Dual functional N-doped TiO2-carbon composite fibers for efficient

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Dual functional N-doped TiO-carbon composite fibers for efficient removal of water pollutants Jinju Zhang, Lei Li, Yanxiang Li, Wangliang Li, and Chuanfang Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02264 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Sustainable Chemistry & Engineering

Dual functional N-doped TiO2-carbon composite fibers for efficient removal of water pollutants Jinju Zhanga, b, c, Yanxiang Lia, b, Lei Li a ,b, Wangliang Lia, b and Chuanfang Yang a, b,* a

State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese

Academy of Sciences, Beijing 100190, China b

CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese

Academy of Sciences, Beijing 100190, China c

University of Chinese Academy of Sciences, Beijing 100049, China

* Corresponding author Tel/Fax: +86 10 82544976 E-mail addresses: [email protected] (J. Zhang), [email protected] (Y. Li), [email protected] (L. Li), [email protected] (W. Li), [email protected] (C. Yang)

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ACS Paragon Plus Environment

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Abstract 3D hierarchically structured porous carbon composite short fibers were prepared by carbonizing in N2 and activating in CO2 micro-cellulose fibers originally tethered with nano-TiO2. The TiO2 nanoparticles grew in situ on cellulose surface and inside-out the body of cellulose during their original hydrolysis synthesis process assisted by microwave irradiation, creating sufficient adsorption sites of hydroxyl groups, and exposing abundant interfaces after heat treatment. The obtained carbonized fibers displayed rapid adsorption of methylene blue, phenol and Cr (VI) and exhibited high adsorption capacity, outperforming commercial activated carbon powders having 50% more surface area. Moreover, the composite effectively photo-catalyzed the oxidation of phenol and the reduction of Cr (VI) under both UV-vis and visible light. The enhanced photo-catalytic activity was believed to be due to (1), the doping of N atoms to the TiO2 crystal lattice resulting from the formation of Ti3+ and oxygen vacancies during the carbonization process; (2), the interfaces formed between TiO2 and the carbon, which promotes charge transfer and inhibit electron-hole recombination. The dual-functional TiO2-carbon composite fibers made by this simple approach with many naturally endowed properties, can also be repeatedly used after regeneration, opening up a sustainable opportunity for their versatile applications for both water and air decontamination.

Keywords: N-doped TiO2; Cellulose; Activated carbon fiber; Adsorption; Photocatalysis

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Introduction Water environment safety has attracted global attention in recent years, and the problem is expected to last for decades. Industrial processes generate poisonous wastewaters containing organic chemicals and heavy metals that have detrimental effect on human health and ecosystems,1 and removing these contaminants more effectively with novel materials and technologies is highly desired. Up till now, technologies such as adsorption,2 photo-catalytic degradation,3,4 membrane separation,5 chemical oxidation6 and electro catalysis7 have all been practiced for contaminant treatment. Adsorption is often preferred for its low cost and high removal efficiency.8 However, drawbacks do exist for thorough adsorption of difficult contaminants such as phenol and chromium (VI).

Photo-catalytic

degradation

has

long

been

recognized

as

an

efficient

and

environmental-friendly technology for decomposition of organic compounds and reduction of Cr (VI) to the less toxic Cr (III).9 A material that can act as both a good adsorbent and a photo-catalyst is expected to greatly enhance the removal efficiency of pollutants. Titanium dioxide (TiO2) nanoparticles (NPs) are such materials that have been extensively studied to decontaminate wastewater.10,11 However, as a photo-catalyst, their applications are limited due to rapid electron-hole recombination rate, low harvest of visible light and difficulty of recovery. To overcome these drawbacks, research works have been carried out to modify TiO2.1,12 Due to their outstanding properties such as high surface area, good conductivity, thermal and mechanical stability, carbon based materials are used as good adsorbents and catalyst supports.13 The combination of TiO2 with porous carbon materials bears synergistic effect, making the composite multifunctional and value-added to find applications in both photocatalysis and 3



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energy-storage. Many efforts have been devoted to immobilizing TiO2 NPs onto carbon-based materials to improve the adsorption and photo-catalytic performance.14-17 The synthesis of TiO2-N-doped carbon composites18 as well as TiO2-anchored activated carbon (AC) were respectively reported,16 in which it was found that pollutants were adsorbed by the carbon firstly, then released to the surface of TiO2 for enhanced degradation. A common approach to extend TiO2’s visible light response is to incorporate with it other elements,18-21 and to this end, N-doping has attracted much attention. According to literature, nitrogen can reduce the band gap of TiO2 either by mixing N 2p states with O 2p states on top of the valence band,

22

or by creating a N-induced

mid-gap level.23 In the report by Zhou et al. regarding the Ti3+/N-co-doped 3D urchin-like black TiO2, the as-prepared samples reduced the band gap of TiO2 from 3.20 eV to 2.43 eV, which significantly enhanced their photo-catalytic activity. In fact, the co-doping of N and Ti3+ exerts a synergistic effect in narrowing the bandgap of TiO2 to a large extent.24 In this work, TiO2-tethered carbon composite fibers were fabricated by in situ growing nano-TiO2 crystals on cellulose fibers, followed by carbonization and activation of the formed composite fibers. TiO2 nanocrystals were quickly synthesized and tethered on the big scaffolds-cellulose fibers by hydrolysis of titanium oxysulfate under microwave irradiation. After carbonization and activation treatment of the TiO2-cellulose composites, porous carbon-TiO2 fibers were obtained with hierarchical structure and high surface area. Meanwhile, N atom was naturally and successfully incorporated into TiO2 lattice to form the N-Ti-O bond, accompanied with the formation of Ti3+ and oxygen vacancies during the carbonization process under nitrogen atmosphere. These consequently elevated the separation of photo-generated electrons and holes,25 resulting in enhanced photo-catalytic activity of the TiO2 for the oxidation of phenol and reduction of Cr (VI). In addition, the TiO2-carbon composite fibers’ high specific surface area and large pore volume lead 4


