Noble-Metal-Assisted Fast Interfacial Oxygen Migration with Topotactic Phase Transition in Perovskite Oxides

Abstract: Transition-metal perovskite oxides constitute a series of functional material systems for electronics, catalysis and energy-conversion processes, in which oxygen migration and evolution play a key role. However, the stable metal-oxygen (M-O) bond forms large energy barrier inhibiting ion diffusion. Therefore, seeking efficient and facile approaches to accelerate oxygen kinetics has become a significant issue. Here, the interaction (interfacial charge transfer and cooperative bonding) between noble metal (Pt, Ag) and perovskites oxide (SrCoO3-delta) is employed to weaken (M-O) bond and decrease the energy barrier of oxygen migration. Noble metal layers serving as oxygen pumps can continuously extract oxygen from oxide films to atmosphere. The temperature of topotactic phase transition from perovskite (SrCoO3) to brownmillerite (SrCoO2.5) is remarkably lowered from 200 K to room temperature. Furthermore, this approach can also be applied to SrFeO3 for similar topotactic phase reduction by moderate thermal activation. Our finding paves a promising and general pathway to achieve fast oxygen migration in perovskite oxides, with important application prospect in low-temperature electrodes and high-activity catalysis interface.

• The paper demonstrates that ultrathin Pt and Ag layers lower oxygen migration barriers (down to 0.686 eV or less) in SrCoO3 and SrFeO3, enabling fast, room-temperature topotactic phase transitions.
• The approach combines DFT calculations with XAS, STEM imaging, and magnetotransport measurements to reveal charge transfer effects and structural conversion at the metal/oxide interface.
• The findings offer a substrate-independent strategy to accelerate oxygen kinetics in perovskite oxides, with promising implications for low-temperature electrochemical devices and nanoionics.

Noble-Metal-Enhanced Interfacial Oxygen Migration and Topotactic Phase Transition in Perovskite Oxides

Introduction

Transition metal perovskite oxides, particularly SrCoO$_3$ and SrFeO$_3$, are pivotal materials for applications in electronics, energy conversion, and catalysis, largely due to their tunable oxygen stoichiometry and associated functionalities. The rate-limiting step for such applications is often oxygen ion migration, due to the high energy barrier for oxygen vacancy formation imposed by stable metal–oxygen (M–O) bonds. The paper elucidates a mechanism enabling dramatic reduction of this barrier and realization of fast, bias-free, interfacial oxygen migration and topotactic phase transitions (TPT) in SrMO$_x$ (M = Co, Fe) perovskites, utilizing noble metals (Pt, Ag) as activation layers.

Mechanism of Noble-Metal-Assisted Oxygen Migration

The interfacial interaction between perovskite oxide surfaces and ultrathin noble metal layers is central to the observed phenomena. Unlike previous interface studies focusing on non-reducible oxides, here the noble metals directly participate in the M–O bond weakening via interfacial charge transfer and cooperative bonding:

• Electronic Structure Modification: DFT calculations reveal significant charge transfer from the noble metals to the perovskite surface, populating the antibonding 2$e_g$ orbitals of SrCoO$_3$ and SrFeO$_3$, thereby reducing the M–O bond order and facilitating oxygen release.
• Interfacial Bonding: Noble metals preferentially adhere to surface oxygen atoms, forming strong Pt–O (1.416 eV) and moderate Ag–O (0.711 eV) bonds, which act as surficial oxygen pumps.
• Barrier Reduction: Calculated energy barriers for interfacial oxygen migration are dramatically decreased at noble-metal/oxide interfaces. For instance, the migration barrier in Pt/SrCoO$_3$ is reduced to +0.686 eV (or -0.264 eV with defects), and to -0.33 eV for Ag/SrCoO$_3$, relative to much higher values for unconstrained vacancy formation.

Experimental Realization and Characterization

Comprehensive experimental investigation substantiates the theoretical model:

• Room-Temperature Topotactic Phase Transition: Epitaxial SrCoO$_3$ films capped with ultrathin (1.8 nm) Pt or Ag undergo complete, bias-free transformation to the brownmillerite SrCoO$_{2.5}$ phase at room temperature — the transition point is reduced from the typical ~200 ℃ to 25 ℃. Untreated films remain stable indefinitely under ambient conditions.
• X-ray Absorption and Electron Microscopy: XAS confirms the reduction in Co valence state (from Co$^{4+}$ to Co$^{3+}$) and the suppression of the Co–O hybridization indicative of extensive deoxygenation at the interface. STEM imaging corroborates structural conversion and reveals formation of horizontal and vertical oxygen vacancy channels, as well as amorphization at the noble metal interface due to rapid oxygen movement.
• Magnetotransport and Optical Response: The phase transition is accompanied by a switch from ferromagnetic metallic to antiferromagnetic insulating behavior, and pronounced changes in optical transmittance, all consistent with electronic structure reconstructions driven by oxygen migration.

Extension to SrFeO$_3$ and Broader Implications

• Thermal Activation in More Robust Systems: In SrFeO$_3$, where the Fe–O bond is intrinsically stronger and the deoxygenation barrier higher (3.572 eV vs. 3.115 eV in SrCoO$_3$), noble metal layers alone are insufficient for complete TPT at room temperature but can yield oxygen vacancies. Moderate heating (~150 ℃) enables full TPT, again at much lower temperatures than required in the absence of noble metals.
• Generalizability and Device Prospects: The findings establish a general, substrate-independent mechanism for accelerating oxygen kinetics in perovskite oxides using ultrathin noble metal interfaces. The process is not reliant on applied bias and is compatible with standard thin-film device architectures.

Theoretical and Practical Implications

This research provides direct evidence for the electronically driven interfacial activation of ionic migration in strongly correlated oxides, highlighting the functional importance of noble metal/oxide charge transfer beyond catalysis and traditional metal-support phenomena:

• Electrode and Catalysis Design: The approach promises enhanced performance for SOFC cathodes and oxygen evolution/reduction catalysts, where low-temperature fast oxygen ion conduction is critical.
• Nanoionics: There is potential for designing nanoionics interfaces where structural phase transitions and associated changes in conductivity, magnetism, and transparency are precisely modulated by engineered metal/oxide coupling.
• Material Integration: The results point to the feasibility of room-temperature, bias-free, reversible control of oxygen content, expanding the parameter space for oxide electronics, memristive devices, and energy conversion materials.

Conclusion

The demonstrated noble-metal-assisted interfacial oxygen migration and associated topotactic phase transitions drastically lower the activation energy for oxygen movement in SrCoO$_3$ and SrFeO$_3$ thin films. The underlying mechanism—charge transfer, cooperative bonding, and lowered migration barriers—positions noble metal interfaces as powerful tools for functional oxide engineering. These insights offer clear routes to optimize low-temperature electrochemical devices and catalysis platforms and provoke further investigations of interface-driven phenomena across correlated oxide systems.

For complete details, see "Noble-Metal-Assisted Fast Interfacial Oxygen Migration with Topotactic Phase Transition in Perovskite Oxides" (2111.00663).