Long-term stable NbSe2 nanosheet aqueous ink for printable electronics

Journal Pre-proofs Short Communication Long-term Stable NbSe2 Nanosheet Aqueous Ink for Printable Electronics Hyemin Park, Jun Yeob Kim, Jin Young Oh, Tae Il Lee PII: DOI: Reference:

S0169-4332(19)33158-7 https://doi.org/10.1016/j.apsusc.2019.144342 APSUSC 144342

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

9 August 2019 4 October 2019 8 October 2019

Please cite this article as: H. Park, J. Yeob Kim, J. Young Oh, T. Il Lee, Long-term Stable NbSe2 Nanosheet Aqueous Ink for Printable Electronics, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144342

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Long-term Stable NbSe2 Nanosheet Aqueous Ink for Printable Electronics Hyemin Park1, Jun Yeob Kim1, Jin Young Oh2*, Tae Il Lee1* 1Department

of Materials Science and Engineering, Gachon University, Seongnam, GyeonggiDo 461-701, South Korea 2Department

of Chemical Engineering, Kyung Hee University, Yongin 17104, South Korea

* Corresponding author. E-mail: [email protected], [email protected] Keywords: transition metal dichalcogenide, niobium diselenide (NbSe2), chemical exfoliation, antioxidant, thermoelectric generator Abstract Despite the rapid development of mass production methods of two-dimensional (2D) transition metal dichalcogenide (TMD) materials in liquid media, long-term stability of these materials in aqueous solution is a major issue. In this study, the instability of chemically exfoliated niobium diselenide (NbSe2) nanosheets (NSs) dispersed in aqueous solution is systematically investigated, and it is revealed that the instability originates from the oxidation of NbSe2 NSs in the aqueous solution, which leads to the precipitation of selenium and restacking of NbSe2 NSs, degrading the electrical property of the NbSe2 NSs. On the basis of the results, for the first time, we propose an efficient method to prevent the oxidation of NbSe2 NSs using an antioxidant (L-ascorbic acid) that effectively consumes the oxygen dissolved in the aqueous solution, rendering the NbSe2 NSs antioxidative during the redox reaction in the aqueous solution, and thereby, improving the long-term stability of the NbSe2 NS ink. Finally, we demonstrate the fabrication of a thermoelectric generator by printing process with our long-term

1

stable NbSe2 NS ink. The thermoelectric power factor was nearly maintained up to 87% regardless of ink storage time even for 30 days.

1. Introduction Two-dimensional (2D) transition metal dichalcogenide (TMD) crystalline nanomaterials have been intensively applied in the fields of electronics [1-6] and energy harvesting technology [7-12] because they exhibit excellent performances, which are attributed to their unique physical and chemical properties originating from their atomic scale thicknesses with large surface area of a specific crystal plane. To utilize the strengths of 2D TMD materials, there have been many proposals in various aspects, and in energy storage and conversion fields have especially approached the possibility of industrialization based on the successful development of their liquid phase mass production processes of the 2D TMD.[13] For printing of 2D TMD materials, liquid-phase mass production techniques based on the following two approaches have been developed: mechanical exfoliation using tensile/shear stress and chemical exfoliation by alkali metal intercalation. The mechanical exfoliation method has several advantages: denaturation of raw materials is minimum, the washing step can be avoided, and the solvent used to produce the 2D TMD material dispersion prevents the oxidation of the materials. However, the production yield of 2D TMD monolayers is reported to be extremely low (~10%).[13] On the other hand, a yield of nearly 100% has been achieved via the chemical exfoliation method;[14] nonetheless, the disadvantages of this method are long-term instability of 2D TMD dispersions and oxidation of the materials in aqueous printing inks. Particularly, when the 2D TMD aqueous ink stored for a long time, the 2D TMD materials were oxidized and the TMD monolayers were re-aggregated. Thus, although chemical exfoliation is an effective

