Enhancement Of Electrocatalytic Performance By Electronic Structure Tuning In Single-Walled Carbon Nanotubes

Single-Walled Carbon Nanotubes(SWNTs)comprise metallic(m-)and semiconducting(s-)SWNTs.The diverse electronic properties of SWNTs are dependent on the geometric diversity.This factor not only endues SWNTs with lots of potential applications,but also restrains the commercialization of SWNTs devices,which is due to the requirement of practical devices on single and stable energy band structure.Therefore regulating the electronic properties of SWNTs is critical to the practical application of carbon nanotubes technonology.In my dissertation,a kind of small molecule,which was able to achieve the high-efficiency separation of m-SWNTs and s-SWNTs,was synthesized.The electrocatalytic activity of SWNTs was tuned by the combination with different aromatic molecules.The regulating patterns of electronic properties of SWNTs were inferred.And a reasonable elucidation for the differences on the activity of electrocatalytic CO2 reduction between m-SWNTs,s-SWNTs and TCNQ-doped s-SWNT was proposed.Moreover,by hydrothermal process MoS2/SWNTs composites were prepared.The influence from the concentration of SWNTs on hydrogen evolution reaction(HER)was studied.The details are as follows:(1)The band structures and corresponding densities of states(DOS)between metallic and semiconducting type single-walled carbon nanotubes are very different.The conduction band and valence band of m-SWNTs are connected,while s-SWNTs have band gaps.The DOS of material directly affects electrochemical performance.This section demonstrated that the difference between m-SWNTs and s-SWNTs on electrochemical properties.The experimental results showed that m-SWNTs exhibited better electrocatalytic properties than s-SWNTs,meanwhile s-SWNTs exhibited better supercapacitor performance(120 F/g vs.24 F/g)than m-SWNTs.Furthermore,it was discovered that small organic molecules with different electrical properties can be utilized to modify the electronic structure of SWNTs.With the help of electron-withdrawing molecules,the electronic structure of m-SWNTs was tuned.Hence the conduction band and valence band were separated to form a counterfeit-band.When the electron-donating molecules were employed,the conduction bands and valence bands in s-SWNTs were narrowed.The electronic structure adjusting effect of SWNTs was directly reflected in the electrochemical property change of SWNTs.The interaction of m-SWNTs and TCNQ leads to the impedance change towards s-SWNTs.The electrocatalytic performance turned to worse.In contrast,s-SWNTs displayed better capacitor performance with the interaction of pyrene derivatives(Dom P).Its impedance was similar to m-SWNTs,which had poor capacitance performance but exhibited excellent electrocatalytic properties.This pattern applies to not only SWNTs but also other sp2 carbon materials.(2)The efficient conversion of CO2 to other high value-added products,such as CO,is an important technology to relieve the greenhouse effect and create vast economic profits.In this section,the employment of single chiral SWNTs as the metal-free electrocatalyst and corresponding performance in electrocatalytic CO2 reduction were discussed.The electrocatalytic activities of m-SWNTs,s-SWNTs and TCQN-doped s-SWNTs in different electrolytes were studied.The results showed that their CO2 reduction performance in KHCO3 electrolyte were better than in KCl.The electrocatalytic reduction potential of CO2 for m-SWNTs was as low as-0.54 V vs.RHE.After doping with TCNQ,the catalytic activity of s-SWNTs was improved.XPS analysis of the influence of electronic state density on catalytic properties of SWNTs found that after regulating the electronic structure of s-SWNTs by TCNQ,the density of electron states of s-SWNTs was increased,so the performance for electrocatalytic CO2 reduction immediately was enhanced.The electrocatalytic reduction potential of CO2 was-0.907 V vs.RHE,and the Faradaic efficiency of CO was 20.1% when m-SWNTs was catalyst.Faradaic efficiency of CO was elevated to 15.1% with TCNQ-doped s-SWNTs catalyst.(3)The synthesis of new nanohybrid catalysts with tunable structure and exploring their synergistic effects has drawn intensive attention.In this project,amorphous molybdenum disulfide/single-walled carbon nanotubes(MoS2/SWNTs)composites are synthesized by a facile hydrothermal process with the assistance of L-cysteine.Through varying the mass ratio of sodium molybdate(as in situ Mo source)to SWNTs,this method provides a well-defined pathway to enable the morphological control over the evolution of MoS2 from nanospheres,nanofilaments,nanorods to nanosheets.Among as-obtained samples,the MoS2/SWNTs(1:1)hybrids,which displayed a three-dimensional(3D)architecture morphology consisting of nanorods,exhibited the highest activity in hydrogen evolution reaction(HER).Compared with pure MoS2,the MoS2 nanosheets decorated along the SWNTs have more highly exposed active edges,and the SWNTs in the center of the MoS2/SWNTs nanorods can enhance the conductivity.Through regulating the ratio between MoS2 and SWNTs,we obtained the balance of active sites and conductivity of MoS2/SWNTs.Here,MoS2/SWNTs nanorods exhibited a low overpotential of 195 mV at 20 m A cm-2 and a low Tafel slope of 41 mV dec-1 among the superior MoS2-based HER catalysts.