TiO2 and Its

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Preparation of Highly Ordered Mesoporous Al2O3/TiO2 and Its Application in Dye-Sensitized Solar Cells Jae-Yup Kim, Soon Hyung Kang, Hyun Sik Kim, and Yung-Eun Sung* School of Chemical & Biological Engineering and Research Center for Energy Conversion & storage, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-742, South Korea Received August 7, 2009. Revised Manuscript Received September 30, 2009 Highly ordered mesoporous Al2O3/TiO2 was prepared by sol-gel reaction and evaporation-induced self-assembly (EISA) for use in dye-sensitized solar cells. The prepared materials had two-dimensional, hexagonal pore structures with anatase crystalline phases. The average pore size of mesoporous Al2O3/TiO2 remained uniform and in the range of 6.33-6.58 nm while the Brunauer-Emmett-Teller (BET) surface area varied from 181 to 212 m2/g with increasing the content of Al2O3. The incorporation of Al content retarded crystallite growth, thereby decreasing crystallite size while simultaneously improving the uniformity of pore size and volume. The thin Al2O3 layer was located mostly on the mesopore surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The Al2O3 coating on the mesoporous TiO2 film contributes to the essential energy barrier which blocks the charge recombination process in dye-sensitized solar cells. Mesoporous Al2O3/TiO2 (1 mol % Al2O3) exhibited enhanced power conversion efficiency (Voc = 0.74 V, Jsc = 15.31 mA/cm2, fill factor = 57%, efficiency = 6.50%) compared to pure mesoporous TiO2 (Voc = 0.72 V, Jsc = 16.03 mA/cm2, fill factor = 51%, efficiency = 5.88%). Therefore, the power conversion efficiency was improved by ∼10.5%. In particular, the increase in Voc and fill factor resulted from the inhibition of charge recombination and the improvement of pore structure.

Introduction Mesoporous materials with ordered and uniform pore sizes have received significant attention since their synthesis by Mobil in 19921 due to their numerous advantages, including large surface area, high porosity, and highly accessible inner surface2,3 as well as their potential applications to photocatalysis,4-6 photovoltaic devices,7-9 sensors,10 and lithium-ion batteries.3,11 Particularly, ordered mesoporous TiO2 was synthesized using alkyl phosphate anionic surfactant as a template, but with the serious complication of phosphorus left in the resultant mesoporous TiO2.12 Later on, Stucky’s group2 synthesized pure mesoporous TiO2 with high crystalline character and a well-ordered structure by employing poly(ethylene oxide) (PEO)-based surfactant as a template combined with evaporation-induced self-assembly (EISA). The EISA process utilizes a dilute initial solution mixed with surfactant molecules and inorganic precursors. As the solvent is slowly *Corresponding author: Tel þ82-2-880-1889; Fax þ82-2-888-1604; e-mail [email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (3) Fattakhova-Rohlfing, D.; Wark, M.; Brezesinski, T.; Smarsly, B. M.; Rathousky, J. Adv. Funct. Mater. 2007, 17, 123. (4) Tang, J.; Wu, Y.; McFarland, E. W.; Stucky, G. D. Chem. Commun. 2004, 10, 1670. (5) Bian, Z.; Zhu, J.; Wang, S.; Cao, Y.; Qian, X.; Li, H. J. Phys. Chem. C 2008, 112, 6258. (6) Pan, J. H.; Lee, W. I. Chem. Mater. 2006, 18, 847. (7) Wei, M.; Konishi, Y.; Zhou, H.; Yanagida, M.; Sugihara, H.; Arakawa, H. J. Mater. Chem. 2006, 16, 1287. (8) Zukalova, M.; Zukal, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Gratzel, M. Nano Lett. 2005, 5, 1789. (9) Lancelle-Beltran, E.; Prene, P.; Boscher, C.; Belleville, P.; Buvat, P.; Lambert, S.; Guillet, F.; Boissiere, C.; Grosso, D.; Sanchez, C. Chem. Mater. 2006, 18, 6152. (10) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Adv. Mater. 2003, 15, 624. (11) Wang, K.; Wei, M.; Morris, M. A.; Zhou, H.; Holmes, J. D. Adv. Mater. 2007, 19, 3016. (12) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. 1995, 34, 2014.

