Effect of Precursor Concentration on Structural Optical and Electrical Properties of NiO Thin Films Prepared by Spray Pyrolysis

Undoped nickel oxide (NiO) thin films were deposited on 500C heated glass substrates using spray pyrolysis method at (0.015–0.1M) range of precursor. The latter was obtained by decomposition of nickel nitrate hexahydrate in double distilled water. Effect of precursor concentration on structural, optical, and electrical properties ofNiO thin filmswas investigated. X-ray diffraction (XRD) shows the formation of NiO under cubic structure with single diffraction peak along (111) plane at 2θ = 37.24∘. When precursor concentration reaches 0.1M, an increment in NiO crystallite size over 37.04 nm was obtained indicating the product nano structure. SEM images reveal that beyond 0.04M as precursor concentration the substrate becomes completely covered with NiO and thin films exhibit formation of nano agglomerations at the top of the sample surface. Ni-O bonds vibrations modes in the product of films were confirmed by FT-IR analysis. Transparency of the films ranged from 57 to 88% and band gap energy of the films decreases from 3.68 to 3.60 eV with increasing precursor concentration. Electrical properties of the elaborated NiO thin films were correlated to the precursor concentration.


Introduction
Transition metal oxides (TMOs) have attracted great attention in fundamental research field recently, because of their diverse physical properties, materials science, renewable energy, and microelectronics and nanotechnology fields applications [1][2][3][4].They were used as light-emitting diodes (LEDs) [5], smart windows [6], and electrode material for Liion batteries [7].Such metal oxides include semiconducting or metallic oxides like ZnO, TiO 2 , NiO, Fe 2 O 3 , doped -Fe 2 O 3, and Cr 2 O 3 [8].Due to its high excellent chemical stability nickel oxide (NiO) has received a great deal of interest.Besides its low cost and large specific capacitance, its optical, electrical, and magnetic properties made it as favorite candidate [9].NiO is a very important VIII-VI semiconductor having NaCl-type structure [10].It has wide band gap ranging from 3.6 to 4.0 eV [11].Nonstoichiometric nickel oxide is a good p-type semiconductor owing to its defect structure [12].It was shown that NiO thin films exhibit weak resistivity which maybe is due to the creation of nickel vacancies and interstitial oxygen atoms in their structures [13,14].
In previous works different synthetic routes have been used for NiO thin films deposition including electron beam deposition [15], DC reactive sputtering [16,17], thermal evaporation [18], Chemical Vapor Deposition [19], sol-gel technique [20], pulsed plasma deposition [21], and electrodeposition [22].Among these, we will focus more particularly in this paper on the spray pyrolysis (SP) process, owing to its several advantages such as low cost of the apparatus, large area with high homogeneity, and easy control of structure of the deposited films [23][24][25][26].
In this paper, undoped NiO thin films were deposited on 500 ∘ C heated glass substrates using the spray pyrolysis

Synthesis of Thin
Films.NiO thin films were prepared from a solution of nickel nitrate hexahydrate [Ni(NO 3 )2, 6H 2 O].The precursor was dissolved in double distilled water with adding few drops of nitric acid (HNO 3 ) as stabilizer.The precursor concentration, which served as a starting solution, was varied from 0.015 to 0.1 M. The resulting solution was stirred at room temperature using a magnetic stirrer for half an hour to yield a clear and transparent solution.The obtained blend was used as a stock solution for spray pyrolysis.Microscopic glass slides (Ref 217102 with dimensions of 75 × 25 × 1.1 mm 3 ) were used as substrates.The later were cleaned with alcohol in an ultrasonic bath and distilled water then blowdried with dry air.The heat temperature of the substrates was fixed at 500 ∘ C, the nozzle-substrate distance was kept 45 cm, the spray rate was 2 ml/min, and each spray takes one second, whereas the time interval between two successive sprays was 10 second to avoid the substrate temperature fall.Different concentrated solutions of fixed volume (40 ml) were sprayed separately on heated glass substrates leading to undoped NiO thin films.After deposition, the films were allowed to cool till room temperature.

