C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1

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TiO2 nanoparticles loaded on graphene/carbon compositenanofibers by electrospinning for increased photocatalysis

Chang Hyo Kim a, Bo-Hye Kim b,*, Kap Seung Yang b,c,**

a Department of Advanced Chemicals & Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju, Republic of Koreab Alan G. MacDiarmid Energy Research Institute, Chonnam National University, 300 Yongbong-dong, Gwangju, Republic of Koreac Department of Polymer & Fiber System Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju, Republic of Korea

A R T I C L E I N F O

Article history:

Received 29 October 2011

Accepted 26 January 2012

Available online 4 February 2012

0008-6223/$ - see front matter � 2012 Elsevidoi:10.1016/j.carbon.2012.01.069

* Corresponding author at : Alan G. MacDiaGwangju, Republic of Korea. Tel.: +82 62 530

** Corresponding author at : Department o500-757 Gwangju, Republic of Korea. Tel.: +8

E-mail addresses: bohye@chonnam.ac.kr

A B S T R A C T

Graphene/carbon composite nanofibers (CCNFs) with attached TiO2 nanoparticles (TiO2–

CCNF) were prepared, and their photocatalytic degradation ability under visible light irra-

diation was assessed. They were characterized using scanning and transmission electron

microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy,

and ultraviolet–visible diffuse spectroscopy. The results suggest that the presence of graph-

ene embedded in the composite fibers prevents TiO2 particle agglomeration and aids the

uniform dispersion of TiO2 on the fibers. In the photodegradation of methylene blue, a sig-

nificant increase in the reaction rate was observed with TiO2–CCNF materials under visible

light. This increase is due to the high migration efficiency of photoinduced electrons and

the inhibition of charge–carrier recombination due to the electronic interaction between

TiO2 and graphene. The TiO2–CCNF materials could be used for multiple degradation cycles

without a decrease in photocatalytic activity.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

TiO2 is one of the most promising catalysts because of its supe-

rior photocatalytic performance, easy availability, long-term

stability, and nontoxicity [1,2]. Typically, photoexcited elec-

tron–hole pairs can be generated by irradiation with light with

an energy greater than the band gap energy of TiO2

(Ebg = 3.2 eV for anatase). However, several problems can arise

when TiO2 is applied as a photocatalyst: (1) photogenerated

electron–hole pairs can recombine quickly, which affects the

photocatalytic efficiency [3–7], and (2) TiO2 can only be excited

with ultraviolet (UV) light, which is less than 5% of solar light,

because of its wide band gap [7–9]. These disadvantages of

TiO2 result in a low photocatalytic activity in practical applica-

tion. Therefore, a number of recent studies have focused on the

er Ltd. All rights reserved

rmid Energy Research Ins0774; fax: +82 62 530 177f Polymer&Fiber System2 62 530 0774; fax: +82 62(B.-H. Kim), ksyang@cho

preparation and modification of TiO2 to narrow its band gap and

enhancethephotocatalyticactivityundervisiblelightradiation[10–30].

Among the carbon nanostructures (e.g., C60, carbon nano-

tubes, and graphenes), graphenes offer new opportunities in

photovoltaic conversion and photocatalysis by the hybrid

structures with a variety of nanomaterials, due to their excel-

lent charge carrier mobility, a large specific surface area, and

good electrical conductivity [31–36]. For example, TiO2 com-

bined with graphene acts as an electron trap, which promotes

electron–hole separation and facilitates interfacial electron

transfer [14,24–26,30]. Furthermore, graphene can help con-

trol the morphology of TiO2 nanoparticles because it controls

nucleation and growth of TiO2 nanoparticles and allows for

optimal chemical interactions and bonding between nanopar-

ticles and graphene.

.

titute, Chonnam National University, Yong-Bong dong 300, 500-7579.Engineering, Chonnam National University, Yong-Bong dong 300,530 1779.

nnam.ac.kr (K.S. Yang).

