Sites for High Efficient Photocatalytic Hydrogen Evolution on a Limited ...

Article pubs.acs.org/JPCC

Sites for High Efficient Photocatalytic Hydrogen Evolution on a Limited-Layered MoS2 Cocatalyst Confined on Graphene Sheets―The Role of Graphene Shixiong Min†,‡ and Gongxuan Lu*,† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ The Graduate School of the Chinese Academy of Sciences, Beijing 10080, China S Supporting Information *

ABSTRACT: The development of an advanced cocatalyst is critical for improving the efficiency of the photocatalytic hydrogen evolution reaction. Noble metals such as platinum (Pt) have been identified to be the most active cocatalyst for this reaction; however, due to their low-abundance, high cost, their usage in the scale-up setup is impeditive. Here, we report a high active cocatalyst, limited-layered MoS2 confined on RGO sheets as an alternative of Pt, for hydrogen evolution in dye-sensitized photocatalytic systems. Growing a MoS2 cocatalyst on RGO sheets provides more available catalytically edge sites and thus exhibits much higher activity than large aggregated pristine MoS2 particles under visible light irradiation (≥420 nm). The apparent quantum efficiency (AQE) of 24% at 460 nm over an Eosin Y-sensitized MoS2/RGO photocatalyst has been achieved. In addition, the electrical coupling and synergistic effect between MoS2 and RGO sheets greatly facilitate the efficient electron transfer from photoexcited dye to the active edge sites of MoS2; as a result, the prolonged lifetime of photogenerated electrons and the improved charge separation efficiency have been accomplished, and the photocatalytic hydrogen evolution activity has been enhanced significantly. This work demonstrates that the structural integration of MoS2 with RGO will be a new promising strategy to develop a high efficient and low-cost non-noble metal cocatalyst for solar hydrogen generation. iron complexes.3 However, H2 evolution activity and stability of these catalysts are far from satisfactory due to the inherent instability of their organic ligands, which tend to degrade under reaction conditions. Nevertheless, recently studies have shown that molybdenum disulfide (MoS2) can efficiently catalyze the electrochemical and photochemical HER.4 Both high-resolution scanning tunneling microscopy (STM) and theoretical calculations have revealed that the catalytic activity of MoS2 plates is derived from the undercoordinated sulfur edge sites, while their basal planes remain catalytically inert.4a−c Therefore, nanocrystalline MoS2 with increased edge catalytic sites should be more active than bulk MoS2 materials.4a However, because of its layered nature and high surface energy of the twodimensional structure, MoS2 nanomaterials are prone to stacking together through π−π interaction during the preparation process,4b making a substantial amount of catalytic edge sites on MoS2 blocked, and delivering higher resistance for the electron transfer and diffusion of reactant molecules, and retarding the catalytic reaction.

1. INTRODUCTION Large-scale and sustainable hydrogen evolution by photocatalytic water splitting is currently being vigorously pursued as one of the promising routes to convert and store solar energy efficiently.1 To perform the hydrogen evolution reaction (HER), the photocatalytic system usually requires a high efficient photocatalyst and a hydrogen evolution cocatalyst.1c−e In particular, the photocatalytic HER efficiency significantly relies on the properties of the hydrogen evolution cocatalyst that should provide highly active sites for hydrogen formation and reduce the overpotential of HER significantly.2 A noble metal, such as Pt, has been fully proven to be an effective cocatalyst for this reaction;2 however, the scale-up application of the Pt-based catalysts is still hard because of their scarceness and high-cost. It is therefore mportant to develop new no-noble metal hydrogen evolution cocatalysts that are highly active, stable, and low-cost, especially from the point of view of element abundance. By understanding the mechanisms involved in HER and the structural nature of the active sites for H2 evolution, many significant advances have recently been made in design and artificial synthesis of analogues of the active sites of hydrogenases, such as molecular catalysts based on cobalt, nickel, and © 2012 American Chemical Society

