1. 1 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

2. 2 Photocatalysis TiO 2 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

3. 3 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 TiO 2 because of quantum confinement effect Studying such systems help in developing tunable photocatalytic systems based on our requirements One such system is PbSe/TiO 2

4. 4 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 atm Heated between 180 to 210  C Trioctylphosphine selenium Flask 1Flask 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

5. 5 Preparation PbSe nanocrystal / TiO x 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 H 2 O for 1 min Steps 3 and 4 repeated for additional layers of TiOx Rinsed ethanol PbSe/SnO 2 prepared using tin isopropoxide in isopropanol

6. 6 Structure and compositon of PbSe NC/TiO x 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

7. 7 XPS and EDX spectra Presence of the various elements confirmed by XPS EDX shows that TiO x deposited even on places without PbSe NCs XPS spectra of various samples Various spots where EDX was taken EDX spot B EDX spot A

8. 8 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

9. 9 XRD pattern Only PbSe peaks observed No peaks corresponding to TiO 2 observed Therefore prepared TiO 2 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 TiO 2

10. 10 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 TiO x film alone Degussa P25 film Bulk PbSe/TiO x film PbSe NC film PbSe NC/SnO x film

11. 11 Photocatalysis results AbsorbanceAbsorbance 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

12. 12 Photocatalytic reaction at 400 nmPhotocatalytic reaction at 600 nm Photocatalytic activity at different wavelengths Only PbSe nanocrystal/TiOx shows considerably activity PbSe nanocrystal/TiO x shows 30% degradation at 600 nm No considerable reduction in concentration during photolysis and other control experiments

13. 13 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/SnO 2 was prepared PbSe and PbSe/SnO 2 found to be inactive in visible light It shows electrons transferred to TiO 2 alone responsible for activity

14. 14 TiO 2 degrades by reducing O 2 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

15. 15 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

16. 16 Summmary PbSe nanocrystal/TiOx heterointerfaces can extend the photocatalytic activity of TiO 2 upto 650 nm Various experiments confirm that photogenerated electrons which transfer from PbSe to TiO 2 are responsible for the activity