J. Phys. Chem. C 2009, 113, 15593–15601
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Preparation and Characterization of Mn-Doped Titanates with a Bicrystalline Framework: Correlation of the Crystallite Size with the Synergistic Effect on the Photocatalytic Activity L. Gomathi Devi,* Nagaraju Kottam, and S. Girish Kumar Department of Post Graduate Studies in Chemistry, Central College City Campus, Dr. Ambedkar Street, Bangalore UniVersity, Bangalore-560001, India ReceiVed: February 6, 2009; ReVised Manuscript ReceiVed: July 15, 2009
Mn2+ ion was doped into the sol-gel anatase TiO2 in the concentration range of 0.02-0.1%. Powder X-ray diffraction results revealed that the phase transformation from anatase to rutile proceeds at a slower rate up to an intermediate dopant concentration of 0.06% and accelerates for a higher dopant concentration of 0.1%. In comparison with the previous studies, the phase transition temperature was reduced by 100 °C for the intermediate dopant concentration. The photocatalytic activity of Mn2+ (0.06%)-TiO2 for the degradation of an oxo-fused polycyclic aromatic dye, Indanthrane BR Violet, was almost equal to that of Degussa P-25 under UV light, but under solar light its efficiency was nearly 4 times higher. This enhanced activity can be attributed to the (i) synergistic effect in the bicrystalline framework of anatase and rutile, (ii) small crystallite size, and (iii) high intimate contact between both the phases. The correlation of the crystallite size with the synergistic effect on the photocatalytic activity is studied in detail. 1. Introduction Titanium dioxide is a well-known photocatalyst stable in most of the chemical environments, inexpensive, and of low biological toxicity. These properties make TiO2 a prime important catalyst for the degradation of organic contaminants.1-10 TiO2 has three different phases: anatase, rutile, and brookite. It is commonly believed that anatase is the active phase in the photocatalytic reactions as it has a higher affinity for the adsorption of organic pollutants due to its high degree of hydroxylation compared to the rutile phase. Reports are also available on the photocatalytic activity of the rutile phase. The difference in the activity of rutile and anatase can be attributed to its band edge positions (more positive for rutile) and also to the higher recombination rate of charge carriers in the rutile phase. Oxygen adsorbed on the surface of the catalyst traps conduction band electrons and hence inhibits the recombination reaction. The amount of adsorbed oxygen depends on the degree of hydroxylation on the adsorbing surface. However, the higher calcination temperature which is needed for rutile phase formation will reduce the concentration of surface-adsorbed water and hydroxyl groups, resulting in lower photocatalytic activity. The mixed phase (anatase + rutile) is shown to exhibit higher photoactivity compared to the pure anatase or pure rutile phase, which can be attributed to the synergistic effect between these two phases. Choi et al.11 have reported the photocatalytic activities of quantum-sized TiO2 doped with different transition-metal ions for the oxidation of chloroform and reduction of carbon tetrachloride. They have reported the enhanced photoactivity for Fe3+-, Ru3+-, and Os3+-doped samples which have a stable half-filled electronic configuration. Xu et al.12 have also reported the influence of doping different rare-earth-metal ions into the TiO2 lattice for the decomposition of nitrate. They have reported the enhanced activity of TiO2 doped with Gd3+ ions which also possess a stable half-filled electronic configuration. To the best of our knowledge, the photocatalytic activity of the doped * To whom correspondence should be addressed. Phone: +91-8022961336. Fax: +91-80-22961331. E-mail:
[email protected].
Figure 1. PXRD patterns of pure TiO2 (SG) and manganese-doped TiO2. (The peaks with asterisks correspond to the rutile phase.)
