Annealing effect on physical properties of evaporated molybdenum oxide thin films for ethanol sensing

Accepted Manuscript Title: Annealing effect on physical properties of evaporated molybdenum oxide thin films for ethanol sensing Author: S. Touihri A. Arfaoui Y. Tarchouna A. Labidi M. Amlouk J.C. Bernede PII: DOI: Reference:

S0169-4332(16)32263-2 http://dx.doi.org/doi:10.1016/j.apsusc.2016.10.139 APSUSC 34233

To appear in:

APSUSC

Received date: Revised date: Accepted date:

25-7-2016 16-10-2016 21-10-2016

Please cite this article as: S.Touihri, A.Arfaoui, Y.Tarchouna, A.Labidi, M.Amlouk, J.C.Bernede, Annealing effect on physical properties of evaporated molybdenum oxide thin films for ethanol sensing, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.10.139 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Annealing effect on physical properties of evaporated molybdenum oxide thin films for ethanol sensing S. Touihri a,*, A. Arfaoui a, Y. Tarchouna a, A. Labidi b, M. Amlouk a and J.C. Bernede c a

Unité de physique des dispositifs a semi-conducteurs, Faculté des sciences de Tunis, Tunis El Manar University, 2092 Tunis, Tunisia b Unité de Recherche de Physique des Semiconducteurs et Capteurs, IPEST, BP 51 La Marsa 2070, Tunis, Tunisia c LUNAM, Universite de Nantes, Moltech Anjou, CNRS, UMR 6200, FSTN, 2 Rue de la houssiniere, BP 92208, Nantes F-44322, France *Corresponding

author: E-mail: [email protected]

1

Highlights  Thermally grown molybdenum oxide films are amorphous, oxygen deficient and gas sensing.  Air or vacuum annealing transforms them into a sub-stoichiometric MoO3−x phase while samples annealed at 500 °C in oxygen were crystallized and identified as pure orthorhombic MoO3 phase.  The conduction process and sensing mechanism of MoO3-x to ethanol are attributed to the non-stoichiometry existing in the sample.

Abstract This paper deals with some physical investigations on molybdenum oxide thin films growing on glass substrates by the thermal evaporation method. These films have been subjected to an annealing process under vacuum, air and oxygen at various temperatures 673, 723 and 773 K. First, the physical properties of these layers were analyzed by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM) and optical measurements. These techniques have been used to investigate the oxygen index in MoOx properties during the heat treatment. Second, from the reflectance and transmittance optical measurements, it was found that the direct band gap energy value increased from 3.16 to 3.90 eV. Finally, the heat treatments reveal that the oxygen index varies in such molybdenum oxides showing noticeably sensitivity toward ethanol gas.

Keywords: Molybdenum oxide; Thermal evaporation; Annealing; crystalline structure; optical properties and gas sensor.

1. Introduction In the last few years, a lot of attention and efforts were dedicated to the technologically use of molybdenum oxides in many industrial applications due to their structural optical and electrical properties. The oxides of general formula of MoOx and belonging to oxides family

2

with relatively high oxygen index such as MoO3, WO3 and V2O5 are n or p -type semiconductors depending on various factors as the experimental conditions as well as the excess or deficiency of oxygen inside these prepared materials [1, 2]. The type character plays an important role in the electrical conductivity which is mainly due to this oxygen index [3,4]. A deep knowledge of the physical properties of these oxides based on molybdenum is interesting for its possible use as optical electrochromic material [5-7]. Also, this oxide raised much attention owing to its numerous applications, such as in catalysis [8, 9] and gas sensors [10, 11]. The physical properties of such oxides are mainly governed by the crystal structure, chemical composition and surface morphology. A large variety of molybdenum oxides, MoOx have been reported in literature. Moreover, these materials can crystallize in amorphous state, orthorhombic and monoclinic phases or in a mixture of both. The molybdenum dioxide MoO2 described in monoclinic phases; Mo4+ ion linked to two crystallographically equivalent oxygen atoms in an octahedral environment. Nevertheless, MoO6 octahedral shows very slight distortion in which the ions are linked together by edge-sharing into infinite MoO4 chains and are then condensed via corner-sharing into a three dimensional MoO2 network. On the contrary, the molybdenum trioxide MoO3 is described in an orthorhombic phase. In this structure, the Mo6+ ion is linked to three crystallographically equivalent oxygen atoms in a strongly distorted octahedral environment. The MoO 6 octahedral is then condensed via edge and corner-sharing to give rise to MoO3 sheets [12]. On the other hand, various techniques were used to synthesis these oxides, such as thermal evaporation [13, 14] chemical vapor deposition [15], electrodeposition [16] spray pyrolysis [17], sol gel [18, 19] and DC magnetron sputtering [20,21]. In this work, the thermal evaporation is selected to prepare MoOx thin films because this process leads to a careful control of the film thickness. This paper highlights some physical investigations on the changes of the microstructure and the optical properties of molybdenum oxide thin films prepared by thermal evaporation technique, followed by annealing in vacuum, air and oxygen atmospheres at 673, 723 and 773 K. Indeed, Obtained samples were characterized by means of several physical techniques such X-Ray diffraction (XRD), Raman spectroscopy, scanning microscopy (SEM) and UVVIS spectroscopy. Specific emphasis is put on the sensitivity of such oxide films against ethanol vapor. 3

