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13STRUCTURAL MATERIALS
STRUCTURAL AND VIBRATIONAL PROPERTIES OF SPRAY PYROLYSED MOLYBDENUM OXIDE THIN FILMS
A. Bouzidi, N. Benramdane*, M. Medles*,
S. Bresson , C. Mathieu ,
** ** ***
B. Khelifa , R. Desfeux , M. El Marssi
* Laboratoire d'Elaboration et de Caractérisations des Matériaux, département d'électronique, Faculté des Sciences de l'Ingénieur, Université Djillali Liabes, BP89, Sidi Bel Abbès, 22000, Algérie Université d'Artois, Faculté Jean Perrin, Rue Jean Souvraz, Lens, SP18, 62307, France Laboratoire de Physique de la Matière Condensée, Université de Picardie Jules Verne, 33 rue St. Leu, Amiens, 80039, France
Received: 15 Sept 2007; accepted: 27 Sept 2007
MoO3 thin films were prepared by spray pyrolysis technique by using 0.1 M of molybdenum chloride (MoCl5) dissolved in deionized water on glass substrates heated at different temperatures. Influence of substrate temperature Ts on structural and vibrational properties is discussed; X-ray diffraction characterization revealed that the films are monoclinic for 200 °C and become orthorhombic above 225 °C. Raman spectra of the films were reported and explained the transformation phase.
Keys words: structural properties, vibrational properties, molybdenum trioxide.
Organisation: Djillali LIABES University.
Education: Djillali LIABES University (1989-1994), Magister (1997), Doctorat d'état (2004). Experience: Teaching (maître de conférence) at Djillali LIABES University (1997 - today), member in Scientific research projects (1998 - today)
Main range of scientific interests: thin films, optical properties of semiconductors. Publications:
- 3 publications in Materials Science and Engineering B.
- 2 publications in Solar Energy Materials and Solar Cell.
- 1 publication in Microelectronic Engineering.
- 1 publication in Molecular Physics Reports.
Attouya Bouzidi
Introduction
Transition metal oxide films have a great technical interest for their optical and electronic properties. Indeed, these materials can be switched between two different optical states prompted by photochromic, thermochromic or electrochromic effect [1]. Moreover, a number of these oxides such as MoO3, V2O5, and V6O13 are promising cathode materials for rechargeable lithium batteries [2-4].
MoO3 thin films were prepared by various techniques, such as reactive sputtering [5], chemical vapor deposition [6], pulsed laser deposition [7], oxygen plasma assisted molecular beam epitaxy [8] and flash evaporation [9]. We have used spray pyrolysis technique to fabricate molybdenum oxide thin films. The detailed study of structural and optical properties of MoO3 thin films prepared by spray pyrolysis technique has been reported in a previous work [10] and shows that structural and optical properties of these films depend on substrate temperature.
The temperature dependence of the phonon spectrum has been investigated previously by Julien et al [11], the same authors have studied the substrate temperature dependence of flash evaporated MoO3 thin films properties [9], but as far we know the effect of substrate temperature on vibrational properties of MoO3 thin films prepared with spray pyrolysis technique has not been studied. So, in this paper, our objective is a comparative study of the spray pyrolysed MoO3 vibrational and structural properties according to the substrate temperature.
Experimental details
Thin films were deposited by spray pyrolysis technique on glass substrates at different temperatures varying from 200 °C to 300 °C. Spraying solution of Molybdenum chloride (MoCl5) dissolved in deionized water, with 0.1 M concentration is used. The description of spray pyrolysis technique has been reported previously [12].
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Structural characterization was been carried out at room temperature in the 8 -28 scan mode using a Rigaku Miniflex diffractometer (CuKa1 radiation, X = 1.5406 Á). Raman spectroscopy measurements were performed at room temperature in a backscattering microconfiguration using the 514.5 nm line from an Ar-ion laser focused on the surface as a spot of 1 ^m in diameter and with a power density of ~ 3 MW/cm2. The scattered light was analysed with a Jobin Yvon T64000 spectrometer, equipped with a liquid nitrogen cooled CCD detector. The spectrometer provided a wave number resolution better than 3 cm-1.
