STRUCTURAL AND ELECTRONIC PROPERTIES OF AlVO4
COMPOUND
S. A. Palomares-Sánchez1, Yu. M. Chumakov2, F. Leccabue3, B. E. Watts3, S. PonceCastañeda4.
1
2Institute
Facultad de Ciencias, UASLP. Alvaro Obregón 64,78000 San Luis Potosí, SLP, México.
of the Applied Physics, Academy of Sciences of Moldova. Academy Str. 5, 2028 Kishinev, Moldova
3IMEM/CNR,
4Universidad
Area delle Scienze 37/a, 43010 Loc. Fontanini (Parma) Italy.
Politécnica de San Luis Potosí, Iturbide 140, 78000 San Luis Potosí, SLP, México.
Abstract
The binary oxide AlVO4 has been proposed as a new gas sensor material. An X-ray
investigation, IR spectrum analysis, resistivity and a calculation of the electronic structure of
this compound have been carried out in order to determine its structural, absorptive and
sensor properties. The fragment [Al3V3O23]18- is the chain chosen to study the electronic
properties of AlVO4 in the framework of the frontier orbital theory and the interaction
between molecules has been described by the Molecular Electrostatic Potential (MEP). The
vibration analysis and the simulated infrared spectrum (IR) were calculated for the unit
[Al3V3O23]18- and a comparative analysis with the experimental spectrum of AlVO4 is also
presented.
Introduction
Metal vanadates are technologically important because of their use as catalysators, laser hosts,
masers and phosphors and, as mentioned, as gas sensors. They have triclinic and tetragonal
structures. The general formula is MVO4, where M = Fe, Al, Y, Cr, etc. The ortovanadate
AlVO4 (I) has captured a lot of attention due to their sensor properties to detect NO and NO2.
It is a relative new sensor material derived from V2O5/Al2O3 mixtures. The reported
sensivities of the mixture for NO and NO2 at 400C are 2.54 and 2.71, respectively [1].
Mixtures of AlVO4-ZnO-V2O5 have also been studied in order to establish their sensory
properties to detect NO and NO2 [2]. Several experimental studies have been also carried out
in order to determine its structural, absorptive and sensory properties [2, 3] and preparation
methods of the compound, as bulk and as thin films, have been also reported [3, 4, 5]. Based
in this studies it is no clear, up to now, whether pure AlVO4 is responsible for the sensory
properties or a mixture of AlVO4-V2O5-Al2O4. Therefore to clarify the structural, absorptive
and sensory properties of I the X-ray investigation, IR spectrum analysis, determination of
resistivity and calculation of the electronic structure of the given compound have been carried
out.
The vibration analysis calculation and simulated infrared spectrum (IR) were calculated for
the unit [Al3V3O23]18- to compare with the experimental infrared spectrum. The electric
characterization in air of the films of AlVO4 deposited on alumina is also presented.
Experimental
Preparation of AlVO4. To obtain I, a solution 0.5 M of VO(OiPr)3 and Al(OiPr)3 in toluene is
initially prepared. For the preparation of the solution, 20 ml of toluene are poured in a backer
with 5.1 g of Al(OiPr)3 and stirred until dissolution of the powder. 5 ml of toluene were added
to 6.1 g of VO(OiPr)3 in another backer. Then the two solutions were mixed and stirred to
blend. Afterwards, the solution is maintained in refluxing during six hours in a three-necked
flask in nitrogen atmosphere in order to avoid the atmospheric humidity to induce the
hydrolysis process. The initial yellow solution got dark green at the end of the refluxing.
Finally it is poured in a flask and aforated to 50 ml with toluene. The whole process was
carried out in dry air atmosphere. The solution Al-V-O-toluene and isopropyl alcohol in a
volume ratio 1:5 was heated up to 350°C with a hot plate during 10 min. After that, the
obtained powder was reduced with the help of one mortar. The resultant powders were
calcined in a furnace at 650°C during 120 min.
