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Applied Surface Science 296 (2014) 1–7

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

hotocatalytic oxidation of methyl orange in water phase bymmobilized TiO2-carbon nanotube nanocomposite photocatalyst

inmao Donga,b, Dongyan Tanga, Chensha Li c,∗

School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, ChinaSchool of Sciences/Beijing Key Lab of Plant Resource Research and Development, Beijing Technology and Business University, Beijing 100048, ChinaDepartment of Mechanical Engineering, Tsinghua University, Beijing 100084, China

r t i c l e i n f o

rticle history:eceived 28 July 2013eceived in revised form1 December 2013ccepted 21 December 2013vailable online 28 December 2013

eywords:iO2

arbon nanotube

a b s t r a c t

We developed an immobilized carbon nanotube (CNT)–titanium dioxide (TiO2) heterostructure materialfor the photocatalytic oxidation of methyl orange in aqueous phase. The catalyst material was preparedvia sol–gel method using multi-walled CNTs grown on graphite substrate as carriers. The multi-walledCNTs were synthesized from thermal decomposing of hydrocarbon gas directly on thin graphite plate,forming immobilized 3-dimensional network of CNTs. The nanophase TiO2 was synthesized coating onCNTs to form “coral”-shaped nanocomposite 3-dimensional network on graphite substrate, thus bringingeffective porous structure and high specific surface area, and possessing the merit of dispersive pow-der photocatalysts, which is the fully available surface area, while adapting the requirement for cleanand convenient manipulation as an immobilized photocatalyst. Moreover, the CNT–TiO2 heterostructure

ol–gel processesanocompositeshotocatalysisEM

reduced the electron–hole pair recombination rate and enhanced the photoabsorption and the adsorp-tion ability, resulting in elevating the photocatalysis efficiency. These synergistic effects due to the hybridnature of the materials and interphase interaction greatly improved the catalytic activity, and demon-strated superior photocatalytic performances. Our work can be a significant inspiration for developinghybrid nano-phase materials to realize sophisticated functions, and bear tremendous significance for thedevelopment and applications of semiconductor nano-materials.

. Introduction

Titanium dioxide (TiO2) is one of the most widely stud-ed semi-conductors for environmental protection, self-cleaning,eodorizing and sterilizing applications due to its low cost, abun-ant resource, photocatalytic activity, harmlessness and resistanceo chemical corrosion and photocorrosion [1]. In order to provideigh photocatalytic activity, it is necessary to synthesize TiO2 ofigh specific surface area, crystallinity and porosity, and extended

ife time of the photon-generated electron–hole pairs, etc. Manyfforts have been made to inhibit crystal growth and retard thehase transformation by doping with another inorganic materi-ls, such as SiO2 [2,3], Al2O3 [4], activated carbon [5–10], etc.owever, although inorganic materials can work as the supportf nano-sized TiO2 photocatalysts and concentrate the pollut-nts and intermediates around TiO2, the photodegradation rate is

ntrinsically low due to the limited surface area. In recent years11–20], attention was paid to the fact that carbon nanotubesCNTs) are attractive and promising candidates for improving the

∗ Corresponding author. Tel.: +86 10 62797756.E-mail addresses: lichnsa@mail.tsinghua.edu.cn, lichnsa@mail.163.com (C. Li).

169-4332/$ – see front matter © 2014 Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.apsusc.2013.12.128

© 2014 Published by Elsevier B.V.

