2D Graphene-TiO2 Composite and Its Photocatalytic Application in Water Pollutants

To solve the global water shortages and serious water pollution problems, research on semiconductor photocatalysts has generated significant research attention. The degradation of pollutants by titanium dioxide (TiO2) exceeds other semiconductor materials. However, its wide bandgap restricts the photocatalytic reaction under visible light. The large specific surface area and good thermal conductivity of graphene yielded an effective graphene-TiO2 catalyst combination effective under visible light. 2D graphene-TiO2 composites (2D-GTC) have shown promise, so a study of the preparation methods, mechanism and catalytic effect of different pollutants on this material was undertaken. In this current review, the characteristics of different graphene and TiO2 composites and their preparation methods, as well as the effects of different synthesis methods on the catalyst are introduced. The reaction mechanism of 2D-GTC catalysts, the degradation effects of different pollutants in water are all reviewed.


INTRODUCTION
Water is necessary for life and overcoming water shortages represents one of the bigger challenges worldwide. Also, the discharge of various chemical substances of various concentrations into water by cities and some factories makes water conservation/purification even more important. In response to existing water resource needs, existing wastewater treatment technologies are booming. Common wastewater treatment methods include biological, membrane filtration, chemical precipitation and photocatalysis. Among them, the photocatalysis has advantages in environmental protection, low process cost, energy saving and easy control of the amount of catalyst used during wastewater treatment (Herrmann, 2010). However, there are some disadvantages coexist including difficulty in regenerating photocatalytic materials and lower efficacy in the presence of too many pollutants.
Various photocatalyst materials have been studied for their photocatalysis. Generally, photocatalysis utilizes the light absorption by the semiconductor as well as transitions of electrons from the lower to higher energy levels (electron-hole separation). In this process, carriers may recombine and result in inefficient catalysts. Long-life charge carriers and fewer charge trapping centers (abnormal centers of a physical location can "trap" charge carriers) are important factors for improving photochemical efficiency. ZnS, CdS, TiO 2 , g-C 3 N 4 (graphite carbon nitride) or ZnO (Sudha and Sivakumar, 2015;Hao et al., 2017;Qi et al., 2017;Thirugnanam et al., 2017) are common photocatalyst materials. Among them, titanium dioxide (TiO 2 ) is a well-known photocatalyst whose advantages include low cost, non-toxic and high photocatalytic activity (Kurniawan et al., 2012;Kuwahara et al., 2012). However, titanium dioxide has disadvantages as well, including a rapid recombination rate of photogenerated electron-hole pairs and its low activity in the visible region (Bak et al., 2002;Liu et al., 2010), and low photocatalytic efficiency in the visible light region (Wang et al., 2007;Afzal et al., 2012;Kruth et al., 2014). These shortcomings degrades the quantum efficiency of TiO 2 and limits wide application.
The most outstanding feature of graphene is its huge specific surface area. In addition, graphene is only one atom thick, has  good thermal conductivity (∼5000 W m −1 K −1 ) (Stoller et al., 2008), and high mechanical strength (1 TPa) (Balandin et al., 2008). It can also achieve ballistic transport (electron mobility at room temperature that exceeds 15,000 m 2 V −1 s −1 ) without scattering (Kumar et al., 2017). These characteristics have allowed graphene to achieve outstanding advancements in the fields of photocatalysis and adsorption (Chakraborty et al., 2008). When graphene and TiO 2 are doped to form 2D composite (2D Graphene-TiO 2 Composite, which abbreviated as 2D-GTC in the description below), the appropriate energy level of TiO 2 shifts, and electrons will convert the conduction band of TiO 2 into the energy level of graphene, which ultimately leads to higher wastewater decomposition efficiency. The 2D-GTC not only enhances the adsorption of reactants in the visible range but also makes it easier to transfer and separate charges.

SYNTHESIS OF 2D-GTC
The current synthetic method for 2D-GTC includes the initial preparation of graphene oxide (GO), preparation of nano-TiO 2 and followed by a combination with GO and nano-TiO 2 . Each step in the synthetic process uses different methods to generate different composites. Here is the method currently utilized most often, though other synthetic methods include chemical vapor deposition, hydrothermal method, and ultrasonic treatment were also be reported.

