# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 4 Photonic Applications Based on 2D Bismuth
### Photocatalysis  
The 2D bismuthene has been widely used in photocatalysis applications due to its high carrier mobility [81], tunable band structure [47], and low toxicity [45]. Very recently, [PERSON] et al. demonstrated that 2D bismuth was used as an efficient photocatalyst in the liquid-phase organic transformation, with high product yield [28]. In this work, furan 1a and 4-chlorobenzenediazonium salt 2a were chosen as compounds for the investigation of photocatalytic activity in the photoredox C-Hylation. During the process, the excited electrons of bismuthene are transferred to the aryl diazonium salt and result in the aryl radical. Compared to sole bismuthene, the compound of diazonium salt 2a and bismuthene shows photoluminescence (PL) spectra, with largely decreasing intensity. The phenomenon can be explained by the rapid migration of photocarriers in bismuthene because the average lifetime of the carriers of bismuthene in diazonium salts is much larger than that of sole 2D bismuth (Figure 5a). Thanks to the narrow bandgap of bismuthene, the carrier excitation can occur without external incentives. Therefore, unlike other photocatalyst that require additional conditions to catalyze the C-Hylation of (hetero)arenes, bismuthene can operate at a low temperature (\(-15\,^{\circ}\)C), without utilizing a light source, as well as under dark conditions. Moreover, it is also of note that, unlike other photocatalysts, the reaction process operates normally without complicated equipment, precise environments, or specific light sources [82; 83]. Additionally, bismuthene can not only catalyze C-Hylation with aryl diazonium, but also with other heteroarenes such as furan, thiophene, and pyrrole, which allows the bismuthene to be used in diverse applications [84].  
In 2021, [PERSON] et al. inserted 2D Bi into C\({}_{3}\)N\({}_{4}\)/BiOCl to form a 2D/2D/2D heterostructure photocatalyst, in which bismuthene acts as an effective metal electron mediator (Figure 5b) [27]. As shown in Figure 5c, the type of heterojunction is changed from type-II to the Z-scheme. The tunneling effect of the bismuthene layer promotes the combination of photogenerated electrons in C\({}_{3}\)N\({}_{4}\), with holes in the valence band of BiOCl, significantly improving the efficiency of the photocatalytic reaction. The CH\({}_{4}\)/CO yield of the C\({}_{3}\)N\({}_{4}\)/bismuthene/BiOCl heterostructure is up to 61/71 \(\upmu\)mole\({}^{-1}\)h\({}^{-1}\), which is superior to that of the other reported Bi-based photocatalysts [85; 86; 87]. Apart from carbon dioxide reduction, the bismuthene-based heterostructure can also be applied in other photocatalyst reactions. Very recently, [PERSON] et al. reported that the InTe/bismuthene heterostructure can produce hydrogen from water splitting, achieving a maximum solar-to-hydrogen conversion efficiency (\(\upmu_{\text{ISTH}}\)) up to 16.17% under 9% strain (Figure 5d) [88]. Compared to monolayer InTe nanosheets, the combination with bismuthene can strongly increase the efficiency of the hydrogen evolution reaction (HER). Meanwhile, the variation of Gibbs free energy verifies the feasibility of the thermodynamics of the HERs using the InTe/bismuthene heterojunction.