Synthesis And Photocatalytic Performance Of Hollow Cobalt-based Semiconductor Nanomaterials

Since the industrial revolution,the more utilize of the natural resources such as coal,petroleum and natural gas,greatly promoted the development of human society.But the overuse of fossil energy have raised pollution,global warming and energy deterioration problems,therefore,prompted people take efforts to research the new green energy.Solar energy has the advantages of clean,pollution-free,unlimited and inexhaustible,hence,the development of technology that can efficiently use solar energy is considered to be a feasible way to relief the energy crisis and alleviate environmental pollution.Photocatalysis has been regarded as an environmentally friendly and economical technology that converts solar energy into chemical energy by using semiconductor photocatalysts under light irradiation.In recent years,in addition to its application in wastewater treatment,it has also been widely studied in photocatalytic water reduction for hydrogen,in conversion of carbon dioxide to carbon monoxide,methane and other available chemicals.Due to the weak light absorption capacity,the low photoelectric conversion efficiency and the sluggish surface reaction kenietics of the single semiconductor,it can not be widely available.Therefore it is necessary to construct appropriate photocatalysts to solve the above problems and challenges.This paper mainly focuses on the synthesis of hollow cobalt-based nano-semiconductor materials and lucubrate their photocatalytic properties.It is mainly elaborated from the following three aspects:1.Construction of hollow Co₃O₄ polyhedron via molybdenum doping for boosted photocatalytic CO₂ conversion.In this work,Mo-Co layered double hydroxide hollow polyhedrons(Mo Co-LDH HPs)are prepared by using ZIF-67 as precursor and then sodium molybdate ion etching with water bath.Then,Mo-doping Co₃O₄(Mo-Co₃O₄)HPs are obtained by annealing in air through high temperature.The hollow structure of Mo-Co₃O₄ HPs catalyst is not only beneficial to enhance the refraction and diffuse reflection of light so as to improve the absorption of light,but also reduces the diffusion distance of photogenerated carriers from the bulk to the surface,which effectively improves the separation and transformation of photogenerated electron-hole pairs.The results show that Mo-Co₃O₄ HPs exhibites much better CO₂ reduction performance compares with the Co₃O₄ nanosheets.The Mo-Co₃O₄-2 HPs for CO₂ conversion to CO is 10.08?mol g^-1h^-1,which is 2.9 times than that of Co₃O₄ nanosheets(NSs)(3.46?mol g^-1h^-1).This work provides a new method for the construction of simple hollow doped photocatalyst for CO₂ conversion.2.Construction of sugar-gourd-shaped CdS/Co_1-xS hollow heteronanostructure as an efficient Z-scheme photocatalyst for hydrogen generation.In this work,we report the synthesis of an unusual sugar-gourd-shaped hollow hetero-nanostructure,Co_1-xS hollow polthedrons(HPs)skewered on CdS nanowires(NWs)(CdS/Co_1-xS HHNSs),through a facile two-step strategy of growing ZIF-67 on CdS NWs followed by a sulfidation reaction.The loading amounts of Co_1-xS HPs on an individual CdS NW can be adjusted by adding the different content of the ZIF-67precursor in the reaction.The coupling of the Co_1-xS HPs with the CdS NWs not only broadens the solar light harvesting spectrum from ultraviolet to near infrared region,but also results in abundant Co vacancies as well as lots of active sites.More importantly,in-situ irradiation X-ray photoelectron spectroscopy confirms the direct Z-scheme transfer pathway of photogenerated charge carriers in the heteronanostructure,which facilitates the separation of photogenerated electron-hole pairs and induces strong photoreduction ability due to lower aligned conductor band of Co_1-xS.As a result,the gular-gourd-shaped CdS/Co_1-xS HHNSs are demonstrated as an efficient visible light-driven photocatalyst for H₂ production.The optimized CdS/Co_1-xS HHNSs achieves the H₂ production rate of 13.48 mmol g^-1 h^-1 without any additional cocatalyst.This work inspires the development of unusual hollow hetero-nanostructures as efficient photocatalysts by means of morphology design and interfacial electronic modulation.3.Construction of Cu₂S@NiCo₃O₄ hierarchical nanocages with p-n heterojunction for efficient photocatalytic CO₂ reducation.In this work,we have prepared Cu₂S@NiCo₃O₄ hierarchical nanocages with p-n heterojunction through a multi-step transformation method.First,Cu S nanoboxes(NBs)were obtained by simple partial sulfuration and ion etching of Cu₂O nanocubes(NCs)template.The NiCo₃O₄ NSs coated on Cu₂S hierarchical nanocages(Cu₂S@NiCo₃O₄ HHNBs)with p-n heterojunction were obtained by hydrothermal loading of Ni-Co layered double hydroxide(NiCo-LDH)NSs on Cu₂S NBs and then high temperature annealing in nitrogen atmosphere.In the optimized Cu₂S@NiCo₃O₄HHNBs,the production rates of CH₄ and CO are 62.1 and 1390.9?mol g^-1h^-1,respectively,which are 8.2 and 262.4 times than those of NiCo₃O₄ NSs(7.6 and 5.3?mol g^-1 h^-1).The reason for the improved performance is that the hollow structure improves the reflection/diffuse refraction of light,which effectively improves the absorption of light.In addition,the coupling of p-type semiconductor Cu₂S NBs and n-type semiconductor NiCo₃O₄ NSs can induce a strong interface-built electric field,which can effectively improve the separation of charge carriers at the interface.This work provides a new idea for the design of hierarchical hollow p-n heterostructures as an efficient photocatalyst.