Preparation Of Wall Thickness-tunable AgNPs-nitrogen-rich Carbon Nanotubes For H₂O₂ Sensing And Catalytic Oxygen Reduction

The nitrogen-rich carbon material can enhance the electrocatalytic activity of the material.Nitrogen-containing compounds are usually applied as both a carbon source and a nitrogen source,and the corresponding carbon material with uniform nitrogen doping can be directly obtained by one-step carbonization of nitrogen-containing organics.Carbon nanotubes with an open inner diameter and adjustable wall thickness have more catalytic active sites.As a nitrogen-containing polymer,polydopamine?PDA?is easily synthesized by self-polymerization of dopamine,PDA has strong adhesion,reducing ability,and is facilely converted into a nitrogen-rich carbon material by one-step high-temperature carbonization.The tube-like carbon prepared by the template method will have higher specific surface area,more catalytic active sites,and better conductivity and electron transfer properties.Silver is a common precious metal,which has the advantages of low price and good electrical conductivity.The nitrogen-rich carbon nanotubes combined with precious metal silver can further improve the catalytic activity of the material.In this paper,the nitrogen-rich carbon nanotubes doped with well-dispersed fine silver nanoparticles?AgNPs-NCNTs?were fabricated by carbonization of the silver-polydopamine?AgNPs-PDA?multilayers.The wall thickness of the nanotube is tunable by varying the numbers of the layer-by-layer alternate deposition of Ag and PDA on electrospun polystyrene?PS?fibers.The morphology of AgNPs-NCNTs was studied by different characterization techniques.AgNPs-NCNTs modified electrode was prepared by drop coating method for high sensitivity detection of hydrogen peroxide and oxygen reduction reaction.The main results and conclusions are as follows:PS nanofiber sheets were prepared by electrospinning,and PS was alternately immersed in dopamine and silver nitrate solution to obtain the multi-layers AgNPs-PDA.Finally,AgNPs-NCNTs with different wall thicknesses were obtained by carbonization.Scanning electron microscopy?SEM?and transmission electron microscopy?TEM?demonstrate that the tubular structure is tuable by varying the AgNPs-PDA layers,wall thickness adjustable between 30-90 nm,and the fine silver nanoparticles is uniformly distributed along the nanotube with the size of about 6 nm.When the AgNPs-PDA arrives at two layers,a uniform nano-tubular structure can be obtained.The corresponding AgNPs-NCNTs have wall thickness of about 40 nm,and the BET specific surface area of 245.05 m² g^-1.The elemental imaging analysis of TEM demonstrates uniform distribution of Ag and N elements on the tube surface.Characterization of X-ray photoelectron spectroscopy?XPS?proves that Ag nanoparticles were successfully reduced by PDA.X-ray diffractometry?XRD?indicates the face-centered cubic crystal structure of Ag nanoparticles.XPS further confirms that before carbonization,the N element mainly existed in the AgNPs-PDA sample in the form of pyrrole-N,while N element in the carbonized AgNPs-NCNTs was mainly graphite-N and pyridine-N forms.The AgNPs-NCNTs modified electrodes were prepared by drop-coating method,and their electrochemical properties were analyzed.Cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?demonstrate that the loading of Ag nanoparticles is beneficial to the enhancement of conductivity and catalytic performance.The electrochemical properties of AgNPs-NCNTs depend on the number of layers of AgNPs-PDA,and the performance of modified electrode with two layers was optimal.The AgNPs-NCNTs electrode shows excellent electrocatalytic activity to H₂O₂ reduction,with a low limit of detection?LOD?of 0.03 mM?S/N?3?and a high sensitivity of 1004.9?A mM^-1 cm^-2.The electrodes have good selectivity and anti-interference properties.The reaction mechanism of H₂O₂ reduction was proposed and verified.The reaction process mainly related to the disproportionation of H₂O₂ and the reduction of product O₂,which was verified by the dissolved oxygen?DO?test.Meanwhile,the oxygen reduction reaction?ORR?of AgNPs-NCNTs was studied.It is found that the electrode has excellent electrocatalytic for O₂ reduction.The rotating disk electrode?RDE?experiments verified that O₂ reduction was close to 4-electron reaction.500cycles of cyclic voltammetry scanning proved that the AgNPs-NCNTs composite catalyst was very stable for oxygen reduction.The AgNPs-NCNTs electrode is promising for the applicaton of ORR catalysis.