Cost-Effective Method for Dissolved Oxygen Sensing with Electrodeposited n-Cu₂O Thin-Film Semiconductors

Abstract

Dissolved oxygen (DO) is a crucial parameter in water quality monitoring because it directly affects the health of aquatic ecosystems. This study explored electrodeposited Cu₂O thin-film semiconductors for DO sensing. Cu₂O was chosen for its low cost, eco-friendliness, and non-toxic nature. Cu₂O films were electrodeposited on titanium (Ti) substrates using an acetate bath (0.1 M sodium acetate and 0.01 M cupric acetate) at −200 mV versus Ag/AgCl for 30 min, with a bath temperature of 55 °C, stirred at 50 rpm. The bath pH was systematically adjusted from 5.8 to 6.8 in 0.2 steps using NaOH and Acetic acid. A range of analyses including synchrotron X-ray diffraction (SXRD), scanning electron microscopy (SEM), surface wettability, capacitance–voltage (C-V), Raman spectroscopy, Fourier-transform infrared (FTIR) spectrum, and Electrochemical Impedance Spectroscopy (EIS) was performed to assess their properties and sensing performance. The results showed that Cu₂O films deposited at pH 6.4 exhibited optimal performance for DO sensing, with a strong linear response, marking this pH, deposition time, and temperature as ideal for creating effective DO sensors. This study introduces a novel, cost-effective approach to dissolved oxygen sensing using electrodeposited n-Cu₂O thin-film semiconductors, marking the first application of this material in such sensors and showcasing its potential for scalable and environmentally sustainable sensing technologies.

1. Introduction

The development of advanced sensing technologies is important for addressing water quality challenges [1,2]. Among the critical parameters for assessing water quality, dissolved oxygen (DO) plays a crucial role in maintaining the health of aquatic ecosystems as well as the health of humans [3]. The concentration of DO in water indicates pollution levels and the presence of organic contaminants [4]. There are several types of DO sensors based on the sensing mechanism and these are optical, electrochemical, iodometric titration, and other sophisticated methods [5,6,7]. Optical sensors include fluorescence [8,9], phosphorescence [10,11], and optode [12] sensors which work by identifying changes in the properties of light because of interaction with DO. Galvanic (Clark) [13,14], polarographic [15,16], and amperometric [17] sensors are types of electrochemical sensors that work based on electrochemical reactions to determine DO concentration. The iodometric method also known as Winkler titration is the most used technique for DO testing because it was the first to use a chemical test method [18,19]. The Winkler titration is a classical wet-chemical method that is known to be very accurate [20]. These traditional sensors are often limited by high costs and maintenance scale, and are less ideal for continuous real-time monitoring, driving the need for innovative, cost-effective alternatives. According to the literature, the global market for gas sensing devices is projected to experience significant growth, with an estimated market value of USD 4.7 billion by 2030 [21]. This surge in demand is primarily driven by stricter environmental regulations that are being set across the world. However, traditional sensors may experience some difficulties in retaining their market leadership because of such trends as the development of the Internet of Things (IoT) and the miniaturization of sensor technology.

In recent times, there has been a growing demand for metal oxide thin-film sensors due to their exceptional properties, such as high sensitivity, stability, and versatility in various sensing applications [22]. In previous work, Wittkampf et al. developed a silicon thin-film sensor for measuring dissolved oxygen, utilizing a Clark-type electrochemical sensing mechanism [14]. Their study demonstrated the feasibility of using semiconductor materials, particularly silicon thin films, for precise and reliable dissolved oxygen detection, marking a significant advancement in sensor technology. This work laid the groundwork for exploring alternative semiconductor materials for similar applications. Additionally, Gillanders et al. developed a composite thin-film optical sensor for measuring dissolved oxygen in contaminated aqueous environments, demonstrating the adaptability of thin-film technology in addressing the challenges posed by complex, polluted water systems. Their work highlights the potential of optical sensing techniques, in combination with thin-film materials, to offer high-performance sensors capable of operating in harsh and variable environmental conditions, thereby broadening the scope for practical applications in real-world scenarios [23]. According to the literature, metal oxide thin-film semiconductors are not often used for dissolved oxygen sensing. Moreover, there are no standard approaches for employing Cuprous Oxide (Cu₂O) thin-film semiconductors for this purpose. This gap in research presents a significant opportunity for innovation, and this makes the research on Cu₂O thin-film sensors for DO sensing highly valuable. Cu₂O is a promising material in the field of semiconductor technology, known for its distinctive properties and wide range of applications such as solar cells [24,25]. Cu₂O has gained significant attention as a semiconductor with a direct band gap of approximately 2.1 eV for its potential use in various optoelectronic devices, sensing devices, and thermoelectric devices [26,27,28,29]. Because of the Cu vacancies that were produced in the lattice structure, Cu₂O was referred to as a p-type material until 1986 [30,31,32]. Siripala et al. first reported on the feasibility of n-Cu₂O growth in 1986 using the electrodeposition approach in an acetate bath [33]. Oxygen vacancies are inherently present in n-type materials. These vacancies act as active sites for adsorption.

