Download Why Hydrogen Energy?

Physically and chemically synthesized TiO2 composite
thin films for hydrogen production by
photocatalytic water splitting
Supervisor
Prof. Antonio Miotello
Laboratorio Idrogeno Energia Ambiente
Dipartimento di Fisica
Facoltà di Scienze MM.FF.NN.
Ph. D. Student
Rupali Dholam
Current Energy System
Fossil fuel reserves such as oil, natural gas and coal
have finite reserves and are depleting rapidly.
Environmental Damage of Fossil Fuels:
􀀁 Climate Change
􀀁 Ozone Layer Depletion
􀀁 Acid Rains
􀀁 Air Pollution
􀀁 Oxygen Depletion
Fossil Fuel Production/Demand
(Petroleum and Natural Gas)
Why Hydrogen Energy?
Hydrogen Economy
Global , clean and environment friendly permanent energy system…….
 The advantages of Hydrogen as fuel are :
 It is the lightest element, and has the highest mass-specific energy content among
the fuels: 119.93 MJ/kg, compared to 44.5 MJ/kg for gasoline, at present the
transportation fuel of choice.
 It is ecologically neutral.
 It is the ideal candidate for use in fuel cells, which produce very little emissions.
 Hydrogen is safer than commonly used natural gas because it mixes much faster with
air than either methane or petrol vapors (due to high diffusion coefficient) which make
accidents in the open air less critical.
 The major outcome by combustion of hydrogen is water which also contains
hydrogen.
But production of H2 from fossil fuels lead to increase in green house effect.
To make the life cycle of hydrogen fuel to be clean and renewable it is very important to
produce hydrogen gas from clean and renewable energy sources such as solar and wind.
Transportation
H2
water
Primary
energy
source
Energy
carrier
Electricity generation
Energy system
consumption
water
Catalyst used in Hydrogen production by Water
Hydrogen
Production:
H 2 O  energy  H 2  1/ 2O2
Solar light is used as the energy to required break the water molecule by using
Photocatalyst
2H+ + 2e- → H2(gas)
2h+ + H2O(liquid) → 1/2 O2(gas) + 2H+
Overall Reaction:
2hν + H2O(liquid) → 1/2 O2(gas) +H2(gas)
Reaction takes place when the energy of the photons absorbed by the photoanode are equal to or larger than Et, the threshold energy:
Et = hν = 1.23 eV
Energy Diagram of Photo-electrochemical cell.
Requirement of the photocatalyst:
 Must have energy band gap ~ 2 eV
 Must have high Corrosion and photo-corrosion resistance
 CB of semiconductor must be more negative than redox potential of H2
and VB must be more positive than oxidation potential of O2
 low cost of manufacturing
Best Photocatalyst
TiO2
TiO2 photo-catalyst thin film is been synthesized on conducting ITO glass by Sputtering
(physical method) and Sol-gel (chemical method).
Sputtering (physical method)
Distance between TiO2 metal
oxide target a ITO deposited
on glass substrate
Sol-gel (chemical method).
5 cm
Titanium
TitaniumButoxide
Butoxide
DC Power
150 W
Deposition rate
13 nm / min
Vigorous Stirring for 1hr at RT
Ethanol
Ar gas flow rate / partial
pressure
19 sccm (8 Pa)
Total sputtering operating
pressure
3 x 10-5 Pa
Substrate heating
temperature
RT
TiO2
ITO
Glass
Ethanol
+
Water
+
Nitric Acid
Dropwise addition under
vigorous stirring for 1hr at RT
TiO2 Solution
Again stirring vigorously
for 1 hr.
Resultant TiO2 sol
Result and discussion
XRD
FTIR
Sputter depsoited TiO2 film
Solgel depostied TiO2 film
Intensity (cps)
Absorbance (arb. unit)
Anatase
Rutile
10
20
30
40
50
60
70
2 (deg.)
• Debye-Scherrer equation :

T =  cos 
.
• Crystal size of anatase phase was 6nm
and of rutile phase was 45nm.
Anatase
TiO2
TiO2 as Deposited
TiO2 annealed at 500 C
1400
1200
1000
800
600
400
-1
Wavenumber (cm )
• sol-gel deposited TiO2 film posses
only anatase phase with low crystalline
degree
However anatase phase is most favorable for photocatalytic reaction.
