Nanoscale

PAPER

Cite this: Nanoscale, 2015, 7, 7078

Received 25th November 2014,Accepted 20th December 2014

DOI: 10.1039/c4nr06969b

www.rsc.org/nanoscale

N-doped graphene nanoribbons as efficientmetal-free counter electrodes for disulfide/thiolateredox mediated DSSCs†

Yuhua Xue,a,b Janice M. Baek,‡b Hao Chen,*a Jia Qu*a and Liming Dai*a,b

Nitrogen-doped graphene nanoribbons (N-GNRs) were prepared by thermal treatment of the as-zipped

graphene oxide nanoribbons in NH3 gas. X-ray photoelectron spectroscopy (XPS) measurements revealed

a high nitrogen content up to 6.5 atom% for the as-prepared N-GNRs. This, together with the high

Brunauer–Emmett–Teller (BET) surface area of about 751 cm2 g−1, prompted us to use the N-GNR as the

first low-cost, metal-free counter electrode for disulfide/thiolate redox mediated dye-sensitized solar

cells (DSSCs). Compared with the widely-used platinum electrode, the newly-developed N-GNR counter

electrode showed a dramatically improved power conversion efficiency for DSSCs based on the thiolate/

disulfide redox shuttle. The observed superior cell performance was attributed to the enhanced charge

transfer capability and electrocatalytic activity induced by N-doping of the graphene nanoribbon.

Introduction

Dye-sensitized solar cells (DSSCs) have attracted a great deal ofinterest due to their high energy conversion efficiency andsimple, inexpensive device fabrication.1–5 A typical DSSCdevice consists of a photoanode, counter electrode, and elec-trolyte. The electrolyte, which transfers the electrons from thecounter electrode to the photoanode electrode within the cell,plays an important role in regulating the device performanceof DSSCs. The most commonly used electrolyte in the DSSCs isthe triiodide/iodide (I3

−/I−) redox couple.1,4,5 Although thetriiodide/iodide couple is efficient for high-performanceDSSCs, it suffers from corrosion of the silver-based current col-lector and partial absorption of visible light around 430 nm.Therefore, iodide-free DSSCs have been developed as analternative choice.1,6 Interestingly, Gratzel and co-workers havedeveloped a new electrolyte based on the disulfide/thiolate(T2/T

−) redox couple of negligible optical absorption in thevisible region, and hence with potential applications in trans-

parent DSSCs.7 However, DSSCs with the disulfide/thiolate(T2/T

−) redox couple electrolyte often show a relatively low fillfactor when platinum (Pt) is used as a counter electrode.Besides, Pt is expensive and has a limited reserve in nature. Inthis study, we used nitrogen-doped graphene nanoribbons(N-GNRs) as metal-free electrocatalysts to replace Pt in DSSCsbased on the T2/T

− electrolyte.Graphene nanoribbons (GNRs) are elongated strips of gra-

phene with straight edges, which have attracted considerableattention due to their unique structure and bandgapproperties.8–12 Various methods, including lithographic engi-neering,13,14 chemical vapour deposition (CVD),15 and unzip-ping of carbon nanotubes,8,10,11 have been devised to preparegraphene nanoribbons. In the present study, we preparedN-GNRs by unzipping carbon nanotubes,8 followed by thermalannealing under an ammonia–argon gas mixture. Comparedwith pure carbon nanomaterials, the incorporation of nitrogeninto carbon nanomaterials has been shown to be an effectiveway to controllably alter their electronic and surface character-istics.16,17 As a result, nitrogen-doped carbon nanomaterialshave been demonstrated to show an excellent electrocatalyticperformance for the oxygen reduction reaction (ORR) in fuelcells and triiodide reduction in DSSCs.18–20 While nitrogen-doped carbon nanotubes and nitrogen-doped graphene havebeen extensively studied, nitrogen-doped graphene nano-ribbons have been scarcely reported.21

In this paper we report the preparation of nitrogen-dopedgraphene nanoribbons (N-GNRs) by unzipping carbon nano-tubes,8 followed by thermal annealing under an ammonia–argon gas mixture. The resulting N-GNRs showed a high nitro-

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr06969b‡On leave of absence from La Lumiece School, 6801 N. Wilhelm Rd, LaPorte, IN46250, USA.

aInstitute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology

& Optometry, Wenzhou Medical College, 270 Xueyuan Xi Road, Wenzhou, Zhejiang

325027, China. E-mail: chenhao@mail.eye.ac.cn, jia.qu@163.combCenter of Advanced Science and Engineering for Carbon (Case4Carbon),

Department of Macromolecular Science and Engineering, Case Western Reserve

University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA.