In this study, the synthesis and characterization of Cu₂O thin films were investigated with the view of enhancing the DO sensing properties. Cu₂O was synthesized as n-type and investigated to better understand its potential for DO sensing applications. Cu₂O can be synthesized through various techniques, including chemical vapor deposition, sol-gel processes, and electrodeposition. Chemical vapor deposition (CVD) is a process where Cu₂O is formed by the reaction of gaseous precursors on a substrate which provides high purity and uniformity of coatings with good adhesion. However, CVD is generally carried out at high temperatures and requires sophisticated equipment and hence is not well suited for large-scale or low-cost applications [34]. The sol-gel process involves the preparation of a gel from a solution of metal precursors, followed by conversion of the gel to Cu₂O through heat treatment. This method offers good compositional control and can be used on a range of substrates; however, it is generally multistep and can be restricted by the need for high-temperature calcination to form the desired crystalline phase [35]. Therefore, electrodeposition was chosen as the synthetization method because it is an easy and inexpensive method that allows precise control of the film thickness and morphology [36]. The study looked at the possibility of improving the DO sensing properties of these films by changing the bath parameters systematically. This approach was intended to optimize the material characteristics for enhanced sensor characteristics. For the first time, this study demonstrates that n-Cu₂O thin films can be effectively utilized for DO sensing. Unlike traditional approaches, n-Cu₂O thin films have shown a distinct linear response to changes in DO concentration, establishing their potential as viable and sensitive materials for DO detection. This finding introduces a new application for thin-film technology in electrochemical sensing, underscoring the novel capabilities of n-Cu₂O in this domain. The potential to develop a new type of sensor in this area underscores the importance of the research outcomes, as they could contribute significantly to advancements in both sensor technology and water quality monitoring systems.

2. Materials and Methods

The titanium (Ti) substrates used for electrodeposition were well polished and cleaned using the following steps. Initially, the substrates of size 2 cm × 1 cm were mechanically polished using sandpapers of different grits: 600 and 1200, respectively. The two-step polishing process was aimed at not only improving the surface roughness but also the effective surface area so that better adhesion and uniformity of the subsequently deposited thin film could be achieved. After the polishing process, they were washed with a diluted mild detergent, diluted hydrochloric acid (HCl), and Deionized (DI) water, respectively, to ensure that all the organic impurities and acid residues were washed off the substrates. To further enhance the cleanliness of the substrates, the substrates were then ultrasonicated in a diluted HCl solution for 15 min. The electrodeposition of n-Cu₂O thin films was conducted using a well-defined and controlled procedure. An acetate bath of 100 mL was used as the electrolyte which contained 0.1 M sodium acetate (Sigma–Aldrich, St. Louis, MO, USA, anhydrous, purity ≥ 99.0%) and 0.01 M cupric acetate (Sigma–Aldrich, St. Louis, MO, USA, monohydrate, purity ≥ 99). The deposition was carried out in a three-electrode electrochemical cell with the Ti substrates used as the working electrode. The working electrode potential was maintained at −200 mV with respect to a saturated Ag/AgCl reference electrode using a Hokuto Denko Potentiostat/Galvanostat (HA-151B, Hokuto Denko, Atsugi, Kanagawa, Japan). For the preliminary study, the temperature was systematically varied from 5 °C to 75 °C with steps of 10 °C, and the deposition period was varied from 15 min to 90 min with 5 min intervals. From the initial investigations, it was observed that samples deposited at a bath temperature of 55 °C with a deposition period of 30 min showed better performance for DO. Therefore, for further studies, the deposition bath pH was systematically varied in a range of 5.8 to 6.8 with steps of 0.2 using Sodium Hydroxide (NaOH) (Sigma–Aldrich, St. Louis, MO, USA, purity ≥ 98.0%) and Acetic acid. The pH was measured using a HANNA edge pH meter (HI2020-01, Hanna Instruments, Woonsocket, RI, USA). The bath temperature was maintained at 55 °C, and the deposition time was fixed at 30 min. The acetate bath was continuously stirred at 50 rpm using a Velp heating magnetic stirrer (AREC-X, VELP Scientifica, Usmate Velate, Italy). The platinum electrode was used as the counter electrode. Finally, the samples were washed with DI water immediately after going through the deposition process and then air dried.

