# Morphological, Structural, and Optical Properties of Doped/Codoped ZnO Nanocrystals Film Prepared by Spin Coating Technique and Their Gas Sensing Application

Authors: Neha Verma; Vishal Jagota; Alimuddin; Arnold C. Alguno; Manik Rakhra; Chanthirasekaran K.; Betty Nokobi Dugbakie
Journal: Journal of Nanomaterials
 (2022)

---

## Abstract

In today’s world of electronics, nanomaterial applications pose a challenge. The spin coating approach was used to create nanostructured ZnO with wurtzite structure in a recent study. Antimony doping, aluminum, and antimony codoping with 2.0 percent were used to make these films. The impact of doped and codoped films on structural, optical properties, and morphological has been examined using a variety of characterization approaches. A ZnO nanocrystal with a diameter of 20-30 nm was discovered using XRD (X-ray diffraction). According to SEM (scanning electron microscope) scans, the grain size is in the 80-120 nm region. The use of Fourier transform infrared spectroscopy (FTIR) to detect elemental elements was studied, and the peak at 400-520 cm-1 was identified as ZnO. The optical properties of doped and codoped ZnO were checked, and it was discovered that antimony-doped ZnO has a larger band gap than Al and antimony-codoped ZnO. This proved that ZnO may be used in gas sensors and solar cells. The gas response of a static gas sensor system based on Sb-doped films was measured and compared to Al- and Sb-codoped films in the presence of ethanol vapor.


---

## Body

## 1. Introduction

In recent years, as the world has become more reliant on smart and functional electronic gadgets, the demand for materials with controllable structural, optical, and electrical properties that can also be deposited on transparent substrates has increased. As time goes on, the growing world demands fast speed with technology. This need was fulfilled by GaAs, but this does not lead to complete the requirement for future world. This universe requires a material with inherent features such as a huge band gap and a high exciton binding energy. Following examination, it was discovered that ZnO is one of these compounds with a huge band gap of 3.37 eV and a high exciton binding energy of 60 meV. Such properties make this material attractive which is suitable for various applications such as LED [1], UV detector [2], field emission devices [3], and lasers [4]. The process of preparation and deposition has a significant impact on the morphology of the coatings. It is important to note that the layer was deposited from freshly generated solutions which have unclear characteristics, poor crystalline quality, and low absorption coefficients there in the visible range [5, 6]. It is one of the promising materials which is useful for next-generation optoelectronic devices such as solar cells [7], flat panel displays [8], heat-reflecting mirrors [9], and gas sensors [10].The amazing features of transparent conducting oxide (TCO) thin films have piqued the interest of numerous researchers. Light-emitting diodes, solar cells, flat-panel displays, remote controls, thin-film electronics, gas detectors, piezoelectric devices, varistors, surface acoustic wave devices, and other sectors all use zinc oxide thin film, because of its promise and unique properties, such as high transmittance in the visible area of the electromagnetic spectrum, low resistivity, natural abundance, ease of manufacturing, nontoxicity, and chemical stability [11, 12]. Various ZnO nanoparticles, such as nanowires and nanotubes, have been synthesized using a variety of physical and chemical methods, including radio frequency magnetron sputtering [13], hydrothermal method [14], spray pyrolysis [15], simple heat treatment method, and sol-gel with spin and dip coating [16–18]. It also draws a large number of scholars interested in learning more about its physical and optical features. ZnO is also a semiconductor, with a 60 meV excitation bond length and a 3.3 eV linear energy band gap [19, 20]. Because of its low cost, simplicity of process control, and suitability for the preparation of metal oxides, sol-gel with spin coating is the most advantageous of these techniques.ZnO is an n-type transistor that can be doped to form a p-type semiconductor because of intrinsic defects such as oxygen vacancies and zinc audio ads [21, 22]. To improve the optoelectronic properties, Zn is doped with Al, In, and Ga and is examined by using different groups of researchers [23]. However, it should be noted that the fabrication of p-type ZnO is rather difficult because of the low solubility of acceptor dopants and its self-compensation effect [24]. It has been predicted that codopants of acceptor and donor are essential to enhance the solubility. Sb is insoluble in water; it can be perfectly soluble with the doping of P with n-type element.Yamanoto et al. [25] proved that a ZnO sample with antimony is possible. A lot of researchers have investigated how different dopants should be doped into Zn to improve optical-electronics characteristics and surface defects [1, 7]. There is a great deal of research going on right now to develop intelligent and successful goods. As a result, various devices are required to be developed. The many structural, optical, and acoustic factors, as well as photocatalytic, morphological, thermal, and sensing qualities, must all be kept track of. Ce, N, and P tridoped TiO2/AC had the highest photocatalytic activity. ZnO’s nanostructure is currently being investigated. For piezoelectricity and low-carbon synthesis, ZnO was employed. Chemical and gas sensors have been put to good use. Nanorods have been created using a variety of physical processes, as well as chemical processes like spin and dip coating, spray pyrolysis, hydrothermal pyrolysis, Rf magnetron sputtering, and simple combustion.Doping ZnO films might be achieved by swapping dopant atoms for Zn2+ and O2- atoms. The difference in ionic radius between dopant and host atoms determines the efficacy of dopant atoms (zinc or oxygen) [26, 27]. Chewki Zegadi et. al [28] Sn doping has a noticeable effect on the different properties of ZnO films. Mounir Alhamed et.al [29] discovered that the better films have been obtained with introduction of 2% Al-doped ZnO. Undoped and doped ZnO thin films have been created via radio frequency (RF) magnetron sputtering, spray pyrolysis, ion beam evaporation, RF/DC superconducting magnet sputtering, physical vapor deposition, chemical oxidation, and sol-gel techniques [30, 31]. By adding dopants to pure metal oxide, the particular gas’s sensitivity and selectivity can be increased. ZnO films were deposited on glass substrates with doping and codoping utilizing the spin coating process in this study. The role of ZnO thin films on the morphological, structural, and optical properties was studied. ZnO thin films with a reduced band gap were also discovered to be suitable for electrical applications.


