A porous, crystalline truxene-based covalent organic framework and its application in humidity sensing

Article abstract of DOI:10.1039/c7ta05043g

Truxene is employed as a building block to successfully synthesize novel covalent organic frameworks (COFs). The condensation reaction between truxene (10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluorene, TX) and 1,4-phenylenediboronic acid (DBA) results in a crystalline COF with boron ester linkages (COF-TXDBA) and a surface area of 1526 m² g^-1, as confirmed by powder X-ray diffraction (PXRD) and Brunauer-Emmett-Teller (BET) surface area measurements. This is the first study where nanochannels generated by periodic COF planar layers are shown to ease the interactions of the boron ester linkages with the water molecules for efficient humidity sensing. The COF-TXDBA based % RH sensor exhibits a change of 3 orders of magnitude in impedance in the 11-98% RH range, with response and recovery times of 37 s and 42 s, respectively. The response transients measured by switching COF-TXDBA sensor back and forth in 4 loops of % RH range displays excellent reversibility of the sensor with a deviation of 2.3% in the switching process.

Full text of DOI:10.1039/c7ta05043g

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Tomer, N. Jena, I. Bala, N. Sharma, D. Nepak, A. De Sarkar, K. Kailasam and S. Pal, J. Mater. Chem. A, 2017,
DOI: 10.1039/C7TA05043G.
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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ARTICLE
Truxene based Porous, Crystalline Covalent Organic Frameworks
and it’s Applications in Humidity Sensing
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
b
b
Harpreet Singh, Vijay K. Tomer, Nityasagar Jena, Indu Bala, Nidhi Sharma, Devadutta Nepak,
DOI: 10.1039/x0xx00000x
b
b
a
Abir De Sarkar,* Kamalakannan Kailasam* and Santanu Kumar Pal*
www.rsc.org/
Truxene is employed as a building block to successfully synthesize new covalent organic Framework (COF). The
condensation reaction between Truxene (10, 15-dihydro-5H-diindeno[1,2-a;1’,2’-c]fluorene) (TX) and 1,4-
2
phenylenediboronic acid (DBA) results in boronic ester linkage based crystalline COF-TXDBA with a surface area 1526 m /g
as confirmed by Powder X-Ray Diffraction (PXRD) and Brunauer–Emmett–Teller (BET) surface area measurements. This is
the first report where nanochannels generated by periodic layers of COF planar sheets are shown to ease the interaction
of boronic ester linkage with the water molecule for efficient humidity sensing application. The COF-TXDBA based %RH
sensor exhibits 3 orders of magnitude change in impedance in 11-98 %RH range with response and recovery times of 37 s
and 42 s, respectively. The response transients of COF-TXDBA based %RH sensor by switching back and forth in 4 loops of
sensing shows the excellent reversibility of the sensor with a deviation of 2.3% in the switching process.
1
1-21
22-24
25-28
devices,
luminescence,
CO
2
capture,
proton
energy conversion,
The physical and chemical properties of COFs
conduction,
storage,
Introduction
2
9-31
32-37
38-40
41-43
catalysis,
4
4-47
and sensing.
Past century witnesses the structural evolution of porous materials
in terms of replacing inorganic components by organic building
blocks, achieving larger pores or building rigid to soft dynamic 2D
and 3D skeleton. An important development made in the last
decade is the construction of porous organic polymers such as
hypercrosslinked polymers (HCPs), polymers of intrinsic
microporosity (PIMs), covalent organic frameworks (COFs) and
conjugated microporous polymers (CMPs) becomes the main
content for advanced applications.
inclusively depend upon the building blocks and the covalent
linkage between them. However, a requirement of specific
geometries and rigid structures of building blocks to perform
reticular chemistry and very few kinetically reversible chemical
reactions, which result into crystalline COFs, limits its scope to
explore these applications to commercial aspects. To overcome
these challenges, continuous design of new building blocks to
accelerate this research field is the utmost requirement. Addressing
the mentioned challenges, we hereby introduce Truxene based
building block of COFs, for the first time.
