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Crystallization of Amorphous Molybdenum Sulfide Induced by Electron or Laser Beam and Its Effect on H Evolving Activities 2

Duc N. Nguyen, Linh N. Nguyen, Phuc Dinh Nguyen, Tran Viet Thu, Anh Duc Nguyen, and Phong Dinh Tran J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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The Journal of Physical Chemistry

Crystallization of Amorphous Molybdenum Sulfide Induced by Electron or Laser Beam and Its Effect on H2 Evolving Activities Duc N. Nguyen,a Linh N. Nguyen,a Phuc D. Nguyen,a Tran Viet Thu,b Anh D. Nguyena and Phong D. Trana* a

Department of Advanced Materials Science and Nanotechnology, University of Science and

Technology of Hanoi (USTH), Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam. Contact: [email protected]. Tel: +84 964602146 b

Le Quy Don Technical University, 236 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam

Abstract We report herein investigation on crystallization of amorphous molybdenum sulfide aMoSx induced by electron and laser beam resulting in formation of crystalline molybdenum disulfide c-MoS2. This crystallization occurred in-situ during Transmission Electron Microscopic and Raman analyses of a-MoSx material. It was also found that a-MoSx to cMoS2 phase transformation was not fully beneficial for H2 evolving catalytic performance. c-MoS2

showed better robustness but significant lower catalytic performance.

Furthermore, c-MoS2 was less tolerant to oxidation stress, as the one cause by photogenerated holes within light harvester, compared with a-MoSx catalyst. Thus, a-MoSx is a better candidate for implementation within photocatalysts for overall solar water splitting application.

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INTRODUCTION Molybdenum sufides based materials1-6 are attractive alternatives to platinum catalyst for the hydrogen evolution reaction (HER). Molybdenum sulfides refer to two main families: (i) amorphous molybdenum sulfide denoted as a-MoSx and (ii) crystalline molybdenum disulfide denoted as c-MoS2. The a-MoSx was identified to be a coordination polymer with [Mo3S13]2

−

discrete building block clusters, thus being fundamentally different compared with c-MoS2.7,8 Furthermore, this coordination polymer shows clear structure deformation under excitation of electron beam, e.g. during the scanning transmission electron microscopic (STEM) analysis.9 We hypothesize that this structure deformation can cause a phase transformation of a-MoSx, e.g. into crystalline c-MoS2. We are convinced a detail investigation on the a-MoSx to c-MoS2 phase transformation, induced by electron and laser beam during microscopic and spectroscopic analyses, is relevant in order to avoid confusing interpretation about structure and catalytic activities of these two materials. Indeed, under certain mild synthetic conditions such as electrodeposition in water7,10,11 in ionic liquid solution,12 product obtained seems to be a-MoSx which displays outstanding electrocatalytic properties. However, microscopic and spectroscopic signatures reported, type transmission electron microscopic (TEM) images and resonance Raman spectra, suggest the product is more likely c-MoS2 rather than a-MoSx.11,12 In other words, there is perhaps an ambiguity in interpretation of microscopic and spectroscopic signatures. Furthermore, it is not clear whether a-MoSx or c-MoS2 is more catalytically active.3 The main reason related to this unclear comparison is difficulty in deducing the actual number of active sites within these sulfides. In our recent work, we found that Mo-defect sites, type Mo=O oxo species or Mo−

unsaturated coordination vacant sites, are actual catalytic centres within a-

MoSx material.7 On these catalytic centres, the H2 evolving mechanism is proposed to run through MoV−H intermediates. Very recently, the same was proposed for c-MoS2 catalyst.13,14 The Mo-defect sites are created during the growth of a-MoSx or in the electrochemical activation step associated with the partial corrosion of this material. It is thus interesting to investigate how a crystallization process, being believed to be able fixing defect sites and enhancing the robustness of catalyst, influences on catalytic activity of a-MoSx catalyst.

