1. TiO2 Review 2. Nitrogen Incorporation in TiO2

Presented By:

Anuradha Verma

TiO2 is an effective photocatalyst

Honda–Fujishima -water splitting using TiO2 electrode early

1970s.

- When TiO2 irradiated with UV light- electrons and holes are

generated.

- Photogenerated electrons reduce water to form H2 on a Pt

counter electrode

- Holes oxidize water to form O2 on the TiO2 electrode

(with some external bias by a power supply)

Photocatalyst

Photocatalyst is the substance which can

modify the rate of chemical reaction using light irradiation.

Desirable Properties Drawbacks

Stable in electrolyte with wide pH range

Properly aligned

band edges with the redox level of water

High recombination of

electron/hole pairs

Large bandgap (Eg - 3.27 eV),

so can absorb in UV light.

• Reference Paper:

Doped-TiO2: A Review, Zaleska A. , Recent

Patents on Engineering, 2008, 2, 157-164

Visible light-activated TiO2 can be

prepared by:

- Metal-ion implantation (Cu, Co, Ni, Cr, Mn, Mo, Nb, V, Fe, Ru, Au, Ag, Pt)

- Reduction of TiO2

- Nonmetal doping (N, S, C, B, P, I, F)

- Sensitizing of TiO2 with dyes - Composites of TiO2 with semiconductor having lower band gap energy e.g. Cd-S particles

Super-Hydrophilic

• When the surface of photocatalytic film is exposed to light, the contact angle of the phtocatalyst surface with water is reduced gradually.

• After enough exposure to light, the surface reaches super-hydrophilic,does not repel water at all, so water cannot exist in the shape of a drop, but spreads flatly on the surface of the substrate.

• Enable the dust particles to be swept away following the water stream, thus making the product self-cleaning.

Mechanism TiO2 absorbs UV radiation from sunlight - produce pairs

of electrons and holes.

Electron of VB becomes excited when illuminated by light.

Excess energy of this excited electron promoted the electron to the CB

Creation of negative-electron (e-) and positive-hole (h+) pair.

Stage is referred as the semiconductor's

‘ photo-excitation ' state. Wavelength of the light necessary for photo-excitation

is:

Photocatalytic mechanism is initiated by absorption of photon

hv1 with energy ≥ Eg of TiO2 (~3.3 eV) producing an

electron-hole pair on the surface of nanoparticle

Excited-state electrons and holes-

• can recombine, dissipate the input energy as heat

• Get trapped in metastable surface states

• React with electron donors and electron acceptors

adsorbed on the semiconductor surface or within the

surrounding electrical double layer of the charged

particles.

After reaction with water, these holes can produce hydroxyl

radicals with high redox oxidizing potential.

Metal Doping-

Dispersion of metal nanoparticles in the TiO2 matrix.

Electron can be excited from the defect state to the

TiO2 CB with hv2 .

Benefit of metal doping: - Improved trapping of electrons to inhibit electron-hole recombination during irradiation. - Enhanced photoactivity.

TiO2 Doped with Nonmetals

1. Band gap narrowing: N 2p state hybrids with O 2p states in anatase TiO2 doped with N (energies are very close)

Eg of N-TiO2 is narrowed - Able to absorb visible light.

2. Impurity energy level: TiO2 oxygen sites substituted by nitrogen atom form isolated impurity energy levels above the valence band.

Illumination with visible light excites electrons in the impurity energy level.

Preparation Methods

• ion-assisted sputtering,

• ion-implantation,

• chemical vapor deposition (CVD)

• sol-gel etc.

High-voltage metal ion-implantation method:

• Electronic properties of TiO2 was modified by bombarding with high energy metal ions.

• The metal ions (Cr,V) were injected into deep bulk of the TiO2 with energy 150- 200 keV

• Calcined in oxygen at 450- 475°C.

