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Solar Energy 105 (2014) 236–242

Synthesis of graphitic carbon nano-onions for dye sensitized solar cells

Ian Y.Y. Bu ⇑

Department of Microelectronics Engineering, National Kaohsiung Marine University, 81157 Nanzih District, Kaohsiung City, Taiwan, ROC

Received 14 October 2013; received in revised form 21 February 2014; accepted 11 March 2014

Communicated by: Associate Editor Frank Nuesch

Abstract

This study examines dye sensitized solar cells (DSSCs) incorporated with carbon nano-onions as counter electrodes. The synthesizedcarbon nano-onion nanoparticles were deposited through candle flame synthesis on Cu sputter coated glass substrates. The depositedcarbon nano-onions were extensively characterized through scanning electron microscopy, energy disperse spectroscopy, transmissionelectron microscopy, Raman spectroscopy. It was found that highly graphitic, interconnected carbon nano-onion with mean diameterof around 30 nm can be formed through the proposed method. Cyclic voltammetry and subsequent DSSCs performances indicated thatthe carbon nano-onion based counter electrode exhibit comparable performances to conventionally used Pt and could be scaled up inindustrial production.� 2014 Elsevier Ltd. All rights reserved.

Keywords: Carbon nano-onion; Dye sensitized solar cells; Candle synthesis

1. Introduction

Ever since the discovery of carbon nanotubes (Iijimaand Ichihashi, 1993; Iijima, 1991), there have been inten-sive interests in carbon-based nanomaterials due to theirunique electronic and mechanical properties. The investi-gated carbon based nanomaterials include amorphous car-bon (Robertson and O’reilly, 1987), diamond (Isberg et al.,2002), fullerene (Liu et al., 1998), nanotubes (Saito et al.,1998; Tune et al., 2010), nano-onion (Ding et al., 2005),nanohorns (Yoshitake et al., 2002), nanofibers (Kothariet al., 2008) and graphene (Novoselov et al., 2004) andhave been produced by different deposition techniques suchas chemical vapor deposition (CVD) (Kong et al., 1998),arc discharge (Hutchison et al., 2001) and flame synthesis(Height et al., 2004). Typically, the CVD growth of carbon

http://dx.doi.org/10.1016/j.solener.2014.03.015

0038-092X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +886 972506900; fax: +886 73645589.E-mail address: ianbu@hotmail.com

nanomaterials can be achieved by growth on heated cata-lyst (Fe, Ni or Co) with a flow of carbon containing gases(C2H2 or CH4) (Meyyappan et al., 2003). The proposedgrowth mechanism is vapor–liquid–solid, with the heatedcatalyst saturated with C species that results in precipita-tion and growth of CNTs. Similar mechanism can bemodified to produce graphene by flowing of CH4 over Nior Cu sheets (Reina et al., 2008; Gomez De Arco et al.,2010).

One of the proposed applications for carbon nano-materials are in dye sensitized solar cells (DSSCs) (Xiaand Yanagida, 2011; Nazeeruddin et al., 2011). Typically,DSSCs solar cells are constructed with fluorine doped tinoxide (FTO) conductive glass/TiO2 nanoparticle/Ru-baseddye/iodine based electrolyte/Pt counter electrode/FTO. Inorder to scale up for industrial module production ofDSSCs, the expensive and rare Pt used in counter electrodemust be replaced by an inexpensive alternative material(Kay and Gratzel, 1996). The ideal replacement material

I.Y.Y. Bu / Solar Energy 105 (2014) 236–242 237

must possess high electrical conductivity (to inject electroninto the electrolyte) and sufficient chemical resistance to thehighly corrosive iodine. The obvious candidate is carbon-based nano-materials due to its large effective surface area,high electrical conductivity and chemical stability.