2. Experimental procedure MoO3 thin films were grown on glass substrates by thermal evaporation of MoO3 powder provided by Aldritch with 99.9% purity using a tantalum boat filament. The powder was placed on a baked tantalum filament and evacuated down to a pressure of 10 -5 mb. After the deposition, the obtained films were then annealed using a programmed tubular furnace during one hour under vacuum, air and oxygen environments (the pressure is already of the order of 0.5 bar) at 673, 723 and 773 K. This annealing is an essential process to control the stoichiometry of obtained films by means of oxygen index. The glass substrate is taken at ambient temperature and the average of the thickness of prepared thin films is about 250 nm. Also, it is found that all experiments are reproducible during the different annealing processes. In order to reach the structure of obtained films, X-ray diffraction analysis of all prepared thin films were performed by a copper-source diffractometer (Analytical X Pert PROMPD), with the wavelength (λ = 1.5418Å). Raman spectroscopy measurements were performed at ambient temperature and the spectrophotometer is composed by three monochromators with high retro-diffused resolution (Δσ ≈ 0.25 cm-1) (JOBIN Yvons Horiba T64000). The excitation is carried out using a laser having a wavelength of λ=488 nm under power of 5 mW. Topography of all obtained Mo xOy films was performed via scanning electron microscopy (SEM). On the other hand, the UVVIS Spectroscopy were carried out using a Schimadzu UV 3100 double-beam spectrophotometer in 250- 2500 nm wavelength range. The sensing property of samples under ethanol gas were investigated at an operating temperature ranges from 175 to 250 °C in order to determine the optimal working temperature of these films. The thin films of molybdenum oxide annealed in oxygen atmosphere at 400, 450 and 500 ºC are the layers used for gas sensing studies; these films have two different structures with different stoichiometric state. The sensors are introduced in a test chamber in order to control the sensor temperature under variable gas concentrations. The ethanol concentration in dry air can be calculated by applying the following equation [22].

[C ]%  [

4

xd1 ]  100 xd1  d1  d 2

(1)

Where “x” is the molar fraction of the vapor at Tethanol =30 ºC, given by: x

Pethanol Patm

(2)

With Pethanol is the partial pressure of the vapor at a given temperature Tethanol , and Patm the atmospheric pressure. Heating the sensor is provided by a 250 Watt halogen lamp fed by a stabilized generator (35 V- 45 A). The sensitivity of sensor (S) is defined as [23]: S 

Gethanol  Gair Gair

(3)

Where Gair and Gethanol are the conductance of the film before and after introduction of ethanol respectively.

3. Results and discussion 3. 1. Microstructural study X-ray diffraction analysis of the as-deposited molybdenum oxide thin film reveals amorphous nature of this layer since no diffraction peaks could be observed (figure-1). In contrast to the annealed films under vacuum at 673 and 723 K, (figure 2-a and b), the patterns exhibit several peaks located at 2  =26.0°, 37.2° and 53.5° and assigned to the monoclinic structure of MoO2 according to (JCPDS 78- 1070) card. These peaks correspond indeed to the lattice planes (011), (111) and (220) respectively [24, 25]. When the annealing temperature reaches 773 K, the spectrum showed a well defined peaks corresponding to (410), (660), (1310), and (1320) planes with a strong peak corresponding to (540) preferred orientation of the tetragonal Mo5O14 (MoO2,8), (JCPDS: 74-1415) [26]. The amount of oxygen in MoO2 structure seems to be high after treatment at 773 K in vacuum, leading to the formation of crystalline oxide Mo5O14 variety. This change in oxygen index may be due to the use of statically vacuum and not to dynamic one as well as the annealing temperature which reached 773 K in glass tube (possibly degassed glass phenomenon at this temperature). In addition, the monoclinic structure of MoO2 thin films showed peaks of with low intensities, this probably due to an increase of defects caused by oxygen deficiency, which deteriorate the crystallinity of molybdenum oxide thin films. 5

