Johanna Paajanen*,
Saara Weintraub,
Satu Lönnrot,
Mikko Heikkilä,
Marko Vehkamäki,
Marianna Kemell,
Timo Hatanpää,
Mikko Ritala and
Risto Koivula
Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014, Finland. E-mail: johanna.paajanen@helsinki.fi
First published on 23rd April 2021
Nanoscale SnO2 has many important properties ranging from sorption of metal ions to gas sensing. Using a novel electroblowing method followed by calcination, we synthesized SnO2 and composite SnO2/SiO2 submicron fibers with a Sn:Si molar ratio of 3:1. Different calcination temperatures and heating rates produced fibers with varying structures and morphologies. In all the fibers SnO2 was detected by XRD indicating the SnO2/SiO2 fibers to be composite instead of complete mixtures. We studied the Co2+ separation ability of the fibers, since 60Co is a problematic contaminant in nuclear power plant wastewaters. Both SnO2 and SnO2/SiO2 fibers had an excellent Co2+ uptake with their highest uptake/Kd values being 99.82%/281000 mL g−1 and 99.79%/234000 mL g−1, respectively. Compared to the bare SnO2 fibers, the SiO2 component improved the elasticity and mechanical strength of the composite fibers which is advantageous in dynamic column operation.
For removal of heavy metal ions from aqueous solutions, there are plenty of techniques such as chemical precipitation, ion exchange, adsorption, membrane filtration, electrochemical methods and phytoremediation.29,30 Adsorption has certain advantages over the other methods including high removal efficiency even at low concentrations, cost-effectiveness, simple design and less production of toxic sludge.29,31 Adsorbents comprise both organic and inorganic materials which may be synthetic or of natural origin, such as agricultural by-products. For example, titanium(IV) iodovanadate particles have been utilized for Pb2+ and Hg2+ separation32 and coco-peat biomass for Pb2+, Cd2+, Cu2+ and Ni2+ removal.33 Inorganic submicron and nanofibers could be excellent adsorbents for a variety of industrial applications. Inorganic fibers have a large specific surface area due to their high surface-to-volume ratio and often form porous structures which can lead to a good adsorption capacity. Zirconium dioxide submicron fibers have performed well in flow-through column operation and shown better mechanical stability, less pressure build-up in the column and faster adsorption kinetics than the corresponding granules.34 Submicron and nanofibers can also be an eco-friendly alternative for the purification of nuclear power plant (NPP) wastewaters, since they can markedly reduce the volume of solid radioactive waste that requires a permanent repository.
The most common method to prepare polymeric or inorganic submicron and nanofibers is electrospinning. In this technique, a high voltage is applied to a precursor solution containing a polymer, solvent(s) and for example a metal salt, depending on the desired fiber composition. During the electrospinning process, repulsive electrostatic forces cause the polymer chains to stretch and form fibers that are deposited on a grounded collector. Via post-electrospinning calcination, the polymer may be removed and the final fibrous product, typically a metal oxide, is formed. Different precursor solutions and operational parameters allow the control of the properties and morphology of the fibers.35 Electrospinning is therefore a simple and cost-efficient method and upscalable for industry.36–39 Solution blowing (airbrushing) method in contrast uses pressurized gas to draw fibers from the precursor solution.40 Solution blowing offers even 15 times faster production rate than electrospinning41 but it tends to result in bundles of aligned fibers whereas electrospinning yields an entanglement of individual, mainly un-aligned fibers.42,43 In electroblowing, the fibers are formed by both electrostatic force and air flow and hence the technique combines electrospinning and solution blowing and their advantages. Moreover, electroblowing enables 2.5 times faster solution feeding rate and thus fiber production rate than solution blowing.41,44 Compared to electrospinning, the additional air flow in electroblowing permits the use of more viscous precursor solutions45 and produces fibers with smaller diameters and fewer beads.46 It should be feasible to scale up electroblowing for industry, similarly to electrospinning.
