g-C3N4 Photocatalytic Coating

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Applied Chemistry

Convenient and Recyclable TiO2/g-C3N4 Photocatalytic Coating: Layer-by-layer Self-assembly Construction on Cotton Fabrics Leading to Improved Catalytic Activity under Visible Light Yanyan Wang, Xin Ding, Ping Zhang, Qi Wang, Kang Zheng, Lin Chen, Jianjun Ding, Xingyou Tian, and Xian Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05509 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Industrial & Engineering Chemistry Research

Convenient and Recyclable TiO2/g-C3N4 Photocatalytic Coating: Layer-by-layer Self-assembly Construction on Cotton Fabrics Leading to Improved Catalytic Activity under Visible Light Yanyan Wanga, b, c, Xin Ding a, c, Ping Zhanga, b, c, Qi Wang*c, d, Kang Zheng a, c, Lin Chen a, c, Jianjun Dinga, c, Xingyou Tian*a, c, Xian Zhang*a, c a

Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230088, People's Republic of China

b

c

University of Science and Technology of China, Hefei 230026, People's Republic of China Key Laboratory of Photovolatic and Energy Conservation Materials, Chinese Academy of

Sciences d

Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of

China. *Corresponding

authors:

[email protected]

(X.

Zhang), [email protected]

[email protected] (Q. Wang)， Tel: +86-551-65592752 Fax: +86-551-65393564

1

ACS Paragon Plus Environment

(X.

Tian),

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract In this work, a TiO2/g-C3N4 photocatalytic coating is built on cotton fabrics with a simple layer-by-layer (LBL) self-assembly strategy in which TiO2 and g-C3N4 are alternately assembled on cotton fabrics via electrostatic attraction. The fabrics, as support, disperse TiO2/g-C3N4 powder photocatalyst to expose more active sites. The fabrics also act as an adsorbent to boost the pollutants capture. Consequently, the coated fabrics exhibit an outstanding photocatalytic property applicable for degradation of both liquid Rhodamine (RhB) and gaseous toluene pollutants. The fabric with 7 bilayer coatings shows the best for RhB decomposition with degradation rate of 92.5%. Over 90% toluene could be eliminated by the photocatalytic coating under simulated sunlight irradiation. Besides, the coating fabrics show excellent stability and reusability.

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Keywords: TiO2; g-C3N4; LBL self-assembly; recyclable; high adsorbability

1 Introduction With the advantage of photocatalytic technology, it has been rapidly developed and attracted more and more attention to improve environmental pollution.1,2 Through the role of photocatalysts, lots of pollutants can be degraded by light radiation under ambient conditions.3-5 To date, TiO2 is the most widely used photocatalyst, owning to its unique nature, such as high durability, non-poisonousness and inexpensive.6,7 However, TiO2 has some fatal flaws such as rapid recombination of photon-generated e--h+ pairs and wide band-gap only corresponding to ultraviolet wave range, limiting its practical application.8 Hence, narrow band-gap semiconductors are constantly used to form composites with TiO2 to extend the light absorption range as well as upgrade quantum efficiency with promoted photogenerated charges separation and transfer. For instance, Zhang et al. constructed a d-Ti3C2/TiO2/g-C3N4 ternary nanostructure via simple heat process of g-C3N4 and delaminated Ti3C2 in which the d-Ti3C2 acted as a support layer and resource to glue TiO2 and g-C3N4 as well as played a role of electronic transmission path. Therefore, d-Ti3C2/TiO2/g-C3N4 (4-1-350-1) exhibited an outstanding H2-generation performance. 9 Shen et al. fabricated TiO2/C3N4 heterojunctions on carbon-fiber cloth using thermal polymerization and dipcoating/hydrothermal method, which presented brilliant catalytic performance.10 However, many of the narrow band-gap semiconductors are involved with a complex preparation process, low yield, or introduction of noble metals or heavy metals, limiting their application in the environmental pollution abatement. Recently, g-C3N4 has been widely concerned with a graphite-analogue feature as well as a high degree of green environmental nature without containing any heavy metal element.11-14 3



