Synthesis of Vanadium Oxide Nanofibers with Variable Crystallinity

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Article Cite This: ACS Omega 2017, 2, 7739-7745

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Synthesis of Vanadium Oxide Nanofibers with Variable Crystallinity and V5+/V4+ Ratios Raúl Berenguer,*,† María Olga Guerrero-Pérez, Isabel Guzmán, José Rodríguez-Mirasol, and Tomás Cordero Universidad de Málaga, Andalucía Tech, Departamento de Ingeniería Química, E29071 Málaga, Spain S Supporting Information *

ABSTRACT: Tailoring the morphological, chemical, and physical properties of vanadium oxides (VOx) is crucial to optimize their performance in current and future applications. The present contribution proposes a new route to obtain VOx nanofibers with different V4+/V5+ ratios and crystallinity. The method involves the exclusive electrospinning of water-free NH4VO3-saturated solutions including a reductant. Subsequent air-annealing under suitable conditions yields vanadium oxide fibers of 20−90 nm diameter and 10−50 m2/g surface area. The presence of the reductant gives rise to VOx nanofibers with a considerable proportion of V4+. Then, the right choice of the calcination heating rate and temperature permits to modify the V4+/V5+ ratio as well as the crystalline phase and crystallite size of the fibers. With the proposed methodology, long-range continuous single-phase orthorhombic V2O5 and monoclinic V3O7 nanofibers are obtained.

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INTRODUCTION Vanadium oxides (VOx) constitute a unique family of materials with large diversity of chemical and physical properties, which make them important for basic research and various technological and industrial applications.1−3 Particularly, those with formula VnO2n+1 (V2O5, V3O7, V4O9, and V6O13) present a wide range of layered structures and phases, from amorphous to crystalline, which can reversibly intercalate protons or alkali ions to form electronically conducting materials.4 These materials showing both ionic and electronic conductivity are used as electrode materials in Li-ion batteries (LIBs),4−6 electrochromic devices,7,8 gas sensors,9 supercapacitors,10,11 etc. Another outstanding feature of these oxides is their ability to incorporate oxygen (O) vacancy defects and to form mixedvalence oxides.2,4 These O-vacancies have been found the energetically most favorable adsorption sites for certain reactants,12 and their formation has been related to the ability of oxides to act as catalysts for partial oxidation13−16 and reduction17 reactions. On the other hand, the presence of V4+ in some of these phases (V3O7, V4O9, and V6O13) has been found interesting for different applications. First, although V5+ is nonmagnetic, V4+ has spin S = 1/2.18 Second, the study of several V-based catalytic systems revealed the necessity of having both V5+ and V4+ interconvertible species to obtain active and selective catalysts for several partial oxidation reactions.19,20 For example, previous studies21,22 described how a V2O5-based catalyst becomes active and selective when it has a certain V5+/V4+ ratio and transforms into V3O7. Finally, it was found that the presence of V4+ and the nonstoichiometry © 2017 American Chemical Society

has a critical positive effect on the reaction of VOx with lithium.23 Apart from the oxidation state and crystallinity, the morphology and dimensions of the VOx materials play a key role on their performance. Generally, nanodimensioned and nanostructured VOx shows higher and more accessible surface area than the bulk counterpart, which is highly beneficial for reaction efficiency and kinetics.5,6,10 In addition, the fibrous morphology enhances mass transfer, low pressure drop, ease of handling and reduced interparticle boundaries for several functional applications.24−26 Furthermore, for applications requiring reversible intercalation of ions, like LIBs, the nanoscale promotes facile strain relaxation upon insertion− extraction cycling.4−6 Given such an extraordinary versatility on properties and applications, several studies have been carried out to design new VOx materials and synthetic routes. Higher-temperature routes (T = 350−600 °C), like the thermal decomposition of NH4VO314,19,20 and chemical vapor deposition,3 usually result in dense thermodynamically stable phases,27 mainly orthorhombic α-V2O5, with poor control of the oxidation state or the morphology. By contrast, the use of lower and milder temperature routes (T < 200 °C), like electrodeposition28 and sol−gel27 and hydrothermal29 techniques, respectively, allowing kinetic control of crystal grown reactions, lead to more Received: July 25, 2017 Accepted: October 18, 2017 Published: November 9, 2017 7739

