Mo, S-codoped TiO2 hetero-nanostructure

Accepted Manuscript Title: Novel High Potential Visible-Light-Active Photocatalyst of CNT/Mo, S-codoped TiO2 Hetero-Nanostructure Author: M. Hamadanian M. Shamshiri V. Jabbari PII: DOI: Reference:

S0169-4332(14)01901-1 http://dx.doi.org/doi:10.1016/j.apsusc.2014.08.123 APSUSC 28578

To appear in:

APSUSC

Received date: Revised date: Accepted date:

25-3-2014 28-6-2014 21-8-2014

Please cite this article as: M. Hamadanian, M. Shamshiri, V. Jabbari, Novel High Potential Visible-Light-Active Photocatalyst of CNT/Mo, Scodoped TiO2 Hetero-Nanostructure, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Novel High Potential Visible-Light-Active Photocatalyst of CNT/Mo, S-codoped TiO2 Hetero-Nanostructure

a

Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran

Department of Physical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran

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b

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M. Hamadaniana,b*, M. Shamshirib, V. Jabbaria

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Abstract

The current study deals with synthesize of novel nanophotocatalysts of CNT/Mo,S-codoped

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TiO2 by reacting between titanium isopropoxide (Ti(OC3H7)4) and CNT in aqueous ammonia and subsequent calcining of hydrolysis of the products. The prepared catalysts were characterized by N2 adsorption–desorption measurements, XRD, SEM, TEM, EDX, FT-IR, and

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UV–Vis DRS spectroscopy. SEM and TEM images exhibited uniform coverage of CNT with

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anatase TiO2 nanoclusters. It was also demonstrated that the presence of S and Mo within the TiO2 acts as electrons traps and prevents the charge recombination and also enables the TiO2

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photocatalyst to be active in visible light region. Moreover, the CNT/Mo,S-doped TiO2 nanohybrids

has

been

proven

to

has

a

excellent

photocatalytic

performance

in

photodecomposition of Congored (CR), at which the rate of decomposition reaches 100% in only 20 and 30 min under UV and visible light irradiation, respectively. The enhanced photocatalytic activity was ascribed to the synergetic effects of excellent electrical property of CNT and metalnonmetal codoping. Finally, which to best of our knowledge is done for the first time, we have demonstrated that Mo- and S-doped TiO2 decorated over CNT, or CNT/Mo,S-codoped TiO2, may have high potential applications in photocatalysis and environmental protection with superior catalytic activity under visible light illumination. Keywords: TiO2; CNT; CNT/Mo,S-codoped TiO2; Nanocomposite; Nano-catalyst *

Corresponding author at: Institute of Nano Science and Nano Technology, University of Kashan, Kashan, I. R. Iran. Tel.: +98 31 55912382; fax: +98 31 55912397. E-mail address: [email protected] (M. Hamadanian)

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1. Introduction Carbon nanotube (CNT) has attracted many attentions recently because of its unique electronic, optical, magnetic, mechanical and gas adsorption properties [1,2]. They are

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considered as promising candidates for various versatile applications. Possession of high electrical conductivity along with a high electron storage capacity (one electron for every 32

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carbon atoms), CNT can acts as largely effective electron sink [3,4]. Hence, CNT supported with

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metal oxide semiconductors are expected to show very different physicochemical properties compared to neat CNT [1]. Recently, CNT has been largely used as templates and scaffolds for

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the hybrid assembly of various nanostructures, and are largely reported to significantly enhance photocatalytic activity of metal oxide nanoparticles through the retardation and inhibition of

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charge carriers (electron–hole) recombination [5,6].

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Many studies have shown that TiO2 is one of the most preferable photocatalyst due to its

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physicochemical stability, non-toxicity and availability. However, it is also known that TiO2 photocatalyst has a few defects: firstly, TiO2 has a wide band gap (Eg > 3.2 eV) and it can only

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absorb the UV light (λ < 388 nm) portion of solar light (which is less than 5% of solar light) [7]. This problem practically rules out the use of solar light as the light source. Secondly, the high charge recombination rate of charge carriers (electron–hole pairs) leads to its low photocatalytic efficiency. And this matter also limits its practical use as an efficient photocatalyst [8]. In order to overcome these problems, various techniques have been employed: metal-ion doping [9], reduced TiOx photocatalysts [10], non-metal doping [11], making composites of TiO2 with other semiconductor having a relatively low band gap energy (e.g. Cd-S particles) [12], sensitizing of TiO2 surface with dyes (e.g. thionine) [13] and doping with up-conversion luminescence agent [14].

