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Opto-structural, electrical and electrochromic properties of crystalline nickel oxide thin films prepared by spray pyrolysis
Physica B 311 (2002) 366–375
Opto-structural, electrical and electrochromic properties of crystalline nickel oxide thin films prepared by spray pyrolysis S.A. Mahmouda,*, A.A. Akla, H. Kamalb, K. Abdel-Hadya b
a Department of Physics, Faculty of Science, Minia University, Minia, Egypt Department of Physics, Faculty of Science, Ain Shams University, Cairo, Egypt
Received 27 April 2001; received in revised form 12 September 2001
Abstract Polycrystalline nickel oxide films with preferential growth along (1 1 1) plane were deposited onto glass substrates, maintained at 3501C, by the spray pyrolysis technique using nickel chloride as starting solution. The effect of solution concentration on their structural, electrical, and optical properties was studied. Using X-ray diffraction, the structural characteristics have been studied and due to the high degree of preferred orientation, Voigt analysis of single reflection was used to determine the microstructural properties (crystallite size and microstrain). The refractive index n and the extinction coefficient k have been computed from the corrected transmittance and reflectance measurements over the spectral range 300–2400 nm. Analysis of the absorption versus photon energy curves revealed a direct transition with optical band gap, Eg ; of 3.6 eV and indirect transition within the range 3.97–3.75 eV as solution molarity increases from 0.05 to 0.3 M. The electrochromic behaviour of polycrystalline nickel oxide film were investigated by means of cyclic voltametry in 1 M KOH aqueous solution. Cycling showed significant increase in solar optical modulation reaching a value of 0.23 after 150 cycles. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochromic properties; Nickel oxide thin films; Spray pyrolysis
1. Introduction Nickel oxide is an anodic electrochromic material (EC), which colours upon reduction (ion extraction). It exhibits large dynamic range and high electrochromic efficiency [1]. The phenomenon of anodic coloration of nickel oxide allows potential applications of this material as a counter electrode in conjunction with tungsten oxide as working electrode in assembling EC device. This *Corresponding author. Tel.: +20-2-86346891; fax: +20-268342601. E-mail address: [email protected] (S.A. Mahmoud).
has the advantage of increasing the optical density variation of the device, since both electrode colours and bleach simultaneously. Other important applications of nickel oxide include preparation of alkaline batteries (as a cathode material), antiferromagnetic layers and p-type transparent conducting films [2–5]. Due to the wide band gap (3.6–4.0 eV), it has a wide range of applications in opto-electronics as well as the thermal applications. Nickel oxide films are a new optical recording material of Ni-NiO heterogeneous systems that would realize portable medium by using a laser diode beam for recording and reading. The new medium needs no protection because of its
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 0 2 4 - 9
S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
stability against atmosphere corrosion agents, unlike conventional materials like tellurium [6]. The increased interest in this material, due to its widespread applications, has prompted the researchers to associate measurements of different physical properties. Various methods have been used to prepare nickel oxide films, including thermal evaporation [7], RF magnetron sputtering [8], electrochemically deposited films [9] and chemical vapour deposition [10]. Quite recently [2], thin films of nickel oxide have been deposited onto glass substrates from chemical route containing Ni2+ ion and urea. Some properties of the as-deposited and postheated films have been reported [11]. It is known that EC variation is based on absorption modulation. This may cause a rise in device temperature with subsequent possible deterioration in the device performance. This could be overcome by using crystalline EC materials based on reflection modulation. In this present work, polycrystalline films of nickel oxide have been deposited onto glass substrates using spray pyrolysis nickel chloride as starting solution. Influence of solution concentration on the microstructural, electrical and optical properties has been investigated and discussed. Furthermore, the influences of solution molarity on the electrochromic properties of crystalline nickel oxide films have been investigated. Effect of cycling on the optical modulation has been dealt with. 2. Experimental details Nickel oxide thin films were deposited from aqueous solution of nickel chloride (NiCl2 6H2O), by spray pyrolysis technique. A homemade spray system described previously [12] has been used. The layers have been deposited onto two kinds of substrates: Indium-doped Tin Oxide (ITO)-coated glass and microscopic slides. The substrates have been chemically and ultrasonically cleaned. The overall reaction process can be expressed as heat decomposition of nickel chloride to form clusters of nickel oxide in the presence of water as follows: Heat
NiCl2 þ H2 O - 2HCl þ NiO:
ð1Þ
367
With more nickel ions present on the sprayed droplets, greater clusters were formed upon deposition. Different solution concentrations in the range 0.05–0.30 M were used to prepare these films. The flow rate, substrate temperature, deposition time, nozzle to substrate distance are kept unvaried during the deposition process at 15 cm3/min, 3501C, 40 s, and 40 cm, respectively. The film thickness of the prepared samples was measured using multiple-beam Fizeau fringes at reflection using either white light or monochromatic light (Hg, lg ¼ 546 nm). The coloured interference fringes enabled the determination of the order of magnitude of the fringe shift, while the monochromatic fringe shift as a fraction of order separation has been measured using an eyepiece micrometer. To investigate the structure of the films, a JEOL X-ray diffractometer (model JSDX-60PA) using ( was Ni-filtered Cu:Ka radiation (lKa ¼ 1:5418 A) employed. Continuous scanning was applied with a slow scanning speed and a small time constant. A range of 2y from 61 to 601 was scanned, so that all possible diffraction peaks could be detected. The crystallite size as well as the microstrain were determined using single-order Voigt profile analysis [13,14]. The microstrain was calculated as the fractional change, Dd=d0 ; in the interplanar spacing, d; of the (1 1 1) plane, where d0 is that of the standard powder (ICDD card no. 47-1049). On the other hand, resistivity measurements were performed on films using the four-point probe. The spectral transmittance and reflectance of the films were recorded in the wavelength range from 300 to 2400 nm using Shimadzu UV 3101 PC, UVVIS-NIR double-beam spectrophotometer with reflection attachment of V–N type (incident angle 51). A developed computer program [11] based on solving the exact equations was used to calculate the refractive index, n; and the extinction coefficient, k: The prepared nickel oxide films deposited onto ITO substrates were subjected to electrochemical ion insertion/extraction in an electrochemical solution containing 1 M KOH aqueous solution. A three-electrode cell has been used. Besides the working electrodes (nickel oxide films), a platinum sheet has been used as counter
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electrode. The reference electrode was a saturated calomel electrode (SCE). The solar transmittance, Ts ; of the prepared samples, in the coloration and bleached states was obtained by integrating the measured transmittance TðlÞ; weighted by the solar spectrum at air mass2 [15], R 2400 TðlÞ GðlÞ Ts ¼ 300R 2400 ; ð2Þ 300 GðlÞ where GðlÞ is the solar spectrum at AM2.
