Fe Spin State Research Articles

Accordion-like multilayered titanium nitride enables FeN4-O axial coordination of iron phthalocyanine as efficient catalysts for oxygen reduction reaction

The susceptibility of MXenes to oxidation leads to rapid chemical degradation and functional loss, and hence limits their diverse applications. In this study, we develop a scalable strategy to transform blocky, condensed titanium nitride (TiN) into accordion-like leafy titanium nitride (M-TiN), which exhibits enhanced chemical stability and an increased surface area. When functionalized with iron phthalocyanine (FePc), the resulting M-TiN/FePc composite displays a remarkable electrocatalytic activity towards the oxygen reduction reaction (ORR) due to the formation of FeN4-O axial coordination and a low-to-intermediate Fe spin state, as confirmed by spectroscopy and magnetic measurements. This leads to electron filling in the antibonding orbitals composed of Fe 3dz2 and O2 π* orbitals, significantly improving O2 activation and ORR activity. Significantly, with the M-TiN/FePc composite as the air cathode catalyst, a zinc-air battery achieves a superior power density of 270.31 mW cm−2 and enhanced cyclability compared to that with commercial Pt/C. Results from this study highlight the significance of morphological engineering of TiN in regulating the atomic configuration of adsorbed metal active centers and hence their catalytic activity, in particular, in harsh oxidative conditions.

FeSeS@C cage-in-cage superlattices for peroxymonosulfate activation: Surface acidity regulates Fe spin state

Two-dimensional cage-in-cage carbon-coated FeSeS superlattices with varying degrees of sulfidation (FeSeS@C) are developed for activating peroxymonosulfate (PMS) to effectively degrade diatrizoic acid (DTZ), and the intrinsic origin that govern the activity of FeSeS@C are deeply elucidated. Experimental and theoretical analyses manifested that proper sulfidation led to increased surface acidity of FeSeS@C. The high surface acidity can optimize the exposure and spin state of Fe sites for FeSeS@C-4, a high spin state of Fe (6.27 μB) not only regulating PMS adsorption for enhancing the charge density, but also expediting interfacial charge deliver to trigger the efficient PMS activation. Therefore, among FeSeS@C, FeSeS@C-4 exhibited the best degradation performance for DTZ, with first-order kinetic rate constants (kobs) of 0.232 min−1 and degradation rate of 100 %. This study demonstrates a novel application of cage-in-cage superlattices in environmental remediation and offers new insights into the mechanism of PMS activation by sulfur modification Fe-based catalysts.

Regulating Fe-spin state by doping P heteroatom in Fe-N-C catalyst derived from colloidal nanoparticles encapsulated ZIF-8 to boost oxygen reduction activity

Over the past decades, Fe-N-C carbon materials have been considered as one of the most ideal alternatives to the noble metal catalysts (e.g. Pt/C) for the sluggish oxygen reduction reaction (ORR) in zinc-air batteries and fuel cells. In this research, ZIF-8 cage encapsulation confinement strategy coupled with post P-doping has been employed to design porous P, N and Fe co-doped carbon (FeNC-P) for efficient oxygen reduction reaction (ORR). The zero-field cooling (ZFC) temperature-dependent magnetic susceptibility measurement revealed that incorporation of P in the Fe-N-C catalysts can induce the transition of spin polarization configuration, which promotes the desorption of *OH in the crucial step of the ORR reaction process. The best-performing FeNC-P catalyst displays a 1.00 V (vs RHE) of Eonset (onset potential) and a 0.90 V (vs RHE) of E1/2 (half-wave potential) in an alkaline solution, which exceeds the benchmark Pt/C catalyst. Moreover, it also delivers a high power density of 130 mW·cm−2 in the self-made zinc-air battery, surpassing the benchmark Pt/C catalyst (90 mW·cm−2).