1
-3
Among these organic porous polymers, covalent organic
frameworks (COFs) become very attractive because of their periodic
skeletons and ordered pores as the wide pore-size-distribution of
other amorphous networks may restrict, to some extent, their
Truxene (10,15-dihydro-5H-diindeno[1,2-a;1’,2’-c]fluorene) is a
C3 symmetric, star-shaped π-conjugated polyarene obtained by
trimerization of indan-1-one. It can also be formally considered as
4
further applications. COFs are devised by manipulating reticular
1
,3,5-triphenylbenzene derivative with three methylene clips,
chemistry in which organic building blocks are precisely linked via
5
6
7
responsible for keeping the four benzene rings coplanar and π-
conjugated. This induces a strong electron donating as well as
strong π-π stacking ability, which hints for the Truxene utility as a
building block in the development of advanced functional materials
for various applications. Truxene molecular derivatives have been
widely investigated in supramolecular chemistry, especially in
organic electronics. Truxene based dendrimer, oligomers, and
polymers were produced to use them as photosensitizers,
photoresists, organic lasers, photoluminescence materials, OLEDs,
covalent bonds like boroxine/boronate ester, imine, hydrazone,
8
9
triazine, imide bonds, etc. They are interesting materials from
physics perspective as their graphene-like π system triggers
interactions with excitons, electrons, holes, spins, ions and
1
0
molecules in confined space. Due to their exceptional high surface
area, large porosity and crystalline nature, COFs have emerged as
promising material in applications including semiconductors
48
a.
OFETs and OPVs. Further, octupolar character combined with
polyaromatic and rigid planar structure makes Truxene and its
derivatives a good candidate resulting in more efficient non-linear
optical (NLO) properties than the conventional dipolar
Department of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Mohali, Sector-81, SAS Nagar, Mohali 140306, India.
E-mail: skpal@iisermohali.ac.in
Institute of Nano Science and Technology, Phase 10, Mohali 160062, India.
b.
E-mail: abirdesarkar@gmail.com and kamal@inst.ac.in
4
9,50
molecules.
†Electronic Supplementary Information (ESI) available: Synthesis details, IR and
solid NMR spectra, and XRD. Full synthesis details and characterization of the, TGA,
XRD, band gap measurement, solid-state C{ H} CP/MAS NMR spectra and 1 H
NMR spectra, comparison table. See DOI: 10.1039/x0xx00000x
In spite of their type, porous materials have been always
explored for the sensing applications due to their unique host
matrices containing various functional species. The composition of
13
1
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the porous material, crystalline structure, surface area, 3-(3,4-dimethoxyphenyl)propanoic acid (a) To obtain compound a,
functionalization, stability, and morphology are crucial parameters 3,4-dihydroxyhydrocinnamic acid was refluxed with Me SO and
which affect crucial sensing attributes including sensitivity, dry K CO in acetone under N atmosphere. After 12 hr, the reaction
2
4
2
3
2
51
2 3
selectivity and response/recovery times. In this respect, COFs was stopped and cooled to room temperature. Solid K CO was
possess the above mentioned attributes which makes them removed by filtration and acetone was removed by vacuum
potential candidates for sensing applications. In this context, distillation. After distillation, the residue was dissolved in ethyl
various groups have designed the COF-based sensor for detection acetate. Organic layer was washed with water, brine and dried over
4
4,45
46,47
of explosives
and toxic metal ions.
anhydrous sodium sulphate. A brown coloured organic residue left
Exploiting the inherent crystallinity, enhanced surface area and after vacuum distillation was further refluxed in 50% (v/v) methanol
electron-rich linker groups, the precise detection of relative and 10% NaOH solution till that TLC showed a single spot. Reaction
humidity (%RH) using COFs is a paradigm shift from tradition metal- mixture was cooled and acidified with conc. HCl. A white colour
52
53
54
55
oxide semiconductor, silica, polymer, carbon nitrides and solid was precipitated, which was filtered and dried. Further
5
6
57
relatively new MOFs or inorganic/organic hybrid based %RH purification was done by recrystallization in hexane/ethylacetate
1
sensors. Keeping in mind, the affinity of boron ester linkage solvent mixture. Yield 85.2%; H -NMR (400 MHz, CDCl
3
) δ: 6.82 (d,
towards water molecule and facilitated interaction between them 1H), 6.77 (d, 2H), 3.89 (s, 3H), 3.88 (3H, s), 2.93 (t, 2H), 2.69 ((t, 2H).