Received: September 21, 2012 Revised: November 11, 2012 Published: November 14, 2012 25415

dx.doi.org/10.1021/jp3093786 | J. Phys. Chem. C 2012, 116, 25415−25424

The Journal of Physical Chemistry C

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Na 2MoO4 ·2H 2 O and NH 2 CSNH 2 in aqueous solution containing exfoliated graphite oxide (GO). All of the reagents were of analytical grade and were used without further purification. Graphite oxide (GO) was prepared by a modified Hummers method using natural graphite powder as the starting material (see experimental details and the characterization data of GO (Figure S1) in the Supporting Information).9 For a typical synthesis of the MoS2/RGO nanohybrid, 100 mg of asprepared GO powders were dispersed into 60 mL of H2O and ultrasonicated until clear yellow-brown suspensions were obtained. After that, 1.21 g of Na2MoO4·2H2O and 1.52 g of NH2CSNH2 were dissolved into the above suspensions and stirred for 30 min, and then the mixture solution was transferred into a 100 mL Teflon-lined autoclave and heated in an oven at 220 °C for 24 h. The black products were collected by filtration under vacuum, washed carefully with water and anhydrous ethanol in sequence, and finally dried at room temperatures. For comparison, the blank samples of MoS2 and RGO were prepared in the absence of GO and Mo precursor, respectively, using the same experimental conditions. 2.2. Characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken with a Tecnai-G2-F30 field emission transmission electron microscope operating at an accelerating voltage of 300 kV. Field-emission scanning electron microscopy (FESEM) images were recorded on a JSM-6701F scanning electron microscope operated at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on K-Alpha-surface Analysis (Thermon Scientific) using X-ray monochromatization. X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku B/Max-RB diffractometer with a nickel filtrated Cu Kα radiation operated at 40 kV and 40 mA. FT-IR spectra were measured on a Nexus 870 FT-IR spectrometer from KBr pellets as the sample matrix. Zeta (ζ) potential measurements for catalysts were carried out using a Malvern Zetasizer Nano-ZS 3600 analyzer. Each particle suspension with a concentration of 0.15 mg/mL was initially dispersed in an aqueous NaOH solution (pH 11) and ultrasonicated to ensure a homogeneous dispersion without visual aggregation.5d The pH was subsequently adjusted incrementally by adding 0.5 M HCl to pH 2 under stirring conditions, and approximately 0.75 mL of suspension was injected into the capillary cell at regular pH intervals. UV−vis absorption spectra were obtained with a Hewlett-Packard 8453 spectrophotometer. Photoluminescence spectra were determined by a Horiba Scientific FluoroMax-4 spectrofluorometer. The fluorescence decay times were measured using the Horiba Jobin Yvon Data Station HUB operating in time-correlated single photon counting mode (TCSPC) with the time resolution of 200 ps. Nano LED diode emitting pulses at 460 nm with 1 MHz repetition rate was used as an excitation source.8b 2.3. Photocatalytic H2 Evolution Experiments. The photocatalytic H2 evolution experiments were performed in a sealed quartz flask (152 mL) with a flat window and a silicone rubber septum for sampling. Typically, 20 mg of the prepared MoS2/RGO nanohybrid material was dispersed in 80 mL of a 15% (v/v) triethanolamine-water (TEOA-H2O) solution (pH 7) by ultrasonication for 15 min, and then a calculated amount of dye Eosin Y (EY) was added under stirring conditions. The light source was a 300-W Xe lamp with an optical filter (≥420 nm) to remove the light in the ultraviolet region. Prior to irradiation, the reactant mixture was degassed by bubbling Ar

