Preparation and Characterization of TiO2/g-C3N4/PVDF Composite Membrane with Enhanced Physical Properties

TiO2/g-C3N4/PVDF composite membranes were prepared by a phase inversion method. A comparison of the performance and morphology was carried out among pure PVDF, g-C3N4/PVDF, TiO2/PVDF and TiO2/g-C3N4/PVDF composite membranes. The results of permeability and instrumental analysis indicated that TiO2 and g-C3N4 organic-inorganic composites obviously changed the performance and structure of the PVDF membranes. The porosity and water content of 0.75TiO2/0.25g-C3N4/PVDF composite membranes were 97.3 and 188.3 L/(m2·h), respectively. The porosity and water content of the 0.75TiO2/0.25g-C3N4 membranes were increased by 20.8% and 27.4%, respectively, compared with that of pure PVDF membranes. This suggested that the combination of organic-inorganic composite with PVDF could remarkably improve UTS, membrane porosity and water content.

To date, a variety of strategies have been employed to fabricate hydrophilic PVDF membranes, including coating [17], adsorption [18], plasma treatment [19], blending [20] and surface grafting polymerization [21]. Among these, published research results thus far have been focused on PVDF membranes fabricated by nanoparticle coating, due to the unique electronic, magnetic and optical properties of nanoparticles, which would greatly improve the capabilities of polymers [22][23][24][25]. The nanoparticles introduced into PVDF membranes include polymeric chains and metal oxide. From among the polymeric chains, surface-modified macromolecules have been used as additives in the membrane matrix for anti-fouling applications [26][27][28]. This has been an effective strategy for enhancing anti-fouling properties by mitigating membrane fouling. Generally, coating a thin film and grafting polymer chains on the surface of the membrane have been two typical approaches in previous research [29]. Among metal oxide nanoparticles, TiO 2 has received the most attention, due to its stability and availability [30][31][32][33].
Recently, graphitic carbon nitride (g-C 3 N 4 ) has become a promising candidate for photocatalysis due to its low cost, visible-light response, simple synthesis and high chemical stability [34][35][36][37]. To date, there have been several reports on the synthesis and application of g-C 3 N 4 /PVDF membranes [38][39][40]. In our previous report, g-C 3 N 4 /PVDF membrane was fabricated through a phase inversion method. It has been found that dispersing g-C 3 N 4 into PVDF membrane can change the thermal decomposition process of PVDF membranes.
In this paper, we propose a facial approach for obtaining TiO 2 /g-C 3 N 4 /PVDF composite membranes by a phase inversion method. It is clearly demonstrated that the TiO 2 and g-C 3 N 4 can obviously change the performance and structure of PVDF membranes. Moreover, the effects of organic-inorganic composites on the performance and structure of the PVDF membrane are also investigated.

Materials
Poly(vinylidenefluoride) (PVDF), the membrane material, was purchased from the Shanghai 3F New Materials Co., Ltd., Shanghai, China. N,N -dimethylformamide (DMF), Silane coupling agent (SCA), polyethylene glycol (PEG 6000), Rutile TiO 2 nanoparticles, and Melamine were purchased from Sinopharm chemical reagent Co., Ltd., Shanghai, China. All of the used chemicals were of analytical grade, and were used without further purification.

Preparation of Carbon Nitride (g-C 3 N 4 )
In a typical synthesis, 20 g melamine was transferred to an alumina crucible with a cover and heated to 550 • C in Ar atmosphere for 2 h with a heating rate of 5 • C/min. After undergoing various reactions at high temperature, a light-yellow powder of g-C 3 N 4 was finally obtained in the alumina crucible.

Preparation of Membrane
In a typical synthesis p, 0.6 g PFG-6000 was introduced in 21 mL DMF and stirred for 30 min at 50 • C. After that, 3 g PVDF and 1.5 mL SCA was added into the previous solution and stirred for another 30 min to form a homogenous suspension. The right amount of TiO 2 and g-C 3 N 4 were added and stirred for 4 h. Finally, the final solution was slowly poured on the near end of the glass and a casting knife was placed on one edge of the glass to cast a membrane with a thickness of 2 mm. The obtained samples were denoted as xTiO 2 /yg-C 3 N 4 /PVDF membrane, where x and y refer to the mass ratio of TiO 2 and g-C 3 N 4 .

Contact Angle Measurement (Sessile-Drop Method)
Water contact angle was measured with a Data Physics optical contact angle measuring instrument with the droplet size controlled using a Gilmont syringe (Chengde Dingsheng testing machine testing equipment Co., Ltd., Chengde, China). Distilled water was used for analysis. The advancing angle was measured when water was added to a droplet spreading over the membrane surface. Droplets were in placed in contact with the membrane at several different locations on each membrane sample to obtain a series of contact angle pairs. All measurements were carried out at room temperature.

