Visible light driven H2 production in molecular systems employing

colloidal MoS2 nanoparticles as catalystw

Xu Zong,a Yong Na,b Fuyu Wen,a Guijun Ma,a Jinhui Yang,a Donge Wang,a Yi Ma,a

Mei Wang,bLicheng Sun

bcand Can Li*

a

Received (in Cambridge, UK) 9th April 2009, Accepted 26th May 2009

First published as an Advance Article on the web 15th June 2009

DOI: 10.1039/b907307h

Colloidal MoS2 nanoparticles with diameters of less than 10 nm

were prepared with a simple solvothermal method and

demonstrated high efficiency in catalyzing H2 evolution in

Ru(bpy)32+-based molecular systems under visible light.

Visible light driven H2 production by water splitting represents

one of the ‘‘grand challenges’’ in artificial photosynthesis,

which provides an attractive way of converting solar energy

into a fuel.1 Molecular systems consisting of a transition metal

complex such as Ru(bpy)32+ (bpy = 2,20-bipyridine) as

photosensitizer, an electron transfer relay and a H2 evolution

catalyst have been shown to successfully effect H2 production

under visible light.2,3 The catalysts used in these systems are

usually based on colloidal Pt or Pt complexes.4 Recently,

MoS2 nanoparticles loaded on an inert or photoactive

semiconductor support were reported to show high activity

in catalyzing electrochemical or photocatalyitic H2 evolution.5

However, except for the above few cases, studies on employing

MoS2 as a H2 evolution catalyst are rare, although supported

MoS2 catalysts have been studied extensively in many

H2-involved reactions such as hydrodesulfurization and

hydrogenation.6 Particularly, in the Ru(bpy)32+-based homo-

geneous molecular systems, there are still no reports on using

MoS2 as a H2 evolution catalyst. Moreover, despite the

fact that MoS2 nanoparticles in a colloidal form have been

synthesized with several complex approaches, their application

in photocatalysis is extremely limited.7

We report herein that ‘‘soluble’’ colloidal MoS2 nano-

particles prepared with a simple solvothermal method could

act as an efficient H2 evolution catalyst in a Ru(bpy)32+-based

molecular system for the first time. Under visible light

irradiation, catalytic H2 generation with a turnover number

of 100 based on Ru(bpy)32+ has been successfully realized in a

three-component system consisting of colloidal MoS2,

Ru(bpy)32+ and ascorbic acid (H2A). Moreover, colloidal

MoS2 nanoparticles exhibit superior efficiency than the

supported MoS2/Al2O3 catalyst in catalyzing H2 evolution in

the molecular systems.

Colloidal MoS2 was prepared by solvothermal treatment of

(NH4)2MoS4 dissolved in methanol in the presence of

poly(viny1pyrrolidone) (PVP, K27-33) as a protecting

polymer and N2H4H2O (50%) as a reductant. In a typical

process, (NH4)2MoS4 (2.5–12.5 mmol) was dissolved inmethanol

(25 mL) containing N2H4H2O (0.02–0.1 mL) and an

appropriate amount of PVP. The solution was added to a

Teflon-lined stainless steel autoclave with a capacity of 30 mL

and then heated at temperatures from 373 to 473 K for 3 h to

obtain the colloidal MoS2. The molar ratio of PVP

(monomeric unit) to MoS2 will be changed between 1 and 20

according to the preparation temperature.

