TiO2 nanoparticles doped SiO2 films with ordered mesopore channels: A catalytic nanoreactor

Article abstract of DOI:10.1039/c3dt53265h

Titanium dioxide (TiO₂) incorporated ordered 2D hexagonal mesoporous silica (SiO₂) films on a glass substrate were fabricated for use as a catalytic nanoreactor. Films were prepared using a tetraethyl orthosilicate (TEOS) derived SiO₂ sol and a commercially available dispersion of TiO₂ nanoparticles (NPs) in the presence of pluronic P123 as the structure directing agent. The effect of TiO₂ doping (4-10 mol% with respect to the equivalent SiO₂) into the ordered mesoporous SiO₂ matrix was thoroughly investigated. The undoped SiO₂ film showed a mesostructural transformation after heat-treatment at 350°C whereas incorporation of TiO₂ restricted such a transformation. Among all the TiO₂ incorporated films, TEM showed that the 7 equivalent mol% TiO₂ doped SiO₂ film (ST-7) had an optimal composition which could retain the more organized 2D hexagonal (space group p6mm)-like mesostructures after heat-treatment. The catalytic activities of the TiO₂ doped (4-10 mol%) films were investigated for the reduction of toxic KMnO₄ in an aqueous medium. ST-7 film showed the maximum catalytic activity, as well as reusability. A TEM study on the resultant solution after KMnO₄ reduction revealed the formation of MnO₂ nanowires. It was understood that the embedded TiO₂ NPs bonded SiO₂ matrix increased the surface hydroxyl groups of the composite films resulting in the generation of acidic sites. The catalytic process can be explained by this enhanced surface acidity. The mesoporous channel of the ST-7 films with TiO₂ doping can be used as a nanoreactor to form extremely thin MnO₂ nanowires.

Full text of DOI:10.1039/c3dt53265h

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TiO2 nanoparticles doped SiO2 films with ordered
mesopore channels: a catalytic nanoreactor†
Cite this: DOI: 10.1039/c3dt53265h
Jony Saha, Anuradha Mitra, Anirban Dandapat and Goutam De*
Titanium dioxide (TiO2) incorporated ordered 2D hexagonal mesoporous silica (SiO2) films on a glass
substrate were fabricated for use as a catalytic nanoreactor. Films were prepared using a tetraethyl ortho-
silicate (TEOS) derived SiO2 sol and a commercially available dispersion of TiO2 nanoparticles (NPs) in the
presence of pluronic P123 as the structure directing agent. The effect of TiO2 doping (4–10 mol% with
respect to the equivalent SiO2) into the ordered mesoporous SiO2 matrix was thoroughly investigated.
The undoped SiO2 film showed a mesostructural transformation after heat-treatment at 350 °C whereas
incorporation of TiO2 restricted such a transformation. Among all the TiO2 incorporated films, TEM
showed that the 7 equivalent mol% TiO2 doped SiO2 film (ST-7) had an optimal composition which could
retain the more organized 2D hexagonal (space group p6mm)-like mesostructures after heat-treatment.
The catalytic activities of the TiO2 doped (4–10 mol%) films were investigated for the reduction of toxic
KMnO4 in an aqueous medium. ST-7 film showed the maximum catalytic activity, as well as reusability.
A TEM study on the resultant solution after KMnO4 reduction revealed the formation of MnO2 nanowires.
It was understood that the embedded TiO2 NPs bonded SiO2 matrix increased the surface hydroxyl
groups of the composite films resulting in the generation of acidic sites. The catalytic process can be
explained by this enhanced surface acidity. The mesoporous channel of the ST-7 films with TiO2 doping
can be used as a nanoreactor to form extremely thin MnO2 nanowires.
