Enhanced Visible-Light-Induced Photocatalytic Disinfection of E. coli

Since only about 3∼4% of the solar spectrum can be utilized by pure TiO2, it is of great ... on a Cary 500 UV/vis/NIR spectrophotometer (Varian, Inc...
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Environ. Sci. Technol. 2007, 41, 5050-5056

Enhanced Visible-Light-Induced Photocatalytic Disinfection of E. coli by Carbon-Sensitized Nitrogen-Doped Titanium Oxide Q I L I , † R O N G C A I X I E , ‡,⊥ Y I N W A I L I , † ERIC A. MINTZ,§ AND J I A N K U S H A N G †,‡,* Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China 110016, and High Performance Polymers and Composites Center, Clark Atlanta University, Atlanta, Georgia 30314

Nitrogen-doped titanium oxide (TiON) nanoparticle photocatalysts were synthesized by a sol-gel process, for disinfection using E. coli as target bacteria. Our work shows that the calcination atmosphere has strong effects on the composition, structure, optical, and antimicrobial properties of TiON nanoparticles. Powders calcinated in a flow of N2 atmosphere (C-TiON) contain free carbon residue and demonstrate different structures and properties compared to the TiON powders calcinated in air. Disinfection experiments on Escherichia coli indicate that C-TiON composite photocatalyst has a much better photocatalytic activity than pure TiON photocatalyst under visible light illumination. The enhanced photocatalytic activity is related to stronger visible light absorption of the carbon-sensitized TiON.

Introduction Since the discovery of photoelectrochemical splitting of water on n-TiO2 electrodes by Fujishima and Honda (1) in 1972, semiconductor-based materials have been investigated extensively as photocatalysts for both solar energy conversion (2-5), and environmental applications (6-12). Due to its high chemical stability, good photoactivity, relatively low cost, and nontoxicity, TiO2 has appeared as a leading candidate for industrial use (13). However, its photocatalytic capability is limited only under ultraviolet (UV) light (wavelength, λ, < 400 nm), which provides sufficient excitation of electrons across its relatively wide band gap of 3.2 eV in the TiO2 anatase crystalline phase. Since only about 3∼4% of the solar spectrum can be utilized by pure TiO2, it is of great interest to develop photocatalysts that can yield high reactivity under visible light so that a greater portion of the solar spectrum may be used to provide photocatalytic capability. Several approaches have been developed to extend the absorption band-edge of TiO2 from UV to visible light region, including doping transition metals into TiO2 (14-16), or * Corresponding author e-mail: [email protected]. † University of Illinois. ‡ Chinese Academy of Sciences. § Clark Atlanta University. ⊥ Formerly with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign. 5050

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forming reduced TiOx photocatalysts (17-18). However, for lack of reproducibility and chemical stability, improved visible light photocatalysts are needed for industrial applications. Recently, experiments demonstrated that anionic nonmetal dopants, such as nitrogen (19-24), carbon (25-32), sulfur (33-35), or fluorine (36), can extend the photocatalytic activity of TiO2 into the visible-light region at an improved stability, photocatalytic efficiency, and ease of the doping process (13). The anionic effect was also demonstrated by a recent theoretical investigation (37). Subsequently, the bactericidal effect of anion-doped TiO2 under visible light conditions was demonstrated by Yu et al. of sulfur-doped TiO2 on gram-positive Micrococcus lylae (38). In our previous work (39), nitrogen-doped TiO2 thin films were successfully prepared by ion-beam assisted deposition (IBAD). A characteristic decreasing trend in band gap values of these films was observed within a certain range of increasing N concentrations, and their absorption band-edges extended into visible light region. The photocatalytic behavior of these films was demonstrated on the disinfection of a model gramnegative bacteria Escherichia coli under visible light condition (40). After 1 h illumination, the survival ratio of E. coli was reduced to around 40%. In this article, we synthesized nitrogen-doped titanium oxide (TiON) nanoparticle photocatalysts by a sol-gel process and investigated their antimicrobial behavior. Our work found that the calcination atmosphere has strong effects on the composition, structure, optical, and disinfecting properties of TiON nanoparticles. The powders obtained by calcinating in a flow of N2 atmosphere contain free carbon residue and have different properties compared with TiON powders. This carbon-sensitized nitrogen-doped titanium oxide (C-TiON) composite photocatalyst demonstrates better disinfection effect than TiON photocatalyst on E. coli. After 1 h illumination, the survival ratio of E. coli was around 1% with the treatment of TiON, while it was reduced by an order of magnitude to around 0.1% with the treatment of C-TiON. Thus, the C-TiON composite photocatalyst has a greater potential to provide adequate disinfection effects on microorganism under visible light illumination.

