Plasma-induced defects enhance the visible-light photocatalytic activity of MIL-125(Ti)-NH2 for overall water splitting.

Defect engineering in metal-organic frameworks is commonly performed by using thermal or chemical treatments. Herein, it is reported that oxygen plasma treatment generates structural defects on MIL-125(Ti)-NH 2 leading to an increase of its photocatalytic activity. Characterization data indicates that plasma-treated materials retain most of their initial crystallinity, while exhibiting somewhat lower surface area and pore volume. XPS and FT-IR spectroscopy reveal that oxygen plasma induces MIL-125(Ti)-NH 2 partial terephthalate decarboxylation and an increase of the Ti-OH population. Thermogravimetric analyses confirm the generation of structural defects by oxygen plasma and allowed an estimation of the resulting experimental formula of the treated MIL-125(Ti)-NH 2 solids. SEM analyses show that the oxygen plasma treatment of MIL-125(Ti)-NH 2 gradually decreases its particle size. Importantly, diffuse reflectance UV-Vis spectroscopy and valence band measurements demonstrate that oxygen-plasma treatment alters MIL-125(Ti)-NH 2 band gap and, more significantly, the alignment of highest occupy and lowest unoccupied crystal orbitals. An optimal oxygen plasma treatment to achieve the highest efficiency in water splitting with or without methanol as sacrificial electron donor under UV-Vis or simulated sunlight was determined. The optimized plasma-treated MIL-125(Ti)-NH 2 photocatalyst acts as truly heterogeneous photocatalysts and retains most of its initial photoactivity and crystallinity upon reuse.