Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Rd., Hong KongState Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P. R. ChinaDepartment of Chemical Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan
r t i c l e i n f o
rticle history:eceived 24 December 2013eceived in revised form 25 March 2014ccepted 25 March 2014vailable online 2 April 2014
a b s t r a c t
In this paper, a facile and scalable aqueous process including mild oxidative intercalation, microwaveexfoliation, ultrasonication, drying and Ar-annealing is developed to synthesize the water-solublemicrowave-exfoliated graphene (MEG)/platinum nanoparticles (PtNPs) composite, which has a relativelow defect level and can be readily dispersed in deionized water without adding surfactants. This low costsynthesis method is applicable in many systems, such as supercapacitors, thermal storage, lithium bat-
eywords:ater-solubleicrowave-exfoliated graphene
ompositelexible counter electrodeye-sensitized solar cells
tery and Dye-sensitized solar cells (DSSCs). An efficiency of 6.69% for the MEG/PtNPs composite depositedon ITO PEN as flexible counter electrode (CE) for DSSCs has been obtained, higher than the control devicemade by PVP-Pt as flexible CE.
Graphene-based composite materials, such as graphene-olymer or graphene-nanoparticle composites, have attractedlobal attention due to their unique combination of thermal,echanical, chemical and electronic properties [1–4]. The man-
facturing of such composites requires the graphene sheetsispersed in a media that is compatible with polymers or nanopar-icles. Chemical vapor deposition (CVD) is a method to producehe graphene with continuous areas of pristine monolayer, butot cost-effective for mass production [5,6]. Hummers method
s the most common used aqueous approach for preparation ofeduced graphene oxide (RGO) [7–9]. It uses the strong oxidizerH2SO4/KMnO4) to intercalate graphite layers followed by chemicaleduction (NaBH4 or hydrazine) [10–12]. Then, a good exfoliationerformance of RGO can be obtained in high polarity solvents, such
s N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidoneNMP) [13–16]. However, the harsh oxidization process wouldamage the pristine sp2 carbon-bonded structure to induce many
structural defects, and the use of organic solvent causes the processcomplexity, high cost, and organic contamination. Therefore, froma practical application point of view, the “green” water dispersion ofgraphene materials is preferred [17–19]. Generally, the graphenematerials are intrinsic hydrophobic, which are difficult to be dis-persed in water. Surfactant has been commonly used to keep stableexfoliated graphene sheets in water, but the formation of covalentbond or the presence of the polymeric covering on graphene sur-face may be undesirable for the further surface modification andfunctionalization to form the graphene-based composite materials.Even though many other diazonium salt methods have been devel-oped to synthesize water-soluble graphene which can be dispersedin water without the use of surfactant stabilizers, the employmentof organic solvents influences the solution stability leading to aphase separation when the concentration of graphene increases[20]. Therefore, a facile and cost-effective aqueous method toprepare low-defect graphene-based materials directly stabilizingin water remains highly desirable.
Carbonaceous materials are of great interest since they possess
excellent electrical conductivity and low material cost, as well ashigh specific surface areas, good chemical stability and fair electro-catalytic activity, enabling them to serve as promising candidatesin fabrication of flexible CEs in DSSCs. Graphene [21,22], carbon
anoparticles [23] and CNTs [24] have been studied to replacet-based flexible CEs. Up to now, the moderate electrocatalyticctivities of the flexible carbon-based CEs still cannot match upo those of Pt, showing a relative low energy conversion efficiency<5%) and thus limiting their further applications.
This paper therefore presents an aqueous process to preparehe low-defect and water-soluble microwave exfoliated grapheneanosheet/platinum nanoparticles (MEG/PtNPs) composite by mildxidative intercalation, microwave exfoliation, ultrasonication,rying and Ar-annealing. The composite can be directly dispersed
n water without adding surfactants. Here, platinum nanoparticlesere chosen because platinum is an inert metal and its suspen-
ion is readily prepared without additional purification. A widerray of nanoparticle materials can also be used. This low costynthesis method is applicable in many systems, such as supercap-citors, thermal storage, lithium battery and Dye-sensitized solarells (DSSCs). We demonstrate this potential in the flexible counterlectrode (CE) for DSSCs.
