Scrolling Graphene Nanosheets - ACS Publications - American

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Mimicking a Dog’s Nose: Scrolling Graphene Nanosheets Zhuo Chen,† Jinrong Wang,† Douxing Pan,§ Yao Wang,*,†,‡ Richard Noetzel,‡ Hao Li,‡ Peng Xie,† Wenle Pei,† Ahmad Umar,∥ Lei Jiang,*,† Nan Li,⊥ Nicolaas Frans de Rooij,⊥,# and Guofu Zhou*,‡,⊥,# †

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, People’s Republic of China ‡ National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, People’s Republic of China § Institute of Advanced Manufacturing Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Changzhou 213164, People’s Republic of China ∥ Department of Chemistry, Faculty of Science and Arts and Promising Centre for Sensors and Electronic Devices, Najran University, Najran 11001, Kingdom of Saudi Arabia ⊥ Shenzhen Guohua Optoelectronics Tech. Co. Ltd., Shenzhen 518110, People’s Republic of China # Academy of Shenzhen Guohua Optoelectronics, Shenzhen 518110, People’s Republic of China S Supporting Information *

ABSTRACT: Inspired by the densely covered capillary structure inside a dog’s nose, we report an artificial nanostructure, i.e., poly(sodium p-styrenesulfonate)-functionalized reduced graphene oxide nanoscrolls (PGNS), with high structural perfection and efficient gas sensing applications. A facile supramolecular assembly is introduced to functionalize graphene with the functional polymer, combined with the lyophilization technique to massively transform the planar graphene-based nanosheets to nanoscrolls. Detailed characterizations reveal that the bioinspired nanoscrolls exhibit a wide-open tubular morphology with uniform dimensions that is structurally distinct from the previously reported ones. The detailed morphologies of the graphene-based nanosheets in each scrolling stage during lyophilization are monitored by cryo-SEM. This unravels an asymmetric polymer-induced graphene scrolling mechanism including the corresponding scrolling process, which is directly presented by molecular dynamics simulations. The fabricated PGNS sensors exhibit superior gas sensing performance with reliable repeatability, excellent linear sensibility, and, especially, an ultrahigh response (Ra/Rg = 5.39, 10 ppm) toward NO2. The supramolecular assembly combined with the lyophilization technique to fabricate PGNS provides a strategy to design biomimetic materials for gas sensors and chemical trace detectors. KEYWORDS: graphene nanoscrolls, supramolecular assembly, lyophilization, cryo-SEM, gas sensors he fine sense of smell of dogs, the most familiar macrosmatic animal in nature, is an excellent guide toward ultrasensitive gas detection.1 In fact, the ultrasensitive olfaction of dogs originates from the complex maxilloturbinate covered by dense capillaries inside the dog’s nose.2,3 The capillaries with large surface area endow dogs with an ultrahigh response to various odorants even at extremely low concentrations.4 Inspired by the capillary structure of the dog’s nose, we demonstrate that the performance of gas sensors can be significantly improved via preparing capillary-mimicking artificial nanostructures such as graphene-based nanoscrolls.

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© XXXX American Chemical Society

Graphene nanoscrolls (GNS), which result from scrolling planar graphene nanosheets in a continuous and uniform manner, have been arousing increasing interest as a promising carbonaceous nanomaterial with one-dimensional topology.5−7 GNS inherit many superior properties from graphene such as large specific surface area, strong thermostability, high carrier mobility, and superior mechanical properties.8−10 Moreover, GNS allow more possibilities to be assembled with various Received: November 22, 2017 Accepted: February 14, 2018

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DOI: 10.1021/acsnano.7b08294 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the supramolecular assembly combined with the lyophilization technique to fabricate PGNS and the comparison of the nanoscroll morphology of PGNS and GNS. (a) Addition of PSS to a GO aqueous dispersion. (b) Formation of well-dispersed PSS-rGO nanosheets after the reduction of GO to rGO and the supramolecular assembly of PSS with rGO nanosheets. (c) Fabricated wide-open tubular PGNS. (d) SEM inspection with low magnification of the attached PGNS, which remain separated. (e) Higher magnified SEM image of the wide-open tubular and straight PGNS. (f) SEM inspection with high magnification of the end of PGNS, showing a circular and well-opened orifice. Inset: Suggested mechanism for the formation of the well-opened orifice, where the negatively charged PSS-rGO nanosheets are prevented from stacking due to electrostatic repulsion. (g) SEM inspection with low magnification of the irregularly entangled GNS framework. (h) Higher magnified SEM image of the collapsed and bent “ribbon”-like GNS. (i) SEM inspection with high magnification of the end of the collapsed GNS with no orifice observed. Inset: Suggested mechanism for the tendency of the GO nanosheet to stick together due to strong π−π interaction during the scrolling in the ice sublimation process, resulting in collapsed GNS.

