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Microporous and Mesoporous Materials 114 (2008) 401–409 www.elsevier.com/locate/micromeso
Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies S. Ribbens a,*, V. Meynen a,b, G. Van Tendeloo c, X. Ke c, M. Mertens b, B.U.W. Maes d, P. Cool a, E.F. Vansant a a
University of Antwerpen (UA), Department of Chemistry, Laboratory of Adsorption and Catalysis, Universiteitsplein 1, B-2160 Wilrijk, Belgium b Flemish Institute for Technology Research, VITO, Boeretang 200, B-2400 Mol, Belgium c University of Antwerpen (UA), Department of Physics, EMAT, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium d University of Antwerpen (UA), Department of Chemistry, Organic synthesis, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium Received 30 November 2007; received in revised form 16 January 2008; accepted 17 January 2008 Available online 31 January 2008
Abstract Titanate nanotubes were prepared via a hydrothermal treatment of TiO2 powders (Riedel De Haen) in a basic solution. Morphology and structure of the prepared samples were characterized by high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), XRD, FT-Raman spectroscopy, nitrogen sorption and DSC. The photocatalytic activity was evaluated by photocatalytic oxidation of rhodamine 6G. Trititanate nanotubes (TTNT) with inner pore diameters between 4 and 4.2 nm and surface areas up till 360 m2/g could be synthesized. The synthesis route was modified by introduction of a calcination step, by applying a lower hydrothermal temperature and microwave irradiation in order to increase the photocatalytic activity of the porous photoactive nanotubular materials. Calcination and a softer hydrothermal treatment led to the formation of anatase without affecting the surface area and nanotubular shape of the samples. In this way, the photocatalytic activity of the original trititanate nanotubes could be significantly increased. By making use of microwave assisted synthesis, the photocatalytic activity can also be increased due to the presence of anatase. However, by applying microwave synthesis, a different structure was obtained, nanoribbons (NR) instead of nanotubes, resulting in a decrease in surface area and porosity. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Anatase; Titania nanotubes; Photocatalytic degradation; Mesoporous
1. Introduction Since the discovery of carbon nanotubes in 1991 by Iijima [1], much research has been done on the development of nanoscale tubular materials because of their excellent electronic and mechanical properties. Besides, carbon nanotubes, BN [2], WS2 [3], MoS2 [4], HxNa2 xTi3O7 [5– 7] and TiO2 [8] nanotubes (TNT) have been successfully synthesized by a short, cheap, simple, template free, hydro-
*
Corresponding author. E-mail address:
[email protected] (S. Ribbens).
1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.01.028
thermal synthesis procedure. The formation mechanism of the trititanate and titanium dioxide nanotubes is however still under discussion. According to Kasuga [8] and other researchers [9,10], the acid washing process of the precipitate after hydrothermal treatment is essential for formation of trititanate nanotubes. Several other authors [11,12] found however evidence that TNT’s are formed during hydrothermal treatment. There is a consensus that after chemical bond breaking of the bulk titania, two-dimensional nanosheets are formed which can be converted into nanotubes by a sheet folding mechanism [13,18]. Nowadays, researchers are showing a great interest in TiO2 based materials because of the great photocatalytic properties
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[15,16] of TiO2 and the applicability in environmental purification processes and high-effective photovoltaic cells [17]. Highly photocatalytic active mesoporous nanomaterials are of great importance for industrial applications. However, the known commercial ultra fine titania powders, such as titanium dioxide Riedel De Hae¨nÒ (Honeywell) and AEROXIDEÒ TiO2 P25 (Evonik Industries), can not be used in a simple way in industrial processes. The drawback of commercial titanium dioxide powders is that after reaction the small particles are too difficult to recover by filtration or centrifugation [19]. A second problem is that the particles will agglomerate in aqueous applications resulting in a decreased photocatalytic activity. Also other mesoporous titania based materials do not offer a very good