Porosity Measurement
In order to evaluate the porosity of the membranes, the membranes were placed in an air-circulating oven at 60 • C for 24 h. Then, they were weighed after wiping off surface water with blotting paper.
After that, the wet membranes were placed in an oven at 80 • C for 24 h in order to ensure they were completely dry. The porosity of the membranes (P) was calculated by: where P is the porosity of membrane, W 0 is the wet sample weight (g), W 1 is the dry sample weight (g), A is the square of membrane (cm 2 ) and h is the thickness of membrane (mm).

Characterization
The mechanical strength of the membranes was tested by Instron 5542 Material Testing Instrument (Changchun Kexin Experimental Instrument Co., Ltd., Changchun, China) at room temperature (25 • C) and 80% relative humidity. Fourier Transform Infrared (FT-IR) spectroscopy was performed on a Nexus 870 spectrometer (BRUKER, TENSOR27, Karlsruhe, Germany). Field Emission Scanning Electron Microscopy (S-4800 Hitachi, Tokyo, Japan) was applied to observe the morphology of the resulting membranes. Each sample was clamped at the both ends with an initial gauge length of 100 mm and width of 20 mm. Thermogravimetric Analysis (Simultaneous TGA-DSC, New Castle, DE, USA) was conducted under nitrogen from 30 to 700 • C at a heating rate of 10 • C·min −1 . X-ray diffraction patterns were recorded by X-ray Diffractometer (BRUKER-AXS, Karlsruhe, Germany). Figure 1 depicts the FT-IR spectra of PVDF, TiO 2 /PVDF, g-C 3 N 4 /PVDF and TiO 2 /g-C 3 N 4 /PVDF membranes. The 0.75TiO 2 /0.25g-C 3 N 4 /PVDF membrane was characterized by typical IR patterns of PVDF membrane, indicating that the main chemical skeleton of PVDF membrane had been retained. There is a weak band located at 2917 cm −1 , which is associated with the CH stretching of PVDF structure [41]. The band at 3341 cm −1 is associated with OH stretching vibration of water molecules emanating from the polymer pores [42]. Notably, the absorption band of OH for TiO 2 /PVDF was weakened compared with that of PVDF, which may be caused by the effects of the hydrogen bonds between the fluorine atoms in PVDF and the oxygen atoms in TiO 2 , implying that the TiO 2 had been successfully distributed on the surface of PVDF. Interestingly, the band at 3341 cm −1 appeared in g-C 3 N 4 /TiO 2 /PVDF, which was associated with the stretching mode of N-H of g-C 3 N 4 [43]. This phenomenon was attributed to the g-C 3 N 4 having also been successfully distributed on the surface of PVDF.

Results and Discussion
where P is the porosity of membrane, W0 is the wet sample weight (g), W1 is the dry sample weight (g), A is the square of membrane (cm 2 ) and h is the thickness of membrane (mm).

