Abstract
Fractals are intriguing structures that repeat themselves at various length scales. Interestingly, fractals can also be fabricated artificially in labs under controlled growth environments and be explored for various applications. Such fractals have a repeating unit that spans in length from nano- to millimeter range. Fractals thus can be regarded as connectors that structurally bridge the gap between the nano- and the macroscopic worlds and have a hybrid structure of pores and repeating units. This article presents a comprehensive review on inorganic fabricated fractals (fab-fracs) synthesized in labs and employed as gas sensors across materials, morphologies, and gas analytes. The focus is to investigate the morphology-driven gas response of these fab-fracs and identify key parameters of fractal geometry in influencing gas response. Fab-fracs with roughened microstructure, pore-network connectivity, and fractal dimension (D) less than 2 are projected to be possessing better gas sensing capabilities. Fab-fracs with these salient features will help in designing the commercial gas sensors with better performance.
Highlights
The industrial sector and its related activities have led to various forms of pollution that compounding up with time
The objectives in gas sensing research are usually set to enhance the sensitivity, selectivity, stability, the response time, and recovery time
Their unique morphologies comprise structures ranging from the nanoscale to the macroscale, where the properties change with the length scales involved
Summary
The industrial sector and its related activities have led to various forms of pollution that compounding up with time. Kante et al prepared SnO2 films with fractal morphology by an electrochemical method with a subsequent oxidation process [68] Both groups tested the films for CO gas sensing at different temperatures. The nanostructured dendrites exhibited a higher sensitivity with a detection limit of 200 ppb towards NO2, with rapid response (7 s) and recovery time (12 s) at 5 ppm NO2 at an operating temperature of 140 °C. The dendritic nanostructure allowed the network passage for electron transfer after ammonia molecules interact with the sensing surface It showed an about 5–8 times enhanced response and an improvement in recovery time by about 30–50 times compared to a pristine NiO sensor. The fractal dimensions estimated in the present article show that structures with D in the range of 1.3–1.8 exhibit better gas sensing responses. While the porous continuous network provides a backbone for better and faster charge transport, the unique morphology of fab-fracs offers a better gas–sensor interaction indicated by the fractal dimension
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