Core-Shell WO3-WS2 Nanostructured Thin Films via Plasma Assisted

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Core-Shell WO3-WS2 Nanostructured Thin Films via Plasma Assisted Sublimation and Sulfurization Prabhat Kumar, Megha Singh, and Gade B Reddy ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00136 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Applied Nano Materials

Core-Shell WO3-WS2 Nanostructured Thin Films via Plasma Assisted Sublimation and Sulfurization Prabhat Kumar*, Megha Singh, and G. B. Reddy Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi, India -110016 *E-mail: [email protected] The present work describes the synthesis of WO3/WS2 core-shell nanorods (NRs) using plasma assisted sublimation followed by sulfurization process. In this process, WO3 nanostructured thin films were deposited using resistively heated tungsten (W) strip in the presence of O2 plasma and their subsequent sulfurization at different temperatures in the presence of H2S/Ar plasma. The sulfurization temperature (Ts) was varied from 150 °C to 550 °C in order to obtain the core-shell WO3/WS2 with varying shell thickness. The structural properties of the as-obtained core-shell nanostructures were studied using XRD, Raman spectroscopy, XPS, and morphological properties using SEM, HR-TEM. It is observed that an intermediate oxidation state (5+) of W is formed during the conversion of WO3 (6+) into WS2 (4+). The use of plasma reduced the energy barrier required for sulfurization. The mechanism for sulfurization of WO3 NRs in the presence of reactive species of H2S/Ar plasma is also proposed.

Keywords: Plasma Assisted Sublimation/Sulfurization, Core-shell, Nanorods, Thin films and Interfaces.

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Introduction In recent years a lot of research is focused on renewable energy production (H2) and wastewater treatment. For this, researchers are using the catalytic property of semiconductors. Transition metal dichalcogenides (TMDs) are attaining interest of scientific community due to their catalytic properties1,2, tunable bandgaps3, strong light-matter interaction4 and fascinating electrochemical properies with great stability in the acidic solution5,6, etc. Tungsten disulfide (WS2) is a typical member of transition metal dichalcogenide family having layered crystal structure consisting of SW-S planes of trigonal prismatic geometry connected to each other by van der Waals forces. The band gap of the WS2 semiconductor can be varied by decreasing the number of the layer from bulk to monolayers and shows strong quantum confinement effect7, spin-orbit coupling8, and active catalytic sites making it a very useful catalyst9. But the catalytic efficiency depends upon the generation of electron-hole pairs, which moves to the surface of the particles/thin films and reacts with the reactive species10 present at the interface and subsequently split water in case of hydrogen production or decompose dye present in polluted water. There are a few disadvantages of this particle/thin film type systems, for example, there is a large number of recombination between the electrons and holes. Another disadvantage is the poor conductivity of the WS2. This creates a hurdle for making this process economically viable and efficient. Therefore, in order to improve the catalytic activity of these TMDs, researchers have reported various strategies which can be classified into two categories, i.e. “electronic conductibility engineering” and “active site engineering”11. The electronic conductivity can be increased by doping suitable heteroatoms or by coupling it with conductive species, such as graphene, carbon nanotube, sub-stoichiometric oxides, which facilitate an interfacial charge transport for the proton reduction. In case of active site engineering, the activity of can be increased by: (i) enhancing the reactivity of active sites, (ii)


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increasing the number of active sites, (iii) increasing the surface area of material (in the form of nanostructures). Therefore, to overcome these barriers (recombination and poor conductivity) of WS2, researchers have used oxide/sulfide heterojunction nanostructures12–15. In these structures, the holes are accumulated at the valence band of a semiconductor and the electrons are accumulated at the lower lying conduction band of another semiconductor due to the vectorial transfer of electron and holes from one semiconductor to another as shown schematically in figure 116. This process increases the redox reaction of the adsorbed species on the surface of particles of coupled/heterojunction semiconductor system.

Figure 1: Schematic diagram representing the charge transfer process in a WO3/WS2 system.

Many methods are reported for the synthesis of WO3/WS2 heterojunction. For example, Cao et al. reported WO3/WS2 nanostructures by hydrothermal method followed by gas phase reaction17; Yang et al. synthesized WO3.H2O/WS2 hybrid catalyst using in-situ anodic oxidation process9; Adelifard et al. fabricated nanostructured WS2/WO3 binary compound semiconductor by sulfurization of sprayed thin films18. Although these methods for the synthesis of heterostructures are effective from a growth point of view but most of these methods have shown some disadvantages such as impurity from the precursor in case of the chemical method of synthesis, lack of control over the thickness of WO3/WS2 layer, the requirement of high-temperature, 3 ACS Paragon Plus Environment


stoichiometry control, cost-effectiveness, etc. Therefore, there is a need to develop a new method which can overcome these issues. The structural properties of heterostructures system depend upon the oxide precursor and method used for the synthesis, as observed in our previous work19–21 that orthorhombic MoO3 oxide is converted into monoclinic MoO2 and hexagonal MoS2 after sulfurization, which in turn affects the properties of these systems, and similar results are observed in other reported work22. Since monoclinic WO3 crystal structure is different from orthorhombic MoO3; it is important to study the sulfurization of WO3 in order to synthesize tungsten oxide/sulfide heterostructures and use them as a catalyst. In this work, we demonstrate controlled synthesis of core-shell WO3/WS2 nanorod-structured thin film having high surface area using plasma assisted sulfurization of WO3 nanostructured thin films. Further, the effect of temperature on the degree of sulfurization has been investigated systematically.

