Photocatalysis: semiconductor physics

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1 Photocatalysis: semiconductor physics Carlos J. Tavares Center of Physics, University of Minho, Portugal 1

2 Guimarães Where do I come from?

3 3

4 Guimarães 4

5 Introduction>> Photocatalysis is an active, attractive, and growing area regarding the removal of organic pollutants from water and air. It is based on the photonic activation of a metal oxide (semiconductor) catalyst. In order to increase the photocatalytic efficiency of these photocatalysts, it is important to enhance their specific surface area, crystallinity and the absorption of light from the solar spectra amongst other properties. 5

6 Solar spectrum>> 6

7 Photocatalysis mechanism>> Titanium dioxide is a light-activated catalyst used in the dissociation (cleaning) of organic and inorganic materials. Bahnemann, D., Photocatalytic water treatment: solar energy applications, Solar Energy, 2004, 77,

8 Promotion of an electron to the conduction band, on irradiation by UV light, results in a hole in the valence band essentially a detriment of the electron density that was localized on that orbital, and usually assigned a positive charge to symbolize the loss of negative. The hole is powerfully oxidizing the orbital very much wants to retrieve electron density just lost after light irradiation. It can retrieve this simply by the electron in the conduction band recombining with the valence band recombination is a sum of radiative (i.e. emission may be observed) and non-radiative processes. Based on the energy gap law, the fact that rutile energy levels are closer mean that the non-radiative process is more efficient, and hence recombination is more efficient. Alternative pathways to recombination are possible, and as you can guess, these result in the use of these materials as photocatalysts. The hole has the potential to oxidize water that may be on the surface of the material resulting in the formation of hydoxyl radicals. Hydroxyl radicals are themselves very powerful oxidisers, and can easily oxidize any organic species that happens to be nearby, ultimately to carbon dioxide and water. Meanwhile, upstairs in the conduction band, the electron has no hole to recombine with, since the hole has oxidized surface bound water. It quickly looks for an alternative to reduce, and rapidly reduces oxygen to form the superoxide anion. This can subsequently react with water to form, again, the hydroxyl radical. 8

9 Titanium Dioxide>> TiO 2 holds several advantages: High chemical stability Excellent functionalization Non-toxic Cheap to synthesize Ti O However in most cases, has the disadvantage of requiring UV light to enhance photocatalysis 9

10 SC band-gap>> oxidation potential of OH is 2.8V Michael Grätzel, Nature 414, (2001) 10

Band-gap tuning>> anionic doping http://photochemistryportal.net/ Nakamura R, Tanaka T, and Nakato Y.

11 Band-gap tuning>> anionic doping Nakamura R, Tanaka T, and Nakato Y., Mechanism for visible light responses in anodic photocurrents at N-doped TiO2 film electrodes, J. Phys Chem. B., 2004, 108, (See also Irie, H et al, J. Phys Chem. B., 2003, 107, ). 11

12 Titania Density of States (DOS)>> Energy (ev) Asahi, et al., Science Vol. 293,

13 TiO 2 Band Structure Rutile Anatase R. Sanjines, et al., Journal of Applied Physics 75 (1994)

14 band-gap tuning>> cationic doping By incorporating a small amount of silver (1 5%) results in increased efficiency in photocatalysis. Silver has a Fermi level or electron accepting region at an energy just below the TiO 2 conduction band. Therefore, after light absorption and charge separation, the electron in the conduction band can be effectively trapped by the silver, while the hole oxidizes water and forms hydroxyl radicals, without the threat of recombination. 14

Nanoparticles Nitrogen doping Urea 2-PrOH

15 >>TiO 2 Photocatalytic Nanoparticles synthesis Titanium precursor + H2O:2- PrOH solution Hydrolysis Condensation SOL Thermal treatment 2h 200 C N-TiO 2 Nanoparticles Nitrogen doping Urea 2-PrOH washing and sample drying (60 C, 4h) TiO 2 Nanoparticles 15

