Preparation and Characterization of In2O3-CuO Nanocomposite Thin Films as NH3 Gas Sensor

An NH3 gas sensor was prepared from nanocomposite films of indium oxidecopper oxide mixtures with ratios of 0 , 10 , and 20 Vol % of copper oxide. The films were deposited on a glass substrate using chemical spray pyrolysis method (CSP) at 400 o C. The structural properties were studied by using X-ray diffraction (XRD) and atomic force microscopy ( AFM). The structural results showed that the prepared thin films are polycrystalline, with nano grain size. By mixing copper oxide with indium oxide, the grain size of the prepared thin films was decreased and the surface roughness was increased. The UV-Visible spectrometer analysis showed that the prepared thin films have high transmittance. This transmittance was decreased by mixing copper oxide with indium oxide. The direct optical energy gap ranged 3.5 3 eV, which was decreased with increasing copper oxide concentration. The sensitivity of the prepared gas sensor was measured towards NH3 gas at a concentration of 71ppm with operating temperatures of 100, 150, 200, 250 and 30) o C, according to the change of sensor resistance. This sensitivity of the mixture oxides showed a value of about nine times greater than that of individual indium oxide thin films. The results of the optimum gas sensor properties demonstrated a sensitivity value of 75.06%, response time of 10s, and recovery time of 11 s, at a mixing ratio of 20% of copper oxide and an operating temperature of 200 o C. Keywords;Thin films, optical and structural properties, Nano – composite, In2O3 – CuO, NH3 gas sensor. نم يهنان بكا رتسل ةقيقر ةيذغا صئارخ ةسا ردو ريزحت In2O3-CuO زاغ سدحتسك NH3 * حلاص دماح ، ديسح ميركلادبع ديسحلادبع يميسجلا قا خعلا ، يدامخلا ،رابنلاا ةعماج ،ةفخرلا مهمعمل ةيبختلا ةيمك ،ءايديفلا مدق ةصلاخلا زاغ ذدحتم عيشرت ثحبلا احه يف مت NH3 مهيجنلاا جيدكوا طيمخ نم يهنان بكا ختم ةيذغا نم جيدكوا ( ةيسجح بدشب ساحشلا 0,10,20 Vol% عاهق ىمع بسخم ءاذغلا. ساحشلا جيدكوا نم ) ةيجاجز ج ةرا خح ةجرجب , يرا خحلا يئايسيكلا لمحتلا ةيشقت ماجختسأب 400 oC ماجختساب تسرد ةيبيكختلا صئارخلا , ( ةيشيدلا ةعشلاا دهيح ةيشقت XRD ( ةيرحلا ةهقلا خهجمو ) ( AFM ةقيقخلا ةيذغلاا نا تشيب ةيبيكختلا جئاتشلا . ىلا ىدا مهيجنلاا جيدكوا عم ساحشلا جيدكوا طمخ جشع . يهنان يبيبح مجح تاذ بيكختلا ةدجعتم ةخزحسلا ( فيط نا . ةخزحسلا ةقيقخلا ةيذغلال حطدلا ةنهذخ دايزاو , يهناشلا يبيبحلا مجحلا نارقن UV-Visible spectrometer نأب خهظا ) طمخ عم صقاشتت ةيذافشلا هحه . ةيلاع ةيذافن كمتست ةخزحسلا ةقيقخلا ةيذغلاا نيب حوا ختت تناك ةخشابسلا ةقاطلا ةهجف ميق نا . مهيجنلاا جيدكوأب ساحشلا جيدكوا eV ( 3.53 صقاشتتو، ) ISSN: 0067-2904 Hameed and Al-Jumaili Iraqi Journal of Science, 2021, Vol. 62, No. 7, pp: 2204-2212 0022 لاا زاغ هاجت جيساكلاا نم ةخزحسلا تاددحتسلا ةيددحت نا .طيمخلا يف ساحشلا جيدكوا ةدايز عم اينهم NH3 ( ديكختب 71ppm ميق تطعا جيساكلاا طيمخ ةيددحت ميق نا . ةمواقسلا خيغت للاخ نم اهسايق مت , ) ةسيق تطعا ةيلاثسلا ذدحتسلا صئارخ نا . تا خم عدت دوجحب دخفشسلا مهيجنلاا جيدكولا اسم خبكا ةيددحت ( يوادت ةيددحت 75.06% ( ةباجتسا نمدبو ) 10s ( عاجختسا نمزو ) 11 s ) ساحشلا جيدكولا طمخ ةبدشب اهرجق 20% ليغذت ةرا خح ةجرجبو 200 oC . Introduction Semiconductor metal oxide (SMO) has many advantages, such as low cost, easy for fabrication, low detection limits for gases, in addition to their long life, non-toxicity, and high conductivity. For these properties, it has been used in many different fields and applications, such as solar cells and gas sensors [1]. SMO has excellent sensitivity, rapid response, and long-term stability. Therefore, it has been widely used in the detection of dangerous and explosive gases in the environment [2, 3]. The enhanced