Ytterbium oxide nanofibers: fabrication and characterization for energy applications

Metal oxide nanomaterials are widely used in many applications of renewable energy. Ytterbium oxide (Yb2O3) also attracts attention due to its similar structure to ZnO (~3,26 eV) and TiO2 (~3,36 eV) in terms of bandgap energy. In this study, Yb2O3 belonging to the lanthanide oxide family was produced as nanofiber forms by using the electrospinning technique, which allows for large-scale production, for the first time. The morphological, structural, optical, and phase properties of the produced nanofibers were investigated via XRD, SEM-Mapping, TEM, FTIR, UV-Vis, and XPS characterization techniques. As a result of these analyses, it was determined that the Yb2O3 nanofibers with a diameter of 125 ± 15 nm have a cubic crystal structure and a bandgap of 3.32 eV. The results of this study have shown that the Yb2O3 nanofibers are capable of performing performance evaluations in many different energy conversion applications and others.

within the scope of this study, the obtained nanomaterials formed as irregular nanoparticles rather than nanofiber [20]. In other nanofiber studies, ytterbium has been used as a dopant material in metal oxide nanofibers [21][22][23][24].
It has been understood from the literature review that the synthesis studies of Yb 2 O 3 nanostructures are limited, and there is no report on the production of their well-crystallized nanofiber. The production of homogeneous and highly crystalline Yb 2 O 3 nanofibers by electrospinning has been reported for the first time in this study. Moreover, a detailed bandgap analysis of Yb 2 O 3 nanofiber has also been carried out. These band gap studies show that the obtained highly crystalline nanofibers can be an alternative to ZnO and TiO 2 , which are used extensively in energy applications.

Synthesis procedure of Yb 2 O 3 nanofibers
Yb 2 O 3 nanofibers were produced by the electrospinning technique in parallel with previous studies [25,26]. Typically, 1 mmol YbAc 3 was dissolved in 5 mL DMF and then 400 mg PAN (8% w/v) was added to the solution. This mixture was stirred overnight to obtain a homogeneous solution. The homogenized solution was then placed in the syringe with a 21-gauge needle and the feed rate of the syringe pump was set as 1 mL/h.
The sample collector was fixed vertically opposite the syringe pump and its distance from the needle was adjusted to 15 cm. After the solution is prepared and the system requirements are set as described, voltage with 18 kV was applied to the metallic needle for the required electric field causing the formation of nanofibers and the sample collector was grounded. Thus, YbAc 3 +PAN nanofibers were collected on aluminum foils on the sample collector and these nanofibers were calcined at 500 °C for 1 h. In this way, the polymer used as a template and other volatile substances in the environment were removed and Yb 2 O 3 nanofibers were obtained. The process steps of this technique are shown schematically in Figure 1.

Results and discussions
First of all, powder X-ray diffraction (XRD) was employed as a bulk analysis technique to confirm the phase structure and purity of the Yb 2 O 3 compound. Figure 2 demonstrates the XRD pattern of the Yb 2 O 3 nanofibers. In this structure, the distinctive diffraction signals indicate that Yb 2 O 3 has a cubic structure (JCPDS card no. 043-1037) with space group Ia-3. The unit cell of this structure consists of 44 polyhedra centered on Yb, and in this structure, each ytterbium atom bonds with 6 oxygen atoms ( Figure 2).
The XRD results show that all diffraction peaks are smooth and intense. At the same time, the absence of peaks belonging to another phase structure confirmed the purity of the Yb 2 O 3 nanofibers. Moreover, the crystallite size distributions were calculated from the XRD pattern by the modified Scherrer's equation and Williamson-Hall method, and the graphs of these measurements were given in the supplementary part (Figures S1 and S2). The crystallite size of the particles that make up the nanofibers was estimated as 6.53 nm and 6.74 nm by modified Scherrer's equation and Williamson-Hall method, respectively. Morphological, crystalline, and compositional properties of the nanofibers were studied by SEM, TEM, elemental mapping, and EDX characterization methods. SEM images of nanofibers pre-and postcalcination process were given in Figures 3a-3c, respectively. These results show that nanofibers have homogeneous size distribution. Moreover, it was determined that the diameters of precalcination nanofibers were 450 nm, and after the heat treatment, the diameters of the crystallized nanofibers decreased to 125 nm due to the loss of organic components. As a result of the performed measurements, the diameter distribution graphs of composite and crystalized nanofibers are given in Figures S3 and S4.
The morphological and crystal properties of nanofibers were detailed by TEM and HR-TEM analyses and the obtained results are given in Figures 3d-3f. It can be seen from the TEM images, the produced nanofibers have homogenous size distribution and this result is also consistent with the SEM results. Figure 3e clearly shows that the nanofibers consisted of Yb 2 O 3 nanoparticles with about 9 ± 2 nm. Additionally, this measurement is compatible with crystallite size calculation from the XRD pattern by the modified Scherrer's equation and Williamson-Hall method. These nanoparticles, which are in contact with each other, were sintered with the effect of heat treatment and thus the nanofiber form was preserved.
The crystallinity of nanofibers was also investigated by the HR-TEM image given in Figure 3f. The perfectly and regular arrangement of the atoms indicates that the fabricated nanofibers have high crystallinity. Moreover, the performed calculations from the XRD pattern show that nanofibers have an 82.75% crystallinity percentage, and this estimate is compatible with TEM results. The process steps belonging to the determination of crystallinity percentage were explained in the supplementary part.
Furthermore, the interplanar spacing was calculated as 3.04 Å, and this value corresponds to the (222) crystallographic plane. In the XRD diffraction pattern, the most intense peak at 29.5° corresponds with the (222) plane, which indicates that the nanoparticles contained in the nanofibers are largely arranged in the (222) plane. Therefore, the obtained results from HR-TEM are compatible with XRD analysis. The elemental distribution and composition of the nanofibers were studied by elemental mapping and EDX analysis methods. Figures 4a-4c (Figure 4d), and these values are quite close to theoretical ratios.
The chemical valance state of the nanofibers was analyzed by the XPS spectra. The high-resolution XPS spectra of Yb 4d and O 1s are given in Figure 5. Yb 4d signals were fitted with five singlet peaks by Gaussian profile (Figure 5a). The peaks centered at 184.4 and 198.8 eV were associated with spin-orbit-split 4d 5/2 and 4d 3/2 , respectively, and satellite peaks were also detected at 192.4 and 205.3 eV. The energy difference of spin-orbit-split was found to be 14.4 eV as reported in the literature, and it was attributed to the 3+ valance state of Yb [27,28]. In addition, the peak at 187.3 eV was correlated to the presence of ytterbium oxalate [29]. It was commented that ytterbium oxalate may have occurred from the reaction of YbAc 3 with organic molecules in the polymer due to the effect of applied temperature during the oxidation of nanofibers. The O 1s high-resolution spectra fitted to two singlet peaks was displayed in Figure 5b. As a result of performed fitting, it was determined that the peak centered at 529.1 was related to O 2ions in the Yb 2 O 3 crystal structure, while the peaks at 531.5 eV were caused by the hydroxyl group and oxygen vacancies [30].
The optical properties of nanofibers were investigated by diffuse reflectance spectra and their result is given in Figure 6a. According to this graph, nanofibers have weak reflectivity in the ultraviolet region, while this value increases    towards the visible part and reaches a maximum in the near-infrared region. The obtained data from diffuse reflectance spectroscopy were used to calculate the band-gap and determine its type. Firstly, the absorption (F(R ∞ )) was calculated from the Kubelka-Munk equation given in Equation 1.

