Self-assembled titanium phosphonate nanomaterial having a mesoscopic void space and its optoelectronic application

Article abstract of DOI:10.1039/c3dt32744b

Here we report the synthesis of a new crystalline titanium phosphonate material (HTiP-7) having a self-assembled nanostructure and a mesoscopic void space without the aid of any surfactant or templating agent. The material has been synthesized hydrothermally through the reaction between benzene-1,3,5-triphosphonic acid (BTPA) and titanium(iv) isopropoxide at neutral pH at 453 K for 24 h. This hybrid phosphonate material has been thoroughly characterized by powder X-ray diffraction, N₂ sorption, HR TEM, FE SEM, TG-DTA, FT IR and UV-Vis diffuse reflectance spectroscopic studies. Two very well-known software packages, REFLEX and CELSIZ unit cell refinement programs, are employed to establish the triclinic crystal phase of this hybrid material (HTiP-7). Very tiny nanocrystals of HTiP-7 self-aggregated to form spherical nanoparticles of dimension ca. 25 nm together with a mesoscopic void space and good BET surface area (255 m² g^-1). The framework is thermally stable up to 650 K. The material showed excellent carrier mobility for photocurrent generation in the presence of a photosensitizer molecule (Rose Bengal). To the best of our knowledge this is the first report of a photon-to-electron energy transfer process over a dye doped titanium phosphonate nanomaterial.

Full text of DOI:10.1039/c3dt32744b

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Self-assembled titanium phosphonate nanomaterial
having a mesoscopic void space and its optoelectronic
application†
Cite this: Dalton Trans., 2013, 42, 5140
Malay Pramanik, Astam K. Patra and Asim Bhaumik*
Here we report the synthesis of a new crystalline titanium phosphonate material (HTiP-7) having a self-
assembled nanostructure and a mesoscopic void space without the aid of any surfactant or templating
agent. The material has been synthesized hydrothermally through the reaction between benzene-1,3,5-
triphosphonic acid (BTPA) and titanium(IV) isopropoxide at neutral pH at 453 K for 24 h. This hybrid phos-
phonate material has been thoroughly characterized by powder X-ray diffraction, N2 sorption, HR TEM,
FE SEM, TG-DTA, FT IR and UV-Vis diffuse reflectance spectroscopic studies. Two very well-known software
packages, REFLEX and CELSIZ unit cell refinement programs, are employed to establish the triclinic crystal
phase of this hybrid material (HTiP-7). Very tiny nanocrystals of HTiP-7 self-aggregated to form spherical
nanoparticles of dimension ca. 25 nm together with a mesoscopic void space and good BET surface area
(255 m2 g−1). The framework is thermally stable up to 650 K. The material showed excellent carrier mobi-
lity for photocurrent generation in the presence of a photosensitizer molecule (Rose Bengal). To the best
of our knowledge this is the first report of a photon-to-electron energy transfer process over a dye
doped titanium phosphonate nanomaterial.
Received 10th August 2012,
Accepted 14th January 2013
DOI: 10.1039/c3dt32744b
www.rsc.org/dalton
tetramethylbiphenylene-bis(phosphonate)] have to be used
otherwise some ‘diluting’ agents such as alkyl phosphonic
Introduction
Organic–inorganic hybrid porous materials have attracted acid have to be added for random pillaring.9 To date most of
major research interest over the years and these developments the reports made in this context are focused on the close-
could contribute significantly to the conversion of photon packed arrangements of the organic ligands in the metal–
energy.1 Metal organophosphonate materials are the most organic frameworks.10 So to impede the close packing of the
promising class of hybrid materials due to the fact that there organic scaffolds, here we have used benzene-1,3,5-triphospho-
are numerous possibilities to vary organic components in the nic acid as the organophosphorus source molecule for the syn-
precursor organophosphorous moiety/ligand leading to thesis of self-assembled porous titanium phosphonate
different modes of coordination and functionalization in the nanoparticles without using any additional building blocks or
resulting solid matrix.2 These metal phosphonates have found templates. We have designed the ligand (benzene-1,3,5-tripho-
potential applications in optoelectronics,3 catalysis,4 heavy sphonic acid) as the organic scaffold molecule for the syn-
metal ion adsorption,5 magnetism6 and so on. Thus consider- thesis of organic–inorganic hybrid porous titanium
able attention has been paid to the design of new organophos- phosphonate material to enhance the mechanical stability and
phorous motifs bearing two or more binding sites. Among flexibility of the material by increasing the number of coordi-
them biphosphonic acid and functionalized biphosphonic nation sites (and thus more cross-linking) over aromatic mono-
acids are worth mentioning here.7 However, for biphosphonic phosphonic or diphosphonic acids.10 Apart from this, the π–π
acid or its derivatives the respective metal phosphonates are stacking interaction among the aromatic rings, present in the
organically pillared and close proximity of the aryl pillars pre- hybrid material, may promote the formation of
a self-
vents nanoscale porosity in the framework.8 So when the poro- assembled nanostructure with an inter-particle mesoscopic
sity is targeted, bulkier biphosphonic acid sources [3,3′,5,5′- void space. Designing a mesoporous metal phosphonate
without the aid of any surfactant or templating agent is very
challenging.
