ZrO2-Modified Mesoporous Nanocrystalline TiO2-xNx as Efficient

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Environ. Sci. Technol. 2006, 40, 2369-2374

ZrO2-Modified Mesoporous Nanocrystalline TiO2-xNx as Efficient Visible Light Photocatalysts X I N C H E N W A N G , * ,† J I M M Y C . Y U , * ,‡ YILIN CHEN,† LING WU,† AND XIANZHI FU† Research Institute of Photocatalysis, College of Chemistry & Chemical Engineering, Fuzhou University, Fuzhou 350002, China, and Department of Chemistry and Environmental Science Programme, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong

Mesoporous nanocrystalline TiO2-xNx and TiO2-xNx/ZrO2 visible-light photocatalysts have been prepared by a solgel method. The photocatalysts were characterized by XRD, N2 adsorption-desorption, TEM, XPS, UV/Vis, and IR spectroscopy. The photocatalytic activity of the samples was evaluated by the decomposition of ethylene in air under visible light (λ > 450 nm) illumination. Results revealed that nitrogen was doped into the lattice of TiO2 by the thermal treatment of NH3-adsorbed TiO2 hydrous gels, converting the TiO2 into a visible-light responsive catalyst. The introduction of ZrO2 into TiO2-xNx considerably inhibits the undesirable crystal growth during calcination. Consequently, the ZrO2modified TiO2-xNx displays higher porosity, higher specific surface area, and an improved thermal stability over the corresponding unmodified TiO2-xNx samples.

2. Experimental Section

1. Introduction Photocatalysts absorb light to initiate chemical reactions for generating hydrogen gas from water splitting, or for decomposing harmful environmental contaminants. Numerous photocatalysts have been proposed, but most of them (e.g., TiO2, ZnO, SnO2) can only be activated by ultraviolet irradiation (1). As a result, the abundant visible light in solar spectrum or artificial light sources cannot be utilized. The development of visible-light photocatalysts, therefore, has become one of the most important topics in photocatalysis research (2). The conversion of UV-active semiconductors into visiblelight photocatalysts by substitutional doping of framework heteroatoms is one of the major approaches to exploit visiblelight photocatalysts. A number of visible-light photocatalysts have been developed including V-, Fe-, or Mn-doped TiO2 (3), TiO2-xYx (Y ) N, C, S, or B) (4), N-doped Ta2O5 (5), and Sm2Ti2O5S2 (6). Calcination of the photocatalysts at high temperatures is indispensable for crystallization and for achieving effective doping of cations/anions in the lattices of photocatalysts (particularly in the case of preparing solidsolution visible-light photocatalysts). Unfortunately, such a high temperature treatment often leads to a loss of surface area due to the grain growth. Hence, the photocatalysts retain very low specific surface area after calcination, greatly reducing their light-harvesting capability. * Address correspondence to either author. E-mail: xcwang@ fzu.edu.cn (X.W.); [email protected] (J.C.Y.). † Fuzhou University. ‡ The Chinese University of Hong Kong. 10.1021/es052000a CCC: $33.50 Published on Web 02/25/2006

Incorporating pores/cavities in bulk materials is a common approach to fabricate porous materials with a large surface-to-volume ratio. Methods for the preparation of porous materials include sol-gel (7), membrane-templated (8), and surfactant-templated syntheses (9). Pore and particle sizes in the order of nanometers can be routinely obtained in these syntheses. Such porous architectures with large surface area and interwoven porous network would improve the photoabsorption and the mass-transfer of materials. For instance, macro/mesoporous TiO2 photocatalysts were proven to be much more active than nonporous TiO2 (10). However, the integrity of the porous architecture is difficult to maintain if a catalyst is subsequently doped with heteroatoms at elevated temperatures. The porous framework often collapses when a catalyst is sintered at high temperatures (10). The use of structural stabilizers to promote the antisintering properties of materials has been broadly employed in the production of porous catalysts with sufficient thermomechanical strength for high-temperature applications, such as treating automotive exhaust (11). A similar preparation approach has also been adopted in the fabrication of mixed metal oxide photocatalysts, including TiO2/SiO2, TiO2/ZrO2, and TiO2/Al2O3 (12). This study investigated the role of a potential promoter (ZrO2) in enhancing a visible-light photocatalyst (TiO2-xNx) for the oxidation of gaseous organic compounds. The nitrogen-doped photocatalysts are synthesized by reacting amorphous metal oxide xerogels with an ammonia solution, followed by calcination of the products. The calcination temperature could be as low as 400 °C, which is much lower than the >500 °C required for the conventional nitridation of crystalline TiO2 with NH3 gas (13). Moreover, this solutionbased nitridation is a lot safer to implement, as it avoids the use of a toxic NH3 gas.

