Comparison of the Melon Nanocomposites in Structural Properties

Jul 26, 2011 - Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833. 1. INTRODUCTION. Melon (Figure 1) is a ...
0 downloads 0 Views 3MB Size
Download PDF
ARTICLE pubs.acs.org/JPCC

Comparison of the Melon Nanocomposites in Structural Properties and Photocatalytic Activities Thiam Peng Ang* and Yen Mei Chan Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 ABSTRACT: A simple method was used to prepare SiO2-melon nanocomposites and TiO2-melon nanocomposites. The nanocomposites structure has been extensively characterized and compared by using UV visible spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry analysis (TGA), solid state 13C and 1H nuclear magnetic resonance spectroscopy (13C NMR, 1H NMR), differential scanning calorimetry (DSC), elemental analysis (EA) as well as Fourier transform infrared spectroscopy (FT-IR). It has been found by FT-IR, TGA, 13 C NMR, and EA results that melon was present in the nanocomposites only when the metal oxide does not undergo phase transformation during the sample preparation. In addition, TGA, DSC, XRD, and TEM data seem to imply that the interaction of melon with TiO2 was more extensive than with SiO2. UV visible spectroscopy revealed that the absorption edge of the nanocomposites was located in visible light region and was likely attributed to absorption of melon or melon-metal oxide charge transfer. Hence, melon could function as the photocatalytic active site of the nanocomposites in the degradation of methylene blue under visible light. Interestingly, it was found that the photocatalytic activity difference between the two melon nanocomposites is due to (i) the effectiveness of synergism between the melon and the metal oxide and (ii) the amount of interaction between the melon and the metal oxide.

1. INTRODUCTION Melon (Figure 1) is a thermally stable polymeric compound with the presence of hydrogen that was first discovered by Berzelius.1,2 The name “melon” was subsequently given by Liebig more than 150 years ago.1,2 Although melon can also be used as a precursor to prepare C3N4, this is not commonly done because of its limited degree of polymerization.3 It has been difficult to define a composition for melon despite having an empirical formula of H3C6N9.1 On the basis of this empirical formula, Redemann and Lucas have identified two possible melon structure models in 1940 that contain heptazine (also known as tri-s-triazine or cyameluric) units namely (i) a linear polymer and (ii) a symmetrical triangular form of polymer (Figure 1).1 Interestingly, another structural model based on triazine units have also been proposed.1 Heptazine units were, however, favored by theoretical calculation for graphitic C3N4 (g-C3N4) since it has been shown that they are energetically more stable.4 In addition, it has also been shown that triazines can be converted to the more stable heptazine structures under thermal treatment.1,5 Recently, the 2D structure of melon was revealed for the first time by Lotsch et al. using a wide array of techniques such as electron diffraction, solid-state NMR, and theoretical calculation, thus resolving the long-standing debate pertaining to the identity and composition of melon.1 To date, there are very few reports on the applications of melon and/or its related compound. In comparison, g-C3N4 that share the same heptazine units (monomer) as melon but with a higher degree of polymerization have shown great potential in (i) photocatalytic water splitting under the visible light,6 (ii) CO2 r 2011 American Chemical Society

activitation,7 (iii) hydrogen storage,8 (iv) Fridel-Crafts reaction of benzene,9 and (v) cyclization of functional nitriles and alkynes.10 Furthermore, g-C3N4 has also been utilized as support for Pt Ru anode catalyst in direct methanol fuel cell.11 It was only until recently when Kisch et al. have elegantly synthesized N-modified TiO2 with good photocatalytic activities where a mixture of melem and melon was acting as visible-light sensitizers.12 Moreover, Ang has reported on the preparation and photocatalytic activity of a novel SiO2-melon nanocomposite.13 The photoactivity of this nanocomposite was found to be more superior compared to melon sample with small amount of SiO2. Synergy between melon and SiO2 was suggested to have retarded the electron hole recombination and thus contributed to this enhanced photocatalytic activity. Despite that, all the literature reporting on either the N-modified TiO2 or SiO2-melon nanocomposite seem to focus largely on unveiling the properties of the respective materials and correlating these properties to their catalytic performance. In addition, the mechanism of melon and/or melem formation from urea or thiourea in these systems have also been carefully studied. However, to the best of our knowledge, no work has ever been done on (i) comparing the properties of TiO2-melon nanocomposites and SiO2-melon nanocomposites and (ii) relating the differences in these properties to the observed photocatalytic activity. Thus, for the first time, we comprehensively Received: January 12, 2011 Revised: June 22, 2011 Published: July 26, 2011 15965

