Editorial Feature

Reviewing Titanium Oxide-Based Gas Sensors for VOC Sensing

Reviewed by Megan Craig, M.Sc.Dec 14 2022

Sensors for determining gases and volatile organic compounds (VOCs) have a wide range of applications. Most of these gas sensors contain semiconducting layers that change electrical resistance when exposed to gases and VOCs.

Reviewing Titanium Oxide-Based Gas Sensors for VOC Sensing

Image Credit: dizain/Shutterstock.com

These semiconducting layers are usually made of semiconductors like WO3, MoS2, ZnO, SnO2, and TiO2 TiOn, whereas the most popular dielectric substrates for gas sensors are Al2O3 and SiO2. New titanium-based compounds, such as MXenes, and non-stoichiometric titanium oxides (TiOn), have also been employed in gas sensor development.

The n-type stoichiometric TiO2 semiconductor, which exists in three primary phases (rutile, anatase, and brookite), may be easily changed between these phases using thermal techniques. Gas sensors can then be tuned analytically as a result.

In addition to stoichiometric TiO2 forms, a non-stoichiometric (TiOn) that is frequently referred to as TiO2-x has recently been used in sensor design. Structures of titanium oxide based on certain Magnéli phases (TinO2n-1) show great promise.

Some scientists have recently sought to improve gas-sensor selectivity by designing certain morphologies of semiconducting structures, using core-shell semiconducting nanocomposites, and quantum dots.

The characteristics, production, and modifications of TiO2-based nanostructures; use of titanium-based nanomaterials for energetics and environmental applications; the design of photo-catalysts; and the applicability of titanium oxides and MXenes in sensor design are all covered in recent studies. The progress in developing gas and volatile organic compound (VOC) sensors based on titanium-based oxides is the focus of a review published in the MDPI journal coatings.

Methodology

TiO2 is classified as an n-type semiconducting substance. Many sensors and biosensors are made with TiO2-based heterostructures. All of the most common types of titanium oxide, however, have distinct bandgaps, which are as follows: 3.02 eV for anatase, 3.23 eV for rutile, and 2.96 eV for brookite.

The addition of TiO2 (anatase) intergrowths to the structure of titanium pentoxide (Ti3O5) increases conductivity (Figure 1) as well as several photoluminescence-related properties.

Temperature dependence of electrical resistance (R (T)) for the TiO2-x/TiO2 (400 °C)-based hetero-structure. Temperature was changed in two ways (indicated by black and red arrows): (i) black cycles shows points measured by cooling down, (ii) red squares shows points by increasing temperature. Measurements were performed in vacuum using helium cryostat.

Figure 1. Temperature dependence of electrical resistance (R (T)) for the TiO2−x/TiO2 (400 °C)-based heterostructure. Temperature was changed in two ways (indicated by black and red arrows): (i) black cycles shows points measured by cooling down, (ii) red squares shows points by increasing temperature. Measurements were performed in vacuum using helium cryostat. Image Credit: Ramanavicius, et al., 2022

Gas sensors made on stoichiometric TiO2 exhibit remarkable sensitivity to several gases. It is worth noting that TiO2-based gas sensors can use a variety of detecting processes, which differ the most for determining reducing gaseous chemicals like H2, H2S, NH3, CO, CH3OH, C2H5OH, and others, as well as oxidizing gaseous compounds like O2, NO2, CO2 (Table 1).

Table 1. Characteristics of titanium oxide-based sensors. Source: Ramanavicius, et al., 2022

Sensing Material Working Temperature Gas Concentration Response Value (Ra/Rg) or ((ΔR/Rg) × 100%) Response Time Recovery Time Reference
TiO2 (rutile), Ti8O15 and Ti9O17 mixture 210 °C 12.5–100 ppm (NH3) 1–7% 2 min 8 min [68]
TiOx-NiO 250–350 °C 100 ppm (H2)
100 ppm (NO2)
100 ppm (NH3)		 17 for H2 (250 °C)
16 for NO2 (250 °C)
4 for NH3 (250 °C)		 2 min 2.3 min [69]
β-Ti3O5 150 °C 50 ppm (H2) 11% - - [70]
Ti3O5-TiO2 mixture 25–180 °C 105 ppm (H2O)
118 ppm (methanol)
53 ppm (ethanol)
18 ppm n-propanol
220 ppm (acetone)		 0.5–18% - 4–35 s [1]
TiO2-Ti6O 150–450 °C 2000 pm (H2)
20 ppm (NO2)
500 ppb (O3)
1.6 ppm (acetone)
80 ppm (NOx)		 2.9–348 8–21 s 20–32 s [34]
Ti3+-TiO2 RT 100 ppm (CO) 39% 10 s 30 s [71]
TiO2 150 °C 100 ppm (ethanol) 75.4% 155 s 779 s [72]
TiO2 270 °C 500 ppm (acetone) 9.19 10 s 9 s [73]
TiO2 350 °C 400 ppm (ethanol) 22.9 5 s 7 s [74]
TiO2 RT 200 ppm (NH3) 64 28 s 24 s [75]
α-Fe2O3-TiO2 325 °C 100 ppm (ethanol) 4 46 s 16 s [76]

 

UV irradiation enhances the activity of TiO2/GO-based heterostructures based on the generated n-n heterojunction, as seen in the band diagram in Figures 2a, b. The heterojunction bandgap in the TiO2/GO-based heterostructure is 4.7 eV, which is greater than the 4.4 eV seen in most GO-based structures. Figure 2b indicates that the produced heterostructure is composed of both accumulation and depletion layers.