Initially, to investigate the successfulness of synthetization of the material, Synchrotron X-ray diffraction (SXRD) measurements were performed. To obtain SXRD measurements, electrodeposited Cu₂O thin films were first carefully peeled off from the titanium substrates, and at room temperature (25 °C) were placed into quartz capillary tubes with an inner diameter of 1 mm. The SXRD analysis was then carried out using beamline BL04B2 in a third-generation synchrotron radiation facility, SPring-8, Hyogo, Japan. For the measurements, incident X-rays with a wavelength of 0.2017 Å equivalent to an energy of 61.46 keV were generated using a silicon (220) monochromator. The intensity of the incoming X-ray beam was monitored by an ionization chamber filled with high-purity argon gas (99.99%), ensuring stable beam quality. To prevent X-ray scattering in air, the samples were held in a vacuum-sealed bell jar. Diffracted X-rays were captured across a wide angular range from 0.3 to 57° using four CdTe detectors and three Ge detectors installed in a two-axis diffractometer at BL04B2, facilitating comprehensive data collection. In a two-electrode system, resistance measurements were made on prepared Cu₂O thin films under ambient conditions in 100 mL water volume, with aeration of oxygen at room temperature. Samples were allowed to stabilize for 10 min prior to aerating oxygen. The counter electrode was made from a platinum plate. The performance of the synthesized Cu₂O thin-film sensors was validated and compared with a commercial DO sensor from Atlas Scientific (Long Island City, NY, USA). This patented sensor is based on Arduino, and the design of specific additional circuits was required to connect it to the system as shown in Figure 1. Furthermore, a program was developed to retrieve the DO readings directly to the computer’s COM port. The serial UART communication protocol was used for data transfer, ensuring accurate and real-time monitoring of the DO levels during the experiments. Surface wettability measurements were assessed to examine the morphology of the samples. Furthermore, measurements of capacitance–voltage (C-V) were obtained using the computer-integrated Gamry series G300 potentiostat to determine the type of conductivity and carrier concentration at the voltage range of −0.8 V to −0.4 V. Electrochemical Impedance Spectroscopy (EIS) was performed to investigate the interfacial properties at n-Cu₂O/DI water interface before and after DO sensing. Images captured by scanning electron microscopy (SEM) were analyzed to obtain changes in morphology and evaluate the effects on the film surface after DO sensing. Raman spectroscopy was used to examine the n-Cu₂O thin films’ structural and vibrational characteristics. The Fourier-transform infrared (FTIR) spectrum was used to identify the bonds before and after DO sensing.

3. Results and Discussion

3.1. Synchrotron X-Ray Diffraction (SXRD) Measurements of the Substrate Prior to DO Sensing

Synchrotron X-ray diffraction (SXRD) measurements were obtained for electrodeposited n-Cu₂O to analyze the properties of the deposited material prior to dissolved oxygen sensing. According to Figure 2, the SXRD patterns exhibit only the expected peaks at 2θ = 3.86°, 4.69°, 5.41°, 7.66°, 8.99°, and 9.39°, corresponding to the (110), (111), (200), (220), (311), and (222) planes, respectively, for n-Cu₂O without any evidence for secondary phases such as CuO, Cu, or Cu(OH)₂. This confirms that the high-purity single-phase, polycrystalline Cu₂O thin films were successfully synthesized on the Ti substrate.