Experimental Two-step method was used to synthesize WO3/WS2 core-shell nanostructured thin films (NTFs). Firstly, WO3 nanorods were deposited on glass substrates by plasma assisted sublimation process (PASP) and named as sample W0. The detailed description of the plasma assisted sublimation setup is given in the supplementary file. The deposition parameters for WO3 NTFs were: chamber/partial pressure: 1.5×10-1 Torr, source temperature of 500 °C, and deposition duration 20 min. In the second step, WO3 samples were sulfurized using H2S/Ar (10:90) plasma. The WO3 sample was placed on the top of W-strip. To create H2S/Ar plasma, a high voltage of 1000 V was applied between the aluminum cup-shaped cylindrical electrodes kept apart at 7.5 cm. The other parameters used in the second step were: partial pressure of H2S/Ar gas 6.5×10-1 Torr and sulfurization duration of 60 min. The schematic diagram of the experimental set-up is shown in figure 2. The WO3 samples were sulfurized at 150, 250, 350, 450, and 550 ºC, are referred to as


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sample W1, W2, W3, W4 and W5, respectively. The temperature is controlled by controlling the current (externally by variac connected to the current transformer) through the tungsten strip over which samples are kept as shown in figure 2. To monitor the Ts a thermocouple arrangement is fixed inside the vacuum.

Figure 2: (A) Schematic representation of plasma assisted sulfurization (PSP) set-up, (B): a photograph of the PSP set-up.

Electrochemical Measurements Electrochemical studies were carried out in a 3-electrode cell using a potentiostat (PEC workstation-Zahner Zennium, PP 211). As-prepared WO3/WS2 samples (i.e. W1, W2, W3, and W4) on ITO/Glass substrate worked as working electrode (WE), an Ag/AgCl (3.0 mol/kg KCl) was used as the reference electrode (RE), and a Platinum (Pt) rod worked as the counter electrode (CE). A schematic diagram of the set-up assembly is shown in supplementary figure S2(a), and the actual photograph of the measurement set-up is shown in supplementary figure S2(b). A 0.5 M H2SO4 electrolyte solution was used for all the electrochemical measurements. Linear sweep voltammetry was employed to obtain polarization plots with a fixed scan rate of 5mV/s. The 5 ACS Paragon Plus Environment


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obtained polarization data were calibrated with respect to the reversible hydrogen electrode (RHE) by using equation (E1) given below as: 𝑉𝑅𝐻𝐸 = 𝑉0𝐴𝑔/𝐴𝑔𝐶𝑙 + 𝑉𝑎𝑝𝑙. +0.059 × 𝑝𝐻

… E1

Where, 𝑉0𝐴𝑔/𝐴𝑔𝐶𝑙 is reference electrode potential i.e. 0.197 V, 𝑉𝑎𝑝𝑙. is the applied voltage (i.e. -3.5 to 0) and the pH of 0.5M H2SO4 solution is zero. The electrochemical impedance spectra (EIS) were recorded in the frequency range of 0 to 1 MHz and at the applied potential of 50 mV.

Results and discussions Structural analysis In order to study the structural changes that occurred in WO3 films after sulfurization at different temperatures, X-ray diffractograms of all the samples were recorded as shown in figure 3. The diffractogram of the as-prepared WO3 sample shows intense diffraction peaks at 2θ values of 23.06°, 23.59°, 24.33° and at 47.26° corresponding to (002), (020), (200), and (004) crystal planes along with other low-intensity peaks. All the peaks are well matched with the monoclinic phase of WO3 as reported in the standard JCPDS card no 43-1035, having lattice parameters a=7.297, b=7.539 and c=7.688 Å. X-ray diffractograms of sample W1 and W2 are shown in figure 3(b and c), no changes in the peak positions or relative intensities are observed. This indicates that sulfurization at these temperatures (i.e. 150 and 250 °C) does not bring out any significant structural changes since no peak corresponding to WS2 or any other intermediate phase is observed. When the sulfurization is carried out at 350 °C, a new diffraction peak at 2θ value 14.2° (as shown in the inset of figure 3 (d)) is observed along with the diffraction peaks of WO3 as observed in the XRD pattern of sample W0. This new peak corresponds to the (002) crystal plane


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of WS2 hexagonal phase (marked by * in figure 3(d) according to JCPDS card no. 87-2417), which is further confirmed by the Raman spectrum in later part of the discussion.

Figure 3: X-ray diffraction pattern of (a): WO3, and sulfurized WO3 film at (b): 150 °C, (c): 250 °C, (d): 350 °C, (e): 450 °C, and (f): 550 °C; the XRD peaks marked by (*) are corresponding to WS2.

