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Preparation of graphene/multi-walled carbon nanotube hybrid

and its use as photoanodes of dye-sensitized solar cells

Ming-Yu Yen a,Min-Chien Hsiao a, Shu-Hang Liao a, Po-I Liu a, Han-Min Tsai a,Chen-Chi M. Ma a,*, Nen-Wen Pu b, Ming-Der Ger b

a Department of Chemical Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwanb Department of Applied Chemistry and Materials Science, National Defense University, No. 1000, Shingfeng Road, Bade, Taoyuan County

33448, Taiwan

A R T I C L E I N F O

Article history:

Received 25 November 2010

Accepted 18 April 2011

Available online 27 April 2011

A B S T R A C T

This study employed a solution-based method to prepare a 3-D hybrid material comprising

graphene and acid-treated multi-walled carbon nanotubes (MWCNTs). The adsorption of

MWCNTs on graphene reduces theppinteraction between graphene sheets resulting from

steric hindrance, providing a subsequent reduction in aggregation. Optimal proportions of

MWCNTs to graphene (2:1) enabled the even distribution of individual MWCNTs deposited

on the surface of the graphene. The hybrid 3-D material was incorporated within a TiO2matrix and used as a working electrode in dye-sensitized solar cells (DSSCs). The hybrid

material provides a number of advantages over electrodes formed of either MWCNTs or

graphene alone, including a greater degree of dye adsorption and lower levels of charge

recombination. In this study, DSSCs incorporating 3-D structured hybrid materials demon-

strated a conversion efficiency of 6.11%, which is 31% higher than that of conventional

TiO2-based devices.

Crown Copyright 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Since their initial introduction by Gratzel et al., dye-sensitized

solar cells (DSSCs) have been attracting interest from aca-

demics and practitioners alike[1]. Compared with traditional

silicon-based solar cells, the fabrication costs of DSSCs are

lower and the performance is higher. Despite the 11% solarconversion efficiency already achieved with these cells, any

further improvement in performance would provide consider-

ate benefits[2].

Within the photoanode of a DSSC, photo-induced electrons

must overcome grain boundaries to be collected by a conduc-

tive glass. The low transfer efficiency of photo-induced elec-

trons across a TiO2 matrix represents a major limitation of

such nanostructured photoanodes. The inefficient charge

transfer paths cause photo-induced electrons to recombine

with the oxidizing species or tri-iodide ions present in the

electrolyte, resulting in a decrease in photocurrent and photo-

conversion efficiency. Preventing charge recombination can

therefore improve the photo-induced transfer of electrons.

In a previous study[3], we reported that the introduction of

acid-treated multi-walled carbon nanotubes (acid-MWCNTs)

into the TiO2matrix of the photoanode of DSSCs can improvethe performance of the cell by improving the electron conduc-

tion paths and distribution of the pores. The improvement in

performance thus obtained was limited by the restricted area

of contact between the TiO2nanoparticles and the cylindrical

carbon nanotubes (CNTs).Electron transfer efficiency was also

limited by the resistance of the contacts between interacting

CNTs[4].

Recently, two-dimensional (2-D) structured graphene has

emerged as a conductive nanomaterial showing considerate

0008-6223/$ - see front matter Crown Copyright

2011 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2011.04.062

* Corresponding author: Fax: +886 35715408.E-mail address:ccma@che.nthu.edu.tw(C.M. Ma).

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promise for use in organic solar cells[58]and DSSCs[912].

Graphene has been progressively developing in the applica-

tion of photoanodes[10,11,13]. Yang et al. introduced graph-

ene into a TiO2matrix as a 2-D bridge for the photoanode of

DSSCs. They demonstrated lower recombination rates,

increased efficiency in electron transport, and enhanced light

scattering through the incorporation of graphene into the

photoanode, contributing to improved cell performance [10].

Sun et al. used the coagulation of commercial TiO2 (P25) on

to Nafion-coated graphene to prepare a photoanode with

good interfacial binding. They demonstrated that the intro-

duction of graphene to TiO2-based working electrodes

improves in both dye adsorption and electron lifetime [13].