Experimental Section Chemicals and Materials. Titanium tetraisopropoxide (TTIP, 97%, Sigma-Aldrich, St. Louis, MO) and tetramethylammonium hydroxide (TMA, 25 wt % in methanol, Sigma-Aldrich, St. Louis, MO) were used in this study as precursors to provide titanium and nitrogen sources, respectively. Ethyl alcohol (EtOH, 100%, AAPER Alcohol and Chemical Co., Shelbyville, KY) was used as the solvent. Sol-Gel Process. The preparation of TiON precursor was completed at room temperature in a sol-gel process as following. First, TMA was dissolved in EtOH at a mol ratio at 1:10. The solution was stirred magnetically for 5 min, and then TTIP was added into the solution with the TMA:TTIP mol ratio at 1:5. The mixture was loosely covered and stirring continued until a homogeneous gel formed. The hydrolysis of precursors was initiated by exposure to the moisture in air. The gel was aged in air for several days to allow further hydrolysis and drying. Then, the xerogel was crushed into fine powders and calcinated at various temperatures in either air or a flow of nitrogen for 5 h to obtain the desired nanoparticle photoctalysts. Nanoparticle Photocatalyst Characterizations. BET surface area was measured by N2 adsorption-desorption isotherm with an Autosorb-1 series surface area and pore size analyzers (Quantachrome Instruments, Boynton Beach, 10.1021/es062753c CCC: $37.00

 2007 American Chemical Society Published on Web 06/12/2007

FL). X-ray photoelectron spectroscopy (XPS) measurements were performed with Physical Electronics PHI 5400 X-ray photoelectron spectrometer (Perkin-Elmer Corporation, Eden Prairie, MN) with an Mg K anode (1253.6 eV photon energy, 15 kV, 300 W) at a takeoff angle of 45°. The freecarbon content in these powders was determined by elemental analysis (CHN) with CE 440 (Exeter Analytical, Inc., N. Chelmsford, MA). XRD study was performed with a Rigaku D-Max X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu ΚR (0.15418 nm) radiation at 45 kV and 20 mA to examine TiON powder’s crystal structure and determine the crystallite phase and size. Transmission electron microscopy (TEM) was used to examine the morphology of the powder on a JEOL 2010LaB6 transmission electron microscope (JEOL Ltd., Tokyo, Japan) operated at 200 kV. The UV-vis spectra of these powders were measured on a Cary 500 UV/vis/NIR spectrophotometer (Varian, Inc., Palo Alto, CA). Photocatalytic Inactivation of Bacteria Escherichia coli. Wild type Escherichia coli AN 387 were used for photocatalytic inactivation experiment (40). They were inoculated each time from an agar plate into a 4 mL Luria-Bertani medium. The cells were grown aerobically on a rotary shaker at 37 °C for 18 h. Cells were harvested from overnight culture by centrifugation for 5 min at 4 °C and 6000 rpm, and washed twice using a buffer solution (0.05 M KH2PO4 and 0.05 M K2HPO4, pH 7.0). The cells were resuspended and diluted to a cell suspension (ca.107 cfu/mL) in buffer prior to the use for photocatalytic inactivation. All solid or liquid materials have been autoclaved for 30 min at 121 °C before use. At starting time, aliquot of 3 mL E. coli cell suspension was pipetted onto a sterile 60 × 15 mm petri dish, with the nanoparticle photocatalysts placed in the bottom. Photocatalysts TiON and C-TiON were mixed with the E. coli suspension at a fixed concentration of 1 mg/mL. TiO2 (Degussa P25) powder and carbon black powder were also used in this study for comparison purpose under the same experimental conditions. The covered petri dishes were illuminated by a metal halogen desk lamp which has a glass filter. Zero light intensity was detected below 400 nm. The light intensity striking the cell suspensions was at ca. 1.0 mW/cm2, as measured by a Multi-Sense MS-100 optical radiometer (UVP, Inc., Upland, CA). Under illumination, the suspension temperature increased from room temperature to no more than 40 °C, which did not demonstrate significant influence on the viability of E. coli cells. The suspension was only stirred at the moment of sample withdrawal. At regular time intervals, 20 µL of aliquots of the irradiated cell suspensions were withdrawn in sequence. After appropriate dilutions in buffer solution, aliquots of 20 µL together with 2.5 mL top agar was spread onto an agar medium plate and incubated at 37 °C for 18 h. The number of viable cells in terms of colony-forming units was counted. Analyses were in duplicates and control runs were carried out each time under the same illumination conditions, but without any photocatalytic materials. Tests were also performed in the dark in presence of the photocatalyst for comparison.

