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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.