The Research On The Synthesis Of Pd-Ni Nanowires By Electrochemical Step-edge Decoration

Nanowires have great applicational prospects as functional materials, magnetic materials, electronic materials and sensors etc. Compared with other sensors, nanowire sensors have more advantages, such as a short response time, high sensitivity and high selectivity etc, so they have broad applications in chemistry and biology. Pd-Ni nanowire hydrogen sensors can overcome all the limitations of hydrogen sensors at persent, and have unexampled advantages, so they will become the most useful and the most promising hydrogen sensors. Nanowires synthesized by templates is short, and furthermore, it is difficult for them to be removed from the template successfully, these factors limit their applications in sensors and electronic devices. But step-edge decoration method can synthesize freestanding uniform nanowires, which length can add up to 700 μm and the diameter can be adjusted from 10 nm to 1 μm. What's more, nanowires synthesized on HOPG substrate also can be removed from it successfully. So the synthesis of nanowires on HOPG substrate by step-edge decoration are paied more and more attention to.This thesis focused on the synthesis of Pd-Ni nanowires in which the percentage of nickel ranging from 8% to l5% on HOPG by electrochemical step-edge decoration. In this thesis, the bath system in which palladium and nickel can co-deposit was established. The electrodeposition behaviors of palladium, nickel and their alloy in the baths composed of ammonia complexing agent, were studied by means of polarization curves. The effects of both bath compositions and process parameters on composition of Pd-Ni alloy electrodeposited at low speed were also discussed. On the basis of above researches, Pd-Ni nanowires were synthesized successfully on HOPG by electrochemical step-edge decoration under the control of potentiostatic triple-pulse or galvanostatic triple-pulse. The appropriate nucleation pulse parameters and growth pulse parameters for synthesis of Pd-Ni nanowires on HOPG under the control of potentiostatic triple-pulse were also explored. What's more, the effect of each pulse parameter on the alloy composition was also discussed. The appropriate bath formulas and deposition conditions for synthesis of Pd-Ni nanowires, in which the percentage of nickle ranging from 8% to 15%, were established. In this thesis, inductively coupled plasma atomic emission spectroscopy (ICP-AES) was employed to study the compositions of Pd-Ni alloy. Scanning electron microscopy (SEM) and Energy-dispersive X-ray analysis (EDS) were also employed to characterize the images and composition of Pd-Ni nanowires. The research results are summarized asfollows:1. The co-deposition potential of palladium and nickel from their simple ions baths and their baths composed of ammonia complexing agent was theoretically calculated by the Nernst equation. The calculation results showed that, in the simple ions baths, the difference between palladium deposition potential and nickel deposition potential was so big that it is difficult to gain satisfying Pd-Ni alloy composition. However, in the baths with ammonia complexing agent, the deposition potentials of palldium and nickle closed each other, so they can be co-deposited certainly.2. The polarization curves showed that both polarization and polarizability of nickel were larger than those of palladium, palladium promoted nickel deposition obviously, while nickel inhibited palladium deposition during the co-deposition process. The cyclic voltammogram revealed that palladium deposites drastically at -0.8 V around, nickel deposites drastically at -1.2 V around in single metal baths, respectively, but in the mixture baths of Pd²⁺ and Ni²⁺, Pd and Ni deposite simultaneously at -0.8 V around. The evolution of hydrogen is the main concomitant reaction going with palladium and nickel co-deposition. The higher pH value is, the more hydrogen evolves and the lower current efficiency is. Appropriate pH value of bath is controlled to 8.5 around, so as to improve bath stability and reduce hydrogen evolution.3. The composition of Pd-Ni alloy deposited at low speed is the function of bath formulas and process parameters. The experiment results revealed that Pd-Ni alloy composition mainly depended on Ni/Pd ions mass ratio in bath. The mass percentage of Ni in Pd-Ni alloy linearly increased with rise of the Ni/Pd mass ratio in bath. The Ni content in Pd-Ni alloy decreased when pH value was hoisted. The content of Ni in Pd-Ni alloy increased when current density increased or electrodeposition potential moved negatively. The electrodeposition potential effecting Pd-Ni composition was more sensitive and easier to be controlled. If the bath pH value was adjusted to 8.5 in baths of Ni/Pd ions mass ratio ranging from 0.1 to 0.4, Pd-Ni nanowire arrays in which nickel content ranging from 8% to 15% can be obtained at appropriate deposition conditions.4. Pd-Ni nanowires can be synthesized successfully on HOPG by electrochemical step-edge decoration under the control of potentiostatic triple-pulse or galvanostatic triple-pulse. The longer Pd-Ni nanowire arrays can be synthesized under the conditions of 50 ms⁵00 ms nucleation at the potential ranging from -1.0 V to -2.0 V...