Efficient photocatalytic degradation of acid orange 7 over N-doped ordered mesoporous titania on carbon fibers under visible-light irradiation based on three synergistic effects

Accepted Manuscript Title: Efficient photocatalytic degradation of acid orange 7 over N-doped ordered mesoporous titania on carbon fibers under visible-light irradiation based on three synergistic effects Author: Youji Li Ming Li Peng Xu Shaohua Tang Chen Liu PII: DOI: Reference:

S0926-860X(16)30247-2 http://dx.doi.org/doi:10.1016/j.apcata.2015.01.050 APCATA 15877

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

5-10-2014 18-1-2015 31-1-2015

Please cite this article as: Youji Li, Ming Li, Peng Xu, Shaohua Tang, Chen Liu, Efficient photocatalytic degradation of acid orange 7 over N-doped ordered mesoporous titania on carbon fibers under visible-light irradiation based on three synergistic effects, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.01.050 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.

Efficient photocatalytic degradation of acid orange 7 over N-doped ordered mesoporous titania on carbon fibers under visible-light irradiation based on three synergistic effects

Youji Li*, Ming Li, Peng Xu, Shaohua Tang, Chen Liu

College of Chemistry and Chemical Engineering, Jishou University, Hunan 416000, China

* Corresponding author: Tel.: +86–13762157748; fax: +86–0743–8563911. E-mail address: [email protected] (Y.J. Li)

1

Graphical abstract

Photodegradation of acid orange 7 (AO7) 10.5 NOMT/CFs

Visible

kapp×103 (min–1)

9.0

d101=0.35nm

7.5

Synergic effect

6.0 4.5

NNT/CFs

3.0

NOMT-CFs NNT-CFs NOMT

1.5

NNT

0.0

Highlights  1.N-doped ordered mesoporous titaniawas supported on carbon fibers ((NOMT/CFs).  2.NOMT/CFs have high photoactivity under visible-light irriadiation.  3.Photocatalytic mechanism of NOMT/CFs was proposed.  4.Degradation pathway of AO7 was proposed.  5.Three synergies of N doping, CF support and ordered mesostructure were obtained.

Abstract: N-Doped ordered mesoporous titania on carbon fibers (NOMT/CFs) has been prepared by a liquid-crystal template (LCT) method with the aid of supercritical deposition. The NOMT/CFs product has been characterized by X-ray diffraction analysis, scanning electron and transmission electron microscopies, X-ray photoelectron spectroscopy, N2 sorption, UV/Vis diffuse reflectance spectrophotometry, and photoluminescence. The photoactivity of the obtained product in the degradation of acid orange 7 (AO7) solution under visible-light irradiation has been evaluated. A 2

photocatalytic mechanism involving three synergistic effects of NOMT/CFs is proposed, whereby the titania acts as an electron/hole generator with its ordered mesostructure facilitating electron–hole separation, N-doping enhances visible-light adsorption, and the CFs serve to concentrate the pollutant around the active sites, favor electron transfer, and facilitate convenient recycling. Gas chromatography and mass spectrometry have been used to analyze the intermediates generated in the conversion of AO7, allowing a tentative decomposition pathway to be proposed. The effects of catalyst amount, pH, and initial AO7 concentration have been examined as operational parameters. The photocatalytic reactions follow pseudo-first-order kinetics and are discussed in terms of a modified Langmuir–Hinshelwood model.

Keywords: N-doped ordered mesoporous titania, carbon fiber, synergy, supercritical deposition, visible-light photoactivity

1. Introduction Titanium dioxide, a chemically stable, highly efficient, and nontoxic photocatalyst, has been widely used for water and air purification [1–3]. However, conventional TiO2 photocatalyst can only utilize light of wavelengths shorter than 388 nm (UV range) due to its wide band gap (Eg ≈ 3.2 eV for anatase), which limits its practical application with solar light [4–7]. Many efforts have been made in the last two decades with a view to overcoming this limitation. The two main approaches have been dye sensitization [4] and doping with impurities [57]. It has been reported that the incorporation of nitrogen into the titania crystal lattice can shift the activation energy of the TiO2 photocatalyst into the visible light region [6,8]. Thus, researchers have found that nitrogen-doping technology is important for the application of visible-light-induced photocatalytic TiO2 [9,10]. On the other hand, photo-induced electron–hole pairs have a short recombination time of the order of 10−9 s, whereas the reaction time for TiO2 with pollutants is in the range 10−8–10−3 s [1]. Thus, the rapid recombination of electrons and holes is one of the major reasons for the low photocatalytic activity. To solve this shortcoming, some researchers have recently developed pore-structured titania to provide a high content of “activated centers” for the adsorbed pollutants, because the efficient oxidation of organics is a surface-orientated process in photocatalysis [11,12]. 3

