The breadth and depth of applications for two-dimensional (2D) materials which, by their nature, are nanoscale and layer structured with a high surface to volume ratio, are extensive and rapidly transforming the field of gas sensing. Facile preparation methods, as well as novel electronic and surface properties that enable functionalisation, provide high sensitivity for gases commonly encountered in natural and industrial environments. Gas sensors that are operational at room temperature offer low power demand and thermal safety during analysis. This review focuses on room-temperature gas sensing performance of conductometric devices employing 2D hybrid nanomaterials including layered transition metal dichalcogenides (TMDs) and graphene materials. The effect of hybridisation of 2D nanomaterials on gas sensing performance and their sensing mechanisms are studied. Hybrid forms of 2D materials occur by the addition of metals, or polymers to surface sites, or by combining two or more different materials (i.e. nanocomposites) or by developing a heterojunction layer, which lead to enhanced collective performance. Gas sensing mechanisms depend on the type and format of 2D material but fundamentally involve gas adsorption on an active surface with a resultant change in the resistive property of the material.

A two-dimensional (2D) CeO₂-Pd-PDA/rGO heterojunction nanocomposite has been synthesised via an environmentally friendly, energy efficient, and facile wet chemical procedure and examined for hydrogen (H₂) gas sensing application for the first time. The H₂ gas sensing performance of the developed conductometric sensor has been extensively investigated under different operational conditions, including working temperature up to 200 °C, UV illumination, H₂ concentrations from 50–6000 ppm, and relative humidity up to 30% RH. The developed ceria-based nanocomposite sensor was functional at a relatively low working temperature (100 °C), and its sensing properties were improved under UV illumination (365 nm). The sensor’s response towards 6000 ppm H₂ was drastically enhanced in a humid environment (15% RH), from 172% to 416%. Under optimised conditions, this highly sensitive and selective H₂ sensor enabled the detection of H₂ molecules down to 50 ppm experimentally. The sensing enhancement mechanisms of the developed sensor were explained in detail. The available 4f electrons and oxygen vacancies on the ceria surface make it a promising material for H₂ sensing applications. Moreover, based on the material characterisation results, highly reactive oxidant species on the sensor surface formed the electron–hole pairs, facilitated oxygen mobility, and enhanced the H₂ sensing performance.

A two-dimensional (2D) Dy₂O₃-Pd-PDA/rGO heterojunction nanocomposite has been synthesised and tested for hydrogen (H₂) gas sensing under various functioning conditions, including different H₂ concentrations (50 ppm up to 6000 ppm), relative humidity (up to 25 %RH) and working temperature (up to 200 °C). The material characterisation of Dy₂O₃-Pd-PDA/rGO nanocomposite performed using various techniques confirms uniform distribution of Pd NPs and 2D Dy₂O₃ nanostructures on multi-layered porous structure of PDA/rGO nanosheets (NSs) while forming a nanocomposite. Moreover, fundamental hydrogen sensing mechanisms, including the effect of UV illumination and relative humidity (%RH), are investigated. It is observed that the sensing performance is improved as the operating temperature increases from room temperature (RT = 30 °C) to the optimum temperature of 150 °C. The humidity effect investigation revealed a drastic enhancement in sensing parameters as the %RH increased up to 20%. The highest response was found to be 145.2% towards 5000 ppm H₂ at 150 °C and 20 %RH under UV illumination (365 nm). This work offers a highly sensitive and selective hydrogen sensor based on a novel 2D nanocomposite using an environmentally friendly and energy-saving synthesis approach, enabling us to detect hydrogen molecules experimentally down to 50 ppm.