Effect of the chemical vapor deposition condition on the electrochemically catalytic efficiency for hydrogen evolution reaction in MoS₂ nanoparticles

Nội dung text: Effect of the chemical vapor deposition condition on the electrochemically catalytic efficiency for hydrogen evolution reaction in MoS₂ nanoparticles

1. Science & Technology Development Journal, 24(2):1947-1953 Open Access Full Text Article Research Article Effect of the chemical vapor deposition condition on the electrochemically catalytic efficiency for hydrogen evolution reaction in MoS2 nanoparticles Quyen Le Do, Duc Anh Nguyen* ABSTRACT Introduction: Using the metal organic chemical vapor deposition (MOCVD) method, we have syn- thesized the MoS2 nanoparticles on graphite foil substrates employed as the electrochemical work- Use your smartphone to scan this ing electrodes with highly efficient electrocatalysis for hydrogen evolution reaction (HER). Meth- QR code and download this article ods: The morphological and structural properties of the as-grown MoS2 materials were demon- strated by field emission scanning electron microscope (FESEM) and Raman spectroscopies, while their elemental components were investigated by X-ray photoelectron spectroscopy (XPS). Re- sults: The optimum growth time was acquired to be 11 hours. Thereby such obtained electrode exhibited the maximum HER activity with onset over the potential of 220 mV versus reversible hy- drogen electrode (RHE), and the Tafel slope of 66 mV per decade (mV/dec). Conclusion: Our results suggest a good technique for the research of high-efficient HER electrocatalyst based on atomic- thickness layered materials. Key words: MoS2 nanoparticles, metal-organic chemical vapor deposition, hydrogen evolution reaction INTRODUCTION ity of MoS2. These efforts can be the growth of ver- tical nanoflakes 12, nanobelts 13, mesoporous 14,15, or Hydrogen gas, an excellent source of clean energy, nanoparticles 16. On the other hand, due to low in- Department of Physics, Faculty of has been demonstrated as an ideal replacement for trinsic conductivity in MoS , one can reduce the num- Basic-Fundamental Sciences, Viet Nam hydrocarbon-based and fossil fuels 1,2. Hydrogen gas 2 Maritime University, 484 Lach Tray ber of layers to minimum the charge transfer resis- Road, Le Chan, Hai Phong, Viet Nam. can be conveniently yielded from the electrochemical tance between the exposure surface at the outmost water splitting reaction 2–4. Using this approach, the layer and the electrode 17. In this regard, a small Correspondence hydrogen evolution reaction (HER) that happens on a number of layers was demonstrated as another im- Duc Anh Nguyen, Department of cathode surface can be accelerated by loading an elec- Physics, Faculty of Basic-Fundamental portant expect for highly catalytic HER performance Sciences, Viet Nam Maritime University, trocatalyst on it. However, the best electrocatalysts for in MoS nanostructure. Generally, MoS nanostruc- 484 Lach Tray Road, Le Chan, Hai HER are Pt and its relation noble metals, which has 2 2 ~ Phong, Viet Nam. substantially limited their commercial massive pro- ture with a small number of layers (around 2 4 lay- Email: ducna@vimaru.edu.vn duction 4–6. Thus, developing low-cost electrocata- ers) might be a great alternative of noble-metal-based HER electrocatalysts. However, synthesis a large scale History lysts that possess strong stable and HER performance • Received: 2021-02-21 to be close to Pt-based catalysts is ultimately desirable. of few-layer MoS2 nanostructure directly on conduc- • Accepted: 2021-05-05 tive substantial has been still difficult 18. • Published: 2021-05-13 So far, nanostructures of molybdenum disulfide Here, we used the MOCVD technique to grow MoS2 (MoS2) have been proven promising candidates for DOI : 10.32508/stdj.v24i2.2520 excellent catalytic activity 7–9. Using DFT calculation, nanoparticles directly on conductive graphite, which J. K. Nứrskov’s group first reported that hydrogen ad- was applied for HER electrochemical working elec- trode. This work focused on the dependence of sorption