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TiO2 nanoparticles doped SiO2 films with ordered mesopore channels: a catalytic nanoreactor† 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 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.
Received 19th November 2013, Accepted 23rd December 2013
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
DOI: 10.1039/c3dt53265h
explained by this enhanced surface acidity. The mesoporous channel of the ST-7 films with TiO2 doping
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can be used as a nanoreactor to form extremely thin MnO2 nanowires.
1.
Introduction
Ordered mesoporous SiO2 has attracted a great deal of attention due to its well known potential applications as a catalyst,1 catalyst support,2,3 adsorbent4–6 as well as template7,8 for the synthesis of different shaped nanomaterials. The incorporation of TiO2, a semiconductor having a moderate energy band gap (Eg = 3.2 eV) inside the SiO2 matrix may open up several prospects in the fields of photocatalysis,9–11 self-cleaning,12 antibacterial13,14 and sensors15,16 as TiO2 can promote an interfacial charge transfer processes in the composite.17–19 The photo-catalytic performance of TiO2 in toxic dye degradation improves when it is doped into another semiconducting metal oxide (FeTiO3, PbTiO3, etc.) of similar energy band gap.20,21 In addition, a highly ordered mesostructure could enhance the
Nano-Structured Materials Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata 700032, India. E-mail: [email protected]; Fax: +91-33-24730957; Tel: +91-33-23223403 † 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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activity of embedded NPs by allowing the reactant molecules to transport through regular interconnected pores. Accordingly, different synthetic strategies like post grafting of Ti species in SiO2, substitution of Si4+ by Ti4+ at the SiO2 surface during micellisation in sol–gel,22 hydrolytic23 and non hydrolytic24 routes have been followed in the fabrication of TiO2 doped regular patterned SiO2 matrices. However, these methods are tedious, time consuming and often result in disorganized SiO2 materials. The direct incorporation of a titanium species (using a soluble Ti salt or alkoxide) into a SiO2 sol leads to a homogeneous TiO2–SiO2 mixture with the formation of Si–O–Ti linkages at the sol stage, and therefore TiO2 crystallinity could not nucleate on the mesoporous SiO2 walls after subsequent heat-treatment.9,25 We used a synthetic protocol 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 corresponding powder in technological applications. For this purpose a commercial dispersion of anatase TiO2 NPs was 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
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removing harmful MnO4− from water using a UV reactor. So, our objective was to utilize the film substrate as a reusable catalyst for the degradation of a pollutant, such as KMnO4 in water under ambient conditions. Interestingly in our case, we found formation of MnO2 nanowires instead of spherical clusters27 occurred in the decomposition process which seemed to form along a particular direction in the hexagonally arranged pore channels of the mesoporous SiO2 template.28 Therefore the developed TiO2 NPs incorporated mesoporous films not only acted as a catalyst to decompose the toxic pollutant KMnO4 but also yielded a useful product, MnO2 nanowires, which may find useful applications in preparing cathode materials for batteries.29,30 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 assembly (EISA) process. The effect of TiO2 on the periodicity of the ordered mesoporous structure of the SiO2 films with different Si/Ti ratios (0, 4, 7 and 10 equivalent mol% TiO2) have been thoroughly investigated. The catalytic activities of these films towards the decomposition of KMnO4 under ambient conditions have been investigated, and a possible mechanism has been suggested.
