Chemical modification of niobium layered oxide by tetraalkylammonium intercalation

Article abstract of DOI:10.1590/S0103-50532010000700024

Chemical modification of the layered K₄Nb₆O ₁₇ material was systematically investigated through the reaction of its proton-exchanged form (H₂K₂Nb₆O ₁₇) in alkaline solutions containing tetramethylammonium (tma ⁺), tetraethylammonium (tea⁺) or tetrapropylammonium (tpa⁺) cations. The intercalated amount reaches 50percent (for tma ⁺), 25percent (for tea⁺) and 15percent (for tpa⁺) of the H₂K₂Nb₆O₁₇ negative charge (concerning the exchange at interlayer I) due to the steric hindrance of larger cations. Hexaniobate samples present (020) basal reflections equal to 23.0, 26.3 and 26.5 A? once intercalated respectively with tma⁺, tea ⁺ and tpa⁺. When samples are heated above 200-250 °C, CO₂ evolution is observed; Hofmann elimination reaction is also detected for hexaniobate-tpa⁺ samples. Scanning electron microscopy images show the predominance of plate-like particles; stick-like particles are also observed for samples containing bulky ions. The intercalation reaction is promoted in the order tma⁺ > tea⁺ > tpa⁺, while the formation of a dispersion of colloidal particles is facilitated in the inverse order.

Full text of DOI:10.1590/S0103-50532010000700024

J. Braz. Chem. Soc., Vol. 21, No. 7, 1366-1376, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0
103 - 5053 $6.00+0.00
Chemical Modification of Niobium Layered Oxide by Tetraalkylammonium
Intercalation
a
b
,a
Ana L. Shiguihara, Marcos A. Bizeto and Vera R. L. Constantino*
a
Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748,
5508-000 São Paulo-SP, Brazil
0
b
Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo - Campus
Diadema, Rua Prof. Artur Riedel 275, 09972-270 Diadema-SP, Brazil
A modificação química do material lamelar K Nb O foi investigada sistematicamente
4
6
17
através da reação de sua forma protônica (H K Nb O ) em soluções alcalinas contendo os cátions
2
2
6
17
+
+
+
tetrametilamônio (tma ), tetraetilamônio (tea ) ou tetrapropilamônio (tpa ).A quantidade intercalada
corresponde a 50% (para tma ), 25% (para tea ) e 15% (para tpa ) da carga negativa do H K Nb O
+
+
+
2
2
6
17
(
considerando a troca iônica na região interlamelar I). As amostras de hexaniobato apresentam
+
reflexões basais (020) de 23,0, 26,3 e 26,5 Å quando intercaladas, respectivamente, com tma ,
tea e tpa . Aquecendo-se as amostras acima de 200-250 C, observa-se a liberação de CO ; a
reação de eliminação de Hofmann também é observada para as amostras de hexaniobato-tpa . As
+
+
o
2
+
imagens de microscopia eletrônica de varredura mostram a presença predominante de partículas
em forma de placas; partículas em forma de bastões também são observadas nas amostras contendo
+
+
+
íons volumosos. A reação de intercalação é promovida na ordem tma > tea > tpa , enquanto a
formação de uma dispersão de partículas coloidais é facilitada na ordem inversa.
Chemical modification of the layered K Nb O material was systematically investigated
4
6
17
through the reaction of its proton-exchanged form (H K Nb O ) in alkaline solutions containing
2
2
6
17
+
+
+
tetramethylammonium (tma ), tetraethylammonium (tea ) or tetrapropylammonium (tpa )
cations. The intercalated amount reaches 50% (for tma ), 25% (for tea ) and 15% (for tpa ) of the
H K Nb O negative charge (concerning the exchange at interlayer I) due to the steric hindrance of
+
+
+
2
2
6
17
larger cations. Hexaniobate samples present (020) basal reflections equal to 23.0, 26.3 and 26.5 Å
+
+
+
o
once intercalated respectively with tma , tea and tpa . When samples are heated above 200-250 C,
+
CO evolution is observed; Hofmann elimination reaction is also detected for hexaniobate-tpa
2
samples. Scanning electron microscopy images show the predominance of plate-like particles;
stick-like particles are also observed for samples containing bulky ions. The intercalation reaction
+
+
+
is promoted in the order tma > tea > tpa , while the formation of a dispersion of colloidal particles
is facilitated in the inverse order.
Keywords: layered niobate, hexaniobate, tetraalkylammonium, intercalation
Introduction
use of chemical reactions or processes that enhance the
physicochemical interactions among the counterparts.
Considering inorganic phases, suitable reactions to
increase the interaction with organic species consist in
(i) functionalization of the inorganic surfaces through
covalent bonds with appropriated pendent groups or
(ii) ion exchange of charged species that neutralize
New materials have been prepared combining
chemical species that show unlike properties such as
organic, inorganic and biochemicals in order to develop
systems with improved or unique performance. However,
the design of these hybrid materials requires chemical
strategies to compatibilize so dissimilar species at the
nanoscopic domain. One plausible approach demands the
1
inorganic surfaces by proper ions. Chemical modification
of inorganic nanostructures or nanocrystals has deserved
1
,2
special attention in the recent literature. Organic-
inorganic and biochemical-inorganic hybrid materials
can be explored in diversified studies concerning the
*
e-mail: vrlconst@iq.usp.br

Vol. 21, No. 7, 2010
Shiguihara et al.
1367
2
+
18
preparation of nanostructured thin-films, macro- or
mesoporous solids, biomaterials, biosensors, polymer
nanocomposites, catalysts and photocatalysts, devices for
generation of photocurrent or photoluminescence, and
to generate photocurrent using [Ru(bpy)3] sensitizer,
1
9
photoluminescence from rare earth ions, molecular
hydrogen from water under illumination, protonic
2
0
2
1
+
22
conductor, Li conductor, interstratified materials with
1,2
23
24
hierarchical structures.
layered double hydroxides, acid catalysts, macroporous
2
5
26
Among the inorganic materials, the layered frameworks
canproducenanostructuredhybridmaterialsbyintercalation
of guest species between the layers or reassemblage of the
anisotropic nanosheets produced in an exfoliation process.
