Ti Ratio on the Visible-Light Photocatalytic Activity of P

Sep 23, 2009 - Carlos Sotelo-Vazquez , Nuruzzaman Noor , Andreas Kafizas , Raul Quesada-Cabrera , David O. Scanlon , Alaric Taylor , James R. Durrant ...
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18134

J. Phys. Chem. C 2009, 113, 18134–18141

Effect of the P/Ti Ratio on the Visible-Light Photocatalytic Activity of P-Doped TiO2 Fangfei Li, Yinshan Jiang,* Maosheng Xia, Mengmeng Sun, Bing Xue, Darui Liu, and Xuguang Zhang Key Laboratory of Automobile Materials (Jilin UniVersity), Ministry of Education, and Department of Materials Science and Engineering, Jilin UniVersity, Changchun 130025, China ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: August 14, 2009

P-doped TiO2 nanoparticles were synthesized by the sol-gel method with various H3PO4 amounts. The samples were calcinated at different temperature and charactered by XPS, ICP, XRD, SEM, Raman, FTIR, and UV-vis methods, so that the formation process of phosphate species could be inspected. The XRD results show that P species hinder the particle growth of anatase and increase the anatase-to-rutile phase transformation temperature to more than 900 °C, and a new titanyl phosphate, Ti5O4(PO4)4, was observed in P-doped TiO2 when calcined at 1000 °C. The UV-vis results indicate that the P species is likely to have two different states, leading to the variety of visible-light photocatalytic activity and band gap energy of P-doped TiO2. One state is the “separated phase”. In this state, the P/Ti ratio is very low so that the P species is surrounded by TiO2. The “separated phase” of P species introduces oxygen into TiO2 lattice and hence causes a red-shift of the adsorption band edge of anatase, leading to the increased visible-light photocatalytic activity of P-doped TiO2. The other state is the “congregated phase”. It appears at the micro region where the ratio of P/Ti is high enough to make the TiO2 clusters isolated by P species. The “congregated phase” of P species acts as the interface phase between TiO2 clusters and strongly retards the crystal growth of anatase, resulting in the widened band gap of P-doped TiO2. Furthermore, a possible mechanism was also proposed to explain the formation of the two phases during the sol-gel process. The results indicate that in order to improve the visible-light photocatalytic activity of P-doped TiO2 the percentage of “separated phase” in P species needs to be enhanced. Introduction TiO2 has been one of the most intensively studied and applied photocatalysts because of its high photocatalytic activity, photostability, low cost, and nontoxicity.1-9 However, the wellknown highly photoreactive anatase TiO2 has wide band gap (3.2 eV), which responds only under UV-light irradiation.8,9 In order to make better use of solar energy, many attempts have been made to sensitize TiO2 into the visible-light region, such as doping with nitrogen,10 sulfur,11 phosphorus,12-19 etc.20-22 It has been found that, comparing with other elements, phosphorus can significantly increase the specific surface area of TiO2 and prevent the phase transformation of anatase to rutile, resulting in the enhancement of the photocatalytic activity of TiO2. The mechanism of P doping has been subjected to a considerably smaller number of studies. Because of the various procedures and different preparation conditions, the results were highly diverse between different researchers. Yu et al.12 reported that the structural stability and photocatalytic activity of mesoporous TiO2 increased due to the phosphate modification. The positive effects were explained by the inhibition of crystal growth, the extended band gap energy, large surface area, and the existence of Ti-O tetrahedron in P-doped TiO2. Whereas other researchers only found Ti-O octahedron in bulk P-doped TiO2 by XPS analysis.13,14,17 Shi et al.13 observed a new impurity energy level (around 450 nm) in UV-vis spectra of P-doped TiO2, and the adsorption band edge in this region showed a red-shift with the increasing P content. The photocatalytic * To whom correspondence should be addressed. Phone: 86-043188502234. Fax: 86-0431-88502234. E-mail: [email protected] or [email protected].