Gibbs free energy of edge sites of MoS2 is HER performance on the MOCVD growing condi- close to zero (∆GH ~ 0 eV), suggesting MoS2 prob- Copyright ably as a great HER catalyst 10. The experimental tion, particularly the growth time. We found the sam- â VNU-HCM Press. This is an open- measurements then assured this prediction of HER ple grown in 11h to exhibit the highest HER activity access article distributed under the 10 with the smallest onset overpotential of 250 mV/dec, terms of the Creative Commons performance of MoS2 nanoparticles on graphite , Attribution 4.0 International license. and Au(111) substrates 11. Subsequently, numerous and the Tafel slope of 66 mV/dec. reports have focused on maximizing the exposured MATERIALS - METHODS active-edge-sites, arming to enhance the HER activ- Cite this article : Do Q L, Nguyen D A. Effect of the chemical vapor deposition condition on theelec- trochemically catalytic efficiency for hydrogen evolution reaction inMoS2 nanoparticles. Sci. Tech. Dev. J.; 24(2):1947-1953. 1947
2. Science & Technology Development Journal, 24(2):1947-1953 Figure 1: Schematic illustrationof MOCVD system growing MoS2 nanoparticles. Synthesis of MoS2 nanoparticles were investigated by micro-Raman spectroscopy us- ing a 473 nm excitation source under ambient con- MoS2 nanoparticles were synthesized via MOCVD, as schematically illustrated in Figure 1. Briefly, the ditions. The X-ray photoelectron spectroscopy (XPS) experiment was taken place in a sealed 1-inch di- measurements were carried out using a Theta Prove ameter quartz tube, and then graphite foil electrodes AR-XPS System (Thermo Fisher Scientific). were placed at the center of the heated zone. The HER measurements metal-organic compound of molybdenum hexacar- The three electrodes configuration, including graphite bonyl (MHC, C6MoO6) and diethyl sulfide (DES, rod (i.e., counter electrode), Ag/AgCl (i.e., reference C4H10S) have high equilibrium vapor pressure were electrode), and MoS nanoparticles on graphite foil used as the gaseous precursors. Firstly, the base vac- 2 (i.e., working electrode) was employed to plot the lin- uum (~1 mtorr) was established in the chamber by a ear sweep voltammetry (LSV) and cyclic voltamme- rotary pump. At the same time, the temperature of try (CV). All the electrochemical measurement was MHC and DES bearer was adapted at 25oC and 60oC, put in a 0.5 M H SO electrolyte and was established respectively. Then, a flow of 1 standard cubic cen- 2 4 in an electrochemical workstation (IviumStat, Ivium timeter per minute (sccm) of Ar gas was injected into Tech) to study the HER reactivity. DES bearer to dilution and thus facilitated the DES va- por flow. The temperature of the carrier line was fixed RESULTS at 50 oC, and that of the reaction zone was 550oC. A The morphology of MoS2 nanoparticles was observed mixture of 30 sccm of Ar and 5 sccm of H was con- 2 by FESEM images, as shown in Figure 2. As shown tinuously flowed into the reaction chamber during all in Figure 2a and b, the 6h grown sample is similar growing processes to maintain the working pressure to the blank substrate, indicating that before 6h, the to be 60 torrs. Subsequently, the reaction process was MoS2 has not been formed in the substrate. When the started by introducing 1 sccm mix of (Ar + DES) and growth time reaches 7h (Figure 2c), tiny particles ap- open soft valve of the MHC holder. For a compar- pear with the size of 50-100 nm. According to Figure ative investigation of HER reactivity, MoS2 samples 2d-f, the density of the nanoparticles increased with were synthesized with various deposition times of 7, the growth time, while the size of particles seems to 9, 11, 13 h by a similar deposition condition. be not changed. Figure 3 shows the Raman spectra of the obtained Characterization MoS2 samples. As shown in Figure 3a, two peaks lo- The morphology of MoS2 samples was character- cated at 378.2 and 401.5 are attributed to the two typi- 1 ized by field-emission scanning electron microscopy cal active Raman scattering mode of E 2g and A1g vi- (JSM-6500F, JEOL). The lattice vibrational properties brational modes in 2H phase of MoS2, in which the 1948
