Theory of Oxides for Photoelectrochemical Hydrogen

Production

Muhammad N. Huda, Aron Walsh, Su-Huai Wei, Yanfa Yan, Mowafak M. Al-Jassim and John A. Turner

National Renewable Energy Laboratory,Golden, Colorado-80401

Project ID # PDP34

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THEORYTHEORY

SYNTHESIS

SYNTHESISAN

ALYSIS

ANALYSIS PECPEC

R&D feedback loop

Catalyst Criteria• Structural stability in solution.

• Low cost.

• High catalytic activity.

• Band gap: 1.5 – 2 eV.

• Visible light absorption.

• Band edge alignment.

• Binary oxides satisfy a number of criteria, but are limited by their large band gaps which fail to absorb photons in the visible range.

• Our aim is to explore new oxide systems which may overcome these intrinsic limitations through combining multiple cations.

Vacuum level

-3.0

-4.0

-5.0

-6.0

-7.0

-8.0

0

-4.5

TiO2Rutile

3.0

TiO2Anatase

3.2

SrTiO3

3.2

FeTiO3

2.7

2.8

MnTiO3

3.2

ZrO2

BaTiO3

5.0

Nb2O5

3.4

3.4

KTaO3WO3

2.8

2.2

ZnO3.

2

Fe2O3

SnO2

3.8

GaP

2.3

1.1

Si

SiC

3.0

CdSe

CdS

1.7

2.5

E vs. NHE

H+/H2

-1.0

+2.0

+1.0

+3.0

0

Band Gap Positions in Various Semiconductors

eV

@ pH = 0

Band Engineering is so far the only option!

None of the common oxides meet the criterion.

Q. How many oxides one can try experimentally?

John Turner from January PEC work group meetingUC Santa Barbara

“There are easily 50,000 combinations of ternary oxides and almost 2 million quaternary oxides.”

“A collaboration of theory, synthesis and characterization groupsis necessary to achieve fundamental PEC goals.”

Theory can guide to narrow the range of materials search for PEC.

ConductionBandMinimum of host

Impuritymetal’sd-band

impurityp-band

ValenceBandMaximum of host

Q. What do we mean by “band engineering”?

-Reduce the band gap.-Have the right position of the band-edges.

Band gapReduced gap

VBM pushed up due to hybridization

Reduced gap

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

B

C

N

O

F

Al

Si

P

S

Zn

Ga

As Se Te

MoHf Ta W Re

p-orbital

d-orbital

Zn

Ga

Energy levels of p-and d-orbital for several atoms

For oxides VBM is mostly made of O-p

Density Functional Theory Calculations (DFT)

DFT maps a many variables problem to a single variable problem.

It says, if electronic density is known, all the ground state propertiesof the system can then be uniquely determined.

In principle, DFT is an exact theory.

Approximations enter the theory when density functionals are constructed. Two most popular approximations are:

1. Local density approximation (LDA), and2. Generalized gradient approximation (GGA).

( ) iiieff rv ψεψ =⎥⎦⎤

⎢⎣⎡ +∇− v2

21

[ ] ∫∫∑ −+−

−= drrrEdrdrrr

rrE xcxc

N

ii

DFT )()()()(21 '

'

'

ρνρρρε

DFT is a first-principle quantum mechanical theory

Scope of DFTDFT can give us access to a number of important properties:

• Band structure.

• Optical absorption.

• Defect and doping effects.

• Surface chemistry.

• Structural stability.

We run state-of-art commercial DFT codes (VASP, WIEN2K) along with our own in-house analysis software.

Calculations are performed on both the NREL and NERSC supercomputers.

Figure: Impurity absorption on a metal oxide surface

Oxide Theory Publications (07/08)1. ‘Electronic structure of ZnO:GaN compounds: Asymmetric bandgap engineering’

M. N. Huda, Y. Yan, S.-H. Wei and M. M. Al-Jassim, Phys. Rev. Lett. In Review (2008).2. ‘Density-functional theory study of the effects of atomic impurity on the band edges of

monoclinic WO3’ M. N. Huda, Y. Yan, Chang-Yoon Moon, S.-H. Wei and M. M. Al-Jassim, Phys. Rev. B, In Press (2008).

