1. INTRODUCTION
NO2 gas is a common air pollutant and is mainly emitted during waste incineration and the combustion of fuels in vehicles [1]. NO2 is the primary cause of photochemical smog and acid rain. Further, at concentrations > 1 ppm, it can causes problems in the respiratory and nervous system [2], exacerbating respiratory diseases such as emphysema and bronchitis, and aggravating existing heart conditions [3-5]. In addition, the inhalation of excessive NO2 can lead to pulmonary edema and eventual death [6,7]. Therefore, the detection of this gas is critical from an environmental and safety perspective.
Chemiresistive metal oxides have been widely studied in the domain of gas detection [8,9]. In gas sensors employing these substances, the adsorption of the gas on the sensing layer results in a resistance change due to an alteration in the charge carrier concentration [10,11]. Such sensors exhibit high sensitivity, high selectivity, and fast dynamics, while also being cost-effective [12,13]. In general, for gas-sensing applications, n-type metal oxides have received more attention than p-type alternatives [14]. Therefore, further research on the gas-sensing features of p-type materials is required.
Generally, Cu exists as one of three oxides with different oxidation states: Cu2O, CuO, and Cu4O3 [15]. Copper oxide nanoparticles (NPs) have garnered particular interest because of their potential applications as catalysts [16], thermoelectric materials [17], supercapacitors [18], and gas sensors [19]. For instance, p-type CuO is cost-effective, abundant, chemically stable, and environment-friendly. It has a monoclinic crystalline structure and a band gap energy of 1.22–1.55 eV [20-22]. Accordingly, it has been used in gas sensors [23].
Various methods are available for synthesizing copper-oxide NPs, including hydrothermal synthesis [24], chemical reduction [25], sonochemical reduction [26], laser ablation [27], metal-vapor synthesis [28], electrochemical deposition [29], solid-state reactions [30], microwave irradiation [31], and sol-gel methods [32]. However, few studies have reported the preparation of CuO (nanoparticles) NPs using the microemulsion method [33]. Inverse micelles are nanometer-sized aggregates of surfactants comprising encapsulated water molecules as cores within a bulk nonpolar solvent. The fabrication of inverse micelles typically involves the complete dissolution of the surfactant in an apolar solvent. Subsequently, an aqueous buffer is added to the aforementioned solution. The inverse micelle solution is clear, and homogeneous. The shape, size, and structure of the aqueous cores; the micro viscosity and aggregation number; affect the final products of inverse micelle system [34]. Initially, two separate microemulsions incorporating different reactants are prepared. Upon mixing these microemulsions, as the water inside becomes supersaturated with reactants, nucleation happens on the micelle edges. Subsequently, growth occurs with the diffusion of more reactants via intermicellar exchange [35].
In this study, mesoporous CuOx NPs were synthesized using a simple inverse micelle method and were subsequently annealed at 250, 350, and 450 °C. Gas sensors were then fabricated using the NPs and were tested on both NO2 and H2S gas at 200 °C. The sensor with the NPs that were annealed at the lowest temperature (250 °C) exhibited an enhanced response to NO2 gas.
2. EXPERIMENTAL PROCEDURES
2.1. Synthesis of CuOx NPs
An inverse micelle method reported in a previous study was used to synthesize mesoporous CuO [36]. Nitric acid (7.6 M) was added to a solution of P123 (0.002 M), copper (II) nitrate (0.02 M), and 1-butanol. The mixture was stirred at room temperature until the precursor dissolved into a gel. After drying the gel in oven (120 °C), powders were obtained. The powders were then washed several times with ethanol and centrifuged. Then, they were dried in a vacuum oven (80 °C/12 h). The dried powder was finally calcined in an O2 atmosphere at 250 °C/4 h, 350 °C/3 h, and 450 °C/2 h, to prepare a mesoporous CuOx structure.
2.2. Material Characterization
A phase study was conducted on the synthesized NPs using an X-ray diffractometer (Bruker, Germany; Bruker D8 Advance). High-resolution transmission electron microscopy (HR-TEM, FEI Tecnai F20) was employed to analyze the particle size and morphology of the samples. Energy dispersive X-ray spectroscopy (EDS) incoporated in TEM was used for compositional studies. Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, USA) was employed to study the surface groups. A Brunauer–Emmett–Teller (BET) analyzer (Micromeritics, United States of America, 3Flex) was used to determine the specific surface areas of the powders. An X-ray photoelectron spectroscopy (XPS) system (Physical Electronics, USA, PHI Quantera-II) was employed to analyze the chemical bonding states of the synthesized powders.
