Harnessing the influence of reactive edges and defects of graphene substrates for achieving complete cycle of room-temperature molecular sensing

Au- or Ag-Decorated ZnO-Rod/rGO Nanocomposite with Enhanced Room-Temperature NO2-Sensing Performance

To improve the gas sensitivity of reduced oxide graphene (rGO)-based NO2 room-temperature sensors, different contents (0-3 wt%) of rGO, ZnO rods, and noble metal nanoparticles (Au or Ag NPs) were synthesized to construct ternary hybrids that combine the advantages of each component. The prepared ZnO rods had a diameter of around 200 nm and a length of about 2 μm. Au or Ag NPs with diameters of 20-30 nm were loaded on the ZnO-rod/rGO hybrid. It was found that rGO simply connects the monodispersed ZnO rods and does not change the morphology of ZnO rods. In addition, the rod-like ZnO prevents rGO stacking and makes nanocomposite-based ZnO/rGO achieve a porous structure, which facilitates the diffusion of gas molecules. The sensors' gas-sensing properties for NO2 were evaluated. The results reveal that Ag@ZnO rods-2% rGO and Au@ZnO rods-2% rGO perform better in low concentrations of NO2 gas, with greater response and shorter recovery time at the ambient temperature. The response and recovery times with 15 ppm NO2 were 132 s, 139 s and 108 s, 120 s, and the sensitivity values were 2.26 and 2.87, respectively. The synergistic impact of ZnO and Au (Ag) doping was proposed to explain the improved gas sensing. The p-n junction formed on the ZnO and rGO interface and the catalytic effects of Au (Ag) NPs are the main reasons for the enhanced sensitivity of Au (Ag)@ZnO rods-2% rGO.

Engineering of ZnO/rGO towards NO2 Gas Detection: Ratio Modulated Sensing Type and Heterojunction Determined Response

Nanoscale heterostructured zinc oxide/reduced graphene oxide (ZnO/rGO) materials with p-n heterojunctions exhibit excellent low temperature NO₂ gas sensing performance, but their doping ratio modulated sensing properties remain poorly understood. Herein, ZnO nanoparticles were loaded with 0.1~4% rGO by a facile hydrothermal method and evaluated as NO₂ gas chemiresistor. We have the following key findings. First, ZnO/rGO manifests doping ratio-dependent sensing type switching. Increasing the rGO concentration changes the type of ZnO/rGO conductivity from n-type (<0.6% rGO) to mixed n/p -type (0.6~1.4% rGO) and finally to p-type (>1.4% rGO). Second, interestingly, different sensing regions exhibit different sensing characteristics. In the n-type NO₂ gas sensing region, all the sensors exhibit the maximum gas response at the optimum working temperature. Among them, the sensor that shows the maximum gas response exhibits a minimum optimum working temperature. In the mixed n/p-type region, the material displays abnormal reversal from n- to p-type sensing transitions as a function of the doping ratio, NO₂ concentration and working temperature. In the p-type gas sensing region, the response decreases with increasing rGO ratio and working temperature. Third, we derive a conduction path model that shows how the sensing type switches in ZnO/rGO. We also find that p-n heterojunction ratio (n_p-n/n_rGO) plays a key role in the optimal response condition. The model is supported by UV-vis experimental data. The approach presented in this work can be extended to other p-n heterostructures and the insights will benefit the design of more efficient chemiresistive gas sensors.

Electrospinning Nanoparticles-Based Materials Interfaces for Sensor Applications

Electrospinning is a facile technique to fabricate nanofibrous materials with adjustable structure, property, and functions. Electrospun materials have exhibited wide applications in the fields of materials science, biomedicine, tissue engineering, energy storage, environmental science, sensing, and others. In this review, we present recent advance in the fabrication of nanoparticles (NPs)-based materials interfaces through electrospinning technique and their applications for high-performance sensors. To achieve this aim, first the strategies for fabricating various materials interfaces through electrospinning NPs, such as metallic, oxide, alloy/metal oxide, and carbon NPs, are demonstrated and discussed, and then the sensor applications of the fabricated NPs-based materials interfaces in electrochemical, electric, fluorescent, colorimetric, surface-enhanced Raman scattering, photoelectric, and chemoresistance-based sensing and detection are presented and discussed in detail. We believe that this study will be helpful for readers to understand the fabrication of functional materials interfaces by electrospinning, and at the same time will promote the design and fabrication of electrospun nano/micro-devices for wider applications in bioanalysis and label-free sensors.