E-mail: liming.dai@case.edu

7078 | Nanoscale, 2015, 7, 7078–7083 This journal is © The Royal Society of Chemistry 2015

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gen content up to 6.5 atom% and a large Brunauer–Emmett–Teller (BET) surface area of 751 cm2 g−1. The use of ourN-GNRs as a metal-free counter electrode to replace Pt inDSSCs with a T2/T

− electrolyte led to an energy conversionefficiency even higher than their counterparts with the Ptcounter electrode. The observed superior cell performance wasattributed to an enhanced charge transfer capability and theelectrocatalytic activity induced by N-doping of the graphenenanoribbon.

Results and discussion

Scheme 1a schematically shows the route to N-GNRs by unzip-ping carbon nanotubes according to the reported procedure,8

followed by N-doping via thermal annealing under anammonia–argon gas mixture (see the Experimental section fordetails). The resulting N-GNR was then used as a counter elec-trode in DSSCs with the T2/T

− redox couple electrolyte(Scheme 1b).

Fig. 1a and b show scanning electron microscopy (SEM)images of the resulting N-GNRs under different magnifi-cations. As can be seen, most of the carbon nanotubes havebeen unzipped into strip-like graphene. Fig. 1c shows the

corresponding AFM image, confirming the nanoribbonstructure.

The nitrogen content of N-GNRs was measured by X-rayphotoelectron spectroscopy (XPS). Fig. 2a shows an XPS surveyspectrum, from which the carbon, nitrogen and oxygen con-tents were estimated to be 90.4 atom%, 6.5 atom%, and 3.1atom%, respectively. The high resolution XPS N1s spectrum inFig. 2b shows two component peaks at 398 and 401 eV corres-ponding to the pyridinic nitrogen and the graphitic nitrogen,respectively.

The Raman spectrum of the N-GNR is given in Fig. 2c,which shows a higher D band than the G band with a ratio ofID/IG = 1.07. The observed higher D band than the G band isattributable to the large number of edge and nitrogen-dopinginduced defects intrinsically associated with the N-GNR, pro-viding an advantage for electrocatalysis. The XRD profile ofN-GNR given in Fig. 2d shows a peak at about 2θ = 26° equi-valent to the lattice spacing of 0.34 nm corresponding to the(002) facet of the graphitic carbon. Nitrogen-doping caused alattice expansion as evidenced by the appearance of anotherrelatively weak band at 2θ = 19°, corresponding to an interlayerdistance of 0.47 nm. The surface area was determined by BETmeasurements (Fig. S1†) to be as high as 751 cm2 g−1 forthe N-GNR, which is higher than that of a nitrogen-doped

Scheme 1 Schematic representation of (a) the route to N-doped graphene nanoribbons (i.e., N-GNRs) and (b) disulfide/thiolate redox DSSC withthe N-GNR as a counter electrode.

Fig. 1 (a, b) SEM images of N-GNRs under different magnifications. (c) A typical AFM image of N-GNRs (scale bar: 0.2 μm).

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Fig. 2 (a) XPS survey spectrum of N-GNR. (b) High-resolution XPS N1s spectrum of N-GNR. (c) Raman and (d) XRD spectra of N-GNR.

Fig. 3 (a) Photograph of the N-GNR counter electrode. (b, c) Side-view SEM images and (d, e) top-view SEM images of the N-GNR counterelectrode.

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graphene20 or nitrogen-doped carbon nanotubes. The largesurface is an additional advantage for electrocatalysis at thecounter electrode of a DSSC.

To use the N-GNR as a counter electrode in DSSCs, wetransferred it onto a piece of FTO glass. Fig. 3a shows a photo-graph of the N-GNR counter electrode, which is rather uniformand flat. Fig. 3b and c reproduce a side view of the N-GNRcounter electrode under different magnifications, showing afoam-like structure. The thickness of the N-GNR counter elec-trode can be controlled by regulating the conditions for bladecoating. Fig. 3d and e show the top-view SEM images of theN-GNR counter electrode under different magnifications.