C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1 2473

In this work, carbon nanofibers (CNFs) containing

micropores were used to support TiO2 because CNFs exhibit

a high adsorption capability and adsorption rate due to the

shallow and uniform pore structure. Shallow micropores

open directly to the surface, which results in a large adsorp-

tion capacity and fast adsorption/desorption [37,38]. In addi-

tion, electrospinning has been widely used as a versatile

technique to fabricate various hybrid nanofibers [39,40]. How-

ever, reports have seldom been published on the fabrication

and evaluation of TiO2 nanoparticles loaded on graphene/car-

bon composite nanofibers (TiO2–CCNF) produced by electros-

pinning techniques.

In the present work, TiO2–CCNF catalysts were prepared by

electrospinning to determine if they exhibit improved photo-

catalytic efficiency in the visible spectrum, as illustrated in

Fig. 1. By increasing the photocatalytic activity of the TiO2–

CCNF materials, the current study aims to (i) extend the light

absorption spectrum into the visible region, (ii) reduce

electron/hole pair recombination, and (iii) enhance the

adsorption capability and high adsorption/desorption rate of

organic pollutants, which increases the reaction efficiency

because of the high specific surface area of the CNFs.

2. Experimental

2.1. Materials and methods

The graphenes used in this study were xGNP-C750-grade

materials produced by XG Science, USA. The elementary anal-

ysis of graphene was characterized as 88.68% carbon, 0.79%

hydrogen, 1.11% nitrogen, and 7.65% oxygen using the Mettler

method (Metler-Toledo AG, Switzerland). The graphene with

functional group can be dispersed in organic solvent. The

3 wt.% grpahene (3 wt.% relative to PAN) sample was im-

mersed in dimethylformamide (DMF) and sonicated in a

bath-type sonicator. The PAN (Mw = 150,000, Aldrich Chemical

Co.) was dissolved in the homogeneous graphene-DMF solu-

tion. This mixture was continuously stirred at 60 �C until a

homogeneous solution formed, and it was then cooled to

room temperature. This solution was then spun into nanofi-

ber webs using an electrospinning apparatus (NTPS-35K,

Ntsse Co., Korea) operating at 25 kV. Spinning solutions were

fed through a capillary tip (diameter = 0.5 mm) using a syr-

inge (30 ml). The anode of the high voltage power supply

was clamped to a syringe needle tip and the cathode was con-

nected to a metal collector. During electrospinning, the dis-

Fig. 1 – TiO2–CCNF hybrids and its re

tance between the tip and collector was 17 cm, and the flow

rate of the spinning solution was 3 ml/h. The electrospun fi-

bers were collected on aluminum foil wrapped on a metal

drum rotating at approximately 300 rpm. The electrospun fi-

ber webs were stabilized in flowing air at 280 �C with a heat-

ing rate of 1 �C/min for 1 h. A 30 mL ethanol solution

containing 2.04 g of titanium n-butoxide (Ti(OnBu)4, STREM

CHEMICALS) was diluted with 30 mL of toluene. The stabi-

lized nanofibers (SNF) were dipped in the prepared sol solu-

tion, washed sufficiently with ethanol and water, dried in

air, and carbonized at 800 �C with a heating rate of 5 �C/min

in N2 atmosphere with a flow rate of 200 mL/min. This sample

was identified as TiO2–CCNF. To compare the photocatalytic

activity under visible light irradiation, the TiO2 loaded on

PAN based carbon nanofiber (TiO2–CNF), Graphene/carbon

composite nanofibers (CCNF), and PAN based carbon nanofi-

ber (CNF) samples were synthesized.

2.2. Characterization

The surface structures of the TiO2–CCNF materials were char-

acterized by a Hitachi S-4700 field-emission scanning electron

microscope (FE-SEM) and energy dispersive X-ray spectros-

copy (EDX). The transmission electron micrographs (TEM)

and the selected area electron diffraction (SAED) micrographs

were taken with a TECHNAI-F20 unit (KJ111, Phillips) operat-

ing at 200 kV. The crystalline structure was characterized with

an X-ray diffraction unit (XRD, D-Max-2400 diffractometer)

equipped with graphite monochromatized CuKa radiation

(k = 0.15418 nm). Backscattering Raman measurements were

carried out with a Renishaw in via-Reflex using 633 nm at

room temperature. The chemical state of the surface was

examined by X-ray photoelectron spectroscopy (XPS) using a

VGScientific ESCALAB250 spectrometer equipped with a

monochromatized AlK X-ray source (15 mA, 14 kV). The

surface functionalities of the copolymer were examined by

Fourier Transform Infrared spectroscopy (FT-IR, Nicolet 200

instrument). All the samples were analyzed using the KBr pel-

let technique and scanned in the range from 4000 to 400 cm�1. Specific surface areas were analyzed by the Brunauer–

