Targeted morphology of copper oxide based electrospun nanofibers

Journal Pre-proofs Targeted Morphology of Copper Oxide Based Electrospun Nanofibers Faisal H. Alshafei, Dante A. Simonetti PII: DOI: Reference:

S0009-2509(20)30079-8 https://doi.org/10.1016/j.ces.2020.115547 CES 115547

To appear in:

Chemical Engineering Science

Received Date: Revised Date: Accepted Date:

26 August 2019 3 February 2020 4 February 2020

Please cite this article as: F.H. Alshafei, D.A. Simonetti, Targeted Morphology of Copper Oxide Based Electrospun Nanofibers, Chemical Engineering Science (2020), doi: https://doi.org/10.1016/j.ces.2020.115547

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Targeted Morphology of Copper Oxide Based Electrospun Nanofibers Faisal H. Alshafei1 and Dante A. Simonetti* Chemical and Biomolecular Engineering Department, University of California-Los Angeles, Los Angeles, CA 90095 (USA) 1Current

Address: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena CA 91125 (USA)

Corresponding author. Phone: 310-267-0169. Email: [email protected]

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ABSTRACT: In this work, CuO-based nanofibers were synthesized via electrospinning. Smooth, defectfree fibers with a diameter of 261 ± 63 nm were fabricated and characterized. Thermal treatment under air at 823 K transformed the smooth nanofibers to a network of segmented, macroporous CuO nanoparticles with an average fiber diameter of 160 ± 41 nm and a crystallite size of 55.4 nm. The effects of solution properties (polymer molecular weight, polymer/metal concentration, and solvent identity) and processing conditions (voltage, tip-to-collector distance, extrusion rate, and humidity) were also investigated. Solution properties were found to strongly influence viscosity, conductivity, dielectric constant, density, and surface tension, which invariably affected fiber dimension, morphology and surface structure. Fibers as thick as 536 nm and as thin as 70 nm with cylindrical and fused structures were produced by manipulating the solution properties. Processing conditions were found to moderately affect fiber uniformity and fiber diameter. Key Words: Electrospinning, Nanofibers, Parameters Effect, Copper Oxide, PVP

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1. Introduction Nanotechnology plays a vital role in energy applications as it provides essential improvement potentials for the enhancement of conventional energy sources and development of renewable sources to meet the world’s increased demand for energy.1 The interest in nanotechnology stems from the idea that nanomaterials have the potential to improve energy efficiency across multiple industries through novel and innovative solutions, prompting researchers to extensively research and systematically investigate their unique properties.2,3 The synthesis of nanostructures, particularly ones that are prepared from earth-abundant and inexpensive metals,4 that offer superior properties and catalytic performance is a major challenge across multiple disciplines. Nanofibers are a type of nanomaterials that have been used in a broad spectrum of research and commercial applications,5 including in energy-based applications.6 Various techniques have been exploited for the fabrication of nanofibers including drawing,7,8 template synthesis,9,10 phase separation,11–13 and electrospinning.14–18 Electrospinning, which is an electrostatic fabrication technique for the synthesis of long, continuous fibers, has received heightened interest due to the potential use of electrospun materials in catalysis,19–21 sorption,22,23 and separation,24–26. Unlike other techniques that are used for generating one-dimensional nanostructures, electrospinning is a relatively simple, easy to use, and a cost-effective synthesis method, which is based on uniaxial stretching.2,27 When compared to mechanical drawing, electrospinning is more suitable for producing fibers with ultra-thin diameters (<100 nm) because the elongation process is due to an external applied electric field. Recently, electrospinning has been extended to produce nanofibers made of ceramics,28,29 composite materials,30,31 and oxides.32–41 In fact, a variety of metal oxides have been fabricated

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using this technique and have shown potential in different engineering domains. The addition of metal particles into the polymer produces multifunctional nanofibers with enhanced properties.42,43 Quasi-cylindrical materials produced from electrospinning may be tuned to have high surface areato-volume ratios, superior mechanical properties, interconnected porous networks, and tunable porosities,3,27,44–47 thus, possibly out-performing quasi-spherical particles or powders. Although several metal-polymer combinations have been successfully electrospun, as reported in the literature,2,48,49 the majority of that work has dealt with the application side of things (or simply material synthesis); there is still no comprehensive work that systematically examines the impact of electrospinning synthesis conditions on morphology for metal-polymer systems (e.g., PVPCu(NO3)2). Previous research has shown that the properties of polymer-only fibers formed via electrospinning are determined by synergetic effects of the solution variables, electrostatic forces applied on the solution jet, and electrospinning operational conditions.50–59 Deitzel et al.,2 for example, examined fiber formation of polyethylene oxide (PEO) via electrospinning by changing the concentration of the polymer and the voltage applied; however, many critical parameters such as, polymer molecular weight, solvent properties, extrusion rate, distance between tip-of-needle and collecting plate, and relative humidity were not investigated in that work and the effects of metal addition were not tested. Similarly, previous work by Casper et al.60 focused only on the effects of molecular weight and atmospheric conditions on the surface properties of polystyrene (PS) fibers. Demir et al.61 studied the influence of electric field, temperature, conductivity, and viscosity of the solution on the electrospinning process and properties of polyurethane fibers. Recently, Shao et al.62 investigated the use of poly(vinylidene fluoride) (PVDF) nanofiber as an active layer for making mechanical-to-electrical energy conversion devices and in doing so

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examined the effect of polymer concentration, applied voltage, and spinning distance for that polymer. Moreover, Su et al.63 examined the doping effects of monovalent, bivalent, and trivalent metal ions on the morphological appearance of chitosan/PEO blend nanofibers. To complement the aforementioned literature studies, which were mostly limited to polymeric-only nanofibers, the work presented herein aims to elucidate the effects of several electrospinning parameters on fiber dimension, morphology and surface structure, for nanofibers comprised of Cu(NO3)2 and polyvinyl-pyrrolidone (PVP). PVP was used as a polymer source because it is an important amorphous polymer, which has low toxicity, excellent solubility in most organic solvents, capability to interact with a wide range of hydrophilic materials, and high biocompatibility.64 Additionally, PVP is considered an excellent steric stabilizer or a capping agent for various metal nanoparticles.65 Copper was selected as the metal source because of its broad application in catalysis, energy storage, gas and liquid sensors, and optoelectronic devices.66–71 Copper nitrate was used as a metal source because nitrates tend to dissolve easily in ethanol, which is the preferred electrospinning solvent.52 Ethanol plays a vital role in local polarization during the electrospinning process and has low surface tension, which encourages the formation of smooth, defect-free (i.e., free of micron-sized surface structural defects), and continuous nanofibers.49 The principal factors that influence the electrospinning process, which have been systematically examined in this study, can be divided into two broad categories: (1) solution parameters (polymer molecular weight, polymer/metal concentration, and solvent(s) identity) and (2) processing conditions (applied voltage, distance between tip-of-needle and collecting plate, extrusion flowrate, and relative humidity). The solution parameters were found to intrinsically impact the viscosity, conductivity, surface tension, boiling temperature, density, and dipole moment of the electrospinning solution while processing conditions were found to affect the travel

