Room-temperature sputtered electrocatalyst WSe2 nanomaterials for hydrogen evolution reaction

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Room-temperature sputtered electrocatalyst WSe2 nanomaterials for hydrogen evolution reaction Jae Hyeon Nam , Myeong Je Jang , Hye Yeon Jang , Woojin Park , Xiaolei Wang , Sung Mook Choi , Byungjin Cho , Y. Kim , J. Yang PII: DOI: Reference:

S2095-4956(19)30919-2 https://doi.org/10.1016/j.jechem.2019.11.027 JECHEM 1024

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Journal of Energy Chemistry

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22 October 2019 25 November 2019 28 November 2019

Please cite this article as: Jae Hyeon Nam , Myeong Je Jang , Hye Yeon Jang , Woojin Park , Xiaolei Wang , Sung Mook Choi , Byungjin Cho , Y. Kim , J. Yang , Room-temperature sputtered electrocatalyst WSe2 nanomaterials for hydrogen evolution reaction, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.11.027

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Communication

Room-temperature sputtered hydrogen evolution reaction

electrocatalyst

WSe2

nanomaterials for

Jae Hyeon Nama,1, Myeong Je Jang b,c,1, Hye Yeon Janga, Woojin Parka, Xiaolei Wangd, Sung Mook Choib,*, Byungjin Cho a, a Department of Advanced Material Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju, Chungbuk, 28644, Republic of Korea Surface Technology Department, Korea Institute of Materials Science, 797 ChangwonDaero, Sungsan-gu, Changwon, 51508, Republic of Korea c Advanced Materials Engineering, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon, 34113, Republic of Korea d Department of Chemical and Materials Engineering, University of Alberta, 9211 116 Street NW, Edmonton, Alberta, T6G 1H9, Canada b

1

These authors contributed equally to this work.

E-mail address: [email protected] (Y. Kim), [email protected] (J. Yang), [email protected] ( S. M. Choi).

Keywords:

AB STR ACT

Two dimensional nanomaterials Sputtering WSe2 nanofilm

The low-temperature physical vapor deposition process of atomically thin two-dimensional transition

Electrocatalyst

metal dichalcogenide (2D TMD) has been gaining attention owing to the cost-effective production of

Hydrogen evolution reaction

diverse electrochemical catalysts for hydrogen evolution reaction (HER) applications. We, herein, propose a simple route toward the cost-effective physical vapor deposition process of 2D WSe2 layered nanofilms as HER electrochemical catalysts using RF magnetron sputtering at room temperature (< 27 °C). By controlling the variable sputtering parameters, such as RF power and deposition time, the loading amount and electrochemical surface area (ECSA) of WSe2 films deposited on carbon paper can be carefully determined. The surface of the sputtered WSe 2 films are partially oxidized, which may cause sphericalshaped particles. Regardless of the loading amount of WSe 2, Tafel slopes of WSe2 electrodes in the HER test are narrowly distributed to be ~120–138 mV dec−1, which indicates the excellent reproducibility of intrinsic catalytic activity. By considering the trade-off between the loading amount and ECSA, the best HER performance is clearly observed in the 200W-15min sample with an overpotential of 220 mV at a current density of 10 mA cm−2. Such a simple sputtering method at low temperature can be easily expanded to other 2D TMD electrochemical catalysts, promising potentially practical electrocatalysts.

