carbon nanotube heterojunction arrays for photoinactivation of E. coli in visible light irradiation

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Synthesis of titania/carbon nanotube heterojunction arrays for photoinactivation of E. coli in visible light irradiation O. Akhavana,*, M. Abdolahadb, Y. Abdib,c, S. Mohajerzadehb a

Department of Physics, Sharif University of Technology, P.O. Box 11155–9161, Tehran, Islamic Republic of Iran Nano-Electronics and Thin Film Lab., Department of Electrical and Computer Engineering, University of Tehran, North Kargar Avenue, P.O. Box 14395/515, Tehran, Islamic Republic of Iran c Department of Physics, Tehran University, Tehran, Islamic Republic of Iran b

A R T I C L E I N F O

A B S T R A C T

Article history:

TiO2/multi-wall carbon nanotube (MWNT) heterojunction arrays were synthesized and

Received 5 May 2009

immobilized on Si(0 0 1) substrate as photocatalysts for inactivation of Escherichia coli bac-

Accepted 22 July 2009

teria. The vertically aligned MWNT arrays were grown on 5 nm Ni thin film deposited on

Available online 26 July 2009

the Si by using plasma enhanced chemical vapor deposition at 650 C. Then, the MWNTs were coated by TiO2 using dip-coating sol–gel method. Post annealing of the TiO2/MWNTs at 400 C resulted in crystallization of the TiO2 coating and formation of Ti–C and Ti–O–C carbonaceous bonds at the heterojunction. The visible light-induced photoinactivation of the bacteria increased from MWNTs to TiO2 to TiO2/MWNTs, in which the bacteria could even slightly breed on the MWNTs. In addition, the TiO2/MWNTs annealed at 400 C showed a highly improved antibacterial activity than the TiO2/MWNTs annealed at 100 C. The excellent visible light-induced photocatalytic efficiency of the TiO2/MWNTs/Si film annealed at 400 C was attributed to formation of the carbonaceous bonds at the heterojunction, in contrast to the 100 C annealed TiO2/MWNTs/Si sample which had no such effective bonds.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Photocatalytic degradation of environmental organic pollutants and toxics using nanostructured semiconductors based on metal oxides has received considerable attention, in past few years. Titanium dioxide (TiO2) as a metal oxide photocatalyst is one of the most widely studied photocatalytic material because of its unique properties, such as high photocatalytic activity, biological and chemical inertness, nontoxicity, strong oxidizing activity, long-term chemical stability and low price [1–5] and due to its numerous applications, such as water splitting, air purification, deodorization, self-cleaning, and bacterial degradation [6–11]. However, TiO2 can be excited only under irradiation of UV light at wavelengths <380 nm (which covers only 5% of the solar spectrum) due to its wide

band-gap of 3.2 eV for the anatase structure. Therefore, in order to utilize solar energy more efficiently, developing new TiO2-based structures to absorb visible light is of great interest [12–14]. In this regard, TiO2 samples doped with nonmetal elements, such as nitrogen, sulfur, and carbon, have recently been reported to enhance visible light photocatalytic activity [15–30]. It was found that highly condensed carbonaceous species that formed during calcination were responsible for the photosensitization behavior of visible light-active C-doped TiO2 materials [22]. In other works, chemically modified carbon-substituted TiO2 has been used to facilitate efficient photochemical water splitting under irradiation of visible light [23,24]. On the other hand, carbon nanotubes (CNTs)-based composites, as advanced materials, have attracted much

* Corresponding author: Fax: +98 21 66022711. E-mail address: [email protected] (O. Akhavan). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.07.046