sensing properties of the thin film's composite can be attributed to the p-n heterojunction contact between the semiconductor metal oxides [4]. Moreover, many attempts have been made to enhance the sensitivity and selectivity of the transparent metal oxide sensors by modifying the surface through adding metallic impurities and mixing different metal oxides [4-9]. Indium oxide (In2O3) is an n-type semiconductor with a wide energy gap n (3.53.75 eV). Also, it has high conductivity, low resistance, and ability to modify its surface by doping or mixing with some metals [10]. CuO is a ptype semiconductor, with a narrow energy gap (1.2-1.9 eV), low cost, high catalytic activity, good electrochemical performance, and excellent response to most neutral and reducing gases [11-12]. CuO is studied as a p-type gas sensor material but not used widely as mixed oxide with n-type metal oxide semiconductors, such as In2O3, SnO2, and ZnO for gas sensor applications [13]. Ammonia gas (NH3) is highly toxic, being commonly used in agriculture, food industry, and chemical industry. In addition, it is known to be exceptionally harmful to the human body and the environment. Thus, highly sensitive detection is necessary for its monitoring [14]. Sensitivity tests for the prepared films aim to create optimal conditions for the operation of the NH3 gas sensor [15]. The present study was designed to manufacture an NH3 sensor from nanocomposite thin films made of a mixture of indium oxide and copper oxide deposited on a glass substrate by a simple chemical spray pyrolysis technique. The structural, optical, and sensing properties of 71 ppm NH3 gas of the prepared films were studied. 2. Experimental Details Nano thin films of indium oxide pristine and the mixture of thin films of indium oxide– copper oxide were deposited with different volumetric ratios of copper solution (0, 10, 20 Vol. %), using the chemical spray pyrolysis technique. The materials’ solutions were prepared from indium chloride (InCl3) with a purity of 97% (supplied by CDH) and copper chloride (CuCl2.2H2O) with a purity of 98 % (BDH chemicals Ltd Poole, England). The molarity of solutions was 0.01 M. The slides were cleaned with detergent water and ethanol using an ultrasonic bath for 10 minutes. The solutions were sprayed automatically on a glass substrate at 400 o C using comprised air of 1 bar as a gas carrier at a period time of spraying of 5 s and (20 s) wait alternately, with a deposition rate of 2.5 ml/min. The optical method was used to determine the thickness of the prepared films, which ranged 250-320 nm. The optical and structural properties were studied using UV-Visible spectrometer, XRD, and AFM. The sensitivity properties of the sensor toward NH3 gas were measured at a concentration of 71 ppm and different operating temperatures. 3. Results and discussion 3.1 Structural properties Figure-1 shows the X-ray diffraction of the prepared films having a polycrystalline structure with nanocrystalline size. The single indium oxide films have several crystalline peaks of (211), (222), (400), (431), (440), (622) at the diffraction angles 2q = (21.43, 30.51, 35.37, 45.5, 50.92, 60. 