To synthesize CoSe2 catalysts (i.e. o-CoSe2, c/o-CoSe2, c-CoSe2, and c-S-CoSe2), hierarchical ZIF-67 precursors were first fabricated using a room-temperature anion-exchange approach and subsequently employed as the template (see Experimental Methods, Figure ). As illustrated by fieldemission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) (Figure and Figures ), yolk-shell nano cubic architectures are evident in the as-prepared ZIF-67 precursors. Followed by a conventional selenylation/sulphuration process, the o-CoSe2, c/o-CoSe2, c-CoSe2, and c-S-CoSe2 nanoarchitectures were successfully fabricated. Then, a wide range of characterization techniques were employed to explore the morphological and structural properties of the as-prepared products. From the results (Figure and), the pristine nano cubic architectures of ZIF-67 precursors were well preserved in the as-synthesized CoSe2 samples. Analysis of high-resolution TEM (HR-TEM) characterizations (Figure and Figure ) unravels interplanar spacings of 2.42, 2.52, and 2.92 Å, corresponding to the main exposed (020), (012), and (110) planes, respectively, reflecting the orthorhombic structures of the as-synthesized o-CoSe2 (PDF no. 09-0234, Figure ). Note that the synthetic parameters (i.e. reaction temperature and heating rate) play a key role in the formation of CoSe2 with different crystal phases, i.e., o-CoSe2 (350 °C at 2 °C/min), c/o-CoSe2 (400 °C at 2 °C/min), and c-CoSe2 (400 °C at 5 °C/min), as confirmed by the powder X-ray diffraction (PXRD) patterns (Figure ). Partial sulphur incorporation into CoSe2 resulted in the formation of c-S-CoSe2, and the observed characteristic diffraction peaks in its PXRD pattern (Figure ) show a shift towards a higher 2 θ position compared to the standard c-CoSe2 (PDF no. 09-0234). This infers that the introduction of S ions results in a decrease in the lattice parameters of CoSe2 because of its smaller atomic radius than the Se ions. Furthermore, a reference o-P-CoSe2 sample was synthesized to gain in-depth insights into the intrinsic catalytic mechanisms (Figure ). The PXRD results (Figure ) indicate that the as-prepared o-P-CoSe2 matches well with the o-CoSe2 phase (PDF no. 53-0449).