Graphene-based Humidity Sensors Research Articles

Recent Advances in Graphene-Based Humidity Sensors with the Focus of Structural Design: A Review

Recent Advances in Graphene-Based Humidity Sensors with the Focus of Structural Design: A Review

A flexible humidity sensor constructed by ordered-pore-array of slightly reduced graphene oxide with much enhanced sensing response

Reduced graphene oxide (rGO) flexible film humidity sensor has received increasing attention, but the low sensing response caused by lack of available hydrophilic functional groups is still a limitation. Herein, a slightly reduced graphene oxide (SrGO) ordered-pore-array, fabricated via a monolayer colloid crystal template method, was introduced as a resistive humidity sensor. It was obtained based on adsorption between the GO sheets and the template microspheres, in-situ slight reduction of the GO shells and the removal of template. The reduction way allows the functional groups of GO to be retained as much as possible, and the unique structures (e.g., spherical double surfaces and small through-holes on pore-walls) facilitate the substantial exposure of functional groups, the penetration of water molecules and the utilization of buried functional groups. The available functional groups are thereby efficiently increased, giving the sensor an unprecedented high sensing response, more than 2600 times the maximum response of existing rGO sensors. The sensor also demonstrated excellent practical characteristics, and by detecting a single exhale, it could be employed in quick and quantitative evaluation of human activities and health. This strategy paves a facile and promising route to improve the sensing response and application of graphene-based humidity sensors or gas sensors.

Monitoring Water Absorption and Desorption in Untreated and Consolidated Tuff by a Non-Invasive Graphene-Based Humidity Sensor.

A hybrid montmorillonite (MMT)/reduced graphene oxide (rGO) film was realised and used as a non-invasive sensor for the monitoring of water absorption and desorption in pristine and consolidated tuff stones. This film was obtained by casting from a water dispersion containing graphene oxide (GO), montmorillonite and ascorbic acid; then the GO component was thermo-chemically reduced and the ascorbic acid phase was removed by washing. The hybrid film showed electrical surface conductivity that varied linearly with the relative humidity, ranging from 2.3 × 10-3 S in dry conditions to 5.0 × 10-3 S at 100% RH. The sensor was applied onto tuff stone samples through the use of a high amorphous polyvinyl alcohol layer (HAVOH) adhesive, which guaranteed good water diffusion from the stone to the film and was tested during water capillary absorption and drying tests. Results show that the sensor is able to monitor water content changes in the stone, being potentially useful to evaluate the water absorption and desorption behaviour of porous samples both in laboratory environments and in situ.

Laser Scribing of Graphene Oxide Using Bessel Beam for Humidity Sensing

Laser-scribed graphene oxide (GO) shows great promise for high-performance, cost-effective humidity sensors. However, when using the commonly employed Gaussian beam, the Rayleigh length is relatively short, leading to potential stability issues during large-area processing, especially when defocusing occurs. In this paper, we utilize a diffraction-free Bessel beam to one-step fabricate reduced graphene oxide (rGO) electrodes specifically designed for humidity sensing applications. The effects of defocusing and laser power on the line width and resistance of the fabricated electrodes are investigated, giving the optimal processing parameters for Bessel laser writing of GO. The line width, resistance, and sheet resistance of the rGO electrode are stable at a defocusing distance within ±1.00 mm. Defocusing also proves to be effective in reducing the ablation region during the fabrication process. The temperature and humidity responses of the electrodes are examined, focusing on those fabricated with typical defocusing settings, and the related mechanisms are discussed. Proof-of-principle rGO/GO/rGO humidity sensors are demonstrated, and were one-step fabricated using a Bessel beam with both focusing and defocusing settings. The corresponding humidity response results evidence that rGO humidity sensors can be fabricated using a Bessel beam, even in the defocusing cases. The investigation into the Bessel-beam-based laser fabrication technique offers promising prospects for rapid, flexible, and cost-effective production of graphene-based humidity sensors. Meanwhile, the study of defocusing may enhance the fabrication stability to withstand defocusing conditions effectively.

High-performance humidity sensor based on plasma modified graphene

Humidity monitoring has flourished in many advanced applications, such as environmental monitoring, industries, and medical health. Humidity devices with fast response and high sensitivity are in high demand for complicated application conditions. In this work, we bridge this gap by employing oxygen plasma treatment in a graphene-based humidity sensor. The sensitivity can be regulated from −99.2% to 153.1%, meanwhile retaining a high-speed response with rise and fall times ∼58 ms and ∼661 ms. This work provides a promising strategy for humidity detection with fast response and high sensitivity.

