Resistive-Based Nanostructured CeO₂ Gas Sensors: A Review

Abstract

Air pollution and the emission of toxic gases represent a critical global concern, posing significant threats to human health and environmental stability. Resistive gas sensors are widely employed to detect toxic gases, owing to their cost-effectiveness, high stability, sensitivity, and swift dynamics. Among various sensing materials, comparatively less attention has been paid to CeO₂ despite its good catalytic activity and high stability. In this review paper, we are focusing on CeO₂ gas sensors in pristine, doped, decorated, and composite forms. Using numerous examples, we have shown the great potential of CeO₂ for gas sensing. The main features of CeO₂ as a gas sensor include excellent environmental stability, the abundance of oxygen vacancies, high mechanical strength, cost-effectiveness, and good catalytic activity. However, low electrical conductivity is the main shortage of CeO₂ as a gas sensor. With a high emphasis on the sensing mechanism, we believe that this review paper is highly useful for researchers working in this field.

1. Introduction

Rare earth (RE) oxides have special properties due to the presence of the 4f shell in RE elements. Cerium has the highest abundance among REs (~0.0046 wt% of the Earth’s crust), and it is the least expensive RE element with a ground state valence configuration of 4f¹5d¹6 s² [1,2]. Two common oxides of Ce are Ce₂O₃ and CeO₂. CeO₂ with a pale-yellow color has a fluorite structure where eight O²⁻ anions are coordinated around each Ce⁴⁺ cation at the corners of a cube, with each O²⁻ coordinated by a tetrahedron of four Ce⁴⁺ ions [3]. More details about the structural properties of CeO₂ can be found in [4].

The main point defects in CeO₂ are cerium vacancy ($V_{Ce}^{\prime \prime \prime \prime }$) and oxygen vacancy ($V_O^{\prime \prime }$) and interstitial Ce (${Ce}_i^{\prime \prime \prime \prime }$) and oxygen defects ($O_i^{\prime \prime }$) [5]. Among them, oxygen vacancies are the most important defects for gas sensing applications, resulting in the formation of non-stoichiometric cerium oxide or ceria (CeO_2−x; 0 < x < 0.28), which is an n-type semiconductor with E_g around 2.8–3.1 eV [6]. Oxygen vacancies in CeO_2−x can be formed by reduction of Ce⁴⁺ to Ce³⁺, with the formation of oxygen vacancies as follows [7]:

$${CeO}_2\left(s\right)\to {CeO}_{2-x}(s)+{\displaystyle \frac{x}{2}}{O}_2(g)$$

(1)

There are many non-stoichiometric CeO_2−x phases such as CeO_1.65, CeO_1.717, CeO_1.775, CeO_1.812 [8]. The ionic radius of Ce³⁺ (115 pm) is larger than that of Ce⁴⁺ (101 pm), which leads to lattice expansion relative to stoichiometric CeO₂ [9]. Oxygen vacancy, derived from the reduction of Ce⁴⁺ to Ce³⁺, has a promising effect on the electrical and gas sensing features of CeO₂ [10]. CeO₂ has good mechanical properties, high thermal stability, high ionic conductivity, and photoluminescence in the UV-vis range. It is noteworthy that, despite its high ionic conductivity, CeO_2−x exhibits low conductivity at low temperatures.

CeO₂ is widely used in photocatalysts due to redox characteristics caused by Ce³⁺/Ce⁴⁺ transitions, capacity for oxygen storage, and the presence of oxygen vacancies [11]. In particular, CeO₂ is employed in three-way catalysts in car exhaust to decrease the emission of CO, NO_x, and hydrocarbons from automobiles, owing to its oxygen vacancies and low redox potential between Ce³⁺ and Ce⁴⁺ associated with oxygen vacancies [12]. Vacancies with high mobility are reactive sites for oxygen activation in CeO₂ and are critically important for the oxygen storage [13]. Apart from catalyst applications of CeO₂ [14], it is used in the realization of UV blocking and shielding devices, due to its high absorption cross-section on UV light and high stability upon exposure to UV irradiation [15]. Other applications include corrosion protective coating [16], biomedical applications [17], photodetectors [18], solar cells [19], batteries [20], and supercapacitors [21]. In particular, CeO₂ has also been used in the realization of various types of gas sensors, including electrochemical [22,23], optical [24], plasmonic [25], surface acoustic wave [26], impedance [27], resistive [28,29], and triboelectric nanogenerator [30] gas sensors. Also, due to lower phonon energy and high luminescence quantum yields, CeO₂ is used in cataluminescence gas sensors [31]. Like most of the semiconducting metal oxides [32,33], CeO₂ has good potential for the realization of gas sensors. The main features of CeO₂ as a material for the realization of gas sensors are cost-effectiveness, excellent environmental stability, the abundance of oxygen vacancies, high mechanical stability, and good catalytic activity. However, low electrical conductivity is the main shortage of CeO₂ as a gas sensor.

As indicated in Table S1 [34], resistive gas sensors are among the best types of gas sensors due to their high sensitivity, high stability, fast response time, and low cost. To our knowledge, there is no review paper on the gas sensing properties of CeO₂. This review paper explains different aspects of CeO₂-based resistive gas sensors for the first time. Also, we have included the results of some theoretical studies. The ongoing advancements in sophisticated supercomputers and the improvement of computational power have led to a better understanding of the properties of materials. Given that gas sensor reactions involve adsorption, reaction, and desorption phenomena, in which the bonds are formed or broken, usually accompanied by charge transfer, quantum methods are excellent in examining gas–surface interactions, as they effectively model adsorption mechanisms, calculate adsorption energy, analyze charge transfer, and assess electronic modifications and structural changes following gas adsorption. In this context, density functional theory (DFT) calculations stand out as a robust technique for simulating the gas-sensing capabilities of various materials without adverse effects. DFT is essential for interpreting experimental results at the molecular level and serves as a predictive tool for designing new sensing materials [35,36,37,38].