To tackle climate change, global energy production is increasingly based on clean low-carbon power including nuclear power. However, the production of nuclear energy generates radioactive fission and activation products that pose a risk to human health and environment without proper treatment of nuclear waste effluents. Among the activation products, 60Co caused by steel corrosion is one of the most hazardous ones due to its rather long half-life of 5.3 years and high gamma decay energies of 1.17 and 1.33 MeV.47 Removal of 60Co is therefore essential to diminish workers' radiation exposure, to reduce radioactive emissions to aquatic systems and to create safe waste forms for the final disposal. However, the separation of this radionuclide is quite difficult owing to its low concentration in the liquid waste of NPPs (402 Bq L−1 60Co in floor drain water, Loviisa NPP, Finland).48 The process for its uptake has to be very selective, since generally the liquid waste contains large amounts of inactive metal ions (e.g. 37 mg L−1 Na+ in floor drain water, Loviisa NPP, Finland).48
In this research, we prepared SnO2 and composite SnO2/SiO2 submicron fibers by the electroblowing technique and calcination. We also studied the ability of the fibers to remove 57Co2+ from an aqueous solution. To investigate the effect of SiO2 on both the structure and Co2+ uptake performance of the SnO2 fibers, we added 25 mol% of silicon in the synthesis solution of the composite fibers. The structure and morphology of all the fibers were characterized and their Co2+ removal capability was studied. To the best of our knowledge, this is the first report on the electroblowing synthesis of SnO2 and SnO2/SiO2 submicron fibers and on their Co2+ uptake. This is also the first report on the uptake of metal ions by fibrous SnO2 and SnO2/SiO2.
For the electroblowing, a self-assembled apparatus was used.49 In a typical experiment, 12 mL of precursor solution was withdrawn into a syringe and a 27 G (inner diam. 0.21 mm) needle was attached to the syringe. The syringe was placed on a syringe infusion pump (KD Scientific Legato® 101) and the solution feed rate was set to 15 mL h−1. This feed rate is 4 to 500 times as high as reported for the electrospinning of SnO2 and SnO2/SiO2 fibers.21–24,26,50,51 The needle was pushed through a 3 mm metal adapter of a box enclosing a cylindrical side collector and a planar back collector at 80 cm distance, both being made of a metal wire mesh. The potential difference between the needle and collectors was set to 15 kV with a high voltage power source, and compressed air was delivered through the adapter at a rate of 30 NL min−1. The solution jet erupted from the needle tip was deposited as fibers on the grounded collectors and additional drying air was delivered to the box at a rate of 40 NL min−1 to enhance solvent evaporation and to control the relative humidity within the box (≤20%). The as-electroblown fibrous mats were detached from the collectors and calcined in an air furnace at 400, 450 and 500 °C for 4 hours with a heating rate of 1 °C min−1 in order to remove the polymer and to form the desired ceramic material. The as-electroblown SnO2/SiO2/PVP fibers were also calcined at 400 °C for 4 hours with heating rates of 5 and 10 °C min−1. The yields of SnO2 and SnO2/SiO2 fibers were at best 0.54 and 0.53 g per hour of electroblowing, respectively.
(1) |
According to the TG analysis of the as-electroblown SnO2/PVP and SnO2/SiO2/PVP fibers (Fig. 2), the combustion of the PVP in the SnO2/SiO2 composite fibers is not complete until at 600 °C whereas it is complete at 500 °C in the case of the bare SnO2 fibers. Furthermore, it can be seen that the mass of pure SnO2 is ca. 15% and the mass of pure SnO2/SiO2 is ca. 20% of the mass of the as-electroblown material. The measured weights of the calcined SnO2 and SnO2/SiO2 fibers are in line with this, also for the lowest calcination temperature of 400 °C. This implies a more effective combustion of the polymer in the calcining furnace which is probably due to a higher oxygen content as the furnace atmosphere comprises air instead of air/N2 mixture and a long 4 hour duration of the calcination. Owing to the presence of SiO2, the calcined SnO2/SiO2 composite fiber mats were elastic and could be bent without causing fractures to them. The SnO2 fiber mats were more brittle and fractured when bent. The differences in elasticity between the SnO2 and SnO2/SiO2 fibers are demonstrated in the video “Bending experiments with the SnO2 and SnO2/SiO2 fibers”.
Fig. 2 TG curves of as-electroblown SnO2/PVP and SnO2/SiO2/PVP fibers in air (50 mol%) and N2 (50 mol%, the purge gas). |
FESEM images of the uncalcined SnO2/PVP fibers and SnO2 fibers calcined at 400, 450 and 500 °C are presented in Fig. 3. The average diameter of the uncalcined fibers containing the polymer was 1.5 μm whereas the average diameter of the calcined fibers was 560 nm. The morphology of the calcined SnO2 fibers was similar regardless of the calcination temperature. The fibers seemed to have a uniform structure that consisted of roundish grains approximately 20 nm in diameter, although there was some variation between individual grains. A similar granular structure has also been reported for both dense21,23,26,50 and hollow22,56 electrospun SnO2 nanofibers. The dense character of the fibers in the current study is proved by the FESEM image of a fiber cross-section (Fig. 3e) and was verified by imaging with transmitted electrons (Fig. 4a and b).
