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Reduction of graphene oxide at room temperature with vitamin C forRGO–TiO2 photoanodes in dye-sensitized solar cell

Hui Ding a, Sam Zhang b,⁎, Ji-Tao Chen c, Xiao-Ping Hu d, Zhao-Fu Du a, Yue-Xiu Qiu a, Dong-Liang Zhao a

a Research Institute of Functional Materials, Central Iron and Steel Research Institute, No. 76 Xueyuan Nanlu, Haidian, Beijing 100081, PR Chinab School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporec College of Chemistry and Molecular Engineering, Peking University, No. 5 Yiheyuan Road, Haidian District, Beijing 100871, PR Chinad Research & Development Center, Advanced Technology & Materials Co., Ltd., No. 76 Xueyuan Nanlu, Haidian, Beijing 100081, PR China

⁎ Corresponding author. Tel.: +65 67904400; fax: +65E-mail address: [email protected] (S. Zhang).

http://dx.doi.org/10.1016/j.tsf.2015.02.0380040-6090/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: H. Ding, et al., Redsensitized solar cell, Thin Solid Films (2015)

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Keywords:Graphene flakeTitanium dioxideNanocompositeVitamin CDye-sensitized solar cellsPhotoanodes

Graphene has been used to enhance conversion efficiency in dye-sensitized solar cell (DSSC) through increasedelectronic transportation. Introduction of graphene into DSSC is realized through reduction of graphene oxide(GO) to reduced graphene or RGO in TiO2 nanoparticles in solution. Chemical and hydrothermal reactions aretwo commonly used avenues. In the chemical route, toxic hydrazine and its derivatives are the typical reducingagent. Hydrothermal process is not toxic but requires prolonged heating. In this study, we mix flakes of GOwithTiO2 nanoparticles and use vitamin C to realize the reduction at room temperature. The RGO–TiO2 compositefilms are then used as photoanode in DSSC. A conversion efficiency of 30% increase (to 7.89%) is obtained as com-pared to that of the pure TiO2 photoanode.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Dye-sensitized solar cells (DSSCs) have drawn extensive attentionbecause of their relatively low cost and high photoelectrical conversionefficiency potential [1]. Since the pioneer work of O'Regan and Grätzel[2], DSSCs have been intensively studied during the past two decades.Although 13% conversion efficiency was obtained recently [3], furtherimprovements are still necessary. In general, DSSCs are comprised ofphotoanode (the most frequently used material is anatase TiO2) deco-rated by a monolayer of dye molecules (sensitizers), a platinized coun-ter electrode, and an electrolyte solutionwith a dissolved iodide ion/tri-iodide ion redox couple between the electrodes. The transport progressof photogenerated electrons across the TiO2 nanoparticle network be-comes the major bottleneck of improving the conversion efficiency ofDSSCs [4,5]. If the photogenerated electrons couldn't accomplish thetransportation, they will recombine with the holes and get annihilatedin the TiO2 nanoparticle network [6]. To enhance electron transfer abil-ity and suppress the recombination, some charge carriers materials areintroduced into photoanode to the direct photo electrons [7–11].

Graphene has attracted a lot of attention because of its unique me-chanical, thermal, electrical and optical properties [12–14]. As a 2Dma-terial, graphene is a zero band gap material with a single molecularlayered structure [14]. In graphene, each carbon atom uses 3 of its 4outer orbital electrons to form3 sigma bonds 120° apartwith 3 adjacent

67924062.

uction of graphene oxide at r, http://dx.doi.org/10.1016/j.t

carbon atoms in the same plane, leaving the 4th electron free to move,therefore, electrons in graphene behave just like massless relativisticparticles without crystal lattices' restrictions [15,16]. As such, graphenepossesses excellent electrical conductivities in two dimensions at roomtemperature (more than 200,000 cm2 V−1 s−1) [17]. Since intermolec-ular forces such as physisorption, electrostatic binding, or charge trans-fer interactions between graphene and TiO2 nanoparticles [18], TiO2

nanoparticle can anchor on the graphene flakes thus graphene couldprovide a fast channel for electron transportation [19–22].