2.3. Gas-Sensing Measurement
To fabricate the gas sensors, the prepared samples were mixed with ethanol and drop-coated onto a substrate (SiO2-coated Si) with interdigital Pt electrodes. The gas chamber was inside of a tubular furnace with temperature control. The flow rate was set to 500 sccm using mass-flow controllers, and air was used as the background gas. The resistance values of the sensor were measured in air (Ra) and in the presence of the target gas (Rg), and the sensor’s response to NO2 gas was defined as R = Ra/Rg.
3. RESULTS AND DISCUSSION
3.1. Morphological, Structural, and Compositional Studies
Figures 1 (a), (c), and (e) are TEM views of the mesoporous CuOx samples annealed at 250, 350, and 450 °C, respectively. High-magnification TEM images of the aforementioned samples are presented in Figures 1 (b), (d), and (f), respectively. In general, the particle size of the samples annealed at higher temperatures was slightly larger. For example, for the sample annealed at 250 °C, the approximate size of the NPs was 7–9 nm, while the sizes of the samples annealed at 350 and 450 °C were 10–12 nm and 12–14 nm, respectively. Figures 2 (a)–(c) show HR-TEM views of the mesoporous CuOx powders annealed at 250, 350, and 450 °C, respectively.
For all the samples, the spacings between the parallel fringes were 0.252, 0.232, and 0.274 nm, which matched the spacings of the (002), (111), and (110) crystalline planes of the CuO phase, respectively[37-39].
Figures 3 (a)–(c) show the TEM–EDS elemental mapping of the mesoporous CuOx annealed at 250 °C. Both oxygen and Cu elements are uniformly distributed throughout the samples. The corresponding TEM-EDS spectrum, illustrated in Figure 3 (d), shows peaks related to both O and Cu. The proportions of these elements were 11.22 and 88.78 wt%, respectively. Figures 3 (e)–(g) present the uniform distribution of O and Cu, as shown in the TEM-EDS analysis of the sample annealed at 350 °C. The corresponding TEM-EDS spectrum of this sample shows peaks for O (9.04 wt%) and Cu (90.96 wt%), as observed in Figure 3 (h). Figures 3 (i)–(k) present the TEM-EDS elemental mapping analysis of the mesoporous CuOx sample annealed at 450 °C, highlighting the even distribution of both O and Cu. In addition, Figure 3 (l) shows the TEM-EDS spectrum, where peaks corresponding to O (7.92 wt%) and Cu (92.08 wt%) are present again. Overall, no impurity elements were observed in any of the samples, which reflects the high purity of the materials and the effectiveness of the synthesis procedure. In addition, the compositional analyses of all the samples yielded similar results, again reinforcing the suitability of the employed synthesis route.
Figure 4 (a) shows the XRD patterns of the mesoporous CuOx NPs at various temperatures. All the patterns have peaks that can be indexed to the monoclinic phase of CuO, matching JCPDS File No. 80-1268 [40]. However, with increasing annealing temperature, the intensity of the peaks increased, reflecting the improved crystallinity of CuOx at higher temperatures. Figure 4 (b) discloses the FTIR spectra of the mesoporous CuOx samples. For the sample annealed at 250 °C, the oxalate peaks at 1610, 1360, and 1317 cm-1 corresponding to surfactant–precursor interactions [36] almost disappeared, as did the nitrate ion (from copper nitrate precursor) peak at 1418 cm-1 [36]. In fact, for the samples annealed at higher temperatures, no peaks related to surface groups were observed; this indicates that all organic materials on the surface of the samples were burned during annealing. Figure 4 (c) presents the N2 adsorption–desorption curves of the three mesoporous CuOx samples, and their calculated BET surfaces are compared in Figure 4 (d). For the CuOx annealed at 250, 350, and 450 °C, the BET surface areas were calculated to be 161.85, 39.88, and 22.52 m2/g, respectively. Therefore, the surface area decreased with increasing annealing temperature because of the growth of particles at higher temperatures.