2. General Gas Sensing Mechanism of Resistive Gas Sensors

Like most other gas sensors, such as quartz crystal microbalance [39], in resistive gas sensors, the target gas should be adsorbed on the sensor surface to be sensed; hence, morphology is a key parameter in gas sensors. Obviously, the morphologies with higher surface area are more beneficial for gas sensing applications as they provide more adsorption sites for gas molecules. Furthermore, chemical composition is another affecting parameter on the gas response. In particular, in nanocomposites, interfaces between two materials act as powerful sources of resistance modulation. Defects are another parameter in boosting gas response. They act as favorable sites for gas species, and in this regard, oxygen vacancies are highly favorable for the adsorption of oxygen gas at high temperatures. Sensing temperature itself affects the sensing response, as high temperatures generally increase the rates of gas diffusion and sensing reactions, leading to a higher sensing response. However, the rate of desorption is also high at higher temperatures, and this leads to the occurrence of maximum net adsorption at a particular temperature, known as the optimal sensing temperature. Crystallinity is another important factor, as highly crystalline materials can better provide the pathways for the flow of charge carriers. Finally, the particle size of the sensing material should be considered. When the particle size is very fine, the whole particle can be depleted of electrons in the air, and by exposing it to a reducing gas, a significant amount of resistance modulation occurs, resulting in the appearance of a high sensing signal. These factors are summarized in Figure 1a. When an n-type metal oxide such as CeO₂ is exposed to air, the oxygen with high electron affinity can be adsorbed on the surface of the sensor, resulting in the creation of an electron depletion layer (EDL) where the concentration of electrons is much lower relative to the core region of the sensor. Upon exposure to a reducing gas, it reacts with adsorbed oxygen species, resulting in the release of electrons. Thus, the width of EDL significantly decreases, and this is reflected in a reduction in resistance. In contrast, when an n-type gas sensing is exposed to an oxidizing gas, further electrons are abstracted from the sensor surface, resulting in an expansion of EDL and an increase in resistance. (Figure 1b). Also, corresponding band diagrams are depicted in Figure 1c, where band bending decreases and increases in the presence of reducing and oxidizing gases, respectively [40,41].

3. CeO₂-Based Resistive Gas Sensors

3.1. Pristine CeO₂ Gas Sensors

Pristine CeO₂ gas sensors have been employed for the detection of various gases, including NH₃ [42], oxygen [43], H₂ [44], and other gases. Notably, operating a gas sensor at RT not only significantly increases its stability but also significantly decreases power consumption, which is crucial from an energy-saving perspective. Therefore, Li et al. [45] synthesized CeO₂ nanoparticles (NPs) using a hydrothermal approach at 180 °C for 48 h and subsequently annealed them at 800, 900, and 1000 °C. The NPs annealed at 900 °C had the highest total amount of oxygen vacancies and adsorbed oxygen species. Also, the band gap values of the CeO₂ NPs annealed at the above-mentioned temperatures were 3.28, 3.18, and 3.30 eV, respectively. Among all sensors, the sensor annealed at 900 °C displayed the largest response (R_a/R_g) of 23 to 500 ppm NH₃ gas at room temperature (RT). This improvement was primarily due to the existence of oxygen vacancies, leading to the adsorption of a significant amount of oxygen, and hence, more sensing reactions occurred. In another research, Li et al. [46] synthesized CeO₂ nanorods (NRs) NPs (8–12 nm), and (NSs) (100–140 nm) by the solvothermal method at 100 °C for 24 h and at 160 °C for 8 h, respectively. At 100 °C, the response (R_a/R_g) of CeO₂ NRs gas sensor to 1000 ppm H₂ gas was ~6, while that of CeO₂ NPs gas sensor was only 3. The enhanced performance was attributed to the availability of more adsorption sites on CeO₂ NRs. In fact, the specific surface area (SSA) values of NRs and NPs were 21 and 3.5 m²/g, respectively, resulting in providing of more anchoring sites for H₂ molecules.

Hussain et al. [47] synthesized CeO₂ nanostructures using a hydrothermal route at 160 °C for 12 and 24 h. The SSA of the samples prepared for 12 and 24 h (polyhedron nanostructures) was 65 and 99 m²/g, respectively. The sensor prepared for 24 h indicated a response (R_a/R_g) of 150 to 100 ppm TEA at 100 °C. The sensing mechanism was attributed to high SSA, providing numerous sites for gas adsorption. Furthermore, the exposed facets in the optimal sensor were favorable sites for gas adsorption, and they also accelerated the adsorption of gas molecules. Meanwhile, the dissociation energy of HCHO was less than that of interfering gases, indicating the ease of breaking the C-H-O bond and dissociating into smaller molecules, which then undergo oxidation on the sensor surface.

Polar facets of CeO₂ are highly effective in accelerating the adsorption of oxygen and enhancing the gas response. Bi et al. [48] synthesized CeO₂ nanopolyhedra, NRs, and nanocubes (NCs) by a hydrothermal reaction at 100 °C in the presence of different amounts of NaOH (Figure 2a–d). Amounts of oxygen vacancies in nanopolyhedra, NRs, and NCs were 32.7, 33.8, and 34.7%, respectively. Also, SSA of nanopolyhedra, NRs, and NCs were estimated to be 110.60, 101.70, and 27.25 m²/g, respectively. At 300 °C, the response (R_a/R_g) of gas sensors fabricated from nanopolyhedra, NRs, and NCs was 10, 2, and 150, respectively, to 100 ppm dimethylamine (DMA). In case of {100} and {111} facets, only cerium atoms are terminated in the planes; however, each Ce atom on the {100} facet has two dangling bonds, while in the {111} facet, it has only one dangling bond. This led to a higher adsorption capacity of {100} facets compared to {111} ones. Also, the amount of oxygen vacancies was higher in {100} relative to {110} and {111} nonpolar facets. Furthermore, the stability of {100} was lower than that of {111} and {110} facets, reflecting the higher selectivity of {100} facets towards gas adsorption. Therefore, in the optimal sensor, there were a greater number of exposed {100} facets, along with a higher content of oxygen vacancies, giving rise to a higher response to gas (

Table 1. Comparison of features of different facets in CeO₂.

