Source: https://pubs.rsc.org/en/content/articlehtml/2019/cy/c8cy02168f
Timestamp: 2019-04-24 05:55:01+00:00

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Advancements in the search for efficient H2 evolving electrocatalysts by engineering the structure and electronic properties of bulk materials at the nano regime have had a constructive impact on realizing better performance with ultralow catalyst loading. In this work, we investigate the electrochemical hydrogen evolution catalysis of WS2 quantum dots (QDs) synthesized electrochemically. WS2 QDs were evaluated for a hydrogen evolution reaction (HER) in 0.5 M H2SO4 as a binder-less electrocatalyst, which necessitated a low HER onset of 190 mV and reached −200 mA cm−2 at 473 mV. In contrast, the bulk WS2 exhibited a poor performance, which required 780 mV to drive −50 mA cm−2. The electrochemical disintegration of bulk WS2 into QDs is the key behind such activation.
Among the various non-precious metal chalcogenides, the disulphides of W are one of the frequently studied systems in the literature due to their intriguing layered structure and electrocatalytic HER activities comparable to those of Pt with lower additional overpotential.12 In many studies, it was implied that a reduction in the size of layered bulk WS2 sheets increased the HER activity. As a result, WS2 had been reported in various morphologies at the nanoscale with enhanced activities. Some of the intriguing morphologies of WS2 nanomaterials are nanosheets,13–21 dots,22,23 nanoflakes,24,25 nanoribbons,26–30 nanoplatelets31 and nanorods.32,33 In addition, the same WS2 has also been combined with synergistically enhancing materials like graphene and reduced graphene oxide which showed a further increased performance in HERs.13,15,20,32 Additionally, the role of metal dopants, namely Ni and Co, in WS2 has also been investigated for HERs.34,35 WS2 had such an excellent HER activity that it was applied to photocatalytic and photoelectrocatalytic HER with photo-harvesting materials like Si, TiO2 and CdS.14,19,32,36–38 Other than WS2, carbides, nitrides, phosphides and selenides of W were also reported for HERs.33,36,39–41 This particular observation leads to further work with WS2via tailoring its morphological features and electronic properties at nanoscale. Nanomaterials of WS2 reported in the literature utilized the top-down approach of tailoring bulk WS2 into the desired structure chemically, where the use of salts and surfactants was unavoidable which made those methods very cumbersome and the resultant nanomaterials were almost always anisotropic.19,24,25,29,42,43 Moreover, HER studies on WS2 QDs of sub-nanometer size are not extensively reported, though WS2 QDs have the potential to beat all other nanostructured WS2 catalysts due to the advantages of smaller size and a large number of catalytically active edges while retaining a layered structure.
The motivation of the present work lies in the recently reported one-step electrochemical synthesis of highly mono-dispersed WS2 QDs of size smaller than 5 nm using an applied bias of 2 V for 24 h in propylene carbonate containing LiClO4.44 The same was screened for HER in an acid medium with no binders for electrode fabrication as propylene carbonate itself possessed good adherence to the substrate electrode. The observed HER activity of the WS2 QDs is far better than the bulk WS2 sheets, or other nanostructured WS2 materials and is comparable to Pt/C.
The disintegration of bulk WS2 sheets into highly mono-dispersed WS2 QDs was performed using a three-electrode electrochemical cell. A Pt mesh counter electrode and a Pt wire quasi-reversible reference electrode were used along with a WS2 pellet as working electrode. The WS2 pellet was made with 1 g of bulk WS2 sheets by KBr hydraulic pressure that had a diameter of 13 mm and was about 2.5 mm thick. The electrochemical disintegration was done in a deoxygenated propylene carbonate electrolyte containing LiClO4 (0.1 wt%) at an applied potential of 2.0 V for 24 h, as we reported earlier. The colorless electrolyte solution turned yellow which was the indication of the formation of WS2 QDs. Then, the electrolyte was purified to exclude the residual ions, followed by the collection of the resultant WS2 QDs in propylene carbonate as a colloidal solution. The calculated molar concentration of WS2 in the resultant colloidal solution was 0.012 M.
