Constructing cuprous oxide-modified zinc tetraphenylporphyrin ultrathin nanosheets heterojunction for enhanced photocatalytic carbon dioxide reduction to methane

The application of supermolecular naonostructures in the photocatalytic carbon dioxide reduction reaction (CO2RR) has attracted increasing attentions. However, it still faces significant challenges, such as low selectivity for multi-electron products and poor stability. Here, the cuprous oxide (Cu2O)-modified zinc tetraphenylporphyrin ultrathin nanosheets (ZnTPP NSs) are successfully constructed through the aqueous chemical reaction. Comprehensive characterizations confirm the formation of type-II heterojunction between Cu2O and ZnTPP in Cu2O@ZnTPP, and the electron transfer from Cu2O to ZnTPP through the Zn-O-Cu bond under the static contact. Under the visible-light irradiation (λ > 420 nm), the optimized Cu2O@ZnTPP sample as catalyst for photocatalytic CO2RR exhibits the methane (CH4) evolution rate of 120.9 μmol/g/h, which is ∼ 4 and ∼ 10 times those of individual ZnTPP NSs (28.0 μmol/g/h) and Cu2O (12.8 μmol/g/h), respectively. Meanwhile, the CH4 selectivity of ∼ 98.7 % and excellent stability can be achieved. Further experiments reveal that Cu2O@ZnTPP has higher photocatalytic conversion efficiency than Cu2O and ZnTPP NSs, and the photoinduced electron transfer from ZnTPP to Cu2O can be identified via the path of ZnTPP→ (ZnTPP•ZnTPP)*→ ZnTPP-→ Zn-O-Cu → Cu2O. Consequently, Cu2O@ZnTPP exhibits a shorter electron-hole separation lifetime (3.3 vs. 9.3 ps) and a longer recombination lifetime (23.1 vs. 13.4 ps) than individual ZnTPP NSs. This work provides a strategy to construct the organic nanostructures for photocatalytic CO2RR to multi-electron products.

Construction of graphene oxide-coated zinc tetraphenyporphyrin nanostructures for photocatalytic CO2 reduction to highly selective CH4 product

The zinc-based photocatalysts for CO₂ reduction have attracted increasing attention, however, usually exhibit low CO₂-to-CH₄ selectivity. Here, the graphene oxide (GO)-coated zinc tetraphenylporphyrin (ZnTPP/GO) nanocomposites are successfully synthesized through a simple method. It is found that with the increase of GO content, the crystallinity of ZnTPP nanocrystals enhances with the size decrease, and then the light absorption can easily match with the solar spectrum. The optimal ZnTPP/GO sample exhibits the CH₄ evolution rate of 41.6 μmol g^-1 h^-1 and CH₄ selectivity of >95%, which are higher than those of ZnTPP nanocrystals (7.8 μmol g^-1 h^-1 and 50.3%). The systematic characterizations confirm that the generation of axial coordinated ZnOC bonds between ZnTPP and GO plays a key role in the formation of ZnTPP/GO nanostructure and their synergic effect on photocatalytic CO₂ reduction. The encapsulation of GO on ZnTPP nanocrystals not only promotes the CO₂ adsorption, interfacial reaction, and stability, but also accelerates the separation of photoinduced carriers on ZnTPP (0.1 ps vs. 425.9 ps), the transportation from ZnTPP to GO (2.3 ps vs. 83.6 ps), and their final enrichment on GO. This work provides a new strategy to apply graphene and organic nanomaterials in artificial photosynthesis.