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Although it is widely reported that doping of TiO2 catalyst with both of metals and nonmetals enhance the photocatalytic activity of TiO2 [15], it has recently been reported that visible light activity of TiO2 can be even further enhanced by suitable combination of metals and nonmetal

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ions [16,17]. The TiO2 codoped with metal and nonmetal ions has some common aspects: leads to the TiO2 band gap narrowing and prevention of charge recombination of electron-hole pairs. In

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the previous study, it was reported that TiO2 absorption spectrum can be extended to the visible

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light region by incorporating Cr as metal an S as nonmetal sources into the TiO2 crystallite [15]. CNT/TiO2 nanocomposites has attracted the attention of many researchers due to their

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various applications, inter alia, the purification of polluted water [18,19]. CNT can effectively adsorb contaminants in water and also raise the TiO2 photocatalytic activity by acting as the

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electron traps, thus stabilizing the charge carriers and suppressing their recombination

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(Schematic 1). The coupling of CNT with TiO2 has been demonstrated to provide a synergistic

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and cooperative effect which leads to enhancement of the overall photocatalytic performance. The conductive structure of CNT template is believed to favor separation of the photogenerated

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charge carriers through formation of the heterojunctions (Schottky barrier) at CNT/TiO2 interface. TiO2 is inherently an n-type semiconductor; however, in the presence of CNT, the photogenerated electrons can freely move towards CNT surface, which have a lower Fermi level, causing the excessive valence band holes within TiO2 to migrate to its surface and react with the species. By considering these circumstances into account, TiO2 can effectively behave as the ptype semiconductor, which is largely photoactive. Furthermore, along with a spatial confinement of TiO2, CNT can also provide a large supporting surface areas for TiO2, causing in much faster photocatalytic reactions [20,21]. Our previous study revealed that after acid treatment of CNT, TiO2 nanoparticles can successfully decorated over CNT through precipitation method [1].

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Additionally, the prepared nanocomposite materials have been proven to own excellent optical

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cr

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and photocatalytic performance.

Schematic 1. Schemes of CNTs as photosensitizers: (a) electron injection into the

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conduction band of TiO2, (b) electron back-transfer to CNTs with the formation of a hole in the

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valence band of TiO2 and reduction of the hole by oxidation of adsorbed OH- species

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Although various attempts have been devoted to TiO2 nanoparticles coating over CNT

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substrate, the photocatalytic performance of the CNT/TiO2 heteronanostrcuture photocatalysts are still remained largely unexplored. There is only a few reports on the CNT coverage by metalnonmetal codoped TiO2 for possible applications in water purification [21,22]. To the best of our knowledge, there is no report on synthesis and application of CNT/Mo,S-codoped TiO2 nanophotocatalyst. Due to the synergetic effect of combination of S, Mo and CNT, considerable photocatalytic performance for the CNT/Mo,S-codoped TiO2 on the oxidative decomposition of the organic pollutants is expected. In this study, which to best of our knowledge was done for the first time, the novel visiblelight-active photocatalyst of CNT/Mo,S-codoped TiO2 photocatalysts with a core-shell structure were prepared by a modified sol–gel method. The morphological, optical and chemical analysis 4 Page 4 of 31

showed that the CNT/Mo,S-codoped TiO2 photocatalysts was successfully developed. In addition, the photocatalytic activity of the prepared nanocatalysts was evaluated by photodegradation of Congored (CR) as probe pollutant in aqueous solution and the results are

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discussed and compared.

