Silver Nanoparticle Decorated Carbon Electrodes Optimized For Photoelectrochemistry And Electrochemical Sensing

Recently, silver nanoparticles (AgNPs) have attracted much interest due to their unique optical and catalytic properties and found applications in new energy development as well as trace analysis. In this thesis, AgNPs were deposited on glass carbon electrodes and graphene by means of gas phase cluster beam deposition for non-enzymatic hydrogen peroxide detection and high-efficiency photocurrent generation, respectively. The main research results are as follows:1. The AgNPs fims were fabricated by using gas phase cluster beam deposition. The operation parameters of the source, such as the condensation distance, sputtering power and flow rate of sputtering or buffering gas were systematically investigated to improve the deposition rate of the AgNPs. It was found the deposition rate of the AgNPs can be well controlled by tuning the above parameters and an enhanced deposition rate was realized under a smaller condensation distance and proper sputtering power as well as a special flow rate of sputtering and buffering gas. The morphology, coverage and size distribution of the optimized AgNPs was characterized with transmission electron microscopy (TEM), atomic force microscope (AFM) and selected area electron diffraction (SAED). The results show that the AgNPs fabricated by this method have the merits of well-crystallized, high surface density (1.4×104μm-2), homogeneous size distribution (-6nm), good dispersity and clean surface. Therefore, the technique of gas phase cluster beam deposition is a promising technology for fabricating close-packed AgNPs particles.2. We demonstrate a novel method to coat AgNPs on glass carbon electrodes(GCE) with high adhesion and good dispersity by performing gas phase cluster beam deposition. A nonenzyme sensing platform for stable detection of H2O2was realized from the silver nanoparticle based electrode with an optimized nanoparticle coverage and number density. AgNPs electrodes fabricated with this method can avoid the problems of catalyst aggregation and shedding that would emerge in sensor applications. To examine the effects of the coverage and size distribution of the AgNPs on the reduction of the H2O2for optimizing the electrode conformation, we fabricated AgNPs-GCE electrodes with different deposition durations. The results show the electrode consists of small nanoparticles which are densely closely-spaced but sufficiently isolated from each other has significantly advantages in electroanalysis. As a result, the fabricated AgNP-GCE shows enhanced electrocatalytic activity toward the reduction of H2O2with a much reduced detection limit (typically1×10-6M) and response time (typically less than1second) as compared with those have been previously reported. Moreover, a possible reaction mechanism is proposed based on the experiments.3. Few-layer graphene sheets (FLG) were prepared by splitting expanded graphite (EG) using high-power sonication. FLG fabricated with this method can reduce the noise pollution and the FLG breaking that would emerge in long time low-power sonication. Atomic-level calibrated scanning transmission electron microscopy was used to obtain efficient layer statistics, enabling optimization of the experimental conditions. Herein, heating temperature of EG, ultrasonic power and duration were systematically investigated. This resulted in a two-step splitting mechanism in which the mean number of layers was first reduced to less than20by heating to1100℃and then to a few-layer region by a5-minute104W L-1-power-density sonication. Our method can fabricate FLG sheets with about five-layers on micro-lever at a yield of～1wt%, and we show that the FLG sheets concentration could potentially be improved by evaporating the solvent to give the concentration of up to～5μg mL-1, namely about5×105pieces per milliliter. X-Ray Photoelectron Spectrometer (XPS) and Raman spectroscopic (Raman) analysis confirm the above mechanism and demonstrate that the sheets are largely free of defects and functional groups. As a solution-phase method, this approach may be scaled up for high-efficient fabrication of the FLG.4. Closely spaced Ag nanoparticle arrays (AgNPs) were deposited on graphene sheets attached glassy electrode (FLG-GCE) by means of gas phase cluster beam deposition with controlled coverage and size distribution. This approach fabricated AgNPs-FLG electrode can avoid the overlap of the AgNPs on the electrode emerge during the chemically prepared AgNPs-FLG films were modified on the electrode. The fabricated AgNPs-FLG electrode show enhanced photocurrent generation compared with the FLG electrode. We employ an amperometric technique for high-efficiency photocurrent detection that enables the extensive study of the relationship between the localized surface plasmon (LSP) of AgNPs and the photocurrent enhancement of FLG in detail. The effects of the coverage of the AgNPs, the wavelength and intensity of the incident light, as well as the electrode potential were systematically investigated to improve the photoelectrochemistry performance of the AgNPs/FLG-GCE electrode. Moreover, the possible physical mechanism of the LSP enhanced photoelectrochemistry in the AgNPs-FLG composite film is proposed based on the experiments. Namely, the enhancement of the photocurrent from an illuminated AgNPs-FLG film is determined by the local electromagnetic fields near the AgNPs. On the one hand, they can increase the absorption of the incident light. On the other hand, they can confine the photoexcited electrons within graphene and depress its recombination within graphene and at the graphene/electrolyte interface. Thus, the electron-hole pairs are separated by this field and an enhanced photocurrent is observed.