Since its first preparation through a modified sol–gel process using a phosphate surfactant by David and Jackie [13] in 1995, mesoporous TiO2 has received increasing attention because of its porous structure. This property facilitates photocatalytic degradation of contaminants such as humic substances, phenolic compounds, pesticides, chlorinated compounds, and dyes [14–19]. It has also been found that a porous support serves to concentrate the pollutant around the active sites on mesoporous titania, and is effective in facilitating the separation of electron–hole pairs [20]. In addition, the separation of TiO2 powder from treated wastewater prior to discharge is a time-consuming and expensive process, which has limited the development and application of TiO2 photocatalysis in water treatment. To provide high pollutant concentrations in the vicinity of catalytic centers and to permit convenient separation of the photocatalysts from the catalytic system, many efforts have been made to immobilize photocatalysts on porous supports, such as activated carbon [21], zeolites [22], ceramics [23], glass fibers [24], optical fibers [25], and steel fibers [26]. Compared to powder and particulate supports, fiber supports are more amenable to convenient recycling of composite catalysts [24,26]. Among such fiber supports, carbon fibers (CFs) are ideal substrate materials for applications in the photocatalysis of pollutants because they are long, thin strand materials composed mostly of carbon atoms, which endows them with advantages such as light weight, high tensile strength, manufacturing flexibility, heat resistance, and stability under corrosive conditions [2729]. Although CFs have been used as substrates for the preparation of photocatalysts, most cases have been limited to the immobilization of TiO2 particles without ordered mesostructure or doping [27,28,30]. In this work, with the goals of effectively utilizing visible light, high catalytic performance, and convenient separation of the photocatalysts from the water treatment system, NOMT with an ordered mesostructure and visible-light photoactivity has been supported on CFs by a liquid-crystal template (LCT) method with the aid of supercritical deposition. To explore the possible synergy between N-doping, the ordered mesostructure, and the CF support on the photoactivity of TiO2, the as-prepared photocatalysts have been characterized by various analytical techniques. The performances of these new materials have been tested in the photocatalytic degradation of a dye under visible-light irradiation. The dye under consideration is acid orange 7 (AO7; C16H11N2O4SNa), which is a highly water-soluble, acid orange dye of the anionic monoazo class. It is widely used as a colorant in textiles, leather, and paper. Hence, the photodegradation of AO7 is 4

important with regard to the purification of dye effluents [3]. 2. Experimental 2.1. Reagents Raw carbon fibers used as supports, prepared by activation of polyacrylonitrile, were obtained from Henan (surface area: 1280 m2/g, total pore volume: 0.19 cm3/g). Tetrabutyl titanate, cetyltrimethylammonium bromide, hydrochloric acid, urea, and absolute ethanol were all analytical grade and were purchased from Beijing Zhonglian Chemical Reagent company. The chemical structure of AO7 and its absorption spectrum are given in Supporting Information Fig. S1. Deionized water, purified with an Elga-Pure water purification system, was used to prepare all solutions for the experiments. 2.2. Preparation of photocatalysts NOMT/CFs was prepared by deposition in supercritical CO2, using tetra-n-butyl titanate, a liquid crystal, and urea as the precursor, soft template, and nitrogen source, respectively. First, cetyltrimethylammonium bromide (5 g) was accurately weighed and then completely dissolved in distilled water with stirring for a specified period of time to form the liquid-crystal template (LCT) by a similar methodology to that described elsewhere [18]. Next, tetrabutyl titanate (25 mL) and urea (2 g) were dissolved in ethanol, and the mixture was magnetically stirred (100 rpm) for 2 h. Ambient laboratory temperature (20 °C) was maintained while stirring. A solution containing 35% HCl (3.3 mL) and the obtained liquid crystal was added dropwise over 1 h until a liquid-crystal sol was obtained. After aging the sol for 12 h at room temperature, it was deposited on CFs in supercritical CO2 (at 7.6 MPa and 50 °C). Subsequently, organic material in the resulting composite was extracted in a soxhlet apparatus for 48 h. The composite was dried for 30 min at 100 °C in an oven and then calcined at 500 °C for 1 h in a nitrogen atmosphere to synthesize NOMT/CFs. Simultaneously, another portion of the aged sol was calcined at 500 °C for 1 h to synthesize pure NOMT powder for comparison. To further evaluate the synergistic relationship between NOMT and CFs, and to determine the effect of NOMT on the synergy, N-doped nanostructured TiO2 on CFs (NNT/CFs) was also prepared by deposition in supercritical CO2. The synthetic processes used to obtain the NNT and NNT/CFs were similar to those used for NOMT and NOMT/CFs, respectively, but without the LCT. 2.3. Characterization of the composite photocatalyst 5

The profile of the NOMT/CF photocatalyst was observed by scanning electron microscopy (SEM) (Hitachi S3400N, Japan). High-resolution transmission electron microscopy/selected-area electron diffraction (HRTEM/SAED) was performed with a JEOL (JEM 2100F) microscope. Small-angle X-ray scattering (SAXS) measurements were made on a Nanostar U small-angle X-ray scattering system (Bruker, Germany) using Cu-Kα radiation (40 kV, 35 mA). Wide-angle X-ray diffraction (WAXRD) patterns were recorded on a Bruker D4 X-ray diffractometer with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Nitrogen adsorption–desorption isotherms were used to determine the Brunauer–Emmett–Teller (BET) surface area and pore size distribution (ASAP2010, Micromeritics Co., USA) at 77 K. UV/Vis absorption spectra were recorded at 298 K on a UV/Vis diffuse reflectance spectrophotometer (Shimadzu UV-2100). X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. Photoluminescence (PL) emission spectra were measured using a Spex 500 fluorescence spectrophotometer with 325 nm radiation for excitation. The CF-supported composite was ignited at 900 °C and the TiO2 loading was calculated in terms of ash weight by thermogravimetric analysis (WCT-2C, Optical Instrument Factory, Beijing). 2.4. Photocatalytic reactor and light source To examine the degradation of AO7 in aqueous solution under visible light, experiments were carried out using a cylindrical batch reactor fitted with a water-cooled quartz jacket and an air-sparging motor. A 500 W high-pressure Hg lamp with a UV filter was placed about 20 cm from the reactor. The photon flow entering the reactor was 6 mW cm−2 for visible light irradiation. 2.5. Photocatalytic activity test The experimental procedure involved placing 500 mL of a filtered suspension in the photocatalytic reactor. Air was bubbled through the reaction solution at a rate of 560 mL min−1 to ensure a constant concentration of dissolved oxygen. Thereafter, samples of about 5 mL were withdrawn at specified time intervals during illumination and filtered through 0.45 mm filters. The AO7 concentration in the reaction mixture was monitored by a TU-1810 spectrophotometer by measuring the absorption intensity at λmax=480 nm in a 10 mm quartz cell to evaluate decolorization. The photocatalysis was tested in triplicate and the results are reported as mean values, with an error of less than 5%. The total organic contents (TOC) of the suspensions containing the photocatalyst and the corresponding initial solutions of the dyes were directly 6