1
3
due to enhanced diffusion effect of nanochannels, we subjected the
C -NMR (100 MHz, CDCl ): δ179.03, 148.86, 147.52, 132.74,
3
newly synthesized Truxene-based COF for the detection of %RH at 120.08, 111.57, 111.25, 55.90, 55.81, 35.90, 30.24.
room temperature. As far as we know, this is the first report in
which COFs are used for relative humidity (%RH) sensing and we 5,6-dimethoxy-1H-inden-1-one (b) PPA (86.8 g) was heated at 110
o
hope that this will open a new dimension for COFs real time
application.
C in a 100 ml round bottom flask closed with anhydrous CaCl filled
2
guard tube. Compound a (3.0 g) was added to this hot PPA and
mixed with glass rod and heated at 110 C for 45 mins. Reaction
o
Experimental
mixture was cooled and poured into cold water, which was
extracted with ethyl acetate. After removing the solvent by vacuum
distillation, the crude compound was purified by column
chromatography using hexane and ethylacetate solvent mixture.
Materials and reagents
After purification, compound b was obtained as a yellow solid. Yield
All chemicals and solvents were of analytical grade quality and used
without further purification. 3,4-dihydroxyhydrocinnamic acid,
polyphosphoric acid (PPA), 1,4-phenylenediboronic acid, Boron
1
4
5 %; H-NMR (400 MHz, CDCl
3
) δ: 7.19 (1H, s), 6.90 (1H, s), 3.97
1
3
(
3H, s), 3.91 (3H, s), 3.06 (2H, t), 2.68 (2H, t). C-NMR (100 MHz,
CDCl ) δ: 205.84, 155.37, 150.47, 149.34, 129.88, 107.51, 107.42,
3
3
tribromide (BBr ), potassium carbonate and dimethylsulphate were
1
04.16, 104.08, 56.31, 56.17, 56.03, 36.53, 25.58.
purchased from Sigma- Aldrich (Bangalore, India). Thin-layer
chromatography was performed on aluminium sheets pre-coated
with silica gel (Merck, kiesel gel 60, F254). Column chromatographic
separations were performed on silica gel (60–120 mesh). Structural
2
,3,7,8,12,13-hexamethoxy-10,15-dihydro-5H-diindeno[1,2-a:1',2'-
c]fluorine (c) Polyphosphoric ester (PPE) (10.4 g) was taken in a 25
mL round-bottom flask which was placed in a preheated oil bath at
1
stirred for 5 min maintaining the temperature at 140 C. the
temperature was subsequently increased to 160 C and reaction
mixture was stirred for 25 min. During this reaction condition,
excessive foaming occurred. The reaction mixture was then poured
in acetone and stirred well. A bright pink coloured solid was settled
at the bottom which was collected after repeated centrifugation by
washing with DCM until DCM layer will not become clear. Pink
coloured solid was dried in vacuum and was subjected to next
reaction without purification. (The pink coloured solid obtained was
insoluble in nearly all the solvents available in lab, So with an
assumption that ~99% of this pink solid is compound (c) we
subjected the crude product for the next reaction). Yield 45 %.
