Research Article | Open Access
S. H. Hsieh, S. T. Ho, W. J. Chen, "Silicon Nanowires with MoSx and Pt as Electrocatalysts for Hydrogen Evolution Reaction", Journal of Nanomaterials, vol. 2016, Article ID 6974646, 10 pages, 2016. https://doi.org/10.1155/2016/6974646
Silicon Nanowires with MoSx and Pt as Electrocatalysts for Hydrogen Evolution Reaction
A convenient method was used for synthesizing Pt-nanoparticle//silicon nanowires nanocomposites. Obtained Pt-/silicon nanowires electrocatalysts were characterized by transmission electron microscopy (TEM). The hydrogen evolution reaction efficiency of the Pt-/silicon nanowire nanocomposite catalysts was assessed by examining polarization and electrolysis measurements under solar light irradiations. The electrochemical characterizations demonstrate that Pt-/silicon nanowire electrodes exhibited an excellent catalytic activity for hydrogen evolution reaction in an acidic electrolyte. The hydrogen production capability of Pt-/silicon nanowires is also comparable to /silicon nanowires and Pt/silicon nanowires. Electrochemical impedance spectroscopy experiments suggest that the enhanced performance of Pt-/silicon nanowires can be attributed to the fast electron transfer between Pt-/silicon nanowire electrodes and electrolyte interfaces.
In recent years, renewable energy sources have attracted great scientific interest because of the depletion of the world’s fossil-fuel reserves and growing environmental pollution. Hydrogen, a clean and practically unlimited power supply, is abundant and renewable clean chemical fuel that can replace fossil fuels in the future. Currently, one effective way to prepare hydrogen is by splitting water. Electrochemical water splitting needs high-performance electrocatalysts for the hydrogen evolution reaction (HER) to afford a high current at low overpotential . As is well known, Pt or Pt-based materials have been regarded as the most efficient catalysts [2, 3]. However, such catalysts are too expensive to be widely used. This limitation has motivated great research effort to minimize the use of precious Pt to obtain the same high active HER catalysts [4, 5]. An efficient strategy loads Pt on carbon materials such as graphene . Support materials play an important role in the performance of catalytic activity.
Molybdenum disulfide (MoS2) with a characteristic layered structure is shown to have attractive activities in solid lubricants , lithium battery cathodes , and catalysis . Recently, some reports have demonstrated that could be a good alternative cocatalyst for hydrogen production reaction [10, 11]. Chang et al.  synthesized on the 3D Ni foam deposited with graphene layers which exhibited superior electrocatalytic activity in the hydrogen evolution reaction. The hydrogen evolution rate reaches 302 mL g−1 cm−2 h−1 (13.47 mmol g−1 cm−2 h−1) at an overpotential of V. Recently, Ge et al.  synthesized the layered cocatalysts on the surface of g-C3N4 and the MoS2-g-C3N4 composite samples exhibited stronger absorption in the visible light region. The 0.5 wt% MoS2-g-C3N4 exhibits the highest H2 evolution rate of 23.10 μmol h−1, which is about 11.3 times higher than pure g-C3N4. Shen et al.  reported the MoS2 deposited on UiO-66 modified CdS, and the photocatalytic activity of H2 evolution is increased by up to 60 times than pure CdS.
Photoanodes of photoelectrochemical (PEC) cells have been extensively investigated [15–18], because Si nanowires (SiNWs) can provide several performance and manufacturing benefits . Many methods have been developed for preparing SiNWs, such as chemical vapor deposition (CVD) , electron beam lithography (EBL) , pulsed laser deposition (PLD) , molecular beam epitaxy (MBE) , and wet chemical etching (WCE) . The improvement of electrocatalytic HER efficiency is crucial to effectively increase the surface area for catalyst loading. Hence, research into three-dimensional (3D) electrode structures is emergent . The SiNWs have a high surface area, which can be used as a support material to host catalysts for increasing the number of reaction sites. The promotion of the performance of hydrogen evolution reactions can be modified by metal nanoparticles, such as SiNWs anodes from modifications by metal nanoparticles such as Pt  and Ru . Recently, Huang et al. reported that the photoelectrochemical performance of SiNWs@MoS3 is comparable to the composite of SiNWs and Pt nanoparticles (SiNWs@PtNPs) . Such findings implicitly indicate a promising potential for SiNWs as support materials in electrocatalysis. In our early study, the promotion of the performance of hydrogen evolution reactions can be modified by metal nanoparticles and .