catalyst correlating the electronic configuration of the dopant and crystallite size with the synergestic effect of the mixed phases is not reported at all. This aroused the interest in choosing a transition-metal ion dopant, Mn2+, which has a larger ionic size than Ti4+ ion and can induce oxygen vacancies, facilitating the rutile phase growth. Recently Arroyo et al.,13 Othman et al.,14 and Mohamed et al.15 have reported the formation of mixed phases when Mn2+ is doped into the TiO2 matrix. Though structural characterizations were well explained, its photocatalytic activity under solar light was not explored. In this view, the present research focuses on the photocatalytic activity of Mn2+-TiO2 with a bicrystalline framework of anatase and rutile under UV/solar light, and its efficiency is compared with that of the benchmark catalyst Degussa P-25. An attempt has been made to provide a new physical insight related to the mechanism of charge transfer in the bicrystalline framework of anatase and
10.1021/jp903711a CCC: $40.75 2009 American Chemical Society Published on Web 08/12/2009
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TABLE 1: Summarized Results of PXRD Analysis of All the Photocatalysts photocatalyst
2θ of the crystal plane (101) of anatase (deg)
d spacing of the crystal plane (101) of anatase (Å)
phase composition anatase:rutile
lattice params (Å)
unit cell vol (Å3)
crystallite size (nm), anatase:rutile
P1 P2 P3 P4
25.32 25.34 25.36 25.33
3.50 3.51 3.51 3.50
100:0 100:0 90:10 52:48
a ) b ) 3.78, c ) 9.50 a ) b ) 3.78, c ) 9.51 a ) b ) 3.78, c ) 9.52 a ) b ) 3.78, c ) 9.50
135.97 136.12 136.21 135.74
26.2:0 23.6:0 20.3:20.3 16.6:16.6
rutile by taking into account the theories of previous models. The model compound chosen for the photocatalytic study is Indanthrane BR Violet (IBRV) dye, which is widely used in textiles and is highly carcinogenic. 2. Experimental Section 2.1. Materials. TiCl4 was supplied from Merck Chemicals. MnC2O4, aqueous NH3, and concentrated H2SO4 were of analytical grade. The synthetic dye IBRV was supplied from the textile industry, India, and was used as received. The molecular formula of the compound is C30H20O2Cl2, the molecular weight is 513, and the compound shows λmax at 540 nm. 2.2. Catalyst Preparation. Crystalline anatase TiO2 is prepared by the sol-gel (SG) method.16 A 25 mL volume of diluted TiCl4 with 1 mL of concentrated H2SO4 was taken in a beaker and diluted to 1000 mL. The pH of the solution was maintained at 7-8 by adding liquor ammonia. The gel obtained was allowed to settle down. The precipitate was washed free of chloride and ammonium ions. The gelatinous precipitate was filtered and oven-dried at 100 °C. The finely ground powder was then calcined at 550 °C for 4.5 h. Doped samples were prepared by taking the calculated amounts of anatase TiO2 along with MnC2O4 solution to get Mn contents of 0.02%, 0.06%, and 0.1% and were labeled as P2, P3, and P4, respectively, while undoped TiO2 was labeled as P1. The mixture was ground for 1 h and heated at 120 °C in an oven for 2 h, and the process was repeated four times. The resultant sample was calcined at 550 °C for 4.5 h. The commercially available Degussa P-25 TiO2 was labeled as P25. 2.3. Analytical Techniques. 2.3.1. Powder X-ray Diffraction (PXRD). The PXRD patterns of various titania samples were obtained using a Philips PW/1050/70/76 X-ray diffractometer which was operated at 30 kV and 20 mA using Cu KR radiation as the source with a nickel filter at a scan rate of 2
Figure 2. Calcination temperature and rutile content (%) for all the catalysts versus the concentration of Mn2+. Line A shows the decrease of the calcination temperature for phase transformation from anatase to rutile, and line B shows the increase in the rutile fraction in the samples with an increase in dopant concentration.
deg/min. To get the X-ray diffraction line broadening, the reflection peaks were recorded with a slow scanning speed of 1/2 deg/min. The average crystallite size (D) was calculated using Scherrer’s equation: D ) kλ/(β cos θ), where k is the shape factor (∼0.9), λ is the X-ray wavelength (0.15418 nm), β is the full width at half-maximum (fwhm) of the diffraction line, and θ is the diffraction angle. The rutile fraction in the sample was calculated using the Spurr and Meyer’s equation:17 XR ) (1 + 0.8IA/IR)-1, where XR is the mass fraction of rutile in the prepared samples and IA and IR are the X-ray integrated intensities corresponding to the (101) diffraction plane of anatase and (110) diffraction plane of the rutile phase, respectively. 2.3.2. Diffuse Reflectance Spectroscopy (DRS). DRS spectra of all the photocatalysts in the wavelength range of 200-800 nm were obtained using a Shimadzu-UV 3101 PC UV-vis-NIR spectrophotometer using BaSO4 as the reference standard. The band gaps were calculated by the Kubelka-Munk method. 2.3.3. Scanning Electron Microscopy (SEM). The surface morphology of the photocatalyst was analyzed by the SEM technique with a Quantimet 520 image analyzer in conjuction with a scanning electron microscope (Cambridge Instrument). The particles were dispersed onto a sample holder, gold coated, and viewed through an electron microscope. 2.3.4. Energy-DispersiWe X-ray (EDX) Analysis. An electron microprobe was used in the EDX mode which was employed to obtain quantitative information on the amount of the dopant metal species incorporated into the TiO2 lattice. 