Silicalite-1/glass fibre substrates for enhancing the photocatalytic activity of TiO2

Article abstract of DOI:10.1039/c4ra15850d

Silicalite-1 (S1) coatings were prepared on silica wool substrates by hydrothermal synthesis and subsequently immersed into a Ti-containing sol at a steady rate of 30 cm min^-1. The material was annealed in a furnace at 90 ?°C for 2 h and 550 ?°C for 2 h to create a silica fibre core surrounded by concentric layers of silicalite-1 and TiO₂. The resulting samples were characterised by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) surface area measurement. The photocatalytic activity of the samples was evaluated using the intelligent ink test and during degradation of stearic acid under UVA light (?? = 365 nm). The new coated-fibres were shown to be substantially better photocatalysts than comparable TiO₂ coatings on plain glass fibres. The TiO₂/S1/glass fibres have potential use in air/water cleaning applications.

Full text of DOI:10.1039/c4ra15850d

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Silicalite-1/glass fibre substrates for enhancing the
photocatalytic activity of TiO2
Cite this: RSC Adv., 2015, 5, 6970
*
F. T. Ozkan, R. Quesada-Cabrera and I. P. Parkin
Silicalite-1 (S1) coatings were prepared on silica wool substrates by hydrothermal synthesis and
subsequently immersed into a Ti-containing sol at a steady rate of 30 cm minꢀ1. The material was
annealed in a furnace at 90 ꢁC for 2 h and 550 ꢁC for 2 h to create a silica fibre core surrounded by
concentric layers of silicalite-1 and TiO2. The resulting samples were characterised by X-ray diffraction
(XRD), Raman spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy
(XPS) and Brunauer–Emmett–Teller (BET) surface area measurement. The photocatalytic activity of the
samples was evaluated using the intelligent ink test and during degradation of stearic acid under UVA
light (l ¼ 365 nm). The new coated-fibres were shown to be substantially better photocatalysts than
comparable TiO2 coatings on plain glass fibres. The TiO2/S1/glass fibres have potential use in air/water
cleaning applications.
Received 5th December 2014
Accepted 19th December 2014
DOI: 10.1039/c4ra15850d
www.rsc.org/advances
processes in industry, including adsorption, ion-exchange,
catalysis, etc. Some zeolites have been effectively combined
Introduction
with TiO2, taking advantage of their active participation in
electron-transfer processes, in order to enhance photocatalytic
activity.16–18 These TiO2/zeolite systems have been made in
powder form16,19,20 or as sheets by a papermaking technique.21,22
However, the latter forms still lead to the same technical
disadvantages observed in the use of photocatalysts in powder
form, such as undesirable particle agglomeration and the need
of further separation processes in the slurry material aer the
photocatalytic reaction.
Silicalite-1 (S1) is a well-known MFI-type zeolite and a model
system in studies of hydrothermal crystal growth.23 This zeolite
has been successfully used in combination of TiO2 in the
treatment of volatile organic compounds.24 In that work, the S1
zeolite was grown directly on stainless steel sheets as part of a
membrane-catalyst for the degradation of trichloroethylene.
Here, highly photoactive systems were prepared by direct
hydrothermal deposition of a thin layer of silicalite-1 on both
glass bres (silica wool) and glass slides, followed by the
The immobilisation of photocatalysts has been identied as a
key factor in photoreactor design for environmental applica-
tions.1 The process of ltration and resuspension of powders is
a major disadvantage in the efficient performance of photo-
reactors. Removing photocatalysts in powder form from the
solution can be very difficult and the particles tend to aggregate
during the cycle process, considerably reducing their activity. As
reviewed recently,1 various strategies have been followed in the
structural design of lters involving the use of non-woven
fabric, ceramic foams, porous materials, etc. These supports
are devised to offer high surface-to-volume ratios and – in the
case of those composed by bres, they can be conveniently light,
exible and cost-effective. Woven glass meshes and glass wool
have been widely used as substrates for titanium dioxide
(TiO2)-based materials in photoreactors for water and air
cleaning applications.2–10 The deposition of TiO2 on glass bres
is benecial in terms of active surface area exposed and inter-
facial charge carrier transfer rate, which are key features in the
efficiency of photocatalytic systems. Unfortunately, the immo-
bilisation of TiO2 on plain glass bres reduces the specic
surface area of the photocatalyst relative to the powder form.