In this study, we studied HER activity of Pt- catalysts onto SiNWs under visible light irradiation. We used SiNWs as HER electrode materials and Pt- composite catalysts were grown on the surface of SiNWs. Using Pt- composite catalysts can reduce the loading of Pt and obtain the higher catalytic activity. The Pt-/SiNWs nanocomposite catalysts were synthesized by a two-step method. Step one, the /SiNWs was synthesized by solution and spin-coating methods. Step two, the Pt nanoparticles were deposited on the surface of /SiNWs by a chemical reduction method. The electrocatalytic performance of the electrodes in terms of HER was evaluated using a 0.5 M K2SO4 solution (pH = 1) based on their cathodic polarization curves. The results demonstrate that the Pt-/silicon nanowire electrodes exhibit an excellent catalytic activity for hydrogen evolution reaction in an acidic electrolyte.
2.1. The Fabrication of SiNWs
SiNWs were fabricated with the metal-assisted chemical etching method. A p-type Si as the substrate was first pretreated for surface cleaning, which included the following two steps: (1) cleaning in an acetone solution with ultrasonic vibration for 15 min and (2) washing with deionized water, pickling in a solution of H2SO4/H2O2 (3 : 1) for 15 min and then washing with deionized water.
The cleaned Si was then treated for the preparation of the Si nanowires. The treating process has three steps as follows: (1) immerse in a solution of 1% HF for 30 sec for removal of the oxide layer; (2) immersion in a solution of 8.5% HF and 25 mM AgNO3 for preparation of the Si nanowires on the Si substrate; and (3) immersion in a solution of 30% HNO3 for 15 min for removing the Ag layer from the Si nanowires.
2.2. Preparation of MoSx/Silicon Nanowire (MoSx/SiNW) by the Solution Method
A total of 26 grams of (NH4)2MoS4 was dissolved in 10 mL of dimethylformamide (DMF) and stirred for 30 min by ultrasonic. SiNWs substrate was cut into small pieces (1.25 × 1.25 cm2). To grow the catalysts on the SiNWs surfaces, the SiNWs substrate was immersed in an (NH4)2MoS4/DMF solution for 1 min. The SiNWs substrate with (NH4)2MoS4/DMF was then backed on an oven at 100°C for 10 min the layer was then formed after subsequent annealing at various temperatures (150, 200, 250, 300, and 350°C) in an Ar environment for 2 h. /SiNWs prepared by solution method are denoted by /SiNWs(S).
2.3. Preparation of MoSx/Silicon Nanowire (MoSx/SiNW) by Spin-Coating
A total of 26 grams of (NH4)2MoS4 was dissolved in 10 mL of dimethylformamide (DMF) and stirred for 30 min by ultrasonic. SiNWs substrate was cut into small pieces (1.25 × 1.25 cm2). In a typical experiment, 25 μL of (NH4)2MoS4/DMF solution was dropped onto SiNWs sample and spun at a speed of 600 rpm until DMF was totally evaporated. The layer was then formed after subsequent annealing at various temperatures (150, 200, 250, and 300°C) in an Ar environment for 2 h. /SiNWs prepared by spin-coating method are denoted by /SiNWs(C).
2.4. Preparation of Pt/Silicon Nanowire (Pt/SiNWs) and Pt/MoSx/Silicon Nanowire (Pt-MoSx/SiNWs)
We followed the process below to obtain the Pt deposited on the surface of /SiNWs and SiNWs. Firstly, H2PtCl6·6H2O (50 mg) were dissolved in 40 mL of ethylene glycol by ultrasonic treatment 1 h. Then /SiNWs or SiNWs were added to the above solution. Additionally, the pH value of this mixture was adjusted to 2 using HCl. Finally, the solution reflux at 120°C and 150°C for 3 and 6 h, respectively. Pt-/SiNWs prepared from /SiNWs made by solution and spin-coating method are denoted by Pt-/SiNWs(S) and Pt-/SiNWs(C), respectively.
2.5. Characterization Methods
A 200 kV transmission electron microscope (TEM, JEOL JEM-2010, and JEM 3010) and scanning electron microscope (JEOL, JSM-6510LV) were used for the observation of microstructure and analysis of crystal structure of the SiNWs, Pt/SiNWs, /SiNWs, and Pt-/SiNWs. TEM samples were prepared by depositing a drop of sample dispersion onto the Cu grids coated with a carbon layer.