dx.doi.org/10.1021/jp200324v | J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C

ARTICLE

Figure 2. UV vis spectra of PreCal TiO2-TU, Amp TiO2-TU, SiO2TU, and TU. Figure 1. Two possible structures of melon, (a) symmetrical triangular form of polymer and (b) linear form of polymer.1

compared the structural properties of SiO2-melon nanocomposites with its TiO2 counterparts in this paper. We have found evidence that indicated the formation of nanocomposites only when SiO2 and precalcined TiO2 are used during the preparation. In addition, it was demonstrated that the interaction of melon with precalcined TiO2 was more extensive than with SiO2, while XRD data pointed out that the melon peak intensity was largely compromised when nanocomposite was formed. It is noteworthy that the photocatalytic activity of the nanocomposites was found to be better than the pure melon.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All chemicals were used as received and without any purification. Pure SiO2 sample was synthesized by adding 0.01 mols of tetraethyl orthosilicate (Sigma Aldrich, 98%) into 20 mL of isopropanol. A 20-mL portion of 0.05 M aqueous ammonium carbonate was then added. The mixture was stirred for 2 h before aging overnight at room temperature. The mixture was then centrifuged and washed to obtain a gelatinous precipitate that will then be dried in the oven of 60 °C overnight. Meanwhile amorphous TiO2 sample was prepared by adding 0.01 mols of titanium isopropoxide (Sigma Aldrich, 97%) to 20 mL of ethanol. A 15-mL portion of deionized water was then added and the mixture was stirred for 2 h before aging overnight at room temperature. The mixture was then centrifuged and washed. The obtained white precipitate was subsequently vacuum dried for 3 4 h. Precalcined TiO2 sample was prepared using the same method with calcination in muffle furnace at 400 °C (heating rate = 1.5 °C/min) for 5 h. To prepare SiO2-melon nanocomposite, ca. 0.42 g of pure SiO2 sample and ca. 2.14 g of thiourea (estimated molar ratio of SiO2 to thiourea = 1:4) were uniformly ground and calcined in muffle furnace at 400 °C (heating rate = 1.5 °C/min) for 5 h. TiO2-melon nanocomposite samples were prepared using the same method except ca. 0.42 g of SiO2 sample was replaced by ca. 0.56 g of TiO2 sample (amorphous or precalcined). Pure melon control was prepared by calcining ca. 2.14 g of thiourea in muffle furnace at 400 °C (heating rate = 1.5 °C/min) for 5 h. To facilitate smooth discussion of results, the following abbreviations were used: (i) samples obtained after SiO2 and

