Chemical Physics Letters 493 (2010) 296–298

Contents lists available at ScienceDirect

Chemical Physics Letters

journal homepage: www.elsevier .com/locate /cplet t

Electrochemistry and electrochemiluminescence study of blue luminescentcarbon nanocrystals

Jigang Zhou a,c, Christina Booker a, Ruying Li b, Xueliang Sun b, Tsun-Kong Sham a, Zhifeng Ding a,*

a Department of Chemistry, The University of Western Ontario, London, ON, Canada N6A 5B7b Department of Mechanical and Materials Engineering, The University of Western Ontario, London, ON, Canada N6A 5B7c Canadian Light Source Inc., Saskatoon, SK, Canada S7N 0X4

a r t i c l e i n f o

Article history:Received 6 April 2010In final form 10 May 2010Available online 13 May 2010

0009-2614/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.cplett.2010.05.030

* Corresponding author.E-mail address: zfding@uwo.ca (Z. Ding).

a b s t r a c t

Electrochemistry and electrochemiluminescence (ECL) have been applied to study the blue luminescentcarbon nanocrystals (NCs). It is found that electrons can be injected into the NCs in sequence. This obser-vation suggests that carbon NCs can stably store charge in solution. The positively charged NCs werefound to be more stable than the negatively charged NCs. The red shifted ECL (relative to photolumines-cence) suggests that the ECL emission from carbon NCs originates from surface traps.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Luminescent carbon nanostructures have recently been of inter-est as a new class of fluorophores. They have unique potential inbiological labeling and biomedicine due to their intrinsic environ-mental compatibility [1–7]. They are also attractive building blocksin fabricating large scale optoelectronic devices, such as, solar cells,organic light emitting diodes (OLED) and field emitters [8,9].

To render carbon nanostructures luminescent, the general ap-proach is to functionalize fullerene, carbon nanotube (CNTs), orgraphite nanoparticles, and these methods have been well ex-plored [3,4,6,10–12]. Electrochemical treatment of CNTs has beendemonstrated recently as another avenue to highly efficient blueluminescent carbon nanocrystals (NCs) [1]. To gain insights intothe origin of the luminescence and to facilitate processing, theunderstanding of the electronic structure and the nanostructurestability upon charge transfer in these emitters is crucial. It hasbeen speculated that the sp2 carbon-rich phase in a sp3 matrix con-tributes to the luminescence in the nanostructured carbon thinfilm [13]. Owing to the strong electron accepting ability in fuller-ene and single walled carbon nanotubes (SWCNTs), the visibleluminescence from their derivatives has been attributed to chargetransfer between the amine group in the passivating molecule andthe targeted carbon nanostructures [5,7]. Such interaction is be-lieved to be responsible to the formation of an emissive adduct.The solvatochromic effect of the luminescence of the derivativeto the solvent provides the experimental evidence for this explana-tion [4,5]. Surface state stabilization by passivation has been analternative model in understanding the luminescence mechanism[2,3]. However, the electronic structure of the luminescent carbon

ll rights reserved.

nanostructures has never been reported. Furthermore, the stabilityof the nanostructures upon charge transfer needs to be verifiedexperimentally.

This report applies electrochemistry and electrochemilumines-cence (ECL) to elucidate the electronic structure of blue lumines-cent carbon NCs and the stability of charged carbon nanocrystals.Electrochemistry has been applied to investigate the electronicstructure of these nanomaterials to explore the band gap and bandedge [14]. Electrochemistry of NCs is a charge transfer process atthe interface between the electrode and the NCs that have diffusedto the electrode [15]. Differential pulse voltammetry (DPV), inwhich the current difference before and after the pulse (which issuperimposed on a staircase potential) is collected as the signal,can minimize background charging current [16]; thus it is espe-cially useful in studying low concentration systems. Along withthe electrochemistry, ECL is a light emission process through ener-getic electron transfer between electrochemistry-generated re-agents. ECL of free diffusing NCs in solution can be realizedeither through the annihilation between the reduced and oxidizedNCs or through the hole (electron) injection into the reduced (oxi-dized) NCs by a co-reactant. ECL from NCs can elucidate theirelectronic structures, relative stability of charged NCs, emittingstate properties and light emission mechanism [15,17].

2. Experimental

The carbon NCs were generated by an electrochemical methodreported recently [1]. The NCs were purified upon aqueous extrac-tion before re-dispersement in the electrolyte solution for electro-chemistry and ECL studies. The carbon NCs were dispersed indichloromethane with tetrabutylammonium perchlorate (TBAP)as a supporting electrolyte. The electrochemical property and ECLof freely diffusing carbon NCs were carried out in an air-tight

J. Zhou et al. / Chemical Physics Letters 493 (2010) 296–298 297

electrochemical cell. The cell consisted of a Pt working electrode(A = 0.07 cm2), a Pt coil as the counter electrode, and a silver wireas the quasi-reference electrode (Ag QRE). The cell potential couldbe referred to the saturated calomel electrode (SCE) using Fc/Fc+

couple [18]. Differential pulse voltammograms (DPVs) were ob-tained using an electrochemical workstation, CHI 610A (CH instru-ments, Austin, Texas). The experimental details are as follows:0.05 V pulse height, 60 ms pulse width, 200 ms period and0.02 V/s scan rate. Voltammetric ECL curves were obtained usingthe CHI 610A coupled with a photomultiplier tube (PMT, R928,Hamamatsu, Japan) held at �750 V with a high voltage power sup-ply. ECL intensity was recorded by this PMT. The ECL spectrum wasrecorded by the spectrograph (Spectra Physics, Cornerstone 260M). The photoluminescence (PL) spectrum of the carbon NCs dis-persed in the electrolyte solution was recorded by a Spex Fluorolog3–11 fluorimeter (Horiba Scientific, Japan).

