| Hydrazine is a kind of micromolecule chemical with various applications, such as chemical corrosion inhibitor, reducing agent, herbicides in agriculture, and also as the fuel for the rockets due to its large burning heat. However, its hyper toxicity during production, transportation, usage and residue treatment has drawn much attention, and it is reported that hydrazine can cause neurotoxin and carcinogenic problem even with a little amount. As a result, the way for detecting hydrazine accurately and quickly has become urgent for the scientific research and industrial production. Previously published works for detecting hydrazine methods include titrimetry and spectroscopy, which have complex operation and high cost. So the electrochemical method for detection hydrazine has been a research hotspot.Though the electrochemical method has a simple operation, accurate result and high sensitivity, the high overpotential for detecting hydrazine is a bottleneck for its real application. Choosing a suitable catalyst is a key factor for dissolving the problem. Noble metals such as platinum, gold, palladium and silver have active electric catalytic activity, but the high cost and rareness retard them for further application. Therefore, it is a tendency for exploring a cheap, stable catalyst with high electrical activity. For this purpose, various 3d transition metal oxides like ZnO, MnO2 and NiO have been widely investigated in this area. However, to the best of our knowledge, there is no report to use Co3O4 for electrochemical hydrazine detection, even it is used widely in the catalytic area.This paper mainly includes three aspects as follow:The first part is the synthesis of Co3O4 with different scales and different morphologies. Through changing reaction temperature, reaction time, concentration of complexing agent and surfactant, the precursor of the cobalt source (cobalt sulfate, cobalt chloride, cobalt nitrate and cobalt acetate), different morphologies of Co3O4 (nanoparticles, nanourchin, nanowire, nanoflower and nanoplate) can be obtained, achieving the morphology-controlled of the nanostructures. Furthermore, it reveals the formation mechanism of Co3O4 nanostructures with different morphologies. Then, different morphologies of Co3O4 nanostructure were used in electrochemical hydrazine detection, and the influence of morphology to detection sensitivity, linear range and detection limit was studied. The results show one-dimentional Co3O4 NWs has the best activity for the electrochemical hydrazine detection compared with the other Co3O4 morphologies. Co3O4 NWs has abundant pore structure because it was constructed by connected short nanorods, furthermore, the 3-d network formed by NWs with each other can obviously enhance electrochemical surface area. Finally, in order to improve the conductivity of Co3O4 nanostructure, in-situ synthesizing Co3O4 supported on carbon nanotubes (MWCNTs) and nickel foam which have high conductivity and using them for electrochemical detection of hydrazine. The results show their permance towards the hydrazine detection improved obviously compared with the single Co3O4 due to the improvement of the conductivity and dispersity of the Co3O4 nanostructure. Meanwhile, one-dimentional Co3O4/NiO was prepared through hydrothermal method. The sensitivity has improved obviously compared with Co3O4 NWs in electrochemical hydrazine detection. The specific of innovative results are as follows:(1) The Co3O4 nanoparticles (NPs) were prepared through a hydrothermal method. The results showed the NPs were cubic structure with a diameter of about 6 nm and dispersed homogeneously. It was used as the catalyst for the electrochemical detection of hydrazine and the response time was 7 s with a sensitivity of 22.2μA mM-1. The linear range was 20-400μM and the detection limit was 2.8μM (S/N=3).(2) Different morphologies of Co3O4 can be prepared through the hydrothermal method and the reaction time, reaction temperature, annealing temperature, the quantity of the urea and surfactant (CTAB) all can affect the morphology and structure of the product. We can get assembled porous Co3O4 nano-urchins, nano-wires, nano-flowers, and nano-plates by adopting different anions in the precursor.(3) One dimensional Co3O4 nanowires (NWs) can self-assemble into three-dimensional porous network through interactive connection. Its high crystallinity and special morphology can obviously enhance the electron transfer rate. As a result, it showed high electrochemical activity towards the oxidation of hydrazine with a sensitivity of 28.6μA mM-1, linear range from 20-700μM, and the detection limit was 0.5μM (S/N=3).(4) Carbon materials have excellent physical-chemical activity and high conductivity, especially for the MWCNTs which have high crystallinity. The composite of Co3O4 supported on MWCNTs was prepared and used for the detection of hydrazine. The results showed it has a high activity compared with the single catalyst. The response time was 5 s and the sensitivity was 34.5μA mM-1. Its linear range was 20-1100μM together with a detection limit was 0.8μM (S/N=3).(5) The transition metal oxide NiO has high electrochemical activity like Co3O4. NiO and Co3O4 were combined and the morphology was changed through changing the preparation condition. Compared with the single transition metal oxide, the bimetal oxide has a lower over-potential which is beneficial to the oxidation reaction as well as reducing the electric noise effectively during the detection of hydrazine. The response time was 4 s, the sensitivity was 36.1μA mM-1 and the linear range was from 20μM-0.8 mM along with the detection limit was 0.7μM (S/N=3) when it was used for the electrochemical detection of hydrazine.(6) Nickel foam is a good support material because it has high conductivity and large surface area. The needle like Co3O4 was supported on the nickel foam through a hydrothermal method. The sensitivity of this material has a large enhancement which is two orders compared with the aforementioned catalysts duo to the combination of the special properties of the nickel foam and the high electrochemical activity of the Co3O4. As a result, when it was used for the detection of hydrazine, the response time was 4 s with a linear range from 50μM-0.6 mM. The sensitivity was as large as 5880μA mM-1 and the detection limit was 0.4μM (S/N=3). |
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