Recent Progress of Toxic Gas Sensors Based on 3D Graphene Frameworks

Development of an NO2 Gas Sensor Based on Laser-Induced Graphene Operating at Room Temperature

A novel, in situ, low-cost and facile method has been developed to fabricate flexible NO2 sensors capable of operating at ambient temperature, addressing the urgent need for monitoring this toxic gas. This technique involves the synthesis of highly porous structures, as well as the specific development of laser-induced graphene (LIG) and its heterostructures with SnO2, all through laser scribing. The morphology, phases, and compositions of the sensors were analyzed using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. The effects of SnO2 addition on structural and sensor properties were investigated. Gas-sensing measurements were conducted at room temperature with NO2 concentrations ranging from 50 to 10 ppm. LIG and LIG/SnO2 sensors exhibited distinct trends in response to NO2, and the gas-sensing mechanism was elucidated. Overall, this study demonstrates the feasibility of utilizing LIG and LIG/SnO2 heterostructures in gas-sensing applications at ambient temperatures, underscoring their broad potential across diverse fields.

Designing State-of-the-Art Gas Sensors: From Fundamentals to Applications

Gas sensors are crucial in environmental monitoring, industrial safety, and medical diagnostics. Due to the rising demand for precise and reliable gas detection, there is a rising demand for cutting-edge gas sensors that possess exceptional sensitivity, selectivity, and stability. Due to their tunable electrical properties, high-density surface-active sites, and significant surface-to-volume ratio, nanomaterials have been extensively investigated in this regard. The traditional gas sensors utilize homogeneous material for sensing where the adsorbed surface oxygen species play a vital role in their sensing activity. However, their performance for selective gas sensing is still unsatisfactory because the employed high temperature leads to the poor stability. The heterostructures nanomaterials can easily tune sensing performance and their different energy band structures, work functions, charge carrier concentration and polarity, and interfacial band alignments can be precisely designed for high-performance selective gas sensing at low temperature. In this review article, we discuss in detail the fundamentals of semiconductor gas sensing along with their mechanisms. Further, we highlight the existed challenges in semiconductor gas sensing. In addition, we review the recent advancements in semiconductor gas sensor design for applications from different perspective. Finally, the conclusion and future perspectives for improvement of the gas sensing performance are discussed.

Inorganic Hydrogel Based on Low-Dimensional Nanomaterials

Composed of three-dimensional (3D) nanoscale inorganic bones and up to 99% water, inorganic hydrogels have attracted much attention and undergone significant growth in recent years. The basic units of inorganic hydrogels could be metal nanoparticles, metal nanowires, SiO2 nanowires, graphene nanosheets, and MXene nanosheets, which are then assembled into the special porous structures by the sol-gel process or gelation via either covalent or noncovalent interactions. The high electrical and thermal conductivity, resistance to corrosion, stability across various temperatures, and high surface area make them promising candidates for diverse applications, such as energy storage, catalysis, adsorption, sensing, and solar steam generation. Besides, some interesting derivatives, such as inorganic aerogels and xerogels, can be produced through further processing, diversifying their functionalities and application domains greatly. In this context, we primarily provide a comprehensive overview of the current status of inorganic hydrogels and their derivatives, including the structures of inorganic hydrogels with various compositions, their gelation mechanisms, and their exceptional practical performance in fields related to energy and environmental applications.

