1 Introduction

Over the last few decades, the transparent conducting oxide (TCO) semiconductor films have been extensively studied because they are the main component in high-technological applications such as high performance organic light-emitting diodes, optoelectronic and solar cell devices [1, 2]. The cadmium oxide (CdO) film is one of the most promising TCOs semiconductor having high absorption and emission capacity of radiation in their narrow bandgap energy [3]. In addition, CdO exhibit fascinating features such as with narrow bandgap with high optical transparency in the visible region, and high electrical conductivity, which make it applicable in photoelectric devices, liquid crystal displays, semiconductor lasers, and gas sensors [4,5,6]. On the other hand, indium oxide (In2O3) thin films as another TCO semiconductors have distinctive and unique characteristics such as high optical transmittance, high electrical conductivity, good chemical stability, excellent adhesion to substrates and photochemical properties [7]. Therefore, In2O3 has been used in a wide range of applications including photovoltaic devices, solar energy conversion, and flat panel displays [8, 9]. Over the past few decades, numerous researchers have demonstrated that the metal–oxide–semiconductors can be widely used as sensors to detect various gases [10]. However, the sensing properties of metal-oxide based sensors need to be further enhanced for excellent selectivity and fast response/recovery time to detect low concentration in a harsh environment. Many researchers have reported that the sensitivity and the selectivity of semiconducting metal-oxide based sensors could be improved by introducing suitable dopants or impurities into a semiconductor crystal [11,12,13]. It is well reported in the literature that the optical and electrical properties of TCO semiconductors depend strongly on the external doping level as well as sample preparation conditions [7]. Thus, in order to obtain an optimum characteristic of these type of materials, the dopant type, and its concentration as well as deposition conditions have to be carefully optimized.

A gas sensor device consists quite generally of an active material layer whose physical properties change in the presence of a determinate gas. The change can then be measured to detect the gas concentration [14]. The microscopic chemical interactions are selectively converted into a measurable electrical signal [15]. The improved selectivity and reversibility of the sensing process are the main requirements in the design and fabrication of sensors [16]. Gas sensors based on metal-oxides semiconductors are commonly used in the monitoring of toxic pollutants, not only because of their excellent thermal and physical stability but also due to their ability to provide the necessary sensitivity, selectivity, and stability required for measurements [12]. Despite the high sensitivity of several metal-oxide materials, they often show high resistivity which significantly affects the performance and accuracy of the sensor. For example, nitrogen dioxide (NO2), which is an oxidizing gas, might increase the resistance of the metal-oxide films due to an increase in the O2− ions in the n-type semiconductors [17].

To get a desire metal-oxide-semiconductors based sensors, with higher sensitivity, excellent selectivity, and more stability; many researchers have focused their researches on the analysis of sensing mechanism [11, 18]. Furthermore, many attempts have been made to improve the sensor performance by surface modification, metal-doping, and mixing various metal-oxides [19, 20]. It has previously been established that the mixing of metal-oxides, modifies the electron structures of the compound which result in changes to both the bulk and surface properties. Consequently, the resulting metal-oxides composite can achieve sensitivity and selectivity for gas detection far exceeds those achievable performance with the individual constituent of the composite. Studies of sensory phenomena in metal-oxide composites have shown that there are certain optimum compositions for which sensitivity reach maximum values [21,22,23]. On the other hand, the literature survey reveals that metal oxide gas-sensors composition is an essential factor that affected the surface morphology of the film [24]. Therefore, it is very important to investigate the morphological features of sensing materials which depend primarily on the nature of the components and the processing conditions. In the present investigation, spray pyrolysis technique has been used to prepare a polycrystalline CdO:In2O3 thin films with different concentration of In2O3, to clarify the effect of the components on the structural, morphological, optical and electrical properties, to find the best conditions to enhance the gas-sensing performance towards nitrogen dioxide.

2 Experimental part

Polycrystalline CdO:In2O3 thin solid films were prepared using the spray pyrolysis technique. In this technique, an ionic solution containing the constituent elements of a compound is pulverized in the form of fine droplets. The droplets spray deposited on a preheated substrate and solidify. Based on the thermal decomposition of the precursor, a film of more stable compounds forms and adheres to the heated substrate. The solutions, used in the preparation of CdO:In2O3 thin films are the mixture of 1.1425 g of Cadmium chloride (CdCl2), and 0.88212 g of Indium chloride (InCl3) dissolved separately in 50 ml of distilled water to obtain a molarity of 0.1 M for each solution. The two solutions were mixed with the desired concentration of InCl3 (10, 20, 30 and 40 vol%). The optimized deposition parameters such as nozzle-to-substrate distance (29 cm), spray time (5 s) and the spray interval (50 s) were kept constant during spraying. The substrate temperature was fixed at 300 °C. The temperature of the samples was controlled using a K-type thermocouple with an accuracy of ± 1 °C. The film thickness was around (\(d = 400 \pm 10\;{\text{nm}}\)) was determined by an optical interferometeric method, using He–Ne laser with a wavelength of \(\lambda = 632\;{\text{nm}}\), and by using the formula:

$$d = \frac{{\Delta x}}{x}\frac{\lambda }{2}$$
(1)

where \(\Delta x\) is the distance between two fringes, and \(x\) is fringe width.

