Fabrication of organic–inorganic hybrid membranes composed of poly(vinylidene fluoride) and silver cyanamide and their high photocatalytic activity under visible light irradiation

Abstract

A new hybrid membrane composed of poly(vinylidene fluoride) (PVDF) and silver cyanamide (Ag₂NCN) has been successfully fabricated via an induced phase inversion method by blending Ag₂NCN particles and PVDF casting solution. The rectangular Ag₂NCN particles were well dispersed in PVDF membranes via weak physical interaction, and the Ag₂NCN amount was varied to investigate the effect of the Ag₂NCN particles on the photocatalytic properties of PVDF/Ag₂NCN hybrid membranes. The as-prepared hybrid membranes display excellent photocatalytic activity towards the degradation of Acid Blue 1 under visible light irradiation, showing great potential for practical applications. Particularly, these PVDF/Ag₂NCN hybrid membranes have enhanced photocatalytic properties under acidic conditions, and a possible H⁺-promoted photocatalytic mechanism was proposed in this study. Furthermore, this facile and generic strategy may provide a convenient and powerful method to fabricate other polymer–inorganic hybrid membranes for advanced applications.

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Introduction

The increasing contamination of organic dyes poses a significant threat to the environment and the health of living beings due to the adverse effects of these dyes. Various technologies have been developed for the treatment of organic dyes. Photocatalytic technology, first reported by Fujishima et al. in 1972, is regarded as one of the most competitive techniques due to its excellent activity in the complete mineralization of hazardous organic wastes.^1,2 Many common semiconductor photocatalysts, such as TiO₂, ZnO, WO₃, Fe₂O₃, CdS, BiOCl and Ag₃PO₄,^3–6 have been applied in the photocatalytic degradation of organic contaminants. Among these, TiO₂ is considered to be one of the most active photocatalysts. However, the wide band gap of TiO₂ (3.2 eV) and its inability to be irradiated by visible light have limited its practical applications.^7–9 Therefore, the development of new photocatalysts with high efficiency and visible-light response has been pursued in recent years.¹⁰

In the past decades, the crystallographic study of metal cyanamide has enormously progressed because of the soft, sterically small, and potentially polydentate nature of cyanamide anion (NCN²⁻). The [NCN]²⁻ anion can adopt two electronic forms: the symmetric carbodiimide [N≡C–N]²⁻ form with a double bond distance of 1.22 to 1.24 Å in MNCN (M = Mn, Cu, Fe, Co, Ni, etc.) and M₂NCN (M = Li, Na, K, etc.), and the asymmetrical cyanamide [N≡C–N]²⁻ form with C≡N and C–N bond distances of 1.15 Å and 1.30 Å, respectively, in PbNCN, and 1.12 Å and 1.35 Å in HgNCN.^11–21 Recently, Ag₂NCN has been reported to be a new n-type of semiconductor that can utilize visible light for photocatalysis.²² The bond distances in Ag₂NCN were found to be 1.20 Å and 1.27 Å, indicating the coexistence of the two forms of [NCN]²⁻. It should be mentioned that the asymmetric form of [N≡C–N]²⁻ can endow localized dipoles and dipolar fields to aid the long-range migration of opposite electrons in the photocatalytic process; thus, metal cyanamide is an outstanding material for photocatalysis-driven applications.^22,23 In our previous work,^24,25 we have investigated both Ag₂NCN and Ag₂NCN–TiO₂ nanocomposites in detail for the splitting of water into hydrogen gas and the degradation of methylene blue under visible light. The experimental results showed that the Ag₂NCN–TiO₂ nanocomposites have excellent stability and exhibit higher photocatalytic performance than pure Ag₂NCN due to the efficient separation of photo-induced electrons and holes between Ag₂NCN and TiO₂ in the composites.

It is known that the aggregation of nanosized photocatalysts still limit its re-usability in practical applications. To address this problem, several methods have been developed.^26,27 Fabrication of a hybrid photocatalytic membrane through immobilizing the semiconductors on the porous polymer films has proved to be an effective way to solve these problems. Some organic polymer membranes, such as polysulfone,²⁸ polyether sulphone²⁹ and PVDF,^30–33 have been developed. PVDF membrane is considered to be more attractive due to its chemical and thermal stability, UV light resistance and controllable pore size. A series of PVDF membranes, such as TiO₂/PVDF,³⁰ Zr–SiO₂@TiO₂/PVDF,³¹ M–rGO–TiO₂/PVDF (M = Ag, Pt)³² and ZnIn₂S₄/PVDF–poly(MMA-co-MAA),³³ have been reported.