Highly Selective Room Temperature Detection of NH3 and NOx Using Oxygen-Deficient W18O49-Supported WS2 Heterojunctions

In this paper, we reported the controlled synthesis of tungsten disulfide/reduced tungsten oxide (WS₂/W₁₈O₄₉) heterojunctions for highly efficient room temperature NO_x and ammonia (NH₃) sensors. X-ray diffraction analysis revealed the formation of the oxygen-deficient W₁₈O₄₉ phase along with WS₂. Field-emission scanning electron microscopy and transmission electron microscopy displayed the formation of WS₂ flakes over W₁₈O₄₉ nanorods. X-ray photoelectron spectroscopy showed the presence of tungsten in W⁴⁺, W⁵⁺, and W⁶⁺ oxidation states corresponding to WS₂ and W₁₈O₄₉, respectively. The WS₂/W₁₈O₄₉ heterojunction sensor exhibited sub-ppm level sensitivity to NO_x and NH₃ at room temperature. The heterojunction sensor detected 0.6 ppm NO_x and 0.5 ppm NH₃, with a corresponding response of 7.1 and 3.8%, respectively. The limit of detection of the sensor was calculated to be 0.05 and 0.17 ppm for NH₃ and NO_x, respectively. The cyclic stability test showed that the sensor exhibited high stability even after 24 cycles for the detection of NH₃ and 14 cycles for NO_x. Compared to pristine WO₃ and WS₂, the WS₂/W₁₈O₄₉ heterojunction showed high selectivity toward NO_x and NH₃. The results could be useful for the development of room temperature NO_x and NH₃ sensors.

Progress in Research and Application of Graphene Aerogel-A Bibliometric Analysis

In recent years, graphene aerogel (GA) has been widely used as a 3D porous stable network structure material. In order to identify the main research direction of GA, we use the bibliometric method to analyze its hot research fields and applications from the Web of Science database. First, we collected all relevant literature and analyzed its bibliometrics of publication year, country, institution, etc., where we found that China and Chinese Academy of Sciences are the most productive country and institute, respectively. Then, the three hot fields of fabrication, energy storage, and environmental protection are identified and thoroughly discussed. Graphene aerogel composite electrodes have achieved very efficient storage capacity and charge/discharge stability, especially in the field of electrochemical energy storage. Finally, the current challenges and the future development trends are presented in the conclusion. This paper provides a new perspective to explore and promote the related development of GA.

Ultralight, Elastic, Hybrid Aerogel for Flexible/Wearable Piezoresistive Sensor and Solid-Solid/Gas-Solid Coupled Triboelectric Nanogenerator

Aerogels have been attracting wide attentions in flexible/wearable electronics because of their light weight, excellent flexibility, and electrical conductivity. However, multifunctional aerogel-based flexible/wearable electronics for human physiological/motion monitoring, and energy harvest/supply for mobile electronics, have been seldom reported yet. In this study, a kind of hybrid aerogel (GO/CNT HA) based on graphene oxide (GO) and carboxylated multiwalled carbon nanotubes (CMWCNTs) is prepared which can not only used as piezoresistive sensors for human motion and physiological signal detections, but also as high performance triboelectric nanogenerator (TENG) coupled with both solid-solid and gas-solid contact electrifications (CE). The repeatedly loading-unloading tests with 20 000 cycles exhibit its high and ultrastable piezoresistive sensor performances. Moreover, when the obtained aerogel is used as the electrode of a TENG, high electric output performance is produced due to the synergistic effect of solid-solid, and gas-solid interface CEs (3D electrification: solid-solid interface CE between the two solid electrification layers; gas-solid interface CE between the inner surface of GO/CNT HA and the air filled in the aerogel pores). This kind of aerogel promises good applications for human physiological/motion monitoring and energy harvest/supply in flexible/wearable electronics such as piezoresistive sensors and flexible TENG.

Hierarchical Wrinkles for Tunable Strain Sensing Based on Programmable, Anisotropic, and Patterned Graphene Hybrids