Tan et al. reported on the preparation of a photoanode in

which chemically exfoliated graphene was incorporated into

a TiO2 matrix, using a method of molecular grafting. They

determined that the implanted graphene provided additional

transport pathways for photo-induced carriers, leading to an

increase in photocurrent [11]. Due to high specific surface

area and pp interactions, dispersing graphene is very diffi-

cult; however, improving the dispersion of graphene in the

matrix is critical to its successful application in photoanodes.

Two strategies to improve the dispersion of graphene have

previously been reported, namely covalent functionalization

[1416] and non-covalent functionalization using aromatic

molecules[17,18]. The covalent functionalization of graphene

adversely influences the electrical properties of the matrix,

due to the strength of the chemical bonding and the defects

introduced to the surface of the graphene. The non-covalent

functionalization of graphene with aromatic molecules leads

to poor electron transport at the photoanode, due to the high

resistivity of the contacts[19]. Recently, a 3-D hybrid material

was reported comprising pristine MWCNTs (1-D) and chemi-

cally converted graphene (2-D). In this system, the combined

use of these two materials in the matrix resulted in a twin ef-

fect influencing both the electrical conductivity and electro-

chemical performance of the photoanode. In these reports

[2023], 3-D hybrid material exhibited stable dispersion in

the polar solvents H2O, DMF and hydrazine; however, the fab-

rication procedure was somewhat complex, thereby hinder-

ing reproducibility, and the presence of a residual reducing

agent between the graphene sheets following chemical reduc-

tion was another drawback of this method. 3-D structured

nano-materials have also been prepared by aligning CNTs

onto a graphene surface using chemical vapor deposition

(CVD) at high temperatures (exceeding 750 C)[24,25].

In the study described herein, we report a simple approach

to preparing a 3-D hybrid graphene material using a two-step

solution-based method at room temperature. First, graphite

oxide (GO) was fabricated using Staudenmaiers method

[26], followed by heat treatment (1050 C) for 30 s in an inert

atmosphere to form graphene. The graphene was then dis-

persed in an ethanol solution containing acid-MWCNTs fol-

lowed by sonication for 1 h at room temperature to yield a

novel hybrid material, as shown in Fig. 1. The prepared 3-D

hybrid was then incorporated into TiO2-based photoanodes

Fig. 1 The mechanism of acid-MWCNTs in improving the dispersibility of graphene.

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for application in DSSCs. To the best of our knowledge, this is

the first report of a 3-D hybrid photoanode. The dispersibility

of the hybrid material was investigated using UVVis spec-

troscopy and transmission electron microscopy (TEM). We

also investigated the photoelectric properties of the 3-D

hybrid photoanode, revealing that the performance of this

hybrid photoanode was better than that of previously re-

ported acid-MWCNT- and graphene-containing photoanodes

[3,10,11,13].

2. Experimental section

2.1. Synthesis of graphene

Natural graphite powder (particle size 70lm, Alfa Aesar)

was oxidized using Staudenmaiers method [26] to produce

GO. In this method, graphite (5 g) was first mixed with sulfuric

(87.5 ml) and nitric (45 ml) acid, and the mixture was stirred.

Following the uniform dispersal of graphite, potassium chlo-

rate (55 g) was slowly added to the mixture and the stirringwas continued for at least 96 h to sure complete oxidation.

The mixture was then added to deionized water, and filtered.

The GO was rinsed repeatedly using deionized water and re-

dispersed three times in a 5% HCl (aq) solution. It was then

washed continuously using deionized water until the pH of

the filtrate was neutral. The GO slurry was dried and pulver-

ized twice. The GO was heated to 1050 C in an inert atmo-

sphere and held at this temperature for 30 s to form

graphene. The acid treatment of MWCNTs was performed

according to a nitric acid washing procedure; one gram of

raw-MWCNTs was boiled in 50 ml of concentrated nitric acid

for 4 h. The MWCNTs were then filtered, washed with deion-

ized water several times to remove residual acid, and dried inan oven at 105 C. This was the method used to produce the

hybrid material. MWCNTs and graphene were combined in

various proportions in 10 ml ethanol followed by ultrasonica-

tion for 1 h.

