Cite this: Phys. Chem. Chem. Phys., 2014, 16, 4699
Local geometric and electronic structures of
gasochromic VO
x
films†
Wei-Luen Jang,aYang-Ming Lu,bChi-Liang Chen,cYing-Rui Lu,ad
Chung-Li Dong,*aPing-Hung Hsieh,eWeng-Sing Hwang,eJeng-Lung Chen,a Jin-Ming Chen,aTing-Shan Chan,aJyh-Fu Leeaand Wu-Ching Chouf
VOxfilms were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at
room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VOx films were
amorphous with mixed V4+ and V5+ valences. The amorphous VO
x had a disordered and expanded
lamellar structure resembling that of polymer-intercalated V2O5 gels. VOx films were crystallized into
orthorhombic V2O5 at 300 1C, and the lamellar structure was eliminated at 400 1C. Additionally, the
gasochromic reaction reduced the vanadium valence via intervalence transitions between V5+and V3+. Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V2O5crystalline phase irreversibly into an H1.43V2O5phase. Based on the results of this
study, amorphous VOxwith a lamellar structure is recommended for use in H2gas sensors.
Introduction
Electrochromic thin film material is a rapidly growing field owing to its wide range of applications. Two highly promising commercial applications of this material are energy-saving window materials and hydrogen sensors. Buildings consume approximately 40% of all energy used in daily life, and humans spend an average of 80% to 90% of their lives indoors,1
warranting significant improvements in the energy efficiency of buildings. Windows account for a significant amount of wasted energy in buildings; electrochromic windows represent a viable energy-saving solution.2,3One proposed electrochromic window is made of a laminated thin film structure, in which current is generated in the thin film battery when the sun shines on the window, which darkens it. Despite the unavail-ability of this technology, electrochromic films may serve this
purpose in the future. Additionally, renewable energy resources have received considerable attention, owing to the global energy crisis and extreme climatic conditions worldwide. Hydrogen burns to form water, and no gas is responsible for global warming. Therefore, hydrogen is widely regarded as a promising alternative energy source to replace oil.4,5The risk of
hydrogen exploding in air at concentrations as low as 4% poses a major obstacle to adopting hydrogen energy technology. Safely handling hydrogen is critical to successfully developing a hydrogen economy. Hydrogen leakages at storage or usage sites must be monitored continuously to ensure safety. Hydrogen sensors are characterized by their durability and reliability. Despite the use of electrochromic films in H2 sensors, their
reliable operation depends on replacing the liquid electrolyte regularly.6 Developing a hydrogen sensor that consumes low power and requires negligible maintenance is thus of priority concern. One feasible approach fully exploits the gasochromic properties of electrochromic thin films.7,8 These materials change color when exposed to hydrogen, allowing for the detection of hydrogen by monitoring the changes in their optical transmission. This approach is superior to measuring electrical conductivities for sensing, possibly causing a hydrogen explosion.6
Vanadium oxygen systems comprise many oxide phases, including VO, V2O3, VO2, V6O13, V3O7, and V2O5. Of these,
V2O5has the highest oxidation state and is widely regarded as
an intercalation host for Li or H cations. V2O5has a diverse array
of applications, including Li batteries,9,10optical H2sensors,11–14
aNational Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan. E-mail: [email protected]
b
Department of Electrical Engineering, National University of Tainan, Tainan 70005, Taiwan
c
Institute of Physics, Academia Sinica, Taipei 11529, Taiwan dProgram for Science and Technology of Accelerator Light Source,
National Chiao Tung University, Hsinchu 30010, Taiwan eDepartment of Materials Science and Engineering,
National Cheng Kung University, Tainan 70101, Taiwan fDepartment of Electrophysics, National Chiao Tung University,
Hsinchu 30010, Taiwan
†Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp54773f Received 12th November 2013, Accepted 16th January 2014 DOI: 10.1039/c3cp54773f www.rsc.org/pccp
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and electrochromic devices.15,16V2O5 thin films are yellow in
their bleached state, turning gray or black with the insertion of ions. V2O5for use as an intercalation host is normally prepared
using the sol–gel method.17 Upon the slow removal of the
solvent, V2O5film becomes amorphous with an ordered stacking
of V2O5ribbons.17As is widely assumed, the lamellar structure
provides a channel for the rapid intercalation of ion species. In crystalline V2O5,18phase transformations occur during
inter-calation and de-interinter-calation processes, subsequently forming irreversible g and o phases. These irreversible phases either yield an unsatisfactory cycle performance or reduce the capacity of thin-film Li batteries. In contrast, no phase transformation occurs in amorphous vanadium oxide during Li intercalation. Correspondingly, the structural properties are important to the intercalation properties of the films. Sensor films prepared by vacuum evaporation19,20 are characterized by their sensitivity, easily controlled deposition parameters and uniformity,21,22 allowing for a detailed study of the coloration mechanism with respect to their electronic and atomic structures. Growth para-meters can be fine-tuned to optimize the thin film. Most studies investigate the optical properties of such films.7,23,24Absorption/
transmittance spectra are examined to describe the optical properties of sensor films during reduction/oxidation.23,25 Gasochromic V2O5films have rarely been investigated. To our
knowledge, no study has elucidated the gasochromic properties of sputtered vanadium oxide films in terms of atomic/electronic structures. The gasochromic switching of vanadium oxide is of great interest from both technological and fundamental perspec-tives. In this study, amorphous VOxthin films are prepared by
sputter deposition. Their film structures are modified by thermal annealing. Based on in situ X-ray absorption spectroscopy (XAS), the electronic and local atomic environments are investigated to determine the parameters for fabricating gasochromic films. Additionally, the overall gasochromic properties of sensor films are determined using the shapes of the XAS spectra. Moreover, exactly how the structural parameters affect the gasochromic reaction of the films is examined. Results of this study demon-strate that the amorphous VOxfilm with a lamellar structure and
with a locally distorted pyramid (Py) symmetry is highly promis-ing for use in H2sensors.