2

method for mass production of 2D TMD monolayers, the instability of 2D TMD aqueous inks has not been fully understood, limiting its availability. Recently, Nurdiwijayanto et al. reported that the dispersion instability of chemically exfoliated 2D MoS2 in aqueous solution was closely associated with the reoxidation of the MoS2 nanosheets in aqueous solution, and they proposed a strategy to suppress the reoxidation process—storing the 2D MoS2 solution in an inert atmosphere.[15] Because 2D TMD materials comprise various elements with individual physicochemical properties, the mechanism related to the instability of these materials in aqueous solution should be separately studied. Selenide or telluride-based 2D TMD materials with unique electronic properties are more easily oxidized than the sulfide-based TMD materials; thus, prevention of deterioration of these 2D TMD materials in aqueous solutions is highly important. For example, the chemically exfoliated 2D monolayers of the selenide-based TMD material niobium diselenide (NbSe2) with semi-metallic electrical property[16] has been applied in supercapacitors,[17] p-type thermoelectric devices,[11] and organic electronic devices (as a hole-transporting layer)[18] in the form of a stacked film fabricated by ink jet printing, spray coating or filtration. However, the rapid deterioration of 2D NbSe2 in aqueous solution is a commonly experienced serious obstacle that limits its use in the abovementioned applications; however, no studies on this issue have been reported. In this study, we have systematically investigated the mechanism related to the instability of chemically exfoliated NbSe2 nanosheets (NSs) in aqueous solution and demonstrated a method, for the first time, to effectively improve its dispersion instability. Notably, our method involves the use of the antioxidant L-ascorbic acid (L-AA) commonly called vitamin C. L-AA which is eco-friendly, cheap and non-toxic to human compared to any other chemical used for similar 3

purpose can be potentially used as a stabilizer for long term stability of NbSe2 NS ink for printing process based on that the key agent L-AA for long term stabilizer of NbSe2 NS is.

2. Materials and methods Materials: NbSe2 bulk powder was purchased from Alfa Aesar. n-Butyllithium solution (1.6 M in hexane), hexane (anhydrous, 95%), and L-AA (reagent grade, crystalline) were purchased from Sigma Aldrich. Mixed cellulose ester membranes with 0.1 μm pores were purchased from Advantec. Preparation of NbSe2 NSs: Chemically exfoliated NbSe2 NSs were synthesized by the Li+ intercalation process. First, 3 ml of n-butyllithium and 50 ml of hexane were added to 0.15 g of bulk NbSe2 powder under an N2 atmosphere. The resulting solution was heated at 90 °C and stirred for 48 h. Next, lithium-intercalated NbSe2 (LixNbSe2) was centrifuged at 3000 rpm for 5 min. The excess lithium ions and residues were removed with hexane. Subsequently, LixNbSe2 was exfoliated into monolayers by hydrolysis under ultrasonication in deionized (DI) water for 90 minutes. The resulting solution was centrifuged at 3000 rpm for 15 min several times to remove unexfoliated bulk NbSe2, and the obtained suspension was centrifuged at 10000 rpm for 15 min to remove residual lithium ions, yielding high purity NSs. An appropriate amount of LAA was added to 50 ml of 0.01 wt% NbSe2 NS suspension and stored under an N2 atmosphere. Characterization of NbSe2 NSs: NbSe2 NS suspension (dispersed in DI water) was vacuum filtrated through a filter membrane with 100 nm pores. The optical properties of the NbSe2 suspension were measured in the wavelength range of 300–700 nm using a UV-vis absorption spectrometer (JASCO, V-670). To confirm the chemical composition of the NbSe2 NSs, XPS (K4

alpha, Thermo Scientific, UK) was performed with the monochromatic Al K radiation (1486.6 eV). Moreover, the NSs were characterized by EDS, coupled with TEM (JEM-F200, JEOL). XRD (Rigaku Ultima IV) was performed to examine the crystallinity of the prepared materials. The morphology of NbSe2 NSs was investigated by SEM (JSM-7500F, JEOL) and OM (BX60F5). Thermoelectric device fabrication and thermoelectric property measurement: Thermoelectric devices were fabricated using a modified contact printing method. Typically, a vacuum-filtrated NbSe2 film was printed on a polydimethylsiloxane (PDMS) substrate along the region of hydrophilic surface area formed by a patterned oxygen plasma treatment. The NbSe2 film could be selectively printed on the hydrophobic area of PDMS because the surface of the vacuumfiltrated NbSe2 film was highly hydrophobic. The Seebeck coefficient and thermoelectric output power of the printed NbSe2 thermoelectric devices were investigated with a homemade measuring device, which included a Keithley 2400 sourcemeter, Keithley 2700 multimeter, and Keithley 2182a nanovoltmeter.

