Flexible NO₂ sensors from renewable cellulose nanocrystals/iron oxide composites

Abstract

A bio-friendly nanostructured cellulose nanocrystal (CNC) sheet with iron oxide grown on it acting as a nitrogen dioxide (NO₂) gas sensor was fabricated by hydrothermal method. The structural investigation was done to monitor the growing mechanism of iron oxide on CNC surface using electron microscope as well as physical and chemical characterization methods. The sensing performance test for NO₂ molecules demonstrates that the devices are highly sensitive and fully recoverable at room temperature, which is attributed to the excellent access of nitrogen dioxide molecules to the sensor surface via CNC. The effects of the temperature, durability, and flexibility of sensor are investigated. The reported sensor performance is a huge improvement towards low power consumption and its room temperature operation augurs well for use in various applications.

Introduction

The need for environmental gas monitoring has induced the development of suitable gas-sensitive materials in which the conducting nanocomposites often impart electrochemical sensing either by acting as sensing component or as a matrix for specific immobilization [1], [2]. Among the air pollutants released by the fossil fuel combustion, nitrogen dioxide (NO₂) gas is particularly dangerous as it causes severe respiratory problems in humans, and also results in acid rain. This NO₂ along with other nitrogen oxides act as precursors for harmful secondary air pollutants such as formaldehyde, ozone, smog etc. [3]. In this scenario, detection of these kinds of toxic or hazardous gases has utmost significance especially for environmental monitoring, personal safety protection, and industrial manufacturing [4].

An ideal gas sensor generally requires high sensitivity, rapid response, high reversibility, low cost, simple structure, and capability of being integrated with a variety of substrates specifically at room temperature [5], [6]. Sensors based on conducting polymer nanocomposites out date the traditional chemi-resistive sensors based on semiconducting oxides in lower power consumption, room temperature operability, and high performance [7]. Flexible and stretchable sensors have also became an inevitable part of smart electronics [8] with their wide range of applicability, for instance, structural health monitoring, manufacturing robot skins and testing the leakage of materials under drastic environments [6]. In recent years, researchers have made progress to minimize the challenges existing in the nanocomposite sensors manufacturing and thus to maximize their response and sensitivity [9], [10], [11]. However, there is still significance in developing smart materials with size-controlled specific sensing effects and more specifically in gas sensing, achieving high recovery time by alleviating the adsorbed gas molecules on the sensors is rather important. Moreover, in the case of NO₂ sensors, the short lifetime of the molecules must also be considered while fabricating the sensor materials [12].

‘Green energy and clean Earth’ is today’s common moto and this concept is reflected in almost all areas of research. The demand for renewable materials is particularly important in this scenario. Cellulose, the most abundant renewable biopolymer is used for synthesizing large number of nanocomposites due to its excellent mechanical properties, microporous structure, chemical stability, biological compatibility, and remarkable hydrophilic properties [13], [14], [15]. Micro-fibrillated and nano-fibrillated cellulose made by delaminating amorphous parts of cellulose fibers are used to make non-caloric food thickeners, cosmetic/pharmaceutical products, nanocomposites and biomedical devices [16], [17]. The crystalline particles in cellulose fibers derived by acid hydrolysis possess rigid shape and anisometry and are called cellulose nanocrystals (CNC). The CNCs produced using concentrated sulfuric acid has high crystallinity, stability and mechanical properties with good dispersion [18], [19]. The hydrophilic CNCs promote nucleation and growth of inorganic pigments in an aqueous medium and its nanocomposite has a positive impact on environment along with ubiquity, renewability, and other unique properties.

Magnetic nanoparticles or ferrofluids (in solution) are controlled by magnetic field gradients and are superparamagnetic by virtue of their sizes. Iron oxide (Fe₃O₄, Fe₂O₃) is a magnetic nanoparticle and has good stability, large saturation magnetization (∼90 emu/g), least toxicity, biocompatibility, and ease to synthesize [20]. As functional materials of great industrial and scientific importance, the important polymorphs of iron oxyhydroxides and iron oxide include α-Iron Oxide-OH, γ-Iron Oxide-OH, α-Fe₂O₃, and γ-Fe₂O₃. Iron oxides of both natural and synthetic origin have been used extensively as pigments mainly due to their low price, non-toxicity, opacity and chemical robustness. Recent investigations of iron oxide nanoparticles filled polymers reveal their potential applications in magnetic resonance imaging for molecular diagnosis, targeted drug delivery, hyperthermia anticancer strategy and water treatment [21], [22]. wang et al. reported the synthesis of cellulose/Fe₂O₃ nanocomposite fibers by wet spinning in NaOH/urea aqueous solution and the samples showed high mechanical strength, superparamagnetic properties, and relatively high dielectric constant [5]. However the inter-particle dipolar force causes the nanoparticles aggregation and the resulting large clusters lose their size effect and specific surface area [23].

Various functionalization methods were employed on the metal nanoparticles to improve the performance and sensitivity of their polymer nanocomposites. Flexible and transparent sensors made from carbon nanotube films applicable in plastic electronics [24] also report the segregation of the filler clusters for achieving better performance and influence of structural modification on material properties [25]. Our group has also developed CNC based proximity sensors by ensuring uniform dispersion and high interfacial interactions [26]. In the case of metal nanoparticles, their clusters display full range of reactivity and thus reacting with different molecules in the medium [27]. All these points explain that the influence on sensing performance of polymer composites of magnetic nanoparticles was significant.

Here, we present a highly responsive NO₂ sensor by using the properties of magnetic iron oxide nanoparticles and renewable CNC with applications extending to different fields like recyclable catalysts, medicine, transparent films in magneto-optics, magnetically retrievable oil-adsorbent and functional cellulose composites. In contrast to the previously reported metal nanoparticle composites, here the particles were allowed to grow on the CNC’s surface which ensures better bonding at the interface and finally results in a flexible, and reversible resistive sensor with light weight. In addition, we demonstrate that the temperature influences sensitivity and the renewable NO₂ sensor fabricated is extremely durable.