The crystalline structure of the films under investigation was confirmed by X-ray diffraction (XRD, Shimadzu, DIFRACTOMETER/6000), with CuKα radiation source λ = 1.5406 Å, in the \(2\theta\) range of 20° to 60°.

The morphology of the films was detected by using atomic force microscope (AFM) model (AA3000 Scanning Probe Microscope SPM, tip NSC35/AIBS) from Angstrom Ad-Vance Inc.

The Hall-effect measurements were performed with a computer-controlled system. The values of carrier concentration (\(n_{\text{H}}\)) and Hall mobility (\(\mu_{\text{H}}\)) were calculated, respectively, using the following equations:

$$n_{\text{H}} = \frac{1}{{R_{\text{H}} e}}$$
(2)
$$\mu_{\text{H}} = \sigma \left| {R_{\text{H}} } \right|$$
(3)
$${\text{where;}}\quad R_{\text{H}} = \frac{{V_{\text{H}} }}{I}\frac{d}{H}$$
(4)

Here \(R_{\text{H}}\) is Hall coefficient, \(V_{\text{H}}\) is Hall voltage, \(I\) is constant current, \(\sigma\) is conductivity, \(e\) is a charge of an electron, and \(H\) is an applied magnetic field in Gauss.

In order to measure the NO2 gas sensing properties of the CdO:In2O3 thin solid films, the resistance of the sensor films was measured in air ambient and NO2 gas atmosphere, using digital multimeter Rigol DM3062 data acquisition system. For monitoring the response of the films to NO2 gas, the films were mounted in 250 cm3 homemade airtight container, and the NO2 gas of particular concentration was injected through a syringe. The resistance of the film was recorded before and after exposure to the NO2 gas. Gas sensitivity (s) is defined as the ration of the resistance in the air (Ra) to the resistance in the air containing NO2 gas (Rg), thus the sensitivity of the sensor can be determined from \(S = R_{\text{a}} /R_{\text{g}}\). All the gas-sensing measurements were carried out at various operating temperatures, with 6 V bias voltages, and NO2 gas concentration was fixed at 25 ppm.

3 Results and discussion

3.1 The XRD analysis

Typical XRD pattern of the CdO:In2O3 thin films prepared by pyrolysis method is presented in Fig. 1. The pattern of CdO:10%In2O3 show two diffraction peaks at \(2\theta\) values of 33.28° and 38.54° corresponded to pure CdO, which are well-matched, respectively with the (111) and (200) planes of the cubic CdO phase according to JCPDS card No. 05-0640 [25]. The low intensity of the observed diffraction peaks suggests a randomly distributed orientation of the crystallites. The existence of multiple diffraction peaks in the XRD pattern, confirms the polycrystalline nature of the CdO films [26]. The observed diffraction peaks appeared at \(2\theta\) values 21.18°, 30.48°, 32.40°, 35.03°, 37.32°, 45.36°, and 50.74° have been assigned, respectively, to the lattice planes (211), (222), (231), (400), (330), (341), and (440) of In2O3 according to JCPDS card No. 06-0416 [27]. It can be clearly seen that the intensity of all peak corresponding to In2O3, increase with increasing indium mixture ratios in the prepared thin films. The high diffraction intensity of the peak around \(2\theta = 30.48^{ \circ }\) observed for all films indicates that the (222) direction is a preferred-orientated nature of the In2O3 films. The same behavior was also reported by Khan et al. [28] for nanostructured In2O3 thin films deposited on glass substrates.

Fig. 1
figure 1

The XRD pattern of the prepared CdO:In2O3 thin films

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The mean grain size (G.S.) of the prepared samples was estimated from the full-width at the half-maximum (FWHM) of the highest intense peak at (222) plane, by using Debye–Scherrer formula [29, 30]. The calculated values of grain size are given in Table 1. It can be seen that the grain size of CdO:In2O3 films increased by increasing In2O3 concentration, due to aggregation effects at higher indium oxide concentrations.