alternative, because hydrolysis and condensation processes during their synthesis are hard to control which makes them very expensive to produce [20]. Therefore high quality mesoporous trititanate nanotubes could offer a great solution to the described problems. Unfortunately, these materials show a very low photocatalytic activity [21]. Hence, the purpose of the present work is to enlarge the photocatalytic activity of the TNT by converting the titania and titanate into anatase which shows a very high photocatalytic efficiency. Anatase nanostructures were synthesized from titanate nanotubes by calcination, variations in the hydrothermal conditions and by a microwave assisted synthesis method. 2. Experimental section 2.1. Chemical reagents Cheap commodities were used for the preparation of titanate nanotubes: commercial TiO2 (Riedel De Hae¨n), NaOH pellets (98.5%, Acros), HCl solution (37% solution in water, Acros) and ethanol (pro analysis, Merck). All chemicals were used as received. 2.2. Preparation of trititanate nanotubes Trititanate nanotubes (TTNT) were prepared using a hydrothermal synthesis method identical to the one described by Zhu et al. [22]. The titanium dioxide source used for the preparation of TTNT is a powder of 100% anatase nanoparticles with a surface area of 12 m2/g. In a typical nanotube preparation, 4.5 g TiO2 was dispersed into 80 ml of 10 M NaOH solution under vigorous stirring. Afterwards, the mixture was stirred for 1 h and transferred into an autoclave with an internal volume of 150 ml, followed by hydrothermal treatment at 150 °C for 48 h. After hydrothermal treatment, the solid was recovered by centrifugation. The precipitate was washed three times with deionized water. In the next step, the wet cake was dispersed into 240 ml of 0.1 M HCl solution and stirred for 30 min in order to obtain H-tubes. The solid was recovered again by centrifugation and further washed with 0.1 M HCl until the pH of the solution
reached ca. 6.5–7. The precipitate was separated by filtration and washed three times with water and two times with ethanol. Finally, the washed solid was dried at 100 °C for several days. 2.3. Microwave assisted synthesis Titanate nanomaterials were also prepared by making use of a microwave oven (Mars CEM), instead of a conventional oven, because of the efficient heating process and short reaction time. The synthesis procedure is identical to the one described in Section 2.2, only the hydrothermal treatment differs. Here, the HP-500 vessel (Teflon insert) is placed in an microwave oven for 6 h at 110 °C (fiber optic measurement) (initial power: 300 W). 2.4. Characterization tools N2-sorption: The surface area and porosity of the mesoporous nanomaterials were determined on a Quantachrome Autosorb-1-MP automated gas adsorption system. All the powder samples were outgassed at 150 °C for 16 h. Afterwards N2-sorption was carried out at 196 °C. The Brunauer–Emmet–Teller (BET) method was used to calculate the specific surface area. The volume adsorbed at a relative pressure P/ P0 = 0.95 was used to determine the total pore volume. FT-Raman spectroscopy: Samples were measured on a Nicolet Nexus 670 bench equipped with a Ge detector in a 180 °C reflective sampling configuration using a 1064 nm ND:YAG laser. EPMA (Electron probe micro analysis): A JEOL JXA 733 superprobe was used to measure the sodium content. TEM/HRTEM/SAED: HRTEM were taken on a Philips CM30 at 300 kV where low beam intensity was applied in order to preserve the structure. Diffraction patterns were taken on CM20 at 200 kV. UV–vis DR: UV–vis diffuse reflectance spectroscopy was recorded on a Thermo-electron evolution 500 UV–vis spectrometer equipped with a Thermo-electron RSA-UC40 Diffuse Reflectance cell. UV–vis DR spectra can give information about the band gap energy. XRD: X-ray diffraction patterns (XRD) were collected on a Panalytical X’Pert PRO MPD diffractometer using Ni-filtered Cu Ka radiation. TGA/DSC: Thermogravimetric analysis and differential scanning calorimetry gave information about the thermal stability of the samples, their composition and possible phase transformations. Measurements were executed on a Mettler TG 50 thermobalance wich contains a Mettler M3 microbalance and a TC10A microprocessor. Samples were heated till 800 °C with a heating rate of 5 °C/min. DSC measurements were performed on a SDT2960 module (TA-instruments). Samples were heated at a heating rate of 5 °C/min under N2 atmosphere at a flow rate of 50 cm3/min.