Emmett–Teller (BET) method using a surface area analyzer

(Micrometrics, ASPS2020, USA). To observe the optical proper-

ties of the samples, the photoluminescence (PL) emission

spectra (Laser Raman and PL spectrometer, SPEX1403;

325 nm (55 mW) He–Cd Laser) were measured which give

information about the recombination rate of photogenerated

sponse under visible irradiation.

2474 C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1

carriers. The porosity was investigated from the nitrogen

adsorption isotherm at 77 K (ASAP 2020, Micromeritics,

USA). The specific surface area, the mesopore size distribu-

tion, and the micropore size distribution of the samples were

evaluated using the Brunauer–Emmett–Teller (BET) method

and the Barrett–Joyner–Halenda (BJH) method. Photocatalytic

efficiencies were evaluated by decolorizing an aqueous solu-

tion of methylene blue (MB) under irradiation by a fluorescent

lamp. A 0.1 g sample of the photocatalyst powder was dis-

persed in 100 mL of 15 ppm MB (Aldrich) solution by stirring

with a magnetic stirrer. A 13 W fluorescent lamp (FRX 13EX-

D) with light output in the 400–800 nm spectrum range was

used as the visible light source. The MB concentration was

measured every 30 min using a UV–Visible spectrophotome-

ter (Shimadzu 160-A).

3. Results and discussion

Fig. 2 presents SEM images of TiO2–CCNF and TiO2–CNF hy-

brids prepared with and without graphene. The electrospun

PAN based- and PAN/graphene based nanofibers showed a

smooth surface with an average diameter of 180 and

270 nm, respectively, before coating of TiO2. It was observed

that the nanoparticles were uniformly distributed across the

surface of the TiO2–CCNF materials (Fig. 2a) without aggrega-

tion, whereas the TiO2 on pure PAN/based fibers (TiO2–CNF)

consisted of large, aggregated nanoparticles with diameters

in the range of 80–110 nm (Fig. 2b). The graphene, functional-

ized with carboxylic acid groups, is capable of binding to me-

tal oxide particles, such as TiO2, without aggregation effects

and yields small nanparticles [14]. Goncalves et al. [41]. have

shown that oxygen functionalities at the graphene surface

Fig. 2 – SEM images of (a) electrospun PAN based nanofiber, (b)

TiO2–CCNF.

act as reactive sites for the nucleation and growth of gold

nanoparticles. They observed that the nucleation and growth

of these metal nanoparticles were dependent on the density

of oxygen functional groups in the graphene surface sheets

[42]. TiO2 nanoparticles directly grown on graphene appeared

to exhibit strong interactions, which should lead to advanced

hybrid materials for various applications, including

photocatalysis.

TEM images, depicted in Fig. 3a, were collected to obtain

microstructural information for the TiO2–CCNF materials.

The images indicated that TiO2 nanoparticles, with an aver-

age particle size of 10–30 nm, had been successfully grown

on the surface of CNFs. SAED measurements (Fig. 3b) con-

firmed the nanocrystalline nature of the investigated TiO2

samples. Based on the SAED ring pattern shown in Fig. 3b,

the clear lattice fringes indicate that the nanoparticles have

some crystallinity. In addition, the SAED patterns indicate

that the nanoparticles, identified as TiO2 rutile structures, re-

flected from the (110), (211), and (101) lattice planes as in-

dexed in Fig. 3b. The corresponding elemental mapping of

an individual TiO2–CCNF materials provided information on

the distribution of Ti, O, and C atoms in the fiber (Fig. 3c).