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attention due to their unique electrical and structural properties. Since CNTs present good electron conductions, high surface areas and high adsorption capacities, they can be considered as excellent dopants and supports for TiO2-based materials to be used as photocatalysts. For example, He et al. [31] reported catalytic properties of Pt and Ru decorated TiO2/ CNT catalysts for methanol electro-oxidation. Song et al. [32] studied the effect of heat treatment on the performance of TiO2–Pt/ multi-wall carbon nanotube (MWNT) catalysts for methanol electro-oxidation. And recently, Pang et al. [33] showed that Pt nanoparticle-decorated CNTs/TiO2 electrode remarkably improved catalytic activities for the oxidation of hydrogen peroxide. Concerning photocatalytic activity of CNT–TiO2 composites, Kuo et al. [34] synthesized CNT-grafted TiO2 as an electrically conductive catalyst for oxidation of NO gas under electrical excitation besides UV irradiation. Wang et al. [35,36] used a modified sol–gel method to prepare MWNT–TiO2 composites that exhibited photocatalytic activity under both UV and visible light. Yu et al. [37] studied synthesis of TiO2–MWNT heterojunction arrays on Ti substrate with a controllable thickness of TiO2 layer for photodegradation of phenol. An et al. [38] deposited anatase TiO2 onto MWNTs via hydrolysis of titanium isopropoxide in supercritical ethanol and investigated the photocatalytic activity of the composites by testing their ability to mediate the degradation of phenol under visible light. And very recently, Chen et al. [39] used a modified sol–gel method to enhance visible light-induced photoelectrocatalytic degradation of phenol by MWNT-doped TiO2 electrodes. But, none of these works reported so far have been concentrated on photoinactivation of bacteria by using CNT–TiO2 composites under visible light irradiation. In this work, we have reported synthesis of aligned MWNT arrays coated by a thin TiO2 layer. Photocatalytic activity of the TiO2/MWNT heterojunction arrays immobilized on Si(0 0 1) substrate as photocatalysts for photoinactivation of E. coli bacteria has been investigated under visible light irradiation. Chemical states and bonds of Ti and C atoms contributed at the heterojunction have been also studied.

2.

Experimental details

2.1.

Preparation of samples

Si(0 0 1) wafers were used as the substrates for growth of CNTs by direct current plasma enhanced chemical vapor deposition (DC–PECVD). At first, the wafers were cleaned by the standard Radio Corporation of America (RCA) method (RCA#1 method with NH4OH:H2O2:H2O solution and volume ratio of 1:1:5), rinsed in deionized water and blow-dried by air [40]. The RCA cleaning procedure is an industry standard method for removing contaminants from wafers, the basic procedure of which was developed by Werner Kern in 1965 while working for RCA. Then, a 5 nm Ni thin film was deposited on the Si by using e-beam evaporation. The Ni/Si samples were heat treated at 650 C in H2 environment with flow rate of 35 standard cubic centimeters per minute (sccm) for 10–15 min. Then, the samples were hydrogenated by using a hydrogen plasma treatment with a power density of 5.5 W/

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cm2 for about 5 min in order to produce Ni nanoparticles as the catalyst for the growth of CNTs on the surface. Subsequently, CNTs were grown in an environment containing a mixture of H2 and C2H2 gases with flow rates of 35 and 5 sccm at temperature and pressure of 650 C and 2.8 · 102 Pa, respectively. It was previously shown that use of this method resulted in tip-growth of MWNTs in which the Ni seeds were trapped within the hollow core of the CNTs [41]. The synthesized MWNTs were not purified, because some attempts for their oxygen purification resulted in disintegration of the CNT arrays. Sol–gel technique was utilized to deposit anatase TiO2 thin films on the grown MWNTs. TiCl4 (chemically pure) was added dropwise to absolute ethanol (50 ml) with a volume ratio of 1/20 while it was stirring. Since TiCl4 shows a strong reaction with water and even humid air, usually TiO2 is produced by hydrolysis of chemically pure TiCl4 in absolute ethanol (see, for example, [42,43]). In this work, the TiO2 films were obtained by dip-coating method. The CNT arrays were immersed in the sol for about 1 min and pulled up vertically at a speed of 0.1 mm/s. After drying the samples at room temperature for 24 h, they were subsequently heated at 100 C for 1 h to promote framework crossing-linkage and prevent the films from cracking and mesostructure collapse under the next high temperature treatment. Elsewhere, it was shown that such procedure resulted in formation of mesoporous TiO2 with the specific surface area, the total pore volume and pore diameter of ca. 474 m2/g, 0.49 cm3/g and 8.0 nm, respectively [44]. The TiO2 films were crystallized by heat treatment at 400 C in air for 60 min. Thickness of the TiO2 films deposited on glass substrates was measured about 20 nm by using atomic force microscopy.