In order to overcome these obstacles, few-layered MoS2 nanocatalysts with less stacking and large edge dimension were previously deposited on graphite,4a Au (111),4b carbon paper,4c and carbon nanotubes.4d The obtained samples have shown significantly enhanced performances for electrocatalytic HER. Compared to the above available support materials, graphene, a novel two-dimensional carbon nanomaterial, may emerge as the promising platform for growing of MoS2 materials due to its high surface area, high charge mobility, and good stability.5,6 These unique characteristics of the MoS2/graphene nanohybrid, such as limited-layered MoS2 on graphene with abundant selectively exposed edge sites and the chemical and electrical coupling effects between MoS2 and graphene, have made it to be a novel and advanced nanohybrid catalyst for HER.5a,b Dai and co-workers5c have reported the selective solvothermal synthesis of MoS2 nanoparticles on reduced graphene oxide (RGO) sheets with superior electrocatalytic activity for HER. Girault et al.5d have shown an enhanced H2 evolution activity over mesoporous carbon and graphene supported MoS2 catalysts in biphasic liquid systems. For photocatalytic HER, the presence of graphene can greatly inhibit the recombination of photogenerated charge carriers due to its excellent electron-accepting and transporting properties, thus greatly enhancing the efficiency of photocatalytic HER.7,8 Moreover, stacking MoS2 nanoplatelets on the conductive surface (such as CNTs and graphene) may provides a low resistance connection for the electrons transfer from the substrate to the active sites, which leads to an advantageous catalyst for electrochemical and photochemical reactions.4d Although the effectiveness of these advantages of graphene in improving photocatalytic activity has been extensively demonstrated in several photocatalysts, such as semiconductor/ graphene7 and dye-sensitized graphene/Pt,8 the utilization of the MoS2/graphene nanohybrid as a catalyst for visible light photocatalytic H2 evolution has not been adequately addressed. In addition, MoS2 as a cocatalyst in enhancing the H2 evolution should have been further demonstrated although its activity on TiO2 under ultraviolet light irradiation has been reported.7g In this work, we report the MoS2/RGO nanohybrid catalyst prepared by a facilely hydrothermal method for efficiently photocatalytic HER. This high active catalyst can operate under visible light irradiation by dye sensitization. Here, twodimensional RGO sheets not only provide a confined substrate for selective growth of limited-layer MoS2 cocatalyst with a large number of exposed catalytic sites but also form the interconnected two-dimensional conductive networks for efficiently transferring photogenerated electrons from excited dye to catalytic active sites of MoS2, thus suppressing the recombination processes and enhancing the photocatalytic efficiency of HER. The greatly improved hydrogen evolution activity compared to that of the pristine MoS2 was achieved. A high apparent quantum efficiency of 24.0% at 460 nm was obtained over an Eosin Y (EY)-sensitized MoS2/RGO photocatalyst. This work indicates that the effectively integrated structure and the electronic properties of the MoS2/RGO nanohybrid lead to a highly active advanced catalyst, which will be a promising alternative of noble metals for photocatalytic HER.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the MoS2/Reduced Graphene Oxide (RGO) Nanohybrid. The MoS2/RGO nanohybrid was synthesized by a one-step hydrothermal reaction of 25416



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Figure 1. SEM and TEM images of MoS2 and the MoS2/RGO nanohybrid. Panels A and E are the SEM images for MoS2 and the MoS2/RGO nanohybrid, respectively. B−D and F−H are the TEM and HRTEM images for MoS2 and the MoS2/RGO nanohybrid, respectively. The insets in D and H are the corresponding SAED patterns.

conditions with irradiation light through a band-pass filter (430, 460, 490, 520, 550, or 590 nm). The photon flux of incident light was determined using a Ray virtual radiation actinometer (FU 100, silicon ray detector, light spectrum, 400−

gas for 40 min. The amount of produced hydrogen gas was measured using gas chromatography (Aglient 6820, TCD, 13× column, Ar carrier). The apparent quantum efficiency (AQE) was measured under the same photocatalytic reaction 25417