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time and the overall forces applied on and experienced by the travelling jet. To elucidate compositional, morphological, and structural information about the formed solids, the electrospun nanofibers were characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and high resolution TEM (HRTEM). We find that significant changes in fiber diameter, size distribution, and morphology accompany changes in these parameters. By optimizing and regulating the electrospinning parameters (particularly, polymer/metal concentration, solvent identity, and humidity), smooth, continuous, ultrafine and defect-controlled PVP-Cu(NO3)2 nanofibers were obtained. Thermal treatment was required to transform the as-synthesized PVP-Cu(NO3)2 fibers to CuO nanofibers. 2. Experimental Methods 2.1 Materials and Electrospinning synthesis Polyvinylpyrrolidone (M.W. 1,300,000, 360,000, and 40, 000; Sigma-Aldrich), copper (II) nitrate trihydrate (Sigma Aldrich, 99%), N,N-Dimethylformamide (DMF; Sigma-Aldrich, anhydrous, 99.8%), ethyl alcohol (Sigma-Aldrich, 100%), and 1-propanol (Alfa Aesar, 99+%) were purchased and used without subsequent purification. Polymer solutions were prepared by dissolving PVP (0.7-1.9 g) in solvent (methanol, ethanol, 1-propanol, DI water, or N,N-Dimethylformamide; 23 cm3). The polymer solution was then vortexed (Fisher Scientific Digital Vortex Mixer) at 3000 rpm for 1 h until the polymer was completely dissolved. The solution was left to settle for 10 minutes, transferred to a beaker, and stirred for 0.5 h. The metal containing solution was prepared by dissolving copper (II) nitrate (01.3 g) in 10 cm3 of DI water and stirring the solution for 0.5 h. The copper-containing solution was

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then added dropwise to the polymer containing solution. The solution was stirred for 0.25 h, and then vortexed for 0.5 h at 3000 rpm. The electrospinning solution was placed in a 10 mL syringe (BD 10 mL syringe with Luer LokTM tip) with a hypodermic needle (MonojectTM Standard 30G x ¾”). The distance between the tip of the needle and a stainless-steel collecting plate covered with aluminum foil was between 3656 cm. A Gamma High Voltage Research ES75 power supply was used to apply voltage (25-45 kV) while the metal-polymer solution was extruded through the needle at a rate controlled by a syringe pump (0.25-2.0 cm3 h-1; Kent Scientific Genie Plus). Dry air was circulated inside a 3 m3 chamber at 0.1-4 cm3 min-1 to control the relative humidity (20-60 ± 1%). All electrospinning was carried out at ambient temperature (294 K) and pressure (101 kPa). The collected electrospun fibers (over a 1 h period, unless otherwise noted) were thermally treated under air at 823 K for 4 h at a ramping rate of 2 K min-1. 2.2 Characterization The viscosity and conductivity of the electrospinning solutions were measured using Cannon-Fenske viscometers (size: 50, 100 and 200) and an Oakton conductivity meter (CON 6). The synthesized and thermally treated nanofibers were characterized using a variety of techniques. Powder X-ray diffraction patterns were obtained on an X-ray diffractometer (JEOL JDX-3530 and Philips X-Pert) using Cu Kα radiation of 1.5410 Å to identify the CuO phases and measure average crystallite sizes using the Scherrer equation. Thermogravimetric analysis was conducted on a Perkin Elmer TGA system. Alumina crucibles were used to hold the samples. Gas flow rates were set to 200 standard cm3 min-1. Heating was carried out under in-house dry air or argon (Matheson UHP Argon; 99.999% purity) at a ramping rate of 10 K min-1. Scanning electron microscopy and energy dispersive X-ray spectrometry (SEM/EDS; NOVA 230 Nano SEM) were used to determine

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the morphology and elemental composition of the nanofibers. The mean diameter and size distribution of the fibers were calculated from the SEM micrographs using ImageJ software (n = 250; where, n is the number of fibers that were measured and averaged). The FEI Titan 80-300 kV S/TEM was used to carry out the transmission electron microscopy (TEM) analysis. The TEM/HRTEM sample was prepared by drop casting an ethanolic dispersion of CuO nanofibers onto a carbon-coated Cu grid. XPS was conducted with PHI 3057 spectrometer using Mg Kα Xray source at 1286.6 eV. The XPS experiments were carried out in an ultra-high vacuum system with a base pressure of about 6 ×10-10 Pa. The XPS spectra were recorded in normal emission. For the XPS analyses, the Casa XPS 2.3.14 software was used. All binding energy values presented in this paper were charge corrected to C 1s at 284.8 eV. For the interpretation of the spectra, the literature positions were applied.72 3. Results and Discussion Due to the flexibility and adaptability of electrospinning, the materials that can be synthesized using this method and the applications in which they can be used in are wide in scope. In fact, the electrospinning process can be modified to yield fibers with different properties (e.g., dimensions, structures, morphology, porosity, etc.). Therefore, understanding how the fibers change, specifically, on a morphological-level with respect to these electrospinning parameters, is critical to their utilization. A baseline synthesis procedure (described in Section 2.1 and in Section S1 of the Supporting Information) yielded PVP-Cu(NO3)2 nanofibers that were long, smooth, and defect-free (i.e., free of micron-scale surface structural defects). Several characterization experiments (XRD, SEM, N2 physisorption, TEM, XPS described in detail in Section S1 (Figures S1-S6) of the SI) revealed that these fibers consist of amorphous PVP phases and Cu(NO3)2 species. Thermal treatment in air causes a compositional transition from a polymeric-based

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nanofiber to a CuO nanofiber via the removal of the majority of the PVP matrix and crystallization of CuO. To understand the effects of changing process parameters on the dimensionality and morphology of metal-polymer nanofibers, PVP-Cu(NO3)2 nanofibers were synthesized at various conditions (listed in Section S2 (Table S1) of the SI) using the baseline procedure, and the results of these studies are reported and discussed in the following sections. In this work, key solution parameters were systematically varied by changing the PVP molecular weight and concentration (viscosity), copper nitrate concentration (conductivity), and solvent identity, to examine their effects on PVP-Cu(NO3)2 nanofibers dimensions and morphology. The effects of solution properties are discussed in detail in the main text with some supporting Tables and Figures included in Section S2 of the SI. As in the case of solution parameters, the effects of electrospinning processing conditions (i.e., voltage supplied, extrusion rate, distance between the tip-of-needle and collecting plate, and humidity) were also studied, and the major conclusions from these studies are included in the main text. However, detailed descriptions of the results (and their supporting Figures and Tables) are included in Sections S3S4 of the SI. 3.1 Altering Solution Viscosity via Polymer Molecular Weight and Concentration Polymer molecular weight and polymer concentration are two key factors that influence the properties of electrospun nanofibers. Effectively, they are indicators of the extent of chain entanglements present in the electrospinning solution,73 which ultimately influence viscosity. Changes in viscosity have been shown to strongly affect the morphology of electrospun fibers for various polymer systems.50,53,60,74–82 In this work, three molecular weights of PVP (1,300,000; 360,000; and 40,000) were used to probe the effect of polymer molecular weight on viscosity, and thus, PVP-Cu(NO3)2 nanofibers