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evaporated in adequate humidity, the surfactant molecules form organized micelles with the inorganic precursors remaining on the outer surface. Following removal of the surfactants, the ordered mesostructured materials are obtained. This process is a simple and facile way to prepare ordered mesoporous TiO2 along with a wide range of other surfactants, such as nonionic surfactants which can be easily removed by heat treatment.13 Therefore, the EISA process has been adopted as a general method for the synthesis of ordered mesoporous TiO2. Meanwhile, mixed mesoporous TiO2 including other oxides or dopants has been widely studied due to its additional favorable properties. Lee’s group synthesized mesoporous WO3/TiO2 and found that WO3 improved both the mesopore structure of TiO2 and the photocatalytic activity in decomposing 2-propanol.6 Furthermore, Li’s group mixed mesoporous TiO2 with Bi2O3 in an effort to achieve photocatalyic activity in the visible region.5 In addition, Cr-doped14 as well as La-, Fe-, and Pd-doped mesoporous TiO215 have been investigated while ZrO2,16,17 Ce,18 and Cd,19 etc., were also used to modify the properties of mesoporous TiO2. Until now, research into TiO2-based binary mesoporous oxides with an ordered structure has been concentrated on photocatalytic applications. However, there have been few reports on photovoltaic applications. Pure TiO2 with an ordered mesoporous structure has already been considered as a fascinating photoanode7-9,20 (13) Hung, C.; Bai, H.; Karthik, M. Sep. Purif. Technol. 2009, 64, 265. (14) Yin, J. B.; Zhao, X. P. J. Phys. Chem. B 2006, 110, 12916. (15) Yuan, S.; Sheng, Q.; Zhang, J.; Yamashita, H.; He, D. Microporous Mesoporous Mater. 2008, 110, 501. (16) Zhou, W.; Liu, K.; Fu, H.; Pan, K.; Zhang, L.; Wang, L.; Sun, C. C. Nanotechnology 2008, 19. (17) Elder, S. H.; Gao, Y.; Li, X.; Liu, J.; McCready, D. E.; Windisch, C. F. Chem. Mater. 1998, 10, 3140. (18) Frindell, K. L.; Tang, J.; Harreld, J. H.; Stucky, G. D. Chem. Mater. 2004, 16, 3524. (19) Li, X. S.; Fryxell, G. E.; Engelhard, M. H.; Wang, C. Inorg. Chem. Commun. 2007, 10, 639. (20) Coakley, K. M.; Liu, Y.; McGehee, M. D.; Frindell, K. L.; Stucky, G. D. Adv. Funct. Mater. 2003, 13, 301.

Published on Web 10/16/2009

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due to its large surface area, its uniform nanochannels easily accessed by the electrolyte, and the absence of micropores.7 However, there is still insufficient research on the possible photovoltaic applications of TiO2-based binary oxides with an ordered mesopore structure. There have been reports on dye-sensitized solar cells (DSSCs) using TiO2 electrode coated with other metal oxides, such as Al2O3,21-23 ZnO,22,24 or Nb2O5.25 However, in these studies, the TiO2 electrode did not have an ordered mesopore structure, but a randomly distributed and nonuniform pore structure made by the conventional hydrothermal method. Accordingly, we synthesize highly ordered mesoporous Al2O3/ TiO2 with various compositions of Al content by the EISA process. We selected Al2O3 because its insulating nature can function as an energy barrier to reduce the charge recombination rate in DSSCs, a phenomenon that is well-known.21-23 In order to investigate the potential application of DSSCs, mesoporous Al2O3/TiO2-based photoanodes were fabricated and evaluated for their structural, crystalline, and stoichiometric properties. Furthermore, their photovoltaic properties were examined by focusing on the thin Al2O3 layer.

