Preparation of 1T'-Phase ReS2xSe2 (1-x)(x= 0-1) Nanodots for Highly

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Preparation of 1T'-Phase ReS2xSe2(1-x) (x=0-1) Nanodots for Highly Efficient Electrocatalytic Hydrogen Evolution Reaction Zhuangchai Lai, Apoorva Chaturvedi, Yun Wang, Thu Ha Tran, Xiaozhi Liu, Chaoliang Tan, Zhimin Luo, Bo Chen, Ying Huang, Gwang-Hyeon Nam, Zhicheng Zhang, Ye Chen, Zhaoning Hu, Bing Li, Shibo Xi, Qinghua Zhang, Yun Zong, Lin Gu, Christian Kloc, Yonghua Du, and Hua Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04513 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Preparation of 1T'-Phase ReS2xSe2(1-x) (x=0-1) Nanodots for Highly Efficient Electrocatalytic Hydrogen Evolution Reaction Zhuangchai Lai,1,† Apoorva Chaturvedi,1,† Yun Wang,2,† Thu Ha Tran,1,† Xiaozhi Liu,3,4 Chaoliang Tan,1 Zhimin Luo,1 Bo Chen,1 Ying Huang,1 Gwang-Hyeon Nam,1 Zhicheng Zhang,1 Ye Chen,1 Zhaoning Hu,1 Bing Li,6 Shibo Xi,7 Qinghua Zhang,4 Yun Zong,6 Lin Gu,3,4,5 Christian Kloc,1 Yonghua Du,7* Hua Zhang1* 1

Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.

2

Centre for Clean Environment and Energy, School of Environment & Science, Griffith University, Gold Coast campus, QLD 4215, Australia.

3

School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.

4

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. 5

Collaborative Innovation Center of Quantum Matter, Beijing 100190, China.

6

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), Singapore 138634, Singapore. 7

Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), Singapore 627833, Singapore. E-mail: [email protected].

E-mail: [email protected]. ABSTRACT: As a resource of clean energy, a reliable hydrogen evolution reaction (HER) requires robust and highly efficient catalysts. Here, by combining the chemical vapor transport (CVT) and the chemical Li-intercalation, we have prepared a series of 1T’-phase ReS2xSe2(1-x) (x=0-1) nanodots for the high-performance HER in acid medium. Among them, the 1T’-phase ReSSe nanodot exhibits the highest hydrogen evolution activity with a Tafel slope of 50.1 mV dec-1 and a low overpotential of 84 mV at current density of 10 mA cm-2. The excellent hydrogen evolution activity is attributed to the optimal hydrogen absorption energy of the active site induced by the asymmetric S vacancy in the highly asymmetric 1T’ crystal structure.

INTRODUCTION Rational design and synthesis of efficient electrocatalysts for hydrogen evolution reaction (HER) is crucial for the realization of sustainable hydrogen economy. Recently, tremendous efforts have been devoted into the exploration of electrocatalysts with low overpotential and high efficiency toward water splitting.1-16 The discovery of layered transition metal dichalcogenide (TMD) nanomaterials, such as MoS2,17 as one of the potential candidates to replace commercial Pt-based catalysts toward the costeffective electrocatalytic hydrogen production has triggered increasing research interest on TMD-based catalysts because of their earth abundance and high catalytic activities.18-24 Recently, the edges of 2H-phase TMD nanomaterials have been identified as the active sites toward the electrocatalytic HER,17 resulting in the size engineering of TMD nanomaterials in order to expose more

active sites.4, 18-19, 25-28 Besides the size effect, crystal-phase engineering of TMD nanomaterials has also been proved as one of the feasible ways to promote their catalytic activities. For example, 1T-phase TMD nanosheets with high electrical conductivity have been confirmed as promising catalysts for the electrocatalytic HER because of their fast charge transport kinetics in the catalytic reaction.10, 20, 27, 2931 Furthermore, introducing strains into the basal planes of TMDs can also improve electrocatalytic activities for HER.22, 32-33 For example, strained 1T-WS2 nanosheets exhibited superior electrocatalytic activities toward HER as compared to those without strains.22 Very recently, defect engineering has been proved as another important strategy to further improve the activity of TMD-based electrocatalysts for HER. By introducing S vacancies into the strained 2H-MoS2 nanosheets, their electrocatalytic activity could be greatly enhanced as compared to the 2H-MoS2