With the increase in the Ts to 450 °C and 550 °C (sample W4, W5), the peak corresponding to WS2 is observed at 2θ value of 14.27° along with the peaks of WO3 at their respective position as present in WO3 sample and there is a relative increase in peak intensity related to the WS2 which can be observed in the inset of figure 3(d-f). Figure 4 shows the change in crystallite size of WS2 with the change in Ts. Scherrer’s equation was used for the calculation of average crystallite size using FWHM of the most intense peak corresponding to the WS2 in the XRD pattern. It can be observed that the crystallite size of WS2 is increased from 5.6 nm to 6.1 nm as the temperature is increased from 350° C to 450° C. With further increase in the temperature to 550 °C there is an increase in the crystallite size to 9.4 nm. Simultaneously, the average crystallite size of WO3 7 ACS Paragon Plus Environment


decreases from 24.91 nm to 21.72 nm with increase in Ts. Thus, it can be inferred that the presence of WS2 becomes more pronounced at higher Ts.

Figure 4: Average Crystallite size of WS2 calculated using Scherrer’s equation as a function of Ts.

From the XRD analysis, the following can be inferred, (i): when the Ts is < 350 °C, there is no recordable diffraction peak corresponding to WS2 phase or any other phase. It implies that the signature of WS2 is observed when sulfurization of WO3 films was carried out at 350 °C and above. (ii): With the increase in the Ts, there is increase in the sulfurization of WO3 into WS2 as relative intensity associated with the WS2 enhanced with the increase in Ts, (iii): In all samples (W1-W5) it is also observed that some minor peaks corresponding to WO3 are not observed probably due to the partial reduction of WO3 and formation of tungsten bronze (HxWO3-x). Raman Analysis To investigate the structural changes occurred in the WO3 crystal structure with respect to the Ts, Raman spectrum of all the samples were recorded in the range of 200-1000 cm-1 (given in figure 5(i)). The Raman spectrum of the as-prepared WO3 nanostructured thin film (figure 5(a)) shows intense peaks at 273, 716 and 806 cm-1 along with the other minor Raman peaks at 220 and 326 cm-1. All the peaks correspond to the monoclinic phase of WO3, showing that the sample is pure as no peak corresponding to any other material is found23,24. The sharp and intense peaks suggest 8 ACS Paragon Plus Environment

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that WO3 sample is highly crystalline in nature and corroborates with the XRD results. The Raman peaks present in the spectral range of 750-950 cm-1 are corresponding to the symmetric stretching of (O-W-O) bonds or the antisymmetric stretching bonding of (W-O-W) bonds. Raman peaks present in the mid (200-400 cm-1) and high-frequency range (600-900 cm-1) are attributed to the deformation and stretching modes of WO3 respectively. Raman peaks at 716 and 806 cm-1 correspond to the stretching vibration of the bridging oxygen. The peaks at 273 and 326 cm-1 are due to the bending vibrations in WO325,26. In the Raman spectrum of the sample sulfurized at 150 °C (figure 5 (i)(b)), all the peaks present are corresponding to the monoclinic phase of WO3, and no characteristic peak corresponding to WS2 is observed.

Figure 5 (i): Raman spectra of (a): WO3, and sulfurized WO3 films at temperature (b): 150 °C, (c): 250 °C, (d): 350 °C, (e): 450 °C, and (f): 550 °C, and characteristic peaks of WO3 and WS2 are marked by #, * respectively. (ii): Raman spectra of WS2 characteristics peaks, showing relative change in the peak intensity of 𝐸12𝑔 and A1g peaks.

In sample W2, the characteristics peaks of WS2 27 are noticed along with the Raman peaks of WO3 as seen in the Raman spectrum of sample W2 (figure 5(i)(c)). Raman active peaks of WS2 are present at 350.9, 355 and 417.5 cm-1 (after multi-peak Lorentzian fittings as given in figure 6)



corresponds to the second-order mode of longitudinal acoustic phonon 2LA(M), in-plane vibrational 𝐸12𝑔 (M) mode and out-of-plane A1g mode respectively7. The relative intensities of peaks corresponding to WS2 are less than the intensity of WO3 peaks suggesting that the WO3 is present in majority amount whereas WS2 is present in a minor amount. As, the Ts is increased from 250 °C to 550 °C, there is increase in the relative intensity of the peaks corresponding to WS2 and decrease in the intensity of the peaks corresponding to the WO3 as observed in the Raman spectrum of sample W2, W3 W4 and W5 (see in the figure 5 (i) & (ii)). This depicts that there is an increase in the sulfurization of WO3 as the temperature is increased. It has been reported by A. Berkdemir et al.28 that Raman spectroscopy can be used for the identification of individual and few layers of WS2 using intensity ratio of I2LA/IA1g. In this author reported that the absolute intensity of the 2LA(M) mode increases with decreasing the number of layers, while the intensity of the A1g displays the opposite behavior. The behavior of the A1g mode with decreasing number of layers presumably results from weaker interlayer contributions to the phonon restoring forces. Figure 5(ii) shows the magnified version of Raman spectra in the spectral range of 300-450 cm-1 for further analysis of WS2 layers.

Figure 6: Raman spectra of sample W2 and W5 after peak fits.


