Strain-Driven Phase Transitions in Delafossite Cu_1–<i>x</i>A_<i>x</i>AlO₂: A Key to Enhanced Photo(electro)catalytic Performance

This study investigates the impact of intrinsic strain and phase transitions on the thermodynamic stability and electronic properties of Cu_1-<i>x</i>A_<i>x</i>AlO₂ solid solutions, which are key to their photocatalytic performance. It is demonstrated that Cu_1-<i>x</i>A_<i>x</i>AlO₂ with A = Ag, Au, Pt can form continuous isostructural solid solutions due to relatively small compressive strain, while a substantial increase strain restricts Cu_1-<i>x</i>Pd_<i>x</i>AlO₂ 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, Cu_1-<i>x</i>Li_<i>x</i>AlO₂ exhibits a phase transition at <i>x</i> ≈ 0.333, whereas Cu_1-<i>x</i>Na_<i>x</i>AlO₂ undergoes three distinct phase transitions. Electronic structure analysis indicates that in Cu_1-<i>x</i>A_<i>x</i>AlO₂ (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 Cu_1-<i>x</i>Ag_<i>x</i>AlO₂, with Cu_0.75Ag_0.25AlO₂ 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 Cu_1-<i>x</i>A_<i>x</i>AlO₂ (A = Li, Na) heterostructural solid solutions. Therefore, 3R-Cu_0.97Li_0.03AlO₂ with asymmetric nonlinear dumbbell O-Cu-O demonstrates the highest photocatalytic H₂ production activity, 72.9 times greater than CuAlO₂. In contrast, α-Cu_1-<i>x</i>A_<i>x</i>AlO₂ 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 Cu_1-<i>x</i>A_<i>x</i>AlO₂ solid solutions.