Abstract
A yellow/brown powder of sensitized nanoporous TiO2 was obtained via an hydrolysis with TiCl4 and iodine hydrosol as raw material. I2 nanoparticles in the hydrosol were used as seeds to initiate the nucleation of a precursory TiO2 shell. The hybridized jumbles were further calcinated at different temperatures. The structure, crystallinity, morphology, and other physical-chemical properties of the samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption-desorption isotherms measurements, and UV-vis diffuse reflectance spectroscopy (DRS). The formation mechanism of these sensitized nanoporous TiO2 is discussed. Methylene blue solutions were used as model wastewater to evaluate the visible light photocatalytic activity of the samples. The results indicate that iodine can exist even in high-temperature calcination for iodine being encapsulated in the nanocavities inside TiO2. The degradation of methylene blue (MB) accorded with the first-order reaction model.
1. Introduction
Semiconductor-catalyzed photooxidation using solar light as an energy source is one of the most efficient processes for a rapid and low-cost degradation of organic pollutants. The excellent chemical stability, nontoxicity, and photoactivity of TiO2 has attracted much attention for its potential use as a solid photocatalyst for environmental purification [1β3]. However, because of its large bandgap of 3.20βeV, only a small UV fraction of solar light, about 2-3%, can be utilized to generate electron-hole pairs. Hence, it is crucial to enhance the light harvesting of TiO2 in the visible region which accounts for more than 43% of the total solar energy [4, 5].
In general, two methods are utilized to enhance the light harvest of TiO2 in the visible region. One method is to sensitize TiO2 with color centers that have proper energy levels for photoelectron transfer between the color centers and TiO2. A cost-effective sensitization of TiO2 is typically achieved by adding nonmetals such as I2 [6β8] and S [9, 10]. The other way is to form a doping level through intercalation of proper atoms into anatase form. For example, by doping carbon [11, 12] and nitrogen [13β15] atoms into the TiO2 lattice, a new, weak absorption is generated at lower energies and promising approach for inducing effective Vis-absorption.
In addition to the narrow bandgap, the enhancement of surface area of the TiO2 is another important aspect for the efficiency of the materials because the desired photocatalytic reactions occur at the interfaces of the catalyst-pollute solution [16, 17]. Mesoporous TiO2 with high specific surface area can provide abundant active sites to adsorb reactant molecules and facilitate the accessibility of reactants to the catalysts [18, 19]. Light efficiency can also be enhanced due to the increased specific surface area and multiple scattering [20].
As a photosensitizer, element iodine not only has the absorption ability in the visible light region but also has low toxicity and can be dissolved in many solvent. Therefore, the elemental iodine can be used as a photosensitizer and prepared visible light photocatalytic materials. Titanium(IV) alkoxides have been frequently studied as titanium precursors, but their high cost and the need to handle organic solvents are important drawbacks. Inorganic precursors are preferable, and TiO2 powders have been often prepared by hydrolysis of TiCl4, generally in aqueous acidic solutions [21] and in pure water [22, 23].
Based on the above state-of-the-art designing strategies for TiO2-based photocatalysts, this paper reports the synthesis of high surface area sensitized nanoporous TiO2. The I2 sensitized is confined preferentially in the surface/near surface region of TiO2 structure in order to accommodate hot carriers in the surface region. The I2 nanoparticles in the I2-hydrosol serve as seeds for nucleation of small TiO2 particles formed from the hydrolysis of the TiCl4. This template-free soft synthetic strategy allows the doping and pore formation to be accomplished simultaneously during calcination, while the amount and position of doping elements can be readily controlled under mild conditions.
2. Experimental
2.1. Materials and Sample Preparation
All chemicals used in this study were reagent grade (Tianjin Ruijinte Chemical Reagent Corp.) without further purification. Water used was deionized and doubly distilled. The sensitized porous TiO2 samples were synthesized via the hydrolysis of TiCl4 in iodine hydrosol without using any templates or surfactants, followed by calcination at elevated temperatures.