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Herein we report investigation on the a-MoSx to c-MoS2 crystallization induced by excitation of electron or laser beam and its effects on catalytic H2 evolving activity. EXPERIMENTAL SECTION Generals All chemical compounds were in analytical grade and used as received without further purification. Fluorine-doped tin oxide (FTO) coated glass slides with 14 Ω/sq resistivity and a thickness of 400 nm were purchased from NSG group. FTO electrode was cleaned by subsequent sonication in acetone, isopropanol and ethanol and then dried by a nitrogen gas flux before used. Electrochemical experiments were performed on an Autolab PGSTAT-30 or a Bio-Logic SP 150 potentiostat employing a conventional three electrodes configuration. Customized two compartment electrochemical cell was used. The working electrode was the synthesized catalyst film on 1cm2 FTO or 0.071 cm2 carbon glassy electrode. Reference electrode was an Ag/AgCl 3M KCl while counter electrode was a Pt wire. To determine overpotential value, potential are quoted against the Reversible Hydrogen Electrode (RHE) by using the following equation: Evs. RHE = Evs. Ag/AgCl + 0.059pH + 0.21V a-MoSx powder and thin film were prepared following the (electro)chemical oxidation process described elsewhere.7 a-MoSx to c-MoS2 phase transformation induced by thermal annealing We conducted thermal treatment to determine a-MoSx to c-MoS2 phase transformation temperature. To do so, a-MoSx nanoparticles or thin films deposited on FTO substrates were annealed at different temperatures from 200 to 400°C for 2h under Ar atmosphere (flux rate of 20 sccm) in a tube furnace. Once the annealing was over, annealed samples were cooled down to room temperature under Ar atmosphere prior to be characterized by powder X-ray diffraction and Raman analyses. Raman analysis Raman analysis was conducted on a NRS-7100 Laser Raman Spectrometer (JASCO, Japan) using a 532 nm green line laser. Prior to measurement, wavelength calibration was done using Page 3 of 15



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the bulk Si peak at 520 cm–1 as standard reference. For the a-MoSx samples (both nanoparticles and thin films deposited on FTO substrate), laser power was varied from 0.1 to 3.6 mW (beam area reached to sample was ca. 0.76 µm2) to identify appropriate power inducing a-MoSx crystallization. For the c-MoS2 samples, power of 1.4 mW was used. In all experiments, exposed time was 480 seconds and a 100X objective lens was used. TEM analysis Transmission Electron Microscopic (TEM) analysis was conducted on a TEM, JEOL 2100F machine operated at 200kV and current density of ca. 20 pA.cm 2. a-MoSx samples were either −

nanoparticles or powders collected from thin films deposited on FTO electrode. These powders were then suspended into ethanol and drop casted on the lacey carbon copper grid. For the case of [MoS4](NH4)2, a dilute solution of this compound was drop-casted on the lacey carbon copper grid and air-dried prior analysis. We first quickly record a TEM image on as-prepared sample. The same sample location was then kept excited by the electron beam for a period of two minutes and four minutes before recording the second and third TEM images. We repeat this procedure for several locations of the sample to deduce a general phenomenon. XRD analysis Powder X-ray diffraction pattern of samples annealed at different temperatures was collected at room temperature from 10 to 70° 2θ with a step size of 0.02° using Bruker D8 Advance (XRD, Cu-Kα radiation) equipped with 1D Lynxeye detector. RESULTS AND DISCUSSION We first investigate the crystallization of a-MoSx induced by electron beam during the TEM analysis. Amorphous morphology of the as prepared a-MoSx is proved by TEM imaging (figure 1a). However, keeping continuous electron beam excitation (acceleration voltage of 200 kV and current density of ca. 20 pA.cm 2) for two (2) minutes induced the crystallization, resulting in −

formation of short slabs MoS2. As seen in figure 1b, c-MoS2 was produced in 1-4 layers, average of 2-6 nm in length. Layer to layer space distance of c-MoS2 was found to be 0.62 nm, indicating presence of (002) orientation.15 MoS2 slabs develop both in length and in number of stacked

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layers once electron beam is kept excited for longer period of time (figure 1c). The c-MoS2 crystallinity was also found to be improved with longer light excitation time as evidenced by selected area electron diffraction (SAED, figure 1, insert).

Figure 1. TEM images (insert SAED) recorded on a-MoSx sample after different excitation time by TEM electron beam with acceleration voltage of 200 kV and current density of ca. 20 pA.cm 2 −

showing progressive crystallization to c-MoS2: t0 (a), t0+2min (b) and t0+4min (c).

