Two Electron Transfer Mediated Enhanced Electrochemical Nitrate Detection by Sn‐Doped Cuxo Thin Film Electrode

The unique electrochemical and electronic properties of CuxO (Cu2O/CuO) give it merit to be used in the development of electronic devices. This paper presents the facile chemical route for the development of a sensitive, accurate, and low‐cost nitrate electrochemical sensor based on undoped and Sn‐doped CuxO thin film electrodes. Various characterization techniques are used to study the physical, electro‐chemical, and electronic properties of the material. Sn‐doping resulted in an increase in the number of electron transfers from 1 to 2 as compared to undoped CuxO. The prepared sensor exhibited a high sensitivity of 100.3 µA mm−1 cm−2, a wide linear concentration range of up to 50.0 mm, a fast response time of <5.0 s is observed at a potential of −1.0 V (vs Ag/AgCl), and a very low limit of detection of 0.1 µm (S/N = 3.0) that outperformed those in the recent literature. The electrode is selective for nitrate in the presence of common interference species and exhibited chemical stability and reproducibility.


Nitrate (NO 3
− ) is a naturally occurring inorganic oxyanion involved in the nitrogen cycle and is widely used in the manufacturing of explosives and fertilizers. [1]There are numerous potential sources of nitrate, namely animal waste, [2] septic tanks, [3] and municipal wastewater, [4] among others.Higher levels of nitrate are commonly found in groundwater because of contamination from human activities in agriculture (e.g., through the intensive use of nitrogen-containing pesticides and fertilizers) and industrial (e.g., mining where ammonia-based explosives are used to blast rocks).Nitrate is also identified as a health hazard when ingested in excessive amounts.Numerous health complications like gastric cancer and methemoglobinemia [5] have been associated with high nitrate intake.Some regulations such as those of voltammetry (CV) was used to claim the presence of Cu 2 O and CuO, detailed analysis of their X-ray diffraction (XRD) results in comparison to the reported copper oxide data of the JCPDS file seemed to suggest the existence of a single phase of copper oxide, i.e., CuO.Plus, such high-temperature and long-hour calcination could be in favor of the complete conversion of Cu 2 O to CuO.To gain simplicity and control over the film thickness, Cu x O materials can also be grown directly on bare electrodes via spin-coating of metal-organic complexes (SC-MOC).This synthetic approach has been adopted previously by Chowdhury group for the fabrication of CuO-based electrochemical glucose sensors. [17]However, the co-existence of the Cu 2 O and CuO mixed phases was not reported in their work.In a recent study, Palmer and co-authors [18] developed CuO:NiO thin film by SC-MOC and then converted it partially to Cu 2 O/CuO:NiO using high vacuum plasma-assisted nitrogen doping for enhanced electrochemical properties of the film toward glucose detection.Shaban and co-authors [19] used the sol-gel technique followed by annealing in a hydrogen and nitrogen (ratio 1:9) atmosphere to synthesize Cr and S co-doped Cu 2 O/CuO nanocomposite thin film for improved detection capabilities of CO 2 gas.
The catalytic performance of CuO thin films can further be improved by doping with various metals such as Mn, Fe, Li, Ti, Sn, and Ni.Among these dopants, Sn-doping has been reported to enhance the electrocatalytic properties of CuO.For example, Sayrac et al. investigated the effect of Sn-doping on CuO using the sol-gel drop casting technique.They reported that increasing the concentration of Sn-dopants resulted in the reduction of the bandgap energy of CuO.This was attributed to the substitutional doping of ionic Cu 2+ by ionic Sn 4+ due to similar ionic sizes of 0.72 and 0.69 Å, respectively. [20]Similar observations were also reported by Al Armouzi et al., in which Sn substitutional doping resulted in increasing the conductivity and reduction of the bandgap of the composite.The substitutional doping of Cu 2+ by Sn 4+ into the CuO crystal lattice generated two holes whereby the Sn 4+ dopants gained two electrons resulting in enhancing the ptype nature of the composite. [21]In another study, Sn-doping was observed to increase the number of electron transfers rates owing to the formation of electron clusters around oxygen atoms connected to Sn impurities as compared to undoped V 2 O 5 . [22]t is apparent that Sn-doping can further enhance the electrocatalytic properties of CuO.However, previous researchers have solely focused on the characterization and practical application of Sn-doped Cu 2 O [23] or CuO. [24]Herein, we rationally designed and engineered a Sn-doped Cu x O thin film for the first time for wide linear range and highly sensitive detection of nanomolar nitrate concentration.We have also evaluated and studied the origin of the enhanced electrochemical and electronic properties rising from the Sn doping of Cu x O material.