## 2. Materials and Methods

### 2.1. To Clean the Glass Substrate

Glass slides were used as a substrate. Glass substrates that are used to eliminate contaminants (40×40 mm2) were cleaned ultrasonically in a variety of chemical solutions for 15 min each and then dried for 5-10 minutes in a hot air oven set to 60°C.To explore the characteristics of ZnO nanoparticles made from various source solutions using a simple heat treatment approach, the crystallinity of ZnO nanoparticles was investigated using an X-ray diffractometer (XRD) with a CuK (=0.154 nm) radiation source in the range of 20-600 using JCPDS data (card number 21-1486). Field emission scanning electron microscope (FESEM) was used to examine surface morphology (JEOL-JSM 6100). FTIR spectra (Fourier transform infrared spectra) were obtained from KBr pellets using a Shimadzu FTIR spectrometer (FTIR 8400S, IR Prestige-21), which provides information on the compound’s nature. A Shimadzu spectrophotometer (UV VIS 2600/2700) with a wavelength range of 200-900 nm was used for optical measurements (absorbance transmittance and optical band gap).


### 2.2. To Prepare the Source Solution

At 60°C for an hour, the base solution was stirred with 0.4 M zinc acetate dihydrate dissolved in a chemical solvent like ethanol to emulsify the solute. A milky solution was then discovered. As a stabilizing agent, gradually incorporate mono ethanol amine (MEA) into the solution. Adding antimony, aluminum-antimony, or both at a 2% concentration to the solution ensured that the molarity of MEA in the solution was equivalent to that of Zn acetate. To apply the solution, slowly apply a drop of the prepared solution to the substrate of choice. The timer was set for 30 seconds for the spin coating. The commencement and the end of this period are separated by a period of time. There were 10 seconds where the speed was 1500 rpm and 20 seconds where it was 3000 rpm. Afterward, dried the substrate in a hot air oven at 100°C for 20 minutes after each application of a new layer of paint. Then, for an hour, anneal the films you have just made at 400°C. Figure1 depicts the spin coating experiment in action.Figure 1
Experimental procedure for spin coating technique.
(a)
Clean the glass substrate(b)
Sol-gel solution of ZnO(c)
Spin coat the solution on substrates(d)
Annealing at 400°C for an hour


