A team of researchers recently published a paper in the journal Coordination Chemistry Reviews that reviewed conducting polymers (CPs), specifically CP-based nanostructures, discussing their feasibility in gas sensing applications.
Study: Conducting polymer-based nanostructures for gas sensors. Image credit: Max Zalevsky/Shutterstock.com
Conductive Polymers for Gas Sensing Applications
CPs can be potentially used to develop gas sensors that exhibit gas-material interactions at room temperature without using a microheater and instantly generate a readout signal upon detection of a target gas molecule. CPs are affordable and lightweight and can be fabricated easily. Additionally, the excellent flexibility, print deposition, and low-temperature fabrication process of the CPs offer a good platform for the large-scale development of advanced and flexible gas sensors.
CPs, such as poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene (PTH), polypyrrole (PPY), and polyaniline (PANI) are increasingly being preferred in the gas sensor technology over the traditional inorganic materials to detect volatile organic compounds (VOCs), carbon monoxide, ammonia, and carbon dioxide, which are toxic and hazardous to the environment and human health.
Gas Sensing Mechanism in CP-Based Gas Sensors
CP-based gas sensors display a change in their electrical properties, such as resistance value or conductivity, after the interaction between the polymer and the gas. These sensors can transform the target gas concentration signal into detectable physical signals, such as resistance, current, or voltage. However, the change in the electrical properties depends on the doping level in these sensors.
In CP gas sensors, doping plays a vital role in the gas sensing mechanism. The level of doping in CPs can be easily regulated at room temperature by chemical reactions with different target gases. Moreover, CPs are also undoped/doped through redox reactions.
Doping with an oxidizing agent leads to p-type doping, while doping with a reducing agent leads to n-type doping.
Synthesis of CP-Based Nanostructures
The sensing responses of CP-based gas sensors are primarily dependent on the morphology and structure of CP sensing layers. Thus, different protocols were explored to synthesize three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) CP-based nanostructures to meet the requirements of various gas sensor configurations.
0D CP Nanostructures
0D CP nanoparticles are typically synthesized in micelles. For instance, CP nanoparticles were synthesized through chemical oxidation polymerization of aniline in dodecylbenzene sulfonic acid (DBSA) micellar solution in order to improve the processability and stability of the nanoparticles.
The average size of synthesized PANI particles was 20-30 nanometers and the highest conductivity was 24 S/cm.
1D CP Nanostructures
1D CP nanostructures can be fabricated by chemical polymerization of aniline monomers using an oxidizing agent. Both template-free and template-based protocols can be used for chemical polymerization.
Template-based methods typically use a porous media, such as anodic aluminum oxide (AAO), as a template to guide the synthesis of 1D CP nanostructures, such as PEDOT and PPY, while template-free methods are based on self-assembly progress that involves non-covalent interactions, such as electrostatic interactions. Electrospinning is another effective method to synthesize 1D CP fibers using electrostatic forces.
2D CP Nanostructures
2D CP nanostructures, such as nanofilms and nanosheets with tens of nanometers thickness, have gained attention for their potential in chemical gas sensors owing to their ultra-high surface-to-volume ratio and unique 2D geometry. Electrochemical deposition of CP films on a conductive substrate is the most preferred method to synthesize 2D CP nanostructures, followed by spin coating technology.
3D CP Nanostructures
3D CPs possess highly continuous porous structures and interconnected 3D networks. Thus, the synthesis of 3D CP nanostructures is more challenging compared to 2D or 1D nanostructures.
Sacrificial template methods that involve the use of a regular 3D structure as a replica template are most suitable for the fabrication of complex 3D CP nanostructures. Moreover, these nanostructures can also be synthesized through the combination of the vapor deposition polymerization method and freezing technology, and the electrospinning method.
Structure-Property Relations of CP Nanostructure-Based Gas Sensors
Surface morphology, specifically the nanostructure, plays a crucial role in the gas sensing performance of CP nanostructure-based gas sensors as efficient physical absorption of gas molecules is necessary to effectively detect target gases. Absorption of gas molecules is improved when the sensing layer has a controllable pore size, high pore volume, and large surface area.
Hybridization of CPs for Advanced Gas Sensors
The gas sensing properties are significantly influenced by the material compositions of CP gas sensors. CPs can be hybridized with different inorganic materials, such as metal oxide nanostructures, and nanocarbons, to address several challenges associated with CP gas sensors. These CP-based hybrid gas sensors possess more active sites for enhanced absorption and higher carrier transport owing to the synergistic reaction between the components.
For instance, CPs can be hybridized with nanocarbons to develop an effective gas sensor as nanocarbons possess high environmental stability and exceptional electrical and mechanical properties. Similarly, metal nanoparticles, such as silver, gold, and platinum nanoparticles, can be used for surface modification of CPs to improve the sensing stability and performance of the CP-based gas sensors.
Challenges of CP Gas Sensors
Several challenges associated with CP gas sensors, such as moisture and temperature resistance, are limiting their application and development.
The conductivity of CPs increases with a rise in temperature, which decreases their absorption capacity as organic compounds such as CPs cannot withstand high temperatures as metal oxides. Thus, CP-gas sensors are typically utilized at room temperatures.
Similarly, several CP gas sensors that display sensitivity to humidity often generate a mixed response based on the water vapor and target gas present in a particular environment, which leads to inaccurate sensing results.
Future of CP Gas Sensors
In the last decade, CP-based nanostructures, specifically PANI and PPY, have demonstrated significant potential in gas sensing applications at room temperature. The impacts of humidity and air on the CP gas sensing properties can be eliminated by carefully designing the CPs at deeper highest occupied molecular orbital (HOMO) levels or using additives as crosslinkers.
Hybridization of CPs can eliminate other shortcomings such as cross selectivity and poor reversibility. However, more research is required to achieve a stable sensing response in different humid environments and thoroughly understand the gas sensing mechanism in CP gas sensors in order to employ CPs in practical gas sensing applications.
Reference
Kumar, M., Kumar, R., Zhang, J.et al. (2022) Conducting polymer-based nanostructures for gas sensors. Coordination Chemistry Reviews. https://www.sciencedirect.com/science/article/pii/S0010854522001126?via%3Dihub
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