analyzed using a TOC analyzer (model 820, Sievers, USA) to evaluate mineralization. Dye degradation products were identified by GC–MS (Agilent 7890A–5973N). 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. Morphological and textural properties of NOMT/CFs The morphology of the obtained product was first examined by SEM and TEM. Compared with the smooth surface of pure CFs (Fig. 1a), an image of NOMT/CFs (Fig. 1b) revealed that a titania layer had been immobilized on almost each fiber. Although the titania layer had been extensively deposited on the fibers, the same spatial distribution of CFs was retained in the composite. Therefore, adequate UV irradiation could penetrate the felt-form photocatalyst to a certain depth to reach a three-dimensional environment for the photocatalytic reaction. This feature differentiates the present photocatalyst from current immobilized photocatalysts such as TiO2 immobilized on the surfaces of glass [31], steel plate [32], and ceramic membranes [33], which only provide a two-dimensional surface for photodegradation. Additionally, the CF substrates are uniformly and compactly covered by a large number of irregular-shaped aggregates with dimensions of 0.5–1 μm to form a rough surface, as shown in Fig. 1c. The thickness of the titania layer is around 200 nm (Fig. 1d). However, the aggregation size of NOMT in the composites is smaller than that of pure NOMT, at about 23 μm according to the observed morphology (Fig. S2). TEM characterization provided additional information regarding the interior structure of these architectures. A typical TEM image is shown in Fig. 1e; the NOMT/CFs exhibited a well-ordered channel arrangement in large domains, corresponding to the SAXS pattern (see discussion below). The inset in Fig. 1e shows the SAED pattern, which also displays a well-resolved diffraction ring and many diffraction spots, indicating high crystallinity of the pure anatase phase. An HRTEM image (Fig. 1f) shows a 2D mesostructure, containing ordered mesopores of about 3 nm in diameter, and randomly oriented anatase nanocrystals with well-defined crystallinity with a lattice spacing of 0.35 nm, consistent with the d-spacing of the (101) reflections of pure anatase [14]. 3.1.2. Crystalline structure of samples For NOMT and its composites calcined at 500 °C, the ordered pore structure was characterized on the basis of SAXS measurements in the 2θ range 0.2–1.5º (Fig. 2a). The SAXS patterns of both NOMT and NOMT/CFs show characteristic Bragg peaks [(100) reflections] indicative of ordered 7

pores in the samples. NNT and its composites do not show any diffraction peaks in the 2θ range 0.2–1.5º in their SAXS patterns because of their extremely large pore dimensions and lack of an ordered mesoporous structure [19]. The characteristic (100) reflection of pure NOMT shifted to a 2θ angle larger than that for NOMT/CFs, indicating a decrease in the unit-cell parameter and shrinkage of the framework. This change may be attributed to the close contact of the thin NOMT layers to the CF surfaces, which exhibit hinder effect for titania crystallization. The WAXRD pattern for the NOMT/CFs obtained at a calcination temperature of 500 °C indicated only the anatase phase (Fig. 2b), whereas anatase and rutile phases can coexist in pure NOMT obtained at the same calcination temperature. Generally speaking, the mesoporous TiO2 show low crystal transition temperature from the anatase to the rutile possible due to crystal size, mesoporous morphology, doping, calcination time and so on [3]. At the same calcination condition, crystal transition temperature of pure NOMT is lower than composite NOMT/CFs. This can be attributed to the CFs suppressing the phase transformation of mesoporous TiO2 from anatase to rutile form at high temperatures. which is good agreent with reports[30,34]. Additionally, compared with pure NNT without rutile, the dual-phase nature of NOMT endows it with low thermal stability due to the mesostructure, as described elsewhere [19]. Although the patterns of NOMT/CFs exhibit narrower and more intense diffraction peaks than those of raw CFs, the coexisting TiO2 peaks for NNT/CFs are more distinct. Enhanced anatase diffraction peaks of the (101), (004), (200), (105), (211), and (204) planes for NNT/CFs were observed at 2θ = 25.48, 37.98, 48.28, 54.28, 55.38, and 64.37, respectively (JCPDS No. 21-1272). Meanwhile, there were obvious decreases in the intensity of the distinct diffraction peaks of the CFs for the G(002) (2θ = 24.47°) and G(101) (2θ = 44.29°) planes, suggesting the presence of numerous titania layers on the surface of the support. The crystallite sizes, as calculated from the broadening of the (101) anatase peak by the Scherrer equation, were found to be 10.2 nm (NOMT/CFs), 18.5 nm (NOMT), 16.4 nm (NNT/CFs), and 21.8 nm (NNT) (Table 1). The crystallite sizes of NOMT/CFs and NOMT are thus smaller than those of NNT/CFs and NNT, respectively. This may be attributed to hindering effects of carbon adsorption and LCT decomposition on the growth of the anatase nanocrystals. 3.1.3. Chemical composition of NOMT/CFs XPS

analysis

provided

valuable

insight

into

the

chemical

composition

of

the

carbon-fiber-supported N-doped OMT photocatalyst. An XPS survey spectrum of the 8