Fig. 1 shows the TEM images of colloidal MoS2 nano-

particles prepared at different conditions. The 0.1 mM MoS2colloids prepared at 373, 393 and 423 K consist of particles

with diameters of less than 10 nm (Fig. 1A–C). The MoS2nanoparticles are inter-connected to form nanowire-like

structure gradually with the increase of the concentration of

MoS2 (Fig. 1D–F, Fig. S1, ESIw). The selected area electron

diffraction (SAED) indicates that the MoS2 nanoparticles are

amorphous (data not shown). The diffuse reflectance

absorption spectrum of the as-prepared MoS2 colloid is

essentially featureless, which is much different from that of

the (NH4)2MoS4 methanol solution while very similar with

that of MoS2 nanocrystals prepared with a recently developed

strategy (Fig. 2A).7c The as-prepared MoS2 colloids are

transparent and remain stable for over one month when stored

at air-free conditions (Fig. 2B). Moreover, the colloidal MoS2nanoparticles can form a homogeneous solution with

acetonitrile and H2O as solvents. Therefore, we present a very

simple way to prepare MoS2 colloid that can overcome the

insolubility of the MoS2 nanoparticles.8

Photocatalytic reactions using the freshly-prepared colloidal

MoS2 nanoparticles were then carried out in a 2 : 1 acetonitrile–

methanol solution (150 mL) containing Ru(bpy)32+ as

a photosensitizer (PS) and H2A as a sacrificial reagent.

Photolysis of the reaction solution results in the production

of H2 and the amount of H2 produced increases with the

amount of Ru(bpy)32+ used (Fig. 3a–d). When MoS2 colloids

prepared at 373, 423 and 473 K were used, 446, 466, and

500 mmol of H2 were evolved in a six hour reaction in the

presence of 10 mmol of Ru(bpy)32+, respectively (Fig. 4).

The highest turnover number for H2 evolution was found to

a State Key Laboratory of Catalysis, Dalian Institute of ChemicalPhysics, Chinese Academy of Sciences, 457 Zhongshan Road,Dalian 116023, China. E-mail: canli@dicp.ac.cn;Fax: +86 411 84694447; Tel: +86 411 84379070

b State Key Laboratory of Fine Chemicals, DUT-KTH JointEducation and Research Centre on Molecular Devices,Dalian University of Technology (DUT), 158 Zhongshan Road,Dalian 116012, China. E-mail: symbueno@dlut.edu.cn;Fax: +86 411 83702185; Tel: +86 411 88993886

cDepartment of Chemistry, Royal Institute of Technology (KTH),Teknikringen 30, Stockholm 10044, Sweden.E-mail: lichengs@kth.se; Fax: +46 8 7912333; Tel: +46 8 7908127w Electronic supplementary information (ESI) available: Details ofpreparation and reaction conditions, TEM and rate of H2 evolution.See DOI: 10.1039/b907307h

4536 | Chem. Commun., 2009, 4536–4538 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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be 100 based on Ru(bpy)32+ when using colloidal MoS2

prepared at 473 K. However, when MoS2/Al2O3 catalysts

prepared with two different strategies were used in the

reactions, much smaller amount of H2 were evolved

(Fig. 3e–f). Therefore, ‘‘soluble’’ colloidal MoS2 nanoparticles

demonstrate unique catalytic activity in this ‘‘homogeneous’’

molecular system compared with the traditional MoS2/Al2O3

catalyst which has shown high catalytic activity in hetero-

geneous catalysis. The superior catalytic activity of colloidal

MoS2 nanoparticles may be due to its small particle size and

high dispersibility that will secure more efficient charge

transfer between MoS2 and the reactant.9

To show the important role of colloidal MoS2 in the

photocatalytic reactions, some control experiments have been

performed. As shown in Table 1, no H2 was evolved when only

Ru(bpy)32+ was employed in the absence of colloidal MoS2

(entry 1). Similarly, the H2 production does not occur when

only colloidal MoS2 was used in the absence of Ru(bpy)32+

(entry 2). However, when both Ru(bpy)32+ and MoS2 were

used, a large amount of H2 can be produced (entry 3). When

K2PtCl4 was used as a H2 evolution catalyst, H2 production

can also be achieved while with much lower rates

(entry 4). The fact that H2 is catalytically evolved in the

Ru(bpy)32+-MoS2-H2A system indicates that colloidal MoS2

acts as a H2 evolution catalyst in the Ru(bpy)32+-based

system. Although many factors affecting the rate of H2

Fig. 1 TEM images of colloidal MoS2 prepared at (A) 373 K

(0.1 mM), (B) 393 K (0.1 mM), (C) 423 K (0.1 mM), (D) 373 K

(0.5 mM), (E) 393 K (0.5 mM), and (F) 423 K (0.5 mM). The molar

ratio of PVP (monomeric units) to MoS2 was from 1 to 20 according to

the preparation conditions.

