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A Novel Near-Infrared Antibacterial Material Depending on the Upconverting Property of Er3+-Yb3+-Fe3+ Tridoped TiO2 Nanopowder Wenlan Wang,† Qingkun Shang,*,† Wei Zheng,‡ Hui Yu,† Xuejiao Feng,† Zhidan Wang,† Yabin Zhang,‡ and Guoqiang Li‡ Faculty of Chemistry, North-east Normal UniVersity, 5268 Renmin Street, Changchun 130024, China, and Jilin Academe of Fisheries Science, 1195 Kunshan Road, Changchun, 130033, China ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: June 30, 2010

A novel near-infrared antibacterial material, Er3+-Yb3+-Fe3+ tridoped TiO2 nanopowder, was synthesized and applied to kill Aeromonas hydrophia with a maximal bactericidal rate of 65% in 90 min under 980 nm laser irradiation. However, Fe3+-TiO2 or Er3+-Yb3+-TiO2 nanopowders obtained under the same conditions barely could kill Aeromonas hydrophia. The initial energy applied in the photocatalysis antibacterial process is supplied by the upconversion emission of Er3+ and Yb3+ and the codoping of Fe3+, which has intensified the light absorption intensity and enlarged the absorption range. The analyses of phase structure and the optical properties including XRD, UV-vis diffuse reflectance spectra, and upconversion photoluminescence spectra of Er3+-Yb3+-Fe3+ tridoped TiO2 nanopowder have been performed in detail to reveal the complex energy transfer and change mechanism. According to our experimental results, the mechanism has been demonstrated involving the upconversion emission, multistep excitation by means of intermediate energy level, capture and transfer electrons of Fe3+, and so on. The photocatalystic antibacterial function of Er3+-Yb3+-Fe3+-TiO2 under near-infrared instead of ultraviolet light is also reported. Because of the strong penetrability of near-infrared, this kind of material may be used in photodynamic therapy (PDT) to kill bacteria or tumors directly. 1. Introduction Titania (TiO2) has been extensively studied as a photocatalyst for applications such as water and air remediation, because of its relatively high photocatalytic actively, robust chemical stability, relatively low production costs, and nontoxicity. TiO2 is capable of decomposing a wide variety of inorganic and organic pollutants and toxic material in both liquid and gas phase systems.1,2 Recently, TiO2 has been applied to degrade biological contaminants such as viruses, bacteria, fungi, algae, and cancer cells.3,4 TiO2 has been regarded as a potential agent on photodynamic therapy (PDT).5,6 However, TiO2 is active only under near-ultraviolet irradiation due to its wide band gap energy of 3.0-3.2 eV. It is of interest to find a TiO2-based photocatalyst which is sensitive to visible light in order to make more efficient use of solar energy in practical applications. Various strategies have been pursued including doping with various ions such as transition metal, rare earth, noble metal, and nonmetal.7-10 Chio et al.11 evaluated the effects of 13 different metal ions doping on the visiblelight photoreactivity of TiO2. They found that visible-light photocatalytic activity was influenced by the fraction of rutile in M-TiO2, charge-carrier recombination, and interfacial chargetransfer rate constants. Recently, upconversion emission in terms of absorption of two or more lower-energy photons followed by emission of a higher-energy photon has attracted much attention due to available low-cost near-infrared laser diodes. Erbium(III) is suitable for the upconversion of infrared to visible light because of a favorable electronic level scheme with equally spaced, long* To whom correspondence should be addressed. Phone: +86- 43185099787. Fax: +86-43185099762. E-mail: [email protected]. † North-east Normal University. ‡ Jilin Academe of Fisheries Science.

