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

Nanotechnology PAPER • OPEN ACCESS An effective formaldehyde gas sensor based on oxygen-rich three-dimensional graphene To cite this article: Shu Zhang et al 2022 Nanotechnology 33 185702 View the article online for updates and enhancements. You may also like Metal-assisted-chemical-etching of silicon nanowires for templating 3D graphene growth towards energy storage in microsystems Jinhua Li, Nguyen Van Toan, Zhuqing Wang et al. - Optimization of CVD parameters on 3D graphene foam structures with response surface methodology (RSM) Sibel Kasap, Mehmet Bahadr Acar and Dilek Çakrolu - Advances in research on 2D and 3D graphene-based supercapacitors Johannes Ph. Mensing, Chatwarin Poochai, Sadanan Kerdpocha et al. - This content was downloaded from IP address 158.196.184.115 on 08/07/2022 at 06:47 An effective formaldehyde gas sensor based on oxygen-rich three-dimensional graphene Shu Zhang 1,2,17 , Jinbo Pang 1,17,∗ , Yufen Li 1,17 , Bergoi Ibarlucea 3,4,17 , Yu Liu 5,6 , Ting Wang 7,8 , Xiaoyan Liu 1 , Songang Peng 9,10 , Thomas Gemming 11 , Qilin Cheng 1 , Hong Liu 1,12,∗ , Jiali Yang 1 , Gianaurelio Cuniberti 3,4,13,14,∗ , Weijia Zhou 1 and Mark H Rümmeli 5,6,11,15,16,∗ 1 Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong, Institute for Advanced Interdisciplinary Research (iAIR), University of Jinan, Shandong, Jinan 250022, People’s Republic of China 2 School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong, Jinan 250022, People’s Republic of China 3 Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische Universität Dresden, Dresden D-01069, Germany 4 Center for Advancing Electronics Dresden, Technische Universität Dresden, Dresden D-01069, Germany 5 College of Energy, Soochow Institute for Energy and Materials Innovations, Soochow University, Suzhou 215006, People’s Republic of China 6 Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, People’s Republic of China 7 State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, No.3501 Daxue Road, Jinan 250353, People’s Republic of China 8 School of Bioengineering, Qilu University of Technology, Shandong Academy of Science, Jinan 250353, People’s Republic of China 9 High-Frequency High-Voltage Device and Integrated Circuits R&D Center, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, People’s Republic of China 10 Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, People’s Republic of China 11 Institute for Complex Materials, Leibniz Institute for Solid State and Materials Research Dresden, PO Box 270116, Dresden, D-01171 Germany 12 State Key Laboratory of Crystal Materials, Center of Bio & Micro/Nano Functional Materials, Shandong University, 27 Shandanan Road, Jinan 250100, People’s Republic of China 13 Dresden Center for Computational Materials Science, Technische Universität Dresden, Dresden D-01062, Germany 14 Dresden Center for Intelligent Materials (GCL DCIM), Technische Universität Dresden, Dresden D-01062, Germany 15 Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie Sklodowskiej 34, Zabrze 41-819, Poland 16 Institute of Environmental Technology (CEET),VŠB-Technical University of Ostrava, 17. Listopadu 15, Ostrava 708 33, Czech Republic E-mail: [email protected],[email protected],[email protected] and m. [email protected] Received 15 December 2021, revised 20 January 2022 Accepted for publication 24 January 2022 Published 10 February 2022 Nanotechnology Nanotechnology 33 (2022)185702 (12pp)https://doi.org/10.1088/1361-6528/ac4eb4 17 These authors contributed equally. ∗ Authors to whom any correspondence should be addressed. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s)and the title of the work, journal citation and DOI. 0957-4484/22/185702+12$33.00 Printed in the UK © 2022 The Author(s). Published by IOP Publishing Ltd1 Abstract 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 highperformance 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 HNO 3 solution—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. Supplementary material for this article is available online Keywords: 3D graphene, chemical vapor deposition, chemiresistors, oxygen plasma treatments, gas sensing Introduction Graphene is an ideal two-dimensional (2D)material with unique electrical and chemical properties [1]. These include extremely high Young’s modulus and fracture stress [2], high electrical conductivity [3,4], excellent