Influence of precursor solution volume on the properties of spray deposited α-MoO3 thin films

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 5817–5823 www.elsevier.com/locate/ceramint

Influence of precursor solution volume on the properties of spray deposited α-MoO3 thin films B. Kannan, R. Pandeeswari, B.G. Jeyaprakashn Centre for Nanotechnology & Advanced Biomaterials and School of Electrical & Electronics Engineering, SASTRA University, Tamilnadu, Thanjavur 613401, India Received 28 August 2013; received in revised form 28 October 2013; accepted 5 November 2013 Available online 12 November 2013

Abstract α-MoO3 thin films were deposited onto a glass substrate with 0.01 M ammonium heptamolybdate tetrahydrate as a precursor salt and deionized water as solvent using spray pyrolysis technique. The influence of precursor solution volume on the structural, morphological, optical and electrical properties were analysed and reported. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: α-MoO3; Thin films; Spray pyrolysis; Lamellar pattern

1. Introduction Molybdenum trioxide (MoO3) is one of the transition metal oxides, which attracted scientific community owing to its exotic properties and finds potential applications in diversified fields such as sensors [1], catalysis [2], batteries [3], optical switches [4], chromic material [5], smart window [4] and memory material [6]. MoO3 exist in three polymorphic phases [7] namely stable orthorhombic α-MoO3, meta-stable monoclinic β-MoO3 and hexagonal ν-MoO3. The α-MoO3 phase has layered structure and stack along (0 k 0) direction [1]. Furthermore, MoO3 possess versatile morphological structures of nanobelts, nanorods, nanowires, etc., offering a high surface to volume ratio. This significant surface property propelled the researchers to focus for the development of MoO3 as a gas sensor for the past few decades [1,8–12]. The α-MoO3 thin films have been deposited by different physical and chemical methods such as sputtering [12], thermal evaporation [1], atomic layer deposition [13], spray pyrolysis [10], hydrothermal method [14], sol gel-spin coating [15], etc. n

Corresponding author. Tel.: þ91 9865421411. E-mail addresses: [email protected], [email protected] (B.G. Jeyaprakash).

Nirupama et al. [16] reported the formation of nanostructured α-MoO3 thin film using magnetron sputtering under biased condition. Dhanasankar et al. [17] reported sol gel-dip coated MoO3 thin film has orthorhombic crystal structure. Sunu et al. [18] investigated the ammonia sensing properties of MoO3 thin films at high operating temperature. Rahmani et al. [1] described the formation of long rectangular crystallites of α-MoO3 thin films by thermal evaporation method and discussed its morphology dependent NO2 sensing behaviour at high temperature. Bouzidi et al. [19] represented the spray deposition of MoO3 thin films and made a study on its structural and optical properties as a function of substrate temperature. In the present work, α-MoO3 thin films were deposited onto the glass substrate by spray pyrolysis technique and the effect of precursor solution volume on structural, morphological, optical and electrical properties were studied and reported. 2. Materials and methods 2.1. Thin film preparation The α-MoO3 thin films were deposited onto the pre-cleaned glass substrates having a dimension of 12 mm
25 mm
1.45 mm. The precursor solution was prepared by dissolving

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.022

B. Kannan et al. / Ceramics International 40 (2014) 5817–5823

a 0.01 M of ammonium hexamolybdate tetrahydrate ((NH4)6 Mo7O24  4H2O) (Sigma-Aldrich, 99.98%) in deionized water. The precursor solution was then sprayed as fine mist over the pre-heated glass substrate using home-build spray pyrolysis unit [20]. The substrate temperature was fixed to 250 1C and controlled using a thermostat with an accuracy of 7 1 1C. The spray gun was mounted at an angle of 451 with the distance between substrate and spray nozzle fixed to 40 cm. The spraying time and successive spray intervals were fixed to 5 s and 75 s respectively. These optimised deposition parameters were maintained to prepare different films by varying the precursor solution volume from 10 mL to 50 mL insteps of 10 mL. The pyrolytic reaction in the formation of the molybdenum trioxide from the precursor solution is conveyed in Eq. (1). All the films were allowed to cool at room temperature on completion of deposition. The resultant films were subjected to further characterisation studies.

600 500

Thickness (nm)

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400 300 200 100 10

20

30

40

50

Soultion volume (mL) Fig. 1. Variation of film thickness as a function of precursor solution volume.

(020)

ðNH4 Þ6 Mo7 O24 4H2 O - 7MoO3ðsÞ þ 6NH3ðgÞ þ 7H2 OðgÞ Δ250 1C

Intensity (a.u.)

