Formation of Tungsten Oxide Nanowires by Electron-Beam-Enhanced

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C: Physical Processes in Nanomaterials and Nanostructures

Formation of Tungsten Oxide Nanowires by ElectronBeam- Enhanced Oxidation of WS Nanotubes and Platelets 2

Miroslav Kolíbal, Kristyna Bukvisova, Lukas Kachtik, Alla Zak, Libor Novak, and Tomas Sikola J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019

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The Journal of Physical Chemistry

Formation of Tungsten Oxide Nanowires by Electron-BeamEnhanced Oxidation of WS2 Nanotubes and Platelets Miroslav Kolíbal 1,2*, Kristýna Bukvišová1, Lukáš Kachtík2, Alla Zak3.4, Libor Novák5 and Tomáš Šikola 1,2

1Institute

of Physical Engineering, Brno University of Technology, Technická 2, 616 69 Brno, Czech Republic 2CEITEC BUT, Brno University of Technology, Purkyňova 123, 612 00 Brno, Czech Republic 3 Holon Institute of Technology, Faculty of Sciences, PO Box 305, IL-5810201 Holon, Israel 4 Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel 5 Thermo Fisher Scientific Brno. Vlastimila Pecha 12, 627 00 Brno, Czech Republic Abstract Oxidation of van der Waals-bonded layered semiconductors plays a key role in deterioration of their superior optical and electronic properties. The oxidation mechanism of these materials is, however, different from non-layered semiconductors in many aspects. Here, we show a rather unusual oxidation of tungsten disulfide (WS2) nanotubes and platelets in a high vacuum chamber at a presence of water vapor and at elevated temperatures. The process results in formation of small tungsten oxide nanowires on the surface of WS2. Utilizing real-time scanning electron microscopy we are able to unravel the oxidation mechanism, which proceeds via reduction of initially formed WO3 phase into W18O49 nanowires. Moreover, we show that the oxidation reaction can be localized and enhanced by an electron beam irradiation, which allows for on-demand growth of tungsten oxide nanowires.

*[email protected]

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1. Introduction Similar to carbon-based materials, the transition metal dichalcogenides (TMDs) are layered materials which can be synthesized in a form of fullerene-like nanoparticles, nanotubes and two-dimensional layers.1,2 Each layer comprises of three monolayers where a transition metal (Mo, W) atomic layer is sandwiched in between two chalcogenide layers (S, Se, Te). This class of nanomaterials has recently gained a lot of attention, because semiconducting TMDs possess a band gap that varies with the number of van der Waals-bonded layers,3 thus being very promising in nanoelectronic and optoelectronic applications.4-6 In addition, WS2 nanoparticles with a fullerene-like structure were found to possess excellent tribological behavior.7 They are commercially exploited for lubrication as additive to oils, greases, metal working fluids and, as well as, in polymer composites8 with rapidly expanding market-share. Addition of minute amounts of WS2 nanotubes to polymer matrices was found to endow remarkable improvements in thermal stability, strength and fracture toughness to nanocomposites,9,10 offering them numerous applications. Despite being much more chemically stable than other emerging 2D materials (e.g. phosphorene) due to the absence of dangling bonds on their surface, a prolonged exposure of TMD-based nanodevices to atmospheric conditions may, nevertheless, result in deterioration of electronic and/or optical properties due to oxidation.11,12 Viewed from a different perspective, complete oxidation products - the transition metal oxides (TMOs) - are semiconducting materials with a large bandgap, being also attractive for many applications, e.g.

electrochromic

devices,13

hydrogen

evolution14

and

hydrogenation,15

photoelectrochemistry16 and other.17,18 Given that these materials are envisioned to outperform and potentially replace the semiconductors currently in use, the knowledge of the TMDs oxidation mechanism into TMOs (either unintentional or demanded) in different environments is essential for their future utilization. 2 ACS Paragon Plus Environment

