Time-Resolved Sensitivity of a Cadmium-Doped Copper Oxide

Due to its strong affinity for chlorine gas, Cd can potentially form CdCl 2 . Cd-doped CuO thin films are sensitive to chlorine gas, as careful inclusion of Cd in the CuO crystal structure modifies the energy gap depending on the dop - ant concentration. We employed spray pyrolysis to deposit Cd-doped CuO on fluorine tin oxide (FTO) while heat - ing the substrate to 500 °C. The X-ray diffraction (XRD) analysis revealed that Cd was interstitially incorporated in the CuO lattice structure, as verified by scanning electron microscopy (SEM) imaging. The photoluminescence study demonstrated that increasing the Cd concentration in CuO resulted in higher emission intensity, providing valuable insights into Cu 2+ and O 2-energy levels. Exposing a Cd-doped CuO thin film to chlorine gas modifies the bandgap, depending on the Cd concentration. The fluctuation in the bandgap energy of copper oxide doped with cadmium indicates the chlorine gas concentration nearby. Time-resolved measurements for the I-V characteristics of the thin film revealed considerable current variation during the exposure to chlorine gas.


INTRODUCTION
Metal oxides have recently emerged as a captivating area of interest in materials research [1,2].One such oxide is copper oxide (CuO), a ptype semiconductor boasting an optical band gap (Eg) ranging from approximately 1.2 to 1.9 eV.CuO has garnered considerable attention due to its distinct properties, positioning it as a potential candidate for diverse applications like gas sensors [3,4,5], optoelectronic devices [6,7,8], solar cells [9,10,11], supercapacitors [12,13] and cells [14,15].Previous studies have explored the enhancement of CuO chemical properties by investigating the impact of doping transition metals such as Li [16], Ni [17], Zn [18], Mn [19], and Cd [20], among others.However, limited research has been conducted on Cd-doped CuO.Various deposition techniques have been employed by some researchers, including the hydrothermal method [20,21].Additionally, alternative approaches such as the gel method [22], simple chemical precipitation method [23], and spray pyrolysis technique [24] have also been utilized.Among these methods, the spray pyrolysis technique is cost-effective and straightforward since it does not necessitate high-quality substrates.Moreover, this approach provides benefits, like regulation of film placement and the capability to apply films across regions [25,26].
The main objective of this research is to improve the optoelectronic properties of Cd-doped CuO specifically for gas sensing applications.The focus is on incorporating Cd into the crystal structure of CuO in a controlled manner to decrease the energy gap of the thin films, thereby improving their sensing capabilities.The deposition of Cd-doped CuO onto FTO substrates was achieved using the spray pyrolysis technique while heating the substrate to 500 °C to facilitate the introduction of Cd into the interstitial sites.This intentional adjustment in the composition would enable the future detection of chlorine gas.Cd's strong affinity for Cl and its deliberate incorporation selectively disrupt specific bonds in CuO, allowing for the preferential combination of Cd 2+ with any expected Cl 2-.

Chemicals and materials
Copper acetate monohydrate (99% purity) [Cu(CH 3 COO) 2 .H 2 O] and cadmium chloride (laboratory reagent) [CdCl 2 .H 2 O] were employed as the copper and cadmium precursors, respectively.These substances were utilized in their original form without further treatment or modifications.

Samples preparation and materials synthesis
Cd-doped CuO thin films were fabricated using the spray pyrolysis technique on FTO-onglass substrates.Before drying with nitrogen (N 2 ) gas, the FTO glass substrates underwent a cleaning process involving methanol, ethanol, and acetone, then rinsing with deionized (DI) water.The reaction procedure involved dissolving 0.1 M of Cu(CH 3 COO) 2 .H 2 O and CdCl 2 .H 2 O into 40 ml of DI water at room temperature (RT).The solutions were stirred until a homogeneous mixture was achieved.Subsequently, three different Cd concentrations (0.1%, 0.2%, and 0.4%) were prepared.During the deposition process, the distance between the substrate surface and the spray nozzle was adjusted and maintained at approximately 15 cm.The deposition operation involved the controlled flow of the solution from the spray nozzle at a constant rate of around 0.84 ml/min.At the same time, the temperature of the substrate was maintained at 500 °C.After deposition, the fabricated films were allowed for 2 hours to cool down to RT.The specific parameters and conditions employed for preparing Cd-doped CuO thin films are summarized in Table 1.