For comparison, 3 mg of bulk WS2 sheets and Pt/C 20 wt% catalysts were mixed in separate vials by sonication with a 1 mL solution of water, isopropyl alcohol and a 5% Nafion ionomers suspension taken in the volume ratio of 7 : 2.5 : 0.5 for 20 min. About 68.5 μL of the resultant catalyst ink was drop casted on the CFP substrate electrode and dried overnight at ambient conditions and in the dark before the electrochemical studies. The corresponding mass loading of these catalysts was 0.205 mg cm−2. To fabricate the WS2 QDs/CFP electrode, the same volume of 68.5 μL of the colloidal solution of the WS2 QDs in propylene carbonate was used. This corresponded to a loading of 0.0132 mg cm−2, as per the calculated molar concentration, which is 15.47 times lower than the loadings of both the bulk WS2 and Pt/C catalysts. This particular electrode took a slightly longer time (18 h) to dry before it could be used as a working electrode.
The X-ray diffraction (XRD) pattern (Fig. 1a) of the WS2 QDs synthesized by electrochemical disintegration at 2 V shows distinct peaks for the planes of (002), (004), (100) and (106), as per the reference ICDD card number of 08-0237, which confirmed that the synthesized WS2 QDs had retained a layered structure.19,43–45 Other cubical ((10n), n = 1, 3, 4 and 5) planes that are usually observed with bulk WS2 are absent with the WS2 QDs, except (106), which further indicated that the bulk WS2 sheets were rigorously disintegrated into QDs, mainly with a layered 2D structure which is highly anticipated for a better HER performance. Moreover, a shift of 2.1° at the (002) plane with the WS2 QDs indicates that the interlayer distance has also been increased as a consequence of the electrochemical disintegration. Such an increase in the interlayer distance is always advantageous in HER electrocatalysis as it leads to an increased electrochemically accessible surface area (ECSA). The morphology of the synthesized QDs was observed by transmission electron microscopy (TEM) and the corresponding TEM micrograph (Fig. 1b) shows the clear presence of WS2 QDs that are distinguishably separated from one another. The measured size (Fig. 1c) of the WS2 QDs was mainly in the range of 2 to 5 nm and the average diameter of the synthesized WS2 QDs is 3.5 ± 1 nm. This particular observation supports our claim of the formation of the WS2 QDs by the chosen method of electrochemical disintegration. The TEM micrograph obtained at a high magnification shows clear lattice planes, as can be seen in Fig. 1d. The measured d spacings were 0.2 nm and 0.27 nm which correspond to the miller indices of (006) and (100), respectively. This is in excellent agreement with the XRD analysis which confirms that the synthesized WS2 QDs are crystalline. The electron diffraction (ED) pattern of these WS2 QDs exhibits a typical dot plus ring feature indicating the presence of small and larger crystalline particles, which is in accordance with the microscopic and XRD analyses. Three predominant rings (highlighted) observed in the ED pattern were calibrated to assign the corresponding diffraction planes according to the literature where it was found that all three rings were of mainly (00n) planes in which n is 4, 6 and 10. This is also in agreement with the same ICDD card number of 08-0237. The combined microscopic and diffraction analyses thus confirm that the bulk WS2 sheets were successfully disintegrated into WS2 QDs of mainly particles smaller than 5 nm with some larger sheets of size 11 to 12 nm as counterparts.
Fig. 1 XRD pattern of the synthesized WS2 QDs (a), TEM micrograph showing the WS2 QDs (b), particle size distribution histogram (c) of the WS2 QDs, HRTEM micrograph (d) showing clear lattice planes with assigned miller indices and the ED pattern (e) of the WS2 QDs.