## 2.1. To Clean the Glass Substrate

Glass slides were used as a substrate. Glass substrates that are used to eliminate contaminants (40×40 mm2) were cleaned ultrasonically in a variety of chemical solutions for 15 min each and then dried for 5-10 minutes in a hot air oven set to 60°C.To explore the characteristics of ZnO nanoparticles made from various source solutions using a simple heat treatment approach, the crystallinity of ZnO nanoparticles was investigated using an X-ray diffractometer (XRD) with a CuK (=0.154 nm) radiation source in the range of 20-600 using JCPDS data (card number 21-1486). Field emission scanning electron microscope (FESEM) was used to examine surface morphology (JEOL-JSM 6100). FTIR spectra (Fourier transform infrared spectra) were obtained from KBr pellets using a Shimadzu FTIR spectrometer (FTIR 8400S, IR Prestige-21), which provides information on the compound’s nature. A Shimadzu spectrophotometer (UV VIS 2600/2700) with a wavelength range of 200-900 nm was used for optical measurements (absorbance transmittance and optical band gap).


## 2.2. To Prepare the Source Solution

At 60°C for an hour, the base solution was stirred with 0.4 M zinc acetate dihydrate dissolved in a chemical solvent like ethanol to emulsify the solute. A milky solution was then discovered. As a stabilizing agent, gradually incorporate mono ethanol amine (MEA) into the solution. Adding antimony, aluminum-antimony, or both at a 2% concentration to the solution ensured that the molarity of MEA in the solution was equivalent to that of Zn acetate. To apply the solution, slowly apply a drop of the prepared solution to the substrate of choice. The timer was set for 30 seconds for the spin coating. The commencement and the end of this period are separated by a period of time. There were 10 seconds where the speed was 1500 rpm and 20 seconds where it was 3000 rpm. Afterward, dried the substrate in a hot air oven at 100°C for 20 minutes after each application of a new layer of paint. Then, for an hour, anneal the films you have just made at 400°C. Figure1 depicts the spin coating experiment in action.Figure 1
Experimental procedure for spin coating technique.
(a)
Clean the glass substrate(b)
Sol-gel solution of ZnO(c)
Spin coat the solution on substrates(d)
Annealing at 400°C for an hour


## 3. Gas Sensing Measurements

The produced films’ gas sensing capabilities were determined using the static gas sensing setup depicted in Figure2. The temperature control device and sensing films were mounted in a steel housing. The gas is concentrated by injecting a certain volume of ethanol gas into the chamber. It is injected with the help of injection. The film’s response was measured with the aid of a personal computer equipped with LabVIEW software. By employing the relationship, one may determine the sensor response.
(1)S=Rg−RaRa.Figure 2
Gas sensor set up.


