Easy preparation of cobalt oxide/copper oxide composites for gas sensing application

This paper describes the preparation of Co3O4/CuO composites with vary CuO contents (Co3O4/CuO = 1.0wt%/0.50–1.0wt%) using a simple sol–gel process. According to SEM analysis, the composite samples exhibit a round-like morphology. XRD analysis revealed the formation of CuxCoyOz mixed oxide nanostructures. The composite materials were tested as thick films in conductometric devices for ammonia gas sensing at the optimal temperature of 150 °C. The response of Co3O4/CuO (1.0wt%/1.0wt%) composite was found much higher compared to pure Co3O4 and CuO NPs, suggesting that a synergic interaction occurs between the two metal oxide components in improving ammonia gas sensing capability. According to the findings reported, the design of Co3O4/CuO composite heterojunctions by the simple sol–gel process adopted might be an effective way to increase gas sensing toward ammonia gas at mild temperature.


Introduction
Industrialization caused air pollution, prompting the public and scientific community to seek improved detection technologies [1].One of the primary research issues addressing industrial and environmental safety is the development of gas sensors [2].The sensing process entails altering the characteristics of materials in response to analyte concentration [3].This is often caused by physical or chemical adsorption [4].Because of their enormous surface areas, nanosized materials are predicted to play an important role in both adsorption and sensor response.In general, gas sensors are based on several detecting techniques, such as chemiresistive [5] or optical [6].Chemiresistor sensors are among the most popular due to their ease of use and inexpensive cost.The fundamental challenge for these sensors typology is the enhancement of their sensitivity and selectivity [5,6].
Over the last three years, a massive amount of numerous pollutants produced from various home and industrial sources have caused major difficulties for ecosystem and human health [7,8].Among these, ammonia (NH 3 ) gas is not only a widespread air contaminant, but it is also projected to play a key role in the emerging hydrogen society [9,10].Because of its toxicity and negative environmental consequences, ammonia gas must be monitored for leaks in applications such as industrial facilities and transportation systems [11].Given this context, there has been an increase in demand for NH 3 gas sensors that are not only very sensitive, but also simple and low-cost [12].
Metal oxides (MOXs) are regarded as the best materials for detecting harmful gases by conductometric sensors.Spinel cobalt tetroxide (Co 3 O 4 ) has two oxides, Co 2 O 3 and CoO, with high oxygen concentrations and so exhibits p-type semiconducting properties.Co 3 O 4 has been used as a cobalt-based gas sensor, and it contains Co 3+ and Co 2+ [13].Stable Co 3 O 4 NPs that are inexpensive in cost and thermally stable, as well as their heterostructures with CuO, have been synthesized as an efficient and environmentally friendly process.Co 3 O 4 NPs with graphene as a support were proposed to improve gas sensing efficiency, stability, and performance for detection of ammonia [14].
Cupric oxide, on the other hand, is frequently used in environmental applications due to its exceptional gas sensing capability and small bandgap energy.Co 3 O 4 and CuO, both of which exhibit p-type semiconductor characteristics, were suitable for making p-p heterojunctions to be used in gas sensing applications [15].
MOX's composite heterostructures are an excellent way to improve gas sensitivity.For example, Cu 2+ ions anchoring into octahedral sites for spinel cobaltite synthesis are intriguing due to the presence of mixed-valence metal ions, which provide better gas sensing capability and electrical conductivity compared to one-component oxides [16][17][18].The interaction between support and active components is one of the most important factors in controlling gas sensing performances [19].
Based on these premises, it was thought to further improve the sensing performances by forming heterojunctions with other metal oxides.In recent years, several Co 3 O 4 -based gas sensors have been developed.To improve gas sensing performance and realise novel functionalities, scientists have made substantial efforts to synthesize metal oxides-based nanocomposites [20].Because of their large surface areas, these hetero-structures have gotten a lot of attention [21].Their performances and gas detection capability depend on the synthesis method adopted.For example,  [28].Here, the preparation was carried out by using the two step-processes schematized in figure 1.
In the first step, pure Co 3 O 4 and CuO NPs were synthesized.Co 3 O 4 was prepared adding 0.2 g of Co (NO 3 ) 2 ×9H 2 O to 200 ml of deionized H 2 O and stirred for 30 min.Then, NaOH solution was added drop by drop until the pH reaches 12, and stirring continuously for 4 h, while increasing the temperature of the hot plate to 250 °C.The resulting material was washed 3-4 times with deionized water, then dried in an oven at 110 °C for the entire night and calcined for 5 h at 550 °C for Co 3 O 4 phase refinement.CuO was prepared adding 15.2 g of Cu (NO 3 ) 2 ×6H 2 O to 200 ml of deionized water, stirred at room temperature for 30 min.5M NaOH solution was dropped into the copper nitrate solution until the pH was 9, stirred at 80 °C for 4 h and finally dried and calcinated at 550 °C for 4 h to obtain CuO NPs.
To synthesize Co 3 O 4 /CuO hetero-structures at different CuO concentrations (0.50-1.0 wt%) and Co 3 O 4 concentration (1.0 wt%), typically, 1 g of synthesized Co 3 O 4 NPs were dispersed in 10 ml of concentrated HCl using ultra sonication for 15 min, and then the calculated dose of CuO was progressively added to the solution while stirring for 60 min.The resulting solution (see figure 1) was then combusted at 110 °C for 4 h, until the liquid entirely dried, before being grounded.

