Photocatalytic Activity of Sol−Gel-Derived Nanocrystalline Titania

Under this condition, the MB dye concentration (C0) remained unchanged even after ... Figure 1 Typical SEM images of the sol−gel-derived nanocrystal...
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J. Phys. Chem. C 2007, 111, 7612-7622

Photocatalytic Activity of Sol-Gel-Derived Nanocrystalline Titania K. V. Baiju, S. Shukla,* K. S. Sandhya, J. James, and K. G. K. Warrier Ceramic Technology Department, Materials and Minerals DiVision (MMD), National Institute for Interdisciplinary Science and Technology (NIST) (Formerly Regional Research Laboratory (RRL)), Council of Scientific and Industrial Research (CSIR), Industrial Estate P.O., Pappanamcode, ThiruVananthapuram, Kerala 695019, India ReceiVed: January 18, 2007; In Final Form: March 27, 2007

Nanocrystalline titania (TiO2) powders have been synthesized via sol-gel, using an alkoxide precursor, under different processing conditions, and their photocatalytic activity has been investigated as a function of processing and material parameters through the decomposition of the methylene blue (MB) dye under exposure to the ultraviolet (UV) radiation (λ ) 200-400 nm) in an aqueous solution. The nanocrystalline TiO2 powders with different morphology, crystallinity, average nanocrystallite size, surface area, and phase structure are obtained by controlling the ratio of molar concentrations of water and alkoxide (R) within the range of 5-60 and calcining the as-synthesized amorphous powders at higher temperatures (400-800 °C). The nanocrystalline TiO2 powders have been characterized using the scanning electron microscope (SEM), X-ray diffraction (XRD), and the Brunauer, Emmett, and Teller (BET) surface area measurement techniques while their photocatalytic activity was monitored using a UV-visible spectrometer. The photocatalytic activity of sol-gel-derived nanocrystalline TiO2 is observed to be a function of R and calcination temperature. The maximum photocatalytic activity is observed for the largest R value and the intermediate calcination temperature as an optimum effect produced by the variation in the morphology, the average nanocrystallite size, the surface area, the phase structure, and the crystallinity of the powders. The dependence of photocatalytic activity on the average nanocrystallite size reveals the existence of a critical size (∼15 nm), below and above which the photocatalytic activity is observed to be reduced. The observed photocatalytic characteristics of sol-gel-derived nanocrystalline TiO2 have been explained based on the existing mechanism associated with the photocatalytic decomposition of organic molecules using semiconductor oxides.

Introduction Titania (TiO2) is a well-known, wide-band-gap, n-type semiconductor oxide used as a photocatalyst for the removal of highly toxic and non-biodegradable pollutants normally present in air and wastewater via photocatalysis, which is a low temperature, non-energy intensive process for the chemical waste remediation,1 involving the migration of a photon-induced electron (e-) and hole (h+) to the particle surface, which serve as redox sites for the destruction of the surface-adsorbed pollutants. Being more photocatalytically active, chemically stable, environmentally friendly, and cheaper, TiO2 has been the most promising one for the photocatalysis compared to other semiconductors such as tin oxide (SnO2),2 zinc oxide (ZnO),3 ceria (CeO2),4-6 and cadmium sulfide (CdS).7 Nanocrystalline TiO2 has been used in the thin film,8-10 thick film,11 and powder12-24 forms for the measurement of the photocatalytic activity by using the various synthesis approaches such as chemical vapor deposition (CVD),8 sputtering,9,10 plasma spraying,11 coprecipitation,12 microemulsion,13 hydrothermal,14 and sol-gel.15-24 Among the various synthesis approaches, the sol-gel technique has received more attention due to the ease of controlling various material parameters such as the powder morphology, the surface area, the average nanocrystallite size, the crystallinity, and the phase structure, which significantly affect the photocatalytic activity of nanocrystalline TiO2. * To whom correspondence should be addressed. Phone: +91-4712515282. Fax: +91-471-2491712. E-mail: [email protected].

It was demonstrated earlier that25,26 the nanocrystalline ceramic oxides such as zirconia (ZrO2) could be synthesized, with different average nanocrystallite size, morphology, and phase structure, by varying the key sol-gel processing parameters such as the ratio of number of moles of water and the alkoxide precursor (R) and the calcination temperature. Hence, in the present investigation, we use the sol-gel process, which utilizes an alkoxide precursor, to prepare the nanocrystalline TiO2 powders with controlled morphology, surface area, average nanocrystallite size, crystallinity, and phase structure by varying the R and the calcination temperature. We also study their effect on the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2. Such systematic study demonstrating the effect of both R and the calcination temperature on the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 is presently not available in the open literature. Experimental Section Chemicals. Titanium(IV) isopropoxide (Ti[OC3H7]4) and anhydrous 2-propanol were purchased from Alfa Aesar, U.S.A. and methylene blue (MB) (AR Grade) was purchased from Qualigens Fine Chemicals, India. All of the chemicals were used as received without any further purification. Sol-Gel Processing. The nanocrystalline TiO2 powders were synthesized via sol-gel process using the hydrolysis and the condensation of titanium(IV) propoxide in an anhydrous alcohol medium. For this purpose, a measured quantity of water was first dissolved in 125 mL of 2-propanol. A second solution

