Transcript 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.
water Primary energy source Transportation
H 2
Energy carrier Electricity generation water Energy system consumption
Catalyst used in Hydrogen production by Water
Hydrogen Production:
H
2
O
energy
H
2 1 / 2
O
2 Solar light is used as the energy to required break the water molecule by using
Photocatalyst
2H + + 2e
→
H 2 (gas)
2h + + H 2 O(liquid) → 1/2 O 2 (gas) + 2H +
Overall Reaction:
2hν + H 2 O(liquid) → 1/2 O 2 (gas) +H 2 (gas
) Reaction takes place when the energy of the photons absorbed by the photo anode are equal to or larger than E t , the threshold energy: E t =
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
TiO 2
TiO 2 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 TiO 2 metal oxide target a ITO deposited on glass substrate 5 cm Ethanol DC Power 150 W Vigorous Stirring for 1hr at RT Deposition rate 13 nm / min 19 sccm (8 Pa) Ethanol + Water + Nitric Acid Dropwise addition under vigorous stirring for 1hr at RT Ar gas flow rate / partial pressure Total sputtering operating pressure Substrate heating temperature TiO2
ITO Glass
3 x 10 -5 Pa RT TiO 2 Solution Again stirring vigorously for 1 hr.
Resultant TiO 2 sol
Result and discussion XRD Anatase Rutile Sputter depsoited TiO 2 film TiO 2 as Deposited TiO 2 annealed at 500 C FTIR
Solgel depostied TiO 2 film
Anatase TiO 2 10 20 30 40 50
2
• Debye-Scherrer equation : T = cos 60 70 .
• Crystal size of anatase phase was 6nm and of rutile phase was 45nm.
1400 1200 1000 800 600 400
Wavenumber (cm -1 )
• sol-gel deposited TiO 2 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 TiO 2 film deposited by sputtering is at higher wavelength (~ 388nm) • Absorption edge of TiO 2 film deposited by sol-gel is at wavelength (~ 370nm) 100 80 • 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 TiO 2 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 60 40 20 0 300 (Solgel deposited TiO 2 ) (Sputter depsoited TiO 2 ) 400 Wavelength (nm) 500 • Band gap is obtained by fitting absorption edge of UV-Visible spectra by following equation ln T = ln T 0 - C (
w
E g
)
w
600
Different Thickness of ITO (nm)
30 50 100 150 250 350 Photocatalytic Activity.
Photo-Voltage measured in distilled water (volt) Sputter deposited TiO 2 (1000 nm) light off light on Solgel deposited TiO light off 2 (100 nm) light on
0.130
0.170
0.196
0.686
0.720
0.583
--- 0.122
--- ---- 0.558
---- 0.246
0.140
--- 0.577
0.543
---- 0.170
0.159
0.134
0.553
0.649
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 TiO 2 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 Tin Titanium 200 300 400
(b)
500 0 100 200 300 400 500
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 o C.
• 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.
TiO 2 /ITO Pt Electrical contact Apparatus to measure Separate evolution of H 2 and O 2 NaOH (1M) H 2 SO 4 (1M) 1.6
1.4
1.2
1.0
0.8
0.6
Light OFF 0.4
0
Sputtering depsoited ITO (50 nm)
/
TiO 2 (1000 nm)
Light ON Light ON 50 Light OFF 100
Time (min)
Light OFF 150 1.6
1.4
1.2
1.0
0.8
0.6
Solgel depsoited ITO (250 nm)
/
TiO 2 (100 nm)
Light ON Light ON 200 0.4
0.2
Light OFF 0 Light OFF 50 100 Time (min) 150 Light OFF 200
Hydrogen measurement 300 250 200 150 100 (Sputter deposited TiO 2 film ) (Solgel deposited TiO 2 film) Light Off 50 0 0 5 10 15 20 25 Time (hrs) •
The H 2 generation rate for sputtered deposited sample TiO 2 for sol gel film it was 4.3
±
0.1µmole/h.
was 12.5
±
0.1
µmole/h and
•
Due to band gap (3.2eV) ,impurity level contributed by stoichiometric defect, the sputtered deposited TiO 2 film leads to higher production of H 2 than sol-gel film .
Conclusions Two different kinds of TiO 2 films were prepared using RF sputtering and the other one by sol –gel 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(TiO 2 ) 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 TiO 2 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 TiO 2 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.