Department of Materials Science, Indian Association for the Cultivation of Science,
On the other hand, considerable effort has been made for
Jadavpur, Kolkata 700 032, India. E-mail: msab@iacs.res.in
the modification of the TiO2 surface with a variety of phospho-
nic acids to control the interfacial and bulk properties of the
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3dt32744b
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resulting hybrid material such as hydrophobicity, porosity, reported organic sensitizer loaded porous TiO2 materials; the
optical and electronic properties.11 However, the homogeneous increment of photocurrent for HTiP-7RB is ca. 32 times more
incorporation of phosphonic acid groups into porous TiO2 is than RB-meso-TiO2 and ca. 19 times more than MT-1-RB.19
really an exigent problem.12 The inhomogeneity of the organic There are a few reports on the synthesis of ordered meso-
phosphonic acid groups in the porous TiO2 materials can be porous titanium phosphonate materials from various aliphatic
easily minimized in the hybrid titanium phosphonate by using phosphonic acid scaffolds.15 But for designing materials with
bridging organophosphorus precursor molecules, where two enhanced optoelectronic property, the presence of a chromo-
or more phosphate groups are attached to the same organic phoric aromatic moiety in the porous framework is essential.
fragment. Thus, designing a new bridging organophosphorus However, to date no such report is available. Thus we design
precursor molecule and employing it as a precursor for the HTiP-7, which carries benzene-1,3,5-triphosphonic acid as the
synthesis of hybrid titanium phosphonate materials having organophosphorus scaffold for the high photon-to-electron
high surface areas is quite challenging.
energy transfer process. To the best of our knowledge this is
In this context it is relevant to mention that porous tita- the first report of the self-assembled mesoporous titanium
nium(IV) phosphonate materials are intensively used in cataly- phosphonate using benzene-1,3,5-triphosphonic acid as the
sis,13 selective ion adsorption,14 proton conductivity,15 photo- organophosphorous precursor and its excellent photoresponse
degradation16 and so on. However, to date there is no report of upon dye loading.20
porous titanium phosphonate based nanomaterials in the
photo-induced enhancement of electrical conductivity. We
have recently reported
(MTiP-1) material using a non-ionic surfactant templating
route. However, the material has relatively low surface area
a porous titanium(IV) phosphate
Experimental section
Materials
(193 m2 g−1), large mesopore size (7.03 nm) and is devoid of Commercially available 1,3,5-tribromobenzene (98%), triethyl-
an organic scaffold in the framework.17 So our aim was to phosphite (98%), 1,3-diisopropylbenzene and titanium-iso-
develop a metal phosphonate material with large surface area propoxide (97%) were purchased from Sigma Aldrich, USA.
bearing an aromatic organic bridging moiety and compara- Nickel(II) bromide (99%) was purchased from Avra Chemicals,
tively smaller pore aperture so that the guest photosensitizer India.
molecule (organic dye) can hold strongly at the surface of the
Synthesis of benzene-1,3,5-triphosphonic acid. 9.45 g of
porous cavity. There are two ways to achieve this goal: firstly we 1,3,5-tribromobenzene was dissolved in 75 ml 1,3-diisopropyl-
can synthesize an ordered mesoporous titanium phosphonate benzene by heating at 120 °C under a nitrogen atmosphere.