3.2. Resistance Variation

In order to observe the response for DO, the variation in resistance was measured with different DO concentrations. The sensing ability of the n-Cu₂O thin films deposited at different bath pH levels, investigated by monitoring the resistance changes of the films exposed to different concentrations of DO, is shown in Figure 3. Notably, the thin film deposited at pH 6.4 demonstrates the highest slope, approximately 45 degrees, indicating a strong linear correlation between DO concentration and resistance. This suggests that the thin film deposited at pH 6.4 for a period of 30 min with a deposition bath temperature of 55 °C exhibits the highest sensitivity to DO, as evidenced by its greater resistance change per unit of DO concentration.

According to the literature, n-type Cu₂O thin films can be successfully deposited in acetate baths within the selected pH range. Beyond this range, particularly at lower pH values, Cu is deposited instead of Cu₂O, while at higher pH levels, Cu₂O films show p-type properties [37,38]. In this research, we observed that the p-Cu₂O thin films did not exhibit a noticeable response to dissolved oxygen, while the resistance of the n-type Cu₂O thin films increased significantly upon exposure to dissolved oxygen. As a result, our study focused on n-type Cu₂O thin films, and all subsequent characterizations were carried out using these films. Therefore, the bath pH was maintained between pH 5.8 and pH 6.8 for optimal sensor performance.

3.3. Wettability

To study the surface morphology of the samples, the surface wettability of the electrodeposited Cu₂O thin films was evaluated through the sessile drop method [39]. In this technique, a syringe was employed to dispense water droplets onto the surface of the sample. Water droplets were allowed to settle on the surface of the film for approximately 15 min prior to measurement. The contact angle was then averaged from measurements taken from three individual droplets on each sample surface. A high-resolution digital microscope was used to capture images of the droplets on the surface. Once the images were taken, contact angles were measured using Hiview Software 2.0. According to the literature, a surface is classified as ‘wetting’ if the contact angle formed by the liquid is less than 30°. Surfaces with contact angles between 30° and 89° are considered ‘partially wetting’, while surfaces with contact angles of 90° or greater are categorized as ‘non-wetting’ [24,40]. Figure 4 revealed that samples deposited at bath pH 6.4 exhibited the lowest contact angle, indicating superior wettability suggesting higher surface energy, and greater interaction between the surface and water. This indicates that the surface becomes more hydrophilic under these conditions, likely due to the formation of a smoother and more uniform layer of Cu₂O [41]. As a result, the enhanced wettability at pH 6.4 contributes to improved interaction with aqueous environments, positively impacting the performance of the thin film as a DO sensor. It was observed that after DO sensing, the contact angle decreased across all pH levels. Notably, the most significant reduction in contact angle was recorded at pH 6.4.

3.4. Capacitance–Voltage (C-V) Measurements (Mott–Schottky)

The C-V measurements were obtained in order to study the conductivity type and doping density of the material before DO sensing and after DO sensing in a photo-electrochemical cell containing 0.1 M sodium acetate aqueous electrolyte. Figure 5a,b show a graph of C-V measurements, which provide a positive gradient, confirming the n-type conductivity of synthesized Cu₂O before and after DO sensing, respectively, without changing the conductivity type of Cu₂O during the sensing mechanism. The Mott–Schottky relation is expressed as follows:

$${\displaystyle \frac{1}{c_{sc}^2}}={\displaystyle \frac{2}{e\epsilon {\epsilon }_0{A}^2{N}_D}}\left(V-V_{fb}-{\displaystyle \frac{kT}{e}}\right)$$

(1)

In this equation, $c_{sc}$ is the capacitance of the space charge region, $\epsilon$ is the dielectric constant of the semiconductor, ${\epsilon }_0$ is the permittivity of free space, A is the area of the working electrode, $N_D$ is the doner density, V is the applied potential, V_fb is the flat band potential, k is the Boltzmann constant, T is the absolute temperature, and e is the electronic charge. By obtaining the gradients, the donor densities were also calculated and those were 4.92421 × 10¹⁶ and 5.7581 × 10¹⁵ before and after DO sensing, respectively.