In this work, a facile synthesis of Ag₂NCN decorated PVDF membranes was successfully developed. The structure and visible-light-driven photocatalytic properties of the Ag₂NCN decorated PVDF membranes with various amounts of Ag₂NCN particles were investigated. The Ag₂NCN particles dispersed homogeneously in the PVDF membranes, and the obtained photosensitive PVDF membranes demonstrated very high photocatalytic efficiency for the degradation of Acid Blue 1; this is attributed to the highly delocalized electronic state in the Ag₂NCN composites and the enrichment of organic wastes in the porous PVDF polymer membrane. The simple physical blending of Ag₂NCN particles and PVDF casting solution, followed by an induced phase inversion method, could also be applied as a universal pathway to fabricate other functional membranes and enable their large-scale applications in environmental protection.

Experimental section

Chemicals

Cyanamide aqueous solution (50 wt%) was purchased from Aladdin Company (China). Poly(vinylidenefluoride) (PVDF) powder was purchased from Mosu Science Equipment Corporation (Shanghai, China). Silver nitrate, polyvinyl pyrrolidone, Acid Blue 1 and N,N-dimethyl formamide (DMF) were obtained from Sinopharm Chemical Reagent Corporation (Shanghai, China). All the chemicals were of analytical grade and were used without further purification; 18 MΩ cm⁻¹ deionized water was used throughout the experiments.

Preparation of PVDF/Ag₂NCN hybrid membranes

The Ag₂NCN particles were synthesized firstly through chemical precipitation according to our previous work.²⁴ In a typical synthesis, 125 mL of NH₃·H₂O (1.5 M) was added to 20 mL of AgNO₃ (0.25 M) under stirring to obtain a transparent solution. After that, 5 mL of H₂NCN aqueous solution (1 M) was added to the above solution at room temperature, and a yellow precipitate was produced. The suspension was maintained at room temperature for 0.5 h under vigorous stirring. Finally, the precipitate was centrifuged, washed thoroughly with distilled water, and dried at 60 °C under vacuum conditions.

PVDF/Ag₂NCN hybrid membranes were fabricated through an induced phase inversion method. Typically, a certain quantity of as-prepared Ag₂NCN particles were fully dispersed in 100 mL of DMF solution containing 14 wt% PVDF and 2 wt% PVP. Then, the mixture was cast onto glass plates using a Dip Coater device. After that, the glass plates were immersed into a coagulation water bath. Finally, the hybrid films were formed, washed and stored in distilled water. The synthesized hybrid membranes were assigned as PVDF/Ag₂NCN-x; the x values indicated the addition amounts of Ag₂NCN particles, whose values varied from 1.0 wt% to 6.0 wt%. For comparison, the pristine PVDF film was prepared through the above method but without Ag₂NCN addition, and 2 wt% PVP decorated Ag₂NCN particles were also prepared by dispersing Ag₂NCN particles in 100 mL of DMF containing 2 wt% PVP, which was labeled as Ag₂NCN–PVP.

Characterizations

The surface morphologies of the membrane samples were characterized using a SSX-50 scanning electron microscope (SEM, Shimadzu, Japan). Power X-ray diffraction (PXRD) patterns were acquired with a Rigaku XRD D/max-2500PC instrument (Cu Kα, tube voltage of 50 kV and tube current of 100 mA). X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi instrument (Thermo Fisher Co., USA) using an Al-Kα monochromatic X-ray (1486.6 eV) source. FT-IR spectra were recorded using a VERTEX 70 Fourier transform infrared spectrophotometer (Bruker, Germany), and the samples were dispersed in anhydrous KBr. Thermo-gravimetric curves were acquired on a TA instrument (TGA/DSC/1600LF, Mettler Toledo, Switzerland); the sample was heated in the temperature range of 25 to 800 °C under nitrogen with a heating rate of 5 °C min⁻¹.