Preparation of Graphene Oxide (GO)
The GO is usually obtained by modifying graphene, and among them the choice of graphene is very important. As shown in Figure 3, graphene can be divided into single-layer graphene, few-layer graphene, and multi-layer graphene, according to the number of layers and thickness of graphene. A single-layer graphene is an ideal membrane structure, and the pore size can be adjusted with a porous distribution structure of atoms. The structure of few-layer graphene is smooth, neat, flat and defectfree, which can be stacked layer by layer for membrane separation experiments. Multi-layer graphene is usually stacked by single-layer graphene to form a film with a thickness of about 3-9 nm. After stacking, vivid nanochannels can be formed to penetrate gas and liquid. The sub-nano-level layer spacing in this tightly packed interlocking plate can be controlled to obtain the desired flux and separation selectivity. Graphene is usually obtained by mechanical peeling through block graphite (the "Scotch tape" method), chemical preparation and chemical vapor deposition (Schniepp et al., 2006;Stankovich et al., 2007;Xu et al., 2008;Li et al., 2008;Lomeda et al., 2008;Si and Samulski, 2008;Lotya et al., 2009;Behabtu et al., 2010). The synthesis of GO was first demonstrated in 1859 and involved the addition of potassium chlorate to a graphite slurry dissolved in fuming nitric acid (Brodie, 1859). In 1898, Staudenmaier (Staudenmaier, 1898) improved that method by dissolving graphite in concentrated sulfuric acid and fuming nitric acid, and adding chlorate several times during the reaction. Although this was only a minor improvement, it represented a more convenient and feasible way to make GO. The Hummers method, which is the most widely developed now, was published in 1958. The method oxidized graphite using KMnO 4 and NaNO 3 in concentrated H 2 SO 4 . However, this method also produces several gases (such as N 2 , N 2 O 4 , and Cl 2 ), some of which are toxic. Due to this, many researchers sought to improve the Hummers methods (Marcano et al., 2010). The reaction equations of Hummers and two improved Hummers were shown in Figure 4. The two different improved Hummers method are safer than the original Hummers method due to the lack of heat and toxic gases released during the reaction and represents a significant synthetic step forward. The GO produced by the improved Hummers method oxidized more readily and did less damage to the graphite base surface.

Preparation of Nano-TiO 2
TiO 2 is an extensively studied photocatalyst. Due to different reaction processes for TiO 2 , the final form and photocatalytic effects are also different. Common forms include nano-sheets, nano-tubes, nano-spheres, nano-rods among other forms. After dissolving the raw materials in ethanol or deionized water, they are mixed by stirring or ultrasonic waves and placed in a polytetrafluoroethylene autoclave to react. The product is centrifuged (or suction filtered) and dried to obtain the titanium dioxide product. Nano TiO 2 can be obtained by the method above, and the time and temperature in the reaction can be controlled to obtain different forms of titanium dioxide Li et al., 2020).
Different synthetic methods produce different TiO 2 crystal structures and morphologies. TiO 2 has three different crystal structures: rutile, anatase, and brookite. Among them, rutile TiO 2 is the most stable, and other types readily convert to rutile TiO 2 at high temperatures. Most of the TiO 2 used to prepare 2D-GTC is anatase TiO 2 or mixed crystals such as P25 (20% rutile phase and 80% anatase phase mixed). The heterojunction formed between different crystal phases facilitates the transfer of photogenerated electrons from one phase to another and reduces eh + recombination, which promotes the generation of free radical groups. The rutile phase is located at 27.1°of the {110} crystal plane, and the anatase phase is located at 24.2°of the {110} crystal plane. At low graphene levels during the synthesis of 2DGT, the 24.2°peak easily overlaps with the anatase peak (Rong et al., 2015).

Combination With GO and Nano-TiO 2
The commonly used methods for combination with GO and nano-TiO 2 to form 2D-GTC including stirring (or ultrasonic) mixing, sol-gel method and hydrothermal/solvothermal methods et al. (Mishra and Ramaprabhu, 2011;Zhang et al., 2012c). Different methods have an impact on the morphological characteristics and structure of the catalyst and influence the way that TiO 2 adheres to the GO surface.
Stirring (or ultrasonic) mixing: Stirring (or ultrasonic) mixing is the simplest way to synthesize 2D-GTC which relies on van der Waals forces for bonding, so the interaction between the various components is very poor. In the reaction, GO (concentration: 0.01-10% by weight) is mixed with TiO 2 . Experimental results showed the addition of TiO 2 altered the size of the GO colloid, which decreased accordingly (Park et al., 2011).
Sol-gel method: This method is another simple technique to prepare 2D-GTC; the 2D-GTC produced by sol-gel not only rely on van der Waals forces but also covalent oxygen bridges and hydroxyl bridges formed between GO and the metal center (Zhang et al., 2010).
Hydrothermal/solvothermal methods: The primary benefit associated with hydrothermal/solvothermal syntheses comes