photocatalyst exhibited prominent peaks of carbon, oxygen, and titanium, but a relatively feeble peak due to nitrogen, as shown in Fig. 3a. The O1s spectrum can be resolved into two peaks, one at 530.6 eV and the other at 529.1 eV (Fig. 3b). The former can be attributed to surface hydroxyl groups and the latter to the Ti-O-Ti lattice oxygen of TiO2 [35]. A high-resolution XPS spectrum of C1s on the composite photocatalyst is shown in Fig. 3c, which can be fitted by three peaks at binding energies of 289.4, 286.1, and 284.6 eV, respectively. This implies that three different chemical environments of carbon existed in the products. The weak peaks at 286.1 and 289.4 eV are usually assigned to the oxygen-bound species C-O and C(O)O, and the strong peak at 284.6 eV is ascribed to elemental carbon [30,36,37]. Due to surface oxidation during the preparation process, the oxidized active sites on the CF surface can contribute to the deposition and growth of N-doped OMT. The presence of nitride dopant is confirmed by the N1s core level peaks. Two peaks at binding energies of 399.6 and 401.6 eV are observed, as shown in Fig. 3d. The first major peak can be attributed to the nitrogen introduced into the O–Ti–N structure [38], indicating that some O atoms were substituted by N atoms, consistent with the visible-light activity of doped TiO2. The latter peak can be assigned to the presence of interstitial N, being characteristic of NO or NO2 in the N-doped TiO2 sample [39,40]. The molar ratio of N to Ti in all N-doped samples was about 2.3% based on XPS analysis. Additionally, nitrogen adsorptiondesorption isotherms and the corresponding pore size distribution curves were measured to investigate the porous structure of the as-prepared samples (see Fig. S3). The obtained samples exhibited type IV isotherms characteristic of mesoporosity. The NOMT/CFs products showed higher surface area (756 m2/g) than that of NNT/CFs (473 m2/g) due to the covering N-doped OMT on the surface of the CFs. Although the mean pore diameter of NOMT/CFs (1.4 nm) was lower than that of NNT/CFs (1.7 nm), the total pore volume of NOMT/CFs (0.11 mL/g) was larger than that of NNT/CFs (0.08 mL/g) owing to the generation of more pores by the association of the titania layers with ordered mesoporous structure. 3.1.4 Optical absorption properties The absorption edge and band-gap energies of the as-prepared samples were determined by UV/Vis diffuse reflectance spectroscopy. Diffuse reflectance spectra of the N-doped samples are illustrated in Fig. 4a. All spectra clearly show a red-shift of the absorption edge towards the visible region due to the lower energy between the valence band and conduction band of TiO2. Notably, the 9

absorption intensities of the composites were higher than that of pure N-doped titania at UV and visible wavelengths. This difference can mainly be attributed to the black hue of CFs, and therefore their strong absorption over the entire range of wavelengths employed. The absorption intensities of the NOMT/CFs were higher than those of NNT/CFs at UV and visible wavelengths, which may be attributed to the mesoporous morphology. There was no shift toward longer wavelengths for the absorption band of the composites in comparison with that of N-doped titania, indicating that a small-particle quantum size effect was absent in the composites. The pure NOMT showed a small shift toward longer wavelengths in comparison with NNT. This behavior of NOMT is presumably due to its mesoporous structure and the significant surface effects. The band gaps Eg were calculated using the following equation:

( Ah )1 / 2  h  E g

(1)

where A is the absorption intensity and hν is the photon energy. Band gaps Eg were obtained from the intersection of the tangents to the straight portions of plots of (Ahv)1/2 against the energy of exciting light (hv), as shown in Fig. 4b. Band gaps of the obtained samples are listed in Table 1. The anatase form of the N-doped sample exhibited a relatively low band gap compared with that of pure TiO2, which has a band-gap energy of 3.2 eV [4]. This observation clearly indicates that the TiO2 doped with urea could generate additional energy levels above or below the valence band of TiO2 [35]. Although Ti-C bonds were formed, as indicated by XPS analysis (Fig. 3c), which is beneficial for band-gap narrowing as in the case of carbon-doped TiO2 [10,41], the CF support did not further narrow the band gap of the as-prepared N-doped TiO2. 3.2. Photocatalysis 3.2.1.Photocatalytic degradation of AO7 under visible-light irradiation Prior to detailed evaluation of the photocatalytic activities of the obtained products, it was noted that the concentration of AO7 in aqueous solution decreased even without illumination because of adsorption. However, the decrease with each catalyst was only 826% of the original AO7 concentration (Fig. S4). For comparison, it can be seen in Fig. 5a that AO7 self-photodegradation was almost negligible under visible-light irradiation in the absence of any catalyst. Evidently, P25 showed negligible photocatalytic activity in the degradation of AO7. However, all of the N-doped samples exhibited good catalytic efficiency for AO7 degradation due to the enhanced visible-light

10

adsorption. This is consistent with the photocatalytic mechanism of N-doped anatase under visible light reported in the literature [9,10]. Although pure NNT showed a certain photocatalytic effect (25%) for AO7 degradation, the photocatalytic activity (44%) of NOMT was greater than that of NNT because of its porous structure with a great number of “activated centers”, providing effective separation of electron–hole pairs [16]. The rate of AO7 decolorization was further accelerated by NOMT-CFs and NNT-CFs due to extensive adsorption of the dye molecules on the CFs, facilitating their photocatalytic decomposition by TiO2. Additionally, the photocatalytic activity of NNT-CFs was evidently lower than that of NOMT-CFs, mainly due to the mesostructured titania of the latter. Furthermore, the photocatalytic activity of the composite was higher than that of a simple mixture of the components because of the intimate contact between the CFs and NOMT, which is beneficial for electron transfer [42]. The suppression of exciton recombination in the presence of CFs is evidently also an important factor in the improved activity of titania besides the high adsorptivity of dye molecules, which showed good agreement with their PL intensities (see Fig. S5). On the basis of the above analysis and the relevant band positions of pure and N-doped TiO2, a photocatalytic mechanism for the action of NOMT/CFs is proposed, as illustrated in Fig. 5b. As mentioned above, the band gap of pure TiO2 is 3.2 eV, and excitation can only be achieved with UV light of wavelengths less than 388 nm. After N-doping, the band gap was obviously decreased and new impurity levels were introduced between the conduction and valence bands of TiO2. Therefore, NOMT/CFs has a narrower band gap with a change in the light absorption band from the near-UV to the visible-light range. The process of visible-light photocatalytic oxidation of AO7 is described as follows: N–TiO2 + h → hVB+ + eCB−

