Browsing by Author "Li, Jiebo"

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    Ion Segregation in Aqueous Solutions
    (American Chemical Society, 2012) Bian, Hongtao; Li, Jiebo; Zhang, Qiang; Chen, Hailong; Zhuang, Wei; Gao, Yi Qin
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    Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces
    (Springer Nature, 2016) Li, Jiebo; Qian, Huifeng; Chen, Hailong; Zhao, Zhun; Yuan, Kaijun; Chen, Guangxu; Miranda, Andrea; Guo, Xunmin; Chen, Yajing; Zheng, Nanfeng; Wong, Michael S.; Zheng, Junrong
    Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces.
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    Vibrational Energy Dissipation in Condensed Phases Investigated by Multiple Modes Multiple Dimensional Vibrational Spectroscopy
    (2014-08-12) Li, Jiebo; Zheng, Junrong; Kolomeisky, Anatoly B.; Kono, Junichiro
    The methodology of ultrafast multiple-mode multiple-dimensional vibrational spectroscopy has been developed and applied to investigate the vibrational energy dissipation in condensed phase. In particular, experiments have been focused on the studies of vibrational energy relaxation and mode-specific vibrational energy transfer in both heterogeneous and homogeneous phases. This thesis presents two distinctive vibrational energy dissipation pathways for molecules absorbed on the typical heterogeneous metal nanoparticle surfaces. On 2-10 nm platinum and palladium nanoparticles, it was found that the electronic excitation-mediated vibrational energy dissipation (~2ps) was at least one order magnitude faster than direct vibration-vibration relaxation (50ps). This electronic energy damping is accompanied by low frequency thermal energy generation on metallic surfaces. This electronic mediated pathway dominates until the electronic property of the particle is altered by reducing size to ~1nm. The energy relaxation pathway also could be altered by changing the chemical nature of the metallic nanoparticle. These findings are of fundamental importance to ultimately understanding the nature of heterogeneous catalysis. This thesis also demonstrates mode-specific vibrational energy exchange between ions in electrolyte solution. (i) Interactions between model molecules representing different building-blocks of proteins and thiocyanate anions in aqueous solutions are studied. The binding affinity between the thiocyanate anions and the charged amino acid residues is about 20 times bigger than that between water molecules and the amino acids, and about 5~10 times larger than that between the anions and neutral backbone amide groups. (ii) Ion segregation was also investigated by mode-specific vibrational energy exchange between thiocyanate anions. In aqueous solutions, it was found that “structure maker” ions, such as F-, would stay in the “water phase” and thereby promote aggregation of the SCN- in an “ionic phase”. “Structure breaker” ions, such as I-, would break the ionic SCN- phase. (iii) Mediated by combination band, vibrational energy flow down from thiocyanate to ammonium was used to confirm that ion pair is formed between ammonium and thiocyanate in aqueous solutions. Investigations of these microscopic structures and dynamics of aqueous salt solutions experiments will add depth to our understanding of general macroscopic properties of electrolyte solutions.