Polyethyleneimine–Chromium Oxide Nanocomposite Sensor with Patterned Copper Clad as a Substrate for CO₂ Detection

Abstract

Polyethyleneimine (PEI) and chromium oxide (Cr₂O₃) with different weight percentages were chosen for sensing carbon dioxide (CO₂). Four divergent varieties of sensors with different concentrations of Cr₂O₃ in PEI were fabricated by drop-casting the sensitive films on prepared interdigitated electrodes (IDE) from copper clad. X-ray, absorbance, morphological, and compositional studies were carried on Cr₂O₃ nanoparticles by x-ray diffractometry (XRD), UV–Visible spectrometry, and field-emission scanning electron microscopy (FESEM). Response proficiency for all the fabricated sensors was meticulously examined at room temperature. Solitary proficiencies of resistance versus gas concentration, sensitivity, repeatability, and precise response time and recovery time measurements were examined. It was epitomized that the appropriate weight ratio of PEI and Cr₂O₃ was critical for CO₂ sensing. A reasonable correlation between the sensing responses of the developed sensors to carbon dioxide under nitrogen was achieved.

Introduction

Sensors are devices or a subsystem that detect changes in the surrounding environment, and send the collected data to the remaining electronic blocks for processing.^1,2,3 Sensors are used in several day-to-day devices, such as light sensors, which help with the auto-brightness function for the display. This is an essential function, because the light sensor detects the light levels of the environment in which the user is using the device and can adjust the brightness automatically. Most people are unaware of its countless applications.^4,5,6 Applications of sensors can be seen in automotives, aircraft, machinery, manufacturing, and various other sectors of everyday life. A wide variety of sensors measures material physical and chemical characteristics. Some of the examples include vibrational sensors, electrochemical sensors, gas sensors, and fluid viscosity measurement sensors.^7,8,9,10

Gas detectors or sensors are electronic gadgets that sense and recognize various categories of gases. Primarily, they are used to identify and measure the gas concentration of explosive or toxic gases.^11,12,13 These types of sensors are used in manufacturing industries to detect gas leakages and smoke, and in homes to monitor the concentration of carbon monoxide or carbon dioxide. Gas sensors vary in their ability to sense and detect range and size. Gas sensors have to be calibrated more frequently than other kinds of sensors, as they are in frequent contact with atmospheric air and other kinds of gases.^14,15,16 Gas sensors detect the metal oxide change in electrical resistance when interacting with target gases. Gas sensors have a wide range of applications in forecasting and avoiding many possible hazards, and must conform to safety standards in industrial and domestic environments. They need to be located precisely to detect the level of accumulation of any gas before it becomes hazardous.^17,18 The spatial and temporal distribution of CO₂ at the earth–atmosphere interface, and at the boundary layer above it, is of great importance for soil, agricultural, and atmospheric sciences. In general, root respiration and soil microbial activity are the main sources of high CO₂ concentrations at this border layer. Enhanced detection of CO₂ gas concentration could lead to a better understanding of agricultural productivity and transport mechanisms at this crucial interface.^19,20,21,22 Accurate monitoring of CO₂ can improve modeling and decision-making to enhance agricultural productivity, thus allowing farmers to meet rising food demands, while also making their lives more accessible.²³ Using a gas sensor to detect CO₂ concentration in the atmosphere inside a structure is a cost-effective way of maintaining adequate air circulation for our comfort and avoiding over-ventilation, which can increase heating and cooling expenses.^24,25,26 Thin films play a vital role in sensing and other applications.^27,28 Polyethyleneimine (PEI), reduced graphene oxide, and cerium oxide can be used as potential materials for CO₂ sensing at room temperature.^29,30 The nano-architectonics concept is supposed to involve the architecting of functional materials using nanoscale units based on the principles of nanotechnology.³¹ A graphene and metal oxide combination has also shown effectiveness in gas sensing.³² This work mainly focuses on the potentiality of the PEI–Cr₂O₃ nanocomposite for carbon dioxide sensing with varying percentages.

Experimental

Precipitation Method for Chromium Oxide Synthesis

Chromium sulfate was used as a precursor material or prospective source of chromium, while ammonium hydroxide was used as a precipitating reagent in the production of chromium oxide (Cr₂O₃). Figure 1 shows a schematic of Cr₂O₃ synthesis, which typically involves five steps,

• Step 1. 250 mL of 0.1 M chromium sulfate solution were dissolved in distilled water and aqueous ammonia was prepared.