DOI: 10.1021/acsomega.7b01061 ACS Omega 2017, 2, 7739−7745

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open metastable VOx phases,27 multiple nanoforms (nanofibers, nanotubes, etc.), and certain control of the V4+/V5+ ratio.29 Nonetheless, because water is used as solvent or present in most cases, these routes are generally restricted to “doublelayer” δ-type gels, xerogels or aerogels (HxV4O10·nH2O or V2O5·nH2O), or crystalline monoclinic δ-V2O54 and orthorhombic H2V3O8 (V3O7·H2O)30 phases. In addition, only VOx nanoforms of low aspect ratio can be obtained. Some research groups have demonstrated that the combination of previous routes with electrospinning successfully solves the problem of discontinuity on VOx nanomaterials.5,6,31−35 However, the properties of the resulting VOx fibers (VFs) are greatly limited by the solution formulation needed for the success of the electrospinning process. Significantly, the preparation of continuous VOx fibers with either diameter below 100 nm or variable vanadium oxidation states is still challenging. In fact, the reported methods to produce VOx phases with mixed V4+/V5+ oxidation state, like V3O7 or V4O9, are quite complex and still date from the 1960s of the last century.36−38 In this work, we propose a new approach to produce VOx nanofibers, allowing variable +4/+5 oxidation state and crystallinity, by electrospinning and subsequent calcination of water-free NH4VO3 solutions in the presence of a suitable reductant. This two-step synthesis procedure is outlined in Figure 1A. Ammonium metavanadate (NH4VO3) was used as vanadium precursor. The benefits of using this compound, instead of much more expensive organic-based precur-

sors,6,31−34 to obtain electrospun VOx nanofibers were highlighted by Mai et al.5 Nevertheless, this salt is insoluble in many of the conventional polar solvents. Thus, when using water as solvent, the authors did not obtain a homogeneous material but instead obtained fibers made up of NH4VO3 nanoparticles embedded into the poly(vinyl alcohol) matrix. Because of this inhomogeneity, the size of the V 2 O 5 nanoparticles after air-annealing, was determined by that of insolubilized NH4VO3. Similarly, by using this route the vanadium oxidation state was not controllable.

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RESULTS AND DISCUSSION Preparation of VOx Nanofibers. Ethanol is an excellent alternative to water as a solvent in electrospinning processes. Basically, it gathers a relatively high dielectric constant and volatility, promoting the yield of regular fibers;39 it can be sustainably produced from biomass, and it is relatively cheap and not harmful. However, the solubility of NH4VO3 in ethanol is very low.40 The addition of oxalic acid to the precursor solution greatly increased this solubility, leading to homogeneity and sufficient consistency of the resulting fibers after calcination. At the same time, this compound played a key role as vanadium reductant (see below). Finally, poly(vinylpyrrolidone) (PVP) and another solvent, dimethylformamide (DMF), were introduced to improve the spinnability of the ethanolic solutions. A freshly prepared spinnable precursor solution, with composition NH4VO3/ethanol/DMF/PVP = 1:9:9:4 and the necessary oxalic acid, was electrospun into NH4VO3/PVP nanofibers (Figure 1A). Details on this procedure can be found in the Experimental Section. TEM characterization reveals that this material consists of long-range continuous and defect-free fibers (Figure 1B), with diameters ranging between 20 and 90 nm (Figure 1C). The diameters of these precalcined fibers are smaller and show narrower diameter distribution than those reported in the literature prepared by similar procedures.5,6,10,31−34 On the other hand, the lack of characteristic features in the selected area electron diffraction (SAED) pattern (inset of Figure 1C) indicates that these fibers are amorphous. For the preparation of the different VOx nanofibers, the electrospun NH4VO3/PVP fibers were heattreated under air atmosphere at different conditions previously studied by thermogravimetry (TG) (see Supporting Information (SI)). TEM Characterization of VOx Nanofibers. The fibers calcined at the lowest temperature (VF-300-0.2) keep practically the regular morphology (Figure 2A,B) of the precalcined ones (Figure 1). When the calcination is carried out at 350 °C and 0.2 °C/min (VF-350-0.2), fibers with more irregular shape and diameter are obtained, probably due to a greater decomposition of the polymer matrix (Figure 2D,E). The magnified images clearly show the presence of dark spots or grains in these fibers (Figure 2E). The increase in the heating rate (HR) to 0.5 °C/min (VF-350-0.5) results in continuous and regular fibers made up of interconnected and well-defined small particles/crystallites of ca. 15−40 nm (Figure 2G,H). Finally, a greater increase in the temperature of up to 400 °C (VF-400-0.2) produces fibers with larger crystallites and diameter, which make them less discernible (Figure 2J,K). Textural Properties of VOx Nanofibers. N2 adsorption− desorption isotherms at −196 °C of the VOx nanofibers (SI) indicated that the samples are mainly macroporous, with Brunauer−Emmett−Teller (BET) surface areas in the range