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17 >>Band-gap measurement [F(R)h ] 0.5 (arb. units) Vis UV 3,0 3,5 4,0 4,5 h (ev) TiO 2 TiO 2 tangent TiO 2 100ºC TiO 2 100ºC tangent TiO 2 200ºC TiO 2 200ºC tangent [F(R)h ] 0.5 (arb. units) Vis UV 3,0 3,5 4,0 4,5 h (ev) TiO 2 TiO 2 tangent N-TiO 2 (1g) N-TiO 2 (1g) tangent N-TiO 2 (4g) N-TiO 2 (4g) tangent Sample Band Gap (ev) TiO2 RT 3.45 Sample Band Gap (ev) TiO2 200 C 3.20 TiO2 100 C 3.24 N-TiO2-1g 3.12 TiO2 200 C 3.20 Increasing synthesis temperature Photocatalytic activation at higher wavelengths N-TiO2-4g 2.90 Increasing dopant (urea) mass 17

18 >>Photocatalytic degradation analysis Nanoparticles immersed in a Methylene Blue solution UV light (LED 365 nm) Concentration (C/C0) 1,0 0,8 0,6 0,4 TiO2 N-TiO2 (1g) N-TiO2 (4g) 0, Time (min) Greater photocatalytic activity under UV radiation in the N-doped samples 18

19 UV-Vis reactor for photocatalysis experiments>> 19

>>Photocatalytic degradation analysis Nanoparticles immersed in a Methylene Blue solution Xenon lamp (400-900 nm) Concentration (C/C0) 1,0 0,8 0,6 0,4 0,2 TiO 2 N-TiO 2 (1g)

20 >>Photocatalytic degradation analysis Nanoparticles immersed in a Methylene Blue solution Xenon lamp ( nm) Concentration (C/C0) 1,0 0,8 0,6 0,4 0,2 TiO 2 N-TiO 2 (1g) N-TiO 2 (4g) Time (min) Mass of doping material significantly influences the photocatalytic behavior of the TiO2 nanoparticles under visible light 20

21 >>Quantification of *OH radicals Photocatalytic generated radical Coumarin + OH 7-Hydroxycoumarin Luminescent compound Intensity (counts) hydroxycoumarin 0 min 30 min 60 min 90 min 120 min 180 min 240 min 300 min 360 min 2000 UV light nm Wavelength (nm) 21

22 Crystallinity assessment (TiO 2 NPs)>> 100 Intensity (arb.units) data Rietveld simul. Spacegroup: I41/amdS Cell Volume (Å^3): Crystallite Size (nm): 13.2 Lattice parameters a (Å): c (Å): (º) 22

23 BET Analysis>> Cumulative Pore Volume (cc/g) NTiO 2 -ph3 Surface Area = m²/g Pore Volume = cc/g Pore Radius Dv(r) = إ V dv(logr) Pore Radius (angstrom) dv(logr) 23

Introduction Magnetron sputtering is very versatile tool

for optical, semiconductor, wear and corrosion

24 Introduction Magnetron sputtering is very versatile tool for the physical vapour deposition (PVD) of thin films for optical, semiconductor, wear and corrosion prevention, photovoltaic, self-cleaning, biomedical applications, to name a few. These coatings are performed in high vacuum and exhibit properties unlike the same bulk materials. 24

25 Sputtering chamber>> 25

26 HI-ERDA: level of N-doping>> Bragg peak height (arb. units) Energy (arb. units) scattered 35 Cl O N C Atomic fraction / % C,H O Ti glass N Depth / at./cm 2 26

27 XPS: detection of substitutional nitrogen 1.19 at% N 0.72 at% N C KVV N O1s Counts / a.u. C1s Ti2p O KVV Ti LMM 400 ev 396 ev N1s peak Ti O N1s Binding Energy / ev atomic β-n states binding energy difference (O1s-Ti2p 3/2 )= =72.5 ev >71.4 ev!! 21

28 Enhancement in UV-visible catalysis after thermal annealment on glass substrates>> k / min -1 0, , , ,00050 UV Vis ln(c/co)=-kt Absorbance / a.u Red 41 dye 1E-5M 0min 30min 60min 120min 180min 240min 300min 360min 0, , N 2 :O 2 flow rate / sccm / nm 28

29 Thank you for your attention

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A Plasmonic Photocatalyst Consisting of Silver Nanoparticles Embedded in Titanium Dioxide. Ryan Huschka LANP Seminar February 19, 2008

A Plasmonic Photocatalyst Consisting of Silver Nanoparticles Embedded in Titanium Dioxide Ryan Huschka LANP Seminar February 19, 2008 TiO 2 Applications White Pigment Photocatalyst Previous methods to