About non metal doped TiO

(5) Di Valentin C. at al. J. Phys. Chem. B 109 (2005) 11414 UNIVERSITA’ DEGLI STUDI DI TORINO

TiO ₂ visible light sensitization by non metal elements doping.

About non metal doped TiO

.

TiO2represents one of the most common oxides employed in photocatalys both in the liquid and gas phase, even though its wider use is partially hampered by the band gap energy ( 3.2 eV for the Anatase) which limits TiO2applications to the presence of U.V.

light⁽¹⁾. In recent years great efforts have been spent to increase its visible light absorption. While an initial attention was paid to TiO2doping by transition metal ions, more recently many research groups point to introduce P-block elements like ( N, C, S, F, … ) in the oxide matrix^(2,3). Despite the efforts spent till now to investigate the non metal-doped TiO2system many aspects remain yet not clear as the chemical nature of the non metal containing sites correlated to the visible light absorption properties and also a full satisfactory description of the light absorption mechanism has not been reached yet.

Synthesis and spectroscopic features.

Stefano Livraghi

, Maria Cristina Paganini

, Anna Malgorzata Czoska

, Elio Giamello

,

Cristiana Di Valentin

, Emanuele Finazzi

, Gianfranco Pacchioni

.

1)

1) DipDip. Chimica IFM, Università degli Studi Torino, via Giuria 7, 1012. Chimica IFM, Università degli Studi Torino, via Giuria 7, 10125 Torino, Italy 5 Torino, Italy and NIS Centre of Excellence Nanostructured

and NIS Centre of Excellence Nanostructured InterfacesInterfacesand and SurfacesSurfaces.. 2)

2) DipDip. di Scienza dei Materiali,. di Scienza dei Materiali,Università degli Studi Università degli Studi MilanoMilano--BicoccaBicocca, via R. Cozzi, 53 , via R. Cozzi, 53 --20125 Milano20125 Milano

EPR has evidenced the existence of paramagnetic bulk species intimately interacting with the TiO2lattice whose spin-Hamiltonian parameters are in excellent agreement with those calculated for both substitutional (Ns) and interstitial (Ni) N centres ( generically named Nb) in TiO2(4).

EPR evidences of N-doped TiO

and correlation with DFT characterization.

67 67 87 87 54 ρ ρpp

11.8 11.8 14.5 14.5 13.0 a aisoiso

33.4 33.4 1.8 1.8 0.2 0.2 N Nintint

38.2 38.2 2.8 2.8 2.5 2.5 N Nsubsub

32.3 4.4 2.3 Exp

A A33 A A22 A A11

Nb(Sub.)

Nb(Int.)

(4) S. Livraghi at al. Chem. Comm. 4, (2005), 498

Theoretical and experimental EPR data comparison for the species Nb•.

TiO2ÆTiO2-x+ x Vo + ½x O2

ANATASE RUTILE 4.2 eV 4.3 eV 0.6 eV 1.0 eV TiO2-2xN2xÆTiO2-3xN2x+ x Vo + ½x O2

In both cases (Nsand Ni) nitrogen doping favours the oxygen vacancies formation

Vacancies formation correspond to electrons injection into the TiO2lattice which consequence is Ti³⁺generation ( Ti⁴⁺+ e^-→Ti³⁺).

the “Dives in Misericordia”

church in Roma TiO2coating of the

Church’s walls A street in Segrate (Mi)

TiO₂coating of the road surface Two examples of TiO₂

outdoor application.

These applications use sun light to activate photocatalitic processes but they are partially hampered because of UV light represents 5%

only of the incoming solar radiation

E

V.B.

C.B.

E

V.B.

C.B.

Ti³⁺

V.B.

C.B. Ti³⁺

V.B.

C.B.

Nb•

paramagnetic Nb−

diamagnetic

20G TiO2⇌ TiO2-x+ x Vo + ½x O2 +xe^- 20G

reduction

oxidation

Ti⁴⁺+ e^-ÆTi³⁺

Ti³⁺+ Nb•ÆTi⁴⁺+ Nb-

Nb•ÅNb-

as produc ed

red 1 ox 1 red 2 ox 2

red 3 ox 3

red 4 ox4 --

cycle 4 cycle 3 cycle 2 cycle 1

I (a.u.)

Thermal treatment

N_b^•

3220 3290 3360 3430

N-doped TiO

2

Sim.