An interesting and simple approach to promote roughness
on glass bres is the interpolation of high-surface area coating
materials previous to the deposition of TiO2. Conveniently,
many zeolites can be grown as continuous lms on brous
materials.11–15 Zeolites are crystalline microporous materials
that have been used in many applications and chemical
Fig. 1 Schematic diagram illustrating the deposition steps described in
this work. A first layer of silicalite-1 zeolite is deposited on glass fibres
(silica wool) in situ during hydrothermal synthesis and then calcined in
two steps at 90 ꢁC (2 h) and 550 ꢁC (2 h). A second layer was then dip-
coated from sol–gel solution and finally annealed at 550 ꢁC.
Materials Chemistry Research Centre, Department of Chemistry, University College
London, 20 Gordon Street, London WC1H 0AJ, UK. E-mail: i.p.parkin@ucl.ac.uk
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deposition of TiO2 as synthesised using a sol–gel method. The surface bound hydroxyl groups.25 Annealing of the samples at
ꢁ
process is described in the schematic diagram presented in 550 C ensured the removal of any remaining solvents, alkoxy
Fig. 1. The silica wool is commercially available and consists of and hydroxyl groups and seemingly improved the packing of the
long amorphous silica bres with diameter of ca. 5–20 mm.13 Ti–O–Ti polymer system into a crystalline anatase network.
Aer the incorporation of the zeolite layer, a signicant change
A series of TiO2/silicalite-1/glass bre samples were
in surface area resulted in a drastic enhancement of photo- prepared, as shown in Table 1. The samples are denoted as (1–4)
catalytic activity, as evaluated during the degradation of stearic A and (5–8)B according to the hydrothermal conditions used for
acid and the reduction of resazurin dye.
the synthesis of the silicalite-1/glass bre materials (Table 1).
Results and discussion
Synthesis and characterisation of TiO2/silicalite-1/glass bre
materials
The silicalite-1 zeolite was synthesised via one- and two-step
hydrothermal methods adapted from the literature.13 The
specic details of the synthesis are given in the Experimental
section. Tetrapropylammonium hydroxide (TPAOH) and tet-
raethyl orthosilicate (TEOS) were used as template agent and
silicate sources, respectively. The reaction conditions are given
in Table 1. The glass bres were included in the hydrothermal
treatment for the deposition of the resulting silicalite-1
material.
The synthesis of TiO2 was carried out following a sol–gel
method based on the hydrolysis of a titanium precursor and its
subsequent polymerization into a Ti–O–Ti network.25 The
silicalite-1/glass bre material was dip-coated into the sol–gel
(withdrawal rate of 30 cm minꢀ1) so that a thin layer of the
titania precursor is deposited on the substrates surface and the
gelation of the sol is substantially accelerated. During
dip-coating, aggregation, gelation and drying occur in seconds
to minutes comparing to bulk sol–gel systems.26,27 The deposi-
tion of TiO2 on plain glass wool was also carried out by
dip-coating and used as reference material. The lms are
chemically bonded to the surface of the glass bres via a
condensation reaction of the titanium alkoxide/hydroxide with
Table 1 Synthesis conditions of the silicalite-1 material prepared by
one- and two-step hydrothermal synthesis. The gel types used in the
synthesis refer to: (A) TPAOH (2.0 g), TEOS (10.4 g) and water (15.1 ml)
and (B) TPAOH (1.2 g), TEOS (10.4 g) and water (15.1 ml). The BET
surface areas indicated were obtained from the annealed samples after
deposition of the TiO2 layer. The BET results for plain fibres and direct
TiO2-coated fibres were 5 and 10 m2 ꢀ1, respectively
g
Hydrothermal synthesis
conditions
1 step
2 step
Samples
Gel
T (ꢁC)
t (h)
T (ꢁC)
t (h)
BET (m2 gꢀ1
)
1A
2A
3A
4A
1B
2B
3B
4B
A
A
A
A
B
B
B
B
170
130
130
130
170
130
130
130
6
4
3
22
6
4
—
—
3
2
—
—
3
73
57
58
109
58
46
150
170
—
—
150
170
—
Fig. 2 X-ray photoelectron spectroscopy survey spectra of a typical
TiO2/S1/glass fibre material. The XPS high resolution spectra consist of
characteristic Ti 2p3/2 and 2p1/2 doublet and O 1s peaks.
