Research Of Fabrication And Hydrogen Sensing Property Of Pd-Ni Alloy Nanostructures

Due to unique properties of optics, electricity, catalysis and good biocompatibility, recently, nanostructure material has been widely studied and applied in electronic equipment, sensor and so on. With the development of the research, fabrication methods of nanostructure material are increasing. This dissertation mainly gives an overview of the development of template method, self-assembly method, chemical vapor deposition and electrodeposition. Hydrogen gas sensor is an important application field of nanostructure material. Hydrogen sensors based on metal, alloy, metallic oxide and semiconductor nanostructure are introduced. Palladium based nanometer degree sensor because of small volume, applied in high pressure and without restriction by environmental, is one of the hydrogen sensors which are applied widely and researched deeply. However, the defects of pure palladium, such as easy deformation and phase transition, promote us to research palladium based alloy hydrogen sensitive materials, such as Pd-Ni alloy nanostructure. Owing to durability, quick response, difficulty of phase transition and deformation and H2S poison resistance of Pd-Ni alloy nanostructure, it is one of the important materials for high performance hydrogen sensor.Pd-Ni alloy nanowires with controllable composition and morphology are fabricated via electrodeposition on highly oriented pyrolytic graphite. In order to assemble a sensor easily, Pd-Ni alloy nanofilms and nanoparticles are synthesized on polyacrylonitrile carbon fibers and Pd-Ni alloy dendritic nanowires are synthesized on Au/Pt microelectrodes. Morphology, structure and composition of the alloy deposits were characterized by scanning electron microscope, energy dispersive X-ray and X-ray diffraction. The processes of three electrodepositions were described. The Pd-Ni 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 hydrogen sensors based on nanofilm, nanoparticles and nanowires were researched, respectively. The main results were gained as following:(1) In the electrolyte composed of70mmol·dm-3Pd(NH3)4Cl2+30mmol·dm-3NiSO4+0.2mol·dm-3NH4C1, pH8.5, Pd-Ni alloy nanowire array were fabricated successfully on fresh high oriented pyrolytic graphite by electrochemical step edge decoration. XRD spectrum indicated that the Pd-Ni nanowires have the alloy structure of face-centerd-cubic with high alloying degree, which mainly displays crystal plane {111}.(2) Through adjusting the electrodeposition parameters in nucleation and growth, the diameter, composition and morphology of the nanowires can be controlled. Nucleation affects nanowire deposition rate more than growth. When the deposition potential was between-0.35--0.88VSCE, the Ni content in deposit was between8-15wt%. Generally, by controlling the growth potential between-0.35--0.5VSCE, nanowires obtained were parallel and smooth. Using low concentration electrolyte (7mmol·dm-3Pd(NH3)4Cl2+3mmol·dm-3NiSO4+0.2mol·dm-3NH4Cl, pH8.5), less than100nm elegant Pd-Ni alloy nanowires were obtained.(3) The pre-treatment of carbon fibers is:to degum at400℃for60min, to clean in degreaser with ultrasonic washer for30min and to coarsen at40℃for60min. After degumming, degreasing and coarsening, carbon fiber surface is appropriate as substratum for electrodeposition.(4) Pd-Ni alloy nanoparicles on carbon fibers were fabricated with single pulse method, the nanoparticle density increased with deposition overpotential going up. Pd-Ni alloy nanofilms were fabricated via three pulse method. The tow nanostructrues with8-15wt%Ni content was obtained at the potential between-0.48--1.04VSCE.(5) In the electrolyte composed of3mmol·dm-3Pd(NH3)4Cl2+7mmol·dm-3NiSO4+0.2mol·dm-3NH4Cl, pH8.5, firstly deposited for5s applying13VPP,300Hz AC field then self-assembled for10min under13Vpp,300kHz,300nm dendritic nanowires with8-15wt%Ni content were synthesized at room temperature.(6) The three nanostructures of nanofilm, nanopaticles and dendritic nanowire can be successfully assembled into a hydrogen sensor, respectively. In0-2.8%and3.6-6%H2circumstance, the sensitivity of nanofilm sensor increases with hydrogen concentration rising, but in2.8-3.6%H2, it decreases with hydrogen concentration increasing. Sensitivity of nanopaticle sensor increases with hydrogen concentration rising between0-6%range. Response time decreases and recovery time lengthens with the increase of hydrogen concentration. Hydrogen sensor assembled by dendritic nanowires shows high sensitivity, quick response and recovery. The nanoparticles sensor is more sensitive than nanofilm sensor. When oxygen in air exists, the sensor is first recovered to baseline state.(7) The action process of hydrogen with Pd-Ni alloy nanostructure is surface chemical adsorption→surface infiltration→interior diffusion→desorption, and the mechanism is adsorption-solution-diffusion-formation into H2-desorption. Response current of the Pd-Ni alloy nanofilm sensor and nanoparticle sensor based on carbon fibers is related to the synthesis of Pd alloy volume expansion, formation of PdHx and change of energy barrier at the alloy/carbon fiber interface; for the sensor assembled by dendritic nanowires, response current depends on Pd alloy volume expansion and formation of PdHx.