Methods to improve catalytic performance in Electrospun Fibers

Electrospun nanofibers with its high specific surface area are very attractive for applications where performance is related to surface area. Further, in fibrous form, the catalytic material can be self-supporting unlike nanoparticles which have high surface area individually but when they aggregate, the specific surface area drops. Although nanofibers with catalytic properties have shown good performance, there are still ways to further improve it. This may come about by further increasing the porosity of the fiber, introducing confinement effect by means of hollow fibers or adding a second material to complement the catalytic function.

Increase fiber porosity

Where there are more surface area for reaction, catalytic rate will generally increase as a result of that. Tests by Li et al (2012) showed that smaller grain size nanofibers exhibits greater specific surface area and better self-photosensitization of Rhodamine (RhB). Since increasing the surface area of the nanofibers would enhance the performance of nanofibers this has led to several researches into introducing pores to the nanofibers. Liu et al (2012) mixed carbon nanosphere into TiO₂ precursor solution before electrospinning. The resultant nanofibers which contained the nanospheres undergo calcination to form TiO₂ nanofibers and during this process, the nanospheres were burnt off, leaving behind porous TiO₂ nanofibers. Comparing the photocatalytic performance of porous TiO2 nanofibers, solid TiO₂ nanofibers and TiO₂ nanoparticles, porous TiO₂ works the best but solid nanofibers showed the poorest result. Therefore, the better performance can be attributed to the high surface area of porous TiO₂.

Foaming agents may be added to facilitate the formation of mesoporous fibers. Foaming agents added to the solution will decompose into vapor phases during electrospinning and the escaping vapors will create homogeneous and thoroughly mesoporous fibers. Hou et al (2015) used foaming agent, diisopropyl azodiformate (DIPA) to mix into a solution of polyvinylpyrrolidone (PVP) and butyl titanate (TBOT). CTAB and paraffin oil was later added to the solution for the formation of titanium dioxide (TiO₂) mesoporous tubes. Following sintering, the mesoporous TiO₂ tubes were tested for its photocatalytic activity for the evolution of hydrogen. The mesoporous TiO₂ tubes were able to exhibit significantly higher photocatalytic activity at a rate of 499 µmol g^-1.h^-1 compared to P25 nanoparticles which was at 198 µmol g^-1.h^-1. The photocatalytic stability in the mesoporous TiO₂ tubes was also greater than P25 nanoparticles with no noticeable drop in its activity after 3 cycles of recovery and re-use.

(a) Schematic illustration for the formation of mesoporous TiO2 hollow fibers via the foaming assisted electrospinning. (b) A typical SEM image of the calcined products under a low magnification. (c-e) Typical SEM images of the calcined products under higher magnifications and different views. (f) A representative XRD pattern of the calcined products. [Hou et al. Scientific Reports 2015; 5: 15228. This work is licensed under a Creative Commons Attribution 4.0 International.]

Hollow Fibers

Electrospun nanofibers may be made into hollow nanofibers using several means such as using the nanofibers as a template or through self-organization of the ions such that hollow tubes are formed after sintering. Hollow fibers will have a higher specific surface area compared to solid nanofibers. Better catalytic performance of hollow fibers may also be attributed to the confinement of the reacting substances within the lumen. Chen et al (2017) tested the catalytic efficiency of electrospun solid, hollow and fiber in tube antimony-doped tin oxide nanofibers (ATONF) with electrodeposited Pt nanoparticles. Solid, hollow and fiber in tube electrospun ATONFs were formed by controlling the heating rate during calcination of its precursors. Following electrodeposition of Pt nanoparticles, it was found that for hollow tube ATONFs, the Pt nanoparticles were located in the hollow channels of the nanofibers instead of on the exposed surface. While this phenomenon requires further investigation, it may be due to lower nucleation energy on the concave inner wall of the tube. Catalytic activity on oxygen reduction reaction (ORR) was the highest on the Pt nanoparticles coated hollow ATONFs compared to Pt nanoparticles coated solid ATONFs and conventional antimony-doped tin oxide powders. Higher ORR in hollow ATONF versus solid ATONF despite having the same Pt loading and nanoparticles size may be attributed to enhanced probability of O₂ collision onto the catalytic Pt surface within the confines of the nanotube.

Composite fibers

Additives may be added to the electrospun nanofibers to improve the catalytic performance of the material. Dai et al (2010) fabricated a layered composite inorganic fibrous membrane containing Pt nanoparticles. First, TiO₂ nanofibers were formed by sintering its electrospun precursor fibers. This is then coated with polyvinyl pyrrolidone (PVP) stabilised Pt nanoparticles followed by another layer of SiO₂ nanoparticles. The presence of SiO₂ nanoparticles was found to stabilise the Pt nanoparticles and prevent their aggregation during sintering at temperatures above 350 °C. Catalytic activity of the composite was determined using methyl red for the model reaction. It was shown that even with the SiO₂ sheath over the Pt nanoparticles layer, methyl red molecules were able to penetrate through the porous sheath layer for catalytic reaction.

