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Article

Effect of Annealing Time on Structure, Morphology, and Optical Properties of Nanostructured CdO Thin Films Prepared by CBD Technique

by
Khalid Ridha Kadhim
and
Raghad Y. Mohammed
*
Department of Physics, College of Science, University of Duhok, Duhok 42001, Kurdistan Region, Iraq
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(9), 1315; https://doi.org/10.3390/cryst12091315
Submission received: 27 August 2022 / Revised: 9 September 2022 / Accepted: 12 September 2022 / Published: 18 September 2022
(This article belongs to the Special Issue Optical and Electrical Properties of Low-Dimensional Crystalline Materials)
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Abstract

:
Nanostructured cadmium oxide (CdO) thin films were deposited onto glass substrates using the chemical bath deposition (CBD) technique. Different deposition parameters such as deposition time, bath temperature, pH, and CdSO4 concentration have been considered to specify the optimum conditions to obtain uniform and well-adherent thin films. The thin films prepared under these optimum conditions were annealed for different times (20, 40, and 60 min) at 300 °C, where no previous studies had been done to study the effect of annealing time. The XRD analysis showed that the as-deposited film is Cd(OH)2 with a hexagonal phase. While all the annealed films are CdO with a cubic phase. The crystallite size increases with the annealing time. However, the strain, dislocation density, and the number of crystallites were found to be decreased with annealing time. SEM images of annealed films showed a spherical nanoparticle with an average of particle size 80–46 nm. EDX analysis revealed that the ratio of cadmium to oxygen increases with increasing the annealing time to 40 min. The optical characterization shows that the transmittance is in the range of 63–73% and the energy gap is in the range of 2.61–2.56 eV. It has been found that the transmittance increased and the energy gap decreased with the annealing time. The prepared CdO films can be used in photodegradation applications to remove pollutants from water.

1. Introduction

Recently, transparent conducting oxides (TCOs) have attracted great significance in multiple areas of research due to their distinct properties such as high optical transmittance in the visible and NIR regions of the electromagnetic spectrum, wide bandgap energy, and unique electrical properties [1,2,3,4,5]. Among them, the CdO is an n-type metal oxide semiconductor from the II-VI group with an FCC crystal structure [6,7,8,9,10] that has a direct band gap from 2.2–2.7 eV [11,12]. These distinctive properties have made CdO one of the important candidate materials for multiple applications such as solar cells [13,14] photodetectors [15,16,17], gas sensors [18,19], and photocatalysis [20,21]. Cadmium oxide is very toxic by inhalation, oral, and dermal materials, but we use it due to its unique properties, various applications, and different features [22,23,24]. CdO thin films have been prepared by using different deposition techniques such as thermal evaporation [25], electron beam evaporation [26], pulsed-laser deposition [27], DC magnetron sputtering [28], RF magnetron sputtering [29], successive ionic layer adsorption and reaction (SILAR) [30], spray pyrolysis [31], sol-gel [32], and chemical bath deposition (CBD) [33,34,35,36]. CBD, also known as chemical solution deposition, is a simple, low-cost, and widely used technique to deposit TCO thin films using a simple setup. The substrate is placed inside a beaker containing a supersaturated solution of aqueous precursors such as mineral salts, complexing agents, and a pH stabilizer. CBD has numerous benefits, such as the ability to operate at atmospheric pressure and low temperatures, it does not need sophisticated instruments such as vacuum systems, can be used to deposit a large number of materials, the ability to deposit wide areas, and the possibility of adjusting the properties of thin films by controlling the deposition conditions [16,37,38].
These days, people are becoming more and more aware of the essential significance of a secure and healthy environment. As a result, detecting and treating hazardous chemicals is crucial. For water treatment, many techniques have been tested [21,39,40,41]. The photocatalytic method stands out among them as the approach to purge pollutants from aqueous solutions that offers the most promise. In this arena, nanotechnology is important, and semiconductor nanoparticles have already been shown to be useful for treating water [42,43]. The two most popular metal oxide-based semiconductor materials are ZnO and TiO2 metal oxides [44,45,46,47,48,49]. Much lower data have been reported for the CdO system [50,51].
On the other hand, CBD has some drawbacks such as wastage of materials due to the homogeneous reaction in the reaction bath, and not being able to work at high temperatures (90 °C and above) unless the deposition time is short due to evaporation of the aqueous solution [38,52], and the required thin films have not been obtained directly, as in this work, where the annealing process is required to convert the deposited thin films from Cd(OH)2 to CdO thin films. Additionally, Cd(OH)2 is an important precursor that can be converted to other important compounds such as CdS and CdSe by reaction with appropriate additives [53,54]. Moreover, it is considered a commonly utilized semiconductor in optoelectronics and solar cell applications due to its wide bandgap energy (2.75–3.5 eV), high optical transmittance in the visible range of the electromagnetic spectrum, and high electrical conductivity compared to some else semiconductors [55,56,57].
Furthermore, to the best of the researchers’ knowledge, there is no previous report on the effect of annealing time on the properties of CdO thin films prepared by the CBD technique.
Therefore, the novelty and importance of this work include the preparation and investigation of the effect of annealing time on structural, morphological, and optical properties of CdO thin films obtained by the CBD technique. Moreover, this includes the optimization of major deposition parameters to obtain uniform and well-adherent thin films. Additionally, the ability of the prepared CdO films will be examined for photodegradation application to remove pollutants from water, and this work is currently in progress (MSc student’s work).