Experimental Section Preparation of Mesoporous Al2O3/TiO2. Mesoporous Al2O3/TiO2 was prepared by the sol-gel reaction and an evaporation-induced self-assembly (EISA) process. First, an initial solution was prepared at room temperature by dissolving titanium tetraisopropoxide (TTIP, Aldrich, 97%) in isopropanol followed by the addition of a small amount of aqueous HCl (Samchun Chemicals, 35 wt %). TTIP is unstable and reactive with water vapor in air, but it becomes stable when dissolved in isopropanol. The aqueous HCl solution should not be diluted further because a more dilute solution can facilitate a condensation reaction to cause precipitation. After addition of 35 wt % aqueous HCl, the solution remained perfectly transparent. After the solution was stirred for 1 h, aluminum tri-sec-butoxide (Aldrich, 99.99%) was added with vigorous stirring and followed by further stirring for 1 h. This solution was then mixed with Pluronic P123 (EO20PO70EO20, BASF) dissolved in isopropanol and stirred for 3 h more. The molar ratios of the constituents were as follows: TTIP/P123/HCl/ H2O/isopropanol = 1:0.02:0.6:2.27:28.4. Following this, the solution was poured into a Petri dish and dried at room temperature by air exposure (the relative humidity was maintained at 50-60%). The dried samples were aged at 40 °C for 48 h and then at 120 °C for 24 h. The samples were then calcined at 350 °C for 4 h in air to remove the block copolymer template. Electrode Assembly. The prepared mesoporous Al2O3/TiO2 powder was dissolved in ethanol and grinded by an ultrasonic processor (VCX130, SONICS) for 30 min. Ethyl cellulose (Kanto Chemical Co., 30 wt % versus TiO2 weight) and R-terpineol (Kanto Chemical Co.) were then added to the mixture as a binder and solvent, respectively. The prepared paste was deposited on an optically transparent conducting glass (F-doped SnO2, Pilkington TEC Glass, 8 Ω/square, transparency 77% in the visible range) by the doctor blade technique. This prepared TiO2 film was sintered at 525 °C in air for 1 h. The resulting electrodes were sensitized with an ethanol solution consisting of 3  10-4 M cis-bis(isothiocyanato)bis(2,20 -bipyridy-4,40 -dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (Ru 535-bisTBA, Solaronix) for 12 h at (21) O’Regan, B. C.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant, J. J. Phys. Chem. B 2005, 109, 4616. (22) Diamant, Y.; Chappel, S.; Chen, S. G.; Melamed, O.; Zaban, A. Coord. Chem. Rev. 2004, 248, 1271. (23) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (24) Wang, Z. S.; Huang, C. H.; Huang, Y. Y.; Hou, Y. J.; Xie, P. H.; Zhang, B. W.; Cheng, H. M. Chem. Mater. 2001, 13, 678. (25) Chen, S. G.; Chappel, S.; Diamant, Y.; Zaban, A. Chem. Mater. 2001, 13, 4629.

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40 °C in an oven. The Pt-coated counter electrodes were prepared by thermal decomposition.26 A drop of 10 mM H2PtCl6 in 2-propanol was spread on the F-doped SnO2 glass by spin-coating followed by heating at 400 °C for 15 min in air. The dye-adsorbed TiO2 electrodes (active area 0.25 cm2) were next assembled with a Pt-coated counter electrode using thermal adhesive film (Surlyn, thickness: 50 μm). The redox electrolyte used consisted of 0.6 M 1-methyl-3-propylimidazolium iodide (MPII), 0.1 M LiI, 0.05 M I2, and 0.5 M tert-butylpyridine in methoxypropionitrile (MPN). Characterization of Mesoporous Al2O3/TiO2. High-resolution transmission electron microscopy (HR-TEM; JEOL JEM-2010) was used to examine the morphology and mesopore structures. The crystalline phases of mesoporous Al2O3/TiO2 were confirmed using high-power X-ray diffraction (XRD; Rigaku D/MAX 2500 V diffractor) with Cu KR radiation. The Brunauer-Emmett-Teller (BET) surface area and the Barret-Joyner-Halenda (BJH) pore-size distribution were characterized by nitrogen adsorption-desorption isotherms at 77 K (Micrometritics ASAP 2010). Surface electronic states and molar ratios of components were investigated by X-ray photoelectron spectroscopy (XPS; Thermo SIGMA PROBE) using an Al KR X-ray source in an UHV system with a chamber base pressure of ∼10-10 Torr. Molar ratios of bulk components were measured by energy-dispersive X-ray spectrometer (EDX; Bruker-AXS QUANTAX). The photovoltaic performance was measured using a 500 W xenon lamp (XIL model 05A50KS source measure units and an AM 1.5 filter) at a power of 100 mW/cm2. The incident light intensity was adjusted with a silicon reference solar cell produced by NREL.