material (having pore width compatible with the guest photo- The reaction mixture was cooled to 70 °C and anhydrous
sensitizer molecule) bearing a phosphonic acid as the precur- nickel(II) bromide (1.5 g) was added to the reaction mixture
sor molecule through the surfactant templating pathway or under vigorous stirring under an inert atmosphere. Then the
secondly designing the porous organic–inorganic hybrid tita- reaction mixture was heated gradually to 180 °C and triethyl
nium phosphonate using aromatic phosphonic acid as a phos- phosphite (22.5 ml) was added for 3 h. The heating was contin-
phorus precursor molecule and having nanoscale porosity ued for the next 6 h at the same temperature under an inert
generated through the self-assembly of tiny nanocrystals via atmosphere. Then the volatile component and 1,3-diisopropyl-
the non-templating pathway for the inclusion of the photo- benzene were distilled off, and a dark viscous residue was
sensitizer molecule. Now the removal of templates or structure obtained. The compound (hexaethyl-1,3,5-benzenetriphospho-
directing agents (SDA) keeping the order structure unper- nate) was purified over a column of silica gel eluted with
turbed to produce ordered mesoporous titanium phosphonate CHCl3 [CHCl3 : MeOH = 10 : 1 (v/v)]. The colorless hexaethyl-
is really an exigent problem for the phosphate based meso- 1,3,5-benzenetriphosphonate was obtained by solvent evapor-
structured materials. Thus, we choose the latter strategy to ation and the yield was 5.85 g (40%). Then 4.5 g of the pro-
develop a novel hybrid titanium phosphonate material having duced ester was hydrolyzed with 30 ml water in the presence of
a mesoscopic void space for insertion of the photosensitizer 30 ml concentrated HCl for 12 h. The hydrolyzed solution was
molecule. Here we report a new self-assembled mesoporous evaporated to dryness and the residue was dissolved in 15 ml
titanium phosphonate nanomaterial by using 1,3,5-benzene distilled water and decolorized by activated charcoal, then the
triphosphonic acid as the organophosphorus source under filtrate was evaporated under reduced pressure to get a white
hydrothermal conditions. We tactically entrapped a photosen- solid, benzene-1,3,5-triphosphonic acid (BTPA)21 having a
1
sitizer molecule18 (organic dye, Rose Bengal) into the meso- yield of 2.7 g (92%). The compound was characterized by H,
scopic void space of the titanium phosphonate nanoparticles. 13C, 31P NMR and IR spectroscopy. 1H NMR (500 MHz, D2O)
The observed photocurrent for dye loaded porous titanium δ = 8.07 ((t, 3JPH) 15 Hz); 13C NMR (500 MHz, D2O) δ = 135.23;
phosphonate (HTiP-7RB) is 13.5 times more than hybrid tita- 136.07; 31P NMR (500 MHz, D2O) δ = 12.87. IR (KBr, cm−1) νmax
nium phosphonate (HTiP-7) and 3500 times more than pure 3387, 3088, 2925, 2319, 1142, 1001, 943, 691, 536, 470.
titanium phosphate synthesized in the absence of any aro-
Synthesis of the self-assembled porous titanium(IV) phos-
matic organophosphorus precursor molecule. We compare phonate HTiP-7. In the typical synthesis of HTiP-7, 1.272 g
this result of photoconductivity value with our previously (4.0 mmol) of benzene-1,3,5-triphosphonic acid was dissolved
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JEOL JEM 2010 high resolution transmission electron micro-
scope (HR TEM) operated at an accelerating voltage of 200 kV.
Nitrogen adsorption/desorption isotherms were obtained by
using a Beckman Coulter SA 3100 surface area analyzer at
77 K. 1H NMR and 13C experiments were carried out on a
Bruker DPX-300 NMR spectrometer. Thermogravimetric analy-
sis (TGA) and differential thermal analysis (DTA) of the sample
were carried out in a TGA instrument thermal analyzer TA-SDT
Q-600 under a continuous flow of nitrogen with a heating/
cooling rate of 10 °C min−1. UV-visible diffuse reflectance
spectra were recorded on a Shimadzu UV 2401PC with an inte-
grating sphere attachment. BaSO4 was used as the background
standard. For the electrical measurements, firstly pellets of
each of the samples of 1 cm diameter were made and two plati-
num electrodes (of ca. 50 nm thickness) were attached on each
pellet in the circular form of diameter 2 mm through a
shadow mask at a separation of 4 mm. The DC current
between the two electrodes was measured using a Keithley
source meter (model 2420). The photocurrents were measured
by illuminating with a 150 W xenon lamp light source
(Newport Corp. USA; model no. 69907).
