VOCs Sensing by Metal Oxides, Conductive Polymers, and Carbon-Based Materials

Abstract

This review summarizes the recent research efforts and developments in nanomaterials for sensing volatile organic compounds (VOCs). The discussion focuses on key materials such as metal oxides (e.g., ZnO, SnO2, TiO2 WO3), conductive polymers (e.g., polypyrrole, polythiophene, poly(3,4-ethylenedioxythiophene)), and carbon-based materials (e.g., graphene, graphene oxide, carbon nanotubes), and their mutual combination due to their representativeness in VOCs sensing. Moreover, it delves into the main characteristics and tuning of these materials to achieve enhanced functionality (sensitivity, selectivity, speed of response, and stability). The usual synthesis methods and their advantages towards their integration with microsystems for practical applications are also remarked on. The literature survey shows the most successful systems include structured morphologies, particularly hierarchical structures at the nanometric scale, with intentionally introduced tunable “decorative impurities” or well-defined interfaces forming bilayer structures. These groups of modified or functionalized structures, in which metal oxides are still the main protagonists either as host or guest elements, have proved improvements in VOCs sensing. The work also identifies the need to explore new hybrid material combinations, as well as the convenience of incorporating other transducing principles further than resistive that allow the exploitation of mixed output concepts (e.g., electric, optic, mechanic).

ZnO Structures with Surface Nanoscale Interfaces Formed by Au, Fe2O3, or Cu2O Modifier Nanoparticles: Characterization and Gas Sensing Properties

“…Amongst a host of available gas sensor technologies based on nanomaterials, metal oxide semiconductors (MOX) based gas sensors are very promising due to their good sensitivity, small dimensions, compatibility with microelectronic fabrication processes, and potentially low cost. These devices have shown sensitivity to various gases (e.g., CO, NO 2 , NH 3 , H 2 S) and vapors (alcohols, ketones, aldehydes, aromatic compounds), although their performance in terms of limits of detection and selectivity is still a subject of improvement [ 2 , 3 , 4 ].…”

“…These improvements are generally connected with the increment of adsorption centers at the surface and the band energy structure modification due to the formation of Schottky-barrier-like junctions (metal–MOX) or heterojunctions (MOX–MOX) [ 7 ]. Consequently, the interest in finding synthetic methods that allow for crystalline MOXs with tuned second-phase constituents (metals or MOXs) has kept constant in the gas sensing field, as well as the use of methods that provide scalability to these materials and compatibility with state-of-the-art microfabrication processes for possible multi-purpose microelectronic devices [ 3 , 7 ].…”

“…Zinc oxide (ZnO) is an n-type semiconductor with a wide band-gap (E G = 3.3 eV), high chemical and thermal stability, and tunable surface morphology due to its stable wurtzite structure [ 8 , 9 ]. This MOX, in its pristine form and modified with noble metals (e.g., Au, Ag, Pt, or Pd) [ 10 , 11 , 12 , 13 ] or another MOX (e.g., CuO) [ 14 ], is one of the most representative gas-sensitive MOXs with proved sensitivity to gases, such as CO, H 2 , H 2 S, and NO 2 , and vapors such as ethanol, methanol, and acetaldehyde [ 3 ]. ZnO was synthesized previously by different methods, including wet- and vapor-chemical routes, delivering films and powders with varied forms, e.g., nanoplates, nanospheres, nanowires, nanorods, nanotubes, nanoflowers, nanofibers, nanoneedles, and nanoribbons [ 3 , 4 ].…”

“…This MOX, in its pristine form and modified with noble metals (e.g., Au, Ag, Pt, or Pd) [ 10 , 11 , 12 , 13 ] or another MOX (e.g., CuO) [ 14 ], is one of the most representative gas-sensitive MOXs with proved sensitivity to gases, such as CO, H 2 , H 2 S, and NO 2 , and vapors such as ethanol, methanol, and acetaldehyde [ 3 ]. ZnO was synthesized previously by different methods, including wet- and vapor-chemical routes, delivering films and powders with varied forms, e.g., nanoplates, nanospheres, nanowires, nanorods, nanotubes, nanoflowers, nanofibers, nanoneedles, and nanoribbons [ 3 , 4 ]. Recently, we developed a method to deposit ZnO by aerosol-assisted chemical vapor deposition (AACVD), which offers significant advantages over other CVD techniques, including atmospheric pressure and relatively low-temperature process, as well as the possibility of tuning the material morphology and structure [ 15 , 16 ].…”

“…The surface modification of MOX structures with second-phase constituents can also be achieved by different methods [ 7 ]; amongst them, AACVD [ 18 , 19 , 20 ] and impregnation [ 21 , 22 ] methods proved effective to modify structures with both metals and MOXs. Recently, for instance, AACVD was employed to modify ZnO with iron and copper [ 3 ], although the gas sensing performance of these systems was not evaluated. Similarly, impregnation procedures based on the reduction of a chemical inorganic precursor were used to incorporate Ag and Au particles over ZnO [ 23 , 24 ].…”