enhancement of P doping was attributed to the increased surface area, the narrowed band gap, and the decrease in the recombination rate of photogenerated electrons and holes. In contrast, Ko˜ro¨si et al.16,17 found a blue-shift of adsorption edge in P-doped TiO2. They ascribed the higher band gap energy of P-doped TiO2 to the quantum-size effect and the chemisorbed phosphate. The anatase-to-rutile phase transformation was also inhibited because the surface-bound phosphate hindered the crystallite growth of anatase. Another interesting idea was brought out by Yu et al.18,19 They suggested that phosphorus dissolved in anatase and formed an interstitial solid solution. These structural defects protected the anatase phase of TiO2 at low temperature. When the temperature was extremely high (>900 °C), the P species inside anatase began to migrate to the surface of TiO2 and caused the anatase-to-rutile phase transformation. But they did not show the details about the migration process of phosphorus in the solid solution. Although the experimental phenomena and the explanations seem discrepant and complicated, their consistent viewpoint confirms that the surface phosphate-like species shows dominant effect on the visible-light photocatalytic activity of P-doped TiO2. Therefore, in order to reveal the anfractuous effect of P species in TiO2, further studies should be carried out to understand the formation process and the properties of the surface congregated phosphate. Therefore, in this article P-doped TiO2 nanoparticles with various phosphorus contents were synthesized by the sol-gel method and their visible-light photocatalytic activities were examined. Furthermore, a possible mechanism was carried out to explain the relationship between the P/Ti ratio and the visible-light photocatalytic activity of P-doped TiO2.

10.1021/jp902558z CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

Visible-Light Photocatalytic Activity of P-Doped TiO2

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TABLE 1: Chemical Composition of P-Doped TiO2 Samples Determined by ICP, FTIR, and XPS Analyses sample name

P/Tiatom ratio calculated

P/Ti atom ratio ICP

P/Ti atom ratio FTIRa

P/Ti atom ratio XPS

P2-400 P5-400 P10-400 P20-400 P50-400

0.02 0.05 0.10 0.20 0.50

0.0163 0.0514 0.1061 0.2299 0.5177

0.0215 0.0567 0.0938 0.2003 0.5203

0.0684 0.1411 0.1992 0.3636 1.0508

a This P/Ti ratio was determined by the intensity ratio between P-O vibration band (at 1043 cm-1) and the anatase main band (at 460 cm-1) in FTIR spectra.

Experimental Section Catalyst Preparation. A certain amount of Ti(OC4H9)4 and absolute ethanol were mixed by stirring. The obtained mixture was added dropwise to H3PO4 solution with various concentrations during vigorous stirring. The volume proportion between Ti(OC4H9)4, ethanol, and H2O is 1:6:8. The suspension was stirred for 4 h, followed by aging at room temperature for 72 h. The obtained gel was dried under infrared ray irradiation. Then the xerogel was ground into fine powders and calcined at various temperatures for 2 h. The resultant powders were washed by deionized water 5 times and then dried at 120 °C. For comparison, pure TiO2 was also prepared by the same procedure without the addition of H3PO4. The symbols: Pxx-yyy will be labeled hereafter, where xx describes the molar percentages of P/Ti in the precursors and yyy is the calcination temperature. For example, P5-400 represents the P-doped TiO2 whose calculated molar ratio of P/Ti is 5% and whose calcination temperature is 400 °C. In order to describe the samples expressly, the sample names and their corresponding P/Ti ratios are also summarized in Table 1. Sample Characterization. The crystal structures of the samples were examined by an X’Pert PRO X-ray diffractometer (XRD; PANalytical, Netherlands). The specific surface areas were measured by N2 adsorption at 77K using NOVA-1000e (Quantachrome, Boynton Beach, FL). The bond vibrations were analyzed by FTIR spectrometer (Nexus 6700, Thermo, Waltham, MA) and Raman spectrometer (Invia, Renishaw, UK, λ ) 632.8 nm). The UV-vis spectra were recorded in a UV-3600 UV-vis-NIR spectrophotometer (Shimadzu, Japan). The SEM images were taken by Quanta 200 (FEI, Hillsboro, OR). The Ti and P contents in various P-doped TiO2 were determined by ICP spectrometry (VISTA-MPX, Varian, Palo Alto, CA). The XPS analysis was taken by ESCALAB 250 (Thermo, Waltham, MA). Photocatalytic Activity Determination. Methyl orange (MO) was chosen as the target pollutant for photocatalytic activity tests, since it was a typical azo dye and difficult to degrade. During the photocatalytic degradation experiments, 10 mg of photocatalyst (P-doped TiO2 or pure TiO2) was added into 10 mL of MO aqueous solution, whose initial concentration was 20 mg/L. The solution was irradiated by white light lamps (2 × 150 W, Philips, major wavelengths in the range 400-700 nm). The concentration of MO was monitored by T6 spectrophotometer (Pgeneral, China) when the irradiation time is up to 1, 2, 3, and 4 h. The decolorization rate (R) of MO was calculated by R ) dC/dt ) C0/A0 · dA/dt. A0 is the initial absorbance value of MO solution at 463 nm, C0 is the initial concentration of MO, and C0/A0 is a constant. Therefore, R was determined by the slope of A vs t for the first 4 h. It should be note that the slight dark absorbance of various samples were