Characterization
The samples were characterized using a wide-angle XRD with Cu-Kα radiation (λ = 1.54056Å) to investigate the crystalline phases.The XRD spectrum was collected in the 2 theta ranges of 10°-100°, at a scanning rate of 3°min −1 with a step of 0.03°.The materials' morphology was examined using a field emission scanning electron microscopy (FESEM).The composition of samples was determined by means of EDX analysis.

Sensor fabrication and gas sensing measurements
The gas sensor was manufactured by dispersing 1mg of Co 3 O 4 /CuO powders, sonicated for 30 min with 1 ml of deionized water to obtain a paste (figure 2).The paste was then printed onto the surface of ceramic substrate devices to produce a sensing layer, which was then allowed to dry at room temperature.For the electrical and sensing measurements, the sensor was mounted onto a holder with four electrical contacts, two of which regulate the sensor's working temperature, while the other two ones are used to measure the resistance.
Sensing tests were performed in a home-made apparatus, inserting the sensor holder in a chamber of the volume 10 ml, where the carrier (dry air) and the gases to be tested, coming from certified bottles, were introduced at an overall flow rate of 100 ml min −1 .
The response of the sensors is defined as Rg/Ra, where Ra and Rg are the resistances of the sensors in the dry air and target gases, respectively.The response and recovery periods are the timeframes necessary for the sensor to attain 90% of the total resistance change.Responses were collected on four identical gas sensors using the same manufacturing technique for each sample, and the findings were then averaged.Co 3 O 4 sample is characterized by the presence of very small and round particles, while particles with an irregular shape and larger size are observed on CuO sample.It can be noted clearly the different morphology of Co 3 O 4 /CuO composite particles from that of the pure metal oxides samples.The composite samples appear to be constituted by aggregated of tiny round nanoparticles.
EDX spectroscopy analysis confirmed the presence of Co, O, and Cu elements in the composite samples.No other significant peaks, attributed to foreign elements, were detected and identified, suggesting that the synthesized Co 3 O 4 /CuO hetero-structure samples were of good purity.
XRD patterns of Co 3 O 4 /CuO heterostructures with varying CuO concentrations revealed that the diffraction peaks are not coincident with those presents on pure Co 3 O 4 and CuO NPs.This suggests the formation of new mixed Co-Cu oxide phases.Indeed, due to the multiple oxidation states of the two Co and Cu elements, the possibility to form mixed phases, with many Co/Cu elemental ratios, is high.The main diffraction peaks observed on the composite samples were attributed to the CuCoO 2 and CuCo 2 O 4 phases.However, the presence of other Cu x Co y O z mixed oxide phases, cannot be excluded.
In exploring the Co 3 O 4 /CuO preparation in terms of alloy or composite formation it is required looking at the ionic radii.Substitutional alloys are generally formed when the two metallic components have similar ionic radii, otherwise solubility is more limited.The ionic radius of Cu 2+ is 0.73 Å, while that of Co 2+ is 0.65 Å [30] and these similar values would also allow forming a substitution alloys formation, as confirmed by XRD spectra above reported.The low intensity of diffraction peaks is an indication that the composite nanoparticles are constituted of low crystallinity and/or small particle size.The decrease of the crystallinity can be due to a contraction of crystal lattice because of substitution of the cobalt ions by copper ions, considering that the ionic radius of copper and cobalt are largely different (i.e., 0.73 Å and 0.62Å for Cu 2+ in octahedral and tetrahedral coordination, respectively, against 0.65 Å and 0.57 Å for Co 2+ at low-spin octahedral sites and tetrahedral sites, respectively).This likely originated an elevated stress which hindered the ordered growth of the composite nanoparticles.

