Performances of Nano-Structured Cu2O Thin Films Electrochemically Deposited on ITO Substrates in Lactate Bath as Liquid Petroleum Gas Sensors

The investigations are focused on structural and surface morphological features and their influence on electrical and wetting characteristics against liquid petroleum gas (LPG) sensing of synthesized nanostructured cuprous oxide (Cu2O) thin films grown by electrochemical deposition (ECD) on indium tin oxide (ITO) glass substrates in lactate bath (pH 10). p-Cu2O films revealed a cubical-tetrahedron nano-scale polycrystalline grain distribution (average grain size: 64.2 nm, inter planer spacing: 0.24 nm) that exhibited dominance in the crystallographic plane (111). LPG sensing evaluations of p-Cu2O/ITO done at 70 °C constant temperature with 100% LPG at 5 cc min−1 flow rate, showed excellent gas sensor response, recovery, and stability over time due to its moderate wetting behavior (average contact angle: ∼86°). AC impedance measurements carried out at room temperature demonstrated an ionic to electronic conduction variation from 100 Hz to 1 MHz, due to adsorption/desorption of LPG molecules (CnH2n+2) and oxygen species on the nano-particles surface. Cu2O/ITO flat band potential was found to be higher than its’ ambient state after exposure to LPG with an increment in acceptor density. Overall, this efficient LPG sensing could be attributed to the interfacial properties of Cu2O thin films and ITO substrates, that provide an optimized fabrication process of nano-structured Cu2O thin films.

Early detection and monitoring of flammable, toxic, and exhaust gases used in industrial and domestic environments such as Liquid Petroleum Gas (LPG) demands gas sensors of higher selectivity and sensitivity with a suitable base material consisting of ideal surface and bulk properties.Solid-state gas sensor development has been pursued in semiconducting, thermal conducting, electrochemical, catalytic combustion, and other techniques by past research.Here Semiconductor Metal Oxide (SMO) is one of the most investigated materials used as gas sensors due to their advantages in structural and morphological features.Recent studies on SMO focus on less than 100 nm particle size range to emerge unique electrical and chemical properties for enhanced sensing. 1 And also about how their geometry affects the movement of electrons and holes in these semiconductor nanomaterials.
Most of the SMO sensing materials are crystalline in nature.Their adjacent grains are interconnected by necks and form larger aggregates connected by grain boundaries.At ambient sensor conditions, ionosorbtion 2,3 takes place at SMO surfaces by absorbing oxygen in the air and driving the adsorbed oxygen molecules to trap surface electrons in the conduction band of SMO.As a result, O 2 , O -, and O 2-are generated at the surface under relevant temperatures as given in Eqs.1-4 O ads 1 This electron-depleted surface charge region (space charge layer) creates a potential barrier at the surface. 4In polycrystalline sensing materials, electronic conduction happens via grain-to-grain contacts depending on the potential barrier of adjacent grains called the Schottky barrier. 5ased on the type of conductivity of the SMO surface layer, carrier concentration gets increased or decreased.A potential barrier induced by the decrease in major carrier (electron) concentration increases n-type SMO resistance.On the other hand, p-type SMO displays reducing resistance with the formation of active oxygen species, which strengthens the hole concentration by trapping electrons from the surface.When a reducing gas such as LPG is introduced to the ionosorbed oxygen molecules on n-type SMO films, the surface-trapped electrons get released.This decreases the film sensor resistance. 5Figure 1 schematically shows this LPG sensing mechanism of p-Cu 2 O thin films deposited on ITO substrates.
1][22][23][24][25][26][27][28] At present, a variety of techniques exist to synthesize Cu 2 O with different microstructures on selected substrates, including thermal oxidation with copper sheets at 200 °C/1050 °C, chemical vapor deposition with alumina slides at 300 °C, magnetron DC sputtering on glass at 693 K, radio, anodic oxidation on glass at 280 °C29-33 etc.However, in many respects, such as ease of film fabrication and low cost, low processing temperature, controlled crystallization with surface morphology, roughness, and wettability in a large area, high deposition rate, 34 Cu 2 O thin films produced by electrochemical deposition are highly suitable for gas sensors, solar cells, photo-electrochemical cells, and catalytic applications.Most of the said metal-oxide gas sensors perform at high temperatures.Since only limited works have been done in low temperature operated gas sensors, identifying the other variables affecting gas sensitivity has been a major challenge.For these reasons, this work reports the conditions for the successful deposition of Cu 2 O nanostructured thin films on Indium Tin Oxide (ITO) substrates by electrodeposition technique, emphasizing structural, morphological, wettability and electrical features and their influence in LP gas sensing at low operating temperatures.