Figure 1. XRD patterns of TiO2 and P-doped TiO2 calcined around the anatase-to-rutile phase transformation temperature. The symbol “P” denotes the titanyl phosphate formed at high temperature.

also monitored, and were deduced from the corresponding A-t photodegradation curves before calculation. Results and Discussion Concentrations of P Species Distributed in TiO2. Table 1 summarizes the P contents of P-doped TiO2 determined by various methods. As shown, the P/Ti ratios determined by ICP and FTIR analyses are identical with those of calculated ones, indicating the added phosphoric acid reacts completely with titania at the applied preparation conditions. However, the XPS results display higher values than other methods, indicating the surface P content is much richer than the body content, and the origins of such difference will be discussed in the following text. Moreover, the ratios of (P/Ti)surface and (P/Ti)total decrease with the increasing total P content, which is consistent with the previous report.17 XRD Studies. Figure 1 shows the XRD patterns of TiO2 and P-doped TiO2 calcined at different temperatures. As shown, the initial temperature of anatase-to-rutile phase transformation for pure TiO2 and P-doped TiO2 is around 600 and 900 °C, respectively. Moreover, considering different amount of rutile phase in P2-900, P5-900, and P10-900, the anatase-to-rutile phase transformation is further delayed when the total content of P is relatively high in P-doped TiO2. It is evident that the anatase-to-rutile phase transformation is remarkably inhibited by P species. All of the P-doped TiO2 samples transform to rutile phase at 1000 °C, and a new titanyl phosphate species comes into being at the same time. In previous studies, the observed hightemperature phosphate in P-doped TiO2 is TiP2O716,17,19 or (TiO)2P2O7.11,19 Whereas, in this study, the appeared phosphate (P50-1000) shows four main XRD peaks at 2θ ) 27-29° (27°, 27.4°, 27.8°, and 28.4°, respectively). The peak at 2θ ) 27.4° indentifies the rutile phase.16 The other three peaks correspond to the main peaks of (TiO)2P2O719 or Ti5O4(PO4)4.23 The less intense reflection peaks (at 23.1°, 24.7°, 25.3°, 30.4°, 34.4°, 44.8°, 55.7°, 57.5°, 58.6°, and 73°, respectively) also could not distinguish the two species from each other. As known, the color of (TiO)2P2O7 is white and Ti5O4(PO4)4 is yellow.24 In this study, the colors of P-doped TiO2 samples calcined at 1000 °C are all

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Li et al. TABLE 2: Crystallographic Parameters of P-Doped TiO2 Determined by (101) XRD Peak Position of the Anatase Phase

Figure 2. Specific surface areas and average crystallite sizes (inserted) of the samples calcined at various temperature. The average crystallite sizes are calculated from the broadening degree of the (101) XRD peak of anatase phase.

Figure 3. SEM images of various P-doped TiO2 calcined at 500 °C. (A) P2-C500; (B) P10-C500; (C) P20-C500; (D) P50-C500.

yellow, and similar phenomena are also reported by other researchers.12,14 Therefore, the obtained high-temperature phosphate phase is considered to be Ti5O4(PO4)4, which is an intergradation phase in the phase diagram of TiO2/TiP2O7/ TiPO4.24 Thus, the P50-1000 might still have a small amount of anatase phase determined by the peak at 2θ ) 25.3°. The specific surface areas and average crystallite sizes of the samples calcined at various temperatures are summarized in Figure 2. As shown, with the increase of calcination temperature, the measured specific surface areas oppose to the calculated crystallite sizes, indicating the crystallite growth and agglomeration of anatase during calcination. Comparing with pure TiO2, P-doped TiO2 has larger specific surface area and smaller crystallite size. Moreover, the SEM images (shown in Figure 3) also suggest that the interstitial spaces between P-TiO2 particles increase with the total P content, consisting with the results of specific surface area. In addition, XRD studies (Figure 2 inserted) indicate that the crystallite sizes of P-doped TiO2 are smaller than 16 nm at temperatures below 700 °C, and decrease with the increasing P content. That means the P species markedly slows down the crystal-growth rate of anatase. As reported, in nano-TiO2 film where the particle growth was restricted in two dimensions, the anatase-to-rutile phase transformation was delayed to 1220-1320 K.25 Such transformation

sample

a (Å)

b (Å)

c (Å)

volume of cell (Å3)