2.2.

Characterization of samples

Surface morphology of the CNTs was characterized by T-Scan scanning electron microscopy (SEM) apparatus. Phase formation and crystalline properties of the samples were investigated by X-ray diffraction (XRD) obtained by using an X’Pert PRO MRD (PANalytical) analyzer with a Cu Ka radiation source. A Philips-CM200 transmission electron microscopy (TEM) apparatus operated at 200 kV was also used to study the structure of the grown TiO2/MWNTs. For sample preparation, two TiO2/MWNTs/Si films were rubbed on each other (polished samples). One of them was washed by methanol. Then the filtered MWNTs were washed by a diluted HNO3 solution and again dispersed in methanol. Then, a holey copper grid was dipped in the solution for TEM imaging. The other sample was used for X-ray photoelectron spectroscopy (XPS). XPS was employed to study the chemical state of the polished TiO2/MWNTs/Si sample. XPS data were obtained using a hemispherical analyzer with an Al Ka X-ray source (hm = 1486.6 eV) operating at a vacuum better than 10 7 Pa.

2.3.

Antimicrobial test

The antibacterial activity of the TiO2 thin film, the MWNT arrays and the TiO2/MWNTs/Si sample against the Escherichia coli (E. coli, ATCC 25922) bacteria was studied using the

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so-called antibacterial drop-test. Dimension of all the samples used in this experiment was 10 mm · 10 mm. Before the microbiological experiment, all glass wares and samples were sterilized by autoclaving at 120 C for 15 min. The microorganisms were cultured on a nutrient agar plate at 37 C for 24 h. The cultured bacteria were added in 10 ml saline solution to reach the concentration of bacteria to 108 colony forming units per milliliter (CFU/ml) corresponding to MacFarland scale. A portion of the saline solution containing the bacteria was diluted to 106 CFU/ml. For the antibacterial drop-test, each sample was placed into a sterilized Petri dish. Then 100 ll of the diluted saline solution containing E. coli were spread on surface of the sample. Then it was irradiated by a 110 mW/cm2 mercury lamp at room temperature (any irradiation below 400 nm was removed by using a cutoff filter). After elapsing a considered time period, the bacteria were washed from surface of the sample using 5 ml phosphate buffer solution (PBS) in the sterilized Petri dish. Then 10 ll of each bacteria suspension was spread on a nutrient agar plate and incubated at 37 C for 24 h for counting the surviving bacterial colonies by using an optical microscope. Number of killed bacteria was calculated by subtraction number of the surviving bacteria on the sample from number of surviving bacteria counted on a control sample (here, the Si substrate) at the same conditions.

3.

Results and discussion

SEM images of the vertically aligned CNT arrays grown on the Ni/Si thin film have been shown in Fig. 1. It is seen that the CNTs were uniformly grown on the Ni catalyst layer. The diameter of the CNTs was measured in the range of 90– 250 nm and their length was evaluated in the range of 2– 4 lm. In addition, SEM analyses showed that the CNT arrays coated about 30% of the film surface. XRD patterns of the uncoated MWNT arrays and the TiO2/ MWNTs/Si(0 0 1) films annealed at 100 and 400 C have been