3
22
2
—
44
51
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Fig.
3 X-ray diffraction patterns of a typical TiO2/S1/glass fibre
material (black line). The peaks from both anatase TiO2 (full circles) and
silicalite-1 zeolite (full diamonds) are indicated. The diffraction signal
from the glass substrate is included for reference (grey line).
Fig. 4 Scanning electron microscopy images of (a) glass fibre, (b) TiO2
coating on plain glass fibre, (c) and (d) silicalite-1 coating on glass fibre
(samples 2A and 3B, respectively).
Correspondingly, the TiO2/silicalite-1/glass bre samples are
denoted as TiO2/1A, TiO2/2A, etc.
shown in Fig. 4. The SEM analysis revealed that the direct
deposition of TiO2 on glass wool was highly heterogeneous and
partial delamination was clearly observed across the bre
(Fig. 4(b)). In general, the microstructural properties of the
silicalite-1 layer were similar in all the samples synthesised in
this work, consisting of rectangular plates.35,36 However, the
coating of the bres in the latter case was strongly depended on
the synthesis conditions. As a case study, Fig. 4(c) and (d) show
typical zeolite-coated bres in samples 2A and 3B, respectively.
It can be observed that, despite the higher concentration of
precursor template agent (TPAOH) and longer annealing times
involved in the synthesis of sample 2A (Table 1), sample 3B
showed rougher coatings and larger zeolite particles, likely due
to the high temperature conditions (170 ꢁC) used during the
second hydrothermal treatment compared to sample 2A
The chemical composition of the glass bre surface treated
with silicalite-1 and TiO2 was characterized by XPS analysis.
Fig. 2(a) shows the XPS survey spectra of the as-deposited
samples. In the quantitative analysis of the lms, carbon,
oxygen, titanium and silicon were detected. The limitations
used for tting the Ti 2p XPS spectra refer to the relation
between integral intensities of these doublet components 2 ꢂ
2p1/2 ¼ 2p3/2 (Fig. 2(b)). The two doublet separation was
5.76 eV.28 The peaks were assigned to Ti 2p1/2 and 2p3/2, located
at binding energies of 464.3 and 458.6 eV respectively.29,30 Si 2p
peaks were found at 103.5 eV, coming from glass bre and/or
silicalite-1 (Fig. 2(a)).31 The XPS analysis of the O 1s peak
(Fig. 2(c)) showed two different environments for oxygen,
namely Si–O–Si labelled at 532.9 eV and either Ti–O–Ti in the
TiO2 lattice or H–O bonds of the surface hydroxyl groups at
530.0 eV.32,33 XPS showed only the expected elements Ti, Si, O
with some residual carbon.
The structural properties of the as-deposited TiO2/silicalite-
1/glass bre materials were studied by XRD and Raman spec-
troscopy (Fig. 3). Although the amorphous nature of the glass
bres dominated the XRD patterns (broad feature at 5ꢁ/2q),
sharp reection peaks conrmed the presence of both anatase
TiO2 and silicalite-1 compounds on the glass wool.13 The
intense reections in the range of 3–4ꢁ/2q and 10–11ꢁ/2q are due
to the silicalite-1 MFI structure.13 Most of the rest are due to
TiO2 anatase phase, with the dominant characteristic (101) peak
at 11.5ꢁ/2q (Fig. 3). There was no evidence of the rutile form in
the XRD patterns. Further structural analysis was attempted
using Raman spectroscopy. The characteristic bands of anatase
TiO2 were clearly observed at 144 (Eg), 197 (Eg), 400 (B1g), 525
ꢁ
(150 C).