2.2. Fabrication of the DSSCs

Various composite pastes were prepared for doctoral blading

as described below. TiO2 colloid (prepared according to a re-

lated method[27]), nanostructural material (loading amount:

0.1 wt.%), ethyl cellulose (10 wt.%, TCI Chemical Co., USA) as

a binder, a-terpineol (Showa Chemical Industry Co., Japan) asa surfactant, and anhydrous ethanol (99.5%, Acros Co., USA)

were mixed. Dispersed solutions were obtained using ultra-

sonic horn. The solution was concentrated in an rotary evap-

orator to a final concentration of 20 wt.%, thus the paste was

obtained. To prepare the DSSC photoanodes, fluorine-doped

tin oxide glass (TEC-7, 7Xsquare1, Hartford Glass Co.,

USA) plates were coated with a layer of paste. The coated

electrodes were heated in an air atmosphere at 500 C for

30 min. The thickness and active cell area of the TiO2 film

were approximately 10lm and 0.16 cm2, respectively. Four

composite photoanodes were prepared, each containing

MWCNTs (denoted electrode 1, Ctube 100, CNT Co. Ltd.,

Korea), hybrid material (electrode 2, 0.07 wt.% MWCNTs

and 0.03 wt.% graphene), graphene (electrode 3) or prinstine

TiO2 (electrode 4), respectively. Prior to the fabrication of the

DSSCs, the electrodes were sensitized by soaking for 24 h in

a 3 104 M solution of ruthenium dye (cis-dithiocyanato-

N,N 0-bis(2,20-bipyridyl-4-carboxylicacid-4 0-tetra-butylammo-

nium carboxylate) ruthenium(II); N719, Solaronix SA, Swit-

zerland) in acetonitrile/t-butyl alcohol (v:v= 1:1). The

sensitized electrodes were then immersed in acetonitrile

for 12 h. The DSSC comprised a sensitized working electrode,

a platinized counter (Pt) electrode and an electrolyte with a

25lm hot-melt sealing foil (SX1170-60, Solaronix SA, Swit-

zerland) between each layer. The electrolyte used in this

study consisted of 0.6 M BMII, 0.1 M guanidinium thiocya-

nate, 0.03 M iodine, and 0.5 M TBP in acetonitrile/valeronitri-

le (volume ratio: 85:15)[27].

2.3. Characterization of the dispersion of carbon materials

and composite working electrodes

We investigated the dispersibility of the carbon materials inethanol. We first obtained a calibration curve by measuring

(at 270 nm) the adsorption (Varian, Cary 50, Varian, USA) of

the solution of graphene and MWCNTs at a known concentra-

tion[28]. The tested carbon materials were then suspended in

ethanol with various ratios of MWCNTs to graphene (10 ml)

and sonicated for 1 h. The solutions were centrifuged at

3000 rpm for 15 min, whereupon the absorption of the super-

natants was measured at 270 nm [19,20]. The morphology of

the composites working electrode was then studied using

scanning electron microscopy (SEM) (Hitachi S-4700I, Japan),

and the microstructures of the carbon materials was also

investigated using transmission electron microscopy (TEM)

(JEOL 2100F, Japan) at an accelerating voltage of 200 kV. Thequantity of dye adsorbed into the electrode was determined

by desorbing the dye from the composites into a 0.1 M KOH

aqueous solution, and measuring the absorbance of the

resulting solution using UVvis spectroscopy (Varian, Cary

50, Varian, USA). Photoluminescence (PL) spectra were ob-

tained by using an F-4500 Luminescence Spectrometer (Hit-

achi, Japan). The photocurrentvoltage characteristics of the

electrodes were also measured using a 2400 digital source me-

ter (Keithley, USA) under illumination with a Class A sunlight

simulator at 100 mW cm2 (91160A, AM 1.5, Oriel, Newport

Corporation, USA), equipped with an AM 1.5G filter (81088A,

Oriel, Newport Corporation, USA) and a 300 W xenon lamp

(6258, Oriel, Newport Corporation, USA). The intensity of thesimulated incident light was calibrated to 100 mW cm2 using

a reference Si solar cell, calibrated at the National Renewable

Energy Laboratory (NREL) (USA) institutes. Electrochemical

impedance spectra (EIS) were obtained using a potentiostat/

galvanostat equipped with a frequency response analysis

(FRA) module (PGSTAT 302N, Autolab, EcoChemie, Nether-

lands), under illumination of 100 mW cm2. The scanned fre-

quency range was from 105 to 102 Hz, with an applied voltage

of 10 mV. Impedance spectra were analyzed using an equiva-

lent circuit model with Autolab FRA software (v4.9, Eco

Chemie B.V.). The incident photon-to-current conversion effi-

ciency (IPCE) was measured using an IPCE instrument (Model

QEX7, PV measurement Co., USA).

