Browsing by Author "Zheng, Junrong"
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Item Catalytic Oxidation Properties of Palladium-decorated Gold Nanoparticles(2014-10-06) Zhao, Zhun; Wong, Michael S.; Gonzalez, Ramon; Zheng, JunrongBimetallic palladium gold (PdAu) catalysts have been shown to be superior to monometallic ones in many reactions, but the reasons for the enhancement are not thoroughly understood. In this work, palladium decorated gold nanoparticles (Pd-on-Au NPs) are used as structured model catalysts, allowing for the precise control of both size and metal distribution with Pd surface coverage (sc%). By testing reactions on a range of these catalysts, we hope to gain insight into the active site for a given reaction. In hydrodechlorination of perchloroethene (PCE), Pd surface coverage was found to be the key factor in catalyst activity, with the optimum at 80 sc%. A complete mechanistic model that coupled mass transfer processes with the surface reactions was further developed, consistent with the observed product profiles. Carbon supported Pd-on-Au NPs were tested for liquid phase glycerol oxidation for the first time. The best catalyst (80 sc%) had an initial TOF of ~6000 h-1, >10 times more active than Au/C and Pd/C. Catalytic activity, selectivity, activation energy and deactivation rate constant exhibited strong volcano-shaped dependences upon Pd sc%. Ex situ XANES results showed no to little change in surface Pd-O% for Au based catalysts, suggesting the possibility of Au suppressing Pd oxidation during reaction. Ex situ EXAFS results further confirmed the core-shell structures of 60 and 150 sc% Pd-on-Au/C catalysts via Punnett square analysis, and also ascertained no to little change in their oxidation states and coordination numbers post glycerol oxidation. EXAFS observations correlate with kinetics results, and lead to the conclusion that catalysts with a larger amount of 3-D Pd ensembles are more prone to oxidize during glycerol oxidation, making them less resistant to deactivation. Finally, Pd-on-Au/C catalysts were tested for room temperature formic acid decomposition. In situ XAS revealed that core-shell structures of 60, 150 and 300 sc% Pd-on-Au NPs maintained while oxidized Pd species was partially reduced during reaction. Catalyst with higher fraction of 3-D Pd ensembles showed much higher dehydrogenation activity than those with mostly 1-D or 2-D, correlating to the proposed mechanism that the dehydrogenation pathway is favored over metal terrace sites.Item CVD Grown Graphene-Based Materials: Synthesis, Characterization and Applications(2015-04-22) Ma, Lulu; Ajayan, Pulickel M; Lou, Jun; Zheng, Junrong; Vajtai, RobertGraphene draws a lot of attention due to its exceptional electrical, mechanical, thermal, optical and chemical properties. However, its zero bandgap is a limitation for electronics applications and its two-dimensional (2D) nature is a limitation for large scale, volumetric and macroscopic applications. Doping graphene with heteroatoms and creating graphene hetero-structures are two approaches herewith suggested to sidestep the above limitations. The illustration of thus approaches begins with the chemical vapor deposition (CVD) growth of graphene in the form of either atomically thin films or 3D porous structures; which involves the synthesis of several structures such as nitrogen-doped graphene, graphene-carbon nanotube hybrids, in-plane graphene-boron nitride heterostructures, and graphene-molybdenum carbide hybrids. The analysis of impurities in CVD grown graphene at the atomic scale and the measurement of fracture toughness of graphene will then follow. Furthermore, the potential applications of as-synthesized materials like field emitters, supercapacitors, and catalysts for water splitting are discussed.Item Developing Multiple Dimensional Vibrational Spectroscopy for Molecular Conformation Determinations and New Catalysts Design(2017-04-12) Zhang, Yufan; Zheng, JunrongUnderstanding molecular structures can play a critical role in revealing many important chemical and biological phenomena. X-ray crystallography and NMR methods, two of the most common structural tools, cannot reveal all unknown molecular structures due to the restriction of sample form or low time resolution, respectively. Motivated by the need of a new approach to reveal structures that are currently not feasible by these traditional techniques, this thesis has developed a uniquely designed ultrafast multiple-mode multiple-dimensional vibrational spectroscopy (MDVS) to conquer the limitations of traditional methods. The MDVS technique has also been applied to study molecular structures under various condensed phases, and facilitate the understanding of various chemical mechanisms in this thesis. Chapter 1 gave a brief review of the current common structure tools that have been used by chemists. Their advantages and disadvantages were compared to bring up the challenges that a new generation of structure tool needs to conquer. Chapter 2 gave a detailed description of the new design of our multiple dimensional vibrational spectroscopy and its theoretical basis on how it can be used to reveal molecular structures. Chapter 3 gave an example of how the MDVS reveals the molecular conformations for molecules in various condensed phases. Chapter 4 showed another application of MDVS in revealing an intermediate’s structure during a real catalytic reaction for hydrogen generation. Chapter 5 was inspired by the potential application of formic acid as a great hydrogen chemical storage material demonstrated in Chapter 4, we continued working on developing and optimizing hydrogenation catalysts with even better efficiency and long lifetime.Item Electron-phonon interactions in MoS2ﾠprobed with ultrafast two-dimensional visible/far-infrared spectroscopy(AIP Publishing LLC., 2015) Guo, Xunmin; Chen, Hailong; Wen, Xiewen; Zheng, JunrongAn ultrafast two-dimensional visible/far-IR spectroscopy based on the IR/THz air biased coherent detection method and scanning the excitation frequencies is developed. The method allows the responses in the far-IR region caused by various electronic excitations in molecular or material systems to be observed in real time. Using the technique, the relaxation dynamics of the photo-excited carriers and electron/phonon coupling in bulk MoS2 are investigated. It is found that the photo-generation