2.3.5. BET Analysis. The specific surface areas of the doped and undoped samples were determined by NOVA-1000 version 3.70. 3. Photocatalytic Degradation Procedure The experiments were performed in an open glass reactor whose surface area is 176 cm2. A medium-pressure 125 W mercury vapor lamp was used as the UV source. The photon flux of the light source was 7.75 mW/cm2 as determined by ferrioxalate actinometry, and the output of the mercury lamp was in the range of 350-400 nm with peaks around 370 nm. A series of experiments were conducted to optimize the catalyst dosage (150 mg) and dye concentration (10 ppm). In a typical experiment, 250 mL of 10 ppm IBRV dye solution containing 150 mg of the catalyst was magnetically stirred in dark conditions for 15 min to establish an adsorption/desorption equilibrium. The difference in the concentration of the substrate before and after stirring gives the amount adsorbed on the catalyst surface. All the experiments were carried out under constant stirring in the presence of atmospheric oxygen. Experiments using solar light irradiation were carried out from 11 a.m. to 2 p.m. during the summer season at Bangalore, India. The latitude and longitude are 12.58 N and 77.38 E, respectively. The average sunlight intensity was found to be around 1200 W m-2. The intensity of the solar light was concentrated by using a convex lens, and the reaction mixture was exposed to this concentrated sunlight. To compare the photocatalytic activity of all the catalysts, the experiments were simultaneously
Mn-Doped Titanates with a Bicrystalline Framework conducted to avoid the error arising due to the fluctuations in solar intensity. The filtrates collected at different time intervals were analyzed by the UV-vis spectroscopic technique using a Shimadzu UV-1700 pharmaspec UV-vis spectrophotometer. 4. Characterization of the Catalysts 4.1. PXRD Studies. 4.1.1. Influence of the Mn2+ Dopant on the Phase Transformation. Figure 1 shows the PXRD pattern of Mn2+-TiO2. The peaks at 25.34° and 27.42° correspond to the crystal planes (101) of anatase and (110) of rutile planes, respectively. P1 showed only the anatase phase, and significant changes in the phase structure of the photocatalysts were observed on doping with Mn2+ ions. The influence of the dopant on the phase transformation of the catalyst can be well explained considering the changes caused by the inclusion of the dopant. These changes are strongly dependent on the charge and ionic radius of the dopant. The dopant with a lower charge than Ti4+ can alter the concentration of oxygen vacancies depending on its position in the TiO2 matrix; it can replace Ti in the TiO2 lattice or can occupy an interstitial position, which in turn depends on its ionic size and concentration.18 The rutile content in the doped sample increases with the dopant concentration. At a lower dopant concentration (e0.02%), the catalyst exhibits only the anatase phase (P2). With an increase in the dopant concentration from 0.02% to 0.06%, a small reflection at 2θ ) 27.42° corresponding to the rutile phase appears. P3 showed a higher anatase:rutile (90:10) ratio, and P4 with a higher dopant concentration showed a higher rutile: anatase (52:48) ratio (Table 1). These observed structural changes can be analyzed in the following way. (1) The ionic radius of Mn2+ (0.80 Å) is greater than that of 4+ Ti (0.68 Å). Therefore, Mn2+ cannot act as an interstitial impurity in the TiO2 matrix. Hence, Mn2+ ions can possibly act as a substitutional impurity at Ti4+ lattice sites. The higher ionic radius of the Mn2+ ion produces a localized charge perturbation in the TiO2 lattice. The introduction of substitutional dopant metal ions with a valancy less than +4 with a higher ionic size would induce oxygen vacancies at the surface of anatase grains, which favors bond rupture, ionic rearrangement, and structure reorganization for the formation of the rutile phase.19 The anatase to rutile phase transformation is generally considered as a nucleation growth process during which the rutile nuclei are formed within the anatase phase.19 (2) The calculated average grain sizes of various samples are given in Table 1. The anatase grain size decreases with an increase in the Mn2+ dopant concentration. This indicates that the Mn2+ dopant has an inhibition effect on the anatase grain growth which favors an increase in the total grain boundary energy for the doped TiO2. This excess energy acts as the driving force for rutile grain growth, promoting the phase transformation. The decrease in the crystallite size of the doped samples leads to an increase in the specific surface area. This would increase the density of surface state defects on the surface of anatase grains. The rutile nucleation is thus accelerated at these surface defect sites.19 These results suggest that phase transformation from anatase to rutile takes place in a smooth way at an intermediate concentration (0.06%) and the process accelerates at a higher dopant concentration (g0.1%). (3) For a higher dopant concentration, the crystallite size is found to be 16.6 nm for both the phases. The sample with small crystallite sizes would contain a larger number of lattice defects. The atoms at these defects have a higher energy than those in the main lattice and can act as nucleation sites for the formation of the rutile phase at the surface of anatase crystallites.