lived excited states. Potential applications in optical devices, such as color display, optical data storage, biomedical diagnostics, and temperature sensors, have been developed on the basis of up-conversion emission of Er3+-Yb3+ codoped materials.12,13 Red and green upconversion emission in Er3+-Yb3+ codoped nano-TiO2 under 980 nm bump laser excitation had been reported.14-16 In this paper, we investigated the photocatalytic activity of Er3+-Yb3+codoped nano-TiO2 with upconversion emission property under 980 nm bump laser excitation. It is very beneficial to the application of TiO2 on medical fields such as photodynamic therapy (PDT), because infrared light can penetrate through the skin of animals and promote the photocatalytic reaction in subcutaneous tissue or muscle. In order to improve its photophysical response, Fe3+ ion was selected to dope with Er3+ and Yb3+ ions together. Our aim is to combine the effect of rare earth and transition metal ions to obtain a new nearinfrared antibacterial material. The results show that Er3+-Yb3+Fe3+ tridoped TiO2 nanopowder synthesized by the sol-gel method can be used to kill bacteria under 980 nm laser irradiation. This photocatalytic antibacterial property of Er3+Yb3+-Fe3+ tridoped TiO2 under infrared instead of ultraviolet light will widen its applications on photocatalytic fields and decrease the damage of ultraviolet light on human beings and our environment greatly. 2. Experimental Section 2.1. Preparation of Samples. Er3+-Yb3+-Fe3+ tridoped TiO2 nanoparticles were prepared from the controlled hydrolysis of tetra-n-butyl titanate (Ti(OBu)4). TiO2 sols were prepared by dropwise addition of 3.4 mL of Ti (OBu)4, which had been dissolved in 16.0 mL of absolute ethanol, into 1.0 mL of deionized distilled water containing 2.0 mL of acetic acid under vigorous stirring at room temperature. Then, the ethanol solution

10.1021/jp102320x  2010 American Chemical Society Published on Web 07/23/2010

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of Er(NO3)3, Yb(NO3)3, and Fe(NO3)3 in the molar ratio Er3+: Yb3+:Fe3+:Ti4+ ) 0.02:0.02:0.002:1 was slowly added. After continuously stirring for 90 min, the resulting transparent solution was let to age for 7 days and then vacuum-dried. The obtained powder was calcined at various temperatures from 950 to 1050 °C for 4 h under air. Er3+-Yb3+ codoped TiO2, Fe3+ single doped TiO2, and pure TiO2 nanoparticles were fabricated following the same procedure. Tetra-n-butyl titanate (analytical pure), Er(NO3)3 (99.95% purity), and Yb(NO3)3 (99.99% purity) were all purchased from Aladdin Company (China). 2.2. Characterization. Crystal structure patterns of samples were examined by X-ray diffraction (XRD) using a RigaKu D/max-rA Diffractometer with Cu KR radiation. UV-vis diffuse reflectance spectra (DRS) were obtained on a CARY 500 spectrophotometer. Upconversion emission spectra were measured with a Jobin-Yvon Lab Ram Raman spectrometer by using 980 nm semiconductor solid laser excitation. TEM images were recorded with TEM-2100F transmission electron microscopy. 2.3. Determination of Photocatalytic Antibacterial Activity. Photocatalytic antibacterial activities of the array of synthesized TiO2 samples, pure TiO2, Fe3+ single doped TiO2, Er3+-Yb3+ codoped TiO2, and Er3+-Yb3+-Fe3+ tridoped TiO2 nanopowders were quantified by measuring the concentration of bacteria. Aquatic pathogenic Aeromonas hydrophia supplied by Jilin Academe of Fisheries Science was chosen as experimental bacterial strain. A VA-1-DC-980 semiconductor solid laser was used as an emission lamp-house. Experiments were conducted at a constant temperature of 22 °C, pH 7.0, and 90 mw output power of irradiation light. The slurry of antimicrobial (1.0 mg/mL) and Aeromonas hydrophia (106cfu/ml) were mixed under stirring. Before irradiation, the suspension was stirred in the dark for 30 min to obtain a state of sorption equilibrium between bacteria with antimicrobial. Usually, antimicrobial adheres to the surface of bacteria due to physical interaction during mixing and stirring process. Time-sequenced sample aliquots were collected from the reactor during the time course of illumination for analysis of the population of Aeromonas hydrophia. A viable concentration of Aeromonas hydrophia was measured with the spreading plate method. The average of three parallel experiments was used to evaluate the photocatalytic activity of the sample. All solution and materials were sterilized by autoclaving beforehand. 3. Results 3.1. Structural Property. XRD patterns of pure TiO2, Fe3+TiO2, Er3+-Yb3+-TiO2, and Er3+-Yb3+-Fe3+-TiO2 calcinated at 1050 °C are shown in Figure 1. The phase structure of Fe3+TiO2 is the same as that of pure TiO2. XRD peaks corresponding to the rutile phase appear. It means the phase structure cannot be changed by doping Fe3+ ions. No obvious diffraction peaks that could be attributed to the metal-ion dopants were observed. The crystal structures of Er3+-Yb3+-TiO2 and Er3+-Yb3+-Fe3+TiO2 indicate a mixture phase of anatase and pyrochlore, rutile and pyrochlore, respectively. The appearance of the pyrochlore phase (Er2Ti2O7 or Yb2Ti2O7)15 suggests that Er3+ and Yb3+ have been fully integrated into the basic structure of TiO2. It can be assumed that Fe3+ ions are likely substituted in Ti4+ sites within TiO2 because the ionic radii of Fe3+ ion (0.64 Α0) are similar to those of Ti4+ (0.68 Α0). In contrast, Er3+ and Yb3+ ions are most likely to be found as dispersed metal oxides within the crystal matrix or dispersed on the surface of TiO2 because of the