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3. Results and discussion

3.1. Dispersion of the hybrid materials

In our previous work [3], we showed that the aggregation of

acid-MWCNTs seriously restricts the charge transport and

adsorption of dye by the photoanode. In the present study,

acid-MWCNT-graphene hybrids, composed of various ratios

of the two components, were prepared to investigate their

microstructure.

As shown in Fig. 2, both the pristine MWCNTs and the

graphene were found to form unstable suspensions following

sonication, leading to aggregation and subsequent precipita-

tion. The aggregation of pristine MWCNTs and graphene are

known to be due to a lack of hydrophilic groups in their struc-

ture, prohibiting interaction with the polar solvent. Although

the presence of hydrophilic groups (e.g. hydroxyl, epoxy, or

carboxyl) on the surface of graphene and edges of graphene

can improve dispersion, they are generally insufficient to al-

low complete dispersion due to the large hydrophobic surface

area of graphene [29]. In contrast, Fig. 1 shows the nearly

complete dispersion of both acid-MWCNTs and hybrid mate-

rials in solution following sonication. After one week, much of

the suspended acid-MWCNT material had aggregated and

precipitated. This was due to the attraction produced by the

large hydrophobic specific surface of the CNTs, even after

acid-treatment[20]. On the other hand, the hybrid materials

exhibited stable dispersion with no precipitation occurring

at room temperature, even after several weeks. p-Stacking

interactions between the p-conjugated aromatic surface of

the graphene and the wall of the acid-MWCNTs binds the

two components, while the hydrophilic surfaces of the acid-

MWCNTs maintain the dispersion of the construct in solu-

tion. These finding show that the incorporation of acid-

MWCNTs can significantly improve the dispersion of graph-

ene in ethanol.

To optimize the dispersion of acid-MWCNT and graphene

hybrids, a series of hybrid materials composed of various ra-

tios of acid-MWCNTs and graphene were prepared. Hybrid

carbon materials of different compositions (0.1 mg) were sus-

pended in ethanol (10 ml) and sonicated for 1 h. After centri-

fugation, we measured the absorption of the supernatants at

270 nm[19,20]and compared the results using a calibration

curve generated from solutions with known concentrations

of carbon material, thereby allowing the dispersion of each

of the hybrids to be determined.

Fig. 3(a) shows that the dispersion of acid-MWCNTs (1 101 mg ml1) was higher than that of graphene (8 104 mg ml1) by three orders of magnitude, as illustrated in the photo-

graph inFig. 1. These two carbon materials exhibited very dif-

ferent dispersive characteristics, resulting from differences in

their dimensions and surface properties. The inset ofFig. 3(a)