Results and Discussion Figure 1 shows the HR-TEM images of several mesoporous Al2O3/TiO2 powder samples. Pure mesoporous TiO2 (Figure 1a) had a two-dimensional hexagonal pore structure and was highly ordered. Figure 1b represents an image of pure mesoporous TiO2 viewed normal to the channel axis of the mesostructure. From the high-resolution image (Figure 1c), pore size was measured as ∼7 nm while the (101) lattice plane (fringe spacing ∼ 0.35 nm)27 clearly showed pore walls consist of highly crystallized anatase phase. Figure 1d presents the selective electron diffraction (SAED) pattern of pure mesoporous TiO2, indicating its diffraction rings exactly correspond to the anatase phase. Furthermore, the pore structures of 1 and 2 mol % mesoporous Al2O3/TiO2 were the same as that of pure mesoporous TiO2 (Figure 1e,f). Figure 2 shows the XRD spectra obtained from mesoporous Al2O3/TiO2 powder. The peak position of mixed oxides was nearly same as pure mesoporous TiO2, thereby implying both contain the crystallized anatase phase. However, the broadness of the peak became larger as more Al was incorporated. In addition, the average crystallite size using Scherrer’s equation28 was measured to be about 8.7, 7.6, and 7.0 nm for pure TiO2, 1 mol % Al2O3/ TiO2, and 2 mol % Al2O3/TiO2, respectively. This is attributed to the retardation of crystallite growth upon addition of Al. Figure 3 shows the N2 adsorption-desorption isotherms and the BJH pore-size distribution of mesoporous Al2O3/TiO2 powder. These curves exhibit a type IV isotherm with a type H1 hysteresis loop, the typical characteristic of mesoporous materials.29 The type IV isotherm has a hysteresis loop due to capillary condensation of N2 gas in the pore. The type H1 hysteresis loop (26) Papageorgiou, N.; Maier, W. F.; Gratzel, M. J. Electrochem. Soc. 1997, 144, 876. (27) Kang, S. H.; Choi, S. H.; Kang, M. S.; Kim, J. Y.; Kim, H. S.; Hyeon, T.; Sung, Y. E. Adv. Mater. 2008, 20, 54. (28) Cullity, B. D. Elements of X-Ray Diffraction, 2nd ed.; Addison-Wesley Pub. Co.: Reading, MA, 1978. (29) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic Press: London, 1982.

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Figure 3. N2 adsorption-desorption isotherms (a) and BJH poresize distribution from the adsorption branch isotherm (b) of pure mesoporous TiO2 as well as 1 and 2 mol % Al2O3/TiO2 calcined at 350 °C. Figure 1. TEM images (a-c) and SAED pattern (d) of pure mesoporous TiO2, 1 mol % Al2O3/TiO2 (e), and 2 mol % Al2O3/ TiO2 (f) calcined at 350 °C.

Figure 2. XRD spectra of pure mesoporous TiO2 as well as 1 and 2 mol % Al2O3/TiO2 calcined at 350 °C.

shows a narrow shape with steep adsorption branches. For type H1, (pa/p0)2 < pd/p0 is satisfied (pa pressure during adsorption; pd pressure during desorption). This type of hysteresis loop indicates a narrow pore size distribution in the mesoporous sample.30,31 The BET surface area was 181, 199, and 212 m2/g for pure TiO2, 1 mol % Al2O3/TiO2, and 2 mol % Al2O3/TiO2, respectively. The BJH average pore diameter was 6.58, 6.38, and (30) Kuznetsova, T. F.; Rat’ko, A. I. Colloid J. 2004, 66, 709. (31) Chandrasekar, G.; You, K. S.; Ahn, J. W.; Ahn, W. S. Microporous Mesoporous Mater. 2008, 111, 455.