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Compositing TiO2 with g-C3N4 was regarded as an efficient way to elevate photocatalytic property and extend light absorption range of TiO2. Tong et al. prepared TiO2/g-C3N4 nanocomposites by mineralization process, and the best sample could degrade most of RhB within 50 min under simulated sunlight irradiation.15 Zhong et al. also built a C3N4/TiO2 heterostructure by a solvothermal method to obtain excellent photocatalytic performance for H2 evolution;16 However, there are still some limitations on the TiO2/g-C3N4 photocatalytic system. One of them is that their photocatalytic activity is still unsatisfied partly due to the low absorption performance of powder catalysts to the pollutants. Though most of efforts have focused on the adjustment of the band gap of photocatalysts composites,17-20 the excellent adsorption performance is beneficial to the sufficient contact of the photocatalyst with the pollutants, thereby effectively promoting the photocatalytic reaction. Another limitation is that it is difficult to realize recycling utilization of photocatalytic system, especially for the powder photocatalysts. Obviously, the unrecyclable feature not only is detrimental to sustainable development but also generally leads to secondary pollution. Very recently, Xiong et al. prepared a reusable 3D δ-Bi2O3 aerogel for radioiodine removal by a hydrothermal method, but the process was comparatively complicated.21 Hence, there is an urgent need for a facile strategy to construct recyclable photocatalytic system which is also suitable for composite materials. LBL self-assembly technique, referring to alternately deposition of reverse charge electrolytes on substrates, has been considered as a promising method to prepare multi-functional coatings.22,23 The technique has favourable industry prospect because of its facile operational process, low cost and environment-friendliness. Considering three-dimensional porous materials have excellent adsorption property to pollutants because of their unique structure and versatility, cotton fabrics with loose porous structure is expected to load the TiO2/g-C3N4 powder photocatalyst by LBL 4


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self-assembly technique to achieve a satisfying pollutant adsorption capacity, an improved photocatalytic activity, as well as a facile recovery of catalysts. Here, TiO2/g-C3N4 functional coating was successfully fabricated on the cotton fabrics through a LBL self-assembly technology. All the coated fabrics exhibit an outstanding photocatalytic performance for both liquid RhB and gaseous toluene pollutants. 92.5% RhB is degraded by 7 BL within 25 min under visible light irradiation and over 90% toluene can be removed by this TiO2/g-C3N4 functional coating under simulated sunlight irradiation. Furthermore, the coating fabrics display excellent stability and reusability under recycling degradation experiment.

2 Experimental 2.1 Materials Cotton fabrics were gained from Huacheng Textiles Industry Co. Ltd. TiO2 nanoparticles (P25) were acquired from Degussa. Potassium alginate (PA) was got from Qingdao Bright Moon seaweed Group Co. Ltd. Melamine was purchased from Tianjin Guangfu Technology Development Co. Ltd. HCl and H2SO4 was acquired from Shanghai Chemical Co. Ltd. Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) was purchased from J&K Scientific Ltd. tert-Butanol (t-BuOH) was acquired from Shanghai Chemical Co. Ltd. p-Benzoquinone (BQ) was obtained from Aladdin.

2.2 Preparation of g-C3N4 samples Bulk g-C3N4 was obtained through facile heating melamine treatment according to a literature.24 In detail, a crucible was need to place 10g melamine, and then a specific heat treatment process is indispensable to produce yellow g-C3N4. Later, the obtained g-C3N4 cooled to room temperature before ground by mortar. Considering many reports confirmed that g-C3N4 nanosheets 5



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exhibited better photocatalytic property than corresponding bulk one,25-27 we further synthesized corresponding nanosheets through chemical exfoliation method according to a literature.27 Firstly, a certain amount of bulk g-C3N4 and H2SO4 (98 wt% ) were mixed to stir for 8 hours. Then the mixture was subjected ultrasonic treatment for 4 hours. Finally, the sediment was obtained by centrifuging for 10 min with 8000 r/min to neutral before freeze-drying.

2.3 LBL self-assembly process The TiO2 cationic solution was acquired via adjusting the pH value to 4 with 1 M HCl solution under stirring to form a stable colloid solution. In order to improve the dispersivity, 0.1 g g-C3N4 nanosheets was firstly added into 200 mL 0.1 wt% PA solution,28 and then ultrasonic-treated for 30 min. Finally, the anionic g-C3N4 solution was obtained when the pH value was 4. In the Figure 1, the corresponding assembly process was presented. Firstly, in order to remove impurities, all fabrics were washed and then dried. Secondly, it was immersed in TiO2 cationic solution and g-C3N4 anionic solution alternately. After each end of immersion, it should be rinsed and dried. The same step was performed until the specified numbers (2, 5, 7 BL) were obtained. The TiO2/g-C3N4 powder composite was obtained for comparison: the above-mentioned TiO2 cationic solution and g-C3N4 anionic solution were mixed under the stirring, and then centrifuged to obtain sediment. At last, this sediment was dried with vacuum oven.