The inuence of annealing time on the zeolite coating
quality was further studied by comparison of samples 2A and 4A
(Fig. 5(a) and (b), respectively). Sample 2A was synthesised
following a two-step hydrothermal process (Table 1), with an
initial annealing period of 4 h at 130 ꢁC and a second one of 3 h
at 150 ꢁC, whilst sample 4A was produced aer a single step of
22 h at 130 ꢁC. It was interesting to note that, whereas the
morphology of both materials was apparently similar, the cor-
responding surface areas where signicantly different, accord-
ing to BET surface analysis. The BET area of plain glass bres
was 5 m2 gꢀ1 and it increased to ca. 10 m2 gꢀ1 aer deposition of
TiO2. In general, it was observed that the zeolite samples
prepared from gel A had a higher surface area than those
produced from gel B under the same conditions. The BET areas
of the TiO2/S1/bre systems are shown in Table 1. A general
decrease in surface area was observed aer the TiO2 coating,
which is likely due to the obstruction of zeolite pores in the dip-
coating process. Sample 4A showed the highest surface area and
underwent the longest hydrothermal treatment, 130 ꢁC over
22 h, during the silicalite-1 preparation process using high
TPAOH concentration (Table 1). It was interesting to note that
sample 4B, synthesised under similar temperature conditions
(A1g + B1g) and 634 (Eg) cmꢀ1 34 However, the Raman spectrum
.
of silicalite-1 is typically weak and a strong uorescence
hindered the assignment of the zeolite bands. No evidence of
rutile or any other phases were identied.
SEM was used to study the morphology of the uncoated and
coated glass bre surfaces with silicalite-1 and TiO2 coatings, as
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Fig. 5 Scanning electron microscopy images of a range of TiO2/sili-
calite-1/fibre samples: (a) sample 2A, (b) sample 4A and (c) and (d)
sample 4B.
Fig. 7 Selected photographs taken during the resazurin ink test on
TiO2/S1/fibre, TiO2/S1/glass slide and TiO2/glass slide materials under
UVA irradiation (1 mW cmꢀ2) over 150 min.
as those used for sample 4A but roughly half the amount of
TPAOH, showed substantially reduced surface area, despite the
apparently similar morphology of both materials (Fig. 5(c) and (d)).
The TiO2 lm thickness was measured by using cross-section
SEM images of the samples. The TiO2 layer was clearly observed
in unevenly coated bres (Fig. 6(a)). The thickness ranged
between 350–1300 nm and the average was ꢃ650 nm. The
cracks observed in the TiO2 coating (Fig. 6(b)) occurred during
annealing, as due to thermal expansion coefficient differences
between the TiO2 layer and the substrate.37
photocatalytic testing was difficult to control. Therefore, the
different TiO2 and S1 layers were deposited on glass slides for
the test, allowing fair comparison between the materials and
highlight the effect of using the S1 substrate in the system. As
expected, the colour of the ink changed from blue to pink
during irradiation of all TiO2-containing samples (Fig. 7).
Further irradiation degraded the ink system and the colour
disappeared. No photo-reduction of the resazurin dye was
observed for glass wool or silicalite-1/glass bre materials in the
absence of TiO2. It was interesting to note that, while the
reduction of the ink is only noticeable aer ꢃ1 h irradiation
(1 mW cmꢀ2) on TiO2 deposited on plain glass, the reaction
readily occurred within 15 min in the case of the TiO2/S1/glass
slide. At the same time, clear signs of reduction were noticed for
the TiO2/S1/bre system within the initial minutes of irradia-
tion (Fig. 7).