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In case of sample W2 (see figure 6(i)), the peak intensity ratio of 2LA (M) mode to the peak intensity of A1g is ~ 0.83, suggesting the 2-3 layers of WS2. As the temperature is increased in the case of sample W5, the relative intensity of 2LA (M) mode is less compared to the intensity of A1g mode (see in figure 6(ii)). The calculated intensity ratio of I2LA/IA1g is ~0.48 in sample W5, indicating that increase in the WS2 from few layers to the bulk. The HR-TEM micrographs also showed ~ 3-4 monolayers of the WS2 in case of sample W2, which is in agreement with the above Raman results. Similarly, the Raman results of sample W5 is in corroboration with the HR-TEM findings, where more than 16 monolayers (which corresponds to bulk nature) are observed. Molina-S´anchez et al.29 reported the relationship between the number of monolayers of WS2 and the Raman active modes (A1g and 𝐸12𝑔) and found that that the A1g mode increases in frequency with an increasing number of layers while the 𝐸12𝑔 mode decreases. They demonstrated that the weak interlayer interaction is the main cause of the frequency increasing (i.e. for A1g) with the number of layers. And the decrease of the 𝐸12𝑔 phonon frequency is associated with a stronger dielectric screening of the long-range Coulomb interaction in few-layer and bulk. In the present case, the difference in the position of the A1g mode and 𝐸12𝑔 is increased from 62.5 cm-1 to 64.8 cm-1, further giving an indication that WS2 layers have been increased from few layers to bulk. SEM Analysis To investigate the morphological changes occurred after the sulfurization of WO3 samples, scanning electron micrographs were recorded for all samples. The SEM of the as-deposited WO3 sample is shown in figure 7(a), in which nanorods with the average thickness of 150 nm having sharp needle-like tip are observed. To measure the length of the nanorods side micrographs are also recorded (see supplementary file figure S3) and is found to be ~ 3 microns. Along with these nanorods, there is a common background of small nanorods. After the sulfurization, we can’t 11 ACS Paragon Plus Environment


observe much change in surface morphology of samples sulfurized at different temperatures, as in all samples nanorods with the common background of small nanorods are observed.

Figure 7: SEM images of (a): as-deposited WO3 nanorods, and WO3 nanorods sulfurized at different temperatures (b): 150 °C, (c): 250 °C, (d): 350 °C, (e): 450 °C, (f): 550 °C. Inset in each micrograph is an actual photograph of the sample.

EDS analysis To study further, the change in the composition of NTFs, EDX spectra of samples W0-W5 were recorded (shown in supplementary file, figure S5(a-f)). The aim of this characterization is to evaluate the change in the relative percentage of tungsten, oxygen, and sulfur at different Ts. The average atomic percentages of different elements are given in figure 8 (and in Table S1 given in supplementary file). From figure 8, it can be observed that the atomic percentage of W almost remained constant, whereas the atomic percentage of O and S was changed with the change in the 12 ACS Paragon Plus Environment

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Ts. The O (%) decreased with the increase in the Ts and S (%) increased with the increase in the sulfurization temperature.

Figure 8: A graphical plot of the atomic percentage change in Mo, O (shown in red colour) and S elements vs sulfurization temperature.

The quantitative results show the at. % of oxygen element decreased from 76.4% to 75.5%, 67.1%, 62.9%, 58.3% and 60.6% in case of sample W1, W2, W3, W4, and W5 respectively. Whereas in case of the sulfur element the relative at. % is increased from 0.0% to 0.9%, 6.8%, 10.4%, 14.7% and 15.6% respectively for sample W1, W2, W3, W4 and W5. This indicates that with the increase in the Ts, there is an increase in the sulfurization of WO3 NTFs. HR-TEM Analysis Further, to study the structural changes occurred at atomic level after the sulfurization process, high-resolution transmission electron micrographs were recorded in bright field mode. In the HRTEM micrograph of WO3 sample (shown in figure 9(a)), a typical nanorod is observed having a length of 400 nm with the diameter of 50 nm. The fringe pattern recorded from the encircled region in figure 9(a) shows the lattice spacing of 0.38 nm, which is corresponding to the (002) crystal plane of WO330 (shown in figure 9(b)). The recorded fringe width matches with the calculated value of interplanar spacing from the XRD data of (002) crystal plane. Inset in figure 9(b), depicts 13 ACS Paragon Plus Environment


the SAED pattern obtained from the encircled region in figure 9(a) and the sharp diffraction spots indicate that WO3 is present in the highly crystalline form.

Figure 9: HR-TEM micrograph of (a): WO3 nanorod, (b): High resolution image recorded form encircled region as shown in (a), (c): sulfurized nanorod at 250 °C, (d): sulfurized nanorod at 350 °C, (e) High resolution image recorded form encircled region as shown in (d), (f): sulfurized nanorod at 450 °C, (g): sulfurized nanorod at 550 °C; (h): High resolution image recorded form encircled region as shown in (g).