Fig. 2 (A) Diffuse reflectance absorption spectra of MoS2 colloid and

(NH4)2MoS4 methanol solution. (B) Photographs of (NH4)2MoS4methanol solution (left), freshly-prepared MoS2 colloid (middle),

and MoS2 colloid kept in the autoclave for a month (right). The

molar ratio of PVP (monomeric units) to MoS2 was from 1 to 20

according to the preparation conditions.

Fig. 3 Photocatalytic H2 evolution from a 2 : 1 acetonitrile–methanol

solution (150 mL) containing colloidal MoS2 (12.5 mmol) prepared at

423 K and (a) 20, (b) 15, (c) 10, and (d) 5 mmol of Ru(bpy)32+(PF6)2

and H2 evolution from a solution containing Ru(bpy)32+(PF6)2

(10 mmol) and MoS2/Al2O3 prepared from (e) (NH4)2MoS2/Al2O3

and (f) MoO3/Al2O3 precursors. 0.01 mol H2A was used as the

sacrificial reagent. The amount of MoS2 was the same for colloidal

MoS2 and MoS2/Al2O3. The irradiation was carried out using 300 W

Xe lamp with a Pyrex-glass filter (l 4 420 nm). The photolysis

solutions were immersed in a water bath kept at 293 K.

Fig. 4 Time course of photocatalytic H2 evolution from a 2 : 1

acetonitrile–methanol solution (150 mL) containing Ru(bpy)32+(PF6)2

(10 mmol), H2A (0.01 mol) and colloidal MoS2 (12.5 mmol) prepared at

(a) 373, (b) 423, and (c) 473 K. The irradiation was carried out using

300 W Xe lamp with a Pyrex glass filter (l 4 420 nm). The photolysis

solutions were immersed in a water bath thermostated at 293 K.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 4536–4538 | 4537

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evolution have not been optimized for the colloidal MoS2 and

Pt catalyst in the present reactions, the above results demon-

strate that colloidal MoS2 may be used as a potential catalyst

in the photocatalytic H2 production reactions.

It was reported that H2A can act as a proton source and its

ascorbate anion (HA�) can function as a reductive quencher

for excited-state of Ru(bpy)32+ to Ru(bpy)3

+.10 Based upon

the above results, in the Ru(bpy)32+–MoS2–H2A three-

component system, photocatalytic H2 production reaction

may proceed via a reductive quenching mechanism. As

illustrated in Fig. 5, after the absorption of light, the excited

states of Ru(bpy)32+ is quenched by HA� to produce Ru(bpy)3

+.

Electrons are then transferred from Ru(bpy)3+ to the colloidal

MoS2 where the protons are reduced to form molecular H2.

In summary, we have demonstrated that ‘‘soluble’’ colloidal

MoS2 nanoparticles could act as an efficient catalyst for

proton reductions in a Ru(bpy)32+-based molecular system.

The colloidal MoS2 nanoparticles were found to catalyze the

reduction of protons more efficiently than the traditional

MoS2/Al2O3 catalyst. The present work demonstrates the

possibility of using colloidal MoS2 as a H2 evolution catalyst

in the molecular systems. Moreover, as the bridges between

homogeneous and heterogeneous catalysts, studies on

colloidal MoS2 nanoparticles are expected to stimulate

research on employing molybdenum-sulfur complexes for light

energy conversion, which holds tremendous potential for

theoretical advances and practical applications.

This work was financially supported by the National Key

Basic Research and Development Program (Grant No.:

2009CB220010), and Programme Strategic Scientific Alliances

between China and the Netherlands (Grant No.:

2008DFB50130).