2.6. Photoelectrochemical Measurements
Electrochemical measurements were performed at a standard three-electrode electrochemical cell with a Jiehan (made in Taiwan) electrochemical workstation. The Jiehan electrochemical workstation is a single compact unit that consists of hardware that is capable of ±10 V scan ranges and 1000 mA current capability. The workstation has measurement resolution and accuracy down to 50 μV for the reference electrode and 500 pA for the working electrode with 0.1% of reading accuracy. SiNWs, /SiNWs, Pt/SiNWs, and Pt-/SiNWs were working as electrodes. A saturated calomel electrode (SCE, Hg/Hg2Cl2 (sat. KCl)) was used as the reference and a platinum plate was used as the counter electrode. The visible light irradiation proceeded by a solar simulator under the one-sun global solar spectrum of air mass (AM) 1.5 (under 100 mWcm−2). H2SO4 solution containing 0.5 M K2SO4 (pH = 1) aqueous solution was used as the electrolyte. The working electrodes were assembled into a homemade electrochemical cell, with only a defined area (1 cm2) of the front surface of sample exposing to solution during measurements. Silver paste was applied to the backside of SiNWs samples enabling Ohmic contact. Photocurrent density-potential (-) relations were measured at scan rate of 50 mV/s. The electrochemical impedance spectroscopy measurements were performed at 50 mV versus SCE in the frequency range of 1–105 Hz.
3. Result and Discussion
3.1. Characterization of SiNWs, MoSx/SiNWs, Pt/SiNWs, and Pt-MoSx/SiNWs
The surface morphology of Si nanowires is prepared in a solution of 8.5% HF + 25 mM AgNO3 for 30 minutes as Figure 1(a) shows. The Si nanowires are bundled up into clusters, which occurs naturally for high aspect ratio vertical nanowires due to surface tension . Figure 1(b) shows the cross-sectional scanning electron microscope image of the SiNWs array, showing large-area aligned SiNWs perpendicular to the Si surface. The length of Si nanowires is about 9 μm and they are relatively uniform in length. The diameter distribution of the nanowires is in the range of 80–250 nm.
To find the optimum reducing time for obtaining catalyst Pt particles with a smaller size and a better distribution on SiNWs, reduction reactions of H2PtCl6·6H2O in ethylene glycol were carried out at 120 and 150°C for 3 and 6 h, in which the weight of H2PtCl6·6H2O is 50 mg/100 mL (0.5 g/L). A typical cross-sectional SEM image of the Pt/SiNWs using chemical reduction at 120°C for 3 h is shown as in Figure 2(a). Figures 2(b)–2(e) show cross-section SEM images (lower part) of catalyst Pt particles reduced on silicon nanowires at 120 and 150°C for 3 and 6 h, respectively. These images show that catalyst Pt particles have sizes of about 20 nm, which is nearly independent for the temperature and time of reflux.
Figure 3(a) shows TEM image of SiNWs subjected to prepared by spin-coating deposition. The thin layer is uniformly coated on the SiNWs. Figure 3(b) shows a HRTEM image of layer on the sidewall of a SiNWs. The lattice fringes with a spacing of 0.318 nm were clearly visible in SiNWs, which was in good agreement with the spacing of (111) planes of Si. In contrast to SiNWs, well-defined lattice fringe is absent in the layer, indicating that is amorphous. The thickness of layer is about 4.4 nm.
The Pt-/SiNWs was also characterized by TEM, and the images of Pt-/SiNWs samples are prepared by solution method annealing at 250 and 300°C for 2 h as Figure 4 shows. The illustration shows the selected area electron diffraction pattern. It can be seen that the Pt particles in Figures 4(a) and 4(b) do not uniform distribute on the surface of /SiNWs. Figures 4(c) and 4(d) show high-resolution TEM images of Pt particles and layer on the surface of SiNWs. The lattice fringes with a spacing of 0.196 nm were clearly visible in Pt particles, which was in good agreement with the spacing of (200) planes of Pt (Figure 4(c)). The particle size of Pt is about 4–8 nm. Additionally, (111) lattice fringe is also present in the SiNWs, and the amorphous layer lies on the sidewall of SiNWs. Figure 5 shows the images of Pt-/SiNWs samples prepared by the spin-coating method annealing at 250°C for 2 h. Figure 5(a) shows the low-magnification TEM image of Pt-nanoparticle- (NP-) decorated /SiNWs, revealing that the congregate Pt nanoparticles are also randomly located at the silicon nanowire surface. The diameter of Pt nanoparticles is typically 6–10 nm, and amorphous layer also lies on the sidewall of SiNW, as in Figure 5(b).
