Defect Engineering in Strained Low-Dimensional ABO3 Perovskite Nanoparticles for Next-Generation Energy Storage

The realization of renewable energy is dependent on the advancement of multi-functional materials for energy storage devices. As an example, the substantial progress observed in Li-ion batteries is a result of the discovery and subsequent industrial commercialization of cathode materials such as LiFePO4 (LFP). Although these materials possess relatively high specific capacity (~170 mAh/g), their low room-temperature electronic conductivity has been identified as a limitation for future high-performance batteries. Metastable SrBO3, (SBO, B = Nb, Ta, Mo, etc.) perovskite nanoparticles (NPs) with metallic properties offer an alternative route to improve the room-temperature electronic conductivity of LFP cathodes. Therefore, this work aims to demonstrate how the optoelectronic properties of metastable SBO perovskite NPs can be leveraged for applications in advanced energy storage. The layered film architecture is taken advantage of in order to synergistically couple the metallic conduction of the SBO perovskite internal layer with the high Li-ion conductivity of the olivine top layer to obtain improved electrochemical performance. Despite the recent attention, the synthesis of SBO NPs using traditional wet-chemical methods result in B-site cations stabilized in highly oxidized states (i.e. Nb5+, Ta5+, Mo6+, etc.), rather than the desired 4+ valency. These over-oxidized states, present as surface/bulk defect states, suppress the expected optoelectronic responses. For this reason, the engineering of these defect states to recover the optoelectronic properties of metastable SBO perovskites is the main objective of this work. To address this challenge, the facile oxygen-controlled CP/MSS method was developed. The low-pressure environment reduces the partial pressure of oxygen during the crystallization process which allows for the simultaneous intercalation of Sr ions and suppression of defect states. Finally, a reducing post-treatment allows for further inhibition of these defect states, which triggers a change in the powder color (white, insulating to colored, metallic). These findings highlight the potential application of these materials as conductive scaffolds that otherwise would not be possible with traditional solution-based methods. As a proof of concept, a LiFePO4 top layer is deposited onto these conductive SBO perovskites to demonstrate their potential application in Li-ion batteries. Incorporation of the conductive scaffold significantly improves the charge transport properties of LFP, highlighting the promising electrochemical potential of these engineered nanomaterials. Ultimately, this ability to modify the charge transport response using these conductive scaffold materials will contribute to the design/development of next-generation energy storage and conversion technology

Stabilizing the B-site oxidation state in ABO perovskite nanoparticles

The stabilization of the B-site oxidation state in ABO3 perovskites using wet-chemical methods is a synthetic challenge, which is of fundamental and practical interest for energy storage and conversion devices. In this work, defect-controlled (Sr-deficiency and oxygen vacancies) strontium niobium(iv) oxide (Sr1-xNbO3-δ, SNO) metal oxide nanoparticles (NPs) were synthesized for the first time using a low-pressure wet-chemistry synthesis. The experiments were performed under reduced oxygen partial pressure to prevent by-product formation and with varying Sr/Nb molar ratio to favor the formation of Nb4+ pervoskites. At a critical Sr to Nb ratio (Sr/Nb = 1.3), a phase transition is observed forming an oxygen-deficient SrNbO3 phase. Structural refinement on the resultant diffraction pattern shows that the SNO NPs consists of a near equal mixture of SrNbO3 and Sr0.7NbO3-δ crystal phases. A combination of Rietveld refinement and X-ray photoelectron spectroscopy (XPS) confirmed the stabilization of the +4 oxidation state and the formation of oxygen vacancies. The Nb local site symmetry was extracted through Raman spectroscopy and modeled using DFT. As further confirmation, the particles demonstrate the expected absorption highlighting their restored optoelectronic properties. This low-pressure wet-chemical approach for stabilizing the oxidation state of a transition metal has the potential to be extended to other oxygen sensitive, low dimensional perovskite oxides with unique properties

Modifying Metastable SrBO (B = Nb, Ta, and Mo) Perovskites for Electrode Materials

The presence of surface/deep defects in 4d- and 5d-perovskite oxide (ABO, B = Nb, Ta, Mo, etc.) nanoparticles (NPs), originating from multivalent B-site cations, contributes to suppressing their metallic properties. These defect states can be removed using a H/Ar thermal treatment, enabling the recovery of their electronic properties (i.e., low electrical resistivity, high carrier concentration, etc.) as expected from their electronic structure. Therefore, to engineer the electronic properties of these metastable perovskites, an oxygen-controlled crystallization approach coupled with a subsequent H/Ar treatment was utilized. A comprehensive study of the effect of the post-treatment time on the electronic properties of these perovskite NPs was performed using a combination of scattering, spectroscopic, and computational techniques. These measurements revealed that a metallic-like state is stabilized in these oxygen-reduced NPs due to the suppression of deep rather than surface defects. Ultimately, this synthetic approach can be employed to synthesize ABO perovskite NPs with tunable electronic properties for application into electrochemical devices

Critical Coupling of Visible Light Extends Hot-Electron Lifetimes for H2O2 Synthesis

Devices driven by above-equilibrium hot electrons are appealing for photocatalytic technologies, such as in situ HO synthesis, but currently suffer from low (\u3c1%) overall quantum efficiencies. Gold nanostructures excited by visible light generate hot electrons that can inject into a neighboring semiconductor to drive electrochemical reactions. Here, we designed and studied a metal-insulator-metal (MIM) structure of Au nanoparticles on a ZnO/TiO/Al film stack, deposited through room-temperature, lithography-free methods. Light absorption, electron injection efficiency, and photocatalytic yield in this device are superior in comparison to the same stack without Al. Our device absorbs \u3e60% of light at the Au localized surface plasmon resonance (LSPR) peak near 530 nm-a 5-fold enhancement in Au absorption due to critical coupling to an Al film. Furthermore, we show through ultrafast pump-probe spectroscopy that the Al-coupled samples exhibit a nearly 5-fold improvement in hot-electron injection efficiency as compared to a non-Al device, with the hot-electron lifetimes extending to \u3e2 ps in devices photoexcited with fluence of 0.1 mJ cm. The use of an Al film also enhances the photocatalytic yield of HO more than 3-fold in a visible-light-driven reactor. Altogether, we show that the critical coupling of Al films to Au nanoparticles is a low-cost, lithography-free method for improving visible-light capture, extending hot-carrier lifetimes, and ultimately increasing the rate of in situ HO generation