Titanium Dioxide Nanomaterials

Titanium DioxideNanomaterials
TOC
i. Introduction
ii. Synthetic Methodsiii. Properties
iv. Modifications
v. Applications
vi. Summary
2011. 07.26 (Tue)Kouji Yasuda (M1)

Titanium Dioxide Nanomaterials

i. Introduction2

Titanium Dioxide Nanomaterials

Where TiO2 is Used?

pigment

toothpaste

paintsunscreen

ointment3

Titanium Dioxide Nanomaterials

TiO2 as Photo Catalysis

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TiO2 as Photo Catalysis

1, TiO2 absorbs Ultraviolet radiation from sunlight.
2, Generated negative-electron (e-) and positive-hole (h+) pair.
3, Water-splitting reaction occurred.
4, Organic pollutant is split up to harmless products.
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Titanium Dioxide Nanomaterials

Why TiO2 Nanomaterials Are Attracted now?
 Among the unique properties of nanomaterials, themovement of electrons and holes in semiconductor
nanomaterials is primarily governed by the well-known
quantum confinement, and the transport properties related
to phonons and photons are largely affected by the sizeand geometry of the materials.
 It has possibility to help solving many serious
environmental and pollution problems.
 It also bears tremendous hope in helping ease the energy
crisis through effective utilization of solar energy basedon photovoltaic and photocatalytic water-splitting devices.
 As another functionality of TiO2 nanomaterials, it has
many elements adsorbent capacity.
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Applications of TiO2 Nanomaterials

adsorbents

photovoltaics

photocatalysis

electrochromics

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ii. Synthetic Methods
i. Sol-Gel Method
ii. Micelle and Inverse Micelle Methodiii. Sol Method
iv. Hydrothermal Methodv. Solvothermal Method

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Sol-Gel Method 1This process normally proceeds via an acid-catalyzed hydrolysis
step of titanium(IV) alkoxide follwed by condensation.
The development of Ti-O-Ti chains is favored with low content ofwater, low hydrolysis rates, and excess titanium alkoxide in thereaction mixture.
3-dimensional polymetric skeltones with close packing result
from the development of Ti-O-Ti chains.

Gerard M. Pajonk. et al., Chem. Rev. 2002, 102. 4243

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Titanium Dioxide Nanomaterials

Sol-Gel Method 2

Autocraving at 200 oC
2500 pKa, 5 hTi(OnBu)

4 as the titanium

source in butanol.

TiO2 nanotube synthesized byusing anodic aluminum oxide
templates (AAO)

A.; Moritz, T. et al., Eur. J. Inorg. Chem. 1999, 235.

AAO

Zeng, H. C. et al., Chem. Mater. 2002, 14, 1391 10

Titanium Dioxide Nanomaterials

Sol-Gel Method 3

TEM images of TiO2 nanoparticles prepared by hydrolysis of Ti(OR)4 in the presence oftetramethylammonium hidroxide. Muramatsu, A. et al., J. Colloid Interface Sci. 2003, 259, 53

Amines are used as the shape controllers of the TiO2 nanomaterials act assurfactants.
Amines include triethanolamine (TEOA), diethylethlenetriamine,ethylenediamine, trimethylenediamine and triethylenetetramine.
The morphology of the TiO2 nanoparticles changes from cuboidal toellipsoidal at pH above 11.
The shape control is attributed to the tuning of the growth rate of thedifferent crystal planes of TiO2 nanoparticles by the specific adsorption ofshape controllers to these planes under different pH conditions.

11Titanium Dioxide Nanomaterials

Micelle and Inverse Micelle Method

TEM images of the shuttle-like and roundshaped TiO2 nanoparticles.
Cheng, H. et al., J. Mater. Chem.2002, 12, 3677
 Experimental procedure for thesynthesis of TiO2 nanoparticles
using W/O microemulsionmediated method.

 The values of H2O/surfactant,
H2O/titanium precusor, ammonia
concentration, feed rate, and reaction
temperature were significant parametersin controlling TiO2 nanoparticle size andsize distribution.
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Sol Method The sol method is the nonhydrolytic sol-gel processes and usually involves thereaction of titanium chloride with a variety of different oxigen donor molecules,e.g., metal alkoxide or an organic ether.

To control particle size Increased nucleophilicity (ore size) of the halide resulted in smaller anatasenanocrystals, e.g., average sizes ranged from 9.2 nm for TiF4 to 3.8 nm for TiI4.
 Reaction in pure trioctylphosphine oxide (TOPO) was slower and resulted insmaller particles, while reactions without TOPO were much quicker and yielded
mixtures of brookite, rutile, and anatase with average particle sizes greaterthan 10 nm.

