Hybrid Integration of Carbon Nanotubes and Transition Metal

Subscriber access provided by UNIV OF LOUISIANA

Functional Nanostructured Materials (including low-D carbon)

Hybrid Integration of Carbon Nanotubes and Transition Metal Dichalcogenides on Cellulose Paper for Highly Sensitive and Extremely Deformable Chemical Sensor Woo Sung Lee, and Jungwook Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03296 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Hybrid Integration of Carbon Nanotubes and Transition Metal Dichalcogenides on Cellulose Paper for Highly Sensitive and Extremely Deformable Chemical Sensor

Woo Sung Lee and Jungwook Choi*

School of Mechanical Engineering, Yeungnam University 280 Daehak-ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea *E-mail: [email protected]

KEYWORDS: carbon nanotube, transition metal dichalcogenide, cellulose substrate, deformable device, sensor-on-paper

ABSTRACT Sensitive and deformable chemical sensors manufactured by a low-cost process are promising as they are disposable, can be applied on curved, complex structures, and provide environmental information to users. Although many nanomaterial-based flexible sensors have been suggested to meet these demands, their limited chemical sensitivity and mechanical flexibility pose challenges. Here, a highly deformable chemical sensor is reported with improved sensitivity that integrates

multiwalled

carbon

nanotubes

(CNTs)

and

nanolayered

transition

metal

dichalcogenides (TMDCs) on cellulose paper. Liquid dispersions of CNTs and TMDCs are

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

absorbed and dried on porous cellulose for sensor fabrication, which is simple, scalable, rapid, and inexpensive. The cellulose substrate enables reversible three-dimensional folding and unfolding, bending down to 0.25 mm, and twisting up to 1800° (~628.4 rad m−1) without degradation, and the CNTs maintain a percolation network and simultaneously provide gas reactivity. Functionalization of the CNTs with TMDCs (WS2 or MoS2) greatly improves the sensing response to exposure to NO2 molecules by more than 150%, and the sensor can also selectively detect NO2 over diverse reducing vapors. The measured NO2 sensitivity is 4.57% ppm−1, which is much higher than that of previous paper-based sensors. Our sensor can stably and sensitively detect the gas even under severe deformation such as heavy folding and crumpling. Hybrid integration of CNTs and TMDCs on cellulose paper may also be used to detect other harmful gases and can be applicable in low-cost portable devices that require reliable deformability.

INTRODUCTION Recent advances in materials and manufacturing processes have enabled the realization of soft electronics, optoelectronics, and sensors with high performance comparable to that of conventional rigid devices.1 Various organic and inorganic materials with micro/nanoscale dimensions have been integrated onto elastomeric substrates for electronic skin, soft robotics, flexible displays, energy storage, and wearable sensors.2,3 These devices can be mounted on nonplanar surfaces and exhibit compelling performance; however, they usually require sophisticated fabrication processes and have limited flexibility.4 Considering the increasing demand for low-cost and highly deformable devices,5,6 achieving reliable foldability and


2

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


twistability would make it possible to improve the functionality of devices and to provide new uses for them for integration into more complex components. Cellulose paper is an alternative that can overcome the limitations of conventional flexible substrates. Owing to its low cost, lightweight, disposability, biodegradability, and deformability, various applications ranging from flexible electronics to electrochemical devices have been successfully demonstrated.4,5,7–10 In addition, because of its unique features, cellulose paper can serve as a scaffold for printing, filtration, and deposition of functional materials for flexible sensors.11–17 In particular, integration of nanomaterials on cellulose paper can be an ideal approach to flexible chemical sensors because of their porosity, large surface area, and bendability,

as

demonstrated

by

carbon

nanotubes

(CNTs),18–23

graphene,24,25

and

nanoparticles.26–28 Among many nanoscale materials and structures, carbon nanomaterials have attracted significant interest as a sensing material because of their excellent material properties.29 Carbon nanomaterials integrated on paper offer flexibility and responsiveness to diverse chemical species; however, their sensitivity to harmful gases such as NO2 is less than ~1.5% ppm−1,22,24,25 which is lower than that of other nanostructured flexible sensors. Thus, it is necessary to develop gas sensors having high chemical sensitivity while exploiting the advantages of the paper substrate. Here we present the first use of a paper-based, extremely deformable gas sensor using integrated

one-dimensional

(1D)