Efficient generation of singlet oxygen for photocatalytic degradation of antibiotics: Synergistic effects of Fe spin state reduction and energy transfer

Singlet oxygen (1O2) exhibits excellent degradation performance and stability in removing antibiotics from an aqueous environment. However, its production is limited by free radical competition. This study proposed a photocatalytic method employing ferrihydrite (Fh) as the base, coated with K-doped g-C3N4 and modified by AgCl (Ag/AgCl), leading to the KC3N4/AgCl/Fh composite for efficient 1O2 production. Compared with Fh, the loading of KC3N4 reduced Fe (III) spin, facilitating the rapid conversion of H2O2 to 1O2. The addition of Ag promoted the intersystem crossing process, decreasing the singlet-triplet energy gap from 0.0708 to 0.0458 eV, thereby effectively generating more triplet excitons to convert O2 into 1O2 in a targeted manner. The KC3N4/AgCl/Fh achieved a degradation rate of 98.10 % for enrofloxacin in 30 min, showing high effectiveness and stability. This work suggests a new way to create Fh-based photocatalysts for the efficient production of 1O2.

Rapid Surface Reconstruction of Pentlandite by High-Spin State Iron for Efficient Oxygen Evolution Reaction.

Triggering rapid reconstruction reactions holds the potential to approach the theoretical limits of the oxygen evolution reaction (OER), and spin state manipulation has shown great promise in this regard. In this study, the transition of Fe spin states from low to high was successfully achieved by adjusting the surface electronic structure of pentlandite. In situ characterization and kinetic simulations confirmed that the high-spin state of Fe promoted the accumulation of OH- on the surface and accelerated electron transfer, thereby enhancing the kinetics of the reconstruction reaction. Furthermore, theoretical calculations revealed that the lower d-band center of high-spin Fe optimized the adsorption of active intermediates, thereby enhancing the reconstruction kinetics. Remarkably, pentlandites with high-spin Fe exhibited ultra-low overpotential (245 mV @ 10 mA cm-2 ) and excellent stability. These findings provided new insights for the design and fabrication of highly active OER electrocatalysts.

Rapid Surface Reconstruction of Pentlandite by High‐Spin State Iron for Efficient Oxygen Evolution Reaction

AbstractTriggering rapid reconstruction reactions holds the potential to approach the theoretical limits of the oxygen evolution reaction (OER), and spin state manipulation has shown great promise in this regard. In this study, the transition of Fe spin states from low to high was successfully achieved by adjusting the surface electronic structure of pentlandite. In situ characterization and kinetic simulations confirmed that the high‐spin state of Fe promoted the accumulation of OH− on the surface and accelerated electron transfer, thereby enhancing the kinetics of the reconstruction reaction. Furthermore, theoretical calculations revealed that the lower d‐band center of high‐spin Fe optimized the adsorption of active intermediates, thereby enhancing the reconstruction kinetics. Remarkably, pentlandites with high‐spin Fe exhibited ultra‐low overpotential (245 mV @ 10 mA cm−2) and excellent stability. These findings provided new insights for the design and fabrication of highly active OER electrocatalysts.

Modulating the Fe spin state in FeNC catalysts by Ru nanoparticles to facilitate the oxygen reduction reaction

FeNC is a promising non-precious metal catalyst that can replace platinum-based catalysts in proton-exchange membrane fuel cells (PEMFCs) and zinc–air battery applications.

Modulating Fe spin state in FeNC catalysts by adjacent Fe atomic clusters to facilitate oxygen reduction reaction in proton exchange membrane fuel cell

Iron,nitrogen-codoped carbon (FeNC) has emerged as promising alternatives to precious metals for oxygen reduction reaction (ORR). Herein, we demonstrate that the ORR activity of FeNC can be markedly enhanced by the incorporation of adjacent Fe few-atom clusters (FeAC), where the octahedral field of FeAC boosts the splitting of the parallelogram field of FeN4, and facilitates the transition from high-spin (t2g3eg2) Fe(III)N4 to medium-spin (t2g5eg1) Fe(II)N4 and hence the interaction with the π* antibonding orbitals of oxygen. This leads to a remarkable ORR performance due to optimized desorption of the OH* intermediate on FeN4, with a half-wave potential of +0.80 in 0.1 M HClO4, in comparison to that with only FeN4 single-atom moieties. In H2-O2 fuel cell tests, a high peak power density of 0.80 W cm−2 is obtained. Results from this work highlight the significance of spin engineering in the manipulation and optimization of the ORR activity of single-atom catalysts.