Figure 2. Resonance Raman analysis carried out on a-MoSx employing a 532 green laser source at different laser power: 0.6 mW (red trace), 1.2 mW (blue trace) and 3.6 mW (green trace)

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The crystallization of a-MoSx was also induced by laser beam during resonance Raman characterization. At low laser powers of less than 1.0 mW (laser beam reaches samples at area of ca. 0.78 µm2), we observed characteristic signatures of a-MoSx being a coordination polymer made of [Mo3S13]2 discrete building blocks. Bridging and sharing disulfide ligands ν(S−S)br/sh −

were found at 555 cm 1 while terminal disulfide ligand ν(S−S)t was found at 525 cm 1. Mo−S −

−

bonds were characterized by typical vibrations ν(Mo3−µS) at 450 cm 1 and ν(Mo−S) at 382−284 −

cm 1 (figure 2, red trace).7,16 Increasing laser power to 1.2 mW or higher results in crystallization −

of a-MoSx, thus generates crystalline MoS2. In these cases, characteristic vibrations of c-MoS2 at 402 cm 1 (A1g mode) and 380 cm 1 (E12g mode) were observed.17 The intensity of these peaks was −

−

increased by employing higher laser power indicating better crystallinity for the c-MoS2 obtained (figure 2, blue and green traces).18 In the latter case, the crystallization is completed as characteristic vibrations of (S-S)2- ligands within a-MoSx are no longer observed. New vibration at 460 cm 1 is obivious that can be assigned for sulfur-based byproduct.19,20 −

Thus, it is evident that excitation by high voltage electron beam or high power laser beam generates heat at narrow excited area of a-MoSx material. The local temperature can reach few hundreds degree Celsius, e.g. over 680 °C for a free standing Si nanocrystal illuminated by 514 nm laser having power of 0.33 mW.21 Thus, this heating effect induced by electron or laser beam might be the origin of the a-MoSx crystallization described this above. Indeed, independent thermal treatment experiment under inert Ar atmosphere showed a-MoSx to c-MoS2 phase transformation happening at low temperature of ca. 350°C (see supplementary information and figures S1, S2 for details). However, it is also possible that the a-MoSx to c-MoS2 phase transformation is a result of direct interaction of a-MoSx material with high energy electron or laser beam rather than just the induced thermal effect. A direct SnS2 to SnS phase transformation was reported wherein sulfide ligands within SnS2 were reductively removed due to interaction with electron beam (type electron energy of 80 keV and beam current of ca. 20 pA/cm2).22 We note that this phase transformation is an internal redox reaction described in eq.1. SnIVS2 → SnIIS + S0

(eq.1)

µMoIV3−S2 → µMoIV3 + S0 + 2e −

−

(eq.2)

MoIV(S−S)t2 + 2e → 2MoIV(S2 ) + 2S2−

−

−

(eq. 3) Page 6 of 15


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Indeed, within the a-MoSx, [Mo3S13]2 clusters are connected in unfolded polymer chain or more −

randomly in a two dimension branched structure.7 Thus, a-MoSx polymer structure can be considered as ready for construction of c-MoS2. Apparently, this can be achieved by moving out the apical sulfide ligand via an oxidation reaction (eq. 2) and reducing disulfide ligands into sulfide ligands (eq. 3) (figure 3). The latter can help to complete the coordination sphere of defect-containing Mo centers. Indeed, during annealing process at 350°C or above, we observed generation of yellow sulfur-based species which is condensed at the end of annealing chamber. We propose the a-MoSx to c-MoS2 crystallization is an internal redox reaction which is induced by high energy electron beam, laser beam or appropriate thermal treatment. Interestingly, we found that reduction of [MoVIS4](NH4)2 generating crystalline c-MoS2 is also readily induced by the same electron beam during TEM characterization (supporting information figure S3). Thus, these results suggest a rapid methodology to synthesize highly pure c-MoS2 by employing electron beam of TEM equipment or laser beam with either readily available [MoS4](NH4)2 or easily prepared a-MoSx as precursors.

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Figure 3. Schematic presentation of the a-MoSx to c-MoS2 crystallization induced by TEM electron beam, laser beam or thermal treatment

We are then interested in investigating impacts of the crystallization onto H2 evolving electrocatalytic activity. We employed a-MoSx thin film (ca. 40 nm thickness) loaded on FTO electrode at loading of 6.2 × 10–8 mol Mo catalyst per cm2 and c-MoS2 thin film obtained from annealing the same a-MoSx film at 400°C for 2h in Ar atmosphere. The only side product of this thermal annealing is sulfur-based species. Thus, these a-MoSx and c-MoS2 films have identical number of Mo sites. It was clear that the crystallization process reduces the number of Mo=(O)1,2 oxo and Mo− defect sites. Based on resonance Raman analysis, characteristic vibrations of Mo=(O)1,2 oxo species in region 850− 950 cm 1 were not observed indicating absence (or low concentration) of −