Experimental
VOxfilms were deposited on a Corning 1737 substrate by using
a radio-frequency reactive magnetron sputtering system with a vanadium target. The working power was fixed at 150 W and the sputter deposition was performed at a gas pressure of 1.33 Pa in pure O2. Thickness of the deposited films was controlled at
200 nm. The as-deposited VOx films were then annealed at
200 (AA200), 300 (AA300), and 400 1C (AA400) in air for 5 min. Next, a thin (B5 nm) Pt layer was deposited on the as-deposited and annealed VOx films as a hydrogen catalyst. During the
gasochromic reaction, the films were exposed to pure hydrogen for 2 h at 101.325 kPa and 25 1C. Experiments involving synchrotron-based approaches were undertaken at the National
Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The crystalline structures of the films were determined by grazing incidence X-ray diffraction (GIXRD) and X-ray powder diffraction (XRD) at BL01C2. The electronic and atomic struc-tures of the as-deposited and annealed VOxfilms were studied by
XAS. The synchrotron XAS were recorded two to three times to ensure the data reliability and reproducibility (Fig. S1–S5, ESI†). The V L2,3-edges and O K-edge XAS were recorded at the
soft-X-ray beamline BL20A. A homemade flow gas cell was used to perform in situ (polarized) V K-edge XAS measurements. The angles of incidence between the photon beam and the sample surface were 201, 451, and 901. V K-edge XAS spectra were collected at the hard-X-ray beamline BL17C. The resolutions for hard-X-ray and soft-X-ray spectroscopic measurements were set to 0.5 and 0.2 eV, respectively.
Results and discussion
Fig. 1 illustrates the GIXRD patterns of the as-deposited and annealed VOxfilms. Reference samples of polycrystalline V2O5
(orthorhombic), VO2 (B), and VO2 (M) films were prepared.
Fig. 1(a) shows their spectra. Crystalline V2O5was obtained by
thermally annealing as-deposited VOxat 500 1C in air for 2 h.
VO2(B) was obtained by thermally annealing as-deposited VOx
at 360 1C in a 90% Ar–10% H2mixture for 3 h. VO2(M) was
obtained by annealing VO2(B) at 500 1C in N2for 1 h. Structures
of the standards were confirmed by GIXRD. The as-deposited VOx yielded only a broad peak at B61. This low-angle peak
originated from the stacking structure along the c-axis and was indexed as (001).26 The shift of the peak to a higher 2y angle coincided with the decrease in the full width at half maximum (FWHM) when increasing the annealing temperature to 200 and 300 1C. Diffraction peaks that correspond to the ortho-rhombic V2O5phase (JCPDS card no. 41-1426) were found at 300,
Fig. 1 GIXRD patterns of (a) reference samples and (b) as-deposited and annealed VOxfilms.
400 and 500 1C. According to the GIXRD patterns, the as-deposited VOxand AA200 were amorphous. Additionally, the films readily
dissolved in water, yielding a clear yellowish solution, verifying the amorphous nature of the films, as suggested by Livage et al.17,27 The broad peaks of the as-deposited VO
x, AA200,
and AA300 are related to the (001) peaks of materials with lamellar structures.17,28 Moreover, the interlayer distances determined from the Bragg’s law are 14.7, 12.83, and 11.29 Å, respectively.
Many studies have attempted to improve the electrochemical properties of V2O5xerogel powders by changing the procedure
for drying bulk materials or adding the inert ceramic material.17,26 The objective is to modify the final products morphologically and structurally to enhance their electrochemical performance, including their ionic conductivity. However, controlling the final product quality from the liquid solution is rather difficult. Herein, sputtered VOxfilms were prepared and the annealing
temperature was controlled to obtain different annealed films with different degrees of amorphicity. With only a few studies describing amorphous vanadium oxide films prepared by sputter deposition,18,21,29to our knowledge, no information is
available on their lamellar structures. Similar lamellar struc-tures have been identified in V2O5nH2O gels, which comprise
ribbon-like species.17Previous studies have suggested that this ribbon-based structure is a bilayered structure.30–32A related study found ordered stacking of the V2O5structure when the gel
was deposited onto a flat surface with the slow removal of the solvent.17 The XRD pattern of the ordered xerogel generally includes a strong (001) peak with other sets of (00l) peaks. The interlayer distance depends on the water content (11.5 Å for V2O51.8H2O and 8.8 Å for V2O50.5H2O).17,28Additionally, the
intercalation of polymers substantially reduces the (001) inten-sity and eliminates some (00l) peaks.29,30 Intercalation also increases the interlayer distance and FWHM of the (001) peak. The vanadium oxide films in this study have interlayer dis-tances that are comparable to (or even greater than) that in V2O5:xH2O. However, these films do not contain water
mole-cules since sputtering occurs in a vacuum and should not involve water. In V2O5:xH2O, removing the water molecules
reduces the interlayer distance. This effect was not observed in this study, which demonstrates that maintenance of the large interlayer distance does not depend on molecular water. The GIXRD pattern of as-deposited VOx herein resembles that of
polymer-modified V2O5gels, implying that this large interlayer
spacing arises from the intrinsic properties or some sputtered species are intercalated into the layers of as-deposited VOx.