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Figure 1. The LBL self-assembly construction process of coating.

2.4 Measurements and characterization Here, scanning electron microscope (SEM) (FEI Corporation, USA) was employed to observe the morphologies of the coated samples and record Ti, N and O elemental distribution. The morphology of g-C3N4 nanosheet was viewed by a transmission electron microscopy (TEM, JEM-2010). The crystal phases were detected by X-ray diffraction (XRD, X’Pert, PA Nalytical Netherlands). DUV-3700 ultraviolet-visible spectrophotometer (SHIMADZU Co., Ltd, Japan) was used to characterize light absorbing ability of coated fabrics. Fourier transform infrared spectra (FTIR) of g-C3N4 was investigated. The X-ray photoelectron spectra (ESCALAB 250, Thermo-VG Scientific) was used to analyze the surface elemental compositions. A fluorometer (FluoroMax-4) was utilized to record the photoluminescence (PL) spectra of coated fabrics. Coating growth was characterized by Thermogravimetric analysis (TGA, PerkinElmer, USA). Zeta potential value was measured by Zetasizer3000HSa (Malvern). EIS was employed to provide the information of charges transfer and recombination via an electrochemical workstation (CHI660D, chenhua, 7



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shanghai). Electron spin resonance (ESR) spectrometer (JES-FA200) was used to investigate photocatalytic mechanism. Photocatalytic performance of powder photocatalysts, uncoated fabric and coated fabrics were all evaluated by a fading experiment of Rhodamine (RhB) and degradation of gas toluene. In detail, 35 mg powder photocatalysts and 1 g fabrics sample were added in 35 mL RhB (1.25*10-5 mol/L), respectively, then stood in black box for one hour to insure saturation adsorption. Later, a certain amount of reaction RhB was collected for concentration test of RhB with a certain time interval in which a xenon lamp (300 W ) with a cutoff filer (λ > 420nm) (PLS-SXE 300, Beijing Perfectlight Co., Ltd.) was employed to motivate the reaction. Gas toluene was decomposed in a 350 mL closed quartz tube. Firstly, 0.58g fabric and 0.2 μL toluene were all placed into the tube, then hot treatment was needed to vaporize toluene. Before irradiation, this tube was placed in black box to insure saturation adsorption. 500 μL reaction gas was taken out at certain intervals and a gas chromatographic was used to detect the change of the concentration of toluene. To validate the practical application of the coated fabrics, degradation experiment was implemented under the real sunlight irradiation as well in Hefei City.

3 Results and Discussion 3.1 Characterization of g-C3N4 nanosheets After chemical exfoliation, the graphite-analogue g-C3N4 turns into g-C3N4 nanosheets, their morphologies are observed by TEM, as shown in Figure S1a and Sb. Obviously, both of them possess sheet features, which are well agreed with other reports.29, 30 Moreover, g-C3N4 nanosheets present a smaller size than the bulk one. The insets in Figure S1 are the corresponding photographs. Obviously, the volume of g-C3N4 nanosheets is larger than the bulk one with same weight, implying 8


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bulk g-C3N4 being successfully converted into smaller g-C3N4.

Figure 2. (a) XRD pattern, (b) FTIR spectra, (c) C 1s XPS and (d) N 1s XPS, (e) the UV-vis spectra and (f) PL spectra of g-C3N4 samlpes

The structure and surface elemental compositions of g-C3N4 samples are explored. From the XRD pattern (Figure 2a), two main diffraction peaks are discovered in the bulk g-C3N4, corresponding to (100) and (002) crystal planes, which are attributed to stacking of planar repeating units and the interlayer stacking, respectively.31-33 It is obviously that the two diffraction peaks in g-C3N4 nanosheets become weaker and wider than the bulk one, implying that the ordered arrangement of stacking of planar repeating units and interlayer stacking structure are destroyed. This result confirms that the bulk g-C3N4 have been converted to subsize g-C3N4 nanosheets, which agrees well with the TEM result. For FTIR (Figure 2b), typical characteristic peaks of g-C3N4 samples are discovered, in which the peaks at 808 cm-1, 1251-1421 cm-1 and 1571-1632 cm-1 are corresponding to the triazine ring breathing mode, C-N stretching of CN heterocycles, C=N stretching, respectively. There is no evident change of the peaks between them. 34-36 Figure 2c and d present the XPS of C 1s and N 1s, in which the C 1s spectrum is fitted into two peaks located at 9