As energy consumption has exponentially increased, fossil fuels, one of the main energy sources over the last century, have been regarded as a serious issue because they are not only limited in resources but also cause serious environmental pollution owing to the emission of carbon dioxide. Sustainable and environmentally friendly energy sources have been investigated. Particularly, hydrogen is one of the most ideal energy sources to replace fossil fuels because it possesses the highest energy density per weight as well as serves as infinite and environmentally friendly energy that does not emit CO2 [1–3]. However, the sustainable and large-capacity production of hydrogen is essential. Water electrolysis can be adopted as a promising approach to generate hydrogen [4]. The hydrogen evolution reaction (HER) requires a high-efficiency catalyst to reduce the overpotential [5–10]. In this regard, platinum group metals exhibit excellent intrinsic catalytic activity for electrocatalysts [11–13]. However, these noble metal catalysts are expensive; thus, many non-noble metal materials have been developed as HER electrocatalysts [14–18]. Meanwhile, two-dimensional transition-metal dichalcogenides (TMDs) have attracted a tremendous amount of attention because of their superior electrochemical reactivity, low cost, and abundant components [19–23]. The HER catalytic activity of MoS2 and WSe2, the most well-known 2D semiconductors, was previously reported [24–28]. Several approaches have been proposed to demonstrate the WSe2 catalyst for HER. First, wetprocessing-based WSe2 flakes were exfoliated from bulk WSe2 using aromatic intercalation [29], partly limiting its high yield and uniform large area deposition. Even if different solution-based synthesis methods [30,31] and chemical vapor depositions [32–34] could enable the large-area synthesis of WSe2, such methods require high temperature and relatively complicated processing sequences. Furthermore, the precise and delicat e control of the loading amount for determining the performance optimization of the electrode catalyst is highly essential, which can be achieved by the process development of thickness-controllable 2D layered nanomaterials. Thus, the simple one-step low-temperature sputtering deposition of 2D layered nanomaterials can potentially be preferable for energy applications involving HER. Herein, we employed the magnetron-sputtering-based low-temperature deposition of 2D WSe2 nanomaterials on carbon paper for HER applications. The electrocatalytic performance of 2D WSe2 HER was systematically optimized with respect to the correlation between the loading amounts and surface area of 2D WSe2 catalysts, which are controlled by sputtering power and deposition time. Specifically, as the sputtering power and time increase, the loading

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amount increases, but the electrochemical surface area (ECSA) decreases. The most optimized deposition condition of 2D WSe2 was found in the 200W15min sample, exhibiting an overpotential of 220 mV at a current density of 10 mA cm −2 and a Tafel slope of 126.3 mV dec−1. The simple sputtered WSe2 can achieve high efficiency of electrochemical reactions for hydrogen production, accelerating a route toward the industrial application of WSe2 electrocatalyst. WSe2-layered nanofilms were deposited on carbon paper and SiO 2 substrates using a magnetron sputter with a 2 inch WSe2 target. All deposition processes were conducted at room temperature without any annealing process during the sputtering process. RF powers of 100 W and 200 W were applied to the WSe2 target under an Ar (99.999%) flow rate of 10 sccm. The base and working pressures of the sputtering chamber were monitored to be ~1.0 × 10−6 Torr and 3.0 × 10−3 Torr, respectively. The substrates for the deposition were placed on a rotating holder to achieve film homogeneity. The WSe2 target was pre-sputtered for 10 min, and then, the film thickness and morphology could be readily manipulated by the processing parameters o f power and deposition time. The thicknesses of the sputtered WSe2 layered nanofilms were measured by the linear scan mode of a contact surface profiler (alpha step, P7 Stylus Profiler, KLA-Tencor). The crystallization of the sputtered WSe2 was analyzed by X-ray diffractometer (XRD, D/Max-2500VL/PC, Rigaku) with a Cu-Kα radiation of 0.15406 nm and, in particular, the XRD test environment was operated under an accelerating voltage of 40 kV and a tube current of 200 mA by utilizing different divergence slit sizes (1, 1/2, 1/4, and 1/6). The 2  angle varied from 20° to 80° at a resolution of 0.02° dec−1. The Raman spectrum was measured using a 532 nm green laser. The morphology of the sputtered samples was examined by scanning electron microscopy (SEM, Crossbeam 540, Carl Zeiss) with a field emission electron beam operated at 2 kV. Energy dispersive X-ray spectroscopy (EDS, XFlash 6130, BRUKER) was performed at 15 kV. Binding energies of the sputtered WSe2 were analyzed by X-ray photoelectron spectroscopy (XPS, PHI-Quantera-II, Ulvac-PHI) with monochromatic Al-Kα radiation (1486.6 eV). XPS depth profile data were obtained under an Ar ion beam (3 kV) at a sputtering rate of 1.5 Å s−1. Electrochemical measurements were evaluated using a potentiostat/galvanostat (VSP-5, Bio-Logic, France) in a standard three-electrode electrochemical cell in a 0.5 M H2SO4 solution at 298 K. A graphite rod and Ag/AgCl (saturated KCl) electrode were used as the counter and reference electrodes, respectively. The sputtered WSe2 electrodes (1 × 1 cm2) were used as working electrodes. Linear sweep voltammetry (LSV) was conducted from 0 to –0.4 V (vs. reversible hydrogen electrode, RHE). For comparison, commercial Pt/C electrode (0.2 mg cm −2 20% Pt on Vulcan − carbon paper electrode, Fuel Cell Store) with an area of 1 × 1 cm2 was also examined. H2SO4 electrolytes were purged with nitrogen for 30 min to remove the dissolved oxygen from the electrolyte solution. The potentials of the working electrodes were calibrated as VRHE = VAg/AgCl (0.199) 0.059 pH, where VRHE is the potential vs. RHE, VAg/AgCl is the potential vs. Ag/AgCl electrode, and pH is the pH value of the electrolyte. The potential values from all the electrochemical measurements were taken with 85% iR compensation. Fig. 1(a) is a schematic illustration of WSe2 layered nanofilm deposited under RF magnetron sputtering conditions. WSe2 nanofilms were prepared directly on carbon paper or SiO2/Si substrate at room temperature by the sputtering physical vapor deposition method. Sputtering powers of 100 and 200 W were selected, and the deposition time was set to ~5, 9, 15, and 18 min. Eight samples (100W-5min, 100W-9min, 100W-15min, 100W-18min, 200W5min, 200W-9min, 200W-15min, and 200W-18min) were deposited, and then, their physical and electrochemical properties were compared. Fig. 1(b) shows the excellent linearity between the WSe2 thickness and deposition time for each power condition. The thickness of WSe 2 was checked through the alpha step by intentionally making a step of the WSe2 nanofilm using a shadow mask. The thicknesses of WSe2 nanofilms deposited at 100 W varied from 36.4 to 137.2 nm, whereas it ranged from 86.4 to 258.9 nm at 200 W. This indicates that the RF power was very effective at controlling the thickness of the WSe2 nanofilms, which would allow the manipulation of the corresponding loading amounts of WSe2 to be considered for the HER test. XRD analysis was performed to confirm the presence and crystallinity of WSe 2. Fig. 1(c) illustrates the XRD patterns of the WSe2 film/carbon paper sputtered at a power of 200 W. XRD patterns of the carbon paper are displayed as black squares, showing the (002), (011), (004), and (110) planes. The peak intensity of WSe2 on the carbon paper is relatively weak, such that by adjusting the divergence slit, distinguishable peak intensities of WS e2 could be obtained (Fig. S1 in Supporting Information). The (100), (103), and (110) planes, marked as a red circle symbol, indicate WSe 2 with a typical hexagonal 2D layered structure (refer to ICSD No. 98-004-0752 from Inorganic Crystal Structure Database). As the sputtering time increases, the corresponding peak intensities of the WSe2 become more evident, indicating that the thickness of the WSe2 layer and crystallite size increase with sputtering time. More details are discussed below regarding the SEM analysis.