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to much faster adsorption rate and higher adsorption capacity of MB, phenol and Cr (VI), as compared to commercial activated carbon powders. The possible mechanisms associated with the enhanced adsorption and photo-catalytic properties of TiO2-carbon composite fibers were therefore analyzed. It should be noted that carbon-supported TiO2 and N-doped TiO2 are well studied in literature for photocatalytic applications. However, the strategies or routes that people take to make such materials are dramatically different. In this work, we grew TiO2 in-situ first on cellulose fibers through a facile hydrolysis process with the aid of microwave irradiation to obtain a mesoporous surface structure consisting of nano-TiO2 particles. By doing so, TiO2 are better attached to the cellulose surface to begin with as the raw material for further carbonization and activation. The final products obtained are naturally doped with nitrogen during the process without the need to use other non-benign nitrogen sources such as ammonia. In addition, we have proved that Ti3+ is also resulted from the process due to the immediate availability of carbon and the presence of hydrogen formed due to the decomposition of cellulose. The conversion of Ti4+ to Ti3+ again happens naturally, which eliminates the need of using other strong reducing agent such as sodium borohydride. Consequently, the simple one-step process endows the final materials with many desired properties for both adsorption and photo-catalysis, an obvious distinction from other approaches reported thus far, and an illustrative example of a close-loop technology development effort with sustainable chemistry and engineering, from materials design conception, to simple process control and the materials end use targeting a better environment.

Experimental Section Materials 5



10~30 µm diameter UF500 cellulose fibers with cutting length of 3 mm were provided by Beijing Ronel Engineering Materials Company. Titanium (IV) oxysulfate solution was purchased from Aldrich. To determine Cr (VI) concentration on a UV-vis spectrophotometer, guaranteed reagents including diphenylcarbazide, potassium dichromate, sulfuric and nitric acid were used and acquired from Sinopharm Chemical Reagent Co. Ltd (China). Phenol and methylene blue used as the simulate pollutants were analytical grade, obtained from Beijing Chemical Works, China.

TiO2-cabon composite fabrication In a general procedure as illustrated in Figure 1, TiO2 NPs were uniformly grown on cellulose by the hydrolysis of titanium oxysulfate in a single pot under microwave irradiation according to our previous report (Figure 1a).19 The obtained TiO2/cellulose composites were put in a tube furnace and calcined under nitrogen atmosphere at 800 ºC (heating rate 5 ºC/min) for 2 h, resulting in the carbonized TiO2-cellulose composite designated as TiO2-C-N (Figure 1b). In the carbonization process, the nano-T iO2 crystal planes would further re-crystallize and grow, forming irregular nano-sheet aggregates. Fourth, switching N2 to CO2, the TiO2-C-N was subsequently activated in the tube furnace at 850 ºC for 2 h with gas flow rate of 30 mL/min, yielding the final product of TiO2-tethered carbon composite fibers named as TiO2-C-A, as shown in Figure 1c.

Figure 1 Schematic illustration of TiO2-C-A fabrication process 6


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As the control experiments, pure cellulose fibers were carbonized and activated under the identical conditions to produce carbonized and activated fibers, which are donated as C-N and C-A, respectively.

Characterization Field emission scanning electron microscopy (FE-SEM, JSM-6700F, Japan) was used to characterize the prepared samples. Transmission electron microscopy (TEM, JEM-2100F, Japan) and high resolution transmission electron microscopy (HR-TEM) were also used to identify the fine features of the materials. TEM samples were prepared by dispersing 20 mg TiO2-C-A composite in 30 mL ethanol with vigorous stirring for 10 h; then a few drops of the dispersion were taken and put onto a copper grid coated with carbon, then the sample was dried in air before observation with the TEM.

The

specific

surface

area

of

the

materials

was

determined

by

using

the

Brunauer-Emmett-Teller (BET, Autosorb-IQ-MP, America) method conducted at liquid nitrogen temperature (77 K), and the pore volume was determined by the Barrett-Joyner-Halenda (BJH) method. Thermal behavior of the materials from room temperature to 800 °C at a heating rate of 5 °C/min was tested thermogravimetrically (TGA, TG-DTA6300, Japan), and nitrogen was used as the carrier gas. The micro gas chromatography (Mico-GC, INFICON, 3000) was used to analyze the gas content from the pyrolysis of cellulose. The electron paramagnetic resonance (EPR, E500, Bruker) spectra of the synthesized materials were recorded at 77 K with an EPR spectrometer working in the X-band. X-ray diffraction photometer (XRD, Smartlab, Japan) was used to analyze the materials’ crystallinity, and the instrument was operated at 45 kV and 200 mA using a scanning rate of 5 °/min under CuKα irradiation. Surface information of the materials including defect and oxidation state analysis was obtained with X-Ray photoelectron spectroscopy (XPS, ESCALAB 7



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250Xi, America). The C1s peak corresponding to 284.8 eV was used to calibrate and correct the final binding energy scale. The fibrous composite samples were also analyzed using Fourier transform infrared spectroscopy (FT-IR, T27-Hyperion-Vector22, Bruker) to determine their chemical changes. To do so, the dried samples were mixed with KBr powder and pelletized, and the analysis was conducted at a spectra range of 400-4000 cm-1. The UV-vis diffuse reflectance spectra (DRS, Varian Cary 5000, USA) of the prepared samples were also obtained, and BaSO4 was used as the reference sample, the recorded range of the spectra was 200-800 nm. The concentration of phenol byproducts was analyzed with high performance liquid chromatography (HPLC, Agilent Technologies 1260 Infinity II,USA) equipped with an Eclipse Plus C18 column and a UV-Vis detector, and the mobile phase used was a mixture of acetonitrile and water (50%/50%, v/v).