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15595 A plot of the calcination temperature and rutile content versus dopant concentration is shown in Figure 2. The plot suggests that the Mn2+ dopant significantly reduces the phase transformation temperature. It is worth noting that diffraction peaks corresponding to Mn or its oxides were not detected in the PXRD pattern due to the complete solubility of Mn2+ ions in the TiO2 matrix. Phase transformation to rutile for P1 starts at 650 °C and is complete at 700 °C. For P2, this phase transformation occurs at 600 °C, while for the P3 phase transformation occurs at 550 °C. These results suggest that the phase transformation is promoted by Mn2+ ions, which also reduce the phase transition temperature for the formation of the rutile phase. Similar reports were made by Arroyo et al.13 However, in their work, though the rutile content in the doped samples increased with the dopant concentration, the phase transition temperature was not reduced accordingly. Their results showed that the rutile fraction (>50%) dominated in the doped titanates when the samples were calcined at 700 °C for a dopant concentration of 0.5 mol %, but in the present work, the rutile phase dominated (>50%) in the samples calcined at 550 °C for a dopant concentration of 0.1%. The phase transition temperature for the formation of the rutile phase was reduced by 100 °C compared to that with undoped TiO2 in the present work, while Arroyo et al.13 reported a decrease of 50 °C13 and Kim et al.20 reported an increase of 100 °C for the formation of the rutile phase. Shiwen et al.21 reported the formation of the rutile phase at 780 °C for the TiO2-MnO2 catalyst. However, it is not clear that the rutile phase formation was favored either due to Mn2+ incorporation or due to the high calcination temperature. Amores et al.22 reported the formation of the rutile phase at 970 °C for the supported manganese oxide XMn-TiO2 (X ) 0.5, the theoretical monolayer fraction). From these results, it can be concluded that the present research work effectively reduces the phase transition temperature for the formation of the rutile phase at very low Mn2+ concentration. 4.1.2. Determination of Lattice Parameters. With an increase in the Mn2+ content (from 0.02% to 0.06%), the diffraction peak of the crystal plane (101) of anatase shifts to a higher angle and the d spacing value decreases. Since the ionic size of Mn2+ (0.80 Å) is larger than that of Ti4+ (0.64 Å), an increase in the unit cell volume is observed due to the substitution of Mn2+ ions.23 The variation in the lattice parameter was reflected in the elongation of the c axis. Since only the c dimension changes while a ()b) remains almost constant for the range of dopant concentration, it can be concluded that Mn2+ substitutes Ti4+ preferentially on the bcc and fcc in the anatase structure.18 The change in the lattice parameter may also indicate the oxygen vacancies which are usually induced along with the incorporation of the impurity. However, a higher dopant concentration (0.1%) shows a slight decrease in the lattice parameter. The anatase crystallite sizes calculated using the (101) reflection are 26.2, 23.6, 20.3, and 16.6 nm for P1, P2, P3, and P4 respectively. The doped sample showed a nearly 36.64% decrease in crystallite size compared to P1. The rutile crystallite size was similar to the anatase crystallite sizes for P3 and P4. Thus, the incorporation of the Mn2+ ion inhibited the crystallite growth of both the phases. The mixed phase and their corresponding crystallite sizes in metal/nonmetal (iodine) doped titanates as reported by several researchers are listed in Table 2.14,15,19,24-28 The substitution of Mn2+ ions in the TiO2 lattice follows Vegard’s law, which states that a “change in the unit cell dimension will be linear with respect to the change in the dopant concentration”.
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TABLE 2: Research Group, Preparation Methods, Photocatalyst, Phase Composition Anatase:Rutile, and Anatase and Rutile Crystallite Sizes As Reported by Several Researchers in Doped Titanates research group
preparation method
photocatalyst
phase composition anatase:rutile
crystallite size (nm), anatase:rutile
present research Othman et al.14 and Mohamed et al.15 Chao et al.19 Xiao et al.24 Gunlazuardi et al.25 Jongsomjit et al.26 Lopez et al.27 Cheng et al.28
solid-state reaction sol-gel sol-gel combustion methods impregnation method wetness impregnation solid-state reaction two-step template hydrothermal
Degussa P-25 Mn2+-TiO2(0.06%) Mn-TiO2 (SG) (10 wt %) Ag-TiO2 (10 mol %) Sm-TiO2 (0.5 mol %) Cu-TiO2 (3 atom %) Co-TiO2 (20 wt %) La-TiO2 (0.2 atom %) iodine-doped TiO2
80:20 90:10 44:56 not mentioned 48.39:51.61 73.4:26.6 81:19 22:78 57:43
20:23 20.3:20.3 48:51 30:155 13.8:13.6 20:26 not reported 31:43 10:16
4.2. UV-Vis Absorption and DRS Studies. The band gap is governed by the crystallite size and the defects in the TiO2 network. Sanchez.et al.29 suggested that small band gaps were caused by a stoichiometric deficiency of Ti:O ratios. The doped samples showed a clear red shift due to the creation of defect levels by the dopant, and the magnitude of the red shift depends on the dopant concentration. The band gap was lowered by 0.5 eV compared to that of P1. The band structure in P1 consists of the 3d level of titanium and 2p level of oxygen, which form the conduction and valence bands, respectively. The absorption edge at 380 nm in P1 corresponds to the transition within these band gap states. The band gap narrowing can be attributed to the substitution of Mn2+ ions, which introduces an electronic state within the band gap of TiO2. When the concentration of the dopant was increased from 0.02% to 0.06%, the extent of red shift increased from 426 to 454 nm and a large blue shift was observed for a higher dopant concentration (g0.1%) as shown in Figure 3. 4.3. Induced Defect States by the Dopant in the Pristine TiO2. The variable oxidation states and the ionic radii of manganese are Mn2+ (0.80 Å), Mn3+ (0.66 Å), and Mn4+ (0.60
Å). Probable defect states can be represented using the Kroger and Vink notation.30,31 Assuming Mn4+ occupies the lattice position of Ti4+ in the TiO2 matrix, the following reaction is obtained:
MnO2 T MnTi + 2Oo (Mn in +4 oxidation state)
(1) Assuming Mn3+/Mn2+ occupies the lattice position of Ti4+ in the TiO2 matrix, it induces doubly ionized/two singly ionized oxygen vacancies:
Mn2O3 T 2MnTi′′ + 3Oo + Vo••/2Vo• (Mn in +3 oxidation state) (2) MnO T MnTi′′ + Oo + Vo••/2Vo• (Mn in +2 oxidation state) (3) Charge compensation can also occur by interstitial oxygen, which is less probable due to the higher ionic size of oxygen:
Mn2O3 T 2MnTi′′ + Oi′′ + 2Oo (Mn in +3 oxidation state) (4) MnO T MnTi′′ + Oi′′ (Mn in +2 oxidation state)
(5)
Figure 3. UV-vis absorption spectra of photocatalysts: (A) P1, (B) P2, (C) P3, (D) P4.