thiourea are mixed and calcined—SiO2-TU, (ii) sample obtained when precalcined TiO2 and thiourea are mixed and calcined— PreCal TiO2-TU, (iii) sample obtained when amorphous TiO2 and thiourea are mixed and calcined—Amp TiO2-TU and (iv) sample obtained after thiourea was calcined—TU. 2.2. Characterization. The catalyst surface was analyzed using XPS, a VG Scientific ESCALAB 250 where a monochromatic Al kR X-ray source at 1486.7 eV was used. The binding energies (BE) of all elements were calibrated using C 1s peak of adventitious C at 284.8 eV. XPS spectra were recorded at θ = 90° for X-ray source. The crystal phase of the catalysts was analyzed using an XRD (Bruker D8 Advance) operated at a voltage of 35 kV and a current of 40 mA. The diffraction pattern was taken in the Bragg’s angle (2θ) range from 1.5° to 90° at room temperature. FT-IR spectra were determined on KBr disks using a FT-IR Spectrophotometer (Biorad Excalibur Series FTS 3000MX) while solid state UV vis spectra of the catalysts were recorded in BaSO4 using Shimadzu UV-2550 with slit width of 1 nm. All solid-state MAS 1H and 13C NMR spectra were recorded with a Bruker Ultrashield AVANCE 400WB (400 MHz) spectrometer with the spin rate of 5000 s 1 as well as with acquisition time of 10 s for 13C NMR measurement and 5 s for 1H NMR measurement. The size and morphology of the catalysts were determined by transmission electron microscopic (TEM) measurement. A small amount of sample was first sonicated in ethanol and a drop of suspension was then dropped and dried over a copper grid. Electron micrographs were taken with a Tecnai, TF20 Super Twin transmission electron microscope at an accelerating voltage of 200 kV. The enthalpy change and percentage weight loss of our photocatalysts was determined using a TA Instruments Q500 TGA analyzer (TGA-DSC mode) under air (flow rate = 60 mL/min) at a heating rate of 20 °C/min while their nitrogen weight percent were analyzed by a EuroVector EA 3000 series elemental analyzer. 2.3. Photocatalytic Activity Measurement. The photocatalytic activities of the samples were measured by the degradation of aqueous methylene blue under visible light. About 50 mg of the sample was first added to a reactor that contained ∼2  10 5 M of aqueous methylene blue. The mixture was then stirred for 1 h in the dark. This is followed by irradiation of visible light obtained from 300 W Xe lamp with a cutoff filter (λ g 420 nm). The concentration of methylene blue was monitored based on its UV vis absorption peak at 664 nm. 15966

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C

Figure 3. FTIR spectra of PreCal TiO2-TU, Amp TiO2-TU, SiO2-TU, and TU.

ARTICLE

Figure 5. Solid-state 13C NMR spectra of PreCal TiO2-TU, Amp TiO2-TU, SiO2-TU, and TU.

Figure 6. Typical thermogravimetric graphs of PreCal TiO2-TU, Amp TiO2-TU, SiO2-TU, and TU in the temperature range from room temperature to 1000 °C. Figure 4. XPS S 2p peak for PreCal TiO2-TU, Amp TiO2-TU, and SiO2-TU.

3. RESULTS AND DISCUSSION 3.1. Effect of the Nature of the Metal Oxide on the Formation of Melon. Figure 2 presents the solid-state UV vi-

sible spectra of SiO2-TU, Amp TiO2-TU, PreCal TiO2-TU, and TU. Although all samples have absorption in the visible light region, only the onset of Amp TiO2-TU seemed to be located at a different position (lower wavelength). In addition, the spectrum profiles for SiO2-TU and TU are similar and we believe that the visible light absorptions for both samples were attributed to the presence of melon. However, the visible light absorption found in PreCal TiO2-TU was due to charge-transfer band where electron in melon was transferred to TiO2 as proposed by Kisch et al.12c The presence of melon in all 3 samples can be verified by FTIR spectra as depicted in Figure 3. In fact, almost all of the melon FTIR peaks located at ca. 806 cm 1(ring-sextent out-of-plane bending), 1253 cm 1[ν (C N)chain of the bond between the heptazine ring and NH group], 1318 cm 1[ν (C N)chain of the bond between the heptazine ring and NH group], 1411 cm 1[δ (NH)], 1477 cm 1 [ν (ring)], 1633 cm 1[δ (NH2) conjugated with heptazine ring or OH vibration] and 3150 cm 1 [ν (NH)] were detected in all samples except Amp TiO2-TU.1,3 Indeed, besides TiO2 peaks, only surface sulfate peaks (∼1047 cm 1 and

