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Photo-Promoted Platinum Nanoparticles Decorated MoS2@Graphene Woven Fabric Catalyst for Efficient Hydrogen Generation Xiao Li, Li Zhang, Xiaobei Zang, Xinming Li, and Hongwei Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01903 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016
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Photo-Promoted Platinum Nanoparticles Decorated MoS2@Graphene Woven Fabric Catalyst for Efficient Hydrogen Generation Xiao Li,1,2 Li Zhang,1 Xiaobei Zang,1 Xinming Li,3 Hongwei Zhu1,2* 1
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China 2 Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China 3 National Center for Nanoscience and Technology, Zhongguancun, Beijing 100190, China *Corresponding author. Email:
[email protected]. Abstract Hydrogen production from water splitting has been considered as an effective and sustainable method to solve future energy related crisis. Molybdenum sulfides (e.g., MoS2) show promising catalytic ability in hydrogen evolution reaction (HER). Combining MoS2 with conductive carbon-based materials has aroused tremendous research interest recently. In this work, a highly efficient multiple-catalyst is developed for HER by decorating Pt nanoparticles (Pt NPs) on MoS2@graphene protected nickel woven fabrics (NiWF) substrate, which comprises the following components: i) Graphene protected NiWF acts as the underlying substrate, supporting the whole structure; ii) MoS2 nanoplates serve as a central and essential photosensitive component, forming a heterostructure with graphene simultaneously; iii) Based on the intrinsic photoluminescence effect of MoS2, together with the photoelectric response at the MoS2/graphene interface, Pt NPs are successfully deposited on the whole structure under illumination. Particularly and foremost, this work emphasizes on discussion and verification of the underlying mechanism for photo-promoted electroless Pt NPs deposition. Due to this assembly approach, the usage amount of Pt is controlled at ~5 wt. % (~0.59 at. %) with respect to the whole catalyst. MoS2@Substrate with Pt NPs deposited under 643 nm illumination, with the synergistic effect of MoS2 active sites and Pt NPs, demonstrates the most superior electrocatalytic performance, with negligible overpotential and low Tafel slope of 39.4 mV/dec. Keywords: Graphene; MoS2; Platinum; Hydrogen Evolution Reaction; Photoluminescence 1 ACS Paragon Plus Environment
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Introduction Fossil fuels are hydrocarbons, and they all produce carbon dioxide, nitrogen oxides and even ash, probably leading to undesirable climate change. Furthermore, the reserved fossil fuels are finite, which will not realize sustainable development.1 Hence, one typical critical issue today is developing a scalable and renewable energy carrier to meet increasing global demands for energy. Recently, hydrogen becomes a promising alternative source to address the existing environmental emissions and energy crisis.1,2 The process of water splitting to produce hydrogen requires high activity and stable electrocatalysts to reduce the overpotential. Recently, molybdenum sulfide materials (e.g., MoS2) have attracted intensive attention for hydrogen evolution reaction (HER) due to their valuable metal-free composition and competitive catalytic performance in acidic electrolytes.3-7 In 2007, Jaramillo, et al. verified that the edge sites of nano-particulate MoS2 are indeed the active sites.8 Hence, one should maximize the use of MoS2 edges to obtain high activity electrocatalysts, which requires to fully expose active edge sites on a high surface area substrate.9 Jaramillo, et al. prepared vertically aligned core-shell MoO3-MoS2 nanowires for HER, facilitating the charge transport and enhancing the hydrogen production efficiency in an integrated architecture.10,11 Although many breakthroughs have been achieved for MoS2 type materials in HER,6,7 their low conductivity is a matter of concern, which largely restrain their catalytic properties. Carbon based materials seem an inspiring option due to their high conductivity and extraordinary stability in electrochemical reactions,12 further HER activity enhancement could be realized by designing multicomponent catalysts.13-15 Hence, taking advantage of graphene as the