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Preparation of Catalysts. TiO2/ZrO2 composite xerogels (Zr/Ti molar ratio ) 0.08) were prepared according to the literature (14). The xerogels were treated with an ammonia solution (37%), followed by calcination at 400-600 °C for 4 h. This converted the white xerogels into yellow solids. As a comparison, pure TiO2 and ZrO2 xerogels were also nitridated by using this method. The nitridation product of TiO2 was yellow and was denoted as TiO2-xNx, whereas for the ZrO2 the product was white. Since the amount of ZrO2 was small in the TiO2/ZrO2 hybrid system, the nitridation product of TiO2/ZrO2 was termed as TiO2-xNx/ZrO2 for simplicity. Characterization. X-ray diffraction (XRD) diagrams were collected in θ-θ mode using a Bruker D8 Advance X-ray diffractometer (Cu KR1 irradiation, λ ) 1.5406 Å). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were recorded on a JEOL 2010F microscope. Nitrogen adsorption-desorption isotherms were collected at 77 K using Micromeritics ASAP 2010 equipment. UV-vis spectra were recorded on a Varian Cary 500 Scan UV-visible system. FT-IR spectra on pellets of the samples mixed with KBr were recorded on a Nicolet Magna 560 FT-IR spectrometer. X-ray photoelectron spectra (XPS) were recorded on a PHI Quantum 2000 XPS System with a monochromatic Al KR source and a charge neutralizer; all the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Activity Testing. The catalyst (0.28 g) was packed into a 30 × 15 × 2 mm fixed bed plane reactor operated in a singlepass mode. A 450 W high-pressure mercury lamp (λ > 300 nm) with a 450 nm cutoff filter was used as a visible light VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. UV/vis absorption spectra of the TiO2-xNx and TiO2-xNx/ ZrO2 photocatalysts calcined at 400, 500, and 600 °C. source. The temperature of the reactor under illumination was 30 °C. Ethylene diluted in water-saturated zero air (12) was used to afford a reactant stream (flow rate: 20 mL/min). The initial concentrations of ethylene and carbon dioxide in the stream were 227 and 0 ppm, respectively. Analysis of the reactor effluent was conducted with a Hewlett-Packard 6890 gas chromatograph (12). Fresh catalyst was used for each run, and ethylene was found to be thermally stable in the reactor without illumination.

3. Results and Discussion Figure 1 shows the UV-visible diffuse reflectance spectra of the TiO2, TiO2-xNx, and TiO2-xNx/ZrO2 samples. Unlike the pure TiO2, TiO2-xNx and TiO2-xNx/ZrO2 have extended absorption bands to the visible light spectrum. Since the nitrided ZrO2 absorbs only ultraviolet light (Figure 1b), the visible light absorption in the TiO2-xNx and TiO2-xNx/ZrO2 samples must originate from doping of nitrogen into titania (13). It should be noted that the visible-light absorption decreases for both TiO2-xNx and TiO2-xNx/ZrO2 when they are sintered at 500 °C. However, the decrease in absorption is more pronounced for TiO2-xNx than for TiO2-xNx/ZrO2. The addition of ZrO2 obviously helps maintain a high degree of visible-light absorption during calcination at 500 °C. Figure 1b also shows the UV-visible spectrum of an ammonia-treated ZrO2 sample. The absorption edge of the nitrided ZrO2 is at ∼300 nm, corresponding to a band gap energy of ∼4.3 eV. This value is smaller than that (5.0 eV) of pure ZrO2, suggesting the doping of nitrogen into zirconia 2370

