Facile Synthesis of N-Doped WS2 Nanosheets As an Efficient And

catalysts

Article Facile Synthesis of N-Doped WS2 Nanosheets as an Eﬃcient and Stable Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media

Arpan Kumar Nayak 1 , Enhbayar Enhtuwshin 2, So Jung Kim 2 and HyukSu Han 2,*

1 Department of Physics: School of Advanced Sciences, Vellore Institute of Technology, Vellore 632014, India; [email protected] 2 Department of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Korea; [email protected] (E.E.); [email protected] (S.J.K.) * Correspondence: [email protected]; Tel.: +82-2-450-3997

Received: 30 September 2020; Accepted: 20 October 2020; Published: 26 October 2020

Abstract: Transition metal chalcogenides have been widely studied as a promising electrocatalyst for the hydrogen evolution reaction (HER) in acidic conditions. Among various transition metal chalcogenides, tungsten disulﬁde (WS2) is a distinguishable candidate due to abundant active sites and good electrical properties. Herein, we report a facile and selective synthetic method to synthesize WS2 with an intriguing two-dimensional nanostructure by using cysteine (C3H7NO2S) as a chemical agent. In addition, nitrogen can be incorporated during chemical synthesis from cysteine, which may be helpful for enhancing the HER. The electrocatalytic activity of N-doped WS2 exhibits a promising HER in acidic conditions, which are not only higher than W18O49 nanowires and hex-WO3 nanowires, but also comparable to the benchmark Pt/C. Moreover, excellent electrocatalytic stability is also demonstrated for acidic HER during long-term tests, thus highlighting its potential use of practical applications as an electrolyzer.

Keywords: water splitting; hydrogen evolution reaction; electrocatalyst; tungsten sulﬁde

1. Introduction Hydrogen is a promising future energy resource, which has the potential to replace fossil fuel stocks [1,2]. The electrochemical electrolyzer has become a crucial energy conversion system to produce hydrogen in a cost-effective and environmentally friendly way [3–5]. Compared to the conventional alkaline electrolyzer, the proton exchange membrane (PEM) electrolyzer has attracted great attention from researchers due to a much higher energy conversion efficiency. With this in mind, electrocatalysts for the hydrogen evolution reaction (HER) in acidic media becomes of paramount importance for efficient electrochemical water splitting using a PEM electrolyzer [6]. Noble metals such as Pt or Pd are currently marked as the best HER electrocatalysts in acidic media [7]. However, the scarcity and high cost of novel metals block their practical use. As such, low-cost electrocatalysts with a high performance need to be developed in order to replace the benchmark novel-metal-based electrocatalysts for the HER. Tungsten-based compounds (i.e., chalcogenides or oxides) are reported as promising materials for acidic HER [8–10]. A multivalent W in the oxides or chalcogenides can give rise to a high probability of electrochemical reactions with electrochemical robustness in harsh acidic conditions. Especially, tungsten chalcogenides such as WSx can exhibit promising electrocatalytic activity due to abundant active sites where both the metal and chalcogenide atoms are electrocatalytically active [11–16]. In addition, N dopants, possessing a high affinity to transition metals, can further enhance interfacial charge transfer in WSx. The incorporated N atoms can modulate the local electronic structure of

Catalysts 2020, 10, 1238; doi:10.3390/catal10111238 www.mdpi.com/journal/catalysts CatalystsCatalysts 20202020,, 1010,, 1238x FOR PEER REVIEW 22 of 99

interfacial charge transfer in WSx. The incorporated N atoms can modulate the local electronic transitionstructure of metal transit atoms.ion metal Thus, atoms. water Thus, dissociation water dissociation can be much can be more much favorable more favorable in N-doped in N-doped WSx comparedWSx compared to the to pristine the pristine WSx, WS resultingx, resulting in an in enhanced an enhanced HER HER [15]. [15]. InIn thisthis context,context, herehere wewe reportreport aa facile syntheticsynthetic methodmethod forfor N-dopedN-doped WSWS22 as a high high-performance-performance HER electrocatalystelectrocatalyst inin acidicacidic media.media. OurOur chemicalchemical methodmethod couldcould selectivelyselectively synthesizesynthesize WSWSxx oror WOWOxx by simply changing chemical agents. In addition, an intriguing two two-dimensional-dimensional nanosheet structurestructure waswas obtainedobtained forfor WSWS22 with a successful incorporation of N via a one one-step-step hydrothermal reaction using cysteinecysteine (C(C33H7NO2S)S) as as a a chemical chemical agent agent.. The The best best sample sample showed showed a low overpotential of 130 mV for 2 generating aa currentcurrent densitydensity ofof 1010 mAmA cmcm−−2 with exce excellentllent catalytic stability upup toto 1212 hh inin 0.50.5 MM HH22SO4..