Preliminary electrical and gas sensing studies
The synthesized samples of cobalt oxide, copper oxide, and copper loaded cobalt oxide nanocomposites were then used to develop conductometric sensors.First, these devices were tested evaluating their electrical baseline resistance at different temperatures.Baseline resistance is a significant parameter for evaluating the sensor's features.Data on the pure oxides have shown that Co 3 O 4 is more resistive compared to CuO, at all temperatures investigated (see figure 5).At the lower CuO loading, this did not modify much the resistance compared to Co 3 O4, but when the loading of CuO in Co 3 O 4 was comparable (1.0wt% versus1.0wt%), a very low baseline resistance was registered, which is advantageous for sensor development.
The sensor baseline decreased as the temperature rises, due to the expected influence of temperature on carrier concentration.Temperature also affects the gas sensing properties.It influences the equilibrium of gas adsorption and desorption on the surface of sensing MOX's semiconductors, as well as electron mobility  between the conduction and valence bands.Preliminary tests (not shown) were designed for evaluating the operating temperature response to NH 3 of the gas sensor based on Co 3 O 4 /CuO contents (1.0wt%/1.0wt%).The higher response was observed at the operating temperature of 150 °C.
Figure 6 depicts the effect of Co 3 O 4 /CuO hetero structures on NH 3 gas sensing performance when the gas concentration is 400 ppm at the optimal temperature of 150 °C.It is noteworthy that the response of Co 3 O 4 /CuO hetero-structures composites in various proportions are always greater than pure Co 3 O 4 and CuO.
It is well known that heterostructure consisting of different active materials usually exhibits enhanced reaction kinetics with respect to individual ones, which could efficiently improve the gas sensing performance.In case of the CuO/Co 3 O 4 heterostructure, the heterojunctions formed present strengthening effects on the gassensing response to reducing gases [28].
Further, another mechanism which can enhance the sensing performance of heterostructure is through charge carrier separation.In a n-p or p-n junction, the electric field created across the depletion region, much like, pull electrons in one direction and holes in the opposite direction.This charge 'directing' can also exist in pp junctions, such as in Co 3 O 4 -CuO, through balancing of Fermi energies, and lead to enhanced sensitivity [31].

NH 3 sensing
After the selection of the best sensor for NH 3 , we carried out a series of experiments to investigate its performances.The dynamic response characteristic curves for 1.0wt% Co 3 O 4 / 1.0wt% CuO composite sensor at pulses of NH 3 gas at different concentrations (0-800 ppm), at the optimal temperature of 150 °C, are shown in figure 7(a).
The response of the fabricated sensor is highly stable and reversible, i.e., the resistance increases when NH 3 is injected and decreases when the carrier gas is re-introduced.
The increase in sensor response with the increase of NH 3 gas concentration is shown in figure 7(b).When the concentration is increased from 0 up to 600 ppm, the trend between response and concentration appears to be highly linear (R = 0.95).When the gas concentration is increased further, the gas response starts to a plateau, indicating that the sensor reaches the saturation for this high NH 3 concentrations.Figure 7(c) reports the calibration curve in the log-log plot.The linearity of the response can be used to extrapolate the limit of detection (around 50 ppm) of the developed sensor.
The dynamics of the sensor is also evaluated.Table 1 shows a comparison of the Co 3 O 4 /CuO composite-based sensor with previous Co 3 O 4 sensors for NH 3 gas detection reported in the literature.Different forms of Co 3 O 4 such as nanorods or sheets, doped or dispersed on various supports (CNTs, graphene, etc) have been considered.The sensor operating temperature, concentration of target gas, sensor response, and response and recovery time are also compared.Based on all these parameters, the performance of our composite sensor is comparable to the other cobalt oxide-based sensors developed for ammonia monitoring, with the additional advantage to be easily fabricated by a mass effective sol-gel process and avoiding expensive materials.
After three test cycles, there was no discernible variation in the response to 200 ppm NH 3 , suggesting that the sensor has also acceptable repeatability and signal stability which are important factor for practical applications.Selectivity study is shown in figure 9.For this, we have compared the response to target gas, NH 3 , with the one obtained by some typical interfering gases, such as H 2 , NO 2 and acetone.The sensor based on the 1.0wt% Co 3 O 4 /1.0wt%CuO composite exhibits significantly high selectivity for NH 3 versus hydrogen and nitrogen oxide.Acetone instead give some interferences, and this will be a factor to be evaluate and overcome for practical applications.