2. Experimental Details

2.1. Materials

CdO thin films were prepared by using the CBD technique. Cadmium sulfate (CdSO4) salt was used as a Cd2+ ions, ammonium hydroxide (NH4OH) as a complexing agent, and hydrogen peroxide (H2O2). Ammonium hydroxide acted as both a complexing agent and a pH stabilizer in the alkaline medium. Furthermore, it represented a source of OH− ions, while H2O2 plays an important role in avoiding spontaneous precipitation of any solid phase in the reaction. In this work, all utilized chemical materials were bought from Sigma-Aldrich company without further purification. Distilled water has been used for synthesis and treatment processes.

2.2. Substrate Cleaning

The soda-lime glass slide with the dimension of (25 × 75 × 1) mm3 was used as the substrate for CdO thin film synthesis. Before the synthesis, the glass substrates have been cleaned by soaking them in chromic acid (H2CrO4) for 24 h, washed with distilled water, and ultrasonically cleaned with acetone (C3H5OH) for 15 min. Finally, dried in air at room temperature, and kept in a desiccator [58]. The mentioned cleaning process provides good adhesion, growth nucleation center, and uniform synthesis of thin films [59].

2.3. Deposition of CdO Thin Films

CdO thin films were prepared using 20 mL of CdSO4, an appropriate amount of NH4OH, and H2O2. The volume was completed with distilled water to 100 mL using the CBD technique (Figure 1) in an alkaline solution. The various bath compositions that have been used to prepare solutions are shown in Table 1.
The solution is stirred for a few minutes to obtain a homogeneous mixture. The color of the CBD solution changed from colorless to a white solution at the beginning of NH4OH addition, as shown in Figure 2. With continued addition of the appropriate amount of NH4OH, the solution will return to clear and colorless. The substrates were soaked in the prepared solution at a small angle to the vertical.
The substrates were taken out from the chemical bath at the end of the specified deposition time. It was ultrasonically cleaned with distilled water to remove the precipitated powder layer, then dried in air and kept in a desiccator. After that, the whitish Cd(OH)2 films were annealed at 300 °C in the furnace to convert Cd(OH)2 to CdO thin films through the evaporation of H2O. The whitish films were found to turn brownish, which indicates the formation of CdO thin films as shown in Figure 3.
Thin film thickness (d) was measured using the optical interferometer technique. Which is based on the interference of the reflected light beam from the surface of the film and the bottom of the substrate. He-Ne Laser with a wavelength of 632.8 nm is used. The thickness is measured using the formula:
d = Δ x x · λ 2
where: x is the width of the fringe, Δx is the distance between two fringes, and λ is the wavelength of the laser light [60].
The X-Pert Pro PANalytical X-ray diffraction (XRD) (Malvern Panalytical Ltd., Malvern, UK) was used to investigate the structural properties of the CdO thin films with CuKα radiation with a wavelength of (1.5406 A°) in the 2θ range (10–80°). The surface and morphology properties of the CdO thin films were studied by MIRA 3 TESCAN scanning electron microscopy (SEM) (TESCAN ORSAY HOLDING, a.s., Kohoutovice, Czech Republic). Energy dispersive X-ray diffraction (EDX) (TESCAN ORSAY HOLDING, a.s., Kohoutovice Czech Republic) is linked to the SEM to study the elemental composition. The JENWAY 6850 UV/V (Jenway, Bibby Scientific Ltd, Stone, UK) is spectrophotometer in the range of 300–1100 nm is used to study the optical transmission spectrums of the CdO thin films.
The lattice constant (a) for the cubic structure is calculated using the below relation [61].
a = d h 2 + k 2 + l 2
where (h, k, l) are Miller indices and d the inter planner spacing.
The crystallite size (D) is calculated using the Debye–Scherrer equation [62].
D = 0.9 λ β c o s θ
where (λ) is the wavelength of the X-rays (1.54Å), β is the full width at half-maximum (FWHM) of the peak intensity and θ is the Bragg angle.
The dislocation density (δ) and strain (ε) were determined by Equations (4) and (5), respectively [63,64].
δ = 1 D 2
ε = β c o s θ 4
The number of crystallites per unit area (N) of the films is determined using the relation [62].
N = t D 3
where (t) is the thickness of the films.
The absorption coefficient (α) of thin films was measured using the below equation:
α = 1 d l n ( 1 T )
where d is the thickness and T is the transmittance of the thin film.
The energy band gap (Eg) has been calculated using the Tauc equation:
( α h v ) = A ( h v − E g ) n
where A is a constant, hv is the photon energy, and n is an index that takes the values 1/2, 2, 3/2, and 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect transition, respectively. In this study, the value of the obtained absorption coefficient is found to be greater than 104 cm−1, which supports the nature of the direct bandgap of the obtained CdO thin films [65].