shown in Fig. 2. The TiO2 layer of the samples used for the XRD analysis was prepared in conditions by which the TiO2 deposition on a glass substrate resulted in the film thickness of 100 nm. For a better comparison, the intensity of all the XRD patterns was normalized to the intensity of the Si(0 0 4) peak. Beside the peaks corresponding to Si(0 0 2) and (0 0 4) of the substrate, Fig. 2a shows also two weak peaks at 26.1 and 44.1 which can be assigned to diffraction from C(1 0 0) and C(0 0 2) planes of the MWNTs, as similarly reported in another work [45]. Appearance of some weak peaks corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (2 0 4) planes of anatase phase of TiO2 indicated initiation of crystallization of the TiO2 coating at 100 C (Fig. 2b). Comparing Fig. 2b and c indicates that the crystallization of the TiO2 anatase phase, as a favorable structure with better photocatalytic functional properties than its rutile phase, was developed at 400 C. In order to confirm formation of the TiO2/MWNT heterojunction nanotubes, the details of the uniform parts of the nanotubes obtained from the polished sample were observed by TEM, as shown in Fig. 3. The CNT wall and the TiO2 coating with about 10 nm thickness are clearly seen in Fig. 3. The TEM image of the CNT shows an unexpected structure for CNTs, particularly for MWNTs. In fact, during the TEM analysis, we found that it was not possible to distinguish the boundary of CNT and TiO2, probably due to multi-walled structure of the CNTs and/or due to deposition of a TiO2 layer thicker than 10 nm on the MWNTs, as the latter was also reported by Yu et al. [37]. On the other hand, it was previously shown that our grown MWNTs were tip-grown CNTs with the Ni seeds trapped within a wide hollow core of the CNTs [41]. It was also shown that plasma hydrogenation can totally or partially remove the Ni seeds from the CNT tips. In this work, some of the Ni seeds were removed from the tip of the MWNTs by the HNO3 solution applied during the TEM sample preparation. In fact, the TEM image shows that washing the MWNTs by the HNO3 solution resulted in removing the Ni seed from the tip of the CNT, remaining a wide hollow core at tip of

Fig. 1 – SEM images of the CNT arrays grown on Ni/Si(0 0 1) thin film at 650 C. The inset shows the CNTs in a greater magnification.

(a) 20

Si(004)

TiO2 (204)

(a) BE Intensity (arb. unit)

(b)

TiO2 (105) TiO2 (211)

C(100) TiO2 (200)

TiO2 (004)

Si(002)

(c)

30

40

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60

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TiO 2(101) C(002) + C(002)

Intensity (arb. unit)

TiO2 (101)

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70

280

80

282

2θ (degree) Fig. 2 – XRD patterns of (a) the grown MWNTs/Si(0 0 1) and the TiO2(100 nm)/MWNTs/Si(0 0 1) films annealed at (b) 100 and (c) 400 C.

284 286 288 Binding Energy (eV)

290

292

458.1 eV

(b) BE Intensity (arb. unit)

463.7 eV

455

280

282

284

286

460

288

465

290

470

292

Binding Energy (eV)

Fig. 3 – TEM image of a TiO2/MWNT heterojunction nanotube at tip of the CNT, after removing the Ni seed from tip of the CNT.

the CNT, probably thinning and smoothing the TiO2 coating on tip of the CNT, and finally, observing the boundary of the TiO2 and CNT at the tip of the CNT (not at the other places). This means that the thickness of the TiO2 coating at the other places was also thicker than the TiO2 thickness at the tip of the CNTs. It should be noted that some TiO2 nanoparticles produced in the sol–gel dip-coating process are also distinguishable in the smoothed and thinned TiO2 coating, as marked in the image. Fig. 4 shows the high-resolution XPS spectra of C and Ti core levels for the polished TiO2/MWNTs/Si samples annealed at 100 and 400 C. The inset figure presents the Ti(2p) binding energy region. There are two bands in this region. The bands centered at binding energies of 463.7 and 458.1 eV were attributed to the Ti(2p1/2) and Ti(2p3/2) spin-orbital splitting photoelectrons in Ti4+, respectively. In addition, the slitting between bands was found 5.6 eV, consistent with presence