A typical procedure for preparing the samples is described as follows. First, 0.80βg iodine powders were dissolved in 40βmL absolute ethanol and slowly added drop-wisely into 100βmL of distilled water at 60Β°C with continuous stirring. The I2 hydrosol was formed after stirring for 0.5βh and then cooled to room temperature. 8βmL TiCl4 was carefully added into 40βmL-deionized water with gentle stirring in ice-water bath to avoid a drastic hydrolysis of TiCl4 in water (designated solution A). Solution A was then added dropwise with vigorous stirring into 100βmL of I2 hydrosol at room temperature, then the mixture solution was heated to 40Β°C and keet for 2βh under closed conditions. After that, a 20% (w/w) solution of ammonia was added dropwise with vigorous stirring until pH = 7, then the mixed solution was cooled down to room temperature and aged further to precipitate growth. Finally, the precipitates were centrifuged, washed with distilled water three times, and dried in an oven at 50Β°C. The precipitates obtained were calcined at different temperatures in air for 3βh. The samples after heat treatment were denoted as IT-T, where the second T indicates the calcination temperatures (in Β°C). In addition, commercial P25 (~80% anatase and ~20% rutile, Deguass) was used as a reference.
2.2. Characterization
The phases of the final products were identified using X-ray diffractometer (XRD) (Rigaku D/max-2500VPC) with a Ni-filtered Cu KΞ± radiation at a scanning rate of 0.02Β°βsβ1 from 20Β° to 70Β°. The morphology of the samples was observed with transmission electron microscope (TEM, Hitachi model H800) using an accelerating voltage of 100βkV. Ultraviolet-visible (UV-vis) diffuse reflection spectra (DRS) were recorded in the range 200β800βnm at room temperature (Shimadzu UV-2550 UV-vis spectrophotometer). Porous structure and BET surface area were characterized by an N2 adsorption-desorption isotherm (ASAP-2020 Micromeritics Co.). Note that the samples were degassed at 180Β°C prior to the BET measurements. The pore volume and pore diameter distribution were derived from desorption branches of the isotherms by the Barrett-Joyner-Halenda (BJH) model. The BET surface area was calculated from the linear part of the Brunauer-Emmett-Teller (BET) plot.
2.3. Adsorption and Photocatalytic Activity Measurements
The adsorption capability and visible light photocatalytic activity of the sensitized porous TiO2 were evaluated by measuring the adsorption and decomposition rate of methylene blue (MB) solution at room temperature. For comparison, the same measurements were also performed on the commercial P25. For a typical absorption and photocatalytic experiment, a total of a 200βmg sample was added to 150βmL MB aqueous solution (5.0 Γ 10β4βmol/L) in a custom-made quartz reactor. The MB concentration was monitored by the UV-vis spectroscopy during the entire experiment. The solution was irradiated by visible light supplied by a 300βW tungsten-arc lamp (Zhejiang Electric Co. Ltd.) with a glass filter (transparent for βnm). After 5 minute intervals during adsorption and visible light illumination, about 3βmL aliquots were taken out and centrifuged to remove the trace particles. The absorbance of the centrifuged solution was measured in the range 500β800βnm using a UV-vis spectrophotometer (Shimadzu UV-2550).
3. Results and Discussions
3.1. Sample Structure
The porous nanostructures were prepared via the hydrolysis of TiCl4 in I2 hydrosol, followed by calcinated in air at elevated temperatures for 3βh. Powder X-ray diffraction (PXRD) is used to investigate the changes of structure and crystallite sizes of the prepared porous samples at different calcination temperatures. Figure 1 shows the XRD patterns of the samples after being calcinated at various temperatures from 200Β°C to 600Β°C. The XRD pattern for the sample obtained at 200Β°C indicated the sample predominantly amorphous titania besides trace amounts of anatase. The diffraction peaks at 2ΞΈ = 31.6Β°, 33.5Β° and 36.0Β° should be the iodine (JCPDS no. 71-1370). As the calcination temperatures increase, the XRD results show the pure anatase TiO2 phase (JCPDS file no. 21-172), indicating that the crystallization of the samples is achieved. In addition, as the calcination temperature increases, the peaks assigned to anatase become sharper and more intense due to the formation of larger grains as summarized in Table 1. The average crystallite sizes of sensitized nanoporous TiO2 are 4.1, 7.2, 9.8, and 13.2βnm for IT-300, IT-400, IT-500, and IT-600, respectively, which are estimated from the full width at half maximum of the diffraction peak using the Scherrer equation.