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2. Experimental

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2.1. Chemicals and materials

Carbon Nanotube (CNT, outer diameter is ~10 nm, length ~1.5 μm, BET surface are of 250-

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300 m2/g, purity 90%, KNT-GP-BT) was kindly prepared from Grafen Chemical Industries Co and was purified by refluxing in 2.6 M nitric acid for 36 h before use. Ammonium

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heptamolibdate tetrahydrate ((NH4)6Mo7O24.4H2O), thiourea, Titanium(IV) isopropoxide,

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glacial acetic acid, ethanol, sodium dodecylbenzesulfonate (NaDDBS), nitric acid, congored

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(CR), and aqueous ammonia (NH4OH) were obtained from Merck Co., Germany. Aqueous solutions were prepared with deionized water that was prepared by an ultra pure water system

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type smart-2-pure made of TKA company of Germany.

2.2. Synthesis of CNT/TiO2 Hetero-Nanostructure CNT/TiO2 nanocomposites were prepared by using acid-treated CNTs and the subsequent reaction with titanium isopropoxide (TTIP) as TiO2 precursor via the surfactant wrapping sol–gel method [23]. In this regard, sodium dodecylbenzesulfonate (NaDDBS) was used as the CNT surface functionalizing second agent, to provides uniform, continuous, mesoporous anatase TiO2 layer onto the CNTs [24,25].

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In the synthesis process, acid-treated CNTs were dispersed into 0.5 wt% NaDDBS aqueous solution (Millipore grade) and sonicated for 12 h. The concentration of MWCNTs was 14.5 g L−1 within the mixture. The stable CNTs suspension was then mixed with 20 mL ethanol for 30 min

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to obtain a uniform suspension. Then, specific amount of titanium precursor (TTIP) (CNT:TiO2 mass ratio of 1.0) was mixed with 15 mL ethanol and glacial acetic acid under stirring for 30 min

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and the resulting solution was added drop-wise into the CNT suspension under violent stirring

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and the obtained mixture was further stirred for 2 h at room temperature. Aqueous ammonia solution was then added drop-wise to complete the hydrolysis of the TTIP (pH 9) to form an

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amorphous TiO2 shell onto the CNTs core (Schematic 2). At last, 10 mL ethanol was added into the reaction pot and the mixture was stirred for 30 min. The suspension was intercepted by three

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cycles of centrifugation and ethanol washing. The samples were dried at 60 ◦C for 12 h in oven,

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and then annealed at 500 ◦C for 30 min to obtain crystalline CNT/TiO2 core–shell

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nanocomposites. The intactness of the CNT structure after annealing at 500 ◦C was also

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confirmed by TEM analysis of the composites.

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Schematic 2. Synthetic scheme for the preparation of CNTs/TiO2 nanocomposites by the surfactant wrapping sol–gel method.

2.3. Synthesis of CNT/S-doped TiO2 Hetero-Nanostructure The all steps in synthesis of CNT/S-doped TiO2 are similar to the synthesis method of CNT/TiO2 nanocomposites but in TTIP hydrolysis step, thiourea as sulfure precursor (S:TiO2 molar ratio of 0.05%) was added to ethanol, and then in mixture with glacial acetic acid was added to TTIP pot for hydrolysis, and other steps were repeated the same as in preparation of CNT/TiO2 hetero-nanostructure [26]. 7 Page 7 of 31

2.4. Synthesis of CNT/Mo-doped TiO2 Hetero-Nanostructure The (NH4)6Mo7O24.4H2O as molibdinium precursor (Mo:TiO2 molar ratio of 0.06%) was

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dissolved into 200 ml dionized water and CNT/TiO2 nanocomposites were subsequently added to this solution [27]. Then, the mixture was exposure to nitrogen atmosphere for prevention from

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formation of MoO2. Afterward, the lid of reaction container was closed and exposed to the UV

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light irradiation for photochemical reaction and coating of TiO2 surface by Mo nanoclusters. The suspension of nanocomposite was intercepted by centrifugation and dionized water washing and

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dried at 70 ◦C for 10 h in oven.

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2.5. Synthesis of CNT/Mo,S-codoped TiO2 Hetero-Nanostructure

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To prepare CNT/Mo,S-codoped TiO2, instead of CNT/TiO2, the CNT/S-doped TiO2 was

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used in 2.4 section and the other steps were repeated as the same, and CNT/Mo,S-codoped TiO2

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nanocomposites was prepared.