## 4. Results and Discussion

XRD spectra of ZnO doped and codoped thin films were displayed in Figures3(a) and 3(b). Table 1 shows the structural parameters of ZnO. All the diffraction peaks exhibits well-defined reflection pattern at 2θ =23.38°, 28.39°, 39.80°, 41.65°, and 51.33° corresponding to (001), (110), (101), (102), and (103) planes, respectively. JCPDS data (card number 21-1486) shows that the XRD pattern corresponds to ZnO patterns with wurtzite structure. This indicates that films are polycrystalline in nature.Figure 3
X-ray diffractogram: (a) antimony doped and (b) aluminum- and antimony-codoped ZnO-based nanoparticles.
(a)(b)Table 1
Structural parameters of ZnO.
(hkl)2θ (0)FWHMCosθD (nm)Lattice spacingd (nm)Antimony-doped ZnO(001)23.380.5630.97914.290.339(110)28.390.4030.92920.170.180(101)39.800.3480.94024.070.202(102)41.650.3470.93424.300.153(103)51.330.3570.90121.960.098Aluminum- and antimony-codoped ZnO(100)31.700.6670.96116.280.449(001)34.370.5240.95517.210.205(101)36.230.4660.95020.230.301(102)47.490.4280.91524.210.177(110)56.560.3880.88024.960.153(103)62.790.3510.85325.030.123(112)68.010.3440.82927.400.099(201)69.100.3280.82627.680.095As a result of increased crystallinity, the plane’s position may have shifted to the left. Zn ions’ lattice space distribution is critical to the optoelectronic application, as demonstrated by this variation in lattice properties.It was determined using the Scherrer’s formula:(2)D=KλβcosƟ,where D denotes the crystal size, K denotes the constant, i.e., 0.89=0.154 nm, the mean wavelength of CuK1 radiation, is the full width half maximum and is the Bragg’s angle in radians.Lattice spacing is deduced by the following equation:(3)1d2=4h2+hk+l23+l2c2.The crystal size lies in the range of 14-30 nm.The produced ZnO films were evaluated for surface morphology using a FESEM (JEOL-JSM 6100), as shown in Figure4. It is noted that ZnO films for an antimony-doped sample do not exhibit discernible grains. Grain growth has been reported to be improved in the codoped sample. ZnO films have had their grain size raised from 25 to 50 nm. This rise in grain size implies that the films have a less dense grain packing, indicating that they are of high quality. The sample doped with (Sb) exhibited nanorods of a lower size. Once the codoping (Al + Sb) is complete, the shape of the nanorods becomes apparent. Having, a wide range of morphological characteristics helps to improve gas sensing. According to XRD, crystal size increases as the dopant concentration changes. SEM images show that the films were perfectly smooth and free of cracks on the order of nanometers. In general, the crack phenomenon was caused by variation in different conditions like thermal annealing temperature, substrate used, and thickness. SEM images reveal that homogenous distribution of ZnO nanoparticles and no agglomeration is observed.Figure 4
Surface morphology of (a) antimony doped and (b) Al-Sb codoped.
(a)(b)Doping and codoping have a lot of rod-shaped morphology, as shown in Figure4. The morphologies round, sticks, drips, and others can be seen clearly. The varied geometries and temporal rates of gas sensing properties have a significant impact on the ZnO nanoparticles, as seen in the FESEM images [25].The Fourier transform infrared spectroscopy (FTIR) using KBr pallets was employed to determine the phase change and functional groups of samples (Shimadzu). Different FTIR spectra peaks were found in the region 4020-400 cm-1 in Figure 5. The bands between 2405 and 3055 cm-1 were caused by the O-C-O bond [15]. The bands between 1200 and 1500 cm-1 represents the C=O species’ bending and stretching vibrations and peaks near the range 630-750 cm-1. Stretching and bending vibrations is due to antimony doping [32].Figure 5
FTIR spectra of doped/codoped ZnO films.The bands located near 749 cm-1 is due to Sb doping, whereas Al and Sb codopant band is located near the 816 cm-1 [33]. The peaks around 400-500 Cm-1 were indicating the formation of stretching mode of ZnO, its COOH and OH group. [34].From FTIR spectra, it was also observed that the transmittance decreases with doping and lower transmittance was observed in case of codoped ZnO films. Table2 shows the details of peaks contained by FTIR Spectra.Table 2
Explanation of peaks contained by FTIR spectra.
Region (cm-1)Mode of vibration400-500Zn-O bond (its COOH and OH group)540-670Al1200-1500C=O2400-3050O-C-OThe optical spectra of antimony-doped and Al- and antimony-codoped samples were measured in the range 200-750 nm (Shimadzu 2600/2700). The highest absorption was seen in the wavelength region of 350-380 nm in Figures6 and Figure 7. Use the following formula to determine the absorbance spectrum from transmitted spectra.
(4)Absorbance=2–log10T.Figure 6
Absorbance spectra of Sb and Sb-Al-codoped ZnO nanoparticles.Figure 7
Transmittance spectra of Sb and Sb-Al-codoped ZnO nanoparticles.When Al and antimony are codoped, the UV peak absorption intensity is higher than when antimony is doped. It has been discovered that as the amount of codopant increases, the UV peaks increase as well. Changes in crystalline quality and surface morphology correspond to changes in optical characteristics in the visible area [29]. The higher the surface roughness, the more light scatters on the surface. The fluctuation of band gap with electron affinity is shown in Figure 6. It educates us about the amount of energy released by a single atom during the process of acquiring electrons, which can be used for gas detection as well as photocatalytic activity [30, 31].Figure8 represents the extinction coefficient K with wavelength in the region 300-700 nm. The absorbance coefficient (α) of ZnO films were calculated by using an equation
(5)K=αλ4πwhere α is the absorption coefficient and λ is the wavelength.Figure 8
(a, b) Extinction coefficient of ZnO nanoparticles.
(a)(b)Absorbance coefficient can be found by using the formula(6)α=lnTt.According to the equation below, the optical band gap and an indirect transition semiconductor’s absorbance coefficient are linked.(7)αhν=Ahν−Egn.WhileEg stands for optical band gap, the ZnO semiconductor is multifunctional due to its direct band gap [29], where hv is for energy and Eg stands for optical band gap. Figure 9 plots (hv)2 vs. energy show the optical band gap of doped and codoped ZnO. In contrast to codoped ZnO, which exhibited a lower band gap, band gaps of 3.00 and 3.15 eV were discovered in doped ZnO. The band gap of ZnO thin films doped with antimony and Al and Sb codoped [35, 36] was measured at 3.00-3.30 eV, which agrees with our measurement of 3.00-3.30 eV. The optical band gap and refractive index are shown in Table 3.Figure 9
(a, b) A plot (αhν)2 versus energy (eV) of Sb, Al-Sb codoped ZnO.
(a)(b)Table 3
Optical band gap and refractive index.
Sample ZnOTechnique usedBand gap (eV)Refractive indexSb-doped ZnOSol-gel3.221.9962Al- and Sb-codopedSol-gel3.102.0199A boost in carrier concentration, which raises the optical band gap, causes a shift in the Fermi level and blockage of some of the lowest states. There are two perspectives on this: as the distance between the valence and conduction bands widens, the least amount of energy necessary to excite an electron from one band to the other grows. Because Pauli’s notion prevents double-occupied states, donor electrons will occupy the lowest conduction band states in doped crystals. These alterations are referred to as the Burstein-Moss effect [37]. The refractive index was in the range of 1.9-2.0, which corresponded to the refractive index reported for ZnO films made using the sol-gel process [38].