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2.5. Measurement of photocatalytic activity The photocatalytic activity was tested by photodegradation of a cationic dye (rhodamine-6G) in aqueous solution. 8 mg of the catalyst was added to a suspension of 25 ml 4 10 5 M rhodamine-6G and stirred for 30 min without UV irradiation so that an adsorption–desorption equilibrium could be established between the rhodamine-6G solution and the catalyst surface. Then, the solution was irradiated for 1 h with UV light (wavelength 365 nm) emitted by a 100 Watt Hg-lamp (Sylvania Par 38). During this illumination, samples with a volume of 5 ml were taken out of the suspension at fixed intervals (10 min) and analyzed by UV–vis spectroscopy (Thermo-electron evolution 500, double beam UV–vis spectrometer). The absorbance was measured at 526 nm with water as a reference. The obtained value was converted to concentration by applying the Lambert–Beer’s law. 3. Results and discussion 3.1. Characterization of prepared nanotubes
Fig. 1. (a) HRTEM : Assymetric, scrolled up Na-TTNT with open ends and inner pore diameters of 4–4.2 nm and interlayer distance of 0.74 nm, (b) TEM/HRTEM: Hydrogen-TTNT, Introduction of structural defects after acid washing procedure and interlayer distance of 0.62 nm.
5000 4500 4000 3500 Intensity
Fig. 1a shows that after hydrothermal treatment of the TiO2/NaOH mixture, a well formed, tubular shaped nanomaterial could be obtained. These prepared multiwall structures have a different number of shells with interlayer distances of about 0.74 nm and an inner pore diameter of 4–4.2 nm. The fact that the nanotubes are not symmetric confirms the existence of a rolling up mechanism of nanosheets during hydrothermal treatment [16]. This observation is in agreement with the results obtained by Du [16] and Wang [17]. Further, these nanotubes are open at both ends what makes the inner pore accessible. This is in contrast to most carbon nanotubes which have caps connected to their ends what makes them inaccessible. The crystal structure of this tubular morphology was investigated with X-ray diffraction. The XRD pattern of the prepared samples, shown in Fig. 2a is identical to that of Na2Ti3O7 [14] (Na-TTNT, sodium trititanate nanotube). No diffractions corresponding to anatase, rutile or brookite can be detected. The peaks of the XRD profile are quite broad due to the small nanometer size dimensions of the tubes. The trititanate crystals are build up by the interconnection of three TiO6 octahedra which share edges. These chains of octahedra join at the corners to form a stepped, zigzag ribbon layered structure [7] (Fig. 3). Between these negatively charged Ti3 O27 layers, sodium cations are located [23]. The sodium content was determined by making use of EPMA which confirmed the presence of 12,5 wt% Na sodium. When the samples were washed with an aqueous 0.1 M HCl solution, sodium could no longer be detected by EPMA. This implies that all sodium ions are exchanged by protons. Zhang et al. [24] has proven by ab initio calculations, based on the density functional theory, that although the Na2Ti3O7 structure is very stable, sodium ions can be replaced by protons. This is possible since the
3000 2500
b
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a
500 0 0
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2 Teta
Fig. 2. XRD pattern : (a) Na-TTNT, (b) H-TTNT.
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S. Ribbens et al. / Microporous and Mesoporous Materials 114 (2008) 401–409 Table 1 Porosity characteristics and photocatalytic performances of Na-TTNT and H-TTNT Sample name
SBET (m2/g)
Vp (cm3/g)
Ads.a (10 5M)
Cat.b (10 5M)
Conv.c (10 5M)
Na-TTNT Na-TTNT C350 H-TTNT H-TTNT C350
205 194 333 345
0.52 0.55 0.76 0.88
2.20 2.45 3.00 1.57
0.15 0.16 0.58 1.27
2.54 2.62 3.58 2.85
a Prior to UV irradiation, an adsorption–desorption equilibrium between the catalyst surface and the dye is established. The concentrations given in Tables 1 and 2 represent the concentration of rhodamine 6G that is absorbed by the sample. b After 30 min, the samples are irradiated with UV-light and photocatalytic oxidation reactions take place. Here the concentrations in Tables 1 and 2 represent the amount of dye that is photodegradated. c The conversion gives an idea about the total concentration of rhodamine 6G that has been taken out of the solution by adsorption or photodegradation.