2.6. Nanocatalysts Characterization Phases analyses of the prepared samples were carried out by powder X-ray diffraction (XRD, Philips X’pert Pro MPD, Holland) using graphite-filtered Cu Ka (k = 0.154 nm) radiation in the range of 2θ from 10° to 90° with a 2θ step size of 0.02° and a measuring time of 0.8 s per point. The morphology was revealed by a transmission electron microscope (TEM) (Philips EM208, Holland) and scanning electron microscope (SEM, Philips XL- 30ESM, Holland) equipped with an energy dispersive X-ray detector (EDX, EDAX Genenis- 4000, USA) operated at 25 kV with spot size 4. UV-Vis absorption spectra of the samples were obtained with a UV-Vis

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spectrophotometer (Perkin Elmer Lambda2S, Germany) between 200 and 800 nm with the scan rate 5 nm s–1. Also, to analyze the band gap edge shift of pure TiO2, CNT/TiO2, CNT/Mo-doped TiO2, CNT/S-doped TiO2, and CNT/Mo,S-codoped TiO2, UV spectra of the samples were

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recorded on Shimadzu UV–Visible spectrometer (Shimadzu 1800, Japan). FT-IR spectra of the samples were recorded on a Nicolet Magna IR 550 spectrometer, USA. The Brunauer–Emmett–

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Teller (BET) surface area (SBET) of the synthesized TiO2 powder was analyzed by nitrogen

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adsorption apparatus (Quantachrome Instruments, USA) using the adsorption data in the relative pressure (P/P0) range of 0.05–1.0. Desorption isotherm was used to determine the pore size

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distribution via the Barret–Joyner–Halender (BJH) method with cylindrical pore size. The nitrogen adsorption volume at the relative pressure (P/P0) of 0.9837 was used to determine the

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pore volume and average pore size.

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2.7. Evaluation of Photocatalytic Performance of the Catalysts The photocatalytic activity of the prepared catalysts was examined using Congored (CR) as

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probe pollutant. The degradation reaction was carried out in a slurry photocatalytic reactor. The photocatalytic reaction was carried out with 100 mL aqueous Congored solution containing 50 mg catalyst. This mixture was aerated for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV source and 25 cm away from visible sources of 400W Osram lamps. The quartz vessel and the light sources were placed inside a black box to prevent UV leakage. The experiments were done at room temperature and after specific times, adequate amounts of the mixture were taken, centrifuged and analyzed with an UV–Visible spectrometer.

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3. Result and Discussion 3.1. BET Analysis Fig. 1 shows the nitrogen adsorption–desorption isotherms of the synthesized TiO2 powders

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calcined at 500 ◦C. At high relative pressure between 0.55 and 1.0, the curve demonstrated a hysteresis loop, exhibiting presence of mesopores. Fig. 2 shows pore size distribution of the

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synthesized TiO2 powders calcined at 500 ◦C. TiO2 powders exhibited average pore sizes about

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11.64 nm [28,29]. It can be seen that the pore diameter ranges from 1.7 to 140 nm. In addition, findings showed that TiO2 powders own a large surface area of 102.123 m2/g and pore volume of

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0.109 cm3/g.

Figure 1.

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Figure 2.

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3.2. Crystalline Structure Analysis

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The X-ray diffraction patterns (Fig. 3) exhibit that the all prepared catalysts contain the nanocrystalline anatase TiO2 structure (2ϴ= 25.2, 37.76, 48.02, 54.05, 55.03, 62.80, 68.85,

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70.19, and 75.07) [26,30]. No characteristics peaks of CNTs were observed. This may be ascribed to the overlap of the violent peaks anatase phases of TiO2 (0 0 1) and CNTs (0 0 2). The average crystalline size (D in nm) of pure and doped TiO2 nanoparticles was determined from XRD patterns according to the Scherrer’s equation using the peak at 25.4◦ as follows: D = Kλ/β cos θ

(1)

where D is the crystalline size, λ the wavelength of X-ray radiation (0.1541 nm), K the constant usually taken as 0.89, and β is the peak width at half-maximum height after subtraction of equipment broadening, 2θ = 25.4◦ for the anatase phase of TiO2. Average crystal size of pure

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TiO2, S-doped TiO2, Mo-doped TiO2 and CNT/Mo, S-codoped TiO2 were found to be around about 12–17 nm.