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6.33 nm for pure TiO2, 1 mol % Al2O3/TiO2, and 2 mol % Al2O3/ TiO2, respectively, while the pore volumes were 0.305, 0.325, and 0.344 cm3/g. The full width at half-maximum (fwhm) of each peak in the BJH pore-size distribution was measured.17 The fwhm was 2.26 nm for pure TiO2, was 2.10 nm for 1 mol % Al2O3/TiO2, and was 1.37 nm for 2 mol % Al2O3/TiO2. These results show the pore diameters remained nearly identical while the BET surface area, pore volume, and the uniformity of pore size improved with increasing the content of Al2O3. This can be explained by the suppression of crystallite size by incorporated Al, which is already confirmed by the XRD data. Generally, the mesoporous structure becomes distorted and disordered upon crystallization at high temperature. Therefore, the suppression of crystallite growth by the incorporation of Al can sustain the mesoporous structure, simultaneously resulting in a larger surface area and improved pore structure. Following the procedure of DSSC fabrication, we prepared the TiO2 film on the FTO-coated substrate. Figure 4 shows the XRD spectra of TiO2 film annealed at 525 °C. For the comparison, 4 and 8 mol % Al2O3-incorporated TiO2 films and pure Al2O3 powder were also prepared following the same procedure. All films except for pure Al2O3 contained the pure anatase phase and included peaks that broadened in size as additional Al precursor was incorporated. We also confirmed that other TiO2 phases (rutile, TiO2-B) were absent by Raman spectra (results not shown here). In addition, as shown in Figure 4b, the main anatase (101) peak shifts slightly to a higher angle as content of the Al precursor increases. The d-spacing of anatase (101) and crystallite sizes are summarized in Table 1. The variation in d-spacing can be considered as a result of the incorporation of Al into the TiO2 lattice. However, the different radii and valence Langmuir 2010, 26(4), 2864–2870

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Article Table 1. Influence of Al Content on Lattice Spacing of Anatase (101) and Crystallite Sizes for Samples Calcined at 525 °C sample

d-spacing (A˚)

crystallite size from XRD (nm)

TiO2 1 mol % Al 2 mol % Al 4 mol % Al 8 mol % Al

3.529 3.527 3.528 3.524 3.523

13.4 10.5 9.7 8.2 7.6

Figure 4. (a) XRD spectra of pure mesoporous TiO2 and Al2O3/ TiO2 with various levels of Al and pure Al2O3 calcined at 525 °C. (b) Local view of anatase (101) peak.

of fully oxidized Ti4þ and Al3þ prevent a common lattice structure such as Al2TiO5, which can be formed only in high temperatures (>1200 °C), from existing.32 Several papers have suggested Al-incorporated TiO2 or Al2TiO5 can be synthesized at lower temperatures using specific methods such as a nonhydrolytic sol-gel process or sol-gel synthesis with alkoxides stabilized by acetylacetone.33,34 However, these methods also require the temperature of the annealing process to be above 700 °C. Furthermore, the peak shift in the XRD spectra in Figure 4b is very slight relative to the doped TiO2,35,36 indicating that a small amount of Al was incorporated the TiO2 lattice and mostly segregated in the form of Al2O3. However, we could not detect Al2O3 crystalline peaks for Al2O3/TiO2 samples. This may be due to the amorphous-like phase of Al2O3 as shown by pure Al2O3 in Figure 4a, which is represented by only one peak with small intensity. Therefore, it is likely that the XRD peak of the Al2O3 phase in the Al2O3/TiO2 samples was too small to detect. (32) Omari, M. A.; Sorbello, R. S.; Aita, C. R. J. Appl. Phys. 2006, 99, 123508. (33) Andrianainarivelo, M.; Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Vioux, A. Chem. Mater. 1997, 9, 1098. (34) Innocenzi, P.; Martucci, A.; Armelao, L.; Licoccia, S.; Di Vona, M. L.; Traversa, E. Chem. Mater. 2000, 12, 517. (35) Durr, M.; Rosselli, S.; Yasuda, A.; Nelles, G. J. Phys. Chem. B 2006, 110, 21899. (36) Choi, Y. J.; Seeley, Z.; Bandyopadhyay, A.; Bose, S.; Akbar, S. A. Sens. Actuators, B 2007, 124, 111.