UV irradiation can overcome this disadvantage by increasing the depletion layer in TiO2 and improving the accumulation layer in GO (Figure 2c). This lowered GO creates a p-n junction with TiO2, which has a narrower junction width at the contact (Figure 2d).

Band diagram of TiO2/GO hetero-structure (a) before the formation of hetero-structure, (b) after the formation of hetero-structure, (c) when UV irradiation is applied, and (d) when UV irradiation is switched off. (‘e’ is an electron; ‘h’ is a hole).

Figure 2. Band diagram of TiO2/GO heterostructure (a) before the formation of heterostructure, (b) after the formation of heterostructure, (c) when UV irradiation is applied, and (d) when UV irradiation is switched off. (‘e’ is an electron; ‘h’ is a hole). Image Credit: Ramanavicius, et al., 2022

Results

Both adsorption and desorption events, as well as the formation of new chemical bonds between the sensing TiO2 layer and adsorbed gas molecules, electrostatically affect the upper layer of the semiconducting TiO2 layer. The conductivity of this layer changes due to the depletion and enrichment of this layer by charge carriers.

Figure 3 depicts a typical analytical signal obtained by the adsorption and desorption of analyte gas.

Representation of typical analytical signal for amperometric gas sensors. It should be noted that the duration of signal development and the regeneration of sensors highly depends on sensing material and sensing gases.

Figure 3. Representation of typical analytical signal for amperometric gas sensors. It should be noted that the duration of signal development and the regeneration of sensors highly depends on sensing material and sensing gases. Image Credit: Ramanavicius, et al., 2022

In TiO2-layers based on nano- or micro-particles, several distinct forms of electrical conductivity have been reported, including (i) Charge transmission via TiO2-particles is intrinsic, and (ii) charge transfer across the borders between particles is limited. As a result, the volume-concentration of these boundaries, as well as the particle size/shape, is crucial in constructing such sensors (Figure 4).

Representation of conductivity mechanism of semiconductor particle-based structure during the determination of CO gas. (A) Structure before the interaction with CO, (B) structure during the interaction with CO.

Figure 4. Representation of conductivity mechanism of semiconductor particle-based structure during the determination of CO gas. (A) Structure before the interaction with CO, (B) structure during the interaction with CO. Image Credit: Ramanavicius, et al., 2022

By combining metal oxide-based structures with conducting polymers, researchers could fine-tune various features of the produced composite material for specific purposes. As a result, several innovative techniques for synthesizing diverse morphologies TiO2 (Figure 5) and their composites with conducting polymers have been developed during the last decade.

Different morphology TiO2 structures used in design of a gas sensors. (A) Nanorods, (B) thin film, (C) nanoparticles, (D) nanotubes.

Figure 5. Different morphology TiO2 structures used in design of a gas sensors. (A) Nanorods, (B) thin film, (C) nanoparticles, (D) nanotubes. Image Credit: Ramanavicius, et al., 2022

Discussion

Most studies have shown that conducting the polymer polyaniline (PANI) via TiO2-based structures can boost their sensitivity to certain gases and VOCs. PANI modification of TiO2-based structures boosts the sensitivity of proposed ammonia sensors, according to a large number of studies.

Although all typical PANI forms (including leucoemeraldine and pernigraniline) may be employed to modify TiO2-based sensing layers, an emeraldine form of PANI appears to be the most appropriate. In addition to the most prevalent TiO2-forms (anatase and rutile), a new trend has emerged based on the use of nonstoichiometric titanium suboxides modified with conducting polymers such as PANI, polypyrrole (Ppy), polythiophene (PTH), and others.

Most (titanium oxide)/PANI-based sensors register analytical signals by measuring sensing layer resistance. There have been several publications on using quartz crystal microbalances (QCM) in the design of (titanium oxide)/PANI-based sensors. Trimethylamine, hydrazine, and NH3 were all detected using TiO2-PANI composite-based QCM sensors.

Conclusion

Gas sensors were designed using a variety of stoichiometric titanium oxide-based devices. However, selectivity issues persist in these sensors, which can be addressed by doping titanium oxide-based layers with dopants or forming heterostructures based on the combination of various semiconducting compounds, as well as adjusting the sensor’s working temperature.

Sensors made from composite materials based on titanium oxide modified with conducting polymers, such as PANI, are particularly promising, among many others.

Recent research has proven that TiO2/TiO2-x-based heterostructures may be successfully employed in the construction of gas sensors, with the detecting capabilities of these sensors being simply modified by modifying the TiO2/TiO2-x-ratio heterostructures.

Furthermore, TiO2/TiO2-x-based heterostructures are strong conductors, allowing them to operate in a “self-heating” mode and achieve temperatures adequate for determining certain gaseous chemicals.

The use of TiO2/TiO2-x-based heterostructures in the design of gas sensors is expected to become more common. Still, the main challenge in this research direction remains the control of stoichiometry and morphology of the TiO2/TiO2-x-based structure, which is critical for the sensitivity and selectivity of the designed gas sensors.

Journal Reference:

Ramanavicius, S., Arunas Jagminas, A. and Ramanavicius, A (2022) Gas Sensors Based on Titanium Oxides (Review). Coatings, 12(5), p.699. Available Online: https://www.mdpi.com/2079-6412/12/5/699/htm.

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