Structural and optical properties of electrodeposited molybdenum oxide thin films

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1 Applied Surface Science 252 (2006) Structural and optical properties of electrodeposited molybdenum oxide thin films R.S. Patil a, M.D. Uplane b, P.S. Patil c, * a The New College, Kolhapur, India b Department of Electronics, Shivaji University, Kolhapur , India c Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur , India Received 7 September 2005; received in revised form 6 October 2005; accepted 7 October 2005 Available online 11 November 2005 Abstract Electrosynthesis of Mo(IV) oxide thin films on F-doped SnO 2 conducting glass (10 20/V/&) substrates were carried from aqueous alkaline solution of ammonium molybdate at room temperature. The physical characterization of as-deposited films carried by thermogravimetric/differential thermogravimetric analysis (TGA/DTA), infrared spectroscopy and X-ray diffraction (XRD) showed the formation of hydrous and amorphous MoO 2. Scanning electron microscopy (SEM) revealed a smooth but cracked surface with multi-layered growth. Annealing of these films in dry argon at 450 8C for 1 h resulted into polycrystalline MoO 2 with crystallites aligned perpendicular to the substrate. Optical absorption study indicated a direct band gap of 2.83 ev. The band gap variation consistent with Moss rule and band gap narrowing upon crystallization was observed. Structure tailoring of as-deposited thin films by thermal oxidation in ambient air to obtain electrochromic Mo(VI) oxide thin films was exploited for the first time by this novel route. The results of this study will be reported elsewhere. # 2005 Elsevier B.V. All rights reserved. Keywords: Molybdenum oxide; Electrodeposition; Thin films; Structural and optical properties 1. Introduction A rapid technological progress in the mobile miniaturized devices and power saving options for intellectual architecture of buildings demand the search for new materials as electrodes in portable rechargeable batteries and as chromogenic coatings. The layered structure of transition metal oxides is a suitable host for ion intercalation/deintercalation necessary in these applications and molybdenum(iv and VI) oxide is the promising candidate. The possible use of MoO 2 as a commercial anode for lithium batteries was reported in 1987 by Auborn and Barberio [1]. A rheological phase reaction method to obtain MoO 2 with singlephase monoclinic structure suitable for anode is reported by Liang et al. [2]. The structural and morphological characteristics of MoO 2 were found to play an important role in the cycling stability. * Corresponding author. Tel.: ; fax: address: psp_phy@unishivaji.ac.in (P.S. Patil). Use of MoO 3 thin films as a cathode in a secondary lithium battery, which undergoes reversible lithium intercalation at ambient temperatures is reported by Julien et al. [3]. A layered oxygen deficient orthorhombic MoO 3 (a-phase) and monoclinic MoO 3 (b-phase) exhibit pronounced optical switching upon thermal, photo or electric excitations. This optical modulation (colouration/bleaching) is effectively used in many applications like smart windows [4 6], antidazzling coatings [7] and display devices [8 9]. Thus, the synthesis of Mo oxide in the desired form seems to be of prime importance. The structure tailoring of deposited material can lead to the required end application. Molybdenum oxide thin films have been prepared by a variety of physical, chemical and electrochemical techniques. Ponomarev et al. [10] reported cathodic electrodeposition of hydrated molybdenum oxide thin films from aqueous ammonium molybdate bath. Electrodeposition of molybdenum oxide thin films from aqueous Li 6 Mo 7 O 24, (NH 4 ) 6 (Mo 7 O 24 ), Li 2 Mo 7 O 4 solutions and Mo dissolved in H 2 O 2 is also reported [11,12]. MoS 3 is electrodeposited [13] from aqueous (NH 4 )MoS 4 and subsequent oxidation between 400 and 550 8C was carried /$ see front matter # 2005 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 out. MoS 3 is converted to MoO 2 rapidly and then to MoO 3 very slowly. In this paper, a novel route of electrosynthesis of MoO 2 thin films is reported. Cathodic reduction of molybdate from aqueous bath is carried out to deposit thin films of hydrous MoO 2. The deposition potential and current density were estimated from cathodic polarization curves drawn for different bath compositions. The compositional, structural and optical properties of as-deposited and argon annealed thin films were studied. 