56o), respectively. The predominant peak is at (222) with a cubic structure, in accordance to the card (JCPDS 01-088-2160), which is in agreement with the results reported by Zhi-HongMaa et al. [10] and Wojtyła et al. [16]. The results also showed that, when adding 10 and 20 % of copper oxide to indium oxide, the predominant peak shifts to the larger angles and the previously appeared peak (431) was missing. Also, there is no compound between the two oxides, which indicates the formation of the composite between the two oxides. The crystalline size was calculated using the DebyeScherer equation. It was found that the addition of copper oxide leads to a decrease in the crystal size as Hameed and Al-Jumaili Iraqi Journal of Science, 2021, Vol. 62, No. 7, pp: 2204-2212 0022 compared with individual indium oxide thin film, as shown in Table-1. Figure 1The X-ray diffraction of the prepared films Figure-2 shows the atomic force microscopy (AFM) images of the prepared thin films. The results show that indium oxide has an average grain size of 107.5 nm and average roughness of 3.13 nm. Adding 10 and 20 % copper oxide to indium oxide leads to a decrease in the grain size to 82.25 nm and 45.27 nm, respectively, along with an increases the roughness and the root mean square, as shown in Table-1. The shape of the grains changes from spherical and elliptical at 10 % copper oxide to spherical at 20 % copper oxide, due to the nanocomposite formation of the mixed films, as shown in figure 2. This behavior corresponds to the XRD result and an agreement with that described by Suhai et al. [5] and Sofi et al. [17]. Figure 2-AFM images of the prepared films Hameed and Al-Jumaili Iraqi Journal of Science, 2021, Vol. 62, No. 7, pp: 2204-2212 0022 Table 1-The values of crystalline size, grain size, average roughness, and root mean square of the prepared films


Introduction
Semiconductor metal oxide (SMO) has many advantages, such as low cost, easy for fabrication, low detection limits for gases, in addition to their long life, non-toxicity, and high conductivity. For these properties, it has been used in many different fields and applications, such as solar cells and gas sensors [1]. SMO has excellent sensitivity, rapid response, and long-term stability. Therefore, it has been widely used in the detection of dangerous and explosive gases in the environment [2,3]. The enhanced sensing properties of the thin film's composite can be attributed to the p-n heterojunction contact between the semiconductor metal oxides [4]. Moreover, many attempts have been made to enhance the sensitivity and selectivity of the transparent metal oxide sensors by modifying the surface through adding metallic impurities and mixing different metal oxides [4][5][6][7][8][9]. Indium oxide (In 2 O 3 ) is an n-type semiconductor with a wide energy gap n (3.5-3.75 eV). Also, it has high conductivity, low resistance, and ability to modify its surface by doping or mixing with some metals [10]. CuO is a ptype semiconductor, with a narrow energy gap (1.2-1.9 eV), low cost, high catalytic activity, good electrochemical performance, and excellent response to most neutral and reducing gases [11][12]. CuO is studied as a p-type gas sensor material but not used widely as mixed oxide with n-type metal oxide semiconductors, such as In 2 O 3 , SnO 2 , and ZnO for gas sensor applications [13]. Ammonia gas (NH 3 ) is highly toxic, being commonly used in agriculture, food industry, and chemical industry. In addition, it is known to be exceptionally harmful to the human body and the environment. Thus, highly sensitive detection is necessary for its monitoring [14]. Sensitivity tests for the prepared films aim to create optimal conditions for the operation of the NH 3 gas sensor [15]. The present study was designed to manufacture an NH 3 sensor from nanocomposite thin films made of a mixture of indium oxide and copper oxide deposited on a glass substrate by a simple chemical spray pyrolysis technique. The structural, optical, and sensing properties of 71 ppm NH 3 gas of the prepared films were studied.