3. Results and Discussion

3.1. Thickness and Growth Rate

The formation of Cd(OH)2 layers occurs when the ionic product of Cd2+ and OH− exceeds the solubility product of Cd(OH)2 ( Ksp = 7.2 × 10−15 at 25 °C ) [66]. Figure 4 shows the variation of the thickness and growth rate of CdO thin films for different parameters (deposition time, bath temperature, CdSO4 concentration, pH of the bath, and annealing time).
From Figure 4a, it can be seen that there are four stages for the growth of CdO thin films. The first stage is before 10 min, which is known as the initial induction phase, in which no clear growth is observed. This is because the development of the film formation requires the availability of sufficient concentration of Cd2+ ions [67]. The second stage (10–40 min) is known as the main phase, where the change in growth is approximately linear with the deposition time. The thickness of CdO thin films obviously increases with the increase in the deposition time (td) from 10–40 min due to the large growth rate (10.50–5.68 nm/min). The third stage (40–50 min), is known as the termination phase, in which the reaction starts to slow and finally stops. In this stage, the growth rate becomes very small (4.84 nm/min); therefore, a slight increase in thickness is observed, indicating an almost saturated state. This is due to the consumption of the precursor over time [68]. In the fourth stage (50–60 min), a decrement in thickness is observed. This decrement can be attributed to the porous formation of an outer layer and peeling off from the substrate [69]. These results are in good agreement with previous studies reported by Rane YN et al. [70].
Figure 4b shows that the thickness and growth rate of the films increases with the increasing bath temperature (Tb) at a specific deposition time. This is because heating provides the kinetic energy of the ions, which leads to an increase in the number of collisions, and thus the formation of Cd(OH)2, whereas, upon reaching the saturation point at 70 °C, the thickness then starts to decrease when the temperature reaches 80 °C due to the desorption and dissolution of pre-formed Cd(OH)2 [71]. Similar results have been reported by Ahmed HS et al. when investigating the effect of bath temperature on the thickness and growth rate of chemically deposited Cu2S thin films [72].
As shown in Figure 4c, the thickness and growth rate increase from 123 to 256 nm and 3.075 to 6.4 nm/min, respectively, with the increasing CdSO4 concentration from 0.05 to 0.2 M. This is due to the fact that the reaction rate is proportional to the concentration of the reacting ions. In the first stage, when the CdSO4 concentration is low, the number of Cd2+ ions is not enough to combine with all the available OH− ions. However, in the second case when the concentration is high, more Cd2+ ions are available, which combine with OH− to form Cd(OH)2, which in turn is deposited on the glass substrate and obtains a thin film with a larger thickness [60]. A saturation state was reached with the concentration of 0.25 M. These results were in good agreement with previous results observed by Salunkhe RR et al. [19].
Figure 4d shows that the thickness and growth rate increase with increasing the pH value from 10 to 10.5. As increasing the pH value of the solution means adding more NH4OH, which in turn provides more OH− ions that react with Cd2+, thus increasing the reaction rate. However, a decrease in terminal thickness was observed by increasing the pH value of the solution further. This is due to the overly rapid reaction between OH− and Cd2+ ions and precipitation of bulk Cd(OH)2 in the solution [73]. These results are in good agreement with the previous study done by Arandhara G et al. [74].
It was found that the optimum deposition parameters are: td = 40 min, Tb = 70 °C, CdSO4 conc. = 0.15 M, and pH = 10.5. The film deposited under these conditions completely covers the surface of the substrate and has good adhesion, which is not removed when wiped or washed with distilled water. The annealing process of the sample under optimum conditions was carried out at 300 °C for different annealing times (tA = 20, 40, 60 min).
Figure 4e shows that the film thickness increases from 200 nm for the as-deposited film to 232 nm by annealing for 20 min despite the evaporation of H2O. This is may be due to the phase change from hexagonal Cd(OH)2 to cubic CdO. However, the thickness of the thin films decreases from 241 to 198 nm when increasing the annealing time from 20 to 60 min, respectively. This is attributed to the progress in the evaporation of H2O molecules, the reduction of grain boundaries with time, and the improvement of the crystal structure of the thin films [75].