The HR-TEM micrographs of WO3 sample sulfurized at 250 °C shown in figure 9(c), shows a typical core-shell type nanorod. The shell is having lattice fringe width of ~0.64 nm, which is 14 ACS Paragon Plus Environment

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corresponding to (002) crystal planes of WS2 31. In the core, the spacing between adjacent lattice layers is observed to be 0.38 nm, which is attributed due to the (002) crystal planes of WO3. The number of WS2 layers observed is ~2-3 in number, which is in agreement with the results obtained in Raman analysis. It is to be noted that in XRD pattern no peak related to WS2 is observed due to few layers of WS2. When the sulfurization is carried out at 350 °C in case of sample W3, increase in the number of WS2 layers to ~ 5-6 monolayers is observed as seen in the HR-TEM micrographs (figure 9 (d & e)). With the increase in the Ts, increase in the thickness of the WS2 shell is observed. The HR-TEM micrograph of sample W5 is shown in figure 9(g-h), depicts that the thickness of WS2 increased to ~12 nm. From, the HR-TEM results, it can be concluded that reactive species of sulfur present in plasma starts to sulfurize WO3 and form WS2 at low temperature as low as 250 °C and with the increase in the temperature reactive sulfur species are able to penetrate deep inside WO3 nanorods and are able to convert WO3 into WS2. In figure 10(a-b), we can see the bright field and dark field mode HAADF STEM images consisting of two nanorods crossing each other and forming X-like structure.

Figure 10: (a) bight-field TEM image of WO3 sulfurized at 550 °C (sample W5) and corresponding (b): HAADF-STEM images, (c): EDS spectrum (d-g): EDS elemental mapping images showing the presence of W (Lα), S (Kα) and O (Kα) atoms. 15 ACS Paragon Plus Environment


Compositional analysis of nanorods obtained in sample W5 was carried out using high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) with EDS elemental mapping and shown in Figure 10(a-g). In the EDAX spectrum of the nanorods, characteristic peaks corresponding to W(Lα, M), S(Kα) and the O(Kα) lines are obtained and confirms that nanorods consist of W, S, and O only. No peak related to any other element is observed in the EDS spectrum confirming suggesting the high purity of nanorods. Figure 10(d-g), shows the spatially resolved W, S, and O elemental maps obtained from mapping the area in image 10(a). Individual elemental map with the elements color coded as red (W), green (S), and blue (O) are shown along with the combined element map. The elemental mapping depicts that sulfurization occurred uniformly as sulfur is observed uniformly distributed on the nanorods. XPS analysis X-ray photoelectron spectra of the sample W0,W2, and W5 was carried used to study the changes that occurred in the oxidation state of W present in WO3 after sulfurization. XPS survey scan of sample W0 shows characteristics peaks related to the W (4f) and O (1s) along with the carbon peak C(1s) and confirming that the sample is free from impurities. All data is calibrated with C (1s) peak which was recorded at a constant binding energy of 284.6 ± 0.1 eV in all samples. Figure 11(a) shows the core level scan of W (4f) present in sample W0, from which it can be observed that a doublet peak is present at a binding energy of 35.82 and 38.02 eV corresponding to the W (4f7/2) and W (4f5/2) respectively with the orbital splitting of 2.20 eV. The peak positions and spinorbital splitting is a characteristic feature of tungsten in 6+ oxidation state as expected in case of WO332,33. W (4f) and S (2p) present in the XPS spectra of sample W2 is shown in figure 11(b and d), the change in the oxidation state of tungsten can be clearly observed as W (4f ) peak is shifted towards lower binding energy indicating a reduction in the oxidation state of W.


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Figure 11: X-ray photoelectron core spectra of W (4f) of (a): WO3 films and films sulfurized at temperature (b): 250 °C, (c): 550 °C, (d): no S-peak is observed in the case of sample W0, (e & f): core level scan of sulfur 2p doublet peaks corresponding to WO3 sulfurized at 250 °C and 550 °C.

The W (4f) peak is Gaussian fitted into a doublet peak keeping the ratio of 4f7/2 and 4f5/2 constant, i.e., 4:3 and fixed position and taking Shirley background into consideration. A pair of doublet peaks was obtained after fitting. First doublet peak with peak position at binding energy of 32.72 and 34.93 eV is corresponding to the W (4f7/2) and W (4f5/2) respectively with spin-orbital splitting of 2.21 eV, this doublet is due to the tungsten in 4+ oxidation state and confirms the presence of WS234, second doublet of W (4f) peak is present at 35.52 and 38.06 eV corresponding to the W (4f7/2) and W (4f5/2). This second doublet peak of W (4f) is having slightly lower binding energy 17 ACS Paragon Plus Environment


compared to the peak of W (4f) present in WO3; this could be possibly due to the following two reasons. First, due to the presence of HxWO3, which could have been formed due to the interaction of WO3 with H ionic species present in plasma. Secondly, due to the creation of the oxygen vacancies, which were created in the pristine WO3 after its sulfurization in the presence of the reactive species of H2S/Ar. Therefore, from the above peak positions, it can be deducted that tungsten is present in between the 5+ to 6+ oxidation state35,36. In W core level spectra of the sample W2, we also observe a peak is present at 39.04 eV which is corresponding to W (5p3/2). Along with W-peak, a doublet of S (2p) peak is also present at a binding energy of 162.17 and 163.38 eV with the spin-orbital splitting of 1.21 eV which is attributed due to the S (2p3/2) and S (2p1/2) respectively35. With the increase in Ts to 550 °C (in case of sample W5), the peaks corresponding to the S (2p) and W (5p3/2) are obtained at their respective positions and no change can be noticed in the position of binding energy. In sample W5, the thickness of WS2 is much more compared to the sample W2 (as observed in HR-TEM images) this led to the non-detection of the substoichiometric phase of WO3, as this phase is present at the interface of WO3/WS2 core-shell nanostructure and below the MoS2 shell. The stoichiometric ratio calculated from the S (2p) and W (4f) is found to be ~2, confirming that the surface has been converted into WS2. Wettability The core-shell nanostructures of tungsten oxide/sulfide find uses in various applications such as membranes, batteries, coatings, nanoelectronic devices, hydrogen evolution reactions (HER), and biosensors, all applications where these materials necessarily comes in contact with aqueous media37–39. Most of these applications depend upon the various properties of the used material such as the degree of crystallinity, chemical composition, and surface topology. For example, in catalytic applications, especially photo induced H2 evolution, and dye degradation, the active sites and wettability are dependent on the surface energy. Therefore, understanding how water interacts 18 ACS Paragon Plus Environment