. Experimental
.1. Preparation of water-soluble MEG/PtNPs composite:
45 ml of concentrated H2SO4 (96%, Sigma Aldrich) and 5 ml ofydrogen peroxide (30%, Prolabo) were mixed as a mild oxidizer.
g of natural graphite flakes (Sigma Aldrich) were added into 50 ml2SO4/H2O2 mixture and then stirred for 120 min at room temper-ture. The powders were rinsed several times with deionized waterntil the pH value reached around 7. After filtering and drying,he acid-intercalated graphite flakes were irradiated in microwaveven (700 W) for 60s to obtain MEG nanosheets. 0.3 mg/ml as-ade MEG nanosheets and 0.3 mg/ml sodium dodecyl sulfonate
SDS, Fluka) were added into 30 mL of dilute PVP-PtNPs solution250 ppm) followed by 2 hrs probe sonication (750 W) to fragment
EG flakes to MEG nanosheets. The PVP-PtNPs solution with MEGanosheets were dried out on hot plate followed by annealing inube furnace under Ar atmosphere at 450 ◦C for 30 min to obtain
EG/PtNPs composite powders. The composite powders were theneadily re-dispersed into deionized (DI) water by ultrasonicationor 15 min to form stable suspension.
.2. Fabrication of flexible MEG/PtNPs CE:
After centrfugation at 12000 rpm for 20 min to remove large-ize particles, the homogeneous suspension was vacuum-filteredhrough the porous cellulose filter membrane (0.22 um pore size,
hatman) to form a MEG/PtNPs composite layer. The compositeayer was weighted pressed and transferred onto ITO PEN (15 �/�)y using isopropanol to remove the trapped air at contact interface.he cellulose filter membrane was then dissolved by acetone vaporath. Lastly, the sample was immersed in acetone liquid bath toemove the residuals. The thickness of obtained MEG/Pt NPs com-osite film is about 4.80 �m. For the comparison, the MEG/PtNPsuspension was diluted by DI water to achieve 3.8 and 1.42 �mlms.
.3. Fabrication of flexible MEG CE:
0.3 mg/ml as-made MEG nanosheets and 0.3 mg/ml SDS were
dded into 30 ml deionized water followed by 4 hrs probe sonica-ion (750 W) to fragment MEG flakes to MEG nanosheets. Then,
EG layer was transfered onto ITO PEN as DSSCs CE by using thebovementioned procedure.
cta 132 (2014) 186–192 187
2.4. Fabrication of flexible PtNPs CE:
A two-step dip-coating process was utilized to prepare PtNPsCE. ITO PEN was first immersed in 1% cationic conditioner (ML 371,OM Group) for 5 min at 60 ◦C and then was immersed in PVP-PtNPssuspension (750 ppm) for 10 min at 40 ◦C. After each step, the ITOPEN was rinsed with deionized water, dried in open air, and finallytreated by UV (275 W, TK-110-H01, Kingo) for 10 min to decomposethe capped PVP.
2.4.1. DSSCs cell assembly and characterization:The TiO2 photoanode for DSSC was prepared as follows: FTO
glass (10 �/�, 3.1 mm thick, Nippon Sheet Glass) was immersedin a 2% PK-LCG545 (Parker Corp.) at 50 ◦C for 30 min with son-ication to clean the surface followed by deionized warer rinse.Then, the nano-TiO2 paste (particle size 20 nm, product, Eternal)was screen-printed on FTO glass repeatedly until the film thick-ness reached 10 �m. Finally, a 4 �m light scattering film (PST400,CCIC) was screen-printed on nano-TiO2 film. The bilayer film wasthen sintered at 450 ◦C for 30 min in furnace. Dye impregnation wasdone by immersing the TiO2 photoanode in a 0.4 mM N719 (D719,Everlight Chemical Industrial Corp.) ethanol solution at room tem-perature for 12 h. The effective area of the TiO2 photoanode is 0.16cm2. The dye-adsorbed TiO2 photoanode and the flexible CE (PtNPs,MEG or MEG/PtNPs) were stacked face-to-face and sealed witha 30 �m-thick thermal-plastic Surlyn spacer (SX1170-25, Sola-ronix). A proper amount of liquid electrolyte (0.6 M PMII, 0.05 M I2,0.1 M LiI, 0.5 M TBP in AN/VN) was injected into the gap betweenthe two electrodes. EIS was measured by scanning the symmet-ric dummy cell (CE/electrolyte/CE) with a potentialstat (CHI 660E)from 100 kHz to 0.1 Hz with 5 mV amplitude at open-circuit condi-tion. The current-voltage (IV) curve of DSSC cell was measured witha computer-controlled digital source meter (Keithley 2400) underexposure of a standard solar simulator (PEC-L01, Pecell) under 1sun illumination (AM 1.5G, 100 mW•cm−2).