3. Results and discussion Nickel oxide films were grown on a glass substrate with different molarities of the starting solution. The variation of the thickness of the deposited films, as obtained interferometrically, as a function of the solution molarity is shown in Fig. 1. It is clear that, as the solution concentration increases, the amount of material that participates in forming the deposited film increases with subsequent increase in the film thickness.
Fig. 1. The variation of the nickel oxide film thickness as a function of the solution molarity.
Each data point represents the mean value of five measurements taken at different locations on the film, the calculated error was found to be 76% as represented by bars. 3.1. Structural properties X-ray diffraction (XRD) patterns for layers deposited at glass substrates (Tsub ¼ 3501C) are presented in Fig. 2 as a function of solution molarity (0.1–0.3 M). Films prepared from 0.05 M solution proved to be amorphous. By calculating the interplanar spacing of the lines in the investigated X-ray patterns and matching the results with ICDD card no. (47-1049), it is concluded that the lines in the patterns [1 1 1] and [2 0 0] are of the NiO phase with a cubic structure. The calculated lattice parameters were found to be a ¼ b ¼ c ¼ 0:416 mm. The films show preferred growth along the [1 1 1] direction, as the solution molarity increases, while an enhancement in the preferred orientation along both [1 1 1] and
Fig. 2. X-ray diffraction patterns for nickel oxide films onto glass substrate as a function of solution molarity (Tsub ¼ 3501C).
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369
[2 0 0] plane is observed. The increase in the intensity of the peaks may be attributed to either the grain growth associated with largest thickness, or the increase in the degree of crystallinity by increasing the solution molarity or both. The full width at half maximum (FWHM) was found to decrease markedly with solution molarity. Such a decrease reflects the decrease in the concentration of lattice imperfections due to a decrease in the internal microstrain within the film and/or an increase in the crystallite/domain size (CS). Considering that the size broadening is the sole effect [16], the CS, was determined using Voigt method for single-line profile analysis. The parameters required for determining CS and the magnitude of microstrains are the Lorentzian (bfL ) and Gaussian (bfG ) integral breadths of the sample profile, bf : According to the apparent CS, e is e¼
1 bfL
ð3Þ
Fig. 3. The dependence of the CS of nickel oxide films on the solution molarity.
and the microstrain jej is given by jej ¼
Dd 1 ¼ cot Y Dð2YÞ: d0 2
ð4Þ
The microstrains are equivalent to a variation in dspacing within domains by the amount that depends on the elastic constants of the material and the nature of internal stresses. Fig. 3 shows the variation of the CS calculated from the (1 1 1) reflection as a function of the solution molarity. The CS is by definition measured in the direction normal to the reflection plane, i.e. in the /1 1 1S direction, and consequently, perpendicular to the substrate. Therefore, the observed increase in the CS may be interpreted in terms of a columnar grain growth. 3.2. Electrical properties The electrical dark resistance of the nickel oxide films was measured using the conventional fourpoint probe assuming homogeneous conduction throughout the film depth, while the change of the sheet resistance with the starting solution molarity is shown in Fig. 4. It is clear that the sheet resistance decreases slightly, within the same order
of magnitude, with increasing solution concentration and tends to be constant, acquiring a value of 1.2 108 O/cm2 at higher values of solution concentrations. This is attributed to the variation of film thickness associated with the increase in molarity. Variation of sheet resistance Rs with the reciprocal of the film thickness (1=d) yielded a straight line as shown in Fig. 5. This linearity implies that the solution concentration (film thickness) has no influence on the resistivity, since Rs ¼ r=d; the resistivity is calculated as 1.1 kO cm. In order to estimate the value of the activation energy Ea of crystalline nickel oxide films, the variation of the film resistivity, which deposited at Tsub ¼ 3501C and 0.1 M, was recorded versus the temperature in the range from 300 to 440 K. This range has intrinsic semiconducting transport phenomenon which applies Arrhenius-like law [17,18] between the resistivity r and temperature TðKÞ in which the resistivity dependence on temperature is approximated to r ¼ AeEa =2kT ;
ð5Þ
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Fig. 4. The relation between sheet resistance of nickel oxide films and solution molarity.