XRD Analysis
The XRD was used to confirm the coalescence of the cubic Cu 2 O and monoclinic CuO phases as well as Sn-doped Cu x O. Figure 1 demonstrates the XRD diffraction peaks of pristine Cu x O and Sn-doped Cu x O.The diffraction patterns of Cu x O (black) at 31.8, 33.9, 35.6, 38.1, and 38.8 0 correspond to the crystal directions of CuO (110), [26] CuO (−111), [26] Cu 2 O (111), [27] CuO (111), [28] and Cu 2 O (200), respectively. [29]The peak labeled crystal planes displayed the most intense XRD signals with a nearly equal magnitude as the preferred growth orientations.No other impurity phases were detected.The occurrence of a mixed phase in this study, unlike previously, [17] resulted from the adjustment of the spin-coating time.The decreased spin-coating time and multiple spin-coating led to a thicker film (≈500.0nm).This resulted in a higher mass loading and therefore an increased number of Cu 2+ ions that were favorable to the partial oxidation of Cu 2 O to CuO.The influence of Sn doping (red) on the crystal structure of pristine Cu x O thin film can be seen with a) the enhanced growth of CuO along the (110) axis, b) diminishing of the peak splitting at 2 = 38.8 0showing phase transition of Cu 2 O from cubic to the tetragonal shape, [30] and c) the disappearance of the CuO (−111) and (111) XRD peaks, denoting a loss of crystallinity and the introduction of amorphous domains due to lattice distortion. [31]hese significant crystallographic changes of the CuO peaks after Sn doping seemed to suggest that Sn 4+ had entered the CuO crystal lattice through substitutional doping.This is most likely as the ionic size of Sn 4+ (0.69 Å) is close to that of Cu 2+ (0.72 Å).No Sn 0 , SnO, SnO 2 , or residual SnCl 2 phase was found, meaning that Sn was successfully incorporated into the lattice structures of Cu x O.The XRD results were compared to JCPDS card no.48-1548.

Scanning Electrone Microscope Analysis
The morphology of pristine Cu x O and Sn-doped Cu x O thin films was studied using scaning electron microscope (SEM).As depicted in Figure 2a, the surface morphology of pristine Cu x O thin film showed the predominance of regularly shaped, single, and clustered ice cube-like morphology.After Sn doping (Figure 2b), one could pick up the emergence of irregularly shaped and pseudocubic structures, which demonstrated structural changes and a decrease in crystallinity as substantiated by the XRD results of Sn-Cu x O thin film.See the magnified SEM micrographs in Figure S1 (Supporting Information).

