Electronic Supplementary Material

High flux photocatalytic self-cleaning nanosheet C3N4 membrane supported by cellulose nanofibers for dye wastewater purificationLilong Zhang1,2, Ge Meng3, Guifang Fan1, Keli Chen2, Yulong Wu1,4 (), and Jian Liu5

1 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China 2 The Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Jilin 130012, China 3 Department of Chemistry, Tsinghua University, Beijing 100084, China 4 School of Chemical Engineering and Technology, Xinjiang University, Urumqi 830046, China 5 College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China Supporting information to https://doi.org/10.1007/s12274-020-3256-0

1 Experimental Section

1.1 Preparation of bulking C3N4 and nanosheet C3N4

The bulking C3N4 was synthesized through a thermal condensation of melamine (AR, Sinopharm Chemical Reagent Co., Ltd.). Melamine (5 g) was heated to 550 °C for 4 h in a tube furnace (heating rate: 2 °C/ min) under nitrogen condition (20 ml/min). The bulking C3N4 was obtained after naturally cooling the reaction system to 25 °C.

As Figure 1 (a) shows, 2 g (oven-dried) melamine and 1 g (oven-dried) CNFs (home made detail information can been seen in our previous work [S1] ) were dissolved in 400 mL deionized water. After quick freeze-drying using liquid nitrogen, water was removed by freezing dryer. The dry solid was transferred into a tube furnace and kept at 550 °C for 4 h under an N2 atmosphere with a heating ramp of 2.3 °C/min. g-C3N4 containing carbonaceous components was prepared. The carbonaceous components contained in the NS C3N4 powder was removed by air combustion at 500 °C for 2 h.

1.2 Preparation of cellulose composite membranes

Adulterated membranes

NS C3N4 (1 g) was separated in a moderate amount (approximately 600 mL) of deionized water by ultrasonic treatment at a power of 1500 W for 30 min, and then the moderate volumes of the NS C3N4 mixtures were adjusted with water to 1000 mL.

Adulterated and coverage membranes were prepared. For the adulterated membranes, a certain volume of C3N4 suspension (i.e., 4-24 mL) was poured into a 0.03 g CNF colloidal solution (containing 0.03 g CNF).

The different volumes of the CNF/NS C3N4 mixtures were adjusted to 20 mL with water and then completely mixed by ultrasonic treatment with a power of 300 W for 5 min. To prevent CNFs loss during the membrane production process, as shown in Figure S1, a sintered glass filter with a nanofiltration membrane (D = 4 cm, 0.45 μm PE) and then suctioned under 0.05 MPa vacuum for 12 h to obtain wet microfiber composite membranes.

Coverage membranes

Similar to the adulterated membranes, the coverage membranes were prepared after the CNFs membranes were completely produced, and a certain volume of NC C3N4 solution was poured on the surface of prepared CNFs membranes. Then, the excess water was sucked dry under 0.05 MPa vacuum.

1.3 Photocatalytic experiments

To determine the optimal film thickness and NS C3N4 addition amount, the rejection rates of membranes with different CNFs thicknesses and different NS C3N4 contents were evaluated by vacuum filtration of rhodamine B (RhB) under 85 bar in a self-designed reactor (as shown in Figure S6). The dynamic changes of the RhB concentration were determined by the UV-vis characteristic absorption peak at 552 nm. The calculation and equation of the rejection rate, the flux, and the degradation efficiency of the dye were shown in support information.

The dynamic changes of the RhB concentration were determined by the UV-vis characteristic absorption peak at 552 nm. The calculation and equation of the rejection rate was calculated by the following equation:

)Ao

At-(1100%rateRejection

where A0, and At are the concentration and absorbance of RhB after membrane filtration at filtration times of 0 and t. Address correspondence to wylong@tsinghua.edu.cn

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The flux (J; in liters hour−1 m−2 bar−1) is calculated by the following equation:

VJA t P

=´ ´

where V is the volume of the filtered water (in liters), A is the effective area of membrane filtration (in square meters), t is the filtration time (in hours), and P is the suction pressure across the membrane (in bars). Here, A is the effective membrane filtration area instead of the membrane area, and the effective filtration area is calculated based on the membrane area and porosity of the support membrane. In the present experiments, the filtration area of the filter holder is 1.16 E-3 m2, and the porosity of the support membrane (PC membrane; pore size, 220 nm) is 10% (Ling et al., 2016). Thus, the effective surface area is 1.16 E-4·m2.

To comprehensively evaluate the potential for photocatalytic degradation of dyes, cationic (RhB) and anionic (methylene blue (MB)) dyes were chosen for photocatalytic degradation experiments. Before light irradiation, the RhB and MB solutions (initial concentration of 10 mg/mL) were stored in the dark for 1 h to achieve the adsorption and desorption balance. Then, 15 mL of the aqueous solution was filtered through the membrane using a vacuum filtration setup (85 bar).

After certain time intervals of visible light irradiation (500 W long-arc Xe lamp, reaction environmental temperature maintained at approximately 25°C by an air conditioner, and illumination distance of approximately 20 cm), 3 mL of solution was collected for UV analysis.

The degradation efficiency of the dye was calculated by the following equation:

%100Ao

At

Co

Ct efficiencyn Degradatio

where C0, Ct, A0, and At are the concentration and absorbance of RhB and MB after membrane filtration and photocatalytic degradation at reaction times of 0 and t.

