How the Environment Encourages the Natural Formation of Hydrated V2O5.

Herein, we report the microscopic and spectroscopic signatures of the hydrated V2O5 phase, prepared from the α-V2O5 powder, which was kept in deionized water inside an airtight glass container for approximately 2.5 years. The experimental results show an evolution of the V4+ component in V 2p3/2 core energy level spectra, and a peak corresponding to σ-OH- bond appeared in the valence band spectra in the hydrated V2O5 powder sample due to the water intercalation. Vanadium metal oxide particles were found to be self-nucleated into micro/nanorods after a long period of exposure to an extremely humid environment. The distinct features in the spectra obtained with high-resolution transmission electron microscopy, micro-Raman scattering, and X-ray photoelectron spectroscopy confirmed the presence of structural water molecules for the first time in the long-aged naturally hydrated V2O5 phase.

Introduction

The structural instability of the orthorhombic α-V2O5 (space group Pmmn) phase has emerged as a constraint for the multi-cation battery design. The interplanar distance of 4.36 Å between [VO5–VO5] polyhedra chains in α-V2O5 may not be able to host reversibly large ionic radius cations into its crystal volume for a long-lifetime. For this reason, it may undergo irreversible structural deformation after a certain number of charge/discharge cycles. V2O5 has a wide range of applications in electrochromic, photochromic devices, gas sensors (ammonia, ethanol, pH sensor, NO2, H2O2, etc.), biosensors (urea, glucose, gene sequence, methylglyoxal etc.) smart glass windows in satellites, and aerospace vehicles.[1−4] The device performance may be reduced due to the degradation or permanent modifications in the V2O5 material in contact with humidity for a long-lifetime. However, the hydrated V2O5, which has water gradients in its structures, can be a suitable material for the long life application where a humid environment is the working condition. The hydrated V2O5 can be synthesized by the several methods such as sol–gel, hydrothermal, electrochemical, xerogel, etc.[5−11] In the past, numerous experimental and theoretical studies have been reported on the impact of water molecules into the V2O5 local structure, its electronic, electrical properties, multications insertion/extraction mechanisms, charge transfer, and energy band gap tuning.[12−15] Water insertion into the V2O5 structure breaks the symmetry and inverts the [VO5–] polyhedra along the c-axis direction resulting in an increase of the interplanar distance up to ∼15 Å.[16] The presence of structural water (free protons, i.e., H+ ions) may improve the electronic or ionic conductivity.[17] Thus, spectroscopic investigation of the chemisorbed and intercalated water gradient into the V2O5 local structure becomes imperative to study the reference signatures. We demonstrate here, how the V2O5 modifies its structure after having been kept in water for more than two years (∼2.5 years) at ambient conditions. The reported experimental evidence in this article is based on chemical reactions occurring at natural environmental conditions typical for the long lifetime application of the reported material.

Results and Discussion

Scanning electron microscopy (SEM) images of reference and hydrated V2O5 powder (see Figure a) show the significant surface morphology modifications, where the micron and submicron sized reference V2O5 powder particles (Figure b) merged to form wrinkled, porous, and self-nucleated nano/microrod mesh-like structures in the hydrated V2O5 sample (Figure e). The dimensions (length/width) of the microparticles of the reference V2O5 powder sample was calculated in the range from 0.2 to 2.0 μm, while the nanorods in the hydrated V2O5 sample have a diameter ranging from ∼30 to 130 nm. The flakes/meshes-like morphological features in the hydrated V2O5 sample (Figure e) have a thickness ranging from ∼30 to 50 nm. The bright field HRTEM images of reference (Figure c) and hydrated (Figure f) V2O5 samples support the findings of the SEM results. From the lattice images (Figure d,g,h), the measured interplanar spacing corresponding to the (002) plane, was calculated to be ∼0.57 and 0.64, 0.89 nm for the reference and hydrated V2O5 phases, respectively. The flakes/meshes showed less d-spacing values than the nanorod-like structures in the hydrated V2O5 sample. This implies that the flakes/meshes formation was an intermediate stage of hydration process before the micro/nanorods formation, which had a high gradient of water inside the structure. A substantial increase of interplanar spacing along the c-axis is evidence of water molecules intercalating between the layered V2O5 structure, resulting in an expansion of the crystal volume. Thus, both SEM and HRTEM results corroborate that a long exposure of α-V2O5 to deionized water modifies its structure into the hydrated V2O5 (δ-V2O5).