• Step 2. Liquid aqueous ammonia was added dropwise until the pH of the reaction mixture reached 10 under continuous stirring on a magnetic stirrer. When the pH came close to 10, precipitation of the reaction mixture started, and was a maximum at 10.

• Step 3. The obtained precipitation was collected and filtered by vacuum filtration and washed with distilled water.

• Step 4. The filtered residue was dried in a hot air oven for 24 h at 80°C.

• Step 5. The obtained mass was crushed using pestle and mortar to obtain Cr₂O₃ powder.

Fig. 1

Schematic of the synthesis of chromium oxide.

Experimental

Interdigitated Electrode (IDE) Preparation by Copper Clad for PEI–Cr₂O₃ Nanocomposite Sensor

Copper clad, permanent markers, ferric chloride powder, a cutter, and a glass Petri dish were required to prepare the interdigitated electrodes (IDE) by copper clad. For the IDE substrates, copper clad is used as the base material. Ferric chloride powder acts as an etching agent, and permanent markers were used as the etch resistant while pattering the copper clad to form the IDE. Figure 2 depicts the process of preparing and coating of the IDE from the copper clad, consisting of four steps,

• Step 1. The copper-clad (Tech Delivers) was carefully cut into four pieces with the dimensions of 2 cm × 1 cm.

• Step 2. The IDE pattern was uniquely marked onto the four pieces of copper clads with the help of a permanent marker.

• Step 3. The patterned copper clads were selectively etched for 15 min in ferrous chloride solution (etching solution).

• Step 4. The patterned copper clads were removed from the etching solution and gently washed with diluted acetone in order to remove the pattern drawn with a marker. IDEs were produced after several washes with distilled water.

Fig. 2

(a) Schematic of preparing IDE from copper clad, (b) images of preparing IDE from copper clad.

PEI–Cr₂O₃ Nanocomposite Sensor Fabrication

PEI was combined with varying concentrations of 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% of Cr₂O₃ for four different sensors to develop a sensitive layer for the precise detection of CO₂. The coating of the sensitive layer on the IDE is shown in Fig. 3. The following steps are involved,

• Step 1. Stirring Initially, 2 wt.% PEI was dissolved in distilled water at 50°C, and, once the PEI was entirely dissolved, the polymer was ready for the next step.

• Step 2. Homogenizing 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% Cr₂O₃ were added to four test tubes (blending tubes), each containing 25 mL of PEI polymer solution, and the mixture was well combined with a homogenizer to achieve a homogeneous solution.

• Step 3. Drop-casting With the help of a micropipette, a homogeneous solution of PEI–Cr₂O₃ was drop-cast onto the prepared IDE.

• Step 4. After allowing adequate time for the droplets to spread properly, the drop-cast film on the IDE was dried in a hot air oven.

Fig. 3

(a) Schematic of the preparation of a sensitive layer (PEI–Cr₂O₃) and coating on the IDE, (b) image of homogenized PEI–Cr₂O₃ nanocomposite, (c) image of PEI–Cr₂O₃-coated sensor.

Figure 4 shows the functional components in the PEI–Cr₂O₃ nanocomposite sensor, which contains the four main elements of the sensitive layer of PEI–Cr₂O₃ to detect CO₂, a substrate, an electrode that serves to supply electric potential, and wires for connecting the sensors to the devices.

Fig. 4

(a) Schematic of fabricated CO₂ sensor (PEI–Cr₂O₃), (b) image of the sensor.

PEI–Cr₂O₃ nanocomposite sensor setup and measuring of CO₂Gas

Figure 5 shows an instrumental setup used for testing the PEI–Cr₂O₃ nanocomposite sensor. Primarily, two major gases were used in the study:

• 1. Carbon dioxide (CO₂) gas served as the target gas

• 2. Nitrogen (N₂) gas was used as the carrier, purge, and dilution gas instead of air, to avoid the probable effect of moisture and oxygen present in the air.

Fig. 5

Schematic of the instrumental setup.

The results were examined at room temperature in an atmosphere of nitrogen gas. A mass flow controller was used to accurately control the concentration of CO₂ and N₂ .