thermal conductivity [5], low contact resistance [6], high mobility [7], large specific surface area, and high light transmittance and flexibility [8]. Therefore, graphene can be developed and applied in various fields [9], such as high-quality composite materials [10], biomedical and drug delivery [11], transistors [12,13], integrated circuits [14],flexible electronics [15]and energy storage devices [16]. Owing to the excellent properties of 2D graphene [17], its three-dimensional (3D)counterpart is widely used in gas molecular sensors owing to its unique 3D nanoporous structure [18], and feasible surface functionalization [19]. The resistance of the graphene-based chemiresistor [20]changes with the introduction of gases, which is the gas-sensing mechanism. Compared with other carbon nanomaterials [21], graphene has the advantages of high conductivity and a large, theoretical, specific surface area (3523 m 2 g −1 )[22]. These facilitate the effective adsorption of gas molecules. Graphene shows excellent prospects for gas-sensing applications [23]. Three-dimensional graphene interacts with different compositions and structures of gas adsorbents [24]in diverse ways. The gaseous molecule adsorbs onto graphene by weak Van der Waals interactions; hence, the resistance of graphene can be monitored by uncomplicated electrical equipment [25]. Owing to its high-quality lattice structure [26], 3D graphene possesses inherently low electrical noise, which avoids large charge fluctuations compared with carbon nanotubes [27].In addition, chemiresistor-based sensing formats possess the advantages of simple equipment, easy fabrication, and direct measurement [28]. With the improvement in human living standards and increasing attention to environmental protection, air quality, and atmospheric pollution, more significant requirements for gas monitoring have been introduced [29]. Air pollution indoors and inside vehicles threatens human health and has become a common concern worldwide. Among the existing hazardous gases, formaldehyde is a common representative example [30]. It is a colorless and soluble irritant gas, which is volatile in adhesive decoration materials such as wallpaper. High concentrations of 20 to 100 ppm are detrimental to health and well-being [31,32], while long-term exposure to lower levels can cause allergies, carcinogenesis, and mutations [33,34]. Among all contacts, children (leukemia rate) and pregnant women (abortion rate)are particularly sensitive. Therefore, an effective formaldehyde gas sensor is an immediate safety requirement. Among the existing formaldehyde sensors, some of them rely on amperometric techniques, requiring either UV irradiation [35]or enzymes [36]as receptors, which are prone to conformational changes and, therefore, show poor long-term stability or require special storage conditions. Indeed, conventional semiconductor sensor requires high operation temperature [37–39]or external UV-light activation [35]. The graphene has the advantage of high conductivity at low operation temperature, which often was blended with semiconducting oxides for improving the surface area and conductivity. Indeed, graphene/metal oxides based composites could improve the sensitivity of formaldehyde sensor because of the electron transfer channels provided by the metal oxide such as SnO [40], SnO 2 [39,41,42], TiO 2 [43–45], and ZnO [46–48]and ZnSnO 3 [49]. Besides, the graphene has formed composites with polymers [50–52],Si nanowires [53]and MoS 2 [54,55]to serve as formaldehydesensing materials. However, the oxygen plasma treated 3D graphene has yet applied in formaldehyde sensing. Therefore, we employed 3D graphene with different treatments and compared their structure-performance relationships. In this study, we applied three strategies to treat 3D graphene, including untreated, HNO 3 etching, and oxygen plasma treatment and their application in chemiresistors for gas sensing using formaldehyde as an example. 