3. Results and discussion

A typical DPV for carbon NCs is displayed in Fig. 1 where the re-sponse for the supporting electrolyte alone is compared. Discretereduction steps are exhibited in the negative potential region. Theycorrespond to the single electron injections into NCs. Similar DPVresponses have been reported on Si NPs [15] and thiol-capped me-tal clusters. The sequenced charging at 0.03/�0.03 V, �0.44/�0.66 V, �0.94/�1.12 V (anodic/cathodic) and �1.41 V reflectsthe chemical stability of the charged NCs. Interestingly, thesehighly reproducible reduction waves have peak-to-peak separa-tions between 0.6 V and 0.3 V. This discrete charging can be attrib-uted to the quantized double-layer charging in a small sized NCwhere the sub-attofard (aF) capacitance of the NC gives rise to acharging energy much larger than the kBT [15] (0.026 eV at roomtemperature). This separation reflects the extra energy (called Cou-lomb capacitive charging energy) required to add another electroninto the charged NCs. The capacitance of individual NCs in thisstudy should be comparable to Si NCs considering the similar sizes;hence the energy is expected to be close in both situations. Thebroadening reduction peaks of the carbon NCs are not as well re-solved as those of their Si counterparts. This could be attributedto the relatively large size distribution deviation in carbon NCs(2.8 ± 0.5 nm) [1] than that in Si NCs (2.77 ± 0.37 nm) [15]. Furthermore, the reduction at �1.4 V is fairly reversible as seen in thesymmetric anodic and cathodic peaks. An irreversible oxidation

Fig. 1. Differential pulse voltammograms (DPVs) for carbon NCs in 0.1 M tetrabu-tylammonium perchlorate (TBAP) dichloromethane solution. The dotted curve isthe response of the blank supporting electrolyte solution.

peak at 1.5 V is observed as well. It may reflect the position ofthe highest occupied molecular orbital (HOMO).The band gap ofthe NCs can be determined to be larger than 2.95 eV from the PLpeak at 420 nm as shown in Fig. 4. Therefore, the position of thelowest unoccupied molecular orbital (LUMO) must be higher than�1.4 V. It can thus be proposed that the reduction peaks (�0.0 and�1.2 V) before �1.4 V are associated with surface states. Thereductions through the surface states are not entirely reversibleas seen by the non-symmetric anodic and cathodic peaks. Rich sur-face states are expected on these carbon NCs as seen by the excita-tion energy dependent PL spectrum [1]. Such surface states can bethe localized traps for the electrons and holes.

When the potential is scanned or pulsed between �2.3 V and+2.3 V as shown in Figs. 2 and 3, significant light emission (ECL)is detected on the working electrode. ECL only turns on at a poten-tial higher than �2.0 V, which is much larger than the energy ofreversibly injecting electrons into NCs (�1.4 V). It has been re-ported that ECL only occurs when the potential is beyond thethreshold at which the electron and hole can be injected into SiNPs [15]. This can be related to the energetic feature of ECL in gen-erating exited states. When the potential scan is reversed, a wideECL peak is observed. Interestingly, only very weak ECL transientis observed at the positive region. As discussed in the DPV ofNCs, positively charged and negatively charged NCs can be gener-ated when the potential is above the threshold. If the ECL isthrough the annihilation of oppositely charged NCs, the chargedNCs have to be chemically stable long enough to meet each otherand undergo the energetic electron transfer reaction. It can thusbe concluded that the positively charged NCs are more stable thannegatively charged NCs. This is not too surprising, since in generalpositive moieties can be stabilized by the solvent sphere via polar-ization. However, it has been observed in CdTe [18] that a co-reac-tant mechanism is possible to generate the excited NCs throughthe injection of a hole from the CHCl� radical into negativelycharged NCs. This co-reactant mechanism will also account for amore intense ECL transient in the negative potential region as ob-served in this study. A stable ECL transient is detected when thepotential is fixed at �2.3 V in the pulsing situation as seen inFig. 3. This stable ECL is expected as it has been seen in the poten-tial scanning situation in Fig. 2. Thus the behavior observed inFig. 3 is consistent with the chemical stability of the positivelycharged NCs and/or the co-reactant mechanism. It may also reflectthe fast diffusion characteristic of carbon NCs. In contrast, the fastdecaying transient dominates in previously reported ECL fromfreely diffused nanomaterials [15,18]. The similarly stable ECLtransient can be obtained from the NC thin film where reactionsare not limited by diffusion [19]. Interestingly, a sharp ECL tran-sient is exhibited with a very short lifetime at the beginning of

Fig. 2. ECL-potential curve of carbon NCs in 0.1 M tetrabutylammonium perchlo-rate (TBAP) dichloromethane solution. The potential scanning rate is 0.5 V/s.

