Two-Dimensional WO3 Nanosheets Chemically Converted from

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Two-dimensional WO Nanosheets Chemically Converted from Layered WS for High-Performance Electrochromic Devices 2

Ashraful Azam, Jungmo Kim, Junyong Park, Travis G. Novak, Anand P. Tiwari, Sung Ho Song, Bumsoo Kim, and Seokwoo Jeon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02150 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Two-dimensional WO3 Nanosheets Chemically Converted from Layered WS2 for High-Performance Electrochromic Devices Ashraful Azam1, Jungmo Kim1, Junyong Park2, Travis G. Novak1, Anand P. Tiwari1, Sung Ho Song3, Bumsoo Kim1, and Seokwoo Jeon1,* 1

Department of Materials Science and Engineering, KAIST Institute for The Nanocentury,

Advanced Battery Center, KAIST, Daejeon 34141, Republic of Korea 2

School of Materials Science and Engineering, Kumoh National Institute of Technology, Gumi,

Gyeongbuk 39177, Republic of Korea 3

Division of Advanced Materials Engineering, Kongju National University, Chungnam 330717,

Republic of Korea *Correspondence should be addressed S.J. (email: [email protected]) KEYWORDS: Layered material, tungsten oxide, intercalation, oxidation, electrochromic device

ACS Paragon Plus Environment

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ABSTRACT: Two-dimensional (2D) transitional metal oxides (TMOs) are an attractive class of materials due to the combined advantages of high active surface-area, enhanced electrochemical properties, and stability. Among the 2D TMOs, 2D tungsten oxide (WO3) nanosheets possess great potential in electrochemical applications, particularly in electrochromic (EC) devices. However, feasible production of 2D WO3 nanosheets is challenging due to the innate 3D crystallographic structure of WO3. Here we report a novel solution-phase synthesis of 2D WO3 nanosheets through simple oxidation from 2D tungsten disulfide (WS2) nanosheets exfoliated from bulk WS2 powder. The complete conversion from WS2 into WO3 was confirmed through crystallographic and elemental analyses, followed by validation of the 2D WO3 nanosheets applied in the EC device. The EC device showed color modulation of 62.57% at 700 nm wavelength, which is 3.43 times higher than the value of the conventional device using bulk WO3 powder, while also showing enhancement of ~ 46.62% and ~ 62.71% in switching response-time (coloration and bleaching). The mechanism of enhancement was rationalized through comparative analysis based on the thickness of the WO3 components. In the future, 2D WO3 nanosheets could also be usable for other promising application such as sensors, catalysis, thermoelectric, and energy conversion. KEYWORDS: Layered Material, Tungsten Oxide, Intercalation, Oxidation, Electrochromic Device


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Atomically thin, two-dimensional (2D) nanosheets have been extensively studied to improve material properties and device performance since Coleman et. al. reported a liquid exfoliation method capable of simply forming a wide variety of 2D nanosheets.1 However, unlike transition metal dichalcogenides (TMDs) and other materials (e.g. graphene, BP, BN) having a layered crystal structure, most transition metal oxides (TMOs) are difficult to directly exfoliate into 2D nanosheets due to their strong atomic bonding in the out-of-plane direction in a crystal system.2-7 Despite some of the 2D TMO nanosheets being very promising in certain electrochemical applications that require high surface area8-10 and fast ion diffusion11, 12, the absence of direct exfoliating methods of 2D TMO nanosheets from layered bulk materials limits the use of such materials.

For example, tungsten oxide (WO3) is the most promising material, especially in the critical applications of electrochromic (EC) devices because of its high color contrast13-15, fast switching time16-18 and good chemical stability19. In general, the degree of optical modulation, expressed as contrast, of WO3 powder-based EC devices depends on the number of ions and electrons inserted into or extracted from an active film consisting of randomly stacked WO3 powders in response to the external potential. The color change is based on the following reversible reaction: Bleached state + + ↔ (Colored state)

(1)

Where A+ represents H+, Li+, Na+, or K+ ion.20-22 The optical switching time of such EC devices is inversely proportional to the ion diffusivity (D) inside the active film, which can be accelerated by reducing the thickness of the WO3 powders constituting the active film.