organic molecules11,12 and inorganic nanoparticles,13−15 realizing a versatility for wide applications especially in functional integration as sensors and electronic devices. GNS can be fabricated by several techniques such as chemical vapor deposition (CVD),8 low-temperature chemical exfoliation,16 and templated assembly.14,15,17 However, effective fabrication technologies are urgently demanded to overcome high energy consumption, complicated fabrication processes, and especially the difficulty of upgrading in large-scale fabrication. In 2014, Gao et al.18 developed a lyophilization technique to fabricate well-controlled GNS with a facile fabrication process, promoting a different strategy to transform planar graphene nanosheets to nanoscrolls. Those GNS were fabricated based on bare raw materials such as graphene oxide (GO) or mild reduced graphene oxide (rGO); thus they were probably not thoroughly reduced due to the irreversible aggregation of rGO nanosheets. As a result, the obtained GNS did not exhibit the innate superior electronic properties of graphene and lacked functionalization, while the GNS were generally shrivelled and entangled with each other in nonuniform dimensions.18−22 Obviously, it remains a challenge

to fabricate high-quality graphene-based nanoscrolls with uniform structure and feasibility of functionalization for practical application. Beyond that, the distinct scrolling behavior of GNS during the lyophilization process has not been visually clarified due to the difficulty in observation through the ice. Fortunately, the increasingly mature cryoelectron microscopy technique makes it possible to distinctly investigate the true nature of graphene-based nanoscrolls.23−25 In this work, we present the supramolecular assembly combined with the lyophilization technique to prepare functionalized graphene-based nanoscrolls with desirable morphology and uniform dimensions. As expected, the capillary-like nanoscrolls reveal significant improvement in NO2 sensing, which is competitive with existing graphene-based sensing materials. By performing cryo-scanning electron spectroscopy (SEM), we clearly observe the distinct morphologies of the graphene-based nanosheets in each scrolling stage during the lyophilization process and unravel the corresponding scrolling mechanism, which is proved in detail by molecular dynamics (MD) simulations. B


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Figure 2. Characterization of the dimensions and demonstration of successful assembly of PSS with the nanoscrolls. (a) Length statistics of the PGNS. (b) Diameter statistics of the PGNS. (c) FTIR spectrum of PGNS, bare rGO, and GO. (d) XPS spectrum of the PGNS and bare rGO. (e) FESEM image of a typical PGNS structure. (f) Corresponding FESEM-EDS mapping of S Ka1.

RESULTS AND DISCUSSION Aromatic molecules such as benzene, naphthalene, and anthracene can noncovalently functionalize graphene nanosheets via π−π interactions due to the similar π-conjugated structure, without destroying the intrinsic electronic properties of graphene.26−28 Here, an aromatic polymer, poly(sodium pstyrenesulfonate) (PSS), with benzene ring structure is selected to disperse into a GO aqueous dispersion (Figure 1a). After the reduction by hydrazine hydrate, most of the oxygen-containing groups of GO are removed to form rGO. Meanwhile the PSS is successfully assembled with the rGO nanosheets to form the starting material (PSS-rGO) through π−π interactions (Figure 1b).29,30 Unlike the aggregated rGO nanosheets, the PSS-rGO nanosheets are well dispersed in the aqueous phase. The dispersion remains stable with a homogeneous black appearance for at least a year (Supporting Information, Figure S1e). Such a phenomenon relies on three facts: (1) the intrinsically hydrophilic sulfonic groups (−SO3−) of PSS contribute to the

excellent dispersity of the PSS-rGO nanosheets. (2) The −SO3− groups of PSS with extra negative charges enhance the interlaminar static-repulsion forces of PSS-rGO and prevent the aggregation.26,31 (3) In the case of (poly)aromatic molecules, the PSS molecules have a strong tendency to stack onto graphene. The supramolecular assembly is governed by the π−π interactions between the PSS molecules and graphene sheets. This also implies that the π−π interactions among graphene sheets (which can cause the restacking of the sheets) are thermodynamically hindered.32 Hence, the PSS-rGO nanosheets are well separated from each other in an aqueous dispersion, revealing a low bending rigidity, thus facilitating the subsequent shape transformation into graphene-based nanoscrolls. Starting from the PSS-rGO dispersion, the lyophilization process to prepare the PSS-rGO nanoscrolls (PGNS) contains three steps. First, the dispersion is poured into a glass bottle and then immersed in liquid nitrogen to become a fast frozen C