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Figure 3.

3.3. Scanning Electron Microscopy (SEM) Analysis

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Fig. 4 shows SEM micrographs of the pure TiO2, CNT/TiO2, Mo-doped TiO2, S-doped TiO2,

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and CNT/Mo,S-TiO2 calcined at 500 °C. The grains of the TiO2 nanoparticles covering over CNTs are found to be uniform, globular and slightly agglomerated. The observation indicates

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formation of mesoporous titania over CNTs support, which may be beneficial to enhances the adsorption of reactants due to its great surface roughness and high surface area. Fig. 4 also

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reveals that the incorporation of Mo and S does not cause any change in the morphology of the

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CNT/TiO2 catalysts.

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Figure 4.

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3.4. Transmission Electron Microscopy (TEM) Analysis Fig. 5 shows TEM images of Mo, S-codoped titania catalyst in which the catalyst have a crystallite sizes around 10-12 nm which is in good agreement with the XRD results. Fig. 5 also shows a thin, uniform, mesoporous and nanometer-scale titania layer coated over CNT. Fig. 5 also reveals that the codoping of Mo and S does not leave any change in the shape of the TiO2 catalyst. Figure 5.

3.5. EDX analysis

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Fig. 6 shows the corresponding EDX results of the Mo,S-codoped TiO2. EDX analysis confirmed the incorporation of both of Mo and S in the nanocomposite. Indeed, the EDX spectrum of Fig. 6 provides the direct proof that TiO2 is successfully doped with Mo

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nanoclusters.

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Figure 6.

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3.6. UV–Visible Diffuse Reflectance (DRS) Spectra

The DRS spectra of the prepared materials are shown in Fig. 7. DRS results show a red shift

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in the absorption onset value in the case of Mo- and S- doped titania compared to the pure TiO2, which indicates that addition of dopant decreases the band gap value of TiO2 photocatalyst. Non-

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metal elements have been proved to be beneficial doping elements in the TiO2 through mixing

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their p orbital with O2p orbital to reduce the band gap value of TiO2, but doping of various

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transitional metal ions into TiO2 could shift its optical absorption edge from UV into visible light range without a prominent change in its band gap [31,32]. The absorbance extension toward

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visible light region increases the number of photogenerated charge carriers (electrons and holes) to participate in the photocatalytic reaction, which results in considerable enhancement of the photocatalytic activity of TiO2 [33].

Figure 7.

3.7. Chemical Structure Analysis FT-IR spectra of CNT, TiO2, CNT/TiO2 with CNT:TiO2 molar ratio of 0.5 and CNT/TiO2 doped with 5% S, 0.06% Mo, and 5% S-0.06% Mo are presented in Fig. 8 [34]. It can be seen that pristine CNT has no peaks in its spectrum. But, after treatment with acids, some groups were

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introduced onto CNT surface. In the case of pure TiO2 and TiO2 composites, findings exhibit a stretching vibration band of the O-H and bending vibration of the absorbed water molecules about 3350-3450 and 1650-1700 cm-1, respectively, which confirms the presence of hydroxyl

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ions in the structure of the catalyst [34]. The peak at 650–700 cm-1 is ascribed to the Ti–O

on the quantity of the surface OH and no additional peaks appear.

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Figure 8.

