Copper Oxide/ Functionalized Graphene Hybrid Nanostruc- tures for Room Temperature Gas Sensing Applications

Oxide semiconductors are conventionally being used as sensing materials in gas sensors, limiting the detection of gases at room temperature (RT). In this work, a hybrid of copper oxide (CuO) with functionalized graphene (rGO) is proposed to achieve gas sensing at RT. The combination of high surface area and presence of many functional groups in CuO/rGO hybrid material makes it highly sensitive for gas absorption and desorption. To prepare the hybrid material, a copper oxide suspension synthesized using copper acetate precursor is added to the graphene oxide solution during its reduction using ascorbic acid. Material properties of CuO/rGO hybrid and its drop-casted thin films are investigated using Raman, FTIR, SEM, TEM, and four-point probe measurement systems. We find that the hybrid material is enriched with oxygen functional groups (OFGs) and defective sites along with electrical conductivity (~1.5 kΩ/□). The fabricated QCM (quartz crystal microbalance) sensor with a thin layer of CuO/rGO hybrid, demonstrates a high sensing response which is twice the response of the rGO-based sensor for CO2 gas at RT. We believe that the CuO/rGO hybrid can be highly suitable for existing and future gas sensors used for domestic and industrial safety.


Introduction
Over the past few decades, the environment is heavily filled with toxic, inflammable, and harmful gases due to the unceasing development of industries and deforestation [1]. The level of these gases is excessively increased in the atmosphere after industrialization. For a safe and healthy environment, the detection of these gases is seriously demanded [2][3][4]. The need for an effective gas sensor is then extremely important. Numerous gas sensors based on the IDE (interdigitated electrode) [5][6][7], FET (field-effect transistor) [8,9], MEMS (microelectromechanical systems) [10,11], and QCM [12][13][14] have been developed. Among them, the QCM-based gas sensor is very effective owing to its attractive features such as accuracy, high sensitivity, and fast response. Also, QCM can be easily integrated with other electronic components [15,16].
In these gas sensors, metal oxide semiconductors such as zinc oxide (ZnO) [17], nickel oxide (NiO) [18], Tin oxide (SnO2) [19,20], titanium dioxide (TiO2) [21], and copper oxide (CuO) [22] have been fabricated and used as the sensing materials. Among numerous metal oxide semiconductors, CuO has gained more attention due to its low cost, non-toxicity, facile preparation, and high surface reactivity [23,24]. CuO is a p-type semiconductor and has a narrow bandgap of 1.2-1.9 eV and exhibits excellent properties that have been utilized in a number of applications such as electrode materials for lithium-ion batteries, thin-film electrodes [25], catalysts [21], and gas sensors [26].
However, these CuO sensing materials need a high temperature to operate, which limits the gas sensing at room temperature.
The incorporation of CuO with carbon-based materials can be an effective approach to realize reliable gas sensors at room temperature (RT). Carbon-based two-dimensional (2D) material like graphene has outstanding electronic, mechanical, and thermal properties as well as a large surface-to-volume ratio [27][28][29]. Graphene has been used as a sensing material for RT detection of various gases such as nitrogen dioxide (NO2), sulfur dioxide (H2S), ammonia (NH3), and hydrogen (H2) [30][31][32][33]. But lower electrical conductivity of graphene thin films also limits their sensors to achieve high performance.
Here, we report CuO/ functionalized graphene (rGO) hybrid nanostructure as a promising sensing material for QCM-based sensors for detecting the gas at room temperature. In synthesis, initially, the CuO nanoparticles (NPs) are prepared using copper acetate as a precursor and then mixed with functionalized graphene oxide solution. The functionalized graphene oxide is achieved using the chemical reduction method by ascorbic acid. The sensing thin films of the synthesized hybrid material are developed by the drop-cast method. The synergistic effect of CuO and graphene in the hybrid structure can help to achieve the detection of gas at room temperature. The CuO/rGO hybrid offers tremendous properties such as enhanced surface reactivity, good electrical conductivity, and large surface area. The CuO/rGO hybrid and rGO-based gas sensors are prepared to investigate the CO2 gas sensing at room temperature.

