J. Korean Powder Metall. Inst., Vol. 24, No. 4, 279-284, 2017

DOI: 10.4150/KPMI.2017.24.4.279

ISSN 1225-7591(Print) / ISSN 2287-8173(Online)

279

The Synthesis and Photocatalytic activity of Carbon Nanotube-mixed

TiO2 Nanotubes

Chun Woong Park, Young Do Kim, Tohru Sekinoa, and Se Hoon Kimb,*

Department of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of KoreaaThe Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan

bMaterials Convergence & Design R&D Center, KATECH, 303 Poongsero, Poongsemyeon, Cheonan-si,

Chungnam 31214, Republic of Korea

(Received August 11, 2017; Revised August 21, 2017; Accepted August 22, 2017)

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Abstract The formation mechanism and photocatalytic properties of a multiwalled carbon nanotube (MWCNT)/TiO2-

based nanotube (TNTs) composite are investigated. The CNT/TNT composite is synthesized via a solution chemical

route. It is confirmed that this 1-D nanotube composite has a core-shell nanotubular structure, where the TNT surrounds

the CNT core. The photocatalytic activity investigated based on the methylene blue degradation test is superior to that

of with pure TNT. The CNTs play two important roles in enhancing the photocatalytic activity. One is to act as a

template to form the core-shell structure while titanate nanosheets are converted into nanotubes. The other is to act as

an electron reservoir that facilitates charge separation and electron transfer from the TNT, thus decreasing the electron-

hole recombination efficiency.

Keywords: CNT-TNT composite, Hydrothermal synthesis, Photocatalyst, TiO2 nanotube, Core-shell structure

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1. Introduction

Titanium dioxide (TiO2) has unique physicochemical

properties and then is widely investigated and used in

many applications such as photocatalyst, Li-battery anode,

dye-sensitized solar cell, antimicrobial coating, gas sensor,

hydrogen storage, and biocompatible surface due to their

high photocatalytic activity, natural abundance, chemical

stability and nontoxicity [1, 2].

Recently, TiO2-based nanotubes have attracted much

attention due to the unique combination of physico-

chemical properties and low-dimensional nanostructures

of TiO2 [3-5]. It has been prepared by replica- or tem-

plate-assisted methods [5-9], templateless methods via a

solution chemical synthesis [4, 6, 9-11], hydrothermal

treatment [12, 13], and electrochemical anodic oxidation

[14-16]. Among these, the solution chemical method,

first reported by Kasuga et al. [4, 10], has the advantage

of simple and low-cost fabrication of nanostructured crys-

tallite materials. This synthesis easily allows to obtain

TNTs with diameter of around 10 nm and to hybridize

with other materials by relatively low-temperature pro-

cessing which is based on the refluxing TiO2 powder in

the high concentration alkaline solution (10~20 M) at

100-150oC. On the other hand, carbon nanotubes (CNTs)

[17, 18] are also well known one-dimensional (1-D) nano-

materials, and that not only can they facilitate the elec-

tron-hole separation but also enhance the light absorption

of the photocatalysts [12, 19], hence the synthesis of

CNT-TiO2 nanoparticles are reported in some studies [13,

20]. In spite of unique characteristics of these two 1-D

nanomaterials, as far as the authors know, there is no

report on the synthesis and characterization of highly

structure-controlled hybrids of TNT and CNT with core-

shell morphology.

In this study, we demonstrate a simple route to fabri-

cate the 1-D nanostructured core-shell composite of CNT

and TNT for enhancing the photocatalytic properties.

This composite was synthesized via the solution chemi-

cal synthesis route. The microstructural characteristics of

*Corresponding Author: Se Hoon Kim, TEL: +82-41-559-3377, FAX: +82-41-559-3288, E-mail: shkim@katech.re.kr

280 Chun Woong Park et al.

Journal of Korean Powder Metallurgy Institute (J. Korean Powder Metall. Inst.)

as-synthesized CNTs and TNTs composite, compared

with pure TNTs, were investigated by several analytical

techniques such as X-ray diffraction, scanning and trans-

mission electron microscopy, thermo-gravimetric analy-

sis, Raman spectroscopy, and the formation mechanism

of as-prepared composite was discussed. The enhanced

photocatalytic activity of CNT and TNT composite was

also discussed by the results of adsorption and degrada-

tion of methylene blue.