Layered niobates are an important class of inorganic
materials that have some interesting characteristics such as
semiconducting property, photosensitivity, acidic sites, high
aspect ratios, chemical stability in a large range of pH and
high loading capability. They are constituted of negative
niobate, biosensors based on hemoglobin and other
2
7
materials, as reviewed recently.
In addition, niobate nanosheets in colloidal dispersion
can undergo a curling process forming tubular or scrolled
inorganic nanoparticles under soft chemical conditions.
Niobate particles with such morphology can form porous
inorganic-inorganic composites with aluminosilicate
or can incorporate cationic porphyrin and complexes
that are mimics of copper oxidases. Nanoscrolls having
[Ru(bpy)3] as the sensitizer and edta as the sacrificial
electron donor can produce molecular hydrogen from water
28
2
9
3
0
3
1
2
+
layers of NbO octahedral units (linked by corner and/or
6
3
edge) and potassium cations between adjacent layers.
3
2
Chemical modification of the niobate interlayer region
and surface properties based on functionalization reactions
can be achieved using the proton exchanged phases.
Organic groups covalently bonded to the interlayer and
external surfaces of niobate materials were obtained by
reaction between the acidic Nb-OH group and n-alcohols
such as methanol, ethanol and others with longer alkyl
splitting with visible light.
In the present work, we conducted the chemical
modification of the layered niobate of K Nb O
composition through reaction of its proton-exchanged
form in tetraalkylammonium hydroxide solutions. Single
crystal X-ray diffractometry studies of hexaniobate phases
(M Nb O , where M = K , Rb or Cs ) indicate that the unit
cells are composed of four stacked layers along the b-axis
directionandeachlayerisconstitutedofdoubleNbO chains
in which one octahedron out of two is periodically missing
4
6
17
+
+
+
4
6
17
4
chains, polyethers of CH (OCH CH ) OH (1 ≤ m ≤ 4)
3
2
2 m
5
6
composition, trifluoracetate and silanes forming Nb-O-Si
6
7
bonds.
3
3
As mentioned above, another strategy to perform the
chemical modification of niobate materials comprises the
intercalation of organic cations. In the pioneer work of
in the second chain. The unit cell of anhydrous K Nb O
4 6 17
is orthorhombic (a = 7.83Å, b = 33.21 Å, c = 6.46 Å)
and the stacked negative layers yield two distinct interlayer
regions, usually designated I and II, as seen in Figure 1. The
regions are crystallographically distinct and also display
unusual intercalation properties (perhaps due to the fact
that only interlayer region I can be hydrated, which is
important for occurrence of exchange). Depending on the
cation electrical charge and the experimental conditions,
8
Lagaly and Beneke, interlayer simple inorganic cations
were exchanged by n-alkylammonium (C -C ) ions. Later
4
18
on, other papers were published about the intercalation of
9
33
n-alkylammonium and also of substituted alkylammonium
1
0
ions (hydroxyl and diamines) into layered niobate
materials.
3
4,35
One important motivation for the replacement of simple
cations such as potassium by organic ions is to overcome
the high layer charge density of niobates that precludes
the intercalation of bulky species. As the organic modified
derivatives are more reactive than the pristine solids, they
are used as a pre-intercalated precursor for preparation
of niobates intercalated with bulky species such as
ion-exchange reactions take place only at interlayer I.
As a result of this peculiar feature, the cation-exchanged
product has a layered structure with a second staging
arrangement. Due to the high crystallinity of layered
hexaniobate, this phenomenon can be experimentally
3
4,36
detected using X-ray diffractometry
determination of exfoliated particles thickness.
K Nb O ·3H O structural data from atomic force
or by the indirect
1
3,16,17,28
1
1
12
13
porphyrins, rhodamine and anthocyanines, as well as
4
6
17
2
1
4
for adsorption of organic compounds like phenols.
microscopy (AFM) have shown a preferential surface
cleavage at the water-containing interlayer I; cleavage at
interlayer II is unlikely. According to that, hydrogen bonds
established by water molecules in interlayer I are weaker
Chemical modification of niobate by the intercalation
route is mainly driven by the fact that the pre-intercalation
37
15
withpolyethermonoaminesurfactant, tetraalkylammonium
16
+
ion (from tetrabutylammonium hydroxide solutions), and
than the ionic bonds between the K ion and oxygen atoms
17
n-alkylammonium ion promotes the exfoliation of layered
niobate. This process produces a colloidal dispersion of
inorganic nanosheets, suitable for preparation of thin films
in interlayer II (Figure 1).
Potassium hexaniobate material can be changed
to the protonic form when suspended in acid aqueous

1
368
Chemical Modification of Niobium Layered Oxide by Tetraalkylammonium Intercalation
J. Braz. Chem. Soc.
+
by tba , since the size and charge of the organic cation
preclude the balance of all niobate negative layer charge.
In this study, the investigation was extended to other
+
tetraalkylammonium (R N , where R = methyl, ethyl,
4
propyl) hydroxide solutions, in order to investigate the
+
+
influence of the carbon chain and the R N /H -niobate molar
4
ratio on the intercalation reaction into proton exchanged
hexaniobate material. Solid samples were analyzed by
powder X-ray diffractometry (XRD), thermogravimetric
analyses coupled to mass spectrometry (TGA-MS),
elemental analysis, infrared (FTIR) and FT-Raman
spectroscopies and scanning electron microscopy (SEM).
To our knowledge, this is the first time that a systematic
study about modification of the hexaniobate structure
and properties through reaction in tetraalkylammonium
hydroxide solutions is performed.
Figure 1. Representation of the K Nb O ·3H O structure. In the right hand
side, solid and dashed lines among the interlayer species and the NbO6
units represent the ionic interactions and hydrogen bonds.
4
6
17
2
+
solution. Under mild conditions, K ion exchange is partial
1
6,36
(
H K Nb O );
considering the chemical properties
x
4-x
6
17
of interlayers I and II mentioned above, it is usually
+
+
proposed that H /H O ions are present at interlayer I.