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Figure 5. XPS spectra of pure mesoporous TiO2 and Al2O3/TiO2 with various levels of Al calcined at 525 °C.

In addition, XPS measurement was performed to survey the chemical states of each element in the Al-incorporated TiO2 film. Figure 5 shows the XPS spectra of pure mesoporous TiO2 and the different ratios of Al-incorporated TiO2 film. The binding energy (BE) of Ti 2p, O 1s, and Al 2p were in the range of 458.6-458.4, 529.9-529.7, and 73.9-74.0 eV, respectively. These values are in agreement with reference ones of pure TiO2 and Al2O3.34 On the other hand, peak positions were slightly shifted due to the incorporation of Al species into the TiO2 lattice. To investigate in more detail what form the Al species in the TiO2 lattice exist as, the O 1s core level peaks were compared. DOI: 10.1021/la902931w

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Figure 6. XPS spectra of O 1s for pure mesoporous TiO2 and 8 mol % Al2O3/TiO2 calcined at 525 °C. Table 2. Concentration of Aluminum As Measured by XPS Spectra and EDX Spectra for Pure Mesoporous TiO2 and Al2O3/TiO2 with Various Levels of Al Calcined at 525 °C measured concentration by XPS (mol %)

measured concentration by EDX (mol %)

sample

Al

Ti

Al/(Al þ Ti)

Al/(Al þ Ti)

1 mol % Al 2 mol % Al 4 mol % Al 8 mol % Al

1.8 2.2 2.3 4.7

18.4 18.6 16.5 13.6

8.9 10.8 12.2 25.7

1.2 1.8 3.8 8.3

The O 1s peaks of 1 and 2 mol % Al2O3/TiO2 were not dramatically different with that of pure TiO2. But, as shown in Figure 6, the O 1s peak was clearly modified in 8 mol % Al2O3/ TiO2. The O 1s peak of 8 mol % Al2O3/TiO2 is broader, and the band centered at 531.7 eV becomes noticeable. It was confirmed that the peak at 531.7 eV represents the oxygen in Al2O3 or C-O bond.37 Moreover, quantitative analysis of the elemental atomic percentage shows that the carbon content of pure TiO2 (21.7%) and 8 mol % Al2O3/TiO2 (23.0%) are almost identical (the remaining carbon content may be from ethyl cellulose used to make a viscous paste). Therefore, this additional O 1s peak suggests the presence of Al2O3.34 The relative concentrations of Ti and Al at surface of samples were measured by XPS spectra and listed in Table 2. For the 1 and 2 mol % Al2O3/TiO2 samples, it was difficult to accurately calculate the relative concentrations because of the extremely small amount. We therefore obtained XPS spectra three times for the 1 and 2 mol % Al2O3/TiO2 samples and calculated the amount of Al by averaging the measured values. In addition, the relative concentrations of bulk were measured by EDX and are listed in Table 2. The relative concentration of Al measured by XPS was 3.1-7.4 times higher than the values measured by EDX. It implies that more Al species existed on the mesoporous oxide surface, as XPS is a technique mainly for surface analysis. This result is similar to a previous paper6 in which Pan et al. also reported the distribution of incorporated W species in highly ordered mesoporous WO3/TiO2 films. There they concluded that W species were present on the surface of the mesoporous film, which was confirmed by quantitative XPS spectra analysis. They further mentioned that formation of WO3 on the surface was attributed to differences in crystallization temperature between TiO2 and WO3. That is, at low temperature (