For resistive oxygen sensors, the sensing mechanism directly depends on the type of conductivity of the sensor material. For n-Cu₂O materials, oxygen species adsorb onto the material surface, and bond by taking free electrons from the material during the time of the sensing mechanism. Hence, this reduces the donor density of n-Cu₂O material after DO sensing. Consequently, it leads to the formation of an electron depletion region with a high potential barrier due to the different relative Fermi level positions of n-type material and the redox potential of oxygen-dissolved DI, as shown in Figure 6a,b. The formation of this type of potential barrier at the n-Cu₂O material/oxygen-dissolved DI interface impedes the movement of free carriers via those interfaces, increasing the resistance of the material. In contrast to the flat band situation, the upward band bending is caused by the negative charge trapped through the n-Cu₂O/oxygen dissolved DI interface. When the oxygen concentration of DI water is increased further, the redox potential is also further increased. Simultaneously, more oxygen is absorbed onto the sensor material surface, leading to an increase in the energy gap between the n-Cu₂O semiconductor material and the redox potential of oxygen-dissolved DI water. Then, the potential barrier at the n-Cu₂O/oxygen dissolved DI interface further increased, while increasing the resistance.

3.5. Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) measurements were performed for the investigation of interfacial properties at the n-Cu₂O/DI water interface, as shown in Figure 7. The semicircle of Nyquist plots is related to the formation of the space charge layer at the n-Cu₂O/oxygen-dissolved DI water interface due to the adsorption of oxygen species onto the surface as discussed in the previous section. According to the Nyquist plots, a semicircle was observed before DO sensing because of the potential barrier (band bending) at the n-Cu₂O/DI water interface due to already dissolved oxygen in DI water. However, the semicircle behavior of the Nyquist plot increased significantly because of the further increase in the potential barrier due to the attachment of more oxygen species onto the surface when the oxygen concentration is further increased. As a result, it can be observed that overall resistance has increased.

3.6. Scanning Electron Microscope (SEM)

To further study the surface morphology of the samples, Scanning Electron Microscope images were observed. Surface morphology is one of the most important parameters that influence the performance of sensing properties of thin-film semiconductors. Figure 8 shows the SEM images obtained for the optimum Cu₂O sample before DO sensing and after DO sensing. It is evident that the films produced are homogeneous, polycrystalline, and have good coverage. The SEM images clearly reveal that before exposure to DO, the samples exhibit well-defined shapes with distinct edges, indicating a uniform and ordered structure. However, after DO sensing, the samples appear to have a more densely packed structure. These changes in morphology may be attributed to the formation of hydroxyl (OH) bonds and the adsorption of oxygen species onto the surface of the Cu₂O thin films.

3.7. Synchrotron X-Ray Diffraction (SXRD) for Post-Sensing DO

Synchrotron X-ray Diffraction (SXRD) analyses were conducted on samples synthesized under optimal conditions and analyzed both before and after DO sensing. As shown in Figure 9, the SXRD spectra prominently display the characteristic peaks of Cu₂O. In addition, peaks corresponding to CuO are also observed. In the Cu₂O structure, copper exists in the Cu⁺ oxidation state, whereas in CuO, copper transitions to the Cu²⁺ oxidation state. This shift from Cu⁺ to Cu²⁺ occurs as oxygen adsorbs the material, leading to the formation of a small amount of CuO. This confirms the creation of CuO bonds during the sensing process.

3.8. Raman Spectroscopy

To analyze the structural and vibrational properties of the Cu₂O thin films, Raman spectroscopy was employed before and after DO sensing. The Raman spectra in Figure 10 exhibited consistent peaks at 109.0, 148.0, 218.0, 416.0, and 630.0 cm⁻¹, both with and without exposure to DO samples. The peak at 109 cm⁻¹ corresponds to the translational vibrations of Cu atoms in the lattice, while the peak at 148 cm⁻¹ is attributed to Cu–O bond vibrations. Peaks at 218 and 416 cm⁻¹ are associated with various phonon modes of the Cu₂O crystal structure, and the peak at 630 cm⁻¹ represents stretching vibrations of the Cu₂O lattice [42,43,44,45]. Any shifts in peak positions or new peaks were not identified, clearly indicating that the chemical composition of the Cu₂O thin films remained unchanged after DO sensing. This confirms that no oxidation to CuO (which would appear around 280 and 330 cm⁻¹) occurred during the sensing process. This consistency in the Raman spectra demonstrates that Cu₂O retains its phase purity and does not undergo chemical degradation, making it a stable material for DO sensing applications.