CNTs

and

two-dimensional

(2D)

transition

metal

dichalcogenides (TMDCs) to improve the chemical sensitivity. As surface functionalization using heterogeneous, multidimensional materials improves the sensing response,30–32 1D CNT networks on cellulose paper were decorated with 2D TMDCs. Semiconducting TMDCs have attracted great interest owing to not only their layer-number-dependent properties,33 but also


3


Page 4 of 29

their high physical and chemical reactivity.34,35 Thus, the high chemical sensitivity of CNTs and TMDCs and charge transfer between them upon adsorption of chemical species are expected to afford significantly improved sensitivity, and the cellulose substrate provides high deformability. In this work, we demonstrate a highly sensitive, foldable, and twistable chemical sensor based on a hybrid of 1D CNTs and 2D TMDCs (WS2 or MoS2) on cellulose paper. Fabrication requires only absorption and evaporation of CNT and TMDC dispersions on cellulose, so it is simple, scalable, rapid (within 30 min), and inexpensive (~$0.23 per sensor for chemicals and materials). Cellulose paper provides the porosity, three-dimensional (3D) foldability, and extreme twistability needed for robust flexible sensors, and the CNTs offer a percolation pathway and gas sensitivity. When CNTs on cellulose are functionalized with WS2, the response to 10 ppm NO2 is improved by ~150%. The versatility of our approach is further demonstrated by functionalizing CNTs with MoS2 or by detecting other chemical species, in which case the sensing behavior is opposite to that for NO2. In addition, the sensor can be reversibly bent (down to a 0.25 mm bending radius) and twisted (up to 1800°) with high cyclic durability and can stably and sensitively detect the gas even under heavy crumpling and creasing. Our sensor not only outperforms previously reported low-sensitive paper-based chemical sensors, but also provides high stability against severe mechanical deformation.


4

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION

Figure 1. Multidimensional, heterogeneous integration of 1D CNTs and 2D TMDCs on cellulose paper. (a) Schematic of preparation process of CNT-TMDC-integrated cellulose paper for sensitive and deformable chemical sensor. The cellulose paper is sequentially dip-coated in CNT and TMDC dispersions and dried until a percolation network forms. The entire process is simple and rapid, so it is suitable for producing low-cost disposable sensors. The nanomaterial-coated paper is self-standing, and an elastic substrate can optionally be used as a support or protection layer. (b) Photograph of nanomaterial-coated, large-area (>100 cm2) cellulose paper, showing process scalability. The paper can be easily cut into various shapes and sizes. (c) 3D origami boat folded from the nanomaterial-coated paper to connect LED circuit. (d) Photographs of the bent and twisted cellulose paper mounted on a PDMS substrate, demonstrating its deformability.

Figure 1a schematically illustrates fabrication of the cellulose-paper-based, CNT-TMDC hybrid sensor. CNT and TMDC powders were sonicated, centrifuged, and finally stably dispersed in N,N-dimethylformamide (DMF) at concentrations of ~2 and ~0.2 mg mL−1,


5


Page 6 of 29

respectively (see Experimental Section for details on material preparation). We used multiwalled CNTs (outer diameter, ~15 nm) for all the experiments and WS2 nanolayers (number of layers, two to three) as a representative TMDC; the number of WS2 nanolayers was confirmed by transmission electron microscopy (TEM) (Figure S1, Supporting information). The UV–visible absorption spectrum of the WS2 dispersion clearly indicated layered WS2 with excitonic signatures at 463, 538, and 642 nm, which correspond to those in a previous report (Figure S2a,b, Supporting information).36 The CNT dispersion showed an absorption peak around 272 nm (Figure S2c,d, Supporting information). Pristine cellulose paper was repeatedly dip-coated in the CNT dispersion and dried until a percolation network formed and the measurable electrical resistance was obtained; the coated paper was then dip-coated in the WS2 dispersion and dried. As a result of repetitive coating processes, both CNTs and WS2 were bounded on the surface of cellulose paper possibly by van der Waals interactions. An elastic substrate such as polydimethylsiloxane (PDMS) can optionally be used to support the nanomaterial-coated cellulose paper. The entire fabrication process takes only ~30 min, including dip-coating, drying, and wiring electrical connections. Owing to the process scalability, nanomaterial-coated largearea paper more than 100 cm2 in size can be prepared (Figure 1b), and it can be scaled up simply by increasing the quantity of dispersed nanomaterials and size of the cellulose paper. Because of the paper substrate, the sensor can be creased, folded, and unfolded to form 3D self-standing structures; if necessary, it can be trimmed and perforated easily. Figure 1c shows a 3D origami boat folded from the nanomaterial-coated paper, which can be used to connect a light-emitting diode (LED) circuit. The deformability of the cellulose paper under bending and twisting is shown in Figure 1d.


