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J. Phys. Chem. C 2010, 114, 9510–9517
I2-Hydrosol-Seeded Growth of (I2)n-C-Codoped Meso/Nanoporous TiO2 for Visible Light-Driven Photocatalysis Pu Xu,† Jun Lu,‡ Tao Xu,*,§ Shanmin Gao,*,†,| Baibiao Huang,| and Ying Dai| School of Chemistry and Materials Science, Ludong UniVersity, Yantai 264025, Shandong, P. R. China, Department of Physics and Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115, and State Key Laboratories of Crystal Materials, Shandong UniVersity, Jinan, 250100, P. R. China ReceiVed: February 23, 2010; ReVised Manuscript ReceiVed: April 18, 2010
We report a template-free process to fabricate (I2)n-C-codoped meso/nanoporous TiO2 nanocrystallites. I2 nanoparticles in I2-hydrosol were used as seeds to initial the nucleation of a precursory TiO2 shell formed by the hydrolysis of organotitanium molecules. The hybridized jumbles were further calcinated at different temperatures. The resulting (I2)n-C-codoped meso/nanoporous TiO2 structure was confirmed by transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) isotherm adsorption of N2. UV-visible diffuse reflectance spectroscopy (DRS) showed that (I2)n-C-codoped porous TiO2 samples have a strong absorption between 400-700 nm. The formation mechanism of (I2)n-C-codoped meso/nanoporous TiO2 spheres is discussed. Under visible light radiation, our samples exhibit superior ability in the photocatalytic degradation of methylene blue to that of the only C-doped TiO2 or to the commercial P25 TiO2 nanoparticles. The origin of the enhanced visible light-light driven photocatalytic degradation is attributed to the continuous states in the band gap of TiO2 introduced by I2 doping. Factors that contribute to the enhanced photocatalytic reaction are also studied, including the enhanced thermostability and conductivity of anatase TiO2 due to C doping, the location of I2 doping, as well as the morphology of the TiO2 structures. 1. Introduction Semiconductor-catalyzed photo-oxidation using solar light as energy source is one of the efficient processes for rapid and low-cost degradation of organic pollutants. Among various semiconducting materials reported so far as photocatalysts, TiO2 has attracted much attention for environmental purification due to its low cost, high photostability and environmental benign properties.1-4 Generally, in the TiO2-catalyzed photo-oxidation process, the photoexcited electrons in the conduction band (CB) of TiO2 (-4.2 eV vs vacuum) can be readily captured by O2 to form superoxide anions (O2-) as well as other oxygen species, whereas the holes in the valence band (VB, -7.4 eV vs vacuum) reside below the highest occupied molecular orbitals (HOMO) of most organic pollutes, allowing aggressive oxidizing (ripping electrons off) the organic pollutes.1-4 However, a major drawback that impedes the practical applications of pristine TiO2-based photocatalytic oxidation is the fact that the large band gap of TiO2 (∼3.2 eV) only partially overlaps with the solar spectrum in the UVA (400-320 nm) and UVB (320-290 nm) regions. As a consequence, only UV light ( 400 nm). For a typical photocatalytic experiment, a total of 200 mg of the PIT-T sample was added to 200 mL of methylene blue aqueous solution (5.0 × 10-4 mol/ L) in a custom-made quartz reactor. The methylene blue concentration was monitored by UV-vis spectroscopy during the entire experiment. Prior to irradiation, each suspension was
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Figure 2. Wide-angle (a) and small-angle (b) XRD patterns of I2-Ccodoped meso/nanoporous TiO2 after thermal treatment at 200, 400, 600, and 800 °C (A: anatase; R: rutile; B: brookite).