For annealed thin films in air, obtained spectra for different heat treatment temperatures at TR = 673, 723 and 773 K are shown in Figure-3. The treated films at TR = 673 K (figure-3a) are crystallized, with the presence of a main peak at 2  = 12.76 ° parallel to other peaks appearing at 2  = 23.32 °, 25.7 °, 27.33° and 38.97°. It is noted that these additional peaks persist for the annealed films at 723 K with an increase in intensity which indicates good crystallization state of treated layer according to JCPDS: 05-0508 card related to MoO3 orthorhombic phase [27]. However, for films treated at 773 K (figure-3c), the spectrum shows a noticeably change in structure from orthorhombic to tetragonal corresponding to Mo5O14 (MoO2,8) phase and most of the peaks of orthorhombic MoO3 disappeared and only the main peak persists in 2  = 12.76° with a significant improvement of its intensity. This is probably due to a possible change of oxygen index inside the film. Also, these results suggest the coexistence of both the orthorhombic and the tetragonal phases and the percentage of each phases change therefore with the annealing temperature. Moreover, X-ray diffraction patterns of molybdenum oxide thin films annealed in oxygen atmosphere at 673, 723 and 773 K are shown in figure-4. We note the appearance of (200), (101), (400), (201), (210), (600) and (501) peaks corresponding to the orthorhombic phase of MoO3. These are in agreement with those found in the literature [28, 29]. It is also worth noting that annealed layers at 673 and 723 K display the same other peaks corresponding to tetragonal structure of Mo 5O14 witch disappear at annealing of 773 K and no other phases are detected at this temperature indicating that all Mo6+ ions are saturated and the stoichiometry is then obtained. The increase of annealing temperature promotes an improvement of the intensity and this behavior is probably due to the increase in the crystallite size. On the other hand, during the annealing process, oxygen diffuses into the bulk of the film, causing partial filling of oxygen vacancies which completely disappear at annealing temperature of 773 K, and thus improved stoichiometry. The same results are reported previously by Al-Kuhaili et al [30]. Indeed, the full width at half maximum (FWHM) of the preferred orientation was used to estimate the average of the crystallite size via DebyeScherrer formula [31]:

D

k  cos 

(4)

6



Where k is the constant (equal to 0.9),  is the X-ray wavelength (   1.5418 A );  is the peak full width at half maximum;  is the Bragg angle. Table I summarized these values. The increase in crystallite size with oxygen atmosphere reveals a real enhancement of the crystallinity and decreases the grain boundary discontinuities. Moreover, the Raman spectroscopy is used to reach more information about the microstructure and the lattice defects. Figure-5, shows the Raman spectra of the as deposited and the annealed molybdenum oxide thin films in vacuum at 673, 723 and 773 K. The first spectrum of amorphous film was uniform since no peaks appear [32]. On the contrary, monoclinic MoO2 corresponding to layer annealed at 673 K was detected with Raman bands at 208, 363, 463, 492, 569,593 and 741 cm-1, with a road wing of Rayleigh scattering, due to the metallic state of MoO2 and the Raman band at 741 cm-1 may be attributed to the Mo-O stretching vibrations [33, 34]. These peaks remain for the layer annealed at 723 K with the appearance of a new peak located at 909 cm-1 of Mo5O14 attributed to the terminal Mo-O vibration. While the tetragonal Mo5O14 (films annealed at 773 K) gave Raman bands at 371, 875, 917 and 950 cm-1. These results regarding both MoO2 and Mo5O14 are consistent with those reported elsewhere [35]. Raman spectra of the annealed molybdenum oxide thin films in air ambient at 673, 723 and 773 k are shown in figure-6. The orthorhombic MoO3 for film annealed at 673 K was detected with low intensities Raman bands at 371, 720, 818, 958 and 1100 cm-1, the Raman bands at 818 and 1100 cm-1 correspond to MoO3 orthorhombic phases and the one assigned to 958 cm-1 corresponds to tetragonal Mo5O14. With annealing at 723 and 773 K, the Raman bands at 371 and 720 cm-1 disappeared and a new peak appears at 569 cm-1 corresponding to monoclinic MoO2. For the layers treated in air, there is a mixture of Raman bands related to unstable structures of MoO2, Mo5O14 and MoO3. Molybdenum suboxides are further characterized by additional bands in the Mo=O stretching vibrational regime at 950, 875 and 724cm-1. Orthorhombic MoO3 (spectrum of molybdenum oxide thin films annealed in oxygen at 673,723 and 773 K showed in figure-7) was detected with Raman bands located at 286, 336, 457, 661, 818, 951and 994 cm-1. It is noted that for films heated under air and oxygen atmospheres, Raman peaks became large which is tributary to the appearance of michrocrystallites as seen by figure-7. The schematic drawing of the atomic motions for the Raman bands characteristic the orthorhombic structure of MoO 3 is presented in figure-8. This 7

figure presents the atomic motions of the Raman vibrations of the Raman bands observed at 336, 818 and 994 cm-1 of the stable orthorhombic structure of MoO3 thin films. The Raman signals of Mo5O14 structure at 368 and 942 cm-1 for the samples annealed at 673 and 723 K have a lower intensity but it still detected, and disappeared with annealing at 773 K [36, 37]. In general, the peak at 993 cm-1 corresponds to the stretching vibration of the molybdenum oxygen double bands along the a-axis, which indicates the formation of the layered structure due to the fact that these units are only present in orthorhombic MoO 3 phase. Raman frequencies of monoclinic MoO2, tetragonal Mo5O14 and orthorhombic MoO3 are summarized in table II. This finding shows that parallel to Mo5O14 oxide, the possible presence of other entities such as MoO3 and MoO2 is still to be verified by means of other physical characterizations as XPS and so on. This is in good agreement with XRD results and matches well with those previously reported [38, 39]. In the same way, with increasing annealing temperature, the Raman bands become more resolved, and this was also consistent with XRD study. This change of intensity ratios and band shifts in Raman of MoO3-x reveal that the scattering tensor of MoO3 is changing under oxygen-deficient conditions, and the deeper color of MoO3-x, may be responsible for the reduced efficiency of Mo suboxides as described in Raman study described above.

