Control and Characterization of Titanium
Dioxide Morphology: Applications in
Surface Organometallic Chemistry
By
Gabriel Jeantelot
In Partial Fulfilment of the Requirements
For the Degree of
Masters of Science
KAUST Catalysis Center (KCC) King Abdullah University of Science and Technology
Thuwal, Kingdom of Saudi Arabia May 2014
2
EXAMINATION COMMITTEE APPROVALS FORM This thesis of Gabriel Jeantelot is approved by the examination committee:
Committee Chairperson Dr Jean Marie Basset
Committee Co-chairperson Dr Samy Ould-Chikh
Committee member Dr Kazuhiro Takanabe
Committee member Dr Aurélien Manchon
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© May 2014 Gabriel Jeantelot
All Rights Reserved
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Abstract
Control and Characterization of Titanium Dioxide Morphology: Applications in Surface Organometallic
Chemistry
Surface Organometallic Chemistry leads to the combination of the high activity
and specificity of homogeneous catalysts with the recoverability and practicality of
heterogeneous catalysts. Most metal complexes used in this chemistry are grafted on
metal oxide supports such as amorphous silica (SiO2) and γ-alumina (Al2O3). In this
thesis, we sought to enable the use of titania (TiO2) as a new support for single-site
well-defined grafting of metal complexes. This was achieved by synthesizing a special
type of anatase-TiO2, bearing a high density of identical hydroxyl groups, through
hydrothermal synthesis then post-treatment under high vacuum followed by oxygen
flow, and characterized by several analytical techniques including X-ray diffraction,
transmission electron microscopy, infrared spectroscopy and nuclear magnetic
resonance. Finally, as a proof of concept, the grafting of vanadium oxychloride (VOCl3)
was successfully attempted.
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Acknowledgements
I would like to thank Prof. Jean-Marie Basset and Dr. Samy Ould-Chikh for
their advisory along my thesis, as well as Dr. Julien Sofack-Kreuzer for his precious
help with labwork in general and the use of high vacuum lines in particular, Dr. Edy
Abouhamad for performing the nuclear magnetic resonance analyses we required, and
the general KAUST Catalysis Center community for welcoming me as one of its
members.
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Table of Contents
Copyright…………………………………………………………………………...3
Abstract……………………………………………………………………………..4
List of Abbreviations……………………………………………………………..7 List of Figures……………………………………………..……………………….8 I – Context…………………………………………………………………....……10 1) Uses of TiO2 in conventional heterogeneous catalysis……...…………….10 2) Uses of TiO2 in photocatalysis……………………………………………..15
3) Targeted catalysts and photocatalysts……………………………………...18 4) Application of Surface Organometallic Chemistry to Titanium Dioxide….20
II - Properties of Titanium Dioxide………………………………....…….….23 1) General properties……………………………………………………….…23 2) Surface chemistry of the different crystalline facets exposed on anatase….24 3) Synthesis of anatase nanocrystals with morphology control………….…...29
III - Objectives and Strategy……………………………………………….….35
IV - Experimental Section……………………………………………………...36 1) Hydrothermal syntheses…………………………………………………....36 2) Dehydroxylation protocol…………………………...………………….….38 3) Grafting protocol………...………………………………………………...38 4) Analytical techniques……………………………………………………....39
V - Results and Discussion……………………………………………………..41 1) Morphology and bulk properties…………………………………………..41
2) Textural properties………………………………………………………....43 3) Sample stability during dehydroxylation post-treatment…………………..44 4) Characterization of surface chemistry……………………………………..48 5) Fluorine removal…………………………………………………………...57 6) Grafting…………………………………………………………………….61
VI – Conclusion…………………………………………………………………..66
Bibliography………………………………………………………………………67
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List of Abbreviations
BET: Brunauer-Emett-Teller physisorbtion theory DFT: Density Functionnal Theory DRIFTS: Diffuse Reflectance Infrared Fourier Transform Spectroscopy FT-IR: Fourier Transform Infrared Spectroscopy HETCOR: Heteronuclear Correlation HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital MAS: Magic Angle Spinning NMR: Nuclear Magnetic Resonnance ODH: Oxydative Dehydrogenation SOMC: Surface Organometallic Chemistry TGA: Thermogravimetric Analysis XRD: X-Ray Diffraction XRF: X-Ray Fluorescence
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List of Figures
Figure 1: General scheme of propane oxidative dehydrogenation and side reactions.
Figure 2: General scheme of photocatalytic water splitting.
Figure 3: Surface vanadium species of interest.
Figure 4: Surface chromium species of interest.
Figure 5: General scheme of surface-bound metal complexes.
Figure 6: Infrared spectra of SBA silica dehydroxylated at 200°C and 700°C under
high vacuum
Figure 7: Clean anatase {101} surface
Figure 8: Clean anatase {001} surface
Figure 9: Hydroxylated anatase {001} surface
Figure 10: Wulff construction of an anatase crystal
Figure 11: Titanium dioxide morphology variation in hydrothermal syntheses
Figure 12: X-ray diffractograms of samples {001}-anatase and {101}-anatase
Figure 13: Transmission electron micrographs of {001}-anatase and {101}-anatase
Figure 14: N2 physisorbtion isotherms and BJH plots of {001}-anatase and {101}-
anatase
Figure 15: X-ray diffractograms of {001}-anatase-NH4OH in function of
dehydroxylation temperature
Figure 16: Average (001) and (100) coherent domains of {001}-anatase-NH4OH in
function of dehydroxylation temperature
Figure 17: Transmission electron micrographs of {001}-anatase-NH4OH in function
of dehydroxylation temperature
Figure 18: BET surface area of {001}-anatase-NH4OH in function of dehydroxylation
temperature
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Figure 19: Thermogravimetric analysis of {001}-anatase-NH4OH
Figure 20: 19F NMR of {001}-anatase-NH4OH in function of dehydroxylation
temperature
Figure 21: 1H NMR of {001}-anatase-NH4OH in function of dehydroxylation
temperature
Figure 22: Double-quanta 1H NMR of {001}-anatase-NH4OH in function of
dehydroxylation temperature
Figure 23: Double-quanta and triple-quanta 1H NMR of {001}-anatase-NH4OH
dehydroxylated at 200°C
Figure 24: 1H-19F heteronuclear correlation NMR of {001}-anatase-NH4OH
dehydroxylated at 300°C
Figure 25: DRIFTS spectra of {001}-anatase-NH4OH in function of dehydroxylation
temperature
Figure 26: DRIFTS spectra of {001}-anatase-NH4OH before and after isotopic
exchange with D2O
Figure 27: XPS spectra of {001}-anatase and {001}-anatase-NH4OH
Figure 28: IR spectra of {001}-anatase depending on surface fluoration
Figure 29: IR spectra of {001}-anatase before and after VOCl3 grafting
Figure 30: 1H NMR spectra of {001}-anatase before and after VOCl3 grafting
Figure 31: RAMAN spectra of {001}-anatase before and after VOCl3 grafting
Figure 32: 51V NMR spectra of {001}-anatase after VOCl3 grafting
Figure 33: proposed structure for the surface vanadium species obtained by grafting
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I - Context
Titania, or titanium dioxide, is a widely used material, produced industrially in
scales of several million tons each year,1 mostly for its optical properties: it is indeed
the most widely used white pigment, owing to its very high refractive index (2.496 for
rutile vs. 2.419 for diamond) and high reflectivity in the visible range. Its current
applications are mainly as a pigment for paints and coatings, plastics and papers.
In the recent years, an increasing interest has been brought towards its chemical
properties: the oxygen atoms in titanium dioxide are labile, their removal leading to
oxygen vacancies and reduced Ti3+.2 As a result, it can take part in a number of oxidative
processes used in heterogeneous catalysis. Furthermore, being a semiconductor, it can
also be used in photocatalytic processes.
1) Uses of TiO2 in conventional heterogeneous catalysis
Several heterogeneous catalytic systems use titanium dioxide as support. Examples of
such systems include:
a) Common reactions
-Nitrogen oxide selective catalytic reduction by ammonia, catalysed by TiO2-
supported metal oxides, is used for the reduction of NO emissions from fixed sources,
by reducing it with ammonia to gaseous N2, according to the following equation:
NO+NH3+1/4 O2 → N2+1.5 H2O
Several catalytic systems are available for this reaction, many of them using
metal oxides supported on TiO2, such as Vanadium oxide and mixed Vanadium-
Tungsten oxides,3–7 or Manganese and Manganese-Cerium mixed oxides.6,8–10
11
- Gas phase and liquid phase Wacker process consist of the oxidation of
ethylene to acetaldehyde or terminal alkenes to ketones, according to the following
equation:
Several catalytic systems are available for this reaction, most using Pd nanoclusters
supported on V2O5-impregnated metal oxides, such as silica, alumina and titania.86-88
Among those supports, titania has been shown to provide higher activities, as well as
slower deactivation, owing to the high resistance to sintering of the vanadium oxide
monolayer on titania88.
