3.1 Atomic diversity free homonuclear DAC heterogenization and
the Stabilities of homonuclear DAC systems
Since both heteronuclear TM atoms exhibit high activity in the eNRR
process, it is hard to detect which active site is more sensitive for
N2 capture and activation, especially when considering
both heteronuclear induced atomic diversity and intrinsic properties’
variety in the same system, which may seriously impede the underlying
electronic mechanism finding. In fact, heteronuclear induced atomic
difference may contribute more to the eNRR process when the intrinsic
properties keep high activity and low distinction, especially for the
radius variety of each TM atom. Hence, the atomic diversity should be
minimal to get more sensitive detection to differentiate intrinsic
electronic influences. However, it is hard to remove the impact of this
effect in heteronuclear TM DAC. For example, the physical radius of TM
atoms varies from 1.24 to 1.60 Å,
as summarized in our previous work46, which could
deteriorate when considering the Van der Waals radius (1.40 to 2.09
Å)47. After constructing heteronuclear DAC (see inFigure 1(a) ), the dual TM atoms present a ’large and small
size’ active site, where the
N2 molecule from the solution is seriously affected by
the atomic diversity, thus insufficient for intrinsic mechanism finding.
The best strategy to minimize this effect is to keep the dual TM atoms
far away from each other to form a single atom active site and follow
the famous ’donation/back-donation’ mechanism48. Or
enlarge the intrinsic properties to ignore the existing effect, e.g.,
metal-free dual atom following the ’capture-backdonation-recapture’
mechanism36.
Nevertheless, those methods are unsuitable for investigating the dual TM
atoms systems. It is worth noting that localizing atomic size to the
same structural properties could be a better strategy to remove the
undesired effect, such as homonuclear DAC, while the intrinsic
properties remain constant at the same time, leading the
N2 molecule to show no sensitivity to each TM atom.
Besides, according to our recent studies, the captured
N2 molecule keeps its equilibrium state and impedes the
hydrogenation process in B-B, C-C metal-free homonuclear
DAC49. Hence, the unfunctionalized homonuclear DAC is
also not suitable for mechanism finding. To solve this dilemma, we
constructed homonuclear dual TM atoms coordinating with various
coordinate environments to differentiate the intrinsic properties. In
this system, the heteronuclear induced atomic difference effect is
entirely removed, and the intrinsic properties are varied with each TM
atom simultaneously. More importantly, a single-element catalyst could
form the homonuclear DAC with various intrinsic properties when
considering the complicated loading environment. Meantime, it is
essential to select an ideal substrate for homonuclear TM loading. Based
on previous studies of the coordination effect atomic catalyst, the
C3N monolayer is an ideal candidate substrate for
homonuclear TM atoms doping28, 50. Advantageously,
several possible defective sites such as
monocarbide
defect, mononitride defect, dicarbide defect, and diatomic defect
consisting of a C and N atom can be formed on
C3N(Figure S1 ) with defected formation energy
of 4.61, 4.82, 4.83, and 6.71 eV51, respectively,
which is lower than the wide-studied monovacancy and divacancy in
graphene (8.09 and 8.20 eV), indicating that the easer synthesis of the
defective sites in C3N nanosheet52.
Furthermore, when considering various doping sites (Figure
1(b) ), the bond length between two TM atoms could be well-tuned, which
is vital for subsequent analysis. For the C3N monolayer,
after complete optimization, the obtained lattice constant is 4.86 Å,
consistent with the previous reports 28, 51, 53,
demonstrating the precision of this study. Then, a 3×3×1 supercell of
the C3N is adopted for further investigation, and four
candidate sites
(C2N-C3,
C3-CN2,
C4-C3, and
C2N-C2N2) are filtrated
finally since the asymmetric vacant geometry and none-dangling bonds of
them, which provide diverse sites for TM embedding (Figure
1(b) ). Finally, 23 kinds of homonuclear dual TM atoms from Ti to Au
(except for Tc) are used to dope in site1,
site2, site3, and site4within 92 systems. To ascertain the stability of the homonuclear TM
atoms doped systems (TM2@sitex, x = 1,
2, 3, 4), the formation energy (Eform) of the
TM2@sitex (x = 1, 2, 3, 4) is
systematically investigated initially, which is defined as:
Eform = (ETM2-C3N – EV- 2ETM)/2 (2)
where ETM2-C3N, EV, ETMis the total energy of the TM2@sitex (x
= 1, 2, 3, 4) system and defective system, as well as the TM atoms in
their bulk phase, respectively. When the Eform< 0 eV, the doped systems can be considered thermodynamic
stability. As depicted in Figure 1(c) and Table S1 ,
most of the considered systems show negative Eform,
indicating their thermodynamic stability. In contrast, the systems that
own positive Eform would be excluded for further
exploration, including Zr, Nb in Site2 and Ti, Cu, Zr,
Nb, Hf in Site3 since their unstable configuration and
the easy aggregation in defective C3N substrate. Apart
from the formation energy of the doped system, the dissolution potential
(Udiss) is further investigated to test the
electrochemical stability of the rest systems, and it can be defined as
follows 54:
Udiss = Udiss-bulk –
Eform/(nee) (3)
in which Udiss-bulk, ne represents the
dissolution potential of the bulk metal as well as the number of
transferred electrons during the dissolution process. It can be regarded
as electrochemical stability of the
TM2@sitex (x = 1, 2 ,3, 4) when
Udiss > 0. As demonstrated inFigure 1(d) , all the doped systems show electrochemical
stability except for Ti, V, Zr, Nb, Hf, Ta in Site1, Ti,
V, Cr, Mn, Zr, Nb, Mo, Hf, Ta in Site2, Ti, V, Mn, Zr,
Nb, Hf, Ta in Site3 and Ti, Zr, Nb, Hf, Ta in
Site4. Notably, most of the former TM atom anchored
systems exhibit electrochemically instability, while the latter TM atoms
doped system shows high stability in an electrochemical view. This
phenomenon mainly originates from the high activity and high dissolution
potential in their bulk phase of the TM atom, thus leading the doped
systems to prefer to dissolve in the solution. Eventually, a total of 72
candidates are selected for the following investigations.