3. 2. Morphological study SEM micrographs in figure 9, 10 and 11 display the influence of the different annealing conditions on the particle size and morphology of the thin films. The untreated layer obtained from a powder of molybdenum oxide has a homogeneous and uniform morphology since the as deposited layers have no cracks and perfectly adheres to the substrate [40] (figure-9 a). The morphology of layers annealed under vacuum are presented in figure 9. The films treated at 673 K, show a spherical grain with size lying in 100 - 200 nm domain, the orientation of these grain are randomly, some whole grains do not share a common border and so are surrounded by empty spaces. At a higher temperature (figure 9-b), the shape of the grains are typically similar, but there is a net decrease in empty spaces. When the temperature reaches 773 K (figure 9-d) the morphology is not the same appearance of the lamellaes surrounded with small grain sizes). This is consistent with X-ray diffraction analysis detailed previously [41]. 8

The samples treated under air ambient were disturbed by clusters of circular shape with various diameter which occur during the annealing process, figure 10-a. In addition, it is observed that the number of clusters increases with the temperature, figure 10-b. This may be due to metallic behavior of prepared films. Finally, the thin films annealed in an oxygen atmosphere at 723 and 773 K show the presence of cracks which probably due to an aggressive oxidation (figure 11.a and 11.b) [42, 43]. Only the effect of two temperature are studied because with 400 and 450°C and under oxygen and air atmospheres, no changes in structure and morphology have been noted. 3.3. Optical study A preliminary characterization of the optical properties of molybdenum oxide thin films is important for giving inputs to the subsequent detailed analysis. This study is based on the transmission measurements, reflection and optical absorption as a function of incident wavelength depending on the heat treatment conditions (temperature and oxygen pressure). Figure-12 shows that the transmission coefficient of the untreated layer transmits less light in the visible (49.61%) compared with the annealed films in air and oxygen atmosphere. Moreover, from the transmission and band gap features for the as-deposited thin films, it can be seen the possible presence of both α and β MoO3 phases. For layers annealed in air ambient at different temperatures T R = 673, 723 and 773K, it is noted that all transmission curves have the same shape. The second point is the absence of interference oscillations which probably due to the destruction of multiple reflections of radiation caused by the roughness effect seen by SEM observations described above. When the layers are treated in oxygen atmosphere at different temperatures figure-13, it is found that there is no evolution regarding interference oscillations but we observe that the transmission coefficient increases with temperature. Its value is of the order of 70% for the treated layer at 673 K and passes to 90% for the treated at 773K. This increase in the transmission coefficient may be the irregularity of oxygen stoichiometry; these values are in agreement with those given in the literature regarding MoO3 thin films obtained by chemical vapour deposition [44]. The absorption measurements in the UV-visible range near IR, were made using a conventional spectrophotometer. The principle of optical absorption based on the measurement of the intensities of the transmitted radiation through the sample and this coefficient is given by:

9

  d  ln

1 T ( )

(5)

Where α is the absorption coefficient, d is the layer thickness and T is the transmission coefficient. The band gap energy width values were extracted from absorption edge of every sample, and we have assumed that molybdenum oxide thin films are characterized in terms of the band gap energy variations. In the case of a direct transition, the optical band gap energy Eg is given by the following relation [45]:

(h ) 2  A(h  Eg )

(6)

Where A is a constant, h is the photon energy and Eg is the optical band gap energy. The variation of (h ) 2 vs h were plotted as shown in figure-14. The extrapolation of the linear part of the absorption gives values for Eg = 3.16 and 3.55 eV. Also, figure-15, 16 shows the changes (h ) 2 as a function of the energy h thin films respectively MoO3 annealed in air and oxygen atmosphere at TR = 673, 723 and 773 K. The experimental values of figure-15 shows that the band gap energy of the as-deposited thin films is around 3.65 eV, G. E. Buono-Core et al [46] found the same values of the band gap energy for amorphous films of molybdenum oxide prepared by photochemical metal-organic deposition. The samples annealed in air are not affected by the annealing effect. However, the Eg of the films increased from 3.75 to 3.89 eV as the annealing temperature in oxygen increased from 673 to 773 K. This may be due to oxygen vacancy in such films. The band gap energy of treated films under both oxygen and air has higher values than those of treated under vacuum. This is mainly due to an enhancement of the crystalline state as well as to the formation of stoechiometric films with oxygen index close to 3. All the results are listed in table III. This increase is likely due to the effect of reduced oxygen vacancies and the restoration of the stoichiometry. Indeed, when the stoichiometry is reached, the defects in the surface and volume of these oxide decreases. With increasing the oxygen, the degree of ionisation also increases. These results of optical band gap are also in agreement with those reported elsewhere [47]. The variation in the optical band gap for the annealed films may be attributed to a variation of oxygen ion vacancies concentration and the shape of the adsorption spectrum can be related to such oxygen vacancies defining different energy levels in the energy gap of 10