P2-400 P5-400 P10-400 P20-400 P5-300 P5-500 P5-700 P5-900

3.78 3.787 3.8 3.785 3.784 3.79 3.788 3.79

3.78 3.787 3.8 3.785 3.784 3.79 3.788 3.79

9.44 9.46 9.44 9.514 9.458 9.47 9.503 9.513

134.58 135.68 136.04 136.3 135.48 136.25 136.37 136.65

temperature region seems similar to our P-doped TiO2. Therefore, it is creditable to attribute the delay of anatase-to-rutile transformation to the considerably inhibitive effect of P species on the crystal growth of anatase. Table 2 lists the crystallographic data of anatase in various samples. At identical calcination temperature (400 °C), the unit cell volume expands with the increase of total P content, and similar results are also observed by Yu.19 Moreover, slight lattice expansion also appears with the increase of calcination temperature. As XPS studies revealed,12-14,17 in P-doped TiO2 phosphorus existed in P5+ state and no Ti-P bonds were found, indicating the replacement of Ti4+ by P5+ in the TiO2 lattice. Considering that the ionic radii of P5+ (0.35 Å) is much smaller than Ti4+ (0.68 Å),19 the replacement of Ti4+ by P5+ should lead to shrinkage of unit cell based on a geometric point of view, which is opposite to the experimental results. Therefore, the observed expansion of unit cell volume might ascribe to the redundant oxygen introduced by P5+ for its electrostatic balance. Raman Analysis. Apart from XRD, Raman measurements were carried out. As shown in Figure 4, below 900 °C P-doped TiO2 (P5) is in pure anatase phase and completely transforms to rutile at 1000 °C. With the increase of calcination temperature, the Raman peaks become sharp and intensified, and the anatase main peak (152 cm-1, Eg) shifts to a lower position. This is due to the rise of particle size during calcination. It has been pointed out that decrease in particle size would lead to blue shift and decrease of Raman peaks, which is much more pronounced for 142 cm-1 of anatase.26,27 Therefore, the Raman spectra indicate that the anatase crystallite particles grow and congregate with increasing temperature, which is consistent with the XRD results mentioned above. New Raman peaks at 1035, 1047, and 1073 cm-1 are observed at 900 °C, which correspond to P-O asymmetric and symmetric stretching vibrations,28,29 indicating the generation of titanyl phosphate. These peaks are very weak and show no corresponding crystal structures in XRD patterns, suggesting that the phosphate is tiny and poor crystalline. When the calcination temperature rises to 1000 °C, these new peaks increase and split with the increasing content of P. When the ratio of P/Ti is 10% (P10-1000), additional peaks appear at 277, 310, 347, 981, 1014, and 1098 cm-1, respectively. In the case of the highest total P content (P50-1000), the Raman spectrum is dramatically changed. The main peak at 722 cm-1 corresponds to the infinite chains of -Ti-O-Ti-O-Ti- in titanyl phosphate structure,28,29 which is consistant with the XRD results. FTIR Spectra. Figure 5 displays the FTIR features of P-doped TiO2. For the unheated samples (Figure 5A), bands at 1626 and 3420 cm-1 are assigned to the δ(H-OH) and O-H stretching vibrations, respectively.10,12,14,16,17,28 The intensities of these bands increase with total P content, and similar phenomena

Visible-Light Photocatalytic Activity of P-Doped TiO2

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Figure 4. Raman spectra of TiO2 and P-doped TiO2 calcined at various temperatures. The symbol “P” denotes the titanyl phosphate formed at high temperature.

Figure 5. FTIR spectra of P-doped TiO2 with various P content calcined at 120 (A), 400 (B), 900 (C), and 1000 °C (D), respectively.

are also reported,12,14,16,17 indicating that P species improves the water adsorbing ability of TiO2 due to its large surface area.10,12 Comparing with anatase, P-doped TiO2 display a new single peak at 1043 cm-1, and its intensity is proportional to the total

content of P. Previous studies have assigned this peak to the vibrations of Ti-O-P bond12,14 or PO4 groups.15,17,18,30 However, the band shape of this peak differs from the P-O vibrations in H3PO4 aqueous solution,18,31,32 inorganic phosphate,28,30,33-35 and

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Figure 6. UV-vis spectra and the λg of P-doped TiO2 with various P content at different states.