XPS Analysis
XPS experiments were performed on pristine Cu x O and Sn-Cu x O thin films to examine their surface chemistry.Figure 3a shows the full survey XPS spectrum of pristine Cu x O and Sn-Cu x O.All spectra were deconvoluted using OriginPro software and referenced at 284.8 eV for adventitious carbon.More attention was devoted to the analysis of the Cu 2p 3/2 , Sn 3d 5/2-3/2 , and O 1s spectra.As shown in Figure 3b, the deconvolution of the Cu 2p 3/2 XPS spectrum of pristine Cu x O thin film showed two spectral peaks centered at 932.8 and 934.6 eV for Cu + (Cu 2 O) [32] and Cu 2+ (CuO), [33] respectively.While the peak at 953 eV was well assigned to Cu 2p 1/2 of Cu + . [34]This thus confirmed the presence of both Cu 2 O and CuO phases (Cu 2+ /Cu + = 0.7/1.4) on the surface of the film and corroborated with XRD results.Figure 3c shows the XPS of the Cu 2p 3/2 bands of Sn-Cu x O, Sn doping caused noteworthy spectral changes of the Cu 2p 3/2 XPS band compared to pristine Cu x O thin film.Specifically, the Cu 2+ /Cu + spectral ratio dropped to 0.4/1.4,indicating a decrease in the number of Cu 2+ ions due to substitutional Sn 4+ ions (total atomic % = 1.3%) as was also observed from XRD analysis.In addition, the replacement of a single Cu 2+ cation by one Sn 4+ ion necessitated the generation of a Cu 2+ -vacancy due to the charge difference between the host and guest ions to maintain the condition of electroneutrality (see Equations ( 1) and ( 2)).The migration of Cu 2+ ions to the surface after Sn 4+ substitution is proposed to be responsible for the enhanced growth of CuO along the (110) axis (see Equation ( 3)).This is in agreement with the XRD data of Sn-Cu x O.For site balance, substitutional Sn 4+ ion and resulting Cu 2+ -vacancy liberate two free electrons as per Equation ( 4) (these free electrons can tune the electronic properties of Sn-Cu x O) and two free holes (see Equations ( 5) and ( 6)), respectively.The formation of Sn 4+ -induced Cu 2+ vacancies accounted also for the reduction of the number of Cu 2+ cations as a result of a decrease in the electronic density, as can be deduced from Figure 3c.Comparing Figure 3b,c, no change in the amount of Cu + cations was observed after Sn doping.To ascertain the oxidation states of Sn (Figure 3d), deconvolution of the Sn 3d 5/2-3/2 bands was performed, which accused of the presence of both Sn 4+ and Sn 2+ ions in the spectral ratio of 0.4/1.0.This also confirms that Sn 4+ substituted Cu 2+ as observed.On a 100% basis of the total number of elements present on the surface of Cu x O and Sn-Cu x O thin films, the depleted amount of Cu 2+ ions (by 0.3%) in Sn-Cu x O was found to be disproportional to that of Sn 4+ (0.4%), and this suggested that the excess dopant atoms, i.e., Sn 4+ and Sn 2+ , possibly occupied the interstices of the cubic Cu 2 O and/or monoclinic CuO phase(s).This could then explain the phase transition (from cubic to tetragonal geometry) mentioned earlier for Cu 2 O on XRD.
The O 1s XPS results of Cu x O in Figure 3e showed peaks at 529.7, 531.2, and 532.9 eV which were assigned to the lattice oxygen (O 2− = O Latt ) of CuO, lattice oxygen (O 2− ) of Cu 2 O/carbonaceous oxygen, and chemisorbed oxygen (O Ads ), respectively.Correction of the O 1s XPS photopeak at 531.2 eV could be applied to separate the contribution of the Cu 2 O lattice oxygen and that of carbon-related oxygen. [35]Table S1 (Supporting Information) (electronic supplementary document) presents the XPS (ratio) results of O 1s, Cu 2p, and Sn 3d.The ratio of O Ads /O Latt (2.4/2.3) was used to probe the surface oxygen vacancies of the CuO phase.Sn doping of pristine Cu x O resulted in the concomitant decrease and increase of the O 1s XPS peaks at 530.0 and 533.2 eV, respectively, as shown in Figure 3e,f.This strange and unexpected change was rationalized to be the result of an augmented amount of oxygen vacancies (O Ads /O Latt = 4.3/1.1) in pristine Cu x O after the substitution of Sn 4+ by Cu 2+ .Furthermore, the increase in oxygen vacancies is often related to an increase in low-valent cations as previously observed. [36]However, this is not the case here.Although more oxygen vacancies were formed, the number of Cu + ions remained unchanged.It can be postulated that the generation of oxygen vacancies (due to interaction between holes [produced from Cu 2+ ] and nearby lattice oxygen species) is always succeeded by the release of two electrons.These electrons recombine with the produced holes, thus preventing the increase of Cu + ions through electron trapping in the 3d orbitals of Cu 2+ .It can also be seen that the XPS peaks of Sn-Cu x O thin film at 530.0 and 533.2 eV appear positively shifted (by 0.3 eV) compared to those of Cu x O thin film.This is due to reduced electron density emanating from the decrease of the amount of CuO lattice oxygen.