Figure 1. TEM images of I2 hydrosol (a), the precursor of I2-C-codoped TiO2 (b), and I2-C-codoped TiO2 after thermal treatment at 200 (c), 400 (d), 600 (e), and 800 °C (f).
magnetically stirred in the dark for 60 min to ensure an adsorption-desorption equilibrium between methylene blue and photocatalysts, which was then irradiated by visible light. After 5 or 10 min intervals during visible light illumination, about 3 mL aliquots were taken out and centrifuged to remove the trace particles. The absorbance of the centrifuged solution was measured in the range 500-800 nm using a UV-vis spectrophotometer (Shimadzu UV-2550). 3. Results and Discussion 3.1. Sample Structure. 3.1.1. Particle Morphology Studied by Transmission Electron Microscopy (TEM). (I2)n-C-codoped TiO2 was prepared via the hydrolysis of TBOT in I2 hydrosol without any addition of templates or surfactants, followed by a heat treatment in air at elevated temperatures for 3 h. Figure 1 shows the TEM images of I2 hydrosol and the (I2)n-C-codoped TiO2 samples before and after heat treatment at different temperatures. The average particle size of I2 in the hydrosol is estimated to be 5.4 nm with a wide particle size distribution (Figure 1a). After TBOT was hydrolyzed in I2 hydrosol, the newly formed particles showed an average size of 17 nm, as presented in Figure 1b. No porous structure was observed at this point. It is the subsequent heat treatment that creates the mesoporous structures, as shown in Figure 1c-e, which clearly show that each individual spherical particle (secondary particle) consists of a large number of much smaller nanoparticles (primary particles). The four samples including PIT-200, PIT400, PIT-600, and PIT-800 were characterized by TEM as shown in Figure 1c-f, respectively. The apparent common feature of the four samples is that they all consist of pseudo spherical TiO2 particles with diameter approximately 50 nm.
Furthermore, each individual sphere in PIT-200, PIT-400, and PIT-600 is composed of a large number of much smaller and loosely packed TiO2 nanoparticles (a few nanometers in diameter), termed as primary nanoparticles. As a result, the interstitial voids among these primary nanoparticles constitute a short-range disordered nanoporous structure, which is termed as primary nanopores. In comparison, the voids among the large spheres (termed as secondary mesoparticles) also create pores, termed as secondary mesopores. The morphology of the primary pores gradually changes as the size of the primary nanoparticles increases with the increasing temperature of heat treatment. Eventually, the primary pores collapse as the primary nanoparticles fused together at 800 °C (sample PIT-800), whereas the secondary pores remains as shown in Figure 2f. 3.1.2. Crystal Structure Studied by X-ray Diffraction (XRD). Powder X-ray diffraction (PXRD) is used to investigate the changes of structure and crystallite sizes of the as-prepared (I2)nC-codoped TiO2 at different stages of the heat treatments. Figure 2a shows the wide-angle XRD patterns of the samples before and after heat treatment at various temperatures from 200 to 800 °C. The sample before thermal treatment appears to be amorphous for TiO2 phase, which is probably due to the fact that hydrolysis of TBOT is relatively incomplete at room temperature, and large amounts of unhydrolyzed alkyls remain in the xerogel powders. As a result, the adsorbing of unhydrolyzed alkyls onto the surface of the as-formed TiO2 particles could prevent further crystallization of TiO2.32 The three low diffraction peaks should be the iodine (JCPDS No. 71-1370). Upon heat treatment, the wide-angle XRD results of all PIT-T samples showed an anatase TiO2 phase, indicating that the crystallization of the samples is achieved. At low temperature, except for those of the dominant anatase TiO2, another XRD peak (2θ ) 30.9°) with low intensity is detected for PIT-200 sample, which can be abscribed to (121) diffraction plane of brookite TiO2 phase. It worth to note that no rutile phase is detected even for sample calcined at 600 °C (PIT-600), indicating an excellent thermal stability of our samples against phase transformation, which can be ascribed to the carbon doping.28 For sample PIT-800 which is calcined at 800 °C, the predominant phase is still anatase TiO2, which is accompanied by a small portion of rutile phase. Since the heat treatment is a necessary step for doping and for the formation of bimodal meso/nanoporous structure, the high thermal stability of our samples is a favorable feature for maintaining anatase (photoactive phase) under high temperature treatment.27 In addition, as the calcination temperature increases, the peaks assigned to
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TABLE 1: Physicochemical Properties of PIT Samples from N2 Sorption Analysis and XRD Patterna sample
SBET (m2/g)
pore volume (cm3/g)
average pore size (nm)
crystal size (nm)
PIT-200 PIT-400 PIT-600 PIT-800
226.8 186.6 118.2 21.21
0.388 0.343 0.309 0.254
5.486 8.586 5.249 36.268
4.3 6.1 7.8 15.2
a
BET surface areas were calculated by the multipoint BET method from the linear part of the BET plots. Single point absorption total pore volumes were obtained from the volume of N2 adsorbed at P/P0 ) 0.995. Average pore diameters were estimated using the desorption branch of the isotherm and the BJH formula. Crystal size was determined form the XRD pattern using the Scherrer equation.