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Figure 1. XRD patterns of (a) TiO2, (b) Fe3+-TiO2, (c) Er3+-Yb3+TiO2, and (d) Er3+-Yb3+-Fe3+-TiO2 calcined at 1050 °C.

Figure 2. XRD patterns of Er3+-Yb3+-Fe3+-TiO2 calcined at various temperatures: (a) 950 °C; (b) 1000 °C; (c) 1050 °C.

relatively large size difference between Er3+, Yb3+ (0.89 or 0.86 Α0) and Ti4+. The anatase-to-rutile (A-R) phase transformation of pure TiO2 normally occurs at more than 400 °C.17 It is observed that Er3+ and Yb3+ doping altered the temperature of the A-R phase transformation of TiO2. The inhibition by these dopants has been explained in terms of formation of Ti-O-Er or Ti-O-Yb bonds at the interface, since they could be located primarily on the surface of TiO2 because of the relatively large differences in the ionic radii resulting in inhibited crystal grain growth. Some previous studies reported that doping with Ce, La, or Y ions also inhibited the A-R phase transformation.18-20 XRD patterns of Er3+-Yb3+-Fe3+-TiO2 samples that were calcined at different temperatures are shown in Figure 2. Anatase remained as the dominant phase at 950 °C. A rutile peak at 2θ ) 27.5° appeared at 1000 °C. The rutile peak was clearly dominant at 1050 °C, while the anatase peak at 2θ ) 25.7° disappeared at this temperature. It implies that the anatase phase is substituted gradually by the rutile phase with increasing calcination temperature. At 1050 °C, the anatase phase cannot been seen. The pyrochlore phase appeared at 1000 and 1050 °C. The fraction of rutile, anatase, and pyroclore and average particle sizes determined

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TABLE 1: Structure and Optics Properties of Modified TiO2 Photocatalystic Antimicrobial undoped TiO2 or modified TiO2

phase composition %a

average sizeb (nm)

threshold wavelength of absorptionc (nm)

band gap energyd (eV)

TiO2-1050 °C Fe3+-TiO2-1050 °C Er3+-Yb3+-TiO2-1050 °C Er3+-Yb3+-Fe3+-TiO2-1050 °C Er3+-Yb3+-Fe3+-TiO2-1000 °C Er3+-Yb3+-Fe3+-TiO2-950 °C

rutile rutile 8.7% pyroclore + 91.3% anatase 8.7% pyroclore + 91.3% rutile 9.7% pyroclore + anatase + rutile anatase