2. Results and Discussion 2. Results and Discussion

Figure1 shows X-ray di ﬀraction (XRD) patterns of W18O49, WO3, WS2 (cysteine 1:2, 0.01 M), Figure 1 shows X-ray diffraction (XRD) patterns of W18O49, WO3, WS2 (cysteine 1:2, 0.01 M), and and WS2 (cysteine 1:4, 0.02 M). XRD results reveal that phase-pure W18O49 (JCPDS: 01-084-1516), WS2 (cysteine 1:4, 0.02 M). XRD results reveal that phase-pure W18O49 (JCPDS: 01-084-1516), WO3 WO3 (JCPDS: 033-1387), and WS2 (JCPDS: 008-0237) were successfully synthesized using our method. (JCPDS: 033-1387), and WS2 (JCPDS: 008-0237) were successfully synthesized using our method. No Nosecondary secondary phases phases or impurities or impurities were werepresented presented for all forsamples. all samples. Notably, Notably, selective selective synthesis synthesis for each for each phase could be achieved by only adding thiourea or cysteine to the starting WCl6 solution. phase could be achieved by only adding thiourea or cysteine to the starting WCl6 solution. This Thishighlights highlights the practical the practical potential potential of our of our synthetic synthetic method method for for preparing preparing tungsten tungsten-based-based compounds. compounds.

FigureFigure 1. 1.XRD XRD patterns patterns of of ( a(a)W) W1818OO4949,(, (b) WO3,,( (cc)) WS WS22 (cysteine(cysteine 1:2), 1:2), and and ( (dd)) WS WS22 (cysteine(cysteine 1:4). 1:4).

SEM images images of of WW1818O49 are shown in FigureFigure2 .2. A A homogeneoushomogeneous nanowire nanowire morphology morphology was was observed forfor W W1818OO4949.. AA thickness thickness on on the the order order of a of few a tensfew of tens nanometers of nanometer was observeds was observed for the W for18O the49, andW18O no49, signiﬁcantand no significant aggregation aggregation of nanowires of nanowires occurred during occurred the d solvothermaluring the solvothermal synthesis (Figure synthesis2b). After(Figure the 2b). addition After the of thioureaaddition (CHof thiourea4N2S) during (CH4N the2S) during wet-chemical the wet synthesis,-chemical asynthesis, pristine nanowirea pristine morphologynanowire morphology was well retainedwas well while retained the crystallinewhile the phasecrystalline was changedphase was from changed W18O 49fromto hexagonal W18O49 to WOhexagonal3 (Figure WO3).3 With(Figure respect 3). With to the respect morphology, to the morphology, the nanowire the thickness nanowire was thickness slightly increased was slightly to aboutincreased a few to hundred about a nanometers few hundred for WO nanometer3 compareds for to WO that3 ofcompared W18O49 (Figure to that3 b). of Substantial W18O49 (Figure changes 3b). inSubstantial morphology change occurreds in morphology when cysteine occurred (C3H when7NO2 S)cysteine instead (C of3H thiourea7NO2S) instead (CH4N 2ofS) thiourea was added (CH to4N the2S) was added to the WCl6 solution during wet-chemical synthesis (Figure 4). An intriguing nanoflower