photocatalytic efficiency of TiO2 due to the contribution of theirhigh and accessible specific areas, exceptional electronic, adsorp-tion, mechanical and thermal properties, chemical inertness andstability [21]. TiO2 is an n-type semiconductor and the mainprocess in photocatalysis is activated by photon absorption andelectron–hole generation. CNTs have the unique hollow and lay-ered structure, the electron–hole pairs could transport freely alongthe cylindrical nanostructure, which thus suppresses the recom-bination rate of the photo-generated electron–hole pairs [22].Therefore, an enhancement of the photocatalytic properties ofTiO2 could be achieved in conjunction with CNTs, which promoteelectron–hole pair separation and migration. It is a process thatleaves an excess of holes in the valence band and the photogenera-ted electrons move freely from the conduction band of the TiO2 tothe electron-accepting CNT surface. TiO2 effectively behaves as ap-type semiconductor in TiO2-CNT nanocomposites. Additionally,the superior adsorption property of nanocomposite catalyst andthe improved suppression of the recombination of the charge car-riers contribute to the higher photocatalytic activity [13,16]. On

the other hand, the current TiO2-CNT nanocomposite photocata-lysts are generally made in dispersive powder state [11–20,23–30].The wide range applications for environmental remediation andcleanup requires the development of immobilized photocatalytic

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Y. Dong et al. / Applied S

ystems to guarantee certain advantages [31]: no requirementor separation or filtration steps, thin films or plates being easilydopted and configured to continuous flow systems, no aggregationf particles, and a electrical polarization as far as possible. However,s the real surface of the semiconductor that is available for the pro-ess is reduced by comparison to the corresponding exposed areaf a slurry system, due to the immobilization, the photocatalyticfficiency diminishes [32,33]. In this work, in order to reach the per-ormance that employ immobilized systems without damnifyinghe benefits of the semiconduct catalyst phase in powder systems,e studied to develop an immobilized CNT-TiO2 nanocompos-

te photocatalyst which was prepared by coating nanophase TiO2n the CNTs immobilized on graphite substrate. The immobilizedNTs were synthesized from thermal decomposing hydrocarbonas directly on thin graphite plate.

Compared to other methods of synthesizing CNTs, such as arcischarge [34] and laser vaporization [35], the catalytic synthe-is of CNTs is simpler, more reproducible, low cost and largecale production method [36,37]. Decomposition of hydrocarbons,hich an be abundantly supplied in the world, for the growth ofNTs is an attractive route and many CNTs have been obtainedy the decomposition of CH4 [38], C2H2 [39], CO [40], benzene41], and polyethene [42], etc. In our work, the bulk growth ofNTs was obtained by the catalytic decomposition of hydrocar-on gas over metal nano-catalyst deposited on thin graphite plate.ome important advantages, which are benefit for the productionf downstream products, are demonstrated: the production effi-iency is higher and the cost is lower relative to the other methods.he synthesized CNTs were stably fixed on the graphite plate with aigh purity, and thus need no further purification before utilization.ore important, the CNTs were grown on the graphite plate with

ontrollable diameters and formed “shrubbery”-like 3-dimensionaletwork, which had effective inter-spaces for mass transport andccessible surfaces for the deposition of hetero-nanophase, andhus is suitable for coating other nanophase materials on CNT-wallsy chemistry methods.

We synthesized nanophase TiO2 coated on this CNTs grown onhin graphite plate via sol–gel method to prepare immobilized CNT-iO2 nanocomposites. The immobilized CNT-TiO2 nanocompositesormed “coral”-liked 3-dimensional net-work on graphite substratehich had fully available surface of high specific area, effectiveore-sizes and mutually connected pore spaces, and thus can fullyxert the photocatalytic capability of nanophase semiconductingatalyst as those in dispersive powder state. The immobilized pho-ocatalyst composites demonstrated high photocatalysis capability,onvenient manipulation, and can be produced with high effi-iency, large batch and low cost.

