Plasma‐Induced Defects Enhance the Visible‐Light Photocatalytic Activity of MIL‐125(Ti)‐NH 2 for Overall Water Splitting

[EN] Defect engineering in metal-organic frameworks is commonly performed by using thermal or chemical treatments. Herein we report that oxygen plasma treatment generates structural defects on MIL-125(Ti)-NH2, leading to an increase in its photocatalytic activity. Characterization data indicate 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)-NH2 partial terephthalate decarboxylation and an increase in 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)-NH2 solids. SEM analyses show that oxygen plasma treatment of MIL-125(Ti)-NH2 gradually decreases its particle size. Importantly, diffuse reflectance UV/Vis spectroscopy and valence band measurements demonstrate that oxygen plasma treatment alters the MIL-125(Ti)-NH2 band gap and, more significantly, the alignment of highest occupied 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)-NH2 photocatalyst acts as a truly heterogeneous photocatalyst and retains most of its initial photoactivity and crystallinity upon reuse.
S.N. thanks the Fundacion Ramon Areces (XVIII Concurso Nacional para la Adjudicacion de Ayudas a la Investigacion en Ciencias de la Vida y de la Materia, 2016), the Ministerio de Ciencia, Innovacion y Universidades RTI 2018-099482-A-I00 project, the Generalitat Valenciana grupos de investigacion consolidables 2019 (ref: AICO/2019/214) project, and the AVI project (INNEST/2020/111) for financial support. Financial support by the European Union (LoterCO2M), Spanish Ministry of Science, Innovation and Universities (Severo Ochoa and RTI2018-098237-B-C21), and Generalitat Valenciana (Prometeo 2017-083) is also gratefully acknowledged.