Enhanced visible-light-driven photoactivities of single-walled carbon nanotubes coated with N doped TiO₂ nanoparticles

Abstract

A two-step hydrothermal method is used to prepare N doped single-walled carbon nanotube–TiO₂ (SWCNT–N/TiO₂) hybrids with different contents of SWCNTs from 1.25 wt% to 10 wt%. The photocatalysts were characterized by X-ray diffraction analysis, UV-vis diffuse reflectance spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The UV-vis diffuse reflectance spectra show an apparent enhancement of absorption throughout the visible light region. Raman spectroscopy further confirms the chemical interaction between N/TiO₂ and SWCNTs in the hybrid. SWCNT–N/TiO₂ hybrid presents a superior photocatalytic activity to DWCNT–N/TiO₂ (N doped double-walled carbon nanotube–TiO₂) and MWCNT–N/TiO₂ (N doped multi-walled carbon nanotube–TiO₂) based on the results of the photocatalytic degradation of sulfathiazole under visible light irradiation, which is attributed to the synergetic effects of SWCNT modification and N doping. N doping induces visible light absorption by either introducing localized electronic states within the band gap or contributing electrons to the valence band and metal-like SWCNT can act as an electron conductor to facilitate the fast injection of the photogenerated electrons into the conduction band of N/TiO₂, leading to significantly enhanced visible-light-driven photocatalytic activity.

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1. Introduction

Photocatalysis is a potential technology in meeting cleaning energy demands and environmental remediation.^1,2 Recently, many researchers have applied photocatalysis to remove organic pollutants in aqueous suspension and demonstrated that the photocatalyst displayed a good removal effect on those pollutants.^3–5 By far, TiO₂ has emerged as the most promising photocatalyst because of its high oxidative power, photostability, low cost, and nontoxicity. However, the band gap energy (E_g = 3.2 eV) of the anatase TiO₂ is a major disadvantage for its application, since the photocatalytic process can only be initiated under UV irradiation (which is only approximately 4% of solar radiation). Moreover, the low quantum efficiency caused by fast recombination of photogenerated h⁺–e⁻ pairs is another drawback when TiO₂ is used to be a photocatalyst. In overcoming the above disadvantages, modification on TiO₂ has drawn a lot of attention from researchers. Doping with metal/nonmetal elements and surface modification with semiconductor of narrow band-gap are both typical approaches to generate visible-light-driven photoactivity from TiO₂. Among the nonmetal dopants, nitrogen has been considered as an effective dopant since N doped TiO₂ (N/TiO₂) was synthesized firstly by Asahi et al.⁶ N/TiO₂ has a new energy band gap lower than pristine TiO₂ owing to the mixing of the 2p nitrogen level with the oxygen 2p orbital to form the valence band. Consequently, the rate of recombination of h⁺–e⁻ pairs is higher, which remains a serious obstacle to achieving high visible-light-induced photocatalytic activity. Several studies have investigated techniques to reduce the electron–hole combination rate via metal coating/doping.^7,8 Noble metal nanoparticles can effectively enhance the charge carrier separation by forming a Schottky barrier on the surface of TiO₂. In addition, generation of surface plasmon resonance can help to shift the absorption of the TiO₂ in the visible region.⁸

In the recent years, carbon nanotubes (CNTs) has received growingly considerable attention on the field of photocatalysis due to its unique electronic and physical properties,^9,10 which could act as an electron acceptor because of the conductive structure of CNTs scaffolds and hence induced efficient charge transfer.^11,12 It is expected that CNTs may also act as effective electron sinks because of its high electrical conductivity and electron storage capacity which was demonstrated by Yu et al.^13,14 Wang et al.¹⁵ argued that CNTs was utilized to be a photosensitizer which can enhance the visible-light-driven photocatalytic activity of TiO₂. However, we reported the functionalized single-walled carbon nanotubes (SWCNTs) coated with TiO₂ nanoparticles via hydrothermally method where SWCNTs act as an electrical conductor rather than photosensitizer, achieving the improved photoactivity.¹⁶ Although, studies always focused more on the multi-walled carbon nanotube–TiO₂ (MWCNT–TiO₂) than SWCNT–TiO₂,^13,17,18 SWCNTs possess a high specific surface area and provides better catalyst-support (dispersal and connection) than MWCNTs, which can achieve a better understanding of the proposed mechanisms.¹⁹ As a novel material, SWCNT–TiO₂ has been fabricated by various ways.²⁰ Zhou et al.²¹ prepared SWCNT–TiO₂ by a facile sol–solvothermal method suggesting that TiO₂ and SWNCTs linked compactly through ester bonds and thus improved their interfaces which could diminish rate of h⁺–e⁻ pair's recombination. Vajda et al.²² synthesized SWCNT–TiO₂ by the aid of ultrasonication method and got the optimal composition of the SWCNT–TiO₂ for the degradation of phenol under UV irradiation. However, the enhancement on the photoactivity of TiO₂ after the introduction of SWCNTs is limited under visible light irradiation and the modification of SWCNT–TiO₂ using nitrogen anion to promote photocatalytic activity in the visible light region is less reported.