3. Results and discussion The schematic in Figure 1a shows the formation of long term stable aqueous NbSe2 NS ink. Aqueous dispersions of chemically exfoliated NbSe2 NS were prepared by a well-known method.[11] As the aging time of the NbSe2 solution increases to 30 days, the NbSe2 NS solution turns to reddish brown from black (Figure S1). The optical property transformation of the NbSe2 NS solution can be attributed to the oxidation of NbSe2 NSs by the oxygen dissolved in the aqueous solution, and this leads to a significant change in the energy band of NbSe2 NSs, which is related to their crystal structure and composition. The decrease in the intensity of the main 5

UV-vis absorption peak of the NbSe2 NS solution (Figure S1) with aging time supports the changed energy band. The reduction of the peak intensity is attributed to the destruction of the NbSe2 NS crystal structure by oxidation, because the absorption peak is related to the energy band structure of NbSe2 NSs.[19] To verify oxidation-induced degradation of NbSe2 NSs, the antioxidant L-AA was added to the NbSe2 NS solution, because the water-soluble L-AA produces electrons and hydrogen ions through the reversible redox reaction (Figure 1), effectively consuming the oxygen dissolved in the aqueous solution, and thereby preventing oxidation of NbSe2 NS in aqueous solution (O2 + 4H+ + 4e- → 2H2O). Thus, the NbSe2 NS solution with L-AA (0.3 M) exhibits the original color

(black) even after 30 days (Figure S1). The retention of NbSe2 NS solution color after the addition of L-AA implies that NbSe2 NSs are structurally and compositionally well preserved. It is to be noted that to achieve dispersion stability of NbSe2 NSs in aqueous solution, we used various antioxidants and tested their performances. The results of the solution dispersion tests performed in the presence of the antioxidants show that only L-AA can retain the initial dispersion condition of the solution for several days (see Figure S2). To further confirm the effect of L-AA, the electrical properties and morphology of the NbSe2 NSs with and without L-AA were investigated. Compared to the color of an inorganic material, its electrical conductivity is more sensitive to the changes in its internal structure; therefore, degradation of NbSe2 NSs can be confirmed from the changes in their electrical conductivity. First, the resistance, which reflects electrical conductivity, of the NbSe2 NS film deposited by vacuum filtration of the NbSe2 NS solution was measured as a function of the L-AA content up to 0.3 M, as shown in Figure 1b. As the aging time increases, the resistance of the NbSe2 NS film without L-AA rapidly decreases. The R0/R ratio of the NbSe2 NS film without L-AA drops to 6