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these defects within c-MoS2 material (figure S2). Consistently, c-MoS2 electrode showed only a catalytic event without any pre-peaks once polarized in a pH 7 electrolyte solution. It is in very shape contrast to that observed for an a-MoSx electrode showing two pre-peaks assigned to electrochemical reduction of MoV(S−S) and (MoV=O/H+) defect sites prior the catalytic event (figure S4).7 Formation of crystalline c-MoS2 also results in disappearance of disulfide ligands (eq.3). As shown in Raman analysis, characteristic vibrations of disulfide ligands at 555 and 525 cm 1 are −

no longer observed (figures 2 and S2). Actually within c-MoS2 terminal disulfide ligand is expected as stabilizing ligand for the Mo-edge atoms.23 However, its extremely low amount raises difficulty to evidence its presence by spectroscopic analyses. Indeed, to the best of our knowledge, other than Sulfur atoms visualization evidenced by Lausen et al.,23 spectroscopic signatures of this terminal ligand have never been reported. Lacking of disulfide ligands makes c-MoS2 more robust. Raman analysis revealed that c-MoS2 is intact under H2 evolving conditions (figure S2). This is in very shape contrast compared with aMoSx. Indeed, electrochemical reduction of disulfide ligands within a-MoSx results in partial corrosion of this material but also creates novel Mo active sites, type Mo− .7 Thus, consecutive cathodic potential polarizations enhance catalytic activities of a-MoSx electrode (figure S5). This is not the case for c-MoS2 which shows steady activities under similar treatment conditions (figure S6). Because of lacking of both Mo-defect sites (type Mo=(O)1,2 oxo and Mo−

sites) and disulfide

ligands, c-MoS2 presents better robustness but lower catalytic performance compared with aMoSx equivalent. In a pH 7 buffer solution, both a-MoSx and c-MoS2 catalyse HER with modest onset overpotential requirement of ca. 150 mV. However, Tafel slop value of 130 mV decade 1 −

was deduced for c-MoS2 catalyst while smaller value of 60 mV decade 1 was calculated for a−

MoSx catalyst (figure S7). In other words, at a given overpotential applied, c-MoSx generates much lower catalytic current compared with a-MoSx.

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Figure 4. Consecutive cyclic voltammograms recorded on c-MoS2 (blue trace) and a-MoSx (red trace) electrodes immersed in a pH 7 phosphate buffer solution. Potential scan rate of 20 mV s 1. −

Insert: Evolution of catalytic current obtained at -0.70 V vs. NHE in function of number of oxidative polarization

Furthermore, c-MoS2 was found to be less supportive to oxidation stress that causes a rapid decomposition. Indeed, both c-MoS2 and a-MoSx catalysts have been used as co-catalysts for activating semiconductor-based H2 evolving photocatalysts such as CdS,24 Si,11,25,26 Cu2O,27 ZnCdS.28 For instance, these photocatalysts are conditioned to function by providing an applied potential or a sacrificial electron donor to supress photogenerated holes. Thus, oxidation stress is not yet an issue. However, the ultimate goal of the artificial photosynthesis research is to create a bias−free photocatalyst or photoelectrochemical cell for the overall water splitting. It means HER catalyst used, being a reductive catalyst, should have capabilities to support oxidation stress caused by photogenerated holes. That can be achieved by employing noble metal based HER catalysts as reported by Domen and coll.29 Nevertheless, identifying noble metal free catalysts which can support oxidative stress represents an attractive challenge. We found that irrespective of pH values of electrolyte solution, c-MoS2 is oxidized at significant higher potential than that recorded for a-MoSx material. In pH 7 phosphate buffer solution, c-MoS2 displays a huge Page 10 of 15


