Flexible room-temperature formaldehyde sensors based on rGO film and rGo/MoS2 hybrid film

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

MoS2-NiO nanocomposite for H2S sensing at room temperature

The layered 2-D materials, such as molybdenum disulfide (MoS2), are among the most promising candidates for detecting H2S gas at very low concentrations. Herein, we have designed a series of novel nanocomposites consisting of MoS2 and NiO. These materials were synthesized via a simple hydrothermal method. The microstructure and morphology of nanocomposites were studied using different characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET) analysis, and X-ray photoelectron spectroscopy (XPS). These nanocomposites were used as gas sensors, and the highest response (6.3) towards 10 ppm H2S was detected by the MNO-10 gas sensor among all the tested sensors. The response value (Rg/Ra) was almost three times that of pure NiO (Rg/Ra = 2). Besides, the MNO-10 sensor exposed good selectivity, short response/recovery time (50/20 s), long-term stability (28 days), reproducibility (6 cycles), and a low detection limit (2 ppm) towards H2S gas at RT. The excellent performance of MNO-10 may be attributed to some features of MoS2, such as a layered structure, higher BET surface area, higher active sites, and a synergistic effect between MoS2 and NiO. This simple fabrication sensor throws a novel idea for detecting H2S gas.

Effect of Different Solvents on Morphology and Gas-Sensitive Properties of Grinding-Assisted Liquid-Phase-Exfoliated MoS2 Nanosheets

Grinding-assisted liquid-phase exfoliation is a widely used method for the preparation of two-dimensional nanomaterials. In this study, N-methylpyrrolidone and acetonitrile, two common grinding solvents, were used during the liquid-phase exfoliation for the preparation of MoS₂ nanosheets. The morphology and structure of MoS₂ nanosheets were analyzed via scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The effects of grinding solvents on the gas-sensing performance of the MoS₂ nanosheets were investigated for the first time. The results show that the sensitivities of MoS₂ nanosheet exfoliation with N-methylpyrrolidone were 2.4-, 1.4-, 1.9-, and 2.7-fold higher than exfoliation with acetonitrile in the presence of formaldehyde, acetone, and ethanol and 98% relative humidity, respectively. MoS₂ nanosheet exfoliation with N-methylpyrrolidone also has fast response and recovery characteristics to 50-1000 ppm of CH₂O. Accordingly, although N-methylpyrrolidone cannot be removed completely from the surface of MoS₂, it has good gas sensitivity compared with other samples. Therefore, N-methylpyrrolidone is preferred for the preparation of gas-sensitive MoS₂ nanosheets in grinding-assisted liquid-phase exfoliation. The results provide an experimental basis for the preparation of two-dimensional materials and their application in gas sensors.

Noble-Nanoparticle-Decorated Ti3C2T x MXenes for Highly Sensitive Volatile Organic Compound Detection

Two-dimensional transition-metal carbides and nitrides (MXenes) have been regarded as promising sensing materials because of their high surface-to-volume ratios and outstanding electronic, optical, and mechanical properties with versatile transition-metal and surface chemistries. However, weak gas-molecule adsorption of MXenes poses a serious limitation to their sensitivity and selectivity, particularly for trace amounts of volatile organic compounds (VOCs) at room temperature. To deal with these issues, Au-decorated MXenes are synthesized by a facile solution mixing method for room-temperature sensing of a wide variety of oxygen-based and hydrocarbon-based VOCs. Dynamic sensing experiments reveal that optimal decoration of Au nanoparticles (NPs) on Ti3C2T x MXene significantly elevates the response and selectivity of the flexible sensors, especially in detecting formaldehyde. Au-Ti3C2T x gas sensors exhibited an extremely low limit of detection of 92 ppb for formaldehyde at room temperature. Au-Ti3C2T x provides reliable gas response, low noise level, ultrahigh signal-to-noise ratio, high selectivity, as well as parts per billion level of formaldehyde detection. The prominent mechanism for Au-Ti3C2T x in sensing formaldehyde is elucidated theoretically from density functional theory simulations. The results presented here strongly suggest that decorating noble-metal NPs on MXenes is a feasible strategy for the development of next-generation ultrasensitive sensors for Internet of Things.