Adsorption experiments To investigate the adsorption capacity of the prepared samples, the simulated pollutants of a cationic dye (MB), an organic contaminant (phenol) and a heavy metal (Cr (VI)) with different concentrations were used, respectively. The adsorption experiments were carried out at neutral pH as follows: the adsorbent material (20 mg) was added to the simulated pollutant solution (40 mL) stirred magnetically at 25 °C in dark. At regular time intervals, 1 mL sample was withdrawn with a syringe and the entrained solid adsorbent, if any, was removed by a 0.22 µm nylon syringe filter. The filtrate was then analyzed on a UV-Vis spectrophotometer (Lab Tech Co., Ltd) to evaluate the adsorption capacity. The absorbance values of MB, phenol and Cr (VI) were recorded at a wavelength of 665, 270 and 540 nm, respectively. The amount of the pollutants adsorbed Q (mg/g) at a certain time t was calculated using Equation (1). Q = (C 0 - C e )V / M

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Equation (1)

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where C0 (mg/L) and Ce (mg/L) are the initial and time dependent concentrations of the pollutant in the solution, respectively; V (mL) is the volume of the solution and M (g) the total mass of the adsorbent.

Photo-degradation experiments The photo-catalytic activity of the TiO2-tethered carbon composite fibers was assessed by photo-degradation of phenol, MB dye, and photo-reduction of Cr (VI) solution. The experimental setup and procedure can be found elsewhere as follows.26 A 300W Xenon lamp (CEL-NP 2000, Au light Co. Ltd., China) was used as the light source. A typical photo-catalytic process was conducted at 25 oC under UV-visible light (> 365 nm) and visible light (> 420 nm) irradiation, and 40 mL mixtures of TiO2-tethered carbon composite fibers (0.02 g) with phenol, MB or Cr (VI) solution were added to the reactor. Before the photo-catalytic reaction, the reactor was put in dark and stirred magnetically to establish adsorption/desorption equilibrium. After that, a single UV-vis light (> 365 nm) or visible light (> 420 nm) from top of the reactor was allowed to shed on the mixture. The distance between the bottom of the lamp and the surface of the mixture was 10 cm. 1 mL sample was drawn from the reactor at different times for analysis. The sample was filtrated through a 0.22 µm nylon syringe filter and the filtrate was collected. It was then analyzed to determine the remaining pollutant concentration. As the control experiments, photo-degradation or reduction of phenol, MB and Cr (VI) by the TiO2-free active carbon composite fibers, a commercial active carbon powder and commercial P25 TiO2 powder was also performed at the same experimental conditions.

Results and Discussion Synthesis and characterization of TiO2-cabon composite (TiO2-C-A) 9



The fabrication procedure of TiO2-carbon composites is illustrated in Figure 1. Cellulose fibers were chosen as a self-template to produce the fibrous carbon materials anchored with TiO2 NPs. Comparing with the original TiO2-CF composite (Figure S1), the resultant TiO2-C-A composite exhibits a porous morphology with the original structure of the inherent cellulose microfibers maintained as their short fragments, as is shown in the SEM of Figure 2a and its inset. Meanwhile, as shown in Figure 2b, fine TiO2 NPs of 41 nm uniformly attach to the surface of the carbon composite fibers. TEM further confirms the porous fibrous structure of the nano-TiO2 tethered carbon composite fibers (Figure 2c), and irregular pores with diameter of approximately 30-60 nm can be observed. The HR-TEM micrograph of the TiO2-C-A in Figure 2d displays clear lattice spacing ascribed to the planes of rutile (110) and anatase TiO2 (101), and the corresponding fringe distance is measured to be 0.325 nm and 0.352 nm, respectively.

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Figure 2 (a and b) SEM images of TiO2-C-A. (c) TEM image of an individual TiO2-C-A composite short fiber. (d) HR-TEM image of the lattice spacing of TiO2 on the activated carbon fiber surface.

To understand the crystal phase of the TiO2-carbon composite formed, XRD analysis was conducted and the diffraction patterns of the prepared TiO2-C-N and TiO2-C-A are given in Figure 3. The peaks at 25.3o, 37.8 o, 48.0 o, 53.9o, 55.1o and 62.7o correspond to the (101), (004), (200), (105), (211) and (204) reflection planes of anatase TiO2, while the peaks at 27.4o, 36.1o, 41.2o, 44.1o and 54.3o are indexed to the (110), (101), (111), (210) and (211) planes of rutile TiO2, respectively, consistent with the standard spectrum (JCPDS# 21-1272 and 21-1276). It can be seen that both the carbon composites show two kinds of crystal phases but with different phase ratios. The anatase/rutile molar ratio can be determined by the Spurr and Myer’s method using diffraction intensity of anatase (101) plane and that of rutile (110) plane. The calculated mass ratio is 85/15 and 21/79 for TiO2-C-N and TiO2-C-A, respectively, indicating further heat treatment in CO2 atmosphere leads to crystal phase transformation from anatase to rutile. No peaks corresponding to graphite carbon can be evidently identified in the XRD pattern, indicating the carbon in the composite is amorphous.19

Figure 3 X-ray diffraction patterns of TiO2-C-A and TiO2-C-N composites

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The pyrolysis process of TiO2-CF under nitrogen and TiO2-C-N under CO2 was respectively analyzed with TGA. As shown in Figure 4a, the composite of TiO2-CF shows a 3-stage weight loss under N2 atmosphere. The first stage is from room temperature to 100 oC, in which adsorbed water evaporates leading to a small weight loss of about 4.2 wt%.27 The second stage ( 250 to 350 oC) has a significant weight loss of 74 wt%, indicating the carbonization process of cellulose that undergoes dehydration, depolymerization and decomposition.28 The third stage from 350 to 800 oC causes a weight loss of only 6.5 wt%, which is a result of carbon residue degradation, as well as TiO2 crystal phase transformation. The heat transformation process of TiO2-C-N under CO2 atmosphere is shown in Figure 4b, where the total weight loss is found to be about 22.8%, which indicates that activation process results in less weight loss. From these results, the yield of the active TiO2-tethered carbon is estimated to be around 13 wt %.