Efficient singlet oxygen nanogenerator with low-spin iron for selective contaminant decontamination

The hydrogen peroxide (H2O2)-based photo-Fenton-like was a promising technology for the generation of singlet oxygen (1O2), which was highly desirable in water purification. However, the yield of 1O2 in traditional radical system was low due to the radical competition. Targeted construction of the non-radical system could solve the above issue, but it has been a challenge. In this study, we targetedly regulated the Fe spin state to efficiently generate 1O2 in a nonradical path. The experimental and theoretical results demonstrated that low-spin FeIII enhanced H2O2 adsorption, directly extracted electrons from H2O2 to Fe center, and rapidly cleaved O–H bond to accelerate the recombination of *OOH with H2O2 for efficient 1O2 generation. This unique nonradical process differed obviously from the traditional radical pathway for 1O2 generation. The developed 1O2-dominant 50Fh/BiOI/H2O2/vis system degraded 98.4% of TCH, which was higher than that in •OH-dominant Fh/H2O2/vis system (84.2%) and it had excellent resistance to various substances, including inorganic ions, dissolved organic matter and real water matrix (tap water and secondary effluent).This study enhanced our understanding of spin-state-dependent photo-Fenton-like reaction and inspired the targeted development of a new-generation selective wastewater treatment technology.

Spin state-dependent in-situ photo-Fenton-like transformation from oxygen molecule towards singlet oxygen for selective water decontamination

The development of 1O2-dominanted selective decontamination for water purification was hampered by extra H2O2 consumption and poor 1O2 generation. Herein, we proposed the reconstruction of Fe spin state using near-range N atom and long-range N vacancies to enable efficient generation of H2O2 and sequential activation of H2O2 into 1O2 after visible-light irradiation. Theoretical and experimental results revealed that medium-spin Fe(III) strengthened O2 adsorption, penetrated eg electrons to antibonding p-orbital of oxygen, and lowered the free energy of O2 activation, enabling the oxygen protonation for H2O2 generation. Thereafter, the electrons of H2O2 could be extracted by low-spin Fe(III) and rapidly converted into 1O2 in a nonradical path. The developed 1O2-dominated in-situ photo-Fenton-like system had an excellent pH universality and anti-interference to inorganic ions, dissolved organic matter, and even real water matrixes (e.g., tap water and secondary effluent). This work provided a novel insight for sustainable and efficient 1O2 generation, which motivated the development of new-generation selective water treatment technology.

Fe spin states and redox processes in Schiff base type complexes

Fe spin states and redox processes in Schiff base type complexes

Regulation of Atomic Fe-Spin State by Crystal Field and Magnetic Field for Enhanced Oxygen Electrocatalysis in Rechargeable Zinc-Air Batteries.

Highly-active and low-cost bifunctional electrocatalysts for oxygen reduction and evolution are essential in rechargeable metal-air batteries, and single atom catalysts with Fe-N-C are promising candidates. However, the activity still needs to be boosted, and the origination of spin-related oxygen catalytic performance is still uncertain. Herein, an effective strategy to regulate local spin state of Fe-N-C through manipulating crystal field and magnetic field is proposed. The spin state of atomic Fe can be regulated from low spin to intermediate spin and to high spin. The cavitation of dxz and dyz orbitals of high spin Fe(Ⅲ) can optimize the O2 adsorption and promote the rate-determining step (*O2 to *OOH). Benefiting from these merits, the high spin Fe-N-C electrocatalyst displays the highest oxygen electrocatalytic activities. Furthermore, the high spin Fe-N-C-based rechargeable zinc-air battery displays a high power density of 170 mW cm-2 and good stability.