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Figure 3. Schematic with corresponding cryo-SEM images and MD simulations of the scrolling behavior from PSS-rGO nanosheets to nanoscrolls at different states in the lyophilization process. (a) Fast freezing to the solid phase by liquid nitrogen; the PSS-rGO nanosheets vertically align in the ice. (b) Spontaneous scrolling of the PSS-rGO nanosheets along with the sublimation of ice. (c) Transformation from planar nanosheets to nanoscrolls after the PSS-rGO nanosheets are entirely freed from the ice. The top-view cryo-SEM image of a typical (a1) PSS-rGO nanosheet that has not yet scrolled, (b1) PSS-rGO nanosheet in the process of scrolling, and (c1) nanoscroll with the edges overlapped. The thermodynamic scrolling process of the NaPSS-rGO-H2O system and the corresponding energy variation (here Na+ is used to balance the number of whole electric charges). (d) Initial state of the system with a planar PSS-rGO nanosheet. (e) Variation of the total energy with time, where three stages are presented: I, dissolution; II-, scrolling; and III, relaxation. (f) Final state of the nanoscroll. See also the movie in the Supporting Information showing the whole thermodynamic scrolling process.

(Supporting Information, Figure S1b) and also verified as nanoscroll structures after the lyophilization process. However, the morphology of the GO nanoscrolls differs a lot compared to that of the PGNS. As revealed in the low-magnification SEM image (Figure 1g), the majority of the GO nanosheets are transformed to GNS, while a few planar nanosheets still remain. In addition, the GNS are irregularly bent and interlaced with each other to form a netlike framework. Further, we investigated the differences between PGNS and GNS at the microscale. For PGNS, a wide-open tubular and straight nanoscroll structure is observed (Figure 1e). The end of the PGNS reveals a round and well-opened orifice (Figure 1f). The highly perfect nanoscroll morphology is attributed to the supramolecular modification by PSS. The −SO3−-containing polymers entirely cover the PSS-rGO nanosheets with negative charges. Thus, the inner walls of the PSS-rGO nanosheets are prevented from sticking together due to electrostatic repulsion during the lyophilization process (Figure 1f inset) to form wellopened nanoscrolls. Moreover, we verified that the fabrication

solid. Afterward, the bottle is immediately transferred into a lyophilizer for sublimation. The PSS-rGO nanosheets are scrolling gradually at this stage. Finally, the nanosheets accomplish the scrolling and transform into PGNS after complete dehydration (Figure 1c). The fabricated PGNS exhibit the desirable nanoscroll structure with a generally straight morphology. Almost all PSS-rGO nanosheets have been successfully transformed. Although the nanoscrolls attach to each other due to the high density, they are not bent or entangled and remain separated (Figure 1d). To investigate the influence of PSS on the formation of the PGNS structure, we tried bare rGO and GO dispersions to fabricate nanoscrolls, performing the same experimental steps. For the rGO dispersion, aggregation is unavoidable and, thus, the lyophilized foams are shrunken and collapsed (Supporting Information, Figure S1d). Stacked rGO layers are formed instead of nanoscrolls (Supporting Information, Figure S2c). For the well-dispersed GO nanosheets, fluffy foams with ultralow density are formed D


