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Novel amorphous molybdenum selenide as an efficient catalyst for the hydrogen evolution reaction Quyen T. Nguyen, Phuc Dinh Nguyen, Duc N. Nguyen, Quang Duc Truong, Tran Thi Kim Chi, Thuy Ung, Itaru Honma, Nguyen Quang Liem, and Phong D. Tran ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18675 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Applied Materials & Interfaces

Novel amorphous molybdenum selenide as an efficient catalyst for the hydrogen evolution reaction Quyen T. Nguyen,a Phuc D. Nguyen,b Duc N. Nguyen,a Quang Duc Truong,c Tran Thi Kim Chi,b Thuy Thi Dieu Ung,b Itaru Honma,c Nguyen Quang Liem,b* and Phong D. Trana,d* a

Department of Advanced Materials Science and Nanotechnology, University of

Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi 100000, Vietnam. Email: [email protected] b

Institute of Materials Science, Vietnam Academy of Science and Technology, 18

Hoang Quoc Viet, Hanoi 100000, Vietnam. Email: [email protected] c

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,

Sendai 980-8577, Japan d

Department of Chemistry, College of Natural Sciences, Hanyang University, Seoul

04763, Republic of Korea. Email: [email protected]

ACS Paragon Plus Environment

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Abstract Amorphous molybdenum selenide nanopowder, obtained by refluxing Mo(CO)6 and Se precursors in dichlorobenzene, shows several structural and electrochemical similarities to the amorphous molybdenum sulfide analogue. The molybdenum selenide displays attractive catalytic properties for the hydrogen evolution reaction in water over a wide range of pH. In a pH 0 solution, it operates with a small onset overpotential of 125 mV and requires an overpotential of 270 mV for generating a catalytic current of 10 mA/cm2. Compared with the molybdenum sulfide, the selenide analogue is more robust in a basic electrolyte. Therefore, the molybdenum selenide is a

potential

candidate

for

incorporating

within

an

electrolyser

or

a

photoelectrochemical cell for water electrolysis either in acidic, neutral or alkaline medium.

Keywords: Water electrolysis, catalysis, molybdenum selenide, hydrogen, cluster


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1. INTRODUCTION Platinum is the best catalyst for the hydrogen evolution reaction (HER), a key reaction of the (solar) water splitting process. However, due to its low abundancy, employment of platinum catalyst in a technological device like a water electrolyser or a photoelectrochemical cell is not viable.1 Efforts are being mobilized in order to identify potential alternatives made of earth abundant elements to replace platinum.2-4 Among reported catalysts, the transition metal chalcogenides display outstanding catalytic performance, high robustness and interesting capability to be hybridized with semiconducting light harvesters for creation of hybrid photocatalysts.3, 5, 6 For the case of molybdenum sulfides, both amorphous molybdenum sulfide a-MoSx and polycrystalline molybdenum disulfides MoS2 have been intensively investigated.3, 7-9 It was found that the preparation conditions strongly influence onto the nanostructure and performance of these sulfide catalysts. For the selenide analogue, few investigations on the potential application of the polycrystalline MoSe210-12 and MoSe2-based composites13-16 for the HER were readily available. However, less findings were achieved for the case of amorphous molybdenum selenide. Saadi et al. reported on the preparation of an amorphous molybdenum selenide, named MoSe3, from (NH4)6Mo7O24 and Na2Se precursors in an acidic solution,17 following a similar process to that had been employed for preparing MoS3 analogue by Benck et al.18 The as-prepared MoSe3 was not active but its catalytic performance was remarkably enhanced by simply conditioning the catalyst under the H2-evolving conditions. The actual active species was assigned to an amorphous molybdenum selenide whose composition is close to MoSe2. When the steady performance was reached, it


