# A Membrane Modified with Nitrogen-Doped TiO2/Graphene Oxide for Improved Photocatalytic Performance
## 1 Introduction  
Membrane separation is a promising separation and purification technology for pollutants. It is efficient and convenient [1]. However, during the treatment of wastewater by membrane separation, some problems had to be faced, such as low water flux, low rejection and poor contamination resistance. Photocatalysis is considered to be a low-cost and sustainable method to decompose organic pollutants, which can directly convert organic pollutants into inorganic small molecules such as H2O and CO2. However, there are some problems, such as catalyst separation, and so forth. In this case, the photocatalytic-membrane coupling membrane separation and photocatalysis can not only maintain the original advantages of the photocatalytic and membrane separation technology, but also solves and alleviates the problems that restrict the development of the two technologies [2; 3; 4]. Among the many photocatalysts, the TiO2 photocatalyst is widely used in the field of water treatment because of its advantages of being cheap, being non-toxic, having strong photochemical stability and not causing secondary pollution [5; 6]. The incorporation of TiO2 nanoparticles into water filtration membranes enhances their flux, contaminant removal and contamination resistance [7]. However, a photocatalytic membrane made of TiO2 alone has a very low photocatalytic performance under sunlight [8]. This problem has seriously hindered the widespread use of TiO2 as a photocatalyst. At present, the modification methods of TiO\({}_{2}\) include metal doping, non-metal doping, dye sensitization and semiconductor recombination. Nitrogen-doped TiO\({}_{2}\) is a non-metal modification. Nitrogen, instead of oxygen vacancies, introduces nitrogen into the TiO\({}_{2}\) gap, which effectively reduces the energy gap between the valence and the conduction band, significantly improving the photocatalytic activity of TiO\({}_{2}\)[9; 10; 11]. The excellent electron transfer properties of GO are considered to be ideal for expanding the photoresponse range of TiO\({}_{2}\)[12]. GO can be used as an electron transporter for TiO\({}_{2}\) nanoparticles so that the lifetime of electron-hole pairs can be significantly improved [13]. In addition, functional groups such as a carboxyl group and a hydroxyl group in GO can improve the hydrophilicity when used as an additive [14]. PVDF is an ideal membrane separation material due to its good thermal and chemical stability. However, the hydrophobicity of PVDF materials makes it easy to adsorb and deposit contaminants during application, causing membrane fouling and reducing the membrane life [15]. Therefore, it is usually necessary to hydrophillically modify PVDF. There are two main modification technologies for PVDF microfiltration membranes. One is the modification of the membrane body and the other is the modification of the membrane surface. Bulk modification is the modification of the raw material before membrane formation by blending. Surface modification is modification by implanting a hydrophilic group on the surface of the membrane. The bulk modification method has a limited hydrophilic effect, it makes it difficult to disperse the modifier, has a low photocatalytic activity and causes the physical-chemical properties to be prone to change. The surface modification hydrophilic effect is a significant improvement from this as it does not affect the physical-chemical properties of the membrane body and the modified membrane exhibits excellent photocatalytic properties [16]. There have been many studies on nitrogen-doped TiO\({}_{2}\)/GO composite photocatalysts, the photocatalytic performance was remarkable but it was difficult to recycle the photocatalysts. In some research, the surface modification of the ultrafiltration membrane by TiO\({}_{2}\)/GO was discussed and the results showed that the photodegradation performance of the modified membrane was improved but that the modification process was slightly complicated [3; 17].