3.2. Structural Properties

Figure 5 represents the XRD patterns of the as-deposited and annealed thin films with different annealing times.
Figure 5a represents the XRD diffraction patterns of the as-deposited thin film, prepared under optimum conditions, showing four peaks at 2θ = 28.945°, 33.524°, 48.184° and 57.243°, which correspond to planes (100), (101), (102), and (003), respectively. All these peaks refer to the hexagonal phase of Cd(OH)2, as compared with the standard data of Cd(OH)2 (JCPDS file no. 31-0228). Similar results were obtained by Ahmed HH [75].
Figure 5b–d represents diffraction patterns for films that have been annealed for 20, 40, and 60 min, respectively, each showing a preferential crystal orientation that corresponds to the diffraction peak (111), which is denoting the prevalent crystal growth along the (111) plane. In addition, four minor peaks refer to planes (200), (220), (311), and (222), which correspond to the angles (2θ) shown in Table 2. These five peaks indicate the cubic polycrystalline phase of CdO compared with the JCPDS file (no. 05-0640). The disappearance of the hexagonal phase of Cd(OH)2 indicates that the H2O has completely evaporated and the film is completely converted to CdO. These observed results are in good agreement with previous studies reported by Ahmed HH, Asmial RA et al., and Jassim SA [75,76].
The diffraction intensity increases with increasing the annealing time from 20 to 40 min. Higher intensity and sharper peaks of the diffraction can be attributed to an increase in the grain size, and an improvement in crystallinity and surface texture, which is useful for photocatalytic activity application [4,30,64]. The decrease in FWHM with increasing annealing time could be due to the grain coalescence at a long annealing time [77]. However, the diffraction intensity began to decrease clearly when the annealing time reached 60 min. The low peak intensity may be due to the small film thickness [73]. It is also observed that the shifting of peaks toward lower values of 2θ with increasing annealing time indicates the decrement in lattice strain [74].
Table 2 shows a comparison between calculated and standard parameters of CdO thin films annealed at different times.
The inter-planner spacing (d) was obtained from the XRD data and found to be consistent with the standard values. The lattice constant (a) has been calculated by Equation (2). A shift in the lattice constant towards it is the standard value (4.695 nm) observed when the film is annealed for 40 min. This indicates an improvement in the quality of the crystalline thin films [76].
The crystallite size (D), dislocation density (δ), strain (ε), and the number of crystallites per unit area (N) have been calculated for the preferential orientation plane (111) using Equations (3)–(6), respectively, and tabulated in Table 3.
The crystallite size is found to be in the range of 14.03–33.65 nm, and it is increased as the annealing time increases, as shown in Figure 6a. This denotes an improvement in the crystallization of the films [74]. From Figure 6b,c, one can see that the strain (ε) and dislocation density (δ) decreased with the increasing annealing time, which indicates a higher degree of crystallization and the increment in grain size (D) of the thin films [78,79].