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 Nanostructured & Amorphous Inc. Potassium permanganate (KMnO4) was obtained from Sarabhai M chemicals private limited. Millipore Milli-Q water (resistivity ∼18.2 MΩ cm) was used. 2.2
Synthesis of TiO2 NPs doped SiO2 films
TiO2–SiO2 composite sols were prepared in the following way. 5 g of TEOS was first mixed with 11.05 g of ethanol in a round bottom flask and then a mixture of 1.5 g of water and 0.012 g of concentrated HCl was poured drop wise into the reaction mixture. This solution was kept in stirring conditions for 15 min and refluxed for 1 h to complete the hydrolysis of TEOS. The final molar ratio of TEOS, ethanol, water and HCl was 1 : 10 : 3.45 : 1.3 × 10−2. An anatase dispersion was used to incorporate the TiO2 NPs into the sol. Before using the TiO2 dispersion, a certain quantity was taken from the as received sample and diluted with isopropanol. This stock solution was then used to estimate the TiO2 content quantitatively. For this purpose, 2 ml of this stock was first slowly evaporated to dryness followed by being heat-treated at 700 °C in a Pt crucible and estimated gravimetrically. Four different sets of sols were prepared by varying the TiO2 content (0, 4, 7 and 10 equivalent mol%) with respect to SiO2 and designated as ST-0, ST-4, ST-7 and ST-10, respectively. Accordingly, a calculated
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amount of stock anatase dispersion in isopropanol was added into the mixture of 3 g of hydrolyzed SiO2 sol and 0.3193 g of P123 (0.013 mol per mol of SiO2) to obtain the ST-4, ST-7 and ST-10 sols, respectively. A sol without anatase dispersion (ST-0) was also prepared as a control sample. The films were deposited on soda-lime silica glass substrates from their respective sols by a dip-coating technique with a lifting speed of 21 inch min−1. Similar films were also deposited on pure silica glass and polished silicon wafers for UV-visible and FTIR studies, respectively. The as prepared films were aged at room temperature for 10 h followed by drying at 90 °C for 14 h, and finally heating at 350 °C (ramp 2 °C min−1) in air for 1 h. 2.3 Inductively coupled plasma (ICP) analysis for TiO2 estimation At first, we converted all the TiO2 incorporated sols (used for coating deposition) into gels under ambient conditions, dried them at 60 °C followed by heat-treatment at 500 °C to prepare the organic free TiO2–SiO2 composite powders. Taking precisely measured amounts of the composite powders from each set of samples (i.e. 4, 7 and 10 equivalent mol% TiO2 doped 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. 2.4
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 measured using a Cary 50 scan spectrophotometer. FTIR spectra of the 350 °C heated films deposited on both side polished silicon (Si) wafers were obtained from a Nicolet 380 spectrometer. Raman spectra of the scratched off film-powders were recorded using a Renishaw inVia micro Raman spectrometer. The refractive index (RI) of the films deposited on the Si wafers was measured by a J. A. Woolam Co. Inc. spectroscopic ellipsometer and the data were reported at 633 nm (model M2000). FESEM of the heat-treated samples was analyzed by a ZEISS SUPRA 35VP field-emission scanning electron microscope. For TEM studies the scratched off films samples were placed on a carbon coated Cu grid and analyzed using a Technai G2 30ST (FEI) transmission electron microscope operating at 300 kV. 2.5
Catalytic tests
The TiO2 NPs doped ordered mesoporous thin films heattreated 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 contents 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 heattreated 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) and (200) planes maintaining the hexagonal symmetry.31,33 This observation indicates structural shrinkage with a mesostructural transformation of the ST-0 film along the (100) orientation 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
Table 1 Nominal and estimated (by ICP) amount of TiO2 present in the heat-treated TiO2–SiO2 powders
Sample names
TiO2 (mol%) (Nominal)
TiO2 (mol%) (Found by ICP)
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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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 (shown by a red line) is expanded (×5) for visual clarity.
with a 1/d spacing ratio 1 : 1.70 : 2.65 confirming the existence 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
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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 heattreatment. 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 heattreated 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 undertaken to observe the signature of the TiO2 species in the SiO2 films (shown in Fig. S2†). It shows that gradual TiO2 incorporation 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 hexagonally 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 heattreated (350 °C) mesoporous SiO2 films with varied TiO2 contents. 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 symmetric 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 centered 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 linkages. We used pluronic P123 surfactant as a structure directing agent for the synthesis of the ordered mesoporous SiO2 and TiO2–SiO2 films. It is to be noted that the EO (ethylene oxide) and PO ( propylene oxide) groups associated with pluronic P123 surfactant start to decompose and eliminate at ∼145 and 300 °C, respectively. So, we heat-treated all the films at 350 °C for 1 h to ensure the complete elimination of the
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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 containing films (ST-4, ST-7 and ST-10) are higher than the corresponding 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
The Raman study of the films is shown in Fig. 3. It shows that mesoporous SiO2 (ST-0) has a broad weak absorption in the region of 250–500 cm−1. However, in the case of the TiO2 loaded films (ST-4, ST-7 and ST-10) three fundamental Raman vibrational peaks of TiO2 at 152, 517, and 637 cm−1 were observed and can be assigned as Eg (1), (A1g + B1g) and Eg (2) 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))
Fig. 3 Raman spectra of TiO2 incorporated mesoporous SiO2 films heat-treated at 350 °C showing gradual increase of Eg(1) band of TiO2 with increasing TiO2 content.