even the PO4 group adsorbed on TiO2.31,32 As reported,18,28,31-33 the P-O stretching vibrations show three strong peaks at 900-1190 cm-1. The observed band (1043 cm-1) is ascribed to the Vas(P-O) of the phosphate unit,28,32 but the Vs(P-O) at 960 cm-1 and Vas(P-O) at around 1100 cm-1 are missing. Considering no PdO bond (1260-1450 cm-1) is found,12,14,34 it is reasonable to ascribe the single band at 1043 cm-1 to the P-O vibration mode in Ti-O-P matrix. In addition, its single band shape also indicates that, comparing with ordinary PO4 groups, the P-O ligands in anatase must be identical and increased in symmetry. The broad shoulder around 818-693 cm-1 corresponds to the infinite chains of -Ti-O- in the structure.28,33 Thus, the intensity ratio between this shoulder band to anatase main band (at 460 cm-1)18 reflects the cross-linking degree of -Ti-Obonds in the sample. When calcined at 400 °C (Figure 5 B), this intensity ratio increases, indicating the aggregation of Ti-O ligands during heat treatment. At the same time, during the course of calcination from 120 to 400 °C, the increase degree of this intensity ratio is in inverse proportion to the total content of P. That means the P species inhibits the junction of Ti-O ligands in the structure, which is responsible for the decrease in the crystalline degree and particle size of anatase (as shown by XRD analysis). Also, such junction inhibition might be the reason for the restraining effect of P species on the anatase-torutile phase transformation. Moreover, since the growth of Ti-O chain is broken by P, the P species would appear at the breakpoints of Ti-O chains. These breakpoints of Ti-O chains are the main participators in the formation of surfaces or interfaces of TiO2; therefore, a greater amount of P would be observed on the surface, which might be the reason for the higher P/Ti ratio detected by XPS than by ICP method. When the calcination temperature increases to 900 °C (Figure 5 C), the band shape of P-O bond is significantly changed. Except for P2-900, all of the samples display Vs(P-O) at 958 cm-1 and Vas(P-O) at 1118 cm-1, respectively.28,33 That means the coordination conditions of P in TiO2 have been changed into ordinary PO4 forms during calcination, indicating the formation of phosphate as confirmed by Raman spectra. Moreover, new adsorption bands at 1213 and 926 cm-1 are observed in P20-900 and P50-900, which are ascribed to the asymmetry stretching vibrations of bridging PO2 and P-O-P bond, respectively.32 And the symmetry stretching vibration of P-O-P at 752 cm-1 also appears in P50-900.12 These results indicate that the cross-linking between P-O ligands comes into effect at 900 °C especially when the total P content is higher than 10%. In addition, the P-O peak position of P20 and P50 shifts to higher value (1064 cm-1) due to the effect Ti.33 All of the above facts indicate that the coordination and position of P

Figure 7. Photocatalytic activities of different P-doped TiO2 calcined at various temperatures.

species in anatase are remarkably changed with the increase of total P content and calcination temperature. As shown in Figure 5 D, all of the samples transform to rutile (515 cm-1)18 at 1000 °C, consistant with the XRD and Raman results. In P10-1000 and P50-1000, the position of P-O-P stretching shifts to lower wavenumber (921 cm-1) due to the increase length of -P-O- chains,32 indicating the further crosslinking between P-O ligands. The original P-O bond vibration (1043 cm-1) in P2 and P5 splits and shifts to 1028 and 991 cm-1 at 1000 °C. The PO2 group (1229 cm-1) is also observed in P5-1000, identifying the generation of new phosphate with low polymerization degree. UV-Vis Comparison. The UV-vis diffuse reflectance spectra of P-doped TiO2 calcined at 400 °C are displayed in Figure 6 (the left graph). The adsorption band edge (λg) shifts to red with increasing P content except for P50-400, due to the lattice expansion of anatase. As our XRD studies revealed (Table 2), the unit cell volume of anatase expands with increasing calcination temperature and total P content. In addition, previous studies11,15,16,18 have shown that the λg of P-doped TiO2 displayed red shift with the increase of calcination temperature, indicating the relationship between the red shift of λg and the lattice expansion of anatase. Therefore, considering that the unit cell volume expands with increasing P content, it is reasonable to ascribe the observed red shift of λg to the aberration of anatase lattice caused by P doping. However, when the total content of P is too high (P50-400), the λg reverses to the lowest value indicating the widened band gap. There are two imaginable reasons for such distinct blue-