3. Results and discussions
Figure 1a is a process flow diagram showing the fabrication ofa warter-soluble composite of MEG/PtNPs. The detail processingconditions are given in the Experimental Section. Figure 1b is aphotograph showing that hydrophobic MEG nanosheets wouldfloat on the deionized water while the water-soluble MEG/PtNPscomposite can be dispersed well in the deionized water with-out surfactants. Figure 1c displays MEG/PtNPs composite layercoated on ITO PEN under bending deformation. There is no obvi-ous peeling or cracks occurring after bending, manifesting thestrong interfacial adhesion. Here, we use H2SO4/H2O2 [25] as amild oxidative to intercalate the graphite flakes with no need forchemical reduction, which can avoid structural defects and exces-sive oxygen containing functionalities (epoxide, hydroxyl, carbonyland carboxyl groups). The microwave rapid heating was then usedto exfoliate the intercalated graphite to obtain MEG nanosheets[26]. As seen in Figure 2a, the graphite powders are dramaticallyexpanded yielding a black and fluffy worm-like MEG nanosheets.The conjugated structure in the graphitic basal plane prefers toform crumpled and curved graphite sheets closely interconnectedwith each other to reach a thermodynamic stability [27]. The as-made MEG nanosheets are hydrophobic so that sodium dodecylsulfonate (SDS) is added as a surfactant stabilizer to well dis-perse MEG nanosheets in an aqueous PVP-PtNPs solution [28].
Subsequently, ultrasonication process is used to fragment theloose-packed MEG nanosheets in the PVP-PtNPs solution. Similarly,the stable graphene suspension was obtained by ultrasonicatingMEG nanosheets in deionized water with SDS. Figure 3a is TEM
188 P. Zhai et al. / Electrochimica Acta 132 (2014) 186–192
F site;
M n ITO
ithifpaisarbddecMpt
Fdr
ig. 1. (a) Schematic illustration for fabricating water-soluble MEG/PtNPs compoEG/PtNPs well dispersing in the DI water (right); (c) MEG/PtNPs composite film o
mage of MEG nanosheets and their selected area electron diffrac-ion (SAED), showing a unique feature for the monolayer grapheneaving a typical carbon hexagonal diffraction pattern with the
ntensity ratio of I{1100}/I{2110} > 1[29]. The diffraction patternor the few-layer graphene with a typical Bernal stacking is alsorovided in Figure 3b. TEM image in Figure 2b shows that PtNPsre uniformly grafted on the MEG nanosheets. In the correspond-ng SAED presented in Figure 2c, the hexagonally arranged whitepots demonstrate the well-defined diffraction of MEG nanosheetsnd the yellow rings are the diffraction pattern of PtNPs. The high-esolution TEM image of PtNPs shows ambiguous lattice fringeefore Ar annealing due to the coverage of PVP polymer (Figure 3c-). In contrast, Figure 2d shows 5 nm PtNPs with clear interplanaristance of 0.226 nm between (111) planes, which implies anffective PVP removal after Ar annealing. In Figure 2e, HRTEM
haracterization shows a perfect graphene crystalline structure forEG nanosheets. The Fast Fourier transformation (FFT) hexagonal
attern shown in the inset reveals the typical six-fold symme-ry feature. The corresponding inverse FFT in Figure 2f gives an
ig. 2. (a) FESEM image of worm-like MEG flakes with inset showing the high magnificaiffraction pattern of MEG/PtNPs nanosheet; (d) HRTEM image of Pt nanoparticles; (e) HRTegion; (f) Inverse FFT of square region; (g) FESEM image of 3D network structure of MEG
Photographs of (b) MEG flakes floating on the DI water (left) and water-solublePEN.
evidence for a large area free of structural defects. The suspen-sion was then vacuum-filtered through the cellulose membraneto form a random stacked and close-packed MEG/PtNPs compositelayer with a three dimensional (3D) network structure, as shown inFigure 2 g. The composite layer in the average thickness of 4.8 �mwas then weighted pressed and transferred onto ITO PEN as DSSCsCE without further heat treatment [30], as shown in Figure 2 h. Auniform distribution of PtNPs on MEG nanosheets can be observedby the elemental mapping (shown in Figure 4).