Figure 1. (A) Scheme of the preparation procedure of VOx nanofibers. Inset: photographs of the NH 4 VO 3 −poly(vinylpyrrolidone) (PVP) fiber mesh in the crucible before and after calcination (example, VF-350-0.2 sample). (B, C) Transmission electron microscopy (TEM) images at different magnifications (inset: selected area electron diffraction (SAED) pattern) of the electrospun NH4VO3−PVP fibers. 7740


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Figure 3. V 2p core-level XPS spectra of the different VOx nanofibers.

of the VOx nanofibers, which display the characteristic V 2p1/2 and V 2p3/2 spin−orbit components separated at 7.4 ± 0.5 eV. The V 2p3/2 peaks of the XPS spectra were deconvoluted to estimate the contribution of vanadium species. For deconvolution, only the contributions of V5+ (around 517 eV) and V4+ (close to 516 eV) were considered,1,41 obtaining a good fitting of the experimental results. Although the presence of some V3+ in the samples should not be ruled out, this was supported by the X-ray diffraction (XRD), Raman, and SAED results, in which only V5+/V4+ mixed phases were found (see below). The V/O and V4+/V5+ atomic ratios of the different samples are collected in Table 1. The increase in the calcination temperature causes an increase in the surface V/O atomic ratio, surpassing that of V2O5 from 350 °C. Thus, the exact chemical formula of the different materials can be calculated as VO3.0, VO2.2, VO2.1, and VO1.8, for VF-300-0.2, VF-350-0.2, VF-3500.5, and VF-400-0.2, respectively, although part of the accounted oxygen may come from the remaining organic matrix. The analysis of the V 2p core-level spectrum revealed the presence of a varying amount of V4+. Extraordinarily, the fibers calcined at 300−350 °C and 0.2 °C/min display a remarkable surface V4+/V5+ ratio (ca. one-third). This coexistence of V4+ and V5+ species is characteristic of mixed-valence VOx. Given that the initial oxidation state of vanadium in NH4VO3 is V5+, the large contribution of V4+ may be associated with the reducing power of the organic compounds, i.e., oxalic acid42 and PVP,43 present in the electrospun NH4VO3/PVP fibers. The well-known reducing effects of oxalic acid on the synthesis

Figure 2. TEM images and SAED patterns of VF-300-0.2 (A−C), VF350-0.2 (D−F), VF-350-0.5 (G−I), and VF-400-0.2 (J−L).