1
13
characterisation of the compounds was carried out by H NMR,
C
°
40 C. To this preheated PPE compound (b) (3.0 g) was added and
NMR (Bruker Biospin, Switzerland, Avance-iii 400 and 100 MHz
spectro- meters, respectively), infrared spectroscopy (Perkin Elmer
°
°
Spectrum
Photoluminescence (excitation wavelength 404 nm) and HRMS
Water g-2 synapt gases). NMR spectra were recorded using
deuterated chloroform (CDCl ) & deuterated dimethylsulfoxide
AX3),
UV-Vis
spectrophotometer
3000+,
(
3
6 2
(dmso-d ) and tetramethylsilane as an internal standard. N
adsorption-desorption isotherms were measured on Nova
Quantachrome at 77 K. Before measurements, the samples were
degassed overnight under vacuum at 120 °C. The specific surface
0
area was calculated in the pressure range of 0.01 < p/p < 0.25 using
the BET micropore assistant surface area method. Density
functional theory (DFT) method was employed to approximate the
pore size distributions of COF-TXDBA sample. Total pore volume
Hexahydroxy truxene (d) The pink coloured solid obtained in the
previous step was insoluble in nearly all the solvents available in
lab, So with an assumption that ~99% of this pink solid is compound
2 0
was obtained by the amount of N adsorbed at P/P ~ 0.9 and 0.95,
assuming that adsorption on the external surface was negligible
compared to adsorption in pores. SEM studies were done by using
Jeol 7600F model. TEM measurements were carried out using high
resolution transmission electron microscope (HRTEM) instrument
(
c) we subjected the crude product for demethylation and got
1
demethylated product hexahydroxy truxene d. Yield 45 %; H-NMR
(
(
1
400 MHz, DMSO-d
3H, s), 3.55 (H O, s), 4.04 (6H, s). C-NMR (100 MHz, CDCl
45.11, 137.28, 135.17, 132.04, 112.94, 109.47, 35.61.
6
) δ: 9.04 (6H, s), 7.35 (3H, s), 7.13 (3H, s), 3.90
(
JEOL) at an acceleration voltage of 200 kV.
1
3
2
3
) δ:
Synthesis
Synthesis of COF-TXDBA (e) Compound d (150 mg, 1 eqv.) and was
dissolved in 30 ml dioxane and stirred till that it is not dissolved
completely. Two equivalents of 1,4-phenylenediboronic acid was
Synthetic route to achieve the hexahydroxy funactionalized Truxene
is given in scheme S1† and synthesis process of all the intermediate
is given below in detail.
added to solution and subjected to sonication, make
a
2
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homogeneous mixture. After this 6 ml mesitylene was added to this Thermogravimetric analyses (TGA) revealed that architectural
and subjected to sonication again. Finally 0.2 ml methanol was structure of COF-TXDBA is stable upto 410 °C with minimal weight
added to this 5:1 solution of dioxane and mesitylene. This reaction loss. The weight loss at lower temperatures < 200 °C is attributed to
°
mixture was reflexed at 110 C under Nitrogen atmosphere. After the evaporation of THF and dioxane present in the nano channels
three days precipitate was filtrate and washed with dry dioxane of COF-TXDBA (Fig. S3).†
followed by THF several times. Hence, obtained light brown
°
coloured COFs are dried under vacuum oven at 150 C. The The structure and crystallinity of COF-TXDBA has been analysed
synthesis scheme of COF-TXDBA is shown in Fig. 1.
using density functional theory (DFT) approach. Two possible
structural arrangement, in which two truxene units can linked to
diboronic acid unit (Scheme S2)†, has been considered to build the
periodic crystal lattices, Symmetric-COF-TXDBA (s-COF-TXDBA) with
symmetric honeycomb like pore and Asymmetric-COF-TXDBA (a-
COF-TXDBA) with asymmetric pore shape (Scheme S3)†. The
periodic crystal lattices with interlayer spacing of 3.74 Å have been
built upon the optimized geometries of s-COF-TXDBA and a-COF-
TXDBA using 6-311G basis set with B3LYP exchange-correlation
5
8
functions, as implemented in Gaussian 09 package. These layers
were stacked with 12Å interlayer offset to match the pore size
obtained from physisorption measurements. The simulated PXRD of
s-COF-TXDBA in its near eclipsed geometry with interlayer upset of
1
2Å resembles more likely the experimental PXRD patterns at low
angle regime with a sharp peak at 3.2°, reflection arising from (100)
plane which infers good crystallinity and long range molecular
ordering structure of COF-TXDBA with subsequent peaks from
(
110), (200) and (210) respectively.