Table 1 Change in BET surface area for the different titanium phosphonates
synthesized at different pH and temperaturesa
Sample
name
Synthetic
pH
Temperature
(°C)
Surface area
(m2 g−1
)
HTiP-7
6–7
1–2
10
6–7
6–7
180
180
180
120
150
255
83
123
188
218
HTiP-7A
HTiP-7B
HTiP-7C
HTiP-7D
a Reaction time = 24 h.
in 10 ml double distilled water. Simultaneously, 6.818 g
(24.0 mmol) of titanium-isopropoxide was dissolved in 10 ml
dry iso-propanol in another dry beaker. The synthetic gel was
prepared by very slow addition of the former acid solution to
the alcoholic titanium-isopropoxide solution. The as syn-
thesized gel was aged under stirring conditions overnight and
then transferred into a Teflon-lined autoclave and kept at
453 K for 24 h. The autoclave was cooled to room temperature.
The final suspension was centrifuged at 5000 RPM to collect
the titanium(IV) phosphonate nanoparticles. Then the product
was suspended in dry ethanol and again centrifuged to get the
desired solid product (HTiP-7) and dried in a high vacuum
pump for 5 h. The final product yield was 3.86 g. Keeping the
molar ratio of Ti and BTPA and reaction time unaltered, we
Results and discussion
Powder X-ray diffraction
synthesized
a few more titanium phosphonate samples
The small angle powder X-ray diffraction pattern for HTiP-7 is
shown in Fig. 1. One broad peak centered at 2θ = 2.64° could
be attributed to the self-assembly of tiny titanium phospho-
nate nanoparticles.22 The interparticle distance corresponding
to this broad diffraction is ca. 3.34 nm. The wide angle X-ray
powder diffraction pattern for the HTiP-7 material is shown in
Fig. 2. The very strong and sharp peaks in the wide angle
(HTiP-7A, HTiP-7B, HTiP-7C and HTiP-7D) by varying the pH
of the synthetic gel and hydrothermal temperature (Table 1).
Preparation of the sample for photoconductivity
measurements
For the impregnation of Rose Bengal (RB) at the surface of the
self-assembled mesoporous titanium phosphonate (HTiP-7)
material, 0.25 g of RB was dissolved in 25 ml absolute ethanol
and then 1.0 g of HTiP-7 was dispersed into this solution with
continuous stirring. After 3 h, the resultant content was fil-
tered and washed with absolute ethanol to remove the adhered
RB bound to the external surface. Then the RB impregnated
titanium phosphonate (HTiP-7RB) material was dried at room
temperature overnight and used for photoconductivity
measurements.
Instrumentation
Powder X-ray diffraction (PXRD) patterns were recorded on a
Bruker D-8 Advance diffractometer operated at 40 kV and
40 mA and calibrated with a standard silicon sample, using
Ni-filtered Cu Kα (λ = 0.15406 nm). Fourier transform infrared
(FT IR) spectra of the materials were recorded using a Nicolet
MAGNA-FT IR 750 Spectrometer Series II. Elemental analyses
were carried out by using a Shimadzu AA-6300 atomic absorp-
tion spectrophotometer (AAS) fitted with a double beam mono-
chromator and a Perkin-Elmer 2400 Series-II CHN analyzer. A
JEOL JEM 6700F field emission scanning electron microscope
(SEM) was used for the determination of the morphology of
powder samples. The pore structure was evaluated by using a Fig. 1 Small angle powder XRD pattern of HTiP-7.
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Table 3 Indexing of triclinic crystal structure of HTiP-7
h
k
l
2θ (°)
d (Å)
0
1
1
0
1
1
0
1
1
1
0
2
2
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
1
1
1
−1
3
−3
1
−3
0
−3
−3
1
1
5
5
5
3
1
5
4
−7
7
1
0
0
2
1
2
1
−1
−5
3
6
−2
2
−7
7
−1
−2
2
7
9
5
8
4
7.732
7.883
11.424
11.214
8.513
8.319
7.533
6.833
4.492
4.334
4.174
3.851
3.654
3.549
3.512
3.121
3.009
2.832
2.793
2.681
2.504
2.377
2.286
2.081
1.924
1.895
1.831
1.768
10.390
10.633
11.746
12.954
19.671
20.488
21.286
23.098
24.358
25.090
25.356
28.599
29.684
31.584
32.045
33.407
35.848
37.946
39.412
43.477
47.223
48.006
49.810
51.679
Fig. 2 Wide angle XRD pattern of HTiP-7.
diffraction pattern provide evidence of long range periodicity
in the porous architecture. None of the peaks can be attributed
to the typical crystalline TiO2 peaks; hence the crystallinity of
the material is due to the long range ordering of the pure tita-
nium(IV) phosphonate framework.23 When alcoholic titanium(IV)
isopropoxide solution is added to the aqueous phosphonic
acid solution, there is competition between the water molecule
and the phosphonic acid groups to coordinate with the posi-
tively charged titanium. Strong coordinative interaction of a
highly water soluble organophosphorus precursor (benzene-
1,3,5-triphosphonic acid) with titanium(IV) could prohibit the
formation of oxide material.24
Here we have used REFLEX software for indexing the crystal
planes of highly crystalline porous titanium(IV) phosphonate
nanoparticles and the extent of matching is verified with the
CELSIZ unit cell refinement program. The wide angle X-ray
diffraction pattern of HTiP-7 matches well with the triclinic
crystal phase with unit cell parameters a = 11.328(0.018) Å, b =
14.184(0.005) Å, c = 22.227(0.010) Å, α = 94.486°(0.073), β =
97.279°(0.096), γ = 93.265°(0.065) (Table 2). The very high unit
cell volume (V = 3523.863 Å3) together with the quite small
standard deviation (ESD = 5.59) is in good agreement with the
triclinic crystal phase of HTiP-7. The reflections from different
planes (hkl) of the polycrystalline material and the correspond-
ing d spacings are given in Table 3.