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284.7 eV and 288.1 eV, respectively. The peak at 284.7 eV is attributed to the sp2 C-C bonds, while the peak at 288.1 eV is assigned to sp2 hybridized carbon atom in N-containing heterocyclic rings, which are both the main carbon structure of g-C3N4.37 The N 1s spectra is fitted into two peaks locating at 398.8 eV and 400.4 eV, respectively, corresponding to the sp2-hybridized aromatic six-membered rings (C-N=C) and the tertiary N (N-(C)3) groups.38 There is no obvious chemical bond change, implying that surface elemental composition of the g-C3N4 is not destroyed after the chemical exfoliation treatment. The optical property of the g-C3N4 samples have been examined by DRS spectra (Figure 2e) and PL spectra (Figure 2f). The absorption edge of g-C3N4 nanosheets exhibits obvious blue shift from 477 nm to 430 nm contrasting to bulk g-C3N4, corresponding to the changing of bandgap from 2.60 eV to 2.88 eV. This phenomenon is mainly attributed to the size decrease of g-C3N4. From the PL spectra of g-C3N4 nanosheets, there is an obvious blue shift in emission peaks which is in line with the change of bandgap. 3.2 The growth process of the photocatalytic coating The as-prepared g-C3N4 nanosheets are then deposited on cotton fabrics with TiO2 nanoparticles to construct a three-dimensional photocatalytic system. Considering that the electrostatic force is the main driving force of the LBL self-assembly process in this work, we examined Zeta potential value of the TiO2 and g-C3N4 in different pH values, and the corresponding curves were shown in the Figure S2. It can be found that there is a reverse potential with pH value ranging from 3 to 5, and a maximum difference of the reverse potential is obtained when pH value is 4. Consequently, both of the colloidal solution of pH=4 were selected for the assembly experiment. The morphologies of uncoated and coated fabrics are exhibited in Figure 3. The typical cotton fiber can be clearly seen in all the samples, meaning a small amount of coating would not affect the 10


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fiber structure. The surface of uncoated fiber is smooth, while coated fabrics show rough surface after the introduction of self-assembly coating. The loading amount of the coating enhances with assembly number raising. In addition, mapping results of Ti and N for the samples can further confirm the uniform distribution of the photocatalytic coating (TiO2 and g-C3N4) and that the coating mass gradually increases as the number of bilayers raises. The result indicates that the TiO2/g-C3N4 coating is successfully constructed on the cotton fabrics. Furthermore, it is noted that the TiO2/g-C3N4 photocatalyst mainly covers on the surface of fibers themselves and there is still plenty of space among fibers after coating treatment. The loose and porous structure is beneficial to full interaction between the photocatalyst and contaminants, promoting the process of photocatalytic reaction.

Figure 3. The SEM photographs of samples and their Ti, N, O mapping.

Meanwhile, TGA is performed to explore the coating growth process, and the corresponding curves are shown in Figure 4a. Obviously, there exist mass reductions for all the fabric samples due to the combustion of fabrics.39 The residue of untreared, 2 BL, 5 BL and 7 BL samples at 700 °C are 0.028, 0.065, 0.083 and 0.095 %, respectively, which increase gradually as the number of bilayers raises. Comparing with the TiO2 avoiding of decomposed (Figure S3), the g-C3N4 almost 11



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completely decay at 700 °C. Therefore, the difference of the residue between the coated fabrics and the uncoated fabric can be considered as the assembly of TiO2 coating. The coating mass of TiO2 can be seen a function of the self-assembly bilayer number, which was presented in Figure 4a. The coating mass almost increases linearly as the bilayer number raising. Crystal structures of all fabrics are investigated via X-ray diffraction, and the result is shown in Figure 4b. The typical peaks of crystalline cellulose at 14.8°, 16.6°and 22.6° are discovered in all samples, corresponding to (1-10), (110) and (200) crystal planes,40 respectively, suggesting that the crystal structure of cellulose is not destroyed after the self-assembly coating treatment. The intensity of these peaks becomes weak gradually due to the increasing coating amount on fiber surface. The diffraction peaks of anatase phase of TiO2 are discovered at 25.3° and 48.0° in the coated samples corresponding to (101) and (200) crystal planes,41 and the peaks intensities increase gradually with the coating growth. These consequences prove that the photocatalytic coating is favourably constructed on cotton fabrics during the LBL self-assembly process. In addition, it is also notable that the typical crystalline peaks of g-C3N4 nanosheets are not discovered, which can mainly be attributed to possible overlapping of the (002) crystal plane of g-C3N4 nanosheets at 27.4° with the (101) crystal plane of TiO2 at 27.3°.