shows that the dispersion of the hybrid materials depends

strongly on the ratio of acid-MWCNTs to graphene. The

microstructure of hybrid materials composed of various ra-

tios of acid-MWCNTs to graphene was investigated using

TEM. When plotted against acid-MWCNT content, the disper-

sibility of the hybrids exhibited three distinct patterns of

behavior. With low concentrations of acid-MWCNTs (ratios

of acid-MWCNTs to graphene from 0.5 to 6), the dispersion

of the hybrid material in ethanol increased with an increase

in the proportion of CNT. As shown inFig. 3(b), when the pro-

portion of acid-MWCNTs to graphene was 2:1, the individual

acid-MWCNTs appears to be evenly separated on the surface

of the graphene due to interaction with the p-conjugated aro-

matic domain on the basal plane of the graphene. The at-

tached acid-MWCNTs formed a steric barrier hindering the

interaction of the graphene sheet with other sheets, thereby

preventing aggregation. Moreover, the hydrophilic groups of

the attached acid-MWCNTs increased the dispersion of the

graphene through interaction with the polar solvent. How-

ever, when higher concentrations of acid-MWCNTs were used

in the hybrids (ratios of 612), the trend of an increase in dis-

persion with increasing acid-MWCNTwas reversed. As shown

in Fig. 3(c), excess acid-MWCNTs were not individually ad-

sorbed onto the surface, but formed bundles or clusters with

a self-aggregated morphology[20]. At the highest tested con-

centrations of acid-MWCNTs (ratios of 1216), the trend of in-

creased dispersion with an increase in acid-MWCNT content

in the hybrid materials was re-established. However, Fig. 3(d)

shows that under these conditions the supernatants were

comprised entirely of acid-MWCNTs, with excess CNTs fully

coagulated on the surface of the graphene in the form of clus-

ters, with all of the graphene located in the precipitate during

centrifugation. It is noteworthy that hybrid materials com-

posed of three different ratios (2, 10, and 14) exhibited very

different microstructural morphology (as seen by TEM), from

those with a similar degrees of dispersibility (as seen by UV

Vis spectroscopy). The ratio of acid-MWCNTs to graphene in

hybrid materials has a significant influence on the surface

morphology of graphene, allowing the control of dispersive

characteristics. The hybrid material exhibiting optimal uni-

form dispersion in solution was achieved at a ratio of 2:1.

As well as influencing the degree of dispersion, acid-

MWCNTs adsorbed on a graphene surface might be expected

to increase the area of the surface available for anchoring to

Fig. 2 Photograph showing the various different carbon

materials dispersed in ethanol (2 mg in 10 ml ethanol, 1 h

sonication). The photographs were taken 1 week after the

preparation of the dispersed carbon materials.

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TiO2 particles, thereby increasing dye adsorption. On the

other hand, the presence of wrinkles on the surface of the

graphene could suppress the conductivity of the basal plane,

due to local bending/curvature effects[30]. The incorporation

of acid-MWCNTs could be expected to connect each p-conju-

gated domain to the graphene, resulting in improved conduc-

tivity. For this reason, we used a hybrid material with a ratio

of 2:1 to prepare composite materials with TiO2particles, for

further investigation of this material as a photoanode.

3.2. Effects of incorporating carbon materials into the TiO2matrix on the surface morphology of the composite film

Several reports[3134]have shown that TiO2particles can be

dispersed effectively through the incorporation of carbon

materials into the TiO2matrix, yielding a coarse surface and

a highly porous morphology. Here, we discuss the influence

of TiO2matrices on dye adsorption and microstructural mor-

phology following the incorporation of acid-MWCNTs, graph-

ene, and hybrid materials into the matrix. UVVis spectra of a

dye solution of 0.3 mM and dye solutions formed by the

desorption of dye from composite films using a 0.1 M KOH

aqueous solution are shown in Fig. 4. When compared with

a pristine TiO2 film (electrode 4), it became evident that a

greater quantity of dye was adsorbed by the composite films,

with the composite film of hybrid carbon material showing

the greatest levels of dye adsorption. We propose that this is

due to the high specific surface area of CNTs and graphene,

providing a large surface area for the anchoring of TiO2particles.

Interestingly, the quantity of dye adsorbed by the electrode

containing graphene (electrode 3) was similar to that of the

electrode containing acid-MWCNTs (electrode 1). Although

Fig. 3 (a) Plot showing the limiting dispersion of acid-MWCNTs and graphene. The inset is a plot of dispersibility with

various ratio of hybrid materials at concentration of 0.1 mg ml1 in ethanol after centrifugation (3000 rpm/15 min). The

transmission electron microscopy images (TEM) of the hybrid materials in various proportions of acid-MWCNTs to graphene:

(b) 2:1; (c) 10:1; and (d) 14:1.

Fig. 4 The UVVis spectra of 0.3 mM dye solution and dye

desorbed from composite working electrodes.

