Titanium Oxide-based Core-shell Structure Derived From MOF-like Structure And Its Photocatalytic Performance

With the development of human society,people's demand for energy has beensignificantly increasing during recent decades.Therefore,non-renewable resources such as coal and oil have been facing energy depletion.H₂?hydrogen energy?is a kind of green energy with high energy,clean,renewable and zero pollution.Up to recent decades,semiconductor photocatalytic hydrogen production technology has attracted wide attention because of its advantages such as simple method,low cost,zero pollution and so on.In recent years,MOF-like structures and metal organic frameworks?MOFS?materials have obtained widespread attention in the field of catalysis.However,there is not enough research on the photocatalysis of oxides derived from MOF and MOF-like structural materials.As a typical catalyst,the main problems faced by TiO₂ in photocatalytic decomposition of water to hydrogen are the rapid recombination of phot-ogenerated electron-hole pairs and the low spectral utilization of TiO₂.We note that the core-shell heterostructure constructed by TiO₂ combined with other semiconductors can effectively inhibit the photo-generated electron-hole pair recombination and significantly enhance the photocatalytic ability of semiconductors.Therefore,we plan to use MOF and MOF-like structures as precursor templates to prepare titanium oxide-based core-shell structures derived from MOF and MOF-like structures,and apply the composite structures that inherit the characteristics of precursors to photocatalytic decomposition of water and hydrogen evolution.?1?The robust photocatalytic hydrogen evolution?PHE?from water needs an effective photo-generated charge spatial separation and enough contact between reactant and catalyst,but the synthesis of catalysts with the characteristics remains a challenge.Therefore,we designed to uniform Ly wrap the thin layer of TiO₂ on the porous ZrO₂octahedron to form a core-shell heterostructure.In this system,Ui O-66-NH₂,one of popular MOF with Zr as metal node,has been chosen as the precursor template due to its plentiful pores,special morphology,as well as the rich NH₂ groups.The ZrO₂@TiO₂-500 has high specific surface(52.4 m²·g^-1).Besides,the intimate contact of TiO₂ shell with ZrO₂ core facilitates the separation and migration of photoinduced carriers,exposing more active sites for the surface photocatalytic hydrogen evolution reaction.The spectrum and electrochemical characterization further exhibit the extended life of photon-generated carrier and easy reactant transfer.The photocatalytic hydrogen evolution rate of ZrO₂@TiO₂-500 is 39.7 mmol·h^-1·g^-1,which is much higher than that of ZrO₂(0.8 mmol·h^-1·g^-1)and TiO₂(7.6 mmol·h^-1·g^-1).?2?At present,the synthesis of semiconductor catalysts with high efficiency of photo-generated electron-hole pair separation remains a challenge.In response to this work,we report a novel method for in-situ synthesis of hexagonal double-shell tube heterostructure of C₃N₄@nitrogen-doped TiO₂?abbreviated as CN@TiO₂?for photocatalytic hydrogen evolution.In this system,the supramolecular precursor not only serves as a template for synthesizing the hexagonal tube,but also a precursor for synthesizing CN,and it will dope a small amount of nitrogen on TiO₂ during the pyrolysis in the formation of CN.The CN@TiO₂ has a high specific surface area(44.7m²·g^-1),which can provide sufficient space for surface catalytic reactions.The reflection of light on the inner wall of a hollow hexagonal tube can improve the utilization of solar energy by the catalyst.In addition,the close contact between the TiO₂ shell and the CN core facilitates the separation and migration of light-induced carriers,thereby improving the efficiency of photocatalytic hydrogen evolution.Compared with CN and TiO₂,the photocatalytic hydrogen evolution rate of CN@TiO₂ is 10.10 mmol·h^-1·g^-1?the apparent quantum efficiency of the catalyst is 6.7%under the light of 420 nm wavelength?,which is 4 times and 15 times that of CN and TiO₂,respectively.?3?At present,the research on TiO₂ is mainly focused on anatase and rutile,but there are few studies on the composite structure of amorphous TiO₂ and TiO₂ with different crystallinity.Therefore,we use the highly crystalline TiO₂ derived from NH₂-MIL-125?Ti-MOF?as the precursor template,and tetrabutyl titanate is hydrolyzed on its surface to form a layer of amorphous TiO₂ nanosheet,resulting in a highly crystalline TiO₂/amorphous TiO₂ core-shell structure?abbreviated as C-TiO₂@A-TiO₂?,which is applied to photocatalytic decomposition of water to produce hydrogen.The synthesized C-TiO₂@A-TiO₂ has good hydrogen evolution efficiency(11.43 mmol·h^-1·g^-1),and the increase of hydrogen evolution rate can be attributed to its large specific surface area.At the same time,the homogenous core-shell interface can promote the migration and separation of photo-generated carriers and inhibit the recombination of electron-hole pairs.