Fig. 3 FESEM images at low (a) and high (b) magnification of uncalcined SnO2/PVP fibers and SnO2 fibers calcined at 400 °C (c to e), 450 °C (f and g) and 500 °C (h and i). |
Fig. 4 TE images at low and high magnification of SnO2 fibers (a and b) and SnO2/SiO2 composite fibers (c and d) calcined at 500 °C. |
FESEM images of the uncalcined SnO2/SiO2/PVP fibers and SnO2/SiO2 fibers calcined at 400, 450 and 500 °C with a heating rate of 1 °C min−1 as well as at 400 °C with heating rates of 5 and 10 °C min−1 are presented in Fig. 5. The average diameters of the uncalcined and calcined fibers were 1.2 μm and 580 nm, respectively. Irrespective of the calcination temperature, the appearance of the fibers calcined with a heating rate of 1 °C min−1 was the same (Fig. 5b–h). The fibers seemed to have a smooth core that was rather sparsely covered with roundish grains approximately 30 nm in diameter. A rather similar structure comprising a smooth core and granular surface has been reported previously for SnO2/SiO2 nanofibers, although with a higher proportion of Si from 50 to 83 mol% compared to ours of 25 mol%.24 As for the fibers calcined with the faster heating rates, they also had a smooth core sparsely covered with grains that were quite angular in shape and rather large (Fig. 5k–n). The average diameter of the grains was ca. 200 nm in the fibers calcined with heating rates of 5 and 10 °C min−1, although there was some variation between individual fibers. Among the fibers calcined at 500 °C with the slow heating rate of 1 °C min−1, there were also some fibers with a special morphology with the fiber core covered with large, irregular grains of 500 nm to 1 μm in diameter (Fig. 5i and j). FESEM (Fig. 5h) and TE imaging (Fig. 4c and d) confirmed that the structure of the SnO2/SiO2 composite fibers was dense like that of the bare SnO2 fibers.
EDX elemental maps of a single SnO2/SiO2 fiber calcined at 500 °C are shown in Fig. 6. EDX spectra recorded from both inner and outer section of the smooth part of the fiber are presented in Fig. 7. A TE image of a thin inner section of the fiber is shown in Fig. 8. For the EDX measurement, a portion of the fiber was FIB milled away to expose a flat longitudinal cross-section surface from which the spectra were recorded. For the TE imaging, equal longitudinal portions from both sides of the fiber were FIB milled away leaving a thin slab in the middle of which the imaging was done. The elemental maps prove the presence of Sn, Si and O in the fiber but on the basis of them it is difficult to tell any difference between the distribution or the concentration of Sn and Si in the fiber. The EDX spectra of the center and outer smooth parts of the fiber are quite identical and they both show the presence of Sn. This implies that the core of the SnO2/SiO2 composite fibers in this study is different from the single-phase amorphous SiO2 core reported for the SnO2/SiO2 composite fibers with a higher proportion of Si.24 The TE image from inside the fiber supports this as it suggests that the inner part of the fiber is not homogeneous but consists of distinct nanoscale domains, possibly small SnO2 grains embedded in a SiO2 matrix. Previously it has been proved that more than ca. 1 mol% of SnO2 cannot be dissolved in SiO2 but SnO2 forms crystalline nanoclusters dispersed in amorphous SiO2.57,58
Fig. 7 EDX spectra of a single SnO2/SiO2 composite fiber calcined at 500 °C measured at the longitudinal cross-section surface. Ga peaks are due to the ion beam used for fiber milling. |
EDX elemental maps of the SnO2/SiO2 composite fibers calcined at 400 °C with a heating rate of 10 °C min−1 are presented in Fig. 9. As can be seen, Sn is more concentrated at the sites where the large grains are located while Si is more concentrated at the smooth part of the fibers. This implies that the large grains on the surface of the fibers consist primarily of SnO2. More evidence is provided by EDX spectra (Fig. 10) recorded at the smooth part (spectrum 1) and at a surface grain (spectrum 2) of the fibers. In the spectrum recorded at the grain there are strong signals for Sn and a very weak signal for Si while in the spectrum recorded at the smooth part the case is the opposite, i.e. the signal for Si is very intense and the signals for Sn are quite low.
