The rational synthesis of metastable inorganic solids, which is a grand challenge in solid-state chemistry, requires the development of kinetically controlled reaction pathways. Topotactic strategies can achieve this goal by chemically modifying reactive components of a parent structure under mild conditions to produce a closely related analogue that has otherwise inaccessible structures and/or compositions. Refractory materials, such as transition metal borides, are difficult to structurally manipulate at low temperatures because they generally are chemically inert and held together by strong covalent bonds. Here, we report a multistep low-temperature topotactic pathway to bulk-scale Mo2AlB2, which is a metastable phase that has been predicted to be the precursor needed to access a synthetically elusive family of 2-D metal boride (MBene) nanosheets. Room-temperature chemical deintercalation of Al from the stable compound MoAlB (synthesized as a bulk powder at 1400 °C) formed highly strained and destabilized MoAl1–x B, which was size-selectively precipitated to isolate the most reactive submicron grains and then annealed at 600 °C to deintercalate additional Al and crystallize Mo2AlB2. Further heating resulted in topotactic decomposition into bulk-scale Mo2AlB2–AlO x nanolaminates that contain Mo2AlB2 nanosheets with thickness of 1–3 nm interleaved by 1–3 nm of amorphous aluminum oxide. The combination of chemical destabilization, size-selective precipitation, and low-temperature annealing provides a potentially generalizable kinetic pathway to metastable variants of refractory compounds, including bulk Mo2AlB2 and Mo2AlB2–AlO x nanosheet heterostructures, and opens the door to other previously elusive 2-D materials such as 2-D MoB (MBene).
The electrocaloric effect (ECE) in BaTiO3 is simulated using two different first-principles based effective Hamiltonian molecular dynamics methods. The calculations are performed for a wide range of temperatures (30-900 K) and external electric fields (0-500 kV/cm). As expected, a large adiabatic temperature change, ∆T , at the Curie temperature, TC, is observed. It is found that for single crystals of pure BaTiO3, the temperature range where a large ∆T is observed is narrow for small external electric fields (< 50 kV/cm). Large fields (> 100 kV/cm) may be required to broaden the effective temperature range. The effect of crystal anisotropy on the ECE ∆T is also investigated. It is found that applying an external electric field along the [001] direction has a larger ECE than those along the [110] and [111] directions.
Fe–N–C single-atomic metal site catalysts (SACs) have garnered tremendous interest in the oxygen reduction reaction (ORR) to substitute Pt-based catalysts in proton exchange membrane fuel cells. Nowadays, efforts have been devoted to modulating the electronic structure of metal single-atomic sites for enhancing the catalytic activities of Fe–N–C SACs, like doping heteroatoms to modulate the electronic structure of the Fe–N x active center. However, most strategies use uncontrolled long-range interactions with heteroatoms on the Fe–N x substrate, and thus the effect may not precisely control near-range coordinated interactions. Herein, the chlorine (Cl) is used to adjust the Fe–N x active center via a near-range coordinated interaction. The synthesized FeN4Cl SAC likely contains the FeN4Cl active sites in the carbon matrix. The additional Fe–Cl coordination improves the instrinsic ORR activity compared with normal FeN x SAC, evidenced by density functional theory calculations, the measured ORR half-wave potential (E 1/2, 0.818 V), and excellent membrane electrode assembly performance.
a b s t r a c tThe perovskite crystal BaTiO 3 is modeled using a first-principles based effective Hamiltonian and molecular dynamics simulations are performed to estimate the pyroelectric response. The electrocaloric temperature change, DT, is calculated for different temperatures and externally applied electric fields. It is found that it is possible to achieve a large DT, around 5-6 K, for a relatively small electric field gradient, less than 100 kV/cm, if the applied fields have a small absolute magnitude.
Fe–N–C single‐atom catalysts (SACs) exhibit excellent peroxidase (POD)‐like catalytic activity, owing to their well‐defined isolated iron active sites on the carbon substrate, which effectively mimic the structure of natural peroxidase's active center. To further meet the requirements of diverse biosensing applications, SAC POD‐like activity still needs to be continuously enhanced. Herein, a phosphorus (P) heteroatom is introduced to boost the POD‐like activity of Fe–N–C SACs. A 1D carbon nanowire (FeNCP/NW) catalyst with enriched Fe–N4 active sites is designed and synthesized, and P atoms are doped in the carbon matrix to affect the Fe center through long‐range interaction. The experimental results show that the P‐doping process can boost the POD‐like activity more than the non‐P‐doped one, with excellent selectivity and stability. The mechanism analysis results show that the introduction of P into SAC can greatly enhance POD‐like activity initially, but its effect becomes insignificant with increasing amount of P. As a proof of concept, FeNCP/NW is employed in an enzyme cascade platform for highly sensitive colorimetric detection of the neurotransmitter acetylcholine.
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