To enhance photocatalytic activity of inorganic material in visible light, dopants may be added to reduce the energy band gap. Samadi et al (2012) doped electrospun ZnO nanofibers with multi-walled carbon nanotube (MWCNT) and the doped ZnO demonstrated photocatalytic activity in visible light while pure ZnO nanofibers were inactive. Other techniques such as nitridation of TiO₂ have also been used successfully to enhance photocatalytic performance in visible light [Li et al 2012b]. Ag is another element known to enhance the photocatalytic ability of TiO₂. Mishra et al (2012) fabricated electrospun TiO₂ nanofibers doped with Ag nanoparticles. TiO₂ nanofibers were produced by annealing of titania precursor sol. To incorporate Ag nanoparticles, Ag-EtOH solution were added to the titania precursor solution containing polyvinyl pyrrolidone (PVP). Comparing the catalytic performance between pure TiO₂ nanofibers and TiO₂/Ag nanoparticles nanofibers, the latter showed 10.3 times higher than that by pure TiO₂ nanofibers. The enhanced photocatalytic performance was attributed to electron-trapping by Ag nanoparticles to dissociate photogenerated electron-hole pairs. Lu et al (2019) demonstrated the enhancement in the photocatalytic performance of ZnO nanorod projections on polyimide/AG (PI/Ag) fibers when Ag nanoparticles were coupled with the ZnO nanorods. When Ag were embedded in the PI fiber matrix, the photocatalytic degradation rate of methylene blue (MB) was 87% with the ZnO projections. However, when Ag nanoparticles were coupled the ZnO projections on the same PI/Ag nanofiber base, the photocatalytic degradation rate increased to 98%. The synergistic effect of Ag on ZnO has been attributed to the Ag nanoparticles functioning as an electron sink that reduce the recombination of photoinduced electrons and holes which is the same as the effect on electrospun TiO₂ nanofibers doped with Ag nanoparticles reported by Mishra et al (2012). Duan et al (2019) constructed a composite of TiO₂ loaded with Au nanoparticles by electrospinning for photodegradation of rhodamine B (RB). The addition of Au nanoparticles increases the photodegradation of rhodamine B (RB) under UV irradiation, visible and natural lights. The most significant improvement is seen in the degradation of RB in visible and natural lights Under natural light, degradations of RB by using TiO₂/Au nanofibers as photocatalysts reached almost 100% in 270?min irradiation while the degradation by using pure TiO₂ nanofibers was just 40%. The superior performance of TiO₂/Au may be attributed to the plasmonic resonance absorption of Au nanoparticles and effective separation of photogenerated electrons and holes by the TiO₂/Au heterojunction structures. Bairamis et al (2019) doped TiO₂ with g-C₃N₄ to improve its photocatalytic performance on methylene blue under simulated sunlight irradiation. Increasing g-C₃N₄ in TiO₂ increases the degradation of methylene blue. This has been attributed to interfacial transfer of carriers and the corresponding generation of -OH radicals due to the presence of g-C₃N₄. A Z-scheme mechanism was suggested for the generation of -OH radicals by the composite. Photogenerated holes were maintained at the VB of TiO₂ while the excited electrons in the CB of TiO₂ combined with the holes in the VB of g-C₃N₄. Excited electrons on the CB of g-C₃N₄ are scavenged by oxygen while the generated holes at the VB of TiO₂ generated -OH radicals. The trapping of electrons at the CB of TiO₂ by the holes of g-C₃N₄ reduces recombination and this resulted in higher photocatalytic efficiency. Taking advantage of the improved function of hollow fibers, Guo et al (2021) constructed electrospun NaYF₄/Yb/Tm/TiO₂ hollow composite fibers for photocatalytic application. The hollow composite fibers were formed using a co-axial electrospinning setup. The inner core was filled with cyclohexane and paraffin which will be removed during the calcination process to form the hollow core. NaYF₄/Yb/Tm/TiO₂ nanoparticles were added to the cyclohexane and paraffin core during electrospinning. The shell was formed by TiO₂/PVP precursor solution, Following calcination, NaYF₄/Yb/Tm nanoparticles were observed to line the inner wall of the hollow electrospun fibers. For calcinated pure TiO₂ hollow fibers, the inner wall was smooth. Photocatalytic performance of the fibers were tested on rhodamine B (RB). The maximum absorption peak was found to be at light wavelength of 550 nm. After 2 h of reaction, the degradation efficiency of the hollow TiO₂ fibers containing NaYF₄/Yb/Tm nanoparticles was at 99.12% which is significantly better than hollow pure TiO₂ fibers with a degradation efficiency of 83.79%.

Morphology characterization and diameter distribution of nanoparticle and nanofibers. (a, b) Cross-sectional SEM images of TiO2 and NaYF4/Yb/Tm/TiO2 core-shell hollow fiber (c, d) proving the existence of NaYF4/Yb/Tm nanoparticles on the inner surfaces. (e, f) Size statistic of nanoparticles and SEM image of core-shell fibers [Guo et al 2021].