The sensors were kept inside a sealed chamber while nitrogen gas was passed through to mitigate the effect of humidity on the capability of the sensors. Also, the gas inflow rate was kept constant at 300 mL/min so that a strong reference line could be obtained quickly. The electric resistances of the four separate sensors against CO₂ were determined by a Keithley 2700 model multimeter (Keithley Instrument). A personal computer collected the data or equivalent information with appropriate hardware and software. The following are some of the steps taken to evaluate the sensors' performance.

• Step 1. CO₂ and N₂ were considered as the target and carrier gases.

• Step 2. The gas concentration was precisely controlled by a mass flow controller.

• Step 3. The sensors were kept in a sealed test chamber.

• Step 4. N₂Gas was passed to reduce the effect of humidity on the sensors' operational capability. CO₂ gas was also introduced to the test chamber via a gas inlet.

• Step 5. The gas inflow rate was kept constant at 300 mL/min to quickly obtain a strong reference line.

• Step 6. A Keithley 2700 model multimeter was used to test the electric resistance of the sensors against CO₂

• Step 7. Data collection and equivalent information acquisition was by PC.

Characterization

FESEM and EDX Analysis of Cr₂O₃ and PEI–Cr₂O₃

Cr₂O₃ samples were scanned at 30 KX and 50 KX with an electron energy of 5.00 kV. The surface analysis of a Cr₂O₃ and PEI–Cr₂O₃ nanocomposite is shown in Fig. 6, and the morphology of the surface of the Cr₂O₃ appears to be agglomerated with flower-like structures. At the nanometer scale, agglomerated and flower-like structures are visible, and Fig. 6 a and b shows morphological FESEM images at 500- and 100-nm scales. Due to the great purity of the produced Cr₂O₃, energy dispersive x-ray analysis of the Cr₂O₃ reveals only chromium and oxygen in its spectrum. Energy dispersive x-ray investigation of the PEI–Cr₂O₃ nanocomposite revealed the presence of chromium and carbon. The x-ray spectra and composition of the Cr₂O₃ and PEI–Cr₂O₃ nanocomposite analysis are shown in Fig. 7 and Tables I and II, respectively.

Fig. 6

FESEM images of (a) Cr₂O₃ nanoparticles at 500-nm scale, (b) Cr₂O₃ nanoparticles at 100-nm scale, and (c) PEI–Cr₂O₃ nanocomposite at 100-nm scale.

Fig. 7

EDX analysis of (a) Cr₂O₃ nanoparticles (b) PEI–Cr₂O₃ nanocomposite.

Table I Elemental composition of Cr₂O₃ nanoparticles

Table II Elemental composition of PEI–Cr₂O₃ nanocomposite

UV–Visible Spectrophotometry Analysis of Cr₂O₃

The UV–visible spectra analysis of the synthesized Cr₂O₃ shows strong absorption peaks at 298.4 nm, 420.8 nm, and 584 nm, which are shown in Fig. 8, corresponding to Cr₂O₃. The band gap of the Cr₂O₃ was obtained by the Tauc plot, as shown in Fig. 9. The band gaps obtained from the Tauc plots for the absorption peaks of 298.4 nm, 420.8 nm, and 584 nm are 2.093 eV, 2.955 eV, and 4.155 eV, respectively.

Fig. 8

UV–visible spectra of synthesized Cr₂O₃.

Fig. 9

Tauc plot of Cr₂O₃.

X-ray Diffractometer Analysis of Cr₂O₃

The XRD pattern of Cr₂O₃ is shown in Fig. 10, revealing a crystalline nature and peaks at 24.501, 34.501, 37.001, 42.501, 50.001, and 54.501, ensuring the synthesis of Cr₂O₃. The interplanar distance of cerium oxide was calculated using Eq. 1, and the relevant results of each peak are shown in Table III. The average crystallite size of Cr₂O₃ was calculated using Eq. 2 for a maximum peak at 34.501 and Dc = 13.81 nm:

$$ d = \frac{n \lambda }{{2 \sin \theta }} $$

(1)

where d is the interplanar distance, n the order of reflection (n = 1), λ the wavelength of characteristic x-rays (0.15418) and θ the x-ray incidence angle or Bragg's angle (11.1088).

$$ \begin{aligned} Dc = & \frac{0.94 \lambda }{{\beta \cos \theta }} \\ = & \frac{{0.94 \left( {0.15418} \right)}}{{\left( {0.01099} \right) \cos \left( {17.2505} \right)}} \\ = & \frac{0.1449}{{\left( {0.01239} \right) 0.95501}} \\ = & \frac{0.1449}{{0.01049}} \\ = & 13.81 nm. \\ \end{aligned} $$

(2)

Fig. 10

XRD pattern of Cr₂O₃.