The most stable form13Titanium Dioxide Nanomaterials

Hydrothermal and Solvothermal Method

Schematic diagram of hydrothermal (solvothermal) synthesis setup: (1) stainless steel
autoclave (2) precursor solution (3) Teflon liner (4) stainless steel lid (5) spring14

Titanium Dioxide Nanomaterials

Hydrothermal Method(left figure) SEM images of TiO2 nanowires with the inset showing a TEM image of a single
TiO2 nanowire with a [010] selected area electron diffraction (SAED) recorded perpendicular
to the long axis of the wire. Zhang, L. D. et al., Chem. Phys. Lett. 2002, 365, 300.
(a) HRTEM images of TiO2 nanotubes. (b) Crosssectional view of TiO2 nanotubes.
N. Wang, et al., Applied Physics Letters, 2003, 82, 281
 Hydrothermal synthesis is normally conducted in steel pressure vessels called
autoclaves with or without Teflon liners under controlled temperature and/or pressure
with the reaction in aqueous solutions.
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Solvothermal Method(a) HRTEM image of an individual nanowire. (b) The
corresponding HRTEM image of the local plane of thenanowire from (a). The fringes with a spacing of 0.35 nm
correspond to (101) planes.
Liu, C. et al., New J. Chem. 2005, 29. 969

TEM micrographs and electron
diffraction patterns of products
prepared from solutions at the weightratio of (precursor / solvent /surfactant ) 1:5:3.
Seo, H. J. et al., J. Cryst. Growth 2003, 257. 309

 The solvothermal method is almost identical to the hydrothermal method except thatthe solvent used here is nonaqueous.
 The temperature can be elevated much higher than that in hydrothermal method.
 The solvothermal method normally has better control than hydrothermal method ofthe size and shape distributions and the crystallinity of the TiO2 nanoparticles.

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iii. Properties
i. Structural Properties
ii. Electronic Properties
iii. Photon-Induced Electron and Hole.

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Titanium Dioxide Nanomaterials

Structural Properties

Anatase

RutileRutileAnataseMorphology predicted for anatase (top), with (a) hydrogenated surfaces, (b) hydrogen-rich surface
adsorbates, (c) hydrated surfaces, (d) hydrogen-poor adsorbates, and (e) oxygenated surfaces, and forrutile (bottom), with (f) hydrogenated surfaces, (g) hydrogen-rich surface adsorbates, (h) hydrated
surfaces, (i) hydrogen-poor adsorbates, and (j) oxygenated surfaces.
 Two structures, rutile and anatase, can be described in term of chains of TiO6octahedra, where each Ti4+ ion is surrounded by an octahedron of six O2- ions.
 They demonstrated that, under different pH conditions from acid to basic, the phasetransition size of a TiO2 nanoparticle varied from 6.9 to 22.7 nm, accompanied withshape changed of the TiO2 nanopaeticles.
Curtiss, L. A. et al., Nano Lett. 2005, 5. 1261

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Titanium Dioxide Nanomaterials

Electronic Properties(a)(b)

(c)

(b)

(a)

(A) Total and projected densities of states (DOSs) of the anatase TiO2 structure. (B) Molecularorbital bonding structure for anatase TiO2: (a) atmic levels; (b) crystal field split levels; (c)finalinteraction states.
 The DOS (density of states) is decomposed into Ti eg, Ti t2g (dyz, dzx, and dxy), O pσ (inthe TiO3 cluster plane), and O pπ (out of the TiO3 cluster plane) components.
 It is well-known that for nanoparticles the band gap energy increase and the energy
band becomes more discrete with decreasing size.
19Freeman, A. J. et al., Phys. Rev. B 2000, 61. 7459

Titanium Dioxide Nanomaterials

Photon-Induced Electron and Hole 1






Reaction (1) is the photon absorption process.
Reactions (2) – (6) are photocatalytic redox pathways.
Reactions (7) – (9) represent the recombination channels.
Reactions (3) & (4) are the competition pathways for holes, leading to bound OH radicals
and O vacancies, respectively.
 The reverse of reaction (4) generates O adatom intermediated upon exposing defective
surfaces to O2-(g).
 Because photocatalytic TiO2 surfaces are extensively hydroxylated, the dynamics ofdefect creation and healing may depend not only on bulk composition and morphology,
but also on the nature and density of Ti-OH species.
Hoffmann, M. R. et al., J. Phys. Chem. B 2000, 104. 9842

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Titanium Dioxide Nanomaterials

Photon-Induced Electron and Hole 2

Scheme of UV induced charge separation in TiO2. Electrons from the valence band can either betrapped (a) by defect states, which are located close to the conduction band (shallow traps), or (b) in
the conduction band where they produce absorption in the IR region.