anatase become sharper and more intense due to the formation of larger grains as summarized in Table 1. The average crystallite sizes of (I2)n-C-codoped TiO2 are 4.3, 6.1, 7.8, and 15.2 nm for PIT-200, PIT-400, PIT-600, and PIT-800, respectively, which are estimated from the full width at half-maximum of the diffraction peak using the Scherrer equation. This trend is also in consistence with our TEM observation. Figure 2b shows the small-angle XRD patterns of the same samples. The strong diffraction peak at about 1° for samples PIT-200, PIT-400, and PIT-600 indicates the presence of a typical short-range disordered microstructure framework, which is not observed in sample PIT-800. The peaks in small-angle range normally suggest the existence of a large lattice plane distance related to the porous structure. This fact demonstrates that our mesoporous TiO2 possesses relatively high stability against thermal collapse. Further elevation of the temperature leads to the collapse of the porous structure, which is evidenced by the disappearance of diffraction peaks at small-angle range for sample PIT-800. The collapse of porous structure in sample PIT-800 is in agreement with the TEM study (Figure 1f), showing the diminished porosity inside the particles. 3.1.3. Specific Surface Area (SSA) and Pore Size Distribution Measured by BET. The pore size and specific surface area of the samples are further characterized by nitrogen adsorptiondesorption isotherm measurements. Figure 3a presents the nitrogen adsorption-desorption isotherms. The results indicate a hierarchically bimodal pore-size distribution for samples PIT200, PIT-400, and PIT-600.33 The first hysteresis loops in Figure 3a for samples PIT-200, PIT-400, and PIT-600 located at relative low pressure are associated with the framework-confined primary nanopores consisting of the interstitial voids among the smaller nanoparticles. The shape and position of this loop are slightly different, depending on the temperatures of heat treatment. For samples PIT-200 and PIT-600, the position of the first loop is located at 0.4 < P/P0 < 0.6, while for sample PIT-400, the first loop is located at 0.5 < P/P0 < 0.8. The monolayer adsorption completes when the relative pressure reaches 0.6, 0.8, and 0.5 for samples PIT-200, PIT-400, and PIT-600, respectively, which indicates that sample PIT-400 has a greater primary pore size than that of samples PIT-200 and PIT-600. The second hysteresis loop located at 0.85 < P/P0 < 1.0 is related to secondary pores composed of the voids among large secondary particles. It should be pointed out that for sample PIT-800, only one hysteresis loop at high relative pressure range (0.9-1.0) is observed, which is associated with the adsorptiondesorption in the secondary pores consisting of the voids among large particles.34 This is consistent with the small-angle XRD and TEM results.
Figure 3. N2 adsorption-desorption isotherms (a) and the corresponding BJH pore size distributions (b) of the as-prepared PIT samples.
