The Applications Of Synchrotron-Based X-ray Spectroscopy On Photoelectric Energy Conversion Material Studies

In recent years,nano-materials show prospective applications in the field of photoelectric energy conversion.The characterization methods have evolved from the nature in macroscopic to the microscopic and the static state to the in-operando real-time dynamic monitoring.Understanding the function mechanism of nano-materials and the change of its chemical structure under working condition in the scale of the microscopic level is a topic that needs to be thoroughly studied.Synchrotron radiation sources with high flux,tunable energy,high resolution and element selectivity have become a promising approach to learn the mechanism of functional nano-materials with different morphology and the electronic variation during the synthesis process.Development of novel photoelectric energy conversion nano-material with high-performance can be benefit from the fundamental knowledge of the electronic structure.In this dissertation,synchrotron-based X-ray spectroscopies were used to study the inorganic semiconductor nano-materials for photoelectrochemical catalytic,as well as organic molecule in organic light-emitting diode device.The relationship between the performance and electronic structures of several nano-material systems can be understood in the following works:?1?The TiO₂ flower ball microstructure which was self-assembled by anatase?116?single crystal nano-faces with significantly enhanced UV and visible photocatalytic activity was studied by the X-ray absorption spectroscopy?XAS?and emission spectroscopy?XES?.The combination of XAS and XES measurement enables one to obtain the band gap with high accuracy.The XAS reflects the electronic density of state?DOS?of the conduction band?CB?,while the XES spectrum detects the DOS of the valence band?VB?.The band position could be obtained quantitatively by getting the first derivative extreme value of XAS and XES respectively,corresponding to the energy difference between unoccupied and occupied O 2p states.The 1.82 eV d-d excitation obtained from the Ti L-edge RIXS resolved the existence of 3d electron within the TiO₂ flower ball structure,the defect state between the band gaps leading to the valence band expanded upward.Moreover,in the X-ray excited optical luminescence?XEOL?spectra,the 675 nm?corresponding to 1.93 eV?luminescence confirms the existence of a mid-bandgap state.The band energy alignment of TiO₂ flower ball with?116?facets was confirmed by multiple approaches.Comprehensive X-ray spectroscopy results combined with laboratory UV-Vis spectra provide us fundamental knowledge about the enhanced visible light response of the nano-material.?2?By an aqueous deposition process,cobalt phosphate?CoPi?,an oxygen evolution catalyst is exclusively photo-deposited on the tips of the nano-pyramidal bismuth vanadate?BiVO₄?arrays.Compared to the 3d0 configuration of bulk BiVO₄,direct observation of spontaneous electron enrichment in the top tips of nano-pyramidal BiVO₄ arrays was achieved by performing synchrotron-based angular dependent XAS.The intrinsic 3d electron enriched tips produced a potential for the diffusion of photoexcited holes,which mitigate the recombination with photoexcited electrons.Meanwhile,the distinctive nano-pyramidal BiVO₄ morphology provides an active reaction site for CoPi deposition.The combination of the TEM,XRD results and DFT calculations suggests that the selective deposition of CoPi is owing to the?112?facets formation on the top tips of the nano-pyramids,which is in consistent with the spectra findings.Moreover,the existence of 3d electron resolved by V L-edge resonant inelastic X-ray scattering?RIXS?map and the metal-to-ligand charge transfer?MLCT?feature observed from the cobalt site consolidates the charge transfer process within the?112?facets enriched top tips,which facilitates the visible light-excited charge separation and results in an enhanced PEC performance.?3?Nickel hydroxide incorporated with 10-20%Fe atom is the most active catalyst electrode material for an oxygen evolution reaction?OER?in alkaline medium so far.However,the mechanism of Fe doping for improving the performance remains to be an open question.By performing in-situ X-ray spectroscopy measurement,we could probe the electronic structure of Ni-Fe composite hydroxide directly during the OER reaction,as well as the interaction between Fe and Ni sites within Ni-Fe composite hydroxide electrode.From the Ni K-edge extended X-ray absorption fine structure?EXAFS?after Fourier transform,a reversible atomic distance variation during the CV cycle of the OER reaction was resolved.We propose that the presence of Fe3+ alters the chemical environment of nickel hydroxide,other than pure nickel hydroxide getting aged easily,resulting in higher oxygen evolution efficiency.?4?A classic Alq₃-based OLED device was systematically studied by synchrotron-based X-ray spectroscopy and an in-operando approach to trace the real-time chemical evolution was successfully developed.The electronic structure and energy level alignment in both Alq₃ thin film and real devices have been obtained by XAS and XES.The NPB molecule substrate template effect and the after-effects of heated metal electrode vapor diffusion,which orients the quinoline ligand during the fabrication process,were elaborated.By measuring the N K-edge XAS spectra of OLED device under the working condition,the Mg atoms reacted with the Alq₃ evidently,instead of simply chemical absorbing into the Alq₃ organic molecular layer like the as-synthesized device.Combined with the simulation and DFT calculation results,we found that at the interface,Mg migrates into the Alq₃ organic layer and even replaces the A1 cation centers of the molecules when under external bias,forming the unstable Mgq3 species.This Mgq3 leads to the degradation of the entire OLED device.This first-stage in-operando soft X-ray investigation helps understand the chemical degradation mechanisms of OLED devices and helps derive strategies to develop more effective and stable devices.?5?Based on an in-situ characterization technology by synchrotron X-ray,a blue-light thermally activated delayed fluorescence?TADF?materials containing activate groups with high-efficiency was well investigated.We will try to explain the electronic structure of light-emitting organic molecules after the electron or hole are injected under the operation,and the formation of excitons.By performing XAS characterization we obtain the electronic configuration and bonding differences of the target light-emitting molecules under the different conditions.By the state-of-the-art in-situ approach to study the light-emitting molecules in real OLED devices,the real-time electronic structure changes before and after its failure were obtained.Combined with the bond decomposition energy calculation,the explanations for the failure mechanism of OLED devices was provided.