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.