23, 24

To

improve both optical contrast and switching time of EC devices simultaneously, it is highly recommended to employ nanostructured materials such as 2D WO3 nanosheets, which can significantly increase the surface area and ion diffusivity inside the active film.25-28 Due to the absence of direct exfoliating methods of 2D WO3 nanosheets from WO3 bulk powders, L. Liang et. al. reported an indirect method for forming 2D WO3.2H2O nanosheets with an average lateral size of less than 500 nm, exfoliated from the hydrated tungstic acid.29 However, to the best of our


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knowledge, no method has been reported to produce large 2D WO3 nanosheets directly from a layered material without hydration.

In this work, we propose a strategy for fabricating highly-crystalline and high-aspectratio 2D WO3 nanosheets for EC devices through sequential exfoliation and oxidation of layered WS2 powders for the first time. This direct exfoliation method from layered materials can produce extremely large 2D WO3 nanosheets with representative lateral size up to over 20µm, which is much larger than any previously reported results. After size separation using a centrifuge, about 82% of the 2D WO3 nanosheets have a thickness below 10 nm (approximately 10 layers or less). Colloidally dispersing 2D WO3 nanosheets in solution enables simple casting to form a uniform thin film (~650±12.75 nm), which can be used as a highly sensitive active film with the high surface area in EC devices. The achieved optical contrast of EC devices using thickness-separated (< 20 nm) WO3 nanosheets reaches 62.57% (at 700 nm wavelength), which represents an improvement of 243% over the EC device using commercial WO3 powders under the same condition. The developed EC device also shows much faster optical switching time for bleaching (6.97s) and coloring (10.74s) of the active film compared to EC device made of WO3 powders (18.69s and 20.12s, respectively). The device also showed exceptional stability, with no significant fading of capacitance after 1000 cycles, proving the robustness of the developed EC device using 2D WO3 nanosheets. Figure 1a shows a schematic illustration of the fabrication process of chemically converted WO3 nanosheets starting from bulk WS2. We first exfoliated partially oxidized WS2 (WSxOy) nanosheets from WS2 intercalation compounds in NMP. The WS2 intercalation compound is formed by intercalating (figure S1) hydrated sodium potassium tartrate salt into layered WS2 bulk powder via grinding followed by heating in the autoclave in an inert atmosphere. (Detailed procedure can be found in the experimental section). Subsequently, the WSxOy nanosheets were simply re-dispersed and stirred for 48 hours in 4M nitric acid (HNO3) to attain fully converted into WO3 nanosheets. During this process of oxidation, sulfur atoms of WS2 nanosheets were substituted with the oxygen atoms and transformed into WO3 nanosheets. Oxidation of WS2 nanosheets faced less constraint along the out-of-plane direction, compared to bulk WS2.30 Therefore, the transformation of WS2 nanosheets to WO3 nanosheets, during


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oxidation has higher degrees of freedom, which makes the transformation process highly favorable. The inset shows the digital images of solutions of the respective types of nanosheets, where the color of the solution changes from greenish-yellow to completely yellow after the conversion process due to the widening of bandgap (figure S2) of the material.

Figure 1b is a representative atomic force microscopy (AFM) image of the converted WO3 nanosheets. The AFM analysis shows that majority (over 82 %) of the WO3 nanosheets has the thickness less than 10 nm, while the lateral size ranges from 1 to 20 µm (figure 1c, figure S3). Moreover, WO3 nanosheets are disperse in water, ethanol and acetone and can be used as an ink to form layer-by-layer thin film for various applications. The WO3 nanosheet ink was coated over indium tin oxide (ITO) glass to form the thin film for EC device (figure 1d), in which a layer-by-layer structure was verified with the scanning electron microscopy (SEM) image in figure 1e.

In order to verify the complete conversion of WS2 powder into WO3 nanosheets, we conducted comparative analyses on the crystallographic and chemical bonding properties of the WS2 powder, WSxOy nanosheets, WO3 nanosheets, and WO3 powder. Figure 2a shows the Xray diffraction (XRD) analysis, which confirms the crystallographic change occurring during the fabrication process of WO3 nanosheets. The peak at 14.4°, which corresponds to the (002) peak of pristine hexagonal phase WS2 (Figure S4), can be clearly seen in the WSxOy nanosheets, but it is completely absent after the oxidation process (WO3 nanosheets). WO3 nanosheets show XRD peaks at 23.02°, 23.51°, and 24.27°, which corresponds to the (002), (020), and (200) planes respectively. These peaks match the typical characteristic XRD peaks observed in pure WO3 31, 32 with monoclinic phase (JCPDS card number 43-1035), implying that the conversion from WSxOy nanosheets into WO3 nanosheets is accomplished (Figure S4). However, WO3 nanosheets show broader XRD peaks (Figure S5), which are slightly shifted towards smaller angles compared to pure WO3 powder. The broadening of the peaks in both WSxOy and WO3 nanosheets is due to the increased randomness of the interlayer spacing, which frequently occurs when thin nanosheets are sampled into a thin film for the XRD analysis.33 The Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses in figure 2b and figure 2c prove the successful conversion of WS2 into WO3 by showing the change in the chemical