In this study, we synthesized for the first time a series of SWCNT–N/TiO₂ hybrid by directly growing TiO₂ on the surface of SWCNTs with hydrothermal treatment and then substantial calcination using an N source. The effects of the type of the CNTs (SWCNTs, MWCNTs, and double-walled carbon nanotubes (DWCNTs)) and the contact between CNTs and N/TiO₂ on the visible-light-responsive photoactivity of the CNT–N/TiO₂ hybrids were extensively investigated according to the experimental results of photocatalytic degradation of sulfathiazole under visible light irradiation.

2. Experimental

2.1 Materials

SWCNTs were purchased from Chengdu Organic Chemicals (CAS, China). All other reagents, including Titanium sulfate (Ti(SO₄)₂), cetyltrimethylammonium bromide (CTAB), sodium chloride, ethanol, nitric acid and urea were commercially available in analytical purity and used as received.

2.2 Methods

2.2.1 Preparation of SWCNT–N/TiO₂ hybrid. SWCNTs should be pretreated prior to synthesizing the SWCNT–TiO₂ to introduce carboxyl and hydroxyl groups. The detail is shown below: SWCNTs were suspended in 40 mL deionized water and concentrated nitric acid (volume ratio 7 : 1), heat stirred for 2 h, followed by rinsing with deionized water until pH value of supernatant became neutral. Next, the dissolved Ti(SO₄)₂ was added to the functionalized SWCNTs, CTAB and deionized water (m(Ti(SO₄)₂) : m(CTAB) : m(water) = 1 : 0.12 : 100). After stirring for 30 min, the resulting mixture was decanted into a 50 mL stainless steel autoclave for 72 h of hydrothermal treatment at 100 °C. When the suspension cooled down to room temperature, it was centrifuged and rinsed with deionized water and ethanol for 2–3 times. In order to remove the CTAB, ion-exchange treatment was performed by mixing the obtained material with a water and ethanol (molar ratio 1 : 1) solution of sodium chloride under stirring at ambient temperature for 24 h, then dried at 80 °C overnight. At last, the obtained powder was calcined at 400 °C for 2 h in Muffle furnace.

The preparation process for the 2.5 wt% SWCNT–N/TiO₂ goes as follows: 0.5 g of SWCNT–TiO₂ and 3.5 g of urea were dispersed in 3 mL deionized water and 1 mL ethanol. The mixture was stirred for 1 h, dried at 80 °C to jelly, and finally calcined at 450 °C for 2 h to obtain SWCNT–N/TiO₂ hybrid. The contents of SWCNTs in the hybrid were controlled at 1.25, 2.5, 5.0, and 10.0 wt%.

2.2.2 Characterization of SWCNT–N/TiO₂ hybrid. X-ray diffraction (XRD) patterns were recorded on the X-ray diffractometer (Bruker Inc., Germany) using Cu Ka radiation source at 35 kV. The UV-vis diffuse reflectance spectroscopy (DRS) of powder solids were collected by a UV-vis spectrophotometer UV-3100 (Shimadzu, Japan) equipped with an integrating sphere using BaSO₄ as the reference sample. Raman spectroscopy was performed by using a Confocal Raman Microspectroscopy (Renishaw inc., UK). Transmission electron microscope (TEM) images were obtained on a JEM-2010FEF electron microscope (JEOL, Japan). Detailed surface images were obtained by means of field emission scanning electron microscope (FESEM) observation on a JEOL-6700F electron microscope. X-ray photoemission spectroscopy (XPS) was carried out by a XSAM800 spectrometer (Kratos Analytical, UK).