Received 19th November 2013,
Accepted 23rd December 2013
DOI: 10.1039/c3dt53265h
www.rsc.org/dalton
activity of embedded NPs by allowing the reactant molecules
to transport through regular interconnected pores. Accord-
1. Introduction
Ordered mesoporous SiO2 has attracted a great deal of atten- ingly, different synthetic strategies like post grafting of Ti
tion due to its well known potential applications as a catalyst,1 species in SiO2, substitution of Si4+ by Ti4+ at the SiO2 surface
catalyst support,2,3 adsorbent4–6 as well as template7,8 for the during micellisation in sol–gel,22 hydrolytic23 and non hydro-
synthesis of different shaped nanomaterials. The incorpor- lytic24 routes have been followed in the fabrication of TiO2
ation of TiO2, a semiconductor having a moderate energy band doped regular patterned SiO2 matrices. However, these
gap (Eg = 3.2 eV) inside the SiO2 matrix may open up several methods are tedious, time consuming and often result in dis-
prospects in the fields of photocatalysis,9–11 self-cleaning,12 organized SiO2 materials. The direct incorporation of a tita-
antibacterial13,14 and sensors15,16 as TiO2 can promote an nium species (using a soluble Ti salt or alkoxide) into a SiO2
interfacial charge transfer processes in the composite.17–19 The sol leads to a homogeneous TiO2–SiO2 mixture with the for-
photo-catalytic performance of TiO2 in toxic dye degradation mation of Si–O–Ti linkages at the sol stage, and therefore TiO2
improves when it is doped into another semiconducting metal crystallinity could not nucleate on the mesoporous SiO2 walls
oxide (FeTiO3, PbTiO3, etc.) of similar energy band gap.20,21 In after subsequent heat-treatment.9,25 We used a synthetic proto-
addition, a highly ordered mesostructure could enhance the col for developing a TiO2 nanoparticles (NPs) doped SiO2 film
with a hexagonal ordered mesostructure on a glass substrate
because of the easy handling of a film compared to the corres-
ponding powder in technological applications. For this
purpose a commercial dispersion of anatase TiO2 NPs was
Nano-Structured Materials Division, CSIR-Central Glass and Ceramic Research
Institute, 196, Raja S. C. Mullick Road, Kolkata 700032, India.
E-mail: gde@cgcri.res.in; Fax: +91-33-24730957; Tel: +91-33-23223403
added directly into the SiO2 sol in the presence of P123. The
resulting sol was used to prepare the TiO2 NPs doped ordered
mesostructured SiO2 on glass. In this regard, it is to be noted
that applications of TiO2 have been mostly explored in the
presence of unsafe UV light.9–11,26 For example, Tada and
coworkers26 applied anatase TiO2 particles as catalyst for
†Electronic supplementary information (ESI) available: High angle XRD (Fig. S1)
and optical spectra (Fig. S2) of films, FTIR, Raman & TEM studies of the TiO2
dispersion used in this work (Fig. S3), spectral evolution during the catalytic
decomposition of KMnO4 in the presence of the ST-7 film, pseudo first order
plot with photos of KMnO4 and MnO2 solutions (Fig. S4) and spectra of the
KMnO4 solution in the presence of the TiO2 dispersion (Fig. S5). See DOI:
10.1039/c3dt53265h
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−
removing harmful MnO4 from water using a UV reactor. So, amount of stock anatase dispersion in isopropanol was added
our objective was to utilize the film substrate as a reusable into the mixture of 3 g of hydrolyzed SiO2 sol and 0.3193 g of
catalyst for the degradation of a pollutant, such as KMnO4 in P123 (0.013 mol per mol of SiO2) to obtain the ST-4, ST-7 and
water under ambient conditions. Interestingly in our case, we ST-10 sols, respectively. A sol without anatase dispersion (ST-0)
found formation of MnO2 nanowires instead of spherical clus- was also prepared as a control sample.
ters27 occurred in the decomposition process which seemed to
The films were deposited on soda-lime silica glass sub-
form along a particular direction in the hexagonally arranged strates from their respective sols by a dip-coating technique
pore channels of the mesoporous SiO2 template.28 Therefore with a lifting speed of 21 inch min−1. Similar films were also
the developed TiO2 NPs incorporated mesoporous films not deposited on pure silica glass and polished silicon wafers for
only acted as a catalyst to decompose the toxic pollutant UV-visible and FTIR studies, respectively. The as prepared
KMnO4 but also yielded a useful product, MnO2 nanowires, films were aged at room temperature for 10 h followed by
which may find useful applications in preparing cathode drying at 90 °C for 14 h, and finally heating at 350 °C (ramp
materials for batteries.29,30
2 °C min−1) in air for 1 h.
In this paper, we have reported the synthesis of anatase
TiO2 NPs doped ordered mesoporous SiO2 films, with varying
concentrations of TiO2, through an evaporation induced self
2.3 Inductively coupled plasma (ICP) analysis for TiO2
estimation
assembly (EISA) process. The effect of TiO2 on the periodicity At first, we converted all the TiO2 incorporated sols (used for
of the ordered mesoporous structure of the SiO2 films with coating deposition) into gels under ambient conditions, dried
different Si/Ti ratios (0, 4, 7 and 10 equivalent mol% TiO2) them at 60 °C followed by heat-treatment at 500 °C to prepare
have been thoroughly investigated. The catalytic activities of the organic free TiO2–SiO2 composite powders. Taking pre-
these films towards the decomposition of KMnO4 under cisely measured amounts of the composite powders from each
ambient conditions have been investigated, and a possible set of samples (i.e. 4, 7 and 10 equivalent mol% TiO2 doped
mechanism has been suggested.
