Journal of the American Chemical Society
Article
istics (green circles) of the catalyst. This outstanding catalytic
performance and particular memristive characteristics show
high promise for use in NRR applications.
the MRS, the decreased e occupancy at the Fe site leads to a
g
decreased adsorption energy, while the Re site demonstrates
the opposite behavior, resulting in the conversion of active
sites.
According to the octahedral ligand field, the structural
transition induced by electric stimulation can be explained by
the detailed orbital analysis in Figure 4c. Regarding the
electronic structure of the HRS (Figure 4d), the asymmetric
To compare the bonding characteristics of Fe−N and Re−
N, we calculated the crystal orbital Hamilton population
(COHP) in Figure 5d. Compared with the Fe−N bonding
contribution for the HRS, an obvious antibonding state from
the Re−N bond appears at the Fermi level, indicating that the
electrons from the Re 5d orbital are partially donated to the
unfilled π* orbital. The stability of the Re−N bond is lower
2
2
Fe−S units make the 3d orbital split into higher Fe d
and
1
x −y
2
lower Fe d orbitals in the valence bands, whereas a flat
2
z
2
2
2
conductance band is composed of Fe d
and Fe dz
2
x −y
1
orbitals, creating a 0.272 eV band gap. Most electrons prefer
to occupy the bonding state (marked by black arrows) formed
than that of the Fe−N bond, thus forcing *N to be active at
2
the Re site for subsequent hydrogenation. Regarding the MRS,
2
by dz orbital hybridization, but this nonequivalent electron
occupation makes the Fe −S bond length (2.31 Å) and Fe −S
Fe−N bonds transform into an antibonding state from a
1
2
bond length (2.35 Å) different. When superfluous carriers
bonding state at the Fermi level; thus, the adsorbed *N on the
2
induced by electrical stimulation are intentionally implanted to
Fe site can be activated more easily. These detailed orbital
analyses confirm that the active site conversion induced by
structural transition plays a vital role in enhancing NRR
performance.
2
2
2
reconstruct the antibonding state of Fe d
and Fe dz
2
x −y
1
orbitals, the difference between the Fe −S and Fe −S
1
2
decreases, resulting in a smaller band gap and a parabolic
conduction band (Figure 4e). Thus, the hybridization between
Fe −S and Fe −S is enforced to generate the MRS structure,
CONCLUSION
1
2
■
and the remaining electrons will hybridize with each other to
form additional symmetric metallic bonds (pink bond between
In this work, we have demonstrated that electrical stimulation
can lead to an intriguing structural transition to boost NRR
performance, and this method is inspired by the learning
networks of spiking neurons. Notably, the optimized catalytic
activity can be remembered and recovered repeatedly through
a simple charging operation. Our findings offer new insight for
boosting NRR performance in electrocatalysis, photocatalysis
and photoelectrochemical catalysis. Because of the variety of
candidate materials, it is feasible for this strategy to be widely
adopted to optimize catalytic performance.
Fe and Fe ), as shown in the right panel of Figure 4c. The
1
2
orbital-resolved bands in Figure 4d show that the energy
2
2
2
splitting between the d
and d orbitals in the HRS (Δ1 and
x −y
z
Δ2) is much larger than that of the MRS in Figure 4e, which
demonstrates the orbital hybridization priority in the MRS
with electrical stimulation (right panel of Figure 4c). In
addition, the decreased band gap in the MRS endows the
To elaborate the bonding features, we compare the orbital-
EXPERIMENTAL SECTION
Preparation of the Ultrathin FeReS Nanosheets. During the
■
resolved DOS of N activation at the Fe and Re sites of the
2
3
MRS in Figure 5a. In the conduction band region, the spin-
synthesis procedure, 100 mg of iron acetylacetonate was suspended in
10 mL of deionized water with vigorous stirring. With ultrahigh purity
argon (Ar, 99.999%) protection, 10 mL of aqueous solution including
18.5 mM sodium perrhenate and 0.65 M thiourea were injected into a
Pyrex reactor. The rate of the syringe pump was controlled at 2 mL
down energy levels of the Fe 3d orbitals and N π* orbitals are
2
coordinated well with each other, creating the appearance of
unoccupied 3d-π*. Strong 3d orbital localization provides a
large exchange stabilization energy at the valence band tail
−
1
min . The solution was refluxed at 278 K for approximately 1 h while
stirring to ensure that the solution dispersed adequately. To obtain
qualified ultrathin FeReS3 nanosheets, a sodium citrate aqueous
solution (2 mL, 0.1 wt %) was steadily added dropwise with Ar
protection before the hydrothermal process. The solution was sealed
in a Teflon-lined stainless steel autoclave (25 mL) for 15 h at 523 K.
The supernatant was removed when the autoclave cooled to room
temperature. The black precipitates were rinsed thoroughly with
absolute ethanol and deionized water and freeze-dried under vacuum.
The crystallization and purity of FeReS3 was improved using an
annealing procedure. Ceramic boats were inserted together with the
nanosheets into a horizontal quart tubular reactor. The reactor was
cleaned with 500 sccm Ar for 2 h before the heating process began.
The furnace was maintained at 537 K for 1 h with a 150 sccm Ar flow,
which helped prevent oxidation. The pressure under the whole
(
marked by purple shadow), which has obvious advantages for
promoting charge transfer from the N δ orbital to the partially
2
occupied Fe 3d orbital. After orbital hybridization, the 3d-δ
bonding state is approximately 0.66 eV smaller than that of N-
δ orbitals, thus forcing *N to be absorbed and activated.
2
Regarding the Re site, the electronic states of the 5d orbital
becomes delocalized, which weakens the interaction between
*
N and the Re site. If the electron occupied at the N δ orbital
2
2
is transferred to the unoccupied 5d orbital to form a 5d-δ
bonding state, more energy needs to be provided, leading to a
higher N−N bond order. Therefore, a higher dissociation
barrier occurs at the Re site instead of the Fe site. Regarding
the N molecule, a lone-pair δ orbital can donate electrons to
2
−
2
the e orbital reaction site, whereas the N π* orbital receives
g
2
annealing process was reduced to less than 1 × 10 Torr.
Electrochemical NRR Measurements. The electrochemical
(EC) measurements were conducted with a CHI 660D electro-
electrons from the t2g orbitals (Figure 5b).
Generally, the catalytic interaction between N and the
2
chemical workstation at room temperature using a N -saturated 0.5 M
reaction site can be reflected by the process of the “acceptance-
donation” of electrons, where controlling the unfilled and
occupied d orbitals of the active site plays a determining role in
2
K SO solution. The two independent compartment cells were
2
4
connected with a Celgard 3401 membrane. Before the NRR test, the
Celgard membrane was sequentially heated in boiling water, H 5%
aqueous solution, dilute H SO (0.5 M) and ultrapure water for 120
3
6
O
2 2
accelerating the NRR. N2 molecules prefer a tilted
2
4
configuration, allowing their π* orbital to overlap with the eg
min, respectively. A graphite rod (Alfa Aesar, 99.9995%), modified
glassy carbon electrode, and saturated Ag/AgCl electrode were
utilized as the counter, working and reference electrodes, respectively.
Stable polarization curves could be achieved after several cycles. The
orbital of the metal site. As a result, e occupancy discloses the
g
strength of the bond between the adsorbate and catalyst
surface (Figure 5c). As a structural transition from the HRS to
5
383
J. Am. Chem. Soc. 2021, 143, 5378−5385
Zhou, Gang