0.05 in 30 days; notably, the R0/R ratios of the NbSe2 NS films with L-AA are higher than that of the control NbSe2 NS film. For the NbSe2 NS film with 0.3 M of L-AA, the R0/R ratio is 0.87 even after 30 days. In addition to the changes in the electronic properties, as discussed above, morphological changes are observed for the NbSe2 NSs, indicating their dispersion instability. Figure 1c shows the optical microscopy (OM, left side) and scanning electronic microscopy (SEM, right side) images of dried aqueous solution of NbSe2 NSs aged for 30 days. The images show microscale aggregates of NbSe2 NSs and deposits of unknown nanoparticles on the aggregated NbSe2 NSs. On the other hand, the dried aqueous solution containing NbSe2 NSs and L-AA (0.3 M) is relatively uniform without any submicron particles. This result indicates that the morphological change is related to the oxidation mechanism or some other degradation mechanism. First, to identify the unknown nanoparticles observed in the SEM image of the NbSe2 NSs, the microstructure of the NbSe2 NSs aged in aqueous solution for 30 days was analyzed by TEM. Figure 2a shows the bright field image and selected area electron diffraction (SAED) pattern (recorded from the region marked with a white square) of the NbSe2 NS sample. The image shows an NbSe2 NS with a surface area of a few microns; in addition, several nanoparticles are observed on the NS surface. The SEAD pattern shows two different sets of spots, which are marked with red and yellow dotted circles. The diffraction spots marked with the yellow and red dotted circles (Figure 2a) can be indexed to trigonal prismatic (2H)-NbSe2 (along the [0001] zone axis) and trigonal (t)-Se (along the [0001] zone axis), respectively; the t-Se crystal is rotationally tilted by ~1.5° with respect to the basis of crystallographic direction of the 2H-NbSe2 NS. Therefore, the bright field image indicates the presence of several t-Se nanocrystals on the (0001) surface of the 2H-NbSe2 NS with a small lattice mismatch. 7

To determine the chemical composition of the nanoparticles observed on the NbSe2 NS, TEM energy-dispersive spectroscopy (EDS) elemental maps were obtained from the area marked with a white square in Figure 2b; points 1 and 2 were further investigated, as shown. The area mapping image shows that the nanoparticles are mostly composed of selenium, while the NbSe2 NS is composed of both niobium and selenium. The chemical compositions of the particles and NbSe2 NS, at points 1 and 2, respectively, were determined (see Figure S3). The chemical composition of the particle (point 1) is as follows: 11.95 at%, selenium 84.97 at%, and niobium 3.07 at%, and The atomic ratio of selenium to niobium (point 2) in NbSe2 NS is 1.84, indicating selenium deficiency in the NS due to oxidation. Thus, the result implies that the NbSe2 NS undergoes partial oxidation, leading to the formation of niobium oxide and precipitation of selenium nanoparticles on the NS surface. This chemical reaction can be expressed by the following equation: 2NbSe2 + 5O2 → 2Nb2O5 + 4Se. Moreover, the effect of L-AA (0.3 M) addition on the morphological change of NbSe2 NSs in aqueous solution was investigated by TEM; the TEM image is shown in Figure 2c. Notably, no particles are observed on the NbSe2 NS, and the SAED pattern can be indexed to 2H-NbSe2. The chemical composition of the well-preserved NbSe2 NS was determined from the TEM EDS element maps obtained from the area marked with a white square and point 3, as shown in Figure 2d. The area element mapping image indicates uniform distribution of niobium and selenium. The chemical composition of the well-preserved NbSe2 NS determined at point 3 by TEM EDS is as follows: oxygen 48.1 at%, selenium 34.43 at%, and niobium 17.47 at% (see Figure S3). The ratio of selenium to niobium in the NbSe2 NS is 1.97, indicating that the chemical composition of NbSe2 NS is well-maintained in aqueous solution even after 30 days.

8

From the TEM results, we can determine the mechanism related to the degradation of the NbSe2 NS in aqueous solution and the role played by L-AA in protecting it against degradation. However, these results are not applicable to the entire NbSe2 NS sample because TEM is a tool for localized inspection in the nanoscale. Therefore, the changes in the chemical composition of the NbSe2 NSs were investigated using the microscale analysis tool X-ray photoelectron spectroscopy (XPS). Full spectra of the XPS profiles of NbSe2 NSs were shown in Figure S4. Specific peaks related with niobium (Nb), selenium (Se), and oxygen (O) in the full spectra were presented in Figure S5, and the peak were fitted using Casa XPS software for peak deconvolution. Information about all peaks was summarized in Table 1. The fitted XPS profiles of NbSe2 NSs with the Nb4+ (3d5/2) and Nb5+ (3d5/2) peaks were presented in Figure 3a. The green, red, and blue peaks are assigned to 1T-phase NbSe2 (1T-NbSe2), 2H-phase NbSe2 (2HNbSe2), and Nb2O5, respectively. The Nb2O5 peak intensities for the NbSe2 NS sample aged for 30 days are significantly higher than those for the pristine NbSe2 NS sample. Moreover, for the aged sample, the Nb2O5 peaks are considerably stronger than the other peaks On the other hand, the intensities of the Nb2O5 peaks for the NbSe2 NS sample stored with L-AA (0.3 M) for 30 days are similar to those for the pristine NbSe2 sample, indicating that L-AA effectively protects the NbSe2 NSs against oxidation in aqueous solution. In addition, the selenium XPS profile for the NbSe2 NSs is shown in Figure 3b. The orange, pink, and purple peaks are assigned to Se2-, amorphous phase Se (a-Se), and trigonal phase Se (t-Se), respectively. The a-Se and t-Se peak intensities for the NbSe2 NS sample stored for 30 days are significantly higher than those for the NbSe2 NS sample stored with L-AA and the pristine NbSe2 NS sample. Thus, the XPS and TEM results indicate that the NbSe2 NSs are prone to oxidation in aqueous solution and L-AA