2.2.3 Photocatalytic activity of the SWCNT–N/TiO₂ hybrid. The experiment on the photocatalytic degradation of sulfathiazole over SWCNT–N/TiO₂ hybrids in aqueous solution was carried out in a photocatalytic degradation system where a glass reactor was surrounded by circulating water. A 300 W Xenon lamp (PLS-SXE300C, Beijing) with a 420 nm cutoff filter was used as the visible light source, with an average light intensity of 600 μW cm⁻². In each experiment, SWCNT–N/TiO₂ hybrid was dispersed in 50 mL of 10 mg mL⁻¹ sulfathiazole aqueous solution with a load of 1 g L⁻¹. The suspension was equilibrated for 30 min on a rotating mixer without light, and then exposed to visible light irradiation. At given irradiation time intervals, the mixture was centrifuged at 4000 rpm for 3 min. The supernatant was analyzed with UV-vis spectrophotometer (UV-3100, Shimazu, Japan) by recording the absorbance characteristic band at 282 nm.

3. Results and discussion

3.1 Structure and morphology of the SWCNT–N/TiO₂ hybrid

The effects of N doping and SWCNTs modification on TiO₂ crystallization was investigated by XRD (shown in Fig. 1). The diffraction peaks at 2θ values of 25.33°, 37.91°, 48.03°, 54.47°, 55.0°, 62.97° can be indexed to the (101), (004), (200), (105), (211) and (204) planes of anatase phase, respectively.²³ This phenomenon may indicate that dopant did not alter the crystallite structure of TiO₂ but nitrogen anions have moved into either the interstitial positions (Ti–O–N) or the substitutional sites (O–Ti–N) of the TiO₂ crystal structure.^24,25 In addition, the characteristic peaks of SWCNTs at 26.6° and 43.5° are not detected in the 1.25 wt% to 10 wt% SWCNT–N/TiO₂ hybrids, which can be attributed to the good dispersion of SWCNTs in the hybrid after surface functionalization.¹⁶ The primary crystallite size of N/TiO₂ and 1.25 wt% to 10 wt% SWCNT–N/TiO₂ hybrids (presented in Table 1) was calculated by the Scherrer equation:

d = (0.89 × λ)/β × cos θ

where d is crystal size, λ is the X-ray wavelength (0.1548 nm for Cu K_α radiation), β is the full width at half-maximum, θ is the diffraction angle associated with the (101) peak. The small grain of TiO₂ nanoparticles in the SWCNT–N/TiO₂ may be attributed to N doping and the introduction of SWCNTs. First, incorporation of nitrogen in TiO₂ crystal lattice result in a decrease in the interplanar distance due to a larger ionic radius (1.46 Å) of N³⁻ ion than O²⁻ ion (1.40 Å).²⁵ Moreover, the slow rate of grain growth for TiO₂ in the SWCNT–N/TiO₂ hybrid may be attributed to the introduction of SWCNTs into N/TiO₂, which retards direct contact of the grains. Nevertheless, the particle sizes initially increase and then decrease with increasing concentration of SWCNTs. These experimental results are ascribed to the quantity of contacts N/TiO₂ and SWCNTs, which is influenced by the concentration and the self-aggregation of SWCNTs.¹⁶

Fig. 1 X-ray diffraction patterns of TiO₂, N/TiO₂, 5 wt% SWCNT–TiO₂ and SWCNT–N/TiO₂ hybrids with different contents of SWCNTs.

Table 1 Summary of the physicochemical properties of TiO₂, N/TiO₂, 5 wt% SWCNT–TiO₂, and SWCNT–N/TiO₂ hybrids

Samples	Crystallite size (nm)	λ (nm)	Eg (eV)
TiO2	18.1[16]	400	3.1
5 wt% SWCNT–TiO2	11.2[16]	428	2.90
N/TiO2	9.6	530	2.34
1.25 wt% SWCNT–N/TiO2	14.6	449	2.76
2.5 wt% SWCNT–N/TiO2	10.2	561	2.21
5 wt% SWCNT–N/TiO2	11.4	432	2.87
10 wt% SWCNT–N/TiO2	10.1	504	2.40

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The DRS spectra of TiO₂, N/TiO₂, SWCNT–TiO₂, and SWCNT–N/TiO₂ with different contents of SWCNTs are shown in Fig. 2a. The spectrum for TiO₂ has an absorption edge at 400 nm, corresponding to a band-gap energy of about 3.1 eV. Compared with pure TiO₂, N/TiO₂, SWCNT–TiO₂ and SWCNT–N/TiO₂ hybrids contain less TiO₂, which leads to a weaker absorption in the UV region. Although the absorption thresholds of SWCNT–N/TiO₂ hybrids are near to that of TiO₂, all SWCNT–N/TiO₂ hybrids have better absorption in the visible light region (>400 nm). Compared with the 5 wt% SWCNT–TiO₂, a greater adsorption property of 5 wt% SWCNT–N/TiO₂ hybrid is obtained from the DRS spectra. This phenomenon may demonstrate that nitrogen anions substituted for oxygen positions in the TiO₂ lattice, thereby forming a mid-gap level between the valence and conduction bands of TiO₂.²⁵