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an increase in the surface area of graphene should result in

an increase in the surface area on which to anchor TiO2par-

ticles, strong ppinteraction between the graphene surfaces

leads to a considerable degree of aggregation with reduced

porosity, thereby decreasing the area available for dye adsorp-

tion. The electrode made of hybrid material (electrode 2)

showed the highest levels of dye adsorption, consistent with

the morphology of the hybrid material, as discussed above.

The acid-MWCNTs distributed on the graphene surface gen-

erated a larger anchoring surface area for TiO2, thereby

enhancing the quantity of dye that could be adsorbed. The

3-D structure of the hybrid material minimizes aggregation

leading to a more uniform distribution of pores.

In a previous study[3], we demonstrated that the degree to

which dye is adsorbed is determined by roughness and spe-

cific surface area. We postulated that the microstructure of

hybrid composites was influenced by the formation of more

porous structures. Fig. 5 shows SEM images of composite

films incorporated with carbon materials. The composite

films clearly exhibit a rough, coarse morphology, which im-

proves their ability to adsorb dye. Due to the formation of

bundles and stacked sheets of carbon material, the surface

of the composite films (electrodes 1 and 3) exhibited a flatter

morphology than that of hybrid composite films. The 3-D

structure of the hybrid material results in high surface asper-

ity in electrode 2. Furthermore, electrode 2 may also possess

greater surface area and a more uniform distribution of pores,

leading to increased dye adsorption. The low porosity and

roughness of electrodes 1 and 3 inhibited dye uptake, proba-

bly due to the tendency of MWCNTs and graphene to aggre-

gate. Due to the many favorable properties of hybrid

material of electrode 2, including large surface area, rough-

ness, and a high degree of dye adsorption, we next incorpo-

rated this electrode to investigate the performance of DSSC

cells.

3.3. Photoelectrical properties of the composite working

electrode

For DSSCs, photoinduced current depends on the quantity of

dye adsorbed on to the electrode [35]. Electronhole (e/h+)

charge recombination significantly reduces the efficiency of

Fig. 5 An illustration of the suggested microstructure of the composites. The SEM micrographs are used to describe the

morphology of the composites: (A) electrode 1 (MWCNTs); (B) electrode 2 (MWCNTs and graphene); (C) electrode 3 (graphene);

and (D) electrode 4 (pristine TiO2).

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photoelectron conversion[3,36,37]. Many authors have shown

that structured carbon materials, including carbon black,

CNTs, and graphene facilitate the transfer of photoelectrons

from the TiO2matrix to a conductive glass and suppress elec-

tronhole (e/h+) charge recombination [3,11,13,36,3840]. In

the present study, we investigated the effects of acid-

MWCNTs, graphene, and hybrid materials in a TiO2 matrix

on the conductive path of the charge transfer. The charge

recombination of the composite electrodes was investigated

using PL spectra (Fig. 6).

Pristine TiO2 film exhibited a broad, intense PL emission

band at approximately 520 nm. The composite electrodes

showed a broad peak at 520 nm, but this had a significantly

lower intensity than that of the pristine TiO2. As the TiO2crys-

tals absorbed photons, excitons were generated, separating

into electrons and holes. The electrons and holes immedi-

ately recombined, resulting in emission. However, the

incorporation of a carbon material into the TiO2matrix sup-

pressed the recombination process, due to the high electrical

conductivity of the carbon materials [3,41]. The emission

intensity of electrode 2 at 520 nm was the lowest of the elec-

trodes tested, which we suggest was due to the 3-D conduc-

tive path and highly anchored surface. This indicates the

efficient collection of photoelectrons by the conductive glass.

To study the effects of conduction path on charge transfer

in the composite electrodes, we studied the transient photo-

current of the DSSCs. The Jsccurve was recorded during one

switched cycle of illumination at 100 mW cm2.Fig. 7shows

that in the case of the DSSC with a hybrid composite elec-

trode, the photoelectrons disappeared more rapidly than they

did in a DSSC with an electrode made with a pristine TiO2ma-

trix. The DSSC with a MWCNT electrode dispersed electrons

more rapidly than the one with the graphene electrode, indi-

cating that the aggregated graphene provided recombination

sites, resulting in a decrease in electron transfer. Electrode 2

provided the DSSC with the most rapid dispersion of photo-

electrons when no photovoltage was applied. This indicates

that photoelectrons are less readily trapped at the surface of

electron trapping sites in the hybrid composite than they

are in pristine TiO2.

