4.2.4. Semiconductor/Carbon Heterojunctions

Carbonaceous nanomaterials have been widely employed for the design of novel photocatalysts due to their advantages of high surface area, good conductivity, and chemical stability. In general, the most commonly used carbonaceous materials for combining with semiconductors involves the carbon dots (CDs), carbon nanotubes (CNTs), and graphene [78].

The CDs as typical nanocarbon materials have been widely used to enhance the photocatalytic activity of semiconductors owing to their intriguing optical and electronic properties, low chemical toxicity, adjustable photoluminescence, and the distinct quantum effect [79]. For example, Long et al. [80] used carbon dots (CDs) to couple with the BiOI with highly exposed (001) facets to form a composition of CDs/BiOI. Furthermore, the obtained CDs/BiOI composite exhibited a greatly improved photocatalytic activity for the degradation of organic dyes in water. It has been proved that the incorporated CDs in the semiconductor formed a CDs/BiOI heterojunction, which was able to construct numerous electron surface trap sites and was beneficial for enhancing the visible light absorption range as well as the charge separation. Recently, Zhao et al. [81] reported the fabrication of carbon quantum dots (CQDs)/TiO2 nanotubes (TNTs) composite via an improved hydrothermal method. The CQDs were incorporated on the surface of the TNTs, and played a vital role in improving the visible light photocatalytic performance of the composite. As shown in Figure 14, they demonstrated that there were three advantages for the formation of CQDs/TiO2: (i) the CQDs could effectively trap the photo-generated electrons from TNTs and suppress the recombination of electron-hole pairs, (ii) the up-conversion photoluminescence property of CQDs could further improve the visible light utilization efficiency of CQDs/TNTs, and (iii) the hetero-structure formed between the CQDs and the TNTs could prolong the life of the photogenerated electron and hole pairs.

**Figure 14.** Schematic illustration indicating the photocatalysis mechanism of the CQDs/TNTs photocatalyst. Adapted with permission from Reference [81]. Copyright (2018) Elsevier.

CNTs are typical nanocarbon materials with highly sp2-ordered structures, and thus exhibit an excellent metallic conductivity, which could form a Schottky barrier junction between the CNT and semiconductors; as reported before, the Schottky barrier junction could effectively increase the recombination time of electron–hole pairs [78,82]. Moreover, CNTs could accept electrons in the composite system with semiconductors due to its large electron-storage capacity, which is beneficial for retarding or hindering the electron–hole recombination. As a result, a variety of semiconductor-CNT composite photocatalysts have been developed. For example, Miribangul et al. [83] prepared a TiO2/CNT composite via a simple hydrothermal method. The influence of the CNT concentration in the TiO2-CNT composites on their photocatalytic activity was investigated and the 0.3 wt % CNT content in TiO2/CNT composite could offer the highest photocatalytic degradation of Sudan (I) in UV–vis light. Apart from the TiO2, some of the other semiconductors can also be employed to composite with CNT, such as the CNT/LaVO4 composite photocatalyst developed by Xu et al. [84]. As shown in Figure 15, with the presence of CNT, the photocatalytic activity of a CNT/LaVO4 composite was greatly improved due to the synergistic effect between CNT and LaVO4, therefore the corresponding photocatalytic degradation rate of CNT/LaVO4 composite for organic contaminant is 2 times that of pure LaVO4.

**Figure 15.** Schematic illustration indicating the reaction mechanism of the photocatalytic procedure of CNT/LaVO4 composite catalyst. Adapted with permission from Reference [84]. Copyright (2019) Elsevier.

*Catalysts* **2019**, *9*, 122

Recently, graphene as a newly developed nanocarbon material has attracted numerous research attention in the area of photocatalysts due to its extraordinary physical properties, including superior charge transport ability, unique optical properties, high thermal conductivity, large specific surface area, and good mechanical strength [78,85,86]. Up to now, a myriad of attempts has been carried out to couple the graphene with various semiconductors to further improve their photocatalytic activity. According to the previous reports, the first graphene composite semiconductor for photocatalysis was prepared by Zhang and co-workers [87]. They fabricated a TiO2 (P25)-graphene composite with a chemically bonding structure via a one-step hydrothermal reaction. As reported, there are three contributions of the graphene for the photocatalytic activity: (i) enhancing the adsorption capacity of pollutants, (ii) extending light absorption range, and (iii) improving charge transportation and separation efficiency. As a result, the photodegradation of the obtained TiO2 (P25)-graphene composite for methylene blue was significantly improved, and was superior to that of the bare P25 and the commonly reported P25-CNTs composite. After that, numerous photocatalysts based on the composite graphene-semiconductors have been invented for the treatment of wastewater. Most recently, in order to overcome the limitation of the poor homogenous dispersion of graphene, as shown in Figure 16, Isari et al. [88] created a ternary nanocomposite catalyst (Fe-doped TiO2/rGO) derived from Fe-doped TiO2 and reduced graphene oxide via a simple sol-gel method. They proved that the band gap of Fe-TiO2/rGO could be significantly decreased compared with that of the pristine TiO2, and the obtained Fe-doped TiO2/rGO exhibited an effective decontamination performance for rhodamine B in water.

**Figure 16.** Schematic illustration demonstrating the photocatalytic mechanism of Fe-TiO2/rGO catalyst for contaminant under solar irradiation. Adapted with permission from Reference [88]. Copyright (2018) Elsevier.
