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Application of Graphene in Photocatalysis
1.Background
Graphene has many unique properties, such as excellent conductivity, large specific surface area, good mechanical and chemical stability and flexible single-layer structure. When it is combined with semiconductor photocatalyst, it can not only act as a good adsorbent to adsorb organic pollutants, but also improve the separation efficiency of photo carriers in the photocatalyst, thus improving the overall photocatalytic efficiency.
2.Classification of graphene based photocatalysts
Graphene can improve the photocatalytic performance of semiconductors in three ways:
1. Form a 2D-2D heterojunction. By forming a heterojunction with another 2D material, the contact area between graphene and graphene is increased, thereby improving the transmission of photogenerated electrons and reducing the recombination rate. Xiang et al. [1] prepared TiO2 / graphene and graphene / g-c3n4 composites as 2D-2D materials, which showed enhanced light efficiency under visible light. Hou et al. [2]synthesized triple junction C3N4 / n-doped graphene MoS2, which showed high photocatalytic efficiency for the oxidation of methylene blue (MB) and the reduction of Cr (VI) under visible light.
2. Increase bonding. Studies have shown that this is very important for the regulation of photocatalytic efficiency of materials [3]. The strong interface effect enhances the charge transport on the surface, thus reducing the probability of electron hole recombination. Qiu et al. [4] synthesized the ternary composite material TiO2 / gas (3dgraphene aeroneneneba gel) and observed a greater catalytic activity than a single semiconductor in the degradation of methyl orange (MO). Similarly, Li et al. [5] treated cyanimide and graphene oxide to thermally prepare g-c3n4 / RGO nanocomposites. The C-O-C covalent bond formed between the two makes the band gap adjustable and indirectly improves the photocatalytic efficiency of the nanocomposites.
3. Introduce interface media. The function of interface medium is to improve the separation efficiency of photogenerated electrons across the interface, so as to realize the effective utilization of electronic conductivity. For example, Bai et al. [6] prepared Cu2O / Pd / RGO composites using precious metal Pd as the interface medium. The role of Pd nanoparticles is to collect holes from Cu2O and transfer them to the surface of graphene, so as to achieve the effect of inhibiting electron hole pair recombination.
3.Application of graphene based photocatalyst
Photodegradation
Selective separation or oxidation of target organics from mixtures is a crucial step in the water treatment process[7,8], so selective photocatalysis is of great significance. Many Semiconductor Photocatalysts such as TiO2 usually exhibit very low photocatalytic selectivity for the desired products, which is due to the non selectivity of hydroxyl radicals with strong oxidation [9]. Recently, Yu et al demonstrated that the selectivity of TiO2 for photodegradation of pollutants can be easily adjusted by changing the pH value of aqueous solution and the crystal surface exposed by semiconductors[9,10]. The photodegradation effect of positively charged fluorinated TiO2?hollow microspheres on negatively charged methyl orange (MO) is better than that of positively charged methylene blue (MB) [10]. In contrast, hollow TiO2 microspheres prepared by NaOH washing or calcination had better photodegradation effect on MB than mo. In addition, Song [11] and others used the structural effect of RGO to construct a highly efficient plasma Ag / Ag2CO3 RGO photocatalyst for photocatalytic oxidation of pollutants. The obtained Ag / Ag2CO3 RGO composite exhibited higher reaction activity than Ag2CO 3 and Ag2CO3 go composite in the photocatalytic oxidation of organic pollutants.
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Fig. 1 reaction mechanism diagram and photodegradation effect diagram of Ag / Ag2CO3 RGO photocatalyst [11]
Water splitting
The photocatalytic decomposition of water into H2 and O2 using solar energy and semiconductors has attracted wide attention and has been proved to be a promising method to solve solar energy storage and low-cost and green production of H2 fuel [12]. However, from the perspective of Gibbs free energy change, photocatalytic water splitting is more difficult than photocatalytic degradation. In recent years, various heterogeneous catalysts (such as metal oxides, sulfides, nitrides, organic dyes, etc.) have been widely used in photocatalytic hydrogen production.
Graphene as a cocatalyst in hydrogen evolution reaction can greatly improve the photocatalytic activity of metal sulfides. For example, Yu and his colleagues loaded 0.25wt% RGO on zn0.8cd0.2s (Fig. 46a), and found that the photocatalytic hydrogen production performance of the whole system was improved by 4.5 times [13], even higher than that of zn0.8cd0.2s loaded with 1wt% Pt (Fig. 46b). The reason may be that RGO acts as a cocatalyst, which can effectively promote the separation of photogenerated electrons, and can also increase more reaction active sites, which can accelerate the hydrogen production rate (Fig. 46C). In addition, graphene itself has also been shown to exhibit some photocatalytic activity in some specific water splitting reactions. Yeh et al. [14] showed that nitrogen doped graphene oxide carbon quantum dots (NGO QDs) exhibited good catalytic activity in the water splitting reaction under visible light.
? Fig. 2 (a) mechanism diagram of photogenerated carrier separation in rgo-zn0.8cd0.2s system;(b) Mechanism diagram of hydrogen production under simulated sunlight [13]
CO2?reduction
Since Inoue and his colleagues [15]first demonstrated the photocatalytic CO2 reduction technology in 1979, in the past 40 years, significant progress has been made in the research of using efficient and feasible semiconductor catalysts to reduce CO2 [16, 17]. Among these semiconductor materials, the combination of RGO / go and semiconductor has become an important means to improve the efficiency of semiconductor photocatalytic CO2 reduction. It was found that there was a close linear relationship between the CO2 adsorption capacity of the composite photocatalyst and the RGO content, but not the specific surface area. Adding 2wt% RGO to CdS nanoparticles can increase the adsorption capacity of CdS nanoparticles for CO2 by three times, which may be the result of π – π interaction between graphene and CO2 molecules [18] . Interestingly, the adsorption site of CO2can also be used as the active site for its photocatalytic reduction. In addition, by introducing fixed (nitrogen doped) and mobile (polyethyleneimine grafted) amino groups, the adsorption capacity and catalytic activity of nano carbon cocatalysts for CO2 were further improved [19,20].
Fig. 3 mechanism diagram of reduction of CO 2 by RGO – CDs photocatalyst under visible light [18]
References
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[2] Hou Y, Wen Z, Cui S, et al. Constructing 2D porous graphitic C3N4 nanosheets/nitrogen‐doped graphene/layered MoS2 ternary nanojunction with enhanced photoelectrochemical activity[J]. Advanced materials, 2013, 25(43): 6291-6297. https://doi.org/10.1002/adma.201303116.
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