Experimental
3
The segregation of proton ions has been validated through
experiments where the protonic hexaniobate is used as
precursor to obtain the exfoliated material. Characterization
Chemicals
17,18,28
of exfoliated particles byAFM
permits to estimate the
Nb O was obtained from Companhia Brasileira de
2
5
particles thickness and distinguish particles constituted
by two layers containing sandwiched K ions. Using
Metalurgia e Mineração (CBMM, Brazil) and K CO3
2
+
was supplied by Merck. Aqueous solutions containing
+
high resolution electron microscopy (HRTEM) images
of restacked hexaniobate obtained from a dispersion of
exfoliated particles, we observed bright lines related to
organic species intercalated in region I and dark lines about
25% tetramethylammonium (tma ) hydroxide and 25%
+
tetraethylammonium (tea ) hydroxide were obtained
-
1
from Acros Organics, while the 1 mol L solution of
+
tetrapropylammonium (tpa ) hydroxide was obtained from
13
1
4Å thick. This value is very close to that obtained by the
Aldrich. All reagents were used as received.
sum of two hexaniobate layers thickness and the interlayer II
+
occupied by K ions (4.1 Å plus 9.3 Å; see Figure 1). This
Preparation of H K Nb O
17
2
2
6
result corroborates the proposal of the partial exchange
+
+
of H /H O ions only into interlayer I and the posterior
K Nb O was prepared by heating a stoichiometric
4 6 17
3
2-
o
exfoliation producing bilayers of [K Nb O ] composition.
mixture of Nb O and K CO at 1100 C for 10 h (ceramic
2
6
17
2 5 2 3
1
1
Ellipsometric data of hexaniobate films prepared by layer-
by-layer technique also allow inferring the thickness of
exfoliated particles and support the assumption of the low
reactivity of interlayer II.
Studies of the reaction between the hexaniobate protonic
form (H K Nb O ) and alkylammonium hydroxide
method), as previously described. The isolation of the
orthorhombic hexaniobate crystal structure with d = 9.4Å
040
+
was confirmed by XRD. The H -exchanged form was
prepared by refluxing a suspension of K Nb O in HNO
16
4
6
17
3
-
1
6 mol L for 3 days. Afterwards, the isolated material was
characterized by XRD and thermogravimetric analyses
(TGA) as previously reported. These data confirmed
the isolation of the acidic hexaniobate of composition
H K Nb O ·H O with d = 8.0 Å.
x
4-x
6
17
3
6
solutions are restricted to the tetrabutylammonium
+
16,24,38
(
tba ) hydroxide solution.
H K Nb O can react
x 4-x 6 17
in tetraalkylammonium hydroxide solutions producing
water and two kinds of materials: (i) intercalated
materials having the alkylammonium ion between niobate
layers, (ii) dispersions of niobate nanosheets. We have
demonstrated that, in tba hydroxide solutions, about
5 wt.% of the sample is delaminated and that bulky
tba cations are not able to intercalate in hexaniobate.
The material recovered from the colloidal dispersions of
hexaniobate nanosheets has only 10% of H ions exchanged
2
2
6
17
2
040
+
Preparation of hexaniobate-R N (R=methyl, ethyl, propyl)
4
samples
+
+
6
Tetraalkylammonium (R N ) hydroxide solutions
4
+
+
39
+
+
containing molar ratios R N /H -niobate (where H -niobate
4
+
represents the total amount of H ion in H K Nb O )
2
2
6
17
+
of 0.25, 0.50, 0.75 and 1.0 were mixed with 0.50 g of

Vol. 21, No. 7, 2010
Shiguihara et al.
1369
H K Nb O and the final volume of the suspensions was
as the proposed chemical formulas, are reported in
the Supplementary Information (SI, Tables S1-S3).
Figure 2 shows the relation between the nominal and
experimental value of x regarding the general formula
2
2
6
17
3
9
adjusted to 250 mL, as reported in a previous study.
Capped flasks containing fixed amounts of H K Nb O
17
2
2
6
in (R N)OH aqueous solution were maintained under
4
+
+
+
stirring at room temperature for two weeks. The shaker
was switched off overnight.After that, the flasks were kept
without stirring for one day, and the opaque supernatants
were separated from the deposited solids (i.e., the sediment
at the bottom of the flasks) using a Pasteur pipette.
Afterwards, the deposited solid fractions were washed with
water and dried under vacuum in a desiccator with silica gel.
(R N ) H K Nb O . It was noticed that, for tma , tea
4 x 2-x 2 6 17
+
and tpa ions, intercalation between the hexaniobate layers
occurred, but only a limited amount of the organic cation
+
+
can replace the H /H O ions. Besides, the amount of the
3
layers negative charge neutralized by the organic ions
decreases with the increasing carbon chain length of the
tetraalkylammonium cation, almost certainly due to the
superior steric hindrance of larger ions. The content of
These solid samples are here abbreviated as R N(XX)dep,
4
+
+
+
+
where XX is the R N /H -niobate ratio used in the
organicammoniumionsincreaseswiththeriseintheR N /H
4
4
experiment and “dep” means “deposited”. In other words,
molar ratio, and the intercalated amount reaches 50%
(R N = tma ), 25% (R N = tea ) and 15% (R N = tpa )
4 4 4
of the negative charge of H K Nb O , concerning the
2 2 6 17
exchange reaction only at the interlayer space I.
+
+
+
+
+
+
+
regarding the general formula (R N ) H K Nb O , the
4
x
2-x
+
2
6
17
+
value of x is ranging from 0.5 (when R N /H = 0.25) to
4
+
+
2
.0 (when R N /H = 1).
4
In order to estimate the percentage of niobate in the
deposited solid, the mass of the solid that remained at the
bottom of the flasks (subtracted the organic content) was
compared to the mass of H K Nb O used in the experiment
2
2
6
17
(0.50 g).