Fig. 4 presents the XRD patterns of the TiO2–CCNF and

TiO2/CNF materials. The broad peak located between 20�and 30� was attributed to the (002) plane of the carbon struc-

ture. The diffraction peaks of TiO2 were also clearly observed

in Fig. 4a and b. The peak locations for TiO2 are cited from the

Joint Committee on Powder Diffraction Standards (JCPDS)

database. The peaks located at 25� Correspond to the (101)

plane of the anatase phase (JCPDS 21-1272), and the peaks lo-

cated at 28o, 36o, 41o, and 54� Correspond to the (110), (101),

(111), and (211) planes of the rutile phase (JCPDS 21-1276),

electrospun PAN/graphene based nanofiber, (c) TiO2–CNF, (d)

Fig. 3 – (a) TEM image of the TiO2–CCNF hybrids. (b) SAED patterns of the TiO2 nanostructures. (c) Elemental mapping of the

individual TiO2–CCNF hybrids.

Fig. 4 – XRD patterns of the (a) TiO2–CCNF and (b) TiO2/CNF.

C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1 2475

respectively [43]. All products were confirmed to be a mixture

of the anatase and rutile phases. The anatase phase content

for all products was calculated from the XRD patterns using

the following equation: Xa = [1 + 1.26(Ir/Ia)]�1, where Xa is the

share of anatase in the mixture and Ir and Ia are the integrated

intensities of the (101) reflection of anatase and the (110)

reflection of rutile, respectively. TiO2–CCNF has a 1:1 ratio of

anatase to rutile, whereas the ratio for TiO2–CNF is 1:4. There-

fore, this result indicates that the addition of graphene to TiO2

can effectively suppress the transformation of TiO2 from an

anatase to a rutile-type phase and prevents the growth of

TiO2 particles.

The Raman spectrum (Fig. 5) indicates the presence of TiO2

crystalline nanoparticles and showed three Raman bands:

144(B1g) for anatase and 431(Eg)/612(A1g) for the rutile structure

ranging between 100 and 800 cm�1 [44]. Fig. 5 shows that the

two structural types of TiO2 coexist, which is consistent with

the reported XRD pattern. In addition to different TiO2 modes,

the broad D-band (defect-induced mode) at 1340–1360 cm�1

and G band (E2g2 graphite mode) at 1570–1590 cm�1 were

observed. The intensity ratio of D band to G band (ID/IG) is pro-

posed to be an indication of disorder in TiO2–CCNF, TiO2–CNF,

and CNF, originating from defects associated with vacancies,

grain boundaries, and amorphous carbons. It is seen that

there is little change in the ID/IG of TiO2–CCNF (1.0547) com-

pared with TiO2–CNF (1.0551) and CNF (1.0522). This result

seems to indicate that graphitic (sp2) nature of the carbon in

the photocatalysts remained the same after loading with TiO2.

XPS analysis can provide valuable insight into the surface

structure of the CCNF-supported TiO2 photocatalyst. The Ti2p

photoelectron peaks of TiO2–CCNF materials are presented in

Fig. 6a. The peaks at 465.23 and 459.62 eV represent the Ti2p1/2

Fig. 5 – Raman spectrum of TiO2–CCNF hybrids.

2476 C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1

and Ti2p3/2 of Ti4+, respectively, and the Ti3+ peaks at

461.90 eV (Ti2p1/2) and 458.58 eV (Ti2p3/2) were also observed.

However, in the case of TiO2–CNF materials (Fig. 6b), the peaks

of Ti2p1/2 and Ti2p3/2 of Ti4+ were present, whereas the peaks

of Ti3+ in Ti2O3 or TiO were barely observed. The Ti3+ species

were generated for the TiO2/CCNFs, which might be due to

the carboxyl groups by graphene. Some of these groups com-

bine with titanium through chelation, and then the positions

of the oxygen in TiO2 were occupied by carboxyl groups. The

chelation effect reduce the valence of titanium, whereas in-

crease the content of Ti3+ [46]. Generally, the Ti3+ state on

the TiO2 surface plays a dominant role in the photocatalytic

activity because it can trap photogenerated electrons and

leave behind unpaired charges to promote photoactivity.

