Ultra-thin carbon nanofibers based on graphitization of near-field electrospun polyacrylonitrile

Efficient wide-spectrum one-dimensional MWO4 (M = Mn, Co, and Cd) photocatalysts: Synthesis, characterization and density functional theory study

Broadening the absorption region to near-infrared (NIR) light is critical for the photocatalysis due to the larger proportion and stronger penetration of NIR light in solar energy. In the present paper, one-dimensional (1D) MWO4 (M = Mn, Co, and Cd) materials synthesized by electrospinning technique, were studied by combining the density functional theory (DFT) with experiment results, which possessed the enhanced light absorption capability within the range of 200-2000 nm. It was proved that in the ultraviolet-visible (UV-Vis) region, the absorption bands of CoWO4 and MnWO4 samples were attributed to the metal-to-metal charge transfer mechanism, while the absorption of CdWO4 sample may be referable to the ligand-to-metal charge transfer mechanism. In the near-infrared (NIR) region, the absorption of CoWO4 and MnWO4 primarily originated from the d-d orbital transitions of Mn2+ and Co2+. The photocatalytic experimental results showed that the degradation rates for bisphenol A (BPA) over CoWO4, MnWO4, and CdWO4 photocatalysts under UV-Vis/NIR light irradiation for 140 min/12 h were 78.8 %/75.9 %, 23.8 %/21.3 %, 12.8 %/8.7 %, respectively. This research offers the novel insights into the precise construction of tungstate catalytic systems and contributes to the advancement of UV-Vis-NIR full spectrum photocatalytic technology, and lays a foundation for a cleaner and more environmental-friendly future.

Fabrication of a three-dimensional micro/nanocarbon structure with sub-10 nm carbon fiber arrays based on the nanoforming and pyrolysis of polyacrylonitrile-based jet fibers

To produce a three-dimensional micro/nanocarbon structure, a manufacturing design technique for sub-10 nm carbon fiber arrays on three-dimensional carbon micropillars has been developed; the method involves initiating electrostatic jetting, forming submicron-to-nanoscale PAN-based fibers, and maximizing the shrinkage from polyacrylonitrile (PAN)-based fibers to carbon fibers. Nanoforming and nanodepositing methods for polyacrylonitrile-based jet fibers as precursors of carbon fibers are proposed for the processing design of electrostatic jet initiation and for the forming design of submicron-to-nanoscale PAN-based fibers by establishing and analyzing mathematical models that include the diameter and tensile stress values of jet fibers and the electric field intensity values on the surfaces of carbon micropillars. In accordance with these methods, an array of jet fibers with diameters of ~80 nm is experimentally formed based on the thinning of the electrospinning fluid on top of a dispensing needle, the poking of drum into an electrospinning droplet, and the controlling of the needle-drum distance. When converting thin PAN-based jet fibers to carbon fibers, a pyrolysis method consisting of the suspension of jet nanofibers between carbon micropillars, the bond between the fibers and the surface of the carbon micropillar, and the control of micropillar spacing, stabilization temperature, and carbonation rate is presented to maximize the shrinkage from PAN-based fibers to carbon fibers and to form sub-10 nm carbon fiber arrays between three-dimensional carbon micropillars. The manufacturing design of a three-dimensional micro/nanocarbon structure can produce thin PAN-based jet nanofibers and nanofiber arrays aligned on micropillar surfaces, obtain shrinkage levels reaching 96% and incorporate sub-10 nm carbon fibers into three-dimensional carbon micropillars; these actions provide new research opportunities for correlated three-dimensional micro/nanocarbon structures that have not previously been technically possible.

Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

Crystalline carbon nanowire arrays were fabricated taking advantage of near-field electrospinning and stress decyanation. A novel fabrication method for carbon nanowires with radii ranging from ~2.15 µm down to ~25 nm was developed based on implementing nitrogen pretreatment on the silica surface and then aligning polymer nanofibers during near-field electrospinning at an ultralow voltage. Stress decyanation was implemented by subsequently pyrolyzing a polymer nanofiber array on the silica surface at 1000 °C for 1 h in an N₂ atmosphere, thus obtaining a crystalline carbon nanowire array with a nanostructured surface. Various crystalline nanostructures were fabricated on the nanowire surface, and their electrochemical performance was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Crystalline carbon wires with diameters ranging from micrometers to submicrometers displayed carbon nanoelectrode-like behavior with their CV curve having a sigmoidal shape. A highly crystalline carbon nanowire array showed distinct behavior, having a monotonically increasing straight line as its CV curve and a semicircular EIS spectrum; these results demonstrated its ultrastable current, as determined by electron transfer. Furthermore, nanocrystalline-structured carbon wires with diameters of ~305 nm displayed at least a fourfold higher peak current density during CV (4000 mA/m²) than highly crystalline carbon nanowires with diameters of ~100 nm and porous microwires with diameters of ~4.3 µm.

Unraveling the electron transfer rates of highly crystalline carbon nanowires with surface oxides

In the development of glassy carbon fiber toward graphene fiber, highly crystalline carbon wires have attracted attention. More importantly, a charge cannot be accommodated at the surface of highly oriented pyrolytic graphite as it would be in a metal. In this work, we demonstrate that enhancing the decyanation reaction rate and reducing the nanowire diameter to below the crystallite size (≲50 nm) greatly contribute to the microstructure transformation of carbon from low crystalline glassy carbon to crystalline micro-structure. Using silica surfaces to limit the shrinkage of electrospun nanofibers during oxidation and carbonization, enhances the conversion of alcohol groups to normal carbonyl groups on the surface of the carbon wires derived from PAN fibers deposited with near field electrospinning (NFES). Cyclic voltammograms (CVs) on the carbon nanowires reveal that the enhancement of alcohol groups to normal carbonyl groups slows down the rapid electron transfer on glassy carbon electrodes. Using electrochemical impedance spectroscopy (EIS), we also establish that the electron transfer on the surface of highly crystalline carbon nanowires almost completely depends on the presence of oxygen groups. The highly crystalline structure of nanoscale carbon wires with a large amount of normal carbonyl groups exhibits an ultra-low electron transfer rate (less than 1.2 μm s^-1), showing the ability to make the charges reside on the highly crystalline carbon nanowires. The straight line in CV allows for EIS measurements at high alternating current voltages, improving upon the non-linearity of traditional electrochemical cells by overcoming the stochastic errors and the lower signal-to-noise ratio for ultra-sensitive biomolecule detection (≤25 mV). The latter could spur the development of a new generation of electrochemical cells and biomedical signal measurements.