10−50 m2/g and a mesoporous volume from 0.01 to 0.08 cm3/ g (Table 1). These surface areas are quite high for bulk oxide materials and may be easily accessible considering the ultrafine diameter of the obtained nanofibers.26 These properties are very attractive for multiple applications. Considering that the dimensions and morphology of the different fibers are quite similar, the observed differences on ABET may be explained in terms of the porous structure (shape, size, number of pores, cracks, etc.) developed upon thermal decomposition of the different precursors in the fibers. Specially, a large number of pores can be formed in the organic matrix. This would explain the highest ABET found for the sample (VF-300-0.2) with higher residuary C (see below) and its marked decrease when further decomposition occurs (VF-350-0.2). On the other hand, a complete removal of this matrix may reduce the diameter and leave empty voids around the nascent VOx crystals, explaining the relative increase of ABET for VF-350-0.5 and VF-400-0.2. X-ray Photoelectron Spectroscopy (XPS) Characterization of VOx Nanofibers. Figure 3 shows the V 2p spectra

Table 1. Textural Parameters, Surface Composition, and Main Crystalline Data of the VOx Nanofibers N2 adsorption

XPS

XRD

sample

ABET (m2/g)

Vmeso (cm3/g)

V/O

V4+/V5+

structure

crystal size (Å)

VF-300-0.2 VF-350-0.2 VF-350-0.5 VF-400-0.2

48.5 12.4 27.7 40.0

0.08 0.01 0.04 0.08

0.330 0.450 0.473 0.553

0.303 0.362 0.096 0.060

amorphous V3O7 V2O5 V2O5

143.0 287.0 557.1

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temperature and heating rate. All these results are in line with XPS and the TG study (SI). X-ray and Electron Diffraction. XRD (Figure 5) and SAED (Figure 2) characterizations reflected the crystalline

of mixed-valence supported vanadium oxide catalysts occur in the absence of PVP. Therefore, it is expected that the role of oxalic acid as a reductant in the reported preparation method must be important. Nevertheless, appropriate controlled experiments should be carried out to discern the reduction action of each organic compound. Remarkably, the rise in the heating rate or calcination temperature reduces the proportion of V4+, so fibers with different V4+/V5+ ratio are obtained. Considering the vanadium oxidation state, the chemical formula would be V(IV)0.23V(V)0.77O3.0, V(IV)0.27V(V)0.73O2.2, V(IV)0.09V(V)0.91O2.1, and V(IV)0.06V(V)0.94O1.8, for VF-3000.2, VF-350-0.2, VF-350-0.5, and VF-400-0.2, respectively. Apart from V and O, residuary C and N were detected by XPS on the surface of all the calcined samples (see SI). These species were assigned to unreacted PVP or NH4VO3 and could affect the properties of the VOx nanofibers. The obtained results show that the concentration of these elements decreased with the calcination temperature and heating rate, suggesting that a suitable adjustment of the variable calcination parameters may permit their complete elimination. Raman Analysis. Figure 4 shows the Raman spectra of the samples. VF-400-0.2 and VF-350-0.5 (see SI) spectra display

Figure 5. X-ray diffractograms of the different samples and standard patterns.

nature of samples calcined at or above 350 °C. By contrast, the absence of diffraction features (Figures 5 and 2C) pointed out the amorphous structure of fibers yielded at lower temperatures. Particularly, the Bragg angles of diffraction peaks of VF350-0.2 sample (Figure 5) fit well with those of the V3O7 pattern (JCPDS 98-000-2338), which crystallizes in the monoclinic structure with C2/c symmetry. This agrees with the SAED pattern of this material (Figure 2F), showing ringlike features corresponding to some of the characteristic crystallographic planes of V3O7. Interestingly, the theoretical V/O atomic ratio for this phase (0.430) is quite close to that estimated by XPS (Table 1). On the other hand, the X-ray (Figure 5) and electron diffraction patterns (Figure 2I,L) of VF-350-0.5 and VF-400-0.2 match with that of orthorhombic (Pmnm) V2O5 phase (JCPDS 01-085-0601). These results closely agree with Raman and XPS data. It is noteworthy to highlight the high reproducibility of the method to obtain these materials (SI). It is interesting that the crystalline phases of samples calcined with different heating rates (VF-350-0.2 and VF-350-0.5) are entirely different. This is assigned to the different reaction kinetics: the faster treatment avoids the yield of a significant proportion of V4+, leading to V2O5 phase, whereas a much slower heating seems to permit the reduction of V5+ into V4+ and the formation of V3O7 phase. This occurs if the final temperature is not too high because slowly heating up to 400 °C (VF-400-0.2) results in complete oxidation of vanadium and the formation of V2O5 phase. On the other hand, anhydrous

