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.