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with and wets such core-shell nanostructured thin films surfaces is important for the above applications. For this purpose, the contact angle was measured for all samples using water as the aqueous media. The measurement for water contact angle repeated at five different positions.

Figure 12:Contact angle measurement for (a) WO3, and after its sulfurization at temperature: (b) 150 °C (c) 250 °C, (d) 350 °C, (e) 450 °C, and (f) 550 °C.

Figure 12 shows the water contact angle measurement of WO3 samples before and after the sulfurization at different temperatures. Generally, 90° is considered the critical value among hydrophilic (CA < 90°) and hydrophobic (CA > 90°) activity. The WO3 sample is highly hydrophilic as no droplet of water is formed on the surface so that measurement can be done, but after the sulfurization, the surface properties changed from highly hydrophilic to hydrophobic in nature. This change in the wettability is due to the change in the surface material since after sulfurization WS2 is present at the surface, which is hydrophobic in nature. The surface of WO3 is hydrophilic nature due to the surface Oxygen defect and attraction to the adsorbed hydroxyl group from the atmosphere. It can be clearly observed that as the Ts increased more hydrophobic the surface becomes.



Hydrogen evolution reaction (HER) The polarization curve of the HER between −0.35 and 0.0 V vs RHE (iR not-corrected) for WO3 and WO3-WS2 core-shell nanostructured thin films obtained after sulfurization of WO3 at 150, 250, 350, 450 °C is shown in figure 13(a). The un-sulfurized WO3 and sample W4 exhibited poor catalytic activity for HER (having smaller cathodic currents at higher potential). WO3 sample showed extreme instability under testing conditions, to a visible degradation of the film from the surface of the substrate. For sample W2, at an overpotential of -200 mV, a current density of 5.5 mAcm-2 was observed, which is much higher than that for the WO3 (i.e. 1.3 mAcm-2). At overpotential of 300 mV, the current density observed in sample W0, W1, W2, W3 and W4 was found to be 2, 10.6, 16.2, 11.7, and 0.03 mAcm-2 respectively. Sample W2 showed the highest cathodic current density (i.e. 16.2 mAcm-2), which is much higher than the WO3 nanorod sample (i.e. 2 mAcm-2). On comparing the current density of the sample W2 and W0, an 8-fold increase in the current density (~ 2 to 16 mAcm-2) is observed, and sample W2 showed an early onset for HER. The small onset potential and higher cathodic current density suggest the superior HER of sample W2. From the HER data analysis, it can be said that initially the catalytic activity of the sulfurized WO3-WS2 core-shell nanostructured sample increased with the increase in the sulfurization temperature and reaches a maximum value at 250 °C and then drops sharply. This is due to the following facts. First, at low Ts, a thin layer of sulfide (WS2) catalyst containing rich defects is formed, which can provide more active sites and are beneficial to HER activity. Secondly, the sub-stoichiometric phase present at the interface of the WO3 and WS2 facilitated the charge transfer for the proton reduction. But, with increasing sulfidation temperature, WS2 layers grew thicker, which affected the catalyst impedances (as we know that WS2 is semiconducting and have poor electrical conductivity) and led to the variation of HER performances.


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Figure 13: (a) Polarization curves of WO3, W1, W2, W3 and W4 samples, (b) Corresponding Tafel plots of sample WO3, W1, W2, and W3, (c) Impedance spectra of the WO3 and W1-W4 samples, (d) stability of sample W2 after 100 cycles.

Tafel slope is another essential parameter to investigate the HER properties of the catalysts and to know the kinetics behind the process. Tafel slope is calculated from the polarization curve and shown in figure 13(b). The linear region is fitted with Tafel equation given as: 𝜂 = 𝑎 + 𝑏 log (𝑗) Where 𝜂 is overpotential, j is current density, b is Tafel slope and a is the intercept related to the current density. The sample W2 showed lowest Tafel slope of 174 mV/dec. and WO3 showed maximum Tafel slope of 310 mV/dec, further supporting the superior electrochemical efficiency of core-shell nanostructures compared to WO3 sample.