The notations Vo, Vo•, and Vo•• represent neutral and singly and doubly ionized oxygen vacancies. Oo is oxygen occupying an oxygen lattice site. MnTi is a manganese ion at a titanium lattice site, and the superscript bullet symbol represents the deficiency in the charge. When Mn is in the +4 oxidation state, its ionic radius is almost equal to that of the Ti4+ ion. Hence, creation of oxygen vacancies or charge imbalance in the TiO2 lattice is minimal or negligible. Charges are compensated by lattice defects such as oxygen vacancies (ionized/neutral) when Mn is in the +2 oxidation state due to its higher ionic radius compared to that of the host Ti4+ ion. These results suggest that doping with a lower valent transition-metal ion with a higher ionic size induces more oxygen vacancies. 4.4. FT-IR Analysis. The following conclusions are made from FT-IR analysis. (1) The catalysts showed strong absorption bands at 482 and 560 cm-1, which can be assigned to the Ti-O bond in the TiO2 lattice.
Mn-Doped Titanates with a Bicrystalline Framework TABLE 3: BET Specific Surface Area of the Photocatalystsa
a
photocatalyst
specific surface area (m2/g)
P25 P1 P2 P3 P4
52 18 21 26 22
P25 is reported as claimed by the supplier.
(2) The peaks at 850 and 915 cm-1 may be assigned to stretching vibrations of O-O for peroxo groups and vibrations of the Ti-O-O bond. (3) The bands at 3432 and 1655 cm-1 can be attributed to the vibrations of surface-adsorbed water molecules and the Ti-OH bond, respectively.32 (4) On doping, the band at 915 cm-1 disappears, which suggests the possible interaction of the Mn2+ dopant with Ti-O-O bonds. (5) Catalyst P3 showed three prominent bands at 528 and 605 cm-1 corresponding to the anatase framework. The third band at 463 cm-1 corresponds to the rutile framework of the O-Ti-O bond.14 (6) The band at 463 cm-1 increased in its intensity significantly for P4 due to the presence of a large fraction of rutile phase in the sample (which is in agreement with the PXRD results). (7) An increase in the dopant concentration reduces the intensity of the 3432 and 1655 cm-1 bands. These bands were highly broadened for doped samples due to their smaller crystallite size.33 This decrease in the band intensity suggests the possible interaction of the dopant with surface hydroxyl groups of pristine TiO2. 4.5. SEM, EDX, and BET Analysis. SEM analysis showed a decrease in the particle size with an increase in the dopant concentration. EDX analysis confirmed the presence of the dopant in the TiO2 lattice. BET analysis showed an increase in
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15597 the surface area for the doped samples compared to P1 excluding P25, which may be due to the introduction of additional nucleation sites by the dopant. The decrease in the particle size may additionally contribute to the enhancement in the surface area (Table 3). 5. Photocatalytic Activity 5.1. Preliminary Adsorption in the Dark. The IBRV solution was stirred along with photocatalysts (150 mg) in the dark for 15 min. The amount of adsorption was calculated by comparing the concentration of the dye before and after stirring. The percentage of dye adsorption was calculated from (1 C/C0) × 100, where C0 and C are the initial and residual concentrations of IBRV, respectively. The adsorptive capacity of IBRV on the photocatalyst surface at pH 5.5 is in the following order: P3 > P2 > P1 > P4. 5.2. Photocatalytic Degradation of IBRV in an Aqueous Suspension. The electron-hole pairs are generated when TiO2 is illuminated with light of energy equivalent to its band gap. The generated charge carriers take part in the following series of redox reactions. (1) Preliminary act of absorption:
TiO2 + hν f eCB- + hVB+
(6)
(2) Possible traps for holes. (a) Surface-adsorbed hydroxyl ions:
hVB+ + ΟΗads- f ΟΗads•
(7)
(b) Surface-adsorbed water molecules:
hVB+ + Η2Οads f ΟΗads• + Η+
(8)
1 hVB+ + Η2Οads f Ο2 + 2Η+ 2
(9)
(c) Electron donor (D) species:
hVB+ + Dads f Dads+
(10)
(3) Possible traps for electrons. (a) Surface shallow traps:
eCB- + TiIVΟH f TiIIIΟH
(11)
(b) Lattice deep traps:
eCB- + TiIV f TiIII
(12)
(c) Electron acceptor (A) species:
eCB- + Αads f Αads-
(13)
(4) Recombination reaction: Figure 4. Surface charges above and below the isoelectric point showing the electrostatic attraction and repulsion of the IBRV dye molecules on the photocatalyst surface.