Degradation of Antibiotics
Antibiotics are used frequently because they can be used for more than just human treatment. However, overuse of antibiotics can contaminate desoxyribonucleic acid (DNA) and lead to bacterial resistance in an organism (Wang et al., 2016;Chang et al., 2017;Zheng et al., 2018). Many countries have explicit regulations on the use and residual levels of antibiotics. The limits of tetracycline antibiotics in China and the EU specify the content of edible animal muscle tissue should contain less than 100 μg kg −1 . The United States has set clear chlortetracycline limits in animal muscle tissue at 200 μg kg −1 . However, antibiotic levels in current water bodies are not within the range of water quality indicators. Antibiotics themselves are extremely durable, difficult to degrade, and often discharged in the original form from the human body (Xu et al., 2019). Therefore, the removal of antibiotics in wastewater has become a problem we must solve. Tetracyclines (TCs) are a class of broad-spectrum antibiotics produced by actinomycetes, including tetracycline (TC), tetracycline hydrochloride (TCH), chlortetracycline (CTC) and oxytetracycline (OTC) etc. The structure of tetracycline antibiotics all contain naphthophenyl skeleton ( Figure 5). Li et al. (2017) obtained a TiO 2 -rGO-TiO 2 photocatalyst by a sol-gel method to degrade, and found under the same degradation conditions, the degradation efficiency of TCH increased by 22.8% using ultraviolet light and 32.8% under simulated sunlight than a pure TiO 2 catalyst. The two main factors that promote the photocatalytic reaction are the number of pores and reactive oxygen species (ROS), and the addition of GO reduces the band gap energy of TiO 2 . Li et al. (2017) also used TiO 2 particles compounded on the surface of GO sheet to form a 2D-GTC catalyst and applied it to remove chlortetracycline (CTC) from water. Those results showed that the removal efficiency of 2D-GTC (10 mg/L) on CTC in real sewage approached 100%. Wang et al. (2019) also reported the removal rate of oxytetracycline (OTC) by hydrothermal synthesised 2D-GTC nearly reached 100% under visible light, and found during the photocatalytic reaction, h + /e − pairs are generated and the appearance of h+ is determined to be the main reason for OTC removal.

PHOTOCATALYTIC MECHANISM AND INFLUENCING FACTORS OF 2D-GTC Photocatalytic Reaction Influencing Factors of 2D-GTC
GO is a structural material with an aliphatic sp 3 carbon structure, an aromatic sp 2 structure and contains an oxygen intercalation group. Some studies have conducted theoretical calculations and model studies and found that GO can fix gases (even gases as small as helium) because the gap in its aromatic ring is blocked by the π orbital cloud (Bunch et al., 2008). One simulation found that single-layer GO has a high salt removal rate, approximately two to three orders of magnitude higher than other permeable membranes (Cohen-Tanugi and Grossman, 2014), and its water flux can reach 400-4,000 m 2 h −1 bar −1 . Moreover, singlelayer GO is selective to certain pollutants (O'Hern et al., 2014). The GO structure has sp 3 hybridized carbon atoms and a topological network, so it is easy to wrinkle and obtain corrugated nanosheets. These GO films also have outstanding adsorption capacity because the oxidized area of GO can divide the next layer, and its non-oxidized area allows a liquid material to flow without friction. Nano multilayer GO nanosheets are highly compatible, whose uneven structure improves the mobility and contact efficiency of the reactants, and have unique permeability and unique molecular transmission characteristics (Wakabayashi et al., 2008;Han et al., 2013;Ji et al., 2015). In addition, the nano-layered GO has the ability to transport water, oil or oil/water mixture at the same time.
Although the GO has so many characteristic advantages, it does not have a photocatalytic effect by itself. It is still TiO 2 that plays a photocatalytic role in 2D-GTC. When the energy captured by TiO 2 exceeds the photon of its band gap (anatase: ∼3.2 eV or rutile: ∼3.0 eV), the photoelectron eis excited to the conduction band (CB) and is in the valence band (VB) and holes are left on the top, resulting in electron-hole pairs (Di Valentin et al., 2004). Figure 6 details the reaction of TiO 2 during photocatalysis to generate reactive oxygen species (ROSs). Holes in the VB oxidize the adsorbed water or OH − groups to produce hydroxyl groups (OH) while CB electrons reduce O 2 molecules. Producing superoxide (O 2-) (2.5, 2.6). O 2can generate hydroperoxy radicals (HO 2 ), it can also generate H 2 O (2.7-2.9) after further protonation (Chong et al., 2010). The electrons captured by O 2 after a direct, two-electron reduction generate H 2 O 2 as seen in 2.10 (Zheng et al., 2019). The role of ROSs is the photocatalytic oxidation of pollutants. As shown in Figure 6, the CB potential (-0.1 eV) of TiO 2 is more negative than typical oxygen anions, which confirms the thermodynamic photocatalytic reduction of TiO 2 (Hou et al., 2018). In an ideal situation, the corresponding half-reaction of photogenerated electrons that reduce oxygen anions is the oxidation of water (Eq. 2.11). However, this was not the case in the actual reaction process. The competition between e − and oxygen anions (CB electrons) reduces the light reduction activity of the catalyst (Marinho et al., 2017). The OH formed in the reaction oxidizes the ions generated and results in a low catalytic performance (Zuo et al., 2015). Formation of H 2 as a by-product consumed a large amount of CB electrons, the H 2 was generated when the oxygen anions were removed (see Eq. 2.12). Therefore, to eliminate side reactions, it was necessary to generate such ROS and side products as little as possible. The oxidization of VB holes and reduction of CB electrons occur simultaneously, so if one of the two reaction pathways is blocked, the charge accumulates on the catalyst surface. Similarly, improvements to one process accelerate the other processes (Jiang et al., 2012).
Equation Water splitting: 2H + +2e -→ H 2 (2.11) and 2H 2 O+4h + → 4H + +O 2 (2.12) The combination of GO and TiO 2 promotes their respective advantages, so that 2D-GTC becomes a photocatalyst with great application potential. Studies have shown that, under visible light, TiO 2 transfers photogenerated electrons to GO and simultaneously transfers holes from GO to TiO 2 , which causes adsorbate oxidization. Pollutants adsorb onto the TiO 2 surface (Wang et al., 2012a), the delayed recombination of photogenerated electron-hole pairs allows organic pollutants to undergo redox reactions with more h + (Geng et al., 2013). Active oxygen appears when photogenerated electron-hole pairs form. Radical O 2anions, hydroxyl radicals (•OH) and h + in these reactive oxygen species (ROSs) react with organic pollutants to cause mineralization (Chen and Liu, 2017;Reshak, 2018).