SiO2 powders) in teflon containers, first SiO2 was evaporated
as gaseous silicic acid by treating with 6 ml of a mixture of HF
(ICP grade, 40 vol%) and HClO4 (ICP grade, 70 vol%) in v/v =
5 : 1 at 50 °C. The process was repeated one more time to
ensure the complete removal of SiO2. Afterwards, the SiO2 free
residue was dissolved in 3 ml of 5 vol% HNO3, evaporated to
dryness at 50 °C and finally dissolved in 50 ml of 5 vol%
aqueous HNO3 in volumetric flasks. This final solution was used
to estimate the Ti content by ICP analysis. Three sets of analyses
were done for each sample and the average data were reported.
2. Experimental
2.1 Chemicals
Tetraethyl orthosilicate (TEOS) and pluronic P123 (EO70PO20EO70,
Mav = 5800) were purchased from Sigma-Aldrich. Ethanol was
supplied by Merck and HF (40%), HClO4 (70%), HNO3 (65%)
and HCl (35.4%) were obtained from Ranbaxy. The anatase
TiO2 dispersion in isopropanol was purchased from Nano-
structured & Amorphous Inc. Potassium permanganate (KMnO4)
was obtained from Sarabhai M chemicals private limited.
Millipore Milli-Q water (resistivity ∼18.2 MΩ cm) was used.
2.4 Characterization
Low and high angle XRD patterns of the film samples were
recorded using a Rigaku Smart lab diffractometer operating at
9 kW using Cu Kα radiation (λ = 1.5405 Å). UV-visible spectra
of the coatings deposited on silica glass substrates were
2.2 Synthesis of TiO2 NPs doped SiO2 films
TiO2–SiO2 composite sols were prepared in the following way. measured using a Cary 50 scan spectrophotometer. FTIR
5 g of TEOS was first mixed with 11.05 g of ethanol in a round spectra of the 350 °C heated films deposited on both side
bottom flask and then a mixture of 1.5 g of water and 0.012 g polished silicon (Si) wafers were obtained from a Nicolet 380
of concentrated HCl was poured drop wise into the reaction spectrometer. Raman spectra of the scratched off film-powders
mixture. This solution was kept in stirring conditions for were recorded using a Renishaw inVia micro Raman spectro-
15 min and refluxed for 1 h to complete the hydrolysis of meter. The refractive index (RI) of the films deposited on the
TEOS. The final molar ratio of TEOS, ethanol, water and HCl Si wafers was measured by a J. A. Woolam Co. Inc. spectro-
was 1 : 10 : 3.45 : 1.3 × 10−2. An anatase dispersion was used to scopic ellipsometer and the data were reported at 633 nm
incorporate the TiO2 NPs into the sol. Before using the TiO2 (model M2000). FESEM of the heat-treated samples was ana-
dispersion, a certain quantity was taken from the as received lyzed by a ZEISS SUPRA 35VP field-emission scanning electron
sample and diluted with isopropanol. This stock solution was microscope. For TEM studies the scratched off films samples
then used to estimate the TiO2 content quantitatively. For this were placed on a carbon coated Cu grid and analyzed using a
purpose, 2 ml of this stock was first slowly evaporated to Technai G2 30ST (FEI) transmission electron microscope oper-
dryness followed by being heat-treated at 700 °C in a Pt cruci- ating at 300 kV.
ble and estimated gravimetrically. Four different sets of sols
were prepared by varying the TiO2 content (0, 4, 7 and 10 equi-
2.5 Catalytic tests
valent mol%) with respect to SiO2 and designated as ST-0, The TiO2 NPs doped ordered mesoporous thin films heat-
ST-4, ST-7 and ST-10, respectively. Accordingly, a calculated treated at 350 °C were used as catalysts for the catalytic
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decomposition of KMnO4 in water at room temperature. The
reaction was monitored by a Cary 50 scan spectrophotometer
fitted with a peltier temperature controller. For this purpose
two pieces of the heat-treated (350 °C) films of dimension
2 cm × 1 cm were dipped in 3 ml of 0.12 mM KMnO4 solution.
The concentration of KMnO4 was estimated by the titration of
permanganate with oxalic acid. The degradation of KMnO4
was also monitored in the absence of the film catalyst. At the
end of the reaction, the films were taken out from the solution,
washed with water followed by drying at 60 °C for 10 min and
used for the next cycle. Control experiments were carried out
with the equivalent amount of TiO2 dispersion used for the
sol/film preparation.