3.2. Electrocatalytic Properties of SiNWs, MoSx/SiNWs, Pt/SiNWs, and Pt-MoSx/SiNWs
Figure 6 shows the typical photocurrent density () versus potential () curves for photoelectrochemical measurements made with the naked p-SiNWs array electrode and the naked planar p-Si photocathode in a stirred solution containing 0.5 M K2SO4 and H2SO4 (pH 1). A three-electrode configuration is used with a SiNWs and planar Si photocathode, a Pt counter electrode, and a standard calomel reference electrode (SCE). The SiNWs and planar Si photocathode were irradiated with a solar simulator at a constant light intensity of 100 mW/cm2. Dashed line curve represents naked n-SiNWs electrode and solid line the naked planar n-Si electrode. The naked planar Si exhibited a very small photocurrent density because without an electrocatalysts present on the Si surface, electron transfer from the Si surface to H+ was very slow. In contrast, the SiNWs photocathode generated a higher photocurrent. The excellent antireflection ability and higher surface area of SiNWs are responsible for the large increase in photocurrent and high optical absorption owing to multiple scattering of light in the nanowire array .
Platinum has been demonstrated as highly efficient electrocatalysts remarkably promoting the photoelectrochemical hydrogen production from SiNWs . In the presence of Pt nanoparticles on the SiNWs surface, H2 generation from the photoelectrodes was greatly enhanced due to the facile reduction of H+ at the Pt electrocatalysts . The effect of reflux temperature and time for Pt decorated on SiNW was investigated. Figure 7 shows the - measurements for a Pt decorated with SiNW photoelectrode. It is shown that the photocurrent exhibits a maximum for the sample annealed at 120°C for 3 hr and the photocurrent slightly reduces after prolonged Pt deposition time (6 hr). The photocurrent also decreases with the further increase in annealing temperature. In addition, the onset potential () is not evidently affected by Pt deposition time and temperature. The highest photoelectrochemical performance of Pt-SiNWs resulted from reflux at 120°C for 3 h. SEM images (Figure 2) show that the size and number density of Pt nanoparticles are almost the same for all conditions (reflux at 120°C for 3 and 6 h and 150°C for 3 and 6 h). The decreased photocurrent may be due to prolonged Pt deposition on SiNWs and would result in the formation of porous silicon layer with large resistance, which reduced the photocurrent .
Figures 8(a) and 8(b) show the polarization curves for the /SiNWs prepared by solution method and spin-coating, respectively, annealing at different temperatures. It was observed that HER efficiency of the sample annealing at 150°C is slightly higher than for the sample annealing at temperatures in the range of 200–300°C between the applied potential of −0.35 V and −0.9 V (versus SCE). This reveals that the photoelectrochemical performance of /SiNWs is not affected by annealing temperature below 300°C.
Figure 9 shows the polarization curves for the Pt-/SiNWs prepared by solution method at different annealing temperatures for 2 h. The performance of Pt-/SiNWs is influenced by the annealing temperature. When the annealing temperature increases from 150°C to 300°C, the photoelectrochemical performance of Pt-/SiNWs increases with increasing temperature; the current density of Pt-/SiNWs increased from ca. 5 mA/cm2 to 18 mA/cm2, which was the optimum HER efficiency obtained by solution method. When the annealing temperature increases from 300°C to 350°C, the photoelectrochemical performance of Pt-/SiNWs decreased with increasing temperature; the current density of Pt-/SiNWs decreased to 16 mA/cm2. Chang et al. report that the /graphene is grown on Ni foams . The materials are grown by the thermolysis of ammonium thiomolybdates at different temperatures (100, 120, 170, 200, 250, and 300°C) in a CVD chamber. They observed that the HER efficiency exhibits a maxima at = 120°C and it decreases with the further increase in annealing temperature. This also evidences that the structure prepared at 300°C is close to MoS2 and the structure of the obtained at lower temperatures such as 100, 120, and 170°C is stoichiometrically close to Mo2S5. In our samples, the transmission electron microscopy (TEM) analyses reveal that all the materials obtained in the temperature range of 150 to 350°C are amorphous. The photoelectrochemical performance of Pt-/SiNWs decreasing with increasing temperature may attribute to the stoichiometry change of the .