Catalysts 2020, 10, 1238 3 of 9

WCl6 solution during wet-chemical synthesis (Figure4). An intriguing nanoflower morphology was observed on the surface of the WS nanoparticles (Figure4b). The WS (cysteine 1:2) nanoparticles Catalysts 2020, 10, x FOR PEER REVIEW2 2 3 of 9 showed aCatalysts spherical 2020, 10 shape, x FOR PEER with REVIEW a diameter of about 300~500 nm. Interestingly, two-dimensional3 of 9 (2D) nanosheetsmorphology with a thickness was observed of on a few the surface nanometers of the WS were2 nanoparticle homogeneouslys (Figure 4b). The grown WS2 (cysteine on the surface1:2) of the morphologynanoparticle swas showed observed a spherical on the surface shape ofwith the a WS diameter2 nanoparticle of abous (tFigure 300~500 4b). nm. The Interestingly, WS2 (cysteine two 1:2)- WS (cysteine 1:2) nanoparticle. For the WS (cysteine 1:4) sample, a nearly identical morphology 2 nanoparticledimensionals (2D) showed nanosheets a spherical with shapea thickness with2 aof diameter a few nanometers of about 300~500were homogeneous nm. Interestingly,ly grown two on- with WS2 dimensionalthe(cysteine surface of 1:2)(2D) the wasWSnanosheets2 (cysteine observed with 1:2) (Figurea nanoparticle. thickness5), of except Fora few the nanometers thatWS2 (cysteine the particle were 1:4) homogeneous sample, size was a nearly slightlyly grown identical on increased to about 500~700themorphology surface nm. of Thesewith the WS WS results2 2(cysteine (cysteine indicate 1:2) 1:2) nanoparticle. was that observed cysteine For (Figure the (CWS 5)3,2H except(cysteine7NO that2S) 1:4) the in sample, theparticle starting a size nearly was WCl identical slightly6 solution can morphologyincreased to aboutwith WS 500~7002 (cysteine nm. 1:2)These was results observed indicate (Figure that 5) cysteine, except that(C3H the7NO particle2S) in the size starting was slightly WCl6 facilitate the formation of a tungsten sulfide phase (i.e., WS2) in a 2D nanosheet structure during the increasesolutiond can to aboutfacilitate 500~700 the formation nm. These of aresults tungsten indicate sulfide that phase cysteine (i.e., WS(C3H2) 7inNO a 22DS) innanosheet the starting structure WCl6 hydrothermal reaction, whereas thiourea (CH4N2S) promotes the one-dimensional (1D) growth of solutionduring the can hydrothermal facilitate the formation reaction, whereasof a tungsten thiourea sulfide (CH phase4N2S) (i.e., promotes WS2) in the a 2D one nanosheet-dimensional structure (1D) tungsten oxideduringgrowth phases theof tungsten hydrothermal such oxide as WO phasesreaction,3 or such W whereas18 asO WO49. thiourea3 Hence, or W18O (CH49 a. selectiveHence,4N2S) promotesa selective synthesis the synthesis one with-dimensional with phase phase and (1D) and morphology control forgrowthmorphology tungsten-based of tungsten control oxide fornanomaterials tungstenphases such-based as can WO nanomaterials3 be or W realized18O49. Hence, can via be a our realized selective simple via synthesis our and simple facile with phase and synthetic facile and method. morphologysynthetic method. control Notably, for tungsten a well-definedbased nanomaterials 2D nanoflower can morphology be realized of viaWS our2 (cysteine simple 1:2) and may facile be Notably, a well-defined 2D nanoflower morphology of WS2 (cysteine 1:2) may be of importance for syntheticof importance method. for enhancingNotably, a electrocatalyticwell-defined 2D activity nanoflower due to morphology the high surface of WS area2 (cysteine for electrochem 1:2) mayical be enhancing electrocatalytic activity due to the high surface area for electrochemical reactions. ofreactions. importance for enhancing electrocatalytic activity due to the high surface area for electrochemical reactions.

Figure 2. FESEM (a) low and (b) high resolution images of W18O49 nanowires synthesized using Figure 2. FESEM (a) low and (b) high resolution images of W18O49 nanowires synthesized using WCl6 Figure 2. FESEM (a) low and (bWCl) high6 in resolution ethanol at image 180 °Cs offor W 2418 Oh.49 nanowires synthesized using in ethanol at 180 ◦C for 24 h. WCl6 in ethanol at 180 °C for 24 h.

Figure 3. FESEM (a) low and (b) high resolution images of WO3 nanowires synthesized using WCl6 Figurein ethanol 3. FESEM and thiourea (a) low at and 180 ( °Cb) highfor 24 resolution h. images of WO3 nanowires synthesized using WCl6 Figure 3. inFESEM ethanol( and) low thiourea and at (b 180) high °C for resolution 24 h. images of WO3 nanowires synthesized using WCl6 in ethanolCatalysts and 20 thiourea20, 10, x FOR at PEER 180 REVIEW◦C for 24 h. 4 of 9

Figure 4. FESEM (a) low and (b) high resolution images of WS2 nanosheets (cysteine 1:2) synthesized Figure 4. FESEM (a) low and (b) high resolution images of WS2 nanosheets (cysteine 1:2) using WCl6 in ethanol and cysteine at 180 °C for 24 h. synthesized using WCl6 in ethanol and cysteine at 180 ◦C for 24 h.