2. Experimental R.S. Patil et al. / Applied Surface Science 252 (2006) The electrodeposition was carried under galvanostatic condition for optimized bath conditions with the help of electrodeposition cell and a self-designed power supply. The electrodeposition cell consisted of a corning glass container of 100 cm 3 capacity, high purity graphite plate of 6 cm 5 cm 0.5 cm as a counter electrode, fluorine doped tin oxide (FTO) coated glass plate of 4 cm 1cm 0.1 cm as working electrode (substrate) and SCE as a reference electrode. An aqueous 0.05 M ammonium molybdate bath (ph 9) was prepared by dissolving appropriate weight of AR grade MoO 3 powder in a few cubic centimetre of warm ammonia (NH 3 ) and then diluted to desired volume with double distilled water (H 2 O). The fluorine doped tin oxide coated conducting glass was used as a substrate for electrodeposition of MoO 2 thin films. These conducting uniform glasses having a sheet resistance of 10 20/V/& and about 95% transparency were prepared using conventional spray deposition technique. Just before the electrodeposition, the FTO glass substrates were dipped for a few seconds into sulphochromic acid and rinsed with double distilled water. This treatment increases the adhesion of the electrodeposit to the substrate and allows the thicker deposition without peeling. Thin films, under galvanostatic mode, were deposited for different deposition times at 1 ma cm 2 current density. Thermogravimetric/differential thermogravimetric analysis (TGA/DTA) analysis of the scratched film was carried on TA (USA) STD-2960 (simultaneous DSC-TGA) instrument. X-ray diffraction (XRD) was carried on Philips PW-3710 diffractometer with Cu ka radiation. Scanning electron microscopy (SEM) and EDAX analyses were made on Cambridge Stereo Scan-250-MK3 and JEOL-JSM 6360 models. Infrared spectrographs were obtained with Perkin-Elmer IR spectrophotometer model-783 in the range cm 1. Optical absorption was carried out in the range run with Hitachi-330 UV vis-nir spectrophotometer. Film thickness was estimated using weight difference method. The estimated values were corrected for density of the film (0.8r). 3. Results and discussion Though the electrodeposition of elemental Mo is difficult, cathodic reduction of the molybdate (MoO 4 2 ) ions in an aqueous alkaline bath is quite easy. The resulting films are X-ray Fig. 1. Film thickness (without density correction) variation with deposition time. amorphous and hydrous with composition MoO 2 n (OH) 2n. The degree of hydroxylation and hydration depends upon growth conditions [14]. As-deposited thin films under optimized conditions were well adherent to the substrate and transparent brown in colour. The film thickness increases with time of deposition, attains a maximum value and then decreases as shown in Fig. 1. During first 30 s of deposition time, the mass of electrodeposit increases as per the Faraday s law. When the deposition time is increased beyond 30 s, the rate of dissolution dominates the rate of deposition and hence the thickness decreases. The transparency decreased with increase in thickness. The reaction mechanism proposed for the formation of hydrous MoO 2 thin film is as follows: MoO 3 þ 2NH 3 þ H 2 O )ðnh 4 Þ 2 MoO 4 (1) ðnh 4 Þ 2 MoO 4 ) 2NH 4 þ þ MoO 4 2 MoO 4 2 þ 2e þð4 þ 2nÞH þ þ 2nðOHÞ, MoO 2 n ðohþ 2n þð4 þ 2nÞH 2 O (3) Thermogravimetric (TG) and differential thermogravimetric (DT) curves in dry N 2 atmosphere for scratched sample are as shown in Fig. 2. The TG curve indicates weight losses at three different temperatures. The DT curve shows three endothermic peaks at 120, 200 and 260 8C, indicating the presence of weakly (zeolitic) bounded, strongly bonded and structural water species. Exothermic peak at about 360 8C indicate the process of crystallization in the sample. Thus, the TGA/DTA study revealed the formation of hydrous molybdenum oxide thin films. The infrared spectroscopy gives clear evidence on the bonding system in the material. The infrared absorption modes are related to dipole moments of the functional groups in the material. A comparison between IR spectra of crystalline bulk and that of thin film reveals the degree of hydroxylation and hydration in the deposited sample. (2)

