Physically and chemically synthesized TiO2 composite

1
Physically and chemically synthesized TiO2
composite thin films
for hydrogen production by
photocatalytic water splitting
Ph. D. Student Rupali Dholam
Supervisor Prof. Antonio Miotello
Laboratorio Idrogeno Energia Ambiente Dipartimento
di Fisica Facoltà di Scienze MM.FF.NN.
2
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
3
Fossil Fuel Production/Demand (Petroleum and
Natural Gas)
4
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.

5
But production of H2 from fossil fuels lead to
increase in green house effect.
6
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
Electricity generation
Primary energy source
water
Energy carrier
Energy system consumption
7
Catalyst used in Hydrogen production by Water
Hydrogen Production
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
2h? H2O(liquid) ? 1/2 O2(gas) H2(gas)
Overall Reaction
Reaction takes place when the energy of the
photons absorbed by the photo-anode are equal to
or larger than Et, the threshold energy
Et h? 1.23 eV
8
Energy Diagram of Photo-electrochemical cell.
9
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
10
TiO2 photo-catalyst thin film is been synthesized
on conducting ITO glass by Sputtering (physical
method) and Sol-gel (chemical method).
Sputtering (physical method)
Sol-gel (chemical method).
Distance between TiO2 metal oxide target a ITO deposited on glass substrate 5 cm
DC Power 150 W
Deposition rate 13 nm / min
Ar gas flow rate / partial pressure 19 sccm (8 Pa)
Total sputtering operating pressure 3 x 10-5 Pa
Substrate heating temperature RT
TiO2
11
Result and discussion
XRD
FTIR

• Debye-Scherrer equation
• .

• sol-gel deposited TiO2 film posses
• only anatase phase with low crystalline
• degree

• Crystal size of anatase phase was 6nm
• and of rutile phase was 45nm.

However anatase phase is most favorable for
photocatalytic reaction.
12
SEM
Sputtered deposited film
Sol-gel deposited film

• Sol-gel film is quite compact with thickness
• 135nm

• Typically dense columnar structure with
• diameter around 30-50nm is observed in
• sputtered film

13
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

• Band gap is obtained by fitting absorption edge
  of UV-Visible spectra by following equation

14
Photocatalytic Activity.
Different Thickness of ITO (nm) Photo-Voltage measured in distilled water (volt) Photo-Voltage measured in distilled water (volt) Photo-Voltage measured in distilled water (volt) Photo-Voltage measured in distilled water (volt)
Different Thickness of ITO (nm) Sputter deposited TiO2 (1000 nm) Sputter deposited TiO2 (1000 nm) Solgel deposited TiO2 (100 nm) Solgel deposited TiO2 (100 nm)
Different Thickness of ITO (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.

15
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.

16
Electrical contact
Apparatus to measure Separate evolution of H2
and O2
17
Hydrogen measurement

• The H2 generation rate for sputtered deposited
  sample TiO2 was 12.5 0.1µmole/h and
• for sol gel film it was 4.3 0.1µmole/h.
• 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 .

18

• Conclusions

• Two different kinds of TiO2 films were prepared
  using RF sputtering and the other
• one by solgel method for hydrogen
  production by water splitting in photo-
• electrochemical 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 sputter-
• deposited 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.