(NiO) thin films - pspatil.com

Applied Surface Science 199 (2002) 211–221

Preparation and characterization of spray pyrolyzed nickel oxide (NiO) thin films P.S. Patil*, L.D. Kadam Department of Physics, Thin Film Physics Laboratory, Shivaji University, Kolhapur 416 004, India Received 3 April 2002; accepted 18 June 2002

Abstract A simple and inexpensive spray pyrolysis technique (SPT) was employed to deposit nickel oxide (NiO) thin films from hydrated nickel chloride salt solution on to glass substrates. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques were used to study the thermal characteristics of the precursor salt. The effect of the volume of sprayed solution on structural, optical and electrical properties was studied using X-ray diffraction (XRD), infrared (IR), optical absorption, electrical resistivity and thermoelectric power (TEP) techniques. It is found that increase in the volume of sprayed solution leads to the increment in film thickness and amelioration of crystallinity of the film, consequently the band-gap energy wanes from 3.58 to 3.4 eV. It also affects resistivity and TEP of the film. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Spray pyrolysis; Nickel oxide; Thin films; Characterization

1. Introduction Nickel oxide (NiO) is the most exhaustively investigated transition metal oxide. It is a NaCl-type antiferromagnetic oxide semiconductor. It offers promising candidature for many applications such as solar thermal absorber [1], catalyst for O2 evolution [2], photoelectrolysis [3] and electrochromic device [4]. Nickel oxide is also a well-studied material as the positive electrode in batteries [5]. Pure stoichiometric NiO crystals are perfect insulators [6]. Several efforts have been made to explain the insulating behavior of NiO. Appreciable conductivity can be achieved in NiO by creating Ni vacancies *

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or substituting Li for Ni at Ni sites [6]. Most attracting features of NiO are: (1) excellent durability and electrochemical stability, (2) low material cost, (3) promising ion storage material in terms of cyclic stability, (4) large span optical density, and (5) possibility of manufacturing by variety of techniques. Nickel oxide thin films have been prepared by various techniques that involve: vacuum evaporation [7], electron beam evaporation [8], rf-magnetron sputtering [9,10], anodic oxidation [11], chemical deposition [12–14], atomic layer epitaxy [15], sol–gel [16] and spray pyrolysis technique (SPT) [17–19]. Although SPT has been employed in the past to deposit NiO films through acetylacetonate [17] and nitrate [18] routes, their characterization have sparsely been carried out. Aqueous solutions are commonly used in SP system to deposit thin films due to ease of handling, safety, low cost and availability of a wide

0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 3 9 - 5

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P.S. Patil, L.D. Kadam / Applied Surface Science 199 (2002) 211–221

range of water-soluble metal salts. The solute must have high solubility to increase the yield of the process. Metal chlorides have highest water solubility relative to other metal salts and are used for the industrial production of several oxides and ferrites. The corrosive nature of the product gases and the adverse effect of residual chlorine on properties decrease the general attractiveness of these salts but the technology for handling such systems is available. Other metal salts such as nitrates, acetates and sulfates can also introduce impurities, which may adversely affect subsequent processing or properties and phase development. The low solubility of metal acetates and high decomposition temperature of metal sulfates limit the use of these salts. With the previously mentioned effect, the present paper focuses on use of aqueous solution of nickel chloride to deposit NiO thin films on glass substrates by spray pyrolysis technique, and their structural, optical and electrical properties studies. Thin films of variable thickness were grown by changing the volume of sprayed solution. Thickness of the thin film plays an important role in determining the film properties unlike a bulk material and almost all film properties are thickness-dependent. Adler et al. studied thickness-dependent magnetic properties of NiO epitaxial film grown on MgO (1 0 0). The Neel temperature strongly reduced from the bulk value even for the 20 monolayer films [20]. Reproducible properties are achieved only when the film thickness and the deposition parameters are kept constant. In many applications, particularly in the case of optical devices such as interference filters, antireflection coating, etc., the success of fabrication depends only on the deposition of specific thickness. In other cases even though a specific thickness may not be strictly necessary, a good control of it will still be imperative. Consequently, this paper focuses on the effect of volume of sprayed solution on NiO film thickness and its effect on structural, electrical and optical properties.

