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Date : 14-02-09

Speaker : G. Magesh

Visible light photocatalytic activity of PbSe nanocrystal/TiOx films

Reference:C. Wang, K. Kwon, M. L. Odlyzko, B. H. Lee, M. Shin, J. Phys, Chem. C., 111 (2007) 11734

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Photocatalysis

• TiO2 is the best known photocatalyst

• Functions only in UV light

• Sensitization with dyes, doping metals and non-metals and composites with metal nanoparticles are attempted to increase visible light absorption

• Sensitization with small bandgap semiconductors also attempted

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Sensitization with small bandgap semiconductors

• Semiconductors other than CdS have not much been attempted because of photostability problems in aqueous solution

• Semiconductors on reduction of their size can have suitable bandposition for charge transfer to TiO2 because of quantum confinement effect

• Studying such systems help in developing tunable photocatalytic systems based on our requirements

• One such system is PbSe/TiO2

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Preparation PbSe nanocrystals

Lead acetate in phenyl ether, oleic acid and trioctyl phosphine

Phenyl ether

Vacuum 85 C 1 h

Cooled to 45 C in Ar atmHeated between 180

to 210 C

Trioctylphosphine selenium

Flask 1 Flask 2

Contents of flask 1 added rapidly to flask 2

Temperatures between 110 to 130 C

Growth time 1-10 mins

Vacuum 85 C 1 h

Cooled to RT

PbSe nanocrystals with bandgap between 0.53 to 1.03 eV

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Preparation PbSe nanocrystal / TiOx photocatalyst

Thin film PbSe prepared by drop casting a hexane solution of PbSe on glass slides

Immersed in 3-mercapto-1,2-propanediol in ethanol for 12h

Dipped in Titanium(IV) isopropoxide in 1:1 toluene:ethanol for 10 mins

Hydrolyzed in H2O for 1 min

Steps 3 and 4 repeated for additional layers of TiOx

Rinsed ethanol

PbSe/SnO2 prepared using tin isopropoxide in isopropanol

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Structure and compositon of PbSe NC/TiOx films

• 5 times larger TiOx deposited in first two layers than subsequent layers

• Because initially surface area of PbSe is large

• Depositing layers reduces surface for deposition of further layers

• Thickness of each layer ~ 1nm

• Total thickness of film ~ 10 nm

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XPS and EDX spectra

• Presence of the various elements confirmed by XPS

• EDX shows that TiOx deposited even on places without PbSe NCs

XPS spectra of various samples

Various spots where EDX was taken

EDX spot B

EDX spot A

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TEM images of PbSe before and after TiOx

deposition

TEM images and UV\Visible\Near IR spectrum

• TEM images show that structure and size distribution of the PbSe remains uniform after TiOx deposition

• UV\Visible\Near IR spectrum shows that optical properties of PbSe retained during film fabrication

UV\Visible\Near IR spectrum

Solid line: PbSe film\ with 3 layer of TiOx

Dotted line: PbSe in hexane

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XRD pattern

• Only PbSe peaks observed

• No peaks corresponding to TiO2 observed

• Therefore prepared TiO2 films are amorphous.

• Although crystallinity is preferred for high activity annealing at high temperature will lead to sintering of PbSe

• Increase in particles size will lead to shift in bandposition of PbSe and therefore no sensitization will be possible

• Therefore annealing was not carried out

XRD pattern of PbSe\TiOx films, standard anatase and rutile TiO2

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Photocatalysis experiment

Source : 200 W Xe arc lamp

Rhodamine 6G : 0.01 mM in water

Schematic representation of photocatalytic setup

Thin film catalyst placed in quartz cuvette containing dye

Irradiation source kept perpendicular to spectrophotometer beam

Degradation monitored by UV-Vis absorbance

Prior to irradiation slides dipped in Rh 6G for 10 h

Control experiments carried out under similar condition

• Dye alone

• TiOx film alone

• Degussa P25 film

• Bulk PbSe/TiOx film

• PbSe NC film

• PbSe NC/SnOx film

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Photocatalysis resultsAbsorbance

Energy level alignment

Degradation with time using 4 nm PbSe and 400 nm light

• Bulk PbSe cannot show activity since CB of PbSe lower in energy than TiOx

• CB of PbSe (< 8 nm in diameter) raises above CB of TiOx due to quantum confinement effect as shown in figure

• CB of PbSe (< 4 nm in size) lies at 0.2 eV above CB of TiOx

• Photogenerated electrons of PbSe are expected to fall into the CB of TiOx

• Optimum activity observed with 3 or 4 layers of TiOx

• Loss of activity at more TiOx layers is due to surface reaction sites being distant from where charges are generated

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Photocatalytic reaction at 400 nm Photocatalytic reaction at 600 nm

Photocatalytic activity at different wavelengths

• Only PbSe nanocrystal/TiOx shows considerably activity

• PbSe nanocrystal/TiOx shows 30% degradation at 600 nm

• No considerable reduction in concentration during photolysis and other control experiments

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• Rhodamine has an oxidation potential of +1.225 V (vs Ag)

• Valence band of PbSe will be at a potential of +0.18 V (vs Ag)

• Holes formed in PbSe cannot oxidize Rhodamine

• To rule out that PbSe is responsible for degradation PbSe/SnO2 was prepared

• PbSe and PbSe/SnO2 found to be inactive in visible light

• It shows electrons transferred to TiO2 alone responsible for activity

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• TiO2 degrades by reducing O2 to OH radicals which reacts with various molecules

• Nitrogen purging completely quenches the degradation of rhodamine by PbSe NC/ TiOx

• This shows that only electrons transferred to TiOx responsible for degradation

Mechanism involved

Mechanism involved

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Activity dependence on particle size and wavelength

Activity was observed with light upto 650 nm

Activity dependence on wavelength of light used Activity dependence on wavelength of light and size of PbSe

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Summmary

• PbSe nanocrystal/TiOx heterointerfaces can extend the photocatalytic activity of TiO2 upto 650 nm

• Various experiments confirm that photogenerated electrons which transfer from PbSe to TiO2 are responsible for the activity