Reported graphene–TiO2 composites are made mostly by mechani-calmixing of graphene oxide (GO) and titaniumdioxide, followed by re-duction of GO to obtain graphene (RGO) via chemical reduction orhydrothermal reaction. In the most common case, chemical reductionof the GO was carried out using hydrazine or hydrazine hydrate as thereducing agent [23,24], where commercially obtained TiO2 (P25) nano-particles were added into a GO aqueous solution to stir ultrasonicallyand magnetically into a homogeneous mixture. This mixture was thenscreen-printed or doctor-bladed into TiO2–GO thinfilms followed byfir-ing at 40 °C under hydrazine vapor for 24 h to allow reduction of GO totake place [4,25]. As hydrazine or hydrazine hydrate is highly poisonousand explosive [24], this process is environmentally unsafe especiallywhen large quantities are involved. Hydrothermal reduction is nottoxic but requires heating in autoclaves [20,26,27] at 160 °C [20] or150 °C [28] for hours.

Recent researches show that vitamin C (VC) reduces graphene oxideto graphene at room temperature [24,29,30]. This opens up a new ave-nue towards nontoxic graphene oxide reduction at room temperature.The average conductivity of the RGO reduced with VC is comparable

oom temperature with vitamin C for RGO–TiO2 photoanodes in dye-sf.2015.02.038

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with that reduced using hydrazine or hydrothermal method [24,31].Herein, we explore this facile approach of making RGO and directlymix it into DSSC for better energy conversion. Flakes of GO are reducedto graphenewith VC as a reducing agent in an aqueous solution throughsimple stirring and centrifugation to form RGO–TiO2 nanocomposite asphotoanode in DSSC. Experiments show that the DSSC efficiency isenhanced by 30%.

2. Experimental

The GO reduction and fabrication of RGO–TiO2 DSSC photoanodewas described in Fig. 1. Graphene oxide (GO) powders with diameterof 500 nm–3 μmand 99% of 1–3monolayers in thicknesswas purchasedfrom Jining LeaderNano Tech L.L.C., and commercial TiO2 (P25) nano-particles of diameter of around 21 nm is purchased from Degussa Co.,Ltd. in Germany, fluorine-doped tin oxide (FTO) conducting glass witha sheet resistance of 15 Ω cm−2 was bought from Nippon Sheet GlassCo., Ltd., Japan, cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4-carboxylicacid-4′-tetrabutylammonium carboxy late) ruthenium (II) (N-719)waspurchased from Solaronix Co., Ltd., Switzerland. All other chemical re-agents used in the experiments were analytical reagents, purchasedfrom Sinopharm Chemical Reagent Beijing Co., Ltd., without additionalpurification before use.

Fig. 2.XRD patterns of (a) GO and RGO, (b) RGO–TiO2mixwith different amounts of RGO.

2.1. Preparation of the RGO–TiO2 nanocomposite

30 mg solid state GO was dissolved in 100 mL deionized water, andsonicated for 2 h until the solution became clear andnoprecipitate pres-ent. Then 3 g of TiO2 powder was added into the GO dispersions andstirred for 1 h in ultrasonic horn for a homogeneous suspension.500 mg of VC was added to the solution. The as-prepared solution wasvigorously stirred for 48 h at room temperature. As the color of the so-lution turned from yellowish to grayish, GO reduced to graphene.Then, centrifugation was conducted several times with water and etha-nol to remove remaining VC and other chemicals from the solution.Finally, the remains were dried in a vacuum dryer at 65 °C. As such,RGO–TiO2 nanocomposite with 1.0 wt.% RGO was prepared. 0.25, 0.5,0.75, 1.25 wt.% RGO–TiO2 nanocomposites were prepared in the samemanner.

Fig. 1. Fabrication process for

Please cite this article as: H. Ding, et al., Reduction of graphene oxide at rsensitized solar cell, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.t

2.2. Preparation of the RGO–TiO2 based photoanode

RGO–TiO2 pasteswere synthesized following the Grätzelmethod forpreparing TiO2 [32]. Before preparing the photoanode films, FTO

RGO–TiO2 photoanodes.