Gas Sensors Based on Mechanically Exfoliated MoS2 Nanosheets for Room-Temperature NO2 Detection

The unique properties of MoS2 nanosheets make them a promising candidate for high-performance room temperature gas detection. Herein, few-layer MoS2 nanosheets (FLMN) prepared via mechanical exfoliation are coated on a substrate with interdigital electrodes for room-temperature NO2 detection. Interestingly, compared with other NO2 gas sensors based on MoS2, FLMN gas sensors exhibit high responsivity for room-temperature NO2 detection, and NO2 is easily desorbed from the sensor surface with an ultrafast recovery behavior, with recovery times around 2 s. The high responsivity is related to the fact that the adsorbed NO2 can affect the electron states within the entire material, which is attributed to the very small thickness of the MoS2 nanosheets. First-principles calculations were carried out based on the density functional theory (DFT) to verify that the ultrafast recovery behavior arises from the weak van der Waals binding between NO2 and the MoS2 surface. Our work suggests that FLMN prepared via mechanical exfoliation have a great potential for fabricating high-performance NO2 gas sensors.

UV-Activated MoS2 Based Fast and Reversible NO2 Sensor at Room Temperature

Two-dimensional materials have gained considerable attention in chemical sensing owing to their naturally high surface-to-volume ratio. However, the poor response time and incomplete recovery at room temperature restrict their application in high-performance practical gas sensors. Herein, we demonstrate ultrafast detection and reversible MoS₂ gas sensor at room temperature. The sensor's performance is investigated to NO₂ at room temperature, under thermal and photo energy. Incomplete recovery and high response time of ∼249 s of sensor are observed at room temperature. Thermal energy is enough to complete recovery, but it is at the expense of sensitivity. Further, under photo excitation, MoS₂ exhibits an enhancement in sensitivity with ultrafast response time of ∼29 s and excellent recovery to NO₂ (100 ppm) at room temperature. This significant improvement in sensitivity (∼30%) and response time (∼88%) is attributed to the charge perturbation on the surface of the sensing layer in the context of NO₂/MoS₂ interaction under optical illumination. Moreover, the sensor shows reliable selectivity toward NO₂ against various other gases. These unprecedented results reveal the potential of 2D MoS₂ to develop a low power portable gas sensor.

Enhancement of charge transfer between graphene and donor-π-acceptor molecule for ultrahigh sensing performance

In this work, we report the formation of a supramolecular assembly of graphene with a donor-π-acceptor (D-π-A) molecule to detect low concentration NO₂. 5-Aminonaphthalene-1-sulfonic acid (ANS) was used herein to π-π stack with reduced graphene oxide (rGO), the resulting π-conjugated bridge being linked by a donor unit (-NH₂) and an acceptor unit (-SO₃H). The prepared ANS-rGO shows the highest response (R_a/R_g = 13.2 to 10 ppm NO₂) so far among the reported organic molecule modified graphene materials, and excellent selectivity and reliable reversibility at room temperature. Furthermore, as revealed through the charge density difference calculation, it is the effective enhancement of charge transfer between ANS and graphene that should be responsible for the sharp improvement of NO₂ gas response of the material. Thus, for the first time, we demonstrate that supramolecular assembly of a D-π-A molecule and graphene provides a facile and effective approach to fabrication of high performance graphene-based gas sensors.

Transfer of CVD-grown graphene for room temperature gas sensors

An easy transfer procedure to obtain graphene-based gas sensing devices operating at room temperature (RT) is presented. Starting from chemical vapor deposition-grown graphene on copper foil, we obtained single layer graphene which could be transferred onto arbitrary substrates. In particular, we placed single layer graphene on top of a SiO₂/Si substrate with pre-patterned Pt electrodes to realize a chemiresistor gas sensor able to operate at RT. The responses to ammonia (10, 20, 30 ppm) and nitrogen dioxide (1, 2, 3 ppm) are shown at different values of relative humidity, in dark and under 254 nm UV light. In order to check the sensor selectivity, gas response has also been tested towards hydrogen, ethanol, acetone and carbon oxide. Finally, a model based on linear dispersion relation characteristic of graphene, which take into account humidity and UV light effects, has been proposed.