3 Results and discussion
Prior to any other characterization we estimated the TiO2 con-
tents in all the heat-treated (500 °C) TiO2–SiO2 composite
powders by ICP analysis. The results are presented along with
their nominal compositions in Table 1. ICP analysis showed
the presence of 3.63, 6.34 and 9.94 mol% TiO2 with respect to
equivalent SiO2 in the ST-4, ST-7 and ST-10 films, respectively.
3.1 XRD analysis
Low angle XRD was performed to analyze the mesostructures
of the dried (90 °C) and heat-treated (350 °C) films (Fig. 1).
The dried ST-0, ST-4 and ST-7 films exhibited similar patterns
(Fig. 1, panel a) showing four peaks at about 0.76, 1.3, 2 and
2.83° 2θ maintaining a 1/d spacing ratio of 1 : 1.70 : 2.65 : 3.72
and can be indexed as the (100), (110), (210) and (310) planes,
respectively, of a 2D hexagonal symmetry.31,32 In the case of
ST-10 only broad low angle XRD peaks were observed (Fig. 1a).
So a higher amount of TiO2 NPs present in ST-10 is expected
to disturb the ordered hexagonal pore structure.10 Low angle
XRD of the corresponding heat-treated (350 °C) films are
shown in panel b of Fig. 1. In the case of ST-0 (undoped heat-
treated SiO2) a clear change in the low angle pattern was
observed. The heat-treated ST-0 shows strong peaks at ∼0.98°
(∼d = 9.0 nm) and ∼2° 2θ (∼d = 4.41 nm) due to only the (100)
Fig. 1 Low angle XRD patterns of TiO2 doped mesoporous SiO2 films
heat-treated at 90 °C (a) and 350 °C (b), respectively, with increasing
TiO2 content. A portion of the low angle region of the XRD pattern
and (200) planes maintaining the hexagonal symmetry.31,33 (shown by a red line) is expanded (×5) for visual clarity.
This observation indicates structural shrinkage with a meso-
structural transformation of the ST-0 film along the (100) orien-
with a 1/d spacing ratio 1 : 1.70 : 2.65 confirming the existence
tation after heat-treatment. However, the heat-treated (350 °C)
TiO2 doped SiO2 films, up to 7 equivalent mol% (ST-4 and
ST-7 films, in Fig. 1b) retained similar (100), (110) and (210)
diffraction patterns to those of the corresponding dried films
of hexagonal unit cells. The heat-treated films also showed a
slight shift of XRD peaks towards higher angles compared to
the dried films due to a shrinkage of mesopores. So, it is
suggested that the presence of TiO2 in the case of heat-treated
TiO2–SiO2 does not allow structural changes except for a slight
shift towards higher 2θ values due to shrinkage of the pores,
thereby stabilizing the mesostructures.32 We observed that for
incorporation of TiO2 up to 7 equivalent mol%, the (110) and
(210) reflections (at 1.66°; d = 5.32 nm and 2.61° 2θ; d =
3.38 nm, respectively) were more resolved, suggesting a better
organization of the mesostructure. The unit cell parameter, a0
(= 2d100/√3) was calculated to be ∼10.4 nm in the annealed
Table 1 Nominal and estimated (by ICP) amount of TiO2 present in the
heat-treated TiO2–SiO2 powders
TiO2 (mol%)
(Nominal)
TiO2 (mol%)
(Found by ICP)
Sample names
ST-4
ST-7
ST-10
4
7
10
3.63 0.14
6.34 0.16
9.94 0.20
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ST-7 film. This suggests about a 22.38% (at 90 °C, d100
=
11.6 nm, a0 = 13.4 nm) shrinkage of pore width during heat-
treatment. For further loading of TiO2 (ST-10) it was observed
that the peaks became broader at ∼0.89° and 1.4° 2θ
suggesting deterioration of the ordered structure.
High angle XRD patterns (20–90° 2θ) of the 350 °C heat-
treated samples are shown in Fig. S1.† ST-0 shows a broad
hump extending from 20 to 28° 2θ due to amorphous SiO2.
With a gradual increase of TiO2, the diffraction peaks for
anatase TiO2 become visible as shown in Fig. S1.† ST-10 shows
peaks at ∼25°, 48°, 55° and 62.5° 2θ corresponding to the
(101), (200), (211) and (204) planes of anatase TiO2 (JCPDS #
00-021-1272), respectively.