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diameter and morphology. In these experiments, three electrospinning solutions were prepared using similar amounts of PVP in each, corresponding to the aforementioned molecular weights. The other electrospinning parameters were the same as those described in Table S1 of the supporting information. Table S2 shows the viscosity and conductivity at room temperature (298 K) for solutions made with PVP MW=1,300,000 (56.23 cP), MW=360,000 (44.79 cP) and MW=40,000 (6.23 cP), indicating the effect of molecular weight changes. Despite showing a strong influence on viscosity, changes in molecular weight did not have a significant impact on conductivity as the recorded conductivity values were 16.66 mS cm-1 for PVP MW=1,300,000, 16.32 mS cm-1 for PVP MW=360,000, and 15.96 mS cm-1 for PVP MW=40,000. Table S2 also shows the average diameter as well as the standard deviation of PVPCu(NO3)2 nanofibers. The average diameter and standard deviation of the formed fibers were measured using ImageJ software (n=250). The trend observed from the SEM images shows that molecular weight has an instrumental effect on fiber dimensionality as well as on bead formation (Figure S7). Specifically, when the molecular weight of the polymer was reduced (thus, reducing viscosity), the nanofibers average diameter decreased. The PVP (MW=1,300,000) solution resulted in the formation of fibers that had an average diameter and standard deviation of 277 ± 79 nm whereas the PVP (MW=360,000) solution resulted in the formation of fibers that were 168 ± 38 nm (Table S2). The lower molecular weight solution, PVP (MW=40,000), did not form any fibers, as this solution’s viscosity (6.23 cP) was too low to electrospin. Figure S8 shows the diameter distribution of PVP-Cu(NO3)2 nanofibers. When the molecular weight of PVP was reduced from 1,300,000 to 360,000 it was observed that the diameter distribution of the fibers was lowered. This is probably attributed to jet splitting (or branching). It’s been suggested previously

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that the formation of branches or splitting in jets and fibers occurs in more viscous polymeric solutions.83,84 In this work, one by-product of altering the molecular weight of the polymer that was observed in the synthesized PVP-Cu(NO3)2 fibers under SEM was structural defects. In this set of experiments, structural defects with diameters in the micron-range (1-4 µm) formed when the molecular weight of PVP was lowered from 1,300,000 to 360,000 (Figure S7). Specifically, the solution that contained PVP (MW=360,0000) had both spindle-like and spherical defects. Structural defects of that magnitude may deleteriously impact the reactivity and/or performance of the formed fibers in certain applications, and thus, avoiding their formation may be critical for the fibers to work. Therefore, when working with lower molecular weight polymers, higher concentrations of the polymer are generally required to increase viscosity and stabilize the electrically charged jet. Although the choice of molecular weight was shown to influence viscosity and thus fiber diameter and morphology, concentration is another way to alter a solution’s viscosity and extent to which it produces fibers. To examine the effect of PVP (MW=1,300,000) concentration on the morphology of PVP-Cu(NO3)2 nanofibers, six different masses of PVP (0.7, 0.9, 1.1, 1.3, 1.5, and 1.9 g) were added to the electrospinning solution to yield solutions with the following polymer concentrations: 0.0212, 0.0273, 0.0333, 0.0394, 0.0455, and 0.0576 g cm-3, respectively (Table S3). Additionally, appropriate masses of Cu(NO3)2 were added to each solution to so that the PVP to Cu(NO3)2 weight ratio remains at 1 to 1 in all the electrospinning solutions. (All other electrospinning parameters were the same as listed in Table S1.) Figure 1 shows the measured dynamic viscosity values of the six solutions. The trend observed in Figure 1 reveals that viscosity increases exponentially as a function of PVP

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(MW=1,300,000) concentration (y = 9.5e44.6x, where “y” is viscosity (cP) and “x” is PVP concentration (g cm-3)). Gomes et al.85 investigated the synthesis of polyacrylonitrile (PAN) at different processing conditions and reported a similar exponential trend for viscosity as a function of polymer concentration for PAN (1-8 %) in DMF. In addition to the observed viscosity changes, increasing the PVP concentration in the electrospinning solution resulted in proportional increases in conductivity (because the metal concentration in this section was also varied to maintain a PVP to Cu(NO3)2 weight ratio of 1:1). The conductivity values measured experimentally for the six solutions were: 11.21 mS cm-1 (0.012 g cm-3), 12.32 mS cm-1 (0.0273 g cm-3), 14.49 mS cm-1 (0.0333 g cm-3), 16.66 mS cm-1 (0.0394 g cm-3), 18.00 mS cm-1 (0.0455 g cm-3), and 22.10 mS cm-1 (0.0576 g cm-3) (Table S3). Figure 1 shows how PVP-Cu(NO3)2 nanofiber dimensionality (average diameter and standard deviation) is influenced by the polymer concentration. Nanofiber diameter was found from the SEM images using ImageJ (Figure S9). The data in Figure 1 certainly shows a strong correlation between PVP (MW=1,300,000) concentration (as well as viscosity) and PVPCu(NO3)2 nanofiber diameter. When the PVP concentration was 0.0212 g cm-3, the PVP-Cu(NO3)2 nanofibers had an average diameter of 99 nm. Indeed, the electrospinning of this solution (0.0212 g cm-3) resulted in the formation of ultrathin nanofibers because of the relatively low viscosity of the solution (23.7 cP). Increasing the PVP concentration effectively increased the viscosity of the electrospinning solution. Therefore, when the PVP concentration was increased from 0.0212 to 0.0273 g cm-3, the viscosity increased accordingly from 23.70 to 31.75 cP, yielding fibers that were 190 nm (Figure 1). This trend was observed for all six concentrations that were tested, and agrees with the literature.47

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Region 2: thin, smooth, and cylindrical fibers (no beads) Region 1: ultrathin fibers with beads