- Ethane oxidation to acetic acid can be performed using titanium dioxide
supported molybdenum and vanadium, either simply impregnated on the support 90,91,
or as Keggin-structured heteropolyacids containing both Mo and V atoms89
- Methane oxidation to methanol has been carried out in hydrothermal
conditions, using hydrogen peroxide as the oxidizer, and a catalytic system consisting
of Au-Pd nanoparticles supported on titanium dioxide.21,22
- Steam reforming of methane can also be achieved, yielding hydrogen and
carbon monoxide through the following equation :
CH4 + H2O � CO + 3H2
Most catalytic systems used for this reaction are constituted of nickel nanoparticles on
a metal oxide support, and are progressively deactivated by coke deposition93. Recent
works have shown that the use of titanium dioxide as a support strongly reduces coke
deposition during the reaction.92
R C ½ O2
CH2
H
R CCH3
O
+
12
b) Focus on alkane oxidative dehydrogenation
Alkane oxidative dehydrogenation is the conversion of alkanes or alkyl groups
into alkenes, using an oxidizer such as O211–16 or CO2
17,18. The general scheme of the
reaction in the presence of dioxygen is the following:
CxHy + ½O2 → CxHy-2 +H2O
i) Thermodynamics
Currently, alkanes conversion to alkenes is done by catalytic dehydrogenation,
an endothermic process (ΔH°=124 kJ.mol-1 for propane conversion to propylene and
hydrogen) that has to be carried out at high temperatures (~600°C), in which an alkane
molecule will dissociate into an alkene and a dihydrogen molecule.
The use of an oxidizer to generate H2O instead of H2 makes the reaction
exothermic (ΔH°=-161.6 kJ.mol-1 for propane and oxygen conversion to propylene and
water), allowing it to be performed at lower temperatures,
Figure 1 : general scheme of propane oxidative dehydrogenation and side reactions
H2C CH3 H2C CH3
CO, CO2
k1
k3k2
CH3 CH3
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The main challenges in achieving that reaction are both overcoming the low
reactivity of the C-H bond (ΔH298=411 kJ/mol for propane weakest (terminal) C-H
bond94) and avoiding C-C bond activation (ΔH298=372 kJ/mol for propane C-C94),
which would lead to complete oxidation to CO and CO2.
This reaction has mostly been studied for two alkanes: ethane and propane,
yielding respectively ethylene and propylene. Three families of catalytic systems are
available for each.
ii) Catalysts
-Noble metal catalysts
Noble metal catalysts generally consist of platinum, alone113,115,116 or in
association with other metals (Pt/Sn99,110,112,115,116, Pt/Rh111, Pt/Ga112), deposited on a
ceramic monolith (alumina, silica). Other metals such as Pd114, Rh113 and Ru113 have
also been used. These catalysts have achieved yields up to yields up to 20% for propane
conversion to propylene, and up to 50% for ethane to ethylene.110
-Non reducible metal oxides
These catalysts generally consist of an alkali earth metal oxide, such as CaO or
MgO97,117,118, promoted by alkali metal oxides (e.g Li)117,118, halogens (e.g Cl)117 or rare
earths (e.g Dy)117. They generally operate at temperatures above 600°C.
Yields up to 23.8%117 have been reported for propane oxidative
dehydrogenation to propylene with these catalysts.
Alkali-earth doped rare earth oxides such as SrxLaNdOy have also shown
activity for ethane oxidative dehydrogenation. 96
14
-Reducible transition metal oxides
It is on this family of catalysts that most alkane oxidative dehydrogenation
studies have been conducted. They include:
-Supported reducible oxides (Vanadium11,12,14,19,20, Molybdenum15,20,
Chromium13,16 supported on TiO2 or Al2O3)
-Metal vanadates and metal molybdates (such as Mg2V2O798)
-Mixed oxides (such as Mo1V0.6Nb0.12Ox95 or TiO2-ZrO2
18)
They work at generally lower temperatures than noble metal and non-reducible
oxide catalysts (<600°C). V/TiO2 is therefore a system of high interest for alkane ODH.
iii) Mechanism and active sites on V/MOx
A Mars Van Krevelen mechanism is generally proposed as the dominant
mechanism on those catalysts.119 In such a mechanism, propane would adsorb on the
metal oxide surface. The H-abstraction is done by a lattice oxygen, and followed by a
β-H elimination with an adjacent oxygen leading to propylene. The two hydroxyl
groups generated in the previous steps will then condense, eliminating a water molecule,
and the reduced catalyst surface is finally reoxidized by oxygen from the gas feed.
In the case of supported vanadium oxide catalysts, different oxygen species are
proposed as active sites for alkane oxidative dehydrogenation: V=O isolated oxygens19,
V-O-V bridging oxygens109,137 or V-O-Support bridging oxygens138.
+ O* + HO*R-CH2-CH2-O*
R-CH=CH2
HO* + HO* H2O
+ O*R-CH2-CH3 R-CH2-CH3 O*
R-CH2-CH3 O*
R-CH2-CH2-O*
+ O*
½ O2O*
+ HO*
+ *
+ *
+ *
* active site
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2) Uses of TiO2 in heterogeneous photocatalysis
a) Photocatalytic oxidation of organic compounds
Due to its semiconductor properties, TiO2 can also take part in a number of
photocatalytic processes. One of them is the complete photooxidation of organic
compounds by dioxygen, breaking them down to water, CO2 and HCl for those
containing chlorine. This reaction is of high interest for water and air depollution and
the elaboration of self-cleaning surfaces. It has been shown to occur with aromatic
compounds (benzene, toluene, xylenes, phenol, salicylic acid)23–31 ; alcohols (methanol,
ethanol, 2-propanol)25,29,32 ; carboxylic acids (formic, acetic),29,33–35 chloroalkanes
(chloroform, dichloroethane, trichlorethylene)23,36–38 and alkanes (methane, ethane,
propane, isobutane, cyclohexane)39–41. They can be achieved on TiO2 alone,28,29,31,33–41
or in the presence of cocatalysts, such as surface bound halides,23–25 silver
nanoparticles,34 and supported vanadium oxide26,27,30,32 or molybdenum oxide32.
When only a limited amount of oxygen is present in the reaction medium, the
selective oxidation of hydrocarbons to ketones and aldehydes can be achieved. The
photocatalytic conversions of toluene to benzaldehyde42,43, styrene to benzaldehyde43
and cyclohexane to cyclohexanone44,45 have been reported. All those syntheses used
pure titanium dioxide, with no cocatalyst.
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b) Photocatalytic water splitting
Finally, a photocatalytic reaction
of high interest in which titanium dioxide
can take part is the photocatalytic splitting
of water into hydrogen and oxygen46. Its
slightly negative conduction band
potential (approx. ̠ 0.52 V vs SHE)47
makes its valence band electrons more
reducing than the H+/H2 couple (0.00V vs
SHE), allowing it to perform proton
reduction to dihydrogen when a proper cocatalyst is present on its surface. Its high
valence band potential (approx. +2,53 V vs SHE)47 makes conduction band holes more
oxydizing than the O2/H2O couple (+1.229 V vs SHE) allows it to perform water
oxydation to dioxygen with other cocatalysts. There are three main limiting factors to
titanium dioxide’s efficiency in photocatalytic water splitting :
1) Its excessively high bandgap, only allowing it to absorb light in the UV
region of the spectrum (with a 3-3.1 eV bandgap, only 3.3% of sunlight photons can
theoretically be absorbed, placing a theoretical upper limit on dihydrogen production
to 6.5 mol.m-2.h-1 in direct sunlight).
2) Direct recombination of photogenerated electron-hole pairs, without taking
part in water oxidation or reduction, therefore lowering the efficiency with which
absorbed photons are used. 100
3) Fast backwards reaction100 : water splitting into hydrogen and oxygen being
Figure 2 : general scheme of photocatalytic water splitting.
Potential (V vs SHE)
-1
0
1
2
3
H+
½ H2
½ H2O
¼ O2
TiO2 Conduction band
TiO2 Valence band
H+/H2
H2O/O
2
TiO2
H2O
e-
e-hν
Water reductioncatalyst
Water oxydationcatalyst
e-
H2O H
2 + ½O
2
hν
Overall splitting equation :
17
thermodynamically unflavoured without energy from an external source, the opposite
reaction can spontaneously proceed, consuming the obtained reaction products.
To overcome those limiting factors, different strategies are available:
-Limited absorption of solar light by TiO2 is enhanced by dye sensitization. It is
performed by grafting a stable dye on the surface of titanium dioxide with a higher
HOMO, slightly higher LUMO, and a lower overall HOMO-LUMO gap, allowing an
increased light absorption by the photocatalyst, and potentially lowering electron-hole
recombination. The absorption of a photon by the dye generates an electron-hole pair.