Amount of Oxygen Vacancies	Stability	Adsorption Capacity
{100} > {110} > {111}	{111} > {110} > {100}	{100} > {110} > {111}

).

Hollow nanostructures have the advantage of high SSA, in which both inside and outside of the nanostructure are suitable sites for gas adsorption. Lyu et al. [49] synthesized hollow CeO₂ microspheres (300 nm) using a hydrothermal approach at 130 °C for 24 h using a SiO₂ template. At 260 °C, the sensor gave a response [(ΔR)/R_a) × 100] of 270% to 100 ppm acetone. The SSA of synthesized powders was 137 m²/g, providing numerous sites for acetone gas adsorption. Based on DFT calculations, the band gap of the sensor was modulated from 2.75 eV to 1.90 eV upon exposure to acetone, due to the transfer of electrons from acetone to the sensor.

Quantum dots (QDs) with ultrafine size and unique electrical properties are highly promising for sensing applications [50]. Rohit et al. [51] synthesized CeO₂ quantum dots (QDs), nanopebbles, with sizes of 3, 15, and 30 nm, respectively, using a hydrothermal approach at 110 °C for 5 h. Among fabricated sensors, the sensor fabricated from CeO₂ QDs revealed a response (R_a/R_g) of 4.2 to 10 ppm H₂ gas at 220 °C. It was related to the formation of more homojunctions between CeO₂ QDs relative to other morphologies. Furthermore, the size of CeO₂ QDs was smaller than the Debye length of CeO₂, resulting in depletion of the entire QDs in the presence of air, and by exposure to H₂ gas, a remarkable modulation of resistance led to large resistance modulation in the presence of gas relative to other gas sensors (Figure 3).

3.2. Doped CeO₂ Gas Sensors

Doping of cations with different valences into the CeO₂ lattice causes the formation of structural defects. For instance, when acceptor cations are doped into the CeO₂ lattice, oxygen vacancies are formed to maintain charge neutrality [52]. Dong et al. [53] synthesized Ni-doped CeO₂ flower-like structures through a precipitation method. The samples were coded as Ce_1−xNi_xO_2−x, where x = 0.01, 0.02, 0.03, 0.06. The size of pristine CeO₂ flower-like structures was 3 µm, and upon Ni²⁺ doping, lattice distortion and contraction occurred, which eventually changed the initial morphology of CeO₂. At 200 °C, the sensor with a composition of Ce_0.97Ni_0.03O_1.97 revealed an enhanced response (R_a/R_g) of 3.2 to 500 ppb H₂S gas. While SSA of pristine CeO₂ was 49 m²/g, it increased to 66 m²/g for the optimal gas sensor. With Ni doping, the number of Ce³⁺ increased while that of Ce⁴⁺ decreased, which eventually led to formation of more oxygen vacancies. They acted a favorable site for oxygen gas adsorption at sensing temperature. In situ DRIFTS measurement (Figure 4a,b) confirmed that H₂S diffused into the sensing layer and adsorbed onto the basic sites to produce S²⁻ ions. Subsequently, they were oxidized to SO₂ by Ni³⁺ with the formation of Ni²⁺ and oxygen vacancies. In next step, Ni²⁺ was oxidized, and oxidation of Ce³⁺ to Ce⁴⁺ occurred by oxygen gas. Based on DFT calculations, H₂S had lower energy to be adsorbed on the {110} facet of the gas sensor.

Tofu with low-fat and high-protein is easily deteriorated during storage, with the release of NH₃. Thus, monitoring of NH₃ released during storage is a reliable way to evaluate the freshness of Tofu [54]. In-doped CeO₂ nanostructures were obtained using a hydrothermal approach at 160 °C for 12 h [55]. As a result of In-doping, oxygen vacancies were generated inside the CeO₂ lattice. The sensor gave an ultra-high response (R_a/R_g) of 5412.93 to 1000 ppm NH₃ gas at RT. Also, little change was observed in the response in a highly humid environment. Furthermore, the sensor had very fast dynamics, with a response time and recovery time of 1.3 and 1.4 s, respectively. Based on the DFT study, the In-doped CeO₂ sensor had the highest adsorption energy for NH₃, relative to other gases. Furthermore, NH₃ had the highest net charge transfer due to the polarity of the gas. The authors tested the sensor against tofu stored at 4 °C, 27 °C, and 37 °C every 5 h, and it successfully detected increased emission of NH₃ from tofu over time.

Wu et al. [56] obtained pristine and Ce-doped (0.17, 0.43, and 0.85 at%) TiO₂ nanostructures using a hydrothermal route at 120 °C for 1 h. The SSA of the samples with 0, 0.17, and 0.43 at% Ce-doping were 213.2, 148.7, and 267 m²/g, respectively. The sensor with 0.43 at% Ce-doping yielded a response (R_a/R_g) of 25 to 20 ppm NH₃ at RT, surpassing that of the pristine sensor by more than five times. The improved response was not only related to the high SSA of the sensor but also due to the presence of the largest amount of oxygen vacancies in the optimal sensor. During the spoilage process, fish’s fatty acids will be decomposed, and NH₃ can be released from the fish. The authors successfully detected the release of NH₃ from fish stored at RT. In another study [57] related to Al-doped CeO₂, the enhanced response to NO₂ gas was due to the generation of oxygen vacancies as a result of Al-doping.