1
00
Fig. 1 Synthetic scheme of Truxene based COF, COF-TXDBA; (a) Truxene (10, a)
5-dihydro-5H-diindeno[1,2-a;1’,2’-c]fluorene); (b) 1,4-phenylenediboronic
acid; (c) and (d) shows the colour change of reaction mixture due to COF
formation; (e) Schematic representation of COF-TXDBA.
1
(a)
110
Pawley refined
(
Rwp = 4.11%, R = 2.66%)
p
Expt. PXRD
Difference
200
210
Performance test for %RH sensing
Simulated PXRD
The humidity sensing measurements were performed on
Interdigitated electrodes (13.4mm x 7mm x 0.5mm) having 5 tracks
of Ag-Pd electrodes. Prior to measurements, a dilute paste of
sample in ethanol was dropped onto the electrodes and dried at 60
°
C. The controlled environments for %RH were prepared by using
air-tight chambers containing saturated aqueous salts solution of
LiCl, MgCl •6H O, MgNO •4H O, NaCl, KCl and K SO generating
1%, 33%, 54%, 75%, 84% and 98% relative humidity, respectively
2
2
3
2
2
4
1
2θ (degree)
at 25 °C. The electric impedance was measured using a two-probe
Keithley source meter. The measurements were carried out at 1V
AC voltage with frequency varying from 100 Hz.
(
b)
b)
Results and Discussion
13
1
The as-synthesized COF-TXDBA was subjected to FTIR and C{ H}
CP/MAS NMR characterization techniques to obtain the bonding
structural information. FTIR showed the strong bands at 1398, 1354
−1
and 1295 cm which is attributed to the asymmetric B−O stretches,
confirming the formation of boronate ester links. Furthermore, for
a boronic–based COF, characteristic band corresponding to C−B
stretching vibration in the boronate esters and C−O stretching
vibration in the catechol moieties were observed at approximately Fig. 2 Simulated PXRD of upset geometry of COF-TXDBA with Pawley
−
1
refinement;(a) Simulated PXRD patterns Experimental PXRD diffraction and
PXRD patterns after Pawley refinement;(b) Top view of eclipsed geometry of
COF-TXDBA upset by 12Å in order to account for the experimental pore size
distribution.
1
080 and 1241 cm , respectively as shown in Fig. S1.† We have
13
1
further characterized the resulting COF-TXDBA by solid-state C{ H}
CP/MAS NMR spectroscopy. The spectra displayed a dominant
aliphatic carbon signal centred at 36 ppm that originates from the
methylene bridging units. In the aromatic region, it displays four
broad signals at 148, 135, 126 and 106 ppm indicating the ten non-
equivalent carbons of the COF structure in Fig. S2.†
However, deviation at higher angle has been observed moving from
eclipsed packing to staggered packing. This infers the geometry of
COF-TXDBA is more likely towards the eclipsed form of s-COF-
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TXDBA. PXRD of both s-COF-TXDBA and a-COF-TXDBA in their as these hollow nanorods show similarity, when Fullerene C
eclipsed and staggered geometries have also been analysed (Fig. S5, crystalized from 2 mM solution in xylene solvent. It should be
6
0
6
0
S6, S7 and S8).†
noted that, Truxene can also be formally considered as a building
block of Fullerene C60, because of its alternative six and five
Finally s-COF-TXDBA with interlayer upset of 12Å have been member ring containing skeleton. So, we can expect that it can
considered for Pawley refinement with experimental PXRD result show the similar behaviour and appearance of similar nanorods
5
9
using Reflex module of Accelrys’s Materials Studio 7.0 package to with our ongoing work. As it is observed from the TEM images, Fig.
