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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.
20 30 40 50 60
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Inte
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a.u
.)
2 (Degree)
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SrTiO3
* :Substrate
:WO3
:SrTiO3
:SrWO4
[11
2]
*o
*
*
*
*
*
*
*
*
*
*SrTiO
3/WO
3
WO3
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