Strain-Driven Phase Transitions in Delafossite Cu1-xAxAlO2: A Key to Enhanced Photo(electro)catalytic Performance.

Abstract

This study investigates the impact of intrinsic strain and phase transitions on the thermodynamic stability and electronic properties of Cu1-xAxAlO2 solid solutions, which are key to their photocatalytic performance. It is demonstrated that Cu1-xAxAlO2 with A = Ag, Au, Pt can form continuous isostructural solid solutions due to relatively small compressive strain, while a substantial increase strain restricts Cu1-xPdxAlO2 to forming only limited solutions. For A = Li, Na, the formation of heterostructural solid solutions is facilitated by structural motif alterations, accommodating significant differences in ionic radii and A-O bond characteristics. Specifically, Cu1-xLixAlO2 exhibits a phase transition at x ≈ 0.333, whereas Cu1-xNaxAlO2 undergoes three distinct phase transitions. Electronic structure analysis indicates that in Cu1-xAxAlO2 (A = Ag, Au), d10-d10 closed-shell interactions dominate, enabling tunable band gaps with varying solubility. Nevertheless, increased intrinsic strain in metal sublattices, as seen in A = Pd and Pt, shifts antibonding states to the Fermi level, inducing a semiconductor-to-metal transition. Experimental evidence confirms that Ag+ ions modulate the band gaps and carrier dynamics in Cu1-xAgxAlO2, with Cu0.75Ag0.25AlO2 exhibiting heightened photoelectrochemical activity and a 38.5-fold enhancement in H2 production rate over CuAlO2. Additionally, the coordination environment changes between alkali metals and O, induced by phase transitions, effectively tune the band edge positions and carrier dynamics of Cu1-xAxAlO2 (A = Li, Na) heterostructural solid solutions. Therefore, 3R-Cu0.97Li0.03AlO2 with asymmetric nonlinear dumbbell O-Cu-O demonstrates the highest photocatalytic H2 production activity, 72.9 times greater than CuAlO2. In contrast, α-Cu1-xAxAlO2 with a smaller CuO6 octahedral splitting energy exhibits increased band gaps, resulting in diminished photocatalytic activity. This research underscores that strain-driven phase transition provides an additional control factor and new mechanism for regulating the photo(electro)catalytic activity of Cu1-xAxAlO2 solid solutions.