3.2 UV-visible study
A UV-visible study of the heat-treated (350 °C) films was under-
taken to observe the signature of the TiO2 species in the SiO2
films (shown in Fig. S2†). It shows that gradual TiO2 incorpor-
ation into SiO2 films from ST-4 to ST-10 causes an effective
increase in absorbance in the 200–350 nm region while ST-0
(SiO2 film without TiO2) does not exhibit absorption in this
region. The TiO2 incorporated films exhibit an absorption
band near the 200–250 nm range due to the ligand (O) to
metal (Ti) charge transfer (LMCT) in tetrahedral co-ordination
[TiO4].34 The absorption in the extended tail part in the
270–320 nm range arises due to the polymeric penta or hexa-
gonally coordinated Ti species formed through the hydration
of tetra coordinated Ti sites.35
3.3 FTIR study
Fig. 2 shows the infrared spectra of the dried (90 °C) and heat-
treated (350 °C) mesoporous SiO2 films with varied TiO2 con-
tents. The dried films exhibited the most intense peak at
1070 cm−1 representing the symmetric stretching vibration of
Si–O–Si. The peaks appearing at 960, 800 and 450 cm−1 are
due to the stretching vibration of Si–OH, and Si–O–Si sym-
metric stretching and deformation, respectively.11,22,36 The
appearance of C–H/CH2 related peaks at 2800–3000 and
1350–1450 cm−1, respectively, indicate the presence of the
organic P123 surfactant in the dried film and the peaks cen-
tered at 3410 cm−1 can be assigned as –OH groups associated
with the SiO2/TiO2 surface.37 It is to be noted that all the TiO2
doped SiO2 films cured at 90 °C (ST-4, ST-7 and ST-10) showed
a broad peak near ∼650 cm−1 representing the signature of Ti–
O–Ti bonds38 which is also observed in the case of the TiO2
dispersion used for doping (see Fig. S3a†). We did not observe
any clear characteristic Si–O–Ti related bands in the FTIR
spectra of the dried films; however it is expected that the TiO2
NPs were present in the mesoporous SiO2 with covalent lin-
Fig. 2 FTIR spectra of TiO2 incorporated mesoporous SiO2 films: (a)
dried at 90 °C and (b) heat-treated at 350 °C.
surfactant from the SiO2/TiO2–SiO2 films.10,32 In cases of all
the heat-treated (350 °C) films, the FTIR peaks (Fig. 2b)
remained the same except for the disappearance of the C–H/
CH2 peaks in the 2800–3000 and 1350–1450 cm−1 regions,
indicating the complete removal P123 surfactant from the
films. It may be noted here that the relative intensity of the
surface –OH groups of the heat-treated (350 °C) TiO2 contain-
ing films (ST-4, ST-7 and ST-10) are higher than the corres-
ponding pure (ST-0) films. Further, a weak shoulder peak
observed near 910 cm−1 could be due to the Si–O–Ti linkages.
So, it may be concluded from the FTIR study that TiO2 NPs are
embedded and the surface of TiO2 is bonded with SiO2.
3.4 Raman study
kages. We used pluronic P123 surfactant as a structure The Raman study of the films is shown in Fig. 3. It shows that
directing agent for the synthesis of the ordered mesoporous mesoporous SiO2 (ST-0) has a broad weak absorption in the
SiO2 and TiO2–SiO2 films. It is to be noted that the EO (ethyl- region of 250–500 cm−1. However, in the case of the TiO2
ene oxide) and PO (propylene oxide) groups associated with loaded films (ST-4, ST-7 and ST-10) three fundamental Raman
pluronic P123 surfactant start to decompose and eliminate at vibrational peaks of TiO2 at 152, 517, and 637 cm−1 were
∼145 and 300 °C, respectively. So, we heat-treated all the films observed and can be assigned as Eg (1), (A1g + B1g) and Eg (2)
at 350 °C for 1 h to ensure the complete elimination of the bands, respectively.10,39 It is to be noted here that bulk anatase
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Table 2 Refractive index (nf) and volume fraction porosity (P) of the
heat-treated (350 °C) TiO2–SiO2 films. Films of similar compositions
were prepared without using the surfactant and heat-treated at 500 °C
to obtain the respective dense films. The refractive index values of these
dense films were considered as n0 (see text and eqn (1))
Samples
n0
nf
P (%)
ST-0
ST-4
ST-7
ST-10
1.4461
1.4744
1.4939
1.5195
1.2848
1.3658
1.3892
1.4304
33.17
20.39
18.70
14.89
Considering the RI values of the dense and porous films,
the volume fraction porosities of the TiO2 incorporated meso-
porous SiO2 films were evaluated and are presented in Table 2.
It may be pointed out here that the SiO2 film prepared without
surfactant showed a RI value of 1.4461 after heat-treatment at
500 °C which is very close to its theoretical density (99.2%).
The RI values of the dense TiO2 incorporated SiO2 films and
their corresponding porous films (ST-0, ST-4, ST-7 and ST-10)
are given in Table 2. It has been observed that with increasing
amounts of the high RI material TiO2, the RI value increases
gradually. The overall volume fraction porosity of the films can
Fig. 3 Raman spectra of TiO2 incorporated mesoporous SiO2 films
heat-treated at 350 °C showing gradual increase of Eg(1) band of TiO2 be determined using the above equation and is presented in
with increasing TiO2 content.