The hierarchically bimodal pore-size distribution is further confirmed by its corresponding pore-size distribution, which is determined from the Barret-Joyner-Halenda (BJH) desorption isotherms as shown in Figure 3b. The results suggested that samples PIT-200, PIT-400, and PIT-600 showed a bimodal poresize distribution consisting of smaller primary pores and larger secondary pores. For the samples PIT-200, PIT-400, and PIT600, the average pore diameters of primary pores are 3.2, 4.7, and 3.3 nm, and the average pore diameters for secondary pores are 42, 46, and 52 nm, respectively. However, PIT-800 sample only showed the presence of secondary pores with an average pore diameter of 38 nm, indicating the thermal collapse of the primary pores, namely the fuse of the primary nanoparticles at high temperature. Again, this result agrees with the small-angle XRD and TEM results. Table 1 summarized the physical properties of the (I2)n-Ccodoped meso/nanoporous TiO2 samples obtained after heat treatment at various temperatures. The pore volume and specific surface area decrease with the increase of the heat treatment temperatures. These meso/nanoporous structures with high specific surface area are of particular interest, since they can provide more active sites to adsorb pollute molecules. Consequently, this may enhance catalytic activity, as demonstrated later in the measurement of photocatalytic activity on degradation of methylene blue. 3.1.4. X-ray Photoelectron Spectra (XPS). XPS is a surface sensitive technique and is used to confirm the presence and chemical states of I2, C, and Ti in our samples. Figure 4a shows the XPS survey spectra of the samples calcined at 400 and 600 °C, indicating the presence of Ti, O, C, and I. The Ti 2p highresolution XPS spectrum (Figure 4b) shows two peaks at binding energies of 458.1 eV (Ti 2p3/2) and 463.9 eV (Ti 2p1/2). The Ti 2p peak shows a slight deformation on the lower side of the binding energy, corresponding to the different oxidation states of Ti, which can be well fitted into two peaks of Ti4+ and Ti3+.35 The existence of surface Ti3+ can form a defect and act as a hole trap to promote charge transfer and thus suppress the recombination of electron-hole pairs.36 There are two peaks in the C 1s high-resolution XPS spectrum (Figure 4c) at the binding energies of 285.7 and 289.5 eV. The peak at 285.5 eV is an adventitious signal due to elemental
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Figure 6. Schematic of the formation mechanisms for the bimodal (I2)n-C-codoped meso/nanoporous TiO2 nanoparticles. Figure 4. XPS spectra of PIT calcined at 400 and 600 °C: survey spectrum (a) and high resolution XPS spectra for Ti 2p (b), C1s (c), and I 3d (d).
Figure 5. Photos of (I2)n-C-codoped meso/nanoporous TiO2 samples (PIT-T), the sample prior to heat treatment, and only C-doped meso/ nanoporous TiO2 sample (T-400).
carbon from the carbon tape used in the measurements,37 whereas the peak at 289.5 eV could be assigned to the formation of carbonate species which can induce the narrowing of the band gap of the doped titania.38 It suggested that carbon may substitute for some of the lattice titanium atoms and form a Ti-O-C structure.39 The carbon contained in a titanium alkoxide precursor could be incorporated into the lattice of TiO2. The XPS spectrum of the I 3d region (Figure 4d) shows doublet peaks at 620.5 eV (I 3d3/2) and 630.5 eV (I 3d5/2), which is equivalent to those in molecular I2.40,41 In addition, with the increase of the temperature during heat treatment, the concentration of iodine molecules on the surface of the PIT samples decreased due to sublimation of I2 at high temperature, which is also confirmed by the color change of the samples during the heat treatment. The color of the samples changed from brown, to slight brown and fawn, and finally to white when the temperature increased from 200 to 800 °C, as shown is Figure 5. 3.1.5. Proposed Formation Mechanism of I2-C-Codoped Porous TiO2. Different from the conventional doping method, in which the iodine (or its precursors) are added to the matrix