Tungsten, a refractory metal due to its high valence and small ionic radius (r_W6+ = 62 pm), can be doped into CeO₂ to adjust the Ce³⁺/Ce⁴⁺ ratio, which increases the amount of oxygen vacancies, resulting in enhanced gas sensing output. Duan et al. [58] synthesized W⁶⁺-doped (1–12 at%) CeO₂ NPs using a hydrothermal route at 160 °C for 24 h. After doping, the morphology remained unchanged, while the porous nature increased. In particular, the SSA (96.32 m²/g) of 7 at% W⁶⁺-CeO₂ was higher than that of pristine CeO₂ (58.63 m²/g), offering more adsorption sites for gas molecules. W-doping regulated the oxygen vacancies by changing the concentration of Ce³⁺. For the sample with 7 at%, W⁶⁺-doping revealed the highest amount of oxygen vacancies. At 225 °C, the sensor with 7 at% W-doping revealed the highest response (R_a/R_g) of 18.6 to 10 ppm H₂S in the presence of 80% RH. Based on DFT calculations, the adsorption energy of H₂S on 7 at% W-doped CeO₂ was almost four times more negative than that of the pristine sensor, demonstrating higher sensitivity of the doped sensor relative to the pristine sensor. A linear discriminant analysis (LDA) algorithm was constructed for classifying various concentrations of H₂S and ethanol. An accuracy of 89% was obtained through cross-validation. Also, as proof of application, a total of 15 breath samples were collected from healthy people, and some of the samples were mixed with 100 ppb of H₂S in bags to simulate halitosis in exhaled samples. Furthermore, in some simulated halitosis exhaled samples, 200 ppb of ethanol was mixed. The fabricated device was able to identify the samples containing TEA even in the presence of ethanol with 100% accuracy.

Xylene is a toxic and carcinogenic VOC, and thus, the development of xylene gas sensors is important [59]. Qin et al. [60] developed a series of Ru-doped (0.12–1.16 wt%) CeO₂ nanosheets synthesized using a solvothermal method. The incorporation of Ru effectively modulated the morphology from NRs to porous NSs. The WO₃/Ru-doped CeO₂ bilayer sensor was constructed by using WO₃ NFs as the lower sensitive layer and Ru-doped CeO₂ as the upper catalytic layer. At 160 °C, the response (R_a/R_g) of the sensor with 0.9 wt% Ru dopant to 5 ppm xylene was 37, which was higher than that of the WO₃ NF sensor. The combination of online mass spectrometry and DFT calculations was employed to validate the enhanced sensing performance arising from the synergistic mechanism between the catalytic and sensing materials. In another study [61], it was reported that Ru doping in CeO₂ resulted in enhanced response to formaldehyde due to the catalytic effect of Ru and the creation of oxygen vacancies.

Jin et al. [62] synthesized Gd-doped CeO₂ NPs using a chemical synthesis method. The responses of the pristine CeO₂ gas sensor to 20 ppm H₂S gas were 1.542, while the response of the Gd-doped gas sensor to the same gas concentration was 3.489. The enhanced response of the GDC gas sensor to H₂S gas was mainly related to the formation of oxygen vacancies as a result of Gd-doping in CeO_2, and good selectivity to H₂S was attributed to the sensing temperature, the higher reactivity of H₂S relative to other gases, and the small bond energy of H-SH.

The production of H₂ and the utilization of biomass for sustainable energy conversion and storage require the availability of gas sensors with the ability to discriminate between H₂ and CO gases. Baier et al. [63] synthesized mesoporous Cu-doped CeO₂ NPs with large SSA and uniform porosity by a nanocasting method. Both Cu and Ce elements existed in both Cu⁺/Cu²⁺ and Ce³⁺/Ce⁴⁺ oxidation states. While the pristine CeO₂ sensor revealed poor performance without selectivity, the Cu-doped sensor revealed a larger response to CO than to H₂ and low cross-sensitivity to humidity. Thus, Cu had a promising role for selective sensing behavior of the gas sensor, due to its high catalytic activity towards CO gas oxidation.

Various dopants have been used in CeO₂ gas sensors, and the researchers have reported enhanced gas sensing capability after doping. Mainly related to the creation of oxygen vacancies in the CeO₂ lattice. However, no specific dopant has been reported for a specific gas.

3.3. Noble Metal Decorated CeO₂ Gas Sensors

Noble metals have high catalytic activity towards the oxidation of gases and VOCs [64]. Thus, noble metals generally enhance the gas sensing properties of CeO₂, due to their catalytic effect as well as electronic sensitization [65,66,67,68]. Generally, resistive sensors have weak selectivity, and a good approach to increasing their selectivity is to apply a filter on top of the sensing layer. In physical filters, interfering gases cannot pass through the apertures of the filter, and only desired gas molecules pass through the filter barrier. In fact, filters act as molecular sieves [69]. In contrast, chemical filters either convert interfering gases into inert substances or enhance the activity of the desired gas [70]. Li et al. [71] applied a catalytic filter using Pt-loaded CeO₂ over a porous SnO₂ layer. The special design avoided the problems of element diffusion and low adhesion caused by the poor contact between the sensitive film and the filter film. The filter was able to oxidize CO and ethanol into CO₂ and H₂O and activate CH₄ during the diffusion process. As a result, it promoted the breakage of C-H bonds in CH₄ and enhanced the interaction with SnO₂ sensing layer. Optimal sensor with 2 wt% Pt loading in filter layer, revealed a high response (R_a/R_g) of 12.4 to 1000 ppm CH₄ at 350 °C. Lower amount of Pt loading cause limited catalytic activity, while higher amount of Pt content led to agglomeration of Pt NPs and again decreased effective catalytic activity of Pt.