2. Materials and Methods

The commercial anatase-phase TiO2 (Wako pure chemi-

cal, Japan, 99%) powder and MWCNTs (Sigma-Aldrich,

USA, OD=10-20 nm, ID=5-10 nm, >95%) were used as

starting materials. Next, MWCNT-TNT composite, hereaf-

ter designated CNT-TNT, which had molar ratio of 5% of

CNT to TNT was synthesized via a soft hydrothermal

route. CNTs were dispersed in distilled (DI) water for 30

min by ultrasonic and magnetic stirring to achieve homo-

geneous dispersions. TiO2 powder was added to the CNT

dispersions, and then NaOH was added to reach 10 M

NaOH solution, and the mixture was refluxed at control

temperature of 135oC for 24 h. The resultant product was

washed with DI water and was treated 0.1 M HCl. The

treated product was washed again with DI water until the

solution conductivity reached below 10 μS/cm. Finally, the

synthesized powder was dried in an oven at 70oC for 48 h.

The phase analysis of synthesized samples was con-

ducted by powder X-ray diffractometer (XRD, D8 Advance,

Bruker AXS GmbH, Karlsruhe, Germany) using Cu Kα

radiation. Field emission scanning electron microscopy

(FESEM) images were acquired using Nova NanoSEM 450

(FEI, Hillsboro, USA). Microstructure observation and com-

position analysis were performed by a transmission elec-

tron microscopy (TEM, JEM-2100F, JEOL, Japan) with

an accelerating voltage of 200 kV equipped with an

energy dispersive spectroscopy (EDS). The thermogravi-

metric (TG, TG-DTA2000SA, MAC Science, Japan)

analysis was performed under a heating rate of 1oC/min up

to 800oC with Air flow of 1 l/min. RAMAN spectra of

samples were acquired by NRS-2000A (JASCO, Japan).

The adsorption and degradation properties of TNT and

CNT-TNT samples were analyzed using methylene blue

(MB, Wako Pure Chemical Industries, Japan). 20 mg of

each sample was dispersed in 100 ml of 10 mg/l MB solu-

tion. The adsorption was measured for 4 h in the dark, and

then the degradation was evaluated in the UV light irradia-

tion (mercury–xenon lamp with a center wavelength of

365 nm, UVF-204S, SAN-EI Electric, Japan) at room

temperature. The MB solution was analyzed with UV-vis

spectrophotometer (UV mini-1240, Shimadzu, Japan) by

measuring the absorption band of 663 nm.

3. Results and Discussion

The solution chemical synthesis process successfully

provided dark grayish powdery products. As shown in

Fig. 1, FESEM observation was performed to confirm

the morphologies of TNT and CNT-TNT samples. Fig.

1(a) shows a typical morphology of as-synthesized TNT

sample. The nanotubes were bundled each other from

Fig. 1. FE-SEM images of (a) as-synthesized pure TNT and

(b) CNT-TNT composite.

The Synthesis and Photocatalytic activity of Carbon Nanotube-mixed TiO2 Nanotubes 281

Vol. 24, No. 4, 2017

several tens to several hundreds. Their outer diameter

was approximately 10 nm and length was below 1 μm as

is well-known. Contrastively, nanotubes of CNT-TNT

composite were well-separated each other as presented in

Fig. 1(b). They had outer diameter of 20~40 nm and

maximum length of several micrometers. In case of the

adding CNTs, the specific surface area was presumed to

be slightly decreased to about quarter because the diame-

ter of nanotubes was increased from 10 nm to more than

20 nm. It could be inferred that CNTs affected to the

morphology of TNTs during the alkaline hydrothermal

synthesis. However, CNT could not be detected easily

through FESEM observation.

Fig. 2 shows the XRD patterns of the TiO2 powder as

raw material and the TNTs synthesized with and without

CNTs. Whereas the raw material TiO2 powder was

shown as an anatase phase (JCPDS 21-1272), the as-syn-

thesized TNT and CNT-TNT composite did not exhib-

ited clear and sharp crystallite peaks. In fact, it was

known that pure TNT synthesized by an alkaline hydro-

thermal route was formed to H2TinO2n+1 or NaxH2-xTi-

nO2n+1 (n=3, 4) [11]. In this study, observed diffraction

peaks of the pure TNT at around 2θ of 10o, 24.5o and

48.5o corresponding to the (200), (110), and (020) planes

were in good agreement with the monoclinic H2Ti4O9·H2O

(JCPDS 36-655) having four edge-sharing TiO6 octahe-

dra in the unit cell. On the other hand, the XRD pattern

of CNT-TNT sample (2θ ~ 8.4o, 10.6o, 12.4o, 25o and

48.5o corresponding to (001), (200), (20 ), (110), and

(020) planes, respectively) seemed to be monoclinic

H2Ti3O7·nH2O because two peaks of (001) and (20 )

planes were more similar to pattern of the H2Ti3O7

(JCPDS 47-561) having three edge-sharing TiO6 octahe-

dra. The typical CNT peak that should exist around 25o

could not be confirmed due to its small amount and peak

overlap with TNT peak of (110) plane, and some peaks

which were indexed to (001), (200), and (20 ) planes,

were shifted to lower angles as compared with an anhy-

drous H2Ti3O7. It is well-known that the interlayer spac-

ing of TNT is expanded with increasing the amount of

adsorbed water because this adsorbed H2O exists in the

one-dimensional tunnel and skeletal crystal structure of

nanotubes [21]. Thus, it was speculated that the peaks of

(001), (200), and (20 ) planes were shifted due to

adsorbed H2O of sample.