Experimental
Electrochemical deposition of nanostructured Cu 2 O films.-Before the deposition of Cu 2 O films, ITO substrates were subjected to surface treatment with an acetone bath and sonicated for several minutes with distilled water to improve Cu 2 O film adhesion to the ITO surface.Selection of the potentiostatic conditions and deposition z E-mail: doneranji@gmail.com;vpper@ou.ac.lk; gksen@ou.ac.lk; knban@ou.ac.lk period also depends on the substrates' characteristics.During the electrodeposition, an area of ∼1 × 1 cm 2 of the ITO substrate was immersed in an electrolyte solution containing 3.5 M lactic acid (Sigma-Aldrich, purity − 99.0%) and 0.45 M cupric sulfate CuSO 4 (Sigma-Aldrich, purity − 99.0%).The pH of the electrolyte was brought to pH 10 using 4 M NaOH (Sigma-Aldrich, purity-98.0%).Applied deposition time varies from 45-60 min with the thickness of the ITO to obtain a particular thickness of the Cu 2 O film.The Cu 2 O thin films were potentiostatically electrodeposited on an ITO glass in a lactate bath at −450 mV versus Ag/AgCl reference electrode in a three-electrode electrochemical cell with a platinum plate as the counter electrode for 45 min. 16Here the electrolyte temperature was maintained constant by continuing the deposition in a water bath at 60 °C.After deposition, films were thoroughly cleaned with distilled water and allowed to dry in air at room temperature. 35 2 O thin film characterization.-Thephase of cuprous oxide and the average crystal size were monitored using the X-ray Diffraction (XRD) (Regaku Ultima-IV) operating at 40 kV and 30 mA tube voltage and current with Cu-Kα (λ = 1.5418Å) as the source of X-rays in the 20°−80°2Ɵ range.The significant effect of the film grain size on the sensitivity and conductance of Cu 2 O films and the inter-planer spacing (d) between the particles can be analyzed by considering the diffraction peaks observed with the XRD analysis by Debye-Scherrer formula and Braggs law given in Eqs.5-6.Where "λ" is the wavelength of X-ray (0.1541 nm), "β" is the FWHM (full width at half maximum) of the prominent peak, "θ" is the diffraction angle corresponding to the peak, and "D" is the mean grain diameter size of the film.
The surface structure and morphology of deposited Cu 2 O thin films were observed using a Scanning Electron Microscope (SEM) (Zeiss EVO 15 lS).The type of conductivity of the deposited Cu 2 O films was verified using spectral response measurements and Motts-Schottky plots.An AUTOLAB Model PGSTAT 302 potentiostat/galvanostat was used to conduct Mott-Schottky analysis using three electrode chemical cell containing Cu 2 O deposited ITO substrate as the working electrode, platinum counter electrode, and Ag/AgCl reference electrode in aqueous 0.1 mol L −1 sodium acetate solution as supporting electrolyte.Mott-Schottky and Electrochemical impedance spectroscopic (EIS) analysis were done by varying the voltage −0.5 V to 1.5 V and frequency 1 Hz-0.1 MHz respectively at both stages before and after introducing the electrochemically deposited Cu 2 O thin film for LP gas sensing.The sessile drop method with double distilled water (parts of 5 μl) was used to measure the contact angles of the film at ambient atmospheric conditions.A digital microscope (2MP 1000x 8 LED USB Digital Microscope Endoscope) focused on the film contacting surface obtained an average contact angle of three separate distilled water droplets after allowing them to settle on the film surface for ∼10 min. 35 gas response measurements.-Gold-platedcontact probes placed 1 cm apart on the film surface with optimal contact pressure measure the electrical resistance variation of the Cu 2 O film with and without LPG.Film temperature was maintained constant by mounting the films on a heating platform inside a stainless steel gas perfusion chamber.The thermocouple on the bare end of the substrate monitored the sensing temperature, ensuring zero leakage of LPG.The output electrical resistance of the film was processed using a computer interfaced Dual measurement multimeter (GW INSTEK GDM-9060/9061) at 2 s time intervals. 35At the beginning of each measurement, the sample chamber was flushed with Nitrogen air, and the substrate was allowed to stabilize at the optimal temperature of 70 °C.At this point, thin film resistance (Ra) was stabilized in its ambient state.Next, LPG was introduced till the resistance reached a steady state maximum.Both resistance levels were measured with ±0.01 tolerance.With 100% LPG exposure, this variation takes typically 30-40 s duration.Finally, the gas flow was stopped to let the thin film recover its ambient state.Gases were applied through blue silica gel as a humidity-controlling measure to avoid external moisture getting inside the sealed chamber via valves.