Table III Interplanar distance analysis results of Cr₂O₃

Results and Discussion

Resistance versus Gas Concentration Studies of PEI–Cr₂O₃ Samples

Resistance versus gas concentration of CO₂ was measured for the PEI–Cr₂O₃ nanocomposite sensors of 0.25, 0.50, 0.75, and 1.00 wt.%. Figure 11 shows that, as the wt.% of PEI–Cr₂O₃ increases, the resistance of the sensor reduces, due to the combination of PEI and Cr₂O₃ and its interaction with CO₂ and N₂ , but the resistance of the sensor increases for the individual wt.% for a varied concentration of CO₂. Resistance versus gas concentration for 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% PEI–Cr₂O₃ nanocomposite sensors are shown in Table IV. The resistance of 0.25 wt.% PEI–Cr₂O₃ sensors is lower than that of other wt.%.

Fig. 11

Resistance versus gas concentration of (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, and (d) 1.00 wt.% of Cr₂O₃ in PEI samples.

Table IV Analysis of resistance against gas concentration results of PEI–Cr₂O₃ nanocomposite sensors

Sensitivity Analysis of PEI–Cr₂O₃ Samples

Sensitivity is one of the key exposition descriptors for detecting materials to identify CO₂. The sensitivity of PEI–Cr₂O₃ nanocomposite sensors was evaluated using Eq. 3, where the sensor's sensitivity increases with an increase in wt.% of filler material. As seen in Fig. 12, the sensor's sensitivity grew gradually until it reached 1000 ppm CO₂ gas, after which it declined. Table V shows the sensitivity analysis results for 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% PEI–Cr₂O₃ nanocomposite sensors for different concentrations of CO₂ at the ppm level. The maximum sensitivity% of 1.23 was obtained for the 1.0 wt.% PEI–Cr₂O₃ sample and the minimum (0.90) for the 0.25 wt.% sample. Based on the sensitivity results, the maximum sensitivity was obtained for all wt.% at 1000 ppm of CO₂. As a result, experiments such as repeatability for three cycles, as well as response and recovery times for the entire cycle, were carried out at 1000 ppm CO₂.

$$ Sensitivity \; = \;\frac{\Delta R}{{R_{0} }}*100. $$

(3)

Fig. 12

Sensitivity plots of (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, and (d) 1.00 wt.% of Cr₂O₃ in PEI samples.

Table V Sensitivity analysis results of PEI–Cr₂O₃ nanocomposite sensors

The sensing mechanism of the PEI–Cr₂O₃ nanocomposite CO₂ sensor is as shown in Fig. 13, and involves the interaction of amino groups of PEI at room temperature to form carbamates by a reversible reaction, whereas, for Cr₂O_3, it is by physisorption. Desorption was carried out with the aid of nitrogen.

Fig. 13

Sensing mechanism of PEI–Cr₂O₃ nanocomposite CO₂ sensor.

Repeatability Studies of PEI–Cr₂O₃ Samples

PEI–Cr₂O₃ nanocomposite sensors with 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% repeatability were investigated (Table VI). As illustrated in Fig. 14, PEI–Cr₂O₃ nanocomposite sensors were subjected to 1000 ppm CO₂ for three cycles, each of which took 8 min (4 min for adsorption and 4 min for desorption). Table VII shows the repeatability results of PEI–Cr₂O₃ nanocomposite sensors. The repeatability of 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% of PEI–Cr₂O₃ nanocomposite sensors were similar. Hence, it can be determined that the repeatability of 1.00 wt.% sensors was better than the others.

Table VI Sensitivity of the nanocomposites for CO₂ sensors

Fig. 14

Repeatability plots of (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, and (d) 1.00 wt.% of Cr₂O₃ in PEI samples.