Fig. 6. The dependence of lnðrÞ versus the reciprocal absolute temperature.
can be obtained from the slope of the plot between logarithmically scaled resistivity and reciprocal absolute temperature, Fig. 6, was found to be 0.78 eV. This value is in agreement with the value estimated from the work of Pejova et al. [2]. In Fig. 6, some differences between experimental data and straight line fitting in the range of 100–1401C were observed, which is attributed to water losses. Thermogravitometry measurements on nickel oxide films prepared by electrochemical technique showed similar finding characterized by a small mass decrease around 1001C [18]. Also, TGA measurements during heating in N2 or O2 of colloidally precipitated nickel oxide films, indicated a mass decrease around 1001C that attributed to water loss [2]. Fig. 5. The dependence of sheet resistance on the reciprocal of the film thickness for films prepared from different solution molarities.
where A is constant, k is Boltzmann constant, T is the absolute temperature and Ea is the activation energy. The estimation of activation energy, which
3.3. Optical properties Measurements of spectral transmittance and reflectance of the sample showed that the films are transparent with transmittance exceeding 80% all over the spectral range 300–2400 nm as shown in Fig. 7. Films that were prepared from higher
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371
Fig. 7. Transmission and reflection spectra of nickel oxide films for different solution molarities.
Fig. 8. The dependence of a on the wavelength in the ultraviolet range for nickel oxide films prepared at different solution concentrations.
solution concentration showed a relatively lower transmittance. This is attributed to the increase in the film thickness, with subsequent increase in absorption. In the thicker films, the onset of absorption edge became less sharp, this is due to the fact that, as the concentration increases, bigger clusters are deposited; and the scattered radiation became remarkable due to the surface roughness. The absorption coefficient a was obtained through formula (6), where d is the film thickness in nm, RðlÞ and TðlÞ are the reflectance and transmittance at the specified wavelength l: 104 f1 RðlÞg2 aðlÞ ¼ log10 : ð6Þ d TðlÞ
formula [19],
Fig. 8 shows the spectral variation of the absorption coefficient, a; in the range of 300–400 nm, with 5 nm step, at different solution molarities. The results show an absorption band in the UV region. The optical band gaps, Eg ; for the as-deposited films were calculated on the basis of the optical spectral absorption using the well-known
a¼
Aðhn Eg Þm ; hn
ð7Þ
where A is constant, hn is the incident photon energy, and m depends on the nature of band transition; m ¼ 12 or 32 for direct allowed and direct forbidden transitions, and m ¼ 2 or 3 for indirect allowed and indirect forbidden transitions. It was noticed from Fig. 9a that for m ¼ 12; the extrapolation of the linear part gives the value of the indirect energy gap. The values show concentration dependence varying from Egind ¼ 3:97 to 3.75 eV for solution molarity varying from 0.05 to 0.3 M. This may be attributed to the variation of film thickness as well as the improved crystallinity. In addition, the extrapolation of the linear part of ðahnÞ1=2 versus hn plot is shown in Fig. 9b, the direct energy gap is deducted as Egd ¼ 3:6 eV. The values of direct and indirect transition agree with the reported values of band gap [2,20,21]. A comparative study with the same previous finding is given in Table 1.
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372
Fig. 9. The dependence of (a) ðahnÞ2 and (b) ðahnÞ1=2 on the incident photon energy ðhnÞ for samples prepared at different solution molarities.
Table 1 A review of some published data on the optical energy gap and its comparison with the present findings Ref.
Eg (eV)
Transition type
Comment
Preparation technique
[2]
3.6
Direct energy gap
Annealed at 3501C
Solution growth
[3]
3.25 3.75 4.0
Direct energy gap
Annealed at 3801C Annealed at 5501C Bulk
Solution growth
4.0 3.6–4.0 3.15–3.5 2.5–3.7
Indirect Indirect Indirect Indirect
Bulk Different substrate temperature Ts ¼ 30024801C For different distance between nozzle and substrate
3.6 3.97–3.75
Direct energy gap Indirect energy gap
[5] [22] [23]
Present work
energy energy energy energy
gap gap gap gap
Ts ¼ 3501C Thickness dependent
The optical transitions observed by different workers are mostly in agreement with one another as far as the gap width in the range 3.6–4.0 eV is concerned. However, regarding the character of the transition, some authors found it to be direct while others found it to be indirect with similar
Thermal evaporation Thermal evaporation Spray pyrolysis Spray pyrolysis
and unsimilar values. Our results given in Table 1 confirm both the direct and indirect characters of the optical transition. The calculation of the refractive index of the samples was calculated using our developed programme based on solving the exact equation
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373
et al. [26] reported a value of D2.05 at l ¼ 576 nm for NiOx films prepared at low pressure using the reactive e-beam evaporation. 3.4. Electrochromic properties
Fig. 10. The dependence of refractive index, n; on the wavelength for samples prepared from different solution concentrations.