1129 cm 1)14,15 and peak attributed to NO+ (∼2047 cm 1)15 were found in the latter. This suggested that TiO2 was interstitially doped by nitrogen species with sulfate species adsorbed onto its surface when a mixture of amorphous TiO2 and thiourea was calcined at 400 °C.15 Undeniably, the presence of sulfate species was indicated by a XPS S 2p (Figure 4) peak that was located at ∼169 eV.15 Similar XPS peak with smaller intensity can also be observed for PreCal TiO2-TU although the corresponding FTIR peaks relating to sulfate species is not discernible. No such peak is observed for SiO2-TU. It is noteworthy that the solid state 13C NMR spectra (Figure 5) for SiO2-TU, PreCal TiO2-TU, and TU contain peaks located at ∼167 ppm and 158 ppm, thus demonstrating that melon in these samples consists of heptazine units.1,16 However, the resolution of the NMR measurement could be not fine enough to observe the split peak at ∼167 ppm in this work as previously identified by Jurgens et al. and Lotsch et al.1,16 In comparison, melon that contains only triazine units should exhibit one splitted peak positioned at ca. 163 168 ppm.5e,16 Again, melon was absent in Amp TiO2-TU since no 13C NMR peak was observed for this sample. The presence of melon in SiO2-TU, PreCal TiO2-TU, and TU was further confirmed by TGA and EA analysis. In the previous work, the second transition in the thermographs (Figure 6) is 15967

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C

ARTICLE

Table 1. N wt%a and Surface Area of PreCal TiO2-TU, SiO2-TU, TU, and Amp TiO2-TU sample

N wt%

surface area (m2/g)

PreCal TiO2-TU

58.6

95

SiO2-TU TU

59.6 58.8

65 6

8.7

179

Amp TiO2-TU a

N wt% = (N wt% derived from elemental analysis)/(wt% loss at the 2nd TGA transition).

attributed to the removal of melon present in the sample.13 Comparing the amount of weight loss in the second transition with the N wt% measured from elemental analysis, the nitrogen content of the species in this transition for SiO2-TU, PreCal TiO2-TU, and TU is calculated to be ∼59% (Table 1) which is close to the nitrogen weight percent of melon shown in the literature.3 Expectedly, we did not obtain the same value for Amp TiO2-TU thus indicating the absence of melon. In fact, there is hardly any weight loss for Amp TiO2-TU during the TGA analysis and the small mass drop, probably due to desorption of sulfate species, started at a higher temperature compared to the other samples. Thus, all of these observations agree well with our FTIR and 13C NMR results. On the basis of our results, it seems that melon can only be found when only SiO2 and precalcined TiO2 are used. In comparison, N-doping instead of melon is observed whenever TiO2 is amorphous. We believed that, during calcination with thiourea at 400 °C, amorphous TiO2 is turning to anatase while no phase transformation occurred for SiO2 and precalcined TiO2. As such, it is possible that, during this process, the bonds in amorphous TiO2 are dissociated and reformed in different conformations to form anatase while dehydration has also concurrently occurred.17 Thus, amorphous TiO2’s surface hydroxyl groups cannot be utilized as thermal catalyst for conversion of isocyanic acid to cyanamide.12c Consequently, no melamine and then thus melon can possibly be formed. Instead, interstitial nitrogen doping and surface sulfate group were resulted. However, when the metal oxides (SiO2 and precalcined TiO2) involved do not undergo any phase transformation, their structures were locked so that the interaction of surface OH group and isocyanic acid was not disrupted to form melon. However, melon can also be prepared directly from thiourea without the presence of an oxide as long as a lid was fully closed on the crucible during calcination. This will likely prevent the sublimation of melamine (intermediate) so that melon can be produced at the end of calcination.18 3.2. Precalcined TiO2—A Better Interactor with Melon. The second transition in the TGA thermographs has also pointed out that there are more melon present in SiO2-TU as compared to PreCal TiO2-TU. However, the onset of the second transition seems to shift toward lower value when precalcined TiO2 was used to form the nanocomposite. No such difference was spotted between TU and SiO2-TU although the transition ended at a higher temperature for TU. As such, this could imply that the presence of precalcined TiO2 has a larger impact on the thermal stability of melon in the nanocomposite than SiO2. Figure 7 presents the DSC thermographs of Amp TiO2-TU, PreCal TiO2-TU, SiO2-TU, and TU. Of the four samples, only Amp TiO2-TU showed no distinct peak, thus further signifying the absence of melon. There is a large exothermic peak located at ∼689 °C for TU which we assigned as melon combustion peak.

