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Interfacial Charge Transfer of Functional Nanomaterials: Manipulation of Electronic Structures and Electrocatalytic Activities

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Electronic structures of the nanomaterials play an essential role in the applications in catalysis because all the catalytic reactions are involved in the adsorption/desorption of reactants, intermediates and products which can be described by the Gibbs binding energies. My dissertation research focused on fundamental studies of the interfacial charge transfer of functional nanomaterials and use them to manipulate the electronic structure in electrocatalysis design.

My first work (Chapter 2) studied the intraparticle charge delocalization via the conjugated metal-ligand interfacial bonds. In this study, metal nanoparticles of 5d metals (Ir, Pt, and Au) and 4d metals (Ru, Rh, and Pd) were prepared and capped with ethynylphenylacetylene and the impacts of the number of metal d electrons on the nanoparticle optoelectronic properties were examined and found that intraparticle charge delocalization was enhanced with the increase of the number of d electrons in the same period with palladium being an exception. Whereafter, we extended similar chemistry to semiconductor nanoparticles, including silicon nanoparticles (SiNPs, Chapter 3) and titanium dioxide nanoparticles (TiO2, Chapter 4). Firstly, stable SiNPs were functionalized by ethynylferrocene as capping ligands by taking advantage of the unique chemical reactivity of acetylene moieties with silicon hydride on the nanoparticle surface, forming Si-CH=CH- interfacial bonds under UV photoirradiation. Electrochemically, the nanoparticles exhibited only one pair of voltammetric waves in the dark, suggesting a lack of effective electronic communication between the particle-bound ferrocenyl moieties, because of the low conductivity of the nanoparticle cores; whereas, under UV photoirradiation (365 nm), two pairs of voltammetric peaks were observed, with a potential spacing of 125 mV, suggesting that the nanoparticles behaved analogously to a Class II compound. This was ascribed to photo-enhanced electronic conductivity of the nanoparticle cores that facilitated intervalence charge transfer of the particle-bound ferrocene moieties. In the case of TiO2, it was first time functionalized by n-alkyne ligands such as octyne (HC8), ethynylferrocene (EFc), and ethynylpyrene (EPy). Experimental studies and first-principles calculations suggest the formation of M-O-C≡C- core-ligand linkages that lead to effective interfacial charge delocalization, in contrast to hopping/tunneling by the conventional M-O-CO- interfacial bonds in the carboxyl-capped counterparts. This results in the formation of an interfacial state within the oxide bandgap and much enhanced sensitization of the nanoparticle photoluminescence emissions as well as photocatalytic activity, as manifested in the comparative studies with TiO2 nanoparticles functionalized with EPy and pyrenecarboxylic acid (PyCA). Furthermore, when TiO2 nanoparticles were functionalized by EFc, the photo-gated intervalence charge transfer was observed due to the photo-enhanced electrical conductivity of the TiO2 cores that served as part of the chemical linkage bridging the ferrocenyl moieties. These results highlight the significance of the unique interfacial bonding chemistry by acetylene anchoring group in facilitating efficient charge transfer across the metal and semiconductor nanoparticle interfacial linkage and hence the fundamental implication in their practical applications.

Another series of my research is to use the charge transfer engineering we learned about to design electrocatalysts. Firstly, a new type of HER catalysts (Chapter 5) was developed where ruthenium ions were embedded into the molecular skeletons of graphitic carbon nitride (C3N4) nanosheets of 2.0 ± 0.4 nm in thickness taking advantage of the strong affinity of ruthenium ions to pyridinic nitrogen of the tri-s-triazine units of C3N4. Significantly, the Ru-C3N4 hybrid materials exhibited apparent electrocatalytic activity towards HER with an overpotential of only 140 mV to achieve the current density of 10 mA/cm2, a low Tafel slope of 57 mV/dec, and a large exchange current density of 0.072 mA/cm2. This suggests the activity was most likely due to the formation of Ru-N moieties where the synergistic interactions between the carbon nitride and ruthenium metal centers facilitated the adsorption of hydrogen. This was strongly supported by results from density functional theory calculations. Subsequently, ruthenium ions are incorporated into graphitic carbon nitride/reduced graphene oxide (rGO) hybrids forming Ru-C3N4/rGO (Chapter 6) composites through RuN coordination bonds with a Ru ions loading of 1.93 at.%. The introducing of Ru ions leads to electron redistribution within the materials and dramatically enhances the HER performance, as compared to C3N4, C3N4/rGO, and Ru-C3N4, with an overpotential of only 80 mV to reach the current density of 10 mA/cm2, a Tafel slope of 55 mV/dec, and an exchange current density of 0.462 mA/cm2. This performance is highly comparable to that of Pt/C, and ascribed to the positive shift of the conduction band of the composite, where the charge carrier density increases by a factor of about 250 over that of C3N4, leading to a lower energy barrier of hydrogen evolution. Beside C3N4, black phosphorus was also chosen as a supporting substrate for metal nanoparticles because of the unique electronic structure which has a lot of lone pair electrons. In this case (Chapter 7), thin-layered black phosphorus (TLBP) was used as a unique supporting substrate for the deposition of metal nanoparticles (MNPs, M = Pt, Ag, Au), and the interfacial charge transfer from TLBP to MNPs was confirmed by X-ray photoelectron spectroscopic measurements and density functional theory calculations. In comparison to the carbon-supported counterparts, Ag-TLBP and Au-TLBP showed enhanced ORR performance, while a diminished performance was observed with Pt-TLBP. This was consistent with the predictions from the “volcano plot”. Results from this study suggest that black phosphorus can serve as a unique addition in the toolbox of manipulating electronic properties of supported metal nanoparticles and their electrocatalytic activity.

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