Polycrystalline nickel oxide films were grown by spray pyrolysis onto ITO substrate to investigate the dependence of the electrochromic performance on solution molarity. Typical samples, deposited under the above conditions, were subjected to electrochromic investigations in 1 M KOH aqueous solution. In the first cycle, the variation of solar optical modulation (DTs ¼ Tsb Tsc ) with solution molarity is shown in Fig. 11. With the increase of solution molarity, DTs initially increases up to 0.1 M, then decreases for a higher solution molarity. This can be attributed to the transformation of some of the nano-crystalline nickel oxide molecules to Ni (OH)2 that act as the active species wherein ion insertion transforms the phase from or to one of Ni (III) phases [14,15] according to the Bode reaction scheme [27].
by iteration [24]. The method is basically a bivariate search technique based on minimizing ðDRÞ2 ¼ jRC RX j2 ; ðDTÞ2 ¼ jTC TX j2
ð8Þ
simultaneously, where the suffices C and X referred to calculated and experimental values, respectively. In the present work, Tomlin functions [25], namely ð1 þ RÞ=T and ð1 RÞ=T; were used as two simultaneous variables instead of T and R: The spectral variation of the refractive index, n; of crystalline nickel oxide films deposited at different solution molarities is shown in Fig. 10. The solid line represented the mean value indicating the decrease of n from 2.6 at l ¼ 400 nm to 1.4 at l ¼ 2400 nm. The n values decrease with l showing normal dispersion according to Couchy’s formula. The solid line shows that the averaged values with probable error varies from 3% in the visible region to 6%, as l increases. Such an error is acceptable in experimental determination of n from spectral measurements of R and T: Bange
Fig. 11. The variation of solar optical modulation of polycrystalline nickel oxide films with different solution molarities. At the first cycle.
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Fig. 12. The effect of cyclic voltammetry on the solar modulation of polycrystalline nickel oxide films.
For a subsequent investigation, nickel oxide film has been prepared at the optimum condition of solution molarity, namely at 0.1 M. The spectral transmittance has been measured in the coloration/bleaching states at a potential of 72 V. Fig. 12 shows that the solar optical modulation increases markedly with the number of cycles reaching a value of 0.23 after 150 cycles. In this stage the injected charge during bleaching is equal to that extracted upon coloration reaching stable electrochromic performance. It is to be noted that such aging procedure (cycling) had no effect on the electrochromic characterization of the amorphous NiOx prepared by electrochemical deposition [11]. Fig. 13 shows the XRD of nickel oxide films deposited onto ITO glass substrates in the asdeposited (a), bleached (b), and coloured (c) states after 150 cycles. The variation of the peak characteristics is clear as shown in the enlarged display. It is evident that cycling (as well as ion insertion) increases the amorphousness of the film as it appeared in the increase of the peak width and in the decrease of peak intensity.
Fig. 13. XRD patterns of polycrystalline nickel oxide films onto ITO substrate after 150 cycle, (a) as-deposited film, (b) bleached state and (c) coloured state.
4. Conclusion Polycrystalline nickel oxide films were prepared onto heated glass and ITO substrate kept at 3501C by spray pyrolysis technique. Physical and electrochromic properties of the prepared films at different solution molarities were investigated. The obtained results lead to the following conclusion: 1. The films show a preferred growth along the [1 1 1] direction. As increase the solution molarity, enhance the peak intensity and crystallite size increases. The observed increase in the CS may be interpreted in terms of a columnar grain growth. 2. The sheet resistance decreases slightly with increasing solution molarity and tends to be constant, acquiring a value of 1.2 108 O/cm2 at higher values of solution molarity representing the bulk resistivity.
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3. The values of optical energy gap show varying concentration dependence as indirect transition, from Egind ¼ 3:9723:75 eV for solution molarity 0.05–0.3 M, respectively. The direct energy gap is deducted as Egd ¼ 3:6 eV. 4. The dispersion curves shows normal dispersion following Couchy’s formula. 5. With the increase of solution molarity, solar optical modulation, DTs initially increases up to 0.1 M, then decreases for higher solution molarity. 6. Solar optical modulation increases markedly with number of cycles reaching a value of 0.23 after 150 cycles. 7. Both cycling and ion insertion increases the amorphousness of the polycrystalline nickel oxide films as revealed by XRD.
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, New York, 1995. [2] B. Pejova, T. Kocareva, M. Najdoski, I. Grozdanov, Appl. Surf. Sci. 165 (2000) 271. [3] A.J. Varkey, A.F. Fort, Thin Solid Films 235 (1993) 47. [4] E. Jujii, A. Tomozawa, H. Torii, R. Takayama, Jpn. J. Appl. Phys. 35 (1996) L328. [5] H. Sato, T. Minami, S. Takata, T. Jamada, Thin Solid Films 236 (1993) 27. [6] A. Had, R. Nishikawa, Jpn. J. App. Phys. 33 (1994) 3952. [7] P. Lunkenheimer, A. Loidl, C.R. Ottermann, K. Bage, Phys. Rev. B 44 (1991) 59278.