Three-Dimensional Crumpled Graphene-Based Nanosheets with Ultrahigh NO2 Gas Sensibility

It is well-established that the structures dominate the properties. Inspired by the highly contorted and crumpled maxilloturbinate inside dog nose, herein an artificial nanostructure, i.e., 3D crumpled graphene-based nanosheets, is reported with the simple fabrication, detailed characterizations, and efficient gas-sensing applications. A facile supramolecular noncovalent assembly is introduced to modify graphene with functional molecules, followed with a lyophilization process to massively transform 2D plane graphene-based nanosheets to 3D crumpled structure. The detailed morphological characterizations reveal that the bioinspired nanosheets exhibit full consistency with maxilloturbinate. The fabricated 3D crumpled graphene-based sensors exhibit ultrahigh response (R_a/R_g = 3.8) toward 10 ppm of NO₂, which is mainly attributed to the specific maxilloturbinate-mimic structure. The sensors also exhibit excellent selectivity and sensing linearity, reliable repeatability, and stability. Interestingly, it is observed that only 4 mg of graphene oxide (GO) raw materials can produce more than 1000 gas sensors, which provides a new insight for developing novel 3D biomimetic materials in large-scale gas sensor production.

Stanene based gas sensors: effect of spin–orbit coupling

B@, N@, and B–N@stanene for NO₂ gas sensors.

Two-dimensional nanomaterial-based field-effect transistors for chemical and biological sensing

This review highlights the recent progress in graphene-, 2D transition metal dichalcogenide-, and 2D black phosphorus-based FET sensors for detecting gases, biomolecules, and water contaminants.

Study on adsorption and desorption of ammonia on graphene

The gas sensor based on pristine graphene with conductance type was studied theoretically and experimentally. The time response of conductance measurements showed a quickly and largely increased conductivity when the sensor was exposed to ammonia gas produced by a bubble system of ammonia water. However, the desorption process in vacuum took more than 1 h which indicated that there was a larger number of transferred carriers and a strong adsorption force between ammonia and graphene. The desorption time could be greatly shortened down to about 2 min by adding the flow of water-vapor-enriched air at the beginning of the recovery stage which had been confirmed as a rapid and high-efficiency desorption process. Moreover, the optimum geometries, adsorption energies, and the charge transfer number of the composite systems were studied with first-principle calculations. However, the theoretical results showed that the adsorption energy between NH3 and graphene was too small to fit for the experimental phenomenon, and there were few charges transferred between graphene and NH3 molecules, which was completely different from the experiment measurement. The adsorption energy between NH4 and graphene increased stage by stage which showed NH4 was a strong donor. The calculation suggested that H2O molecule could help a quick desorption of NH4 from graphene by converting NH4 to NH3 or (NH3)n(H2O)m groups, which was consistent with the experimental results. This study demonstrates that the ammonia gas produced by a bubble system of ammonia water is mainly ammonium groups of NH3 and NH4, and the NH4 moleculars are ideal candidates for the molecular doping of graphene while the interaction between graphene and the NH3 moleculars is weak.

Metal Decoration Effects on the Gas-Sensing Properties of 2D Hybrid-Structures on Flexible Substrates

We have investigated the effects of metal decoration on the gas-sensing properties of a device with two-dimensional (2D) molybdenum disulfide (MoS₂) flake channels and graphene electrodes. The 2D hybrid-structure device sensitively detected NO₂ gas molecules (>1.2 ppm) as well as NH₃ (>10 ppm). Metal nanoparticles (NPs) could tune the electronic properties of the 2D graphene/MoS₂ device, increasing sensitivity to a specific gas molecule. For instance, palladium NPs accumulate hole carriers of graphene/MoS₂, electronically sensitizing NH₃ gas molecules. Contrarily, aluminum NPs deplete hole carriers, enhancing NO₂ sensitivity. The synergistic combination of metal NPs and 2D hybrid layers could be also applied to a flexible gas sensor. There was no serious degradation in the sensing performance of metal-decorated MoS₂ flexible devices before/after 5000 bending cycles. Thus, highly sensitive and endurable gas sensor could be achieved through the metal-decorated 2D hybrid-structure, offering a useful route to wearable electronic sensing platforms.

Effect of Water Vapor and Surface Morphology on the Low Temperature Response of Metal Oxide Semiconductor Gas Sensors

In this work the low temperature response of metal oxide semiconductor gas sensors is analyzed. Important characteristics of this low-temperature response are a pronounced selectivity to acid- and base-forming gases and a large disparity of response and recovery time constants which often leads to an integrator-type of gas response. We show that this kind of sensor performance is related to the trend of semiconductor gas sensors to adsorb water vapor in multi-layer form and that this ability is sensitively influenced by the surface morphology. In particular we show that surface roughness in the nanometer range enhances desorption of water from multi-layer adsorbates, enabling them to respond more swiftly to changes in the ambient humidity. Further experiments reveal that reactive gases, such as NO₂ and NH₃, which are easily absorbed in the water adsorbate layers, are more easily exchanged across the liquid/air interface when the humidity in the ambient air is high.