Results and Discussion
Surface morphology and nano-crystal structure.-Theoperating characteristics of solid-state gas sensors depend on both receptor and transducer functions. 2Semiconductor metal oxides with considerably small grain sizes and porous nature provide optimal surface morphology for such applications.Figures 2a-2b illustrate the top view of SEM micrographs obtained for the ITO/Cu 2 O thin films electrochemically deposited for 45 min to get the optimum film thickness in this study.The Cu 2 O surface has a well-defined grain distribution in nano-scale with considerably homogeneous, cubical-tetrahedron crystals.The surface adhesive nature of ITO has contributed to a pinhole-free compact nanostructured Cu 2 O polycrystalline morphology that creates a large contacting surface area for adsorbed analytes.This crystallographic structure facilitates more surface oxygen vacancies for the diffusion of LPG, which improves the kinetics of the chemical reaction on the p-Cu 2 O surface and shows possible enhanced gas sensing recovery. 4n Fig. 3 Cu 2 O Grain size and adhesive film nature.-Theelectrical conductance and gas sensitivity of the Cu 2 O film have a significant dependency on its average grain size.A smaller grain size results in high sensitivity, but a significant reduction in grain size decreases the structural stability of the film due to the small crystallites' deleterious nature on the temporal stability of the sensing platform. 35he size dependence of gas sensitivity is explained by V. E. Bochenkov and G. B. Sergeev. 3 Recent studies on the gas sensitivity of nanostructured metal oxides [2][3][4] suggest a visible increase when the grain size D < 2 L, where L is the thickness of the surface charge layer.This is attributed to the higher grain contribution in creating the space charge layer causing the film conduction via grain   boundaries, grain necks, and grains itself (Fig. 4).Table I explains the grain size (D) and d-spacing calculations of each p-Cu 2 O hkl plane observed in the thin films deposited on ITO substrates.The identified smaller average grain size of ∼64 nm drives a possible higher resistance response and gas sensitivity based on electronic conduction models observed in Fig. 9 with the semicircles presented.
Average contact angle, grain size variation, and the corresponding surface energy evaluate a thin film's wetting nature.The average contact angles measured for p-Cu 2 O thin films deposited on ITO substrates have shown the probability of stable partially wetting behaviors according to the contact angle variations presented in former studies. 36Figure 5 shows the average contact angle measurement of electrodeposited Cu 2 O (pH 10) thin films grown on ITO substrates.
LPG sensing performance.-Theresistance variations of the Cu 2 O films electrochemically deposited on ITO were measured at ambient atmospheric conditions (Ra) and in the presence of 5 cc min −1 100% LPG (Rg) concentration (Fig. 6).The time taken by the sensor to reach 90% of its maximum resistance response with LPG and to recover 90% of the ambient resistance were recorded.Upon the adsorption of LPG molecules on the film surface, the prestable resistance (∼0.1 MΩ at 70 °C) of the film increased and came to a saturation after a few seconds (30-40 s).But as it can be seen with the steeper slope in the line, film recovery is faster than the response at 70 °C.The percentage change in the gas response is given by Eq. 7.For p-type Cu 2 O films > Rg Ra.
Effect of LP exposure on carrier density.-Asemiconductor material that perfectly abides by the Mott-Schottky plots can be characterized according to Eq. 8 below.and T are the space charge layer capacitance of the film at potential V, the electron charge, permittivity of the free space, the relative permittivity of Cu 2 O free carrier concentration of the semiconductor, flat band potential, Boltzmann constant, and absolute temperature respectively.The Mott-Schottky analysis defines the conductivity type of electrochemically deposited Cu 2 O thin films having negative slopes to be with p-type conductivity.The linear regions of Mott-Schottky plots observed for a selected Cu 2 O film, before and after it was introduced to LPG, are illustrated in Fig. 7. Referring to the plots, flat band potential (V FB ) and the free carrier concentration 37,38 has been calculated using the following simplifications of Eq. 8.The obtained flat band potentials (V FB ) and the free carrier concentrations measurements both before and after LPG exposure are listed in Table II.The LPG exposure time was limited to ∼2 min at room temperature (300 K).In both instances, Cu 2 O thin film on ITO shows a p-type conductivity with holes as the major carriers.
