Journal of Nanomaterials
Volume 2016 (2016), Article ID 6974646, 10 pages
http://dx.doi.org/10.1155/2016/6974646

Research Article

Silicon Nanowires with MoSx and Pt as Electrocatalysts for Hydrogen Evolution Reaction

S. H. Hsieh,¹ S. T. Ho,² and W. J. Chen²

¹Department of Materials Science and Engineering, National Formosa University, 64 Wenhua Road, Huwei, Yunlin 632, Taiwan
²Graduate School of Materials Science, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin 64002, Taiwan

Received 9 June 2016; Revised 7 September 2016; Accepted 18 October 2016

Academic Editor: Vincenzo Baglio

Copyright © 2016 S. H. Hsieh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [1]. 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 [6]. 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 [7], lithium battery cathodes [8], and catalysis [9]. Recently, some reports have demonstrated that could be a good alternative cocatalyst for hydrogen production reaction [10, 11]. Chang et al. [12] 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. [13] 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. [14] 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 [19]. Many methods have been developed for preparing SiNWs, such as chemical vapor deposition (CVD) [20], electron beam lithography (EBL) [21], pulsed laser deposition (PLD) [22], molecular beam epitaxy (MBE) [23], and wet chemical etching (WCE) [24]. 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 [12]. 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 [25] and Ru [26]. Recently, Huang et al. reported that the photoelectrochemical performance of SiNWs@MoS₃ is comparable to the composite of SiNWs and Pt nanoparticles (SiNWs@PtNPs) [27]. 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 [28].

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. Experimental

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 [29]. 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.

(a)

(b)

Figure 1: (a) Surface morphology and (b) cross-sectional scanning electron microscope images of the SiNWs array.

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.

(a)

(b)

(c)

(d)

(e)

Figure 2: (a) A typical cross-sectional SEM image of the PtNPs/SiNWS using chemical reduction at 120°C for 3 h. The cross-section SEM images (lower part) of catalyst Pt particles reduced on silicon nanowires at (b) 120°C for 3 h, (c) 120°C for 6 h, (d) 150°C for 3 h, and (e) 150°C 6 h.

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.

(a)

(b)

Figure 3: (a) TEM image of SiNWs subjected to prepared by spin-coating deposition. (b) High-resolution TEM image of /SiNWs.

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).

(a)

(b)

(c)

(d)

Figure 4: TEM images of Pt-/SiNWs samples prepared by solution method annealing at (a) 250 and (b) 300°C for 2 h. High-resolution TEM images of Pt-/SiNWs samples annealing at (c) 250 and (d) 300°C for 2 h.

(a)

(b)

Figure 5: (a) TEM image and (b) high-resolution TEM image of Pt-/SiNWs samples prepared by spin-coating method annealing at 250°C for 2 h.

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 [30].

Figure 6: The typical photocurrent density () versus potential () curves for (a) the naked p-SiNWs array electrode and (b) the naked planar p-Si photocathode.

Platinum has been demonstrated as highly efficient electrocatalysts remarkably promoting the photoelectrochemical hydrogen production from SiNWs [25]. 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 [25]. 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 [30].

Figure 7: The - measurements for a Pt decorated with SiNWs photoelectrode.

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.

(a)

(b)

Figure 8: The polarization curves for the /SiNWs prepared by (a) solution method and (b) spin-coating.

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 [12]. 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 .

Figure 9: The polarization curves for the Pt-/SiNWs prepared by solution method at different annealing temperatures for 2 h.

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.

Figure 10: The - measurements for planar Si, SiNWs, MoS₂/SiNWs(S), MoS₂/SiNWs(C), Pt/SiNWs, Pt-MoS₂/SiNWs(S), and Pt-MoS₂/SiNWs(C) electrodes.

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 [27]. The charge transfer resistance is related to the electrocatalysis kinetics and a lower value corresponds to a faster reaction rate [31]. 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 [27]. 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.

(a)

(b)

Figure 11: (a) Nyquist plots the electrochemical impedance spectroscopy response of the planar Si, SiNWs, MoS₂/SiNWs(S), MoS₂/SiNWs(C), Pt/SiNWs, Pt-MoS₂/SiNWs(S), and Pt-MoS₂/SiNWs(C). (b) Typical results of fitting model to the experimental results of (a).