10.1021/jp070452z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/05/2007

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was prepared in which 0.1 M titanium(IV) propoxide was dissolved completely in 125 mL of 2-propanol. Both of the solutions were sealed immediately and stirred rapidly using the magnetic stirrer to obtain the homogeneous solutions. Different solutions were prepared by varying the R within the range of 5-60. The water part of the solution was then added dropwise to the alkoxide part under the continuous stirring. As a result of hydrolysis of titanium(IV) propoxide due to the reaction with water, change in the color of the solution from colorless to white was visible. The time required for the observable color change was, however, different depending on the processing conditions. After the complete addition of the water part of the solution to that of the alkoxide part, the resulting solution was stirred overnight before drying in the furnace at 80 °C for the complete removal of the solvent and the residual water. The dried powders were then calcined at 400, 600, and 800 °C for 2 h for the crystallization of amorphous TiO2 powders. Characterization. The morphology and the particle size distribution of the calcined nanocrystalline TiO2 powders were studied using the scanning electron microscope (SEM) (JEOL JSM-5600LV, Japan) operated at 15 kV. The crystalline phase, evolved after the calcination of the amorphous TiO2 powders, was determined using the X-ray diffraction (XRD) (Rigaku, Japan). The broad-scan analysis was typically conducted within the 2-θ range of 10-80° using the Cu KR (λ ) 1.542 Å) radiation. The narrow scan analysis was conducted within the 2-θ range of 20-30° as it contained the strongest lines for the anatase ((101)A) and the rutile ((110)R) phases of TiO2 and was subsequently used to determine the average TiO2 nanocrystallite size using Scherrer’s equation

DXRD )

0.9λ β cos θB

(1)

where DXRD is the average nanocrystallite size (nm), λ the X-ray wavelength (Cu KR, 1.542 Å), β the full-width at half-maximum intensity (in radian), and θB the half of diffraction peak angle (2θB). The surface area and the average nanoparticle size of the nanocrystalline TiO2 powders, processed under different R values and calcined at different temperatures, were measured using the Brunauer, Emmett, and Teller (BET) surface area measurement technique (Micrometrics Gemini 2375 surface area analyzer) via nitrogen (N2) adsorption, using a single-point method, after degassing the nanocrystalline TiO2 powders in flowing N2 at 200 °C for 2 h. The average nanoparticle size was determined using the measured BET surface area via the following equation

6000 DBET ) FS

(2)

where DBET is the average nanoparticle size (nm), F the powder density (g/cm3), and S the specific surface area (m2/g) measured via BET method. (Note: The surface area, determined using the multipoint method, was comparable with the one determined using the single-point method). Photocatalytic Activity Measurement. The photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders was studied by monitoring the degradation of the MB dye in an aqueous solution containing the nanocrystalline TiO2 powders, under continuous stirring and exposure to the ultraviolet (UV) radiation. An amount 75 mL of aqueous solution was prepared by completely dissolving 0.0064 µmol/L of MB dye and then dispersing 0.4 g/L of sol-gel-derived nanocrystalline

TiO2 powders in the deionized water. The resulting suspension was equilibrated by stirring in the dark (without exposure to the UV radiation) for 1 h to stabilize the adsorption of the MB dye over the surface of the nanocrystalline TiO2 powders. The stable aqueous suspensions was irradiated with the UV light, under the continuous magnetic stirring, using the Rayonet photoreactor (The Netherlands) containing 15 W tubes (Philips G15 T8) as the UV source, which emitted the UV radiation with the wavelength within the range of 200-400 nm (corresponding to the photon energy range of 3.07-6.14 eV). Following the UV radiation exposure, 3 mL of aqueous suspension was taken out of the UV chamber for each 10 min time interval for a total of 1 h of UV radiation exposure for obtaining the absorption spectra. The nanocrystalline TiO2 powder was then filtered out from the sample suspension using a centrifuge (R23, Remi Instruments India Ltd.), and the filtered solution was then examined using a UV-visible spectrometer (Shimadzu, Japan, UV-2401 PC) to study the degradation of the MB dye in the aqueous solution under the UV radiation exposure in the presence of the sol-gel-derived nanocrystalline TiO2 powders, which act as a photocatalyst. The absorption spectra of the MB dye solution were obtained within the range of 200-800 nm as a function of UV radiation exposure time for the nanocrystalline TiO2 powders processed under different conditions. The intensity of absorbance peak (A) of the MB dye solution, located at 656 nm, was taken as a measure of the residual concentration of MB dye (C). The UV-visible absorbance spectrum of the solution without the addition of nanocrystalline TiO2 powders, prior to the UV radiation exposure, was also recorded as a reference spectrum corresponding to the initial MB dye concentration (C0). The normalized residual concentration of MB dye was then obtained using the relationship of the form

() ( ) C C0

)

MB

Atime)t Atime)0

(3)