The deconvoluted XPS Cu 2p core-level spectra of the mesoporous CuOx annealed at 250, 350, and 450 °C are presented in Figures 5(a), (c), and (e), respectively. In all cases, peaks related to Cu2O and CuO appeared in the XPS spectra. For the sample annealed at 250 °C, the main peak was related to Cu2O, whereas with increasing annealing temperature, the CuO peaks became dominant. Because XPS can be used to detect elements at a depth of 10 nm, both Cu2O and CuO phases appear to have been present on the surface of samples. However, the sample annealed at 250 °C contained more Cu2O than CuO, whereas the other two samples contained more CuO than Cu2O. The deconvoluted O 1s core-level spectra of the CuOx annealed at 250, 350, and 450 °C are presented in Figures 5 (b), (d), and (f), respectively. In all cases, the O 1s peak can be fitted to three separate peaks at binding energies of 529.6, 531.3, and 533 eV, which are considered to be related to lattice oxygen, adsorbed oxygen species, and oxygen vacancies, respectively [41].
3.2. Gas-Sensing Studies
For NO2 gas (2–10 ppm) at 200 °C, Figures 6 (a), (c), and (d) display the transient resistance graphs of the gas sensors fabricated using the CuOx samples annealed at 250, 350, and 450 °C. The sensors annealed at 350 and 450 °C showed no response to the NO2 gas.
However, the sensor annealed at 250 °C could detect the NO2 gas; as shown in Figure 6 (b), its response values (R) for 2, 4, 6, 8, and 10 ppm of NO2 gas were 1.101, 1.219, 1.31, 1.354, and 1.907, respectively. In addition, none of the sensors exhibited a meaningful response to H2S gas (Figure 7).
Therefore, considering that the sensor annealed at 250 °C could detect NO2 gas while exhibiting no response to H2S gas, it achieved high selectivity to NO2 gas in the presence of H2S.
3.3. Gas-Sensing Mechanism
When gas sensors are exposed to air, oxygen is initially adsorbed on their surfaces. Owing to its high electron affinity, the oxygen then attracts the electrons from the conduction band of the sensing layer. The related reactions are as follows [42]:
Therefore, a hole accumulation layer (HAL) was initially formed on the surface of the mesoporous CuOx NPs constituting the sensors fabricated in this study. After subsequent exposure to NO2 gas, the NPs were oxidized further due to the powerful oxidizing nature of the gas [43]. In fact, additional electrons were transferred from the sensor to the NO2 gas; hence, the width of the HAL increased, leading to a reduction in the sensor resistance. The relevant equations are as follows [44]:
The sensor fabricated using the mesoporous CuOx annealed at 250 °C showed the strongest response to the NO2 gas. Because the operating temperatures of all the gas sensors were identical, this factor can be excluded when comparing the behaviors of the three sensors. Thus, the enhanced response of the sensor annealed at 250 °C is assumed to be related to its higher surface area (161.85 m2/g) relative to the other two sensors. Because a higher surface area affords more adsorption sites, a larger amount of gas can be adsorbed on the sensor surface.
In addition, based on the XPS results, the Cu2O and CuO phases co-existed in the annealed samples. Therefore, in regions where these phases were in close contact, the electrons likely moved from CuO, which has a work function of 5.3 eV [45], to Cu2O, which has a work function of 4.6 eV [45], to equate the Fermi levels on both sides of the phase boundary. In an air atmosphere, this causes band bending and the formation of potential barriers at the phase boundary. In a NO2 atmosphere, more electrons are captured by the NO2 gas, leading to a change in the height of the potential barriers. This finally leads to modulation of the sensor resistance and contribute to the sensing signal.
4. CONCLUSIONS
In this study, mesoporous CuOx NPs were synthesized using an inverse micelle method for gas-sensing applications. After annealing at 250, 350, and 450 °C, the NPs were characterized using HRTEM, XRD, FTIR, and XPS. The findings showed that increasing the annealing temperature increased the sizes of the NPs while reducing their surface areas. Additionally, the crystallinity of the NPs improved at higher annealing temperatures. Based on the XPS results, both the Cu2O and CuO phases co-existed on the NP surface. The sample annealed at 250 °C exhibited a significantly higher surface area (161.85 m2/g) than the samples annealed at 350 °C (39.88 m2/g) and 450 °C (22.52 m2/g) did. Gas sensors fabricated with the NP samples were tested for sensitivity to both NO2 and H2S gases at 200 °C. The sensor fabricated with the sample annealed at the lowest temperature (250 °C) exhibited an enhanced response to NO2 gas but no response to H2S, whereas the other two sensors showed no meaningful response to either gas. The improved response of the former is considered to be due to its high surface area, the oxidizing nature of NO2 gas, and the co-existence of both CuO and Cu2O phases.
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