The electron is injected in the conduction band of TiO2, while the hole is be filled by
water oxidation on the dye.100
Currently, this is done using organic dyes (such as [Ru(dcbpy)2dpq]2+ 101 where dcpy is
2,2’-bipyridine-4,4’-dicarboxylate and dpq is 2,3-bis-(2 pirydyl quinoxaline).)
However, when U.V light is present, titanium dioxide is capable of photocatalytically
decompose organic compounds to CO2 and H2O, which will in this case result in the
degradation of the dye.108 A way of avoiding it would be through the use of inorganic
dyes, such as supported chromium oxides120,121.
-Depositing a water reduction cocatalyst on the surface of Titanium dioxide can
improve the energetic efficiency by increasing the water H2 formation rate compared to
recombination kinetics. Such a catalyst must allow the quick transfer of electrons from
the titanium conduction band, and as rapidly use them for water reduction, since
increasing water splitting rate will favour it over electron-hole recombination.
Examples of such catalysts are nanoparticles of gold, palladium, platinum, rhodium, tin,
chromium oxide or molybdenum disulfide48-50.
-Similarly, water oxidation cocatalysts will allow a faster use of holes in the
18
valence band for oxygen production. Such catalysts include such as ruthenium oxide,
iridium oxide, manganese oxide, or cobalt oxide.51–54
3) Targeted catalysts and photocatalysts
a) Alkane oxidative dehydrogenation catalysts
Surface vanadium species have
shown on multiple times both high activities
and selectivities in alkane oxidative
dehydrogenation11,12,14,19,20,94,95,98,99. Similar
efficiencies have been reported with
molybdenum species15,20,98,95. Achieving the
deposition of identical well defined vanadium or molybdenum sites on the surface of
TiO2 could provide useful insight into the exact nature of active sites involving these
elements, and lead to catalysts with improved activity and selectivity. In particular,
tetrahedral vanadium(V) oxide dimers and isolated tripodal vanadium V monomers
have been cited as possible active species.19,105,109,137
b) Photocatalytic water splitting inorganic dyes
Surface octahedral chromium (III)
species have been shown to allow titanium
dioxide to perform photocatalysis in the
visible light domain.120,121 However, this
gain of activity in the visible light was also
accompanied by a loss of activity in the UV range, that could be due to the formation
Figure 3 : targeted surface vanadium species. Left : isolated tripodal tetrahedral vanadium (V) oxide. Right : tetrahedral vanadium (V) oxide dimer.
O O O
V
O
O O
OV
O
O O
V
TiO2
Figure 4 : examples of surface chromium species of interest.
O O O
Cr
O O
TiO2
OH2
OH2H
2O
Cr
OH2
H2O
OHH2O
19
of chromium recombination centers. The single-site surface grafting of chromium could
allow the exclusive obtention of visible light sensitizing species while avoiding the
generation of surface recombination sites.
A known way of obtaining such identical and well defined grafted surface
species on a metal oxide surface is Surface Organometallic Chemistry (SOMC).
4) Application of Surface Organometallic Chemistry to titanium
dioxide
a) General principles of Surface Organometallic Chemistry (SOMC)
Surface Organometallic Chemistry
(SOMC) is a field of chemistry that studies
metal complexes bound to the surface of a
metal oxide material via oxygen atoms
bridging between the metal complex and
metal oxide support.56,57 Therefore, said
complexes are immobilized on the surface
of the material, enabling the use of
organometallic complexes in heterogeneous catalysis, especially highly electron-
deficient metal complexes that would otherwise cluster together.57,58
In order to acquire a good comprehension of catalytic reaction mechanisms
involving these complexes and achieve high selectivity, one must perform single-site
catalysis, meaning all metal complexes must be chemically identical.
Figure 5 : general scheme of a surface-bound metal complex
MS
O
Mc
R
X
OO
O
Reactive species
Active site: Metal center and ligands
Surface ligand : the surface is considered as a standard organometallic chemisttry ligand
20
b) SOMC requirements
Surface hydroxyl groups are the main grafting sites used for SOMC: graftings
are performed through a nucleophilic substitution reaction between the –OH group and
a ligand of a metal complex, such as a metal alkyl, alkoxide or halide ; with departure
of an alkane, alcohol or hydrogen halide. Therefore, the presence of a single type of
OH group on the surface of a material often means a single type of surface-bound
complex will be obtained, achieving single-site grafting.
In order to be suitable for surface grafting of organometallic complexes, a metal
oxide must therefore bear surface OH groups (preferably all identical), and no other
available nucleophile, such as physisorbed water. Amorphous silica is a suitable
material for SOMC57, and is currently the most used metal oxide for SOMC. Although
this material presents different hydroxyl groups, the selectivity for isolated silanols (Fig.
Figure 6 : Infrared spectrum of SBA silica dehydroxylated under dynamic vacuum (10-8 bar) at 200°C
(black) and 700°C (blue).
The sharp peak at 3740 cm-1 is due to O-H stretching from isolated silanols. The broad peak centered
around 3600 cm-1 is due to O-H stretching from vicinal and geminal silanols.The absorbance has been
normalized to match the isolated silanol peaks intensities
21
3) is controlled by temperature and water partial pressure. At room temperature, silica’s
surface bears mostly vicinal and geminal hydroxyls, along with physisorbed water. By
heating it up under dynamic vacuum (~10-8 bar), one can selectively remove these
species : physisorbed water is removed at 120°C, vicinal and geminal silanols are
mostly present up to 500-600°C59, and at 700 °C, isolated silanols become the largely
majoritary species, as shown by the infrared spectrum in figure 3. Surface hydroxyl
coverage is then of 1.1 OH.nm-2.
If the natures of grafting sites are well known, the amorphous nature of the
support leads to a random disposition of hydroxyl groups, and therefore varying
interactions between grafted complexes, or even complexes bound to the silica surface
by a different number of oxygen atoms (monpodal, bipodal or tripodal). A way to
control the arrangement and distances between grafted complexes as well as their
podality would be through the use of crystalline materials, owing to their fixed and
repetitive geometry. However, the surface of crystalline materials usually displays
varying surface chemical properties, depending on the exposed crystalline facet60:
atoms present on different facets display different geometrical arrangements and
degrees of coordination.
Another metal oxide used for SOMC is γ-alumina. However, this material often
bears different available crystalline facets for which few means of morphological
control are known122. Also, different hydroxyl groups are present on a same facet,
preventing grafting in a well-defined manner.102,103 It is, however, one of the only
crystalline materials on which surface organometallic graftings have been achieved.123
Presently, ways of controlling the morphology of several titanium dioxide
allotropes are known, and knowledge of the surface hydroxyl groups present on their
facets is available61–65. These studies show titanium dioxide could be a suitable
22
candidate for the preparation of a crystalline material on which single-site surface
organometallic chemistry can be performed.
II – Properties of titanium dioxide
1) General properties
Titanium dioxide is a transition metal oxide of which three different crystalline
allotropes are known : anatase, rutile and brookite. All three can be obtained pure by
sol-gel synthesis. Rutile is the most thermodynamically stable allotrope, whereas
anatase is the kinetically favored allotrope using sol-gel syntheses at basic pH. Brookite
is a relatively uncommon allotrope. In all cases, bulk oxygens are bound to three
titanium atoms, and bulk titanium atoms are hexacoordinated. Titanium centers of rutile
show a D2h symmetry, and oxygens show a C2v symmetry. Similarly, Titanium centers
of anatase show D2d symmetry, oxygens also show a C2v symmetry. Brookite is far less
symmetrical, with all Ti-O bonds around a same Ti atom having different lengths, which
confers it a Cs symmetry. Two different types of oxygen atoms are present in its
structure, both showing a Cs symmetry.
rutile anatase brookite
Crystalline
system : Tetragonal Tetragonal Orthorhombic
Unit cell :
Table 1 : crystal structures of titanium dioxide allotropes
23
To the best of our knowledge, hydrothermally synthesized rutile samples only bear {110}
and {111} facets.31,68–70 Of those two facets, water chemisorption has only been studied
on the first one. Most of those studies seem to show water does not dissociate on that
facet, or is in an equilibrium between dissociative and non-dissociative
chemisorption.71 Therefore, this allotrope does not seem fit for our purposes. In the case
of brookite, no means of morphology control are known. It will therefore not be studied
here.
Regarding anatase, three different facets are frequently seen: the {101}, {100}
and {001} facets. Several theoretical studies investigating the chemisorption of water
on each of those facets are available. There are known means of efficiently controlling
the morphology of anatase using hydrothermal syntheses, and therefore the proportions
in which each facet is present. For these reasons, the focus of the following work will
be on the surface chemistry of anatase allotrope.
