Mechanistic insights into in situ generation and transformation of CO2 towards Mn-Ti carbonate-oxide fused cuboids: A comparison between hydrothermal and microwave synthesis

The formation of metal carbonate-oxide microarchitectures from C-O bond-free precursors remains poorly understood mechanistically, despite their importance in energy and environmental applications. Here, we introduce a mechanistically supported hydrothermal and microwave method for synthesizing MnCO 3 based fused cuboidal structures using hexamethylenetetramine (HMTA) as both a structure-directing agent and an in situ CO 2 source. Decomposition of HMTA in the reaction system releases CO 2 and other intermediate fragments, allowing carbonate crystallization without any external carbonate additives. The structural development varies with the synthesis method, such that hydrothermal processing results in crystalline MnCO 3 /TiO 2 combined with surface-decorated Ti 4+ -O. In contrast, microwave synthesis yields crystalline MnCO 3 /Mn 3 O 4 with surface decoration by amorphous Ti 4+ -O species with no Ti 3+ signatures. The control experiment without HMTA generated anatase TiO 2 with traces of crystalline Mn oxides and Ti-O fragments, with no MnCO 3 detected. To the best of our knowledge, this study provides the first mechanistic demonstration of latent CO 2 driven formation of carbonate-oxide fused cuboids with Ti 4+ -O surface richness and establishes two mechanistically known potentially scalable routes for carbonate-oxide heterophases. As a proof of concept, these carbonate-oxide composites have been employed in photocatalysis; however, optimization is necessary to achieve higher photoactivity.