Visible-Light Photocatalytic Activity of P-Doped TiO2 shift of λg. The first one is the quantum-size effect. As known, the value of λg displays blue-shift with the decrease of cluster size especially for the particles smaller than 10 nm.12,17 Such size region is comparable to the calculated average crystallite sizes of P-doped TiO2 calcined at 400 °C (as shown in Figure 2, inset). At the same time, as revealed by FTIR, the TiO2 clusters are separated by P species especially when the total content of P is higher than 20%. Thus, it is evident that in sample P50-400 the nanosized TiO2 particles are surrounded by P species, resulting in the quantum-size effect and hence the blueshift of λg. The other reason is the larger band gap of the surface covered P species especially when the calcination temperature is high. As revealed by XPS, the P/Ti ratio on the surface of P50-400 is up to 1.05, which is high enough to reach the P/Ti level of some phosphates, such as the (TiO)2P2O7 and Ti5O4(PO4)4. At such low calcination temperature (400 °C), these surface covered P species are in poor crystallization and the organizing conditions of P-O ligands are different from ordinary PO4 groups of phosphates (according to the XRD and FTIR results). Therefore, we name them as phosphate-like materials. If the calcination temperature is increased high enough, these phosphate-like materials will convert into titanyl phosphates inevitably. The as-formed titanyl phosphates, such as (TiO)2P2O7 and Ti5O4(PO4)4, usually have larger band gap than TiO2,36 leading to the blue-shift of λg. The wet gels of P-doped TiO2 are also investigated, and the band edges of P-doped TiO2 in two different states are compared in Figure 6 (the right graph). Unlike the calcined powder, the λg of wet gel increases monotonously with the total P content, suggesting that before dehydration the effect of P species is simply red shift of λg. When the total P content is higher than 20%, the red-shift degree grows much slower, and excessive P only shows very tiny red-shift effect. Such remarkable differences between wet gel and calcined powder of P-doped TiO2 attribute to the absence of titanyl phosphates-like material in the wet gel. As known, the wet gel has enough water to dissolve P species; thus, the P amount on TiO2 matrix of wet gel is insufficient to encapsulate TiO2 clusters, leading to the absence of quantum-size limitation and the lack of blue-shift effect. Therefore, in the calcined powders of P-doped TiO2, the observed band edges are the cooperating results of two opposite effects (red-shift effect and blue-shift effect). In view of the final efficacy of calcined powders, when the total P content is low enough, the P species is isolated in the body of TiO2; thus, the main effect of P doping is the expansion of anatase lattice and red-shift of λg. While when the total P content is extremely high, the P species tends to link with each other and forms phosphatelike material in between TiO2 particles, leading to the quantumsize limitation and the widened band gap of TiO2. In our applied synthetic conditions, the critical content of P species for this turning point is around 20% as revealed in Figure 6. If the total P content keeps increasing, the blue-shift effects would surpass the red-shift effects, just like P50-400, resulting in the observed blue-shift of λg. Supposing that the above two diverse effects of P doping are two distinct P phases, then the original difference between the two phases is the content of P species. On the one hand, when the P/Ti ratio is lower than a critical value, the P-O ligands are surrounded or separated by Ti-O ligands, and no P-O-P bonds are observed (as FTIR studies revealed). Because of the electrostatic diversity, P species introduce oxygen into anatase lattice (as XRD studies revealed), leading to the redshift of λg. Therefore, we call such a phase the “separated phase” of P species, which is sensitive under visible light. On the other hand, when P/Ti ratio oversteps the critical value, the P-O

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18139 ligands are so close to each other to generate P-O-P chains during calcination (as FTIR studies shown), resulting in the congregation of P species. These phosphate-like materials tend to aggregate onto the surfaces or interfaces of TiO2 (as revealed by XPS and ICP analysis) and form interface phase in between TiO2 particles. Therefore, we entitle this phase as “congregated phase” of P species, which displays blue-shift effect to λg of P-doped TiO2. Both of the two phases bring structural defects into TiO2, resulting in the resistance in crystal growth of anatase and the delay of anatase-to-rutile phase transformation. Photocatalytic Activities. The visible light photocatalytic activities of P-doped TiO2 are determined by the decoloration rates (R) of MO at the initial 4 h. As shown in Figure 7, the photocatalytic activity of P-doped TiO2 varies with the calcination temperature and the total P content. When the calcination temperature increases to 1000 °C, the photocatalytic activity of P-doped TiO2 drops sharply due to the anatase-to-rutile phase transformation (taking into account XRD, Raman, and FTIR results). Moreover, for sample P20 and P50, the photocatalytic activity of anatase is in inverse proportion to the total P content because of the bloom of “congregated phase”. Under such a high P/Ti ratio, the P species is prone to be “congregated phase”, which shows no visible-light activity (as revealed by UV-vis analysis), leading to the decrease in photocatalytic degradation rate of P-doped TiO2. However, samples with relatively low content of P show random photocatalytic activities and display no accordance with the XRD, Raman, FTIR, or UV-vis results. This might attribute to the different ratio between “separated phase” and “congregated phase” of P species. According to XPS, ICP, and FTIR analyses, the surface P/Ti ratio is much higher than inner, indicating that the P species might form “congregated phase” on the surface and leave the “separated phase” inside. With increasing calcination temperature, the regulation of Ti-O lattice takes place gradually, and the P species also begins cross-linking (as FTIR revealed), indicating the possible conversion of “separated phase” into “congregated phase”. Thus, the absolute amount and the ratio between the two phases would alter obviously with the variation of calcination temperature. As discussed, only “separated phase” has visible-light catalytic activity, and the surface conditions controlled by “congregated phase” are also important for the photocatalytic process, both of which are responsible for the complicated final performance of P-doped TiO2. Proposed Mechanism. On the basis of the above results, a possible mechanism is proposed in Scheme 1. When dropped into water, the Ti(OC4H9)4 begins to hydrolyze rapidly and forms Ti(OH)4. Then the Ti-OH aggregates with Ti-OH or P-OH and forms Ti-O-Ti or Ti-O-P bonds as follows (shown in Scheme 1 A)