. Experimental procedure

.1. The growth of CNTs on thin graphite plate

A thin graphite plate with the dimension of 5 cm × 5 cm × 0.5 cmas first annealed in air at 500 ◦C for 15 min. The NiO nanoparti-

les were deposited onto the graphite plate by radiofrequency (rf,4.5 MHz) magnetron sputtering using a metal-oxide target of NiOrom Super Conductor Materials Inc. with a purity of 99.99% and

cm in diameter. The sputtering was carried out at room temper-ture, and the argon deposition pressure was 0.2 Pa. The distanceetween the substrate and the target was 10 cm, and the rf poweras changed between 55 and 170 W. Catalysts were reduced in situ

uring the synthesis of CNTs. The graphite plate deposited with NiOanoparticles, which were the catalyst precursor, was put in theiddle of a horizontal furnace, and heated to 460 ◦C under nitro-

en. When nitrogen was substituted by hydrogen at flow rate of

Science 296 (2014) 1–7

250 ml/min for 20 min, the catalyst precursor was reduced to beNi nanoparticles, which were the catalyst for synthesizing CNTs.After heating the catalyst to 660 ◦C in hydrogen, propylene wasintroduced with a flow rate of 350 ml/min and the hydrogen inputwas reduced to half. The decomposition reaction proceeded for30 min. Then the product was cooled to room temperature undernitrogen to obtain CNTs grown on graphite plate (CNT/GP).

2.2. Preparation of the TiO2-CNT nanocomposites immobilized onthin graphite plate (TiO2/CNT/GP)

The CNT/GP was annealed in air at 550 ◦C for 15 min, then washeated in concentrated nitric acid at the boiling point tempera-ture for 40 min, then purified by distilled water till pH value beingneutrality and finally dried at 100 ◦C for 24 h.

Butyl titanate (kermel), H2O, isopropyl alcohol and HNO3 withthe molar ratio of 1:1.5:20:0.16 [43] were used as starting mate-rials for preparing immobilized TiO2-CNT nanocomposites. Butyltitanate (15 g) was solved in isopropyl alcohol, the CNT/GP was putinside the solution and stirred in a closed beaker for one hour, themixture of H2O and HNO3 was slowly dropped into it within 30 minunder stirring. The formed sol was kept at room temperature untilit formed gel. Lastly, TiO2/CNT/GP was obtained by drying the gel inan oven at 45 ◦C, annealed at 500 ◦C for one hour in the protectionof Ar gas. For comparison, the TiO2 particles immobilized on thingraphite plate (TiO2/GP) were prepared under the same conditionswithout the synthesized CNTs grown on graphite plate.

2.3. Photocatalytic degradation experiments

A pyrex reactor with the length, width, and depth being 22 cm,8 cm and 8 cm respective, was utilized. The volume of methylorange aqueous solution with the initial concentration of 30 mg/Lfilled in the reactor was 600 mL. The immobilized photocatalyst wasassembled on the bottom of the reactor. Two 6 W UV strip lampswith the length of 21 cm and a wavelength centered at 365 nm(Philips, TUV 6W/G5) were used as the light source above thesolution surface. The distance between the light sources and thephotocatalyst was about 7 cm. Constant agitation of the solutionwas insured by a magnetic stirrer placed at right angle from thereactor basis.

The solution was first stirred in dark for 120 min before irra-diation to reach equilibrated adsorption as deduced from thesteady-state concentrations. Then UV-irradiation started to ini-tiate the photocatalysis reaction. Microsamplings were drawnevery 5 min from the solution, their absorbency were mea-sured by a “721 UV/vis spectrophotometer” at the maximumabsorptive wavemeter of methyl orange (465 nm), and used fordetermining the time-dependent change of the dye concentra-tion.

2.4. Materials characteristics

The structure and characteristics were analyzed by trans-mission electron microscopy (TEM, JEOL-200CX), high resolutiontransmission electron microscopy (HRTEM, H-9000NAR), fieldemission scanning electron microscopy (FE-SEM, AMRAY-1910),X-ray diffraction (XRD, Rigaku Dmax �A X-ray diffractometer withCu-K� radiation, � = 0.154178 nm) and RM2000 fiber confocalRaman spectroscope. The specific area and the pore volume of the

the employed apparatus is SORPTOMATIC1990. ThermoQuest ItaliaS.P.A. UV–vis absorption spectra were recorded on a ShimadzuUV-2450 double-beam recording spectrophotometer. Fluorescencespectra were obtained on a Shimadzu RF-5301.