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0 10 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060 Concentration of PVP (MW=1,300,000) in electrospinning solution (g cm-1) Figure 1. Dynamic viscosity of the electrospinning solutions (fitted using an exponential curve) and average diameter of PVP-Cu(NO3)2 nanofibers at six concentrations of PVP (MW=1,300,000). Each electrospinning solution was prepared using an appropriate amount of PVP and copper nitrate to maintain a copper nitrate to polymer weight ratio of 1:1, 23 cm3 of ethanol, and 10 cm3 of DI water. Electrospinning was conducted at 30 kV, 1.0 cm3 hr-1, 46 cm, and 20% relative humidity. Furthermore, based on the SEM images shown in Figure S9, Figure 1 was divided into three distinct regions: region 1 (0.0212 g cm-3), region 2 (0.0273, 0.0333, and 0.0394 g cm-3) and region 3 (0.0455 and 0.0576 g cm-3). Region 1 comprised of PVP-Cu(NO3)2 fibers that were ultrathin, but contained structural defects in the form of spherical or spindle-like beads. These defects likely formed because of capillary instabilities, which were caused by the reduction in the overall electrical charges experienced by the jet (due to low viscosity).27 Region 2 in Figure 1 comprised of PVP-Cu(NO3)2 fibers that were continuous, cylindrical, smooth, and free of micronsized surface defects. The nanofibers in this range were synthesized from solutions having optimal PVP MW=1,300,000 viscosity (~30-55 cP) and as such possessed good intermolecular interactions. Lastly, region 3 comprised of fibers having large diameters and ribbon-like structures 13

(Figure 1). The fibers in this region were prepared from solutions that contained high PVP concentrations (0.0455 and 0.0576 g cm-3) and as such, high viscosities (69.06 and 123.17 cP). Ribbon-like structures occur because at high polymer concentrations, the skin of the fibers solidify before all solvent molecules diffuse through, thus, forming hollow tubes, which later collapse.27 The results in Figure 1 demonstrate the importance of maintaining an optimal PVP concentration (region 2) to allow the electrospinning solvent ample time to evaporate to form smooth and cylindrical PVP-Cu(NO3)2 fibers, with minimal defects. In addition to its influence on diameter size, polymer concentration (much like PVP molecular weight) affected the distribution of PVP-Cu(NO3)2 nanofibers. Diameter distribution is a key and a real challenge in the synthesis of nanofibers via electrospinning because electrospun nanofibers are rarely uniform.52 Figure S10 shows the diameter distribution of the PVP-Cu(NO3)2 nanofibers that were electrospun. The results show that in order to obtain a narrow fiber diameter distribution, lower PVP concentrations (thus, lower viscosity) are required. The results in this section prove the importance of controlling viscosity as a solution parameter by means of changing PVP molecular weight and/or concentration. Indeed, by combining the results from the PVP molecular weight study (3 experiments) and the PVP concentration study (6 experiments), it can be shown that a linear relationships exists between PVP-Cu(NO3)2 nanofiber diameter and viscosity, irrespective of how the viscosity is altered (Figure 2). By simply changing the solution viscosity, PVP-Cu(NO3)2 fibers as thin as 99 nm and as thick as 540 nm were electrospun.

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Average Nanofiber Diameter (nm)

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0.058

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Figure 2. Average PVP-Cu(NO3)2 diameter as a function of dynamic viscosity. The electrospun fibers were synthesized using the method described in Section S1 and by varying the PVP concentration (0.021, 0.027, 0.033, 0.039, 0.045, 0.058 g cm-3; purple) and PVP molecular weight (1,300,000 and 360,000 g mol-1; green). Each electrospinning solution was prepared using an appropriate amount of PVP, 1.3 g of copper nitrate to yield the desired metal nitrate to PVP ratio (percentage), 23 cm3 of ethanol, and 10 cm3 of DI water. Electrospinning was conducted at 30 kV, 1.0 cm3 hr-1, 46 cm, and 20% relative humidity.

3.2 Controlling Solution Conductivity via Metal Concentration The previous section demonstrated that PVP-Cu(NO3)2 nanofibers properties (e.g., diameter and morphology) were influenced by the viscosity of the electrospinning solution. In this section, the effect of metal concentration (or simply, conductivity) is investigated by changing the weight ratio of Cu(NO3)2 to PVP in the electrospinning solution. Indeed, of all the electrospinning parameters that were studied and reported in the literature, the effect of metal to polymer ratio (or simply, metal concentration) was the least researched.50,61,86,87 To probe the metal concentration effect on PVP-Cu(NO3)2 fiber dimension and morphology, five solutions were prepared in which 1.3 g (0.0394 g cm-3) of PVP (MW=1,300,000) 15

was added while the weight ratio (weight percentage) of Cu(NO3)2 to PVP was changed to: 0% (0 g), 25% (0.325 g), 50% (0.65 g), 75% (0.975 g), and 100% (1.3 g). In this work, more emphasis was placed on the lower metal loadings (since the prior art shows that lower metal concentrations correspond to thinner and rougher fibers).88 Thin and rough fibers are generally desired especially in catalysis-based applications where rough fibers are believed to be more reactive. As in the previous section, electrospinning was conducted under the conditions described in Section S1. In this section, the metal-and-polymer-containing solution was electrospun for 12 h before the PVPCu(NO3)2 nanofibers were characterized (SEM) and thermally treated at 823 K for 4 h to form CuO nanofibers. Table S4 lists the viscosity and conductivity measurements of the five solutions described in the previous paragraph. The results indicate that adding Cu(NO3)2 to a PVP-containing solution does not significantly alter viscosity, as the viscosity of the 0% and 100% solutions were 52.76 to 56.23 cP, respectively; however, conductivity increased linearly as a function of copper nitrate concentration, as adding more metal effectively increased the number of ions present in the electrospinning solution (Figure 3). The solution that contained PVP only (0%), for instance, had a conductivity of 0.023 mS cm-1 while the solution with 1 to 1 Cu(NO3)2 to PVP ratio (100%) had a conductivity of 16.66 mS cm-1. Figure 3 shows the average diameter of PVP-Cu(NO3)2 and CuO nanofibers. The reported average diameters and standard deviations were measured using ImageJ, using the SEM images in Figure S11 (PVP-Cu(NO3)2) and Figure S12 (CuO). The SEM images show that when no metal was added to the electrospinning solution (0%), the PVP-Cu(NO3)2 fibers had a fused (i.e., crosslinked) structure, with an average diameter of 288 ± 66 nm. Crosslinking (or fusing) occurs due to inadequate drying of the fibers during the electrospinning process.52,89 The low conductivity

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(0.023 mS cm-1) of the electrospinning solution and high water content (as discussed in the next section) are responsible for the inadequate drying, and thus, the crosslinked structure. When the metal concentration is increased by increasing the Cu(NO3)2 to PVP weight ratio from 0 to 25%, the conductivity increased from 0.023 to 4.79 mS cm-1, respectively. This increase in conductivity caused the average diameter to drop from 288 ± 66 nm (0%) to 168 ± 47 nm (25%) (Figure 3). Fundamentally, conductivity is related to the number of charges carried by an electrospinning jet.90,91 Since electrospinning requires the charges on the surface of the jet to repulse to start the stretching process of the solution, having more charges (by increasing the number of copper and nitrate ions present in the solution) increases the stretching of the jet, which results in thinner fibers. These findings agree with the work of Qiu et al.35 in which they synthesized nickel and zinc oxide nanofibers at two metal concentrations and observed a decrease in diameter size when the metal nitrate concentration was lowered.