3. Science & Technology Development Journal, 24(2):1947-1953 Figure 2: Morphological analysis. FESEM images of baregraphite foil (a), and MoS2 nanoparticles (b-f) syn- thesized for 6,7, 9, 11, 13 h. The scale bar is 1 àm. Figure 3: Raman spectra of MoS2 nanoparticles ongraphite substrate (a). The zoom-in of the Raman spectra of samples grown in 7hto 13h. 1 first one (E 2g) is attributed to the in-plane vibration 3d region. As can be seen, the two olive-fitted peaks of Mo-S bond, while the other one (A1g) is corre- located at 229.65 and 232.8 eV are corresponding to 19 4+ 4+ sponding to the out-of-plane vibration of S atoms . Mo 3d5/2 and Mo 3d3/2 energy levels in MoS2, Moreover, the frequency separation between these respectively 21. Besides, the two small peaks (orange − two peaks of 23.5 cm 1 suggested that the number of fitted curves) overbed at 231.8, and 235.9 eV are as- layers was between 3 and 4 layers 17,20. Although the cribed to the Mo6+ states, indicating the formation growth time is much different (alternating between of a small amount of molybdenum oxide due to par- 7h~11h), the number of layers of samples does not tial oxidation of MHC throughout the deposition pro- change, reflected by the non-shift in the Raman peaks cess 22,23. Additionally, two blue fitted peaks that position (Figure 3b). were detected at 162.5 and 163.7 eV (see Figure 4b) 2− 2− The X-ray photoelectron spectroscopy (see Figure 4a- are attributed to the S 2p3/2 and S 2p1/2 energy 23 b) was constructed to study the elemental bonding states in MoS2, respectively . Thus, all these data states of as-grown MoS2 nanoparticles. Figure 4a il- confirmed the chemical elemental composition of the lustrated the high-resolution of XPS spectra in Mo fabricated materials. 1949
4. Science & Technology Development Journal, 24(2):1947-1953 Figure 4: Chemical composition analysis. The high-resolution XPS spectroscopy of as-grown MoS2 nanoparti- cles superimposed by fits(red lines) for (a) Mo 3d energy levels range: Mo4+ (olive trace),and Mo6+ (orange trace); 2− (b) S 2p energy levels range: S of MoS2 (blue trace). 2 The HER performance was firstly examined by the lin- the maximum Cdl of 2.21 mF/cm , which is consider- ear sweep voltammetry (i.e., polarization curves), as ably larger than that of the rest ones. Thus, although illustrated in Figure 5a. As can be seen, there was the morphology and the density of MoS2 nanoparti- a considerable enhance of HER activity as the CVD cles of 11h and 13h samples are almost similar, the for- growth time increase from 7h to 11h. The 11h sample mer reveals a significantly higher performance than revealed the highest performance with the onset over- the latter. Nevertheless, these calculations of the elec- potential of approximately 250 mV vs. RHE, which trochemically active surface area of catalysts were well was considerably smaller than the 7h, 13h, and bare consistent with the above polarization curves. samples. The Tafel plots can also be used to evalu- Finally, the stability test for the HER catalytic activ- ate the electrocatalytic activity for HER, in which the ity of 11h sample was investigated by the transient smaller obtained Tafel slope corresponds to the higher chronopotentiometry measurement with a working HER reactivity. Figure 5b exhibited the correspond- current density of -5 mA/cm2, as depicted in Fig- ing Tafel plots of the MoS2 nanoparticles synthesized ure 5e. During a period of 20h, we observed no signif- at different times. Even though the 9h sample exhib- icant variation of overpotential, demonstrating great ited a similar onset overpotential with the 11h sam- stability. In addition, the nominal modification of po- ple, its Tafel slope of 91 mV/dec was much higher than larization curves before and after the durability char- that of the 11h sample (66 mV/dec). However, when acterization verified the superior working stability of we further expanded the deposition time until 13h, as-obtained MoS2 nanoparticles (Figure 5f). the performance experienced