Metal oxide semiconductor Gas Sensors with Different Morphologies

Metal oxide semiconductor Gas Sensors with Different Morphologies: History

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Subjects: Materials Science, Ceramics

Contributor:

Jin-Young Kim

There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. In this articles we have discussed different morphologies of metal oxide gas sensors.

semiconductor
metal oxide
morphology
gas sensor
sensing mechanism

1. Quantum-Dot-Based Gas Sensors Using MOSs

Morphology engineering is an important approach for enhancing the gas-sensing features of MOS-based gas sensors, and MOS materials with different morphologies have been explored for gas-sensing purposes. The main goal of using novel morphologies for sensing studies is to increase the surface area. The larger the surface area, the greater the availability of adsorption sites, resulting in the adsorption of more gas molecules on the surface of the gas sensor, along with the appearance of a higher sensing signal. For sensing studies, zero-, one-, two-, and three-dimensional morphologies have been explored. Quantum dots (QDs) are zero-dimensional (0D) semiconductor nanoparticles, where the motion of charge carriers is confined in all three directions. Thus, the energy difference between energy bands changes depending on the size of the particle, and it is possible to tune the band gap of QDs by changing their size. Accordingly, the electrical properties of QDs can be engineered by controlling the dot size [23]. Owing to the extremely small size of QDs, when the QDs are exposed to air, they are depleted of electrons, and subsequent exposure to the target gas leads to extensive modulation of the electrical resistance, leading to a high sensing signal. In addition, owing to their large resistance variation and good electrical properties, most QD-based gas sensors can work at low temperatures.

2. Nanowire-Based Nanomaterial Gas Sensors

Among different morphologies, nanowires (NWs) with aspect ratios greater than 20 [28] are highly popular for sensing applications, owing to their high surface area, straightforward preparation methods, ease of gas sensor fabrication, high and rapid response to the target gas, long-term stability, and high crystallinity [29,30]. Accordingly, several studies have realized the enhanced gas-sensing capacity of NW-based gas sensors [31,32]. In [33], various methods to synthesize NWs are discussed in detail.

Among different MOSs with NW morphology, SnO₂ NWs are very popular for sensing purposes [34,35,36,37]. SnO₂ has a band gap of 3–4 eV, depending on the synthesis method, size, surface states, and impurities [38]. However, most studies have reported a band gap of 3.7 eV for this MOS [39]. SnO₂ is the most stable n-type oxide, and is extensively used in the preparation of semiconductor gas sensors. Its high electron mobility (~160 cm²/V·s) makes it a key choice for sensing applications [19,40]. In addition, SnO₂ NWs exhibit good optical transparency and conductivity [41]. Wang et al. [42] performed one of the first studies on SnO₂ NWs for the detection of H₂ gas, and compared the results with those of SnO₂ nanorods (NRs). It was reported that owing to the higher surface area of NWs, size effects when the diameter of the NWs was less than the Debye length of SnO₂, and the small electrode gap of the SnO₂ NW gas sensor, a larger response was observed for the SnO₂ NWs at 300 °C than that for SnO₂ NRs. Notably, not only SnO₂ NWs, but also other MOS NWs, have been employed as gas sensors [43,44]. The construction of n–n and p–n junctions is a commonly employed strategy for improving the sensitivity of NW-based gas sensors [45]. Owing to the generation of these heterojunctions, the resistance of gas sensors is higher than that of the pure sensing material, and the subsequent modulation of resistance in the presence of target gases generates a high sensing signal for heterojunction gas sensors [46].

Branched NWs (e.g., nanoforests or nanotrees) are a special type of NWs that have recently been used for sensing applications. Their 3D morphology and numerous homo- or heterojunctions make the branching NWs highly popular for sensing studies. With high surface area, branched NWs can detect extremely low amounts of gases. Branched NWs also offer an increased number of carrier paths and improved conduction between the NW branches and backbones [47,48,49].

3. Nanofiber-Based MOS Gas Sensors

Nanofibers (NFs) are another type of 1D material used in MOS gas sensors. Similar to NWs, NFs have an aspect ratio of more than 20. However, their surfaces have many nanograins, which result in a further increase in the surface area [78]. There are various methods to synthesize NFs; however, NFs are commonly produced using the electrospinning (ES) technique. The ES technique consists of a spinneret section, a high-voltage power supply, and a metal collector [79]. During electrospinning, NFs are formed from a liquid jet and subsequently elongated under a high voltage. The NF formation process consists of the following: (i) onset of jetting and development of a rectilinear jet, (ii) bending deformation along with solvent evaporation to form solidified NFs, and (iii) NF collection [80]. In a simplified view, a sol is initially prepared in this technique, and is subsequently poured into a syringe. Upon applying a high voltage to the sol with a defined viscosity, it is possible to collect extremely fine and long NFs on the collector placed at a fixed distance from the syringe. After calcination of the synthesized NFs at an appropriate temperature for sufficient time, the final diameter decreases due to evaporation of the solvent and organic materials [79].