Figure 2 shows the TEM images of IT-T samples before and after being calcinated at different temperatures. The average particle size of the precursor is estimated to be 21βnm, as presented in Figure 2(a). No porous structure was observed at this point. The wormhole-like irregular mesopores for sensitized porous TiO2 can be observed in Figures 2(c)β2(f). The sample obtained at 200Β°C shows the noncrystalline nature of amorphous (Figure 2(b)), but there were porous structure. The amorphous structure is consistent with the XRD result, and the porous structure is caused by the I2 and H2O gas escape at heat treatment process. It is the subsequent heat treatment that creates the mesoporous structures. The apparent common feature of the four samples after heat treatment at 300Β°C, 400Β°C, 500Β°C and 600Β°C is that they all consist of pseudospherical TiO2 particles. Furthermore, each individual sphere is composed of a large number of much smaller and loosely packed TiO2 nanoparticles (about 4~10 nanometers in diameter). As a result, the interstitial voids among these primary nanoparticles constitute a short-range disordered nanoporous structure. We also notice that the size of the primary nanoparticles grows with the increasing calcination temperature.
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Pore size and specific surface area of the as-prepared samples were characterized by nitrogen adsorption-desorption isotherm measurements. Figure 3 shows the results obtained from N2 adsorption-desorption isotherm studies and analyzed by BET method for surface area and BJH pore size distribution. The surface area and porosity have been measured for all of the IT-T samples, and the results are given in Table 1. Calcination treatment showed a significant influence on the isotherm behavior of the samples. The VI-type isotherms for sample IT-200 and IV like Isotherm for samples IT-300, IT-400, IT-500, and IT-600 shown in Figure 3(a) demonstrate typical characteristics of mesoporous structure in the materials [24]. From the isotherms, it can be seen that the monolayer adsorption is complete when the relative pressure reaches 0.7 for sample IT-600. However, the monolayer adsorption for other samples is not complete until reaching a relative pressure of 0.8 for sample IT-300 and 0.78 for IT-400. This indicates that sample IT-600 possesses much smaller pore size than other samples. Although mesopores contribution could be observed in Figure 3(b) for IT-600, overall pore volume measured is 0.148βcm3gβ1, indicating that the contribution from mesoporous is small. Less mesoporosity in IT-600 may be attributed to the large size of the particles. This result agrees with the XRD and TEM results.
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Table 1 summarizes the BET-specific surface area and the apparent average pore size of our samples. The pore volume and specific surface area decrease with increasing calcination temperature. However, the samples still maintain relatively high specific surface area and pore size even for sample IT-600, which is calcined at 600Β°C. Consequently, this may enhance catalytic activity, as demonstrated later in the measurement of photocatalytic activity on degradation of methylene blue.
3.2. Possible Formation Mechanism of Sensitized Nanoporous TiO2
Different from the conventional doping method, in which the iodine (or its precursors) is added to the matrix 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 4, is proposed to interpret the formation of the sensitized nanoporous TiO2. When TiCl4 solution was added to I2 hydrosol, the heat of the exothermic reaction explosively generated the formation of orthotitanic acid (Ti(OH)4) to become a titania precursory layer on the I2 nanoparticles [23]. These primary TiO2 precursors formed as a shell on the surface of I2 particles. In other words, the I2 particles appear as nucleation sites for TiO2 spheres via the hydrolysis of TiCl4. At room temperature, the hydrolysis of TiCl4 is relatively slow. When the starting solution was heated in an isothermal water bath of 40Β°C for 2βh and add ammonia solution, it became supersaturated and precipitated. After drying the precursor turn into -H2TiO3. The subsequent calcinated of the precipitated at elevated temperatures leading to a series of reactions among I2 and TiO2 precursors with the presence of O2. We believe that the -sensitized nanoporous TiO2 is a product of the following synergetic processes, including (1) the I2 sublimes and the thermal decomposition H2TiO3 resulting gaseous products pulverize the TiO2 shell, leading to the formation of the primary pores. (2) Part of the I2 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 [7]. (3) The amorphous TiO2 crystallizes into anatase TiO2.
3.3. UV-Vis Diffuse Reflectance Spectra
Figure 5 collects the UV-Vis diffuse reflectance spectra of the as-synthesized samples, the P25, as well as the I2 hydrosol. In comparison to the pure TiO2, the spectra of sensitized nanoporous TiO2 samples exhibit a strong broad absorption band between 400 and 750βnm, covering nearly the entire visible range. This band is centered around 500βnm, which is the spectral range where iodine absorbs [7]. Apparently, the absorbance between 400 and 700βnm decreases for sample prepared at higher temperatures. This can be ascribed to the additional loss of I2 by sublimation at higher temperatures. But the sample which is obtained at 600Β°C still has a strong absorption in the visible region. Nonetheless, the absorption in the visible-light region implies that the prepared sensitized nanoporous TiO2 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. We believe that, with the increase of the temperature during heat treatment, the concentration of iodine molecules on the surface of the IT 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 puce, to brown, slight brown and fawn, and finally to light gray when the temperature increased from 200 to 600Β°C.