Results and discussion
Structural properties X-ray diffraction patterns of molybdenum films prepared at different substrate temperatures are given in Fig. 1. At 200 °C, the pattern exhibits (011) and (200) peaks with low intensity. These peaks are indexed by comparing the experimental data (measured interreticular distances dm) with the JCPDS card No. 471081, corresponding to the p-MoO3 (monoclinic phase, P2/c space group (No. 13)). This phase has been obtained by thermal treatment of spray dried powders of aqueous molybdic acid solutions [13] and by pulsed laser deposition [7].
w
Q
£
30 40 50 2 e, Deg
Fig. 1. X-ray Diffraction spectra of spray pyrolysed samples prepared for different substrate temperatures with 0.1M of spray solution MoCl5
At 225 °C, the diffraction spectrum shows a large band suggesting a disorder in the structure due probably to the P-MoO3 and a-MoO3 mixture.
At 250 °C and 275 °C, the (0k0) peaks predominate indicating a preferential growth and suggesting layered
structure of the films. The grains have the b--axis perpendicular to the substrate surface. Indeed, the a-MoO3 can be described as a layered structure in which each layer is built up of MoO6 octahedra at two levels connected in the direction to c axis by edge and corner sharing so as to form zig-zag rows, in the direction to the a axis the octahedron are connected by corners sharing [14] (see Fig. 2). Our result is similar to that obtained in literature for flash evaporated MoO3 thin films [9].
At the higher temperature 300 °C, the pattern exhibits several peaks in different directions indicating the thin films polycrystalline nature. A good agreement is observed between the inter-reticular distances of thin films deposited in the range from 250 to 300 °C and those of JCPDS file (card N° 05-0508) corresponding to the orthorhombic phase (a-MoO3). The experimental lattice parameters are a = 3.973 A, b = 13.902 A and c = 3.692 A [10], which are in good agreement with the literature data (a = 3.962 A, b = = 13.858 A and c = 3.697 A) [15]. Moreover, the grain size increases with increasing substrate temperature, its values are about 21 nm for the range (250-275 °C) and 26 nm for 300 °C [10]. The a-MoO3 spray pyrolysed films are light colored at low temperature and become deeply greyish when the substrate temperature increases, This fact suggests that the number of defects increases due to oxygen vacancies formation in the films and induces a decrease of the optical gap of MoO3 [9, 16].
In order to confirm our hypothesis that the large band observed on the diffraction spectrum at 225 °C is attributed to the mixed phase during the transformation from p-MoO3 to a-MoO3 and to affine our obtained results, we present the study of spray pyrolysed MoO3 vibrational properties according to the substrate temperature.
b
Mo-Oj: 1,67 Â; Mo-O2: 2,25 Â; Mo-O3: 1,95 Â; Mo-O4: 1,73 Â; Mo-O5: 1,95 Â; Mo-O6: 2,33 Â
Fig. 2. Structure of a-MoO3 (zig-zag chains of octahedra [14])
J
LrJ
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27
Raman micro-spectroscopy The obtained Raman spectra in the range 80-1100 cm-1 for samples prepared by spray pyrolysis at substrate temperatures varying from 200 °C to 300 °C, are shown in Fig. 3 and 4.
100
200
300
Raman shift, cm-1
400
500
Fig. 3. Raman spectra in the frequency range 80 to 550 cm'1 of MoO3 films prepared at different substrate temperature
600
700
800
900
1000
1100
Raman shift, cm"
Fig. 4. Raman spectra in the frequency range 550 to 1100 cm'1 of MoO3 films prepared at different substrate temperature
Vibrational study in 80-550 cm'1 spectral region In the range 80-550 cm-1, we can observe that the vibrational behaviors of the different samples are not the same. Thus, the spectra can be separated on three parts: the first concerns the vibrational behavior of the sample prepared at 200 °C, the second part amounts to Ts = 225 °C and the thin films prepared at Ts = 250 to 300 °C correspond to the last part. All the observed bands in Fig. 3 and 4, are summarized and compared with previous literature data in Table 1.
For the sample prepared at Ts = 200 °C, in the spectral range 80-550 cm-1, we can only observe some very weak peaks below 200 cm-1, above this frequency value we notice weak peak at 355 cm-1. This band has been observed for p-MoO3 phase [7, 17] and can be assigned by deformation bending mode 5OMo3. Moreover, in Fig. 5, we present the Raman spectrum of this sample in the range 72 to 80 cm-1. A resolved doublet at the bands 76 and 77 cm-1, is observed. The peak at 76 cm-1 has a width at half maximum smaller than the second peak. However, we know the existence of a plasma ray in this spectral range. So, it is reasonable to assign the band at 76 cm-1 to this ray plasma and the band at 77 cm-1 to our material. In the literature [7], the band at 77 cm-1 corresponds to a vibrational mode of p-MoO3. This result must be confirmed by the spectral study on the range 550-1100 cm-1.