The flat band potential of Cu 2 O film after exposure to LPG once is higher than its original state.But the LPG interaction has drastically increased the free carrier density of a 1 cm 2 area.When LPG is introduced, the interaction of hydrocarbons ( ) + C H n 2n 2 and chemisorbed oxygen increases the adsorption rate of the film surface.The given additional pressure produces gaseous species and H 2 O as a result (Eq.11).
Since the p-Cu 2 O layer deposited on ITO shows low wettability, formed water molecules depart and create a potential barrier. 39At high temperatures (180 °C), these water molecules are removed from the surface thereby, decreasing sensor resistance back to its original state.This water vapor concentration and pressure are high at low operating temperatures (70 °C).Therefore an increase in resistance in the Cu 2 O film can be observed with LPG.
Electrical characteristics by EIS.-The electrical properties such as concentration of electroactive species, charge transfer, adsorption and diffusion of species, mass transfer of p-Cu 2 O/ITO interface, and the film could be observed via EIS measurements by monitoring the current response to an AC voltage.These features are characterized by a fit and simulated equivalent electrical circuit to the measured impedance spectra, which consists of resistors, capacitors, constant phase elements (CPE), etc, that are connected in series or parallel.Figure 8 schematically shows the impedance measurement across the Cu 2 O film surface deposited on ITO.0][41][42][43] Here in the EIS measurement, the Nyquist plot interprets the imaginary component Z" versus the real component Z'   AC Impedance spectroscopy study was carried out in 100 Hz to 1 MHz frequency range with an oscillating voltage amplitude (V RMS ) of 10 mV within a current range of 10 nA to 100 mA at room temperature.Nyquist and bode plots were obtained for the process under zero forward bias DC voltage.Impedance measurements were taken across the film surface by placing two identical Fluorine Doped Tin Oxide (FTO) substrates contacting the film surface with their conducting sides.The gap between the FTOs was kept at ∼1 mm distance, with equal proportions of the FTOs on the film.However, longer the distance, the film cannot flow a smaller current across the film surface, resulting in a distorted impedance spectrum.As the figure inset explains, impedance response is weaker in low frequencies and way more consistent and strong in higher frequencies due to white noise.Figure 9a shows the Nyquist plot of the p-Cu 2 O under normal atmospheric conditions and while it was introduced to a 5 cc min −1 LPG flow rate for 1-2 min duration at room temperature.During the AC impedance measurement, the surface roughness variations and non-homogeneity in the deposited polycrystalline p-Cu 2 O film furnish a double-layer capacitance having non-ideal capacitive behavior. 45Radius of the non-centered deformed semicircles on the Z' axis implies the total impedance of the film to be increased with the exposure of LPG from low (right) to high (left) frequencies.This explains the observed reducing electronic conductivity (Fig. 6) of the p-Cu 2 O film surface with the LPG adsorption at room temperature.3][44] However, the film reaches a pure resistive state within this range without LPG contact.The equivalent circuit models of the film at both stages of LPG are represented in Figs.9b, 9c show reasonable concordance with these simulated results.
Here, R S (∼0) is the series resistance of the ohmic contacts between the film surface and FTO.The combination of parallel circuits at both LPG stages represents the deformation of the semicircular arks to the start of second semi-circles.As discussed earlier, grain size impact on film conductance is clearly visible with LPG, which exhibited two arcs at low and high frequencies corresponding to the grain and grain boundary effects, respectively. 3The corresponding conductance models are illustrated in Fig. 10.Without LPG the film behaves as a parallel connection of capacitances (C 1 and C 2 ) formed with series connections of charge transfer resistance components (R 1 and R 2 ) and constant phase elements (CPE 1 and CPE 2 ).But with LPG, constant phase elements are in parallel with R 1 and R 2 .
The significant impedance modulus in Fig. 11a, and negative phase angles observed in Fig. 11b, at high frequency ranges of p-Cu 2 O/ITO films exposed to LPG are associated with a capacitive nature which lessens at lower frequencies, thereby revealing a resistive nature and almost zero phase change corresponding to the ionic conduction of H + and OH -ions on the p-Cu 2 O film surface.The discussed pure resistive state of the film in the absence of diffusion ions resulted in an LPG response (Fig. 8) can be identified with the increasing phase angle reaching a zero in Fig. 11b.