### 4.1. Gas Response Measurement

From a static gas sensing device, the gas sensing properties of ZnO nanoparticles were examined. In terms of ethanol gas exposure sensitivity,S=Rgas/Rair is used. Using ethanol at 70 parts per million (ppm), the device is heated to 350°C for the temperature range tested. Figure 10 depicts the gas sensitivity of ZnO films with Al- and Sb-codoped ZnO. Codoped ZnO nanoparticles were discovered to have strong ethanol gas sensitivity. This is in conformity with the findings of [39]. This time around, though, sensitivity was up to 69 percent.Figure 10
Response vs. temperature plot of codoped ZnO sample.Gas sensing application mechanism has been proposed. Electrical resistance is mostly determined by absorbed gas. When the synthesized samples are exposed to the air, oxygen molecules are absorbed, and several ionic species On (O2-, O2, and O2+) are formed [40–43]. It is these ions that contribute to form the host surface depletion layer. The instrument’s sensitivity is enhanced by the presence of these ionic species. When an electron trapped in an oxygen adsorbate is exposed to a donor, the potential barrier decreases and the conductivity rises (reducing gas). One of the most critical components of the gas sensing system to grasp is how test gas molecules are adsorbing and desorbing. Sample surface ionic species are formed. Here, it is evident that, at 205°C, codoped sample provides the best sensing response. The prepared films have lightning-fast response and recovery times of 125 and 166 seconds, respectively. Chemical resistance is a key component of the sensing system, as is chemiresistance [44, 45]. The type of gas molecules and the predominant carriers influence the change in resistance of the metal oxide coating. ZnO’s surface is often coated with oxygen from the atmosphere. ZnO films’ surface electrons can be captured by these oxygen species, forming oxygen species. In order to decrease conductivity, the depletion layer is created by these oxygen species. In contrast, The oxygen adsorbate retains one electron, which returns to the ZnO layer, lowering the potential barrier and increasing conductivity when the sensor is exposed to a reducing atmosphere [46–48]. As a result, gas sensor performance is enhanced.The reaction of the gas sensor increases as the temperature changes. The following is an explanation of the sample’s ethanol detecting process. Adsorption is a surface flaw that results in the formation of ionic species (O2− and O−) on the sample surface. Equations (8)–(10) explain the kinetic reaction before and after ethanol exposure [49].
(8)O2gas↔O2ads,(9)O2ads+e−↔O2−ads,(10)O2−ads+e−↔2O−ads.