Pd is a highly active noble metal for the dissociation of hydrogen gas [72,73]. Wang et al. [74] synthesized CeO₂-ZnO NRs composite by a hydrothermal approach at 180 °C for 5 h, and then Pd (0.5, 1.5, and 2.5 wt%) was decorated on it using an impregnation method. At RT, the sensor with 1.5 wt% Pd loading revealed the enhanced response (R_a/R_g) of 300 to 1000 ppm H₂ gas. A lower amount of Pd did not provide sufficient catalytic activity, whereas a higher amount resulted in the agglomeration of Pd NPs, a decrease in effective SSA of Pd, and a decrease in catalytic activity. Also, Schottky junctions were formed at interfaces between Pd and CeO₂, resulting in the generation of potential barriers for the flow of electrons in air at these interfaces. Upon exposing to acetone and release of electrons, the height of Schottky barriers significantly decrease, resulting in generation of a sensing signal. Meanwhile, Pd can adsorb H₂ and form PdH_x with different electrical characteristics than metallic Pd.

3.4. Composite CeO₂ Gas Sensors

Composite sensors based on intimate contact of CeO₂ with other materials have been used for the identification of various gases in different composite systems [75,76,77,78,79,80,81,82,83]. Since CeO₂ has high resistance at RT, it is difficult to realize pristine CeO₂ gas sensors that can operate at RT. Therefore, adding graphene with high conductivity to CeO₂ can reduce resistance, and the resultant sensor can work at RT. Naganabonia et al. [84] synthesized graphene-CeO₂ nanocomposite and fabricated a sensor on a cellulose paper as substrate, having features such as non-toxicity, low cost, light weight, abundance, biocompatibility, and mechanical flexibility. The synthesized nanocomposite had a flower-like morphology comprised of plenty of nanosheets (NSs). At RT, the sensor showed a response (R_a/R_g) of 1.5% to 10 ppm CO gas with a response time of 120 s and a recovery time of 240 s. Also, the performance of the sensor after bending 300 times under a curvature radius of 10 mm remained unchanged, demonstrating the good flexibility of the sensor. The sensing mechanism was related to the formation of p-n heterojunctions between graphene and CeO₂, the high surface area of graphene, and the presence of defects on graphene, acting as favorable sites for gas adsorption. Although the sensor was able to work at RT, its performance was not satisfying for practical applications due to low sensitivity and long dynamics.

Oxygen vacancies are highly beneficial for gas sensing applications; however, their amounts must be optimized to acquire the best sensing performance. Zhang et al. [85] synthesized CeO₂-graphene nanocomposite through a hydrothermal approach at 200 °C for 24 h. The response to NO₂ increased when the concentration of Ce³⁺ increased from 14.6 wt% to 50.7 wt%, and then the response decreased. At RT, the optimal sensor exhibited a response [(ΔR)/R_a) × 100] of 10.6% to 5 ppm NO₂ gas. According to DFT calculations, when the amount of Ce³⁺ ions was less than 50.7 wt%, the surface of CeO₂ became metallic and the Fermi level shifted upward due to the existence of low-electronegativity Ce³⁺ ions. This led to a decrease in the height of the Schottky barrier between CeO₂ and graphene, improving interfacial charge transfer and boosting the sensor response. However, a deep energy level was induced when the amount of Ce³⁺ was > 50.7 wt%, and the Fermi level pinning occurred at the interface. It resulted in a decrease in the density of free electrons, giving rise to an increase in the height of the Schottky barrier and a decrease in the gas sensing response.

Conducting polymers (CPs) with high electrical conductivity, low density, simple synthesis process, low sensing temperature, and compatibility with flexible substrates are highly popular materials for gas sensing usages [86,87]. Among them, polyaniline (PANI) is highly popular owing to its reversible doping/de-doping, mechanical flexibility, high electrical conductivity, environmental stability, and the ease of synthesis [88]. Accordingly, it has been used in combination with CeO₂ for gas sensing purposes [89]. Liu et al. [90] synthesized PANI-CeO₂ nanocomposite and prepared a thin film sensor on a flexible polyimide (PI) substrate. CeO₂ NPs were wrapped in PANI, resulting in a decrease in agglomeration among them. At RT, the PANI-CeO₂ sensor revealed a higher response [(ΔR)/R_a) × 100] of 107% to 10 ppm NH₃ than that of the pristine PANI sensor with a response [(ΔR)/R_a) × 100] of 82% to 10 ppm NH₃, confirming the promising role of CeO₂ NPs for enhanced gas response. Also, after bending for 500 cycles and bending under an angle of 40°, no noticeable changes in the sensing performance was observed, confirming excellent flexibility of the sensor, due to the high mechanical flexibility of PANI and PI substrate as well as good adhesion of sensing layer to substrate through electrostatic interaction which resulted in tightly and uniformly adhesion of film to PI substrate. The NH₃ abstracted holes from =NH⁺—and =NH₂⁺—groups of PANI, resulting in conversion from emeraldine salt (ES) form to emeraldine base (EB) form (Figure 5a), increasing the resistance of the sensor. Furthermore, the modulation of depletion layers in p-n junctions in the presence of NH₃ gas remarkably contributed to the improved sensing performance (Figure 5b). The protonation degree of PANI was improved by the addition of CeO₂ NPs, which led to an increase in the amount of =NH⁺ groups, which provided more adsorption sites for NH₃ molecules.