Figure 7. Differential Scanning Calorimetric graphs of PreCal TiO2TU, Amp TiO2-TU, SiO2-TU, and TU in the temperature range from room temperature to 1000 °C.

Similarly, there is also one exothermic peak present in PreCal TiO2-TU and SiO2-TU that was located at ∼521 and 651 °C, respectively. Assuming that all of the observed exothermic peaks are related to the combustion of melon, it seems to suggest that the presence of either SiO2 or TiO2 can lower the combustion temperature. Since melon is polymeric in nature, our DSC observations suggested that melon in nanocomposite is presented in a less ordered structure as compared to pure melon. Previously, it has been discussed that the OH groups on the surface of SiO2 and precalcined TiO2 can catalyze the formation of melon. Subsequent condensation between the heptazine amino group (from melon) and surface OH group (from SiO2 or TiO2) could be resulted and Ti N (or Si N) bonds were generated.12 In fact, it has been concluded that during the TiO2 modification with urea, melem-, and melon-based heteroaromatic compounds were formed prior to the formation of Ti N bond between amino group of these compounds and TiO2 surface hydroxyl groups.12c Therefore, any interaction between melon and the metal oxides in this work would possibly involve the breaking of metal oxygen bond, N H bond as well as C N bond (link the individual heptazine units in melon) and the formation of metal nitrogen bond. On the basis of the bond strengths of H N, N Si, O Si, N Ti, and O Ti,19 the resulting enthaply change is estimated to be less endothermic when TiO2 interact with melon. As such, it is more feasible for TiO2 to interact with melon when compared to SiO2. Indeed, on the basis of our TGA and DSC results, TiO2 interacted better with melon’s heptazine units than SiO2 thus resulting in lower combustion temperature. The interaction involved surface OH group of SiO2 and melon could be less significant and thick overlayers of melon remained uncoordinated since DSC exothermic peak positions between SiO2-TU and TU are close to each other. That could also explain the larger weight change experienced by SiO2-TU during TGA analysis. Further evidence can be provided by TEM images where it can be clearly observed that TiO2 particles are more well-dispersed among the polymeric melon structures (Figure 8a). In comparison, SiO2 particles are less dispersed in melon (Figure 8b) and agglomerate of SiO2 particles without melon can also be spotted in some parts of the microgrid (Figure 8c). However, only polymeric melon is present in TU (Figure 8d) while no melon was observed in Amp TiO2-TU (Figure 8e). 15968

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C

ARTICLE

Figure 8. TEM micrographs of (a) PreCal TiO2-TU, (b) SiO2-TU, (c) agglomerate of SiO2 particles in SiO2-TU, (d) TU, and (e) Amp TiO2-TU.

In addition, the XRD diffractogram shown in Figure 9 has shown that there is a broad overlapped peak located between 20° and 30° for SiO2-TU. Two prominent peaks at ca. 27° and 13° were observed for TU with two smaller splitted peaks located at ∼25° and 12°, respectively. The former peak (∼27°) has often been indexed as (002) peak for graphitic materials and the stacking distance was calculated to be ∼0.327 nm (the layer packing can be visualized by the fine lines of TEM image shown in Figure 10).1,2 Compared to the packing in crystalline graphite (about 0.353 nm), the packing density of melon perpendicular to the layer is higher, probably due to the localization of electrons and stronger binding between the layers.1,2 In fact, we have assumed that the melon layer consist of a 2D array of hydrogen

bonded melon strands that were arranged in zigzag manner.1,2 The melon layers were then stacked via either π π stacking1 or van der Waals forces4 (Scheme 1). As such, we attributed the higher combustion temperature of TU recorded in our DSC analysis to these strong interaction between and within the individual melon layers. The peak at ∼13° is related to in-plane structural packing motif such as the hole-to-hole distance of the nitride pore in the crystal1,2 while the presence of the split peaks could suggest that TU contained a small amount of melem.1,12,16 It is noteworthy that PreCal TiO2-TU (as well as Amp TiO2-TU) exhibited only TiO2 anatase peaks with no melon XRD peaks observed. This agrees well with our TGA results that the amount 15969