Figure 4 (a) TG curve of TiO2-CF composite calcined in nitrogen atmosphere (b) TG curve of TiO2-C-N composite calcined in CO2 atmosphere.

The specific surface area, average pore size and pore volume of the obtained samples were determined by the measured nitrogen adsorption-desorption isotherms. The results are shown in Table 1. The CO2 activated samples of C-A and TiO2-C-A possess higher surface area than that of the non-activated C-N and TiO2-C-N composites, indicating CO2 activation process has induced the 12


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generation of new pores, as evidenced by the increased pore volumes listed in Table 1. However, regarding all the activated carbon materials of interest here, the surface area of TiO2-C-A (1097.8 m2/g) is lower than that of C-A (1275.1 m2/g) and the commercial AC (1642.3 m2/g) without TiO2. Although there exist mesopores among the TiO2 NPs, the particles may block some pores of the substrate carbon derived from cellulose that offsets the surface area contribution by the pores created by the bridging of the particles. All the adsorption-desorption isotherm curves shown in Figure 5 are of type IV, indicating the mesoporous nature of the carbon materials. In addition, the pore size distribution of the samples further verify the existence of the mesoporous structure. As shown in Figure 5b, the pore size is mainly concentrated in the mesopore range of 2-10 nm, which is in accordance with the type (IV) nitrogen adsorption-desorption isotherms curves shown in Figure 5a for all the samples. Such porous network structure gives rise to more transport channels for adsorption of organic/inorganic pollutants to the interior of the material for more interfacial contact, which benefits both adsorption and catalysis.

Figure 5 (a) The nitrogen adsorption-desorption isotherms and (b) the pore size distribution of TiO2-C-A, TiO2-C-N, C-N, C-A and AC

Table 1 BET surface area, pore volume and average pore size of TiO2-C-N, TiO2-C-A, C-N, C-A and AC 13



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Sample

BET surface area (m2/g)

Pore volume (cm3/g)

Average pore size (nm)

TiO2-C-N

501.8

0.29

6.61

TiO2-C-A

1097.8

0.63

3.41

C-N

379.8

0.19

3.07

C-A

1275.1

0.84

3.39

AC

1642.3

1.50

3.39

XPS analysis was conducted to examine the chemical bonding and surface composition of C-A and TiO2-C-A samples, and the result is shown in Figure 6. Signals of C and O elements are displayed in the spectra survey for both samples, and the characteristic peak of Ti and N elements can be found for TiO2-C-A (Figure 6a). Considering the possible existence of sulfuric species in the samples, the S 2p peak signals of TiO2-C-A was recorded with XPS, and the result is displayed in Figure S2. It is seen that the binding energy peak is negligible, indicating the absence of S on the sample. The XPS spectrum of the C 1s peak is given in Figure 6b, which can be deconvoluted to three peaks positioned at 284.7, 286.0 and 287.5 eV, corresponding to C=C-C, C-OH and C=O, respectively.29 The surface group peak ratio of C-OH/C=O for TiO2-C-A is 0.69 in the semi-quantitative area fashion, while that of C-A is 0.19. It indicates that the TiO2 tethered carbon composite is more enriched with -OH. The O 1s spectra for samples of TiO2-C-A and C-A could be divided to three peaks concentrated at 530.7, 531.7 and 532.8 eV (Figure 6c), respectively,30 among them the peak of 532.8 eV could be originated from surface oxygen in the C-O bond of the carbonized fibers. The peaks at 530.7 and 531.7 eV can be assigned to oxygen O2- group in the TiO2 crystal phase, and surface OH groups binding with Ti atoms and the C=O bond, respectively.31 In addition, compared with TiO2-C-A, the O 1s spectra for C-A show a relatively low intensity, which 14


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could be attributed to the lower oxygen content on the surface of the activated carbon material. The main peak centered at 532.8 eV for C-A is mainly contributed by the formed C-O bond. Simultaneously, the peak at 530.7 eV of TiO2-C-A is mainly characteristic for Ti-O bond on the surface. The surface species involving N were obviously detected as shown in the N 1s spectra in Figure 6d. The N 1s binding energy peak of TiO2-C-A is broad and strong, stretching from 397.4 to 403.7 eV; however, the peak for C-A is ignorable, which is an evidence of the absence of N on the C-A surface. For TiO2-C-A, three peaks at 401.3, 400.8 and 399.6 eV can be deconvoluted, respectively. The peaks at 401.3 and 399.6 eV are assigned to substitutional N (O-Ti-N),32-34 the third major peak at 400.8 eV is ascribed to interstitial N (Ti-O-N),35 suggesting the presence of multi-forms of binding energy of N atoms in the environment. The N-doping might be attributed to two reasons: first, the pyrolysis of TiO2-cellulose composite in the tube furnace during carbonization releases H2 (confirmed by the micro gas chromatography analysis of the collected gas products from the carbonization process shown in Table S1), which acts as a reducing agent to convert Ti4+ to Ti3+ and generate the oxygen vacancies. Then the N atoms diffuse into the amorphous shell of TiO2, taking these oxygen vacancies to form the N-doped TiO2 (TiO2-N).36 Second, the high temperature carbonization process can convert the substrate cellulose fibers to carbon (C) that will also play as a reducing agent to reduce Ti4+ to Ti3+, and then form the Ti-N bond under N2 atmosphere by the carbothermal reduction process.37 And as a result, nitrogen is introduced into TiO2 lattice forming N-Ti-O and/or Ti-N-O bonds. The N incorporation was further confirmed by Ti 2p XPS, two similar chemical states of TiO2 are separated after deconvolution of Ti 2p spectra of TiO2-C-A and P25 (Figure 6e). The typical Ti 2p spectrum of pure TiO2 (P25) is fitted to two peaks at 458.9 and 464.6 eV corresponding to the binding energy of Ti4+ 2p2/3 and Ti4+ 2p1/2, respectively. While for the activated sample of TiO2-C-A, an obvious shift of 0.5 eV to lower 15