Fig. 3. Schematical representation of Na-TTNT.
strain energy. The relaxation of these tensed bond lengths and angles, originating from the curvature of the layers, leads to small differences in nanotubes dimensions which are responsible for the increase in surface area. Further, it can be seen in N2-sorption that both samples show a type H3 hysteresis loop (IUPAC classification), confirming the presence of mesopores (2–50 nm). Moreover, the observed hysteresis loop approaches P/P0 = 1 suggesting the presence of macropores (>50 nm) what can be clearly seen in the pore size distributions of these samples (Fig. 4b). This is due to the formation of aggregates of the trititanate nanotubes what can be clearly seen in the pore size distributions of these samples (Fig. 4b).
1200 0.012
dv (r)
Volume (cc/g) STP.
800
H-tube a Na-tube
b
0.010
1000
0.008 0.006 0.004
600
0.002 0.000
400
0
100
200
300
r (Å)
200 0 0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 4. (a) N2-sorption isotherm at 196 °C: Na-TTNT and H-TTNT, (b) pore size distribution: Na-TTNT and H-TTNT.
sodium ions are only weakly bonded to the negatively charged Ti3 O27 layers. While the Na–O bond length in ˚ , the bond length of H–O in Na2Ti3O7 is above 2 A ˚ H2Ti3O7 is about 1 A. Therefore, the proton exchange process is irreversible. HRTEM (Fig. 1b) proves that the morphology of the original alkali trititanate is preserved, although some defects can be observed on the outer surface of the nanotubes are observed. TEM shows that the outer surface of the Na-TTNT is smooth, while the outer surface of the H-TTNT is clearly not. N2-sorption results (Fig. 4, Table 1) reveal that there is a significant difference in specific surface area (205 m2/g Na-TTNT and 333 m2/g HTTNT). This can be explained by taking into consideration that the large sodium cations with big hydration spheres are exchanged by small, poorly hydrated protons during the acid treatment. The ion exchange process results in a smaller interlayer distance (H-TTNT:0.62 nm, NaTTNT:0.74 nm as determined by TEM) and a decrease in
3.2. Photocatalytic activity of H-tubes and Na-tubes The photocatalytic activity of Na-TTNT and H-TTNT was tested under UV-irradiation by decomposition of rhodamine 6G. From Fig. 5 it is clear that more rhodamine 6G is adsorbed on the H-TTNT compared to Na-TTNT due to the increased surface area. Moreover, it can be seen that more rhodamine 6G is degradated on H-TTNT than on Na-TTNT. This indicates that the photocatalytic activity of H2Ti3O7 nanotubes is high compared to Na2Ti3O7 nanotubes. This can be attributed to three altered properties, namely band gap energies, defect sites and surface areas. First, the lower photocatalytic activity can be explained by the high band gap energy of the sodium tube which can be calculated from the UVDR spectrum (Fig. 5). The spectrum shows that sodium tubes can absorb light with wavelengths up till 360 nm what corresponds to a band gap energy of 3.5 eV. On the other hand, hydrogen tubes can absorb light with much higher wavelengths, up till 375 nm (band gap 3.3 eV). This means that the hydrogen nanotubes are more photo efficient. Secondly, the contact with acid leads to dehydration and small rearrangements of the structural units of the Na-TTNT [18,24]. Structural defects can be observed in H-TTNT (Fig. 1b), but not in
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H tube Na tube
Absorbance
0.78
H-1 c350 Anatase
0.58
0.38
0.18
Fig. 6. DSC spectrum of H-TTNT. 320
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350
360
370
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420
Wavelength (nm)
Fig. 5. UV–vis DR: anatase, H-TTNT, H-TTNT C350, Na-TTNT.