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C

ARTICLE

Figure 9. XRD diffractograms of of PreCal TiO2-TU, Amp TiO2-TU, SiO2-TU, and TU.

Scheme 1. Cartoons That Illustrate (a) the Interaction of Melon Layers with SiO2 Particles in SiO2-TU, (b) the Interaction of Melon Layers with TiO2 Particles in PreCal TiO2TU and (c) Melon Layer Stacking in TUa

Figure 10. High-resolution TEM micrograph depicting the layer packing in the melon.

Figure 11. Plot of C/Co vs time for PreCal TiO2-TU, Amp TiO2-TU, SiO2-TU, TU, PreCal TiO2-melon, and SiO2-melon. Photolysis of MB was included for comparison.

3.3. Factors Contributing to the Differences in Photocatalytic Activity of the Melon Nanocomposites. The photoa

The melon layer consists of 2D array of hydrogen bonded melon strands.1

of melon present in PreCal TiO2-TU is small. In addition, it is also possible that the better interaction between precalcined TiO2 and heptazine units of melon has disrupted the melon layer stacking and also the hydrogen bonding network between the melon strands. Consequently, it compromised the interaction between and within the melon layers and thus lowered the temperature for melon combustion in PreCal TiO2-TU. In the case of SiO2-TU, melon peak at ∼27° is present although it overlapped substantially with the broad band relating to amorphous SiO2. As such, there is a larger amount of melon present in SiO2-TU and a sizable portion of the melon stacking was left undisturbed by SiO2 (Scheme 1). In this manner, our DSC analysis only recorded a small difference in melon combustion temperature between SiO2-TU and TU.

catalytic activity of SiO2-TU, PreCal TiO2-TU, Amp TiO2-TU, and TU were evaluated from methylene blue (MB) degradation under the visible light (Figure 11). It can be clearly observed that both SiO2-TU and PreCal TiO2-TU have outperformed Amp TiO2-TU and TU even though Amp TiO2-TU has the largest surface area as shown in Table 1. This has clearly pointed out that surface area is not the main factor in affecting the photocatalytic activity among all samples. However, between the former 2 nanocomposites, PreCal TiO2-TU can degrade MB more effectively than SiO2-TU. In addition, photolysis of MB (Figure 11) seems to contribute little to the overall activity. As such, all of these observations suggested that both melon and metal oxides need to be present to give a better MB photodegradation. In order to verify that melon and metal oxide must interact chemically to exhibit good photocatalytic activity, we have physically mixed melon and metal oxide (PreCal TiO2melon and SiO2-melon) with a weight ratio based on our TGA results and evaluated their performance. On the basis of the 15970

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C photocatalytic activities shown in Figure 11, it is clear that metal oxide need to form M N bond with heptazine units of melon in order to demonstrate good activity. In fact, the presence of the

Figure 12. Solid state 1H MNR spectra of SiO2-TU, PreCal TiO2-TU, and TU.

Figure 13. Plot of MB peak position vs time for PreCal TiO2-TU, Amp TiO2-TU, SiO2-TU and TU.