9

effectively prevents oxidation of NbSe2 NSs, imparting chemical stability to the NbSe2 NSs in aqueous solution. Thus far, we focused on the elemental composition change of NbSe2 NS in aqueous solution, resulting in the reduction of conductivity, apparent color shift, and precipitation of selenium. Reminding the Figure 1c, we should consider the formation of several tens of micrometer sized particles after 30 days of aging, which originates from re-staking by losing negative charge of NbSe2 NS. [20-22] The degree of dispersion of nanomaterials in liquid media is a typical longterm stability evaluation indicator for application as inks, nanomaterials should exhibit high dispersibility. The restacking of exfoliated NSs in aqueous solution is a practical issue that must be overcome for long-term storage. To determine the extent of restacking of NbSe2 NSs aged for 30 days, XRD was performed; the XRD patterns are shown in Figure 3c. The XRD pattern of NbSe2 powder is provided for reference. The pristine NbSe2 NS film formed by vacuum filtration shows the dominant (0002) peak because the NbSe2 powder fully exfoliates into NbSe2 NSs with {0002}-oriented surfaces in aqueous solution. As observed, the crystallinity of the NbSe2 NS film prepared with the NbSe2 NS solution stored for 30 days is significantly higher than that of the pristine NbSe2 NS film. The enhanced crystallinity is attributed to an increase in the thickness of the NbSe2 NSs caused by restacking of exfoliated NbSe2 NSs in aqueous solution. The negatively charged NbSe2 NSs formed by lithium-assisted chemical exfoliation become electrostatically neutral in aqueous solution because the oxygen dissolved in the solution forces the transfer of the negative charge to water molecules; this results in the disappearance of electrostatic repulsive forces among the NbSe2 NSs, which leads to restacking of neutrally charged NbSe2 NSs in aqueous solution via van der Waals forces. The corresponding reaction can be described by the following equation: NbSe2 ― 10

(H2O) ―1/2H2

NbSe20 + OH ― .[23] Notably, even

after 30 days, the crystallinity of the NbSe2 NSs stored with L-AA (0.3 M) is lower than that of the NbSe2 NSs without L-AA (similar to that observed for pristine NbSe2 NSs). This result indicates that L-AA effectively preserves the electrostatic repulsive forces among the NbSe2 NSs by maintaining their negatively charged state in aqueous solution; the chemical reaction by which the dispersibility of the NbSe2 NSs is maintained can be described by the following equation: NbSe20 + OH