(2)

O2 + eCB− → O2•−

(3)

O2•− + H2O → •OH + OH−

(4)

hVB+ + OH− → •OH

(5)

•OH + AO7 → · · · → CO2 + H2O

(6)

Firstly, electrons and holes are generated under visible light irradiation. The electrons can then react with molecular O2 on the surface of TiO2 to form a superoxide anion radical (O2•−). This O2•− can then interact with adsorbed H2O to produce •OH radicals, the main species responsible for the degradation of pollutants, such as AO7 in the present case. Meanwhile, holes can be captured by 11

the surface OH− to also form •OH radicals. From the above equations, it can be inferred that OMT played an important role in preventing the recombination of photogenerated electron and hole pairs, because its ordered mesoporous structure provides a large number of “activated centers”. This increases the likelihood of electron-hole pairs on the surface of the catalyst reacting with adsorbed O2 and H2O, respectively, to form O2∙ and ∙OH. Additionally, CFs also facilitate channeling of electrons due to their graphite structure [5]. Hence, the separation rates of photogenerated charge carriers are effectively improved. Meanwhile, many dye molecules are adsorbed on the CF support, leaving them suitably predisposed in the photocatalytic environment, thereby enhancing the photocatalytic activity. To verify the practical applicability of NOMT/CFs, recycling experiments on the photocatalytic decoloration of AO7 solution were carried out under visible-light irradiation. As shown in Fig. 5c, the efficiency of photocatalytic decoloration was maintained without any significant decline after five cycles. Thus, NOMT/CFs displayed great potential for practical application in wastewater treatment using solar light. The excellent reusability of NOMT/CFs may stem from the good binding property between the NOMT layer and CFs. As shown in Fig. S6, two SEM images of NOMT/CFs after five cycles show no detectable differences in comparison with those of the corresponding fresh samples (Fig. 1b and c), which is beneficial for the stability of the photocatalytic performance. Additionally, the fibrous structure of the catalyst is very beneficial for its separation from the treated wastewater. To further examine the photoactivity of the obtained samples under visible light, the degree of AO7 mineralization was analyzed in the presence of various photocatalysts, as shown in Fig. 5d. Photodegradation of AO7 roughly followed pseudo-first-order kinetics. The rate constants for AO7 degradation with NOMT/CFs, NNT/CFs, NOMT-CFs, NNT-CFs, NOMT, and NNT were found to be 9.2, 4.1, 2.4, 1.3, 1.1, and 0.5×103 min1, respectively. Among all of these catalysts, NOMT/CFs show the highest photocatalytic activity for AO7 degradation under visible-light irradiation. This can be attributed to three synergistic effects between enhanced visible-light absorption of N-doping, high adsorptivity of dye molecules, and fast electron transport in CFs and effective separation of electron–hole pairs in OMT. 3.2.2 Possible pathways for conversion of AO7 In general, for TiO2 under UV irradiation, the oxidative activity of TiO2 particles is mostly attributed to species formed through reactions initiated by photogenerated electron–hole pairs, 12

such as ·OH, other peroxyl radicals, and valence-band holes [42]. The intermediates generated in the conversion of AO7 using NOMT/CFs under visible light were identified by GC–MS, as illustrated in Table 2. Additionally, a possible pathway for AO7 photocatalytic degradation by NOMT/CFs is proposed in Fig. 6. A representative GC–MS trace is also presented in Fig. S7. Overall, the photocatalytic degradation of AO7 by NOMT/CFs can be described in terms of a series of consecutive degradation steps. AO7 is firstly decomposed to aromatic intermediates, further oxidized to ring-opened products, and finally mineralized to CO2, H2O, and inorganic salts [43]. 3.2.3. Effects of the NOMT/CFs dosage The reaction rate, which is a function of catalyst content, is an important parameter [44]. It is well known that the absorption of light and adsorption of the reactant on the titania surface are limiting factors in such photocatalysis. For a certain light intensity, the amount of photons impinging on the photocatalyst is finite. At contents up to about 1 g L1 (or saturated adsorption of photons), the degradation rate increases with increasing catalyst content simply because more catalyst provides more activated centers and can adsorb more of the reactant. However, a further increase in catalyst dosage beyond 1 g L−1 may result in deactivation of activated molecules through collision with ground-state molecules. At dosages higher than 1 g L−1, NOMT/CF aggregation (particle–particle interactions) may commence, thereby lowering the effective surface area of the catalyst and thus the adsorption of the reactant. Additionally, the turbidity of the mixture is increased, which greatly reduces the amount of light transmitted through the solution.

3.2.3. Effects of pH Fig. 8 shows the variation in rate constant of AO7 mineralization with pH. Interestingly, the order of reaction rates was pH 7 > pH 9 > pH 5 > pH 11 > pH 3. Strong acid or alkali was not appropriate for decomposing AO7 because the amount of hydroxyl ion adsorbed on TiO2 is influenced by the pH [45]. At various pH values, the main pathways for producing •OH are probably different; therefore, the rates of •OH production are different. An increase in pH can be expected to increase the number of OH− ions on the TiO2 surface, and •OH could be formed when OH− combines with photogenerated holes [Eqs. (5) and (7)] [40]:

13

hVB + H 2 O(ads) → •OH + H+

(7)

In acidic solution, the photocatalytic degradation of AO7 is probably due to the formation of •OH according to the following reactions [Eqs. (3), (8)–(10)]:

 O2( ads) + H+ → HO2•

(8)

2HO2• → O2 + H2O2

(9)

H2O2 +  O2( ads) → •OH + OH− + O2

(10)