ARTICLE

chemical interaction between the metal oxide and melon in the nanocomposites can be further supported by solid state 1H NMR spectroscopy (Figure 12). It seems that if uncoordinated N H bonds were present in the nanocomposites, a hump might be located at the same position with the peak corresponding to TU. Nonetheless, we did not make such observation, thus suggesting that chemical interaction between metal oxides and melon has indeed occurred. It is noteworthy that MB peak position in UV vis spectra (Figure 13) is experiencing a blue-shift with time for all samples except TU. This indicated that MB is demethylated when SiO2-TU, PreCal TiO2-TU, and Amp TiO2-TU were used as photocatalysts.15 In addition, it can also be observed that PreCal TiO2-TU gives the largest blue-shift in MB peak position thus concurs with our photocatalytic result that PreCal TiO2-TU can degrade MB most effectively. In the previous work, the role of SiO2 in a SiO2-melon nanocomposite was assessed and concluded that SiO2 had played an important role in MB degradation even though it is not photocatalytically active.13 It was believed that the defects present in SiO2 could possibly accept the electron from the LUMO of melon after melon was photoactivated under the visible light (Scheme 2). In addition, it was recently pointed out that the holes present in the tri-s-triazine cores were stabilized due to delocalization.12b As a result, the electron hole recombination was retarded and this contributed to the good photocatalytic activity of this material. Similarly, in this case, there are energy levels present in precalcined TiO2 that could accept the electrons from the LUMO in the melon and thus retard the electron hole recombination. The difference between precalcined TiO2 and SiO2 is that while the latter possesses only limited number of defects-generated empty energy levels to accept the electron, the former utilized its conduction band. Indeed, DFT study done by Wang et al. have shown that the LUMO of polymeric melon is located at a higher energy level than conduction band of TiO2.6a As a result, it could be more feasible for the electrons in the melon LUMO level to transit to conduction band of precalcined TiO2 (Scheme 2). Consequently, at any point in time, there are more electron hole pairs in PreCal TiO2-TU than in SiO2-TU upon light irradiation and this might contribute to the better activity found in PreCal TiO2-TU.

Scheme 2. Cartoons Illustrating the Flow of Electrons from Melon to (i) SiO2 and (ii) Pre-calcined TiO2 during Visible Light Irradiation

15971

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972

The Journal of Physical Chemistry C Moreover, it was observed from our TEM, TGA, and DSC results that the interaction between TiO2 and melon in PreCal TiO2-TU is more prominent compared to that between SiO2 and melon in SiO2-TU. A more widespread interaction between melon and an oxide will eventually lead to more electron electron pairs and thus better MB photodegradation. In addition, Table 1 has also indicated that PreCal TiO2-TU has a larger surface area than SiO2-TU. Consequently, the former is likely to offer more active sites during visible light irradiation and thus better photocatalytic activities.

4. CONCLUSIONS SiO2-melon and TiO2-melon nanocomposite were prepared using a simple method. It was observed that the melon in all samples present in heptazine units while DSC and TEM results pointed out that the interaction between TiO2 and melon is more extensive compared to that found in SiO2 and melon. In addition, melon can only be found in the nanocomposite when SiO2 and precalcined TiO2 were used. The presence of amorphous TiO2 will only result in the formation of nitrogen doping in TiO2 and surface sulfate species. It is imperative for both oxide and melon to be present and interact chemically to demonstrate better photocatalytic activity. Precalcined TiO2-melon nanocomposite shows more superior activity than SiO2-melon nanocomposite because of (i) more effective electron transfer from LUMO of melon to conduction band of TiO2 and (ii) more extensive TiO2 and melon interaction to generate more electron hole pairs.

