3.2. Adsorption of metal single atom on the surface of LaTaON2
Based on the relaxed bulk structure, (100), (010) and (001) surfaces are built using (1×1) sized periodic slabs including eight atomic layers. The computed surface energies are 2.24 m/Å2 for (100), 1.46 m/Å2 for (010) and 2.12 m/Å2 for (001). The smaller the surface energy, the more stable the surface. As can be found, the stability of (010) is notably higher than other two surfaces. Hence, (010) surface is chosen to act as the support for loading the metal cocatalysts. As shown in Fig. 3, there are two possible terminations exposed with La and Ta atoms for (010) surface of LaTaON2, which are named as La-termination and Ta-termination, respectively. To find the stability tendency of metal single atom cocatalysts on the surface, we initially consider seven adsorption sites on the La-termination and eight adsorption sites on the Ta-termination, as displayed in Fig. S1. After the optimization, some initial structures undergo important displacement-relaxation procedure. Consequently, three and four stable adsorption sites are found on La-termination and Ta-termination, respectively, which are highlighted in Fig. 3. The relative energies of varied adsorption structures are suggested in Fig. 4. For La-termination, all the studied metal single atoms preferentially adsorb on the site of topN. While for Ta-termination, the most stable adsorption site for all the metals is located above the La atom in the sublayer, hollowLa.
The adsorption energy is computed for the most stable structure of each metal adatom on LaTaON2 (010) surface by Equation (1):49
E ads =EM@ surfaceE surfaceE M (1)
where EM@ surface,E surface, and E M represent the energy of a surface adsorbed with metal single atom, the energy of a clean surface and the energy of a free metal atom, respectively. Based on the definition, the more negative the adsorption energy, the more stable the structure. The most stable structures and the corresponding adsorption energies are presented in Fig. 5, and the less stable structures are shown in Fig. S2. The La- and Ta-termination attached with Pt, Ru and Ni adatom are denoted as Pt@La, Ru@La, Ni@La, Pt@Ta, Ru@Ta and Ni@Ta, respectively. As can be seen from Fig. 5, the adsorption energies are in the range from -7.10 eV to -3.48 eV, which reveal that there are strong interactions between metal adatoms and the surface terminations. For the same termination, the adsorption of Ru on the surface is the strongest, followed by Pt and Ni. The adsorption of a metal atom on Ta-termination is stronger than that on La-termination, which is mainly due to the formation of more new metal-nonmetal bonds in the former.
The adsorption of single metal atoms leads to the deformation of local structures, especially the displacement of the atoms in the top two layers. For La-termination, the predicted bond lengths are 1.937 Å for Pt-N, 1.783 Å for Ru-N, 1.737 Å for Ni-N, which become smaller with the increase in the number of valence electrons layers of metal adatoms. The adsorption of the metal atom causes the N atom right below it to move upwards. The calculated N-Ta bond lengths are 2.089 Å in Pt@La, 2.080 Å in Ru@La and 2.070 Å in Ni@La, longer than 1.969 Å in pure La-termination. For Ta-termination, the metal adsorbate forms four covalent bonds with the adjacent nonmetal atoms in each ease. The bond lengths of M-O are 2.088 Å in Pt@Ta, 2.154 Å in Ru@Ta and 1.936 Å in Ni@Ta. The average distances of M-N bonds are 2.085 Å in Pt@Ta, 1.973 Å in Ru@Ta and 1.945 Å in Ni@Ta. Two Ta atoms neighboring to the metal adatom move to the inner of the surface due to Coulomb repulsion between them. It turns out that the bond lengths between the Ta atom and the sublayer O atom are computed to be 1.945 Å in Pt@Ta, 1.940 Å in Ru@Ta and 1.944 Å in Ni@Ta, which are shorter than that in pure Ta-termination (1.953 Å).
To study the influence of metal adsorption on electronic structures of LaTaON2 surface, we have calculated DOS of each adsorption structure and exhibited the partial density of states (PDOS) for La 5d , Ta 5d , O 2p , N 2p and M nd states in Fig. 6. For all of six systems, the VBM is primarily composed of N 2p states and the CBM mostly consists of Ta 5d states, which are similar with the band features of bulk. The adsorption of metal single atoms does not cause any impurity states in the forbidden gap, which can become the recombination center of photogenerated electrons and holes, and are adverse to the enhancement of photocatalytic activity. The majority of Pt 5d , Ru 4d and Ni 3d states are located in the lower energy region of the VBM and the minor of them are located in the top of the VBM or the CBM.
The Bader charge analyses show that the effective charges of adatoms are -0.74 e for Pt@La, -0.57 e for Ru@La, -0.45 e for Ni@La, 0.24 e for Pt@Ta, 0.82 e for Ru@Ta and 0.56 e for Ni@Ta. These results reveal that the metal adatoms receive electrons from La-termination while they lose electrons to Ta-termination. The remarkable difference in the direction of the charge transfer between the adatom and the surface can be explained by that the larger negative charges of the nitrogen (by 0.17 e) and oxygen atoms (by 0.20 e) on La-terminated surface than Ta-terminated surface results in a poorer oxidizing ability in the former than in the latter.50 The charge redistribution at the interface between the adatom and LaTaON2 surface are visualized by the charge density difference in Fig. 7 , where the yellow and cyan contours represent the accumulation and depletion of charge densities, respectively. M@La (M=Pt, Ru and Ni) systems demonstrate the similar character, that is, the attachment of metal single atoms mostly promotes the localized changes in the charge densities of metal adatom and the connected N atom, which is reified as the transfer of charge density from the N atom to the metal adatom. The redistribution of charge is found to be more delocalized in M@Ta than in M@La (M=Pt, Ru and Ni) since the embedded metal atom forms more bonds in the former than in the latter. As to all the M@Ta (M=Pt, Ru and Ni) systems, a large area of the charge depletion appears around the metal adatom and the charge accumulation region are mainly located at the neighboring nonmetal atoms. The direction of charge transfer is consistent with the prediction of Bader charge.
Work function is the required energy of removing one electron from the Fermi level to the vacuum level, which plays an important role in the charge transfer and the band bending in the interfacial structure. To explore the effect of adsorbing metal single atoms on the work function of LaTaON2 (010) surface, we have calculated the difference of work functions between the surface with an adsorbate and the pristine surface (ΔΦ). A positive value indicates that the work function of the surface adsorbed with a metal atom is larger than that of the pristine surface. The predicted values of ΔΦ are 0.15 eV for Pt@La, 0.10 eV for Ru@La, 0.06 eV for Ni@La, -0.03 eV for Pt@Ta, -0.04 eV for Ru@Ta, -0.05 eV for Ni@Ta. Our results demonstrate that depositing metal atoms on different termination of LaTaON2 can slightly tune the work function of the photocatalytic system. Loading metal single atoms on Ta-terminated surface leads to the decrease of work function, which is favorable to the escape of electrons from the surface of semiconductor.