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generated a catalytic current of 10 mA/cm2 at an applied overpotential of 250 mV, thus being comparable with the top-tier MoS2 catalysts. In this work, we present a novel amorphous molybdenum selenide catalyst, hereafter denoted as MoSea, that displays attractive catalytic activity for the HER over a wide range of pH solution. In pH 0 electrolyte, the MoSea requires 270 mV overpotential to generate catalytic current of 10 mA/cm2 and displays a Tafel slop of 60 mV/decade. Moreover, the MoSea showed some structural and electrochemical similarities to the amorphous molybdenum sulfide analogue. 2. EXPERIMENTAL SECTION 2.1 Synthesis of amorphous molybdenum selenide (MoSea). Molybdenum hexacarbonyl (Mo(CO)6, 98%), selenium powder 100 mesh (Se, 99,99%), 1,2-dichlorobenzene anhydrous (99%), n-hexane anhydrous (95%), carbon disulfide anhydrous (CS2, 99%) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH, 99%) (pellets) was purchased from Merck. All the chemicals and solvents were used as received without any purification. We prepared MoSea following the process described by Hibble et al.19 Mo(CO)6 (0.5 g) and Se powder (0.9 g) were added into 30 mL of 1,2-dichlorobenzene in a 100 mL round bottom flask fitted with a reflux condenser. The mixture was heated under reflux for 4h with vigorous stirring. When the reaction was over, the mixture was cooled to room temperature. Black precipitate was collected by centrifugation at an angular rate of 6000 rpm. The product was then intensively washed with carbon disulfide (CS2), n-hexane, and finally dried in air. 2.2 Physical and chemical characterizations Morphology of the MoSea nanoparticles was characterized by the Field Emission Scanning Electron Microscopy (FESEM), using a Hitachi S-4800 equipment. The


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close morphology and structure of these nanoparticles were analysed by High Resolution Transmission Electron Spectroscopy using a HRTEM JEOL JEM-2100 equipment with an electron beam of 200 kV. Powder X-ray diffraction analysis was conducted employing the XPERT – PRO PANalytical X-ray diffraction (XRD) with the X-ray source Cu Kα 0.15406nm. Raman spectrum was collected using a microRaman system (HORIBA Scientific, Japan). The powder sample was excited by using a 532 nm green laser. Scans were taken on an extended range of (100–3000 cm-1) with an exposure time of 60 s. The MoSea powder was pressed on a glass slide for observation. The sample was viewed by using a red laser apparatus under the maximum magnification of x100. FTIR spectrum of MoSea/KBr pellet was collected using the Thermo Scientific™ Nicolet™ iS™50 Spectrometer. Chemical composition of the MoSea was analysed by Energy Dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). EDX was carried out employing an excitation beam of 15 kV. XPS analysis was conducted on ULVAC PHI 500 (Versa Probe II) equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. 2.3 Electrochemical characterizations All electrochemical measurement was conducted employing a potentiostate Metrohm Autolab PG302N. A conventional two compartments, three electrodes configuration was used. The reference electrode was an Ag/AgCl/1M KCl while the counter electrode was a Pt plate. The working electrode was either a MoSea-loaded glassy carbon (S 0.071 cm2), a MoSea-loaded fluorine doped tin oxide electrode (FTO, S 0.2826 cm2) or a MoSea-loaded Au/Quartz electrode (S 5.22cm2).


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The reference electrode was regularly calibrated following the procedure described elsewhere.20, 21 Potentials are quoted against the Normal Hydrogen Electrode (NHE) or the Reversible Hydrogen Electrode (RHE) (equations 1,2). E vs. NHE = E vs. Ag/AgCl + 0.21 (V)

(equation 1)

E vs. RHE = E vs. Ag/AgCl + 0.21 + pH x 0.059 V

(equation 2)

