XRD patterns of the as synthesized BPA c BPA

The surface area and the pore-size distribution were examined by adopting nitrogen-sorption analysis (Fig. 3). In order to see the effect of the agglomerate structure on the photocatalytic activities, two distinct types of agglomerates were prepared: (i) pristine BPA and (ii) crushed BPA (c-BPA) that Calhex 231 was processed by ball milling. Their properties are summarized in Table 1. The specific surface area was calculated by employing the Brunauer–Emmett–Teller (BET) adsorption model. For both types of samples, H1-type hysteresis [38] and type IV isotherms with a sharp capillary-condensation step were observed at high relative pressures. This suggests the presence of mesopores in both types of samples (Fig. 3a). The estimated pore size and the total pore volume are 6.53 nm and 0.1676 cm3 g−1 for BPA, and 6.92 nm and 0.1937 cm3 g−1 for c-BPA according to the Barrett–Joyner–Halenda (BJH) model (Table 1). After the ball-milling process, the volume of smaller mesopores (3–10 nm) decreases while stomata of larger mesopores (10–80 nm) increases, with a broadening in the size distribution (Fig. 3b). The specific surface area is 42.0 ± 0.5 m2 g−1 for BPA and 42.2 ± 0.3 m2 g−1 for c-BPA, respectively, which remains unchanged. The significant increase in the volume of large mesopores and the pore size after the ball-milling treatment indicates that the ordered arrangement of BP nanoparticles in the BPA structure is disrupted (at least, partly) by the ball-milling process. The fact that the ball milling did not induce any change in the XRD spectrum of BPA (See Fig. 2a) implies that the ball milling process was mild enough not to change the crystalline structure of BPA but disrupted only the aggregate structure.