Fig. 1. (a) Schematic illustration of the synthesis for WSe2 film deposited by RF magnetron sputtering method. (b) Plot of thickness of WSe2 thin films as a function of deposition time for different RF powers of 100 and 200 W. Each sample was deposited for 5, 9, 15, and 18 min. (c) X-ray diffraction (XRD) patterns of the WSe2 sputtered at 200 W for 5, 9, 15, and 18 min on carbon paper. (d) Comparison of Raman spectra of sputtered WSe2 for the 200W15min (red) and 100W-15min (blue) samples. Using WSe2 film on the SiO2/Si substrate for the clarity of peak intensity, Raman spectra were investigated to not only confirm the existing vibration stretching modes of the sputtered WSe2 nanofilms but also compare the crystallinity between the 100 and 200 W samples. The E12g and A1g modes of WSe2 (~250 and 260 cm−1) were overlapped and, thus, appeared as only one broad peak (Fig. 1d). The corresponding phonon frequencies of E 12g and A1g for WSe2 indicate in-plane W-Se vibrational modes and out-of-plane Se-Se vibrational modes, respectively. So low signal corresponding to the E12g might induce the large difference in peak position. The peak intensity was remarkably increased for the sample with higher power (200 W). Higher Raman peaks usually result from thicker 2D film or higher crystallinity [35–36]. However, the WSe2 Raman peak intensity was almost the same, regardless of the deposited thickness for the 200 W RF samples, indicating that the crystallinity is more dominant in relatively large grain samples rather than the thickness itself (Fig. S2 in Supporting Information). Therefore, higher sputtering power causes an increase in the crystallinity of WSe 2. Fig. 2(a–h) depict scanning electron microscopy (SEM) images for the as-deposited WSe2 nanofilm on carbon paper with different deposition powers and times. The shape and size of the nanostructures significantly influence the performance of the electrocatalyst, as its el ectrocatalytic activity is closely related to the quantitative concentration of highly active sites exposed on the surface [37–39]. In particular, we used carbon paper with a rough surface as