4. Conclusions

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.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this article.

Acknowledgments

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.

References

M. G. Walter, E. L. Warren, J. R. McKone et al., “Solar water splitting cells,” Chemical Reviews, vol. 110, no. 11, pp. 6446–6473, 2010.

R. Palaniappan and G. G. Botte, “Effect of ammonia on Pt, Ru, Rh, and Ni cathodes during the alkaline hydrogen evolution reaction,” Journal of Physical Chemistry C, vol. 117, no. 34, pp. 17429–17441, 2013.

W. C. Sheng, H. A. Gasteiger, and Y. J. Shao-Horn, “Hydrogen oxidation and evolution reaction kinetics on platinum: acid vs alkaline electrolytes,” Journal of the Electrochemical Society, vol. 157, no. 11, pp. B1529–B1536, 2010.

I. E. L. Stephens and I. Chorkendorff, “Minimizing the use of platinum in hydrogen-evolving electrodes,” Angewandte Chemie—International Edition, vol. 50, no. 7, pp. 1476–1477, 2011.

X. Huang, Z. Zeng, S. Bao et al., “Solution-phase epitaxial growth of noble metal nanostructures on dispersible single-layer molybdenum disulfide nanosheets,” Nature Communications, vol. 4, article 1444, 2013.

G.-R. Xu, J.-J. Hui, T. Huang, Y. Chen, and J.-M. Lee, “Platinum nanocuboids supported on reduced graphene oxide as efficient electrocatalyst for the hydrogen evolution reaction,” Journal of Power Sources, vol. 285, Article ID 20920, pp. 393–399, 2015.

T. W. Scharf, P. G. Kotula, and S. V. Prasad, “Friction and wear mechanisms in MoS₂Sb₂O₃/Au nanocomposite coatings,” Acta Materialia, vol. 58, no. 12, pp. 4100–4109, 2010.

D.-A. Zhang, Q. Wang, Q. Wang, J. Sun, L.-L. Xing, and X.-Y. Xue, “High capacity and cyclability of hierarchical MoS₂/SnO₂ nanocomposites as the cathode of lithium-sulfur battery,” Electrochimica Acta, vol. 173, pp. 476–482, 2015.

C. T. Tye and K. J. Smith, “Hydrodesulfurization of dibenzothiophene over exfoliated MoS₂ catalyst,” Catalysis Today, vol. 116, no. 4, pp. 461–468, 2006.

J. Zhang, L. Zhao, A. Liu, X. Li, H. Wu, and C. Lu, “Three-dimensional MoS₂/rGO hydrogel with extremely high double-layer capacitance as active catalyst for hydrogen evolution reaction,” Electrochimica Acta, vol. 182, pp. 652–658, 2015.

F. Li, L. Zhang, J. Li et al., “Synthesis of Cu–MoS₂/rGO hybrid as non-noble metal electrocatalysts for the hydrogen evolution reaction,” Journal of Power Sources, vol. 292, pp. 15–22, 2015.

Y.-H. Chang, C.-T. Lin, T.-Y. Chen et al., “Highly efficient electrocatalytic hydrogen production by MoSx grown on graphene-protected 3D Ni foams,” Advanced Materials, vol. 25, no. 5, pp. 756–760, 2013.

L. Ge, C. Han, X. Xiao, and L. Guo, “Synthesis and characterization of composite visible light active photocatalysts MoS₂-g-C₃N₄ with enhanced hydrogen evolution activity,” International Journal of Hydrogen Energy, vol. 38, no. 17, pp. 6960–6969, 2013.

L. Shen, M. Luo, Y. Liu, R. Liang, F. Jing, and L. Wu, “Noble-metal-free MoS₂ co-catalyst decorated UiO-66/CdS hybrids for efficient photocatalytic H₂ production,” Applied Catalysis B: Environmental, vol. 166-167, pp. 445–453, 2015.

A. P. Goodey, S. M. Eichfeld, K.-K. Lew, J. M. Redwing, and T. E. Mallouk, “Silicon nanowire array photoelectrochemical cells,” Journal of the American Chemical Society, vol. 129, no. 41, pp. 12344–12345, 2007.