Table 2. This shows a gradual decrease of porosity from
∼33.2% to 14.9% due to the inclusion of TiO2 NPs in the SiO2
films.
TiO2 shows Eg (1) at 143 cm−1, (A1g + B1g) at 514 cm−1 and
3.6 Electron microscopy analyses of ST-7 film
Eg (2) at 637 cm−1 20
In our case, the shift of the Eg (1) and
.
Cross-sectional FESEM and TEM studies were undertaken to
visualize the thickness and structure of the most optimized
and organized TiO2 doped mesoporous SiO2 film (ST-7) heat-
treated at 350 °C. The cross-sectional FESEM image is shown
in Fig. 4a. As shown in Fig. 4a, the thickness of the heat-
treated ST-7 film is around 565 nm. TEM analysis of this film
was performed along the [100] and [110] zone axes to ensure
symmetry of the pores. Fig. 4b–e show the results of the TEM
studies. When the incident electron beam of TEM was aligned
along the [100] direction, two types of channels of diameters
close to 6 and 9 nm (Fig. 4b) were observed. These values are
close to the d spacing of the (110) (d = 5.32 nm) and (100) (d =
9 nm) reflections, as observed in the low angle XRD (ST-7 in
Fig. 1; panel b). Fig. 4c shows the TEM image of the honey-
comb like structure of the hexagonal symmetry. Two dark
spots in the FFT at different places in the inset of Fig. 4b also
confirm the presence of the (100) and (110) planes of the 2d
hexagonal p6mm symmetry. Further, along the [110] direction,
an array of several unidirectional porous SiO2 channels of
about 6 nm width (appearing as a white contrast in Fig. 4d)
was observed and found to match with the d spacing value of
the (110) reflection. A close view of the matrix in Fig. 4e
(A1g + B1g) bands towards higher wavenumbers could be due to the
interaction of the TiO2 NPs with the SiO2.22 It can be noticed
from Fig. 3 that with an increase in TiO2 content the Raman
peak intensity at 152 cm−1 of Eg(1) gradually increases. The
full width at half maximum (fwhm) of the peak at 152 cm−1 is
found to be 30 cm−1 in all the TiO2 incorporated films. Using
this fwhm (30 cm−1) data the crystallite size of the incorpor-
ated TiO2 NPs is estimated to be 5 nm.40 As a control experi-
ment the Raman spectra of the TiO2 dispersion (Fig. S3b†) was
also recorded which gives a size of the Ti crystallites to be
close to 6 nm. TEM of the TiO2 dispersion also revealed exist-
ence of NPs in the range of 3.5 to 7.5 nm with an average size
distribution of 5.8 nm (see some encircled NPs in Fig. S3c†
and their size distribution in Fig. S3d†).
3.5 Ellipsometric analysis
The RI of the films was measured by ellipsometry and the
volume fraction porosity (P) of the films was evaluated using
the Lorenz–Lorentz equation:
2
2
2
2
1 ꢀ P=100 ¼ ðnf ꢀ 1Þðn0 þ 2Þ=ðn0 ꢀ 1Þ=ðnf þ 2Þ
ð1Þ
where nf is the RI value of the heat-treated films at 350 °C; n0 revealed porous channels of 9.0 nm of the (100) plane similar
is the RI value of the corresponding dense films having similar to Fig. 4b. In the TEM images (Fig. 4b–e) the incorporated
compositions. The respective dense films were prepared from TiO2 NPs were not distinguishable, suggesting that the NPs
the TiO2 dispersion mixed SiO2 sols without using a surfactant were encapsulated in the SiO2. FFT pattern in the inset of
and heat-treated at 500 °C.
Fig. 4d exhibiting two spots representing the (110) and (210)
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the decomposition of an aqueous KMnO4 solution in ambient
conditions. KMnO4 exhibits two sets of characteristic UV-
visible peaks at (i) 315 and 350 nm and (ii) 505, 525 and
565 nm corresponding to the electronic excitation from the
non-bonding orbital of oxygen to the anti-bonding orbital of
manganese and oxygen as observed in the molecular orbital
− 26
diagram of MnO4
.