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Journal of the American Chemical Society nanosheets with exclusive strain.33 The vacancy-induced catalytic performance enhancement is also shown in porous 1T-MoS2 nanosheets and 1T-MoSSe nanodots (NDs), indicating the important role of defects in TMDs for the HER.19, 28 However, due to the symmetric nature of trigonal prismatic 2H structure and octahedral 1T structure, only one type of anionic vacancies can be formed. Different from the 2H and 1T structures, the highly asymmetric 1T’-phase TMDs may result in different types of anionic vacancies, because there are different types of chalcogen atoms in the crystal space in terms of their positions. However, the synthesis of 1T’-phase TMDs still remains a big challenge.34 Since the ReS2xSe2(1-x) possesses 1T’ phase,35 it could be an ideal model catalyst to investigate the effect of asymmetric vacancies for HER. Here, by combining the chemical vapor transport (CVT) and the chemical Li-intercalation method, the large-scale production of water-dispersed, ultrasmall, inherent 1T’-phase ReS2xSe2(1-x) NDs have been prepared. As a proof-of-concept application, the prepared ReS2xSe2(1x) NDs with high-density exposed active sites were used as highly efficient catalysts toward the electrocatalytic HER. Due to the intrinsic distorted 1T’ structure, alloying effect and the unsaturated coordinated Re sites caused by the asymmetric S vacancies, the ReSSe NDs exhibit superior HER performance with a low overpotential of -84 mV at current density of 10 mA cm-2, a Tafel slope of 50.1 mV dec-1, and good long-term stability. To the best of our knowledge, the electrocatalytic HER performance of the obtained ReSSe NDs is among the best as compared to the reported TMD nanomaterials.

resolution TEM (HRTEM) image was used to confirm the crystal structure of ReSSe NDs. As shown in Figure 1d, the lattice fringe of 0.26 nm is consistent with the (002) plane of ReSSe. The atomic-resolution high angle annular dark-field scanning TEM (HAADF-STEM) image further reveals the distorted 1T’ crystal structure of the ReSSe NDs (Figure 1e).35 The EDS in TEM mode was used to confirm the chemical composition of the ReSSe NDs (Figure 1f), showing evident signals of Re, S and Se elements. Moreover, the UV-Vis spectrum of the brown aqueous solution of prepared ReSSe NDs gives a smooth line without any obvious absorption peak, which is similar with the reported ReS2 nanosheets (Figure 1g).36 The Zeta potential of the ReSSe ND solution was measured to be around -29.7 mV, indicating the negatively charged surface of ReSSe NDs, which stabilized the NDs in aqueous solution (inset in Figure 1g). The atomic force microscopy (AFM) characterization confirmed that the height of ReSSe NDs is 1.2 ± 0.6 nm (Figure 1h,i), indicating their ultrathin thickness.

RESULT AND DISCUSSIONS Synthesis and characterizations of 1T’-phase ReSSe NDs. The ultrasmall ReS2xSe2(1-x) NDs with distorted 1T’ structure were prepared by the combination of CVT and chemical Li-intercalation method (Figure 1a). First, ReS2xSe2(1-x) (x=0-1) bulk crystals were prepared by the CVT method (Figure S1), including ReS2, ReS1.8Se0.2, ReS1.4Se0.6, ReSSe, ReS0.6Se1.4, ReS0.2Se1.8, and ReSe2. In order to get ReS2xSe2(1-x) NDs in high yield, the prepared big ReS2xSe2(1-x) crystals were subjected to ball-milling to obtain ReS2xSe2(1-x) powder, which was then immersed in nbutyllithium solution to form the Li-intercalated compounds, followed by the sonication process in de-ionized (DI) water. Finally, after the high-speed centrifugation, uniform ultrasmall ReS2xSe2(1-x) NDs were obtained. The as-prepared ReSSe bulk crystals were characterized by the scanning electron microscopy (SEM) (Figure S2a,b), showing a small crystal size of a few hundred nanometers. Energy dispersive X-ray spectroscopy (EDS) (Figure S2c) was used to confirm the chemical composition of the obtained ReSSe bulk crystals, and the result is close to the designed stoichiometric ratio of Re, S and Se. The X-ray diffraction (XRD) pattern (Figure S2d) is consistent with the reference XRD patterns of ReS2 and ReSe2. Transmission electron microscopy (TEM) images (Figure 1b-c) clearly reveal that the size of the obtained ReSSe NDs is 1.7 ± 0.4 nm (inset in Figure 1b). The high-

