1. Introduction
In recent years, hydrogen generation via photocatalytic water splitting
based on heterogeneous photocatalysts has received much attention and
become a promising strategy for solving the global energy crisis and
environmental issues.1-4 Most semiconductor-based
photocatalysts with relatively high activity and excellent chemical
stability are composed of transition metal oxides containingd 0 and d 10 metal ions.
However, their large band gaps made them only active under the
ultraviolet light irradiation, which makes up only a small part of the
solar energy spectrum. Hence, in order to enhance the utilization of
solar energy, extending the light absorption range of heterogeneous
photocatalysts has become a hot research topic in this field. In this
respect, the synthesis of perovskite-type transition metal oxynitrides
with chemical formula AB(O,N)3 (A=Ca, Sr, Ba, La; B=Ti,
Nb, Ta) has drawn much attention, not only because they have
photocatalytic activities under visible light irradiation caused by the
higher energy level of N 2p than O 2p at the valence band maximum (VBM),
but also because they can accomplish the overall water splitting
reaction in principle due to their suitable band positions relative to
the oxidation and reduction potentials of
water.5-9
In the family of perovskite oxynitrides, LaTaON2 is an
attractive photocatalyst for splitting water because of its broad
absorption in the visible light region and suitable band edge
positions.10-14 LaTaON2 generally
exhibits poor photocatalytic activity due to its high defect
concentration and grain boundaries. To enhance the photocatalytic
activity, various strategies were used in the
LaTaON2-based materials, such as doping metals, loading
suitable cocatalysts, and forming solid solutions or
heterojunctions.15-22 In 2006, Liu et al . found
that LaTaON2 with Pt or Ru as a cocatalyst showed high
activity for reduction of water to H2 in the presence of
ethanol as a sacrificial electron reagent.23 A
remarkable enhancement in production efficiency was observed when both
Pt and Ru were present. Zhang et al . observed the progress of
H2 evolution from an aqueous methanol solution over
Pt-loaded LaTaON2 flux under visible-light
irradiation.24 In 2017, Si et al . investigated
the behavior of different cocatalyst on LaTaON2photoanode for decomposing water.25 Their results
showed that Ni-based oxides lead to higher improvement than other
oxidation cocatalysts on the photoelectrochemical performance, and
severe recombination of photogenerated carriers in bulk or poor
electronic conductivity may be main factors limiting the
photon-to-current conversion efficiency in LaTaON2photoanodes. Hojamberdiev et al . found that with Pt and
CoOx as cocatalysts, LaTaON2 showed
higher photocatalytic activities and photoanodic response of HER and OER
than PrTaON2 mainly due to a less amount of intrinsic
defects and reduced tantalum species.26 In 2018, Huanget al . reported that after the modification of
CoOx, the solar photocurrent of LaTaON2was an order of magnitude larger than the previously-reported value,
which primarily resulted from bulk defect control and interface
engineering 27. In 2019, Wang et al. synthesized facet-controlled LaTaON2 with low defect
concentrations through selecting LaKNaTaO5 as the
appropriate oxide precursors, which exhibited the photocatalytic
activity of HER four times greater than that obtained from the samples
modified with a Rh cocatalyst.28 In 2022, Xu et
al . synthesized a platy LaTaON2 by a one-pot
nitridation route, which showed a six-fold increase in photocatalytic
H2 evolution activity compared with the conventional
LaTaON2 powder after loading with a Pt
cocatalyst.29 In the visible-light-driven Z-scheme
overall water splitting system, Pt/LaTaON2 served as
H2-evolving photocatalyst.
As can be seen, metal cocatalysts are essential components in
LaTaON2 photocatalytic system for highly efficient
hydrogen production since they are beneficial to promoting the
separation/transfer of photoinduced carriers and reducing the reaction
energy barrier.30 However, some critical questions
about cocatalysts still need to be addressed, such as the stable
location of cocatalysts on the semiconductor surface, the effect of
loading cocatalysts on the electronic structure, the relationship
between the type of cocatalysts and the reaction activity, and so on.
The structural complexity of nanoscale cocatalysts makes it difficult to
obtain the relevant information above in experiments. In this respect,
density functional theory (DFT) calculations have been proven to be a
favorable supplement to experimental measurements in revealing the
structure-property relationship and reaction mechanism of various
photocatalytic systems.31, 32 In this work, we have
theoretically simulated LaTaON2 loaded with metal (M)
cocatalysts (M=Pt, Ru and Ni) presenting as a single
atom.33, 34 We have constructed different surface
terminations based on the relative stable structure of bulk
LaTaON2, searched the most stable adsorption site of
metal cocatalysts, studied the electronic structures of photocatalytic
systems and the mechanism of HER, and explored the role of cocatalysts
in influencing the reaction activity.
Computational methods
Spin-polarized DFT calculations were carried out by utilizing the
projected augmented wave method implemented in the Vienna ab initio
simulation package.35-38 The valence electrons were
expanded into a set of plane waves with a cutoff of 450 eV. The
Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient
approximation was applied to represent the exchange-correlation
potential.39, 40 The convergence criterion of
electronic and ionic loops were set to be 10-4 eV and
10-2 eV/Å, respectively. According to the experimental
measurements in literatures,41, 42we chose the orthorhombic crystal structure with the space groupImma to model bulk LaTaON2. There are 20 atoms in
the unit cell with the lattice parameters of a =5.716 Å,b =8.067 Å, c =5.746 Å, and α = β = γ =90˚. A 7×5×7 Monkhorst–Pack k -point sampling was used to
calculate the structural and electronic properties of bulk. The PBE +U method was utilized to calculate the electronic properties, in
which only the difference Ueff between the
exchange J and Coulomb U parameters was considered. Here,
we used a Ueff value of 5 eV for Ta 5d orbitals
to obtain the similar band character to experimental
measures.22, 26, 42
Stoichiometric slab models containing even atomic layers were
constructed for simulating the low-index surfaces of bulk
LaTaON2. In the perpendicular direction to each surface,
a vacuum layer of 15 Å was added to eliminate spurious interactions
between periodic images. The dipole correction was used in calculations
to compensate the depolarizing field resulting from the asymmetry of the
slab. The surface energy was computed to evaluate the stability of
surface terminations with Monkhorst–Pack k -point samplings of
5×7×1 for (100), 7×7×1 for (010) and 7×5×1 for (001), respectively. To
model the (2×2) supercell with adsorbates, the atoms in the bottom two
layers were fixed at bulk positions and other parts were fully relaxed,
and a 3×3×1 k -point mesh was applied to perform the structural
relaxation and self-consistent calculations.