Experimental Details
Nano thin films of indium oxide pristine and the mixture of thin films of indium oxide-copper oxide were deposited with different volumetric ratios of copper solution (0, 10, 20 Vol. %), using the chemical spray pyrolysis technique. The materials' solutions were prepared from indium chloride (InCl 3 ) with a purity of 97% (supplied by CDH) and copper chloride (CuCl 2 .2H 2 O) with a purity of 98 % (BDH chemicals Ltd Poole, England). The molarity of solutions was 0.01 M. The slides were cleaned with detergent water and ethanol using an ultrasonic bath for 10 minutes. The solutions were sprayed automatically on a glass substrate at 400 o C using comprised air of 1 bar as a gas carrier at a period time of spraying of 5 s and (20 s) wait alternately, with a deposition rate of 2.5 ml/min. The optical method was used to determine the thickness of the prepared films, which ranged 250-320 nm. The optical and structural properties were studied using UV-Visible spectrometer, XRD, and AFM. The sensitivity properties of the sensor toward NH3 gas were measured at a concentration of 71 ppm and different operating temperatures. Figure-1 shows the X-ray diffraction of the prepared films having a polycrystalline structure with nanocrystalline size. The single indium oxide films have several crystalline peaks of (211), (222), (400), (431), (440), (622) at the diffraction angles 2q = (21.43, 30.51, 35.37, 45.5, 50.92, 60. 56º), respectively. The predominant peak is at (222) with a cubic structure, in accordance to the card (JCPDS 01-088-2160), which is in agreement with the results reported by Zhi-HongMaa et al. [10] and Wojtyła et al. [16]. The results also showed that, when adding 10 and 20 % of copper oxide to indium oxide, the predominant peak shifts to the larger angles and the previously appeared peak (431) was missing. Also, there is no compound between the two oxides, which indicates the formation of the composite between the two oxides. The crystalline size was calculated using the Debye-Scherer equation. It was found that the addition of copper oxide leads to a decrease in the crystal size as  Adding 10 and 20 % copper oxide to indium oxide leads to a decrease in the grain size to 82.25 nm and 45.27 nm, respectively, along with an increases the roughness and the root mean square, as shown in Table-1. The shape of the grains changes from spherical and elliptical at 10 % copper oxide to spherical at 20 % copper oxide, due to the nanocomposite formation of the mixed films, as shown in figure 2. This behavior corresponds to the XRD result and an agreement with that described by Suhai et al. [5] and Sofi et al. [17].

3-2. Optical properties
Optical examinations of the prepared indium oxide-copper oxide mixture showed a high transmittance within the wavelengths of visible light, as shown in Figure-3. The transmittance of the prepared individual indium oxide films reached 74% at a wavelength of 650nm, as shown in Figure-3. The transmittance was decreased to 66% at 20% of copper solution and a wavelength of 650nm. The reason for the decrease in transmittance after adding copper oxide is the high absorption of copper oxide within the visible wavelengths, which corresponds to the small value of the optical energy gap. These results have good agreement with those of Mahdi et al. [5]. The direct optical energy gap of the prepared films was calculated from the relationship between the energy values of the photon and 2, as shown in Figure-4. The value of the direct energy gap for In 2 O 3 was equal to 3.5 eV. Then, it was decreased with increasing copper oxide concentration to 3.3eV) for 10%, and to 3 eV for 20%. The decrease in the direct energy gap value of the mixture In 2 O 3 -CuO films together with the increase in the ratio of copper oxide concentration occurred due to the small energy gap value of copper oxide.