X-Ray analysis and IR spectrum. X-ray powder diffraction data were collected for each
specimen from 3 to 90° 2 with a step width of 0.05°and a count time of 2.0 s/step. We used a
Philips PW 1050/25 modified diffractometer with CuK radiation. Analysis of X-ray
spectrum of the powder indicates the sole presence of AlVO4, Figure 1.
AlVO4 (Powder)
20
40
60
80
2 (Deg.)
Figure 1. X-ray diffraction spectrum of AlVO4 powder.
The Rietveld refinement was carried out in order to find the structural parameters of this
compound. In spite of the fact that the actual structure of AlVO4 has not been determined, the
use of the structural parameters of the FeVO4 as initial model for the Rietveld refinement
produces a very good fitting between the experimental and simulated spectrum. Based in this
fact, it is possible conclude that the AlVO4 is isostructural with the FeVO4. The MAUD
program was used to refine the spectrum [6]. As initial model, the triclinic structure (space
group P-1) and atomic positions reported in [5] were assumed. Table 1 shows the refined
structural parameters for this compound compared with the corresponding ones found in the
literature. After the refinement, the obtained atomic positions (Table II) were used to choose
coordinated of the atoms of the chosen chain to analyze the electronic properties of AlVO4.
Table I. Refined cell parametes of AlVO4 (Space group P-1).
Ref. [5]
Ref. [7]
Ref. [8]
This work
a (nm)
b (nm)
c (nm)
 ()
 ()
 ()
0.6538
0.7756
0.9131
96.170
107.230
101.400
0.6480
0.7750
0.9084
96.848
105.825
101.399
0.6544
0.7760
0.9142
96.26
107.22
101.43
1000
900
800
700
600
537
619
889
Absorbance (a. u.)
1100
582
773
916
1010
963
AlVO4
1200
457
Table II. Atomic positions of Al, V and O in AlVO4.
x/a
y/b
z/c
Uiso
0.78440964 0.7075605
0.38718462
2.743629
0.44425398 0.8947806
0.20253097
4.6296287
0.9650142
0.2920213 0.0045775394 0.26217282
0.008724525 0.9938662
0.2518021
4.773237(
0.21272993 0.60352296( 0.34498516(
3.1804833
0.52802247 0.29755414 0.11548059
2.2879663
0.6251935 0.48470527 0.28008288
0.8289297
0.25638348 0.40600768( 0.4148904(
-6.808943
0.08994505 0.7043704
0.4378164
1.7460103
0.13070291 0.06775913 0.42920256
10.777374
0.5194325
0.7908758
0.36824155
4.345036
0.7584226 0.88695985
0.2714941 -0.056205746
0.5234637 0.13606311
0.2353987
-4.544215
0.18672179 0.8836565
0.19102962
13.443152
0.3739295
0.7283855 0.045751486
-1.077644
0.27008364 0.2989211 0.059253845 -1.2430751
0.9311518 0.11064107 0.13166831
12.190901
0.049221464 0.49520302 0.14970364
-3.6944354
730
711
Atom
Al1
Al2
Al3
V1
V2
V3
O1
O2
O3
O4
O5
O6
O7
O8
O9
O10
O11
O12
0.6471
0.7742
0.9084
96.848
105.825
101.399
500
400
-1
Wavenumber (cm )
Figure 2. Infrared spectrum of AlVO4 powder taken between 1200 cm-1 and 400 cm-1.
The experimental IR spectrum was obtained with a Nicolet Avatar 360 FT-IR spectrometer by
applying the KBr disc technique, Figure 2.
Thin film deposition. For thin film deposition a solution of isopropyl alcohol and the Al-Vtoluene solution in proportion 3:1 and 5:1 vol. was prepared; also the pure Al-V-toluene
solution was used for the deposition of films. A 0.2 µm filter attached at one syringe was used
in order to avoid the deposition of solid particles in the film. Three different thin films
(ALVO9, ALVO10 and ALVO11), deposited on polycrystalline alumina, were prepared with
the parameters shown in Table III. The deposited films were left dry during 48 hours before
the annealing.