While using X-rays to promote electrons from core levels to partially filled and empty states, XAS spectroscopy is a local element-specific probe of the electronic and geometric struc-tures of a material. This method is thus a highly effective means of studying vanadium oxide-based materials that exhibit various symmetry distortions and valence states. In a purely ionic representation of the V2O5ground state, the O 2p orbital
is completely filled (O2 ) whereas the V 3d state is unoccupied (V5+, 3d0 configuration). The covalent V–O interaction causes a significant contribution of the O 2p orbitals above the
Fermi level, in an energy region dominated by the V 3d orbitals. With perfect Ohsymmetry, the V 3d orbitals are split into two
sub-groups, i.e. t2gat the lower energy and egat the higher one,
by crystal field splitting. Deviations from the octahedral environment are particularly strong for V atoms in the V2O5
crystal structure and it is associated with further splitting of the 3d t2g and 3d eg orbitals. The t2g-like orbital is split into the
lower 3dxy orbital component and the higher 3dxz and 3dyz
orbital components; the eg-like orbital is also split into 3dx2 y2
and 3dz2components. In this study, the electronic structure and
vanadium valence of the as-deposited and annealed VOxfilms
were determined from the XAS spectra of the V L- and O K-edge, as shown in Fig. 2(a) and (b), respectively.
In the V L-edge XAS in Fig. 2(a), spin–orbital coupling splits the V 2p core hole into 2p3/2and 2p1/2levels, yielding the V L3
and V L2edges at approximately 518 and 525 eV, which arise
from V2p3/2- V 3d and V2p1/2- V 3d transitions, respectively.
The spin orbit splitting between the L3and L2peaks is around
6.7 eV for the AA500 film, which is similar to that of single crystal V2O5.33,34Notably, the L3XAS of crystal V2O5 includes
several recognizable features, A2–D2. As mentioned above, the
peaks in the L3XAS arise mainly from the crystal field splitting
of the final 3d orbitals. The nature of the L2 transitions
resembles that of the L3 transitions. However, the spectral
resolution in the V L2-edge was lower than that in the V L3-edge.
The L2 peak is broadened by a Coster–Kronig Auger decay
process into a 2p3/2hole, subsequently increasing the number
of decay channels and ultimately shortening the lifetime of the excited state. Simultaneously, the peak is broadened by time–energy uncertainty. Therefore, the V L2-edge is normally
less informative than the L3-edge in terms of elucidating the
electronic structure.
Of the fine structures in the V L3-edge of crystalline V2O5, the
very weak A2feature in the low-energy range originates from the
Fig. 2 (a) V 2p and (b) O 1s XAS spectra of as-deposited and annealed VOx
films and standards (VO2(M), VO2(B), and V2O5).
mixed configurations that arise from the transitions toward 3d orbitals dominated by both the central V atom and the V neighbor atoms. This feature can be described in terms of molecular orbitals that comprise both a central V 3dxy
compo-nent and 3d t2g-like components of the neighboring V atoms.35,36
Approximately 1.4 eV above A2is B2, which is a transition that is
characteristic of the other two t2g-like components, 3dxz and
3dyz. The increased intensity of this peak B2 is caused by an
increase in the 3d contribution of the excited V atoms relative to that of the peripheral V atoms in the final states.35,36The C2
and D2 peaks are associated with the eg-like components.
According to a finite cluster model calculation, the C2 peak
has a higher percentage of 3dx2 y2 character of the central
V atom while the D2peak is derived from transitions that are
primarily associated with the 3dz2component of that atom.34,36
Although the assignments of peaks A2 and B2 contradict an
earlier study that assigned them to transitions from V 2p to V 4s,37 disordered vanadium oxides such as powdered and polycrystalline samples never generate two peaks A2 and B2.