Moreover, from Table 3, one can see a clear decrement in N with the increasing annealing time. It is decreased from (61.95 to 4.15) × 1011 crystallite/cm2 where the annealing time increased from 20 to 60 min, respectively as shown in Figure 6d. This is attributed to the coalescence of the grains with each other through reducing the number of grain boundaries as the annealing process continues. These results are in agreement with the results previously reported by O. Vigil et al., K. M. Saleh when studying the effect of annealing time on the structural properties of CdO films prepared by spray pyrolysis technique [80,81].
In this work, the most interesting structural properties were obtained at 40 min of annealing time. It was found that the film is peeled off from the substrate when the annealing time reaches 60 min.

3.3. Morphology and Elementary Composition

Figure 7 shows SEM images of the as-deposited and annealed thin films at different times.
It can be clearly seen that all the thin films consist of spherical-like nanoparticles entirely covering the substrate surface. The morphology appears to be a dense, porous structure in the form of clusters with some cracks appearing in the annealed films. The appearance of cracks may be due to removing the hydroxide phase and/or densification of the CdO thin films during the heat treatment [82,83].
The average particle size is found to be about 243 nm for as-deposited thin films and 80, 59, and 46 nm for annealing times at 20, 40, and 60 min, respectively. the formation of agglomerates was also observed when the annealing time reached 40 min, with the agglomerates increasing with the annealing time increasing to 60 min. The obtained spherical shape with the porous surface and small particle size with the existence of cracks may provide better photocatalytic efficiency due to the increasing reaction surface [21,84].
Figure 8 shows the elemental composition of CdO thin films annealed at different times using EDX.
The analysis confirms the presence of Cd and O in as-deposited and annealed films. The presence of Si, C, Ca, Na, and Mg peaks are attributed to the glass substrate [85]. One can see that the Cd percentage is low in the as-deposited film. The inset of Figure 8 shows that the Cd percentage increased from 54.7% for as-deposited to 72.4% and 73.5% for (20 and 40) min of annealing time, respectively. However, it decreases to 59.5% by peeling the film from the substrate when the annealing time reached 60 min. Inversely, the percentage of O decreases by annealing the films at 20 and 40 min. This is due to the evaporation of the H2O molecule and the transition from the Cd(OH)2 to CdO. These results confirm the XRD results.
Table 4 shows the average atomic ratio of Cd/O as a function of the annealing time. A high percentage of oxygen is observed, which is attributed to the alkaline aqueous solution and/or may be incorporated into the films from the atmosphere during the annealing process [77]. The phenomenon of high concentration of oxygen was also reported previously by (U. T. Nakate et al., A. K. Sharma et al.) [86,87].
Additionally, Figure 9 shows EDX mapping images of the distribution of Cd and O in the films annealed at different times. It can be observed that the Cd and O elements are homogeneously distributed along all the films. The most promising surface morphology and better elemental composition of the CdO thin film were obtained with the annealing time of 40 min.