Gas detection and activation temperature
When investigating the behavior of a film doped with Cd on an FTO substrate as a sensor for chlorine gas, we can observe changes in the sensing response over time and at different activation temperatures [22].
Activation energy -by adjusting the temperature, we can notice variations in the activation energy of the sensing process.The current flowing through the film may demonstrate temperature tendencies, such as increasing or decreasing with rising temperatures.Determining the activation energy allows us to understand how the sensing mechanism works.
Time response -upon exposure to chlorine gas, the current passing through the film may exhibit time-related patterns.Initially, current changes might indicate a response to the gas.However, over time, factors like diffusion, surface reactions, or alterations in the properties of the film could cause stabilization or gradual changes in the current.
Chlorine gas can interact with the interstitially doped Cd in CuO thin film and change the thin film's electrical properties.However, this interaction depends on the activation temperature of the sensor.A ceramic electric heating plate has provided the temperature source with a maximum temperature of 90 °C.

CHARACTERIZATION TECHNIQUES
The crystal structure of Cd-doped CuO thin films was characterized using X-ray diffraction (XRD) analysis.The XRD measurements were performed on an Ultima IV instrument (Rigaku, Japan) equipped with Cu-Kα radiation (wavelength = 1.54060Å) and operated at a voltage of 40 kV.The obtained XRD data was processed using MATCH!Software from Crystal Impact.A UV-visible-near-infrared (UVvis-NIR) spectrophotometer (Lambda 750, PerkinElmer) was employed to inspect the optical properties of the films.The photoluminescence emission spectrum was obtained at room temperature using a Spectro fluorophotometer (RF-5301pc, Shimadzu).A field emission scanning electron microscope (Quanta 250, FEI, A constant DC voltage of 1.1 V was maintained during the gas detection with a base current of 0.759 A. The sensor activation temperature was provided using a ceramic electric heating plate operated by 220 V AC power.

XRD investigation
The study of Cd-doped CuO at Cd concentrations 0.1%, 0.2%, and 4% revealed the XRD pattern displayed in Figure 1 The decrease in intensity of the CuO diffraction lines can be attributed to the Cd diffraction centers' dominance over CuO [28,29].The basic Scherrer equation [27] (1) estimated the crystallite size G.Where k is Scherrer constant and is taken at 0.9, FWHM is the full width at half maximum of the most intense peak.The d-spacing and the grain (crystallite) size were influenced by the increase in Cd concentration and are listed in Table 2.A decrease in the dspacing was observed as the Cd concentration increased.This reduction in interplanar spacing is attributed to incorporating Cd between the atomic planes, resulting in strain within the crystalline structure.However, it should be noted that the grain size exhibited a slight decrease as the Cd concentration increased.There could be a few reasons for the slight reduction in grain size with higher Cd concentration in Cd-doped CuO thin films.In "Crystal Growth Inhibition" [30], Cd atoms in the CuO lattice may inhibit the crystal growth, leading to smaller grain sizes.In addition, Cd impurities may alter the surface energy [31,32] of the growing film, affecting the grain boundaries and resulting in smaller grain sizes.Higher Cd concentrations may also intensify film stress and strain, impeding grain growth and leading to smaller grain sizes.

𝐺𝐺 = 𝑘𝑘 𝜆𝜆 𝐹𝐹𝑊𝑊𝑊𝑊𝑊𝑊 cos 𝜃𝜃
(1) ( where: G is the grain size, FWHM is the full width at half maximum of the selected maximum at the specified Muller indices (hkl), k is the shape factor taken 0.9, 2θ is the diffraction angle as a function of (hkl), and λ is the X-ray wavelength (1.5406 Å).

SEM and surface morphology characterization
The examination of surface morphology revealed that the Cd ions did not replace the Cu ions within the structure of CuO.Instead, they occupied interstitial positions, evidenced by surface characterization of the thin film.Cd manifested as bright grains on the surface, in contrast to CuO.Furthermore, the investigation of surface properties indicated that the average particle size for CuO: Cd 0.0.1% was approximately 155 nm ± 15.For Cd 0.2%, the average particle size increased to around 275 nm ± 18, while for CuO: Cd 0.4%, the average particle size was approximately 376 nm ± 20.The top surface views of the thin film for each of these sizes can be observed in Figure 2A, 2B, and 2C, while an inclined view at a 50° angle to the surface is depicted in Figure 2D, 2E, and 2F.Overall, the thin film surface displayed high uniformity and homogeneity.

