1. Introduction

A chemical sensor is defined as a device that transforms chemical information into an analytically useful signal ^[1]. Generally, chemical information originates from the chemical reaction of the analyte with the active part of the sensor that induces a variation of the properties of the sensor. However, not only the chemical reaction but also physical processes (adsorption-desorption), mechanisms of electrical conductivity of sensitive material, etc., contribute to the overall sensitivity of the sensor ^[1]. Chemical/gas sensors are identified as excellent candidates for the detection and quantification of chemical/gas compounds due to their direct electronic interface, fast response, high sensitivity, and low cost of production ^[1],^[2],^[3],^[4],^[5],^[6],^[7],^[8],^[9]]. In general, chemical/gas sensors are based on oxidation or reduction of the active material by the surrounding atmosphere. Numerous types of chemical/gas-sensing materials including metal oxide semiconductors ^[2],^[3],^[4],^[5],^[6], conducting polymers ^[7], conducting polymer composites ^[8],^[9], carbon nanomaterials ^[10], metal oxide/polymer composites ^[11],^[12], and various transduction mechanisms have been reported over the past decades.

Metal oxide (MOX) semiconductors may be divided in two different groups depending on their majority carriers. For instance, in n-type MOX semiconductors, such as ZnO, In 2O 3, Fe 3O 2, TiO 2, WO 3, SnO 2, the majority carriers are electrons, while in p-type, NiO, Co 3O 4, Cr 2O 3, Mn 3O 4, CuO, the majority carriers are holes ^[13],^[14],^[15]. In contrast to n-type MOX gas sensors, p-type MOX ones are less studied, and the reported works are still in a premature stage of development. Figure 1 shows the results of p-type and n-type MOX gas sensors in Web of Knowledge on 30 June 2021 with the keyword “metal oxide gas sensor.” Only 13% of articles report on the p-type MOX gas sensors out of a total of 24,234 articles concerning MOX gas sensors. Irrespective of the number of reported works, p-type metal oxides have shown excellent performances in chemical/gas sensing owing to the majority of charge carriers, their conduction paths, resistance to humidity influence on sensing performances, superior catalyst properties, long-term stability, and lower open circuit resistance in contrast to n-type MOX gas sensors ^[16]. Furthermore, oxygen is excellently adsorbed at p-type MOX surface at low temperature compared to n-type. Thereby p-type MOX gas sensors could potentially be used for the sensing of volatile organic compounds (VOCs) since they are highly oxidized by adsorbed oxygen at the surface compared to surface lattice oxygen ^[17],^[18],^[19]. Moreover, the excellent gas-sensing performances at low working temperatures in p-type MOX gas sensors have paved the interest for the detection and quantification of VOCs ^[16]. Thereby, the number of scientific research works was gradually increased on p-type MOX as shown in Figure 2 .

Figure 1. The reported scientific research on n-type and p-type metal oxide gas sensors in Web of Knowledge on 30 June 2021.

Consequently, scientific works on Co3O4 gas-sensing applications have been gradually increased ( Figure 2 ) due to tremendous sensitivity, response/recovery, and stability even at lower and elevated temperatures, superior electrical and chemical properties together with their abundance  ^[20],^[21]. Similarly, various nanostructures of Co3O4, nanoparticles ^[22],^[23], nanowires ^[24],^[25],^[26],^[27],^[28],^[29], nanorods^[30],^[31], nanosheets ^[32],^[33], nanocubes ^[34], nanoneedles ^[35], hollow microspheres ^[36],^[37],^[38], urchin-like structures ^[39],^[40],^[41], have been reported in the literature for the detection of CH3OH, C2H5OH, HCHO, CH4, CH 3COCH3, C6H6, C6H5CH3, C6H5(CH3)2, CO.

Figure 2. The advancement of p-type MOX gas sensors by yearly from the search on Web of Knowledge on 30 June 2021.

However, the significant improvements in the application of one dimensional (1D) nanostructures of Co 3O 4, on chemical/gas sensing have been attributed to the superior adsorption—desorption of chemical compounds which results in an outstanding response and reliability for chemicals/gases detection ^[42],^[43]. Additionally, the higher surface energy, crystalline quality, large intrinsic resistance modulation, number of reactive sites, and large surface to volume ratio of 1D nanostructures have significantly improved the gas-sensing performances ^[14],^[44]. 

2. Material and Sensing

The key parameters that can be useful when discussing chemical/gas sensors are response, selectivity, response time, recovery time, stability, gas detection limit, and operating temperature. A comprehensive but brief understanding on each parameter is useful prior to the detailed discussion on chemical/gas sensors.