2) Surface chemistry of the different crystalline facets exposed
on anatase
The four exposed facets of anatase obtained by hydrothermal synthesis are the {101},
{001}, the relatively unusual {100}, and the {110}, which, to the best of our knowledge,
has only been reported once.72
24
a) Surface chemistry of the {101} facet
― Nature and density of surface groups on dehydroxylated surface
The {101} facet is the most
common (~90% of thermodynamic
anatase surface). It exhibits both TiV and
TiVI (5 atoms.nm-2 for each) as well as µ2-
O (5 atoms.nm-2) and µ3-O (10 atoms.nm-
2) sites. It has the smallest density of
undercoordinated atoms of all facets, and
consequently the lowest surface energy, calculated around 0.4 - 0.49 J.m-2 when fully
dehydroxylated.65,73,74
According to DFT calculations, water molecules are chemisorbed on this
surface, most likely without dissociation, even though dissociated and non-dissociated
states could compete within less than 7 kJ.mol-1.61,73
― Thermal desorption of water
This facet would be the easiest to dehydroxylate, losing all water at 180°C under
1 atm of water, and being already dehydroxylated at ambient temperature under 10-2
atm of water vapor.61,73
― Nature and density of surface groups on hydroxylated surface
For coverage up to 5 H2O.nm-2, oxygen free doublets from the water molecules
Figure 7 : Clean anatase {101} surface. TiVI and TiV are respectively light and dark blue, μ3O and μ2O are respectively light and dark red. Dashed lines are bonds towards the bulk
25
molecularly adsorb non dissociatively on undercoordinated TiV atoms, leading to
surface TiV-OH2 groups. For higher coverages, these sites being saturated, water
molecules develop hydrogen bonds with the surface µ2-O sites . Saturation coverage is
then reached at 10.1 H2O.nm-2 61,73
It has also been shown by DFT that water could absorb dissociatively on defect
sites of this facet, generated by missing surface oxygen atoms when the material is
partly reduced.106,107
― Experimental evidences
The aforementioned DFT calculations predicts that the O-H stretching
vibrational motions of chemisorbed water on this surface should cause two infrared
absorption bands, respectively at 3665 and 3646 cm-1, since both hydrogen atoms of a
chemisorbed water molecule are in different positions relative to surface atoms. The H-
O-H bending mode is predicted around 1565-1646 cm−1 .61 Experimentally, two bands
at 3670-3676 and 3640-3642 cm-1 have been observed on anatase samples73, as well as
a third one at 1620 cm-1, showing similar wavenumbers to those calculated by DFT.
Furthermore, the intensity of these bands has been shown to increase relatively to other
O-H stretching bands on samples bearing a higher proportion of {101} facet. 73
Thermally programmed desorption studies conducted on thermodynamic
anatase103,104 have shown two desorption peaks, one at 370 K for coverages above 2.2
H2O.nm-2, and one at 470 K at all coverages. This can be explained by physisorbed
water desorption from the 101 facet at 370 K, followed by chemisorbed water
desorption at 470 K
Scanning Tunneling Microscope observations of hydroxylated {101} anatase
surface have shown non-dissociated water molecules adsorbed next to each other, along
TiV rows124.
26
b) Surface chemistry of the {001} facet
― Nature and density of surface groups on dehydroxylated surface
The 001 facet seems to be by far
the most interesting for our purposes. It
shows a high density of TiV sites (6,74
atoms.nm-2), along with µ2-O and µ3-O
sites (also 6,74 atoms.nm-2 for each). Its
surface energy is the highest of all three considered surfaces, at 0.88-0.98 J.m-2,65,73,74
due to its high density of insaturated atoms.
― Thermal desorption of water
This facet would retain water at much higher temperatures than the {101} : at
pH2O/P0 =10-3 , it would still bear 3.5 H2O.nm-2 at 550 K (277 °C), and would not be
dehydroxylated before 700 K (427°C). A 3.5 H2O.nm-2 coverage should in theory be
achieved at temperatures between 120 and 400°C at 1 atm water
― Nature and density of surface groups on hydroxylated surface
For coverages up to 3.5 H2O.nm-2, it bears
exclusively µ1-OH groups from
dissociative adsorption followed by lattice
Ti-O bond breaking : The formed OH
groups are in pairs, bonded by a strong
hydrogen bond, and replacing a surface
Figure 9 : partly hydroxylated (001) surface at 3.5 H2O/nm². Dashed lines are bonds towards bulk atoms. Titanium atoms are in blue, oxygen atoms are in red.
Figure 8 : Clean anatase {001} surface. Dashed lines are bonds towards bulk atoms
27
µ2-O.
― Experimental evidences
The predicted O-H vibrational wavenumbers for the surface hydroxyls
described above are 3746-3751 cm-1 for the isolated proton, and around 2300 cm-1 for
the hydrogen-bonded proton, although doubts exist regarding that last value. IR spectras
of anatase bearing some {001} facets have shown a peak measured at 3736 cm-1, related
to free O-H stretching, that increases with the proportion of {001} facet. The 2300 cm-
1 band has, however, never been reported. 61,73
Therefore, this surface seems ideal for organometallic grafting, especially
bipodal : all chemisorbed water is dissociated, surface hydroxyls would be grouped
in pairs, and the distance between the two titanium atoms beneath an OH pair is
of 3.8 Å, versus 3.1 Å for the Si-Si distance on silica, which should allow the
grafting of slightly larger metal atoms. The hydroxyl coverage is also very high, at
7 OH/nm2.
28
3) Synthesis of anatase nanocrystals with morphology control
a) General principle of hydrothermal synthesis
The sol-gel process being the only method through which efficient control of
the morphology of anatase has been achieved, all samples will be synthesised through
this method.
The way hydrothermal precipitation proceeds from metal alkoxydes is through
their hydrolysis followed by the condensation, nucleation and growth of the resulting
hydrolysis products to lead to the desired oxides.
-Hydrolysis
When a metal alkoxide M(OR)x is placed in the presence of water, the
nucleophillic character of water will lead to substitutions between the alkoxide ligands,
in ways depending on the acidity of the solution : At very low pH, aquo ligands (M-
OH2) will be present in the coordination sphere, and will be increasingly deprotonated
into hydroxo (M-OH) ligands at higher pH. Aquo or hydroxo ligand addition can also
lead to an increase in metal coordinance when it is thermodynamically favoured.
Ti OR H2O+ Ti OH HO+ R
29
-Condensation
Complexes bearing hydroxo ligands can condense through two different
mechanisms :
Olation occurs when a hydroxo ligand of a metal centre will form a bridging
OH group between two metals with the departure of a highly labile aquo group :
Oxolation occurs in the absence of aquo ligands being initially present : a
hydroxo ligand can still form a bridging OH group between two metal centres. The
proton present on the bridge is then transferred to another non-bridging hydroxo ligand,
which departs the metal as a water molecule:
-Nucleation
Once a hydroxyl group bearing metal complex able to polymerize are present
in the solution, whether the growth of a crystallite from an oligomeric specie is
thermodynamically favoured depends on two factors :
-Its bulk energy : species in the bulk of a crystalline phase are more stabilized
than in solution. Generating more bulk species by condensation is therefore
thermodynamically favoured.
-Its interfacial energy : undercoordinated species on the surface of a crystal are
less stabilized than species in solution. The increase in particle surface that
Ti TiOH H2O+ Ti TiOH
H2O+
Ti TiOH HO+ Ti TiOH
H2O+
OH
Ti TiO
30
condensation causes is therefore thermodynamically unfavoured.
Consequentlly, the condensation of dissolved species into small particles with a
high surface/volume ratio is thermodynamically unfavoured, until the particle reaches
a critical size where its bulk energy overcomes its surface energy and growth becomes
favorable. The formation of such particles, or nuclei, is called nucleation. It takes place
once the metal concentration in solution is high enough above saturation. From then on,
growth happens by addition of metal complexes from the solution on the metal oxide
surface.
b) Equilibrium shape of a crystal
Particle growth favors the most stable facets :
the crystal’s morphology will tend towards the shape
offering the lowest overall surface energy possible.
Said surface energy is given by the following
equation :
∆Gi=∑(j)γjA j
With ∆Gi (J) the difference between the
energy of a crystal made of i molecules and the energy
of the same number of molecules in the bulk of an
infinite crystal ; γj (J.m-2) the surface energy of the jth
facet of the crystal, and Aj (m2) the area of that facet.
Therefore, the crystal’s shape is calculated from the energies of its different
possible facets through what is known as the Wulff construction: for each possible facet,
a vector is drawn, starting from the center of the crystal and normal to that facet, with
Figure 10 : Projection along the [010] axis of the Wulff construction of an Anatase crystal. Red : (001) plane (0.98 J.m-2), Green : (101) plane (0.44 J.m-2), Blue : (100) plane (0.53 J.m-2)
{001}
{100}
0.9
8 J
.m-2
0.53 J.m-2
31
a length proportional to the facet’s energy. The remaining volume, enclosed within all
planes, is the ideal shape of the crystal. Consequently, morphology control is achieved
by selective stabilization of the desired facet. This can be done through the addition of
species in solution with the ability to selectively adsorb on the Lewis acidic and Lewis
basic sites that surface undercoordinated atoms constitute, changing its surface energy.