Table 1 XRD parameters for CdO:In2O3 thin films, at highest intense peak at (222)
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3.2 Morphological analysis

In the extreme case of thin films, the surface roughness may be in the order of the film thickness and can affect all physical properties of the film such as optical, electrical, mechanical, magnetical, and gas sensor properties [23]. Figure 2 depicts the two- and three-dimensional atomic force microscopy (AFM) images for CdO:In2O3 thin films deposited on a glass substrate with a thickness of about 400 nm. It can be seen clearly that all films exhibit a granular structure, distributed almost homogeneously on a crack-free surface. The finer morphology and roughness of the films can be seen. The average grain size and surface roughness analysis of the grown CdO:In2O3 thin films varied significantly with In2O3 concentration, as tabulated in Table 2. The surface roughness of the films can be attributed to the grain growth of different sizes [31]. It is well reported in the literature that, the high specific surface area of sensing material (maximum roughness) usually has positive effects on the gas-sensing performance [17]. Thus the sample with 20% of In2O3 expected to have improved sensitivity due to its higher roughness.

Fig. 2
figure 2

AFM images of CdO:In2O3 thin films with various In2O3 concentration

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Table 2 Average grain size, average roughness and average r.m.s roughness values of CdO:In2O3 thin films
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3.3 Transmission spectra analysis

The effect of In2O3 concentration on the optical properties of CdO films was identified from the optical transmittance spectrum, which is the most direct and simplest technique to investigate the band structure of semiconductor materials [32, 33]. Figure 3 shows the optical transmittance spectra of CdO:In2O3 thin films with different vol% of In2O3 and in the wavelength ranging from 300 to 1100 nm. The transmittance spectra of all samples under investigation exhibit high transmittance values in the visible and near-infrared wavelengths in the order of 90%. Such films would have potential applications in optoelectronic devices. The decreased in transmittance with increasing of In2O3 mixing ratio, can be referred to the increase in the absorbance within visible and near-infrared regions of the spectrum. Moreover, in the transparent metal-oxides films, the metal-to-oxygen ratio in the film decides the percentage of the optical transmittance. Generally, a metal-rich thin film shows less transparency [34,35,36]. Hence the decrease in optical transmittance for higher In2O3 concentration samples might also be attributed to the increase in metal to oxygen ratio, (Cd + In)/O. The transmittance spectrum of CdO:In2O3 thin films is characterized by a sharp increase in the wavelength range 310-380 nm, which is an identification of good crystallinity nature of these films [37]. This is in agreement with the results of the XRD patterns shown in Fig. 1.

Fig. 3
figure 3

A transmission as a function of the wavelength for CdO:In2O3 thin films

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3.4 The optical band gap calculation

The optical band-gap energy of CdO:In2O3 thin films with various In2O3 contents were calculated by plot the variation of (αhυ)2 with photon energy (hυ), as shown in Fig. 4. The straight line in Fig. 4 is an indicator of the presence of direct allowed optical transition. According to Tauc’s relation, the intercepts of the best fit line of the plot on hυ-axis give the value of direct allowed optical band-gap [38, 39]. The extracted optical band-gap for CdO:In2O3 thin films were found to be 3.60, 3.42, 3.40, and 3.30 eV, for 10, 20, 30, and 40 vol% mixed ratio of In2O3, respectively. There was shifting towards lower energies with increasing In2O3 concentration. The addition of In2O3 may have led to the formation of localized levels within the forbidden band gap, which contributes to increasing the number of electrons that reach the conduction band-gap [40]. The wide direct band-gap of the samples under investigation makes these films good material for potential applications in optoelectronic devices such as solar cell, and photodetectors.

Fig. 4
figure 4

Dependence of (αhυ)2 on photon energies (hυ) for CdO:In2O3 thin films at various In2O3 concentrations

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3.5 Hall effect measurements

The Hall effect measurement is a useful tool to provide the basic electrical parameters to find the suitability of metal-oxide-semiconductor for particular applications [41]. The Hall measurements were performed at room temperature for CdO:In2O3 thin films deposited on a glass substrate at a temperature of 300 °C with different In2O3 contents. The conductivity (σ), Hall coefficient (RH), carrier concentration (nH), and carrier mobility (μH) were calculated for each film, and the values are arranged in Table 3. The negative sign of Hall coefficients for all CdO:In2O3 thin films confirmed the n-type nature conductivity of this system. This is caused due to the existence of defects such as oxygen vacancies and/or intrinsic interstitial cadmium atoms, which can be easily ionized [42, 43]. The induced electrons from this process contribute to the conduction of electricity, causing CdO to act as an n-type semiconductor.