MoO3. Their energy position in the band gap depends on the applied temperature and these centers are in the forbidden gap and form a narrow donor band below the conduction band. The reactivity of oxygen ions with Mo atoms is improved, leading to the formation of nearly stoichiometric MoO3 thin films [29]. The variation of band gap energy matches well the colorless of treated films. Indeed, it is noted that the annealed films under oxygen atmosphere are transparent while the untreated ones as well as vacuum treated films exhibit blue colour [4]. 3. 4. Gas sensing property Gas sensing characteristics of molybdenum oxide thin films annealed in oxygen atmosphere at 400, 450 and 500 ºC toward 0.1 and 0.5 % of ethanol vapor were measured at different operating temperature ranging from 175 to 250 ºC. Figure - 17 shows the dynamic responses of MoO3 thin films tested for duration of 1 min at every operating temperature. The conductance increased upon exposure to ethanol vapor and recovered completely to the initial value upon the removal of ethanol. The sensor responses were quite stable and reproducible for repeated test cycles. Besides, the molybdenum oxide annealed at 500 ºC detects the vapor of ethanol only for a concentration of 0.5% which not the case for the other annealed films. The film annealed at 500 ºC in oxygen atmosphere is stoichiometric since no vacancies in oxygen appear and the structure is so stable so the layer do not affect by any outside interaction. Figure - 18 compare the sensitivity of molybdenum oxide toward 0.1 and 0.5 % of ethanol at working temperature of 175-250 ºC. The highest response of MoO3 was found for annealing at 400 ºC at 200 and 225 ºC and these experimental results are tributary to oxygen vacancy for this temperature and are consistent with XRD analysis conducted on such films [48, 49]. This is probably correlated with the changes brought by the defects states in the structure of the film related to oxygen deficiencies. This shows that the sub-oxides are good sensor materials towards these reducing gases under these operating conditions. The response is also related to the reducing ability and the adsorbing ability of detected gas on surface of the material and these results are in good agreement with the observed in the literature by Illyaskutty et al. [50]. This author proves that the formation of oxygen vacancies generates free electrons to the bulk, hence conductance increases with ethanol vapour concentration. This same group [51] found that the oxygen vacancies can also result in an improved specific surface area of the sensor surface and enhance the adsorption of analyte species over the oxide layer. 11

Conclusion The annealing temperature and oxygen content affect significantly the crystalline structure of MoxOy materials as they generate important changes in their optical properties. The structural shows that the as-deposited films were amorphous. Also it shows a transparent color with a band gap around 3.65 eV, while the annealed layers in vacuum at 673 and 723 K presents the MoO2 monoclinic phases with blue color and the gap assumes values around 3.16 eV. The vacuum annealing caused a coloration of the films and a reduction of the band gap associated with the presence of oxygen vacancies which promotes the reduction of oxygen atoms in the oxide structure and the coloration is due to the presence of different oxidation states, such as Mo4+ Mo5+ and Mo6+. When the annealing temperatures reach 773 K in air the structure transformed to tetragonal of Mo 5O14 phases which contain excess metal atoms. With annealing in oxygen atmosphere, the percentage of instable suboxide phases starts diminishing and simultaneously, the more stable MoO3 orthorhombique phases appears; the grain size of this structure increase to values of the order of 213 Å and this sample`s gap increase up to 3.89 eV. It was found also, that the stoichiometric films have a poor response for ethanol gas while the substoichiometric of films exhibit a good response for an operating temperature from 200 to 225 ºC. Further studies are in progress to test such oxide films in photocatalysis.

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[24] S. Wang, C. An, Y. Zhang, Z. Zhang and Y. Qian « Ethanothermal reduction to MoO2 microspheres via modified pechini method», Journal of Crystal Growth 293(2006) 209-215. [25] J. Baltrusaitis, B. Mendoza-Sanchez, V. Fernandez, R. Veenstra, N. Dukstiene, A. Roberts and N. Fairley « Generalized molybdenum oxide surface chemical state XPS determination via informed amorphous sample model», Applied Surface Science (2014). [26] G. A. Zenkovets, G. N. Kryukova, V. Yu. Gavrilov, S. V. Tsybulya, V. A. Anufrienko, T. A. Larina, D. F. Khabibulin, O. B. Lapina, E. Rodel, A. Trunschke, T. Ressler and R. Schlogl «The structural genesis of a complex (MoVW)5O14 oxide during thermal treatements and its redox behavior at elevated temperatures», Materials Chemistry and Physics 103(2007) 295-304. [27] Rabindar K. Sharma and G. B. Reddy« Effect of substrate temperature on the characteristics of α-MoO3 hierarchical 3D microspheres prepared by facile PVD process», Journal of Alloys and Compounds 598 (2014) 177-183. [28] R. Martinez Guerrero, J. R. Vargas Garcia, V. Santes and E. Gomez « Preparation of molybdenum oxide thin films by MOCVD», Journal of Alloys and Compounds 434-435 (2007) 701-703. [29] O.M. Hussain and K.S. Rao « Characterization of activated reactive evaporated MoO3 thin films for gas sensor applications», Materials Chemistry and Physics 80 (2003) 638–646. [30] M. F. Al-Kuhaili, S. M. A. Durrani, I. A. Bakhtiari and A. M. Al-Shukri «Optical constants and thermocoloration of pulsed laser deposited molybdenum oxide thin films», Optics Communications 283 (2010) 2857-2862. [31] P. Scherrer, Nachr, Ges. Wiss, Göttingen, Math-Phys. Klasse 26 (1918)98. [32] Y. A. Yang, Y. W. Cao, B. H. Loo, J. N. Yao « Microstructures of electrochromic MoO3 thin films colored by injection of different cations», Physics chemical B 102 (1998) 9392–9396. [33] George H. Smudde and Peter C. Stair «The oxidation of Mo (100) studied by XPS and surface Raman spectroscopy: the onset of MoO2 formation and the formation of surface polymolybdate», Surface Science 317 (1994) 65-7. [34] Adrian . A. Bolzan, Bredan. J. Kennedy and Christopher. J. Howard «Neutron powder diffraction study of molybdenum and tungsten dioxides», AUSTRALIAN JOURNAL OF CHEMISTRY 48 (1995)1473-1477. [35] M. Dieterle and G. Mestl Howard «Raman Spectroscopy of Molybdenum oxides part II, Resonance Raman spectroscopic characterization of the molybdenum oxides Mo4O11 and MoO2 », Physical Chemistry 4 (2002), 822–826.