-

+ H + + e ― → NbSe2 ― + H2O. The hydrogen ion (H+) produced by

L-AA in aqueous solution captures the hydroxyl ion (OH-) and the electron from L-AA imparts negative charge to the NbSe2 NS, which leads to the formation of a water molecule and NbSe2-, thereby facilitating long-term dispersion stability of the NbSe2 NSs in aqueous solution. To fabricate an electronic device with NbSe2 NS inks, long-term stable dispersion of NbSe2 NSs is required. During fabrication, the chemical composition and thickness of NbSe2 NSs in the ink have to be maintained for normal operation and high performance of the electronic device. Thus, storage of NbSe2 NS inks without degradation of NbSe2 NSs is highly important in a practical fabrication process. To confirm the applicability of our strategy to preserving NbSe2 NSs in aqueous inks, we fabricated thermoelectric devices by spray coating with the ink, as shown in Figure 4a. Inks were prepared with or without L-AA, and thermoelectric devices were fabricated with these inks, which were aged for 1, 3, 7, 10, 20, and 30 days. Figure 4b and 4c present the variations in the Seebeck coefficient (S) and power factor (PF = S2σ, where σ is conductivity) of the thermoelectric devices fabricated with and without L-AA, respectively, as a function of the aging time. The thermoelectric properties (S and PF) of the device fabricated with the NbSe2 NS ink stored with L-AA (0.3 M) are nearly maintained, regardless of the storage time ( up to 30 days), while those of the device fabricated with the NbSe2 NS ink stored without LAA strongly depend on the storage time. 11

4. Conclusions In summary, we investigated the instability of chemically exfoliated NbSe2 NSs in aqueous solution and the related mechanism to develop long-term stable NbSe2 NS inks. We observed that the well-dispersed NbSe2 NSs were venerable to oxidation by oxygen dissolved in the aqueous solution, leading to formation of niobium oxide and precipitation of selenium nanoparticles on the NS surfaces. In addition, the negatively charged NbSe2 NSs formed by lithium-based chemical exfoliation easily lost their charge to water molecules via an oxidation reaction; consequently, the NbSe2 NSs became electrostatically neutral and restacked, forming thick NbSe2 sheets, because of instability of the NbSe2 NSs in aqueous media. To inhibit the action of oxygen dissolved in the aqueous solution and achieve long-term stability, the antioxidant L-AA was added into the NbSe2 NS solution. L-AA effectively consumed the dissolved oxygen, and hydrogen ions and electrons were generated during its redox reaction, rendering NbSe2 NSs antioxidative and significantly enhancing their dispersibility in aqueous solution. Finally, with our long-term stable NbSe2 NS ink, we fabricated thermoelectric devices by printing process; their initial thermoelectric properties (S and PF) were nearly maintained, regardless of the ink storage time (up to 30 days). We believe this result become a trigger to develop a preservative for various long-term stable TMD nanosheet inks.

References [1] Bernardi, M., Palummo, M., Grossman, J. C., Nano Lett. 13 (2013) 3664-3670.

12

[2] Lee, H. S., Min, S. W., Chang, Y. G., Park, M. K., Nam, T., Kim, H., Kim, J. H., Ryu, S., Im, S., Nano Lett. 12 (2012) 3695-3700. [3] Withers, F., Del Pozo‐Zamudio, O., Mishchenko, A., Rooney, A. P., Gholinia, A., Watanabe, K., Taniguchi, T., Haigh, S. J., Geim, A. K., Tartakovskii, A. I., Novoselov, K. S., Nat. Mater. 14 (2015) 301-306. [4] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, M. S. Strano, Nat. Nanotechnol. 7 (2012) 699-712. [5] Lee, S., Zhong, Z., Nanoscale 6 (2014) 13283-13300. [6] Wi, S., Kim, H., Chen, M., Nam, H., Guo, L. J., Meyhofer, E., Liang, X., ACS Nano 8 (2014) 5270-5280. [7] Chia, X., Eng, A. Y. S., Ambrosi, A., Tan, S. M., Pumera, M., Chem. Rev. 115 (2015) 11941-11966. [8] Wang, H., Feng, H., Li, J., Small 10 (2014) 2165-2181. [9] Stephenson, T., Li, Z., Olsen, B., Mitlin, D., Energy Environ. Sci. 7 (2014) 209-231. [10] Gao, M.‐R., Xu, Y.‐F., Jiang, J., Yu, S.‐H., Chem. Soc. Rev. 42 (2013) 2986-3017. [11] Oh, J. Y., Lee, J. H., Han, S. W., Chae, S. S., Bae, E. J., Kang, Y. H., Choi, W. J., Cho, S. Y., Lee, J.‐O., Baik, H. K., Lee, T. I., Energy Environ. Sci. 9 (2016) 1696-1705. [12] Voiry, D., Yang, J., Chhowalla, M., Adv. Mater. 28 (2016) 6197-6206. [13] Niu, L., Coleman, J. N., Zhang, H., Shin, H., Chhowalla, M., Zheng, Z., Small 12 (2016) 272-293. [14] Chhowalla, M., Shin, H. S., Eda, G., Li, L.‐J., Loh, K. P., Zhang, H., Nat. Chem. 5 (2013) 263-275. [15] L. Nurdiwijayanto, R. Ma, N. Sakai and T. Sasaki, Inorg. Chem. 56 (2017) 7620-7623. 13