Photocatalytic Reaction Influencing Factors of 2D-GTC
Many factors affect the photocatalysis of titanium dioxide and graphene composites, such as the mass ratio of graphene to titanium dioxide, pH, and light intensity. During photocatalysis, the primary photocatalytic material is titanium dioxide, and the addition of GO increases the number of pores. The degradation efficiency of pollutants initially increases but decreases with an increasing weight percentage of GO (Zhou et al., 2011;Wang et al., 2012b), because the addition of GO can guide nucleation of TiO 2 . At the same time, carrier recombination decreases and TiO 2 crystals are evenly distributed. The introduction of pores leads to an attraction between the photocatalyst and the adsorbent. As the weight ratio of GO increases, the relative levels of TiO 2 decrease which lowers carriers induced by photocatalysis as well as the photocatalytic reaction sites of TiO 2 . Also, high levels of TiO 2 crystals cause agglomeration and reduce the number of catalytic sites. To combat this, the catalyst can be used multiple times by washing or high-temperature combustion (Wang et al., 2012a). Alteration of pH levels also led to different degradation effects. For some antibiotics, a higher pH initially increased the degradation effect though it subsequently decreased. The degradation effect maximized at a pH of 7 (for some antibiotics).

CONCLUSION
Graphene and TiO 2 can form composites through a variety of different composite methods, and a variety of pollutants, such as dyes, antibiotics can be removed through photocatalytic reactions by 2D-GTC. TiO 2 is a good semiconductor with multiple catalytic sites. Compounding GO and TiO 2 produces a large number of pores that provide space for adsorption so that liquid contaminants can diffuse onto the interface, which further increases degradation. Therefore, the addition of GO increases the photocatalytic active sites of titanium dioxide. In addition to the mass ratio of GO, the pH and the form of different materials also play vital roles in the catalytic effect. 2D-GTC materials not only treat water pollutants under ultraviolet light but also perform photocatalytic reactions under sunlight.
Although a long time has passed since the discovery of 2D-GTC materials as photocatalysts, and its charge carrier transport, photo-generated electrons and photocatalytic properties are remarkable. But in its normal form it can theoretically stretch indefinitely across its width. In order to make the 2D-GTC material a practical engineering material, there are still challenges in improving the efficiency of photocatalysis and the cost of catalyst preparation, so there is still a lot of work to be done in the further.

AUTHOR CONTRIBUTIONS
XZ, The manuscript writing; XZ, The second author, The mechanism of reactions and manuscript review; YW, The third author, Document collection and sorting; ZW, The Corresponding author, The management and manuscript conduction.