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[36] D. S. Kim, I. E. Wachs and K. Segawa «Molecular Structures and Reactivity of Supported Molybdenum oxide», Catalysts Journal of Catalysis 146, 268-277 (1994) [37] L. O. Aleman-Vazquez, F. Hernandez-Perez, J. L. Cano-Dominguez, A. RodriguezHernandez and J. L. Garcia-Gutierrez «Binder effect on the catalytic activity of MoO3 bulk catalyst reduced by H2 for n-heptane hydroisomerization», Fuel (2013). [38] G. Mestl, C. Linsmeier, R. Gottschall, M. Dieterle, J. Find, D. Herein, J. Jager, Y. Uchida and R. Schlogl Journal of Molecular Catalysis A 162 (2000)455. [39] L. Kihlborg, Ark Kemi 21 (1963) 427. [40] R. S. Patil, M. D. Uplane and P. S. Patil «Electrosynthesis of Electrochromic Molybdenum Oxide Thin Films with Rod-Like Features», International Journal of ELECTROCHEMICAL SCIENCE 3 (2008) 259-265. [41] C. Gretener, J. Perrenoud, L. Kranz, C. Baechler, S. Yoon, Y. E. Romanyuk, S. Buecheler and A. N. Tiwari «Development of MoOX thin films as back contact buffer for CdTe solar cells in substrate configuration», Thin Solid Films 535(2013) 193-197. [42] Y. X. Li, K. Galatsis, W. Wlodarski, M. Passacantando, S. Santucci, P. Siciliano and M. Catalano «Microstructural characterization of MoO3-TiO2 nanocomposite thin films for gas sensig», Sensors and Actuators B 77(2001) 27-34. [43] A. Pardo and J. Torres «Substrate and annealing temperature effects on the crystallographic and optical properties of MoO3 thin films prepared by laser assisted evaporation», Thin Solid Films 520 (2012) 1709-1717. [44] F. Urbach «The Long-wavelength edge of photographic sensitivity and of the electronic absorption of solids», Physical Review 92 (1953) 1324. [45] A. Abdellaoui, G. Leveque, A. Donnadieu, A. Bath and B. Bouchikhi «Iteratively derived optical constants of MoO3 polycrystalline thin films prepared by CVD», Thin Solid Films 304(1997) 39-44. [46] G. E. Buono-Core, G. Cabello, A. H. Klahn, A. Lucero, M. V. Nunez, B. Torrejon and C. Castillo «Growth and characterization of molybdenum oxide thin films prepared by photochemical metal-organic deposition (PMOD)», Polyhedron 29(2010) 1551-1554. [47] G. E. Buono-Core, , A. H. Klahn, C. Castillo, E. Munoz, C. Manzur, G. Cabello and B. Chornik « Synthesis and characterization of thin molybdenum oxide films prepared from molybdenum dioxo tropolonate precursors by photochemical metal-organic deposition (PMOD) and its evaluation as ammonia gas sensors», Journal of Non-Crystalline Solid 387 (2014) 21-27. 16

[48] Shouli Bai, Song Chen, Liangyuan Chen, Kewei Zhang, Ruixian Luo, Dianqing Li, Chung Chiun Liu « Ultrasonic synthesis of MoO3 nanorods and their gas sensing properties», Sensors and Actuators B 174(2012) 51-58. [49] K. Galatsis, Y. X. Li, W. Wlodarski, E. Comini, G. Sberveglieri, C. Cantalini, S. Santucci, M. Passacantando «Comparison of single and binary oxide MoO3, TiO2 and WO3 sol-gel gas sensors», Sensors and Actuators B 83 (2002) 276–280. [50] Navas Illyaskutty, Heinz Kohler, Thomas Trautmann, Matthias Schwotzer, V.P. Mahadevan Pillai « Hydrogen and ethanol sensing properties of molybdenum oxide nanorods based thin films: Effect of electrode metallization and humid ambience », Sensors and Actuators B: Chemical 187 (2013) 611–621. [51] Navas Illyaskutty, Heinz Kohler, Thomas Trautmann, Matthias Schwotzer, V.P. Mahadevan Pillai « Enhanced ethanol sensing response from nanostructured MoO3: ZnO thin films and their mechanism of sensing », Journal of Materials Chemistry C 1 25(2013) 3976-3984.