a degeneration with the onset overpotential and the corresponding Tafel slope DISCUSSION enlarged to ~350 mV vs. RHE and 88 mV/dec, respec- As mentioned, the MoS2 nanoparticles that recently tively. Generally, the MoS2 nanoparticles deposited have been considered as a promising candidate for in 11h exhibited the optimum electrochemically cat- highly efficient electrocatalytic for HER were fabri- alytic activity for HER. cated. Remarkably, our MOCVD method supported To evaluate the density of the active site of cata- a direct growth of MoS2 nanoparticles on conductive lysts, the cyclic voltammetry (CV) plots in a non- graphite foil electrodes which simplified the material Faradic potential range were conducted at the scan preparation. The electrode surficial phenomena were rates changing between 10 and 70 mV/s, as shown in tested without any extra transfer process. In this way, Figure 5c. Then, the double-layer capacitance (Cdl) it also naturally avoided the electrical loss contacts for obtained from the linear fitting the dependance of the the electrochemical measurements. For catalytic HER average current density versus scan rates (Figure 5d) activity, the electrochemically active sites play an es- was demonstrated to be proportional to the active site sential role. Frequently, the active sites locate at the density 24. As can be seen, the 11h sample exhibited edge-sites rather than at the basal plane. As a result, 1950
5. Science & Technology Development Journal, 24(2):1947-1953 Figure 5: Electrocatalytic HER activities of MoS2 nanoparticles. (a, b) iR-corrected LSVs and Tafel plots, re- spectively. (c, d) Cyclic voltammetry of the 11h sample at various scan rate, and the linearfitting of the average capacitive current density versus the scan rate for MoS2 sample with different growth time, respectively. Stability 2 of the obtained MoS2 sample, (e) Potential vs. time plot, conducted at -5 mA/cm for 11hsample, (f) LSVs of the initial 11h sample (magenta) and after applying bias states (dashed black). ragged particles in nanoscale size are more favorable nanoparticles was similar; therefore, the catalytic ac- than a uniform continuous film. In addition to the tivities essentially only depended on the density of morphological aspects, the thickness of MoS2 mate- particles. Meanwhile, the growth time leaded to the rial is another essential factor affecting total perfor- change of particle density. Thus, for the samples mance. The layer number ranging from 3 to 4 was grown from 7 to 11h, a considerable enhancement in demonstrated as the best option for HER reactivity 16. the electrocatalytic activity can be easily understood Interestingly, the fabricated MoS2 particles in this as extending the density of MoS2 nanoparticles. How- work well matched with above requirements. ever, we assert that a further extending growth time In this study, under the same reaction temperature, (≥ 13h) should not be employed to acquire the opti- carrier gas concentration, and precursors’ flow rate, mum performance. We attributed the best HER per- the morphology (i.e. size and layer number) of MoS2 formance of 11h samples to the maximum of active 1951
6. Science & Technology Development Journal, 24(2):1947-1953 site density compared to the other ones, which was AUTHORS CONTRIBUTIONS then proved by the fact that it has the highest value of Q. L. D designed and performed the experiments. D. electrochemical double-layer capacitance (C ). One dl A. N analyzed data and wrote the manuscript. All au- possible reason for lower performance in the more ex- thors have given approval to the final version of the tended growth sample was a transition from the elec- manuscript. trochemical active-Mo-edge sites to the inert S-edge sites through extra growth time 25. Such extra growth ACKNOWLEDGMENTS time might fulfill some S-vacancies existing near the This work is funded by Vietnam Maritime University active-Mo-edge sites. This mechanism should be fur- under grant number: “DT20-21.94”. ther confirmed in the following research topic. 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