Changing the temperature at which the particle grows can also affect the chemisorption
equilibriums of species on its surface, thereby affecting its morphology.65
In the case of the anatase polymorph, The three lowest energy known facets are
the {101}, {001} and {100} facets, with calculated respective energies of 0.44 ; 0.53
and 0.98 J.m-2 when dehydrated with no species coordinated to them.65,73 These values
compare to the densities of undercoordinated atoms on each facet (respectively 5,0 ; 5,5
and 6,74 TiV.nm-2), and the Wulff construction obtained from those values also seems
to be in reasonable agreement with experimentally measured shapes of thermodynamic
anatase, as it would predict crystals with 98% (101) surface, 2% (001) surface, and no
(100) surface (shown in fig. 9).
32
c) Methods for the controlled synthesis of faceted anatase
nanocrystals.
A way of controlling the
morphology of hydrothermally prepared
anatase crystals is by addition of
fluorides in solution. Fluorides can be
absorbed on any facet’s TiV atoms,
thereby lowering their energy. However,
their stabilizing effect is much more
important on the {001} facet, and, at low
temperatures, the {100} facet. It is
therefore possible, by using high enough
fluoride concentrations, to obtain anatase with majoritary {001} facets.42,63,65,72,75–78
Aminoacids have also been successfully used for controlling the morphology of
anatase : Histidine has shown to slightly favour the {001} facet, while Glutamine and
Asparginin lead to almost exclusively {101} facets. 79
Methods for increasing the proportion of {001} facet are summarized in table 2.
Figure 11 : titania morphology variation after hydrothermal synthesis depending on synthesis temperature and fluoride concentration. Adapted from J.Mater.Chem., 22, (2012) 23906-23912
33
Table 2: Hydrothermal and solvothermal synthesis techniques for {001} majoritary anatase
Fluorides being by far the most documented and efficient way to control the
morphology of titanium dioxide in a way that favours the desired 001 facet, our
synthesis method will be the hydrolysis of tetrabutyltitanate by HF/H2O.This process
has previously been determined to lead to around 89% of 001 facet on the obtained
anatase.75
Reference Starting material
Solvent Morphology control agent
Other chemicals present
Hydrolysis temperature
Highest {001} surface
proportion
42 TiF4 tBuOH, BzOH
(F- from TiF4) / 160°C 65%
72 Ti metal H2O HF H2O2 180°C ND 75
128
129 Ti(OBu)4 / HF / 180°C 89%
65 TiCl4 H2O NaBF4 HCl 160°C 55% 76 TiF4 H2O HF / 180°C 47% 77 TiF4 iPrOH HF HCl 180°C 64%
78 TiF4 1-
octadecene 1-octadecanol (F- from TiF4)
Oleic acid 290°C ~100%
80 TiF4 BuOH HF HCl 210°C 98.7%
130 TiF4 Diethylene
glycol (F- from TiF4) 180°C 90%
131 Ti(OiPr)4 Acetic acid
1-butyl-3-methylimidaz
olium tetrafluoro
borate
H2O 25°C 31.4%
132 TiF4 H2O (F- from TiF4) Disodium
EDTA 200°C 22.8%
133 Ti metal H2O HF 180°C ND
79 TiCl4 H2O Histidine HCl,
NaOH 60°C ND
34
III – Objectives and Strategy
Our long term objective is to achieve stable, well-defined and uniform grafting
of metal complexes on the surface of titania. We must therefore obtain a specific titania,
with a high surface area and bearing an important density of identical TiV-μ1OH surface
hydroxyl groups.
As the {001} facet of anatase seems to possess the desired surface chemistry
towards water chemisorption, showing exclusively dissociative chemisorption and high
coverage and stability of the resulting hydroxyl groups, it will be the main focus of our
efforts.
In order to minimize the influence of other facets and obtain a high density of
surface hydroxyls, we will first attempt to synthesize pure anatase, with a high surface
area, dominated by the {001} facet.
We will then apply different pre-treatments to the obtained anatase samples in
order to selectively remove its surface physisorbed water without causing sintering or
morphology reconstruction. Sample stability and surface chemistry will be studied
depending on the pre-treatment conditions. For comparison, thermodynamic anatase
will be studied in the same conditions. Finally, as a proof of concept, we will attempt
the grafting of metal complexes, such as VOCl5.
35
IV – Experimental section
1) Hydrothermal syntheses
a) {001}-anatase
Anatase with majoritary {001} facets exposed was successfully synthesised
through the following method75: tetrabutyl orthotitanante (Ti(OBu)4), 560 mL, was hy-
drolysed with 90 mL 47% hydrofluoric acid in water. Hydrolysis ratio was 2 H2O : 1
Ti : 1.2 F. Product then underwent hydrothermal treatment in a 1 l autoclave at 180°C
for 24h for the crystallization to occur. Once the hydrothermal phase was finished, the
product was rinsed three times with 500 mL water, to remove the large excess of HF.
This sample will be referred to as “{001}-anatase”.
In order to remove remaining surface fluorides, sample {001}-anatase-H2O was
prepared by repeatedly rinsing sample {001}-anatase with milli-Q water, until the con-
ductivity of the rinsing solution became lower than 10 µS.cm-1.
Sample {001}-anatase-NH4OH was prepared by rinsing sample {001}-anatase
ten times with a 1M aqueous solution of ammonia, followed by repeated rinsing with
milli-Q water, until the conductivity of the rinsing solution became lower than 10
µS.cm-1.
Samples {001}-anatase-base-100°C were prepared by autoclaving 5 g of sam-
ple {001}-anatase-NH4OH in 500ml of a 1M aqueous base at 100°C for 24h, followed
by repeated rinsing with milli-Q water, until the conductivity of the rinsing solution
became lower than 10 µS.cm-1.The base was chosen among NH4OH, KOH and NaOH.
They produced respectively the samples {001}-anatase-NH4OH-100°C, {001}-ana-
tase-KOH-100°C and {001}-anatase-NaOH-100°C
36
b) {101}-anatase
Thermodynamic anatase (with majoritary {101} facets) was successfully syn-
thesized through the following method75 :
A 55 mL titanium oxychloride starting solution was made by hydrolyzing dropwise 20
ml TiCl4 with 25 ml H2O in an ice bath (H2O:Ti 2.5:1). Separately, a 545 mL solution
of 8.4 M NH4OH and 0.9 M NH4Cl was prepared. Both solution were then mixed under
vigorous stirring. Precipitation immediately occurred. Final pH was measured at 10.36
The precipitate was then centrifuged out, and placed back in suspension in 600
mL milli-Q water. Measured pH was 10.002, conductivity was 18.2 mS.cm-1. The sus-
pension was then autoclaved at 180°C for 24h for the crystallization to occur. The prod-
uct was then repeatedly rinsed and centrifuged until the conductivity of the rinsing so-
lution was below 10 µS.cm-1. This sample will be referred to as “{101}-anatase”.
2) Dehydroxylation protocol
Dehydroxylations were performed by placing the sample in a quartz tube
with an adapter on each end and a frit in the middle. A dynamic vacuum of 10-8 bar is
then applied using a high vacuum line, and the sample is heated at the desired
temperature using a tubular oven. The tube is then opened in a glove box, where some
of its content can be separated for further characterization.
Due to the labile nature of titanium dioxide’s lattice oxygens, samples
obtained after dehydroxylation were partly reduced. Reoxidations were therefore
performed by applying to the same setup a flow of 20%O2 and 80% Ar mix, dried over
a molecular sieve.
37
3) Grafting protocol
Grafting was performed by placing 0.3g of the titanium dioxide support in
suspension in 10 ml of dry pentane under an inert atmosphere. An excess of vanadium
oxychloride (5 ml) is then added to the suspension. The reaction medium is left under
agitation for 24 hours. The pentane containing the excess metal is then filtered out using
a cannula, and the support is washed five times using additional dry pentane. The
sample is then dried at room temperature under dynamic vacuum (10-8 bar) for 12 hours,
and finally collected and kept under inert atmosphere and protected from UV light.
4) Analytical techniques
DRIFTS infrared spectras were acquired using a Nicolet 6700 FT-IR spectrometer with
an airtight argon-filled IR cell with ZnSe windows.
