From Layered Borate to Porous-Layered Aluminoborate

Two novel borates, KCaB₅O₈(OH)₂ (1) and K₂Cs₃[{Al₂[B₅O₉(OH)]₂}{B₅O₈(OH)₂}] (2), have been synthesized under medium-temperature solvothermal conditions. 1 is a 2D B-O monolayer with rectangular eight-membered ring (8-MR) windows built by B₅O₁₀(OH)₂ clusters acting as 4-connected nodes, in which two types of B₅O₁₀(OH)₂-based chains ([B₅]-I and [B₅]-II) strictly alternately connect to form a 2D wavy layer. The adjacent layers in 1 stack in the simple -AAA- mode. Compound 2 features a 3D porous-layered aluminoborate (ABO) built by three types of structural building units (SBUs) including AlO₄ units and B₅O₉(OH) and B₅O₈(OH)₂ clusters, in which the AlO₄ units and B₅O₉(OH) clusters connected to form the ABO layers, and the B₅O₈(OH)₂ clusters as 2-connected nodes link the adjacent ABO layers to make the 3D porous layer and further stack in the -AAA- mode. Structure changes from layer 1 to the porous layer 2 show the effectiveness of the synthesis strategy by inducing AlO₄ units into the B-O network system, resulting in significant implications for enriching the structural chemistry of borates and ABOs. The UV-vis diffuse reflectance measurements show that both 1 and 2 have wide UV transparency windows below 200 nm. Density functional theory (DFT) calculations indicate the top of the valence band (VB) and the bottom of the conduction band (CB) for both compounds determined by the B-O interaction. This reveals that the introduction of Al as a framework heteroatom into the B-O system can still maintain the broadband gap advantage of borates.