Conclusions
In summary, Cu 2 O electrochemically deposited on ITO has a grain distribution in nano-scale with considerably homogeneous and cubical tetrahedron crystals.The surface adhesive nature of ITO has contributed to a pinhole-free compact nano-structured Cu 2 O polycrystalline morphology that creates a large contacting surface area for adsorbed analytes exhibiting promising LP gas sensitivity and enhanced gas sensing recovery.Evidence supporting the explanation of these measurements was further clarified by the contact angle measurement and resulting partial wetting nature.The characterized film was subjected to gas sensing evaluations at a constant temperature of 70 °C which ultimately confirmed that ITO can improve gas sensor performances with their moderate wetting behaviors.Under stable sensing conditions, Cu 2 O/ITO exhibited enhanced LPG response, recovery, and stability over time.After the exposure to LPG, the films' flat band potential and acceptor density of a 1 cm 2 area were identified to be higher than their original state.AC impedance spectroscopy was carried out at room temperature.With the diffusion/adsorption of LPG species at room temperature impedance characteristics were recorded to vary from ionic to electronic conduction on the p-Cu 2 O film surface from low to high frequencies.In comparison, all the experimental results highlighted the impact of LPG on the interfacial properties of Cu 2 O thin films deposited on ITO substrates.

Figure 1 .
Figure 1.Schematic interpretation of semiconductor metal oxide LP gas sensing mechanism.
, the XRD patterns of p-Cu 2 O (pH 10) films electrodeposited on ITO substrates in an aqueous electrolyte solution of 3.5 M lactic acid and 0.45 M cupric sulfate, at a constant temperature of 60 o C for 45 min duration is shown.The presented peaks corresponding to crystalline planes of (110), (111), (200), (220), and (311) are tallying with the standard JCPDS card PDF file no.05-0667, which confirms the observed thin films to consist of Cu 2 O nanoparticles.The crystallographic plane (111) is dominant in the acquired p-Cu 2 O thin films influenced by ITOs' conductivity.The presence of relatively low peak intensities of (110), (200), (220), and (311) Cu 2 O planes result in a polycrystalline nature in the observed thin films.This clarifies the cubical-tetrahedron grain distribution in Fig. 2.

Figure 2 .
Figure 2. SEM morphological variations of electrodeposited Cu 2 O (pH 10) thin films grown on ITO substrates in different magnifications.

Figure 4 .
Figure 4. Grain size effect on conductance models of p-Cu 2 O thin film grown on ITO.

Figure 5 .
Figure 5. Average contact angle of electrodeposited Cu 2 O (pH 10) thin films grown ITO substrates in lactate bath.

Figure 6 .
Figure 6.LPG response variation of electrodeposited Cu 2 O (pH 10) thin films grown ITO substrates in lactate bath.

Figure 7 .
Figure 7.The linear regions of Mott-Schottky plots were observed for a selected Cu 2 O (pH 10) thin film grown ITO substrates before and after it was introduced to LPG.

Figure 8 .
Figure 8. Schematic view of the impedance measurement across Cu 2 O film surface deposited on ITO.

Figure 9 .
Figure 9. (a) Comparison of complex plane impedance plots of p-Cu 2 O thin film grown on ITO and respective equivalent circuits simulated (b) without LPG and (c) with LPG.Inset of (a) gives the corresponding experimental and the fit and simulated impedance curves (given in solid lines).

Figure 10 .
Figure 10.Complex plane impedance plot corresponding to the grain effect and conductance models (given in inset) of p-Cu 2 O thin film grown on ITO exposed to LPG.

Figure 11 .
Figure 11.Comparison of (a) Bode impedance magnitude plots and (b) Bode phase plots observed for a selected Cu 2 O (pH 10) thin film grown on ITO with and without it was introduced to LPG.

Table I .
Grain size and d-spacing calculations of p-Cu 2 O thin films deposited on ITO substrates.

Table II .
Flat band potentials and carrier densities of Cu 2 O thin films deposited on ITO substrates before and after LPG exposure.