2. Experimental procedure Nickel oxide thin films were prepared by spraying a 0.05 M solution of nickel chloride in doubly distilled water onto the pre-heated amorphous glass substrates

kept at 350 8C. The spray rate was kept constant at 8 ml/min. Films of various thickness were obtained by spraying 30, 45, 60 and 75 ml, 0.05 M solution of nickel chloride and the films were denoted as samples N30, N45, N60 and N75, where subscripts denote the volume of the solution sprayed. Film thickness was measured by using weight difference method considering the density of the bulk nickel oxide. As the density of thin films was certainly lower than the bulk density, the actual film thickness would be larger than the estimated values. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the nickel chloride ((NiCl26H2O), AR grade, purity 98%) was carried out using TA instruments (USA) SDT 2960 (simultaneous DCS–TGA). The structural, optical and electrical characterization of the films deposited at optimized preparative parameters was carried out. The X-ray diffractometer (Philips PW 1710) with Cu Ka radiation was used for structural studies in the range of 2y ¼ 10–1008. The infrared (IR) spectrum of as-deposited films was recorded using a Perkin-Elmer IR spectrophotometer model 783 in the spectral range 400–4000 cm1. The pellets were prepared by mixing KBr with nickel oxide powder collected by scratching of the film from glass substrate in the ratio 300:1 and then pressing the powder between two pieces of polished steel. To determine band-gap energy of the films, optical absorption study was carried out in the wavelength range 300–850 nm, using a Hitachi 330 spectrophotometer. A two-probe resistivity unit was used to measure the electrical resistivity of the films in the temperature range 300–550 K. The thermoelectromotive force (thermo-emf) was measured as a function of temperature in the range of 300–475 K.

3. Results and discussions 3.1. Thermal decomposition of NiCl26H2O The decomposition behavior of the precursor for NiO films (NiCl26H2O) was studied by TGA and DTA at a scan rate of 10 8C/min, in the temperature range 0–1000 8C. Fig. 1A and B shows the TGA and DTA analysis of NiCl26H2O, respectively, in air atmosphere. It is clearly depicted that the loss of water takes place in four endothermic peaks at 66, 132.5,


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chloride into nickel oxide. This continues up to 590 8C. The calculation of the amount of residue (31.5%) clearly indicates the formation of NiO phase, which remains stable up to about 1000 8C. Therefore, in the study, the decomposition temperature for NiO film was selected such that it is greater than 280 8C. Reports on decomposition of nickel acetate, nickel nitrate and nickel sulfate were given by others [21] and stated that decomposition characteristics depend on initial ingredient. The onset of formation of NiO also depends on other parameters such as particle size, total mass taken for the characterization, temperature scan rate and ambient atmosphere. 3.2. Formation of nickel oxide thin films Nickel chloride solution was sprayed onto the preheated glass substrates, which undergoes evaporation, solute precipitation and pyrolytic decomposition, thereby resulting in the formation of nickel oxide thin films according to the following reaction: 350 C

NiCl2 6H2 O ) NiO þ 2HCl " þ 5H2 O "

Fig. 1. (A) Thermogravimetric analysis of NiCl26H2O; (B) differential thermal analysis of NiCl26H2O.

195.5 and 242 8C. The weight loss at 66 8C is due to expulsion of physisorbed water, whereas above 100 8C the weight loss is mainly due to removal of water of crystallization. The total weight loss corresponding to removal of water is calculated to be about 46%. The regular weight loss starts at about 280 8C, which is due to the slow decomposition of the sample and can be related to the transformation of nickel

(1)

The as-prepared films were gray in color, uniform and strongly adherent to the substrates. The thickness of the films was varied from 0.028 to 0.26 mm. The values of the film thickness are given in Table 1. It is observed that the thickness of the film increases with volume of the sprayed solution. However, this increase in thickness is not exactly proportional to the volume of sprayed solution. This may be due to the variation in deposition efficiency, which may have resulted from the diminished mass transport to the substrate and due to gas convection, pushing the droplets of the precursor away. The nickel oxide films thus formed were further characterized by X-ray diffraction (XRD), IR spectroscopy,

Table 1 Effects of volume of sprayed solution on film thickness and properties of NiO thin films prepared by spray pyrolysis technique Serial no.