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lower temperature than the top, and a vertical temperature gradient is generated. Therefore, the ice crystals grow vertically and drive the PSS-rGO nanosheets to turn vertically until the end of the ice nucleation, and the nanosheets are finally unidirectionally embedded in the ice. The schematic of the fast freezing process and the vertical temperature gradient measurement are shown in Figure S8 in the Supporting Information. On the basis of the vertical orientation of the PSS-rGO nanosheets, the spontaneous scrolling behavior is clearly understood. During the layer-by-layer sublimation of ice, the vertical PSS-rGO nanosheets begin to be exposed gradually, and sublimation induces the nanosheets to scroll (Figure 3b). The process was directly observed by cryo-SEM in Figure 3b1. As the sublimation continues, the vertical PSS-rGO nanosheets scroll simultaneously until they are entirely freed from the ice with the edges overlapped (Figure 3c) to transform them from planar nanosheets to nanoscrolls (Figure 3c1). From the evidence of the cryo-SEM in each state during the lyophilization process, we demonstrate that the scrolling process is triggered during the sublimation to form edgeoverlapping nanoscrolls instead of free scrolling and forming random scrolls. The intriguing scrolling behavior is directly visualized during the lyophilization process of graphene-based nanosheets. To elucidate the scrolling process of the PSS-rGO nanosheets, a systematic MD simulation was carried out. The models of Coulombically stabilized assemblies39 and the related studies of graphene self-assembly on droplets, polymers, and carbon nanotubes (CNTs) inspired us in building the model of our MD simulations.40,41 It is feasible and reasonable to reveal a relatively macroscopic phenomenon via simulating at a microscopic size instead of a macroscopic size;42 thus the models here were set up at orders of magnitude smaller size than the experiment, still in good qualitative agreement with the available experimental data. The computational model is shown in Figure 3d, consisting of one rGO nanosheet (with a length of 7 nm and width of 21 nm), three PSS chains (with the degree of polymerization N = 20), and 5000 H2O molecules. The detailed simulation procedure is shown in the Supporting Information. Initially, the whole system is in a frozen state, which can be viewed as a liquid nitrogen environment. To simulate the sublimation, a rapid warming process was conducted, so that the frozen state could be released according to the minimum energy principle. Such a reviving process is very quick, and it can reach the minimum point of total energy at 0.025 ns. The corresponding configuration is presented at the end of the first stage (I) in Figure 3e. As the temperature rises, the kinetic energy increases and contributes to the total energy, while the PSS adhere closely to the rGO under the adhesive force from van der Waals and electrostatic interactions. Also, the scrolling energy from the increased curvature, which is directly produced from the asymmetric distribution of PSS on the opposite sides of the rGO nanosheet, contributes to the increased total energy. Due to the onedimensional feature of the PSS, the rGO is induced to assemble into a nanoscroll, as shown in Figure 3e at the end of the second stage (II). It can be concluded that for the large multilayered rGO with numerous PSS in the practical coscrolling process the complex scrolling configuration can be viewed as the superposition of the PSS-rGO system discussed above. Moreover, due to the strong hydrophilic interaction of the PSS, the scrolling behavior of the PSS-rGO nanosheet is accelerated with the departure of H2O molecules during the

of PGNS is sensitive to the ionic strength of the solution. When increasing the ionic strength by adding NaCl,33 nanoscrolls are no longer obtained (Supporting Information, Figure S3), whereas the GNS are more like thin “ribbons” (Figure 1h), which is clearly verified by the high-magnification SEM image of the end of the GNS (Figure 1i). There are obvious wrinkles wrapped over the “ribbon”, but no orifice is observed; thus the thin “ribbon” is likely a collapsed nanoscroll like the previous reported ones.19−21 That is because the GO nanosheets have a tendency to stack together due to the strong π−π interaction during the scrolling in the ice sublimation process (Figure 1i inset), and finally the inner GO sheets stack together to form the collapsed nanoscroll. To sum up, PGNS can be effectively fabricated in large scale and exhibit a well-opened nanoscroll structure with a tubular and straight shape. In contrast to GNS, we confirm that PSS is key to support the aimed at structure. Further, we evaluated the statistics in length and diameter to elucidate the regularity of PGNS. As revealed in Figure 2a, the length peaks in the range of 8−18 μm with a percentage of 82.4% (8−13 μm accounts for 45.9% and 13−18 μm for 36.5%), and the average length is 13.5 μm. The diameter of around 660 nm contains a large proportion of 88.4% (Figure 2b). Thus, the PGNS are prepared with uniform dimensions, which can be easily upgraded to large-scale fabrication. The statistics in length and diameter were conducted based on Leica images (Supporting Information, Figures S4 and S5). Several techniques were adopted to verify the successful and uniform assembly of PSS on the graphene nanoscrolls. Fourier transform infrared (FTIR) spectroscopy analysis of the PGNS provides direct evidence of the successful surface modification with PSS (Figure 2c), showing the characteristic SO stretching vibration band at 1620 cm−1 and the phenyl ring absorption peak at 1400 cm−1, along with the 1196−1032 cm−1 C−C vibration of the carbon skeleton of the PSS chain. The pure PSS FTIR spectrum is shown in Figure S6 in the Supporting Information. The two characteristic hydroxyl and carboxyl peaks of the GO at 3410 and 1722 cm−1, respectively, are attenuated significantly in the spectrum of rGO and PGNS, indicating that most oxygen-containing functional groups have vanished due to the effective reduction with hydrazine hydrate. X-ray photoelectron spectroscopy (XPS) is in accordance with the FTIR results, revealing an obvious S 2p peak at 168.4 eV that is assigned to the −SO3− groups, indicating the successful modification with PSS (Figure 2d). Further, the field emission scanning electron microscopy (FESEM) image of a typical PGNS (Figure 2e) and the corresponding energy dispersive Xray spectroscopy (EDS) mapping verify the uniform coverage of the nanoscrolls with PSS (Figure 2f), revealing the excellent potential of the facile modification of graphene-based materials using supramolecular assembly. In order to gain clear knowledge of the spontaneous scrolling behavior of the PSS-rGO nanosheets, we tracked the intermediate states during the lyophilization process by cryoSEM. In contrast to previous reports,18,22 most PSS-rGO nanosheets are uniformly distributed in the ice with the basal plane oriented in the direction perpendicular to the ice sublimation surface (Figure 3a), which was verified through the top-view cryo-SEM image (Supporting Information, Figure S7). A typical nanosheet that has not yet scrolled is shown in Figure 3a1. This phenomenon is explained by the vertical nucleation and growth of ice crystals.34−36 Behaving as a typical thermal conductor,37,38 the glass bottle is immersed vertically into the liquid nitrogen for fast freezing. The bottom of the bottle is at a E