Figure 4. In situ Raman spectra under dehydrated conditions of VF300-0.2, VF-350-0.2, and VF-400-0.2 (practically identical to that of VF-350-0.5, see SI) nanofibers.

clear bands at 143, 283, 302, 405, 480, 526, 698, and 994 cm−1, which are characteristic of pure V2O5.44 By contrast, the Raman spectrum of VF-350-0.2 sample is quite different. Although it also shows the features of V2O5 phase, their intensity is considerably lower than in VF-400-0.2 and VF-350-0.5 samples. Regarding that V2O5 is a high Raman scatter; these facts suggest that V2O5 may be a minority phase in this sample. On the other hand, the spectrum also shows exclusive features. The band near 1020 cm−1, that redshifts upon hydration, is characteristic of V5+O stretching mode in supported vanadium oxide species.19,45 The bands near 900 cm−1 have been associated with the stretching V4+O bond.19,44 VF-300-0.2 and VF-3500.2 samples show two broad bands near 1350 and 1580 cm−1, which are assigned to C−H and CO and/or C−N bonds, respectively.45,46 This indicates that a considerable part of PVP has not been decomposed at 300 °C. The intensity of these bands decrease or they disappear with the calcination 7742


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the air gas flowed at 150 cm3(STP)/min. Once the final temperature was reached, the crucible was slowly extracted and let to cool down at the lab atmosphere. The effect of the heating rate (HR = 0.2; 0.5 and 1.0 °C/min) and final temperature (T = 300, 350, and 400 °C) on the calcination of the resulting fibers (denoted as VF-T-HR) was analyzed by different techniques. Characterization. The diameter and morphology of the different fibers were characterized by transmission electron microscopy (TEM). TEM images and selected area electron diffraction (SAED) patterns were obtained by a Philips CM200 microscope at an accelerating voltage of 200 kV. Nonisothermal thermogravimetric analyses were carried out in a CI Electronics balance (model MK2) under air atmosphere (150 cm3 (STP)/min). The sample mass was approximately 10 mg and the sample temperature was increased from room temperature up to 900 °C at 10 °C/min. The textural properties were analyzed by N2 adsorption− desorption at −196 °C in a Micromeritics ASAP2020. Samples were previously outgassed for 8 h at 150 °C under vacuum. From the N2 adsorption−desorption isotherm, the apparent surface area (ABET) was determined by applying the BET equation and the mesopore volume (Vmeso) was determined as the difference between the adsorbed volume at a relative pressure of 0.95 and the micropore volume (Vt) calculated by the t-method.48 X-ray photoelectron spectroscopy (XPS) analyses were obtained by using a 5700 C model Physical Electronics apparatus, with Mg Kα radiation (1253.6 eV). For the analysis of the XPS peaks, the C 1s peak position was set at 284.5 eV and used as a reference to position the other peaks. The fitting of the XPS peaks was done by least squares using Gaussian− Lorentzian peak shapes. X-ray diffraction (XRD) patterns of samples were acquired by a Philips X’Pert PROMPD diffractometer using Cu Kα radiation (λ = 1.5406 Å). The Xray generator was set at 45 kV and 40 mA. The crystallite size of the V2O5 crystalline samples was accurately calculated by the Rietveld refinement method using the Williamson−Hall equation,49 whereas that of V3O7 was determined by the Scherrer equation.50 Raman spectra were collected with a single-monochromator Raman spectrometer (Renishaw Micro-Raman System 1000) equipped with a cooled charge-coupled device detector (−73 °C) and an edge filter, which removes the elastic scattering. The samples were excited with the 488 nm line from an Ar+ laser operating at 10 mW. The spectral resolution was ca. 4 cm−1, and the spectra acquisition consisted of 10 accumulations of 30 s each. The spectra were obtained under dehydrated conditions (50 mL/min synthetic air, ca. ambient temperature −120 °C) in a hot stage (Linkam TS-1500).