Chemical Sensing of 2D Graphene/MoS2 Heterostructure device

We report the production of a two-dimensional (2D) heterostructured gas sensor. The gas-sensing characteristics of exfoliated molybdenum disulfide (MoS2) connected to interdigitated metal electrodes were investigated. The MoS2 flake-based sensor detected a NO2 concentration as low as 1.2 ppm and exhibited excellent gas-sensing stability. Instead of metal electrodes, patterned graphene was used for charge collection in the MoS2-based sensing devices. An equation based on variable resistance terms was used to describe the sensing mechanism of the graphene/MoS2 device. Furthermore, the gas response characteristics of the heterostructured device on a flexible substrate were retained without serious performance degradation, even under mechanical deformation. This novel sensing structure based on a 2D heterostructure promises to provide a simple route to an essential sensing platform for wearable electronics.

Defect/oxygen assisted direct write technique for nanopatterning graphene

High resolution nanopatterning of graphene enables manipulation of electronic, optical and sensing properties of graphene. In this work we present a straightforward technique that does not require any lithographic mask to etch nanopatterns into graphene. The technique relies on the damaged graphene to be etched selectively in an oxygen rich environment with respect to non-damaged graphene. Sub-40 nm features were etched into graphene by selectively exposing it to a 100 keV electron beam and then etching the damaged areas away in a conventional oven. Raman spectroscopy was used to evaluate the extent of damage induced by the electron beam as well as the effects of the selective oxidative etching on the remaining graphene.

Impact of distributions and mixtures on the charge transfer properties of graphene nanoflakes

Many of the promising new applications of graphene nanoflakes are moderated by charge transfer reactions occurring between defects, such as edges, and the surrounding environment. In this context the sign and value of properties such as the ionization potential, electron affinity, electronegativity and chemical hardness can be useful indicators of the efficiency of graphene nanoflakes for different reactions, and can help identify new application areas. However, as samples of graphene nanoflakes cannot necessarily be perfectly monodispersed, it is necessary to predict these properties for polydispersed ensembles of flakes, and provide a statistical solution. In this study we use some simple statistical methods, in combination with electronic structure simulations, to predict the charge transfer properties of different types of ensembles where restrictions have been placed on the diversity of the structures. By predicting quality factors for a variety of cases, we find that there is a clear motivation for restricting the sizes and suppressing certain morphologies to increase the selectivity and efficiency of charge transfer reactions; even if samples cannot be completely purified.

Additive-free synthesis of In₂O₃ cubes embedded into graphene sheets and their enhanced NO₂ sensing performance at room temperature

In this report, we developed an additive-free synthesis of In2O3 cubes embedded into graphene networks with InN nanowires (InN-NWs) and graphene oxide (GO) as precursors by a facile one-step microwave-assisted hydrothermal method. In absence of GO, the InN-NWs maintained their chemical composition and original morphology upon the same treatment. At varying mass ratios of InN-NWs and GO, the different morphologies and distributions of In2O3 could be obtained on graphene sheets. The uniform distribution, which is usually considered favorable for enhanced sensing performance, was observed in In2O3 cubes/reduced graphene oxide (rGO) composites. The room-temperature NO2 sensing properties of the In2O3 cubes/rGO composites-based sensor were systematically investigated. The results revealed that the sensor exhibited a significant response to NO2 gas with a concentration lower to 1 ppm, and an excellent selectivity, even though the concentrations of interferential gases were 1000 times that of NO2. The enhanced NO2 sensing performances were attributed to the synergistic effect of uniformly distributed In2O3 cubes and graphene sheets in the unique hybrid architectures without the interfering of extra additives.

Ultrasensitive and selective nitrogen dioxide sensor based on self-assembled graphene/polymer composite nanofibers

Reduced graphene oxide (rGO) sheets were self-assembled onto the surfaces of electrospun polymer nanofibers to form an ultrathin coating. These rGO/polymer composite nanofibers were used to fabricate nitrogen dioxide (NO2) sensor. This sensor can be performed at room temperature, and it exhibited a high sensitivity of 1.03 ppm(-1) with excellent selectivity and good reversibility. Furthermore, the limit of detection was experimentally measured to be as low as 150 ppb, and this value is much lower than the threshold exposure limit proposed by American Conference of Governmental Industrial Hygienists (200 ppb).