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In layered materials, the oxidation is a more complex phenomenon as compared to conventional non-layered semiconductor materials where it is successfully explained by e.g. the diffusion-based Deal-Grove model.19 It has already been established that the oxidation of layered TMDs starts at grain boundaries and edges, where the outer layer integrity can be easily attacked by the radicals responsible for oxidation (mostly O* and hydroxyl groups).20,21 Then, in contrast with traditional non-layered semiconductors, the oxidation reaction front is moving at a constant speed,22 as the interfaces between van der Waals-bonded layers are opened to oxygen transport.23 On top of that, Yamamoto et al.24 have shown that by a careful choice of process conditions the oxidation of WSe2 flakes can be performed in a self-limited regime, leaving only the outermost layer oxidized and thus offering a superior control over the oxidation process. However, corrugated surface is commonly observed upon oxidation if the reaction is performed by annealing at elevated temperature25,26 or plasma treatment.27 It is worth mentioning that the family of tungsten oxides, like WO3, WO2, as well as substoichiometric phases (W18O49, W20O58 etc.),28 are different in their structure. Hence, different morphology of the oxidation products should be expected depending on the oxidation procedure. In this paper, we report that oxidation of tungsten sulfide (WS2) nanotubes and platelets proceeds in a different, more complex way. Instead of a thin oxide film the reaction results in formation of oxide nanowires. Such a process resembles the growth of W18O49 nanowires by oxidation of tungsten metal, which is known for more than 60 years.29 Despite the intensive research, there exists no general conclusion on the growth mechanism. The in-situ microscopic studies reported the formation of nanoparticles during initial stage of oxidation30 and it was hypothesized they are necessary prerequisite for nanowire formation.30,31 Several papers claim that an amorphous WO3, hydrated WO3 (WO3.nH2O) or WO2 are intermediate phases preceding formation of substoichiometric oxide (W18O49).30,32-34 It has been already



proposed that the formation of nanoparticles is related to the intermediate phase and these serve as sites for W18O49 nanowire nucleation by root-growth mechanism.30,31,34,35 However, this mechanism cannot explain the formation of nanowires apart from the material source, which is frequently observed.33 Indeed, this would imply vapor transport of the growth material to the nanowire top. Here, by combining in-situ real time electron microscopy and post-growth spectroscopy techniques we are able to shed more light on the mechanisms behind the nanowire formation. Our results strongly indicate that both nanowire formation mechanisms proposed in the literature for oxidation of tungsten metal are valid also for WS2, with one or another being predominant depending on the process conditions. Additionally, we show that the growth can be enhanced by an electron beam irradiation, offering the opportunity to site-selectively fabricate the oxide nanowires on demand. These findings not only shed new light onto the oxidation process of transition metal dichalcogenides, but also demonstrate a promising method for prototyping e.g. novel nanophotonic devices. 2. Methods Synthesis of inorganic multiwall nanotubes (MWINT) of WS2. The MWINT-WS2 were grown according to the growth mechanism reported in detail in Ref. 36. WS2 nanotubes were synthesized via multi-step, but one-pot reaction process carried out at high temperature (840900 °C) and at the atmosphere of H2/H2S gases, using a unique quartz reactor. The main two steps are the growth of tungsten oxide nanowhiskers and their following sulfidization. Supporting Figure S1 displays the SEM and TEM micrographs of such nanotubes. The slow diffusion-controlled sulfidization reaction (second step) starts from the nanowhisker surface and proceeds sequentially towards the inner core (outwards – inwards). According to this growth mechanism, the obtained WS2 nanotubes maintain the size of the as-prepared nanowhiskers (first step), resulting in 5-30 microns long and 30-120 nm thick nanotubes, consisting of 10-50 layers. The nanotubes’ hollow core is associated to the difference in 4 ACS Paragon Plus Environment

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specific gravity of tungsten oxide (7.15 g/cm3) vs. tungsten sulfide (7.5 g/cm3), while the size of the whisker dictates the nanotube dimensions. Tungsten sulfide platelets were purchased from Alfa Aesar. Growth of the oxide nanowhiskers on the WS2 nanotubes’ and platelets surface. The nanotubes (or platelets) were first dispersed in isopropanol and upon several ultrasonication steps were drop-casted on a silicon substrate. The oxidation of WS2 was performed in a high vacuum chamber (base pressure 5×10-4 Pa) equipped with an inlet valve for water vapor dosing and a sample holder. For heating the sample to a desired temperature we have used a PBN (pyrolytic boron nitride) heating element (Momentive). The sample temperature was calibrated before the experiment by a thermocouple securely clamped to the sample surface and the results were correlated with pyrometer measurements. Real-time observations in an SEM were done utilizing a Reactor connected to a common SEM stage and equipped with an on-chip heater.37 Gas was injected into a differentially pumped reaction space (2 l volume) above the heating element. The observation of the process by an electron beam is possible via a pressure-limiting aperture. Temperature of the sample was calculated from the MEMS chip resistance, and this calibration was verified by correlating the calculated temperature with well-known phase transitions (Au melting point etc.). Despite this effort, we estimate the error in temperature measurement to be relatively high (up to 10%) due to a temperature gradient across the heating element.38 For pressure measurement we have utilized the Pirani gauge-like effect, so that the changes of the heating power with pressure inside the reactor were correlated with the previously calibrated powerpressure curve measured in an environmental SEM mode of the microscope for identical heater temperature and corrected for the fact that the heater is cooled down by the gas from one side only.