Notes and references

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3 J. I. Goldsmith, W. R. Hudson, M. S. Lowry, T. H. Anderson andS. Bernhard, J. Am. Chem. Soc., 2005, 127, 7502; H. Ozawa,M. Haga and K. Sakai, J. Am. Chem. Soc., 2006, 128, 4926;P. Du, J. Schneider, P. Jarosz and R. Eisenberg, J. Am. Chem.Soc., 2006, 128, 7726; M. Elvington, J. Brown, S. M. Arachchigeand K. J. Brewer, J. Am. Chem. Soc., 2007, 129, 10644; S. Rau,B. Schafer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich,H. Gorls, W. Henry and J. G. Vos, Angew. Chem., Int. Ed., 2006,45, 6215; P. W. Du, K. Knowles and R. Eisenberg, J. Am. Chem.Soc., 2008, 130, 12576.

4 L. L. Tinker, N. D. McDaniel, P. N. Curtin, C. K. Smith,M. J. Ireland and S. Bernhard, Chem.–Eur. J., 2007, 13, 8726;K. Sakai and H. Ozawa, Coord. Chem. Rev., 2007, 251, 2753.

5 B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen,J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Norskov,J. Am. Chem. Soc., 2005, 127, 5308; T. F. Jaramillo,K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch andI. Chorkendorff, Science, 2007, 317, 100; X. Zong, H. Yan,G. Wu, G. Ma, F. Wen, L. Wang and C. Li, J. Am. Chem. Soc.,2008, 130, 7176.

6 E. Furimsky, Catal. Rev., 1980, 22, 371; M. Daage andR. R. Chianelli, J. Catal., 1994, 149, 414; F. Bataille,J. L. Lemberton, P. Michaud, G. Perot, M. Vrinat, M. Lemaire,E. Schulz, M. Breysse and S. Kasztelan, J. Catal., 2000, 191, 409.

7 (a) M. W. Peterson, M. T. Nenadovic, T. Rajh, R. Herak,O. I. Micic, J. P. Goral and A. J. Nozik, J. Phys. Chem., 1988,92, 1400; (b) E. Boakye, L. R. Radovic and K. Osseo-Asare,J. Colloid Interface Sci., 1994, 163, 120; (c) H. Yu, Y. Liu andS. L. Brock, Inorg. Chem., 2008, 47, 1428.

8 M. N. Tahir, N. Zink, M. Eberhardt, H. A. Therese, U. Kolb,P. Theato and W. Tremel, Angew. Chem., Int. Ed., 2006, 45, 4809.

9 J. Kiwi and M. Gratzel, Nature, 1979, 281, 657.10 G. M. Brown, B. S. Brunschwig, C. Creutz, J. F. Endicott and

N. Sutin, J. Am. Chem. Soc., 1979, 101, 1298; C. V. Krishnan andN. Sutin, J. Am. Chem. Soc., 1981, 103, 2141.

Table 1 Photocatalytic H2 production at various conditionsa

Entry Ru(bpy)32+ (mmol) Catalyst H2 evolved (mmol) TONc TONd

1 10 — — — —2 — MoS2

b — — —3 10 MoS2

b 466 93 754 10 K2PtCl4 164 33 26

a Photocatalytic H2 production from a 2 : 1 acetonitrile–methanol solution (150 mL) containing Ru(bpy)32+(PF6)2, H2A (0.01 mol) and catalyst

(12.5 mmol) in 6. b The colloidal MoS2 prepared at 423 K was used. c Based on PS. d Based on catalyst. The irradiation was carried out using

300 W Xe lamp with a Pyrex-glass filter (l 4 420 nm). The photolysis solutions were immersed in a water bath thermostated at 293 K.

Fig. 5 Proposed reaction mechanism for photocatalytic H2 evolution

in the Ru(bpy)32+–MoS2–H2A three-component system.

4538 | Chem. Commun., 2009, 4536–4538 This journal is �c The Royal Society of Chemistry 2009

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