3H. Ding et al. / Thin Solid Films xxx (2015) xxx–xxx

conducting glass was immersed into a 40 mM TiCl4 aqueous solution at70 °C for 30 min, and then a 0.6 × 0.6 cm2

film was coated on it bydoctor-blading technique. After drying at 125 °C, the RGO–TiO2 filmwas heated up at 5 °C per minute to 500 °C and held for 3 h under thenitrogen atmosphere to remove the organic component. The RGO–TiO2 nanocomposite electrode was prepared after an extended post-treatment using a TiCl4 aqueous solution followed by a sintering processat 450 °C in air for 30min. Finally, the resulting electrodewas immersedinto an N719 ethanol solution for 24 h to form the sensitizedphotoanode. For comparison, two kinds of electrodes are also prepared:a pure TiO2 electrode and another made of 0.75 wt.% as-received GOmixed with TiO2.

2.3. Fabrication of the DSSC

The solar cell was assembledwith a Pt counter electrode and the dyesensitized RGO–TiO2 photoanode. The standard redox electrolyte was asolution of 0.60 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M

Fig. 3. (a) TEM image of graphene oxide flake and (b) magnified TEM image of grapheneoxide flake's edge.

Fig. 4. SEM image of cross-section of the RGO–TiO2 nanocomposite film.


iodine (I2), 0.10 M guanidinium thiocyanate (GSCN) and 0.50 M4-tert-butylpyridine (TBP) in the mixture of acetonitrile andvaleronitrile (volume ratio: 85:15). The solar cell was assembled intoa sandwich type cell and sealed by using Surlyn thermoplastic frame.

2.4. Characterization

The crystal structures of the GO, the RGO and the RGO–TiO2 werecharacterized by X-ray diffraction (XRD) (D8 Advance, Bruker,

Fig. 5. The SEM image of reduced graphene at top of the RGO–TiO2 nanocomposite film(a) and in the cross-section (b).




Germany) with Cu Kα radiation (40 kV, 40 mA, λ = 0.0541 nm) at astep size of 0.02° (0.2 s per step). High-resolution transmission electronmicroscopy (TEM) images were obtained on a JEM-2010 (JEOL Ltd.,Japan) at 200 kV. The RGO–TiO2 film's morphologies and cross sectionscanning electron microscopy (SEM) images were obtained by aS4800 SEM instrument (HITACHI, Japan) at 15 kV. The reductioneffect of VC on GO was obtained on a Raman spectrometer (LabRamHR800, HORIBA Jobin Yvon, France) equipped with a 514 nm laser andFourier transform infrared spectroscopy (FT-IR) (Nicolet iS10 FT-IRSpectrometer, Thermo Scientific, China). The current density–voltage(J–V) characteristics of cells were measured by the Keithley 4200 semi-conductor characterization system (Keithley Instruments, USA) underAM 1.5 simulated illumination (Newport 91160, 300 W xenon lamp,

Fig. 6. SEM images of RGO–TiO2 powder with different contents of RGO: (a) pur


USA). Electrochemical impedance spectrum (EIS) was conducted onAutolab Potentiostat 30 system (Metrohm, Switzerland) with a fre-quency range from 100 kHz to 0.1 Hz in the condition of 10 mV ACamplitude under 100 mW·cm−2 irradiation.

3. Results and discussion

3.1. Crystal structure

Fig. 2 shows the XRD patterns of the original GO, the RGO, the TiO2

nanoparticles and the RGO–TiO2 with different contents of RGO.Fig. 2(a) illustrates that the GO had only one peak at around 10° 2θ.After reduction, the peak was widened and moved to around 25° (c.f.,

e TiO2; (b) 0.25 wt.%; (c) 0.5 wt.%; (d) 0.75 wt.%; (e) 1.0 wt.%; (f) 1.25 wt.%.