Figure 1. Schematic diagram of the synthetic procedure and characterizations of ReSSe NDs. (a) Simulated structure of ReSSe and schematic diagram of the synthetic process of ReSSe NDs. (b) Low-magnification TEM image of the prepared ReSSe NDs. Inset: Size distribution of ReSSe NDs. (c) HRTEM image of the prepared ReSSe NDs. (d) HRTEM image of individual ReSSe nanodots. (e) HAADF-STEM image of a single ReSSe nanodot showing typical 1T’ structure overlapped with the simulated structure. (f) EDS spectra of the prepared ReSSe NDs obtained under TEM mode. (g) UV-Vis spectra of the diluted solution of ReSSe NDs. Inset: Photograph of the ReSSe ND solution. (h) AFM image of the prepared ReSSe NDs. (i) Statistical analysis of the height of 110 ReSSe NDs measured from AFM images.

Besides the ReSSe NDs, other ReS2xSe2(1-x) NDs, including ReS2, ReS1.8Se0.2, ReS1.4Se0.6, ReS0.6Se1.4, ReS0.2Se1.8 and ReSe2, have also been successfully prepared via the similar method. As shown in Figure S3, all the prepared ReS2xSe2(1-x) crystals have similar XRD patterns with a gradual shift of some main peaks between ReS2 and ReSe2 bulk crystals, indicating their good crystallinity and structure accuracy.37 The ground powders of ReS2, ReS1.8Se0.2, ReS1.4Se0.6, ReS0.6Se1.4, ReS0.2Se1.8, and ReSe2 exhibited simi-



lar crystal sizes with that of ReSSe crystal (Figure S4). The exact experimental chemical compositions of these prepared bulk crystals were further characterized by EDS (see Table S1), showing high consistence with the designed stoichiometric ratios. Using the similar chemical Li-intercalation method, a series of ReS2, ReS1.8Se0.2, ReS1.4Se0.6, ReS0.6Se1.4, ReS0.2Se1.8 and ReSe2 NDs were prepared (see Figure S5). Furthermore, the high-resolution HAADF-STEM images of ReS2 NDs clearly reveal the distorted 1T’ structure with only the visible Re atoms showed in the image, resulting from the large Z contrast difference between Re atom and S atom (Figure S6a,b). While for the ReSe2 NDs, both Re and Se atoms are clearly shown in the HAADF-STEM images, which also exhibit the similar distorted 1T’ structure (Figure S6c,d). Moreover, the X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were performed to investigate the surface properties and structure information of the prepared ReSSe NDs. As known, the chemical Li-intercalation of bulk TMD crystals can produce many defects, such as S vacancies, during the exfoliation process.27-28 Obviously, from the XPS spectra (Figure 2a), about 0.3 eV lower binding energy of Re 4f in the ReSSe NDs was observed as compared to the ReSSe bulk crystals, indicating that the Re atom is partially reduced in the NDs. However, the binding energies of S 2p and Se 3p in the ReSSe NDs and ReSSe bulk crystals are almost unchanged (Figure 2b). It reveals that some anionic vacancies, i.e., S and Se vacancies, are formed in ReSSe NDs. To further investigate the dominant types of vacancy, the prepared ReSSe NDs were characterized by XANES and EXAFS. As shown in the Re L3-edge XANES spectra (Figure 2c) and the corresponding k3χ(k) oscillation curve (Figure S7), the ReSSe NDs exhibit obvious difference as compared to the bulk crystals, which is also verified by the Fourier transformed EXAFS (FT-EXAFS) (Figure 2d). As compared to the bulk crystals (2.12 Å), the peak position corresponding to the Re-S/Se coordination for ReSSe NDs slightly shifted to the higher radial distance (2.15 Å), indicating that there are more Re-Se bonding than Re-S bonding in the ReSSe NDs. Furthermore, the peak intensity of ReSSe NDs decreased significantly, i.e., around 50%, as compared to the ReSSe bulk crystals. These differences between the prepared ReSSe NDs and bulk crystals clearly reveal the structural change and size reduction of ReSSe NDs. Detailed structure parameters are summarized in Table S2. As compared to the bulk crystals, which has the same coordination number (N) of 3 for both Re-S and Re-Se, the Re-S and Re-Se bonds in ReSSe NDs exhibit unsaturated coordination, giving a lower N of ~1.4 for Re-S and ~1.9 for Re-Se, respectively. The unsaturated coordination and structural distortion of ReSSe NDs indicate that there are highly exposed Re atoms at edge sites and vacancies after the vigorous exfoliation process. Note that in obtained ReSSe NDs, the Re-S bond has a much lower coordination number (N=~1.4) as compared to Re-Se bond (N=~1.9), implying that the chemical Liintercalation and exfoliation process can partially remove