29.6 35.5 29.4 33.6 28.1 21.5

431 527 400 473 439 429

2.88 2.35 3.10 2.62 2.82 2.89

a Calculated using the equation A% ) 100/(1 + 1.265IA/IR), where IR, IA, and IP are the intensity of the rutile peak at 2θ ) 27.4°, the anatase peak at 2θ ) 25.3°, and the pyroclore peak at 2θ ) 15.14°, respectively. b Determined by XRD using the Scherrer formula. c Calculated referring to the literature18 according to the UV-vis spectrum. d Calculated using the equation band gap (Eg) ) hc/λ, where Eg is the band gap energy (eV), h Plank’s constant, C the light velocity (m/s), and λ the wavelength (nm).

TABLE 2: Cell Parameters of TiO2 with Different Doping Ions under Different Calcination Temperatures rutilea

a

pyrocloreb

undoped TiO2 or modified TiO2

a)b

c

TiO2-1050 °C Fe3+-TiO2-1050 °C Er3+Yb3+-TiO2-1050 °C Er3+Yb3+Fe3+-TiO2-1050 °C Er3+Yb3+Fe3+-TiO2-1000 °C Er3+Yb3+Fe3+-TiO2-950 °C reference value of TiO2

4.58890 4.59796

2.96043 2.95960

4.59045 4.59210

2.95907 2.95928

4.593

2.959

anatasec

a)b)c

a)b

c

10.05877 10.05387 10.11570 10.05728 10.058

3.77979

9.51495

3.78230 3.81458 3.785

9.52469 9.24335 9.513

b

Ref: Natl. Bur. Stand. (U.S.) Monogr. 25, 7, 83 [1969]. Ref: Knop, Brissi et al., Can. J. Chem., 43, 2812 [1965]; Roth, J. Res. Natl. Bur. Stand. (U.S.), 56, 17 [1956]. c Ref: Natl. Bur. Stand. (U.S.) Monogr. 25, 7, 82 [1969].

through the widths of half-height in terms of the Scherrer equation are listed in Table 1. The average particle sizes were 29.6, 35.5, 29.4, and 33.6 nm for pure TiO2, Fe3+-TiO2, Er3+-Yb3+-TiO2, and Er3+-Yb3+-Fe3+-TiO2, respectively. As the calcination temperature increases, the particle size of Er3+-Yb3+-Fe3+-TiO2 changes from 21.5 to 33.6 nm. Referring to the phase composition of these

samples, we found that the dominant rutile phase may lead to the particle size increasing, whereas the anatase phase is just the opposite. Doping Fe3+ ions had a significant influence on the particle size increasing due to promoting rutile phase generation. The inhibition of anatase-to-rutile (A-R) phase transformation by Er3+ and Yb3+ doping decreases the particle size.

Figure 3. TEM images of (a) TiO2, (b) Fe3+-TiO2, (c) Er3+-Yb3+-TiO2, and (d) Er3+-Yb3+-Fe3+-TiO2 powders.

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Figure 4. UV-vis diffuse reflectance spectra for TiO2 samples calcined at 1050 °C: (a) Er3+-Yb3+-TiO2; (b) Fe3+-TiO2; (c) Er3+-Yb3+-Fe3+TiO2; (d) TiO2.

