The electrochemical N2 reduction reaction (NRR) under ambient conditions is attractive in replacing the current Haber-Bosch process toward sustainable ammonia production. Metal-heteroatom-doped carbon-rich materials have emerged as the most promising NRR electrocatalysts. However, simultaneously boosting their NRR activity and selectivity remains a grand challenge, while the principle for precisely tailoring the active sites has been elusive. Herein, we report the first case of crystalline two-dimensional conjugated covalent organic frameworks (2D c-COFs) incorporated with M–N4–C centers as novel, defined, and effective catalysts, achieving simultaneously enhanced activity and selectivity of electrocatalytic NRR to ammonia. Such 2D c-COFs are synthesized based on metal-phthalocyanine (M = Fe, Co, Ni, Mn, Zn, and Cu) and pyrene units bonded by pyrazine linkages. Significantly, the 2D c-COFs with Fe–N4–C center exhibit higher ammonia yield rate (33.6 μg h–1 mgcat –1) and Faradaic efficiency (FE, 31.9%) at −0.1 V vs reversible hydrogen electrode than those with other M–N4–C centers, making them among the best NRR electrocatalysts (yield rate >30 μg h–1 mgcat –1 and FE > 30%). In situ X-ray absorption spectroscopy, Raman spectroelectrochemistry, and theoretical calculations unveil that Fe–N4–C centers act as catalytic sites. They show a unique electronic structure with localized electronic states at Fermi level, allowing for stronger interaction with N2 and thus faster N2 activation and NRR kinetics than other M–N4–C centers. Our work opens the possibility of developing metal–nitrogen-doped carbon-rich 2D c-COFs as superior NRR electrocatalyst and provides an atomic understanding of the NRR process on M–N x –C based electrocatalysts for designing high-performance NRR catalysts.

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Exfoliation of atomically thin layers from non-van der Waals bulk solids gave rise to the emergence of a new class of two-dimensional (2D) materials, such as hematene (Hm), a structure just a few atoms thick obtained from hematite. Due to a large number of unsaturated sites, the Hm surface can be passivated under ambient conditions. Using density functional theory calculations, we investigate the effects of surface passivation with H and OH groups on Hm properties and demonstrate that the passivated surfaces are energetically favorable under oxygenrich conditions. Although the bare sheet is antiferromagnetic and possesses an indirect band gap of 0.93 eV, the hydrogenated sheets are half-metallic with a ferromagnetic ground state, and the fully hydroxylated sheets are antiferromagnetic with a larger band gap as compared to the bare system. The electronic structure of Hm can be further tuned by mechanical deformations. The band gap of fully passivated Hm increases monotonically with biaxial strain, hinting at the potential applications of Hm in electromechanical devices.

Oxygen electrocatalysis is vital for advanced energy technologies, but inordinate challenges remain due to the lack of highly active earth-abundant catalysts.

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In this paper, the revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) is used to investigate the interaction between hydrogen with different concentrations and gallium vacancies in β-Ga2O3. The hydrogen can compensate a gallium vacancy by forming hydrogen-vacancy complex. A gallium vacancy can bind up to four hydrogen atoms, and formation energies decrease as the number of hydrogen atoms increases. Hydrogen prefers to bind with three coordinated oxygen. The bonding energy and annealing temperatures of complexes containing more than two hydrogen atoms are computed, and show relatively high stability. In addition, vacancy concentrations increase with the increasing vapor pressures. This paper can effectively explain the hydrogen impact in β-Ga2O3.

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In the present work, we report the 25 MeV oxygen irradiation effects in n-type single crystal β-Ga 2 O 3 at different fluences. We demonstrate that the symmetric stretching modes and bending vibrations of GaO 4 and GaO 6 units are impaired upon increasing O irradiation fluence. Blue and green photoluminescence (PL) emission bands are found to be mainly associated with galliumoxygen divacancies, gallium vacancies and oxygen interstitials. The increase of optically active centers at low fluence and the PL quenching at high fluence are ascribed to the reduction of carrier density and the production of non-radiative recombination centers, respectively. The results envisage the possibility of obtaining pre-designed spectral behaviors by varying the oxygen irradiation fluence.