For single crystals, the shoulder is pronounced.37,38The smear
of the fine structures can be viewed as an indicator of the degree of disorder. This disorder may affect the 3d contribution from the central and neighboring V atoms. Notably, the separa-tion of C2and D2is less clear in the spectrum of the VOxsample
than in that of the crystalline V2O5sample, probably owing to
the higher degree of disorder in the first case.34,38Additionally, C2(3dx2 y2) from VOxand AA200 is more intense than that from
crystalline V2O5perhaps because the central V atom is closer to
the basal plane; the dx2 y2component is thus stronger. Above
results verify the amorphous nature of the as-prepared VOxfilm
and its crystallization upon thermal annealing, as revealed by GIXRD. This nature may also be owing to the association of V4+ with VO2. In the V 2p XAS spectra, the L3 peak of the VO2
standards shifts to a lower photon energy than that of the V2O5
standard, as indicated by the upward and downward-pointing arrows in Fig. 2(a). The shift is owing to the increase in the occupancy of electrons, which enhances the screening of the core holes, thereby lowering the absorption energy. The L3
main peaks of VOx, AA200, and AA300 are relatively broad,
and the energy range covers the L3 main peaks of the VO2
and V2O5 standards. Additionally, the characteristics of V4+
gradually disappear as the annealing temperature increases. The V 2p XAS spectra of AA400 and AA500 are almost the same as that of the V2O5 standard, which is consistent with the
GIXRD results. Experimental results indicate that, in addition to the as-deposited VOxfilms having a +4 and +5 mixed valence,
thermal annealing reduces the amount of V4+. The spectral profile depends not only on the charge state but also on the orbital symmetry, as revealed in greater detail by the O K-edge XAS discussed later.
Fig. 2(b) presents the O K-edge XAS. The O 2p orbitals are greatly mixed with the corresponding V 3d orbitals, giving rise to a widely spread and considerably intense absorption signal. This finding clearly indicates the significant covalent character of the V–O bonds in the film. The spectra of the all samples exhibit clear similarities, and each one can be divided into
two regions. The first region is the pre-edge from the absorp-tion edge up to 5–7 eV and includes two distinct structures at 529 (E2) and 531 eV (F2) that are split into O 2p–V 3d(t2g) and O
2p–V 3d(eg) states, respectively, by the crystal field effect. More
specifically, the most intense peak at low energy in the O K pre-edge spectra has a t2g-like structure and corresponds to the
p interactions between the O 2pxylevels and V 3dxy, 3dxz, and
3dyzlevels;34,36the high-energy eg-like structure arises from s
interactions between O 2pz orbitals and V 3dx2 y2 and 3dz2
orbitals that point directly toward the oxygen ligands.34,36 Accordingly, the pre-edge spectrum is highly sensitive to changes in the charge states and orbital symmetry upon thermal anneal-ing. As is expected, the crystallographically distinct oxygen atoms differ slightly in the overlapping O 2p–V 3d hybridization, thereby affecting the spectral profile and orbital energy. Hence, the modification of the O K-edge upon thermal annealing is attributed to an increase in the number of crystallographically inequivalent oxygen sites. The second region appears at higher energies and exhibits a highly complex structure. This region is associated with the mixture of O 2p states with V 4sp states, and is pushed up in energy by the increase in strength of the O 2p–V 4sp interactions (peak G2). The G2 peak in VOx, and AA200 is
shifted to a higher energy than those of AA300, AA400 and AA500 because the 3d orbitals become more extended with an increas-ing electron occupancy, resultincreas-ing in a greater overlap with the O 2p orbitals and a larger splitting. This finding implies that the valence is lower in VOxand AA200 than the other films, which
corresponds to the observed shift of the V L3-edge in as-prepared
or AA200 films to a lower energy.
According to Fig. 2(b), t2g (E2) and eg(F2) structures of the
sputtered films are similar to those of the standards. Closely examining the energy separation, DE(Eeg Et2g), of these films
reveals that the DE decreases with an increasing annealing temperature; in addition, their values equal that of V2O5above
300 1C. The energy separation, DE, yields information about ligand-field splitting, which is influenced by the local coordina-tion of vanadium atoms. As mencoordina-tioned above, in Ohsymmetry,
the 3d orbital is split into t2gand egstates. In V2O5with a C4v
crystal field, the t2gorbital is further split into e (dxz, and dyz)
and b2(dxy) states; in addition the egorbital is split into a1(dz2)
and b1 (dx2 y2) states.39 Generally, 10Dq refers to the energy
splitting between the t2g and eg orbitals, whereas 4Ds + 5Dt
and 3Ds 5Dtare the energy splits DEt2g(e and b2) and Deg
(a1and b1). Therefore, the energies of the e, b2, a1and b1levels
in C4vsymmetry are 6Dq+ 2Ds 6Dt, 6Dq+ 2Ds 1Dt, 4Dq+
2Ds 1Dt, and 4Dq 1Ds + 4Dt, respectively.39 The energy
separations DEt2g(e and b2) are too small to be resolved in the
X-ray absorption spectrum, but they can be identified by the cluster model calculation with a single particle approximation.39
Notably, Deg(a1and b1) is approximately 1.4 eV, allowing for the
contributions of (dz2) and (dx2 y2) to be determined from the
deconvoluted XAS spectrum.
Fig. 3 presents the deconvolution of the pre-edge region of the O K-edge. The inset displays the variations of the t2gand eg
(dz2 and dx2 y2) states for all of the films. The inset shows
changes in the relative weights of the dz2and dx2 y2structures,
suggesting the atomic rearrangement of these films. The fact that the weight of dz2 is lower in relation to dx2 y2 in
as-deposited VOx than in AA500 suggests that the central
V atom is closer to the basal plane.34 Therefore, the distance between the central V and vanadyl oxygen is larger, explaining why the dz2contribution is smaller and the dz2energy (as observed in
the deconvoluted XAS) is lower. Notably, the 3d occupancy number significantly affects the t2gpeak, and egis closely related
to the structural symmetry. The t2gpeak from VOxand AA200 is
less intense than in the films that are annealed above 300 1C. Thus, the change in the intensity of the t2gpeak, the spectral
profile and DE confirm not only that V4+ is present in the
as-deposited VOx, but also that V4+ is present in the form of
VO2(B). Similarities in the 4sp region support this inference, as
shown in Fig. 2(b). The mixture of VO2 (B) in the O 1s XAS
spectra may contribute to the disorder in the film. Therefore, the decrease in the interlayer distance and the FWHM after thermal annealing, as revealed by GIXRD, is related to the rearrangement of the sputtered atoms and the decrease in the amount of VO2(B) in the films.