Previous research have already shown that several allo-tropes of carbon, multiwalled carbon nanotubes (Kyawet al., 2011; Anwar et al., 2013; Sayer et al., 2010), dopedcarbon nanotubes (Kyaw et al., 2011; Tantang et al.,2011), graphene (Kavan et al., 2011; Cruz and Pacheco,2012; Guai et al., 2012), carbon black (Li et al., 2009)and glassy carbon (Xu et al., 2011), have been successfullyintegrated into DSSCs. Through the utilization of carbonnano-material as counter electrode in DSSCs, acceptablesolar cell performances of �6% has been achieved (Namet al., 2010). The barrier that prevents a full industrial uti-lization with most of these nanomaterials are the high pro-duction costs associate with expensive manufacturingequipments. Candle synthesis of carbon nanomaterials isa promising process technique due to its simplicity andcontinuous production in atmosphere without complexapparatus (Merchan-Merchan et al., 2010). Already, highquality MWCNTs has been obtained by flame synthesisof Co catalyst coated substrate via commercially availablecandles (Li and Hsieh, 2007a).

From processing point of view, a catalyst-less based pro-cess is preferred due to reduced contamination and lowerprocess cost. In this study, carbon nano-onions were syn-thesized by flame synthesis technique on Cu coated glasssubstrate. The carbon nano-onions were characterizedthrough scanning electron microscopy (SEM), transmis-sion electron microscopy (TEM), Energy dispersive spec-troscopy (EDS) electrochemical measurements andRaman spectroscopy. Subsequently, the carbon nano-onions were utilized as counter electrode in DSSCs anddemonstrated its potential as a replacement for conven-tionally used pt.

2. Experimental

Fluorine doped tin oxide (FTO) and Corning glass sub-strates were cleaned by ultrasonic agitation in acetone, iso-propanol (IPA) and distilled water, for 10 min within eachsolvent. Around 10 nm of Cu thin film was deposited onthe substrates using a 2-in. Direct Current (DC) magne-tron-sputtering coater using a 99.9% pure Cu target. Thesubstrates were introduced into the sputter chamber (withtarget-to-substrate distance at 9 cm) and turbo-pumpeddown to base pressure of around 5 � 10�6 mTorr. Subse-quently, high purity argon gases were injected into thechamber using mass flow controllers. For the Cu deposi-tions, the working pressure, power, temperature and dura-tion were set to �5 � 10�4 mTorr, 33 �C, 120 W and 30 s,respectively. Common household candles (Shin Hong DarMetal Design Ltd.) were used as source of carbon for thisstudy. Typically, such candles consist of 90% alkanes withcarbon numbers range from C18 to C40. It is important to

note that the size and thickness of the obtained carbonnanomaterial could depend on the ingredients used duringthe manufacturing of candles (e.g. paraffin, beeswax andtallow). The samples were held around 5 cm from the sur-face of the candle, which is around 600 �C (Li and Hsieh,2007b) for a period of 30–120 s.

The Scanning electron microscope (SEM) image andfilm composition of the samples was obtained using anFEI Quanta 400 F Environmental Scanning ElectronMicroscope (ESEM) equipped with Energy dispersiveSpectroscopy EDS. FT-IR spectra were recorded using aFT-IR Bruker IFS 66 V/S spectrometer in a frequencyrange of 4500–500 cm�1. A Dongwoo Macro Raman spec-trometer/PL system was used to obtain Raman spectra andevaluate the quality of the carbon film. Electrical resistivityof the carbon nano-onion paste was determined by scrap-ing carbon nano-material by doctor blade method. Theresultant carbon solids were dispersed in IPA solutionand spin coated onto glass substrate to for uniform thinfilm. Four probe measurements were performed to deter-mine the sheet resistance of the films. Cyclic voltammetrymeasurements were conducted in a conventional three-elec-trode set-up in 3-methoxypropionitrile solution containing1 M 1.3-dimethylimidazoliumiodine, 0.5 M 4-tert-butyl-pyridine, 0.15 M iodine, and 0.1 M Guanidine thiocyanate(Sigma Aldrich). DSSCs were fabricated by doctor blademethod �15 lm of TiO2 (Degussa) onto pre-cleaned fluo-rine doped tin oxide. The TiO2 photoanodes were dye-sen-sitized by immersion into a 5 � 10�4 M ethanoic solutionof ruthenium-based N719 dye for at least 1 day. After thedye impregnation process, the photoanodes were washedwith ethanol to remove excess dye. Two types of counterelectrodes were fabricated: conventional Pt coated FTOand carbon nano-onion/Cu/Corning substrate. Pt counterelectrodes were fabricated by spin coating “Eversolar Tai-wan” Pt solution onto cleaned FTO substrates and bakedat 450 �C for 10 min to remove organic residues, whereascarbon-onion based counter electrodes were fabricated byaforementioned method for 90 s on copper coated Corningglass. DSSCs were constructed by sandwich a Suryn sheetbetween the TiO2 photoanode and Pt coated glass. Thenthe DSSCs cell were sealed by hot compress the Suryn sheetwith redox couple electrolyte, 0.1 M LiI, 50 mM I2, 0.6 M1,2-dimethyl-1,3-propylimidazolium iodide, 1 M tert-butylpyridine, and 3-methoxypropionitrile, injectedbetween the counter and photoanode. Current density–voltage (J–V) characteristics were measured using a Keith-ley 2400 source-measure unit, under illumination (100 mW/cm2), provided by a solar simulator (Science-tech). Cyclicvoltammetry was performed in a three-electrode systemin an acetonitrile solution of 0.001 M I2, 0.01 M LiI and0.1 M LiClO4.