Table 2 shows the lattice parameters of TiO2 with different doping ions under different calcination temperatures. Compared with the reference value of rutile, anatase, and pyrochlore TiO2, the notable change of lattice parameters demonstrated that crystal lattices of the as-prepared samples were locally distorted by doping and calcination. TEM images of the samples of TiO2, Fe3+-TiO2, Er3+-Yb3+TiO2, and Er3+-Yb3+- Fe3+-TiO2 calcined at 1050 °C are shown in Figure 3. It can be found that particle agglomeration is obvious in pure TiO2 and Fe3+-TiO2. This agglomeration can be greatly improved by doping with rare earth ions. The particle sizes of these samples are also consistent with that of the XRD test. 3.2. UV-vis Absorption Property. Figure 4 shows the UV-vis diffuse reflectance spectra of TiO2, Fe3+-TiO2, Er3+Yb3+-TiO2, and Er3+-Yb3+-Fe3+-TiO2 calcined at 1050 °C. TiO2 and Er3+-Yb3+-TiO2 are characterized by sharp absorption edges at about 430 nm (Ebg ∼2.88 eV) and 400 nm (Ebg ∼3.10 eV). Compared to pure TiO2, a blue shift of absorption edge for Er3+Yb3+-TiO2 is exhibited, which may be considered as small size effect and clusters of nanomaterial. However, Fe3+-TiO2 shows extended absorption spectra into the visible region over the range 400-700 nm. It can be explained in terms of the excitation of 3d electrons of Fe3+ ion to the conduction band of TiO2, which is called metal to conduction band charge transfer, according to their respective energy levels. It may also originate from defects associated with oxygen vacancies that give rise to colored centers. Jina Choi11 concluded that the generation of new energy levels due to the injection of impurities within the band gap coupled with the generation of oxygen vacancies by metal ion doping may contribute to the observed visible-light absorption of the M-TiO2 samples. The 473 nm absorption edges of Er3+-Yb3+-Fe3+-TiO2 shows a red shift relative to 431 nm of pure TiO2 but are not up to 527 nm of Fe3+-TiO2. It indicates that rare earth ions may partly counteract the effect of Fe3+ ion on the absorption property of TiO2. In other words, doping Fe3+ had a great influence on the absorption properties of TiO2 although rare earth existed simultaneously. In addition, some absorption peaks in the visible region such as 486, 521, 650, 795, and 974 nm were assigned to 4F7/2 f 4I15/2, 2H11/2 f 4I15/2, 4 F9/2 f 4I15/2, 4I9/2 f 4I15/2, and 4I11/2 f 4I15/2 transitions of Er3+ ions, respectively. The absorption on 974 nm is also assigned to the transitions of 2F5/2 f 2F7/2 of Yb3+ ion.14 It was enhanced by adding Fe3+ ions. From the absorption spectra of Fe3+-Er3+-

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Figure 5. UV-vis diffuse reflectance spectra for Er3+-Yb3+-Fe3+-TiO2 calcined at (a) 950 °C, (b) 1000 °C, and (c) 1050 °C.

Figure 6. Upconversion photoluminescence spectra for Er3+-Yb3+TiO2 under 980 nm excitation calcined at (a) 950, (b) 1000, and (c) 1050 °C.

Yb3+-TiO2 at different calcination temperatures shown in Figure 5, we can find that the absorption on 974 nm increases as the calcination temperature increases. The absorption band edge and band gap energy of these samples are calculated according to their absorption spectra and listed in Table 1. 3.3. Upconversion Photoluminescence Property. Figure 6 shows the upconversion photoluminescence spectra of Er3+Yb3+-TiO2 obtained at different calcination temperatures. A 980 nm diode laser is used as the excitation source. The green emission at 520-570 nm and red emission at 640-690 nm can be found clearly. The red emission is stronger. According to the energy level diagram of Er3+ and Yb3+ ion, they are assigned to the transitions (2H11/2,4S3/2) f 4I15/2 and 4F9/2 f 4I15/2 or 4 F7/2 f 4I13/2 for Er3+ in TiO2 nanocrystals, respectively. In addition, a small blue emission at 486 nm can be seen also, which derived from the 4F7/2 f 4I15/2 transition of Er3+. The intensities of upconversion emission increase with an increase of the calcination temperature from 950 to 1000 °C and then decrease from 1000 to 1050 °C. Figure 7 shows the upconversion photoluminescence spectra of Er3+-Yb3+-Fe3+-TiO2 excitated by a 980 nm laser. The emission intensity decreases as the calcination temperature

A Novel Near-Infrared Antibacterial Material

Figure 7. Upconversion photoluminescence for Er3+-Yb3+-Fe3+-TiO2 under 980 nm excitation calcined at 950, 1000, and 1050 °C.