Cyclic Voltammetry Studies
Figure 4a shows the cyclic voltammograms of Sn-Cu x O thin film swept in the range of −1.2-0.1 V at the scan rate of 30.0 mV s −1 in blank and 1.0 mm nitrate-containing KOH.Without nitrate addition (blank), two cathodic peaks were found at −0.34 and −0.75 V, which were attributed to the pairs of Cu(II)/Cu(I) (see Equation ( 7)) and Cu(I)/Cu(0) (see Equation ( 8)), respectively.The corresponding anodic peak currents were noticed at −0.125 and −0.34 V, respectively.These redox peaks confirmed the coalescence of the Cu 2 O and CuO phases.In 1.0 mm nitrate, the magnitude of the cathodic and anodic peak currents of the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples decreased significantly due to analyte adsorption, and the reduction peak current of nitrate appeared at −1.0 V (vs Ag/AgCl).The fact that nitrate electroreduction took place shortly after the reduction peak current of the Cu(I)/Cu(0) couple suggested that the metallic Cu 0 provided activation for the reduction process.The outcome of the CV experiment revealed that Sn-Cu x O showed the best electrocatalytic response with the highest cathodic peak current of NO 3 − occurring at −1.0 V.No redox peaks were observed at bare FTO.The improved performance of Sn-Cu x O/FTO compared to that of pristine Cu x O/FTO was assigned to the incorporation of Sn into the pristine Cu x O lattice.Specifically, the surface free electrons supplied from the substitution of Cu 2+ by Sn 4+ could participate in conduction during nitrate electroreduction and result in an enhanced current signal of Sn-Cu x O thin film. 2CuO Figure 4b exhibits the CV curves of Sn-Cu x O thin film in 0.1 m KOH solution with successive additions of nitrate.One could observe the linear increase of cathodic peak current with nitrate concentration varying from 1.0-4.0mm (R 2 = 0.996), and this confirmed the nitrate-sensing capability of the Sn-Cu x O thin film.In addition, the cathodic peak potential shifted negatively with increasing nitrate concentration due to nitrate adsorption on the electrode surface. [36]The inset in Figure 4b demonstrates the linear relationship between the peak current density of NO 3 − and the nitrate concentration.Figure 4c illustrates the effect of scan rate on the cyclic voltammogram (CV) of Sn-Cu x O thin film in 1.0 mm nitrate solution at the scan rate range of 25.0-200.0mV s −1 .The inset in Figure 4c demonstrates the linear rela-tionship between the cathodic peak current of NO 3 − and the scan rate (R 2 = 0.9886).This is inherent to an adsorption-controlled process.The cathodic peak potential shifted in the negative direction with increasing scan rate due to the chemical irreversibility of the electroreduction process. [37]The product ( c n c ) was found from Equation ( 9) [38] (to calculate the standard electron-transfer rate constant k 0 ) with E p and E p/2 as the cathodic peak and halfpeak potentials (V), respectively.Hence, ( c n c ) in the case of pristine Cu x O and Sn-Cu x O was found to be 0.96 and 0.76, respectively.
The k 0 values for pristine Cu x O and Sn-Cu x O were calculated from Equation (10) [39] and were found to be 337 and 391 s −1 , respectively.
where E 0′ is the formal electrode potential (V), obtained using the cyclic voltammograms of pristine Cu x O and Sn-Cu x O in Figure 4c, [40] R is the gas constant (8.314J mol −1 K −1 ), F is the Faraday's law constant (96 485 C), T is the temperature in K, and v is the scan rate in V s −1 . c is the transfer coefficient, and n c is the number of electrons transferred in the rate-determining step.
The number of transferred electrons was further calculated using the cathodic peak current ratio of nitrate from the cyclic voltammograms of Sn-Cu x O and pristine Cu x O as described in Equation ( 13), derived from the mathematical manipulation of Equations ( 11) and ( 12) for the adsorption-controlled irreversible process: [37] where I PC/1 and I PC/2 are the cathodic peak currents of Sn-Cu x O and pristine Cu x O, respectively, from their respective cyclic voltammograms.We observed a two-fold enhancement in the reduction peak current of nitrate at the Sn-Cu x O electrode.Q c/1 and Q c/2 are the integrated peak areas of the Sn-Cu x O and pristine Cu x O electrodes, respectively, from their CV spectra.Γ is the surface concentration of the electroactive analyte.The number of electrons transferred for pristine Cu x O and Sn-Cu x O was found to be 1 and 2, respectively.The mechanism of nitrate electroreduction at pristine Cu x O and Sn-Cu x O was proposed as follows: [41] NO 3 − first adsorbed onto the active sites of the electrochemically produced Cu 0 species (see Equations ( 14), ( 15), (18), and ( 19)), which in turn mediated the electroreduction of NO 3 − by supplying electrons to produce cuprous ions (see Equations ( 16) and ( 20)).Finally, the Cu + ions instantly got reduced to restore the metallic Cu by a flow of current from the external circuit (e − ext ).This is similar to the incipient hydrous oxide adatom mediator model [42] (see Equations ( 17) and ( 21)).The main difference in nitrate reduction pathway between the sensors was the one-and two-electron transfer for pristine Cu x O (Equations ( 14)-( 17)) and Sn-Cu x O (Equations ( 17)-( 21)), respectively, leading to two different products, i.e., NO 2 (for pristine Cu x O) and NO 2 − (for Sn-Cu x O).The increased number of electrons transferred in the case of Sn-Cu x O was assigned to the excess/surface free electrons derived from substitutional Sn 4+ doping as was shown by the XPS analysis.To outline the electronic contribution of Sn 4+ doping, one electron (e − surf ) was added to the left-hand side of Equation ( 20) for a total of two transferred electrons (i.e., one from the metallic Cu and the other from the dopant).
Nitrate reduction mechanism over Cu x O thin film