• Photocatalysts work effectively for decomposition of NO into N2 and O2 under visible light (> 450 nm)

Fe-doped-TiO2

Prepared by: hydrothermal method

Degradation of dye in aqueous solution under UV and visible light

V-doped TiO2

• Prepared by sol-gel method

• Red-shift in the UV-vis spectra and has higher activity in photodegradation of dyes under visible light than pure TiO2

N-TiO2 powders

• Higher photocatalytic activity for oxidation of CO and C2H6 than

standard TiO2 in the visible region

Advantages of Using Sol Gel Method

• Does not require complicated instruments

• Provides simple and easy means for preparing nano-

size particles.

• The incorporation of an active dopant in sol during

gelation stage allows the doping elements to have a

direct interaction with support, therefore, material

possesses catalytic or photocatalytic properties.

Surface Doping

Metal-doped nanoparticles, utility as

Stabilizing ingredients within cosmetics to prevent degradation from sun light

Agriculture, horticulture and veterinary medicine

Coatings for plastics

Environmentalprotection.

Reference Paper- Nitrogen Incorporation in TiO2: Does ItMake a

Visible Light Photo-ActiveMaterial? Viswanathan B., Krishanmurthy, K. R.; International Journal of Photoenergy, 2012

Heteroatom (S, C, F, P, B etc) substitution (doping / implantation)

o Generates extra allowed energy levels in

the wide band gap of TiO2

Promote absorption of visible light photons

Alternate pathways for the electron-hole

recombination

What we know from this paper

1. Chemical nature of the substituted and interstitial nitrogen

2. Net effect observed in shifting the absorption edge of the semiconductor

3. Net changes observed in the photocatalytic activity of substituted systems

State of N in titania and its effectiveness

in extending the light absorption edge

depend upon way it is introduced

(preparation methods/techniques)

Preparation Methods of N-TiO2

• Sol-gel method

• Reaction with ammonia

• Plasma Treatment

• DC magnetron sputtering of TiN followed by oxidation.

• Electrochemical Anaodization

• Low ion implantation method

Fujishima et al, 2008 showed plasma-enhanced CVD

yields substitutional N while sol-gel method, annealing in NH3 and chemical methods produces interstitial N.

XPS Technique

• Probes the core level binding energies

of the constituent species

• Value of the binding energy is a

reflection of the valence state and

charge density around each of the

atoms.

What is Concluded? -Valence state of N− anion

But some reports only Ti–N bonding.

1. N− -then the valence state of Ti has to be different from Ti4+, but

not accounting for the valence state of Ti.

2. N-1s-binding energy ∼396-397 eV, present when the N content

in substituted systems is very small.

Increasing N content peak ∼400 eV appears which is normally

considered to be due to chemisorbed molecular species or

interstitial N or due to the nitrogen of the precursor species

employed for N substitution in TiO2.

3. N - assume anionic states (as is generally believed) then

nitrogen-1s-binding energy should be ∼ 394 eV ,can also

be expected on the basis of electronegativity difference

between that of Ti and N.

N- cationic state, it should be ∼ 400 eV which is less

likely on the basis of size and charge.

Ti–N bond- assume covalent character, the observed

nitrogen-1s-binding energy can vary with extent of loading

and possibly account for the variations in binding energies.

4. The species like Ti-N and Ti-O-N shown by XPS not by

XRD

Means that the surface layers have a non-native

configuration as compared to the native configuration

that is present in the bulk of the material.

-photocatalytic activity of the surface should be different

from bare TiO2

5. XPS peaks at 396–398 eV – substitutional N

400–402 eV - interstitial N

Though the exact chemical nature is not clear.

Theoretical Studies on N Substitution

Calculation of the density of states:

1. N 2p states give rise to allowed energy states

just above the VB.

2. 3d states of the metal provide allowed energy

levels near the CB.

3. Transition from the allowed 2p states of

nitrogen to the conduction band accounts for

the visible light absorption

4. N 2p and O 2p states hybridize and resulting

in narrowing of Eg.

No clearance in what state nitrogen is introduced in TiO2.

No correlations exist between the method adopted for N incorporation and the type of N in the lattice (substitutional or interstitial)

Photocatalytic N- TiO2

N-doped TiO2 have not shown considerable enhancement of the

decomposition of water by increasing absorption in the visible range