Fig. 4 – Peak deconvolution of C(1s) XPS core level of the polished TiO2/MWNTs/Si films post annealed at (a) 100 and (b) 400 C. The inset of (b) shows Ti(2p) core level of that film.

of the normal state of Ti4+ in the anatase TiO2 film. In Fig. 4a and b, we have attributed the band located at 285.0 eV to the C–C and C@C bonds. In addition, the bands at 286.3, 287.3 and 289.0 eV were assigned to the C–O, C@O and O–C@O bonds, respectively [38,39]. Moreover, the band at 283.7 eV observed only for the sample annealed at 400 C was assigned to the presence of Ti–C bond [28,39,46]. These results also suggest presence of Ti–O–C carbonaceous bonds at the interface. The synthesized MWNT arrays were not purified. Hence, amorphous carbon contamination could be existed in the film of MWNTs. In addition, elsewhere, using TEM it was shown that disorders considerably existed in the grown MWNTs [41]. Therefore, observation of the Ti–C signal in the deconvoluted XPS spectrum of the TiO2/MWNTs annealed at 400 C can be attributed to formation of Ti–C bond between the TiO2 coating and the disorders of the MWNT arrays and/or between the TiO2 coating and the amorphous carbon. The improvement in the visible light photo-induced antibacterial activity of the TiO2/MWNTs annealed at 400 C

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relative to the corresponding activity of the TiO2/MWNTs annealed at 100 C (indicated in the following) confirmed formation of Ti–C bond between the MWNTs (at least, the disorders of the MWNTs) and the TiO2 coating. These results showed that post annealing at 400 C resulted in formation of a heterojunction at the interface of the MWNT and the TiO2. Recently, it was also shown that for TiO2–MWNT heterojunction arrays the annealing temperature of 400 C is a suitable temperature for connection between CNTs and TiO2 at their interface [37]. The photocatalytic activity of the TiO2/MWNTs/Si samples, as the catalysts for photoinactivation of E. coli bacteria in visible light, was investigated and compared to corresponding activity of the bare MWNTs (the uncoated MWNTs) and an anatase TiO2 thin film (the pure TiO2 thin film with thickness of about 100 nm deposited on a glass substrate and post annealed at 400 C in air). As a bench mark, the antibacterial activity of the Si substrate (as a control sample), the bare MWNTs and the pure TiO2 thin film has been shown in Fig. 5. The time evolution of the bacteria on the Si substrate in dark showed that the number of the bacteria slightly decreased on this surface, in our experimental conditions. In fact, in this case only 17 ± 12% of the bacteria was killed on the Si substrate in 80 min indicating nearly no antibacterial activity. But, time evolution of the bacteria on the bare MWNT arrays showed a small increase in the number of the bacteria on the surface, in dark. In fact, it was found that number of bacteria on the MWNT arrays increased 13 ± 5% in 1 h. It was previously shown that single-wall carbon nanotubes (SWNTs) in CNT-coated filters can inactivate 87 ± 7% of E. coli bacteria in 1 h, while MWNTs can inactivate only 31 ± 6% of the bacteria in the same conditions [47]. In fact, it was suggested that the lower antibacterial activity of MWNTs relative to SWNTs can be assigned to the differences in length, diameter, surface area, chemical reactivity and elec-

c) 0.8

1.4

0.6

b) 0.4

1.2

0.2

Ratio of viable bacteria

Normalized ratio of killed bacteria

1

a)