of TiO2 (or precursors of TiO2), we adopt a counter strategy to add precursors of TiO2 into the matrix of I2 nanoparticles formed in the I2 hydrosol. A possible multistep mechanism, illustrated in Figure 6, is proposed to interpret the formation of the (I2)nC-codoped meso/nanoporous TiO2. First, when TBOT was added to I2 hydrosol at room temperature, the Ti(OBu)4 molecules adsorbed onto the surface of I2 nanoparticles and slowly hydrolyzed to become a titania precursory layer on the I2 nanoparticles. These primary TiO2 particles formed as a shell on the surface of I2 particles. In other words, the I2 particles appears as nucleation sites for TiO2 spheres via the hydrolysis of TBOT. This is confirmed by the TEM images in Figure 1, panels a and b, showing that the particle size of I2 hydrosol increase upon the hydrolysis of Ti(OBu)4 in I2 hydrosol due to the formation of precursory shell of (I2)n-C-codoped TiO2 on the I2 particles. At room temperature, the hydrolysis of Ti(OBu)4 is relatively slow and incomplete. Therefore, a large amount of unhydrolyzed alkoxyls still remains in the xerogel powders.42 The subsequent heat treatment of the xerogel at elevated temperatures leads to a series of reactions among I2, alkyls, and amorphous TiO2 with the presence of O2. We believe that the meso/nanoporous anatase TiO2 doped with I2 and C is a product of the following synergetic processes: (1) The I2 sublimes and a portion of the alkyls are oxidized into CO2 and H2O. Consequently, the resulting gaseous products pulverize the TiO2 shell, leading to the formation of the primary pores. (2) Part of the I2 along with some carbon pyrolyzed from the alkyls attaches to surface of the TiO2 primary particles. The further oxidation of I2 is more or less prevented by the shell of primary TiO2 nanoparticles and the remaining Ti-OR groups. (3) The amorphous TiO2 crystallizes into anatase TiO2 (Figure 6) under heat treatment. (4) Finally, the aggregation among secondary particles results in the secondary pores consisting of the voids among these secondary particles during the heat treatment. 3.2. Photophysical, Electrical, and Photocatalytic Oxidation Studies. 3.2.1. UV-Vis Diffuse Reflectance Spectra. Figure 7 collects the UV-vis spectra of the prepared (I2)n-Ccodoped meso/nanoporous TiO2 samples (PIT-T), the undoped TiO2 sample (P25), as well as the only C-doped meso/ nanoporous TiO2 (T-400). In comparison to the undoped P25 and the only C-doped T-400 samples, the spectra of I2-Ccodoped TiO2 samples (PIT-200, PIT-400, PIT-600, and PIT800) obtained at 200, 400, and 600 °C exhibit a strong broad
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Figure 7. UV-vis DRS of only C-doped TiO2 (T-400), P25 and I2C-codoped TiO2 denoted as PIT-T).
absorption band between 400-700 nm, covering nearly the entire visible range. This band is centered around 500 nm, which is the spectral range where iodine absorbs.22,43 PIT-400 showed the maximum visible light absorption compared to the other samples. Apparently, the absorbance between 400-700 nm decreases for sample prepared at higher temperatures, especially for PIT-800. This can be ascribed to the additional loss of I2 by sublimation at higher temperatures. Nonetheless, the absorption in the visible-light region implies that the prepared codoped samples can be activated by visible light and that more photogenerated electrons and holes can be created and they can participate in the photocatalytic oxidation reactions. 3.2.2. Electrical ConductiWity Measurement. The conductivity of the samples were also measured and summarized in Table 2. Apparently, the carbon-doped samples show much higher conductivity than bare TiO2 (sample P25) which is bare TiO2 and contains no carbon. Especially for PIT-400 and PIT-600 samples, their conductivity is ∼5 orders of magnitude greater than P25, in agreement with the recent report on the enhanced conductivity of TiO2 nanotubes through carbon doping and modification.30 This boosted conductivity may significantly improve the charge carrier transport in the bulk of the TiO2 particles to their external surface region, thus enhancing the photocatalytic activity that occurs at the particle-solve interface. Therefore, we further conducted the photocatalytic study. 3.2.3. Photocatalytic ActiWity on the Decomposition of Methylene Blue and FT-IR Analysis. Aqueous solutions of methylene blue have been contacted with PIT-T samples, with only C-doped meso/nanoporous TiO2 (i.e., T-400 sample) and with bare TiO2 nanoparticles (P25) for comparison. The decomposition profiles of methylene blue over PIT-T samples, T-400, and P25 under visible light (λ > 400 nm) are collected in Figure 8. The concentration of methylene blue decreases slowly for all TiO2 samples analyzed before the light is turned on, as shown in the left part of Figure 8a. However, it is still noticeable that the concentrations of methylene blue in solution containing PIT-T samples are lower than those in the presence of P25. This trend agrees with the fact that the (I2)n-C-codoped meso/nanoporous PIT-T samples possess more specific surface area and thus adsorb more methylene blue molecules and cause more depletion of methylene blue in solution phase. Compared to the only C-doped meso/nanoporous TiO2 and bare TiO2 (P25), significantly enhanced photocatalytic oxidation