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[8] Y. Ushio, A. Ishikawa, T. Niwa, Thin Solid Films 280 (1996) 233. [9] S.A. Aly, S.A. Mahmoud, M. Abdel-Rahman, K. AbdelHady, Egypt, J. Sol. 21 (1998) 59. [10] T. Maruyama, S. Arai, Solar Energy Mater. Solar Cells 30 (1993) 257. [11] S.A. Mahmoud, S.A. Aly, M. Abdel-Rahman, K. AbdelHady, Physica B 293 (2000) 125. [12] S.A. Mahmoud, Egypt. J. Sol. 19 (1996) 221. [13] J.I. Langford, J. Appl. Cryst. 11 (1978) 10. [14] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd Edition, Wiley, New York, 1974. [15] C.G. Ribbing, Introduction to Solar Selective Coating. International School on Material Science and Solar Energy, Cairo & Alexandria, March 1983. [16] T.S. Hutchison, D.C. Baird, The Physics of Engineering Solids, Wiley, New York, 1968. [17] J.P. Bromberg, Physical Chemistry, Allyn, Boston, 1984. [18] M.C.A. Fantini, G.H. Bezerra, C.R.C. Carvalho, A. Gorenstein, Proc. Soc. Photo-Opt. Instrum. Eng. 81 (1991) 1536. [19] S.I. Cordoba-Torresi, A. Hugot-Le Goff, S. Joiret, J. Elactrochem. Soc. 138 (1991) 1554. [20] I. Hamberg, C.G. granqvist, J. Appl. Phys. 60 (1980) R123. [21] Z.M. Jarzebski, Oxide Semiconductors, Pergamon Press, Poland, 1973. [22] W. Paatsch, J. Phys. C 5 (1977) 151. [23] P. Puspharajah, S. Radhakrishna, J. Mater. Sci. 32 (1997) 3001. [24] N. El-Kadry, M.F. Ahmed, K. Abdel Hady, Thin Solid Films 274 (1996) 120. [25] S.G. Tomlin, J. Phys. D 1 (1968) 1667. [26] K. Bange, F.G.K. Bancke, B. Metz, Proc. Soc. Photo-Opt. Insturm. Eng. 1016 (1988) 170. [27] H. Bode, K. Dehmelt, J. Witte, Z. Anorg, Allg. Chem. 366 (1969) 1.
Opto-structural, electrical and electrochromic properties of crystalline nickel oxide thin films prepared by spray pyrolysis S.A. Mahmouda,*, A.A. Akla, H. Kamalb, K. Abdel-Hadya b
a Department of Physics, Faculty of Science, Minia University, Minia, Egypt Department of Physics, Faculty of Science, Ain Shams University, Cairo, Egypt
Received 27 April 2001; received in revised form 12 September 2001
Abstract Polycrystalline nickel oxide films with preferential growth along (1 1 1) plane were deposited onto glass substrates, maintained at 3501C, by the spray pyrolysis technique using nickel chloride as starting solution. The effect of solution concentration on their structural, electrical, and optical properties was studied. Using X-ray diffraction, the structural characteristics have been studied and due to the high degree of preferred orientation, Voigt analysis of single reflection was used to determine the microstructural properties (crystallite size and microstrain). The refractive index n and the extinction coefficient k have been computed from the corrected transmittance and reflectance measurements over the spectral range 300–2400 nm. Analysis of the absorption versus photon energy curves revealed a direct transition with optical band gap, Eg ; of 3.6 eV and indirect transition within the range 3.97–3.75 eV as solution molarity increases from 0.05 to 0.3 M. The electrochromic behaviour of polycrystalline nickel oxide film were investigated by means of cyclic voltametry in 1 M KOH aqueous solution. Cycling showed significant increase in solar optical modulation reaching a value of 0.23 after 150 cycles. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochromic properties; Nickel oxide thin films; Spray pyrolysis
1. Introduction Nickel oxide is an anodic electrochromic material (EC), which colours upon reduction (ion extraction). It exhibits large dynamic range and high electrochromic efficiency [1]. The phenomenon of anodic coloration of nickel oxide allows potential applications of this material as a counter electrode in conjunction with tungsten oxide as working electrode in assembling EC device. This *Corresponding author. Tel.: +20-2-86346891; fax: +20-268342601. E-mail address: [email protected] (S.A. Mahmoud).
has the advantage of increasing the optical density variation of the device, since both electrode colours and bleach simultaneously. Other important applications of nickel oxide include preparation of alkaline batteries (as a cathode material), antiferromagnetic layers and p-type transparent conducting films [2–5]. Due to the wide band gap (3.6–4.0 eV), it has a wide range of applications in opto-electronics as well as the thermal applications. Nickel oxide films are a new optical recording material of Ni-NiO heterogeneous systems that would realize portable medium by using a laser diode beam for recording and reading. The new medium needs no protection because of its
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 0 2 4 - 9
S.A. Mahmoud et al. / Physica B 311 (2002) 366–375
stability against atmosphere corrosion agents, unlike conventional materials like tellurium [6]. The increased interest in this material, due to its widespread applications, has prompted the researchers to associate measurements of different physical properties. Various methods have been used to prepare nickel oxide films, including thermal evaporation [7], RF magnetron sputtering [8], electrochemically deposited films [9] and chemical vapour deposition [10]. Quite recently [2], thin films of nickel oxide have been deposited onto glass substrates from chemical route containing Ni2+ ion and urea. Some properties of the as-deposited and postheated films have been reported [11]. It is known that EC variation is based on absorption modulation. This may cause a rise in device temperature with subsequent possible deterioration in the device performance. This could be overcome by using crystalline EC materials based on reflection modulation. In this present work, polycrystalline films of nickel oxide have been deposited onto glass substrates using spray pyrolysis nickel chloride as starting solution. Influence of solution concentration on the microstructural, electrical and optical properties has been investigated and discussed. Furthermore, the influences of solution molarity on the electrochromic properties of crystalline nickel oxide films have been investigated. Effect of cycling on the optical modulation has been dealt with. 2. Experimental details Nickel oxide thin films were deposited from aqueous solution of nickel chloride (NiCl2 6H2O), by spray pyrolysis technique. A homemade spray system described previously [12] has been used. The layers have been deposited onto two kinds of substrates: Indium-doped Tin Oxide (ITO)-coated glass and microscopic slides. The substrates have been chemically and ultrasonically cleaned. The overall reaction process can be expressed as heat decomposition of nickel chloride to form clusters of nickel oxide in the presence of water as follows: Heat