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Figure 4. (a) Digital and SEM images of a typical fabricated PGNS sensor, maintaining the nanoscroll structure on IEs. (b) I−V characteristics of the PGNS sensors. (c) Response curves of PGNS, PSS-rGO w/o L, rGO w/L, and rGO w/o L sensors toward 10 ppm of NO2. (d) Successive response curve of PGNS sensors to different NO2 concentrations ranging from 1 to 10 ppm and (e) corresponding linear fit of the responses as a function of NO2 concentration. Error bars for the data points lie within the symbols themselves. (f) Selective response of the PGNS sensors toward 10 ppm of NO2 and 1000 ppm of interferential gases, including Cl2, NH3, ethanol, acetone, and methanol. (g) Threecycle response curve of PGNS sensors upon exposure to 10 ppm of NO2. (h) Aging test of PGNS sensors toward 10 ppm of NO2 for 6 months. All the gas sensing measurements were conducted at room temperature under 45−65% relative humidity.

the electrical contact plays a negligible role in the sensing process.43 Schottky barriers are absent between the PGNS and IEs,44 ensuring the accuracy of the gas sensing measurement in our work. The gas sensing system was employed to monitor the resistance variation of the fabricated sensors. The corresponding response is defined as Response = Ra/Rg, where Ra and Rg denote the resistance captured in an atmosphere of air and NO2, respectively. Details about the fabrication of the gas sensor and the gas sensing tests are described in the Materials and Methods. As shown in Figure 4c, the detailed dynamic gas sensing transients reveal that the PGNS (i.e., PSS-rGO with lyophilization (PSS-rGO w/L)) sensor exhibits a dramatically enhanced response (Ra/Rg = 5.37) toward 10 ppm of NO2 gas compared to other graphene-based materials, i.e., PSS-rGO without lyophilization (PSS-rGO w/o L), rGO with lyophilization (rGO w/L), and rGO without lyophilization (rGO w/o L). The enhanced response of PGNS is mainly attributed to the specific nanoscroll structure and the functionalization with PSS molecules. The mechanism analysis for gas sensing and the response enhancement of PGNS is discussed in detail in the

sublimation process. Once the scrolling is accomplished, the system comes into the relaxation stage to adjust itself, accompanied by slight bending, twisting, shrinking, and expanding due to the thermal motion at a finite environmental temperature. Thus, the total energy decreases slightly at the third stage (III) in Figure 3e, and the final configuration presented in Figure 3f is established. A video of the whole thermodynamic scrolling process is shown in the Supporting Information, which can explain our experimental observation visually. A drop and dry method was adopted to fabricate the gas sensors. First, a 20 μL dispersion of each sensing materials was dropped on the Ag−Pd interdigitated electrodes (IEs), which are supported by a ceramic substrate. After drying, a thin sensing film was formed on the IEs, and the gas sensor was ready for testing. The digital and SEM images of a typical fabricated PGNS-based sensor are displayed in Figure 4a. It is obvious that the PGNS uniformly cover the IEs, and the nanoscroll structure remains. The current versus voltage (I−V) curve of the PGNS sensor displays excellent linearity between −1 and 1 V (Figure 4b), revealing good ohmic contact between the PGNS and the IEs. In other words, the result indicates that F