Elemental verification
By employing an energy-dispersive X-ray (EDX) technique, we could analyze a specific film region and determine the elemental composition of Cd-doped CuO.The EDX investigation showed an absorption edge at 3.134 keV, corresponding to the L α transition.The elemental composition analysis confirmed that the Cd-doped CuO consisted solely of the targeted elements.The elements ratios were 30.02,65, and 4.98 wt% for oxygen, copper, and cadmium, respectively.

Optical characterization
Transmittance spectroscopy was conducted on thin films of Cd-doped CuO to investigate their optical characteristics at the underlying Cd concentrations.Figure 3A illustrates notable variations in transmittance among the three films.It is  (2) (2)   where: K is an energy-independent constant, a is the absorption coefficient, h is the Planck constant, Eg is the bandgap energy, and n indicates the direct or indirect bandgap energy.n=1/2 for allowed indirect bandgap and is 2 for allowed direct bandgap It revealed that the Cd-doped CuO thin films exhibited direct band gaps, with values of 1.61 eV, 1.45 eV, and 1.29 eV for Cd concentrations of 0.1%, 0.2%, and 0.4%, as seen in the Figure 3B.
Interestingly, although Cd does not replace the Cu ions within the CuO crystal structure, the Cd introduction reduced the thin film's band gap energy.The material's electronic structure is modified by introducing Cd atoms, which have electronic properties different from Cu, into the CuO crystal lattice.The Cd impurities introduced new energy levels within the band structure.These new energy levels created by Cd atoms can lower the conduction band's energy, raise the valence band's energy, or both [33].As a result, the band gap energy of the Cd-doped CuO thin film is reduced compared to the pure CuO material.

Photoluminescence characterization
The investigation of photoluminescence in Cd-doped CuO thin films demonstrated that the presence of Cd in CuO crystal lattice with increasing concentration enhanced the emission intensity of the film.This increase in emission intensity can be observed in Figure 4A.The emission intensity enhancement by adding more Cd to the CuO structure can be ascribed to defect-related emission centers [34].Cd impurities can introduce defect states or vacancy sites within the CuO crystal lattice.These defect sites can serve as emission centers, promoting radiative transitions and contributing to the enhanced emission intensity observed in the film.In a careful study of the deconvoluted photoluminescence spectra for the Cd concentration of 0.1%, a single emission line belonging to Cu 2+ was observed at approximately 382.60 ± 2.0 nm.This emission corresponds to the transition 0 [213311] 7/2 → 1 [239440] 7/2 [35].Also, an emission line of O 2-was observed at approximately 431.78 ± 3.5 nm, corresponding to an energy level of 2.87 eV in the conduction band.Adding more Cd concentration to the structure changed the emission line's position for oxygen ions.In addition, two Cu 2+ emission lines were observed at 381.35 ± 0.1 nm and 392.35 ± 0.1 nm for a Cd concentration of 0.2%.For a Cd concentration of 0.4%, the Cu 2+ emission lines were observed at 381.6 ± 1.0 nm and 393.85 ± 1.6 nm.The shifts in Cu and O emission lines are attributed to the increase in Cd concentration.The emission lines [36] and their variations are listed in Table 3.The emission lines due to Cu 2+ and O 2-are depicted in Figure 4B, 4C, and 4D.The emission centers of Cu 2+ and O 2-are responsible for the emission enhancement in Figure 4A.

