| Recently the preparation of efficient and steady hydrogen sensing materials has gained more and more attentions, because of the small sizes, large specific surface characterisitics and unique properties of optics, electricity, catalysis of the nanowires. This dissertation mainly gives an overview of the development of metal nanowires electrodeposition and the applications of various nano-based hydrogen sensing materials. The alloy nanostructures could reduce the "hydrogen embrittlement" caused by hydrogen absorbing of pure palladium and prevent poisoning and lengthen the life. However, the most attention has been focused on alloy nanofilm materials right now, the sensitivity of alloy nanofilm was controlled by the thickness of nanofilm, character of fundus and so on. Thereby, the preparation of alloy nanowire-based sensor became a new hotspot.Pd-Ag alloy nanowires with controllable composition and morphology were fabricated via electrodeposition on highly oriented pyrolytic graphite. Pd-Ag and Pd-Au alloy nanowires were obtained by single or dual frequency sine alternating current electrodeposition between gold microelectrodes. Morphology, structure and composition of the alloy deposits were characterized by scanning electron microscope (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD) and transmission electron microscope (TEM). The processes of three electrodepositions were described. The Pd-based alloy nanostructures can be assembled into hydrogen sensor and their hydrogen sensing properties were measured. Through applying5mV controlled in electrochemical workstation, the hydrogen sensitive property was detected. The main results were gained as following:(1) In the electrolyte composed of0.5mmol-dm3Pd(NO3)2+0.033mmol·dm3AgNO3+2.0mol·dm-3NH4NO3, pH2-3, the alloy nanowire arrays with the diameters from60～150nm have been obtained under the optimum condition with the overpotential range of120-150mV and deposition time200～350s, the content of Ag is16～25wt.%The nucleation-growth mechanism of Pd-Ag alloy nanowires electrodeposited under overpotential100-150mV can be described by instantaneous nucleation on the active sites and diffusion-controlled3-D growth.(2) The dendritic Pd-Ag alloy nanowires was obtained between gold microelectrodes by single frequency AC electrodepositon in the electrolyte composed of1mmol-dm-3Pd(NO3)2+1mmol·dm3AgNO3,pH2～3. According to the results of cyclic voltammetry and AC voltammetry, it shows that the reaction was reversible and occurred in the form of diffusion contol. With the frequency increasing, the Ag content in Pd-Ag alloy nanowires was increased and the nanostructure became dendritic from amorphism. On the basis of the results of XRD and TEM, Pd-Ag alloy nanowires was single-crystalline with a face centered cubic (fcc) lattice. And the single-crystalline is with trunk grown along the<111> direction and two group of branches grown along the<200> and<111> directions, respectively.(3) Different Pd-Au alloy nanostructures were obtained between gold microelectrodes by dual frequencies AC electrodepositon in the electrolyte composed of1mmol·dm-3AuCl3·HCl·4H2O+5mmol·dm-3PdCl2+10mmol·dm-3HEPES pH3～4. The deposition of the nanowires was found to depend on the magnitude of the applied ac field, and we found that in each case a minimum threshold of electric field is necessary to initiate the process. The best range of low frequency for nucleation was200～300Hz. Along with the frequency increasing, the Pd-Au nanostructure grew in one dimensional direction. The main diffraction peak of XRD is<111> planes, which shows the growth direction of nanowires is<111>. Pd-Au alloy nanowires was single-crystalline with a face centered cubic (fcc) lattice. And this single-crystalline is with trunk grown along the<111> direction and two group of branches grown along the<200> and<111> directions, respectively.(4) The chances of nucleation from aqueous solution to gold microelectorde was lower because of the high n-DEP in the method of single frequency AC depsotion. In the two-step DEP process developed herein, the number of the nucleus would be high since the ac electric field is vibrated at a low frequency, approximately close to be a dc field. When the frequency was increased sharply at second-step, the continuous growth of the nanowires would be possible due to the lower activation energy barrier required for the second-step in-phase electrodepositions.(5) The Pd-Ag, Pd-Au alloy nanowires obtained between microelectrodes can be successfully assembled into a hydrogen sensor. In the same condition of Pd alloy hydrogen sensor, Pd-Au based sensor has wider response range of hydrogen concentration and shorter response time. The sensors showed excellent reproductionity and stability. The action of hydrogen with palladium alloy nanostructure is the mechanism of solution and diffusion, which is a process of surface chemical adsorption, surface infiltration diffusion and desorption. For the sensors assembled by nanowires between microelectrodes, the hydrogen sensing mechanism depends on Pd alloy volume expansion and formation of Pd-H.The innovations in the dissertation as bellow:(i) The smooth and parallel Pd-Ag alloy nanowires arrays with60～150nm diameter were obtained by single pulse electrodeposition on HOPG. There were less factors that affect this method, so it was very simple. The mechanisms of nucleation and growth were proposed to explame the deposition of nanowires on the step of HOPG.(ii) The fabrication of Pd alloy nanowires by dual frequency AC electrodepositon between microelectrodes from electrolyte was also an innovation. When single frequency AC electrodepositon was applied, the success ratio of nanowire assembly was low in the high frequency, but the amorphism depositions were obtained easily in the low frequency. The method of dual frequency AC electrodepositon with instantaneous nucleation in the low frequency at first, then progressive growth in the high frequency was suggested for the first time to solve the problem of low success ratio. Less-branch nanowires was obtained using this method. The problem of connection between nanowires with Cu wires was also resolved.(iii) The new pattern Pd alloy nanowires were fabricated by AC electrodeposition for the first time for sovling the lower hydrogen sensitivity due to "hydrogen embrittlement" caused by hydrogen absorbing of pure palladium.(iv) The mechanism of dual frequency AC electrodepositon was proposed to understand the growth low of alloy nanowires. |
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