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Figure 4. (a) The TGA curves and (b) XRD patterns of all fabrics; (c) XPS survey spectrum and (d) N 1s of 7 BL coated fabric.

Further, XPS characterization of the typical 7 BL coated fabric is performed to investigate the existence of g-C3N4 and shown in Figure 4c and d. In the survey spectrum, the sharp photoelectron peaks at 458, 530, 399 and 286 eV are corresponding to Ti, O, N and C element, respectively. The N 1s spectra is fitted into two peaks locating at 398.8 eV and 400.6 eV, which are corresponding to the sp2-hybridized aromatic N in six-membered rings (C-N=C) and the tertiary N (N-(C)3) groups in g-C3N4, respectively.38 The above analysis confirms g-C3N4 are successfully constructed on cotton fabrics.

3.3 Photocatalytic performance of photocatalytic coating Since absorptive capacity of photocatalyst to pollutants is a key factor to affect the catalytic 13



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capacity during the photocatalytic process,24 RhB is chosen to explore the adsorption ability of samples and the corresponding adsorption curve is presented in Figure 5. The adsorption values of the TiO2 nanoparticles, TiO2/g-C3N4, uncoated fabric, 2 BL, 5 BL and 7 BL are 10.68%, 13.11%, 53.21%, 53.89%, 50.54% and 59.41%, respectively. Obviously, the adsorption capacity of fabric samples is all much bigger than that of the powder photocatalysts. For example, the adsorption capacity of 7 BL is much larger than TiO2/g-C3N4 powder photocatalyst. The result demonstrates that fabrics act as a superior carrier to endow powder photocatalysts an outstanding adsorption capacity, which would be beneficial to accelerating photocatalytic reaction.

Figure 5. The adsorption curve of TiO2 nanoparticles, TiO2/g-C3N4 powder, uncoated fabric, 2 BL, 5 BL and 7 BL for RhB.

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Figure 6. (a) The UV-vis spectra, (b) PL spectra and (c) Nyquist plots of fabrics.

The light utilization ability of cotton fabrics are also examined by UV-vis absorption spectra and PL spectra. From the Figure 6a, it is found that all of coated fabrics exhibit an obviously improved absorbance from 250 to 430 nm with the introduction of photocatalytic coating. There exist slight redshift with the increasing of bilayer number, suggesting that extra light can be used to degrade pollutants by coating fabrics. From the PL spectra (Figure 6b), it is found that emission peaks intensities weaken as the bilayers number raising, indicating that recombination of carriers is greatly suppressed, which is favourable for improving quantum efficiency. Identically, EIS plots of the coated fabrics (Figure 6c) are also utilized to evaluate charge transfer and recombination capability. The semicircles in the typical Nyquist plots represent charges transfer resistance (Rct) between photocatalyst and electrolyte interface. The semicircle decreases gradually as the increasing bilayer number, implying that the charge transfer and restraint charge recombination have promoted sequentially. This result is well consistent with the above PL spectra. In terms of the practical application of coated fabrics, both liquid pollutant and gas pollutant are conducted to explore photocatalytic degradation ability of coated fabrics. For the degradation of liquid pollutant, RhB is chosen and the corresponding degradation curve (Figure 7a) under the visible light irradiation was described. Pure RhB solution is very stable and not degraded under the light irradiation. However, 45.5% RhB is degraded by TiO2 nanoparticles within 25 min attributing to the existence of heterojunction in anatase and rutile phase. When coupling TiO2 with g-C3N4 nanosheets, this composite shows greater photocatalytic activity (76.7%) than the separate TiO2. 15