By optimization of the ES technique parameters—such as the applied voltage, distance between the needle tip and the collector, flow rate, and temperature—it is possible to tune the diameter of the resulting NFs [81,82]. In particular, the viscosity of the solution can significantly affect the formation of long NFs [83]. If the viscosity of the ES solution is not adjusted to the desired value, NFs with beads are formed [83].

Furthermore, with a special type of ES technique, known as coaxial ES, it is possible to directly synthesize C–S NFs [84,85]. Many MOSs have been successfully synthesized using the ES technique for gas-sensing studies [86]. Similar to C–S NWs, the thickness of the shell layer should be adjusted to obtain the maximum response to the target gas. For example, Kim et al. [87] reported that in SnO₂–Cu₂O C–S NF gas sensors with different shell thicknesses, the sensor with a shell thickness of 30 nm gave the highest response to CO gas—it showed a response to 5–10 ppm of CO gas at 300 °C.

4. Nanotube-Based MOS Gas Sensors

Metal oxide nanotubes (NTs) have been studied less compared to their NW and NF counterparts [95]. This may be due to the difficulty in the synthesis of MOS NTs. Various techniques can be used for synthesizing MOS NTs. However, the most common method is the use of a hard or soft sacrificial template. Hard templates have been prepared using silica, carbon, and polystyrene beads, while soft templates have been fabricated from bubbles, polymer vesicles, emulsion droplets, and surfactant micelles. The core can be removed by dissolving in a solvent or calcination to generate hollow structures [96]. The main features of gas sensors based on hollow structures such as NTs are (i) wall permeability, (ii) wall thickness, and (iii) wall chemistry [97]. Although thinner NTs are expected to provide greater surface area and more adsorption sites for gas molecules [98], the presence of defects can sometimes alter the sensing properties. Hazra et al. investigated the effects of shell thickness on the sensing characteristics of TiO₂ NTs. Interestingly, the sensor with the thinnest wall and largest surface area did not show the highest response to the target gas, due to the lower amount of oxygen vacancies. The device with a thicker wall and sufficient oxygen vacancies exhibited the highest response [99]. In conclusion, MOS-based NTs for gas-sensing studies offer a high surface area owing to the hollowness of their structure, which can provide many adsorption sites for gas molecules.

5. Nanorod-Based MOS Gas Sensors

Nanorod-based gas sensors are another popular category of MOS materials for gas-sensing studies, owing to their unique electrical properties and high surface area provided by the NR morphology. The hydrothermal method provides a convenient, fast, versatile, and low-cost route for constructing well-ordered nanorod arrays [100,101,102,103]. However, other methods—such as chemical bath deposition (CBD), which is economical, straightforward, and scalable—can also be used to prepare NRs from different materials [104]. Chemical vapor deposition [105,106], metal–organic chemical vapor deposition [107,108], and pulsed laser deposition [109] are other methods for obtaining NRs in MOSs. NR-based MOS materials are popular for sensing studies because of their ease of production, high surface area, and compositional versatility.

6. Nanosheet-Based MOS Gas Sensors

Two-dimensional MOS nanostructured materials with a sheet-like morphology are popular for gas-sensing studies due to their high surface area. Only a few 2D MOS (MoO₃, WO₃, and SnO₂) analogs of graphene are known. Two-dimensional (2D) layered MOSs have strong in-plane bonds and weak van der Waals forces between layers, and gas molecules can be adsorbed on these sites. Thus, 2D layered MOSs can be employed for the production of gas sensors [110]. Apart from the naturally 2D layered MOSs, other MOSs can also be synthesized in a nanosheet-like morphology. For instance, Choi et al. [111] synthesized porous ZnO nanosheets using a solvothermal technique for NO₂ detection. The sensor exhibited a high response to 74.68–10 ppm of NO₂ gas at 200 °C. The width of the EDL increased in the presence of NO₂ gas, contributing to the sensing signal. Moreover, homojunctions were formed, and changes in the height of the homojunctions led to resistance modulation in the presence of NO₂ gas. Furthermore, the highly porous morphology and high specific surface area (11.51 m²/g) contributed to the increased availability of active sites for NO₂ gas molecules [111].