## 4.1. Gas Response Measurement

From a static gas sensing device, the gas sensing properties of ZnO nanoparticles were examined. In terms of ethanol gas exposure sensitivity,S=Rgas/Rair is used. Using ethanol at 70 parts per million (ppm), the device is heated to 350°C for the temperature range tested. Figure 10 depicts the gas sensitivity of ZnO films with Al- and Sb-codoped ZnO. Codoped ZnO nanoparticles were discovered to have strong ethanol gas sensitivity. This is in conformity with the findings of [39]. This time around, though, sensitivity was up to 69 percent.Figure 10
Response vs. temperature plot of codoped ZnO sample.Gas sensing application mechanism has been proposed. Electrical resistance is mostly determined by absorbed gas. When the synthesized samples are exposed to the air, oxygen molecules are absorbed, and several ionic species On (O2-, O2, and O2+) are formed [40–43]. It is these ions that contribute to form the host surface depletion layer. The instrument’s sensitivity is enhanced by the presence of these ionic species. When an electron trapped in an oxygen adsorbate is exposed to a donor, the potential barrier decreases and the conductivity rises (reducing gas). One of the most critical components of the gas sensing system to grasp is how test gas molecules are adsorbing and desorbing. Sample surface ionic species are formed. Here, it is evident that, at 205°C, codoped sample provides the best sensing response. The prepared films have lightning-fast response and recovery times of 125 and 166 seconds, respectively. Chemical resistance is a key component of the sensing system, as is chemiresistance [44, 45]. The type of gas molecules and the predominant carriers influence the change in resistance of the metal oxide coating. ZnO’s surface is often coated with oxygen from the atmosphere. ZnO films’ surface electrons can be captured by these oxygen species, forming oxygen species. In order to decrease conductivity, the depletion layer is created by these oxygen species. In contrast, The oxygen adsorbate retains one electron, which returns to the ZnO layer, lowering the potential barrier and increasing conductivity when the sensor is exposed to a reducing atmosphere [46–48]. As a result, gas sensor performance is enhanced.The reaction of the gas sensor increases as the temperature changes. The following is an explanation of the sample’s ethanol detecting process. Adsorption is a surface flaw that results in the formation of ionic species (O2− and O−) on the sample surface. Equations (8)–(10) explain the kinetic reaction before and after ethanol exposure [49].
(8)O2gas↔O2ads,(9)O2ads+e−↔O2−ads,(10)O2−ads+e−↔2O−ads.


## 5. Conclusions

The codoped ZnO thin films had a significant impact on ethanol gas detection properties, as demonstrated in this study. At 210°C, the maximum ethanol sensitivity of 69 percent was obtained. Hexagonal crystal structure can be discovered using XRD on samples with crystal sizes in the nm range. Doping and codoping were shown to modify the surface morphology of the film using SEM. In the 350-380-nm wavelength range, there is a lot of absorption. The crystal size ranges from 14 to 30 nm, whereas the grain size ranges from 25 to 50 nm. Al doping is responsible for the bands near 749 cm-1, while the Al and Sb codopant bands are found near 816 cm-1. The formation was indicated by the peaks around 400-500 cm-1. The band gap of doped ZnO is higher than that of codoped ZnO, according to the optical properties. The findings of this research reveal that the films created have a lot of potential as gas sensors.



---

*Source: 6250706-2022-05-16.xml*