2 Nanotechnology 33 (2022)185702 S Zhang et al Experimental details Synthesis of 3D graphene by chemical vapor deposition (CVD) The Ni foam was washed with HCl (19 vol%), then washed with deionized water. The pre-treated Ni foam was placed in a quartz boat and deposited in the center of the furnace (figure S1 (available online at stacks.iop.org/NANO/33/185702/ mmedia)). First, the tubular furnace was vacuum pumped to 10 Pa to remove air and water. Second, the fixed carrier gas rate was 270/30 ml min −1 Ar/H 2 after high-speed cleaning with a large flow of Ar/H 2 to atmospheric pressure. It was heated at 70 °C min −1 and annealed at 1028 °C for 15 min. Further, 20 sccm CH 4 as growth gas was injected for 1 h, then rapidly cooled to room temperature. Freestanding 3D graphene without Ni First, 3D graphene fabricated by CVD was soaked in 2M FeCl 3 mixed solution to remove the Ni substrate. The fully etched 3D graphene was then transferred to deionized water (mixed with HCl acid)for 2 h. Here, the HCl solution was diluted to 0.37 wt% with deionized water. Then, the soaking in diluted HCl was repeated for three times for thoroughly removing the residual Fe species. Indeed, the titer by the transfer recipe has been well established in our group for thorough removal of Fe removal, i.e. no emergence of Fe atoms over graphene in TEM images [17,56], which have been commonly observed by the Cs-corrected atomic resolution TEM imaging [57,58]. Eventually, the freestanding 3D graphene was dried naturally at 25 °C for storage, characterizations, and device fabrications. Posttreatment of 3D graphene Three approaches were employed for graphene posttreatment (Table S1). First, non-treatment was conducted on the 3D graphene. The second treatment was the HNO 3 etching of 3D graphene. The synthesized 3D graphene was submerged in HNO 3 (34 wt%)for 24 h. Third, the 3D graphene was treated with oxygen plasma (Diener Electronic, model: Atto-BLS). Initially, the chamber was vacuumed at 20 Pa. Then, oxygen at a flow rate of 10 ml min −1 was introduced into the chamber. Further, the oxygen plasma was generated at an RF power (13.56 MHz)of 90 W and treated for 5–30 min. The plasma-induced defects in 3D graphene were characterized using Raman spectroscopy. Eventually, 15 min was found to be optimal for completely functionalizing graphene with oxygen. Material characterization Optical microscopy (Olympus BX53MRF-S)wasusedtoobtain snapshot optical micrographs. Raman spectra and mapping were performed using 532 nm excitation wavelength Raman spectroscopy (Horiba Labram HR800). The present chemical bonds or functional groups were determined using a Fourier transform infrared spectrometer (Bruker VERTEX 70 FT-IR). The phase structure was tested using x-ray diffractometry (Thermo Fisher ARL Equlnox 3000). The surface morphologies were investigated using a scanning electron microscope (Hitachi Regulus8100). The lattice structure, selected area electron diffraction, and elemental analysis were conducted using a transmission electron microscope (JEOL JEM-2100)integrated with energy-dispersive x-ray spectroscopy. Device fabrication Three types of post-treated 3D graphene (2×2cm 2 )were transferred to glass slides with patterned Au electrodes (50 nm Au). Gold electrodes were fabricated on glass slides using an electron-beam evaporator (HHV ATS 500). The 3D graphene was aligned and adhered to bridge to two adjacent Au electrodes (figure S9). Gas-sensing examination A simple gas chamber was used to test for the presence or absence of resistive 3D graphene based sensors. A 25 ml formaldehyde solution (Sigma-Aldrich, 38%)was dropped onto a hot plate (30 °C)to generate formaldehyde vapor. The relative humidity was maintained at 30%, and the temperature was maintained at 25 °C. The concentration of the targeted gas molecule was measured from the mean of the static liquid distribution, which was calculated using the following equation: r =´´´ ´ ´CV MV 22.4 1000 ppm, 1 2 Ф where C(ppm)is the concentration of the target gas, Фis the volume fraction of the target gas molecule, ρ(gml −1 )is the density of the liquid, V 1 (ml)is the volume of the test liquid, V 2 (l)is the volume of the test chamber, and M(g mol −1 )is the molecular weight of the test liquid. The time-dependent current curves of the gas sensor were collected using a source measurement unit (Keithley 2400). The response and recovery times were determined from the time-dependent current curves of the gas sensors. The response time of the sensor was determined when the resistance (in gas)dropped to 90% of the pristine resistance (in air) during the adsorption process. The recovery time was determined when the resistance dropped to 90% upon the desorption of gas molecules. Results and discussion Three different treatments of 3D graphene after growth were investigated and compared in this study. These labels are provided in Table S1, and each experiment in the text and graphics employs the same label. The 3D graphene treatments were non-treatment, oxygen plasma, and etching in HNO 3 solution. We first discuss the appearance and morphology of the 3D graphene observed by optical microscopy (figure 1)to confirm the homogeneity of the large area of the synthesized 3D graphene. 