−5
12
−1
0
8
Spectroscopic study
The FT IR spectral bands between 4000 cm−1 to 400 cm−1 of
HTiP-7 are shown in Fig. 3. The peak centered at 554 cm−1 is
the characteristic peak for the Ti–O stretching vibration. No
additional peak in the range <500 cm−1 confirms the absence
of Ti–O–Ti bonding in the porous framework. The peak cen-
tered at 1040 cm−1 is due to the phosphonate (P–O–Ti) stretch-
ing vibration. The absence of a set of bands at 943 cm−1 and
1142 cm−1 confirms the absence of free P–OH and PvO in the
porous matrix. Hence, it is expected that all P–O bonds are
coordinated to titanium forming –P(O–Ti)3 units24 and the
Table 2 Unit cell parameters of HTiP-7
Parameters
Deviations
a = 11.328
b = 14.184
c = 22.227
α = 94.486
β = 97.279
γ = 93.265
V = 3523.863
0.018
0.005
0.010
0.073
0.096
0.065
ESD = 5.590
Fig. 3 FT IR spectrum of HTiP-7.
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number of surface defect sites (P–OH) is very low. The charac-
teristic peaks at 1406 and 1588 cm−1 confirm the presence of a
benzene ring in the porous architecture.21 A prominent peak
at 1632 cm−1 could be attributed to the surface adsorbed water
molecules.
Solid state MAS NMR studies
Solid state 13C CP MAS and 31P MAS NMR experiments gener-
ally provide information about the chemical environment
around C and P nuclei in the hybrid metal phosphonate
framework. In Fig. 4, the 13C CP MAS NMR of HTiP-7 is shown.
It exhibits two peaks at 130.7 ppm and 134.7 ppm with almost
similar intensity, indicating the presence of two types of
carbon with equal number (shown in the inset of Fig. 4). The
31P MAS NMR of HTiP-7 is shown in Fig. 5. Only one peak at
−1.11 ppm signifies that only the 111 type of bonding is
present around P nuclei. This information further proves the
presence of only –P–(O–Ti)3– units around phosphorous
nuclei.25 These spectroscopic results further suggested the
presence of an organic moiety in the porous titanium(IV) phos-
phonate framework of HTiP-7.
Fig. 5 31P MAS NMR spectrum of HTiP-7.
Electron microscopic analysis
The characteristic HR TEM image of HTiP-7 is shown in
Fig. 6A. It is seen from this image that the hybrid titanium
phosphonate nanoparticles with uniform spherical mor-
phology (particle size ca. 23–25 nm) are spread throughout the
specimens. A closer view of the individual nanoparticle
(Fig. 6B) revealed that white spots of dimension ca. 1.5 nm are
distributed throughout the matrix of the nanoparticles (white
spots marked with a white circle). The presence of these micro-
pores in the framework of HTiP-7 could be attributed to the
cross-linking of the organophosphorous ligand and titanium.
The self-assembled nanostructure in the material is also seen
in this high resolution image, where the individual tiny
Fig. 6 HR TEM images of HTiP-7: low (A) and high (B) magnification; UHR-TEM
images of HTiP-7: low (C) and high (D) magnification.
nanoparticles adhere to each other, maintaining a uniform
distance between them. This self-assembled nanostructure is
the origin of one broad small angle X-ray diffraction peak as
shown above (Fig. 1). Further, the presence of medium and
large size mesopores is quite clear from the high resolution
electron microscopic image shown in Fig. 6B. Large interparti-
cle mesopores of dimension ca. 4.8–5.0 nm are encircled with
red circles in Fig. 6B. Further, the UHR-TEM analysis of
HTiP-7 clearly proves the presence of medium and large size
Fig. 4 13C MAS NMR spectrum of HTiP-7.
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Fig. 8 FE SEM image of HTiP-7.
