Nano-structured MoS2 and WS2 for the Solar Production of Hydrogen

Thomas F. JaramilloDept. of Chemical Engineering

Stanford University

GCEP Research Symposium:New Research Directions in a Rapidly Evolving Global Energy Landscape

Stanford UniversityStanford, CA

2 October 2009

Solar Fuels by Photoelectrochemistry (PEC)

Artist: Mr. Zhebo Chen

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

PEC water-splitting: materials challenges

Many materials “issues” to consider…• Absorbance………. dictated by bandgap → 1.23 eV < Eg < 2.5 eV• Charge Transport… dictated by crystallinity, doping, and morphology• Band Structure Energetics…. sufficient to split water (Eg > 1.23 eV)

and must have appropriate redox potentials for H2 & O2 evolution • Surface electrocatalysis… must maximize interfacial charge transfer. • Stability, Cost, Non-toxicity.

Nowotny, J.; Sorrell, C.C.; Bak T.; Sheppard, L.R. Solar Energy 2005, 78, 593‐602

Bak et. al., Int. J. Hydrogen Energy,vol 27 (2002) 991-1022

Common semiconductors

A. Nozik and R. Memming, J. Phys. Chem., v100 (1996) 13061-13078.

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)• CO2 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?

What’s known about bulk MoS2 and WS2 for PEC

• 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. E0H+/H2.

• Surface: Not catalytically active for either H2 evolution or O2 evolution.

• Photoanodic corrosion is problematic.

• Inexpensive and earth-abundant materials.

Efficiency

Durability

Potential solution: Nanostructuring!

Potential solution: p-type materials may be cathodically stable.

Cost

Surface science and electrochemistry for HER

1 - Preparation and STM imaging of MoS2 on Au(111) in UHV conditions.

-0.3 -0.2 -0.1 0.010-6

10-5

10-4

10-3

10-2

10-1

Cur

rent

(A/c

m2 ge

omet

ric)

Voltage vs. RHE

H2SO4 pH 0.24 (N2 saturated) 5 mV/s

High Coverage MoS2

"Blank" sulfided Au(111)

Low coverage MoS2

2 - Measure electrochemical activity of the MoS2 previously characterized.

The approach

• Prepare a sample set of MoS2 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

MoS2 edge

470 Å x 470 Å 470 Å x 470 Å 60 Å x 60 Å

Electrochemical measurements

-0.25 -0.20 -0.15 -0.10 -0.05 0.00

-300

-200

-100

0

MoS2

550 οC

Blank

i (µA

/ cm

2 geom

etric

)

E (V vs. NHE)

MoS2

400 οC

-0.25 -0.20 -0.15101

102

103

Reference Electrode (SCE)

Counter Electrode(Pt mesh)

Working electrodeSTM imaged MoS2 on Au(111)

MoS2 on the HER volcano

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Pt(111)

Au

Nb Mo

WCoNi

IrRe Pd

Pt

Rh

Au(111)

Cu

i 0 (A/c

m2 )

∆GH* (eV)

Ag

MoS2

T.F. Jaramillo, K.P. Jørgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff; Science 317 (2007) 100.

Tailoring Electronic Band Structure

Bulk Materials

NanostructuredMaterials

8 nmCdSe

2 nmCdSe

CdSe: a “classic” exampleof quantum confinement

Bawendi et. al., MIT

Synthesizing Metal Sulfides

Synthesize metal

Synthesize metal oxide

Synthesize metal sulfide

Sulfidization of Mo/W or WOx/MoOx at 150°C in 10% H2S / 90% H2.

400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Electrodeposited MoO3-x sulfidizedin 10%/90% H2S/H2

Abs

orba

nce

/ a.u

.

Wavelength (nm)

Untreated 100oC 150oC 200oC 400oC

Increasingtemperature

Electrodeposited thin films on fluorine-doped tin oxide (FTO)

Mo/MoOx W/WOx

Synthesis of supported MoS2 nanoparticles

Nanoparticle synthesis by wet chemistry

PS/P2VP ratios (m.w.):27700/430032500/780081000/21000172000/42000

Metal precursor:P2VP0.05-1.0

Sulfidization of MoOx nanoparticles to MoS2

240 238 236 234 232 230 228 226

232.6Mo(VI) 3d3/2

Cou

nts

/ a.u

.