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bonding. The Raman spectrum of WSxOy nanosheets exhibits main peaks at 351 cm-1 and 418.5 cm-1, while a minor peak at 805 cm-1 can also be observed.34 The main peaks respectively correspond to the in-plane mode (E12g) and out-of-plane (A1g) vibration mode in WS2, and the minor peak corresponds to W-O-W stretching mode in WO3. The peak at 805 cm-1 grows more evident in Raman spectrum of the converted WO3 nanosheets, where additional peaks are observed at 272 cm-1, 326 cm-1 and 712 cm-1 and ascribed to O-W-O bending mode and W-O-W stretching mode.35, 36 The transition in the bond configuration can be verified more clearly in the XPS result. Tungsten has the oxidation state of 4+ in the case of WS2, and of 6+ in the case of WO3.37, 38 The W4f scan of respective materials in figure 2c shows that the peaks corresponding to W4+ are completely removed in the converted WO3 nanosheets, while the peaks corresponding to W4+ and W6+ co-exist in the WSxOy nanosheets. The S2p scan in figure S6 also shows complete removal of S peaks in WO3 nanosheets, suggesting that all sulfur atoms have been substituted with oxygen atoms. Moreover, XPS survey (Figure S7) analysis reveals that the oxygen to tungsten ratio is 2.98 and 3.07 in WO3 powder and WO3 nanosheet respectively, indicating that there is no oxygen vacancy in WO3 nanosheet. The change in the bonding configuration can also be directly observed by analyzing the crystallographic structure of the nanosheets with transmission electron microscopy (TEM). Figure 2d is a TEM image of a WSxOy nanosheet before the chemical conversion. The complex fast Fourier transform (FFT) pattern shown as the inset implies that the nanosheet is comprised of multiple patches with different crystal structures. The high-resolution TEM images in figure d-(i) and figure d-(ii) show that the WSxOy nanosheet is comprised of quilted patches with the hexagonal structure and monoclinic structure, each corresponding to the crystal structure of WS2 and WO3. It should be noted that the lattice parameters of the WO3 region are 0.382nm and 0.394 nm, which are larger than the conventionally known value (i.e. 0.37nm and 0.38 nm).39 This can be attributed to the existence of WS2 regions, which can cause the deviation of the oxygen atom from its intrinsic position, similar to what had been observed in partially oxidized MoS2 nanosheet3. After the WSxOy nanosheet is converted into WO3 nanosheet, the hexagonal region is completely removed, as confirmed by the TEM image and FFT pattern in figure 2e. Moreover, the lattice parameter is restored to the intrinsic values of 0.373 nm and 0.377 nm, implying that the effect from the WS2 region is removed.


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In order to validate the mechanism for the high efficiency of the WO3 nanosheet film as the EC layer, we attempted to find the performance dependency on the thickness of the WO3 nanosheets. We compared synthesized 2D WO3 nanosheets with the commercial WO3 powder with an average particle size of 131 nm (figure S8). Furthermore, we separated the successfully converted WO3 nanosheets into two regimes of thickness; i) below 20 nm (thin nanosheets), and ii) 20-50 nm (thick nanosheets); and analyzed their morphological and electrochemical properties. We separated the WO3 nanosheets by simple centrifugation (above 2012×g and between 2012 ×g and 1398×g), where the supernatant region was comprised of thin WO3 nanosheets, and the sediment region was comprised of relatively thick WO3 nanosheets (figure S9). The AFM analyses in figure 3a verify the successful separation of the WO3 nanosheets into the respective thickness regimes.