S atoms from ReSSe NDs rather than Se atoms, thus generating more S vacancies in the basal plane of ReSSe NDs.

Figure 2. High-resolution XPS and X-ray absorption characterization of prepared ReSSe bulk crystals and NDs. (a, b) Re 4f (a) and S 2p and Se 3p spectra (b) of ReSSe bulk crystals and obtained ReSSe NDs. (c, d) XANES spectra (c) and Fou3 rier transformed (FT) k -weighted χ(k)-function of the EXAFS spectra (d) for Re L3-edges of ReSSe bulk crystals and ReSSe NDs.

Electrocatalytic activity of ReSSe NDs on HER. Previous studies demonstrated that the S vacancy and the 1T/1T’ structure are highly desirable for electrocatalytic hydrogen production.10, 27, 29-30, 33 As a proof-of-concept application, our prepared 1T’-ReS2xSe2(1-x) NDs were also used for electrocatalytic HER. All the catalytic experiments were carried out on a standard three-electrode electrochemical configuration in H2-saturated 0.5 M H2SO4 aqueous solution, using ReS2xSe2(1-x) ND-modified glassy carbon electrode (GCE), Ag/AgCl (3 M KCl) and graphite rod as the working electrode, reference electrode and counter electrode, respectively. The iR-corrected polarization curves of ReSSe, ReS2 and ReSe2 NDs in comparison with the commercial 10% Pt/C catalyst are demonstrated in Figure 3a, showing the onset potentials of -196, -72 and -32 mV for ReS2, ReSe2 and ReSSe NDs, respectively. Impressively, the obtained ReSSe NDs achieve a low overpotential of -84 mV at current densities of 10 mA cm-2, which is much better than that of ReS2 NDs (-320 mV) and ReSe2 NDs (-123 mV). The corresponding Tafel slopes of ReSSe NDs is 50.1 mV dec-1 (Figure 3b), suggesting that the catalytic reaction happened through the combination of Volmer-Tafel and Volmer-Heyrovsky mechanisms, in which the Volmer reaction is the ratedetermining step. For comparison, other compositions of ReS2xSe2(1-x) NDs were also subjected to electrochemical test under the same conditions (see Figure S8a,b), further revealing that ReSSe NDs are the most active catalyst toward electrocatalytic HER. The electrochemical impedance spectroscopy was also conducted to study the electrode kinetics under HER process (Figure 3c and Figure S8c,d). The Nyquist plots give the impedances of ReS2