Electrochemical Impedance Spectroscopy (EIS)
EIS experiments were performed to probe the charge-transfer resistances (R CT ) of the Sn-Cu x O and pristine Cu x O electrodes at their interfaces during electrolysis using a three-electrode setup controlled by the Metrohlm Autolab PGSTAT204 potentiostat with pre-installed NOVA 2.1 software.From an EIS spectrum, the electron-transfer kinetics and diffusion characteristics can be determined.The semicircle portion at the high-frequency region represents the electron-transfer resistance (R CT ) of the electrode, and the linear part at lower frequencies indicates the diffusion-limited process.EIS study was conducted using 5.0 mm EIS Spectrum Analyser was used to fit the data (See the Randles equivalent circuits in Figure 5b).The decreased charge-transfer resistance of Sn-Cu x O thin film was attributed to the excess free electrons from the Sn doping, which  participated in electronic conductivity.The preceding statement agreed with the increased electron-transfer rate of Sn-Cu x O thin film compared to that of pristine Cu x O thin film.

Chronoamperometric Study of the As-Prepared Electrode
The sensing performance of Sn-Cu x O thin film was further evaluated using chronoamperometry.Figure 6 shows the chronoamperogram of Sn-Cu x O thin film in 0.1 m KOH under mild stirred conditions near −1.0 V (vs Ag/AgCl).The corresponding inset displays the plot of current density versus nitrate concentration within the two-step linear concentration range from 0.0-15.0mm and 15.0-50.0mm.The first linear regression equation (up to 15.0 mm) is I p (mA cm −2 ) = −0.10027C (mm)-0.811,where I p is the current density (mA cm −2 ) and C is the nitrate concentration (mM) (R 2 = 0.9956).The sensitivity of the device was found to be 100.3μA mm −1 cm −2 (up to 15.0 mm).The second linear regression equation (up to 50.0 mm) is I p (mA cm −2 ) = −0.015C (mm)-2.145, the sensitivity of the device is found to be 15.0 μA mm −1 cm −2 .The limit of detection (LOD) was obtained from the formula 3/S with  being the standard deviation of the current response and S the slope of the calibration curve.The LOD was calculated to be 40.0nm (S/N = 3), which was considerably lower than the maximum allowable NO 3 − concentration in drinking water according to the WHO regulation and confirmed the practical application of Sn-Cu x O sensor.The sensor exhibited the lowest detection limit over a wide linear range compared to those listed in Table 1 with the exception of one or two cases. [10]

Reproducibility, Stability, and Shelf Life
Figure 7a,b presents the reproducibility and stability of the Sn-Cu x O sensor investigated using CV in 0.1 m KOH electrolyte containing 1.0 mm nitrate, respectively.Four independent electrodes were tested, and the relative standard deviation (RSD) of the current responses was obtained to be 2.6%, and this small value confirmed the good reproducibility of the developed electrode.Furthermore, the shelf life of the sensor was studied over one month with the electrode tested twice per week.During the shelf-life study, the electrode was kept open to air with an average humidity of 78.0% (during the month of winter in Cape Town, South Africa).The results showed an RSD value of 4.7% (data not shown), attesting to the good durability and moisture insensitivity to the moisture of the sensor.This is very important for long-term storage of electrodes.The electrochemical stability/repeatability of the as-prepared sensor was evaluated over four repetitive CV scans, and only a standard deviation of 1.3% was observed (Figure 7b).