Fig. 7 SAED pattern of HTiP-7.
mesopores throughout the specimen (Fig. 6C). A closer view of
a small part of this specimen revealed that very tiny nanocrys-
tals of dimension ca. 3.42 nm are self-aggregated to form
medium size mesopores (ca. 3.0 nm) together with an inter-
particle distance of ca. 3.4 nm (Fig. 6D). Interparticle spacing
as estimated through small angle powder XRD (Fig. 1) nicely
resembles the electron microscopic data. The elemental
mapping of the material with EDX analysis (Fig. S1†) clearly
proves the presence of carbon, phosphorus, titanium and
oxygen in the porous framework. The high crystallinity of the
material has been supported by its well-resolved selected area
electron diffraction (SAED) pattern shown in Fig. 7. The diffrac-
tion spots suggest the formation of a triclinic crystal phase of
the hybrid material with indexed crystal planes. The FE-SEM
image of the material is shown in Fig. 8. From this image it is
clear that the hybrid titanium phosphonate material is com-
posed of uniform spherical nanoparticles of dimension
ca. 25 nm. Thus, the electron microscopic analysis reveals that
the tiny microporous nanocrystals of ca. 3.5 nm size are self-
assembled to form uniform aggregated spherical nanoparticles
of ca. 25 nm dimension and mesoscopic void space in the
hybrid material.26
Fig. 9 TG (a) and DTA (a1) profiles of HTiP-7 and TG (b) and DTA (b1) profiles
of HTiP-7RB.
correspond to the burning of organic fragments of the hybrid
framework. The breaking of C–C, C–P and C–H bonds
extended up to 900 K. The 20.4% weight loss of the material in
this temperature interval is mainly due to the burning of the
carbon fragment of the hybrid material. We have also carried
out variable temperature powder XRD analysis of the titanium
phosphonate material to understand the thermal stability
(Fig. 10). As seen from the figure, the structure is very stable at
Thermal analysis
The stability of the hybrid titanium(IV) phosphonate and the 450 K. At 650 K a considerable decrease in the intensity of
dye loading capacity of the material have been determined some of the major diffraction peaks is observed, whereas at
from the TG-DTA profile under air flow. The TG and DTA plots 750 K many significant diffraction peaks disappear. At 900 K
of the samples are shown in Fig. 9 (a and a1 plots correspond the crystallinity of the materials decreases hugely with some
to the TG and DTA profiles of HTiP-7 and b and b1 correspond additional weak peaks around 2θ values 25.3°, 37.7°, 48.1°
to those of HTiP-7RB). The TGA plot of HTiP-7 shows three dis- which could be attributed to the formation of anatase TiO2 at
tinct weight losses. The first 5.2% weight loss up to 373 K is higher temperature. The first 5.2% weight loss up to 373 K
attributable to the loss of intercalated and adsorbed water remained the same for HTiP-7Rb also. Now the second and
molecules in the sample. The second and third weight losses third weight losses of HTiP-7RB correspond to the burning of
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Fig. 11 N2 adsorption/desorption isotherm of HTiP-7. Adsorption points are
marked by filled circles and those for desorption by open circles. NLDFT pore
size distribution is shown in the inset.
Fig. 10 Temperature variation XRD of HTiP-7.
organics in the hybrid framework and the adsorbed dye mole-
cules. The breaking of bonds extended up to 900 K. Now the
30.5% weight loss in the temperature interval is due to the
burning of carbon fragments of the hybrid material and
adsorbed Rose Bengal molecules. Comparing the weight loss
of HTiP-7 without dye loading, it can be concluded that
HTiP-7 has 10.1% dye loading capacity, which is good to
explore its potential in the photon-to-electron energy transfer
process.
adsorption takes place and a strong capillary condensation is
observed at the relative pressure of 0.60–0.97, suggesting the
presence of appreciable amounts of mesopores in the material.
The isotherm does not saturate in the higher relative pressure
also, suggesting the presence of a small to large range of meso-
pores in the material. Due to the presence of applicable
amounts of mesopores in HTiP-7, at the higher relative
pressure (P/P0 > 0.7) the capillary evaporation and conden-
sation do not take place at the same pressure, which leads to
production of a hysteresis loop.29 The Brunauer–Emmett–
Teller (BET) surface area of the material is 255 m2 g−1 with a
total pore volume of 0.4635 cc g−1. The pore size distribution
of the material has been done from the non-local density func-
tional theory (NLDFT)30 and is shown in the inset of Fig. 11.
The pore size distribution peak is centered at 1.4, 3.0, and
4.8 nm, suggesting that the material contains the pore in the
supermicropore to large mesopore region. This result agrees
very well with the HR TEM analysis described above. The mole-
cular dimension calculated by using MOPAC-2012 software
suggested that the size of the Rose Bengal molecule is 1.2 nm.