6

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Characterization of the nanomaterial-integrated cellulose paper. (a,b) SEM images of the 1D CNT-2D WS2 on cellulose microfibers. The CNTs completely cover the surfaces of the cellulose fibers, forming percolation pathways, and the WS2 is deposited locally on them. Enlarged SEM image of red box in (a) is shown in (b). (c,d) Raman spectra of pristine cellulose (green line), CNT-coated cellulose (red line), and CNT-WS2-coated cellulose (blue line). Raman spectra collected from the CNTs are deconvoluted into four curves. The CNTs exhibit D and G


7


Page 8 of 29

peaks (~1350 and ~1577 cm−1, respectively), and WS2 displays in-plane (E12g) and out-of-plane (A1g) modes at 350 and 418 cm−1, respectively. Magnified Raman spectra in (d) are from region enclosed by dotted line in (c). (e–g) XPS spectra of CNT-WS2-coated cellulose. The C 1s (e), W 4f (f), and S 2p (g) core-level XPS spectra indicate successful integration of CNTs and WS2 without significant degradation during sonication, dispersion, coating, and drying.

The scanning electron microscopy (SEM) images in Figure 2a,b show the cellulose microfibers, on which the CNTs form percolation pathways, and WS2 nanolayers locally deposited on them. The cellulose microfibers maintain their network after multiple dip-coating processes until the CNTs completely cover their surfaces (Figure S3, Supporting information). Raman spectra of the cellulose paper before and after CNT and WS2 coating were collected under 532 nm excitation (Figure 2c). Distinct Raman signatures of multiwalled CNTs were fitted into four Lorentzian curves, which can be assigned to D, D’’, G, and D’ peak located at ~1350, ~1469, ~1577, and 1612 cm-1 (red line in Figure 2c). The intensity ratio between D and G peaks of the CNTs before and after WS2 coating is ~0.96 and ~1.05, respectively. The comparable intensity of the D and G peaks indicates the presence of defects and disorder in the sp2hybridized carbon structure, which may have originated during sonication and dispersion of the CNTs. Note that these defective CNTs could be useful for improving the gas sensitivity, as imperfections provide chemisorption sites for NO2 molecules.37 In the Raman spectrum measured after WS2 coating (blue line in Figure 2c), the distinctive Raman signatures of WS2 at 350 cm−1 (in-plane mode, E12g) and 418 cm−1 (out-of-plane mode, A1g) emerge over the cellulose peaks (Figure 2d). The difference in peak position and the intensity ratio of E12g and A1g are ~68 cm−1 and ~1.29, respectively, indicating that there are two to three layers of WS2.38 The Raman spectra clearly demonstrate successful integration of CNTs and WS2 on the cellulose paper.