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oxidation event started at ca. +0.2 V and peaked at +0.82 V vs. NHE. This was attributed to the complete destructive oxidation of c-MoS2 producing inactive MoO3.30 Thus, the c-MoS2 catalyst may not be stable at the surface of photocathodes if these electrodes are poised at a potential above +0.2 V vs. NHE at any moment of the operation. In very shape contrast, two less intense oxidation events were oberved at −0.15 V and +0.25 V vs. NHE for the a-MoSx electrode (figure 4). We tentatively attribute these events to irreversible oxidation of terminal and bridging disulfide ligands, respectively.31 Oxidative polarization causes destruction of these two catalysts. However, the destruction happens much faster within c-MoS2 than within a-MoSx. After three potential scans, activities of c-MoS2 are completely supressed while 70% of catalytic current is recovered for a-MoSx under identical conditions (figure 4, insert). CONCLUSIONS Crystallization of amorphous a-MoSx into c-MoS2 is rather easy. This phase transformation is readily happened during microscopic (TEM) and spectroscopic (resonance Raman) analyses due to interaction of a-MoSx with electron beam and laser beam, respectively. The same in-situ crystallization was also observed for other amorphous sulfides, like a-CoWSx,32 a-CoMoSx (see supporting information figure S8).25 Thus, to gain insights into structure of amorphous metal sulfides, it is critical to identify appropriate conditions, e.g. good laser power, so these material analytical tools are not destructive. a-MoSx to c-MoS2 crystallization is not fully a beneficial strategy in regards of catalytic H2 evolving performance. Indeed, c-MoS2 displays better robustness e.g. no corrosion issue. However, a-MoSx displays better catalytic activities (e.g. higher catalytic currents at a given applied potential) and is more tolerant to oxidation stress compared with c-MoS2. Thus, for creation of bias free photocatalysts or photoelectrochemical cells for the overall water splitting application, a-MoSx represents a better candidate compared with c-MoS2. ASSOCIATED CONTENT Supporting Information The Supporting Information is avaiable free of charge on the ACS Publication website at DOI: XRD and Raman analysis of a-MoSx and c-MoS2 materials; TEM analysis of [MoS4](NH4)2 and of a-CoMoSx; Electrochemical properties of a-MoSx and c-MoS2 materials. Page 11 of 15



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AUTHOR INFORMATION Corresponding Author: Email: [email protected]. Tel: +84 96 46 021 46 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Foundation for Science and Technology Development (NAFOSTED, project code 103.99-2015.46). 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 2 3

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Chen, Y.; Tran, P. D.; Boix, P.; Ren, Y.; Chiam, S. Y.; Li, Z.; Fu, K.; Wong, L. H.; Barber, J. Silicon Decorated with Amorphous Cobalt Molybdenum Sulfide Catalyst as an Efficient Photocathode for Solar Hydrogen Generation. ACS Nano 2015, 9, 3829-‐3836. Seger, B.; Laursen, A. B.; Vesborg, P. C. K.; Pedersen, T.; Hansen, O.; Dahl, S.; Chorkendorff, I. Hydrogen Production Using a Molybdenum Sulfide Catalyst on a Titanium-‐Protected n+p-‐Silicon Photocathode. Angew. Chem. Int, Ed. 2012, 51, 9128-‐9131. Morales-‐Guio, C. G.; Tilley, S. D.; Vrubel, H.; Gratzel, M.; Hu, X. Hydrogen Evolution from a Copper(I) Oxide Photocathode Coated with an Amorphous Molybdenum Sulphide Catalyst. Nat. Commun. 2014, 5, 3059. Doi: 10.1038/ncomms4059 Nguyen, M.; Tran, P. D.; Pramana, S. S.; Lee, R. L.; Batabyal, S. K., Mathews, N.; Wong, L. H., Graetzel, M. In Situ Photo-‐Assisted Deposition of MoS2 Electrocatalyst onto Zinc Cadmium Sulphide Nanoparticle Surfaces to Construct an Efficient Photocatalyst for Hydrogen Generation. Nanoscale 2013, 5, 1479-‐1482. Maeda, K.; Xiong, A.; Yoshinaga, T.; Ikeda, T.; Sakamoto, N.; Hisatomi, T.; Takashima, M.; Lu, D.; Kanehara, M.; Setoyama, T.; Teranishi, T.; Domen, K. Photocatalytic Overall Water Splitting Promoted by Two Different Cocatalysts for Hydrogen and Oxygen Evolution Under Visible Light. Angew. Chem. Int. Ed. 2010, 49, 4096-‐4099. Bonde, J.; Moses, P. G.; Jaramillo, T. F.; Norskov, J. K.; Chorkendorff, I. Hydrogen Evolution on Nano-‐Particulate Transition Metal Sulfides. Farad. Discuss. 2009, 140, 219-‐231. Note: The MoS2 to MoO3 oxidation involves 4-‐18 electrons while (S-‐S)2-‐ to 2S0 involves only 2 electrons. Futhermore, MoS2 oxidation completely occurs after one potential polarization while (S-‐S)2-‐ oxidation happens progressively. These facts can explain for a huge difference in peak areas for these oxidation events. Tran, P. D.; Chiam, S. Y.; Boix, P.P.; Ren, Y.; Pramana, S. S.; Fize, J.; Artero, V.; Barber, J. Novel Cobalt/Nickel–Tungsten-‐Sulfide Catalysts for Electrocatalytic Hydrogen Generation from Water. Energy Environ. Sci. 2013, 6, 2452-‐2459.

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Crystallization of Amorphous Molybdenum Sulfide ... - ACS Publications

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