determine the lattice parameters, which results the unit-cell 3e to 3h, the edges of the images (especially 3f) at different
parameters as, a=b= 35.88 Å, c= 3.74 Å, α = β = 90°, and γ = 120°. magnifications show the scrambled sheet-like structural
Pseudo-Voigt function for peak broadening and Berrar-Baldinozzi morphology which is in agreement with the XRD data and in-line
function for asymmetry correction have been considered with DFT calculation results.
simultaneously to improve profile fitting, the refinement converged
with residuals; Rwp = 4.11% and Rp = 2.66%, resulting hexagonal
space group of P6/M as shown in Fig. 2.
1000
2
2
1
1
500
000
500
000
Desorption
Desorption
Adsorption
Adsorption
8
6
4
2
00
00
00
00
0
a
b
d
f
5
00
0
0
50 100 150 200 250 300 350 400
Pore width (A)
Pore width (Å)
c
0
.0
0.2
0.4
0.6
0.8
1.0
Re t e P ssure P
Rl ae l ai vt ive Pr er essure p // Pp
0
o
Fig. 4 Nitrogen isotherms at 77 K for COF-TXDBA and NLDFT pore size
distributions of COF-TXDBA (Inset).
The porous features of COF-TXDBA were determined by
measuring nitrogen gas (N ) adsorption at 77 K, revealing reversible
e
2
adsorption and desorption curves typical of type-IV isotherms with
a sharp uptake below p/p0 = 0.01 Figure 3. The significant
adsorption at p/p < 0.01 shows the presence of micropores with a
0
narrow distribution which shows the uniform condensation of the
monomers to form a boronate ester COF. Further the step observed
between 0.01 < p/p < 0.25 indicates condensation in mesopores.
0
The absence of hysteresis in adsorption-desorption curves is a
common nature of hexagonally aligned 1D mesopores materials, as
g
h
shown by the COF-TXDBA. It exhibit the surface area of around
2
1
526 m /g calculated in the pressure range of 0.01 < p/p
0
< 0.25
using the BET micropore assistant surface area calculations. Density
functional theory (DFT) method was employed to approximate the
pore size distributions of COF-TXDBA employing a cylindrical pore-
oxide surface model, which yielded average pores sizes of 23 Å.
Deviation of pore size from theoretical value (ideal pore size of 31 Å
Fig. 3 SEM image of the COF-TXDBA (a); A higher magnification image of a is predicted for the eclipsed form) can be taken into account due to
6
1,62
square portion (yellow highlight) of the Figure 2a (b); Nanorods structures of the significant offsets between the stacking layers.
the same COF at lower (X 10000) (c) and higher magnification (X 50000) (d),
There is a
steep increase of the adsorbed nitrogen at high relative pressures
p/p < 0.95), which is often assigned to outer surface area or
0
respectively. TEM images of COF-TXDBA at different magnifications (e-h)
(
interparticle voids of smaller COF-TXDBA particles, which was
confirmed by DFT pore size distribution analysis. The total pore
Scanning electron microscopy images at the different
magnifications in Fig. 3a and 3b reveals a uniform spongy
morphology of COF-TXDBA sample. However, as shown in Figure 3c
and 3d, formation of few hollow nanorods was also observed.
Attempts were made to enrich this nanorods phase by changing the
dioxane and mesitylene ratio. It was observed that the systematic
increase in the mesitylene ratio from 0 to 50% initially increases the
crystallite size and promote the formation of vesicles of random
size and shape along with the nanorods (Fig. S9).† Further work is
going on to optimize the conditions to enrich the nanorod phases,
3
-1
0 0
volumes calculated at p/p = 0.95 is about 0.35 cm g and at p/p =
3
-1
0
.99 is about 0.45 cm g . These crystalline features of the COF-
TXDBA samples and high surface area attributes were utilised for
humidity sensing application. Up to our best knowledge this is the
first report to use a COF for humidity sensing applications so far.