To analyze more detail, TG analysis was conducted

Fig. 3 shows the TG curves of TNT and CNT-TNT sam-

ples. Some steps were observed at 30-150, 150-250, 250-

400oC for TNT and CNT-TNT, and at 550-700oC only

for CNT-TNT. The first step up to 150oC was desorption

of H2O, which was existed in interlayer space of nano-

tube wall. The weight loss of CNT-TNT composite in

this step was approximately 5.1% corresponding to

0.78H2O in H2Ti3O7·nH2O. Pure TNT sample had the

weight loss of 5.3% that was equivalent to one mole of

H2O in H2Ti4O9·nH2O. The weight losses of second and

third steps from 150 to 450oC meant that titanate (H2Ti-

nO2n+1) was changed to TiO2 through a dehydration. In

these steps, the weight losses were approximately 5.4 and

1

1

1

1

Fig. 2. XRD patterns of raw TiO2, as-synthesized TNT and

CNT-TNT composite.

Fig. 3. Thermogravimetric (TG) curves of TNT and CNT-

TNT composite.

282 Chun Woong Park et al.

Journal of Korean Powder Metallurgy Institute (J. Korean Powder Metall. Inst.)

6.6% for pure TNT and CNT-TNT samples, respectively.

Also, the calculated dehydration amounts of H2Ti4O9 and

H2Ti3O7 are 5.3 and 6.9%, respectively. From the results

of XRD and TGA, it was considered that TNT and CNT-

TNT were formed as H2Ti4O9·H2O and H2Ti3O7·0.78H2O

each. Moreover, the weight loss at 550-700oC for CNT-

TNTs was confirmed to be occurred by burning the

MWCNTs corresponding to 5mol.%.

Fig. 4 shows the RAMAN spectra of TNT and CNT-TNT

samples. The RAMAN band at 190 cm-1 was attributed to

the anatase TiO2 mode [11] and bands at 275 cm-1, 448 cm-1,

670 cm-1, 840 cm-1 were generally assigned to the TNTs [2,

22, 23]. It was reported that the features at 275 cm-1,

448 cm-1, 670 cm-1 were originated from Ti-O-Ti vibra-

tion and the band around 840 cm-1 with a broad and weak

intensity was related to Ti-O-H vibration [23, 24]. How-

ever, the origin of TNTs bands is still not revealed clearly.

The inset in Fig. 4 shows the D-band at 1350 cm-1 and G-

band at 1580 cm-1 of CNT for the CNT-TNT composite,

showing CNT could remain without any serious damage

during the high concentration alkali treatment.

To reveal the formation mechanism of CNT-TNT com-

posite, FESEM and TEM observations were conducted.

Although the formation mechanism of pure TNT in

NaOH aqueous solution has been argued between some

researchers till now, many researchers agree that the tita-

nate nanosheets are formed and then are rolled to nano-

tubes [25-28]. FESEM image of the CNT-TNT sample

which was taken during an alkaline hydrothermal synthe-

sis was shown in Fig. 5(a). It can be confirmed that the

titanate sheet-like products wrapped around the CNTs

before rolling to titanate nanotubes. Fig. 5(b) shows TEM

image of the as-synthesized CNT-TNT sample. The result

of TEM-EDS line-scanning revealed that the CNT and

TNT were consisted by the core-shell nanotubular struc-

ture, where the TNT shell surrounded the CNT core. It

could be confirmed that the inner and outer diameters of

TNT which had the tubular structure were ~20 nm and

~25 nm, respectively. The diameter of TNT on CNT-TNT

composite was increased compared with that of the pure

TNT sample which had the outer diameter of ~10 nm.

According to the formation mechanism of TNT, it was

consequently speculated that the increment of diameter

Fig. 4. Raman spectra of TNT and CNT-TNT composite.

(Inset) D and G bands of CNT for CNT-TNT composite.