3.4. Optical Properties

Figure 10 shows the optical transmittance (T) spectra for as-deposited and annealed CdO thin films in the range of 300–1100 nm.
It has been found that the transmittance decreases when the film is annealed compared with the as-deposited film. This decrement is due to the increase in thickness during the transition from the hydroxide phase to the oxide phase. The maximum T values of CdO films after the annealing equal 63%, 71%, and 73%, corresponding to annealing times of 20, 40, and 60 min, respectively, in the visible region (625–835 nm). The increase in T with the annealing time is due to the decrease in thickness due to the evaporation of H2O molecules and the increase in crystallite [74,88]. Moreover, the increase in transmittance can be attributed to lower scattering effects due to reducing the crystal defects, structural homogeneity, and crystallinity improvement [89]. In addition, the increase in transmittance may be due to increased carrier concentration and lower bandgap values [79]. These results confirm XRD and EDX results.
To find the energy bandgap Eg, the absorption coefficient (α) was calculated from Equation (8). The band gap was determined by plotting a graph between (αhv)2 and (hv). The energy bandgap was estimated through the extrapolation of the linear portion of the curve to (hv) at zero (αhv)2 and is shown in Figure 11 [90].
Figure 12 shows the variation of Eg under different annealing times.
It is found that the Eg of the as-deposited thin film is 3.30 eV, which is matched with a previously reported value for Cd(OH)2 thin film Eg (3.2–3.5 eV) [55]. However, Eg decreases from 2.61–2.56 eV when increasing the annealing time from 20 to 60 min, respectively. This is due to the increase in the crystallite size and crystallinity improvement [91]. The decrease in the Eg could be due to the increased oxygen content of the film due to the annealing process [79]. Some defects such as vacancies and atoms adsorbed on crystal surfaces, etc., if some of their energy levels in the bandgap are close to the lower part of the conduction band, the absorption associated with these defects will cause a reduction in the absorption of CdO, resulting in a decrement in the bandgap value [92].
Moreover, the calculation of the band gap from the Tauc diagram is considered the most convenient method for calculating the energy gap in much of the literature; however, there is an error rate of about 0.03 as the maximum, depending on the technique and accuracy of drawing the tangent line [92,93]. The results confirm XRD and EDX results. These results are in agreement with the results previously reported by (A. A. P. Frit et al., V. Soliya et al., and A. Bouraiou et al.) when studied the effect of annealing time on the optical properties of other thin films prepared by different methods [94,95,96].

4. Conclusions

Nanostructured CdO thin films were successfully deposited by the CBD technique. It has been observed that the deposition parameters strongly affected the thickness, growth rate, and quality of CdO thin films. It has been observed that the thickness increased with increasing the deposition time and bath temperature from 10 to 50 min, and 50 to 70 °C, respectively. After that, the thickness starts to decrease as the deposition time and bath temperature reach 60 min and 80 °C, respectively. Additionally, the thickness increased with increasing CdSO4 concentration from 0.05 to 0.25 M. The growth rate was found to decrease with the deposition time and increases with the bath temperature and precursor concentration. Moreover, the thickness and growth rate increased with increasing pH from 10 to 10.5. However, they started decreasing as the pH value reached 11. Furthermore, it was found that the thickness of thin films decreases with increasing the annealing time. It is noteworthy that the saturation state was observed only in the case of precursor concentration.
It was found that the optimum deposition parameters are: deposition time 40 min, bath temperature 70 °C, CdSO4 concentration 0.15 M, and pH value 10.5. Additionally, the best film quality is obtained at 40 min of annealing time.
XRD showed a hexagonal phase of polycrystalline Cd(OH)2 for as-deposited thin films. However, all the annealed films are polycrystalline CdO with a cubic face and a preferential orientation (111). The peak intensity and crystallite size were found to increase with annealing time. However, the FWHM, strain, dislocation density, and the number of crystallites were found to be decreased with the annealing time.
SEM images of annealed films showed homogeneous surfaces in the form of spherical nanoparticles with an average particle size of 80–46 nm. It decreased with the annealing time, and the agglomerated nanoparticles increased as the annealing time increased. EDX analysis revealed that the best atomic stoichiometry of Cd/O was obtained at 40 min of annealing time.
Optical analysis shows that the transmittance of CdO thin films increases and the energy gap decreases when increasing the annealing time.