SEM
Sputtered deposited film
Sol-gel deposited film
• Typically dense columnar structure with
diameter around 30-50nm is observed in
sputtered film
• Sol-gel film is quite compact with thickness
135nm
UV-Visible spectroscopy :
• Absorption edge of TiO2 film deposited by
sputtering is at higher wavelength (~ 388nm)
• Absorption edge of TiO2 film deposited by
sol-gel is at wavelength (~ 370nm)
• The energy band gap of chemically prepared
sample is 3.4eV which is higher than theoretical
value for anatase (3.2eV) and rutile (3.0eV)
• Sputtered deposited TiO2 band gap is 3.21 eV.
The absorption edge also contains shoulder at
2.85eV indicating presence of impurity energy
level in the band gap
Transmittance (%)
100
80
(Solgel deposited TiO2)
60
(Sputter depsoited TiO2)
40
20
0
300
400
500
Wavelength (nm)
• Band gap is obtained by fitting absorption edge of UV-Visible spectra by following equation
ln T = ln T0 - C
(w  E g )
w
600
Photocatalytic Activity.
Different
Thickness of
ITO (nm)
Photo-Voltage measured in distilled water (volt)
Sputter deposited TiO2 (1000 nm)
Solgel deposited TiO2 (100 nm)
light off
light on
light off
light on
30
0.130
0.686
----
-----
50
0.170
0.720
0.122
0.558
100
0.196
0.583
----
-----
150
0.246
0.577
0.170
0.553
250
0.140
0.543
0.159
0.649
350
----
-----
0.134
0.707
• In case of sputtered deposited film Voc shows maximum value when placed on
thinner ITO films as compared to thicker films.
• Since the conductivity is inversely proportional to the thickness of ITO film,thus
deposition of TiO2 on thinner ITO films (30 and 50nm) gives better electrical contact
and favoring better photo-voltage.
• Thickness of ITO film can be decreased further to enhance the photo-voltage value but
this will result incomplete coverage of the substrate.
(a)
0
100
200
300
400
Tin
Titanium
0
100
500
(b)
200
300
400
500
Thickness (nm)
Composition analysis along the cross-section of the sol-gel deposited TiO2 thin film (a) before and (b) after heat-treatment at 500 oC.
• Reverse behavior is observed for sol-gel deposited thin film which showed
increase in Voc by increasing ITO thickness up to certain value.
• Heat treatment on sol-gel film causes diffusion of Ti atoms into ITO layer
up to depth of 120-150nm that will change the peculiar properties of ITO
which results in low Voc for thinner thickness.
• For thicker ITO film (250 and 300nm) the Ti atoms partially diffused in to
the ITO film thus preserving its properties and shows better Voc.
Electrical contact
TiO2/ITO
Apparatus to measure
Separate evolution of H2
and O2
Pt
NaOH (1M)
H2SO4 (1M)
1.6
1.6
Solgel depsoited ITO (250 nm)/TiO2 (100 nm)
Sputtering depsoited ITO (50 nm)/TiO2 (1000 nm)
Light ON
Light ON
1.4
Light ON
1.4
Light ON
1.2
Voltage (V)
Voltage (V)
1.2
1.0
1.0
0.8
0.6
0.8
Light OFF
Light OFF
0.4
Light OFF
0.6
0.2
Light OFF
Light OFF
Light OFF
0.4
0
50
100
Time (min)
150
200
0
50
100
Time (min)
150
200
Hydrogen measurement
300
(Sputter deposited TiO2 film )
(Solgel deposited TiO2 film)
H2 concentration ( moles)
250
200
Light Off
150
100
50
0
0
5
10
15
20
25
Time (hrs)
• The H2 generation rate for sputtered deposited sample
for sol gel film it was 4.3 ±0.1µmole/h.
TiO2 was 12.5 ±0.1µmole/h and
• Due to band gap (3.2eV) ,impurity level contributed by stoichiometric defect, the sputtered
deposited TiO2 film leads to higher production of H2 than sol-gel film .
 Conclusions
 Two different kinds of TiO2 films were prepared using RF sputtering and the other
one by sol–gel method for hydrogen production by water splitting in photoelectrochemical cells.
 Depositions were performed on electrical conducting ITO whose electrical properties
play vital role to reduce the photon energy loss.
 The photo-anodes(TiO2) have been characterized by several techniques to infer on
their optical and compositional properties.
 The observed differences in hydrogen production have been attributed to the
peculiarities in absorption properties of the two TiO2 films that in the case of sputterdeposited films are more prone to absorb radiation because of the produced
defects during the deposition process.
Publication :
Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting.
R. Dholam, N. Patel, M. Adami, A. Miotello ,International Journal of Hydrogen Energy, 33 (2008) 6896-6903.