Volume of sprayed solution (ml)

Thickness (mm)

Grain size (nm)

Band-gap energy Eg (eV)

Electrical resistivity at 300 K (104 O cm)

Activation energy Ea (eV)

Thermoelectric power (mV/K)

1 2 3 4

30 45 60 75

0.028 0.048 0.10 0.23

14 14.5 15 17

3.58 3.55 3.49 3.40

1.0 1.9 3.0 9.0

0.35 0.36 0.38 0.39

101 89 85 76

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optical absorption, electrical resistivity and TEP measurement techniques. 3.3. XRD studies The nickel oxide thin films were analyzed by X-ray diffraction technique to study structural identification and changes in the crystallinity. The XRD patterns of nickel oxide thin films of different thickness are shown in Fig. 2. It is found that all the samples were polycrystalline consisting of NiO cubic phase, comprising a strong reflection along (1 1 1) plane and a

weak reflection along (2 2 0) plane. The ‘d’ values of the XRD reflection were compared with the standard ‘d’ values taken from the ASTM diffraction data file (no. 4-0835). Good agreement between the observed and standard ‘d’ values confirms that the material deposited is NiO (cubic). The magnitude of major XRD peak corresponding to (1 1 1) plane and value of area under the peak slightly ameliorates with increase in film thickness. No other additional peaks corresponding to other phases have emerged. This suggests that NiO phase is stable and its formation is independent of film thickness. It is also seen that the positions of (1 1 1) diffraction peaks change among samples by about 0.278. This may be due to the change in internal stress of the films of various thickness during film growth. The grain size of the crystallites (mean crystallite diameter) was calculated for major reflex (1 1 1) using the well-known Scherrer’s formula (Eq. (2)), by assuming the factors, viz. instrumental broadening, distortion of lattice, etc. are common among all the samples: D¼

0:9l b cos y

(2)

where l is wavelength (1:5406 1010 m), b the full width in radian at half maximum of the peak, and y is Bragg’s angle of the XRD peak. The grain size was found to vary between 14 and 17 nm as the film thickness was changed from 0.028 to 0.23 mm. The values are given in Table 1. 3.4. Infrared spectroscopy

Fig. 2. XRD patterns of NiO thin films of various thickness.

Nickel oxide thin films were studied by means of IR spectroscopy, which gives information about the phase composition and the way in which oxygen is bound to the metal ions. IR transmission spectrum of the sample N75 of the nickel oxide thin film in the range between 400 and 4000 cm1 is shown in Fig. 3. It consists of four intensive bands at 470, 1020, 1600 and 3420 cm1. The band at 470 cm1 can be assigned to nickel– oxygen interaction. The existence of other bands clearly indicates that the sample N75 consisted of chloride ions, water molecules and/or hydroxide ions. The results of thermal analysis already indicated that NiCl26H2O can be completely decomposed to NiO at

Fig. 3. The IR spectrum of powder scratched from NiO thin film (N75).

Fig. 4. Variation of optical density (at) vs. wavelength (l) for all the samples.

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temperatures higher than 590 8C. Hence, at the pyrolysis temperature (350 8C), thermal decomposition proceeds imperfectly and NiCl2 thereby remains in the film, leading to evolution of transmission band at 1020 cm1. The presence of water molecules or hydroxide ions (at 1600 and 3420 cm1) at 350 8C deposition temperature were not anticipated from the thermal analysis results as both physisorbed and chemisorbed water molecules expel well below this temperature. Therefore, their presence in IR spectrum

may be due to absorption of water during mixing and pelleting with KBr. 3.5. Optical absorption The variation of optical density (at) with photon energy (hn) for the NiO films with different thickness is shown in Fig. 4. It is found that absorption coefficient decreases with decrease in photon energy, with sharp decrease near the band edge in the visible

Fig. 5. Variation of (ahn)2 vs. hn for all the samples.


region. For all the samples, absorption coefficient was of the order of 104 cm1. Nickel oxide is a high band-gap semiconductor with the absorption edge in the UV region and no absorption in the visible region. The presence of Ni3þ ions in the oxide lattice shows charge transfer transition, with the consequent absorption in the visible region [22]. NiO films deposited by us also show absorption in the visible region. There may be three possible explanations for the above process. The first one is that the main stoichiometry of the film is NiO and Ni2O3 is present as a minority phase that could not be detected by XRD. The second possibility is that two adjacent divalent nickel ions become Ni3þ due to charge transfer process caused by presence of nickel

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vacancy. Third possibility is that the stoichiometry of the film is NiO, but excess oxygen together with the hydrogen may be present in the film as OH groups. Recorded optical data were further analyzed to calculate the band-gap energy of the NiO films using classical relation. Fig. 5 shows the plot of (ahn)2 versus hn for NiO films with different thickness. The optical band-gap is obtained by extrapolating the straight-line portion of the plot at a ¼ 0. The value of the optical band-gap shifts towards lower energy and the slope of the plot decreases when the film thickness increases. The value of band-gap energy changes from 3.58 to 3.4 eV with increase in the film thickness. Varkey and Fort reported Eg ¼ 3:25 eV for NiO films prepared by solution growth technique [13]

Fig. 6. Variation of log r vs. 1/T for all the samples.

