Origin of Pressure‐Induced Metallization in Cu₃N: An X‐ray Absorption Spectroscopy Study

Abstract

High‐pressure (0–26.7 GPa) Cu K‐edge X‐ray absorption spectroscopy is used to study possible structural modifications of anti‐perovskite‐type copper nitride (Cu3N) crystal lattice. The analysis of X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS), based on theoretical full‐multiple‐scattering and single‐scattering approaches, respectively, suggests that at all pressures the local atomic structure of Cu3N remains close to that in cubic Pmtrue3¯m phase. Therefore, the transition to metal state above 5 GPa, observed previously using pressure‐dependent electrical resistance and optical absorption measurements, is explained by the band gap collapse due to a decrease of the unit cell volume. We found that the lattice parameter of Cu3N is reduced by ≈2% upon increasing pressure up to 26.7 GPa, and the structure is restored upon pressure release.

Production of C2/C3 Oxygenates from Planar Copper Nitride-Derived Mesoporous Copper via Electrochemical Reduction of CO2

“…The Cu K-edge XANES spectrum of Cu 3 N was also compared with the spectra collected for Cu 2 O, CuO, and Cu­(OH) 2 (Supporting Information, Figure S4). The absorption edge of Cu 3 N is located between those of the metallic Cu and CuO, indicating the oxidation state of copper ions in Cu 3 N to be +1, which is consistent with expectations. − The assignment of the +1 oxidation state is also supported by XPS (Figure S5), which demonstrates no satellite peaks in the Cu 2p spectrum of the as-prepared Cu 3 N.…”

“…A second smaller peak at 2.1 Å was also observed. As the expected second shell of Cu 3 N is at 2.35 Å, this feature is more likely related to underlying metallic copper. ,− The combined XAS and EXAFS results demonstrates the successful incorporation of N atoms into Cu to form Cu 3 N.…”

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

“…Moreover, this setup provides much better beam stability since no moving components are involved. This makes it attractive for studies of very small samples, such as samples under extreme conditions (high temperatures, high pressures, strong magnetic fields), allowing one to extract unique information about in situ transformations of materials. − The applications of this method for the studies of catalysts under working conditions, are, however, limited by the fact that the data collection in fluorescence mode is normally not possible. Moreover, the quality and homogeneity of the sample are even more critical than in transmission measurements at a conventional XAS beamline because X-ray photons with different wavelengths travel through different parts of the sample.…”

Direct Reaction between Copper and Nitrogen at High Pressures and Temperatures

“…31−33 Under compression, the narrow band gap (0.6 eV) of Cu 3 N closes by ≈5 GPa, and no structural phase transitions are observed up to 26.7 GPa. 34 Silver also forms an explosive azide (AgN 3 ), 35 and attempts at forming stable gold nitrides have been limited to ion-irradiated surfaces. 26,28 Here, using a combination of high pressure and high temperatures, we have explored the synthesis of novel copper− nitrogen compounds.…”

“…The coinage metals (group 11: Cu, Ag, and Au) have so far shown a limited propensity to form nitrogen-bearing compounds despite intense efforts utilizing an array of synthesis techniques including nitrogen-ion irradiation. − Copper is known to form two nitrogen-containing compounds: copper azide (CuN 3 ), a highly sensitive explosive, and copper nitride (Cu 3 N), a stable semiconductor finding applications in a wide range of areas from lithium-ion battery electrodes to conductive ink and solar energy harvesting. − Under compression, the narrow band gap (0.6 eV) of Cu 3 N closes by ≈5 GPa, and no structural phase transitions are observed up to 26.7 GPa . Silver also forms an explosive azide (AgN 3 ), and attempts at forming stable gold nitrides have been limited to ion-irradiated surfaces. , …”