3 Nanotechnology 33 (2022)185702 S Zhang et al The 3D graphene synthesized by CVD had a large 3D framework structure (figure 1(a)) and high surface uniformity (figure 1(b)). The 3D graphene treated by oxygen plasma maintained the intrinsic 3D framework property (figure 1(d)) and homogeneous surface uniformity (figure 1(e)). This illustrated that the posttreatment of oxygen plasma did not affect the morphology of 3D graphene. Besides, the HNO 3 treatment has caused negligible changes in morphology and structure of 3D graphene (figure S6). Raman spectroscopy is a powerful tool for exploring the properties of graphene. The quality and purity of the synthesized 3D graphene was determined by Raman spectroscopy. It was also used to analyze the number of layers of the grown graphene films on the substrate of the Ni foam. The Raman spectrum illustrated the monolayer property through peaks at ca. 1580 cm −1 for the G mode and ca. 2700 cm −1 for the 2D mode (figure 1(c)). The emergence of the D mode (ca. 1350 cm −1 )indicated graphene defects (figure 1(f)). Hence, 3D graphene treated by oxygen plasma may lead to defects due to surface functionalization. After the posttreatment with oxygen plasma, 3D graphene possessed oxygen-containing groups. For example, carbonyl and epoxy groups, this is further discussed with the infrared spectra. These structural defects were determined by Raman spectroscopy. The Raman spectra of 3D graphene with different oxygen plasma treatment times are shown in figure 1(g). Oxygen functionalization often occurred at the surface and the edges of the 3D graphene [59]upon the introduction of plasma. Specifically, no oxygen was incorporated into the interlayer spacing of few-layer graphene [60]. Therefore, surface oxygen functionalization could achieve a saturable condition on the graphene surfaces. We used the D/G ratio in the oxygen-plasma-treated graphene to determine the saturation of oxygen functionalization. Oxygen Figure 1. Morphology and Raman spectra of 3D graphene with and without oxygen plasma treatment. (a),(b)The optical microscopic images of non-treated 3D graphene (over Ni foam)with different magnifications. (c)The Raman spectrum of non-treated 3D graphene. The peak positions were assigned for D mode (ca. 1350 cm −1 ), G mode (ca. 1580 cm −1 )and 2D mode (ca. 2700 cm −1 ).(d),(e)The optical microscopic graphs and (f)Raman spectrum of 3D graphene after treatment with oxygen plasma. The Raman spectra and D/G ratio of oxygen plasma-treated 3D graphene. (g)The Raman spectra of oxygen plasma-treated 3D graphene for different durations of (from bottom to top)0, 5, 10, 15, 20 min. (h)Statistics of the D/G ratio of the oxygen plasma-treated 3D graphene after different treatment times. 4 Nanotechnology 33 (2022)185702 S Zhang et al saturation occurred when the D/G ratio ceased to increase with prolonged plasma treatment. After 15 min of oxygen plasma the D/G ratio (0.10)of graphene stabilized (figure 1(h)). Pristine 3D graphene does not show D mode in Raman spectrum (figure 1(c)). With 5 min oxygen plasma treatment, the graphene exhibits significant D mode (figure 1(g)). After extending the oxygen plasma duration from 5 min to 20 min, the D/G intensity ratio increases to 0.1 (figure 1(h)). Further oxygen plasma treatment, i.e. for 30 min, does not induce larger D/G ratio. Therefore, we selected an oxygen plasma duration of 15 min for the graphene treatment and subsequent device fabrication. To show the crystal quality, we compared pure 3D graphene and Ni-supported 3D graphene by x-ray diffraction (figure 2). The purity of the 3D graphene was analyzed after etching. The 3D graphene supported by the Ni framework (figure 2(a)) showed an obvious face-centered cubic peak of Ni metal at ca. 2θ=44.4°(111), ca. 2θ=51.7°(200), and ca. 2θ=76.3°(220), respectively. The XRD graph of the pure 3D graphene (figure 2(b)) after etching exhibits one peak (002). This confirmed the success of etching as no Ni residue or other impurities remained. The XRD pattern of the synthesized 3D graphene