7. Three-Dimensional MOS Gas Sensors

Hierarchical nanostructures with three-dimensional (3D) morphology are composed of zero-, one-, two-, and three-dimensional morphologies, such as nanorods, nanotubes, or nanosheets [22]. Hierarchical nanostructures with high surface area provide more adsorption sites for target gases. In addition, with pores or channels in their structure, the diffusion of gases is facilitated, resulting in better interaction of the target gas with deep parts of the sensing material [113]. Vapor-phase growth and hydrothermal techniques are the two most-used methods to synthesize hierarchical nanostructures [22]. Owing to these advantages, hierarchical morphologies of MOSs are widely used for sensing studies [114,115].

For instance, Guo et al. [116] used a 3D hierarchical hollow structure of CuO/WO₃ for p-xylene-sensing studies. The sensor showed a good response to 6.36–50 ppm of xylene gas at 260 °C. The sensing mechanism was related to the unique 3D hierarchical structure with a high surface area of 23.4962 m²·g⁻¹, the formation of defects in the interfaces between CuO and WO₃, and the formation of p–n heterojunctions between CuO and WO₃. Recently, Yu et. al. [117] reviewed MOSs with hierarchical structures for gas-sensing studies.

8. Noble-Metal-Decorated MOS Gas Sensors

Gas-sensing properties can be improved by loading MOSs with small amounts of appropriately chosen noble metals. Noble metals such as Au, Pt, Pd, Ag, and Ru are widely used for the decoration or functionalization of the MOS surfaces to improve the overall performance of the resulting gas sensors.

It should be noted that the noble metals need to be dispersed as finely as possible on the surface of the MOSs. Agglomeration of noble metals on the surface of the sensing layer can lead to a poor response of the gas sensor. Generally, noble metals affect the gas-sensing performance via two well-known mechanisms: The chemical sensitization mechanism, which occurs via the spillover effect, is a commonly known phenomenon in catalytic chemistry. In this case, the noble metal activates the target gas to facilitate catalytic oxidation of gas. Noble metals increase the gas sensitivity as they increase the rate of chemical processes [119]. In electronic sensitization, because noble metals generally have a larger work function than MOSs, electrons are transferred from the MOS to the noble metals, leading to the formation of Schottky barriers and contraction of the EDL or HAL inside the MOS. In the target gas atmosphere, the width of the EDL and the HAL changes, leading to significant modulation of the sensor resistance. Among noble metals, Pd is well known for H₂ gas detection [120], owing to its excellent catalytic activation ability for hydrogen through the hydrogen spillover effect [121]. When a sensor decorated with Pd NPs is placed in an atmosphere containing H₂ molecules, hydrogen can easily dissociate into hydrogen atoms on the Pd surface; in the spillover effect, hydrogen atoms move to the neighboring MOS surfaces, leading to additional reactions between atomic hydrogen and adsorbed oxygen [122]. Other noble metals also exhibit good catalytic activities toward different gases. For example, Ag is often used for H₂S detection because of the generation of Ag₂S upon exposure to H₂S gas [123]. Pt and Pd also have good catalytic activities toward C₇H₈ and C₆H₆ gases, respectively [124,125].

9. Hybrid MOS Gas Sensors

A good strategy to enhance the gas-sensing characteristics of MOS-based gas sensors is using another material in combination with the MOS. Hybrid MOS nanocomposites are either (i) composites of MOS and carbon materials such as graphene, graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotubes, or (ii) composites of MOS and conductive polymers (CPs). Graphene, which comprises sp² carbon atoms, is utilized in gas sensors due to its high charge carrier mobility (200,000 cm² V⁻¹ s⁻¹) and large surface area (2630 m² g ⁻¹) [126]. The first ever graphene gas sensor was introduced in 2007 [127]. Owing to its single-layer or few-layer nature, graphene can even interact with a single molecule. Pristine graphene can easily agglomerate owing to the surface interactions. Furthermore, graphene has no band gap, hindering its gas-sensing usage [128]. Thus, rGO, with its many functional groups—such as -OH and –O—as well as defects, is a better choice for gas-sensing studies. Although GO can be used for sensing applications, it has a very high resistance owing to the presence of oxygen-based functional groups, limiting its use in sensing studies [129]. rGO can be synthesized from GO using chemical reduction, thermal reduction, and UV light reduction methods [130,131]. Many papers on gas sensors with hybrids of MOSs and carbon allotropes have been reported. However, for sensing studies, rGO is the most effective allotrope of carbon in its hybrid form. This is because of its large surface area, presence of many defects, and high concentrations of charge carriers with high mobility—all of which are beneficial for sensing. Accordingly, several studies have been reported on hybrid MOS–rGO composites with the ability to operate at room temperature [132,133].

