Highly efficient photocatalysts by pyrolyzing a Zn–Ti heterometallic metal–organic framework

Abstract

A series of carbon composite materials containing metal oxides (ZnO and/or TiO x ) were synthesized by pyrolyzing a Zn-Ti heterometallic metal-organic framework (MOF) as the precursor. The photocatalytic activities of these composites were evaluated by the photodegradation of methylene blue (MB) in aqueous solutions. The chemical composition and porosity of prepared composites can be facilely tuned by varying the pyrolysis temperature. The sample being pyrolyzed at 1000 °C exhibited a dramatically increased surface area due to the carbothermal reduction of Zn 2+ into metallic Zn followed by vaporization as an extra pore-forming mechanism. Because Zn and Ti are adjacent in the heterometallic secondary building units (SBUs) of the MOF precursor, the catalytically active TiO x sites generated during pyrolysis would remain in the pores and channels formed by Zn vaporization and were readily accessible to MB leading to a huge increase of photocatalytic activity. Our work reveals a novel mechanism to generate porosity in photocatalysts facilitating the access of substrate to catalytically active sites which can be readily applied to other MOFs containing low boiling point metals.

Ag‐Enhanced TiO2–x/C Composites with Metal‐Organic Frameworks as Precursors for Photodegradation of Methyl Blue

“…ZTOF‐1 is a heterometallic metal‐organic framework incorporating Zn 2+ and Ti 4+ , . The PXRD of synthesized ZTOF‐1 matches well with previously reported data , , . ATC‐P4 and ATC‐P6 identify the X‐ray diffraction (PXRD) peak of ZnO (JCPDS card 75–0576).…”

“…At the same time, several PXRD peaks representing a mixture of TiO (JCPDS card 65–2900) and Ti 2 O 3 (JCPDS card 10–0063) was found for the sample being pyrolyzed at 1000 °C. In previous study,, , Ti 4+ cations in the SBUs of MOFs can be carbothermally reduced to TiO and Ti 3 O 5 by pyrolysis at 1000 °C, because of the synergic effect involving Zn reduction and carbothermal reduction.…”

“…It can be clearly seen that the pair of peaks of Ti 2p3/2 and 2p1/2 appeared at 458.6 and 464.4 eV, respectively, and the splitting of the 3d doublet is 5.8 eV, which can be assigned to Ti 4+ 2p peaks of TiO 2 , . For ATC‐P10, Ti 3+ and Ti 2+ can be easily identified from the XPS spectrum, indicating the existence of Ti 2 O 3 and TiO proved by PXRD data . The presence of Ti 3+ and Ti 2+ species implies that Ti 4+ ions can occupy electrons from nearby oxygen vacancies ,…”

“…One is P25 TiO 2 that is commercially available with mixed anatase/rutile crystallites commonly used as the benchmark photocatalyst . The other one is ZTOF‐1‐P10 that is carbon composite photocatalyst materials containing metal oxides (ZnO and/or TiO x ) . Before the illumination, the dark adsorption was checked for 30 min to attain adsorption‐desorption equilibrium (Figure a).…”

“…However, because of the labile coordination bonds in MOFs, using MOFs directly as photocatalysts has the risk of material decomposition . Alternatively, in our previous studies, we have demonstrated the preparation of the carbon composites containing ZnO and reduced TiO 2 (TiO 2– x ) by pyrolyzing Ti‐containing MOF ZTOF‐1 as precursors . Thanks to the pores and channels formed by Zn vaporization, MB can be readily accessible to the catalytic activity sites of TiO 2– x generated during pyrolyzing MOFs, leading to a huge increase of photocatalytic activity.…”