When the WO3 nanosheets are made into a film for the EC layer, 2D permeable channels are opened in between the nanosheets, which should determine the surface area exposed to the electrolyte. On the other hand, thin film fabricated with WO3 powder is predicted to have the less exposed surface area, due to the relatively higher thickness of WO3 powder, compared to the WO3 nanosheets. Calculated electrochemical surface area (Figure S10) of bulk WO3 powder, thick WO3 nanosheets, and thin WO3 nanosheets was 6.85 mF/cm2, 23.71 mF/cm2, and 32.40 mF/cm2 respectively. As the average layer number of the nanosheets is lowered, the number of exposed surfaces increases at the same weight amount of WO3, thus resulting in the increase of the active surface area of WO3 nanosheets. The increase of the surface area of active material has the direct effect on the electrochemical properties of the EC layer. The increased exposure area should result in the reduction of coloration time by providing a larger amount of open access points for Li-ions, thus shortening the overall diffusion length required for Li-ions to fully intercalate into the WO3 components in the EC layer. The average thickness of the WO3 nanosheets also affects the diffusion rate of Li-ions during the coloration and bleaching of EC layer.

To calculate the diffusion coefficient constant of WO3 powder and WO3 nanosheets, the anodic peak current and the cathodic peak current (figure S11) of cyclic voltammetry (CV) was plotted as a function of the square root of the scan rates(figure 3b,c). Figure 3b and Figure 3c,


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respectively, show the change in the diffusion coefficient of the Li-ions during intercalation and de-intercalation. The diffusion coefficient for the intercalation (coloration) process in WO3 powder is 1.89 × 10-10 cm2 s-1, which increases by approximately 2.5 times in thick WO3 nanosheets (6.6×10-10 cm2/s), and by 5.3 times in thin WO3 nanosheets (1.01×10-9 cm2/s). A similar trend is also observed in the case of the de-intercalation (bleaching) process, where the diffusion coefficient of WO3 powder, thick WO3 nanosheets, and thin WO3 nanosheets are 6.41×10-11 cm2/s, 2.56×10-10c m2/s and 5.85×10-10 cm2/s, respectively. Thin WO3 nanosheets with higher active surface area create more open 2D permeable channels and could reduce diffusion paths29. Therefore, the diffusion coefficient of thin WO3 nanosheets possesses the highest value compared to the bulk WO3 powder and thick WO3 nanosheets. The combination of increased surface area and diffusion coefficient in EC layer formed with thin WO3 nanosheets is expected to result in the enhancement in EC performance.40

The performance was analyzed of EC devices fabricated from bulk WO3 powder, thick WO3 nanosheets, and thin WO3 nanosheets, all with same film thickness of 650±12.75 nm and under same condition. Figure 4 a-c shows the optical transmittance spectra under the applied potentials of 3 V (bleached state) and -3 V (colored state) for bulk WO3 powder, thick WO3 nanosheet, and thin WO3 nanosheet devices. The amount of optical modulation tends to increase as the thickness of individual WO3 components decrease. The device made from thin WO3 nanosheets exhibits supreme optical modulation of 62.57 % at the visible wavelength of 700 nm, while those made from bulk WO3 powder and thick WO3 nanosheets, respectively, show smaller modulation of 18.21 % and 46.19%. The high contrast optical modulation in the thin WO3 nanosheet device (3.43 times larger than bulk WO3 powder device) is most likely to be the result of the increased amount of ion intercalated into the active materials.41 As analyzed above, the active surface area and ion diffusion rate become higher as the average thickness of WO3 decreases, leading to enhanced susceptibility of the WO3 layer to the ion intercalation. It is noteworthy that we observed the best optical modulation for the WO3 layer with thickness of (650±20.75) nm (Figure S13). Figure 4d is the digital image of a thin nanosheet WO3 EC device showing high-contrast coloration change during +3v (bleached state) and -3V (colored state). It is also worth noting that the optical transmittance in the near-infrared region (wavelength of 1100nm) drops down to 2.41% in the thin WO3 nanosheet-based device. The transmittance in the