3 8052 R.S. Patil et al. / Applied Surface Science 252 (2006) Fig. 2. TGA/DTA curves of the powder scratched from as-deposited sample. The film with maximum thickness (S 25 ) was scratched and mixed with KBr powder. Small pellet was prepared and IR spectrum was recorded in cm 1 range. Fig. 3 shows the IR spectrum for as-deposited sample. The absorption peak at 1600 cm 1 is caused by H O H deformation, while the broad absorption peak at about 3350 cm 1 is caused by O H stretching. A strong absorption peak at 1400 cm 1 is due to vibration of Mo OH bond [15]. Thus, the presence of these three peaks indicates the formation of hydrous thin films with probable composition MoO 2 n (OH) 2n. A weak absorption peak near by 650 cm 1 indicates vibration mode of Mo O Mo bond. These observations are similar to those reported for molybdenum oxide thin films [16] and for crystalline bulk sample [17]. The X-ray diffraction patterns between 2u range from 108 to 1008 of (a) FTO substrate; (b) as-deposited film and (c) film annealed in dry argon at 450 8C for 1 h, are as shown in Fig. 4. The reduction in the observed peak intensities in pattern (b) as compared to that in pattern (a) and absence of any new peak in pattern (b) indicates the X-ray amorphous nature of asdeposited films. Due to annealing in argon, crystallization of amorphous film takes place and the X-ray pattern indicates enhancement in the peak intensities with formation of few new peaks as seen in pattern (c). Thus, the microcrystalline growth initiated along the polycrystalline substrate planes is enhanced as the temperature increases. Similar growth mechanism was observed in case of thin films deposited on stainless steel and titanium substrates. The analysis of diffraction pattern (c) revealed a monoclinic rutile structure of MoO 2. A rutile structure is similar to defect perovskite but with edge sharing MoO 6 octahedra, which are distorted. The XRD data is given in Table 1. The observed d values are in good agreement with standard d values from JCPDS file no for monoclinic MoO 2. The scanning electron micrographs for as-deposited S 5,S 15 and S 25 (suffix denotes time of deposition) samples of increasing thickness are as shown in Fig. 5(a c) at the same magnification. These micrographs reveal relatively smooth and completely covered surface for sample S 5, while the cracks are seen in the samples S 15 and S 25. It is also observed that the size of cracked domains decreases with thickness, which indicates that small thickness films are more adherent to the substrate. The cracking Table 1 XRD data for argon annealed sample S 15 exhibiting monoclinic MoO 2 structure Fig. 3. IR spectrum for as-deposited sample. Serial number 2u (degree) Observed d value (Å) Standard d value (Å) Plane (h kl) (1 0 0) (1 1 0) (0 2 0) (2 0 0) (1 0 2) (3 1 0) (2 0 2) (1 4 0)

4 R.S. Patil et al. / Applied Surface Science 252 (2006) Fig. 4. X-ray diffraction patterns: (a) FTO coated glass; (b) as-deposited film (S 15 ); and (c) argon annealed film (S 15 ). in thin films is attributed to drying shrinkage in case of hydrous films. It is reported that films having thickness greater than 0.2 mm are prone to cracking [18]. Similar results are also observed in cathodically deposited zirconia (ZrO 2 ) thin films [19]. The scanning electron micrographs for S 15 (suffix denotes time of deposition) sample annealed in dry argon at 450 8C for 1 h are as shown in Fig. 6 at increasing magnification. These micrographs reveal a cracked surface with enhanced crystal growth along the border of domain. The crystallites tend to align perpendicular to the substrate (i iii). The observed growth between the cracks (iv) supports the layer-by-layer or Frankvan der Merwe growth mechanism [20]. In this growth mechanism the reducing ionic species (adatoms) are incorporated at the large number of planer sites available on the substrate and then diffuse over the surface towards steps and kinks. The coalescence of these adatoms leads to the first monolayer. The process is repeated and subsequent layers are formed. The absorption spectra of thin films were recorded in the wavelength range run at room temperature. The absorption coefficient estimated using predetermined thickness values was found to be of the order of 10 5 cm 1. In order to know the type of optical transitions in the asdeposited thin films a graph of (ahn) 2 versus hn was plotted. A linear rise near absorption edge indicated direct allowed transitions. Extrapolation of the straight-line portion to the zero absorption (a = 0) gave the value of band gap energy (E g ). Fig. 7 shows these plots for samples S 5,S 15 and S 25 (suffix denotes time of deposition) of increasing thickness, while the values of thickness and corresponding band gap energy for all the samples are shown in Table 2.