1H MAS solid-state NMR spectra were acquired on a 600 MHZ NMR spectrometer
with 2.5mm rotor under 20 KHz MAS spinning frequency, number of scans=8,
repetition delay = 5s
2D 1H-1H double quantum and 1H-1H triple quantum NMR spectras were acquired on
a 600 MHz NMR spectrometer under 22KHz MAS spinning frequency with a back-to-
back recoupling sequence, number of scans =32, repletion delay =5 s number of t1
increments =128, with the increment set equal to one rotor period of 45.45 μs
19F MAS solid-state NMR spectra were acquired using direct polarization (DP) on a
38
600 MHZ NMR spectrometer with 2.5 mm rotor under 20 KHz MAS spinning
frequency, number of scans=2000, repetition delay = 15s
2D CP/MAS 1H-19F HETCOR NMR spectra were acquired with contact times of 0.2
ms under 20 kHz MAS, number of scan per increment = 2000, repetition delay = 4 s,
number of t1 increments = 32, line broadening = 80 Hz
51V MAS NMR as determined by variation of the spinning frequency was acquired on
900 MHz NMR spectrometer under a 20 KHz and 18 KHz MAS spinning frequency,
repetition delay =0.25s
X-ray diffractograms were acquired on a Bruker D8-Advance diffractometers
Thermogravimetric Analysis was performed using a Mettler Toledo TGA/DSC1
STARe System. Ramp rate : 10°C/min, air flow : 50 ml/min
BET surface area and nitrogen adsorption measurements were done using a
Micromeritics ASAP 2420 surface area and porosity analyser.
39
V – Results and discussion
1) Morphology and bulk properties
The XRD patterns of {101}-anatase and {001}-anatase-NH4OH are diepicted
on figure 12. For both samples, XRD showed exclusively anatase diffraction peaks
(Fig.1) . The particles dimensions, measured along the [004] and [200] directions, were
respectively 12 nm and 69 nm for sample {001}-anatase-NH4OH, against 35 and 24
nm for sample {101}-anatase, indicating the expected anisotropy.
20 40 60 80
36.0 36.5 37.0 37.5 38.0 38.5 39.0 39.5 46.5 47.0 47.5 48.0 48.5 49.0 49.5 50.0
{103} {112}
{101}-anatase {001}-anatase-NH
4OH
Inte
nsity
(A
.U)
2θ (deg)
{004} {200}
Figure 13: X -ray diffractograms of the as-synthesised samples
Sample {001}-anatase {101}-anatase
ε[001] (nm) 12 35
ε[100] (nm) 69 28
Table 3: Particle dimensions along the [001] and [100] axis of the particles, determined by XRD.
40
Transmission Electron Microscopy showed for both samples that the {001} and
{101} facets were the only facets exposed, as shown in figure 2 : Miller indexes of
different spots on fast Fourrier transforms can be deduced from the distances between
atom planes. The only facets present on the particles are perpendicular to the [001] and
[101] directions. Particle maximum width and length measured along the [100] and
[001] axes are respectively 20 and 29 for sample {101}-anatase, and 4.7 and 43 nm for
sample {001}-anatase-NH4OH, showing 81% {001} surface has been obtained.
The difference between the dimensions of sample {001}-anatase measured on
TEM micrographs and those determined by X-ray diffraction can be explained by the
observed stacking of particles with their atom planes aligned, increasing the observed
coherent domain on X-ray diffraction.
Figure 14 : a and b : TEM images of respectively {101}-anatase and {001}-anatase,
with facet attributions based on Fast Fourier Transforms.
Sample {101}-anatase {001}-anatase
Avg. {100} width (TEM) (nm) 20 43
Avg. {001} width (TEM) (nm) 29 4.7
Theoretical surface area (TEM) (m²/g) 54 130
Table 4 : Particle dimensions measured by TEM and calculated surface area
41
2) Textural properties
Textural properties of the samples were investigated by nitrogen physisorption.
The surface areas of {001}-anatase and {101}-anatase were determined to be respec-
tively 92 m2.g-1 and 58 m2.g-1. Pore volumes were respectively of 0.54 and 0.34 cm3g-1
The measured surface area of {001}-anatase, 30% lower than that calculated
from crystallite dimensions, can be explained by the fact that particle aggregation ren-
ders part of the surface inaccessible to nitrogen molecules.
BJH plots show that most pores on sample{001}-anatase are approxiamtely 40
nm wide, corresponding with the length of anatase platelets, while the H1 hysteresis
shows no slit-like pores. The porous volume would therefore be located between plate-
let aggregates.
Most pores of sample {101}-anatase are within 20 to 30 nm, corresponding with
particle dimensions. The porous volume would therefore be located between individual
crystallites.
0.0 0.2 0.4 0.6 0.8 1.00
100
200
300
400
500 {001}-anatase {101}-anatase
Qty
ads
orbe
d (c
m3 /g
)
P/P0
0 200 400 600 800 10000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
dV/d
D (
cm3 /g
)
Pore diameter (Å)
{001}-anatase {101}-anatase
Figure 15 : N2 physisorption isotherms (left) and BJH plots on desorption branch (right) of {001}-anatase and {101}-anatase
42
3) Sample stability during dehydroxylation post-treatment
In order to remove physisorbed water from the obtained samples, a dehydrox-
ylation post-treatment has to be applied. Similarly to what is done to silica, samples
were heated under dynamic vacuum (10-8 bar). The stability of sample {001}-anatase-
NH4OH under those conditions is studied here.
X-ray diffractograms showed no changes in the coherent domains of the sam-
ples for dehydroxylation temperatures up to 400°C, followed by a morphology recon-
struction starting between 400°C and 500°C. This is shown by an increase in the co-
herent domains on both the {100} and {001} directions, as well as an increase in the
e(001)/e(100) ratio, indicating an evolution of morphology towards that of thermody-
namic anatase. No rutile formation could be seen.
This was confirmed by TEM images, on which anatase crystallites show distinct
octahedral geometries after dehydroxylation at 500°C and reoxidation. In addition to
that, sample surface reconstruction can be seen starting between 350°C and 400°C, as
indicated by the presence of clear patches on the crystallites. No changes can be seen
below 350°C.
43
20 40 60 80
500°C 400°C 350°C 300°C 200°C 100°C
Inte
nsity
(A
.U)
2θ
Dehydroxilation temperature :
Figure 16 : X-ray diffractograms of {001}-anatase-NH4OH after heating under dynamic vacuum (10-8 bar) for 20h at varying temperature, and reoxidation for 10h in 20% O2 in Ar at 200°C
0 100 200 300 400 50014
16
18
20
22
24
26
(004
) co
here
nt d
omai
n [n
m]
e(004) e(200) e(004)/e(200) ratio
Post-treatment temperature
48
50
52
54
56
58
60(1
00) co
here
nt d
omai
n [n
m]
Stable morphologyMorphologyreconstruction
0.0
0.1
0.2
0.3
0.4
Rat
io
Figure 17 : Average coherent domains along the (001) and (101) directions of {001}-anatase-NH4OH (measured from XRD data) in function of dehydroxylation temperature
44
Figure 18: TEM images of {001}-anatase-NH4OH a) at 25°C, and after
dehydroxylation at b) 200°C, c) 300°C, d) 350°C, e) 400°C and f) 500°C
a) b)
c) d)
e) f)
45
Nitrogen adsorbtion shows a decrease in sample surface area starting between
350°C and 400°C, followed by a decrease in total porous volume starting between 400
and 500°C. This can be explained by an increase in particle aggregation between 350°C
and 400°C, followed by the morphology reconstruction previously shown by TEM and
XRD between 400°C and 500°C.
0 100 200 300 400 5000
10
20
30
40
50
60
70
80
90
100
BET surface area Total porous volume
Temperature (°C)
SB
ET (m
2 /g)
0.0
0.2
0.4
0.6
0.8
1.0
Vto
t (cm
3 /g)
Figure 19 : Evolution of sample {001}-anatase-NH4OH surface area and porous volume in function of dehydroxylation temperature under vacuum (20h, 10-5 bar)
Thermogravimetric analysis shows an important weight loss between room tem-
perature and 120°C, which can be explained by the departure of physisorbed water. It
is followed by a slower weight loss between 120°C and ~550°C, which we attribute to
water elimination from the condensation of surface hydroxyl groups. A small increase
in weight loss slope is seen between 450°C and 500°C. This can be attributed to mor-
phology reconstruction, causing a decrease in the number of surface hydroxyl groups.
46
200 400 600
-4
-2
0W
eigh
t los
s (%
)
Temperature (°C)
Phy
siso
rbed
w
ater
loss
Water loss from surfacehydroxyl condensation
Figure 20 : Thermogravimetric analysis of sample {001}-anatase-NH4OH (ramp rate : 10°C/min, flowing air 10ml/min)
4) Characterization of surface chemistry
a) NMR spectroscopy
Several NMR techniques were used to determine the nature of the different
surface species present on the {001}-anatase-NH4OH sample : 19F NMR, 1H NMR,
1H double and triple quantum NMR, and 1H-19F heteronuclear correlation NMR.