1.4 Optimal film thickness and NS C3N4 addition amount for membrane production

To obtain the optimal film thickness and NS C3N4 addition amount, the rejection rate and flux for RhB solution were determined for different thicknesses. As shown in Figure S7, with increasing thickness, the rejection rate increases. When the volume reaches 6 mL (0.5 wt%) or higher, the rejection rate slightly increases. At the same time, the flow curve also forms a plateau at the 6 mL point. This means that with a higher CNF addition amount, a better rejection effect cannot be achieved. Therefore, the optimal film thickness is approximately 4 μm (6 mL CNF solution).

The optical properties of the prepared CNF/NS C3N4 membranes were further studied based on the 4 μm thick CNF support. It can be clearly found in Figure S8 that with increasing NS C3N4 addition amount, both the coverage and adulterated membranes exhibit increased rejection rates for the RhB solution. When the addition amount increases to 10 mL, the rejection rate no longer increases for the coverage membrane and slightly increases for the adulterated membrane. However, the coverage membranes have a more pronounced increase in the rejection rate and perform better than the adulterated membranes in terms of the flux. Compared with the 80% maximum rejection rate for the adulterated membranes, the coverage membranes reach a 95% rejection rate. Therefore, the coverage style membrane has greater competitive advantages in terms of fabrication cost and wastewater treatment efficiency. This kind of production process was chosen for further investigation in this paper.

2 CHARACTERISATION

Morphology of the CNFs/C3N4 membranes

The surface and cross-section morphology of the CNFs/C3N4 membranes were visualized by scanning electron microscope (Nova nanosem 450, FEI Co., Ltd., USA) with beam accelerating voltage of 10 kV. For the record, the samples were fractured under liquid nitrogen prior to the cross-section visualization. Samples were then sputter-coated (the coating thickness is c.a 10 nm).

The binding morphology of CNFs support and C3N4 powder was performed by transmission electron microscopy (TEM) analysis with using a field emission electron microscope (JEM-2100F, JEOL, Japan) at an accelerating voltage of 50 KV, coupled with an X-ray spectrometer (XFlash 5030T, Bruker, German).

X ray diffraction characterization of C3N4 powder and NS C3N4

XRD measurements were performed on a XRD diffractmeter (Panalytical B.V., Almelo, Holland) with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA. The samples were dried in a vacuum oven with P2O5 at 35 oC for 48 h and then fine powder for eliminating the influence from the crystalline orientation. The signal data were recorded in the region of 2 θ from 10° to 90°.

X-ray photoelectron spectroscopy of C3N4 powder and NS C3N4

X-ray photoelectron spectroscopy (XPS) data was obtained from Thermo ESCALAB 250 using 150 W Al Kα radiation. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon.

FT-IR spectroscopic characterization of the bulking C3N4 powder and NS C3N4.

FT-IR spectra were carried out on a FTIR spectrometer (VERTEX 70, Bruker Co., Germany). The samples were cut into powder and then vacuumdried for 24 h before measurement. The test specimens were prepared by the Faix’s method [S2].

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3 SUPPLEMENTARY FIGURES AND TABLES

Figure S1 Image of NS C3N4 and the height profile along the line in the AFM image

Figure S2 (a) Nitrogen adsorption−desorption isotherms at 77 K and (b) pore size distribution plots calculated by BJH method of the bulking C3N4

and NS C3N4

Figure S3 UV-vis spectra of the (a) RhB (b) MB liquid in the flow reactor during the membrane photodegradation process

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Figure S4 SEM cross-section of adulteration CNFs/C3N4 membrane and its distribution of C, N and O elements.

Figure S5 SEM cross-section of coverage CNFs/C3N4 membrane and its distribution of C, N and O elements.

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Figure S6 Self-made flow reactor of membrane’s photo-degradation process

Figure S7 (a) Linear relationship between the volume of the CNFs solution (0.5 wt%) and membrane thickness; (b) UV−vis absorption changes; (c) rejection rate and (d) flow capacity for RhB solution in filtration using different thickness membranes.

Figure S8 UV-vis spectra of liquid vacuum filtered through membranes with different nanosheet C3N4 addition amounts: (a) coverage and (b) adulterated membranes. (c) Rejection rate and (d) flux for different nanosheet C3N4 addition amounts at the same CNF thickness

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Figure S9 Potential degradation mechanisms of dye waste-water

Table S1 Molecular structure and properties of dyes used in this work [S2-S4]

Dye name Molecular formula Relative molecular mass Reject rate Flow capacity

(liter∙hour−1∙m−2∙bar−1)

Methylene Blue 374 98.5% 200

Rhodamine B

479 98.8% 160

The Methylene blue (MB) and Rhodamine B (RhB) are photosensitizers, which can be degraded by a self sensitization mechanism

under visible light, as shown in Figure S8. The adsorbed dye molecules will be in an unstable excited state, which can form electronically excited oxygen atoms with strong oxidizability. The charge can also transfer from the excited MB/RhB molecule to the photocatalyst and finally form electrons and holes [S5]. The electron can react with the dissolved oxygen in water, forming the superoxide radical (·O2

-) with high oxidation [S6]. The holes can react with the hydroxyl radical (·OH) with high reactivity in solution. Finally, the dye molecules will be degraded after a series of reactions in the existence of ·O2

- and ·OH [S7].

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