During continuous UV irradiation, photogenerated electrons were either trapped at
localized sites, giving paramagnetic Ti3+ centers, or remained in the conduction band.
Photoredox chemistry occurring at the particle surface emanated from trappedelectrons and holes rather than from free valence band holes and conduction bandelectrons.

Yates, J. T., Jr. et al., J. Phys. Chem. B 2005, 109. 606121

Titanium Dioxide Nanomaterials

iv. Modifications
i. Metal-Doped TiO2 Nanomaterials
ii. Nonmetal-Doped TiO2 Nanomaterials

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Metal-Doped TiO2 Nanomaterials

Schematic diagram to illustrate the ohotoexcitation process
under visible light of metal-doped TiO2(a) Ti1-xVxO2 , (c) Ti1-xCrxO2
 The one of the goals for improvement of theperformance of TiO2 nanomaterials is to increase
their optical activity by shifting the onset of theresponse from the UV to the visible region.

DOS of the metal doped TiO2(Ti1-xAxO2: A = V, Cr, Mn, Fe, Co, or Ni)
Gray: total DOS, Black: dopant’s DOS
Asai, K. et al., J. Phys. Chem. Solids 2002, 63, 3466.

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Titanium Dioxide Nanomaterials

Nonmetal-Doped TiO2 Nanomaterials(A) Total DOSs of doped TiO2 and (B) the projected DOSs
into the doped anion sited, culculated by FLAPW, for thedopants F, N, C, S and P located at a substitutional site for
an O atom in the anatase TiO2 crystal (eight TiO2 units percell). Ni-doped stands for N doping at an interstitial site,and Ni+s-doped stands for doping at both substitutional andinterstitional sites. R. Asahi, et al., Science 2001, 293, 269.

Reflectance measurements showing
the red shift in optical response dueto the nitrogen doping of TiO2nanoparticles.
Gole, J. L. et al., Nano Lett. 2003, 3, 1049.

 Recent theoretical and experimental studies have shown that the desired band gap
narrowing of TiO2 can also be achieved by using nonmetal dopants.
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Titanium Dioxide Nanomaterials

v. Applications
i. Photocatalytic Applications
ii. Photovoltaic Applications
iii. Photocatalytic Water Splitting
iv. Electrochromic Devices
v. Polyfunctional Adsorbent

25Titanium Dioxide Nanomaterials

Photocatalytic Applications 1Elements constructing heterogeneous photocatalysts
 Most metal oxide photocatalysts consist of metal cations with d0 and d10 configurations.
 Their conduction bands for the d0 and d10 metal oxide photocatalysts are usuallycomposed of d and sp orbitals, respectively, while their valence bands consist of O 2p
orbitals.
A. kudo, et al., Chem. Soc. Rev. 2009, 38, 253.

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Titanium Dioxide NanomaterialsPhotocatalytic Applications 2
Various parameters and quantum yields of the photocatalytic hydrogenation reactions ofCH3CCH with H2O over Rutile-Type (top) and Anatase-Type (bottom) TiO2 at 300 K.
Quantum yield = (number of photoformed products) / (number of incident photons).

RutileAnatase

Y. Kubokawa. et al., J. Phys. Chem. 1987, 91. 4305

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Photocatalytic Applications 3

Photocatalytic properties of TiO2-xNx samples(solid circles) compared with TiO2 samples(open squares). Decomposition of methylene
blue as a function of the cutoff wavelength ofthe optical high-path filters under fluorescentlight between 350 and 520 nm, compared withthe results under BL illumination with theintegrated in the UV range.
R. Asahi, et al., Science 2001, 293, 269.

The main function of the TiO2
photocatalyst is to adsorb organic andinorganic contaminants. Under theinfluence of the UV lamp they later get
decomposed into CO2 and H2O.

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Photovoltaic Applications

Principle of operation and energy level schemeof the dye-sensitized nanocrystalline solar cell.
 S : Photoexcitation of the sensitizer The dye molecule is regenerated by theredox system, which itself isregenerated at the counter-electrode byelectrons passed through the load.
Gräzel, M. et al., J. Photochem. Photobiol. A: Chem
2004, 164, 3

Electron transfer in the operating photovoltaic device.