Photocatalytic activity of TiO2/silicalite-1/glass systems
The photocatalytic activity of the TiO2/S1/glass systems was
initially screened using the intelligent ink test under UVA illu-
mination (1 mW cmꢀ2).38 The details of this test are given in the
experimental section. It is based on the irreversible photore-
duction of resazurin dye molecule to resorun, which involves a
colour change from blue to pink, in the presence of a sacricial
electron donor (in this case, glycerol).38 The results of the ink
test are shown in Fig. 7. The amount of TiO2 deposited on the
glass bres and also the amount of wool used in the
Further photocatalytic testing was carried out during the
photo-induced mineralisation of a model organic pollutant,
octadecanoic (stearic) acid. The degradation reaction is
expressed by eqn (1) and typically monitored using infrared
spectroscopy.39
TiO hn $ E
=
2
bg
CH3ðCH2Þ CO2H þ 26O2
18CO2 þ 18H2O
ƒƒƒƒƒƒƒƒƒƒ!
16
(1)
The photocatalytic reaction rate is estimated from integrated
areas of typical C–H bands of the acid at 2958, 2923 and
2853 cmꢀ1. In the calculation of the reaction rate, linear
regression is estimated from the initial zero-order kinetics
reaction steps (30–40%). An estimation of the number of
Fig. 6 Scanning electron microscopy images highlighting (a) the
different aspects of the TiO2 coating (black arrow) and the silicalite-1
coating (red arrow) and (b) cracks on the TiO2 coating as a result of the
annealing procedure.
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lm was 6 times higher than that of the pure anatase TiO2
layer on glass, as expected from the corresponding increase in
surface area.
This efficient, cost-effective photocatalytic system is
perceived as a promising material for environmental applica-
tions, in particular as a lter for air cleaning photoreactors.
Experimental
All chemicals used in this work were purchased from Sigma-
Aldrich: glass (silica) wool, TPAOH (tetrapropylammonium
hydroxide) 1 M in H2O, TEOS (Tetraethyl orthosilicate),
$99.0%; acetylacetone, 99%; isopropanol, 99.5%; titanium(IV)
n-butoxide, 97%; butan-1-ol, 99.8%; acetonitrile, 99%;
resazurin, 92%; glycerol, 99.5%; hydroxy ethyl cellulose (average
Mv ¼ 90 000); stearic acid, $95%.
Fig. 8 (a) Curves of integrated areas of stearic acid IR bands and (b)
corresponding photocatalytic activities, given formal quantum effi-
ciencies, during UVA irradiation (3.7 mW cmꢀ2) of a TiO2/S1/glass
sample and respective individual components, as indicated. The cor-
responding areas of acid on plain glass (no photocatalyst) are included
for reference (empty symbols).
molecules of stearic acid degraded on the lm can be obtained
using a conversion factor (1 cmꢀ1 h 9.7 ꢂ 10+15 molecules)
reported in the literature.39 The photocatalytic activity of the
catalyst is widely expressed in terms of formal quantum effi-
ciency (FQE), dened as the number of acid molecules degraded
per incident photon.
Synthesis of the TiO2/silicalite-1/bre materials
In the synthesis of silicalite-1, 10 mmol of TPAOH (tetrapropy-
lammonium hydroxide) were mixed with 50 mmol of TEOS
(tetraethyl orthosilicate) in 15 ml of distilled water under strong
stirring conditions for 30 min. The mixture was then heated in
teon-lined stainless steel autoclave at different temperatures
and time periods, as indicated in Table 1. For the synthesis of
TiO2, 50 mmol of titanium n-butoxide were added to a solution
of acetylacetone (25 mmol) in 32 ml of butan-1-ol (350 mmol)
under strong stirring conditions during 1 hour. The solution
turned from clear, colourless to pale straw yellow without any
precipitation. 3.6 ml of an aqueous solution of isopropanol
(150 mmol) were mixed with the titanium n-butoxide solution.