656nm

A photocatalysis experiment, without any addition of TiO2 photocatalyst, was also performed with otherwise exactly the same experimental conditions to confirm the stability of the MB dye under the UV radiation exposure in the absence of photocatalyst particles. Under this condition, the MB dye concentration (C0) remained unchanged even after irradiating the sample for a total of 1.5 h. Results Powder Morphology and Average Aggregate Size. Typical SEM micrographs of the sol-gel-derived nanocrystalline TiO2 powders, calcined at 400 °C, are presented as a function of R in Figure 1. At the lowest R value, the TiO2 powder appears to form large aggregates of ∼5 µm average size having nearspherical shape. These aggregates are further seen to be made up of spherical particles of size ∼2 µm, as marked by the arrows in Figure 1a. Relatively smaller submicron-sized spherical TiO2 particles, surrounding the big-sized aggregates, are also visible in the micrograph. With increasing R, Figure 1b and c, the average aggregate size is qualitatively observed to decrease (∼3 µm). Moreover, the small spherical particles (∼1-2 µm) forming the larger near-spherical aggregates are more clearly revealed under these processing conditions. With the highest R value investigated here, Figure 1d, the average aggregate size and the average particle size forming the aggregates are noted to decrease further to ∼1-2 µm and ∼500-700 nm, respec-

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Figure 1. Typical SEM images of the sol-gel-derived nanocrystalline TiO2 powders, in an aggregated form, having different average aggregate sizes (DSEM) depending on the processing conditions, (a) R ) 5, (b) R ) 15, (c) R ) 30, and (d) R ) 60. All powders are calcined at 400 °C for 2 h.

Figure 3. Variation in the improvement in the crystallinity (defined here as the ratio (I600/I400)A, where I400 and I600 are the linear intensities of the main anatase peak (101)A in the XRD spectra presented in Figure 2 obtained for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C, respectively) as a function of R. Figure 2. Typical broad-scan XRD patterns within the 2-θ range of 10-80°, obtained for the sol-gel-derived nanocrystalline TiO2 powders processed under the conditions, (a) R ) 5 and (b) R ) 60. The powders are calcined at different temperatures, (i) 400, (ii) 600, and (iii) 800 °C. A is anatase, and R is rutile.

tively. Over all, both the average size of the aggregates and the particles forming the aggregates tend to decrease with increasing R within the investigated range. Phase Evolution Behavior and Average Nanocrystallite Size. Typical XRD broad-scan analyses of the sol-gel-derived TiO2 powders, synthesized under different R values, are presented in Figure 2a and b for different calcination temperatures. The TiO2 powders in the as-synthesized condition are amorphous for all R values investigated here; however, they crystallize into the anatase phase after the calcination at 400 °C, as identified by the comparison of the obtained XRD spectra with the JCPDS data file # 21-1272. For both R values, the main anatase peak (101)A appears to be less intense and relatively broader after the calcination at 400 °C. After the calcination at 600 °C, the anatase-to-rutile phase transformation is not observed in Figure 2. However, the intensity of the main anatase peak increases, and it becomes narrower after the calcination at 600 °C. The increase in the

intensity of the main anatase peak reveals an improvement in the relative crystallinity (defined here as the ratio (I600/I400)A, where I400 and I600 are the linear intensities of the main anatase peak (101)A after the calcination at 400 and 600 °C, respectively) of the sol-gel-derived nanocrystalline TiO2 powders after the calcination at 600 °C. The variation in the relative crystallinity as a function of R, for the sol-gel-derived nanocrystalline TiO2, is shown in Figure 3. It is noted that, although the relative crystallinity is improved for the entire range of R investigated here, (that is, (I600/I400)A greater than unity), it decreases with increasing R. By further increasing the calcination temperature to 800 °C, abrupt and complete anatase-to-rutile phase transformation has been observed, Figure 2, as confirmed by comparing the obtained XRD spectra with the JCPDS data file # 211276. No trace of the anatase phase could be detected after the calcination at 800 °C. The variation in the average nanocrystallite size (DXRD) as a function of R, as determined using Scherrer’s equation, eq 1, is presented in Figure 4 for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C. After the calcination at 400 °C, the average nanocrystallite size is observed to increase with R. Although the average nanocrystallite size is noted to be larger for the calcination temperature of 600 °C relative to

Sol-Gel-Derived Nanocrystalline TiO2

Figure 4. Variation in the average nanocrystallite size (DXRD), determined using eq 1, as a function of R for the sol-gel-derived nanocrystalline TiO2 powders calcined at different temperatures, (i) 400 and (ii) 600 °C.

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Figure 6. Variation in the average nanoparticle size (DBET), determined using eq 2, as a function of R obtained for the sol-gel-derived nanocrystalline TiO2 powders calcined at different temperatures, (i) 400 and (ii) 600 °C.

Figure 5. Variation in the BET surface area (SBET) as a function of R obtained for the sol-gel-derived nanocrystalline TiO2 powders calcined at different temperatures, (i) 400 and (ii) 600 °C.

that for the calcination temperature of 400 °C, it tends to decrease with increasing R within the investigated range at higher calcination temperatures. Thus, the nature of the variation in the average nanocrystallite size as a function of R reverses with increasing calcination temperature from 400 to 600 °C. In other words, the amount of increase in the average nanocrystallite size is observed to be relatively larger for lower R values. Surface Area and Average Nanoparticle Size. The variation in the measured BET surface area as a function of R for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C is presented in Figure 5. For all R values, the BET surface area after the calcination at 400 °C appears to be larger than that after the calcination at 600 °C. The surface area is noted to decrease with increasing R after the calcination at 400 °C; however, the trend is reversed after the calcination at 600 °C. As a result, the difference in the surface area is observed to decrease with increasing R. The variation in the average nanoparticle size (calculated using eq 2) as a function of R obtained for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C, is presented in Figure 6. The average nanoparticle size is seen to be larger at a higher calcination temperature. After the calcination at 400 °C, the average nanoparticle size is observed to increase marginally with increasing R; however, the trend is reversed after the calcination at higher temperature. Comparison of Figures 4 and 6 shows that, at lower calcination temperature, the average nanoparticle size calculated using the BET surface area is comparable with the average nanocrystallite size determined using Scherrer’s equation. In Figure 6, similar to the observation made in Figure 4, the amount of increase in the average nanoparticle size is noted to be much larger for lower R values relative to that observed for larger R values. Photocatalytic Activity. Typical UV-visible absorbance spectra of the MB dye solution, obtained by irradiating the

