Nano-structured MoS 2 and WS 2 for the Solar Production of Hydrogen

Transcription

1 Nano-structured MoS 2 and WS 2 for the Solar Production of Hydrogen Thomas F. Jaramillo Dept. of Chemical Engineering Stanford University GCEP Research Symposium: New Research Directions in a Rapidly Evolving Global Energy Landscape Stanford University Stanford, CA 2 October 2009

2 Solar Fuels by Photoelectrochemistry (PEC) Artist: Mr. Zhebo Chen

3 Conclusions & Perspectives Chemical fuels have extraordinary energy density. The key is to develop a method to synthesize such fuels sustainably. There are big challenges in converting solar energy into fuels, challenges involving semiconductor properties and the catalytic properties of surfaces. The results from this research show that quantum confinement may be a viable approach to tailoring the electronic structure of supported, nano-scaled semiconductors for PEC

4 PEC water-splitting: materials challenges Bak et. al., Int. J. Hydrogen Energy, vol 27 (2002) Nowotny, J.; Sorrell, C.C.; Bak T.; Sheppard, L.R. Solar Energy 2005, 78, Many materials issues to consider Absorbance. dictated by bandgap 1.23 ev < E g < 2.5 ev Charge Transport dictated by crystallinity, doping, and morphology Band Structure Energetics. sufficient to split water (E g > 1.23 ev) and must have appropriate redox potentials for H 2 & O 2 evolution Surface electrocatalysis must maximize interfacial charge transfer. Stability, Cost, Non-toxicity.

5 Common semiconductors A. Nozik and R. Memming, J. Phys. Chem., v100 (1996)

6 Broad research questions in PEC Semiconductors How do you tune electronic band structure? How do you improve charge transport? Surfaces How do you improve catalytic activity for reduction reactions? hydrogen evolution reaction (HER) CO 2 reduction to hydrocarbons and alcohols How do you improve catalytic activity for water oxidation, i.e. the oxygen evolution reaction (OER)? How do you stabilize the surface and mitigate corrosion?

7 What s known about bulk MoS 2 and WS 2 for PEC Efficiency Bandgap: 1.2 ev allows for significant solar absorption but is too narrow to split water without an applied bias. Band alignment: Mismatched CB ~ 0.4 ev too low vs. E 0 H+/H2. Surface: Not catalytically active for either H 2 evolution or O 2 evolution. Durability Potential solution: Nanostructuring! Photoanodic corrosion is problematic. Potential solution: p-type materials may be cathodically stable. Cost Inexpensive and earth-abundant materials.

8 Surface science and electrochemistry for HER 1 - Preparation and STM imaging of MoS 2 on Au(111) in UHV conditions. 2 - Measure electrochemical activity of the MoS 2 previously characterized. Current (A/cm 2 geometric ) High Coverage MoS 2 Low coverage MoS 2 "Blank" sulfided Au(111) H 2 SO 4 ph 0.24 (N 2 saturated) 5 mv/s Voltage vs. RHE

9 The approach Prepare a sample set of MoS 2 nanoparticles on Au(111) with variations in: coverage: controlled by Mo deposition rate / time. particle size: controlled by sintering at elevated temperatures. Stig Helveg, Ph.D. Thesis, Århus University (2002). 400 ºC 550 ºC 550 ºC MoS 2 edge 470 Å x 470 Å 470 Å x 470 Å 60 Å x 60 Å

10 Electrochemical measurements 0 i (µa / cm 2 geometric ) Blank MoS ο C MoS ο C Reference Electrode (SCE) Counter Electrode (Pt mesh) E (V vs. NHE) Working electrode STM imaged MoS 2 on Au(111)

11 MoS 2 on the HER volcano 10-1 i 0 (A/cm 2 ) Nb W Mo Re Ni Co Pd Pt Rh Pt(111) Ir MoS 2 Cu Au Au(111) Ag G H* (ev) T.F. Jaramillo, K.P. Jørgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff; Science 317 (2007) 100.

12 Tailoring Electronic Band Structure Bulk Materials CdSe: a classic example of quantum confinement 2 nm CdSe 8 nm CdSe Nanostructured Materials Bawendi et. al., MIT

13 Synthesizing Metal Sulfides Synthesize metal Synthesize metal oxide Synthesize metal sulfide Sulfidization of Mo/W or WO x /MoO x at 150 C in 10% H 2 S / 90% H 2. Mo/MoOx W/WOx Electrodeposited thin films on fluorine-doped tin oxide (FTO) Absorbance / a.u. 1.4 Electrodeposited MoO 3-x sulfidized in 10%/90% H 2 S/H Increasing temperature Untreated 100 o C 150 o C 200 o C 400 o C Wavelength (nm)

14 Synthesis of supported MoS 2 nanoparticles Nanoparticle synthesis by wet chemistry PS/P2VP ratios (m.w.): 27700/ / / /42000 Metal precursor:p2vp