After electrochemical measurement, MoSea electrode was dipped into DI water several times to remove physio-adsorbed species, then dried under N2 flux prior to use for microscopic and spectroscopic characterizations. Catalyst ink was prepared by homogeneously dispersing MoSea nanoparticles (3 mg) in 1 mL of a H2O/EtOH solvent mixture (4/1 v/v) together with 1 µL of Nafion 5% (wt%) as the binder with help of ultrasonication. The catalyst was loaded onto the surface of a glassy carbon or a FTO electrode at the loading density of 212 µg/cm2 by depositing aliquots of the catalyst ink. The electrodes were first dried naturally in air, then in a vacuum oven prior to use. Electrochemical property of the MoSea electrode was assayed in a H2SO4, citrate buffer, phosphate buffer, borate buffer or NaOH solution adjusted to different pHs. Prior to electrochemical measurement, the electrolyte solutions were saturated with research grade N2 gas to remove the O2-dissolved. 2.4 QCM analysis We used a Stanford research system QCM200 for the Quartz crystal microbalance analysis. The crystal sensor is a 5 MHz, AT-cut, α-quartz with circular patterned gold electrodes on both sides (S 5.22cm2). To enhance the adhesion of MoSea catalyst onto the large Au electrode surface, higher Nafion binder content of 100 µL of Nafion 5% was added into 1 mL of the catalyst


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ink. 5 µL of the catalyst ink was dropped onto the patterned Au surface of the electrode. The resultant electrode, namely MoSea/Au/Quartz, was first dried naturally in ambient condition and then further dried in vacuum oven at 80 oC for 1 hour. The electrode was then cooled down naturally and ready for the experiment. The QCM analysis was conducted in a pH 0 H2SO4 being free of O2. Prior to measurement, the MoSea/Au/Quartz electrode was immersed into the electrolyte and hold without applying potential until it reached the steady weight (e.g. after a period of 2h). Subsequently, potential polarization with a slow potential scan rate of 0.5 mV/s was applied to the electrode from the open-circuit voltage to -0.3V vs. RHE while the electrode weight was simultaneously tracked by QCM analysis. 3. RESULTS AND DISCUSSION 3.1 Physical and chemical characterization of MoSea nanopowder Scanning electron microscopic (SEM) analysis carried out on the MoSea nanopowder revealed nanospherical shape, sized of ca. 30-50 nm in diameter, with well-defined border suggesting low degree of agglomeration (figure 1a). Transmission electron microscopic analysis showed the amorphous characteristic (figure 1b). It was further confirmed by powder X-ray diffraction analysis, showing only patterns of excess Se precursor (figure S1).


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Figure 1. Characterization of MoSea by SEM (a), TEM (b) and Raman spectroscopy with an excitation by a 532 nm green laser (c).

Figure 2. XPS analysis conducted on an as-prepared MoSea catalyst electrode (a, b) and on the same electrode when it reached its steady catalytic performance (c, d).


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The chemical composition was first characterized by Energy Dispersive X-ray spectroscopy. It revealed a Mo:Se atomic ratio of 1:4.6 (figure S2), being very close to the result reported by Hibble et al.19 This composition is much richer in Se content than a possible MoSe2. We then employed X-ray Photoelectron Spectroscopy analysis to identify the chemical state of Mo and Se elements within the MoSea nanopowder. Deconvolution of the Se 3d envelop revealed two sets of doublet. The set at lower binding energies, Se 3d5/2 of 54.4 eV and Se 3d3/2 of 55.3 eV, is attributed to Se2ligand (figure 2b, blue trace).16 The set at higher binding energies is assigned to the excessive Se0 element (figure 2b, red trace). On the basis of XPS analysis, the atomic concentration of Se0 element was calculated to be 14% of the total Se-based species. Putting beside the Se elemental impurity, a Mo:Se ratio of 1: 3.9 was deduced for the actual MoSea. This ratio is very close to the Mo:S ratio of 1:4, determined for the amorphous molybdenum sulfide a-MoSx.22 The deconvolution of Mo 3d envelop shows a doublet with Mo 3d5/2 peaked at 228.4 and Mo 3d3/2 peaked at 231.5 eV, being characteristics for Mo4+ species (figure 2a, blue trace).16, 17, 23 The Se 1s single peak was kept at 230 eV (figure 2a, green trace).17 It is obvious that species with higher oxidation state like MoO3 oxide,17, 24 Mo=O oxo,22 or MoOxSy25 are not present within the MoSea powder even if it was handled in open-air for longtime. However, in the Raman scattering analysis, we observed two peaks at 820 cm-1 (Ag, νs Mo=O stretch) and 991 cm-1 (Ag, νas Mo=O stretch), being characteristics for molybdenum oxide species.26 It indicates a partial oxidation of MoSea powder under the excitation of the Laser beam in ambient condition. We note that an oxidation of a-MoSx into MoOx under a laser beam was recently reported.27 Characteristic signatures of crystalline MoSe2 at Raman shifts of 240-243 cm-1 (A1g mode) and 290 cm-1 (E2g mode) were not observed.12, 23, 28 Thus, we excluded the co-presence of crystalline