Planar Si, SiNWs, /SiNWs(S), /SiNWs(C), Pt/SiNWs, Pt-/SiNWs(S), and Pt-/SiNWs(C) are used as HER electrodes. The - measurements for different electrodes are shown as in Figure 10. As can be seen, the /SiNWs(C) electrode displays a current density of 13 mA/cm2 when potential is applied at −0.7 V versus SCE. In comparison, the current density of the Pt//SiNWs(C) sample is 18.5 mA/cm2. In addition, the cathodic current density of Pt//SiNWs(S) (15.5 mA/cm2) at −0.7 V (versus SCE) is also much higher than for /SiNWs(S) (9 mA/cm2) and Pt/SiNWs (8.5 mA/cm2). Therefore, the enhanced HER activity of Pt//SiNWs is due to the synergetic effect between layer and Pt nanoparticles. The performance of /SiNWs(C) is slightly higher than /SiNWs(S) sample. deposited via spin-coating method can slightly enhance photocurrent of SiNWs. The enhancement may be attributed to the more uniform layer using the spin-coating method.
HER kinetics at the electrode/electrolyte interface was investigated with electrochemical impedance spectroscopy (EIS) techniques. Figure 11 presents the Nyquist plots of the EIS response of the planar Si, SiNWs, /SiNWs(S), /SiNWs(C), Pt/SiNWs, Pt-/SiNWs(S), and Pt-/SiNWs(C), where the vertical axis is the real part (Z′′) and the horizontal axis is the imaginary part (Z′) of the impedance. In order to derive a physical picture of the electrode/electrolyte interface and the processes occurring at electrode surface, experimental EIS data were modeled using fit analysis software provided with the impedance system and an electrical equivalent circuit was obtained based on one time constant. The EIS data are fitted to the equivalent circuit shown in the inset of Figure 11(a), where represents the total resistance of the electrolyte solution and the electrode, CPE is the capacitance phase element at electrode/electrolyte interface, and is the charge transfer resistance at electrode/electrolyte interface . The charge transfer resistance is related to the electrocatalysis kinetics and a lower value corresponds to a faster reaction rate . Semicircle of the Pt-/SiNWs is smaller than the /SiNWs and SiNWs indicating a lower charge transfer resistance in the Pt-/SiNWs (Figure 11(b)). The fitted value of is about 218, 258, 379, 423, and 720 Ω for Pt-/SiNWs(C), Pt-/SiNWs(S), /SiNWs(C), /SiNWs(S), and SiNWs, respectively. The lower charge transfer resistance is in favor of higher photocurrent production, which is in good agreement with our - curves described above. The Pt-/SiNWs exhibits enhanced photocurrent in comparison with pristine SiNWs, /SiNWs, and Pt/SiNWs. The high HER activity Pt-/SiNWs can be attributed to the synergetic effect between nanolayer and Pt nanoparticles play the important role in the enhancement of HER activity, which is confirmed EIS results and other reports . Pt-/SiNWs related to current electrocatalysts for hydrogen evolution can utilize less Pt. In addition, the enhanced photoelectrochemical performance of Pt-/SiNWs can be attributed to the relatively small charge transfer resistance at Pt-/SiNWs/electrolyte interface.
In summary, the Pt-/SiNWs nanocomposite was fabricated by means of a chemical reduction method on /SiNWs. The obtained Pt-/SiNWs exhibits enhanced photoelectrochemical performance in comparison with SiNWs, /SiNWs, and Pt/SiNWs. When applying potential at −0.7 V versus SCE, the current densities of the Pt//SiNWs(C), Pt//SiNWs(S), /SiNWs(C), /SiNWs(S), and Pt/SiNWs samples are 18.5 mA/cm2, 15.5 mA/cm2, 13 mA/cm2, 9.0 mA/cm2, and 8.5 mA/cm2, respectively. In this study, Pt-/SiNWs exhibits enhanced photocurrent in comparison with pristine SiNWs, /SiNWs and Pt/SiNWs. The high HER activity Pt-/SiNWs can be attributed to the synergetic effect between the nanolayer and Pt nanoparticles play the important role in the enhancement of HER activity, which is confirmed by EIS results and other reports. Pt-/SiNWs related to current electrocatalysts for hydrogen evolution can utilize less Pt. The enhancement of photoelectrochemical performance can be attributed to the fast electron transfer between Pt-/silicon nanowires electrode and electrolyte interface.
The authors declare that there is no conflict of interests regarding the publication of this article.
The authors are grateful for the financial support by the Ministry of Science and Technology of the Republic of China under Contracts MOST 103-2221-E-224-015.
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