Interference Study
The selectivity of the Sn-Cu x O sensor was evaluated using chronoamperometry in 0.1 m KOH electrolyte with consecutive addition of a steady amount of nitrate at an applied overpotential of −1.0 V in the presence of five-times higher concentrations of interferents such as nitrite, chloride, hydrogen carbonate, sulphate, calcium, and magnesium ions that could coexist with nitrate in water.As shown in Figure 8, the anions caused no significant interference to the cathodic current signal of NO   possibly because they are more hydrophilic based the Hofmeister series, allowing nitrate to easily reach the electrode surface. [51]However, the addition of Ca 2+ /Mg 2+ ions decreased the reduction spike current of NO 3 − probably due to electrostatic interaction or precipitation reaction with the hydroxide ions (OH − ) of the supporting electrolyte.This indicated that the developed sensor was selective toward nitrate and thus can be efficiently utilized in various media.

Determination of Nitrate in Real Sample
The recovery study of nitrate in real sample matrices, such as borehole water (somerset west area, Cape Town, South Africa), Mowbray river water (Mowbray area, Cape Town, South Africa), and drinking tap water (Supplied by the city of Cape Town municipality to Bellville area, Cape Town, South Africa) was conducted using a standard addition method to demonstrate the practical application of the Sn-Cu x O sensor.The test results are summarized in Table 2.It can be concluded that the Sn-Cu x O sensor showed good recovery in the range of 97.8-111.2%.This highlights the practical application of the developed sensor.

Conclusion
We have successfully prepared and Sn-doped, for the first time, Cu 2 O and CuO mixed thin film via spin-coating metal-organic complexes for electrochemical detection of nitrate.Nitrate electroreduction at pristine Cu x O was found to be mediated by the Cu/Cu 2 O couple and turned out to be a one-electron transfer process with NO 2 as the reduction product.The enhanced electrocatalytic activity of Sn-Cu x O thin film toward nitrate electroreduction compared to that of pristine Cu x O was indexed to the excess/surface free electrons from Sn 4+ and its substitution of Cu 2+ , which increased the number of transferred electrons and resulted in a two-electron transfer process with NO 2 − as the product.The Sn-Cu x O sensor exhibited a high sensitivity of 100.3 μA mm −1 cm −2 at a potential of −1.0 V (vs Ag/AgCl), a wide linear concentration range of up to 50.0 mm, a fast response time of <5.0 s, and a limit of detection of 40.0 nm that outperformed in the reported literature.