Na-TTNT. When the initial bonds in H2Ti3O7 are broken during acid treatment structural defects sites, oxygen vacancies and Ti3+, can be formed which lead to a higher surface activity [25]. Indeed, oxygen vacancies act as electron traps and Ti3+ sites act as hole traps. Separation of electron/hole pairs by trapping diminishes recombination resulting in an increased photocatalytic activity. Another aspect responsible for the higher photocatalytic activity is the higher surface area of the hydrogen nanotubes. When the number of hydroxyl groups is higher, more water-, oxygen- and dye-molecules can be adsorbed and therefore more active sites will be obtained. Indeed, an increased amount of adsorbed water and oxygen will lead to more reactive compounds that can be formed, resulting in a higher photocatalytic activity [25]. Namely, oxidation of water molecules by photoinduced positive holes leads to hydroxyl radicals and reduction of adsorbed oxygen by photo electrons results in superoxide radical anions which are directly responsible for the photodegradation. Further, it is believed that Ti–O–Na and Ti–OH bonds react with diluted acid and that new Ti–O–Ti bonds are formed. At this stage, the metastable anatase phase was claimed to be formed at low temperatures [8]. However no bands of anatase could be observed in FT-Raman spectroscopy or XRD. 3.3. Improvement of photocatalytic activity by calcination Anatase is known as one of the best photocatalysts for decomposing organic pollutants in water and air. Section 3.2 revealed that H-nanotubes, consisting of trititanate, have only a small photocatalytic activity, which is far less than the photocatalytic activity of pure anatase particles (conversion 3.410 5 M). To improve the photocatalytic efficiency of the prepared nanomaterials, it would be interesting if the crystal phase (H2Ti3O7) could be transformed into an anatase structure. DSC was used to investigate at which temperature there is a possible phase transforma-
tion. The DSC spectrum (Fig. 6) shows three strong endothermic peaks. The first peak at 80 °C is characteristic for the evaporation of adsorbed water molecules. The second and third peak at 350 and 600 °C are a clear indication for a phase transformation into other crystal structures. These structures are identified with FT-Raman spectroscopy. Fig. 7e clearly shows that upon heating extra peaks become visible in addition to the original peaks of H2Ti3O7 (Fig. 7d). The peaks at Raman shifts 150, 399, 519, 638 cm 1 (see Fig. 7a) reveals the presence of anatase. At the temperature of 350 °C, there is no complete transformation into anatase because the spectrum still shows peaks characteristic to the structure of H2Ti3O7. The fact that the hydrogen trititanate structure transforms to an anatase structure and not to the thermodynamic more stable rutile structure can be explained by the fact that anatase and trititanate structures have common features: the edge sharing observed in an anatase structure is the same as in a hydrogen trititanate structure which is composed out of connected chains in wich TiO6 octahedra share edges. At 600 °C transformation to another crystal structure is observed which is identified as rutile. Although, during
e Arbitrary Raman intensity
-0.02 310
d
c b a 700
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500 400 300 Raman shift (cm-1)
200
100
Fig. 7. FT-Raman spectroscopy: (a) Anatase reference spectrum, (b) NaTTNT nc, (c) Na-TTNT C 350 °C, (d) H-TTNT nc, (e)H-TTNT C 350 °C.
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-0.00005 -0.0001
a
dw/dt
0
b
-0.00015 -0.0002
30 80 130 180 230 280 330 380 430 480 530 580 630 680 730 780 Temperature (°C)
Fig. 8. DTG obtained by 5 °C/min under flowing O2: (a) Na-TTNT nc, (b) H-TTNT.