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MoSe2 within the MoSea. Assuming that the current amorphous MoSea has a coordination polymeric structure made of [Mo3Se13]2- building block, being similar to the polymeric structure of a-MoSx constituted of [Mo3S13]2- building block,22 we tentatively assigned the band at 552 cm-1 to vibration of (Se-Se)2- ligand, band 462 cm-1 to Mo3-µSe bond while the remained bands at 371.6, 308.5 and 249.5 cm-1 to Mo-Se bonds within the [Mo3Se13]2- cluster. Further works are on-going in order to confirm this spectroscopic assignment. For instance, Raman analysis on a [Mo3Se13]2cluster is not available for reference even it was successfully synthesized and its crystal structure was solved.29 Putting together these available results, we can conclude that the MoSea is a new amorphous phase of molybdenum selenide that contains some Se element impurity. The crystalline MoSe2 phase was definitively not present within the MoSea. Indeed, the MoSe2 phase was co-generated with MoSea only when Se precursor amount was depleted. At Mo(CO)6:Se molar ratio of 1:3, SEM analysis revealed the co-presence of both spherical nanoparticles and nanosheets (figure S3a). TEM analysis clearly showed layer structure of MoSe2 slabs with characteristic layer space of 0.7 nm (figure S3b).14, 30 3.2 Electrochemical property of the MoSea nanopowder We then assayed the catalytic performance of MoSea nanopowder for the HER. In an O2-free pH 0 H2SO4 electrolyte solution, the as-prepared MoSea catalyst loaded on a glassy carbon disk electrode showed two small pre-catalytic events at ca. 0 V and 0.15 V vs. RHE, prior to the catalytic H2-evolving event at the onset potential of -0.20 V vs. RHE (figure 3, black trace). These two pre-catalytic events were only observable for the first potential polarization but completely suppressed in the subsequently polarizations. Interestingly we observed a clear enhancement of the


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catalytic performance upon repeating potential polarizations. Indeed, the steady performance was achieved by repeating three potential polarizations from the open circuit voltage, e.g. +0.4V, to -0.4V vs. RHE with a slow potential scan rate of 2 mV/s. Alternatively, it was achieved by holding the catalyst under the H2-evolving conditions at -0.3V vs. RHE for 30 min. At the steady state, MoSea required small onset overpotential of ca. 125 mV to operate and an overpotential of 270 mV to generate a catalytic current of 10 mA/cm2 (figure 2, red trace). This performance is comparable to that achieved for the amorphous molybdenum selenide developed by Saadi et al.17 This performance also puts the current MoSea among the promising MoSe-based catalysts (table 1).

Figure 3. Catalytic performance of a MoSea catalyst electrode assayed in a pH0 electrolyte. The black trace presents the first potential polarization on an as-prepared MoSea electrode while the red trace shows the polarization on the same electrode when it reaches its steady activity. The potential scan rate was 2 mV/s.