Binding Energy / eV

Mo(VI) 3d5/2

235.7

236 234 232 230 228 226 224 222

226.1

228.9

232.0S 2s

Mo(IV) 3d3/2

Cou

nts

/ a.u

.

Binding Energy (eV)

Mo(IV) 3d5/2

sulfidize

sulfidize

MoOx MoS2

Varying MoS2 Nanoparticle Size with PS/P2VP

• Atomic force microscopy (AFM) of MoS2 nanoparticles in various sizes synthesized using a Mo(OCOCH3)2 precursor.

32500/7800 81000/2100027700/4300 172000/42000

Increasing molecular weight block copolymer (PS/P2VP, units: Da)

MoS2 Nanowebs

• Alternatively, we can use a MoCl3 precursor to make interconnected nanowires or “web” structures

200 nm

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.

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Abs

orba

nce

/ a.u

.

hν / eV

Nanoparticle Absorbance

Bulk film

Decreasing nanoparticle size

1.0 1.5 2.0 2.5 3.0 3.5 4.0

(Ahν

)1/2

hν / eV

Tauc Plot for Nanoparticles

Decreasing nanoparticlesize

Bulk film

Bandgap Enlargement in MoS2

In pursuit of p-type MoS2

SMoNb

Nb5+:Mo4+S2

Mo0.95Nb0.05S2

1. Drop cast 5 mol-% NbCl5 + 95 mol-% Mo powder in H2O2

TCOMo0.95Nb0.05O32. Anneal in air at 400oC for 1 hour.

3. Sulfidize in 10% H2S / 90 % H2 at 450oC for 4 hours.

TCOMo0.95Nb0.05Ox(OH)yClz

TCO

Synthetic route

Photoactivity of p-type MoS2

ON

OFF

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6-0.5

-0.4

-0.3

-0.2

-0.1

0.0

E (V) vs. Hg/HgSO4I (

µA)

20 mV/s

0 10 20 30

-0.45

-0.44

-0.43

ON

OFF

t (s)

I (µA

)

P.S.

RuO2Nb-MoS2

0.1 M H2SO4

Eapp = -1.5 V

Cathodic photocurrent indicates p-MoS2

Several Targeted Nanostructures

22

Nanoparticles Ultra-thin Nanofilms

Nanowires 3-dimensional double-gyroid

Double Gyroid Mesostructures

Double gyroid silica template Metal electrodeposition Etch away silica template(2% HF 4hr)

Goal: Synthesize a MoS2 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

Double Gyroid Pt on FTOSEM

SEM

Double gyroid Pt mounds on FTO

Pulsed electrodeposition:-2.0V for 50ms, 1s between pulses

FTO

Double Gyroid Pt on FTO

High resolution SEM and TEM show the porous double gyroid Pt structure

SEM

TEM

FTO

RuO2 Counter Electrode DevelopmentOxygen Evolution Reaction (OER)

2H2O → O2 + 4H+ + 4e- Eo = 1.229 V vs. SHE

Low Overpotential for OER Unstable

E (V) vs. SHE

I (m

A/c

m2 )

Ru / FTOPt

0.8 1.0 1.2 1.4 1.6

0

1

2

3

1.8E (V) vs. SHE

I (m

A/c

m2 )

20 mV/s

1.60.8 1.0 1.2 1.4

0

2

4

6

8

10

Ru was electrodeposited at -0.6 V (vs. Ag/AgCl) for 5 minutes in aq. 5 mM RuCl3 + 0.1 M KCl.

RuO2 Counter Electrode Development

Significant improvement in stability by annealing.

Cycle Number

Cur

rent

Effi

cien

cy (%

)

No annealing

After Annealing

0 20 40 60 80 100

0

20

40

60

80

100

400oC, 1 hr

E (V) vs. SHE

I (m

A/c

m2 )

20 mV/s

100 cycles

1.60.8 1.0 1.2 1.4

0

2

4

6

Electrodeposited Ru/FTO was annealed in air at 400oC for 1 hour.

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

Accelerating materials development for photoelectrochemical (PEC) hydrogen production:

Standards for methods, definitions, and reporting protocolsZ. 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 ResearchFocus Issue: January 2010

29

Acknowledgements

Mr. Zhebo Chen [nanoparticulate & nanowebbed MoS2]Dr. Jakob Kibsgaard [double gyroid structures]Dr. Shin-Jung Choi [p-MoS2 & RuOx]