TiO2 shows Eg (1) at 143 cm−1, (A1g + B1g) at 514 cm−1 and Eg (2) at 637 cm−1.20 In our case, the shift of the Eg (1) and (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 incorporated TiO2 NPs is estimated to be 5 nm.40 As a control experiment 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 existence 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: 1 P=100 ¼ ðnf 2 1Þðn0 2 þ 2Þ=ðn0 2 1Þ=ðnf 2 þ 2Þ
ð1Þ
where nf is the RI value of the heat-treated films at 350 °C; n0 is the RI value of the corresponding dense films having similar compositions. The respective dense films were prepared from the TiO2 dispersion mixed SiO2 sols without using a surfactant and heat-treated at 500 °C.
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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 mesoporous 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 be determined using the above equation and is presented in 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. 3.6
Electron microscopy analyses of ST-7 film
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) heattreated at 350 °C. The cross-sectional FESEM image is shown in Fig. 4a. As shown in Fig. 4a, the thickness of the heattreated 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 honeycomb 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 revealed porous channels of 9.0 nm of the (100) plane similar to Fig. 4b. In the TEM images (Fig. 4b–e) the incorporated TiO2 NPs were not distinguishable, suggesting that the NPs were encapsulated in the SiO2. FFT pattern in the inset of Fig. 4d exhibiting two spots representing the (110) and (210)
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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). 3.7
Catalytic activity
We investigated the catalytic activities of the anatase TiO2 NPs doped mesoporous SiO2 films (ST-0, ST-4, ST-7 and ST-10) in
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the decomposition of an aqueous KMnO4 solution in ambient conditions. KMnO4 exhibits two sets of characteristic UVvisible 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 diagram of MnO4−.26 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 infinity, 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 morphology 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 K0.29MnO2 (see inset of Fig. 6b). So, the TEM observation supports 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 is expected to be leached from the SiO2 film under the experimental 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 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
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TiO2 nanoparticles doped SiO2 films with ordered mesopore channels: a catalytic nanoreactor† 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 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.
Received 19th November 2013, Accepted 23rd December 2013
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
DOI: 10.1039/c3dt53265h
explained by this enhanced surface acidity. The mesoporous channel of the ST-7 films with TiO2 doping
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can be used as a nanoreactor to form extremely thin MnO2 nanowires.
1.
Introduction
Ordered mesoporous SiO2 has attracted a great deal of attention due to its well known potential applications as a catalyst,1 catalyst support,2,3 adsorbent4–6 as well as template7,8 for the synthesis of different shaped nanomaterials. The incorporation of TiO2, a semiconductor having a moderate energy band gap (Eg = 3.2 eV) inside the SiO2 matrix may open up several prospects in the fields of photocatalysis,9–11 self-cleaning,12 antibacterial13,14 and sensors15,16 as TiO2 can promote an interfacial charge transfer processes in the composite.17–19 The photo-catalytic performance of TiO2 in toxic dye degradation improves when it is doped into another semiconducting metal oxide (FeTiO3, PbTiO3, etc.) of similar energy band gap.20,21 In addition, a highly ordered mesostructure could enhance the
Nano-Structured Materials Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata 700032, India. E-mail: [email protected]; Fax: +91-33-24730957; Tel: +91-33-23223403 † 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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activity of embedded NPs by allowing the reactant molecules to transport through regular interconnected pores. Accordingly, different synthetic strategies like post grafting of Ti species in SiO2, substitution of Si4+ by Ti4+ at the SiO2 surface during micellisation in sol–gel,22 hydrolytic23 and non hydrolytic24 routes have been followed in the fabrication of TiO2 doped regular patterned SiO2 matrices. However, these methods are tedious, time consuming and often result in disorganized SiO2 materials. The direct incorporation of a titanium species (using a soluble Ti salt or alkoxide) into a SiO2 sol leads to a homogeneous TiO2–SiO2 mixture with the formation of Si–O–Ti linkages at the sol stage, and therefore TiO2 crystallinity could not nucleate on the mesoporous SiO2 walls after subsequent heat-treatment.9,25 We used a synthetic protocol 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 corresponding powder in technological applications. For this purpose a commercial dispersion of anatase TiO2 NPs was 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
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removing harmful MnO4− from water using a UV reactor. So, our objective was to utilize the film substrate as a reusable catalyst for the degradation of a pollutant, such as KMnO4 in water under ambient conditions. Interestingly in our case, we found formation of MnO2 nanowires instead of spherical clusters27 occurred in the decomposition process which seemed to form along a particular direction in the hexagonally arranged pore channels of the mesoporous SiO2 template.28 Therefore the developed TiO2 NPs incorporated mesoporous films not only acted as a catalyst to decompose the toxic pollutant KMnO4 but also yielded a useful product, MnO2 nanowires, which may find useful applications in preparing cathode materials for batteries.29,30 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 assembly (EISA) process. The effect of TiO2 on the periodicity of the ordered mesoporous structure of the SiO2 films with different Si/Ti ratios (0, 4, 7 and 10 equivalent mol% TiO2) have been thoroughly investigated. The catalytic activities of these films towards the decomposition of KMnO4 under ambient conditions have been investigated, and a possible mechanism has been suggested.