eCB- + hVB+ f TiΟ2 + energy
(14)
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TABLE 4: Influence of the pH on the Adsorption Percentage, Decolorization Percentage, and Rate Constant for the Degradation of the IBRV pH
adsorption (%)
decolorization (%)
k × 10-2 (min-1)
2 5.5 9.0
15 10 1
56 100 12
1.1 2.0 0.3
In bare TiO2 suspensions, electron-donating species are water molecules, hydroxyl ions, and IBRV dye molecules (in the present case), while the electron scavenger is the oxygen molecule. Therefore, all the experiments are carried out in the presence of atmospheric oxygen. The dopant Mn2+ serves as charge carrier traps in the doped samples. 5.3. Effect of pH. pH is a complex parameter as it is related to several factors such as the (i) ionization state of the surface, (ii) size of the particle aggregates formed, (iii) nature of the dye, and (iv) extent of the substrate adsorption on the catalyst surface. The point of zero charge (pzc) of TiO2 is widely reported to be pH ≈ 6.25. Thus, the TiO2 surface will be positively charged at pH < 6.25 and negatively charged at pH > 6.25 (Figure 4). The interpretation of pH effects on the degradation process is very difficult as it includes various factors such as (i) electrostatic interactions between the catalyst surface and dye molecules and (ii) reaction of charged radicals such as superoxide, hydroxy radicals, etc. formed on the catalyst surface with the dye molecules. It can be seen that, as the pH increases, the extent of IBRV adsorption decreases. These results can be better understood by taking into account both the surface states of titania and the ionization state of the dye. The hydroxylated TiO2 surface can be protonated under acidic medium and deprotonated under alkaline conditions as represented in eqs 15a and 15b, respectively.34
TiOH + Η+ T TiOΗ2+ (in acidic medium)
(15a) TiOH + OΗ- T TiΟ- + Η2Ο (in alkaline medium) (15b) The degradation reactions were carried out at different pH conditions of 2.0, 5.5, and 9.0. It can be seen that adsorption is quite high under acidic pH and is negligible under alkaline conditions (Table 4). This may be due to the fact that, under acidic pH, TiO2 behaves as a strong Lewis acid due to the surface positive charge. The dye is a fused polynuclear aromatic compound with extensive π electron conjugation which can easily form a stable complex by donating electrons to the vacant d orbital of titanium. In other words, the IBRV dye is a strong Lewis base and can easily adsorb on the positively charged catalyst surface. This favors the adsorption of the dye under acidic conditions, while in the alkaline conditions, this complexation process is not favored presumably because of two factors: (i) competitive adsorption by hydroxyl and the dye molecule, (ii) Coulombic repulsion due to the negatively charged catalyst with the dye molecule. The rate of the reaction decreases under strong acidic conditions (pH 2). This may be due to the higher extent of adsorption of dye molecules, which prevents the photoexcitation of semiconductor particles by masking the surface, thereby reducing the generation of free radicals. However, a low rate constant was observed under alkaline pH 9.0 due to the negligible dye adsorption, leading to a poor
Figure 5. (a) -(log C/C0) versus time for the degradation of IBRV under UV light: (A) P1, (B) P2, (C) P3, (D) P4, (E) P2. (b) -(log C/C0) versus time for the degradation of IBRV under solar light: (A) P1, (B) P2, (C) P3, (D) P4, (E) P25.
TABLE 5: Calculations of the Degradation Percentage, Rate Constant (k), and Process Efficiency (Φ) for the Degradation of IBRV under UV Light photocatalyst
degradation (%)
k × 10-2 (min-1)
Φ × 10-6a (ppm min-1 W-1 cm-2)
P1 P2 P3 P4 P25
68 82 100 80 100
1.3 1.4 2.0 1.6 2.0
10.30 12.42 15.15 12.12 15.15
a Process efficiency Φ ) (C0 - C)/tIS (ppm min-1 W-1 cm-2), where C0 is the initial concentration of the substrate and C is the concentration at time t, C0 - C denotes the amount degraded (ppm), I is the irradiation intensity, which is equal to 125 W, S denotes the solution irradiated plane surface area (cm2), and t represents the irradiation time (min).