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ACS Nano Table 1. Comparison of NO2 Sensing Performance of Typical Graphene-Based Sensors materials and structure 49

2D graphene/MoS2 heterostructure 3D mesoporous rGO/SnO250 MoS2/graphene hybrid aerogel51 3D scaffold rGO/polymer52 N and Si co-doped graphene nanosheets53 rGO network covered 3D micropillar54 single-layer graphene channel55 RGO/porous PEDOT56 flexible Ag-S-RGO45 In2O3 cubes/rGO composites57 holey rGO nanosheets58 sulfonated rGO44 graphene/polymer nanofibers59 3D SiO2@graphene frameworks60 rGO/In-SnO2 nanohybrids61 PGNS (this work) a

method

NO2 (ppm)

responsea

operating temperature

photolithography and ion etching hydrothermal and lyophilization freeze-drying and annealing electrospinning high-temperature annealing photoetching CVD in situ polymerization deposition gravure print microwave-assisted hydrothermal method hydrothermal reduction chemical modification self-assembly electrostatic self-assembly one-pot aqueous method supramolecular assembly and lyophilization

5 100 3 4.5 21 5 10 10 10 10 12.5 10 5 50 10 5 10

7% 12% 15% 18% 26% 28% 28% 33% 45% 50% 54% 58% 61% 69% 75% 75% 81%

150 °C 55 °C 200 °C 100 °C RTb RT 100−165 °C RT RT RT RT RT RT RT RT RT RT

For convenience of comparison, the evaluation of response is converted as Response =

ΔR Ra

=

|R a − R g | Ra

=

|R a − R g| Ra

(%). bRT represents room temperature.

Notably, no distinct degradation of the response is observed, and the response/recovery times also remain stable, proving the high stability of the PGNS sensor. In fact, the high response toward NO2 of the PGNS sensor is competitive with the existing graphene-based gas sensors as summarized in Table 1. Here, it is noteworthy that the PGNS sensor operates at room temperature with an ultrahigh response, which makes it more versatile for practical gas detection with low energy consumption.46−48

Supporting Information together with a mathematic model analysis. In practical gas sensing applications, linearity of response as a function of gas concentration is also of important significance. Figure 4d exhibits the successive dynamic-sensing response (Ra/Rg) of the PGNS sensor toward 1−10 ppm of NO2. It is clear that the response amplitude increases monotonically with the rising of NO2 concentration. A simple linear regression fit is applied to find that the PGNS sensor reveals an excellent linear detection range from 1 to 10 ppm with the corresponding response (Ra/Rg) measured from 2.25 to 5.48 (Figure 4e). The PGNS sensor also exhibits excellent selectivity to NO2 gas, as shown in Figure 4f. For convenience of comparison, here the evaluation of the response is converted as Response =

ΔR Ra

CONCLUSIONS In summary, inspired by the capillary structure inside a dog’s nose, functionalized graphene-based nanoscrolls, i.e., PGNS, were fabricated for application as highly sensitive gas sensors. An effective supramolecular assembly combined with the lyophilization technique was developed to massively transform the planar PSS-rGO nanosheets to uniform PGNS of high quality. By performing cryo-SEM, we revealed the distinct morphological changes in each scrolling stage during the lyophilization process and clarified the corresponding scrolling mechanism supported by MD simulations. Due to the important role of the functional PSS molecules, the PGNS exhibited a wide-open tubular morphology with uniform dimensions. The as-prepared PGNS gas sensors revealed an ultrahigh response, superior selectivity and sensing linearity, reliable repeatability, and stability. The supramolecular assembly combined with the lyophilization establishes an effective processing technique to prepare graphene-based nanoscrolls with the potential for large-scale production.

