultant brown GO powder was stored as a solid until required at which time a given quantity was suspended in solution with the aid of mild sonication. The RGO-TiO2 composite was made by mixing suspensions of commercial TiO2 powder (Ishihara Sangyo Kaisha Ltd., ST-01 anatase, average particle size 7 nm) with the GO suspension in a sealed sample tube at various ratios and irradiating the suspension with UV light for one to several hours, depending on the volume, with stirring in argon atmosphere. During illumination, the solution would change from brown to black, indicating reduction. After reduction, the product was either dried in vacuum at room temperature for Raman and X-ray photoelectron spectroscopy (XPS) measurements, or drop- or spin-cast onto conducting fluorinated tin oxide (FTO) slides for electrical characterization. XPS analysis was performed using an ESCALab220i-XL probe (VG Scientific) with monochromated Al KR radiation (hν =1486.6 eV). Analysis was carried out in a vacuum chamber (1 kHz), the response of each film is unique. (The response above 1 kHz represents the bulk conductance of the H2SO4 electrolyte in series with the thin film.) Thus, information about the films themselves is obtained from the region between 1 mHz and 1 kHz. At frequencies below 100 Hz, the RGO-containing films are significantly more conductive than the bare TiO2 film with the RGO-TiO2 (0.5 mg) film being roughly five to ten times more conductive than TiO2 film. Higher amounts of RGO (1.0 and 1.5 mg) lead to conductivities a few times that of the 0.5 mg film. These results correlate with our finding that the optimum RGO/ TiO2 ratio for photocurrent enhancement is roughly 0.7:4 mg. Higher RGO ratios do not produce higher photocurrents because the enhanced conductivity of these films cannot compensate for the intrinsically lower photogeneration rate of the TiO2 particles caused by the light-blocking effect of the additional RGO. At ultralow frequencies (∼1 mHz), the conductivity is dominated by the interface between the film and the FTO. The higher conductance of the RGO-TiO2 films in this region indicates that the RGO not only enhances conductivity within the film but also facilitates conduction between the film and the FTO substrate. Nyquist plots of the film are shown in the inset of Figure 7. As the amount of RGO is increased the radius of the Nyquist arc decreases. The radius of each arc is correlated with the charge transfer ability of the corresponding film; the larger the radius the lower the film’s ability to transfer charge. Thus, TiO2 with its near-vertical impedance arc (large phase angle) exhibits poorer charge transfer ability than the film containing RGO.36 A simple equivalent circuit model consisting of a series of RC circuits (with the capacitors modeled as constant phase elements) was used to determine the apparent electron lifetime for each film from the above measurement.37 The lifetimes were 0.10, 0.25, 0.46, and 0.45 μs for TiO2, TiO2/0.5, TiO2/1.0 and TiO2/1.5 mg RGO, respectively. Thus, the more efficient charge transport in RGO-TiO2 films 6008
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The Journal of Physical Chemistry C resulted in longer electron lifetimes, as suggested by the current transient analysis.
’ CONCLUSION Integrating RGO into TiO2 nanoparticle films improves the electrochemical performance of these films by providing a lowresistance conduction pathway through the film. Photocatalytic reduction of GO to RGO is an ideal method for incorporating RGO with TiO2 because it facilitates complete dispersion of TiO2 on the RGO sheets, maximizing the charge-transfer capability of TiO2 to RGO. EIS measurements reveal that the electron lifetime in TiO2 films containing RGO is approximately four times that of pure TiO2 films, and that RGO also facilitates conduction between the nanoparticle film and an FTO substrate. The excellent integration of TiO2 and RGO enabled by the photocatalytic GO reduction, combined with the electron lifetime enhancement of the RGO itself, allows the fabrication of RGO-TiO2 photovoltaic cells that exhibit approximately ten times the photogenerated current of TiO2-only films. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors would like to thank Dr. Bill Gong for assistance with the XPS measurements and the Australian Research Council for funding this work. ’ REFERENCES (1) Ko, Y.-S.; Kim, M.-H.; Kwon, Y.-U. Bull. Korean Chem. Soc. 2008, 29, 463. (2) Lee, G.-W.; Bang, S.-Y.; Lee, C.; Kim, W.-M.; Kim, D.; Kim, K.; Park, N.-G. Curr. Appl. Phys. 2009, 9, 900. (3) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 8607. (4) Hu, L.; Dai, S.; Weng, J.; Xiao, S.; Sui, Y.; Huang, Y.; Chen, S.; Kong, F.; Pan, X.; Liang, L.; Wang, K. J. Phys. Chem. B 2007, 111, 358. (5) Snaith, H. J.; Gr€atzel, M. Adv. Mater. 2007, 19, 3643. (6) Kongkanand, A.; Domínguez, R. M.; Kamat, P. V. Nano Lett. 2007, 7, 676. (7) Kanai, Y.; Grossman, J. C. Nano Lett. 2008, 8, 908. (8) Hasobe, T.; Hattori, S.; Kamat, P. V.; Urano, Y.; Umezawa, N.; Nagano, T.; Fukuzumi, S. Chem. Phys. 2005, 319, 243. (9) Fukuzumi, S.; Kojima, T. J. Mater. Chem. 2008, 18, 1427. (10) Vietmeyer, F.; Seger, B.; Kamat, P. V. Adv. Mater. 2007, 19, 2935. (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. Nature 2009, 457, 706. (12) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (13) Kim, S. R.; Parveza, M. K.; Chhowalla, M. Chem. Phys. Lett. 2009, 483, 124. (14) Zhang, H.; Lv, X.; Li, Y.; Wang, Y.; Li, J. ACS Nano 2010, 4, 380. (15) (a) Ng, Y. H.; Lightcap, I. V.; Goodwin, K.; Matsumura, M.; Kamat, P. V. J. Phys. Chem. Lett. 2010, 1, 2222. (b) Ng, Y. H.; Iwase, A.; Kudo, A.; Amal, R. J. Phys. Chem. Lett. 2010, 1, 2607. (c) Ng, Y. H.; Iwase, A.; Bell, N. J.; Kudo, A.; Amal, R. Catal. Today 201110.1016/j. cattod.2010.10.090. (16) William S. Hummers, J.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.
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