Nanostructured SrTiO3/WO3 Heterojunction Thin

Films for Efficient Photoelectrochemical Water

Splitting for Hydrogen Generation

Surbhi Choudhary

Supervisor: Prof. Sahab Dass

Dept. of Chemistry

Dayalbagh Educational Institute Dayalbagh, Agra

Outline

Introduction

Bilayered Thin Films in PEC Water Splitting

Experimental Study

Preparation

Characterization

PEC Study

Results &Conclusion

Bilayered Photoelectrodes possess:

Better absorption in the visible region.

Effective separation of charge carriers.

Inbuilt electric field at the interface, reduces recombination of charge carriers.

Applied external bias promotes transfer and separation of photogenerated charge carriers.

Introduction

Why SrTiO3 and WO3

Tungsten Oxide (WO3) [Eg :- ~ 2.7 eV]

Advantages:

Basically n-type in nature due to presence of oxygen

vacancies within the material.

Favorable transport properties

Chemically inert and Photostable

Problem: Non optimal band-edge alignment of conduction

band with redox potential of water.

Strontium Titanate (SrTiO3) [Eg :- ~ 3.5 eV] Advantages:

Remarkable stability in strong acidic/alkaline solutions.

Large negative flatband potential (Vfb= -0.2V), due to which electrolysis of water is possible without any applied bias. Properly aligned band edges with the redox level of water

Problem: Low photoconversion efficiency due to large

band gap and high resistivity.

SrTiO3/WO3 Bilayered films

Bilayered semiconductors in this study, typically composed of two semiconductors, one with a wide band gap (SrTiO3) and another with a moderate band gap (WO3).

WO3 is responsible for sensitizing SrTiO3 semiconductor through hole injection.

The energy layers in the SrTiO3/WO3 semiconductor can cover visible spectrum thereby offering synergistic effect.

Inbuilt electric field at the SrTiO3/WO3

heterojunction facilitates charge carrier transfer easily across the interface of the heterojunction.

3.2eV

2.7eV

1.23 eV

H+/H2

O2/H2O

ITO

WO3

SrTiO3

e-

h+

Energy band diagram of SrTiO3/WO3

Heterojunction on ITO substrate

Preparation of Bilayered SrTiO3/WO3 thin film:

Sol-gel spin-coating Technique

Characterization Techniques:

X-ray Diffraction (XRD)

UV-Visible Absorption Spectrophotometery

Scanning Electron Microscopy (SEM)

Photoelectrochemical (PEC) Study on Bilayered thin film

Experimental Study

Thin Film Preparation

Precursor solution of WO3 obtained by sol-gel spin-coating method1

Precursor solution of SrTiO3 obtained by sol-gel spin-coating method2 , deposited on pre-deposited WO3 films

Multiple layer of precursor solution was spin coated on two third portion of ITO substrate

Bilayered thin films of SrTiO3/WO3

Annealed the films at 450oC for 1 hr

Annealed the films at 600oC for 1 hr

Optimizing Parameters of the Study: No. of layers of the two materials or Thickness

1. Luo et al, J. Phys. D: Appl. Phys. 40 (2007) 1091–1096. 2. Solanki et al, Int. J of Hydrogen Energy 36 (2011) 5236-5245.

XRD Study

XRD pattern of Tungsten Oxide, Strontium Titanate and Bilayered (SrTiO3/WO3) thin films

Films are crystalline in nature.

XRD pattern for SrTiO3 and WO3 confirmed the cubic and orthorhombic phase respectively.

Average particle size of SrTiO3 and WO3 was 45 nm and 39 nm respectively.

SrWO4 in Bilayered films: due to the diffusion of Ti4+ (0.605 Å)

and W6+ (0.60 Å) ions at the interface. Similar ionic radii making easy substitution of ions at the interface.

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Optical Characterization

Tauc plot of Tungsten Oxide and Bilayered (SrTiO3/WO3) thin films. Inset shows Tauc plot

for Strontium Titanate.

Sample ID

Band Gap

(eV)

SrTiO3 3.52

WO3 2.98

SrTiO3 / WO3

2.87

2.0 2.5 3.0 3.5 4.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

(h)1

/2

Energy (eV)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Energy (eV)

(h)2

SrTiO3

WO3

SrTiO3/WO

3

Mode of transition for : - SrTiO3 - Direct nature ((αhυ)2 versus hυ is linear)

WO3 & Bilayered films - Indirect nature ((αhυ)1/2 versus hυ is linear)

Surface Morphology

FE-SEM image of SrTiO3/WO3

Homogeneous WO3 film deposited on ITO substrate. Good film-to-substrate adhesion of WO3 film was observed. Rough surface with aggregation of grains is shown in upper SrTiO3 film.

SrTiO3 layer shows porous structure that may be beneficial for the diffusion of electrolytes and the effective scattering of incident light.

I-V Characteristics

Photocurrent density versus applied potential curve for Strontium titanate, Tungsten oxide

and Bilayered thin film.

Sample ID

Photocurrent Density (mAcm-2)

0 V/SCE

0.9 V/SCE

SrTiO3 0.046 0.059

WO3 0.002 0.018

SrTiO3/WO3 0.079 0.088

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

0.00

-0.02

-0.04

-0.06

-0.08

-0.10

-0.12

P

ho

tocu

rren

t D

en

sit

y (

mA

/cm

2)

Voltage (V)

SrTiO3

WO3

SrTiO3/WO

3

Maximum Photocurrent density of SrTiO3/WO3 Bilayered films = 0.079 mA/cm2 at 0 V/SCE, which is approx. two times higher than that of SrTiO3

(0.046 mA/cm2) and much higher than that of WO3 (0.002 mA/cm2) at zero bias.

Increase in absorbance of WO3 towards visible region or red shift from 420 nm to 432 nm was observed. The nanostructured porous morphology of upper most SrTiO3 layer provided more surface area for interaction with electrolyte (increased liquid-semiconductor contact area).

The increase in photocurrent of SrTiO3/WO3 is attributed to increased absorption and improved charge separation across the interface of two oxide layers leading toward the reduced combination of photogenerated charge carriers.

Conclusion

Thank You