Table 1
Experimental frequencies (cm-1) and assignment of the Raman active modes of orthorhombic MoO3
This work (cm-1) E. Haro-Poniatowski et al [7] Eda [18]
995, s 995, s 995
818, vs 819, vs 819
666, w 667, w 666
471, w 471, w 471
378, m 379, m 378
364 , vw 365, w 366
337, m 337, m 338
291, s 291, s 291
284*, m 283, m 283
244, m 245, m 246
218, w 217, w 217
197, w 198, w 197
158, s 158, m 159
128, s 129, w 129
116, m 115, m 117
98, m 98, m 100
83, w 82, s 84
Assignement
[19]
Ag, Big v O=Mo Ag, B1g, v OMo2 B2g, B3g, v OMo3 Ag, B1g, v OMo3 B1g,S O=Mo Ag, S O=Mo Ag, B1g, S OMo3 B3g, S O=Mo B2g, S O=Mo B3g, S OMo2 Ag, S OMo2
B2g, S OMo2
' Other deformation modes
w = weak, m = medium, s frequency value
strong, vs = very strong, fitted
72
74
76
Raman shift, cm"1
78
80
Fig. 5. Raman spectra in the frequency range 72 to 80 cm'1 of ¡5-MoO3 films prepared at 200 °C
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When the substrate temperature reaches 225 °C, some weak broad peaks are observed at 158, 238, 281, 336, 372 and 447 cm-1, a well resolved doublet appears at 116 and 124 cm-1 and medium peak is observed at 148 cm-1. The bands at 238, 281, 336, 372 cm-1 are near to the ones observed in literature for a-MoO3 [17, 18, 19] (Table 1). In comparison with the previously Raman studies on p-MoO3 or a-MoO3, it seems that the weak broad peak at 447 cm-1 has not indexed. However, it can explain the disorder in the structure. For the third part (Ts = 250 to 300 °C), we always observe in the Fig. 3 a medium peak, nearby to 337 cm-1. A strong peak with shoulder appear near 290 cm-1 in the case of the sample prepared at 300 °C, whereas we notice a broad peaks at same frequency value for 250 and 275 °C, all theses bands are due to the Raman-active bending modes [19]. In order to analyse the peak shape, lorentzian fitting is used in 270 to 300 cm-1 spectral region (Fig. 6), the curve fittings confirm the existence of shoulder bands (fitted frequencies values labeled * in Table 1), it is clearly that the 5Mo=O vibrations corresponding to frequencies of 284 and 291 cm-1 are closely dependant to substrate temperature and the intensity ratio (1291/1284 ) increases as function of Ts.
13 TO
4—I
(Л £= <D
£= TO £ to
or
~ 1 I 1 г
250 260 270 280 290 300 310
Raman shift, cm-1
Fig. 6. Analysis of the peaks 284 and 291 cm'1 as a function of substrate temperature
The line shifted from 149 cm-1 to 153 cm-1 (Fig. 3, c, d) is assigned to B1g mode (translational rigid MoO4 chain mode, Tb), when substrate temperature reaches 300 °C, a strong band at 158 cm-1 is observed and assigned to Ag mode (translational rigid MoO4 chain mode, Tb).
The resolved doublet at 116 and 124 cm- is always observed with a shift of 5 cm-1 for 124 cm-1 band at 300 °C, theses bands are assigned respectively to B2g, B3g modes (translational rigid MoO4 chain mode, Tc). We can notice that the different modes for MoO3 thin film prepared at 300 °C are more resolved than films prepared at 250 or 275 °C.