interfacial charge transfer process. Therefore, the most favorable pH for degradation of the IBRV dye is found to be 5.5. 6. Correlation of the Crystallite Size with the Synergistic Effect on the Photocatalytic Activity The plots of -(log C/C0) versus time using all the photocatalysts for the degradation of IBRV are shown in Figure 5, and the corresponding rate constant and process efficiency are shown in Tables 5 and 6. The efficiencies of P25 and P3 under UV light were almost the same despite the fact that the surface area of P25 was almost twice that of P3. Furthermore, the
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TABLE 6: Calculations of the Degradation Percentage, Rate Constant (k), and Process Efficiency (Φ) for the Degradation of IBRV under Solar Light photocatalyst
degradation (%)
k × 10-2 (min-1)
Φ × 10-7 (ppm min-1 W-1 cm-2)
P1 P2 P3 P4 P25
2 80 100 65 25
2.7 3.8 2.3 0.84
6.3 24.24 30.03 19.69 7.57
catalyst P3 with a bicrystalline framework of anatase and rutile showed a higher efficiency compared P2, which had only the anatase phase, although both the catalysts had similar surface areas. Hence, we conclude that the surface area has no obvious relation to the enhanced photocatalytic activity in the present study. The enhanced activity of P3 under both UV and solar light was attributed to the bicrystalline framework of anatase and rutile, which suggests the existence of a synergistic effect. It is well-known that this pair of polymorphs can effectively reduce the recombination of the photogenerated charge carrier to enhance the photocatalytic activity.35-57 Under UV excitation, anatase in the mixed phase gets activated as it is a good absorber of UV light photons. The transfer of electrons takes place from the conduction band edge of anatase to the trapping sites of rutile. Thus, rutile serves as a passive electron sink, hindering the recombination in the anatase phase and the hole originating from the anatase transfer to the surface, which accounts for the enhanced activity of P25.58,59 A similar mechanism takes place in the case of P3. In contrast to P25, P3 possesses an impurity level of the dopant which is about 0.5 eV below the conduction band of anatase, which is even lower than the conduction band of rutile itself. Hence, subsequent electron transfer from the rutile trapping site to the impurity level further favors the charge separation, which might account for the higher activity of P3 (Figure 6). The band gap of rutile is favorable for visible light excitation as the conduction band edge of rutile lies 0.2 eV below the conduction band edge of anatase. Under visible light excitation, the photogenerated electron from the conduction band of rutile transfers to trapping sites of the anatase phase, which can be considered as an antenna effect by the rutile phase60 (Figure
Figure 7. Charge transfer in a mixed phase under solar light: (A) in Degussa P-25 (as per ref 60), (B) in P3.
7). Subsequent transfer of electrons to lattice trapping sites of anatase further separates the charge carriers effectively. The lattice trapping sites of anatase have an energy 0.8 eV less than that of the conduction band edge of anatase.61 Thus, by competing with the recombination, the charge separation activates the catalyst and the hole originating from the rutile valence band participates in the oxidative degradation of organic pollutants. The dopant Mn2+ has a unique stable half-filled electronic structure (3d5). If Mn2+ is assumed to trap electrons, its stable half-filled electronic configuration is disturbed. The trapped electron may thus be transferred either to an oxygen molecule, which forms a superoxide radical, or to a dye molecule, leading to the formation of a dye radical. By this process Mn will retain the +2 stable oxidation state of the half-filled electronic configuration.
Mn2+ + e- f Mn+
(16)
Mn+ + Ο2ads f Mn2+ + Ο2•-
(17)
Mn+ + dyeads f Mn2+ + dye-
(18)
Alternatively, manganese can also attain the stable half-filled electronic configuration by trapping a valence band hole.
Mn+ + h+ f Mn2+
(19)
If Mn2+ is assumed to behave as a hole trap, its stable halffilled electronic configuration is again disturbed. The trapped holes may be transferred to a hydroxyl anion adsorbed on the catalyst surface, leading to the formation of a hydroxyl radical, or it can also be transferred to an adsorbed dye molecule to form a dye radical. Figure 6. Charge transfer in a mixed phase under UV light: (A) in Degussa P-25 (as per ref 58), (B) in P3.
Mn2+ + h+ f Mn3+
(20)
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Mn3+ + OH- f Mn2+ + OH•
(21)
Mn3+ + dyeads f Mn2+ + dye+
(22)
Alternatively, Mn3+ can also trap a conduction band electron and be reduced to Mn2+.
