TiO2

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

TiO2

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