More efficient formation of hydroxyl radicals occurs in alkaline solution via the shorter route

[Eq. (7)]. On the other hand, AO7 has a sulfonate group in its structure, which is negatively charged under alkaline conditions; therefore, AO7 may not be effectively adsorbed on the photocatalyst surface in alkaline solution [46]. The above findings indicated that the most efficient photodegradation of AO7 probably occurred at neutral pH. 3.2.4. Effects of the initial AO7 concentration and kinetics of photocatalytic mineralization The efficiency of the mineralization process was found to decrease with increasing concentration of AO7 (Fig. 9). This may have been a result of blocking of the photocatalytically active sites [47]. The modified Langmuir–Hinshelwood kinetic model [37] is generally utilized for such photo-oxidation processes, and the degradation rate is expressed by the following equation: 1 1 C   0 K kr k S kr

(11)

where C0 is the initial concentration of the reactant (mg L−1), kr is the reaction rate constant (mg L−1 min−1), ks is the adsorption coefficient of the reactant (L mg–1), and K is the observed apparent pseudo-first-order rate constant (min−1). It was found that the Langmuir–Hinshelwood kinetic model [Eq. (11)] adequately describes the kinetic behavior of AO7 degradation by NOMT/CFs, because a linear plot of reciprocal of rate (1/kapp) against initial AO7 concentration (C0) showed a high R2 of 0.98. The Langmuir–Hinshelwood equation could also be applied for the degradation of AO7 by other catalysts, as shown in Fig. 9b. As indicated in Fig. 9b, all of the samples showed photocatalytic activity, and the rate constants kr and absorption equilibrium constants ks determined in this way were dependent on the nature of the N-doped titania and the synergy between CFs and titania. 14

Values of kr with NOMT were higher than those with NNT. This can be attributed to the fact that more pores are formed and more activated centers are located on the external surface of the catalyst, making them more accessible to the target substrate. Therefore, the better dye removal performance may be attributed to the porous structure of the titania, which is consistent with the variation in ks. By addition of CFs, both K and kr of mixtures are obviously increased and are much higher than those of pure N-doped titania. This may be due to the fact that the CFs serve to concentrate the AO7 molecules around the active sites and facilitate electron transfer. However, the positive impact of CFs on the photocatalytic efficiency greatly depends on the combined characteristics of CFs and titania. Although ks of composites is lower than that of mixtures, K and kr of the former were higher than those of the latter because close contact between CFs and titania is more beneficial to electron transfer and migration of the adsorbed AO7 to the active sites of titania. Additionally, the high photoactivity (K=9.2×103 min1, kr=0.015 mg L1 min–1) and adsorption capacity (ks=4.26 L mg–1) of NOMT/CFs can be attributed to more activated centers, faster electron transfer, and easier migration of the adsorbed AO7 as a result of the mesostructure and close contact. 4. Conclusions With the aim of effectively utilizing visible light and convenient application of the photocatalyst in water treatment systems, NOMT/CFs has been successfully fabricated by a combined method of supercritical deposition and liquid-crystal templating. The NOMT/CF composite showed high photocatalytic activity for AO7 degradation under visible-light irradiation due to three synergistic effects arising from effective separation of electron–hole pairs, enhanced visible-light adsorption, and high adsorptivity of dye molecules. Meanwhile, the strong binding between the NOMT layer and the CFs leads to excellent recyclability of NOMT/CFs in practical applications for wastewater treatment using solar light. A photocatalytic mechanism for the action of NOMT/CFs and a degradation pathway for AO7 have been proposed. Conditions of 1 mg L−1 AO7, pH 7, and 2 g L−1 NOMT/CFs were identified as optimal for AO7 degradation. The NOMT/CFs system has good potential for commercial application. The photocatalytic degradation processes can be described in terms of a modified Langmuir–Hinshelwood model for the surface reaction between the dyestuff and the oxidizing agent. The values of the adsorption equilibrium constants, ks, and the rate constants, kr, were certainly dependent on the catalyst porperties. For 15

NOMT/CFs with the highest rate constant, kr and ks were 0.015 mg L1 min–1 and 4.26 L mg–1, respectively.

References [1] S.Mamoru, M.Nicholas, A.Anne, L.Vincent, P.Cesar, B.Oualid, B.Paul, J.Mater. Res. 28 (3)(2013)354-361. [2] J.Choina, H.Kosslick, C.Fischer, G. U.Flechsig, L.Frunza, A.Schulz, Appl. Catal. B 129(5)(2013)589-598. [3] M. Faycal Atitar, Adel A. Ismail, S.A. Al-Sayari, Detlef Bahnemann, D. Afanasev, A.V. Emeline, Chem. Engineer. J., 264 (2015) 417–424 [4] M. M. Maitani, C.H. Zhan, D. Mochizuki, E. Suzuki, Y.J. Wada, Appl. Catal. B 140–141(2013)406-411. [5] P. Chen, L. Gu, X.D. Xue, M.J. Li, X.B. Cao, Chem. Commun., 46(2010)5906-5908. [6] N. Pugazhenthiran, S. Murugesan, S. Anandan, J. Hazard. Mater. 263(2)(2013) 541-549. [7] F.T. Li, Y. Zhao, Y.J. Hao, X.J. Wang, R.H. Liu, D.S. Zhao, D.M. Chen, J.Hazard.Mater.239–240(2012) 118-127. [8] X.W. Cheng, X.J. Yu, Z.P. Xing, Appl. Surf. Sci. 258(7)(2012)3244-3248. [9] S.Z. Hu, F.Y. Li, Z.P. Fan, J. Hazard. Mater. 196(2011) 248-254. [10] D. Dolat, N. Quici, E. Kusiak-Nejman, A.W. Morawski, G. Li Puma, Appl.Catal.B 115–116(2012)81-89. [11] F.D. Mai, W.L. W. Lee, J.L. Chang, S.C. Liu, C.W. Wu, C.C. Chen, J. Hazard. Mater. 177(1–3)(2010)864-871. [12] M.R. Bayati, A.Z. Moshfegh, F. Golestani-Fard, Appl.Catal.A 389(1–2)(2010)60-67. [13] M. A.David, Y. Y.Jackie, Angew Chem. 107(18) (995)2202-2206. [14] C. X.He, B. Z.Tian, J. L.Zhang, Microporous Mesoporous Mater. 126(1–2)(2009)50-57. [15] J. G.Yu, G. H.Wang, B.Cheng, M. H. Zhou, Appl. Catal. B 69(3–4)(2007)171-180. [16] D. S.Kim, S. J.Han, S.Y. Kwak, J. Colloid Interface Sci. 316(1)(2007)85-91. [17] T. C.An, J. K.Liu, G. Y. Li, S. Q.Zhang, H. J.Zhao, X. Y.Zeng, G. Y.Sheng, J.Fu, Appl. Catal. A 16