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Figure 3. Conductivity of five electrospinning solutions (fitted using a linear line), and average diameter of PVP-Cu(NO3)2 and CuO nanofibers (before and after thermal treatment at 823 K and 101 kPa for 4 h) at different Cu(NO3)2 to PVP (MW=1,300,000) weight ratios. Each electrospinning solution was prepared using 1.3 g of PVP, an appropriate amount copper nitrate to 17

yield the desired metal nitrate to PVP ratio (percentage), 23 cm3 of ethanol, and 10 cm3 of DI water. Electrospinning was conducted at 30 kV, 1.0 cm3 hr-1, 46 cm, and 20% relative humidity.

As the Cu(NO3)2 to PVP weight ratio was further increased from 25 to 50 to 75%, the average fiber diameter increased from 168 ± 47 nm to 175 ± 56 nm to 199 ± 57 nm, respectively, instead of decreasing (Figure 3). It’s likely that this moderate increase in PVP-Cu(NO3)2 diameter going from 25 to 50 to 75% was due to the competition between viscosity, which slightly increases with metal concentration and encourages the formation of thicker fibers, and conductivity, which increases with metal concentration and encourages the formation of thinner fibers. Figure 3 also shows that increasing the Cu(NO3)2 to PVP ratio from 3:4 to 1:1 resulted in a significant increase in fiber diameter (261 ± 63 nm). Figure S13 shows the diameter distribution of the five PVP-Cu(NO3)2 fibers discussed in this section. It was found that increasing the metal concentration from 25% to 100% resulted in a slight increase in the PVP-Cu(NO3)2 diameter distribution, which is likely due to the slight increase in viscosity, which encourages jet splitting (as discussed previously). Even though the effect of Cu(NO3)2 to PVP weight ratio on PVP-Cu(NO3)2 nanofiber diameter was modest prior to thermal treatment, as shown in Figure 3, the fibers after thermal treatment in air at 823 K and 101 kPa for 4 h showed inordinate sensitivity to this electrospinning synthesis parameter. In fact, the CuO fiber diameter increased almost linearly between 25 to 75% (copper nitrate to PVP weight ratio), changing from 142 ± 40 to 394 ± 128 nm, respectively (Table S4). Also, the gap between the nanofiber diameter before and after thermal treatment was found to be larger at the higher metal nitrate to PVP ratios than at the lower ones, reaching 433 nm (after thermal treatment) compared to 261 nm (before thermal treatment) at the 100% case compared to 142 nm (after thermal treatment) and 168 nm (before thermal treatment) at 25%. Figure S12 shows the SEM images of CuO fibers after thermal treatment. The SEM images reveal that the CuO fibers appear to comprise of small, agglomerated CuO particles. The PVP fibers that contained no metal (0%) burnt off completely when thermally treated under air at 823 K. 18

Figures S13-S14 show the diameter distributions of the PVP-Cu(NO3)2 and CuO nanofibers. The results demonstrate the importance of lowering the metal nitrate concentration in the electrospinning solution. By lowering the Cu(NO3)2 concentration, the formed CuO fibers not only had smaller average diameters, but also smaller distributions (Figure S13). All the CuO nanoparticles synthesized in this section suffered from some degree of particle agglomeration upon thermal treatment, which was expected due to the elevated thermal treatment conditions used in this work.92 Generally, the CuO fibers that contained a higher concentration of Cu(NO3)2 suffered from a higher degree of particle agglomeration and were larger in size, as evidenced by SEM (Figure S12). Moreover, it was found that the thermal treatment of a thick layer of PVP-Cu(NO3)2 nanofibers (after 12 hours of electrospinning as shown in Figure S12) as opposed to a thin layer (after 1 hour of electrospinning as shown in Figure S2) for the same electrospinning solution (1:1 copper nitrate to PVP ratio) encouraged the formation of larger and more agglomerated CuO particles, which affected the nanofibers morphology and porosity (overall void space and macro-porosity).

3.3 Influence of Solvent Properties and Composition 3.3.1 Single Polymer Solvent Systems The choice of solvent is critical for the transformation of the electrospinning solution to solid fibers.90 An amalgam of solvents have been used and reported in the literature for electrospinning nanofibers; they include formic acid, dimethyl formamide (DMF), chloroform, water, methanol, dichloromethane, tetrahydrofuran, and acetic acid.93,55 When it comes to PVPbased fibers, ethanol has been the most commonly used solvent due to its properties (e.g., low boiling temperature, low surface tension, high dielectric constant, and low viscosity). To investigate the effect of solvent choice on PVP-Cu(NO3)2 morphology, five solutions were prepared using three alcohols (methanol, ethanol, and 1-propanol), water, and DMF. These solvents acted as single polymer solvents (23 cm3) while DI water remained as the Cu(NO3)2 solvent (10 cm3). All other electrospinning parameters remained the same as discussed in Section

19

S1. Methanol, ethanol and 1-propanol were specifically selected in this work because of their properties (shown in Table 1) and because they all contain the same functional group (-OH), and as such, comparisons can be made regarding the interactions between the solvent and PVP/Cu(NO3)2. The data shows that the three alcohols have roughly similar dipole moment and surface tension values; however, their boiling temperatures, densities, and dielectric constants differ. Water has a considerably high dielectric constant (80.1) when compared to the other solvents while DMF has a high dipole moment (3.82 D).

Table 1. Key properties (boiling temperature, surface tension, dielectric constant, density, viscosity, and dipole moment) of the solvents used in the fabrication of PVP-Cu(NO3)2 nanofibers. Solvents

Boiling Surface Temperature Tension (K) (mN/m)

Dielectric Constant at 293 K

Density (g/cm3)

Viscosity Dipole at 300 K Moment (cP) (D)

Methanol

337.85

22.6

32.6

0.792

0.56

1.69

Ethanol

351.55

22.2

24.3

0.789

1.10

1.66

1-Propanol

370.15

23.7

20.1

0.803

1.96

1.68

Water

373.15

72.8

80.1

0.998

0.89

1.87

N,N-Dimethylformamide

426.15

37.1

36.7

0.944

0.96

3.82

20

Figure 4 shows the average diameter and standard deviation of the PVP-Cu(NO3)2 nanofibers synthesized using the polymer solvents mentioned in the previous paragraph. Methanol, ethanol, and 1-propanol resulted in the formation of fibers that were 78 ± 23 nm, 261 ± 63 nm, and 363 ± 151 nm, respectively. The trend exhibited by the three alcohols suggest that the diameter of the PVP-Cu(NO3)2 fibers was affected by dielectric constant and viscosity. Methanol had the highest dielectric constant (32.6) and lowest viscosity (0.56 cP), and consequently, it had the thinnest nanofibers. Of the three alcohols, 1-propanol had the highest viscosity (1.96 cP) and lowest dielectric constant (20.1), as shown in Table 1, and as such it had the thickest fibers. Dielectric constant has been shown previously to affect fiber diameter, as both conductivity and dielectric constant relate to the charge density in solution.56,58,93 The fibers that were formed using 1-propanol also had a fused structure (Figure 4). From the data in Table 1, it can be rationalized that fusing occurred because 1-propanol had the highest boiling temperature (370.15 K) of the three alcohol solvents, and thus, did not dissolve upon reaching the collecting plate at the tested experimental conditions.