Author Contributions

Conceptualization, R.Y.M.; methodology, R.Y.M.; software, K.R.K.; validation, R.Y.M. and K.R.K.; formal analysis, R.Y.M.; investigation, R.Y.M.; resources, R.Y.M.; data curation, K.R.K.; writing—original draft preparation, K.R.K.; writing—review and editing, R.Y.M.; visualization, R.Y.M. and K.R.K.; supervision, R.Y.M.; project administration, R.Y.M.; funding acquisition, University of Duhok. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the University of Duhok.

Acknowledgments

The acknowledgment is directed to the University of Duhok for their support.

Conflicts of Interest

First the authors declare no conflict of interest. Second, the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Crystals 12 01315 g001 550
Figure 1. Experimental setup of CBD technique.
Figure 1. Experimental setup of CBD technique.
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Crystals 12 01315 g002 550
Figure 2. Different stages of chemical bath solution where (a) aqueous solution of CdSO4, (b) immediately upon adding 1 mL of NH4OH, (c) after adding the appropriate amount of NH4OH, (d) after adding H2O2 and distilled water, and (e) after 3–5 min of heating to 50 °C.
Figure 2. Different stages of chemical bath solution where (a) aqueous solution of CdSO4, (b) immediately upon adding 1 mL of NH4OH, (c) after adding the appropriate amount of NH4OH, (d) after adding H2O2 and distilled water, and (e) after 3–5 min of heating to 50 °C.
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Crystals 12 01315 g003 550
Figure 3. The photography images of thin films: (a) before, and (b) after the annealing process.
Figure 3. The photography images of thin films: (a) before, and (b) after the annealing process.
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Crystals 12 01315 g004 550
Figure 4. The variation of thickness and growth rate of CdO thin films with (a) deposition time, (b) bath temperature, (c) CdSO4 concentration, (d) pH of the bath, and (e) annealing time.
Figure 4. The variation of thickness and growth rate of CdO thin films with (a) deposition time, (b) bath temperature, (c) CdSO4 concentration, (d) pH of the bath, and (e) annealing time.
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Crystals 12 01315 g005 550
Figure 5. XRD patterns of as-deposited and annealed thin films where t (a) = (b) 20 min, (c) 40 min, and (d) 60 min.
Figure 5. XRD patterns of as-deposited and annealed thin films where t (a) = (b) 20 min, (c) 40 min, and (d) 60 min.
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Crystals 12 01315 g006 550
Figure 6. Variation of structural values with annealing time. (a) Crystallite size, (b) strain, (c) dislocation density, and (d) the number of crystallites per unit area.
Figure 6. Variation of structural values with annealing time. (a) Crystallite size, (b) strain, (c) dislocation density, and (d) the number of crystallites per unit area.
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Crystals 12 01315 g007 550
Figure 7. SEM photographs for (a) as-deposited, and annealed thin films at (b) 20 min, (c) 40 min, and (d) 60 min.
Figure 7. SEM photographs for (a) as-deposited, and annealed thin films at (b) 20 min, (c) 40 min, and (d) 60 min.
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Crystals 12 01315 g008 550
Figure 8. EDX of as-deposited and annealed thin films at different annealing times. (a) As-deposited, (b) 20 min, (c) 40 min, and (d) 60 min.
Figure 8. EDX of as-deposited and annealed thin films at different annealing times. (a) As-deposited, (b) 20 min, (c) 40 min, and (d) 60 min.
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Crystals 12 01315 g009 550
Figure 9. EDX mapping analysis of Cd and O elements with various annealing times: (a,b) as-deposited, (c,d) 20 min,(e,f) 40 min, and (g,h) 60 min.