XPS analysis was performed using Kratos Axis Supra utilizing monochromated Al K radiation (1486.6 eV) and a hemispherical analyzer with the pass energy set to 20 eV. The energy scale was calibrated with respect to the adventitious carbon peak at 284.8 eV. TEM analysis (as well as EDX and EELS) were done at a Titan G2 microscope operated at 60 keV to minimize the beam-induced damage.

3. Results and Discussion

Fig.1: Microscopic characterization of processed WS2 nanotubes and platelets. SEM images of a) pristine nanotube (100 nm diameter); nanotube oxidized b) at 300 °C , c) 400 °C and d) 510 °C under 130 Pa water vapor for 20 minutes (scale bar in d) is 100 nm). The black arrows in b) mark the initial oxidation sites, which become visible due to different contrast (revealing the chiral angle of the tube – see text). The nanotube in e) was oxidized at 400 °C for 2 hours. Inset in a) shows a HRTEM image of the layered structure of the WS2 nanotube, confirming the interlayer distance of 6.3 Å. High resolution TEM analysis of the oxide 6 ACS Paragon Plus Environment

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nanoparticles and nanowires formed at the nanotube surface is shown in f) and g), respectively. The oxidation of platelets proceeds in similar way (h-j); scale bars are 50 nm.

3.1 Oxidation at high pressure & temperature A series of oxidation experiments was performed in a high vacuum chamber utilizing water vapor (2 - 130 Pa) as an oxidant at different temperatures (300 – 580 °C). WS2 nanotubes dispersed on a silicon or SiC substrate were treated for 20 minutes at 300 °C, 400 °C and 510 °C. SEM images of the resulting morphology are shown in Fig. 1. In agreement with other reports,20,21 the oxidation indeed could start at defects (Fig. 1b), but we have found out their formation along chiral lines, where the increased secondary electron contrast suggests initiation of protrusions’ growth. Interestingly, this oxidation mechanism thus reveals the chiral angle (folding angle) of the nanotube, which has been so far accessible utilizing electron diffraction only.39 After higher temperature oxidation (400 °C), the nanotubes are decorated with nanoparticles (Fig. 1c). High-resolution transmission electron microscopy image (HRTEM) in Fig. 1f shows that these nanoparticles emerge from the 1-2 outer layers of the nanotube. If the oxidation is conducted at even higher temperature (510 °C), the nanowires appear as protrusions from the surface of the nanotubes (Fig. 1d). The nanowires have the diameter usually ranging from 7 nm to several tens of nanometers and lengths of tens to a few hundreds of nanometers (occasionally, up to micrometer long nanowires were seen). We have also observed that such nanowires can be formed on the nanotube surface, lying along the axis or along the chiral angle, after a prolonged oxidation at lower temperatures (400 °C, 2 hours – Fig. 1e). These results indicate that under the present experimental conditions the nanoparticles are thermodynamically unstable and formation of nanowires is preferred. Increasing the temperature resulted in longer nanowires, while decreasing the



pressure results in their lower concentration on the nanotube surface. HRTEM imaging revealed that the nanowires are made of a single crystal (Fig. 1g) with an interlayer distance of 0.38 nm, which is close to the 0.378 nm between {010} planes of W18O49 nanowires growing in the [010] direction.40 In addition to oxidation of nanotubes we have performed the oxidation of WS2 platelets to unravel any effects of the nanotube surface curvature on the oxidation mechanism. As shown in Fig. 1(h-j) the reaction proceeds in the same way as in the case of nanotubes oxidation. Nanowires are formed preferentially at the edges20,21, and after prolonged oxidation period also on the surface of platelets (Fig. 1j). Nanoparticles can be spotted as an intermediate phase (Fig. 1i). The following experiments were performed with WS2 nanotubes, since their geometry allows for better view of the processes in an SEM.