Figure 5a-b show the X-ray photoelectron spectra (XPS) of C1speak for MEG nanosheets and MEG/PtNPs composite powders. Asseen, Fig. 5a only has a pure C-C signal while Fig. 5b comprises the C-C, C-N and C-O signals where the corresponding peaks are centeredat 284.56, 285.20 and 286.82 eV, respectively[31]. The main C-Cpeak corresponds to the graphite-like sp2 C, suggesting most of the
C atoms in the composite are arranged in a conjugated honeycomblattice. The occurrence of C-O group is introduced by the fabricationprocesses such as mild oxidation and annealing and the C-N cova-lent bond manifests that carbon in the pyrrodine group is bonded to
tion image of the square; (b)TEM image of MEG/PtNPs nanosheet; (c) the electronEM image of graphene (MEG/PtNPs nanosheet). Inset shows a FFT pattern of square/PtNPs composite film; (h) FESEM side-view of the composite film.
P. Zhai et al. / Electrochimica Acta 132 (2014) 186–192 189
F hite sn
aFtsatAeltaTacpi
ig. 3. TEM images of (a) monolayer; (b) few-layer MEG and their SAEDs from the wanosheet before annealing; (d) HRTEM image of PVP-capped Pt nanoparticles.
nitrogen atom. The N 1s (pyrrodine nitrogen) peak in the inset ofig. 5b indicates that MEG is not only grafted by PtNPs but also func-ionalized by PVP polymer after dispersing MEG in the PVP-PtNPsolution followed by drying and Ar annealing. As noted, PVP wouldttach on the graphene surface via pyrrolidone rings, which canurn graphene from the water-repellent to the water-soluble[32].s a result, the hydrophilic pyrrodine functionality contributes tonhance the interfacial adhesion between MEG/PtNPs compositeayer and ITO PEN and the following Ar annealing tends to preservehe characteristics of graphene and PtNPs (electrocatalytic activitynd electrical conductivity) by removing the unnecessary polymer.he XPS spectra of Pt4f is characterized by two pairs of doublets,s shown in Figure 5c. The major peaks at 71.33 eV and 74.71 eV
orrespond to the zero valence state of Pt metal, and the minoreaks at 72.33 eV and 75.86 eV are correlated with Pt (II) species. It
s evident that the most of PtNPs can be maintained in the metallic
quare marked in left images with the inset displaying the intensity; (c) MEG/PtNPs
state after fabrication. Raman spectroscopy was employed to inves-tigate the structural defects of the graphene based on its D band(defect-related, breathing mode of A1g) and the G band (the in-plane bond stretching motion of C sp2 atoms, E2g mode). As shownin Figure 5d, the ID/IG intensity ratio of 0.67 for MEG/PtNPs com-posite is significantly lower than that of 2.14 for the previouslyreported RGO/PtNPs composite[33], demonstrating a relatively lowdefect generation via our process.
Herein, we demonstrate the better performance of MEG/PtNPscomposite as flexible CE for DSSCs. The interfacial electrochemicalbehaviors between CE and electrolyte were examined by electro-chemical impedance spectroscopy (EIS) using a symmetric dummycell (CE/electrolyte/CE). The Nyquist plots of MEG CE, MEG/PtNPs
CE, and PtNPs CE are shown in Figure 6a-b, which all include apoint of series resistance and two semicircles. The series resistance(Rs) is correlated to the electron transportation from CE to current
190 P. Zhai et al. / Electrochimica Acta 132 (2014) 186–192
Fig. 4. SEM images of (a) MEG/PtNPs composite film on ITO/PEN with the inset of Pt nanocluters; (b,d) carbon, oxygen and platinum elemental mapping of the compositefilm.
Fig. 5. The C1s peak in the XPS spectra of (a) MEG flakes and (b) MEG/PtNPs, the inset shows the N 1s peak in composite; (c) The Pt4f peak in the XPS spectra of MEG/PtNPs;(d) Raman spectra of MEG flakes and MEG/PtNPs.