Figure 5. FESEM (a) low and (b) high resolution images of WS2 nanosheets (cysteine 1:4) synthesized

using WCl6 in ethanol and cysteine at 180 °C for 24 h.

X-ray photoelectron spectroscopy (XPS) was performed on the WS2 (cysteine 1:2) sample to study chemical and electronic structures of the surface (Figure 6). High-resolution XPS scans were measured on W 4f, S 2p, O 1s, and N 1s. The presence of W–S bonds in WS2 (cysteine 1:2) was confirmed because W 4f3/2 and W 4f1/2 peaks were observed at 32.18 and 34.38 eV, respectively (Figure 6a) [14]. Two doublets were observed for W 4f3/2 and W 4f1/2 peaks, with the corresponding satellite peaks at 33.18 and 35.08 eV indicating the existence of W4+ in the hexagonal WS2 phase. In the S 2p spectra, the XPS peaks for W–S were observed at ~161.88 eV (Figure 6b). The XPS peaks in the S 2p spectra near 163.18 and 164.68 eV are associated with S–O and S–OH bonds, respectively, which can be formed during solvothermal reaction using ethanol as a solvent [14]. The high-resolution XPS spectra of the O 1s core level is shown in Figure 6c. The prominent peak at binding energies of 531.78 eV can be ascribed to O–W6+ or O–W5+ bonds, respectively, which may be presented in the surface (oxy)hydroxide layers [6,17]. Notably, the high-resolution N 1s XPS spectra showed the presence of N in the WS2 (cysteine 1:2) sample, which could have originated from cysteine (C3H7NO2S) during the synthesis. The amount of nitrogen in the sample was determined as 2.41 at% by analyzing the relative peak area in the XPS spectrum. In addition, the deconvoluted N 1s peak revealed that the incorporated nitrogen is primarily present as quaternary nitrogen (~401.48 eV) with an additional sp2 hybridized nitrogen component (~399.8 eV). Given that pyridinic nitrogen can act as an active site for electrocatalytic reactions or promote charge transfer between the adsorbents and the catalyst surface, improved catalytic performance can be expected for the WS2 (cysteine 1:2) sample owing to the pyridinic nitrogen [18–22].

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Catalysts 2020, 10, 1238 4 of 9 Figure 4. FESEM (a) low and (b) high resolution images of WS2 nanosheets (cysteine 1:2) synthesized

using WCl6 in ethanol and cysteine at 180 °C for 24 h.