Characterization methods
Elemental analysis (C, H, N) were conducted on a Perkin
Elmer2400analyzerattheInstitutodeQuímica(Universidade
de São Paulo - USP). XRD patterns of powdered samples
were recorded on a Rigaku diffractometer model Miniflex
using Cu-K radiation (1.541 Å, 30 kV and 15 mA).
a
Mass spectrometry-coupled thermogravimetric analyses
(
TGA-MS) were recorded on a Netzsch thermoanalyser
Figure 2. Experimental versus nominal values of X in the general formula
+
+
model STA 490 PC Luxx coupled to an Aëolos 403C
mass-spectrometer, under synthetic air (50 mL min )
(R N ) H K Nb O , where R N = tetraalkylammonium cation.
4
x
2-x
2
6
17
4
-
1
o
-1
at 10 C min . Fourier transform infrared spectra (FTIR)
were recorded on a Bomem spectrophotometer, model
MB-102, with a reflectance accessory; the samples were
diluted in solid KBr. Fourier-transform Raman (FT-
Raman) spectra were recorded in a FT-Raman Bruker
FRS-100/S spectrometer using 1064 nm exciting radiation
X-ray diffraction patterns of the protonic niobate and
+
the R N -intercalated niobates are shown in Figure 3. The
4
XRD pattern of H K Nb O shows a basal spacing (d )
2
2
6
17
040
of 8.0 Å and the absence of the 020 diffraction peak, as
3
0,36,39,40
expected.
The calculated d040 value is consistent
with the intercalation of one water molecule in the protonic
8
+
(
Nd:YAG laser Coherent Compass 1064-500N) and a Ge
phase. After the intercalation of R N cations, the peak
4
detector. Scanning electron micrographs of gold-coated
samples were recorded on a Leo 440i microscope with
a SiLi detector (secondary electrons) at the Instituto de
Geociências (Universidade de São Paulo - USP).
attributed to the 020 interplanar distance appears, as well
as the second, third and fourth order reflections. The
exception is observed with tba , for which the diffraction
+
pattern indicates that this cation does not intercalate into
the hexaniobate. The shifts in the diffraction peaks of the
+
Results and Discussion
niobate treated with tba , as compared to the H K Nb O
2
2
6
17
diffraction peaks, are not related to the alkylammonium
intercalation; the corresponding increase in the basal
spaces is too small for tba cation intercalation. The peak
Carbon, nitrogen, hydrogen and water contents of
all samples isolated from the H K Nb O suspension
+
2
2
6
17
in tetraalkylammonium hydroxide solution, as well
shifts can be attributed to the presence of H K Nb O
17
2
2
6

1
370
Chemical Modification of Niobium Layered Oxide by Tetraalkylammonium Intercalation
J. Braz. Chem. Soc.
phases containing different amounts of interlayered water
39
40
molecules. Abe et al. also observed a proton-exchanged
phase having a 040 basal spacing of about 9 Å.
Figure 4. Schematic representation of the acid-base reaction between the
hexaniobate acidic form and tetraalkylammonium hydroxide solutions.
HRTEM image showed that the dark line thickness is
2
-
compatible with [K Nb O ] layers.
2
6
17
+
The height of adjacent layers containing confined R N
4
cations can be determined by subtracting the values of the
hexaniobate layer thickness (4.1 Å) and the basal spacing
of a potassium filled interlayer (9.3 Å) from the calculated
b unit cell parameter, as indicated in Figure 4. Thus the
o
+
estimated gallery heights are 9.6 Å for tma , 12.6 Å for
+
+
tea and 12.8 Å for tpa . These values could be compared
to the radius of the organic cations in order to analyze the
arrangement of the ions between the layers. However, there
is a divergence of about 30 to 50% in the reported values
+
41
estimated on various scales for R N radii. In addition,
Figure 3. XRD patterns of H K Nb O , tma(0.50)dep, tea(0.50)dep,
4
2
2
6
17
tpa(0.50)dep and tba(0.50)dep.
hydrated radius should be assumed in the case of these
cations, since TGA and elemental analysis data confirm
the hydrated nature of the hexaniobate isolated samples
+
Isolated hexaniobate-tma samples present basal
reflections (0k0) equal to 23.0 Å (020), 11.5 Å (040) and
+
(Table 1). Water molecular dynamics simulation of tma
+
+
7
.79 Å (060), while hexaniobate-tea solids show basal
and tea solutions having chloride or bromide ions suggests
spacings of 26.3 Å (020), 13.0 Å (040), 8.64 Å (060) and
that the distance between the nitrogen atom of the cation
and the oxygen atom of the water in the first shell is about
4.5-5 Å and also that water (as well as the anion) prefers
the space between the alkyl groups. The hydration of
tetraalkylammonium ions is affected by the counterion,
which in this work is the hexaniobate layer (a macroanion).
Therefore, the gallery heights calculated from XRD data
are in agreement with the above mentioned works.
+
6
.48 Å (080). For the hexaniobate-tpa samples, basal
reflections are 26.5 Å (020), 13.1 Å (040) and 8.64 Å
060). Considering the 0k0 reflections where k = 2, 4, 6,
the mean values of the b unit cell parameters are 46.4 Å,
4
2
(
o
5
2.0Å and 52.4Å for the systems intercalated respectively
+
+
+
with tma , tea and tpa . Based on the chemical properties
of hexaniobate interlayers I and II, and also the fact that
the precursor H K Nb O has the H /H O ions located
+
+
By analyzing the basal spacing observed for the three
organic cations (Figure 3), it becomes apparent that the
2
2
6
17
3
in interlayer I, we propose that the organic cations are
preferentially intercalated in region I, as illustrated in
Figure 4. Hence, proton-exchanged hexaniobate mixed with
hydroxide solutions of small tetraalkylammonium cations
generate mainly intercalated materials (reaction A in
+
carbon chains in the tpa ions are bent inside the interlayer
+
region, since it is expected a radius bigger than that of tea
ions. The energy required to convert a trans-trans∙trans-
trans (tt·tt) conformer into a trans-gauche∙trans-gauche
+
-1 43
Figure 4), while bulky ions such as tba favor the formation
(tg·tg) conformer is low (about 1 kcal mol ). Raman
of niobate nanosheets (reaction B in Figure 4).
spectroscopy has been used to monitor (tt·tt) and (tg·tg)
+
+
The intercalation of R N ions only at interlayer I was
conformers of the tea ion through the spectral changes
4
1
3
-1
evidenced in a previous study, in which the tea(0.50)
sample was used as a precursor to intercalate flavylium
cations (anthocyanins) between hexaniobate layers. The
in the -CH rocking mode at 860-880 cm . In the case
3
of hexaniobate-tetraalkylammonium samples, it is not
possible to perform the Raman spectral analysis of the

Vol. 21, No. 7, 2010
Shiguihara et al.