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700 nm; sensitivity, 10−50 μV μmol−1·m−2·s−1). The AQE was calculated from the ratio of the number of reacted electrons during hydrogen evolution (equivalent to nH2) to the number of incident photons np according to eqs 1 and 2,8a,b where t is the irradiation time (s), S is the effective light irradiation area (m2), and Q is the photon flux of the incident light (μmol (photons) m−2 s−1). np = t × S × Q

number of reacted electrons × 100 number of incident photos 2n H2 = × 100 np

0.62 nm, corresponding to the (002) plane of hexagonal atomic lattices.4a In strong contrast to pristine MoS2 particles, the TEM image of the MoS2/RGO nanohybrid shown in Figure 1F demonstrates that the MoS2 grown on RGO sheets appear as loose and soft agglomerates of several nanometers. Figure 1G indicates that these MoS2 agglomerates consist of limited-layer MoS2 nanosheets, and most of them are intimately laid flat on the underlying RGO sheets. The HRTEM image of the MoS2/ RGO nanohybrid (Figure 1H) reveals that the thickness of MoS2 grown on RGO is significantly reduced to about 3−5 layers, and the sheets tend to bend and their edges are ragged as a result of minimizing the surface energy. These results suggest that the presence of RGO can greatly inhibit the restacking of MoS2 during its nucleation and growth,5 and, as a result, limited-layered MoS2 with abundant exposed edge sites can be confinedly grown on the surface of RGO sheets. This conclusion is further confirmed by the zeta (ζ) potential difference of the above samples in aqueous solution. As reported by Girault et al.5d the ζ-potential is a useful qualitative probe to determine the relative abundances of sulfur edge sites for MoS2-based catalysts. As discussed above, an increased exposure of negatively charged sulfur edge atoms is expected for the MoS2/RGO nanohybrid compared to pristine MoS2. From Figure 2, more negative ζ-potentials in the pH range 2 to

(1)

AQY[%] =

(2)

2.4. Electrochemical and Photoelectrochemical Measurements. All the electrochemical and photoelectrochemical measurements were performed on an electrochemical analyzer (CHI660A) in a homemade standard three-electrode cell.8b Platinum foil was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The supporting electrolyte was 15% (v/v) TEOA mixed with 0.1 M Na2SO4 aqueous solution. The surface areas of the working electrode exposed to the electrolyte was about 1.6 cm2. For the preparation of working electrodes for electrochemical measurements, a homogeneous catalyst ink was first prepared by dispersing 4 mg of catalyst material and 80 μL of a 5 wt % Nafion solution in 2 mL of H2O by ultrasonication, and then 400 μL of catalyst ink dispersions was drop-coated directly onto the precleaned indium tin oxide (ITO) glass surface (ca. 1.96 cm2) by microsyringe and placed on a hot plate to speed drying. The cathodic polarization curves were obtained using the linear sweep voltammetry (LSV) technique with a scan rate of 1 mV·s−1. For the preparation of working electrodes for photoelectrochemical measurements, above catalyst film electrodes were further sensitized by adding 400 μL of EY aqueous solution (4.0 × 10−4 M) onto the as-prepared electrode surface and drying. A 300-W Xe lamp equipped with an optical cutoff filter of 420 nm and a band-pass filter of 520 nm was used for excitation.

Figure 2. Zeta (ζ) potentials of MoS2, the MoS2/RGO nanohybrid, and RGO as a function of pH in aqueous dispersions.

11 for the MoS2/RGO nanohybrid than pristine MoS2 are observed, which strongly support that the presence of more MoS2 sulfur edge sites on RGO sheets, which providing more reaction sites for protons reduction, thus enhancing the photocatalytic H2 evolution activity (see below). Crystalline structures of RGO, MoS2, and the MoS2/RGO nanohybrid are characterized by X-ray diffraction (XRD). As shown in Figure 3A, both MoS2 and the MoS2/RGO nanohybrid exhibit the same typical hexagonal structure (JCPDS 37-1492) of crystalline MoS2, and all the diffraction peaks are in accordance with the selected area electron diffraction (SAED) patterns in Figure 1D and Figure 1H. The primary peak for both samples appears at 2θ = 14.06° corresponding to the diffraction from (002) plane of MoS2 with d-spacing of 0.62 nm, which is consistent with HRTEM results, indicating that the layered crystallinity nature of MoS2 still retains in the MoS2/RGO nanohybrid, albeit with the presence of RGO sheets.5a,b Particularly, it is worth noting that the characteristic diffraction peak at 2θ = 24.98° corresponding to the (002) plane of RGO nanosheets is absent in the XRD and SAED pattern of the MoS2/RGO nanohybrid, which suggests that the stacking between adjacent RGO sheets is suppressed due to the selective growth of MoS2, which is consistent with the previously reported results by Dai and Chen et al.5a−c As a