Figure 5. FESEM (a) low and (b) high resolution images of WS2 nanosheets (cysteine 1:4) synthesized Figure 5. FESEM (a) low and (b) high resolution images of WS2 nanosheets (cysteine 1:4) using WCl6 in ethanol and cysteine at 180 °C for 24 h. synthesized using WCl6 in ethanol and cysteine at 180 ◦C for 24 h. X-ray photoelectron spectroscopy (XPS) was performed on the WS2 (cysteine 1:2) sample to X-raystudy photoelectron chemical and spectroscopy electronic structures (XPS) of wasthe surface performed (Figure on 6). theHigh WS-resolution2 (cysteine XPS 1:2)scans sample were to study chemical andmeasured electronic on W 4f, structures S 2p, O 1s, of and the N surface 1s. The (Figure presence6 ). of High-resolutionW–S bonds in WS2 XPS(cysteine scans 1:2) were was measured confirmed because W 4f3/2 and W 4f1/2 peaks were observed at 32.18 and 34.38 eV, respectively (Figure on W 4f, S 2p, O 1s, and N 1s. The presence of W–S bonds in WS2 (cysteine 1:2) was conﬁrmed 6a) [14]. Two doublets were observed for W 4f3/2 and W 4f1/2 peaks, with the corresponding satellite because Wpeaks 4f3 /at2 and33.18 Wand 4f 35.081/2 peaks eV indicating were observedthe existence at of 32.18 W4+ in and the hexagonal 34.38 eV, WS respectively2 phase. In the (Figure S 2p 6a) [ 14]. Two doubletsspectra, were the XPS observed peaks forfor W–S W were 4f3 /observed2 and W at 4f~161.881/2 peaks, eV (Figu withre 6b). the The corresponding XPS peaks in the satelliteS 2p peaks spectra near 163.18 and 164.68 eV are associated with S–4O+ and S–OH bonds, respectively, which can at 33.18 and 35.08 eV indicating the existence of W in the hexagonal WS2 phase. In the S 2p be formed during solvothermal reaction using ethanol as a solvent [14]. The high-resolution XPS spectra, the XPS peaks for W–S were observed at ~161.88 eV (Figure6b). The XPS peaks in the S spectra of the O 1s core level is shown in Figure 6c. The prominent peak at binding energies of 531.78 2p spectraeV near can be 163.18 ascribed and to O 164.68–W6+ or eV O– areW5+ bonds, associated respectively, with S–Owhich and may S–OHbe presented bonds, in the respectively, surface which can be formed(oxy)hydroxide during layers solvothermal [6,17]. Notably, reaction the high using-resolution ethanol N 1s XPS as aspectra solvent showed [14 ].the Thepresence high-resolution of XPS spectraN in ofthe the WS O2 (cysteine 1s core 1:2) level sample is, shown which could in Figure have originated6c. The from prominent cysteine (C peak3H7NO at2S) binding during energies the synthesis. The amount of nitrogen6+ in the sample5+ was determined as 2.41 at% by analyzing the of 531.78relative eV can peak be area ascribed in the XPS to O–W spectrum.or In O–W addition,bonds, the deconvoluted respectively, N 1s peak which revealed may that be the presented in the surfaceincorporated (oxy)hydroxide nitrogen is layers primarily [6 ,present17]. Notably, as quaternary the high-resolutionnitrogen (~401.48 eV) N with 1s an XPS additional spectra sp2 showed the presence ofhybridized N in the nitrogen WS2 (cysteine component 1:2) (~399.8 sample, eV). Given which that could pyridinic have nitrogen originated can act fromas an active cysteine site for (C 3H7NO2S) during theele synthesis.ctrocatalytic Thereactions amount or promote of nitrogen charge transfer in the between sample the was adsorbents determined and the as catalyst 2.41 at%surface, by analyzing improved catalytic performance can be expected for the WS2 (cysteine 1:2) sample owing to the the relativepyridinic peak nitrogen area in [18 the–22 XPS]. spectrum. In addition, the deconvoluted N 1s peak revealed that the incorporated nitrogen is primarily present as quaternary nitrogen (~401.48 eV) with an additional sp2 hybridized nitrogen component (~399.8 eV). Given that pyridinic nitrogen can act as an active site for electrocatalytic reactions or promote charge transfer between the adsorbents and the catalyst surface, improved catalytic performance can be expected for the WS2 (cysteine 1:2) sample owing to the pyridinic nitrogen [18–22]. The electrocatalytic HER performance of W18O49, WO3, WS2 (cysteine 1:2), and WS2 (cysteine 1:4) was tested in 0.5 M H2SO4 solution using a three-electrode measurement system with a rotating disk 2 electrode (RDE, glassy carbon) and a loading amount of 0.28 mg cm− . For comparison, the bare glassy carbon electrode (GCE) and the benchmark Pt/C (20 wt %) catalyst were also tested. Linear sweep 1 voltammetry (LSV) was performed at a scan rate of 2 mV s− while the RDE was continuously rotated at 1600 rpm to remove any bubbles generated during the measurements. All the measured potentials were iR-compensated, which were then referenced to a reversible hydrogen electrode (RHE). Figure7a shows LSV polarization curves for the measured samples. An overpotential to deliver 10 mA 2 − cm− (η10) is widely used as a ﬁgure of merit to compare catalytic activity for the HER. Among the tested samples, the WS2 (cysteine 1:2) sample exhibited the lowest η10 (130 mV), indicating its high electrocatalytic activity for the HER in acidic media, which is comparable to that of the benchmark Pt/C sample (80 mV). The η10 values of W18O49, WO3, and WS2 (cysteine 1:4) were determined as 233, 181, and 143 mV, respectively. Figure7b shows the calculated Tafel slope for the tested samples. 1 Notably, the Tafel slope of WS2 (cysteine 1:2) (45 mV dec− ) exhibited a much lower value compared 1 to the control samples, except for Pt/C (31 mV dec− ). This reveals that the WS2 phase with a 2D nanosheet morphology is advantageous to obtain a superior electrocatalytic activity for the acidic HER compared to the W18O49 or WO3 phases with a 1D nanowire morphology. Two major factors could be responsible for the superior electrocatalytic HER performance of the WS2 (cysteine 1:4) and Catalysts 2020, 10, 1238 5 of 9