Fig. 2: Chemical analysis of the oxidation products. a) High-angle annular dark field (HAADF) image and EDX mapping of a nanoparticle (50 nm diameter) formed at the nanotube surface. b) EDX maps taken at nanowire formed upon prolonged oxidation. c, d, e) 8 ACS Paragon Plus Environment

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Detail of the W 4f peak measured by XPS on c) pristine nanotubes (exhibiting sulfide-related doublet W4+ only), d) nanotubes treated at 400 °C under 130 Pa H2O vapor atmosphere (exhibiting an additional doublet related to oxide, W6+) and e) nanotubes treated at 510 °C under 130 Pa H2O vapor atmosphere (with additional components related to sub-oxides, W4+ and W5+). The vertical line in c-e) marks the position of the sulfide-related W4+ peak component for pristine nanotubes.

The acquired chemical element-resolved energy dispersive X-ray spectroscopy (EDX) maps (see Fig. 2a,b) clearly show a very large oxygen content in both nanoparticles and the nanowires, indicating they are tungsten oxide. To identify the stoichiometry of the oxides we have conducted X-ray photoelectron spectroscopy (XPS) analysis of the oxidized nanotubes (see Fig. 2c-e). The lateral resolution of XPS does not allow to perform the analysis with required spatial resolution. However, we note that even after annealing at high temperature, which results in formation of nanowires, the layered structure of WS2 nanotubes is still easily recognizable in TEM (see Supporting Figure S2), thus suggesting minimum overall oxidation. Based on this observation we believe it is plausible to conclude that the new features in the XPS spectra of processed nanotubes can be ascribed to the nanoparticles and nanowires formed upon oxidation. XPS spectra of pristine WS2 nanotubes show a W4f doublet with a binding energy of 32.51 eV (W4f7/2), in agreement with recent studies associated with WS2.41 Detailed spectra of the processed nanotubes reveal that the ones oxidized at 400 °C (with nanoparticles formed, Fig. 1c) show a new doublet, which is related to W6+, suggesting that the nanoparticles stoichiometry corresponds to the WO3 phase. Two additional doublets are detected for nanotubes oxidized at 510 °C (with nanowires formed, Fig. 1e), indicating W4+ and W5+ oxidation states. The occurrence of these components is indicative of a reduced WO3. Further quantification of the oxide stoichiometry is challenging given the longtime



ongoing debate on the physical explanation of W5+,42-45 which should be preferentially associated with defects (oxygen vacancies) in the reduced oxide. Nevertheless, XPS analysis supports the conclusion derived from TEM that the nanowires are substoichiometric tungsten oxide phase - W18O49. Additionally, the sulfide-related W4+ component of the W 4f peak of oxidized nanowires exhibits a shift towards lower binding energy, which has been commonly ascribed to formation of sulfur vacancies in WS2.46 To check the stability of the WS2 nanotubes and to decouple the effects of temperature and oxidation atmosphere we have performed a control experiment with annealing of the nanotubes under high vacuum conditions (5×10-4 Pa) up to 550 °C. Consistent with previous reports,47 the nanotubes do not exhibit any signs of oxidation under SEM inspection and the W 4f peak does not shift, suggesting that the formation of sulfur vacancies is related to the chemical reaction between water molecules and tungsten disulfide.

Fig. 3: Real-time SEM observation of nanoparticle and nanowire formation. a) WS2 nanotube imaged at 430 °C under PH2O = 5 Pa. The formation of NPs is very fast (next frame). The scale bar is 50 nm. b) The nucleation of the nanowire starts at the nanoparticle which is located at the nanowire bottom during subsequent growth (nanoparticle is in contact with the nanotube surface). c) The nucleation of a nanowire is abrupt, followed by rapid onedimensional elongation. d) At higher temperatures, it is occasionally seen that the nanoparticle (marked by arrows in the first image) is consumed during growth, and the nanowire continues to grow even after the nanoparticle is fully consumed. The scale bars in