Therefore, the increasing Ti3+ density promotes effective seg-

regation of electron and cavity, interface charge transfer as

well as lowered the probability of compounding cavity, and

then increases the photocatalysis performance.

Fig.7 shows the high-resolution XPS spectra of the C1s and

O1s region on the surface of TiO2–CCNF. The deconvolution of

the C1s XPS spectrum (Fig. 7a) revealed three Gaussian curves

Fig. 6 – XPS spectra of the deconvoluted Ti(2p) peaks

centered at 285.0, 286.3, 287.4, and 289.1 eV, which can be as-

signed to graphitic carbon (C–C and CHn), alcohol or ether car-

bon (C–OH, C–O–C), carbonyl groups (C@O), and carboxyl or

ester carbon (O@C–O), respectively. The O1s core level spec-

trum (Fig. 7b) was deconvoluted into two peaks, indicating

the presence of C@O or Ti–O bond of TiO2 (530.5 eV) and

C–O in ether (O�2 , 532.2 eV) [46]. Atomic ratios of titanium to

oxygen for TiO2–CCNF and TiO2–CNF can be calculated

through the rough stoichiometry using the XPS data. The

Ti:O ratios of TiO2–CCNF and TiO2–CNF were determined to

be 1:2.63 and 1:2.50, respectively. However, it has been sup-

posed that the titanium-oxygen ratio is less than 1:2. There

is an overall increase in the Ti:O ratio, which is consistent

with the increase in the proportion of oxygen in the photocat-

alysts. It means that there may be more oxygen species such

as carboxylic acid group of graphene in the TiO2–CCNF photo-

catalyst surface besides the crystal lattice oxygen [46,47].

The FT-IR spectrum of the TiO2–CCNF was reported in

Fig. 8. We can observe absorption at 3439.08 cm�1 (OH broad-

ened band of either alcoholic or hydroxyl groups), 1612.49/

1454.33 cm�1 (C@C and C–C stretching bands in the aromatic

range, carbonyl), and 1000–1300 cm�1 (C–O stretching band of

ether). In particular, the intensity of SNF is larger than that of

TiO2–CCNF, which represent indirectly that TiO2 nanoparticles

are grown on surface of CCNF with the aid of the oxygen func-

tionalities as reactive sites at the graphene surface. Hence,

the XPS and FT-IR spectra show the presence and decrease

of the oxygen functional groups, such as –C–O–, –C@O, and

–O–C@O, on the TiO2–CCNF catalyst after carbonization.

The photoluminescence (PL) emission spectroscopy has

been widely used to study the transfer behavior of the photo-

generated electrons and holes in semiconductor materials, so

that it can reflect the separation and recombination of photo-

generated carries. [48,49]. The measured PL-emission spectra

of TiO2–CCNF, TiO2–CNF, and CNF in the range of 300–850 nm

are presented in Fig. 9. The spectra are broad (extending from

400 to 700 nm) and centered around 485 nm (2.56 eV) for TiO2–

CCNF and 497 nm (2.50 eV) for TiO2–CNF. The intensity of the

PL-spectra decreased in the following order for the photocat-

alysts: CNF, TiO2–CNF, and TiO2–CCNF; this result indicates

that the reduction of the PL-intensity shows the diminution

for the (a)TiO2–CCNF and (b) TiO2–CNF materials.

Fig. 7 – Deconvolution of (a) C1s, (b) O1s core levels of TiO2–CCNF according to XPS spectra.

Fig. 8 – FT-IR spectra of the TiO2–CCNF and SNF.

C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1 2477

of the electron/hole pairs recombination process. Generally,

the lower PL intensity suggests the lower the recombination

rate of photogenerated electron–hole pairs, which leads to

the high the photocatalytic activity of semiconductor

Fig. 9 – Photoluminescence spectra of (

photocatalysts [50]. Therefore, the low PL intensity for TiO2–

CCNF indicates that the photocatalytic activities of TiO2 may

be improved due to the interactions between the excited elec-

tron of TiO2 particles and the graphene.