The electrochemical impedance spectroscopy (EIS) measurements were carried out, and results are given in figure 13 (c). The obtained results showed that minimum resistance (series resistance Rs ~ 25Ω) is obtained for the sample W2, which resulted in maximum current density and lower onset potential. This is due to the thinner semiconducting WS2 layers. The results indicate that the reduced impedance promoted the charge transfer between the electrode and the catalysts, thereby contributing to the higher HER rate. In case of sample W3 and W4, the series resistance is very high due to increase in the thickness of the WS2 layers, another possible increase in the Rs is due to the loss/change in the resistance of the ITO substrate at a higher temperature of sulfurization. Whereas in the case of the sample W1 the series resistance is high due to exposure of WO3 surface, as less sulfurization occurred at low temperature. To study the stability, sample W2 is tested for 100 cycles in the potential range of -0.35 to 0 with increased scanning rate. The obtained results manifested good stability as observed in obtained results (figure 13(d)). In Conclusion, it can be said that the HER catalytic activity of the as-fabricated oxide-sulfide coreshell WO3-WS2 heterostructures increased compared to WO3. This is due to the following reasons. First, the underlying WO3 is partially reduced as observed in the XPS analysis, which facilitated the charge transfer for a proton reduction reaction. Second, these nanostructures are in the form of nanorods which have a very high surface to volume ratio, thus exposing more catalytic actives sites of WS2. Third, WS2 shell acted as a protective layer for the underlying WO3 core from the acidic electrolyte environmental in HER. Therefore, the obtained results suggest that this method can be utilized for the synthesis of oxide-sulfide core-shell nanostructures and modify their properties by changing the thickness of the sulfide layer.


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Reaction Mechanism The WO3 has commonly monoclinic crystal structure composed of corner-shared WO6 octahedra, as shown in figure 14. Every WO6 octahedra have a W metal atom at center surrounded by six corner-shared oxygen atoms. This results in a highly stable structure with very few sites available for reaction. It is one of the reasons that the formation of WS2 from WO3 requires quite high energy. Since the tungsten-oxygen bond energy is very high (W=O: 672 ± 41.8 kJ/mol)35 making it difficult for the oxygen-sulfur exchange reaction as compared to the other oxide like molybdenum oxide (MoO3) where this exchange can take place at relatively low temperature. The crystal structure of MoO3 is the layered type, which facilitates the movement of species into the structures and reduces the barrier for its sulfurization (see the supplementary file, figure S6-7). The use of plasma environment and moderate temperature helps to initiate the reaction of sulfurization by bringing down the required energy barrier. Equation 1 shows that in the absence of plasma, i.e., when the sulfurization is carried out using H2S gas only, the net Gibbs free energy change (ΔG) at 527 °C is +203.03 kJ.mol-1. The positive ΔG indicates that even at high temperature the reaction is not thermodynamically favorable whereas when the sulfurization is carried out in the presence of the H2S plasma (reactions (2-4)) ΔG is negative indicating that conversion of WO3 into WS2 is favorable. Chemical Reaction

Gibbs free energy change (ΔG) in kJ mol-1 +203.03 -2722.88 -3804.02 -2722.88

2WO3 + 4H2S → 2WS2 + 4H2O + O2 WO3 + 2S ― + 2H + →WS2 + H2O + O2 WO3 + 4S + + 4e ― →2WS2 + 3O2 WO3 + 2S ― + 2H + →WS2 + H2O + 3O2

Equation number … (1) … (2) … (3) … (4)

Sulfurization of WO3 can be understood as a two-step process. The first step is the formation of substoichiometric WO3, which is achieved by the formation of the tungsten bronze phase of WO3.



This is formed in the presence of H-ions environment amply provided by dissociated H2S gas in the plasma. These bronzes are represented as HxWO3. Migration of H atoms in monoclinic WO3 has been studied theoretically by various groups40,41. The schematic diagram shown below depicts the movement of the H in the WO3 crystal structure.

Figure 14: Schematic representation of sulfurization of WO3 to WS2.

According to these studies, the first position that H attached itself is terminal oxygen named Ot in figure 14; this is further facilitated as dangling bond presents itself as a defect site. Once the bronze is formed, the stoichiometry of WO3 shifts to sub-stoichiometric due to thermodynamically favored product formation of H2O and lowering the energy barrier for the sulfurization of WO3 into WS2. It can also be interpreted as a reduction in the oxidation state of WO3 from 6+ to 5+ as observed in the XPS results, promotes the reaction of sulfur with W atom present in substoichiometric WO3. This is the foremost and necessary step for the sulfurization of the WO3, also reported by various other researchers like Ampe et al. showed the formation of tungsten bronze 24 ACS Paragon Plus Environment

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H0.10WO3 by reacting WO3 with H2S and Vlies et al. showed that sulfur could only be incorporated into the WO3 only if there is about 20% of tungsten centers are reduced from 6+ to 5+ oxidation state35,42. In the second step, two reactions take place simultaneously. On the one hand, tungsten bronzes get further reduced as a result of the release of H2O. This release is accomplished by strong hydrogen bond formation between H-OH which elongates the W-O bond41. This elongated bond is weakened, and the release of Hydrogen molecule takes places under the influence of applied energy (temperature and plasma). The above reaction leads to the creation of the oxygen vacancies in the WO3 crystal and forming crystallographic shear planes commonly referred to as CS planes and paving the path for the further sulfurization of WO3. Formation of HxWO3-x has not been detected in samples. Therefore, it is inferred that reduced WO3 is followed immediately by filling up of oxygen vacancies by S species from the plasma surroundings. This filling of vacancies is thermodynamically favorable due to the formation of a more stable phase of WS2, and the reactive species of sulfur from plasma reaches deep inside WO3 nanostructure due to the concentration gradient. Therefore, it can be inferred that H2S plasma environment not only provides reactive species of S and H but also lowers the energy required for the reaction to take place, thereby reducing the temperature at which sulfurization takes place, as noticed in the obtained results.