showed a diffraction peak at 2θ=26.6°(figure 2(b)). The estimated layer spacing of graphene was 0.335 nm by the Bragg equation 2dsin θ=nλ, which was the result of the preferred orientation of the graphene reflection. A noticeable reflection of the (002)peak demonstrated that the grown 3D graphene was arranged regularly along the stacking direction. Compared with the non-treatment of 3D graphene, the 3D graphene after oxygen plasma treatment (figure 2(c)) showed the same single strength peak at the crystal plane of (002), which indicated that the posttreatment of oxygen plasma did not destroy the initial crystal structure. In addition, we did not see the emergence of the GO peak at around 10 degrees [61]. Again, the oxygen-plasma treatment does not change the crystal structure of graphene, i.e. without the formation of graphene oxide. In addition, the HNO 3 treatment did not cause change in crystal structure of graphene (figure S8)as XRD data show. To further study the surface morphology characteristics of 3D graphene, scanning electron microscopy (SEM)was used for more detailed observation and characterization. After etching the Ni framework, 3D graphene retained the interconnected 3D supporting structure of the original Ni foam template (figure 2(d)) and a large hollow tube (figure 2(f)). The pore size of the 3D graphene was mainly distributed in the range of 300–500 μm(figure 2(e)), which was consistent with the diameter of the hole of the Ni foam. Therefore, the synthesized 3D graphene possessed structural integrity and size stability. The crystal structure and characteristics of the synthesized 3D graphene after treatment with oxygen plasma were analyzed through imaging and electron diffraction of the transferred 3D graphene (figure 3). The low-magnification TEM graph (figure 3(a)) demonstrated the smooth homogeneity of the surface of the 3D Figure 2. Diffraction spectra and surface morphology of 3D graphene. (a)The x-ray diffraction (XRD)spectrum of the 3D graphene supported by Ni foam. The XRD spectra of (b)3D graphene (after Ni removal)and (c)3D graphene (after Ni etching)treated by oxygen plasma. (d)–(f)SEM micrographs for oxygen treated 3D graphene (after Ni removal). In panel (f), a hollow tube of 3D graphene was presented. 5 Nanotechnology 33 (2022)185702 S Zhang et al graphene after treatment with oxygen plasma. The SAED (figure 3(b)) exhibited a [100]lattice plane with six-fold symmetry of the 3D graphene crystal. The TEM micrograph of graphene with fringes (figure 3(c)) showed the 3D graphene layers (ca. 8–10), which was illustrated by the micro-nanocrystalline surface (figure 3(d)). The surface atomic diagram of the multilayer graphene demonstrated the distribution of the epoxy and carbonyl groups (figure 3(e)). The TEM data show negligible difference for pristine 3D graphene (figure S2)and HNO 3 treated 3D graphene (figure S3), compared to the oxygen plasma treated sample. Now we come to discuss the defects and the oxygen contents of graphene by three types of treatments. CVDgrown graphene over Ni foam was free of defects, i.e. none D mode in Raman spectrum. The oxygen plasma seems to introduce sp 3 type defects, e.g. hydroxyl or epoxy groups in our experiments (figure 3(e)). Indeed, the vibrational modes and chemical environments of these oxygen-related bonds were confirmed later as FT-IR (figure 4)and XPS data (figures S4 and S5)indicate. The structure disorder in graphene, often termed defects, e.g. sp 3 type, could be induced with plasma treatments [62,63]. Indeed, the plasma could introduce defects both at edges and on the basal planes as tipenhanced Raman spectroscopy mapping shows [64]. Besides, vacancy-type defects were often observed with heavily ionsirradiated graphene surfaces [65–68]. The graphene edges contribute to the D modes under Raman spectroscopic characterizations [69–71]. Figure 3. Structural, elemental, and diffraction analysis of the oxygen-plasma-treated 3D graphene. (a)Low-magnification transmission electron microscope graph of graphene (transferred)over a Quantifoil grid. (b)Selected area electron diffraction (SAED)pattern of the graphene. (c)TEM micrograph of graphene with fringes and (d)high-magnification TEM micrographs showing the layer stacks of the graphene. (e)The surface atomic diagram of the multilayer graphene. (f)Energy-dispersive x-ray spectrum of the oxygen plasma-treated 3D graphene. (g)TEM graph showing the stacked layers of few-layer 3D graphene. (h)The intensity profile of the interlayer spacing of graphene in panel (g). 