The elemental composition of the synthesized WS2 QDs was analyzed by energy dispersive X-ray analysis (EDS) (Fig. S1 in ESI†) which indicates the presence of W and S for various shells such as W M, W L, S L, and S K. It also indicates the presence of some C and O peaks that are probably from the adsorbed O and from the residual solvent (propylene carbonate) that could have resided on the fabricated thin film during the analysis. The results of this EDS analysis confirmed that the synthesized WS2 QDs are free from other impurities, including Cl and Li, and are ready for their use in HER studies. Nonetheless, as the synthesized WS2 materials are QDs, their optical properties were also examined with UV-visible and photoluminescence (PL) spectroscopic techniques. Fig. S2a and b in ESI† show the UV-vis spectrum of bulk WS2 and the synthesized WS2 QDs in propylene carbonate. The UV-visible spectrum of bulk WS2 powder shows three broad characteristic peaks A, B and C at 620 nm, 530 nm and 460 nm, respectively. The peaks A and B correspond to the excitonic absorption due to the direct bandgap transitions at the K point of the Brillouin zone. The absorption located at around 460 nm is attributed to the optical transition from the valence band to the conduction band. However, the synthesized WS2 QDs showed a hump at 350 nm which is clear cut evidence for the formation of WS2 QDs. This is also in agreement with earlier findings.43–45 The spectrum of the bulk WS2 sheets had another broad and distinct peak around 620 nm which is absent the in case of WS2 dots and QDs.43–45 Since fluorescence is an intrinsic property of QDs, the PL characteristics of the electro-synthesized WS2 QDs were also analyzed. Fig. S3 in ESI† is the PL spectrum of the WS2 QDs in propylene carbonate, acquired by excitation with monochromatic radiation of a light with 350 nm, which was chosen because of a distinct peak observed in the UV-vis spectroscopic analysis for the same QDs. From the PL spectrum, it was observed that the WS2 QDs had shown an excitation wavelength dependent photoluminescence, which matches with earlier findings.43–45 Both UV-vis and PL spectral analyses have also ascertained the formation of WS2 QDs by the electrochemical disintegration of bulk WS2 sheets.
X-ray photoelectron spectroscopy (XPS) measurements were carried out to confirm the elemental composition. The deconvoluted XP spectra of W 4f, S 2p, C 1s and O 1s states are presented in Fig. 2a–d, respectively. The W 4f spectrum exhibits a doublet due to spin–orbit splitting, as can be seen in Fig. 2a. The first peak, located at 33.05 eV, is due to the 4f7/2 sublevel and the second peak, located at 35.41 eV, is due to the 4f5/2 sublevel. The occurrence of these peaks at these binding energies, with a separation of 2.36 eV, is clear cut evidence for the existence of W4+ in the synthesized sample, which is also in agreement with earlier reports of WS2.19,43–47 Interestingly, no sign of W6+ is observed here. In addition, one more peak with a relatively poor intensity was observed at 39 eV and assigned to the 5p3/2 state of W in the 4+ oxidation state. This further understates the tendencies of forming higher oxidation state oxysulfides of WS2. The observed peak features of W 4f and W 5p3/2 also resonate with earlier reports of WS2.19,43–47 Similarly, the XP spectrum of the S 2p state (Fig. 2b) upon deconvolution yielded two distinct peaks which are also due to the known spin-orbit coupling effect. The observed peaks are located at the binding energy values of 162.35 and 164.11 eV. The occurrence of these peaks at those binding energy values agrees well with the S in the 2− oxidation state.19,43–47 In addition, though there are C and O, there were no indications of C–S or S–O functionalities. This observation is also in agreement with our earlier report.44 The high resolution XPS spectrum of the C 1s state, provided in Fig. 2c, revealed the presence of more than one type of C. A careful deconvolution of the C 1s spectrum showed four distinct peaks located at 284.8, 285.3, 286.3 and 288.9 eV, respectively, for the presence of C–C, C–H, C–O–C and O–C O functionalities. All of these functional groups are present in the propylene carbonate solvent. Likewise, the high-resolution XP spectrum of the O 1s state (Fig. 2d), acquired for the same, also revealed similar results. The acquired raw spectrum for the O 1s state was deconvoluted into two distinct peaks located at 531.8 and 532.7 eV. These peaks correspond to the presence of O–C O and C–O–C functionalities, respectively.44 These two functional groups are present in the propylene carbonate used as the solvent for the electrochemical disintegration of bulk WS2 sheets into WS2 QDs. Overall, the XPS analysis indicated that the formed QDs are only WS2, and not any higher oxidation state sulphide of W, along with some residual solvent that still resided on the fabricated thin film sample.