Figure 9. EDX mapping analysis of Cd and O elements with various annealing times: (a,b) as-deposited, (c,d) 20 min,(e,f) 40 min, and (g,h) 60 min.
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Crystals 12 01315 g010 550
Figure 10. Transmittance spectra of as-deposited and annealed thin films at different times.
Figure 10. Transmittance spectra of as-deposited and annealed thin films at different times.
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Crystals 12 01315 g011 550
Figure 11. The energy gap of as-deposited and annealed thin films where: (a) as-deposited, (b) 20 min, (c) 40 min, and (d) 60 min.
Figure 11. The energy gap of as-deposited and annealed thin films where: (a) as-deposited, (b) 20 min, (c) 40 min, and (d) 60 min.
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Crystals 12 01315 g012 550
Figure 12. Variation of energy gap as a function of annealing time and crystallite size.
Figure 12. Variation of energy gap as a function of annealing time and crystallite size.
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Table
Table 1. Chemical bath compositions.
Table 1. Chemical bath compositions.
Bath No.12345
CdSO4 conc.(M)0.150.150.05–0.25
By step 0.05		0.150.15
pH10.510.510.510–11.5
By step 0.5		10.5
td (min)10–60
By step 10		40404040
Tb(°C)7050–80
By step 10		707070
Table
Table 2. The observed and standard values of some XRD parameters for CdO thin films.
Table 2. The observed and standard values of some XRD parameters for CdO thin films.
tA (min)hklObserved 2θ (°)Standard 2θ (°)Observed d (Å)Standard d (Å)Observed a (Å)Observed I (%)Standard I (%)
2011132.92433.022.7202.7124.712100100
20038.32438.2862.3482.3494.69717.9488
22055.19355.2601.6641.6614.70812.9143
31165.88265.9121.4171.4164.7027.5728
22269.29669.2901.3551.3554.6937.6913
4011133.04133.022.7112.7124.696100100
20038.31138.2862.3492.3494.6999.3688
22055.30355.2601.6611.6614.6987.7043
31165.78665.9121.4191.4164.7084.7828
22269.15969.2901.3571.3554.7027.9713
6011132.80933.022.7292.7124.728100100
20038.25438.2862.3522.3494.70614.8488
22055.36155.2601.6591.6614.69417.6543
31165.75065.9121.4201.4164.71110.7328
22269.31069.2901.3551.3554.6938.8113
Table
Table 3. Calculated values of the structural parameters of CdO thin films at different annealing times.
Table 3. Calculated values of the structural parameters of CdO thin films at different annealing times.
tA (min)2θ (°)FWHM β (2θ°)Grain Size, D (nm)Micro Strain, ε (×10−3)Dislocation Density, δ × 1010 (Lines/cm2)N/Unit Area (×1011 Crystallites/cm2)
2032.9240.590414.0252.47150.83561.95
4033.0410.295228.0591.23512.70110.28
6032.8090.246033.6511.0298.8314.15
Table
Table 4. Compositional analysis of CdO thin films at different annealing times.
Table 4. Compositional analysis of CdO thin films at different annealing times.
Annealing Time (min)Cd Atomic %O Atomic %Cd/O
As-deposited16.4445.50.36
2028.1942.940.66
4029.9444.060.68
6019.5939.770.5
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Kadhim, K.R.; Mohammed, R.Y. Effect of Annealing Time on Structure, Morphology, and Optical Properties of Nanostructured CdO Thin Films Prepared by CBD Technique. Crystals 2022, 12, 1315. https://doi.org/10.3390/cryst12091315

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Kadhim KR, Mohammed RY. Effect of Annealing Time on Structure, Morphology, and Optical Properties of Nanostructured CdO Thin Films Prepared by CBD Technique. Crystals. 2022; 12(9):1315. https://doi.org/10.3390/cryst12091315

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Kadhim, Khalid Ridha, and Raghad Y. Mohammed. 2022. "Effect of Annealing Time on Structure, Morphology, and Optical Properties of Nanostructured CdO Thin Films Prepared by CBD Technique" Crystals 12, no. 9: 1315. https://doi.org/10.3390/cryst12091315

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