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13667 Fe3+-TiO2 prepared at the same temperature. However, the bactericidal effect of Er3+-Yb3+-Fe3+-TiO2 is obvious. More than 40% bacteria are killed after 40 min radiation with a 980 nm laser. The maximal bactericidal rate is 65% in 90 min. It also can be seen in Figure 9 that the spots (Aeromonas hydrophia) number decreased after 40 min. The bacterial survival ratio changes little in the presence of Er3+-Yb3+-Fe3+-TiO2 or 980 nm laser light solely. This indicated that the bactericidal action of Er3+-Yb3+-Fe3+-TiO2 cannot take place without the existence of light. Fe3+-TiO2 has no bactericidal action because it cannot absorb 980 nm light. Due to the recombination of photoelectrons and photoholes to produce red and green upconversion emission for Er3+-Yb3+-TiO2, no energy can be absorbed by the TiO2 matrix to kill bacteria. Compared to Fe3+-TiO2 and Er3+-Yb3+TiO2, the absorption at 980 nm light for Er3+-Yb3+-Fe3+-TiO2 is the strongest but the upconversion emission almost disappeared. This suggested that most of the excited electrons derived from upconversion emission have been eliminated or trapped by means of a nonradiative process including a photocatalystic antibacterial process. 4. Energy Transfer Mechanism

Figure 8. Bacterial survival rate of different conditions: (a) 980 nm light + Er3+-Yb3+-Fe3+-TiO2; (b) 980 nm light + Fe3+-TiO2; (c) 980 nm light + Er3+-Yb3+-TiO2; (d) Er3+-Yb3+-Fe3+-TiO2; (e) 980 nm light.

increases from 950 to 1050 °C. The green and red emission of Er3+ can be observed at 950 °C only. They almost disappeared at 1000 and 1050 °C. It seems that doping Fe3+ ions lead to a quenching of upconversion emission.11 One possible explanation is excited electrons of Er3+ are trapped by Fe3+ or TiO2 when they return to the ground state, which inhibited the recombination of photoproduced electrons and holes and caused the energy self-absorption of Er3+-Yb3+-Fe3+-TiO2. On the other hand, the quenching of luminescence is probably related to the phase composition of Er3+-Yb3+-Fe3+-TiO2. When the anatase phase is unique for the sample calcinated at 950 °C, the upconversion emission is significant. For samples calcinated at 1000 and 1050 °C, rutile and pyrochlore phases appeared and even substituted anatase phase. The mixed phase composition results in more crystal defects and oxygen vacancies which may capture more photoelectrons and thus reduce their recombination with photoinduced holes.21 3.4. Photocatalytic Antibacterial Activity. We used Er3+Yb3+-Fe3+-TiO2 calcined at 1050 °C to kill Aeromonas hydrophia under 980 nm laser excitation. Figure 8 shows the survival ratio of Aeromonas hydrophia at different times. It can be seen that the bacteria almost cannot be killed by Er3+-Yb3+-TiO2 and

