Ni
+ by PH 3 treatment. The synthetic process of TMPs from NH 4 H 2 PO 2 or
(NH 4 ) 2 HPO 2 precursors is similar to that of the phosphate route.
In addition, there is another method to form TMPs from inorganic phosphate. In
general, the forming of TMPs from a specific metal phosphate precursor needs to go
through a deoxidization of metal phosphate from P–O bond to metal–phosphorus
bond [51]. Generally speaking, this deoxidization process is achieved under the H 2
or H 2 /Ar atmosphere, in which H 2 atmosphere can facilitate the reduction process
from metal phosphate to metal phosphide. Taking NaH 2 PO 2 source as an example,
TMPs can be synthesized via a first hydrothermal method to prepare metal phosphate
by mixing up NaH 2 PO 2 with metal chloride or nitrate. In these cases, some surfactants can be selected to form specific morphology in this step. In the following step,
the metal phosphate was heated to 150–500
C to form TMPs. In this way, CoP,
MoP, and RuP with different nanostructures can be obtained for the potential
application in photocatalysis [52].
16.3 Effect of P on Photocatalytic Hydrogen Evolution
Reactions
TMPs can be viewed as P element doped into the crystal lattice of transition metal.
Until now, six transition metal (Ni, Co, Fe, Mo, Mn, Cu) can be formed into TMPs,
and these TMPs can be applied into photocatalytic hydrogen evolution reactions.
Other transition metals (Zn, Ti) also can be formed into TMPs, but these TMPs are
not suitable for photocatalytic hydrogen evolution reactions owing to their property
of easy hydrolysis in an aqueous solution.
16.3.1 The Role of P
It has been certified that P element plays a positive role in photocatalytic hydrogen
evolution reactions. Fu et al. [53] prepared phosphorus-doped hexagonal tubular
carbon nitride (P-TCN) with a layered microstructure through the hydrothermal
method (Fig. 16.7a). In this case, P from phosphorus acid squeezed into graphitic
carbon nitride (GCN) skeleton to obtain the P-TCN. Compared with pure TCN,
P-TCN has a more narrow bandgap and negative conductive band position
(Fig. 16.7b–c). Thus, an enhanced photocatalytic hydrogen activity over P-TCN
can be observed (Fig. 16.7d). Photochemical measurement reveals that P element
introduces a low hydrogen evolution over potential (Fig. 16.7d).
In addition, density functional theory (DFT) calculations have been proved that P
atoms play vital roles in hydrogen evolution reactions [54, 55]. P atoms with more
electronegativity can withdraw the electrons from metal atoms [56]. Moreover, it is
384
16 Transition Metal Phosphide As Cocatalysts for Semiconductor-Based. . .
+ by PH 3 treatment. The synthetic process of TMPs from NH 4 H 2 PO 2 or
(NH 4 ) 2 HPO 2 precursors is similar to that of the phosphate route.
In addition, there is another method to form TMPs from inorganic phosphate. In
general, the forming of TMPs from a specific metal phosphate precursor needs to go
through a deoxidization of metal phosphate from P–O bond to metal–phosphorus
bond [51]. Generally speaking, this deoxidization process is achieved under the H 2
or H 2 /Ar atmosphere, in which H 2 atmosphere can facilitate the reduction process
from metal phosphate to metal phosphide. Taking NaH 2 PO 2 source as an example,
TMPs can be synthesized via a first hydrothermal method to prepare metal phosphate
by mixing up NaH 2 PO 2 with metal chloride or nitrate. In these cases, some surfactants can be selected to form specific morphology in this step. In the following step,
the metal phosphate was heated to 150–500
C to form TMPs. In this way, CoP,
MoP, and RuP with different nanostructures can be obtained for the potential
application in photocatalysis [52].
16.3 Effect of P on Photocatalytic Hydrogen Evolution
Reactions
TMPs can be viewed as P element doped into the crystal lattice of transition metal.
Until now, six transition metal (Ni, Co, Fe, Mo, Mn, Cu) can be formed into TMPs,
and these TMPs can be applied into photocatalytic hydrogen evolution reactions.
Other transition metals (Zn, Ti) also can be formed into TMPs, but these TMPs are
not suitable for photocatalytic hydrogen evolution reactions owing to their property
of easy hydrolysis in an aqueous solution.
16.3.1 The Role of P
It has been certified that P element plays a positive role in photocatalytic hydrogen
evolution reactions. Fu et al. [53] prepared phosphorus-doped hexagonal tubular
carbon nitride (P-TCN) with a layered microstructure through the hydrothermal
method (Fig. 16.7a). In this case, P from phosphorus acid squeezed into graphitic
carbon nitride (GCN) skeleton to obtain the P-TCN. Compared with pure TCN,
P-TCN has a more narrow bandgap and negative conductive band position
(Fig. 16.7b–c). Thus, an enhanced photocatalytic hydrogen activity over P-TCN
can be observed (Fig. 16.7d). Photochemical measurement reveals that P element
introduces a low hydrogen evolution over potential (Fig. 16.7d).
In addition, density functional theory (DFT) calculations have been proved that P
atoms play vital roles in hydrogen evolution reactions [54, 55]. P atoms with more
electronegativity can withdraw the electrons from metal atoms [56]. Moreover, it is
384
16 Transition Metal Phosphide As Cocatalysts for Semiconductor-Based. . .