Figure 7. Typical UV-visible absorbance spectra obtained for the MB dye aqueous solution irradiated with the UV radiation (λ ) 200400 nm) for different time intervals. The spectra are obtained for solgel-derived nanocrystalline TiO2 powders, processed under different conditions, (a) R ) 5, T ) 600 °C and (b) R ) 60, T ) 600 °C; (i) 0, (ii) 10, (iii) 20, (iv) 30, (v) 40, (vi) 50, and (vii) 60 min.

sample with the UV-visible radiation having the wavelength within the range of 200-800 nm, are shown in Figure 7. With increasing irradiation time within the range of 0-60 min, the intensity of the main absorbance peak at 656 nm is noted to decrease continuously, suggesting the degradation of the MB dye. The degradation rate is qualitatively observed to be larger for higher R value. The variation in the normalized MB dye concentration as a function of irradiation time obtained for the nanocrystalline TiO2 powders, synthesized under different processing conditions, is presented in Figure 8a-c for three different calcination temperatures. The phase evolution behavior as observed in Figure 2 suggests that the photocatalytic behavior of the sol-gelderived nanocrystalline TiO2 powder presented in Figure 8a and b corresponds to the anatase TiO2, while that shown in Figure

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Baiju et al. 800 °C is inferior to that of those calcined at 400 and 600 °C. Interestingly, the difference in the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C tends to reduce with increasing R. Discussion Sol-Gel Processing of Nanocrystalline TiO2 Powders. Nanocrystalline TiO2 powders have been synthesized, in the present investigation, via sol-gel process by reacting the alkoxide precursor with a controlled amount of water in an anhydrous alcohol medium. The hydrolysis and the condensation reactions, which are responsible for the formation of TiO2 particles, can be summarized as26

Hydrolysis: Ti(OC3H7)4 + 4H2O f Ti(OH)4 + 4C3H7OH (4) Condensation: Ti(OH)4 f TiO2 + 2H2O

(5)

Net Reaction: Ti(OC3H7)4 + 2H2O f TiO2 + 4C3H7OH (6)

Figure 8. Variation in the residual MB dye concentration (obtained using eq 3) as a function of UV radiation exposure time obtained for the sol-gel-derived nanocrystalline TiO2 powders calcined at different temperatures, (a) 400, (b) 600, and (c) 800 °C. The powders were processed under different R values, (i) R ) 5, (ii) R ) 15, (iii) R ) 30, and (iv) R ) 60.

8c is related to the rutile TiO2. It is observed that the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 increases with increasing R within the investigated range of 5-60, and this behavior is independent of the nature of the phase involved. However, comparison of Figure 8a and b with Figure 8c shows that the photocatalytic activity of the anatase TiO2 is much better relative to that of the rutile TiO2. The variation in the normalized MB dye concentration as a function of irradiation time for the sol-gel-derived nanocrystalline TiO2 powders calcined at different temperatures is presented in Figure 9a-d for the various R values within the investigated range of 5-60. For all R values, the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders calcined at 600 °C is observed to be better than that of the powders calcined at 400 °C. Moreover, the photocatalytic activity of the nanocrystalline TiO2 powders calcined at

Nanocrystalline TiO2 powders have been synthesized with different R values within the range of 5-60 and then calcined at different temperatures after drying at 80 °C. The assynthesized and calcined powders are observed to be in an aggregated form having different average aggregate size, average nanoparticle size, and average nanocrystallite size depending on the processing conditions. (Note: The definitions of latter three parameters have been provided in Figure 10, which take into account the SEM and the atomic force microscope (AFM) observations made in this investigation and as reported by others27.) From the net reaction presented in eq 6, it appears that the nucleation rate of TiO2 particles increases with increasing R, and as a result, the average nanocrystallite size and the average nanoparticle size should decrease with increasing R. In addition to this, with increasing R, although the hydrolysis reaction, eq 4, is driven in the forward direction, the condensation reaction, eq 5, is driven in the reverse direction, which suggests more dissolution of TiO2 particles, which may effectively reduce the average size of nucleated TiO2 particles within the sol with increasing R. However, due to the formation of a large number of nuclei with increasing R, the growth rate of the TiO2 nanocrystallites is also possibly enhanced for higher R values during the calcination at 400 °C. This is reflected in an increase in both the average nanocrystallite size and the average nanoparticle size with increasing R after the calcination at 400 °C. As discussed later, the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 °C is seen to be greatly effected by this variation in the average nanocrystallite size as a function of R. Interestingly, the nature of the variation in the average nanocrystallite size and the average nanoparticle size as a function R is reversed after the calcination at 600 °C. Both of the parameters tend to decrease with increasing R under this processing condition. It appears that the variation in the average aggregate size (DSEM) as a function of R, as qualitatively observed in Figure 1, is responsible for this reverse trend. We note that, at lower R values, the average aggregate size is much larger, which effectively increases the coordination number (defined here as the number of nearest neighbors surrounding a nanoparticle within the aggregate) of the nanoparticles within the aggregates. This is highly conducive in enhancing the diffusion kinetics in the large-sized aggregates. As a result, the growth rate is possibly much higher in the large-sized ag-