Table 3 Hall measurements of CdO:In2O3 thin films at different In2O3 concentration
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3.6 Gas-sensing performance

Sensing tests aimed to find the optimal conditions of operation of sensors for NO2 gas. It is well accepted that doping can improve the gas-sensing performance of the metal-oxide semiconductor-based sensors for selected gas [44]. Figures 5, 6, 7 and 8 show the resistance-time variation of CdO:In2O3 thin film sensors with different In2O3 concentrations, at various testing temperatures. The NO2 gas concentration was fixed at 25 ppm. The moment at which the gas turn-on and turn-off are monitored on the figures. It can be seen from these figures that the values of electrical resistance for all samples continuously increases with increasing operating temperature. Also, it is evident that the sensor resistance increased when the films were exposed to the NO2 gas for all temperatures, and a maximum sensor response was achieved at about 200 °C. This is in consistency with the result reported by Ferro et al. [45] for sprayed CdO:ZnO thin film-based NO2 gas sensors. They noted that the maximum conductance was achieved in the temperature range 200–230 °C, they attribute this behavior to the increase of the valence electrons concentration produced by thermal excitation.

Fig. 5
figure 5

The variation of resistance with time for CdO mixed 10% In2O3

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Fig. 6
figure 6

The variation of resistance with time for CdO mixed 20% In2O3

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Fig. 7
figure 7

The variation of resistance with time for CdO mixed 30% In2O3

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Fig. 8
figure 8

The variation of resistance with time for CdO mixed 40%In2O3

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Two different models mainly describe the sensing mechanism of metal-oxide semiconductor based sensors: the ionosorption model and the oxygen-vacancy model. The ionosorption model considers the changes in the electric surface potential that result from the gas adsorption, ionization and redox reactions. The oxygen-vacancy model focus on the reaction between gas molecules and oxygen vacancies [11, 46]. In the present study, the NO2 gas molecules are adsorbed on the surface of the CdO:In2O3 film due to oxidizing nature of NO2 molecules, resulting the electrons transfer occurrence from sensing element to the adsorbed NO2 gas on the surface of the film and forming (NO2)−. The process leads to the reduction in electron density of the materials, and hence an increase in the value of resistance upon exposure to NO2 gas [20, 47].

It can be seen in Figs. 5, 6, 7 and 8, that the films responsivity increases as the temperature increases from room temperature to a 200 °C and this shows a typical negative temperature coefficient of resistance (NTCR). When the temperature exceeds 200 °C, the sensor film sensitivity is slowly decreased which led to the positive temperature coefficient of resistance (PTCR). This behavior reveals that the interaction between NO2 gas molecules and the metal-oxide surface is thermally activated. The sensing properties of the film can be determined by considering the processes of adsorption, reaction, and desorption of gas molecules on the sensing surface [48]. The optimum operating temperature depends on the sensing materials and the kind of gases to detect, this could result from the change of the adsorption and desorption rates of gas molecules on the surface of the sensing element [12]. Thus, the maximum sensor response at 200 °C is due to an increase in the reaction rate of NO2 molecules on the sensing surface, which might be attributed mainly to enhance in the adsorption rather than desorption rate. The variation of the sensitivity with the operating temperature for CdO mixed with different concentrations of In2O3 is shown in Fig. 9. It is obvious that CdO thin films mixed with 20% In2O3 exhibit maximum sensitivity towards NO2 gas at a temperature of 200 °C. For most metal-oxide gas sensors, the high operating temperature is due to the reaction temperature of O− [12]. According to Pozos et al. [49], the sensitivity of the metal oxide based sensors will increase as the effective surface area increased, which directly related to the roughness of the film which caused a bigger contact area for detecting gas. Thus the maximum sensitivity of CdO–20%In2O3 film can be attributed to the increase in both particle size and surface roughness as confirmed by XRD and AFM measurements.

Fig. 9
figure 9

The sensitivity as a function of operating temperature for CdO mixed with different concentrations of In2O3

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4 Conclusions

Metal-oxide NO2 gas sensor based on CdO:In2O3 thin films deposited by spray pyrolysis techniques has been extensively studied. The XRD analysis revealed that the films are polycrystalline in nature with a preferred-orientation along the (222) diffracted plane. The surface morphology and the optical band gap of CdO:In2O3 thin films were affected by the In2O3 concentration. The Hall effect measurements confirm the n-type nature of CdO:In2O3 thin films. The structure of CdO:In2O3 exhibits high sensitivity with rapid response/recovery which is one of the main features of this sensor. The maximum sensitivity for the film contented 20 vol% of In2O3 towards NO2 gas has been obtained at 200 °C.