Nanostructured MoS2 and WS2 for the Solar Production of Hydrogen Investigators Thomas F. Jaramillo, Assistant Professor, Chemical Engineering, Stanford University; Zhebo Chen, Graduate Student, Chemical Engineering, Stanford University; Jakob Kibsgaard, Post-doctoral Researcher, Chemical Engineering, Stanford University. Abstract

We have successfully synthesized MoS2 nanoparticles of various sizes using a reverse micelle encapsulation method, and studied their opto-electronic properties for the synthesis of fuels by photoelectrochemistry (PEC). The nanoparticles exhibit a size dependent blueshift in both the direct and indirect band gaps relative to each other and to bulk films. In pursuing these studies, we developed a low temperature sulfidization process using a custom H2S/H2 gas mixture that enables Mo to be sulfidized at temperatures as low as 150ºC. The sulfidized nanoparticles are fully stable when exposed to air and do not spontaneously re-oxidize. The development of nanowire morphologies have yielded the production of anodic alumina oxide templates with pores below the size necessary for quantum confinement. We have also performed initial synthesis of a nanoscaled bi-continuous double-gyroid template. Both of these templating methods require the optimization of molybdenum and tungsten electrodeposition, which we have achieved. Opto-electronic studies of the supported nanoparticles show significant bandgap enlargement from 1.2 eV to approximately 2 eV, as we had originally targeted for PEC applications. This exploratory work has shown that nanostructured MoS2 is indeed a promising material for the synthesis of fuels from sunlight.

Introduction The fact that the sun provides us with nearly 10,000 times more energy than we

consume strongly compels us to find a cost-effective way to harness solar power. Photoelectrochemical (PEC) hydrogen production is one promising approach (see Figure 1), first demonstrated with TiO2 in 1972 [1]. Since then, research in this area has aimed to improve device efficiency in several ways, including semiconductor doping/alloying [2-6], tandem absorbers [7], and dye-sensitization [8-10]. Despite these efforts, there exists no material system that can simultaneously satisfy all the materials criteria required for cost-effective PEC hydrogen production; new materials with new properties are needed. This proposal aims to transform research in this area by engineering inexpensive semiconductors such as MoS2 and WS2 at the nano-scale in order to tailor their surface and bulk properties for efficient PEC water-splitting. The fundamental, cross-cutting knowledge gained regarding the chemical, catalytic, optical, and electronic phenomena

Figure 1. PEC H2 production. Courtesy: John Turner, NREL.

within these nano-scale dichalcogenide semiconductors will contribute broadly to a number of different scientific fields such as photovoltaics and heterogeneous catalysis, facilitating a broad mix of technological innovations.

Hydrogen has emerged as a contender among clean-burning fuels. While significant challenges remain in its storage and use, its high specific energy content and clean combustion make it an attractive option. Hydrogen already plays an enormous role in our chemical and fuel industries with an annual production of 30 million tons (15 Tera-moles) [11]. Unfortunately, most commercial H2 is produced by steam reforming [2] – an energy-intensive, fossil fuel based process with substantial greenhouse gas emissions. Whether H2 produced by “green” methods such as solar PEC water-splitting is best used as a clean fuel, as a reagent for greener synthesis of foods/chemicals, or for synthesis of higher-value liquid fuels is a question that economics will ultimately answer. Storing an intermittent energy source such as the sun as hydrogen is inherently flexible and will help reduce greenhouse gas emissions on a terrestrial scale.

Background

The PI, Jaramillo, recently took part in a techno-economic study of hydrogen production by photocatalysis and photoelectrochemistry, sponsored by the DOE. As described in the published report, if PEC R&D targets can be met in the laboratory regarding efficiency, stability, and material cost, a large-scale solar-to-hydrogen plant can be constructed with H2 costs at the plant gate as low as $1.60/kg H2. Note that 1 kg H2 is equivalent in energy content to 1 gallon of gasoline. This outcome further inspires R&D efforts in this field [12]. During the course of this 1-year project, no major breakthroughs in device efficiency have been reported by our group or others. The world-record remains at 12.4%, the GaInP2/GaAs device reported by NREL in reference [7]. Nevertheless, incremental advancements have been made on other devices by the conventional approaches described above: doping/alloying, tandem cells, and dye sensitization.

