M. Reza Mohammadizadeh

Department of Physics, University of Tehran

Doping & Vacancy in Solids

Outline

Important parameters in doping & vacancy study

Choose one compound (TiO2)

Some examples:

Hydrogenated Titania

Hydrogenated Titania surface

Summary

2

Doping / vacancy study Concentration convenient supercell

symmetry

interstitial

Doping

Substitutional

Neutral / charged

Location relaxation

in depth (inside the bulk)

Surface # of layers

Vacuum thickness

Reconstruction

Which (001), (111), …

Which termination

Stability temperature

time (diffusion)

Correct magnetic ordering

Concentration convenient supercell symmetry

ABO3 ABO2.5 A2B2O5

Doping : interstitial / substitution

Neutral / Charged

H ions in TiO2 with d= 150 nm, 10 keV, 4 keV

Location relaxation

in depth (inside the bulk)

Surface # of layers

Vacuum thickness

Reconstruction

Which (001), (111), …

Which termination

H ions in TiO2 with d= 150 nm, 10 keV, 4 keV

Abundance

H ions in TiO2 with d= 150 nm, 10 keV, 4 keV

O vacancy

Ti vacancy

Which exactly doping/vacancy

)101)، )100)، )001.)

Which direction?

c

b

a

Oا

Ti

Which termination?

-724.4

-724.3

-724.2

-724.1

-724

-723.9

-723.8

-723.7

0 5 10 15 20 25 30 35 40

En

erg

y (

Ry

)

Vacuum (Bohr)

-723.8600

-723.8400

-723.8200

-723.8000

-723.7800

8 12 16 20 24 28 32

vacuum

After

relaxation

z

6842/1 Å

4969/0Å

6118/1 Å

Before

relaxation

z

Å 9936/1

Å 4101/0

Å 5835/1

O

Ti

Slab choosing

0.348

0.3485

0.349

0.3495

0.35

0.3505

0.351

0.3515

0.352

0 1 2 3 4 5 6 7

Su

rfac

e E

ner

gy

(eV

/(A

)2)

Number of Slabs

𝜎 =1

𝐴[𝐸𝑠𝑙𝑎𝑏 − 𝑛𝐸𝑏𝑢𝑙𝑘]

Å 807/4

Surface reconstruction

(1x4)

Stability temperature

time (diffusion)

Correct magnetic ordering

[J. Phys. Chem. Lett. 6, 4627 (2015)]

Choose one compound (TiO2)

Removal of organic pollutants, purifying of water, air Self-cleaning/deinfecting coatings (bacteria, viruses, cancer cells) Anti fogging surfaces Photoelectrochemical cells, solar cells Photocatalytic splitting of water, H-production White pigment Sunscreen Additives in foods

TiO2

In ambient conditions

[E. Shojaee, et al., Phys. Rev. B 83 (2011) 174302]

[M. Abbasnejad, et al., Appl. Phys. Lett. 100 (2012) 261902]

[M. Abbasnejad, et al., Europhys. Lett, 97 (2012) 56003]

The calculated bulk modulus confirms that the experimentally claimed structure

(cubic fluorite phase) can be Pca21-TiO2.

Photocatalytic activity

22

2

2).(

OOe

OHHOHh

ehhTiOPhotoCat

cb

vb

cbvb

hydroxyl

superoxide

23

Examples CN¯ + h+ → CN•

2CN• → (CN)2

(CN)2 + 2OH•¯ → CN¯ +CNO ¯ +H2O

CNO¯ + 8OH•¯+ 8 h+ → NO3¯ + CO2 + 4H2O

NO decomposition & NO2

2NO2 + 2OH•¯→ H2 + 2NO3-

NO + O2•¯ → NO3

-

Anatase & Rutile

Photoinduced superhydrophilicity UV

24

TiO2; transition metal oxide

Photoactivated

Thermal & Chemical stability

Cheap

Nontoxic

Various applications

Eg ~ 3.2 eV

4th metal & 9th element; abundance

Red shift

387.5 nm

28

[X. Chen, et al., Science, April 2011, 1200448] in a 20 bar H2 atmosphere at about 200 C for 5 days

Temperature enhanced DC Plasma

29

Current (mA) Time (min) Temp. (oC) Pressure

(mTorr)

H Flow Rate (Lit/min)

05 60 25-150-250-

300-350

20 0.2

H doping with DC plasma

[M. Bagheri, et al., Appl. Sur. Sci. 350 (2015) 43]

Hydrogen Irradiation with MBM

Multi-Casp Magnetic Field

[S. Hidari, et al., Appl. Phys. A 121 (2015) 149]

Computational Details

Spin-polarized Density Functional Theory Calculations

PBE Exchange Correlation Functional

Hubbard U Correction for Ti 3d orbital and O 2p orbital electrons

Norm conserving Ti (3s, 3p, 3d, 4s) and O Pseudopotentials

MP grid of 6×6×2 K-points

Energy Cutoff of 100Ry for the Wave function

www.quantum-espresso.org.