Results and Discussion Composition of Sol-Gel TiON Nanoparticle Powders. XPS investigation was first conducted to determine the composition of obtained TiON nanoparticle powders. Figure 1a shows two representative XPS survey spectra of TiON powders calcinated at 500 °C in air and a flow of N2, respectively. It confirms the existence of N, O, and Ti in these powders. Multiplex high-resolution scans over N 1s, O 1s, and Ti 2p spectral regions are shown in Figure 1b-d, respectively, which can be used to determine the relative element composition ratio. The N 1s peak shifting from β N 1s peak position at 397

eV (shown in Figure 1b) is caused by the surface adsorbates during the sol-gel process. The present of N below the surface demonstrates the substitution of O atoms by N atoms in anatase TiO2 and slightly smaller N/Ti atomic ratio was observed between the surface and below surface XPS analysis. Details can be found elsewhere in our previous work (41). The N/Ti atomic ratio of these powders is summarized in Figure 2a. It is clear that with the increase of calcination temperature, the N/Ti atomic ratio decreases for samples calcianted both in air and in N2, while the N2 atmosphere shows a beneficial effect on retaining a higher final N concentration in the obtained powders. Due to the widespread of carbon in the environment, its content in TiON powders cannot be estimated from the XPS spectra, although the C 1s peak can be observed clearly in XPS survey spectra (as shown in Figure 1a). Elementary analysis was then carried out on these powders to determine the free carbon concentration in them (shown in Figure 2b). When calcinated in air, the free carbons in the powder can be burned out with the increase of the calcination temperature, leaving only 0.1 wt % residue in the obtained powders at 600 °C. When calcinated in N2, however, the residual carbon concentration in these powders is kept at ∼1.0 wt % when calcinated at 500 °C or above, about 10 times higher than that in powders calcinated at 600 °C in air. We also found that these powders have very different colors. For powders calcinated in air, they change from gray to almost white with the increase of the calcination temperature from 500 to 600 °C. For powders calcinated in N2 atmosphere, however, all three samples have the similar dark color. The only difference between these two sets of powders is that they were calcinated in different atmospheres. When calcinated in air, organics in the xerogel were decomposed in the calcination process and burned out by the reaction with oxygen in the air, which results in only a small carbon residue in the obtained powder. When calcinated in N2, however, the decomposed organics could not be burned out due to the lack of oxygen. They can only be converted to carbon, creating a high residual free carbon content. The free carbon plays a major role in determining the structure and light absorption properties of these powders. Thus, powders calcinated in N2 can be regarded as a novel kind of C-TiON composite photocatalyst. Structure of Sol-Gel TiON Nanoparticle Powders. Figure 3a shows the X-ray diffraction patterns of TiON powders obtained by calcinating xerogels in air at various temperatures. These powders have an anatase-type structure within the temperature range investigated (from 500 to 600 °C), and no rutile phase is observed. With the increase of the calcination temperature, the reflectance intensity increases, indicating a better crystallization. Figure 3b shows the X-ray diffraction patterns of C-TiON powders obtained by calcinating xerogels in a flow of N2. Interestingly, these powders show significantly different crystal structure patterns, compared with those powders calcinated in the air. Although the dominated structure in these powders is still the anatasetype, minor peaks assigned to the rutile-type structure appear when the calcination temperature is at 500 °C or higher. This temperature is lower than the one usually required for rutile phase to appear in TiO2 powders. In a recent study by Lin et al. (31) on uniform carboncovered TiO2, they found that pure TiO2 treated at 600 °C has an anatase-type structure, while rutile-type structure appears after being calcinated at 700 °C. However, for carbon-covered TiO2, no rutile phase appears at both 600 and 700 °C. So they concluded that the presence of carbon on the TiO2 surface inhibits its phase transition. While this conclusion was valid based on their experimental work, it is clear that under our experimental conditions, the presence of free carbon in these powders is the cause of the difference on crystal structure VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. XPS survey spectra (a) and multiplex high-resolution scan over N 1s (b), O 1s (c), and Ti 2p (d) spectral regions associated with TiON calcinated in air at 500 °C for 5h (black line), and TiON calcinated in a flow of N2 at 500 °C for 5h (red line), respectively. patterns between powders calcinated in air and N2. The appearance of rutile phase in powders calcinated in N2 indicates that free carbon promotes the phase transition in the calcination process by acting as a nucleation seed, which is similar to the observation by Rinco´n et al. (32) on sol-gel TiO2 sensitized by nanometric carbon blacks. The crystallite size of these powders can be obtained from the largest XRD peak (appearing at 2θ ∼ 25.5° for anatase phase, and at 2θ ∼ 27.5° for rutile phase) by the Scherrer’s formula (42):