Synthesis of CuO/ Functionalized Graphene Hybrid and Thin Film Development
The schematic representation of CuO/rGO hybrid synthesis is shown in scheme 1. The solution is further washed with water and the solid product was separated using centrifugation. Next, 30 ml Graphene oxide (0.5 mg/ml) was dissolved in 100.00 mL of DI water and vigorously stirred for 10 min to form a well-mixed suspension. Then, 100 mg ascorbic acid (AA) is carefully added to the suspension. Afterward, 0.02 g of obtained CuO powder is added immediately to this GO suspension followed by vigorous stirring at 80 °C for 4 hrs. The suspension is further washed with ethanol and water (1:1 ratio). The solid product was later collected after the centrifugation followed by drying in the oven at 60 °C.
For comparison, the rGO is also synthesized using our previous work [34]. In brief, 15 ml of GO was added in 30 ml of DI water followed by ultrasonication for 15 min to acquire a uniform aqueous dispersion. Then, 100 mg of ascorbic acid was carefully added to the GO suspension, then stirred for 1 h at 65 ºC under RT environment conditions. The change in color of GO suspension from brown to black suggested the accomplishment of functionalized graphene oxide. Scheme 1. Schematic representation of CuO/functionalized graphene (rGO) hybrid synthesis.
The thin films of synthesized rGO suspensions and CuO/rGO hybrid were developed by the drop-casting method. Before thin-film development, the target substrates such as glass and SiO2 (300 nm)/Si were washed in acetone, isopropyl alcohol, DI water, and ethanol using ultrasonication for 15 min in each solvent.
For sensor fabrication, thin sensing layers of CuO/rGO hybrid and rGO materials were drop casted on a silver (Ag) electrode of 10 MHz QCM (WTL International China, Shenzhen, China). Before that, the QCM resonators were cleaned properly using acetone, isopropyl alcohol (IPA), DI water, and ethanol. A custom-made QCM based gas sensing setup is used to get the sensing performance. Before sensing the rGO and CuO/rGO coated QCM were dried at room temperature.

Characterization
The structural and morphological properties of synthesized materials were For gas sensing, rGO and CuO/rGO hybrid coated QCMs were placed inside the QCM holder. In the typical arrangement, the QCM holder is connected to the frequency counter. When gas is pursed to QCM sensors, the analyte gas molecules get absorbed at the material surface and the frequency of QCM gets changed. The frequency counter counts the frequency change. The response and recovery curve can be observed in the attached computer. The sensitivity (S) can be calculated using equation (10)

FTIR Analysis
FTIR spectrum is used to identify the presence of functional groups and chemical compounds. The FTIR spectra of GO, rGO, and CuO/rGO hybrid are shown in Figure 1.
The FTIR spectrum of GO exhibited strong peaks at 3182 cm -1 , 1620 cm -1 , and 1044 cm -1 , attributing to the stretching and deformation of -OH and -COOH functional groups as well as the adsorbed and inhibited water molecules to atmospheric moisture [35,36]. The After the formation of the CuO/rGO hybrid, the peaks for the functional group were found to be shifted to 3198 cm -1 (hydroxyl), 1570 cm -1 (carboxyl), and 1023 cm -1 (epoxy). This suggests the oxygen functional groups (OFGs) particularly hydroxyls have emerged during the formation of a hybrid nanostructure. However, a weak peak for hydroxyls was found in the rGO nanostructure. In CuO/rGO hybrid spectrum, some additional peaks at 560 cm -1 , 576 cm -1 , and 607 cm -1 were also observed, signifying the vibrations of the Cu-O bond [37]. The presence of these peaks is also attributed to the stabilization of CuO nanoparticles through the residual OFGs present in rGO and shows the presence of copper nanoparticles along with rGO [38]. Figure 1. FTIR spectra of GO, rGO, and CuO/rGO hybrid.