Conclusions In summary, synthesis of WO3/WS2 core-shell nanostructured thin films has been carried out successfully using O2 and H2S/Ar plasma. In the first step, WO3 nanorods have been deposited on glass substrate using plasma assisted sublimation process using O2 plasma, and in second step these WO3 NTFs are sulfurized at controlled temperature using H2S/Ar plasma at different temperatures from 150 °C to 550 °C increase in steps of 100 °C. The effect of temperatures on the extent of sulfurization of WO3 nanorods, the changes in the crystallinity, and morphology after the sulfurization has been studied. Synthesized core-shell nanostructured thin films were examined 25 ACS Paragon Plus Environment


with SEM, XRD, Raman, XPS, EDX-mapping and HR-TEM with STEM techniques. It has been found out that the surface of films sulfurized at Ts ≥ 250 °C has been converted into WS2 and the content of WS2 increased with the increase in Ts. XPS analysis showed that an intermediate state of W (5+) might be present during the process of sulfurization due to the creation of oxygen vacancies. It has been inferred that the mechanism behind the formation of WS2 from WO3 is taken place with a two-step process. In the first step, the creation of oxygen vacancies in WO3 by the reactive species present in plasma and in second step converting WO3-x into WS2 by a redox reaction. Therefore, it is concluded that temperature of sulfurization plays an essential role in order to control the thickness of the WS2 shell. The obtained WO3/WS2 core-shell nanostructured thin can be used in different applications such as H2 production, dye degradation in polluted water, solar energy, etc.

Acknowledgements The authors acknowledge the use of the central research facility and the nanoscale research facility at I.I.T. Delhi, India. The authors also acknowledge the use of XPS facility at CeNSE, IISc. Bangalore, India.

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Graphical Abstract

Figure 1: Schematic diagram representing the charge transfer process in a WO3/WS2 system.

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Figure 2: (A) Schematic representation of plasma assisted sulfurization (PSP) set-up, (B): a photograph of the PSP set-up.

Figure 3: X-ray diffraction pattern of (a): WO3, and sulfurized WO3 film at (b): 150 °C, (c): 250 °C, (d): 350 °C, (e): 450 °C, and (f): 550 °C; the XRD peaks marked by (*) are corresponding to WS2.



Figure 4: Average Crystallite size of WS2 calculated using Scherrer’s equation as a function of Ts.

Figure 5 (i): Raman spectra of (a): WO3, and sulfurized WO3 films at temperature (b): 150 °C, (c): 250 °C, (d): 350 °C, (e): 450 °C, and (f): 550 °C, and characteristic peaks of WO3 and WS2 are marked by #, * respectively. (ii): Raman spectra of WS2 characteristics peaks, showing relative 1 change in the peak intensity of 𝐸2𝑔 and A1g peaks.


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Figure 6: Raman spectra of sample W2 and W5 after peak fits.

Figure 7: SEM images of (a): as-deposited WO3 nanorods, and WO3 nanorods sulfurized at different temperatures (b): 150 °C, (c): 250 °C, (d): 350 °C, (e): 450 °C, (f): 550 °C. Inset in each micrograph is actual photograph of the sample.



Figure 8: A graphical plot of atomic percentage change in Mo, O (shown in red colour) and S elements vs. sulfurization temperature.


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Figure 9: HR-TEM micrograph of (a): WO3 nanorod, (b): High resolution image recorded form encircled region as shown in (a), (c): sulfurized nanorod at 250 °C, (d): sulfurized nanorod at 350 °C, (e) High resolution image recorded form encircled region as shown in (d), (f): sulfurized nanorod at 450 °C, (g): sulfurized nanorod at 550 °C; (h): High resolution image recorded form encircled region as shown in (g).



Figure 10: (a) bight-field TEM image of WO3 sulfurized at 550 °C (sample W5) and corresponding (b): HAADF-STEM images, (c): EDS spectrum (d-g): EDS elemental mapping images showing the presence of W (Lα), S (Kα) and O (Kα) atoms.

Figure 11: X-ray photoelectron core spectra of W (4f) of (a): WO3 films and films sulfurized at temperature (b): 250 °C, (c): 550 °C, (d): no S-peak is observed in case of sample W0, (e & f): core level scan of sulfur 2p doublet peaks corresponding to WO3 sulfurized at 250 °C and 550 °C.


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Figure 12: Contact angle measurement for (a) WO3, and after its sulfurization at temperature: (b) 150 °C (c) 250 °C, (d) 350 °C, (e) 450 °C, and (f) 550 °C.

Figure 13: (a) Polarization curves of WO3, W1, W2, W3 and W4 samples, (b) Corresponding Tafel plots of sample WO3, W1, W2, and W3, (c) Impedance spectra of the WO3 and W1-W4 samples, (d) sstability of sample W2 after 100 cycles.



Figure 14: Schematic representation of sulfurization of WO3 to WS2.


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Core-Shell WO3-WS2 Nanostructured Thin Films via Plasma Assisted

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