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Figure 9. Variation in the residual MB dye concentration (obtained using eq 3) as a function of UV radiation exposure time obtained for the sol-gel-derived nanocrystalline TiO2 powders processed under different conditions, (a) R ) 5, (b) R ) 15, (c) R ) 30, and (c) R ) 60. The powders were calcined at different temperatures, (i) 400, (ii) 600, and (iii) 800 °C.

Figure 10. Schematic representation of the average nanocrystallite size, average nanoparticle size, and average aggregate size, as defined in this investigation, which correspond to DXRD, DBET, and DSEM, respectively.

gregates, which leads to larger average nanocrystallite size and average nanoparticle size for lower R values after the calcination at 600 °C. Hence, in Figures 4 and 6, the nature of the variation in the average nanocrystallite size and the average nanoparticle size as a function R is reversed after increasing the calcination temperature from 400 to 600 °C. A relatively larger increase in the average nanocrystallite size and the average nanoparticle size, as observed for lower R values, relative to that observed for higher R values is, hence, attributed to a larger average aggregate size (DSEM) at lower R values. As a consequence, the BET surface area of the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 °C is observed to be much larger than those calcined at 600 °C, and the reverse trend in the nature of its variation as a function of R for these two calcination temperatures, Figure 5, is hence justifiable. It is further noted that the crystallinity of the sol-gel-derived nanocrystalline TiO2 powders increases with increasing calcina-

tion temperature from 400 to 600 °C. This has been associated with better diffusion kinetics at a higher calcination temperature, which in turn leads to the removal of crystal defects, the residual amorphous-to-anatase phase transformation, the narrowing of the main anatase peak, an increase in the average nanocrystallite size and the average nanoparticle size, and an improvement in the band structure.11,17,18,28,29 In the present investigation, the diffusion kinetics appears to degrade with increasing R possibly due to the decrease in the average coordination number of the nanoparticles as a result of a decrease in the average aggregate size (DSEM) with increasing R. As a consequence, the relative crystallinity of the sol-gel-derived nanocrystalline TiO2 powders is seen to decrease with increasing R, Figure 3, which has a strong influence in determining the nature of the variation in the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders as a function of R, as discussed later. Metastable Anatase Phase Stability at Room Temperature in Sol-Gel-Derived Nanocrystalline TiO2. For bulk TiO2, the rutile phase is more stable than the anatase phase at room temperature. However, it has been observed that, with the reduction in the nanocrystallite size, the rutile phase gets transformed to the metastable anatase phase when the nanocrystallite reduces below a critical size.16,30-34 On the basis of the thermodynamic considerations, the critical size (D′AfR) for the stabilization of the metastable anatase phase in a single, isolated TiO2 nanocrystallite is given by the relationship of the form30

D′AfR )

(2t′ + 3)M (∆G0f )AfR

×

(

)

γR γ A FR FA

(7)

7618 J. Phys. Chem. C, Vol. 111, No. 21, 2007 wher, M is the molecular weight of TiO2 (80 g/mol), (∆G0f )AfR is the change in the volume free energy associated with the anatase-to-rutile phase transformation for the bulk TiO2 at room temperature (∼6 kJ/mol), t is the proportionality constant between the surface stress and the surface free energy for the bulk TiO2 (∼3.5), γR and γA are the surface free energies of the rutile (1.91 J/m2) and the anatase (1.32 J/m2) phases, respectively, and FR and FA are the densities of the rutile (4.26 g/cm3) and the anatase (3.84 g/cm3) phases, respectively. Substituting these values into eq 7, the critical size for the anatase-to-rutile phase transformation, at room temperature, is calculated to be ∼14 nm. It appears that, in the present investigation, the metastable anatase phase has been stabilized at room temperature within the sol-gel-derived nanocrystalline TiO2 powders after the calcination at 400 and 600 °C. The average nanocrystallite size, after calcination at these temperatures, varies within the ranges of 8.5-12.5 and 18-22 nm, respectively. The average nanocrystallite size range, after the calcination at 400 °C, appears to be below the critical size of ∼14 nm. Hence, in this case, the stabilization of the metastable anatase phase within the solgel-derived nanocrystalline TiO2 is in accordance with the thermodynamic considerations. However, the average nanocrystallite size range for the sol-gel-derived nanocrystalline TiO2 powders calcined at 600 °C is above the critical size of ∼14 nm. The metastable anatase phase stabilization, at room temperature, within the TiO2 nanocrystallites of size greater than 14 nm has also been reported by others.16,33,34 It is to be noted that the critical size of ∼14 nm is calculated only for a single, isolated TiO2 nanocrystallite. In practice, the nanocrystallites form aggregates which modify their interfacial energy values than those considered in eq 7. As reported for other system such as ZrO2, the change in the interfacial energy values may increase the critical size for the metastable phase stabilization.25,35 Moreover, the anatase-to-rutile phase transformation is known to be accompanied by 10% decrease in the volume. If we assume that the initial anatase-anatase interface boundaries within the aggregates are the coherent boundaries, then considerable restraint would be imposed to the anatase-to-rutile phase transformation process, which may also lead to an increase in the critical size for the phase transformation.36 In addition to this, since the anatase-to-rutile phase transformation process is initiated via the interface nucleation process,37,38 the interface structure also plays an important role in determining the critical size for the phase transformation.16 Hence, the room-temperature stabilization of the metastable anatase phase within the TiO2 nanocrystallites of sizes greater than 14 nm is attributed here to the possible modification in the interfacial energy, the presence of strain energy, and the change in the interface structure due to the aggregation of TiO2 nanocrystallites. After the calcination at 800 °C, the anatase-to-rutile phase transformation has been noted, which is mainly due to the significant amount of growth in the average nanocrystallite size, which was, in fact, so extensive that it could not be measured using the XRD and the BET methods. The metastable anatase phase stability within the sol-gel-derived nanocrystalline TiO2 powders at lower calcination temperatures and its transformation to the more stable rutile phase at higher calcination temperatures have significant influence in determining the dependence of the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 as a function of R and calcination temperature, as discussed in the following sections. Mechanism of Photocatalytic Decomposition of MB Dye Using Nanocrystalline TiO2. Before we explain the photo-