ð1Þ 2.2. Characterisation The thickness of the film was obtained from stylus profilometer (Mitutoyo SJ 301). Structural studies were carried out using X-ray diffractometer (XRD, D8 Focus, Bruker, Germany) with CuKα1 radiation of wavelength 1.5406 Å at a generator setting of 30 mA and 40 KV. The optical studies were carried through UV–visible spectrophotometer (Perkin Elmer Lambda 25). Surface morphology of the films was studied from Scanning Electron Microscope (SEM, FEI Quanta-300, Icon analytical, Japan) and Field Emission Scanning Electron Microscope (FE-SEM, JEOL-6701F, Japan). The microstructure and roughness on the film surface were inferred from Atomic Force Microscope (AFM, Park System, XE-170, Germany). The surface electrical resistance was studied using four probe technique. 3. Results and discussion 3.1. Film thickness Fig. 1 shows the increase in film thickness from 120 nm to 580 nm as the volume of the precursor solution increased from 10 mL to 50 mL. The films deposited with precursor solution volume less than 10 mL had a discontinuous nature, while the film obtained from the precursor solution volume greater than 50 mL shows a powdery deposition. Hence, the precursor solution volume was fixed in the range of 10 mL to 50 mL. 3.2. Structural studies The XRD pattern of as-deposited MoO3 thin films obtained from different precursor solution volume is shown in Fig. 2. The observed peaks were indexed with respect to JCPDS 050508 and indicates α-MoO3 phase. The strong reflection peaks

(040)

(060) (110) (021)

(0100)

50 mL 40 mL 30 mL 20 mL 10 mL 10

20

30

40

50 60 2θ(deg.)

70

80

Fig. 2. XRD pattern of α-MoO3 thin films for varying precursor solution volume.

Table 1 Obtained texture co-efficient values for strong peaks with different precursor volumes. Precursor solution volume (mL) 10 20 30 40 50

Texture co-efficient (TC) (020)

(040)

(060)

(0100)

3.0589 3.9456 4.6415 2.9353 9.0946

0.4454 0.4775 0.6217 0.4325 0.8708

0.2859 0.3374 0.4361 0.3168 0.5945

0.2096 0.2054 0.2133 0.1689 0.2674

(0 k 0) with k¼ 2, 4, 6 proves the existence of the lamellar structure. No structural rearrangement and impurity peaks were found in the films obtained from different precursor solution volumes and α-orthorhombic phase retains in all the films. As the precursor solution volume increased, a slight shift in 2θ value was observed for the preferential peak. However, crystallite orientation is not altered. This clearly indicates that the growth of crystallite orientation is independent of precursor solution volume. Furthermore, the texture coefficient (TC) is

B. Kannan et al. / Ceramics International 40 (2014) 5817–5823 76 74

FWHM (deg.)

0.128 72 0.127 70 0.126 68 0.125 66

Crystallites size (nm)

0.129

0.124 64 10

20

30

40

50

Soultion volume (mL) Fig. 3. FWHM and crystallites size as a function of precursor solution volume.

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used to quantify the preferential orientation of the film deposited at different volume of precursor solution [20] using the following Eq. (2): TC ¼

I ðhklÞ =I oðhklÞ   ð1=NÞ ∑ I ðhklÞ =I oðhklÞ

ð2Þ

N

where I is the measured intensity, Io is the JCPDS intensity and N is the number of diffraction peaks. The estimated texture co-efficient values with respect to the precursor solution volume are given in Table 1. It indicates, texture co-efficient value is high for (0 2 0) plane in all the films deposited from different volume of precursor solution. The crystallite size was

Fig. 4. Scanning electron micrograph of (a) 10 mL, (b) 20 mL, (c) 30 mL, (d) 40 mL, (e) 50 mL and (f) field emission scanning electron micrograph for 50 mL deposited MoO3 films (inset: cross sectional view of single rectangular nanocrystallite).

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estimated using the well-known Scherrer formula: D¼

Sλ β cos θ

ð3Þ

where D is the crystallite size (nm), S is the shape factor (0.9), λ is the wavelength of X-rays (1.5406 Å), β is the full width at half maxima (FWHM in deg), θ is the diffracting angle (deg). The precursor solution volume dependent FWHM and crystallites size of the film for the preferential plane (0 2 0) is shown in Fig. 3 and indicates an increasing trend of crystallite size as the solution volume increased. This is because, as the solution volume increases, the amount of solutes (i.e. ammonium hexamolybdate

tetrahydrate) reaching the substrate is also increased to form thin films. Therefore, the electrostatic interaction between the solute particles becomes greater thereby increasing the probability to gather more solute particles together to form large crystallites [21–23]. The FWHM value decreases from 0.129 to 0.124 causing the corresponding crystallite size to increases from 65 nm to 75 nm respectively. 3.3. Morphological studies The scanning electron micrographs of the films obtained from different precursor solution volumes are shown in Fig. 4

Roughness (nm)

400

300

200

100

0 10

20

30

40

50

Precursor Solution Volume (mL)

Fig. 5. Topography image of (a) 10 mL, (b) 20 mL, (c) 30 mL, (d) 40 mL, (e) 50 mL and (f) precursor solution volume dependent surface roughness of α-MoO3 films.

B. Kannan et al. / Ceramics International 40 (2014) 5817–5823

was determined by extrapolating the liner region in the plot of (αhν)2 versus photon energy. The estimated band gap value and average transmittance value are shown in Table 3. The band gap found to decreases from 3.38 eV to 3.03 eV with an increase in the precursor solution volume and is due to increase in crystallite size [25]. 3.5. Electrical studies The surface electrical resistance (R) of the α-MoO3 thin film was studied from 35 1C to 140 1C using four probe measurement technique and the plot is shown in Fig. 7. It represent 10 mL 20 mL 30 mL 40 mL 50 mL

7 6

Absorbance (a.u.)

