Synthesis And Research On The Photochromic And Piezochromic Luminescence Of Two-Dimensional-Based Optical Nanomaterials

The discovery of layered graphene synthesized by British scientist Andre K.Geim and Konstantin Novoselov open the door to the research on other two-dimensional materials.Recentlly,tremendous efforts have been devoted to the investigation of two-dimensional nanomaterials,graphene analogues such as graphitic carbon nitride（g-C₃N₄）,boron nitride（BN）,transition metal sulfides（TMDs）and transition metal oxides（TMOs）have been widely employed for biosensing,supercapacitors,catalysis and optical imaging applications with unique structure,superior chemical and physical properties.The properties of materials mainly rely on their unique structures and growth forms.Compared to their bulk materials,two-dimensional nanomaterials exhibit superior optical and electrical properties resulting from the enhanced quantum confinement and edge effects.The currently synthetic methods of optical nanomaterials using two-dimensional materials as sources suffer from a series of defects including expensive raw materials,complicated procedures and serious pollution etc.To actualize the preparation method of two-dimensional-based optical nanomaterials with green simplicity and excellent products performance still remain a challenge.In this work,we study the two-dimensional-based optical nanomaterials from preparation method,product properties and applications:（1）Graphitic carbon nitride nanosheets（g-C₃N₄ NSs）and quantum dots（QDs）were synthesized via one-pot ethanol-thermal method in the presence of KOH,and the synthetic mechanism was explored.Transmission electron microscopy（TEM）images show the layered structure of g-C₃N₄ NSs,and the average particle size of g-C₃N₄ QDs was about 3.25 nm.There were large amounts of hydrophilic groups on the surface of g-C₃N₄ NSs and QDs measured by Fourier-infrared（FT-IR）spectroscopy and X-ray electronspectroscopy（XPS）.Excitation-dependent photoluminescence behavior was observed by photoluminescence（PL）spectra.Based on its unique nanostructure,good water solubility and biocompatibility,g-C₃N₄ QDs could be employed for bio-imaging applications.（2）As one kind of fluorescent probes,fluorescent nanomaterials can detect temperature,pH,metal ions and oxygen value of environment by the change of intensity and color of fluorescence.Nitrogen–doped carbon quantum dots（N-C QDs）were synthesized via one-pot ethanol-thermal method in the presence of hydrogen peroxide（H₂O₂）and ammonia（NH₃·H₂O）.The morphology and chemical composition of N-C QDs were conducted by TEM,AFM and XPS measurements.Excitation-dependent ultraviolet and excitation-independent visible two emissive photoluminescence were observed by PL spectra of N-C QDs.Based on the phenomenon in which the PL emissive intensity and peak of N-C QDs varied by tuning the pressure,its color changed from bright blue to dark green with increasing pressure and the partical reversibility of PL peak was tested as pressure releases,the piezochromic N-C QDs could be used in mechanosensors,photoelectronic devices and data storage systems.（3）Tungsten trioxide（WO₃）QDs and molybdenum oxide（MoO_x）nanoparticles（NPs）were synthesized via one-pot ethanol-thermal method in the presence of H₂O₂ using tungsten disulfide（WS₂）and molybdenum disulfide（MoS₂）as precursors.By controlling the amount of H₂O₂,size-controlled WO₃QDs were obtained.Based on the fluorescence quenching of WO₃ QDs by ferric ion(Fe³⁺)and its unique nanostructure,excellent water solubility and good biocompatibility,WO₃ QDs could be used for the efficient,sensitive detection of Fe³⁺and bio-imaging applications.By controlling the synthesis time,MoO_x NPs with different phases including MoO₂,MoO_3-x and MoO₃ NPs can be obtained.Along with Mo⁴⁺gradually oxidized to Mo⁵⁺and Mo⁶⁺,the color of MoO₂,MoO_3-x and MoO₃ NPs dispersion was dark blue,light green and yellowish.Due to the localized surface plasmon resonance（LSPR）effect,MoO₂ NPs exhibited intense near-infrared absorption,which results in their application as a surface Raman enhanced scattering（SERS）substrate to detect trace amounts of Rhodamine 6G（R6G）,and the detection limit was as low as 5×10^-8 M,the maximum enhancement coefficient（EF）was up to 1.10×10⁷.Combined with the photothermal conversion efficiency of MoO₂ NPs up to 61.3%and excellent water solubility and biocompatibility,MoO₂ NPs could also be used as a photothermal therapy agent for cancer cells.