Response may be described as the fraction of the resistance on the sensing material when interacting with the analyte gas (Rg ) and the one in air (Ra ). However, response vastly relies on the crystallite size, porosity, operating temperature, and film thickness. Selectivity determines the ability to identify the target gas in a mixture of gases and can be tuned with the operating temperature. However, MOX chemical/gas sensors may respond in similar manners toward different gas molecules ^[45]. Hence the selectivity is one of the paramount features when describing the sensing performances. Stability is the ability of the active material to keep its properties, such as electrical resistance in the case of conductometric sensors, constant over time. Response time is an evaluation of the dynamics of the sensor to achieve a stable value of the monitored sensor parameter, for example, the time to accomplish 90% of the final resistance of the sensor when interacting with the gas. Usually, the electrical resistance of the conductometric chemical/gas sensors changes when interacting with the analytes molecules as shown in Table 1. In general, a dynamic response curve shows the resistance/conductance variation of the sensor with time. While the recovery time may be calculated as the time interval necessary to get back to 90% of the resistance value in the air before the gas introduction as the airflow is restored. These parameters can be calculated from the dynamic response plot as shown in Figure 3. The detection limit is the least concentration of analyte gas that may be detected by the sensor. Lastly, working temperature is the temperature where the sensor is operating.

Table 1. Variation of the resistance in MOX upon the interaction of reducing or oxidizing gases.

Figure 3. Schematic illustration of a dynamic response curve of a conductometric chemical/gas sensor.

In general, bottom-up or top-down approaches are used for the growth of nanostructures. The experimental setup for bottom-up approaches may be low cost producing high crystallinity and purity nanostructures, but their alignment may be extremely difficult. Top-down approaches suffer from long preparation times, while they easily form structures directly on flat substrates. Concerning the growth of Co₃O₄ nanostructure as chemical/gas sensors, several growth techniques such as hydrothermal, electrospinning, and solvothermal were widely employed. 

3. Overview of Reported 1 D Nano-Structured Co3O4 Gas Sensors

3.1 Sensing Toward Ethanol (C₂H₅OH)

Ethanol is a VOC that is vastly present in the daily life of human beings in means of food, beverages, fuel-processing, pharmaceutical, as well as in many laboratories and industries for diverse research and applications ^[47]. However, long-term exposure to C2H5OH has been identified as the main reason for serious health effects such as lethargy, irritation to skin and eye, difficulty breathing, coma, liver damage, and intoxication. Moreover, C2H5OH as an alcoholic drink has been identified as one of the root causes for rising traffic accidents around the world ^[48]. For this reason, almost all countries have legalized the consumption limitation of C2H5OH. For example, in Italy, the highest ethanol level that is allowed in the breath of drivers is 130 ppm (0.05% in the blood) while in the USA, 208 ppm (0.08% in the blood) ^[49]. Hence, detection and quantification of C2H5OH in the environment as well as in human breath have become a necessity. Before discussing the C2H5OH-sensing applications it would be interesting to understand its sensing mechanism at MOXs surface. When ethanol is interacting with the near-surface irrespective of the type of MOXs sensor, these molecules are chemisorbed on the MOXs surfaces. Afterward, chemisorbed ethanol molecules start to react with the adsorbed oxygen species resulting in the formation of H2O and CO2. Hence the trapped electrons are liberated back to the MOXs altering the sensor resistance. The chemical reaction between C2H5OH and adsorbed oxygen species on the near-surface of the MOXs is described as in (1–2) regardless of the type of the MOXs ^[49].

C2H5OH (ads) + O¯ (ads) → CH3CHO (ads) + H2O + e− (1)

CH3CHO (ads) + 5O− (ads) → 2CO2 +2H2O + 5e− (2)

Electrospinning has been employed for the preparation of nanofibers (NFs) for the detection of C2H5OH by Yoon et al.^[50]. The sensors have shown superior sensitivity (Rg /Ra = 51.2) toward 100 ppm of C2H5OH compared to CO, C3H8, and H2 at 301 °C before decreasing to 19.2 at 336 °C. While the recovery time was decreasing as the operating temperature increases due to the thermal boosting of oxygen ionization.

3.2 Sensing toward Acetone (C₃H₆O)

A superior sensitive and selective performance toward acetone has been reported by Choi et al. on Co₃O₄ NFs composite with Ir NPs and graphene oxide (GO) sheets in 2014 [66]. Co₃O₄ NFs, have been grown by electrospinning technique while the GO sheets as well as Ir NPs were grown by the polyol method [68]. The composites NFs (1 wt% Ir-GO-Co₃O₄) were able to detect acetone even at 120 ppb in highly humid ambient conditions (90% RH) demonstrating great potentialities for acetone analysis in human breath [66]. In addition, the sensors have shown excellent selectivity toward acetone other than the usual breath biomarkers; pentane, NO₂, NO, NH₃, and CO. One of the reasons for these excellent performances is the combined catalytic effect of GO sheets and Ir NPs. A second one is the hole accumulation layer thinning on the surface of Co₃O₄ NFs due to Ir NPs together with the effective electronic sensitization by the GO sheets. In addition, the rapid transfer of the carrier to the sensing electrodes, due to the highly conductive GO sheets, contributed to improving the performances.