Fig. 9 EDX elemental maps of SnO2/SiO2 composite fibers calcined at 400 °C with a heating rate of 10 °C min−1. |
Fig. 10 EDX spectra measured at the smooth part (spectrum 1) and surface grain (spectrum 2) of a single SnO2/SiO2 composite fiber calcined at 400 °C with a heating rate of 10 °C min−1. |
Fig. 11 X-ray diffraction patterns of the SnO2 fibers calcined at 400, 450 and 500 °C. Cryst. refers to average crystallite size in this and Fig. 12 and 13. |
Fig. 12 X-ray diffraction patterns of the SnO2/SiO2 fibers calcined at 400, 450 and 500 °C with a heating rate of 1 °C min−1. |
Fig. 13 X-ray diffraction patterns of the SnO2/SiO2 fibers calcined at 500 °C with a heating rate of 1 °C min−1 and at 400 °C with heating rates of 5 and 10 °C min−1. |
Fibers | Calc. param. | Crystallinity | Av. cryst. size (nm) | Wt% |
---|---|---|---|---|
SnO2 | 400 °C, 1 °C min−1 | High/low | 11/3 | 63/37 |
SnO2 | 450 °C, 1 °C min−1 | High/low | 10/2 | 93/7 |
SnO2 | 500 °C, 1 °C min−1 | High/low | 71/12 | 87/13 |
SnO2/SiO2 | 400 °C, 1 °C min−1 | Low/high | 2/14 | 83/17 |
SnO2/SiO2 | 450 °C, 1 °C min−1 | Low/high | 2/11 | 80/20 |
SnO2/SiO2 | 500 °C, 1 °C min−1 | High/low | 243/11 | 79/21 |
SnO2/SiO2 | 400 °C, 5 °C min−1 | High/low | 230/15 | 74/26 |
SnO2/SiO2 | 400 °C, 10 °C min−1 | High/low | 111/9 | 74/26 |
The crystallites in the SnO2 fibers calcined at 400 and 450 °C are in average 11 and 10 nm in size, respectively, whereas in the corresponding SnO2/SiO2 fibers they are only 2 nm in size. The silica in the composite fibers probably acts as a crystal growth inhibitor, as is also the case with SnO2 nanoparticles embedded in a SiO2 network.59 It should be noted though that the refinement results of the broad bumps in this data (Fig. 12) leading to below 10 nm sized crystallites represent rather the lack of long range order than an actual precise size, since severe overlapping of the peaks showing poor crystallinity undermines the accuracy of the results. In both the bare and composite fibers, the average crystallite size increases substantially as the calcination temperature is increased and in the composite fibers the increase is quite steep (Table 1). It is known that the crystallite size of ceramic submicron fibers depends on the calcination temperature.44 In the bare SnO2 fibers, the crystallite growth is probably accompanied with a grain growth at the higher temperatures21 although no prominent difference in grain size between the fibers calcined at 400, 450 and 500 °C was observed on the basis of the FESEM analysis (Fig. 3). As for the composite fibers calcined at 500 °C with a heating rate of 1 °C min−1 and at 400 °C with heating rates of 5 and 10 °C min−1, both the high calcination temperature and fast heating rates could explain the large crystallite sizes of 243, 230 and 111 nm, respectively (Table 1). It is likely that the large, irregular grains on the fibers (Fig. 5i–n) consist mainly of SnO2 that is known to form various shapes in nanoscale.50,60 High calcination temperatures and fast heating rates may influence both the size and shape of crystals,61 which is reflected in the morphology of grains comprising the crystals.