This work Expt.

a (Å) 3.78 3.78

C (Å) 9.53 9.51

Eg(eV) 3.27 3.2

[J. Am. Chem. Soc 118, 6716 (1996)]

H-doping & Oxygen vacancy in Anatase TiO2 (bulk)

Adiabatic Charge Transition Levels

34

Thermodynamic Behavior

35

Phase Diagram

36

Electronic Structure

High Concentration

(0.125 )

Low Concentration

(0.0625 )

37

nTi

nH

nTi

nH

Electronic Structure(High Concentration)

38

[S. Ataei, et al., J. Phys. Chem. C (2016) in press.]

[001] surface of Anatase TiO2

(1×4) anatase surface

0

1

2

3

4

5

G X M

En

erg

y (

eV

)

EF

0

50

100

150

EFTotal

0 0.5

1 1.5

Surface Ti-d

0 1 2 3

E

lectr

on

ic d

en

sity

of

state

s (S

tate

s/eV

)

Deep Ti-d

0 0.5

1 1.5

Deep O-p

0 1 2 3

-4 -2 0 2 4 6

Energy (eV)

Surface O-p

Bonds Doping energy* in perpendicular

arrangement (eV)

Doping energy in parallel

arrangement (eV)

H-O2c 0.83 1.78

H-O3c 1.52 1.54

Deep H-doped 1.70 2.24

*The difference energy between the doped system and the clean one with an additive H.

The doping energy of the six calculated H-doped systems.

-4

-2

0

2

4

6

G X M

En

erg

y (

eV)

EF

50 100

Electronic density of states (states/eV)

EF

H-doped

0 50 100 150

EF

pure

0 50

100 150

EFTotal

0 0.5

1 1.5

H-s

0 1 2

Surface Ti-d

0 0.5

1 1.5

E

lectr

onic

densi

ty o

f st

ate

s (S

tate

s/eV

)

Deep Ti1-d

0 0.5

1 1.5

Deep Ti2-d

0 0.5

1 1.5

Deep O-p

0 1 2 3

-4 -2 0 2 4 6

Energy (eV)

Surface O-p

-4

-2

0

2

4

6

G X M

En

erg

y (

eV

)

EF

50 100

Electronic density of states (States/eV)

EF

O vacancy

0 50 100 150

EF

Clean

0 50

100 150

EFTotal

0 1 2 3

Surface Ti1-d

0 1 2 3

Surface Ti2-d

0 1 2

E

lectr

onic

densi

ty o

f st

ate

s (S

tate

s/eV

)

Deep Ti1-d

0 0.5

1 1.5

Deep Ti2-d

0 0.5

1 1.5

Deep O-p

0 1 2 3

-4 -2 0 2 4 6

Energy (eV)

Surface O-p

-4

-2

0

2

4

6

G X M

Ene

rgy

(eV

)

EF

50 100

Electronic density of states (states/eV)

EF

H-doped and O vacancy

0 50 100 150

EF

Pure

0 50

100 150

EFTotal

0 0.5

1 1.5

H-s

0 1 2

Surface Ti-d

0 0.5

1 1.5

E

lectr

onic

densit

y o

f sta

tes

(Sta

tes/

eV

)

Deep Ti1-d

0 1 2

Deep Ti2-d

0 0.5

1 1.5

Deep O-p

0 1 2 3

-4 -2 0 2 4 6

Energy (eV)

Surface O-p

0.52 eV

-4

-2

0

2

4

6

G X M

En

erg

y (

eV

)

EF

50 100

Electronic density of states (States/eV)

EF

H-deep doped and O vacancy

0 50 100 150

EF

Clean

0 50

100 150

EFTotal

0 0.5

1 1.5

H-s

0 1 2

Surface Ti1-d

0 1 2 3

Surface Ti2-d

0 1 2

E

lectr

onic

densi

ty o

f st

ate

s (S

tate

s/eV

)

Deep Ti-d

0 0.5

1 1.5

Deep O-p

0 1 2 3

-4 -2 0 2 4 6

Energy (eV)

Surface O-p

[M. Sotudeh, et al., AIP Advances 4 (2014) 027129]

16.5

17

17.5

18

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0

-0.61 -0.22

2A

g (

Ry

)

mO (Ry)O

-po

or

O-r

ich

Clean

H doped

O vacancy

O vacancy with H-surface doped

O vacancy with H-deep doped

Summary Interstitial hydrogen is the most stable defect.

The positively charged Hi is thermodynamically stable with respect to pure Titania.

Different kinds of defects and also the concentration, correspond to different electronic structure.

There are midgap states located 0.7-0.9 eV below the CBM, in presence of neutral defects with lower

concentration.

Our results explain discrepancies between different experiments.

High concentration of HO as a particularly promising system for photocatalytic applications.

50

Doping / vacancy study Concentration convenient supercell

symmetry

interstitial

Doping

Substitutional

Neutral / charged

Location relaxation

in depth (inside the bulk)

Surface # of layers

Vacuum thickness

Reconstruction

Which (001), (111), …

Stability temperature

time (diffusion)

Correct magnetic ordering

SRL

Thank you for your attention

53