crystalline phases are obtained in all cases because of the absence of water in the spinnable precursor solution. Table 1 also includes the average crystallite size of the different samples calculated from XRD patterns. As can be observed, the crystallite size gradually increases with the calcination temperature or heating rate, enabling to obtain a similar crystalline phase, for example V2O5, with different crystallite sizes. Such a difference in crystallite sizes is also deduced from the different intensity of SAED patterns of these samples (Figure 2F,I,L). Consequently, the obtained results show that the right choice of heating conditions allows obtaining VOx nanofibers with different V4+/V5+ ratios, crystalline phase, and crystal size.

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CONCLUSIONS

In summary, long-range continuous single-phase orthorhombic V2O5 and monoclinic V3O7 nanofibers have been prepared by electrospinning of water-free NH4VO3-saturated solutions including oxalic acid. The efficient incorporation of this compound, which acts as NH4VO3 solubilizer and reductant, results in VOx nanofibers with a considerable proportion of V4+. Further thermal treatment of these fibers permits to decrease the V4+/V5+ ratio. This opens the possibility to obtain long-aspect fibrous materials with different crystalline phases that are difficult to obtain by conventional methods. By the optimization of the parameters of this synthetic route, e.g., the nature and concentration of the reductant, the calcination conditions, etc., it is expected to obtain other interesting mixedvalence VOx materials. Hence, the reported procedure is considered a powerful tool for the design of new materials for catalytic, optical, and electrochemical applications.

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EXPERIMENTAL SECTION Materials. All chemicals were analytical grade and used as received. Ammonium metavanadate (NH4VO3) (99%, Sigma Aldrich), ethanol (J.T. Baker), poly(vinylpyrrolidone) (PVP) (Sigma Aldrich), anhydrous dimethylformamide (DMF) (99.8%, Sigma Aldrich), oxalic acid dihydrate (H2C2O4· 2H2O) (Scharlau). Preparation Procedure. In a typical synthesis, 0.25 g of NH4VO3 was dissolved in 8.5 g of solvent (ethanol/DMF = 1:1). To increase the salt solubility, oxalic acid was added and the mixture was ultrasonicated at 45 °C, obtaining a brownyellow solution. This substance acts as a chelating agent forming (NH4)2[VO(C2O4)2].47 The amount of oxalic acid was the minimum necessary to increase the solubility of NH4VO3 because any excess entails an excessive conductivity of the solution as well as the corrosion of the needle. After complete dissolution, 1 g of PVP was added to the solution, which was magnetically stirred for 2 h to obtain a homogeneous darkgreen gelled solution suitable for electrospinning. For the electrospinning process, the precursor solution was flowed at 0.5 cm3/h through the capillary tip of a single spinneret downward to a square plate used as a fiber collector. Both the spinneret and the collector were made of stainless steel. The tip-to-collector distance was 15 cm, and the electrical potential difference was 16 kV (the collector was at −8 kV and the tip at +8 kV). The electrospun NH4VO3/PVP nanofibers were collected in the form of a nonwoven mesh. For the preparation of the different VOx fibers (VFs), the electrospun NH4VO3/PVP fibers were heat-treated under air atmosphere at different conditions in a horizontal furnace, with

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01061. Experimental details on the thermal behavior and characterization of VOx nanofibers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +34 965902364. Fax: +34 96 590 3464. 7743


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Raúl Berenguer: 0000-0002-8627-4939 María Olga Guerrero-Pérez: 0000-0002-3786-5839 Present Address †

Instituto Universitario de Materiales, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain (R.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded by the Spanish Ministry of Economy and Competitiveness (MINECO) and FEDER for project CTQ2015-68654-R as well as the Erasmus Mundus programme (EurasiaCat EMA2/S2). Dr. R.B. thanks MINECO a “Juan de la Cierva” position (IJCI-2014-20012). Authors thank Prof. Miguel A. Bañares (ICP-CSIC, Madrid) for his help with Raman spectroscopy.

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Synthesis of Vanadium Oxide Nanofibers with Variable Crystallinity

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