P. Zhai et al. / Electrochimica Acta 132 (2014) 186–192 191
F PtNPs CEs and PtNPs CEs and (b) zoom in the high-frequency region; (c) Nyquist plot oft DSSCs with flexible MEG CEs, MEG/PtNPs CEs and PtNPs CEs.
chet3itcprbFcpMtPP
Table 1Photovoltaic characteristics of the DSSCs based on PtNPs, MEG, MEG/PtNPs counterelectrodes (The film thickness for MEG and MEG/PtNPs is about 4.8 �m).
Rs(�*cm2) Rct(�*cm2) Voc(V) Jsc(mA/cm2) FF �(%)
MEG 6.12 14.29 0.69 13.16 0.50 4.49
Fa
ig. 6. Nyquist plots of the symmetric dummy cells for flexible (a) MEG CEs, MEG/hickness-dependent MEG/PtNPs CEs; (d) Current density-voltage characteristic of
ollector [34]. For PtNPs CE prepared by two-step dip-coatingaving a “discontinuous PtNPs islands” microstructure (Figure 7),lectron conduction passing through ITO PEN is inevitable. In con-rast, for MEG/PtNPs CE, the electrons can be transported throughD network structure of conductive MEG nanosheets, resulting
n the reduced Rs. As noted, MEG/PtNPs CE performs lower Rs
han the reported RGO/PtNPs CE due to its improved conductivityontributed by the low defect generation via mild oxidativerocess. The first semicircle corresponds to the charge transferesistance (Rct) of I3-/I− redox reaction, which can be obtainedased on the proposed equivalent circuit model as presented inig. 6a. As listed in Table 1, the Rct are 14.29, 1.13 and 0.72 �m2 for MEG, MEG/PtNPs and PtNPs CEs, respectively. MEG CEerforms a relative high Rct and is hardly comparable to PtNPs CE.
EG/PtNPs CE achieves an improved Rs and an acceptable Rct due
o a combination of the MEG’s good electrical conductivity and thetNPs’ high electrocatalytic activity. ICP-AES was used to evaluatet amount between PtNPs CE and MEG/PtNPs CE after 24 hours of
ig. 7. FESEM images of (a) bare ITO PEN; (b) Pt nanoclusters on ITO PEN prepared by
immersing the samples in 1 ml aqua regia. The Pt amount for PtNPsCE and MEG/PtNPs CE are 205 ppm and 216.5 ppm, respectively.As shown in Figure 6c, the catalytic abilities of MEG/PtNPs filmsare thickness-dependent. As observed, the increased film stresswith increasing the film thickness over 6 �m causes the film topeel off so that 4.8 �m was chosen due to its good mechanical
and electrocatalytic properties. Figure 6d shows the photocurrentdensity-voltage characteristic curve of DSSCs under simulatedsolar illumination (100mW/cm2, AM 1.5G). The DSSC energy
the two-step dip-coating process with the inset displaying the EDX of the square
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92 P. Zhai et al. / Electrochim
onversion efficiency of PtNPs CE is 6.29% with short-circuiturrent density (Jsc) of 12.09 mA/cm2, open-circuit voltage (Voc)f 0.73 V, and fill factor of 0.70. The poor electrocatalytic activityf MEG CE causes low energy conversion efficiency of 4.49%. Asxpected, MEG/PtNPs CE having good electrical conductivity andigh electrocatalytic activity achieves high energy conversionfficiency of 6.69% with Jsc of 12.75 mA/cm2, Voc of 0.73 V, and fillactor of 0.71, which outperforms PtNPs CE.
. Conclusions
In summary, a facile and scalable preparation of stable aque-us suspension of MEG/PtNPs composite has been developed.raphene exfoliated via mild oxidation and rapid microwave irradi-tion owns relative low structural defects. The PVP-PtNPs solutionith MEG nanosheets were dried on hot plate followed by Ar
nnealing to obtain MEG/PtNPs composite, which can be readilyispersed in water without adding surfactants due to the graftedydrophilic pyrrodine group. One application of using this water-oluble composite was demonstrated in the flexible CE for DSSCso achieve a high energy conversion efficiency of 6.69%. Our resultsnd a way to fabricate the water-soluble and low-defect MEG com-osite and may find applications in many systems.
cknowledgements
This work was supported by the General Research Fund fromesearch Grants Council of Hong Kong Special Administrativeegion, China, under Award Number: HKU 719512E.
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