Results Our investigation of nanostructured transition metal dichalcogenides for solar

production of hydrogen has resulted in the synthesis of molybdenum and tungsten nanoparticles in various, controlled sizes, the production of anodic alumina oxide templates with pores in the range necessary for quantum confinement, optimization of molybdenum and tungsten electrodeposition and sulfidization, as well as the development of a low-temperature sulfidization procedure as well as a synthetic route for a double-gyroid mesoporous templating agent.

Synthesis of well-defined nanoparticles was the first crucial step in this project. We developed of a low-temperature sulfidization procedure that was an important step in the synthetic route as we were able to mitigate nanoparticle sintering. Using a custom mixture of H2S and H2 gas, we have optimized the sulfidization temperature for molybdenum and tungsten at 150ºC and 400ºC, respectively. The low temperature required for the sulfidization of molybdenum in particular minimizes sintering effects that would cause the nanoparticles to coalesce into larger structures.

We have successfully synthesized molybdenum and tungsten nanoparticles in the 5-25 nm range using a reverse micelle encapsulation method. Four different molecular weight poly(styrene-block-2-vinylpyridine) polymers were used in the synthesis. The resulting nanoparticles exhibited a trend in size consistent with the molecular weight of

the vinylpyridine block, as confirmed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The particle sizes could be tuned between 5-25 nm.

Figure 1. AFM images of MoS2 nanoparticles on Si(100). The molecular weight ratios of styrene:vinylpyridine for the block copolymer used in the synthesis were (a) 27700/4300, (b) 42500/7800, (c) 81000/21000, (d) 172000/42000. Images are 100µm x 100 µm.

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Figure 2. XPS of the Mo(IV) prepared using (a) 27700/4300molecular weight polymers.

Following oxygen plasma

ray photoelectron spectroscopboth molybdenum and tungstereactor as described above. Xcomponent, Figure 2. This wato be stable in air and water.

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3d5/2 and 3d3/2 and S (2s) peaks of MoS, (b) 42500/7800, (c) 81000/21000, and

treatment to remove the polymer fromy (XPS) confirmed fully oxidized (6n. Molybdenum nanoparticles were thPS confirmed complete MoS2 composs a critical result as the nanoparticula

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the nanoparticle, x- valent) states for n sulfidized in our tion with no oxide MoS2 was shown

Ultraviolet-visible (UV-Vis) spectroscopy confirmed a size-dependent blue-shift in the absorption onset of nanoparticulate MoS2 versus bulk films, Figure 3. This showed that quantum confinement can in fact be achieved with MoS2, up to ~ 2 eV which was the bandgap target of the investigation.

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Figure 3. (a) UV-Vis absorption data of MoS2 film and nanopartfluorine-doped tin oxide. A blueshift in the absorption onset is observsize. Tauc plots indicate an increase in both the (b) direct and (c) indir

The bulk of the effort in this project was focused on proving th

with MoS2 nanoparticles; however significant progress was made in dfor the other structures – namely nanowires and a double-gyroid meTemplating methods were employed to produce the latter two morphthe synthesis procedure is to (1) synthesize a porous template – t

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icles supported on ed with decreasing ect band gaps.

e concept/principle eveloping methods soporous structure. ologies. In general, he negative of the

desired structure, (2) electrodeposit Mo/MoO3 into the template, and (3) remove the template by etching. Below we describe our efforts to produce these structures.

A general procedure for the electrodeposition of molybdenum and tungsten has been optimized in our laboratory, Figure 4. The results from this work are directly applicable to the development of nanostructures from the anodic alumina and double-gyroid templating methods we investigated for this project. This procedure could also potentially be extended to future studies of ultrathin films that may exhibit one-dimensional quantum confinement. XPS confirmed MoS2 and WS2 for these films after a H2/H2S furnace treatment to 150 ºC and 400 ºC, respectively.

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Figure 3. Profilometry of (a) M Anodized aluminum oxide temp

13 nm, Figure 5, which is below Mobe needed to refine anodization pasize control of the pores. This willusing the electrodeposition method

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lates have been produced with pores on the order of S2’s Bohr exciton radius of 15 nm. Future work will ameters in order to improve the monodispersity and ultimately be followed by development of nanowires hat we have optimized.

