| Graphitic carbon nitride(g-C3N4) has shown great potential in photocatalysis and electrocatalysis field for oxygen reduction reaction(ORR), but these problems including the high recombination ratio of carries,insufficient usage efficiency of sunlight and poor electron transfer ablity, have restricted its further applications. Fabricating g-C3N4 based heterojunction is an effective method for enhanced photocatalytic activity. The separation and migration of photogenerated carriers can be promoted by the internal field between heterojunction and therefore, the recombination probability can be reduced. The electron transfer ablity in g-C3N4 is so poor that it must be loaded on certain supporting material to insure better activity for ORR. Therefore, the development of more stable and advanced supporting material is highly desired.The paper is mainly focused on the two studies as follows. Firstly, to prepare a heterojunction photocatalyst with large surface area, the electrospinning technique combined with thermal treatment are used to synthesis g-C3N4/Ti O2 nanofibers. The factors in the preparation process are optimized; the composition, structure and photocatalytic performance of g-C3N4/Ti O2 nanofibers are discussed. Sencondly, to prepare a g-C3N4 supported electrocatalyst for ORR, the electrospinning, curing, high-temperature pyrolysis, and infiltration-thermal condensation are applied to synthesis g-C3N4/Si C ultrafine fibers. The factors in the preparation process are optimized. The composition and structure of g-C3N4/Si C ultrafine fibers are discussed. The electrocatalytic performance of g-C3N4/Si C ultrafine fibers for ORR is investigated and the impact of post thermal treatment is also studied.It is found that the g-C3N4 pieces content should be less than 0.2 g to electrospin green fibers of g-C3N4/Ti O2. The prepared nanofibers after 500℃ calcination are composed of g-C3N4, Ti O2 and N-Ti O2. The diameter is in the range of 100~200nm, with sheets-like structure embedded in and/or covered on the nanofibers. The g-C3N4/Ti O2 nanofibers exhibit excellent photocatalytic performance under simulated solar light, for which a degradation rate of 0.0311 min-1 is achieved from photocatalytic degradation of rhodamine B(which is 3.5 and 7.1 times of pure g-C3N4 and Ti O2 nanofibers, respectively) and a H2 production rate of 323.7 μmol h-1 g-1 can be reached from water splitting(which is 17.3 and 5.6 times of pure g-C3N4 and Ti O2 nanofibers, respectively). Also, the value can be enhanced to 8931.3 μmol h-1 g-1 when minim Pt is used as cocatalyst. The enhanced photocatalytic performance can be ascribed to the fabricated heterjunction structure between g-C3N4 and Ti O2(N-Ti O2), which provides a facile route for the migration and separation of photoinduced carriers. Besides, the mesoporous NFs with large surface area can provide more surface-active sites.The solvent should be the mixture of xylene: DMF: acetone=7:2:1(v:v:v), the relative humidity should below 70% to electrospin green fibers for porous Si C ultrafine fibers. The fabricated Si C fibers are hierarchically porous structure with a diameter of 3~4 μm. After entrapment with g-C3N4, the g-C3N4/Si C ultrafine fibers present enhanced activity for ORR than pristine Si C ultrafine fibers and g-C3N4. Post modification of g-C3N4/Si C ultrafine fibers can change the chemical state of N element, and thus influence the electrocatalytic activity for ORR. The sample treated at 700℃ displays better activity with a peak potential of-0.388 V and a pek current density of 2.30 m A cm-2 in the CV curve. The hierarchically porous Si C ultrafine fibers as a support can greatly enhance the mass and electron transfer ability and thus endow the prepared supported electrocatalys better catalytic activity for ORR. |
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