It was observed that in the presence of
the catalyst films (Fig. S4a†) the set (ii) peaks disappeared
gradually with the development of a peak centered at ∼363 nm
due to the formation of MnO2 NPs.27 The appearance of a
strong peak for MnO2 suggests the catalytic conversion of
KMnO4 to MnO2 took place instead of physical adsorption
onto the surface of the ST-7 film. The progress of the reaction
was evaluated by the gradual change of peak at 565 nm with
respect to time.27,41 The reaction does not progress noticeably
in the absence of the catalyst except the growth of a peak at
−
310 nm which is due to the formation of MnO42− from MnO4
(inset of Fig. S4a†).27 The plot of ln(A565 – Aα) vs. time (where
A565 and Aα are the absorbance of solution at time t and infin-
ity, respectively) gives a straight line with a positive slope
(Fig. S4b†) and thus is expected to follow pseudo first order
kinetics. From the slope of the curve, the rate constant value of
each set of experiments was determined and is presented in
Fig. 5a. As the incorporation of TiO2 in SiO2 has some positive
influence on the catalytic reaction, we expected that with an
increase in TiO2 content in the films i.e. from ST-0 to ST-10,
the rate of the decomposition would become faster. But we
noticed the rate only increased up to 7 mol% TiO2 doped SiO2
with a maximum rate constant (kST-7 = 0.224 h−1) compared to
the most TiO2 rich ST-10 film (kST-10 = 0.206 h−1). This may be
due to the structural disorder of the ST-10 film as observed in
the XRD analysis. It is noteworthy here that the catalytic
efficiency of the ST-4 film (kST-4 = 0.105 h–1) does not improve
significantly compared to the undoped SiO2 film (kST-0
=
0.088 h–1). We believe that the primarily mesoporous mor-
phology of the film matrix with optimized TiO2 content in
ST-7 helped the degradation process to be faster. We checked
the reusability of the ST-7 film catalyst, and found that the
activity of the catalyst did not deteriorate even after the fourth
cycle (shown in Fig. 5b). We noticed that as the decomposition
reaction proceeds, the pink KMnO4 solution transformed into
the MnO2 NPs and the resulting solution became intensely
yellow in colour due to the appearance of a broad strong peak
at ∼363 nm. The yellow coloured solution was drop casted
onto a glass slide for GIXRD analysis. This showed (Fig. 6a)
two sets of characteristic peaks for crystalline MnO2 (JCPDS #
01-073-6999) and K0.29MnO2 (JCPDS # 01-074-7889). TEM
imaging of the sample (Fig. 6b) confirmed the formation of an
extremely thin nanowire-like structure. The SAED pattern
shows the spots of the crystalline planes of MnO2 and
Fig. 4 FESEM and TEM studies of the ordered mesoporous ST-7 film
(350 °C): (a) cross-sectional FESEM image showing the thickness of the
film; (b and c) low resolution TEM images viewed along [100] and (d and
e) [110] zone axes. The FFT patterns obtained from (b) and (d) are shown
in the respective insets with labelling of lattice planes.
planes of hexagonal symmetry which is also in accordance
with the low angle XRD results of ST-7. Considering the nature
of the bright field images, FFT patterns and low angle XRD
results, it can be concluded that the hexagonal symmetry of
the p6mm space group has been retained in the 7 mol% TiO2
NPs doped heat-treated SiO2 film (ST-7).
K0.29MnO2 (see inset of Fig. 6b). So, the TEM observation sup-
ports the GIXRD results. EDX analysis (Fig. 6c) revealed the
presence of Mn, O and K in the sample, further strengthening
the GIXRD results. A small amount of Si observed in the EDX
3.7 Catalytic activity
We investigated the catalytic activities of the anatase TiO2 NPs is expected to be leached from the SiO2 film under the experi-
doped mesoporous SiO2 films (ST-0, ST-4, ST-7 and ST-10) in mental conditions during KMnO4 decomposition. The
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Fig. 6 (a) XRD pattern revealing characteristic Bragg reflections of
MnO2 and K0.29MnO2 after decomposition of KMnO4 in the presence
ST-7 film, (b) TEM image showing nanowires and the SAED pattern with
diffraction rings (inset) corresponding to MnO2 and K0.29MnO2, and (c) EDS
analysis confirming the presence of Mn, O and K from nanowires.
A small Si peak is expected due to leaching from the film catalyst. Cu and
C are from the carbon coated Cu grid used in the TEM study. (d) HRTEM
image of a single MnO2 nanowire with characteristic lattice fringes.