Characterization
The mechanical strength of the membranes was tested by Instron 5542 Material Testing Instrument (Changchun Kexin Experimental Instrument Co., Ltd., Changchun, China) at room temperature (25 °C) and 80% relative humidity. Fourier Transform Infrared (FT-IR) spectroscopy was performed on a Nexus 870 spectrometer (BRUKER, TENSOR27, Karlsruhe, Germany). Field Emission Scanning Electron Microscopy (S-4800 Hitachi, Tokyo, Japan) was applied to observe the morphology of the resulting membranes. Each sample was clamped at the both ends with an initial gauge length of 100 mm and width of 20 mm. Thermogravimetric Analysis (Simultaneous TGA-DSC, New Castle, DE, USA) was conducted under nitrogen from 30 to 700 °C at a heating rate of 10 °C·min −1 . X-ray diffraction patterns were recorded by X-ray Diffractometer (BRUKER-AXS, Karlsruhe, Germany). Figure 1 depicts the FT-IR spectra of PVDF, TiO2/PVDF, g-C3N4/PVDF and TiO2/g-C3N4/PVDF membranes. The 0.75TiO2/0.25g-C3N4/PVDF membrane was characterized by typical IR patterns of PVDF membrane, indicating that the main chemical skeleton of PVDF membrane had been retained. There is a weak band located at 2917 cm −1 , which is associated with the CH stretching of PVDF structure [41]. The band at 3341 cm −1 is associated with OH stretching vibration of water molecules emanating from the polymer pores [42]. Notably, the absorption band of OH for TiO2/PVDF was weakened compared with that of PVDF, which may be caused by the effects of the hydrogen bonds between the fluorine atoms in PVDF and the oxygen atoms in TiO2, implying that the TiO2 had been successfully distributed on the surface of PVDF. Interestingly, the band at 3341 cm −1 appeared in g-C3N4/TiO2/PVDF, which was associated with the stretching mode of N-H of g-C3N4 [43]. This phenomenon was attributed to the g-C3N4 having also been successfully distributed on the surface of PVDF. The morphologies of PVDF, TiO 2 /PVDF, g-C 3 N 4 /PVDF and g-C 3 N 4 /TiO 2 /PVDF membranes were investigated through SEM, and the results are presented in Figure 2. Before SEM, the sample membranes have to be dried. Generally, three different drying methods have been proposed to dry membranes, including room temperature-oven drying, ethanol-hexane drying, and freeze-drying [44,45]. In this paper, room temperature-oven drying is used. The membranes are firstly dried at room temperature for 12 h, and then dried in an oven of 120 • C for 6 h. Compared with the porous and coarse structure of PVDF membrane (Figure 2a), both TiO 2 /PVDF and g-C 3 N 4 /PVDF (Figure 2b,c) membranes show a smooth structure with circular and dark voids of uniform dimensions. Notably, the g-C 3 N 4 /TiO 2 /PVDF membrane (Figure 2d) shows decreased but more regular and uniform voids. The morphologies of PVDF, TiO2/PVDF, g-C3N4/PVDF and g-C3N4/TiO2/PVDF membranes were investigated through SEM, and the results are presented in Figure 2. Before SEM, the sample membranes have to be dried. Generally, three different drying methods have been proposed to dry membranes, including room temperature-oven drying, ethanol-hexane drying, and freeze-drying [44,45]. In this paper, room temperature-oven drying is used. The membranes are firstly dried at room temperature for 12 h, and then dried in an oven of 120 °C for 6 h. Compared with the porous and coarse structure of PVDF membrane (Figure 2a), both TiO2/PVDF and g-C3N4/PVDF (Figure  2b,c) membranes show a smooth structure with circular and dark voids of uniform dimensions. Notably, the g-C3N4/TiO2/PVDF membrane (Figure 2d) shows decreased but more regular and uniform voids. To further study the physical strength and durability of the membrane, the Ultimate Tensile Strength (UTS) was measured, and the results are shown in Tables 1-3. As expected, 0.75TiO2/0.25g-C3N4/PVDF membrane showed the highest UTS value. Values for TiO2/PVDF, g-C3N4/PVDF, and 0.75TiO2/0.25g-C3N4/PVDF membranes were measured as 7.5, 7.1 and 8.7 MPa, respectively. It is clear that the addition of TiO2 and g-C3N4 to PVDF improved the mechanical strength of the composite membranes. It is well known that membranes with macro-void morphologies often show inferior mechanical properties [42,46]. It was assumed that the increased UTS in TiO2/g-C3N4/PVDF composite membrane was attributable to the decreased voids in the membrane structure. Furthermore, 0.75TiO2/0.25g-C3N4/PVDF composite membrane exhibited the highest tensile strength, which could be attributable to there being fewer voids in the membrane structure.