backbone for MoS2, shedding light to further HER development.16-18 In 2011, Dai, et al. synthesized a hybrid structure of MoS2 nanoparticles on reduced graphene oxide (RGO) sheets, successfully coupling the adequate catalytic edge sites of MoS2 with highly conductive graphene network.19 In 2013, Li, et al. obtained a three-dimensional (3D) catalyst by growing MoSx on graphene-protected Ni foams, which demonstrated highly efficient hydrogen production property.20 They proved that the amorphous states of bridging S22- or apical S2- afforded the superior performance, which was in accordance with the result reported by Hu, et al.3,20 2 ACS Paragon Plus Environment
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Notwithstanding the impressive progress, the gained HER activity of these nonmetal catalysts is still not comparable to that of commercial Pt based catalysts (summarized in Table S1). Hence, one compromise solution is to reduce Pt consumption in designing catalysts. Pt nanoparticles (Pt NPs) with large surface to volume ratio have shown attractive properties and seem to be an alternative approach.21-23 Many strategies have been reported for Pt NPs deposition. In 2013, Huang, et al. reported a solution-phase epitaxial growth of Pt nanostructures on MoS2, Pt NPs on MoS2 hybrid material with 36 wt. % Pt content, showing small overpotential and low Tafel slope to be 40 mV/dec. This method was innovative, while the whole process was relatively time-consuming and the Pt content was in a high level.22 Hou, et al. used electro-deposition to synthesize Pt NPs on MoS2/carbon papers, which was commonly used and facile, but it totally relied on electricity supply.23 Fortunately, early in 2002, Dai, et al. has proposed a spontaneous reduction of Au or Pt ions on the side-walls of carbon nanotubes, which was attributed to the relative potential levels between nanotubes and metal ions. This process was different from other electroless deposition procedure that needed catalyst, meeting the requirements of simplicity and energy conservation.24 Intrigued by the electroless Pt NPs deposition method reported by Dai’s group in 2002,24 we develop a suitable substrate material by combining MoS2 and graphene on Ni woven fabrics (NiWF) to load Pt NPs under illumination. Multilayer graphene can isolate NiWF from the outside electrolyte. Graphene on NiWF is abbreviated as Substrate for short. MoS2 nanoplates are indispensable photosensitive component, which not only form heterogenous interfaces with the underlying graphene, but exhibit intrinsic photoluminescence (PL) effect under certain wavelength ranges. In addition, MoS2 itself contributes to the HER activity enhancement. Based on the above design, photo-promoted Pt NPs deposition can be realized and the underlying mechanism is disclosed and validated. Due to this coupling concept, Pt consumption is controlled low to be ~5 wt. % (~0.59 at. %) with respect to the whole catalyst. Finally, MoS2@Substrate with Pt NPs deposited under 643 nm wavelength (denoted as Pt NPs(643)@MoS2@Substrate) demonstrates challenging HER performance. The overpotential is only -0.056 V (vs. RHE) at the current density of 10 mA/cm2, extremely close to that of commercial Pt based catalysts.
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Results and Discussion Structural Characterizations. The schematic illustration for the photo-promoted preparation of Pt NPs@MoS2@Substrate catalyst is demonstrated in Scheme 1. First, multilayer graphene protected NiWF was synthesized through a typical atmospheric pressure chemical vapor deposition (APCVD) process as previously reported,25 which acts as the substrate in this work. Detailed transmission electron microscope (TEM) characterizations for multilayer graphene are shown in FigureS1(a)~(c). Figure S1(b) is a selected area electron diffraction (SAED) pattern of the area marked in the yellow box (Figure S1(a)). Dotted-ring distribution indicates a high degree of crystalline order of the graphene edge. Enlarged TEM view in Figure S1(c) shows typical multilayer graphene morphology. Figure S1(d) exhibits the photographs of NiWF before (left) and after (right) the APCVD growth of graphene. One notable change was that the silver-grey color of NiWF becomes dark-grey after this procedure. Multilayer graphene could protect the underlying NiWF thoroughly. To prepare MoSx@Substrate, a