calcinations, there is a change in crystal structure, morphology and surface area are preserved what can be seen as deduced from TEM (not shown) and N2-sorption (Table 1). In contrast to the hydrogen nanotubes, no crystal phase transformation is observed in the FT-Raman spectrum (Fig. 7b and c) for the sodium nanotubes. This result is confirmed by TG analysis (Fig. 8). Here only 2 regions of weight loss can be distinguished for sodium tubes whereas 3 regions were observed for hydrogen tubes. In sodium tubes, first, adsorbed water is removed from the surface at 30–100 °C. When the temperature is further increased, intercalated water molecules are removed. Because sodium tubes have strongly hydrated intercalated sodium ions, the weight loss in the region between 100 and 200 °C is larger than that of the poorly hydrated protons in the hydrogen nanotubes. The region at 200–350 °C is quite different for both structures. Here, the hydrogen tubes show a clear weight loss due to further dehydration resulting in a phase transformation. (H2Ti3O7 ? TiO2 + H2O). Such dehydration is not observed for the sodium tubes. Based on Raman spectra, no anatase is formed in the calcined Na-TTNT because Na-TTNT has a much higher thermal stability than H-TTNT, while there is a good stabilisation of the different layers by intercalated sodium ions. Because of this stabilization no phase transformation can take place [26,27]. The transformation of part of crystal phase into anatase seriously increases the photocatalytic activity (Table 1) of the hydrogen nanotubes with about 118%. In contrast to these hydrogen nanotubes, the photocatalytic activity of sodium nanotubes can not be significantly improved because of the absence of anatase. Although the photocatalytic activity of the H-TTNT could be increased by calcination, there is a clear decrease in adsorption capacity although no decrease in surface area is observed. Most likely this can be explained by the amount diminished of surface OH-groups which are the adsorption sites for the rhodamine 6G molecules. 3.4. Improvement of photocatalytic activity by hydrothermal treatment It is already mentioned that Na-TTNT are formed from bulk TiO2 during hydrothermal treatment. So the tempera-
ture of the hydrothermal treatment is a key parameter in the formation of the nanotubes. During this process, recrystallisation takes place and anatase nanoparticles are transformed into a nanotubular trititanate crystal structure. It would be interesting if an optimum temperature could be found under which pure anatase nanotubes are formed. Therefore, the synthesis was executed at different temperatures: 60, 110 and 150 °C (H-TTNT 60 °C, H-TTNT 110 °C, H-TTNT 150 °C). Samples prepared at 60 °C are identified as pure anatase particles with low surface areas and small adsorption capacities (SEM pictures not shown). Hydrothermal treatment is insufficient at this temperature to destruct the original bulk TiO2 resulting in the disability to form nanotubes. TEM images (Fig. 9A) show that at 100–110 °C nanotubes can be formed with surface areas comparable with those of the nanotubes prepared at 150 °C. There is however an important difference in crystal phase. FT-Raman (Fig. 10) reveals that the samples prepared at 110 °C contain a mixture of both anatase and trititanate. A selected area electron diffraction pattern of nanotubes was taken to get more information about the anatase crystal phase. The diffraction pattern of the nanoscale tubular materials can be assigned to H2Ti3O7, so the nanotubular materials don’t contain anatase (See Fig. 9C) [28]. This suggests that there have to be anatase particles in sample as a separate phase, what could confirmed with TEM (Fig. 9B) and SAED (see supporting information Fig. 2). The photocatalytic activity of the three samples was tested and results are shown in Table 2. Two opposite effects can be deduced from this table: first, the adsorption capacity and total conversion increases at elevated temperatures. Secondly, the photocatalytic activity decreases at elevated temperatures. As already mentioned, low hydrothermal temperatures do not result in the formation of high surface nanotubular materials, so the adsorption capacity of these samples is low. While the original bulk anatase crystals are still present, high photocatalytic activities are observed (Table 2). Due to the absence of the nanotubes, these materials do not offer any advantages compared to the fine powder materials. At 110 °C highly porous nanotubes could be formed which increases the adsorption capacity. Based on the presence of highly photocatalytic active anatase particles, the samples prepared at 110 °C show an increase in photocatalytic activity of 100% compared with the nanotubes prepared at 150 °C. 3.5. Increase of the photocatalytic activity by microwave assisted synthesis Pure anatase, mesoporous nanomaterials could not be obtained directly without calcination when a conventional hydrothermal treatment at low temperatures was performed. Therefore, the effect of heating via microwave irradiation has been investigated. By applying microwave heating for a short irradiation time, the reaction mixture is heated rapidly and more uniformly through direct molecular interaction with EM radiation (absorption of
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Arbitrary Raman Intensitity
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c
b a 700
600
500
400
300
200
100
Raman shift (cm-1)
Fig. 10. FT-Raman spectroscopy H-TTNT synthesized at: (a) 60 °C, (b) 110 °C, (c) 150 °C.
