Metastability and Photoelectrochemical Properties of Cu₂SnO₃ and Cu_2–xLi_xTiO₃: Two Cu(I)-Based Oxides with Delafossite Structures

Abstract

Metastable, p-type Cu(I)-based semiconductors were synthesized using cation-exchange reactions between delafossite-type layered precursors and CuCl flux, yielding Cu 2 SnO 3 (I) and Cu 2−x Li x TiO 3 (II, x min ∼ 0.4). These represent the first reported crystalline semiconductors found in the Cu−Sn−O or Cu−Ti−O chemical systems (and not currently predicted within any materials databases), with their kinetic stabilization requiring a relatively low reaction temperature of ∼475 °C. Both phases crystallize in the monoclinic crystal system in the space group C2/c, exhibiting edge-shared hexagonal "MO 3 " (M = Sn or Ti) layers that also contain octahedrally coordinated Li(I)/Cu(I) cations. These layers are bridged by linearly coordinated Cu(I) cations. Magnetic susceptibility measurements confirm the +1 oxidation state of the copper cations. The optical band gaps were found to be indirect and to significantly red shift with the Cu(I) content, down to ∼2.31 eV for I and ∼1.46 eV for II. Electronic structure calculations show that the decreased band gaps can be attributed to a higher energy valence band derived from the filled 3d 10 orbitals of the Cu(I) cations, which most notably arise from the octahedrally coordinated Cu(I) cations within the layers. Total energy calculations reveal an increasing metastability with respect to decomposition to Cu 2 O and SnO 2 or TiO 2 as a result of occupation of the intralayer sites by Cu(I) cations. In both phases, their edge-shared hexagonal layers lead to highly dispersive conduction bands and small electron effective masses of ∼0.51 m e for I and ∼0.41 m e for II. Polycrystalline films of both were deposited onto fluorine-doped tin oxide slides and exhibited p-type photocurrents under 100 mW cm −2 irradiation in the range of ∼50 to 250 μA cm −2 . This study thus reveals new fundamental relationships between the origin of metastability in Cu(I)-oxide semiconductors, i.e., octahedral coordination, and enhanced optical and photoelectrochemical properties.

Cu-Induced Interfacial Water Engineering of SnO2 for Durable and Highly Selective CO2 Electroreduction

“…This phenomenon probably originates from the disturbance of distorted Cu−O octahedra on the Sn−O bond (Figure S6). 22,38 The Cu-induced structure evolution of SnO 2 was further confirmed by Fourier transform infrared (FTIR) results. As shown in Figure 2b, the stretching vibration of O− Sn−O (570 cm −1 ) gradually splits and an obvious peak appears at 640 cm −1 with the increase of Cu content.…”

Simple to Quadruple Perovskite Transformation by Coordination Switching upon Solid-State Ion Exchange of NaNbO3

“…38,39 The formation enthalpy of ionic salt byproducts often provides the thermodynamic driving force for the reaction, 40,41 while the molten salt acts as an in situ flux and facilitates ion diffusion, which leads to faster reaction rates than conventional solid−solid reactions. 42 The heat effect may indicate the nature of the metathesis. In the case of the NaNbO 3 −CuSO 4 reaction, no exothermic peak is observed in the differential scanning calorimetry (DSC) curve (Figure S4), indicating the absence of highly exothermic events.…”

“…The solid-state ion-exchange reaction between NaNbO 3 and CuSO 4 is a double-decomposition or metathesis reaction, which is kinetically slow and requires a few hours to complete, unlike some of the self-propagating metathesis (SPM) reactions that are highly exothermic and give product formation in minutes or seconds. , While SPM have been frequently used for the rapid synthesis of refractory compounds, , many metastable oxides were synthesized using slow metathesis reactions. , The formation enthalpy of ionic salt byproducts often provides the thermodynamic driving force for the reaction, , while the molten salt acts as an in situ flux and facilitates ion diffusion, which leads to faster reaction rates than conventional solid–solid reactions . The heat effect may indicate the nature of the metathesis.…”