The sol remained clear but deepened in yellow colour aer
stirring for an hour, which was followed by the addition of
acetonitrile (40 mmol) and stirred for another hour. The sol–gel
Fig. 8(a) shows the curves of integrated areas during UVA
irradiation (3.7 mW cmꢀ2) over 20 h. As it can be seen, the acid
is very stable under UVA light over the test period. As expected,
the results conrmed the photocatalytic behaviour of the lms
observed in the preliminary ink testing. Complete mineralisa-
tion of the stearic acid was only observed on the TiO2/S1/glass
system over the course of the experiment. It was interesting to
note that the silicalite-1 material deposited on plain glass
(no TiO2) also showed some photocatalytic activity beyond
instrumental error (Fig. 8(a)). This is in contrast with former
studies involving silicalite-1, where no photocatalytic activity
was observed in the degradation of trichloroethylene.24 The
corresponding formal quantum efficiencies (FQE) estimated are
shown in Fig. 8(b). The FQE of the TiO2/S1/glass system was 6
times that of the TiO2 layer deposited on glass. The enhanced
activity was attributed to the roughening effect of the zeolite
layer rather than to the zeolite intrinsic surface area, since it is
unlikely that the underpinning nanoporosity of the zeolite will
affect the photocatalysis performance of the system.
ꢁ
was nally le overnight. The material was rst dried at 90 C
for 2 hours and then calcined at 550 ꢁC for another 2 hours, and
was allowed to cool down to room temperature overnight.
Characterization techniques
X-ray diffraction patterns were carried out using a Stoe diffrac-
˚
tometer with monochromated Mo Ka1 radiation (l ¼ 0.7093 A)
in transmission mode over the angle range 2–40ꢁ/2qꢁ. In the
XRD analysis, the samples were ground gently and sieved in
order to isolate the coating materials from the amorphous glass
bres. Raman spectroscopy was performed using a Renishaw
Conclusions
A range of TiO2/silicalite-1 (S1)/glass bre materials were Raman system 1000 equipped with a helium–neon laser
hydrothermally prepared under different synthesis conditions (l ¼ 514.5 nm). Scanning electron microscopy (SEM) analysis
(temperature, annealing time and precursor concentration), as was achieved on gold-sputtered samples using a Jeol JSM-6301F.
indicated in Table 1, and their photocatalytic activity correlated The SEM images were obtained using SEMAfore soware. A
with surface roughness. The deposition of the S1 zeolite on pinch of the samples were taken and coated with a lm layer of
glass bres (silica wool) was carried out in situ during the gold to avoid charging. An X-ray photoelectron spectroscopy
hydrothermal process and the TiO2 layer was dip-coated from a (XPS) was carried out using a Thermo Scientic K-Alpha
sol–gel mixture and calcined at 90 ꢁC for 2 h and 550 ꢁC for 2 h. instrument with monochromatic Al-Ka source for the analysis
The photocatalytic activity of the lms was rst screened during of chemical composition and oxidation states of the samples.
photoreduction of the resazurin dye system (“intelligent ink”) High resolution scans were obtained for Ti(2p), C(1s), O(1s) and
under UVA irradiation and also evaluated during mineralisation Si(2p). C 1s was recorded for calibration of the spectrometer.40
of stearic acid. The photocatalytic activity of the TiO2/S1/glass The peaks were modelled using CASAXPS soware with binding
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Resazurin ink was prepared following a procedure from the
literature.39 The ink consisted of 0.3 g of hydroxy ethyl cellulose
(HEC) polymer, 3 g of glycerol and 0.04 g of resazurin dye in an
aqueous solution that was aged for 24 hours at 3–5 ꢁC. An
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aerosol spray-gun was lled with this indicator ink and used to
ˇ´
´
´
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coat the specied surface area of the samples evenly. The pho-
tocatalytic reduction of resazurin was observed by digital
photographic methods. For the stearic acid test, a thin layer of
the acid was deposited by dip-coating the lms into a 0.05 M
chloroform solution. The lms were irradiated under UVA
illumination (l ¼ 365 nm) using BLB lamps (Vilber-Lourmat,
2 ꢂ 8 W, 1 and 3.7 mW cmꢀ2 for the ink and stearic acid
tests, respectively). The mineralisation of stearic acid bands at
2700–3000 cmꢀ1 was monitored using a Perkin-Elmer RX-I
Fourier Transform Infrared Spectrometer.
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Acknowledgements
Kevin Reeves, Steven Firth and Martin Vickers are thanked for
assistance with SEM, Raman spectroscopy and XRD instru-
ments. FTO is funded by The Turkish Ministry of National
Education.
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