Table 2 Porosity characteristics and photocatalytic performances of H-TTNT
Fig. 9. (A) HRTEM: H-TTNT 110 °C, (B) HRTEM: presence of anatase particles, (C) SAED from particles: Characteristic diffraction pattern of anatase.
microwaves which are transformed into heat). Microwave synthesis gained a lot of attention in the past decade recent,
Sample name (°C)
SBET (m2/g)
Vp (cm3/g)
Ads. (10 5 M)
Cat. (10 5M)
Conv. (10 5M)
H-TTNT 60 H-TTNT 110 H-TTNT 150 H-TNT MW 110 H-TTNT MW 150
57 377 333 194 305
0.13 0.72 0.76 0.55 0.67
0.77 2.22 3.00 1.28 3.35
1.27 1.16 0.58 1.03 0.44
2.03 3.37 3.58 2.32 3.79
since it has been claimed that reaction rates can be accelerated, reaction times can be shortened, yields can be improved and reaction pathways can be activated or suppressed [29]. In the previous section, it was shown that at 110 °C nanotubes could be formed with anatase in the structure without necessity of a calcination step. Nevertheless, the anatase was present as a separate phase next to the H-TTNT. Therefore, the reaction mixture in the microwave oven was kept at 110 °C for comparison (H-TNT MW 110 °C). The morphology of the microwave prepared samples is studied with TEM (Fig. 11). The low magnification image (Fig. 11A) suggests the formation of a certain nanostructure whereas HRTEM (Fig. 11B) further reveals a fine lattice inside the nanostructure instead of a wall structure with a porous interior. These structures are recognised in literature as a nanoribbon which does not have an inner pore. This absence of the internal pores implies that the surface area is a lot lower than the surface area and pore volume of tubular nanomaterials as was seen in N2sorption (Table 2). From FT-Raman spectroscopy (See supporting information Fig. 2), it is clear that these nanoribbons have the typical pure anatase (TiO2) crystal structure without the presence of trititanates. The
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Fig. 11. H-nanostructures prepared by microwave synthesis: (A) TEM and (B) HRTEM.
photocatalytic activity of the low surface, anatase nanoribbons is much higher (77%) than the photocatalytic activity of the corresponding high surface, H2Ti3O7 nanotubes prepared at 150 °C (H-TTNT MW 150 °C) (Table 2). The formation mechanism of these anatase nanoribbons is still unclear. Yu et al. [30] observed already the transition of titanate nanoribbons to anatase nanoribbons by a post hydrothermal treatment with water. It is possible that due to the rapid molecular and more uniform by microwave irradiation heating by microwave irradiation anatase nanoribbons are formed directly. The exact reason why nanoribbons instead of nanotubes are formed is presently unresolved. Especially remarkable is that nanotubes are when the microwave assisted synthesis is performed at 150 °C with the same characteristics as the nanotubes prepared using an indirect classical heating source. This implies that high surface hydrogen nanotubes can be prepared in a very fast way (6 h instead of 48).
ventional hydrothermal treatment. Also here the photocatalytic activity was enhanced due to the presence of anatase. However the adsorption capacity was lower since the porosity of the nanoribbons was much smaller. In summary, it is possible to synthesize mesoporous nanomaterials in a cheap and simple way, with high photocatalytic activity. Acknowledgments Prof. B. Van Der Veken and J. Janssens are gratefully acknowledged for performing the DSC measurements. Vera Meynen and B.U.W. Maes acknowledges the FWO Flanders for financial support. This work is part of the NoE project ‘‘Inside Pores” and the CRP project funded by the special fund for Research of the University of Antwerpen. Appendix A. Supplementary data
4. Conclusions Here, we reported on the synthesis of sodium and hydrogen nanotubes are really photocatalytically active, however, the photoactivity is quite low compared to anatase. In order to increase the photocatalytic activity the initial synthesis route was modified by calcination, softer hydrothermal treatment and microwave synthesis at lower temperature. All three modifications resulted in an increase of photocatalytic activity of the initial trititanate nanotubes. Calcination and a softer hydrothermal treatment did not affect the porous properties and nanotubular shape of the materials. Since all modifications result in the formation of partial or total transformation into anatase, the photocatalytic activity can be significantly increased. A decrease in adsorption capacity was observed because of the decreased OH surface groups on the surface upon anatase formation. By the use of microwave irradiation, pure anatase nanoribbons could be synthesized which had a different morphology than the samples prepared with a con-
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