Fig. 4(a) displays the V L-edge XAS of the films following a gasochromic reaction. Inserting hydrogen changes the color of the film and causes its spectral features to undergo a chemical shift to a lower energy; in addition, its line shapes to become more like those of VO2. The negative shift of the V L-edge with a
drop in valence can be understood as being caused by a decrease in the attractive potential of the nucleus and a weak-ening of the repulsive Coulomb interaction of the core with the other electrons in the compound. When the film adsorbs H2,
the electron may enter the 3d orbital, thereby reducing the V charge state. The additional electron lowers the charge state of the vanadium ions, causing a shift of the main peak of the V L3-edge to a lower energy. Notably, the L-edge spectrum in
the colored state is less intense than the spectrum in the bleached state. This reduction in intensity can be viewed as a drop in the number of 3d-unoccupied states, implying an increase in the charge in the t2g state of the d orbital upon
coloration. The calculation of the band structure by C. He´bert et al. suggests that a reduction in valence and a change in the structural symmetry influence the crystal-field effect.40,41 Therefore, the change in intensity also reflects a modification in the relative weight of the xy- (in-plane) and z (out-of-plane) characteristics of 3d-orbital transitions in C4v symmetry.
This modification is owing to a highly anisotropic layered structure of the film. The difference between the 3d band splitting in the bleached and colored films is related to the variations in the structural geometries and electronic configu-rations, and the subsequent difference between the line profiles of XAS spectra.
Fig. 4(b) presents the corresponding O K-edge. Notably, the most significant change between the colored and bleached states in the O K-edge region is that the intensity of the t2g
structure is lower in the colored film than that in the bleached films. This phenomenon reflects the extra electron occupancy in the t2g orbital that is associated with the incorporation of
hydrogen atoms. The d-orbital of V5+is formally d0(t2g0eg0) and
V4+is formally d1(t2g1eg0), explaining why the intensity ratio of
t2g and eg is directly related to the 3d orbital occupancy. The
drop in t2gof the films upon the gasochromic reaction is caused
by the reduction of V5+, and is consistent with the V L-edge. The extra electron occupancy may affect the 3d(t2g)-O hybridization,
alter the orbital orientation and, ultimately, distort the lattice structure. The above results suggest that the changes in orbital orientation are attributed to the modification of the lattice structure from the distorted Oh-like symmetry (C4v) of the V2O5
thin film in the bleached state to the more symmetrical Oh-like
structure of the film in the colored state, which correlates well with the V K-edge results.
Fig. 3 Experimental pre-edge of O K-edge XAS for V2O5and deposited
VOxfilms and those fitted by Gaussian profiles (red circle). Inset (left) shows
the fitted data using Gaussian profiles for t2gand egstates. Inset (right)
represents the integrated area of (a1/b1) and t2gpeaks.
Fig. 4 (a) V L2,3-edge and (b) O K-edge of XAS of VOxfilms in bleached
states (solid lines) and colored states (hollow symbol).
Unlike the V L-edge that is used primarily to probe V 3d states (atomic in nature), V K-edge spectral features represent transitions from the core levels to 4p-like states that are less localized than the 3d states. Thus, as is expected, the V K-edge provides further insight into energy band dispersion that arises from the solid-state effects by probing the more delocalized levels that are derived from V 4p orbitals. Although sensitive to the valence and the local atomic surroundings of the absorbing element, the features at the V K-edge XAS are also influenced by constructive and destructive interferences (i.e. single and multi-ple scattering) from the neighbors around the central vanadium atoms. Such interferences are caused not only by several electronic many-body effects (e.g., quadrupolar transitions), but also by changes in the medium- and long-range environ-ments. While the main transition at the K-edge is electric dipolar-allowed (1s to 4p), the pre-edge peak area is correlated mainly with the transition from the 1s core level to empty 3d states.42The electronic transition in the pre-edge area becomes allowed when the inversion center is lost. In an Oh-like
struc-ture, the loss of symmetry allows for partial overlapping and mixing of the unfilled d states of the metal with the 4p orbital of the metal via V ligands (2p states).43 Normally, the pre-edge peak intensity is virtually zero when the symmetry around the absorber is regular octahedral; this intensity is higher when the symmetry is tetrahedral.44Therefore, the pre-edge intensity is a clear fingerprint of the change in symmetry. Additionally, this intensity is used to evaluate qualitatively the alteration of the local structure of vanadium. A rather intense pre-edge peak is obtained from the vanadinite with tetrahedral V5+, and the pre-edge peak becomes very weak when the symmetry is regular octahedral, as it does for coulsonite.44