Table VII Repeatability studies results of PEI–Cr₂O₃ nanocomposite sensors

Response and Recovery Time Analysis of PEI–Cr₂O₃ Samples

The whole cycle involving adsorption and desorption of PEI–Cr₂O₃ nanocomposite sensors has been examined in terms of response and recovery times. Sensors were exposed to a CO₂ concentration of 1000 ppm until they reached saturation values, followed by desorption with the aid of N₂ until they reached the initial resistance values. It can be seen from Fig. 15 and Table VII that the response time and recovery time of complete cycles of 0.75 wt.% and 1.00 wt.% sensors were about 14 min, which was longer than for the 0.25 wt.% (12 min.) and 0.5 wt.% (13 min) sensors.

Fig. 15

Response and recovery time plots of (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, and (d) 1.00 wt.% of Cr₂O₃ in PEI samples.

Ta and Tb Plots Studies of PEI–Cr₂O₃ Samples

T_a and T_b represent the response time and recovery time of the PEI–Cr₂O₃ nanocomposite sensors, respectively, where the response time is defined as the time taken to reach 9% of equilibrium after the injection of the test gas relevant to changes in the electrical resistance (Table VIII).

Table VIII Analysis of PEI–Cr₂O₃ nanocomposite sensor response and recovery times

The recovery time is defined as the time taken to reach 1% of the electrical resistance when the test gases were removed from the sensor environment. Table IX shows the values of time and $\Delta_{R} /R_{0}$ for the PEI–Cr₂O₃ samples sensors with 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.%.

Table IX T_a and T_b analysis results of PEI–Cr₂O₃ nanocomposite sensors

Figure 16 shows T_a and T_b plots of the PEI–Cr₂O₃ nanocomposite with individual wt.%, and a consolidated plot involving all the results of the 0.25, 0.50, 0.75, and 1.00 wt.% sensors. From Fig. 16 and Table X, it is concluded that the 0.25 wt.% PEI–Cr₂O₃ nanocomposite sensor shows a quick response time of 20 s and a recovery time of 22 s, compared to the other sensors.

Fig. 16

Ta and Tb plots of (a) 0.25 wt.%, (b) 0.50 wt.%, (c) 0.75 wt.%, and (d) 1.00 wt.% of Cr₂O₃ in PEI samples.

Table X Individual T_a and T_b of PEI–Cr₂O₃ nanocomposite sensors for different wt.%

Conclusions

In this empirical research, the pertinent materials chosen for the efficient sensing of CO₂ were PEI and Cr₂O₃ used with diverging weight percentages at room temperature. In addition, PEI–Cr₂O₃ nanocomposite film of diverse Cr₂O₃ concentrations was fabricated by drop-casting sensitive films on IDE. The IDEs were created by the copper clad to serve as a substrate. X-ray diffractometry studies showed the crystalline nature and peaks at 24.501, 34.501, 37.001, 42.501, 50.001, and 54.501, which ensures the synthesis of Cr₂O₃. The interplanar distance of the synthesized Cr₂O₃ varied from 0.16 nm to 0.36 nm, and the crystallite size of the maximum at 34.501 is 13.81 nm. The UV–visible spectra analysis of the synthesized Cr₂O₃ shows strong absorption peaks at 298.4 nm, 420.8 nm, and 584 nm. The band gaps obtained from the Tauc plots for the absorption peaks were 2.093 eV, 2.955 eV, and 4.155 eV. FESEM investigation of the agglomerated flower-like shape of the Cr₂O₃ nanoparticles was at 500 and 100 nm dimensions. EDAX spectra of the Cr₂O₃ nanoparticles showed peaks relevant to chromium and oxygen due to the high purity of the Cr₂O₃ nanoparticles. Due to the interaction of PEI and Cr₂O₃ with CO₂ and N₂, the resistance of the sensors reduces as the wt.% of PEI–Cr₂O₃ increases. The sensitivity of the PEI–Cr₂O₃ sensors increased gradually until the CO₂gas of 1000 ppm, and, afterwards, it decreased deliberately, and a maximum sensitivity% of 1.23 was obtained for the 1.0 wt.% PEI–Cr₂O₃ sample. The repeatabilities of the 0.25 wt.%, 0.50 wt.%, 0.75 wt.%, and 1.00 wt.% of PEI–Cr₂O₃ nanocomposite sensors were similar. Hence, it can be determined that the repeatability of the 1.00 wt.% sensor was better than the others. The 0.25 wt.% PEI–Cr₂O₃ nanocomposite sensor shows a quick response time of 20 s and a recovery time of 22 s.