Surface-modified Ti-MOF/TiO2 membrane and its gas-sensing characteristics

Abstract

A titanium metal–organic framework/titanium dioxide (Ti-MOF/TiO2) composed membrane was fabricated by light inductive growth of Ti-MOF ([Ti2-(TpA)2-NDI] n ; TpA = terephthalic acid, NDI = 1,8,4,5-naphthalene-tetracarboxdiimide) on a titanium dioxide film composite optical waveguide (COWG) substrate. The Ti-MOF/TiO2 membrane conforms to a mesoporous structure with a 24 nm pore size and a 108 nm thickness through continuous growth for 40 min under 340 nm ultraviolet light illumination. Under ambient conditions, in terms of COWG sensors, Ti-MOF/TiO2 exhibited the greatest response to ethylenediamine (EDA), followed by nitrogen dioxide (NO2), methylamine and trimethylamine, when exposed to 15 types of benzenes, amines and acidic gases. To improve response selectivity, the Ti-MOF/TiO2 film was modified with 1,3,3-trimethylindolinonaphthospirooxazine (SP), forming an SP@Ti-MOF/TiO2 COWG. This modified COWG showed improved selectivity response by showing robust response to EDA and a negligible response to others. When an EDA gas molecule was adsorbed on the surface of the membrane, charge transfer between them preferentially occurred, leading to a change in the optical parameter. The surface-modified SP@Ti-MOF/TiO2 COWG showed a fast (3 s) and reversible response with a wide detection range (0.1–1000 parts per billion) to EDA gas without the interference of benzene, toluene, xylene, styrene and chlorobenzene; acidic gases; and other amines.