The N-GNR counter electrode thus prepared was then testedin DSSCs with the T2/T

− electrolyte. T− (i.e., 5-mercapto-1-methyltetrazole ion) and T2 (i.e., the dimer of T−) were syn-thesized following the published method and confirmed byNMR (Fig. S2†).1 Using the T2/T

− redox couple electrolyte, wehave prepared several DSSCs with either Pt or N-GNR as thecounter electrode. Three N-GNR counter electrodes with theN-GNR foam thickness of 7, 13, and 19 μm (designated as:N-GNRs-1, N-GNRs-2, and N-GNRs-3, respectively) were tested.Fig. 4a shows the obtained photocurrent density–voltage ( J–V)curves. The corresponding numerical data, including the shortcircuit current ( Jsc), open circuit voltage (Voc), fill factor (FF),and power conversion efficiency (PCE), are listed in Table 1. Ascan be seen in Table 1, the PCE for DSSCs with the N-GNRcounter electrodes is much higher than that of the DSSC withPt as a counter electrode. The FF for the DSSC with Pt as a

counter electrode is only 0.36, which is much lower than thoseof DSSCs with the N-GNRs as counter electrodes. Furthermore,the short circuit current for DSSCs based on the N-GNRcounter electrode increased with increasing the N-GNR thick-ness, leading to an increased power transfer efficiency(Table 1). The incident photon to charge carrier efficiency(IPCE) for DSSCs with the N-GNRs-3 and Pt counter electrodes,respectively, is given in Fig. 4b, which, as expected, shows arelatively high IPCE for the DSSC with the N-GNR counter elec-trode. This is consistent with the electrochemical impedancespectroscopy (EIS) measurements on the asymmetric dummycells with Pt and N-GNRs as the counter electrodes, respecti-vely. As seen in Fig. 4c, the semicircles of the Nyquist plots forthe two N-GNR electrodes are both smaller than that of the Ptelectrode, indicating smaller charge transfer resistances forthe N-GNR electrodes. The charge transfer resistances calcu-lated from the EIS spectra are 12.7, 74.1, and 559.6 Ω for theN-GNRs-3, N-GNRs-1, and Pt electrodes, respectively, showinga strong dependence on the electrode thickness. The series

Fig. 4 (a) Photocurrent density–voltage (J–V) curves for DSSCs using T2/T− as the electrolyte redox couple and platinum or N-GNR as the counter

electrode. (b) The incident photon to charge carrier efficiency (IPCE) of DSSCs with Pt and N-GNR as the counter electrodes. (c, d) Nyquist plots ofasymmetric dummy cells with Pt and N-GNR counter electrodes with different scales.

Table 1 Jsc, Voc, FF and PCE for DSSCs with the N-GNRs, and Pt as thecounter electrodes, respectively

Counter electrode Jsc [mA cm−2] Voc [V] FF PCE [%]

Pt 13.20 0.65 0.36 3.09N-GNRs-1 (7 μm) 10.82 0.62 0.57 3.82N-GNRs-2 (13 μm) 12.47 0.63 0.59 4.64N-GNRs-3 (19 μm) 13.43 0.64 0.59 5.07

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resistances of the N-GNR counter electrodes are also smallerthan that of the Pt counter electrode (Fig. 4d).

In order to investigate the role of nitrogen-doping to theDSSC performance, we also compared the N-GNR counter elec-trodes with the reduced graphene oxide nanoribbon (rGOR)counter electrode without nitrogen-doping. The photocurrentdensity–voltage ( J–V) curves for DSSCs with the Pt, rGOR, andN-GNR counter electrodes (i.e., N-GNRs-3), respectively, areshown in Fig. 5a while the corresponding numerical data ofthe Jsc, Voc, FF and PCE are presented in Table 2. As can beseen in Table 2, the PEC of the DSSC with the N-GNRs-3counter electrode is much higher than that of the DSSC withrGORs as a counter electrode. Furthermore, the resistances ofthe N-GNR, rGOR and Pt electrodes were also calculated fromthe EIS measurements on asymmetric dummy cells. Theobtained Nyquist plots were given in Fig. 5b, which shows alower charge-transfer resistance and series resistance for theN-GNR counter electrode with respect to the rGOR counterelectrode.

