Photocatalytic performance of disordered titanium-based hollow nanosheet metal−organic frameworks in wastewater treatment

Background: The low long-range order in the crystallinity of metal−organic frameworks (MOFs), commonly known as disordered MOFs, can lead to critical property variations. However, controlling the synthesis conditions for a reproducible outcome is somehow cumbersome, especially if this control is accompanied by morphology engineering to be utilized in photocatalysis where band structure, band gap, surface area, and porosity matter. Methods: Here, we report a crystal and structure-directing approach for the morphology engineering of NTU-9-like MOF aggregated particles (Ti-DHTA_AP) (composed of Ti⁴⁺ and 2,5-dihydroxyterephthalic acid (DHTA)) into a hollow nanosheet (Ti-DHTA_HNS) morphology. Triethylamine (TEA), here, acts as a structure-directing agent (SDA), and monodispersed polystyrene (PS) as a hard template. The three obtained Ti-DHTAs were eventually investigated in the photocatalytic removal of organic contaminants (dye and pharmaceuticals) and comprehensively characterized by (photo)electrochemical approaches. Significant Findings: The hollow nanosheet-architected Ti-DHTA_HNS with a superior photocatalytic activity than the other morphologies also exhibits an overall photocatalytic removal (synergic adsorption-photodegradation) of ∼6.5-fold higher than the commercial TiO₂ (P25) under the visible light irradiation, with a degradation turnover (dTON) of 27 mmol h⁻¹ g_cat⁻¹. Ti-DHTA_HNS also shows promising results in the photocatalytic removal of dye and pharmaceutical wastewater. Photoelectrocatalytic characterizations were provided to compare the photocatalytic performance of synthesized Ti-DHTAs (e.g., in cyclic chronoamperometry (CA), 6-fold higher photoresponse than Ti-DHTA_AP). Nyquist plots further exhibit that the charge transfer resistance (R_ct) of the unmodified Ti-DHTA_AP is ∼10-fold higher than Ti-DHTA_HNS under visible light illumination. Furthermore, the actual water samples and the reusability of Ti-DHTA_HNS were investigated. The addition of the radical scavenger agent can confirm the presence of varoius active radicals during the degradation, and hence, the formation of hydroxyl radicals was probed by adding o-phenylenediamine as a trapping agent. During methylene blue (MB) photodegradation, the LC-MS analysis exhibits acetoacetic acid formation (m/z = 102.03) as the dominant intermediate.