Photocatalytic experiments

The photocatalytic degradation of Acid Blue 1 was carried out in an aqueous solution at ambient temperature under visible light irradiation from a 300 W xenon lamp with a 420 nm cutoff filter. Briefly, 0.5 g of PVDF/Ag₂NCN-x composite film was suspended in 100 mL of Acid Blue 1 aqueous solution (10⁻⁵ M), the pH value of which was pre-adjusted to 3.0, 4.0 and 5.0, respectively, using glacial acetic acid. The obtained suspension was transferred into a glass reactor with a top quartz window and a water cooling jacket, which was maintained at 5 °C during the photocatalysis. The distance between the reaction bottle and light source was maintained at 10 cm, and the light irradiation density was 94.30 mW cm⁻². The concentrations of Acid Blue 1 at different times were analyzed with a UV-visible spectrophotometer (Lambda 35, Perkin Elmer, USA) at a wavelength of 630 nm. The photocatalysts were recovered after the experiments and dried for the characterization and recycling experiments.

Results and discussion

Characterization of PVDF/Ag₂NCN-x hybrid membranes

Fig. 1(a)–(h) shows the SEM images of single Ag₂NCN particles and the PVDF/Ag₂NCN hybrid membranes with increasing Ag₂NCN addition from 1% to 6%. It can be seen that the rectangular Ag₂NCN particles are well dispersed in the PVDF membranes. The average particle size of Ag₂NCN was maintained at about 900 nm in length and 500 nm in width. The pore size and shape of the PVDF films varied as the amount of Ag₂NCN increased. Comparing Fig. 1(f) and (h) with Fig. 1(b) and (d), when the Ag₂NCN addition increased to 4% and 6%, the finger-like pores of PVDF were increased, and the length of the pores increased. It has been demonstrated before that the existence of nanoparticles, such as TiO₂ particles, can act as crystal nuclei to favor heterogeneous nucleation of PVDF films rather than homogeneous nucleation.^34–36 Thus, as shown in Fig. 1, with increasing nanoparticle amounts, the surface roughness and the pore sizes of PVDF increased. The SEM results also proved that the Ag₂NCN particles were successfully decorated in the inner pores of the PVDF matrix.

Fig. 1 Top (a, c, e and g) and cross-sectional (b, d, f and h) view SEM images of PVDF/Ag₂NCN hybrid membranes with Ag₂NCN additions of 1% (a and b); 2% (c and d); 4% (e and f) and 6% (g and h).

Fig. 2(a) shows the PXRD patterns of the PVDF/Ag₂NCN hybrid membranes, and the same X-ray diffraction patterns were observed for all the hybrid membranes. As shown in Fig. 2(a), the diffraction peaks appeared at 2θ = 19.30°, 20.04° and 25.45°, attributed to the diffraction from (020), (110) and (022) of α-phase PVDF films.³⁵ The remaining diffraction was in agreement with the standard data of monoclinic Ag₂NCN (JCPDS no. 70-523). The PXRD results also proved that the Ag₂NCN microparticles have been well decorated in the PVDF membranes.

Fig. 2 PXRD patterns (a) and TG curves (b) of PVDF/Ag₂NCN-x hybrid membranes (x = 1, 2, 4, and 6, indicating the weight percent of Ag₂NCN particles (%) used in the synthesis mixture).

TG curves of the PVDF/Ag₂NCN hybrid membranes with various amounts of Ag₂NCN are shown in Fig. 2(b). For comparison, the TG measurement of pristine PVDF film without Ag₂NCN particles was also conducted. A main weight loss was observed between 300 °C and 500 °C, corresponding to the decomposition of PVDF. The weight loss for PVDF and PVDF/Ag₂NCN hybrid membranes with Ag₂NCN amounts of 1%, 2%, 4%, and 6%, were 93.02, 90.56, 87.72, 80.16 and 72.51%, respectively. The calculated amounts of Ag₂NCN were estimated to be 2.46, 5.3, 12.86 and 20.86%, respectively, for the as-prepared PVDF/Ag₂NCN hybrid membranes with different ratios of 1, 2, 4 and 6% Ag₂NCN particles used in the syntheses.

In order to investigate the interaction between the PVDF film and Ag₂NCN particles, the FT-IR spectra of PVDF and PVDF/Ag₂NCN hybrid membranes were measured and are shown in Fig. 3. For PVDF, the absorptions of 1394 cm⁻¹ and 1174 cm⁻¹ were assigned to the deformation and stretching vibrations of –CF₂;³⁷ the absorption of 873 cm⁻¹ was a characteristic absorption of α-phase PVDF;^37,38 the band at 1671 cm⁻¹ was assigned to the stretching vibration of C=O for PVP or residual DMF; and the absorption at 1062 cm⁻¹ could be attributed to the stretching vibration of –OH, indicating that some –C–F bonds were substituted by –OH groups.^37,38 For comparison, the PVDF/Ag₂NCN-x hybrid membranes showed similar IR absorptions to those of pure PVDF, except that the characteristic absorption of [NCN]²⁻ appeared at 1980 cm⁻¹.^39,40 The FT-IR spectra implied that there was no chemical bond between the PVDF and Ag₂NCN particles, and weak physical interaction played an important role in the formation of the hybrid membranes.