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This could be interpreted as that combination TiO2 with g-C3N4 nanosheets can extend light response range which enhances catalytic activity under visible light, as well as promote the separation of photo-generated carriers through the heterojunction between them. Although uncoated fabric does not possess ability of degradation for RhB under the same light irradiation, it exhibits strong adsorption capacity for RhB derived from abundant pore structure, as we discussed above. With the construction of the assembly coating on the cotton fabrics, all of coated fabrics have more superior degradation capacity than TiO2/g-C3N4 powder photocatalyst. Furthermore, the catalytic capacities of coated fabrics increase progressively as assembly number raising. The coated fabric with 7 bilayers number exhibits the best pollutant degradation ability in which 92.5% RhB was degraded within 25 min, demonstrating that there is an advantageous synergistic effect between photocatalyst and fabrics. Our superb photocatalytic system mainly benefits from two aspects, the one, during the LBL self-assembly process, TiO2 and g-C3N4 could easily form heterojunction by electrostatic interaction which efficiently degrades pollutants by suppressing carriers recombination. The other, cotton fabrics act as not only support for photocatalyst to avoid aggregation and expose more active sites, but also as an adsorbent of RhB for quick capturing pollutants facilitating light degradation reaction. Furthermore, the reaction rate is assessed and the degradation rate constant is calculated from formula calculation: ln(C/C0)= κt,42 in which the C, C0, κ and t represent instantaneous and initial concentration of RhB solution, reaction rate constant and reaction time, respectively. From the Figure 7b, it can be found that the photocatalytic reaction is well conformed to quasi-first-order kinetic. The κ of TiO2, TiO2/g-C3N4, 2 BL, 5 BL and 7 BL are 0.018, 0.053, 0.052, 0.059 and 0.065 min-1, respectively. Significantly, the κ of TiO2/g-C3N4 is higher than TiO2, demonstrating that coulping TiO2 with g-C3N4 is an efficient strategy to upgrade photocatalytic performance. Further, with the incorporation of assembly coating, the reaction rate constant obviously increases and κ value of the 5 BL and 7 BL are both larger than that of TiO2/g-C3N4 powder, though degradation rate constant of 2 BL slightly decreases due to too small coating amount. 16


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Figure 7. (a) The degradation curves of RhB under visible light irradiation, (b) the corresponding kinetic curves, (c) the degradation curves for the toluene under simulated sunlight irritation, (d) the corresponding kinetic curves

As a typical volatile organic compounds (VOCs), toluene is also used to assess the photocatalytic activity of coated fabrics under simulated sunlight irritation. From the Figure 7c, it is found that the pure cotton fabric does not possess any ability of degradation for toluene, while there is a new the adsorption-desorption balance after 30 minutes of illumination which also appears in TiO2/g-C3N4 degradation curve. Unexpectedly, the powder photocatalyst of TiO2/g-C3N4 presents a lower photocatalytic activity than TiO2 within 50 min of illumination. It possibly ascribes the truth that holes on the valence band (VB) of TiO2 possess a higher oxidability than those on g-C3N4. Because there exist transfers of holes from TiO2 to g-C3N4 for TiO2/g-C3N4 with irradia

tion, so

the holes on TiO2 showing stronger photocatalytic ability than TiO2/g-C3N4. Interestingly, with the 17



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construction of TiO2/g-C3N4 photocatalytic coating, coated fabrics exhibit excellent degradation ability for toluene. What’s more, all the coated fabrics poccess a similar catalytic performance in which the degradation rate of 2 BL, 5 BL and 7 BL are 93.64%, 91.72% and 92.97%, respectively. The similar degradation abilities of coated fabrics can be attributed to the fact that the photocatalytic coating participating in degradation gas reaction is limited to surface layer, which is good agreement with the low absorptive capacity of the coated fabrics for toluene. Simultaneously, the reaction rate constant of TiO2 nanoparticles, TiO2/g-C3N4, 2 BL, 5 BL and 7 BL are calculated and the corresponding κ values are 0.050, 0.007, 0.068, 0.060 and 0.059 min-1, respectively. Obviously, the reaction rate constants of coated fabrics are great bigger than powder photocatalysts. These results confirm that coating powder photocatalyst on cotton fabric is a nice method to upgrade photocatalytic efficiency. Recyclability is indispensable for realizing the practical application of photocatalyst. Since the stability and reusability of the coated fabrics are also investigated by recycling experiment with RhB, and the 7 BL coated fabric is selected as the testing sample. From the Figure 8a, there is almost no change in photocatalytic activity after 4 cycles, implying of the excellent stability of the assembly coating attributing to the electrostatic interaction between the photocatalyst and cotton fibers. Meanwhile, the photocatalytic system can be easily recycled after filtrating the cotton fabrics from the reaction solution for next using. The composition and morphology of coated fabrics after degradation reaction are also investigated. As the information given by Figure 8b, the similar diffraction peaks of crystalline cellulose and TiO2 are discovered before and after the reaction. The morphology of coated fabric (Figure 8c-d) show that both of the fabrics retain a rough surface which is thoroughly covered by the coating material, and there is no obvious distinction in the morphology for the coated fabric before and after degradation reaction. These results further verify 18


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that the coating material on the fibers surface possesses robust stability.