Mn3+ + e- f Mn2+
(23)
These processes not only accelerate the interfacial charge transfer process, but also enhance the generation of highly reactive oxidative species such as superoxide and hydroxyl radicals. In any case, efficient charge separation caused by the mixed phase structure can activate the catalyst, and the holes originated can effectively participate in the degradation mechanism. For such an interparticle electron transfer to be possible, the two polymorphs must be in close contact. The intimate contact between the two polymorphs critically depends on their crystallite size. Hong et al.62 prepared iodine-doped titania with mixed phases of anatase and rutile by calcining the sample at 500 °C, which showed lower activity compared to iodine-doped anatase titania. The low activity was due to the large rutile crystal size, which resulted in poor intimate contact between these two phases, which failed to demonstrate its structure advantage. Hence, it is crucial to maintain the crystallite size of both the phases, which enables the mixed phase for efficient charge transfer. P25 possesses an anatase:rutile ratio of 80:20 and consists of individual crystallites of anatase and rutile, with rutile being slightly larger than the anatase. In contrast to P25, sample P3 has an anatase:rutile ratio of 90:10, and the crystallite size is 20.3 nm for both the phases. Since the crystallite sizes of both the phases are the same, it can be speculated that both the polymorphs are in intimate contact compared to the situation for P25. The charge carriers generated in rutile titania with a small particle size as in P3 possess much stronger redox ability compared to those generated in the rutile titania with a larger particle size (P25). Furthermore, excited energetic electrons in the conduction band of small-sized rutile titania can be more effectively transferred to the lower energy lattice trapping sites of anatase. According to Gray’s results,63 such an interfacial mixed polymorph structure would contain a surplus amount of tetrahedral Ti4+ sites which can act as reactive electron-trapping sites. These isolated tetrahedral Ti4+ sites are more active than octahedrally coordinated Ti4+ sites as in bulk TiO2. These tetrahedral Ti4+ sites could serve as catalytic hot spots at the anatase/rutile interface and thus avail the mixed polymorph nanocrystals into an effective photocatalytic relay for solar energy utilization.64 Hence, we believe that these tetrahedral Ti4+ sites contribute to the increased activity of the mixed phase relative to the pure anatase (P2). The small crystallite size in P3 reduces the diffusion path length for the charge carriers, from the site where they are photoproduced to the site where they react. Reduction in this diffusion path length results in reduced recombination of charge carriers, resulting in an enhanced interfacial charge transfer process. Therefore, such an intimate contact between the mixed polymorph with the smaller crystallite will have a core of rutile crystallites interwoven with bound anatase crystallites, thus accelerating the transfer of electrons from rutile to neighboring anatase sites or to the
impurity level created by the dopant. However, it is vital that the existence of a synergistic effect between the mixed polymorphs is not universal and there exists an optimum value for both the phases to show enhanced activity. In the present case, the optimum value of the anatase:rutile ratio is 90:10. The induced oxygen vacancies by the Mn2+ along with the other defects could act as trapping centers to capture photoinduced electrons, effectively inhibiting the recombination. Moreover, oxygen vacancies could promote the adsorption of oxygen by the strong interaction between the photogenerated electrons trapped at these sites with the adsorbed oxygen. Thus, the created defects and oxygen vacancies favored the photocatalytic reactions.24 With the dopant concentration around 0.1%, the crystallite size for both the phases is found to be 16.6 nm. Most of the charge carriers in these crystallites are generated sufficiently close to the surface. As a result, the photogenerated charge carriers may quickly reach the surface, resulting in a faster surface recombination reaction. This is also due to the excess trapping sites in the sample and lack of a driving force to separate these charge carriers. In the catalyst with a smaller crystallite size, surface charge carrier recombination outweighs the interfacial charge transfer process. Since Mn2+ serves as a trapping site for both an electron and a hole, the possibilities of trapping these charge carriers will be high at a higher dopant concentration, and this trapped charge carrier pair may recombine through quantum tunneling.65 Therefore, there is a need for an optimal dopant concentration in the TiO2 matrix to get an effective crystallite size for the highest photocatalytic efficiency. Beyond the optimum dopant concentration, the rate of recombination starts dominating the reaction in accordance with the following equation:
KRRR exp(-2R/a0)
(24)
where KRR is the rate of recombination, R is the distance separating the electron and hole pairs, and a0 is the hydrogenic radius of the wave function for the charge carrier. As a consequence, the recombination rate increases exponentially with the dopant concentration because the average distance between the trap sites decreases with increasing number of dopants confined within a particle. Furthermore, it is suggested that Ti4+ in the TiO2 with a higher fraction of rutile is more difficult to reduce to Ti3+, which suggests that trapping sites might serve as recombination centers, which is in agreement with eq 24. 7. Conclusions The influence of Mn2+ ion incorporation into the TiO2 lattice on the phase transformation was studied in detail. Phase transformation proceeds at a smoother rate for a dopant concentration of 0.06% and accelerates at higher dopant concentrations (0.1%). The photocatalytic activities of these catalysts were compared with that of the commercially available Degussa P-25 for the degradation of IBRV, an oxo-fused polycyclic aromatic dye under both UV/ and solar light. The defect states introduced by the dopant serve as trap sites for the charge carriers under both UV and solar light. The photocatalytic activity of Mn2+ (0.06%)-TiO2 was almost equal to that of Degussa P-25 under UV light, while its efficiency was 4 times greater than that of Degussa P-25 under solar light. This was attributed to the synergistic effect between the mixed phases. Furthermore, the stable half-filled electronic configu-
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