3.4. Photocatalytic Activity on the Decomposition of Methylene Blue
Methylene blue (MB) is a brightly colored blue cationic thiazine dye and is often used as a test model pollutant in semiconductor photocatalysis. A blank experiment was also carried out. The pure MB solution cannot decompose without catalyst. Figure 6(a) shows the adsorption and photodegradation ability on MB solutions of our IT-T samples in comparison to P25. Prior to the turn-on of the light, the concentrations of MB in the presence of the IT-T samples deplete much faster than P25. This is especially seen for IT-200 sample; nearly 70% of the dye molecules were adsorbed within 30βmin. We ascribe this effect to the much larger surface area of the IT-T samples than that of P25 as characterized and discussed above. For IT-T samples, their adsorption capacity increases with the decrease of the calcination temperatures. This can be credited to the reduced surface area.
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Furthermore, Figure 6(a) shows, under visible light illumination, the MB solutions containing the IT-T samples underwent significant degradation and became nearly transparent within 50βmin. In contrast, the MB solutions containing P25 show very limited degradation. It should be noted that IT-400 sample showed the highest photocatalytic oxidation on methylene blue. This is because the crystalline of IT-400 is well and has fewer defects, so the recombination of electron and hole is very low. In addition, the sample still has larger surface area, so IT-400 has good absorption capacity.
The photocatalytic activity of the powders can be quantitatively evaluated by comparing the apparent reaction rate constants. It is well known that photocatalytic oxidation of organic pollutants in aqueous suspension follows the Langmuir-Hinshelwood model [25], and it can be described as follows [26]: where is the concentration of MB aqueous at reaction time , is the initial MB concentration, is apparent rate constant, and is reaction time. The variations in as a function of irradiation time are given in Figure 6(b). Since all the curves can be fitted roughly to a straight line, the photocatalytic degradation reaction can be assumed to follow first-order kinetics [27]. The rate constant for the photodegradation of the MB using IT-400 is 21 times higher than that of P25. All together, we can conclude that IT-T samples exhibited a strong photocatalytic activity for decomposition of MB under visible light irradiation.
The cyclic stability test under the same conditions of IT-400 sample was also examined and shows excellent photocatalytic activity in degrading MB solution, as shown in Figure 7. No significant decrease in its photocatalytic oxidation activity is observed after being used 6 times, which makes it very promising for practical application.
3.5. Proposed Mechanism for the Enhanced Photocatalytic Activity
The enhanced photocatalytic oxidation activity in IT-T samples is a product of several factors. (1) The sensitized of introduces continuous states residing in between the VB and CB of TiO2 [28]. As schematically elucidated in Figure 8, we believe these I-induced continuous states can either accept visible light-excited electrons from the VB of TiO2 and/or provide visible-light excited electrons from these I2-induced states to the CB of TiO2. (2) Due to the nature of our preparing method, the sensitized 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 hot carrier in the surface region where the desired photodegradation reactions occur [6, 29]. (3) Because the photo-degradation occurs at the surface of TiO2, the organic pollute molecules must be preconcentrated at the TiO2 surface in order to react with the trap hot carriers and the reactive radicals. Thus, the high surface areas of the nanoporous TiO2 enhance the adsorption of methylene blue molecules onto the surface of TiO2 particles. This is evidenced by the pronounced more and faster deletion of methylene blue molecules in solution under darkness (Figure 6(a)) in comparison to P25 sample. In addition, 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 [30].
4. Conclusions
In summary, a unique template-free synthetic strategy is used to prepare sensitized nanoporous TiO2 through a hydrolysis-calcination process. I2 hydrosol is used as nucleation center and sensitizer, while the TiCl4 was used as the precursor for TiO2. The calcination temperature has a crucial role in the amount of sensitized nanoporous TiO2. The calcination enhanced the phase transformation of the TiO2 powders form amorphous to anatase phases and crystallization of anatase. The photocatalytic activity of the as-prepared samples was higher than that of P25 for the degradation of MB. The sample calcined at 400Β°C shows the highest photocatalytic activity in the decomposition of methylene blue under visible light due to the enhanced absorption in visible region and the large specific surface area.
Acknowledgments
S. Gao and B. Huang thank the National Basic Research Program of China (973 Program, no. 2007CB613302), Doctoral Foundation of Shandong Province (2007BS04040), and Project of Shandong Province Higher Educational Science and Technology Program (J10LF02).