Synthesis Of Near-infrared,core/shell Quantum Dots For Photoelectrochemical Hydrogen Generation

Colloidal semiconductor quantum dots?CQDs?are nanomaterials with size/shape/composition-dependent optical properties.Among various kinds of CQDs,CQDs with near-infrared?NIR?optical properties exhibit great potential for various applications.The broad NIR light absorption of theses CQDs is favorable to improve the light harvesting and efficiency of photovoltaic devices and the tunable NIR photoluminescence?PL?emission matches the wavelength range of biomedical window and optical communication.NIR CQDs have been employed to fabricate optoelectronic devices including solar cells,light-emitting diodes and photodetectors etc.Moreover,NIR CQDs are used for biological imaging and biological detection as well.However,to date,plenty of NIR CQDs still contain highly toxic chemical elements?e.g.lead,cadmium,and mercury?,which are harmful to human health and the natural environment,and are not favorable for future commercialization of QDs-based optoelectronic devices and biological applications.NIR,environmental-friendly CQDs?e.g.CuInS/Se CQDs?have been developed in recent years,while their multicomponent nature leads to the easily formation of surface defects/trap states.These surface defects/traps can deteriorate the quantum yield efficiency and stability of CQDs,thus hindering the development of high performance and long-term stable CQDs-based optoelectronic devices.To address these problems,we focus on developing the NIR core/shell structured CQDs,we systematically studied the growth,optical properties,electron-hole wavefunction and photoelectrochemical?PEC?application of these CQDs,as summarized below:?1?NIR,environmentally friendly CISeS/ZnS core/shell QDs were synthesized by cation exchange method and used for high efficiency PEC hydrogen production.Morphological characterization shows that this cation exchange technique results in the formation of a thin ZnS shell on the surface of CISeS core QDs.Optical properties shows the NIR optical absorption and photoluminescence?PL?spectrum of as-synthesized CISeS/ZnS core/shell QDs.The PL intensity and PL lifetime indicates that the ZnS shell effectively passivate the surface defects/traps on CISeS QDs.These NIR,heavy metal-free QDs were used as photosensitizers to fabricate QDs-sensitized photoanode by electrophoretic deposition?EPD?method.The morphology of QDs-sensitized photoanode shows that these NIR,environmentally friendly CISeS/ZnS core/shell QDs possess intimate contact with TiO₂,which is beneficial to electron transfer and transport.The element mapping of the photoanode indicates that the QDs are uniformly distributed on the TiO₂ nanoparticles.Taking account of the ultraviolet photoelectron spectroscopy?UPS?and the absorption band energy of the CISeS/ZnS core/shell QDs,the precise energy band alignment of QDs/TiO₂ is evaluated to be favorable for the separation and transport of electron-hole pairs.Finally,the NIR,heavy metal-free CISeS/ZnS core/shell QDs-sensitized photoanode was used as working electrode in a PEC cell.Under standard one sun illumination?AM 1.5G,100 mW/cm²?,such QDs-based PEC cell exhibits a saturated photocurrent density of⁵.3 mA/cm²,which is higher than the saturated photocurrent density?².57 mA/cm²?of bare CISeS QDs based PEC cell.It indicates that the ZnS shell effectively passivate the surface defects/traps of the CISeS QDs for reduced non-radiative recombination,thus enhancing the device performance.In addition,CISeS/ZnS core-shell QDs-based PEC cell exhibited higher device stability as compared to bare CISeS QDs-based device.?2?NIR,heavy metal-free CuInSe₂/CuInS₂“giant”core/shell QDs?g-QDs?were synthesized by sequential cation exchange method and used for PEC hydrogen generation.Morphology investigation demonstrates the formation of CuInS₂ thick shell with wurtzite?WZ?phase.Optical characterization shows that this type of NIR,environmental-friendly g-QDs possess tunable NIR absorption and PL spectra.The NIR,heavy metal-free g-QDs exhibit a red-shift of PL peaks and prolonged lifetime with increasing shell thickness,indicating the formation of“Quasi type-II”band structure,in which the electrons delocalized into the shell region while the holes are well confined in the core region.By choosing appropriate physical parameters and solving the Schr?dinger equation of this g-QDs,the electron-hole wavefunction distribution as a function of shell thickness is calculated.The simulation results also demonstrated the“Quasi type-II”band structure of g-QDs,which is consistent with the optical properties.Finally,a PEC cell based on such NIR and“green”g-QDs was fabricated by EPD technique,showing a saturated photocurrent density of³ mA/cm² under standard one sun illumination?AM 1.5G,100mW/cm²?.By comparing the stability of PEC device based on g-QDs with various shell thickness,it is demonstrated that the g-QDs with thicker shell possess higher photo/chemical stability in solar-driven PEC hydrogen production.?3?NIR-emitting,pyramidal shaped CISeS/CdSeS/CdS g-QDs were successfully synthesized by using a facile two-step method and employed for PEC hydrogen production.Morphology characterization demonstrates the pyramidal structure of as-synthesized g-QDs,which was etched to be quasi-octahedral shape during initial growth stage and restored to be pyramidal shape with subsequent growth of the shell.The CdSeS/CdS shell of as-prepared g-QDs was proved to possess zinc blende?ZB?structure.Optical properties of g-QDs exhibit NIR PL emission with peaks located at⁸30 nm,as well as excellent PL quantum yield?PLQY,17%?.As-synthesized pyramidal-shaped g-QDs show extremely long PL lifetime?²?s?which prolonged with increasing shell thickness,indicating that the pyramidal shape can induce efficient electron-hole separation and the g-QDs possess a“Quasi type-II”band structure.Theoretical simulation calculates the spatial electron-hole wave function distribution of as-synthesized g-QDs with various shell thickness,demonstrating that the pyramidal-shaped g-QDs exhibit direction-dependent electron-hole wave function distribution and"Quasi type-II"band structure.These g-QDs were subsequently used to fabricate PEC cells,showing saturated photocurrent as high as⁵.5 mA/cm² and outstanding device stability under one sun illumination?AM 1.5 G,100 mW/cm²?.These results indicate that this type of pyramidal-shaped g-QDs possess efficient electron-hole separation and are promising for high performance PEC hydrogen generation.