As pointed out before, the {100} facet of CeO₂ has higher reactivity relative to both {111} and {110} facets, and thus, controlling the growth condition of CeO₂ is essential to achieve the highest sensing performance. Ema et al. [91] synthesized {100} CeO₂ NCs/SnO₂ NSs and {111} CeO₂ nanooctahedron/SnO₂ NSs composites for gas sensing studies. The {100} facet in CeO₂ not only had a higher amount of oxygen vacancies, but also more dangling bonds. Accordingly, it had less stability relative to the {111} facet, facilitating more adsorption of oxygen and gases. At 550 °C, the response (R_a/R_g) of {100} CeO₂ NCs/SnO₂ NSs gas sensor to 20 ppm acetone was 14.8, while that of {111} CeO₂ NCs/SnO₂ NSs sensor was only 1.7. The high response was related to the high gas adsorption and enhanced oxidation capacity of the metastable {100} facet of CeO₂. Furthermore, heterojunctions between SnO₂ and CeO₂ were formed, resulting in the generation of potential barriers, acting as robust sources of resistance modulation.

SnO₂ is a highly sensitive metal oxide, due to its high mobility of electrons. In this context, Motaung et al. [92] synthesized SnO₂-CeO₂ nanocomposite via a hydrothermal method at 200 °C for 10 h. The SSA of the synthesized composite was 47 m²/g. The sensor revealed a high response (R_a/R_g) of 1323 to 60 ppm H₂ gas at 300 °C. The formation of n-n CeO₂-SnO₂ heterojunctions and high SSA of the sensor accounted for the enhanced response to H₂ gas. Also, plenty of oxygen vacancies resulted in improved adsorption of oxygen and enhanced sensing response. In another study performed by Oosthusian et al. [93], the promising role of oxygen vacancies in the CeO₂ gas sensor towards enhancement in response to NO₂ and H₂S gases was demonstrated.

Generally, the response of a resistive gas sensor decreases in the presence of humidity, and it is a difficult task to obtain a high response in a highly humid environment. In fact, in a highly humid environment, hydroxyl or water molecules can be adsorbed on the surface of the sensor, resulting in a decrease in available sites for incoming gas molecules [94]. Accordingly, the response decreases in the presence of humidity. However, some approaches, such as coating with a hydrophobic layer [95] or defect engineering [96], can decrease the humidity influence on the gas sensing response. Zhu et al. [97] synthesized a hydrophobic CeO₂-SnO₂ nanocomposite by depositing the CeO₂ layers with thicknesses in the nanoscale range onto the SnO₂ films through a sputtering method. The sensor manifested high performance in terms of TEA. At 200 °C, it revealed a response (R_a/R_g) of 22.5 to 20 ppm TEA in a highly humid environment. As a proof of study, a portable, wireless sensor was fabricated for the real-time monitoring of TEA released from a spoiled fish.

H₂ with a flammable and explosive nature is considered a highly efficient and green source of energy, producing only water vapor upon combustion [98]. Nevertheless, it is a colorless, tasteless, and odorless gas that cannot be detected by human senses, highlighting the urgent need for the development of reliable hydrogen gas sensors [99]. Therefore, Hu et al. [100] synthesized CeO₂ (0.5–6 at%)-In₂O₃ hollow composite spheres (diameter 0.5–1.5 μm; wall thickness 20–30 nm) using a hydrothermal route at 160 °C for 12 h. The sensor with 2 at% CeO₂ manifested a response (R_a/R_g) of 20.7 to 50 ppm H₂ at 160 °C, while that of the pristine sensor was only 6.9. The enhanced response was related to the formation of n-n heterojunctions between CeO₂ and In₂O₃ and the generation of potential barriers at the interface areas. In an H₂ atmosphere, the height of potential barriers significantly decreased, contributing to the generation of a sensing signal (Figure 6).

Furthermore, Ce⁴⁺ adsorbed the electrons, resulting in the generation of Ce³⁺ ions and oxygen vacancies as follows:

$$2{Ce}^{4+}+O_O^x\to 2{Ce}^{3+}+V_O^{..}+{\displaystyle \frac{1}{2}}{O}_2$$

(2)

By subsequent release of electrons via oxygen vacancies, more oxygen species were adsorbed on the sensor surface, and hence, more sensing reactions occurred.

Metal oxides with nanowire (NW) morphology offer a high surface area for gas adsorption and gas sensing reactions. Therefore, Yuan et al. [101] synthesized CeO₂ nanodot-decorated WO₃ NWs via a hydrothermal route followed by thermolysis. It revealed a response (R_a/R_g) of 1.30 to 500 ppb acetone based on a miniaturized microelectromechanical system device. The improved performance was related to the formation of WO₃-CeO₂ heterojunctions, the presence of a large amount of oxygen vacancies, large SSA, availability of a large number of adsorption sites, and high conductivity of WO₃.

Neri et al. [102] synthesized CeO₂-Fe₂O₃ nanocomposite by a co-precipitation method. Thanks to the lower electronegativity of the Ce⁴⁺ ion relative to Fe³⁺, CeO₂ was more basic than Fe₂O₃. Therefore, the basicity of the nanocomposite increased with the addition of CeO₂. At 400 °C, the sensor with 50 mol% CeO₂ revealed a response of 4 (I_g/I_a) to 100 ppm methanol. Based on the profile methanol conversion, the Fe₂O₃ sample completely converted methanol at 230 °C, while the composite sample with 50 mol% CeO₂ completely converted methanol at 210 °C. The oxidation of methanol was related to the number of basic sites on the sensor surface. On CeO₂-Fe₂O₃ nanocomposite, methanol molecules at low temperature, adsorbed via oxygen to the coordinatively unsaturated cations (Me = Fe³⁺, Ce⁴⁺) acting as Lewis acids, and the hydrogen of the O–H group adsorbed on lattice oxygen acting as basic sites.

