highly active sites of the surface, which enhances the rate of the HER and makes it
comparable to that of the [NiFe] hydrogenase. The first experimental work for TMPs
is reported by Zhang’s group [39]. They prepared nanoporous FeP nanosheets
through anion-exchange method, and the resulting sample shows high
electrocatalytic activity toward hydrogen evolution reaction with low overpotential
and a small Tafel slope. After that, various TMPs (Cu 3 P, CoP, Ni 2 P, Mo 3 P) as highly
efficient electrocatalysts have been investigated in hydrogen evolution reactions
[57, 63].
In photocatalytic hydrogen evolution process, the semiconductors have large
surface overpotential, which need to be compensated by extra photovoltage.
According to the excellent performance of TMPs in electrocatalysis, it can be
deduced that TMPs can lower down the overpotential of semiconductor and have
great potential as cocatalyst in photocatalytic hydrogen evolution reactions.
16.4.2 Explanation of TMPs As Cocatalysts
A highly efficient cocatalyst can not only rapidly extract the photo-generated
electrons from photocatalyst to its surface but also catalyze H 2 evolution on its
surface by using these trapped electrons. In general, the overall hydrogen evolution
reactions pathway can be summarized as three-state diagram, including an initial
state H
+ + e
À
, an intermediate state adsorbed H*, and a final product 1/2H 2
[13, 64]. The Gibbs free energy of the intermediate state, |ΔG H* |, is regarded as a
key indicator of the hydrogen evolution reaction activity for various cocatalysts. The
most desirable value of |ΔG H* | should be zero. For instance, platinum (Pt), as a wellknown and excellent cocatalyst in photocatalytic hydrogen evolution reaction,
shows a near zero of ΔG H* % À0.09 eV [65]. According to TMPs, the Gibbs free
energy value of |ΔG H* | can be calculated by DFT calculations.
As early as 2005, Liu et al. [54] used DFT calculations to indicate that the P atoms
with a small negative charge from the surface of TMPs can not only trap hydrogen
protons but also provide high activity for the dissociation of H 2 . Wang’ s group [55]
presented comparative analysis of Mo, Mo 3 P, and MoP as cocatalyst for hydrogen
evolution reaction, which indicates that phosphorization can potentially modify the
properties of the metal and different degrees of phosphorization lead to distinct
activities and stabilities (Fig. 16.9). Gibbs free energy of ΔG H* of the P-terminated
surface on (001)-MoP is rounded to the nearest zero. When H coverage increases
from 1/4 ML to full coverage, ΔG H* gradually turns from À0.36 to 0.54 eV, which
suggests that P can bond hydrogen at low coverage when H desorbed at high
coverage. This behavior is similar to hydrogen deliverer, which also existed in the
S-edges of MoS 2 . This good hydrogen evolution activity is determined by the low
value of |ΔG H* |.
Wu et al. [66] unraveled that the electrocatalytic process for TMPs plays a vital
role in the heterostructure photocatalytic H 2 evolution system for the first time.
Electrocatalysts were used as the excellent cocatalysts to facilitate the charge
388
16 Transition Metal Phosphide As Cocatalysts for Semiconductor-Based. . .
comparable to that of the [NiFe] hydrogenase. The first experimental work for TMPs
is reported by Zhang’s group [39]. They prepared nanoporous FeP nanosheets
through anion-exchange method, and the resulting sample shows high
electrocatalytic activity toward hydrogen evolution reaction with low overpotential
and a small Tafel slope. After that, various TMPs (Cu 3 P, CoP, Ni 2 P, Mo 3 P) as highly
efficient electrocatalysts have been investigated in hydrogen evolution reactions
[57, 63].
In photocatalytic hydrogen evolution process, the semiconductors have large
surface overpotential, which need to be compensated by extra photovoltage.
According to the excellent performance of TMPs in electrocatalysis, it can be
deduced that TMPs can lower down the overpotential of semiconductor and have
great potential as cocatalyst in photocatalytic hydrogen evolution reactions.
16.4.2 Explanation of TMPs As Cocatalysts
A highly efficient cocatalyst can not only rapidly extract the photo-generated
electrons from photocatalyst to its surface but also catalyze H 2 evolution on its
surface by using these trapped electrons. In general, the overall hydrogen evolution
reactions pathway can be summarized as three-state diagram, including an initial
state H
+ + e
À
, an intermediate state adsorbed H*, and a final product 1/2H 2
[13, 64]. The Gibbs free energy of the intermediate state, |ΔG H* |, is regarded as a
key indicator of the hydrogen evolution reaction activity for various cocatalysts. The
most desirable value of |ΔG H* | should be zero. For instance, platinum (Pt), as a wellknown and excellent cocatalyst in photocatalytic hydrogen evolution reaction,
shows a near zero of ΔG H* % À0.09 eV [65]. According to TMPs, the Gibbs free
energy value of |ΔG H* | can be calculated by DFT calculations.
As early as 2005, Liu et al. [54] used DFT calculations to indicate that the P atoms
with a small negative charge from the surface of TMPs can not only trap hydrogen
protons but also provide high activity for the dissociation of H 2 . Wang’ s group [55]
presented comparative analysis of Mo, Mo 3 P, and MoP as cocatalyst for hydrogen
evolution reaction, which indicates that phosphorization can potentially modify the
properties of the metal and different degrees of phosphorization lead to distinct
activities and stabilities (Fig. 16.9). Gibbs free energy of ΔG H* of the P-terminated
surface on (001)-MoP is rounded to the nearest zero. When H coverage increases
from 1/4 ML to full coverage, ΔG H* gradually turns from À0.36 to 0.54 eV, which
suggests that P can bond hydrogen at low coverage when H desorbed at high
coverage. This behavior is similar to hydrogen deliverer, which also existed in the
S-edges of MoS 2 . This good hydrogen evolution activity is determined by the low
value of |ΔG H* |.
Wu et al. [66] unraveled that the electrocatalytic process for TMPs plays a vital
role in the heterostructure photocatalytic H 2 evolution system for the first time.
Electrocatalysts were used as the excellent cocatalysts to facilitate the charge
388
16 Transition Metal Phosphide As Cocatalysts for Semiconductor-Based. . .