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b-d) are 20 nm and the process temperatures were b) 560 °C, c) 520 °C and d) 580 °C under the pressure of around PH2O = 6 Pa. To clarify the oxidation mechanism, we set up an experiment utilizing a Reactor (see Methods for further details) placed into a high-resolution scanning electron microscope (SEM) chamber, allowing us to observe the surface morphology of the WS2 nanotubes during oxidation at high-temperature and high-pressure conditions. The image sequences in Fig. 3 show the formation of nanoparticles (Fig. 3a) and their transformation into nanowires (Fig. 3b,c,d). The formation of the nanoparticle is difficult to capture within the timeframe of our experiment in detail, because this is a rather fast process. We have observed that some of the nanoparticles evaporate or migrate along the surface, while many act as a nucleation sites for nanowire growth (Fig. 3b,c,d). The nuclei, initially visible as bright protrusions in an SEM image, quickly elongate to form a nanowire with the nanoparticle at its bottom. This morphology is resembling the root-growth mode, identified in several papers as the mechanism responsible for tungsten oxide nanowire growth.30,31,34,35 At temperatures above 500 °C it is more frequently seen that the nanowire growth is accompanied by shrinking of the nanoparticle (Fig. 3c,d). Interestingly, the nanowires elongate even when the nanoparticle is consumed (Fig. 3d), indicating the existence of an additional growth supply apart from the root-growth mechanism. This is further supported by observation of nanowire nucleation apart from the nanotubes at increased process temperature (see Supporting Figure S3). Additionally, the image sequence in Fig. 4 shows growth of a nanowire which has nucleated at the substrate (i.e. not in contact with the WS2 nanotube) and elongates on both ends without presence of any nanoparticle, which convincingly rules out the root-growth mechanism in this case. The nanowire formation is inevitably accompanied by the decomposition of the mother WS2 phase. The details of this process are revealed in Supporting Figure S4 where an image 11 ACS Paragon Plus Environment


sequence shows that the outer layers of the nanotube are decomposed during the oxidation process, allowing the nearby nanowire to continue its elongation.

Fig. 4: SEM image sequence showing the oxide nanowire growing on both ends without a nanoparticle attached to it and without a direct contact with the WS2 nanotube (T = 560 °C, PH2O = 2 Pa). The yellow dashed lines in the last image mark the positions of both nanowire ends in the first frame. The scale bar is 50 nm. Based on these observations a complex oxidation mechanism could be unraveled. The shift of the sulfide-related W4f peak in the XPS spectra upon oxidation (see Fig. 2) towards lower binding energy indicates the formation of sulfur vacancies. Since the control experiments have shown that the WS2 nanotubes are stable if annealed under high vacuum at temperatures used for the oxidation experiments (no change detected by XPS even after annealing at 550 °C under 5×10-4 Pa), the sulfur vacancies result most likely from formation of S-O or S-O-H bonding at a presence of water. Thus, the adsorption of water molecules on the nanotube surface is followed by a scission of a W-S bonds.11 This process leads to the formation of either SO2 or H2S volatile molecules, which explains why we have not detected any trace of S-O or S-H bonding by XPS (see Supporting Information Fig. S5). The remaining partiallybonded tungsten is prone to oxidation and reacts with water vapor leading to WO3 formation. The oxidation reaction front then spreads from the initiation location (Supporting Figure S4). However, instead of a continuous WO3 layer the formation of WO3 nanoparticles is observed (Fig. 1b,c,f,i), which is facilitated by clustering of mobile species, most probably (WO3)n (n=3-5).48 This hypothesis is supported by the direct observation of nanoparticles coarsening in real-time SEM (Supporting Information, Fig. S6) as well as by the contrast oscillations of