21

Figure 4. Solution properties (density and conductivity), fiber properties (average diameter, standard deviation, and morphology), and structural defects (type and diameter) of PVP-Cu(NO3)2 fibers prepared using different polymer solvents: (A) methanol, (B) ethanol, (C) 1-propanol, and (D) N,N-dimethylformamide (DMF). Each electrospinning solution was prepared using 1.3 g of PVP, 1.3 g of copper nitrate, 23 cm3 of solvent, and 10 cm3 of DI water. Electrospinning was conducted at 30 kV, 1.0 cm3 hr-1, 46 cm, and 20% relative humidity. Even though the results in Figure 4 for the three alcohols indicate the effect of boiling temperature, dielectric constant, and viscosity, on fiber diameter and morphology, the data for water and DMF suggest that density (which affects the kinematic viscosity of a solution) and surface tension also play a role in the suitability of a solvent for electrospinning. When water was tested as a polymer solvent, no fibers were formed. Instead, discrete droplets were collected on the 22

aluminum foil. Water, when compared to the other solvents, has a considerably high boiling temperature (373.15 K). Indeed, the high boiling temperature of water and its low volatility are likely the two main reasons why no fibers formed when water was used as a solvent. Water also has high surface tension (72.8 mN m-1), the highest of all the solvents tested (Table 1). It’s been suggested previously that the surface tension of a fluid favors the formation of sphere-like shapes that have smaller surface areas per unit volume.27,56 Therefore, solutions that have high surface tensions such as, water, essentially resist the jet formation process (i.e., forming a Taylor cone).52 Unlike water, DMF formed crosslinked (yet beaded) PVP-Cu(NO3)2 fibers that had an average diameter of 82 ± 29 nm (Figure 4). At similar experimental conditions, the average diameter of the PVP-Cu(NO3)2 nanofibers formed from the DMF- (82 nm) and methanolcontaining (78 nm) solutions were roughly similar. DMF has a dielectric constant and viscosity value of 36.7 and 0.96 cP, respectively, which are close to those of methanol (32.6 and 0.56 cP) Table 1). As mentioned previously and reported in the literature,58,94–96 solvents that have high dielectric constants and low viscosities tend to form thinner fibers. The main differences between the fibers formed from methanol and DMF as seen from the SEM images in Figure 4 lies in morphology. Since DMF has a high boiling temperature, DMF did not adequately evaporate prior to reaching the collecting plate, thus, forming fused fibers as opposed to cylindrical, uniform fibers. Figure S15 shows the diameter distributions of PVP-Cu(NO3)2 nanofibers. Both methanol and DMF had narrow diameter distributions compared to those of ethanol and 1-propanal (widest diameter distribution). The results in this section show the complexity of the electrospinning process and the involvement of many factors (e.g., solubility, dielectric constant, surface tension, boiling temperature, and viscosity) in the selection of a solvent, which is consistent with previous

23

reports.50,55–58,94–98 Because each solvent has a unique set of properties—some that are suitable for the electrospinning process while others might be deleterious, often, to optimize the effect of solvent on fiber diameter and morphology, more than one solvent is used. This approach of using co-solvents is studied for a binary solvent system (ethanol and water) and a ternary solvent system (water, ethanol and DMF) in the next two sections. 3.3.2 Binary Solvent System (Ethanol and Water) In the previous section, five solvents (methanol, ethanol, 1-propanol, water and DMF) were examined as possible solvents for PVP while the Cu(NO3)2 solvent (water) remained unchanged. Those results demonstrated the suitability of ethanol and water when used as co-solvents for the electrospinning of PVP-Cu(NO3)2 fibers. In this section, instead of having a polymer solvent (23 cm3) and a metal solvent (10 cm3) as two separate solvents, one binary solvent system of a total volume of 33 cm3 was used. To investigate the effect of a binary solvent system comprising of ethanol and water on the morphology of PVP-Cu(NO3)2, the ethanol to water volumetric ratio in the electrospinning solution was varied (100:0, 70:30, 50:50, 30:70, and 0:100). All other electrospinning parameters remained the same as described in Section S1. Figure 5 shows the SEM images of the PVP-Cu(NO3)2 nanofibers synthesized at the five volumetric ratios listed previously. When the ethanol to water ratio was 100:0 (pure ethanol solution of 33 cm3), the solution gelled when the metal-containing solution was added to the PVP solution. As such, this solution was not electrospun. The solutions that contained 30:70 and 100:0 ethanol to water volumetric ratios also did not electrospin. The reason these two solutions failed to electrospin was because they comprised of mostly water, which in the previous section was shown to have high surface tension. Indeed, the results demonstrate that water is an inadequate electrospinning solvent for the synthesis of PVP-Cu(NO3)2 nanofibers at these experimental

24

conditions (solution parameters and processing conditions). The two solutions that electrospun and formed fibers were the ones that comprised of 70:30 and 50:50 ethanol to water. The 50:50 solution resulted in the formation of thinner fibers (70 ± 21 nm) (Figure 5). The 70:30 solution, on the other hand, formed fibers with diameters of 260 ± 63 nm (Figure 5).