NiCl2 þ H2 O - 2HCl þ NiO:
ð1Þ
367
With more nickel ions present on the sprayed droplets, greater clusters were formed upon deposition. Different solution concentrations in the range 0.05–0.30 M were used to prepare these films. The flow rate, substrate temperature, deposition time, nozzle to substrate distance are kept unvaried during the deposition process at 15 cm3/min, 3501C, 40 s, and 40 cm, respectively. The film thickness of the prepared samples was measured using multiple-beam Fizeau fringes at reflection using either white light or monochromatic light (Hg, lg ¼ 546 nm). The coloured interference fringes enabled the determination of the order of magnitude of the fringe shift, while the monochromatic fringe shift as a fraction of order separation has been measured using an eyepiece micrometer. To investigate the structure of the films, a JEOL X-ray diffractometer (model JSDX-60PA) using ( was Ni-filtered Cu:Ka radiation (lKa ¼ 1:5418 A) employed. Continuous scanning was applied with a slow scanning speed and a small time constant. A range of 2y from 61 to 601 was scanned, so that all possible diffraction peaks could be detected. The crystallite size as well as the microstrain were determined using single-order Voigt profile analysis [13,14]. The microstrain was calculated as the fractional change, Dd=d0 ; in the interplanar spacing, d; of the (1 1 1) plane, where d0 is that of the standard powder (ICDD card no. 47-1049). On the other hand, resistivity measurements were performed on films using the four-point probe. The spectral transmittance and reflectance of the films were recorded in the wavelength range from 300 to 2400 nm using Shimadzu UV 3101 PC, UVVIS-NIR double-beam spectrophotometer with reflection attachment of V–N type (incident angle 51). A developed computer program [11] based on solving the exact equations was used to calculate the refractive index, n; and the extinction coefficient, k: The prepared nickel oxide films deposited onto ITO substrates were subjected to electrochemical ion insertion/extraction in an electrochemical solution containing 1 M KOH aqueous solution. A three-electrode cell has been used. Besides the working electrodes (nickel oxide films), a platinum sheet has been used as counter
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electrode. The reference electrode was a saturated calomel electrode (SCE). The solar transmittance, Ts ; of the prepared samples, in the coloration and bleached states was obtained by integrating the measured transmittance TðlÞ; weighted by the solar spectrum at air mass2 [15], R 2400 TðlÞ GðlÞ Ts ¼ 300R 2400 ; ð2Þ 300 GðlÞ where GðlÞ is the solar spectrum at AM2.
3. Results and discussion Nickel oxide films were grown on a glass substrate with different molarities of the starting solution. The variation of the thickness of the deposited films, as obtained interferometrically, as a function of the solution molarity is shown in Fig. 1. It is clear that, as the solution concentration increases, the amount of material that participates in forming the deposited film increases with subsequent increase in the film thickness.
Fig. 1. The variation of the nickel oxide film thickness as a function of the solution molarity.
Each data point represents the mean value of five measurements taken at different locations on the film, the calculated error was found to be 76% as represented by bars. 3.1. Structural properties X-ray diffraction (XRD) patterns for layers deposited at glass substrates (Tsub ¼ 3501C) are presented in Fig. 2 as a function of solution molarity (0.1–0.3 M). Films prepared from 0.05 M solution proved to be amorphous. By calculating the interplanar spacing of the lines in the investigated X-ray patterns and matching the results with ICDD card no. (47-1049), it is concluded that the lines in the patterns [1 1 1] and [2 0 0] are of the NiO phase with a cubic structure. The calculated lattice parameters were found to be a ¼ b ¼ c ¼ 0:416 mm. The films show preferred growth along the [1 1 1] direction, as the solution molarity increases, while an enhancement in the preferred orientation along both [1 1 1] and
Fig. 2. X-ray diffraction patterns for nickel oxide films onto glass substrate as a function of solution molarity (Tsub ¼ 3501C).
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[2 0 0] plane is observed. The increase in the intensity of the peaks may be attributed to either the grain growth associated with largest thickness, or the increase in the degree of crystallinity by increasing the solution molarity or both. The full width at half maximum (FWHM) was found to decrease markedly with solution molarity. Such a decrease reflects the decrease in the concentration of lattice imperfections due to a decrease in the internal microstrain within the film and/or an increase in the crystallite/domain size (CS). Considering that the size broadening is the sole effect [16], the CS, was determined using Voigt method for single-line profile analysis. The parameters required for determining CS and the magnitude of microstrains are the Lorentzian (bfL ) and Gaussian (bfG ) integral breadths of the sample profile, bf : According to the apparent CS, e is e¼
1 bfL
ð3Þ
Fig. 3. The dependence of the CS of nickel oxide films on the solution molarity.