Humidity sensing
4
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The %RH dependent impedance response of the prepared COF-
TXDBA in the 11-98 %RH range is shown in Fig. 5a. As can be seen,
at low %RH, the sample exhibit very high impedance, however, as
the %RH increases, a dramatic decrease in impedance was
observed. The COF-TXDBA exhibits ∼3 orders of magnitude change
in impedance in 11-98% RH range. Hysteresis is another important
attribute to estimate the reliability of sensor and is defined as the
maximum difference between the processes of adsorption and
desorption. Generally, a sensing material undergoes hysteresis
effect at increasing and decreasing %RH. The hysteresis was
measured by switching the sensor between the closed chambers
(
a)
(b)
Time (s)
Time (s)
Fig. 6 Response and recovery curves of COF-TXDBA (a). Repeated response
and recovery characteristics of COF-TXDBA measured at consecutive
intervals of time in four loops (b).
(11 – 98 %RH and 98 – 11 %RH). The hysteresis curves for COF-
TXDBA based humidity sensor are shown in Fig. 5b, where solid
circle line represents the process of adsorption (11-98 %RH) and
hollow circle line represents the process of desorption (98-11 %RH).
The maximum humidity hysteresis error was ~2.5% for COF-TXDBA
in the range of 11–98 %RH.
To study the effect of humidity on the stability of COF-TXDBA,
PXRD patterns were recorded for 5 loops after every cycle (11 → 98
→
11 %RH) of measurement. The sample was scratched out from
(a)
(b)
the sensing substrate and dried at 100 °C for 10 mins. As can be
seen in Fig S10a, the crystallinity of the COF-TXDBA remains intact
during the test experiments as can be seen from characteristic
sharp peak of COF peak at 3.2º in PXRD. However very low intensity
extra peaks also appeared near 2ϴ= 18º and 25º, which shows very
few boronic ester bond are dissociating during the experiment. The
†
SEM image of COF-TXDBA (Fig S10b) after fifth cycle of humidity
sensing experiment also confirms that the morphology of the COF
was retained and confirms the excellent stability of the material.
Thermogravimetric analyses (TGA) of COF- the sensing experiment
shows nearly 10% weight loss after 250 °C as compare to as
synthesized COF-TXDBA, which can be attributed to formation of
small amount of oligomers during interaction of water molecule
Relative Humidity (%)
Relative Humidity (%)
Fig. 5 Humidity sensing curves showing decrease in impedance with increase
in %RH (a). The hysteresis curve showing adsorption-desorption responses
measured in the 11-98 %RH range (b).
and COF-TXDBA Which also confirm that condition used during the
.
†
The response and recovery time are other important parameters to sensing experiment is not affecting the COF much (Fig. S3) . The
evaluate the performance of humidity sensor and defined as the PXRD patterns as a result of time-dependent stability test for COF-
time taken by a sensor to achieve ~90% of total impedance change TXDBA were recorded by placing the sample in chamber containing
5
2,
in case of the process of adsorption and desorption, respectively.
98 %RH for 10 min, 20 min, 30 min and 50 min time interval. PXRD
5
3
Fig. 6(a) shows the response and recovery curve of COF-TXDBA was recorded after drying the sample at 100 °C for 10 mins. The
measured continuously in the lowest and highest humidity PXRD patterns of COF-TXDBA measured as a result of time
environment. The response time (humidification from 11-98 %RH) dependent stability test in Fig S11 shows that the sample can
was about 37 s and the recovery time (desiccation from 98-11 %RH) sustain its long range order and structural morphology in high
was around 42 s.The response transients of COF-TXDBA based %RH humidity conditions (98 %RH) as well. The BET measurement study
sensor were measured by switching back and forth the sensor in 4 for COF-TXDBA after exposing the sample to 98%RH and thereafter
loops of 11 %RH and 98 %RH, respectively Fig. 6b. It was observed
drying overnight (120 °C) shows that the porous feature of the COF
that the impedance of the sensor retains its original value, when
humidity is restored to the initial state, which indicates the
excellent reversibility of the sensor. The standard deviation in
process of switching of sensor response was 2.3%. A comparison of
humidity sensing performance with those of published literature in
Table – T1† shows that our material exhibit satisfactory %RH
sensing attributes as compared to than that of conventional %RH
sensors under the similar test conditions.