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near-IR region is directly related to the amount of heat transmission through the device in the infrared region, implying that the EC device made of thin WO3 nanosheets is also an excellent candidate for potential applications regarding thermal management. The superiority of WO3 nanosheets is also observed in switching response time. To investigate the EC device switching response time, the corresponding in-situ coloration/bleaching transmittance change was measured at a wavelength of 700 nm with alternately applying the potential of ± 3.0 V for 30 s (figure 4e, figure S14). The color-switching response-time is defined as the time required for 90% of the transmittance change in between the steady bleached and colored states.42 In the thin nanosheet-based EC device, the color-switching response-time is 6.97s for bleaching and 10.74s for coloration, which is faster than both thick nanosheet WO3 based (15.78s, 14.91s) and WO3 powder based EC devices (18.69s, 20.12s). The fast color-switching time can also be explained through the combinatorial effect of increased active surface area and ion diffusion rate, proving the advantage of utilizing 2D WO3 nanosheets over bulk WO3 powder. Moreover, the use of WO3 nanosheets results in the enhanced cyclability of the EC device. To investigate the cyclability, chronoamperometry tests (figure 4f) were performed for all three sets of devices. After 1000 cycles of device testing, it was observed that thin nanosheets-based EC device showed 94% retention in the current density, which revealed robustness of electrochromic operation, while the bulk-based EC device retains only 13%. It is evident that high-aspect-ratio, flat nanosheets of WO3 can form a uniform and adhesive film, and provide an easy access of ions compared to a device made of bulk irregular WO3 particles, which ensures the device stability and long cycle life.

In summary, we propose a simple and novel route for the fabrication of high aspect ratio WO3 nanosheets from bulk WS2 powder by a method of exfoliation and subsequent chemical oxidation. This is the first ever report of a synthesis route of 2D transition metal oxide starting from bulk transition metal chalcogenides. The chemically converted large lateral size and highcrystalline WO3 nanosheets have an average thickness of approximately four layers and maintain high crystallinity. Furthermore, we fabricated EC devices using the WO3 nanosheet inks prepared by this method. The color modulation and the switching time of bleaching and coloration from thin (< 20 nm) WO3 nanosheet EC device results in an improvement of 243%, 46.62%, and 62.71%, respectively, over the commercial WO3 powder EC device prepared


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through the same method. The enhancement of EC property is due to the higher exposed active surface area of 2D WO3 nanosheets, which creates faster ion diffusion inside the EC thin film device. In conclusion, we believe the presented route for the fabrication of WO3 nanosheets opens a new horizon for fabricating other 2D like transition metal oxides such as MoO3, V2O5, and etc. from their respective bulk transition metal dichalcogenides for various applications such as sensors, solar cell, catalysis, energy storage, thermoelectric and so on.

ASSOCIATED CONTENT Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION: Corresponding Author: *E-mail: [email protected]

Notes The authors declare no competing interests.

ACKNOWLEDGMENTS This research was supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier (NRF-2013M3A6A5073173), NanoMaterial Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (NRF2016M3A7B4900118), and NanoMaterial Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (NRF-2017M3A7B4041989).


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FIGURES AND CAPTIONS

Figure 1 Fabrication of 2D WO3 nanosheets film. (a) Schematics of 2D WO3 nanosheets fabrication steps from bulk WS2 powder. (b) Representative AFM image and height profile of fabricated WO3 nanosheets. (c) Lateral size distribution of fabricated WO3 nanosheets. (d) Schematics of WO3 EC device, inset layer by layer film formation of WO3 nanosheets. (e) SEM image of cross sectional image 2D diffusion channels of WO3 film (scale bar: 200 nm ).


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Figure 2 Characterization of conversion of WS2 to WO3 nanosheets. comparative analysis of

WS2 powder, WSxOy nanosheets, WO3 nanosheets, WO3 powder: (a) XRD analysis, (b) Raman spectroscopy analysis, (c) XPS analysis: W4f Scan. (d-f) HRTEM analysis: before and after oxidation of WSxOy nanosheets.


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Figure 3 Thickness dependent separations of WO3 nanosheets and WO3 film Diffusion coefficient constant calculation. (a) Thickness distribution of fabricated WO3 nanosheets after thickness separated via centrifugation process. (b-c) Diffusion coefficient constant measurement of WO3 powder and WO3 nanosheets using the anodic peak current and the cathodic peak current as a function of the square root of the scan rates.


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Figure 4 EC device Performance test. Optical transmittance spectra change under potentials of 3 V (bleached state) and -3 V (colored state) of EC Device made of: (a) WO3 Powder, (b) Thick WO3 nanosheets, (c) Thin WO3 nanosheets respectively. (d) Digital image of EC device at bleached state and colored state. (e) Comparison of switching response time of EC Device made of WO3 powder and WO3 nanosheets for the colored and bleached states measured at +3.0 V and – 3.0 V for 30 s with a wavelength of 700 nm. (f) Stability test of EC devices for 1000 reversible cycle at +3.0 V and – 3.0 V for 30 s interval.


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Two-Dimensional WO3 Nanosheets Chemically Converted from

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