Fig. 8. FT-IR spectra of GO, TiO2 and RGO–TiO2.


RGO spectrum). The shift of peak suggested that the GO had been partlyreduced to graphene: the RGO had smaller interlayer spacing than theGO because the oxygen-containing groups were partially removed[22]. Fig. 2(b) shows that the peaks from the RGO–TiO2 are originatedfrom the anatase and rutile TiO2 phases, and the relative content of an-atase and rutile phase is about 4:1, which are in excellent agreementwith that of TiO2. There is no obvious peak change at various contentsof RGO–TiO2, indicating that the TiO2 nanoparticles maintain their crys-tal lattice structure of TiO2 after going through the reduction process.The content of RGO is too little to be detected by XRD.

3.2. Morphology of the RGO–TiO2 films

Fig. 3 has TEM images of the graphene oxide before the reduction.Fig. 3(a) shows the lateral size of graphene oxide flakes, which revealedthat the graphene oxide's lateral size was more than 2 μm. Fig. 3(b) isthe magnified TEM image of graphene oxide and the layers are clearlyseen at the edge of the GO flakes: mostly 3 layers. Fig. 4 is an SEMimage of the cross-section of the RGO–TiO2 composite film displayinga film thickness of about 14.5 μm. Fig. 5 is the SEM image of an enlargedview of the top and a cross-section of the RGO–TiO2 photoelectrodefilm, revealing a goodmix between the RGO (flakes) and the TiO2 nano-particles (round particles). With an increase in amount of RGO in theRGO–TiO2 nanocomposite, the stock of graphene flakes becomes moreapparent, as observed from the SEM image in Fig. 6. The graphene flakesmay still not be completely covered by TiO2 nanoparticles even whenthe amount of RGO goes beyond 1 wt.%.

3.3. Removal of oxygen groups

Fig. 7 is the Raman spectra of GOwith TiO2 nanoparticles before andafter reduction by vitamin C. The Raman spectra exhibit two peaksaround 1358 and 1605 cm−1 corresponding to the D band and the Gband. The D band comes as a breathing mode of k-point phonons ofA1g symmetry and G band is generally assigned to the E2g phonon ofsp2 bonds of carbon atoms [33]. As compared to GO–TiO2, there is an in-crease in intensity ratio of ID/IG in RGO–TiO2. This agrees well with theRaman spectra of the GO reduced by VC [24] or hydrazine [34], indicat-ing that the reduction did take place.

Fig. 8 shows the FT-IR spectra of the TiO2, the GO and the RGO–TiO2.The GO peaks at C–O (1060 cm−1), C–O–C (1230 cm−1), C–OH(1410 cm−1), C_O (1720 cm−1), and a hump at 3000–3500 cm−1

which is attributed to the O–H stretching vibrations of the C–OH groupsandwater. In RGO–TiO2, the peaks at 1060, 1230, and 1740 cm−1 disap-peared, indicating that these oxygen-containing functional groups are

Fig. 7. Raman spectra of GO with TiO2 before (bottom) and after (top) being reduced viavitamin C.


removed. The RGO–TiO2 composite has a broad absorption at lowwavenumbers (below 1000 cm−1), that is attributed to the Ti–O–Tibonds in TiO2. The peak around 1630 cm−1 is contributed by the skele-tal vibration of the reduced graphene flakes.

3.4. Effect of the RGO–TiO2 photoanode

The effects of the incorporation of RGO were evaluated and listed inTable 1 with regard to short circuit photocurrent density (JSC), open-circuit photovoltage (VOC), fill factor of the cell (ff) and the overall con-version efficiency (η). The electrode performance is illustrated by J–Vcurves under AM 1.5 condition shown in Fig. 9. It is observed fromTable 1 that a proper amount of the RGO in the RGO–TiO2 photoanodeenhances the energy conversion efficiency of the DSSCs. With0.75 wt.% RGO, the photoelectric performs the best in this experimentsetting with a conversion efficiency of 7.89%, an increase of 30.2% ascompared to pure TiO2-based DSSC. The overall conversion efficiencywas calculated from JSC, VOC, ff and the intensity of the incident light(Pin), [35] namely,

η ¼ JSC � VOC

Pin� ff : ð1Þ

Consequently, the efficiency of the cell is determined by those threeparameters. Fig. 10 shows the trends of VOC and JSC with different con-tents of RGO in the photoanodes.