NH 3 sensing mechanism
The NH 3 sensing mechanism of the Co 3 O 4 /CuO sensor can be explained as follows.When the sensor is exposed to air, O 2 molecules are adsorbed in different forms depending on the operating temperature.Physical and ionisation adsorption of O 2 can occur, with the latter favored as the temperature increase [36].Equations  Oxygen adsorption modulate the electronic behavior of the sensor; indeed, oxygen drain electrons from the p-semiconductor's surface (see arrows direction in figure 10(a)), increasing the holes concentration.As the electrical carrier concentrations are enhanced, the composite material is in a low-resistance condition [37].
When the composite is exposed to NH 3 , it is adsorbed on the sensing layer.Due to its electron donating character and the reaction with the adsorbed oxygen (equation ( 4 electrons are produced and returned on the surface of the composite (note the reversed direction of arrows in figure 10(b)).This process lead to the decrease of the resistance in n-type semiconductors [38], while for p-type ones it causes a decrease of the holes concentration and, consequently, to the increase the resistance.
The response of Co 3 O 4 /CuO composite to NH 3 gas is higher than that of pure Co 3 O 4 , which is ascribed to surface defects that promote gas molecules adsorption.The surface of the Co 3 O 4 /CuO composite contains more flaws with high adsorption energy than pure Co 3 O 4 .These high energy defects contain significant chemical adsorption sites for NH 3 , and their response towards this gas was stronger than pure Co 3 O 4 .At the same time, this causes lattice oxygen enrichment.Increased lattice oxygen can boost reactivity, which may  As before discussed, junctions formed when different metal oxides are mixed, are responsible of the gas sensing mechanism.However, previous research is mainly limited to p-n heterojunctions, and few studies related to p-p hetero metal oxide junctions are reported.Our data demonstrated that the formation of p-p heterojunctions in CuO-Co 3 O 4 improve the response of sensors to NH 3 .It can be assumed that introducing CuO, a mechanism occurs in which the majority of charge carriers come from the wide-bandgap materials to narrow-bandgap materials, establishing a low potential barrier.This also explain the increase of conductivity as CuO loading is increased.Further, CuO can capture NH 3 molecules, suggesting that CuO is the active center to improve the NH 3 sensing performance.

Conclusion
In summary, we used a sol-gel combustion approach to synthetize Co 3 O 4 /CuO nanocomposites.We accomplished hetero-structure modulation by altering the proportion of CuO in Co 3 O 4 nanoparticles.The results demonstrate that the sensor based on Co 3 O 4 /CuO (1.0wt%/1.0wt%)hetero structures has good performances towards the detection of NH 3 gas.The improved sensing capabilities could be attributable to the p-p hetero junctions with abundant active sites and lattice oxygens which enhance chemical reactivity of the Co 3 O 4 /CuO composite.

Figure 2 .
Figure 2. Fabrication process of the conductometric sensor by using pure Co 3 O 4 and CuO, and Co 3 O 4 /CuO.