X-ray diffraction spectrum of the ALVO9 sample is shown in figure 3. Also the spectrum of
powder of pure AlVO4 is shown (lower graph). It is possible to associate some peaks of the
spectrum of the film to the main peaks of the AlVO4, indicating that the compound is present
n the film.
Table III. Preparation of thin films of AlVO4 on polycrystaline alumina
Film
A. isopropil/Al-V-touene ratio 1st Deposition Annealing
ALVO9
0:1
3000 rpm/1´ 650 C/20´
ALVO10
5:1
3000 rpm/1´ 650 C/20´
ALVO11
3:1
3000 rpm/1´ 650 C/20´
X-ray diffraction spectrum of the ALVO9 sample is shown in figure 3. Also the spectrum of
powder of pure AlVO4 is shown (lower graph). It is possible to associate some peaks of the
spectrum of the film to the main peaks of the AlVO4, indicating that the compound is present
on the film.
Alumina
substrate
AlVO4 on -Al2O3
AlVO4 puro
20
40
2 (Deg.)
Figure 3. X-ray spectra of the thin film of AlVO4 compared with the corresponding to AlVO4 powder.
Electric characterization. Gold electrodes were evaporated on the surface of the films in
order to perform the electric characterization. The electrical characterization was achieved by
the van der Paw method. The samples were attached to one heater block collocated in a
vacuum chamber and then the resistivity was obtained at some given temperatures. The
temperature ranges from 290 K up to roughly 402 K. Figure 4 shows the plot of ln() versus
the inverse of temperature, 1/T.
7,2
ln(rho)
6,8
6,4
ALVO10
ALVO11
6,0
5,6
5,2
0,0024
0,0026
0,0028
0,0030
0,0032
0,0034
0,0036
-1
1/T (K )
Figure 4. Plot of ln() vs. 1/T of the samples ALVO10 and ALVO11.
Theoretical Analysis
A quantum-chemical study of the electronic structure of AlVO4 and the simulation of the IR
spectrum were performed in order to investigate the sensor properties of this compound to
detect NO- and NO2- groups. The [Al3V3O23]18- chain is chosen to study the electronic
properties of I, Figure 5.
Figure 5. [Al3V3O23]18- chain used to study the electronic properties of AlVO4.
Atomic coordinates were taken after Rietveld refinement of the experimental spectrum of I.
The [Al3V3O23]18- consists of two different pentagonal (AlO5-) and one hexagonal (AlO6-)
polyhedrons and three different tetragonal (VO4-) polyhedrons. This study was performed in
the framework of the frontier orbital theory that considers the interaction between Highest
Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO)
[9]; for reagents, these orbitals have the smallest energy separation and they can overlap most.
The electronic structure of AlVO4 has been explored by ZNDO/1 method [10] in the
framework of the HyperChem 7.0 program and then the vibration analysis was performed.
Atomic coordinates were taken after Rietveld refinement of the experimental spectrum of I.
The [Al3V3O23]18- consists of two different pentagonal (AlO5-) and one hexagonal (AlO6-)
polyhedrons and three different tetragonal (VO4-) polyhedrons. This study was performed in
the framework of the frontier orbital theory that considers the interaction between Highest
Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO)
[9]; for reagents, these orbitals have the smallest energy separation and they can overlap most.
The electronic structure of AlVO4 has been explored by ZNDO/1 method [10] in the
framework of the HyperChem 7.0 program and then the vibration analysis was performed.
The contour maps of the HOMO and LUMO for [Al3V3O23]18- are represented in figure 6 and
7, respectively. The region of the highest density (regardless of sign) is generally the site of
electrophilic attack; the oxygen atoms, which are connected to six-coordinated atoms of Al
and one of the V atoms, are such sites. The interaction between molecules was conveniently
characterized with the aid of the molecular electrostatic potential (MEP).
The results of the electrostatic potential calculations can be used to predict initial attack
positions of protons or other ions. The MEP distribution analysis of [Al3V3O23]18, figure 8,
shows that the reaction sites are located mainly in the regions of the (AlO6-) polyhedrons.