Figure 5. SEM images, average diameter and standard deviation of PVP-Cu(NO3)2 nanofibers synthesized at different volumetric ratios of ethanol to water: (A) 70:30, and (b) 50:50. Each electrospinning solution was prepared using 1.3 g of PVP, 1.3 g of copper nitrate, and 33 cm3 of ethanol and water in the appropriate volumetric ratio. Electrospinning was conducted at 30 kV, 1.0 cm3 hr-1, 46 cm, and 20% relative humidity. The results indicate that water encourages the formation of thinner PVP-Cu(NO3)2 fibers (when a small amount of it is added as a part of a binary solvent system) because of its high dielectric constant (80.1) and low viscosity (0.89 cP) (Table 1). When added in small volumetric ratios to ethanol, water effectively increases the dielectric constant of the PVP and copper nitrate electrospinning solution and lowers its viscosity, resulting in the fabrication of ultrathin fibers. The trend observed in this work for a co-solvent comprising of water and ethanol agrees with the findings of Fong et al.,50 who reported that increasing the ethanol to water ratio formed smoother and larger poly(ethylene oxide) fibers. 3.3.3 Ternary Solvent Systems (Ethanol, Water and DMF) 25

A ternary solvent system of a total volume of 33 cm3 is investigated in this section. DMF was shown in this work to form very thin, yet fused fibers. Since ethanol and water were proven to yield smooth and defect-free PVP-Cu(NO3)2 fibers when they were added as a binary solvent system, they were combined in this section with DMF in different volumetric ratios to test their ability to form ultrathin PVP-Cu(NO3)2 fibers. Five solutions were prepared with the following ethanol to water to DMF volumetric ratio: (70:0:30, 70:15:15, 70:22.5:7.5, 50:0:50, and 50:25:25). In all these electrospinning solutions, ethanol made up the volumetric majority of the ternary solvent system. All other electrospinning parameters remained the same as described in the SI. Figure 6 shows the SEM images and average diameters of the PVP-Cu(NO3)2 nanofibers. Solutions that contained only ethanol and DMF (70:0:30, 50:0:50) resulted in the formation of crosslinked and relatively thick fibers, as the average diameter of the PVP-Cu(NO3)2 fibers prepared from these electrospinning solutions were 338 ± 126 and 259 ± 104 nm, respectively. The high boiling temperature (426.15 K) of DMF affected the evaporation rate of the electrospinning solution, resulting in fused fibers (Table 1). On the other hand, the solutions that contained ethanol, water and DMF (70:15:15, 70:22.5:7.5, 50:25:25; ethanol to water to DMF volumetric ratio) resulted in the formation of mostly smooth and defect-free PVP-Cu(NO3)2 nanofibers. As shown in Figure 6, the solution that contained 50:25:25 (ethanol to water to DMF ratio) had the thinnest PVP-Cu(NO3)2 fibers of the five solutions tested (108 ± 31 nm). The results in this section indicate that DMF when combined with ethanol and water results in the fabrication of thin yet crosslinked nanofibers. Although most of the electrospinning literature has focused on the synthesis and application of smooth and cylindrically-shaped nanofibers, crosslinked fibers are preferred in many biomedical applications due to their enhanced mechanical properties.99

26

Figure 6. SEM images, average diameter and standard deviation of PVP-Cu(NO3)2 nanofibers synthesized at different volumetric ratios of ethanol to water to DMF: (A) 70:0:30, (B) 70:15:15, (C) 70:22.5:7.5, (D) 50:0:50, and (E) 50:25:25. Each electrospinning solution was prepared using 1.3 g of PVP, 1.3 g of copper nitrate, and 33 cm3 of ethanol, water, and DMF in the appropriate volumetric ratio. Electrospinning was conducted at 30 kV, 1.0 cm3 hr-1, 46 cm, and 20% relative humidity. 3.4 Controlling Electrospinning Processing Conditions for the Synthesis of Smooth and Defect-Controlled PVP-Cu(NO3)2 Fibers Sections 3.1-3.3 demonstrated that polymer molecular weight, polymer concentration, metal concentration, and solvent choice substantially affect the intrinsic electrospinning solution properties (e.g., viscosity, dielectric constant, surface tension, conductivity, boiling temperature, dipole moment, and density), which have been shown to influence the morphology of the synthesized PVP-Cu(NO3)2 fibers. The electrospinning processing conditions (voltage supplied, extrusion rate, distance between the tip-of-needle and collecting plate, and humidity) were also systematically varied to explore their effects on nanofiber morphology. Our results demonstrate 27

that factors which do not directly and strongly influence the evaporation of solvents (voltage, extrusion rate, and tip-of-needle and collecting plate distance) do not have substantial effects on fiber morphology (see details in Section S3 of the SI). However, the relative humidity under which electrospinning occurs impacts fiber morphology and size to similar extents as the solution properties as we discuss these effects in the following section. Interactions between the surrounding atmosphere (in the form of relative humidity) and the electrospinning jet have been reported to influence the morphology and properties (e.g., mechanical strength) of the synthesized nanofibers.60,76,100–105 In this section, the standard PVP and Cu(NO3)2 solution was electrospun at different relative humidity levels (20, 30, 40, 50 and 60%). All other parameters were the same as those described in Section S1. Figure 7 and Table S8 show the average diameter of PVP-Cu(NO3)2 nanofibers, which were measured using ImageJ software (n=250), as a function of relative humidity. The data collected from the measurements made on the SEM images (Figure S25 in Section S4) shows that when relative humidity was increased from 20 to 50%, the average fiber diameter decreased from 271 ± 81 nm to 63 ± 21 nm, respectively (Table S8). Indeed, Figure 7 demonstrates a strong correlation between relative humidity (between 20 and 50%) and fiber diameter. The diameter of PVP-Cu(NO3)2 nanofibers was sensitive to the relative humidity in the electrospinning chamber due the absorption of water from the atmosphere to the surface of the jet/fiber.101 These water molecules interfered with the solidification process of the charged jet (effectively hampering it), which led to inadequate drying, which invariably influenced the PVP-Cu(NO3)2 fiber morphology. By absorbing more water, the viscosity of the jet changes—the same way a lower PVP concentration would result in thinner nanofibers and more structural defects (as discussed previously).

28

Nanofiber Diameter (nm)

400 350 300 250 200 150 100 50 0 10

20

30 40 50 Relative Humidity (%)

60

70

Figure 7. Average diameter of PVP-Cu(NO3)2 nanofibers at five relative humidity levels (20, 30, 40, 50, and 60 %). Each electrospinning solution was prepared using 1.3 g of PVP, 1.3 g of copper nitrate, 23 cm3 of ethanol, and 10 cm3 of DI water. Electrospinning was conducted at 30 kV, 46 cm, and 1.0 cm3 hr-1. Figure S25 shows the SEM images of the PVP-Cu(NO3)2 nanofibers. At 20% relative humidity, smooth, continuous and defect-free nanofibers were formed. As the relative humidity was increased, however, the SEM images show that although the diameter of the spun fibers decreased (up until 50% relative humidity), the PVP-Cu(NO3)2 fibers formed had numerous and sizable defects. More defects formed as the humidity was increased from 30 to 40 to 50 to 60%. The structural defects that formed at these humidity levels were spindle-like and spherical in shape, and their density and sizes increased with increasing humidity. For instance, at 30% humidity, spindle like structures with diameters of 0.4 µm and spherical beads with diameters of 1.2 µm were present in the fibrous structure (Table S8). When the humidity was 60%, however, there were no spindle-like defects but the spherical defects were 3.7 µm in diameter (on average) and they covered the fiber structure almost entirely (Figure S25). 29