Construction and Performance of a Nanocellulose–Graphene-Based Humidity Sensor with a Fast Response and Excellent Stability

Humidity sensors have been widely applied in environmental monitoring, respiratory monitoring, electronic skin, noncontact switch, etc. However, weak interface interactions between a flexible substrate and conductive fillers will deteriorate the sensing performance. Herein, a flexible, rapid-response, and durable humidity sensor was proposed via employing cellulose nanofiber (CNF)-dispersed graphene (Gr) as a humidity sensing layer, which could be easily filtered onto a coessential CNF film (CNFF) to form the humidity sensor. CNFs as a dispersant and bonder strengthened the adhesion of graphene on the surface of the CNFF substrate. Simultaneously, CNFs as a reservoir provided abundant adsorption sites to capture water molecules and promoted electron transfer from adsorbed water molecules to graphene, further amplifying electrical signals by a hygroscopic expansion of the sensing layer under humidity conditions. Benefiting from this unique composition and structure, the prepared humidity sensor exhibited a rapid response (45 s) and recovery time (33 s), low hysteresis (4%), wide RH detection range (15–99%), and long-term stability (15 days). Further, the sensor could monitor the humidity of human skin and human breath, indicating a versatile noncontact humidity sensing function by altering its component proportion of CNF-to-Gr on demand.

Influence of Sweat on Joint and Sensor Reliability of E-Textiles

This article addresses reliability under the sweat of interconnection techniques for the mounting surface mounted device (SMD) components and fully printed humidity sensors onto conductive stretchable textile ribbons. Samples underwent testing for the effect of ageing by artificial sweat on their electrical resistance using both alkaline and acidic artificial sweat. The best results in terms of electrical resistance change were obtained for samples soldered to the conductive fibers interwoven in the ribbon. However, this method can damage the ribbon due to the high temperature during soldering and significantly reduce the mechanical properties and flexibility of the ribbon, which can lead to a limited service life of samples. On the other hand, adhesive bonding is a very interesting alternative, where the above-mentioned properties are preserved, but there is a significant effect of sweat ageing on electrical resistance. The results of fully printed graphene-based humidity sensors show that, for the intended use of these sensors (i.e., detection of changes in moisture on the human body), usage of the samples is possible, and the samples are sufficiently reliable in the case of sweat degradation. In addition, the response of the sensor to humidity is quite high: 98% at a relative humidity of 98%.

Ultrafast humidity sensor based on liquid phase exfoliated graphene

Humidity sensing is important to a variety of technologies and industries, ranging from environmental and industrial monitoring to medical applications. Although humidity sensors abound, few available solutions are thin, transparent, compatible with large-area sensor production and flexible, and almost none are fast enough to perform human respiration monitoring through breath detection or real-time finger proximity monitoring via skin humidity sensing. This work describes chemiresistive graphene-based humidity sensors produced in few steps with facile liquid phase exfoliation followed by Langmuir–Blodgett assembly that enables active areas of practically any size. The graphene sensors provide a unique mix of performance parameters, exhibiting resistance changes up to 10% with varying humidity, linear performance over relative humidity (RH) levels between 8% and 95%, weak response to other constituents of air, flexibility, transparency of nearly 80%, and response times of 30 ms. The fast response to humidity is shown to be useful for respiration monitoring and real-time finger proximity detection, with potential applications in flexible touchless interactive panels.

Research Progress of Graphene-Based Flexible Humidity Sensor.

Graphene is a new type of carbon material with a flexible, two-dimensional structure. Due to the excellent stability of its lattice structure and its mechanical flexibility, graphene-based materials can be applied in flexible humidity sensors. At present, the application of graphene-based flexible humidity sensors in the fields of medical care and environmental monitoring is attracting widespread attention. In this review, the basic properties of graphene oxide (GO) and reduced graphene oxide (rGO) as moisture-sensitive materials and methods for their preparation were introduced. Moreover, three methods for improving the performance of moisture-sensitive materials were discussed. The working principle of different types of graphene-based humidity sensors were introduced. The progress in the research on graphene-based flexible humidity sensors in four respects: Human respiration, skin moisture, human sweat, and environmental humidity were discussed. Finally, the future research, following the development trends and challenges, to develop the potential of integrated, graphene-based flexible humidity sensors were discussed.

Effect of substrate topography for graphene-based humidity sensors

Chemical vapor deposition graphene (CVDG) and electrochemical exfoliated graphene (ECG) were transferred and spin-coated on the nanoporous alumina substrate (NS) and flat alumina substrate (FS) to investigate the effect of substrate topography for graphene-based humidity sensors. The electrical resistance change of two graphene types on different substrate topography could be measured to estimate the environmental relative humidity. The relationship between graphene types, substrate topography, wettability, and humidity sensing performance was studied and discussed. The results of the water contact angle of CVDG on NS/FS and ECG on NS/FS were 77.0 ± 5.0°, 88.8 ± 4.9°, 55.9 ± 7.0°, and 59.8 ± 3.2°, respectively. The response, linearity and hysteresis characteristics of four graphene-based humidity sensors were measured and estimated. The ECG on NS humidity sensor shows the best performance in regards to the response, linearity and hysteresis.