3. ‘Electronic, energetic and chemical effects of intrinsic defects and Fe-doping of CoAl2O4 ’ A. Walsh, S.-H. Wei, Y. Yan, M. M. Al-Jassim, J. Phys. Chem. C, In Review (2008).

4. ‘Nature of the bandgap in In2O3 revealed by first-principles calculations and X-ray spectroscopy’ A. Walsh, J. L. F. DaSilva, S.-H. Wei et al., Phys. Rev. Lett. 100, 167402 (2008).

5. ‘Doping asymmetry in wide-bandgap semiconductors: Origins and solutions ’ Y. Yan and S.-H. Wei, Phys. Stat. Sol. B 245, 641 (2008).

6. ‘Structural, magnetic, and electronic properties of the Co-Fe-Al oxide spinel system: DFT calculations’ A. Walsh, S.-H. Wei, Y. Yan, M. M. Al-Jassim, J. A. Turner, M. Woodhouse and B.A. Parkinson, Phys. Rev. B 76, 165119 (2007).

Co-Fe-Al Oxide System (I)

Figure: Schematic inkjet salt printing (left) and measured photoresponse (right), relative to the p-type and n-type standards.

• Large scale theoretical screening of new oxide systems is limited by the lack of a single property defining a good PEC material.

• We take our initial lead from high-throughput experimental screening by the Parkinson group at Colorado State University1.

1. M. Woodhouse et al., Chem. Mater. 17, 4318 (2005); Chem. Mater. 20, 2495 (2008).

Co-Fe-Al Oxide System (II)• Experimental screening found a ternary oxide from the Co-Fe-Al system to exhibit good p-type behavior with a band gap ~1.7 eV.

• XRD showed the spinel structure over a large compositional range.

Figure: Spinel crystal structure, with two cations sites, labeled A (tetrahedral) and B (octahedral).

• Previously reported stable Co-Fe-Al phases:

Co3O4

Co2AlO4

CoAl2O4

Fe3O4

Fe2AlO4

FeAl2O4

Fe2CoO4

CoFe2O4

Co-Fe-Al Oxide System (III)• Our initial theoretical study explored the chemical trends along the

Co-Fe-Al binary and ternary spinel phase diagram.

• We found that as Al is substituted into Co3O4, it has preferential occupation for the octahedral spinel sites. This results in an increase in

the fundamental band gap from ~1 eV up to ~ 2 eV for the CoAl2O4 end compound.

Likely observed composition

Co-Fe-Al Oxide System (IV)• To understand the origin of conductivity in these poorly explored materials, we investigated the formation of intrinsic defects and Fe-

doping in the prototype CoAl2O4.

• Cation vacancies are low in energy, while oxygen vacancies are high.

•This confirms the p-type nature and suggests oxygen rich growth.

Low formation energy acceptor defects

Co-Fe-Al Oxide System (V)• Fe is found to readily substitute on both the Co and Al sites, due to

the small steric mismatch and +2/+3 oxidation states of Fe.

• Deep donor/acceptor levels are formed corresponding to Fe(II)↔Fe(III) transition.

• These band can contribute to lower energy photon absorption.

Deep mid-gap levels

Co-Fe-Al Oxide System (VI)• The drawback of cobalt spinels is the weak absorption in the

visible range arising from the nature of the d-d optical transitions.

Figure: Calculated absorption spectra.

• To overcome these limitations we are currently investigating isovalent cation substitution.

• Based on changes in the electronic energy levels on transition from Al to Ga to In, we predict a dramatic increase in visible light absorption.

Experimental verification of these predictions are in progress.

Band engineering of ZnO for PECZnO Structure

-4 -3 -2 -1 0 1 2 3 4 50 .0

0 .5

1 .0

1 .5

2 .0

2 .5

3 .0

DO

S

E n e rg y in e V

T o ta l D O S Z n -d O -p Z n -p O -s

-8

-6

-4

-2

0

2

4

6

8

Ener

gy (e

V)

L M Γ A Hd-bands

Absorption coefficients:ZnO

0

5

10

15

20

25

0 1 2 3 4 5 6

Energy (eV)

Alp

ha (1

04 /cm

)

x-polarizedz-polarized

Goal is to improve absorption in this region

ZnO:N

400 500 600 700 800 900 1000 1100 12000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1 ZnO ZnO:N at 80 W ZnO:N at 100 W ZnO:N at 120 W ZnO:N at 150 W ZnO:N at 200 W Zn3N2

Wavelength (nm)

Abs

orba

nce

K.‐S. Ahn, Y. Yan, and M. Al‐Jassim, J. Vac. Sci. Technol. B, L23 (2007)K.‐S. Ahn, et al . J. Electrochemical Society, 154 B956‐B959 (2007) 

Excellent absorption Bad photocurrentBad photocurrent

What is the reason:

-N substitution creates a deep level.

