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.