(cysteine 1:2): (i) The higher electrical conductivity of the tungsten sulfide phase compared to the tungsten oxide phase can enhance charge transfer during the HER. (ii) A well-defined 2D nanosheet morphology can secure a high electrochemically active surface area for the HER. In addition, the higher catalytic activity of the WS2 (cysteine 1:2) in comparison to the WS2 (cysteine 1:4) can be attributed to the smaller particle size, leading to a higher surface area. The electrocatalytic stability of the WS2 (cysteine 1:2) was tested using a chronoamperometric (CA) study. Additionally, the HER performance of WS2 (cysteine 1:2) is compared with other recently reported catalysts based on transition metal chalcogenides in Table1. The WS 2 (cysteine 1:2) showed outstanding HER performance in terms of low overpotential and the Tafel slope, highlighting its potential as a promising HER catalyst. Figure7c presents CA data for the WS2 (cysteine 1:2) sample for 10 h under the applied voltage of 0.14 VRHE in 0.5 M H2SO4. Notably, no noticeable current decrease was observed during the CA test for 10 h. Moreover, the LSV polarization curve was also measured for the WS2 (cysteine 1:2) sample after a continuous CA test for 10 h (Figure7d). Almost identical LSV curves were obtained before and after the CA test. These results highlight the excellent electrocatalytic stability of the WS2 (cysteine 1:2) sample for the HER in acidic conditions. These results demonstrate that the WS2 (cysteine 1:2) can be a promising electrocatalyst with high activity and stability for the acidic HER. In addition, our facile synthetic method to simultaneously control materials’ phases and nanostructures is of great significance for designing new functional catalytic nanomaterials. Catalysts 2020, 10, x FOR PEER REVIEW 5 of 9

Figure 6. XPS spectra of WS2 nanosheets (cysteine 1:2) for (a) W 4f, (b) S 2p, (c) O 1s, (d) N 1s, and (e) Figure 6. XPS spectra of WS2 nanosheets (cysteine 1:2) for (a) W 4f, (b) S 2p, (c) O 1s, (d) N 1s, and survey scan. (e) survey scan.

The electrocatalytic HER performance of W18O49, WO3, WS2 (cysteine 1:2), and WS2 (cysteine 1:4) was tested in 0.5 M H2SO4 solution using a three-electrode measurement system with a rotating disk electrode (RDE, glassy carbon) and a loading amount of 0.28 mg cm−2. For comparison, the bare glassy carbon electrode (GCE) and the benchmark Pt/C (20 wt %) catalyst were also tested. Linear sweep voltammetry (LSV) was performed at a scan rate of 2 mV s−1 while the RDE was continuously rotated at 1600 rpm to remove any bubbles generated during the measurements. All the measured potentials were iR-compensated, which were then referenced to a reversible hydrogen electrode (RHE). Figure 7a shows LSV polarization curves for the measured samples. An overpotential to deliver −10 mA cm−2 (η10) is widely used as a figure of merit to compare catalytic activity for the HER. Among the tested samples, the WS2 (cysteine 1:2) sample exhibited the lowest η10 (130 mV), indicating its high electrocatalytic activity for the HER in acidic media, which is comparable to that of the benchmark Pt/C sample (80 mV). The η10 values of W18O49, WO3, and WS2 (cysteine 1:4) were determined as 233, 181, and 143 mV, respectively. Figure 7b shows the calculated Tafel slope for the tested samples. Notably, the Tafel slope of WS2 (cysteine 1:2) (45 mV dec−1) exhibited a much lower value compared to the control samples, except for Pt/C (31 mV dec−1). This reveals that the WS2 phase with a 2D nanosheet morphology is advantageous to obtain a superior electrocatalytic activity for the acidic HER compared to the W18O49 or WO3 phases with a 1D nanowire morphology. Two major factors could be responsible for the superior electrocatalytic HER performance of the WS2 (cysteine 1:4) and (cysteine 1:2): (i) The higher electrical conductivity of the tungsten sulfide phase compared to the tungsten oxide phase can enhance charge transfer during the HER. (ii) A well-defined 2D nanosheet

