Study On Synthesis And Photocatalytic Activity Of Carbon Dot-composited Metal Oxides

The design and development of new photocatalysts with environmental friendliness and high efficiency are very important for the sustainable conversion of solar energy,the synthesis of green fuels,and organic chemicals.At present,many kinds of nano-semiconductor materials have been used to prepare photocatalysts.However,there are still many problems,including limited visible light absorption efficiency,surface carrier accumulation due to limited transfer efficiency,etc.As commonly used photocatalytic semiconductor materials,metal oxides contain various species and show stable chemical properties,but their catalytic performance always is limited by a wide bandgap and high carrier recombination rate.Therefore,strategies such as composition changing,doping,and heterostructure construction need to be adopted to regulate the metal oxides.Carbon dots(CDs),as a new type of carbon nanomaterial,exhibit excellent visible light absorption and rapid separation efficiency of photogenerated carriers.Moreover,the abundant functional groups,heteroatoms,and defects on the surface endow them with good binding and reactivity with metal ions.Therefore,it is an effective strategy to improve the band structure of metal oxides and enhance the photocatalytic activity by introducing CDs into the synthesis.Nano-composites with various phases and structures could be obtained by controlling the formation process of metal oxides and CDs.Based on this assumption,composites with coal-based CDs and several metal oxide composites were prepared in this paper.Besides,the mediated effect of CDs on the formation of CDs-composited metal oxide was discovered.Furthermore,the photocatalytic activity of the composites was studied under different organic systems and a wide p H range,and the important role of CDs in the photocatalytic mechanism was illustrated.The specific research contents are as follows:CDs@Cu O_x composite nanomaterials was synthesized quickly and conveniently at room temperature by utilizing the residual H₂O₂ of the preparation process of anthracite-derived CDs.It was found that CDs showed stabilizing effect to copper peroxide(Cu O₂),which could generate the self-supplying hydrogen peroxide in the solution and trigger the photocatalytic activity of CDs@Cu O_x.The experimental results showed that CDs@Cu O_x had excellent photo-response,and the highest catalytic oxidation reaction rate was 35 times that of Cu O_x.Besides,it also revealed a wider p H applicability range compared with the photo-Fenton system which also needs the participation of hydrogen peroxide.The study revealed that the high catalytic activity of CDs@Cu O_x was achieved through the improvement of energy band structure and the cyclic transformation of hydrogen peroxide.The·O₂^-?H₂O₂?O₂?·O₂^-reaction closed loop was formed and effectively eliminated the accumulation of carriers.Moreover,CDs accelerated the carrier transfer,assisted in fixing oxygen on the surface,and inhibited the recombination of e^-and h⁺,speeding up the chemical reaction cycle effectively.Inspired by the effect of self-supplying hydrogen peroxide,a green and stable CDs-composited calcium peroxide(Ca O₂/CDs)was synthesized in PEG200 solvent according to the principle of hetero-nucleation.The heterogeneous interface between Ca O₂ and CDs was identified by transmission electron microscopy.It was found that the release rate of self-supplying hydrogen peroxide in the complex was affected by the illumination and p H of the solution.The introduction of CDs expanded the absorption range of the composite to the visible spectrum,significantly promoted the generation of photogenic carriers,and improved the yield of self-supplying H₂O₂ under the illumination.From the view of the photocatalytic effect,Ca O₂/CDs exhibited a 2.1 times higher photocatalytic rate than Ca O₂,and the photocatalytic activity remained when p H extended to the alkaline range.Meanwhile,the good recycling performance confirmed its practicability.The energy structure test and free radical experiment revealed that the Type-?heterostructure formed by Ca O₂ and CDs could promote the effective separation of photocarriers and improve the activation efficiency of self-supplying H₂O₂.The generated·O₂^-dominated the ROS in the catalytic oxidation reaction.To regulate the photocatalytic activity of transition metal oxides and broaden the catalytic application,a-Ni O_x/CDs composite was prepared in CDs/H₂O₂ system combining with the active site effect of transition metal in catalytic reduction reaction.The characterization showed its amorphous structure with long-range disorder and short-range order.The main components were shown to be Ni O and Ni₂O₃,and the content of Ni₂O₃ in a-Ni O_x/CDs was higher than in a-Ni O_x.CDs were densely and uniformly embedded in the amorphous structure,which separated the amorphous domain into smaller structures and shorten the transmission distance of photogenerated electrons.The results showed that a-Ni O_x/CDs composite not only show the photocatalytic oxidation ability to TMB,but also showed the photocatalytic reduction activity to p-NP.With 3 mg composite and the visible light of 100 m W·cm^-2,the conversion rate of 0.11 m M p-NP reaches 98%within 20 min.The catalytic activity of a-Ni O_x/CDs was positively correlated with the light intensity.Besides,the photocurrent and electrochemical experiments showed that a-Ni O_x/CDs had good photo-response and fast carrier transferability.The result of free radicals showed that h⁺and·O₂^-participated in photocatalytic oxidation,and e^-participated in photocatalytic reduction.The reaction mechanism could be induced that CDs act as hole acceptors,while Ni²⁺/Ni³⁺site as the electronic receiver to transfer electrons by the conversion between Ni²⁺and Ni³⁺,thus ensuring the continuous catalytic reaction.In addition,due to the reactive oxygen species produced by the composite,a-Ni O_x/CDs showed excellent bacteriostatic performance with illumination,indicating its potential application in the fields of biomedical and sewage sterilization.