Ti-OH + HO-Ti- f Ti-O-Ti + H2O P-OH + HO-Ti- f P-O-Ti + H2O Since no PdO bond is observed in FTIR spectra, the PdO must also have been reacted with Ti-OH as follows (shown in Scheme 1 B)

≡P ) O + HO-Ti≡ f ≡P - OH LO-Ti≡ ≡P-OH + HO-Ti≡ f ≡P-O-Ti ≡ +H2O LO-Ti≡ LO-Ti≡ Thus, in the TiO2 matrix, the fresh TiO2 clusters are joined by P5+, as shown in Scheme 1B. It should be note that, at this

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SCHEME 1: Proposed Mechanism for the Assembling Conditions of P Species in TiO2 during the Sol-Gel Processa

a

9 denotes the microcosmic region where the P/Ti ratio is relatively high.

point, only part of the P species could combine with TiO2. Other P species still dissolves in water, and its amount depends on the doping ratio of P/Ti and the concentration of H3PO4. Until the drying process, these free P species are forced to combine with TiO2 on the surface, leading to the higher P/Ti ratio on the surface than inner side. Before calcination, in spite of various doping amount of P, the P-O ligands display similar band shape and band position in FTIR spectra (Figure 5A), indicating the formation of a P-O-Ti bonds. However, in spite of uniform P-O-Ti bonds, the P-doped TiO2 displays different visible-light photocatalytic activities due to the different P/Ti ratio and distinct phases of P species. As mentioned above, if the microregion has very low P/Ti ratio, P-O ligands are prone to be isolated by TiO2; thus, the “separated phase” will form. The “separated phase” causes the lattice expansion of anatase and hence the red shift in λg of P-doped TiO2. When the P/Ti ratio is high enough, the TiO2 clusters will be encapsulated by P species and the “congregated phase” will be formed. The bloom of “congregated phase” forms the interface phase between TiO2 clusters, which inhibits the crystal growth and phase transformation of anatase, leading to the blue-shift of λg. The “separated phase” and “congregated phase” can coexist together in P-doped TiO2 and convert by increasing total P content. Since the surface P/Ti ratio is much higher than interior (as revealed by XPS, ICP, and FTIR analyses), when the “congregated phase” emerges at the surface, the “separated phase” might still remain inside. Furthermore, with increasing doping amount of P, the “separated phase” of P species in TiO2 clusters also increases. When the P/Ti ratio reaches a critical value, the amount of P-O ligands are so large that the P species is no longer to be encapsulated by Ti-O ligands, resulting in the decay of “separated phase” and the generation of “congregated phase”. Apart from total P/Ti ratio, the characters of sol-gel process, such as the cluster size and size distributions, also will affects the resultant phases of P species. For example, when the cluster size makes a great difference, the smaller cluster would be crushed into the interstice between larger clusters (as shown in Scheme 1C). At this microregion, the surfaces of different clusters are concentrated, leading to the higher P/Ti ratio. Thus, the content of P differs in the microcosmic regions even in the same synthetic system. That means the “separated phase” and “congregated phase” of P species are coexistent, in spite of the total P/Ti ratio in the sample. The only difference is that the “separated phase” is primary when the total P content is relatively low, and vice versa. Since only the “separated phase” shows visible-light activity, the percentage of “separated phase” in P species is a key factor in improving the photocatalytic activity of P-doped TiO2 under visible light. On the basis of the above discussions, there are at least two factors that affect the ratio between “separated phase”

