Detection of Carbon dioxide

Carbon dioxide (CO₂) is a colorless, odorless, tasteless, and heat-trapping gas that is naturally present in the atmosphere. Usually, it generates from the extraction and burning of fossil fuels including coal, natural gas, and oil [1]–[3]. Besides, wildfires and natural processes like volcanic eruptions are also key sources that generate CO₂ [4]. The consumption of fossil fuel as the traditional energy source to fulfill energy requirements adds to 40% of total CO₂ gas emissions globally [5]–[7]. CO₂ is the foremost byproduct produced by industries. Most of the industrial processes are highly energy intensive and account for one-third of the world’s total energy usage. The CO₂ emissions mainly from chemical, cement, paper, iron, steel, and petrochemical industries are gradually increasing worldwide [8]. Cement is the major industrial producer of CO₂, accounting for 15-35% of total CO₂ emissions, compared to 10-15% from coal-fired power plants and 5% from gas-fired power plants [9]. The construction industry contributes 5% of the total CO₂ emission. Moreover, the continuous demand for manufactured goods and infrastructure dramatically increases CO₂ gas emissions. It is anticipated that if CO₂ gas emission by industries remains unattended, total CO₂ emissions are projected to be increased by 90% by 2050 compared to 2007 globally. Particularly in Malaysia, the annual CO₂ emission was reported as 272.62 million tons in 2020, which was 0.78% annual share of total CO₂ gas emission globally as shown in Figure 1.1 (a) [1], [10]. Recently, NASA  reported that the CO₂ gas level increased to 419.48 ppm in September 2022 (see Figure 1.1 (b)), which was ~280 ppm during the pre-industrialization era (the 1950s) [4]. Although CO₂ at 419 ppm (0.04%) is not hazardous under normal environmental conditions and prolonged exposure to such CO₂ level is not immediately life-threatening, it may have health consequences for healthy individuals as well as sensitive populations [2]. The continuous rise in CO₂ gas concentration in the atmosphere can trigger adverse effects on human, plant, and animal health.

According to the world health organization (WHO), 90% of individuals inhale in a polluted environment, causing up to seven million people to die every year. CO₂ does not only cause asphyxiation by hypoxia, but it also causes respiratory arrest within 1 min at a high concentration [3], [12]. In addition, CO₂ absorption is the main cause of lung Cancer and heart disease [13], [14]. One-third of fatalities globally occur due to increasing CO₂ absorption. Moreover, increased CO₂ concentrations in the atmosphere cause the surge in daily temperature change resulting in global warming which is a serious problem that has altered the whole human life. From a human health point of view, high concentration of CO₂ gas is hazardous and cause wheezing (>20000 ppm), tremors and loss of consciousness (>100000 ppm), and death (>250000 ppm) [2], [6]. As, CO₂ gas is 1.5 times heavier than air, making it accumulate near the ground. Owing to this, the confined places become vulnerable to CO₂ build-up. In these confined places, there is a high probability of displacement of oxygen by CO₂. In confined places such as offices, the low concentration (<1000 ppm) of CO₂ may harm the occupants’ health and comfort, resulting in their performance degradation. Therefore, monitoring indoor and outdoor CO₂ gas will be effective in alleviating natural and anthropogenic hazards [3], [15], [16].

For monitoring of indoor and outdoor CO₂ quality in the air, optical (nondispersive infrared sensors – NDIR) CO₂ sensors have been typically utilized. These sensors are effective in detection but they are usually complex, bulky, expensive, require skilled operators, and their response easily gets affected by humidity, and fog [2], [17]–[19]. Alternatively, different state-of-the-art chemiresistive CO₂ sensors have been explored widely owing to their quality such as their smaller size, easy fabrication, low cost, and low power consumption [20]. In these gas sensors, the sensing layer is the heart. The sensing layer is developed using different advanced materials. Functionally, the sensing material interacts with analyte gas molecules (adsorption/desorption) and the variations in the surface properties of the sensing material are mainly responsible for the sensing response. Numerous organic and inorganic [5], [21], [22] have been employed for CO₂ sensing. Among these materials, metal oxides (MOS) such as cadmium oxide (CdO), zinc oxide (ZnO), tungsten oxide (WO₃), titanium dioxide (TiO₂), iron oxide (Fe₂O₃), copper oxide (CuO), nickel oxide (NiO), manganese oxide (Mn₂O₃), Tin oxide (SnO₂), and barium titanate (BaTiO₃) are primarily used for gas sensing [23]–[26].  These MOS-based CO₂ sensors are inexpensive, highly portable, and suitable for mass production, exhibiting good response (from a few seconds to minutes) and sensitive to a wide range of target gas concentrations [27]. However, the MOS-based sensors require high temperatures (>200 ⁰C) and high voltage to operate (10 – 100 V) [22], cannot be used at room temperature. Moreover, the lifetime of the MOS-gas sensor is reduced with high power consumption. Owing to these limiting factors, the MOS sensors are less suitable for gas detection at room temperature (RT) [28].

For RT CO₂ detection, various low-dimensional materials such as conducting polymers [29], metal carbides [30], and carbon materials [15]  have been explored. Among them, carbon-based materials such as graphene have demonstrated impressive sensing performance owing to their outstanding properties at room temperature such as high electronic mobility (200000 cm²/V.s), high thermal conductivity (2000 – 6000 W/m k), high electronic mobility (200000 cm²/V.s) [31], [32], high electrical conductivity (106 – 108 S/ m), high surface area (300 – 700 m²/g), high mechanical properties (1.03 – 1.06 TPa) [22], [33] and good environmental stability [34]. In addition, graphene is a competitively low-cost choice because it is efficient, cheap, suitable for mass production, and easy to integrate into electronic and control devices [11]. Therefore, graphene materials have high potential in contributing towards the development of fast and efficient room temperature CO₂ gas sensors.