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@ surface –E surface – E 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.