1371
Table 1. TGA-MS analysis under air: weight losses (wt.%) and identification of main released species
event 1
endo
event 2
endo
event 3
exo
event 4
exo
o
o
o
H K Nb O
17
23-117 C
117-242 C
242-500 C
2
2
6
Dm = 1.40%
Dm = 1.69%
Dm = 1.90%
m/z = 18 (H O)
m/z = 18 (H O)
m/z = 18 (H O)
2
2
2
o
o
o
o
tma(0.50)dep
tea(0.50)dep
tpa(0.50)dep
23-140 C
140-250 C
250-450 C
450-850 C
Dm = 3.20%
m/z = 18 (H O)
Dm = 2.00%
m/z = 18 (H O)
Dm = 4.58%
m/z = 18 (H O)
m/z = 44 (CO2)
Dm = 3.83%
m/z = 18 (H O)
m/z = 44 (CO2)
2
2
2
2
o
o
o
o
23-135 C
135-200 C
200-457 C
457-800 C
Dm = 2.82%
m/z = 18 (H O)
Dm = 1.17%
m/z = 18 (H O)
Dm = 5.71%
m/z = 18 (H O)
m/z = 44 (CO2)
Dm = 3.60%
m/z = 18 (H O)
m/z = 44 (CO2)
2
2
2
2
o
o
o
o
23-150 C
150-215 C
215-450 C
450-800 C
Dm = 2.50%
Dm = 0.48%
Dm = 3.90%
Dm = 1.30%
m/z = 18 (H O)
m/z = 18 (H O)
m/z = 18 (H O)
m/z = 18 (H O)
2
2
2
2
m/z = 41 (propylene)
m/z = 44 (CO2)
m/z = 43 (CH CH CH )
3
2
2
m/z = 44 (CO2)
conformers because their bands are superimposed with
the strong niobate layers bands, as shown further on.
and symmetrical stretching modes of methylene and
methyl groups. The band related to the bending d (CH )
s
2
+
The broadening observed in the XRD pattern of tea and
mode is sensitive to the amount of tetraalkylammonium
ions in the samples, once its intensity raises in the order
+
tpa -intercalated niobates might be related to phases with
+
+
+
different water contents and not to phases with different
tma > tea > tpa , agreeing with elemental analyses and
+
R N content or arrangement inside the interlayer region,
4
since the saturation of tetraalkylammonium cations inside
the layers does not depend on the charge balance alone.
It should be possible to estimate the amount of cation
required to neutralize the negative charge of hexaniobate
+
if the R N radius (or its cross sectional area) was known.
4
However, as discussed above, the cation radius depends on
its hydration degree and also on the conformer stabilized
into the layered material. The negative charge density of
2
+
the niobate layer is 12.6 Å /charge, and any R N radius
4
value already reported gives an area larger than that
occupied by one hexaniobate negative charge. Thus, the
steric hindrance precludes the neutralization of the whole
charge of the inorganic structure by tetraalkylammonium
cations supporting only one positive charge.
FTIR spectra of the deposited solids show absorptions
bands of water, ammonium ions and niobate group
4
4,45
(
3
Figure 5), as follows:
a strong and broad band at
-1
-1
300 cm and a medium-weak band at 1650 cm assigned
to n (O-H) and to d (H O), respectively; bands at 1480 cm
-1
as
s
2
assigned to d (CH ) of the organic cations; a strong and
s
2
-1
broad absorption band in the 900-600 cm region related
to the NbO units. Bands ascribed to the organic species
6
do not appear in the spectrum of the tba(0.50)dep sample,
39
because the intercalation does not occur. FTIR spectra
+
of R N -hexaniobate samples present weak bands in the
4
Figure 5. FTIR spectra of (a) H K Nb O , (b) tma(0.50)dep, (c) tea(0.50)
dep, (d) tpa(0.50)dep and (e) tba(0.50)dep.
2
2
6
17
-
1
3
000-2800 cm range, attributed to the asymmetrical

1
372
Chemical Modification of Niobium Layered Oxide by Tetraalkylammonium Intercalation
J. Braz. Chem. Soc.
+
-1
TGA data. The influence of R N ions on the stretching
of slightly distorted octahedra) and 300-200 cm (bending
4
4
5
-1
modes of octahedral NbO groups could provoke
modes of the Nb-O-Nb linkages). The band at 900 cm
shifts to 938 cm after the replacement of K by H /H O
In the potassium form,
the interlayer cations interact with the oxygen atoms of
the inorganic layers and also with water molecules.
Replacement of K by protons increases the bond order
of the short and terminal Nb-O bonds because H ions
are probably shielded by the hydration sphere, precluding
interaction with the layers. As can be seen in Figure 6,
the partial substitution of protons by tma provokes the
appearance of a new band at about 900 cm , suggesting
an increase in the Nb=O bond order. The shoulder at about
940 cm in the spectrum of tma(1.0)dep can be attributed to
the antisymmetric stretching vibration of the C N group.
4
When the samples of hexaniobate saturated with the other
tetraalkylammonium ions are considered (tea(1.0)dep and
tpa(1.0)dep), the disappearance of the band at 938 cm and
the growth of a band at 900 cm are also noticed. Hence,
6
-
1
-1
+
+
+
modifications in the 900-700 cm region, but they are
not perceived in the infrared spectra. As discussed ahead,
Raman spectra are more responsive to the presence of
organic species.
Figure 6 shows the Raman spectra of K Nb O ,
H K Nb O and hexaniobate samples containing a low
and the highest amount of intercalated tma ions. In the
000-2500 cm region, the FTIR spectra of hexaniobate-
tetraalkylammonium systems are dominated by intense
and broad bands arising from water, while the Raman
spectra are nearly free from such interference. Bands
related to C-H stretching modes are observed at 3032,
3
3
4,47
ions, as reported previously.