Results and Discussion
To determine the hydrophilicity/hydrophobicity of the membrane surface, the contact angle was measured. As shown in Table 1, the contact angle of the as-prepared membrane with TiO2 and g-C3N4 was decreased. It is well known that the membrane wettability of the membrane is influenced by the membrane material, as well as the surface porosity and roughness [47,48]. After adding TiO2 and g-C3N4, the surface porosity sharply increased. In other words, adding the TiO2 and g-C3N4 leads to a higher surface porosity and results in a decreased contact angle of the membrane. To further study the physical strength and durability of the membrane, the Ultimate Tensile Strength (UTS) was measured, and the results are shown in Tables 1-3. As expected, 0.75TiO 2 / 0.25g-C 3 N 4 /PVDF membrane showed the highest UTS value. Values for TiO 2 /PVDF, g-C 3 N 4 /PVDF, and 0.75TiO 2 /0.25g-C 3 N 4 /PVDF membranes were measured as 7.5, 7.1 and 8.7 MPa, respectively. It is clear that the addition of TiO 2 and g-C 3 N 4 to PVDF improved the mechanical strength of the composite membranes. It is well known that membranes with macro-void morphologies often show inferior mechanical properties [42,46]. It was assumed that the increased UTS in TiO 2 /g-C 3 N 4 /PVDF composite membrane was attributable to the decreased voids in the membrane structure. Furthermore, 0.75TiO 2 /0.25g-C 3 N 4 /PVDF composite membrane exhibited the highest tensile strength, which could be attributable to there being fewer voids in the membrane structure.
To determine the hydrophilicity/hydrophobicity of the membrane surface, the contact angle was measured. As shown in Table 1, the contact angle of the as-prepared membrane with TiO 2 and g-C 3 N 4 was decreased. It is well known that the membrane wettability of the membrane is influenced by the membrane material, as well as the surface porosity and roughness [47,48]. After adding TiO 2 and g-C 3 N 4 , the surface porosity sharply increased. In other words, adding the TiO 2 and g-C 3 N 4 leads to a higher surface porosity and results in a decreased contact angle of the membrane.  Membrane porosity and water content play an important role in membrane performance. As shown in Table 1, the porosity and water content of the 0.75TiO 2 /0.25g-C 3 N 4 /PVDF composite membranes were 97.3 and 188.3 L/(m 2 ·h), respectively. This can be ascribed to the improvement in the hydrophilicity of the composite membranes because of the addition of TiO 2 and g-C 3 N 4 .
To further explore the pyrolysis properties of the 0.75TiO 2 /0.25g-C 3 N 4 /PVDF composite membrane, a temperature-domain TGA was conducted, and the results are shown in Figure 3. It is clearly shown that the TiO 2 /g-C 3 N 4 /PVDF composite membrane exhibited a higher thermal decomposition temperature than that of the pure PVDF membrane, indicating the better thermal stability of the composite membrane, which could potentially be attributed to the physical and chemical interactions between PVDF chains and g-C 3 N 4 surface functional groups.   Membrane porosity and water content play an important role in membrane performance. As shown in Table 1, the porosity and water content of the 0.75TiO2/0.25g-C3N4/PVDF composite membranes were 97.3 and 188.3 L/(m 2 ·h), respectively. This can be ascribed to the improvement in the hydrophilicity of the composite membranes because of the addition of TiO2 and g-C3N4.
To further explore the pyrolysis properties of the 0.75TiO2/0.25g-C3N4/PVDF composite membrane, a temperature-domain TGA was conducted, and the results are shown in Figure 3. It is clearly shown that the TiO2/g-C3N4/PVDF composite membrane exhibited a higher thermal decomposition temperature than that of the pure PVDF membrane, indicating the better thermal stability of the composite membrane, which could potentially be attributed to the physical and chemical interactions between PVDF chains and g-C3N4 surface functional groups. The crystal structures of the g-C 3 N 4 , PVDF and g-C 3 N 4 PVDF membranes were characterized by XRD. As shown in Figure 4, two pronounced peaks at around 13.0 • and 27.4 • were observed in the XRD patterns of g-C 3 N 4 , corresponding to the in-plane structural packing motif and the interlayer stacking of aromatic systems, respectively [34]. The XRD patterns of the g-C 3 N 4 /PVDF membrane showed a weak peak of g-C 3 N 4 , which could be attributed to the low g-C 3 N 4 content. The crystal structures of the g-C3N4, PVDF and g-C3N4 PVDF membranes were characterized by XRD. As shown in Figure 4, two pronounced peaks at around 13.0° and 27.4° were observed in the XRD patterns of g-C3N4, corresponding to the in-plane structural packing motif and the interlayer stacking of aromatic systems, respectively [34]. The XRD patterns of the g-C3N4/PVDF membrane showed a weak peak of g-C3N4, which could be attributed to the low g-C3N4 content.

Conclusions
A TiO2/g-C3N4/PVDF composite membrane was fabricated by a phase inversion method. A comparison of the performance and morphology was carried out between pure PVDF membrane and PVDF composite membranes with different mass ratios of TiO2 and g-C3N4. It was clearly shown that the TiO2 and g-C3N4 were able to obviously change the performance and structure of PVDF membranes. Contact angle measurements and SEM images showed that the addition of 0.75 g TiO2 and 0.25 g g-C3N4 to PVDF membrane greatly improved hydrophilicity due to the decreased number of voids. The porosity and water content of the 0.75TiO2/0.25g-C3N4/PVDF composite membranes were 97.3 and 188.3 L/(m 2 ·h), respectively. The porosity and water content of the 0.75TiO2/0.25g-C3N4 membrane were increased by 20.8% and 27.4%, respectively, compared with those of the pure PVDF membrane. These results provide a novel way of improving membrane morphology, structure and stability without influencing the separation function.