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Photocatalytic fixation of nitrogen to ammonia represents an attractive alternative to the Haber-Bosch process under ambient conditions, and the performance can be enhanced by defect engineering of the photocatalysts, in particular, formation of shallow energy levels due to oxygen vacancies that can significantly facilitate the adsorption and activation of nitrogen. This calls for deliberate size engineering of the photocatalysts. In the present study, pyrochlore Bi2Ti2O7 quantum dots and (bulk-like) nanosheets are prepared hydrothermally by using bismuth nitrate and titanium sulfate as the precursors. Despite a similar oxygen vacancy concentration, the quantum dots exhibit a drastically enhanced photocatalytic performance toward nitrogen fixation, at a rate of 332.03 µmol g-1 h-1, which is 77 times higher than that of the nanosheet counterpart. Spectroscopic and computational studies based on density functional theory calculations show that the shallow levels arising from oxygen vacancies in the Bi2Ti2O7 quantum dots, in conjunction with the moderately constrained quantum confinement effect, facilitate the chemical adsorption and activation of nitrogen.

Photocatalytic fixation of nitrogen to ammonia represents an attractive alternative to the Haber–Bosch process under ambient conditions, and the performance can be enhanced by defect engineering of the photocatalysts, in particular, formation of shallow energy levels due to oxygen vacancies that can significantly facilitate the adsorption and activation of nitrogen. This calls for deliberate size engineering of the photocatalysts. In the present study, pyrochlore Bi2Ti2O7 quantum dots and (bulk‐like) nanosheets are prepared hydrothermally by using bismuth nitrate and titanium sulfate as the precursors. Despite a similar oxygen vacancy concentration, the quantum dots exhibit a drastically enhanced photocatalytic performance toward nitrogen fixation, at a rate of 332.03 µmol g−1 h−1, which is 77 times higher than that of the nanosheet counterpart. Spectroscopic and computational studies based on density functional theory calculations show that the shallow levels arising from oxygen vacancies in the Bi2Ti2O7 quantum dots, in conjunction with the moderately constrained quantum confinement effect, facilitate the chemical adsorption and activation of nitrogen. [ABSTRACT FROM AUTHOR]

This paper explores testing the equality of two covariance matrices under high‐dimensional settings. Existing test statistics are usually constructed based on the squared Frobenius norm or the elementwise maximum norm. However, the former may experience power loss when handling sparse alternatives, while the latter may have a poor performance against dense alternatives. In this paper, with a novel framework, we introduce a double verification test statistic designed to be powerful against both dense and sparse alternatives. Additionally, we propose an adaptive weight test statistic to enhance power. Furthermore, we present an analysis of the asymptotic size and power of the proposed test. Simulation results demonstrate the satisfactory performance of our proposed method. [ABSTRACT FROM AUTHOR]

Fe−N−C catalysts with single‐atom Fe−N4 configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton‐exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N4 catalysts. The integration of Fe−N4 configurations with highly uniform Co4 ACs on the N‐doped carbon substrate (Co4@/Fe1@NC) is realized through a "pre‐constrained" strategy using Co4 molecular clusters and Fe(acac)3 implanted carbon precursors. The as‐developed Co4@/Fe1@NC catalyst exhibits excellent ORR activity with a half‐wave potential (E1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm−2 in a H2−O2 fuel cell test. First‐principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N4 that modified with Co4 ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy‐related catalysis. [ABSTRACT FROM AUTHOR]