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and Eg ¼ 3:6 was reported for NiO films deposited using spray pyrolysis technique with nickel nitrate solution. Many researchers have studied the absorption of photon energy for NiO [12,18]. Reported bandgap energy value for the nickel oxide is in the range of 3.4–3.8 eV, which is in good agreement with our report. This suggests that the optical transition in nickel oxide takes place through direct inter-band transition. Change in the optical band-gap energy with thickness may be attributed to the changes in homogeneity and crystallinity of the film, caused by difference in experimental conditions, mainly the amount

of spraying solution, spray rate and cooling of the substrates during deposition. 3.6. Electrical resistivity The room temperature electrical resistivity was calculated and found to vary from 1 104 O cm for N30 to 9 104 O cm for N75. Increase in the resistivity with increase in thickness of the film may be attributed to non-stoichiometry of the NiO films and surface states. Nominally pure and stoichiometric nickel oxide has room temperature resistivity of the order

Fig. 7. Variation of log r vs. (T0/T)1/4 for all the samples.


of 1013 O cm [25]. Low electrical resistivity in our nickel oxide thin film may be due to formation of nonstoichiometric film with excess oxygen or less oxygen, which produces doped sample. In these cases, an increase in resistance can be expected with increase in thickness [24]. Electrical resistivity of nickel oxide thin films have been studied by many researchers [9,12,13,19,20,23], and reported resistivity is in the range of 10–106 O cm. Fig. 6 shows the variation of electrical resistivity with temperature for NiO films with different thickness. For all the samples, it is observed that resistivity decreases with increase in temperature and supports the semiconducting nature of NiO films. The thermal activation energy (Ea) is calculated by using Arrhenius equation. The estimated activation energy was found

219

to depend on the material form (single crystal or polycrystal) and on the temperature range and its value ranges from 0.6 to 1.9 eV [24]. In case of oxides, the activation energy is the thermal energy required to hop the charges (electrons or holes) from one site to the other. Estimated values of ‘Ea’ are comparable to the reported values for NiO films [9,10]. It is observed that the activation energy increases slightly with an increase in the film thickness. This increase in ‘Ea’ may be attributed to varying dislocation and stoichiometry, caused by difference in experimental conditions, viz. amount of spraying solution, spray rate and cooling of the substrates during deposition. Fig. 7 shows the variation of log r with (T0/T)1/4. This plot yields a straight line, which suggests that the

Fig. 8. Variation of thermo-emf vs. temperature difference (dT) for all the samples.

220


temperature dependence of electrical resistivity is consistent with the relation a T0 r ¼ r0 exp (3) T where, T0 ¼ 7 107 K and a depends on kind and degree of disorder of the considered system and on temperature. In doped crystalline semiconductor, the values of a are 0.25 and 0.5 [24,26]. This result evinces for variable range hopping of charge carriers between randomly distributed localized electronic states in the NiO samples. Since the material NiO has not been prepared as pure stoichiometric, extrinsic conduction dominates rather than intrinsic conduction. Impurity or vacancies present in the material creates carriers in NiO.

lytic decomposition characteristic (TGA and DTA) of a precursor NiCl26H2O was used to derive the deposition temperature. The film thickness increases from 0.028 to 0.23 mm as the volume of spraying solution was varied from 30 to 75 ml. The XRD studies show that all the films deposited were NiO (cubic phase) with orientation along (1 1 1) direction. Increase in film thickness leads to a slight improvement in the crystallinity. The IR studies of a typical film indicated presence of NiO phase with some amount of hydration and chloride ions. The optical band-gap energy of NiO film decreases from 3.58 to 3.4 eV with increase in film thickness. The room temperature electrical resistivity of all the samples was of the order of 104 O cm, thicker film being more resistive. Thermo-emf measurements showed that NiO films were of p-type nature.

3.7. Thermo-emf measurement Thermo-emf measurement was used to evaluate thermoelectric power (TEP) and nature of the material being studied. From the sign of thermoelectric-emf it has been found that all the samples exhibit p-type conductivity. Fig. 8 shows the variation of thermo-emf with temperature difference. The plot shows that thermo-emf increase with increasing temperature difference. This may be attributed to the increase in mobility and/or concentration of charge carrier with rise in temperature. Small change in induced emf is observed with increase in film thickness. This may be due to slight change in average grain size of crystallite with increase in film thickness [27]. The values of thermoelectric power are given in Table 1. In general, the small values of TEP (