Fig. 5(a) and (c) display the V 1s XAS spectra of the films before and after gasochromic reaction. Fig. 5(b) compares these spectra. In the gasochromic reaction, the films are exposed to hydrogen at 101.325 kPa and 25 1C for 2 h. All spectra were
obtained at an angle of incidence of 451. The insets present photographs of the films before and after the gasochromic reaction. According to Fig. 5(a), the spectra of VOxand AA200
before gasochromic reaction are almost the same, and they converge to the characteristic spectrum of crystalline V2O5 at
temperatures above 300 1C. According to Fig. 5, the V K-edge is highly sensitive to the local symmetry of the vanadium site. All spectra include a pre-edge A5, which is well known to be a
formally forbidden 1s–3d electronic transition, which becomes dipole-allowed as the full local Ohsymmetry is reduced. Two
factors influence the pre-peak. First, distortion of the symmetry from Ohtends to significantly enhance the pre-edge intensity.43,44
Second, the emptiness of the p states in the p–d hybridized orbitals increases as the number of d electrons decreases, subsequently raising the pre-edge peak.42 Pre-edge A5in both
VOx and AA200 is less pronounced than that in the more
crystalline AA300 and AA400, owing to the change in the vertical asymmetry of the apical V–O bond, which is confirmed by the results for xerogel, aerogel and cystalline V2O5.45The pre-peak
intensity of crystalline V2O5 surpasses that of xerogel and
aerogel. Notably, xerogel has V–O apical bonds at 1.58 and 2.31 Å, and aerogel at 1.6 and 2.31 Å.45These values differ by less than those of crystalline V2O5, which are 1.58 and 2.79 A.
This fact explains why the pre-edge peak of VOx and AA200
has a lower intenstiy. The main feature B5 is associated with
the continuum states and arises from multiple scattering resonances of the photoelectrons. The symmetry of the four basal oxygens that surround the vanadium sites influences the intensity and shape of the edge resonance.46,47The intensity and shape of the edge resonance of the VOxand AA200 differ
from those of the crystalline AA300 and AA400, as revealed by peak B5in Fig. 5(a). This occurrence follows from a different
local structural arrangement of the VO6units in VOxand AA200
from that in crystalline AA300 and AA400.
The gasochromic reaction shifts the absorption edge to a lower photon energy and reduces the pre-edge intensity, as displayed in Fig. 5(b). The gasochromic reaction coincides with the change in the color of the film from yellow to gray. The spectral and color changes in AA400 are relatively minor. The color becomes black upon exposure to hydrogen forB72 h, as discussed in a later section. Peak B5following the absorption
jump is associated with scattering in the plane of the pyramid base.46,47 Therefore, the main differences between the films before and after the gasochromic reaction arise from the fact that the rearrangement of the atoms of the base causes less distortion in the colored state than in the bleached state. Peak A5originates from the multiple scattering pathways along the
axis of the pyramid43,47and, therefore, depends on the posi-tions of the apical V–O. Interestingly, the colored states of VOx
and AA200 have markedly different spectral profiles from that of the bleached state. For AA400, the spectral profiles in the colored and bleached states closely resemble each other. How-ever, although the profile of AA300 in the bleached state resembles that of AA400, those in colored states differ from each other, suggesting that color switching is closely related to the amorphous character.
Fig. 5 V K-edge XAS spectra of films (a) before (black lines) and (c) after (red lines) gasochromic reaction. Insets show photographs of films before and after gasochromic reaction. (b) Overlay of (a) and (c) for comparison.
To study the effect on the vanadium oxidation state and/or symmetry upon annealing, the pre-edge feature is extracted and compared with various standard vanadium oxides. The con-tribution of the pre-peak is obtained by subtraction of the baseline. Fig. 6(a) presents the pre-peak region of the sputtered films along with that of the standards with different crystal symmetries. Clearly, the spectral profiles of all of the films resemble each other and also resemble that of Py(V5+),
indicat-ing that vanadium is most likely in the form of V5+ with pyramid symmetry. Notably, the drop in intensity is accompa-nied by an enhancement of the contribution on the low-energy side (as denoted by the star in Fig. 6(a)), implying the possible presence of pyramidal V4+, which is consistent with the above speculation based on the V L-edge. Fig. 6(b) displays the change in the pre-peak region of the films upon H2 adsorption. The
pre-edge region in Fig. 6(b) demonstrates that for AA400, the charge state of V remains unchanged by the gasochromic reaction, but that the structural symmetry is modified from Py(V5+) to Oh(V5+), and that for the other films, the structural
symmetry is modified from Py(V5+) to a mixture of Py(V4+) and
Oh(V4+). The above results reveal that in the as-deposited and
annealed films (below 300 1C), gasochromism changes not only the charge state but also the symmetry.
Exactly how annealing affects the films is more closely examined. The lower part of Fig. 7(a) presents the spectral differences between VOxand annealed films in bleached states.