Thin Films Characterization.
Structural characterizations are carried out using X-ray diffraction (XRD) with an X-ray diffractometer (BRUKER-AXS type D8) equipped with X'Pert High Score under Cu K ( = 1.5406Å) radiation whereas the scanning range of (2) was between 30 ∘ and 55 ∘ .Bonding information was obtained by Fourier transformed infrared (FT-IR) spectroscopy apparatus (Shimadzu IR-Infinity1); the scan of the measurements ranged from 400 to 4000 cm −1 .Optical transmittance of the deposited thin films was measured in the range of 300-900 nm by using an UV-visible spectrophotometer (SHUMATZU 1800).To affirm the p-type conductivity and its variation with precursor concentration, Seebek effect and four-point probe were processed on sheets having 2 × 1 cm 2 for all the samples.The measurement of the thickness () was done by gravimetric weight difference method, by assuming constant density of NiO as 6.7 gm/cm 3 .The film thickness  is given by [50] where Δ is the difference in mass before and after deposition,  is the bulk density, and  is the area of the film.

Structural Properties
3.1.1.X-Ray Diffraction.X-ray diffraction (XRD) patterns of NiO thin films elaborated by the spray pyrolysis were recorded for different precursor concentration as 0.015, 0.03, 0.04, 0.05, and 0.1 M and shown in Figure 1.The as-deposited thin film at 0.015 M shows an amorphous structure in nature; it indicates the poor crystallinity of the film.This may be due to the incomplete formation of the NiO film at this concentration [51].With increasing of precursor concentration from 0.03 to 0.05 M, the commencement of crystallization was observed.It is seen from XRD patterns that only single peak appears at 2 = 37.24 ∘ , which is attributed to the (111) diffraction peak and clearly indicated that the NiO phase exists under its face centered cubic (fcc) structure, which is in good agreement with Joint Committee on Powder Diffraction Standards (JCPDS card number 47-1049) [52].From XRD patterns, with respect to limit detection, no other impurity peaks were detected.The thin film that can be seen, deposited at 0.1 M, has higher and sharper (111) diffraction peak indicating an improvement in crystallization of this film compared to other ones deposited at lower concentrations.It is worth noting that other peaks at (200) appeared in films when the concentration of solution exceeds 0.03 M. In order to obtain more structural information, different structural parameters such as lattice constants , interplanar spacing  ℎ , grain size , mean strain , and dislocation density  of NiO thin films coated on various precursor concentration are calculated.In the case of cubic structure, the lattice constant  for NiO a phase is calculated using the following equation [53]: where ℎ are Miller indices and  ℎ is the interplanar spacing given by Bragg's law [54]: where  is the order of diffraction taken equal unity (first order),  is the wavelength of the radiation ( = 1.54056Å) for CuKa radiation, and  is the Bragg diffraction angle of peak in degree.The grain size  of the films was calculated for (111) plane by using Scherrer's equation [55]: where  is the full width at half maximum (FWHM) corresponding to diffraction angle  and  is the X-ray wavelength of the incident radiation (1.54056 Å).
The mean strain  in NiO thin films can be estimated via the formula [56] where  and  0 are the lattices constants of NiO thin films and the standard lattice constant of the bulk material ( 0 = 4.1769 Å) according to standard card (JCPDS, number 47-1049).The dislocation density , which is defined as the dislocation line per unit volume, has been calculated using the following relation [57]: Table 1 summarizes variations of different structural parameters of elaborated NiO thin films with various precursor concentrations.Variation of grain size along the (111) plan as a function of precursor concentration is reported in Figure 2. As can be seen from this figure when precursor concentration covers the average 0.03-0.1 M, the grain sizes of the thin films were about 15.20, 20.27, 18.55, and 37.08 nm for samples deposited using 0.03 M, 0.04 M, 0.05, and 0.1 M, respectively.The film prepared at 0.1 M concentration presents the high value of grain size (37.08 nm) revealing the nano structure of the product.The increase in the grain size indicates a decrease in the lattice defects, which in turn reduces internal strain and dislocation density [58] as it was seen in Table 1.3(a)-3(d) represent the surface morphologies of the samples with different concentration of the precursor (0.015-0.1 M).As can be seen thin film elaborated with 0.015 M precursor concentration reveals that NiO mater starts to grow on the substrate but the latter is not