17

Figure captions

Intensity (a.u)

films as-deposited

0

10

20

30

40

50

60

2(°)

Figure-1: X-ray diffractogram of as-deposited molybdenum oxide thin films on glass substrate.

18

0

10

20

30

220

111

Intensity (a.u)

011

(a) Molybdenum oxide annealed in vacuum at 673 k

40

2(°)

50

60

0

10

20

30

2(°)

220

111

Intensity (a.u)

011

(b) Molybdenum oxide annealed in vacuum at 723 K

40

50

60

0

10

1310 1320

660

410

Intensity (a.u)

540

(c) Molybdenum oxide annealed in vacuum at 773 K

20

30

40

50

60

2()

Figure-2: X-ray diffractogram of molybdenum oxide thin films annealed in vacuum at different temperatures: (a) 673K, (b) 723K and (c) 773K.

19

10

20

060

021

110

Intensity (a.u)

020

040

(a) Molybdenum oxide annealed in air at 673 K

30

40

50

60

2()

10

021

110

060

Intensity (a.u)

020

040

(b) Molybdenum oxide annealed in air at 723 K

20

30

40

50

60

2()

660

1310

Intensity (a.u)

540

(c) Molybdenum oxide annealed in air at 773 K

0

10

20

30

40

50

60

2()

Figure-3: X-ray diffractogram of molybdenum oxide thin films annealed in air at different temperatures: (a) 673K, (b) 723K and (c) 773K. 20

(a) Molybdenum oxide annealed in flowing oxygen at 673 K

(74-1415) Mo5O14

0

10

20

30

1141 600

210

331

820

400

401

Intensity(a.u)

200

(65-2421) MoO3

40

50

60

200

(b) Molybdenum oxide annealed in flowing oxygen at 723 K 2(°)

(74-1415) Mo5O14

0

10

600

820

610

040

Intensity (a,u)

(65-2421) MoO3

20

30

40

50

60

2(°)

0

10

20

600 501

210

101 400 201

Intensity (a.u)

200

(c) Molybdenum oxide annealed in flowing oxygen at 773 K

30

40

50

60

2()

Figure-4: X-ray diffractogram of molybdenum oxide thin films annealed in flowing oxygen at different temperatures: (a) 673K, (b) 723K and (c) 773K.

21

875.83 917 950 741.7

569.6 593.6

909.6

743.1

723 K

673 K

569.6

463.3 494.2

361.9 363.3

463.3 492.8

208.3

Raman intensity (a.u)

371.15

773 K

asdeposited

200

400

600

800

1000

1200

Raman shift (cm-1)

371.5 200

400

600

951 958.4 958.4

721.1

673 K

820.8

723 K

569.1

Raman intensity (u.a)

773 K

818.5

820.8

569.1

Figure -5: The Raman spectra of as-deposited and annealed molybdenum oxide thin films in vacuum at 673, 723, 773 K.

800

1000

1200

Raman shift (cm-1)

Figure -6: The Raman spectra of as-deposited and annealed molybdenum oxide thin films in air at 673, 723, 773 K.

22

Raman Intensity (a.u)

993.9 1010.6

951.7

661.9

336.8

457.4

286.2

816.4

773 K

993.9 902.1 942.5

662.8 720.4

673 K

200

400

600

994.6

902.8 943.2

664.2 720.4

588.3

334.5 369.4

217.2 279.6

819.8

594.3

334.5 368.7

279.6

818.5

723 K

800

1000

1200

-1

Raman shift (cm )

Figure -7: The Raman spectra of annealed molybdenum oxide thin films in oxygen at 673, 723,773 K.

336 cm-1 Ag , δ O-Mo-O bend

818 cm-1 Ag νs Mo=O stretch

994 cm-1 Ag νas Mo=O stretch

Figure -8: : Schematic drawing of the atomic motions for the characteristic Raman bands of orthorhombic MoO3.

23

Figure-9: SEM micrographs of molybdenum oxide thin films (a) as-deposited and annealed in vacuum for 1h at (b) 673, (c) 723k and (d) 773k.

24

Figure-10: SEM micrographs of molybdenum oxide thin films annealed in air for 1h at (a) 673 and (b) 773k

Figure-11: SEM micrographs of molybdenum oxide thin films annealed in oxygen atmosphere for 1h at (a) 673 and (b) 773k

25

100

molybdenum oxide annealed in air

Intensity (a,u)

80

60

as deposited T= 673 K T= 723 K T= 773 K

40

20

0 500

1000

1500

2000

2500

(nm)

Figure-12: The spectral transmittance (T) and reflectance (R) for molybdenum oxide thin films annealed in air at T = 673, 723 and 773K.