3-3. Gas sensor characteristics
The sensitivity of individual indium oxide and that mixed with copper oxide films toward 71ppm of NH 3 gas was calculated through the following relationship [7]: where Ra and Rg are the electrical resistance values of the sensor in air and gas, respectively. Figures-(5, 6, 7) show the change in the electrical resistance of the sensor with the gas reaction time at different temperatures. Figure-5 shows the change in the electrical resistance verses time of indium oxide sensor towards NH3. Ammonia gas is one of the reducing gases, while indium oxide is considered an n-type semiconductor. Exposing this sensor to ammonia gas leads to a decrease in the electrical resistance value for the sensor from which the sensitivity was calculated, and its value was (4.92%), while the response time was 12s and the recovery time was14 s at an operating temperature of 100 o C. Also, there was no response to the sensor at room temperature and when the operating temperature was increased to 250 o C. The sensitivity was equal to 5.47% while the response time was equal to 4s and the recovery time was equal to 10s. When mixing a proportion of 10% copper solution, the sensitivity reached8.81%), the response time was 20s and the recovery time was 11s at an operating temperature of 200 o C . By observing the results, it is concluded that the sensor is thermally activated, as the reaction between the surface of the sensor and ammonia gas increases with increasing temperature. The mixing ratio of 20% of the copper solution clearly led to an improvement in the sensitivity of the sensor, the response time, and the recovery time. The sensitivity of the sensor at 20% of copper solution reached 15.18%, with a response time of 20s and a recovery time f 8s, at an operating temperature of 100 o C. When the operating temperature was increased to 200 o C, the sensitivity increased to 75.06%, while the response time was 10s and recovery time was 11 s, as shown in Table-2.     From the observation of the figure we conclude that the optimal sensitivity of the prepared films to NH 3 gas was at the mixing ratio of 20% of copper oxide. The reason is the contact p-n heterojunction between CuO and In 2 O 3 . Also from Figure-8, it is observed that the optimum operating temperature was decreased from 200 o C for 0% and 10% to 180 o C for 20% mixing ration. This means that the optimum mixing percentage between oxides is 20%, which led to improving the sensitivity properties of the thin film. The sensitization mechanism for most of the semiconducting metal oxides works on the principle of changing the conductivity of the sensor materials due to the interaction between the gas and the oxygen ions adsorbed on the surface ( -O 2 , O -, O 2-), which appears when the sensor is exposed to atmospheric oxygen. This leads to the formation of an area of depletion as a result of extracting electrons from the conduction band, thus increasing the barrier of sensor and thereby increasing the sensor resistance [18]. When the sensor is exposed to ammonia gas (NH 3 ), the gas reacts with oxygen ions and the electrons are released to the conduction band, which leads to a decrease in the electrical resistance of the sensor, according to the following equations [19]: -----------(2) ------------- (3) This reaction requires activation energy, which means that it requires the provision of heat. For this reason, the In 2 O 3 -CuO mixture sensor does not obtain a response at room temperature or at lower temperatures, while the response is as high as possible when the actual thermal energy (100 o C) is given for the reaction, reaching to the optimum temperature. Also, we note that above this temperature, the response was decreased, due to the desorbed of oxygen ions from the surface of the sensor [7,[20][21][22].

Conclusions
The sensor was prepared at 400 o C from depositing indium oxide-copper oxide mixture nanocomposites film by using the chemical spray pyrolysis method. The XRD and AFM analyses were used to study the structural properties, which indicated that the prepared films are polycrystalline. They had grain size that was decreased with increasing the copper mixing ratio, while the surface roughness was increased. The optical properties were examined by using UV-Visible spectrometer and the results showed that the transmittance decreases with increasing the mixing ratio of the copper solution. The direct optical energy gap ranged between 3.5 and 3 eV and was decreased with increasing the mixing ratio of the copper solution. The sensitivity of the prepared thin films was measured towards NH 3 gas at a concentration of 71ppm. The sensitivity of the mixture showed a higher value than that for individual indium oxide, with lower response time and recovery time. The optimum sensitivity value and the operating temperature were achieved at a 20% mixing ratio of copper oxide.