3
3
+
4
6
17
+
2
2
6
17
+
-
1
48
4
+
-1
-1
-
1
+
46
2
2
2
1
984, 2928 and 2822 cm for (tma)dep samples; 2998,
948 and 2897 cm for (tea)dep samples; and 3004, 2973,
938 and 2880 cm for (tpa)dep samples. The bands at
453 and 757 cm are assigned to the CH bending mode
-
1
-
1
-
1
-1
3
+
-1
and the symmetric stretching vibration of the C N group,
4
4
6
+
respectively.
independently of the R N ion intercalated, the band related
to Nb=O shifts to 900 cm . The same decrease in intensity
and displacement of the band at 938 cm is observed
when H K Nb O is dehydrated, suggesting that, after
elimination of the water molecules, H ions can establish
a strong interaction with oxygen atoms, decreasing the
4
-1
The very intense bands in the FT-Raman spectra showed
in Figure 6 are characteristic of hexaniobate as follow:
50-800 cm (Nb-O terminal stretching mode of highly
-
1
-1
9
2
2
6
17
-1
+
distorted NbO octahedra), 700-500 cm (Nb-O stretching
6
4
8
Nb-O bond order.
TGA and C, H, N analysis data show that the amount
+
of water in the interlayer region decreases when R N
4
cations are intercalated. Hence, the presence of the organic
species should promote the enhancement of the interaction
+
+
between H and Nb=O. The R N cations can be localized
4
more distant of Nb-O terminal groups situated on the layer.
TGA and DTG-MS curves (in air) of H K Nb O and
2
2
6
17
the samples saturated with tetraalkylammonium cations
are showed in Figures 7 and 8 respectively. It is possible to
identify four main thermal steps of weight loss, as indicated
Figure 6. FT-Raman spectra of K Nb O , H K Nb O , tma(0.25)dep
Figure 7. TGA curves of H K Nb O , tma(0.50)dep, tea(0.50)dep and
4
6
17
2
2
6
17
2
2
6
17
and tma(1.0)dep.
tpa(0.50)dep.

Vol. 21, No. 7, 2010
Shiguihara et al.
1373
Figure 8. DTG-MS curves of (a) H K Nb O , (b) tma(0.50)dep, (c) tea(0.50)dep and (d) tpa(0.50)dep.
2
2
6
17
in Table 1. According to mass spectrometry data, the first
under the present experimental conditions. Hofmann
+
and the second events (from room temperature to about
elimination reaction does not occur for tma but it could
o
+
2
00-250 C) correspond to release of the water molecules.
happen for tea .Also, ammonia or nitrogen oxides have not
The dehydration step at low temperature (up to about
been detected for any of the samples under the experimental
conditions described in this work.
o
1
40-150 C) can be assigned to the interparticles water
+
release; the dehydration process at higher temperatures
event 2) should be due to the liberation of interlayer water
SEM images of some hexaniobate-R N deposited
4
(
solids show the existence of particles with different
shapes according to the amount and nature of the
tetraalkylammonium cation used. In the case of hexaniobate
molecules. The water content in the hexaniobate modified
by organic cation intercalation shows a tendency to decrease
+
+
+
+
+
in the order tma > tea > tpa (Table 1, event 2), which
can be explained by the increase of the nonpolar surface
intercalated with tma and tea , the images of all samples
show a clear predominance of plate-like particles, as
expected for the non-exfoliated layered hexaniobate.
However, the presence of some stick-like particles mixed
with the plate-like ones is observed in the SEM images
4
9
of the tetraalkylammonium ions in the same order.
o
Above ca. 250 C, the H K Nb O sample undergoes a
2
2
6
17
dehydroxylation reaction (event 3) producing Nb O ,
2
5
36
+
+
K Nb O and one water molecule per formula.
of samples containing tpa (Figures 9a and 9b) and tba
(Figure 9c). The amount of stick-like particles increases
2
4
11
For the hexaniobate samples intercalated with
+
+
tetraalkylammonium cations, besides the dehydroxylation
of the layers, event 3 also comprises the partial oxidation of
guest ions, evidenced by the evolution of carbon dioxide.
when the R N /H molar ratio used in the experiments is
4
raised, as shown in Figures 9a and b.
For hexaniobate deposited solids obtained after contact
+
+
Mass spectra of gases released from hexaniobate-tpa
with tba solutions for two weeks, spherical particles
samples during the third step of weight loss also present
peaks assignable to propylene (most intense characteristic
fragment appears at m/z 41) and the propyl fragment
are also observed in minor quantities. With the images
registered till now it is not possible to ascertain if these
specific particle shapes are constituted of a single particle or
an aggregation of smaller ones. The reason for the existence
of these different kinds of particle shapes (influenced by
the tetraalkylammonium cation) is not yet understood and
it is now under investigation in our laboratory.
(
CH CH CH -), suggesting that a Hofmann elimination
3
2
2
reaction also occurs:
+
+
(
CH CH CH ) N → (CH CH CH ) N + H + CH CH=CH
3
2
2
4
3
2
2
3
3
2
The weight of the solids deposited was evaluated for
+
Tripropylamine molecules can be protonated when
the three hexaniobate-R N systems and revealed values of
4
+
+
+
produced and kept between the layers, undergoing
progressive Hofmann elimination reaction and/or oxidative
degradation under air. It is noticed that, for the smallest
cations, release of olefins and trialkylamine is not observed
about 90, 86 and 40% for tma , tea and tpa respectively.As
reported previously in the same experimental conditions
used in this work, tba ions kept 35 wt.% of particles in
the deposited condition (while 65 wt.% of niobate particles
3
9
+

1
374
Chemical Modification of Niobium Layered Oxide by Tetraalkylammonium Intercalation
J. Braz. Chem. Soc.
hexaniobate-tetraalkylammonium samples are washed in
order to remove the non-intercalated cations, i.e., when the
ionic strength decreases. The XRD pattern of a hexaniobate
gel-like sample in the wet state does not show diffraction
peaks (data not shown), suggestingthat removingthe ions by
washing can lead to a long-range swelling of the layers and
a disorganized arrangement of the hexaniobate sheets. The
gel-like aspect was mainly observed for the hexaniobate-
+
tea samples. Bearing in mind the four tetraalkylammonium
+
ions focused in the present work, tea has an intermediate
character when the ion surface polarity and the electrostatic
+
49
interactions established by the R N ions are considered.