and “congregated phase”, viz. the total P/Ti ratio and the size distributing of clusters in TiO2 sol. The above mechanism could further explain some experimental phenomena. A good case in point is the variety of hightemperature phases. Because of the differences in procedures, P/Ti ratio, and preparation conditions by different researchers, the resultant “congregated phase” at high temperatures is varied, such as Ti5O4(PO4)4, (TiO)2P2O7,11,19 and TiP2O7.16,17,19 Among those, our Ti5O4(PO4)4 shows the lowest P/Ti ratio. That means the P species is more homogeneous in our resultant samples, which benefits the increase of “separated phase” at low total P content. Unfortunately, all of these high-temperature phases show no visible-light activities,36 which hinders the photocatalytic performance of the overcalcined samples under visible light. In the previous studies, an interesting phenomenon is that P-doped TiO2 displays a red-shift of λg if titanyl procedure meets with water and P species synchronously,13,14,18 whereas the P-doped TiO2 displays a blue-shift of λg if the titanyl procedure is first hydrolyzed before the addition of P species.11,12,16,17 This could be explained as follows. The hydrolyzation speed of titanyl procedure is very quick, and the cross-linking of Ti-O network happens quite rapidly. If the titanyl procedure has already hydrolyzed in pure water, the later additional P species miss the chances of bonding with inchoate TiO2 clusters and could not be wrapped into TiO2 framework to form the “separated phase”. They could only bond to the surfaces of the aggregated TiO2 clusters and are prone to spread on the cluster surfaces and interfaces to form the “congregated phase”. Thus, the “congregated phase” is the dominating P species in such asprepared P-doped TiO2. As discussed, the “congregated phase” corresponds to the blue-shift of λg, while the “separated phase” acts inversely. Therefore, if the titanyl procedure is prehydrolyzed before the P doping, the P species appears to cause a blueshift of λg and widened band gap of anatase. Conclusions P-doped TiO2 were synthesized by the sol-gel method with H3PO4 additions. Considering the great difference in P/Ti ratio, the conditions of P species in TiO2 matrix are defined as two different states, namely the “separated phase” and “congregated phase”. In the “separated phase”, the P species is encapsulate by Ti-O ligands. The “separated phase” emerges at the microcosmic regions where the P/Ti ratio is relatively low and acts as dominating P species when the total P content is lower than 20%. Because of the stronger electrostatic force and the smaller ionic radii of P, the “separated phase” introduces oxygen into TiO2 lattice and causes a red-shift of λg. The “separated phase” also produces structural defects in the TiO2 matrix, which hinders the crystal growth of anatase. All of these effects increase the photocatalytic activity of P-doped TiO2 under visible light.

Visible-Light Photocatalytic Activity of P-Doped TiO2 On the other hand, when the P/Ti ratio in the microcosmic regions surpasses a crucial value, TiO2 clusters are surrounded by P species, and hence the “congregated phase” is formed. It is the dominating P species when the total content of P is relatively high. The “congregated phase” hinders the enlarging of Ti-O chains during calcination and forms the interface phase between TiO2 clusters. Since the crystal growth of anatase is retarded strongly, the superabundant “congregated phase” causes the higher specific surface area of TiO2 and the delay of anataseto-rutile phase transformation. Furthermore, the quantum-size effect brought by “congregated phase” leads to the blue-shift in λg of anatase. When the calcination temperature increases, the P species in “congregated phase” begins cross-linking and forms new titanyl phosphate at high temperature, such as Ti5O4(PO4)4, (TiO)2P2O7, and TiP2O7 depending on the ratio of P/Ti in the “congregated phase”. Thus, the “congregated phase” is sensitive only under UV light. Therefore, the ratio between the “separated phase” and “congregated phase” in P species is a pivotal factor in dominating the photoresponse range and photocatalytic activity of P-doped TiO2. In order to improve visible-light photocatalytic activity of P-doped TiO2, a greater amount of “separated phase” is preferred. And effective measures to reduce the amount of “congregated phase” are as follows: the relatively low total P/Ti ratio, the homogeneous cluster size distributions in TiO2 sol, and the simultaneity of hydrolyzation process and P doping process. Acknowledgment. The authors gratefully thank the financially supports of National Natural Science Foundation of China (Grants No. 50574043 and 40772028). References and Notes (1) Kang, C.; Jing, L.; Guo, T.; Cui, H.; Zhou, J.; Fu, H. J. Phys. Chem. C 2009, 113, 1006. (2) Mahmoodi, N. M.; Arami, M. J. Photochem. Photobiol. B 2009, 94, 20. (3) Kansal, S. K.; Singh, M.; Sud, D. J. Hazard. Mater. 2008, 153, 412. (4) Kansal, S. K.; Singh, M.; Sud, D. Desalination 2008, 228, 183. (5) Mahmoodi, N. M.; Limaee, N. Y.; Arami, M.; Borhany, S.; Mohammad-Taheri, M. J. Photochem. Photobiol. A 2007, 189, 1. (6) Mahmoodi, N. M.; Arami, M.; Limaee, N. Y.; Gharanjig, K. J. Hazard. Mater. 2007, 145, 65.

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