Li et al. [103] synthesized CeO₂-(5 and 10 at%) CdS composite NWs with a diameter of 30 nm for gas sensing applications. While CdS showed a response (R_a/R_g) of 20 to 100 ppm acetone gas at 210 °C, 5 at% CeO₂ decoration led to a significant enhancement in the response (R_a/R_g) of 52 at 160 °C. The improved performance was related to the formation of CeO₂-CdS heterojunctions and high SSA as a result of the decoration of CeO₂ on CdS NWs.

Silica (SiO₂) can stabilize metal oxide NPs, and due to the presence of numerous hydroxyl groups on the surface of silica, it has a high ability to adsorb water molecules. Wang et al. [104] synthesized SiO₂ modified (3, 8, and 14 mol%) CeO₂ NPs using a sol–gel method followed by a hydrothermal method at 150 °C for 10 h. Among the fabricated sensors, that with 8 mol% SiO₂ revealed a high response [(ΔR)/R_a) × 100] of 3245% to 80 ppm NH₃ gas at RT. However, the response time was 750 s and the recovery time was 2750 s, both of which were too long for practical applications. Also, in a humid environment, the response increased with the increase in RH from 30% to 70%. In fact, due to the abundant hydroxyl groups on SiO₂, many water vapor molecules were adsorbed on the sensor surface, and subsequently, NH3 gas reacted with water molecules to produce NH4+ and OH− ions, resulting in a decrease in resistance in the presence of humidity.

Resistive gas sensors are even able to detect gases at ppt level [105]. In this regard, Fang et al. [106] synthesized CeO₂-decorated CuO NSs with Cu to Ce proportions of 1:0, 100:1, 20:1, 5:1, and 0:1 via a hydrothermal approach at 120 °C for 16 h. The thickness of CuO NSs was approximately 45 nm. The sensor with a Cu to Ce ratio of 20:1 revealed an enhanced response (R_a/R_g) of 6.96 to 20 ppm NO₂ gas at 100 °C. It was also able to theoretically detect 600 ppt NO₂ gas. This improvement was primarily due to the formation of CeO₂-CuO heterojunctions and the presence of the highest amount of oxygen vacancies in the optimal sensor, which acted as highly favorable sites for gas adsorption. In addition, the optimal sensor had a high SSA of 49 m²/g, meaning it was able to provide plenty of adsorption sites for NH₃ gas molecules.

Li et al. [107] synthesized hierarchical CeO₂-ZnO composites via a solvothermal route at 90 °C for 4 h. The Optimal sensor exhibited a high response (R_a/R_g) of 92 to 100 ppm TEA at 200 °C. Also, upon mixing TEA with other VOCs, the response did not change significantly, implying excellent selectivity of the sensor. The C-N bond in the TEA molecule has low bond energy, making it easily broken, which results in enhanced selectivity to TEA. Furthermore, the response was almost the same upon an increase in humidity to 75%. The creation of CeO₂-ZnO heterojunctions and the special morphology of sensing materials, which offered numerous adsorption sites, both affected the sensing performance.

Bi₂O₃ is a semiconductor with non-toxicity, chemical stability, abundance, and the ease of synthesis [108]. Meng et al. [109] synthesized hierarchical Bi₂O₃-CeO₂ core–shell nanocomposites via hydrothermal reaction at 200 °C for 24 h. Molar ratios of Bi₂O₃ to CeO₂ were 1:1, 1:2, 1:3, and 2:1. A lot of CeO₂ NSs were grown on Bi₂O₃ as a core with a size of 2.5 µm. At RT, the sensor with a Bi₂O₃ to CeO₂ of 1:2 manifested the highest response [(ΔR)/R_a) × 100] of 137% to 100 ppm HCHO, which was 6.6 times that of Bi₂O₃ (20.7%). The boosted response stemmed from the formation of a sufficient number of heterojunctions at interfaces between Bi₂O₃ and CeO₂. However, an excessive amount of CeO₂ resulted in the full encapsulation of Bi₂O₃. It led to the development of a compact CeO₂ overlay on the Bi₂O₃ surface, which partially limited the interaction between HCHO and Bi₂O₃, resulting in a reduced response. Also, based on DFT calculations, the adsorption of formaldehyde by the Bi₂O₃-CeO₂ composite was significantly higher relative to that of Bi₂O₃ sensor due to a larger adsorption energy.

MoSe₂ belongs to the TMD group with a high surface area and high mobility of electrons. It has several phases, and the effect of phase changing during gas sensing applications should be investigated. Singh et al. [110] investigated the effect of phase transition on the sensing features of MoSe₂-CeO₂ nanocomposite to distinguish NH₃ and TEA gases at RT. When the 2H phase of MoSe₂ was dominant, the sensor revealed more selective behavior to NH₃, whereas when the 1T phase was dominant, the sensor showed more selectivity to TEA gas. The sensing performance was related to the high SSA of the sensor, formation of n-n heterojunctions, the presence of oxygen vacancies in CeO₂, high conductivity of WS₂, and the synergistic effect of CeO₂ and WS₂ in combined form.

DMA is a breath biomarker for Parkinson’s disease [111], and hence, the detection of this VOC in exhaled breath is important. Wu et al. [112] synthesized CeO₂-coated Ti₃C₂T_x MXene/carbon nanofiber for DMA detection at RT. The high conductivity of MXene, along with C–Ti–O bonds and a sp² hybridized carbon structure, increased the effective surface active sites for the fabricated gas sensor. Furthermore, Ce³⁺ ions accelerated the formation of surface-active oxygen species, while the MXene-CeO₂ heterojunction expanded the width of EDL inside of CeO₂, and upon exposure to DMA gas and release of electrons, significant resistance modulation occurred. Furthermore, the response of the sensor was almost constant even in a highly humid environment with relative humidity of 80%, making it highly useful for DMA analysis in exhaled breath.