Tunable spin and conductance in porphyrin-graphene nanoribbon hybrids

Recently, porphyrin units have been attached to graphene nanoribbons (Por-GNR) enabling a multitude of structures. Here we report first-principles calculations of two prototypical, experimentally feasible, Por-GNR hybrids, one of which displays a small band gap relevant as electrodes in devices. Embedding a Fe atom in the porphyrin causes spin-polarized ground state (S = 1). Using density functional theory and nonequilibrium Green’s function, we examine a 2-terminal setup involving a Fe-Por-GNR between small band gap, Por-GNR electrodes. The coupling between the Fe-d and GNR band states results in a Fano anti-resonance feature in the spin transport, making the conductance highly sensitive to the Fe spin state. We demonstrate how mechanical strain or chemical adsorption on the Fe give rise to spin-crossover to S = 2 and S = 0, directly reflected in the transmission. Our results provide a deep understanding which can open an avenue for carbon-based spintronics and chemical sensing.

Regulating the Fe-spin state by Fe/Fe3C neighbored single Fe-N4 sites in defective carbon promotes the oxygen reduction activity

Non-noble iron-based single-atom catalysts (Fe-N-C) require more accessible active sites and rapid mass transportation, and spin state regulation of iron atoms is also key but challenging to synergistically improve the zinc-air battery (ZAB) performance. Thus, here we indicate that by pre-preparing a 3D nitrogen-doping carbon-sheet network from in situ gas-molecule cutting of bulk MOF and then adsorbing iron into defects and pores of this preformed matrix, we achieve a Fe-N-C catalyst with 14.78 wt.% iron existed as single-atom Fe-N4 sites neighboured Fe/Fe3C nanostructures. Electrons transfer to Fe3C/FeN4 from the adsorbed Fe atom and forms an electron-rich region around Fe3C, achieving the spin-state modulation of the d-band center of Fe atom in FeN4 through charges transfer. The calculation proves that d-band centers of spin-up/down for Fe in Fe-Fe3C/FeN4 are closer to the Fermi level than that of Fe3C/FeN4 (-1.51/-0.72 eV vs. -1.47/-0.65 eV), implying Fe site in Fe-Fe3C/FeN4 is more chemically active and is beneficial to the adsorption and reaction of O2 when involved in ORR. This catalyst delivers excellent ORR activity and the primary ZAB performance with the energy density (ED) of up to 1002 mWh g-1 Zn at 50 mA cm−2 and the maximum power density of 212 mW cm−2, especially in which it displays an excellent durability with 92.7% ED retention during a ∼150 h continuous discharge. These results highlight the significance of metal-atoms spin-state modulation by coupled active metal-species in hybrid catalysts for electrochemical energy devices.

Tuning Fe-spin state of FeN4 structure by axial bonds as efficient catalyst in Li-S batteries

In lithium-sulfur (Li-S) batteries, the root-cause solution is to accelerate the polysulfide conversion to their end products by electrocatalysts. Herein, we put forward a molecular spin engineering to break the symmetry of Fe-N-C and achieve the spin modulation for triplet-to-singlet conversion by the ligand field theory. The Fe-N-C shows an 3d-electronic structure of (dxy)2(dxz)2(dyz)2 and dramatically speeds up the LPSs conversion kinetics. According to the density functional theory (DFT), there is an upshift of energy levels after the adsorption of LPSs on Fe center. As a result, the optimized catalyst delivers a capacity fading rate of 0.026% per cycle at 2 C. In addition, a high capacity of 1042 mA h g−1 is achieved with satisfied capacity retention with the sulfur loading of about 4 mg cm−2. This strategy provides a novel route for the regulation of Fe-spin state and the exploration of catalytic effect in Li-S batteries.

Precise Regulation of Iron Spin States in Single FeN4 Sites for Efficient Peroxidase-Mimicking Catalysis.

