Design of dispersant for highly concentrated one-dimensional Nb2Se9 inorganic molecular chains from bulk crystal

We determined the optimum dispersant to separate bulk Nb2Se9 material into 1D chain units. The Nb2Se9, which had a negative zeta potential (−43.3 mV), showed acidic characteristics and strongly bonded with the amine head of octadecyl amine through a charge transfer (from the amine to Se atoms) reaction. The steric hindrance of the octadecyl tail resulted in excellent dispersion of Nb2Se9 (down to nanometre-sized mono-chains).

We determined the optimum dispersant to separate bulk nb 2 Se 9 material into 1D chain units. The nb 2 Se 9 , which had a negative zeta potential (−43.3 mV), showed acidic characteristics and strongly bonded with the amine head of octadecyl amine through a charge transfer (from the amine to Se atoms) reaction. the steric hindrance of the octadecyl tail resulted in excellent dispersion of nb 2 Se 9 (down to nanometre-sized mono-chains).
Low-dimensional materials have opened new areas of research, and their advanced applications have attracted the interest of the materials community. In particular, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDCs), and black phosphorous have been attracting much attention due to their unique physicochemical and electromagnetic properties, and many studies have been conducted on transistors, superconductors and optoelectronic devices [1][2][3][4][5] . Unlike the low-dimensional material obtained by simply reducing the size from the bulk material, the above-mentioned 2D materials are obtained by physically or chemically exfoliating the material having the weak interlayer binding by van der Waals (vdW) force. The resulting 2D materials have unique structural features, such as atomic thicknesses, a lack of dangling bonds on their surfaces, and flexibility.
Recently, the authors successfully prepared a novel 1D bulk crystal, Nb 2 Se 9 . The crystal was synthesised by a chemical reaction between Nb and Se in an evacuated quartz ampoule, could be reproduced in large quantities, and was stable in air 21,22 . These properties are essential for use in subsequent processes and device applications. It is important to isolate inorganic nanoscale chains from this bulk crystal to study the material properties or device applications [23][24][25] . For this purpose, the authors dispersed Nb 2 Se 9 crystals in a solvent because the solvent exfoliation method can be used to obtain large sample quantities simply and hence, has been widely used for dispersing CNT bundles 26,27 and 2D materials 28 . However, it was very difficult to prepare a high-concentration solution by dispersing Nb 2 Se 9 crystals with only a solvent in our experiment 29 . Furthermore, the surface characteristics of the Nb 2 Se 9 crystals and the functional groups that can be adsorbed effectively remain unexplored. In this study, we designed the chemical structure of a dispersant for the nano-dispersion of Nb 2 Se 9 crystals to prepare a highly www.nature.com/scientificreports www.nature.com/scientificreports/ concentrated solution of Nb 2 Se 9 nanowires and to verify whether single molecular chains could be obtained from this solvent. This not only provides basic information on the surface of Nb 2 Se 9 , but it can also provide guidelines for further chemical reactions utilizing it.