Maafa et al (2021) constructed a core-shell inorganic nanofiber by electrospinning a precursor solution of poly(vinyl alcohol), zinc acetate dihydrate, and cadmium acetate dihydrate followed by calcination. The resultant inorganic nanofiber was found to exhibit a Cd-rich core and Zn-rich shell. Such composite nanofiber has a much higher photocatalytic than pure ZnO nanofiber with methylene blue (MB) photo-degradation at 98% and 42% under 210 min of sunlight Irradiation for CdO/ZnO core/shell nanofibers and ZnO nanofibers, respectively. The better photocatalytic performance of CdO/Zn nanofibers over ZnO nanofibers may be attributed to lower band gap of CdO/Zn. With CdO/ZnO core/shell nanofibers, the generated electrons from irradiation of the surface ZnO were transferred to the conduction band of the CdO core. This reduces the recombination rate of the generated electron-hole pairs. The photo-generated holes and electrons would facilitate the formation of hydroxyl free radicals from water which oxidizes and degrades organic pollutants.

Illustration of Z-scheme photocatalytic mechanism for the g-C3N4 [Bairamis et al 2019].

Photocatalytic Material Location and Distribution

The location of the photocatalytic material on the electrospun membrane may have a significant influence on its photocatalytic activity. AlAbduljabbar et al (2021) tested the efficacy of degrading methyl orange with TiO₂ nanoparticles either adhered to the surface of electrospun polyacrylonitrile (PAN) fibers or blended within the PAN fibers. The electrospun PAN membrane with surface coated TiO₂ nanoparticles was constructed by electrospraying a suspension of TiO₂ nanoparticles over the prefabricated PAN nanofibers membrane. Comparing their photocatalytic activity, it was shown that PAN nanofibers membrane with surface coated TiO₂ nanoparticles has a much higher activity at 92% against the TiO₂/PAN blended sample at 49.6%. The difference in photocatalytic activity can be attributed to the greater exposure of surface TiO₂ nanoparticles to methyl orange. Having TiO₂ nanoparticles on the surface also increases the surface roughness and contributes to increased surface area of the membrane.

For electrospun fibers to be an effective carrier for catalytic particles, the nanoparticles have to be uniformly dispersed throughout the matrix. To construct nanofibers for uniform dispersion of negatively charged platinum nanoparticles, Li et al (2017) electrospun amino-functionalized polyacrylonitrile (PAN) nanofibers by electrospinning a PAN and 3-aminopropyltriethoxysilane (APS) mixed precursor solution. The electrospun fibers were dipped into platinum sol followed by reduction to form platinum nanoparticles (PtNPs). The positive-charged PAN/APS nanofibers were able to bind with the negative-charged platinum nanoparticles (PtNPs) through electrostatic interaction. A high density of PtNPs was found to be uniformly distributed on the surface of the electrospun nanofibers.

Placement of photocatalytic material

Electrospun fibers are commonly used as carriers for photocatalytic nanoparticles. As a carrier, factors such as the location of the photocatalytic material and the bonding between the photocatalytic material and carrier may have a strong influence on the photocatalytic activity. AlAbduljabbar et al (2021) used electrospun chitosan nanofibers as the carrier for TiO₂ nanoparticles. Two methods of loading TiO₂ nanoparticles were being investigated. The first method involves surface functionalization of chitosan nanofibers followed by electrospraying to deposit TiO₂ nanoparticles on the surface of the nanofibers. In the second method, TiO₂ nanoparticles were mixed into a chitosan solution and electrospun into fibers. Comparison of their photocatalytic performance was determined by the degradation of methylene blue dye. With the TiO₂ nanoparticles on the surface of functionalized nanofibers, the photodegradation rate under UV light was 89.30% and a reaction rate constant k at 0.0088 min^-1. For TiO₂ nanoparticles that were blended into the chitosan nanofibers, the degradation efficiency under UV was only 40.26% and the reaction rate constant k was 0.0016 min^-1. This significant difference in performance may be attributed to TiO₂ nanoparticles on the surface of the fibers being more exposed to the UV light and in direct contact with the methylene blue molecules as compared to the blended system where the TiO₂ nanoparticles were embedded within the carrier matrix material. The band gap of the TiO₂ nanoparticles on the surface of functionalized nanofibers was also found to be smaller than the TiO₂ nanoparticles/chitosan blended fibers. Surface roughness of chitosan nanofiber with TiO₂ nanoparticles on its surface is also higher and this increases the exposed reaction site with the dye molecules.

Environmental condition

The environmental condition where the photodegradation takes place has a significant influence on its rate. Higher reaction temperature generally increases the photodegradation rate. This is due to faster electron-hole formation and faster movement of the dye molecules. However, an increase in temperature beyond 80 °C may be detrimental as the electron-hole recombination rate increases [Maafa et al 2022].

Another factor is the light intensity on the photocatalyst. An increase in light intensity increases the photons striking the photocatalyst and generating more electron-hole pairs. Maafa et al 2022 showed that with their electrospun derived carbon /NiTiO₃/ TiO₂ nanofibers membrane, the photodegradation of Methylene blue (MB) increases with increasing light intensity.

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