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.