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The geometry, stability and electronic properties of hydrogenated Janus MoSSe monolayer (MoSSe-Hx) (x=0-16) were investigated by first-principles calculations. Energetically, Sets is the most favorable and stable site for H adsorption. Janus MoSSe-Hx is predicted to be stable because its formation energy is lower than graphane and H2S, and the adsorption energy is lower than H atoms adsorbed on graphene and MoS2. The analysis of the electronic density distribution and bond population also show that Janus MoSSe-Hx forms stable structure. Then, the variations of electronic structure of Janus MoSSe-Hx were investigated. With the increase of the adsorbed numbers of H atoms, the density of state (DOS) of Janus MoSSe monolayer changed. It can be found that the DOS of Janus MoSSe-H2 has additional peaks in the band gap, which are introduced from H 1s doping states. With the increase of H atoms adsorbed, there are more H 1s doping states in the band gap. Correspondingly, the band gap of the Janus MoSSe-Hx monolayer gradually decreases. The band gap of pristine Janus MoSSe is 1.47 eV, and the band gap of 2 H adsorbed on Janus MoSSe is 1.41 eV. When 16 H atoms are adsorbed on Se side of Janus MoSSe, the band gap is 0.25 eV. Also, the work function of the Janus MoSSe-Hx monolayer was observed to decrease as the number of adsorbed H atoms increased. After hydrogenation process, the Se atoms transfer electrons to H atoms and Mo atoms, Mo atoms neutralize part of electrons, which leads to the weakening of the electric field pointing outward from Mo atoms to vacuum in the Janus MoSSe. As a result, the work function of the pristine Janus MoSSe is 5.24 eV, and the work function of Janus MoSSe-Hx (x=16) decreases to 3.70 eV.

We investigated the improvements in the photocatalytic properties of Bi4O5Br2 after Mn-doping via first principles calculations. Based on the formation energy calculation, transition metals replaced Bi atoms with a coordination bond of 4. Importantly, the Mn-doped Bi4O5Br2 has the lowest band gap among transition metal-doped Bi4O5Br2. The key factors underlying the improvements in photocatalytic efficiency are estimated as follows. First, the band gap decreased from 2.38 eV in the pristine case to 1.88 eV in the six Mn-doped Bi4O5Br2 case resulting in a shift of the photon absorption edge to lower energy. Second, the absorption coefficient in the visible light range obviously increases with Mn-doped Bi4O5Br2. Third, the relative mass ratio of photoinduced electrons and holes increases with Mn concentration resulting in a higher efficiency of charge carrier separation. Therefore, it is reasonable to believe that the photocatalytic efficiency of Bi4O5Br2 can be improved via Mn-doping. Our work provides a reasonable rationale for choosing Mn as a dopant—this can help experimental work and explain the improvements in photocatalytic efficiency.