3.3. Hydrogen evolution reaction mechanism
In the photocatalytic reactions, loading proper metal cocatalysts is one
of the most effective methods of enhancing the HER performance since
they can act as the reactive sites, promote the charge separation and
decrease the reaction barrier.30 In the present work,
we only focus on the thermodynamic process of the surface reaction. HER
is generally considered to be a three-state process and can be described
by the following equation:
H+ + e - → 1/2
H2(g) (2)
The Gibbs free energy of the initial state is equal to that of the final
state under the standard electrode voltage.51 Thus,
the catalytic activity of HER is mainly correlated with the Gibbs free
energy change (ΔGH) of the process from the initial
state to the adsorptive state, which can be calculated based on Equation
(3):52
ΔG H = ΔE H + ΔZPE −T ΔS H (3)
where ΔE H is the adsorption energy of a H atom
attached to the active site of the surface, ΔZPE is the
difference in the zero-point energies of an adsorbed species and a
gaseous phase, and ΔS H is equal to the negative
value of half the entropy of H2 in the gas phase under
the standard conditions.53 The more positive the value
of ΔG H, the more difficult the adsorption of the
H atom on the surface. While the more negative the value of
ΔG H, the more difficult the desorption of
H2 from the surface. Hence, ΔG Hshould be close to zero for the ideal catalyst of
HER.54
For both terminations of clean surface, M@La and M@Ta systems (M=Pt, Ru
and Ni), we have examined possible adsorption sites of hydrogen,
including the exposed La, Ta, O, N and metal adatoms. The optimized
structures are summarized in Fig. 8 and the corresponding values of
ΔG H are utilized to draw the free-energy diagram
of HER. As we know, the ideal catalyst for the HER is regarded as
ΔG H=0, since both adsorption and desorption steps
are thermoneutral.55 In other words, the closer
|∆GH| is to zero, the better the
catalysts. As shown in Fig. 8(a), for pure BaO-termination, the
calculated values of ΔG H are 0.36 at the N site,
0.84 eV at the La site and 1.28 eV at the O site, which indicate that
the hydrogen adsorption on this termination is endergonic and the N site
is more active than metal sites in the HER. For pure TaON-termination,
the calculated values of ΔG H are -0.63 at the N
site, -0.60 eV at the O site and 1.78 eV at the Ta site, which suggest
that the hydrogen adsorptions on the nonmetal atoms is energetically
much more favorable than that on the Ta atom.
Fig. 8(b), 8(c) and 8(d) illuminate that the calculated values of
ΔG H varies in the site of hydrogen adsorption,
ranging from -0.77 to 0.99 eV for Pt@La, from -0.17 to 1.01 eV for
Pt@Ta, from -0.77 to 0.89 eV for Ru@La, from -0.18 to 1.12 eV for Ru@Ta,
from -0.68 to 0.84 eV for Ni@La, and from -0.05 to 0.99 eV for Ni@Ta.
For M@La (M=Pt, Ru and Ni), the value of ΔG H at
the N site is remarkably closer to zero than those at other sites, which
shows that the N site is the most active site in the HER. The adsorption
of hydrogen on the metal adatom is quite stable since the values of
ΔG H are -0.68 eV for Ni@La and -0.77 eV for both
Pt@La and Ru@La. The strong adsorption is unfavorable to the following
desorption step. The hydrogen adsorptions on the O atom and La atom are
relatively difficult since the process is endergonic. As to M@Ta (M=Pt,
Ru and Ni), both the N and O atoms binding to the metal adatom can be
the active sites of HER since the computed values of
|ΔG H| at two sites are
comparable and closer to zero than those at other sites. The adsorption
of hydrogen on the metal adatom and La atom is energetically unfavorable
due the larger positive values of ΔG H. Our
results demonstrate that the active sites of HER are the exposed
nonmetal atoms on the surfaces with and without single metal adatoms.
The adsorption of Pt, Ru and Ni single atom on the
LaTaON2 (010) surface activates the adjacent nonmetal
atoms and significantly decreases the value of
|ΔG H|.