Fig. 3 FT-IR spectra of the PVDF/Ag₂NCN-x hybrid membranes (x = 1, 2, 4, and 6, indicating the weight percent of Ag₂NCN particles (%) used in the synthesis mixture).

PVP is a good pore-forming reagent in the fabrication of porous polymer membranes to control the porous structures of the membranes.^41–43 In our experiments, as shown in Fig. S1,† a determined amount of PVP (2%) is essential for synthesizing porous PVDF films with uniform pore structures. In order to study whether there is a interaction between PVP and Ag₂NCN, a FT-IR investigation of the PVP decorated Ag₂NCN particles was also conducted. As shown in Fig. S2,† a weak band at 1676 cm⁻¹ could be assigned to the stretching vibration of C=O for PVP or residual DMF, which overlapped with the band at 2005 cm⁻¹ attributed to NCN²⁻. The IR peaks at 1270 and 1191 cm⁻¹ are due to the symmetric stretching vibration and deformation vibration of [NCN]²⁻. The above results implied that a weak physical interaction may exist between PVP and Ag₂NCN.

Fig. 4 shows the XPS spectra of PVDF/Ag₂NCN-1 hybrid membranes. As shown in Fig. 4(a), the full-scale XPS pattern indicated the existence of Ag, C, F, N and O. The binding energies of C 1s, Ag 3d, N 1s, O 1s and F 1s were 284.2, 368.8, 399.5, 530.9 and 685.8 eV, respectively. The high-resolution XPS spectrum of Ag 3d (Fig. 4(b)) presented binding energies of Ag 3d_5/2 (368.3 eV) and Ag 3d_3/2 (374.4 eV), respectively. According to ref. 44–46, the 3d peaks of metallic Ag are located at 367.9 and 373.9 eV, while those for the Ag(I) ions are centered at 369.4 and 375.6 eV. Therefore, it was suggested that a small amount of metallic Ag may exist in the PVDF/Ag₂NCN-1 hybrid membrane, which was probably produced by a slight photo-induced reduction of Ag(I) during the synthesis process. However, the amount of metallic Ag must be very small, as it was not detected in the XRD pattern.

Fig. 4 Broad XPS spectrum (a) and Ag 3d XPS spectrum (b) of the PVDF/Ag₂NCN-1 hybrid membrane.

Photocatalytic properties and photocatalytic mechanism of PVDF/Ag₂NCN-x hybrid membranes

A control experiment was first conducted by irradiating the Acid Blue 1 solution without adding photocatalyst; the results are shown in Fig. S3.† It was observed that the concentration of Acid Blue 1 remained constant on exposure to irradiation. Fig. 5(a) shows the photocatalytic degradation kinetics of Acid Blue 1 under visible light irradiation catalyzed by PVDF/Ag₂NCN-x hybrid membranes. Before the light irradiation, an adsorption equilibrium under dark conditions was achieved in the first 30 min. It was found that the concentration of Acid Blue 1 did not change evidently under dark conditions and decreased dramatically when visible light was induced. All the hybrid membranes presented excellent photocatalytic efficiency, and nearly 100% of Acid Blue 1 could be degraded within 40 min. Among these hybrid membranes, the hybrid membranes with 1% and 2% Ag₂NCN addition presented higher photocatalytic activity than other hybrid membranes with more Ag₂NCN addition (4% or 6%). Taking into account the SEM results of these hybrid membranes, the decreased photocatalytic activity for the hybrid membranes with higher Ag₂NCN content might be caused by the aggregation of Ag₂NCN microparticles in the PVDF matrix, which blocked some pores of the PVDF and inhibited the pre-concentration of organic dyes around the Ag₂NCN photocatalysts.

Fig. 5 (a) Photodegradation kinetics of Acid Blue 1 by the PVDF/Ag₂NCN hybrid membranes; (b) control photocatalytic experiments using pure Ag₂NCN, PVP decorated Ag₂NCN, and single PVDF films with and without the addition PVP as photocatalysts; (c) pH-dependent experiments for the PVDF/Ag₂NCN-1 hybrid membrane; (d) pH-dependent experiments for pure Ag₂NCN particles.