For DSSCs, the VOC values are obtained as the difference between theelectrons' Fermi level in TiO2 under the illumination and the Nernst po-tential of the redox couple in the electrolyte [35]. The VOC value of thecells does not show drastic change with different amounts of RGO asshown in Table 1. It means that the Fermi level in TiO2 does not shiftmuch after stirring with VC. Despite the fluctuations, the VOC stayabove 0.7 V in those low RGO concentrations (below 1.0 wt.%). When

Table 1Photovoltaic parameters for DSSCs with anode of TiO2 with varying amount of RGO/GO.

Anode with wt.% RGO

JSC(mA·cm−2)

VOC

(V)ff η

(%)Rs

(Ω)Rct

(Ω)Cμ

(μF)τ(ms)

0 12.2 0.715 0.695 6.06 8.22 47.97 445.45 21.370.25 13.3 0.704 0.704 6.59 8.52 44.14 499.54 22.050.50 14.2 0.702 0.716 7.14 8.12 41.85 542.09 22.690.75 15.2 0.708 0.733 7.89 8.27 40.94 659.75 27.011.00 12.9 0.698 0.693 6.24 9.11 39.31 547.37 21.521.25 11.8 0.687 0.673 5.46 8.10 35.65 519.50 18.520.75 wt.%as-received GO

11.6 0.693 0.682 5.48 8.35 59.33 303.22 17.99



Fig. 9. J–V curves of DSSC with varying amount of RGO/GO.


adding a large number of RGO, graphene layersmay be stacked togetherandnot completely covered byTiO2 nanoparticles, as shown in Fig. 6(e–f).Because graphene has the excellent electrical conductivity, therewill bea large number of recombination between the interface of graphene andelectrolyte. Moreover, more recombinationmeans less electron density,and it would cause a shift of the Fermi level [36], leading to a decrease inVOC (Fig. 10).

The short-circuit current is another fundamental parameter for effi-ciency of DSSC. A good short-circuit current is based on a good chargeinjection efficiency from the excited dye to TiO2, the excellent charge-transport property and large recombination resistance [28]. For theRu-complex N-719 used here, the injection from the excited dye mole-cule to the conduction band (CB) of the TiO2 is fast enough to completethe other processes [37–39]. Therefore, the charge collection and trans-portation efficiency of the injected electrons becomes a key factor to JSCof the cell. The remarkable electrical transport property of graphene in-dicates that it is a good conductor, and it is beneficial to capture andtransport electrons from the CB of TiO2. That is to say, graphene wouldact as a high-way to speed up electron transportation and lead todecrease the recombination in the TiO2 electrode. Compared withthe pure TiO2 electrode, the JSC of DSSC with 0.75 wt.% RGO is15.2 mA·cm−2, which increased by 24.6%, respectively. The electricalconductivity of GO is billions of times smaller than that in RGO [40,41]. Without high conductivity, GO can't capture and transport elec-trons from the CB of TiO2. Compared with the 0.75 wt.% RGO–TiO2

Fig. 10. Trends of VOC and JSC with different contents of RGO.


cell, the JSC of DSSC with 0.75 wt.% as-received GO is 11.6 mA·cm−2,which decreased by 28.2%, respectively. As shown in Fig. 10, JSC ofRGO–TiO2 anodes first increases to the maximum value and then de-creases. The increase can be explained by the remarkable electricaltransport property of graphene,which is beneficial to capture and trans-port charges from TiO2. The electrons transferred from the CB of TiO2

can transport quickly to FTOwithout recombination or back-transfer re-action, as shown in Fig. 11. Since graphene has a continuous band struc-ture like metal [13,14], electrons transferred from TiO2 into graphenecan stay at continuous energy levels near the Fermi level. Whengraphene contacts with the electrolyte, graphene accelerates the reac-tion back to tri-iodide [10]. With too much RGO, the flakes over-stackeach other, and thus are not able to be fully covered by TiO2 nanoparti-cles, as such, back transfer reactions at the interface between RGO andelectrolyte increase, which, in turn, eliminates the benefit of highconductivity of RGO.