Notably, AA200 and VOx differ negligibly and are thus not
considered in Fig. 7. According to the theoretical polarized XAS of V2O5, the components along the z axis and in the basal
plane contribute to the pre-edge region. However, the remark-ably intense pre-peak appears only if a short V–O bond is present along the z-axis,43,47 emphasizing the importance of the vanadyl oxygen. The curve that is fitted with a linear combination of the calculated z-component and the calculated xy-component is compared with the experimental data.43,47 The fitting curve agrees with the experimental data to some
extent (Fig. 7(a)). The ratio of z- to xy-components in the difference spectra of (AA400-VOx) and (AA300-VOx) are 1 : 1
and 1 : 2, respectively. This finding suggests that increasing the thermal annealing temperature shortens the apical V–O bond, explaining why the z-component is enhanced. Similarly, an attempt is made to determine how H2adsorption affects the
films. Fig. 7(b) presents the spectral differences in the color states. The calculations demonstrate that the weights of the z and xy components are in the ratio 2 : 1 for (AA400-VOx) and
1 : 3 for (AA300-VOx). The greater xy-component in AA300
implies that the central V atom is forced to move closer to the basal plane upon coloration.
The small changes in the spectral profiles and color indicate that the coloration rate of AA400 declines when the lamellar structure is removed by thermal annealing. Changes in the V 1s XAS spectra suggest that the gasochromic reaction is closely related to a decrease in the oxidation state of vanadium and an increase in its octahedral symmetry. Above results thus reveal that the coloration rate is strongly correlated with the formation of a lamellar structure. Additionally, increasing the annealing temperature increases the weight of the z-component, revealing that H2adsorption causes structural modulation. The increase
in the strength of the z-component suggests the importance of the apical oxygen.
Vanadium sites in the V2O5gels are highly asymmetric and
the V–O distances vary greatly.47While observed in gels on a flat substrate, spontaneous texturation causes strong anisotropy in physical properties.46 A lamellar structure may provide a channel for the rapid intercalation of ion species in V2O5gels,
Fig. 6 Pre-edge region of V K-edge XAS of VOx, AA200, AA300 and
AA400 in (a) bleached states and (b) colored states.
Fig. 7 Experimental spectral differences between VOxand annealed films
and those fitted by linear combination of xy and z components in (a) bleached states and (b) colored states.
and the formation of a lamellar structure may lead to anisotropy in the film.28,46Polarized absorption spectra separate the con-tributions of neighboring atoms in different directions. K-edge XAS spectra are more prone to solid-state effects (i.e. energy band dispersion) and are much more affected by polarization than are L-edge spectra. This study also examined how the lamellar structure affects the intercalation rate and anisotropy in sputtered VOxby using polarized V K-XAS (Fig. 8(a)). This figure reveals that,
with an increasing angle of incidence of X-rays, the pre-edge feature is reduced and the main peak is enhanced. Fig. 8(b) displays the V-polarized K-XAS spectra of oriented V2O51.6H2O
for comparison. As shown in the inset, the spectra vary substan-tially with the angle of incidence, which is consistent with the literature.46Variations of the as-deposited VOxare smaller than
those of V2O51.6H2O xerogel, and all spectra resemble that of
V2O51.6H2O xerogel at an angle of incidence of 451. Experimental
results indicate that sputtered VOxfilms have only weak
aniso-tropy, which is related to the disorder of the lamellar structure. The absence of anisotropy in the V K-edge in AA400 verifies that polycrystallinity is responsible for the weaker anisotropy in the L-edge, as shown in Fig. 8(c).
Gasochromic material must behave reliably and repeatedly if it is to be used in practice. Consequently, the failure of a film of such material should be examined to more thoroughly under-stand it and to help engineer the material. Therefore, in this study, an in situ experiment is performed on films in a sample holder filled with hydrogen to prevent the out-diffusion of hydro-gen during data collection. The films are exposed continuously to hydrogen until they are hydrogen-saturated; all films become black afterB72 h. Hydrogen-saturated VOxis then studied using
in situ polarized V K-XANES. Fig. 9 shows the spectra that were
obtained using three experimental geometries. Clearly, the spectra resemble each other and no anisotropic effect is observed in the hydrogen-saturated VOxfilm.
The inset in Fig. 9 compares the V K-XAS spectra of hydrogen-saturated VOxwith those of the references (V2O3, VO2, and V2O5).
The main absorption edge of the hydrogen-saturated VOx
signifi-cantly overlaps that of the V2O3 standard, revealing that the
vanadium valence is approximately +3. Many vanadium-based electrochromism/gasochromism-related studies have described the transformation of pentavalent to tetravalent vanadium by the gasochromic reaction.48 Trivalent vanadium has seldom been examined, yet has been found in LixV2O526and HxV2O5.49 The
presence of trivalent vanadium in the VOxfilm may be attributed
to the over-reaction of the film with hydrogen during coloration. The overlapping V-polarized K-XAS spectra reveal that hydrogen-saturated VOxis isotropic, indicating that the gasochromic reaction
equalizes the V–O distances, subsequently lowering the distortion and increasing the octahedral symmetry in the films. Additionally, the inter-diffusion of hydrogen atoms reduces the interlayer dis-tance and reduces the lamellar character of the structure. The elimination of anisotropy is probably related to the increased octahedral symmetry and the breaking up of the lamellar structure. Upon coloration, the hydrogen diffuses into the interlayer. Owing to the large interlayer space, small ions like hydrogen can move freely in the lamellar structure. However, excessive gasochromism causes an excess of hydrogen in the structure; this hydrogen is very likely irreversible in forming the HxVOyphase.