Fig. 3. ECL transient with applied potential pulsing between +2.3 V and �2.3 V.

Fig. 4. ECL spectrum (integration time of 1 min) of carbon NCs in 0.1 M tetrabu-tylammonium perchlorate (TBAP) dichloromethane solution obtained by scanningpotential between +2.3 V and �2.3 V with a scanning rate at 0.5 V/s along with ECLrecoding with a blank electrolyte solution. Photoluminescence spectrum (PL) ofthese carbon NCs in the electrolyte solution is also plotted.

298 J. Zhou et al. / Chemical Physics Letters 493 (2010) 296–298

the positive potential. This observation reinforces the relativelyhigh stability of the positively charged carbon NCs. Pulsing carbonNCs between �2.3 V and +2.3 V, the ECL efficiency has been deter-mined to be about 0.06% relative to diphenylanthracene (DPA). Ifperoxide was used and the NCs were pulsed between 0 V and�2.3 V, the efficiency was enhanced by a factor of 10. Thisenhancement was expected considering that a co-reactant systemcan overcome problems associated with the short life time ofcharged NCs [15].

The ECL spectrum is shown in Fig. 4 and is compared with thePL spectrum. The ECL spectrum is broader relative to PL spectrum.The ECL peak position at 470 nm is red shifted about 50 nm fromthe PL peak position. A shoulder at 420 nm (maximum PL emis-sion) can be seen in the ECL spectrum. Most ECL from semiconduc-tor nanomaterials have been observed to originate from surfacestates, which were often significantly red shifted from the PL peaksby as much as hundreds of nanometers since these defect states are

located in the band gap [15,17]. It is worthy mentioning that theefficient blue ECL emission from carbon NCs is also technologicallyrelevant.

In conclusion, we have demonstrated that electrochemistry andECL can be applied to elucidate the electronic structure, the stabil-ity of charged NCs and the emission mechanism in the luminescentcarbon nanocrystals. Our results show that electrons can be in-jected into the NCs in sequence, which suggests that carbon NCscan store charge stably in solution. The positively charged NCshave been found to be more stable than the negatively chargedNCs. The red shifted ECL spectrum relative to PL suggests thatthe ECL origin in carbon NCs is through the surface traps.

Acknowledgements

Works done at UWO were financially supported by NESRC, CFI,OIT, OPT, ADF, PREA, and CRC (T.K. Sham). J.G. Zhou is grateful toOGSST support.

References

[1] J. Zhou, C. Booker, R. Li, X. Zhou, T.-K. Sham, X. Sun, Z. Ding, J. Am. Chem. Soc.129 (2007) 744.

[2] Y.-P. Sun et al., J. Am. Chem. Soc. 128 (2006) 7756.[3] Y. Lin et al., J. Phys. Chem. B 109 (2005) 14779.[4] J.X. Cheng, Y. Fang, Q.j. Huang, Y.J. Yan, X.Y. Li, Chem. Phys. Lett. 330 (2000)

262.[5] Y. Sun, S.R. Wilson, D.I. Schuster, J. Am. Chem. Soc. 123 (2001) 5348.[6] F. Zhang, Y. Fang, J. Phys. Chem. B 110 (2006) 9022.[7] G. Schick et al., J. Am. Chem. Soc. 121 (1999) 3246.[8] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787.[9] X. Zhou et al., J. Am. Chem. Soc. 129 (2007) 1476.

[10] L. Qu, X. Peng, J. Am. Chem. Soc. 124 (2002) 2049.[11] J.E. Riggs, Z. Guo, D.L. Carroll, Y.-P. Sun, J. Am. Chem. Soc. 122 (2000) 5879.[12] S.R. Cordero, P.J. Carson, R.A. Estabrook, G.F. Strouse, S.K. Buratto, J. Phys.

Chem. B 104 (2000) 12137.[13] S.J. Henley, J.D. Carey, S.R.P. Silva, Appl. Phys. Lett. 85 (2004) 6236.[14] S.K. Haram, B.M. Quinn, A.J. Bard, J. Am. Chem. Soc. 123 (2001) 8860.[15] Z. Ding, B.M. Quinn, S.K. Haram, L.E. Pell, B.A. Korgel, A.J. Bard, Science 296

(2002) 1293.[16] A.J. Bard, L.R. Faulkner, Electrochemical Methods, New York, 2001.[17] N. Myung, Z. Ding, A.J. Bard, Nano Lett. 2 (2002) 1315.[18] Y. Bae, N. Myung, A.J. Bard, Nano Lett. 4 (2004) 1153.[19] Y. Bae, D.C. Lee, E.V. Rhogojina, D.C. Jurbergs, B.A. Korgel, A.J. Bard,

Nanotechnology 17 (2006) 3791.