Exp.

(N_b^•) g₃

B/Gauss EPR spectrum and related simulation of species Nb•.

This poster is devoted to rationalize paramagnetic species in N and F-doped TiO

end their effects on the oxide electronic structure

EPR evidences of F-doped TiO

. EPR evidences of N,F-codoped TiO

.

DFT results nicely fit with EPR behaviours of species Nbduring cycles of thermal Red-Ox treatment ⁽⁷⁾.

DFT elaboration shows that most stable electronic configuration in both cases ( Nsor Ni) corresponds to diamagnetic condition. Experimental results confirm that both Nb•and Nb-coexist in N-TiO2(6).

(6) S. Livraghi at al. J. Am. Chem. Soc. 128 (2006) 15666

3300 3350 3400 3450 3500 3550

F-doped TiO2

O^-g= 2.005//

g⊥= 2.013

g_//= 1.961 g⊥= 1.991Ti³⁺

sim.

exp sim.

x5exp

B/Gauss ^{3300 3350 3400 3450 3500}

sim.

exp.

(N_b^•) g₃ N,F-codoped TiO₂ Ti³⁺

B/Gauss EPR spectrum and related simulation of T3+and O-in F-doped TiO2.

Unpaired electron ≡ EPR active

P-block doped TiO2were synthesized via Sol-Gel method using NH4Cl, NH4F or HF as non metal dopants sources. All samples show anatase structure and N doped materials show yellow colour and are able to absorb visible light while F doped system is white as the pure TiO2. Photocatalytic activity of yellow materials has been tested by methylen blue degradation.

400 500 600

TiO₂ N,F-TiO₂ N-TiO₂ F-TiO₂ F(R∞)

λ (nm)

TiO₂N-TiO2N,F-TiO2 F-TiO₂

white pale yellow yellow white

Excitations from these localized states explain the visible light absorption of the yellow N-TiO2 material and the consequent activity in visible light⁽⁵⁾.

EPR spectrum and related simulation N, F-codoped TiO2.

EPR has evidenced that F doping leads to simultaneous stabilization of Ti³⁺ centres and a small and negligible amount of second signal ascribed to O^- species. Ti³⁺ species is unambiguously located in the bulk ( the species is not affected by any dipolar broadening effect when the spectra are recorded in O2atmosphere ).

Preliminary theoretical calculations confirm Ti³⁺

stabilization due to fluorine doping⁽⁸⁾.

(7) Di Valentin C. at al. ChemPhys. (2007) in press (1) O. Carpat al. Prog. Solid st. Chem. 32 (2004) 33 (2) R Asahiat al. Science 293 (2001) 269 (3) N. Serponeat al. J Phys. Chem. B 110 (2006) 24287

Unlike from what observed in N-TiO2, in N,F codoped TiO2 the most stable electronic configuration does not corresponds to a diamagnetic state.

0 1 2

N,F-Anatase N-Anatase Anatase Methylen Blue I/I0 (arb.u.)

Time (h)

Ti³⁺ is an unreactive centre. In the presence of an electron scavenger as O2 no superoxide anion (O2-) species was observed.

Ti³⁺

V.B.

E C.B.

In N,F-TiO2 EPR has evidenced the same species observed in the single- doped cases (Ti³⁺ centres and Nb•

species). Also in this case both species are located in the bulk oxide and Ti³⁺does not react with oxygen

Ti³⁺

V.B.

C.B.

Nb•

E

Methylen blue degradation under Blue light (437nm) of Nitrogen containing materials.

TiO₂ photocatalytic mechanism.

Photons excite electrons from valence band to conduction band. Electrons (e^-) and holes (h⁺) can react with adsorbed molecules.

400 500 600

F(R∞)

λ (nm) TiO2 p-block doped TiO2 Vis

U.V

+ -

D D⁺ A

A^- C.B.

V.B.

Visible region U.V.

region

⎯P-block doped TiO₂

⎯TiO₂

(8) See Di Valentinet al. Communication (this conference)

•Both dopant elements lead to intra band gap states. Ti³⁺is not able to give rise electron transfer neither on N states or to adsorbed species.

•Yellow colour of N,F codoped TiO2 arise from intra bend gap states due to nitrogen as in the case of N-doped system.

• Role of fluorine related states in photocatalytic propertiesÆ work in progress….