6 Nanotechnology 33 (2022)185702 S Zhang et al The pristine 3D graphene contains the minimum oxygen content (0.4 at%)as EDX data show (figure S2(e)), which corresponding to hydroxyl group (figure 4(a)). Then, the 3D graphene treated by oxygen plasma is less (figure 3(f)), i.e. 0.7 at%, corresponding to the hydroxyl groups as indicated by infrared spectra (figure 4(b)). In addition, the oxygen content of three-dimensional graphene etched by nitric acid is the highest (0.8 at%),(figure S3(e)) which corresponds to the enhancement of C–O group in the peak shown by infrared transmittance spectrum (figure S7(a)). The hydroxyl groups are preferred for the rapid response to formaldehyde gas molecules compared to other oxygen-containing groups (discussed later in the section of sensing mechanism). The TEM graph exhibited typical graphitic fringes along smooth edges, which indicated multilayer graphene features (figure 3(g)). The measurement of the interlayer spacing of the 3D graphene was ca. 0.33 nm (figure 3(h)). We compared the infrared spectra of the 3D graphene with and without oxygen plasma. The strong absorption peak of O–H was at ca. 3450 cm −1 (figure 4(a)). This was caused by the hydrogen bond adhering to the graphene surface during the plasma etching. This was caused by the hydrogen bond adhering to the graphene surface during the plasma etching [59,72]. Compared to the non-treated 3D graphene, the posttreatment with oxygen plasma sample showed an absorption peak at ca. 2700 cm −1 (−CHO), and a stronger stretching vibration peak of C=O at 1625 cm −1 (figure 4(b)) [73].In addition, the absorption peaks emerge at ca. 2900 cm −1 (including two peaks at 2920 and 2850 cm −1 ), which are assigned as the C–H stretching modes [74,75]. Therefore, oxygen-containing functional groups were introduced across the surface of 3D graphene (figure 4(e)). These oxygen-containing groups were more likely to adsorb formaldehyde gas Figure 4. Infrared spectra of 3D graphene without (a)and with (b)oxygen plasma treatment. Oxygen plasma leads to the formation of epoxy groups and carbonyl radicals at the edges and on the surface of graphene. The atomic configuration of graphene during plasma treatment (c) pristine graphene, (d)initial oxygen plasma, and (e)complete plasma treatment. The oxygen-containing groups are distributed over the surface of graphene, such as the carbonyl, aldehyde, and epoxy groups. (f)A typical SEM micrograph of the oxygen plasma-treated 3D graphene. 7 Nanotechnology 33 (2022)185702 S Zhang et al molecules, which was consistent with the performance tests for the detection of formaldehyde molecules (discussed later). We examined the gas-sensing performance of the three types of post-treated 3D graphene. The electric current of the sensor was tested at a fixed voltage (0.1 V). Liquid formaldehyde was injected and dropped onto a hot plate (30 °C) (figure S10). At high temperatures, formaldehyde volatilized into gaseous molecules, which filled the gas test chamber (figure 5). A large specific surface area and high quality of 3D graphene have been demonstrated in the detection of gas molecules. The analysis of response and recovery confirmed the gas sensitivity of the 3D graphene-based gas sensor at a formaldehyde concentration of 11 ppm at 25 °C. The untreated 3D graphene exhibited a stable response and excellent repeatability for exposure to a formaldehyde concentration of 11 ppm. This demonstrated that the sensor Figure 5. Gas-sensing performances of 3D graphene with and without oxygen plasma treatment. (a)The gas sensor testing platform schematic includes a test chamber, electric measurement equipment (source measurement unit), and a gas introducing unit. (b)The photograph of the gas sensor based on the oxygen plasma-treated 3D graphene bridging two Au electrodes. (c),(d)Response curve of the gas sensor based on the untreated 3D graphene. The response is the quotient of the Ra/Rg. Ra denotes the resistance of 3D graphene in open air. Rg denotes the resistance of 3D graphene with introducing the target gas. (e),(f)The response performances of the gas sensor based on oxygen plasma-treated 3D graphene. Different concentrations of formaldehyde molecules were introduced during the sensor examination. 8 Nanotechnology 33 (2022)185702 S Zhang et al