versatile electrophoretic deposition in ammonium thiomolybdates ((NH4)2MoS4) solution was adopted (Figure S1(e)). Moreover, annealing at 400 oC was conducted to improve the crystallinity of MoSx and remove organic residuals, finally obtaining MoS2@Substrate. From the scanning electron microscopic (SEM) image of Substrate in Scheme 1, the wrinkles of multilayer graphene can be clearly observed, which is due to the different shrinkage coefficient of metal and carbon materials during rapid cooling. After this electrophoretic process at the applied potential of 2 V for 5 min, MoSx nanoplates in anomalous shapes are distributed along the surface topography of Substrate. MoSx(400 o
C)@Substrate, also named MoS2@Substrate, has the similar configuration to that of
MoSx@Substrate, as shown in Scheme 1. Finally, Pt NPs were deposited on MoS2@Substrate (Pt NPs@MoS2@Substrate) by a direct redox reaction24 promoted by xenon light illumination in hydrochloroplatinic acid (H2PtCl6) solution (The setup is shown in Figure S1(f)). Pt NPs(643) refers to the Pt NPs loading on MoS2@Substrate under the illumination of 643 nm light. Typical crisscross-pattern structures (aperture: 100 µm, wire diameter: 70 µm) are described in the optical microscopy (OM) images (Figure 1(a)). Enlarged SEM image is displayed in Figure 1(b). Energy dispersive X-ray spectroscopy (EDS) analysis is conducted 4 ACS Paragon Plus Environment
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for the area marked in the yellow box in Figure S2(a). Peaks for C, Mo, S, Pt, O and Ni elements are clearly shown in Figure S2(b) and the corresponding element contents (weight (%) and atomic (%)) are summarized in Table S2. These results provide qualitative and half-quantitative analysis to the existing Pt NPs and MoS2 nanoplates on the Substrate. The weight content of Pt NPs is ~5 % with respect to the whole catalyst, relatively reducing the consumption of Pt. Moreover, X-ray photoelectron spectroscopy (XPS) atomic content (Pt, S, Mo, C, and O) are provided in Table S3, which shows Pt (at. %) is ~0.59 %. TEM characterizations of Pt NPs@MoS2@graphene composite film is exhibited in Figure 1(c), with Pt NPs and MoS2 nanosheets distributed on the graphene backbone. Due to their similar sheet-like morphology, the interlayer spacing was measured for analysis. For the small quantity of MoS2, its interlayer spacing is 0.65 nm, larger than that of graphene (~0.35 nm), as pointed out in the inset of Figure 1(c). Enlarged high-resolution TEM view is shown in Figure 1(d), showing distinguishable lattice fringes for Pt NPs. The size of Pt NPs is about 3 nm, and their orientation is primarily (111) (Figure S3). To reveal the reduction state of Pt NPs on MoS2@Substrate, XPS spectra are recorded and peak differentiation results for Mo (3d), S (2p) and Pt (4f) elements are displayed in Figure 2(a-c). Pt NPs(643)@MoS2@Substrate sample is chosen for analysis. In Figure 2(a), the characteristic peaks for Mo4+ at 229.8 eV and 232.9 eV are depicted. Besides, the peak observed at 235.9 eV is attributed to the 3d3/2 binding energy of Mo6+, indicating a certain degree of oxidation due to the formation of MoO3. In addition, the peaks at 162.5 eV and 163.6 eV in Figure 2(b) are assigned to S (2p). In Figure 2(c), peaks for Pt0, PtII and PtIV species are plotted, which can be speculated that the reduction of [PtIVCl6]2- follows a disproportionation reaction ([1] and [2]) and probably a sequential reduction ([1] and [3]).26 The formula can be described as follows:
[Pt Cl ] → [Pt Cl ] + 2Cl
[1]
2[Pt Cl ] ↔ [Pt Cl ] + Pt + 2Cl
[2]
[Pt Cl ] → Pt + 4Cl
[3]
The peaks for PtIV state at about 76.9 eV and 78.5 eV are extremely low,27 indicating its high reduction level and the dominant role of sequential reduction (Eq. [1] and Eq. [3]). These 5 ACS Paragon Plus Environment
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analysis confirm the successful formation of Pt NPs and MoS2 on the surface of Substrate. Raman spectroscopy is a convenient characterization method to evaluate the quality and layer number for layered materials. In Figure 2(d), Raman spectra of Substrate, MoS2@Substrate and Pt NPs(643)@MoS2@Substrate are plotted respectively. For multilayer graphene, typical D, G and 2G peaks are located at 1350 cm-1, 1580 cm-1 and 2700 cm-1, respectively. The intensity ratio of 2G peak to G peak (I2G