Fig. 2 (a) UV-vis diffuse reflection spectra of TiO₂, N/TiO₂, SWCNT–TiO₂ and the SWCNT–N/TiO₂ hybrids with different contents of SWCNTs; (b) plot of (αhν)^1/2 versus photon energy (hν) according to the DRS in (a).

The band gap energy of TiO₂, N/TiO₂, SWCNT–TiO₂, SWCNT–N/TiO₂ hybrids were calculated by plotting (αhν)^0.5 versus hν where α is the absorption coefficient (Fig. 2b). The results suggest that both SWCNT modification and N doping could narrow the band gap of TiO₂, which lead to a broader optical absorption range; however, the band gap initially narrow and then enlarge with the increase of loading amount of SWCNTs (shown in Table 1), this conclusion indicates that whether the decrease in band gap enhances photocatalytic activity or not depends on the wavelength of incident light.²⁶

Fig. 3 shows the SEM and TEM images of 2.5 wt% SWCNT–TiO₂ and 2.5 wt% SWCNT–N/TiO₂. As shown in Fig. 3a and b, the acid-treated SWCNT morphology remain the original structure and is densely covered with a large amount of TiO₂ or N/TiO₂ regardless of nitrogen doping or not, suggesting that TiO₂ or N/TiO₂ was coated on the outer surface of the SWCNTs according to the field emission theory.²⁷ On the other hand, the N/TiO₂ nanoparticles are homogeneously distributed on the surface of acid-treated SWCNTs (Fig. 3c) owing to the effect of surface functional groups like carboxyl and hydroxyl, which can contribute to a high specific surface area.²⁸ Meanwhile, lattice fringes can be clearly seen in TEM image (Fig. 3d) indicating that the d spacing for TiO₂ (101) planes is 0.35 nm consistent with XRD results. Close interfacial contact between N/TiO₂ and the SWCNTs is observed, which is advantageous for transferring electrons and retarding the recombination of photogenerated h⁺–e⁻ pairs, thus improving the photocatalytic activity.²¹

Fig. 3 SEM images of the 2.5 wt% SWCNT–TiO₂ (a) and 2.5 wt% SWCNT–N/TiO₂ (b); TEM images of the 2.5 wt% SWCNT–N/TiO₂ (c and d).

Raman spectrum of TiO₂, SWCNTs, 2.5 wt% SWCNT–TiO₂, and 2.5 wt% SWCNT–N/TiO₂ are presented in Fig. 4. The characteristic bands at 402.1, 517.4, 636.2 cm⁻¹ in the 2.5 wt% SWCNT–N/TiO₂ hybrid corresponding to B_1g, A_1g, E_g modes of anatase, respectively; no Raman peak due to TiN can be observed in the 2.5 wt% SWCNT–N/TiO₂ hybrid which is in agreement with XRD pattern. Furthermore, our results show an obvious red-shift for the E_g mode of the 2.5 wt% SWCNT–N/TiO₂ hybrid and 5 wt% SWCNT–TiO₂, indicating a decrease nanoparticle size according to the report of Zhang et al.²⁹ However, compared with 5 wt% SWCNT–TiO₂, the Raman peaks of 2.5 wt% SWCNT–N/TiO₂ hybrid shift to lower frequency due to lattice expansion, which confirm the presence of the dopant anion in the crystal lattice.²⁵ The characteristic D-band (disordered mode) at 1353 cm⁻¹ and G-band (tangential mode) at 1580 cm⁻¹ of SWCNTs could be observed, indicating the existence of SWCNTs in the hybrid. The relative intensity ratio of the D-band to the G-band is known as an index of graphitization to determine the CNTs microstructure.³⁰ The I_D/I_G ratio of the hybrid is 0.11 similar to the pure SWCNTs,¹⁶ which demonstrates N doping and hydrothermal treatment did not damage the graphitization structure of the 2.5 wt% SWCNT–N/TiO₂ hybrid.

Fig. 4 Raman spectrum of TiO₂, SWCNT, SWCNT–TiO₂ and SWCNT–N/TiO₂ hybrid.