The photocatalytic activities of the TiO2–CCNF, TiO2–CNF,

CCNF, and CNFs were evaluated by measuring the decolor-

ation rate of MB under visible light irradiation (Fig. 10) at

18 �C. The experimental results showed that the MB decom-

position rate in the samples occurred in the following order:

TiO2–CCNF > TiO2–CNF � CCNF > CNF. The concentration re-

duced rapidly over the first 30 min, and then the decomposi-

tion rate slowed down or stopped after the amount of time for

all of the samples. As seen in Fig. 10a, the TiO2–CCNF showed

the best photocatalytic activity under visible light irradiation.

The use of the TiO2–CCNF catalyst yielded nearly a 100%

reduction in the first 30 min of irradiation. The MB decompo-

sition reaction followed pseudo-first-order kinetics, and the

kinetic constants (k, min�1) were calculated and are pre-

sented in the inset in Fig. 10a. The superior photocatalytic

activity of TiO2–CCNF materials was also demonstrated in

the degradation of MB. The average rate constant for the

TiO2–CCNF (k = 0.16 min�1) is much larger than those for

TiO2–CNF (k = 0.012 min�1), CCNF (k = 0.011 min�1), and CNFs

a) TiO2–CCNF, (b) TiO2–CNF, (c) CNF.

Fig. 10 – (a) Photocatalytic degradation of MB monitored as the concentration change versus irradiation time (inset: average

reaction rate constant (min�1) for the photodegradation of MB with free TiO2). (b) Photocatalytic activity of the TiO2–CCNF

hybrids for MB degradation over five degradation cycles.

2478 C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1

(k = 0.003 min�1). Additionally, the stability of the TiO2–CCNF

materials was examined by following the degradation of MB

during a five-cycle experiment. After each run, the TiO2–CCNF

photocatalyst was evacuated for 30 min and was reused in the

next run. As shown in Fig. 10b, the photocatalytic degradation

of MB with TiO2–CCNF phtocatalysts under visible light irradi-

ation was consistently effective over five degradation cycles.

In general, the efficiency of the photodegradation by TiO2

depends on the illuminated catalyst surface area in contact

with the solution and on the mass transfer to the catalyst sur-

face [37,38]. Nitrogen adsorption isotherms and pore size dis-

tributions for TiO2–CCNF, TiO2–CNF, and CNF are shown in

Fig. 11. The adsorption isotherms (Fig. 11a) of TiO2–CNF and

CNF show typical type I behavior representing the micropo-

rous adsorption, and the adsorption of nitrogen was nearly

complete at a low relative pressure (P/P0 < 0.1). The adsorption

isotherms of TiO2–CCNF exhibit combined type I and type II

characteristics. Hysteresis at a relative pressure higher than

Fig. 11 – (a) Nitrogen adsorption isotherms at 77 K, (b) Differenti

pore diameter.

P/P0 = 0.5 was observed, which was typical type II behavior

of mesoporous adsorption and micropore filling were ob-

served at a low relative pressure of P/P0 < 0.1, which was

typical type I behavior [51]. The pore size distribution in

TiO2–CCNF, TiO2–CNF, and CNF samples based on the BJH

method show a broad distribution of mesopores, with sizes

ranging from 2–50 nm (Fig. 11b). In particular, TiO2–CCNF

sample has a much larger pore volume around 3.8 nm than

the other samples, suggesting that the presence of mesopores

can lead to adsorption of MB molecular easily. Because the

molecular size of MB is 1.36 · 0.47 · 0.24 nm [20], and it can

access pores with diameters larger than 1.5 nm [52]. Further-

more, as shown in Table 1, the pore characteristics of the

photocatalysts decreased in the following order: TiO2–CCNF,

CNF, and TiO2–CNF; this result indicates that the exposed

CNFs surface with high surface area will be functioned as

centers of condensing substrates with a physical adsorption

process. Therefore, rapid decomposition of MB in the

al pore volume of various photocatalysts as a function of the

Fig. 12 – Adsorption behavior of MB on TiO2–CCNF and TiO2–

CNF.