Figure S26 shows the diameter distribution of the PVP-Cu(NO3)2 fibers at different relative humidity levels. The figure shows that, in general, increasing humidity decreases the diameter distribution of the fibers. Interestingly, as humidity was further increased from 50 to 60%, the fiber diameter as well as distribution increased rather than continue on a decreasing trajectory (Figure S26). This might be due to the presence of structural defects, which became abundant and significant in terms of density and size (3.7 µm) at 60% relative humidity, as discussed in the previous paragraph (Table S8). The results in this section show that in the case of PVP-Cu(NO3)2 fibers (prepared from PVP, copper nitrate, ethanol and water) humidity encouraged the absorption of water into the electrospinning jet, which decreased the diameter of the synthesized fibers. Nazarati et al.76 investigated the effects of humidity on poly(carbonate urethane) (PCU) fibers. The authors reported an increase in fiber diameter with increased relative humidity (5 – 75%), which disagrees with the results in this section. The contradiction in the effect of humidity on morphology between our work and theirs can be explained by looking at the properties of the polymer and solvent selected in each work. PVP, which was used in this work, is more soluble in water than PCU. When water insoluble polymers like PCU are used in electrospinning, the water vapor present in the air is not absorbed by the polymer and instead it precipitates (rather than get absorbed). The precipitation of water molecules on the jet as it undergoes the stretching process hinders the elongation of the polymer or metal-polymer jet and evaporation of the solvent, which leads to the formation of fibers with larger diameters. The results in this section prove that polymer hydrophobicity and solvent miscibility are good predictors of whether humidity will have a positive or a negative effect on the synthesized fiber diameter. Given these results, humidity can be used as a tool for regulating the morphology of metal-polymer nanofibers.

30

4. Conclusion Electrospinning was shown in this work to be an easy and effective technique for the fabrication of continuous, smooth and defect-controlled PVP-Cu(NO3)2 fibers. Thermal treatment of the synthesized PVP-Cu(NO3) nanofibers (prepared from the standard solution) at 823 K was shown to alter the morphology of the spun fibers (261 ± 63 nm) from smooth and continuous to a connected network of segmented and agglomerated CuO nanoparticles, with a crystallize size of 55.4 nm and an average CuO fiber diameter of 160 ± 41 nm. SEM and TEM analysis further showed that the thermally treated CuO fibrous structure was macroporous. Parameters that influence the electrospinning process (solution properties and processing conditions) were investigated to determine their effects on PVP-Cu(NO3)2 nanofiber dimension and morphology. Viscosity, conductivity, dielectric constant, surface tension, boiling temperature, and density were altered by systematically changing the PVP-Cu(NO3)2 solution properties (polymer molecular weight, polymer/metal concentration, and solvent identities). The experimental results demonstrated that fiber morphology and dimension were affected by the choice of electrospinning material (polymer, metal and solvent(s)). PVP molecular weight and concentration were found to strongly impact the viscosity of the PVP-Cu(NO3)2 solution (thus, fiber morphology), forming fibers with diameters as thin as 100 nm and as large as 540 nm. Cu(NO3)2 concentration was found to greatly affect the conductivity of the spun solution. Although metal concentration did not greatly affect the morphology of PVP-Cu(NO3)2 fibers (prior to thermal treatment), particularly at the low metal to polymer weight ratios, the thermally-treated CuO showed great sensitivity towards this parameter. The investigation of solvent choice showed that ultra-thin, defect-free (i.e., free of micron-sized surface defects) nanofibers can be fabricated with the right choice of solvent or by combining solvents in different volumetric ratios. Methanol

31

and water (78.31 nm), DMF and water (82.89 nm) and ethanol and water (70.32 nm) were found to be promising binary solvent systems for the synthesis of PVP-Cu(NO3)2 fibers with diameters less than 100 nm. Processing conditions (voltage, distance between tip-of-needle and collecting plate, and extrusion flowrate) were found to only moderately affect the morphology, diameter and distribution of the PVP-Cu(NO3)2 fibers, especially when they were within an acceptable range to permit electrospinning. When they were outside this range, structural defects in the form of spindle-like fibers and spherical beads were observed. Humidity (a processing parameter), though, was found to greatly influence the morphology of PVP-Cu(NO3)2 nanofibers. Highly defective, yet very thin (63.4 nm) fibers, were formed at elevated humidity levels (50 %). Although many challenges exist in the electrospinning process, this work has demonstrated that the properties of the synthesized PVP-Cu(NO3)2 fibers could be greatly altered when their morphologies can be precisely controlled and regulated. This work offers insight into a better control over the electrospinning process for the customization of metal-polymer fibers with specified morphologies, for various applications. 5. Supporting Information Discussion of baseline synthesis procedure and formation of CuO structures from PVPCu(NO3)2 fibers. EDS spectra of CuO nanofibers; summary tables including, solution properties (dynamic viscosity, conductivity), PVP-Cu(NO3)2 fiber appearance (average diameter, 1 standard deviation, and structure description), and structural defects (type and average diameter); SEM images of PVP-Cu(NO3)2 and CuO nanofibers; nanofiber diameter distribution and normal distribution plots. Discussion of the effects of electrospinning processing conditions (voltage supplied, extrusion rate, distance between the tip-of-needle and collecting plate, and humidity).

32

6. Acknowledgements The authors acknowledge financial support from the Henry Samueli School of Engineering and Applied Sciences and the Office of Equity, Diversity, and Inclusion at UCLA. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. The authors also acknowledge the Molecular & Nano Archaeology (MNA) Laboratory at UCLA Materials Science Department for use of the SEM, the J.D. McCullough Laboratory of X-ray Crystallography at UCLA Chemistry Department for use of the XRD, and the California NanoSystems Institute (CNSI) at UCLA for use of the TGA and TEM/HRTEM. The authors acknowledge Dr. Gen Chen for assistance with TEM/HRTEM; Derrick Rosales for assistance with the TGA; Sara Azzam for assistance with XRD; and Richa Ghosh, Zubin Mishra, and Caitlyn Anderson for assistance with conducting the electrospinning experiments. F.A. acknowledges Aramco R&D for supporting his M.S. studies at UCLA. 7. References (1)

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Graphical Abstract

PVP-Cu(NO3)2 Nanofibers

CuO Nanofibers Thermal

Treatment

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Highlights    

Properties of Cu-based nanofibers can be tuned via electrospinning conditions Polymer molecular weight and concentration impact morphology via solution viscosity Cu(NO3)2 concentration influences fiber diameter via solution conductivity Sensitivity to processing conditions is low

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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CRediT author statement Dante Simonetti: Conceptualization, Methodology, Writing- Review & Editing, Supervision Faisal H. Alshafei: Conceptualization, Methodology, Investigation, Writing- Original draft preparation

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