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 Nanostructured & Amorphous Inc. Potassium permanganate (KMnO4) was obtained from Sarabhai M chemicals private limited. Millipore Milli-Q water (resistivity ∼18.2 MΩ cm) was used. 2.2
Synthesis of TiO2 NPs doped SiO2 films
TiO2–SiO2 composite sols were prepared in the following way. 5 g of TEOS was first mixed with 11.05 g of ethanol in a round bottom flask and then a mixture of 1.5 g of water and 0.012 g of concentrated HCl was poured drop wise into the reaction mixture. This solution was kept in stirring conditions for 15 min and refluxed for 1 h to complete the hydrolysis of TEOS. The final molar ratio of TEOS, ethanol, water and HCl was 1 : 10 : 3.45 : 1.3 × 10−2. An anatase dispersion was used to incorporate the TiO2 NPs into the sol. Before using the TiO2 dispersion, a certain quantity was taken from the as received sample and diluted with isopropanol. This stock solution was then used to estimate the TiO2 content quantitatively. For this purpose, 2 ml of this stock was first slowly evaporated to dryness followed by being heat-treated at 700 °C in a Pt crucible and estimated gravimetrically. Four different sets of sols were prepared by varying the TiO2 content (0, 4, 7 and 10 equivalent mol%) with respect to SiO2 and designated as ST-0, ST-4, ST-7 and ST-10, respectively. Accordingly, a calculated
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amount of stock anatase dispersion in isopropanol was added into the mixture of 3 g of hydrolyzed SiO2 sol and 0.3193 g of P123 (0.013 mol per mol of SiO2) to obtain the ST-4, ST-7 and ST-10 sols, respectively. A sol without anatase dispersion (ST-0) was also prepared as a control sample. The films were deposited on soda-lime silica glass substrates from their respective sols by a dip-coating technique with a lifting speed of 21 inch min−1. Similar films were also deposited on pure silica glass and polished silicon wafers for UV-visible and FTIR studies, respectively. The as prepared films were aged at room temperature for 10 h followed by drying at 90 °C for 14 h, and finally heating at 350 °C (ramp 2 °C min−1) in air for 1 h. 2.3 Inductively coupled plasma (ICP) analysis for TiO2 estimation At first, we converted all the TiO2 incorporated sols (used for coating deposition) into gels under ambient conditions, dried them at 60 °C followed by heat-treatment at 500 °C to prepare the organic free TiO2–SiO2 composite powders. Taking precisely measured amounts of the composite powders from each set of samples (i.e. 4, 7 and 10 equivalent mol% TiO2 doped 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. 2.4
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 measured using a Cary 50 scan spectrophotometer. FTIR spectra of the 350 °C heated films deposited on both side polished silicon (Si) wafers were obtained from a Nicolet 380 spectrometer. Raman spectra of the scratched off film-powders were recorded using a Renishaw inVia micro Raman spectrometer. The refractive index (RI) of the films deposited on the Si wafers was measured by a J. A. Woolam Co. Inc. spectroscopic ellipsometer and the data were reported at 633 nm (model M2000). FESEM of the heat-treated samples was analyzed by a ZEISS SUPRA 35VP field-emission scanning electron microscope. For TEM studies the scratched off films samples were placed on a carbon coated Cu grid and analyzed using a Technai G2 30ST (FEI) transmission electron microscope operating at 300 kV. 2.5
Catalytic tests
The TiO2 NPs doped ordered mesoporous thin films heattreated 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 contents 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 heattreated 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) and (200) planes maintaining the hexagonal symmetry.31,33 This observation indicates structural shrinkage with a mesostructural transformation of the ST-0 film along the (100) orientation 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
Table 1 Nominal and estimated (by ICP) amount of TiO2 present in the heat-treated TiO2–SiO2 powders
Sample names
TiO2 (mol%) (Nominal)
TiO2 (mol%) (Found by ICP)
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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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 (shown by a red line) is expanded (×5) for visual clarity.