8

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We also characterized the elemental composition of the CNT-WS2-coated cellulose paper using X-ray photoelectron spectroscopy (XPS), as shown in Figure 2e–g. The core-level C 1s spectrum of the CNTs (Figure 2e) was deconvoluted into multiple peaks. The major peak, which is centered at 284.4 eV, originates from sp2-hybridized carbon. Other peaks at 285.2, 285.9, 287.4, and 291.0 eV can be assigned to sp3 bonding, C–O–, C=O, and –COO–, respectively, in agreement with previous studies.39 The sp3 and oxidized carbon structures can be considered as defect sites that may promote molecular adsorption; this result is also consistent with the presence of the D peak in the Raman spectrum. The W 4f core-level XPS spectrum has a 4f7/2 and 4f5/2 doublet at binding energies of ~32.68 and 34.98 eV, respectively (Figure 2f). In addition, the binding energies of S 2p3/2 and 2p1/2 are approximately 162.38 and 163.58 eV (Figure 2g). The XPS spectra of WS2 deposited on the CNTs show a slight blue shift compared to that in a previous report on pristine WS2.31 As the shift in binding energy is correlated with a change in the Fermi level, this blue shift can originate from n-type doping (an increase in the electron concentration) of WS2.40 The reason could be hole transfer from WS2 to the CNTs driven by the Fermi level difference between them, which is further verified by measuring the resistance change of the CNTs as a function of the number of WS2 coatings.


9


Page 10 of 29

Figure 3. Electrical characterization of cellulose paper (100 mm2) with respect to number of (a) CNT and (b) WS2 coatings. Increasing the number of CNT coatings on the cellulose paper dramatically decreases the resistance, and the CNT network serves as both an electrical conductor and a chemiresistor. With three CNT coatings, the resistance becomes measurable (1.625 MΩ), and it finally saturates down to 10.7 kΩ. Increasing the number of WS2 coatings on the CNT-coated cellulose paper also decreases the resistance. Note that the effect of the dispersing solvent (DMF) on the resistance of the CNTs was excluded. Locally deposited WS2 enabled charge transfer to the CNTs, resulting in a decrease in the resistance to 2.5 kΩ. Insets in (a) and (b) show the corresponding linear I–V curves.

Increasing the number of CNT coatings dramatically decreases the resistance of the cellulose paper (Figure 3a). The initial resistance was not measurable (>1 GΩ), and it became saturated down to ~10.7 kΩ after several cycles of CNT coating and drying. As DMF, which we used as a solvent to disperse the nanomaterials, could interact with the CNTs, the CNT-coated cellulose paper was immersed in pure DMF and dried as a control experiment. Possibly owing to the interaction between the nanomaterial and solvent,41,42 the resistance of the CNTs increased as the number of interactions with DMF increased (Figure S4a, Supporting Information). However, when the CNT-coated cellulose was immersed and dried in the WS2-DMF dispersion, the increasing trend of the resistance was suppressed, indicating that the locally deposited WS2 ACS Paragon Plus Environment

10

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decreased the resistance of the CNT network (Figure S4b,c, Supporting Information). Thus, interaction of the CNTs with DMF can be excluded. The resistance change is plotted versus the number of WS2 coatings in Figure 3b. CNTs and WS2 reportedly behave as a p-type semiconductor when exposed to ambient air,43 and as shown by the XPS studies, the Fermi level difference between CNT and WS2 could transfer holes from WS2 to CNT, increasing the number of carriers in the CNTs and ultimately decreasing the resistance (Figure S5, Supporting Information). The insets in Figure 3a,b show the corresponding linear I–V curves with respect to the number of nanomaterial coatings. Note that the fabrication process is highly reproducible, as there is minimal sample-to-sample variation (Figure S6, Supporting Information).


11


Page 12 of 29

Figure 4. NO2 sensing response of CNT-WS2-cellulose paper. (a) Transient sensing response of CNT- and CNT-WS2-coated paper under repeated exposure to 10 ppm NO2 in air environment. The resistance decreases upon exposure to NO2 in both cases, and functionalization of the CNTs with WS2 greatly improves the response (|ΔR/R0|), by more than 150%. (b,c) NO2-concentration-


12

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dependent response of the CNT-WS2 hybrid from 0.1 to 0.6 ppm (b) and from 1 to 10 ppm (c). The response increases in proportion to the NO2 concentration. (d) Sensitivity (response per ppm) of the sensor which is well fitted to the Langmuir adsorption model (red line). Alternatively, the sensitivity can also be interpreted as two linear regions at 0.1–2 ppm (R2=0.985) and 2–10 ppm (R2=0.953) (blue lines). The high sensitivity (4.57% ppm−1) at low NO2 concentrations (

Hybrid Integration of Carbon Nanotubes and Transition Metal

Recommend Documents