Equation 1
where F(R ∞ ) is absorption and R is reflection.
The absorption value was used to estimate the approximately band-gap (3.21 eV), which is determined by the graph of d[ln(F(R ∞ )hʋ)]/d[hʋ]-hʋ (Figure 6b). The m exponent was assigned from the slope of the ln(F(R ∞ )hʋ)-ln(hʋ-Eg) plot ( Figure 6c) obtained from Tauc's equation (Equation 2). The m value was calculated as 0.6 and this shows that Yb 2 O 3 nanofibers have direct transition type [31,32].
where hʋ is photon energy, A is energy-independent constant, Eg is optical band gap and m is exponent a value that determines bandgap types.
The optical band-gap of nanofibers was estimated as 3.32 eV by fitting the linear part of the graph of (F(R ∞ )hʋ) 2 vs photon energy given in Figure 6d.
FTIR spectroscopy was carried out to determine the presence of organic molecules in crystalized Yb 2 O 3 nanofibers. FTIR results of crystalized nanofiber, polymer composite, and PAN are given comparatively in Figure 7. The FTIR results of PAN and composite nanofibers show that the characteristic peaks at 2935, 2245, 1448, 1360, 1242, and 1072 cm -1 are matched to the bond vibration of organic molecules belonging to PAN. The bands at 2935, 2245, and 1448 cm -1 were attributed to stretching of the C-H of the CH 2 , the C≡N of the acrylonitrile group, and the C=O of the carbonyl group, respectively. The peak centered at 1448 cm -1 occurred from CH 2 tensile vibration, while the peak at 1360 cm -1 was ascribed to the C-H stretching of aliphatic CH groups. Furthermore, the C-H and C-O bond vibrations were observed at 1242 and 1072 cm -1 , respectively [33,34]. After the calcination process, these characteristic peaks of PAN disappeared due to the decomposition of organic molecules under the influence of heat treatment. Moreover, the peak at 568 cm -1 which is seen in the FTIR result of Yb 2 O 3 nanofibers was assigned to bond vibration between Yb with O atoms.

Conclusion
Highly crystalline, pure, and homogeneous Yb 2 O 3 nanofibers were fabricated by the electrospinning technique and presented to the literature for the first time with this study. The produced nanofibers were characterized by various methods such as XRD, SEM, TEM, elemental mapping, EDX, XPS, UV-Vis spectroscopy, and FTIR. As a result of the analyses, it was seen that the nanofibers consist of nanoparticles with 9 ± 2 nm, which have a pure and cubic crystal structure. Moreover, the chemical composition of the nanofibers was calculated as Yb 2.19 O 2.81 by EDX. It was determined with optical measurements and calculations that the nanofibers have a direct band transition type and a band-gap value of 3.32 eV. The obtained characterization results show that the high crystalline Yb 2 O 3 nanofibers can be used in many energy applications such as solar cells, hydrogen production, oxygen evolution-reduction reactions, and fuel cells. Moreover, it is predicted that produced nanofibers may contribute to an increase in application efficiencies because they have a larger contact surface area than other nanomaterials with different morphology used in these applications.

Supporting Information
The particle size distribution was estimated by modified Scherrer equation and Williamson-Hall method which are given in the supporting information. 1

Percentage of crystallinity
The percent crystallinity of nanofibers was calculated from the XRD pattern using the following equation.
where X c is the degree of crystallinity, A c and A a are the areas of crystalline and amorphous peaks in the XRD pattern, respectively.