D ) 0.9λ/βcosθ

(1)

where λ is the average wavelength of the X-ray radiation (Cu KR, λ ) 0.15418 nm), β is the line-width at half-maximum peak position, and θ is the diffracting angle. Our calculation indicates that that powders calcinated in air and in N2 differ on their crystallite size (see Table 1 in Supporting Information for details). For powders calcinated in air, their crystallite size shows a steady and significant increase with the increase in calcination temperature. When calcinated in air at 600 °C, it is ∼70% larger than that calcinated in air at 500 °C. For powders calcinated in N2, although their crystallite size also shows an increase with calcination temperature increase, its increase is much smaller compared with that calcinated in air. When calcinated in N2 at 600 °C, it is only 30% larger than that calcinated in N2 at 500 °C for anatase phase, and 15% larger for rutile phase. The crystallite size of powders calcinated in air is much larger than their counterpart calcinated in N2 under the same calcination temperature, 5052

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and the size difference increases with the increase of the clacination temperature. The lattice constants for these powders were also determined from XRD diffraction patterns (see Table 2 in Supporting Information for details). For the tetragonal anatase phase, powders calcinated in air have a shorter a-axis and longer c-axis than powders calcinated in N2, which means that the crystal lattice in powders calcinated in air is “slimmer” than that in powders calcinated in N2. The BET surface specific areas of these powders are ∼55 m2/g, corresponding to an average particle diameter ∼20 nm. Figure 4 shows the TEM image of the sample calcinated in a flow of N2 at 550 °C. TEM observation demonstrates that the sample consists of nanosized particles with nonuniform shapes and that particle size is similar to the results of the BET measurements. Optical Properties of Sol-Gel TiON Nanoparticle Powders. The optical properties of these TiON/C-TiON nanoparticle powders were investigated by the diffuse reflectance spectra measurement. The optical absorbance can be approximated from the reflectance data by the Kubelka-Munk function, as given by eq 2:

F(R) )

(1 - R)2 2R

(2)

where R is the diffuse reflectance (43). Figure 5a shows the light absorbance (in term of KubelkaMunk equivalent absorbance units) of TiON powders ob-

FIGURE 3. X-ray diffraction patterns of TiON powders obtained by (a) calcinating xerogels in air at various temperatures for 5 h, and (b) calcinating xerogels in a flow of N2 at various temperatures for 5 h, respectively. (Note, blue line for calcination temperature 500 °C, red line for 550 °C, and black line for 600 °C.)

FIGURE 2. N/Ti atomic ratio (a) and free carbon weight concentration (b) in obtained powders with various calcination temperature in air (black square), or in a flow of N2 (red square), respectively. tained by calcinating xerogels in air at various temperatures for 5 h, compared with the light absorbance of TiO2 (Degussa P25) powder. P25 shows the characteristic spectrum with the fundamental absorbance stopping edge at 400 nm. TiON powders, however, show a clear shift into the visible light range (>400 nm) as expected. With the decrease of the calcination temperature, which means that N content increases in the powder, more visible light absorbance is observed. To determine the semiconductor band gap, the Tauc Plot (43) ((F(R) × hv)n vs hv) is constructed from Figure 5a and presented in Figure 5b, which shows the linear Tauc region just above the optical absorption edge. For a direct band gap semiconductor, n equals 0.5. Extrapolation of this line to the photon energy axis yields the semiconductor band gap, which is the key indicator of its light harvesting efficiency under sun light illumination. As expected, for Degussa P25 powder its band gap is ∼3.20 eV, which is consistent with the very poor visible light absorbance ability shown in Figure 5a. TiON powders show a smaller band gap, which explains the visible