with a 1/d spacing ratio 1 : 1.70 : 2.65 confirming the existence 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
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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 heattreatment. 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 heattreated 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 undertaken to observe the signature of the TiO2 species in the SiO2 films (shown in Fig. S2†). It shows that gradual TiO2 incorporation 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 hexagonally 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 heattreated (350 °C) mesoporous SiO2 films with varied TiO2 contents. 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 symmetric 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 centered 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 linkages. We used pluronic P123 surfactant as a structure directing agent for the synthesis of the ordered mesoporous SiO2 and TiO2–SiO2 films. It is to be noted that the EO (ethylene oxide) and PO ( propylene oxide) groups associated with pluronic P123 surfactant start to decompose and eliminate at ∼145 and 300 °C, respectively. So, we heat-treated all the films at 350 °C for 1 h to ensure the complete elimination of the
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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 containing films (ST-4, ST-7 and ST-10) are higher than the corresponding 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
The Raman study of the films is shown in Fig. 3. It shows that mesoporous SiO2 (ST-0) has a broad weak absorption in the region of 250–500 cm−1. However, in the case of the TiO2 loaded films (ST-4, ST-7 and ST-10) three fundamental Raman vibrational peaks of TiO2 at 152, 517, and 637 cm−1 were observed and can be assigned as Eg (1), (A1g + B1g) and Eg (2) 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))
Fig. 3 Raman spectra of TiO2 incorporated mesoporous SiO2 films heat-treated at 350 °C showing gradual increase of Eg(1) band of TiO2 with increasing TiO2 content.
TiO2 shows Eg (1) at 143 cm−1, (A1g + B1g) at 514 cm−1 and Eg (2) at 637 cm−1.20 In our case, the shift of the Eg (1) and (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 incorporated TiO2 NPs is estimated to be 5 nm.40 As a control experiment 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 existence 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: 1 P=100 ¼ ðnf 2 1Þðn0 2 þ 2Þ=ðn0 2 1Þ=ðnf 2 þ 2Þ
ð1Þ
where nf is the RI value of the heat-treated films at 350 °C; n0 is the RI value of the corresponding dense films having similar compositions. The respective dense films were prepared from the TiO2 dispersion mixed SiO2 sols without using a surfactant and heat-treated at 500 °C.
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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 mesoporous 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 be determined using the above equation and is presented in 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. 3.6
Electron microscopy analyses of ST-7 film
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) heattreated at 350 °C. The cross-sectional FESEM image is shown in Fig. 4a. As shown in Fig. 4a, the thickness of the heattreated 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 honeycomb 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 revealed porous channels of 9.0 nm of the (100) plane similar to Fig. 4b. In the TEM images (Fig. 4b–e) the incorporated TiO2 NPs were not distinguishable, suggesting that the NPs were encapsulated in the SiO2. FFT pattern in the inset of Fig. 4d exhibiting two spots representing the (110) and (210)
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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). 3.7
Catalytic activity
We investigated the catalytic activities of the anatase TiO2 NPs doped mesoporous SiO2 films (ST-0, ST-4, ST-7 and ST-10) in
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the decomposition of an aqueous KMnO4 solution in ambient conditions. KMnO4 exhibits two sets of characteristic UVvisible 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 diagram of MnO4−.26 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 infinity, 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 morphology 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 K0.29MnO2 (see inset of Fig. 6b). So, the TEM observation supports 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 is expected to be leached from the SiO2 film under the experimental 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 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
