Figure 5. DFT calculations of MoSe2 based electrocatalyst; Fully relaxed geometry. (a) 1T/2H MoSe2, (b) 1T MoSe2, (c) 2H MoSe2. 1T/2H MoSe2 with Pt cluster and the optimized geometry when H atom adsorbs different Pt positions; (d) Pt1, (e) Pt2, (f) Pt3, (g) Pt4. Grey, light purple, light green, and white ball indicate Pt, Mo, Se, and H, respectively, (h) Charge density difference of 1T/2H MoSe2; yellow and blue contour are charge accumulation and depletion regions, respectively, (i) Gibbs free energy of HER with different active sites, (j) Partial density of states for Se 4p and H 1s; Ep indicates p-band center, (h) Partial density of states for Pt 5d; Ed indicates d-band center.
We conducted density functional theory (DFT) to reveal the role of heterostructure of 1T/2H mixed phases of MoSe2 and the 1T/2H mixed phase MoSe2 surface with Pt cluster. The fully relaxed MoSe2 structures, which are 1T/2H MoSe2, 1T MoSe2, 2H MoSe2, and 1T/2H MoSe2 with Pt, respectively, were prepared for calculating the Gibbs free energy of hydrogen adsorption (ΔGH), as illustrated inFigure 5 (a)–(g). As depicted Figure 5(h), the charge density accumulation was found for the interface between 1T and 2H MoSe2, resulting in the enhanced HER activity.[21] The HER activity can be estimated by ΔGH value. When the ΔGH value approaches 0 eV, the HER activity increases owing to the optimal balance between the adsorption and desorption reaction of H atoms on the active sites.[22] The ΔGH values of 1T/2H MoSe2 of point 1, point 2, and point 3 are 1.76, 0.93, and 1.81 eV, respectively (Figure S20 (a) and (b) in Supporting Information). Their corresponding structures are shown in Figure 5(a) and Figure S20(c) and (d) in the Supporting Information. Figure 5(i) shows ΔGH values of 1T MoSe2 (1.62 eV), 2H MoSe2 (2.27 eV), 1T/2H MoSe2 (0.93 eV), 1T/2H MoSe2 with Pt1 (–0.28 eV), 1T/2H MoSe2 with Pt2 (–0.24 eV), 1T/2H MoSe2with Pt3 (–0.18 eV), 1T/2H MoSe2 with Pt4 (–0.61 eV). The 1T/2H MoSe2 exhibits the lower ΔGHvalues for the than those of individual 2H MoSe2 and 1T MoSe2 phases, indicating that the presence of heterointerfaces between 1T/2H phases in MoSe2 enhances the HER activity. The improved HER activity may be attributed to the electron accumulation at the heterointerface, where optimized ΔGH was observed at the interface between 1T MoS2 and 2H MoS2.[21] Furthermore, the presence of the Pt cluster improves HER activity of Se site at the heterointerface, showing lower Gibbs free energy of 0.88 eV (Figure S21(a) ).
To investigate relationship between electronic structures and HER performance, we calculate partial density of states (PDOS) of 1T/2H MoSe2 with Pt, 1T/2H MoSe2, 1T MoSe2, and 2H MoSe2 for Se atom and H atom, as shown in Figure 5 (j). The bonding strength of H atom at active site can be confirmed by p-band center (Ep). When Ep is upshifted to Fermi energy (EF), the bonding strength is increased.[23] The Ep of 1T/2H MoSe2 with Pt, 1T/2H MoSe2, 1T MoSe2, and 2H MoSe2 for Se 4p were ­­–2.67, –2.75, –3.89, and –4.13 eV, respectively. Upshift of PDOS of H 1s to EF indicates the increased H atom activation, as observed in 1T/2H MoSe2. Therefore, the electron accumulation at the heterointerface, improved the bonding strength between Se and H atom, leading to more favorable H activation which contributes to the increased HER activity. In addition, the Pt cluster promotes the HER activity at the heterointerface, which attributed to the modulation of electronic structure, as shown in Figure 5(j).
When d-band center (Ed) is close to EF, the bonding strength of the adsorbate is enhanced. When H atom is adsorbed on different Pt sites, ΔGH of Pt1, Pt2, Pt3, and Pt4 are –0.28, –0.24, –0.18, and –0.61 eV, respectively. Ed values of above sites are –2.71, –2.81, –2.94, and –1.84 eV, as depicted in Figure 5 (i) and (k), indicating that the Pt1, Pt2, and Pt3 sites near the Se atoms are favorable for hydrogen adsorption and desorption. Although the Pt4 site shows the low HER activity, its site has higher HER activity than 1T MoSe2, 2H MoSe2, and 1T+2H MoSe2. Additionally, as shown in Figure S22 , Pt3 sites of 1T MoSe2+Pt and 2H MoSe2+Pt exhibit reduced Gibbs free energies (–0.24 and –0.23 eV), indicating the interaction between Pt and Se moderately control bonding strength of Pt and H regardless of the MoSe2 slab types. 1T+2H MoSe2 shows the lowest ΔGH, indicating that heterostructure is more favorable for HER compared to 1T MoSe2 and 2H MoSe2 when the Pt cluster is decorated onto the substrate. Based on DFT results, the high HER activity of the 1T/2H MoSe2+Pt cluster is attributed to the synergistic effect of the heterostructure of 1T/2H MoSe2 and the Pt cluster.
In summary, we synthesize 1T/2H mixed phase MoSe2heterostructure directly grown on a carbon paper using a hydrothermal method. Pt cluster is decorated on the heterostructure using an in-situ electrochemical deposition approach. This two-step approach enhances the interfacial adhesion and reduces the interfacial resistance between the carbon paper and the electrocatalyst compared to that of using MoSe2 powder with binder. As a result, directly grown sample (MoSe2-I-36h) exhibits higher HER activity than the 1T/2H MoSe2 powder (MoSe2-P-36h) due to direct bonding between the substrate and electrocatalyst which improves charge transfer. In addition, the HER activity is significantly improved when the small amount of 0.15 wt% Pt was decorated on the heterostructure of 1T/2H MoSe2. From DFT results, we found that the heterostructure interface (phase boundary between 1T and 2H MoSe2) act as a site with low ΔGHwhich would act as a primary active region in the electrocatalyst. Such reduction in ΔGH at the phase boundary is attributed to the electron accumulation, improved H activation, and enhanced bonding strength between Se and H atom. In addition, the decorated Pt cluster act as not only an independent active site but also tuning the electronic structure both Pt and Se active site which enhanced the HER activity.