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the reaction front in Fig. S4, suggesting rapid morphology changes (e.g. sudden increase of intensity of the reaction front contour between 75 s and 95 s followed by a decrease at 125 s). This reasoning is in agreement with previous reports, which speculate that the nanoparticles are either amorphous or liquid at the relevant process temperatures.30,31 Due to the presence of hydrogen in the system, it is plausible to assume that WO3 nanoparticles are reduced.49 It is not clear at this point if the reduction to substoichiometric oxide proceeds with the assistance of molecular hydrogen (WO3 + H2 →WO3-x + xH2O + (1x)H2) or due to hydrated tungsten trioxide (WO3-x·nH2O).50 Nevertheless, the local supersaturation with WO3-x is the largest at the nanoparticles surface (in direct contact with the hydrogen supply), making it the most suitable place for nucleation of the new reduced solid phase. It would be highly desirable to perform real-time high resolution TEM study of the nucleation process; here, based on the already published works51,52 we can only speculate that reduction of WO3 and nucleation of the suboxide is accompanied by rearrangement of the [WO6] octahedra (basic building blocks of the oxide structure) into the shear planes characteristic for the suboxides. Our microscopic data suggest that the growth continues by the nucleation of subsequent layers at the base of the nanowire (root-growth mechanism). This results in nanowire elongation. However, this is not the only one mechanism responsible for the nanowire growth. We have provided a clear experimental evidence that the other pathway for the material supply during nanowire elongation is the vapor phase. The sublimation of tungsten trioxide is known to be enhanced by water53 and the WO3-x·nH2O clusters have been detected previously in the outgoing gas flux stream over heated WO3 powder, suggesting they are volatile at elevated temperatures.28,54 Thus, if oxidation is performed at higher temperatures (according to our results above 500 °C) the rate of vaporization of WO3-x·nH2O increases accordingly. Additionally to the root-growth mechanism, the growth species are supplied by direct condensation at the nanowire tip or by



surface diffusion along the nanowire sidewall to the tip, where the chemical potential is the smallest.55 The tip represents the fastest growing facet, which results in strongly anisotropic (one-dimensional) growth.55 The interplay between the two regimes (root-growth and vaporsolid) is critically dependent on the process temperature. At higher temperatures the latter one gets more enhanced, given the volatility of the one of suboxide WO3-x phases and WO3x·nH2O

clusters at elevated temperatures.48

3.2 Electron beam effects By carefully comparing the regions where the real-time SEM observation was done with the ones not exposed to the electron beam, we found that the energetic electrons affect the growth in a significant way. While the morphology of the nanowires remains the same irrespective whether the nanowire was irradiated during the growth or not, the electron beam exposure results in accelerated nanowire growth – the nanoparticles are smaller, denser and the nanowires start to grow sooner and are notably longer on electron-beam-exposed areas. The effect is clearly visible in Fig. 5a, where the nanowire length evolution in time was captured using periodically changing electron doses. Increasing the dose results in acceleration of the growth. In order to unravel the mechanism beneath, we have performed several experiments shown in Fig. 5b-d. The electrons with energies used in this work (5-15 keV) are unlikely to directly create vacancies in WS2 given a large knock-off energy necessary for sulfur displacement (93 keV),56 even at an increased temperature as predicted by the theoretical simulations.57 This is verified by the experimental data shown in Fig. 5b, as the W4f doublet measured on a WS2 nanotubes annealed in high vacuum (without water vapor) at 480 °C for 40 minutes does not shift (no additional s-vacancies formation) if the nanotubes are continuously exposed to a 12 keV electron beam during annealing. The very same experiment performed under 130 Pa water vapor (Fig. 5c) shows an appearance of new peaks indicative of an increased rate of WS2 oxidation under the electron beam irradiation. Note that even in 14 ACS Paragon Plus Environment

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this case the electron beam exposure does not result in an additional shift which could be attributed to the formation of sulfur vacancies solely by the electron beam (both the W 4f peaks overlap despite the electron beam exposure, Fig. 5c). Additionally, Supporting Figure S7 indicates that the growth is accelerated mostly by secondary electrons, thus supporting the conclusion that primary electrons play only a minor role. At this point, it is plausible to conclude that the electrons either promote formation of the growth species, e.g. by enhancing the decomposition rate of water molecules, or make the tungsten disulfide more prone to oxidation. With the aim of distinguishing between the two possible mechanisms we have designed an experiment shown in Fig. 5d. A WS2 nanotube is first exposed by 12 keV electrons (dose: 5.6 pC/nm2, T = 480 °C, PH2O = 2 Pa) and subsequently left under these conditions without e-beam exposure for 60 minutes. The nanowires nucleate preferentially on the e-beam exposed areas and subsequently grow, while the rest of the nanotube is left almost intact (note that at lower pressures, used in this experiment (P=2 Pa), the nucleation rate of nanowires is significantly lower). Hence, this result is inconsistent with a common picture of electron-beam-induced chemistry (decomposition of physisorbed precursor molecules, e.g. water), but favors the latter mechanism mentioned above. It has been suggested just recently58 that electrons may provide energy to overcome the energetic barrier for chemisorption of adsorbates (related e.g. to S-O or –H bond formation), thus allowing desorption of the reaction products (SO2 or H2S) and weakening of the substrate atoms bonds. Unlike other mechanisms of surface “activation” by electron beams reported in the literature59 the one revealed here does not involve direct vacancy formation by primary electrons. Instead, it is driven by thermally-activated surface chemistry and the energetic contribution of electrons is just added to the thermal energy. By increasing the temperature, the oxidation proceeds spontaneously even without the electron beam. The experiment in Fig. 5d shows also that the