Fig. 2 (a–d) XPS high resolution spectra of the W 4f, S 2p, C 1s and O 1s states, respectively, obtained with WS2 QDs thin films.
To examine the electrocatalytic HER activities of WS2 QDs, bulk WS2 and Pt/C 20 wt. catalysts were modified over carbon fiber paper (CFP) substrate electrodes 0.5 cm in width, 5 cm in length on only one side and an area of 1 cm−2. The remaining area of the substrate electrode was masked by insulating sticky tape, leaving some part of the substrate electrode on the other end which was destined for making an electrical connection with the electrochemical workstation. All electrochemical characterizations were done with a conventional three-electrode electrochemical cell containing Ar purged 0.5 M H2SO4 where an Hg/Hg2SO4 reference electrode and graphite counter electrodes were used. As a primary evaluation parameter, linear sweep voltammograms (LSVs) for WS2 QDs/CFP, WS2 bulk/CFP and Pt/C/CFP were recorded at a slower scan rate of 5 mV s−1. The same were corrected for the iR drop and the potential scales were converted to reversible hydrogen electrode (RHE), following earlier reports.48–50 The resultant iR compensated LSVs are given as Fig. 3a. The LSV profiles exhibit an improved HER activity of WS2 QDs which increased several fold compared to that of bulk WS2. This is clearly due to the reduction in size and the retention of a HER favoring layered structure having (00n) planes with more edges and corners. Moreover, it is also perceivable that the activity of the WS2 QDs is comparable to the state-of-the-art Pt at a higher overpotential region with a low overpotential difference. Specifically, the WS2 QDs required 190 mV to start the H2 evolution from that of Pt/C whereas the bulk WS2 required 310 mV for the same. The measured difference in the overpotentials at the current density of −50 mA cm−2 between WS2 QDs/CFP and WS2 bulk/CFP is 430 mV which is very high, indicating the advantage of disintegrating the bulk WS2 sheets into QDs for energy efficient HER electrocatalysis. Moreover, at the current density of −200 mA cm−2, the difference in the overpotential between Pt/C/CFP and WS2 QDs/CFP is just 338 mV. Interestingly, the activity observed with the WS2 QDs is 15.47-fold lower for catalyst loading than both bulk WS2 and Pt/C catalysts. These observations indicate that the electrochemical disintegration of bulk WS2 not only enhanced the activity but also reduced the catalyst loading. The reproducibility of the LSV features was confirmed by repeated measurements and the plot of j against overpotential (Fig. S4 in ESI†) also showed minimum error at each data point.
Fig. 3 (a) LSVs of CFP, WS2 bulk/CFP, WS2 QDs/CFP and Pt/C/CFP interfaces acquired at 5 mV s−1 in 0.5 M H2SO4. (b) Corresponding Tafel plots of the same. (c) Nyquist plots of CFP, WS2 bulk/CFP, WS2 QDs/CFP and Pt/C/CFP interfaces at onset overpotentials. (d) Plot of scan rate vs. double layer charging current density difference for CFP, WS2 bulk/CFP and WS2 QDs/CFP interfaces.
Fig. 4 (a) Response of WS2 QDs in chronoamperometric analysis, which was carried out in 0.5 M H2SO4 at the overpotential of 265 mV (b) pre- and post-CA LSVs showing significant activation in the HER activity of the same after CA study. (c) LSVs of WS2 QDs recorded before and after 1000 cycles of rapid CV cycling at 200 mV s−1.