Thus, considering the contour maps of the HOMO and distribution of the MEP, we can
suppose that interaction between [Al3V3O23]18- and NO2 group would occur according to the
scheme represented in figure 9.
Figure 6. HOMO for [Al3V3O23]18-
Figure 7. LUMO for [Al3V3O23]18-
The vibrational frequencies of the simulated infrared spectrum (IR) for [Al3V3O23]18- are
derived from the harmonic approximation, which assumes that the potential surface has a
quadratic form. Table IV shows the main atomic groups (Al-O and V-O bonds) that
contribute to the theoretical normal modes of vibration. Negative values of the frequency
were also found in the calculated IR spectrum that indicate that our model is not a minimum
energy structure.
Table IV. Frequencies of theoretical normal modes of vibration. Al(h) and Al(p) indicates hexagonal and
pentagonal coordinated Al, respectively. Units of wavenumbres are given in cm -1.
1139.2
1123.2 1117.4 1103.8 1092.1 1068.8 1053.41 1018.4
971.0
Al(p)/V2 Al(h)/V3 Al(h)
V3
V2
V2
Al(h)/V1 Al(h)/V1
966.9 950.3 948.1 918.9 888.5 748.8 540.1 505.63
Al(h)
Al(h)/V1 Al(h) V3
V2 Al(h)/V1
At the same time, only for this model the self-consistency has been successfully achieved;
therefore, the positive frequencies can be used for interpretation of the experimental IR
spectrum. Figure 10 shows the experimental and calculated IR spectrum as well as the
deconvolution curves. Vertical lines indicate the position of the simulated frequencies and
their relative intensity.
Figure 8. MEP distribution for [Al3V3O23]18-
The deconvolution was made using Gauss functions in order to fit the experimental spectrum
using the theoretical positive frequencies. The deconvolution was performed by keeping fixed
the position of the calculated frequencies while the width and height of the functions were
freed. Our model only reproduces the trend of the experimental spectrum, especially in the
interval from 540.1 cm-1 till 918.9 cm-1, because a thorough simulation is not possible due to
the complex crystal structure of the AlVO4.
Figure 9. The possible interaction between AlVO4 and NO2- group.
E x p e r im e n ta l
G e n e ra te d
.
.
.
.
4 5 0
.
.
.
6 0 0
W
7 5 0
.
..
9 0 0
a v e n u m b e r (c m
.
.
.
..
. .
1 0 5 0
-1
1 2 0 0
)
Figure 10. Experimental and calculated IR spectrum of AlVO 4. Vertical lines indicate the position of the
frequency of normal modes of vibration of the calculated IR. The simulated deconvolution functions are also
shown.
Conclusions
AlVO4 was prepared using the sol-gel route, both as powder and as thin film. The thin films
were deposited on alumina and their electrical characterization was performed. Very similar
resistivity temperature coeficients were obtained for the films prepared with different
disolutions of the sol and toluene. The Rietveld refinement was made to the X-ray spectrum
of the powder and the atomic positions of Al, O and V obtained were used as input for the
quantum-chemical analysis in order to find the contour maps of HOMO and LUMO of the
chain [Al3V3O23]18-. It is found that, on the base of the given model, we can assume that
during the absorptive processes the NO2- groups form the second coordination sphere of sixcoordinated atoms of Al.
The very good agreement between the refined and the calculated X-ray spectra let conclude
that the structure of AlVO4 is isostructural with that of the FeVO4. The vibrational spectrum
was also calculated and compared with the experimental one. Taken only the positive
frequencies, it is possible to reproduce the experimental IR of AlVO4 in the range from 600
cm-1 to 1200 cm-1. A complete simulation is not possible because the structure of AlVO4 is
not simple.
Calculations of the vibrational spectrum of NO and NO2 adsorbed on AlVO4, to be compared
with experimental results, are under way.
Acknowledgments
The autors want to thank the Laboratorio de Materiales of the Facultad de Ciencias, UASLP
for the obtention of IR spectra.
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