binding energy for both the Ti spectra is observed. This could be because of the strong interaction between the host ion Ti4+ and nitrogen, which implies the possible lattice changes of TiO2.38 In addition, four peaks cam be fitted for Ti 2p spectrum (TiO2-C-A) as shown in Figure 6e, corresponding to the binding energy of Ti4+ 2p1/2, Ti3+ 2p1/2, Ti4+ 2p3/2 and Ti3+ 2p3/2 at 464.2, 463.2, 458.5 and 457.8 eV.39,40 In order to confirm the above results, EPR spectra were used to detect the Ti3+ and oxygen vacancies in the sample of TiO2-C-A, and the result is shown in Figure S3. A strong EPR signature was detected for this sample at g = 2.003, which could be assigned to the unpaired electrons trapped by oxygen vacancies (Ov),41 and which is usually assigned to a species containing a nitrogen atom trapped in TiO2 lattice.42 The redox couple (Ti4+ + e →Ti3+; O2- - e →O-) is always produced by the Ov as the neighbor to Ti3+ defects , thus this appearing signal is an indication that Ti3+ defects exist in the bulk of the sample.43,40 Such a result further indicates the development of Ti3+ species is originated from the redox reaction of TiO2 and H2 (or C) during the high temperature treatment process of the raw material. Therefore, it can be concluded that during the carbonization process under nitrogen atmosphere, N atoms not only partially fill the oxygen vacancies in TiO2 as oxidized nitrogen in the form of Ti-O-N linkage, but also create O-Ti-N bond by getting into TiO2 lattice as anionic nitrogen. With such nitrogen doping, TiO2’s photo-catalytic performance is expected to be enhanced under visible light. Figure 6f displays the FTIR spectra of the carbon materials at 400-4000 cm-1. The broad peaks near 1625 cm-1 and 3300 cm-1 presented in both C-A and TiO2-C-A are attributed to stretching vibration of O-H bond and adsorbed water molecules,44 which has been confirmed by the TGA measurement in Figure 4a. For the spectrum of TiO2-C-A, the band at 1638 cm-1 is associated with the bending vibration of surface hydroxyl groups (Ti-OH).42 The peaks at 400-800 cm-1 are ascribed to bending vibrations of Ti-O bonds for TiO2-C-A.32 Furthermore, the small adsorption peaks at 16


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1045 cm-1 for TiO2-C-A due to the low doping concentration of N can be assigned to the vibration of the N-Ti-O bond,43 which further verifies the XPS results that N is doped into TiO2 lattice.

Figure 6 (a) XPS spectra of TiO2-C-A and C-A. (b) The C 1s spectra comparision of TiO2-C-A and C-A. (c) The 17



O 1s binding energy comparision of TiO2-C-A and C-A. (d) The N 1s peak comparision of TiO2-C-A and C-A. (e) The Ti 2p spectra of TiO2-C-A and pure TiO2. (f) FTIR spectra of TiO2-C-A and C-A.

Adsorption Adsorption of MB The adsorption performance was investigated by using MB, a typical cationic dye, as a simulate contaminant. The batch adsorption experiments were all conducted at 25 ºC in dark and the adsorbent dose was 0.5 g/L. It is always desired the adsorbent removes the target pollutant as quickly as possible. In this work, five kinds of adsorbents including TiO2-C-A, C-A, TiO2-C-N, C-N and the commercial AC were used to adsorb MB with initial concentration of 200 mg/L. The influence of contact time on the adsorption was investigated. As shown in Figure 7a, the adsorption preceded rapidly, and all the adsorption curves present three stages (Figure 7a). In the first stage (0-2 min), the adsorption happened rapidly and a sharp adsorption capacity increase was observed, especially in the first 20 seconds. Then the adsorption became slower in the second stage (2-40 min) until an approximate plateau was reached at 80 min (the third stage), indicating adsorption equilibrium was reached. Accordingly, the subsequent adsorption experiments for MB were all conducted for 80 min. As shown in Figure 7b, the adsorption kinetics curves fitted with pseudo-second kinetic model43 quite well with correlation coefficient R2 >0.998. The related fitting parameters were calculated and listed in Table S2. In addition, it is worth noting that TiO2-C-A exhibits the highest adsorption rate and adsorption capacity, which is mainly attributed to its porous carbon structure and the electrostatic interaction between the rich hydroxyl groups on the surface of TiO2-C-A and the organic pollutant. The result also implies that surface area is not the only determining factor for the adsorption 18