So the photosensitizer molecule (RB) can fit inside the porous
architecture composed of micro- and mesopores of HTiP-7.
For the samples synthesized under strongly acidic conditions
(pH ∼ 1) some of the phosphonic acid source remains proto-
nated, hence the cross-linking and formation of the three
dimensional porous framework (HTiP-7A) is considerably dis-
turbed. This is reflected in its low BET surface area (83 m2
g−1). On the other hand for the synthesis of titanium phospho-
nate under relatively strongly alkaline conditions, as the pH of
the medium increases (pH ∼ 10) a handsome amount of tita-
nium could be converted to titanium oxide (TiO2), which
blocks the pores of the HTiP-7 framework. Hence, the BET
surface area of the resulting sample HTiP-7B is also low
(123 m2 g−1) compared to HTiP-7. The observed optimum
temperature for the maximum number of cross-linking of Ti
Molecular formula of HTiP-7
TG-DTA analysis of the sample HTiP-7 indicates the presence
of 20.4 wt% organics in the material. On the other hand the
CHN analysis data suggested the presence of 16.8 wt% C and
1.27 wt% H in this porous framework. Further, the atomic
absorption spectroscopy (AAS) study revealed the presence of
25.9 wt% titanium in HTiP-7. From the FT IR and solid state
13C and 31P MAS NMR study, the presence of benzene-1,3,5-tri-
phosphonic acid in the porous framework is confirmed.
Taking all these elemental analysis results into consideration,
the molar ratio of carbon : titanium in HTiP-7 is 2.59. Thus,
the probable molecular formula of the porous architecture of
HTiP-7 can be written as Ti9[C6H3(PO3)3]3.89·xH2O. Hence the
formula of HTiP-7 can be expressed as Ti9(BTPA)4·xH2O.
N2 absorption/desorption
The porosity and surface area of the hybrid material are deter-
mined from the N2 adsorption/desorption isotherm at 77 K.
The adsorption/desorption isotherm of the material is shown
in Fig. 11. The isotherm can be classified as a type IV iso-
therm27 with an H3 hysteresis loop.28 If we analyze the iso-
therm very closely, then we can see that at the pressure range
(P/P0) 0.1–0.2 the curve is all about flat, characteristic of a type
I isotherm indicating the presence of relevant amounts of
micropores in the material. After (P/P0) 0.2, the multilayer
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and benzene-1,3,5-triphosphonic acid is 180 °C. At lower temp-
eratures (120 and 150 °C), somehow the cross-linking between
organo phosphonic acid and titanium is hampered, which is
the probable reason behind the relatively lower surface area of
the samples HTiP-7C (188 m2 g−1) and HTiP-7D (218 m2 g−1).
Corresponding adsorption/desorption isotherms and pore size
distributions are shown in ESI (Fig. S2 and S3†), which also
suggested lack of mesopore contribution compared with
HTiP-7 (Fig. 11).
UV-visible spectroscopy
The UV-visible reflectance spectra of the samples are shown in
Fig. 12. The bulk titanium phosphate (synthesized by using
H3PO4 as the P source under identical synthesis conditions)
showed absorption maxima at 258 nm, while hybrid titanium
phosphonate (HTiP-7) showed absorption maxima at 303 nm.
The corresponding band gap energies are 3.42 and 2.82 eV,
respectively. Basically HTiP-7 is the system where one free
hydroxyl group (–OH) attached to phosphorous of bulk tita-
nium phosphate has been replaced by a large conjugated
organic system (benzene-1,3,5-triphosphonic acid). Due to the
incorporation of a π-conjugated system into the bulk system
the energy gap between HOMO and LUMO is decreased com-
pared to bulk titanium phosphate, which has been reflected in
the corresponding UV-Vis reflectance spectrum and the
decrease in band gap energy.31 Respective band gap energies
for the materials are shown in Fig. 13. On the other hand, a
complicated UV-Vis reflectance spectrum arises for dye encap-
sulated HTiP-7 material with the absorption maxima at 557,
520 and 312 nm. The respective band gap energy is 2.05 eV.
The red shift of the UV-Vis spectrum of HTiP-7RB from
303 nm of HTiP-7 could be attributed to the interaction of the
titanium phosphonate molecular orbital by Rose Bengal
through RB → HTiP-7 charge transfer.32
Fig. 13 Band gap energies of BTP, HTiP-7, HTiP-7RB.