3.9. Fourier-Transform Infrared (FTIR)

The Fourier-transform infrared (FTIR) spectrum of Cu₂O was acquired using the KBr pellet technique to verify the presence of several bonds following DO sensing. The analysis was conducted over a wavenumber range of 4000 to 400 cm⁻¹. Figure 11 shows the FTIR spectra of Cu₂O samples before and after DO exposure. The characteristic peak at 623 cm⁻¹ corresponds to the stretching vibrations of the Cu-O bond [46]. The presence of this vibrational mode is consistent with the expected structural properties of Cu₂O, further validating the electrodeposition process and confirming the integrity of the copper oxide phase in the synthesized samples. Peaks in the 3000 to 3700 cm⁻¹ range in the spectrum after DO sensing and in the 3000 to 3550 cm⁻¹ range in the spectrum before DO sensing are attributed to the stretching vibrations of the OH⁻ group from adsorbed water molecules [47]. The presence of these peaks in both before and after DO is likely due to the high surface area of the nanostructured materials, which promotes the absorption of atmospheric moisture. However, the extended range in the spectrum after DO sensing indicates an increase in OH⁻ stretching vibrations. The peak at 1390 cm⁻¹ in the prior to DO sensing graph can be attributed to the presence of carbonate (CO₃²⁻) species, likely due to the adsorption of atmospheric carbon dioxide (CO₂) onto the surface of the thin films, leading to the formation of copper carbonate (CuCO₃) or related species. However, these surface species do not adversely affect the practical functionality of the sensor. The performance of the sensor is predominantly governed by the interaction between dissolved oxygen and the Cu₂O thin film during the sensing process. This suggests that exposure to air may have influenced the surface chemistry of the films. After the DO sensing process, the FTIR spectrum reveals that the peak at 1390 cm⁻¹, previously attributed to the carbonate (CO₃²⁻) peak, is no longer present. This suggests that the carbonate species have been either removed or reacted during the sensing process. The exposure to DO, along with the interaction with water, may have resulted in the dissolution or displacement of the carbonates. Additionally, it is possible that surface reactions involving DO led to the degradation or transformation of the carbonate species, contributing to the absence of this peak in the post-sensing spectrum. The peak observed at 1638 cm⁻¹ in the post-DO sensing spectrum is indicative of the H-O-H bending vibrations of adsorbed water molecules. This indicates the presence of moisture on the film surface, which may have been adsorbed during the DO sensing process. Moreover, the appearance of more peaks in the 1400 to 1700 cm⁻¹ range in the post-DO sensing spectrum strongly emphasizes the increased formation of Cu–OH bonds after DO exposure [48,49]. This further confirms the interaction of adsorbed water molecules with Cu atoms during the DO sensing process. Additionally, the film may undergo surface modifications due to the formation of hydroxyl (OH) bonds. When the film is in contact with both water and oxygen, Cu(OH)₂ may form at the surface. Cu(OH)₂ is an insulating compound, and its presence creates a barrier that impedes electron flow, contributing further to the observed increase in resistance. However, this process predominantly affects the surface layer of the film, resulting in surface morphological changes [50].