Figure 10 shows the probable energy transfer mechanism for Er3+-Yb3+-Fe3+-TiO2 in photocatalystic sterilization under 980 nm excitation. According to its UV-vis absorption property, the photocatalytic reaction can happen only under irradiation less than 473 nm. The upconversion emission of Er3+ is at 520-570 and 640-690 nm, which cannot excite the electrons on the valence band of TiO2. It seems impossible for the photocatalytic reaction. However, the results of the photocatalytic bactericidal experiment are indubitable. This suggests that there must be some intermediate energy levels in the band gap of Er3+-Yb3+-Fe3+-TiO2. They are introduced as the replacement of Ti4+ by Fe3+ in the lattice.11 Considering the energy of green and red upconversion emission, we estimate these energy levels may be located at 2.18-2.39 or 1.80-1.94 eV over the valence band of TiO2. Combined with the visible absorption property of Fe3+-TiO2 (listed in Table 1), the intermediate energy level may lie at 2.35 eV. Thus, electrons on the valence band of TiO2 can be excited by green or red upconversion emission of Er3+ to these intermediate energy levels first, and then excited to the conduction band of TiO2. Thus, photogenerated electrons and holes with strong oxidizing and deoxidizing properties are obtained, which can react with the cell wall, cell membrane, and components in cell and destroy them directly when Er3+Yb3+-Fe3+-TiO2 adhered to the surface of bacteria. At the same time, the photogenerated electrons and holes can combine with oxygen in water or H2O and OH- absorbed on the surface of Er3+-Yb3+-Fe3+-TiO2 and produce O2- · , · OOH, H2O2, and active hydroxyl radical. These groups with very strong oxidizing power can damage bacteria in suspension even though they do not contact with Er3+-Yb3+-Fe3+-TiO2. There is another possible explanation. If the excited state electrons of Er3+ are captured before they return to the ground state, the upconversion emission will disappear. Fe3+ ions have an ability to capture and transfer electrons. The energy level 2 H11/2 of Er3+ is almost parallel to the intermediate energy level introduced by Fe3+ doping. It is very convenient for Fe3+ to trap the excited state electrons of Er3+ and transfer them to another acceptor such as O2 on the surface of TiO2. This active oxygen can change to superoxide radical or hydroxyl radical with very strong oxidizing power. They can degrade organic compounds completely.

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Figure 9. Pictures of survival Aeromonas hydrophila cells at different irradiation times for Er3+-Yb3+-Fe3+-TiO2 calcined at 1050 °C under 980 nm excitation.

Figure 10. Possible energy transfer mechanism of Er3+-Yb3+-Fe3+-TiO2 under 980 nm excitation.

In addition, the distortion and oxygen vacancy of the TiO2 lattice caused by the doping ions Er3+, Yb3+, and Fe3+ are beneficial to not only the generation of more Ti3+ oxidation centers on the surface of TiO2 but also the transition of photoelectrons and photoholes. These distortions can effectively prevent the recombination of photoelectrons and photoholes and increase the active specific surface area. At the same time, more oxygen in water can be absorbed to the surface of TiO2 with the increase of oxygen vacancy. As a result, the photocatalytic antibacterial activity of samples is improved and enhanced greatly. 5. Conclusions A series of Er3+-Yb3+-Fe3+-TiO2 samples have been obtained by the sol-gel method. The effects of doping rare earth ions and transition metal ions on the phase structure of TiO2 are different due to their different ionic radius and position in the lattice of TiO2. The phase transition of TiO2 from anatase to

rutile as the calcination temperature increases can be prevented by doping rare earth ions Er3+ and Yb3+, which entered the lattice space of TiO2 and caused the new pyrochlore phase generation. The absorption property of TiO2 including absorption band edges and band gap energy can be changed greatly by doping Fe3+ ions because of the substitution of Fe3+ to Ti4+ in the lattice. The green and red upconversion luminescence has been found in Er3+-Yb3+-TiO2, but this emission can be quenched by Fe3+ ion doping and phase structure changing. We use a novel near-infrared antibacterial material, Er3+-Yb3+-Fe3+TiO2 nanopowder, to kill Aeromonas hydrophia. The maximal bactericidal rate of 65% is obtained under 980 nm laser irradiation in 90 min. According to our photocatalytic antibacterial experiment, we proposed the possible energy transfer mechanism of Er3+-Yb3-Fe3+-TiO2. The electrons on the valence band of TiO2 can be excited by green or red upconversion emission of Er3+ to the conduction band of TiO2 via intermediate energy levels derived from Fe3+. The bacteria absorbed on the