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(15). The composition of the nitrided TiO2/ZrO2 is therefore expressed as TiO2-xNx/ZrO2-yNy. Because of the small amount of ZrO2 in the sample, the formula TiO2-xNx/ZrO2 is used in this paper. The N2-sorption isotherms (Figure S1, Supporting Information) of the TiO2-xNx and TiO2-xNx/ZrO2 samples show that, except for TiO2-xNx-600, all isotherms exhibit stepwise adsorption and desorption (type IV isotherms), indicative of a solid mesoporous structure (16). The corresponding BJH pore-size distribution plots show that the TiO2-xNx and TiO2-xNx/ZrO2 samples possess a mean pore diameter of ∼3.5 nm with a narrow distribution (fwhm ) 0.5 nm). These results suggest that the TiO2-xNx and TiO2-xNx/ZrO2 are typical mesoporous materials. Further TEM observations (Figure 3a) reveal that the mesoporous structure is a honeycomb structure. Such architecture is particularly useful as heterogeneous photocatalysts because it offers additional transport routes for both reactants and light. The specific surface area of the mesoporous TiO2-xNx400 was found to be 145 m2/g (Table S1, Supporting Information). After sintering at 500 °C, the surface area decreased drastically to 23 m2/g. This was due to a partial collapse of the porous framework. Further heating at 600 °C led to the complete collapse of the mesoporous structure. The zirconia-modified TiO2-xNx samples, however, had significantly higher surface area than the corresponding TiO2-xNx. The specific surface area of TiO2-xNx/ZrO2-400 was 220 m2/g. It remained as large as 185 and 130 m2/g even after calcination at 500 and 600 °C, respectively. It has been reported that the presence of secondary phase zirconia particles in hydrous transition metal oxides could restrict advancing of grain boundaries and densification at elevated temperatures by providing dissimilar boundaries (11), giving rise to a high chemical interface in the resultant material after calcination. The modified samples preserved the high porosities of the systems even after thermal treatment. The zirconia-promoted TiO2-xNx retained their porosity of ∼40% virtually irrespective of the sintering temperature (Table S1, Supporting Information). However, for the TiO2-xNx, the porosity decreased considerably from 39.5% to 0.3% when the temperature was increased from 400 to 600 °C. The addition of zirconia also inhibits phase transformation and crystal growth in the solids. Figure 2a and b show the XRD patterns of the TiO2-xNx and TiO2-xNx/ZrO2 samples fired at different temperatures. At the sintering temperature of 400 °C, the two samples display dominantly the anatase crystal phase. The broad peaks of the 101 reflections confirm the nanocrystalline nature of anatase in the two samples. The crystal sizes determined from peak broadening suggest that the anatase crystallites are 7 and 5 nm for TiO2-xNx-400 and TiO2-xNx/ZrO2-400, respectively. The anatase in the TiO2-xNx samples undergoes anatase-to-rutile phase transformation and subsequent crystal growth beginning at 500 °C. This transformation is completed at 600 °C, where the catalyst is 100% rutile. However, such a phase transformation does not occur for the ZrO2-midified TiO2-xNx/ZrO2 samples. No rutile phase is observed in the modified system even when the system is heated at 600 °C. The crystal size of anatase in the modified systems is well maintained at ca. 6 nm virtually regardless of sintering temperature. These results are promising since nanocrystalline anatase has long been considered as the more photoactive form of titania (1). Figure 3a and b are TEM images of TiO2-xNx and TiO2-xNx/ ZrO2 samples sintered at 500 °C. A honeycomb porous structure with fine particulate morphology in three dimensions is observed in the image of TiO2-xNx/ZrO2-500 (Figure 3a), indicating that the mesoporosity is partly due to the interparticle porosity. The electron diffraction (ED) patterns of the selected area on TiO2-xNx/ZrO2-500 show several weak Debye-Scherrer rings (Figure 3a, inset), corresponding to

FIGURE 2. XRD patterns of (a) TiO2-xNx and (b) TiO2-xNx/ZrO2 catalysts sintered at 400, 500, and 600 °C.

FIGURE 3. TEM images of (a) TiO2-xNx/ZrO2-500 and (b) TiO2-xNx500, as well as HRTEM images of (c) TiO2-xNx/ZrO2-500 and (d) TiO2-xNx-500. The insets in (a) and (b) are the electron diffraction patterns of the corresponding samples. reflections of the polycrystalline anatase TiO2 nanoparticles. For the TiO2-xNx-500, no honeycomb structure was observed (Figure 3b), and larger particles were produced. The ED patterns (inset, Figure 3b) show both strong DebyeScherrer rings and complicated bright spots, indicating the coexistence of large anatase and rutile crystallites. These results reveal that the addition of ZrO2 stabilizes the porous structure and inhibited the anatase-to-rutile phase transformation, in good agreement with XRD and N2-sorption measurements. Figure 3c and d are the HRTEM images of TiO2-xNx and TiO2-xNx/ZrO2 samples sintered at 500 °C. The image shows that the TiO2 nanoparticles in TiO2-xNx/ZrO2-500 were aggregated and polycrystalline in nature (Figure 3c). The grain size can be confirmed to be about 6 nm. Furthermore, the lattice fringes of d ) 3.5 Å match that of the (101) crystallographic planes of anatase-TiO2, consistent with the XRD analysis. In the absence of ZrO2, the anatase-TiO2 would be converted to rutile-TiO2, as shown in Figure 2b. In addition, the crystal size of TiO2 in the TiO2-xNx/ZrO2-500 is much larger than that in the TiO2-xNx-500. This provides direct evidence that the addition of ZrO2 can effectively inhibit the excessive crystal growth of nanocrystalline TiO2 during