19F NMR showed a single peak at -123.45 ppm that does not seem to be
affected in any way by post-treatment temperature. It is believed, from XPS results
shown in figure 27 and for reasons detailed below, that this peak corresponds to
surface fluorides.
47
Figure 21 : 19F NMR of sample {001}-anatase-NH4OH after dehydroxylation (20h,
10-8bar, varying temperature) and reoxydation (20h, flowing 20% O2 in Ar, 200°C)
Proton NMR, on the other hand, shows a main peak which intensity decreases
as post-treatment temperature increases. It is, however, still present after the sample
has been dehydroxylated at 500°C. Its chemical shift, starting at 5.67 ppm,
progressively increases up to 6.98 ppm as the post-treatment temperature is increased.
A smaller peak is also present at 0.8 ppm, and shows no noticeable change in
shift or intensity.
48
Figure 22: 1H NMR of {001}-anatase-NH4OH after dehydroxylation (20h, 10-8bar,
varying temperature) and reoxydation (20h, flowing 20% O2 in Ar, 200°C)
In order to get more informations on the nature of those peaks, 2D 1H-1H
correlation (double quantum and triple quantum) NMR spectras were acquired (fig. 11).
They show that on the samples where post-treatment temperature did not exceed 100°C,
protons on the 5.67 ppm peak are within correlation distance from each other (<5Å),
indicating the contribution of species bearing two protons in close proximity. The
correlation dissapears when the post-treatment temperature is of 200°C or above. The
0.8 ppm peak also appears in double quantum NMR, regardless of post-treatment
temperature.
49
Triple quantum NMR of the {001}-anatase-NH4OH sample after post-treatment
at 200°C shows that the peak at 0.8 ppm is due to species bearing at least three protons.
Remaining butanol from the hydrolysis of Ti(OBu)4 could explain its presence.
Figure 23 : Double quantum 1H NMR of {001}-anatase-NH4OH: a) Room temperature
sample, and after dehydroxylation at b) 100°C, c) 200°C, d) 300°C followed by
reoxydation at 200°C in flowing 20% O2 in Ar.
Figure 24 : 1H NMR of {001}-anatase-NH4OH dehydroxylated at 200°C under vacuum
and reoxidized :double quantum (left) and triple quantum (right)
a) 25°C b) 100°C
c) 200°C d) 300°C
50
To determine whether surface hydroxyl groups were in physical proximity with
fluorine groups, proton-fluorine heteronuclear correlation spectras were acquired.
No correlation was observed on {001}-anatase-NH4OH after post-treatment at
200°C. However, the more fluorine-rich sample {001}-anatase-H2O showed a 19F-1H
correlation with protons exhibiting a 5.8 ppm shift, while its 19F spectrum still showed
the same single peak at -123.45 ppm. (fig. 13)
The value of the 1H NMR main peak’s chemical shift (5.67 ppm) falls in the
range of what is expected for physisorbed water. Indeed, no additionnal peak is
observed when no dehydroxylation post-treatment above 100°C was applied to the
sample, and water is therefore present, as confirmed by double quantum NMR.
However, the peak is still present after the sample was dehydroxylated at temperatures
from 200°C and up to 500°C, where no physisorbed water can be present, as confirmed
by double quantum NMR. Therefore, this peak should be constituted of at least two
contributions : that of physisorbed water (when post-treatment temperature are below
200°C), and that of chemisorbed water.
19F-1H correlation NMR of the {001}-anatase-H2O after dehydroxylation at
200°C shows that protons of the 5.8 ppm peak of that sample are within correlation
distance (5 Å) of the fluorine atoms. Since no noticeable difference in the nature of the
fluorine and hydrogen atoms between the dehydroxylated {001}-anatase-H2O and
{001}-anatase-NH4OH can be seen from either their 1H and 19F NMR spectras, as well
as their IR spectras (shown below), it can be supposed that the absence of 1H-19F
correlation observed on {001}-anatase-NH4OH could be caused by an excessively
weak signal from the fluorine atoms rather than excessive distance between them. The
51
increase in number of fluorine atoms on the surface of {001}-anatase-H2O would then
make the correlation strong enough to be visible.
Figure 25: 1H-19F heteronuclear correlation of {001}-anatase-H2O after
dehydroxylation (20h, 10-8bar, 300°C) and reoxidation (10h, flowing 20%O2 in Ar,
200°C)
52
b) Infrared spectroscopy
Infrared spectras of sample {001}-anatase-NH4OHwere made after the samples
were dehydroxylated at 100, 200, 300, 350, 400 and 500°C ; and then reoxidized at
200°C in a flow of 20% oxygen in argon.
The main features of these spectras are one peak at 3666 cm-1 that can be at-
tributed to surface O-H stretching,61,73 and a broad peak at 2388 cm-1. The baseline
change of the sample dehydroxylated at 500°C is due to incomplete reoxidation of the
oxygen vacancies
Other features are a minor O-H stretching peak at 3717 cm-1 and, two small
peaks at 3487 and 3397 cm-1 that can be attributed to N-H stretching from residual
ammonia,127 a peak at 1618 cm-1, that can be attributed to H-O-H bending from phy-
sisorbed water (below 200°C) or non-dissociatively chemisorbed water 61,73. All peaks
below 1600 cm-1 do not appear on sample {001}-anatase-H2O, which was not rinsed
with ammonia. They can therefore be attributed to nitrogen species: a peak at 1560 cm-
1 can correspond to surface NH2 bending from residual ammonia125, and a peak at 1352
cm-1 could be explained by the presence of nitrate ions from the oxidation of said am-
monia126.
The breadth of the 3150 cm-1 would indicate protons engaged in hydrogen bonds
with other nearby atoms. That could be due to surface hydroxyl groups bonding with
surface oxygen atoms, with surface fluorines, or with each other. Another explanation
would be the presence of titanium vacancies in the bulk and on the surface of TiO2,
charge-compensated by protons.
53
4000 3500 3000 2500 2000 1500 1000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
500°C 400°C 350°C 300°C 200°C 100°C
Abs
orba
nce
Wavenumber (cm-1)
Dehydroxylation temperature :
4000 3500 3000 2500 2000 1500 10000.0
0.5
1.0
1.5
1353 cm-1
1620 cm-1
300°C
3152 cm-1
3397 cm-1
3487 cm-1
Abs
orba
nce
Wavenumber (cm-1)
3666 cm-13717 cm-1
Figure 26 : Top : Infrared spectras of {001}-anatase-NH4OH after dehydroxylation (20h, 10-8bar, varying temperature) and reoxydation (10h, flowing 20%O2 in Ar, 200°C)
Bottom : Infrared spectrum of {001}-anatase-NH4OH after dehydroxylation (20h, 10-8bar, 300°C) and reoxydation (10h, flowing 20%O2 in Ar, 200°C)
To determine the accessibility of protons constituting each IR peak, an
isotopic exchange experiment was conducted: the sample {001}-anatase-NH4OH was
first dehydroxylated at 300°C in a dry flow of 20% Oxygen in Argon. A first infrared
spectrum was acquired. We then proceeded to the isotopic exchange with D2O: the
54
sample temperature was brought down to 150°C, and the oxygen/argon flow was first
bubbled in deuterium oxide before passing through the sample. The exchange was al-
lowed to occur for 24 hours. The sample was then dehydroxylated again at 300°C in a
dry flow of 20% oxygen in argon. A second infrared spectrum was acquired. Both spec-
tras are reported in figure 15.
The results show that every peak previously attributed to O-H stretching (seen
here at 3715 ; 3661 and 3170 cm-1), as well as the peak attributed to H-O-H bending
(seen here at 1622 cm-1) all shifted simultaneously at respectively 2737, 2701, 2388 and
1569 cm-1. This would indicate that all protons on the sample are accessible to deuter-
ium cations.
4000 3500 3000 2500 2000 15000
1
2
3
3715 cm-1
1266 cm-1
1432 cm-1
1510 cm-1
1569 cm-1
2388 cm-1
2701 cm-1
1560 cm-1
1622 cm-1
3170 cm-1
Before isotopic exchange After isotopic exchange
Abs
orba
nce
(offs
et)
Wavenumber (cm-1)
3661 cm-1
3715 cm-1
2737 cm-1
Figure 27 : In-situ DRIFTS spectrum of {001}-anatase-NH4OH after dehydroxylation at 300°C under flowing 20% O2 in Ar (black) ; followed by isotopic exchange of with D2O, and dehydroxylated again at 300°C under flowing 20% O2 in Ar (red)
55
5) Fluorine removal
As the synthesis was carried out in a fluorinated medium, it is expected that
remaining fluorine atoms will still be present on the surface
Particle dimensions Width (along (001) axis) : 4,68 nm
Length (along (100) axis) : 42,8 nm
Surface area
Total surface area 134 m2/g
001 surface area 109 m²/g (81%)
101 surface area 25 m²/g (19%)
Surface fluorides
Maximum surface fluorides (One F on each TiV)
1,47 mmol.g-1 / 2,8 %wt.