5 8054 R.S. Patil et al. / Applied Surface Science 252 (2006) Fig. 5. SEM samples (a) S 5 ; (b) S 15 ; and (c) S 25. Fig. 6. SEM sample S 15. Argon annealed at 450 8C (i) 1500; (ii) 11,000; (iii) 43,000; and (iv) 55,000. Table 2 Thickness and band gap values for all samples Serial number Sample Thickness without correction (nm) Thickness with correction (nm) 1 S S S S S S Band gap energy (ev) The band gap energy first increases and then decreases with increase in film thickness. This is attributed to variation in refractive index (n) of the film due to porosity and disorder in the film structure; this observation is consistent with Moss rule [21] E g ¼ 0:95 n 4 (4) The values of band gap energy for as-deposited and argon annealed samples S 5, S 15 and S 25 (suffix denotes time of

6 R.S. Patil et al. / Applied Surface Science 252 (2006) Fig. 7. (ahn) 2 vs. hn curves samples (a) S 5 ; (b) S 15 ; and (c) S 25. Table 3 Band gap narrowing upon argon annealing Serial number Sample Band gap before annealing (ev) 1 S S S Band gap after argon annealing (ev) Acknowledgements One of the authors (R.S. Patil) wishes to acknowledge UGC, New Delhi, India for the award of teacher fellowship under IXth plan. The authors acknowledge financial support from UGC- DRS (SAP) Programme ( ), sanctioned by UGC, New Delhi, India and DST-FIST Programme. deposition) as shown in Table 3 indicate that the band gap decreases after annealing. This band gap narrowing upon crystallization is attributed to structural ordering and is also reported for thin films prepared by other methods [22]. 4. Conclusions This study reveals the possibility of large area deposition of stable MoO 2 thin films by relatively simple and economical electrodeposition method. The as-deposited films are X-ray amorphous and annealing in dry argon at 450 8C for 1 h results into polycrystalline monoclinic MoO 2. TG/DT analysis and IR spectra confirmed the hydrous nature of the electrodeposit. Post deposition argon annealing treatment produces MoO 2 that may be suitable for anode material in microbatteries, while airannealing treatment can produce MoO 3 suitable for electrochromic applications. Further investigations in this direction are underway in our laboratory. References [1] J.J. Auborn, Y.L. Barberio, J. Electrochem. Soc. 134 (1987) 638. [2] Y. Liang, S. Yang, Z. Yi, X. Lei, J. Sun, Y. Zhou, Mater. Sci. Eng., B 121 (2005) 152. [3] C. Julien, G.A. Nazri, J.P. Guesdon, A. Gorenstein, A. Khelfa, O.M. Hussain, Solid State Ionics 73 (3 4) (1994) 319. [4] C.M. Lampert, Circuits Devices March (1992) 20. [5] M.A. Habib, S.P. Maheswari, J. Appl. Electrochem. 23 (1993) 44. [6] C.G. Granqvist, A. Azens, A. Hjelm, L. Kullman, G.A. Niklasson, D. Ronnow, M.S. Mattsson, M. Veszelei, G. Vaivars, Sol. Energy 63 (1998) 199. [7] C. Trimble, M. devries, J.S. Hale, D.W. Thomson, T.E. Tiwald, J.A. Woolam, Thin Solid Films 26 (1999) 355. [8] S. Morizono, JEE September (1982) 42. [9] K. Matshiro, JEE March (1986) 78. [10] E.A. Ponomarev, M. Neumann-Spallart, G. Hodes, C. Lévy-Clément, Thin Solid Films 280 (1996) 86. [11] J.N. Yao, B.H. Loo, K. Hashimoto, A. Fujishima, J. Electroanal. Chem. 290 (1990) 263. [12] A. Bhattacharaya, C.Y. Lee, F.H. Pollak, D.M. Schleich, J. Non-Cryst. Solids 91 (1987) 235. [13] G. Laperriere, B. Marsan, D. Belanger, Synth. Met. 29 (1989) 201.

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