Fourier Transform Infrared (FT-IR) Analysis.
To further support the XRD results, the quality and composition of elaborated NiO thin films were investigated by FT-IR spectroscopy.Analysis through FT-IR spectroscopy was done because its spectra are powerful tool to provide information on the nature of obtained metal oxides.In general oxides and hydroxides of metal nanoparticles give absorptions peaks in wave number region below 1000 cm −1 that arises from interatomic vibrations [32,59,60].Figures 4(a)-4(e) show the FT-IR spectrum of NiO thin films with varying precursor concentration from 0.015 to 0.1 M, respectively.As indicated in the spectra, for all samples characteristics peaks at 407, 437, 475, 500, 520, 560, 607, 668, and 820 cm −1 correspond to Ni-O stretching vibration bonds as reported in literature [29,34,35] but with use of others deposition methods.To give confirmation to the observed 820 cm −1 absorption strength vibration, in the case of 0.05 and 0.1 M sprayed sample (Figures 4(d) and 4(e)), calculated wave number (in cm −1 as units) using Hook's law in simple harmonic oscillator model was done.As well known, Hook's law is illustrated by the following formula [61]: where  is the light velocity,  is the bond force constant, and  is the reduced mass.Thus the position of specific band absorption of nickel oxide powders is determined on the basis of the bond force constant  (= 5 ⋅ 10 5 dyne⋅cm −1 ) and the reduced mass  ( = 1/ Ni + 1/ O ).This formula yields to theoretical NiO stretching vibration equal to 821 cm −1 which is closely equal to the observed 0.05 and 0.1 M sprayed samples, as seen in Figures 4(d) and 4(e), and corresponds to the bulk Ni-O vibration mode.

Optical Properties.
Figure 5 shows the optical transmittance spectra recorded in the wave length range 300-900 nm.For all thin films prepared at various precursor concentrations, spectra show two regions: the first one at wavelength higher than 400 nm showing practically an average transmission between 57 and 88% and revealing a decrease by increasing the precursor concentrations and, based on calculated film thickness (), transparence was found to be depending on thickness of samples reveling Beer-Lambert low as it was  shown in Table 3.The second region is at wavelength lower than 400 nm for which transparence decreases rapidly for all samples exhibiting the onset fundamental absorption due to the transition between the valence band and the conduction band [62].
For more investigation, absorption coefficient () of the NiO thin films was determined from the transmittance measurements using Swanepoel's method [63,64]: where  and  are the thickness and transmittance of the film, respectively.Energy gap values depend in general on the films crystal structure (disorder and its regularity) [65].Also () and the incident photon energy (ℎ]) are related each other as they are given in Tauc's equation [66]: where A is a constant,   is the band gap of the material, and  is an exponent which depends on the type of transition;  = 1/2; 2; 3/2; and 3 correspond to allowed direct, allowed indirect, forbidden direct, and forbidden indirect, respectively.Direct energy gap was determined by plotting a graph between (ℎ]) 2 versus ℎ] and extrapolation of straight line to (ℎ]) 2 = 0 gives the direct band gap values of different concentrated sprayed NiO thin films as seen in Figure 6.The calculated band gap values are listed in Table 3. Obtained band gap values are in good agreement with results carried out in literature [67,68].A disorder, so called Urbach tails, which is related to width of the localized states available in the optical band gap of the NiO thin films and affected its structure, was done for further investigation.This width, of the localized states, is related directly to a similar exponential tail for the density of states near band edges and can be expressed by the following relation [69]: where ℎ] is the photon energy,  0 is constant, and   is the Urbach energy which refers to the width of the exponential absorption edge.Figure 7 shows the variation of ln  versus photon energy (ℎ]) for the films.  values were calculated from reciprocal of the straight line slopes, as shown in the Figure 7, and illustrated in Table 3.As can be seen from Table 3, the energy band gap is averaged in 3.68-3.60eV.The decrease in the optical band gap with precursor concentration may be attributed to the increase in grain size and decrease in structural disorder in the films as observed from Urbach energy analysis.In general it was shown that the band gap of all samples is inverted to Urbach energy [70] and the grain size is in respect to equation driven from the well known formula given by the following relation [71]: this relation can be rewritten as follows: where   is the band gap of the film,   is the band gap of bulk form of the material, and  * and R are the electron effective mass and the radius of the nanocrystallites (or grains).Also this relation can be expanded as a series by neglecting the higher order of terms (since (2ℏ 2  2 / * )/   2 ≪ 1) yielding to the final expression, cited above, giving the variation of   as function of radius of crystallite size and confirms what is found in this study and observed from Figure 2.