Catalysts 2020, 10, x FOR PEER REVIEW 6 of 9

Catalysts 2020morphology, 10, 1238 can secure a high electrochemically active surface area for the HER. In addition, the 6 of 9 higher catalytic activity of the WS2 (cysteine 1:2) in comparison to the WS2 (cysteine 1:4) can be attributed to the smaller particle size, leading to a higher surface area. The electrocatalytic stability of Tablethe 1. Comparison WS2 (cysteine of 1:2) the was catalytic tested HERusing properties a chronoamperometric by WS2 (cysteine (CA) study. 1:2) to Additionally, recently reported the HER catalysts in 0.5performance M H2SO4. of WS2 (cysteine 1:2) is compared with other recently reported catalysts based on transition metal chalcogenides in Table 1. The WS2 (cysteine 1:2) showed outstanding HER 1 performanceMaterials in terms of Overpotential low overpotential (mV) and theTafel Tafel Slope slope (mV, highlighting dec− ) its potentialReference as a promising HER catalyst. Figure 7c presents CA2 data for the WS2 (cysteine 1:2) sample for 10 h under WS2 (cysteine 1:2) 130 @ 10 mA cm− 45 This work the applied voltage of 0.14 VRHE in 0.5 M H2SO4.2 Notably, no noticeable current decrease was observed N-WS2-H2 197 @ 100 mA cm− 69 15 during the CA test for 10 h. Moreover, the LSV2 polarization curve was also measured for the WS2 (cysteineN-doped 1:2) MoS sampl2 e after164 a continuous @ 10 mA cmCA− test for 10 h (Figure71 7d). Almost identical LSV 19 curves 2 werePd-WS obtained2 NF before and175 after @ 10 the mA CA cm test.− These results highlight54 the excellent electrocatalytic 20 2 stabilityV0.0065-WS of the2/ CCWS2 (cysteine148 1:2) @ 10sample mA for cm −the HER in acidic condition72s. These results demonstrate 21 2 thatTe-doped the WS2WS (cysteine2 1:2)213 can @be 10 a promising mA cm− electrocatalyst with94 high activity and stability 22 for the acidic HER. In addition, our facile synthetic method to simultaneously control materials’ phases and nanostructures is of great significance for designing new functional catalytic nanomaterials.

Figure 7. (Figurea) Linear 7. (a) sweepLinear sweep voltammetry voltammetry (LSV) (LSV) polarization polarization curves curves measured measured (iR-compensated) (iR-compensated) at a at a scan rate of 2 1mV s−1 in 0.5 M H2SO4. (b) Calculated Tafel slopes of the samples. (c) CA curve of WS2 scan rate of 2 mV s− in 0.5 M H2SO4.(b) Calculated Tafel slopes of the samples. (c) CA curve of nanosheets (cysteine 1:2) measured in 0.5 M H2SO4 for 10 h. (d) LSV curves of WS2 nanosheets WS2 nanosheets(cysteine (cysteine 1:2) before 1:2)and after measured the CA intest. 0.5 All M current H2SO density4 for 10is based h. (d on) LSVthe geometric curves ofarea WS of 2thenanosheets (cysteine 1:2)electrode. before and after the CA test. All current density is based on the geometric area of the electrode. Table 1. Comparison of the catalytic HER properties by WS2 (cysteine 1:2) to recently reported 3. Experimentalcatalysts Procedure in 0.5 M H2SO4. Materials Overpotential (mV) Tafel Slope (mV dec−1) Reference −2 3.1. Synthesis of W18O49 NanowiresWS2 (cysteine 1:2) 130 @ 10 mA cm 45 This work The material was synthesized using a solvothermal technique. In a typical synthesis, 0.005 M of WCl6 (Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 80 mL ethanol (Sigma-Aldrich, St. Louis, MO, USA) at room temperature and stirred for 30 min at a rotation speed of 400 rpm. The above solution was transferred to a 100 mL autoclave chamber. Then, the Teﬂon-linked stainless steel autoclave was heated at 180 ◦C for 24 h in a vacuum oven. The product was collected by centrifuging and washing several times using ethanol and distilled water. Finally, the sample was dried at 60 ◦C overnight in a hot air oven. Catalysts 2020, 10, 1238 7 of 9

3.2. Synthesis of Hex-WO3 Nanowires

During the synthesis of W18O49, 0.01 M of thiourea (CH4N2S, Sigma-Aldrich, St. Louis, MO, USA) was added to the WCl6-ethanol solution and stirred for 30 min with a rotation speed of 400 rpm at room temperature. Then the Teﬂon-lined stainless-steel autoclave was heated at 180 ◦C for 24 h in a vacuum oven. The blue-colored precipitate powder was collected by centrifuging and washing several times using ethanol and distilled water. Finally, the sample was dried at 60 ◦C overnight in a hot air oven.