CPs have tunable conductivity and high flexibility in synthesis and processing. However, because of the high affinity of CPs for moisture, they are unstable, and generally exhibit poor sensitivity and selectivity for different gases in their pristine form. The use of hybrid nanocomposites with CPs and MOSs could result in the development of room-temperature gas sensors [134]. Accordingly, hybrids of CPs and MOSs have been used to enhance the sensitivity of nanostructured sensors at low or room temperature [135,136]. One application of hybrids of MOSs and CPs is the development of flexible and wearable gas sensors. Flexible gas sensors are generally deposited on flexible substrates, and need to work at low temperatures, e.g., room temperature. Since composites of MOs and CPs can function at low temperatures, they are good candidates for such applications.

10. Comparison of Performance of Gas Sensors with Different Morphologies

Table 1 summarizes the performance of gas sensors with various morphologies. Different morphologies and compositions have been used for the detection of various gases at different temperatures. Moreover, pristine gas sensors show lower responses and higher sensing temperatures, while composite or decorated sensing materials show better performance at lower temperatures.

Table 1. Gas-sensing properties of different sensors with various morphologies.

Sensing Material	Morphology	Gas	Conc. (ppm)	T $(° C)$	Response (R_a/R_g) or (R_g/R_a)	Ref.
TiO₂/SnO₂	QD	NO₂	1	300	2.62	[25]
SnO₂	QD	C₄H₁₀	8219.2	25	8.09	[26]
SnO₂	QD	C₂H₅OH	300	N/A *	215	[27]
Pt-SnO₂	NW	C₇H₈	100	300	55	[32]
In₂O₃/SnO₂	NW	NO₂	5	300	25	[34]
In₂O₃	NR	CO	400	350	3.5	[35]
SnO₂	NW	NO₂	5	200	180	[36]
SnO₂	NW	LPG	2000	350	21.8	[37]
SnO₂	NW	H₂	1000	300	3.3	[42]
ZnO	NW	NO₂	20	225	95	[43]
SnO₂	NW	C₂H₅OH	100	300	50.6	[48]
ZnO	NW	NO₂	5	300	106	[49]
ZnO-SnO₂	NR	C₂H₅OH	100	275	18	[50]
Ni/ZnO	NW	p-xylene	5	400	42.44	[52]
Gr/ZnO	NW	C₂H₅OH	20	125	23	[53]
Bi₂O₃/SnO₂	NW	NO₂	2	250	56.92	[54]
Pt/CeO₂	NW	CO	200	25	3	[56]
SnO₂	NW	NO₂	500	300	17	[57]
SnO₂	NW	C₂H₅OH	50	350	6.7	[58]
NiO/NiFe₂O₄	Nanotetrahedrons	HCHO	50	240	19	[61]
ZnO/SnO₂	NW	C₂H₅OH	200	400	280	[62]
ZnGa₂O₄/ZnO	NW	NO₂	10	250	23	[63]
ZnO/WO₃	NW	H₂	1000	25	6.45	[64]
Ga₂O₃/SnO₂	NW	C₂H₅OH	1000	400	66	[65]
Au/SnO₂–ZnO	NW	CO	0.1	300	26.6	[67]
Pt/W₁₈O₄₉	NW	H₂	1000	200	0.528	[70]
Ag/SnO₂	NW	H₂S	0.5	N/A *	21.2	[73]
Rh/SnO₂	NF	C₃H₆O	50	200	60.6	[78]
ZnO/In₂O₃	NF	C₂H₅OH	100	225	31.87	[85]
SnO₂/Cu₂O	NF	NO₂	10	300	5	[87]
In₂O₃	NF	CO	100	300	5.4	[89]
Pd/SnO₂	NF	HCHO	100	160	18.8	[92]
Co₃O₄/ZnO	NF	HCHO	100	220	5	[93]
TiO₂/ZnO	Hemitube	NO₂	25	25	1.23	[98]
ZnO	NR	NO₂	5	250	200	[100]
ZnO	NR	C₂H₅OH	250	400	7	[101]
α-Ag₂WO₄	NR	C₃H₆O	20	350	3.6	[103]
ZnO	NR	O₃	2.5	575	850	[106]
ZnO	NS	NO₂	10	200	74.68	[111]

* N/A: No available data.

This entry is adapted from the peer-reviewed paper 10.3390/chemosensors10070289

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