The structures of the hydrogen-saturated films are studied using XRD. Fig. 10 presents the results. Unlike AA300 and AA400 (whose XRD spectra are shown in Fig. 1), AA300 and AA400 in hydrogen-saturated films yield no peak that corresponds to the orthorhombic V2O5phase. Instead, tiny phase transitions to the
H1.43V2O5phase are found in the films. The small peak at 28.41
cannot be identified. The inset shows the GIXRD patterns of
Fig. 8 V polarized K-XAS spectra of (a) as-deposited VOx, (b) V2O51.6H2O
xerogel, and (c) AA400 film for incidence angles of 201, 451, and 901.
Fig. 9 V polarized K-XAS spectra of hydrogen-saturated VOxfor incidence
angles of 201, 451, and 901. Inset shows V K-XAS spectra of hydrogen-saturated VOx (incidence angle of 451) and standards (V2O3, VO2 (M),
and V2O5).
AA400 for various durations of exposure to hydrogen. The inset reveals an amorphization process, in which the peaks that correspond to orthorhombic become V2O5progressively weaker
with an increasing exposure time, and disappear by 72 h. Moreover, the phase transition is not found in as-deposited VOx and AA200, suggesting that VOx and AA200 are highly
promising for gasochromic applications.
As is generally accepted the color change that occurs upon the intercalation of hydrogen is caused by a change in the valence of the cation. However, its mechanism remains unclear. According to our results, the intercalation of hydrogen causes a considerable structural rearrangement and a large change in bonding in the orthorhombic V2O5. Additionally, the difference between the
amorphous VOx and orthorhombic V2O5 crystalline phase is
related to intrinsic structural differences. As is widely assumed, the stereochemistry of crystalline orthorhombic V2O5 is
deter-mined by deformed octahedral VO6, which serves as the building
block of the V2O5 structure. The deformed VO6octahedra form
warped layers that are connected to each other by VQO–V bridges of length 4.37 Å.50 Only weak van der Waals interactions are assumed to exist between the layers. Moreover, the intercalated hydrogen attaches to the oxide matrix to form –OH groups51or –OH2groups.52 No evidence of metal–hydrogen bonds is
avail-able. Also, hydrogen forms –OH2 groups at the apices of the
square pyramid that surrounds each vanadium ion.52 Correspond-ingly, the layered structure of V2O5 is broken and the VQO–V
bridges can no longer be established. The breaking up of chemical bonds subsequently causes a phase transformation into another stable phase, such as H1.43V2O5. According to related
studies, amorphous V2O5 gels have a ribbon-like structure that
comprises two V2O5 sheets facing each other at a distance of
2.8 Å.30–32The structure resembles that of the layered phase of AgxV2O5, in which the vanadyl oxygen does not form a chemical
bond with another layer. The intercalated hydrogen may attach only to the vanadyl oxygen without significantly affecting the chemical bonding. Hence, no phase transformation is observed
in the amorphous V2O5. Without knowledge of the fundamental
electronic and atomic structures of the gasochromic films and the changes of those structures upon the adsorption of H2, better
engineering of the film for more practical purposes is impossible. Growth parameters must be finely tuned to optimize the thin film. The XAS spectrum is exploited herein as an indicator of the overall gasochromic properties of sputtered films. Moreover, we recom-mend using a film with a small amount of V4+ and a lamellar structure for applications. Above results significantly contribute to efforts to develop a reliable and inexpensive hydrogen sensor technology, which is critical to ushering in a hydrogen economy.
Summary
VOxthin films with lamellar structures were successfully prepared
by sputter deposition. The geometric and electronic structures of these smart films were characterized by XAS measurements at the V K-edge, V L-edge and O K-edge. XAS lineshapes, peak positions, and intensities yield valuable information on the local electronic structure, the transition metal oxidation state, the ligand type, and site symmetry. The films had a V4+and V5+mixed valence, and V4+ was present in the form of VO2(B). Analytical results indicate that
the intercalation of sputtered species and the mixing of VO2(B) are
responsible for the disordered and expanded lamellar structure. The gasochromic reaction causes films with the orthorhombic V2O5crystalline phase to undergo an amorphization process and a
partial transformation into an irreversible H1.43V2O5phase.
Addi-tionally, the gasochromic reaction reduces the vanadium valence and alters the as-deposited VOx from slightly anisotropic to
isotropic. The gasochromic rate is strongly correlated with the lamellar structure. Color switching upon the intercalation of hydrogen is not only caused by the valence change of the cations, but also accompanied by the local atomic rearrangement. Much of the physics of this mechanism remains to be explored. Our results further demonstrate that the local atomic structure significantly influences the gasochromism. Owing to its stable structure and a high coloration rate, amorphous VOxfilm with a
lamellar structure is recommended for use in H2 gas sensors.
Results of this study significantly contribute to efforts to improve the reaction rate and durability of vanadium-oxide-based hydro-gen sensors by elucidating their atomic and electronic structures.
Acknowledgements
This study was financially supported by the National Science Council of Taiwan under grants NSC 101-2112-M-213-004-MY3, 102-2112-M-001-004-MY3, 99-2221-E-024-003, and 97-2221-E-006-006-MY3.
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