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The double-gyroid nanostructure required the construction of a custom dip coater with humidification control, which we have successfully developed. Humidity can be controlled over a range of 20-70% (optimal double-gyroid formation occurs between 40-60%). We have produced SiO2 double-gyroid template films in our laboratory, and subsequently filled them with Pt by electrodeposition. Etching the SiO2 template in HF produces a thin-film of mesoporous, double-gyroid Pt, Figure 6. Our initial attempts to replicate this structure with molybdenum have not been as successful, however we are continuing our efforts in this area.

Figure 6. SEM and TEM images of a double-gyroid Pt mesoporous structure synthesized by electrodeposition into an SiO2 template, followed by template etching in HF.

Conclusions The work conducted in this exploratory project has answered two important

questions: (1) Quantum confinement can be achieved in supported nanoparticles of MoS2, and (2) the bandgap of MoS2 can be tuned to ~ 2 eV which is the ideal target for PEC applications. Significant progress was also made in developing new routes to other morphologies of nanostructured MoS2 – namely nanowires and a double-gyroid mesoporous structure. Our results have confirmed that this approach is promising indeed for manipulating semiconductor band structure for the synthesis of fuels from sunlight – a technology that could lead to substantial reductions in emissions of greenhouse gases by means of producing clean, solar-derived fuels.

Publications 1. 217th National Meeting of the Electrochemical Society, Vancouver, B.C.

“Nanostructured MoS2 for Solar Hydrogen Production”, Z. Chen and T.F. Jaramillo, April 2010.

2. 239th National Meeting of the American Chemical Society, San Francisco, CA. “Nanostructured MoS2 for Solar Hydrogen Production”, Z. Chen and T.F. Jaramillo, March 2010.

3. University of California at Santa Cruz (invited). “Nano-scaled materials for the synthesis of fuels from sunlight.” T.F. Jaramillo, February 2010.

4. University of California at Berkeley (invited). “Nano-scaled materials for the synthesis of fuels from sunlight.” T.F. Jaramillo, December 2009.

5. 2009 American Institute of Chemical Engineers Annual Meeting, Nashville, TN. “Nanostructured MoS2 for the Photoelectrochemical (PEC) Production of Hydrogen.” T.F. Jaramillo, November 2009.

6. University of Texas at Austin (invited). “Nano-scaled materials for the synthesis of fuels from sunlight.” T.F. Jaramillo, October 2009.

7. 2009 Global Climate Energy Project Research Symposium, Stanford, CA. “Nanostructured MoS2 and WS2 for the Solar Production of Hydrogen.” T.F. Jaramillo, October 2009.

8. University of California, Berkeley, Nanosciences and Nanoengineering Institute (BNNI), Nanoscale Science and Engineering (NSE) Seminar, Berkeley, CA. “Designing nano-scaled, non-precious metal catalysts for hydrogen evolution” T.F. Jaramillo, March 2009.

9. Chevron Corporation, Richmond, CA, March 2009. “Solar Fuels by Photoelectrochemistry (PEC)” T.F. Jaramillo, March 2009.

References

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Hydrogen Energy 2005, 30, 521-44. 3. Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E. Solar Energy 2005,

78, 581-592. 4. Anpo, M.; Dohshi, S.; Kitano, M.; Hu, Y.; Takeuchi, M.; Matsuoka, M. Annual

Review of Materials Research 2005, 35, 1-27. 5. Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243-2245.

6. Jaramillo, T.F.; Baeck, S.-H.; Kleiman-Shwarsctein, A.; Choi, K.-S.; Stucky, S.D.; McFarland, E.W.; J. Comb. Chem., 2005, 7, 264.

7. Khaselev, O.; Turner, J.A. Science, 1998, 280, 425. 8. Grätzel, M. Nature 2001, 414, 338-344. 9. Grätzel, M. Chemistry Letters, 2005, 34, 8. 10. Cesar, I., Kay, A., Gonzalez Martinez, Jose A., Grätzel, M., J. Amer. Chem. Soc.,

2006, 128, 4582-4583. 11. Moon, S. C.; Matsumura, Y.; Kitano, M.; Matsuoka, M.; Anpo, M. Research on

Chemical Intermediates 2003, 29, 233-256. 12. B.D. James, G.N. Baum, J. Perez, K.N. Baum, “Technoeconomic Analysis of

Photoelectrochemical (PEC) Hydrogen Production”, DOE Report (2009) Contract # GS-10F-009J.

Contacts Thomas F. Jaramillo: jaramillo@stanford.edu Zhebo Chen: zchen7@stanford.edu Jakob Kibsgaard: kibs@stanford.edu