Submillimeter-Long WS2 Nanotubes: The Pathway to Inorganic Buckypaper

WS2 nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS2 nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000–5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W5O14 nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W18O49 nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS2 nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS2 nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.


Supplementary Experimental part
Powder X-ray diffraction X-ray powder diffraction (XRD) was performed using TTRAX III (Rigaku, Tokyo, Japan) theta-theta diffractometer.The set-up was equipped with a rotating copper anode X-ray tube operating at 50 kV/200 mA.The samples were prepared on a zero-background Si substrate.
They were scanned using the Cu Kα line (wavelength 1.54 Å) in specular diffraction mode (θ/2θ scans) from 10-80° (2θ) with a step size of 0.02° and a scan rate of 0.5° per min.

Electron microscopy
Scanning electron microscopy (SEM) analyses were performed using Thermo Fisher Scientific Apreo 2S SEM.
Cross-sections of the nanotubes for transmission electron microscopy (TEM) were prepared using a Thermo Fisher Scientific Helios -NanoLab 660.The intended region of interest was covered with a layer of amorphous carbon using electron beam-assisted deposition.An additional protecting layer was subsequently added using ion beam-assisted deposition.A chunk of deposited material containing the nanotube cross-section was transferred to a TEM copper grid according to a standard lift-out procedure.
TEM analysis was performed using a Thermo Fisher Scientific Talos F200i operated in high resolution (HRTEM) mode at a high accelerating voltage of 200 kV and a beam current of 1 nA.HRSTEM cross-sectional analysis was done using Thermo Fisher Scientific Talos F200X equipped with X-CFEG at 200 kV and beam current 50 pA.TEM images were post-processed using the Velox and ImageJ software.

Electrical Characterization
The ultralong WS 2 nanotubes were meticulously transferred onto a boron-doped Si wafer, which was covered with a SiO 2 layer approximately 115 nm thick.Selected nanotubes were contacted by electron beam lithography, utilizing an AR-P 679.04 (PMMA) resist (∼800 nm thick).For the electrical contacts, Cu (∼350 nm) and Au (∼20 nm) layers were evaporated using Ti (∼10 nm) as an adhesion layer (total thickness ∼ 380 nm) onto the developed sample, followed by a lift-off process.The electrical characterization was done at room temperature and atmospheric pressure utilizing a Cascade Microtech MPS 150 probe station and Keithley 4200-SCS parameter analyzer.The diameter of the tubes varies slightly along their growth axis (by approx.25 % maximum, as inferred from SEM images).Hence, the mean diameter value was used for the calculation of current density and conductivity.TEM analysis revealed that the nanotube's interior constituted approximately 70 % of WS 2 layers and 30 % of hollow cavity space.The calculations were performed accordingly to this ratio of hollow space and WS 2 walls.
Control leakage current measurements on the contacted nanowires have shown four orders of magnitude lower values within the investigated voltage range.

Photoluminescence measurement
PL measurements of the long WS 2 nanotubes were carried out on a Horiba LabRAM HR evolution micro-Raman system built around an Olympus BXFM modular microscope.The measurements were done using a 532 nm laser through 100x MPlanFL N NA = 0.9 (RT -100% = 2.5mW, 50% = 1.25mW, 10% = 0.225mW), 50x LMPlanFL N NA = 0.5 (LWD) (LT -10% = 0.4mW), and 10X MPlan N NA = 0.25 (RT -100% = 8mW, 50% = 4mW, 10% = 0.75mW) objectives.The PL spectra were dispersed on a 600gr/mm grating in an 800mm spectrograph with high resolution and low stray light.The low-temperature measurements utilized a Linkam LN2 THMS temperature-controlled stage.   .The diffractograms overlap to some degree with the major diffractions being identical to both structures.Therefore, XRD analysis alone was not able to give a definitive answer on the nature of the crystalline phase of the nanowhisker.HRSTEM analysis of the nanowhisker cross-section provided a definite confirmation of the W 5 O 14 phase.Possibly, the particles forming the bulk of the agglomerate (see Figure S3) belong to the W 20 O 58 phase, whereas the ultralong nanowhiskers on the surface are W 5 O 14 .

Figure S1 .
Figure S1.Photography of ampule before a) and after b) reaction.a) the dark blue/black powder of H 0.23-0.33WO 3 was evenly spread in the ampule.b) after the reaction the material was bright blue and in a compact layer slightly fixed to the ampule.

Figure S2 .
Figure S2.Powder X-ray analyses of the powder precursor a) and resulting nanowhisker layer b).a) The hydrogen tungsten bronze used as a precursor consisted of two tetragonal phases with different stoichiometries H 0.23 WO 3 (ICDD PDF 04-016-3510) and H 0.33 WO 3 (ICDD PDF 00-006-0706).b) The high-temperature transformation resulted in the monoclinic phases W 5 O 14 (ICDD PDF 00-041-0745) and W 20 O 58 (ICDD PDF 04-007-0501).The diffractograms overlap to some degree with the major diffractions being identical to both structures.Therefore, XRD analysis alone was not able to give a definitive answer on the nature of the crystalline phase of the nanowhisker.HRSTEM analysis of the nanowhisker cross-section provided a definite confirmation of the W 5 O 14 phase.Possibly, the particles forming the bulk of the agglomerate (see FigureS3) belong to the W 20 O 58 phase, whereas the ultralong nanowhiskers on the surface are W 5 O 14 .

Figure S3 .
Figure S3.Low magnification SEM image of the agglomerates with the W 5 O 14 nanowhiskers on the surface.

Figure S4 .
Figure S4.Low-magnification SEM image of the WS 2 nanotube film.Clearly, some of the nanotubes reach a fraction of a millimeter.

Figure S5 .
Figure S5.TEM analysis of the ultralong WS 2 nanotube fragment ripped out from the web during the grid preparation.The nanotube was inspected along its profile showing its morphological consistency.The tips of the fragment are jagged due to the tearing from the web.

Figure
Figure S6.a) TEM image of the curved nanotube.b) magnified portion of the curved nanotube from a) showing wrinkles in the concave layers buckled under a compression strain.The yellow arrow indicates the slight peeling off of the outer layers and minuscule wall cavity formation.c) magnified WS 2 layer wrinkle

Figure S7 .
Figure S7.Electron microscopy measurements of various other WS 2 morphologies in the sulfidated layer.a) SEM image of a mixture of WS 2 nanotubes, belts, and whiskers, usually found between the particles in the layer.In a number of cases the morphology of the specimen changes along its length, e.g. from a nanotube to a nanobelt and back, primarily induced by bending and strain.A similar example can be observed in b) marked by the yellow arrow.The nanotube cavity collapses into a WS 2 nanowhisker and later reappears again.c) example of WS 2 nanotube filled in the core by additional WS 2 layers.d) segment of the WS 2 nanobelt formed by bending and collapsing of the WS 2 nanotube.

Figure S8 .
Figure S8.SEM image of wet-laid regular WS 2 nanotubes (app.5 m long) showing a filtration cake, which was unable to arrange itself into a self-sustained paper or felt.When scratched, the layer disintegrated to powder, leaving a distinct black stain on the office paper.This showed the poor integrity of the film in comparison with the self-supporting paper prepared with ultralong WS 2 nanotubes (see Figure4in the main text).