Result and Discussion
Nb 2 Se 9 is composed of chain-shaped molecular units with strong covalent bonds (Nb atoms are decorated by Se atoms) that are assembled in a crystal structure via weak vdW attractions between chains (Fig. 1a). During dispersion, mono-chains can be exfoliated from the 3D crystal due to the weak interactions between chains (Fig. 1b). Single crystalline Nb 2 Se 9 was grown via a chemical reaction between Nb and Se in an evacuated quartz ampoule. When the Nb-Se mixture at 700-800 °C was cooled to room temperature, dark grey needle-shaped bulk crystals were formed, and XRD analysis confirmed that the material had a well-crystallised Nb 2 Se 9 phase (Fig. 1c). Figure 1d shows the SEM image of the Nb 2 Se 9 crystals prepared in this study. It was observed that some Nb 2 Se 9 was naturally exfoliated in the form of a chain. Consequently, the material synthesised in this study could be dispersed as 1D units using a suitable dispersant.
In general, the structure of a dispersant is composed of head and tail groups. To obtain nanoparticles dispersed at a high concentration using a dispersant, the following two conditions must be satisfied 30 . First, the head groups of the dispersant must be firmly fixed to the surfaces of the nanoparticles. Second, the tail groups of the dispersants should be highly solubilised to strongly repel the nanoparticles through steric hindrance. In this experiment, 2-propanol (IPA) was used as the main solvent of the Nb 2 Se 9 dispersant because it showed good dispersing performance in a test experiment using only the solvent. Three types of head groups were considered for the Nb 2 Se 9 dispersion: an acidic carboxylic acid group, a neutral hydroxyl group, and a basic amine group. The octadecyl-alkyl group, which is highly soluble in IPA and has a linear chain structure (for strong steric hindrance), was selected as the tail of the dispersant. Figure 2 shows the structure of the dispersants used in this study.
Nb 2 Se 9 crystals in solution were dispersed with the dispersant by sonication, and then were centrifuged to obtain a well-dispersed supernatant with large and un-exfoliated particles removed. Digital photographs of the dispersed solution before and after centrifugation are shown in Fig. 3a. The dispersions using no dispersant (only IPA) showed a moderate Tyndall effect, indicating that some exfoliation occurred. However, when the carboxylic acid and hydroxyl groups were used as the head groups of the dispersant, the Tyndall effect was reduced. The strongest Tyndall effect was found in the sample using the amine head group of the dispersant. The solubility and nano-dispersion of Nb 2 Se 9 chains can be analysed using UV-vis-NIR spectroscopy (Fig. 3b). The Nb 2 Se 9 with the amine head group showed the strongest absorption, indicating that a nano-dispersion of Nb 2 Se 9 was obtained. The concentration of the dispersion in octadecylamine was measured using inductively coupled plasma (ICP) mass spectrometry to be 79.2 μg mL −1 , which was about 1.6 times larger than for the sample without a www.nature.com/scientificreports www.nature.com/scientificreports/ dispersant (50.1 μg mL −1 ). When octadecylamine was used, the colloidal dispersion maintained for 7 days, which was slightly better than the 5 days maintaining without the dispersant (Fig. S1).
The adsorption of an organic polymer on inorganic materials in organic solvents can be explained through the acid-base theory (acidic polymers bind strongly to basic particles, while basic polymers bind strongly to acidic particles) [31][32][33] . Therefore, to understand the dispersion behaviour of Nb 2 Se 9 crystals, it is necessary to understand the acid-base characteristics on the surface of Nb 2 Se 9 . The surface potential of Nb 2 Se 9 in IPA was measured by a zeta potential meter and found to have a negative value of −43.3 mV (Fig. 4a), indicating that the surface of Nb 2 Se 9 has acidic characteristics. Because Nb 2 Se 9 has its own stable surface without dangling bonds, acid properties are induced by electrons localized on the surface of the material, not by various functional groups. Therefore, the acidic Nb 2 Se 9 was better dispersed when the amine head group with a basic characteristic was used in the dispersant rather than the neutral hydroxyl or acidic carboxyl head group. This adsorption mechanism of the amine head group on the Nb 2 Se 9 surface was confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Figures 4b-d present the XPS core level spectra (N 1s, Se 3d, and Nb 3d, respectively) to examine the charge  www.nature.com/scientificreports www.nature.com/scientificreports/ transfer reaction between the amine head group of the dispersant and Nb 2 Se 9 . First, the N 1s peak appeared clearly in the dispersed samples using octadecyl amine as a head. Second, a higher binding energy shift of about 0.5 eV occurred at the Se 3d core level due to n-type doping when electrons were transferred from the amine head group to the electronegative Se atoms in the Nb 2 Se 9 chains. When n-type doping occurs, the binding energy tends to increase because the Fermi energy level shifts to the conduction band and moves away from the core level. Since Nb atoms are surrounded by Se atoms in the Nb 2 Se 9 chains, a higher binding energy shift also occurred at the Nb 3d core level, but the XPS data were not clear enough to compare to Se 3d. Based on these results, we conclude that the strong adsorption of the amine head group to Se atoms in the Nb 2 Se 9 chains via charge transfer contributes to improving the dispersion of the Nb 2 Se 9 chains.
The dispersed Nb 2 Se 9 was spin-coated on SiO 2 /Si substrates, and the sizes of the nano-chains were analysed using atomic force microscopy (AFM). The results are shown in Fig. 5a for pure IPA and Fig. 5b for octadecyl amine used as dispersant. These results confirm that mono-chains (~1 nm scale in AFM) of Nb 2 Se 9 can be obtained by using an octadecyl amine dispersant instead of pure IPA. The effect of the dispersant can be more clearly seen in the statistical values of the AFM measurements (Fig. 5c,d). The average diameter of the Nb 2 Se 9 chains in the pure IPA dispersion was about 10.4 nm, while that in the octadecyl amine dispersant was less than half that size, with a value of 4.0 nm. Also, the aspect ratio (length / diameter) of the chains when octadecyl amine dispersant was used was more than double that of the pure IPA dispersion (531.39 for pure IPA dispersion and 1002.74 for octadecyl amine dispersant).