Gas detection by Cd-doped CuO thin film
The heating plate has been attached to the glass side of the FTO-on-glass substrate using thermal paste, as seen in Figure 5A and 5B are contact probes.A constant DC voltage of 1.1 Volts was applied to the thin film, and a minimum current of 0.759 A passed through the thin film simultaneously.Two CASSYLAB modules were used to supply the biasing voltage and to record the current with time, and the 2 nd module was used for measuring temperature with time.The composition was put into an electrochemical cell with a cap providing a closed circumference with an inlet switch for the gas supply.
Starting with an activator temperature of 27°C and at a running time of about 50 s, the activator power supply was started simultaneously with the running time.The system recorded the current against running time, and a thermocouple  recorded the accompanying activator temperature.We allowed the gas to flow constantly during a specific time interval and cut it off.We repeated the on-off gas allowance routine three times, deonted in the Figure 6 by thee-ONs, until the activator temperature reached its maximum value, and then we stopped the run.The timetemperature-gas flow current dependence was recorded and displayed in Figure 6.In addition, the current intensity exhibited an activation temperature dependence.The sensor's responsivity to the gas flow increased as the activation temperature increased.The response time can be approximately 5.4 seconds, and the recovery time can be 10 seconds.
It can be inferred that the Cd concentration of 0.4 in the CuO matrix is the most effective in detecting or responding to the specific gas being tested, the maximum response was about 0.002 A. This finding suggests that the higher concentration of Cd in the CuO material enhances its gassensing properties, potentially due to increased surface area or improved interaction with the gas molecules.Other lower Cd concentrations did not give a response to the gas, so they were not included in the results.

CONCLUSIONS
The deposition of Cd-doped CuO on an FTOon-glass substrate was successfully achieved using the spray pyrolysis deposition method at a temperature of 500 °C.X-ray diffractio analysis confirmed that Cd did not replace Cu in the CuO structure but occupied interstitial sites within the lattice.The scanning electron microscopy technique was employed to investigate and analyze the surface morphology and thickness of the Cddoped CuO films, revealing uniform thickness and a homogeneous composition across the films.
UV-visible spectroscopy was employed to investigate the absorption properties of the Cddoped CuO films with varying concentrations of Cd.It was observed that the absorption decreased as more Cd was added to the CuO, indicating changes in the material's light absorption characteristics.Additionally, the band gap energies of the Cd-doped CuO films decreased as the Cd concentration increased.
Photoluminescence investigations were conducted, and it was found that the emission intensity increased with higher Cd concentrations in the CuO films.The positions of the emission lines associated with Cu 2+ ions were not preserved upon adding Cd to the thin films.
These findings suggest the potential use of Cd-doped CuO films as gas sensors, particularly for detecting chlorine gas.Cd exhibits a strong affinity for chlorine gas, resulting in variations in the energy gap of the material.These variations can be seen by an external I-V circuit, highlighting the potential of Cd-doped CuO films for gas sensing applications.
The lower concentrations of Cd showed no response to the gas passage.However, the 0.4 Cd concentration revealed a noticeable influence on the Current passing through the thin film, indicating the suitable dopant concentration for this sensor to be affected by the chlorine gas.
, with the FTO diffraction pattern subtracted.According to the Crystallography Open Database card COD # [96-410-5686], the Cd-doped CuO unit cell is monoclinic.No phase change occurred with the addition of more Cd concentrations.However, the CuO diffraction lines (110), (002), (111), and (021), which appeared at the diffraction angles (2θ = 33.25,35.76, 38.59, and 55.61°), had their positions slightly red-shifted due to the interstitial corporation of more Cd into the CuO crystal.Additionally, the diffraction intensities decreased by increasing Cd concentration.

Figure 1 .
Figure 1.The X-ray diffraction (XRD) pattern of thin films of CuO doped with different concentrations of Cd (0.1%, 0.2%, and 0.4%) was measured at room temperature

Figure 3 .
Figure 3. (A) The transmittance spectrum of the underlying Cd concentrations, measured at roome temperaturej, (B) Tauc plot for the Cd-doped CuO thin films

Figure 4 .
Figure 4.The PL spectra, measured at room temperature, of the Cd-doped CuO thin films

Figure 5 .Figure 6 .
Figure 5. Schematic of the Cd-doped CuO thin film deposited on FTO with ceramic heating plate attached

Table 1 .
Deposition parameters of Cd-doped CuO thin films FP 2012/14) examined the surface morphology of the prepared films.Two Leybold Didactic GMBH sensor-CASSY were used for current I versus time t and temperature T measurements.

Table 2 .
The Cd-doped CuO lattice parameters

Table 3 .
The emission lines, obtained at room temperature, of the Cu 2+ and O2-