3. RESULTS AND DISCUSSION The MoS2/RGO nanohybrid was synthesized by a one-pot hydrothermal reaction of Na2MoO4·2H2O and NH2CSNH2 in exfoliated graphite oxide (GO) aqueous suspensions at 220 °C for 24 h. During this hydrothermal process, GO was converted to RGO, and Mo salt was reduced to well-crystallized MoS2 on RGO sheets using NH2CSNH2 as both S resource and reductant. Here, the various oxygen-containing groups on the GO surface should facilitate the nucleation and following growth of limited-layered MoS2 on RGO.5 Figure 1A and E shows the scanning electron microscopy (SEM) images of resulting MoS2 and the MoS2/RGO nanohybrid, respectively. Compared to featureless pristine MoS2 composed of large solid aggregates with a size of several micrometers, the MoS2/RGO nanohybrid shows a flowerlike nanostructure consisting of 2D nanoflakes. Figure 1B and C shows the low-magnification transmission electron microscopy (TEM) images of pristine MoS2, which clearly indicate the aggregated nature of bulk MoS2 materials. The high-resolution TEM (HRTEM) image in Figure 1D shows that basic sheets of pristine MoS2 are prone to stacking together (tens of layers) with an interlayer distance of 25418



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Figure 4. (A) XPS survey spectra of MoS2, MoS2/RGO, and RGO. (B) C1s, (C) Mo3d, and (D) S2p scan spectra of MoS2 and MoS2/ RGO. Figure 3. (A) Powder X-ray diffraction (PXRD) patterns of MoS2, the MoS2/RGO nanohybrid, and RGO. (B) N2 adsorption isotherms and the corresponding pore size distribution (inset) for MoS2 and the MoS2/RGO nanohybrid.

RGO show a dominated CC peak with a small peak related to oxygen-containing groups, confirming the effective reduction of GO to RGO (Figure 4B) during the hydrothermal process,10 which is further proved by FTIR analysis (see Figure S2 in the Supporting Information). The Mo3d XPS spectra in Figure 4C show that the binding energies of Mo3d5/2 and Mo3d3/2 for MoS2/RGO occur at 229.26 and 232.36 eV, respectively, which are indicative of reduction of Mo6+ to Mo4+ and the formation of MoS2. These values are slightly higher than that of pristine MoS2, for which the Mo3d5/2 and Mo3d3/2 peaks appear at 229.13 and 232.33 eV, respectively. At the same time, the S2p spectra of MoS2/RGO shown in Figure 4D exhibit higher binding energies of S2p3/2 and S2p1/2 (162.11 and 163.31 eV) as compared to MoS2 (162.03 and 163.18 eV).5a−c This result indicates that there is a strong interaction between MoS2 and RGO sheets in the nanohybrid, driven from the probable electron transfer and delocalization of firmly contacted MoS2 layer and RGO sheets.5a In the nanohybrid, the electrons of the S atom layer and its adjacent carbon layer could form a coelectron cloud.5a The high concentration of electrons between the MoS2 layer and the RGO layer could greatly enhance the electronic conductivity of the nanohybrid and were beneficial to the transfer of photogenerated electrons during the photocatalysis. Figure 5A shows the time courses of H2 evolution catalyzed by RGO, MoS2, and the MoS2/RGO nanohybrid sensitized by Eosin Y (EY) as a photosensitizer in aqueous TEOA solution under visible light irradiation (≥420 nm). Control experiments reveal that no H2 gas could be detected either in the absence of photosensitizer or light irradiation, suggesting that this reaction is photocatalytical. EY-RGO catalyst shows a very low activity (