FIGURE 4. TEM image of powders obtained by calcinating xerogels in a flow of N2 at 550 °C. light absorbance ability they demonstrate. With the increase of the N content (decrease of the calcination temperature from 600 to 500 °C), TiON powder’s band gap decreases (from ∼3.00 to 2.84 eV), which attributes to the better visible light absorbance observed. VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (a) Optical absorbance (in term of Kubelka-Munk equivalent absorbance units) of TiON powders obtained by calcinating xerogels in air at various temperatures for 5 h, compared with the optical absorbance of Degussa P25 powder. (b) Tauc Plot constructed from Figure 4a. Band gap values are determined from the extrapolation of the linear Tauc Region line to the photon energy abscissa. (Note, blue line for calcination temperature 500 °C, red line for 550 °C, black line for 600 °C, and green line for Degussa P25 TiO2 powder.) Figure 6a shows a typical light absorbance spectrum of C-TiON powders obtained by calcinating xerogels in N2, compared with the light absorbance of Degussa P25 powder. In the whole visible light range, this powder shows a huge absorbance, which is similar to what Wang et al. (30) recently reported on samples of multiwalled carbon nanotubes (MWNT) and TiO2 composites. To achieve such a huge absorbance in the whole visible light range, a very high initial MWNT/TiO2 ratio (40%) was used in their study. However, there is only ∼1 wt % free carbon in our powders, indicating a much better sensitization effect for free carbon from pyrolysis than intentionally added from outside sources. For these C-TiON powders, due to the free carbon content, N-doping does not play the major role in determining their optical properties. Figure 6b shows the Tauc Plot constructed 5054

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FIGURE 6. (a) Optical absorbance (in term of Kubelka-Munk equivalent absorbance units) of TiON powders obtained by calcinating xerogels in N2 at 500 °C for 5 h, compared with the optical absorbance of Degussa P25 powder. (b) Tauc Plot constructed from Figure 5a. Band gap values are determined from the extrapolation of the linear Tauc Region line to the photon energy abscissa. (Note, red line for TiO2, and black line for C-TiON.) from Figure 6a. Here n ) 2 was chosen on constructing the Tauc Plot to get the linear Tauc region, which indicates that this C-TiON powder is an indirect band gap semiconductor (43). The band gap determined is ∼1.30 eV. A similar result has recently been reported by Rinco´n et al. (32) on their work of sol-gel TiO2 sensitized by nanometric carbon blacks. Although their reported Tauc Plot curve is not exactly the same shape as our constructed Tauc Plot curve, they found an exact same band gap value as our finding, 1.3 eV. By combining the effect from both nitrogen-doping and carbonsensitization, these C-TiON nanoparticle powders have the potential to work as a photocatalyst as well as or possibly better than results reported recently on C-TiO2 photocatalysts (30-32). Photocatalytic Inactivation of Bacteria Escherichia coli. The photocatalytic activity of both TiON and C-TiON (calcinated at 500 °C) photocatalysts were demonstrated by