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stretching vibration. Compared with pure TiO2, the doping of S and Mo seems to have no effect

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3.8. Evaluation of Photocatalytic Activity of CNT/Mo,S-codoped TiO2 Catalyst Photodecomposition of Congored (CR) in the presence of CNT/TiO2, CNT/S-doped TiO2,

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CNT/Mo-doped TiO2, and CNT/Mo,S-codoped TiO2 nanophotocatalysts under UV and visible

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irradiation was evaluated and corresponding results are depicted in Figs. 9 and 10. During the

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photocatalytic process, the absorption of photons by the photocatalyst leads to the excitation of electrons from the valence band of TiO2 to its conduction band which leads to generation of

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electron–hole pairs. The electron in the conduction band is captured by oxygen molecules dissolved in the suspension and the hole in the valence band can be captured by OH− or H2O species adsorbed onto the surface of the catalyst, to produce the hydroxyl radical. Hydroxyl radicals then oxidize the pollutants. Thus, recombination of photogenerated electrons and holes is one of the most significant factors that deteriorate the photoactivity of the TiO2 [35]. CNTs with its semiconductor nature can be expected to serve as electron acceptors from an excited semiconductor photocatalyst [1]. In order to study the photocatalytic performance of TiO2/CNT nanostructure, degradation of CR aqueous solution was investigated. The photodegradation rate (R) of CR under UV and visible light irradiation over TiO2/CNT

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photocatalysts is shown in Fig. 9. Comparing the histograms of the photocatalytic degradation of CR over TiO2 and TiO2/CNT (Fig. 9), the TiO2/CNT0.5 hetero-nanostructure has a higher degradation rate compared to the pure TiO2. Indeed, due to their higher specific surface area, the

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TiO2/CNT composites are more active than pure TiO2 nanoparticles for adsorption of organic substance which leads to high percentage photodegradation of the substance. However, this

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phenomenon is not observed in term of TiO2/CNT1.0, due to high ratio of CNT to TiO2. In

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addition, as it is obvious from the results, the photocatalytic performance of TiO2/CNT0.5 is much higher compared to the commercial TiO2 (P25) nanoparticles.

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Figure 9.

Incorporation of S and Mo in TiO2 has a profound effect on the decomposition efficiency of

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the nanocatalysts. S specie reduces TiO2 band gap by forming new energy states within the inter-

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gap region while Mo can effectively trap the photogenerated electrons, thereby enhancing

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electron–hole separation on the surface of TiO2 photocatalyst and raising its photocatalytic performance [36]. As it is obvious from Figs. 10, the photocatalytic activity of Mo- and S-doped

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TiO2 catalysts is much higher than that of pure TiO2. The role of Mo and S elements is due to either one or combination of the following factors: (i) improved light absorption of Mo,Scodoped TiO2 compared to undoped TiO2 (ii) alleviation of the surface poison phenomenon; and (iii) trapping of both hole (h+) and electron (e−) to reduce the recombination rate of these charge carriers during the TiO2 photo-excitation [36]. Figure 10. Fig. 10 demonstrates that CNT/Mo,S-codoped TiO2 has higher catalytic activity compared to the undoped TiO2 or TiO2 doped with sulfur and molibdinium, under UV and visible light irradiation.

The

promising

photocatalytic

activity

of

CNT/Mo,S-codoped

TiO2

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nanoheterostructure may be due to the synergetic effect of the codoping of sulfur and molibdinium and using CNTs as TiO2 catalyst support. Wang et al. reported on photodecomposition of phenol by TiO2 and 20% CNT/TiO2 nanocomposites and the obtained

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values were 0.68 and 1.55 × 10-3 min-1, respectively [37]. Oh et al. reported on the catalytic degradation of the contaminant methylene blue (MB) in an aqueous solution containing 5%

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CNT/TiO2 photocatalysts, under UV light irradiation and reported the rate value of 9.5 × 10-3

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min-1 which was largely higher than 8.0 × 10-3 min-1 obtained for pure TiO2 [38]. Tian et al. synthesized CNT/TiO2 nanostructures via solvothermal and sol–gel methods with and the

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resulting catalyst was used for visible light driven degradation of MB [39]. The sol–gel CNT/TiO2 (20%) significantly enhanced the photodegradation of MB, compared to pure TiO2.