4
+
While tma (that shows high surface polarity) produces the
most highly hydrated samples and tpa (or tba ) ions are
bulky enough to promote a high delamination degree, tea
+
+
+
ions have the better hydrophilicity/size relation to form a
gel-like system. It is possible to infer that the gel is not
constituted by exfoliated particles but by disorganized and
long-range swelled particles. The tea(0.50)dep sample in
the gel-like form was used to intercalate flavylium cations
(
anthocyanins) and the SEM and TEM images confirmed
1
3
that the hexaniobate particles were not exfoliated. The
particles morphology is different when compared to the
images of hexaniobate-flavylium obtained from a colloidal
dispersion of exfoliated nanosheets.
Conclusions
+
+
Intercalation of the tetraalkylammonium tma , tea
+
and tpa ions in H K Nb O is promoted by an acid-base
2
2
6
17
reaction, but steric hindrance of the cations precludes the
+
neutralization of all interlayered H ions. Regarding the
Figure 9. SEM images of (a) tpa(0.25)dep, (b) tpa(0.75)dep and (c)
tba(0.75)dep.
+
general formula (R N ) H K Nb O , the X value reaches
4
x
2-x
2
6
17
+
+
about 1.0 for tma systems, 0.56 for tea samples and 0.31
for tpa materials. X-ray diffraction patterns indicated that
+
develop a dispersion of nanosheets). Taking these results
and Figure 4 into account, it is possible to claim that process
the gallery height of adjacent layers containing confined
+
+
+
+
A (intercalation) is promoted in the order tma > tea > tpa ,
while process B (exfoliation) is facilitated in the inverse
R N species is compatible with a monolayer of hydrated
4
+
ions and, in the case of samples with tpa ions, the carbon
+
+
+
+
order: tba > tpa >> tea > tma . Tetraalkylammonium
ions are symmetric species and the positive charge is
located on the nitrogen atom, what means that electrostatic
attraction between the organic cation and the negative
charged inorganic surface decreases if the cation diameter is
increased. Hence, two factors can be considered important
to drive the formation of niobate nanosheets from acidic-
hexaniobate in tetraalkylammonium hydroxide solutions:
chain should be in a trans-gauche conformation. FTIR and
FT-Raman spectra present bands related to water molecules,
organic cations and the inorganic framework. The band
assigned to the stretching of Nb=O group (pointing out to
the interior of interlayer region) is shifted to low frequency
due to an increase in the interaction between the niobyl
+
group and the H ions. The presence of tetraalkylammonium
ions decreases the H O content between the layers; water
2
+
…
+
the R N electrostatic attraction to the inorganic layers and
molecules can shield the Nb=O H interaction.
TGA and DTG-MS curves support this interpretation,
since the water content in the modified hexaniobate
4
the steric hindrance.
Between the extreme situations (intercalated or
exfoliated samples), another process was perceived. A
system with a gel-like aspect is observed when some
+
+
+
decreases in the order tma > tea > tpa , i.e., when the
nonpolar surface of the tetraalkylammonium ions increases.

Vol. 21, No. 7, 2010
Shiguihara et al.
1375
o
When heated above 200-250 C, organic ions experience an
oxidativedecompositionproducingcarbondioxide;inthecase
5. Takeda, Y.; Momma, T.; Osaka, T.; Kuroda, K.; Sugahara, Y.;
J. Mater. Chem. 2008, 18, 3581.
+
of niobate samples containing tpa , a Hofmann elimination
is also observed. Hexaniobate-R N deposited solids have
6. Takeda,Y.; Suzuki, H.; Notsu, K.; Sugimoto, W.; Sugahara,Y.;
Mater. Res. Bull. 2006, 41, 834.
+
4
plate-like particles, as expected for the non-exfoliated
layered hexaniobate. However, stich-like particles are also
observed when H K Nb O is kept in solutions containing
7. Tahara, S.; Takeda, Y.; Sugahara, Y.; Chem. Mater. 2005, 17,
6198; Nakato, T.; Hashimoto, S.; Chem. Lett. 2007, 36, 1240.
8. Lagaly, G.; Beneke, K.; J. Inorg. Nucl. Chem. 1976, 38, 1513.
9. Nedjar, R.; Borel, M. M.; Raveau, B.; Z. Anorg. Allg. Chem.
1986, 540, 198; Nedjar, R.; Borel, M. M.; Raveau, B.; J. Solid
State Chem. 1987, 71, 451; Jacobson, A. J.; Lewandowski, J.
T.; Johnson, J. W.; J. Less-Common Met. 1986, 116, 137; Abe,
R.; Ikeda, S.; Kondo, J. N.; Hara, M.; Domen, K.; Thin Solids
Films 1999, 343-344, 156.
2
2
6
+
17
+
the larger tpa and tba ions. Considering the amount of
hexaniobate that is retained at the bottom of the suspensions
and the amount that is delaminated (the supernatant colloidal
dispersion), it is plausible to state that the intercalation
+
+
+
reaction is promoted in the order tma > tea > tpa ,
while formation of niobate nanosheets is facilitated in the
+
+
+
+
inverse order: tba > tpa > tea > tma .
10. Yakabe, S.; Nakato, T.; J. Mater. Sci. 2003, 38, 3809.
11. Bizeto, M. A.; Faria, D. L. A.; Constantino, V. R. L.; J. Mater.
Sci. 2002, 37, 265.
+
Samples containing intercalated tea ions form a gel-
like system when washed to remove the non-intercalated
ions dissolved in water. Experimental data suggest that
the gel phase is not constituted by exfoliated particles but
by disorganized and long-range swelled particles. This
fact was interpreted as a consequence of the intermediate
12. Shinozaki, R.; Nakato, T.; Microporous Mesoporous Mater.
2008, 113, 81.