Y. Dong et al. / Applied Surface Science 296 (2014) 1–7 3

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are mutually connected and the pore-spaces are fully available, leadto the surface area of nanophase TiO2 coated on CNTs being fullyavailable. It is indicated in Table 1 that the TiO2/CNT/GP sample

Table 1Specific surface area (Sp) and pore volume (Vp) of the CNT/GP, TiO2/GP andTiO2/CNT/GP.

Sample S (m2/g) V (cm3/g)

ig. 1. (a) SEM image of the CNTs grown on graphite substrate, and (b) HRTEM imagef CNTs showing the multi-walled nanotubes composed of well graphitized layers.

. Results and discussion

.1. Characterization of the CNT/GP sample

The graphite plate was first annealed in air at 500 ◦C for 15 min,ts surface was etched by the air and became more rough, so that theatalyst particles can be effectively dispersed on it. The CNTs wererepared by the decomposition of propylene at 600 ◦C to 700 ◦Cver the catalyst pre-reduced at about 500 ◦C. The purity of CNTsas approximate to 98 percent. SEM image (Fig. 1(a)) shows that

he CNTs were grown on graphite substrate to form continuous-dimensional network as shrubbery. The shrubbery-liked CNT-etwork should be led by the crooked and entangled shapes ofhe grown CNTs. HRTEM observation shows the synthesized multi-alled CNTs with outer diameters ranging from 20 nm to 30 nmad many rope-like structures, as shown in Fig. 1(b).

Fig. 2 shows the Raman shift spectrum of the CNTs, whichas measured at room temperature with a frequency range

00–3600 cm−1. Two sharp peaks are present at 1586 cm−1 (Geak) and 1334 cm−1 (D peak). These correspond to the character-

stic peak of the graphite structure with a small basal domain size45]. Also there is a weaker peak around 2644 cm−1 which indicatesD or D* peak. Those features, including the wavenumbers andelative integrated intensities, are close to those for multi-walledarbon nanotubes [46].

The growth of CNTs during above producing process includedhe following steps: at first the feed hydrocarbon molecules weredsorbed and decomposed on certain surface active sites of the

atalyst metal particles to form carbon species; and then, somef the surface carbon species dissolved into the bulk and dif-use through the metal particle from the front face (i.e. the

etal–CNTs interface), then, some of the surface carbon species

Raman sh ift ( c m )

Fig. 2. Raman spectrum of the synthesized CNTs.

dissolved into the bulk and diffused through the metal parti-cle from the front face (i.e. the metal–gas interface) to the rearface (i.e. the metal–CNT interface), where carbon was depositedin the form of the CNTs [47]. Some defect structures with pen-tagons and heptagons might be formed in CNT-walls duringthe deposition of carbon species, resulted in a curly growth ofCNTs. Different from the method of catalytic pyrolysizing met-allorganics (catalyst precursors) and carbon precursors, whichsynthesized densely align-assembled CNTs [48]. This shrubbery-liked 3-dimensional CNT-network had feasible sized mesh-poresand enough interspaces for mass transport, and almost fully avail-able surface areas, thus can become effective carrier for thecoating of hetero-nanophase materials through chemistry pro-cesses.

3.2. Characterization of the TiO2/CNT/GP and TiO2/GP samples

The XRD patterns of the sol–gel prepared TiO2/GP sample andthe TiO2/CNT/GP sample are given in Fig. 3(a) and (b). The peaksin all the diffraction patterns of the two correspond to the anataseTiO2 (JCPDS no. 21-1272). Fig. 3(b) indicates the two peaks of (0 0 2)and (1 0 0) reflections of CNTs [49,50]. By the thermogravimetricanalysis (PERKIN-ELMER TGA7), the measured ratio of CNTs in TiO2-CNT composite was about 13 wt%.