[16] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, Two-dimensional atomic crystals, Proc. Natl Acad. Sci. 102 (2005) 10451-10453. [17] Bissett, M. A., Worrall, S. D., Kinloch, I. A., Dryfe, R. A. W., Electrochim. Acta 201 (2016) 30-37. [18] Gu, X., Cui, W., Song, T., Liu, C., Shi, X., Wang, S., Sun, B., ChemSusChem 7 (2014) 416. [19] G. E. Myers and G. Montet, J. Appl. Phys. 41 (1970) 4642. [20] B. K. Miremadi, T. Cowan, S. R. Morison, J. Appl. Phys., 69 (1991) 6373. [21] M. Acerce, D. Voiry, M. Chhowalla, Nat. Nanotech., 10 (2015) 313-318. [22] X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, Adv. Funt. Mater., 15 (2015) 1393-1403. [23] A. S. Golub, Ya. V. Zubavichus, Yu. L. Slovokhotov, Yu. N. Novikov, Russ. Chem. Rev. 72 (2003) 123-141.

14

Figure 1. (a) Schematic illustration of overall concept for redox stable NbSe2 NSs dispersed in aqueous solution. L-AA is used as a reducing agent to prevent oxidation of NbSe2 NSs in aqueous solution. Inset: unit cell of NbSe2 monolayer with trigonal prismatic structure and equation describing L-AA redox reaction. (b) Variation in the normalized resistances of NbSe2 NS films with different L-AA contents (0.05 M to 0.3 M) as a function of aging time of NbSe2 NS solution. (c) OM and SEM images of dried NbSe2 NS solution without (top) and with (bottom) L-AA after aging for 30 days.

15

c

a

with L-AA (0.3M) NbSe2 NS SAED

SAED

50nm

500nm

500nm

NbSe2 NS

50nm

d

b 1

3

2 500nm

1 μm

100nm

100nm

Nb

100nm

1 μm

100nm

O

100nm

Se

100nm

100nm

Nb

100nm

100nm

O

100nm

Se

Figure 2. Bright field TEM images (left) and SAED patterns (right) of NbSe2 NSs (a) stored without and (c) with L-AA in aqueous solution for 30 days. Scanning TEM EDS element mapping images of NbSe2 NSs obtained from the area marked with white squares in (b) Figure 2a and (d) Figure 2c.

16

Figure 3. XPS profiles of NbSe2 NSs stored in aqueous solution for 30 days (a) without and (b) with L-AA. (c) XRD patterns of NbSe2 powder and NbSe2 NS films.

17

Figure 4. (a) Schematic of thermoelectric device fabrication by printing process

using the

developed NbSe2 NS ink. Variations in the (b) Seebeck coefficient and (c) power factor of NbSe2 NS-based thermoelectric devices as a function of aging time.

18

Graphical abstract

Research highlights ‧Long-term stable niobium diselenide (NbSe2) nanosheet aqueous ink for printable electronics is demonstrated using an antioxidant (L-ascorbic acid, L-AA). ‧L-AA efficiently protects the NbSe2 nanosheet against oxidation, aggregation, and decomposition in aqueous solution. ‧Thermoelectric generators fabricated by printing process of the NbSe2 nanosheet aqueous ink preserve their thermoelectric properties, regardless of the aging time of the ink (up to 30 days).

19