and the microstrain jej is given by jej ¼
Dd 1 ¼ cot Y Dð2YÞ: d0 2
ð4Þ
The microstrains are equivalent to a variation in dspacing within domains by the amount that depends on the elastic constants of the material and the nature of internal stresses. Fig. 3 shows the variation of the CS calculated from the (1 1 1) reflection as a function of the solution molarity. The CS is by definition measured in the direction normal to the reflection plane, i.e. in the /1 1 1S direction, and consequently, perpendicular to the substrate. Therefore, the observed increase in the CS may be interpreted in terms of a columnar grain growth. 3.2. Electrical properties The electrical dark resistance of the nickel oxide films was measured using the conventional fourpoint probe assuming homogeneous conduction throughout the film depth, while the change of the sheet resistance with the starting solution molarity is shown in Fig. 4. It is clear that the sheet resistance decreases slightly, within the same order
of magnitude, with increasing solution concentration and tends to be constant, acquiring a value of 1.2 108 O/cm2 at higher values of solution concentrations. This is attributed to the variation of film thickness associated with the increase in molarity. Variation of sheet resistance Rs with the reciprocal of the film thickness (1=d) yielded a straight line as shown in Fig. 5. This linearity implies that the solution concentration (film thickness) has no influence on the resistivity, since Rs ¼ r=d; the resistivity is calculated as 1.1 kO cm. In order to estimate the value of the activation energy Ea of crystalline nickel oxide films, the variation of the film resistivity, which deposited at Tsub ¼ 3501C and 0.1 M, was recorded versus the temperature in the range from 300 to 440 K. This range has intrinsic semiconducting transport phenomenon which applies Arrhenius-like law [17,18] between the resistivity r and temperature TðKÞ in which the resistivity dependence on temperature is approximated to r ¼ AeEa =2kT ;
ð5Þ
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Fig. 4. The relation between sheet resistance of nickel oxide films and solution molarity.
Fig. 6. The dependence of lnðrÞ versus the reciprocal absolute temperature.
can be obtained from the slope of the plot between logarithmically scaled resistivity and reciprocal absolute temperature, Fig. 6, was found to be 0.78 eV. This value is in agreement with the value estimated from the work of Pejova et al. [2]. In Fig. 6, some differences between experimental data and straight line fitting in the range of 100–1401C were observed, which is attributed to water losses. Thermogravitometry measurements on nickel oxide films prepared by electrochemical technique showed similar finding characterized by a small mass decrease around 1001C [18]. Also, TGA measurements during heating in N2 or O2 of colloidally precipitated nickel oxide films, indicated a mass decrease around 1001C that attributed to water loss [2]. Fig. 5. The dependence of sheet resistance on the reciprocal of the film thickness for films prepared from different solution molarities.
where A is constant, k is Boltzmann constant, T is the absolute temperature and Ea is the activation energy. The estimation of activation energy, which
3.3. Optical properties Measurements of spectral transmittance and reflectance of the sample showed that the films are transparent with transmittance exceeding 80% all over the spectral range 300–2400 nm as shown in Fig. 7. Films that were prepared from higher
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Fig. 7. Transmission and reflection spectra of nickel oxide films for different solution molarities.
Fig. 8. The dependence of a on the wavelength in the ultraviolet range for nickel oxide films prepared at different solution concentrations.
solution concentration showed a relatively lower transmittance. This is attributed to the increase in the film thickness, with subsequent increase in absorption. In the thicker films, the onset of absorption edge became less sharp, this is due to the fact that, as the concentration increases, bigger clusters are deposited; and the scattered radiation became remarkable due to the surface roughness. The absorption coefficient a was obtained through formula (6), where d is the film thickness in nm, RðlÞ and TðlÞ are the reflectance and transmittance at the specified wavelength l: 104 f1 RðlÞg2 aðlÞ ¼ log10 : ð6Þ d TðlÞ
formula [19],
Fig. 8 shows the spectral variation of the absorption coefficient, a; in the range of 300–400 nm, with 5 nm step, at different solution molarities. The results show an absorption band in the UV region. The optical band gaps, Eg ; for the as-deposited films were calculated on the basis of the optical spectral absorption using the well-known
a¼
Aðhn Eg Þm ; hn
ð7Þ
where A is constant, hn is the incident photon energy, and m depends on the nature of band transition; m ¼ 12 or 32 for direct allowed and direct forbidden transitions, and m ¼ 2 or 3 for indirect allowed and indirect forbidden transitions. It was noticed from Fig. 9a that for m ¼ 12; the extrapolation of the linear part gives the value of the indirect energy gap. The values show concentration dependence varying from Egind ¼ 3:97 to 3.75 eV for solution molarity varying from 0.05 to 0.3 M. This may be attributed to the variation of film thickness as well as the improved crystallinity. In addition, the extrapolation of the linear part of ðahnÞ1=2 versus hn plot is shown in Fig. 9b, the direct energy gap is deducted as Egd ¼ 3:6 eV. The values of direct and indirect transition agree with the reported values of band gap [2,20,21]. A comparative study with the same previous finding is given in Table 1.
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Fig. 9. The dependence of (a) ðahnÞ2 and (b) ðahnÞ1=2 on the incident photon energy ðhnÞ for samples prepared at different solution molarities.
Table 1 A review of some published data on the optical energy gap and its comparison with the present findings Ref.