Fig. 2. SEM images of WSe2/carbon paper prepared at different RF sputtering powers and times: (a) 100W-5min; (b) 100W-9min; (c) 100W-15min; (d) 100W-18min; (e) 200W-5min; (f) 200W-9min; (g) 200W-15min; (h) 200W-18min. (i) SEM image of the 200W-15min sample and corresponding elemental EDS mapping images for W, Se, O, and C atoms.

the electrode substrate (Fig. S3 in Supporting Information). The rough surface of the carbon paper usually provides a greater contact area against the electrolyte, which is beneficial for improving the HER catalytic performance [40]. To optimize the HER catalyst performance of the WSe2/carbon electrode, SEM analysis was performed for a series of samples with different powers and times. As shown in Fig. 2(a–h), the morphology of all WSe2 film catalysts exhibited spherical-shaped particles rather than a 2D layered crystal structure [34]. It is highly likely that this is due to the co-existence of WSe2 and WO3 compounds [41]. As the deposition time increases, the WSe2 nanostructures evolve in the form of a coarse particle. Meanwhile, rather than the deposition time, the RF power appeared more effective at controlling the loading amount of WSe 2 particles. For the 200W-15min sample, the corresponding EDS mapping images of elemental W, Se, O, and C are shown in Fig. 2(i). Both elemental W and Se are uniformly distributed over a large area of the carbon paper substrate. The existence of elemental O was attributed to the partial oxidation of the WSe 2 surface. Meanwhile, elemental C results from the carbon paper substrate.

Fig. 3. X-ray photoelectron spectroscopy (XPS) data measured at the surface and 5 nm depth of the 200W-15min WSe2 sample: (a) W 4f; (b) Se 3d; (c) O 1s. XPS depth profiling was performed to analyze the chemical compounds of the sputtered WSe 2 nanofilms on the SiO2 substrate. The difference in the binding energy states between the surface and 5 nm depth for the 200W-15min sample was compared. In Fig. 3(a), the featured peaks at 32.1 and 34.2 eV are attributed to the doublet of W 4f7/2 and W 4f5/2 of WSe2, respectively. The 35.6 and 37.8 eV peaks correspond to the W 4f7/2 and W 4f5/2 doublet of WO3, respectively. There exists a considerable amount of WO 3 at the surface, whereas its percentage becomes lesser at 5 nm below the surface. As shown in Fig. 3(b), the Se peak can be deconvoluted to the 54.2 and 55.3 eV peaks, corresponding to the doublet of Se 3d5/2 and Se 3d3/2, respectively. The O 1s peak involving WO3 is clearly remarkable at the surface but negligible at 5 nm below the surface (Fig. 3c). The comparison of the XPS depth profiling results for two samples with different powers (100 or 200 W for 15 min) is also shown in Fig. S4 in Supporting Information. Regardless of the type of sample, WO3 and WSe2 co-exist at the surface, whereas WSe2 becomes substantially dominant below the surface. The plasma-based sputtering process occasionally causes the non-stoichiometry of Se-deficient WSe2-x; thus, the broken W-Se bonds are converted into W–O bonds if they are exposed to oxygen-bearing molecules. This is the reason why the surface of the sputtered WSe2 becomes partially oxidized.

Fig. 4. (a) Polarization curves of 200W-5, -9, -15, and -18min WSe2 films, Pt/C and carbon paper substrate. (b) Tafel plots of 200W-5, -9, -15, and -18min WSe2 electrodes and 40 wt% Pt/C. (c) Cycle voltammetry curves of the 200W-15min electrodes at scan rates from 5 to 40 mV s−1. (d) Calculated ECSA of 200W-5, -9, -15, and -18min WSe2 electrodes.