15 Sulfidization of MoO x nanoparticles to MoS 2 sulfidize MoOx MoS 2 Counts / a.u. Mo(VI) 3d 5/ Mo(VI) 3d 3/ sulfidize Counts / a.u. Mo(IV) 3d 5/ Mo(IV) 3d 3/ S 2s Binding Energy / ev Binding Energy (ev)

16 Varying MoS 2 Nanoparticle Size with PS/P2VP Atomic force microscopy (AFM) of MoS 2 nanoparticles in various sizes synthesized using a Mo(OCOCH 3 ) 2 precursor / / / /42000 Increasing molecular weight block copolymer (PS/P2VP, units: Da)

17 MoS 2 Nanowebs Alternatively, we can use a MoCl 3 precursor to make interconnected nanowires or web structures 200 nm

18 Blue-shift in UV-Vis absorption A blue-shift in absorbance from ultraviolet-visible spectroscopy indicates quantum confinement of the indirect bandgap up to 1.8 ev in nanoparticles and 1.6 ev in nanoweb structures. Absorbance / a.u. Nanoparticle Absorbance Bulk film Decreasing nanoparticle size hν / ev (Ahν) 1/2 Tauc Plot for Nanoparticles Bulk film Decreasing nanoparticle size hν / ev

19 Bandgap Enlargement in MoS2

20 In pursuit of p-type MoS 2 Nb 5+ :Mo 4+ S 2 Synthetic route S Mo Nb 1. Drop cast 5 mol-% NbCl mol-% Mo powder in H 2 O 2 Mo 0.95 Nb 0.05 O x (OH) y Cl z TCO 2. Anneal in air at 400 o C for 1 hour. Mo 0.95 Nb 0.05 O 3 TCO 3. Sulfidize in 10% H 2 S / 90 % H 2 at 450 o C for 4 hours. Mo 0.95 Nb 0.05 S 2 TCO

21 Photoactivity of p-type MoS mv/s P.S. I (µa) OFF -0.4 ON E (V) vs. Hg/HgSO 4 Nb-MoS M H 2 SO 4 RuO OFF I (µa) ON E app = -1.5 V t (s) Cathodic photocurrent indicates p-mos 2

22 Several Targeted Nanostructures Nanoparticles Ultra-thin Nanofilms Nanowires 3-dimensional double-gyroid 22

23 Double Gyroid Mesostructures Double gyroid silica template Metal electrodeposition Etch away silica template (2% HF 4hr) Goal: Synthesize a MoS 2 double gyriod structure for PEC Results so far: Synthesize double gyroid silica template using a tri-block copolymer poly(ethylene oxide)-poly(propylene oxide)-alkane surfactant. Electrodeposit Pt (for structure characterization) in porous silica template SEM/TEM of Pt double gyroid structure

24 Double Gyroid Pt on FTO SEM SEM FTO Double gyroid Pt mounds on FTO Pulsed electrodeposition: -2.0V for 50ms, 1s between pulses

25 Double Gyroid Pt on FTO SEM FTO High resolution SEM and TEM show the porous double gyroid Pt structure TEM

26 RuO 2 Counter Electrode Development Oxygen Evolution Reaction (OER) 2H 2 O O 2 + 4H + + 4e - E o = V vs. SHE Low Overpotential for OER Unstable 3 10 I (ma/cm 2 ) Ru / FTO Pt I (ma/cm 2 ) mv/s E (V) vs. SHE E (V) vs. SHE 1.6 Ru was electrodeposited at -0.6 V (vs. Ag/AgCl) for 5 minutes in aq. 5 mm RuCl M KCl.

27 RuO 2 Counter Electrode Development Significant improvement in stability by annealing. I (ma/cm 2 ) mv/s 100 cycles Current Efficiency (%) No annealing After Annealing 400 o C, 1 hr E (V) vs. SHE Electrodeposited Ru/FTO was annealed in air at 400 o C for 1 hour. Cycle Number

28 Conclusions & Perspectives Chemical fuels have extraordinary energy density. The key is to develop a method to synthesize such fuels sustainably. There are big challenges in converting solar energy into fuels, challenges involving semiconductor properties and the catalytic properties of surfaces. The results from this research show that quantum confinement may be a viable approach to tailoring the electronic structure of supported, nano-scaled semiconductors for PEC

29 Accelerating materials development for photoelectrochemical (PEC) hydrogen production: Standards for methods, definitions, and reporting protocols Z. Chen, T. F. Jaramillo, T. G. Deutsch, A. Kleiman-Shwarsctein, A. J. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E. W. McFarland, K. Domen, E. L. Miller, J. A. Turner, H. N. Dinh Journal of Materials Research Focus Issue: January

30 Acknowledgements Mr. Zhebo Chen [nanoparticulate & nanowebbed MoS 2 ] Dr. Jakob Kibsgaard [double gyroid structures] Dr. Shin-Jung Choi [p-mos 2 & RuO x ]