The presence of crystallites of two different sizes in the SnO2/SiO2 composite fibers is reasonable in the light of the electron microscopy and EDX analysis results. Based on EDX analysis, both Sn and Si are present in the core of the composite fibers but the surface grains consist primarily of SnO2 (Fig. 7, 9 and 10). It is seen in the TEM image (Fig. 8) that the core of the fibers comprises different nanoscale domains probably of SnO2 and SiO2. As discussed, SiO2 in the core of the fibers presumably prevents SnO2 crystallite growth. This might cause the presence of small SnO2 crystallites and grains in the core of the fibers. The SnO2 crystallites can grow more freely on the surface of the fibers resulting in larger crystallite and grain sizes. For the SnO2/SiO2 fibers calcined at 400 and 450 °C with a heating rate of 1 °C min−1 the relatively high amount of small 2 nm crystallites (83 and 80 wt%, respectively, Table 1) and the moderate size of the larger crystallites (14 and 11 nm, respectively, Table 1) are probably due to the low calcination temperature and slow heating rate. As regards the bare SnO2 fibers, the presence of the smaller crystallites (3, 2 and 12 nm for the fibers calcined at 400, 450 and 500 °C, respectively, Table 1) might be at least partly due to some PVP residues hindering crystal growth. This may especially be the case for the fibers calcined at the lowest temperature of 400 °C for which the fraction of the small crystallites, 37 wt%, is the highest of the SnO2 fibers. The average crystallite sizes of 11, 10 and 12 nm (SnO2, Table 1) as well as 14, 11 and 11 nm (SnO2/SiO2, Table 1) for the fibers calcined at 400, 450 and 500 °C with slow heating, respectively, coincide with some SnO2 grain sizes seen in the FESEM images (Fig. 3d, e, g, i and 5c, e, g). Previously it has been found that the crystal size of granular polycrystalline SnO2 nanofibers is approximately the same as the size of the SnO2 grains meaning that each SnO2 grain is possibly a single crystal.26 Perhaps this is the case for some SnO2 grains in the fibers of this study as well.
Fibers | Calc. param. | Uptake (%) | Kd (mL g−1) |
---|---|---|---|
SnO2 | 400 °C, 1 °C min−1 | 99.51 ± 0.02 | 173000 ± 3000 |
SnO2 | 450 °C, 1 °C min−1 | 99.81 ± 0.02 | 272000 ± 4000 |
SnO2 | 500 °C, 1 °C min−1 | 99.82 ± 0.01 | 281000 ± 5000 |
SnO2/SiO2 | 400 °C, 1 °C min−1 | 99.69 ± 0.02 | 163000 ± 3000 |
SnO2/SiO2 | 450 °C, 1 °C min−1 | 99.18 ± 0.02 | 58100 ± 1000 |
SnO2/SiO2 | 500 °C, 1 °C min−1 | 99.28 ± 0.02 | 69700 ± 1300 |
SnO2/SiO2 | 400 °C, 5 °C min−1 | 99.70 ± 0.02 | 168000 ± 3000 |
SnO2/SiO2 | 400 °C, 10 °C min−1 | 99.79 ± 0.02 | 234000 ± 4000 |
In regard to the SnO2 and SnO2/SiO2 fibers calcined with a heating rate of 1 °C min−1, the calcination temperature had a different effect on their Co2+ uptake performance. Calcination temperatures of 500 and 400 °C produced the best adsorption performance for SnO2 and SnO2/SiO2 fibers, respectively (Table 2 and Fig. 14). Lower calcination temperature tends to produce smaller crystallites and thus increase the surface area of the material, which, in turn, improves its adsorption properties.44 Therefore it is surprising that the SnO2 fibers calcined at 500 °C possessing 71 nm crystallites perform better than the SnO2 fibers calcined at 400 and 450 °C possessing 11 and 10 nm crystallites, respectively. One possible reason is that the lower calcination temperatures leave some PVP residues in the material blocking some of the adsorption sites. However, despite their different crystallite size, the Kd value of the SnO2 fibers calcined at 450 °C is almost as high as that of the fibers calcined at 500 °C. The crystallite size doesn't seem to be a determining factor in the Co2+ uptake performance of the SnO2 fibers.