Fig. 5 (a) Comparative catalytic studies showing the rate constant (k)
−
values of the reduction of MnO4 in the presence of TiO2 incorporated
Accordingly, the calculated amount of TiO2 dispersion
(mixed with a small amount of water) was added to the
aqueous KMnO4 solution maintaining the same concentration
of MnO4− as that of the experiments carried out with the films
(see section 2.5). The reactions were observed for up to 6 h
under stirring conditions. In both cases no reactions were
observed, and as a result, no peak around 363 nm appeared
due to MnO2 NPs (see Fig. S5†). A slight decrease in intensity
mesoporous SiO2 films (ST-0, ST-4, ST-7 and ST-10) and absence of film
catalyst; (b) rate constant values of ST-7 film in four consecutive cycles
using the same catalyst films.
diameter of a single nanowire was found to be extremely thin,
of the order of <2 nm, as observed from the TEM image
(Fig. 6b). As the nanowires are very thin it is difficult to obtain
clear HRTEM images. Nevertheless, the HRTEM image pre-
sented in Fig. 6d shows the (111) plane of MnO2 corres-
ponding to a d value of 0.233 nm. The formation of nanowires
−
of the MnO4 bands was observed due to the adsorption of
KMnO4 onto the TiO2 surface.42 In these control reactions we
observed a depletion of the TiO2 peak (∼230 nm) due to the
hydrolysis of TiO2 NPs (confirmed by a blank reaction i.e. TiO2
NPs in water shows a similar depletion of the TiO2 peak; see
inset of Fig. S5†) forming the Ti–OH species.43
−
may be explained as follows: (1) inclusion of MnO4 into the
pores through capillary interactions, (2) interaction of MnO4
−
with hydrophilic surface –OH groups attached to the inner
pore walls followed by reduction, (3) nucleation of MnO2 and
(4) growth of MnO2 nanowires inside the 2D hexagonal pore
channels of the mesoporous TiO2–SiO2 films. As the diameters
of the nanowires (<2 nm) are much less than the pore channel
diameters, the nanowires easily come out of the channels
during the catalytic process with mechanical stirring.
To understand the mechanism of the KMnO4 reduction,
control experiments were performed using the TiO2 NP dis-
persion used in the sols as dopants to see if TiO2 itself has any
effect on the decomposition. The TiO2 content of the ST-7
films was estimated, and two control experiments were per-
formed using (i) the equivalent amount of TiO2 and (ii) a 4
fold concentration of TiO2 relative to that present in the ST-7
films used for the catalysis reaction (see section 2.5).
Therefore, TiO2 NPs alone do not act as a catalyst in the
decomposition of KMnO4. Although Raman analysis and XRD
showed the signature of TiO2 crystals in the film, TEM did not
show any distinct particles. The FTIR study showed a weak
band near 910 cm−1 most likely due to Si–O–Ti linkages (this
band is weak because the overall concentration of Si–O–Ti is
very low; see Fig. 2b). So, we can consider the surfaces of the
embedded TiO2 NPs are bonded with the SiO2. This is quite
logical because the surfaces of the TiO2 NPs will be hydrolyzed
to Ti–OH during the sol stage (see the control experiment;
inset of Fig. S5†) and condense with Si–OH to form Ti–O–Si.
FTIR analysis of the 350 °C heated TiO2 doped SiO2 films
(Fig. 2b) shows the presence of very intense FTIR peaks
centered at around 3410 and 960 cm−1 due to the surface
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hydroxyls (Si–OH) compared to the undoped SiO2 film. This at room temperature. The ordered SiO2 films doped with TiO2
revealed an enhanced surface acidity in the TiO2 doped meso- NPs could thus be designated as a catalytic nanoreactor for the
porous SiO2 films. This could happen due to two reasons. synthesis of MnO2 nanowires which is expected to be useful in
First, it is known that a Ti atom in an octahedral field arrange- fuel cell battery applications.
ment can distribute its four valence electrons over six bonds
with oxygen. As pointed out by Anderson and coworkers,44 this
would cause a charge imbalance on both O and Ti atoms.
Under this situation the O atom would experience −1/3 elec-
Acknowledgements
tronic charge in excess; and as a result this would facilitate for- JS and AM thank CSIR, India for fellowships.
mation of more –OH groups. Second, the embedment of TiO2
NPs in SiO2 would restrict silanol (Si–OH) condensation to
some extent during the heat-treatment stages. So, in the case
of TiO2 incorporated films, lots of Si–OH groups will not be
Notes and references
able to come closer to condense, and remain available. On the
contrary, in the case of a pure SiO2 film, no such restrictions
result in easy silanol condensation, and as a result, ST-0 pos-
sesses significantly less –OH groups after heat-treatment. The
appearance of a weak –OH stretching peak in FTIR supports
this (see ST-0 in Fig. 2b). In the cases of TiO2 NPs incorporated
films, the FTIR study shows existence of very intense peaks
relating to the –OH groups after heat-treatment (Fig. 2b).
Therefore, it can be understood that TiO2 in SiO2 films can
increase the number of Si–OH groups significantly and make
the surface acidic in nature.
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