XPS measurements were carried out to further determine the interaction of SWCNTs and N/TiO₂ nanoparticles. Fig. 5a presents C 1s, O 1s, Ti 2p, Ti 3s, Ti 3p and N 1s XPS spectrum of 5 wt% SWCNT–N/TiO₂ hybrid. It is observed that C 1s spectrum (Fig. 5b) can be fitted to the three peaks, located at 284.6, 285.7, 288.6 eV, which are ascribed to C–C, C–O, and C–O–O bonds, respectively. The existence of these polar groups demonstrates that the surface of SWCNTs was oxidized to some extent, which is beneficial for the adsorption of the precursor molecules and the nucleation of N/TiO₂ on the surface of SWCNTs.³¹ Fig. 5c displays the XPS spectrum of Ti 2p for the 5 wt% SWCNT–N/TiO₂ hybrid, which indicates the two peaks at 458.9 and 464.8 eV corresponding to the photo-splitting electrons Ti 2p_3/2 and Ti 2p_1/2, respectively. The binding energies of two peaks presented a blue-shift compared to the CNT/TiO₂,³² which is attributed to the substitution of a less electronegative nitrogen atom in the place of oxygen in spite of the strong interaction existed between N/TiO₂ nanoparticles and SWCNTs.³¹ Furthermore, the O 1s peak for the hybrid appears at 530.0 eV (Fig. 5d), suggesting the presence of the substitutional N (O–Ti–N) rather than interstitial N (Ti–O–N).²⁵ The N 1s spectrum of the hybrid has a main peak at 399.0 eV (Fig. 5e), which is greater than the typical binding energy of 396.9 eV in TiN,³³ which can be attributed to the 1s electron binding energy of the N atom in the environment of O–Ti–N. This shift to a higher energy is ascribed to the formation of O–Ti–N structure when nitrogen substitutes for oxygen in TiO₂, and hence the electron density around nitrogen is less than in TiN.²⁵

Fig. 5 XPS spectra for the SWCNT–N/TiO₂ hybrid: survey (a), C 1s (b), Ti 2p (c), O 1s (d), and N 1s (e).

3.2 Photocatalytic activity

To investigate the contact between the SWCNTs and N/TiO₂ on the visible-light-induced photoactivity of the SWCNT–N/TiO₂ hybrid, homemade TiO₂, N/TiO₂, 5 wt% SWCNT + N/TiO₂ (a simple mixture of SWCNTs and N/TiO₂ at a weight ratio of 1 : 19) and a series of SWCNT–N/TiO₂ hybrids synthesized by the hydrothermal method were examined in a solution of sulfathiazole under visible light irradiation. The results of degradation efficiency are illustrated in Fig. 6. It is obvious that sulfathiazole was barely decomposed in the absence of photocatalyst. This phenomenon indicates that sulfathiazole elimination by the hydrolysis and/or photolysis can be neglected under visible light irradiation. Photocatalytic degradation of sulfathiazole over homemade TiO₂ was limited because of the low photoactivity of TiO₂ under visible light.³⁴ Simultaneously, the enhancement on the photoactivity of TiO₂ after the introduction of SWCNTs was inconspicuous. According to Zhu et al.,²⁸ a metal possesses a higher work function than that of the n-type semiconductor (such as TiO₂), electrons can flow from the semiconductor into the metal for adjusting the Fermi energy levels. In general, as-synthesized SWCNTs are composed of one-third metal and two-thirds semiconductor ones; therefore SWCNTs cannot act as photosensitizer to improve the visible-light-induced photoactivity.³⁵ Nevertheless, N/TiO₂ exhibits a high photoactivity, since substitutional nitrogen forms new state that lies just above the valence band of TiO₂ which causes band gap narrowing enabling it to absorb visible light.³⁶ Moreover, the 5 wt% SWCNT + N/TiO₂ showed a slower removal rate of sulfathiazole than the 5 wt% SWCNT–N/TiO₂ hybrid. According to these experimental results, a possible explanation for the higher photoactivity of 5 wt% SWCNT–N/TiO₂ hybrid is the intimate interaction of N/TiO₂ with the SWCNTs and the formation of localized electronic states in the band gap caused by N doping.³⁷ In our study, the 5 wt% SWCNT–N/TiO₂ hybrid presented superior photoactivity to 5 wt% DWCNT–N/TiO₂ hybrid and 5 wt% MWCNT–N/TiO₂ hybrid (Fig. 6). The phenomenon is different from the previous study, according to the report of Liu et al.¹⁰ SWCNT–TiO₂ presented less photoactivity than DWCNT–TiO₂ and MWCNT–TiO₂. As we know, the structure of CNTs plays an important role in the photocatalytic process. SWCNTs' architecture provides a network to disperse N/TiO₂ nanoparticles. An increase of photoconversion efficiency represents the beneficial role of the SWCNTs as conducting scaffold to facilitate the fast injection of the photogenerated electrons on the CB of N/TiO₂.³⁸ Furthermore, the photocatalytic activity of the hybrids with varying SWCNTs content was also investigated. As shown in Fig. 7 and 5 wt% SWCNT–N/TiO₂ demonstrated the highest photoactivity with a degradation rate of approximately 100% after reaction of 1 h. By contrast, the effect of 10 wt% SWCNT–N/TiO₂ was not favorable since the excessive SWCNTs can act as recombination center for h⁺–e⁻ pairs, resulting in depressed photoactivity.¹⁶

