Light/thermal Excitation Reaction Controllable Synthesis Of Transition Metal Nanocomposites

Synthesis of the functional compounds with controllable structures and performance by manipulating the chemical reactions has attracted sustained attention in the field of nanomaterial preparation.However,the stimulation of chemical reactions typically involve ambient energy owing to the depth of the potential well.As a kind of energy medium,the electromagnetic radiations?EMRs?are generally employed to induce the chemical reactions,since they can provide sufficient energy to the reactants.Among which,the microwave,ultraviolet?UV?,and infrared?IR?are regarded as the common candidates,since their frequencies coincide with molecular resonance;on the other hand,their relatively low radiation energy can greatly reduce the radiation damage and the power consumption.In this work,we employed the light/thermal induced reactions to fabricate the transition metal compounds ZnO and Mo₂N?C?;we examined the morphologies,crystal structures,molecular structures and chemical elements of the resultant samples by FESEM,TEM,XRD,Raman and XPS,respectively;according to the characterization results,we investigated the effect of UV and thermal?IR?radiation on ZnO nucleation and growth as well as the bond dissociation of g-C3N4 molecule,respectively.?1?In the first section of this work,we employed the UV light?wave length 365nm?to irradiate the TiO₂ nanotube substrate immersed in Zn?NO3?2 aqueous solution,and obtained ZnO/TiO₂ nano-composites with controllable morphologies;we investigated the effect of the UV irradiation,the concentration of Zn?NO3?2 solution and the annealing time of TiO₂ nanotube substrate on the morphologies of resultant ZnO nanostructures based on the FESEM,TEM,XRD and Raman characterization results;we also discussed the photochemical reactions occurred on TiO₂ substrate surface under UV radiation and analyzed the nucleation and growth of ZnO crystal during the reactions using the photochemical theory.The FESEM characterization definitely elucidates that the substrate surface fabricated a uniform nanoporous layer constructed by the massive nanosheets with 8 h substrate annealing,1 molL-1 Zn?NO3?2 solution,and 12 h UV irradiation;the nanoporous structures will be evolved into the nanoflower structures with reaction temperature at 343 K.The XRD analysis indicates that the TiO₂ nanotubes mostly belong to?101?plane of the anatase phase,while the ZnO nanostructures index to?101?plane of the hexagonal wurtzite phase.Besides,the Raman spectra exhibits the inherent non-polar E2?high?and E1?LO?phonon modes of hexagonal wurtzite ZnO.Furthermore,we discussed the mechanism of ZnO crystal nucleation and growth on TiO₂ nanotube surface,which is essentially determined by the density of surface electrons,the distribution of absorbed O₂ molecules,the concentration of Zn2+ ions and H2 O molecules,and the reaction temperature.The emergence of oxygen vacancies enhance the electron density and the ability of molecule absorption,and the inducing effect of photons on the growth orientation of ZnO nanosheets will be weakened with increasing reaction temperature,which result in the evolution of ZnO crystal morphology from nanoporous layer?2D?to spherical nanoflower?3D?.?2?In the second section of this work,we employed C2H4N4 to synthesize g-C3N4 precursor under high temperature,and we finally obtained Mo₂N/Mo₂ C nano-composites by g-C3N4 reacting with ?-Mo O3.we investigated the phase transitions of the reactants under high temperature according to the TEM,XRD,DSC,Raman and XPS characterization results;we also calculated the potential energy,the molecular orbitals,and the effect of temperature on g-C3N4 bond dissociation using the density functional theory?DFT?and the model of crystal thermal expansion.The TEM images reveal that the resultant samples are mainly spherical Mo O₂ crystal,and partial Mo₂ N and Mo₂ C crystals embed in Mo O3 lattice with the dominant growth of?111?and?002?planes,respectively.According to the XRD and DSC characterization,the Mo O3 crystal evolves into a thermodynamically favored Mo O₂ crystal at 748 K with the dominant growth of?-111?plane,which described as a exothermic reaction;the?100?and?002?planes of g-C3N4 crystal will vanish at 798 K and 873 K,respectively,which is an endothermic reaction;the Mo₂ N crystal emerge at923 K with an endothermic process,while the Mo₂ C crystal appear at 998 K.The Raman and XPS characterization emphasize that the dissociation of Mo-O and C-N bonds rely on the reaction temperature,and the terminal C-N bond of g-C3N4 molecule is considerably reactive under high temperature.The DFT simulation results indicate that the localized ? bonds that involve sp2 hybridization among adjacent C and N atoms promote to the stability of the planar melem ring;yet the delocalized ? bonds that contributed by the parallel pz orbitals of C and N atoms give rise to the highest occupied molecular orbitals?HOMO?and the lowest unoccupied molecular orbitals?LUMO?.In particular,a comparison of the HOMO corresponding to the formation and scission of the terminal C-N bond revealsthat the distribution of ?-electron can be transferred from the symmetric center to the edge of melem ring,contributing to a considerably high chemical activity of the terminal C and N atoms;yet the distribution of ?-electron on the LUMO has barely altered even the interaction of C-N bond is completely free.Moreover,the discussions based on crystal thermal expansion emphasize that the drastic atomic vibration substantially contribute to the lattice cleavage under high temperature,and the dissociation temperature can be essentially ascribed to its equilibrium dissociation energy.