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Table 1. H2-evolving catalytic activity of some selected molybdenum selenide catalysts Entry

Catalyst

Onset

Overpotential to Tafel slope

overpotential

generate

(η η/mV)

jcat

10

Refs.

(mV/decade) 2

mA/cm

(η η/mV) 1

Amorphous MoSea

125

270

60

This work

2

Amorphous

150

250

80

17

75

160

80

11

80−110

170-180

45-66

31

150

210

56

10

70-115

182-222

69-76

16

molybdenum selenide 3

Porous

MoSe2

nanosheets 4

MoS2(1−x)Se2x nanoﬂakes

5

MoSe2−NiSe

6

MoSe2 nanosheets/C fiber cloth

7

MoSSe

200

235

63

15

8

MoSe2

170

280

95

10

9

MoSe2

210

310

>70

15

10

MoSe2

200

>450 mV

105-120

12

220

300

82

11

vertically

aligned

layers 11

MoSe2 nanosheets

In order to identify potential reason being responsible to the obvious catalytic enhancement observed, we tracked the change of the MoSea catalyst under the potential polarization. We first tracked the evolution of catalyst weight in function of potential applied by employing a Quartz Crystal Microbalance analysis (QCM). Similar to the observation on the MoSea-coated glass carbon electrode, the MoSea/Au/Quartz electrode showed two reduction pre-catalytic events at ca. +0.05 V ACS Paragon Plus Environment

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and -0.10 V prior to emerging of the H2-evolving catalytic event at onset potential of 0.15V vs. RHE (figure 4a, red trace). These reduction potentials represent anodic shifts compared with those recorded for a MoSea-coated glassy carbon electrode. This shifting is likely due to a favor electronic communication between the MoSea catalyst and gold electrode support.32,

33

We found that each pre-catalytic event caused a

decreasing of catalyst weight of ca. 15% (figure 4a, blue trace). In total, ca. 28% of catalyst weight was lost after the first potential polarization. This result suggests an electrochemical reductive corrosion process. The same reductive corrosion was recorded for the a-MoSx catalyst.34 In the latter case, the corrosion was attributed to the irreversible reduction of a-MoSx into MoS2-like material. However, it could be also the origin of the creation of Mo-vacant or molybdenum oxysulfide defects.22

Figure 4. Evolution of MoSea catalyst weight under the H2-evolving conditions tracked by QCM analysis (a). SEM images collected on an as-prepared (b) and on a steady MoSea catalyst (c).

We then employed XPS analysis to track the chemical changes of the MoSea catalyst. Indeed, when the catalyst reached its steady performance, an obvious change was


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recorded. Together with MoIV species, novel Mo species having higher binding energy, Mo3d5/2 peaked at 231.5 eV, appeared (figure 2c, red trace). We attributed these species to molybdenum oxysulfide defects like MoIV/VOxSy.25 These defects account for 43% of Mo sites as deduced on the basis of Mo3d envelop deconvolution. We also noted that content of both Se2- and Se0 element is reduced during the electrochemical treatment. In the steady MoSea catalyst, Se0 element still accounts for 15% of total Se species. The composition of the steady catalyst MoSea is Mo: Se of 1: 2.3, deduced on the basis of integration of all Mo species and Se2- species. Saadi et al reported on a remarkable catalytic enhancement of an amorphous molybdenum sulfide under the H2-evolving condition, in the same pH 0 electrolyte solution.17 In that selenide, high content of molybdenum oxide impurity was incorporated during the preparation. Thus, under H2-evolving conditions, the dissolution of this oxide into the acidic electrolyte induced a remarkable enhancement of the electrode porousness. As a consequence, an obvious increasing of the number of active sites exposed to electrolyte was achieved. Together with the reduction of MoVI/ MoV species into MoIV species, the change in catalyst morphology induced a remarkable improvement of the catalytic performance. However, for our current MoSea material, morphology and electrochemical surface area remained unchanged (figures 4b, 4c and S4). Thus the catalytic enhancement was attributed to the changes in the chemical nature of the MoSea catalyst, namely the increasing of Mo:Se atomic ratio from 1:3.9 to 1:2.3 and the creation of MoIV/VOxSy species due to the electrochemical corrosion under the H2-evolving conditions. However, further works are needed in order to reveal the actual implication of the MoIV/VOxSy defects within the catalytic H2-evolving cycle. For instance, in pH 0 electrolyte, MoSea catalyst operated with a Tafel slope of 60 mV/decade (figure 3).