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behavior observed. Langmuir and Freundlich isotherm models45, 46 were investigated to describe the adsorption. Langmuir adsorption fitting curves for MB are shown in Figure 7c together with Freundlich fittings, and the corresponding fitting data are provided in Table S3. The adsorption capacity increases with increasing initial MB concentration. Based on the fitting curves and correlation coefficients, Langmuir isotherm shows a better fit for the adsorption of MB onto the as-prepared carbon materials, suggesting monolayer adsorption. The maximum adsorption capacity of the five adsorbents follows the order of TiO2-C-A > C-A > AC > TiO2-C-N > C-N, indicating the benefits of activation treatment of the carbon materials to gain more surface areas. In addition, the hydroxyl groups on TiO2 nanoparticles offer more active sites and accelerate the adsorption process. As a result, the maximum adsorption capacity is about 100 % larger than the commercial AC whose surface area is 50 % higher. The performance of C-A is also better than the commercial AC surprisingly. The value of the Freundlich parameter 1/n (between 0 and 0.5) for TiO2-C-A, C-A and AC also indicates the favorability of the occurred adsorption.47

19



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Figure 7 (a) Adsorption equilibrium curves of MB (initial concentration of 200 mg/L) with various adsorbents. (b)

Pseudo-second order kinetics plots and fitting lines for MB adsorption. (c) Langmuir and Freundlich adsorption isotherm fittings for MB adsorption.

Furthermore, a performance comparison is made between TiO2-C-A and other adsorbents reported in literature for MB removal and the result is shown in Table 2. It is very encouraging to see that the material developed in this work outperforms all the others in both adsorption capacity and the rate of adsorption. The adsorption is at least 25% faster, and the adsorption capacity is at least 43 % higher, indicating the huge advantage of the current material for MB removal.

Table 2 Comparison of the adsorption performance of different adsorbents for MB

Adsorbent

Qm (mg/g)

Equilibrium time (min)

Reference

TiO2-C-A

432

40

This work

Biomass-derived activated carbon

303

60

48

Amine-functionalized UiO-66

204

50

49

Active carbon

270

>500

50

Polyacrylonitrile fiber treated

161

60

51

20


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Regeneration Based on the above adsorption results, we further investigated the cycling capacity of the TiO2-tethered active carbon composite fibers by using 0.2 M 40 mL HCl as the de-sorption reagent for MB. The desorbed material was then used to adsorb MB (200 mg/L) again. As shown in Figure S4, the adsorption capacity of TiO2-C-A shows little reduction (maintains over 92%) after 3 cycles of adsorption-desorption, suggesting the material’s good potential for repeated uses.

Photocatalytic Performance Optical band gap analysis UV-Vis spectra were recorded to investigate the optical photo absorption and band gap of P25, TiO2-C-N and TiO2-C-A with integrating sphere of 200-800 nm. As shown in Figure 8a, all the samples exhibit strong UV absorption and the absorption edge appears at ～400 nm. This exactly conforms to pure TiO2 light adsorption characteristics. However, the N doped TiO2-carbon composites exhibit a wider and stronger absorption of visible light around 500 nm. As reported in literature,52 N and Ti3+ doping can improve TiO2 valence band edge that induces visible light absorption as a result. Compared to the pure TiO2 (P25), it is observed that TiO2-C-N and TiO2-C-A obviously have better light absorption in both the UV light and visible light regions, attributed to the N/Ti3+ doping and the existence of oxygen vacancy. In addition, the color of a material is also a related factor to judge its photo-absorption ability.44 We observed that after carbonization and activation treatment, the white colored composite changed to black for TiO2-C-N and light black for TiO2-C-A, an indication of better photo-absorption. The bandgap was determined and the result is shown in Figure 8b. The values presented are surprisingly interesting, although they may not indicate exactly the actual rigid shift of the valence 21



band (VB) or the conduction band (CB) of TiO2, they can be explained as a consequence of electronic transitions, from the elevated valence band to the lowered conduction band.53 For the pure TiO2 (commercial P25), the band gap value is 3.20 eV.54 However, in our case, the possible Urbach tail absorption contributes significantly to the enhancement of the absorption of photons (known as sub-band gap adsorption) with energies lower than the intrinsic band gap.55,56 For the N/Ti3+ doped TiO2-carbon composite, because of the formation of N-Ti-O bond, N atom in the TiO2 lattice reduces the electron density due to the substitution of the highly electronegative O atom in the intial O-Ti-O structure, which induces the valence band shift. Meanwhile, the existence of Ti3+ that changes the conduction band to the lower energy edge, will couple with the sensitization effect of the carbon substrate,57 to form a new energy band level that further promotes the light absorption capacity in the visible region. The new formed energy level (known as mid gap states) is found to be as low as 2.32 eV and 2.12 eV for TiO2-C-N and TiO2-C-A, respectively, as shown in Figure 8b. Similar results were also reported in literature regarding low band gap black colored Ti3+/ N-doped TiO2.53 Additionally, the narrower band structure may be caused by the crystal phase transformation from anatase to rutile. Previous study suggested that the increase in calcination temperature would result in phase ratio increase of rutile to anatase, which can further decrease the band gap value of the resultant photocatalyst.58,59 Therefore, it is anticipated that the narrower band gap of the prepared TiO2-carbon composites will display enhanced photocatalytic performance.

22


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Figure 8 (a) UV-visible absorption spectrum and (b) the band energy of P25, TiO2-C-N and TiO2-C-A.