Conclusions

In summary, we have synthesized nitrogen-doped graphenenanoribbons with a high nitrogen content up to 6.5 atom%.The resulting N-GNR was demonstrated to be efficient counterelectrode material, better than the platinum, for DSSCs with thedisulfide/thiolate (T2/T

−) redox couple electrolyte. Comparedwith a Pt counter electrode, the N-GNR counter electrodespossess very low charge transfer resistances. This, together withthe large surface area and large number of the edge/doping-induced defects, makes N-GNRs very efficient counter electrodesfor high-performance DSSCs of practical significance.

Experimental sectionPreparation of nitrogen-doped graphene nanoribbons

Graphene oxide nanoribbons were synthesized by chemicalunzipping of carbon nanotubes according to the publishedprocedure.8 The as-prepared graphene oxide nanoribbons inwater solution were freeze dried for 3 days to obtain the gra-phene oxide nanoribbon foam.

Nitrogen-doped graphene nanoribbons were then preparedby annealing the freeze-dried graphene oxide nanoribbonfoams in a quartz tube furnace under an ammonia–argon gasmixture. Typically, the graphene oxide nanoribbon foam wasfirstly placed in a horizontal quartz tube and an ammonia–argon gas mixture (NH3 : Ar = 400 s.c.c.m. : 400 s.c.c.m.) wasintroduced into the tube. Then, the furnace was heated upfrom room temperature to 800 °C within 1 hour, and kept at800 °C for another hour. Thereafter, the quartz tube was air-cooled from the outside to room temperature while keepingthe ammonia–argon mixture gas flow rate inside of the quartztube unchanged. Finally, ammonia was turned off and thesample was taken out from the quartz tube.

Fabrication of DSSCs

The N-GNR counter electrode was prepared by mixing theN-GNR with 5% (wt) PEO solution, which was then coated onthe FTO glass and heat treated at 400 °C under air for 3 hours.The Pt counter electrode was prepared by coating the Pt cata-lyst (Pt-Catalyst T/SP, Solaronix) on a FTO glass and calcinationat 400 °C for 30 min.

Preparation of the TiO2 photoanode

The FTO glass was cleaned by ultrasonication in the scouringagent, isopropanol and ethanol for 10 min, respectively. Afterdrying, FTO was further cleaned by the UV-O3 system at roomtemperature for 18 min. DSSC was prepared by blade coatingof the TiO2 paste (Ti-Nanoxide T, Solaronix) on the freshlycleaned FTO glass substrate using the 3M Scotch Magic tape tocontrol the thickness. The TiO2 paste was then calcinated at450 °C for 1 h. The resulting electrode was immersed in asolvent mixture of acetonitrile and isobutanol (v/v = 1/1) con-taining 0.5 mM N719 dye (Solaronix) for 24 h to form the

Table 2 Jsc, Voc, FF and PCE for DSSCs with the Pt, rGOR, and N-GNRas the counter electrodes, respectively

Counter electrode Jsc [mA cm−2] Voc [V] Fill factor Efficiency [%]

Pt 13.20 0.65 0.36 3.09rGORs 13.88 0.61 0.46 3.89N-GNRs-3 13.43 0.64 0.59 5.07

Fig. 5 (a) Photocurrent density–voltage (J–V) curves for DSSCs using T2/T− as the electrolyte redox couple and platinum, rGORs and N-GNRs-3,

respectively, as the counter electrodes. (b) Nyquist plots of asymmetric dummy cells with the Pt, rGOR and N-GNR counter electrodes, respectively.

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sensitized photoanode. Finally, the DSSC cell was fabricatedby packaging the photoanode and the counter electrode, andsealed by 60 µm think sealant film (SX1170-60, Solaronix Co)after an electrolyte (Iodolyte AN-50 from Solaronix) was added.