FIGURE 7. Survival ratio of E. coli cells vs visible light (>400 nm) illumination time with TiO2 (blue square), carbon black (brown square), TiON (black square), and C-TiON (red square), respectively. The cell suspension had an initial concentration ca. 107 cfu/mL. their bactericidal effect on the viability of E. coli cells. The results are compared to those obtained using a TiO2 (Degussa P25) photocatalyst. From the above analysis, the N2 atmosphere provides two potential benefits on our powder’s photocatalytic disinfection ability. One is the higher residual N content in the obtained photocatalyst, which will contribute to a better visible light absorption and subsequent higher photocatalytic bactericidal efficiency (39-40). The other is the carbon-sensitization effect provided by the remarkable amount of free carbon residue in the C-TiON powder, as demonstrated in the previous reports on various carbonTiO2 photocatalysts (30-32). Photocatalytic inactivation of E. coli was conducted by exposing the cells suspended in buffer solution with various photocatalysts under visible light (λ > 400 nm) for varying time intervals. The survival ratio of E. coli were determined by the ratio of Nt/N0, where N0 and Nt are the numbers of colony-forming units at the initial and each following time interval, respectively. The high free carbon residue in the C-TiON composite photocatalyst can absorb light heavily, which may cause a relatively large heat buildup near its surface, subsequently increasing the media temperature around it. Such local heating at the microscopic level near the carbon particle surface may contribute to the total bactericidal effect. Thus, carbon black powder was also used to investigate the possible heat buildup effect caused by free carbon in C-TiON composite photocatalyst and its consequent effect on the survival of E. coli. Tests in the dark showed no bactericidal effect, indicating that our materials themselves are not toxic to E. coli bacteria cells. The E. coli survival ratio under various treatments is demonstrated in Figure 7. As expected, TiO2 shows no bactericidal effect under visible light illumination due to its relatively wide band gap and the consequent limitation on its photocatalytic capability only under UV light (λ < 400 nm). Carbon black powder shows a very weak bactericidal effect during the whole illumination time. After 1 h illumination, there is still ∼80% E. coli cells survive. Both TiON and C-TiON nanoparticle photocatalysts show a clear bactericidal effect, which is consistent with their decreased band gap and consequently visible light absorption ability. An overall decreasing trend in the survival ratio with an increase of visible light illumination time was observed for E. coli

cells treated by either TiON or C-TiON nanoparticle photocatalysts. After 1 h visible light illumination, ∼1% E. coli survived under TiON treatment, while only ∼0.1% (an order of magnitude less) E. coli survived under C-TiON treatment. E. coli solution under the carbon black powder treatment has the highest temperature increase from room temperature to ∼40 °C during the illumination, but only a very weak bactericidal effect was observed. Compared with carbon black powder, the free carbon amount in C-TiON is much less, which suggests that the heat buildup effect caused by the free carbon will only play a minor role for the bactericidal effect of the C-TiON. Thus, it is clear that the enhanced bactericidal effect of C-TiON is not from the simple addition of the bactericidal effect of TiON and the free carbon. Its better photocatalytic activity under visible light illumination can be attributed to the combination of higher N content and the carbon-sensitization effect from its high free carbon residue. Until now, there was no standard to follow to compare the disinfection efficiency and the experimental conditions differ greatly among various research groups. Compared with some reports on the disinfection of E. coli bacteria using TiO2 and UV light illumination (11-12), our photocatalysts demonstrate comparable disinfection efficiency under visible light illumination. In summary, nitrogen-doped titanium oxide (TiON) nanoparticle powders were synthesized by sol-gel process. A series of studies were carried out on the effects of calcination temperature and atmosphere on the composition, structure, and properties of the TiON nanoparticles. The increase of the calcination temperature promotes the crystallization, while decreasing N-doping content in the obtained powders. When calcinated in air, the TiON powders have the anatasetype phase and the residual carbon content in these powders is small (from 500 to 600 °C). Their light absorbance shifts into the visible light range due to the N-doping. On the other hand, powders made by calcinating in N2 atmosphere show very different properties due to the higher free carbon residue content. Rutile-type phase begins to appear at a relatively low calcination temperature (500 °C). In the whole visible light range, these C-TiON powders show a large absorbance, which cannot be simply attributed to N-doping. The enhanced visible light absorption of the C-TiON composite photocatalyst results in a better photocatalytic activity than TiON photocatalyst under visible light illumination in the photocatalytic inactivation of E. coli.

Acknowledgments We thank Dr. Z. R. Yue in University of Illinois at UrbanaChampaign for the assistance in the BET measurements, and Dr. C. H. Lei in the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign for TEM observations. The support for this study was provided by the Center of Advanced Materials for the Purification of Water with Systems, National Science Foundation, under agreement no. CTS-0120978, and by the National Basic Research Program of China through grant no. 2006CB601201. XPS, XRD and TEM measurements were carried out at the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, which is partially supported by the U.S. Department of Energy under grant DEFG02-91ER45439.

Supporting Information Available Two tables show the crystallite sizes and lattice constants of obtained TiON powders. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review November 18, 2006. Revised manuscript received April 10, 2007. Accepted May 8, 2007. ES062753C