3. Results and discussion

Fig. 1(a–d) shows the SEM images of carbon nanomate-rials synthesized on Cu coated glass substrates for 30 s,

Fig. 1. SEM images of the deposited carbon nano-onion at (a) 30 s, (b) 60 s, (c) 90 s and (d) 120 s (scale bar 2 lm).

238 I.Y.Y. Bu / Solar Energy 105 (2014) 236–242

60 s, 90 s and 120 s. Unlike previous study of candle flamesynthesis of CNTs via Co catalyst (Li and Hsieh, 2007c),only carbon nano-onions were found on the substrate.Fig. 1(a) shows the deposition for 30 s at higher magnifica-tion and reveals the nano-onions with diameter of around20–50 nm. Individual carbon nano-onion was found to beinter-connected to each other near the contact points.From the SEM image, it can be observed that the coverageof carbon nano-onion increases with increase in depositiontime. Clearly, the role of Cu is different from previouslystudy using Co catalyst. Firstly, candle synthesis of carbonnanomaterials from Co catalyst resulted in deposition ofcarbon nanotubes (Li and Hsieh, 2007c), in contrast Cuthin film resulted in carbon nano-onions. Furthermore,Co catalyst tends to peel off with increase in depositiontime, whereas Cu catalyst appears stable within the investi-gated deposition time.

Fig. 2 shows the EDX composition study of the carbonnano-onion/Cu coated glass substrate. The correspondingEDX analysis reveals the composition of nano-onion oncopper coated substrate consists of C (40.10 at%), O(11.36 at%) and Cu (48.54 at%). For EDS analysis, thedetection error is 0.1 at%. In general, copper forms twotypes of oxides: Cu2O and CuO, with very different opto-electronic properties. Cu2O is a p-type semiconductor withhigh optical transmittance, whereas CuO is a highly con-ductive n-type semiconductor with an opaque appearance.Although Cu2O is the native oxide of copper, it is difficultto obtain stoichiometric Cu2O, as it is more energeticallyfavorable to form CuO (Ray, 2001). As oxygen tends toetch carbon at temperature >450 �C (Bu et al., 2011), some

of the oxygen is believed to be bonded to Cu to form CuO.Fig. 2(b) shows the FTIR analysis of the deposited sample.The FTIR analysis suggests that in addition to the synthe-sis of carbon nano-onions, there appears to be incompleteburning of the candles with the presence of CO and OHgroups, which correlates well with other studies (Liuet al., 2007; Zhang et al., 2014).

Base on the aforementioned mechanism, the formationof carbon nano-onions is believed to be a competitivemechanism between growth from supply of carbon sourceand etching by oxygen. The role of Cu can be attributedto a surface precipitation growth mechanism due to lowsolubility of C in Cu (Li et al., 2009).

Detailed structure properties of the synthesized carbonnano-onion were investigated through TEM and presentedin Fig. 3(a and b). HRTEM image shows a typical multi-shell. Unlike carbon nanomaterials prepared by arc dis-charge method (Imasaka et al., 2006), that consisted ofhomocentric structure. The crystallographic planes of theobtained carbon nano-onions are not as well-defined. Thespiral structure can be identified to distribute randomlyat sections of the nano-onion. It should be noted that somemulti-core particles is also be observed. An importantfeature is that carbon nano-onions via candle synthesisare formed without encapsulation of catalyst. The absenceof catalyst is a distinct process advantage as it eliminatesthe additional removal purification process. Generally,the catalyst plays an important role in carbon nanomaterialsynthesis. Carbon are absorbed and decomposed into themetal catalyst particle. Consequently, catalyst is encapsu-lated by graphitic carbon. The discontinuous curved lines

Element Wt% At%CK 12.85 40.10OK 04.85 11.36CuK 82.30 48.54

Matrix Correction ZAF

(a)

(b)

Fig. 2. Representative EDS composition analysis of carbon nano-onion/Cu coated glass substrate.