350(2)(2008)237-243. [18] Anders E.C. Palmqvist, Curr. Opin. Colloid Interface Sci. 8 (2003) 145-155. [19] K.Zimny, T.Roques-Carmes, C.Carteret, M. J.Stebe, J. L.Blin, J.Phys.Chem.116(11) (2012)6585-6594. [20] EP.Reddy, L.Davydov, P.Smirniotis, Appl. Catal. B, 42(1)(2003)1-11. [21] M.H. Baek, W.C. Jung, J.W. Yoon, J.S. Hong, Y.S. Lee, J.K. J. Ind. Engineer. Chem.19(2)(2013)469-477. [22] Sanly Liu, May Lim, Rose Amal, Chem. Engineer. Sci. 105(2014)46-52. [23] Sittidej Teekateerawej, Junichi Nishino, Yoshio Nosaka, J. Photochem. Photobiol. A, 179(3)(2006)263-268. [24] C.H. Lin, J.W. Lee, C.Y. Chang, Y.J. Chang, Y.C. Lee, M.Y. Hwa, Surf. Coat. Technol. 205(2010) S341-S344. [25] E. Taboada, I.Angurell, J. Llorca, J. Catal. 309(2014)460-467. [26] H. F.Guo, M.Kemell, M.Heikkil, M. Leskel, Appl. Catal. B 95 (3-4)(2010)358-364. [27] K. D. Nilay, J. Mol. Catal. A Chem. 337(1-2)(2011)33-38. [28] P.Fu, Y.Luan, X. Dai. P. F.Fu, Y.Luan, X. G. Dai, J. Mol. Catal. A 221(1)(2004)81-88. [29] G.H. Jiang, X. Li, Z.Wei, T.T. Jiang, X.X. Du,W.X.Chen, Powder Technol. 260 (2014) 84–89 [30] J. W.Shi, H. J.Cui, J. W.Chen, M. L.Fu, B.Xu, H.Y.Luo, Z. L.Ye, J. Colloid Interface Sci.388(1) (2012)201-208. [31] E.Aubry, J.Lambert, V.Demange, A.Billard, Surf. Coat. Technol. 206(23)(2012)4999-5005. [32] L.Axel, G.Thierry, P.Sébastien, B. R.Elisabeth, Appl. Catal. A 391(1-2)(2011)43-51. [33] Y.Q.Zhu, Q.Xie, F. J.Chen, X. F.Fan, Y. J. Feng, Sci Adv. Mater. 4(12)(2012)1191-1199. [34] W. X.Guo, F.Zhang, C. J.Lin, Z. L.Wang, Adv. Mater. 24(35)(2012)4761-4764. [35] X.W. Cheng, X.J. Yua, Z.P. Xing, Appl. Surf. Sci. 258 (2012)3244-3248. [36] Q. Peng, Y. Li, X. He, H. Lv, P. Hu, Y. Shang, C. Wang, R. Wang, T. Sritharan, S. Du, Compos. Sci. Technol. 74 (2013) 37-43. [37] L. Gu, J.Wang, H. Cheng, Y. Zhao, L. Liu, X. Han, ACS Appl. Mater. Interfaces 5 (2013)3085-3086. [38] N.C. Saha, H.G. Tompkins, J. Appl. Phys.72 (1992)3072-3079. [39] J.M. Du, G.Y. Zhao, Y.F. Shi, H. Yang, Y.X. Li, G.G. Zhu,Y.J. Mao, R.J. Sa, W.M. Wan, Appl. 17

Surf. Sci. 273 (2013) 278-286. [40]T.M. Breault, B.M. Bartlett, J. Phys. Chem. C 116 (2012)5986-5994. [41] M. E. Hassan, L.C. Cong, G.L. Liu, D.W. Zhu, J.B. Cai, Appl. Surf. Sci. 294(2014)89-94. [42] B.Wang, R.Karthikeyan, X.Y. Lu, J. Xuan, M.K.H. Leung, J.Hazard.Mater. 263(2013) 659-669. [43] H.Z. Zhao, Y. Sun, L.N. Xu, J.R. Ni, Chemosphere 78 (2010) 46-51. [44] J. Choina, H. Kosslick, C. Fischer, G.U. Flechsig, L. Frunza, A. Schulz, Appl. Catal. B 129 (2) (2013) 589-598. [45] N. Daneshvar, M.H. Rasoulifard, A.R Khataee, F. Hosseinzadeh, J. Hazard. Mater. 143 (1-2) (2007) 95-101. [46] A.Troupis, T.M.Triantis, E.Gkika, A.Hiskia, Appl. Catal. B 86(1-2)(2009)98-107. [47] A. Idris, N. Hassan, R. Rashid, A.F. Ngomsik, J. Hazard. Mater. 186(1)(2011)629-635.