Baiju et al.

Figure 11. (a) Schematic representation of the molecular structure of the MB dye; (b) the mechanism of the photocatalytic activity using the nanocrystalline semiconductor oxides (such as TiO2). In (b), the MB dye was considered as a model organic molecule.

catalytic activity of the sol-gel-derived nanocrystalline TiO2 powders having different morphologies, average nanocrystallite sizes, surface areas, crystallinity, and phase structures, as observed in this investigation, we briefly review the mechanism of the photocatalytic decomposition of the MB dye using the nanocrystalline TiO2.39-43 The chemical structure of the MB dye is schematically shown in Figure 11a. The MB dye has a cationic configuration in an aqueous solution, which results in its adsorption through the Coulombic interaction with the OH- ions present on the surface of TiO2 nanocrystallites.39 As described schematically in Figure 11b, when the MB dye suspension is irradiated with the UV radiation, the e-/h+ pair is created within the TiO2 nanocrystallites due to ejection of an electron from the valence band into the conduction band, leaving behind a hole in the valence band (charge-carrier generation). The generated holes may react with the surface-adsorbed OHions forming the OH• radicals. The OH• radicals may also be formed by the reaction of dissolved oxygen (O2) with the generated electrons and the protons forming the hydrogen peroxide (H2O2) as an intermediate product, which subsequently gets decomposed to the OH• radical by releasing the OH- ion into the aqueous solution. The overall reaction may be summarized as (interfacial charge transfer)39,42

TiO2 + hν f TiO2 + e- + h+

(8)

OH- + h+ f OH•

(9)

O2 + e- f O2-

(10)

O2- + H+ f HO2

(11)

2HO2 f H2O2 + O2

(12)

H2O2 + e- f OH• + OH-

(13)

The OH• radicals thus formed are mainly responsible for the degradation of the MB dye through its successive attacks via formation of several other intermediate products. The degradation of the MB dye mainly begins with the cleavage of the

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C-S+dC functional group since this group is responsible for the adsorption of the MB dye on the surface of TiO2 nanocrystallites.39 The overall reaction, which results in the decomposition of the MB dye into carbon dioxide (CO2) gas, nitrate (NO3-) ions, sulfate (SO4-) ions, protons, and water, may be summarized as43 UV,TiO2

C16H18N3S+ + 102OH• 98 16CO2 + 3NO3- + SO42- + 6H+ + 57H2O (14) The efficacy of the above mechanism in decomposing the MB dye depends on the effectiveness of the photocatalytic process in transferring the photoinduced e-/h+ pair from the particle volume to the particle surface and subsequently to the surface-adsorbed species. The generated e-/h+ pair, hence, must migrate to the particle surface as a separate entity; however, if the TiO2 nanocrystallite size is relatively larger, which increases the travel distance for the e-/h+ pair, then they may recombine within the particle volume before reaching to the particle surface (volume charge-carrier recombination)40

h+ + e- f Heat

(15)

On the other hand, if the nanocrystallite size is relatively smaller, the generated e-/h+ pair may escape to the particle surface and get trapped at the active surface sites before undergoing the volume charge-carrier recombination process (surface chargecarrier trapping)

e- + >TiIVOH f (>TiIIIOH)

(16)

h+ + >TiIVOH f (>TiIVOH•)+

(17)

where >TiOH is the hydrated surface functional group, (>TiIIIOH) is the surface-trapped conduction band electron, and (>TiIVOH•)+ is the surface-trapped valence band hole. The surface-trapped charge carriers may get transferred to the surface-adsorbed species via the following reactions (interfacial charge transfer)

(>TiIIIOH) + oxd f (>TiIVOH) + oxd•-

(18)

(>TiIVOH•)+ + red f (>TiIVOH) + red•+

(19)

where oxd and red are the surface-adsorbed oxidant and reductant species, respectively. However, if the nanocrystallite size is too small, the surface-trapped charge carriers may get annihilated by the subsequent photoinduced e-/h+ pair before the interfacial charge-transfer process takes place (surface charge-carrier recombination)

e- + (>TiIVOH•)+ f (>TiIVOH)