(%). The PGNS sensor exhibits a

much higher response toward NO2 (ΔR/Ra = 81%, 10 ppm) than other interferential gases including Cl2 (ΔR/Ra = 21%, 1000 ppm), NH3 (ΔR/Ra = 17%, 1000 ppm), ethanol (ΔR/Ra = 14%, 1000 ppm), acetone (ΔR/Ra = 8%, 1000 ppm), and methanol (ΔR/Ra = 7%, 1000 ppm), even though the NO2 concentration is 100 times lower than the others. The results indicate that the PGNS sensors are extremely sensitive to NO2. Similar to the previously reported mechanism,29,44,45 the high selectivity of the PGNS sensor probably results from two facts: (1) The −SO3− groups of PSS possess a strong adsorption capacity especially for NO2 gas molecules. (2) NO2 is a typical strong electron-withdrawing gas, while the interferential gases exhibit weak electron-withdrawing properties or -donating capacity and are, thus, unable to cause a significant resistance change. Therefore, the selectivity of the PGNS sensors for NO2 gas detection is high. To investigate the repeatability and stability of our sensing device, the PGNS sensor was exposed to 10 ppm of NO2 for three successive cycles. An average response (Ra/Rg = 5.39) with a small standard deviation of 8% is measured (Figure 4g), verifying the reliable repeatability of the PGNS sensor. Moreover, an aging test was carried out monthly for half a year (Figure 4h). The sensor reveals a high stable response toward 10 ppm of NO2 within an 11% standard deviation of its average value (Ra/Rg = 5.37) over the whole period of time.

MATERIALS AND METHODS Preparation of GO Dispersion. GO flakes (purchased from XianFeng NANO Co., Ltd.) were ultrasoniated in deionized (DI) water for an hour and then mildly sonicated for 40 min to prepare a 0.2 mg/mL GO dispersion. Preparation of rGO Dispersion. A 75 μL amount of ammonia (30%) was added to 20 mL of GO dispersion (0.2 mg/mL) to regulate the pH to 12.5. Then 10 mL of hydrazine hydrate (1 μL/mL) was used to chemically reduce the GO dispersion in an oil bath for 1 h at 95 °C with gentle stirring. After cooling to ambient temperature, the dispersion was dried by vacuum filtration and then redispersed into 20 G


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ACS Nano mL of DI water under mild sonication to prepare a 0.2 mg/mL rGO dispersion. Preparation of PSS-rGO Dispersion. An 80 mg amount of PSS (Purchased from Alfa Aesar) was added into 20 mL of a GO dispersion (0.2 mg/mL). After 5 min of mild sonication, 10 mL of hydrazine hydrate (1 μL/mL) was added dropwise under gentle stirring, and the dispersion was then kept at 80 °C for 1 h in an oil bath. After cooling to ambient temperature, the resultant dispersion was rinsed three times by vacuum filtration with DI water and finally redispersed into 20 mL of DI water under mild sonication to get a homogeneous PSS-rGO dispersion (0.2 mg/mL). Lyophilization of GO, rGO, and PSS-rGO Dispersion. The asprepared GO, rGO, and PSS-rGO aqueous dispersions were poured into glass bottles (10 mL) and vertically immersed into liquid nitrogen for 5 min to be solid frozen. The samples were immediately transferred into the hanging bottle of a lyophilizer and kept at around −10 °C under 20 Pa for 48 h to be completely dehydrated. Fabrication of Gas Sensors. A drop and dry method was applied in the gas sensor fabrication. First, the lyophilized rGO powders and PGNS were redispersed into DI water with the same concentration (0.2 mg/mL) of rGO w/o L and PSS-rGO w/o L. Afterward, 20 μL of each dispersion was dropped on the surface of the IEs, which were fabricated by jetting Ag−Pd paste on ceramic substrates through a metal-jetting system (Supporting Information, Figure S9). Then the IEs were dried on a heating holder in air at 50 °C for 2 min. Finally, the gas sensors were available for the subsequent tests. Gas Sensing Tests. All the gas sensing tests were performed by a gas sensing system that was designed to monitor the resistance changes of the fabricated gas sensors under a dc bias voltage of 4.5 V. Prior to the sensor measurements, pure air gas was inflated into the gas chamber to create a stable air environment; then quantitative NO2 (Cl2) with testing concentration was injected as the test started, and the gas chamber was opened when the test ended. For the injection of volatile gases including ammonia, ethanol, acetone, and methanol, their vapor streams can be generated by heating the corresponding volatile compounds in a crucible inside the chamber. All the gas sensing tests were performed at room temperature (25 °C) in the range of 45−65% relative humidity, and the final data were obtained based on five measurements. Characterizations. The general morphology of GNS and PGNS was examined by SEM (Quanta 250 FEG, FEI, Czechia). The morphology of the scrolling PSS-rGO nanosheets embedded in ice was characterized by cryo-SEM (Helios NanoLab G3 UC, Czechia). The length and diameter of the PGNS were measured by confocal optical microscopy (Leica DCM8, Germany). The surface elemental composition and characteristic functional groups of the samples were analyzed by XPS (ESCALAB 250 photoelectron spectrometer, Thermo Fisher Scientific, USA) and FTIR (Thermo Scientific Nicolet iN10, USA). FESEM (JSM-7500F, JEOL, Japan) in combination with EDS mapping (Oxford Instruments, UK) was performed to verify the uniform coverage of PSS on the nanoscrolls. I−V characteristic curves were measured by an SA6101 electrical analysis system (Sinoagg Co., Ltd., China). The applied voltage was varied from dc −1 to 1 V in steps of 0.05 V. All the gas sensing measurements were performed by the Intelligent Gas Sensing Analysis System (CGS-1TP, ELITE TECH. Beijing).