It is important now to show the morphological and chemical robustness of the studied WS2 QDs after prolonged HER studies. XRD, TEM and XPS analyses have been done to show such results. For XRD analysis, the WS2 QDs/CFP electrode was taken directly after chronoamperometry studies. The obtained diffraction pattern is provided in Fig. S8 in ESI.† We can see three distinguished peaks corresponding to graphitic and graphenic carbons of the CFP substrate. Other than these carbon peaks, no other peaks for WS2 or the other phases are observed. This could be due to the very low amount of catalyst that we loaded on the substrate. Fig. 5a–c show the TEM micrographs of the WS2 QDs after the HER studies. For the TEM analysis, the catalyst was re-dispersed in propylene carbonate again to make TEM specimens. From Fig. 5a–c, we can see that the WS2 QDs have retained their morphological characteristics by having average sizes of less than 5 nm. This strongly proves the morphological robustness of our WS2 QDs and suggests that they could be an efficient and robust alternate to the noble metals based HER electrocatalysts. Moreover, the SAED pattern of the WS2 QDs was also acquired and calibrated as shown in Fig. 5d. From this, the existence of (004), (006) and (0010) basal planes can be seen. This is in exact agreement with the pre-HER SAED results and suggests that the structural characteristics of the WS2 QDs are retained after HER studies. Fig. 5e shows the particle size distribution histogram that suggests that the average size of the WS2 QDs was also retained at 3.5 nm, which is more proof for the structural robustness of the WS2 QDs in HER studies.
Fig. 5 (a–c) TEM micrographs of WS2 QDs after HER studies. (d) Corresponding SAED pattern. (e) Particle size distribution histogram showing the average particle size.
Having confirmed the structural robustness, it is now time to analyze the chemical stability of the WS2 QDs after HER studies. To do this, a detailed XPS analysis was carried out after the HER studies, taking the WS2 QDs/CFP electrode directly. Fig. 6a–d show the high resolution XPS spectra of the W 4f, S 2p, C 1s and O 1s states, respectively. Fig. 6a shows a notable difference from that of Fig. 2a. The exposure to an acidic environment for a considerably longer period of time resulted in the oxidation of W4+ to W6+ in considerable proportion. However, most of the WS2 QDs retained their chemical nature. Similarly, the S 2p high resolution spectrum (Fig. 6b) had also shown the oxidation of S2− to S4+ to a notable extent. Both results suggest that the surface oxidation of W and S in an acidic environment, even under reductive atmosphere, is irresistible. Other XPS spectra, such as C 1s (Fig. 6c) and O 1s (Fig. 6d), had shown the presence of C C, C–C, C–O, C O, S–O and H2O adsorbed moisture in it, as expected. The overall post-HER material characterization studies have revealed that our WS2 QDs are morphologically robust and also chemically stable to a significant extent.
Fig. 6 (a–d) XPS high resolution spectra of W 4f, S 2p, C 1s and O 1s of WS2 QDs/CFP electrode after chronoamperometry in acid, respectively.
In summary, we have demonstrated how efficiently a simple electrochemical disintegration in a non-aqueous solvent can be utilized to abnormally enhance the moderately active bulk WS2 into a high performance HER electrocatalyst upon transforming to WS2 QDs. A detailed electrochemical characterization indicated that the WS2 QDs are a far better HER electrocatalyst than bulk WS2, which required 430 mV lesser overpotential than the latter for −50 mA cm−2. Moreover, the WS2 QDs required just 338 mV extra than that of Pt/C to drive −200 mA cm−2, which is a significant number in HER electrocatalysis, with a lower Tafel slope of 90 mV dec−1. Furthermore, an excellent stability without any added binders is another advantage of making WS2 QDs in propylene carbonate.
S. A. and M. O. V. acknowledge CSIR, India for SRF award and K. K. acknowledges UGC, India for SRF award.
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‡ These authors have contributed equally.

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