Characterization

X-ray photoelectron spectroscopy (XPS) measurements werecarried out on a VG Microtech ESCA 2000 using a monochro-mic Al X-ray source (97.9 W, 93.9 eV). The Raman spectra wererecorded using a Raman spectrometer (Renishaw) with a514 nm laser. The thermogravimetric analysis was performedon a TA instrument with a heating rate of 10 °C. X-ray diffrac-tion (XRD) was measured using a Miniflex II Desktop X-raydiffractometer. Scanning electron microscopy (SEM) imageswere taken on a JEOL JSM-6510LV SEM.

Acknowledgements

This work was supported financially by the AFOSR (FA-9550-12-1-0069), DOD-Army (W911NF-11-1-0209), NSF(CMMI-1400274, IIP-1343270), NSFC-NSF (DMR-1106160),CWRU-WMU (CON115346), NSFC (51202167), and the “Thou-sand Talents Program” of China.

References

1 S. Yanagida, Y. H. Yu and K. Manseki, Acc. Chem. Res.,2009, 42, 1827, and references cited therein.

2 H. J. Snaith and L. Schmidt-Mende, Adv. Mater., 2007, 19,3187.

3 T. W. Hamann, R. A. Jensen, A. B. F. Martinson, H. VanRyswyk and J. T. Hupp, Energy Environ. Sci., 2008, 1, 66.

4 M. K. Nazeeruddin, E. Baranoff and M. Gratzel, Sol. Energy,2011, 85, 1172.

5 B. Oregan and M. Gratzel, Nature, 1991, 353, 737.6 J. Y. Cong, X. C. Yang, L. Kloo and L. C. Sun, Energy

Environ. Sci., 2012, 5, 9180.

7 M. K. Wang, N. Chamberland, L. Breau, J. E. Moser,R. Humphry-Baker, B. Marsan, S. M. Zakeeruddin andM. Gratzel, Nat. Chem., 2010, 2, 385.

8 D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii,J. R. Lomeda, A. Dimiev, B. K. Price and J. M. Tour, Nature,2009, 458, 872.

9 X. L. Li, X. R. Wang, L. Zhang, S. W. Lee and H. J. Dai,Science, 2008, 319, 1229.

10 D. B. Shinde, J. Debgupta, A. Kushwaha, M. Aslam andV. K. Pillai, J. Am. Chem. Soc., 2011, 133, 4168.

11 L. Y. Jiao, X. R. Wang, G. Diankov, H. L. Wang andH. J. Dai, Nat. Nanotechnol., 2010, 5, 321.

12 H. L. Zhang, C. Y. Zhang and D. D. Shi, Adv. Mater. Res.,2011, 148, 1737.

13 M. Y. Han, B. Ozyilmaz, Y. B. Zhang and P. Kim, Phys. Rev.Lett., 2007, 98, 206805.

14 L. Tapaszto, G. Dobrik, P. Lambin and L. P. Biro, Nat. Nano-technol., 2008, 3, 397.

15 J. Campos-Delgado, J. M. Romo-Herrera, X. T. Jia,D. A. Cullen, H. Muramatsu, Y. A. Kim, T. Hayashi,Z. F. Ren, D. J. Smith, Y. Okuno, T. Ohba, H. Kanoh,K. Kaneko, M. Endo, H. Terrones, M. S. Dresselhaus andM. Terrones, Nano Lett., 2008, 8, 2773.

16 H. B. Wang, T. Maiyalagan and X. Wang, ACS Catal., 2012,2, 781.

17 Y. C. Ma, A. S. Foster, A. V. Krasheninnikov andR. M. Nieminen, Phys. Rev. B: Condens. Matter, 2005, 72,205416.

18 K. P. Gong, F. Du, Z. H. Xia, M. Durstock and L. M. Dai,Science, 2009, 323, 760.

19 L. T. Qu, Y. Liu, J. B. Baek and L. M. Dai, ACS Nano, 2010,4, 1321.

20 Y. H. Xue, J. Liu, H. Chen, R. G. Wang, D. Q. Li, J. Qu andL. M. Dai, Angew. Chem., Int. Ed., 2012, 51, 12124.

21 R. Cruz-Silva, A. Morelos-Gomez, S. Vega-Diaz, F. Tristan-Lopez, A. L. Elias, N. Perea-Lopez, H. Muramatsu,T. Hayashi, K. Fujisawa, Y. A. Kim, M. Endo andM. Terrones, ACS Nano, 2013, 7, 2192.

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