freezing process and temperature gradient measurement; metal-jetting system and interdigitated electrode fabrication process; MD simulation methods; mechanism analysis for gas sensing and response enhancement of PGNS (PDF) Video of the whole thermodynamic scrolling process (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yao Wang: 0000-0002-0713-5018 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 51673007, 51773069, 51561135014), National Key Basic Research Program of China (2014CB931800), Startup Foundation from SCNU (No. 8S0134), Guangdong Innovative Research Team Program (No. 2013C102), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No. 2017B030301007), MOE International Laboratory for Optical Information Technologies, and the 111 Project. REFERENCES (1) Onodera, T.; Toko, K. Towards an Electronic Dog Nose: Surface Plasmon Resonance Immunosensor for Security and Safety. Sensors 2014, 14, 16586−16616. (2) Craven, B. A.; Neuberger, T.; Paterson, E. G.; Webb, A. G.; Josephson, E. M.; Morrison, E. E.; Settles, G. S. Reconstruction and Morphometric Analysis of the Nasal Airway of the Dog (Canis Familiaris) and Implications Regarding Olfactory Airflow. Anat. Rec. 2007, 290, 1325−1340. (3) Dawes, J. D.; Prichard, M. M. Studies of the Vascular Arrangements of the Nose. J. Anat. 1953, 87, 311. (4) Widdicombe, J. Physiologic Control. Anatomy and Physiology of the Airway Circulation. Am. Rev. Respir. Dis. 1992, 146, S3−S7. (5) Viculis, L. M.; Mack, J. J.; Kaner, R. B. A Chemical Route to Carbon Nanoscrolls. Science 2003, 299, 1361−1361. (6) Mpourmpakis, G.; Tylianakis, E.; Froudakis, G. E. Carbon Nanoscrolls: A Promising Material for Hydrogen Storage. Nano Lett. 2007, 7, 1893−1897. (7) Shi, X.; Pugno, N. M.; Cheng, Y.; Gao, H. Gigahertz Breathing Oscillators Based on Carbon Nanoscrolls. Appl. Phys. Lett. 2009, 95, 163113. (8) Chen, X.; Li, L.; Sun, X.; Kia, H. G.; Peng, H. A Novel Synthesis of Graphene Nanoscrolls with Tunable Dimension at a Large Scale. Nanotechnology 2012, 23, 055603. (9) Xie, X.; Ju, L.; Feng, X.; Sun, Y.; Zhou, R.; Liu, K.; Fan, S.; Li, Q.; Jiang, K. Controlled Fabrication of High-Quality Carbon Nanoscrolls from Monolayer Graphene. Nano Lett. 2009, 9, 2565−2570. (10) Pan, D.; Wang, C.; Wang, T. C.; Yao, Y. Graphene Foam: Uniaxial Tension Behavior and Fracture Mode Based on a Mesoscopic Model. ACS Nano 2017, 11, 8988−8997. (11) Fang, Q.; Zhou, X.; Deng, W.; Liu, Y.; Zheng, Z.; Liu, Z. Nitrogen-Doped Graphene Nanoscroll Foam with High Diffusion Rate and Binding Affinity for Removal of Organic Pollutants. Small 2017, 13, 1603779.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08294. Digital images of GO, rGO, PSS-rGO dispersions, and corresponding lyophilized foams; SEM images of PGNS, rGO w/o L, rGO w/L, and PSS-rGO w/o L; SEM images of PGNS under enhanced ionic strength; Leica images of PGNS; Leica image of PGNS; FTIR spectrum of PSS powders; top-view cryo-SEM image of vertically distributed PSS-rGO nanosheets; schematic of fast H


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Scrolling Graphene Nanosheets - ACS Publications - American

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