The SEM image of TiO2/GP sample, as shown in Fig. 4(a), illus-trates rough, sponge like structured TiO2 coat on graphite substrate.Though TiO2 particles are well dispersed to form a network with aneven distribution of mountains and valleys, they stack together tomake large part of particle surfaces being overlapped each otherand very little inter-space of the network being available. TheSEM image of TiO2/CNT/GP sample, as shown in Fig. 4(b), demon-strates that nanophase TiO2 particles densely coated on the CNTsgrown on the graphite substrate, form “coral”-liked nanocompos-ite 3-dimensional network immobilized on graphite substrate. Thepores in this “coral”-liked nanocomposite 3-dimensional network

p p

CNT/GP 202.67 0.972TiO2/GP 69.58 0.052TiO2/CNT/GP 173.34 0.758

4 Y. Dong et al. / Applied Surface Science 296 (2014) 1–7

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ig. 3. XRD patterns of the TiO2/GP sample (a), and the TiO2/CNT/GP sample (b). (hich carried the TiO2/CNT). (For interpretation of the references to color in figure

as higher BET specific surface area and pore volume comparedo TiO2/GP sample, this improvement of specific surface area and

ore volume should be owe to the scaffold effect of CNT- network,s indicated in Table 1.

The TEM micrograph of CNT-TiO2 nanocomposites (Fig. 4(c))hows TiO2 nanoparticles homogeneously and densely spread on

bstrate was removed. The peaks marked by green bars were led by the glass plated, the reader is referred to the web version of the article.)

the surfaces of CNTs. A HRTEM micrograph of the heterostructurebetween CNTs and TiO2 nanoparticles is presented in Fig. 4(d), quite

well crystallized and organized nanophase TiO2 is observed on theCNT surface. Furthermore, the interface between CNTs and TiO2is clearly seen, indicating that TiO2 nanoparticles, whose sizes areabout 10 nm, are well attached on the outermost shell of CNTs.

Y. Dong et al. / Applied Surface

Fig. 4. (a) A SEM image of the TiO2/GP sample, (b) a SEM image of the TiO2/CNT/GPsample, (c) a TEM image of the nano TiO2 coated CNTs scraped from the TiO2/CNT/GPsample, and (d) a HRTEM image of the nano TiO2 coated CNTs scraped from theTiO2/CNT/GP sample.

Science 296 (2014) 1–7 5

3.3. Effect of photocatalytic degradation

The UV–vis absorption spectra of the prepared samples, includ-ing TiO2 particles which were removed from the GP substrate,TiO2-GP composite which was the TiO2 particles together withthe top GP layer removed from the GP substrate, CNTs and TiO2-CNTs composite are shown in Fig. 5. The samples show an intenseabsorption in UV region due to electron promotion of TiO2 fromthe valence band to the conduction band. There is a weak differ-ence of absorption intensity between TiO2 and TiO2-GP. The CNTssample exhibits a stronger absorption in the total spectra range,the reason not only lies in its characteristic as a carbon material,but also due to its greatly higher specific area compared to the GP.The TiO2-CNTs composite exhibits the strongest absorption in UVregion, indicates that the presence of CNTs with the heterostruc-ture to nano TiO2 phase strengthen the absorption capability of UVlight. In order to study the effect of CNTs on the recombination ofe−/h+ produced by TiO2, the photoluminescence spectra (PL) aredetected, as shown in the inset of Fig. 5. TiO2 shows a strong PLemission band. Compared with the spectrum of TiO2, it is foundthat the PL peak of the TiO2-CNTs composite in the same wave-length range is much lower than that of TiO2. The reduction of PLintensity in the TiO2-CNTs composite indicates the decrease of theradiative recombination process. Thus, with the assistance of CNTs,the recombination of e−/h+ excited by TiO2 under UV light can bedecreased and the photon efficiency can be increased.