ARTICLE

Y. D.; Fu, X. Z.; Antonietti, M. J. Am. Chem. Soc. 2009, 131, 1680. (c) Maeda, K.; Wang, X. C.; Nishihara, Y.; Lu, D. L.; Antonietti, M.; Domen, K. J. Phys. Chem. C 2009, 113, 4940. (d) Chen, X. F.; Jun, Y. S.; Takanabe, K.; Maeda, K.; Domen, K.; Fu, X. Z.; Antonietti, M.; Wang, X. C. Chem. Mater. 2009, 21, 4093. (e) Yan, H. J.; Yang, H. X. J. Alloys Compd. 2011, 509, L26. (7) Goettmann, F.; Thomas, A.; Antonietti, M. Angew. Chem., Int. Ed. 2007, 46, 2717. (8) Yang, S. J.; Cho, J. H.; Oh, G. H.; Nahm, K. S.; Park, C. R. Carbon 2009, 47, 1585. (9) Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. Angew. Chem., Int. Ed. 2006, 45, 4467. (10) Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. New J. Chem. 2007, 31, 1455. (11) Kim, M.; Hwang, S.; Yu, J. S. J. Mater. Chem. 2007, 17, 1656. (12) (a) Mitoraj, D.; Kisch, H. Angew. Chem., Int. Ed. 2008, 47, 9975. (b) Mitoraj, D.; Beranek, R.; Kisch, H. Photochem. Photobiol. Sci. 2010, 9, 31. (c) Mitoraj, D.; Kisch, H. Chem.—Eur. J. 2010, 16, 261. (13) Ang, T. P. Catal. Commun. 2009, 10, 1920. (14) Yang, Q. J.; Xie, C.; Xu, Z. L.; Gao, Z. M.; Du, Y. G. J. Phys. Chem. B 2005, 109, 5554. (15) Ang, T. P.; Toh, C. S.; Han, Y. F. J. Phys. Chem. C 2009, 113, 10560. (16) Jurgens, B.; Irran, E.; Senker, J.; Kroll, P.; Mller, H.; Schnick, W. J. Am. Chem. Soc. 2003, 125, 10288. (17) Madras, G.; McCoy, B. J. Cryst. Growth Des. 2007, 7, 250. (18) Yan, S. C.; Li, Z. S.; Zou, Z. G. Langmuir 2009, 25, 10397. (19) Kerr, J. A.; Stocker, D. W. In CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: New York, 2000; 9 54.

’ AUTHOR INFORMATION Corresponding Author

*Tel./Fax: +65-67963832/+65-63166182; E-mail: ang_thiam_ [email protected].

’ ACKNOWLEDGMENT The research was supported by ICES and Agency for Science, Technology and Research in Singapore. The authors thank Dr. P. K. Wong, Prof. Jianyi Lin, Dr. Armando Borgna, and Dr. Martin Karl Schreyer for their great support and advice. The authors would also like to thank Ms. Zhan Wang, Ms. Li Li Ong, and Ms. Wenting Zhu for the XPS analysis, the solid state NMR analysis and elemental analysis, respectively. ’ REFERENCES (1) Lotsch, B. V.; D€oblinger, M.; Sehnert, J.; Seyfarth, L.; Senker, J.; Oeckler, O.; Schnick, W. Chem.—Eur. J. 2007, 13, 4969. (2) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; M€uller, J.-O.; Schl€ogl, R.; Carlsson, J. M. J. Mater. Chem. 2008, 18, 4893. (3) Komatsu, T. Macromol. Chem. Phys. 2001, 202, 19. (4) Sehnert, J.; Baerwinkel, K.; Senker, J. J. Phys. Chem. B 2007, 111, 10671. (5) (a) Jurgens, B.; Irran, E.; Schneider, J.; Schnick, W. Inorg. Chem. 2000, 39, 665. (b) Jurgens, B.; Irran, E.; Senker, J.; Knoll, P.; Muller, H.; Schnick, W. J. Am. Chem. Soc. 2003, 39, 665. (c) Lotsch, B. V.; Schnick, W. Chem. Mater. 2005, 17, 3976. (d) Lotsch, B. V.; Schnick, W. Chem. Mater. 2006, 18, 1891. (e) Lotsch, B. V.; Schnick, W. Chem.—Eur. J. 2007, 13, 4956. (f) Holst, J. R.; Gillan, E. G. J. Am. Chem. Soc. 2008, 130, 7373. (6) (a) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen., K.; Antonietti, M. Nat. Mater. 2009, 8, 76. (b) Wang, X. C.; Maeda, K.; Chen, X. F.; Takanabe, K.; Domen, K.; Hou, 15972

dx.doi.org/10.1021/jp200324v |J. Phys. Chem. C 2011, 115, 15965–15972