The Effect of Thin Film Fabrication Techniques on the Performance of rGO Based NO2 Gas Sensors at Room Temperature

Reduced graphene oxide (rGO) has attracted enormous interest as a promising candidate material for gas detection due to its large specific surface areas. In our work, rGO films were fabricated on a large scale using dip-coating and spin-coating methods for the detection of nitrogen dioxide (NO2) gas at room temperature. The influence of different test environments on the sensing performance, including the test atmosphere, gas flow and gas pressure was evaluated. The response time of the dip-coating method was 573 s with a long recovery period of 639 s and for the spin-coating method, the response time and recovery time was 386 s and 577 s, respectively. In addition, the spin-coated sensor exhibited high selectivity to NO2, with the response increasing by more than 20% (for 15 ppm NO2) as compared with that for HCHO, NH3, and CH4. Our results indicated that the spin coating method was more suitable for rGO-based gas sensors with higher performance.

Flexible Ceramic Film Sensors for Free-Form Devices

Recent technological innovations, such as material printing techniques and surface functionalization, have significantly accelerated the development of new free-form sensors for next-generation flexible, wearable, and three-dimensional electronic devices. Ceramic film sensors, in particular, are in high demand for the production of reliable flexible devices. Various ceramic films can now be formed on plastic substrates through the development of low temperature fabrication processes for ceramic films, such as photocrystallization and transferring methods. Among flexible sensors, strain sensors for precise motion detection and photodetectors for biomonitoring have seen the most research development, but other fundamental sensors for temperature and humidity have also begun to grow. Recently, flexible gas and electrochemical sensors have attracted a lot of attention from a new real-time monitoring application that uses human breath and perspiration to accurately diagnose presymptomatic states. The development of a low-temperature fabrication process of ceramic film sensors and related components will complete the chemically stable and reliable free-form sensing devices by satisfying the demands that can only be addressed by flexible metal and organic components.

An effective formaldehyde gas sensor based on oxygen-rich three-dimensional graphene

Three-dimensional (3D) graphene with a high specific surface area and excellent electrical conductivity holds extraordinary potential for molecular gas sensing. Gas molecules adsorbed onto graphene serve as electron donors, leading to an increase in conductivity. However, several challenges remain for 3D graphene-based gas sensors, such as slow response and long recovery time. Therefore, research interest remains in the promotion of the sensitivity of molecular gas detection. In this study, we fabricate oxygen plasma-treated 3D graphene for the high-performance gas sensing of formaldehyde. We synthesize large-area, high-quality, 3D graphene over Ni foam by chemical vapor deposition and obtain freestanding 3D graphene foam after Ni etching. We compare three types of strategies-non-treatment, oxygen plasma, and etching in HNO₃solution-for the posttreatment of 3D graphene. Eventually, the strategy for oxygen plasma-treated 3D graphene exceeds expectations, which may highlight the general gas sensing based on chemiresistors.

Alkali metal-incorporated spinel oxide nanofibers enable high performance detection of formaldehyde at ppb level

Sensing material with high sensitivity, excellent selectivity and ultra-low detection limit is crucial for monitoring formaldehyde, which is a kind of hazardous gas to human health at very low concentration. Some one-dimensional semiconductor metal oxides show acceptable responses towards formaldehyde. However, the detection limit and selectivity of these sensors are still not satisfied, especially at ppb level. Herein, alkali metals (K, Na) doped CdGa₂O₄ nanofibers with excellent formaldehyde sensing performance are prepared by an electrospinning method. These nanofibers have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance spectroscopy (EPR), elemental mapping and other techniques. As a result, the sensor based on 7.5 at.% K doped CdGa₂O₄ gives remarkably improved formaldehyde sensing properties compared with that of pristine CdGa₂O₄. The greatly increased sensitivity and selectivity should be attributed to the increased chemisorbed oxygen and the enhanced basicity caused by the additional alkali metal, respectively. All in all, the 7.5 at.% K doped CdGa₂O₄ is a good candidate for the rapid detecting formaldehyde at ppb level.