The adsorption abilities of TiO2/CNT/GP and TiO2/GP to themethyl orange in solution were first measured. The adsorptionratios of methyl orange by TiO2/CNT/GP and TiO2/GP are shownin Fig. 6. It is indicated that the TiO2/GP has lower capabil-ity of adsorbing methyl orange in aqueous solution whereas theTiO2/CNT/GP has evident effect of adsorbing methyl orange. Thiscan be attributed to the superior adsorption property of CNTs[16,19,51] and the large effective surface area of the TiO2/CNT/GP,which correlates to a strong adsorption ability.

Fig. 7(a) demonstrates the concentration change of methylorange during the photocatalytic degradation by the TiO2/CNT/GPand TiO2/GP respectively at different temperature. The experimen-tal data indicated that the photocatalytic degradation of methylorange is consistent with the first-order reaction: −ln C/C0 = kt,where C0 is the initial methyl orange concentration, C is the con-centration at a certain time, and t is time. k is the apparent reactionrate constant, it is not only determined by Arrhenius formula, butalso is influenced by some other properties of the reaction system[12,14,18], such as the specific area and porosities of the photo-catalyst, the photon absorption and scattering properties, etc. Themeasured apparent reaction rate constants of the photodegrada-tion of methyl orange by the TiO2/CNT/GP and TiO2/GP respectiveat different temperature are shown in Fig. 7(b). Fig. 7 indicates thatthe photodegradation rate increases as the temperature rising. Thereason lies in that the subsequential oxidation–reduction reactionsled by the photocatalysis can be obviously influenced by tempera-ture. Moreover, higher temperature can accelerate the exchange ofreactants, and enhance the conductivity of photo-induced carriersin solid phase, results in the efficiency of migration and transport ofthe photo-induced carriers in TiO2 and CNTs being increased, hencepromoted the redox reactions. But the photocatalysis capabilitiesfor methyl orange degradation of TiO2/CNT/GP are evidently higherthan that of TiO2/GP. The reason should be as follows:

First, CNTs enhance the disassociation efficiency of photogen-erated holes and electrons in TiO2 semiconductor. CNTs are capableof forming a Schottky barrier at the CNT–TiO2 interface, where

there is a space charge region. TiO2 is an n-type semiconductor, inthe presence of CNTs, photogenerated electrons may move freelytoward the CNT surface, which may have a lower Fermi level. Thusthe CNT acts as an electron sink. This leaves an excess of holes

6 Y. Dong et al. / Applied Surface Science 296 (2014) 1–7

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ig. 6. Absorption of methyl orange by the immobilized photocatalyst of TiO2/GPnd TiO2/CNT/GP respectively. Temperature: 24 ◦C.

n the valence band of the TiO2, which can migrate to the sur-ace and react. The TiO2 therefore effectively behaving as a p-typeemiconductor [52]. A continuous CNT network ensures higherlectrical conductivity and electron storage capacity, thus may acts extremely effective electron sinks. The CNT-network grown onhe graphite substrate formed a continuous conducting networkf cylindrical nanostructure, let the electron–hole pairs to trans-ort freely along these network paths, and effectively suppresseshe recombination rate of the photo-generated electron–hole pairs.

oreover, the nanoscale TiO2 phase coated on CNT surfaces (asndicated in Fig. 4) with high aspect ratio may also play a role inetardation of electron–hole recombination [53], and this effect isromoted by the continuous network of CNT-scaffolds. In addi-ion, the high aspect ratio and nanoscale geometry allows electronccumulation at the ends of CNTs [54], providing highly effec-

ive reduction sites; secondly, CNTs can effectively adsorb methylrange in the aqueous solution, as shown in Fig. 6, to result in theethyl orange being concentrated around the CNTs carried TiO2

hases, and the photocatalysis reaction being promoted; thirdly,

46 ◦C; c: 67 ◦C) and TiO2/CNT/GP (d: 24 ◦C; e: 46 ◦C; f: 67 ◦C). (b) Apparent reactionrate constants of TiO2/GP catalyst and TiO2/CNT/GP catalyst at various temperature.