Figure 1

Reference V2O5 (Sigma-Aldrich) and hydrated V2O5 powder (kept into deionized water after ∼2.5 years) samples images (a), scanning electron microscopy (SEM) images (b,e), bright field and high-resolution transmission electron microscopy (HRTEM) images (f,c), and filtered lattice images of the reference V2O5 (d) and hydrated V2O5 (g,h) samples, respectively. The power spectrum generated from the lattice images is shown in the insets. HRTEM lattice images were acquired on the selected area (drawn by solid line squares) as shown in respective bright field images of the reference and hydrated V2O5 samples. The magnified SEM images of reference and hydrated V2O5 samples are shown in inset images of Figure 1b,e, respectively.

Figure a illustrates the basic electronic structures of α-V2O5 and hydrated V2O5. The Raman spectrum of the hydrated V2O5 exhibits an enhancement of the peak intensity corresponding to the bridging O1–V–O3/V–O1–V vibrational bonds (ranging from 200 to 700 cm–1) and two additional Raman bands appearing at ∼769 and 888 cm–1 (V–OH vibrational bond) Raman shifts as compared to the reference V2O5 sample (see Figure b).[18,19] Consequently, the Raman shift of vanadyl bonds (V=O2) was found blue-shifted along with the peak broadening in the hydrated V2O5 sample spectra, which may be due to the electrostatic interaction of water molecules or OH– ions along the c-axis. These electrostatic forces may reduce the V=O2 bond length, as a consequence of the volume expansion of the hydrated V2O5 unit cell, compared to the reference V2O5. The appearance of the additional Raman shifts, peak broadening, enhancement in peak intensity, and blue shift, indicates evidence of the presence of structural water into the hydrated V2O5 sample.[8] The HRTEM and Raman scattering results provide evidence of water molecule intercalation inside the V2O5 crystal structure at ambient conditions (the natural formation of hydrated V2O5). Furthermore, X-ray photoelectron spectroscopy (XPS) a surface sensitive technique was employed to verify the intercalated water into V2O5 (first layer of intercalated water molecules) by an escape depth of the electrons at a kinetic energy of oxygen (O 1s) and valence band core energy levels.

Figure 2

Electronic structures of reference and hydrated V2O5 phase (a), micro-Raman scattering spectra (b), and X-ray photoelectron spectroscopy (XPS) core energy level spectra of the reference and hydrated V2O5 powder samples, V 2p3/2 (c), O 1s (d), and VB (e), respectively.

Figure panels c and d show the O 1s, V 2p3/2, and VB core energy level XPS spectra of the reference and hydrated V2O5 powder samples. In the V 2p3/2 spectra (Figure c), the components at binding energies (BEs) of ∼516.3 ± 0.1 eV (fwhm = 1.8 eV) and 517.3 ± 0.1 eV (fwhm = 1.0 eV) are associated with V4+ and V5+ oxidation states, respectively.[20] The V4+ component contribution was increased in the hydrated V2O5 spectra compared to the reference V2O5 powder sample spectra, which may be due to the water molecules interaction with the [VO5–VO5] atomic layers and resulting in the reduction of V5+ to the V4+ oxidation state.[17] However, a residual presence of V4+ on the surface of reference V2O5 powder sample (Figure c) was due to the possible V6O13 secondary phase.[21,22] In general, the reduction of vanadium (V5+) in the V2O5 system occurs due to the loss of loosely bound lattice oxygen atoms. Furthermore, in O 1s core level spectra (Figure d), the BEs at value of ∼529.9 ± 0.1 eV (fwhm = 1.2 eV), 530.6 ± 0.1 eV (fwhm = 1.2 eV), 531.7 ± 0.1 eV (fwhm = 1.5 eV), and 532.8 ± 0.1 eV (fwhm = 2.3 eV) are assigned to vanadium–oxygen (V–O), V–OH, and H2O chemical bonds in the reference and hydrated V2O5 samples, respectively.[23] The H2O-related O 1s component is totally absent in the reference sample while it is clearly present in the hydrated one, providing a direct proof of the water presence. Due the dielectric property of the V2O5 (energy band gap = ∼2.3 eV), the VB spectra are dominated by the O 2p contribution,[24] associated with the three types of lattice oxygen bonds with vanadium, that is, V–O, O–V–O/V–O–V, and V=O in the V2O5 structure. The V 3d conduction band appeared at the BE of ∼1.1 eV below the Fermi energy (EF) level, due to the O 2p–V 3d hybridization (Figure e). The V 3d component appeared because of the presence of the V4+ state, resulting in an increase of the electron density in the conduction band.[25−27] Most notably, the VB spectra of the hydrated V2O5 sample depicted the enhanced intensity (increased by the factor ∼2.0) of V 3d band along with an additional peak at the BE of ∼10.6 ± 0.3 eV assigned to the σ-(OH–) chemisorbed bonds.[20] The O 2p line shape was slightly affected (become narrower) by the water molecules interaction in the hydrated V2O5 sample spectra.