Table 2. Gas sensing properties of CeO₂-based gas sensors.

Sensing Material	Gas	Conc (ppm)	T(°C)	Response (Ra/Rg) or (Rg/Ra) or [(ΔR)/Ra) × 100]	Ref
CeO2 NPs	NH3	500	RT	23	[45]
CeO2 NRs	H2	1000	100	6	[46]
CeO2 nanostructures	TEA	100	100	150	[47]
CeO2 nanopolyhedra	DMA	100	300	150	[48]
hollow CeO2 microspheres	Acetone	100	260	270%	[49]
CeO2 QDs	H2	10	220	4.2	[51]
Ni-doped CeO2	H2S	0.5	200	3.2	[53]
In-doped CeO2 nanostructures	NH3	1000	RT	5412.93	[55]
Ce-doped TiO2 nanostructures	NH3	20	RT	25	[56]
W6+-doped CeO2 NPs	H2S	10	225	18.6	[58]
Ru-doped CeO2 NSs	Xylene	5	160	37	[60]
Pt loaded CeO2 -porous SnO2	CH4	1000	350	12.4	[71]
Pd-decorated CeO2-ZnO NRs composite	H2	1000	RT	300	[74]
graphene-CeO2 nanocomposite	CO	10	RT	1.5%	[84]
CeO2-graphene nanocomposite	NO2	5	RT	10.6%	[85]
PANI-CeO2 nanocomposite	NH3	10	RT	82%	[90]
{100} CeO2 NCs/SnO2 NS	Acetone	20	550	14.8	[91]
SnO2-CeO2 nanocomposite	H2	60	300	1323	[92]
CeO2-SnO2 nanocomposite	TEA	20	200	22.5	[97]
CeO2-In2O3 hollow composite spheres	H2	50	160	20.7	[100]
CeO2 nanodot-decorated WO3 NWs	Acetone	0.5		1.30	[101]
CeO2-Fe2O3 nanocomposite	Methanol	100	400	4	[102]
CeO2-CdS composite NWs	Acetone	100	160	52	[103]
SiO2 modified CeO2 NPs	NH3	80	RT	3245%	[104]
CeO2-decorated CuO NSs	NO2	20	100	6.96	[106]
CeO2-ZnO composites	TEA	100	92	92	[107]
Bi2O3-CeO2 core-shell nanocomposites	HCHO	100	RT	137	[109]

compares the gas sensing properties of CeO₂-based gas sensors. Overall, they have been successfully used for the detection of various gases at various temperatures down to RT. The detection of NH3 and TEA is crucial for specific food industry applications, including tofu and fish spoilage. On the other hand, detecting other toxic gases and VOCs, such as CO, NO₂, H₂S, acetone, and xylene, is primarily associated with environmental monitoring and industrial applications. They are generally found in our environment and various industries. Hence, the development of gas sensors for the detection of these gases is important.

4. Conclusions and Outlooks

We reviewed the gas sensing features of CeO₂-based gas sensors. Ce is the most abundant rare earth element, and thus, CeO₂ is a relatively cheap material for gas sensing fabrication. CeO₂-based gas sensors have been successfully used for the realization of high-performance gas sensors to detect various gases at different temperatures down to RT. Although pristine CeO₂ gas sensors have demonstrated the capability to detect various gases, their performance can be further increased by various strategies such as doping, noble metal decoration, and nanocomposite formation.

Doping with cations can result in the creation of oxygen vacancies within the lattice of CeO₂. They serve as desired sites for adsorption of oxygen and thus can increase the sensing reactions with gases. Noble metals with catalytic activity and electronic sensitization are also highly effective in enhancing the sensing characteristics of CeO₂ gas sensors. They not only increase the selectivity but also decrease the sensing temperature. Thanks to the generation of Schottky junctions between noble metal and CeO₂, a significant modulation of the resistance occurs, resulting in the generation of a large sensing signal. Nanocomposites are another promising architecture for sensing improvement, due to the generation of numerous heterojunctions between different components of the composite. Thus, the amount of resistance change sources increases significantly. CeO₂, in combination with other materials such as CP and MXenes, holds strong potential for the development of flexible gas sensors, which are highly required for new applications such as wearable applications where rigid sensors cannot be used.

Future directions in the field of CeO₂ sensors are as follows: (i) decreasing sensing temperature by illumination of sensing material with UV light, (ii) working the gas sensor in self-heating mode to decrease sensing temperature, and (iii) increasing the selectivity by decoration of bimetallic noble metals on the surface of CeO₂. Bimetallic NPs have higher catalytic activity as well as more stability relative to single noble metals [113] (iv) functionalization with self-assembled molecules (SAM) to increase selectivity of the gas sensor. By selecting an appropriate SAM with a high tendency towards reaction with specific gases, the selectivity can be tuned towards desired gases [114] (v) synthesis of CeO₂ in morphologies with high SSA to offer more adsorption sites for gas molecules. (vi) In general, microelectromechanical systems (MEMS)-based gas sensors have important features like small sizes, extremely low power consumption, long-term reliability, and reduced batch fabrication cost. Since CeO₂-based gas sensors work at high temperatures, an elevated temperature is often needed to activate the sensing mechanism. Therefore, the appropriate electrothermal and structural design of a microheater, i.e., having fast response, uniform temperature distribution over sensing area, and minimal residual/thermal-stress-induced membrane deflection, is very important for MEMS-based CeO₂ gas sensors [113,114,115,116]. (vii) In addition, AI-assisted sensing, multi-gas arrays, and non-resistive hybrid sensor platforms are promising strategies to significantly improve sensing performance in terms of sensitivity and selectivity.