activity on the decomposition of methylene blue under visible light is observed for (I2)n-C-codoped meso/nanoporous PIT-T (T < 600 °C) samples, as shown in the right part of Figure 8a. The methylene blue solutions containing the PIT-T samples were almost fully degraded and became transparent within about 30 min. It should be noted that the PIT-400 sample showed the highest photocatalytic oxidation on methylene blue. This is consistent with the results from UV-vis spectra which showed that PIT-400 sample exhibits the highest absorption intensity in the visible region. For this reason, the PIT-400 sample was further selected for a cyclic stability test under the same conditions, and the results are presented in Figure 8b. No significant loss in the photocatalytic oxidation activity is observed even after being used six times. As an additional evidence for the visible-light-driven photooxidation activity exhibited by the PIT-400 sample, FTIR spectra were collected to study the surface chemistry of the photocatalysts. The FTIR spectra of PIT-400 before and after degradation and the pure methylene blue IR spectrum are presented in Figure 9. The FTIR spectra of sample PIT-400 before degradation (curve a) show several characteristic peaks. The absorption at 3524 cm-1 is assigned to the stretching vibration mode of the hydroxyl groups chemisorbed on the TiO2 surface and dissociated or molecularly adsorbed water.27 The weak peak observed at 1670 cm-1 is assigned to the deformation vibration for H-O-H bonds,44 which is normally recognized as an important factor affecting photocatalytic activity.45 The greater the number of surface hydroxyl molecules, the faster the photocatalytic reaction.46 The peak below 1000 cm-1 corresponds to the titania crystal lattice vibration. The distinct FTIR spectrum of PIT400 after degradation (curve b) and that of the pure methylene blue (curve c) strongly suggests that the degradation is due to the photocatalytic degradation instead of any physisorption of methylene blue on the catalyst. 3.2.4. Proposed Mechanism for the Enhanced Photocatalytic ActiWity. The enhanced photocatalytic oxidation activity in (I2)n-C-codoped meso/nanoporous TiO2 samples (PIT-T) is a product of several factors. (1) The doping of (I2)n introduces continuous states residing in between the VB and CB of TiO2.22-24 As schematically elucidated in Figure 10, we believe these I-induced continuous states can either accept visible lightexcited electrons from the VB of TiO2 and/or provide visiblelight excited electrons from these I2-induced states to the CB of TiO2. (2) Due to the nature of our doping method, the doped I2 must be preferentially located in the surface region of the TiO2 nanoparticles. Consequently, the I-induced defective states are preferentially concentrated in the surface region instead of the bulk of the TiO2, which facilitate the trap of carriers in the surface region where the desired photodegradation reactions occur. (3) The carbon doping enhances the conductivity of TiO2, so that the photoinduced carriers can rapidly transfer to the surface region and participate in the desired photo-oxidation reactions. (4) The highly crystalline anatase phase also promotes the transfer of photoelectrons from bulk to surface, thus inhibiting charge recombination in the bulk of TiO2.4 (5) Because the photodegradation occurs at the surface of TiO2, the organic pollute molecules must be preconcentrated at the TiO2 surface in order to react with the trapped carriers and the reactive radicals. Thus, the carbon doping and the increased surface area of the meso/nanoporous TiO2 together enhance the
TABLE 2: Conductivity of the Pellets of Samples sample conductivity
P25 6.9 × 10
-9
PIT-200 s/cm
8.8 × 10
-8
s/cm
PIT-400 5.5 × 10
-4
s/cm
PIT-600 5.9 × 10
-4
s/cm
PIT-800 7.4 × 10-5 s/cm
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Figure 8. (a) Photodegradation of methylene blue solutions by using PIT, P25, and only C-doped TiO2 (T-400) as photocatalysts under visible light irradiation in neutral suspension. (b) Recycling test of PIT-400.