100

Molybdenum oxide annealed in oxygen

80

Tann =673 K

R/T(%)

60

Tann =723 K Tann =773 K

40

20

0 500

1000

1500

2000

2500

(nm)

Figure-13: The spectral transmittance (T) and reflectance (R) for molybdenum oxide thin films annealed in flowing oxygen at T = 673, 723 and 773K.

26

110

18

1.0x10

Tann vacuum= 723K Tann vacuum= 673K

17

8.0x10

2

(h) (ev)

Absorption (%)

2

90

70

50

17

6.0x10

Eg=3.16 eV Eg=3.55 eV

17

4.0x10

17

2.0x10

0.0 0

1

2

3

energy (eV)

4

5

30

10 250

500

750

1000

1250

Wavelength (nm)

1500

1750

2000

Figure-14: UV-Vis of molybdenum oxide annealed in vacuum at 673 and 723K, the insert is the plot of (αhν)2 vs photon energy.

10

6x10

molybdenum oxide annealed in air 10



h eV/cm)

2

5x10

as deposited TR=673 K

10

4x10

TR=723 K

10

3x10

TR=773 K

10

2x10

10

1x10

0 1

2

3

4

5

h(eV)

Figure-15: Variations of (αhυ) 2 versus (hυ) for molybdenum oxide thin films annealed in air at T = 673, 723 and 773K.

27

10

6.0x10

as-deposited Tr= 673K

molybdenum oxide annealed in flowing oxygen

Tr= 723K Tr= 773K

10

2

h) (eV/cm)

2

4.0x10

10

2.0x10

0.0 1

2

3

h(eV)

4

5

Figure-16: Variations of (αhυ) 2 versus (hυ) for molybdenum oxide thin films annealed in flowing oxygen at T = 673, 723 and 773K.

28

(Tann= 400 C) 2.0x10

-1

Response ( )

200 C 225 C 250 C

Cethanol =0.1%

10

10

1.5x10

10

1.0x10

9

5.0x10

0.0 0

1

2

3

4

5

6

7

time (min)

6

9x10

(Tann= 450 C) Cethanol =0.1%

6

8x10

-1

Response ( )

175 C 200 C 225 C 250 C

6

7x10

6

6x10

6

5x10

6

4x10

0

1

2

3

Time (min)

29

4

5

6

7

200 C 225 C 250 C

(Tann= 400 C)

10

1.4x10

10

Cethanol =0.5%

1.2x10

-1

Response ( )

10

1.0x10

9

8.0x10

9

6.0x10

9

4.0x10

9

2.0x10

0

1

2

3

4

5

6

time(min)

7

1.0x10

175 C 200 C 225 C 250 C

(Tann= 450 C)

-1

Response ( )

Cethanol =0.5% 6

8.0x10

6

6.0x10

0

1

2

3

time (min)

30

4

5

6

9

1.6x10

(Tann= 500 C)

175 200

c ethanol = 0.5

9

1.2x10

225

-1

Response ( )

250 8

8.0x10

8

4.0x10

3

4

5

6

Time (min)

Figure - 17: Dynamic response of annealed molybdenum oxide thin films at 400, 450 and 500°C in oxygen exposed to 0.1 and 0.5 % for a period of 1 min at a various operating temperature from 175 to 250 C.

31

molybdenum oxide thin films annealed in oxygen 8

Tann= 400 C (0,1%) Tann= 450 C (0,1%) Tann= 400 C (0,5%)

Sensitivity

6

Tann= 450 C (0,5%) Tann= 500 C (0,5%) 4

2

175

200

T(C)

225

250

Figure - 18: Sensitivity of annealed molybdenum oxide thin films at 400, 450 and 500°C in oxygen exposed to 0.1 and 0.5 % for a period of 1 min at a various operating temperature from 175 to 250 C.

32

Table I: Crystallites mean diameter D versus annealing temperature.

Tann (K)

673

723

773

D (nm) under vacuum

67

73

160

D (nm) in air ambient

164

106

188

D (nm) in flowing oxygen

208

223

213

Table II: Raman frequencies of MoO2, Mo5O14 and MoO3. Monoclinic MoO2 Raman Vibration Bands mode 362 m-MoO2 493 m-MoO2 742 Mo-O(I) stretch

Tetragonal Mo5O14 Raman Bands Vibration mode 371 δ(O=Mo) 917 MoO3-x 950 νas (O=Mo)

33

Orthorhombic MoO3 Raman Vibration Bands mode 336 δ(O-Mo-O) 818 ν(OMo2) 994 ν (OMo)

Table III: Effect of annealing temperature on the molybdenum oxide layers band gap energy. Tann (K) Eg (eV) annealed in air Eg (eV) annealed in flowing oxygen

As-deposited

673

723

773

3.65

3.73

3.75

3.90

3.65

3.75

3.81

3.89

34