(a)–(e). It shows the formation of rectangular nanocrystallites onto the surface of the films. The field emission scanning electron image shown in Fig. 4(f) also indicates the formation of lamellar type crystallites. The cross-section of a single rectangular crystallite shown as an inset in Fig. 4(f) indicates a stack of nanolayers with thickness of 30–40 nm. The crystallite size found to increases as the precursor solution volume increased and can also be confirmed from the observed increase in the intensity of (0 k 0) plane in XRD results. The observed size of the rectangular crystallites was found to be 750 nm in length and 80 nm in breadth. The surface topography of α-MoO3 films obtained from different precursor solution volumes is displayed in Fig. 5(a)–(e). The growth of the layered structure was evident from the observed images. Furthermore, Fig. 5(f) represents the surface roughness of αMoO3 film as a function of precursor solution volume and found to increases from 38 nm to 355 nm as the solution volume increases from 10 mL to 50 mL. It may be due to the growth of nanocrystallites and formation of large grains on the surface of the film [24]. Similar results have been reported earlier [24–26]. The surface profile parameters such as peak valley line (Rpv), root mean square roughness (Rrms), average roughness (Ra), skewness (Rs), kurtosis (Rk) are displayed in Table 2.

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5 4 3 2 1 0

3.4. Optical studies

450

600

750

900

1050

900

1050

Wavelength (nm) 60

10 mL 20 mL 30 mL 40 mL 50 mL

50

Transmittance (%)

The optical properties of the α-MoO3 thin films were measured within the wavelength range of 300–1100 nm. The visual appearance of the as-deposited MoO3 films showed a light bluish colour and homogenous deposition. The optical absorbance and transmittance spectrum of α-MoO3 thin film obtained from different precursor solution volumes are shown in Fig. 6(a) and (b). An increase in film thickness supports an increase in the absorbance value. The optical transmittance spectrum for the film deposited from 10 mL of precursor solution showed a transparency of 41%, and decreased to 9% as the precursor solution volume increased. This decrease in optical transmittance may due to polycrystalline nature and high surface roughness of the film. Furthermore, the random orientation of the crystallites as observed in FE-SEM result may scatter the incident wave in the entire visible spectrum and giving rise to a non-saturated transmittance (%) in the visible region as indicated in Fig. 6(b). Similar results were also reported earlier for magnetron sputtered NiO and ZrO2 thin film materials [27,28]. The optical band gap of the film

40

30

20

10

0 450

600

750

Wavelength (nm) Fig. 6. Optical, (a) absorbance and (b) transmittance spectrum of α-MoO3 thin films.

Table 2 The comparison table for surface profile parameters of different solution volume deposited films. Solution concentration (mL)

10

20

30

40

50

Surface profile parameters Rpv (nm) Rq (nm) Ra (nm) Rsk Rku

240 37 28  0.025 2.963

876 157 123 0.199 3.140

977 169 134 0.061 3.192

1297 177 137 0.584 3.604

2149 355 272 0.610 3.625

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Table 3 Average optical transmittance and optical band gap as a function of precursor solution volume. Solution volume (mL)

Average transmittance (%)

Band gap (eV)

10 20 30 40 50

41 25 15 10 9

3.38 3.35 3.23 3.11 3.03

Resistance (x10 11 Ω Ω)

0.75

10 mL 20 mL 30 mL 40 mL 50 mL

0.60 0.45 0.30 0.15 0.00 20

40

60

80

100

120

140

Temperature(o C) Fig. 7. Surface electrical resistance of α-MoO3 films as a function of temperature.

that, all the α-MoO3 thin films has an increasing trend of electrical resistance up to  50 1C, which may be due to scattering of the available charge carriers in the film. However thermal excitation of charge carriers occurs beyond 50 1C and leads to decrease in film resistance and indicating a semiconducting type. Furthermore, it also indicates the transition of nanostructured α-MoO3 from conducting to wide band gap semiconducting nature with an electrical resistance in the order of 1011 Ω. 4. Conclusion In summary, α-MoO3 films were deposited successfully by home-built spray pyrolysis method. No significant change in crystallites orientation and phase has been found due to change in precursor solution volume. Furthermore no significant change in electrical resistance was observed in the films. However, the size of the rectangular crystallites, optical transparency and surface roughness strongly depends on precursor solution volume. These changes can be utilised to apply the desired nanostructured size and shape of α-MoO3 film for different applications. References [1] M.B. Rahmani, S.H. Keshmiri, J. Yu, A.Z. Sadek, L. Al-Mashat, A. Moafi, K. Latham, Y.X. Li, W. Wlodarski, K. Kalantar-zadeh, Gas sensing properties of thermally evaporated lamellar MoO3, Sens. Actuators B 145 (2010) 13–19. [2] J. Haber, E. Lalik, Catalytic properties of MoO3 revisited, Catal. Today 33 (1997) 119–137.

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