Fig. 3. (a) HR TEM image of the carbon nano-onion and (b) TEM image of the same sample showing the intercalation.

I.Y.Y. Bu / Solar Energy 105 (2014) 236–242 239

on the HRTEM is assumed to be associated with the for-mation of the various carbon rings during the growth ofcarbon nano-onions. It is well known that carbon atomscan arrange in several types of graphitic carbon rings.The flat graphitic layer is composed of carbon arrangedin hexagonal configurations (Oku et al., 2004). Arrange-ment in pentagonal and heptagonal carbon rings causesthe network to bend upwards and downwards respectively(Oku et al., 2004; Chen et al., 2001). The formation of

heptagonal and pentagonal atom rings is the cause for car-bon nanotube bending and closure cap. Pentagonal atomicrings are basis of formation of perfect bucky-ball C60.However, if the heptagonal carbon rings are also synthe-sized, the carbon sheet will be wrinkled (Chen et al.,2001). In such case, carbon nanostructure exhibit discon-tinuous lines in the TEM image. From previous studieson carbon nano-onion formation, it can be attributed totwo different mechanisms. In the case for arc discharge

(a)

(b)

Fig. 4. (a) Raman spectra of the synthesized carbon nano-onion (coloredversion of this graph can be found online) and (b) extracted ID/IG valuefrom the Raman spectra as function of deposition time. (For interpreta-tion of the references to color in this figure legend, the reader is referred tothe web version of this article.)

Fig. 5. Cyclic voltammogram of I2/I� for the Pt coated FTO substrateand carbon nano-onion on Cu/glass substrate.

240 I.Y.Y. Bu / Solar Energy 105 (2014) 236–242

synthesis, the formation of curvature graphite sheets occursduring the cooling of carbon (Banhart and Ajayan, 1996).On the other hand, during the thermal treatment of amor-phous carbon, it transforms into multiple shells of crystal-line graphite structure (Kuznetsov et al., 1994). Fig. 3(b)shows the HR-TEM image of the sample at lower magnifi-cation. It can be observed from Fig. 3(b) that the depositedfilms are intercalated with diameter range 20–30 nm. Thesmall diameter of the deposited carbon nano-onion meansmore surface area is available for reaction and thereforebeneficial toward DSSCs application. TEM image alsoindicated that the carbon nano-onions are intercalatedwithout the need of additional of binders such as TiO2.Past studies have revealed that around 5 wt% of TiO2 isrequired to ensure good carbon binding (Kay and Gratzel,1996). Although TiO2 addition is necessary to ensure goodadhesion of carbon nanomaterial on FTO surfaces, it doesnot contribute to the catalytic process. Furthermore, com-pared with carbon, TiO2 nanoparticles are resistive thatresult in higher charge resistance and degrade DSSCperformances.

Raman spectroscopy is a powerful and non-destructivemethod to evaluate graphitic carbon materials. Fig. 4(a)shows the Raman spectra of carbon nano-onion with incre-mental deposition time with characteristics for sp2 carboninduced (ID) 1345.3 cm�1 and graphite (IG) 1571.58 bands.Where the ID peak is associated with the dangling bondsvibrations of carbon atoms for the in-plane terminationsof disordered graphite and the IG peak has been assignedfor vibrations in all sp2 bonded carbon atoms in a two-dimensional hexagonal lattice, respectively. From theRaman spectra the integrated ID/IG ratio (with standarddeviation errors) can be extracted to evaluate the qualityof the carbon material.

Fig. 4(b) shows the extracted integrated ID/IG ratio asfunction of deposition time. It can be observed that theratio decreases with increasing deposition time and con-firms an improvement in the degree of graphitization andreduction of disorder. The relatively high ID/IG ratio sug-gests that the obtained carbon nano-onion are composedof partially crystallized graphite in good agreement withTEM data.