For single-use processes, traditional sensors are not ideal as they must be directly inserted into the system, increasing the risk of contamination and leakage. The solution to this issue is the use of single-use sensors, which significantly reduce the risk of contamination and eliminate the need for cleaning and sterilization between uses [51,52,53]. The thin-film DO sensors in this study are designed for single-use applications, functioning similarly to disposable diabetic test strips. Once the sensor is exposed to DO, it undergoes surface and structural changes necessary for detection and cannot revert to its initial state. This single-use characteristic limits the sensor’s ability to monitor DO over extended periods. Despite this limitation, these thin-film sensors are still cost-effective as the low-cost fabrication process of these sensors offers a significant advantage. To evaluate the performance of the fabricated sensor, a comparison was made with a commercially available patented Atlas Scientific dissolved oxygen sensor. The results showed that the Cu₂O films deposited at pH 6.4 exhibited nearly identical readings for dissolved oxygen measurements. Based on this comparison, we can confidently conclude that our grown samples can be effectively utilized as dissolved oxygen sensors for commercial applications. Compared to commercially available DO sensors, which are often costly and involve complex instrumentation, these disposable thin-film sensors offer a significant advantage in terms of both affordability and simplicity. This cost-effectiveness, combined with their ease of fabrication, positions these sensors as a practical and accessible alternative for situations where large-scale deployment or frequent sensor replacement is required, such as in environmental water quality assessments or spot checks.

This study presents a groundbreaking approach to dissolved oxygen sensing using electrodeposited n-Cu₂O thin-film semiconductors, a novel and cost-effective method that has not been explored in previous research. The innovative aspect of this work lies in the development of Cu₂O thin films through electrodeposition, a process that allows for precise control of material properties and is highly scalable. While numerous techniques exist for measuring dissolved oxygen, ranging from electrochemical sensors to optical methods, the use of thin films for this purpose remains relatively unexplored. In particular, there is no prior evidence in the literature regarding the application of Cu₂O thin films for dissolved oxygen sensing.

Cu₂O, with its semiconducting properties and environmentally friendly characteristics, presents a promising material for developing innovative sensing platforms. This study represents a pioneering effort to utilize Cu₂O thin films for dissolved oxygen detection. Through systematic optimization of the deposition process, including precise control of the pH in the acetate bath, the study successfully fabricated stable, high-quality n-Cu₂O thin films with excellent structural and electronic properties tailored for sensing applications. The remarkable response of these thin films, particularly the significant increase in resistance upon exposure to dissolved oxygen, showcases their potential for low-cost, reliable sensing applications.

The findings highlight the potential of n-Cu₂O thin films as effective and reliable sensors for dissolved oxygen, filling a critical gap in the field. This research not only addresses the lack of methods employing Cu₂O thin films for dissolved oxygen sensing but also sets the foundation for future development of advanced, cost-effective, and sustainable sensing devices. These discoveries represent a significant advancement in sensor technology and open new possibilities for commercial and environmental monitoring applications.

4. Conclusions

In conclusion, electrodeposited n-Cu₂O has proven to be a cost-effective solution for DO sensing in water quality monitoring. In this study, initially, Synchrotron X-ray diffraction (SXRD) analysis confirmed that the fabricated Cu₂O thin films are single-phase and polycrystalline, while Mott–Schottky plots validated their n-type conductivity. Resistance measurements in a two-electrode system identified optimal conditions for DO sensing, with results showing a linear increase in resistance corresponding to higher DO concentrations. This resistance change is attributed to band bending at the Cu₂O surface when exposed to DO. Surface wettability measurements revealed that films grown under optimal conditions demonstrated superior hydrophilicity compared to other samples, enhancing their interaction with aqueous environments. Electrochemical Impedance Spectroscopy (EIS) indicated an increase in charge transfer resistance after DO exposure, providing the reason for an overall increase in resistance. SEM imaging confirmed the films’ homogeneity, polycrystalline nature, and consistent surface coverage, with no material change post-sensing. Further analysis using SXRD (performed post-sensing) and Raman spectroscopy confirmed phase stability, while FTIR spectra showed new bond formations following DO exposure which affected the increase in resistance in response to DO concentration. This study highlights that films deposited at pH 6.4 with a 30 min deposition time and bath temperature of 55 °C exhibit an optimal linear correlation between DO levels and resistance, presenting an efficient, novel, and cost-effective method for DO sensing using Cu₂O thin films. The findings of this study highlight the optimized n-Cu₂O thin films’ superior response to dissolved oxygen, a breakthrough in thin-film sensor technology, further underscoring their potential to replace or complement existing methods with a more scalable, cost-effective, and environmentally friendly solution. The innovation of this work lies in uplifting the unique semiconducting properties of Cu₂O thin films, optimized through electrodeposition, to address the gap in existing methodologies for dissolved oxygen sensing where thin-film technology remains largely underexplored.