This means:

Localized wavefunctions.

Stronger recombination centers.

-1.5 -1.0 -0.5 0.0 0.5 1.0

-0.4

-0.2

0.0

0.2

Parti

al D

OS

Isovalent doping/alloying would improve the situation.

N-p

Two ways of making a ZnO-GaN composite system*:

1. ZnO:GaN super lattice

2. Random alloy system

Zn-O and Ga-N are isovalent.

*Electronic structure of ZnO:GaN compounds: Asymmetric bandgap engineering,M. N. Huda, Y. Yan, S.-H. Wei and M. M. Al-Jassim, Phys. Rev. Lett. In Review (2008).

At large number of layers, bandgap is smaller.

(ZnO)n(GaN)n Super-lattice

2.4

2.6

2.8

3

3.2

3.4

2 3 4 5 6 7 8 9 10 11 12

n, number of ZnO (GaN) layer

Ban

dgap

(eV

)Lowest band gapFor n > 12

Most stable interface

Band Offset ZnO/GaN

3.5

4.5

5.5

6.5

7.5

8.5

Ene

rgy

(eV

) VBM

CBM

Band offset 0.722 eV

ZnO

GaN

Measured value is 0.80 eV*

The smallest bandgap is only at theinterface

VBM (valence band maxima) is localized at GaNCBM (conduction band minima) is localized at ZnO.

ZnO:GaN random Alloy Dilute regime

Ga-N in host ZnO (or vice versa) tend to cluster together.

For example, second pair Ga-N doping in already Ga-N doped ZnO is only 0.068 eV.

Band gap decrease further as the cluster size increase.

-3 -2 -1 0 1 2 3 4 50.00

0.05

0.10

0.15

0.20

0.25 -3 -2 -1 0 1 2 3 4 50

3

6

9

12

Ato

mic

p-D

OS

Energy (eV)

N-p Ga-d O-p Zn-d

Tota

l DO

S

Host Eg reduction

Ga-N in ZnO 0.410

Zn-O in GaN 0.102

Band gap reduction is asymmetric:

ZnO host GaN host

EgEg

VBMZnO

HOSGaN

LUSGaN

CBMZnO

HOSZnO

VBMGaN

LUSZnO

CBMGaN

0.7 eV

Can be explained by confinement effects:

Question: Why?

ZnO and GaN has similar band gap,Band edges and are isovalent.

Random alloy

Super-lattice

Calculated total absorption coefficient spectra of superlattice of (ZnO)8(GaN)8 (solid blue curve) and (ZnO)1-x(GaN)x random alloys (x=0.0740).

SupperSupper--lattice or Randomlattice or Random--alloy?alloy?

This shows that the random alloy approach is better for optical absorption

1 2 3 4 50

10

201 2 3 4 5

0

10

201 2 3 4 5

0

10

20

Energy (eV)

Abs

orpt

ion

Coe

ffici

ents

(104 /c

m)

x-polarized y-polarized z-polarized

Pure ZnO

Ga-N substitution

2Ga-2N substitution

*Kwang-Soon Ahn, Yanfa Yan, Sudhakar Shet, Todd Deutsch, John Turner, and Mowafak Al-JassimAppl. Phys. Lett. 91, 231909 (2007).

Improvedphoto-current

Current Theory work

•The focus is now on Cu-based delafossites.

•Explaining the observed PEC results in Fe2O3 and to improve its performance.

•Ternary oxides, such as, CuIn2O4 and CuGa2O4 and new alloys from them.

•Search for other new oxides.

• Isovalent metal substitution in spinel structure has shown dramatic improvement in absorption.

•Optical absorption and photo-current properties can be improved by charge passivated doping.

• ZnO is a better host than GaN to improve the photo-response in dilute regime.

•Theory can be used as a predictive tool, as well as to understand the physics, to design new materials.

Conclusions