Mind the Interface Gap: Exposing Hidden Interface Defects at the Epitaxial Heterostructure between CuO and Cu₂O

Abstract

Well designed and optimized epitaxial heterostructures lie at the foundation of materials development for photovoltaic, photocatalytic, and photoelectrochemistry applications. Heterostructure materials offer tunable control over charge separation and transport at the same time preventing recombination of photogenerated excitations at the interface. Thus, it is of paramount importance that a detailed understanding is developed as the basis for further optimization strategies and design. Oxides of copper are nontoxic, low cost, abundant materials with a straightforward and stable manufacturing process. However, in individual applications, they suffer from inefficient charge transport of photogenerated carriers. Hence, in this work, we investigate the role of the interface between epitaxially aligned CuO and Cu 2 O to explore the potential benefits of such an architecture for more efficient electron and hole transfer. The CuO/Cu 2 O heterojunction nature, stability, bonding mechanism, interface dipole, electronic structure, and band bending were rationalized using hybrid density functional theory calculations. New electronic states are identified at the interface itself, which are originating neither from lattice mismatch nor strained Cu−O bonds. They form as a result of a change in coordination environment of CuO surface Cu 2+ cations and an electron transfer across the interface Cu 1+ −O bond. The first process creates occupied defect-like electronic states above the valence band, while the second leaves hole states below the conduction band. These are constitutional to the interface and are highly likely to contribute to recombination effects competing with the improved charged separation from the suitable band bending and alignment and thus would limit the expected output photocurrent and photovoltage. Finally, a favorable effect of interstitial oxygen defects has been shown to allow for band gap tunability at the interface but only to the point of the integral geometrical contact limit of the heterostructure itself.

Integrated In Situ Fabrication of CuO Nanorod-Decorated Polymer Membranes for the Catalytic Flow-Through Reduction of p-Nitrophenol

“…The corresponding Tauc plots depict a band gap energy of approximately 1.5 eV for all membranes, very close to the theoretical value of 1.42 eV for CuO . The low band gap energies also indicate the absence of trace Cu 2 O, which would lead to a widening of the energy gap. , …”

Role of Interfacial Morphology in Cu2O/TiO2 and Band Bending: Insights from Density Functional Theory

“…Theoretical calculations have been employed for a long time to study the electronic properties of interfaces. While earlier methods leaned on more basic techniques such as effective dipole models, tight-binding schemes, or empirical rules, in the recent few decades, DFT has been utilized as an advanced computational to study band offset and interfacial dipole in heterostructures. , Despite the valuable contributions of DFT studies in this area, several limitations persist. The majority of investigations lack precise predictions due to the absence of explicit heterostructure calculations. − Additionally, many studies tend to focus on arbitrary surface orientations of the individual components rather than considering the most dominant orientations from experimental observations, and more importantly, arbitrarily chosen interfacial morphologies, which may not accurately sketch the full picture of all the possible configurations and their effect on band bending. − …”

“…While earlier methods leaned on more basic techniques such as effective dipole models, 21 tight-binding schemes, 22 or empirical rules, 23 in the recent few decades, DFT has been utilized as an advanced computational to study band offset and interfacial dipole in heterostructures. 24 , 25 Despite the valuable contributions of DFT studies in this area, several limitations persist. The majority of investigations lack precise predictions due to the absence of explicit heterostructure calculations.…”