As shown in Fig. 4(a), LSV was measured in 0.5 M H2SO4 to evaluate the catalytic activity of the WSe2 electrodes sputtered at 200 W for 5, 9, 15, and 18 min. To compare the catalytic activity, we measured commercial Pt/C as a noble catalyst, which has the onset potential of 40 mV at a current density of

10 mA cm−2. The 200W-5, -9, -15, and -18min samples for WSe2 electrodes exhibited overpotentials of 302, 279, 220, and 247 mV at 10 mA cm −2, respectively. Overall, high electrocatalytic performance results from numerous defects and catalytic active sites on the surface [42–44]. The stability test was carried out at a constant current of 10 mA cm-2 for 10 h for Pt/C and 200W-15min electrode, as shown in Fig. S6. Durable electrocatalysts have a small increase in potential during stability test, and Pt/C shows a potential change of 12 mV in stability tests. In the case of the 200W-15min electrode, the potential increased by approximately 50 mV until 5 h of stability test, and since then has been showing stable potential changes to 10 h. SEM images of the 200W-15min and Raman spectra data were compared before and after the stability test, as shown in Fig. S6(a). The Raman spectra of the 200W-15min showed no change in the position of the peak before and after the stability test, and no significant change in the particle size or shape was observed in the SEM images. The Tafel plots are shown in Fig. 4(b) to analyze the intrinsic catalytic activity for HER. The Tafel slope value of the WSe 2 electrodes narrowly ranged from 123.5 to 138.9 mV dec−1. This implies that the intrinsic catalytic activity of the fabricated WSe 2 electrodes maintains a small variation with increasing thickness. Additionally, we analyzed the LSV of the WSe2 electrodes deposited at 100 W power (Fig. S5 in Supporting Information). The relatively low sputtering rate at 100 W did not provide sufficient WSe2 catalyst layer for HER and had low Raman crystallinity, resulting in lower catalytic activity than 200 W samples. The electrochemical surface area (ECSA) was also studied for the samples with variable sputtering time. The ECSA was calculat ed by using the following equation (1): ⁄

(1)

where is the double-layer capacitance and refers to the flat-planar capacitance of the metal surface, which generally uses a value of 40 μF cm −2 [45]. Fig. 4(c) shows the cycle voltammetry curves of the 200W-15min sample at scan rates from 5 to 40 mV s−1 in H2SO4. The values of WSe2 electrodes at 200 W for 5, 9, 15, and 18 min were 0.335, 0.248, 0.239, and 0.2 mF, respectively. The calculated ECSAs of WSe 2 electrodes at 200 W for 5, 9, 15, and 18 min were 8.37, 6.04, 5.98, and 4.99 mF cm−2, respectively. The ECSA values of the WSe2 electrodes tended to decrease with increasing sputtering time. As confirmed by the analysis of the SEM images, the decrease in ECSA was due to the growth of WSe 2 particles with sputtering time. The optimized catalytic activity of the WSe 2 electrodes can be finally determined by considering the trade-off between the catalyst loading amount and the ECSA. The WSe2 electrode with the sputtering power of 200 W for 15 min in our test exhibited the most optimal HER catalytic performance.

In this study, we explored WSe2 nanofilm/carbon paper for HER applications using a room-temperature RF sputtering technique. The simple deposition approach could effectively and controllably determine the loading amount and ECSA by adjusting the sputtering power and deposition time. Rather than the deposition time, RF power was more effective at further enhancing the HER catalytic activity. With increasing sputtering time at fixed power, a tradeoff between the loading amount and ECSA was found. This was due to the growth of spherical-shaped particles with increasing deposition time, which was confirmed by SEM image analysis. In the electrochemical test for HER applications, the Tafel slopes of WSe 2/carbon paper electrodes ranged narrowly from 120 to 138 mV dec−1, indicating intrinsic electrocatalytic activity. The most excellent HER performance was observed in the 200W-15min sample, exhibiting an overpotential of 220 mV at a current density of 10 mA cm −2. Our facile sputtering deposition can be easily expanded to other 2D TMD target materials, providing practical electrochemical catalysts for HER applications.

Acknowledgments This study was supported by the Fundamental Research Program of the Korean Institute of Materials Science (Grant PNK6130) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT; Ministry of Science and ICT) (No. 2017R1C1B1005076). This research was also financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the National Innovation Cluster R&D program (P0006704_Development of energy saving advanced parts)

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TOC 0

WSe2 target Ar plasma

i (mA cm-2)

W Se

-25 -50 200W-9min 200W-12min 200W-15min 200W-18min Pt/C Carbon paper

-75

Substrate

-100 -0.4

-0.3

-0.2

-0.1

0.0

E (V vs. RHE)

The 2D WSe2 nanomaterial was simply deposited on carbon paper by low-temperature magnetron sputtering and optimized by adjusting the loading amount by controlling the sputter power and time.