S. Yae, I. Nakanishi, Y. Nakato, N. Toshima, and H. Mori, “Preparation of a Langmuir-Blodgett layer of ultrafine platinum particles and its application to n-Si for efficient photoelectrochemical solar cells,” Journal of the Electrochemical Society, vol. 141, no. 11, pp. 3077–3081, 1994.

D. Kumar, S. K. Srivastava, P. K. Singh, M. Husain, and V. Kumar, “Fabrication of silicon nanowire arrays based solar cell with improved performance,” Solar Energy Materials and Solar Cells, vol. 95, no. 1, pp. 215–218, 2011.

E. A. Dalchiele, F. Martín, D. Leinen, R. E. Marotti, and J. R. Ramos-Barrado, “Single-crystalline silicon nanowire array-based photoelectrochemical cells,” Journal of the Electrochemical Society, vol. 156, no. 5, pp. K77–K81, 2009.

E. A. Dalchiele, F. Martín, D. Leinen, R. E. Marotti, and J. R. Ramos-Barrado, “Synthesis, structure and photoelectrochemical properties of single crystalline silicon nanowire arrays,” Thin Solid Films, vol. 518, no. 7, pp. 1804–1808, 2010.

T. Stelzner, M. Pietsch, G. Andrä, F. Falk, E. Ose, and S. Christiansen, “Silicon nanowire-based solar cells,” Nanotechnology, vol. 19, no. 29, Article ID 295203, 2008.

R. Juhasz, K. Kylmänen, A. Galeckas, and J. Linnros, “Size-reduced silicon nanowires: fabrication and electrical characterization,” Materials Science and Engineering C, vol. 25, no. 5–8, pp. 733–737, 2005.

A. M. Morales and C. M. Lieber, “A laser ablation method for the synthesis of crystalline semiconductor nanowires,” Science, vol. 279, no. 5348, pp. 208–211, 1998.

L. Schubert, P. Werner, N. D. Zakharov et al., “Silicon nanowhiskers grown on Si substrates by molecular-beam epitaxy,” Applied Physics Letters, vol. 84, no. 24, pp. 4968–4970, 2004.

K.-Q. Peng, Y.-J. Yan, S.-P. Gao, and J. Zhu, “Synthesis of large-area silicon nanowire arrays via self-assembling nanoelectrochemistry,” Advanced Materials, vol. 14, no. 16, pp. 1164–1167, 2002.

I. Oh, J. Kye, and S. Hwang, “Enhanced photoelectrochemical hydrogen production from silicon nanowire array photocathode,” Nano Letters, vol. 12, no. 1, pp. 298–302, 2012.

L. Zhu, Q. Cai, F. Liao, M. Sheng, B. Wu, and M. Shao, “Ru-modified silicon nanowires as electrocatalysts for hydrogen evolution reaction,” Electrochemistry Communications, vol. 52, pp. 29–33, 2015.

Z. P. Huang, C. F. Wang, L. Pan, F. Tian, X. X. Zhang, and C. Zhang, “Enhanced photoelectrochemical hydrogen production using silicon nanowires@MoS₃,” Nano Energy, vol. 2, no. 6, pp. 1337–1346, 2013.

S. H. Hsieh, S. T. Ho, and W. J. Chen, “Silicon nanowires with MoSx and Pt as electrocatalysts for hydrogen evolution reaction,” in Proceedings of the International Thin Films Conference, Tainan, Taiwan, November 2015.

K. Q. Peng, Y. Xu, Y. Wu, Y. J. Yan, S.-T. Lee, and J. Zhu, “Aligned single-crystalline Si nanowire arrays for photovoltaic applications,” Small, vol. 1, no. 11, pp. 1062–1067, 2005.

K.-Q. Peng, X. Wang, X.-L. Wu, and S.-T. Lee, “Platinum nanoparticle decorated silicon nanowires for efficient solar energy conversion,” Nano Letters, vol. 9, no. 11, pp. 3704–3709, 2009.

D. M. Hou, W. J. Zhou, X. J. Liu et al., “Pt nanoparticles/MoS₂ nanosheets/carbon fibers as efficient catalyst for the hydrogen evolution reaction,” Electrochimica Acta, vol. 166, Article ID 24593, pp. 26–31, 2015.