Photoconductivity
Bulk titanium phosphate (BTP), HTiP-7 and dye-loaded hybrid
titanium phosphonate HTiP-7RB were kept in the dark over-
night before measuring the dark current.33 Linearity in the I–V
characteristic curve proves the ohmic behavior34 of platinum
contacts on the samples (Fig. 14). From Fig. 14 it is clear that
HTiP-7RB conducts much more current than HTiP-7 and BTP
in the dark. Now to measure the light driven photoconductivity
of the materials, we excited the surface of the materials (BTP,
HTiP-7 and HTiP-7RB) with white light. In Fig. 15 we have
shown the growth/decay current for the materials at 10 V bias.
As soon as the white light irradiates the surface of materials,
Fig. 12 UV-Vis reflectance spectra of BTP, HTiP-7, HTiP-7RB.
Fig. 14 I–V plot of BTP, HTiP-7, HTiP-7RB.
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Fig. 15 Growth and decay current of BTP, HTiP-7, HTiP-7RB.
Scheme 1 Schematic representation of electron flow in the dye loaded HTiP-7
the current reaches 3.48 × 10−4 A for organic dye doped tita-
nium phosphonate, 5.09 × 10−5 A for hybrid titanium phos-
phonate and 1.39 × 10−6 A for bulk titanium phosphate. Hence
the photogenerated current ΔI (photocurrent minus dark
current)19 for photosensitizer doped titanium phosphonate is
0.35 × 10−4 A, while for hybrid titanium phosphonate it is 0.26
× 10−5 A and 0.1 × 10−6 A for bulk titanium phosphate. These
results indicate that dye doped self assembled mesoporous
titanium phosphonate nanoparticles (HTiP-7RB) generate 13.5
times more photocurrent than hybrid titanium phosphonate
(HTiP-7) and 3500 times more photocurrent than bulk tita-
nium phosphate (BTP). The interest in interfacial electron
transfer pathways is increasing day by day due to the exciting
solar cell application of the various TiO2 based nanomater-
ials.35 The mechanism of electron transfer involves the adsorp-
tion of various sensitizer molecules on the surface of
semiconductor materials with a wide band gap.36 The incor-
poration of a sensitizer molecule into the porous semiconduc-
tor materials helps visible light driven photoconductivity.37
Now when we irradiate HTiP-7RB with light, the optical exci-
tation of the RB injects an electron from its excited state to the
conduction band of the porous hybrid titanium phospho-
nate.38 This interfacial electron transfer process is much faster
than the decay of the excited state of RB to its ground state.39
The injected electron to the conduction band of titanium
phosphonate is transported by diffusion to the oxidized dye
and completes the total electrical circuit through the platinum
electrode (Scheme 1).40 But when we excite the hybrid titanium
phosphonate and bulk titanium phosphate with white light,
no such electron transfer process takes place without the trans-
fer of an electron from the valence band to the conduction
band of the materials. Due to the large band gap (3.2 eV) for
bulk titanium phosphate, it produces minimum photocurrent
(0.1 × 10−6 A) compared to hybrid titanium phosphonate nano-
particles. The band gap energy and photocurrent for hybrid
in the presence of light.
phosphonate enhance its visible light driven generation of
photocurrent.
Conclusion
A new mesoporous hybrid titanium(IV) phosphonate nano-
material has been synthesized by using benzene-1,3,5-triphos-
phonic acid as the organophosphorus source in the absence of
any template molecule. The origin of the small angle XRD
peak and mesoporosity in this material is due to the self-
assembly of tiny nanocrystals of titanium phosphonate and
the surface interactions of the aggregated nanoparticles. The
material has been assigned with a new triclinic crystal phase.
It showed high BET surface area (255 m2 g−1) together with a
pore volume of 0.46 cc g−1. Porosity of the material has been
utilized for the impregnation of the photosensitizer molecule
(dye) inside the pores. The photocurrent generated by sensi-
tizer entrapped titanium phosphonate material is quite higher
than other titanium oxide based nanomaterials. Thus, we can
design different hybrid titanium phosphonate materials
bearing organic functionalities to enhance the efficiency of the
photon-to-electron energy transfer process. Alongside, the
hybrid titanium phosphonate material with nanoscale porosity
may have potential biological applications in drug delivery and
in lithium ion batteries.
Acknowledgements
titanium phosphonate is 2.98 eV and 0.26 × 10−5 A. Thus, exci- MP and AKP thank CSIR, New Delhi for respective Senior
tation of electrons from the photosensitizer molecule and elec- Research Fellowships. AB wishes to thank DST, New Delhi for
tron transfer inside the pores of semiconductor titanium instrumental support through the DST Unit on Nanoscience.
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