Good fill factor is based on a low total series resistance of the cells. AsTiO2 nanoparticles incorporate with RGO and RGO is an excellent con-ductor, the interior resistance of the cells can be reduced to a lowerlevel to achieve a higher fill factor (Table 1). As shown in Fig. 9 andTable 1, the PCE is decreased by ~10% in the anode incorporated with0.75 wt.% as-received GO as compared with pure TiO2 anode. In thecase of directly using the as-received GO to mix with TiO2 particles,the benefit of high conductivity of RGO does not exist. The electronmo-bility of GO is about 0.1–10 cm2 V−1 s−1 [40] that is as slow as the TiO2

nanoparticles [42]. The presence of GO, on the hand, interrupts the con-nection between TiO2 nanoparticles and increases the number of inter-faces therefore increases the interior resistance in the cell, and thusresults in a decrease in ff (Table 1).

The interfacial electrical properties of DSSCs are characterized by theelectrochemical impedance spectrum (EIS). Fig. 12 shows that there arethree main semicircles observed in the EIS. The equivalent circuit of theDSSCs shown in Fig. 12 explains the transportation and recombinationprocesses of the photogenerated electrons in the photoanode. The fittedRs is the series resistance, including the sheet resistance of the FTO glass,the contact resistance of the cell and the transfer resistance of the elec-trolyte. Rct is the charge transfer resistance of the recombination be-tween the TiO2 nanoparticles and I3

− in the electrolyte, and Cμ is thechemical capacitance of the TiO2 film. RPt and CPt are the charge transferresistance and the double-layer capacitance at the counter electrode I3

−

at the electrolyte/Pt/FTO glass interface [43,44].From Table 1, Rs values of the various films are almost the same,

whichmeans that the resistance of TiO2/FTO interface has no change de-spite of differentmaterials with or without RGO. However, the Rct of thefilms assemble with RGO is smaller comparing with that of the pureTiO2 photoanodes, indicating enhanced electron transportation be-tween the TiO2 nanoparticles and I3

− in the electrolyte after mixingwith RGO. This is a result of the outstanding electrical conduction prop-erty of graphene. In DSSCs, electron life-time (τ) is an important param-eter to explain the difference of energy conversion efficiency in different

Fig. 11. Energy diagram of DSSC incorporating with graphene.



C PtC

RctsR PtR

Fig. 12. EIS measurements of DSSCs with various RGO/GO–TiO2 electrodes.


samples, extractable from the Rct and Cμ using τ=CμRct with fitting theEIS curves [35]. Obviously RGO–TiO2 composites have longer electronlife-time except at too much RGO (1.25 wt.% RGO). At 0.75 wt.% τ =27.01 ms, being the longest in Table 1. This result is consistent withthe result of short-circuit current and conversion efficiency in Fig. 9.

4. Conclusion

With vitamin C as a reduction agent, direct reduction of grapheneoxide (GO) into reduced graphene (RGO) to form RGO–TiO2 nanoparti-cle photoanode for dye sensitized solar cell (DSSC) takes place at roomtemperatures with only stirring and centrifugation. Mixing of RGO inthe DSSC photoanode improves the energy conversion efficiency. At0.75 wt.% RGO obtained the conversion efficiency obtained is 7.89%,which is 30.2% increase as compared to its no RGO counterpart(6.06%). This increase is attributed to RGO's higher conductivity forimproved photoelectron transfer and reduced charge recombination.

Acknowledgments

This work was supported by National Natural Science Foundation ofChina (No. 51301041 and No. 51202035) and the CISRI Project on DSSC(No. 11020990A).

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