Recently, electrochemical impedance has been used to

analyze electrode kinetics of DSSCs [4246]. In general, the

impedance spectra of DSSCs form three semicircles, present-

ing the three different reaction steps in the electrochemical

process, namely the electrochemical reaction at the Pt sur-

face, the charge transfer at the TiO2/dye/electrolyte interface,

and the diffusion of I=I3 in the electrolyte.Fig. 8and its inset

show the Nyquist plot and equivalent circuit of the DSSCs

employing various working electrodes, and the results are

presented in Table 1. The series resistance values observed

in DSSCs with composite working electrodes were substan-

tially lower than those from a DSSC using pristine TiO2 as

an electrode. The presence of carbon materials improved

the conductivity of the electrode, implying that a decrease

in the value of Rs can improve the fill factor (FF) of DSSCs,

yielding cells with higher performance[47]. Moreover, the Ny-

quist plots of the DSSCs with composite working electrodes

were reduced in the middle frequency region due to the

Fig. 6 Photoluminescence (PL) spectra of pristine TiO2and

carbon materials/TiO2composite films.

Fig. 7 Transient photocurrent spectra of DSSCs with

pristine TiO2and carbon material/TiO2composite working

electrodes.

Fig. 8 Impedance spectra of DSSCs with pristine TiO2and

carbon materials/TiO2composite working electrodes

measured at 100 mW cm2. The inset show the equivalent

circuit of the device.

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presence of carbon material, providing further evidence that

the introduction of carbon materials can improve the conduc-

tive path of the TiO2 matrix. Meanwhile, an increase in the

surface area for charge transfer could also contribute to an in-

crease in electron transfer. Interestingly, the charge transfer

resistance of the graphene electrode was lower than that of

the MWCNT electrode. The larger area per unit mass of graph-

ene provided more conductive paths than CNTs[25]. However,

hybrid materials provide even more conductive pathways for

photo-induced electrons than either the MWCNT or graphene

electrodes. In this system, the graphene provided a conduc-tive pad in its large surface area, while the high aspect ratio

of the CNTs connected the isolated graphene pads, yielding

a more extensive conductive network in the composite work-

ing electrode. The CNTs may also help to smooth the wrin-

kles of graphene, improving the electrical conductivity of

the graphene and increasing the electron transfer at the inter-

face. This analysis is consistent with the results from photo-

luminescence and transient photocurrent spectroscopy

presented herein.

The introduction of hybrid materials into TiO2-based

working electrodes substantially improves electrical conduc-

tivity, effectively reducing the charge recombination at the

TiO2/dye/electrolyte interface, which would in turn be ex-pected to improve the photoelectron conversion efficiency

of DSSCs in these electrodes.

3.4. Analysis of DSSC performance using various working

electrodes

To establish the utility of the hybrid composite working elec-

trodes, we investigated the IPCE of DSSCs with various work-

ing electrodes, to identify the effects of the composite on the

generation of photocurrent. As expected, a substantial

improvement in IPCE was observed for the DSSCs that had

composite working electrodes (Fig. 9). This increase in IPCE

was due to the high degree of dye adsorption and low charge

recombination observed for the TiO2matrices containing car-

bon materials. Interestingly, the IPCE spectra of the hybrid

and graphene-containing electrodes showed an improvement

in the conversion efficiency in the red region of light (600

700 nm). This suggests that the pores introduced through

the incorporation of graphene may act as light-harvesting

centers [10]. The impact of low optical absorbance of the

dye at longer wavelengths (Fig. 4) was attenuated by the pores

in the electrodes (electrodes 2 and 3), which improved the

light path to enhance the absorption of the dye, thereby fur-

ther boosting cell performance[48].