thermal processing. The excessive crystallite growth is believed to be the main reason that accounts for the destruction of the mesopore structures in the surfactanttemplated metal oxide gels upon thermal treatment (18). Furthermore, Figure 3d shows the presence of well-ordered lattices of TiO2, whereas for the ZrO2-modifed sample lessordered lattices with characteristic “chevron-like” appearance (Figure 3c, inset) are observed. The appearance may be attributed to the substitution of oxygen atoms in metal oxides by heteroatoms (e.g., N or C). Similar “chevron-like” structures induced by heteroatom doping have been reported for the carbon-doped molybdenum oxides (19). The surface electronic state of nitrogen in the samples was examined by XPS. Figure 4 shows the spectrum for the N1s region. For the sample sintered at 400 °C, the N1s peaks for both TiO2-xNx and TiO2-xNx/ZrO2 are centered at 400 eV. This is greater than the typical binding energy of 396.9 eV in TiN (20). This can be attributed to the 1s electron binding energy of the N atom in the environment of O-Ti-N. When nitrogen replaces the oxygen in the O-Ti-O structure, the electron density around N is reduced. Thus, the N1s binding energy in an O-Ti-N environment is higher than that in an N-Ti-N environment. The peaks corresponding to N1s in NH3 (398.8 eV) or nitrates (e.g. 408 eV) were not observed, indicating the effective substitutional doping of nitrogen in TiO2 matrix (21). At an elevated temperature of 500 °C, the intensity of the N1s peak decreases significantly for the TiO2-xNx, indicating the loss of N-species during the high-temperature sintering. Additionally, the peak is much broader than that of TiO2-xNx400. This implies that some of the nitrogen doped in TiO2 is converted to oxidized nitrogen moieties (e.g., NO+, NO2-, and NO3-). The formation of these moieties could be due to the fact that the sample was sintered at the presence of air. Such conversion was much more significant at 600 °C, as evident by shifting of the N1s peak to ∼405 eV (a typical BE of NO2- on a metal oxynitride surface) (22). Because only the doped-nitrogen (N-Ti-O) can extend the optical absorption of TiO2 to visible light region, the decrease in the amount of lattice-nitrogen (Table S1, Supporting Information) in the TiO2-xNx sample led to the decrease in the visible-light absorption (Figure 1a). Similarly, when the ZrO2-modified TiO2-xNx was sintered at 500 °C the N1s peak was also decreased and broadened, but the changes were slight as compared with those in TiO2-xNx-500. Apparently, the addition of ZrO2 can effectively stabilize the nitrogen in the TiO2 matrix. This was also confirmed by the UV-visible results in Figure 1b. Upon further heating of the TiO2-xNx/ZrO2 sample at 600 °C, the N1s peak at 399.8 eV still remained, but additional weak peaks VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. XPS spectra of TiO2-xNx and TiO2-xNx/ZrO2 catalysts heated at 400, 500, and 600 °C.

FIGURE 5. Visible-light-induced catalytic conversion of ethylene on TiO2-xNx and TiO2-xNx/ZrO2 catalysts sintered at 400, 500, and 600 °C at ∼405 and ∼408 eV appeared. Fairbrother et al. recently examined surface nitrogen species on an iron nitirde annealed at the presence of oxygen by XPS measurements. They confirmed the formation of NO+, NO2-, and NO3- on the surface of iron nitirdes (22). We therefore conclude that the additional peaks at ∼405 and ∼408 eV come from the oxidized nitrogen species of NO2- and NO3-, respectively. Together, the optical, textural, and surface electronic state characterizations confirm that the thermal treatment of ammonia-adsorbed TiO2 and TiO2/ZrO2 xerogels has incorporated nitrogen into the mesoporous TiO2 nanoparticles, giving rise to the clear red-shift in the optical response of TiO2. The introduction of ZrO2 considerably stabilizes the porous structure of the N-doped TiO2 during thermal treatment, rending the materials a large surface-to-volume ratio. Additionally, the modification also stabilizes the nitrogen in the N-Ti-O structure. These are desirable structural and chemical features for N-doped TiO2 photocatalysts. Figure 5 shows the visible-light-driven photocatalytic activity of the TiO2-xNx and TiO2-xNx/ZrO2 samples calcined 2372