conclusion
In summary, we designed the ideal dispersant to exfoliate Nb 2 Se 9 inorganic molecular chains as 1D units from bulk crystals based on the acid-base theory. Zeta potential measurements confirmed that Nb 2 Se 9 had acidic surfaces (negative zeta potential of about −43.3 mV). It was confirmed that octadecyl amine, with an amine head group with a basic property, bound more strongly to the Nb 2 Se 9 surfaces than the acidic carboxyl or neutral hydroxyl head groups due to charge transfer between the amine and Se atoms, resulting in excellent dispersion to the nanometre level. The dispersant determined in this study is expected to be widely used for the study of new 1D Nb 2 Se 9 materials. and Se (99+ %, Alfa Aesar). A mixture of the starting elements in a molar ratio of Nb:Se = 1:200 was pelletised and then sealed in a 15 cm-long quartz tube with a neck in the middle of the tube. The evacuated quartz tube was heated to 800 °C for 72 h (at 5.5 °C h −1 ) and then naturally cooled to room temperature. Then, the unreacted Se flux was removed by inverting the quartz tube, causing the flux to fall to the other side of the tube, followed by heating in a box furnace at 250 °C for 12 h. Finally, the residual Se was sublimed in a tube furnace at 250 °C for 24 h under an Ar atmosphere. The resulting material was composed of grey needle-shaped crystals.
Dispersion. First, 10 mg of the obtained Nb 2 Se 9 was immersed in 20 mL of IPA. It was initially sonicated for 5 min with 2 s/2 s on/off intervals in a probe sonicator (VC 505, Sonics & Materials, Inc.) to crush the large crystals coarsely. After the first sonication, 10 mL of dispersant/IPA solution with a concentration of 1 mg mL −1 was added. Then, the solution containing the dispersant was sonicated again in a bath sonicator (B2005S-68K, 68 kHz, 200 W, KODO Technical) for 3 h. After the two ultrasonic steps, the solution was centrifuged at 6000 rpm for 10 min to remove the insufficiently dispersed chains. Finally, 10 mL of the supernatant solution was used for further analysis. characterization. Field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4300SE) was performed for morphological characterisation of Nb 2 Se 9 . Powder XRD (Mac Science, M18XHF22) was employed with Cu-K α radiation (λ = 0.154 nm). A Zetasizer Nano-ZS90 from Malvern Instruments, Ltd. (Worcestershire, UK) and UV-vis spectrophotometer (Agilent Technologies lnc., Agilent 89090A) were used to measure the zeta potentials and UV absorption of the dispersions. X-ray photoelectron spectroscopy (XPS) data was obtained using an ESCALAB250 from Thermo. The samples were prepared by a filtering method using an Anodisc filter (Whatman) with a pore size of 100 nm to form a solid film. To evaluate the morphologies of the exfoliated nanowires, atomic force microscopy (AFM, Park systems, NX10) was employed in non-contact mode. The samples were prepared by spin-coating on SiO 2 /Si wafers.

Data Availability
The data that support the findings of this study is available from the corresponding author upon request.