Fig. 5. FESEM and TEM images of CNT-TNT composite; (a)

during an alkaline hydrothermal synthesis and (b) as-

synthesized. (Inset) Line profile of the TEM-EDS corresponding

to yellow line in (b).

The Synthesis and Photocatalytic activity of Carbon Nanotube-mixed TiO2 Nanotubes 283

Vol. 24, No. 4, 2017

was because while titanate nanosheet was formed to

nanotube, this nanosheet wrapped around the CNT tem-

plate having a diameter of ~10 nm, resulting in forming

the core-shell structure.

To evaluate the photocatalytic activity of synthesized

CNT-TNT composite, the MB removal test was per-

formed under the dark and UV light irradiation condi-

tions. Fig. 6 depicts the adsorption and degradation

properties of MB by TNT, and CNT-TNT samples. The

amount of adsorbed MB by CNT-TNT was slightly lower

than that by TNT. The lower amount of adsorbed MB on

CNT-TNT sample could be ascribed to the decrease to

about quarter of the specific surface area of TNT due to

increasing the diameter and wall thickness of synthe-

sized nanotubes as mentioned before. However, the deg-

radation rate of MB by CNT-TNT sample under UV light

irradiation after 4 h was dramatically higher than that by

TNT. This superiority of CNT-TNT was considered that

CNT acted as a reservoir to trap electrons emitted from

TNT during the photocatalysis process, and thereby the

electron-hole pair recombination could efficiently be sup-

pressed, resulted in improvement of the photocatalytic

activity [12, 19].

To enhance photocatalytic properties of TiO2, many

researches have been performed to load noble metal

nanoparticles (NPs) such as Pt on TiO2 surface to facili-

tate the co-catalytic effect of the metal NPs [29, 30].

Recently, Pt NPs loaded TNT has also attracted much

attention due to their potential as high-performance cata-

lyst [31-35]. In the case of metal (Pt) NPs loaded TiO2, it

is argued that photocatalytic degradation of organic mole-

cules in TiO2 surface is promoted by the reduction and

oxidation reaction by various radicals such as O2•-, OH•

and so on which are generated by the photogenerated

electrons and holes [35, 36]. The Pt NPs plays a major

role in realizing charge separation to hold electrons while

the TiO2 acts individual molecular adsorption as well as

reaction site by providing photogenerated holes for radi-

cal formation as h+ + OH- → OH• [36]. In the present

CNT-TNT composite, CNT core might accumulate elec-

trons and TNT shell sufficiently acts as reaction site of

MB degradation. In this regard, TNT surface can fully

utilized for this sequence due to the core-shell structure.

This seems to be the similar effect that was pointed out

for the site-selective loading of Pt into inside of TNTs,

which exhibited better photocatalytic activity for oxida-

tion of acetaldehyde gas than those of Pt NPs loaded on the

outer surface of TNTs [31]. However, it is considered that

CNT might be more desirable as electron accumulation

agent because of higher electronegativity of carbon than

platinum. These facts hence imply us that the core-shell

CNT-TNT has suitable nanostructure with the enhanced

photocatalytic activity.

4. Conclusions

One dimensional CNT-TNT composite was success-

fully synthesized via the solution chemical route. FESEM

and HRTEM results showed that CNT-TNT composite

was formed by the core-shell nanotubular structure with

outer diameter of 20~40 nm and maximum length of sev-

eral micrometers, where the TNT surrounded the CNT

core. It was speculated that the titanate nanosheet, com-

posed by H2Ti3O7·0.78H2O, might roll-up to the CNT

template.

In the photocatalytic activity of samples, although the

MB adsorption property of CNT-TNT was slightly

decreased, as compared with that of pure TNT, the MB

degradation under UV light irradiation was significantly

increased due to the appropriate nanoarchitecture and

resultant function sharing of CNT core and TNT shell. It

was thus considered that such a low-dimensional core-

shell nanotube consisting of semiconductor oxide (TNT)

and conductive carbon (CNT) may exhibit better photo-

Fig. 6. Adsorption and degradation of MB for TNT and

CNT-TNT under dark and UV light irradiation.

284 Chun Woong Park et al.

Journal of Korean Powder Metallurgy Institute (J. Korean Powder Metall. Inst.)

catalytic activity as CNT played a role in suppressing the

electron-hole pair recombination of photocatalysts.

Acknowledgments

This research was supported in part by the JSPS under

the Grant-in-Aid for JSPS Fellows (No.23-01332) and

under the Grants-in-Aid for Scientific Research (A) (No.

22241017). This research was also supported in Basic

Science Research Program through the National Research

Foundation of Korea (NRF) funded by the Ministry of

Education (NRF-2016R1A6A1A03013422).

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