the higher surface area of our prepared nanocomposite photocata-lyst (as indicated in Table 1) can enhance its apparent photoactivitydue to the higher interface for the redox reaction and the higherrate of photon absorption, which is shown in Fig. 5. In addition,the unique structure of the prepared immobilized photocatalystTiO2/CNT/GP is also propitious to enhancing photocatalysis capa-bility. Different from those immobilized CNT-TiO2 photocatalystswhich were fabricated by sticking CNT-TiO2 powder on a sub-strate to form a compacted film of CNT-TiO2 composites, alsodifferent from those aligned CNTs densely grown on substrates,whose heterojunctions with other nanophase materials may onlybe formed on the tops of CNTs [55], our prepared CNTs grownon graphite substrate formed “shrubbery”-like 3-dimensional net-work, the synthesized nanophase TiO2 were uniformly and denselycoated on the CNT-walls to form “coral”-liked nanocomposite net-

work immobilized on graphite substrate, as shown in Fig. 4(b). This“coral”-liked nanocomposite network of photocatalyst exhibitedhigh and accessible specific surface area, pore volume (as indicated

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n Table 1), and available inter-spaces. The mutual connected poresith enough available volume formed a channel network to let

he solution freely flow through to promote the exchange of reac-ants. Without the CNTs as scaffolds, the synthesized TiO2 particlesightly stacked together, result in low specific area, pore volumend unavailable pore spaces, as indicated in Fig. 4(a) and Table 1.

In the future work, some methods will be developed tourther enhancing photocatalysis capability of the immobilizediO2/CNT/GP photocatalyst. Firstly, the novel surfactant wrappingol–gel method will be adopted in order for coating a uni-orm and well-defined nanoscale TiO2 layer on CNTs [12,14,18].anophase of TiO2 coating CNT-walls with a higher coverage

ate and controlled thickness should contribute to a closer con-act of heterophase, diminish the losses of photons by absorptionnd scattering by the bare CNT phase, enhance the efficienciesf disassociation and migration of the photogenerated carri-rs, and increase the available specific area of the TiO2-CNTsanocomposites, thus should effectively improve the photocata-

ysis performance. Secondly, controlling the composition ratio iniO2-CNTs is crucial to obtain an optimal synergistic effect betweenNTs and TiO2, for reason that it can influence the photo absorp-ion and scattering, the size and the uniform distribution degree ofhe nanophase TiO2, etc. [12,14,15,18]. Therefore, the effect of theNTs/TiO2 ratio on the photocatalysis performance will be studied.hirdly, the immobilized TiO2/CNT/GP photocatalyst can be directlysed as the electrode for photo-electro-catalytic process [12,18]ue to the semiconducting scaffolds (CNTs) and substrate (graphitelate), the efficiency of photocatalysis will be further enhanced byombining a photo-electro-catalytic system.

. Conclusion

Nanocarbon-TiO2 systems have been widely investigated andre promising materials for future high activity photocatalysts16,56,57]. In order to adapt the requirements for effectivehotocatalysis capability and immobilization of photocatalyst sys-em based on nanocarbon-TiO2, we developed an immobilizediO2-CNTs nanocomposite. The CNTs synthesized from thermalecomposing hydrocarbon gas grew on thin graphite plate toorm “shrubbery”-like 3-dimensional network. The nanophase TiO2as synthesized by sol–gel method to be coated on CNT-walls

nd formed “coral”-liked nanocomposite network immobilizedn graphite substrate, which had high and accessible specificrea, effective pore-sizes and available inter-spaces, thus canreatly enhance and fully exert the photocatalytic capabilitys a nano-photocatalyst. The prepared immobilized TiO2-CNTsanocomposite demonstrated superior photocatalysis perfor-ance while adapt the application need of clean and convenientanipulation, and the batch production process with low cost. In

he future, it is promising to greatly improve the performance ofur immobilized TiO2-CNTs photocatalyst by combining the newlyovel techniques of preparing TiO2-CNTs nanocomposites.

cknowledgements

This work was supported by the Major State Basic Researchevelopment Program of China, Grant No. 10332020.

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