oxidation reaction can be locally enhanced, which allows development of strategies for siteselective growth of oxide nanowires.

Fig. 5 Electron beam effect on the oxidation process and site-selective formation of tungsten oxide nanowires. a) Oxide nanowire elongation at 550 °C under 4 Pa of water vapor captured under periodically changing imaging conditions, clearly illustrating the growth enhancement by the electron beam exposure. Each point represents a single image taken using an electron dose of 0.21 fC/nm2, so that the average electron flux alternates between 16 ACS Paragon Plus Environment

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0.01 and 0.04 fC/nm2s. b,c) Detail of the W4f peak measured by XPS on WS2 nanotubes annealed b) under high vacuum (5×10-3Pa) at 480 °C (open black circles – unexposed by the e-beam, full red dots – exposed by a 12 keV electron beam) and c) under 130 Pa of water vapor (other conditions are the same as in b)). d) The WS2 nanotube is first exposed by a 12 keV electron beam (dose 5.6 pC/nm2) at T=480 °C and PH2O = 5 Pa. After 60 minutes under the same experimental conditions (without e-beam exposure) the nanowires are formed close to the exposed areas. The scale bar is 100 nm.

4. Conclusions In conclusion, we have studied oxidation of WS2 nanotubes at elevated temperature in the presence of water vapor, which results in formation of small tungsten oxide nanowires on the surface of nanotubes. The occurrence of bright contrast along chiral angle lines on the WS2 nanotubes’ surface at the beginning of the process, as observed in the SEM images, is indicative of the oxidation sites initiation in the form of protrusions. The chiral angle lines are the most sensitive sites on the surface of nanotubes, and we believe the formation of S-O or SO-H bonding starts here. The adsorption of water molecules on the nanotube surface is followed by a scission of a weakened W-S bond along these sites. This observation possibly allows determination of the nanotube chiral angle just from the SEM images. The initial protrusions in the shape of semi-spherical nanoparticles were identified as the WO3 phase, which further reduced to W18O49 due to presence of hydrogen in the system. The reduction of trioxide was accompanied by the growth of one-dimensional nanocrystals, resulting in formation of nanowires forest on the surface of WS2 nanotubes. Our real-time SEM observations show that the 1D nanowhiskers grow via root-growth mechanism, however, with increasing process temperature also vapor-solid growth at the nanowire tip gets significantly enhanced. Moreover, here we demonstrated that the electron beam-induced chemistry is 17 ACS Paragon Plus Environment


advantageous in initiating and accelerating the growth, allowing preparation of the oxide nanowires site-selectively. Given the recently reported appealing functional properties of tungsten oxide, like e.g. evidence of room temperature single photon emission sources,60 the reported mechanism is attractive for prototyping of nanophotonic devices.

Acknowledgement The authors greatly acknowledge R. Tenne for discussions of the results and Ricardo Egoavil for assistance with TEM measurements. This research has been financially supported by the Grant Agency of the Czech Republic (16-16423Y), Ministry of Education, Youth and Sports of the Czech Republic under the projects CEITEC 2020 (LQ1601) and Ceitec Nano+ (CZ.02.01/0.0./.0.0./16_013/0001728 under the program OPVVV) and Horizon 2020 Research and Innovation Programme under the Grant Agreement No 810626. K.B. was supported by Thermo Fisher Scientific student scholarship. Part of the work was carried out with the support of CEITEC Nano Research Infrastructure (ID LM2015041, MEYS CR, 2016–2019). A.Z. acknowledges the Israel Science Foundation (ISF) for the financial support of this research.

Supporting information. Details of XPS fitting and additional SEM characterization of the nanowires, including real-time image sequence of nanoparticles’ formation.

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Formation of Tungsten Oxide Nanowires by Electron-Beam-Enhanced

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