The Study Of Visible Light Response Quantum Dots And Their Photocatalytic Applications

Quantum dots?QDs?are well known as a kind of semiconductor nanocrystals with the size smaller than 10 nm in three dimensions.In recent years,quantum dots have attracted many interests due to their properties including large specific surface area and tunable band gap,and they have exhibited great potentials in applications of photocatalysis.Compared with conventional semiconductor photocatalysts,quantum dots possess rich surface reaction sites,and their photoelectric properties can be easily altered by tailored their particle size.Among various quantum dots,CdTe quantum dots are regarded as an ideal candidate in photocatalytic reactions,due to their advantages such as suitable band structure,strong light harvesting capacity,easy preparation and low cost.However,these quantum dots lack of effective catalytic sites on their surface,and the generated electrons in conduction band are easily recombined with holes in valence band,which limit their applications.In this paper,we study the CdTe based photocatalyst for hydrogen generation and CO₂ reduction.By surface engineering strategies such as doping and coating a active shell layer material,they exhibited a higher photocatalytic activity and an effective charge separation efficiency.The specific research results are displayed as follows:?1?Photocatalytic hydrogen?H₂?production from water provides an intriguing approach for alleviating the fossil fuel crisis.As one of the most promising light harvesters,colloidal quantum dots?QDs?,however generally exhibit low photocatalytic activity toward H₂ evolution,due to the lack of catalytic sites on their surface.In this section,water soluble Ni²⁺doped CdTe quantum dots?Ni:CdTe?photocatalyst were prepared and studied.By analyzing the quantum dots with TEM and XRD,we found that the incorporation of Ni²⁺did not affect their crystal structure and crystal size,indicating that Ni²⁺was uniformly doped into the structure of CdTe quantum dots.Additionally,by analyzing their absorption spectrum?UV-vis?and cyclic voltammetry?CV?,we found that after doping metal ions into CdTe,their band structure has no obvious change,while their photocatalytic hydrogen generation rate are markedly improved,indicating that Ni²⁺ions can provide CdTe with the catalytic sites.In addition,the Ni²⁺ions in CdTe can also facilitate the electron-hole separation,as evidenced by fluorescence and photocurrent measurements.Since the recombination rate of carriers is slowed down,more electrons can be used to split water,and the holes are captured by the sacrificial agent,conferring CdTe a good durability during illumination.Furthermore,the near surface Ni²⁺ions can function as active sites for H₂ generation during the photocatalytic water-splitting.The formation rate of H₂ exceeded 27.3 mmol/g/h under visible light irradiation,which is approximate to 110 times and 13 times higher than that of pristine CdTe and CdTe+Ni²⁺system under the same condition,respectively.This work may offer a perspective approach to develop efficient QDs-based photocatalysts for visible light driven H₂ evolution.?2?Solar-driven CO₂ reduction is an intriguing approach to mitigate greenhouse gas emissions and tackle the fossil fuels crisis.The photocatalytic reaction generally proceeded at the interface between catalyst and CO₂?or H₂O?,thus a strong adsorption of substrate and high catalytic activity on the catalyst are impending desired.Meanwhile,a suitable photocatalyst for CO₂ reduction should also meet the requirements including the expanded light-harvesting range and accelerated charge separation.However,the current catalysts can hardly possess the above features simultaneously.Herein,we report that modifying CdTe/CdS type II core/shell quantum dots?QDs?with ZnO activate layer can overcome these limitations to achieve a highly efficient photocatalyst in CO₂ reduction.Taking advantages of the superior optoelectronic properties in type II QDs,the CdTe/CdS QDs exhibit extended visible light harvesting and high spatial charge separation?the electrons and holes are separated spatially due to the energy level staggered?.Importantly,the localized electrons in CdS shell can be effectively trapped by ZnO coating layer,and substantially reduce the adsorbed CO₂ on ZnO.In this system,the CdTe/CdS QDs sever as the light harvester,and the ZnO coating layer may potentially offer the catalytic sites toward CO₂ reduction.As a result,the ZnO modified CdTe/CdS QDs achieve high selectivity for the photocatalytic reduction CO₂ and H₂O?100%to CO and CH₄?with the activity 9.0?mol/g/h and 6.8?mol/g/h.The versatile method reported here offers new opportunities for exploring QDs-based photocatalysts.