Strain-Driven Phase Transitions in Delafossite Cu 1- x A x AlO 2 : 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- x A x AlO 2 solid solutions, which are key to their photocatalytic performance. It is demonstrated that Cu 1- x A x AlO 2 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- x Pd x AlO 2 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- x Li x AlO 2 exhibits a phase transition at x ≈ 0.333, whereas Cu 1- x Na x AlO 2 undergoes three distinct phase transitions. Electronic structure analysis indicates that in Cu 1- x A x AlO 2 (A = Ag, Au), d 10 -d 10 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- x Ag x AlO 2 , with Cu 0.75 Ag 0.25 AlO 2 exhibiting heightened photoelectrochemical activity and a 38.5-fold enhancement in H 2 production rate over CuAlO 2 . 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- x A x AlO 2 (A = Li, Na) heterostructural solid solutions. Therefore, 3R-Cu 0.97 Li 0.03 AlO 2 with asymmetric nonlinear dumbbell O-Cu-O demonstrates the highest photocatalytic H 2 production activity, 72.9 times greater than CuAlO 2 . In contrast, α-Cu 1- x A x AlO 2 with a smaller CuO 6 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- x A x AlO 2 solid solutions.