Fig. 6 Photocatalytic degradation efficiencies of sulfathiazole over different photocatalysts under visible light irradiation ((a) photolysis, (b) TiO₂, (c) 5 wt% SWCNT–TiO₂, (d) 5 wt% SWCNT + N/TiO₂, (e) N/TiO₂, (f) 5 wt% MWCNT–N/TiO₂, (g) 5 wt% DWCNT–N/TiO₂, (h) 5 wt% SWCNT–N/TiO₂).

Fig. 7 Photocatalytic degradation efficiencies of sulfathiazole over SWCNT–N/TiO₂ hybrids with different contents of SWCNTs under visible light irradiation. ((a) 10 wt% SWCNT–N/TiO₂, (b) 1.25 wt% SWCNT–N/TiO₂, (c) 2.5 wt% SWCNT–N/TiO₂, (d) 5 wt% SWCNT–N/TiO₂).

Based on the experimental findings and the discussion above, the proposed mechanism for the enhanced photoactivity of SWCNT–N/TiO₂ hybrid is summarized (Fig. 8). N/TiO₂ exhibits a relatively lower energy band gap than that of pure TiO₂ for electrons to emit from the up-shifting Fermi level into the SWCNT, which results in the metal-like SWCNT possessing an excess negative charge and the VB of semiconductor (N/TiO₂) an excess positive charge.²⁷ Therefore, SWCNT–N/TiO₂ hybrid can form a semiconductor–metal junction called a Schottky barrier which offers an effective route of decreasing recombination of photogenerated h⁺–e⁻ pairs.²⁹ The electrons on SWCNTs can further generate superoxide radical from oxygen. Simultaneously, the produced holes can be transferred from the valence band to the surface of N/TiO₂ where they can directly oxidize organic pollutants or indirectly oxidize them through the formation of hydroxyl radicals.^15,39

Fig. 8 Proposed mechanism for photocatalytic degradation of sulfathiazole over the SWCNT–N/TiO₂ hybrid; S: sulfathiazole; S_ox: oxidized sulfathiazole.

4. Conclusions

N doped and single-walled carbon nanotube modified TiO₂ (SWCNT–N/TiO₂) hybrid was synthesized via a two-step hydrothermal method and analyzed using XRD, DRS, SEM, TEM, Raman, and XPS techniques. SWCNTs in the hybrid were coated with N/TiO₂ nanoparticles, which decrease the crystallization degree of the anatase and generate an intimate contact between the SWCNTs and N/TiO₂. Compared with N/TiO₂ and SWCNT–TiO₂, the high visible-light-induced photoactivity can be rationalized by the synergetic effects originated from both SWCNT modification and N doping. Moreover, SWCNT–N/TiO₂ hybrid showed better photocatalytic activity than DWCNT–N/TiO₂ and MWCNT–N/TiO₂ in the photocatalytic degradation of sulfathiazole under visible light irradiation. The experimental results demonstrate that the SWCNTs act more as an electrical conductor than a photosensitizer, which efficiently suppress charge recombination, resulting in improving interfacial charge transfer, and eventually enhance the photoactivity of SWCNT–N/TiO₂ hybrid.

Acknowledgements

The research was financially supported by the National Natural Science Foundation of China (21307035), Open Foundation of Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and Ministry of Education, South-Central University for Nationalities (CHCL12004), Fundamental Research Funds for Central Universities of China (2013PY112), and the National Training Programs of Innovation and Entrepreneurship for Undergraduates (201210504130).

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