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pH titration at a catalytic rate of 0.2 mA/cm2 showed a slope of 54 mV/pH (figure 5b). This result suggests a rate limiting step involving of 1H+ and 1e-.35 Similar phenomenon was recorded for the amorphous a-MoSx catalyst under identical conditions.22 For the latter catalyst, we have proposed a mechanism wherein the H2evolution can either follow a homolytic or heterolytic pathway from the MoV-H hydride species that was generated from the MoIV center via a 1H+, 1e- process. However, alternative mechanism where protonation occurred on the bridging disulfide ligand and oxo ligand served as a proton relay was also possible.36, 37

Figure 5. Potential polarization on an as-prepared MoSea catalyst electrode immersed in electrolyte solutions with different pHs. The potential scan rate was 2 mV/s (a). pH titration was conducted on a steady MoSea catalyst electrode at a constant catalytic rate of 0.2 mA/cm2 (b). We noted that MoSea catalyst showed similar electrochemical property in different pHs. Regardless of pH value, catalytic pre-peaks were observed prior to the H2evolving catalytic event (figure 5a). These peaks were then disappeared in subsequent potential polarizations. Remarkably, when the MoSea catalyst already reached its steady performance, it was stable for long-term operation (figures S5, S6). Even in a pH 11 alkaline electrolyte, we did not observe any degradation of catalytic activity of the steady MoSea catalyst. This robustness represents a clear advantage of ACS Paragon Plus Environment

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the current MoSea catalyst compared with the amorphous a-MoSx analogue. It was known that in the similar alkaline solution, the a-MoSx was rapidly and completely dissolved, generating soluble [Mo3S13]2- and other [MoS] clusters.22 4. CONCLUSION To conclude, the novel amorphous molybdenum selenide MoSea is an efficient and robust catalyst for the HER over a wide range of pH solutions. This material can be easily prepared by a scalable process employing commercially available precursors, namely Mo(CO)6 and Se. The MoSea showed several structural and electrochemical similarities to the amorphous molybdenum sulfide a-MoSx analogue. Like a-MoSx, the MoSea catalyst is activated under the H2-evolving conditions thanks to an electrochemical corrosion process that induced the generation of molybdenum oxygensulfide species. These species likely play a key role in the catalytic H2evolving cycle either as an electron reservoir or as a proton relay, or even as both. Compared with the a-MoSx analogue, the current MoSea is more robust in alkaline solutions. Thus, the MoSea catalyst is an attractive candidate for incorporating within an electrolyzer for water electrolysis or in a photoelectrochemical cell for solar water splitting, especially when the neutral or basic working medium is desired, e.g. when light harvesters are made of semiconductors like CuO, Cu2O, Cu2S that are chemically unstable in an acidic solution. ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publications website at DOI: The document consists of further detail on physical, chemical and electrochemical characterization of MoSea catalyst


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] ORCID Phong D. Tran: 0000-0002-9561-6881 Notes The authors declare no competing financial interest ACKNOWLEDGEMENTS This work was supported by the National Foundation for Science and Technology Development (NAFOSTED, project code 103.99-2015.46) and Office of Navy Research Global (Grant code N62909-16-1-2191). The authors acknowledge University of Science and Technology of Hanoi (USTH) and Institute of Materials Science, Vietnam Academy of Science and Technology for facilities supports. REFERENCES 1.

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