In a complementary manner, the microscopic and spectroscopic data analysis depicted the natural phenomenon of H2O molecules intercalation into the V2O5 local structure occurred by the kept into the deionized water for a long time period without any applied external electrical potential or other electrochemical forces. The mixed oxidation of V5+ and V4+ are associated with the V2O5·xVO2·nH2O phase in case of the hydrated V2O5 phase.[28] The free electron density in the V 3d state of the hydrated V2O5 or reduced V2O5, contributed from VO2± phase. The structural water increased the interplanar distance between the [VO5–VO5] polyhedra, calculated from HRTEM data, resulting in an increase of overall crystal volume of the hydrated V2O5 phase as proved by HRTEM diffraction analysis. The phase modification from α-V2O5 (∼990 cm–1) to δ-V2O5 (∼1010 cm–1) occurring due to an electrostatic interaction force from water molecules was observed.

Conclusion

The spectroscopic and microscopic signatures confirmed the hydrated V2O5 phase. These results may be considered as the reference signatures of the naturally aged hydrated V2O5 material. The reported results are important to distinguish the naturally hydrated V2O5 and hydration by the artificial methods in future related studies. The zero cost and natural method to produce the hydrated V2O5 nano/microrods, flake-like structure may replace the synthetic experimental ways. The self-nucleation of vanadium metal oxides micro/nanorods, like biological things growing in nature, is a new concept of material growth for the material science community.

Experimental Section

SEM images were captured by a ZEISS Gemini FE-SEM Sigma 300 instrument with a field emission gun at the Physics division, University of Camerino, Italy. Micro-Raman scattering spectroscopy (μ-RS) spectra were acquired by the HORIBA IHR320 apparatus equipped with a 50× objective lens (probing depth 500–800 nm) micro-Raman spectroscopy with an argon-ion green laser light source (λ = 532 nm) and grating system at 600 lines/mm at the Physics division University of Camerino, Italy. A constant value of the laser power (∼5 mW) was applied to avoid the internal thermal effect on both the reference V2O5 powder and the hydrated V2O5 sample. High resolution transmission electron microscopy (HRTEM) images and the selective area electron diffraction (SAED) pattern were using a JEOL 2010 UHR field emission gun microscope operated at 200 kV with a measured spherical aberration coefficient (Cs) value of 0.47 ± 0.01 at the TASC laboratory, CNR-IOM, science park, Trieste, Italy. The TEM sample preparation of the reference and hydrated powder samples was followed by ultrasonication for 30 min in ethanol and deionized water, respectively. Later on, the powder samples were drop cast on the carbon coated Cu grid with 200 μm2 mesh size. Scanning photoelectron microscopy (SPEM) at the ESCA microscopy beamline at the Elettra Synchrotron Light Center, Trieste, Italy, was employed to study the electronic structure modification before and after water intercalation into the V2O5 structure by analyzing the core energy level and valence band spectra. For the SPEM experiments performed in an ultrahigh vacuum (UHV) (∼10–10 mbar) chamber, the powder samples were sandwiched between TEM grids, which were mechanically mounted between stainless steel clips to avoid any contamination and substrate effect. An incident photon energy of 650.0 eV (calibrated by Au 4f) was used to acquire the V 2p, O 1s, and valence band (VB) spectra. The hydrated V2O5 powder sample was subjected to heat treatment at 80 °C temperature inside the UHV chamber for 12 h, to remove the surface contamination and adsorbed water contents from the powder surface.