The catalytic activity and selectivity of single-atomsites catalysts is strongly dependent on the supports structure and central metal coordination environment. However, the further optimization of electronic configuration to improve the catalytic performance is usually hampered by the strong coordination effect between the support and metal atoms. Herein, it is discovered that enzyme-mimicking catalytic performance can be enhanced at the fixed coordination single-atom Fe sites by regulating the Fe spin states. The X-ray absorption fine structure, 57 Fe Mössbauer spectrum, and temperature-dependent magnetization measurements reveal that the spin states of Fe in single FeN4 sites can be well manipulated via changing the pyrolysis temperature. The intermediate-spin Fe sites catalyst (t2g 4eg 1) demonstrates a much higher peroxidase-mimicking activity in comparison with high-spin structure (t2g 3eg 2). More importantly, the based enzymes system realizes sensitive detection of H2 O2 and glucose by colorimetric sensors with high catalytic activity and selectivity. Furthermore, theoretical calculations unveil that the intermediate-spin FeN4 promotes the OH* desorption process, thus greatly reducing the reaction energy barrier. These findings provide a route to design highly active enzyme-mimicking catalysts and an engineering approach for regulating spin states of metal sites to enhance their catalytic performance.

Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal-Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study.

The porous Hoffman-type 3D lattice Fe(pz)[NiII(CN)4] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

Boosting Nitrogen Reduction to Ammonia on FeN4 Sites by Atomic Spin Regulation

Understanding the relationship between the electronic state of active sites and N2 reduction reaction (NRR) performance is essential to explore efficient electrocatalysts. Herein, atomically dispersed Fe and Mo sites are designed and achieved in the form of well‐defined FeN4 and MoN4 coordination in polyphthalocyanine (PPc) organic framework to investigate the influence of the spin state of FeN4 on NRR behavior. The neighboring MoN4 can regulate the spin state of Fe center in FeN4 from high‐spin (d xy 2 dyz 1 dxz 1 dz2 1 dx2−y2 1) to medium‐spin (dxy 2 dyz 2 dxz 1 dz2 1), where the empty d orbitals and separate d electron favor the overlap of Fe 3d with the N 2p orbitals, more effectively activating N≡N triple bond. Theoretical modeling suggests that the NRR preferably takes place on FeN4 instead of MoN4, and the transition of Fe spin state significantly lowers the energy barrier of the potential determining step, which is conducive to the first hydrogenation of N2. As a result, FeMoPPc with medium‐spin FeN4 exhibits 2.0 and 9.0 times higher Faradaic efficiency and 2.0 and 17.2 times higher NH3 yields for NRR than FePPc with high‐spin FeN4 and MoPPc with MoN4, respectively. These new insights may open up opportunities for exploiting efficient NRR electrocatalysts by atomically regulating the spin state of metal centers.

Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity

Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity

Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity

As low-cost electrocatalysts for oxygen reduction reaction applied to fuel cells and metal-air batteries, atomic-dispersed transition metal-nitrogen-carbon materials are emerging, but the genuine mechanism thereof is still arguable. Herein, by rational design and synthesis of dual-metal atomically dispersed Fe,Mn/N-C catalyst as model object, we unravel that the O2 reduction preferentially takes place on FeIII in the FeN4 /C system with intermediate spin state which possesses one eg electron (t2g4eg1) readily penetrating the antibonding π-orbital of oxygen. Both magnetic measurements and theoretical calculation reveal that the adjacent atomically dispersed Mn-N moieties can effectively activate the FeIII sites by both spin-state transition and electronic modulation, rendering the excellent ORR performances of Fe,Mn/N-C in both alkaline and acidic media (halfwave positionals are 0.928 V in 0.1 M KOH, and 0.804 V in 0.1 M HClO4), and good durability, which outperforms and has almost the same activity of commercial Pt/C, respectively. In addition, it presents a superior power density of 160.8 mW cm−2 and long-term durability in reversible zinc–air batteries. The work brings new insight into the oxygen reduction reaction process on the metal-nitrogen-carbon active sites, undoubtedly leading the exploration towards high effective low-cost non-precious catalysts.