Raman Analysis
Raman spectrum of CuO/rGO hybrid is presented in Figure 2 and compared with spectra of GO, and rGO. The D-peaks for GO, rGO, and CuO/rGO hybrid were found at the Raman shift of ~1355 cm -1 , ~1353 cm -1 , and 1350 cm -1 respectively. The G-peaks for GO, rGO, and CuO/rGO hybrid were observed at the Raman shift of ~1586 cm -1 , 1600 cm -1 , and 1594 cm -1 respectively (see figure 3(a)). Typically, the D-peak appears from the defects in sp 3 carbon atoms whereas G-peak correlates to the sp 2 carbon atoms [39,40].
The prepared rGO and CuO/rGO material inhibit intense D-peak and G-peak, which is attributed to the presence of defects in the graphene layer. The D-peak of rGO and CuO/rGO hybrid was observed to be more intense as compared to G-peak, indicating the formation of sp 3 graphitic domains and the formation of new defects in the structure during the reduction process [35,41]. The occurrence of two additional peaks at low-frequency Raman shift ~322 cm -1 and ~678 cm -1 , indicates the Ag and Bg modes of the vibration of CuO [42]. The 2D peaks were observed at ~2679cm -1 , ~2689 cm -1 , and ~2701 cm -1 for GO, rGO, and CuO/rGO hybrid, respectively. The 2D-band also known as G'-band, originates from the second-order mode of the D-band [43]. The higher wavenumber position and the lowest peak height of the 2D-band of CuO/rGO hybrid suggest the existence of more number of graphene layers as compared to rGO. Moreover, it expresses that the Cu nanoparticles located between graphene layers, work as a spacer to hinder the agglomeration of graphene layers. formation of higher graphitic domains (smaller spatial dimensions) [44]. This indicates a decrease in the average size of the sp 2 carbon domain upon the reduction of GO. Similar results are also reported by Cheng et al. [45]. The higher intensity of D-peak shows the presence of a large amount of OFGs that are favorable for the physisorption of analyte gas molecules. Reduced graphene oxide consists of graphene layers and OFGs attached at its basal plane and edges [35]. The I2D/IG ratios for GO, rGO, and CuO/rGO hybrid were found to be 0.093, 0.18, and 0.13 indicating the presence of few layers of graphene in the CuO/rGO hybrid.
The deconvoluted Raman spectra (Lorentz fit) of GO, rGO, and CuO/rGO hybrid are shown in Figure 4. The height of the D band, G band, 2D band, and D+G band is shown in Table 1. It was observed that the intensity of the D-peak of rGO and hybrid were lower than the D-peak of GO, attributing to the presence of fewer OFGs in the rGO and hybrid material. Also, the intensity of G-peak was found to be lower than that of D-peak in rGO and hybrid material, showing that these materials have higher defects [46,47].

SEM and TEM Analysis
The structure and morphology of the synthesized materials can be studied using SEM and TEM. Figure 5(a) shows the SEM image of rGO, which consists of wrinkles on its surface. Figure 5    response of hybrid sensing film is possibly due to the presence of more OFGs at its surface as indicated by FTIR spectra. The CO2 gas was also exposed to GO coated QCM sensor at RT. The prepared GO coated QCM sensor showed no response upon CO2 gas exposure possibly due to the unavailability of active sites on its surface. GO has mainly hydroxyl and epoxide groups at its basal plane and it loses sp 2 hybridization during the oxidation process which makes GO an insulating material at room temperature [35].
The comparison between response time, recovery time, and sensitivity of rGO and CuO/rGO hybrid-based gas sensors is presented in Figure 7(b). The response time (Tres) was observed to be 43 s and 48 s for CuO/rGO hybrid and rGO thin film-based sensors, respectively, for 500 ppm CO2 gas at RT. The recovery time of hybrid and rGO-based sensors was observed to be 24 s and 21 s, respectively. The CuO/rGO hybrid-based sensor showed more recovery time than that of the rGO-based sensor. It is possibly due to the formation of hydroxyl groups. The gas sensing characteristics, frequency shift, response time, and recovery time strongly depend on the interactions between the oxygen functional group (OFGs) and analyte gas molecule [12,48]. The hybrid requires a higher time to recover due to the formation of hydroxyl bonds. Hydroxyl bonds are mainly attached to the basal plane however carboxyl and carbonyls are mainly attached to the edge plane of graphene [35]. These hydroxyls work as trap charges for the analyte gas molecule. Although the rGO and CuO/rGO hybrid thin film-based sensors showed a good response towards CO2 gas, both sensors demonstrated baseline drift because the adsorbed CO2 molecules do not desorb completely during the desorption process. The calculated sensitivity of hybrid and rGO-based gas sensors was found to be 15 Hz/µg and 1 Hz/µg for 500 ppm of CO2 gas.

Conclusion
In this work, we have successfully synthesized the CuO/functionalized graphene hybrid material. The thin films of material were developed by the drop-casting method on the substrates. The material properties were examined using advanced spectroscopy and microscopy. In hybrid nanostructure, a wrinkled and folded graphene surface with randomly distributed CuO NPs of size 10-40 nm was observed. The hybrid material was also found to have a large amount of OFGs and defective sites on its surface. The electrical conductivity of CuO/rGO hybrid thin film was measured to be ~1.5 kΩ/□. The functionality of the CuO/rGO hybrid was investigated as a sensing layer in the QCM sensor for gas detection at room temperature. The CuO/rGO hybrid-based gas sensor realized a high sensing response (Δf ~440 Hz, Tres ~43 s) for 500 ppm CO2 gas at room temperature. The sensing performance of the CuO/rGO hybrid-based gas sensor was compared with that of the rGO-based sensor. A 2-fold improved sensing response was achieved in comparison to the rGO-based sensor. The selectivity of the CuO/rGO hybrid will be investigated in our upcoming work. This study shows the interesting possibilities of CuO/rGO hybrids in future gas sensing devices.