Vibrational study in 550-1100 cm'1 spectral region In Fig. 4, a, the more intense peaks at 849 cm-1 and 774 cm-1 can be assigned to the stretching vibrations of Mo-O bonds in the expected corner-sharing octahedral Mo-O-Mo and the weak peak observed at 901 cm-1 indicates a stronger distortion of the MoO6 octahedra. The monoclinic MoO3 contains two crystallographically independent octahedra, shown the disorder at the Mo and O sites; its ReO3 structure is analogous to WO3. Indeed the Raman spectrum of MoO3 thin film prepared at 200 °C (Fig. 3, a and Fig. 4, a) is similar to WO3 as described in the literature [7, 13]. This spectrum exhibits other very weak bands at 818 and 957 cm-1 that are determined by lorentzian fit of the experimental spectrum. The very weak band at 818 cm-1 corresponds to stretching vibration of Mo-O-Mo suggesting the slight presence of a-MoO3 in thin film deposited at 200 °C. For Ts = 200 °C, the band at 957 cm-1 was observed at 951 cm-1 by S.H. Lee et al [5] for sputtered amorphous MoO3. The authors assigned to Mo=O stretching mode of terminal oxygen atoms possibly on the surfaces of the cluster, these terminal oxygen bonds are created by breaking of Mo-O-Mo bonds at the corner-shared oxygen, which are common to two octahedron [13], T.Ono et al [20] have observed this band at 955 cm-1 for MoO3 catalyst exchanged with O18 that assigned to vibration of Mo-O18 bond.
When the substrate temperature reaches 225 °C, the peaks being particularly broad, indicate the poor crystallization of film, confirming the X-ray diffraction measurements. The 848 cm-1 and 774 cm-1 characteristic peaks of p-MoO3 becomes very weak and the band at 957 cm-1 becomes more intense due to disorder increase in film. According to Ts = 200 °C, additional peaks at 819 cm-1 and 992 cm-1 appear, these modes characterizing the a-MoO3 are assigned respectively to Mo-O(2)-Mo and Mo=O(1) stretching vibrations. That's why it is reasonable to think that at Ts = 225 °C, we have a mixture of a-MoO3 and p-MoO3. The weak band observed at 924 cm-1 is not mentioned by other experimental works and it can be explained by film disorder. Above 225 °C, further the stretching vibrations modes at 819 and 994 cm-1, a weak band is observed at 666 cm-1, that assigned to OMo3 bridging stretching vibrations. With the increasing substrate temperature, a shift toward the mode frequencies characteristic of orthorhombic phase (a-MoO3) is observed and the bands become well resolved. The band at 994 cm-1 is more intense which confirm the layered structure of the film. The bands corresponding to orthorhombic phase are assigned according to a-MoO3 single crystal studied by
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Py and Maschke [19] and summarized in Table 2, our results are in good agreement with those reported in literature [7, 9, 17-19].
Table 2
Experimental frequencies of Raman active modes of MoO3 spray pyrolysed thin films deposited at different substrate temperatures
ß-MoO3 [7] This work, °C a-MoO3 [19]
200 225 250 275 300
76 (s) 76
83 83 83 85 83
91 91
90* 93 94 98 99
116 116 115 116 116 116
130 (w) 124 124 126 128 129
148 149 153 154
158 158 158
176
194
195* 195 195 197 198
214* 214 217 218 217
237 (w) 238 239 241 244 246
283 (w) 281 279* 280* 284* 283
287* 288* 291 291
310 (w)
336 337 336 337 338
349 (m) 355
365
372 373 374 378 379
391 (w)
414 (w)
447
471 473
662 666 666 666
774 (s) 774 784*
819* 819 818 818 818 819
849 (vs) 849 855*
904 (m) 901
924
957 960
992 992 994 995 995
w = weak, m = medium, s = strong, vs = very strong, fitted frequency value
Conclusion
MoO3 thin films were prepared by spray pyrolysis technique on glass substrate with a temperature variation from 200 to 300 °C. The films exhibit respectively a monoclinic structure at low temperature and an orthorhombic structure at high temperature. The structural results reveal that the films structure changes with substrate temperature.
The samples were characterized also by Raman spectroscopy; the results confirm the temperature dependence of the nature of the film. At 200 °C the Raman frequencies correspond to p-MoO3. A mixture of monoclinic and orthorhombic structure is observed at
225 °C, the spectrum is particularly broad for this temperature, which explains the poor film crystallization and the absence of peaks in XRD diagram. Above 250 °C, all frequencies Raman corresponding to a-MoO3 are observed and confirm the XRD results.
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International Scientific Journal for Alternative Energy and Ecology № 5 (61) 2008
© Scientific Technical Centre «TATA», 2008
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