Figure 9. FTIR spectra of PIT-400 and pure methylene blue: (a) PIT400 before degradation, (b) PIT-400 after degradation, and (c) pure methylene blue.
Figure 10. Proposed visible light-driven photocatalytic mechanism on the I2-C-codoped TiO2 nanoparticle.
adsorption of methylene blue molecules onto the surface of TiO2 particles. This is evidenced by the more pronounced and faster deletion of methylene blue molecules in solution under darkness (Figure 8) in comparison to P25 sample. In addition, a large surface area can accommodate more surface-adsorbed water and hydroxyl groups that act as photoexcited hole traps and produce hydroxyl radicals for the degradation of organic pollute molecules.22,47,48 Thus, we believe the temperature and doping method are keys for achieving the enhanced visible light-driven photocatalytic oxidation. First, the temperature has a diametric opposing effect on the crystal structure and morphology of the TiO2.22 On one hand, low temperature help maintain the photoactive anatase phase and prevent the sintering of the nanoparticles (loss of surface area). On the other hand, higher temperature can enhance the crystallinity of the anatase TiO2, but also causes the collapse of primary pores. This effect explains the observed fact that the sample PIT-200 does not exhibit the best photocatalytic activity, despite of its relatively largest specific surface area in
comparison to other PIT-T samples (Table 1). The reason is that PIT-200 is more amorphous than other PIT-T samples as evidenced by its much broader XRD anatase peaks than the corresponding peaks for PIT-400 and PIT-600 samples (Figure 1). Vise versa, a higher temperature causes the sintering of the TiO2 nanoparticles, leading to the collapse of the pores as in the case of the PIT-800 sample. Therefore, the optimization in the temperature during heat treatment is of great importance to achieve good photoactivity. Second, the amount and location of I2 doping have a very complicated effect on the photocatalytic activity. A small amount of I2 can not create adequate continuous states for the generation of electron-hole pairs. In the case of a high amount of I2 doping, if the doped I2 is mainly distributed in the bulk of the TiO2, it can cause more lattice deformation and trap states in the bulk of TiO2. This will lower the photocatalytic oxidation. On the contrary, if the doped (I2)n is preferentially located in the surface and near surface region, the trap states can be accessed by organic pollutes to initiate the desired photo-oxidation reactions. The experimental parameters that are relevant to the amount and location of I2 doping include the nature of the precursors, the way they are doped in the TiO2, as well as the doping temperature. In this work, we add the precursors of TiO2 into the I2 hydrosol to confine the location of I2 doping preferentially in the surface region of TiO2. The following heat treatment is a necessary step for driving the (I2)n into the TiO2 matrix and for the formation of anatase TiO2. The temperature of the heat treatment must be optimized, because high temperature favors the crystallinity of TiO2, the pyrolysis of alkyls, and the entering of I2 into the surface lattice of TiO2, but it also leads to the loss of I2 due to sublimation. Therefore, the highest photocatalytic activity found in PIT-400 is an optimization of all factors discussed above. 4. Conclusions Bimodal meso/nanoporous I2-C-codoped TiO2 with high crystallinity and high surface area has been synthesized through a simple counter-doping strategy, in which organotianium precursory molecules are added into the matrix of I2 nanoparticles formed in I2-hydrosol to achieve uniform mixing at the nanoscopic level. The I2 nanoparticles serve as seeds, on which the primary TiO2 nanoparticles aggregate during heat treatment. The interstitial voids between the primary TiO2 nanoparticles construct the primary nanopores, while the voids between the jumbles of the primary nanoparticles lead to the formation of the secondary mesopores. The resulting I2-C-codoped meso/ nanoporous TiO2 shows enhanced visible light-driven photo-
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