A Novel Near-Infrared Antibacterial Material surface of Er3+-Yb3+-Fe3+-TiO2 are oxidized or deoxidized by photoholes and photoelectrons produced by this multistep excitation. The ability to capture and transfer electrons of Fe3+ and the lattice distortion caused by doping ions are also important to the photocatalytic antibacterial activity of Er3+Yb3+-Fe3+-TiO2. The work in this paper opens a new application field of nano-TiO2 doped with rare earth and transition metal ions on the photocatalystic antibacterial process by near-infrared radiation but not ultraviolet light. Acknowledgment. This work has been financially supported by the Science and Technology Foundation of Jilin Province (20090593), Jilin Environmental Protection Agency (2008-22), and Changchun City (2007GH26) of China. Part of the research was finished in Jilin Academe of Fisheries Science. References and Notes (1) Chen, C. H.; Kelder, E. M.; Schoonman, J. Thin Solid Films 1999, 342, 35. (2) Serpone, N., Pelizzetti, E., Eds. Photocatalysis-Fundamentals and Applications; Wiley Interscience: New York, 1989. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Tayade, R. J.; Kulkarni, R. G.; Jasra, R. V. Ind. Eng. Chem. Res. 2006, 45, 5231. (5) Cai, R.; Kubota, Y.; Shuin, T.; Sakai, H.; Hashimoto, K.; Fujishima, A. Cancer Res. 1992, 52, 2346.

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13669 (6) Zhang, A. P.; Sun, Y. P. World J. Gastroenterol. 2004, 10, 3191. (7) Zhang, X. W.; Lei, L. C. Mater. Lett. 2008, 62, 895. (8) Liang, C. H.; Hou, M. F.; Zhou, S. G.; Li, F. B.; Liu, C. S.; Liu, T. X.; Gao, Y. X.; Wang, X. G.; Lu, J. L. J. Hazard. Mater. 2006, 138, 471. (9) Niishiro, R.; Kato, H.; Kudo, A. Phys. Chem. Chem. Phys. 2005, 7, 2241. (10) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffman, M. R. J. Phys. Chem. B 2004, 108, 17269. (11) Choi, J.; Park, H.; Hoffmann, M. R. J. Phys. Chem. C 2010, 114, 783. (12) Sivakumar, S.; van Veggel, F. C.; Raudsepp, M. J. Am. Chem. Soc. 2005, 127, 12464. (13) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Ge, Y.; Yang, W. J.; Chen, D. P.; Guo, L. H. Nano Lett. 2004, 4, 2191. (14) Shang, Q. K.; Yu, H.; Kong, X. G.; Wang, H. D.; Wang, X.; Sun, Y. J.; Zang, Y. L.; Zeng, Q. H. J. Lumin. 2008, 128, 1211. (15) Chen, S. Y.; Ting, C. C.; Hsieh, W. F. Thin Solid Films. 2003, 434, 171. (16) Patra, A.; Friend, C. S.; Kapoor, R. P.; Prasad, N. Chem. Mater. 2003, 15, 3650. (17) Pan, C.; Li, Q.; Qiao, Q. D. Sci. Technol. Chem. Ind. 2004, 12, 26. (18) Jing, L. Q.; Sun, X. J.; Xin, B. F.; Wang, B. Q.; Cai, W. M.; Fu, H. G. J. Solid State Chem. 2004, 177, 3375. (19) Ghosh, S. K.; Vasudevan, A. K.; Rao, P. P.; Warrier, K. G. Br. Ceram. Trans. 2001, 100, 151. (20) Sibu, C. P.; Kumar, S. R.; Mukundan, P.; Warrier, K. G. K. Chem. Mater. 2002, 14, 2876. (21) Borgarello, E.; Kiwi, J.; Gratzel, M. E.; Pelizzetti, M.; Visca, M. J. Am. Chem. Soc. 1982, 104, 2996.

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