Photo-catalytic activity evaluation The photocatalytic properties of the photocatalysts were evaluated by photo-reduction of Cr (VI) and photo-oxidation of phenol under UV-vis and visible-light. Prior to photo-catalysis, the Cr (VI) solution added with the adsorbent/catalyst of interest was thoroughly mixed with constant stirring in dark to achieve adsorption-desorption equilibrium, and commercial P25 was used as the benchmark. The evolution of Cr (VI) concentration defined by C/C0 is shown in Figure 9a. After the adsorption equilibrium was reached at 80 min, the mixture was exposed to UV-vis irradiation. Compared with the pristine carbon materials and P25, the TiO2 tethered carbon composites show higher adsorption capacity, which could be attributed to the formed hydroxyl groups bonded to TiO2 that participate in the complexation with Cr (VI) via either electrostatic interaction or chelation, or both. Additionally, the adsorption kinetics curves were well fitted with pseudo-second kinetic model (R2 > 0.994) as shown in Figure S5a, indicating the domination of chemisorption. Figure 9a displays the Cr (VI) reduction efficiency of all the samples during 100 min of UV-Vis light irradiation. Specifically, TiO2-C-A shows the highest catalytic activity with 97% reduction of Cr (VI) within 20 min, and TiO2-C-N is 3 times slower (60 min) to 23



reach the same reduction efficiency, while the removal by P25 is only 65% after 80 min irradiation, and all the other materials show no photo-activities. In addition, the kinetics plots were used to compare the photo-catalytic performance more intuitively, as shown in Figure 9b. The Cr (VI) reduction followed a pseudo-first-order reaction expressed by a simplified Langmuir-Hinshelwood model. According to the calculated apparent rate constants, the reaction rate of TiO2-C-A is about 1.5 times higher than TiO2-C-N, and 9.7 times higher than P25. The higher adsorption capability, together with the synergistic effect of the nano-TiO2 structure and the porous activated carbon, play a key role in promoting the material’s photo-catalytic activity in converting Cr (VI) to Cr (III). Similar to photo-catalytic reduction of Cr (VI), the photo-catalytic oxidation of phenol presents the same kind of trend (Figure 9c). Both the adsorption performance and photo-catalytic activity of the activated TiO2 tethered carbon composite (TiO2-C-A) surpass all the other materials, and the adsorption kinetics can also be explained by the pseudo-second kinetic model (R2 > 0.988) as shown in Figure S5b.The photo-catalytic degradation of phenol also followed pseudo-first-order kinetics according to the linear trend of C/C0 versus the irradiation time (Figure 9d), with which the apparent rate constants were calculated correspondingly. The catalytic-activity of TiO2-C-A exceeds by 4.6 times and 9.5 times that of TiO2-C-N and P25, respectively, while all the pristine carbon materials show none catalytic activity. To reveal the intermediate species of phenol decomposition and evaluate the mineralization properties, HPLC and TOC analyses were carried out. As shown in Figure S6, the TOC removal efficiency is a function of reaction time, after 80 min UV-vis light illumination, about 87% of the TOC is removed due to the photo-catalytic oxidation with TiO2-C-A. This indicates that part of the phenol and its byproducts could be mineralized during the photo-catalytic process. The byproducts formed during the degradation process of phenol were tracked with HPLC, and benzoquinone, catechol and hydroquinone were found to be the major ones. 24


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Figure 10a shows the concentration variation of the detected byproducts during the process of photo-catalytic degradation of phenol using TiO2-C-A as the catalyst. The concentration of each individual byproduct increases with irradiation time initially and then decreases. The possible photo-catalytic degradation pathways for phenol could be understood in that, the hydroxyl radicals are first generated on the photo-catalyst surface, then react with phenol and yield hydroquinone, followed by the reaction with OH- to produce benzoquinone. As the photo-oxidation continuous, the benzene ring will eventually be cut open, leading to formic acid formation, which is ultimately mineralized to become CO2 and H2O.60 Similar photocatalytic experiments of MB degradation were also conducted and the results are given in Figure S7. Again, sample TiO2-C-A still outperforms other materials in light of photocatalytic efficiency.

Figure 9 Photo-catalytic activity of (a) Cr (VI) reduction and (c) phenol oxidation with different samples under 25



UV-Vis irradiation. Variation of ln (Ct/C0) versus irradiation time for (b) Cr (VI) reduction and (d) phenol oxidation.

The photo-reduction of Cr (VI) and photo-oxidation of phenol using TiO2-C-N, TiO2-C-A and P25 were also evaluated using visible light with wavelength above 420 nm, and the result is shown in Figure S8. Obviously, both TiO2-C-N and TiO2-C-A demonstrate much stronger photo-catalytic activity than P25, with TiO2-C-A standing out; P25 exhibits negligible activity. After 3 hours of visible light illumination, 70% and 48% of Cr (VI) are reduced and 58% and 31% of the phenol are oxidized by TiO2-C-A and TiO2-C-N, respectively. To check the reusability of the N doped TiO2-C composite (TiO2-C-A and TiO2-C-N) prepared herein for long-term photo-catalytic applications, cycle experiments were carried out under UV-vis and the results are shown in Figure 10b. No striking loss of photo-activity can be observed after either three times of photo reduction of Cr (VI) or three times of photo oxidation of phenol, suggesting the TiO2-C-A has durable photo-catalytic performance that is essential for repeated use.

Figure 10 (a) By-product concentration variation with time during phenol degradation using TiO2-C-A as the

catalyst under UV-visible light irradiation. (b) The recyclability of TiO2-C-A for Cr (VI) reduction and phenol oxidation under UV-vis light irradiation.