of excited carriers occurs within two hundred fs and the relaxation of the carriers is tens of ps. The electron-phonon coupling between the excitations of electrons and the phonon mode E1u of MoS2 is also directly observed. The electron excitation shifts the frequency of the phonon mode 9 cm−1 higher, resulting in an absorption peak at 391 cm−1 and a bleaching peak at 382 cm−1. The frequency shift diminishes with the relaxation of the carriers.Item The opposite effects of sodium and potassium cations on water dynamics(Royal Society of Chemistry, 2017) Zhang, Qiang; Chen, Hailong; Wu, Tianmin; Jin, Tan; Pan, Zhijun; Zheng, Junrong; Gao, Yiqin; Zhuang, WeiWater rotational dynamics in NaSCN and KSCN solutions at a series of concentrations are investigated using femtosecond infrared spectroscopy and theory. Femtosecond infrared measurements, consistent with previous NMR observations, detect that sodium slows down while potassium accelerates the water O–H bond rotation. Results of reported neutron scattering measurements, on the other hand, suggested that these two cations have similar structure-breaking effects on water, and therefore should both accelerate water rotation through the presumably dominating large-amplitude angular jump component. To explain this discrepancy, theoretical studies with both classical and ab initio models were carried out, which indicate that both ions indeed accelerate the large-amplitude angular jump rotation of the water molecules, while the observed cation specific effect originates from the non-negligible opposite impact of the sodium and potassium cations on the diffusive rotation of water molecules.Item Theoretical investigation of biological transport processes using discrete state stochastic models and simulations(2013-05-20) Uppulury, Venkata; Kolomeisky, Anatoly B.; Diehl, Michael R.; Zheng, JunrongBiological processes take place far away from equilibrium and are of interest to uncover the basic principles governing these phenomena. In this endeavor, molecular transport across channels and transport of cargos by molecular motors are studied. Functioning of a normal cell is contingent upon import of important biomolecules via narrow passage ways called ‘Channels’. Due to entropic barriers the incoming molecules often face hindrance to cross the channels successfully and reach specific locations inside cells. But in real systems the channels utilize special binding sites to accelerate this process. For a set of in-vitro experiments studying molecular flow across channels in presence of the special attractive binding sites, explicit analytical results are derived and they evince how modification of the free energy barrier could achieve quicker translocation; it has been shown that the nature of interactions between molecule and channel, their spatial distribution, strength of interactions and inter-molecular interactions in the channel all contribute to the complex process of translocation. In another study, cargo transport driven by molecular motors is studied. Cells utilize special enzyme molecules called ‘molecular motors’ that convert chemical energy into mechanical motion to transport cargos. Often multiple motors drive transport due to viscous nature of cell medium and other crowding effects in cells. In such systems the extent to which multiple motors share applied load and in turn how they influence the properties of the system is not well understood. From in-vitro measurements pertinent to well-defined structural assemblies and quantitative modeling treatments, it has been shown that the extent to which the motors collectively drive transport depends on the basic biophysical properties of the motors and the loading conditions of the optical trap. Specifically, kinesin motors are seen to negatively cooperate and cooperate positively only under high forces. Further, the microscopic origins of cooperativity is investigated which depend on biochemical interactions of the constituents within the complex and to some extent on mechanical properties. The mechanisms explain how the extent of cooperativity is related to the motor mechanochemistry, how different energy transport modes are manifested during cargo transport and what factors affect cooperative behaviors among multiple motors during transport.Item 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, JunrongEnergy 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.Item Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2ﾠheterostructures(Springer Nature, 2016) Chen, Hailong; Wen, Xiewen; Zhang, Jing; Wu, Tianmin; Gong, Yongji; Zhang, Xiang; Yuan, Jiangtan; Yi, Chongyue; Lou, Jun; Ajayan, Pulickel M.; Zhuang, Wei; Zhang, Guangyu; Zheng, JunrongVan der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2ﾠheterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.Item Ultrafast probes of electron–hole transitions between two atomic layers(Springer Nature, 2018) Wen, Xiewen; Chen, Hailong; Wu, Tianmin; Yu, Zhihao; Yang, Qirong; Deng, Jingwen; Liu, Zhengtang; Guo, Xin; Guan, Jianxin; Zhang, Xiang; Gong, Yongji; Yuan, Jiangtan; Zhang, Zhuhua; Yi, Chongyue; Guo, Xuefeng; Ajayan, Pulickel M.; Zhuang, Wei; Liu, Zhirong; Lou, Jun; Zheng, JunrongPhase transitions of electron-hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics. To investigate interfacial dynamical transitions of charged quasiparticles, however, remains a grand challenge. By employing ultrafast mid-infrared microspectroscopic probes to detect excitonic internal quantum transitions and two-dimensional atomic device fabrications, we are able to directly monitor the interplay between free carriers and insulating interlayer excitons between two atomic layers. Our observations reveal unexpected ultrafast formation of tightly bound interlayer excitons between conducting graphene and semiconducting MoSe2. The result suggests carriers in the doped graphene are no longer massless, and an effective mass as small as one percent of free electron mass is sufficient to confine carriers within a 2D hetero space with energy 10 times larger than the room-temperature thermal energy. The interlayer excitons arise within 1 ps. Their formation effectively blocks charge recombination and improves charge separation efficiency for more than one order of magnitude.Item 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, JunichiroThe 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.