Experimental Section
Materials: All chemicals were purchased from Sigma-Aldrich South Africa and used as received without further purification.All reagents used were of analytical grade.Deionized water was used throughout the experiment.Potassium hydroxide (KOH) with a concentration of 0.1 m was used as an electrolyte.Copper oleate (99.9%), toluene (99.5%), tin chloride dihydrate (98%), and ethanol (95%) were used as received.
Electrode Fabrication: A two-step spin coating method (Figure 9) was used in the production of undoped and Sn-doped Cu x O thin films.Typically, 0.18 g of copper oleate was dispersed into 1.0 mL of toluene before sonication of the mixture at 45.0 kHz for 30-45 min to achieve homogeneity.An amount of 0.02 g tin chloride dihydrate was also dissolved in 0.5 mL of ethanol and then sonicated for 10 min.Both solutions were mixed.Thereafter, 50.0 μL aliquot of the homogeneous solution (80:20% v/v; copper oleate: tin chloride dihydrate) were spin-coated on 1.0 cm 2 of a precleaned and cut-to-size FTO slide at 2500 rpm for 10 s and then 4000 rpm for 50.0 s (the spin-coating time was decreased in this study compared to previously). [17]The spin-coated liquid thin films were calcined subsequently at 350 0 C for 10 min.The formation mechanism of pristine Cu x O thin film could be proposed as 1) decarboxylation of the oleate ligands with the release of two electrons [25] (Equations ( 11) and ( 22)); 2) complete reduction of cupric ion to metallic copper by the released electrons (Equation ( 23)); 3) complete oxidation of metallic Cu species to cuprous oxide (Equation ( 24)); 4) finally, partial oxidation of cuprous oxide to cupric oxide (Equation ( 25)).The stoichiometric ratios of the final reaction step were deduced from XPS analysis and presented in previous section.
Cu ( The spin coating and calcination steps were repeated, respectively, and alternately until four layers of deposition were reached.The prepared electrodes were labeled as Cu x O (undoped) and Sn-Cu x O (Sn-doped).
Physical Characterization: The crystal structure of Cu x O and Sn-Cu x O thin films were examined by XRD (PANalytical X'Pert PRO PW3040/60) technique where XRD patterns were obtained in the 2 range of 20-40°u sing Cu-K radiation of wavelength  = 0.154 nm with the step size of 0.02°s −1 .SEM micrographs of the thin films were obtained by using Tescan MIRA3 RISE SEM, a high-resolution Field Emission SEM that was associated with low kV imaging Nova Nano SEM.XPS measurements were carried out on Cu x O and Sn-Cu x O with a spatial resolution of <3.0 μm using KRATOS AXIS X-ray photoelectron spectrometer at UNISA (Florida Science Campus), South Africa.
Electrochemical Evaluation: All electrochemical measurements were carried out using Autolab PGSTAT 302N potentiostat with pre-installed NOVA 2.1 software.The conventional three-electrode setup was adopted with Ag/AgCl/KCl (3.0 m) as the reference electrode, Platinum wire as the counter electrode, and Cu x O/Sn-Cu x O as the working electrode.Cyclic voltammetry (CV) and chronoamperometry (CA) experiments were performed at room temperature in 40.0 mL of 0.1 m KOH.EIS study was conducted using 5.0 mm K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] as the redox probe in 0.1 m KCl over the frequency range of 1.0-100.0kHz and ac voltage amplitude of 100.0 mV.The Randles equivalent circuits were obtained using EIS Spectrum Analyser.
Statistical Analysis: Peak fitting software was utilized to establish the correlation between XPS intensity and the specific feature.To assess the statistical coefficient of determination (R2) in the response sensor, linear regression was employed.To ensure statistical significance, experiments were conducted in triplicates when necessary.For the determination of the limit of detection, standard deviations from ten replicate measurements of the blank and responses at a low concentration were utilized for statistical analysis.Descriptive statistics were used to derive the mean and standard error in the analysis where necessary.

Figure 1 .
Figure 1.XRD diffraction patterns of pristine Cu x O (black) and Sn-doped Cu x O (red).

Figure 2 .
Figure 2. SEM micrographs of a) pristine Cu x O and b) Sn-doped Cu x O.

Figure 3 .
Figure 3. Full-scan XPS spectrum of a) pristine Cu x O and Sn-Cu x O, b) Cu 2p of pristine Cu x O, c) Cu 2p of Sn-Cu x O, d) Sn 3d of Sn-Cu x O, e) O 1s of pristine Cu x O f) O 1s of and Sn-Cu x O.

Figure 4 .
Figure 4. Electrochemical sensor performance of a) bare FTO, pristine Cu x O/FTO, and Sn-Cu x O/FTO in the presence and absence of 1.0 Mm NO 3 , b) the effect of NO 3 concentration (insert:), and c) effect of scan rate, [CV performed in 0.1 m KOH

K 4 [
Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] as a redox probe in 0.1 m KCl within the frequency range of 1.0-100.0kHz and ac voltage amplitude of 100.0 mV. Figure 5a shows the resulting Nyquist plot of pristine Cu x O and Sn-Cu x O thin films obtained at a formal potential of 0.2 V (vs Ag/AgCl).The Sn-Cu x O electrode exhibited a smaller semicircle diameter (R CT = 50.0Ω) than that of the pristine Cu x O electrode (R CT = 70.0Ω).

Figure 5 .
Figure 5. a) Nyquist plots of pristine Cu x O and Sn-Cu x O and b) schematic of the equivalent circuits used for fitting.[R1 = Solution resistance; R2 = Charge Transfer resistance, R ct ; CPE = constant phase elements; W = Warburg impedance].

Figure 7 .
Figure 7. a) Reproducibility and b) stability studies of the Sn-Cu x O sensor investigated using CV in 0.1 m KOH electrolyte containing 1.0 mm nitrate.

Figure 8 .
Figure 8. Interference study of Sn-Cu x evaluated using chronoamperometry in 0.1 m KOH electrolyte.

Figure 9 .
Figure 9. Schematic of the electrode fabrication process.

3 − Table 1 .
Comparison of the performance factors between Sn-Cu x O and the reported literature.

Table 2 .
Practical application of Sn-Cu x O.