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Journal of the American Chemical Society (678.8 Ω), ReS1.8Se0.2 (398.7 Ω), ReS1.4Se0.6 (55.4 Ω), ReSSe (14.4 Ω), ReS0.6Se1.4 (46.9 Ω), ReS0.2Se1.8 (26.9 Ω) and ReSe2 (23.1 Ω) NDs at working potential of -0.202 V (vs. RHE), which shows the same trend with their catalytic activities, indicating that a faster Faradic process results in a superior HER kinetics. Importantly, the ReSSe NDs showed great long-term stability, with only negligible degradation on HER performance after continuous 20,000 cycling test (Figure 3d). The tested ReSSe NDs were then subjected to the TEM characterization, showing similar dispersity and crystallinity with the untested ReSSe NDs (Figure S9a,b). This confirms that ReSSe NDs remained stable after the long-term stability test. To the best of our knowledge, the catalytic performance of our obtained ReSSe NDs is among the best as compared to other reported state-of-the-art TMD-based nanostructured catalysts to date (see Table S3).

Figure 3. Electrocatalytic HER performance of ReSSe NDs. (a) Polarization curves (iR-corrected) of the commercial 10% Pt/C, bulk ReSSe, and chemically exfoliated ReS2, ReSSe and ReSe2 NDs used as catalysts in 0.5 M H2SO4 aqueous solution. (b) The corresponding Tafel slopes of the catalysts derived from (a). (c) Nyquist plots of bulk ReSSe, ReS2 NDs, ReSSe NDs, ReSe2 NDs at working potential of -0.202 V (vs. RHE). Insets: (Bottom-left) the corresponding fitting equivalent circuit, where Rs represents the uncompensated resistance, Rp represents the charge transfer resistance, and CPE is the value of the argument of the constant phase element. (Top-right) the enlarged plot of the area indicated with a red dash square. (d) Durability test of ReSSe NDs. The polarization curves were recorded before and after 20000 potential cycles in 0.5 M H2SO4 aqueous solution from 0 to 0.202 V (vs. RHE).

Mechanism for the enhanced catalytic activity towards HER. To further verify the catalytically active sites and understand the mechanism of the high-performance of ReSSe catalysts, we systematically studied the Gibbs free energies (ΔGH*) of the adsorption of the intermediate hydrogen atom (H*) on the edges and vacancies of ReS2, ReSe2 and ReSSe, by virtue of extensive first-principles density functional theory (DFT) calculations (Figure 4 and Figure S10). More details about the calculation