remain intact after this treatment. However, a decrease in the
surface area was observed for the treated COF (1048 m²/g) as
compared to the as synthesized COF (1526 m²/g). The long-term
stability in response of the COF-TXDBA sensor was measured by
†
recording the sensor impedance at each %RH for 70 days (Fig S12) .
A standard deviation of just 2.3% which is calculated as the average
value of impedance at each %RH level accounts for the excellent
stability of the COF-TXDBA material.
Thus the high surface area and ordered micropores/ mesopores in
this COF-TXDBA are effectively utilized for addressing any target
oriented, here in, effective humidity sensing which further opens up
the broad role the COFs can play in different applications. These
functional designed COFs will be good candidates for being
explored in sensing of different other analytes like VOCs.
Mechanism: The reason for this good response towards humidity
sensing is due to the presence of boronic ester linkages, which
interacts with the water molecules in the planar sheets of COF
backbone. Due to the high surface area and ordered micropores/
mesopores in this COF-TXDBA, a large number of boronic ester
active site is exposed for the interaction of water molecule. Under
the conditions of low %RH, the little amount of water molecules are
adsorbed on the surface of sensor and form first chemisorbed layer
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ARTICLE
Journal Name
through Lewis acid-base interaction, where empty orbitals of boron
atom interact with the electron lone pair of oxygen atom of water
Acknowledgements
molecule. This little amount of water molecules forms
a
This work was carried out with the financial support from IISER,
Mohali and INST, Mohali. Dr. SK Pal is grateful for INSA Medal for
Young Scientist 2015 and the financial support from INSA bearing
Sanction No. SP/YSP/124/2015/433. K Kailasam thanks DST-SERB
for the Early Career Research Award (ECR), File
discontinuous layer and causes hinder in the free movement of
water molecules as a result high impedance was observed under
low %RH conditions. However, high %RH pushes the system for
multilayer adsorption of water molecules and helps in the
formation of continues array of water molecules in nanoporous
channels. This continues array of water molecules provides a No. ECR/2016/001469 and DST-Nano Mission funded Technology
favourable condition for the process of charge transportation Project, File No. SR/NM/NT-06/2016. Dr. V K Tomer and Dr. D
+
+
followed the Grotthuss chain reaction (H O + H O → H O + H O) Nepak thank INST Mohali for the postdoctoral fellowship and Mr. N
2
3
3
2
and an abrupt increase in conductivity of the sensor was
Jena and Ms. N Sharma also thank INST Mohali for the doctoral
fellowship. We are grateful to NMR, HRMS, SEM and SAXS/WAXS
facility at IISER Mohali. We sincerely thank Mr. Indrajit Singh for
helping with SEM measurements at IISER Mohali. We also thank
6
3
observed. Under high %RH conditions, the electric field ionizes the
+
water molecules to produce hydronium ions (H
3
O ) as charge
+
+
carriers, which upon hydration produces protons (H O → H O + H )
3
2
and hopping of these protons between adjacent water molecules
become easier at high %RH.
2
INST, Mohali for providing N Physisorption and TEM facility. ADS
and NSJ remain highly thankful to Center for Development in
Advanced Computing (CDAC), Pune for providing its
supercomputing facilities on PARAM-YUVA-II.
In addition to the sensing attributes, we tried to explore the
optical properties of COF-TXDBA samples for their application in the
related areas. The diffuse reflectance spectra show the sample has
wide absorbance in the visible region centred at around 450 nm
with the significance tail absorption from 800 nm onwards, see Fig.
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Table of Contents:
Sensing mechanism illustrating proton hopping between the water molecules physisorbed on the COF-TXDBA
surface