3.3. Synthesis of Hex-WS2 Nanosheets

During the synthesis of hex-WO3, 0.01 M and 0.02 M of cysteine (C3H7NO2S, Sigma-Aldrich, St. Louis, MO, USA) was used by replacing thiourea (CH4N2S), keeping rest of the parameters ﬁxed. The black-colored precipitate powder was collected by centrifuging and washing several times using ethanol and distilled water. Finally, the sample was dried at 60 ◦C overnight in a hot air oven.

3.4. Structural and Chemical Characterizations An X-ray diffractometer (XRD, D/MAX RINT-2000, Rigaku, Tokyo, Japan) was employed to investigate the crystal structures of the samples using Cu Kα X-rays (1.54 Å) at 40 kV and 40 mA. Field-emission scanning electron microscopy (FE-SEM, JSM-7600F, Jeol, Tokyo, Japan) was used to study the morphology of the samples. The electronic structure and valence states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha+, Thermo Fisher Scientific, Waltham, MA, USA) with 100 and 20 eV for the XPS survey and high-resolution scans, respectively.

3.5. Electrochemical Characterization The electrochemical properties of the samples were studied using a standard three-electrode system (Autolab PGSTAT, Metrohm Autolab, Utrecht, Netherlands). A rotating disk electrode (RDE, Meetrohm Autolab, Utrecht, Netherlands) conﬁguration was used for all electrochemical tests. A 0.5 M H2SO4 electrolyte was used for the electrochemical tests. A quantity of 5 mg of the catalyst powder was mixed well in a solution of 1 mL of water and ethanol with a volume ratio of 3:1. A Naﬁon® solution (5 wt %, 30 µL, Sigma-Aldrich, St. Louis, MO, USA) was added to secure a rigid bind between catalyst powder and the electrode surface. Five microliters of as-prepared catalyst ink was drop-coated on the glassy carbon (GC) RDE electrode with a diameter of 3 mm. Then, the electrode was dried overnight at room temperature. The RDE electrode as the working electrode, Ag/AgCl (3.0 M KCl, Meetrohm Autolab, Utrecht, Netherlands) as the reference electrode, and graphite rod as the counter electrode were used for all electrochemical tests. Voltage calibration was performed to change the 1 measured working potential to the reversible hydrogen electrode (RHE). A scan rate of 2 mV s− was used for the linear sweep voltammetry.

4. Conclusions

W18O49, WO3, WS2 (cysteine 1:2), and WS2 (cysteine 1:4) were synthesized via a one-step facile solvothermal method. The phase-selective synthesis was achieved by simply changing starting precursors such as cysteine (C3H7NO2S) or thiourea (CH4N2S). XRD and SEM results conﬁrm that cysteine (C3H7NO2S) facilitates the formation of tungsten sulﬁde phases in a 2D nanosheet morphology, while thiourea (CH4N2S) promotes the formation of tungsten oxide phases in a 1D nanowire structure. The electrocatalytic activity of the samples was tested for the acidic HER, and the WS2 (cysteine 1:2) showed the best catalytic performance among the tested samples, comparable to that of the benchmark Pt/C catalyst. In addition, excellent electrocatalytic stability was demonstrated for the WS2 (cysteine 1:2) using a long-term stability test. These results reveal that our facile chemical synthetic method is powerful for the design of high-performance electrocatalysts with controlled phase and morphology.

Author Contributions: Conceptualization, A.K.N. and H.H; methodology, H.H.; formal analysis, E.E. and S.J.K.; investigation, A.K.N.; writing—original draft preparation, A.K.N. and H.H.; writing—review and editing, H.H.; Catalysts 2020, 10, 1238 8 of 9 supervision, A.K.N. and H.H.; project administration, A.K.N. and H.H.; funding acquisition, A.K.N. and H.H. All authors have read and agreed to the published version of the manuscript Funding: This research received no external funding. Acknowledgments: We thank for Heejun Chung for fruitful discussions on this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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