Nanoheterostructure Construction and DFT Study of Ni-Doped In2O3 Nanocubes/WS2 Hexagon Nanosheets for Formaldehyde Sensing at Room Temperature

A high-performance formaldehyde sensor based on nickel (Ni)-doped indium trioxide (In₂O₃)/tungsten disulfide (WS₂) nanocomposite was demonstrated. An epoxy substrate served as matrix of the Ni-In₂O₃/WS₂ nanocomposite sensor. The material properties of self-assembled Ni-In₂O₃/WS₂ nanoheterostructure were fully characterized and confirmed. The formaldehyde-sensing properties of the Ni-In₂O₃/WS₂ composite were tested at 25 °C. Compared to the In₂O₃, WS₂, and their composite, the Ni-In₂O₃/WS₂ sensor demonstrated significant improvement on the formaldehyde-sensing performance, including a low detection limit of 15 ppb, good selectivity, repeatability, fast detection rate, and a fair logarithmic function toward formaldehyde concentration. The dramatically enhanced sensing performance of Ni-In₂O₃/WS₂ film sensor can be attributed to the Ni ion doping and synergistic interfacial incorporation of In₂O₃/WS₂ heterojunction. The sensitive mechanism of the Ni-In₂O₃/WS₂ film sensor toward formaldehyde is explored through density functional theory (DFT) simulation. This work verified that the synthesis of Ni-doped In₂O₃/WS₂ nanofilm provides a new avenue to develop promising hybrids for formaldehyde sensing.

Challenges and recent advancements of functionalization of two-dimensional nanostructured molybdenum trioxide and dichalcogenides

Atomically thin two-dimensional (2D) semiconductors are the thinnest functional semiconducting materials available today. Among them, both molybdenum trioxide and chalcogenides (MT&Ds) represent key components within the family of different 2D semiconductors for various electronic, optoelectronic and electrochemical applications due to their unique electronic, optical, mechanical and electrochemical properties. However, despite great progress in research dedicated to the development and fabrication of 2D MT&Ds observed within the last decade, there are significant challenges that affected their charge transport behavior and fabrication on a large scale as well as there is high dependence of the carrier mobility on the thickness. In this article, we review the recent progress in the carrier mobility engineering of 2D MT&Ds and elaborate devised strategies dedicated to the optimization of MT&D properties. Specifically, the latest physical and chemical methods towards the surface functionalization and optimization of the major factors influencing the extrinsic transport at the electrode-2D semiconductor interface are discussed.

A highly sensitive gas sensor employing biomorphic SnO2 with multi-level tubes/pores structure: bio-templated from waste of flax

Metal oxide gas sensors with porous structures are widely used in numerous applications ranging from health monitoring and medical detection to safety; in this study, we report a highly sensitive SnO2 gas sensor with a multi-level tube/pore structure prepared via biomimetic technology using flax waste as a bio-template and a simple wet chemical process combined with subsequent annealing. Indeed, MLTPS not only maintained and improved the excellence of porous structure gas sensing materials with abundant active sites and large surface-to-volume ratios, but also overcame the deficiency of the lack of gas diffusion channels in porous gas sensing materials. Thus, this novel multi-level tube/pore SnO2 gas sensor exhibited significantly enhanced sensing performance, e.g. an ultra-low response concentration (250 ppb), a high response (87.9), a fast response (9.2 s), a low operating temperature (130 °C) and good stability, for formaldehyde. On the basis of these results, via the reuse of agricultural waste, this study provides a new concept for the low-cost synthesis of environmentally friendly and effective multi-level tube/pore gas sensor materials.