Figure 8. (a) The cycle experiment of 7 BL coated fabric for RhB degradation, (b) the XRD patterns of 7 BL before and after cycle experiment, (c) and (d) the morphology before and after degradation reaction of 7 BL

The practical application performance of these photocatalytic coatings in degradation of environmental pollutants, the 7 BL coated fabric is also used to degrade RhB solution under the real sunlight irradiation in Hefei City. In the Figure 9a, the absorption spectra of the reaction solution and the corresponding photographs are presented. Evidently, the peaks intensities gradually decrease within 4 h and the corresponding colour of RhB solution fades away until colourless. In addition, the colour of 7 BL coated fabric changes to white as well, implying that the disappeared RhB is degraded instead of absorbed by the coated fabrics. Meanwhile, this work is also compared with other typical TiO2/g-C3N4 photocatalytic systems, 15, 43-45 as shown in Table S1. Although there are some differences in terms of experimental condition and evaluating system, the coating system exhibits obvious advantage in photocatalytic degradation of pollutants. 19



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Figure 9. (a) The absorption spectra of RhB under the real sunlight, (b) the trapping experiment of 7 BL for RhB degradation, (c) ESR spectra of DMPO-·OH adducts and (d) DMPO-O2-· adducts

3.4 Exploration of the photocatalytic mechanism For understanding the photocatalytic mechanism of this system, trapping experiment is carried out through EDTA-2Na, t-BuOH and BQ to trap h+, OH· and O2-·, 46 respectively. From the Figure 9b, the catalytic capacity of 7 BL coated fabric presents a slight decline after the adding of EDTA-2Na and t-BuOH, respectively. While it decreases much as the introducing of BQ. These results suggest that the O2-· is the mainly oxidative species for the coating system when the light source is visible light. The ESR spin-trap signals of coated fabrics with DMPO are also recorded to explore the photocatalytic mechanism. From the Figure 9c, it is found that there exist typical peaks 20


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of DMPO-·OH in aqueous dispersions under simulated sunlight irritation, while the signals intensity drastically reduce when changing the light source to visible light. The corresponding signals are not detected in dark. The peaks of DMPO-O2-·(Figure 9d) are detected under both the visible light and simulated sunlight irritation in which the these signals with simulated sunlight irritation are stronger, suggesting that more O2- · generate under simulated sunlight irritation. Identically, no signals of DMPO-O2- · are seen in dark. These results demonstrate that certain light irritation is essential to generate the active species (·OH, O2-·). Furthermore, the ESR result also confirms that O2- · is the mainly oxidative species under visible light irritation, being in good accordance with the result of trapping experiment. Another conclusion is that ·OH and O2- · are generated on the coated fabrics under simulated sunlight irritation, in which O2-· accounts for a larger proportion.

Figure 10. The photocatalytic reaction mechanism of coated fabrics for pollutions degradation.

According to the above discussion, possible photocatalytic mechanism and the corresponding schematic diagram are proposed in Figure 10. Firstly, there is ample space among the fabrics which 21



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are composed of fibers wrapped by the photocatalytic coating and the pollution molecular can be adsorbed to the surface of fiber in which the pollution will contact sufficiently with photocatalyst. Secondly, catalytic surface reactions would proceed under the light radiation. As for g-C3N4, photo-generated e- and h+ trend to recombine, only a small fraction participate catalytic surface reactions, leading to a low activity. When the coated fabrics are exposed to visible light, g-C3N4 will be excited to form photo-generated e--h+ pairs by the photon whose energy is greater than or equal to Eg of g-C3N4. Comparing with TiO2, g-C3N4 exhibits lower CB position, so e- will transfer to CB of TiO2 through the heterojunction between them. It results in efficiently promoting separation and transfer of carriers, as well as improving photocatalytic performance. The e- on the CB of TiO2 may directly reduce O2 near photocatalysts surface into O2·- which is verified as the major oxidative species under visible light. The h+ is left on the VB of g-C3N4 and directly reacts with pollutants. Since the VB of g-C3N4 (+1.4 eV) is lower than Eθ (OH·/H2O) (2.68 eV),47 suggesting that oxidizing H2O into OH· is hard for h+ which is in good accordance with the weak DMPO-·OH signal. Moreover, the degradation mechanism of toluene is presented under simulated sunlight irradiation. Both TiO2 and g-C3N4 are excited accompanying charges separation and transfer. Through the heterojunction between them, the e- on the CB of g-C3N4 will transfer to TiO2 forming active species with surface O2. The photo-generated h+ trend to transfer from the VB of TiO2 to g-C3N4, and there is still part of h+ on TiO2 will directly decompose pre-adsorbed toluene. For above mechanism analysis, the outstanding photocatalytic property of coated fabrics is explained in two ways. One is the g-C3N4/TiO2 heterojunction formation with electrostatic force. The other one is that the cotton fabrics act as not only support for photocatalyst to avoid aggregation and expose more active sites, but also as an adsorbent for quickly capturing pollutants facilitating light degradation reaction. 22