A: Photon absorption
B: Electron transfer from S*
C: Electron conduction
D: Reduction of S*

Tang, J. et al., Nature 2003, 421, 616

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Photocatalytic Water Splitting

The principle of water splitting using asemiconductor photocatalyst.
The bottom level of the conduction
band has to be more negative thanthe reduction potential of H+ / H2 (0V vs. NHE).
The top level of the valence bandhas to be more positive than theoxidation potential of O2 / H2O (1.23V).

Kudo, A. et al., Catal. Surv. Asia 2003, 7, 31.

Strategy of the development of photocatalysts
with a visible light response.
1.

2.
3.

To form a donor level above a valenceband by doping some element into
conventional photocatalysts with wideband gaps such as TiO2.
To create a new valence band employing
some element.
To control the band structure by making asolid solution.
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Titanium Dioxide Nanomaterials

Electrochromic Devices

1sPrinciple of signal amplification by a TiO2 nanocrystalline film.

 The device could be switched back and forth between the coloress and coloredstates within 1 s.
 First reduction potential : 0.2 – 0.6 V
 Second reduction potential : 0.2 – 0.4 V
Gräzel, M. et al., Displays 1999, 20, 137.

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Polyfunctional Adsorbent ~Crystal Structure~

Layered Hydrazinium Titanate (N2H5)0.5Ti1.87O4 (LHT-9)
Projections onto (right) (001) and (left) (100). Edge-faced stacking of corner-sharing TiO6 octahedra(yellow) results in the formation of interlayer “pseudochannels” directed along the c axis. These
pseudochannels are half-occupied by hydrazinium ions (blue spheres depict the front row and grayish
spheres the back row) tailored in such a way that NN bonds are directed along the pseudochannels. The
relative sizes of the shown structural units are consistent with their actual size ratios.
 New nanohybrid compound combining the redox functionality of hydrazine, the ionexchange properties of layered titanate, the large surface area of quasi-two-dimensional
crystalities, surface Brønsted acidity, and the occurrence of surface titanyl bond.
 It possesses a high uptake capacity of ~50 elements of the periodic table.
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Wulf, D. et al., J. Am. Chem. Soc 2011, 133 (24), 9516.

Titanium Dioxide Nanomaterials

Polyfunctional Adsorbent ~Periodic Table~

Highlighted are the elements that can be adsorbed by nanocrystalline LHT-9 in aqueous solutions.
Depending on the nature of each element, different adsorption mechanisms can be proposed. The
adsorption capacity of each element is given in atom % relative to Ti.
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Wulf, D. et al., J. Am. Chem. Soc 2011, 133 (24), 9516.

Titanium Dioxide Nanomaterials

Polyfunctional Adsorbent ~Functionalities~
 Redox Properties
 LHT-9 is an effective reducing agent that has the redox properties of hydrazine
superimposed onto the adsorption properties of the nanocrystalline titanate matrix,
resulting in the phenomenon of reductive adsorption.
 EX) 4HAuIIICl4 + 6(N2H5)0.5Ti1.87O4 → 4Au0↓ + 6H0.5Ti1.87O4 + 3N2↑ + 16HCl
 The irreversibility of the majority of reduction reactions under the given redox
conditions, LHT-9 can be used for cumulative extraction of reducible elements fromtheir solutions.

 Ion Exchange Properties
 LHT-9 exhibits high rates of ion-exchange and protonation reactions.
 EX) 0.25RbICl + (N2H5)0.5Ti1.87O4 → (N2H5)0.25RbI0.25Ti1.87O4 + (N2)↑↑ + (HCl) The majority of exchange products however, were found to contain hydrazinium.

 Surface Acidity Properties Brønsted acidity is likely caused by protonation ofbridging Ti-O-Ti oxygen atoms located at loose
corners of TiO6 octahedra exposed at the surfacesof the titanate layers.

Wulf, D. et al., J. Am. Chem. Soc 2011, 133 (24), 9516.

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vi. Summary

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Summary
 TiO2 has various functionalities not apply only to be used for whitesource of paints and cosmetics expected to reflect UV light, they also
has light sensitivity and modified it has adsorption capacity.
 TiO2 nanomaterials has significant differences by nanoparticle sizeand geometry.
 It is necessary to select suitable reaction condition to synthesize
objective particle shape materials.
 Photocatalytic and photovoltaic tec. are begun to use various fields,
for example, solar panel, pollutant decomposition, superhydrophilicmaterials and water splitting, etc.
 Although TiO2 is NOT used industrially since less effectivity, today I
introduced, some solubility to improve light effectivity are studying
with all one's heart!
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