The performance of cells with DSSCs was measured using

simulated sunlight (1 Sun AM 1.5). Fig. 10 and Table 2show

the characteristics of the DSSCs fabricated using various

working electrodes. Under illumination, the DSSC with thehybrid composite electrode exhibited a short-circuit photo-

current (Jsc) of 11.27 mA cm2, an open-circuit voltage (Voc)

of 0.78 V and a FF of 0.70, yielding a conversion efficiency (g)

of 6.11%. For the DSSC with a graphene-containing electrode

fabricated using the same method, the values of Voc, Jsc, FF

and g were 0.75 V, 10.27 mA cm 2, 0.69% and 5.35%. The com-

posite working electrodes showed a marked improvement in

photocurrent compared with the pristine TiO2 electrode.

Table 1 Series resistance (Rs) and charge transfer resis-tance (R2) of the DSSCs with various working electrodes.

Electrode no. Rs(Xcm2) R2(Xcm

2)

1 19.0 182 14.0 13.563 17.7 13.71

4 21.8 25.34

Fig. 9 The IPCE spectra of DSSCs with various working

electrodes.

Table 2 The characteristics of the JVcurves of DSSCs withvarious working electrodes.

Electrodeno.

Jsc(mA cm2) Voc(V) FF g(%)

1 10.16 0.77 0.70 5.502 11.27 0.78 0.70 6.113 10.27 0.75 0.69 5.35

4 8.61 0.78 0.67 4.54

Fig. 10 Photocurrent densityvoltage characteristics of the

different electrodes described herein.

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The high specific surface area and improved electrical paths

provided by the structured carbon materials, led to high cell

performance and a decrease in resistance to charge transport

in the DSSC devices. The observed impedance also indicated

that the lowerRsled to an increase in FFas a result of the in-

creased electrical conductivity of the TiO2film[47]. However,

the Voc of the DSSC prepared using the graphene-containing

electrode was lower than that in the DSSC prepared using a

pristine TiO2electrode. This may have been the result of the

poor adhesion of the sensitized TiO2 to the surface of the

graphene, which could lead to increased charge recombina-

tion. An increase in the dark current consequently caused a

decrease inVoc[49]. DSSC prepared using the hybrid compos-

ite electrode showed a remarkable improvement in its perfor-

mance characteristics compared with those of the pristine

TiO2 electrode, providing a 31% increase in photocurrent

and 35% improvement in conversion efficiency, without any

decrease in Voc. By combining the advantages of graphene

and acid-MWCNTs, we achieved both good adhesion at the

interface and a high occupied surface area in the matrix,

thereby producing the high performance observed in the

experiment.

The incorporation of hybrid carbon materials within a 3-D

architectural structure has been shown to increase dye

adsorption, enhance the electrical conductive path through

the generation of a more uniform pore structure, and de-

crease charge recombination in the composite working elec-

trode. The use of hybrid composites as working electrodes

in DSSCs shows great potential, warranting further

investigation.

4. Conclusion

We have described a novel method with which to improve the

dispersive characteristics of thermally-reduced graphene

through the incorporation of acid-MWCNTs. TEM results

and the observed dispersion of hybrid materials support our

contention that the dispersion of graphene can be controlled

by varying the acid-MWCNT loading, thereby lending a un-

ique microstructure to the hybrid material SEM imagery and

observations of dye adsorption indicate that both the surface

morphology and distribution of pores can be improved

through the incorporation of our structured carbon material.

Furthermore, the use of acid-MWCNTs has been shown to im-

prove the surface potential of graphene, improving the

attachment of TiO2 particles. Analysis of the photoelectricproperties of the electrodes showed a reduction in charge

recombination and enhanced electrical conductivity, thereby

leading to improved cell performance. The photocurrent den-

sity of DSSCs using a hybrid material photoanode was in-

creased by 35% and the conversion efficiency was increased

by 31%, compared to DSSCs using a pristine TiO2photoanode.

This was due to the increased quantity of adsorbed dye, re-

duced charge transfer resistance and an enhanced electron

transport rate, resulting from the incorporation of hybrid

material. Our findings demonstrate that 3-D structured hy-

brid materials possess great potential for applications related

to DSSCs, and merit further investigation.

Acknowledgements

The authors are grateful to the National Science Council, Tai-

wan, Republic of China under contract NSC-100-ET-E-007-001-

ET, for financial support, and to the National Defense Univer-

sity for the supplement of graphene.

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