method are described in the Supplementary Information. As proved by the XPS, XANES and FT-EXAFS results, there are more S vacancies than Se vacancies in the prepared ReSSe NDs. Therefore, we considered the S vacancy in ReSSe NDs as the main factor here. Note that due to the highly asymmetric structure of the 1T’ phase, there are two possible ways of losing the chalcogenide atoms to form four types of vacancy site, i.e., the high-site S vacancy (HS-V), low-site S vacancy (LS-V), high-site Se vacancy (HSe-V) and low-site Se vacancy (LSe-V) (see Figure S10). In addition, the total energies of ReS2, ReSe2 and ReSSe with different types of vacancies were also calculated, revealing that ReS2, ReSe2 and ReSSe with low-site S/Se vacancy are more stable because they have the lower energy as compared to the high-site S/Se vacancy counterparts (Table S4). Therefore, the low-site S/Se vacancies and the edge sites can be the main active sites toward HER (Figure 4a). On the edge sites, the ΔGH* value is 0.92, 0.43 and 0.52 eV for ReS2, ReSe2 and ReSSe, respectively, suggesting that the unsaturated coordinated Re atom associated with Se atom at the edge sites is most reactive (Figure 4b). If the H atom adsorbs on the vacancy, the ΔGH* value is further reduced in all systems. This is because each H atom interacts with only one unsaturated coordinated Re atom at the edge site, but with two unsaturated coordinated Re atoms at the vacancy site. As such, the different structures between edge and vacancy lead to the much different ΔGH* values. The ΔGH* values are 0.46, 0.28 and 0.00 eV for ReS2 LS-V, ReSe2 LSe-V and ReSSe LS-V, respectively (Figure 4b). The ΔGH* values for ReS2 HS-V, ReSe2 HSe-V and ReSSe HS-V were also calculated for comparison, which are -0.33, -0.40 and -0.37 eV, respectively (Figure 4b). Based on the theory proposed by Norskov and coworkers, the optimal ΔGH* value of desired electrocatalysts (e.g., Pt) should be around 0.00 eV.38 In this work, it was found that the ΔGH* for H bonded with the unsaturated coordinated Re atoms in the prepared ReSSe LS-V site has the optimal value of 0.00 eV, which is similar as that of Pt, suggesting its highest catalytic activity toward HER. Additionally, the unsaturated coordinated Re atoms in ReS2 have the highest ΔGH* value, indicating their lowest activity. Our theoretical trend matches the experimental observations very well. Our results also confirm that the alloying effect between S and Se atoms in ReSSe can enhance the catalytic activity of the basal plane of 1T’-ReSSe for HER. This can also explain why ReSSe NDs exhibited the better performance as compared to ReS2 and ReSe2 NDs. Based on the calculation results, it could be concluded that the significantly enhanced catalytic activity of prepared ReSSe NDs might originate from the excess of charge densities around the unsaturated coordinated Re site next to the low-site S vacancy. The exposed unsaturated coordinated Re atoms endow these ultrasmall NDs with abundant catalytically active sites for the electrochemical HER, which may also be found in other highly asymmetric 1T’-phase TMD nanomaterials.



Figure 4. Theoretical calculation results of ReS2, ReSe2 and ReSSe with different types of S/Se vacancies. (a) Simulated models of an H atom bonded with the ReS2 edge, ReSe2 edge, ReSSe edge, ReS2 LS-V, ReSe2 LSe-V and ReSSe LS-V. (b) Calculated free energy (ΔGH*) versus the reaction coordinate of HER in the edge sites and vacancy sites of 1T’-ReS2, 1T’-ReSe2 and 1T’-ReSSe NDs.

CONCLUSIONS In summary, a facile and feasible method via combining CVT and chemical Li-intercalation method has been used for the preparation of ultrasmall 1T’-ReS2xSe2(1-x) NDs from the corresponding powders, including ReS2, ReS1.8Se0.2, ReS1.4Se0.6, ReSSe, ReS0.6Se1.4, ReS0.2Se1.8, and ReSe2. Through the fine-tuning of the chemical composition of ReS2xSe2(1-x) bulk crystals, the 1T’-ReSSe NDs with highdensity active edge sites and active low-site S vacancies exhibited the enhanced electrocatalytic activity toward HER as compared to ReS2 and ReSe2 NDs, exhibiting a low overpotential of 84 mV at current density of 10 mA cm-2, a Tafel slope of 50.1 mV dec-1 and excellent long-term durability. Our work may open a new way to the rational design and synthesis of highly efficient 1T’-phase TMD catalysts with asymmetric structure via the defect engineering at the atomic level.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” This file includes materials, detailed experimental methods, Figure S1-S11 and Table S1-S4.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Author Contributions †

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by MOE under AcRF Tier 2 (ARC 19/15, No. MOE2014-T2-2-093; MOE2015-T2-2-057; MOE2016T2-2-103; MOE2017-T2-1-162) and AcRF Tier 1 (2016-T1-001147; 2016-T1-002-051; 2017-T1-001-150; 2017-T1-002-119), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Techno-

logical University, Singapore, for use of their electron microscopy (and/or X-ray) facilities. This work was supported by National Program on Key Basic Research Project (2014CB921002) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB07030200) and National Natural Science Foundation of China (51522212, 51421002, and 51672307). The computational research was undertaken on the supercomputers in National Computational Infrastructure (NCI) in Canberra, Australia, which is supported by the Australian Commonwealth Government, and Pawsey Supercomputing Centre in Perth with the funding from the Australian government and the Government of Western Australia.

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