Synthesis of Copper Oxide-Based Nanofibers Via Electrospinning and Their Utilization in the Removal of Sulfur Compounds from Gaseous Streams

Electrospinning is a fast-growing synthesis technique that allows for the fabrication of one-dimensional inorganic-organic nanofibers from solutions containing a metal precursor and/or a polymer. Fibers prepared via this method may be tuned to have high surface areas, superior mechanical properties, and a tunable porosity, which makes them ideal candidates in applications involving separation, catalysis, and reactive sorption. Although the electrospinning process is simple, versatile, and adaptable, a myriad of parameters ought to be regulated and optimized to fabricate ultrafine, smooth, continuous, and defect-controlled fibers for use as sorbents in the removal of toxic sulfur compounds.

In this work, polyvinylpyrrolidone-copper (PVP-Cu) nanofibers were electrospun from a solution containing PVP (MW=1,300,000, 360,000, or 40,000 g/mol) and copper nitrate trihydrate. Four key solution properties (polymer concentration, polymer molecular weight, metal concentration, and solvent identity) were systematically varied to investigate the effect of these parameters on PVP-Cu fibers morphology. The electrospinning solutions were prepared by dissolving PVP (0.0212-0.0576 g/cm³) in a solvent (methanol, ethanol, 1-propanol, or water). The copper precursor was dissolved in DI water while the copper nitrate to PVP weight ratio was varied from 1:4 to 1:1. The collected nanofibers were heat treated under air at 823 K for four hours at ramping rate of 2 K/min. The heat treatment was necessary to remove the PVP matrix from the fiber structure and form copper oxide (CuO). Various characterization techniques were employed in this work: thermogravimetric analysis (TGA) was used to assess the extent removal of the polymer upon heat treatment, X-Ray diffraction (XRD) was used to identify CuO phases and average crystallite sizes, scanning electron microscopy (SEM) was used to analyze nanofiber morphology, and both electron dispersive spectroscopy (EDS) as well as X-ray photoelectron spectroscopy (XPS) were used to determine the elemental composition of the fibers. Diameter size and distribution of PVP-Cu fibers were found using ImageJ software by means of converting pixels to length measurements using the SEM images scale bar.

This research effort shows that by varying the solution properties, PVP-Cu nanofibers with diameters ranging from approximately 50 to 550 nm can be formed with either ribbon-like or circular cross sections. Fused PVP-Cu nanofibers also formed in this work when the solvent was not allowed to evaporate properly. The solution viscosity, conductivity, and density as well as the solvent dielectric constant were found to be responsible for these morphological changes in PVP-Cu fibers.

After identifying the optimal solution properties ranges that yielded smooth, continuous and defect-free fibers, three CuO fibers with different mean diameters (70, 350, and 650 nm) and crystallite sizes were catalytically tested as sorbents for the removal of sulfur compounds. The fixed bed sorption reaction at room temperature demonstrated the sensitivity of fiber diameter (and thus crystallite size) on the amount of sulfur sequestered. The small reactive domains of the electrospun nanofibers and the thermodynamically favorable reaction between CuO and sulfur at low temperatures make CuO nanofibers an attractive option for the removal of sulfur compounds from gaseous streams.