Fig. 14 Distribution coefficient of 30 Bq mL−1 57Co2+ on SnO2 and SnO2/SiO2 fibers as a function of calcination temperature and heating rate in 0.01 M NaNO3 at pH 6.0 (2 g L−1 SnO2 or SnO2/SiO2). |
In the SnO2/SiO2 fibers calcined with a heating rate of 1 °C min−1 the Co2+ uptake correlated better with the crystallite size, since the fibers calcined at 400 °C and with the smallest 2 nm crystallites performed the best (Table 2 and Fig. 14). It should be noted that although the fibers calcined at 450 °C also possessed 2 nm crystallites, their sorption and Kd values were lower. One reason for the weaker Co2+ uptake performance of the SnO2/SiO2 fibers calcined at 450 and 500 °C with a heating rate of 1 °C min−1 could be that due to the higher calcination temperature, they have fewer surface hydroxyl groups that are assumed to play a significant role in the uptake process. As for the fibers calcined at 500 °C, the peculiar morphology of some of the fibers (Fig. 5i and j) and large crystallite size of 243 nm that decreases the surface area, may also impair their Co2+ uptake performance. The SnO2/SiO2 fibers calcined at 400 °C with the fast heating rates of 5 and 10 °C min−1 exhibited excellent Co2+ uptake despite their large crystallite sizes. This might be explained by the roundish shape and quite sparse distribution of their SnO2 grains (Fig. 5k–n) compared to the angular and rather tightly packed SnO2 grains of the fibers calcined at 500 °C (Fig. 5i and j). The roundish and sparsely distributed surface SnO2 grains might be better accessible to adsorbing ions in an aqueous solution. Moreover, the lower target temperature of 400 °C may cause more hydroxyl groups remaining on the surface of the fibers enhancing the uptake of Co2+.
The pH of the solution may have a major impact on the uptake properties of a material because it affects both the speciation of the adsorbate and the surface charge of the adsorbent. As for Co2+, it exists as Co(H2O)62+ in the pH range of 2 to 9.64 It can be inferred from the Kd values (Fig. 15), that the SnO2 fibers adsorb Co2+ the best in the neutral to mildly basic pH region, from pH 6 to 9, and they reach the highest Kd value at the pH of 7. This is in accordance with previous research7 and promising for the use of the fibers in purification of NPP waste waters, since the pH of the primary coolant water in NPPs is about 7. The point of zero charge (pHpzc)of SnO2 lies at a pH of 4,7 and above this pH the surface charge of pure SnO2 is negative. The Co2+ uptake by the rutile structured SnO2 is evidently based on electrostatic forces65 which explains the good uptake in the neutral to mildly basic pH region where the charges of the Co2+ species and the surface of the SnO2 fibers are the opposite. The adsorption of Co2+ on hydrous SnO2 most probably occurs via substitution of H+ ions of the surface water molecules or hydroxyl groups by Co2+ ions.4–6,66 An assumed ion exchange reaction between hydrous SnO2 and Co2+ is illustrated in Fig. 17.5
Fig. 15 Sorption and distribution coefficient of 30 Bq mL−1 57Co2+ on SnO2 fibers calcined at 500 °C as a function of pH in 0.01 M NaNO3 (2 g L−1 SnO2). |
Fig. 17 An assumed ion exchange reaction between Co2+ and a proton of surface hydroxyl group of SnO2.5 |
A good selectivity of the adsorbent is crucial when a trace amount of a specific ion is separated from a solution containing much higher concentrations of other ions. Thus, we examined how the Co2+ adsorption on SnO2 fibers is influenced by Na+ and Ca2+ ions that are among the most common cations in natural and nuclear waste waters with concentrations of 0.47 and 0.01 M in sea water, respectively.67 As Fig. 16 reveals, the Co2+ uptake remains high in the presence of Na+ ions irrespective of their concentration. By contrast, there is a marked weakening of the uptake in the CaCl2 solution at concentrations of 0.01 M or higher. With both of the ions, the interfering effect increases with concentration up to 0.1 M, after which there is no further decreasing effect on the uptake of Co2+. The good selectivity of the fibers for Co2+ over Na+ is promising for their use in decontamination of radioactive waste water, since Na+ is the most abundant coexisting ion in nuclear waste effluents.67
Both bare SnO2 and composite SnO2/SiO2 fibers had a high Co2+ uptake with SnO2 fibers exhibiting slightly more efficient Co2+ separation. Calcination temperature and heating rate affected the Co2+ uptake by the fibers. Among the SnO2 fibers, the fibers calcined at 500 °C performed the best. Among the composite fibers, the fibers calcined at 400 °C performed the best, and within them the fibers calcined with a heating rate of 10 °C min−1 were superior to the fibers calcined with heating rates of 1 and 5 °C min−1. The SnO2/SiO2 composite fibers were more elastic and durable and easier to handle than the somewhat brittle SnO2 fibers which makes them ideal for use in flow-through separation columns. Overall, on the basis of our results, the mechanical strength of SnO2 submicron fibers can be enhanced by adding a moderate amount of SiO2 without compromising their adsorption performance too much. This approach might also be extended to other ceramic submicron and nanofibers that require improved mechanical properties in various applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra01559a |
This journal is © The Royal Society of Chemistry 2021 |