18

f d101=0.35nm

2 nm

Fig. 1. SEM image of carbon fibers (a), SEM images of NOMT/CFs at low and high magnifications (b–d), TEM image of NOMT/CFs at low magnification (e) with selected-area electron diffraction (SAED) pattern and its high-magnification TEM image with the d-spacing (f).

19

Fig. 2. SAXS (a) and WAXRD (b) patterns of samples.

20

Fig. 3. XPS spectrum of the as-prepared NOMT/CFs (a) and its high-resolution XPS spectra of O1s (b), C1s (c), and N1s (d).

21

1.2

2.5

2.0

0.8 0.6

(Ahv)1/2

Absorbance (a.µ)

1.0

NOMT/CFs NNT/CFs NOMT NNT

0.4

1.5 NOMT/CFs NNT/CFs NOMT NNT

1.0

0.5

0.2 0.0 300

350

400 Wavelength

450

500

0.0 2.5

2.7

2.9

3.1

3.3

3.5

3.7

3.9

Photon energy hv (eV)

(nm)

Fig. 4. UV/Vis diffuse reflectance spectra of samples (a) and plots of (Ahv)1/2 versus photon energy (b).

22

1.

CB e-

NNT

8 0.

NOMT NNT-CFs

CFs O2• EV NOMT, O2p h+ OH-EV Anatase, h+ ∙OH O2p VB

NOMT-CFs

4 0.

a

NNT/CFs

2 0.

NOMT/CFs

0

3

6

9

12

15

18

0

0

0 Time 0 (min) 0 0

21

24

0

0

1.0

3. 1st cycle

2nd cycle 3rd cycle 4th cycle 5th cycle

Ln (TOC0/TOC)

0.6 0.4

02. 52.

0.0

50. 4

6

8

10

Time (hr)

12

14 16

18 20

NOMT/CFs

NNT/CFs

51. 00.

2

d

01.

0.2

0

y = 0.0092 x, R2 = 0.98 y = 0.0041x, R2= 0.99 y = 0.0024x , R2= 0.98 y = 0.0014x, R2= 0.98 y = 0.0011x, R2= 0.99 y = 0.0005x, R2 = 0.99

53.

0.8 c

O2 e-

2.8eV

UVlight

6 0.

A/A0

eEC

3.2eV

A/A0

Visible light

P25

0 0.

0

b

Photolysis

0

NOMT-CFs NNT-CFs NOMT NNT

0

7

14

0

0

21

28

Time0 (min) 0

35

42

0

0

Fig. 5. The photocatalytic degradation of AO7 aqueous solution under visible light; its color removal rates (a), catalytic mechanism (b), cycling runs (c), and mineralization degree (d) over NOMT/CFs; catalyst content = 1 g/L, pH 7, AO7 content = 1 mg/L.

23

Fig. 6. A possible pathway for the degradation of AO7 in the visible-light photocatalysis system by using NOMT/CFs as catalyst.

24

10

Rate constant K×103 (min–)

9 8 7 6 5

[AO7] = 1mg/L

4

pH = 7

3 0.5

1

2

3

NOMT/CFs (g/L)

Fig. 7. Effect of NOMT/CFs dosage on the degradation rate constant of AO7.

25

9 8 7 6

(min–1)

Rate

constant

K×103

10

[AO7] = 1mg/L

5

[NOMT/CFs]= 1 g/L

4 3 3

5

7 pH

9

11

Fig. 8. Effect of pH on the degradation rate constant of AO7.

26

10

500

9

450 y = 68.11x + 15.97 400

R2 = 0.98

7

350

6

300

5

250

[NOMT/CFs] = 1g/L pH = 7

4

1/Kapp (min–)

Kapp×103 (min–1)

8

200

3

150

2

100 01

2

3

4

5

6

7

AO7 concentration (mg/L)

b NNT-CFs

5

ks (L∙mg –1)

NOMT/CFs

16 14

NNT/CFs 4

12 10

NOMT-CFs 3 2

8 NNT

6

NOMT

kr ×103 (mg∙L1∙min–1)

6

4 1

2

0

0 Samples

Fig. 9. Effect of initial AO7 concentration on kapp and 1/kapp (a) and the ks and kr values of different samples (b).

27

Table 1. An overview of the texture parameters of NOMT and NNT supported on CFs in comparison with those of pure NOMT and NNT and their physical mixtures with CFs. Content of Surface Total pore Average pore Crystallite Band-gap Samples

size (nm)d

energy (eV)e

1.4

10.2

2.84

–

–

21.8

–

125

0.02

2.5

18.5

2. 84

6.4

473

0.05

1.7

16.4

2.90

6.4

1245

–

–

18.5

–

100

53

0.04

2.2

21.8

2.90

TiO2

area

volume

diameter

(wt.%)a

(m2/g)b

(cm3/g)c

(nm)c

6.4

756

0.06

6.4

1080

NOMT

100

NNT/CFs NOMT+

NOMT/C Fs NNT+CF s

CFs NNT a

TiO2 content was calculated by TG with an error of less than 1%.

b

The BET surface area was determined by the multipoint BET method.

c

The total pore volume and average pore size were calculated by the BarrettJoynerHalenda

method (adsorption branch). d

The average crystallite size of TiO2 was determined by XRD using the Scherrer equation.

e

The band-gap energies were calculated using the equation: (Ahv)1/2 = hv-Eg.

28

Table 2. Reaction intermediates of AO7 degradation identified by GC–MS. Identified reaction Photocatal Identified

Photocatal

Compound

reaction

ytic

intermediates

(min)

intermediates

ytic

time Compound

(min) ·

sulfanilic acid

1-amino-2-naph thol

O3S

N H2

25

50

p-benzoquinone

75

2-naphthalenol

100

1,2-naphthalene dione

ninhydrin

2-formylbenzoi c acid

hexylacetic acid

time

125

150

175

200

29