Graphene oxide humidity sensor built entirely by additive manufacturing approaches

Graphene is emerging as an excellent material choice for high-performance electrical and electrochemical devices such as humidity sensors. The rapid technological evolution in additive manufacturing (AM) enables freedom to design and process multiple materials for scalable fabrication of sensors. Herein, we introduce a hybrid AM approach to fabricate a graphene-based humidity sensor and study its sensing performance. A powder-bed binder-jetting AM technique is used to build a porous 3D-structure of thermally reduced graphene oxide as the humidity sensing element. In parallel, a material extrusion AM approach is used to make a silicone-based hollow cube as the housing of the graphene structure. The AM fabricated sensor shows high sensitivity when tested in the relative humidity (RH) range of 6.4–97.3% with a rapid response time of 7 s at 45% RH due to the open porous structure formed by the binder-jetting approach. Sensing performance was investigated at low and medium RH in the temperature range of 25–80 °C and the device demonstrated a negligible temperature dependence. The presented graphene-based humidity sensor also shows good repeatability in RH measurements.

Recent Advances in Graphene-Based Humidity Sensors.

Humidity sensors are a common, but important type of sensors in our daily life and industrial processing. Graphene and graphene-based materials have shown great potential for detecting humidity due to their ultrahigh specific surface areas, extremely high electron mobility at room temperature, and low electrical noise due to the quality of its crystal lattice and its very high electrical conductivity. However, there are still no specific reviews on the progresses of graphene-based humidity sensors. This review focuses on the recent advances in graphene-based humidity sensors, starting from an introduction on the preparation and properties of graphene materials and the sensing mechanisms of seven types of commonly studied graphene-based humidity sensors, and mainly summarizes the recent advances in the preparation and performance of humidity sensors based on pristine graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, and a wide variety of graphene based composite materials, including chemical modification, polymer, metal, metal oxide, and other 2D materials. The remaining challenges along with future trends in high-performance graphene-based humidity sensors are also discussed.

Graphene-based humidity sensors: the origin of alternating resistance change

The response of a graphene-based humidity sensor is considered as a function of film structures. Analysis of the resistance changes due to water molecule adsorption on the graphene or multi-layer graphene (MLG) surface is performed for films with different structures and resistivities from hundreds of ohms/sq to hundreds of kilo-ohms/sq. The results revealed possible increase, decrease and non-monotonous behavior of resistance with changes in film structure. Adsorption of water molecules at grain boundary defects is assumed to lead to an increase in film resistivity due to the donor property of water and the p-type conductivity of graphene. Another type of conductive center with a higher capture cross-section is realized in the case of water molecule adsorption at edge defects in MLG films (the formation of conductive chains with ionic conductivity). If these chains form a continuous network the film resistivity decreases. The result of the competition between the opposite effects of the conductivity compensation and formation of the water-based conductive chains depends on the film structure and determines the response of humidity sensors. Sensor sensitivity is found to increase when only one type of defect determines water adsorption (edge defects or grain boundary defects).

(Invited) Emerging Graphene Device Technologies

Graphene has extraordinary electronic properties including a high carrier mobility [1], a Young’s modulus over 1 TPa [2], [3] and the ability to detect the presence of single molecules [4]. Here we explore a number of potential applications based on graphene including sensors, transistors, and supercapacitors. Its remarkable mechanical properties have made it an ideal candidate for nanoelectromechanical system (NEMS) applications. Specifically, its use as a pressure sensor has been thoroughly explored [5], [6]. Graphene has shown an extraordinary sensitivity to changes in pressure, drastically outperforming conventional silicon sensors as well as carbon nanotubes [5]. These devices function through the piezoresistive effect and the underlying physics has been examined in detail [5]. Fig. 1a shows the structure of the graphene pressure sensor along with a typical resistance response of the device to changes in pressure. Graphene based humidity sensors have been demonstrated, providing a comprehensive overview of their sensing properties [7]. These sensors have rapid response and recovery times and have a linear signal response over a wide humidity range. Fig. 1b shows a humidity sensor device and corresponding resistance change when exposed to a variety of gasses. Results are adapted from [7]. Two types of graphene based transistors have additionally been explored. The first example is a graphene field effect transistor (GFET) that has been fabricated and examined showing soft current saturation [8]. The other example is the graphene base transistor (GBT) which was first experimentally demonstrated in 2013 [9]. The difference between the GBT and the GFET are expressed in Fig. 1c and Fig. 1d. In the case of the GBT, charge transport occurs perpendicular to the graphene by tunneling through an emission barrier interface (EBI), then transported through the graphene layer (acting as the base), and lastly through the interface between the base and the collector (BCI). The GBT device structure and function is illustrated in a schematic and simplified band diagram (Fig. 1c). These GBTs have since had their performance improved by the use of an intermediate trap assisted tunneling layer between the EBI and BCI [10]. For GFETs, charge transport occurs along the graphene layer between the source and the drain and the current is electrostatically controlled by the gate (Fig. 1d). The combination of graphene’s electronic properties with this high surface to volume ratio makes graphene an ideal material for use in energy storage devices. Graphene based supercapacitors have been demonstrated which employ ink-jet printed graphene films to provide high surface area [13]. Fig. 1e and Fig. 1f shows ink-jet printed graphene on two substrates. Fig. 1e is an optical image of printed graphene on glass while Fig. 1f is a scanning electron microscope (SEM) image of ink-jet printed graphene on SiO2. These devices can be integrated with other graphene devices in a back end of the line (BEOL) integration scheme. Graphene’s integratability has promising implications for the heterogeneous integration of multiple devices on the same chip.