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4. Conclusions In summary, we demonstrate an excellent photocatalytic activity coating prepared by alternately depositing TiO2 nanoparticles and g-C3N4 nanosheets on cotton fabrics via a LBL self-assembly strategy. In this system, the fabrics support and disperse photocatalyst to expose more active sites. Meanwhile, the loose and porous structure of fabrics provides strong adsorption ability to pollutions and impels active species to quickly capture pollutants to degrade. As a result, the coated fabrics present brilliant photocatalytic performance for both liquid and gas pollutants decomposition. A maximum degradation rate of 92.5% for RhB could be reached by the 7 BL and all the coated fabrics can degrade over 90% toluene within 50 min under simulated sunlight irradiation. At the same time, the coating fabrics show excellent stability and reusability after recycling degradation experiment. This work demonstrates that our TiO2/g-C3N4 photocatalytic coating system is a promising alternative for degradation of both liquid and gas environmental pollutants and provides a available strategy to promote the application of photocatalysts for environmental modification. Supporting Information The TEM of g-C3N4 samples, Zeta potential value of TiO2 colloidal solution and g-C3N4 colloidal solution under different pH values, TGA curves of TiO2 and g-C3N4 nanosheets, Comparison coating system with other TiO2/g-C3N4 works for the photocatalytic activity.

Acknowledgements This work was supported by Anhui Provincial Natural Science Foundation (1708085MB46), the CASHIPS Director's Fund (YZJJ201523), National Natural Science Foundation of China (11575253), Anhui Provincial Natural Science Foundation for Distinguished Young Scholars of China (1608085J03), Anhui Provincial key research and development plan (1704a0902017). 23



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Figure 1. The schematic diagram of LBL self-assembly process. 553x457mm (300 x 300 DPI)


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Figure 2. (a) XRD pattern, (b) FTIR spectra, (c) C 1s XPS and (d) N 1s XPS, (e) the UV-vis absorption spectra and (f) PL spectra of bulk g-C3N4 and g-C3N4 nanosheets 594x351mm (300 x 300 DPI)



Figure 3. The SEM photographs of samples and their Ti, N, O mapping.


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Figure 4. (a) The TGA curves and (b) XRD patterns of uncoated and coated fabrics; (c) XPS survey spectrum and (d) N 1s of 7 BL coated fabric. 406x351mm (300 x 300 DPI)



Figure 5. The adsorption curve of TiO2 nanoparticles, TiO2/g-C3N4 powder, uncoated fabric, 2 BL, 5 BL and 7 BL for RhB. 190x188mm (72 x 72 DPI)


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Figure 6. (a) The UV-vis absorption spectra, (b) PL spectra and (c) Nyquist plots of uncoated and coated fabrics. 600x181mm (300 x 300 DPI)



Figure 7. (a) The degradation curves for the RhB under visible light irradiation, (b) the corresponding kinetic curves, (c) the degradation curves for the toluene under simulated sunlight irritation, (d) the corresponding kinetic curves 387x339mm (300 x 300 DPI)


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Figure 8. (a) The cycle experiment of 7 BL coated fabric for RhB degradation, (b) the XRD patterns of 7 BL before and after cycle experiment, (c) and (d) the morphology before and after degradation reaction of 7 BL 402x308mm (300 x 300 DPI)



Figure 9. (a) The degradation of RhB under the real sunlight irradiation, (b) the trapping experiment of 7 BL for RhB degradation, (c) ESR spectra of DMPO-•OH adducts in aqueous dispersions and (d) DMPO-O2-• adducts in methanol dispersions 393x369mm (300 x 300 DPI)


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Figure 10. The photocatalytic reaction mechanism of coated fabrics for pollutions degradation. 555x375mm (300 x 300 DPI)


g-C3N4 Photocatalytic Coating

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