26


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Further Discussion Based on all the results of adsorption and photo-catalysis described earlier, TiO2-C-A excels all the other materials with a large margin. This can be due to the following reasons. First, the adsorption kinetics for MB is fast and accompanied by the non-liner sorption isotherms, indicating pore filling mechanisms.61 Also, the adsorption of Cr (VI) and phenol is very fast as well by this material in dark before the photo-catalytic experiments starts. Second, as magnified in Figure 11a and b, the carbon has rich porous structure, caused partially by the interface formation between TiO2 and the substrate. It seems TiO2 NP grow inside out the body of cellulose, resulting in more pores after heat treatment as can be clearly observed in Figure 11b. Such structure provides increased surface area for adsorption and is beneficial for charge transfer and spatial charge separation to enhance photo-catalytic activities. In addition, the large amount of surface hydroxyl groups of TiO2-C-A provided by TiO2 (confirmed by XPS) offers more active adsorption sites,62 which accelerates the adsorption kinetics.63-65 Accordingly, the excellent adsorption performance is always welcomed as a perquisite for superior photo-induced catalytic activity. The enhancement in photo-catalytic oxidation of the prepared TiO2-C-A is attributed to three important reasons. First, both TiO2 and porous carbon have good affinity to the target molecules, at the same time, the electrically conductive carbon support plays a key role for effective electron capture and transfer. Second, the nitrogen atoms doped in the TiO2 lattice structure after high temperature carbonization leads to the change in the band edge position of TiO2, and the existence of Ti3+ defects and oxygen vacancies, which later promote electron excitation under visible light range.63 Third, the high temperature activation treatment induces the crystal phase transformation from anatase to rutile, and also improves the crystallinity of TiO2 that enhances the material’s visible light absorption. 27



The possible mechanism of photo-catalytic oxidation of phenol and reduction of Cr (VI) is shown in Figure 11c. As a result of nitrogen doping, the band gap is narrowed due to the existence of substitutional (N 2p located states and interstitial N-O band) levels located above the valence band edge of TiO2 in the prepared TiO2-C-A, and the increased molar phase ratio of rutile to anatase.66 TiO2 is activated directly by UV-vis light irradiation to generate electron (e-)/hole (h+) pairs (Eq. (2)), and at this time the carbon could act as an electron reservoir. It traps electrons emitted from the conduction band of TiO2 (process I), leading to increased efficiency of charge separation. Then the electron and Ti3+ can reduce O2 to produce superoxide radicals anions ∙O2(process II, Eq. (3)), which would further lead to the generation of oxidative H2O2 molecule (Eq. (4)). At the same time, the photo-generated holes in the valence band will move to TiO2 surface to react with the hydroxyl groups, forming the very active hydroxyl radicals ∙OH (process III Eq. (5)). The generated oxidative substances, such as ∙OH, h+ and H2O2 would oxidize and decompose phenol to small molecule (process IV, Eq. (6)), and the e- would reduce Cr (VI) to the non-toxic Cr (III) (Eq. (7)).67

28


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Figure 11 (a) and (b) Detailed porous structure of TiO2-C-A at high SEM magnification. (c) The postulated

mechanism of UV-vis light irradiated TiO2-C-A for the reduction of Cr (VI) and the oxidation of phenol.

Equation (2)

T iO 2  hv → h + + e − O 2 + e − → O 2− •

Equation (3)

2 O 2− • + 2 H

→ H 2O2 + O2

Equation (4)

2 H 2 O + h + → 2 • OH + H +

Equation (5)

h + / H 2 O 2 / •O H + phenol → decomposition product

Equation (6)

Cr ( IV ) + e − → Cr ( III )

Equation (7)

+

Conclusion A hierarchically structured N-doped TiO2-carbon composite was fabricated by carbonization and activation of nano-TiO2-tethered cellulose fibers synthesized via microwave-assisted hydrolysis of TiOSO4. Nitrogen atoms were successfully doped into the TiO2 lattice to form O-Ti-N bond in the carbonization process under N2, and CO2 activation resulted in the creation of well-developed pore structure for the end carbon materials. The as-prepared N-doped TiO2-carbon had unique

29



three-dimensional network morphology with high surface area and abundant reactive sites, which led to extremely fast adsorption of MB dye, as compared to commercial active carbon and other adsorbents reported in literature. In addition, the material also showed superior photo-catalytic ability than others for phenol decomposition and Cr (VI) reduction under UV-vis irradiation. It is believed the material’s enhanced photo-activity is partially delivered by the formed TiO2 and carbon interfaces that promote charge transfer and spatial charge separation. Meanwhile, the doped N atoms, the formed Ti3+ and oxygen vacancies narrow the bandgap of TiO2 by creating mid-gap states, promoting electron excitation under visible light range. It is foreseen that such a dual-functional carbon material, developed from a very simple fabrication process, will have great potential for environmental remediation by acting as a sustainable material.

Supporting Information SEM image of the original TiO2-CF composite fiber; S 2p peak signals of TiO2-C-A detected by XPS; yield of gas products from TiO2/CF pyrolysis under nitrogen atmosphere; electron paramagnetic resonance spectra of TiO2-C-A; MB adsorption kinetics fitting parameters; MB adsorption isotherms fitting parameters; TiO2-C-A regeneration performance for MB adsorption; Cr (VI) and phenol adsorption kinetics plots and fitting lines; TOC removal efficiency during phenol degradation with TiO2-C-A under UV-visible light; photo-catalytic activity of MB degradation subject to UV-vis irradiation and the degradation kinetics; photo-catalytic activity for phenol oxidation and Cr (VI) reduction under visible light

30


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Acknowledgement This work is supported by the Major Science and Technology Program for Water Pollution Control and Treatment (Grant No. 2015ZX07202-013) and the Chinese Academy of Sciences under the talent program

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Abstract Graphic

High performance N-doped TiO2-carbon composite fibers with dual functionality were synthesized by a simple and sustainable process for water pollution control.

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Dual functional N-doped TiO2-carbon composite fibers for efficient

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