Cyclic voltammetry was performed to evaluate the elec-trochemical catalytic activities of the carbon nano-onionand pt to reduce triiodide (scanning from �0.8 to 0.8 v).Fig. 5 shows the cyclic voltammograms of I2/I� for thept and carbon nano-onion coated FTO substrate. Twopairs of redox peaks were observed in both counter elec-trode. The relative negative pair is assigned to the redoxreaction (1) and the positive one is assigned to redox reac-tion (2)

I�3 þ 2e� ¼ 3I� ð1Þ3I2 þ 2e� ¼ 2I�3 ð2Þ

The size and shape of the peaks indicate the catalyticperformance of the material against the redox couple.Two pairs of oxidation and reduction peaks were found

I.Y.Y. Bu / Solar Energy 105 (2014) 236–242 241

from carbon nano-onion based counter electrode similar tothose in Pt electrode. In addition, the carbon nano-onionbased counter electrode exhibits a large current densitycompared with Pt, demonstrating a large electrode activearea, that leads to high electrochemical catalytic activity.

Subsequently, DSSCs were fabricated using carbonnano-onion and Pt as counter electrode, respectively.Fig. 6 shows the photocurrent density–photovoltage (J–V) curves of the fabricated DSSCs and summarized withinFig. 6. The power conversion efficiency of the carbon nano-onion counter electrode was comparable to that of the Ptcounter electrode at 3.39 ± 0.6% as to 4.37 ± 0.25% forPt based counter electrode. DSSCs fabricated using carbonnano-onion counter electrodes showed similar values of fillfactor (F.F.), 6% less open circuit voltage and around 25%less short circuit current. The high conductivity of the car-bon nano-onion has resulted in reduced series resistancewithin the device and hence yielded comparable fill factorto Pt counter electrode. The slight shift in Voc of carbonnano-onion counter electrode DSSC can be attributed toshift of I�/I3

� redox energy level (Durr et al., 2006).Although CV measurements suggests carbon nano-onionexhibits higher redox reactivity, the fabricated DSSCsshows lower power conversion efficiency. Previous theoret-ical study has suggested that surface defects, such as edge–plane defects, are responsible for the catalytic activity ofcarbon (Banks et al., 2005). The calculated data was con-firmed by experimental study that introduced defects onthe surface of CNTs using ozone (Trancik et al., 2008).The carbon nano-onions investigated in this study containsignificant amount of defects, with the appearance of dis-continuous shells from the TEM image. Therefore, it isbelieved that the combination of high surface area anddefect-rich surface resulted in the high DSSC performance.Clearly, the reduction in short circuit current in carbononion based DSSCs contributed the most toward low effi-ciency. Past studies have revealed that the short circuitcurrent is influenced by the composition of the dye,

Fig. 6. Shows the photocurrent density–photovoltage (J–V) curves of thefabricated DSSCs using pt and carbon nano-onion counter electrodes.

effective photoanode surface area and electrolyte. Noneof these parameters were changed between the Pt and car-bon nano-onion samples during the study. One other pos-sible cause for the reduced performance in carbon nano-onion-based DSSC could be the substrate used. In the pres-ent study, carbon nano-onions were deposited on Cucoated Corning substrate with resistivity �23 X/square,whereas, Pt were deposited on FTO coated glass substrate(resistivity �15 X/square). It is well-know that the perfor-mances of the DSSCs are strongly dependant on the resis-tivity of the film (Lee et al., 2010; Bu, 2014a,b). Thedifferences in resistivity can affect the electron transfer pro-cess and influence the Jsc of the DSSC device, similar mech-anism has been observed in other TCO-less DSSC study(Lee et al., 2010).

4. Conclusion

In this paper, we have successfully demonstrated amethod for high yield production of carbon nano-onionthrough candle combustion synthesis. The deposited car-bon nano-onion was extensively characterized by SEM,TEM and Raman spectroscopy. It was found that highlygraphitized carbon nano-onion can be synthesized ataround 90 s. Further increase in deposition time was foundto degrade the graphitization of the nanomaterial. The syn-thesized carbon nano-onion was used as counter electrodein DSSCs and demonstrated comparable power conversionefficiency to commonly used Pt.

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