Electrical Properties.
It is worth noting that NiO is known as a p-type semiconductor due to the Ni +2 vacancies which is the considered defect for its p-type conductivity [11,72].Seebek effect was processed on all the samples and confirmed their p-type conductivity.The room temperature electrical conductivity () was calculated using four-point probe.Figure 8 represents the electrical conductivity and the film thicknesses plotted as function of the precursor concentration; as can be seen from this plot the conductivity of the NiO thin films decreases from 0.16 Ω −1 cm −1 for thin film deposited with 0.015 M to a feeble value (0.11 Ω −1 cm −1 ) at 0.04 M then increases to reach 0.53 Ω −1 cm −1 at 0.1 M. The electrical conductivity () was correlated to the film thicknesses revealing the same behavior upon the precursor concentration.

Conclusions
Undoped nickel oxide (NiO) thin films were deposited on 500 ∘ C heated glass substrates using spray pyrolysis method.Structural studies using X-ray diffraction (XRD) show when precursor concentration is over 0.015 M formation of NiO under cubic structure with single diffraction peak along (111) plane at 2 = 37.24 ∘ .Furthermore (111) peak intensity becomes more significant, when precursor concentration reaches 0.1 M, leading to an increment in NiO crystallite size close to 37 nm indicating nanosize structure of the films.SEM images reveal that beyond 0.04 M as precursor concentration the substrate becomes completely covered with NiO and thin films exhibit formation of nano agglomerations at the top of the sample surface.FT-IR analysis of spectra confirmed the existence of Ni-O bonds vibrations modes in the composition of product.Transmittance spectra showed that transparency of the films ranged from 57 to 88% and band gap energy of the films decreases from 3.68 to 3.60 eV with increasing precursor concentration whereas their thickness averaged from 170 to 210 nm.The electrical conductivity () of the elaborated NiO thin films, having p-type character, was correlated to the film thicknesses revealing the same behavior upon the precursor concentration.

Figure 1 :
Figure 1: X-ray patterns of NiO.Thin films prepared at 500 ∘ C with different precursor concentration.

Figure 2 :
Figure 2: Variations of grain size and band gap (  ) of NiO thin films as function of precursor concentration prepared at 500 ∘ C.

Figure 3 :
Figure 3: SEM Images of NiO thin films prepared at 500 ∘ C with different precursor concentration, (a) samples prepared with 0.015, (b) with 0.03, and (c) with 0.04 M, (d) with 0.1 M.

Figure 4 :
Figure 4: FT-IR spectra of NiO thin films prepared at 500 ∘ C with different precursor concentration, (d, e) samples prepared with 0.05 and 0.1 M, (a, b, c) with 0.015, 0.03, and 0.04 M.

Figure 5 :
Figure 5: Spectral transmittance plots of NiO thin films prepared at 500 ∘ C with different precursor concentration.

− 2 )Figure 6 :
Figure 6: Band gap (  ) estimation from Tauc's relation of NiO thin films prepared at 500 ∘ C with different of precursor concentration.

Figure 7 :
Figure 7: Variation of band gap (  ) and Urbach energy (  ) of NiO thin films as function of precursor concentration of NiO thin films prepared at 500 ∘ C.

Figure 8 :
Figure 8: Variations of conductivity and thickness (t) of NiO thin films as function of precursor concentration prepared at 500 ∘ C.

Table 1 :
Values of Bragg angle 2, lattice constants , full width at half maximum , grain size , mean strain , and dislocation density  deduced from the (111) peak of NiO thin films prepared at 500 ∘ C with different of precursor concentration.

Table 2
summarizes obtained results in this study in agreement with other results carried out in literature [27-49, 52].

Table 2 :
The assignments of the most important bands in the FT-IR spectra of NiO thin films sprayed at 500 ∘ C with different of precursor concentration.SamplesPositions (cm −1 ) in this work in respect to the concentration Assignments Positions (cm −1 ) in other works

Table 3 :
Values of thickness t, average transmittance, optical band gap energy   , and Urbach energy   of NiO thin films prepared at 500 ∘ C with different of precursor concentration.