0 0

20

40

60

80

100

1 120

Time (min)

tronic characteristics of their CNTs [47]. Meanwhile, different purities and functionalization of carbon-based materials, different cell culture media, and a variety of cell types can be resulted to various antibacterial activities of the carbon–based materials. For example, it was observed that E. coli bacteria can breed on carbon fibers [48]. In this work, we have also observed that the grown MWNTs not only showed no antibacterial activity but also the E. coli bacteria could slightly grow on surface of the film of the MWNTs. In addition, we found that oxygen purification of the grown MWNTs resulted in disintegration of the CNT arrays grown on the film surface. Therefore, presence of the defects and disorders would be considerable in the structure of the grown MWNTs, as shown elsewhere by TEM [41]. To observe improving the antibacterial activity of the TiO2/MWNTs, the corresponding activity of pure TiO2 thin film was firstly checked in dark, as shown in Fig. 5c. It is seen that the synthesized TiO2 thin film showed an antibacterial activity, so that 60 ± 16% of the bacteria was killed on the TiO2 surface in 60 min. It was previously reported that although TiO2 is capable of producing toxic of reactive oxygen species in the presence of UV light, it has a potential toxicity under dark condition suggesting the additional mechanisms (as yet undetermined) for its toxicity [49]. In other work, Liu et al. [5] also showed that about 40% of E. coli bacteria were killed in contact to mesoporous TiO2 thin films within 20 min in dark, in agreement with our result. The same antibacterial activities were also found for these samples under the visible light irradiation (not shown here), indicating no considerable effect of the visible light irradiation on the antibacterial activity of the samples. Fig. 6 shows the antibacterial activity of the TiO2/MWNTs/ Si films annealed at 100 C in dark and under the visible light irradiation. It is seen that the antibacterial activity of the TiO2/MWNTs/Si films is weaker than the activity of the TiO2 thin film. For example, only 40 ± 24% of the bacteria was killed on surface of the TiO2/MWNTs/Si film in 60 min in dark. This indicates that although the effective surface area of the TiO2 coating in the TiO2/MWNTs are much higher than the surface

1 Normalized ratio of killed bacteria

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(b) 0.8

0.6

(a)

0.4

0.2

0 0

Fig. 5 – (a) Normalized ratio of the killed bacteria on surface of the Si substrate as a control sample (h), (b) ratio of the viable bacteria on surface of the bare MWNT arrays (s) and (c) normalized ratio of the killed bacteria on surface of the pure TiO2 thin film annealed at 400 C (d).

20

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60

80

100

120

Time (min) Fig. 6 – Normalized ratio of the killed bacteria on surface of the TiO2/MWNT samples annealed at 100 C; (a) in dark and (b) under visible light irradiation.

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area of the pure TiO2 film, the TiO2 coating in the TiO2/ MWNTs could not completely prevent the growth of bacteria on the surface of the MWNT arrays. However, the visible light irradiation significantly increased the antibacterial activity of the TiO2/MWNTs/Si film, so that in this case, 75 ± 9% of the bacteria was killed in 60 min. This result showed that use of TiO2/MWNT composition can enhance the photocatalytic performance of the TiO2 coating. After formation of the TiO2/MWNT heterojunction in the TiO2/MWNTs/Si film at 400 C, the visible light photo-induced antibacterial activity of the film was excellently increased, as shown in Fig. 7. For example, in this case, 94 ± 2% of the bacteria was killed on the surface of the TiO2/MWNTs/Si film annealed at 400 C in 60 min under the visible light irradiation, while in dark only 52 ± 18% of the bacteria was killed on the film surface in the same period of time. It was also found that after 120 min irradiation, 99.8% of the bacteria were killed indicating the strong antibacterial activity of the TiO2/MWNT heterojunction under the visible light irradiation. This means that the TiO2/MWNT heterojunction can reduce number of viable bacteria to less than 10 CFU (a requirement for the clean environments) in a reasonable short time. It was found that the data presented in Figs. 5–7 can be fitted by an exponential equation such as N = (1 e–at) in which N is the normalized ratio of the killed bacteria on surface of the samples in terms of time (t). In this regard, a can be considered as the rate of killing the bacteria on surface of the different samples. To have a better quantitative comparison of the antibacterial activity of the samples, the values of a calculated for dark and visible light conditions have been listed in Table 1. The substantial improvement in the photocatalytic performance of the TiO2/MWNTs/Si annealed at 400 C than the pure TiO2 thin film (as a conventional photocatalyst in which the photogenerated charge pairs migrate randomly and most of them recombine before they arrive at the photocatalyst surface [50]) and also than the TiO2/MWNTs/Si annealed at