For comparison, the photo-degradation of Acid Blue 1 catalyzed by pure Ag₂NCN, 2% PVP decorated Ag₂NCN, and original PVDF films with and without PVP was conducted under same conditions. As shown in Fig. 5(b), for the original PVDF films synthesized without PVP, photocatalytic degradation activity was not detected. In comparison, for the pure PVDF films synthesized with PVP, a 17.62% degradation rate was found, which may be correlated with its uniform porous adsorption under visible light irradiation. The PVP decorated Ag₂NCN particles also presented better photocatalytic activity than the pure Ag₂NCN particles; the degradation rates of Acid Blue 1 within 50 min were 93.61% and 75.12% for Ag₂NCN–PVP and Ag₂NCN, respectively. The results proved that the improved photocatalytic efficiency for the hybrid membranes was mainly due to the synergistic effect of the composite structure. For the PVDF/Ag₂NCN hybrid membranes, the organic dye molecule was able to pre-concentrate around the Ag₂NCN particles due to the porous nature of PVDF, which helped to accelerate the photocatalytic process.

The effect of pH on the photocatalytic efficiency was also studied. Fig. 5(c) shows the degradation percentage of Acid Blue 1 at different pH values using the PVDF/Ag₂NCN-1 hybrid membrane as a photocatalyst. Obviously, the photocatalytic activity of the hybrid membrane was significantly affected by the pH conditions, and the maximum photocatalytic activity was obtained at pH 3.0. In comparison, the same pH-dependent photocatalysis was also conducted using pure Ag₂NCN particles, and Fig. 5(d) showed the experiment results. It is obvious that the photocatalytic activities of pure Ag₂NCN particles were also sensitive to the pH of the solution; at pH 3.0, the best photocatalytic efficiency was observed, with nearly 100% of Acid Blue 1 being decomposed in 90 min.

The UV-Vis spectrum of Ag₂NCN was measured and is shown in Fig. 6(a). The Ag₂NCN microparticles had strong absorption capacity in both the UV and visible light ranges, and their band gap can be calculated according to the formula:

Fig. 6 (a) UV-Vis absorption spectrum of Ag₂NCN particles; (b) plot of the calculated (A × hν)² as a function of light energy (hν); (c) Mott–Schottky plot of the as-prepared Ag₂NCN microparticles; (d) effects of scavengers on the photodegradation of Acid Blue 1 by Ag₂NCN particles.

Fig. 6(b) presents the calculated value of (A × hν)² as a function of light energy (hν), and the associated direct band gap was 2.31 eV. The Mott–Schottky plot of the as-prepared Ag₂NCN microparticles were also measured (Fig. 6(c)), and the obtained potential of the conduction band (CB) and valence band (VB) of Ag₂NCN versus saturated calomel electrode (SCE) were −0.97 V and 1.34 V, respectively. To further investigate the photocatalytic active species for the Ag₂NCN photocatalysts, the electron and hole trapping experiments were carried out using methanol as a hole scavenger and hydrogen peroxide as an electron scavenger, respectively, and the results are shown in Fig. 6(d). It could be observed that the photodegradation process was decelerated when hydrogen peroxide was added. Furthermore, the addition of methanol did not affect the photodegradation efficiency. These results implied that electrons rather than holes play an important role in the photocatalytic process for the Ag₂NCN photocatalysts.

Based on the above results, the band structure of Ag₂NCN and the photocatalytic process are proposed in Fig. 7. The potential of the photo-generated holes of Ag₂NCN (1.34 V) is lower than the potential of OH⁻/˙OH (2.37 V). As a result, the holes could not interact with OH⁻; thus, the photo-generated electrons play an important role in the photocatalytic reaction. Based on the photocatalytic mechanism reported for other photocatalysts,⁴⁷ the photocatalytic process for the PVDF/Ag₂NCN hybrid membranes was proposed and is shown in Fig. 7(b). First, the photo-generated electrons reacted with an adsorbed oxygen molecule and produced ˙O₂⁻; then the ˙O₂⁻ might be translated to ˙OOH or accept electrons and be converted into ˙OH in the presence of H⁺. ˙OH and ˙OOH are very active radicals with strong oxidation capacities, leading to the decomposition of the organic dyes.