(20)

h+ + >TiIIIOH f (>TiIVOH)

(21)

It appears that, for the optimum photocatalytic activity, the rate of volume and surface charge-carrier recombination processes should be minimum, while that of the interfacial charge-transfer process should be maximum. As a result, the photocatalytic activity of the nanocrystalline TiO2 has been shown to be highly dependent on the nanocrystallite size.40,44 As described schematically in Figure 12, the photocatalytic activity of nanocrystalline TiO2 increases with decreasing average nanocrystallite size due to an increase in the surface area, which increases the number of active surface sites for enhancing the rate of the interfacial charge-transfer process. This

Figure 12. Schematic representation of the variation in the photocatalytic activity of nanocrystalline semiconductor oxides (such as TiO2) as a function of nanocrystallite size.

is further aided by the decrease in the rate of the volume chargecarrier recombination process with decreasing nanocrystallite size. These factors overcome the negative effect of the surface charge-carrier recombination process, the rate of which tends to increase with decreasing nanocrystallite size. As a result, the photocatalytic activity increases due to an enhanced rate of the interfacial charge-transfer process with decreasing nanocrystallite size. Nevertheless, the photocatalytic activity does not increase continuously with decreasing nanocrystallite size. A critical size (D*) is reached below which the photocatalytic activity begins to decrease.40,44 (Note: This critical size, D*, related to the photocatalytic activity, is different than that, D′, related to the room-temperature metastable anatase phase stabilization in the nanocrystalline TiO2). It appears that, below D*, although the rate of the volume charge-carrier recombination is less effective in annihilating the photoinduced e-/h+ pair, the rate of the surface charge-carrier recombination process becomes a dominant process. This reduces the rate of the interfacial charge-transfer process, and hence, the photocatalytic activity decreases below D*. The existence of the critical size of ∼10-12 nm has been experimentally demonstrated in the literature.40,44 This dependence of the photocatalytic activity on the average nanocrystallite size has also been observed in this investigation and will be discussed in the following section. Photocatalytic Activity of Sol-Gel-Derived Nanocrystalline TiO2. It is well-known that the photocatalytic decomposition of the organic molecules follows the Langmuir-Hinshelwood kinetics, which may be represented as39,45

dC ) kappC dt

(22)

where dC/dt represents the rate of change in the MB dye concentration, t the UV radiation exposure time, kapp the apparent first-order reaction rate constant, and C the concentration of the MB dye. The solution to the above integration may be obtained as

()

ln

C0 ) kappt C

(23)

where C0 is an initial MB dye concentration. Since the photocatalytic activity of the present sol-gel-derived nanocrystalline TiO2 powders may be represented by kapp,44 we determine its value under different processing conditions from the slope of the graphs as typically shown in Figure 13. The variation in kapp as a function of average nanocrystallite size has been plotted in Figure 14, while its variation as a function of R and calcination temperature is shown in Figure 15. It

7620 J. Phys. Chem. C, Vol. 111, No. 21, 2007

Figure 13. Typical plots for determining the kapp (using eq 23) for the sol-gel-derived nanocrystalline TiO2 powders processed under the conditions of R ) 30; (i) T ) 400 °C, (ii) T ) 600 °C, and (iii) T ) 800 °C.

Figure 14. Variation in kapp as a function of the average nanocrystallite size obtained for the sol-gel-derived nanocrystalline TiO2 powders. The data points below D* correspond to the powders calcined at 400 °C, while those above D* correspond to the powders calcined at 600 °C. All of the data points correspond to the anatase TiO2.

Figure 15. Variation in kapp as a function of R obtained for the solgel-derived nanocrystalline TiO2 powders calcined at different temperatures, (i) 400, (ii) 600, and (iii) 800 °C.

appears from Figure 14 that kapp increases first with the decreasing average nanocrystallite size of the anatase TiO2. From the fitted curve, it is deduced that kapp reaches the maximum for the D* of ∼15 nm and then decreases with decreasing average nanocrystallite size below 15 nm. (Note: The actual data points reveal the D* of ∼12 nm, which is slightly less than that obtained from the fitted curve.) Thus, an inversion phenomenon has been clearly observed in this investigation for the variation in the kapp and, hence, in the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 as a function of average nanocrystallite size, which can be well-explained via the mechanism of the photocatalytic activity using the semiconductor oxides as discussed earlier, Figure 12. (Note: Although the crystallinity of the data points below D* is less than that of those above D*, matching the crystallinity of these

