# MoSe\({}_{2}\) with Ultra-Fine Pt Decoration for Efficient Photodegradation
## 1 Introduction  
Photodegradation of organic pollutants in natural water has been studied for several decades [1, 2]. By employing sunlight and semiconducting materials, redox reactions could be taking place to degrade and remove those dissolved pollutants, especially organic dyes [3] such as methylene blue, methyl orange, bisphenol A (BPA), etc. It provides a green and sustainable route for natural water purification. Among so many semiconducting candidates, transition metal dichalcogenides (TMDs) are known for their suitable bandgaps for visible light absorption and utilization [4, 5]. As layered two-dimensional materials, TMDs are formed by covalence bonds in layers and van der Waals forces between layers [6]. Such a layered structure also brings an adjustable bandgap which can be easily manipulated by controlling the layer numbers [7]. They usually have active edge sites due to various types of defects but a relatively inert basal surface [8, 9]. Thanks to their host ability for incorporating dopants, especially noble metal nanoparticles, the light-matter interaction as well as the photocatalytic performance could be improved. In this case, members of the TMD group are currently featured as photocatalysts for hydrogen evolution and organic-pollutant decomposition in water [10]. It is believed that the TMDs would hopefully provide a solution towards current fossil fuel deficiency and natural water pollution problems. As a typical member of the TMD family, molybdenum diselenide (MoSe\({}_{2}\)) has similar atomic arrangements to molybdenum disulfide (MoS\({}_{2}\)) [11], and has been widelyinvestigated for applications such as transistors, gas sensors, photodetectors and photocatalysts [12; 13]. For example, n-MoSe\({}_{2}\) transistors perform well in detecting the streptavidin molecule, due to the controllable high reverse current [14]. As for photocatalytic ability, researchers proposed the hydrothermal synthesis method to prepare MoSe\({}_{2}\) for hydrogen evolution reactions and organic dye degradations [15]. MoSe\({}_{2}\) is stacked into layers of Se-Mo-Se sandwich planes through the van der Waals force. The typically tunable bandgap also exists in MoSe\({}_{2}\), making it suitable for light absorption among a wide range of wavelengths. It should be mentioned that MoSe\({}_{2}\) has relatively smaller band gap than MoS\({}_{2}\), which brings more efficient photoexcitation [16]. In the photocatalytic applications, researchers proposed several routes to improve its total activities. For instance, the typical core-shell structures are beneficial for strong chemical activities which could be achieved by incorporating carbon nanotubes, MoS\({}_{2}\) nanoflakes, and transition metals, especially noble metals [17; 18; 19; 20; 21]. The optical absorption of MoSe\({}_{2}\) has been successfully adjusted by phase engineering, providing another choice for obtaining efficient photocatalytic performance [22]. Moreover, it is also possible to form Z-scheme nanocomposites by introducing other semiconducting materials [23; 24]. Among the above methods, noble-metal doping is an effective and controllable way. Noble metal nanoparticles could be easily anchored to MoSe\({}_{2}\) edges due to the commonly existing Se defects. However, it usually requires sophisticated synthesis to activate the inert MoSe\({}_{2}\) basal planes [8]. For instance, there should be a pre-synthesis to prepare MoSe\({}_{2}\)@mesoporous hollow carbon spheres, and the following step to obtain the incorporation of Pt nanoparticles [25]. Herein, we report a feasible and straightforward synthetic method to introduce Pt nanoparticles onto MoSe\({}_{2}\). The Pt nanoparticles are well formed and their sizes are controlled within 2.3 nm, which could be called ultra-fine Pt nanoparticles. The synthesized MoSe\({}_{2}\)-Pt nanocomposites exhibit great structural and chemical stabilities. The introduced Pt nanoparticles provide plenty of active sites for light absorption and photocatalysis.