Max. (001) surface fluo-rides
1,25 mmol.g-1 / 2,4 %wt.
Max. (101) surface fluo-rides
0,22 mmol.g-1 / 0,4% wt.
Elementary analysis showed that the sample {001}-anatase, on which no wash-
ing was applied initially contained 6.5% wt. fluorine. This very high value could be
explained by the possible presence of physisorbed HF on the surface of the sample, or
by the presence of fluorides in the bulk of the material.
To determine whether all fluorides are on the surface or some are in the bulk,
an XPS analysis was performed. According to several publications, XPS of fluorinated
TiO2 allows to differentiate surface fluorides from bulk fluorides. The first give an F 1s
signal at 684.1 eV, while the second give an F 1s signal at 689.6 eV.65,76,77,81–83
56
The spectrum obtained from the as-synthesised sample showed a single peak at
684.1 eV, indicating only surface fluorides are present, in amounts large enough to
completely saturate the surface.
Two different methods for removing fluorides from the surface of titanium di-
oxide have been reported. The first one is calcination at 600°C for 90 minutes63,77,84,85.
It was attempted by spreading the {001}-anatase in an aluminum oxide crucible,
and heating it in a muffle furnace at 600°C for 90 minutes, under static air, with a ramp-
ing rate of 5°C/min, then letting it cool down in open air. XRD analysis showed the
morphology was highly altered by the treatment, making it inapplicable to the samples
studied here.
The second method is rinsing with an aqueous base24,75,82, to substitute fluoride
ions by hydroxyl ions. Here, aqueous ammonia was chosen as the base, as it is easier to
remove from metal oxide surfaces than alkali metals. The sample was rinsed 10 times
with a 1M solution of ammonia, then with water until σ < 10 μS.cm-1, to remove the
ammonia. As a comparison standpoint, an other sample was thoroughly rinsed with
pure water only, until σ < 10 μS.cm-1.
Elemental analysis showed {001}-anatase-H2O, rinsed exclusively with water
contained 2.6% wt. fluorine, a value in accordance with the one predicted in table 2 for
fully fluorinated TiO2. The ammonia-rinsed sample {001}-anatase-NH4OH contained
1% wt. fluorine. This value could fit with the fluorination of half of the titanium atoms
present on the 001 face. XPS again showed a single peak at 684.1 eV, though of lower
intensity than previously.
57
695 690 685 680 675
6000
8000
{001}-anatase {001}-anatase-NH4OH
Inte
nsity
(A
.U)
Binding energy (eV)
684.1 eV F 1s
Figure 28 : XPS spectra of {001}-anatase and {001}-anatase-NH4OH
Further defluoration could be achieved by autoclaving the obtained {001}-ana-
tase-NH4OH at 100°C in solutions of other bases. The effect of those treatments on
fluoride concentration are reported below, in table 3.
58
Sample name Rinsing method Fluorine content (wt. %)
Other ele-ments (wt.%)
{001}-anatase No rinsing 6.5
{001}-anatase-H2O
Rinsing with water until σ<10 µS.cm-
1 2.6
{001}-anatase-NH4OH
Rinsing with 1M ammonia (10x), then water until σ<10 µS.cm-1
1.0 N : 0.14
{001}-anatase-NH4OH-100°C
Rinsing with 1M ammonia (10x), then autoclaving with ammonia (100°C, 24h), then rinsing with water until σ<10 µS.cm-1
0.69 N : 0.10
{001}-anatase-KOH-100°C
Rinsing with 1M ammonia (10x), then autoclaving with 1M KOH (100°C, 24h), then rinsing with water until σ<10 µS.cm-1
0.61 K : 0.33
{001}-anatase-NaOH-100°C
Rinsing with 1M ammonia (10x), then autoclaving with 1M NaOH (100°C, 24h), then rinsing with water until σ<10µS.cm-1
0.42 Na : 0.15
To determine the effect of surface fluorination on surface hydroxyls, the infrared
spectra of sample {001}-anatase-H2O ; {001}-anatase-NH4OH and {001}-anatase-
NaOH-100°C were recorded after dehydroxylation at 300°C and reoxydation.
The results, reported in figure 17, show that lowering the fluorine concentration
first increases the intensity of the 3666 cm-1 O-H stretching band. The 3717 cm-1
stretching band then starts appearing at ~1% wt. fluorine, and increases in intensity as
the fluorine concentration is lowered to 0.42% wt.
59
4000 3500 3000 25000.0
0.5
1.0
1.5
3152 cm-13674 cm-1
{001}-anatase-NaOH-100°C (0.42% F) {001}-anatase-NH4OH (1.0 % F)
{001}-anatase-H2O (2.6 % F)A
bsor
banc
e
Wavenumber (cm-1)
3717 cm-1
Figure 29 : Infrared spectras of {001}-anatase- NaOH-100°C, {001}-anatase-NH4OH and {001}-anatase-H2O after dehydroxylation (20h, 10-8 bar, 300°C) and reoxydation (10h, flowing 20%O2 in Ar, 200°C)
6) Grafting of VOCl 3
Metal grafting on the surface of {001}-Anatase was done using vanadium (V)
oxychloride (VOCl3), following the protocol described above, in part IV.3.
Elemental analysis shows a 1.81% wt. vanadium loading, with 2.4 wt.% chlo-
rine. This corresponds to a 1.9 Cl:V molar ratio, which could indicate an exclusively
monopodal grafting of vanadium.
Infrared spectroscopy (DRIFTS) of the {001}-anatase-NH4OH sample shows
that grafting exclusively occurred on the OH groups constituting the 3666 cm-1 peak,
60
which disappears completely. For reasons yet to be explained, the intensity of the broad
peak at 3152 cm-1 increases after grafting.
4000 3500 3000 2500 2000 1500 10000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5 After Grafting Before Grafting
Abs
orba
nce
(offs
et)
Wavenumber (cm-1)
Figure 30 : Infrared spectras of {001}-anatase-NH4OH before and after VOCl3 grafting
1H NMR of the sample shows a decrease and shift in the main peak, going from
5.7 to 6.8 ppm. This would indicate that the protons from the 3666 cm-1 IR peak are
contributing to the 5.7 ppm NMR peak, without being the only contribution to it. The
0.8 ppm peak remains unchanged.
61
Figure 31 : 1H NMR of {001}-anatase-NH4OH before and after VOCl3 grafting
To further characterize the nature of the grafted vanadium species, Raman and
51V NMR spectras of the sample were acquired.
Raman spectroscopy shows the apparition of a single peak at 1026 cm-1, which
can be attributed isolated V=O stretching135.
51V NMR shows a single peak at -581 ppm with multiple rotation bands. This
chemical shift is within the expected range for inorganic vanadium V(V) compounds
(for comparaison, [VO4]3- and [V2O7]4- complexes show shifts of respectively 535 ppm
and 559 ppm, while VOCl3 is the standard defined as a 0 ppm shift.).136
62
300 400 500 600 700 800 900 1000 1100 1200
Eg
B1g
+A1g
After grafting Before grafting
Inte
nsity
(A
.U, s
moo
thed
)
Wavenumber (cm-1)
1026 cm-1
B1g
Figure 32 : Raman spectrum of {001}-anatase-NH4OH before and after VOCl3 grafting
Figure 33 :Magic Angle Sample Spinning 51V NMR of {001}-anatase-NH4OH after
VOCl3 grafting at 20 and 18 KHz spinning rate
63
Figure 34: Proposed structure for the surface vanadium species obtained by grafting
O
V
O
O
Cl
Cl
Tis
OO
O
O
64
VI – Conclusion
The long term objective of this research is to achieve stable, well-defined and
uniform grafting of metal complexes on the surface of well-defined planes on titania.
We have first obtained a specific isotrope of titania (anatase), with a high surface area
(92 m2/g ) dominated by the {001} facet (81%) and bearing a relatively important
density of TiV-μ1OH surface hydroxyl groups (according to IR data).
We then applied different pre-treatments to the obtained anatase samples in
order to selectively remove its surface physisorbed water without causing sintering or
morphology reconstruction. Sample stability and surface chemistry have been studied
depending on the pre-treatment conditions. For comparison, thermodynamic anatase
has been studied in the same conditions. Finally, as a proof of concept, we have
attempted successfully the grafting of VOCl3. The surface structure, according to
analyses, is a monopodal Ti-O-[V(V)(O)Cl2].
The research will continue in several directions:
Improvement of the fluorine removal process to obtain clean fully defluorinated
samples
Study of the reactivity of this supported in propane oxidative dehydrogenation.
Study of the photocatalytic activity of this material for water splitting with several co-
catalysts.
65
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