Fig. 7 (a) The sketching map of the band structure of the Ag₂NCN microparticles; (b) the proposed photocatalytic mechanism for the PVDF/Ag₂NCN hybrid membranes.

Photocatalytic stability of PVDF/Ag₂NCN hybrid membranes

To further study the photocatalytic stability of the PVDF/Ag₂NCN hybrid membranes, recycling photocatalytic tests over the PVDF/Ag₂NCN-1 hybrid membrane were conducted for 5 cycles, and the results are shown in Fig. 8. It was observed that the degradation efficiency of Acid Blue 1 was nearly 100% in the first two rounds; however, the degradation decreased for the second cycle, and the degradation time also increased to 40 min. In the 3^rd, 4^th and 5^th cycles, the degradation efficiency decreased even further, to 91.2, 83.5 and 77.2%, respectively, in 90 min.

Fig. 8 Recycling photodegradation kinetics of Acid Blue (1) by the PVDF/Ag₂NCN-1 hybrid membrane.

The morphology and crystalline structure of the PVDF/Ag₂NCN hybrid membranes after photocatalytic tests were measured, and the results are shown in Fig. 9(a) and (b). Compared with the structure of the hybrid membranes before photocatalysis, the morphology and the crystalline structure of the hybrid membranes did not change. The XPS spectra of the PVDF/Ag₂NCN-1 hybrid membranes after photocatalysis are also shown in Fig. 9(c) and (d). As shown in Fig. 9(c), the full-scale XPS pattern indicated the existence of Ag, C, F, N and O elements. The high-resolution XPS spectrum of Ag 3d (Fig. 9(d)) showed that the binding energies of Ag 3d_5/2 and Ag 3d_3/2 were 368.2 and 374.4 eV, respectively. Compared with that of the samples before photocatalysis (Fig. 4), the element composition of the hybrid membranes and the chemical state of Ag element did not change during the photocatalysis. The above results suggested the good structural stability of these hybrid membranes. The decreased photocatalytic activity of the hybrid membranes in the cycle tests may arise from membrane fouling or a reduction of the activity of the nanophotocatalysts; the former would limit the pre-concentration of Acid Blue 1 around the Ag₂NCN photocatalysts and the latter would directly decrease the photocatalytic efficiency.

Fig. 9 (a) SEM image, (c) broad XPS spectrum and (d) Ag 3d XPS spectrum of PVDF/Ag₂NCN-1 hybrid membrane post photocatalysis; (b) PXRD patterns of PVDF/Ag₂NCN-1, 2, 4 and 6 hybrid membrane post photocatalysis.

Conclusions

Hybrid membranes with high photocatalytic activity have been fabricated by dispersing Ag₂NCN microparticles in poly(vinylidenefluoride) (PVDF) film. Physical blending of Ag₂NCN microparticles and PVDF casting solution, followed by a induced phase inversion method, was applied for the synthesis of the hybrid membranes. The characterization results showed that the average particle size of Ag₂NCN was about 900 nm in length and 500 nm in width, and the pore size and shape of PVDF films varied as the Ag₂NCN addition increased. A weak physical interaction existed between the Ag₂NCN microparticles and PVDF films. In the photocatalytic degradation of Acid Blue 1, the PVDF/Ag₂NCN hybrid membranes presented enhanced photocatalytic activity under visible light irradiation compared with pure PVDF films or Ag₂NCN particles, and nearly 100% of Acid Blue 1 degraded within 40 min. The photocatalytic properties of the hybrid membranes are also improved under acidic conditions, and photogenerated electrons rather than holes play an important role in the photocatalytic process of Ag₂NCN. Thus, a H⁺-promoted photocatalytic mechanism was proposed. The hybrid membranes possess good structural stability, demonstrated by photocatalytic recycle experiments. The above results suggest that PVDF/Ag₂NCN hybrid membranes have great application prospects in the photocatalytic degradation of organic dyes.

Acknowledgements

The authors are very grateful for the financial support of the National Natural Science Foundation of China (No. 21501023, 21401018) and the National Training Program of Innovation and Entrepreneurship for Undergraduates (151012).

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Footnotes

† Electronic supplementary information (ESI) available: SEM images of PVDF membranes fabricated without/with PVP, FT-IR spectrum of 2 wt% PVP decorated Ag₂NCN particles, and the concentration of Acid Blue 1 versus irradiating time without adding photocatalysts. See DOI: 10.1039/c6ra10434g

‡ These authors contributed equally to this work.

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