Normalized ratio of killed bacteria

1

(b)

0.8

(a) 0.6

0.4

0.2

0 0

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120

Time (min) Fig. 7 – Normalized ratio of the killed bacteria on surface of the TiO2/MWNT samples annealed at 400 C; (a) in dark and (b) under visible light irradiation.

Table 1 – The rate of killing E. coli on surface of the different samples in dark and under visible light irradiation. Sample

Annealing Temp. (C)

Rate of killing E. coli (·10 3 min 1) Dark

MWNT arrays TiO2 film TiO2/MWNT

– 400 100 400

0.7 7.1 3.8 5.3

Visible light no change no change 9.3 21.2

100 C can be attributed to the formation of Ti–C and Ti–O–C carbonaceous bonds at 400 C in the TiO2/MWNT heterojunction, as proved by the XPS results. Elsewhere, it was also shown that formation of such carbonaceous bonds is effective on absorption of visible light [39]. Injection of electrons from the photoexcited CNTs to the conduction band of TiO2 generates positively charged CNTs, which capture electrons from the valence band of TiO2, i.e., formation of holes [36,38]. In addition, when TiO2 joints to another semiconductor or metal (e.g., CNT) to make a heterojunction, a space charge layer ranging from several tens to hundreds nanometers would be formed near the interface to equalize Fermi levels. The driving force coming from interior electric field of the space charge layer will separate the photogenerated pairs if the photoexcited electrons can diffuse into the space charge layer. In fact, in the TiO2/CNTs as a photocatalytic system involving heterojunction, the electrons and the holes would be driven to the different directions and so the recombination rate could be reduced. This corresponds to increase in the rate of formation of OH Æ radicals and thereby the more photocatalytic efficiency.

4.

Conclusions

Vertically aligned MWNT arrays were grown by using PECVD at 650 C on 5 nm hydrogenated Ni thin film catalyst on Si(0 0 1) substrate. The MWNTs immobilized on the Si substrate were dip-coated by the sol–gel TiO2 coating. Post annealing of the TiO2/MWNTs at 400 C resulted in crystallization of the TiO2 layer (anatase phase, based on the XRD analysis) as well as formation of Ti–C and Ti–O–C carbonaceous bonds at the heterojunction of TiO2/MWNT (based on the XPS analysis from the polished sample), while for the TiO2/MWNTs annealed at 100 C no such carbonaceous bonds were observed. The antibacterial activity increased from the MWNTs to the TiO2 to the TiO2/MWNTs. For the former, the bacteria could even slightly grow on the surface of the MWNT arrays, which was assigned to the presence of amorphous carbon and the disorders in the structure of the MWNTs. The visible light-induced antibacterial activity of the TiO2/ MWNTs annealed at 400 C was also greater than the corresponding activity for the TiO2/MWNTs annealed at 100 C. Since the Ti–C and Ti–O–C carbonaceous bonds formed at the heterojunction were effective on visible light absorption and photocatalytic activity, it was proposed that they were efficiently contributed in the charge transfer between the photoexcited MWNTs and the TiO2 layer, reduction in the

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electron–hole recombination rate, and consequently, increase in the rate of formation of active OH radicals for degradation of the bacteria.

Acknowledgements OA would like to thank the Research Council of Sharif University of Technology and also the Iran Nanotechnology Initiative Council for financial support of the work.

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