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The proposed mechanism for the enhanced photocatalytic activity of CNT/Mo,S-codoped

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TiO2 nanocomposites is proposed schematically in Schematic 3. Under visible light irradiation,

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the photoexcited electrons from the CNT substrate is injected into the conduction band of Mo,Scodoped TiO2 through Ti–C bonds [1,36]. The positively charged CNT in turn captures electrons

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from valence band of TiO2 leaving the holes behind. The holes can react with the surface hydroxyl ions or water molecules to produce hydroxyl radicals (OH•), while the electrons can be trapped by Mo before they react with the adsorbed oxygen molecules producing superoxide radicals (O2•), which scavenge the water molecules to form greatly reactive OH• radicals. The OH• radicals are very highly oxidizing and are capable of decomposing the CR to water and carbon dioxide [36].

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Schematic 3. Excitonic processes for the CNT/Mo,S-codoped TiO2 catalyst under visible light

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irradiation.

When Mo-doped TiO2 forms a heterojunction structure with another semiconductor (e.g., CNT) a charge space with diameter ranging from several tens of nanometres to hundreds of nanometres can be formed near the junction to equalize the Fermi levels. This will result in reduced band gap energy within the CNT/Mo-doped TiO2 heterojunction. Formation of an interior electric field of charge space within this nanocomposite can separate the photogenerated charge carriers which causes in decline of the charge recombination rate [40]. Finally, it is proposed that the positive effect of Mo,S-codoping coupled with CNTs leads to enhanced photoactivity of TiO2 photocatalyst due to synergistic effects.

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4. Conclusions The sulfur and molibdinium codoped-TiO2 decorated over CNT (CNT/Mo,S-codoped TiO2) as a novel high efficient catalyst with TiO2 particle size of 10–12 nm was successfully prepared

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via modified sol–gel method to degrade Congored as probe pollutant in aqueous solution. It was proposed that photocatalytic performance of CNT/Mo,S-codoped TiO2 materials should be

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optimized at some specific value of CNT content. Incorporation of S2- ions leads to decreases in

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band gap value of TiO2 while Mo acts as a sink for the photogenerated electrons. The photocatalytic activity of the synthesized nanocatalysts demonstrated that CNT/Mo,S-codoped

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TiO2 nanocomposites have much higher photocatalytic activity compared to the pure TiO2 under UV and visible light irradiation. As result, it was proposed that CNT can acts as potential

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photosensitizer besides its role as adsorbents and dispersing agents. In conclusion, our findings

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exhibited that CNT/Mo,S-codoped TiO2 has high potential as photocatalyst and can be a good

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Acknowledgment

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candidate for water and wastewater purification.

Authors are grateful to Council of University of Kashan for providing financial support to undertake this work.

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Figures Captions: Figure 1. N2 adsorption–desorption isotherms of the synthesized TiO2 powders. Figure 2. Pore size distributions of the synthesized TiO2 powders.

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Figure 3. XRD patterns of the synthesized nanocatalysts: CNT/TiO2 at three different molar ratio, CNT/S-doped TiO2, and CNT/Mo,S-codoped TiO2.

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Figure 4. SEM micrographs of the synthesized nanocatalysts: (a,b) pure TiO2, (c,d) S-doped

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TiO2, (e,f) Mo-doped TiO2, and (g,h) Mo,S-codoped TiO2 decorated over CNT.

Figure 5. TEM micrographs of the synthesized CNT/Mo,S-codoped TiO2 nanocatalysts in

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different magnifications.

Figure 6. EDX result of the synthesized Mo-doped TiO2, CNT/TiO2, and CNT/Mo,S-codoped

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TiO2 nanocatalysts.

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Figure 7. UV-Visible DRS analysis of the synthesized nanocatalysts.

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Figure 8. FTIR analysis of CNT (a), TiO2 (b), CNT/TiO2 doped with 0.06% Mo (c), CNT/TiO2 with CNT:TiO2 molar ratio of 0.5 (d), CNT/TiO2 doped with 0.05% S (e), and CNT/TiO2 doped

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with 0.05% S-0.06% Mo (f).

Fig. 9. Photocatalytic degradation results of the prepared nanocatalysts of pure TiO2 and CNT/TiO2 with different molar ratio under UV and visible light irradiation. Fig. 10. Photocatalytic degradation results of the prepared nanocatalysts of Mo and S-doped TiO2 decorated onto CNT under UV and visible light irradiation.

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Graphical Abstract (for review)

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Figure 10

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