3 .
Results and discussions 3.1.Structural and morphological characteristics SEM pictures of the synthesized pure Co 3 O 4 and CuO samples, and Co 3 O 4 /CuO composites are shown in figure 3.

Figure 4 .
Figure 4. XRD patterns of the prepared Co 3 O 4 /CuO hetero structures with different proportions of CuO in Co 3 O 4 .The XRD patterns of pure CuO and Co 3 O 4 are also shown.

Figure 5 .
Figure 5. Baseline resistance of the sensors based on pure oxides and Co 3 O 4 /CuO composites with different Cu/Co ratios.

Figure 8 (
a), depicts the response and recovery time of 1.0wt% Co 3 O 4 /1.0wt%CuO composite to 200 ppm NH 3 .At 150 •C, the response time of 1.0wt% Co 3 O 4 / 1.0wt% CuO sample for NH 3 gas time is 94 s, while the recovery time is 270 s.The as prepared 1.0wt% Co 3 O 4 /1.0wt%CuO composite performed superior NH 3 gas sensing also in terms of response and reaction time, being these values lower compared to the pure Co 3 O 4 -based sensor (figure 8(b)).

Figure 6 .
Figure 6.Response of the sensors to NH 3 based on Co 3 O 4 -CuO with different Cu/Co ratio.The concentration of NH 3 is 400 ppm.Temperature is 150 °C.

Figure 7 .
Figure 7. (a) Dynamic response curves to different NH 3 concentrations of the composite sensor operating at 150 • C; (b).Calibration curves of the sensors tested at different NH 3 concentrations; (c) Log-log plot of calibration curve for the composite sensor computed from results reported in the graphs (a) and (b).

Figure 8 .
Figure 8.(a) Response-recovery curves of the sensor Co 3 O 4 /CuO (1.0wt%/1.0wt%)operating at 150 °C to 200 ppm of NH 3 ; (b) Comparison of the response and recovery times between the pure Co 3 O 4 and the composite-based sensor, respectively.

Figure 10 .
Figure 10.Gas sensing mechanism of Co 3 O 4 /CuO hetero structures in the presence of a) air and b) NH 3 gas.

NO 3 ) 2 ×6H 2 O), sodium hydroxide (NaOH), and cobalt nitrate (Co(NO 3 ) 2 ×9H 2 O). Carlo Ebro Chemical Reagent Co. Ltd. supplied 98% concentrated hydrochloric acid (HCl), which was utilized as received with no additional purification. 2.2. Synthesis of Co 3 O 4 /CuO hetero structures The preparation of Co 3 O 4 /CuO heterostructures has been reported previously
[27]et al synthesised Co 3 O 4 nanowires for carbon monoxide (CO) detection [22], while Wang et al synthesised Co 3 O 4 hollow nanotubes for formaldehyde monitoring [23].Jeong et al used a p-n junction such as Co 3 O 4 -SnO 2 , to develop a sensor for ethanol, toluene and xylene [24].Na et al also developed a sensor using Co 3 O 4 -ZnO for the detection of ethanol and NO 2 [25].Li et al instead considered developing a p-p junction such as Co 3 O 4 -SWCNT that showed good response for NOx [26], while Vishnuraj et al used Co 3 O 4 -CNF for ammonia monitoring[27].For these reasons, we planned to exploit the properties of Co 3 O 4 to increase its performance by developing a p-p heterojunction with CuO to obtain a sensor capable of detecting ammonia.The work describes the design and manufacturing of Co 3 O 4 /CuO hetero-structures utilizing an effective and simple sol-gel technique.The samples nanocomposites have the same Co 3 O 4 content (1.0wt %) and varying CuO contents (0.50 and 1.0wt%).The Co 3 O 4 /CuO hetero structures demonstrated a synergistic impact between Co 3 O 4 and CuO NPs, resulting in significantly improved gas sensing capability toward ammonia (NH 3 ) as compared with the pure Co 3 O 4 .The sensing performance of the Co 3 O 4 /CuO (1.0wt%/1.0wt%) hetero structure was three times quicker than that of the pure Co 3 O 4 NPs.

Table 1 .
Comparison of the performances towards NH 3 of the Co 3 O 4 /CuO sensor with someone found in the literature.