Baiju et al. two sets of data points would only shift their relative positions along the vertical axis without significantly affecting the D*.) Further, the sol-gel-derived nanocrystalline TiO2 powders consist of only the anatase phase after the calcination at lower temperatures (400 and 600 °C), while they consist of only rutile phase after calcination at the higher temperature (800 °C). As observed in Figure 15, the anatase TiO2 has higher photocatalytic activity than that of the rutile TiO2 for all R values investigated here. This may be attributed to the relatively larger average nanocrystallite size and smaller BET surface area of the rutile TiO2 powders as they are formed at a higher calcination temperature (800 °C). In addition to this, the photocatalytic activity of the rutile TiO2 is inherently known to be inferior to that of the anatase TiO2.41 Since the effective mass of the electron in the rutile TiO2 is higher than that in the anatase TiO2, the electron mobility, the electron diffusivity, and the electron flux in the rutile TiO2 have been estimated to be much lower than those in the anatase TiO2.41 This slow electron movement in the rutile TiO2 increases the rate of the e-/h+ recombination process, which in turn reduces its photocatalytic activity. The high rate of e-/h+ recombination process in the rutile TiO2 as compared to that in the anatase TiO2 has been confirmed via time-resolved microwave conductivity (TRMC) measurements.46 In Figure 15, kapp is noted to increase with increasing R for both the anatase TiO2 and the rutile TiO2 powders. It appears that the nature of the variation in kapp as a function of R is independent of the nature of the phase involved. For the nanocrystalline TiO2 powders calcined at 400 °C, the average nanocrystallite size increases with increasing R, Figure 4. From Figure 14, it is observed that this range of nanocrystallite size is located below the D* (∼15 nm). As a result, as the nanocrystallite size increases with increasing R, the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 °C also increases with R, Figure 15. For the sol-gel-derived nanocrystalline TiO2 powders calcined at 600 °C, the average nanocrystallite size is observed to decrease with increasing R, Figure 4. However, this average nanocrystallite size range is located above the D* (∼15 nm), Figure 14. As a consequence, with increasing R as the nanocrystallite size decreases, the photocatalytic activity of the nanocrystalline TiO2 powders calcined at 600 °C increases. It thus appears that, although the variation in the average nanocrystallite size as a function of R is reversed for the sol-gelderived nanocrystalline TiO2 powders calcined at 400 and 600 °C, Figure 4, kapp (and, hence, the photocatalytic activity) increases with increasing R under both of the conditions due to the existence of D* (∼15 nm), as observed in Figure 14. We further attribute the higher kapp and, hence, the enhanced photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powders calcined at 600 °C, Figure 15, relative to that of the powders calcined at 400 °C to their higher crystallinity (that is, the relative intensity greater than unity), Figure 3. The nanocrystalline TiO2 powders calcined at 400 °C possibly contain the residual amorphous phase, which may reduce the photocatalytic activity due to a higher e-/h+ recombination rate as the amorphous phase does not have the band structure required for an effective charge separation.18,28 The calcination at 600 °C results in the residual amorphous-to-anatase phase transformation, thus increasing the crystallinity of the sol-gelderived nanocrystalline TiO2 powders for the entire range of R investigated here. As a result, the photocatalytic activity of the sol-gel-derived nanocrystalline anatase TiO2 powders calcined

Sol-Gel-Derived Nanocrystalline TiO2

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7621 The photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powder is a function of both R and the calcination temperature. The maximum photocatalytic activity is observed for the highest R value and an intermediate calcination temperature within the investigated range. The photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 powder is in accordance with the established mechanism of the photocatalytic decomposition of the organic molecules using the nanocrystalline semiconductor oxides.

Figure 16. Variation in the difference in the residual MB dye concentration as a function of R obtained for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C. The data points are obtained by considering the residual MB dye concentrations after the UV radiation exposure time of 1 h (Figure 9a-d).

at 600 °C is higher than that of the powders calcined at 400 °C, Figure 15. However, since the relative crystallinity decreases with increasing R, Figure 3, kapp’s for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C tend to merge with each other with increasing R, Figure 15. This is further reflected in the decrease in the difference between the residual MB dye concentration with increasing R, after the total UV radiation exposure time of 1 h for the sol-gel-derived nanocrystalline TiO2 powders calcined at 400 and 600 °C, as demonstrated in Figure 16. Over all, the photocatalytic activity of the sol-gel-derived nanocrystalline TiO2 is observed to be a function of both R and the calcination temperature, which are effective in controlling the powder morphology, the average nanocrystallite size, the surface area, the crystallinity, and the phase structure of the sol-gel-derived nanocrystalline TiO2. Various other parameters such as doping, a metal surface catalyst, the pH, and anions also affect the photocatalytic properties of nanocrystalline TiO2.13,47-49 However, such a study for the present sol-gelderived nanocrystalline TiO2 is beyond the scope of this work, but it will be reported in the near future. It is also necessary to mention that, for the precise determination of the critical size related to the photocatalytic inversion phenomenon, the nanocrystalline TiO2 must be synthesized with the narrow size distribution in the nonaggregated form. There is no direct evidence in the open literature for the dependence of the critical size on the aggregation tendency of TiO2 nanocrystallites. However, from the present data and the available literature,40 it appears that the critical size may not be drastically dependent on the aggregation tendency of the TiO2 nanocrystallites, provided that the aggregates contain intra-aggregate porosity (that is, the mesoporous structure), which possibly tends to reduce the effect of aggregation on the surface area and make the internal surfaces available for the decomposition of the dye molecules. Investigation in this direction is currently underway. Conclusions Nanocrystalline TiO2 powders in an aggregated form, having different morphology, average nanocrystallite size, surface area, crystallinity, and phase structure, have been successfully synthesized via the sol-gel technique using an alkoxide precursor and varying the R value within the range of 5-60 and calcining the powders at higher temperatures within the range of 400-800 °C. The sol-gel-derived nanocrystalline TiO2 powder, processed under different conditions, is shown to be an effective photocatalyst at room temperature for the decomposition of the MB dye in an aqueous solution under the UV radiation exposure.

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