Another interesting morphology for chemical compounds detection is one of the hierarchical nanofibers (HNF). Its effective surface area is outstanding and there is a smooth transfer of carriers through nanofibers without additional barriers, due to the connection of the nanosheet-like structures in the nanofiber ^[51]. In 2019 Cao et al. have demonstrated C3H6O sensing with HNFs at an operating temperature of 190 °C. The grown HNFs comprised many nanosheets with a smooth surface almost perpendicular to the nanofiber’s surfaces (thickness of 20–40 nm) ^[51]. However, Zhang et al. have reported the possibility of detecting C3H 6O at an operating temperature of 75 °C with Co3O4 NWs assembled on hollow carbon spheres (Co3O4 NW-HCS) ^[23]. The enhanced sensing performances at low operating temperatures have been attributed to the interaction between hollow carbon spheres and short Co3O4 nanowires. Moreover, the unique porous cavity structure of hollow carbon spheres contributes to the excellent sensing performances by paving a higher number of available sites for easy adsorption and desorption of C3H6O molecules. 

3.3 Sensing toward Carbon Monoxide (CO)

In 2010 Patil et al. have demonstrated Co3O4 NRs sensing of CO with a response of 6.5 to 50 ppm CO at 250 °C ^[15]. The NRs sensors have demonstrated a higher response compared to the commercially available NP sensors due to the stronger bonding between nanoparticles in the NRs beside the higher surface-to-volume ratio. Additionally, the fabricated sensors have shown superior selectivity compared to H 2, LPG, CO2, and ethanol.

However, CO sensing at a low working temperature (100 °C) has been reported by Dou et al. in 2014 [16]. The NFs sensors demonstrated a higher response (~13.5 (R_g/R_a) toward 50 ppm CO. Moreover, the detection of 5 ppm at 100 °C has been reported by the Busacca et al. on NFs Co₃O₄ sensors in the same year [62]. The excellent CO-sensing response of 2.4 (R_g/R_a) together with a response time of 14 s and recovery time of 36 s of NFs was attributed to the higher effective surface area as well as the higher number of oxygen vacancies or defects on the NFs surface. Additionally, the lower crystallite size (about 15 nm) has been identified as one of the potential factors that could enhance sensor performances. However, the sensor response was saturated at a concentration of 20 ppm at 100 °C owing to the formation of carbonates species at a low temperature that deactivates the action of Co₃O₄ catalysts in CO oxidation.

The sensing mechanism can be understood as follows.

4. Conclusions and Outlook

Usually, Co3O4 is a p-type MOX semiconductor with excellent catalytic properties together with higher stability in chemical/gas testing even in humid conditions. Among the different growth techniques of growing the Co3O4 nano structures, hydrothermal, solvothermal, and electrospinning techniques have been vastly employed owing to higher yield, simplicity, and scalability. The excellent catalytic effect of the Co3O4 has been identified as one of the root causes for the achievement of exceptional sensing performance turning to operate at elevated working temperatures as well as the long-term stability. However, Co3O41D nano structures are still capable of detecting few gases such as CO, H2S, NH3, and some VOCs such as C2H5OH , C3H6O, HCHO, C4H 10O, and C3H9O 3P. Among these gases and VOCs, Co3O41D nano structures are highly selective toward C2H5OH and C3H6O. Besides, the sensing performances are highly dependent on the morphology where rougher topology along with the unique structure of Co3O4 NR arrays to a large extent could be the best choice for the superior gas-sensing performances. Nonetheless, low working temperature regimes can be proposed for the chemical/gas sensors based on Co3O4 ID nano structures owing to its excellent catalytic properties. However, a complete and well-controlled characterization of operating temperature on the key factors such as response time, recovery time of the Co3O4 1D chemical/gas sensors would not be far away because of rapid development of the new technologies adapted to nanomaterials.

In conclusion, a reliable material growth process and cheaper production cost of the devices must be achieved for large-scale production of both active material and transducer. Unfortunately, most of the reported sensors in this review article do not provide a significant indication for the possibility of large-scale production. However, various fabrication and characterization strategies have been developed over the past decade to demonstrate the possible application of Co3O4 1D nanostructures as reliable and cheaper chemical/gas sensors. As long as an appropriate optimization of the growth technologies will allow the controlled and reliable production of Co3O4 1D nanostructures together with improvement in the sensor’s performances, the large-scale production and their integration in commercial gas-sensing devices will certainly follow owing to the day by day improvements and progresses in materials research.

This entry is adapted from the peer-reviewed paper 10.3390/chemosensors9080197