Table 1 – Pore characteristicsa of the photocatalysts used inthis study.

Samples BET surfacearea(m2 g�1)

Total porevolume(cm3 g�1)

Average porediameter(nm)

TiO2–CCNF 434 0.23 2.17TiO2–CNF 361 0.15 1.60CCNF 447 0.17 1.58CNF 405 0.18 1.81

a Pore parameters were obtained from N2 adsorption.

C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1 2479

beginning is reasonably supposed to be due to further adsorp-

tion of MB into CNF layer, and the following gradual change in

Fig. 13 – Schematic illustration of phot

color could be due to consequential processes of adsorption

into CNF layer and then decomposition on the surface of

TiO2 particle.

As CNFs and graphene of photocatalysts have some

adsorption capacity for MB, as shown in Fig. 11 and Table 1,

a comparison between adsorption and photocatalytic

degradation of MB was carried out experimentally with and

without visible light irradiation to evaluate the actual photo-

catalytic activity. Under the same conditions, adsorption of

TiO2–CCNF and TiO2–CNF to MB in the dark was also tested

and the results are shown in Fig. 12. When the catalyst is illu-

minated by visible light irradiation, the MB concentration is

much higher decreased than that without visible light irradi-

ation, indicating that MB is rapidly decomposed by the

TiO2–CCNF catalysts photocatalytically with visible light

irradiation.

The results indicate that TiO2–CCNF has a higher efficiency

for the decomposition of MB than the TiO2–CNF, CCNF, and

CNFs. It was confirmed that the introduction of graphene

and CNFs to the TiO2 photocatalyst could increase the decom-

position rate of some organic compounds using a photocata-

lytic process. The reasons for these improvements are

attributed to the following functions, as shown in Fig. 13.

(i) The graphene with carboxylic acid groups is capable of

binding TiO2 without aggregation effects, resulting in

small nanoparticles. As the particle size of TiO2

becomes small, the photogenerated electrons and holes

can easily migrate to reaction sites on the surface,

thereby reducing the recombination probability [13,45].

(ii) In the TiO2–CCNF materials with a mixture of anatase

and rutile structures, the graphene may act as an elec-

tron acceptor (Fig. 13a) for rutile (Ebg = 3.0 eV) and as a

photosensitizer (Fig. 13b) for anatase (Ebg = 3.2 eV),

which prevents the recombination of electron–hole

ocatalysis with TiO2–CCNF hybrids.

2480 C A R B O N 5 0 ( 2 0 1 2 ) 2 4 7 2 – 2 4 8 1

pairs and promotes photodegradation [13]. This phe-

nomenon may be responsible for extending photocata-

lytic activity into the visible light range.

(iii) A large surface area is necessary for the photocatalytic

degradation of organic compounds because adsorption

of the organic compound is an important process. The

exposed fibers with high-surface-area TiO2–CCNF

materials will function as centers of substrate conden-

sation in the physical adsorption process. There the

peculiar properties of CNFs are expected to elevate

the photocatalytic activities of TiO2.

4. Conclusion

Small TiO2 nanoparticles were successfully deposited on elec-

trospun CCNFs by the sol–gel method, and these composites

were very active photocatalysts in the photodegradation of

MB under visible light irradiation. Furthermore, TiO2–CCNF

materials could be recycled without a decrease in the photo-

catalytic activity. The results suggest that graphene acts as an

electron acceptor and a photosensitizer, which causes an in-

crease in the photodegradation rate and reduces electron–

hole pair recombination. In addition, the CNFs’ high surface

area synergistically improved the photocatalytic activity of

TiO2 by enhancing the physical adsorption of the substrate.

Therefore, TiO2–CCNF materials are expected to be a promis-

ing candidate for photocatalytic processes using sunlight

irradiation.

Acknowledgements

This research was supported by the Basic Science Research

Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education, Science and

Technology (No. 2011-0005890) and the Ministry of Education,

Science and Technology (MEST) (K20903002024-11E0100-

04010).

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