Flexible, stretchable, wearable, and stable electronic materials are widely studied, owing to their applications in wearable devices and the Internet of Things. Because of the demands for both strain-insensitive resistors and high gauge factor (GF) strain-sensitive materials, anisotropic strain sensitivity has been an important aspect of electronic materials. In addition, the materials should have adjustable strain sensitivities. In this work, such properties are demonstrated in reduced graphene oxide (RGO) with hierarchical oriented wrinkle microstructures, generated using the two-step shrinkage of a rubber substrate. The GF values range from 0.15 to 28.32 at 100% strain. For device demonstrations, macrostructure patterns are designed to prepare patterned wrinkling graphene at rubber substrate (PWG@R). Serpentiform curves can be used for the constant-value resistor, combined with the first-grade wrinkles. Strip lines can increase the strain-sensing property, along with the second-grade wrinkles. The patterned sensor exhibits improved GF values range from 0.05 to 49.5. The assembled sensor shows an excellent stability (>99% retention after 600 cycles) with a high GF (49.5). It can monitor the vital signs of the throat and wrist and sense large motions of fingers. Thus, PWG@R-based strain sensors have great potential in various health or motion monitoring fields.

Recent Advances in the Synthesis and Application of Three-Dimensional Graphene-Based Aerogels

Three-dimensional graphene-based aerogels (3D GAs), combining the intrinsic properties of graphene and 3D porous structure, have attracted increasing research interest in varied fields with potential application. Some related reviews focusing on applications in photoredox catalysis, biomedicine, energy storage, supercapacitor or other single aspect have provided valuable insights into the current status of Gas. However, systematic reviews concentrating on the diverse applications of 3D GAs are still scarce. Herein, we intend to afford a comprehensive summary to the recent progress in the preparation method (template-free and template-directed method) summarized in Preparation Strategies and the application fields (absorbent, anode material, mechanical device, fire-warning material and catalyst) illustrated in Application of 3D GAs with varied morphologies, structures, and properties. Meanwhile, some unsettled issues, existing challenges, and potential opportunities have also been proposed in Future Perspectives to spur further research interest into synthesizing finer 3D GAs and exploring wider and closer practical applications.

Hydrothermal Synthesis of Nitrogen, Boron Co-Doped Graphene with Enhanced Electro-Catalytic Activity for Cymoxanil Detection

A sample of nitrogen and boron co-doped graphene (NB-Gr) was obtained by the hydrothermal method using urea and boric acid as doping sources. According to XRD analysis, the NB-Gr sample was formed by five-layer graphene. In addition, the XPS analysis confirmed the nitrogen and boron co-doping of the graphene sample. After synthesis, the investigation of the electro-catalytic properties of the bare (GC) and graphene-modified electrode (NB-Gr/GC) towards cymoxanil detection (CYM) was performed. Significant differences between the two electrodes were noticed. In the first case (GC) the peak current modulus was small (1.12 × 10-5 A) and appeared in the region of negative potentials (-0.9 V). In contrast, when NB-Gr was present on top of the GC electrode it promoted the transfer of electrons, leading to a large peak current increase (1.65 × 10-5 A) and a positive shift of the peak potential (-0.75 V). The NB-Gr/GC electrode was also tested for its ability to detect cymoxanil from a commercial fungicide (CURZATE MANOX) by the standard addition method, giving a recovery of 99%.

One-Dimensional Nanomaterials in Resistive Gas Sensor: From Material Design to Application

With a series of widespread applications, resistive gas sensors are considered to be promising candidates for gas detection, benefiting from their small size, ease-of-fabrication, low power consumption and outstanding maintenance properties. One-dimensional (1-D) nanomaterials, which have large specific surface areas, abundant exposed active sites and high length-to-diameter ratios, enable fast charge transfers and gas-sensitive reactions. They can also significantly enhance the sensitivity and response speed of resistive gas sensors. The features and sensing mechanism of current resistive gas sensors and the potential advantages of 1-D nanomaterials in resistive gas sensors are firstly reviewed. This review systematically summarizes the design and optimization strategies of 1-D nanomaterials for high-performance resistive gas sensors, including doping, heterostructures and composites. Based on the monitoring requirements of various characteristic gases, the available applications of this type of gas sensors are also classified and reviewed in the three categories of environment, safety and health. The direction and priorities for the future development of resistive gas sensors are laid out.