Eg (eV)
Transition type
Comment
Preparation technique
[2]
3.6
Direct energy gap
Annealed at 3501C
Solution growth
[3]
3.25 3.75 4.0
Direct energy gap
Annealed at 3801C Annealed at 5501C Bulk
Solution growth
4.0 3.6–4.0 3.15–3.5 2.5–3.7
Indirect Indirect Indirect Indirect
Bulk Different substrate temperature Ts ¼ 30024801C For different distance between nozzle and substrate
3.6 3.97–3.75
Direct energy gap Indirect energy gap
[5] [22] [23]
Present work
energy energy energy energy
gap gap gap gap
Ts ¼ 3501C Thickness dependent
The optical transitions observed by different workers are mostly in agreement with one another as far as the gap width in the range 3.6–4.0 eV is concerned. However, regarding the character of the transition, some authors found it to be direct while others found it to be indirect with similar
Thermal evaporation Thermal evaporation Spray pyrolysis Spray pyrolysis
and unsimilar values. Our results given in Table 1 confirm both the direct and indirect characters of the optical transition. The calculation of the refractive index of the samples was calculated using our developed programme based on solving the exact equation
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et al. [26] reported a value of D2.05 at l ¼ 576 nm for NiOx films prepared at low pressure using the reactive e-beam evaporation. 3.4. Electrochromic properties
Fig. 10. The dependence of refractive index, n; on the wavelength for samples prepared from different solution concentrations.
Polycrystalline nickel oxide films were grown by spray pyrolysis onto ITO substrate to investigate the dependence of the electrochromic performance on solution molarity. Typical samples, deposited under the above conditions, were subjected to electrochromic investigations in 1 M KOH aqueous solution. In the first cycle, the variation of solar optical modulation (DTs ¼ Tsb Tsc ) with solution molarity is shown in Fig. 11. With the increase of solution molarity, DTs initially increases up to 0.1 M, then decreases for a higher solution molarity. This can be attributed to the transformation of some of the nano-crystalline nickel oxide molecules to Ni (OH)2 that act as the active species wherein ion insertion transforms the phase from or to one of Ni (III) phases [14,15] according to the Bode reaction scheme [27].
by iteration [24]. The method is basically a bivariate search technique based on minimizing ðDRÞ2 ¼ jRC RX j2 ; ðDTÞ2 ¼ jTC TX j2
ð8Þ
simultaneously, where the suffices C and X referred to calculated and experimental values, respectively. In the present work, Tomlin functions [25], namely ð1 þ RÞ=T and ð1 RÞ=T; were used as two simultaneous variables instead of T and R: The spectral variation of the refractive index, n; of crystalline nickel oxide films deposited at different solution molarities is shown in Fig. 10. The solid line represented the mean value indicating the decrease of n from 2.6 at l ¼ 400 nm to 1.4 at l ¼ 2400 nm. The n values decrease with l showing normal dispersion according to Couchy’s formula. The solid line shows that the averaged values with probable error varies from 3% in the visible region to 6%, as l increases. Such an error is acceptable in experimental determination of n from spectral measurements of R and T: Bange
Fig. 11. The variation of solar optical modulation of polycrystalline nickel oxide films with different solution molarities. At the first cycle.
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Fig. 12. The effect of cyclic voltammetry on the solar modulation of polycrystalline nickel oxide films.
For a subsequent investigation, nickel oxide film has been prepared at the optimum condition of solution molarity, namely at 0.1 M. The spectral transmittance has been measured in the coloration/bleaching states at a potential of 72 V. Fig. 12 shows that the solar optical modulation increases markedly with the number of cycles reaching a value of 0.23 after 150 cycles. In this stage the injected charge during bleaching is equal to that extracted upon coloration reaching stable electrochromic performance. It is to be noted that such aging procedure (cycling) had no effect on the electrochromic characterization of the amorphous NiOx prepared by electrochemical deposition [11]. Fig. 13 shows the XRD of nickel oxide films deposited onto ITO glass substrates in the asdeposited (a), bleached (b), and coloured (c) states after 150 cycles. The variation of the peak characteristics is clear as shown in the enlarged display. It is evident that cycling (as well as ion insertion) increases the amorphousness of the film as it appeared in the increase of the peak width and in the decrease of peak intensity.
Fig. 13. XRD patterns of polycrystalline nickel oxide films onto ITO substrate after 150 cycle, (a) as-deposited film, (b) bleached state and (c) coloured state.
4. Conclusion Polycrystalline nickel oxide films were prepared onto heated glass and ITO substrate kept at 3501C by spray pyrolysis technique. Physical and electrochromic properties of the prepared films at different solution molarities were investigated. The obtained results lead to the following conclusion: 1. The films show a preferred growth along the [1 1 1] direction. As increase the solution molarity, enhance the peak intensity and crystallite size increases. The observed increase in the CS may be interpreted in terms of a columnar grain growth. 2. The sheet resistance decreases slightly with increasing solution molarity and tends to be constant, acquiring a value of 1.2 108 O/cm2 at higher values of solution molarity representing the bulk resistivity.
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3. The values of optical energy gap show varying concentration dependence as indirect transition, from Egind ¼ 3:9723:75 eV for solution molarity 0.05–0.3 M, respectively. The direct energy gap is deducted as Egd ¼ 3:6 eV. 4. The dispersion curves shows normal dispersion following Couchy’s formula. 5. With the increase of solution molarity, solar optical modulation, DTs initially increases up to 0.1 M, then decreases for higher solution molarity. 6. Solar optical modulation increases markedly with number of cycles reaching a value of 0.23 after 150 cycles. 7. Both cycling and ion insertion increases the amorphousness of the polycrystalline nickel oxide films as revealed by XRD.
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