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 + ΔZPET 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|.