13. Teixeira-Neto,A.A.; Shiguihara,A. L.; Izumi, C. M. S.; Bizeto,
M. A.; Leroux, F.; Temperini, M. L. A.; Constantino, V. R. L.;
Dalton Trans. 2009, 4136.
+
characteristics (surface polarity and ion radius) of the tea
ions compared to the others ions investigated in this study.
14. Nakato, T.; Miyashita, H.;Yakabe, S.; Chem. Lett. 2003, 32, 72;
Wei, Q.; Nakato, T.; Microporous Mesoporous Mater. 2006, 96,
84; Nakato, T.; Kameyama, M.; Wei, Q.; Haga, J.; Microporous
Mesoporous Mater. 2008, 110, 223.
Supplementary Information
Supplementary data (Tables S1-S3) are available free
of charge at http://jbcs.sbq.org.br, as a pdf file.
15. Treacy, M. M. J.; Rice, S. B.; Jacobson, A. J.; Lewandowski,
J. T.; Chem. Mater. 1990, 2, 279.
1
6. Keller, S. W.; Kim, H-N.; Mallouk, T. E.; J. Am. Chem. Soc.
1994, 116, 8817.
Acknowledgments
1
7. Miyamoto, N.; Yamamoto, H.; Kaito, R.; Kuroda, K.; Chem.
Commun. 2002, 2378; Bizeto, M. A.; Constantino, V. R. L.;
Mater. Res. Bull. 2004, 39, 1811.
The authors acknowledge the Brazilian agencies
FAPESP and CNPq for financial support and fellowships.
They also would like to thank Mr. Isaac J. Sayeg, from
Instituto de Geociências (Universidade de São Paulo), for
his attention and suggestions regarding the acquisition
of the SEM images, the Laboratório de Espectroscopia
Molecular (Universidade de São Paulo) for the Raman
spectra and CBMM for the sample of Nb O .
18. Unal, U.; Matsumoto,Y.; Tamoto, N.; Koinuma, M.; Machida,
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1. Kobayashi,Y.; Schottenfeld, J.A.; MacDonald, D. D.; Mallouk,
T. E.; J. Phys. Chem. C 2007, 111, 3185.
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FAPESP helped in meeting the publication costs of this article.

J. Braz. Chem. Soc., Vol. 21, No. 7, S1, 2010.
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Chemical Modification of Niobium Layered Oxide by Tetraalkylammonium
Intercalation
a
b
,a
Ana L. Shiguihara, Marcos A. Bizeto and Vera R. L. Constantino*
a
Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748,
5508-000 São Paulo-SP, Brazil
0
b
Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo - Campus
Diadema, Rua Prof. Artur Riedel 275, 09972-270 Diadema-SP, Brazil
+
Table S1. Estimated composition of hexaniobate-tma deposited solids
Sample
C (%)
2.05
H (%)
N (%)
Water (%)
tma(0.25)dep
1.16
(1.10)
0.60
(0.48)
4.40
(4.44)
+
a
[
(CH ) N ]0.42H1.58K Nb O ·2.4H O
(1.94)
3
4
2
6
17
2
tma(0.50)dep
4.41
(4.43)
1.79
(1.80)
1.29
(1.15)
5.23
(5.38)
+
[
(CH ) N ]0.95H1.05K Nb O ·3H O
3
4
2
6
17
2
tma(0.75)dep
4.85
(4.83)
1.88
(1.93)
1.41
(1.24)
5.19
(5.33)
+
[
(CH ) N ]1.05H0.95K Nb O ·3H O
3
4
2
6
17
2
tma(1.0)dep
4.86
(4.90)
1.85
(1.90)
1.42
(1.16)
4.86
(4.91)
+
[
(CH ) N ]1.05H0.95K Nb O ·2.8H O
3
4
2
6
17
2
a)
experimental data.
+
Table S2. Estimated composition of hexaniobate-tea deposited solids
Sample
C (%)
4.51
H (%)
N (%)
Water (%)
tea(0.25)dep
1.65
(1.38)
0.66
(0.71)
4.94
(4.95)
+
a
[
(CH CH ) N ]0.48H1.52K Nb O ·2.8H O
(4.69)
3
2
4
2
6
17
2
tea(0.50)dep
5.63
(5.39)
2.02
(1.59)
0.82
(0.71)
6.3
(6.2)
+
[
(CH CH ) N ]0.62H1.38K Nb O ·3.7H O
3
2
4
2
6
17
2
tea(0.75)dep
5.18
(5.30)
1.85
(1.41)
0.75
(0.91)
5.55
(5.46)
+
[
(CH CH ) N ]0.56H1.44K Nb O ·3.2H O
3
2
4
2
6
17
2
tea(1.0)dep
5.18
(5.24)
1.85
(1.37)
0.75
(0.72)
5.54
(5.49)
+
[
(CH CH ) N ]0.56H1.44K Nb O ·3.2H O
3
2
4
2
6
17
2
a)
experimental data.
+
Table S3. Estimated composition of hexaniobate-tpa deposited solids
Sample
C (%)
1.36
H (%)
N (%)
Water (%)
tpa(0.25)dep
0.80
(0.56)
0.13
(0.03)
3.00
(3.27)
+
a
[
(CH CH CH ) N ]0.09H1.91K Nb O ·1.6H O
(1.39)
3
2
2
4
2
6
17
2
tpa(0.50)dep
(CH CH CH ) N ]0.25H1.75K Nb O ·1.6H O
3.66
(3.68)
1.21
(1.11)
0.36
(0.28)
2.92
(2.94)
+
[
3
2
2
4
2
6
17
2
tpa(0.75)dep
(CH CH CH ) N ]0.29H1.71K Nb O ·1.5H O
4.22
(4.28)
1.30
(1.16)
0.41
(0.40)
2.73
(2.78)
+
[
3
2
2
4
2
6
17
2
tpa(1.0)dep
4.48
(4.54)
1.36
(1.27)
0.44
(0.49)
2.89
(3.00)
+
[
(CH CH CH ) N ]0.31H1.69K Nb O ·1.6H O
3
2
2
4
2
6
17
2
a)
experimental data.
e-mail: vrlconst@iq.usp.br
*