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at 400, 500, and 600 °C. As expected, the sintering temperature affects the photocatalytic activity of the TiO2-xNx toward oxidizing ethylene in gas-phase. The activity of the TiO2-xNx is initially quite high and decreases rapidly with increasing calcination temperature. At the sintering temperature of 400 °C, the conversion of C2H4 on TiO2-xNx is 28%, but it drops to 7% at 500 °C. Further heating sample at 600 °C would stop the conversion all together. However, when ZrO2 is added into the TiO2-xNx, the effect of sintering temperature on the activity was distinctively different. At the sintering temperature of 400 °C, the conversion of C2H4 on TiO2-xNx/ZrO2 is 32% higher than that of TiO2-xNx-400. Interestingly, the conversion remains about the same (30%) when the temperature is increased to 500 °C. Further heating to 600 °C would only drop the conversion rate to 15%. The measurement of CO2 produced from the reaction also confirms the beneficial effect of ZrO2 modification. The amount of CO2 produced from the reaction on TiO2-xNx-500 was 25 ppm at the steady state, whereas on the TiO2-xNx/ ZrO2-500 sample it is greatly increased to 95 ppm (Figure S2, Supporting Information). The high photocatalytic perfor-

mances of ZrO2-modified TiO2-xNx can be well accounted for by the pore-wall structure and surface chemical properties of the samples. It is known that gas-solid heterogeneous photocatalysis is a surface-based process, and therefore large surface has positive effects on such processes (1). The large surface area provides more surface sites for the adsorption of reactants molecules and, ultimately, making the photocatalytic process more efficient. The mixture of ZrO2 secondary phase into porous TiO2-xNx can drastically inhibit the crystal growth of the catalyst during calcination and, therefore, the porous structure with high surface area can be preserved. Additionally, the preservation of interconnected porous nanonetworks facilitates facile transport of small molecules through the interior space. These partly contribute to the high visible light photocatalytic activity of TiO2-xNx/ZrO2500 compared with TiO2-xNx-500. Moreover, the porous structure is also favorable for the harvesting of visible light due to enlarged surface area and multiple scattering within the solid framework (23). It is also known that excess crystal growth during calcination causes a loss in the surface area of hydrous metal oxide gels. Such growth triggers extensive condensation of surface hydroxyl groups between the original particles, leading to the decrease in surface hydroxyl groups. IR spectra (Figure S3, Supporting Information) show that the TiO2-xNx500 sample possesses weaker adsorption bands of hydroxyl groups than TiO2-xNx/ZrO2-500 does. In a photocatalytic process, the hydroxyl groups capture holes on illuminated photocatalysts and form active hydroxyl radicals, which can then oxidize adsorbed molecules. The trapping of holes can also stabilize the electron-hole pair, prohibiting their undesirable recombination. The beneficial effect of surface hydroxyl groups on heterogeneous photocatalysis has been reported previously (10, 24). Another reason for the high activity of TiO2-xNx/ZrO2-500 is the ability to preserve nitrogen in its lattice. It is known that nitrogen atoms are doped in the surface sublayer of TiO2 nanoparticles (13). This is confirmed by the absence of bulk XRD peaks for TiN in our experiment (Figure 2). The loss of lattice-nitrogen from TiO2-xNx-500 is inevitable during the extensive crystallization and subsequent crystal growth of the photocatalyst. As shown in Scheme S1 in the Supporting Information, the addition of ZrO2 restrains grain growth and thus preserves the lattice nitrogen. So, the amount of latticenitrogen on the surface of TiO2-xNx/ZrO2-500 was larger than that on the TiO2-xNx-500. This was also confirmed by the stronger visible light absorption of the former compared with the latter (Figure 1). Inclusion of N in the lattice of TiO2 adds an impurity level within the band gap of TiO2 and causes the visible-light activity (13). In addition, the oxidation of latticenitrogen during thermal treatment at high temperatures also caused the loss of lattice-nitrogen, as confirmed by the appearance of additional XPS peaks with higher binding energies (Figure 4b) on TiO2-xNx/ZrO2-600.

Acknowledgments The work was financially supported by the National Natural Science Foundation of China (20133010, 20273014, 20473018, 20571015, 20573020, and 20537010) and the Research Grants Council of the Hong Kong Special Administrative Region (No. 402904). X.W. and X.F. thank the Ming River Scholars Program of Fujian Province, P. R. China.

Supporting Information Available Figures S1-S3, Scheme S1, and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review October 10, 2005. Revised manuscript received January 25, 2006. Accepted January 30, 2006. ES052000A